SURFACE PHENOMENA AND COLLOIDS
Giorgio Carboni, march 2002
Translation edited by G.L. Stuart
Introduction to surface phenomena
Soaps and detergents
In this article, we collect a series of laboratory experiments which mainly concern surface phenomena and colloidal systems. Due to their number, these experiments will be briefly described. As you know, our articles do not intend to supply an exhaustive explanation of the topics we deal with, but rather to give rise to a curiosity toward them and to give young people exposure to interesting categories of natural phenomena. We try to achieve these goals through practical and experimental activities in which hands-on involvement plays an important role. The way these activities are presented tends to bring out their amusing aspects and, hopefully, to establish a positive attitude towards the subjects treated. We are convinced that these activities will arouse questions in the participants and that they themselves, through curiosity, will go looking for explanations. We are also convinced that feeding the desire to know the how and why of things, that is, the curiosity spawned within each student is more important and useful than explanations supplied when a student feels no need.
As the explanations are short, we have often supplied links to Internet websites and sometimes a bibliography. Unfortunately, we will not be able to continually update these links, so we have supplied keywords to look for updated information on the Internet using search engines. Do not use all these keywords together, but in the combinations which seem more relevant to you. During these searches, often too much documentation is found, much of which is useless to our search. So, if necessary, add a term like these: school, students, experiment, test, classroom, homemade, homework, science fair, science project, lesson, lesson plan.
WARNING: Some of these experiments can be dangerous. When children are doing them, an adult must always be present to avoid any damage or harm. In any case, we do not assume any liability. As for the safety and the liabilities, we recommend you to read our Warning page.
Have a good time!
INTRODUCTION TO SURFACE PHENOMENA
Why do some insects succeed in skating on water instead of sinking? Why in some cases, does the water sprinkled on a glass surface collect into drops and in other cases spread like a thin film? Why does water climb up a thin tube? Why can you make bubbles with soapy water and not with tap water? For reasons we will see later on, the surface of a substance has special properties. These surface properties are what allow these strange phenomena we have mentioned. Not only that, but the surface is also the place of contact among different substances. In short, the properties of surfaces are so special and important that there is a branch of science, the physics of surfaces, devoted to the study of surface phenomena.
A molecule of a liquid attracts the molecules which surround it and in its turn it is attracted by them (figure 2). For the molecules which are inside a liquid, the resultant of all these forces is neutral and all them are in equilibrium by reacting with each other. When these molecules are on the surface, they are attracted by the molecules below and by the lateral ones, but not toward the outside. The resultant is a force directed inside the liquid. In its turn, the cohesion among the molecules supplies a force tangential to the surface. So, a fluid surface behaves like an elastic membrane which wraps and compresses the below liquid. The surface tension expresses the force with which the surface molecules attract each other. A way to see the surface tension in action is to observe the efforts of a bug to climb out of the water. On the contrary, other insects, like the marsh treaders and the water striders, exploit the surface tension to skate on the water without sinking. Here are some simple experiments using surface tension:
|1 -The floating needle.
Carefully place a needle on the surface of a glass of water. If the water does not completely wet it, you will see the needle float. To avoid your fingers disturbing the surface as you place the needle, you can make a small cradle from wire to hold the needle as you lower it gently on to the surface of the water. Another way to make it easier to float an object heavier than water using only the surface tension is to first float a strip of tissue paper and lay the needle on it. Slowly, the water will soak the strip, which will eventually sink, while the needle will remain on the surface.
Figure 3 - Floating needle. At the bottom of the pot you can see the sunken strip of tissue paper.
2 - Make sulfur powder sink. Sprinkle some sulfur powder over a
glass of water (You can buy sulfur in a hardware store). Sulfur is hydrophobic enough to
float on the water. Add a drop of detergent and you will see the particles of sulfur sink.
This experiment also works with talcum powder which you probably already have in your
http://www.ilpi.com/genchem/demo/tension/ has a short movie on this experiment and a description of the properties of surfactants.
3 - Launch of the needle. With some steel wire, make a ring. Place a needle on the ring and submerge in soapy water. When you extract the ring, two membranes will be formed: one at the left side of the needle and the other at the right side. Now, with a finger burst one of these membranes. The needle will be thrown away by the surface tension of the remaining membrane, which quickly contracts, in an effort to achieve the smallest possible surface area.
|4 - The strength of the soap films.
With some iron wire, make a "U" frame and a slider, as shown by the figure 4. Plunge the frame in soapy water. When you extract it, you will see that the slider will be drawn toward the bottom of the frame by the surface tension of the soap membrane. By holding the slider still with your fingers, you can feel the force of the membrane.
Figure 4 - U-shaped frame with slider. The surface tension of the membrane draws the slider toward left.
5 - Measuring the surface tension. In order to measure the surface
tension of a liquid, you can use an equal-arm analytical balance. As shown by the figures
5 and 6, hang a U-shaped steel wire under one of the two weighing pans (A). By lowering
the A arm and then by lifting it up again, make a membrane to form in the U-shaped frame.
Balance it with some masses on the weighing pan B. At this point, break the film. The
balance will go down by the B side, therefore restore the equilibrium placing some masses
on the side A. The value of these last masses (F) corresponds to the force with which the
membrane tends to close into the liquid. The surface tension (T) is given by the force (F)
divided by the width (W) of the membrane, divided again by two because it is necessary to
keep into account the membrane possess two surfaces. So, T = F/2W. The value of the
surface tension of the distilled water is 7,42 g/m at 20°C and that of ethyl alcohol is
2,27 g/m always at 20°C. We supply to you these values because you will be allowed to
compare with them those you obtain through experimentation. If you do not possess an
analytical balance, you can build one of them. It will not be as exact, but it will allow
you to do these measures. Given the forces which play in this experiment, the balance
should have an accuracy of a hundredth of a gram at least.
http://www.pvri.com/sp/BalBuild.htm How to build a no cost sensitive balance (by Salvatore Previtera)
http://userpages.prexar.com/dwilliamsmaine/scale/scale.html A Home-made Balance Scale (by Dan Williams)
6 - Other method to measure the surface tension. To measure the
surface tension of liquids, you can use a metal wire ring of the diameter comprised
between 3 and 4 cm, instead of the "U" frame we have described. This wire should
be made of platinum, anyway, as this material is costly and not easy to find, use a
stainless steel wire which you can buy in a welding shop or in a hardware store. If you
have difficulty finding a wire of this material, use an iron wire. Its diameter should be
of 1 - 2 mm. Even in this case you should use an analytical balance.
Dip the ring just under the surface of the liquid of which you want determine the surface tension. Level the balance in these conditions. Add some masses on the opposite arm until the ring detaches from the liquid. The surface tension (T) of the liquid will be given by the detachment force (F) you have measured divided by two times the mean circumference (crf) of the ring: T = F/2crf. This factor 2 takes into account the two surfaces of liquid: the internal one and the external one to the ring (figure 8). For reasons of clarity, in the figure the ring has been drawn with the diameter greater than the actual diameter.
http://www.tensiometry.com/STMethods.htm Other methods to measure the surface tension.
7 - With distilled water, verify the good working order of your experimental system.
8 - Determine the surface tension of the tap water.
9 - Determine the surface tension of the tap water to which you have added a little detergent. You will notice that small amounts of surfactants are sufficient to lower the surface tension of the water a lot.
10 - Relationship between the weight of the drops and the surface tension. By a dropper, slowly drop some water of the test 8 and determine the mass of a certain number of drops (ie 30). Do the same thing with the water of the experiment n° 9. Verify if there is a relationship between the mass of the drops and the surface tension of the solutions. Answer: The mass of the drops is proportional to the surface tension of the liquid: M = T/K, where K is a constant which you can determine using distilled water at 20°C of which you know the surface tension. This constant is valuable only for this dropper. Determine the mass of a given number of drops is a method to measure the surface tension of a liquid. In these tests, to obtain a better precision, calculate the mean of a series of measures. Verify if the following relationships are valuable: T1:M1 = T2:M2.
11 - Surfactant powered boats. From a thin wooden or cardboard sheet, cut three little "boats" like those indicated in the figure 9. They must have an opening with a seat for a bit of soap. Place a bit of soap in the seat of a boat and put it in a small basin with water. You will see the boat move quickly forward. With the opening on a side or off-center, the boat will turn. The movement of the boat can be explained by the quick scatter of surfactant molecules on the water surface, so this little boat would move by reaction. Another explanation recalls Marangoni's effect, according to which, in case of a gradient of surface tension from one zone of a liquid to another, there will be established a flow from the zone of low surface tension toward the one of high surface tension. In this case, the boat will be dragged by the movement of the water surface. This amusing experiment can also be done using substances other than soap, provided they have surface active properties. For example, you could place a little drop of detergent on the carving. If you will use a bit of camphor, your boat will sail more quickly and longer. If the stretch of water in which the boat moves is small, like a dish or a small basin, quickly the water surface will be covered by a layer of surfactant molecules and the boat will stop and you will need to change the water to restart the boat. If instead you do these experiments in a pond, you will not have this problem. Try different shapes of boat and of carving, try hot and cold water, different types of soap, etc. The water will quickly soak through the wood or especially the cardboard of your boat and will disable it. Some boards will even sink. To save your fleet, make the little boats waterproof with acrylic paint or flatting. When the paint dries, you will be able to restart the races.
Surface Tension ***
http://teachers.net/lessons/posts/224.html Surface tension on coins
http://www.online-tensiometer.com/oberfl/ Some experiments on the surface tension
http://www.biologylessons.sdsu.edu/ta/classes/lab1/TG.html Properties of Water
http://www.ed.gov/pubs/parents/Science/soap.html Have you ever tried using soap to power a boat?
Internet keywords: surface tension, surface phenomena, surface tension boat, soap boat.
Why does one fabric absorb water well while another seems to refuse it? Why does water collect into large drops on a greasy surface and instead form an adherent film on a clean surface? According to the nature of the liquid and the solid, a drop of liquid placed on a solid surface will adhere to it more or less. To understand this phenomenon it is necessary to take into account the fact that molecules of a liquid are subject to a cohesive force which keeps them united to one another, but there is also an adhesive force which is the force with which the molecules of the liquid adhere to the surface of materials that they contact. When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface, when instead the forces of adhesion are less by comparison to those of cohesion, the liquid tends to "refuse" the surface. In this people speak of wettability between liquids and solids. For example, water wets clean glass, but it does not wet wax.
1 - Measuring the contact angle. Place a drop of a liquid on a smooth surface of a solid. According to the wettability of the liquid in relationship to this solid, the drop will make a certain angle of contact with the solid. With reference to the figure 10, if the contact angle is lower than 90°, the solid is called wettable, if the contact angle is wider than 90°, the solid is named non-wettable. A contact angle equal to zero indicates complete wettability. To measure the contact angle use a protractor and a ruler. Taking a picture of the outline of the drop will make easier and more exact the measurement.
2 - Prominent drops, flat drops. Lay a water drop on a dirty glass plate. For example a glass with a lot of fingerprints. Measure the contact angle. Now wash the plate with water and detergent, then rinse it with care and dry it. Make the test again and compare the contact angle in the two cases.
3 - Misted plate. Breathe on a glass plate which has been washed, but not very well. You will see the plate become misted, this is due to the formation of a myriad of tiny water drops on the surface of the glass.
4 - Water film. With water and detergent, wash a plate of glass well, then rinse it a first time with tap water and then with distilled water and leave it to dry in a place devoid of dust. Now, breathe on it. If the plate of glass is very clean, it will not mist because the water will arrange on the surface as a thin and continuous film of water. This happens because the water has complete wettability toward a clean glass. If the cleaning method above has not cleaned the plate well enough, wipe it with a cotton cloth with some pure acetone in it. Use caution because acetone is inflammable and toxic, so do this operation outdoors and with care.
By studying plants, a German scientist discovered a method to keep surfaces clean or to clean them with less water. You have to cover the surface with a thin layer of wax. This substance has a very low wettability toward the water. It tends to keep clean and it is commonly used to enhance the cleanliness and appearance of buildings and vehicles.
http://www.ksvinc.com/contact_angle.htm Contact Angles
Internet keywords: wettability, interfacial tension, IFT, contact angle
Let us stay in the field of the wettability. Surely you have noticed that water tends to rise near the walls of a glass container. This happens because the molecules of this liquid have a strong tendency to adhere to the glass. Liquids which wet the walls make concave surfaces (eg: water/glass), those which do not wet them, make convex surfaces (eg: mercury/glass). Inside tubes with internal diameter smaller than 2 mm, called capillary tubes, a wettable liquid forms a concave meniscus in its upper surface and tends to go up along the tube (figure 11). On the contrary, a non-wettable liquid forms a convex meniscus and its level tends to go down. The amount of liquid attracted by the capillary rises until the forces which attract it balance the weight of the fluid column. The rising or the lowering of the level of the liquids into thin tubes is named capillarity. Also the capillarity is driven by the forces of cohesion and adhesion we have already mentioned.
1 - The rise of water along a capillary. Immerse a capillary in a glass containing tap water and measure the height (h) of the water column inside it.
2 - Effect of the surfactants. Add a few drops of detergent to the water and measure again. Compare the variation in the height of the water column. You will be able to notice that even small amounts of surfactants produce important effects on the level reached by the water in the capillary.
3 - Effect of the diameter of the capillary. With a tube of glass and a Bunsen burner, make a series of capillary tubes having different diameter. Verify the relationship between the height of the water column and the internal diameter of the capillary. (Answer: the height of the column is described by this formula h=k/r, where h is the height of the column, k is a constant which depends on the surface tension of the liquid and on the contact angle between the liquid and the wall, r is the internal radius of the capillary tube. So, with the same liquid and material of the capillary tube, the height of the column is in inverse proportion to the diameter of the capillary tube. You can determine the value of k for water using distilled water at 20°C.
4 - Try other liquids. Make some other tests with liquids other than water, such as alcohol, oil, etc. and measure the height of the liquid column. This height depends by a number of factors such as the surface tension of the liquid, the contact angle liquid/capillary, the radius of the capillary, the density of the liquid, the acceleration of gravity. In fact, the column attains the height of equilibrium between the ascensional forces and its own weight. Oily substances tend to contaminate inside the capillary, so when changing from one liquid to another, clean the capillary well or replace it. The vegetable world exploits capillarity and osmosis to bring water up to the higher parts of plants. In this way, some trees succeed in bringing this precious liquid up to 120 meters above the ground.
5 - An emergency plant watering system. It is summer and you are going on vacation. You are worried about your potted plants, which risk to remain without water. In fact, even if you have asked your neighbor to water them, you know by experience that after the first day, he will forget, that's just the way he is. Then, try this emergency watering system. It bases itself on the fact that a string is able to carry water among its fibers by capillarity. Place a tank on some bricks and fill it with water. Place the pots round the drum. Cut some pieces of string long enough to reach the bottom of the tank and to be inserted into a pot. Immerse all strings in the water to soak them well. Tie all the strings together at one end and sink this knot to the bottom of the drum with a stone or weight. Now, one at the time, put the free end of each string into a different pot. Each pot has to be served by a string. Test the system before you go on your vacation. You have to verify if it works well, to find the suitable type of string and to proportion the amount of water in the tank to the length of your absence. Try strings made up of fibers of different dimension, of different materials, even in plastic. If the string tends to become encrusted with mineral deposits, add some vinegar to the water. Also try to insert each string in a thin plastic tube. If the water flow is too fast, use a thinner string. Check the effect of some drops of detergent on the flow.
Internet keywords: capillary, capillarity.
SOAPS AND DETERGENTS
How do soaps and detergents work in removing dirt? Soaps and detergents are formed by special molecules, which have a hydrophilic head, which therefore loves to remain in water and a hydrophobic tail, which avoids water and loves fat substances (figure 12 A). Because of their hydrophobic tail, a part of the molecules of detergent collects to the water surface forming a monomolecular layer (figure 12 B), it lowers the surface tension of the water and makes easier its penetration into the fabrics to be cleaned. Within the water, the molecules of detergent collect themselves in micelles and membranes, little aggregates of molecules united by their hydrophobic tail (figure 12 B). When they meet dirt, these molecules surround the particles and insert their tail in them. The hydrophilic heads attract the dirt toward water and with the agitation of the liquid they contribute to remove the dirt from the fabric (figure 12 D). The crown of hydrophilic heads carries the particles of dirt in the water (figure 12 D), where they end up in suspension and then they are rinsed away. Hence, the dirt water contains also greasy particles which have been emulsified. For the same reason, the detergents aid the formation of emulsions. The substances which lower the surface tension of a liquid are called surfactants (from: surface-active agents). The lowering of the surface tension of the water allows the formation of soapy membranes (figure 12 C), foam and soap bubbles. Notice the special arrangement of the surfactant molecules in these membranes.
The phospholipids are molecules like surfactants, they also have a hydrophilic
head and this time two hydrophobic tails. These molecules are the main components of the
membranes of cells. In fact, usually the membranes of cells are made up of two layers of
phospholipids, with the tails turned inward, in the attempt to avoid water. As we know,
the external membrane of a cell contains all the organelles and the cytoplasm. Liposomes
are empty cells which are manufactured by some industries. They are microscopic vesicles
or containers, formed by the membrane alone. They are widely used in the pharmaceutical
and cosmetic fields because it is possible to insert chemicals inside them. You can use
liposomes to contain hydrophobic chemicals such as greasy or oily substances so that they
can be dispersed in an aqueous medium by virtue of the hydrophilic properties of the
membrane of the liposomes.
http://cellbio.utmb.edu/cellbio/membrane_intro.htm Membrane Structure and Function
http://ntri.tamuk.edu/cell/membranes.html Architecture of membranes
Internet keywords: phospholipids membrane, cell membrane
1 - Comparison of the ability of different detergents. Try the efficacy of different detergents for glass or dishes. Soil some microscope slides with the same type of fat. If you do not have microscope slides, use glasses or even ceramic dishes. Clean all the slides with a different detergent, rinse them well and dry them. You can check the level of cleanliness by measuring the contact angle of water drops placed on them. Another method is to measure the reflected light by each slide in the same conditions of illumination by means of an exposure meter: the cleaner slide reflects less light.
Understanding Soaps & Detergents
http://www.kcpc.usyd.edu.au/discovery/9.5.5-short/9.5.5_introsurfactants.html Introduction to Surfactants ***
http://www.ilpi.com/genchem/demo/tension/ Structure and properties of the surfactants and experiment with the sulfur powder.
http://www.ngo.grida.no/reduce/german/klaus/developm/oekonet/p2/p02a.htm Chemical Products in the Houshold
http://www.selah.wednet.edu/SOAR/SciProj2000/LaceyT.html Which Window Cleaner Best Removes Various Substances? A Science Fair
http://www.cem.msu.edu/~reusch/OrgPage/VirtualText/lipids.htm Lipids: Fatty Acids, Soaps and Detergents, Fats and Oils, Waxes, Phospholipids, Eicosonoids, Terpenes, Steroids, Lipid Soluble Vitamins ***
http://www.surfactants.net/ The Surfactants Virtual Library. The most comprehensive and organized source for surfactant information on the Internet.
Internet keywords: detergents, surfactants.
As long as there has been soap, making soap bubbles has been an amusement for children. Everybody has played with soap bubbles as a child. A straw and a glass with soapy water is all that is needed to amuse a child for hours. One child blows bubbles and others run after them and play with or pop them. What astonishes the children is the spherical and perfect shape of the bubbles, their colors, their transparency, their lightness which competes only with that of the butterflies and fairies. By means of thin membranes of soapy water, it is possible to do interesting experiments and amusing games, such as to blow bubbles of different sizes, concentric bubbles, helical bubbles, "solids" supported by frames in metal wire, it is possible to observe and to study the coloured interference figures on the membranes of soapy water, to obtain membranes so thin that they lose all color and become invisible, to obtain membranes measuring some square meters of surface and bubbles of some cube meters of volume, so that you can to trap a friend. And then you will learn to blow cubic bubbles... by using a square straw, of course! No, just kidding! :)
HOW DO THE SOAP BUBBLES FORM?
As Grownups, we pose questions like these: "How do soap bubbles form? Why does soapy water produce foam while pure water does not?". When water sprays from a tap in a small basin, you can see bubbles form, but they burst very soon. This is due to the fact that the surface tension of the normal water is high and it tends to draw the water molecules into the main body of the water, to the point where the thickness of the bubble wall is too thin to remain intact and quickly bursts. Instead, the surface tension of the soapy water is much lower: about a third of the pure water, so the molecules of the bubble are less stressed and it can last longer. Soap and detergents lower the surface tension of water and, as we have said, they are called surfactants. As we have said in the paragraph on the soaps and detergents, the molecules of surfactants have a hydrophilic head and a hydrophobic tail. When these molecules are dissolved in water, they tend to collect on the surface with the tails outward, forming continuous layers (figure 12 B). The membranes of soapy water are made up by three layers: the external two are formed by surfactant molecules and the internal layer is formed by soapy water (figure 12 C). These layers of surfactant molecules are very elastic and they deform easily without breaking. They also slow the evaporation of the water film and so extend the life of the bubbles.
Water is an important ingredient to our recipes. Usually, to produce soap bubbles, people used a mixture of tap water and soap. Unfortunately, the mineral salts which make hard water subtract a part of soap with negative consequences on the formation of the bubbles. In fact, soap reacts with the calcium and magnesium salts, which are in the tap water, forming an insoluble precipitate which subtracts surfactant molecules from the solution. Instead, the detergents react with the mineral salts of the water producing soluble compounds, so detergent are less influenced by the hardness of water. If your tap water is soft, it is OK to use for bubbles. In any case, you will obtain the best results with distilled water.
After the water, the most important ingredient is the base surfactant. There are a lot of surfactants which can be used as detergents and to blow bubbles. Therefore, try some different brands of detergent until you find the best one. Dawn and Joy brand liquid detergents for dishes supplied good results, but try other products if you like.
The presence of water in a soapy film is important to make it last a long time. As time goes by, a part of the water migrates by gravity and reaches the bottom of the film or of the bubble and another part evaporates. In this way, the membrane grows thin, weakens and in the end bursts. To extend the life of bubbles, people add substances which make the water more viscous, slowing its descent toward the bottom. Other substances are added to slow the evaporation of the water. Substances which have these effects are: sugar, honey, glycerin, gelatin, arabic gum, viscous liquid soap. You will have best results if you let the soapy solution rest for a couple of days, but if you are impatient, you can use it immediately. A cold solution makes longer lasting bubbles. For various bubble recipes, look at the links we have put at the end of this section on bubbles.
|1 - How to find the basic surfactant.
To find the main component of your recipe, the base surfactant, obtain some dishwashing detergents, shampoo, bath soap, etc. With water, make a solution in the ratio of 1 to 10 for each surfactant. In a place without wind, blow a bubble of about 7 cm in diameter. Keep it on the straw (figure 13) and measure its duration. Repeat the test 5 times for each detergent so to obtain a more reliable mean value. Obviously, the best detergent is the one which produces bubbles which last longer.
Figure 13 - How to keep the bubbles during the test of duration.
2 - Adjusting the secondary ingredients. A second series of tests will have the purpose of adjusting the recipe in its secondary components, those destined to reduce the evaporation and the fluidity of the water. Follow the same method as you did in point 1.
3 - Blow some bubbles. When the solution is ready, you will be allowed to pass to the further experiments. In the meantime, blow some bubbles and watch them fly, carried by the wind.
4 - How to make bigger bubbles. With some thick iron wire, make a ring of about thirty cm diameter. Immerse it in bubble solution that you have put in a small basin. Moving the ring quickly in the air, you should be able to obtain quite large bubbles.
|5 - Again on the force of the surface tension.
Knot a heavy cotton thread with a slipknot to the ring of the experiment 4. After you have wet the ring in the soapy solution, the ring will be closed by a film. If you burst the membrane inside the loop, you will see it take a circular shape (figure 14). This happens because of the surface tension of the remaining part of the soapy film.
6 - A support for bubbles. To comfortably observe bubbles, it is important they are steady. With some iron wire, make some rings on which to put the bubbles. Leave a stem to each ring so you can insert it into an object or you can shape as a pedestal. To avoid bursting the bubbles you put on it, wet the ring with bubble solution. Wood or velvet can support bubbles for a long time without bursting them, but are harder to fashion into a ring shape.
7 - Study the contact surface among bubbles. On a clean glass or a rigid plastic sheet soaked with solution, place two bubbles in contact each other. Observe the surface of contact. You will see the smaller bubble of the two will tend to bulge into the bigger one. This happens because of the internal pressure of the little bubble is higher than the pressure of the large ones. This also means that two bubbles of equal diameter have a flat contact surface. After having made some bubbles in contact with each other, produce some foam and observe it. Observe that sometimes the shapes of the foam bubbles are the same as that of cells of biological tissues, in other cases the shapes of the cells are different because they have to increase their surface of contact or for other reasons. Note also that the crystals of metals often have the same shape as the foam bubbles. After all, during the solidification of a metal, they are deformable spheres very close each other and which cannot leave empty spaces.
|Figure 15 - Membranes on a cubic frame. These membranes do not arrange on the faces of the cube, but they are in contact each other.||Figure 16 - Membranes on a cubic frame. The cubic central bubble has been placed with a straw.||Figure 17 - Membranes in a pyramidal frame (tetrahedron). Place a bubble in the center.||Figure 18 - Membranes between two rings and having a film in common.||Figure 19 - Tube-shaped membrane between two rings. It has been obtained by breaking the film in common.|
8 - Solid figures made on suitable frames. With some frames made with metal wire, you can create flat, helical films or with many other forms. You can also create quite complex solids (figures 15, 16, 17, 18, 19, 20). To do this you have to dip a suitable frame into the soapy solution. When you will have withdrawn it, you will see the membranes. Usually, people expect these films to form on the faces of the solid, but this does not happen because they tend to keep into contact with each other and to form figures of minimum surface area. Remember that soapy films tend to keep the shape of smallest energy. So, if you will make a tube-shaped membrane, do not be surprised if its diameter will reduce in the middle.
9 - Helical films. To obtain helical films (figure 20), make a helix with a few coils made up of iron wire (like a normal spring), place a piece of wire along the axis of the helix and solder it to the two extremities of the helix.
10 - Regular polyhedral bubbles. What shape frames are necessary to
obtain central bubbles with the shape of an octahedron, a dodecahedron, an icosahedron? It
is possible fabricate them?
Internet keywords: regular polyhedra
You can bet your friends you are able to make cubic soap bubbles. Obviously, they will not believe you. Then you can explain them that this is possible to you by using a square straw. It is very probable they will accept the bet. It will be easy for you to win it by making your cubic bubble inside a cubic frame, as shown in figure 16. Before blowing the bubble, crush the tip of a straw so to obtain a square section. This is part of the bet, but you know that with a normal round straw that the bubble will become cubic due to the frame, not the straw.
Figure 20 - Helical film.
Figure 21 - Frames on metal wire to study the soapy membranes.
The figure 21 shows some frames of metal wire which can be made to study the soap films and to measure the surface tension of liquids. To build them, we have used galvanized iron wire, cut in segments which then we have soldered with tin. You can also try plastic coating these frames by dipping them into tool handle coating products which are sold at hardware stores.
"Why are soap bubbles colored?". The membrane of the soap bubbles are formed by three layers. The external two are both formed by a layer of surfactant molecules with the polar head turned inward, the inner layer is formed by soapy water (figure 12 C). The light which crosses a soap film is in part reflected by the front surface of the membrane and by the back one. The waves of light reflected emerge out of phase, they sum algebraically (interference), giving rise to variations of color. The emerging hue depends on the thickness of the film. These colors are very fine and create beautiful shapes formed by the zones of different color when turbulence is present within the film. In fact, if you gently blow on a film, you can create magnificent designs (figures 1, 22, 23, 24). Over time, due to evaporation or the descent of the water toward the bottom, the thickness of the membrane will have become very thin, the two reflections will fade completely and the bubble will become black against a black background: it will not show colors any more and will become invisible. In that condition, the film will be also very unstable and near bursting.
|Figure 22 - The interference fringes which form as the water flows down by gravity. As the film gets thin at the top it becomes black because its thickness is less than the wavelength of the visible light.||Figure 23 - By gently blowing on the film, you can create beautiful turbulence zones which can be observed and studied. Notice on the top the black zone has widened.||Figure 24 - By blowing again, the figures become more complex and rich with details.|
11 - Colors and shapes of the figures of interference on soap films. The soap membranes are well suited to observe the colors and the turbulences which are created by light air currents. So, by means of a ring on iron wire, make a soap film and examine its colors. Blow lightly on the film to observe the turbulence on its surface (figures 23 and 24). To better see the colors of the membrane, it is worthwhile to observe it against a black background and illuminate it with bright white light. If you keep the frame vertical, you will see the colors change as the film grows thinner. Usually, shortly before bursting, a part of the film will become black. Here some other figures of interference: figure 31, figure 32.
12 - To cross a membrane without bursting it. If you touch a film with a dry finger, the membrane will burst. If you will wet the same finger with the soapy solution, the film will not burst and you will be able to penetrate it.
13 - Plays on the water. Make bubbles in a small basin of water. Look for the conditions which allow to the bubbles to bounce or to alight on the surface without adhere to it. Place a drop of oil on the surface of the water, which will arrange itself on the surface as a monomolecular layer, (eg. stearic acid) and repeat the test. Also an oily hair can deposit a thin, oily layer on the waters surface, when slowly immersed in it.
Creating a Soap Bubble Company (how to get ready a quality soap solution)***
http://www.scri.fsu.edu/~dennisl/CMS/activity/bubbles.html Bubbles ***
http://members.tripod.com/sharing_science/bubbles.html Bubble explorations ***
http://www.exploratorium.edu/ronh/bubbles/bubbles.html Bubble formulae, Bibliography, Internet resources ***
http://www.sme.org/memb/neweek/actsoap.htm Soap Bubbles (recipes)
http://www.freeweb.pdq.net/headstrong/bubble.htm The bubbles page
http://www.bubbles.org/ Bubblesphere ***
http://www.bubbles.org/html/questions/color.htm Why do bubbles have color?
http://wwwchem.csustan.edu/chem2000/Exp5/PURPOSE.HTM Experiments on surface tension ***
http://chemmovies.unl.edu/chemistry/beckerdemos/BD028c.html Soap Films in Action (movies)
http://more.abcnews.go.com/sections/science/DailyNews/soapfilm981019.html Build Your Own Soap Tunnel (study of the vortex on the soap films)
http://eduscapes.com/42explore/bubbl.htm Internet Resouces
http://www.nap.edu/readingroom/books/rtmss/1.101.html Soap Film and Bubbles. Bibliography.
http://www.physics.ohio-state.edu/~brent/papers/General.pdf What makes soap films stable?
Internet keywords: soap bubbles, blowing bubbles, soap films, soapy water, bubble solution, soap bubbles formula / composition / recipe.
In order to do experiments with osmosis, you need to obtain a semipermeable membrane. For this purpose, you can use cellophane, which is a thin transparent film, essentially made up of cellulose and which is often used to pack wrap flowers and gifts. Sometimes, florists also use a plastic which is very like cellophane, but, instead is completely impermeable to the water and which is not suitable for these experiments. How can you distinguish between these two materials? Putting some water on cellophane, you will see it soften, dilate and even the opposite side of the sheet will become moist. This does not happen with the transparent plastic sheet. You can obtain cellophane in a stationery shop. Unfortunately, this material is often covered with a thin layer of water repellent nitrocellulose which prevent the passage of the water. This layer can be removed by immersing the cellophane in a solvent for varnish or perhaps in acetone. Use caution because these solvents are inflammable and toxic.
Another possible source of semipermeable membrane can also be found in certain plastic bags. The plastic is made from starch and is used to produce biodegradable plastic bags for recycling. In some European cities, these plastic bags are used to collect organic wastes. When touched, this plastic is flabby, quite elastic and near rubbery. You can also try the membrane of a chicken egg and other membranes you will find or you are able to fabricate.
Water flows slowly through the membrane. If you limit yourself to closing the bottom of a tube, it will take days to see the level of the inner liquid increase. To accelerate the flow, it is necessary to widen the surface of exchange. It would be necessary to have special flared tubes, which are difficult to find. Instead, you can use a small funnel, which is much easier to obtain.
1 - Diffusion by osmosis. For the first experiment, use distilled water, some sugar, a semipermeable membrane, a beaker, and a support for pipettes. Obtain a flared tube of glass or transparent plastic. Or, as an alternative, a little transparent funnel. The internal diameter of this tube has to be at least one cm. With a rubber band or clamp, attach a piece of membrane on the flared bottom of the tube and then pour the concentrated solution of sugar in the tube. Insert the tube in a beaker and put water into it until you attain the same level of the solution in the tube. After some hours, you should see the level of the liquid in the tube is increased (figure 25). After some time, the level will attain a maximum. If, instead of tap water, you will use distilled water, the phenomenon will be more evident. To render more visible the concentrated solution, you can add a drop of ink or some watercolor. Why does the more concentrated solution rise? As we said, there is a tendency of the two solutions in contact via a semipermeable membrane to reach the same concentration. The more concentrated solution absorbs solvent from the more diluted. In these experiments, the level of the liquid in the tube increases, but not to infinity. It goes up until the pressure of the liquid column attains the equilibrium with the osmotic pressure. The equilibrium pressure between a solution and its solvent is the osmotic pressure of that solution.
2 - Osmotic pressure and density of the solution. Determine the osmotic pressure of some solutions. Verify if it is proportional to the amount of molecules per volume of the solution.
3 - When the dissolved particles are very small. If, instead of the sugar, you will use salt, the osmotic pressure will result very low. This happens because in water the salt dissociates itself into the Na+ and Cl- ions, which are smaller than the molecules of water and they easily pass through the semipermeable membrane.
4 - Osmotic pressure and microorganisms. Place under the microscope a slide with a small drop of water rich in protists, then add a pair of drops of distilled water. At the beginning, the protists will swell and you will see their vacuoles work very hard in the attempt to expel the excess water from their cytoplasm, then you will see their cellule explode, pouring their organelles outward. The cilia of the mouth will continue to beat for long time, even if they are not connected to the body any more.
You can obtain some devices for these experiments on osmosis in a store that stocks
laboratory supplies for chemistry and biology.
http://www.science-projects.com/Osmosis.htm An Experiment on Osmosis
http://www.hinduonnet.com/thehindu/2001/09/15/stories/13151108.htm Osmosis -a different experiment
http://www.purepro.net/kemflo/What-is-RO.htm What is... Reverse Osmosis
Internet keywords: osmosis, semipermeable membrane.
INTRODUCTION TO THE COLLOIDAL SYSTEMS
Let us leave the surface phenomena to enter into the mysterious world of the colloids. A first example of a colloid is gelatin, a strange substance: neither liquid nor solid. It is very elastic and if deformed it returns to its previous shape. Goofy, the friend of Mickey and Donald, learned something about it when, in the Disney film: Mickey and the Beanstalk, he was "walking" on a pudding of the Giant. The emulsion of oil in water is another substance with unusual properties. Unusual are also substances such as foams, aerosols, smokes and fogs, not to mention the solid emulsions and foams. What do all these curious substances have in common? That is what we will see before long. These substances are called colloids and they are in some ways related to the solutions and to the mixtures, even if they do not belong to the former nor latter. To understand what colloids are, it is necessary to know what solutions and mixtures are.
A solution is a homogeneous mixture of two or more substances. When placed in water, many substances dissolve and are called soluble, others do not dissolve and are called insoluble. Salt and sugar easily dissolve in water. If instead you put sand in water, you can mix for as long as you want, but you will not succeed in dissolving the sand. In fact, sand is insoluble in water. In a solution, the material present in greater quantity is defined solvent and that in smaller quantity solute. What does it mean to say that a substance is soluble in another? It means that the molecules of the solute separate each other and they disperse among those of the solvent. Instead, the insoluble substances keep themselves compact and their molecules do not disperse into the solvent. As solvent, we have used the example of water because many solids are soluble in water, but nearly every liquid can be a solvent. And then, why we should limit ourselves to the liquids? Let us generalize the concept of solvent and concede to all substances, solid or liquid or gaseous the possibility to be a solvent. At this point, even the solutes can belong to all of these three states of matter. For example, some solid solutions are the metal alloys such as steel (Fe+C), brass (Cu+Zn), bronze (Cu+Sn). Finally, all gases are completely soluble among each other. Also common are solutions of gases in liquids. For example, carbon dioxide is added to many beverages to make them fizz. In the water of ponds, rivers and seas, gases like oxygen, carbon dioxide and others go into solution in a natural way. The presence of these gases in the water make possible the life of the aquatic organisms.
The solubility of a substance is measured as the maximum amount, in grams,
which can be dissolved in 100 g of solvent. When the solute does not dissolve any more,
but a deposit is formed on the bottom, the solution is defined saturated.
CATEGORIES OF SOLUTIONS
|Gas||Gas||air (nitrogen, oxygen, etc.)|
|Liquid||Gas||moist air (water vapor in air)|
|Gas||Liquid||CO2 in water (sparkling water)|
|Liquid||Liquid||wine (water + alcohol)|
|Solid||Liquid||marine water (salt in water)|
|Gas||Solid||gas in silicates (pumice stone)|
|Liquid||Solid||dental alloys (mercury in cadmium)|
|Solid||Solid||metal alloys (steel, bronze)|
1 - Saturated solution. Determine the content of salt in a saturated solution. In order to not waste too much salt, use only a little water.
2 - To grow crystals. Determining the density of sugar in a saturated solution is not easy because sugar continues always to dissolve. Anyway, make a heavy sugar solution and a saturated solution of salt in water. Put a cotton thread in each of them and wait some days for some crystals to grow. Describe the shape of these crystals. If you like to grow crystals, it is possible to find packets of salts specially chosen to this purpose. Also search the Internet with the words: growing crystals.
3 - Where does sugar go? Put a beaker on a magnetic stirrer, insert the stir bar and fill the container with water up to the top. Slowly, add grains of sugar so they are dissolved by the stir bar as it rotates. Note the amount of sugar you will have put into the water before it overflows. Do the same thing with salt and then with sand. Compare the results and explain the different behaviors.
4 - How to separate salt from sand? Solve this problem: A day, a child who lived on the border of the desert was sent to buy some salt. While he was coming back and he was playing with friends of his own, the bag broke and the sand shed on the sand. For these people the sand was important and costly, so that child would be scold by his parents. How would have you done to recover the precious salt, separating it from the sand?
As we have seen, by mixing sugar with water, a solution is obtained. If instead we mix sand into water, we obtain a mixture. Also by mixing bits of coal and iron filings we obtain a mixture. With a pair of thin tweezers it is possible to take away sand grains from the water or pieces of coal from the filings, but it is not possible to take away singly molecules of sugar from the water because they are too much small. Hence, what distinguishes a mixture from a solution? In a mixture the particles are enough large to be separated by mechanical means such as tweezers or sieves, in a solution this is not possible because the particles which form it are so small that they cannot be seen even with an electron microscope. To separate the components of a solution it is necessary to use physical method like distillation. So, mixtures are formed by quite big particles, solution are formed by very small particles.
1 - A mixture. Make a mixture, for example by using sand and wood sawdust. How could you quickly separate the two components?
2 - Sedimentation speed and size of the particles. As indicated in the experiment on the analysis of the soil composition in the article on the experiments on environmental education and biology, put some water and a sample of earth in a glass or transparent plastic jar. Close the pot and shake it until all the earth is dissolved. Place the jar at rest and observe the different layers of materials. On the bottom, there will be stones and gravel, then thick sand and fine sand. Silt will require half an hour to be deposited, clay will demand 24 hours. Very small particles will remain in suspension, some of them will deposit very slowly, the finest ones instead will never deposit. Some other substances will have gone into solution. It seems the Etruscans collected the very fine clay which deposited after some days to obtain the black color of their earthenware.
3 - To separate particles according their grain size. If you want to separate the thick sand from the finer sand, you can use a sieve. If you want to clean sand from silt and clay, you can use flowing water. With a plastic tube, make water flow into the container of the sand. The water will carry away the smaller particles, while the larger ones will remain in the container. This method exploits the different sedimentation speeds to separate the particles of different grain size. Usually, the sand destined to be put in aquariums is cleaned to avoid water contamination. By using a sieve and with sedimentations and cleanings, produce 100 g of thick sand, 100 g of thin sand, 100 g of silt and 100 g of clay. Remove the water in excess and let all components dry to obtain moist sands, soft silt and clay. Compare the properties of these materials.
4 Observe under the microscope the finest particles. With a microscope, try to measure the size of the particles of silt, clay and of those which remain in suspension in water during your experiments of sedimentation.
We have seen that in the solutions, the molecules of the solute separate each other and disperse among those of the solvent. In the mixtures instead, the molecules do not separate and the particles remain compact. From the point of view of the sizes, solutions are formed by very small particles (single molecules) and the mixtures by quite large particles. In an intermediate position, between mixtures and solutions, there are the colloids. They are dispersions of small particles, but not molecule sized. What distinguishes mixtures from colloids and from solutions is therefore the size of the particles which form them. By convention, a colloid is a dispersion of particles which size is comprised between 0.2 and 0.002 µm (a micrometer, or micron, = 10-6 meters). If the particles are larger than 0.2 µm, we have a mixture, if they are smaller than 0.002 µm, we have a solution. In general, the components of a colloid are formed by small aggregates of molecules, while the components of a solution are single molecules. Anyway, if these molecules are large enough, as it is the case of many macromolecules, their solution will give a colloid. So, the criterion of distinction between colloids and solutions cannot be the presence of single molecules, but as we were saying, the size of the particles which form them.
> 0.2 µm
0.2 - 0.002 µm
< 0.002 µm
According to the dispersing phase, colloids are distingued in gaseous, liquid and solid suspensions. Gaseous suspensions, or aerosol, are smokes and fogs. Smokes are suspensions of solid particles in a gas. Fogs are suspensions of liquid particles in a gas. Sols, gels, emulsions, foams are liquid suspensions. Oily rocks, pumice stones are solid suspensions.
TYPES OF COLLOIDS
|Solid||Gas||Smoke - Aerosol||Smoke|
|Liquid||Gas||Fog - Aerosol||Fog|
|Solid||Liquid||Sol, Gel||Paint, Gelatin|
|Liquid||Solid||Solid emulsion||Oily rocks|
|Gas||Solid||Solid foam||Pumice stone|
The term colloid refers to substances with a glue-like consistency, in which the dispersant phase is therefore liquid. However, do not forget that even substances such as smokes and aerosols, in which the dispersant phase is aeriform and which we can also call gaseous suspensions, are colloids. Finally, even some solid substances, in which the dispersant phase is solid and which we can also call solid suspensions, are colloids too.
Colloids have unusual properties, for example gelatin. Colloidal systems have a high ratio area/volume among the surface of the particles and their volume. In other words, as in the colloids the amount of dispersed particles is very large, their overall surface is very large too and by consequence the interaction of the two phases is important. For example, a cube of 1 cm a side has a surface area of 6 cm2, the material of the same cube divided into little cubes of 0.002 µm of side, has a surface area of 3000 m2. Because of the wide surface of contact between the two phases, often the colloids are studied with the surface phenomena and the discipline which studies them is called surface and colloid science.
A sol is a dispersion of very thin solid particles in a liquid. It has a liquid consistency and resembles a true solution. An aqueous sol appears clear, very similar to common water. Anyway, if you shine an intense beam of light across it, a part of the light will be diffused from the particles which are in suspension. These particles are very small, but they are still enough large to obstruct the light and diffuse it. This phenomenon is called Tyndall effect. You can observe it with sols, but not with true solutions.
1 - Tyndall effect. In a transparent jar, put some clayey earth 1/4 of the volume and water until attain 3/4 of the container. Close the jar with its cap and shake until all the earth is "dissolved". Leave the pot to rest for a day to allow the clay particles to settle. The liquid which is above the sediment should have become clear. Shining an intense bundle of light through the jar, you should see the Tyndall effect. Do the same thing with a glass of pure water and compare the results.
A gel is a dispersion of very thin solid particles in a liquid and it has a gelatinous consistency. Increasing the concentration of the particles, a sol can pass to the state of gel. On the contrary, by diluting a gel you will obtain a sol. So, what makes a sol different from a gel is its fluid or gelatinous consistency. Also the temperature can determine the passage from sol to gel and vice versa. For example, broth gelatin is gelatinous at room temperature, but it becomes liquid when it is heated. Animal gelatin is a reversible gel because depending on the temperature it can pass from gel to sol and vice versa The albumen of eggs instead is not reversible because when heated it coagulates and it does not come back to the state of sol. Silica gel absorbs moisture and keeps its properties with broad concentrations of water. Because its affinity for water it is used as dehumidifier. When left to rest, a sol can spontaneously jell and come back to the state of sol simply by mixing it (eg: aqueous suspensions of kaolin).
1 - Making gelatin. Buy some dry gelatin. Dissolve it in warm water and, with subsequent dilutions, determine what is the minimum concentration of dry gelatin necessary to obtain a normal gelatin at room temperature. Do not keep gelatins a long time because they easily become cultures of bacteria. Store them in a refrigerator and, after a day, throw them away.
2 - Reversibility of the gelatin. By means of the temperature, make some gelatin pass from the gel to sol states and vice versa.
3 - Experiments with vegetable resin. Resins are gels and they possess useful properties. Often, fruit-bearing plants produce gelatinous spheroids which diameter can attain some centimeters. Conifers are important producers of resins and often you can collect drops of resin which hang from their trunk. You can also make an incision on a trunk to obtain some resin. Canada Balsam is a very important resin in optics and in microscopy. It is extracted from the Abies balsamea, a conifer of North America and it is used to glue lenses and to make permanent microscope slides. For their adhesive properties, resins take part to the composition of paints. Collect resin from trees, observe under the microscope the particles which are suspended in it. Dissolve the resin of a fruit-bearing tree in warm water and try to obtain a glue. Dissolve the resin of a conifer in turpentine and assess their adhesive properties.
4 - Experiments with polysaccharides. Polysaccharides are resinous
gums soluble in water. They are used in the fabrication of cosmetics, paper and in a lot
of other applications. Some polysaccharides are edible and are added in creams, yogurts
and in other foods. You can obtain some polysaccharides and experiment with their
properties. In particular, add to them some water and check the consistency, viscosity and
adhesiveness of the substance you will obtain.
Absolutely do not eat polysaccharides, do not inhale their powders and do not use them in recipes for food. If eaten dry, these substances will swell and risk obstruction of the digestive tract. If inhaled, they will swell and risk obstruction of the respiratory airways, causing dangerous problems in breathing. Do not use them in food recipes, but only in experiments. Keep in mind that some polysaccharides are not edible. When hydrated, these substances become culture medium for bacteria, so use them for a short time and then throw them away. An adult must be always present during these tests.
http://saps1.plantsci.cam.ac.uk/worksheets/ssheet22.htm Some Gum Fun (experiments with polysaccharides).
http://food.orst.edu/gums/foegeding.html Hydrocolloids, Vegetable Gums References.
http://class.fst.ohio-state.edu/FST605/lectures/lect20.html Gums and stabilizers (formula and other information).
Internet keywords: polysaccharides, hydrocolloids, experiments, recipes.
5 - Making photographic gelatin. Photographic gelatins have a
suspension of silver halide salts, which are sensitive to the light. When they are still
warm, these gelatins are spread on a transparent plastic film to obtain a photographic
film, or on a card to obtain paper for photographic prints. As shown through the history
of photography, there are many methods to produce photosensitive surfaces, and many of
them do not use silver salts. In the Internet you can find recipes to make photosensitive
films and paper by many techniques. These preparations require the use of substances and
procedures which can be dangerous. Read information on the caution needed. Children must
be guided by an adult who is expert in chemistry.
http://www.cheresources.com/photochem.shtml Chemistry of Photography
http://www.astro.wisc.edu/~mukluk/misc.html Miscellaneous Photographic Formulas and Information.
http://www.tri-esssciences.com/photography_books.htm Photography books of recipes.
William Crawford; The Keepers of Light : A History and Working Guide to Early Photographic Processes (a book).
Internet keywords: photographic gelatin recipe / formula, photography sensitizing processes, photography chemistry.
An emulsion is a dispersion of an insoluble liquid in another liquid. For instance, the oil is not soluble in water. If you pour some oil in a container with water, it will float it and keeps separate from the water. Instead, if you vigorously shake the container, you will obtain a dispersion of small drops of oil in water, however these drops quickly join together, so that in a short time nearly all the oil will return as before. To make the emulsion more stable, before shaking the container, add some detergent. The surfactant molecules will arrange on the surface of the oil drops with the heads outward. As these heads have an electrical charge and as this charge is always the same, the oil drops will repel each other and be unable to return to the homogeneous layer as before. So, surfactants can help you to obtain more stable emulsions. There are special surfactants for emulsions, endowed of a higher capability to stabilize the oil drops than the detergents. There are also emulsifying agents for alimentary use such as lecithin and emulsifiers for industrial purposes which are not edible. Butter is formed by small water drops suspended in fat. Cheese and mayonnaise too are considered emulsions. A lot of creams used both in pharmacy and in cosmetics are emulsions. Fuels emulsified with water have been produced. Emulsified oils are used in machine working to make it easier to cut metals with machine tools. In fact, metal cutting can create an intense heat, which has to be removed if you want to avoid burning the tools. The oil and water in the cutting fluid help remove the heat and make it possible to cut metals efficiently. Milk is another emulsion made up by small greasy drops in an aqueous phase.
|1 - Stability of the emulsions.
Fill two plastic bottles halfway with water, then put 5 cc (about a spoonful) of vegetable oil in each. Only in one of these bottles, put 0.5 cc (about 20 drops) of liquid detergent for dishes. Close the bottles and shake them for a couple of minutes to emulsify the oil, then place them on a table and observe them. The drops of oil will try to reassemble and to surface. By comparing the two emulsions, you will see that the one with detergent will be much more stable (figure 28). In fact, even after a month, the white color of this emulsion indicates that there is a great deal of small oil drops in the liquid, while in the other bottle the liquid is become nearly transparent, this is a sign that near all the oil drops have fused together and surfaced.
2 - Vinegar and vegetable oil. Using a kitchen whisk, emulsify a teaspoon of vinegar with 125 cc of peanut oil or olive oil. The emulsion will result instable.
Figure 28 - The two emulsions of the experiment 1 after 24 hours of rest. In the right bottle, some detergent has produced a more stable emulsion.
3 - Mayonnaise. To the ingredients of the test 2, add an egg yolk and emulsify again. The emulsion will be much more stable. Add some salt and if you want some pepper and you will have obtained a good mayonnaise. If you prefer, you can replace the vinegar with lemon juice. Why is the emulsion stable with the egg yolk? This is due to the presence of lecithin in the egg yolk. Lecithin is a surfactant and the molecules spread on the surface of the oil drops with the hydrophilic head outward. As these heads are electrically charged, the oil drops will repel and their merging is prevented. Lecithin is a phospholipid and it has a structure like that of the phospholipids which form the membranes of cells. Another well known lecithin and which you can find on the market is soy lecithin.
What is lecithin?
Internet keywords: emulsions, mayonnaise recipe, lecithin, soy lecithin, homemade butter, handmade butter, creams
Foam is a dispersion of a gas in a liquid (liquid foams) or in a solid (solid foams). Among the liquid foams, we have the ones produced by soaps and detergents, and various foods such as wine, beer and many others. Among the solid foams we have Pumice stone, earthenware, sponges, expanded plastics like expanded polystyrene and expanded polyurethane. By dispersing helium in a liquid which produced bubbles with very thin walls and which then solidified, some researchers succeeded in fabricating a solid foam lighter than air.
1 - Foam and shape of the bubbles in contact. With a drop of liquid
detergent in a small basin of water, make a foam. Observe the shape of the bubbles which
are in contact each other. With a microscope, observe a thin section of elder pith and
compare it with the foam.
2 - Make a solid foam. Beat egg whites and some sugar, then cook it so to obtain its solidification: you will have obtained a meringue, just an edible solid foam.
Develop a lighter-than-air solid (a discussion)
http://stardust.jpl.nasa.gov/tech/aerogel.html Aerogel, a foam with surprising properties.
OTHER EXPERIMENTS WITH SURFACTANTS AND COLLOIDS
1 - Who can guess more colloids? List the colloids you have in your home or which you know by experience: (milk, mayonnaise, resin, paint, ink, expanded polystyrene, cell cytoplasm, blood serum, etc.).
2 - A half-solid fluid. Put in a cup four spoons of corn starch. Add some water until you have obtained a creamy substance. While mixing, you will notice that this substance has an odd property: if you slowly mix it, it behaves like a liquid, but if you try to mix it fast, it seems solid. By quickly lifting it on a side, you will be also able to remove this cream from the cup, but you will have some difficulties in keeping it in your hands because, even if it moves slowly, it will escape from all sides like a liquid. Liquids which change viscosity with the mixing speed are called dilatant fluids. Also wet sand behaves as dilatant fluid. Sold in the US as a childs toy under the name of Gak or Goo, you can make your own by dissolving 1/2 cup of white glue with 1/2 cup of water, then adding 3 tablespoons of Borax, while stirring well. You will obtain a substance which is apparently solid, but which loses its shape within some minutes, becoming like a liquid puddle... which however you will able to lift it as if it was a carpet.
ATOMIZER FOR AEROSOL
How do atomizers work? There are many models of atomizers or of sprayers like those of pressurized spray paint cans, or those provided with a small pump that you press with a finger, those that work by mean of a rubber syringe or, for industrial uses, by a compressor.
|1 - Anatomy of an atomizer.
trigger spray bottle. Often, these devices breaks so, if you have one of them broken,
dismantle it to try to understand why it does not work any more and try to repair it.
2 - Build an atomizer. To build a small atomizer, take two thin straws and fix them as shown in figure 30. At the end of the horizontal straw, insert a plug with a hole of one mm of diameter. Under the vertical straw, mount a small bottle with water. Now, blow with force in the horizontal straw. The air jet which comes out of the hole will cause an area of low pressure above the vertical can which will draw some water up the straw and blow it away atomizing it. To produce an air jet, you can also use a rubber syringe. Usually, this type of atomizers is used for perfumes, but you can use it also to humidify the leaves of a house plant.
A short introduction to the colloids.
http://dwb.unl.edu/Teacher/NSF/C01/C01Content.html A lot of experiments on water, solutions, etc.
http://www.eduplace.com/science/profdev/handbook/solutions.html Recipes: Solutions and Materials
http://www2.ncsu.edu/ncsu/pams/science_house/learn/CountertopChem/index.html The Science House. Also experiments on colloids and surface tension.
http://www.synthashield.net/vault/colloids.html What are Colloids & Colloidal Suspension? Definition and history of the colloids.
http://www.acs.ucalgary.ca/~schramm/ Laurie's Colloid & Interface Science Page. A lot of useful information and a good bibliography.
http://www.webcrawler.com/education/science_and_nature/chemistry/disciplines/ Interesting links of chemistry and polymers.
http://www.ch.kcl.ac.uk/kclchem/staff/arr/gloss.htm Hypertext Guide to Terms in Colloid and Polymer Science
http://www.solgel.com/educational/glossary.htm Terms and words frequently used in the Sol-Gel area.
http://www.luxurylane.com/thelibrary/index.htm Toiletries Listservice. Making your own Lotions, Creams, Soaps, Personal Care Products, and related subjects. Recipes to make cosmetic products.
http://alex.edfac.usyd.edu.au/methods/science/studentwork/Lifestyle_Chemistry.html Lifestyle Chemistry
Google, Excite, Yahoo With these or other search engines, look for these terms: colloids, colloidal systems, solutions, mixtures, sol, gel, gelatin, emulsion, foam. You will find a deal of interesting information.
Surface phenomena and colloids concern many objects, products and events of our everyday life, which are not immediately explainable with the physics we usually study at school. Having introduced some principles, and suggested some experiments in this field which until now may have been quite mysterious seemed to us useful and important. Not only, but you have also noticed how fascinating these topics are and how amusing is to do laboratory activities with them.
Books on surfactants and colloids.
R. Aveyard, D. A. Haidon: "An Introduction to the Principles of Surface Chemistry", Cambridge Chemistry Texts
C. C. Miller, P. Neogi: "Interfacial Phenomena", Marcel Dekker inc., N.Y. (1985).
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