In this activity, you can:
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Soaps and detergents are made from long molecules that contain a head and tail. These molecules are called surfactants; the diagram below represents a surfactant molecule.
The head of the molecule is attracted to water (hydrophilic) and the tail is attracted to grease and dirt (hydrophobic). When the detergent molecules meet grease on clothes, the tails are drawn into the grease but the heads still sit in the water.
The attractive forces between the head groups and the water are so strong that the grease is lifted away from the surface. The blob of grease is now completely surrounded by detergent molecules and is broken into smaller pieces which are washed away by the water. You can find out more about how detergents work here.
The detergent molecules also help to make the washing process more effective by reducing the surface tension of the water. Surface tension is the force which helps a blob of water on a surface hold its shape and not spread out. The surfactant molecules of the detergent break apart these forces and make water behave, well, wetter!
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Bubbles and soap films are made of a thin layer of water, sandwiched between two layers of soap molecules. You can make giant bubbles by mixing these ingredients together:
Once you've made your bubble solution, you can try our four experiments!
Use your hands to make a hoop-shape. Dip them in the bubble solution and blow gently but firmly. Using this method you should be able to blow bubbles up to about 60 cm in diameter!
Dryness not sharpness breaks bubbles. Blow a large bubble then try putting your fingers inside it. If your hand is wet you can touch and even place your hand inside the bubble without bursting it! A soap bubble is only 1/500,000th of a centimetre thick as it starts to pop!
Make a large hoop of string about 1 metre in diameter and tie 4 small loops at the corners to make handles. Dip this into the soap solution and with a friend pull the handles apart to form a giant soap film. Trying shaking one end and watch the wave travel along the film.
Now try and make a bubble dome, like our diagram below.
Wet a tray or the kitchen work-surface with your bubble solution. Using a straw, blow a large bubble. Push the straw through the original bubble and blow a smaller one inside. See how many smaller bubbles you can make!
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"The introduction of the Dr. Marten size 338 finally allowed Annie to walk on water"
Water has many unusual properties, one of which is the phenomenon of surface tension. As we said earlier, surface tension is the force that prevents a blob of water on a surface from spreading out. Surface tension allows pond skaters and other insects to walk across water and also allows a pin to float.
You can demonstrate this yourself by taking a bowl of water and floating a pin on the surface. Carefully add just one drop of washing-up liquid and see what happens to the pin. It should sink immediately because the detergent molecules break apart the forces holding the water together. The pin is no longer supported and so sinks to the bottom!
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You can measure surface tension yourself by making your own button balance, like the one used by the famous nineteenth century home experimentalist, Agnes Pockels. You will need:
Here's how to build your button balance:
You can set up the balance in one of two ways, as shown in the diagram above. You'll soon find out which one works best for you. The lollystick is used for the lever and the nylon thread has the advantage of not soaking up water and influencing the balance. The piece of card can be suspended from the lollysick with the nylon thread to act as a counterbalance pan. Small squares of graph paper can be used as weights – you can weigh a large number of whole graph paper sheets and work out from this what each small square weighs. To use your balance:
With your button balance, try measuring the surface tension of a range of liquids and comparing them. For example: cold water, salt water, warm water and soapy water. You can also try changing the size of the button used or the material it is made out of.
After completing this section, you should be able to
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Make certain that you can define, and use in context, the key terms below.
Soap making has remained unchanged over the centuries. The ancient Roman tradition called for mixing rain water, potash and animal tallow (rendered form of beef or mutton fat). Making soap was a long and arduous process. First, the fat had to be rendered (melted and filtered). Then, potash solution was added. Since water and oil do not mix, this mixture had to be continuously stirred and heated sufficiently to keep the fat melted. Slowly, a chemical reaction called saponification would take place between the fat and the hydroxide which resulted in a liquid soap. When the fat and water no longer separated, the mixture was allowed to cool. At this point salt, such as sodium chloride, was added to separate the soap from the excess water. The soap came to the top, was skimmed off, and placed in wooden molds to cure. It was aged many months to allow the reaction to run to completion.
All soap is made from fats and oils, mixed with alkaline (basic) solutions. There are many kinds of fats and oils, both animal and vegetable. As previously discussed, fats are usually solid at room temperature, but many oils are liquid at room temperature. Liquid cooking oils originate from corn, peanuts, olives, soybeans, and many other plants. For making soap, all different types of fats and oils can be used &#; anything from lard to exotic tropical plant oils.
Carboxylic acids and salts having alkyl chains longer than eight carbons exhibit unusual behavior in water due to the presence of both hydrophilic (CO2) and hydrophobic (alkyl) regions in the same molecule. Such molecules are termed amphiphilic (Gk. amphi = both) or amphipathic. Fatty acids made up of ten or more carbon atoms are nearly insoluble in water, and because of their lower density, float on the surface when mixed with water. Unlike paraffin or other alkanes, which tend to puddle on the waters surface, these fatty acids spread evenly over an extended water surface, eventually forming a monomolecular layer in which the polar carboxyl groups are hydrogen bonded at the water interface, and the hydrocarbon chains are aligned together away from the water. This behavior is illustrated in the diagram on the right. Substances that accumulate at water surfaces and change the surface properties are called surfactants.
Alkali metal salts of fatty acids are more soluble in water than the acids themselves, and the amphiphilic character of these substances also make them strong surfactants. The most common examples of such compounds are soaps and detergents, four of which are shown below. Note that each of these molecules has a nonpolar hydrocarbon chain, the "tail", and a polar (often ionic) "head group". The use of such compounds as cleaning agents is facilitated by their surfactant character, which lowers the surface tension of water, allowing it to penetrate and wet a variety of materials.
When minute amounts of soaps are put into water, instead of forming simple solutions, the molecules become concentrated at the surface of the water, with the saltlike ends sticking down into the water and the hydrocarbon chains forming a layer on the surface. This arrangement greatly reduces the surface tension of the water and contributes to the startling properties of soap films and bubbles. At higher concentrations, the solutions become turbid as the result of micelle formation. Micelles are spherical aggregates of soap molecules, wherein the hydrocarbon chains form a region of low polarity that is stabilized by having the polar salt ends of the molecules in contact with the water. By gathering the hydrophobic chains together in the center of the micelle, disruption of the hydrogen bonded structure of liquid water is minimized, and the polar head groups extend into the surrounding water where they participate in hydrogen bonding.
The importance of soap to human civilization is documented by history, but some problems associated with its use have been recognized. One of these is caused by the weak acidity (pKa ca. 4.9) of the fatty acids. Solutions of alkali metal soaps are slightly alkaline (pH 8 to 9) due to hydrolysis. If the pH of a soap solution is lowered by acidic contaminants, insoluble fatty acids precipitate and form a scum. A second problem soaps are less effective in hard water, which is water that contains a significant concentration of magnesium, calcium and iron ions. These ions form precipitates with soap molecules, and this precipitate is often seen as a gray line on a bathtub or sink and is often called &#;soap scum&#;. Since soap forms a precipitate with these ions, it means that many of the soap molecules are no longer present in the solution. Therefore, soap will form fewer suds in hard water. &#;Soft water&#; is water that contains very few or no ions that precipitate with soap. Soap will therefore be much more effective in soft water than in hard water.
These problems have been alleviated by the development of synthetic soaps called detergents. Detergents are similar to soaps in that they have a charged head group and a long nonpolar tail group, but they are not prepared from natural fats or oils. Detergents are useful because they do not form precipitates with magnesium, iron or calcium ions, which means that they work in both soft and hard water. By using a much stronger acid for the polar head group, water solutions of detergents are less sensitive to pH changes. Also the sulfonate functions used for virtually all anionic detergents confer greater solubility on micelles incorporating the alkaline earth cations found in hard water. Variations on the detergent theme have led to the development of other classes, such as the cationic and nonionic detergents shown above. Cationic detergents often exhibit germicidal properties, and their ability to change surface pH has made them useful as fabric softeners and hair conditioners. These versatile chemical "tools" have dramatically transformed the household and personal care cleaning product markets over the past fifty years
Sodium Lauryl Sulfate (a non-biodegradable detergent)
After detergents started being widely used, it was discovered that they were not broken down in sewage treatment plants. Many streams and lakes became contaminated with detergents and large amounts of foam appeared in natural waters. Biodegradable detergents were then developed. Shown below is an example of a biodegradable detergent, sodium laurylbenzenesulfonate.
Sodium Laurylbenzenesulfonate (a biodegradable detergent)
Explain how soaps or surfactants decrease the surface tension of a liquid. How does the meniscus of an aqueous solution in a capillary change if a surfactant is added? Illustrate your answer with a diagram.
Adding a soap or a surfactant to water disrupts the attractive intermolecular interactions between water molecules, thereby decreasing the surface tension. Because water is a polar molecule, one would expect that a soap or a surfactant would also disrupt the attractive interactions responsible for adhesion of water to the surface of a glass capillary. As shown in the sketch, this would decrease the height of the water column inside the capillary, as well as making the meniscus less concave.
Draw the structure of calcium stearate, a component of soap scum.
(C17H35COO)2Ca2+
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