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What is Soap?                             

Definition: Soap is a cleansing agent made from the interaction of fats and oils with alkali.


Records mentioning the use of soapy materials date from ancient times. Soap making was common in Italy and Spain during the 8th century. By the 13th century, the soap industry had traveled into France. Most soap was produced by using the tallow of goats with beech ash (furnishing the alkali). The French devised a method of making soap from olive oil. In 1783, a Swedish chemist accidentally simulated the reaction that occurs in the present-day boiling process of soap making. He produced a sweet-tasting substance that is now known as glycerin. In 1823, a French chemist discovered the chemical nature of the ingredients used in soap.


Oils and fats for soap are compounds of glycerin and a fatty acid. When oils are mixed with an alkali, they form glycerin and the sodium salt of the fatty acid. The fatty acids required for soap making are supplied by tallow, grease, fish oils, and vegetable oils. The hardness, lathering qualities, and transparency of soap vary according to the combinations of fats and alkalis used as ingredients. An experienced soap crafter uses many combinations of oils.

How does it work?

Most soaps remove grease and dirt because some of their components are surfactants (surface-active agents). Surfactants have a molecular structure that acts as a link between water and the dirt particles. This loosens the particles from the underlying fibers or surfaces to be cleaned. One end of the molecule is hydrophilic (attracted to water), and the other is hydrophobic (attracted to substances that are not water soluble). This peculiar structure allows soap to adhere to substances that are otherwise insoluble in water. The dirt is then washed away with the soap.

Chemistry 101 - The Scientific Explanation

We have talked a bit about water solubility but have not really discussed why some things are soluble in water while others are not. We can in general divide compounds into ionic compounds (like salt, potash, and lime), polar compounds (like water and alcohol), and non-polar compounds (like fats, oils, and gasoline).

Let us begin by talking about the structure of water. Water molecules consist of 2 hydrogen atoms and an oxygen atom with the oxygen in between the two hydrogens and a bond angle of about 104 degrees. Oxygen is far more electronegative than hydrogen and so it tends to hog more of the electrons. Consequently the water molecule is polar, with a positive charge at one end of the molecule and a negative charge at the other. In the first figure, the molecule on the left shows two hydrogen atoms and an oxygen atom bound together into a water molecule. The molecule on the right shows the distribution of charges on the water molecule. A red color denotes a negative charge, while a blue color denotes a positive charge. The positive end of one water molecule will be strongly attracted to the negative end of another water molecule. When an ionic compound, like sodium chloride, dissolves in water, oxygen (negative) end is attracted to the cations (positive ions) while the hydrogen (positive) end of the molecule is attracted to the anions (negative ions). The solubility of a substance in water is largely determined by the relative strength of the attraction of water to the substance compared to the strength of the attraction between water molecules.

In contrast to oxygen, carbon has almost the same electronegativity as hydrogen and the carbon-hydrogen bond is non-polar. For example, the octane molecule (a component of gasoline) consists of 8 carbon atoms in a chain, with 2 hydrogens attached to the interior carbons and 3 hydrogens on the end carbons. Since the electrons are not hogged by any of the atoms, the molecule is electrically neutral along its entire length. In the second figure, the molecule on the left shows eight carbon atoms and eighteen hydrogen atoms bound together into an octane molecule. The molecule on the right shows the distribution of charges on the octane molecule. No regions of red and blue show up because there are no strongly negative or strongly positive regions in the molecule. Instead, the molecule is green, which denotes neutrality in this figure.

The simplest way to understand solubility is to remember the rule "like dissolves like," that is polar and ionic substances are soluble in polar and ionic substances while non-polar substances are soluble in non-polar substances. Thus salt dissolves in water but not in gasoline. Oil dissolves in gasoline but not water.

Now, living cells need both polar and non-polar substances. The cell uses non-polar substances, fats and oils, to make up the cell membrane which separates the interior of the cell from the exterior. If the cell membrane were soluble in water, it would dissolve away and soon there would be nothing to divide the cell from the non-cell. But in order to get to the cell in the first place, all the parts of the cell must be water soluble because that's how materials get transported from place to place. What nature needs is a non-polar material that can be dissolved, moved around, and then made non-polar again. This material is known as a lipid, or triglyceride.

A lipid consists of two parts, a fatty acid, and a type of alcohol called glycerol, or glycerine. The fatty acid by itself and the glycerol by itself are both water soluble because of the polar oxygen atoms on the ends of these molecules. In a lipid, three fatty acids are bonded to the three oxygens on the glycerol. Although the oxygens are still there, they are now buried way down inside the molecule and the lipid is essentially non-polar and therefore insoluble in water.

Now fatty acids and glycerol may seem pretty exotic, but they are variations on molecules with which we are already familiar. Glycerol (aka glycerine) is simply a tri-alcohol, i.e. an alcohol with three OH groups. It has chemistry similar to that of ethanol. Whereas ethanol is C2H5OH, glycerol is C3H5(OH)3. The chemistry is dominated by the properties of the OH group. Because the OH group is polar, alcohols tend to be soluble in water.

We are also familiar with an organic acid, acetic acid, the acidic component of vinegar. Were as acetic acid is CH3COOH, a fatty acid has formula CnH2n+1COOH. The chemistry is dominated by the properties of the COOH group. Because this group is polar, fatty acids tend to be soluble in water. Octanoic acid, C8H17COOH, is just one of a very large number of fatty acids. In fact, most fatty acids are longer than octanoic acid. Two very common components of lipids are palmitic acid (C15H31COOH) and stearic acid (C17H35COOH). Solid lipids are generally called fats.

All the fatty acids we have discussed so far are saturated, i.e. they have 2n+1 hydrogens for every n carbons. Another class of fatty acids are the unsaturated fatty acids, with less than 2n+1 hydrogens for every n carbons. Oleic acid, for example, has formula C17H33COOH and linoleic acid has formula C17H31COOH.

Saturated fats contain saturated fatty acids and are solids at room temperature. Lard, and butter are examples of saturated fats. Soap made from these fats tends also to be solid at room temperature. Unsaturated fats contain unsaturated fatty acids and are liquids at room temperature. Generally called oils, examples include corn oil and safflower oil. These oils produce liquid soap. While unsaturated fats are generally more healthy than saturated fats, many times a liquid fat is not convenient. For example, margarine is made from unsaturated plant oils (e.g. corn oil) which has been hydrogenated to produce a saturated (solid) fat.

To make soap, we must break the fat into its fatty acid and glycerol constituents. The fatty acid has a long hydrocarbon tail which is soluble in fats, and a polar oxygen end which is soluble in water. Thus a fatty acid in solution acts as a soap by dissolving fats in one end of the molecule and water in the other. When we use a strong base, such as lye to break apart or hydrolyse the fat, the fatty acid is present as a large cation which is polar at one end and non-polar at the other. Just as we can have sodium chloride and sodium carbonate which are soluble in water, we can have sodium octanoate, the sodium salt of octanoic acid, which is also soluble in water.

Let's take a fat derived from palm oil (containing palmitic acid) and hydrolyse it using sodium hydroxide. Saponification is the term applied to the hydrolysis of fats using a strong alkali like lye. The reaction is
[C15H31CO]3[C3H5O3](s) + 3 NaOH(aq) -----> 3 C15H31COONa(aq) + C3H5(OH)3(aq) fat(s) + 3 lye(aq) -----> 3 sodium palmitate(aq) + glycerol(aq)
While this reaction may appear intimidating because of the long formulas, it is, in fact, quite simple. It could be written generally as
[RCO]3[C3H5O3](s) + 3 NaOH(aq) -----> 3 RCOONa(aq) + C3H5(OH)3(aq)
Where "R" is some long carbon hydrogen chain. If you look on a list of ingredients on a soap, you will find things like "sodium stearate," "sodium palmitate," or, generally, "sodium somebiglongnameate." This is simply specifying the particular fatty acids present in the soap.

When fat is introduced to a soap solution, the non-polar tail of the fatty acids dissolves in the non-polar fat, leaving the water-soluble oxygen end at the surface of the fat globule. With enough soap, these fat globules become covered with a water-soluble coating and disperse throughout the solution, as in the last figure. They are not truly dissolved since individual fat molecules are not dispersed in the solution. Rather, we say the fat is emulsified. Notice the glycerol molecule in the upper right hand corner of the figure.

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