What are the uses of saponification

Soaps and surfactants

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Surfactants are molecules that are composed of a polar head and a non-polar residue. Surfactants are soluble in polar and non-polar solvents and are therefore called surface-active substances.

Due to their washing effect, surfactants are mostly used for body and surface cleaning.

Soaps are special surfactants. These are sodium or potassium salts of fatty acids. They have the typical structure of surfactants: a non-polar radical R and a carboxy group as a polar head.

  • Soaps made from the sodium salts of a fatty acid are solid soaps. They are called curd soaps.
  • Soaps that consist of the potassium salts of a fatty acid are liquid soaps and they are called soft soaps.

Manufacture of soaps by saponification

Soaps are made by allowing a fat to react in a saponification process. There are two options: basic and acidic saponification.
The two mechanisms are shown below. The reaction is only shown for one ester group so that it is a bit clearer.

Basic saponification

Basic saponification is a reaction in which a base is involved. To make a soap, caustic soda (making a curd soap) or potassium hydroxide (making a soft soap) is required.

The lone pair of electrons of the hydroxide ion of the alkali attacks the positively charged carbon atom of the ester.

Now three oxygen atoms are bound to one carbon atom, which is why a single bond between the carbon atom and an oxygen atom must be broken. The only thing that comes into question here is the single bond between the carbon atom and the oxygen atom to the left of the carbon atom in question (if we break the one below, we would get back to the beginning, and if we break the top one would result in a very unstable O ^ (2 -) - ion arise).

But now the reaction is not ready. In a further reaction step, the carboxylic acid releases its proton, which is taken up by the alcoholate ion.

But why doesn't the proton just stay where it is? This is because the negative charge created by deprotonation is stabilized by mesomeric boundary structures. With alcoholation there are no mesomeric boundary structures and therefore the negative charge cannot be stabilized. The alcoholation is therefore significantly more unstable than the deprotonated carboxylic acid and this reaction step takes place.

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Acid saponification

Acid saponification, as can be seen from the name, is a reaction in which an acid is involved. This acid acts as a catalyst here. The lone pair of electrons on the carbonyl oxygen atom of the ester group attacks the proton given off by the acid. The attack creates a positive charge on the carbon atom. This reaction step is necessary so that the saponification can function without a base.

The resulting cation can now react with water in a nucleophilic substitution. Here again a molecule is created in which three oxygen atoms are bound to one carbon atom. A single bond must be broken. Here again only the single bond between the carbon atom and the oxygen atom to the left of the carbon atom comes into question. So that this single bond can be broken more easily, the proton is first rearranged from the oxygen atom on the right to the oxygen atom on the left. Due to the resulting positive charge, the oxygen atom is no longer willing to share the single bond. As a result, it can be broken open more easily.

Then the positive charge on the carbon atom has to be balanced. Apart from that, the catalyst, i.e. the proton, has to be restored since a catalyst has to emerge unchanged from the reaction. The proton is split off again and the electron pair can "fold in" again.

The reaction product is a fatty acid. But to get a soap we need a sodium or potassium salt of a fatty acid. To make a soap, we have to react the resulting fatty acid with sodium or potassium hydroxide solution.

Application of soaps through saponification

Reduction of the surface tension

Water has a strong surface tension. This is due to the intermolecular hydrogen bonds. The water molecules attract each other through these interactions. On the surface of the water, this means that the water molecules are drawn inwards. This creates the surface tension.

Adding surfactants to water lowers the intermolecular forces of the water, as the surfactants accumulate between the water molecules. This leads to a lowering of the surface tension of the water.

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Formation of micelles

Although surfactants have a long-chain, non-polar residue, they can be dissolved in water. This is because they form what are known as micelles. These are spherical structures that are created when the non-polar residues attract each other, as intermolecular interactions act between them. The polar heads of the surfactants form an outer shell. As a result of this micelle formation, surfactants can not only attach to the surface of the water, but also dissolve in the water.

Washing effect of soap

If non-polar dirt accumulates on a fiber, it cannot be removed with water because it does not dissolve in water. With the help of surfactants, the dirt can be loosened from the fiber. The following processes take place:

The fiber and dirt surfaces are wetted with surfactants. The non-polar residues of the surfactants interact with the non-polar surfaces, which creates wetting.

Since the surfactants are not arranged next to one another in a linear manner, the charged carboxy groups come into contact with one another. These repel each other, breaking the dirt apart. This creates more space for renewed surfactant deposits.

As before, the charged carboxy groups repel each other. This gradually separates the dirt from the textile fiber.

After the dirt has been completely separated from the textile fiber, so-called dirt-surfactant micelles are created, i.e. the dirt is completely surrounded by surfactants. Surfactants accumulate on the entire surface of the textile fiber, which means that the dirt can no longer accumulate on the fiber. So the dirt is permanently removed.

Disadvantages of soaps

Soaps have some drawbacks, which is why we use synthetic surfactants for washing these days.

  1. Soap molecules form hydroxide ions in water, which means that an alkaline solution is created. This attacks the skin, textile fibers and washing machine components and permanently damages them. In addition, this reaction creates a fatty acid molecule that no longer has any washing effect. The washing effect is also reduced.
  2. Soaps are sensitive to acids. If an acid is added to the soap solution in any form (e.g. because you have wiped up vinegar with a cloth and are now washing it), the washing effect is reduced because the soap molecules react with the acid. This creates a fatty acid molecule that has no washing effect.
  3. Soaps are sensitive to hardness. The washing effect is considerably reduced when using hard water, as the soap molecules react with the calcium ions present and form sparingly soluble salts which no longer have a washing effect. These salts then also accumulate on clothing.

Because of these disadvantages, synthetic surfactants have been developed which do not have these disadvantages. The most common representatives of the synthetic surfactants are the so-called alkylbenzenesulfonates. They have the following structure (where n is a number between 8 and 16):

Alkylbenzenesulfonates react neutrally in aqueous solution, are not sensitive to acids and are only slightly sensitive to hard water.

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Surfactants are not only used as washing substances, but also as emulsifiers. In this process, non-polar particles are surrounded by the non-polar residues of the surfactants, creating spherical structures with a polar shell, as in the case of micelles. Through the polar layer, this structure can then interact with the polar substance and it is dissolved in it.