Soaps and soapless detergents

YOU SHOULD BE ABLE TO:

Explain hydrolysis of esters, including saponification. Compare soapy and soapless detergents.

Saponification is the process of making soap by the hydrolysis of fats and oils using an alkali. For example, the making of soap by heating animal fat or vegetable oil with concentrated sodium bydroxide (NaOH). Soaps are, therefore, the alkali metal (Na, K or Li) salts of fatty acids.

The oil or fat would represent the ester formed from the combination of glycerol with a fatty acid. When sodium hydroxide is added, the ester is hydrolysed to form glycerol, and the sodium ion replaces the hydrogen on the acid to form the sodium salt (a soap).

EXAMPLE

Fat/ Oil (Ester) + sodium hydroxide -----> glycerol + sodium stearate (soap)

The long hydrocarbon chain forms the hydrophobic end which repels or hates water. The ionic part forms the hydrophilic or water-loving end.

Soapless detergents are made from petroleum products treated with concentrated sulphuric acid. Here, crude oil is used instead of animal fats and vegetable oils. In soapless detergents, the hydrophilic end contains a sulphonate ion from the use of the sulhuric acid.

Soaps are usually biodegradable since they are made from plant or animal products, while soapless detergents are nonbiodegradable products that can end of harming the environment. The phosphate levels in some of these detergents influence pollution of water bodies as a result of nutrient enrichment. Nowadays, some detergents are made ‘phosphate free’, which lessens their impact on waterways.

When used with hard water (water having dissolved calcium and magnesium salts), soap reacts with the calcium and magnesium ions to form insoluble precipitates called scum. The presence of scum is unsightly as it sticks to clothes. As scum sticks to clothing, the cleaning action of the soap is affected and, hence, results in the use of more soap to wash with.

Detergents can clean effectively, even in acidic mediums, as they are made from the salts of strong acids (e.g., sulphuric acid). They are more soluble in water and have a stronger cleansing action. Unlike soap, less detergent would be needed when washing, as it is not being used up to form scum. The use of detergents would also result in conserving natural vegetable oils and animal fats, which can serve other purposes. Unfortunately, the fact that they are made from petroleum products will only serve to deplete these non-renewable resources.

REVIEW 34.1

1. How are soaps made? 2. What is the main difference between a soap and a detergent? 3. Why is more soap used when washing with hard water? 4. What are the advantages and disadvantages of using detergents and soaps? 5. Soap making is similar to which reaction of esters?