Soaps and Detergents: Preparation, Structure & Differences

Soaps and Detergents

A substance capable of removing dirt and grease is called a detergent. Both soap and detergents are cleaning agents as they can remove any dirt or grease. Hence both are used to wash clothes in the laundry, clean our body, hair shampoos, cleaning utensils, textile industries, etc. The detergents are of two types, namely,

1. Soapy detergents
2. Non-soapy detergents

The soapy detergents are ‘soaps’, whereas the non-soapy detergents are ‘synthetic detergents’ or ‘detergents.
Let us discuss soap in detail first.

Soap

Soap is the sodium or potassium salt of a long-chain carboxylic acid or glycerol. The carboxylic acids contain chains of \(15\) to \(18\) carbon atoms. Glycerol is an alcohol-containing three hydroxyls \(\left( {{\rm{OH}}} \right)\) group.

Thus, sodium palmitate, sodium stearate, sodium oleate, sodium linoleate are examples of soaps.

Soap: Manufacture

The soap is prepared by heating animal fats or vegetable oils (olive oils, castor oil or palm oil) with sodium hydroxide or potassium hydroxide.

Let us now explain the process of manufacture of soap.

Soap can be manufactured by heating fat or vegetable oils with concentrated sodium hydroxide solution to form soap and glycerol. The reaction involved is as follows:

The process of manufacture of soap is called saponification reaction. Glycerol is a very useful by-product of this reaction as it is used to prepare many organic compounds, explosives and medicines.

Note
1. Animal fats are also used for the preparation of soaps.
2. Vegetable oils such as castor oil, cottonseed, linseed oil, coconut oil, palm oil, olive oil, etc., can also be used to prepare soap.

We shall discuss the preparation of soaps in our homes or school laboratory.

Preparation of Soap in a Laboratory or at Home

Materials Required:

1. Vegetable oil (such as cottonseed, linseed oil, coconut oil or palm oil)
2. Distilled water
3. Sodium hydroxide or Caustic soda \(\left( {{\rm{NaOH}}} \right)\)
4. Sodium chloride or Common salt \(\left( {{\rm{NaCl}}} \right)\)

Procedure:

1. Step 1- Take \({\rm{20}}\,{\rm{mL}}\) of castor oil in a beaker.
2. Step 2- Mix \({\rm{20\% }}\) sodium hydroxide solution in distilled water and add it to the castor oil.
3. Step 3- Dip red and blue litmus paper strips in the solution.
4. Step 4- Now, heat the mixture with constant stirring till a paste of soap is formed.
5. Step 5- To this mixture, add \({\rm{5 – 10}}\,{\rm{g}}\) of common salt.
6. Step 6- Stir the mixture continuously using a glass rod till the soap starts to set.
7. Step 7- Leave the mixture for a day till it cools and becomes solid.
8. Step 8- Cut the solid into desired shapes and sizes.
9. Step 9- You can also add perfumes before the soap sets.

Addition of Sodium Chloride in Soap Making

Most of the soap separates on its own, but some soaps remain in an aqueous solution. To separate the soap from the aqueous solution, sodium chloride is added to the aqueous solution. When \({\rm{NaCl}}\) is added to the aqueous solution, the solubility of soap in common salt decreases, due to which all the soap separates out from the solution as a precipitate. This is called salting out of soaps.

To understand the cleansing action of soap, we should know about its structures. Let us discuss the structure of the soap molecule.

Structure of Soap Molecule

A soap molecule consists of two parts,a long hydrocarbon tail part and a short negatively charged head.

A long hydrocarbon chain being non-polar is insoluble in water but is soluble in oil. Thus, it is hydrophobic and forms the water-repelling tail. A negatively charged head consists of a carboxylate ion which, being polar, is soluble in water and hence is hydrophilic, forming a water-attracting head. These two unique properties of the soap molecules help in cleaning dirty clothes. We shall now discuss micelle formation, which is important for the cleaning action of soap.

Cleaning Action of Soaps

Soaps are widely used as cleansing agents. The cleansing action is mainly due to its ability to emulsify the greasy or oily dirt through micelle formation. The mechanism of micelle formation in soap solution and the cleansing action of soaps are described below.

Micelle Formation in Soap Solution

A soap can be represented as \({\rm{RCOONa,}}\) where R represents a long chain alkyl group. When dissolved in water, soap ionises to give \({\rm{RCO}}{{\rm{O}}^ – }\) ion. The \({\rm{RCO}}{{\rm{O}}^ – }\) ion possesses two parts-long hydrocarbon chain \({\rm{R}}\) (tail), and a polar group \( – {\rm{CO}}{{\rm{O}}^ – },\) as shown in figure(a).

The hydrocarbon tail R is non-polar and hydrophobic, i.e., water-repelling whereas, the \( – {\rm{CO}}{{\rm{O}}^ – }\) group being polar is hydrophilic, i.e. water-loving. Therefore, \({\rm{RCO}}{{\rm{O}}^ – }\) ion orients itself so that the \( – {\rm{CO}}{{\rm{O}}^ – }\) end dips in the water, and the hydrocarbon tail \({\rm{R}}\) orients away from water. The \( – {\rm{CO}}{{\rm{O}}^ – }\) groups of different \({\rm{RCO}}{{\rm{O}}^ – }\) ions tend to stay far away from one another due to the like charges, whereas the R-groups try to approach each other to form a bunch. This leads to the formation of a micelle, as shown in figure (b).

\({\rm{RCOONa}} \to {\rm{RCO}}{{\rm{O}}^ – } + {\rm{N}}{{\rm{a}}^ + }\)

Thus, a soap micelle is a negatively charged colloid particle in which the negatively charged \( – {\rm{CO}}{{\rm{O}}^ – }\) groups are arranged in a spherical manner at the surface. In contrast, the hydrocarbon chains point towards the centre.

The \( – {\rm{CO}}{{\rm{O}}^ – }\) groups present at the surface of the micelle get surrounded by \({\rm{N}}{{\rm{a}}^ + }\) ions which tend to drag the micelle in the bulk of the solution. A micelle may contain as many as \(100\) molecules or more. The minimum concentration of soap at which it forms micelle is called critical micelle concentration or \({\rm{CMC}}.\)

The Cleansing Action of Soaps

Water is not capable of wetting oily or greasy substances. However, the hydrocarbon residue \({\rm{R}}\) of the soap anion \(\left( {{\rm{RCO}}{{\rm{O}}^ – }} \right)\) can do so.

When a dirty cloth due to the deposition of dust and oil or greasy substances is dipped into a soap solution, the hydrocarbon non-polar part \({\rm{R}}\) (hydrophobic) of \({{\rm{RCO}}{{\rm{O}}^ – }}\) ion dissolves as polar impurities of oily or greasy dirt and encapsulates it in the micelle. When the cloth is rinsed with water, the micelles carrying the oily or greasy dirt are washed away. The cleansing action of soap is shown in the figure.

Using Soap as a Cleansing Agent: Limitations

Soft water like distilled water or rainwater forms lathers easily with soap, whereas hard water like river water or tap water does not easily lather with soap. The presence of calcium and magnesium ions makes water hard. These ions form insoluble calcium and magnesium salts with soaps called scum. Scum stick to the fibre of the cloth to be cleaned and makes cleaning difficult. Moreover, a large amount of soap gets wasted in the formation of scum.

Synthetic Detergents

This limitation of using soap as a cleansing agent is overcome by using another class of compounds called detergents.

Examples of detergents

Detergents are usually used to make shampoos and products for cleaning clothes.

Advantages of using Detergent as a Cleansing Agent

The detergents do not form insoluble precipitates with the calcium and magnesium ions in hard water. Thus, these can form lather easily with hard water also. There are various advantages of detergents over soaps when used as a cleansing agent.
1. Detergents, unlike soaps, can be used with both soft and hard water.
2. Detergents are more soluble in water than soaps.
3. Detergents have stronger cleansing action than soaps.

Limitations of Using Detergent as a Cleansing Agent

The major limitation in detergents is that most detergents are not easily degraded by microorganisms like bacteria, i.e., these are generally non-biodegradable. This leads to their accumulation in water and causes water pollution. On the other hand, soaps are biodegradable and hence do not cause water pollution.

Detergent Vs Soap

Synthetic detergents are better cleansing agents than soaps due to the following reasons.

(i) Detergents can be used both in soft and hard water as their calcium and magnesium salts are water-soluble. On the other hand, soaps form insoluble salts with calcium and magnesium ions and cannot be used in hard water.

(ii) The aqueous solutions of detergents are usually neutral. Therefore, they do not damage delicate fabrics and can be used for washing almost all types of fabrics. On the other hand, aqueous solutions of soap are alkaline and damage delicate fabrics. Therefore, soaps cannot be used for washing delicate fabrics.

Biodegradable and Non-Biodegradable Detergents and Pollution

The detergents having a branching in the hydrocarbon tail are not biodegradable and cause pollution in rivers and waterways. The presence of side chains in the hydrocarbon tail prevents bacteria from attacking and breaking the chains. This results in the slow degradation of detergent molecules leading to their accumulation in water.

Efforts are being made to minimise the branching in the detergent molecule to make them biodegradable easily. Since unbranched chains are more easily attacked by the bacteria, the detergents having no branching or minimum branching are easily biodegraded and pollution is prevented.

Difference Between Soap and Synthetic Detergent

Summary

The substances which, along with water, are used for cleaning or for removing dirt are known as detergents. Soap has been used as a detergent for about \(2300\) years. But substances other than soap are also used for cleaning. These are called synthetic detergents or simply detergents. This article learned about the preparation of soaps and detergents, the structure of soap molecules, the cleansing action of soap, the advantages of using detergent as a cleansing agent and the difference between soap and detergent.

FAQs on Detergents and Soaps

Frequently asked questions related to detergents and soaps are listed as follows:

Q.1.What are detergents and soaps?
Ans: Soaps are an anionic class of detergents. The sodium or potassium salts of higher fatty acids such as palmitic acid \(\left( {{{\rm{C}}_{15}}{{\rm{H}}_{31}}{\rm{COOH}}} \right),\) etc. obtained by the saponification process which involves alkaline hydrolysis of glyceryl esters (fats and oils). In a soap molecule, \({\rm{RCOONa,}}\,{\rm{R}}\) part is hydrophobic, whereas \({\rm{CO}}{{\rm{O}}^ – }\) part is hydrophilic. In solution, soaps form a micelle that encapsulates the greasy dirt of clothes and makes them clean. Soaps do not produce lather with hard water.
Synthetic detergents are either sodium salts of long-chain alkyl-substituted benzene sulphonic acids or sodium salts of sulphuric acid esters of long-chain aliphatic alcohols or alkyl ammonium halides. They work equally well with hard water and are therefore regarded superior to soaps.

Q.2. What is the main difference between soap and detergent?
Ans: Detergents are sodium salts of long-chain benzene sulphonic acids or long-chain alkyl hydrogen- sulphate. Soaps are the sodium or potassium salts of a long chain of carboxylic acids.

Q.3. What are soaps and detergents? Give examples?
Ans: Soap is the sodium or potassium salt of a long-chain carboxylic acid or glycerol. Thus, sodium palmitate, sodium stearate, sodium oleate, sodium linoleate are examples of soaps.
A detergent is a sodium salt of a long chain benzene sulphonic acid or alkyl hydrogen sulphate, which has the cleaning property in water. Sodium n-dodecyl benzene sulphonate is an example of a detergent.

Q.4. What is a detergent example?
Ans: Examples of detergents are: sodium \({\rm{n}}\)-dodecyl sulphate \(\left( {{\rm{C}}{{\rm{H}}_3} – {{\left( {{\rm{C}}{{\rm{H}}_2}} \right)}_{10}} – {\rm{C}}{{\rm{H}}_2} – {\rm{SO}}_4^ – {\rm{N}}{{\rm{a}}^ + }} \right)\) and sodium \({\rm{n}}\)-dodecyl benzene sulphonate \(\left( {{\rm{C}}{{\rm{H}}_3} – {{\left( {{\rm{C}}{{\rm{H}}_2}} \right)}_{11}} – {{\rm{C}}_6}{{\rm{H}}_4} – {\rm{SO}}_3^ – {\rm{N}}{{\rm{a}}^ + }} \right).\)

Q.5. How are soaps and detergents prepared?
Ans: Preparation of synthetic detergents
Synthetic detergent is made from hydrocarbons obtained from coal or crude oil (petroleum). Such hydrocarbons are treated with conc. \({{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}.\) A synthetic detergent is the sodium salt of a long-chain benzene sulphonic acid or the sodium salt of a long-chain alkyl hydrogen sulphate.

Q.6. What is the chemical name of soap?
Ans: Sodium stearate is the sodium salt of a long-chain saturated fatty acid called stearic acid \(\left( {{{\rm{C}}_{17}}{{\rm{H}}_{35}}{\rm{COOH}}} \right).\) In sodium stearate, the long alkyl group \({{\rm{C}}_{17}}{{\rm{H}}_{35}}\) and the ionic carboxylate group is \({\rm{CO}}{{\rm{O}}^ – }{\rm{N}}{{\rm{a}}^ + }.\)

Q.7. Are detergents dangerous?
Ans: The detergent in single-use laundry packs is highly concentrated and hazardous. Even a small amount of detergent in a child’s mouth or eyes can cause major breathing and stomach problems, eye irritation, and even coma and death.

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