Detergent and soap

Soap

Soap is a cleansing agent of sodium or potassium salt of long chain fatty acids.

Cleansing agents :- 

Those substances which have ability to remove alien materials from the surface being washed (skin, clothes, floor, utensils, etc) are called cleansing agents.

Types of soap:-

Hard soap :-

A sodium soap is called a hard soap and it is used for washing clothes. It is not used for bathing purpose as it may cause irritation of skin. Sodium stearate [C₁₇H₃₅COONa] is a sodium soap.

Soft soap :-

A potassium soap is called a soft soap and it is used for bathing and washing hands. It is also called toilet soap. Potassium stearate [C₁₇H₃₅COOK] is a potassium soap.

Molecular formula of soap :-

R-COONa or R-COOK

R is a long chain saturated or monounsaturated or polyunsaturated hydrocarbon radical having carbon atoms 15 or above. 
Examples of saturated alkyl radicals R are as follows:-
C₁₅H₃₁, C₁₆H₃₃, C₁₇H₃₅ etc.

Examples of unsaturated alkyl radicals R are as follows:-
C₁₇H₃₃ etc

So soap can be represented by molecular formula like:-
C₁₅H₃₁COONa ( Sodium palmitate salt)
C₁₆H₃₃COONa (Sodium margarate salt)
C₁₇H₃₅COONa (Sodium stearate salt)
C₁₇H₃₅COOK (Potassium stearate) etc

Surfactants :- 

Anionic surfactant.

Due to presence of ion RCOO⁻

Preparation of soap :-

Raw materials:-

(i) Fat or oil
(ii) Caustic alkali (NaOH or KOH)

Process:-

(i) Fat and caustic alkali is mixed.
(ii) They are boiled.
(iii) Fat and caustic alkali react to form soap and glycerol.This process is called saponification.
(iv) Glycerol is more worthy than soap so it may be separated from the soap.

Separation of glycerol :-

Glycerol is more valuable than soap so manufacturers usually separate glycerol from the after saponification product. For this they use sodium chloride (NaCl). NaCl is mixed in the solution containing soap and glycerol. This increases the density of the NaCl solution and also glycerol is even soluble in the NaCl solution but soap is not. In this way glycerol is separated from the soap.  


Manufacturing of soap in the past :-

Soaps, in the past, were manufactured by heating mixture of wood ashes with animal fat. Wood ashes consists of sodium hydroxide. It may be upto 10% by volume.

      Still in rural Nepal and India people uses wood ash obtained from burning wood pellets and dung cake for washing pots, hands and even for brushing teeth.

Chemical reaction :-

Molecular formula of fat
C₃H₅(COOR)₃
So fat are given by molecular formula like 
C₃H₅(COO-C₁₅H₃₁)₃
C₃H₅(COO-C₁₆H₃₃)₃ etc

C₃H₅(COO-C₁₆H₃₃)₃ + NaOH → NaCOOC₁₆H₃₃ (Soap) + C₃H₅(OH)₃ (Glycerol)

Cleansing mechanism of soap :-

A soap molecule has two component called hydrophilic(-COOK) and hydrophobic (-R-) component. Hydrophilic components have strong affinity toward water molecule than the surface being washed while hydrophobic components are repelled by water. As both hydrophilic and hydrophobic components are attached in a soap molecule they helps removing alien particles attached to the surface being washed.

-COONa or -COOK are hydrophilic components of a soap while long chain saturated or monounsaturated or polyunsaturated hydrocarbon radicals of a soap are hydrophobic components.

Limitations of soap :-

Soap is not suitable to use with hard water. Soap reacts with hard water to give precipitation and hence does not produce much lather. The precipite may be seen as stain on clothes and utensils.

Detergent

Detergent is a synthetic cleansing agent. It is more useful than soap in many aspects. It does not give scum in hard water.

      Almost every cleansing products we used in daily life are detergents. It comes in form of cake, gel, liquid, paste and powder. Like laundry washing powder, shampoo, toothpaste, body wash, floor cleaner, utensils cleaner etc.

Types of detergent :-

1.  Ionic detergent

         a. Anionic detergent

         b. Cationic detergent 

2.  Non-ionic detergent

3.  Amphoteric detergent

Anionic detergent :-            

It has its surface active agent negative in charge. It is the most commonly used detergent. Most of the anionic detergents are obtained from sulphation of long chain alkane or long chain alcohol or sulphonation of benzene alkyl chloride.

Examples:-

Ammonium lauryl sulfate (C₁₂H₂₅-O-SO₃NH₃)

Sodium lauryl sulphate (C₁₂H₂₅-O-SO₃Na)

Sodium laureth sulphate (C₁₂H₂₅-O-C₂H₄-O-SO₃Na)

Sodium lauryl benzene sulfonate (C₆H₅-C₁₂H₂₄-SO₃Na)

R-CH₂-O-SO₃⁻  and C₆H₅-R-SO₃⁻ is the anionic active agent carrying negative charge in aqueous solution.

Linear chain anionic detergent is biodegradable while branched chain anionic detergent are less likely to degrade.

Uses:-

Anionic detergent is most widely used and economically manufactured detergent.  

Cationic detergent :-

             It has its surface active agent positive in charge.Though it is a surface active agent it is rarely used as a detergent. It has very poor detergent property. Its molecule is same as the anionic detergent except that its surface active agent is cationic ion quaternary ammonium [N(CH₃)₃⁺] instead of anionic sulfonic group (SO₃⁻) of the anionic detergent. 

2D structural formula:-

R-CH₂-N(CH₃)₃⁺Cl⁻

Example:-

Lauryl Trimethyl Ammonium Chloride [C₁₂H₂₅N(CH₃)₃Cl]

Uses:-

It is used as an emulsifying agent.

Non-ionic detergents :-

Those detergents which don't have any ions as their surface active agents are called non-ionic detergents. These detergents are usually obtained by condensation of long chain alcohol with various molecules of ethylene oxide(C₂H₄O).

Examples of non-ionic detergents :-

Cocamide MEA(Monoethanolamine)

Lauryl polyglycol ether (C₁₁H₂₃-O-CH₂-CH₂-(O-CH₂-CH₂)ₓ-O-CH₂-CH₂-OH)

Uses:-

For making shampoo

Some laundry detergent uses non-ionic in combination with anionic detergent.

Amphoteric detergent :-

            The detergent which has both anionic and cationic surface active agent is called amphoteric detergent.

Examples of amphoteric detergent:-

Dodecyl beta alanine (C₁₂H₂₅-NH-C₂H₄COOH)

Cocamidopropyl betaine (C₁₁H₂₃-CONH-C₂H₄-N(CH₃)₃-CH₂-COOH) or C₁₉H₃₈N₂O₃

Uses:-

Shampoo and laundry detergent.

Why detergent ?

Detergent is suitable to use with hard water as well as with acidic water. Following points illustrates why detergents are preferred over soaps :-

1. Solubility of detergent in water is greater than solubility of soap in water.


2. Detergents don’t give scum in hard water while soaps give scum in hard water. This makes detergent better than soap for washing purpose in hard water.


3. Detergents have stronger cleansing effect than soap.


4. Usage of washing detergents is more economical than soap. That is why for industrial and large amount use detergent is preffered over soap.
5. Detergent is also suitable to use with acidic water.

Differentiate between Soap and Detergent