THE CHEMISTRY OF SOAPS AND DETERGENTS
THE CHEMISTRY OF SOAPS AND DETERGENTS
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Soaps are the sodium and potassium salts of long chain fatty acids.
Soap molecule consists of a long hydrocarbon chain with a carboxylic acid on one end.
Soap has a large non-ionic hydrocarbon group and an ionic group COO-Na+.
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Examples of soaps:
Sodium stearate (C17H35COO-Na+)
Sodium palmitate (C15H31COO-Na+)
Sodium oleate (C17H33COO-Na+)
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Saponification is the process of making soap by the hydrolysis of fats and oils with alkalies.
Soap is made by heating animal fats or vegetable oil with concentrated sodium hydroxide (NaOH).
Fat or Oil + NaOH → Soap + Glycerol
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Hot Process (Boiling process) of soap manufacture:
The saponification of the fat is done by boiling the fat with sodium hydroxide solution in a soap pan or kettle.
Steam is admitted to boil the mixture and maintain agitation.
Boiling is continued until the fat is saponified to about 80%.
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Salting Out: Separation of soap and glycerol.
Soap is insoluble in concentrated salt solution, while glycerol is soluble.
Solid salt or brine is added to the mixture, boiled, and allowed to settle.
Soap is thrown out of solution as a curdy mass and floats to the surface.
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Washing operation to reduce glycerol content and remove impurities.
Soap is boiled with fresh soda lye to complete saponification.
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Finishing: Neat soap is obtained and mixed with ingredients like color and scent before being put in the mold.
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Cold Process:
Saponification is allowed in cold conditions with mechanical stirring.
Heating is initiated to convert soap into a hot liquid, which is then run into frames for saponification.
Glycerol remains in the soap.
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Modern Continuous Process:
Saponification is carried out in a closed vessel at elevated temperature and pressure.
Faster and more economical compared to the open-pan method.
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Direct Neutralization of Fatty Acids:
Soap manufacture by direct neutralization of fatty acids is a recent introduction.
Continuous methods are more economical.
Fatty acids are obtained by hydrolysis of fats in the presence of specific catalysts.
Note: This summary includes all the main ideas and supporting details from the given transcript.
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Hot water is fed into the hydrolyser near the top and fat near the bottom.
Hydrolysis is rapid and complete.
Fatty acids rise to the surface and are drawn out at the top.
Glycerol is removed in water leaving at the bottom.
Fatty acids are pumped to another vessel called neutralizer.
Neutralized with sodium hydroxide or sodium carbonate to form soap.
Twitchell process: hydrolysis of fats using a catalyst of dilute sulphuric acid and aromatic sulphonic acid.
Drying and finishing of soaps obtained is done as described under the Modern Continuous Centrifugation Process.
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A soap molecule has two ends with different properties.
Long hydrocarbon part that is hydrophobic.
Short ionic part containing COO-Na+ that is hydrophilic.
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Working of micelles.
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Mechanism of cleaning action of soaps.
Hydrocarbon ends of soap molecules attach to oil or grease particles on a dirty cloth.
Soap micelles entraps oily particles using hydrocarbon ends.
Ionic ends of soap molecules remain attached to water.
Dirty cloth agitated in soap solution disperses oily particles in water, cleaning the cloth.
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Advantages and disadvantages of soaps.
Advantages:
Eco-friendly and biodegradable.
Disadvantages:
Not suitable in hard water.
Weaker cleansing properties than detergents.
Formation of insoluble salt of Calcium.
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Detergents or syndets.
Detergents are synthetic soap-like cleansing agents.
Introduced in the United States and Great Britain in the 1920s.
Consumption of synthetic detergents exceeds that of soaps.
Syndets account for over 80% of all detergents used in the United States, France, and West Germany.
Consumption of syndets in India is increasing and likely to develop rapidly in the next few years.
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Detergents.
Sodium salts of long-chain benzene sulphuric acids.
Primarily surfactants produced from petrochemicals.
Surfactants lower the surface tension of water.
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Examples of detergents.
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Sodium Lauryl Sulphonate.
Sodium n-dodecylbenzene sulphonate.
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Cleansing action of detergents.
Synthetic detergents have the same molecular structure as soaps.
Cleansing action similar to soaps, forming micelles and emulsification.
Synthetic detergents can lather well even in hard water.
Soluble sodium or potassium salts of sulphonic acid or alkyl hydrogen sulphate.
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Manufacture of detergents.
Sodium Alkyl Sulphates produced from aliphatic long-chain alcohols.
Alcohol sulphated with sulphuric acid, resulting in alkyl hydrogen sulphate.
Neutralization gives the sodium salt.
ABS (Alkyl Benzene Sulphonate) type detergents manufactured using the Fridels Craft reaction.
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Advantages of detergents.
Do not decompose in acidic medium.
More soluble in water than soaps.
Stronger cleansing action than soaps.
Save on natural vegetable oils.
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Disadvantages of detergents.
Many detergents are resistant to biological agents and not biodegradable.
Stable foams in rivers pose a danger to aquatic life.
Inhibit oxidation of organic substances in wastewaters.
Page 34: Constituent of Detergents
Active detergents make up about 20% of detergents
Active detergents enhance the cleaning action of detergents
Sodium sulphate is another 20% of detergents
Sodium sulphate acts as a makeup substance
Inorganic phosphates make up about 30-50% of detergents
Inorganic phosphates can complex with calcium and magnesium ions in hard water
Complexing with calcium and magnesium ions enhances the cleaning action of detergents
Other ingredients like Sodium perborate are added
Sodium perborate acts as a bleaching agent, fluorescent agent, coloring agent, and odorant agent
Page 35: Differences between Soaps and Detergents
Soaps are metal salts of long chain higher fatty acids
Soaps are prepared from vegetable oils and animal fats
Soaps produce scum in hard water due to insoluble precipitates of Ca2+, Mg2+, Fe2+, etc.
Detergents are sodium salts of long chain hydrocarbons like alkyl sulphates or alkyl benzene sulphonates
Detergents are prepared from hydrocarbons of petroleum or coal
Detergents do not produce insoluble precipitates in hard water
Detergents are effective in soft, hard, or salt water
Page 36: How Detergents Cause Water Pollution and Its Remedy
Alkyl Benzene Sulphonate (ABS) detergents were commonly used until the 1960s
ABS detergents were made from a tetramer of propylene
ABS detergents are "hard" or nonbiodegradable
Linear Alkyl Sulphonate (LAS) detergents were introduced in 1966 as a remedy
LAS detergents are "soft" and biodegradable
Modern LAS detergents do not foam