Thursday, 26 February 2015

5.1 Soaps and Detergents

Soaps
■ Soaps

Mostly are water-soluble substances.

Either sodium salts or potassium salts of long-chain fatty acids.

General formula:
RCOONa for sodium soap
RCOOK for potassium soap
R stands for alkyl groups of carbon chain (Each alkyl group have a hydrocarbon chain containing 12 to 18 carbon atoms)

Example:
Examples of soap Formula
Sodium laurate CH3(CH2)10COONa+
Potassium oleoate CH3(CH2)7CH=CH(CH2)7COOK+
Sodium palmitate CH3(CH2)14COONa+
Sodium stearate CH3(CH2)16COONa+
■ The history of soap manufacturing

In ancient time, soap was made by mixing vegetables with ashes of plants that contain potassium carbonate.

In 1791, Nicholas Le Blanc (1742-1806) – discovered a way to produce from sodium hydroxide.


Preparation of Soaps
■ Saponification

The process that is used for making soap.

Involves the hydrolysis process between ester (oil or fat) and concentrated alkali solution
concentrated alkaline solution acts as a catalyst
Fat (from animals) or oil (vegetable oil) is hydrolysed

Big ester molecules are break down to fatty acids and glycerol.
fat + alkali → fatty acid salt (soap) + glycerol
■ During the saponification process:

Animal fat or vegetables fat is heated with sodium hydroxide.

Once the saponification reaction is complete, sodium chloride is added to precipitate the soap.

Sodium chloride is added to reduce the solubility of soap in water and causes the solid soap form from the liquid soap solution.

The soap can be separated from the solution by filtration.
■ Additives added to soap

Colouring and perfumes.

Optical brighteners as whitening agent.

Water softeners that used in areas with hard water.

Abrasives that increase the power of cleansing action.
■ The video below shows the preparation of making soap.
■ The video below shows the preparation of making soap.
Laboratory Activity 5.1.1 : Preparation of Soap by Saponification


Cleansing action of Soaps
■ Structure of soap molecules

Each soap molecule consists of two parts :
Ionic head (hydrophilic) (−COONa+) which are soluble in water.
Long hydrocarbon chain as the tail (hydrophobic) which are soluble in grease or oil.
■ Functions of soap:

Wetting agent – reduce the surface tension of water to allow water spread out and wet the surface of the material.

Emulsifying agent - capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.
■ Cleansing action of soap

The animation below shows the cleansing action of soap.

The head region (hydrophilic part) attracts water molecules and the dissolve in water.
Soap reduces the surface tension of the water.
Spreads out the water and then wet the clothes.

The tail region (hydrophobic end) will attract grease and mix with the particles of grease.
Grease begins to be lifted off the surface on agitation.

The soap chains surround the grease particles.
Attraction of the head end of soap molecule to water.
Washing motion of the clothes pull off the soap and grease.
Grease dirt are removed from clothes surface as emulsion (oil droplets).

The particles of dirt and grease come off the surface and remain suspended in the water when the water is stirred or agitated.
Soap can hold the suspended particles because it can lather (bubbles) with water

Removing dirty water and soap foam will remove grease dirt as well.
■ The video below shows how does a soap work.


Detergent
■ Detergent

Salt that is produced from the reaction between an alkali and a sulphonic acid.

Non-soap cleaning chemicals that made from petroleum fractions.

Having the similar molecular structure of soap.
■ Sodium salt of sulphonic acid

Sodium alkyl sulphate
Example: Sodium lauryl sulphate
CH3(CH2)11–SO3Na+

Sodium alkyl benzene sulphonate
Example: Sodium n-dodecylbenzene sulphonate
CH3(CH2)11–C6H4–SO3Na+
■ Preparation of detergent sodium lauryl sulphate(Sodium alkyl sulphate)

Sulphonation
The long chain alcohol reacts with concentrated sulphuric acid
Water and alkyl sulphonic acid are formed

Neutralisation
Alkyl sulphonic acid is neutralised by adding sodium hydroxide.
Sodium alkyl sulphate (detergent) is formed.
■ Preparation of Detergent Sodium Alkylbenzene Sulphonate

Alkylation
Alkene carbon chain from petroleum react with benzene.
Alkylbenzene is formed.

Sulphonation
Alkylbenzene is reacted with concentrated sulphuric acid.
Alkylbenzene sulphonic acid is formed.

Neutralisation
Sodium hydroxide is added to neutralise alkylbenzene sulphonic acid.
Sodium alkylbenzene sulphonate(detergent) is formed.
■ This video contains information regarding detergents.


Cleansing action of detergent
■ Structure of detergent

Detergent ionise in water to form detergent ions and sodium ions

Detergent ions consist of hydrophilic head(ionic part) and hydrophobic tails (hydrocarbon chain).
alkyl sulphate ions
alkylbenzene sulphonate ions

The structure of detergent is similar to that of soap.
■ Cleansing action of detergent

The cleansing action of detergent is similar to that of soap.

The tail region (hydrophobic tails) is soluble in grease but not in water.

The head region (hydrophilic head) is soluble in water but not in grease.

During the washing action, all the tails molecules will go into the grease and the head dissolve in water. When the water is shaken, all detergents molecules will remove the grease (dirt) away.
■ The video below shows how does a detergent work.


Effectiveness of the cleansing action of soap and detergent
■ Soap cannot be used in hard water

Soap effectiveness is reduced when used in hard water.
■ Hard water

The water that contains dissolved calcium and magnesium salts.
■ Soap in hard water

Soap form scum (non-soluble precipitate) with hard water

Calcium and magnesium ions in hard water will react with soap anions to form scum as shown in the following equations:
RCOO(aq) + Ca2+(aq) → (RCOO)2Ca(s)
RCOO(aq) + Mg2+(aq) → (RCOO)2Mg(s)

The soap will have to react with all the calcium and magnesium ions before it start cleaning actions.

The scum that are formed will leave a clear sediment that can be seen on clothes.
■ Detergents can be used in hard water

Detergents are similar to soap, but they are less likely to form soap scum.
2ROSONa+(aq) + Mg2+(aq) → (ROSO)2Mg(aq) + 2Na+(aq)
2ROSONa+(aq) + Ca2+(aq) → (ROSO)2Ca(aq) + 2Na+(aq)

Detergents are not affected by the presence of minerals in water (hard water), so cleaning action in hard water can be improved by using detergent.


Additives in detergent
■ Detergent additives

give certain characteristics.

provide extra useful for commercial purpose.

increase the washing power.
■ The additives

The table below shows the additives with their functions
Additive Functions
Whitening agent (Sodium perborate) Remove colored dirt stains on white clothes
Florescence agent Makes white clothes look whiter and more sparkling
Biological enzymes Remove protein stains
Phosphate compound (Sodium triphosphate) Helps displace mud from clothes
Softens the water
Drying agent (Sodium sulphate and sodium silicate) Make sure the detergent powder in dry condition.
Stabiliser (Alkyl monoethanolamide) Reduce the amount of bubbles formed
Anticoagulant Prevents the washed-out dirt from sticking back to the clothes
Perfume Give fragrances to clothes after cleaning
Make the washed clothes smell fresh and clean


Comparison between soap and detergent
■ Similarities between soap and detergent

Both are cleansing agents which act as emulsifying agent and wetting agent

Both molecules consist of hydrophilic head (ionic part) and hydrophobic tails (hydrocarbon chain).
■ Soap differs from detergent in some aspects:

Differences Soap Detergents
Made from Natural materials found in animals and plants Synthetic materials from petroleum
Formation Reaction between an alkali and a fatty acid Reaction between an alkali and a sulphuric acid
Effectiveness in hard water No effective in hard water due to formation of scum. Effective in hard water. Do not form scum in hard water
Environment impact Easily broken down by bacteria or biodegradable Not easily broken down by bacteria or not biodegradable. May cause pollution
■ The video below shows the comparison between soap and detergent.

⇲ For exercise(objective and subjective), download for free on Android OS.

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