Detergents (O.o)..?
Detergents are structurally similar to soaps, but differ in the water-soluble portion.
1. Detergents are synthetic cleansing agents made from hydrocarbons obtained from petroleum fractions. Thus, detergents are petrochemicals.
2. Detergents can be classified into three main types, depending on the charge on the detergent ion.
a) Anionic detergents where the head of the detergent particle contains a negatively charged ion.
Example: R – O – SO3-Na+(Sodium alkyl sulphate)
b) Cationic detergents where the head of the detergent particle contains a positively charged ion.
Example:R – N (CH3)3+BR-
c) Non ionic detergents.
Example:R – O – CH2CH2OH
3.There are two types of anionic detergents :
- Detergent molecule with a benzene ring such as sodiumalkylbenzenesulphonate. Positively charged ion Negatively charged ion.
- Detergent molecule without a benzene ring such as sodium alkyl sulphate.We can represent the detergent ion, alkyl sulphate ion as:
R- OSO3
Detergent Preparation Process
1. The detergent,sodium alkyl sulphate can be prepared from alcohols with chain lengths of 12 to 18 carbon atoms in two steps.
Step1: Reaction with concentrated sulphuric acid
Step 2: Neutralisation with sodium hydroxide solution.
2. An example of a long chain alcohol is didecan – 1 –ol,CH3(CH2)10CH2OH. The detergent prepared from dodecan -1 –ol is called sodium dodecyl sulphate (IUPACname) or sodium lauryl sulphate (common name).
CH3(CH2)10CH2O-SO3-Na+.
3.Sodium alkylbenzene sulphinates, were first used in 1940s. It can be prepared in three steps. The starting materials for making this detergents in a long chainalkene, RCH = CH2
, obtained from the cracking of petroleum.
a)Step 1 : Alkylation
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| Alkylation. Sorry For the quality. |
Alkylation is the introduction of the alkyl group to an organic molecule.
b )Step 2 : Sulphonation
Alkylbenzene produced the react with concentrated sulphuric acid acid to form alkylbenzene sulphonic acid.
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| Sulphonation. Sorry for the quality. |
Sulphonation is the introduction of the sulphonic acid group, -SO3H to an organic molecule to form sulphonic acid.
c) Step 3 : Neutralisation
Alkylbenzene sulphonic acid produced reacted with sodium hydroxide to form sodium alkylbenzene sulphonate, the detergent
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| Neutralisation. Sorry for the quality. |
The Structure of Detergent Molecule
When a detergent is dissolved in water, it dissociates to form sodium ions (Na+) and detergent ions. The detergent ions have the same basic structure as the soap ions, that is consist of two parts :
The ‘ head’ is the sulphate group (-OSO3-), which is negatively charged and hydrophilic (dissolves readily in water but not in oils and grease).
The ‘tail’ is the long hydrocarbon chain, which is neutral and hydrophobic(dissolves readily in oils and grease, but not in water).
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The Cleansing Action of Detergent / Soap
1.The cleansing action of soap or detergent depends on their chemical bonding and structures.
- The ionic ‘head’(negatively charged)is soluble in water (hydrophilic) but insoluble in oily layer.
- The long hydrocarbon ‘tail’(neutral)is insoluble in water (hydrophobic) but soluble in oily layer.
2. Oil cannot be washed away from clothing with water because oil (a covalent molecule is insoluble in water.
3. Lifting greasy dirt from the surface cloth. When soap or detergent is added to the dirty surface of a piece of cloth covered with a layer of oil or grease.
- The negatively charged ‘head’ (hydrophilic) of soap ions or detergent ions dissolves in water.
- The hydrocarbon ‘tail’ (hydrophobic) of soap or detergent ions dissolves in the layer of grease.
4. I the water is agitated slightly, the grease begins to be lifted off the surface. This cause by the forces of attraction between the water molecules and the negatively charged heads.
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| Cleansing Action of Soap |
5. On further agitation during washing, the greasy dirt is lifted from the surface.
6.Emulsifying dirt in water
- Soaps and detergents can act as emulsifying agents to emulsify oils and grease.
- The process of emulsification breaks large drops of grease into smaller droplets that floats in water. The greasy droplets repel on another because they carry the same charge. As a result, the grease is suspended in the solution.
- When the cloth is rinsed with the water, the droplet will be carried away.
- The cleaning process become more efficient in the water containing the soap or detergent solution is stirred
Additives in detergents
1.Modern detergents used for washing clothes usually contains a few types of additives to :
- Increase their cleaning power.
- Make them attractive and saleable.
2. Only about 20% of the substances in a detergent are cleansing agents
(sodiumalkyl sulphate or sodium alkylbenzene sulphonate). The other substances are additives.
(sodiumalkyl sulphate or sodium alkylbenzene sulphonate). The other substances are additives.
3. Builders : Sodium tripolyphosphate (Na5P3O10)
- Sodium tripolyphospathe is used to soften hard water. In the presence of sodium tripolyphosphate, Ca2+ ions and Mg2+ions are removed.
- Sodium tripolyphosphate increases the pH value of water. In this way,muddy dirt can be removed.
4.Whitening / bleaching agents : sodium perborate
- Bleaches (bleaching agents) remove coloured stains by oxidation process.When coloured stains are oxidised, the colour will disappear.
- The whitening (bleaching) agents commonly used in detergent are sodiumperborate (NaH2BO43H2O). Sodium perborate decomposes in hot water to release oxygen (an oxidising agent) which is responsible for the whitening(bleaching) action.
- Unlike chlorine, oxygen does not bleach the colour of dyes are not damaging to fabrics. When properly used, the perborate bleaches make fabrics whiter than chlorine bleaches and the colourful dyes of the fabrics do not fade when dirty stains are removed.
- Besides sodium perborate, sodium hypochlorite, (NaCIO) can also be used as bleaches in detergents. The IUPAC name of sodium hypochlorite is sodium chlorate (I).e)Sodium hypochlorite releases chlorine that bleaches with dirty stains.However, high concentrations of chlorine can be quite damaging to fabrics.These bleaches do not work well on synthetic fabrics (polyster fabrics),often causing a yellowing rather than the desire whitening. Also chlorine causes the dyes on fabrics to fade.
5.Biological enzymes : Amylase, lipase, and protease
- Protein stains such blood, milk, and tomato sauce cannot be removed by the ordinary detergents because these types of stains are insoluble in water.
- Biological enzymes in detergents can break down fat and protein molecules in food stains. The fatty acids, glycerol and amino acids produced are soluble in water and are removed during washing.
6.Brighteners
- Figure below shows the action of brighteners. The brighteners absorb the invisible ultra-violet and re-radiate it as blue light.
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- Brighteners make fabrics appear whiter and brighter because the blue light can hide any yellowing on the fabrics. Blue light added to the yellow light reflected on old fabrics make them look white.
7.Drying agents ; Sodium sulphate and sodium silicate
- Anhydrous sodium sulphate and sodium silicate (Na2S2O3) are used as drying agents to ensure that the detergent in powdered firm is always in a dry condition.
8.Stabilisers
- The functions of stabilisers is to prevents the formation of foam.
- In an automatic washing machine, excessive foam can stop the pump working. So, washing powders for automatic washing machine are made using detergents that are good at removing and emulsifying grease, but do not produced foam.
9.Perfumes
- Perfumes are added to make clothes smell fresh and clean.
The Effectiveness of Soaps and Detergents as Cleansing Agents
Advantages of Soaps
- Soaps are effective cleansing agents in soft water, that is water does not contain Mg2+and Ca2+ ions.
- Soaps do not cause pollution problems to the environment. This is because soaps are made from chemical found in animals and plants. This means that soaps are biodegradable, that is they can be composed by the action of bacteria.
Disadvantages of Soaps
- Soaps are ineffective in hard water, that is, water that contains magnesium and calcium salts.
- In hard water, soaps will react with Mg2+and thus, soaps do not lather in hard water.
- Scum is grey solid that is insoluble in water. It consists of magnesium stearate and calcium stearate.
- Soaps are not also effective in acidic water, for example rainwater containing dissolves acids. H+ions from acids will react with soap ions to produce carboxylicacids molecular size that are insoluble in water.
- Stearic acids and other carboxylic acids do not act as cleansing agents because they exist mainly as molecules and do not anionic hydrophilic ends (’head’) that dissolves in water.
Advantages of Detergents
- Detergents are cleansing agents that are effective in soft water as well as hard water. This is because detergents do not form scum with Mg+and Ca2+ions found in hard water.
- The detergents ions (R –O – SO3-and R – SO3-)react with Mg+and Ca2+ ions in hard water. However, the magnesium salts and calcium salts which are formed are soluble in water. Hence, the scum is not formed and the detergents are still active in hard water and lathers easily.
- Detergents are synthetic cleansing agents. This means that the structure of the hydrocarbon chain can be modified to produce detergents with specific properties.Nowadays, different types of detergents have been synthesised for specific uses such as shampoos and dish cleaner.
- Furthermore, detergents are also effective in acidic water because H+ ion is acidic water do not combined with detergents ions.
Disadvantages of Detergents
- Most detergents have branched hydrocarbon chains and are non-biodegradable,that is, they cannot decomposed by bacteria. As a result, non-biodegradable detergents cause water pollution.
- Phosphates in detergents act as fertilisers and promote the growth of water plant sand algae. When the plants die and decay, they will used up the oxygen dissolves in water. This will decrease the oxygen content in water and kill fishes and other aquatic lives.
- Detergents produce a lot of foam in water. The layer of foam that covers the water surface will prevents oxygen from dissolving in water. This condition will cause fish and other aquatic life ti die from oxygen starvation.
- Additives such as sodium hydrochlorite (bleaching agents) releases chlorine gas in water that is acidic. Chlorine gas is highly toxic and kills aquatic life.
