Soap & Detergent

                                                  Soap and Detergent

        In the era of globalization, people are increasingly focusing on their images and health within the advanced technology. The need for efficient washing of skin and clothes pushed many inventors and scientist to create many types of substances that facilitated that need. Because modern detergents found their footing only after the rise of technology and chemistry of 20th century, our ancestors focused their attention on production of various soaps and detergent. Soap is one of the major detergent cleaning agents. It is use to clean your body from bacteria, dirt and grease. There are much type of soap like bar soap (a solid soap which dissolve in water) and liquid soap (need to dissolve in water). Detergent comes from the Latin word detergere meaning to clean; it is defined as a cleansing agent. Therefore, water itself is a detergent. This essay looks at soap and soapless (or synthetic) detergents. Both substances we use every day and have a big market commercially, they affect everyone.

Soaps are sodium or potassium salts of long-chain fatty acids. The general formula of soap can be written as RCOONa / RCOOK where R is an alkyl group usually containing 12 or 18 carbon atoms. –R can be saturated or unsaturated. For example of soap are sodium laurate [CH₃ (CH₂)₁₀COO^-Na⁺], sodium palmitate [ CH₃(CH₂)₁₄COO^-Na⁺], potassium stearate [CH₃(CH₂)₁₆CO O^-K⁺], and potassium oleate CH₃(CH₂)₇CH=CH(CH₂)₇COO^-K⁺]. In the past, soap was made by mixing animal fats with alkaline wood ashes. Large-scale commercial soap-making occurred in 1791 when a French chemist, Nicholas Leblane patented a process for making soda ash or sodium carbonate from common salt. The process yielded large quantities of quality soda ash. Twenty years later Belgian chemist Emest Solvay developed a process to further reduce the cost of obtaining soda ash, thereby reducing the cost of soap manufacturing. Soaps can be made from animal fats and vegetable oils. The animal fat most commonly used are fats from cows and goats. The vegetable oils often are palm oils, olive oils and coconut oil. Soaps are prepared by hydrolysing fats and oils under alkaline condition. The reaction is called saponification. The saponification involves the boiling fats or oil with concentrated sodium hydroxide solution or concentrated potassium hydroxide solution to produce glycerol and the salt of fatty acids which are soaps.  The general equation for this reaction is:

                         Oil  or Fat               +    Alkali       =      Glycerol    +     Soap

The fats or oils are hydrolysed first to form glycerol and fatty acids. The acids then react with an alkali to form the corresponding potassium or sodium salts.

Any cleaning agent that is not soap is a detergent. Detergents are usually made from synthetic resources such as petroleum fractions. They are developed during the Second World War in response to shortage of animal fats and vegetable oils. Detergents are usually sodium salts of sulphonic acid. The general formulae for two common detergents are

                   

A sodium alkyl sulphate                               A sodium alkylbenzene sulphonate 

During the preparation of detergents, a long-chain hydrocarbon obtained from petroleum fraction is converted into organic acid through a series of steps. The organic acid is then neutralised with sodium hydroxide solution to produce a neutral salt which is detergent. Let see one of the example which is preparation of sodium alkyl sulphate. Step1, formation of an organic acid (Sulphonation), a long-chain alcohol reacts with concentration sulphuric acid to form alkyl sulphonic acid. Step 2, neutralisation, the resulting is then converted to sodium salt by a reaction with sodium hydroxide. For preparation of sodium alkylbenzene sulphonate, a long-chain alkene reacts with benzene to form alkylbenzene. The alkylbenzene formed is then reacted with concentrated sulphuric acid to form alkylbenzene sulphonic acid (Sulphonation). The alkylbenzene sulphonic acid is then converted to the sodium salt by a reaction with sodium hydroxide (neutralisation).

          

The cleaning action of both detergent and soap results from their ability to lower the surface tension of water and to emulsify oil or grease and to hold them in suspension in water. This ability is due to the structure of soaps and detergents. In water, sodium soap dissolves to form soap anions and sodium cations. For example, the following chemical equation shows the ionisation of sodium palmitate.

                            CH₃ (CH₂)₁₄COONa CH₃ (CH₂)₁₄COO^-   + Na⁺

A soap anion consists of a long hydrocarbon chain with a carboxylate on one end. The hydrocarbon chain which is hydrophobic is soluble in oils or grease (water-hating). The ionic part is the carboxylate group, which is hydrophilic, is soluble in water (water-loving).

In water, detergent dissolves to form detergent anions and sodium cations. For example, the following chemical equations show the ionisation of sodium alkyl sulphate and sodium alkylbenzene sulphonate.

Similarly, the anion part of a detergent also consists of a hydrophobic part and a hydrophilic part.

                                  

Furthermore, I want to explain about the cleaning action of a soap or detergent on a piece of cloth with a greasy stain. When soap or detergent is add to water, it reduces the surface tension of water. This increases the wetting ability of water. Therefore, the surface of cloth is wetted thoroughly. The hydrophobic part of the soap or detergent anion is soluble to grease. The hydrophilic part is soluble in water. During scrubbing, the water is agitated. This help to pull the grease free from the surface and also break the grease into small droplets. The droplets do not coagulate and redeposit on the surface of the cloth due to the repulsion between the negative charges on the droplets. These droplets are suspended in water forming an emulsion. Rinsing washes away these droplets and leaves the surface clean.

                        

                      

Although soap is good cleaning agent, its effectiveness when used in hard water (is the water that has high mineral content). Hard water contains calcium and magnesium ions. These ions react with the soap to form an insoluble precipitate know as soap scum.

Formation of a soap scum reduces the amount of soap available for cleaning, thus causes wastage of soap. Soaps are only suitable for use in soft water. Soft water does not contain calcium and magnesium ions. Soaps do not form scum with soft water. The effectiveness of cleaning action of soap is also reduced when used in acidic water. The hydrogen ions in acidic water react with soap to form long-chain fatty acids.

           CH₃ (CH₂)₁₆COO^- (aq) [stearate ion]        +    H⁺        =    CH₃ (CH₂)₁₆COOH

Long-chain fatty acids are insoluble in water due to their high reactive molecular masses. This reduces the amount of soap available for cleaning. The effectiveness of the cleaning action of soap is thus reduced. Detergent do not form scum with hard water. They form soluble substances with calcium or magnesium salts. This means detergent can still perform its cleaning action in hard water. Thus, detergent is more effective than soap in hard water. Detergents do not form insoluble substances with hydrogen ions in acidic water. Thus, their cleaning action is not affected.

As the technology becomes more modern nowadays, additives are added to a detergent enhance its cleaning efficiency and to meet the needs of consumers. The additives in detergent and their respective function are shown in the table below:

Additives	Example	Function
Biological enzyme	Amylase, proteases, celluloses, lipases	To remove protein stains such as blood
Whitening agent	Sodium perborate	To convert stains into colourless substances
Optical whitener	Fluorescent dyes	To add brightness and whiteness to white fabrics
Builder	Sodium tripolyphosphate	To enhance the cleaning efficiency of detergent by softening the water
Suspension agent	Carboxymethylcellulose	To prevent the dirt particles removed from redepositing onto cleaned fabrics
Drying agent	Sodium sulphate, sodium silicate	To ensure the detergent in powdered form is always in a dry condition and to enable it be poured easily.
Foam control agent	silicones	To control foaming in detergent.

The cleaning action of soap and synthetic detergents are very similar, they are both surfactants. They act as emulsifying agents and reduce the surface tension of water allowing it to ‘wet’ better. Each has their own disadvantages and advantages. For example, advantage of soap is soaps are eco-friendly and biodegradable. Disadvantages of soap are not suitable in hard water and have weak cleaning action compare to detergent. On the other hand, advantages of detergent are do not decompose in acidic medium and do not form scum in hard water. They have stronger cleaning action than soaps. As detergents are derived from petroleum they save on natural vegetable oils. Disadvantages of detergent are detergents are resistant to the action of biological agents and thus are not biodegradable. Their elimination from municipal wastewaters by the usual treatment is a problem. They have a tendency to produce stable foams in rivers that extend over several hundred meter of surfactants used in their preparation. Thus they pose a danger to aquatic life. They tend to inhibit oxidation of organic substances present in wastewaters because they form a sort of envelope around them. In conclusion, our environment has been affected badly because of products made up of soap and detergent cannot be disposed except through specially designed ways. There are also some irresponsible users who dump the unwanted product of soap and detergent as well as scum, into river and even sea as an easier way to dispose of them. In doing this, the quality of the environments will decline cause by pollution, especially water pollution. So we need to use and dispose the soaps and detergents in the correct way to reduce the environment pollution. I think the production of synthetic detergents is a good thing especially since all the time improvements are being made to make them environmentally friendly.

Thank you for reading.

References

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• http://global.britannica.com/EBchecked/topic/550751/soap-and-detergent
• http://whttp://dwb4.unl.edu/Chem/CHEM869A/CHEM869ALinks/www.sdahq.org/sdalatest/html/soapchemistry1.htm
• www.cleaninginstitute.org/clean_living/soaps__detergents_chemistry_2.aspx
• http://www.cleaninginstitute.org/clean_living/soaps__detergent_history.aspx
• http://www.slideshare.net/muhammedthahir129/soap-and-detergents-15243636
• http://berryberryeasy.com/2011/09/spm-chemistry-form-5-notes-chemicals-for-consumers-part-4/