Chapter 8

Soap and DetergentsSoap and Detergents

INTRODUCTIONSoap is a salt of a fatty acid and mainly used for washing, bathing and Soap s a sa t o a atty ac d a d a y used o was g, bat g a dcleaning.Soaps are obtained by treating vegetable or animal oils and fats with strongly alkaline solution the reaction called saponificationstrongly alkaline solution, the reaction, called saponification

Firstly, is the hydrolysis of esters of the tri-hydric-alcohol (glycerol)and long-chain fatty acids. These esters collectively form a group oforganic compounds called triglycerides.Secondly, a triglyceride is treated with sodium or potassiumy, g y phydroxides, hydrolysis takes place forming glycerol and the sodium orpotassium salts of the fatty acids, that is, soap.•However pure soap is hard and easily oxidized so various additives•However, pure soap is hard and easily oxidized, so various additivesare used for adjusting and to make the soap more aesthetically pleasingproduct.•Glycerin: makes the soap smoother and softer

The Chemistry of Soap and Detergent Functionf h i iAll d d i surfactant as their active All soaps and detergents contain a

cationic . This is an ionic species consisting of a ingredientor anionic ’head’ and a counter ion The tail is wateror anionic head and a counter ion. The tail is water insoluble and the head is water soluble .

The difference in solubility has two important implications: 1- makes the surfactant molecule a wetting agent: the tails migrate to

align themselves with the solid: water interface lowering thealign themselves with the solid: water interface, lowering the surface tension at that point so that it penetrates the fabric better.

2- allows the oily dirt particles to form an emulsion with the water: th t il f f t t l l d il di tthe tails of many surfactant molecules surround an oily dirt particle, forming a micelle with a drop of oil in the centre and the ionic heads of the surfactant molecules pointing outwards.

Surfactants• Hydrophobic: is a substances that don’t dissolve in water

such as organic fats and oilsuch as organic fats and oil.• Hydrophilic: substances that are miscible in water

including hydrogen bonding molecules such as methanolg y g gand acetone.

• Surfactants: large linear molecules that have ah d h bi t il d h d hili h d S h d lhydrophobic tail and a hydrophilic head. Such dualnature molecules is known as surfactant.

• As example see the following figure that shows the• As example, see the following figure that shows thesodium stearate as a typical surfactant.

•Sodium stearate is a typical dual natureMechanism of cleansing soaps

molecule. It has an ionic, hydrophilic polarhead and a hydrophobic non-polar tail. Theyform a molecular monolayer on the surface.yThe polar head groups immerse themselves inthe water while the non polar tails aggregatetogether on the surfacetogether on the surface.•Agitating the solution may cause themolecules to arrange into spherical aggregatescalled micelles in which the hydrophobic tailinward. The polar head lie on the outside ofthe micelle.•The exterior of the micelleis hydrophilic (attracted towater) and the interior iswater) and the interior ishydrophobic(dissolves grease).

Soaps are the product of the reaction between a fat and sodium hydroxide:sodium hydroxide:

fat + 3NaOH → glycerine + 3 soap

Soap is produced industrially in four basic steps. Each step b d l t b t i i i l itmay be done over several process steps, but in principle it

could be done in the four steps outlined here.

Step 1 - SaponificationA mixture of tallow (animal fat) and coconut oil is mixed ( )with sodium hydroxide and heated. The soap produced is the salt of a long chain carboxylic acid.

Step 2 - Glycerine removalGlycerine is more valuable than soap most of it must beGlycerine is more valuable than soap, most of it must beremoved. Some will be left in the soap to help make it softand smooth. Soap is not very soluble in salt water,a d s oo . Soap s o ve y so ub e sa wa e ,whereas glycerine is, so salt is added to the wet soapcausing it to separate out into soap and glycerine in saltwater.Step 3 - Soap purificationA i i di h d id i t li d ithAny remaining sodium hydroxide is neutralised with aweak acid such as citric acid and two thirds of theremaining water removedremaining water removed.Step 4 - FinishingAdditives such as preservatives, color and perfume areadded and mixed in with the soap. Then is shaped intobars for sale.

Step 1 – SaponificationThe raw materials (mixture of Tallow and coconut oil )The raw materials (mixture of Tallow and coconut oil ) are continually fed into a reactor in fixed proportions. The raw materials would be fed along with NaOH solution ginto the reactor.

These ingredients alone would give a low water, high glycerine soap. Soap needs to be about 30% water for ease fluidity (pumping) and even then needs to be held atease fluidity (pumping), and even then needs to be held at around 70oC, so excess lye is added to hydrate the soap and dissolve out some of the glycerine. The lye added isand dissolve out some of the glycerine. The lye added is known as "half spent lye" and is the lye discharged from the washing column (see below). This lye already contains some glycerine, but it is further enriched by that formed in the saponification reaction

The Detergent

Detergents are similar in structure and function to soap,and for most uses they are more efficient than soap and soa d o os uses ey a e o e e c e a soap a d soare more commonly used. But, they use a syntheticsurfactant in place of the metal fatty acid salts used insoaps.

Th d b th i d d li id f d ldThey are made both in powder and liquid form, and soldas laundry powders, hard surface cleansers, dish washingliquids etcliquids, etc.

Most detergents have soap in their mixture of ingredients,but it usually functions more as a foam depressant than asa surfactant.

Detergent powder manufacture

Step 1 - Slurry makingThe solid and liquid raw ingredients (Table 2) are droppedThe solid and liquid raw ingredients (Table 2) are dropped into a large tank known as a slurry mixer. As the ingredients are added, the mixture heats up as a g , presult of two exothermic reactions: 1-the hydration of sodium tripolyphosphate (STP) and 2-the reaction between caustic soda and linear

alkylbenzenesulphonic acid (LAS). Th i t i th f th h t d t 85oC d ti d tilThe mixture is then further heated to 85oC and stirred until it forms a homogeneous slurry.

Step 2 - Spray dryingThe slurry is deaerated in a vacuum chamber and thenThe slurry is deaerated in a vacuum chamber and then separated by an atomiser into finely divided droplets. These are sprayed into a column of air at 425oC, where p y ,they dry instantaneously. The resultant powder is known as ’base powder’.

Step 3 - Post dosingOther ingredients are now added and the air blownOther ingredients are now added, and the air blown through the mixture in a fluidiser to mix them into a homogeneous powder. Typical ingredients are listed belowg p yp g