Why is glycerin used in laundry detergents

Traditional soap production after neutral oil saponification

Traditional soap making used to be a very laborious process, and the exact recipes were well-kept secrets. The import of coconut oil into Europe from 1830 and the possibility of the industrial production of soda using the Solvay process (1861) ushered in the age of modern detergents.

Neutral oil saponification using the cold process.

In the case of neutral oil saponification of fats with sodium hydroxide solution, the reaction proceeds relatively slowly. The process takes a long time. In the traditional cold process, it takes six weeks to complete. In the following example, the sodium hydroxide reacts with a fat molecule made up of an oleic acid residue, a palmitic acid residue and a linoleic acid residue. The fat molecule splits up and glycerine and the three corresponding fatty acid salts sodium oleate, sodium palmitate and sodium linolate are obtained. These salts are typical components of a soap.

Saponification principle

Saponification of a fat molecule made up of an oleic, palmitic and linoleic acid residue.
In the case of the hot process, this process is greatly accelerated at higher temperatures. Subsequent salting out with sodium chloride reduces the solubility of the soap paste. As a result, the soap core separates out as a solid and floating layer on top. The glycerine, which is used as a raw material for other products, remains in the lower liquor.

Salting out curd soap.

The one made from caustic soda and fat Curd soap is still strongly alkaline. For this reason, the process of salting out is repeated several times. If you use potassium hydroxide solution instead of sodium hydroxide solution to saponify the fat, a semi-solid to liquid mass is obtained when salting out Soft soap. Soft soaps are mainly used for cleaning purposes in households, curd soaps for removing stains or for matting fabrics.
Large-scale soap production after fatty acid saponification  

The traditional methods are hardly used in large-scale industrial processes today. Instead, the fats and oils are first split in pressure vessels at 180 ° C by hydrolysis with steam to form free fatty acids. The glycerine produced at the same time is separated off. The fatty acids are purified by a subsequent vacuum distillation at low pressure. When the fatty acids are stirred into a boiling sodium carbonate solution (soda solution), the fatty acids react with the sodium carbonate to form soap, water and carbon dioxide.
2 C17H35COOH + Na2CO3  2 C17H35COON / A+ + H2O + CO2   
Stearic acid + sodium carbonate Sodium stearate + water + carbon dioxide
The advantage of fatty acid saponification is that it can be used to make soap from petroleum. The long-chain paraffins obtained from petroleum can first be converted into fatty acids by catalytic oxidation. In this way, soap can also be made from stearic acid. Stearic acid is a component of candle wax.
C.17H35COOH + NaOH C.17H35COON / A+ + H2O
Stearic acid + sodium hydroxide Sodium stearate + water
However, there are currently enough natural fats and oils available, so that the production of soaps from petroleum is rather insignificant.
Large-scale production of surfactants 
The manufacturing process for modern surfactants involves many, complicated steps. The "classic" way to produce an alkylbenzenesulfonate is made from paraffins obtained from petroleum. These are converted into chloroalkanes in a substitution reaction with chlorine (I). The chloroalkanes obtained in this way can be dehydrogenated to linear alkenes at high temperature using platinum or aluminum catalysts. The elimination reaction that takes place here also produces hydrogen chloride (II). With the aid of a Friedel-Crafts catalyst such as hydrogen fluoride, an alkylbenzene (III) is obtained with benzene.
(I) alkane + chlorine Chloroalkane + hydrogen chloride
(II) chloroalkane Alkene + hydrogen chloride
(III) alkene + benzene Alkylbenzene

With sulfur trioxide, which is obtained, for example, in the production of sulfuric acid, in one Sulfonation the sulfo group -SO3Introduce H into an alkylbenzene molecule. A solution of sulfur trioxide in concentrated sulfuric acid (oleum) is used.

Principle of a sulfonation

The sulfonation of the aromatic by the electrophilic sulfur trioxide is an electrophilic substitution.

To produce a fatty alcohol sulfate (FAS), for example, the fatty alcohol dodecanol can be mixed with a little concentrated sulfuric acid and gently heated. A yellow ester forms after a few minutes. It is neutralized with caustic soda. The formation of foam when the product is shaken with water can be used to prove that a surfactant has formed. When the fatty alcohol is reacted with concentrated sulfuric acid, an ester is initially formed:
CH3- (CH2)n-OH + H2SO4  CH3- (CH2)n-O-SO3H + H2O
Fatty alcohol + sulfuric acid Sulfuric acid alkyl ester + water
When the ester is neutralized with the sodium hydroxide solution, a fatty alcohol sulfate is formed:
CH3- (CH2)n-O-SO3H + NaOH CH3- (CH2)n-O-SO3N / A+ + H2O
Sulfuric acid alkyl ester + sodium hydroxide solution Fatty alcohol sulfate + water
The early heavy duty detergents came on the market in powder form. Above all, good storage, transport and dosing properties were desirable. In the high pressure spray process, the solid and liquid components of the detergent are combined in one Mixing tank mixed with water to form a pumpable pulp. in theSpray tower dry the components by adding hot air at about 300 ° C. The result is a powder that is very soluble in water and produces little dust. The cooling performed. Then it will be in the Storage container stored. Only then does one admit processing the temperature-sensitive ingredients such as bleach, enzymes or perfumes. in the Packing machine the finished detergent is portioned and packaged for storage and sale. The production scheme is shown in simplified form:
In the case of compact detergents, the powder coming from the spray tower is compressed together with the subsequently added components. Granules with a higher bulk density are obtained. To make tabs, the granules are pressed into tablets. It is also possible to produce multi-layer tabs. The upper layer contains sensitive components such as bleaching agents and the lower layer contains bleach activators and enzymes.