Washing powders

Washing powders are combinations of soaps, detergents and other chemicals. Generally washing powders have about 15 to 30% of their weight in synthetic detergents.

The chemicals included in washing powders and their functions are as follows :

Sodium sulphate and sodium silicate are added to keep a powder dry. Sodium triphosphate or sodium carbonate is added to make an alkaline solution. This helps to remove dirt and also soften water. Carboxy methyl cellulose is added for maintenance of dirt particles in suspension state in solution. For obtaining sparkling white clothes it is necessary to use a bleaching agent. A bleaching agent is introduced in a composition as sodium perborate.

1.9 Structure and properties of surface-active substances

An active or washing substance is a basic component of synthetic detergents structure. All washing substances used for synthetic detergents manufacture concern to surface-active substances.

Surface-active substances are substances which at dissolution in water lower its surface tension on a border with air and other bodies.

All surface-active substances are polar compounds consisting from hydrophobic and hydrophilic parts. A hydrophilic part: carboxyl group (СОО^-); sulphatic group (-SO₃^-); nitrogen-containing groups; groups (-СН₂ – CH₂ – O – CH₂ – CH₂ -). A hydrophobic part contains: 1) a straight (unbranched) or branched paraffin chain; 2) a benzol or naphthalen ring with active radicals. A straight paraffin chain usually contains 10-18 carbon atoms.

For example, sodium stearate СН₃ (СН₂)₁₆ -----------------------СООNa

Sodium stearilsulphate CH₃(CH₂)₁₇ -----------------------OSO₃Na

Cetyltrimethylammoniumbromide СН₃

СН₃

СН₃ (СН₂)₁₅ NBr

СН₃

Hydrophobic tail Hydrophilic head

Sodium ion Na⁺

Fig. 1.4. Simplified way of a detergent molecule representation

Arrangement of hydrophilic and hydrophobic parts of a molecule can be various. For example, hydrophilic parts are located on the ends of a molecule, and the average part is hydrophobic:

HO (C₂H₄)_x (C₃H₆O)_y (C₂H₄O)_z H - polypropyleneglycol

Now some thousands of synthetic detergents names are known. However it is necessary to take into account that many kinds of synthetic detergents are produced on the basis of the same surface-active substance, and they differ from each other only by quantity and assortment of other components which are included in their structure. Frequently in many countries various companies produce the same product under different names.

Solubility of surface-active substances depends on the following factors:

1) nature and position of a hydrophilic group;

2) length of a hydrophobic hydrocarbon chain;

3) nature and position of atoms in a hydrophobic part of a molecule;

4) temperature;

5) a type of cation containing in a molecule.

Solubility of anion-active surface-active substances is raised with temperature growth. In compounds with a straight chain solubility is reduced with increase of a chain’s length. Branching of a chain increases solubility.

In non-ionogenic SAS we can observe the inverse relationship: with temperature growth solubility goes down. Turbidity of solution is observed. Temperature of appearance of turbidity is called turbidity point. The longer chain is the higher turbidity point is.

In strongly diluted water solutions soaps and synthetic detergents dissociate:

RCH₂OSO₃Na → Na⁺ + RCH₂OSO₂O^-

Na alkylsulphate Anion of alkylsulphate

RCOONa → Na⁺ + RCOO^-

Soap Anion of fat acid

If at dissolution in water ions of salt form with ions of water a low dissociating molecule such salt partly breaks up to the corresponding base and acid. This phenomenon is called hydrolysis.

Soap is a salt of a weak acid and a strong base and in diluted water solutions dissociates according to the specified circuit. Water dissociates according to the equation:

H₂O → H⁺ + OH^-

In solution RCOO^- anions combine with ions Н ⁺. Low dissociating molecules of a fat acid are formed:

H⁺ + RCOO^- → RCOOH

Ions Н⁺ leave from a system. The solution pH will be alkaline. Thus, soup water solutions have alkaline reaction. For potash soaps hydrolysis degree is more, than hydrolysis degree for sodium soaps.

As against soap, synthetic detergents are salts of a strong acid and a strong base. At water dissolution they are not hydrolyzed at all (alkylbenzolsulphonates, alkylsulphonates), or hydrolyzed at certain conditions (increase of temperature, strongly acid medium). This group includes alkyl sulphates. This property of synthetic detergents is very important at their application in acid mediums for preparation of neutral detergents.

At the dissolution of SAS at determined for each of them concentration sharp change of properties of these solutions is observed: surface tension, conductivity, density, washing ability, etc. These changes are explained by change of solutions’ structure. There are simple molecules and ions in solution, and at the same time there is formation of colloidal particles, which refer to clusters. In diluted solutions small ionic clusters with a high charge are formed, in more concentrated solutions – lamellar clusters. Lamellar clusters are formed from layers of focused molecules.

Hydrophilic groups form one surface of a layer (COONa-groups for soap), СН₃-groups form other surface of the layer. Polar groups are inverted to each other; hydrocarbonic groups CH₃ also are inverted to each other.

Technical products contain compounds having various length of a hydrocarbon chain. In this case mixed clusters are formed. Long chains in them are bent.

Detergent molecules on the surface

Fig. 1.5. Two ways in which detergents behave in water