Control questions
What is surface tension?
To compare surface tension of water with surface tension of liquids with the same molecular weight.
Explain temperature influence on surface tension?
What is positive adsorption?
What is negative adsorption?
Why does water surface tension reduce at dissolution of SAS in water?
How influence the introduction of electrolytes in solutions of soaps and detergents on surface tension?
Call kinds of hydrophilic and hydrophobic surfaces.
Which factors influence on moistening ability of SAS?
Which class of SAS has the best moistening ability?
How does ramifying of hydrocarbon chain influence on wetting ability?
What is an emulsion? Which kinds of emulsions you know?
SAS (soaps, synthetic detergents) are used in quality of emulgators at washing process. Explain the mechanism of action.
How influence on foaming ability the following factors: molecular weight of SAS, temperature, ramifying the hydrocarbon chain?
Which class of surface-active substances has the best foaming ability?
Which values characterize a foaming ability of detergents?
Explain a role of carboxylmethylcellulose in structure of detergents.
1.12 Solubilization
For a long time it is known, that in solutions of soap or other surface-active substances at the certain concentration there is the dissolution of liquid or firm substances, insoluble in pure water. This phenomenon refers to as colloidal solubility or solubilization. Solubilization or colloidal solubility is a spontaneous transition of molecules of insoluble in water substance in a water solution of soap or synthetic washing substance with formation of steady solution.
In case of colloidal solubility of hydrocarbons they penetrate in cluster, into layers between the hydrophobic ends of molecules of washing substances. Molecules of polar character (spirits, amines) penetrate in cluster in parallel between separate molecules of washing substance.
The length of chain, replacement in chain, position of hydrophilic groups influence on solubilization.
For synthetic fat acids the solubilizational ability increases with increase of molecular weight. At sulfates and sulphonates solubilizational ability increases with increase in number of carbon atoms of a chain. Compounds with the ramified chain have the best solubilizational ability, than normal compounds with equal length of a chain.
1.13 Washing action
Washing process is the complex system. Set of factors influence on the mechanism of washing action: interaction between various on the character and the nature pollutions, surfaces, the water, washing substance, electrolytes, mechanical and thermal interaction.
Till now there is no universal theory of washing action, but the major factors playing the important role in washing process are determined, and their value is appreciated.
Surface-active substances are adsorbed on the surface of separation and lower the surface tension that facilitates wetting of hydrophobic surfaces and penetration of washing solution between the cleared surface and pollution that in turn leads to separating of pollution from the cleared surface.
Oil is originally distributed on a surface of a fibre as a layer. Under action of detergent this layer in several places is shifted. First segments are formed, they pass in hemispheres then the droplet is formed. This droplet adjoins to a fabric only on a small plane, is easily separated from a fabric itself or at small mechanical influence and passes in washing bath.
Due to presence of detergents in the washing bath, oil drops are emulsified, being shattered on finer droplets. Ions of SAS are orientation adsorbed on the interface oil - water and form around of each drop of oil the molecular protective environment. The hydrophobic hydrocarbon ends are inverted in oil, and polar groups are directed to water.
It is much more difficult to wash the firm pollutions. They can be led in solution by peptization or to take out in foam by flotation.
The firm pigment pollutions are usually as the conglomerates which have been stuck together by fatty substances. Solutions of detergents moisten the firm pollutions, forming around of separate particles a protective environment, penetrate into crack of particles and weaken binding forces which keep separate particles. Pigment complexes break up. Released particles again are surrounded by focused adsorption layer.
Dispersed pigment pollutions are removed with a fabric by mechanical action and by means of electrostatic repulsion.
Cluster of detergents Sodium ion Grease drops forming an emulsion
molecules in water; the drops carry the same
charge and repel each other
The hydrophilic tails of detergent Fibres of the fabric Negative charges on the fibres
molecules escape from the water repel the charged grease drops
by dissolving in the grease and stop them returning to the
fabric
Fig. 1.7. The process of grease separation from fabric by detergent
Efficiency of removal of pollutions from fabrics also depends on nature fabrics.
Necessary to keep in solution the pollutions, removed from the fabric, not supposing their repeated sedimentation on the fabric. With this purpose in structure of synthetic detergents carboxylmethylcellulose is entered.
The majority of vegetative fibres have a negative charge owing to selective adsorption of hydroxyl ions. This charge will increase if to raise the concentration of hydroxyl ions by addition of alkaline salts. The majority of pollutions in water are charged also negatively. The same charges of particles of pollutions and the surface of fibre cause electrostatic repulsion, but it is insufficiently. In solution of detergent the negative charges of pollution and fibre are increased. As a result of electrostatic repulsion of the same charges pollution or is removed from a fibre, or is loosened, then it can be removed from a fabric by mechanical influence.
Fig.1.8. The removal of grease from a fibre.
Efficiency of detergents depends on class and structure. Primary alkylsulphates (AlkCH2OSO2ONa) are the most effective detergents. Their washing ability at first grows with increasing of molecular weight, and then starts lower owing to bad solubility of heavier compounds.
Secondary alkylsulphates (Alk(Alk*)CHOSO2ONa) concede primary alkylsulphates on the washing ability. Their washing ability depends mainly on position of polar group in a molecule. Washing ability is better if the polar group is situated nearly to the end of SAS molecule.
Washing ability of alkylarylsulphonates (AlkArSO2ONa) depends on nature of aromatic compounds, on length, number and structure of alkyl chains and position of sulphurgroup.
The following factors play the important role in practical washing: water, a washing substance, electrolytes, thermal influence, a nature of fabric, a nature of pollution, the equipment for washing.
Water plays the important role in washing process. Due to the solvent action it can consider as independent detergents concerning water-soluble pollutions, it is especially at the appendix of mechanical work.
The great value has rigidity of water. It is especially shown at application of soap, alkylsulphates with the long direct chain and alkylbenzolsulphonates with the long direct alkyl chain. At application of the listed substances there is the formation of insoluble salts of calcium and magnesium that leads to loss of detergents.
Presence of electrolytes in washing solution allows to achieve the best results at washing with the smaller charge of more expensive than electrolytes washing substances. Used for washing of cotton fabrics the synthetic detergents in general are inconceivable without presence of such electrolyte as sodium tripolyphosphate.
Alkaline electrolytes neutralize sour pollutions and partly saponify fatty pollutions. Silicates prevent sedimentation of iron salts on a fabric. Alkalis also promote the removal of spots at washing by synthetic detergents, and efficiency of removal of spots increases with increase of рН.
Thermal energy is widely used in washing process. Higher temperature allows to apply the washing substances with higher molecular weight, namely they are the most effective. Rising of temperature reduces viscosity of liquid pollutions, increases solubility of soluble pollutions, increases speed of reaction of neutralization of sour linen pollutions of alkaline electrolytes. However in some cases influence of raised temperature can be negative. For some fabrics, for example silk, the high temperature is inadmissible.
The nature of pollutions. Pollutions can be both soluble and insoluble organic and inorganic substances. Water-soluble organic and inorganic pollutions: sugar, starch, flour, urea, the organic acids contained in fruits, albumens, inorganic salts. Soluble compounds are easily removed with water.
Insoluble organic and inorganic pollutions: clay, cement, plaster, soot, lubricant oils, pitches, paints, neutral fats, fat acids. Insoluble compounds are removed much more difficultly, than soluble compounds. Though fats are easily emulsified by detergents, but fat acids of skin fat can form with iron, available in pollutions or in water, iron soaps. These soaps are very difficultly removed and cause yellow colour of linen. Firm pigment pollutions are especially difficultly removed.
Mechanical influence. The role of mechanical influence at washing is reduced not only to friction of products one about another, but it provides also fast wetting of fabric, fast and full distribution of washing solution and its contact with linen.
Influence of intensity of hashing is difficult and depends from:
- duration of washing;
- grade of detergents:
- character of fabric pollutions.
In general increase of intensity of hashing increases removal of pollutions in first minutes of washing, and then practically does not influence.
Optimum time of washing and the water module (the ratio of water weight to linen weight). In washing process there is the removal of pollutions and their repeated sedimentation on a fabric. That pollutions precipitate on a fabric in minimum quantity, some washings are carried out, but each of them should not be long. The majority of pollutions is removed in the first 10 minutes of washing.
The optimum water module for laundries - 5, at domestic washing it grows till 10-15.