1.10 Surface tension
Each molecule in a solution tests an attraction of all molecules surrounding it, therefore resultant of all attraction forces is equal to zero. But on a surface this attraction is shown in unbalanced force which aspires to involve surface molecules inside of a liquid and to give surfaces the minimal sizes.
To involve molecules in a surface layer is required to spend work. This work, related to 1sm2 of again formed surface, turns to a stock of free energy and refers to as surface tension. A unit of measurement: 10-7 j/sm2 or 10-5 n/sm. Water possesses higher surface tension in comparison with majority of liquids with the same molecular weight.
Surface tension decreases with rise in temperature.
At dissolution of SAS the surface tension of water changes. If concentration of molecules of dissolved substance on a surface is more, than in volume the surface tension of solution will be less than a surface tension of water. This phenomenon refers to as positive adsorption.
If concentration of SAS molecules in volume is more than on a surface the surface tension of solution will be more than a surface tension of water. This phenomenon refers to as negative adsorption.
In water solutions the active or polar group is immersed in water, the hydrocarbon part remains not dissolved. It direct outside from a surface of water to a gas phase.
Molecules of SAS are polar molecules. At dissolution in water molecules are adsorbed on the surface. The interface air - water is replaced with the interface air - hydrocarbon. The surface tension of hydrocarbons is much less, than at water. Therefore the surface tension of solution is reduced.
With increase in concentration of solution of washing-up liquid the surface tension of solution is reduced up to some minimal limit, then remains practically constant.
At introduction of electrolytes in soap solutions the surface tension of solution increases. At introduction of electrolytes in solutions detergents the surface tension of solution decreases.
1.11 Physical and chemical action of washing substances and detergent solutions
Wetting. Wetting is the phenomenon observable on border of contact of three phases. One phase is a firm body, and two other phases are liquids or a liquid and a gas.
If on smooth surface of any firm body to put a drop of a liquid it, spreading on a surface, will take the form of a lens. Between a surface of a firm body and a tangent to a surface of a drop in a point of the unit of phases the corner Ө is formed. This corner refers to as a regional corner. The size of corner depends on a surface tension:
1) firm body - air σт.в.;
2) firm body - liquid σ т.ж.;
3) liquid σ ж.в.
σт.в. = σт.ж. + σж.в. Соs Ө
Fig.1.6. The water on the left contains detergents, the water on the right does not.
There are hydrophilic and hydrophobic surfaces. Hydrophilic surfaces are well moistened by water. There are glass, silicates, oxides, sulfates, carbonates, cellulose, starch. Hydrophobic surfaces are badly moistened by water, but well moistened by not polar liquids. There are paraffin, fats, graphite, sulphur.
At Ө <900 Соs Ө > 0 - a substance is moistened by water;
Ө> 900 Cos Ө <0 - a substance is not moistened by water.
Wetting plays the big role in washing process. Clean fibres are easily moistened by water. Penetration inside of a fabric is complicated, as water has the big superficial tension. Practically washing-up liquids are applied to processing the fabrics polluted hydrophobic substances (fats, oils).Moistening ability depends on a class of SAS and their structure. Soaps have the least moistening ability, the greater – alkylsulphates, the best - sulphonates.
Solutions of sodium salts of normal fat acids from С10 and below possess bad moistening ability. At solutions of sodium salts of high-molecular fat acids the maximum of moistening ability is observed. Sodium salts of ramified fat acids possess the best moistening ability, than salts of normal fat acids (exception: sodium miristate). A ramifying of chain improves wettability therefore the compounds with hydrophilic group in the middle of a molecule possess the best wettability, than compounds with hydrophilic group on the end of a molecule.
Emulgation. Emulsion is the system consisting of one liquid (a disperse phase), allocated as droplets in other liquid (the dispersive environment). Distinguish emulsions of type oil in water (the dispersive environment - water) and type water in oil (the dispersive environment - oil).
Dispersions of one liquid in the second are unstable. For preparation of steady emulsions presence of the third phase (emulgator) is necessary.
SAS (soaps, synthetic detergents) are used in quality of emulgators which lower a surface tension between oil and water as a result of positive adsorption.
Except for it SAS stabilize the emulsions. They form a blanket around of each drop. The blanket should possess elasticity and mechanical durability. To such SAS concern the substances forming gel blanket. As stabilizers of emulsions of type water in oil serve oleophilic colloids, and emulsions of type oil in water – hydrophilic colloids. Thin dispersive substances can be stabilizers also.
At use of detergents are formed the emulsions of type oil in water. Dirt usually contains many fatty and oil pollutions. Their removal from a fabric is carried out by using of emulgation by detergents.
Foaming. Foam is disperse system in which bubbles of gas are divided by a thin film of a liquid. Foams can be considered as concentrated emulsions in which a disperse phase instead of a liquid is gas.
Pure liquids do not form foam. Presence of the third component is necessary for formation of foam.
Formation of foam is connected to the big increase of a surface, and it is possible, when the surface tension is small.
Stability of foamy systems is various (from several seconds about several days). For increase of viscosity and giving of elasticity enter special additives, for example, glycerin.
In homological line of sodium salts of normal fat acids foaming ability raises with increase in molecular weight and temperatures. Moderate ramifying of fat acids positively influences on foaming ability of soaps.
Foaming ability of alkylbenzolsulphonates depends on length of alkyl chain, its structures and position of sulphogroup. Foaming ability in beginning raises with increase in length of a normal chain, and then decreases owing to bad solubility of compounds at the big molecular weight. It is experimentally established, that the best foaming ability in hard water possess alkylbenzolsulphonates with alkyl chain at 10-12 carbon atoms at moderate ramified chain.
Foaming ability of non-ionogenic compounds depends on number attached molecules of ethylene, temperatures and concentration of solutions.
Foaming ability is characterized by volume or height of a column of the foam formed under certain conditions, and also stability of foam.
Stability of foam represents a ratio of volume or height of a column of foam through the certain time interval to initial volume or height of a column of foam.
Some researchers accept foaming ability for criterion of washing efficiency. However direct dependence between washing action and foaming ability does not exist. Now for automatic washing machines and machines for washing utensils are created the week foaming detergents possessing high washing action. Addition of soap decreases foaming abilities of synthetic detergents.
Do not overestimate an opportunity of foam to detain on itself the pollution removed from a fabric. Researches have shown that in case of using of soap solutions in foam passes only 10-13 % of pollutions, in solutions of alkylbenzolsulphates - 5-7 %, in solutions of alkylsulphonates - 8-11 %. Other quantity of pollutions remains in a solution.
Foam plays the important role in various processes in a number of industries (in construction, by manufacture of polymers, for suppression of fires, at drilling, flotation, etc.).
Stabilizing action of SAS
In washing process distinguish two opposite actions:
1) Separating of pollution from a fabric;
2) Repeated sedimentation of pollution on a fabric.
Hence, it is necessary, that the detergent not only effectively deleted pollution, but also kept them in solution, interfering of their repeated sedimentation on a fabric. The detergent should possess the stabilizing action interfering repeated sedimentation of pollution on a fabric.
Synthetic detergents are more hydrophilic compounds, than usual fatty soaps. They form less strong adsorption layers and consequently possess smaller stabilizing ability. Owing to the lowered stabilizing action of synthetic detergents at repeated washings white cotton fabrics get a grey shade owing to repeated sedimentation of pollutions. It causes to enter into structure of synthetic detergents special protective colloids, in particular carboksimethylcellulose and other polymeric products (cellulose sulphate, derivatives of starch). Carboxylmethylcellulose gives the best result, than cellulose sulphate. However it is effective only for cotton fabrics.