Theoretical considerations of detergency

Abstract

1. 1. Solid particles are more difficult to remove than oily and fatty dirts. In the case of solid dirts, the smaller the particle size the more difficult is the removal of the dirt, whereas, in the case of oily soils, the surface-active products cause droplet formation followed by soil removal. 2. 2. Looking at a homologous series of any soap, with increasing C-number the optimum wetting action will be reached earlier than the optimum washing action. Branches in the paraffin chain shift both optima to a higher C-number. In this case the absolute value for the wetting action will be increased. A second hydrophilic group in the detergent reduces both wetting action and cleaning action. 3. 3. The C.M.C. of anionic soaps and electroneutral soaps are reviewed because numerous soap properties show a break at or near the C.M.C. There is a break in the detergency curve which is in agreement with the C.M.C. in those cases where the full detergent power is utilized and where the dirt is not of an extremely small particle size. Furthermore, of practical interest is the break in the curves of surface activity, foam characteristics and solubility of oils and Ca-soaps at the C.M.C. 4. 4. In the presence of electrolytes, C.M.C. and critical washing concentration are not necessarily the same. Of basic importance in understanding the detergent action of soap and washing alkalies are the interfacial potentials between the soiled surface, the dirt and the aqueous solution. The kind and the degree of these potentials are dependent on the ions of soap aggregates which are adsorbed preferentially by dirt and substratum. The loosening of the dirt from the substratum takes place because the detergent, due to its polar construction, goes between the dirt and the substratum. Electrostatic forces are the influencing factors which cause the loosening of the dirt; this process will be supported by mechanical movement. In general, the dirt cannot be removed if cationic soaps are used in low concentrations because the surface-active ions produce a positive charge on the dirt. But a cleaning effect with cationic soaps is possible if their concentration is high enough to give the substratum surface a positive charge. 5. 5. The cleaning activity of neutral or alkaline salts is dependent on the charge of their anions. Salts with multivalent cations decrease the cleaning action, because the preferentially adsorbed cations give a positive charge to the dirt as with cationic soaps. At high salt concentrations, agglomeration of the dirt takes place and the pH value also has a very important influence on the cleaning action. Therefore, cleaning action with salts present is dependent on the amount of salt and the kind and degree of charged ion preferentially adsorbed by the dirt and substratum. 6. 6. The increase in cleaning action with concentration of pure soap solutions below the C.M.C. can be explained by the increase in valence of the aggregating soap ions before the C.M.C. is reached and the formation of micelles takes place.