The influence of electrolyte on the mechanism for oily soil removal

We present the results of a paper chromatographic study of the effect of electrolyte on the mechanism by which oily soils are removed with surfactant from a substrate in a state almost without mechanical and/or hydrodynamic actions from the standpoint of surface chemistry. Using a cellulose filter paper as the stationary phase for the substrate, R-f' Values of oleic acid (OA), squalene (SQ) or cholesterol (CPI) as a model of sebum soil have been examined as a function of sodium dodecylsulfate (SDS) concentration in aqueous solution containing NaCl. An increase in the concentration of SDS produced an increase in the R-f' value of the oily soils to give a higher detergency. When NaCl was added, a lower SDS concentration was required to increase the R-f' values in order to produce the removal of the oily soils at a lower SDS concentration. Based on a filter paper capillary analysis, it was suggested that the removal of OA from cellulose fiber occurred due to a molecular complex formation between OA and SDS at a low concentration below the critical micelle concentration (cmc) of SDS. The number of SDS molecules bound to OA decreased with increasing concentration of NaCl. At a concentration above the cmc, the detergency of OA could be explained on the basis of the solubilization of the soil. The addition of NaCl to the SDS solution above the cmc produced an increase in the partition coefficient of OA between the fiber and SDS micelle, and a decrease in the partition coefficient of OA between the micelle and water, to give a lower detergency of the OA.