Behaviour of calcium ions in micellar sodium dodecyl sulphate solution containing solubilized polar organic compounds

Abstract

Polar organic compounds and in particular n-hexanol, when solubilized in sodium dodecyl sulphate (S.D.S.) micelles, have been shown to provide a marked degree of protection against precipitation in the presence of calcium ions. For example, 2 % S.D.S. (0.069 M) containing a [hexanol]/[S.D.S.] mole ratio of 1.25 can tolerate 0.018 M calcium chloride without precipitation at 25°C. In the absence of amphiphile, precipitation of calcium dodecyl sulphate occurs with 0.006 M calcium chloride. Measurements of the sodium ion activity of 2 % S.D.S. with hexanol only and with hexanol + calcium ions have enabled the contribution of each species to be determined. Large increases in sodium ion activity were observed partly due to solubilization of the alcohol and partly due to preferential adsorption of the calcium ions at the micellar surface; these factors have been discussed with reference to the mechanism of the phenomenon. The effect appears to provide a new approach to the subject of soap protection and the practical importance of the phenomenon with respect to detergency and possible application to biological systems has been indicated.