Ionic activities in sodium dodecyl sulphate solutions from electromotive force measurements

Abstract

Measured surfactant ion activities (a₁) and counterion activities (a₂) in solutions of sodium dodecyl sulphate (SDS) in the presence and absence of NaCl are reported. The results show that premicellar association is insignificant. Mean ionic activities of surfactant (a±) above the c.m.c. (critical micelle concentration) agree well with estimates from light scattering. This shows that the surfactant ion electrodes work well in the presence of micelles and that solubilisation of membrane constituents is unimportant. Comparison of a± values with estimates from surface tension studies suggests increasing surfactant adsorption above the c.m.c.

At the c.m.c. in the presence of NaCl we find log a₁+ 0.795 log a₂=–3.185. (i), By putting (c′)²=m₁[m₁+0.2(c–m₁), (ii), where c and m₁ denote total and monomeric surfactant concentration and c′ is the concentration that would give the same activity as that in the solution of interest if no micelles formed, we found that increasing c leads to decreasing m₁ above the c.m.c. An attempt to obtain liquid junction potentials in solutions above the c.m.c. was unsuccessful. Differences between present and previous results are probably attributable to the non-Nernstian response of previous surfactant electrodes.