The effect of sodium chloride on the dynamic surface tension of sodium dodecyl sulfate solutions

Abstract

The dynamic surface tension of sodium dodecyl sulfate (SDS) solutions in the absence and presence of 0.5 M sodium chloride, using a tensiometer based on the principle of maximum bubble pressure, is examined. The surface tension of various concentrations of SDS was measured at 30°C using a variety of bubble rates, and the critical micelle concentration (CMC) determined from the surface tension versus log concentration curves. In the absence of salt, the CMC of SDS at various bubble rates was between 8.67 and 8.88 m M, within 4% of literature values. In the presence of 0.5 M sodium chloride, however, the surface tension and CMCs were lower than those determined in the salt-free systems and not comparable to reported values. The surface tension versus bubble rate plots for the salt-free surfactant systems (0.52, 1.73, 2.77, and 20 m M) had slopes between 0.6 and 0.3 g s −1. For the salt-containing SDS concentration above or at the CMC (1.73, 2.77, and 20 m M), the slopes of the surface tension versus bubble rate curves were between 1.5 and 0.35 g s −1, and decreased with rising surfactant concentration. In the salinized SDS concentrations below the CMC (0.29 and 0.52 m M) the surface tension-versus-bubble rate plots were sigmoid, showing a greater increase in surface tension compared to the other curves.