Micellization study of sodium dodecyl sulfate in water and microemulsion systems by conductivity and counterion-activity measurements

Abstract

The micellization process of sodium dodecyl sulfate (SDS) involved in water S DS , water/SDS/1-pentanol, and water/SDS/1-pentanol/dodecane systems has been followed by means of pNa and conductivity measurements in a wide SDS concentration range lying from concentrations below the critical micelle concentration up to the boundary of the realm of existence of the isotropic monophasic media investigated. In all cases, the plots of free counterion activities and conductivities against the logarithm of SDS concentration show two characteristic breakpoints. The first breakpoint agrees well with the critical micelle concentration of SDS and the second breakpoint, corresponding to an obvious increase of the degree of counterion dissociation a is interpreted as proceeding from a change in the micelle structure and a different distribution between free and bound counterions. In pure SDS solutions, the slopes of the pNa curve above the first and the second breakpoints give α values equal to 0.21 and 0.39, respectively. The addition of 1-pentanol and dodecane acts upon both micellization steps by decreasing the concentration of the breakpoints and by changing the amount of associated counterions. In all systems, the electrical conductivity of the solutions is ensured essentially by the free sodium counterions. The addition of NaCl is followed by a marked increase of associated Na + counterions in the water SDS solutions but the effect is lowered in the ternary and quaternary systems.