SALT EFFECT ON POSTMICELLAR TRANSITIONS - HEAT-CAPACITIES AND VOLUMES OF TERNARY-SYSTEM WATER PLUS SODIUM DODECYL-SULFATE PLUS SALT AT 25-DEGREES-C

Densities and heat capacities of the ternary system water + sodium dodecylsulfate (SDS) + NaCl have been measured in the aqueous domain as a function of NaCl and surfactant concentrations. Apparent molar volumes and heat capacities of both solutes, SDS and NaCl, have been obtained therefrom and analyzed as functions of the concentrations of the two solutes. In the micellar domain, for peculiar concentrations of surfactant or salt, marked variations of the apparent heat capacity of the solute are observed. It is shown they are related to transitions occurring in the micellar solution and influenced by the presence of salt. Particularly, the post-micellar transition, revealed at 0.2 m in SDS solutions from the study of the transfer of alcohols, is enhanced in the presence of the salt and a drastic variation of the transfer heat capacity of NaCl occurred between 0.15 and 0.2 m. Moreover, the ’second CMC’ which had been suggested from conductivity measurements is also evidenced here through apparent heat capacities of the salt at 0.05 m in SDS. It is shown that these two transitions are very sensitive to NaCl, revealing thus their electrostatic nature. The latter, at 0.05 m, is due to the variation of the ionicity of small spherical micelles whereas the former, at 0.2 m, is certainly associated with the variation of β, the ratio of bound counterions to micelles, preceding the sphere-to-rod transition. Moreover, the identical values of heat capacities of transfer obtained with NaCl and NaI and their similar variations with the SDS concentration are also additional evidence of the pure electrostatic nature of the observed transitions. © 1994 by Academic Press, Inc.