COMPARISON OF THE COSURFACTANT CHARACTER OF 1-BUTANOL AND PHENOL IN SODIUM DODECYL-SULFATE AQUEOUS-SOLUTIONS FROM RELATIVE ENTHALPIES OF DILUTION AT 298.15 K

Heats of dilution of 1-butanol and phenol in aqueous micellar solution s of sodium dodecylsulfate (SDS) have been measured at 298.15 K with a dynamic-flow Picker microcalorimeter and used for determining the rel ative apparent molar enthalpies of dilution (L-phi,L-3) of solutes. Th e variation of L-phi,L-3 for both solutes, at low concentrations, as a function of the surfactant concentration provides evidence of a postm icellar transition occurring at 0.2 mol kg(-1) in SDS solutions. In ea ch micellar domain, at concentrations below and above 0.2 mol kg(-1), the limiting apparent molar enthalpies of dilution of the solute (extr apolated to 1/m = 0 from the linear dependence of the inverse of surfa ctant concentration predicted by the pseudophase model), reflect vario us trends. For 1-butanol, both values are close to one another, wherea s for phenol, where the sign of the linear slopes also changes, the li miting values are noticeably different. The latter indicates that a mo dification of the counterion binding, accompanying the postmicellar tr ansition, occurred. For concentrated solute mixtures at fixed surfacta nt composition, an analysis of the variation of the abnormal, relative apparent molar enthalpies of dilution of 1-butanol with its unique co ncentration, shows that 1-butanol is strongly trapped in alcohol aggre gates stabilized by the surfactant and, consequently, acts as a good c osurfactant capable of forming microemulsions. Alternatively, although phenol hydrophobicity is similar to that of 1-butanol, owing to its a cid character, it is mostly solubilized in the aqueous layer of micell es and does not penetrate deeply towards the micellar interior and, he nce, could not be considered as a very efficient cosurfactant. (C) 199 7 Elsevier Science B.V.