Studies on apparent molar volumes and heat capacities of several pluronics (triblock copolymers) in aqueous solutions of sodium dodecyl sulfate at 298.15 and 318.15 K

Abstract

Density and heat capacity measurements were performed in order to investigate the interactions between the ionic surfactant sodium dodecyl sulfate (SDS) and some triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), at 298.15 and 318.15 K. The pluronics L31, L35, 10R5, L64, F68 and P123, were selected as copolymers because of their convenient hydrophilic-hydrophobic ratio and critical micellar temperature. Molar volumes and heat capacities of transfer of either SDS or copolymer from water to an aqueous mixed solution were calculated from the density and heat capacity values and analysed as a function of the surfactant concentration. It was found that the transfer properties depend mainly on the state of the copolymers. For the unassociated copolymers, the molar volumes of transfer of both SDS and copolymer are positive. The initial sharp increase seen in the corresponding plots against surfactant molality signals the formation of a surfactant-copolymer complex up to the saturation of the copolymer. Thereafter, the constant values of transfer volumes of copolymers depend mainly on the length of the poly(propylene oxide) block. For the associated copolymers, negative volumes of transfer of either SDS or copolymer were observed. They are ascribed to interactions between SDS and copolymer micelles that give rise to a rapid breakdown of the aggregates. The profiles of the transfer heat capacity curves show more complicated trends through the critical micellar concentration region, which are interpreted as being due to the large positive contribution of the relaxation terms related to the equilibrium shifts induced by the temperature.