Molar volumes and isentropic compressions of pluronics L64 and P123 in aqueous surfactant solutions, over the critical temperature range of aggregation

Interactions between triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), EOxPOyEOx, and ionic surfactants, have been investigated using density and ultrasound speed measurements over the temperature range 288.15–318.15 K. Two copolymers, denoted L64 (x = 13, y = 30) and P123 (x = 20, y = 69) were selected because their critical micellar temperatures (CMT) could be conveniently covered. The ionic surfactants were the sodium dodecylsulfate, SDS, and the hexadecyltrimethylammonium bromide, CTAB. The transfer molar volumes and transfer molar isentropic compressions of each copolymer from water to aqueous micellar solution were calculated and analysed at a constant copolymer concentration ( w p = 1 % ) for varying surfactant concentrations up to 0.15 mol kg−1. The trends in the variations of the transfer properties of either L64 or P123 gradually decrease from positive to negative values over the range that encompasses the critical micellar temperature. By contrast, they are remaining mostly similar apart from CMT, i.e. the copolymer is present either in monomeric or micellar state, and are weakly dependent on the temperature. The transfer values are assumed to be the sum of two contributions of opposite sign. A positive one arises from the hydrophobic interactions between the monomers and SDS and gives rise to the formation of copolymer–surfactant complexes. A negative one arises from the rehydration of the copolymer molecules expelled from the pluronic micellar core during the disruption of the pluronic micelles with increasing concentration of SDS. This negative contribution has been shown to be rigorously equivalent to the value of the micellisation properties of the pluronics in water. Results obtained in the present work for CTAB solutions reveal seemingly similar mechanisms of binding between the surfactant and the pluronics.