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Abstract
Density and sound velocities were measured in order to investigate the interactions between the ionic surfactants sodium dodecyl sulfate (SDS), dodecyl- (DoTAB) and hexadecyl- (CTAB) trimethylammonium bromides and diblock copolymer S15E63 (where E denotes oxyethylene, and S oxyphenylethylene, and the subscripts denote block lengths) between 20°C and 40°C. The adiabatic and the specific adiabatic compressibilities of the surfactants and the block copolymer/surfactant aqueous dilutions were deduced therefrom. The critical micelle concentrations of the surfactant in the absence and presence of the copolymer were determined for SDS and DoTAB. The adiabatic compressibility data in the presence of the copolymer shows for SDS and DoTAB an increase of the critical micelle concentration of the surfactant, and a clear reduction of the compressibility. The specific adiabatic compressibility is proposed as an experimental observable that could detect and characterize surfactant/copolymer states. This data is considered to be in the form of the sum of two terms: a positive intrinsic contribution that reflects the packing of the copolymer/surfactant chains in the aggregate, and a negative hydration contribution resulting from the solute groups and the surrounding molecules.
Acknowledgements
This project was supported by the Ministerio de Educacion y Ciencia through project MAT2004-02756 and Xunta de Galicia. P.T. and E.C. thanks the Ministerio de Educacion y Ciencia for his Ramón y Cajal position and for his grant, respectivelly. We thank Professors Attwood and Booth for generous gift of block copolymers.