Effects of internal and external pressure on the structure and dynamics of micelles: A FTIR study of sodium and potassium decanoates in D2O1☆

Abstract Infrared spectra of aqueous micellar solutions of sodium and potassium decanoates were measured as a function of external pressure up to and exceeding 50 kbar. Counterion interactions also generate an internal pressure in these micellar systems which strongly affects the features and shape of the infrared spectra. The internal pressure-induced spectral changes are countercation dependent and are discussed in terms of countercation effects on the structural and dynamic properties of these aqueous surfactants. External pressure, on the other hand, induces a structural phase transition from the micellar state to a coagel state, which is also affected by the nature of the countercation. The critical coagelization pressure is 21.2 kbar for sodium decanoate and 22.5 kbar for potassium decanoate. The decanoate molecules in the micellar state are conformationally and orientationally highly disordered. External pressure cannot induce conformational ordering of the methylene chains in the micellar phase; on the contrary, the conformational disorder in this phase increases with increasing pressure. This is the first demonstration that disordered structures can exist in a methylene chain system at pressures as high as 20 kbar. The alkyl chains become fully extended and the reorientational fluctuation of the decanoate molecules are completely damped at the critical coagelization pressures. The structure of the pressure-induced coagel phases is different from that of the corresponding temperature-induced coagel phases due to the different magnitude of the counterion binding force under extreme conditions of temperature or pressure.