Dissolution of a surfactant-water lamellar phase investigated by combining time-lapse polarized light microscopy and confocal Raman spectroscopy

Abstract

HypothesisWhile the phase behavior of aqueous surfactant solutions is usually described in term of the equilibrium microstructures of lyotropic liquid crystals, the transformations which take place when a phase turns into another one, either by changing the concentration or the temperature, are still to be elucidated. A simultaneous determination of concentration and microstructure is at order to elucidate the phase behavior under changing conditions, such as in a dissolution experiment. ExperimentsConfocal Raman micro-spectroscopy and time-lapse polarized light microscopy are combined to study the phase transitions taking place in the dissolution of a common anionic surfactant (sodium laurylethersulfate) in water. FindingsBy comparing Raman concentration profiles and polarized light images, it is found that the aqueous solution, with initial surfactant concentration of 72 wt%, undergoes a sequence of complex microstructural transformations including distortion of the initial lamellar phase, formation of an intermediate striated texture, which can be considered as a precursor of a cubic phase, and a heterogeneous hexagonal phase going through a transition region before turning into a micellar phase. The effects of the sodium counter-ion and of water confinement are also investigated by analyzing the OH-stretching bands.