Water Confined in Lamellar Structures of AOT Surfactants: An Infrared Investigation

Both X-ray diffraction and infrared spectroscopy on water confined between the lamellar bilayers of aerosol-OT (AOT) allowed us to explore the differences in connectivity between bulk and confined water when water layer thicknesses are allowed to vary from 19.0 to 1.5 nm. Compared with previous studies on AOT reverse micelles, the present report in the lamellar mesophase allows one to cancel the micelle curvature contribution, which disrupts the water connectivity, too. The influence of AOT surface on the water connectivity degree was thus quantified alone in the mid infrared region (3000−3800 cm-1) in the OH stretching mode. The OH stretching mode peak can be fitted by three Gaussian peaks that describe three main connecting populations, namely the “multimer” water at c.a. 3585 cm-1, the “intermediary” water at c.a. 3465 cm-1, and the “network” water at c.a. 3320 cm-1. It is shown that interactions with the surfactant heads do not alter the water structure until the water layer shrinks below 4.0 nm and the hydration water molecules per surfactant are found to be equal to 2.6, in good agreement with equivalent studies on micellar systems.