The influence of charge on the structure and dynamics of water encapsulated in reverse micelles

Abstract

Hydrogen-bonded structure and relaxation dynamics of water entrapped inside reverse micelles (RMs) composed of surfactants with different charged head groups: sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (anionic), didodecyldimethylammonium bromide (DDAB) (cationic) and Igepal CO-520 (Igepal) (nonionic) in cyclohexane (Cy) have been studied as a function of hydration (defined by ). Sub-diffusive slow (sub-ns) relaxation dynamics of water has been measured by the time resolved fluorescence spectroscopy (TRFS) technique using two fluorophores, namely 8-anilino-1-naphthalenesulfonic acid (ANS) and coumarin-343 (C-343). The hydrogen bonded connectivity network of water confined in these RMs has been investigated by monitoring the hydrogen bond stretching and libration bands of water using far-infrared FTIR spectroscopy. In addition, the ultrafast collective relaxation dynamics of water inside these RMs has been determined by dielectric relaxation in the THz region (0.2–2.0 THz) using THz time domain spectroscopy (THz-TDS). While TRFS measurements establish the retardation of water dynamics for all the RM systems, FTIR and THz-TDS measurements provide with signature of charge specificity.