Influence of CTAB Reverse Micellar Confinement on the Tetrahedral Structure of Liquid Water.

Abstract

The effect of confinement on the tetrahedral ordering of liquid water plays a vital role in controlling their microscopic structure and dynamics as well as their spectroscopic properties. In this article, we have performed the classical molecular dynamics simulations of four different CTAB/water/chloroform reverse micelles with varied water content to study how the tetrahedral ordering of nanoscale water inside reverse micellar confinement influences the microscopic dynamics and the structural relaxation of water···water hydrogen bonds and its impact on the low-frequency intermolecular vibrational bands. We have noticed from the results obtained from simulated trajectories the lowering trends of tetrahedral ordering of water pools in reverse micellar confinements as we move from bulk to confined and strictly confined environments. We have observed that the order of confinements significantly altered the relaxation pattern of water···water hydrogen bonds present in the nanoscale water pool of reverse micelles. The recrossing related to hydrogen bond dynamics can effectively explain the relaxation pattern of $C_{\mathrm{HB}}^{\mathrm{WW}}\left(t\right)$ under confinement. The Br^-1···water hydrogen bond depicts a much slower relaxation compared to the water···water hydrogen bonds inside reverse micelles. We have also explored the correlation between the tetrahedral ordering of nanoscale water pools and the relaxation of water···water hydrogen bonds with the 50 cm^-1 band for water inside reverse micelles. The computations reported that compared to bulk water, the band appearing at 50 cm^-1 for O···O···O triplet bending is nonuniformly blue-shifted by 18-45 cm^-1 for the nanoscale water pool inside reverse micelles, and the intensity of the band drops from bulk to confined and strictly confined environments, which indicates the reduced tendency of such triplet formation. It is observed that a significant intensity variation at the 200 cm^-1 band correlates with the effect of confinement on the tetrahedral ordering of the water pool inside reverse micelles. So, our observations support the influence of strictly confined environments on the tetrahedral water structure to adopt the quasi-two-dimensional water network and experience restricted longitudinal translations. It is further noticed that the 500 cm^-1 librational band is also found to be blue-shifted by 71-112 cm^-1 for the water pool in reverse micelles, and the extent of the shift being more noticeable for strictly confined environments correlates excellently with the sluggish relaxation of water···water hydrogen bonds in such environments.