Picosecond Solvation and Rotational Dynamics: An Attempt to Reinvestigate the Mystery of Alcohol-Water Binary Mixtures.

Abstract

In this article, we have investigated the anomalous behavior of two alcohol-water (tert-butyl alcohol (TBA)-water and ethanol-water) binary mixtures using femtosecond fluorescence upconversion technique. Recently, Gupta and Patey (Gupta, R.; Patey, G. N. J. Chem. Phys. 2012, 137, 034509(1)-034509(12)) have used four force fields to simulate TBA-water binary mixtures. Surprisingly, two of them do not identify any aggregation of TBA molecules. We have calculated average solvation time of Coumarin 480 (C480) using two different methods. Our results indicate slowdown in solvation time in the mole fraction ranges XT = 0.09-0.15, XT = 0.40-0.46 and XE = 0.06-0.08, XE = 0.20-0.25 for TBA-water and ethanol-water binary mixtures, respectively. Additionally, we have detected another anomalous region at XT ∼ 0.03. Slow solvation responses in the ranges XT = 0.40-0.46 and XE = 0.20-0.25 are probably due to the higher shear viscosity of the medium. However, XT = 0.09-0.15 and XE = 0.06-0.08 are the manifestation of aggregation induced structural transition of alcohol molecules. Hindered rotation of C480 in the ranges XT = 0.04-0.09 and XE = 0.03-0.07 corroborates our solvation dynamics results. From temperature dependent anisotropy measurements, we have shown that aggregation of alcohol molecules increases with increase in temperature.