TOWARD IN SITU MEASUREMENT OF LIQUID DENSITY USING OPTICAL KERR EFFECT SPECTROSCOPY
Bender, John S.
Fourkas, John T
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Optical Kerr effect (OKE) spectroscopy is a widely used technique for probing the low-frequency, Raman-active dynamics of liquids. Although molecular simulations are an attractive tool for assigning liquid degrees of freedom to OKE spectra, the accurate modeling of the OKE and the motions that contribute to it rely on the use of a realistic and computationally tractable molecular polarizability model. Here we explore how the OKE spectrum of liquid benzene, and the underlying dynamics that determines its shape, are affected by the polarizability model employed. We test a molecular polarizability model that uses a point anisotropic molecular polarizability and three others that distribute the polarizability over the molecule. The simplest and most computationally efficient distributed polarizability model tested is found to be sufficient for the accurate simulation of the liquid polarizability dynamics. The high-frequency portion of the OKE spectrum of benzene shifts to higher frequency with decreasing temperature at constant pressure. Molecular dynamics simulations of benzene are used to isolate the effects of temperature and density on the spectrum. The simulations show that, at constant density, the high-frequency portion of the spectrum shifts to lower frequency with decreasing temperature. In contrast, at constant temperature, the high-frequency portion of the spectrum shifts to higher frequency with increasing density. Line shape analyses of simulated spectra under isochoric and isothermal conditions shows that the effects of density and temperature are separable, suggesting that OKE spectroscopy is a viable technique for in situ measurement of the density of van der Waals liquids. OKE spectroscopy is then used to investigate the density of benzene confined in nanoporous silica. The high-frequency portion of the OKE spectrum shifts to the blue with increasing confinement, which is consistent with densification. Molecular dynamics simulations show that the tumbling vibrational density of states of benzene confined in silica pores exhibit behavior similar to that of the OKE spectrum. The dependence of the structure of the simulated liquid with increasing confinement resembles that of the bulk liquid at constant temperature and increasing density, further supporting the premise that benzene is densified upon confinement in silica pores.