Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

Abstract

A review. The water mol. has the convenient property that its mol. polarizability tensor is nearly isotropic while its dipole moment is large. As a result, the low-frequency anisotropic Raman spectrum of liq. water is mostly collision-induced and therefore reports primarily translational motions while the far-IR (terahertz) and dielec. spectrum is dominated by rotational modes. Atomic and globular-mol. liqs. have a zero dipole moment as well as an isotropic polarizability tensor. These spectrum-simplifying properties were exploited in a study of a no. of liqs. and solns. using ultrafast optical Kerr-effect (OKE) spectroscopy combined with dielec. relaxation spectroscopy (DRS), terahertz time-domain spectroscopy (THz-TDS), and terahertz field-induced second-harmonic generation (TFISH) spectroscopy. For room-temp. ionic liqs. (RTILs), liq. water, aq. salt solns., noble gas liqs., and globular-mol. liqs. it was found that, in each case, surprising structure and/or inhomogeneity is obsd., ranging from mesoscopic clustering in RTILs to stretched-exponential dynamics in the noble gas liqs. For aq. electrolyte solns. it is shown that the viscosity, normally described by the Jones-Dole expression, can be explained in terms of a jamming transition, a concept borrowed from soft condensed matter studies of glass transitions in colloidal suspensions.