Molecular diffusive dynamics in complex liquids

Abstract

In simple expanded liquids, atoms/molecules show a two relaxation process, namely vibrations and long range diffusion process, also known as α-process. However, in viscous liquids the atoms/molecules undergo an additional relaxation process and is called fast β-relaxation (Angelani, Cavagna, Giardina, Leuzzi, & Scala, 2008). In this case atoms/molecules rattle around the cage formed by its neighbouring atoms/molecules before the long range diffusion process. Theoretically, Mode- Coupling Theory (MCT) has been very successful in describing the fast β-relaxation process in dense liquids (Würger, March 1994). Experimentally, the fast β-relaxation process has been observed in a variety of molecular liquids by light scattering, neutron scattering and dielectric spectroscopy, etc. In most liquids, the fast β-relaxation process exhibits a simple exponential decay (Han, Akcasu, & A, 2011). Recently, logarithmic decay of correlations for the fast β-relaxation has been observed in orthoterphenyl (OTP), benzophenone (BZP), 2-biphenylmethanol (BPM) and in Salol, in their supercooled state by Optical-Kerr-Effect experiments and in DNA, tRNA and proteins by QENS. One of the striking features of the liquids that show the logarithmic relaxation process is their molecular structure. For example, the molecular structure of Salol, OTP, BZP or BPM shows two benzene rings either end of a chain. Studied have suggested that the unique inter molecular potential is the origin of exotic logarithmic relaxation process in molecular liquids. In this project we have studied the long range diffusion dynamics in one of the molecular liquids, phenyl-salicylate (Salol), that show the logarithmic decay. We have altered the inter-molecular potential of the Salol by dissolving it in Acetone and studied the self-diffusion dynamics by dynamic light scattering. We found that the a-relaxation dynamics of the Salol do not change appreciably in the Acetone even at high diluted state. However, temperature dependence of diffusion coefficient changes from an Arrhenius to non-Arrhenius form. At high dilution the system changes from a strong liquid to a fragile liquid.