8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P2.16Tue 611:23-14:00Dipolar solute rotation in a supercritical polar fluidAmit Das 11 S. N. Bose National Centre for Basic Sciences, S. N. Bose National Centre for BasicSciences, Block JD, Sector III, Salt Lake 700098, Kolkata, IndiaFlurescence anisotropy measurements reveal a non-monotonic density dependence for averagerotation time (τ R ) of a polar solute coumarin 153 (C153) in supercritical (SC) fluoroform (CHF 3 )[1]. The conventional Stokes-Einstein-Debye model, relating τ R to the solvent viscosity, failsto explain the observed density dependence, because the experimental viscosity η increasesmonotonously with density for a fluid, in general. Here, the density dependent τ R is calculated byincorporating the wave vector dependent viscosity of the solvent and the solute-solvent interaction.A molecular hydrodynamic description, verified by molecular dynamics simulation is used for thewave vector dependent viscosity. A justification for the applicability of the present prescriptionis provided by reproducing the experimental η of SC CHF3. Solute-solvent interaction has beenincluded via the torque acting on the rotating solute. Incorporation of wave vector dependentviscosity gives qualitative description of the experimental density dependence of τ R which isfurther improved upon inclusion of solute-solvent interaction, leading to a semi-quantitativeagreement [2] with the experimental data. Recently, we have extended this formalism for a polarliquid solvent where [3] we focus on the modification of the total friction due to the solute-solventdipolar interaction, to point out different regimes where the electrostatic contributions aresignificant. Our calculated rotation times for C153 in several polar ambient liquids agree wellwith the experimental data [4].[1]. N. Kometani et al, J. Phys. Chem. A, 108, 9479 (2004).[2]. A. Das, R. Biswas, J. Chakrabarti, J. Phys. Chem. A, 115, 973 (2011).[3]. A. Das, R. Biswas, J. Chakrabarti, Preprint (2011).[4]. M. L. Horng et al, J. Phys. Chem. A, 101, 1030 (1997).16