Measurement and computational modelling of intermolecular interactions in fluids.

Abstract

The molecular theory of the second light-scattering virial coefficient Bp describing the effects of interacting pairs of molecules on the depolarization ratio p of Rayleigh-scattered light is reviewed, both for interacting linear and nonlinear molecules. The molecular tensor theory of Bp for nonlinear molecules is extended for the first time to include in the scattered intensity p those contributions arising from field gradient effects and induced quadrupole moments in the molecular interactions. The expressions for contributions to Bp are evaluated numerically for the nonlinear polar molecule dimethyl ether. We have used an existing light-scattering apparatus to investigate the pressure-dependence of the depolarization ratio p for dimethyl ether, allowing Bp to be extracted. The measured value is compared with the calculated value, theory and experiment being found to agree to within 9%. This success in modelling Bp for dimethyl ether spurred us on to extend our investigation to the second Kerr-effect virial coefficient BK • The molecular-tensor theory of BK for nonlinear molecules is reviewed, and is applied in this work to dimethyl ether. The calculated BK values generally lie within the uncertainty limits of the available measured data over their full range of temperatures. We have used a recently-commissioned Kerr cell to undertake our own measurement of BK for dimethyl ether at room temperature. This value is in good agreement with the findings of our molecular model, and is in reasonable agreement with the other measured data. This thesis serves to reaffirm recent claims that comprehensive dipole-induced-dipole theories of molecular interaction effects explain the observed phenomena adequately provided one works to higher orders in the molecular tensors so that the series of contributing terms has converged to a meaningful numerical result, and provided the full symmetry of the molecules is allowed for.