Structure of dilute supercritical solutions

Solutions in supercritical solvents have interesting properties such as high diffusivity, low viscosity and enhanced solvent strength, which make them useful. These interesting properties are related, through statistical mechanics, to interesting structures. A considerable amount of recent research is directed to develop an improved understanding of local fluid structure and the molecular interactions between solutes and other components in supercritical fluid mixtures.

This thesis is a contribution to a larger research program, which is a continuing group effort towards the microscopic understanding of the interesting bulk properties in the supercritical fluid mixtures. Approximate theories and molecular simulations provide a means of relating the microscopic world and the macroscopic world through molecular distribution functions. To provide data for testing simu- lations and theories for near-critical states, this research program has undertaken scattering experiments, first of simple noble gas systems, then of more complex molecular systems such as long-chain hydrocarbons in supercritical hydrocarbon solvents.

It was necessary to determine whether the solubility of argon in supercritical neon is sufficient for these scattering experiments. This work included develop- ment and initial testing of techniques for carrying out phase equilibrium studies on a dilute mixture of argon in supercritical neon.

One strong relation between the microscopic world and the bulk properties which has been explored is the truncated virial density expansion, to describe the solubility of solids in near-critical solvents. The analysis is based on the observa- tion that data for a wide variety of binary and ternary systems at conditions that include densities of more than twice the critical follow a straight-line relation given by the third virial equation of state. The implications of this little-used finding are examined in light of theory, correlation and prediction. First, the virial series for mixed systems is examined further. Second, the importance of higher order terms is studied for model Lennard-Jones systems. Third, the second cross virial coefficients from high pressure data, which are the same as those from other low pressure data, are analyzed in terms of the Hayden-O'Connell correlation. Finally, the possibility of predicting the apparent third virial term is examined. The results suggest that there is a fundamental basis for the empirical finding and that molecular theory leads to simple modeling of systems with complex substances in near-critical solvents but full prediction will be difficult to achieve.