Supercritical solute, molecular structure

King, J.W. and Friedrich, J.P. (1990) Quantitative correlations between solute molecular structure and solubility in supercritical fluids. J. Chromatogr. 517, 449-458. 

In this chapter, we will review some of the work that we have been doing in recent years in the context of solvation and dynamical properties in polar and non-polar supercritical solutions using molecular dynamics computer simulations. First we will discuss solvation of alkaloids in SC-CO2 and provide detailed molecular views of the main structural features of the local density augmentation around simple alkaloids... 

The solvation structures which we have determined for the supercritical solutions of LJ molecules appear to be in agreement with the recent theoretical and experimental suggestions of solvent-solute clustering and solute-solute aggregation near the CP. Quantitative testing of theoretical models for solvation structure of supercritical solutions (such as that presented here) may become possible if recent efforts at simulation of supercritical solutions (elsewhere in this volume) prove successful. Clearly, the LJ model used in our theoretical studies to date does not provide an adequate representation of real molecular interactions. However, the method we have demonstrated is potentially capable of application with much more accurate (and complicated) potential functions, and meaningful quantitative interpretation of experiments may then become possible. 

Often, the unique and unusual molecular structure in SCF solutions profoundly affects the reactivity in these systems. Thus, the elucidation of this structure is one of the first requirements for developing a predictive capability for reaction rates and pathways. A powerful spectroscopic technique that has only recently been used for in situ characterization of the molecular structure in supercritical fluid solutions is X-ray absorption fine structure (XAFS). XAFS provides detailed structural information about the number of nearest-neighbor atoms, bond distances, and bond strengths (from the Debye-Waller factor). The application of XAFS to a wide range of SCF solutions provides another powerful tool to explore the detailed structure of SCFs. 

The physical properties of the solutes also play a crucial role in critical fluid extraction processes, particularly with regard to their molecular structure and temperature-dependent properties. The author has offered some insight into predicting solute solubility in supercritical fluids based upon a correlation between a molecular group structure contribution-solubility parameter correlation [11]. Suffice to say, the introduction of polar functional groups into a compound usually results in the need for a higher extraction pressure and/or temperature [12]. The vapor pressures of the solutes to be extracted or... 

In this chapter, we have reviewed some of our own work on solvation properties in supercritical fluids using molecular dynamics computer simulations. We have presented the main aspects associated with the solvation structures of purine alkaloids in CO2 under different supercritical conditions and in the presence of ethanol as co-solvent, highlighting the phenomena of solvent density augmentation in the immediate neighborhood of the solute and the effects from the strong preferential solvation by the polar co-solvent. We have also presented a summary of our results for the structure and dynamics of supercritical water and ammonia, focusing on the dielectric behavior of supercritical water as functions of density and temperature and the behavior of excess solvated electrons in aqueous and non-aqueous associative environments. 

S. T. Cui and J. G. Harris, Ion association and liquid structure in supercritical water solutions of sodium chloride a microscopic view from molecular dynamics simulations, Chem. Eng. Sci. 49,2749-2763 (1994). 

---