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The solution structure of tert-butyl alcohol was investigated as a function of pressure and temperature using high-resolution nuclear magnetic resonance (NMR) spectroscopy. Simulations of the solut...

Dependence of the solvatochromic shift for binary supercritical fluid mixtures as a function of modifier mole fraction, temperature, and pressure is discussed. Carbon dioxide/2-propanol binary fluid mixtures were studied at both subcritical and supercritical conditions. The local composition of the cybotactic region was seen to be enriched in organic cosolvent at low pressures. As the pressure or temperature increased, the local composition of the organic cosolvent decreased. A simple theory is developed to describe the local composition of the fluid in the cybotactic region about the solvatochromic probe molecule.

Solvatochromic probe molecules, with absorbances in the UV region, were used to examine the cybotactic region of pure and mixed supercritical fluid solvent systems. This approach allowed determination of the relative polarity-polarizability of these fluids and comparison with conventional solvents. The position of maximum absorbance was seen to be density dependent. The solvatochromic shift for the most polar fluid studied, supercritical ammonia, was the most sensitive to changes in solvent density, and at moderate densities it approached that observed for tetrahydrofuran. Supercritical CO/sub 2/, N/sub 2/O, and CCl/sub 3/F showed smaller spectral shifts over a comparable density range. In mixed fluid solvents (CO/sub 2/-methanol) the spectral shift approached that seen for pure methanol as the percentage of methanol was increased.

The solvatochromic behavior of nonpolar supercritical fluids, in terms of the ..pi..* Kamlet and Taft polarity/polarizability parameter, is compared to the McRae-Bayliss model. A break in the plot of ..pi..* against the Onsager reaction field (F/sub 0/) is seen for the fluids studied at low reaction field values (low densities). The relationship between ..pi..* and the Onsager reaction field (F/sub 0/) shows two regions of nearly linear behavior. The transition from the low-density region to higher density region, where a smaller dependence of ..pi..* upon (F/sub 0/) is observed, is tentatively attributed to extensive solvent clustering about the solute. This hypothesis is supported by the behavior previously observed for the partial molar volume of the solute in the fluid anti v/sub 2/ infinity as a function of pressure. The rapid change in anti v/sub 2/ infinity is also shown to occur in the same region where the solvatochromic behavior of the fluid changes and the solute's solubility increases dramatically.

The characterization of inorganic submicrometer size particles by sedimentation field flow fractionation (SdFFF) with organic instead of aqueous liquids is described. The difference between particle and liquid densities can be varied with different organic liquids and thus is advantageous in controlling retention times needed to effect the separation as compared with aqueous systems. The retention of inorganic silica particles is compared in both aqueous and nonaqueous systems.