Because the phase behavior of the mobile phase must be known before conclusions fr-om supercritical-fluid chromatography (SFC) can be considered reliable, the phase behaviors of tri-n-butylphosphate/CO2 and acetone/CO2 were thoroughly determined in a variable-volume view cell at conditions applicable to SFC (0-20 mol % modifier, 25-140 degrees C, and 80-415 atm). The chromatographic utilities of the binary fluids were determined with test compounds (condensed tannins and steroids). Although the UV-absorbance detector base-line rise was severe with acetone/CO2, chromatographic performance was not compromised. Standard base-line correction methods were used to produce conventional-looking chromatograms. The chromatographic performance with tri-n-butylphosphate/CO2 was unsatisfactory (erratic retention). Static restrictors (integral, frit, crimped Pt/Ir, linear, and valves) produced erratic flow. Heating the restrictors to 250-400 degrees C did not improve performance. Reasons for the compromise in chromatographic performance are proposed.
A model is presented to predict the depression of the glass transition temperature of a polymer in the presence of a liquid, gas, or supercritical fluid as a function of pressure. It is developed using lattice fluid theory and the Gibbs-Di Marzio criterion, which states that the entropy is zero at the glass transition. Four fundamental types of T(g) versus pressure behavior are identified and interpreted as a function of three factors: the solubility of the compressed fluid in the polymer, the flexibility of the polymer molecule, and the critical temperature of the pure fluid. A new phenomenon is predicted where a liquid to glass transition occurs with increasing temperature, which we define as retrograde vitrification. This retrograde behavior is a consequence of the complex effects of temperature and pressure on sorption. For the limited data which are available for the polystyrene-CO2 and poly(methyl methacrylate)-CO2 SYStems, the predictions of the model are in good agreement with experiment.
In a highly compressible supercritical fluid(SCF), local densities of the solvent and solute about a solute molecule are augmented over bulk values. The influence of this solvent-solute clustering on reactions is examined based on the photolysis of 1,3-diphenylacetone and a new interpretation of the photolysis of iodine. Together these studies indicate that solvent-solute clustering causes the solvent cage effect to be larger than expected based on bulk properties, but smaller than in liquid solvents. New experimental results indicate that the rate of cyclohexenone photodimerization and the regioselectivity to the more polar head-to-head versus the less polar head-to-tail dimer increase sharply as pressure is decreased to the critical point. This unusual result is attributed to solute-solute clustering, which increases the local polarity and the number of encounters between reacting species. Solute-solute clustering is shown to occur in a single phase region, where volume fluctuations are large, just prior to the onset of nucleation and growth of a condensed phase.
A thermodynamic equilibrium between the locally excited state and the twisted intramolecular charge-transfer (TICT) state in p-(dimethylamino)benzonitrile and ethyl p-(dimethylamino)benzoate is used to probe unusual solute-solvent interactions in supercritical trifluoromethane and carbon dioxide mixtures. Well-defined locally excited state fluorescence spectra of the two molecules are obtained through application of principal component analysis. Quantitative resolution of dual fluorescence spectra of the locally excited state and the TICT state is accomplished by using a combination of nonlinear least-squares fitting and principal component analysis-self-modeling, in which a new self-modeling constraint is introduced.