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.
Absorption and emission spectral maxima, bandwidths, and fractional contribution of twisted intramolecular charge transfer states to the observed emission of p-(NN-dimethylamino)benzonitrile (DMABN) and ethyl p-(NN-dimethylamino)benzoate (DMAEB) in supercritical CHF3, CO2, and C2H6 are presented. By examining a wide range of reduced densities from 0.05 to 2.2, we have discovered a characteristic density dependence in the spectral shifts in all three fluids. A model for these spectral effects is proposed, differentiating intermolecular interactions in three distinct regions: gas-phase solute-solvent clustering, clustering in the near-critical region, and ’’liquidlike’’ solvation. Even below a reduced density of 0.5, clustering of solvent about solute is already prevalent.
Solute-solvent interactions of (dimethylamino)benzonitrile and ethyl (dimethylamino)benzoate in mixtures of supercritical trifluoromethane and carbon dioxide are studied using fluorescence spectroscopy. The density dependence of solvation in the mixtures is similar to that in the pure supercritical fluids. The polar component CHF3 in the mixtures clusters preferentially about a solute molecule. This clustering is also density dependent. Bulk and local (microscopic) solvent effects in the in different density regions are rationalized based on the Onsager reaction field model and on the concepts of local density and composition.
The degree of hydrogen bonding and macroscopic thermodynamic properties for pure and mixed fluids are predicted with the hydrogen bonding lattice fluid (LFHB) equation of state over a wide range in density encompassing the gas, liquid and supercritical states. The model is successful for molecules forming complex self-associated networks, in this case pure methanol, ethanol, and water, and the mixture 1-hexanol-SF6. In supercritical water, significant hydrogen bonding is still present despite all the thermal energy and is highly pressure- and temperature-dependent. A fundamental description of pressure and temperature effects on hydrogen bonding is presented for a well-defined case, the formation of a complex between a donor and acceptor in an inert solvent, where no self-association is present. The partial molar enthalpy and volume change on complexation both become pronounced near the critical point, where the density is highly variable with temperature and pressure.