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.
The effect of the strong Lewis base tri-n-butyl phosphate (TBP) on the solubility of benzoic acid and hydroquinone (HQ) in supercritical fluid carbon dioxide is reported. TBP is shown to be a much stronger cosolvent for these solutes than methanol. For example, 2% TBP increases hydroquinone’s solubility by a factor of 250. The principles of chemical reaction equilibria are combined with the Peng-Robinson equation of state in order to model these results. The behavior of the hydroquinone-carbon dioxide-TBP system is shown to be attributable to the formation of an HQ-TBP2 complex having an enthalpy of formation of -18.9 kcal/mol. The performance of this chemical model is compared to that of a recently developed density-dependent local composition (DDLC) model.