A variety of types of thermodynamic properties have been determined at infinite dilution by supercritical fluid chromatography. A key challenge is to identify clearly the retention mechanism. An experimental technique is presented for the measurement of retention due to absorption into a bulk C-18 liquid (stationary) phase, independently of the adsorption on the support. The important effect of the swelling of the liquid phase by the fluid phase is included. Distribution coefficients are presented for naphthalene and phenanthrene between CO2 and the C-18 liquid phase, and used to determine Henry’s constants in the liquid phase and solute partial molar volumes and enthalpies in the fluid phase. In the highly compressible region of CO2 at 35-degrees-C, solute partial molar enthalpies have been found to reach negative values of hundreds of kJ/mol, indicating strongly exothermic solute-solvent clustering.
The Norrish Type I photofragmentation of two dibenzylic ketones (1,3-diphenyl-2-propanone and 1-(4-methylphenyl)-3-phenyl-2-propanone) in supercritical ethane and carbon dioxide proceeds without evidence for cage recombination of the photogenerated radical pair. A statistical mixture of the bibenzyls formed by random coupling of benzyl and p-xylyl radicals was observed and the first order rate constant for the depletion of the reactant was independent of pressure. These results indicate that solvent cage effects are not operative in these low viscosity supercritical fluids even in the near critical region where solute-solvent clustering is presumably maximal.
The azo-hydrazone tautomeric equilibrium of 4-(phenylazo)-1-naphthol is compared in various liquid and supercritical fluid solvents. The less polar azo tautomer is dominant in the dilute gas phase, compressed ethane, and liquid alkanes. In liquid and supercritical CO2, the equilibrium shifts toward the more polar hydrazone, to yield similar amounts of the two tautomers. This shift is attributed to the Lewis acidity and large quadrupole moment of CO2. The dominance of the hydrazone tautomer in fluoroform (> 90%) can be attributed to that solvent’s large dipole moment and ability to act as a strong electron acceptor (hydrogen bond donor). Since acid-base interactions are prevalent at the lowest pressure studied (1000 psia), changes in the equilibrium constant as a function of pressure have been assigned primarily to increases in the nonspecific polar interactions. The large differences in the polarities, acidities, and basicities of these fluids, despite their similar polarizabilities per volume, are of interest for manipulating chemical processes and for practical applications of supercritical fluid science and technology.
Pressure effects on both the curvature and phase behavior of water-in-oil microemulsions (swollen reverse micelles) are predicted with a unified classical and molecular thermodynamic theory developed by Peck et al. (this issue). The theory is used to identify quantitatively the roles of the intramicellar interfacial interactions and micelle-micelle interactions. A supplementary molecular model is used to calculate the strength of attractive intermicellar interactions over a wide range of conditions, based on previous small-angle neutron-scattering data. An important distinction is made between systems with a small water-to-oil ratio and those where the water-to-oil ratio is much larger, on the order of unity. In the latter the micelle radius is controlled primarily by intramicellar interfacial interactions, specifically the enthalpic propane-surfactant tail interactions. For a small water-to-oil ratio, the micelle radius is limited by attractive micelle-micelle interactions. As pressure increases, the radius increases but eventually reaches a maximum governed by the intramicellar interfacial interactions. There is good agreement between the predictions and experiments over a wide range of water-to-oil ratios.
A unified classical and molecular thermodynamic model is developed in order to predict the phase behavior and interfacial properties of spherical water-in-oil microemulsions. A modified Flory-Krigbaum theory is used to describe the interactions between the surfactant tails and solvent, while the ionic head-group interactions are treated with the Poisson-Boltzman equation. The interfacial tension and the bending moment of the interface are calculated explicitly. These values are incorporated into a classical thermodynamic framework that is forced to satisfy the Gibbs adsorption equation on the interface, guaranteeing thermodynamic consistency. Given a surfactant molecular architecture, the model predicts the size of microemulsion droplets as a function of the chain length of the alkane solvent. For bis(2-ethylhexyl) sodium sulfosuccinate (AOT) in the solvents propane through decane, the calculated trends agree with experiment and are explained mechanistically at the molecular level. The microemulsion radius increases for the solvents pentane through propane, an unusual behavior that is explained theoretically.