A lattice fluid self-consistent field theory is used to calculate both the composition and fundamental thermodynamic properties, i.e., the interfacial tension (gamma) and bending moment (c), of spherical interfaces between oil and water. The variation in density throughout the interface is treated by the inclusion of holes in the lattice. This molecular theory is inserted into new classical thermodynamic expressions for gamma and c which take into account the fact that the surfactant tails are anchored to an interface. The detailed description of the composition throughout the interface provides a means to understand the effect of density and radius on the bending moment. The natural curvature and interdroplet interactions are calculated for water-in-propane microemulsions formed with the surfactant Aerosol-OT and compared with experiment.
A pronounced solvent effect on the hydrogen bonding of methanol and triethylamine is observed throughout the gas, supercritical, and liquid states in the relatively inert solvent SF6, based on FTIR spectroscopy. The free energy of hydrogen bonding is stabilized by a decrease in density; i.e., the donor and acceptor are destabilized more than the complex as the solvation is reduced. Also, the hydrogen bond energy becomes stronger. A hydrogen-bonding lattice-fluid (LFHB) model is extended to treat this density dependence, and the calculations are in reasonable agreement with experiment. Near the mixture critical point, the number of hydrogen bonding encounters between the donor and acceptor is enhanced due to solute-solute clustering as expected on the basis of previous experimental and computer simulation studies.
Spectroscopy is used for monitoring a number of processes relevant to solution, extraction and impregnation in supercritical CO2 (scCO2). Examples include: a combined infrared (IR) and ultraviolet study of the interaction between para-hydroquinone (HQ) and tributyl phosphate in scCO2, which reveals hydrogen bonding, detected by the characteristic nu(O-H) IR bands; IR measurement of the solubility of CpMn(CO)3 (CP = eta5-C5H5) in SCCO2 as a function of temperature and pressure; an investigation of the uniformity of supercritical impregnation of CpMn(CO)3 into 4 mm diameter pellets of polyethylene (PE) using Fourier-transform infrared (FTIR) microscopy and FTIR depth profiling by photoacoustic detection; and an IR study of the photochemical reaction of CpMn(CO)3 with N2 with PE film.
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.