Michael E. Paulaitis

Atomistic simulation of water and salt transport in the reverse osmosis membrane FT-30

Abstract Atomistic computer simulations of water and salt (NaCl) transport in the polyamide discriminating layer of the reverse osmosis membrane FT-30 are reported. We find that water transport occurs by a “jump” diffusion process, similar to the diffusion of simple, dissolved gas molecules in amorphous polymer glasses. As expected, lower water mobilities are observed at higher polymer densities. Cross-linking in the polymer matrix leads to an increase in density, which results in a decrease in water mobility, in accordance with experiment. We also observe a lower mobility for Cl − in the hydrated polymer, relative to Na + , which we attribute in part to the larger number of polar groups on the polymer chain that participate in solvating the anion. That the anion limits salt transport in our model FT-30 structures is consistent with experimental observations. Finally, although we find that the presence of salt reduces water mobility in the polyamide, in accordance with experiments; contrary to previous views, this effect is not related to a change in polymer density. Based on estimates of the salt partition coefficient and the diffusion coefficient of salt in the membrane, we conclude that high salt rejection in FT-30 is due in large part to the large difference in water and salt mobilities within the polyamide discriminating layer.

Reaction paths and free energy profiles for conformational transitions: An internal coordinate approach

A new approach is proposed for the determination of transition states and reaction paths for conformational transitions. The method makes use of adiabatic energy surfaces in the space of ‘‘essential’’ degrees of freedom of the molecule. The reduced dimensionality of this space, compared to the full Cartesian space, offers improved computational efficiency and should allow determination of exact reaction paths in systems much larger than those currently amenable to study in Cartesian space. A procedure to obtain reaction paths and free energy profiles in solution is also proposed. The free energy profile along the path in solution is calculated utilizing a free energy perturbation method with constrains and perturbations in internal coordinate space. Applications to a conformational transition of the alanine dipeptide and the folding transition of a model reverse turn in water are presented. For the reverse turn, the sequential flip of dihedral angles reported by Czerminsky and Elber on a similar peptide [...

Double azeotropy in binary mixtures of NH3 and CHF2CF3

Abstract Vapor-liquid equilibrium pressures have been measured as a function of overall composition for binary mixtures of NH3 and CHF2CF3 at constant temperatures of −19.00, 3.23, 35.00, and 49.90°C. The coexistence curves for vapor-liquid equilibrium at each temperature were regressed from these measurements using a modified form of the Peng-Robinson equation of state to calculate molar volumes and the fugacities of each component in the two phases. The coexistence curve at 35.00 and 49.90°C exhibit both a maximum and a minimum in pressure, establishing the existence of double azeotropes at these two temperatures. The double azeotropes are unusual in that they exist at temperatures for which the vapor pressures of the two components are notably different. Double azeotropy is attributed to the weak association of NH3 in the liquid phase, leading to the maximum-pressure azeotrope at high NH3 concentrations, and to similar vapor pressures for unassociated NH3 and CHF2CF3 in nearly ideal (Raoults law) dilute NH3 solutions, which leads to the minimum-pressure azeotrope.

Molecular thermodynamic model for solvent-induced glass transitions in polymer—supercritical fluid systems

A molecular thermodynamic approach is described for predicting polymer glass transition temperatures as a function of the amount of gas sorbed by the polymer. The predictive model is based on a lattice theory of polymer solutions and the concept of order parameters, the use of which has been important in the development of macroscopic models and phenomenological analyses of glass transitions. A general definition of the solvent-induced glass transition is given, and then applied within the framework of this lattice model and its order parameters to predict glass transition temperatures for several polymer-compressed CO2 mixtures. The model is also used to examine a new experimental observation described as retrograde vitrification.

An experimental study of three- and four-phase equilibria for isopropanol—water—carbon dioxide mixtures at elevated pressures

Abstract Compositions and molar volumes of the three phases in liquid—liquid—gas equilibrium are reported for ternary mixtures of isopropanol, water and CO2 at elevated pressures and at temperatures of 50 and 60°C. Phase compositions and molar volumes were also obtained for three-phase, liquid—liquid—liquid equilibrium and four-phase, liquid—liquid—liquid—gas equilibrium at 40°C. Gas—liquid and liquid—liquid critical endpoints, which represent pressure bounds on the liquid—liquid—gas region at 60°C, were determined from observations of critical opalescence. The phase behavior exhibited by the isopropanol—water—CO2 system is quite complex, particularly at conditions near the critical point of CO2. These conditions are well within the range of operating conditions proposed for supercritical-fluid extraction of organic compounds from water using CO2. Therefore, the existence of multiple coexisting phases can be an important factor in designing and operating such extraction processes.

Experimental determination of enhancement factors from supercritical-fluid chromatography

Abstract An experimental technique is described which involves the application of SCF chromatography to predicting enhancement factors in supercritical fluids at elevated pressures. We consider an important step in the development of this application: the determination of solute fugacities in the stationary phase of the chromatograph column. Experimental results are presented for chromatographic retention times of several aromatic hydrocarbons in a column with supercritical CO 2 as the carrier fluid and a stationary phase of octadecylsilica particles. The measurements are used to determine the mechanism of solute retention in the column, and to develop a thermodyanmic correlation for predicting solute fugacities in the stationary phase.

Phase equilibria for tetralin-water and 1-methylnaphthalene-water mixtures at elevated temperatures and pressures

Abstract Christensen, S.P. and Paulaitis, M.E., 1992. Phase equilibria for tetralin-water and 1-methylnaphthalene-water mixtures at elevated temperatures and pressures. Fluid Phase Equilibria, 71: 63-83. Vapor-liquid and liquid-liquid equilibrium phase compositions and vapor-liquid critical points have been measured for tetralin-water and 1-methylnaphthalene-water mixtures at temperatures from 300-400 ° C and pressures up to the mixture critical pressure. A flow experimental technique was used for the measurements of phase compositions to minimize thermal degradation of the hydrocarbon at elevated temperatures and to facilitate sampling at elevated pressures. The apparatus contains a high-pressure view cell equipped with sapphire windows to allow visual observation of phase separation, the number of coexisting phases, and critical opalescence. Observations of critical endpoints for three-phase, vapor-liquid-liquid equilibrium have also been made for both hydrocarbon-water mixtures using a static equilibrium cell. These critical endpoints are shown to be sufficient for classifying the pressure-temperature projection of each binary mixture.

Coupled diffusion and morphological change in solid polymers

Abstract Sorption of low molecular weight solutes in glassy polymers is known to often result in swelling of the polymer phase. The swollen polymer may on occasion crystallize. One thus has a phenomenon where three different rate processes take place simultaneously and are coupled with each other: diffusion, swelling and crystallization. Correspondingly, there are as many as four time scales: the one for diffusion, the swelling time, the crystallization time, and the relaxation time for the mass flux; the last is shown to be crucial to the possible development of discontinuities and quasi-discontinuities in the state of the polymer. A model has been developed to take into account all these phenomena, and experimental data are successfully compared with the model predictions. Specific numerical techniques have been developed to deal with the resulting quasi-linear hyperbolic equations.

A sapphire cell for neutron scattering at elevated pressures

A pressure cell for cold neutron scattering from liquid solutions at elevated pressures is described. The scattering cell is constructed entirely of sapphire in order to transmit neutrons that scatter at angles as large as 29° while being optically transparent. The cell has been hydrostatically tested to 330 bar. In addition, it is surrounded by an aluminum jacket that permits heating and cooling of the sample to within ±0.05 °C of the setpoint from 25 to 35 °C, as measured by an in situ platinum RTD. This cell has been successfully used for scattering from surfactant/D2O/CO2 mixtures at pressures up to 90.7 bar.

The entropy of hydration of simple hydrophobic solutes

Abstract Infinite-dilution partial molar entropies of solvation of simple, monatomic solutes in water are defined in terms of the entropy associated with (1) solute insertion at constant volume and at a fixed position in the solvent, and (2) expansion or contraction of the pure solvent to maintain constant pressure. A statistical mechanical expansion for the entropy of solution in terms of multiparticle correlation functions is applied to this definition to identify three intrinsic contributions to the hydration entropy - solute-solvent pair correlations, rearrangement of solvent in the vicinity of the solute molecule, and expansion or contraction of the pure solvent - which we evaluate for the inert gases in water at 25°C. For the smaller solutes, we find that the solvent reorganization and solvent expansion contributions offset one another such that the entropy of hydration is determined almost exclusively by solute-water pair correlations. The solute-water pair correlation entropy also prevails as the primary factor determining entropies of hydration for the larger solutes; however, solvent reorganization now makes a small, negative contribution to the entropy.