(NMR) analysis, using the tris [3-(trifluoromethylhydroxymethylene)-(ϩ)camphorato]europium(III) complex. The compound was hydrolyzed to the corresponding acid and then the optical purity was enhanced up to 97% ee by cocrystallization with commercially available R-PEA. The optical purity was checked by NMR and circular dichroism spectra:[␣] D 25 ϭ Ϫ63.2 (C ϭ 1.01 g/liter tetrahydrofuran). 5. The isotherms were measured from A ϭ 100 to 5 Å 2

Molecular dynamics simulations of supercritical electrolyte solutions with three different ion–water models are performed to study the anion–cation potential of mean force of an infinitely dilute aqueous NaCl solution in the vicinity of the solvent’s critical point. The association constant for the ion pair Na+/Cl− and the constant of equilibrium between the solvent-separated and the contact ion pairs are determined for three models at the solvent critical density and 5% above its critical temperature. The realism of the aqueous electrolyte models is assessed by comparing the association constants obtained by simulation with those based on high temperature conductance measurements. Some remarks are given concerning the calculation of the mean-force potential from simulation and the impact of the assumptions involved.

Molecular dynamics simulation of a dichain surfactant + water + carbon dioxide (solvent) system is performed to study the structural properties of reversed micelle-like surfactant aggregates formed in the system. The simulations use a detailed and realistic molecular model for the surfactant molecule and explicit representation of the water and solvent molecules to enable quantitative comparisons with a prior experimental (small-angle neutron scattering) study. The results of the simulation are found to be in reasonable agreement with experimental values. The simulations show that the size and shape of the surfactant aggregates depends on their water-to-surfactant ratio. A higher water-to-surfactant ratio results in larger and more spherical aggregates. The two distinct tails of the surfactant molecule exhibit different conformations in carbon dioxide indicating contrasting CO2-philic behavior. The perfluoroalkane tails assume more extended conformation than the alkane tails. The microstructure of the aqueous core reveals that the water molecules in the interfacial region are strongly oriented in response to the electric fields of the anionic headgroups and sodium counterions, while water near the center of the core approaches bulklike properties with the presence of a hydrogen-bonded network.

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Droplet interactions in water-in-carbon dioxide (W/C) microemulsions formed with a perfluoropolyether-based surfactant for droplet volume fractions from 0.05 to 0.10 are studied with small-angle neutron scattering (SANS) to understand the mechanism of microemulsion stability. The water-to-surfactant ratio (W o) is fixed at 12.5. Droplet interactions increase as the upper critical solution pressure is approached with decreasing pressure at constant temperature, increasing temperature at constant pressure, or increasing droplet volume fraction. These interactions are quantified in terms of the structure factor at zero momentum vector, S(0), and the correlation length, ξ, for an Ornstein−Zernicke structure factor, or the square-well depth for the structure factor of a square-well potential. Near the critical solution pressure, the interaction strength (A) approaches the value predicted for a hard-sphere fluid with a van der Waals attractive term. The observed interaction strength between droplets is larger in W/C relative to water-in-oil microemulsions due to stronger tail−tail interactions resulting from the weak solvation by CO2. Overcoming these larger droplet interactions is the key challenge in forming stable W/C microemulsions. Pressure and volume fraction have a negligible effect on the droplet size; however, drop size decreases with temperature.

Articles you may be interested inStatic and dynamic structural analysis of a saturated solution of ZnBr2 in water: Anomalous xray diffraction and molecular dynamics simulations Molecular dynamics simulation of water beween two ideal classical metal walls Molecular dynamics (MD) calculations have been performed to determine equilibrium structure and properties of systems modeling supercritical (SC) water and SC aqueous solutions at two states near the critical point using the simple point charge (SPC) potential model of Berendsen et al. for water. Both thermodynamic and dielectric properties from the simulations for pure water are accurate in comparison with experimental results even though the SPC model parameters were fitted to properties of ambient water. Details of the nearcritical clustering in SC water have been predicted which have not been measured to date. MD studies have also been undertaken of systems that model sodium and chloride ions and neutral argon in SC water at the same states. The first solvation shell in SC water is observed to be similar to that in ambient water, and long-range solvation structures in SC water are similar to those observed for simple SC solvents. An excess of water molecules is observed clustering around ionic solutes which behave attractively and a deficit is observed around neutral atomic solutes which behave repulsively. These results should be helpful in developing a qualitative understanding of important processes that occur in SC water. 5606

One of the primary reasons that supercritical carbon dioxide has thus far failed to achieve its full potential as an environmentally benign alternative to conventional industrial solvents is that few surfactants are known at present for use in CO2. As an initial step toward developing the molecular-level understanding needed to design new surfactants, we report the first molecular simulations of the self-assembly of dichain surfactants in supercritical carbon dioxide into stable, spherical aggregates. These aggregates exhibit the expected characteristics of reverse micelles with aqueous cores, consistent with earlier experimental findings, demonstrating the potential of molecular simulation for modeling such complex systems.

The formation of water-in-carbon dioxide microemulsions with a cationic perfluoropolyether trimethylammonium acetate surfactant, PFPE-C(O)-NH-CH 2 -N + (CH 3 ) 3 CH 3 COO -, is reported over a range of temperatures (25-90°C) and pressures (87.3-415 bar). Spherical droplets are observed by SANS with radii ranging from 16 to 36 Å for water-to-surfactant molar ratios (W o ) from 9.5 to 28. Porod analysis of the SANS data indicates an area of approximately 60 Å 2 /surfactant molecule at the water-CO 2 interface, in reasonable agreement with the value of 72 Å 2 determined from the change in the droplet radius with W o . The CO 2 -phobic functionality between the surfactant headgroup and perfluoropolyether tail reduces CO 2 penetration of the tails, resulting in a smaller area/surfactant than in the case of an anionic perfluoropolyether surfactant [Langmuir 1997, 13, 3934]. A relatively rigid film, with a mean film rigidity (2K + K h ) of approximately 1 k B T, along with the strong partitioning of the surfactant toward CO 2 versus water, lead to the small, rigid, spherical water droplets in CO 2 .