Comp Chem Research Developments

The prediction of phase equilibria of multicomponent mixtures is one of the grand challenges for molecular simulation requiring both accurate force fields and efficient sampling algorithms.

Gas expanded liquids. Expansion of an organic solvent by an inert gas can be used to tune the solvent's liquid density, solubility strength, and transport properties. In particular, gas expansion can be used to induce miscibility at low temperatures for solvent combinations that are biphasic at standard pressure, providing a new route to enhance reaction rates for biphasic catalytic systems. Graduate student Ling Zhang and Professor Ilja Siepmann have used particle-based simulations to investigate the vapor-liquid-liquid equilibria and the microscopic structures for the ternary mixture of n-decane, n-perfluorohexane, and carbon dioxide. From the predicted phase diagram shown above, one can see that the two liquid phases are almost immiscible at atmospheric pressure (horizontal tie line). At elevated pressures, carbon dioxide swells the two liquid phases (the horizontal tie lines move upward to higher carbon dioxide concentrations), and these expanded phases become more miscible. Above the upper critical solution pressure (about 3.3 MPa in good agreement with experiment), there is a single, albeit on a microscopic-level heterogeneous liquid phase.

Polymeric surfactants in supercritical carbon dioxide. Supercritical carbon dioxide has tremenduous potential as a versatile, environmentally benign process solvent. The biggest factor hindering its wide use is its low solvent power, requiring the addition of entrainers or surfactants to enhance the solubility of polar solutes. While partially fluorinated surfactants possess desirable solubility characteristics, their cost is prohibitive and their environmental impact is not fully understood. Therefore, the development of cheaper and more benign hydrocarbon-based polymeric surfactants is highly desirable and necessary for carbon dioxide to become an economically viable solvent for a variety of processes. The search for novel polymeric surfactants is hindered by synthetic challenges and incomplete understanding of the molecular interactions and thermodynamic parameters that control desirable solubility characteristics for polymeric surfactants. Following upon pioneering experimental work by Eric Beckman and co-workers on carbonate ether copolymers, postdoctoral fellow Collin Wick in collaboration with Professors Doros Theodorou and Ilja Siepmann have demonstrated that molecular simulations can be employed to accurately predict the phase equilibria of a CO2-philic hydrocarbon surfactant with CO2 and to show that the accessibility of the carbonyl oxygens plays a major role for the higher solubility of a carbonate poly(ethylene oxide) (CARB-PEO) copolymer compared to a poly(ethylene oxide) (PEO) of similar molecular weight. The findings of this work suggest that the accessible surface area of polar groups (oxygen or fluorine) should be taken into account as a design element for the development of CO2-philic surfactants.

The development of advanced computational strategies for the most challenging problems in chemistry and chemical physics is a theme common to the research endeavors of the Minnesota Computational Chemistry Group, where research includes new theoretical formulations, the development of new computational algorithms, and use of state-of-the-art supercomputers to solve prototype problems to high accuracy and to predict chemically useful results for a wide range of system scales ranging from a few atoms to thousands of atoms.

Financial support from the National Science Foundation, Divisions of Chemical and Transport Systems and of Analytical and Surface Chemistry, is gratefully acknowledged. Part of the computer resources were provided by the Minnesota Supercomputing Institute.