I am interested in the structure, thermophysical properties and technical applications of fluids and amorphous solids. The overall goal of my research program is to provide fundamental understanding on the relationship between molecular-level structure and bulk properties in this broad class of systems. Current research projects address problems in the areas of water and aqueous solutions, metastable liquids, amorphous solids (glasses), biopreservation, and supercritical fluids. The practical applications of our research include the preservation and formulation of commercial pharmaceutical products; the protection of blood plasma proteins; the prevention of hazardous vapor explosions in the paper, natural gas, nuclear, and metals processing industries; the inhibition of gas hydrate formation in natural gas pipelines; the vapor-phase synthesis of materials; the production of biologically active protein powders for therapeutic use; and the development of improved routes to pharmaceutical aerosols and powders.

Water and aqueous solutions. The peculiar properties of water include expansion upon cooling, contraction upon melting, increased fluidity upon compression, anomalously large heat capacity and dielectric constant, and unusually high melting and boiling temperatures. In spite of water’s central role as a matrix for life, as a key determinant of global climate, and as a participant in countless industrial processes, our understanding of its physical properties and that of aqueous solutions is very incomplete. An important activity in my group is the development of statistical-mechanical models of aqueous systems. We apply these theories to problems such as hydrophobic hydration, the phase behavior of supercooled water, the stabilization of biologically significant structures of proteins in solution, and the effect of confinement on the properties of water.

Supercooled liquids and glasses. Glasses are disordered materials that lack the periodicity of crystals but behave mechanically like solids. The glassy state is crucial in the processing of foods and the commercial stabilization of labile biochemicals. The most common way of making a glass is by cooling a viscous liquid fast enough to avoid crystallization. Although this route to the vitreous state – supercooling – has been known for millenia, the molecular processes by which liquids acquire amorphous rigidity upon cooling are not fully understood. We use a combination of molecular simulation and statistical mechanical theory to study fundamental questions on supercooled liquids and the glass transition. Examples include the relationship between a substance’s “energy landscape” (potential energy as a function of particle coordinates) and its viscosity, and the development of quantitative measures of disorder in computer-generated glasses.

Supercritical fluids. All fluids are infinitely compressible at the critical point. Fluids in the vicinity of the critical point can therefore be orders of magnitude denser than gases at ambient conditions, and at the same much more compressible. These and other properties of supercritical fluids can be exploited for a variety of purposes, including the production of novel materials, the destruction of hazardous wastes, and the replacement of toxic solvents for industrial cleaning operations. We are interested in modeling the complex interaction of fluid mechanics, mass transfer, nucleation, and phase equilibrium thermodynamics that underlies supercritical routes to particle formation.

Biopreservation. The commercialization of many valuable biochemicals, such as therapeutic proteins or vaccines, requires the design of formulations that are stable during shipping and long-term storage. Concentrated solutions of carbohydrates in water are widely used in the pharmaceutical industry, generally in vitreous form, for the storage, protection, and formulation of labile biochemicals. Well-known examples include lactose- and sucrose-based glasses. Generally, these matrices are prepared by water removal from an initially dilute solution. The slow crystallization kinetics of technically important carbohydrates allows the solution to become deeply supersaturated, and when its viscosity reaches ca. 10¹³ Poise the material becomes a glass. The molecular mechanisms by means of which vitreous matrices confer stability to proteins and other biochemicals are poorly understood. In our group we use a variety of techniques, including calorimetry, neutron scattering, and molecular simulation, to study the behavior of concentrated sugar-water solutions and the behavior of proteins and nucleic acids in water-soluble glasses. Our goal is to provide fundamental understanding of the physical chemistry of stabilization of biomolecules in vitreous matrices.