Benjamin Widom of Cornell University is supported by an award from the Theoretical and Computational chemistry program for the theoretical study, by the methods of statistical mechanics and thermodynamics, of the energetics and structures of liquids and liquid mixtures, of the phase transitions such fluids undergo and of the structures and structural transformations of the interfaces between fluid phases. One study focuses on the borderline between two types of wetting transition, where a range of first-order wetting transitions and one of second-order transitions join at a point of "tri-critical" wetting; and to study the behavior of the line tension as that tricritical point is approached. Another study focuses on hydrophobic effects.
The studies have broad impact in many important and very different areas of science, technology and society. For example, wetting is a central concern in soil science; it is crucial in industrial flotation and separation systems; it is also crucial for the proper action for the lungs in respiration. Hydrophobic effects have numerous manifestations throughout chemistry and biology.
for Award 0842022 Theory of Liquids and Liquid Mixtures and their Phase Equilibria and Interfaces This project was on the theory of the properties of liquids and liquid solutions and of how those properties arise from the molecular structure of the fluids and the interactions among their molecules. There were two main areas in this research: the structure and properties of the region of contact between two or three such fluid phases (or between fluids and a solid), and on the phenomena called "hydrophobic" effects in the dilute liquid solutions of low-solubility solutes. This research was theoretical but made close contact with experiment and has potentially important applications. The studies of the interfaces between different fluids or fluids and a solid were concerned mostly with the case in which three different ones are in contact, such as a liquid drop resting on another liquid with which it is immiscible (e.g., a drop of oil on the surface of water), and both in contact also with a common vapor. Those three– vapor, liquid drop, and liquid substrate – would then meet in a circular "contact line", and associated with it would be a "line tension". Alternatively, the middle phase may spread at ("wet") the interface between the other two, so no region of common contact of all three; i.e., no three-phase contact line. By changing temperature or chemical composition, one may even observe the change from one structure to the other: a "wetting transition." The studies of the three-phase contact line and line tension have implications for the wettability of small particles, which can be crucial in industrial flotation and separation systems and relevant also in atmospheric and environmental science. Wetting is a central concern in soil science, where it controls the flow of fluid in porous rock; it is crucial for the proper action of the lung surfactant in respiration; and it is important for the action of the mucin proteins on the surface of the eye’s cornea. Line tension effects have been found to be important in the technology for growing nanometer-scale silicon wires. In biological contexts, line tension is an important factor in the budding of membranes and viruses and in the dynamics of endocytosis. We worked out classifications of the various kinds of wetting transitions and of the factors that drive them, and we learned how changes in chemical composition or temperature affect the line tension. Our work on the properties of liquid solutions consisted primarily of studies of solutions in water of substances that are hardly soluble -- although the slight solubility of oils and other substances of low solubility in water is often of crucial importance. It affects the efficiency of chemical separations by distillation and extraction in the chemical process industries, it is a primary factor in drug design in the pharmaceutical industry, it is a concern in water pollution and other environmental problems, and it underlies the most fundamental biological phenomena. It is the relative incompatibility of the oil-like constituents of a protein and the surrounding water that is responsible for the protein’s folding into its proper biologically active form. Low solubility in water is an example of what are known collectively as hydrophobic effects. This phase of the research was directed toward the understanding of their molecular origins and consequences: What makes a substance hydrophobic ? In what other ways does hydrophobicity manifest itself in addition to low solubility ? How does the structure and other properties of the surrounding water itself respond to the presence of dissolved hydrophobic molecules ? What are the physical and chemical consequences of such molecular-scale rearrangements ? We found that for hydrophobic solutes there was an effective attraction between the solute molecules over and above what would have been the attractive force between them in the absence of solvent, but now mediated by the water in which they were dissolved. It is that water-mediated attraction between the dissolved molecules that is responsible for all the observed hydrophobic effects. We also discovered that not only was the effective attractive force between hydrophobic solutes stronger than it would have been in the absence of the water but it is also of much longer range: the solvent-mediated attraction acts over distances much greater than the sizes of the molecules themselves. We characterized the strength and range of those attractive forces quantitatively, and related them to the changes the dissolved molecules induce in the structure of the water layers surrounding them.
