Ph.D., Princeton University, 1992
Colloidal and surface forces determine the behavior of systems with high surface/volume ratios such as suspensions of small particles and geologic porous media (e.g., rocks and sand). Recent work indicates that heterogeneity and anisotropy affect the overall behavior of the system. For example, the thermodynamic behavior of systems containing heterogeneously charged proteins differs markedly from that of systems composed of uniformly charged particles. Colloidal interactions also affect the fate of environmental contaminants because the adsorption strength and specificity of the contaminants depend on the interactions between contaminant species and the surfaces of sediment particles. Because the nature of the sediment varies with location, the transport of contaminants over large distances is actually dictated by surface forces. These two very different applications of colloidal phenomena are described below.
(1) Transport and fate of environmental contaminants.
As long as environmental remediation is required, reliable estimates of the time, equipment, and cost of site clean-up will be needed. Unfortunately, there is a great deal of uncertainty in every step of the remediation process, especially in estimating the effective field-scale transport and adsorption properties. The proper "scale-up" from laboratory sample to contaminated aquifer is the main focus of this research program.
The fate of contaminants depends on the balance between transport phenomena such as convection and diffusion and surface phenomena that affect adsorption. If adsorption dominates, contaminants become immobilized and should not present an immediate hazard. If transport dominates, contaminants are carried along by the prevailing flow of water in an underground aquifer and can be spread over large distances. The strength of adsorption depends on the nature of the sediment, which varies with location due to the geologic processes that formed the soil. The character and spatial distribution of adsorption sites can combine with transport processes in a nonadditive way to concentrate contaminant in locations not predicted by simple models. We plan to exploit the interaction between transport and adsorption to improve remediation efficiency.
Calorimetry is the primary means for characterizing the strength of adsorption while microscopy is used to determine the locations of adsorption sites. From the measured heat of adsorption, the thermodynamics and kinetics of adsorption can be calculated. If the sediment surface is scanned when contaminants are present, adsorption and desorption sites can be identified. An important goal of this work is to correlate the adsorption behavior of the sediment with its geologic structure. Such a connection between structure and behavior is crucial if conclusions are to be transferred to other systems.
This long-term investigation involves several research projects that focus on phenomena at different length scales. For example, adsorption studies on a centimeter-size sample provide data on the short-range geologic structure of sediment and the distribution of adsorption sites. These results are incorporated into computer models of larger blocks of sediment. Predictions from the model are then compared with experimental results. Successive application of this procedure yields a hierarchical model of the effects of geologic structure on contaminant transport.
(2) Colloidal interactions in protein systems.
Crystallization is an important step in the purification of protein mixtures. Protein crystals are also grown for X-ray diffraction studies that probe the three-dimensional structure of the protein. The rate-limiting step in determining the molecular structure is the growth of sufficiently large, ell-ordered crystals. When a new protein is isolated, its crystallization conditions are usually unknown. An exhaustive and tedious search of conditions (ionic strength, pH, solvent mixtures) is performed until suitable conditions are found; this process may take months or years. Any procedure that streamlines the production of suitable crystals would be welcomed by the pharmaceutical industry.
Systems of biological molecules are influenced through both nonspecific interactions (e.g., electrostatic and van der Waals forces) and specific interactions (hydrogen bonds). Although electrostatic interactions are nonspecific, the particular arrangement of charged groups on the molecule is responsible for the function of biomolecules. These electrostatic interactions differ markedly from those of uniformly charged particles. The irregular shape of biomolecules introduces another essential heterogeneity that contributes to biological function and determines the behavior of the system. Computer models that explicitly account for geometric and electrostatic heterogeneity aid the interpretation and design of experiments involving proteins.
Static light scattering experiments suggest that crystallization occurs in a specific range of values for the second virial coefficient: the second virial coefficient should be slightly negative (attraction between the molecules) but not too negative (or else amorphous precipitate forms). Since the second virial coefficient depends only on interactions between pairs of molecules, the behavior of crystallizing systems can be estimated from studies of dilute protein solutions.
Current work focuses on understanding the interactions between protein molecules in solution. Computer models that account for electrostatic and van der Waals effects are used to calculate the interaction potential for two protein molecules. This interaction potential is then used to compute virial coefficients and other thermodynamic properties of the system. The current calculations can identify pH and ionic strength conditions that are likely to provide crystals. Targeted experiments can then be performed to verify the model and obtain crystals.