My research program emphasizes the design and application of solvent phases and solvent interfaces based on pressurized CO2 and “CO2-philic” (i.e., fluorinated) molecules. This research applies my expertise in thermodynamics to nontraditional and emerging fields. The activities of my research program are centered around supercritical fluid and pressurized fluid technologies, with an emphasis on a) biotechnology and bioseparations employing compressed solvents; b) self-assembly and colloidal behavior in CO2 and fluorinated systems; and c) materials synthesis and processing employing compressed CO2 and fluorinated molecules.

Biotechnology and Bioprocessing Applications. Supercritical fluids and compressed solvents are of increasing interest as organic solvent replacements for biotechnology and bioprocessing applications. Their solvent properties are tunable with pressure, allowing for control of enzymatic synthesis and extraction. In addition, the resultant process streams are solvent-free following system depressurization, which addresses a major drawback of organic solvents in bioprocessing. Precipitation processes for micron-sized pharmaceuticals and protein powders based on CO2 technologies have been developed in our laboratory and extended to fluorinated solvent-based precipitation techniques.

Our research team has demonstrated cell growth and biosynthesis in pressurized fermentations, both in the presence and absence of a compressed fluid (ethane or propane) headspace. Further, we have demonstrated membrane-based product recovery of fermentation products using pressurized solvents. Thus, pressurized whole cell synthesis may complement existing approaches which utilize nonaqueous solvents to manipulate product selectivity and enhance product recovery.

More significantly, we have shown that pressurized fermentations demonstrate a measurable and varied respon se in their metabolic phenotype. Using C. thermocellum as a model system, we have established that pressure tunability is not unique to piezophiles but is most likely common to microorganisms at moderate pressure,particularly those which produce gaseous products (e.g., CO2 and H2). In collaboration with a multidisciplinary group of scientists, we are working to develop pressure as a perturbation for probing and directing microbial metabolism. Perturbations of metabolic phenotypes will provide a rationale for genetic, adaptive, and/or environmental approaches to maximizing the commercial viability of biochemical fermentations.

Self Assembly and Colloidal Behavior. The discovery of surfactants that self assemble in the presence of CO2 has transformed the range of solvent properties, and therefore the potential applications, of CO2-based technologies. For example, the polar environment afforded by the aqueous core of water-in-CO2 microemulsions has been used as a nanoreactor for the synthesis of a range of metallic nanoparticles. CO2 pressure is the “switch” that creates these thermodynamically stable dispersions of water in CO2. We recently demonstrated CO2-activated reverse microemulsion formation in fluorinated solvent systems. This approach maintains the advantages of processing in a chemically inert media while benefiting from the pressure-tunable properties of CO2-expanded solvents. The overall goal of our research in this area is to design, manipulate and employ the unique solvent properties of these CO2-philic self assemblies.

The effect of CO2 on self-assembly is not limited to fluorinated or CO2-philic amphiphiles. CO2 also interacts with biological molecules in self assemblies (such as bilayers) and at fluid interfaces, as we have demonstrated for liposomes (model whole cell membranes) and enzymes. I nteractions between biomolecules and fluid interfaces can dictate the biocompatibility and recovered biocatalyst activity in a variety of enzymatic and whole-cell bioprocesses including biphasic biosynthesis, bioseparations, and product formulation. For example, our observation that pressurized CO2 reduces the gel-fluid phase transition of liposomes suggests a processing method to control the permeability and morphology of liposomes formed at ambient temperature.

Materials Synthesis and Processing. An established route to the synthesis of nano-scale ordered porous ceramics is the liquid-phase co-assembly of amphiphiles and reactive metal oxide precursors. In this process, precursors “fossilize” the polar components of the mesophase, and the surfactants are removed to form nanostructured materials with controlled pore sizes and shapes. Potential uses of these inorganic and organic-functionalized materials include catalytic, separation, sensing and nanodevice applications. In a collaborative research effort with Dr. Steve Rankin and Dr. Hans Lehmler (see NIRT grant), a new class of structure directing agents, fluorinated surfactants, is being developed for the design of ordered nanoporous metal oxides and organic-inorganic hybrids. Fluorinated surfactants self assemble more readily than their hydrocarbon analogues. The self assembly behavior of fluorinated surfactants results in low critical micelle concentrations and low surface tensions, and the formation of fluorinated mesophases with a broader range of structures including novel ‘intermediate’ phases.

Our research team has developed methods employing fluorinated surfactant templating to prepare particles, films, and monoliths with all of the common types of ordered arrangements (hexagonal close-packed cylinders, bicontinuous cubic, lamellar) observed with traditional hydrocarbon templates, as well as unusual arrangements such as single-walled hollow particles, multilamellar particles, mesh-phase rods, and radially oriented cylindrical pores. Further we have demonstrated the ability to tailor the pore size of fluorinated surfactant templated materials using CO2 processing. Sol-gel processing of templated materials provides the opportunity to capture self-assembled structures into kinetically stable materials. Thus, capturing the effects of CO2 on self-assembly has practical application (i.e., pore expansion or CO2-directed self assembly) and addresses the limited investigations of the effect of CO2 on surfactant mesophases.