(153a) Area 1a Keynote Address: Thermodynamics for the Design of Smart Surfactants and Ligands in Pharmaceutical, Environmental, and Energy Applications

Smart surfactants and ligands are being designed to (1) perform multiple functions, (2) achieve targeted activity at particular interfaces, and (3) be active at unusual interfaces, for example, in CO2. In pharmaceutical science, one of the key challenges is particle engineering of poorly water soluble drugs to achieve high bioavailability for oral and pulmonary administration. Increasingly, two goals are being pursued simultaneously: (1) control of particle nucleation and growth to achieve the desired particle morphology and (2) rapid wetting and dissolution, and in some cases high levels of supersaturation, both in vitro and in vivo. Studies of fundamental thermodynamic and transport mechanisms in each of these goals are leading to improvements in bioavailability in vivo.

Environmentally benign carbon dioxide-based solvent formulations may replace toxic organic solvents for pharmaceutical, chemical, materials, and microelectronics processing. A variety of these applications use microemulsions, macroemulsions, and inorganic nanocrystal dispersions. Whereas colloids in supercritical fluids typically have been stabilized by expensive fluorinated surfactants, substantial progress is being made using hydrocarbon surfactants with stubby tails and a small fractional free volume. The tails are designed to minimize tail?tail interactions (given the weak solvent strength of CO2) and to block contact between the two phases at the interface. These surfactants stabilize CO2-in-water emulsions or foams needed to control mobility in CO2-enhanced oil recovery, in order to recover up to 60 billion barrels of oil (approximately $6 trillion value). Nonionic methylated branched hydrocarbon surfactants emulsify up to 90% CO2 in water with polyhedral cells smaller than 10 microns, with the potential for excellent mobility control.

An emerging understanding of the role of surfactants in charging and stabilization mechanisms for colloids in low-permittivity solvents (dielect. const. < 5) at atmospheric pressure will help advance a variety of applications including electrophoretic displays and electrophoretic deposition of nanocrystals to form superlattices. On the basis of novel experimental measurements for both hydrophilic and hydrophobic TiO2, a general mechanism is presented to describe particle charging in terms of preferential partitioning of cations and surfactant anions between the particle surface and reverse micelles in the bulk solvent. In addition, electrostatic repulsion has recently been found to stabilize colloids in CO2, including water droplets and TiO2 particles. The design of smart surfactants and ligands for nano- and micron-sized emulsions and particle dispersions is in its infancy, and many new concepts will be developed for pharmaceutical, environmental, and energy applications.