Stabilization of colloidal dispersions in supercritical carbon dioxide
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Over the past decade, compressed carbon dioxide (CO2) has emerged as a possible alternative to traditional organic solvents in various industrial processes. Compared to other compressible fluids, CO2 is particularly appealing since it is inexpensive, relatively non-toxic, non-flammable, and possesses a mild critical point. Unfortunately, the solvent strength of CO2 is limited due to its lack of a permanent dipole moment and weak van der Waals interactions. To accommodate CO2’s limited solvation capacity, emulsions and colloidal dispersions are often stabilized with fluorinated surfactants and polymers. However, even for the most CO2-philic fluoropolymers, pressures above 100 bar are typically required to achieve good solvent conditions. In an attempt to increase the industrial applicability of CO2, a majority of this research is focused on the development of novel approaches to stabilize emulsions and colloids in liquid CO2 at low pressures. The use of solid particles in lieu of classical surfactants is demonstrated to allow for the stabilization of emulsions consisting of water and CO2 at low CO2 densities since stability is not dependent on tail solvation. The stability of these emulsions is shown to be highly dependent on the particle hydrophilicity and its subsequent contact angle at the water-CO2 interface. Concentrated dispersions of inorganic silica particles are stabilized at pressures as low as the vapor pressure through the formation of a cross-linked polymeric shell around the solid core. The presence of the polymeric shell allows for dispersibility by weakening the Hamaker interactions between the core-shell particles. The density-dependent interparticle interactions between these dispersed core-shell nanoparticles are quantified in terms of a diffusional second virial coefficient using dynamic light scattering. Finally, the water-CO2 and the solid-CO2 interfaces are investigated. Using high-pressure pendant-drop tensiometry, the water-CO2 interfacial tension is measured for a family of surfactants in order to investigate the relationship between surfactant molecular architecture and interfacial activity. Measurements of the CO2/water/solid contact angle on well defined homogeneous substrates as a function of CO2 pressure provide fundamental insight into the specific interactions between CO2 and the solid interface.