Stabilization of colloidal dispersions in supercritical carbon dioxide
Abstract
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.
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