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Relationship between interfacial properties and formation of microemulsions and emulsions of water and supercritical carbon dioxide
textThe utilization of supercritical (SC) CO2 as an alternative green solvent
has attracted significant research devotion in the last decade. Its uniqueness lies
on the fact that CO2 is a non-FDA regulated solvent mainly generated as the sideproduct
of industrial process, is easily recyclable, readily available, non
flammable and essentially non toxic. Dense CO2 is non-polar (unlike water), has
weak van der Waals forces (unlike oils) and as such may be considered a third
type of fluid phase in nature, somewhat similar to fluorocarbons. The use of SC
CO2 has expanded into broad technological areas one of which is the stabilization
of water-in-CO2 dispersions that offer new possibilities for separations on the
basis of polarity, and as media for reactions between polar and nonpolar
molecules. The formation of stable emulsions of water-in-CO2 (W/C) so far has
been hampered by the lack of suitable surfactants. The synthesis of various
molecularly engineered surfactants is demonstrated in this study, among which
are polydimethylsiloxane (PDMS)-based block copolymer ionomers, ionic and
nonionic perfluoropolyether (PFPE) and nonionic perfluorooctylmethacrylate
(PFOMA)-based ones. The concentrated W/C emulsions are characterized with
electrical conductivity, optical microscopy and multiwavelength turbidity
technique. The emulsion stability is assessed as a function of formulation
variables that influence the surfactant monolayer curvature, such as temperature,
pH, salinity and pressure. The response of the interfacial activity of the surfactant
to changes in the variables above is monitored with interfacial tension (γ)
measurements and is correlated to emulsion stability. Moreover, salinity is used to
tune the surfactant aggregation characteristics, resulting in spontaneous
microemulsion formation upon crossing the critical microemulsion concentration
(cµc). Based on guidelines provided by γ versus temperature, stable concentrated
(50:50 by mass) C/W miniemulsions consisting of 200 nm droplets are formed
with the phase inversion temperature (PIT) method. Finally, the formation of
unflocculated and stable dilute W/C emulsions is studied with a homologous
series of PFOMA-based nonionic surfactants, and mapping of γ with surfactant
hydrophilicity provide useful pathways for the synthesis of the optimum structure.Chemical Engineerin