Partitioning behavior of silica in the PEG/dextran/H2O aqueous biphasic system

The partitioning behavior of silica particles was investigated in the polyethylene glycol (PEG)/dextran/water system. It was found that both surfactant/solid electrostatic interactions and polymer/solid hydrogen bonding interactions played important roles in determining the solids distribution. Silica partition was highly pH-dependent; the solids transferred from the top PEG-rich phase to the bottom dextran-rich phase with increase in pH, but the particles moved back to the top phase above pH 12. This behavior was interpreted in terms of the pH dependence of PEG/SiO2 hydrogen bonding interaction and the weak acidity of dextran. The addition of sodium dodecylsulfate resulted in the preferential distribution of the particles to the bottom phase above pH 2. In contrast, in the presence of dodecyltrimethylammonium bromide, the solids stayed in the top phase below pH 6; with an increase in pH, the particles transferred from the top phase to the interface, then to the bottom phase. These surfactant effects are attributable to the electrostatic interaction between the surfactants and the solid surface.