Stability of hydrocarbon-in-water emulsions during centrifugation. Influence of dispersed phase composition

Abstract

The coalescence of emulsions of pure hydrocarbons in water stabilized by sodium dodecyl sulfate was studied during centrifugation in order to investigate the influence of composition of the dispersed phase on emulsion stability. The hydrocarbons used as the dispersed phase included nC 6 to nC 17 alkanes, benzene, and various n-alkylbenzenes. Emulsions containing equal volumes of hydrocarbon and aqueous phase were prepared using an aqueous solution of 8.0 × 10 −3 moles/liter of sodium dodecyl sulfate (the critical micelle concentration). The initial particle size of these emulsions was determined by microscopic observation, by the sodium alignate creaming method or by the Coulter counter method. A large fraction of the continuous aqueous phase was separated from the emulsion and all the hydrocarbon droplets were deformed into polyhedra very early during centrifugation. The mechanical stability of these emulsions was found to be dependent upon buoyant pressure, dispersed phase composition and properties and interfacial tension. Plots of φ, the fraction of stable emulsion remaining uncoagulated versus centrifugation time ( t) for n-C 17 to nC 8-in-water emulsions are fitted for all values of φ by the empirical equation φ = φ o t − n , where n varies from 0.14 to 0.17. The aromatic hydrocarbon-in-water emulsions showed a different behavior during centrifugation. These emulsions coagulated slowly during the first 20 min or more after which a drastic increase in coagulation rate was observed. The possible causes for the differences between n-alkane and aromatic emulsions behavior during centrifugation are discussed. The emulsion stability is inversely correlated with spreading coefficient of the hydrocarbon on the aqueous surfactant solution. Hydrocarbons with the most negative spreading coefficients produce the most stable emulsions.