The effect of counterions on coacervation and solubilization in oil-external and middle-phase microemulsions

Abstract

The effect of hydrated radii, valency, and concentration of counterions on the coacervation of aqueous petroleum sulfonate solutions and on the solubilization capacity of oil-external and middle-phase microemulsions was investigated. The critical electrolyte concentration (CEC) for coacervation increased with Stokes' hydrated radii of monovalent counterions. The CEC for CaCl 2 was much lower than that predicted by either the Stokes' hydrated radii or the ionic strength. For mixed electrolytes containing NaCl and CaCl 2, it was concluded from the shift in CEC that 1 mole of CaCl 2 is equivalent to 16 to 19 moles of NaCl. The changes in relative concentrations of NaCl and CaCl 2 for coacervation exhibited additive behavior. The maximum solubilization of brine in oil-external microemulsions occurred at a specific salt concentration. For mixed electrolytes containing NaCl and CaCl 2, the shift in electrolyte concentration for maximum solubilization showed that 1 mole of CaCl 2 is equivalent to about 4 moles of NaCl. These results suggest that the equivalence ratio of CaCl 2 to NaCl is strikingly different in aqueous solutions and oil-external microemulsions. For solubilization in middle-phase microemulsion containing mixed NaCl and CaCl 2, it was concluded from the shift in optimal salinity that 1 mole of CaCl 2 is equivalent to about 16 moles of NaCl. Here also the changes in NaCI and CaC1 2 concentrations showed additive behavior. The equivalence ratio of CaCl 2 and NaCl appears to be independent of oil chain length in the present study. As shown by the equivalence ratio of CaCl 2 to NaCl, the formation of middlephase microemulsions appears to be similar to coacervation of aqueous surfactant solutions and quite different from the solubilization of water in oil-external microemulsions.