MEASUREMENT OF PH AND PK IN O-W MICRO-EMULSIONS

Glass electrode pH measurements have been performed in oil in water microemulsions stabilized by butanol or pentanol and cationic, anionic, and nonionic surfactant. In conjunction with the pH measurements, the pKa of chlorophenol red has been determined in these microemulsions using both cationic and anionic buffer systems. The results indicate that the interphase region of the microdroplet in which the indicator is located has an effective dielectric constant of about 20, and that the intrinsic pKa of a dye is the same in all microemulsions of this type provided both indicator species are in the same average environment. It is further suggested that the effective aqueous concentration of hydrogen ion is related to the geometric mean of the aqueous buffer and microemulsion concentrations, and that a pKa obtained using the average of the aqueous and microemulsion pH values is the most physically meaningful quantity. © 1980 by Academic Press, Inc.