Acid-base equilibria of hydrophilic indicators in water-in-oil microemulsions

Article abstract of DOI:10.1002/bbpc.19900940812

The apparent pK_a values of the positively charged, hydrophilic indicators 2-hydroxy-5-bromo-trimethylanilinium chloride (HBTC) and 2-hydroxy-5-nitro-trimethylanilinium chloride (HNTC) in the presence of water-in-oil microemulsion of the surfactants benzylcetyldimethyl ammonium chloride (BCDC) in benzene and polyoxyethylene (4) dodecyl ether (Brij-30) in heptane were determined spectrophotometrically. The experimental variables were the type of buffer (benzimidazole, phosphate, and tris) and the ratios [water]/[surfactant] and [buffer]/[surfactant]. It is shown that: a) the indicators are confined within the second hydration shell of the nanodroplet, i.e. are not adsorbed at the water/oil interface; b) the use of the starting pH values of the buffer solutions, i.e., those measured outside the micellar domain, to calculate the micellar pK_a values of the indicators is inadequate; c) buffer-independent micellar pK_a values can be obtained if the starting pHs of the buffer solutions were corrected for ion exchange with the counter-ion of the positively charged surfactant, and for the lower polarity of the micelle-solubilized water (relative to bulk water).