EFFECT OF ANIONIC AND NONIONIC WATER-IN-OIL MICROEMULSIONS ON ACID-BASE EQUILIBRIA OF HYDROPHILIC INDICATORS

Apparent pK(a) values (hereafter written pK) for the hydrophilic, negatively-charged indicators 2-hydroxy-5-bromo sodium benzenesulphonate (HBBS), 2-hydroxy-5-nitro sodium benzenesulphonate (HNBS), and 2-(5,7-dinitro-8-hydroxy) sodium naphthalenesulphonate (napthol yellow, NY) in the presence of water-in-heptane microemulsions of bis(2-ethylhexyl) sodium sulphosuccinate (AOT, anionic) and polyoxyethylene (4) dodecyl ether (Brij-30, nonionic) were determined spectrophotometrically. The variables studied were the type of buffer (borate, imidazole, nicotinamide, phosphate, and piperidine) and the ratios [water]/[surfactant] and [buffer]/[surfactant]. Partition experiments between water and heptane and UV-VIS spectra of the indicators in aqueous buffer in the presence of anionic and nonionic aqueous and reversed micelles showed that these indicators are confined within the second hydration shell of the microemulsion, i.e., are not adsorbed at the water-in-oil (W/O) interface. Buffer-independent pK values were obtained after the initial pHs of the solubilized buffer solutions were corrected for the ion exchange with counter-ion of the surfactant (AOT) and for the lower polarity of the micelle-solubilized water (Brij-30 and AOT). The micelle-induced pK shifts were in the range 1.66-2.08 units for HBBS/AOT, 1.17-2.14 units for HNBS/AOT, 0.73-1.31 units for NY/AOT, 1.06-1.40 units for HNBS/Brij-30, and 0.12-0.33 units for NY/Brij-30. The observed pK shifts are explained as due to the effect of two factors, i.e., the lower polarity of the micelle-solubilized water (relative to bulk water, AOT, and Brij-30) and electrostatic perturbation of the - --> - - equilibria by the negatively charged surfactant. The smaller micellar effect on the pK of NY was attributed to the extensive electron delocalization in its conjugate base.