Water Pool pH of AOT-Based W/O Microemulsions at Various Water Contents Estimated by Absorbance Ratio of Pyranine

The dependence of water pilot pH on the water content of the W/O microemulsions was investigated by using the absorhance ratio of pH-sensitive probe pyranine. The W/O microemulsions were prepared with a mixture of an aqueous solution of pH 1-13 containing pyranine, AOT and isooctane at w(0) = [water]/[AOT] = 10, 20, 30 and 40. The water pool pH was estimated by comparing the absorbance ratios given by the W/O microemulsion and the aqueous solutions of pH 1-13. The experimental results suggest that the water pool pH is kept approximately constant its it buffer regardless of the solution pH 4-11, and that the water pool pH increases with decreasing w(0). As all additive factor to affect the absorbance ratio of pyranine. high Na(+) counterion concentration in the water pool was focused oft because the roughly estimated Na(+) concentration increases front 1.4 to 5.6 M with decreasing w(0). It was found by the ratio measurements for NaOH aqueous solutions that the ratio increases at NaOH concentration > 5 M, which suggests that the ratio given by the W/O microemulsions is affected by not only pit, but also high Na(+) concentration in the water pool. It would be correct to assume that the AOT-based W/O microemulsions play a role of the buffer, and that the water pool pH depends on w(0). However, attention should be paid to the possibility that the water pool fill estimated by the absorbance ratio of pyranine may include the effect of high Na(+) concentration in the water pool.