Water pool pH of AOT-based W/O microemulsions at various water contents estimated by absorbance ratio of pyranine

The dependence of water pool pH on the water content of the W/O microemulsions was investigated by using the absorbance 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_o = [water]/[AOT] = 10, 20, 30 and 40. The water pool pH was estimated by comparing the absorbance ratios given by the W/O microemulsions and the aqueous solutions of pH 1-13. The experimental results suggest that the water pool pH is kept approximately constant as a buffer regardless ofthe solution pH 4-11, and that the water pool pH increases with decreasing w_o. As an additive factor to affect the absorbance ratio of pyranine, high Na⁺ counterion concentration in the water pool was focused on because the roughly estimated Na⁺ concentration increases from 1.4 to 5.6 M with decreasing w_o. 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 pH, 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_o. However, attention should be paid to the possibility that the water pool pH estimated by the absorbance ratio of pyranine may include the effect of high Na⁺ concentration in the water pool.