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

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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 w₀ = [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 of the solution pH 4–11, and that the water pool pH increases with decreasing w₀. 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 w₀. 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 w₀. 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.