Scaling Relation between Electrical Conductivity Percolation and Water Diffusion Coefficient in Sodium Bis(2-ethylhexyl) Sulfosuccinate-Based Microemulsion

• Published in: The journal of physical chemistry. B 2003-11, Vol.107 (45), p.12542-12548
• Main Authors: Kataoka, Hiroshi, Eguchi, Taro, Masui, Hirotsugu, Miyakubo, Keisuke, Nakayama, Hirokazu, Nakamura, Nobuo
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp020510a

We investigated the conductivity percolation mechanism of a water-in-oil (W/O) microemulsion composed of sodium bis(2-ethylhexyl) sulfosuccinate (AOT), D2O, and decane using impedance spectroscopy and the pulsed-gradient spin-echo (PGSE) NMR technique. The electrical conductivity (σ) measured as a function of temperature (T) and the volume fraction (Φ) of water droplets show a marked increase near room temperature. The PGSE NMR measurements show that the water diffusion coefficient (D water) is responsible for σ, implying that the main charge carrier is the cation species (Na+) dissolved in and transported with water. Given the relation D exchange ≡ D water − D droplet, where D exchange and D droplet are diffusion coefficients for the exchange of water between water droplets and for the whole host droplet, respectively, a quantitative scaling relation between reduced conductivity σTΦ-1 and D exchange was first confirmed to show asymptotic behavior in which σTΦ-1 ∼ (D exchange)β [β = 3.2 for −10.7 ≤ log (D exchange) ≤ −10.0 and β = 1.3 for −10.0 ≤ log (D exchange) ≤ −9.3]. This reveals that a common mechanism governs both conductivity percolation and the water exchange between droplets in AOT-based W/O microemulsion. The discontinuity of the exponent β at log (D exchange) ca. 10.0 indicates a change in the mechanism of water exchange between the droplets, resulting mainly from Na cation distribution in a droplet.