Transfer Mechanism of Dodecyl Sulfate with Methylene Blue across an Oil/Water Interface Studied by Single-Droplet Injection and Microabsorption Methods

The ion-pair extraction mechanism in a dodecyl sodium sulfate (Na^+DS^-)/Methylene Blue (MB^+Cl^-) system was kinetically investigated by single microdroplet techniques. A single tributyl phosphate microdroplet was injected into an aqueous solution and the extraction rate of MB^+ by DS^- into the single droplet was measured with time using a microabsorption method. The MB^+ concentration in the droplet at the extraction equilibrium was independent of the pH in water, while the extraction rate was highly influenced by the pH. The characteristic pH dependence of the MB^+/DS^- extraction was considered in terms of the mass transfer of MB^+, DS^- and H^+ as individual ions across the oil/water interface.