Ion-pair extraction of an anionic surfactant with a cationic dye

ION-PAIR EXTRACTION OF AN ANIONIC SURFACTANT WITH A CATIONIC DYE 

Ion-pair extraction of an anionic surfactant with a cationic dye such as methylene blue from water into oil is often used for the quantitative analysis of the surfactant in water [57,58]. The surfactant concentration in water is then determined as the dye concentration in the oil or water phase by conventional absorption spectroscopy. Synthetic surfactants such as sulfates and sulfonates completely dissociate in water even at low pH. On the other hand, the association of fatty acid salts (traditional soaps) with H depends on the pH. Therefore, the quantitative analysis of surfactants in water is performed by the ion-pair extraction at various pHs. Although quantitative analysis and thermodynamic studies have been already reported for the anionic surfactant/cationic dye extraction, kinetic analysis of the ion-pair extraction has been rarely reported and the extraction mechanism is not discussed in detail. In this section, we describe the kinetic analysis of the extraction of a dodecyl sulfate anion with methylene blue as a typical example using the single microdroplet manipulation and microabsorption methods [59]. In particular, the pH dependence of the ion-pair extraction is discussed. 

Sigmoidal curves having an induction period are frequently analysed on the basis of the two-step consecutive reaction-type kinetics. In this case, A(r) is given by Equation (6). 

TABLE 9.1. tf and the partitioning ratios of both MB+ (with DS ) and DS (with H+) in the TBP/water system. 

Transfer Mechanism of a Dodecyl Sulfate Anion with Methylene Blue 

Interfacial tension values of the TBP droplet in water in the absence and presence of DS were 8 and 5 mN m at pH 2, indicating that DS slightly adsorbs at the TBP/water interface. However, the interfacial tension was independent of pH so that the characteristic pH dependence of the ion-pair extraction rate in the DS /MB+ system is not ascribed to the adsorption of the solute at the TBP/water interface. The standard ion transfer potentials of DS , Cl , Na, H and MB+ in the water/l,2-dichloroethane system were —0.08, -0.53,0.59,0.55 and 0.4 V, respectively [56]. The data predict that MB+ is extracted with DS . On the other hand, DS will be extracted with H+, Na+ and MB+. At low pH, the concentration of H+ is much greater than that of MB+ in the water phase (lx 10 mol dm ) so that DS is likely to be extracted with H+. By an ordinary extraction experiment in the TBP/water system without MB+, the actual partitioning ratio of DS with H+ (/ s) was determined to be 40 and 4 at pH 2 and 6, respectively (Table 9.1). Therefore, DS is extracted with H+ into the TBP phase in the absence of MB+. 

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