Ion-transfer stripping voltammetry of polyoxyethylene alkyl and alkylphenyl ether surfactants at the organic-gel ∣ water interface and its application to trace analysis

Stripping voltammetric behavior of the transfer of poly(oxyethylene) alkyl and alkylphenyl ether non-ionic surfactants at trace level at the organic-gel ∣ water (O-gel ∣ W) interface has been studied using normal pulse voltammetry (NPV) and pulse amperometry (PA). The method is based on the metal cation-assisted transfer of surfactant into O-gel in an anodic way, that is, pre-electrolysis, to form a metal cation–surfactant complex in O-gel which is followed by its transfer back into the test solution (W) in a cathodic way by NPV or PA. In the presence of Na+ or Ca2+ ion in W, heptaoxyethylene dodecyl ether gave well-defined cathodic stripping current versus potential curves by NPV. The stripping limiting current as determined by PA was proportional to a concentration of the surfactant in W as low as 10 nM. Other poly(oxyethylene) non-ionic surfactants (NSs, DP=10∼38, DP being the degree of polymerization) also gave well-defined stripping voltammograms at the O-gel ∣ W interface in the presence of Ca2+ ion. The stripping limiting currents of NSs of DP larger than 16 were found to depend on DP and the voltammetric conditions, such as the pre-electrolysis potential and the concentration of the assisting cation in W. The result can be explained by theoretical equations in which the formation of a binuclear metal cation–NS complex in addition to a mononuclear one is taken into consideration. Dodecylsulfate and dodecylbenzenesulfonate anionic surfactants in W also gave well-defined voltammograms at the O-gel ∣ W interface. Their half-wave potentials are well separated from those of the cation-assisted transfer of NSs, indicating potential advantages of the voltammetric method for the separate determination of non-ionic and anionic surfactants simultaneously.