Formation of Copper(II) Thiocyanato and Cadmium(II) Iodo Complexes in Micelles of Nonionic Surfactants with Varying Poly(ethylene oxide) Chain Lengths

Abstract

Formation of copper(II) thiocyanato and cadminum(II) iodo complexes in micelles of poly(ethylene oxide) (PEO)-type nonionic surfactants with varying PEO chain lengths of 9.5 (Triton X-100), 30 (Triton X-305), and 40 (Triton X-405) has been studied by titration spectrophotometry and calorimetry at 298 K. In a given surfactant solution, all data obtained were analyzed by assuming formation of ternary complexes MXnYm(2−n)+ (M = CuII,CdII; X = SCN−, I−; Y = surfactant), and the complexes thus form in aqueous phase (m = 0) or in micelles (m = 1). In the CuII–SCN− system, spectrophotometric data obtained by varying concentrations of the surfactant can be explained well in terms of formation of Cu(NCS)2Y in micelles and Cu(NCS)+ and Cu(NCS)2 in an aqueous phase, and it turned out that formation constant of Cu(NCS)2Y increases with increasing PEO chain length. In the CdII–I− system, the formation of CdI3Y− and CdI4Y2− is concluded in micelles, and that of CdI+, CdI3−, and CdI42− in an aqueous phase. Interestingly, formation enthalpies of CdI3Y− and CdI4Y2− become significantly less negative with increasing PEO chain length. This suggests that transfer of the complexes from aqueous solution to a hydrophobic octylphenyl (OP) moiety in micelles is significantly more exothermic than that to a hydrophilic PEO one. Thermodynamic parameters of transfer of CdI3− and CdI42− from aqueous solution to the OP and PEO moieties of micelles have been evaluated.