Thermodynamics of [Co(NCS)(4)](2-) at poly(ethylene oxide) and octylphenylmoieties in micelles of nonionic surfactants

Formation of cobalt(II)-thiocyanato complexes in nonionic surfactant soluti ons of poly(ethylene oxide) type with varying poly(ethylene oxide) chain le ngths of 7.5 (Triton X-114), 30 (Triton X-305), and 40 (Triton X-405) has b een studied by titration spectrophotometry and calorimetry at 298 K. Data w ere analyzed by assuming formation of a series of ternary complexes Co(NCS) (n)Y-m((2-n)+) (Y = surfactant) with an overall formation constant beta (nm ). In all the surfactant systems examined, data obtained can be explained w ell in terms of formation of Co(NCS)(+) and Co(NCS)(2) in an aqueous phase (aq), and Co(NCS)(4)Y2- in micelles, and their formation constants, enthalp ies, and entropies have been determined. The beta (41)/beta (20) ratio incr eases and the corresponding enthalpy becomes significantly less negative wi th an increasing number of ethylene oxide groups. This suggests that micell es of these nonionic surfactants have a heterogeneous inner structure consi sting of ethylene oxide and octylphenyl moieties. Indeed, on the basis of m olar volumes of ethylene oxide and octylphenyl groups, intrinsic thermodyna mic parameters have been extracted for the reaction Co(NCS)(2)(aq) + 2NCS(- )(aq) = Co(NCS)(4)Y2- (Delta (r)G degrees, Delta H-r degrees, and Delta S-r degrees) at each moiety. The Delta (r)G degrees, Delta H-r degrees, and De lta S-r degrees values are -16 kJ mol(-1), -15 kJ mol(-1), and 3 J K-1 mol( -1), respectively, for the ethylene oxide moiety, and -15 kJ mol(-1), -70 k J mol(-1), and -183 J K-1 mol(-1) for octylphenyl. Significantly less negat ive Delta H-r degrees and Delta S-r degrees values for ethylene oxide imply that the hydrogen-bonded network structure of water is extensively formed at the ethylene oxide moiety, and the structure is thus broken around the C o(NCS)(4)(2-) complex with weak hydrogen-bonding ability. (C) 2001 Academic Press.