Mercaptopropyl magnesium phyllosilicate - thermodynamic data on the interaction with divalent cations in aqueous solution

Magnesium phyllosilicate (SILMgSH) was obtained by the sol-gel process involving the reaction of magnesium cation and mercaptotrialkoxysilane in aqueous basic solution at 298 K. The inorganic-organic hybrid was characterized by elemental analysis to give a presence of 4.88 mmol g(-1) of organic chains, and X-ray diffraction (XRD) patterns gave basal peaks associated with an interlayer distance of 1300 pm. The interactions between the terminal thiol groups of the organic chains of a silylating agent attached to magnesium phyllosilicate and the divalent cations copper, nickel, cobalt and zinc in aqueous solution were followed through calorimetric titrations. After subtracting the thermal effect of dilution, the net thermal effects were adjusted to a modified Langmuir equation, and the enthalpy of the interaction calculated to give the following exothermic values: -47.06+/-1.00, -27.63+/-0.50, -21.92+/-0.50 and -14.66+/-0.52 kJ mol(-1), for copper, nickel, cobalt and zinc cations, respectively. The distinct behavior of the copper cation involves the formation of a more organized compound. Gibbs free energy values changes are negative. With the exception of copper, all reactions were entropically favoured and the final products of adsorption presented an increased crystallinity clearly detected by XRD. (C) 2000 Elsevier Science B.V. All rights reserved.