Mercaptopropyl magnesium phyllosilicate Ð thermodynamic data on the interaction with divalent cations in aqueous solution Maria G. Fonseca a , Claudio Airoldi b,* a Faculdade de Cie Ãncias e Tecnologia, Universidade Estadual da Paraõ Âba, Campina Grande, Paraõ Âba, Brazil b Instituto de Quõ Âmica, Universidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas, Sa Äo Paulo, Brazil Received 6 December 1999; received in revised form 25 February 2000; accepted 26 February 2000 Abstract 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 modi®ed 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 ®nal products of adsorption presented an increased crystallinity clearly detected by XRD. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Phyllosilicates; Modi®ed clays; Enthalpy 1. Introduction The interactive processes involving metallic cations with inorganic or organically modi®ed matrices have been explored in recent years [1±7]. Due to the pre- sence of acidic centres on inorganic matrices, bases can be adsorbed on the surface. Also, covalent attach- ment of organic molecules to inorganic surfaces can yield new inorganic±organic hybrids, such as for silica gel, where a great variety of chelating agents are used to complex cations from aqueous or non-aqueous solutions [1,5,8±11]. The properties of these materials are of importance in many applications such as elec- trochemistry, chromatography, pre-concentration of species or cations, and decontamination of water [2,12±15]. With relation to the last feature, investiga- tions are focused on highly selective and speci®c processes for cations such as cobalt, nickel, copper and zinc, which are human hazards in the environment [2]. Exploration of these chemical interactions by spectroscopic techniques and calorimetric procedures Thermochimica Acta 359 (2000) 1±9 * Corresponding author. Tel.: 55-192-39-7766; fax: 55-197-88-3805. E-mail address: airoldi@iqm.unicamp.br (C. Airoldi) 0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0040-6031(00)00490-1