Interaction of cations with SH-modified silica gel: thermochemical study through calorimetric titration and direct extent of reaction determination

The reaction of silica gel with 3-(trimethoxysilyl)propane-l-thiol, MPTS, produced a thiol-modified silica gel, Sil-SH. This material removes silver, mercury, copper, zinc and nickel ions from aqueous solutions. Enthalpy changes that occur in the interactions between Sil-SH and these cations were obtained by a calorimetric titration technique, in which thermal effects and the quantities of cations that interact at each titration point were determined simultaneously. From this procedure, values of the maximum capacity of interaction (N-3), enthalpies of interaction at different coverages of the surface [Delta(int(i))H-m] and integral enthalpy of interaction for a monolayer (Delta(mon)H(m)) have been established. It is shown that this method permits charting the basic sites created upon modification. From Delta(int(i))H-m values, the sequence of the interaction strength between the basic sites of the silica-attached MPTS and the cations is Hg2+ > Ag+ > Cu2+ > Ni2+ > Zn2+, agreement with the concept of hard and soft acids and bases. The influence of some parameters concerning the thermodynamics of solvation of the indicated transition metals has been also taken into account in the investigation of these interaction processes.