Measurement of cation binding to immobilized vanillin by isothermal calorimetry.

Isothermal calorimetry was used to determine enthalpy changes for interaction of divalent cobalt, nickel, copper, and zinc chlorides with silica gel functionalized with vanillin, Sil-Van. The thermal effect, Q(int), and the corresponding amount of cation that interacts, n(int), were obtained in the same experiment. Langmuir expressions for adsorption isotherms were applied to determine the maximum adsorption capacity to form a monolayer, N(mon), and the energy of interaction for a saturated monolayer per gram of Sil-Van, Q(mon). From knowledge of N(mon) and Q(mon), the molar enthalpy of interaction for formation of a monolayer of anchored cations per gram of Sil-Van, Delta(mon)H(m), was determined. Interactions between the Lewis-acidic cations and the donor atom attached to silica are reflected by Delta(mon)Hm values in the order Ni2+ > Cu2+ > Zn2+ congruent with Co2+.