THERMODYNAMIC AND KINETIC INVESTIGATION OF THE PROTONATION OF SILICA-IMMOBILIZED 3-AMINOPROPYLSILANE

Interactions occurring between the amino group of silica-immobilized 3-aminopropylsilane (APS) and surface silanol groups were investigated by thermometric enthalpy titration. Enthalpies of reaction of bound APS were compared with values obtained for solution phase analogs. Enthalpograms from titration of APS-silica gel (APS-SG) in aqueous slurry with HClO4 contain two end points. The first is generated by reaction of APS in solution and species attached to the surface. The amine moiety in each case participates in a hydrogen bond as indicated by the experimental enthalpy of protonation. Analysis of filtrates from APS-SG slurries confirmed that APS is rapidly hydrolyzed from the surface. The second end point is generated by protonation of bound species. Immobilized species exhibit sluggish reaction kinetics requiring rates of titrant addition of 0.02 mmol min-1 to minimize end point errors. The enthalpy of protonation of immobilized APS suggests that adsorption of amino groups to silica silanols is more ionic in nature than that to silane silanol groups. As much as 26% of immobilized aminopropylsilane species, found to be present by DRIFT, are not titratable. Amino groups of these species may be irreversibly bound or protonated by acidic surface silanol groups. © 1991.