Peculiarities of the Protonation Reaction in the Interfacial Layer of a Silica Containing Covalently Bonded -Methylaminopropyl Groups

Abstract

The reaction between an aqueous solution of a strong acid and the methylaminopropyl groups chemically bonded to a silica surface (A-NHCH 3 ) was studied potentiometrically at various temperatures. Equilibrium constants were calculated on the basis of the law of mass action. It was shown that the interaction of acid with A-NHCH 3 cannot be described within acceptable confidence levels by a single constant. At all temperatures studied, two types of chemical reactions had to be assumed: (1) protonation of an associate formed from adjacent amine molecules and (2) reaction of the charged associate with a second proton. The thermodynamic characteristics of the reactions were: ΔH 1.2 = 25 ± 4 kJ/mol and ΔS 1 = −17 J/(mol K), ΔS 2 = +49 J/(mol K). The dynamic change in the structure of the interfacial layer was evaluated from the data obtained. It seems that after the first step in the reaction the surface associates remain unchanged. The second proton promotes a significant change in the geometry of the interfacial layer.