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

Abstract

The reaction between an aqueous solution of a strong acid and the methylaminopropyl groups chemically bonded to a silica surface (A-NHCH3) 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-NHCH3 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: ΔH1.2 = 25 ± 4 kJ/mol and ΔS1 = −17 J/(mol K), ΔS2 = +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.

Influence of the Donor Properties of Silica-immobilized Amines on the Thermodynamic Characteristics of Interfacial Protolytic Reactions

“…Both these facts are not trivial and suggest that reaction of the proton with the bonded amines increases the organization of the interfacial layer. As has been demonstrated earlier (Zaitsev and Oleynik 1999), the entropy value of -17 J/(mol K) for Sg-NHCH l can be attributed to the decrease in the numberof degrees of freedom possessed by the proton in'solution due to its immobilization in the interfacial layer as a result of the protonation of the bondedamines. Additional structurization of the water physically adsorbed on the CMSi surface should also be considered.…”

“…the distribution of the bonded group on the surface and even its geometry. We have recently published results which demonstrate that the reaction between an aqueous solution of a strongacidand methylaminopropyl groupschemically bondedto a silicasurfacecannotbedescribed by a single constant within acceptable confidence levels (Zaitsev and Oleynik 1999). In the present work.…”

“…at 293 K). The first proton reacts at random with the bonded amino groups and increases the energy necessary for the approach of a second proton to a neighbouring bonded amine (Zaitsev and Oleynik 1999). The second step in the reaction causes the destruction of surface associates between the bonded molecules (due to the repulsion of similarly charged bonded alkylammonium cations) and an increase in the disorder of the structured physically adsorbed water.…”

“…In contrast, Sg-Dien has three nitrogen donor atoms which exhibit sharplydifferentbasicities [9.6.8.6and 3.62 whenpresentin aqueoussolution(AllisonandAngelici 1971 »). Since at least two constants are necessary to describe the reaction of acids with amino groups bonded to a silica gel surface (Zaitsev and Oleynik 1999), it was of interest to compare such processes for amines possessing one (Sg-NHCH, and Sg-NHCN) and three (Sg-Dien) binding sites.…”