Influence of the Van der Waals force on the infrared spectra of short aliphatic alkylammonium cations held on montmorillonite

Abstract

The aim of this paper is to show how van der Waals forces may account for profound changes in the IR spectra of alkylammonium saturated montmorillonite and the effect of adsorbed water molecules. IR spectra recorded on alkylammonium montmorillonite films submitted to vacuum and after exposure to water vapour show, in addition to the shifts in the stretching and deformation bands expected when hydrogen bonding is present, marked changes in the intensity of the CH2 and CH3 deformation bands. All these observations are explained by the following model: (1) in the absence of water, the aliphatic chains are tightly held between the oxygen atoms of the silicate layers with their axis parallel to the 001 planes; and (2) hydration causes a rotation of the organic chain of the dialkyl derivatives while it does not affect the orientation of the monoalkylammonium cations. Moreover in the former situation, the symmetric deformation band of CH3 is removed because of the increase of the reduced mass due to the immobilization of the chain between the oxygen atoms.