Abstract: Aliphatic chain molecules may be adsorbed on montmorillonite surfaces with the zigzag of their carbon chain either parallel or perpendicular to the surface. These orientations are designated αI and αI, respectively. Fourier syntheses have too few terms to distinguish clearly between these orientations. The problem is studied therefore by consideration of the basal spacings. These spacings are appreciably less than the sum of the van der Waals cross-section of the molecules plus the thickness of the montmorillonite layer. Chemical bonding or geometrical packing, or both, appear as possible explanations.
Previous studies have shown that CH activity influences the shape of the adsorption isotherms, but infrared studies of the adsorbed molecules have shown no appreciable change in the CH-stretching frequencies. This suggests the importance of the CH activity for organic-organic interaction on the clay surface rather than for clay-organic interaction itself.
The possibility of geometrical packing is studied by means of molecular models and it is shown that an appreciable contraction can occur at a clay-organic interlace owing to a keying of the molecules into the surface structure. This is possible equally well for the αII and the αI orientations.
From the information available, it appears that the αII arrangement is adopted by aliphatic chain molecules containing strongly polar groups in order to place the dipole parallel to the silicate surface, while the αI arrangement is favored by other aliphatic chain molecules possibly because this provides a favorable organic-organic interaction.