On the impact of intermolecular interactions between the quaternary ammonium ions on interlayer spacing of quat-intercalated montmorillonite: A molecular mechanics and ab-initio study

Abstract The work comprises the nature of intermolecular interactions in the organoclays modified with quaternary ammonium ions, the SOMASIF ME100 organofilized with N-octadecyltrimethylammonium ions and the commercially available Cloisite 15A. The potential energy scan was carried out for two large organoclay models with consistent-valence force field and a few others for comparison. Then, the two-body model of the interacting ions in their most and less preferable orientations was build for both systems and the more detailed, electrically neutral model of the two ions adsorbed on montmorillonite was constructed in order to investigate the non-additive interactions. The most of the work was devoted to study ab-initio calculations performed on the Unrestricted Hartree–Fock level of theory, but the electron correlation effects were also considered. The calculated interaction energy was partitioned into exchange, delocalization and electrostatic terms and the latter was distributed into its multipolar expansion. The specific intermolecular order occurring for the most and the least energetically preferred arrangements was discussed in detail with respect to the intermolecular interactions with the special emphasis on the structures underlying the occurrence of three distinctive phases in organoclay samples, for which the X-ray data was present in the literature. The basis-set dependency of the variational-perturbation scheme adopted to decompose the calculated Hartree–Fock interaction energies was examined with the use of cc-pVDZ and 6-31G basis sets. The comparison proved high reliability of the method used, even for the less extended 3-21G basis. The results revealed the presence of the relationship between the sum of two- and three-body delocalization energy terms and the inverse induced dipole moment, calculated for three body system as well as the independent functional relations between the second order electron correlation correction and Hartree–Fock delocalization and inverse Heitler–London exchange energy terms for the system of two N-octadecyltrimethylammonium cations. The equations proposed may help to predict the values of the electron correlation correction and the stabilizing delocalization energy term from the calculations at Hartree–Fock level of theory as well as from the measured module of the induced dipole moment for molecules.