External surface structure of organoclays analyzed by transmission electron microscopy and X-ray photoelectron spectroscopy in combination with molecular dynamics simulations

Abstract

HypothesisInterface properties of organoclay particles can be related directly to type of organic cation, and density and arrangement of organic coating at clay surfaces. ExperimentsThis study provides detailed nanoscale insights on surface structure of hexadecyltrimethylammonium/hexadecylpyridinium-montmorillonite (HDTMA+/HDPy+-M) organoclays by combining several experimental methods (e.g. transmission electron microscopy, TEM, and X-ray photoelectron spectroscopy, XPS) with molecular simulations. FindingsTEM showed a relation between the thickness of the organic coating and the amount of organic cation loading. Furthermore, coating thickness varied for the same sample indicating a heterogeneous surface of clay particles. The changes in elemental composition determined by XPS were correlated with the thickness of the organic coating. High resolution XPS showed changes in binding energies of CN bonds, which were attributed to varying local environment of head groups of organic cations. Classical molecular dynamics (MD) simulations showed a successive transformation of the organic cation coating of the montmorillonite surface from thin disordered monolayers at low up to disordered bilayer or quasi paraffin-type arrangements at high surfactant coverages. For organoclays with low cation loading no significant difference was observed between HDTMA+ and HDPy+. However, at high cation loading, surface packing density was higher for HDTMA+ than for HDPy+.