New insights into the multilevel structure and phase transitions of synthetic organoclays

Abstract

The lamellar structure of unmodified as well as octadecylamine-exchanged fluoromica was revisited using scanning and transmission electron microscopy as well as small-angle X-ray scattering (SAXS). The fluoromica particles were found to form piled, 10 nm-thick stacks of polygonal platelets having planar dimensions in a submicron range. Cross-sectional micrographs of the stacks indicated that they are made of nearly uniformly spaced platelets. Nonetheless, smaller peaks were identified, both in the Fourier transforms of the TEM patterns and in the X-ray diffractograms. They were preceding the main maximum corresponding to the spacing between subsequent platelets (the basal spacing, d_I). Conventional analysis of peak positions would suggest that the silicate contains fractions having basal spacing of d_I and 2 × d_I. Direct comparison of SAXS and TEM patterns indicated that additional peaks in the diffractograms originate exclusively from discontinuities or defects located at the boundaries of stacks and not from the presence of any additional fractions of the fluoromica. Solid-state NMR investigations supported by SAXS and molecular dynamics simulations were utilized to probe the structure and physical transitions that the surfactant molecules intercalated in the galleries of the silicates undergo are based on the γ-gauche effect. During the first heating cycle, a disordering of the octadecylamine molecules is observed followed by a de-interdigitation and formation of a gauche-dominated liquid-like phase at a temperature around 80 °C. These changes cannot be reversed by simply cooling of the hybrids. However, cooling the system back to 25 °C causes the reoccurrence of trans conformers and decreases the mobility of methylene groups, which is accompanied by a change in heat capacity similar to that observed during a glass transition. The latter changes can be observed upon repeating heating/cooling cycles.