Structure and Phase Transitions of Alkyl Chains on Mica

Abstract

We use molecular dynamics as a tool to understand the structure and phase transitions [Osman, M. A.; et al. J. Phys. Chem. B 2000, 104, 4433-4439. Osman, M. A.; et al. J. Phys. Chem. B 2002, 106, 653-662] in alkylammonium micas. The consistent force field 91 is extended for accurate simulation of mica and related minerals. We investigate mica sheets with 12 octadecyltrimethylammonium (C(18)) ions or 12 dioctadecyldimethylammonium (2C(18)) ions, respectively, as single and layered structures at different temperatures with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside preferably above the cavities in the mica surface with an aluminum-rich boundary. The nitrogen atoms are 380-390 pm away from the superficial silicon-aluminum plane. With increasing temperature, rearrangements of C(18) ions on the mica surface are found, while 2C(18) ions remain tethered due to geometric restraints. We present basal-plane spacings in the duplicate structures, tilt angles of the alkyl chains, and gauche-trans ratios to analyze the chain conformation. Agreement with experimental data, where available, is quantitative. In C(18)-mica with less than 100% alkali-ion exchange, the disordered C(18) rods in the island structures [Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913-5917] break at 40 degrees C. At 60 degrees C, the headgroups of the C(18) alkyl chains rearrange on the mica surface, and the broken chain backbones assume a coillike structure. The C(18)-mica obtained on fast cooling of this phase is metastable due to slow reverse rearrangements of the headgroups. In 2C(18)-mica with 70-80% ion exchange, the alkali ions are interspersed between the alkyl chains, corresponding to a single phase on the surface. The observed phase transition at approximately 53 degrees C involves an increase of chain disorder (partial melting) of the 2C(18) ions without significant rearrangements on the mica surface. We propose a geometric parameter lambda for the saturation of the surface with alkyl chains, which determines the preferred self-assembly pattern, that is, islands, intermediate, or continuous. lambda allows the calculation of tilt angles in continuous layers on mica or other surfaces. The thermal decomposition seems to be a Hofmann elimination with mica as a base-template.