Two-Stage Growth of Octadecyltrimethylammonium Bromide Monolayers at Mica from Aqueous Solution below the Krafft Point

The time-dependent growth of octadecyltrimethylammonium bromide (C 18 TAB) monolayers at mica surfaces from aqueous solution, at a temperature below the Krafft point, is investigated with transmission IR spectroscopy, wetting measurements, and atomic force microscopy (AFM). Under these deposition conditions, C 18 TAB adsorbs predominately as the monomer rather than as an aggregated solution species resulting in monolayer films. Formation of the C 18 TAB monolayer is characterized by two distinct growth regimes. A rapid initial adsorption is observed which results in a 2-D liquid phase comprised of randomly oriented alkyl chains. When the films are allowed to assemble for times longer than 24 h, distinct, densely packed, monolayer high islands of C 18 TA + molecules are observed (2-D solid). As the assembly time is increased further, the relative surface coverage of the semicrystalline islands increases substantially. In this growth regime, the mica surface is covered by a two-phase film comprised of the 2-D liquid and solid phases. Comparison of these results to those previously obtained for the adsorption of octadecylphosphonic acid (OPA) on mica clearly shows a distinctly different growth mechanism for the two systems.