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.