2-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 temperatu re below the Krafft point, is investigated with transmission IR spectr oscopy, wetting measurements, and atomic force microscopy (AFM). Under these deposition conditions, C(18)TAB adsorbs predominately as the mo nomer rather than as an aggregated solution species resulting in monol ayer films. Formation of the C(18)TAB monolayer is characterized by tw o distinct growth regimes. A rapid initial adsorption is observed whic h results in a 2-D liquid phase comprised of randomly oriented alkyl c hains. When the films are allowed to assemble for times longer than 24 h, distinct, densely packed, monolayer high islands of C(18)TA(+) mol ecules are observed (2-D solid). As the assembly time is increased fur ther, the relative surface coverage of the semicrystalline islands inc reases substantially. In this growth regime, the mica surface is cover ed by a two-phase film comprised of the 2-D liquid and solid phases. C omparison of these results to those previously obtained for the adsorp tion of octadecylphosphonic acid (OPA) on mica clearly shows a distinc tly different growth mechanism for the two systems.