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The time-dependent growth of octadecyltrimethylammonium bromide (C18TAB) 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, C18TAB adsorbs predominately as the monomer rather than as an aggregated solution species resulting in monolayer films. Formation of the C18TAB 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 C18TA+ 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.

We report a study of the physical and electrochemical properties of two-component self-assembled monolayers (SAMs) composed of both electroactive (4-aminothiophenol, 4-ATP) and electroinactive (n-octadecanethiol, ODT) species. In all of the experiments reported here, relatively short (3 h) assembly times were used to prepare the mixed SAMs. We have characterized the macroscopic composition and the microscopic structure of these SAMs using Auger electron spectroscopy (AES), coulometry, grazing angle Fourier transform infrared spectroscopy, and lateral force microscopy (LFM). The adsorption isotherms determined by AES and coulometry show significant deviation from Langmuirian behavior and are suggestive of phase separation. LFM images obtained at three points near the critical region of the isotherm ([4-ATP]/[ODT] ∼ 4) indicate that these two-component SAMs display complex phase behavior:  At relatively low 4-ATP coverages, the surface consists of small islands of 4-ATP imbedded in an ordered film of ODT. At higher coverages of 4-ATP, however, we find evidence of both separation into distinct phases and mixing of the two components. In a second series of experiments, we demonstrate that phase domains of 4-ATP are electroactive and can be used to carry out localized electrochemistry. That is, the islands of 4-ATP, which are randomly distributed in the ODT matrix, behave as an array of ultramicroelectrodes. Surface-confined 4-ATP molecules can be used to nucleate the growth of polyaniline selectively from the phase separated domains of 4-ATP. We find that if a 4-ATP/ODT mixed monolayer is oxidized in the presence of aniline, nanometer scale polyaniline features are formed. The size and distribution of these features have been characterized by AFM and can be controlled through a combination of monolayer composition and the concentration of aniline in solution.