Monte Carlo Simulation of Amphiphile Self-Assembly during Dip Coating
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Monte Carlo Simulation of Amphiphile Self-Assembly during Dip Coating Stephen E. Rankin3 , Anthony P. Malanoski2 and Frank van Swol1;2 1
Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Blvd. SE, Suite 100, Albuquerque, NM 87106, U.S.A. 2 Department of Chemical and Nuclear Engineering, University of New Mexico AML, 1001 University Blvd. SE, Suite 100, Albuquerque, NM 87106, U.S.A. 3 Department of Chemical and Materials Engineering, University of Kentucky 177 Anderson Hall, Lexington, KY 40506-0046, U.S.A. ABSTRACT Fascinating nanostructured porous materials have recently been synthesized by evaporation-driven coassembly of ceramic precursors and amphiphiles. To expand our understanding of this process, we examine the influence of interfaces on self-assembly process using equilibrium lattice-based Monte Carlo simulations of ternary amphiphile-solvent mixtures. The simulations are able to predict the existence of all significant lyotropic mesophases, including lamellae, hexagonal closepacked cylinders, and cubic phases such as the gyroid. In the presence of walls that attract the majority solvent, the amphiphiles are confined to a smaller region of space, and experience a higher local concentration than the bulk concentration. This can lead to early transitions between mesophases. On the other hand, when the surface repels the majority solvent, amphiphiles tend to adsorb at the walls, and the local effective concentration in solution is lower. This can delay mesophase formation. When the amphiphile concentration is high enough that mesophases form in the bulk solution, however, either type of strongly attracting wall will align the mesophase (lamellae or hexagonal channels) parallel to the walls. In contrast, neutral walls (with no preferential interaction with either component) align mesophases perpendicular to themselves, which could be an interesting route to pores aligned normal to a film. INTRODUCTION In 1992, researchers at Mobil reported the synthesis of a new class of porous materials consisting of ˚ monodisperse, ordered channels [1]. These materials were synamorphous silica with 20 to 200 A thesized by the hydrothermal sol-gel polymerization of silica in the presence of cetyltrimethylammonium bromide (CTAB). Subsequent work has shown that other amphiphilic structure-directing agents including cationic, anionic, and nonionic surfactants can give ordered porous materials with structures reflecting the major classes of self-assembled mesophases (HCP cylindrical channels, lamellar layers, and cubic pore networks) [2]. More recently, Brinker and coworkers have reported that similar materials can be prepared in a wider variety of geometries and with greater control using evaporation-induced self-assembly [3]. In this process, an initially dilute solution of structure directors, silica source, water, and catalyst in ethanol is formed into a desired geometry (e.g., a thin film or a spherical particle) and allowed to dry. The concentration of the non-volatile species drives both self-asse
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