(486b) Monomer Partition Between Surfactant Surface Aggregates And Bulk Aqueous Solution: Influence Of Monomer Lyophilicity

Equilibrium molecular dynamics simulation results for the structure of sodium dodecyl surfactant aggregates at the graphite-water interface [Tummala and Striolo, Langmuir, submitted] were used to study the partition of benzene, aniline, pyrrole, acrylonitrile, and methyl methacrylate between bulk aqueous solution and surface aggregates. We employed all-atom molecular dynamics techniques within the umbrella-sampling algorithm and we reconstructed the free energy landscape as a function of the distance between the center of mass of the organic monomers and the graphite surfaces by implementing the weighted histogram analysis method (WHAM) [Shankar et al., J. Comput. Chem. 13 (1992) 1011]. Relative maxima and minima in the free energy landscape were related to unfavorable and favorable effective interactions between the monomer and the surfactant aggregates, respectively. Our detailed study allows us to highlight the relative importance of monomer-surfactant tails (hydrophobic) and/or monomer-surfactant heads (hydrophilic) interactions towards the adsorption of the organic monomers within the surfactant surface aggregates. We will discuss how our results are crucial for the outcome of template-assisted admicellar polymerization (TAAP) processes [Marquez et al., Langmuir 22 (2006) 8010], in which monomers are first adsorbed within surfactant surface aggregates and then reacted to yield polymeric structures. In particular, we found that for the production of defect-free surface structures it is necessary that the organic monomers are preferentially adsorbed (e.g., corresponding to a minimum in the free-energy landscape) in the region adjacent to the graphite surfaces. Thus our methodology provides a reliable screening for the monomers that could be used successfully in TAAP processes.