(517f) Shape Transitions, Binary Interactions, and Fusion of Surfactant Micelles From Coarse-Grained Molecular Dynamics Simulations
While structural phase transitions in surfactant solutions have been widely studied experimentally, their molecular mechanisms are still not well understood. For example, salts are generally known to promote sphere to rod transition in micelles; however, aromatic hydrophobic salts such as sodium salicylate are known to drive the sphere to rod transition to a much greater extent than simple inorganic salts such as sodium chloride. In this work, molecular dynamics (MD) simulations using coarse-grained (CG) MARTINI force field and explicit CG solvent, validated against atomistic MD studies, are used to represent micellar assemblies of Cetyltrimethylammonium chloride (CTAC) in the presence of sodium salicylate and sodium chloride. Above a threshold concentration of sodium salicylate, sphere to rod transition of the micelle is observed in microsecond timescale simulations. This transition can be attributed to the reduction in the micelle-water interfacial tension induced by the adsorption of the amphiphilic salicylate ions on the micelle corona .
Inter-micelle potential of mean force (PMF) is evaluated using umbrella sampling as a function of molar concentrations of sodium salicylate and sodium chloride to investigate the screening of micelle-micelle repulsive interactions by added salt. Local ionic environment in the vicinity of the micelle corona and the electrical double layer between two approaching micelles are significantly different for sodium salicylate as compared to those for sodium chloride. Strong adsorption of salicylate ions results in a sharp decrease in the inter-micelle repulsion. Consequently, fusion of two spherical micelles to form a cylindrical micelle is observed at sufficiently large concentrations of sodium salicylate. We find qualitative agreement between the inter-micelle PMF obtained from the MD simulations and inter-micelle potentials predicted by the DLVO theory. The simulation results are used to suggest a renormalization of DLVO potentials to accurately represent the interactions between ionic micelles.
Acknowledgement: We acknowledge National Science Foundation grant CBET-1049454 and the New York Center for Computational Sciences at Stony Brook University/Brookhaven National Laboratory for support of this research.
1. A.V. Sangwai and R. Sureshkumar, Coarse-Grained Molecular Dynamics Simulations of the Sphere to Rod Transition in Surfactant Micelles, Langmuir, Article ASAP, http://pubs.acs.org/doi/full/10.1021/la2006315.