Molecular dynamics simulations of ionic interactions with dodecyl sulfate micelles

Specific ion effects on micelles have been addressed by molecular dynamics simulations of Li+, Na+, and NH4+ cations with dodecyl sulfate (DS) micelles in aqueous solution. Of the three systems, the most compact structure for a dodecyl sulfate micelle is observed in the case of Li+ ions, which penetrate considerably deeper into the micelle than either Na+ or NH4+. Comparison of molecular trajectories of micellar solutions and those of methyl sulfate solutions with the same counterion demonstrates that the micelle surface is effective at disrupting the ionic hydration shells to allow direct cation-sulfate interaction. The structure of the second coordination shell of the sulfate groups on the micelle surface is, in turn, affected by the cations involved in contact ion pairs. The 4.9 Angstrom peak in the sulfur-sulfur radial distribution function (rdf) in the LiDS micelle was attributed to direct coordination of two or more headgroups to a lithium ion. A second peak in the rdf is 1.2 Angstrom farther, corresponding to a water molecule intervening between Li+ and one of the sulfates. The sulfur-sulfur rdfs for both NaDS and NH4DS feature single peaks at distances that suggest a lesser role of these cations in headgroup structuring. The direct coordination of the ions to the headgroups requires their dehydration and shows that hydrated-ion radii are inadequate for models of the micelle interfacial region.