Tetraalkylammonium interactions with dodecyl sulfate micelles: a molecular dynamics study.

All-atom molecular dynamics (MD) simulations were performed to study the effects of different tetraalkylammonium (TAA(+)) counterions, including tetramethylammonium (TMA(+)), tetraethylammonium (TEA(+)), tetrapropylammonium (TPA(+)) and tetrabutylammonium (TBA(+)), on dodecyl sulfate (DS(-)) micelles. Structural properties, such as the radius of gyration (Rg), micelle radius (Rs), micelle size, solvent accessible surface area (SASA), carbon and sulfur distribution, hydration numbers, and distribution of polar heads on the micelle surface, were investigated. The simulation results show that the longer the carbon chains of the TAA(+) counterion, the greater the radius of the micelle formed. TMA(+) leads to the most compact structure of the DS(-) micelle among the five studied systems and DS(-) and TAA(+) formed mixed-micelles. There are mainly four interaction patterns between TAA(+) and DS(-) ions, and the pattern in which two alkyl chains of the TAA(+) ion penetrate into the DS(-) micelle is the most favorable one. Based on the preceding analysis, a model based on this MD method is proposed.