Langevin Dynamics Simulations of Cationic Surfactants in Aqueous Solutions Using Potentials of Mean Force

Abstract

A Langevin dynamics (LD) technique is proposed for the simulation of surfactant molecules in aqueous solutions, where no water molecules of the solvent are explicitly treated, but the effects are incorporated using both the self-diffusion coefficients of the solutes and the potentials of mean force between them in water. The present LD technique is employed to simulate (i) the self-assembly of n-decyltrimethylammonium chloride surfactants in water and (ii) the micelles of different sizes in water and in a 0.5 M NaCl solution. In the first simulation starting from the disaggregate configuration, the surfactant aggregates grew by addition of monomers and small aggregates to them rather than by fusion of large aggregates into larger ones within 12 ns: this behavior corresponds to the early stage of surfactant self-assembly and indicates that the system did not completely reach the equilibrium states for the period of several nanoseconds. In the second simulation starting from the spherical micelles, the small micelle of 20 surfactants was unstable and less spherical in water compared with the larger micelles of 30 and 40 surfactants, but it exhibited a stable spherical shape in the NaCl solution where no significant difference among these micelles was found in the shape and the stability. The results are in fair agreement with those from the atomistic model molecular-dynamics simulations and the experimental measurements, demonstrating that our LD technique can capture the characteristics of ionic surfactants in aqueous solutions despite the implicit treatment of water molecules of the solvent.