Binary Interactions and Salt-Induced Coalescence of Spherical Micelles of Cationic Surfactants from Molecular Dynamics Simulations

Abstract

A direct estimation of salt-mediated potential of mean force (PMF) between spherical micelles of cationic surfactants is obtained for the first time using molecular dynamics (MD) simulations. Coarse-grained (CG) potentials benchmarked in an earlier study [Langmuir, 2011, 27(11), 6628-6638] are used to model a binary system of cetyltrimethylammonium chloride (CTAC) surfactant micelles at varying concentrations of sodium chloride (NaCl) or sodium salicylate (NaSal). The shape and structure of micelles are not subject to external constraints. NaSal is significantly more efficient in screening the intermicelle repulsive interactions shown by the PMF compared to NaCl due to a stronger binding of salicylate counterions to the micelle corona. Upon contact with each other, the micelles coalesce in the presence of NaSal to form a cylindrical structure which is stabilized by the adsorbed salicylate anions. Comparison of the PMF with Derjaguin-Landau-Verwey-Overbeek (DLVO) potentials shows qualitative agreement, while the magnitude of PMF is significantly greater than that of the DLVO potentials. To understand this discrepancy, PMF is evaluated by turning off (a) long-ranged electrostatic interactions and (b) solvent polarizability. The above effects are shown to play an important role in determining the solvent-mediated and ion-correlated interactions between the two micelles, which are not explicitly captured by mean-field double layer theories such as DLVO.

Nonmonotonic Scission and Branching Free Energies as Functions of Hydrotrope Concentration for Charged Micelles

“…[13,19,20] Moreover, neither this, nor any other method has been yet proposed that can estimate the branching free energy of the micelles. Although self-assembly, dynamics and rheology of micelles have been heavily studied both experimentally and computationally, [19][20][21][22][23][24][25][26] a method for direct estimation of both micelle scission and branching (free) energy is lacking so far.…”

“…Furthermore, we also compute the branching free energy of CTAC micelles as a function of hydrotrope concentration and use this and the scission free energy to explain the origin of the ubiquitous large peak in the viscosity of this micellar solution with increasing hydrotrope concentration. All CG simulations were performed using the MARTINI [28] force field which has been used extensively [4,[23][24][25] [29,30] to model these surfactant systems, and has been shown to reproduce many experimental observations including the sphere-to-rod transition and shearinduced micelle stretching energy. Details of the simulation protocol are given in the Supporting Information (SI) [31], which includes Refs.…”

The Martini Model in Materials Science

“…Although Martini reproduces well the phase behavior of small surfactants and polymers in aqueous solution, [ 35,109–117 ] predicting morphologies of large‐scale polymer—especially block copolymer—systems remains challenging. In particular microphase‐separated assemblies of copolymers are hugely important in many technological applications.…”

Structural Locality and Early Stage of Aggregation of Micelles in Water: An Adaptive Resolution Molecular Dynamics Study

“…In this perspective, our results are in good agreement with results in the literature for the same system [35] www.advancedsciencenews.com www.advtheorysimul.com and fully consistent with results of larger MD simulations for similar systems. [36]…”