Surfactant molecules self-assemble in aqueous solutions to form various micellar structures such as spheres, rods, or lamellae. Although phase transitions in surfactant solutions have been studied experimentally, their molecular mechanisms are still not well understood. In this work, we show that molecular dynamics (MD) simulations using the coarse-grained (CG) MARTINI force field and explicit CG solvent, validated against atomistic MD studies, can accurately represent micellar assemblies of cetyltrimethylammonium chloride (CTAC). The effect of salt on micellar structures is studied for aromatic anionic salts, e.g., sodium salicylate, and simple inorganic salts, e.g., sodium chloride. Above a threshold concentration, sodium salicylate induces a sphere to rod transition in the micelle. CG MD simulations are shown to capture the dynamics of this shape transition and support a mechanism based on the reduction in the micelle-water interfacial tension induced by the adsorption of the amphiphilic salicylate ions. At the threshold salt concentration, the interface is nearly saturated with adsorbed salicylate ions. Predictions of the effect of salt on the micelle structure in different CG solvent models, namely, single-site standard water and three-site polarizable water, show qualitative agreement. This suggests that phase transitions in aqueous micelle solutions could be investigated by using standard CG water models which allow for 3 orders of magnitude reduction in the computational time as compared to that required for atomistic MD simulations.

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.

The self-assembly of nanoparticles (NPs) with cationic micelles of cetyltrimethylammonium chloride (CTAC) is known to produce stable nanogels with rich rheological and optical properties. Coarse-grained molecular dynamics (MD) simulations are performed to explore the molecular mechanisms underlying this self-assembly process. In an aqueous solution of CTAC surfactants, a negatively charged NP with a zeta potential of less than -45 mV is observed to form a stable vesicular structure in which the particle surface is almost entirely covered with a double layer of surfactants. In comparison, surfactants form a monolayer, or a corona, around an uncharged hydrophobic NP with the tailgroups physically adsorbed onto the particle. In the presence of sodium salicylate salt, such NP-surfactant complexes (NPSCs) interact with rodlike CTAC micelles, resulting in the formation of stable junctions through the opening up of the micelle end-cap followed by surfactant exchange, which is diffusion-limited. The diffusive regime spans several hundred nanoseconds, thereby necessitating MD simulations over microsecond time scales. The energetics of NPSC-micelle complexation is analyzed from the variation in the total pair-potential energy of the structures.

Flow-induced configuration dynamics and scission of rodlike micelles are studied for the first time using molecular dynamics simulations in presence of explicit solvent and salt. Predicted dependence of tumbling frequency and orientation distribution on shear rate S agrees with mesoscopic theories. However, micelle stretching increases the distance between the cationic head groups and adsorbed counter ions, which reduces electrostatic screening and increases the overall energy Φ linearly with micelle length. Micelle scission occurs when Φ exceeds a threshold value, independent of S.

Partial molar volumes (PMVs) for a range of organic solutes in aqueous solution are evaluated from molecular simulations using Kirkwood-Buff theory. Long-range oscillatory variations in the Kirkwood-Buff integrals are suppressed using the techniques of Lockwood, Rossky, and Levy [J.