Coarse-Grained Molecular Dynamics Simulation of Self-Assembly and Surface Adsorption of Ionic Surfactants Using an Implicit Water Model

Abstract

We perform coarse-grained molecular dynamics simulations for sodium dodecyl sulfate (SDS) surfactant using a modification of the Dry Martini force field (Arnarez et al. 2014) with implicit water. After inclusion of particle mesh Ewald (PME) electrostatics, an artificially high dielectric constant for water (ε(r) = 150), and reparameterization, we obtain structural and thermodynamic properties of SDS micelles that are close to those obtained from the standard Martini force field with explicit water, which in turn match those of atomistic simulations. The gains in computational efficiency obtained by removing explicit water allow direct simulations of the self-assembly of SDS in solution. We observe surfactant exchange among micelles and micelle fission and fusion and obtain realistic, equilibrated micelle size distributions at modest computational cost, as well as a transition to cylindrical micelles at high surfactant concentration or with added salt. We further apply this parametrized force field to study the adsorption of SDS onto hydrophobic surfaces and calculate the adsorption kinetics and equilibrium adsorption isotherm. The greatly increased speed of computation of surfactant self-assembly made possible by this Dry Martini method should allow future simulation of competitive adsorption of multiple surfactant species to surfaces, as well as simulation of micellar shape transitions.

A novel multiscale model for the simulation of polymer flash nano-precipitation

“…The main novelty of this work lies on the use of a multiscale approach, capable of describing FNP at three scales: molecular-scale, nanoparticle-scale and macro-scale. It represents therefore an improvement with respect to other similar works (Tatek and Pefferkorn, 2004;Wang and Larson, 2015;Yuan et al, 2010) and it is consistent with other research efforts conducted in this area (Cheng et al, 2010;Li et al, 2006;Rajagopalan, 2001;Yan and Xie, 2013). The three scales communicate among each other, as suggested by other authors (Capretto et al, 2011;Rajagopalan, 2001), and are mod- eled respectively with MD, Smoluchowski PBE and CFD, without resorting to other mesoscale techniques (Li et al, 2006;Yan and Xie, 2013).…”

“…These neglect therefore kinetic effects, which are also well-known to play an important role (Johnson and Prud'homme, 2003b;Lince et al, 2008), in determining, for example, the size and structure of the final nanoparticles (Celasco et al, 2014;Valente et al, 2012a,b). Another important limitation of the developed models is that the nanoparticle formation process is investigated and modeled only at one scale, by using for example molecular dynamics (MD), Monte Carlo (MC) methods and various coarse-grained models (Sun and Yang, 2014;Wang and Larson, 2015;Yuan et al, 2010). While these methods can capture the molecular interactions, they are unable to account for the effect that the fluid flow has on the nanoparticle formation process, particularly important when investigating the scale-up of these processes or their transfer from batch to continuous, as it happens in continuous manufacturing.…”

Hybrid Particle-Field Molecular Dynamics Simulations of Charged Amphiphiles in Aqueous Environment

“…Increasing the SDS concentration (C SDS = 182 mM) led to spheroidal micelles with a prolate character, and eventually to the formation of nanotubular structures (C SDS = 400mM), in agreement with experiment, and previous calculations using different CG models. [66][67][68][69][70] In particular, the morphological transformation from spherical to microtubular micelles observed in our simulations reproduces the one previously observed using the MARTINI CG model, both with explicit or implicit solvation, 68-69 as well as dissipative particle dynamics simulations. 70 SDS aggregation is sensitive to the choice of the dielectric constant.…”

“…Direct influence of the choice of the dielectric on the aggregation properties of SDS was also found in previous studies with the MARTINI force field. [68][69] The aggregation mechanism of SDS observed in our simulations is in qualitative agreement with the kinetic mechanism proposed by Lund et al 71 Regardless of the dielectric, at all concentrations δ rapidly develops to values close to 0, indicating the initial formation of regular spherical structures (Figure 7); for higher concentrations, with ɛ r = 45, δ increases over time, signaling the metamorphosis from spherical to spheroidal (δ ~ 0.2, C SDS = 182 mM) or nano-tubular aggregates (δ ~ 0.4, C SDS = 400 mM). This phenomenon occurs by fusion of smaller spherical units rather than by continuous aggregation of monomeric SDS.…”

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

“…Details of the simulation protocol are given in the Supporting Information (SI) [31], which includes Refs. [32][33][34][35][36]. The scission energy of the micelle was computed using an Umbrella Sampling method where the number of beads in the pre-chosen to the equilibrium state with two end-caps; the beads in these end-caps remain in the scission region until the micelle fragments are driven apart, and the bead count drops towards zero.…”