Modeling Aggregation of Ionic Surfactants Using a Smeared Charge Approximation in Dissipative Particle Dynamics Simulations

Abstract

Using dissipative particle dynamics (DPD) simulations, we explore the specifics of micellization in the solutions of anionic and cationic surfactants and their mixtures. Anionic surfactant sodium dodecyl sulfate (SDS) and cationic surfactant cetyltrimethylammonium bromide (CTAB) are chosen as characteristic examples. Coarse-grained models of the surfactants are constructed and parameterized using a combination of atomistic molecular simulation and infinite dilution activity coefficient calibration. Electrostatic interactions of charged beads are treated using a smeared charge approximation: the surfactant heads and dissociated counterions are modeled as beads with charges distributed around the bead center in an implicit dielectric medium. The proposed models semiquantitatively describe self-assembly in solutions of SDS and CTAB at various surfactant concentrations and molarities of added electrolyte. In particular, the model predicts a decline in the free surfactant concentration with the increase of the total surfactant loading, as well as characteristic aggregation transitions in single-component surfactant solutions caused by the addition of salt. The calculated values of the critical micelle concentration reasonably agree with experimental observations. Modeling of catanionic SDS-CTAB mixtures show consecutive transitions to worm-like micelles and then to vesicles caused by the addition of CTAB to micellar solution of SDS.

Determination of the critical micelle concentration in simulations of surfactant systems

“…azp@princeton.edu as the total surfactant loading is increased above the cmc. This is confirmed by experiments, 12-14 simulations [15][16][17][18][19] and theory. [20][21][22] The free surfactant concentration goes through a maximum near the cmc and then decreases at higher overall loadings.…”

“…This has been shown earlier to be a poor approximation for ionic surfactants. 9,[18][19][20] Here, we show that this approximation can lead to significant errors for nonionic surfactants as well. For example, at T = 7.5, for an NVT simulation at ϕ = 0.0916 (approximately five times ϕ cmc ), the uncorrected oligomer volume fraction is ϕ olig = 0.0146.…”

“…The main reason for this decrease in free surfactant concentration for ionic surfactants is the changing ionic strength of the solution at higher loadings. 13 The correction initially proposed for experiments 12,13 has also been implemented in simulations 9,[18][19][20] and justified from theory. 20 The magnitude of the decrease in free surfactant concentration for nonionic surfactants is significantly smaller than for ionic surfactants and still somewhat controversial.…”

Coarse-grained model of water diffusion and proton conductivity in hydrated polyelectrolyte membrane

“…is bond rigidity, r 0 is equilibrium bond length, and r m is the maximum bond length. Following our recent papers, 56,[65][66][67] in addition to the nearest neighbor (1-2) bonds, we introduced the second neighbor (1-3) bonds in order to control the chain rigidity. Because the polymer conformation is an important factor affecting the segregated structure of hydrated sPS, which is relatively complex (compared to linear chains considered earlier 56,[65][66][67] ) and contains very rigid fragments, we also introduced the harmonic angle potentials between certain pairs of nearest neighbor bonds as shown in Figure 1(b):…”

Dynamic simulations of many-body electrostatic self-assembly

“…Coupled with this is the added requirement that any dynamical study of M particles requires the simultaneous solving of 6M equations of motion at each time step in a simulation. To date, there have been a number of 'coarse grain' studies where the dissipative particle dynamics (DPD) technique has been used to simulate a range of electrostatic assembly processes [18][19][20][21][22][23][24]. For the most part these studies have used pair potentials; however, in a related study, Dahirel et al [25,26] have used an interaction potential that included three-body terms to study the dynamical behaviour of charged particles in aqueous solutions of electrolytes.…”