Determination of the critical micelle concentration in simulations of surfactant systems

Abstract

Alternative methods for determining the critical micelle concentration (cmc) are investigated using canonical and grand canonical Monte Carlo simulations of a lattice surfactant model. A common measure of the cmc is the "free" (unassociated) surfactant concentration in the presence of micellar aggregates. Many prior simulations of micellizing systems have observed a decrease in the free surfactant concentration with overall surfactant loading for both ionic and nonionic surfactants, contrary to theoretical expectations from mass-action models of aggregation. In the present study, we investigate a simple lattice nonionic surfactant model in implicit solvent, for which highly reproducible simulations are possible in both the canonical (NVT) and grand canonical (μVT) ensembles. We confirm the previously observed decrease of free surfactant concentration at higher overall loadings and propose an algorithm for the precise calculation of the excluded volume and effective concentration of unassociated surfactant molecules in the accessible volume of the solution. We find that the cmc can be obtained by correcting the free surfactant concentration for volume exclusion effects resulting from the presence of micellar aggregates. We also develop an improved method for determination of the cmc based on the maximum in curvature for the osmotic pressure curve determined from μVT simulations. Excellent agreement in cmc and other micellar properties between NVT and μVT simulations of different system sizes is observed. The methodological developments in this work are broadly applicable to simulations of aggregating systems using any type of surfactant model (atomistic/coarse grained) or solvent description (explicit/implicit).

Desorption of hydrocarbon chains by association with ionic and nonionic surfactants under flow as a mechanism for enhanced oil recovery

“…Cationic surfactants are, in general, more expensive than anionic surfactants because of the high-pressure hydrogenation reaction required during their synthesis, yet they are often of great commercial importance, such as in corrosion inhibition [11].For ionic surfactants, it is now widely accepted that there are significant changes in the free surfactant concentrations the total surfactant loading is increased above the critical micelle concentration. This is confirmed by experiments [12][13][14], simulations [15][16][17] and theory [18][19][20].The magnitude of the decrease in free surfactant concentration for nonionic surfactants is significantly smaller than that for ionic surfactants and still somewhat controversial [21]. Depending on the nature of the hydrophilic group, synthetic surfactants are classified in four types.…”

DPD Parameters Estimation for Simultaneously Simulating Water–Oil Interfaces and Aqueous Nonionic Surfactants

“…To calculate the critical micelle concentration (CMC) we implemented the approach proposed by Santos et al, 27 who demonstrated that the CMC can be estimated by the constant value of the volume fraction free surfactant (φ oligomer ) in the accessible volume of aqueous phase (water beads in our case) when the total surfactant volume fraction in water (φ)…”

Toward a Standard Protocol for Micelle Simulation

“…Nonetheless, the idealized behavior of micellization ( as a function of surfactant concentration. 19 One may also look for changes in the partial molar volume of the surfactant in isobaric (constant particle number, pressure and temperature) simulations as one increases the total surfactant concentration from below to above the CMC.…”

“…One may perform linear fits of the submicellar concentration as a function of the total surfactant concentration above and below an estimate of the CMC and detect a crossing point. With any of these approaches based on submicellar concentration, even the computation of this concentration may not be straightforward.Santos and Panagiotopolous (2016) have shown that in computing the submicellar concentration as a surrogate for the CMC, one must correct the volume considered to be accessible to the free surfactant by a factor somewhat more than one would expect based on the space occupied by the micellar material 19. The idealized micelle behavior also suggests one may perform a linear fit of the total number of micelles as a function of total surfactant concentration and extrapolate this fit to find the zero intercept, representing the highest total surfactant concentration where no micelles are seen.…”