Simulation insights into the role of antiparallel molecular association in the formation of smectic A phases

DPD simulations of anionic surfactant micelles: a critical role for polarisable water models

We investigate the effects of polarisable water models in dissipative particle dynamics (DPD) simulations, focussing on the influence these models have on the aggregation behaviour of sodium dodecyl sulfate solutions. Studies in the literature commonly report that DPD approaches underpredict the micellar aggregation number of ionic surfactants compared to experimental values. One of the proposed reasons for this discrepancy is that existing water models are insufficient to accurately model micellar solutions, as they fail to account for structural changes in water close to micellar surfaces. We show that polarisable DPD water models lead to more realistic counterion behaviour in micellar solutions, including the degree of counterion disassociation. These water models can also accurately reproduce changes in the dielectric constant of surfactant solutions as a function of concentration. We find evidence that polarisable water leads to the formation of more stable micelles at higher aggregation numbers. However, we also show that the choice of water model is not responsible for the underestimated aggregation numbers observed in DPD simulations. This finding addresses a key question in the literature surrounding the importance of water models in DPD simulations of ionic micellar solutions.

A many-body dissipative particle dynamics parametrisation scheme to study behaviour at air-water interfaces

In this article, we present a general parametrisation scheme for many-body dissipative particle dynamics (MDPD). The scheme is based on matching model components to experimental surface tensions and chemical potentials. This allows us to obtain the correct surface and mixing behaviours of complex, multicomponent systems. The methodology is tested by modelling the behaviour of nonionic polyoxyethylene alkyl ether surfactants at an air/water interface. In particular, the influence of the number of ethylene oxide units in the surfactant head group is investigated. We find good agreement with many experimentally obtained parameters, such as minimum surface area per molecule; and a decrease in the surface tension with increasing surfactant surface density. Moreover, we observe an orientational transition, from surfactants lying directly on the water surface at low surface coverage, to surfactants lying parallel or tilted with respect to the surface normal at high surface coverage. The parametrisation scheme is also extended to cover the zwitterionic surfactant lauryldimethylamine oxide (LDAO), where we provide good predictions for the surface tension at maximum surface coverage. Here, if we exceed this coverage, we are able to demonstrate the spontaneous production of micelles from the surface surfactant layer.

Investigating anionic surfactant phase diagrams using dissipative particle dynamics: development of a transferable model

Dissipative particle dynamics (DPD) provides a powerful coarse-grained simulation technique for the study of a wide range of soft matter systems. Here, we investigate the transferability of DPD models to the prediction of anionic surfactant phase diagrams, taking advantage of fast parameter sweeps to optimise the choice of DPD parameters for these systems. Parameters are developed which provide a good representation of the phase diagrams of SDS (sodium dodecyl sulfate) and three different isomeric forms of LAS (linear alkylbenzene sulfonates) across an extensive concentration range. A high degree of transferability is seen, with parameters readily transferable to other systems, such as AES (alkyl ether sulfates). Excellent agreement is obtained with experimentally measured quantities, such as the lamellar layer spacing. Isosurfaces are produced from the surfactant head group, from which the second moment M of the isosurface normal distribution is calculated for different phase structures. Lyotropic liquid crystalline phases are characterised by a combination of the eigenvalues of M, radial distribution functions, and visual inspections.

Assessing the transferability of common top-down and bottom-up coarse-grained molecular models for molecular mixtures

Assessing the performance of top-down and bottom-up coarse-graining approaches.