Development of a Coarse-Grained Polyoxyethelyne Glycol Non-Ionic Surfactant Model Using the SAFT-? Mie Force-Field for Molecular-Dynamics Simulations

Robust coarse-grained (CG) models which exhibit both quantitative accuracy and representability are needed if one expects to obtain valuable insights from the molecular modelling of complex fluid systems. In this study the SAFT-𝛾force-field methodology is employed to develop a CG surfactant model for use in the molecular simulation of aqueous mixtures of non-ionic alkyl poly(oxyethylene) glycols (POE) surfactants, C_iE_j . In this approach, the effective Mie (generalized Lennard-Jones) interactions between the constitutive chemical moieties (alkyl, ether and hydroxyl groups), and their interactions with water are parameterized using appropriate experimental data for the vapor-liquid equilibria, liquid-liquid equilibria, enthalpy of mixing, and interfacial tension of selected small molecules and mixtures. The model incorporates the use of heteronuclear alkaneand ether sub-models, allowing for a “buildable” POE molecule without further parameterization.

Utilizing the POE surfactant model, C₁₀E₄ is simulated within aqueous systems of varying concentration exhibiting a micellar region, transitioning from spherical to rodlike at low to mid concentration ranges At higher concentrations the model predict the hexagonal, the lamellar and inverse micelle regions, with no additional parameters. Studies of the structures formed are conducted and compared to the corresponding experimental data; showing excellent agreement to the predicted structures from established phase diagrams⁵ including reproduction of the lamellar phase and further replicating the micelle concentration dependence predicted from experiment. This is investigated through a quantitative analysis of micelle/lamellar size, shape and structure, from calculation of parameters such as aggregation number N_A, relative shape anisotropy κ² and bilayer thickness. Further studies are then performed of varying alkyl C_iand ethoxy E_j lengths, and surfactant system characteristics are extracted including te critical micelle concentration.