Development of coarse-grained models of ionic and non-ionic surfactants for the molecular simulation of structural, thermodynamic and dynamical properties

Abstract

Molecular simulations have become a mainstream tool of the physical sciences. In spite of their success in allowing us to understand the behaviour of matter at a molecular scale, current and foreseeable computational power limits the system sizes and time scales of ob- servation. Many common engineering scenarios, e.g., self-assembly of aqueous surfactant solutions, require access to large systems and long time scales. A coarse-graining method- ology, in which atoms and molecules are grouped into beads can be used to capture this behaviour using molecular simulation. However, the development of coarse-grained force fields for use in molecular simulation to study structural, thermodynamic and dynamical properties still remains a challenge. This challenge is the focus of the work presented in this thesis. Coarse-grained force fields obtained in this work were used in molecular simulations for a broad range of systems including: fluid phases of small molecules such as carbon dioxide, linear chains of alkanes, siloxanes and alcohols, to more complex aqueous systems of non- ionic surfactants, electrolytes, and ionic surfactants. The coarse-grained models were developed using a molecular-based equation of state of the Statistical Associating Fluid Theory (SAFT) family, based on the Mie (generalised Lennard-Jones) inter-molecular potential for the interactions between beads. This allows the structural, dynamical, and interfacial properties to be studied directly in molecular simulation. A transferable coarse-grained model for linear alkanes was developed. The carbon chains were used to form the backbone of a variety of organic molecules. Coarse-grained potentials for charged species were also obtained using an electrolyte version of the theory (SAFT-VRE) for use in molecular simulations. The coarse-grained models developed for the linear alkanes and aqueous electrolytes were subsequently used to establish a coarse- grained force field for the aqueous mixture of an important ionic surfactant: sodium dodecyl sulphate. The phase behaviour of the aqueous solutions of sodium dodecyl sulph- ate was studied with the coarse-grained models by molecular dynamic simulations, with emphasis on the structural properties of the different phases. Graphic processing units were also employed to perform large-scale simulations of the coarse-grained SAFT-γ Mie models of aqueous solutions of a non-ionic surfactant: tetraethylene glycol monodecyl ether. Despite the relative simplicity of the coarse-grained force fields developed using the SAFT-γ Mie equation of state, the models were robust and transferable. Properties that have not been considered in the original parameterisation procedure can be predicted, and the results are comparable with the more sophisticated and computationally demanding atomistic and united atom models. Therefore, the methodology developed in this work can be employed in a wide range of industrial and academic applications to help bridge the gap between the microscopic and macroscopic scales.