Computational modelling of adsorption and aggregation in bio oils at multiple length scales

Surfactants in apolar solvents, such as common bio oils self-assemble into various colloidal assemblies, for example reverse micelles. The assemblies may also adsorb at interfaces. The shape and size of these assemblies is sensitive to factors such as surfactant chemistry, concentration, temperature, as well as the presence of co-surfactants, water, and other additives. Although the responses are qualitatively understood in chemical engineering processes, molecular scale understanding of surfactant assembly in apolar solvent environments remains sparse. In this thesis, self-assembly and adsorption of surfactants and colloidal species at solid – liquid interfaces in bio oils is critically assessed using three modelling approaches applicable to research questions at different length and time scales: atomistic detail classical molecular dynamics (MD), coarse-grained dissipative particle dynamics (DPD), and equilibrium state thermodynamics modelling. This combined approach, together with comparison to real chemical systems, allows discerning the effect of individual molecular features and e.g., hydration on adsorption strength and aggregation propensity in bio oils, but also simulation of large scale adsorption and aggregation equilibrium structures. The results presented identify the driving forces of adsorption and aggregation equilibria of bio surfactants, mostly lipids, in common bio oils. On the atomistic scale, hydrogen bonding and electrostatic interactions drive adsorption of single molecules. The interactions are affected by the presence and steric availability of functional groups, as well as, adsorbent surface chemistry and density of adsorption site functional groups. This may result in competitive intra-surface interactions. Water in the bio oil may mediate adsorption via water bridging. The findings show that surfactant aggregate in oil following a step-wise aggregation response, even at low surfactant concentrations and in the absence of water. Addition of water promotes growth and elongation of the aggregates. The surface packing is tunable by adsorbate chemistry, mainly surfactant head group charge, oil water content, and adsorbent surface hydrophilicity. On hydrophilic surfaces, water also acts as a competitive wetting agent. Overall, this thesis provides via multi scale modelling methods guidelines for understanding and manipulating surfactant adsorption/aggregation response in bio oils in terms of molecular architecture, oil water content, and adsorbent chemistry. The work also highlights the challenges and provides solutions associated with modelling self-assembly in apolar environments with sensitive self-assembly and adsorption equilibria.