A SSIMPLE approach to surfactant self-assembly using DPD simulations

Molecules' ability to form supramolecular structures plays a fundamental role for several processes in chemistry, biology and many industrial pplications. Surfactants are among the most common molecules with such ability and depending on their chemical structure and the solution conditions, they can aggregate into a large number of different structures. Molecular modeling has been regarded as a powerful tool to investigate the self-assembly process at microscopic level. However, due to the length and time scale at which these processes occur, all-atom simulations are not a common choice and coarse grain (CG) approaches such as dissipative particle dynamics (DPD), where molecule fragments are merged into a single entity, are preferred. For CG simulations, finding the correct interaction between beads is a crucial step to obtain the correct behaviour of the simulated system. Herein, a new DPD parametrisation method, based on the SSIP (Surface Site Interaction Point) model previously developed in the Hunter group, is proposed. Experimental data were collected using a combination of NMR and US-Vis titrations in order to expand the experimental H-bond scale to positive charged species for future extension of the parametrisation method to charged species. Through a footprint algorithm, the SSIPs description of each bead was obtained from the molecule electrostatic potential surface. This set of points, used in combination with the SSIMPLE (surface site interaction model for the properties of liquids at equilibrium) algorithm provided the transfer free energy, then converted into the required DPD parameters. The method was expanded to several neutral surfactant systems, together with molecular mechanics which provided the intramolecular bead distances and angles. The reliability of the approach was demonstrated comparing experimental properties such critical micelle concentration, aggregation number and micelle shape with those obtained from simulations. Finally, the Setschenow equation, which describes the solubility dependency on the salt concentration, was used to parametrise the interactions between charged and neutral species. These results lay strong foundations for the future development of a solid and general parametrisation method for DPD simulations.