The main objective of this project is to improve the understanding of the molecular mechanisms that govern droplet coalescence and vesicle fusion. Droplet coalescence in oil-in-water and water-in-oil emulsions is crucial in many fields: A key challenge fo r the oil industry is to separate water droplets from the oil phase. Conversely, the food and pharmaceutical industries usually need to prevent separation. The stability of these emulsions is controlled by adding surfactants that self-assemble on the oil/ water interface and modify the coalescence barrier. Choosing surfactants that change this barrier in the desirable direction is a principal problem in emulsion science. Vesicle fusion is fundamentally related to droplet fusion. This process has exciting a pplications such as liposome-based drug delivery as well as being essential for cell life.
We propose here a fundamental study of droplet and vesicle fusion intermediate states. The emphasis will be on understanding how the fusion free energy barriers d epend on the structure of the amphiphiles that constitute the mono- or bilayers. To achieve this, we will use molecular simulations and model amphiphiles that allow for simple variations in structure such as hydrophilic/hydrophobic affinity, chain length, and branching. We will study droplet fusion along a reaction coordinate that includes the initial rupture of the two monolayers, formation of a channel between the droplets, and expansion of its radius. Vesicle fusion proceeds via multiple metastable sta tes. To enable a detailed study of unstable states we will use novel biased sampling methods. We will calculate the free energy along a reaction coordinate and predict the fusion energy barrier. The focus will be on how variations in amphiphilic structure affect the fusion barrier, and how additives can lower this barrier. This knowledge will provide a better basis for choosing and designing surfactants with a desired fusion barrier.