Enhancing Surfactant Transport across the Oil-Water Interfaces by Polymer Competitive Adsorption

Understanding mechanisms of the interfacial transport of amphiphilic molecules plays a crucial role in many fields such as interfacial assembly, self-emulsification, capsule delivery system, etc. However, the mass transfer resistance and energy barrier induced by their interfacial affinity remain unclear. Here, we employ coarse-grained molecular dynamics (CGMD) simulation combined with umbrella sampling method to quantify the energy barriers for sodium dodecyl sulfate (SDS) across the water/dodecane interface covered with different densities of polyethylene glycol (PEG). Furthermore, we describe the diffusion subjected to the interface potential energy using the generalized Langevin equation and calculate the interface diffusion coefficients and retention times by combining MD trajectories. We find that the PEG layer with an appropriate interfacial coverage reduces the desorption energy barrier of SDS up to 23.8%. It differs from the traditional viewpoint that the coverage increases the spatial obstruction for transport. We attribute this phenomenon to the competitive adsorption of SDS and PEG near the interface. The adsorption energy of PEG partly counterbalances the desorption energy of SDS. It can significantly reduce the interaction energy between SDS and both water and dodecane at the interface. Meanwhile, it also eliminates the "flip" of SDS during the transport process, which indicates weakened interfacial constraints and greater molecular freedom. Consequently, the transport kinetics are significantly accelerated, as evidenced by an increase in the interfacial diffusion coefficient on orders of magnitude and a shorter retention time. Our study investigates the short-range interfacial hopping and transfer of amphiphiles with asymmetric phase affinity, providing theoretical insight into the design and regulation of their mass transfer driving forces and rates.