Initial partition and aggregation of uncharged polyaromatic molecules at the oil-water interface: a molecular dynamics simulation study.

Initial partitioning and aggregation of several uncharged polyaromatic (PA) molecules with the same polyaromatic core but different terminal moieties at oil-water interfaces from the bulk oil phase were studied by molecular dynamics simulation. The partition of the PA molecules between the bulk organic phase and oil-water interface was highly dependent on the terminal moiety structure of the PA molecules and aromaticity of the organic phase. The polarity ratio between the oil and water phases showed a significant influence on adsorption of the PA molecules at the oil-water interface. The presence of hydrophobic aromatic moieties in PA molecules hindered the adsorption process. Larger aromatic rings in PA molecules lowered the interfacial activity due to strong intermolecular π-π interactions and molecular aggregation in the bulk oil phase. The presence of a terminal carboxylic functional group on the side chain enhanced the adsorption of the PA molecules at the oil-water interface. The fused ring plane of the uncharged PA molecules was found to preferentially adsorb at the oil-water interface in a head-on or side-on orientation with the polyaromatic core staying in the nonaqueous phase (i.e., the principal plane of the molecule perpendicular to the oil-water interface). The results obtained from this study could provide a scientific direction for the design of proper chemical demulsifiers for PA molecule-mediated emulsions formed under specific process conditions of temperature, pressure, and pH.