Interfacial Layer Properties of a Polyaromatic Compound and its Role in Stabilizing Water-in-Oil Emulsions

Physical properties of interfacial layers formed at the xylene–water interface by the adsorption of a polyaromatic organic compound, N-(1-hexylheptyl)-N′-(5-carbonylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (in brief, C5Pe), were studied systematically. The deprotonation of the carboxylic group of C5Pe at alkaline pH made it highly interfacially active, significantly reducing the xylene–water interfacial tension. Thin liquid film experiments showed a continuous buildup of heterogeneous C5Pe interfacial layers at the xylene–water interfaces, which contributed to the formation of stable W/O emulsions. Continual accumulation and rearrangement of C5Pe aggregates at the xylene–water interface to form a thick layer was confirmed by in situ Brewster angle microscopy (BAM) and atomic force microscopy (AFM). The rheology measurement of the interfacial layer by double-wall ring interfacial rheometry under oscillatory shear showed that the interfacial layers formed from C5Pe solutions of high concentrations were substantially more elastic and rigid. The presence of elastically dominant interfacial layers of C5Pe led to the formation of stable water-in-xylene emulsions.