Non-additive interfacial organization of mixed monoglycerides underlies the composition-dependent stability of water-in-oil emulsions

Mixed food-emulsifier layers often show macroscopic behavior intermediate between the corresponding single-component systems, but whether this reflects simple averaging or distinct molecular reorganization remains unclear. Here, emulsion-separation experiments, dynamic interfacial tension, interfacial dilatational rheology, fluorescence microscopy, and atomistic molecular dynamics simulations were combined to examine monopalmitin (PAL), monoolein (OLE), and their equimolar mixture (MIX). Experimentally, increasing the OLE fraction lowered the triolein/water interfacial tension, strengthened the dilatational elastic response, suppressed water-droplet coalescence, and delayed macroscopic separation. The mean water-droplet area increased in the order OLE-stabilized < PAL/OLE-mixed < PAL-stabilized emulsions. To interpret these trends, we used an oil-free bilayer surrogate representing locally oil-depleted contact zones between approaching emulsifier-covered interfaces, together with oil-containing residual-oil models. Compared with PAL, OLE formed a less densely packed, more hydrated, and more mobile layer. In the MIX system, however, both PAL and OLE exhibited higher orientational order than in the corresponding single-component layers, while mixing-index analysis and two-dimensional radial distribution functions revealed segregation into PAL-rich and OLE-rich regions. These molecular tendencies were largely retained in the presence of residual triolein. Thus, the mixed emulsion was macroscopically intermediate, whereas the mixed PAL/OLE layer underwent non-additive molecular reorganization. The coexistence of PAL-rich ordered regions and OLE-rich mobile regions provides a possible molecular interpretation of the composition-dependent stability of monoglyceride-stabilized W/O emulsions.