Tail unsaturation controls monoglyceride monolayer organization and oil–water interfacial tension

Tail unsaturation is a standard design lever in emulsifier formulation, yet its coverage-dependent link to oil–water interfacial tension remains incomplete. Here we combine pendant-drop tensiometry with atomistic molecular dynamics (MD) using the mechanical (pressure-tensor) route to establish a quantitative structure–property for two monoglycerides that differ only in tail chemistry: monopalmitin (saturated) and monoolein (cis-unsaturated) at the triolein–saline interface. Experiments show that the unsaturated analogue lowers interfacial tension more effectively across comparable surface coverages. MD reproduces the coverage-resolved trends and attributes the difference to packing frustration from the cis double bond, which yields more disordered/fluid monolayers, greater interfacial thickness, enhanced headgroup hydration (higher hydrogen-bond occupancy and larger solvent-accessible surface area), and reduced short-range lateral order relative to the saturated analogue. These results establish a coverage-resolved structure–property linking tail chemistry to interfacial organization and to macroscopic interfacial tension, providing practical guidance for selecting and blending glyceride-based emulsifiers under formulation-relevant conditions. • Unsaturated tails lower oil–water interfacial tension in monoglycerides. • Experiments and molecular dynamics connect tail chemistry to interfacial tension. • Unsaturation yields more fluid monolayers and thicker interfacial films. • Headgroup hydration increases with unsaturation, aiding tension reduction. • A coverage-resolved map links molecular structure to interfacial mechanics.