When the Disperse Phase Crystallizes: How Surfactant Structure Shapes Interfacial Properties.

Commercial oil-water emulsions typically contain a partially crystalline fat phase, which is essential for macroscopic attributes such as creaminess and whippability. While it is well established that the molecular structure of surfactants can accelerate or delay fat crystallization, much less attention has been paid to what happens at the interface during this process. Particularly, the extent to which fat crystallization modifies interfacial rheological properties remains insufficiently understood, despite their relevance for emulsion stability and functionality. This study investigates how cooling-induced crystallization of triglycerides affects interfacial viscoelasticity as a function of surfactant. Surfactants with identical saturated fatty acyl (FA) chains (C18:0) but different headgroups (Tween 60, BrijS20, Span 60), as well as Tweens with varying FA chain lengths (Tween 20: C12:0; Tween 60: C18:0), were examined. To capture differences in molecular similarity, tristearin (TS), tripalmitin (TP), and trilaurin (TL) in MCT oil were used as the fat phase. C18:0-based surfactants promoted interfacial TS crystallization and formed crystalline interfacial networks with increased viscoelasticity. Span 60 generated the strongest elastic response due to its dense interfacial packing and formation of a crystalline emulsifier layer, whereas Tween 60 and BrijS20 produced weaker, less connected structures. The FA chain length controlled the packing density and mobility of the interfacial (sub)layer and thereby the resulting interfacial viscoelasticity upon cooling. Tween 20 formed a thin and highly mobile interfacial layer that disrupted TS and TP crystallization, resulting in weaker and less connected interfacial films compared to Tween 60 (C18:0). Overall, the results show that crystallization of the dispersed fat phase actively reshapes the structure, thickness, and connectivity of the interfacial layer, thereby altering interfacial viscoelasticity. The magnitude of this effect depends on the surfactant headgroup, FA chain length, and their molecular match with the triglyceride phase, which collectively determines the extent to which interfacial networks can form.