(Non)linear\nInterfacial Rheology of Tween, Brij and\nSpan Stabilized Oil–Water Interfaces: Impact of the Molecular\nStructure of the Surfactant on the Interfacial Layer Stability

During emulsification\nand further processing (e.g., pasteurizing),\nthe oil–water interface is mechanically and thermally stressed,\nwhich can lead to oil droplet aggregation and coalescence, depending\non the interfacial properties. Currently, there is a lack of insights\ninto the impact of the molecular structure (headgroup and FA chain)\nof low molecular weight emulsifiers (LME) on the resulting interfacial\nproperties. Additionally, the crystallization/melting of the oil/the\nemulsifier is often neglected within interfacial rheological experiments.\nWithin this study, the stability of interfaces formed by Tween, Span\nor Brij was determined as a function of their molecular structure,\ntaking crystallization effects of the LME into account. The headgroup\nwas kept constant while varying the FA, or vice versa. The interfacial\nfilm properties (viscoelasticity) were investigated at different temperatures\nusing dilatational and interfacial shear rheology. Both the headgroup\nand the FA chain impacted the interfacial properties. For the same\nFA composition, a rather small hydrophobic headgroup resulted in a\nhigher packed interface. The interfacial elasticity increased with\nincreased FA chain length (C12 to C18). This seemed to be particularly\nthe case when the emulsifier crystallized on the interface among cooling.\nIn the case of a densely packed interface, network formation due to\nchain crystallization of the LME’s FA chains occurs during\nthe cooling step. The resulting interface shows predominantly elastic\nbehavior.