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

## TL;DR

This study explores how surfactant structure influences the crystallization of fats in oil-water emulsions and how this affects the stability and texture of the emulsion.

## Contribution

The study reveals how surfactant headgroups and fatty acid chain lengths influence interfacial viscoelasticity during fat crystallization.

## Key findings

- C18:0-based surfactants promote interfacial crystallization and increase viscoelasticity.
- Span 60 forms the strongest elastic interfacial layer due to dense packing and crystalline structure.
- Shorter fatty acid chains, like in Tween 20, disrupt fat crystallization and reduce interfacial film strength.

## Abstract

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.