Effect of surfactant molecular structure on temperature-dependent interfacial tension and shear rheology

Abstract: The interfacial behaviour of surfactants strongly depends on their molecular structure, in
particular the contributions of their polar and non-polar moieties to interfacial adsorption and
packing. The purpose of this study was to assess the temperature-dependent interfacial tension
and rheology of four emulsifiers at different temperatures using drop and force tensiometry as
well as interfacial shear rheology. Four surfactants were evaluated - the monoacylglycerols
glycerol monostearate (GMS) and glycerol monooleate (GMO), and two variants of citric acid
ester of mono and diacylglycerols (CITREM) - one containing an oleic acid tail (OA-CITREM)
and the other a stearic acid tail (SA-CITREM). Monoacylglycerols are distinguished by a small
headgroup consisting of glycerol whereas in CITREMs, the headgroup is citric acid. Surfactants
containing stearic acid will have a higher crystallization temperature than those with oleic acid.
Both CITREMs reduced oil-water interfacial tension to a greater extent than the
monoacylglycerols, which we associated with the presence of their citrated headgroup. The
surfactants with a stearic acid tail (GMS and SA-CITREM) reduced interfacial tension more than
surfactants with the oleic acid tails (GMO and OA-CITREM) when molten at 65°C, suggesting
different contributions to interfacial activity in surfactants containing stearic vs. oleic acid. The
surfactants with a stearic acid tail produced higher interfacial shear moduli than those containing
oleic acid when crystallized at 5°C due to their solidification at the oil-water interface. Overall,
these results demonstrated that surfactant temperature-dependent interfacial activity and shear
rheology are dependent on their molecular structure.