Cationic and anionic
surfactant mixtures can form viscous films
that dominate the rheology and stability of micrometer-sized droplet
suspensions. In this work, we use micropipette aspiration to study
the mechanical properties of mixed surfactant surface films of anionic
sodium dodecyl sulfate (SDS) and cationic dodecylamine hydrochloride
(DAH) on alkane and lipid droplets. For octane droplets, SDS was found
to decrease the surface tension until a minimum of 5 ± 1 mJ/m<sup>2</sup> was reached after the critical micelle concentration (cmc).
The surface viscosity of the droplets was found to be on the order
of 10<sup>–3</sup> mN s/m at an SDS concentration of 10 mM.
An addition of 0.2 mM of DAH was found to increase this viscosity
to a peak of 0.24 ± 0.01 mN s/m. Similar to octane, the surface
tension of dodecane decreased to a value of 7.7 ± 0.4 mJ/m<sup>2</sup> at SDS concentrations above cmc. Unlike with octane, however,
the dodecane droplets had a significant surface viscosity of 0.37
± 0.01 mN s/m when only the 10 mM SDS film was present. An addition
of DAH caused a decrease in this viscosity initially, before rising
to a peak viscosity of 0.45 ± 0.01 mN s/m at a DAH concentration
of 0.15 mM. We speculate that the peaks in viscosities were the result
of the completions of a phase change associated with microcrystalline
SDS/DAH grains growing in the film at the surface of the droplets.
Fluorescence microscopy and visual observations provided further evidence
that these films can show rigid microcrystalline-like structure. Further
work done with soybean oil in the same conditions and with a lipid
film, simulating biological lipid droplets, confirmed that lipid droplets
behave rheologically similar to alkanes in the presence of these mixed
surfactant and lipid films. These results imply that droplet mechanics
may be heavily influenced by the presence of microcrystalline grains
in the oil–water systems with complex surfactant mixtures
