What measurable physical properties allow one to distinguish surfactant-stabilised from Pickering emulsions? Whereas surfactants influence oil/water interfaces by lowering the oil/water interfacial tension, particles are assumed to have little effect...
Emulsions often act as carriers for water-insoluble solutes
that
are delivered to a specific target. The molecular transport of solutes
in emulsions can be facilitated by surfactants and is often limited
by diffusion through the continuous phase. We here investigate this
transport on a molecular scale by using a lipophilic molecular rotor
as a proxy for solutes. Using fluorescence lifetime microscopy we
track the transport of these molecules from the continuous phase toward
the dispersed phase in polydisperse oil-in-water emulsions. We show
that this transport comprises two time scales, which vary significantly
with droplet size and surfactant concentration, and, depending on
the type of surfactant used, can be limited either by transport across
the oil–water interface or by diffusion through the continuous
phase. By studying the time-resolved fluorescence of the fluorophore,
accompanied by molecular dynamics simulations, we demonstrate how
the rate of transport observed on a macroscopic scale can be explained
in terms of the local environment that the probe molecules are exposed
to.
Pea protein isolate (Pisum sativum L., PPI) has been much studied in the last decade because of its potential as a bio-based alternative for surfactants to produce innovative and environmentally friendly emulsion products. PPI is ideal due to its favorable nutritional properties, low allergenicity and low environmental impact. Despite its growing popularity, understanding the stabilisation mechanism of emulsions stabilized with PPI remains a key question that requires further investigation. Here, we use fluorescence lifetime microscopy with molecular rotors as local probes for interfacial viscosity of PPI stabilized emulsions. The fluorescence lifetime correlates to the local viscosity at the oil-water interface allowing us to probe the proteins at the interfacial region. We find that the measured interfacial viscosity is strongly pH-dependent, an observation that can be directly related to PPI aggregation and PPI reconformation. By means of molecular rotor measurements we can link the local viscosity of the PPI particles at the interface to the Pickering-like stabilisation mechanism. Finally, this can be compared to the local viscosity of PPI solutions at different pH conditions, showing the importance of the PPI treatment prior to emulsification.
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