  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 towards the dispersed phase in polydisperse oil-in-water
emulsions. We show that this transport comprises two timescales, 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.
