Surface-Mediated Molecular Transport of a Lipophilic Fluorescent Probe in Polydisperse Oil-in-Water Emulsions

TL;DR

This study investigates molecular transport of a lipophilic probe in oil-in-water emulsions, revealing how droplet size and surfactant type influence transport mechanisms and timescales at a molecular level.

Contribution

It introduces fluorescence lifetime microscopy combined with molecular dynamics simulations to elucidate transport mechanisms in emulsions, highlighting the impact of droplet size and surfactant type.

Findings

Transport timescales vary with droplet size and surfactant concentration.

Transport can be limited by interface crossing or diffusion depending on surfactant.

Local environment influences macroscopic transport rates.

Abstract

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.