In Situ Monitoring of Droplet Behavior in Inverse Microemulsions

Thermoresponsive polymer assemblies are of growing interest in fields ranging from photonics to drug delivery, with their phase transitions often attributed to upper- or lower-critical solution temperatures and cloud-point behaviors. However, the direct imaging of these nanoscale transitions remains underexplored. This study addresses that gap by developing a temperature-sensitive inverse microemulsion system and elucidating its dynamic structural transitions under heating. We present a temperature-sensitive inverse microemulsion system composed of the nonionic surfactants Brij 010 and Span 80. Upon heating within a stable microemulsion temperature range, the decrease in hydrogen bonding between the hydrophilic surfactant head and the dispersed phase results in an initial droplet contraction. Above a critical destabilization temperature, the droplets expand and destabilize as the affinity of the surfactant for the continuous phase increases. This intriguing behavior was observed via dynamic light scattering and liquid-phase transmission electron microscopy, which revealed a rapid and reversible droplet transformation during heating cycles. This versatile inverse microemulsion system also serves as a modular nanoreactor for polymerizations, demonstrated through both conventional radical and photoiniferter polymerization. Our research contributes to the understanding of inverse microemulsions, which offer a platform for precise nanoparticle synthesis.