Surface Tension Measurements Reveal Charge-Driven Surfactant Depletion in Microdroplets Approaching the Rayleigh Limit

Abstract

The net charge on droplets is predicted to play a crucial role in accelerating chemical reactions in microdroplets by either altering reaction energetics or changing surface compositions. However, there are few experimental studies that have explored how the net charge of microdroplets alters their physicochemical properties. Here, we present a new technique to investigate how net charges on levitated microdroplets affect their surface tensions and the partitioning of surfactants to the air-water interface. The technique is validated by measuring how the resonant surface oscillation frequency (ω_obs) of microdroplets changes with increasing net charge. For simple one- and two-component microdroplets, ω_obs predictably decreases with charge. This enables the determination of surface tension of microdroplets (6-20 μm radius) with net charges approaching the Rayleigh limit (QR), which represents the maximum net charge a droplet can stably carry before Coulombic fission. To demonstrate how this new technique can be used to measure charge-driven changes in surface composition, we measured the dependence of surface tension on charge for microdroplets containing cetrimonium bromide (CTAB). For ~8-μm-radius microdroplets containing 0.5 mM CTAB, the surface tension was observed to increase from ~50 to ~65 mN/m as net positive charge on droplets increases from low charge to QR. A similarly-sized change in surface tension is not observed in negatively charged droplets nor droplets with CTAB concentrations larger than the critical micelle concentration. These initial results demonstrate that the net charge on droplets can alter the equilibrium partitioning of charged surfactants to the air-water interface. Ultimately, the new approach to measure the surface tensions of highly-charged microdroplets enables investigations into how charge alters interfacial concentrations of molecules to promote microdroplet chemistry.