Formation of multicomponent surface nanodroplets by solvent exchange

Multicomponent surface droplets that consist of more than one compound are of great interest for fundamental studies of microwetting, evaporation, and dissolution behaviors, as well as for practical applications in high-throughput screening, microcompartmentalized chemical reactions, and microanalytics. In this work, we study the formation of multicomponent surface nanodroplets from heterogeneous nucleation and growth induced by the process of solvent exchange. In our experiments, as a solution of two oils in their good solvent was displaced by a poor solvent of the oils in the standard solvent exchange, binary droplets of oils were produced on an immersed substrate. The concentration of one oil was constant in the initial solution, whereas the other oil was increased gradually. We characterized the ratio of the two oils inside individual binary droplets by an infrared microspectrometer. Our results show that the ratio of two oils within binary nanodroplets could be varied from 0 to 100% by tuning the composition of the initial solution. However, the ratio of the two oils in the droplets did not simply correspond to that in the solution. Rather, we were able to correlate the ratio of the oils in the droplet to the oversaturation level of each oil based on the ternary phase diagram. We further demonstra te that the principle of the oversaturation level also governs the components in ternary nanodroplets formed by solvent exchange. The quantitative understanding in this work is valuable for the formation of multicomponent surface nanodroplets, which may be applied in nanoextraction, microcompartmentalized reactions, and surface functionalization.