Fig. 2. For a generic bimolecular reaction, several chemical–physical processes can be responsible for accelerated reaction rates in ESI/ESSI micro- and nano-droplets compared to the same reaction conducted in bulk solutions. (a) Solvent evaporation and the resulting increased concentration of reagents; (b) partial solvation at the surface and consequent k_surface > k_bulk; (c) physical confinement of reagents at the air–droplet interface and increased concentration at the surface; (d) effects of charge and strong electric fields, together with ion evaporation and gas-phase reactivity. The factors affected by each possible acceleration mechanism are indicated in red.