  Understanding the dynamics of charge exchange between a solid surface and a
liquid is fundamental to various situations, ranging from nanofiltration to
catalysis and electrochemistry. Charge transfer is ultimately determined by
physicochemical processes (surface group dissociation, ion adsorption, etc...)
occurring in the few layers of molecules at the interface between the solid and
the liquid. Unfortunately, these processes remain largely uncharted due to the
experimental challenges in probing interfacial charge dynamics with
sufficiently high spatial and temporal resolution. Here, we resolve at the
single-charge scale, the dynamics of proton charges at the interface between an
hBN crystal and binary mixtures of water and organic amphiphilic solvents (e.g.
alcohol), evidencing a dramatic influence of solvation on interfacial dynamics.
Our observations rely on the application of spectral Single Molecule
Localization Microscopy (sSMLM) to two types of optically active defects at the
hBN surface, which act as intrinsic optical markers for both surface
protonation and interaction with apolar alkyl groups of the organic solvent. We
use sSMLM to reveal interfacial proton charge transport as a succession of
jumps between the titratable surface defects, mediated by the transport of the
solvated proton charge along the solid/liquid interface. By changing the
relative concentration of water in binary mixtures, we evidence a non-trivial
effect on interfacial proton charge dynamics, leading at intermediate water
concentration to an increased affinity of the proton charge to the solid
surface, accompanied by an increased surface diffusivity. These measurements
confirm the strong role of solvation on interfacial proton charge transport and
establish the potential of single-molecule localization techniques to probe a
wide range of dynamic processes at solid/liquid interfaces.
