  We study the solvation and electrostatic properties of bare gold (Au)
nanoparticles (NPs) of $1$-$2$ nm in size in aqueous electrolyte solutions of
sodium salts of various anions with large physicochemical diversity (Cl$^-$,
BF$_4$$^-$, PF$_6$$^-$, Nip$^-$(nitrophenolate), 3- and 4-valent
hexacyanoferrate (HCF)) using nonpolarizable, classical molecular dynamics
computer simulations. We find a substantial facet selectivity in the adsorption
structure and spatial distribution of the ions at the Au-NPs: while sodium and
some of the anions (e.g., Cl$^-$, HCF$^{3-}$) adsorb more at the `edgy' (100)
and (110) facets of the NPs, where the water hydration structure is more
disordered, other ions (e.g., BF$_4$$^-$, PF$_6$$^-$, Nip$^-$) prefer to adsorb
strongly on the extended and rather flat (111) facets. In particular, Nip$^-$,
which features an aromatic ring in its chemical structure, adsorbs strongly and
perturbs the first water monolayer structure on the NP (111) facets
substantially. Moreover, we calculate adsorptions, radially-resolved
electrostatic potentials, as well as the far-field effective electrostatic
surface charges and potentials by mapping the long-range decay of the
calculated electrostatic potential distribution onto the standard
Debye-H\"uckel form. We show how the extrapolation of these values to other
ionic strengths can be performed by an analytical Adsorption-Grahame relation
between effective surface charge and potential. We find for all salts negative
effective surface potentials in the range from $-10$ mV for NaCl down to about
$-80$ mV for NaNip, consistent with typical experimental ranges for the
zeta-potential. We discuss how these values depend on the surface definition
and compare them to the explicitly calculated electrostatic potentials near the
NP surface, which are highly oscillatory in the $\pm 0.5$ V range.

Chemical Physics;Computational Physics