  Solvation of gold nanoparticles passivated with end group functionalized
alkylthiols, namely CH$_3$, NH$_2$ or COOH is studied in solvents of varying
degrees of repulsion-dispersion and electrostatic interactions ranging from
strongly polar SPC/E water to modified hybrid water models, where the
Lennard-Jones contribution to the potential energy is enhanced relative to
SPC/E to completely non-polar, decane. The effects due to solvent
reorganization around the nanoparticle as a function of the ligand and solvent
chemistry are monitored using the nanoparticle-solvent pair correlation
functions and tetrahedral order parameter. The solvent penetration inside the
ligand shell is maximum for decane, indicating better solute-solvent
interaction in decane compared to other solvents. The COOH end group
functionalized nanoparticle breaks the tetrahedral structure of water molecules
more as compared to other nanoparticles used in this study. The ligand
reorganization and its effect on solvation are monitored using radial density
profiles (RDPs), radius of gyration ($R_g$) and ligand asymmetry parameter
($\langle \Delta \rangle$). RDP and $R_g$ values show significant stretching of
ligands in decane than in model waters, which is also consistent with $\langle
\Delta \rangle$. The ligand shell anisotropy for all nanoparticles is maximum
in SPC/E water and minimum in decane. The isotropic potential of mean force
($V_{PMF}(r)$) between two identical end group functionalized nanoparticles
have been calculated in vacuum, SPC/E water and H3.00 modified hybrid water,
which consistently shows attractive well depth. Distance-dependent fluctuation
driven anisotropy has also been examined. The implications for self-assembly of
passivated gold nanoparticles from aqueous dispersions as well as the
dependence of calculated quantities on ligand and solvent chemistry are
highlighted.
