  Fundamental studies of the effect of specific ions on hydrophobic
interactions are driven by the need to understand phenomena such as
hydrophobically driven self-assembly or protein folding. Using
beta-peptide-inspired nano-rods, we investigate the effects of both free ions
(dissolved salts) and proximally immobilized ions on hydrophobic interactions.
We find that the free ion effect is correlated with the water density
fluctuation near a non-polar molecular surface, showing that such fluctuation
can be an indicator of hydrophobic interactions in the case of solution
additives. In the case of immobilized ion, our results demonstrate that
hydrophobic interactions can be switched on and off by choosing different
spatial arrangements of proximal ions on a nano-rod. For globally amphiphilic
nano-rods, we find that the magnitude of the interaction can be further tuned
using proximal ions with varying ionic sizes. In general, univalent proximal
anions are found to weaken hydrophobic interactions. This is in contrast to the
effect of free ions, which according to our simulations strengthen hydrophobic
interactions. In addition, immobilized anions of increasing ionic size do not
follow the same ordering (Hofmeister-like ranking) as free ions when it comes
to their impact on hydrophobic interactions. The immobilized ion effect is not
simply correlated with the water density fluctuation near the non-polar side of
the amphiphilic nano-rod. We propose a molecular picture that explains the
contrasting effects of immobilized versus free ions.
