Cationic and anionic AuNPs were modeled with amine
(NH3
+) and carboxyl (COO−) terminal groups and Na+/Cl−
counterions. For the two systems, RDFs (Figure 3) were found
to be rather similar: The side chains and terminal groups
showed significant flexibility and the water/counterion profiles
had the same characteristics. However, the distance distributions
of terminal groups (Figure 4, Supporting Information)
showed that the NH3
+-terminated alkyl thiols displayed a wider
range of distances (fluctuations with respect to each other), and
the atomic configurations (water/counterions) were significantly
different around the NH3
+ and COO− terminal groups.
The orientation of water was observed to be distinct for both
AuNPs in the first solvation shell, and the AuNPs clearly caused
a long-range effect in the solvent structure. This effect was
particularly strong for counterions, emphasizing the importance
of long-range interactions (electrostatics) in the present system.
The radial electrostatic potential profiles (Figures 4 and 5)
displayed a minimum for AuNP− at about 2.0 nm from the