FIGURE 3 The sodium cations in the interface regions of Au-TMA
nanoparticles. The time evolution of positive interface charge for the
P2-4, P3-3 P4-6and P6-6 systems (A). The contact interface region was
demonstrated as space within a cutoff distance of 1.5 nm around the two
Au-TMA nanoparticles. Snapshots of the detailed interface regions of the
P2-4, P3-3, P4-6and P6-6 systems (B). Whereas, the green spheres
represent sodium ions.
In the next analysis, we systematically compared the electrostatic
interactions in the aggregates by calculating the charge densities among
the four tested systems. The charge distributions were calculated by
summing the number of charges as a function of the distance from the
surfaces of the two gold cores over the last 1500 ns of MD simulations.
It is important to notice that the initial peaks of the charge density
at ~1.5 nm for the four systems correspond to the
positive charges of TMA ligands (Figure 4A). During this analysis, the
peak heights of P2-4 and P3-3systems observed are lower than those of P4-6 and
P6-6 systems which may be attributed to the smaller
sizes of P2-4 and P3-3 anions. The
smaller sizes of these anions also enabled them to partially penetrate
the coating layer of TMA ligands and cause neutralization of the
positive charges in these ligands.
As the positive charges of TMA ligands are further neutralized by the
charges of anions, we also observed valleys for the charge density of
these four systems. It can be described as charge reversal at the
interface of Au-TMA nanoparticles. A detailed examination suggests that
the valley positions are in the order of
P2-4<P3-3<P4-6<P6-6,
which can be attributed mainly to the increase in anion sizes.
Meanwhile, the depths of the valleys are in the same order and
correspond to an increase in negative charges near the surface of Au-TMA
nanoparticles. It can be inferred that the electrostatic attractions
between Au-TMA nanoparticles and surface anions are in the same order of
P2-4<P3-3<P4-6<P6-6.
Final snapshots of systems at the end of the MD simulation are
demonstrated in Figure 4B. In comparison with the P2-4and P3-3 systems, the P4-6 and
P6-6 systems showed much fewer anions in the
solutions, especially for the P6-6 system. This also
employs that the molecular anions absorbed onto the Au-TMA nanoparticles
reverse the surface charge from positive to negative and allow sodium
ions further attracted towards the surface of Au-TMA nanoparticles. It
is then followed by the appearance of second peaks representing charge
accumulations observed at ~2.2 nm. Interestingly, the
heights of the second peaks were in the similar order of
P2-4<P3-3<P4-6<P6-6.
It also consistently indicates an order of electrostatic attractions
between surface molecular anions and nearby cations.