Molecular Characterization of the Surface Excess Charge Layer in Droplets

Charged droplets play a central role in native mass spectrometry, atmospheric aerosols and in serving as micro-reactors for accelerating chemical reactions. The surface excess charge layer in droplets has often been associated with distinct chemistry. Using molecular simulations we have found that this layer is ≈ 1.5−1.7 nm thick and depending on the droplet size it includes 33%-55% of the total number of ions. Here, we examine the effect of droplet size and sign of ions in the structure of the surface excess charge layer by using molecular dynamics. We find that the thickness of the surface excess charge layer is invariant not only with respect to droplet size but also with respect to the nature of the ions and it is not sensitive to fine details of different force fields used in our simulations. We also find that differences in the average water dipole orientation in the presence of positive and negative ions in this layer are reflected in the charge distributions. Within the surface charge layer, the number of hydrogen bonds reduces gradually relative to the droplet interior where the number of hydrogen bonds is on the average 2.9 for droplets of diameter < 4 nm and 3.5 for larger droplets. The decrease in the number of hydrogen bonds from the interior to the surface is less pronounced in larger droplets. In droplets with diameter < 4 nm and high concentration of ions the charge of the ions is not compensated only by the solvent polarization charge but by the total charge that also includes the other free charge. This finding shows exceptions to the commonly made assumption that the solvent “neutralizes” the charge of the ions in solvents with very high dielectric constant. The simulation findings provide molecular insight into the bi-layer droplet structure assumed in the equilibrium partitioning model of C. Enke.