THE INFLUENCE OF CONCENTRATION AND IONIC-STRENGTH ON THE CLUSTER STRUCTURE OF HIGHLY-CHARGED ELECTROLYTE-SOLUTIONS

The structure of 2:2 electrolyte solutions at several concentrations-obtained through stochastic dynamics simulations-is investigated by means of a cluster analysis. It is shown that most of the ions in the systems at the higher concentrations studied (up to 2 M) are linked at short distances, forming large clusters, even though the mean number of neighbours is only about two. As the concentration decreases there is a distinct short and long-range behaviour. The clustering observed at short distances is similar to that obtained in the more concentrated solutions. This means that within this region the charges of the nearby ions are individually perceived. Conversely, at long distances, the effect of the surrounding charges becomes more diffuse; the properties seem to be rather dependent on the mean field and are similar to those of a 1:1 electrolyte at the same ionic strength. Although no neutral pairs seem to be present in the more dilute solution investigated in this paper (0.25 M), the concentration dependence of our results suggests that the Bjerrum picture (and recent theories of the ionic association) should accurately describe the clustering at lower concentrations.