Size-dependent polarizability of mesoscopic ionic clusters during assembly*

Mesoscopic clusters composed of oppositely charged particles are ubiquitous in synthetic and biological soft materials. Using coarse-grained simulations, we demonstrate that the ionic polarizability of electrically neutral ionic clusters, and hence their induced-dipole interactions, vary as charged particles aggregate into the clusters during assembly. By applying an external electrical field, we measure the ionic polarizability as a function of the cluster size, which is characterized by the number of charged particles inside the cluster. Four different models are examined, namely, clusters of binary charges, clusters of neutral polyampholytes, polyelectrolytes in a collapsed conformation, and chloroform-containing polyelectrolyte complexes. The mobility of the constituent charged particles in response to the electrical field is the root cause for the size-dependent polarizability of these mesoscopic clusters, which are fundamentally different from macroscopic objects. Importantly, the findings again emphasize that the effective interaction of the ionic clusters are hierarchical and time-dependent in nature.