Molecular Dynamics Investigation of Giant Clustering in Small-Molecule Solutions: The Case of Aqueous PEHA

The importance of the formation of giant dusters in solution, in nature and industry, is increasingly recognized. However, relatively little attention has been paid to the formation of giant dusters in solutions of small, relatively soluble but nonamphiphilic molecules. In this work, we present a general methodology based on molecular dynamics that can be used to investigate such systems. As a case study, we focus on the formation of apparently stable dusters of pentaethylenehexamine (PEHA) in water. These clusters have been used as templates for the construction of bioinspired silica nanopartides. To better understand clustering in this system, we study the effect of PEHA protonation state (neutral, +1, and +2) and counterion type (chloride or acetate) on PEHA clustering in dilute aqueous solutions (200 and 400 mM) using large-scale classical molecular dynamics. We find that large stable clusters are formed by singly charged PEHA with chloride or acetate as the counterion, although it is not dear for the case with acetate whether bulk phase separation, that might lead to precipitation, would eventually occur. Large dusters also appear to be stable for doubly charged PEHA with acetate, the less soluble counterion. We attribute this behavior to a form of complex coacervation, observed here for relatively small and highly soluble molecules (PEHA + counterion) rather than the large polyions usually found to form such coacervates. We discuss whether this behavior might also be described by an effective SALR (short-range attraction, long-range repulsion) interaction. This work might help future studies of additives for the design of novel bioinspired templated nanomaterials and of giant clustering in small-molecule solutions more generally.

Automated summary

This work presents a general methodology based on molecular dynamics to investigate the formation of apparently stable clusters of pentaethylenehexamine (PEHA) in water. The study shows that large stable clusters are formed by singly charged PEHA with chloride or acetate as the counterion, and large clusters also appear to be stable for doubly charged PEHA with acetate counterion. This behavior is attributed to a form of complex coacervation, observed for relatively small and highly soluble molecules rather than the large polyions usually found in such coacervates.