The Entropic Bond in Nanoparticle and Colloidal Assemblies

Chemical approaches to the study of matter are based on the notion of bonding. Chemical bonds rely on the reconfiguration of electronic structure to bind atoms and molecules. However, many of the characteristics of bonds such as hydrogen bonds, including directionality, cooperativity, and even thermal scale, could be satisfied by “bonds” that are neither quantum mechanical nor electromagnetic in origin. We demonstrate using a simple model of hard nanoplatelets that bonding mediated solely by entropy is analogous to bonding mediated by electron density. We quantify entropic bonding in terms of local entropy density and compute bond lifetimes, finding strong similarities between the behavior of entropic and hydrogen bonds. We demonstrate how to manipulate the structure of bonded states, thereby controlling entropic valence. We find that detailed knowledge of bonding in purely entropic systems can be used to deduce structural information about systems in which assembly is driven by enthalpic interactions arising from traditional chemical bonds. By realizing minimal requirements for bonding in mesoscale systems, our results open up the possibility of classes of systems in which bond properties can be continuously manipulated and designed.