Thermodynamics and Modeling of Collective Effects in the Organic Ligand Shell of Colloidal Quantum Dots

Colloidal nanocrystals are an interesting platform for studying the surface chemistry of materials due to their high surface area/volume ratios, which results in a large fraction of surface atoms. As synthesized, the surfaces of many colloidal nanocrystals are capped by organic ligands that help control their size and shape. While these organic ligands are necessary in synthesis, it is often desirable to replace them with other molecules to enhance their properties or to integrate them into devices. Traditionally, these ligand exchanges have been studied using ¹H NMR. Recently, isothermal titration calorimetry has proven itself to be a highly versatile measurement technique, yielding insights into the thermodynamics of the reaction, including the enthalpy and entropy of the reaction, that are inaccessible via ¹H NMR. The most common technique for analyzing ligand exchange reactions has been to model these data with one-site and two-site Langmuir isotherm models. Unfortunately, a detailed analysis of ¹H NMR and isothermal titration calorimetry data simultaneously demonstrates that these simple models are inadequate for understanding ligand reactions on the surfaces of colloidal nanocrystals.