  Binary compositions of surface ligands are known to improve the colloidal
stability and fluorescence quantum yield of nanocrystals (NCs), due to
ligand-ligand interactions and surface organization. Herein, we follow the
thermodynamics of a ligand exchange reaction of CdSe NCs with alkylthiols
mixtures. The effects of ligand polarity and length difference on ligand
packing were investigated using isothermal titration calorimetry (ITC). The
thermodynamic signature of the formation of mixed ligand shells was observed.
Correlating the experimental results with thermodynamic mixing models has
allowed us to calculate the inter-chain interactions and infer the final ligand
shell configuration. Our findings demonstrate that the small dimensions of the
NCs and the subsequent increased interfacial region between dissimilar ligands,
in contrast to macroscopic surfaces, allow the formation of a myriad of
clustering patterns, controlled by the inter-ligand interactions. This work
provides a fundamental understanding of the parameters determining the ligand
shell structure and should help guide smart surface design toward NC-based
applications.
