  Inorganic nanoparticle cores are often coated with organic ligands to render
them dispersible in apolar solvents. However, the effect of the ligand shell on
the colloidal stability of the overall hybrid particle is not fully understood.
In particular, it is not known how the length of an apolar alkyl ligand chain
affects the stability of a nanoparticle dispersion against agglomeration. Here,
Small-Angle X-ray Scattering and molecular dynamics simulations have been used
to study the interactions between gold nanoparticles and between cadmium
selenide nanoparticles passivated by alkanethiol ligands with 12 to 18 carbons
in the solvent decane. We find that increasing the ligand length increases
colloidal stability in the core-dominated regime but decreases it in the
ligand-dominated regime. This unexpected inversion is connected to the
transition from ligand- to core-dominated agglomeration when the core diameter
increases at constant ligand length. Our results provide a microscopic picture
of the forces that determine the colloidal stability of apolar nanoparticles
and explain why classical colloid theory fails.
