  We report on the colloidal stability of nanoparticles with alkanethiol shells
in apolar solvents. Small angle X-ray scattering and molecular dynamics
simulations were used to characterize the interaction between nanoparticles in
linear alkane solvents ranging from hexane to hexadecane, including
\SI{4}{\nano\meter} gold cores with hexadecanethiol shells and
\SI{6}{\nano\meter} cadmium selenide cores with octadecanethiol shells. We find
that the agglomeration is enthalpically driven and that, contrary to what one
would expect from classical colloid theory, the temperature at which the
particles agglomerate increases with increasing solvent chain length. We
demonstrate that the inverted trend correlates with the temperatures at which
the ligands order in the different solvents, and show that the inversion is due
to a combination of enthalpic and entropic effects that enhance the stability
of the ordered ligand state as the solvent length increases. We also explain
why cyclohexane is a better solvent than hexane, despite having very similar
solvation parameters to hexadecane.
