  Colloidal nanocrystal gels can be assembled using a difunctional "linker"
molecule to mediate bonding between nanocrystals. The conditions for gelation
and the structure of the gel are controlled macroscopically by the linker
concentration and microscopically by the linker's molecular characteristics.
Here, we demonstrate using a toy model for a colloid-linker mixture that linker
flexibility plays a key role in determining both phase behavior and structure
of the mixture. We fix the linker length and systematically vary its bending
stiffness to span the flexible, semiflexible, and rigid regimes. At fixed
linker concentration, flexible-linker and rigid-linker mixtures phase separate
at low colloid volume fractions in agreement with predictions of first-order
thermodynamic perturbation theory, but the semiflexible-linker mixtures do not.
We correlate and attribute this qualitatively different behavior to undesirable
"loop" linking motifs that are predicted to be more prevalent for linkers with
end-to-end distances commensurate with the locations of chemical bonding sites
on the colloids. Linker flexibility also influences the spacing between linked
colloids, suggesting strategies to design gels with desired phase behavior,
structure, and by extension, structure-dependent properties.
