Effects of linker flexibility on phase behavior and structure of linked colloidal gels

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 the 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.

Automated summary

This study demonstrates that the flexibility of the linker plays a key role in determining the phase behavior and structure of the colloid-linker mixture. Different flexibilities of the linker lead to different mixture behaviors, with flexible and rigid linker mixtures phase-separating at low colloid volume fractions, while semiflexible linker mixtures do not. The flexibility of the linker also influences the spacing between linked colloids, suggesting strategies for designing gels with desired properties.