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

TL;DR

This study investigates how linker flexibility affects the phase behavior and structure of colloidal gels, revealing that linker stiffness influences gelation and the prevalence of loop formations, which impacts the material's properties.

Contribution

The paper introduces a toy model demonstrating the significant role of linker flexibility in colloidal gel formation and structure, highlighting the impact of linker stiffness on phase separation.

Findings

Flexible and rigid linkers cause phase separation at low colloid concentrations.

Semiflexible linkers do not induce phase separation, unlike flexible and rigid ones.

Linker flexibility affects colloid spacing, informing gel design strategies.

Abstract

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.