New gel formation mechanism discovered

Colloidal suspensions are important in a range of industries; in fundamental research, for example, they are accessible systems for studying gel transitions. Scientists report in The Journal of Chemical Physics a surprising new mechanism of gel formation in a mixture of colloidal species with modest size ratio. Specifically, they report gel formation in octadecyl-coated silica nanocolloids suspended in decalin, a marginal solvent for the octadecyl chains.

At elevated temperatures the hydrocarbon chains stabilize the colloids in a liquid state, but when cooled the chains collapse, creating an attraction between nanoparticles. When sufficiently strong, this causes gel formation. Rheometry showed that mixing two nanoparticles where one is twice the size of the other resulted in gel formation at much weaker attractions than with particle mixtures of equal size.

The dynamic scattering of X-ray photon correlation spectroscopy permitted tracking of the nanoscale motion in the gel phase. The authors were surprised to find a behavior distinct from depletion-induced gels that form in larger size ratio mixtures. Introducing the short-range attraction drove de-mixing of the larger and smaller particles. However, the two species only partially separated, as the smaller particles became dynamically arrested in a system-spanning network around which the larger nanoparticles continued to move freely. This phenomenon was reproduced using molecular dynamic simulations.

The large response introduced by this subtle microscopic change suggests that small alterations in colloidal characteristics, such as interparticle attraction and size distribution, can be a powerful route to tailor the mechanical structure and flow behavior of nanostructured colloidal solids. “We believe this mechanism can be exploited in technology and might explain observations in other mixtures in nature, such as proteins,” said co-author Robert Leheny.

Source: “Enhanced gel formation in binary mixtures of nanocolloids with short-range attraction,” by James L. Harden, Hongyu Guo, Martine Bertrand, Tyler N. Shendruk, Subramanian Ramakrishnan, and Robert L. Leheny, Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5007038 .