Dual Controlling Factors for Nonmonotonic Viscosity of Cyrene-Water Mixtures

The viscosity of organic solvents governs the migration and mass transport of ions and molecules, thereby exerting a critical influence on the performance of various materials. Unlike polymers, whose viscosity is mainly determined by chain entanglement, small-molecule solutions are dominated by a single factor of intermolecular friction, leading to the expectation of a linear relationship between viscosity and concentration. However, increasing evidence indicates a pronounced nonlinear relationship between viscosity and concentration in aqueous small-molecule solutions. Herein, using Cyrene as a model, our experimental measurements reveal that there is a distinct viscosity peak as a function of Cyrene concentration. Combined with molecular dynamics simulations, we found that two mechanisms jointly govern this nonmonotonic trend: (i) diffusion resistance controls molecular mobility, and (ii) cohesive energy controls intermolecular friction. Prior to the peak (<80 wt % Cyrene), the viscosity is primarily governed by diffusion resistance, in which H₂O forms a hydrogen-bond network that envelops Cyrene, enabling rapid cooperative motion. As H₂O is replaced with Cyrene, the system exhibits a higher diffusion resistance. Overall, the transport is hindered, and the viscosity increases continuously. As the peak is approached, the hydrogen-bond network formed by water is disrupted, and the diffusion resistance reaches a plateau. Consequently, the dominant mechanism transitions from diffusion resistance to cohesive energy. The interactions between the Cyrene molecules cannot compensate for the rapid loss of H₂O-H₂O hydrogen bonds, leading to an overall decrease in cohesive energy and ultimately driving the viscosity decline. This trend can be generalized across various organic solvents, guiding material optimization for mass transport performance.