(158e) The Effects of Ionic Strength on Rheological Properties and Microstructural Changes in Aqueous Mxene (Ti3C2Tx) Dispersions

This work examines the effects of salt addition and concentration on the rheological properties and phase transitions of aqueous MXene (Ti₃C₂T_x) dispersions. MXenes are a relatively new family of two-dimensional (2D) nanomaterials comprised of transition metal carbides and nitrides. Their exceptional electrical, electrochemical, and thermal properties have made them a desirable candidate for fluid phase processing of advanced devices. Understanding nanomaterial rheology and phase behavior is of practical relevance for the improvement of fluid phase manufacturing processes such as direct ink writing. Dispersions of large (~3 μm) MXenes and a bimodal mixture of large and small (~0.3 μm) MXenes were investigated over a range of concentrations. Sodium chloride (NaCl) was added to specific concentrations to investigate the effects of increasing ionic strength on dispersion behavior and properties. Steady shear and small amplitude oscillatory shear rheology were used to understand the change in viscoelastic properties and flow behavior of the dispersions as a function of MXene concentration and ionic strength. Cross-polarized optical microscopy (POM) was utilized to observe changes in microstructure and dispersion morphology as MXene and salt concentrations were increased. Without the addition of NaCl, the large MXene dispersions readily arranged into extended aligned structures, a feature that typically advances fluid phase processability. Bimodal MXene dispersions also formed extended aligned structures, though a higher concentration was needed to achieve similar results to the large MXene dispersions. POM has shown that birefringence is observable in both large MXenes and bimodal MXenes, suggesting that the liquid crystalline phase can be achieved in these additive-free dispersions. The addition of a low concentration of NaCl resulted in an accelerated transition to a gel-like phase for large MXene dispersions, with a similar phase behavior change in bimodal MXene dispersions. Further increasing NaCl concentration caused severe aggregation and discontinuity of microstructure in both large and bimodal MXene dispersions. These results provided a deeper understanding of a new 2D nanomaterial/salt system and the parameters affecting dispersion tunability. The insights achieved from this work provide a foundation for further optimization of MXene printing inks and devices.