Two-dimensional Ti₃C₂Tₓ MXene combines metallic conductivity with solution processability, making it a promising building block for fibres, films, and electrodes whose performance depends on how well the nanosheets are aligned.¹,² Because the flakes are anisotropic charged platelets, they can be coaxed into a lyotropic liquid-crystalline (LC) state in which orientational order develops spontaneously above a threshold concentration, offering a scalable route to alignment without external fields.³,⁴ The onset of this order is set by excluded-volume interactions between plate-like particles, as formalised by Onsager, so that the effective aspect ratio and number density of the ordering unit should dictate the degree of alignment.⁵ Yet for MXene the practical factors that govern the magnitude of order remain poorly defined.
Using combined small- and wide-angle X-ray scattering, we quantify the nematic order parameter (S), the stack-to-stack spacing, and the (002) interlayer stacking of Ti₃C₂Tₓ across concentration, flake aspect ratio, polydispersity, and dispersing solvent. Order grows monotonically with concentration, S rising from 0.25 to 0.63 for large (~1 µm) flakes between 10 and 60 mg/mL. Small (~300 nm) flakes remain isotropic up to 30 mg/mL and reach only 0.58 at 60 mg/mL, confirming that the isotropic-to-nematic threshold is governed by aspect ratio, exactly as the Onsager excluded-volume picture predicts.⁵ Above the transition, a stack-to-stack correlation develops whose spacing contracts with increasing concentration for both flake sizes.
Polydispersity is an equally important control. An initial aqueous wash preferentially depletes the smallest flakes, narrowing the size distribution and thereby sharpening the nematic order, consistent with the disproportionate weight that low-aspect-ratio platelets carry in suppressing alignment. Solvent choice matters in turn: exchanging water for DMSO swells the (002) interlayer spacing from 16.4 to 21.9 Å and exfoliates the flakes toward single sheets. Because a single sheet carries twice the aspect ratio and twice the number density of a two-sheet stack at fixed mass, the solvent, not concentration alone, sets the stacking state and hence the achievable order.
Together, concentration, aspect ratio, polydispersity, and solvent chemistry emerge as the primary controls on liquid-crystalline order in Ti₃C₂Tₓ, each acting through the same excluded-volume mechanism. These results provide quantitative design rules for processing MXene dispersions into aligned, high-performance fibres and films.⁴,⁶