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Zheng et al. Soft Sci. 2026, 6, 32 Page 31 of 57

The creation of MXene spinning solutions (or “inks”) with optimal rheological characteristics is the main
requirement for attaining effective, continuous spinning. Excellent hydrophilicity is provided by the many
-OH, -O, and other functional groups on the surface of MXene nanosheets, allowing for stable dispersion in
polar solvents such water and N,N-dimethylformamide . MXene nanosheets go through a phase transition
[144]
from isotropic to nematic liquid crystalline states when the dispersion concentration rises above a certain
threshold. The Onsager rigid rod theory, which was first developed for rigid rod-like particles and then
expanded to disc-shaped particles like MXene and GO, can quantitatively explain this behavior. According to
the theory, the free energy of the system is dominated by the repulsive volume effect, which is the volume of
space that a particle’s center is pushed from by the existence of another particle. The isotropic phase is
preferred at low concentrations because the entropy gain from random orientation exceeds the entropy loss
from repulsive volume. The repulsive volume per particle rises with concentration, increasing the entropy
cost of random orientation. When a threshold concentration is exceeded, the system partially orients
particles to increase entropy while simultaneously decreasing repulsive volume and increasing overall
entropy. There is no need for interparticle attraction because this transition is solely entropy-driven.

The aspect ratio of the particle has an inverse relationship with the critical concentration (φ ) for this phase
c
transition. The diameter-to-thickness ratio is the important aspect ratio for disc-shaped MXene sheets.
Higher aspect ratios from larger sheet sizes translate into lower φ values. This suggests that lower mass
c
concentrations can be used to reach the nematic phase [Figure 11B and C]. Nematic phase production is
difficult in real-world applications because smaller sheets necessitate much higher concentrations to
accomplish orderly packing, frequently approaching solubility limits. A logical foundation for choosing
MXene flake size in fiber spinning is provided by this theoretical framework. Larger flakes in MXene
dispersions are preferred because they can create a fully formed nematic phase at concentrations that are
appropriate for experimentation. Achieving high-quality fiber spinning and subsequent orientation control
requires this [33,139] .

The rheological conditions for spinning are supported by Onsager theory, which goes beyond phase
behavior. A nematic dispersion that is appropriate for wet spinning needs to behave like a viscoelastic gel,
which is defined by having a storage modulus (G’) that is higher than the loss modulus (G”) and being stable
over a wide frequency range (G’/G” > 1, Figure 11D and E). The ordered structures created inside the
rotating orifice are guaranteed to be maintained throughout solidification because to this mechanical
integrity. As a result, Onsager theory not only clarifies why MXene layers spontaneously organize into a
nematic phase, but it also shows how this phase transition may be used to create processable inks, which in
turn converts ordered structures at the nanoscale into macroscopic fiber characteristics.

The key to wet spinning is to use chemical bath design and fluid shear forces in concert to “freeze” and
further reinforce the ordered structures within the solution once a spinning solution with desirable liquid
crystallinity and rheology has been obtained. The microstructure and characteristics of the fibers are
determined by the solvent exchange kinetics, which are directly influenced by the composition of the
coagulation bath. For example, Zhang et al. found that MXene nanosheets instantly solidified and formed a
porous, open, loose network when pure acetic acid was used as the coagulation bath due to rapid solvent
exchange. The resultant fibers had a conductivity of about 4,048 S·cm , which was relatively low. The
-1
cross-sectional morphology of the fiber in Figure 11F illustrates how the slower exchange rate in a chitosan
solution, on the other hand, allowed the nanosheets to completely rearrange and tightly pack before
solidification, resulting in fibers with a more compact structure, smaller diameter, and significantly enhanced
conductivity (approximately 7,748 S·cm ) . Multivalent cations (such as Mg , Ca , and NH ) can be added
2+
2+
+
-1 [33]
4
to the solidification bath to further improve the mechanical and electrical characteristics of the fibers.
Through electrostatic cross-linking, these ions create strong interlayer connections and efficiently protect the