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Page 26 of 35 Zhang et al. Chem Synth 2023;3:10 https://dx.doi.org/10.20517/cs.2022.40

particles move directionally in an ordered manner, and the driving force to form this state is the attraction-
repulsion balance (i.e., the lowest point of the potential energy) . To achieve orderly motion, the attractive
[173]
potential (E) must be greater than the thermal noise (k T), i.e., E > k T . For particles, the attractive
[174]
B
B
potential (E) can be described using the potential-distance curve [Figure 25]. As the particles approach each
other, the attractive potential gradually decreases. When the distance is shortened to a certain value, the
attractive potential reaches a minimum E , and the attraction between the particles is at a maximum. As the
0
distance continues to decrease, the attractive potential gradually increases and eventually becomes positive,
indicating that the particles begin to exhibit a repulsive force. According to Jiang’s work , the absolute
[174]
value of E is approximately equal to the thermal noise at the critical temperature (T ) of the particles,
C
0
i.e., |E | ~ k T . Therefore, to achieve He superfluidity, due to the low critical temperature of helium (5.20
4
0
B C
K), the temperature must be reduced low enough (< 2.17 K ) to shorten the distance between the He atoms
to ensure a high attractive potential (|E| > k T).
B
As mentioned previously, the attractive potential (E) must be greater than the thermal noise (k T) for a
B
superfluid to be formed. In addition to reducing the temperature required to reach this condition,
superfluidity can be achieved through the space-confinement effect. On one hand, the confined space can
reduce the degree of disorder of the particles; on the other hand, the confined space can help shorten the
particle distance and make it easier to reach the lowest point of the potential energy (E ). Therefore, some
0
molecules (such as water molecules) can exhibit superfluidity at room temperature in confined spaces . As
[174]
shown in Figure 26, when the channel size (D) of the confined space is four times the vdW equilibrium
distance (d ), superfluidity occurs in the middle of the channel. As the channel size decreases to ~2 d , all the
0
0
molecules in the channel enter the lowest potential energy point, inducing molecular superfluidity.
Evidence of water molecule superfluidity has been found in artificial nanochannels. Wu et al. studied the
transport of water molecules in carbon nanotubes (CNTs) and found that their flux was seven orders of
magnitude larger than that of bulk water . Molecular dynamics (MD) simulation showed that with a
[175]
decrease in the size of the CNT channel (from 4.99 to 1.66 nm), the rate of water molecule transport
increased significantly, and the proportion of ordered water molecules in the channel also increased
gradually, indicating ordered fast transport of the water molecules. For psaAWH, the fast water transport
behavior in the micropores shown by some materials may be related to superfluidity. For example, in the
nanoconfined channel of CPOS-6, water in the weak adsorption layer undergoes weak hydrogen bonding
interaction with the strong adsorption layer, ensuring that the channel has a moderate affinity to help
reduce the disorder of the water molecules [Figure 27A] . Consequently, it is possible to achieve rapid and
[29]
orderly transport of the water molecules in the channel, leading to a fast kinetic process. Based on
theoretical simulations, Song et al. controlled the hydrophilic group content and pore size in porous
carbon . In the theoretical calculations in this study, for a certain density of hydrophilic groups in porous
[151]
carbon, the diffusion energy barrier of the water molecules increased with an increase in the number of
hydrophilic groups [Figure 27B]. This may be due to the increased hydrophilicity, which strengthens the
interaction between the pore and water molecules, thus destroying the orderly movement of the water
molecules, thereby increasing the transmission energy barrier. From the above discussion, we can
reasonably infer that water molecule superfluidity can exist in nanoconfined channels, thus promoting the
rapid transport of water molecules. The formation of a superfluid is a feasible strategy for improving the
intracrystalline diffusion of the psaAWH adsorbent.

CONCLUSION AND OUTLOOK
In this review, we introduced the working principles and general processes of psaAWH. Compared with
other freshwater production technologies, psaAWH has the advantages of a wide application range and no