Abstract
Confined water inside carbon nanotubes (CNTs) has attracted a lot of attention in recent years, amassing as a result a very large number of dedicated studies, both theoretical and experimental. This exceptional scientific interest can be understood in terms of the exotic properties of nanoconfined water, as well as the vast array of possible applications of CNTs in a wide range of fields stretching from geology to medicine and biology. This review presents an overreaching narrative of the properties of water in CNTs, based mostly on results from systematic nuclear magnetic resonance (NMR) and molecular dynamics (MD) studies, which together allow the untangling and explanation of many seemingly contradictory results present in the literature. Further, we identify still-debatable issues and open problems, as well as avenues for future studies, both theoretical and experimental.
Highlights
We identify seven broad areas that together form all main aspects of water in carbon nanotubes (CNTs): First, there is a fundamental question of whether water can even enter the CNT channels at all, given that the latter are made of hydrophobic graphene sheets and their nano sizes are expected to reduce the entropy of the uptake process
Turning first to the adequately understood aspects of water’s nature inside carbon nanotubes, it is evident that despite the CNTs’ hydrophobic and restrictive nature, water readily enters the CNTs under ambient conditions, as it does for a wide range of temperature and pressure
For small CNTs with diameter d < 1.0 nm, water forms a single molecular chain near the center of the CNT and diffuses in a single-file fashion due to lack of space. This has been verified with several complementary studies, theoretical and experimental. This single water component results in a lone nuclear magnetic resonance (NMR) peak, which can be readily distinguished from outside water, based on the fact that internal water freezes at a much lower temperature than its bulk counterpart, regardless of the CNT diameter
Summary
Upon water entering the CNTs, one can argue that all its properties deviate from the bulk mainly due to the hydrophobic interactions with the carbon atoms and the effect of nanoconfinement As it is already mentioned, the CNT walls made of pure graphene (i.e., with negligible defects and impurities) exert hydrophobic forces on the water molecules, which generally cause the formation of a depletion layer close to the walls [74], a number of concentric water tubes towards the center of the CNT, plus possibly a chain of stratified water molecules at the very center, depending on the diameter of the CNT [90].
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