Confined water inside single-walled carbon nanotubes: Global phase diagram and effect of finite length

Abstract

Studies on confined water are important not only from the viewpoint of scientific interest but also for the development of new nanoscale devices. In this work, we aimed to clarify the properties of confined water in the cylindrical pores of single-walled carbon nanotubes (SWCNTs) that had diameters in the range of 1.46 to 2.40 nm. A combination of x-ray diffraction (XRD), nuclear magnetic resonance, and electrical resistance measurements revealed that water inside SWCNTs with diameters between 1.68 and 2.40 nm undergoes a wet-dry type transition with the lowering of temperature; below the transition temperature T(wd), water was ejected from the SWCNTs. T(wd) increased with increasing SWCNT diameter D. For the SWCNTs with D = 1.68, 2.00, 2.18, and 2.40 nm, T(wd) obtained by the XRD measurements were 218, 225, 236, and 237 K, respectively. We performed a systematic study on finite length SWCNT systems using classical molecular dynamics calculations to clarify the effect of open ends of the SWCNTs and water content on the water structure. It was found that ice structures that were formed at low temperatures were strongly affected by the bore diameter, a = D - σ(OC), where σ(OC) is gap distance between the SWCNT and oxygen atom in water, and the number of water molecules in the system. In small pores (a < 1.02 nm), tubule ices or the so-called ice nanotubes (ice NTs) were formed irrespective of the water content. On the other hand, in larger pores (a > 1.10 nm) with small water content, filled water clusters were formed leaving some empty space in the SWCNT pore, which grew to fill the pore with increasing water content. For pores with sizes in between these two regimes (1.02 < a < 1.10 nm), tubule ice also appeared with small water content and grew with increasing water content. However, once the tubule ice filled the entire SWCNT pore, further increase in the water content resulted in encapsulation of the additional water molecules inside the tubule ice. Corresponding XRD measurements on SWCNTs with a mean diameter of 1.46 nm strongly suggested the presence of such a filled structure.

Solid−liquid critical behavior of water in nanopores

“…The temperature T and the internal axial pressure P zz (the pressure tensor parallel to the z axis) are controlled using modified Nose-Andersen's method (40). The intermolecular interaction of water is taken to be the TIP4P model (41): Rhe melting temperatures of the model water in carbon nanotubes are in good agreement with experimental results (11,42,43). The interaction between a water molecule and the cylindrical wall is described by the Lennard-Jones potential integrated with respect to the positions of carbon atoms over the cylindrical surface with an assumption of uniform distribution of carbon atoms (1).…”

Diffusion enhancement in core-softened fluid confined in nanotubes

“…In addition, confinement in nanopores and nanotubes have been used also to avoid spontaneous water crystallization below the melting point in an attempt to observe its hypothetical second critical point [32][33][34][35][36] . Simulations employing some of the discussed molecular models for water, namely SPC/E, TIP4P-EW and ST2, confined in nanoscale channels exhibit two complementary effects: the melting temperature of the fluid at the center of the channel decreases and water crystallizes at the channel surface [37][38][39][40][41] . In addition to these thermodynamic properties, the mobility properties of confined water also exhibit an unusual behavior.…”

Exohedral Physisorption of Ambient Moisture Scales Non-monotonically with Fiber Proximity in Aligned Carbon Nanotube Arrays

“…[31,32] By forming clusters of 5 or more water molecules (cluster size of ∼ 1 nm), [31,[33][34][35]] the affinity of the water molecules can be transformed from hydrophillic to hydrophobic, [33] making their interaction with the CNT walls more favorable. While many previous studies have explored the mechanics, kinetics, and energetics of the entry of water molecules into uncapped single walled CNTs, [30,[36][37][38] the likelihood that a water molecule can enter the 3 inner region of a capped multiwalled CNT via wall defects is low (since the openings in the CNT walls are likely smaller than the cluster size), meaning that the exohedral physisorption of water is expected. Wall defects may also lead to the following: higher CNT surface energies [39], which lead to stronger interactions with water molecules [40,41] and enable the separation of salt ions from solution [42]; altered electronic properties, [43][44][45][46] which can lead to a CNT behaving as either a metal or semi-conductor, [43] and can also lead to reversible wetting and de-wetting of water in nanopores.…”