Unexpectedly High Salt Accumulation inside Carbon Nanotubes Soaked in Dilute Salt Solutions

We experimentally demonstrate the formation of salt aggregations with unexpectedly high concentration inside multiwalled carbon nanotubes (CNTs) soaked only in dilute salt solution sand even in solutions containing only traces of salts. This finding suggests the blocking of fluid across CNTs by the salt aggregations when CNTs are soaked in a dilute salt solution with the concentration of seawater or even lower, which may open new avenues for the development of novel CNT-based desalination techniques. The high salt accumulation of CNTs also provides a new CNT-based strategy for the collection or extraction of noble metal salts in solutions containing traces of noble metal salts. Theoretical analyses reveal that this high salt accumulation inside CNTs can be mainly attributed to the strong hydrated cation-$\pi$ interactions of hydrated cations and $\pi$ electrons in the aromatic rings of CNTs.

Figure 1

(a) HAADF-STEM image of the NaCl aggregation in a MWCNT soaked in $0.034M$ NaCl solution. (b) C, O, Na, and Cl element mappings of the square frame marked in (a) are displayed in (i) and (ii), respectively. (c) Proportion of Na, Cl, and O elements in salt aggregations inside different MWCNTs for eight series of experiments. (d) $\mathrm{Na}/\mathrm{O}$ ratios in the salt aggregations as a function of the concentration of the soaked NaCl solution outside the MWCNTs. Here, the concentrations corresponding to a normal saline and seawater are marked by pink and blue rectangles, respectively. The red triangles are the average ratios for several experiments at different concentrations.

Figure 2

(a) HAADF-STEM images of a salt aggregation inside a MWCNT under a high intensity electron beam at different times. Dashed rectangles and lines are used for guidance. (b),(c) TEM images of a segment of MWCNT together with inside salt aggregations in the initial stages and after irradiation for 8 min. The electron-beam irradiation induced structural changes of the aggregation are displayed by the HRTEM images of the insets in (b) and (c). Here, the interplanar distance of the NaCl crystalline is marked in the inset of (b).

Figure 3

(a) Distribution probabilities of ${\mathrm{Na}}^{+}$, ${\mathrm{Cl}}^{-}$, and water molecules along the $z$ direction in the salt solution inside a CNT with a radius of 1.4 nm from MD simulations. Inset: Radial distribution probabilities of ${\mathrm{Na}}^{+}$, ${\mathrm{Cl}}^{-}$, and O atoms inside CNT averaged over the perpendicular coordinate. (b) Snapshot. Green structures depict the CNT; water molecules and ions are shown with oxygen in red, hydrogen in white, ${\mathrm{Na}}^{+}$ in blue, and ${\mathrm{Cl}}^{-}$ in green, respectively.

Figure 4

(a) HAADF-STEM image of a ${\mathrm{PtCl}}_4$ aggregation inside a MWCNT soaked in a dilute ${\mathrm{PtCl}}_4$ solution of $0.001M$. (b) C, O, Pt, and Cl element mappings of the rectangular frame in (a) are displayed in (i)–(iv), respectively.