Effect of temperature on the coupling transport of water and ions through a carbon nanotube in an electric field

Abstract

Temperature governs the motion of molecules at the nanoscale and thus should play an essential role in determining the transport of water and ions through a nanochannel, which is still poorly understood. This work devotes to revealing the temperature effect on the coupling transport of water and ions through a carbon nanotube by molecular dynamics simulations. A fascinating finding is that the ion flux order changes from cation > anion to anion > cation with the increase in field strength, leading to the same direction change of water flux. The competition between ion hydration strength and mobility should be a partial reason for this ion flux order transition. High temperatures significantly promote the transport of water and ions, stabilize the water flux direction, and enhance the critical field strength. The ion translocation time exhibits an excellent Arrhenius relation with the temperature and a power law relation with the field strength, yielding to the Langevin dynamics. However, because of self-diffusion, the water translocation time displays different behaviors without following the ions. The high temperature also leads to an abnormal maximum behavior of the ion flux, deciphered by the massive increase in water flow that inversely hinders the ion flux, suggesting the coexistence of water–ion coupling transport and competition. Our results shed deep light on the temperature dependence of coupling transport of water and ions, answering a fundamental question on the water flux direction during the ionic transport, and thus should have great implications in the design of high flux nanofluidic devices.

Electropumping Phenomenon in Modified Carbon Nanotubes

“…This is abnormal because high flux should correspond to a low translocation time, as exemplified by pristine CNTs. 22,27 The reason is that the increased charge density leads to the increase in the Na + −surface interaction that hinders the motion of Na + inside the CNT. After the maximum peak, the translocation time of Na + exhibits a drastic drop since a majority of Na + are absorbed by the charged surface, and only a small amount of Na + will quickly pass through the CNT, which is also consistent with the low flux in Figure 2a.…”

“…We also note that the ion translocation time normally yields to a power relation with E in pristine CNTs, which can be well described by the one-dimensional Langevin equation. 27,30 Therefore, the surface charge changes the ion dynamical behaviors because it can significantly slow down the ion motion. In Figure 7b, the water translocation time for the NH 3…”

“…38 Thomas and Corry showed that in some cases the thermostat choices can affect the water transport. 39 In our previous work, 27 we compared different ensembles, thermostats, and barostats in the transport of water and ions through a carbon nanotube in electric fields. These choices will slightly affect the flux of water and ions, but the main conclusion will be preserved.…”

“…We used the SPC/E water model, 40 as it can well reproduce the experimental diffusion coefficient and thus predict reliable water flux through CNTs. 27 The Amber 03 force field was used to describe the uncharged carbon with a 1.2 nm cutoff for the Lennard-Jones (L-J) interaction. 41 The ion parameters were taken from previous work.…”

“…However, for pristine CNTs the unidirectional transport efficiency of water cannot be very high since the same number of cations and anions will be in direct competition with each other, which results in an efficiency of less than 40%. 27 With the advance of experimental techniques, the hydrophobicity of CNTs can be tuned by modifying appropriate charged residues or chemical groups, and recent experiments showed a high water flow rate and ion selectivity for such modified CNTs. 23,24 Simulation studies also demonstrated some unique transport dynamics of water in such modified CNTs.…”