Molecular simulation study of temperature effect on ionic hydration in carbon nanotubes

Abstract

Molecular dynamics simulations have been performed to investigate the hydration of Li(+), Na(+), K(+), F(-), and Cl(-) inside the carbon nanotubes at temperatures ranging from 298 to 683 K. The structural characteristics of the coordination shells of ions are studied, including the ion-oxygen radial distribution functions, the coordination numbers, and the orientation distributions of the water molecules. Simulation results show that the first coordination shells of the five ions still exist in the nanoscale confinement. Nevertheless, the first coordination shell structures of cations change more significantly than those of anions because of the preferential orientation of the water molecules induced by the carbon nanotube. The first coordination shells of cations are considerably less ordered in the nanotube than in the bulk solution, whereas the change of the first coordination shell structures of the anions is minor. Furthermore, the confinement induces the anomalous behavior of the coordination shells of the ions with temperature. The first coordination shell of K(+) are found to be more ordered as the temperature increases only in the carbon nanotube with the effective diameter of 1.0 nm, implying the enhancement of the ionic hydration with temperature. This is contrary to that in the bulk solution. The coordination shells of the other four ions do not have such behavior in the carbon nanotube with the effective diameter ranging from 0.73 to 1.00 nm. The easier distortion of the coordination shell of K(+) and the match of the shell size and the nanotube size may play roles in this phenomenon. The exchange of water molecules in the first coordination shells of the ions with the solution and the ion diffusion along the axial direction of the nanotube are also investigated. The mobility of the ions and the stability of the coordination shells are greatly affected by the temperature in the nanotube as in the bulk solutions. These results help to understand the biological and chemical processes at the high temperature.

Molecular simulation study of the effect of inner wall modified groups on ionic hydration confined in carbon nanotube

“…We can find that, for the three cations, their distributions of cos˛present maxima at −1, indicating that the water molecules around a cation tend to have their oxygen atoms pointing to the cation. This observation is consistent with the previous results [14,15]. The values at −1 can reflect the orientation strength of the ionic hydration.…”

“…2, because of the confinement effect of the CNTs, the values of the cation-oxygen RDFs turn to zero as r increases, which is consistent with the observations of previous simulations [14,15]. In Fig.…”

“…During the production stage, the fluctuation of the potential energy of the ensemble is around 1%. The other simulation details are the same as our previous work [14,15].…”

“…The simulation of Shao et al shows that the confinement changes the temperature dependence of the structure order of water molecules in the first coordination shell of K + . In a (10, 10) CNT, the temperature increase results in the order enhancement instead of the order decease that is observed in the bulk solution [14]. Moreover, the pore size effect on the hydration structures of cations was also evaluated [15].…”

Diffusion of water molecules confined in slits of rutile TiO2(110) and graphite(0001)

“…Although a few theoretical works on TiO 2 are related to water [15][16][17][18], few have been focused on the diffusion of water confined in TiO 2 slits. Our previous works of molecular dynamics (MD) showed that the chemical modification of carbon nanotubes would affect the water behaviors [19][20][21][22][23]. It means that the chemical property of the solid surface plays an important role in the structural behaviors of confined water, and will accordingly influence their diffusion behavior.…”