Molecular dynamics simulation of desalination process using polyoxometalate ionic liquid confined in carbon nanotubes

Abstract

In this work, the effect of positioning of [Keggin][emim]3 ionic liquid (IL) into carbon nanotubes (CNTs) with different concentrations has been examined in the desalination process at different external pressures using molecular dynamics (MD) simulations. Another membrane was also modelled composed of four similar CNTs with almost equal space from each other. Using of only Keggin anions into the charged CNT was also studied in the desalination process. To study the mechanism of the water and ions fluxes, the potential of mean force, average number of hydrogen bonds, and self-diffusion coefficient have been also computed and discussed. The results showed that the placing of [Keggin][emim]3 IL into a CNT decreases the water flux (Nwaterns=74at150MPa) relative to the empty CNT (Nwaterns=834). However, the four CNTs configuration shows increase in the water flux (Nwaterns=183at150MPa) than the one CNT system. The placing of one polyoxometallate IL into the CNT increases the salt rejection 10 to 20 times than the empty-CNT system at low and high pressures, respectively. Moreover, positioning of the two ILs results in the complete salt rejection at all pressures. These results indicate that the use of polyoxometalate ILs into the CNT efficiently increases the salt rejection. The self-diffusion of confined water molecules in the single IL-CNT composite (D = 0.73 × 10-9 m2/s) is also smaller than the empty CNT at 150 MPa (D = 1.89 × 10-9 m2/s). The results also indicated that using of the Keggin anions into the charged CNT leads to the more water flux (Nwaterns=250at150MPa) and complete salt rejection relative to the fixing of [Keggin][emim]3 IL into the uncharged CNT. The effects of CNT charge distributions, temperature, and CNT chirality have been also examined on the desalination process.