Influence on the adsorption of phenol on single-walled carbon nanotubes caused by NaCl and an electrostatic field in saline

Abstract

In the present study, the molecular dynamics simulation method was used to investigate the influence on the adsorption capacity of single-walled carbon nanotubes (SWCNTs) to phenol caused by NaCl and an external electrostatic field in saline by calculating the structural parameters, the hydrogen bonding, the interaction energy and the viscosity of the solution system. The results show that when the numbers of added NaCl molecules are ten, twenty and thirty, the coordination number of phenol around the SWCNTs decreases from 150 to 144, 143 and 141, the coordination number of water around phenol increases from 2.133 to 2.257, 2.301 and 3.345, the lifetime of hydrogen bonds between phenol and water increases from 7.02 ps to 7.09 ps, 7.16 ps and 7.22 ps, the interaction energy of SWCNT and phenol decreases from −1571 kJ·mol−1 to −1374.05 kJ·mol−1, −1341 kJ·mol−1 and −1326 kJ·mol−1, and the viscosity of solution increases from 0.70 mPa·s to 0.73 mPa·s, 0.74 mPa·s and 0.76 mPa·s, respectively. These results indicate that the presence of NaCl can decrease the number of phenol molecules around SWCNTs, enhance the hydration of phenol, reduce the phenol's hydrophobic performance, and decrease the interaction between the phenol molecules and the viscosity of the phenol solution, which indicates that the added NaCl can weaken the adsorption capacity of SWCNTs to phenol. Furthermore, when an electrostatic field is applied in saline (with twenty NaCl molecules), the number of salt ions around the SWCNTs decreases, leading to an increase in the adsorption sites for phenol on the surface of the SWCNTs. The electrostatic field can also decrease the interaction energy between water and phenol (from −4662 kJ·mol−1 to −4490 kJ·mol−1) and increase the interaction energy between SWCNT and phenol (from −1361 kJ·mol−1 to −1552 kJ·mol−1); thus, the adsorption between phenol and SWCNT can be enhanced. However, the presence of NaCl suppresses this enhancement caused by the electrostatic field.