Modification of (15,15) 2.034 nm diameter carbon nanotubes with long aliphatic chain and their desalination behavior

In order to imitate the structure of protein aquaporin-4 to improve salt rejection and to keep high water flux, long aliphatic chains were added at entrance or into interior of 2.034 nm (15,15) armchair carbon nanotubes (CNTs) and reverse osmosis membrane was built by simulation in this paper. The potential of mean force, conductance and axial density distributions of salt and water in CNTs was calculated. Results showed that under 200 MPa, salt rejection could be dramatically improved with high water flux (3.8–5.3 times higher than that of (8,8) unfunctionalized CNTs) when CNTs were modified with opposite groups or zwitterion groups at entrance. Certain numbers of short functional groups at entrance together with long aliphatic chain will improve both water permeation and salt rejection. For example, one pair of short chains (CH₂NH₃⁺ and CH₂COO^–) and four 10-ammonioundecanote groups could make higher water flux than that with only four 10-ammonioundecanote groups and kept 100% salts rejection. CNTs with one pair of undecanoate and decan-1-aminium groups or two 6-ammonio-2-methylundecanedioate groups into interior can also obtain 100% salt rejection.