Biomimetic modification of large diameter carbon nanotubes and the desalination behavior of its reverse osmosis membrane

Based on the fact that wide diameter carbon nanotube (CNTs) has higher water conductance and lower cost than narrow one, this paper built a set of RO membranes with modified 1.35nm diameter CNTs (10,10) and calculated their desalinating properties by molecular dynamics simulations. Inspired by the structure of biological membrane which has perfect ion selectivity, different charged and polar groups (CONH₂, NH₃⁺, COO^-, OH) were added to the interior of CNTs. The potential of mean force (PMF), conductance and axial density distributions of water and ions in CNTs were examined under hydrostatic pressure. Results showed that water flux of 1.35nm diameter (10,10) unmodified CNTs was about 3 times of 1.1nm diameter (8,8) CNTs (Chen S. and Corry B., 2009) ^[1]. Compared to CNTs with functional groups at entrance, water flux of CNTs with modified groups in the interior decrease slightly, while salt rejection greatly improved. With certain number, type and position of functional groups (CONH₂, NH₃⁺, COO^-), 100% salt rejection could be achieved without affecting water conductance evidently. Water flux of 100% salt rejection modified CNTs was from 146% to 167% of unmodified (8,8) CNT (Chen S. and Corry B., 2009) ^[1]. Compared to four functional groups (COO^-, CONH₂, OH) in the interior, one functional group added at entrance and four functional groups added in the interior could obtain both higher desalination and high water conductance.