We have functionalized carbon nanotubes with carboxylic acid and zwitterion groups. We have evaluated the effect of functionalization by measuring the sorption of CO2, CH4, and N-2 at 35 degrees C for pressures up to 10 bar. Zwitterion functionalized nanotubes were found to be highly hygroscopic. Thermal gravimetric analysis indicates that water can be desorbed at about 200 degrees C. The adsorption of gases in zwitterion functionalized nanotubes is dramatically reduced when compared with nanotubes functionalized with carboxylic acid groups. The presence of water on the zwitterion functionalized nanotube reduces the sorption even further. Molecular simulations show that three or more zwitterion groups per tube entrance are required to significantly reduce the flux of CO2 into the tubes. Simulations also show that gas phase water is rapidly sorbed into the zwitterion functionalized nanotubes, both increasing the free energy barrier to CO2 entering the tube and also lowering the equilibrium adsorption through competitive adsorption. (C) 2012 Elsevier B.V. All rights reserved.