Density functional theory (DFT) is applied to estimate the gas-phase acidity of several carboxylated SWCNTs, with varying lengths and diameters of the zigzag and armchair tubes. The efficient same level different basis set (SLDB) protocol is used. Deprotonation energy of the open-ended molecular model of NT-COOHs indicates more acidic nature of zigzag-COOH than armchair-COOH. Effect of lengths and diameters on the acidity and stability of acids are discussed. Vibrational analyses reveal differences in carboxylic band (nu(C=O)) in the two forms of acid. The effects of solvents on the acidity and stability are evaluated and discussed. (C) 2008 Elsevier B.V. All rights reserved.