In carbon nanotube immobilized membrane (CNIM), the nanotubes serve as a sorbent that provides additional pathways for solute transport. In this paper we present that carboxylated nanotubes which are significantly more polar and can increase interactions with the water vapor in CNIM to improve desalination efficiency in membrane distillation (MD). The encapsulation of the nanotubes in PVDF prevented the carboxylated nanotubes from making the overall membrane more hydrophilic and thus retain its performance. Overall, desalination was consistently better with carboxylated nanotubes than with unfunctionalized ones with flux reaching as high as 19.2 kg/m(2) h in a sweep gas membrane distillation mode. (C) 2013 Elsevier B.V. All rights reserved.