Abstract: Carboxylic-acid functionalized multiwall carbon nanotubes ( MWCNT-COOH) were first modified by ( CH₃O)₃Si (CH₂ )₃SO₃H by an esterification reaction of the carboxylic-acid group with a methoxy group, then by hydrolyzing N, N-di(2-hydroxyethoxy)ethyl octadecylamine by an ion exchange reaction to form a quaternary ammonium salt as a soft shell of the MWC-NTs. The modified MWCNTs had a larger diameter than the original MWCNTs and their MWCNT content was up to 16. 03 wt %. The loss modulus of the modified MWCNTs is bigger than the storage modulus, which is a characteristic of a solvent-free nanofluid. The modified MWNTs can be well dispersed in deionized water and the resulting dispersion is stable. Their seepage threshold value in water is about 5 mg/ g.