Husein,

In my experience, there are very few good solvents for nanotubes, if any.
Toluene, Ethanol, IPA, Actetone and similar solvents do not work. There are
some which keep them in suspension much more stably than others; the best
one that I have found is Chloroform.  It does keep them in more or less
stable suspension for days. I think most other chlorinated solvents behave
similarly.

It also does depend on what MWNT samples you are using, whether they are
arc-discharge, CVD etc, as the preparation method does determine many things
which affect the solubility, such as length, diameter (which determine how
much they can aggregate, hindering solubility), level of impurities (such as
amorphous carbon, catalytic particles, and other less soluble components)
and number of defects.

Depending on what you want to use them for, an alternative is to try a
surfactant, eg. SDS in water, which I believe should work well, although I
havent tried it myself. It is a commonly used method, although the
surfactant will affect the nanotube's properties.

Hope this helps,

Brendan Mc Carthy
Research Associate
Optical Sciences Center
University of Arizona