Try an ethoxylkated surfactant or co-polymer as we used in these studies 

Colloids and Surfaces. 62 (I 992) I I I-l I8

Adsorption of nonyl phenol propylene oxide--ethylene

oxide surfactants on carbon black and the rheology of

the resulting dispersions

F. Miano”, A. Bailey”, P.F. Luckham” and Th.F. Tadros b-’

Colloids and Surfaces, 68 (1992) 9-16

Adsorption of poly(ethyiene oxide)-poly(propylene

oxide) ABA block copolymers on carbon black and the

rheology of the resulting dispersions

F. Miano”- l, A. Bailey”, P.F. Luckham” and Th.F. Tadrosb

Colloids and Surfaces A: Physicochem. Eng. Aspects 404 (2012) 25– 35

Investigating the effectiveness of PEO/PPO based copolymers as dispersing

agents for graphitic carbon black aqueous dispersions

S. Yasin, P.F. Luckham∗

Hi,

I am not expert in carbon but I have some knowledge in sonication. When trying sonication be aware that the final efficiency depends on a combination of many factors: a type of sonicator (with a specific power and  frequency). If you work with a water bath sonicator, the shape of bath and an immersion depth will significantly affect the efficiency. Shape of sonication vessel and a material are also important. All this just to say that it is possible  that you simply use suboptimal sonication settings.

Try an ethoxylkated surfactant or co-polymer as we used in these studies 

Colloids and Surfaces. 62 (I 992) I I I-l I8

Adsorption of nonyl phenol propylene oxide--ethylene

oxide surfactants on carbon black and the rheology of

the resulting dispersions

F. Miano”, A. Bailey”, P.F. Luckham” and Th.F. Tadros b-’

Colloids and Surfaces, 68 (1992) 9-16

Adsorption of poly(ethyiene oxide)-poly(propylene

oxide) ABA block copolymers on carbon black and the

rheology of the resulting dispersions

F. Miano”- l, A. Bailey”, P.F. Luckham” and Th.F. Tadrosb

Colloids and Surfaces A: Physicochem. Eng. Aspects 404 (2012) 25– 35

Investigating the effectiveness of PEO/PPO based copolymers as dispersing

agents for graphitic carbon black aqueous dispersions

S. Yasin, P.F. Luckham∗

I suspect that mixing with virtually any neat surfactant (e.g. washing-up liquid), stirring vigorously into a paste will allow dispersion into water.  I've used this method for the most hydrophobic of surfaces e.g. PTFE and metals.

You should note that dispersion by sonication only works if particles are stabilized sufficiently against agglomeration. This in mind proceed as suggested above.