Muhammad Kaleem Zeta potential is measured indirectly by mobility measurement in an electrical field. The optical properties of the particle are not required for zeta potential. You may need the RI of the pure continuous/dispersant phase for DLS and zeta potential measurement. Note that interpretation of mobility for CNT's may be a university exercise. Why are you trying to measure ZP of CNT's? Not sure what information it would give....
BTW, what are signal carbon nanotubes? SWCNT means single walled carbon nanotube...