Synthesis, characterization, and their electrokinetic properties of functionalized multi-walled carbon nanotubes

Firstly, MWCNT-OH and MWCNT-COOH were synthesized from MWCNT by oxidation reactions. The samples were characterized by BET, FTIR, DTA/TG, NMR, SEM/EDX, and TEM devices. The electrokinetic properties of MWCNTs were investigated as a function of solid/liquid ratio, suspension pH, and electrolyte concentration, type, and valence. Changes in surface area, presence of carbonyl, and hydroxyl groups, changes in thermal decomposition steps and morphological properties proved that functionalized MWCNT structures were synthesized. While zeta potential did not show a significant change with increasing solid/liquid ratio, it changed with the initial pH of the suspension. While all carbon nanotubes had positive zeta potential at pH = 2, it was determined that zeta potential decreased and changed sign with increasing pH. The isoelectric points of MWCNT, MWCNT-OH, and MWCNT-COOH suspensions are pH 9, 4.2, and 2.8, respectively. While there was no change in the sign of the zeta potential of carbon nanotube suspensions of monovalent electrolytes, the zeta potentials of divalent and trivalent electrolyte suspensions changed sign with increasing concentration. While monovalent electrolytes acted as indifferent electrolytes, divalent and trivalent electrolytes adsorbed specifically on the carbon nanotube surface. The presence of CO₃²⁻ ions in MWCNT suspensions transformed the zeta potential from positive to negative with increasing concentration. In conclusion, since the electrokinetic properties of functionalized MWCNTs vary depending on pH and electrolyte concentration, accurate tuning of these parameters is critical for the stability and performance of MWCNT-based systems in industrial applications.

Graphical Abstract