Surface activity, micellization, and application of nano-surfactants—amphiphilic carbon dots

For nano-surfactants, a new class of surfactants, the unique surface properties, micellization, and application of amphiphilic carbon dots (CDs) have been investigated carefully. Amphiphilic CDs (C12-CDs and C12-SCDs) were synthesized under mild reaction conditions. These CDs exhibit higher surface activity than conventional surfactants (APG12 and SDBS), whereas hydrophilic CDs have negligible surface activity. Above the critical micelle concentration, C12-CDs aggregate into an irregular spherical structure like micelle with an average diameter of about 100 nm, whereas C12-SCDs do not form aggregates. The surface tension of C12-CDs and C12-SCDs reaches the minimum at pH = 3, which is close to the isoelectric point of CDs. As pH increases, the water solubility of CDs increases, and thus the surface tension increases. C12-CDs can still maintain high activity at cNaCl = 250 g/L, but their resistance to CaCl2 is poor. The addition of the sulfonate groups enhances the tolerance of CDs to both NaCl and CaCl2. At cNaCl = 360 g/L and cCaCl2 = 750 g/L, C12-SCDs still have high surface activity. The removal efficiency of oil film and contact angle experiments show that compared to the traditional surfactant SDBS, C12-CDs and C12-SCDs have an excellent wettability alternation due to their abundant hydroxyl, carboxyl, amine, and sulfonate groups. Among them, the wettability alternation of C12-SCDs is better than that of C12-CDs due to their hydrophilic sulfonate groups. Furthermore, the Pickering emulsions stabilized by C12-SCDs have rapid pH and CO2 response performance. In addition, compared to the traditional surfactants SDBS and APG12, CDs have an excellent dispersion performance to MWCNTs. Different from the dispersion mechanism of cationic surfactant CPC (strong electrostatic attraction), CDs are adsorbed on the surface of MWCNTs through their hydrophobic effect and π-π stacking effect to achieve effective dispersion. Among them, because the sulfonate groups have high steric resistance and electrostatic repulsion, the dispersion performance of C12-SCDs is better than that of C12-CDs.