Aqueous dispersion stability of multi-carbon nanoparticles in anionic, cationic, neutral, bile salt and pulmonary surfactant solutions

Abstract

Molecular structure of surfactants governs the dispersion stability of, poorly soluble and mainly hydrophobic, carbon nanoparticles in solvents. A systematic study was carried out to establish the surfactant assisted dispersion mechanism of carbon nanoclusters (size ≈150 nm, zeta potential ≈−15 mV), obtained from soot, in water. An array of surfactants, SDS, CTAB, TX-100, sodium cholate (bile salt) and dipalmitoyl phosphatidylcholine (DPPC, a clinical pulmonary surfactant preparation called Survanta), were used in a wide range of concentration (0.01CMC to 2CMC) to probe the dispersion mechanism. Results revealed that the adsorption of surfactant molecules on the nanoparticle surface was interplay of ionic, hydrophobic and π–π stacking forces. The CTAB molecules (cationic) formed a bilayer on the carbon nanoclusters providing robust dispersion stability whereas SDS molecules (anionic) were poorly adsorbed through hydrophobic interactions. TX-100 molecules (neutral) stabilized the dispersion via hydrophobic and π–π stacking interactions. Sodium cholate, adsorbed on nanoclusters mostly through hydrophobic interaction and generated large asymmetric complexes. DPPC, a gemini surfactant, formed a rigid monolayer around the carbon nanocluster even at nanomolar concentration and provided excellent stability to the dispersion. Binding constant for adsorption onto a hydrophobic surface or being part of membrane/micelles was found to be energetically most favorable for TX-100 followed by CTAB, NaC and SDS in that order. A comparison with carbon nanotubes data indicates that surfactant assisted dispersion stability is sensitive to the size and morphology of the carbonaceous nanostructure.