Experimental and Molecular Dynamics Simulation Study of Specific Ion Effect on the Graphene Oxide Surface and Investigation of the Influence on Reactive Extraction of Model Dye Molecule at Water-Organic Interface

The influence of different inorganic anions (Cl-, Br-, SCN-, NO3-, SO42-, and CH3COO-) and cations (Ca2+, Mg2+, Na+, and NH4+) on the surface potential of graphene oxide (GO) suspension has been investigated both experimentally and computationally. The hydrophilic GO surface has negative surface potential (zeta potential) which can be varied by changing the pH of the suspension as well as by adding external inorganic ions. The surface of GO is hydrophilic in a basic medium and becomes hydrophobic in an acidic medium because of the protonation and deprotonation of the surface functional groups. The presence of inorganic ions affects the electrophoretic mobility of the dispersed phase within the GO suspension and influences its zeta potential. This is due to the formation of a double layer of charge at the interface of the GO and ionic salt solution. Molecular dynamics simulations were used to understand the interactions of ions within the slipping plane of GO, which influences its zeta potential in salt solutions. The results suggested that the influence of the various inorganic ions on the electrokinetic potential of GO is ion-specific and depends on the polarizability of the ions. Having high specific surface area and being amphiphilic and biocompatible, GO was successfully utilized in the reactive extraction technique of methyl blue (MB) dye molecule at the water-toluene interface. The present study demonstrates that the presence of highly polarizable ions increases the zeta potential as well as hydrophobicity of GO, which facilitates the extraction of MB from the aqueous to the organic phase.