Adsorption of sodium dodecyl sulfate on functionalized graphene measured by conductometric titration

We report on the adsorption of sodium dodecyl sulfate (SDS) onto functionalized graphene sheets (FGSs) in an aqueous system, measured at broad SDS and FGS concentration ranges by conductometric surfactant titration. At dilute SDS concentrations (<12 μM in bulk solution), there is evidence of a counterion exchange between hydronium ions (from the dissociation of acidic chemical functionalities on FGS) and sodium ions coadsorbing with dodecyl sulfate monomers onto FGSs. We find that, for FGS with a carbon-to-oxygen ratio of ∼18, monolayer adsorption of SDS on FGS reaches full surface coverage by ∼12 μM SDS. Additionally, the critical surface aggregation concentration (csac) for surface micelle formation on FGS is measured to be ∼1.5 mM SDS. The transition from monolayer adsorption to surface micelle formation appears to occur at a similar SDS concentration on FGSs as on graphite, suggesting there is little difference in the surfactant adsorption behavior on both materials. We estimate that the FGS surface area available for SDS adsorption is ∼600 m²/g, which is significantly less than expected for FGSs in suspension and indicates the presence of regions on FGS on which SDS adsorption does not occur.