The adsorption of sodium dodecyl sulfate (SDS) onto functionalized graphene sheets (FGSs) in an aqueous system is measured at broad SDS and FGS concentration ranges by conductometric surfactant titration. 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 �M SDS. Additionally, the critical surface aggregation concentration (csac) for surface micelle formation on FGS is measured to be ~1.5 mM SDS.The colloidal stability of FGSs, ultrasonically dispersed in aqueous SDS solutions of different surfactant concentrations, is studied by optical microscopy and ultraviolet-visible light absorption. Below ~10 �M SDS, FGSs reaggregate within a few minutes, forming ramified structures in the absence of SDS and increasingly compact structures as the SDS concentration increased. Above ~10 �M, the rate of reaggregation decreased with increasing SDS concentration; above ~40 �M, the suspensions were stable for over a year. Thus, neither surface micelle formation nor dense monolayer coverage is required to obtain stable aqueous FGS dispersions.We support our experimental results by calculating the van der Waals and electrostatic interaction energies between FGSs as a function of SDS concentration, showing that the experimentally observed transition from an unstable to a stable dispersion correlates with a transition from negative to positive interaction energies between FGSs in the aggregated state. Furthermore, our calculations support experimental evidence that the FGS aggregates tend to develop a compact structure over time.

Colloidal stability of functionalized graphene in aqueous sodium dodecyl sulfate solutions: From surfactant adsorption to dispersion behavior

247th National Meeting (2014)

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