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Chapter 6 โ Production of Few Layer Graphene in Aqueous Media for Biological Studies It is known that solvents with surface tensions values of 40-50 mJ/m2 are the best solvents for dispersing graphene, as they reduce the interfacial tension between the solvent and graphene.3 It has been shown by molecular simulations that the last layer of confined solvent molecules around graphene sheets hinders their aggregation.387 Liu et al. have shown that unfunctionalised graphene in organic solvents was either positively or negatively charged and that their stability was mainly due to electrostatic repulsion.71 For NMP and DMF (higher donor number), the charge transfer happens from solvent to graphene, rendering graphene sheets net negatively charged with respect to the solvent, calculated ๐ ~ -30 mV.71 Meanwhile Yi et al. demonstrated that repeated washing and re-dispersal in water of DMF-exfoliated graphene resulted in stable water-dispersed graphene with negligible residual DMF.104 It was found that, as the size of the graphene decreases, the ๐ increases, due to grapheneโs enhanced edge effects (bonding of edge carbon atom to oxygen in water) resulting in higher stability of smaller-sized graphene (๐ > -30mV for <0.09 ฮผm2) in water, with no structural disorder in the basal plane.104 MD simulation studies have also shown that organic solvents such as NMP, due to their higher affinity, form dense layers around graphene surfaces.388 Wetting simulations have also shown that NMP could spread around graphene surfaces, whilst water remains at the hydrophobic contact angle. From these observations, the observed high ๐ values for 0.1 v/v% dialysed dispersions could be attributed mainly to the charge transfer from the residual organic solvents surrounding graphene surfaces (acting like surfactant) and that their colloidal stability was largely due to electrostatic repulsion of the charged graphene sheets. Even though direct evidence of edge functionalisation was not observed in the dialysed samples (no significant broadening in Raman G and D band, no considerable increase in interlayer spacing by XRD), the enhanced edge effects (bonding of edge carbon atom to oxygen in water) of smaller-sized graphene in water, as described by Yi et al., cannot be fully neglected.104, 389 174PDF Image | PRODUCTION AND APPLICATIONS OF GRAPHENE AND ITS COMPOSITES
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