Nanosheet size-dependent rheology, microstructure, adsorption properties of graphene oxide-electrolyte dispersions and adsorbents

The variation of lateral dimensions of GO nanosheets by subjecting them to different ultrasonication time (t = 30, 60, and 120 min) influences the rheology, microstructure, and adsorption capacity of graphene oxide (GO) suspensions. The average storage modulus (G'_p) of GO aqueous dispersions (volume fraction (ϕ_GO) = 0.018) varies as GO-30 < GO-60 < GO-120, although the C/O of GO nanosheets does not change significantly with ultrasonication. Addition of 10⁻⁵ M – 10^-1 M NaCl and MgCl₂ lead to a rise in G'_p as compared to electrolyte-free suspensions with the formation of liquid-like, fragile gels and solid gel-like samples. Independent of time of ultrasonication, solid gels are formed at 10^-1 M of electrolytes. The maximum G'_p is observed in gels of GO-30 with 10^-1 M MgCl₂. We hypothesize that an increase in ultrasonication time causes the formation of new nanosheets with un-functionalized edges. While Na⁺ screens the negative charge of GO causing aggregation, but Mg²⁺ not only screens but also interacts with carboxyl groups of GO nanosheets. Therefore, the degree of Mg²⁺ bridging the GO nanosheets is the lowest in GO-120 and the highest in GO-30 suspensions. The adsorption capacity of methylene blue (MB) on GO-120-electrolyte lyophilized gels is the smallest, in agreement with the weak cross-linking and mechanical strength of these gels.