FIGURE 2Dependence of CG onCP withCG,i keeping constant at a) 3.0 mg mL-1 and b) 25, 50, and 80 mg mL-1. Inset: photograph of an aqueous FLG dispersion produced withCP = 2.0 mg mL-1 andCG,i = 3.0 mg mL-1(therein the Tyndall effect is shown).
increase with increasing CP (0.1−2.0 mg mL-1).25 It was well documented that micelles, especially the large-sized ones, cannot fit into adjacent nanoparticles.44 As a consequence, the osmotic pressure of micelles around nanoparticles generates a strong depletion attraction, causing aggregation of nanoparticles and thus decrease of the dispersion concentration. In the sonication process, the cavitation effect makes it possible that no micelles or only small ones are formed in aqueous solutions until CP is higher than 0.6 mg mL-1. Under the shear-mixing condition, however, the critical CP for formation of micelles with sizes large enough to create effective depletion attraction seems to be improved to higher than 3.0 mg mL-1 (maximal CPexamined in this work), owing to serious micelle breakage brought by the combined high shear, cavitation, and collision effects.3 Higher CPbut absence of large-sized micelles is helpful to facilitate exfoliation, dispersion, and stabilization of FLG, particularly in the cases of higher CG,i . Almost constantCG observed within 2.0−3.0 mg mL-1 of CP is attributed to adsorption of polymers on the graphitic particle surface already reaching saturation and more polymers having little influence on dispersing FLG.
The aqueous solution of CP = 2.0 mg mL-1 was then selected as liquid medium to study the influence of CG,i onCG . As shown in Figure 3a,CG increases monotonously withCG,i in the detected range of 3.0−80 mg mL-1. Such an increase trend is different from that of sonication exfoliation, where CG usually tends to decrease beyond a certain CG,i(10−60 mg mL-1) whose specific value depends on selection of sonication mode and water-soluble stabilizer.25,30,45 It is well established that sound cavitation is the primary driving force for sonication-assisted FLG exfoliation.3 Too highCG,i may block the sound propagation and thus weaken the exfoliation efficiency. In the case of shear exfoliation, however, shearing force dominantly responsible for FLG exfoliation is long-range relative to sound wave, and its transmission is little affected by CG,i (at least within the currently examined CG,irange). Figure 3a also reveals that CGrelates with CG,i in an empirical form of CGCG,i 1.17. This power-law correlation was as well observed in sonication exfoliation with the exponents falling in 0.5−0.8.30,46,47 Much higher value achieved here (1.17) reflects an obviously faster increase of CG withCG,i , indicating high-shear mixing more efficient in FLG exfoliation than sonication. What should be pointed out is that the exponent of 1.17 is realized by merely 20 min oftM and, meanwhile,CG is up to 0.24 mg mL-1. In stark contrast, it usually requires several or even tens of hours to get such a comparableCG by sonication exfoliation.21,48 Despite monotonous increase ofCG withCG,i , a parabolic-like dependence of production yield (CG/CG,i ) onCG,i is noted withCG/CG,i reaching the maximum of 0.38% at CG,i = 50 mg mL-1 (Figure 3b), compared with a quasi-linear decline of CG/CG,i with increasing CG,i in sonication exfoliation.25,30,46 This again demonstrates that the shear exfoliation is superior to sonication exfoliation from the viewpoint of capacity for improvingCG/CG,i .