## 4 Results and Discussion  
### Stabilization of Graphite  
#### 4.1.1 Influence of Dispersant Content on Viscosity  
The rheological behavior of the graphite dispersions was investigated for different PVP and TX-100 contents, respectively. The rheological behavior of a pure PVP and a pure TX-100 solution were also examined. The pH value of the PVP samples was adjusted to 9.0-9.5, since [PERSON] et al. [26] postulated the pH value at the lowest zeta potential to be 9.0, while [PERSON] et al. [30] put it at 9.5. For TX-100, no suitable data regarding pH were found in the literature. Therefore, the pH value of the TX-100 samples was adjusted to the same pH value range as the PVP samples. The viscosity curves are given in Figures 2 and 3. Graphite content refers to the total mass consisting of water and graphite. Dispersant content relates to the graphite content. Unless stated otherwise, this also applies to all further measurements. The standard deviation is based on 61 single point measurements.  
Figure 2: Viscosity as a function of polyvinylpyrrolidone (PVP) content for (**a**) an aqueous dispersion containing 10 m% graphite nanoparticles (see Table 1) and (**b**) a PVP solution without nanoparticles (see Table 2).  
The viscosity curves for both graphite dispersions (PVP and TX-100) show a similar progression. Below, a PVP content of 20 m% or a TX-100 content of 40 m%, respectively, the viscosity of the graphite dispersion gradually improves as the dispersant increases. In this range, the graphite nanoparticles are successively coated by PVP or TX-100 molecules, respectively, in turn weakening the interparticle attraction forces [39]. A PVP content of 20 m% or a TX-100 content of 40 m% indicates the onset of a plateau in viscosity for both pH values. At this point, the added PVP or TX-100 molecules are fully adsorbed at the surface of the graphite particles, marking the highest stability of the dispersion. The viscosity values in the range of the plateau correspond well with those of pure PVP or TX-100 in an aqueous medium (see Figures 2b and 3b). A further increase in PVP or TX-100 content does not lead to any significant change in viscosity and thus enhanced stability. Because the graphite surface is fully coated, the excess PVP or TX-100 molecules remain in the water. Hence, the viscosity of the graphite dispersion is gradually determined by the viscosity of the PVP or TX-100.  
The observations in the viscosity curves are also reflected in the appearance and behavior of the dispersions. The PVP dispersions seemed to be well dispersed with no visible agglomerates. The higher the PVP content, the more homogeneous the dispersion. However, the TX-100 dispersions displayed a number of isolated agglomerates. Furthermore, a foamy behavior of the dispersions was observed as the TX-100 content increased. The differences in the external appearance of the dispersions can be attributed to the formation of micelles. Micelles are formed when the critical micelle concentration of a surfactant in a solution is reached. The hydrophilic parts of the surfactant molecules then align themselves with the adjacent water molecules, whereas the hydrophobic parts cluster together, forming a separate phase [41]. Varying statements can be found regarding the critical micelle concentration of the surfactant TX-10, whereby [PERSON] et al. [41] determined that the value highly depends on the measuring principle. [PERSON] [42] specifies a range of 0.22 and 0.24 mmol/L, which is equivalent to about \(3.95\times 10^{-3}\) m% in the investigated dispersion system. This illustrates that micelles are present, even at marginal TX-100 concentration. In contrast to low molecular weight surfactants, such as TX-100, amphiphilic side-chain polymers, such as PVP, do not exhibit a critical micelle formation [43].