## 3 Results and Discussion  
### Nanopowder Chemical Composition and Nanofluids' Stability  
The employed nanopowders were morphologically and chemically characterized previously [21; 29; 37; 42; 43]. Atomic force microscopy analyses evidenced the structure of nanoplatelets (stacks of graphene sheets), with similar heights per sheet of 3 to 12 nm for S-GnP [42] and of 2 to 18 nm for P-GnP [43]. Transmission electron microscopy (TEM) analyses [21; 37] showed that S-GnP particles are generally larger than P-GnP particles, with longer dimensions up to 590 nm and 500 nm, respectively. Concerning to chemical composition, Energy-Dispersive X-ray Spectroscopy tests stated the existence of C, O and S for S-GnP [29] and C, O and K for P-GnP [37]. Results of elemental analysis for nanopowders are shown in Table 1, establishing the relative amount of each element. For S-GnP, the major presence of C (69.1%) and O (12.6%) was confirmed but the presence of \(\sim\)5% of S and \(\sim\)3% of hydrogen (H) and nitrogen (N) was also detected. In the case of P-GnP, the presence of C (52.4%) and O (22.4%) was reaffirmed and a content of \(\sim\)2% of H was established. The stability of S-GnP and P-GnP aqueous nanofluids was studied previously [21] by DLS analyses of the averaged apparent size of the nanoadditives into the base fluid over the time and by Zeta potential studies. The DLS analyses over time confirmed that the initial characteristics of the dispersion were easily recoverable by a simple mechanical stirring. However, some sedimentation was detected for samples left in completely static conditions after one week, mainly for S-GnP samples [21]. In this work, the polydispersity of the S-GnP and P-GnP dispersions was characterized by DLS measurements through the size distribution by number. S-GnP nanoadditive (Figure 1a) presents higher apparent size than P-GnP nanoadditive (Figure 1b), in accordance with the previously stated information. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Element** & **S-GnP** & **P-GnP** \\ \hline C [\%] & 69.1 & 52.4 \\ H [\%] & 3.1 & 2.2 \\ N [\%] & 3.2 & 0 \\ S [\%] & 5.4 & 0 \\ O [\%] & 12.6 & 22.4 \\ Total [\%] & 93.4 & 77.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of elemental analyses for sulfonic acid-functionalized graphene nanoplatelets (S-GnP) and polycarbonate chemically modified graphene nanoplatelets (P-GnP) nanopowders. Furthermore, as it can be concluded comparing Figure 1a,b, P-GnP presents a narrower size distribution than S-GnP, which is a symptom of higher uniformity of size. This conclusion is reinforced by the obtained values of polydispersity index, which is also a measure of width of the particle size distribution [44; 45]. According to the literature, monodispersed particles present values between 0.01 and 0.5-0.7 while samples with very broad size distribution present values over 0.7 (also a first symptom of unstable dispersions) [44; 45]. Values of 0.41-0.44 and 0.28-0.32 were obtained for the S-GnP and P-GnP nanofluids, respectively. It can be also observed that there are not significant concentration dependences on the polydispersity within the studied nanoadditive loading range. The force of the electrostatic repulsions among the dispersed particles counteracts with that of the attractions that provoke agglomeration and precipitation. These forces are largely dependent on the distance of the pH from the isoelectric point [46; 47]. Zeta potential values of 44 and -53 mV were obtained for S-GnP and P-GnP dispersions in water, respectively [21], synonymous of moderate/good stability according to literature criteria [48]. These values also indicate that the analysed nanofluids are far away from the isoelectric point. At the same time, extremely acid working fluids should be avoided to prevent the corrosion of facilities. Table 2 shows the effect of the nanoparticle addition to the pH. The dispersion of both types of different-functionalized graphene nanoplatelets implicates acidification of the base fluid, soft for P-GnP and moderate for S-GnP.