## 3 Results and Discussion
### Electrokinetic Analysis of Treated Lithotypes  
The results of the streaming current measurements on treated lithotypes (Table 6 and Figure 3) show that a remarkable increase in IEP took place in all cases after the application of the treating products. As one may expect, the largest IEP changes were recorded for stones treated with WNC, the product providing a coating material with the highest inherent isoelectric point (Table 5). Furthermore, the differences among the IEPs of untreated lithotypes (Table 4) ended up being partially masked after application of the products. For example, Obernkirchen stone, with its lowest IEP as an untreated material, once treated showed nearly the same IEP values as the Apuan marble, a clear indication that the surface properties of the stones became dominated by those of the coating material. However, such differences were not completely cancelled out, and the IEP values of the treated stones were always lower than those measured on the coatings prepared on non-porous glass substrate. This suggests that the treating products did not succeed in producing a continuous and uniform coating layer on the mineral surface, including the pore walls of the outermost layer of the porous stone network. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**WNC**} & \multicolumn{4}{c}{**ANC**} \\ \cline{2-9}
**Stone Type** & \multicolumn{2}{c}{**Untreated 1**} & \multicolumn{2}{c}{**Treated**} & \multicolumn{2}{c}{**Untreated 1**} & \multicolumn{2}{c}{**Treated**} \\ \cline{2-9}  & \(\zeta\) & IEP & \(\zeta\) & IEP & \(\zeta\) & IEP & \(\zeta\) & IEP \\ \hline Apuan marble & \(-58\pm 2\) & 3.7 \(\pm\) 0.2 & 24 \(\pm\) 3 & 8.4 \(\pm\) 0.6 & \(-62\pm 3\) & 3.7 \(\pm\) 0.1 & 7 \(\pm\) 6 & 7.51 \(\pm\) 0.06 \\ Balegem & \(-54\pm 5\) & 2.8 \(\pm\) 0.3 & 9 \(\pm\) 5 & 7.7 \(\pm\) 0.2 & \(-61\pm 2\) & 3.1 \(\pm\) 0.1 & \(-22\pm 10\) & 6.4 \(\pm\) 0.3 \\ Obernkirchen & \(-93\)2 & 2.0 \(\pm\) 0.1 & 432  & 8.9 \(\pm\) 0.3 & \(-85\pm 10\) & 2.2 \(\pm\) 0.2 & 1 \(\pm\) 1 & 7.1 \(\pm\) 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Values of \(\zeta\) potential at pH 7 (in mV) and isoelectric points (in pH units) of lithotypes before and after treatment with WNC/ANC. \begin{table}
\begin{tabular}{c c} \hline \hline
**Coating** & **IEP (pH Units)** \\ \hline WNC & 10.3 \(\pm\) 0.7 \\ ANC & 7.9 \(\pm\) 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Isoelectric points (IEP, in pH units) of the two nanocomposites applied on glass slides. The extent to which the isoelectric point of stones is modified upon treatment may not linearly correlate with the actual fraction of surface coverage (SEM micrographs taken to assess the modification of surface morphology are available as Supplementary Materials, Figures S1-S3), a complex parameter of surface modification also affected by the stone porosity features being more likely [32] and, when it applies, by the occurrence of chemical reactions between the stone and the coating material. Indeed, in the case of a porous substrate, the streaming current reflects not only the charge of the EDL from the external surface, but also the contribution from pore walls sensed by the tangential electrolyte flow. Thus, the penetration depth of the treating product and its effectiveness in generating a thin coating layer on the pore walls are additional factors affecting the streaming current measurements. In our case, the results of electrokinetic measurements clearly indicated that, while both coating formulations succeeded in producing a significant modification of the surface chemical properties of the treated stones, the surface distribution of the coating material was less uniform than expected. However, this may not be considered as an indication of poor effectiveness of a water repellent coating material.