## 3 Results and Discussion
### Electrokinetic Analysis of Nanocomposites  
From the results of the streaming current measurements on the two nanocomposite coatings applied onto glass slides (Table 5), quite different IEPs were found, as expected from the different chemical structure of their components. In particular, the IEP of the WNC coating, which is based on an aminoalkyl trialkoxysilane precursor, lies in the alkaline pH region, in agreement with the presence of an amine group, its value being close to those reported in the literature for other materials modified with aminosilane or aminosiloxane compounds [40; 41]. On the other hand, the IEP of the ANC coating, based on an alkyl trialkoxysilane precursor, while being lower than that of WNC, turned out to be quite higher than those reported elsewhere for similar compounds [42]; this mismatch may depend on  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Stone Type** & **Calcite + Dolomite 1** & \(\zeta\) **(pH 7)** & \(\zeta\) **(pH 5)** & **IEP** \\  & **(mol\%)** & **(mV)** & **(mV)** & **(pH Units)** \\ \hline Apuan marble & \(99.6\) & \(-60\pm 4\) & \(-47\pm 4\) & \(3.7\pm 0.1\) \\ Balegem & \(53.1\) & \(-57\pm 6\) & \(-48\pm 8\) & \(3.0\pm 0.3\) \\ Obernkirchen & \(1.2\) & \(-88\pm 10\) & \(-73\pm 5\) & \(2.1\pm 0.2\) \\ Calcite monocrystal & \(100\) & - & \(-39\) & \(3.6\) \\ \hline \end{tabular}  
* 1 from X-ray diffraction measurements [27].  
\end{table}
Table 4: Mineralogical composition (calcite + dolomite content, in mol%), values of \(\zeta\) potential at pH 7 and 5 (in mV) and isoelectric points (IEP, in pH units) of untreated lithotypes.  
Figure 2: Example of titration curves expressing \(\zeta\) potential as a function of pH for the three lithotypes: Apuan marble, Balegem limestone and Obernkirchen sandstone.  
the different surface density of slightly acidic silanol groups resulting from the alkoxysilane hydrolysis not followed by exhaustive sol-gel condensation. In an aqueous environment, silanols are in their hydrolys form and bear a negative charge, thus contributing to reducing the \(\zeta\) potential (and the IEP) of the surface. Depending on the molecular structure of the precursor and on the experimental conditions upon coating application (temperature, humidity, presence of catalytically active acidic or basic species in the formulation or at the surface of the glass substrate that was pre-treated with Piranha solution), the surface density of free silanol groups may change, and the \(\zeta\) potential may be affected accordingly. In addition, the surface density of silanols may change over an extended period of time (much longer than the four weeks of ageing allowed here to stabilize the coating before performing the measurements), as the sol-gel hydrolysis and condensation reactions proceed. Finally, a possibly not negligible contribution of the TiO\({}_{2}\) nanoparticles to the electrokinetic properties of the nanocomposite film surface should also be considered. In fact, TiO\({}_{2}\) has an IEP of 6.2, higher than that of pure silica, with little difference between the two most common crystalline polymorphs anatase and rutile [43]. Streaming current measurements performed on coatings cast on glass slides from the respective. TiO\({}_{2}\)-free silane dispersions gave the same results as the respective fully formulated coatings containing TiO\({}_{2}\) nanoparticles, possibly because of the low concentration of TiO\({}_{2}\) nanoparticles and/or because they become mainly embedded in the organosilica gel network.