## 4 Conclusions  
Electrokinetic measurements, being sensitive to molecular-level modifications and yet averaging the response over a macroscopically-sized surface area, provide information useful for a more accurate understanding of the different contributions (mineralogy and morphology of stone and nanocomposite product formulation) to the ultimate performance of a coating material. In fact, while they cannot provide spatially-resolved information on the chemical composition of a macroscopic surface, their sensitivity to the extent and nature of the actual surface modifications and the mere fact that the measurements are performed with a probing aqueous electrolyte instead of the high vacuum dry conditions required in most microscopy and spectroscopy measurements offer unique possibilities to investigate the properties of porous materials exposed to the outer environment. This is particularly true for deteriorated stones and for evaluating the effectiveness of treatments expected to protect the  
Figure 6: Correlation between the index of surface modification (ISM, Equation (3)) and \(\Delta\theta\) (see Section 2.4.3) for lithotypes after treatment with WNC/ANC. stone from weathering, the latter being largely determined by the interaction of the porous stone system with condensed water. In spite of the potential of the technique, several reasons may have prevented until now its use in the field of stone conservation. Among them, the relatively recent availability of user-friendly commercial equipment for streaming potential and streaming current measurements, which have been mostly used by scientists studying separation membranes, materials for biomedical applications and oil well systems. In addition, the morphological complexity and intrinsic mineralogical variability of natural stones (also resulting from chemical transformations upon aging and from the presence of soluble salts in the stone porous network) require a cautious approach to using this technique in order to take into account the many factors influencing the measurements. Only recently, the first examples of streaming potential technique used for measuring the extent of coverage of inorganic nonporous surfaces by nanoparticle adsorption have been reported [46; 47]. While the studied systems were simpler than those investigated here, a nontrivial use of theory related to the electrokinetic phenomena was involved. With the present work:  
* electrokinetic analysis based on streaming current measurements was shown to be a sensitive and effective tool to characterize the modification of the surface properties of different natural stones upon application of coating materials;
* a correlation was found between the extent of surface modification (according to the empirical ISM parameter) and the effectiveness of the products as determined by the reduction of water absorption by capillarity and of surface wettability for the six investigated lithotype-nanocomposite coating combinations;
* significant differences were observed between the low porosity Apuan marble and the other two more porous lithotypes, as the former prevents to a large extent penetration of more viscous fluids and possibly of nanoparticles, while the difference between products is, to a good extent, levelled out in the case of the most porous lithotypes as a result of product absorption into the stone matrix. In summary, the ISM parameter based on streaming current measurements was shown to provide quantitative information on the quality and uniformity of the modification produced by the application of a surface treatment material on a porous substrate when a very thin (not occlusive of the pore structure) coating layer is required. In particular, the effectiveness of the treatment correlates well with the ISM data unless saturation, that is exhaustive coverage of the surface including the pore walls of the outermost layer probed by the streaming current measurement, is achieved.