Figure and Figure show that for the pure water systems there is a clear correlation between the number of HBs and the surface tension: the interface with the lowest surface tension (water in contact with a positive surface) has the highest number of HBs. Wang et al. found a similar correlation for water wetting mica surfaces. This correlation is valid whether we look at the number of HBs in the first Figure or second layer (Figure ). In line with the correlation identified for pure water, in the electrolyte solutions the number of HBs for the interfacial water in contact with the positively charged surface (the most hydrophilic) is always higher than that calculated for the negatively charged and neutral surfaces, and it decreases with increasing ionic concentration. This correlation holds even if we account for the fact that the amount of interfacial water decreases with increasing salt concentration and calculate the number of hydrogen bonds (HBs) per water molecule Figure . In general, we observe that for each ionic concentration, the number of HBs remains the highest for the positively charged surface, while the number of HBs at the negatively charged surface consistently exceeds that of the neutral surface.