Osmolytes, small molecules synthesized by all organisms, play a crucial role in tuning protein stability and function under variable external conditions. Despite their electrical neutrality, osmolytes action is entwined with that of cellular salts and protons in a mechanism only partially understood. To elucidate this mechanism, we utilize an ultra-high resolution FM- AFM for measuring the effect of two biological osmolytes, urea and glycerol, on the surface charge of silica, an archetype protic surface with pK value similar to that of acidic amino acids. We find that addition of urea, a known protein destabilizer, enhances silica deprotonation and hence surface charge by more than 50%, an effect equivalent to a 4 units increase of pH. Conversely, addition of glycerol, a protein stabilizer, practically neutralizes the silica surface, an effect equivalent to 2 units reduction of pH. Simultaneous measurements of the interfacial liquid viscosity indicate that urea accumulates extensively near the silica surface while glycerol depletes there. Comparison between the measured surface charge and Gouy-Chapman-Stern model for the silica surface teaches that the modification of surface charge is 4 times too large to be explained by the change in dielectric constant upon addition of urea or glycerol. The model hence leads to the conclusion that surface charge is chiefly governed by the effect of osmolytes on the surface reaction constants, namely, on silanol deprotonation and on cation binding.  These findings highlight the unexpectedly large effect that neutral osmolytes may have on surface charging and Coulomb interactions.