Yes.

Another possibility is dissolved carbon dioxide. Water quickly picks up CO2 from the atmosphere and, in the absence of a buffer, will result in a final pH of about 5.5.

Thanks John!

The authors were actually alluding to the nanoparticles possessing a pH and not the dispersion media (water) hence, changes in pH corresponding to changes in the concentration of the nanoparticles.

Is it okay to think of nanoparticles as hydroxyl/hydrogen ions?

pH is a bulk property of the liquid, not the particle. But the properties of the surface of the particle can affect the pH of the liquid.

Consider a silica particle. Chemically, it is quite simple - (SiO2)n with surface -OH groups. Let's call the particle P. The similarity with water is clear: P-OH vs. H-OH. All of the -OH groups of the particle can be considered to behave similarly to water. This means the following equilibrium can exist:

P-OH2+ + -OH P-OH + H20 P-O- + H3O+

just as with water.

Of course, the equilibrium constants will be different than for water.

It isn't just the chemical nature of the particle that can affect pH. So can adsorbed materials.

Prof. John Miller,

Thanks for your logical explanation and analogy!

I totally agree with you.