XPS study of the silica–water interface

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XPS was applied to study the silica–water interface formed in NaCl solution at variable pH (2–9) and ionic strength (10–600 mM). To prevent alteration of the interface, the wet silica pastes were precooled with liquid nitrogen. After measurements under liquid nitrogen, XPS spectra were collected the next day at room temperature to follow changes at the silica surface caused by water loss. XPS data show that, at low ionic strength (<20 mM), Cl − ions could not be detected at the surface even at the lowest pH. Changes of the Na/Si atomic ratio with pH are in close agreement with the adsorption curve of OH − ions. At higher ionic strength (>100 mM), the silica surface’s point of zero charge can be determined from the Na/Cl atomic ratio. At pH 9 the amount of “free” Na + at the surface is independent of NaCl concentration reflecting the formation of SiONa bonds, confirmed by O 1s spectra. Removing water from the sample surface allows determination of a “built-in” potential at the interface. Si 2p and O 1s spectra of the matrix shift to lower BE values whereas Na 1s and Cl 2p lines of the counter-ions shift to higher BE. At higher ionic strength and pH<2, shifts of Si 2p and O 1s spectra change sign, reflecting the formation of positively charged silica surfaces. The absolute value of the photoelectron line shifts for the matrix elements does not exceed 0.25 eV at any pH, whereas the shifts for counter-ions can reach 0.6 eV. The shifts decrease with increasing ionic strength, implying the formation of a more compact interface.