pH effects on molecular adsorption and solvation of p-nitrophenol at silica/aqueous interfaces.

Resonance-enhanced second-harmonic generation (SHG) was used to examine the effects of solution pH and surface charge on para-nitrophenol (pNP) adsorption to silica/aqueous interfaces. During the early stages of monolayer formation, SHG spectra of interfacial pNP showed a single resonant excitation wavelength at approximately 313 nm regardless of solution pH. This resonance wavelength of adsorbed species is lower than the 318 nm excitation maximum of pNP in bulk aqueous solution. Experiments were performed at pHs of 1.0, 5.0, 7.0, and 10.5. Under these conditions, the silica surface carried a surface charge that ranged from slightly positive (pH = 1) to strongly negative (pH = 10.5) due to protonation/deprotonation of surface silanol groups. Over the course of 1-3 h, SHG spectra of pNP evolved so that spectra from interfaces fully equilibrated with solution pH showed two clear resonance features with wavelengths of approximately 310 and 330 nm. These wavelengths imply that adsorbed pNP samples two discrete local solvation environments at the silica/aqueous interface. On the basis of the solvatochromic behavior of pNP in different bulk solvents, the shorter-wavelength feature corresponds to a local environment having an effective dielectric constant of 9.5 (similar to that of dichloromethane), while the longer-wavelength feature lies outside of pNP's standard solvatochromic window. This longer-wavelength result implies an effective dielectric constant greater than that of bulk water or an adsorption mechanism that has pNP adsorbates sharing a proton with surface silanol groups (and adopting an electronic structure that begins to resemble that of its deprotonated form, p-nitrophenoxide). The longer-wavelength feature is weakest in the low-pH systems when the surface is either neutral or slightly positively charged and most prominent at the negatively charged silica/aqueous (pH = 10.5) interface. pNP adsorption isotherms for all systems showed approximate Langmuir behavior. Using concentration-dependent data from both low and intermediate pH led to calculated adsorption energies of -19 ± 2 kJ/mol for all pH values except pH 10.5 where ΔG(ads) was -6 ± 2 kJ/mol. Taken together, these spectroscopic and adsorption studies of pNP adsorption to silica/aqueous interfaces as a function of aqueous pH show that interfacial acid/base chemistry can require hours to reach equilibrium and that the silica surface presents hydrogen-bonding solutes such as pNP with two distinct adsorption sites. The invariance of pNP's SHG spectra to bulk solution pH suggests that pNP solvation is dominated by substrate-solute interactions, with the adjacent solvent having very little influence on adsorbed solute properties.