Resonance-enhanced second-harmonicgeneration (SHG) was used toexamine the effects of solution pH and surface charge on para-nitrophenol (pNP) adsorption to silica/aqueous interfaces. Duringthe early stages of monolayer formation, SHG spectra of interfacialpNP showed a single resonant excitation wavelength at approximately313 nm regardless of solution pH. This resonance wavelength of adsorbedspecies is lower than the 318 nm excitation maximum of pNP in bulkaqueous solution. Experiments were performed at pHs of 1.0, 5.0, 7.0,and 10.5. Under these conditions, the silica surface carried a surfacecharge 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 thatspectra from interfaces fully equilibrated with solution pH showedtwo clear resonance features with wavelengths of approximately 310and 330 nm. These wavelengths imply that adsorbed pNP samples twodiscrete local solvation environments at the silica/aqueous interface.On the basis of the solvatochromic behavior of pNP in different bulksolvents, the shorter-wavelength feature corresponds to a local environmenthaving an effective dielectric constant of 9.5 (similar to that ofdichloromethane), while the longer-wavelength feature lies outsideof pNP’s standard solvatochromic window. This longer-wavelengthresult implies an effective dielectric constant greater than thatof bulk water or an adsorption mechanism that has pNP adsorbates sharinga proton with surface silanol groups (and adopting an electronic structurethat begins to resemble that of its deprotonated form, p-nitrophenoxide). The longer-wavelength feature is weakest in thelow-pH systems when the surface is either neutral or slightly positivelycharged and most prominent at the negatively charged silica/aqueous(pH = 10.5) interface. pNP adsorption isotherms for all systems showedapproximate Langmuir behavior. Using concentration-dependent datafrom both low and intermediate pH led to calculated adsorption energiesof −19 ± 2 kJ/mol for all pH values except pH 10.5 whereΔGadswas −6 ± 2 kJ/mol.Taken together, these spectroscopic and adsorption studies of pNPadsorption to silica/aqueous interfaces as a function of aqueous pHshow that interfacial acid/base chemistry can require hours to reachequilibrium and that the silica surface presents hydrogen-bondingsolutes such as pNP with two distinct adsorption sites. The invarianceof pNP’s SHG spectra to bulk solution pH suggests that pNPsolvation is dominated by substrate–solute interactions, withthe adjacent solvent having very little influence on adsorbed soluteproperties. [ABSTRACT FROM AUTHOR]