RESUMO
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.
RESUMO
Second order nonlinear optical spectroscopy has been employed to examine the organization of four different liquids at the hydrophilic silica/liquid interface. The liquids - cyclohexane, methylcyclohexane, 1-propanol, and 2-propanol - were chosen to isolate how intermolecular forces between the liquid and the substrate competed with steric effects to control liquid structure and solvating properties across the interfacial region. Vibrational sum frequency generation (VSFG) data showed that cyclohexane structure at the silica/liquid cyclohexane interface closely resembled the structure of a cyclohexane monolayer adsorbed to the silica/vapor interface. Methylcyclohexane, however, showed evidence of large structural reorganization between the silica/liquid and silica/monolayer/vapor interfaces. 1-Propanol at a silica/vapor interface formed a well-ordered, Langmuir-like monolayer due to strong hydrogen bonding with the surface silanols and cohesive van der Waals interactions between carbon chains. 1-Propanol at the silica/liquid interface retained the same ordered structure. In contrast, 2-propanol adopted different structures adsorbed to the solid/vapor and at the solid/ liquid interfaces. Specifically, the plane defined by 2-propanol's three carbon atoms changed orientation from being perpendicular to the surface (silica/vapor) to parallel to the surface (silica/liquid). Surface mediated liquid structure affected the solvation of adsorbed solutes. Resonance enhanced second harmonic generation (SHG) data showed that silica/alkane interfaces were significantly more polar than would be expected based on a solute's bulk solution solvatochromic behavior. Both silica/alcohol interfaces exhibited alkane-like polarity, a result that was interpreted in terms of a reduction in hydrogen bonding opportunities for adsorbed solutes.
