Vibrational sum-frequency generation spectroscopy as a molecular-level probe for aqueous, polymer, and solid interfaces
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Interfaces play a crucial role in a variety of physical, chemical, and biological phenomena. Due to the unique local environment, chemical and physical properties observed at the interface are different than in bulk media. This work exploits the highly selective surface layer sensitivity of the second-order phenomenon vibrational sum-frequency generation (SFG) spectroscopy to learn about interfacial chemistry and structure. The surface pKa of substituted benzoic acids and short-chain carboxylic acids are found by SFG spectroscopy and surface tension measurements. Measured surface pKa values are greater than bulk solution values and are attributed to increased surface proton concentrations. Stronger van der Waals interactions are also found to increase pKa. Anomalously large carbonyl stretching SFG intensities are found in a narrow pH range and provide evidence for a cooperative surface adsorption effect between p-methyl benzoic acid and its conjugate base. In SFG studies of poly(N-isopropylacrylamide) (pNIPAM), Hofmeister anion effects are observed for the amide I band. Furthermore, SFG spectra reveal two distinct pNIPAM amide I bands. One peak is centered at 1625 cm-1, consistent with bulk FTIR studies, and a second peak is observed at 1665 cm-1, blue shifted by 40 cm-1. The “bulk-like” peak (1625 cm-1) arises from fully solvated pNIPAM molecules located just below the interfacial layer and is enhanced in the presence of chaotropic anions. The blue shifted peak is attributed to decreased water solvation of pNIPAM molecules at the uppermost surface layer. An odd-even effect is observed in the SFG spectra of n-alkanethiolate self-assembled monolayers (SAMs) on Au and Ag surfaces depending on surface roughness. Odd-even oscillations in SFG signal amplitudes and linewidths were observed for spectra derived from alkyl thiols with either odd or even numbers of carbons for SAMs on flat surfaces (RMS roughness = 0.40 nm), but not on rougher surfaces (RMS roughness = 2.38 nm). This effect is attributed to differences in terminal methyl group orientation for odd or even n. This work demonstrates the capability of SFG spectroscopy to be an effective tool for determining surface number density, equilibrium constants, and molecular orientation for molecules at the interface.
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