Vibrational sum frequency generation (SFG) spectroscopy was utilized to distinguish different populations of water molecules within the electric double layer (EDL) at the silica/water interface. By systematically varying the electrolyte concentration, surface deprotonation, and SFG polarization combinations, we provide evidence of two regions of water molecules that have distinct pH-dependent behavior when the Stern layer is present (with onset between 10 and 100 mM NaCl). For example, water molecules near the surface in the Stern layer can be probed by the pss polarization combination, while other polarization combinations (ssp and ppp) predominantly probe water molecules further from the surface in the diffuse part of the electrical double layer. For the water molecules adjacent to the surface within the Stern layer, upon increasing the pH from the point-of-zero charge of silica (pH ∼2) to higher values (pH ∼12), we observe an increase in alignment consistent with a more negative surface with increasing pH. In contrast, water molecules further from the surface appear to exhibit a net flip in orientation upon increasing the pH over the same range, which we attribute to the presence of the Stern layer and possible overcharging of the EDL at lower pH. The opposing pH-dependent behavior of water in these two regions sheds new light on our understanding of the water structure within the EDL at high salt concentrations when the Stern layer is present.

Historically, different pH dependent behaviour at the mineral oxide/aqueous electrolyte interface has been observed by non-resonant second harmonic generation (SHG) and resonant sum frequency generation (SFG), despite a general understanding that both techniques are dominated by the response of water. Here we compare the two at the silica/aqueous interface at high salt concentration and as a function of pH to shed light on the origins of both measurements. From this comparison and SHG measurements at the silica/air interface, we conclude that SHG originates from the net order of water and the silica substrate, with the latter dominating the observed intensities below pH 6.5. In contrast, SFG is dominated by the higher SF activity, yet lower number density, of waters that contribute in the low wavenumber range, according to molecular dynamic simulations. Furthermore, spectral resolution in SFG of oppositely oriented water populations prevents cancellation of signal making it more difficult to relate SF intensity to the net order of water.

Homo- and heterospectral correlation analysis are powerful methods for investigating the effects of external influences on the spectra acquired using distinct and complementary techniques. Nonlinear vibrational spectroscopy is a selective and sensitive probe of surface structure changes, as bulk molecules are excluded on the basis of symmetry. However, as a result of this exquisite specificity, it is blind to changes that may be occurring in the solution. We demonstrate that correlation analysis between surface-specific techniques and bulk probes such as infrared absorption or Raman scattering may be used to reveal additional details of the adsorption process. Using the adsorption of water and ethanol binary mixtures as an example, we illustrate that this provides support for a competitive binding model and adds new insight into a dimer-to-bilayer transition proposed from previous experiments and simulations.

Surface-specific nonlinear vibrational spectroscopy, combined with bulk solution measurements and imaging, is used to study the surface conditions during the growth of E. coli. As a result of the silica high surface charge density, the water structure at the silica-aqueous interface is known to be especially sensitive to pH and ionic strength, and surface concentration profiles develop that can be appreciably different from the bulk solution conditions. We illustrate that, in the presence of growing cells, a unique surface micro-environment is established as a result of metabolites accumulating on the silica surface. Even in the subsequent absence of the cells, this surface layer works to reduce the interfacial ionic strength as revealed by the enhanced signal from surface water molecules. In the presence of growing cells, an additional boost in surface water signal is attributed to a local pH that is higher than that of the bulk solution.

A new method is proposed to analyze doubly-resonant infrared-visible sum-frequency (DR-SFG) spectra. Based on the transform technique, this approach is free from assumptions about vibronic modes, energies, or line widths, and accurately captures through the overlap spectral function all required aspects of the vibronic structure from simple experimental linear absorption spectra. Details and implementation of the method are provided, along with three examples treating rhodamine thin films about one monolayer thick. The technique leads to a perfect agreement between experiment and simulations of the visible DR-SFG lineshapes, even in the case of complex intermolecular interactions resulting from J-aggregated chromophores in heterogeneous films. For films with mixed H-and J-aggregates, separation of their responses shows that the J-aggregate DR-SFG response is dominant. Our analysis also accounts for the unexplained results published in the early times of DR-SFG experiments.