Geochemical Insight from Nonlinear Optical Studies of Mineral-Water Interfaces.

The physics and chemistry of mineral-water interfaces are complex, even in idealized systems. Our need to understand this complexity is driven by both pure and applied sciences, that is, by the need for basic understanding of earth systems and for the knowledge to mitigate our influences upon them. The second-order nonlinear optical techniques of second-harmonic generation and sum-frequency generation spectroscopy have proven adept at probing these types of interfaces. This review focuses on the contributions to geochemistry made by nonlinear optical methods. The types of questions probed have included a basic description of the structure adopted by water molecules at the mineral interface, how flow and porosity affect this structure, adsorption of trace metal and organic species, and dissolution mechanisms. We also discuss directions and challenges that lie ahead and the outlook for the continued use of nonlinear optical methods for studies of mineral-water boundaries.

Surface-Bulk Vibrational Correlation Spectroscopy.

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.

Simultaneous measurement of magnitude and phase in interferometric sum-frequency vibrational spectroscopy.

We present a visible-infrared sum-frequency spectroscopic technique that is capable of simultaneously determining the magnitude and phase of the sample response from a single set of experimental conditions. This is especially valuable in cases where the phase stability is high, as in collinear beam geometries, as it enables multiple experiments to be performed without re-measuring the local oscillator phase or the reference phase. After illustrating the phase stability achievable with such a geometry, we provide a technique for quantitatively determining the magnitude and phase from a single set of two-dimensional spectral-temporal interference fringes. A complete demonstration is provided for the C-H stretching frequency region at the surface of an octadecyltricholosilane film.

Phase measurement in nondegenerate three-wave mixing spectroscopy.

A detailed model is presented that describes the temporal and spectral interference patterns resulting from phase-recovery infrared-visible sum-frequency spectroscopy. Included in this model are the effects of dispersive elements other than the phase shifting unit placed between the sample and local oscillator signals. This inclusion is critical when considering the interference patterns arising from studies of buried interfaces. Furthermore, in the midinfrared where it is difficult to have high visibility of the fringes, it is demonstrated that local field corrections have a significant effect on the shape of the interference pattern. By collecting and subsequently fitting a two-dimensional interference pattern displaying both temporal and spectral fringes, a complete characterization of all these effects is possible.

Direct Measurement of Photoacoustic Signal Sensitivity to Aerosol Particle Size.

Continuing efforts to quantify the influence of aerosol light absorption upon global heat budgets rely on high-quality measurements of aerosol optical properties. Of the available methods, photoacoustic spectroscopy stands out as a sensitive method for measurements of aerosol absorption with minimal sample modification. Theoretical treatments of photoacoustic aerosol detection have predicted size-dependent damping of the photoacoustic signal as a result of particle thermal inertia. We provide experimental confirmation of this prediction using a single-particle photoacoustic spectrometer, which allows us to measure photoacoustic signals with high sensitivity and size-specificity. Both the magnitude and phase of the photoacoustic response follow the linearized description of the heat flux. The quantification of this effect provides a basis for future, system-specific case studies.

Throwing Salt into the Mix: Altering Interfacial Water Structure by Electrolyte Addition.

Interfacial water commonly distinguishes itself from the bulk phase by adopting a polar, ordered structure. It is well-established that electrolytes can act to perturb this structure; however, the nature of this perturbation remains a topic of interest. In this study, surface- and structure-sensitive nonlinear vibrational spectroscopy is used to monitor electrolyte-induced changes in interfacial water structure. Solution ionic strength was varied over 5 orders of magnitude, and spectra were collected from two mineral surfaces (fused silica and calcium fluoride) and two polymer surfaces (polystyrene and poly(methyl methacrylate)). Analysis of the spectra reveals striking similarities and differences between these four aqueous interfaces; trends in overall intensity do not always follow changes in the spectral shape. Our results reveal the complex interplay between surface charge, ion adsorption, and hydrophobicity in determining interfacial water structure in the presence of dissolved ions.

Molecular-level surface structure from nonlinear vibrational spectroscopy combined with simulations.

Vibrational sum-frequency generation spectroscopy is valued for its ability to selectively probe molecules at a variety of interfaces without the use of extrinsic chromophores. The spectra contain valuable information regarding the molecular structure and the interfacial environment through the observation of vibrational resonances associated with specific moieties. Chemical information is obtained by close inspection of the frequencies of these bands and the amplitude of the response under conditions of different beam polarizations. Such sensitivity motivates the development of techniques that can provide structural details. We illustrate several approaches by which various types of calculations and molecular simulations may be used to enhance the sought structural interpretation of experimental data. By applying these techniques to the adsorbate molecules, interfacial water, and the substrate surfaces themselves, we are able to achieve a holistic picture of the adsorption environment.