Influence of High pH on the Organization of Acetonitrile at the Silica/Water Interface Studied by Sum Frequency Generation Spectroscopy

Efficient Quantum Vibrational Spectroscopy of Water with High-Order Path Integrals: From Bulk to Interfaces

Vibrational spectroscopy is key in probing the interplay between the structure and dynamics of aqueous systems. To map different regions of experimental spectra to the microscopic structure of a system, it is important to combine them with first-principles atomistic simulations that incorporate the quantum nature of nuclei. Here we show that the large cost of calculating the quantum vibrational spectra of aqueous systems can be dramatically reduced compared with standard path integral methods by using approximate quantum dynamics based on high-order path integrals. Together with state-of-the-art machine-learned electronic properties, our approach gives an excellent description not only of the infrared and Raman spectra of bulk water but also of the 2D correlation and the more challenging sum-frequency generation spectra of the water-air interface. This paves the way for understanding complex interfaces such as water encapsulated between or in contact with hydrophobic and hydrophilic materials through robust and inexpensive surface-sensitive and multidimensional spectra with first-principles accuracy.

Liquid flow reversibly creates a macroscopic surface charge gradient

The charging and dissolution of mineral surfaces in contact with flowing liquids are ubiquitous in nature, as most minerals in water spontaneously acquire charge and dissolve. Mineral dissolution has been studied extensively under equilibrium conditions, even though non-equilibrium phenomena are pervasive and substantially affect the mineral-water interface. Here we demonstrate using interface-specific spectroscopy that liquid flow along a calcium fluoride surface creates a reversible spatial charge gradient, with decreasing surface charge downstream of the flow. The surface charge gradient can be quantitatively accounted for by a reaction-diffusion-advection model, which reveals that the charge gradient results from a delicate interplay between diffusion, advection, dissolution, and desorption/adsorption. The underlying mechanism is expected to be valid for a wide variety of systems, including groundwater flows in nature and microfluidic systems.

Reactions at the interface between mineral surfaces and flowing liquids are ubiquitous in nature. Here the authors explore, using surface-specific sum frequency generation spectroscopy and numeric calculations, how the liquid flow affects the charging and dissolution rates leading to flow-dependent charge gradients along the surface.

Intermolecular Interactions at the Silica-Liquid Interface Modulate the Fermi Resonance Coupling in Surface Methanol

The buried solid/liquid interface between hydrophilic fused silica and binary solvent mixtures of acetonitrile (MeCN) and methanol (MeOH) was studied with vibrational sum-frequency generation (vSFG) spectroscopy. Our data showed that at high relative concentrations of methanol, the Fermi resonance peak in the vSFG spectrum is greatly suppressed, and it progressively gains intensity as methanol is diluted with perdeuterated acetonitrile. This phenomenon is quantified by the Fermi resonance coupling coefficient, W, extracted using a two-level model, as well as the experimental intensity ratio, R, of the methyl Fermi resonance band to that of the symmetric stretch. At a 1.0 MeOH mole fraction, W and R values were 10 ± 10 cm^-1 and 0.01 ± 0.02, respectively, whereas at a 0.1 mole fraction, W and R increased to 46 ± 4 cm^-1 and 0.43 ± 0.16, respectively. This indicates that solvation with acetonitrile effectively tunes the Fermi coupling of methanol vibrations at the silica/liquid interface.

Vibrational Sum-Frequency Generation Hyperspectral Microscopy for Molecular Self-Assembled Systems

In this review, we discuss the recent developments and applications of vibrational sum-frequency generation (VSFG) microscopy. This hyperspectral imaging technique can resolve systems without inversion symmetry, such as surfaces, interfaces and noncentrosymmetric self-assembled materials, in the spatial, temporal, and spectral domains. We discuss two common VSFG microscopy geometries: wide-field and confocal point-scanning. We then introduce the principle of VSFG and the relationships between hyperspectral imaging with traditional spectroscopy, microscopy, and time-resolved measurements. We further highlight crucial applications of VSFG microscopy in self-assembled monolayers, cellulose in plants, collagen fibers, and lattice self-assembled biomimetic materials. In these systems, VSFG microscopy reveals relationships between physical properties that would otherwise be hidden without being spectrally, spatially, and temporally resolved. Lastly, we discuss the recent development of ultrafast transient VSFG microscopy, which can spatially measure the ultrafast vibrational dynamics of self-assembled materials. The review ends with an outlook on the technical challenges of and scientific potential for VSFG microscopy.

Application of the Approximate 3D-Reference Interaction Site Model (RISM) Molecular Solvation Theory to Acetonitrile as Solvent

The integral equation formalism based reference interaction site model (RISM) molecular solvation theory is applied to pure liquid acetonitrile and water-acetonitrile binary mixtures of different compositions. Solvate formation of d- and f-block ions by ACN is also calculated to check applicability of the RISM theory. The generalized Amber force field (GAFF) and the universal force field (UFF) parameters were found to be suitable for applications with the RISM theory for acetonitrile solvent. The presence of local microsolvated clusters for water-acetonitrile mixtures is validated in the RISM calculations.

Structure and sum-frequency generation spectra of water on uncharged Q4 silica surfaces: a molecular dynamics study

The structural characteristics and sum-frequency generation (SFG) spectra of water near neutral Q₄ silica surfaces were investigated by molecular dynamics simulations. The interactions of water molecules with atoms of the solid were described by different potential models, in particular by the CLAYFF [Cygan et al., J. Phys. Chem. B, 2004, 108, 1255] and INTERFACE [Heinz et al. Langmuir, 2013, 29, 1754] force fields. The calculations of the contact angle of water have shown that the silica surface modeled with CLAYFF behaves as macroscopically hydrophilic, in contrast to the surface described with the INTERFACE model. The hydrophilicity of CLAYFF stems from too attractive electrostatic surface-water interactions. Regardless of the surface's affinity for water, the aqueous phase has a layered structure in the direction perpendicular to the surface with density fluctuations decaying within a distance of 10 Å from the surface. The orientational ordering of H₂O molecules was found to be more short-range than the density fluctuations, especially for the hydrophobic surfaces. Modeling the SFG spectra has shown that the spectra of all studied hydrophobic silica-water interfaces are similar and have features in common with the spectrum of the water-vapor interface. The spectra fairly agree with experimental results obtained for the silica-water interface at low pH conditions [Myalitsin et al., J. Phys. Chem. C, 2016, 120, 9357]. The spectral response for the hydrophobic interface was computed to primarily arise from the topmost molecules of the first layer of interfacial water. In contrast, the SFG signal from the hydrophilic silica-water interface is accumulated over a greater distance extending for several water layers due to more long-range perturbation of the structure by the surface.