Molecular theory on dielectric constant at interfaces: A molecular dynamics study of the water/vapor interface

Spontaneous Appearance of Triiodide Covering the Topmost Layer of the Iodide Solution Interface Without Photo-Oxidation

Ions containing iodine atoms at the vapor-aqueous solution interfaces critically affect aerosol growth and atmospheric chemistry due to their complex chemical nature and multivalency. While the surface propensity of iodide ions has been intensely discussed in the context of the Hofmeister series, the stability of various ions containing iodine atoms at the vapor-water interface has been debated. Here, we combine surface-specific sum-frequency generation (SFG) vibrational spectroscopy with ab initio molecular dynamics simulations to examine the extent to which iodide ions cover the aqueous surface. The SFG probe of the free O-D stretch mode of heavy water indicates that the free O-D group density decreases drastically at the interface when the bulk NaI concentration exceeds ∼2 M. The decrease in the free O-D group density is attributed to the spontaneous appearance of triiodide that covers the topmost interface rather than to the surface adsorption of iodide. This finding demonstrates that iodide is not surface-active, yet the highly surface-active triiodide is generated spontaneously at the water-air interface, even under dark and oxygen-free conditions. Our study provides an important first step toward clarifying iodine chemistry and pathways for aerosol formation.

On the Fresnel factor correction of sum-frequency generation spectra of interfacial water

Insights into the microscopic structure of aqueous interfaces are essential for understanding the chemical and physical processes on the water surface, including chemical synthesis, atmospheric chemistry, and events in biomolecular systems. These aqueous interfaces have been probed by heterodyne-detected sum-frequency generation (HD-SFG) spectroscopy. To obtain the molecular response from the measured HD-SFG spectra, one needs to correct the measured ssp spectra for local electromagnetic field effects at the interface due to a spatially varying dielectric function. This so-called Fresnel factor correction can change the inferred response substantially, and different ways of performing this correction lead to different conclusions about the interfacial water response. Here, we compare the simulated and experimental spectra at the air/water interface. We use three previously developed models to compare the experiment with theory: an advanced approach taking into account the detailed inhomogeneous interfacial dielectric profile and the Lorentz and slab models to approximate the interfacial dielectric function. Using the advanced model, we obtain an excellent quantitative agreement between theory and experiment, in both spectral shape and amplitude. Remarkably, we find that for the Fresnel factor correction of the ssp spectra, the Lorentz model for the interfacial dielectric function is equally accurate in the hydrogen (H)-bonded region of the response, while the slab model underestimates this response significantly. The Lorentz model, thus, provides a straightforward method to obtain the molecular response from the measured spectra of aqueous interfaces in the H-bonded region.

The dielectric function profile across the water interface through surface-specific vibrational spectroscopy and simulations

The dielectric properties of interfacial water on subnanometer length scales govern chemical reactions, carrier transfer, and ion transport at interfaces. Yet, the nature of the interfacial dielectric function has remained under debate as it is challenging to access the interfacial dielectric with subnanometer resolution. Here we use the vibrational response of interfacial water molecules probed using surface-specific sum-frequency generation (SFG) spectra to obtain exquisite depth resolution. Different responses originate from water molecules at different depths and report back on the local interfacial dielectric environment via their spectral amplitudes. From experimental and simulated SFG spectra at the air/water interface, we find that the interfacial dielectric constant changes drastically across an ∼1 Å thin interfacial water region. The strong gradient of the interfacial dielectric constant leads, at charged planar interfaces, to the formation of an electric triple layer that goes beyond the standard double-layer model.

Polarization-Dependent Heterodyne-Detected Sum-Frequency Generation Spectroscopy as a Tool to Explore Surface Molecular Orientation and Ångström-Scale Depth Profiling

Sum-frequency generation (SFG) spectroscopy provides a unique optical probe for interfacial molecules with interface-specificity and molecular specificity. SFG measurements can be further carried out at different polarization combinations, but the target of the polarization-dependent SFG is conventionally limited to investigating the molecular orientation. Here, we explore the possibility of polarization-dependent SFG (PD-SFG) measurements with heterodyne detection (HD-PD-SFG). We stress that HD-PD-SFG enables accurate determination of the peak amplitude, a key factor of the PD-SFG data. Subsequently, we outline that HD-PD-SFG can be used not only for estimating the molecular orientation but also for investigating the interfacial dielectric profile and studying the depth profile of molecules. We further illustrate the variety of combined simulation and PD-SFG studies.

Polarization-Dependent Sum-Frequency Generation Spectroscopy for Ångstrom-Scale Depth Profiling of Molecules at Interfaces

The three-dimensional spatial distribution of molecules at soft matter interfaces is crucial for processes ranging from membrane biophysics to atmospheric chemistry. While several techniques can access surface composition, obtaining information on the depth distribution is challenging. We develop a noninvasive, polarization-resolved, surface-specific sum-frequency generation spectroscopy providing quantitative depth information. We demonstrate the technique on formic acid molecules at the air-water interface. With increasing molar fraction from 2.5% to 10%, the formic acid molecules shift, on average, ∼0.9  Å into the bulk. The consistency with the simulation data manifests that the technique allows for probing the Ångstrom-scale depth profile.

Dispersion of Complex Refractive Indices for Intense Vibrational Bands. II. Implication to Sum Frequency Generation Spectroscopy

Sharp and intense vibrational bands are characterized with conspicuous dispersion of complex refractive indices. Based on the quantitative data of dispersion in the preceding paper, this paper clarifies the influence of the dispersion on the sum frequency generation (SFG) spectroscopy. As a consequence of the large dispersion, the lineshapes of SFG spectra could be influenced by the frequency dependence of the Fresnel factor as well as the nonlinear susceptibility. This paper argues the relative importance of the two factors in general cases and provides a useful criterion to evaluate their importance. The effect of Fresnel dispersion becomes significant when the SFG spectrum involves a sharp and intense vibrational band as well as a large non-resonant background susceptibility, typically in some solid-liquid interfaces. A possible way to correct the effect of Fresnel dispersion is suggested using the heterodyne measurement.

Ab initiomodelling of interfacial electrochemical properties: beyond implicit solvation limitations

First-principles calculations are an important tool to investigate the complex processes occurring at solid/liquid interfaces which are at the heart of modern technologies. Currently, capturing the whole electrochemical environment at an interface, including the applied potential and solvation, still remains challenging as it necessitates to couple different approaches whose interactions are not fully understood. In this work, a grand canonical density functional theory approach is coupled with solvation models to investigate the electrochemical interfaces under applied potential. We show that a parametrized polarizable continuum model (PCM) which represent solvation in a mean field approach by a continuous polarizable media, possesses catastrophic limitations for the modelling of ionic and charged interfaces. We reveal the origin of PCM instabilities under chemical or electrochemical strong oxidation to be the consequence of a phase transition in the surface Li electronic structure. Thus, PCM undergoes an unphysical response to this phase transition by penetrating within the atomic radius of surface Li atoms. To recover a physical response, an explicit first solvation shell has to be included in addition to the PCM in order to properly describe the electrochemistry of the interface. The Fukui functions show that the first solvation shell becomes involved in the redox process as solvent electron doublet is transferred to the acidic Li⁺. If another explicit solvent layer is added, the interface electrochemical properties become independent of the PCM parameters: in particular capacitance can then be computed from a parameter-free electrochemical approach. This is an important conclusion as the experimental electrochemical capacitance are not easily found and thus the parametrization of the PCM for electrochemical interface can be difficult. This approach can easily be applied to investigate electrochemical properties at the atomic scale and generalized to any electrochemical device for which interfaces play a crucial role.

Dielectric Behavior near a Spherical Ion

The dielectric response of a polar solvent to an ion is analyzed in terms of the bound charge, the net charge that accumulates near the ion as a consequence of the inhomogeneous polarization of the surrounding solvent. We demonstrate that the total bound charge arising in a full molecular treatment is identical to the total bound charge from standard continuum theory. In continuum theory, the bound charge resides in an infinitely thin layer, while in a molecular description the bound charge is spread over a region of finite width. Near simple atomic ions, the width of the bound charge distribution is roughly 1.3 nm. By simulating a sequence of ion charges from 0.1 to 2 e, where e is the magnitude of the electron charge, we analyze the applicability of linear response theory, which has been used by several authors. With increasing charge, the nonlinear response extends to an increasing distance from the ion. However, outside the region containing bound charge, the response is linear and in accord with continuum theory. Previous attempts to assign a dielectric constant to a solvent in the interfacial region are analyzed.

Development of quadrupole susceptibility automatic calculator in sum frequency generation spectroscopy and application to methyl C-H vibrations

Sum frequency generation (SFG) spectroscopy has been established as a powerful interface probe technique based on the electric dipole approximation, while possible signals of quadrupole and bulk origin have also been known for a long time. In this work, we developed a computational tool, namely, Qsac (quadrupole susceptibility automatic calculator), to evaluate the comprehensive contributions of the dipole/quadrupole and interface/bulk in the arbitrary vibrational bands of SFG spectra. The calculations of relevant susceptibility terms are performed on the basis of the theory of energy representation using quantum chemical calculation and molecular dynamics simulation, which allows for semi-quantitative comparison among these terms on the same footing. We applied the Qsac to the methyl C-H stretching bands of organic molecules and found a general trend that the weak asymmetric bands are more sensitive to the bulk contribution than the symmetric ones. The phases of interface and bulk terms tend to cancel in the asymmetric band, which results in the reduced band intensity in the SFG spectra.

Bi-layering at ionic liquid surfaces: a sum-frequency generation vibrational spectroscopy- and molecular dynamics simulation-based study

Room-temperature ionic liquids (RTILs) are being increasingly employed as novel solvents in several fields, including chemical engineering, electrochemistry, and synthetic chemistry. To further increase their usage potential, a better understanding of the structure of their surface layer is essential. Bi-layering at the surfaces of RTILs consisting of 1-alkyl-3-methylimidazolium ([C_nmim]⁺; n = 4, 6, 8, 10, and 12) cations and bis(trifluoromethanesulfonyl)amide ([TFSA]^-) anions was demonstrated via infrared-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulations. It was found that the sum-frequency (SF) signal from the [TFSA]^- anions decreases as the alkyl chain length increases, whereas the SF signal from the r⁺ mode (the terminal CH₃ group) of the [C_nmim]⁺ cations is almost the same regardless of chain length. MD simulations show the formation of a bi-layered structure consisting of the outermost first layer and a submerged second layer in a "head-to-head" molecular arrangement. The decrease in the SF signals of the normal modes of the [TFSA]^- anions is caused by destructive and out-of-phase interference of vibrations of corresponding molecular moieties oriented toward each other in the first and second layers. In contrast, the r⁺ mode of [C_nmim]⁺ does not experience destructive interference because the peak position of the r⁺ mode differs marginally at the surface and in the bulk. Our conclusions are not limited to the system presented here. Similar bi-layered structures can be expected for the surfaces of conventional RTILs, which necessitates the consideration of bi-layering in the design and application.

Static dielectric constant of water within a bilayer using recent water models: a molecular dynamics study

The water confined within a surfactant bilayer is studied using different water models via molecular dynamics simulations. We considered four representative rigid models of water: the SPC/E and the TIP4P/2005, which are commonly used in numerical calculations and the more recent TIP4Q and SPC/ε models, developed to reproduce the dielectric behaviour of pure water. The static dielectric constant of the confined water was analyzed as a function of the temperature for the four models. In all cases it decreases as the temperature increases. Additionally, the static dielectric constant of the bilayer-water system was estimated through its expression in terms of the fluctuations in the total dipole moment, usually applied for isotropic systems. The estimated dielectric was compared with the available experimental data. We found that the TIP4Q and the SPC/ε produce closer values to the experimental data than the other models, particularly at room temperature. It was found that the probability of finding the sodium ion close to the head of the surfactant decreases as the temperature increases, thus the head of the surfactant is more exposed to the interaction with water when the temperature is higher.

Effects of third-order susceptibility in sum frequency generation spectra: a molecular dynamics study in liquid water

When sum frequency generation (SFG) spectroscopy is applied to charged solid/liquid interfaces, the observed SFG signals include both the second-order and third-order polarizations. The latter is called the χ⁽³⁾ effect, which mainly includes induced molecular orientation by electric fields at charged interfaces. We theoretically evaluate the χ⁽³⁾ effect on the SFG spectroscopy of liquid water using molecular dynamics (MD) simulations. The MD simulations enable us to definitely calculate the χ⁽³⁾ susceptibility as a bulk property, and thereby separating it from the usual χ⁽²⁾ effect shown in the SFG spectra. The calculated results of χ⁽³⁾ for liquid water are fairly consistent with the experimental estimates. The present finding is utilized to analyze the spectral change of SFG at the air/water interface under electric fields and at the charged silica/water interface. The present analysis of the spectral changes allows for distinguishing the intrinsic change of the interface structure and the χ⁽³⁾ effect from bulk liquid.

Giant Pockels effect of polar organic solvents and water in the electric double layer on a transparent electrode

The Pockels effect of polar organic solvents and water within the electric double layer on an ITO electrode is studied to find that water has the largest Pockels coefficient, followed in order by methanol, ethanol, and dimethyl sulfoxide.

Vibrational Sum Frequency Generation by the Quadrupolar Mechanism at the Nonpolar Benzene/Air Interface

The interface selectivity of vibrational sum frequency generation (VSFG) spectroscopy is explained under the dipole approximation as resulting from the breakdown of inversion symmetry at the interface. From this viewpoint, VSFG is not expected to occur at the interface consisting of centrosymmetric molecules, because the inversion symmetry is preserved even at the interface. In reality, however, VSFG at the nonpolar benzene/air interface has been observed with traditional homodyne-detected VSFG. Here we report a heterodyne-detected VSFG study of the benzene/air interface. The result strongly indicates that VSFG at this interface cannot be explained within the framework of the dipole approximation. The selection rule and polarization dependence of the observed VSFG signal show that the quadrupole transition plays an essential role because of the field discontinuity at the interface. This finding implies the applicability of interface-selective VSFG to the nonpolar interfaces comprising centrosymmetric molecules, which opens a new possibility of VSFG spectroscopy.