Interface structure of water/[3-palmitoyl-2-oleoyl-d-glycero-1-phosphatidylcholine] (POPC) lipid layer is investigated with molecular dynamics (MD) simulation by analyzing the recent heterodyne-detected vibrational sum-frequency generation (HD VSFG) spectroscopy. The MD simulation clearly reproduced the experimental HD VSFG spectrum of imaginary susceptibility (Im[χ]), which exhibits two positive bands in the OH stretching vibrations of water. With the help of decomposition MD analysis, we found three kinds of interfacial water in relation to the HD VSFG spectrum. The low-frequency positive band is attributed to the water pointing toward the lipid side, whose orientation is influenced by negatively charged phosphate and positively charged choline of POPC. The high-frequency positive band is attributed to the water bonding with the carbonyl groups of the lipid. The gap between the two positive bands indicates the interfacial water pointing toward the bulk water phase in the vicinity of the choline groups.

Microscopic mechanism of ion transport through water–oil interface was investigated with molecular dynamics simulation. The formation/breaking of a water finger during the ion passage was explicitly formulated in the free energy surface. The calculated 2D free energy surface clearly revealed a hidden barrier of ion passage accompanied by the water finger. This barrier elucidates the retarded rate of interfacial ion transfer.

Polarization dependence of sum frequency generation (SFG) spectroscopy has been widely discussed to detect molecular orientation at surfaces. The present work examines the orientational analysis by molecular dynamics (MD) simulation of methanol/water mixture surfaces with varying concentrations. We calculated by MD the surface structure of the solutions and their SFG spectra in the methyl C–H stretching region, and directly analyzed the relations. The MD calculations reported that (i) the SFG signal of the methyl symmetric stretching exhibits a turnover behavior with increasing concentration and (ii) the polarization ratio is almost invariant over the concentration, while (iii) the orientation of the methyl group significantly randomizes in high concentration. The present work elucidated these three findings in a consistent manner by analyzing the MD calculations.

We elucidated the phase-sensitive sum frequency generation (SFG) spectrum of NaOH aqueous solution by molecular dynamics simulation. The MD analysis reproduced the effects of NaOH on the imaginary χ(2) spectrum, characterized with a positive (upward) shift in the amplitude in midfrequency region (3300–3600 cm–1) and a negative (downward) shift in low frequency (3000–3200 cm–1). We found that the positive shift is a consequence of electric double layers, while the negative shift is attributed to the first solvation shell of OH–. The latter mechanism generally holds for electrolyte solutions, though it becomes conspicuous for the NaOH solution in the low-frequency tail region. These perturbation mechanisms evidence that OH– at the surface is fully hydrated and not preferentially exposed to the surface.

Surface acidity/basicity of water has been elusive, due to contradicting observations from various aspects. One effective definition has been proposed by investigating the protonation equilibrium (pKa) of the indicator species at the water surface. We calculated the pKa of trimethylamine at the surface by using quantum mechanics/molecular mechanics (QM/MM) and thermodynamic integration calculations and elucidated the observed pKa shift at the surface. It was revealed that the observed pKa shift reflects specific properties of the indicator species, particularly relative solvation free energy at the surface. The pKa shift appears within ∼5 Å thickness. One has to properly take account of the specific indicators toward deriving a general picture of surface acidity/basicity from protonation reactions.

Major alteration or even destruction of the hydration shell around interacting molecules and ions in solution is an important process that determines how hydrated substances interact. Therefore, the direct observation of structural changes in hydration shells around solutes in close contact with other solutes or surfaces is important for understanding chemical processes that take place in solution. In the work described in this paper, time-resolved IR absorption measurements were performed to study the interaction of hydrated Na+ or tetrapropylammonium cation (Pr4N+) with a hydrophobic CO-covered Pt surface; the adsorption force between cations and the surface was controlled by using an electrochemical system. We found that the hydrophobic hydration shell of Pr4N+ is initially stabilized on the hydrophobic surface, but application of a strong force to the cation approaching CO destroys the water layers between them. This process is rather slow, taking a few hundred milliseconds. Hydrophilic Na+ behaves quite differently from Pr4N+ due to the different structure of its hydration shell. These experimental results are supported by molecular dynamics simulations.

Highlights•Inner pressure at small cavity is rigorously defined and calculated by molecular dynamics.•Validity of Young–Laplace equation and possible maximum pressure in the cavity are clarified.•Liquid argon and water show analogous behavior of the inner pressure.This letter definitely evaluates inner pressures of microbubbles in liquid argon and water by molecular dynamics simulation. The microbubbles are modeled with spherical cavity, which circumvents most uncertainties about microscopic definition of curved surface and related quantities. In both liquids, the inner pressure deviates downward from the Young–Laplace equation when the cavity radius is smaller than two molecular diameters, and takes a maximum, about 900 and 3000 atm, respectively, at the cavity radius being ∼3 Å. The hydrogen-bonding character of water plays little specific role in the deviation of the inner pressure.Graphical abstract

The evaporation and condensation mechanisms of water through a butanol film on sulfuric acid solution are elucidated by molecular dynamics simulation. A previous experiment by Nathanson et al. reported the mass accommodation coefficient α to be almost unity, whereas MD simulation of water scattering on the butanol film on water predicted a value of α significantly smaller than unity. This discrepancy is elucidated by considering the protonated butanol at the sulfuric acid solution surface, which roughens the surface layer, and the low temperature at the supercooled condition.

Microscopic mechanism of phase transfer catalyst (PTC) is investigated by molecular dynamics simulation. As an example of PTC, tetrabutylammonium cation is treated which facilitates Cl-Cl- transfer from water to chloroform phase. Calculated free energy profiles reveal the effects of PTC in relation to the pertinent change of interface structure. The ion pair formation changes the free energy profiles in the late stage of transfer, where large structural fluctuation emerges, called water finger. The PTC controls the formation of water finger and thereby influences on the free energy. These microscopic insight is important for PTC-assisted reactions at the interface.Graphical abstractHighlights► Phase transfer catalyst (PTC) at liquid–liquid interface is analyzed by MD simulation. ► PTC influences on free energy profiles for ion transport in the oil side of interface. ► Formation/break of ‘water finger’ is a key factor to facilitate ion transport.

We elucidated surface structure and sum frequency generation (SFG) spectra of aqueous NaF and Na2SO4 solutions by molecular dynamics (MD) simulation. These electrolyte solutions contain hard ions, and their surfaces are considered to be void of ion species. Nevertheless, the two solutions have shown remarkably different SFG spectra, implying different surface structure. The present MD analysis confirmed that all the ionic species are repelled from the surface, though their experimental SFG spectra are well reproduced. The difference in their surface structure is observed in the subsurface region, where slight charge separation occurs in the Na2SO4 solution. The perturbation in the SFG spectra is not attributed to the topmost layer but to the water orientation in a fairly deep region of the surface.

Liquid benzene emits a significant sum frequency generation (SFG) signal of C–H stretching vibration, though it is composed of centrosymmetric molecules. The present paper theoretically elucidates the SFG spectrum from liquid benzene by considering two mechanisms, symmetry breaking at the interface and bulk contribution. Molecular dynamics and quantum chemical calculations reproduced the observed SFG spectrum and revealed that the two mechanisms are equally significant in the SFG spectrum of liquid benzene. At the interface, the SFG signal arises from local C–H stretching modes via the mixing of IR active and Raman active modes. The local modes are readily induced by the anisotropic environment at the interface. The observed SFG signal should also involve significant amount of quadrupole contribution, which is attributed to the IR active mode of the bulk liquid.

Flexible and polarizable molecular dynamics simulations are carried out to elucidate the structure and vibrational sum frequency generation (SFG) spectra of two water/organic liquid interfaces: water/carbon tetrachloride (CCl4) and water/1,2-dichloroethane (DCE). Preceding experimental spectra by Richmond and co-workers have shown quite contrasting features of the two SFG spectra. The former spectrum is analogous with that of water/vapor interface with two-band structure, while the latter is structureless without feature of the dangling OH bond. The present calculations well reproduced these experimental features, though the orientational structure of interfacial water is found to be qualitatively similar. The structureless spectrum of the water/DCE is elucidated with considerable local interaction between water and DCE molecules at the interface, which results in red shift and broadening of the dangling OH band of the interfacial water directing toward the DCE phase.

Since the basal plane surface of ice was first observed by sum frequency generation, an extraordinarily intense band for the hydrogen(H)-bonded OH stretching vibration has been a matter of debate. We elucidate the remarkable spectral feature of the ice surface by quantum mechanics/molecular mechanics calculations. The intense H-bonded band is originated mostly from the “bilayer-stitching” modes of a few surface bilayers, through significant intermolecular charge transfer. The mechanism of enhanced signal is sensitive to the order of the tetrahedral ice structure, as the charge transfer is coupled to the vibrational delocalization.Keywords: charge transfer; hydrogen bonding network; molecular dynamics; sum frequency generation; vibrational delocalization;

The energetically unfavorable termination of the hydrogen-bonded network of water molecules at the air/water interface causes molecular rearrangement to minimize the free energy. The long-standing question is how water minimizes the surface free energy. The combination of advanced, surface-specific nonlinear spectroscopy and theoretical simulation provides new insights. The complex χ(2) spectra of isotopically diluted water surfaces obtained by heterodyne-detected sum frequency generation spectroscopy and molecular dynamics simulation show excellent agreement, assuring the validity of the microscopic picture given in the simulation. The present study indicates that there is no ice-like structure at the surface—in other words, there is no increase of tetrahedrally coordinated structure compared to the bulk—but that there are water pairs interacting with a strong hydrogen bond at the outermost surface. Intuitively, this can be considered a consequence of the lack of a hydrogen bond toward the upper gas phase, enhancing the lateral interaction at the boundary. This study also confirms that the major source of the isotope effect on the water χ(2) spectra is the intramolecular anharmonic coupling, i.e., Fermi resonance.

Surface structure of aqueous sulfuric acid solution at a typical atmospheric concentration (0.2x, x: mole fraction) is investigated by close collaboration of molecular dynamics (MD) simulation and sum frequency generation (SFG) measurement. The SFG spectra of both O–H and S–O stretching vibrations are provided with different sets of polarization combination. These sets of experimental spectra are consistently elucidated by the MD calculations. In modeling the surface structure, there exists a major uncertainty about local ion composition at the surface region. To address this uncertainty, we performed MD simulations with various assumptions on the local dissociation constants of sulfuric acid, and searched for the condition to be consistent with the experimental spectra. We have thereby concluded that the first acid dissociation of sulfuric acid is almost complete at the surface, while the second dissociation is more strongly suppressed than in the bulk liquid. The present MD simulation elucidates the ion distribution and molecular orientation at the sulfuric acid solution surface, and also the concentration dependence of the SFG spectrum.

The surface structure of aqueous sulfuric acid solution at 0.02x (x is the mole fraction) concentration is investigated by molecular dynamics (MD) simulation with newly developed flexible and polarizable molecular models. The models are applied to calculate the SFG spectrum on the assumption that the acid dissociation of sulfuric acid at the surface is the same as in the bulk. The calculated SFG spectrum well reproduces the experimental one, indicating that the assumption on the acid dissociation is valid in a relatively low concentration range. The density profiles of the constituent ions at the surface region show distinct surface preference in the order of the hydronium cation, bisulfate anion, and sulfate dianion, which thereby forms an electric double layer and strongly perturbs the orientational structure of surface water. The enhancement of the SFG intensity at the hydrogen-bonding OH stretching region and the depletion at the dangling (free) OH region are elucidated from the interfacial structure.

Surface structure of aqueous sulfuric acid solution at a typical atmospheric concentration (0.2x, x: mole fraction) is investigated by close collaboration of molecular dynamics (MD) simulation and sum frequency generation (SFG) measurement. The SFG spectra of both O–H and S–O stretching vibrations are provided with different sets of polarization combination. These sets of experimental spectra are consistently elucidated by the MD calculations. In modeling the surface structure, there exists a major uncertainty about local ion composition at the surface region. To address this uncertainty, we performed MD simulations with various assumptions on the local dissociation constants of sulfuric acid, and searched for the condition to be consistent with the experimental spectra. We have thereby concluded that the first acid dissociation of sulfuric acid is almost complete at the surface, while the second dissociation is more strongly suppressed than in the bulk liquid. The present MD simulation elucidates the ion distribution and molecular orientation at the sulfuric acid solution surface, and also the concentration dependence of the SFG spectrum.