Combined Sum Frequency Generation and Thin Liquid Film Study of the Specific Effect of Monovalent Cations on the Interfacial Water Structure

Abstract

Some salts have been recently shown to decrease the sum frequency generation (SFG) intensity of the hydrogen-bonded water molecules, but a quantitative explanation is still awaited. Here, we report a similar trend for the chloride salts of monovalent cations, that is, LiCl, NaCl, and CsCl, at low concentrations. Specifically, we revealed not only the specific adsorption of cations at the water surface but also the concentration-dependent effect of ions on the SFG response of the interfacial water molecules. Our thin-film pressure balance (TFPB) measurements (stabilized by 10 mM of methyl isobutyl carbinol) enabled the determination of the surface potential that governs the surface electric field affecting interfacial water dipoles. The use of the special alcohol also enabled us to identify a remarkable specific screening effect of cations on the surface potential. We explained the concentration dependency by considering the direct ion-water interactions and water reorientation under the influence of surface electric field as the two main contributors to the overall SFG signal of the hydrogen-bonded water molecules. Although the former was dominant only at the low-concentration range, the effect of the latter intensified with increasing salt concentration, leading to the recovery of the band intensity at medium concentrations. We discussed the likelihood of a correlation between the effect of ions on reorientation dynamics of water molecules and the broad-band intensity drop in the SFG spectra of salt solutions. We proposed a mechanism for the cation-specific effect through the formation of an ionic capacitance at the solution surface. It explains how cations could impart the ion specificity while they are traditionally believed to be repelled from the interfacial region. The electrical potential of this capacitance varies with the charge separation and ion density at the interface. The charge separation being controlled by the polarizability difference between anions and cations was identified using the SFG response of the interfacial water molecules as an indirect probe. The ion density being affected by the absolute polarizability of ions was tracked through the measurement of the surface potentials and Debye-Hückel lengths using the TFPB technique.

Revealing Interfacial Lipid Hydrolysis Catalyzed by Phospholipase A1 at Molecular Level via Sum Frequency Generation Vibrational Spectroscopy and Fluorescence Microscopy

“…The distinct vibrational peak centered at 1660 cm −1 can clearly be observed in the amide I region, as shown in panel a of Figure 5, like the spectrum in panel e of Figure 3 added to destroy the interfacial water layer. 42 The corresponding spectrum is shown in panel c of Figure 5, top one (green), with a clear peak centered at 3300 cm −1 (denoted by the yellow dashed line), indicating that PLA 1 indeed stayed at the interface. The other spectral difference after the addition of the calcium ions was the enhanced C−H stretching signals.…”

“…The CH 3 ss mode centered at 2875 cm –1 and Fermi resonance (FR) centered at 2933 cm –1 , contributed by the alkyl chains of the fatty acids, were also observed. In order to visualize the N–H stretching signal centered at 3300 cm –1 from PLA 1 , the calcium ions were added to destroy the interfacial water layer . The corresponding spectrum is shown in panel c of Figure , top one (green), with a clear peak centered at 3300 cm –1 (denoted by the yellow dashed line), indicating that PLA 1 indeed stayed at the interface.…”

Quantifying Double-Layer Potentials at Liquid–Gas Interfaces from Vibrational Sum-Frequency Generation

“…13 Note that the film thickness measured by optical methods is an equivalent thickness h eq that differs from the real physical thickness d of the water core film, because the thickness of the adsorbate layer contributes to h eq . Although d ≈ h eq is often assumed for simplicity, 22 a more detailed analysis involves the exact value of d = h eq – Δ h corr , where Δ h corr is a correction factor. 18,50,71 We found a value of 3.2 ± 0.6 nm for Δ h corr , wherein the standard deviation is due to approximations for the thickness and the refractive index of the adsorbate layer which are used in the data analysis (Supporting Information).…”

“…In this respect, much progress in molecular-level understanding of aqueous interfaces has been developed due to the fruitful feedback between experiments and simulations. 31,37−44 Although previous studies already used a combination of SFG and TFPB, 12,22 the authors applied SFG mainly to address the structure of the interface but not to extract quantitative information on ϕ 0 . In fact, the relation between the SFG intensity of O−H stretching bands and ϕ 0 has not been discussed quantitatively in these works.…”

Effects of Ca2+ Ion Condensation on the Molecular Structure of Polystyrene Sulfonate at Air–Water Interfaces

“…The nonresonant contribution is caused mainly by electronic excitations at the interface, whereas the resonant contribution arises from molecular vibrations. In the case of charged surfaces, an additional static electric field leads to a third-order contribution χ (3) due to the polarization and polar ordering of water molecules near and at the charged surface. − The Debye length κ –1 and the surface potential ϕ 0 are decisive factors for the magnitude of this contribution. The resonant part depends on the resonance frequency ω k , the bandwidth Γ of the vibrational mode k , and the oscillator strength A k ∝ N ⟨α k μ k ⟩, which is determined by the molecular number density N and the orientational average of the Raman polarizability α k and the dynamic dipole moment μ k : In the case of centrosymmetric materials or isotropic bulk liquids, this orientational average and consequently the oscillator strength equals zero. However, the symmetry break at interfaces gives rise to nonzero contributions to χ (2) that originate purely from interfacial molecules.…”