Counterion Effect on Interfacial Water at Charged Interfaces and Its Relevance to the Hofmeister Series

Specific counterion effects represented by Hofmeister series are important for a variety of phenomena such as protein precipitations, surface tensions of electrolytes solutions, phase transitions of surfactants, etc. We applied heterodyne-detected vibrational sum-frequency generation spectroscopy to study the counterion effect on the interfacial water at charged interfaces and discussed the observed effect with relevance to the Hofmeister series. Experiments were carried out for model systems of positively charged cetyltrimethylammonium monolayer/electrolyte solution interface and negatively charged dodecylsulfate monolayer/electrolyte interface. At the positively charged interface, the intensity of the OH band of the interfacial water decreases in the order of the Hofmeister series, suggesting that the adsorbability of halide anions onto the interface determines the Hofmeister order as previously proposed by Zhang and Cremer (<i>Curr. Opin. Chem. Biol.</i> <b>2006</b>, <i>10</i>, 658–663). At the negatively charged interfaces, on the other hand, the OH band intensity does not depend significantly on the countercation, whereas variation in the hydrogen-bond strength of the interfacial water is well correlated with the Hofmeister order of the cation effect. These results provide new insights into the molecular level mechanisms of anionic and cationic Hofmeister effects

Spectroscopy and Dynamics of the Multiple Free OH Species at an Aqueous/Hydrophobic Interface

Sum frequency generation (SFG) spectra and free induction decay (FID) measurements of the H₂O/octadecylsilane (ODS)/silica interface in the free OH spectral region (∼3700 cm^–1) show spatially inhomogeneous behavior. The SFG spectra and FIDs suggest an inhomogeneous response of the free OH, consisting of at least two distinct species at the interface with short and long coherence times. In most areas of the sample, an OH band at ∼3680 cm^–1 with a short dephasing (<150 fs), assigned to the free OH of water interacting with the hydrophobic methyl group of ODS, was observed in agreement with previously reported SFG spectra of the H₂O/ODS/silica interface. In a small fraction (∼20%) of the sample areas, a more intense peak at ∼3700 cm^–1 was observed in the SFG spectrum characterized by significantly longer dephasing (∼760 fs) in the SFG-FID. Based on the peak position, as well as control experiments on octadecydimethylmethoxysilane (ODMS) monolayers and deuterium substitution experiments at the water/ODS/silica interfaces, two possible assignments for the new feature are provided. The long dephasing can be due to the free OH of the Si–OH of incompletely cross-linked/tethered ODS molecules. Alternatively, a contribution of water molecules trapped in nano pores of silica surface and/or confined between the ODS molecules can explain the long coherence. Either way, the long coherence can be attributed to the OH species decoupled from bulk water

Observation of the Bending Mode of Interfacial Water at Silica Surfaces by Near-Infrared Vibrational Sum-Frequency Generation Spectroscopy of the [Stretch + Bend] Combination Bands

Vibrational sum-frequency generation (SFG) spectroscopy of interfacial water at mineral/aqueous interfaces is extended to the near-IR range containing the low cross section stretch + bend combination bands (ν_comb = ν_OH + δ_HOH) of liquid water at silica surfaces near 5000–5300 cm^–1, for the first time. The assignments of SFG spectra are supported by FTIR and Raman spectroscopic measurements of the bulk water ν_comb modes. The SFG spectra contain significant contributions from two combinations, [ν_s + δ] ≈ 5060 cm^–1 and [ν_as + δ] ≈ 5300 cm^–1. These measurements provide the first, to our knowledge, reported probe of the bending mode of water at buried interfaces. The data suggest that the interfacial water bending mode is blue-shifted from that of bulk water

The Topmost Water Structure at a Charged Silica/Aqueous Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Despite recent significant advances in interface-selective nonlinear spectroscopy, the topmost water structure at a charged silica surface is still not clearly understood. This is because, for charged interfaces, not only interfacial molecules at the topmost layer but also a large number of molecules in the electric double layer are probed even with second-order nonlinear spectroscopy. In the present study, we studied water structure at the negatively charged silica/aqueous interface at pH 12 using heterodyne-detected vibrational sum frequency generation spectroscopy, and demonstrated that the spectral component of the topmost water can be extracted by examining the ionic strength dependence of the Imχ⁽²⁾ spectrum. The obtained Imχ⁽²⁾ spectrum indicates that the dominant water species in the topmost layer is hydrogen-bonded to the negatively charged silanolate at the silica surface with one OH group. There also exists minor water species that weakly interacts with the oxygen atom of a siloxane bridge or the remaining silanol at the silica surface, using one OH group. The ionic strength dependence of the Imχ⁽²⁾ spectrum indicates that this water structure of the topmost layer is unchanged in a wide ionic strength range from 0.01 to 2 M

Three Distinct Water Structures at a Zwitterionic Lipid/Water Interface Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation

Lipid/water interfaces and associated interfacial water are vital for various biochemical reactions, but the molecular-level understanding of their property is very limited. We investigated the water structure at a zwitterionic lipid, phosphatidylcholine, monolayer/water interface using heterodyne-detected vibrational sum frequency generation spectroscopy. Isotopically diluted water was utilized in the experiments to minimize the effect of intra/intermolecular couplings. It was found that the OH stretch band in the Imχ⁽²⁾ spectrum of the phosphatidylcholine/water interface exhibits a characteristic double-peaked feature. To interpret this peculiar spectrum of the zwitterionic lipid/water interface, Imχ⁽²⁾ spectra of a zwitterionic surfactant/water interface and mixed lipid/water interfaces were measured. The Imχ⁽²⁾ spectrum of the zwitterionic surfactant/water interface clearly shows both positive and negative bands in the OH stretch region, revealing that multiple water structures exist at the interface. At the mixed lipid/water interfaces, while gradually varying the fraction of the anionic and cationic lipids, we observed a drastic change in the Imχ⁽²⁾ spectra in which spectral features similar to those of the anionic, zwitterionic, and cationic lipid/water interfaces appeared successively. These observations demonstrate that, when the positive and negative charges coexist at the interface, the H-down-oriented water structure and H-up-oriented water structure appear in the vicinity of the respective charged sites. In addition, it was found that a positive Imχ⁽²⁾ appears around 3600 cm^–1 for all the monolayer interfaces examined, indicating weakly interacting water species existing in the hydrophobic region of the monolayer at the interface. On the basis of these results, we concluded that the characteristic Imχ⁽²⁾ spectrum of the zwitterionic lipid/water interface arises from three different types of water existing at the interface: (1) the water associated with the negatively charged phosphate, which is strongly H-bonded and has a net H-up orientation, (2) the water around the positively charged choline, which forms weaker H-bonds and has a net H-down orientation, and (3) the water weakly interacting with the hydrophobic region of the lipid, which has a net H-up orientation

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

Water Structure at the Buried Silica/Aqueous Interface Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation

Complex χ⁽²⁾ spectra of buried silica/isotopically diluted water (HOD-D₂O) interfaces were measured using multiplex heterodyne-detected vibrational sum frequency generation spectroscopy to elucidate the hydrogen bond structure and up/down orientation of water at the silica/water interface at different pHs. The data show that vibrational coupling (inter- and/or intramolecular coupling) plays a significant role in determining the χ⁽²⁾ spectral feature of silica/H₂O interfaces and indicate that the doublet feature in the H₂O spectra does not represent two distinct water structures (i.e., the ice- and liquid-like structures) at the silica/water interface. The observed pH dependence of the imaginary χ⁽²⁾ spectra is explained by (1) H-up oriented water donating a hydrogen bond to the oxygen atom of silanolate, which is accompanied by H-up water oriented by the electric field created by the negative charge of silanolate, (2) H-up oriented water which donates a hydrogen bond to the neutral silanol oxygen, and (3) H-down oriented water which accepts hydrogen bonds from the neutral silanol and donates hydrogen bonds to bulk water molecules. The broad continuum of the OH stretch band of HOD-D₂O and a long tail in the low frequency region represent a wide distribution of strong hydrogen bonds at the silica/water interface, particularly at the low pH

Partially Hydrated Electrons at the Air/Water Interface Observed by UV-Excited Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Hydrated electrons are the most fundamental anion species, consisting only of electrons and surrounding water molecules. Although hydrated electrons have been extensively studied in the bulk aqueous solutions, even their existence is still controversial at the water surface. Here, we report the observation and characterization of hydrated electrons at the air/water interface using new time-resolved interface-selective nonlinear vibrational spectroscopy. With the generation of electrons at the air/water interface by ultraviolet photoirradiation, we observed the appearance of a strong transient band in the OH stretch region by heterodyne-detected vibrational sum-frequency generation. Through the comparison with the time-resolved spectra at the air/indole solution interface, the transient band was assigned to the vibration of water molecules that solvate electrons at the interface. The analysis of the frequency and decay of the observed transient band indicated that the electrons are only partially hydrated at the water surface, and that they escape into the bulk within 100 ps

Cooperative Hydrogen-Bond Dynamics at a Zwitterionic Lipid/Water Interface Revealed by 2D HD-VSFG Spectroscopy

Molecular-level elucidation of hydration at biological membrane interfaces is of great importance for understanding biological processes. We studied ultrafast hydrogen-bond dynamics at a zwitterionic phosphatidylcholine/water interface by two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy. The obtained 2D spectra confirm that the anionic phosphate and cationic choline sites are individually hydrated at the interface. Furthermore, the data show that the dynamics of water at the zwitterionic lipid interface is not a simple sum of the dynamics of the water species that hydrate to the separate phosphate and choline. The center line slope (CLS) analysis of the 2D spectra reveals that ultrafast hydrogen-bond fluctuation is not significantly suppressed around the phosphate at the zwitterionic lipid interface, which makes the hydrogen-bond dynamics look similar to that of the bulk water. The present study indicates that the hydrogen-bond dynamics at membrane interfaces is not determined only by the hydrogen bond to a specific site of the interface but is largely dependent on the water dynamics in the vicinity and other nearby moieties, through the hydrogen-bond network

Unified Molecular View of the Air/Water Interface Based on Experimental and Theoretical χ⁽²⁾ Spectra of an Isotopically Diluted Water Surface

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 <i>how water minimizes the surface free energy</i>. 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