Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 6744−6749
Neat Water−Vapor Interface: Proton Continuum and the Nonresonant Background Sanghamitra Sengupta,† Daniel R. Moberg,‡ Francesco Paesani,‡,§ and Eric Tyrode*,† †
Department of Chemistry, KTH Royal Institute of Technology, SE-10044 Stockholm, Sweden Department of Chemistry and Biochemistry, Materials Science and Engineering, §San Diego Supercomputer Center, University of California, San Diego 92093, United States
‡
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S Supporting Information *
ABSTRACT: Whether the surface of neat water is “acidic” or “basic” remains an active and controversial field of research. Most of the experimental evidence supporting the preferential adsorption of H3O+ ions stems from nonlinear optical spectroscopy methods typically carried out at extreme pH conditions (pH < 1). Here, we use vibrational sum frequency spectroscopy (VSFS) to target the “proton continuum”, an unexplored frequency range characteristic of hydrated protons and hydroxide ions. The VSFS spectra of neat water show a broad and nonzero signal intensity between 1700 and 3000 cm−1 in the three different polarization combinations examined. By comparing the SF response of water with that from dilute HCl and NaOH aqueous solutions, we conclude the intensity does not originate from either adsorbed H3O+ or OH− ions. Contributions from the nonresonant background are then critically considered by comparing the experimental results with many-body molecular dynamics (MB-MD) simulated spectra.
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targeting the OH stretches of water42−45 and the SH resonant response46,47 of highly acidic solutions (i.e., pH < 1). At conditions closer to neutral pH, the evidence from these surface specific techniques remains, however, inconclusive. Directly targeting the hydrated proton vibrations36 in the broad continuum could help gain the necessary sensitivity. In this study, we present VSFS spectra of the neat water− vapor interface in an extended spectral region that includes all fundamental vibrations. Spectra in the three polarization combinations examined show a nonzero intensity in the characteristic proton continuum region that stretches between 1700 and 3000 cm−1. To determine the origin of this intensity, we first consider resonant contributions, which include the bend + libration combination band of water and the potential adsorption of hydronium or hydroxide ions to the liquid/vapor interface. Contributions from the nonresonant background are then examined by comparing the experimental observations with many body molecular dynamics simulations performed with the MB-pol potential energy function. The starting point for the discussion is the conventional SF |χ(2)|2 spectra of neat water presented in Figure 1 collected under the polarization combinations SSP, SPS, and PPP. Each polarization probes different components of χ(2) providing complementary information on the identity and orientation of surface species.6 Spectra were collected using a femtosecond SF spectrometer that allows for the automated collection of data in extended spectral regions48 (see the Supporting
he interfacial molecular properties of water, the most ubiquitous of all liquids, have a fundamental importance for a vast number of fields in science and technology, ranging from simple wetting phenomena to complex biological and environmental processes.1−4 Due to the asymmetry imposed by the interface, the structure and dynamics of water molecules at the surface generally differ significantly from those in the bulk.5−7 With its intrinsic surface specificity, vibrational sum frequency spectroscopy (VSFS) in its homodyne, phasesensitive, and time-resolved configurations has been the preferred experimental technique to investigate interfacial water molecules.8−21 Concurrent advances in molecular theoretical methods capable of predicting the SF response have also played a critical role in improving our understanding of the interfacial water structure.22−29 Most of these studies, however, have focused on the OH stretching vibrations, with only recent interest in the OH bending19,30−33 and water libration20,26 modes. A region of interest that remains unexplored is the broad continuum between the bending and stretching fundamentals characteristic of hydrated protons,34,35 which also encompasses the bend + libration combination band centered at ∼2130 cm−1. Bulk Raman and IR studies of concentrated acid and base solutions have shown that not only hydrated protons35,36 but also hydrated hydroxide ions37,38 give rise to characteristic broad absorption bands in this “proton” continuum region that are absent in pure water. At surfaces, there is still an intense disagreement on whether protons or hydroxide ions preferentially adsorb to the water− vapor interface.35,39−41 VSFS and second harmonic generation (SHG), have provided indirect evidence for the preferential adsorption of hydronium ions to extended flat interfaces, by © XXXX American Chemical Society
Received: October 5, 2018 Accepted: November 8, 2018 Published: November 8, 2018 6744
DOI: 10.1021/acs.jpclett.8b03069 J. Phys. Chem. Lett. 2018, 9, 6744−6749
Letter
The Journal of Physical Chemistry Letters
groups point on average toward the bulk liquid. The two hydrogen-bonded sub-bands can also be resolved in the PPP spectrum (Figure 1) that is in turn dominated by an intense free OH band. Probing four different χ(2) elements, the features in the PPP spectra depend significantly on the experimental geometry chosen.55 At the angles of incident used here (70° and 55° for the visible and IR beams, respectively), the free OH in the PPP spectrum is relatively enhanced when compared to the SSP spectrum but remains in excellent agreement with the expected ratio following a previous study using multiple experimental configurations.17 The SPS spectrum in the OH stretching region has been less frequently reported in the literature,16,17,42,49 primarily owing to its lower intensity. Besides the free OH at ∼3698 cm−1, additional features not previously resolved can be clearly seen in the spectrum (Figure 1). They appear as relatively narrow dips at ∼3250, ∼3480, and ∼3650 cm−1, together with a small shoulder at ∼3750 cm−1 (see enlarged spectrum in the Supporting Information). The band at ∼3650 cm−1 is reminiscent of the shoulder observed in the imaginary SSP spectrum,52−54 which has been assigned to either an antisymmetric stretch of water molecules participating in three hydrogen bonds (two donors and one acceptor),22 or the Fermi resonance between the free OH and a combination band involving a low frequency intermolecular hydrogen bond vibration (∼200 cm−1).56 The shoulder observed here at ∼3750 cm−1 suggests an alternative explanation. Following gas phase IR studies in small water clusters,57 the two bands are assigned to the symmetric (∼3650 cm−1) and antisymmetric (∼3750 cm−1) OH stretch of water molecules with both OHs free from hydrogen bonds (single or double acceptor water molecules). In the OH bending region (1500−1750 cm−1), the bending mode is clearly resolved in all three polarization combinations, including SPS, which had not been previously reported (Figure 1). The intensity in SPS is, however, much weaker when compared to those of SSP and PPP. The bending modes in not only SSP but also PPP show the characteristic dispersive shape previously reported using a similar experimental geometry.19,30 Central to this study is the spectral region extending between 1700 and 3000 cm−1, where a nonzero intensity is observed in all three polarization combinations (Figure 1). Several alternatives can be proposed to explain its origin. First, we consider potential resonant contributions. IR and Raman spectra of bulk water show the presence of a weak and broad peak centered at ∼2130 cm−1 assigned to the bend + libration combination band58,59 (see inset in Figure 2). However, the limited breadth (
