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Ziyuan Li, Ziwei Chen, Jie Hu, Hao Li, Shan Xi Tian. A New Experimental Method for Investigations on Microstructure of Liquid-Vapor Interface†[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 43-50. doi: 10.1063/1674-0068/cjcp2101002
Citation: Ziyuan Li, Ziwei Chen, Jie Hu, Hao Li, Shan Xi Tian. A New Experimental Method for Investigations on Microstructure of Liquid-Vapor Interface[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 43-50. doi: 10.1063/1674-0068/cjcp2101002

A New Experimental Method for Investigations on Microstructure of Liquid-Vapor Interface

doi: 10.1063/1674-0068/cjcp2101002
More Information
  • Corresponding author: Shan Xi Tian, E-mail: sxtian@ustc.edu.cn
  • Part of special topic of "the New Advanced Experimental Techniques on Chemical Physics".
  • Present address: Department of Physics, Anhui Normal University, Wuhu 241002, China.
  • Received Date: 2021-01-04
  • Accepted Date: 2021-01-18
  • Publish Date: 2021-02-27
  • Many physical, chemical, and biological processes happen in liquid-vapor interface and are profoundly influenced with the local microstructures. In contrast to the liquid bulk, molecular orientation is the remarkable one of asymmetric structural features of the interface. Here we report an experimental method, namely, electron-impact time-delayed mass spectrometry and give a brief review about our recent progresses. This brand-new method not only enables us to have more insights into the interfacial structures, as done with small-angle X-ray and neutron scatterings and vibrational sum frequency generation spectroscopy, but also provides opportunity to explore the electron-driven chemical reactions therein.
  • Part of special topic of "the New Advanced Experimental Techniques on Chemical Physics".
    Present address: Department of Physics, Anhui Normal University, Wuhu 241002, China.
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A New Experimental Method for Investigations on Microstructure of Liquid-Vapor Interface

doi: 10.1063/1674-0068/cjcp2101002

Abstract: Many physical, chemical, and biological processes happen in liquid-vapor interface and are profoundly influenced with the local microstructures. In contrast to the liquid bulk, molecular orientation is the remarkable one of asymmetric structural features of the interface. Here we report an experimental method, namely, electron-impact time-delayed mass spectrometry and give a brief review about our recent progresses. This brand-new method not only enables us to have more insights into the interfacial structures, as done with small-angle X-ray and neutron scatterings and vibrational sum frequency generation spectroscopy, but also provides opportunity to explore the electron-driven chemical reactions therein.

Part of special topic of "the New Advanced Experimental Techniques on Chemical Physics".
Present address: Department of Physics, Anhui Normal University, Wuhu 241002, China.
Ziyuan Li, Ziwei Chen, Jie Hu, Hao Li, Shan Xi Tian. A New Experimental Method for Investigations on Microstructure of Liquid-Vapor Interface†[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 43-50. doi: 10.1063/1674-0068/cjcp2101002
Citation: Ziyuan Li, Ziwei Chen, Jie Hu, Hao Li, Shan Xi Tian. A New Experimental Method for Investigations on Microstructure of Liquid-Vapor Interface[J]. Chinese Journal of Chemical Physics , 2021, 34(1): 43-50. doi: 10.1063/1674-0068/cjcp2101002
  • Basic knowledge about liquid and its interface is an essential prerequisite toward understanding of physical, chemical and biological processes, such as surface catalysis [1], atmospheric aerosol [2, 3], and protein folding [4]. It has been the focus of laboratorial researches for over a century [5-8]. Yet now, molecular-level views on the most familiar water are rudimentary and controversial. Intermolecular interactions in the liquid-vapor interface are distinctly different from those in the bulk, leading to the asymmetric properties of the surface [9, 10]. As an asymmetric feature, molecular orientation in the outermost layer or first molecular layer of the liquid surface [11-15] plays a role in molecular aggregation and assembly [16]. Hydrogen-bonded clusters result in distribution inhomogeneity of the liquid and interfaces of alcohol, water, and other organic molecules. In the liquid-vapor interface, some specific hydrogen-bonded clusters may have a prominent concentration, besides the molecular orientation. Information about these microstructures can be obtained with various experimental methods, such as, small-angle X-ray [14, 17-19] or neutron scattering (SAXS or SANS) [15, 20-22] experiment and vibrational sum frequency generation (SFG) spectroscopy [11-13, 23-25]. Meanwhile, molecular dynamics (MD) simulations [26-28] are required to unscramble the observations and sometime remedy the experimental limitation.

    The SAXS [14, 17-19] or SANS [15, 20-25] experiment is usually performed with a large facility, and optics theory and the sophisticated laser skills are demanded for SFG experiments [13, 23, 29]. Mass spectrometry, frequently used by chemists and analysts, is a more straightforward method to inspect the species from liquid, e.g., liquid chromatography [30] and electrospray [31] mass spectrometry. In principle, they could be applicable to explore the interfacial structures, but unavailable to date. The serious challenge is to disentangle the interfacial species from those of the liquid bulk in a mass spectrum. Recently, we successfully established a new MS method and realized the selective detection of the interfacial species [32]. Using this method, we can map the molecular orientations both in the outermost layer and in the subsurface, and more notably, validate in situ existence of the hydrogen-bonded molecular clusters in the subsurface [32, 33]. Here we briefly review about our recent progresses, as well as the other techniques applied on this topic.

  • X-ray and neutron scattering experiments are analogous, complementary tools for studying microstructural and dynamic properties of condensed matter in different pressures and temperatures. Theories of X-ray and neutron scatterings can be found elsewhere [15, 34], not detailed here. The SAXS or SANS is a kind of coherent elastic interaction in which the difference between incident and scattered wave vectors at angle 2$ \theta $ is small. A typical SAXS/SANS arrangement is schematized in FIG. 1(a). In the SAXS and SANS experiments, the thickness of the molecular layers of the liquid-vapor interface through which the X-rays or neutron passes is controlled imprecisely. The structural features of different local regions such as the outermost layer and the subsurface (several molecular layers underneath the outermost layer) cannot be distinguished, although the information on monolayers at the liquid surface was demonstrated by detecting the reflected X-rays and neutrons [14, 17-19]. Meanwhile, the molecular vapor above the surface definitely interferes the experimental results.

    Figure 1.  Schematics of (a) small-angle X-ray or neutron scattering and (b) vibrational sum frequency generation spectroscopy.

    Infrared and Raman spectroscopies are photoabsorption and photon scattering techniques, respectively. The main problem of infrared spectroscopy for the liquid surface is to extract the very weak signal from the strong absorptions of the surrounding vapor environment and liquid bulk [35, 36]. The inherent difficulty of Raman scattering is its very weak cross-section and correlatively the screening effect of much more intense emissions like elastic Rayleigh scattering and fluorescence [36, 37]. Nevertheless, recent technical improvements have made the conventional infrared and Raman spectroscopies applicable for liquid surface. The most remarkable approach in the interfacial spectroscopy should be the second-order nonlinear optical technology with interface selectivity and sensitivity, i.e., the SFG and its special case, the second harmonic generation (SHG), spectroscopies [11-13, 23-25]. The working principle of SFG can be found in FIG. 1(b), in which two different frequency light beams $ \omega_1 $ and $ \omega_2 $ (usually one is infrared light and the other is visible light) strike the interface and have resonance with the vibrational energy levels of the interfacial molecules, then the photon with $ \omega_1 $+$ \omega_2 $ frequency emits in the decaying from the high-lying to the lower states. A recent review of the SFG theory can be found in Refs.[11, 23].

  • Besides the molecular structures of liquid and interface, the electronic structure also receives much attention since the pioneer study with the X-ray electron spectroscopy (e.g., ESCA [38]). In the ESCA experiment, the liquid sample was carried with a metal thread and introduced into a vacuum chamber [38]. Subsequently, Fenn and co-worker produced a liquid surface in the vacuum with a rotating wheel [39]. Later, a micrometer-sized liquid beam was produced with a microjet [40], in which a stable laminar flow of the cylindrical beam was formed by precisely controlling flow rate. A photoelectron spectrometer of the liquid beam (e.g., with a diameter of 6 μm [41], ) is shown in FIG. 2. Since the liquid sample vaporizes in vacuum, the reaction chamber is usually of 10$ ^{-4} $-10$ ^{-5} $ mbar. As shown in the inset picture of FIG. 2, a skimmer was put close to the laminar flow region (region Ⅰ), while regions Ⅱ and Ⅲ represent the dense vapor and ideal gas regions respectively. The spatial sizes or thicknesses of these regions depend on the cylindrical diameter, liquid volatility, and vacuum evacuation rate.

    Figure 2.  A typical apparatus of photoelectron energy spectrometer (upper) and the zoom-in picture of the liquid beam in vacuum (bottom). In the spectrometer, photoelectrons produced under irradiation of vacuum ultraviolet (VUV) laser pulse pass through a skimmer and their kinetic energies are analyzed with a hemispheric electrostatic analyzer; a liquid nitrogen (LN$ _2 $) cryotrap is used as the liquid dump. In the vacuum, three regions, Ⅰ, Ⅱ, and Ⅲ, are arbitrarily divided around the liquid beam, where $ r_0 $ and $ r $ represent the radius of the liquid beam and the distance from the liquid-vapor interface.

    A lot of efforts have been made to derive the ionization potentials of liquid samples or the binding energy of hydrated electron by using the photoelectron spectrometers of the liquid beam [41-43]. Before escaping from the liquid, the photoelectron collides several times with the surrounding molecules. This process is described with two items, inelastic mean free path (IMFP) and electron attenuation length (EAL), as shown in FIG. 3(a). Only the photoelectron collisions with the surrounding molecules in the liquid are very seldom, EAL$ \approx $IMFP; otherwise, EAL$ < $IMFP. The liquid-beam photoelectron studies indicate that the EAL values rely heavily on the kinetic energy of photoelectrons. In general, the electron-kinetic-energy dependent EAL values show a V-type profile in FIG. 3(b), although the deviations of this profile were proposed [42, 43]. According to this profile, the electron with dozens of eV can go into the liquid with a depth about several angstroms to nanometers. In other words, the depth reached by an incoming electron can be tuned with the kinetic energy of this electron. Above information becomes a key point in the electron collisions with liquid.

    Figure 3.  (a) IMFP is inelastic mean free path and EAL is electron attenuation length in the liquid. (b) The electron EAL is dependent on the electron kinetic energy. The thickness of EAL curve represents the uncertainties derived from experiments or theoretical simulations [42, 43].

  • We began to set up a mass spectrometer to study the electron impacts with liquid beam in 2017, and successfully obtained the mass spectra of the ionic products of liquid ethanol [32]. As shown in FIG. 4, that apparatus is primarily consisted of a pulsed electron gun, a liquid microjet to product a cylindrical beam (with a diameter of 25 μm), a linear time-of-flight (TOF) mass spectrometer (90 cm length), and three liquid nitrogen cryotraps. A set of Helmholtz coils (not shown in FIG. 4) are established outside the vacuum chamber to collimate the electron beam. More details about the operation procedure can be found in Ref.[32], where the time-delay ($ \Delta t $) between the electron beam and the ion extractor pulses is essential to disentangle the ionic products of regions Ⅱ and Ⅲ from those of region Ⅰ. For the ions produced in regions Ⅱ and Ⅲ, (ⅰ) the intensity of each peak in the mass spectra is weakened gradually with the increase of $ \Delta t $, (ⅱ) the peak position shifts to a longer flight time with an interval equal to $ \Delta t $, (ⅲ) a proper $ \Delta t $ facilitates the formation and collection of the protonated parent ion via the ion-molecule reaction. Only the $ \Delta t $ value is long enough or without the voltage pulse of the ion extractor (i.e., $ \Delta t $ = $ \infty $), the ions from the surface of region Ⅰ become to be the predominant signals in the mass spectra. Our onion-peeling strategy in deriving microstructural information of the liquid-vapor interface is based on two facts: the (sub)microsecond time scale (10$ ^{-7} $ s to 10$ ^{-6} $ s) of molecular evaporation and the layer-by-layer evaporation in vacuum.

    Figure 4.  Schematic of our time-delayed TOF (time-of-flight)-QMF (quadrupole mass filter) tandem mass spectrometer.

    Unfortunately, the ionic products from the liquid beam cannot be identified individually with the linear TOF spectrometer. This is distinctly different from what is observed by using the nanosecond laser pulses [44], because the ions formed in the nanosecond-laser ablations are from the micro droplets rather than the liquid surface or bulk. The micro droplets are easily produced from the liquid beam under the long-pulsed laser irradiation due to shock wave effect. To identify the individual ions, in 2019 we supplemented a quadruple mass filter (QMF) at the downstream of the TOF mass spectrometer. As shown in FIG. 4, the flying ions in the TOF tube can be guided into the QMF with an ion bender. This experimental arrangement enables us to detect the ionic products by recording the time-delayed TOF mass spectra and to identify the individual ions by guiding them into the QMF.

    We have two operation modes of this TOF-QMF tandem mass spectrometer, as shown in FIG. 5. At a given $ \Delta t $, all ionic products can be identified with the QMF by applying a DC voltage on the ion bender (the right panel in FIG. 5(b)) or the extremely long width of the ion bending pulse (FIG. 5(a)); the ions of a specific band in the TOF mass spectrum (the upper panel in FIG. 5(c)) can be selectively detected with the QMF by applying the ion bending pulse with a proper width (the bottom panel in FIG. 5(c)). This flexible mode is utilized to detect the ions that we are interested in.

    Figure 5.  (a) Time pulse sequence. (b) Two working modes, QMF mode can be operated with the continuously and selectively (pulsed) bending conditions. (c) The selective detection of some ions in the TOF spectra, the ions of the main band in the TOF spectrum (upper panel) are selected by applying a bending pulse on the ion bender, and then identified with the QMF (as shown in the inserted spectrum in the bottom panel). This band disappears in the TOF spectrum (bottom panel).

  • In the liquid-vapor interface of alcohol, the hydrophobic group, -C$ _n $H$ _m $ ($ n $ = 1, 2, 3, $ \cdots $; $ m $ = 3, 4, 5, $ \cdots $) tends to point out. Thereby, the production of hydrocarbon cations is more preferable than that of the others containing -OH group, in the electron-impact dissociative ionization. In the experiment of the liquid ethanol by using the time-delayed mass spectrometer [32], above intuitive description was proved successfully. The similar results for methanol are presented in FIG. 6 and FIG. 7. Note that these results, as well as those of ethanol [32], were obtained only with the linear TOF mass spectrometer.

    Figure 6.  Time-delayed mass spectra of (a) the gas-phase methanol and (b-d) the ethanol liquid beam. $ \Delta $$ t $ equals 0.0 μs in (a, b), while the spectra in (c, d) are recorded at different delaying time.

    Figure 7.  Intensity variances of CH$ _n $$ ^+ $ ($ n $ = 2, 3) and H$ _m $O$ ^+ $ ($ m $ = 1, 2) produced from liquid beam, as the delay-time of the ion extractor (relative to the electron pulse) increases.

    The mass spectra in FIG. 6 were recorded at the electron-impact energy of 22 eV. Different ions produced in the electron impacts with the gas-phase beam (emitted from a nozzle of gas-phase methanol) can be identified clearly in FIG. 6(a), while the mass-spectral peaks merge into two broad bands in FIG. 6(b) for the liquid beam. Moreover, the HO$ ^+ $ ions (with extremely weak intensity in the gas-phase spectrum), possibly together with H$ _2 $O$ ^+ $ by the reaction of HO$ ^+ $+H, are visible as the right shoulder of the first spectral band. In FIG. 6 (c, d), we focus on this band (comprising CH$ _{2, 3} $$ ^+ $ and H$ _{1, 2} $O$ ^+ $) at different $ \Delta t $ values. For the beginning $ \Delta t $ values, the band shifts to the longer TOF positions and is weakened gradually, as shown in FIG. 6(c). Since $ \Delta t $$ \geq $2.4 μs, the profile of this band is unchanged in FIG. 6(d). According to the criteria (ⅰ-ⅲ) mentioned in subsection Ⅱ C for the gas-phase sample, we conclude that the ions corresponding to this band for the smaller $ \Delta t $ values are primarily produced in regions Ⅱ and Ⅲ above the liquid surface.

    The intensity variances of two sub-bands of this broad band are plotted against the $ \Delta t $ values in FIG. 7, indicating a significant complementarity of these two curves. The crossing point locates $ \Delta t $ = 1 μs, while the CH$ _{2, 3} $$ ^+ $ profile becomes flat and constant as $ \Delta t $$ > $3 μs. This implies that the hydrocarbon cations produced in the electron-impact dissociative ionization of the interfacial methanol molecules go through the aperture of the ion extractor and turn to the predominated signals of the TOF mass spectra for $ \Delta t $$ > $3 μs. Meanwhile, the H$ _{1, 2} $O$ ^+ $ ions produced in the gas phase (i.e., regions Ⅱ and Ⅲ) are largely eliminated after the long delaying time. Although we cannot provide more information (e.g., the distributions of C-H/O angles respective to the surface determined with the SFG spectroscopy) about the orientation of the hydrocarbon group on the surface, the group orientation revealed here is useful enough to understand the microstructure of the liquid surface of the larger molecule.

  • With the help of the combinational usage of the QMF, we can identify the individual ionic products. FIG. 8 shows the time-delayed TOF mass spectra of the electron-impacts with a liquid beam of 1-propanol, and in the surrounding QMF mass spectra, different ions are identified for $ \Delta t $$ > $1.5 μs [33]. Besides the orientations of the hydrocarbon group in the outermost layer and subsurface, the protonated dimer (C$ _6 $H$ _{17} $O$ ^+ $) was detected. In band 4, the cationic dimer is minor, implying that the protonated dimer is much more stable in thermodynamics. Furthermore, two different sources of the protonated dimer were clarified on the basis of the profile variances in terms of $ \Delta t $ and the energetic simulations for the evaporations of the protonated dimer and cationic dimer from the liquid [33]. That study, for the first time, provided a direct evidence about in situ existence of the molecular clusters in the liquid surface [33].

    Figure 8.  Time-delayed TOF mass spectra of 1-propanol liquid-beam (middle) and the QMF mass spectra (surrounding). Bands 1, 2, and 3 consist of sub-bands A and B, C and D, E and F, respectively.

  • Here we summarize our recent progresses on the microstructures of liquid-vapor interfaces by using time-delayed mass spectrometry. This brand-new method is based on a table-top facility, i.e., the TOF-QMF tandem mass spectrometer. Further improvements, in particular, on the detection sensitivity, are being proceeded, and then the unique species, e.g., the molecular clusters with much low concentrations in the liquid surface, are hopefully detected. On the other hand, the electron-driven chemical reactions at different impact energies deserve explorations. The experimental method to study the ion-impacts with the liquid will be also developed in our laboratory. Using above techniques, we can have more insights into the dynamics mechanisms of the formation of salty aerosol on the sea, biological reactions, and molecular assembling on the liquid surface.

  • This work is supported by the Ministry of Science and Technology of China (No.2017YFA0303502) and the National Natural Science Foundation of China (No.21625301). We also thank Ling-ling Chen for her early contribution to the apparatus construction and Dr. Cen-feng Fu at University of Science and Technology of China for the theoretical simulations.

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