Article pubs.acs.org/Langmuir
Surface Electrostatic Immobilization of Thin Layers of Water on Silver Halide. Experimental and Calculated Infrared Spectrum of Cyclic Trimer of Water and a Ponderal Isotope Effect Edward M. Kosower,*,† Gil Markovich,† and Galina Borz† †
School of Chemistry, Tel-Aviv University, Ramat-Aviv, Tel-Aviv 69978, Israel ABSTRACT: Evaporation of water on a planar AgX surface leads to a strongly bound monolayer for which IR spectra display the marker peaks for modest numbers of oligomers. From 700−1800 spectra for each isotopomer, H2O16 and H2O18, a pair was selected with moderate intensity at 1616 cm−1 (a peak reported for the cyclic trimer of water) from the monolayer portion of the experiment. Every selected spectrum had lesser peaks for other oligomers. The sum of a spectroscopic pair reveals the vibrational spectra of the cyclic trimers of H2O16 and H2O18. All fundamentals in the mid-IR are seen including the bending, OH stretching, and intramolecular H-bonding regions, the last never previously recognized. The relative prevalence of cyclic trimer can be attributed to the “low” water concentration on the surface. In addition, a ponderal effect leads to higher concentrations of cyclic trimer in the H2O18 spectra than in the H2O16 spectra and allows observation of combination bands in the H2O18 spectra, representing a new type of isotope effect. The spectroscopic results for the two water isotopomers are much more extensive than those obtained through matrix isolation. Remarkably complete spectra of the cyclic trimer are obtained for the first time, especially for H2O18. DFT calculations with the cyclic trimer on a simplified model for the AgCl surface yield spectra consistent with the experimental spectrum. The technique can be extended to other oligomers of water and many other OH compounds.
1. INTRODUCTION Thin films on surfaces are important in both science and technology, and a detailed knowledge of their internal structure could be important in many fields. Infrared spectroscopy is a technique that can provide detailed molecular information on the composition of such films. The interaction of the molecular components of the films with the surface can improve the quality of the IR spectra by decreasing molecular motion . Thus, the restriction of motion of polar molecules on a polar surface1 (especially water on AgCl2,3) combined with the mutiple reflections obtained for thin layers on planar AgX (silver halide) in a special IR cell4 makes possible the observation of very detailed IR spectra. The use of low signal energy and short scan times allows detection of unstable and transient species. The term “electrostatic immobilization” replaces the previously used “electrostriction”4 as more accurate. We have identified experimentally five of the many possible oligomers of water using thin film infrared spectroscopy of liquid water on planar AgX.4 These are the cyclic pentamer, two cyclic hexamers (chair and boat), and two bicyclic hexamers (books 1 and 2). Some peaks for the cyclic trimer of H2O16 and H2O18 have been obtained in matrix isolation5,6 and cavity ringdown spectroscopic experiments, reported together with calculations and summarized in a review.6 We have now discovered that the last stage of the evaporation of a sample of water produces a tightly bound monolayer of water, evidenced by the average absorbance (ca. 0.02 for H2O16 and 0.03 for H2O18). The diminished quantity © 2012 American Chemical Society
of water on the surface in the monolayer increases the importance of smaller oligomers. We have identified the cyclic trimer of water in the monolayer and found a more complete set of mid-IR peaks for this oligomer than was heretofore observed. Support for the identification comes from a model calculation to be described later. The implications of the oligomer presence on the surface will be taken up at a later time. A ponderal effect7 accounts for greater amounts of H2O18 than H2O16 oligomers as well as for enhanced combination bands, representing a new type of isotope effect. We have discovered that the IR spectra of the cyclic trimer fall into groups for each type of vibration (OH stretching, bending, Hbonding, and combination bands) as was reported4 for the OH stretching vibrations of the pentamer, cyclic hexamers (boat and chair), and bicyclic hexamers (books 1 and 2). We have introduced a new parameter, the span (wavelength range of each type of vibration), that may be characteristic of the oligomers.
2. EXPERIMENTAL SECTION Our experimental procedure has been reported in some detail4 but is now partly repeated with specific details relevant to the present article. The fiberoptic IR cell has been described4,8 in previous papers, but for easy reference we include an illustration in the next section. The cell Received: April 20, 2012 Revised: July 15, 2012 Published: August 29, 2012 13208
dx.doi.org/10.1021/la301613k | Langmuir 2012, 28, 13208−13217
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contains a planar crystalline AgX fiber [AgX, usually AgBr:AgCl, 0.6:0.4]. Planar portion: l 3.9−4.0 cm, w 0.4 cm, d 180 μm. Cylindrical portion: d 0.9 mm. An extremely simple but highly effective method for controlling the signal intensity involves positioning a Styrofoam plate with a hole to admit the IR beam in front of the fiber entry. This made it possible to control the signal intensity at levels between 50 and 3700. The signal intensity is set before the measurement of the background. As far as we know, this is the first time that signal strength has been controlled in infrared spectroscopy.9−13 A whole new world of transient or unstable structures is opened to the spectroscopist. Ultrapure H2O16 (18.2 MOhm, via ultrafiltration, Biolab 232141) and H2O18 (D-Chem, Rehovot, Israel) are used. Typically, in an experiment, a quantity (∼0.4 μL) of water is placed (1 drop, ∼1 s) on the planar portion of the fiber with a micropipet. A slow stream of dry filtered air is directed at the cell surfaces and the cell compartment during sample loading of H2O18 to minimize contamination with H2O16. Recording of the spectra is begun before addition of the sample so that there are no missed points; the collection of data is begun, and then the sample is distributed over the fiber. After the cell is closed, the sample is exposed to a slow flow of dry N2 (1 mL/min) that has been passed through a drying train of calcium chloride, dried molecular sieves 4A, and silica gel containing cobalt indicator. The gas flow is controlled by a Rotaflow valve. The low rate of gas flow means that the thin layer of solvent is removed slowly from the surface of the AgX. The small sample of water is removed in 2−3 min, but not completely, as reported in the present article. The FTIR spectrometer is a Bruker Equinox 55/S equipped with two sources [Globar, mid-IR (MIR); tungsten, near IR (NIR)], two beam splitters [KBr, MIR; CaF2, NIR], and two detectors [DTGS, MCT]. Only the MCT system is used in the MIR for the present research. Infrared spectra are recorded (700−1800 scans, 0.1 s each) over the range from 7000 to 650 cm−1 at a resolution of 4 cm−1, mostly in a continuous manner or with one 0.1 s scan every 10 or 20 s to extend the time of observation without obtaining excessive data sets. After baseline smoothing, the data are transferred to another computer, the output of the Bruker Opus program is converted to a matrix form in which all but one of the wavelength lists are removed by a Matlab program, and the Origin 6.1 program is used for analysis, spectral displays, and plotting. Each scan is regarded as a separate experiment measuring the spectrum of the water film averaged over 0.1 s. As it happens, and as expected from modeling of water on the AgCl surface,2,3 the electrostatic immobilization of the film adjacent to the AgX surface is sufficient to yield excellent spectra as long as the energy input (Sg level) is low, around 100. The temperature of the sample is at ∼20 °C. The crystalline nature of the AgX fiber apparently stabilizes the material, which can be used for months. Preliminary studies on CaF2 planar windows reveal similar results4 over a more limited wavelength range. Thus, AgX fiber is a stable nontoxic substrate useful over a wide IR range.
peaks over a period of time, we can estimate the lifetimes and the rate of formation or dissolution of the oligomers. We will illustrate later some spectra of oligomers with lifetimes near 1 s. By combining two spectra, each with a higher proportion of one oligomer, we can visualize parts of a spectrum that might not be readily identified in a single specrtrum. This approach, akin to “lifetime tagging”,4 allows one to obtain spectra of transient species that cannot be obtained in pure form. The lifetime of transient species must be obtained from continuous series of spectra (scans every 0.1 s), but spectra obtained over longer time periods (one 0.1 s scan every 10 s as mentioned in the Experimental Section) can be used to find spectra with higher concentrations of particular oligomers. We paid attention to the fact that the fluctuations continued after the evaporation of water by the usual slow stream of N2 had apparently been completed (ca. 30−60 s). It was obvious that strongly bound water remained on the surface and was classified as a monolayer from its absorbance. The cell used in this research is illustrated in Figure 1, together with a blank
3. RESULTS AND DISCUSSION Infrared spectra of a small sample (0.4 μL) of water at low light energy (Sg 100) show a whole variety of marker peaks, of which five have been associated with specific oligomers,1 as noted in the Introduction. As we reported earlier,8 there are fluctuations in absorbance that represent rapid changes due to formation and dissociation of oligomers and/or oligomer groups. A brief explanation will help one to understand the nature of fluctuations. A fluctuation is a rapid and irregular change in a property on a time scale that is “short” compared to that being used in the assembly of the data, such as a plot of absorbance against time. In the latter case, a rapid formation and dissolution of absorbing species is indicated. These changes occur on a time scale of fractions of a second but are readily observable by altering the scale of the plot. At each point, we may extract an entire spectrum and choose spectra that happen to contain a higher concentration of interesting oligomers. By following the
Figure 1. A schematic view of the Teflon Cell containing a silver halide (AgX) fiber. The Styrofoam plate is used to admit controlled amounts of the IR beam so as to give the desired signal. The spectrum shown below is a blank that illustrates how low the noise is (due to a small amount of adsorbed H2O16) in the spectroscopic measurements (see later text).
spectrum showing that the “noise” can actually be attributed to water but still less than any of the absorbances with which we are concerned. The blank spectrum is that of strongly bound water (∼
