Diffraction-limited mid-infrared microspectroscopy to reveal a micron-thick interfacial water layer signature

Abstract

The potential of a supercontinuum laser besides synchrotron radiation to conduct high-resolution, diffraction limited FTIR measurements and to record a micron thick interfacial water layer near the solid surface.

Ubiquity of the Micrometer-Thick Interface along a Quartz–Water Boundary

“…16 They speculated that the quartz surface of the pore walls was dominated by siloxane (hydrophobic) bridges, adding the long-range hydrophobic forces as an interfacial player. In a recent paper, 17 using three different and independent experimental setups, we confirmed the presence of such a micrometer-thick interfacial water layer near the surface of quartz.…”

“…The background spectrum of the surrounding quartz matrix was recorded before each set of hyperspectral data to remove the effects of any changes in atmospheric CO 2 and water vapor. All of the acquired spectra were postprocessed, with baseline correction (by subtracting a linear baseline in the interval of 2800–4200 cm –1 ), normalization with Origin Pro version 2021, and removal of interference fringes done by implementation of an algorithm in Igor Pro version 9.0 (see ref for further details of data processing). A statistical treatment of the hyperspectral FTIR data was carried out through non-negative matrix factorization, implementing a script/algorithm in Python version 3.11.1, on the Jupyter Notebook platform.…”

“…16,17,24 With regard to the infrared source, we utilized the unpolarized supercontinuum laser (Coverage, Novae) operating in the mid-infrared range of 1.9−3.9 μm. 25,26 This laser was integrated with the spectrometer through an in-house-designed optical configuration (see ref 17 for more details).…”

“…The inclusions were mapped to obtain a hyperspectral FTIR data set (i.e., x−y distribution of IR absorbances). Using the microbeam provided by the supercontinuum laser setup, 17 it was possible to record how the OH stretching band profile changes, in terms of peak position (wavenumber, in inverse centimeters), peak intensity (corresponding to the absorption), full width at half-maximum (fwhm), or peak area, from one part of the inclusion to another, focusing here on the distance from each beam location to the solid walls.…”

“…In our ultralarge pores, no confinement effect can be expected, so that the pore size has no impact, and the walls are hydrophobic as described previously. 16,17 The basic idea is that the interface spectral signature is large enough to imprint the IR band as long as the volume ratio of the surface layer versus the bulk domain "seen" by the microbeam would be "sufficient" (no number here). From that viewpoint, the band broadening of what we call "bulk water" in this paper with respect to normal water 17 would be a remnant signature of this convolution.…”