Probing interfacial vibrations with IR absorption spectroscopy: from molecular to mesoscopic and macroscopic surfaces

Capturing the vibrational signatures of interfacial molecules is complicated by their low abundance relative to bulk molecules. Molecules at extended flat liquid interfaces can be elegantly singled out and probed by vibrational sum-frequency generation (SFG) spectroscopy as this method is intrinsically surface-specific due to the requirement of broken inversion symmetry. Interfacial molecules in solvation shells at molecular interfaces, on the other hand, exhibit inversion symmetry and vanish in SFG spectroscopy. However, in several cases, the fraction of solvent molecules at such molecular interfaces can be several percent of the total and can be isolated using advanced subtraction methods. Raman-MCR spectroscopy was first developed as a general and accurate method for extracting the Raman spectrum of this fraction of solvent molecules in the solvation shells of solutes. We later adapted the technique to FTIR spectroscopy in the form of ATR spectroscopy. The method is, in essence, a difference spectroscopy, wherein the vibrational spectrum of a solution is considered to be composed of two contributions: (i) that of the bulk solvent, which can be either a pure liquid or a mixture itself, and (ii) that of the solute with its solvation shell, defined as the part of the solvent that is perturbed by the interactions with the solute. In solvation shell spectroscopy, the bulk solvent contribution is removed from the solution spectrum, and the solute-correlated spectrum is then obtained. This solute-correlated spectrum contains the vibrational modes of the solute itself and those of perturbed molecules at the solute-solvent interface, which can reveal information such as the strength and extent of solute-solvent interactions. Here, we discuss advantages and complications of the method in detail using tert-butyl alcohol, a small amphiphilic molecule, as an example, starting in pure water and moving to more complex systems including ionic additives and mixed solvents. We furthermore illustrate how changes in the hydrogen-bond strength in the solvent shell can be quantified. Lastly, we push the spectroscopy to the detection limit and show that solvent interactions can be probed at not only molecular interfaces, but also meso- and even macroscopic interfaces. When done correctly, solvation shell spectroscopy holds great promise to be widely implemented to study solvent interactions in a wealth of different simple and complex systems.