Perturbation of water structure by water-polymer interactions probed by FTIR and polarized Raman spectroscopy

Infrared and polarized Raman spectroscopy are ideal tools to probe changes of water structure caused by the addition of water soluble polymers. In this paper, we investigate how the addition of polyethylene glycol (PEG-4000) and Ucon-4000 changes the water structure at polymer concentration below the solubility limits of these polymers. We measured the IR-spectra of H2O, 5 mol% D2O/95 mol% H2O and 5 mol% H2O/95 mol% D2O in the absence and the presence of various amounts of polymers. The weight concentration was varied in increments of 10% up to 40%. The spectra were self-consistently analyzed by a heuristic model that describes the band profiles of OH/OD stretching vibrations as a superposition of Gaussian bands. Our analysis revealed that the addition of polymers substantially reduces the intensity of the OH and OD bands on the low energy site without causing a corresponding increase on the high energy site. In order to shed some light on the underlying mechanisms governing these changes, we also measured the polarized Raman spectra of 5 mol% D2O/95 mol% H2O without and with 40 wt% PEG. From these spectra we obtained the isotropic and anisotropic Raman scattering profiles. A spectral analysis of the latter and a comparison of the results obtained from the analyses of the IR and Raman spectra strongly suggests that polymer addition causes a decrease of large complexes of water molecules interconnected by hydrogen bonding and at least partial restriction of water mobility imposed by hydrogen bonding between water clusters and the polymer units' oxygen atom as well as the restriction of solvent space owing to the high polymer concentrations investigated. This reduces the coupling of water stretching vibrations to rotational and translation motions of their surrounding molecules and thus the non-Condon contribution to the oscillator strength of OH and OD vibrations alike. Finally, we used previously reported solvent features of water in the polymers solutions, the solvent's acidity, basicity and dipolarity/polarizability and found them to correlate with the polymer-induced spectroscopic changes in the IR-spectra of polymer-water samples. (C) 2018 Elsevier B.V. All rights reserved.