Hydration-shell spectroscopy of biologically relevant and organic aqueous solutions
Abstract
This work focuses on applying Raman Spectroscopy to investigate vibrational spectral features associated with hydration-shell water molecules around various biological and organic solutes. Hydration-shell spectra are obtained by combining variable concentration Raman spectra with Multivariate Curve Resolution (Raman-MCR). Solution Raman spectra are resolved into unperturbed (pure water) and solute-correlated (hydration-shell) components. The resulting hydration-shell spectra of water around alkane, aromatic, and ionic solutes show evidence of water structural changes, including the solute-induced formation of dangling water OH bonds. The appearance of a narrow high frequency OH stretch band closely resembles that of dangling (non-hydrogen bonded) OH groups at macroscopic water-vapor and water-alkane interfaces. The intensity of the observed band is used to estimate the number of nearly free water OH groups around each solute. Hydration-shell water molecules around carboxylic acids, whose head groups are either neutral, R-COOH, or charged, R-COO-, has been explored. The results begin to reveal that substituting head groups from hydroxyl, R-OH to either carboxyl, R-COOH or carboxylate, R-COO- has little to no influence on the dangling OH formation around the neighboring hydrophobic R-group. Furthermore, long chained alkanes of trimethyloctyl ammonium chloride and octanoate, show the dangling OH probability does not scale with exposed surface area, thus suggesting the onset of aggregation or alkane chain folding of the hydrophobic groups below their reported critical micelle concentrations. Raman Spectroscopy is subsequently applied to measuring the influences of alkali halide ions on hydration-shell spectra of water molecules around dissolved nonpolar groups. Solute-induced perturbations of water molecules can be measured by frequency shifts and integrated intensities of the OH, CH, and CC bands. Both the alkali halide anions (F-, Cl -, Br-, I-) and cations (Li+ , Na+, K+) in the hydration-shell show little to no affinity for alkane groups dissolved in water.
Degree
Ph.D.
Advisors
Ben-Amotz, Purdue University.
Subject Area
Biochemistry|Physical chemistry
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