Raman spectroscopic study on the structure of water in aqueous polyelectrolyte solutions

The structure and hydrogen bonding of water in various kinds of aqueous polyelectrolyte solutions were analyzed with contours of O-H stretching of polarized Raman spectra. Effects of chemical properties of the polymers and water domains surrounded by the polymer chains on the relative intensity of collective band (C value) corresponding to a long-range coupling of O-H stretchings were discussed. The C values for various polymer solutions were almost constant in a relatively low molecular weight (M-w) region, and decreased with an increase in M-w value. When the size of the space surrounded by the pseudo-network was sufficiently small, the structure of water in the space was altered to have a relatively lower average number of hydrogen bonds between water molecules than that of bulk water. The number of hydrogen bonds collapsed by the presence of one monomer residue (N value) of polyelectrolyte (sodium polyethylenesulfonate, poly-L-lysine hydrobromide, etc.) with a small M-w was much larger than those for neutral polymers such as poly(ethylene glycol) and poly(N-vinylpyrrolidone). This result indicates that the monomer residues of water-soluble neutral polymers do not disturb the structure of water significantly, whereas electrostriction effect by the polyelectrolyte is quite effective on the structure of water. On the contrary, the N value for poly(2-methacryloyloxyethyl phosphorylcholine) with a small M-w was nearly zero, suggesting that the zwitterionic-type monomer residues do not disturb the hydrogen bonding between water molecules.