Investigating the New Interaction Model between Cu(II) and PVA by the Proton Relaxation Method

An aqueous PVA-Cu²⁺ solution at pH = 3 is in the hydrated form and becomes green at pH ? 6 with a decrease in viscosity. The structure of the copper ion is suggested to be that of a polynuclear complex at pH > 6. For the green solution the polynuclear chains of the copper complex are believed to be surrounded by the PVA chains with the hydrophobic backbones facing toward the inside and the hydrophilic OH groups oriented toward the outside facing the bulk water. The proton spin-lattice relaxation rate 1/T_1p and the spin-spin relaxation rate 1/T₂ of CH and CH₂ in PVA and H₂O for aqueous PVA-Cu²⁺ solutions at pH = 3, can be explained by the two site exchange model in the region of the fast exchange limit. The dipolar correlation time τ_c is dominated by the reorientational process with a dipolar correlation time of 2.11 × 10^?11 s. When the pH rises from pH=3 to pH=12.5, the variation of 1/T₁p and 1/T₂p of CH and CH₂ in PVA with Cu²⁺ ion concentration in aqueous PVA-Cu²⁺ solution at pH=12.5 can be explained in terms of the relaxation by an inclusive model of the polynuclear copper complex and PVA. Furthermore, the frequency (or field) dependence of 1/T₁p, 1/T₂p of CH in PVA for aqueous PVA-Cu²⁺ solution at pH = 12.5 suggests that the dipolar relaxation is dominated by the electron-spin relaxation with the electron spin relaxation time T_1e = 1 ? 2 × 10^?10 s. The invariance of 1/T₁p and 1/T₂p of H₂O with the variation of the Cu²⁺ ion concentration in aqueous PVA-Cu²⁺ solution at pH = 12.5 supports the hypothesis that the water is not directly bound to the Cu²⁺ ion.