Nuclear magnetic relaxation in aqueous solutions of vinylpyrrolidone and polyvinylpyrrolidone

Deuteron magnetic spin-lattice relaxation times have been measured in D₂O solutions of vinylpyrrolidone and polyvinylpyrrolidone as a function of concentration and temperature. For the monomer the results are interpreted in terms of a hydrophobic hydrations effect in which 42 D₂O molecules per solute molecule have a correlation time of 3.2 psec at 298°K. Application of the transition state theory to the temperature dependence gave H^*=21 kJ mole^–1 for the relaxation process. In the case of the polymer it is argued that a hydrophilic hydration effect dominates the observed relaxation. These activation enthalpy at 298°K is 24 kJ-mole^–1. Assuming a hydration number of one D₂O per polymer unit, the correlation time for the bound water is 77 psec at 298°K. Polymer proton spin-lattice relaxation times were measured as a function of frequency, and the results are analyzed in terms of a log normal distribution of correlation times. The median value at 296°K is 1.2 nsec.