Supramolecular structure of the LiPFN-PVP system: F-19 NMR studies

F-19 NMR measurements on lithium perfluorononanoate [LiPFN] in D2O and in solutions of D2O with poly(vinylpyrrolidone) [PVP] at different molecular weights have been carried out. A careful study of literature data and COSY maps allowed the full assignment of the F-19 resonances as a function of concentration and composition of the systems. For the LiPFN/D2O system, analysis of the chemical shifts of the fluorine atoms bound to the different carbons of the LiPFN backbone reveals significant variations correlated to the LiPFN concentration. Such a finding has been explained in terms of the changes in the local dielectric constant caused by the surfactant self-aggregation process. The presence of PVP in solution strongly modifies the trend of the F-19 chemical shifts observed in the binary LiPFN/D2O system. Such a feature, depending both on the position of the fluorine atoms along the surfactant backbone and on the LiPFN concentration, indicates the presence of strong polymer-surfactant interactions. The line width analysis of selected F-19 resonances as a function of the surfactant concentration shows, in addition, the presence of an equilibrium among different species of LiPFN in solution (i.e., free monomers, bound monomers, free micelles, bound micelles). The extreme spread of F-19 chemical shifts allow the spectroscopic observation of separate resonances for free and bound micelles at low temperature. Combining the F-19 NMR results with other detailed information derived from previous H-1 NMR, surface tension, viscosity, and conductometric studies, a structural model for the micelle-polymer supramolecular aggregates has been formulated.