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

F-19 NMR measurements on lithium perfluorononanoate [LiPFN] in D2O and in s olutions of D2O with poly(vinylpyrrolidone) [PVP] at different molecular we ights have been carried out. A careful study of literature data and COSY ma ps allowed the full assignment of the F-19 resonances as a function of conc entration and composition of the systems. For the LiPFN/D2O system, analysi s of the chemical shifts of the fluorine atoms bound to the different carbo ns of the LiPFN backbone reveals significant variations correlated to the L iPFN concentration. Such a finding has been explained in terms of the chang es in the local dielectric constant caused by the surfactant self-aggregati on 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 fe ature, depending both on the position of the fluorine atoms along the surfa ctant backbone and on the LiPFN concentration, indicates the presence of st rong polymer-surfactant interactions. The line width analysis of selected F -19 resonances as a function of the surfactant concentration shows, in addi tion, the presence of an equilibrium among different species of LiPFN in so lution (i.e., free monomers, bound monomers, free micelles, bound micelles) . The extreme spread of F-19 chemical shifts allow the spectroscopic observ ation of separate resonances for free and bound micelles at low temperature . Combining the F-19 NMR results with other detailed information derived fr om previous H-1 NMR, surface tension, viscosity, and conductometric studies , a structural model for the micelle-polymer supramolecular aggregates has been formulated.