Spectroscopic studies of solvation and adsorption properties of PVP

Sammanfattning: The adsorption mechanism of 1-vinyl-2-pyrrolidone (VP), poly(1-vinyl-2- pyrrolidone) (PVP) and azelaic acid on synthetic gamma-aluminium oxide surfaces was investigated using FT-IR, FT-Raman, UV, 1H and 13C NMR spectroscopy. (Liquid-solid state) As a first step the solvation of VP and PVP in solvents of varying polarity, specifically water, ethylene glycol (EG), chloroform and carbon tetrachloride, was studied. The IR and Raman measurements made it possible to establish the structures of the solvated molecules as well as the type of hydrogen bonding. From the UV measurements, the strength of the vinyl double bond was estimated in the different solvents. The 1H and 13C NMR spectra revealed that polar solvents attack the solutes at the carbonyl group of the pyrrolidone ring, whereas nonpolar solvents interact mainly with the vinyl group in VP and with the polymer chain in PVP. From the adsorption experiments of VP and PVP carried out in EG and water, it was concluded that VP could not be used as a model compound for PVP, because the adsorption properties of the two substances are quite different. The PVP adsorption from both, aqueous and EG solutions, was negligible. It was found that the presence of a dicarboxylic acid enhances the adsorption of PVP, due to a hydrophobic interaction between the carbon chains of the polymer and the dicarboxylic acid. This theory is supported by the experiments using dicarboxylic acids with different carbon chain lengths. However, under a critical chain length (C5) of the dicarboxylic acid, the enhancement of the PVP adsorption was diminished. The decreased PVP adsorption on the surface in case of dicarboxylic acids with short chain lengths may be explained by less hydrophobic interaction. The simultaneous adsorption of PVP and azelaic acid was studied as a function of adsorption time, pH and solvent in order to establish a more detailed surface complexation model. It was found that surface-complexation of azelaic acid in aqueous solution starts with mainly outer sphere co-ordination, driven by electrostatic forces, which is transformed to an inner sphere complex in time, forming a covalent bond between the carboxylate and the alumina surface, especially at low pH. At high pH the outer sphere complex is preferred. The pH has a strong influence on the adsorbed amount of azelaic acid and therefore on the adsorbed amount of PVP as well. With increasing pH, the electrostatic attractive forces (the driving force for the first step of the adsorption) decrease, resulting in a reduction in adsorption. The pH seemed to have no effect on the reaction between the PVP and the azelaic acid. In EG solution the complexation model is even more complex since the solvent molecules are competing for the surface sites.