Studies of phase behavior and host-guest binding in aqueous systems containing macromolecules

Abstract

The two dissertation goals were to investigate 1) liquid-liquid phase separation (LLPS) of dendrimer solutions and 2) thermodynamic stability of polymer-porphyrin supramolecular structures in solutions. LLPS is important in separation and materials science. However, no experimental studies have been reported on LLPS of dendrimer solutions. Dendrimers are hyperbranched macromolecules that find applications in these fields. We focused on the LLPS of aqueous solutions of poly(amido amine) dendrimers of forth generation with hydroxyl- (PAMAM-OH) and amino- (PAMAM-NH2) functionality. LLPS was observed in the presence of sodium sulfate. In the case of PAMAM-OH, LLPS switches from being induced by cooling to being induced by heating as the salt concentration increases while, in the case of PAMAM-NH2, LLPS is observed by cooling only.^To explain our findings, a thermodynamic model based on two parameters, describing dendrimer-dendrimer interaction energy and salt salting-out strength, was developed. This model successfully describes the observed LLPS behavior including the unusual thermal behavior of PAMAM-OH. We showed that coupling dendrimer oligomerization with LLPS leads to the formation of globular dendrimer nanoparticles. This provides the basis for the preparation of nanoparticles of low-generation dendrimers, which may circumvent the need of preparing high-generation dendrimers. Porphyrin optical properties can be tuned through polymer-porphyrin host-guest binding in solution, giving rise to the formation of supramolecular structures. The formation, thermodynamic stability and spectroscopic properties of polymer-porphyrin supramolecular structures and their competition with porphyrin self-association were investigated by isothermal titration calorimetry (ITC) and absorption spectroscopy.^Reaction enthalpies and equilibrium constants were measured for meso-tetrakis(4-sulfonatophenyl) porphyrin (TPPS) self-association and TPPS binding to poly(vinylpyrrolidone) (PVP) in aqueous solutions at pH 7 and three temperatures (12-37?C). ITC provides two independent means to determine reaction enthalpies: direct measurements and Van?"t Hoff plot. This was used to assess that self-association of TPPS is limited to the formation of dimers and TPPS binds to PVP in its monomeric state. TPPS dimerization and PVP-TPPS supramolecular structures are enthalpically driven. However, polymer-porphyrin binding was found to be entropically favored compared to dimerization. Finally, we show that TPPS spectrophotometric properties become independent of pH in the presence of a large excess of PVP.