Mixtures of polyelectrolytes (PEs) and oppositely charged surfactants are of significant technological importance and scientific interest. A complex molecular association mechanism in these mixtures leads to the self-assembly of PE-surfactant complexes (PSCs). Such PSCs exhibit a variety of appealing supramolecular structures with useful properties and are found ubiquitously in the food, consumer products, pharmaceutics, and oil industries. Despite their broad use, however, a rational approach to formulation remains challenging because of the complicated association mechanisms that govern the physicochemical properties of PSCs. As such, the goal of this dissertation is to better understand how the thermodynamic and physical properties (e.g. structural, mechanical and adhesion) of PSCs depend on the molecular composition. By identifying the most important molecular interactions and studying their influence, we are able to deduce semi-empirical rules that facilitate the "predictive design" of PSCs with desired properties for use in industrial applications.
Establishing a relationship between molecular-scale interactions and the formulation chemistry is identified as the foundational step towards this goal. A novel methodology of combining visual and isothermal titration calorimetry (ITC) measurements is developed to analyze the equilibrium and metastable formation of PSCs in terms of the strength of surfactant cooperative binding with the polyelectrolyte. An improvement to a two-energy-state adsorption model is derived and the resulting thermodynamic properties, including binding constants and the molar Gibbs free energies, enthalpies, and entropies, identify the relative importance of both hydrophobic and electrostatic interactions in PSC formation. Through a design of experiments varying the independent parameters of surfactant hydrophobicity and PE linear charge density, a power-law relationship between the rescaled cooperative binding strength and the PE's linear charge density is deduced. This correlation is further validated across a broad range of PSC mixtures reported in literature, spanning natural and synthetic PEs including DNA. A semi-empirical model is proposed to enable quantitative prediction of the binding strength parameter from the molecular composition.
A systematic characterization of the microstructures and properties of these model PSCs is then performed via a combination of X-ray scattering, small-angle neutron scattering (SANS), thermal gravimetric analysis (TGA), and the development of a nanoindentation protocol using atomic force microscopy (AFM). More ordered structures result from stronger surfactant binding and these PSCs have increased elasticity. This research enables correlating the PSC properties with the self-assembled microstructure, which in turn depends on the cooperative binding strength and composition—thus, providing guidance for formulating PSCs for a specific end-use. Further, an industrial end-use of PSC adsorption onto emulsion surfaces is explored and found to follow the solution behavior outlined above. This dissertation provides new insights about and guidance for rationally formulating PSC materials for consumer products and biomedicines as well as a better understanding of the thermodynamics and properties of PSCs, including structure, rheology and interfacial adsorption. The methodologies and techniques developed in this dissertation are of additional value as methods available for more broadly investigating other PSCs and related systems.
|Advisor:||Wagner, Norman J.|
|Commitee:||Furst, Eric M., Liu, Yun, Schubert, Beth A.|
|School:||University of Delaware|
|School Location:||United States -- Delaware|
|Source:||DAI-B 75/06(E), Dissertation Abstracts International|
|Subjects:||Polymer chemistry, Chemical engineering, Molecular physics|
|Keywords:||Emulsion, Microstructure, Polyelectrolyte, Property, Surfactant, Thermodynamics|
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