Reversible hydrophobic microdomains in amphipathic polymers: Studies of stimuli-responsive organization and sequestration of a model foulant

Michael Frederick Richardson

Abstract

The objective of this research was to study the stimuli-responsive domain formation of water-soluble polymers and to evaluate the ability of the domains to capture a model hydrophobic foulant. The associative nature of a group of polymers, proteins and polymerizable surfactants was investigated and the stimuli-responsive sequestration of the polymers and surfactants with para-cresol was then studied. The synthetic polymers included a poly(ethylene oxide)-poly(propylene oxide) triblock polymer, PluronicTM F-127 and a n-octyl hydrophobically modified copolymer of maleic anhydride-alt -ethyl vinyl ether. Light scattering studies with each of the polymers revealed stimuli-responsive domain formation. The F-127 copolymers possess a unimer-to-multimer transition with an increase in temperature from 5°C to 25°C. The copolymers showed a transition from an intermolecular aggregate at pH 4.0 to an extended chain polyelectrolyte at pH 8.0. Studies of the uptake of para-cresol demonstrated that the domains formed by each of the polymers are capable of sequestration of large amounts of cresol. The transition to the unimer with temperature or pH change results in a significant reduction in cresol binding. Each of the three proteins, β-lactoglobulin, bovine serum albumin (BSA) and β-casein, undergo stimuli-dependent aggregation processes. Investigations of each with light scattering and fluorescence revealed differences in aggregation number and fluorescence binding with a change in pH or ionic strength of the solution. Sequestration of cresol by each of the proteins is consistent with the observed function of the protein in naturally occurring systems. β-Lactoglobulin has a single hydrophobic binding site and sequesters only a few molecules of cresol with extremely high affinity. BSA in the non-aggregated form binds approximately ten molecules of cresol. In the aggregated state, BSA forms an intermolecular microdomain capable of sequestering a large amount of cresol. β-Casein sequesters cresol in a pH-dependent manner. Above pH 6.0, no binding is observed. Reduction in pH leads to separation into a dispersed phase which associates with relatively large amounts of cresol. A group of twin tail polymerizable surfactants was synthesized to study the influence of domain organization and size on sequestration. The monomers associate above a critical concentration to form domains similar to vesicles of naturally occurring lipids. The monomer aggregates have hydrodynamic diameters greater than 125 nm. The monomer domains are nonpolar and much like small molecule surfactant assemblies. The C 12 monomer exhibits lamellar liquid crystalline phase behavior and the organized monomer aggregates in dilute solution are composed of multilamellar structures. The C12 monomer aggregates sequester large amounts of cresol in domains that are similar to small molecule micelles. (Abstract shortened by UMI.)