Structure of Rod-like Polyelectrolyte-Surfactant Aggregates in Solution and in Adsorbed Layers

Polyelectrolyte-surfactant aggregates (PES) have diverse sets of properties, which could be controlled by a wide range of parameters, both within the aggregates themselves and the surrounding environment. The large portfolio of applications amplifies the need to understand them. A particular system of polyelectrolyte-surfactant aggregate, polycetyltrimethyl vinylbenzoate, denoted pC₁₆TVB, which self-assembles in aqueous solution, is the main focus of the thesis. Its structure is the product of the balance between surfactant head groups – polyelectrolyte charge groups electrostatic interactions and surfactant tails – polyelectrolyte backbone hydrophobic interactions. At neutral solution pH, the structure is one of a core-shell cylinder, with the shell consisting of surfactant heads. The surfactant tails point toward the core center, while the polyelectrolyte also resides in the core, but remains close to the core-shell interface for charge neutralization. As the pH drops to 1.0, the balance is disrupted with the hydrophobic interaction being increasingly dominant while electrostatic interaction is reduced, and the structure transforms into a more commonly seen string-of-pearl, in which the polymer chain connects a series of spherical surfactant micelles. The solution properties are impacted accordingly, becoming viscoelastic while solubilizing 10 times more hydrophobic molecules.

Treating the pC₁₆TVB aggregate as a whole, its adsorption onto oxide nanoparticles surfaces has been analyzed, extended from a previous flat surface adsorption work. Aided by hydrophobic dyes as molecular trackers, the adsorbed thickness has been proven to be a function of the surface curvature, with less curve surface adsorbs more material. The dye loading remains intact after the adsorption, enabling the use of the aggregate as a delivery vehicle for hydrophobic materials in aqueous solution. The resulting adsorption of pC₁₆TVB aggregate onto SiO₂ surfaces has a core-shell sphere structure. Unlike for flat surfaces, in which the adsorption mechanism has been shown previously to consist of two main steps, with some dissociated surfactant molecules adsorbing head first via electrostatic attraction with the surface to create hydrophobic anchor points for further aggregate adsorption, the high bending energy cost and the low aggregate concentration (relative to the total sphere surface area) suspend the adsorption after the initial surfactant adsorption step