Coacervation in mixture of polyelectrolytes and surfactants with opposite charge is common in nature and is also technologically important for applied research including food, cosmetics and pharmaceutics. To better understand the complexation behavior of these systems, we have investigated a systematic variation of model mixtures, to assess the significance of polymer's charge density and surfactant chain's hydrophobicity, holding as many of the auxiliary properties constant. The goal is a quantitative link between molecular interactions, phase behavior, microstructure and coacervate rheological performance. To achieve this, we combined experimental techniques for thermodynamic measurements with methods for microstructure characterizations, and rheology at different structural levels. A model for surfactant binding onto the polymer enables a semi-empirical quantification of the polyelectrolyte/surfactant association behavior, where a master binding behavior was identified in the frame work of the Satake-Yang binding parameters. The interactions are governed by both electrostatic and hydrophobic interactions, with relative importance depending on the polymer's charge density and surfactant alkyl chain's hydrophobicity. The proposed semi-empirical free energy equation with the given contributions of both the charge density as well the surfactant hydrophobicity, provides us with an engineering map describing the self-assembly behavior in coacervation systems. Finally, along with the structural and rheological characterizations, it is shown that increasing the polymer's charge density and surfactant chain's hydrophobicity lead to dramatic changes in coacervate microstructure, which also corresponds to the greater shear and elastic moduli.

Coacervation in model oppositely charged polyelectrolyte/surfactant mixtures: Effects of cationic polymer charge density and alkyl chain surfactant hydrophobicity

245th National Meeting (2013)

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