Abstract
In the search for responsive complexes with potential applications in the formulation of smart dispersed systems such as foams, we hypothesized that a pH-responsive system could be formulated with polyacrylic acid (PAA) mixed with a cationic surfactant, Gemini 12-2-12 (G12). We studied PAA-G12 complexes at liquid–air interfaces by equilibrium and dynamic surface tension, surface rheology, and X-ray reflectometry (XRR). We found that complexes adsorb at the interfaces synergistically, lowering the equilibrium surface tension at surfactant concentrations well below the critical micelle concentration (cmc) of the surfactant. We studied the stability of foams formulated with the complexes as a function of pH. The foams respond reversibly to pH changes: at pH 3.5, they are very stable; at pH > 6, the complexes do not form foams at all. The data presented here demonstrate that foam formation and its pH responsiveness are due to interfacial dynamics.
Highlights
A pH-Responsive Foam FormulatedFormulated with polyacrylic acid (PAA)/GeminiPolyelectrolytes and surfactants are used in a broad number of industries and technologies, both on their own and mixed
To our knowledge, no pH-responsive foams have been formulated with polyelectrolyte-surfactant complexes
Foams stabilized with PAA-G12 are pH responsive; the response is fully reversible, at least after three cycles of NaOH-hydrochloric acid (HCl) additions
Summary
The interfaces saturate with surfactant molecules, at which point spontaneously self-aggregate in mixtures bulk to form micelles. PAA is a weak acid, the charge of the carboxylic groups can be modulated by changing the pH, which could modify the interactions with the oppositely charged G12 surfactant; this in turn could change the structure and properties of the complexes, both in bulk and at interfaces. We hypothesized that this could produce pH-responsive foams and focused on the surface properties of PAA/G12 complexes for the formulation of smart foams. For PAA-G12 complexes, the surface activity at equilibrium is not modified by pH at the surfactant concentrations used
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