Abstract
Emulsions on the scale of 100 s of nm in diameter traditionally achieve long-term colloidal stability (> 1 month) through surfactants or a combination of surfactant and polymer. The stability of emulsions coated with only polyelectrolyte remains largely unknown. Here, we introduce an emulsion system coated with the surface-active polymer, poly(acrylic acid) (PAA) that stabilizes hexadecane-in-water emulsions with a marked dependence on pH and ionic strength. Surface specific vibrational sum frequency scattering spectroscopy (VSFSS) provides a molecular picture of PAA organization at the hexadecane-water droplet interface while dynamic light scattering and zeta potential measures the colloidal stability as a function of pH and salt concentration. We find that a steric layer of polymer alone stabilizes emulsions despite having a low zeta potential (<5 mV) suggesting a mechanism of stability unexplained by traditional theories. In fact, as the magnitude of the zeta potential increases, emulsions become polydisperse and unstable. VSFS spectra reveals that an ordered backbone at pH 2 and pH 4 contribute to colloidal stability while at pH 6, even with the addition of charge-screening cations, PAA prefers to be solvated in water and does not form stable oil droplets.
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