Understanding acid-glycol switchable emulsifiers on the molecular scale

Switchable emulsifiers are commonly studied at the macroscopic scale without significant insight into the molecular properties and processes underpinning their behaviour. This work aims to investigate on the molecular scale a switchable emulsifier comprising poly(methacrylic acid) (PMAA) and poly(ethylene glycol) methacrylate (PEGMA). This emulsifier forms stable oil/water emulsions at pH > 10, but the emulsion becomes unstable at pH < 3 due to droplet aggregation. It has been suggested that H-bonding between the MAA (methacrylic acid) and EG (ethylene glycol) groups is central for the switching behaviour. However, no detailed molecular understanding of the surface of the emulsifier coated droplets has been developed so far. Using sum frequency generation spectroscopy, we compared the spectra of the PMAA/PEGMA polymer deposited on a water surface under acidic (pH 2.5) and basic (pH 11.5) conditions. We found that at pH 2.5, both MAA-EG H-bonds and MAA = MAA cyclic dimers contribute to droplet aggregation in the emulsion. In contrast, at pH 11.5 deprotonation and solvation of the PMAA component lead to a polymer restructuring. As a result, the PMAA chains form a negative shell around the droplets, electrostatically stabilising the emulsion in alkaline conditions.