All-aqueous emulsions stabilized by sporopollenin exine capsules

The efficacy of sporopollenin exine capsules (SpECs) for stabilization of all-aqueous emulsions was assessed. Cytoplasmic substances were removed from Lycopodium clavatum spores to obtain the SpECs. SpECs had a comparable morphology with the parent spores, and a size of ∼31 μm. The all-aqueous emulsions were prepared with polyethylene glycol (PEG) and dextran (Dex) at different PEG: Dex concentration ratios between 2:9 and 18:1. Confocal microscopy imaging showed that the emulsions were Dex-continuous at PEG: Dex concentration ratios of 2:9 and 4:8 and PEG-continuous at the ratios ≥6:7. When the SpECs were initially suspended within the PEG phase, the Dex-continuous emulsions could not be stabilized. The SpECs also failed to stabilize the emulsion with a ratio of 6:7, where the emulsions transitioned to PEG-continuous. However, the SpECs could stabilize a bottom emulsion phase, consisting of Dex-rich droplets within the PEG exterior phase, at concentration ratios ≥8:6. We hypothesized that a Pickering-type stabilization mechanism at the aqueous-aqueous interface of Dex droplets account for the emulsion stability, together with a possible formation of SpEC particle rafts. The emulsion stability at a concentration ratio of 8:6 was dependent on its pH; stable emulsions were formed at pH 7, but at pHs 2 and 4, the emulsions became unstable. These results were attributed to the high ζ-potential (−31 mV) of the SpECs at pH 7. These results show that repulsion between dispersed droplets was more important than the packing of the particles at the interface itself.