Cerato-ulmin hydrophobin-coated air bubbles and oil droplets: Stability, shapes, and interfacial behavior

Abstract

Hydrophobins are small amphipathic proteins: one side shuns water, the other seeks it. Unlike common amphipathic molecules (e.g., sodium dodecyl sulfate or SDS used in soap), hydrophobins are nearly rigid, thanks to an elaborate disulfide crosslinking network that stabilizes their compact, globular structure. Hydrophobins have been called nature’s Janus particles, and nature makes them by the ton in mushrooms and other forms of fungi. The present dissertation research concerns a particular hydrophobin cerato-ulmin (CU). This thesis is directed toward (1) the exploration of the CU’s ability to encapsulate whether gaseous or liquid and the stability of the resulting bubbles and droplets, (2) the understanding of the intermediate structures coated with CU, such as cylindrical and toroidal shapes, (3) the understanding of the interfacial behavior of CU alone and the complex interfacial interaction at air-water and oil-water interfaces, and (4) the exploration of delivering hydrophobic molecules by CU. Firstly, CU is found to stabilize cylindrical microbubbles upon simple agitation of its dilute suspension or sausage-like oil droplets in the presence of nonpolar solvents. No emulsifier or other polymer is required to trap either air or oil, suggesting that CU provides both emulsification and strength. Air or oil can be trapped directly without a fluid carrier. The bubbles or droplets are numerous and remain in suspension long enough for facile study. Secondly, manipulation of pressure in a prescribed sequence introduces shape transitions of the bubbles from cylinders to spheres and ultimately torus. Bending elastic energy and curvature model are used to explain toroidal shapes and their stability. The solid-like CU films are stiff enough to retain the unusual shapes. Thirdly, CU molecules are prone to adsorb to the air-water and oil-water interfaces and the adsorption is irreversible. The interfacial moduli are often about ten times stronger than membranes formed by traditional surfactant molecules, although CU films can be disrupted by SDS above its critical micelle concentration. Finally, the diffusive behavior of small debris ejected from CU air bubbles and hydrophobic particles inside CU oil droplets was characterized by differential dynamic microscopy. The particles freely follow Brownian diffusion soon after encapsulation, but lose their diffusive motion as the solvent evaporates.