Viscoelastic phase separation at cytochrome c solution/capsules interface

Abstract

In this study, quasi 2 D films of cytochrome c capsules have been used to model confinement effects on the protein at air/solution and solid/air interfaces, and the viscoelasticity of the system has been studied using interfacial rheological and quartz crystal microbalance (QCM) measurements. Spread films of the proteins in the form of designed capsules on very dilute solutions of the protein itself have been prepared to study the concentration-dependent viscoelasticity. The effective surface pressure (referred as surface pressure) ⿿ surface concentration (Ͽ-ο) plots in very dilute and semi dilute regime of the spread films of the capsules interacting with the protein in the solution (solid/liquid interface) indicate that the individual protein molecules cause short-range attraction between the capsules through bridging and depletion mechanism of water on the surface which can be tuned by the surface concentration (Surface concentration also often referred as surface density has been used interchangeably in the text). Both dilational and shear rheological studies carried out on the capsule/solution interface suggest that the viscoelastic contributions become pronounced at a critical concentration (ο=0.025⿿0.05mg/m2). Brewster angle microscopy at air/solution interface shows clearly the viscoelastic phase separation which has been confirmed by Quartz crystal microbalance with dissipation at different overtones (n=3, 5, 7⿦). Scanning electron microscopy (SEM) has been used to characterize capsules at the solid/liquid interface and the micrographs show fused capsules. The results suggest that the protein molecules from the solution act as bridging units between the capsules which in turn trigger the liquid to gel-like transition in the spread films which results in the viscoelastic phase.