Balancing soft elasticity and low surface polarity in films of charged BSA capsules at air/fluid interface

Interaction between charged BSA colloids and the buffer at air/fluid interface has been studied using spread films of the capsules of the protein prepared at pH 4.5 and 7.5 (below and above the pI of BSA). Surface pressure-surface concentration plots, interfacial dilational rheology and Quartz crystal microbalance with dissipation have been used to characterize the films. The study shows that below the pI of the protein, the positively charged colloids entrain more water on the surface which leads to partial neutralization of the charges. Results suggest that the charged capsules are elastic due to the strongly adsorbed protein layers that restrict deformation and any small shape fluctuations is likely due to the distortion of the viscoelastic surface layer at pH=4.5. Capsules of BSA behave as 'soft elastic membrane' with interfacial properties lying between that of an elastic membrane and a slightly soluble diffuse capsule with low interfacial tension. Such elastic capsules would find applications in drug delivery and food colloids.