Morphological Changes in Adsorbed Protein Films at the Air−Water Interface Subjected to Large Area Variations, as Observed by Brewster Angle Microscopy

Abstract

Adsorbed films of proteins at the air-water interface have been imaged using Brewster angle microscopy (BAM). The proteins beta-lactoglobulin (beta-L) and ovalbumin (OA) were studied at a range of protein concentrations and surface ages at 25.0 degrees C and two pH values (7 and 5) in a Langmuir trough. The adsorbed films were periodically subjected to compression and expansion cycles such that the film area was typically varied between 125% and 50% of the original film area. With beta-L on its own, no structural changes were observable at pH 7. When a low-area fraction (less than 0.01%) of 20 mum polystyrene latex particles was spread at the interface before adsorption of beta-L, the particles became randomly distributed throughout the interface, but after protein adsorption and compression/expansion, the particles highlighted notable structural features not visible in their absence. Such features included the appearance of long (several hundred micrometers or more) folds and cracks in the films, generally oriented at right angles to the direction of compression, and also aggregates of protein and/or particles. Such structuring was more visible the longer the film was aged or at higher initial protein concentrations for shorter adsorption times. At pH 5, close to the isoelectric pH of beta-L, such features were just noticeable in the absence of particles but were much more pronounced than at pH 7 in the presence of particles. Similar experiments with OA revealed even more pronounced structural features, both in the absence and presence of particles, particularly at pH 5 (close to the isoelectric pH of OA also), producing striking stripelike and meshlike domains. Changes in the dilatational elasticity of the films could be correlated with the variations in the structural integrity of the films as observed via BAM. The results indicate that interfacial area changes of this type, typical of those that occur in food colloid processing, will lead to highly inhomogeneous adsorbed protein layers, with implications for the stability of the corresponding foams and emulsions stabilized by such films. Overall, the experimental results are in broad agreement with the sorts of trends predicted by earlier computer simulations of protein films subjected to such compression and expansion.

Visible and Sub-visible Particle Formation for a Model Bioconjugate

“…The addition of surfactants at levels near or above the CMC may be used to prevent this air-water interface and agitation-induced particle formation (8,18,(22)(23)(24). Morphological differences in the VPs are in line with previously reported literature (25)(26)(27).…”

Stabilization of bubbles and foams

“…The simulations produced remarkably similar results to microscopic images of real protein displacement. Furthermore, Pugnaloni et al [108 • ] extended such types of simulation to examine the response of films of different strength to large-strain compression and expansion and these results are also in agreement with recent BAM studies of adsorbed protein films [65]. Hambardzumyan et al [109] have compared experimental data on the adsorption of β-casein with that for a symmetrical polypropylene oxidepolyethylene oxide tri-block copolymer.…”

“…In fact, the inhomogeneity of adsorbed layers is one important general factor that is increasingly recognized in the field of food colloids, due to further atomic force microscopy (AFM) measurements [60 • ] of films transferred from the A-W interface to solid substrates, fluorescence microscopy of labeled spread films [61,62] and in situ measurements of adsorbed and spread films via Brewster angle microscopy (BAM) [63,64]. Xu et al [65] recently used BAM to show how successive compressions and expansions in situ could generate film inhomogeneity on a macroscopic scale in adsorbed films of individual proteins (β-lactoglobulin and ovalbumin). These changes in film structure were found to be correlated with changes in the overall film dilatational rheology and the extent of protein adsorption.…”

The rheology of polyvinylpyrrolidone-coated silica nanoparticles positioned at an air-aqueous interface

“…parallel light to the incident plane) from a clean air-water interface, reflected light was only measured following deposition of the particle-laden interface, with the reflected light intensity being a function of the particle surface coverage and film thickness. [39] Three electrolyte concentrations (0.01 M, 0.1 M and 0.55 M Na2SO4) in the aqueous sub-phase were considered. The Brewster angle was calibrated for each electrolyte concentration (0.01 M ~53.23°, 0.1 M ~53.27° and 0.55 M ~53.45 o ) to achieve the best image quality (i.e.…”