Dilatational Rheology of β-Casein Adsorbed Layers at Liquid−Fluid Interfaces

Abstract

The rheological behavior of beta-casein adsorption layers formed at the air-water and tetradecane-water interfaces is studied in detail by means of pendant drop tensiometry. First, its adsorption behavior is briefly summarized at both interfaces, experimentally and also theoretically. Subsequently, the experimental dilatational results obtained for a wide range of frequencies are presented for both interfaces. An interesting dependence with the oscillation frequency is observed via the comparative analysis of the interfacial elasticity (storage part) and the interfacial viscosity (loss part) for the two interfaces. The analysis of the interfacial elasticities provides information on the conformational transitions undergone by the protein upon adsorption at both interfaces. The air-water interface shows a complex behavior in which two maxima merge into one as the frequency increases, whereas only a single maximum is found at the tetradecane interface within the range of frequencies studied. This is interpreted in terms of a decisive interaction between the oil and the protein molecules. Furthermore, the analysis of the interfacial viscosities provides information on the relaxation processes occurring at both interfaces. Similarly, substantial differences arise between the gaseous and liquid interfaces and various possible relaxation mechanisms are discussed. Finally, the experimental elasticities obtained for frequencies higher than 0.1 Hz are further analyzed on the basis of a thermodynamic model. Accordingly, the nature of the conformational transition given by the maximum at these frequencies is discussed in terms of different theoretical considerations. The formation of a protein bilayer at the interface or the limited compressibility of the protein in the adsorbed state are regarded as possible explanations of the maximum.

Emulsion Drops with Complex Interfaces: Globular Versus Flexible Proteins

“…[3] Adsorbed proteins modify the properties of liquid interfaces in several ways: (i) they are polyampholytes and decrease the interfacial tension upon adsorption; depending on the pH and the ionic strength, they can also provide charge to the interface; (ii) unlike many traditional surfactants, protein adsorption layers impose local viscoelastic properties onto liquid interfaces. [4][5][6][7][8] Furthermore, adsorption of proteins is effectively irreversible, as demonstrated in subphase exchange experiments: [4,5] only minor desorption and negligible changes in interfacial tension are observed after a protein solution is replaced with protein-free buffer surrounding a protein-covered pendant drop. These effects have been described for a number of proteins of both biological and technical relevance, such as b-casein, [6,7] b-lactoglobulin, [9][10][11][12] bovine serum albumin, [13] or lysozyme.…”

“…[4][5][6][7][8] Furthermore, adsorption of proteins is effectively irreversible, as demonstrated in subphase exchange experiments: [4,5] only minor desorption and negligible changes in interfacial tension are observed after a protein solution is replaced with protein-free buffer surrounding a protein-covered pendant drop. These effects have been described for a number of proteins of both biological and technical relevance, such as b-casein, [6,7] b-lactoglobulin, [9][10][11][12] bovine serum albumin, [13] or lysozyme. [5] While the interfacial chemistry and rheology of adsorption layers formed by these proteins have received increased attention recently, less is known about the role their transport properties play in the context of high This study focuses on the flow behavior of emulsion drops with complex interfaces.…”

“…It is the most hydrophobic protein of the casein family and commonly used as a model flexible polyampholyte. [16] Experimental data for interfacial tensions, adsorption kinetics, and surface pressure isotherms of proteins have been measured by numerous authors, mostly using the Langmuir trough and pendant drop methods, [6,[17][18][19][20] resulting in several suggested adsorption models and equations of state. [15,[21][22][23] Proteins are important gelformers in the bulk, depending on protein concentration, ionic strength, pH, and thermal history.…”

“…[24,25] When a surface-active protein is adsorbed from a dilute solution to a liquid interface, its local concentration in the interfacial region remains drastically increased as compared to the bulk phase concentration. [4][5][6]15] Such interfacial layers exhibit rheological properties similar to those of bulk protein gels, [26][27][28] even if the parent solution of the protein is dilute, Newtonian, and low-viscous. Spread layers of blactoglobulin have been described as a ''soft glasses'' [10,29] and their surface rheology is (at least phenomenologically) similar to colloidal monolayers of mm-sized polystyrene spheres.…”

Microfluidic methods to study emulsion formation

“…%). This may be caused by a higher affinity of protein for oil than for air resulting in a higher interfacial protein concentration with a more compact and protective structure at the oil-water interface (Pradines et al 2009;Maldonado-Valderrama et al 2005). This foam experiment shows that it is possible to form protein-stabilised air bubbles in a microfluidic device and subsequently measure their coalescence stability.…”