Interfacial rheology of protein–surfactant mixtures
Introduction
Mixtures of proteins and surfactants are often used in many technological applications, including food and pharmaceutical industries, cosmetics, coating processes, and so forth. In many of these applications protein–surfactant mixtures are used in the manufacture of processed dispersions. These dispersions contain two or more immiscible phases (aqueous, oil and/or gas phases) in the form of foams and emulsions. Dispersions are inherently unstable systems because of their large interfacial area. The stability of these systems is generally achieved through a protective interfacial layer around the particles (emulsion droplets or foam bubbles). The properties of this interfacial layer are governed by the composition and structure of the adsorbed material and in turn would determine the properties of the dispersion. This review focuses essentially on protein–surfactant mixed films in relation to food dispersion formulations. In food systems, the interfacial layer often comprises both proteins and surfactants (mainly lipids and phospholipids). These surface-active substances are used as emulsifiers or foaming agents because of their amphiphilic character. The optimum use of emulsifiers in food technological applications depends on our knowledge of their interfacial physicochemical characteristics, such as surface activity, amount adsorbed, the kinetics of film formation, structure, thickness, topography, ability to desorb (stability), lateral mobility, interactions between adsorbed molecules, ability to change conformation, interfacial rheological properties, etc. However, in this contribution we will focus on interfacial rheology of protein–surfactant mixtures.
Interfacial rheology is important for food dispersions since the structural and mechanical properties of food emulsifiers at fluid interfaces influence the production, stability and texture of the product [1], [2], [3••], [4•]. Interfacial rheology studies the response of interfacial films to deformation. Interfacial rheology can be defined for both compressional deformation (dilatational rheology, which measures variation of area while maintaining a constant shape) and shearing motion of the interface (shear rheology, which accounts for changes in shape at constant area, provided that the interface is in thermodynamic equilibrium with the bulk solution). These two methods are complementary and focus on different aspects of the interfacial layer. While shear viscosity may contribute appreciably to the long-term stability of dispersions, dilatational rheology plays an important role in short-term stability [1], [2], [5•], [6]. Interfacial shear rheology is most useful for protein and mixed protein surfactant adsorption layers and insoluble monolayers and gives access to interaction forces in two dimensional layers [4•], [5•], [7], [8•], [9••], [10••]. On the other hand, interfacial dilatational rheology is a very sensitive technique to monitor the interfacial structure and concentration of single emulsifiers at the interface [4•] or the relative concentration, the competitive adsorption, and the magnitude of interactions between different emulsifiers at the interface [4•], [10••], [11], [12]. However, these two contributions are usually combined in real process conditions, and therefore the composition, interactions and mechanical behaviour of a deformed film is better interpreted through a synergy of dilatational and shear techniques [2], [5•], [8•], [9••], [13•]. The interfacial shear rheology of protein–surfactant mixtures has been analysed recently by Krägel et al. [9••]. Since the same topic will be analysed in this issue, the emphasis of this review is on interfacial dilatational rheology.
Important for interfacial rheology are (Fig. 1) the native conformation of the protein (random or globular) [14], [15], [16], [17•], [18], [19], [20•], [21], the type of surfactant (i.e., water-soluble or oil-soluble surfactant, ionic or non-ionic) [5•], [8•], [9••], [10••], [13•], [14], [15], [16], [17•], [18], [22•], [23], the nature of the interface (air–water or oil–water) [5•], [8•], [9••], [13•], [24•], [25], [26], the composition of the aqueous media (i.e., pH, ionic strength, etc.) [4•], [10••], [11], [27], [28], the method of formation of the interfacial film (by spreading or adsorption) [4•], [5•], [8•], [9••], [10••], [13•], and the interactions (of hydrophobic and/or electrostatic nature) [5•], [8•], [9••], [13•] and/or displacement of protein by surfactant (including the protein modification due to complexation, competitive adsorption, orogenic displacement [7], [8•], [12], [29], [30], [31], etc.). The analysis of the interfacial rheology of spread mixed emulsifiers at fluid interfaces (including the penetration of a water-soluble or insoluble emulsifier on a spread monolayer) is of practical importance, because these spread monolayers constitute well defined models for the analysis of food dispersions with several advantages for fundamental studies [4•], [10••], [11], [30], [32••], [33], [34••], [35]. The adsorption from mixed protein–surfactant solutions can be studied generally via two different experimental routes; via sequential or simultaneous adsorption. In the former case, the protein is first adsorbed or spread at the interface and the surfactant is then injected directly into the bulk phase. Hence, the interaction between both species is influenced by the conformation adopted by the protein at the interface. Differently, in the simultaneous adsorption experiment, protein and surfactants compete for the interfacial area and the interaction between both species, in the bulk and at the interface, has to be considered. This latter procedure is more directly related to technological processes [5•], [8•], [9••], [13•], [36].
Recently, the analysis of interfacial rheology of protein–surfactant mixtures has been facilitated by the utilisation of complementary traditional instruments (tensiometry and film balance) and new microscopic, nanoscopic, and spectroscopic techniques (Brewster angle microscopy, BAM, atomic force microscopy, AFM, imaging ellipsometry, IE, infrared reflection absorption spectroscopy, IRRAS, surface plasmon resonance, SPR, etc.) suitable for the characterisation of such layered structures, which has provided new insight into the characterisation of mixed interfaces [4•], [10••], [13•], [30], [31], [34••], [35], [37], [38], [39], [40], [41], [42], [43]. In addition, quantitative theoretical models have been derived for describing the equilibrium state as well as the dynamics and dilatational rheology for mixed adsorption layers. The results show that, although the equilibrium state is described by theoretical models adequately, the quantitative analysis of the dilatational viscoelasticity data requires further refinement of the corresponding models [44], [45••].
There has been a great interest in understanding the behaviour of mixed interfacial layers in the last years. Recent reviews summarise the state of the art of this topic focusing in different aspects; the theoretical modelling and thermodynamics [7], [45••], the physicochemical properties of interfacial layers [10••], [11], their role in colloidal stability [3••], [4•], [12], the interfacial shear rheology [9••], and the interfacial dilatational rheology [2]. Accordingly, in the present review we will concentrate on the last five years, complementing the information detailed in previews reviews.
Section snippets
Interfacial dilatational properties of protein–surfactant mixtures
Dilatational rheology implies a deformation in the interfacial area. This technique is sensitive to the intrinsic softness or hardness of the molecules at the interface, as well as to inter-molecule interactions and adsorption. Accordingly, dilatational rheology contains large amount of information that however, is difficult to interpret [28]. The effect of protein–protein, surfactant–surfactant, and protein–surfactant interactions at fluid interfaces (penetrated, adsorbed, coadsorbed, or
Interfacial shear properties of protein–surfactant mixtures
Although interfacial shear rheology implies a change in shape, because no area changes are imposed during shear, this technique is more sensitive to the intermolecular interactions between the adsorbed molecules [9••], [28], it provides additional information so as to infer the structure of mixed protein–surfactant layers [8•].
Implications of interfacial rheology of protein–surfactant mixtures in dispersion formulations
The link between interfacial rheology and dispersion stability is far from straightforward. Proteins and surfactants are surface-active substances used to stabilize both foams and emulsions [3••], [4•], [12]. However, the mechanisms by which such stabilisation occurs are different. Proteins form immobile viscoelastic films at the interface depending on the molecular structure of the protein whereas most low-molecular-weight surfactants form fluid, mobile interfacial layers, with low interfacial
Conclusions
In this review, the mechanical behaviour of protein–surfactant mixed films at fluid interfaces has been analysed, putting more emphasis on the interfacial dilatational rheology. Work so far has shown that interfacial rheology of protein–surfactant mixed films depends on the protein (random or globular), the surfactant (water-soluble or oil-soluble surfactant, ionic or non-ionic), the interface (air–water or oil–water), the interfacial (protein/surfactant ratio) and bulk (i.e., pH, ionic
Acknowledgements
This research was supported by CICYT (Spain) through grant AGL2007-60045, and Consejería de Educación y Ciencia, Junta de Andalucía (Spain), through grant PO6-AGR-01535. JMV acknowledges support from “Ministerio de Educación y Ciencia”, project MAT2007-66662-C02-01 and EU, seventh Framework Programme, Marie Curie Intra-European Fellowship (FP7-PEOPLE-2007-2-1-IEF-LIF-220570).
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