R. Wüstneck

Dilational and shear rheology of adsorption layers at liquid interfaces

Abstract Dynamic properties of interfaces are of increasing interest in science and in practice as they give insight into interactions and processes at interfaces rather than equilibrium properties. The general ideas on mechanical interfacial properties as an important part of dynamic properties were established long ago by Gibbs and Boussinesq. Now on the basis of new techniques, better experiments can be performed which allow a more and more quantitative understanding. The mechanical behaviour of interfaces, modified by soluble adsorption layers or insoluble monolayers of surfactants or polymers, is the subject of many actual studies. Computer-driven instruments using new sensors and very sophisticated methodologies enable us to perform very complex and sensitive measurements which were impossible until recently. Numerous studies of interfacial shear and dilational rheology have been reported and use a large variety of techniques. Shear experiments are most useful for polymer and mixed polymer-surfactant adsorption layers and insoluble monolayers and give access to interaction forces in two-dimensional layers. Dilational interfacial properties however are most frequently studied for soluble adsorption layers of surfactants and mixtures of polymers and surfactants. This overview gives an introduction to the interfacial rheology and discusses some specific theoretical aspects necessary to interpret experiments. Experimental techniques to perform shear and dilational experiments at liquid interfaces are summarised and only the most recent developments are described in more detail. Examples are given to demonstrate how the experiments work and what output can be expected.

Properties of mixed protein/surfactant adsorption layers

The adsorption isotherms, adsorption kinetics and surface rheological properties of β-lactoglobulin, β-casein, in the absence and presence of Tween 20 were measured. To study the adsorption process (isotherms and kinetics) at the water–air interface the pendant drop technique (axial drop shape analysis, ADSA), and ring tensiometry were used. The surface shear rheological parameters were measured with a torsion pendulum set-up. Also, data of the equilibrium film thickness and surface diffusion coefficients obtained from fluorescence recovery after photobleaching (FRAP) measurements are used to understand the competitive adsorption mechanism. The adsorption process and shear rheological behaviour of the studied systems show a rather complex behaviour which depends most of all on the systems composition. At high protein or surfactant content the behaviour is controlled by the main component while for the more mixed systems the adsorption process is complex and consists of partial adsorption, surfactant–protein interaction and protein rearrangement as a function of surface coverage. The results obtained illustrate that all these processes must be taken into account in future new theoretical models to be derived for such systems.

Dynamic surface tension and surface shear rheology studies of mixed β-lactoglobulin/Tween 20 systems

Abstract The mechanical behaviour of β-lactoglobulin (BLG) and BLG/Tween 20 adsorption layers at the air/water interface has been studied by dynamic surface tension and surface shear rheological measurements. The adsorption properties were measured using different surface tension methods (ring tensiometry, drop volume tensiometry and axisymmetric drop shape analysis). The surface shear rheological measurements were performed with a torsion pendulum set-up which simultaneously provides information about the surface shear viscosity and elasticity. The adsorption layer structure is controlled by the interactions between protein and surfactant molecules. The corresponding dynamic surface tension measurements confirm the peculiarities foun adsorption layer. With increasing surfactant concentration at constant protein concentration the mixed surface film gradually approaches a state identical to a pure surfactant adsorption layer, from which the protein is completely repelled.

Adsorption characteristics of mixed monolayers of a globular protein and a non-ionic surfactant.

Abstract Experimental and theoretic studies of the adsorption behaviour for the mixture of globular protein (human serum albumin (HSA)), and non-ionic surfactant (decyl-dimethyl-phosphine-oxide C10DMPO) are performed. The experimental results for the mixtures agree well with a theoretical model which assumes significant differences between the partial molar areas of the protein and the surfactant, and takes into account large unbound charge of the protein molecules. An anomalous surface tension increase of the mixtures at low surfactant concentrations was found experimentally and explained on the basis of a thermodynamic model. The concentration range at which a comparable coverage of the mixed surface layer by protein and surfactant molecules appears is shown to be quite narrow.

Adsorption of protein-surfactant complexes at the water/oil interface.

Interfacial tension measurements have been performed at the water/hexane interface on mixtures of the bovine milk protein β-lactoglobulin and positively charged cationic surfactants (alkytrimethylammonium bromides). The addition of surfactants with different chain lengths leads to the formation of protein-surfactant complexes with different adsorption properties as compared to those of the single protein. In this study, the formation of complexes has been observed clearly for protein-long chain surfactant (TTAB and CTAB) mixtures, which has shown in addition to specific electrostatic interactions the relevance of hydrophobic interactions between surfactant molecules and the protein. The modeling of interfacial tension data by using a mixed adsorption model provides a quantitative understanding of the mixture behavior. Indeed, the value of the adsorption constant of the protein obtained in the presence of surfactants has strongly varied as compared to the single protein. Actually, this parameter which represents the affinity of the molecule for the interface is representative of the hydrophobic character of the compound and so of its surface activity. Even if a more hydrophobic and more surface active protein-surfactant complex has been formed, the replacement of this complex from the interface by surfactants close to their cmc was observed.

Use of pendent drop technique as a film balance at liquid/liquid interfaces

Abstract The pendent drop technique has been extended to investigate insoluble monolayers of the phospholipid dipalmitoyl-phosphatidylcholine at the water/ n -dodecane interface. The change of the drop size enables one to compress and expand the surfactant film. The dynamic interfacial tension of the film obtained by using axisymmetric drop shape analysis (ADSA) as a function of surface area provides thermodynamic data of the monolayer. The isotherm obtained with ADSA shows the same type of dynamic behaviour as measured with the classic Langmuir-Blodgett trough technique. Dynamic behaviour of insoluble monolayers, such as monofilm relaxation can be studied by ADSA more readily than by the common Langmuir film balances because of the uniform temperature of the entire system and uniform interfacial pressure and concentration along the drop surface.

Stress relaxation behaviour of dipalmitoyl phosphatidylcholine monolayers spread on the surface of a pendant drop

Dipalmitoyl phosphatidylcholine (DPPC) monolayers were characterised by surface pressure/area isotherms (π/A) and surface dilational rheological parameters at temperatures 20–40°C. The methods used were the Langmuir trough and the pendant drop micro-film balance. The latter allows accurate measurements at higher temperatures and transient drop deformation. Stable DPPC monolayers were found only for low surface pressures, π 25 mN m−1 and 20°C relaxation experiments give evident of rupturing, brittle monolayer structures. At higher temperatures the monolayers became more fluid-like. π/A-isotherms determined by using both methods principally agree with each other, but show also remarkable differences, which cannot be explained so far satisfactory. Transient drop relaxation experiments were analysed for the short time range (600 s). At 20°C the dilational modulus (er) and the surface dilational viscosity (ξr) passes a stationary maximum at 0.54 nm2 molecule−1 and increase strongly at higher surface coverage, thus indicating crystalline monolayer structure. Increasing temperature from 20 to 30°C causes a rapid decrease of er and ξr and a shift of the stationary maximum to lower surface coverage. No evidence for crystalline structure is found. Further increase of temperature causes er and ξr increase again. This increase is caused by a rising relaxation time, while the elasticity does not change in the same manner. Such intermediate decrease of er and ξr in the range 30–40°C appears to be unusual and can be interpreted as a consequence of strong DPPC interactions and strongly pronounced retardation of monolayer deformation. The study is discussed in connection to the physiology of breathing. For pulmonary surfactants the observed behaviour seems to be understandable. It is however interesting that such complex behaviour is observed for monolayers consisting of DPPC only.

CHARACTERISATION OF THE INITIAL PERIOD OF PROTEIN ADSORPTION BY DYNAMIC SURFACE TENSION MEASUREMENTS USING DIFFERENT DROP TECHNIQUES

Abstract A model is derived for the initial state of the formation of protein adsorption layer. This model considers the balance of protein molecules adsorbed at the drop interface and solved in the bulk of a drop. The mass balance allows estimation of the adsorbed amount, the adsorption layer thickness, and the diffusion coefficient at the end of the so-called induction time. For a globular (human serum albumin) and a native protein (β-casein) these parameters are determined and agree with data from literature. For β-casein the effect of bulk concentration on the degree of denaturation and the minimum adsorption layer thickness are estimated.

PROTEIN ADSORPTION ISOTHERMS STUDIED BY AXISYMMETRIC DROP SHAPE ANALYSIS

Abstract The pendant drop method is used to measure the interfacial tension isotherms of solutions of the model proteins β-casein, β-lactoglobulin both from bovine milk, and human serum albumin. A recently developed theory of protein adsorption at fluid interfaces [V.B. Fainerman, R. Miller, R. Wustneck, J. Colloid Interface Sci. 183 (1996) 26] is used to calculate the equilibrium protein concentration at the interface and in the bulk. Comparison of the results with literature data obtained by plate and ring tensiometry shows that at low initial protein concentrations in a drop and a large surface area, the final protein concentration after the establishment of equilibrium can be significantly decreased. However, this deficiency of the pendant drop method is turned into an advantage, as from the mass balance of the protein at low initial the adsorption can be calculated.

Adsorption kinetics and rheological properties of food proteins at air/water and oil/water interfaces.

The behaviour of foams and emulsions stabilised by protein adsorption layers is controlled by the properties of the air/water or oil/water interface. Due to the dynamic conditions of these systems, the non-equilibrium interfacial properties are dominant, such the formation of adsorption layers and their mechanical resistance. The adsorption kinetics and thermodynamics of the food proteins β-lactoglobulin and β-casein are measured at the water/air and water/tetradecane interface by using drop techniques. The equilibrium adsorption and the adsorption kinetics are discussed on the basis of a new theory which considers changing molar area of adsorbed proteins according to the surface coverage. The shear rheological behaviour of the systems is measured by using an interfacial shear rheometer. The results complement each other and support the idea that the degree of unfolding of proteins depend on the surface coverage and thus changes with time and bulk concentration.