A. V. Makievski

Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface.

The adsorption behaviour of proteins and systems mixed with surfactants of different nature is described. In the absence of surfactants the proteins mainly adsorb in a diffusion controlled manner. Due to lack of quantitative models the experimental results are discussed partly qualitatively. There are different types of interaction between proteins and surfactant molecules. These interactions lead to protein/surfactant complexes the surface activity and conformation of which are different from those of the pure protein. Complexes formed with ionic surfactants via electrostatic interaction have usually a higher surface activity, which becomes evident from the more than additive surface pressure increase. The presence of only small amounts of ionic surfactants can significantly modify the structure of adsorbed proteins. With increasing amounts of ionic surfactants, however, an opposite effect is reached as due to hydrophobic interaction and the complexes become less surface active and can be displaced from the interface due to competitive adsorption. In the presence of non-ionic surfactants the adsorption layer is mainly formed by competitive adsorption between the compounds and the only interaction is of hydrophobic nature. Such complexes are typically less surface active than the pure protein. From a certain surfactant concentration of the interface is covered almost exclusively by the non-ionic surfactant. Mixed layers of proteins and lipids formed by penetration at the water/air or by competitive adsorption at the water/chloroform interface are formed such that at a certain pressure the components start to separate. Using Brewster angle microscopy in penetration experiments of proteins into lipid monolayers this interfacial separation can be visualised. A brief comparison of the protein adsorption at the water/air and water/n-tetradecane shows that the adsorbed amount at the water/oil interface is much stronger and the change in interfacial tension much larger than at the water/air interface. Also some experimental data on the dilational elasticity of proteins at both interfaces measured by a transient relaxation technique are discussed on the basis of the derived thermodynamic model. As a fast developing field of application the use of surface tensiometry and rheometry of mixed protein/surfactant mixed layers is demonstrated as a new tool in the diagnostics of various diseases and for monitoring the progress of therapies.

The analysis of dynamic surface tension of sodium alkyl sulphate solutions, based on asymptotic equations of adsorption kinetic theory

Abstract We analysed existing and newly derived asymptotic solutions of the adsorption kinetic equations for the liquid phase interface in the regions of infinitely small and infinitely great surface lifetimes (t) for the cases of one and of a few surfactants, on non-deforming and deforming surfaces, under non-stationary, stationary and quasi-stationary conditions, assuming either the diffusion adsorption mechanism or mixed adsorption mechanism. It was proved that in the region t → ∞, the adsorption barrier does not influence the dynamic surface tension σ, but the role of surface-active contaminants is significant. In contrast, in the region t → 0, the role of contaminants is small, but the adsorption barrier influences the dynamic surface tension substantially. The dynamic surface tension of sodium alkyl sulphate solutions was measured by the maximum bubble pressure method, in the t range 0.001–10 s. In the region t → ∞ we obtained good agreement of experimental results with asymptotic formulae. The diffusion adsorption mechanism of the surfactant solutions studied was confirmed and we also estimated the concentration values of the surfactant admixtures. Small additions of the more active surfactant sodium tetradecyl sulphate to sodium dodecyl sulphate substantially influences the shape of the σ—t curve in the region t → ∞, increasing (in full accordance with theoretical considerations) the tangent value of the curve inclination of the dependence of σ on t−1/2. In the regions t → 0, long-chained high molecular weight sodium alkyl sulphates adsorb according to the diffusion mechanism, whereas for sodium decyl and dodecyl sulphates the existence of the adsorption barrier was confirmed. We corroborated experimentally the absence of any influence of surfactant admixtures on the values of dynamic surface tension at t → 0.

Drop and bubble shape analysis as tool for dilational rheology studies of interfacial layers.

Drop and bubble shape tensiometry is a modern and very effective tool for measuring dynamic and static interfacial tensions. An automatic instrument with an accurate computer controlled dosing system is discussed in detail. Due to an active control loop experiments under various conditions can be performed: constant drop/bubble volume, surface area, or height, trapezoidal, ramp type, step type and sinusoidal area changes. The theoretical basis of the method, the fitting procedure to the Gauss-Laplace equation and the key procedures for calibration of the instrument are analysed and described.

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.

Adsorption of hydroxypropyl methylcellulose at the liquid/liquid interface and the effect on emulsion stability

Abstract This paper investigates the physico-chemical properties of the aqueous solutions of three different types of hydroxypropyl methylcellulose (HPMC 2208, HPMC 2906 and HPMC 2910) and those of HPMC stabilized oil-in-water emulsions containing medium-chain triglycerides (MCT) as dispersed phase. The mean molecular weight and molecular dimensions of HPMC in aqueous solutions were calculated from the intrinsic viscosity, which was obtained by fitting viscosity data to the Huggins equation. The dynamic adsorption behavior of HPMC at the water/MCT interface has been studied using the axisymmetric drop shape analysis. Measurements of the dynamic interfacial tension give information on the structure of the adsorption layer. The equilibrium interfacial tension at the critical aggregation concentration depends on the HPMC substitution type, whereby HPMC 2910 shows the lowest values. The adsorption process can be described by a diffusion-controlled model. The emulsion viscosity versus phase volume relationship fits well to predicted values from the Krieger–Dougherty equation, indicating that HPMC-stabilized emulsions can be described with the hard-sphere model. The thickness of the adsorbed layer, calculated from the maximum phase volume fraction, decreases with increasing phase volume. The emulsion stability depends on HPMC substitution type. Emulsions stabilized by HPMC 2208 and HPMC 2906 as compared with HPMC 2910 show higher stability of the droplet size when stored in a temperature cycle test for 6 months.

Dynamics of mixed protein–surfactant layers adsorbed at the water/air and water/oil interface

Abstract Drop and bubble shape tensiometry experiments are performed at the water/air and water/hexane interfaces in order to get more information about the differences in the adsorption layer structure of mixed protein/surfactant systems. For mixtures of β-lactoglobulin and sodium dodecyl sulphate the adsorption at the water/air interface is essentially a competitive process between protein/surfactant complexes and free surfactant molecules, while the water/oil interface is essentially covered by the complexes.

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.

DYNAMIC SURFACE TENSION OF AQUEOUS ALKYL DIMETHYL PHOSPHINE OXIDE SOLUTIONS. EFFECT OF THE ALKYL CHAIN LENGTH

Abstract The dynamic surface tension of aqueous solutions of alkyl dimethyl phosphine oxides with different alkyl chain length is studied. The experimental results obtained for the lower homologues (C8–C12) were found to correspond rather satisfactorily with the values predicted by the classical diffusion model. For the higher homologues (C13–C15), however, the measured dynamic surface tensions are higher than the theoretically predicted ones. Fitting would lead to a theoretical diffusion coefficient for, say, C14DMPO, which was 3 to 4 times larger than the physical values. A diffusion-controlled adsorption kinetics theory is proposed which considers two states of CnDMPO at the surface, each with its characteristic different partial molar area value. This theory agrees with the experimental data with typical diffusion coefficients.

Effect of surfactant interfacial orientation/aggregation on adsorption dynamics.

The application of new thermodynamic adsorption isotherms allow to improve the description of surfactant adsorption kinetics based on a diffusional transport. While the consideration of interfacial reorientation corrects apparently too high diffusion coefficients, interfacial aggregation avoids too small diffusion coefficients or the assumption of adsorption barriers. The adsorption kinetics of alkyl dimethyl phosphine oxides is influenced by interfacial reorientation. While the lower homologues (C8-C12) follow the classical diffusion model, the higher homologues (C13-C15) yield diffusion coefficients several times larger than the physically reasonable values. Assuming two different adsorption states, the resulting diffusion coefficients agree with those expected from the geometric size of the molecules. The model also works well for oxyethylated non-ionics, such as C10EO8. As a second example, a good theoretical description is obtained for experiments of 1-decanol solutions when a mean surface aggregation number of n = 2.5 is assumed. The same n was obtained from the description of the equilibrium adsorption isotherm of 1-decanol. Assuming that the transition from one into the other state is controlled by a rate constant (change in orientation, formation or disintegration of two-dimensional aggregates) significant changes in the kinetics curves can result. The use of additional rate constants yields an improved fitting to experimental data.

Adsorption of alkyl dimethyl phosphine oxides at the solution/air interface.

Abstract The surface tension isotherms were measured for homologous series of alkyl dimethyl phosphine oxides using different experimental techniques: maximum bubble pressure technique, drop volume technique and ring tensiometry. The adsorption data obtained for the surfactants with hydrocarbon chains from 8 to 16 alkyl groups are interpreted by a reorientation isotherm assuming two possible partial molar surface areas of surfactant molecules, i.e. states with maximum ( ω 1 ) and minimum ( ω 2 ) surface area. The lower homologues ( n ⩽13) of C n DMPO do not adsorb in the state with the large molar area, while the molecules with long alkyl chains ( n ⩾14) adsorb in both states. The area values determined from the experimental isotherms agree well with those calculated from the molecular geometry.