V. B. Fainerman

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.

Adsorption of surfactants and proteins at fluid interfaces.

Abstract This review presents some new equations of state and adsorption isotherms which describe mixed monolayers of surfactants possessing different molar area values, monolayers which comprise surfactants and proteins capable of reorientation or reconformation. For systems allowing reorientation and aggregation, the effect of self-regulation of the surface layer composition caused by surface pressure is considered. This effect is especially pronounced in protein adsorption layers, where not only the composition, but also the thickness of the adsorption layer depends on surface pressure. This principle was first proposed by Paul Joos for describing surface layers. The results of the proposed models have a direct impact on dynamic surface phenomena. The rate of adsorption for a diffusion-controlled mechanism depends on molecular reorientation or aggregation processes within the surface layer. For protein surface layers mainly conformational changes at the surface determine the rate of adsorption/desorption and other dynamic and mechanical properties of surface layers.

Description of the adsorption behaviour of proteins at water/fluid interfaces in the framework of a two-dimensional solution model

In the framework of the two-dimensional non-ideal solution model, surface layer equations of state, adsorption isotherms and functions of the distribution of protein molecules in respect to different molar area were derived. The thermodynamic analysis was based on Butlers equation for the chemical potentials of the components and a first-order model for the non-ideality of surface layer enthalpy and entropy. For concentrated solutions, aggregation of protein molecules in the surface layer was assumed. The resulting equations satisfactorily describe the measured adsorption and surface pressure isotherms of proteins at liquid/fluid interfaces in terms of a set of constant parameters. The model reflects the well-known differences between proteins and ordinary surfactants: a sharp increase in the surface pressure with concentration beyond a certain protein adsorption; an almost constant surface pressure at higher concentrations and a significant increase in the adsorption layer thickness with increasing adsorption for flexible proteins.

Simple model for prediction of surface tension of mixed surfactant solutions.

A rigorous theoretical model is presented which describes the equilibrium behaviour of a surfactant mixtures at liquid/fluid interfaces. The theory describes mixtures of surfactants with different molar areas and accounts for the non-ideality of the surface layer. The theoretical results are in good agreement with experimental data and support the idea of additivity of the interaction parameters in the surface layer. The rigorous equation of state is transformed into simple relationships for the description of the adsorption behaviour of mixed surfactant systems. The model requires surface tensions of the single surfactant systems or the adsorption isotherms to construct the isotherm of the mixture while no extra interaction parameters between the different compounds are assumed. The model is tested with a number of literature data, such as mixed sodium alkyl sulfates, mixtures of betaine homologues BHB12 with BHB16, non-ionic surfactant mixtures, and anionic-nonionic mixtures (1-butanol with BHB12, and oxethylated decanol (C10EO5) with sodium dodecyl sulfate). The agreement between experimental data and the theoretical calculations is excellent. This approach can be especially important for practical applications of surfactant mixtures for which experimental data are scarce.

Interfacial Properties of Mixed β-Lactoglobulin-SDS Layers at the Water/Air and Water/Oil Interface

The adsorption behavior of the beta-lactoglobuline has been studied in the presence of the anionic surfactant sodium dodecylsulfate (SDS) and compared for two different interfaces, water/air and water/hexane. The fitting of experimental data (adsorption isotherms) by a mixed adsorption model and the determination of structural parameters such as the molecular area occupied by the protein-surfactant complex and the surfactant molecules at the interface allowed to have a better understanding of the composition and as a consequence the behavior of the mixed interfacial layer. The parameters obtained for the mixtures are similar to those obtained separately for the single components, but the comparison of the both interfaces has shown significant differences. Much higher concentration of complex is found at the water/hexane interface, which is the result of a better affinity of the protein for this interface. A higher penetration of the protein into the oil phase and the presence of interactions between protein-surfactant complexes and free surfactant molecules stabilize the interface preventing its replacement by the SDS molecules. Rheological experiments show a decrease of the visco-elastic modulus at both interfaces with increasing SDS concentration. But at the water/oil interface, contrary to the water/air interface at which the replacement of the protein has been clearly observed, this decrease is attributed to changes of complex properties. At high SDS concentrations, an increase of the hydrophilic character due to hydrophobic interactions with the surfactant molecules leads to an increase in the mobility of the complex, which favors its desorption upon increased competition by the surfactant.

Kinetics of adsorption of globular proteins at liquid/fluid interfaces

Abstract A model for the adsorption kinetics of globular proteins at a liquid/fluid interface is described which takes into consideration the transport of molecules in the bulk by diffusion and a process accounting for the transition between the adsorption states of the protein molecules. The model is based on a recently published thermodynamic model for the adsorption isotherm of proteins at a liquid/fluid interface, which considers various adsorption states with different molar interfacial area. The model calculations show the particular features common to protein adsorption kinetics: induction period for low bulk concentrations, steep interfacial tension changes, significant effect of the transition kinetics on the overall adsorption process. A comparison with dynamic surface tension data for β-lactoglobulin at the water/air interface shows good agreement. In full agreement with the generally excepted physical picture, it turns out that the adsorption kinetics is significantly influenced by the transfer kinetics between the different adsorption states of the protein molecule. The estimation of the transfer rate constant yields values of the order of k =10 −3 s −1 .

Equilibrium of Adsorption of Mixed Milk Protein/Surfactant Solutions at the Water/Air Interface

Ellipsometry and surface profile analysis tensiometry were used to study and compare the adsorption behavior of beta-lactoglobulin (BLG)/C10DMPO, beta-casein (BCS)/C10DMPO and BCS/C12DMPO mixtures at the air/solution interface. The adsorption from protein/surfactant mixed solutions is of competitive nature. The obtained adsorption isotherms suggest a gradual replacement of the protein molecules at the interface with increasing surfactant concentration for all studied mixed systems. The thickness, refractive index, and the adsorbed amount of the respective adsorption layers, determined by ellipsometry, decrease monotonically and reach values close to those for a surface covered only by surfactant molecules, indicating the absence of proteins from a certain surfactant concentration on. These results correlate with the surface tension data. A continuous increase of adsorption layer thickness was observed up to this concentration, caused by the desorption of segments of the protein and transforming the thin surface layer into a rather diffuse and thick one. Replacement and structural changes of the protein molecules are discussed in terms of protein structure and surface activity of surfactant molecules. Theoretical models derived recently were used for the quantitative description of the equilibrium state of the mixed surface layers.

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 tensions of surfactant mixtures at the water-air interface

Abstract Studies of dynamic surface tensions γ(t) using an automated maximum bubble pressure method in the time interval from 1 ms to 30 s are performed with mixtures of non-ionic surfactants (Triton X-45, X-100, X-114, X-165, X-305 and X-405) and sodium alkyl sulphates (decyl, dodecyl, tetradecyl and hexadecyl). In the range of longer times t the dependencies γ( 1 √t ) of the single components show linear behaviour, giving evidence for a diffusion controlled adsorption in the absence of disturbing amounts of surface active impurities. Similar results are obtained on the addition of a second component with a sufficiently high surface activity. The adsorbed amount calculated from the ( γ( 1 √t ) ) dependences using a modified Hansen-Joos approximate relation agree well with those obtained from equilibrium surface tension data. The experimental γ( 1 √t ) curves of some surfactant mixtures show small jumps in the range over which displacement of the lower component by the stronger surface active molecules is introduced. These surface tension jumps are used to calculate the rate constants of desorption of sodium decyl and dodecyl sulphate. The obtained value of kad = 50 s−1 is in good agreement with earlier published data for these surfactants.