Advances in Colloid and Interface Science (v.86, #1-2)

Foreword (ix-x).

Human biological liquids comprise various surfactants, which adsorb at liquid interfaces and lead to a variation in surface tension. The adsorption processes involving low molecular weight surfactants, proteins and phospholipids play a vital role in the physiological functions of the human organism, especially if large surfaces are involved (e.g., gas exchange in lungs, metabolism of kidneys, liver and brain). Dynamic surface tensiometric studies of biological liquids like serum and cerebrospinal fluid provide surrogate parameters that reflect surface tension phenomena. We provide dynamic surface tension data of serum and cerebrospinal fluid that were collected from healthy volunteers and patients with rheumatic, neurological or oncological diseases. Our studies indicate that dynamic surface tension data are helpful for diagnostic purposes and for monitoring of therapeutic interventions.
Keywords: Dynamic surface tension; Maximum bubble pressure tensiometry; Biological liquids; Diagnostic measures in medicine; Serum; Cerebrospinal liquid; Human study;

Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface by R. Miller; V.B. Fainerman; A.V. Makievski; J. Krägel; D.O. Grigoriev; V.N. Kazakov; O.V. Sinyachenko (39-82).
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.
Keywords: Mixed adsorption layers; Liquid/fluid interfaces; Adsorption kinetics; Penetration kinetics; Protein/surfactant mixtures; Protein/lipid mixtures; Dynamic interfacial tensions; Surface viscoelasticity; Brewster angle microscopy; Maximum bubble pressure tensiometry; ADSA;

Effect of surfactant interfacial orientation/aggregation on adsorption dynamics by V.B Fainerman; R Miller; E.V Aksenenko; A.V Makievski; J Krägel; G Loglio; L Liggieri (83-101).
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.
Keywords: Surfactant adsorption layers; Interfacial orientation; Interfacial aggregation; Thermodynamic models; Diffusional transport;

The review demonstrates the recent theoretical and experimental progress in the understanding of penetration systems at the air–water interface in shich a dissolved amphiphile (surfactant, protein) penetrates into a Langmuir monolayer. The critical review of the existing theoretical models which describe the thermodynamics of the penetration are critically reviewed. Although a rigorous thermodynamic analysis of penetration systems is unavailable due to their complexity, some model assumptions, e.g. the invariability of the activity coefficient of the insoluble component of the monolayer during the penetration of the soluble component result in reasonable solutions. New theoretical models describing the equilibrium behaviour of the insoluble monolayers which undergo the 2D aggregation in the monolayer, and the equations of state and adsorption isotherms which assume the existence of multiple states (conformations) of a protein molecule within the monolayer and the non-ideality of the adsorbed monolayers are now available. The theories which describe the penetration of a soluble surfactant into the main phases of Langmuir monolayers were presented first for the case of the mixture of the moleucles possessing equal partial molar surfaces (the mixture of homologues), with further extension of the models to include the interesting process of the protein penetration into the monolayer of 2D aggregating phospholipid. This extension was based on a concept which subdivides the protein molecules into independent fragments with areas equal to those of phospholipid molecule. Various mechanisms for the effect of the soluble surfactant on the aggregation of the insoluble component were considered in the theoretical models: (i) no effect on the aggregate formation process; (ii) formation of mixed aggregates; and (iii) the influence on the aggregating process via the change of aggregation constant, but without any formation of mixed aggregates. Accordingly depending on the mechanism, different forms of the equations of state of the monolayer and of the adsorption isotherms of soluble surfactant are predicted. Based on the shape of the experimental Π-A isotherms, interesting conclusions can be drawn on the real mechanism. First experimental evidence has been provided that the penetration of different proteins and surfactants into a DPPC monolayer in a fluid-like state induces a first order main phase transition of pure DPPC. The phase transition is indicated by a break point in the Π(t) penetration kinetics curves and the domain formation by BAM. Mixed aggregates of protein with phospholipid are not formed. These results agree satisfactorily with the predictions of the theoretical models. New information on phase transition and phase properties of Langmuir monolayers penetrated by soluble amphiphiles are obtained by coupling of the Π(t) penetration kinetics curves with BAM and GIXD measurements. The GIXD results on the penetration of β-lactoglobulin into DPPC monolayers have shown that protein penetration occurs without any specific interactions with the DPPC molecules and the condensed phase consists only of DPPC.
Keywords: Penetration; Langmuir monolayers; Adsorption; Amphiphiles; Theoretical models; Brewster angle microscopy;