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Two different approaches have been used in the literature to describe the effects of ionisation of surfactants on the surface pressure, pi. One approach is based on a molecular model for a charged monolayer, in which the mutual repulsion of the long-chain surfactant ions results in an additional surface pressure, piel, calculated with the Gouy-Chapman theory for the formation of a diffuse electric double layer, and with counterion binding in the Stern-Helmholtz layer adjacent to the surfactant monolayer. The other approach regards the surface as a two-dimensional solution defined as a Gibbs dividing surface, which is electroneutral by definition. In this approach, the adsorption of any ion is the sum of its excesses in the monolayer and the electrical double layer; no assumptions are made about the spatial distribution of charges. It has been shown that both models can produce a reasonable description of experimental results obtained for solutions of a single ionic surfactant (RX) with or without inorganic electrolyte (XY). In many cases, measurements of pi vs. mean ionic activity at different salt concentrations (cXY) are found to coincide on a single curve, implying that at given mean ionic activity both adsorption and pi are independent of cXY, i.e. that double-layer contributions to the surface pressure are negligible. In addition, the electroneutral 2-D solution approach has resulted in a simple explanation of several typical features of mixed ionic surfactant solutions, in particular for mixtures of anionic and cationic surfactants. In mixed solutions too, double-layer effects appear to be negligible. We present arguments for such negligibility. One reason is a significant degree of binding between adsorbed surface active ions (R) and counterions (X); another is that for 1:1 electrolytes, the contribution of the diffuse double layer to the adsorption of the combination (RX) vanishes. As a result, it is possible to interpret the same experimental data in terms of both models.

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.

The surface behaviour of spread dipalmitoyl phosphatidyl choline (DPPC), lung surfactant protein C (SP-C), and their mixtures were characterised using a captive bubble surfactometer. The surface tension was determined by using axisymmetric bubble shape analysis. Surface dilatational rheological behaviour was characterised by sinusoidal oscillation of the bubble volume and at frequencies 0.006-0.025 Hz. The pi/A isotherms of DPPC, SP-C, and their mixtures were described with a generalised equation of state. Monolayer cycling of mixed DPPC/SP-C layers yields isotherms with a plateau in the range of 50-53 mN/m. When the surface pressure becomes higher SP-C is squeezed out of the film, but it re-enters the film upon expansion. Surface dilatational elasticities of DPPC films had a maximum at about 30 mN/m. At higher surface pressures, the films became brittle and the elasticity decreased. A slightly pronounced maximum was found at a surface pressure exceeding 55 mN/m. The dilatational viscosity had two distinct maxima, corresponding with those in the elasticity curves, i.e. one before the minimum area demand, and one in the range of over-compression. This was explained by the formation of a second ordered complex structure in the range of film over-compression. SP-C films show continuously increasing dilatational elasticities and viscosities with a maximum at f approximately 0.02 Hz. Mixed monolayers, DPPC+2 mol% SP-C, had dilatational elasticities increasing with surface pressure. In contrast to DPPC alone, an elasticity maximum appeared in the range of the squeeze out plateau. The dilatational viscosity had two distinct maxima as observed for DPPC, whereas the maximum before the squeeze out plateau is very broad like that of SP-C. The viscosity decreased for frequencies higher 0.02 Hz favouring elastic properties of the film. Our data provide experimental evidence that SP-C mixed with DPPC yield higher elasticities and viscosities as compared with films formed by the single components. This behaviour is likely to support breathing cycles, especially for the turn from inspiration to expiration and vice versa.

A theoretical model is discussed which describes the relaxation of an adsorption layer under a harmonic surface area perturbation (oscillating bubble method). The model assumes sufficiently high oscillation frequencies that a diffusional exchange of matter can be neglected. As relaxation process the change in the molar area of an adsorbed molecule is considered, caused for example by changing the orientation at the interface. The model calculations show that the use of a reorientation isotherm first leads to a different dilational elasticity isotherm as compared to that obtained for a Langmuir adsorption model. Moreover, it is shown that the surface relaxation due to molecular orientation changes can influence the viscoelasticity significantly. Qualitative comparisons with literature data show good agreement. Copyright 2001 Academic Press.

Adsorption data in the form of surface tension vs bulk concentration isotherms having strong changes in the slope have often been discussed in the literature in terms of phase transitions in the adsorption layer. It will be shown thermodynamically that these apparent kink points in the isotherms are unreal. A phase transition would cause a completely opposite change in the isotherm. Experimental surface tension data for sodium octanoate and dodecyl ammonium chloride obtained from the literature are reinterpreted on the basis of the Frumkin and reorientation isotherms. Copyright 2000 Academic Press.

The dynamic behavior of a bubble or drop oscillating at the tip of a capillary immersed in a surfactant solution is considered. The pressure variation in the cell and the nonstationary flow in the capillary are taken into account. The amplitude- and phase-frequency characteristics of the system are obtained, which contain information about the relaxation processes at the interface and in the bulk phases. Their dependency on the system geometry, the bulk properties of contacting media, and the viscoelastic properties of the interface is analyzed. Copyright 2000 Academic Press.

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 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.

The review demonstrates the recent theoretical and experimental progress in the understanding of penetration systems at the air-water interface in which 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 results 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 molecules 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 the 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 pi-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 pi(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 pi(t) penetration kinetics curves with BAM and GIXD measurements. The GIXD results on the penetration of beta-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.

A theoretical analysis is given to describe the behavior of an oscillating bubble in a closed measuring cell taking into account the finite liquid compressibility and the cell deformation. The results show that the behavior of a closed liquid cell can differ significantly from that of open cells. For example, under the same conditions two stable meniscus positions can be obtained in a closed cell. In a closed cell a meniscus larger than a hemisphere can be stable even when the gas compartment is open, while in an open cell such a meniscus is always unstable. For closed cells the meniscus can jump between the two equilibrium positions either randomly or under the influence of regular factors, such as external pressure, temperature, and surface tension changes. Relationships are obtained for the description of the pressure response on external harmonic perturbations which make it possible to determine the complex dilatational elasticity as a function of frequency. It is shown that the measured signal can depend on the cell properties, which should be taken into account in the interpretation of experimental data. Copyright 2000 Academic Press.

A convolution-type equation has been derived to describe the behavior of a bubble under periodical pressure oscillations. This equation holds for a diffusion-controlled adsorption mechanism and small disturbances of the equilibrium state, and it describes both the established and transition regimes of bubble oscillation. Systems free of any surfactant and in the presence of a surfactant are considered. The results obtained allow all aspects of surfactant influence on the bubble oscillation resonance to be analyzed. The sharp increase in the bubble oscillation amplitude may result in bubble detachments, even at rather low harmonic pressure oscillations. The presence of surfactant can result in a depression of the resonance amplitudes. Copyright 2000 Academic Press.

The theoretical description of maximum bubble pressure experiments needs to consider inertial properties of the gas and liquid. On the basis of an analysis of the time-dependent pressure gradient inside the capillary, bubble lifetime and bubble deadtime are estimated. Numerical calculations of the derived model equations yield different importent dependencies which allow us to better understand bubble pressure experiments at high bubble frequencies: gas flow velocity as function of time, bubble radius as function of time, pressure drop within the capillary as function of time, and bubble lifetime as function of the initial gas velocity. Copyright 1998 Academic Press. Copyright 1998Academic Press

Aim: Assessment of the dynamic surface tension (DST) of blood serum (BS), synovial fluid (SF) in various courses of rheumatoid arthritis (RA)., Materials and Methods: Forty three patients with RA and 63 apparently healthy individuals were examined. DST of BS and SF was determined in the computer-aided tensiometer and some blood biochemical parameters were also measured., Results: DST of BS in patients with RA were found to be higher than the normal values and some parameters c beta 2 and beta 3) of BS DST did not significantly differ from the normal values whereas others (beta 1 and gamma proved to be much lower. Depending on the disease course variants, there were some differences in these DST changes. There was a feedback between DST parameters and the levels of immunoglobulins, beta 2-microglobulin, and lipids in blood., Conclusion: Assessment of BS and SF DST may be useful in the differential diagnosis of RA, in the determination of its intensity and prognosis, and therapeutical efficiency.

The dynamic drop volume technique has been used to measure the dynamic interfacial tensions of nonionic surfactants (Triton X-45, X-100, X-165, and X-405) at the water/nonane and water/butylbenzene interface. The measurements were performed under the conditions of equilibrium distribution of the surfactant between the two liquid phases established after a sufficiently high number of aqueous solution drops have been formed within a small volume of the oil phase under study. The diffusion kinetics model for the description of the adsorption at the surface of growing drops involves equilibrium surfactant distribution and adsorption from both adjacent phases. Two orientational states of adsorbed surfactant molecule at the interface were assumed in agreement with recently published results at the water/air interface. Good agreement between this theory and the experimental data was obtained.