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The spread layers of lysozyme (LYS) microgel particles were studied by surface dilational rheology, infrared reflection–absorption spectra, Brewster angle microscopy, atomic force microscopy, and scanning electron microscopy. It is shown that the properties of LYS microgel layers differ significantly from those of ß-lactoglobulin (BLG) microgel layers. In the latter case, the spread protein layer is mainly a monolayer, and the interactions between particles lead to the increase in the dynamic surface elasticity by up to 140 mN/m. In contrast, the dynamic elasticity of the LYS microgel layer does not exceed the values for pure protein layers. The compression isotherms also do not exhibit specific features of the layer collapse that are characteristic for the layers of BLG aggregates. LYS aggregates form trough three-dimensional clusters directly during the spreading process, and protein spherulites do not spread further along the interface. As a result, the liquid surface contains large, almost empty regions and some patches of high local concentration of the microgel particles.

The adsorption behaviour of linear poly(oxyethylene) alkyl ether (CnEOm) is best described by a reorientation model. Based on a complete set of experimental data, including the adsorption kinetics, the equilibrium surface tension isotherm and the surface dilational visco-elasticity, the thermodynamic and kinetic adsorption parameters for some CnEOm at the water/air interface were determined. For the study, six CnEOm surfactants were selected (n = 10, 12 and 14 and m = 4, 5 and 8) and were studied by bubble profile analysis and maximum bubble pressure tensiometry. A refined theoretical model based on a reorientation-adsorption model combined with a diffusion-controlled adsorption kinetics and exchange of matter allowed us to calculate the surface layer composition by adsorbing molecules in different orientations. It turns out that at larger surface coverage, the adsorption rate decreases, i.e., the apparent diffusion coefficients are smaller. This deceleration can be explained by the transition of molecules adsorbed in a state of larger molar surface area into a state with smaller molar surface area.

The pulmonary surfactant (PS) is a complex mixture of lipids and proteins dispersed in the aqueous lining layer of the alveolar surface. Such a layer plays a key role in maintaining the proper lung functionality. It acts as a barrier against inhaled particles and pathogens, including viruses, and may represent an important entry point for drugs delivered via aerosols. Understanding the physicochemical properties of PS is therefore of importance for the comprehension of pathophysiological mechanisms affecting the respiratory system. That can be of particular relevance for supporting the development of novel therapeutic interventions against COVID-19–induced acute respiratory distress syndrome. Owing to the complexity of the in vivo alveolar lining layer, several in vitro methodologies have been developed to investigate the functional and structural properties of PS films or interfacial films made by major constituents of the natural PS. As breathing is a highly dynamic interfacial process, most applied methodologies for studying PSs need to be capable of dynamic measurements, including the study of interfacial dilational rheology. We provide here a review of the most frequently and successfully applied methodologies that have proven to be excellent tools for understanding the biophysics of the PS and of its role in the respiratory mechanics. This overview also discusses recent findings on the dynamics of PS layers and the impact of inhalable particles or pathogens, such as the novel coronavirus, on its functionality.

Application of dilational surface rheology, surface tensiometry, ellipsometry, Brewster angle, and transmission electron and atomic force microscopies allowed the estimation of the structure of the adsorption layer of a fullerenol with a large number of hydroxyl groups, C-60(OH)(x) (X = 30 +/- 2). The surface properties of fullerenol solutions proved to be similar to the properties of dispersions of solid nanoparticles and differ from those of the solutions of conventional surfactants and amphiphilic macromolecules. Although the surface activity of fullerenol is not high, it forms adsorption layers of high surface elasticity up to 170 mN/m. The layer consists of small interconnected surface aggregates with the thickness corresponding to two-three layers of fullerenol molecules. The aggregates are not adsorbed from the bulk phase but formed at the interface. The adsorption kinetics is controlled by an electrostatic adsorption barrier at the interface.

Interfacial rheology of adsorbed layers of surfactants, demonstrating the response of the interface to interfacial deformations, plays a key role in formation and stability of foams and emulsions. It also provides insights into complex surfactant systems in different applications; in particular, medical treatments and diagnostics. The response of the interface is mainly determined by the composition of a surfactant system, the equilibrium and kinetic adsorption properties of the included surface-active compounds, and their interaction within the adsorption layer. The subject of on-going investigations is interfacial rheology of surfactant layers in the presence of inorganic ions. Although these ions have no surface activity, they can strongly influence the interfacial rheological properties due to their interaction with the surface-active molecules. This work aims to present recent developments in the interfacial rheology of surfactant adsorbed layers at liquid-fluid interfaces in the presence and absence of salts, highlighting the state of the art of experimental and theoretical works in this area. We highlight drawbacks of recently developed techniques for measuring dilational interfacial properties of surfactant layers, compared with previous techniques. Moreover, this review shows the dearth of research in the ion-specific effect on the interfacial rheology of surfactant layers. This demonstrates the necessity of further investigation of the effect of ion-specificity on interfacial viscoelasticity.

The review discusses recent results on the adsorption of amyloid fibrils and protein microgels at liquid/fluid interfaces. The application of the shear and dilational surface rheology, atomic force microscopy and passive particle probe tracking allowed for elucidating characteristic features of the protein aggregate adsorption while some proposed hypothesis still must be examined by special methods for structural characterization. Although the distinctions of the shear surface properties of dispersions of protein aggregates from the properties of native protein solutions are higher than the corresponding distinctions of the dilational surface properties, the latter ones give a possibility to obtain new information on the formation of fibril aggregates at the water/air interface. Only the adsorption of BLG microgels and fibrils was studied in some details. The kinetic dependencies of the dynamic surface tension and dilational surface elasticity for aqueous dispersions of protein globules, protein microgels and purified fibrils are similar if the system does not contain flexible macromolecules or flexible protein fragments. In the opposite case the kinetic dependencies of the dynamic surface elasticity can be non-monotonic. The solution pH influences strongly the dynamic surface properties of the dispersions of protein aggregates indicating that the adsorption kinetics is controlled by an electrostatic adsorption barrier if the pH deviates from the isoelectric point. A special section of the review considers the possibility to apply kinetic models of nanoparticle adsorption to the adsorption of protein aggregates.

The addition of denaturants strongly influences the surface properties of aqueous myoglobin solutions. The effect differs from the results for mixed solutions of the denaturants and other globular proteins, for example, bovine serum albumin (BSA), lysozyme and β-lactoglobulin (BLG), although the surface properties of the solutions of the pure proteins are similar. The kinetic dependencies of the dynamic surface elasticity of myoglobin solutions with guanidine hydrochloride (GuHCl) reveal at least two adsorption steps at denaturant concentrations higher than 1 M: a very fast increase of the dynamic surface elasticity to approximately 30 mN/m at the beginning of adsorption, and a slower growth to abnormally high values of 250-300 mN/m. At the same time, the surface elasticity of BSA/GuHCl, BLG/GuHCl and lysozyme/GuHCl solutions is a non-monotonic function of the surface age, and does not exceed 50 mN/m close to equilibrium. The high surface elasticity of myoglobin/GuHCl solutions may be associated with protein aggregation in the surface layer. The formation of aggregates is confirmed by ellipsometry and Brewster angle microscopy. The addition of ionic surfactants to protein solutions leads to the formation of myoglobin/surfactant complexes, and the kinetic dependencies of the dynamic surface elasticity display local maxima indicating multistep adsorption kinetics, unlike the corresponding results for solutions of other globular proteins mixed with ionic surfactants. Ellipsometry and infrared reflection-absorption spectroscopy allow tracing the adsorption of the complexes and their displacement from the interface at high surfactant concentrations.

The surface properties of mixed aqueous dispersions of lysozyme and silica nanoparticles were studied using surface-sensitive techniques in order to gain insight into the mechanism of the simultaneous adsorption of protein/nanoparticle complexes and free protein as well as the resulting layer morphologies. The properties were first monitored in situ during adsorption at the air/water interface using dilatational surface rheology, ellipsometry, and Brewster angle microscopy. Two main steps in the evolution of the surface properties were identified. First, the adsorption of complexes did not lead to significant deviations in the dynamic surface elasticity and dynamic surface pressure from those for a layer of adsorbed lysozyme globules. Second, through the gradual displacement of protein globules from the interfacial layer as a result of further complex adsorption, the layer became more dense with much higher dynamic surface elasticity (similar to 280 mN/m compared to similar to 80 mN/m for a pure protein layer). These layers were shown to be fragile and could be easily broken into separate islands of irregular shape by a weak mechanical disturbance. The layer properties were then monitored following their transfer to solid substrates using atomic force microscopy and scanning electron microscopy. These layers were shown to consist of nanoparticles surrounded by a rough shell of protein globules, whereas some particles tended to form filamentous aggregates. This comprehensive study provides new mechanistic and morphological insight into the surface properties of a model protein/nanoparticle system, which is of fundamental interest in colloidal science and can be extended to systems of physiological relevance.

In this communication, the single element version of the fractional Maxwell model (single-FMM or Scottndash;Blair model) is adopted to quantify the observed behavior of the linear interfacial dilational viscoelasticity. This mathematical tool is applied to the results obtained by capillary pressure experiments under low-gravity conditions aboard the International Space Station, for adsorption layers at the hydrocarbon/water interface. Two specific experimental sets of steady-state harmonic oscillations of interfacial area are reported, respectively: a drop of pure water into a Span-80 surfactant/paraffin-oil matrix and a pure n-hexane drop into a C13DMPO/TTAB mixed surfactants/aqueous-solution matrix. The fractional constitutive single-FMM is demonstrated to embrace the standard Maxwell model (MM) and the Lucassenndash;van-den-Tempel model (Lndash;vdT), as particular cases. The single-FMM adequately fits the Span-80/paraffin-oil observed results, correctly predicting the frequency dependence of the complex viscoelastic modulus and the inherent phase-shift angle. In contrast, the single-FMM appears as a scarcely adequate tool to fit the observed behavior of the mixed-adsorption surfactants for the C13DMPO/TTAB/aqueous solution matrix (despite the single-FMM satisfactorily comparing to the phenomenology of the sole complex viscoelastic modulus). Further speculations are envisaged in order to devise combined FMM as rational guidance to interpret the properties and the interfacial structure of complex mixed surfactant adsorption systems.

A study of the dilational surface viscoelastic properties of mixed solutions of lysozyme and denaturing agents with different chemical natures allows us to characterize the changes of protein tertiary structure in the surface layer upon adsorption at the liquid–gas interface. We show that guanidine hydrochloride (GuHCl) and urea influence the dynamic surface properties of lysozyme solutions less than the properties of previously studied solutions of bovine serum albumin and β-lactoglobulin. Although the addition of sodium polystyrene sulfonate (PSS) changes the kinetic dependencies of the surface properties and leads to the formation of large aggregates in the bulk phase, the dependencies of the dynamic surface elasticity on the surface pressure almost coincide with the results for pure lysozyme solutions thereby indicating the preservation of the adsorption layer structure. At the same time, the simultaneous addition of PSS and GuHCl to lysozyme solutions results in a strong synergistic effect: the kinetic dependency of the dynamic surface elasticity becomes non-monotonical and similar to that for solutions of amphiphilic polymers and non-globular proteins. The local maximum of this dependency indicates the destruction of the protein globular structure and formation of the distal region of the surface layer. The simultaneous addition of PSS and urea to lysozyme solutions does not lead to a similar effect. These results confirm different mechanisms of protein denaturation under the influence of urea and GuHCl, and the important role of electrostatic interactions in the latter case.

The dynamic surface elasticity of mixed solutions of bovine serum albumin (BSA) and poly(diallyldimethylammonium chloride) (PDADMAC) was measured as a function of the surface age, polyelectrolyte concentration and solution pH. The addition of polyelectrolyte does not influence the dynamic surface properties of protein solution at pH below the isoelectric point of BSA. At the same time, one can observe significant changes of the kinetic dependencies of the surface pressure and surface elasticity under the influence of polyelectrolyte at higher pH indicating the interaction between the components. The acceleration of the changes of the surface properties can be connected with the decrease of the electric adsorption barrier due to the decrease of the total charge of the protein/polyelectrolyte complex. The local maximum of the kinetic dependencies of the dynamic surface elasticity at pH higher than the isoelectric point indicates partial destruction of the protein tertiary structure at high surface pressures.

The surface properties of the solutions of two derivatives of C60 fullerene with lysine and arginine were investigated by measurements of surface tension, dilational surface rheological properties, characteristics of capillary waves, and by Brewster angle microscopy. It is shown that the adsorption of the derivatives is irreversible and they can form a dynamic network at the interface with almost zero surface pressure and high dynamic surface elasticity, up to ∼100 mN/m. The adsorption layers of C60-Lys and C60-Arg are characterized by strongly different surface rheological properties and distinct morphologies after an external mechanical perturbation due to different numbers of amino acid residues per C60 fullerene.

The dilational rheology for solutions of surface active compounds and their mixtures can be measured with various methods of different frequency ranges. The most frequently used techniques are based on oscillating drops and bubbles. Besides these techniques, also capillary wave damping is a siutable method, in particular at higher frequencies. The analysis of the resulting data is mainly based on a diffusion model derived by Lucassen and van den Tempel many years ago. For mixtures of surfactants or proteins with surfactants only recently a functioning theoretical apparatus was represented and applied to different mixed adsorption layers. The present chapter summarizes the experimental tools and corresponding theories, and provides well selected examples of classical surfactants and mixtures of milk proteins mixed with a number of surfactants.

The dynamic surface elasticity of mixed solutions of β-casein and surfactants of different chemical nature was measured as a function of surface age and surfactant concentration with the aim to elucidate the influence of non-ionic interactions between the components on the dynamic surface properties. Small admixtures of the amphiphilic non-ionic polymer poly( N -isopropylacrylamide) (PNIPAM) did not influence the dynamic surface elasticity and the surface properties of the solution were entirely determined by the adsorbed protein. The corresponding kinetic dependencies of the dynamic surface elasticity have two local maxima. The increase of the polymer concentration resulted in the appearance of a third local maximum in the surface elasticity corresponding to the polymer adsorption. In this case the height and position of the second peak in the kinetic curve of the surface elasticity were changed noticeably indicating interactions between the components. The increase of the surfactant hydrophobic chain length in the case of small additions of cationic surfactants of low molecular weight increased the hydrophobicity of the formed protein/surfactant complex and also led to changes of the shape of the dynamic surface elasticity. The application of the scanning probe microscopy indicated different mechanisms of the β-casein displacement from the liquid surface by the surfactants of low and high molecular weights, correspondingly.

For the understanding of short-time adsorption phenomena and high-frequency relaxations at liquid interfaces particular experimental techniques are needed. The most suitable method for respective studies is the capillary pressure tensiometry. However, under gravity conditions there are rather strong limitations, in particular due to convections and interfacial deformations. This manuscript provides an overview of the state of the art of experimental tools developed for short-time and high-frequency investigations of liquid drops and bubbles under microgravity. Besides the brief description of instruments, the underlying theoretical basis will be presented and limits of the applied methods under ground and microgravity conditions will be discussed. The results on the role of surfactants under highly dynamic conditions will be demonstrated by some selected examples studied in two space shuttle missions on Discovery in 1998 and Columbia in 2003.

Measurements of the concentration and adsorption time dependencies of the dynamic surface elasticity of mixed polyethylenimine/sodium dodecyl sulfate (PEI/SDS) solutions show transitions between a few states of different surface elasticity. In the range of low surface pressures the elasticity is governed by the branched structure of the macromolecules, while at high surface pressures the elasticity exhibits a behaviour typical for other polyelectrolyte/surfactant surface layers.

For the characterisation of interfaces between two immiscible liquids, capillary pressure tensiometry is the most versatile method. To measure short time interfacial tensions it has to be applied under very dynamic conditions for which the following protocols have been developed: 1—continuously growing drop (CGD); 2—pre-aged growing drop (PGD); 3—stopped growing drop (SGD). The CGD is a procedure analogous to the well-known maximum bubble pressure tensiometry for liquid–gas interfaces, however, the processes of drop detachment and formation of the subsequent drop require a rather complex data analysis. For the PGD protocol based on a growing drop with an initially pre-established equilibrium adsorption layer, the complexities are less, but for higher surfactant concentrations it is not optimum. The SGD protocol provides a drop with a fresh surface and the dynamic interfacial tension can be monitored in the absence of hydrodynamic effects. All three protocols complement each other and provide a good set of data for dynamic interfacial tensions at short adsorption times.

The dynamic surface properties of mixed solutions of polyacrylic acid (PAA) and alkyltrimethylammonium bromides (C n TAB) were measured by the oscillating ring method as a function of surface age and surfactant concentration. The concentration dependencies of the surface tension and dilational dynamic surface elasticity were similar to the corresponding results for other polyelectrolyte/surfactant solutions, which have been investigated earlier. The kinetic dependencies of the surface elasticity proved to be non-monotonic for all the investigated surfactants at n = 8, 10, 12, 14, 16 and displayed two local maxima at surfactant concentrations close to the critical micellar concentration (CMC). The first maximum was caused by an aggregate formation in the surface layer while the second maximum occurred simultaneously with a multilayer formation at the gas–liquid interface. The surface tension corresponding to the beginning of the aggregate formation decreased and the surface elasticity in the region of the first local maximum increased with the hydrophobicity of C n TAB/PAA complexes caused by the increasing hydrocarbon chain length of the surfactant. The hydrophobic interactions influenced also the height and position of the second local elasticity maximum. These effects were connected with changes in the rate of adsorption layer and aggregate structure formation. Comparable values of the real and imaginary parts of the dynamic surface elasticity at concentrations for aggregate formation in the surface layer indicated that the relaxation time of the mass exchange between the aggregates and the surrounding adsorption layer is comparable with the period of oscillations, i.e. ≤10 s.

The dynamic surface dilational elasticity, surface pressure, and adsorbed amount of the mixed solutions of beta-lactoglobulin and guanidine hydrochloride were measured as a function of surface age and denaturant concentration. It was shown that the conformational transition from compact globules to disordered protein molecules in the surface layer leads to strong changes in the surface elasticity kinetic dependencies and thereby can be easily detected by measuring the surface dilational rheological properties. The corresponding changes of the kinetic dependencies of the surface pressure and adsorbed amount are not so pronounced but correlate with the results on surface dilational elasticity.

The dynamic dilational elasticity of adsorption layers of the complexes between polyacids and dodecyltrimethyl ammonium bromide were measured by the oscillating ring method as a function of surface age and surfactant concentration. The concentration dependencies of surface tension and dilational surface elasticity are similar to the corresponding dependencies of other polyelectrolyte/surfactant solutions. At the same time, the kinetic dependencies of the dynamic surface elasticity differ significantly from the results for other systems and exhibit in respective concentration ranges one or two local maxima. The first maximum and the associated distortion of the harmonical surface tension oscillations are caused by an aggregate formation in the surface layer. The second maximum occurs simultaneously with a multilayer formation at the liquid surface, which probably results in a new relaxation mechanism of surface stresses. The hydrophobic interactions influence the height and position of the second elasticit...

In this study the effect of particles of nanometric dimensions on the interfacial properties of liquid-liquid systems is addressed. The nanoparticle transfer and the consequent attachment to the fluid interface are mainly governed by their hydrophobic/lipophilic character. For this reason a model particle-surfactant system has been investigated, namely a nanometric colloidal silica dispersion plus CTAB, where the role of the cationic surfactant is varying the particle surface properties adsorbing on them. The interfacial properties of the dispersion against hexane were determined as a function of the surfactant concentration, measuring the interfacial tension and the dilational viscoelasticity versus frequency. These results were then crossed with a study on the behaviour of micrometric oil droplets inside the same dispersions, as well as on the morphology of the respective nanoparticle stabilised emulsions. By this multiple investigation it was possible to find out that diffusion transport of particles from the bulk to the interface plays an important role, as well as the reorganisation of the mixed parlicle-surfactant layer. Moreover from these results the irreversible aspect of the nanoparticle attachment to the fluid interface was also evidenced which, under particular conditions, can provide the formation of a solid-like layer. (c) 2007 Elsevier B.V. All rights reserved.

Dynamic interfacial properties of droplets, as well as mechanical and hydrodynamic processes, play an important role in the formation and stability of emulsions. A better understanding of formation and stability of W/O (and of O/W) macroemulsions requires the acquisition of a deeper knowledge a) of the adsorption dynamics and relaxation processes occurring at single-droplet interfaces and b) of the mechanism of droplet disproportionation and of droplet-droplet coalescence. Thermodynamic models show that molecular processes in adsorption layers, like reorientation or 2D-aggregation, can change the rate of adsorption and the dynamic surface elasticity and viscosity significantly. A change of interfacial (non-equilibrium) composition induces a transient perturbation of interfacial viscoelasticity that influences the contraction of droplets (issuing in Marangoni motion and in possible drop elongation and drop splitting). Also, drop viscoelasticity is a key parameter in the processes of droplet-droplet coalescence. As a significant stage in bringing ahead the scientific comprehension of emulsion behaviour, we conducted measurements of the interfacial dilational modulus and of the interfacial tension relaxation on oscillating millimeter-sized and micrometer-sized drops for water / hydrocarbon systems (as concerns interfacial drop properties, the scaling rule is linear within the macroscopic domain). We used novel experimental methodologies, so that we have access to accurate data on non-equilibrium properties of liquid-liquid interfaces. The poster reports experimental observations of drop viscoelasticity relevant to model hydrocarbon-water-surfactant systems, such as to simulate the composition of the real (complex) emulsions encountered in crude oil industry.

The dynamic surface elasticity of solutions of /?-casein/surfactant mixtures was measured as a function of surface age and surfactant concentration. Small additions of ionic surfactants changed strongly the kinetic dependence of the dynamic surface elasticity of protein solutions. The shape of the kinetic curve depended on the charge of the surfactant ion. The results can be explained by strong electrostatic interactions between charged amino acid residues in the protein and surfactant ions, and different distributions of positive and negative charges along the protein chain. It is shown that small additions of ionic surfactants can probe the mechanism of protein adsorption. The obtained results confirm the consecutive transitions of the relatively hydrophilic and hydrophobic parts of a /3-casein molecule into the subphase as loops and tails. Unlike the case of /3-casein/ionic surfactant solutions, the effect of small additions of a nonionic surfactant with high surface activity can be described as a gradual displacement of /?-casein from the adsorption layer.

The nonmonotonic kinetic dependencies of the dynamic elasticity of adsorbed and spread β-casein layers at the liquid−gas interface have been determined by the oscillating barrier method. While two local maxima in the surface elasticity versus concentration dependence are well documented in literature, these features have not been reported for kinetic curve. The surface elasticity in the time range of the second maximum depended on the β-casein bulk concentration and deviated from the elasticity of spread β-casein layers at the same surface pressures. In parallel to the surface viscoelasticity of PEO−PPO−PEO block copolymers, the experimental findings for β-casein can be explained by a separation of relatively hydrophobic groups of the polypeptide chain during the slow process of protein adsorption.

The dynamic surface elasticity of the aqueous solutions of the copolymer of sodium 2-acrylamide-2-methyl-1-propane sulfonate and N-isopropylacrylamide is measured by the oscillating barrier method as a function of surface lifetime and copolymer concentration. The results obtained are in qualitative agreement with the theory accounting for Coulombic interactions between segments, yet, at the same time, strongly differ from those obtained for sodium poly(styrene sulfonate), thereby confirming earlier conclusion on the effect of non-Coulombic interactions on the surface properties of this polymer solutions.

Oscillating bubble and drop methods were applied to aqueous solutions of poly(dimethyldiallylammonium chloride) and copolymers of sodium 2-acrylamido-2-methyl-1-propansulfonate with acryl amide and N -isopropylacrylamide at various concentrations and ionic strengths. The dynamic surface elasticity was measured as a function of the surface age and non-monotonic kinetic dependencies were observed for some systems. Although solutions of different polyelectrolytes displayed various types of the surface viscoelastic behavior, they all differed strongly from the surface dilational viscoelasticity of non-ionic polymer solutions. At the same time some distinctions with the surface dilational viscoelasticity of sodium poly(styrenesulfonate) solutions were discovered too. They are a consequence of the specific structure of the PSS adsorption films as it had been assumed earlier (B.A. Noskov, S.N. Nuzhnov, G. Loglio, R. Miller, Macromolecules 37 (2004) 2519).

We present experimental results on the dynamic surface properties of aqueous solutions of sodium poly(styrenesulfonate). The dynamic surface elasticity at frequencies from 0.01 up to 500 Hz and at concentrations exceeding 0.1 wt % proved to be significantly higher than the corresponding data for solutions of nonionic polymers. Another peculiarity consisted in extremely slow changes of the surface properties with time. The addition of salt did not lead to significant changes of the surface elasticity, which was in disagreement with the expectation from the SCF theory. The obtained results can be explained if one assumes aggregate formation in the surface layer.

The present analysis shows that the introduction of intrinsic two-dimensional compressibility of a surfactant adsorption layer allows a much better interpretation of experimental data on the limiting (high-frequency) elasticity and characteristic frequency of diffusional relaxation than models using Frumkin's or other adsorption isotherms. The proposed model describes experimental data on surface dilational elasticity, as well as on surface pressure, sufficiently well at low and high surfactant concentrations, assuming the Lucassen and van den Tempel model of surface dilational elasticity is realized.

The simplest singlet-doublet emulsions (SDE) exhibit singlet-doublet quasi-equilibrium (s.d.e.) and slow coalescence between doublets. Orthokinetic coagulation and creaming may be eliminated using low-density contrast emulsions. The investigation of the evolution in time of the total number of droplet in such emulsions at SDE was recommended as a standard method to determine the characteristic times of the elementary acts of coalescence (tau(c)) and doublet fragmentation (tau(d)) [J. Disp. Sci. Technol. 19 (1998) 311]. Further improvement of this method and its automation requires discrimination between singlets and doublets in the bulk, which is possible for droplets of approximately 5 mum in size. Simultaneously, the droplet dimension must not be too large as the fragmentation time increases rapidly with the droplet dimension, while it has to be very small in compare to the Smoluchowski time (tau(sm)) to preserve the condition for SDE. It is shown here that this controversy in the demand to the droplet size can be satisfied by a simultaneous decrease in electrolyte concentration, increase in surface potential and decrease in droplet volume fraction. The calculation of the fragmentation time in a wide range of electrolyte concentration, surface potential and particle radius shows that tau(d) may be very small in compare to tau(sm). (increases at small volume fractions) at such large mini-emulsion droplet dimension as it is necessary for a singlet and doublet discrimination. The second experimental difficulty is the discrimination between doublets in the primary (PD) and secondary minimum (SD) using video microscopy, while there is a huge difference in their kinetic behavior in respect to fragmentation and coalescence. The analysis of the domains for PD only, for SD only, and for a coexistence of SD and PD shows that the characterization of mini-emulsions has to be accomplished using experimental conditions corresponding to the domain of SD only. (C) 2002 Elsevier Science B.V. All rights reserved.

Kinetic dependences of the dynamic dilatational surface elasticity of solutions of the complexes of b-lactoglobulin and bovine serum albumin with dodecyltrimethylammonium bromide show that the destruction of the tertiary protein structure and the partial destruction of the secondary structure in the surface layer occur at lower surfactant concentrations than in the bulk solution.