Transition from spherical to irregular dispersed phase in water/oil emulsions.

Bulk properties of transparent and dilute water in paraffin oil emulsions stabilized with sodium dodecyl sulfate (SDS) are analyzed by optical scanning tomography. Each scanning shot of the considered emulsions has a precision of 1 µm. The influence of aluminum oxide nanoparticles in the structure of the water droplets is investigated. Depending on concentrations of SDS and nanoparticles, a transition occurs in their shape that changes from spherical to polymorphous. This transition is controlled by the SDS/alumina nanoparticles mixing ratio and is described using an identification procedure of the topology of the gray level contours extracted from each images. The transition occurs for a critical mixing ratio of Rcrit ≈ 0.05 which does not significantly depend on temperature and electrolyte concentration. This structural change seems to be a general feature when emulsifying dispersions and most probably involves both interfacial and bulk phenomena.

Characterization of micro- and nanocapsules for self-healing anti-corrosion coatings by high-resolution SEM with coupled transmission mode and EDX.

The observation of morphological details down to the nanometer range of the outer surface of micro-, submicro- and nanoparticles in a high-resolution scanning electron microscope (SEM) was extended with in-depth observation by enabling the transmission mode in the SEM, i.e. TSEM. The micro- and nanocapsules characterized in this study were fabricated as depots for protective agents to be embedded in innovative self-healing coatings. By combining the two imaging modes (upper and in-depth observation) complementing each other a better characterisation by a more comprehensive interpretation of the "consistency" of the challenging specimens, e.g. including details "hidden" beyond the surface or the real specimen shape at all, has been attained. Furthermore, the preparation of the quasi electron transparent samples onto thin supporting foils enables also elemental imaging by energy dispersive X-ray spectroscopy (EDX) with high spatial resolution. Valuable information on the elemental distribution in individual micro-, submicro- and even nanocapsules completes the "3D" high resolution morphological characterization at the same multimodal SEM/TSEM/EDX system.

Impact of globule unfolding on dilational viscoelasticity of beta-lactoglobulin adsorption layers.

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.

Competitive adsorption from mixed hen egg-white lysozyme/surfactant solutions at the air-water interface studied by tensiometry, ellipsometry, and surface dilational rheology.

The competitive adsorption at the air-water interface from mixed adsorption layers of hen egg-white lysozyme with a non-ionic surfactant (C10DMPO) was studied and compared to the mixture with an ionic surfactant (SDS) using bubble and drop shape analysis tensiometry, ellipsometry, and surface dilational rheology. The set of equilibrium and kinetic data of the mixed solutions is described by a thermodynamic model developed recently. The theoretical description of the mixed system is based on the model parameters for the individual components.

Biofriendly nanocomposite containers with inhibition properties for the protection of metallic surfaces.

An attempt to combine two 'green' compounds in nanocomposite microcontainers in order to increase protection properties of waterborne acryl-styrene copolymer (ASC) coatings has been made. N-lauroylsarcosine (NLS) served as a corrosion inhibitor, and linseed oil (LO) as a carrier-forming component. LO is compatible with this copolymer and can impart to the coating self-healing properties. For the evaluation of the protective performance, three types of coatings were compared. In the first two, NLS was introduced in the coating formulation in the forms of free powder and micro-containers filled with LO, correspondingly. The last one was a standard ASC coating without inhibitor at all. Low-carbon steel substrates were coated by these formulations by spraying and subjected subsequently to the neutral salt spray test according to DIN ISO 9227. Results of these tests as well as the data obtained by electrochemical study suggest that such containers can be used for the improvement of adhesion of ASC-based coatings to the substrate and for the enhancement of their protective performance upon integrity damage, whereas the barrier properties of intact coatings were decreased.

Composite interfacial layers containing micro-size and nano-size particles.

Surface layers of micro- and nanoparticles at fluid/liquid interfaces in absence and presence of surfactants are of large importance in the process of re-discovering Pickering systems, i.e. emulsions and foams stabilized by particles. The surface pressure/area isotherms of such layers can provide information about the properties of the used particles (dimensions, interfacial contact angles), the structure of interfacial layers, the interactions between the particles as well as about relaxation processes within the layers. For a correct description of Pi-A isotherms of composite surface layers containing particles the significant difference in size of these particles to that of solvent and surfactant molecules should be taken into account. Corresponding equations can be derived on the basis of the two-dimensional solution theory. The gained equations provide satisfactory agreement with experimental data and predict realistic values for the area of particles at the interface. Also equations of state and of the dilational elasticity for composite surface layers containing particles can be obtained in the framework of the presented methodology.

Ellipsometric study of nonionic polymer solutions.

The thickness and refractive index of adsorption films of poly(vinylpyrrolidone) (PVP) and poly(ethylene glycol) (PEG) were determined by null-ellipsometry at the air-aqueous solution interface. Both parameters, in the same way as the earlier studied dynamic surface elasticity and surface tension, exhibit rather abrupt changes when the concentration approaches the range of semidilute solutions. This behavior can be explained by the worsening of the solvent quality with increasing PEG concentration and by the PVP displacement from the surface by a contamination of high surface activity.

Relation between rheological properties and structural changes in monolayers of model lung surfactant under compression.

rSP-C surfactant monolayers spread on a native physiological model substrate show two plateau regions in the pi/A-isotherm. The first corresponds to the main phase transition in the monolayer from a LE to a LC phase. Its course is non-horizontal because of the complex composition of the lung surfactant. The second plateau, which is much more pronounced, cannot be attributed to a change of the phase state. Brewster angle microscopy images taken in this region show a sharp apparent decrease of the aggregation degree from the LE to the LC state. This process can be considered as a change in the monolayer orientation relative to the direction of the propagated light. Such a change can be the result of monolayer folding and formation of a thicker layer, which is supported by results of rheological measurements. The dilatation elasticity obtained from oscillating barrier and longitudinal wave measurements reveals a pure elastic behaviour with a steep increase in the second plateau region. Because of the insolubility of the pure lipid components, a possible explanation is squeezing protein components of rSP-C or its complexes with lipids out of the monolayer into the bulk.

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.

Experimental studies on the geometrical characteristics determining the system behavior of surface tension autooscillations.

Autooscillation of the surface tension is a phenomenon related to Marangoni instability periodically arising and fading by dissolution of a surfactant droplet under a water-air interface. A detailed experimental investigation was performed to clear up the influence of the system geometry on development and characteristics of autooscillations. It was found that the aspect ratio is an additional dimensionless parameter that determines the system behavior equally to the Marangoni number. The influence of the cell diameter, capillary immersion depth, and droplet radius on the autooscillation period and amplitude was studied as well.

Beta-casein bilayer adsorption at the solution/air interface: experimental evidences and theoretical description.

Ellipsometric and surface pressure studies of beta-casein adsorption layers at the water/air interface support the idea of a model that assumes the formation of a second layer adjacent to the primary adsorption layer. A thermodynamic model describes the concentration behavior of the surface pressure and the adsorbed amount with one and the same set of model parameters over the entire concentration range studied.

From spherical to polymorphous dispersed phase transition in water/oil emulsions.

Optical scanning tomography is used to characterize bulk properties of transparent water-in-paraffin oil emulsions stabilized with hexadecyl-trimethylammonium bromide (CTAB) and silica nanoparticles. A flow of 500 hundred images is used to analyze each scanning shot with a precision of about 1 microm. The role of silica particles in the shape of the water droplets is investigated. Depending on the concentration of CTAB and silica nanoparticles, a transition occurs in their geometry that changes from spherical to polymorphous. This transition is controlled by the ratio R=[CTAB]/[SiO2] and is described using an identification procedure of the topology of the gray level contours of the tomographic images. The transition occurs for Rcrit approximately 3x10(-2) and is shown to correspond to a pH of the dispersed phase of 8.5.

Polyelectrolyte multilayered nanofilms as a novel approach for the protection of hydrogen storage materials.

This work describes the encapsulation of hydrogen storage materials from organic solvents. Due to complex formation the shell provides stability and selective permeability. Specifically, sodium borohydride particles were encapsulated within polymer films by the layer-by-layer self-assembly of oppositely charged polyelectrolytes (polyethyleneimine and poly(acrylonitrile-co-butadiene-co-acrylic acid)). The polymer nanofilm fabrication was performed using dichloromethane as a working media. IR spectroscopy was applied to investigate the chemical interaction between the polyelectrolytes. The multilayer film preparation was verified by Z-potential measurements, scanning electron microscopy, and confocal laser microscopy. The stability of sodium borohydride protected with a polyelectrolyte shell was increased compared to that of the pure material under ambient conditions.

Theoretical evaluation of nano- or microparticulate contact angle at fluid/fluid interfaces: analysis of the excluded area behavior upon compression.

A novel method for the determination of the particle contact angle at the liquid/gas or liquid/liquid interface based on the excluded area concept revealed, in spite of its simplicity, some serious difficulties connected with the exact quantitative particle deposition at the interface and with changes in the particulate contact angle upon binary monolayer compression. The comprehensive theoretical consideration of the contact angle behavior made for such a system allowed considerable improvements: firstly, the prediction of direction of the particles' displacement at surface pressure increase is now possible and hence an unambiguous identification of particle hydrophobicity can be done. Secondly, the analytical relation describing the dependence of the particulate contact angle on the surface tension (surface pressure) was derived, allowing the prediction of whether or not particles of a given hydrophobicity will be expelled from the monolayer at certain surface pressure and of the area relinquished by the displaced particles. Thirdly, the transformation of this relation upon taking into consideration the initial conditions led to a linear dependence between excluded area and normalized surface tension allowing the determination of the particle contact angle and the exact number of deposited particles simultaneously and independently of each other. Finally, the application of the improved approach to the previously collected experimental data yielded reasonable values for both particle contact angle and number of deposited particles.

New method for fabrication of loaded micro- and nanocontainers: emulsion encapsulation by polyelectrolyte layer-by-layer deposition on the liquid core.

A novel approach to the emulsion encapsulation was developed by combining the advantages of direct encapsulation of a liquid colloidal core with the accuracy and multifunctionality of layer-by-layer polyelectrolyte deposition. Experimental data obtained for the model oil-in-water emulsion confirm unambiguously the alternating PE assembly in the capsule shell as well as the maintenance of the liquid colloidal core. Two different mechanisms of capsule destruction upon interaction with the solid substrate were observed and qualitatively explained. The proposed method can be easily generalized to the preparation of oil-filled capsules in various oil/water/polyelectrolyte systems important in the field of pharmacy, medicine, and food industry.

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.

Contact angle determination of micro- and nanoparticles at fluid/fluid interfaces: the excluded area concept.

A novel and simple method for the determination of the contact angle of nano- and microparticles at the liquid/air interface is proposed. The principle is based on the consideration of differences between the pressure/area isotherms of a 2D single-component system of a surfactant and those of binary systems comprised of the same surfactant and the particles to be studied. The theoretical analysis of the contact-angle behavior in this system upon compression allows the prediction of direction of the particles' squeezing out of the surface layer and therefore the distinction between the particles with high contact angle (Theta(p) > 90 degrees) and low (Theta(p) < 90 degrees) hydrophobicity. The application of this method to microparticles of two different hydrophobicities demonstrates good agreement between the obtained contact angles and the corresponding degrees of hydrophobicity given by the particle provider.

Dynamic surface properties of polyelectrolyte/surfactant adsorption films at the air/water interface: poly(diallyldimethylammonium chloride) and sodium dodecylsulfate.

The dynamic surface elasticity, dynamic surface tension, and ellipsometric angles of mixed aqueous poly(diallyldimethylammonium chloride)/sodium dodecylsulfate solutions (PDAC/SDS) have been measured as a function of time and surfactant concentration. This system represents a typical example of polyelectrolyte/surfactant complex formation and subsequent aggregation on the nanoscale. The oscillating barrier and oscillating drop methods sometimes led to different results. The surface viscoelasticity of mixed PDAC/SDS solutions are very close to those of mixed solutions of sodium polystyrenesulfonate and dodecyltrimethylammonium bromide but different from the results for some other polyelectrolyte/surfactant mixtures. The abrupt drop in surface elasticity when the surfactant molar concentration approaches the concentration of charged polyelectrolyte monomers is caused by the formation of microparticles in the adsorption layer. Aggregate formation in the solution bulk does not influence the surface properties significantly, except for a narrow concentration range where the aggregates form macroscopic flocks. The mechanism of the observed relaxation process is controlled by the mass exchange between the surface layer and the flocks attached to the liquid surface.

Surface-pressure isotherms of monolayers formed by microsize and nanosize particles.

The thermodynamic model of a 2D solution developed earlier for protein monolayers at liquid interfaces is generalized for monolayers composed of micro- and nanoparticles. Surface pressure isotherms of particle monolayers published in the literature for a wide range of particles sizes (between 75 microm and 7.5 nm) are described by the theoretical model with one modification. The calculations of surface pressure pi on area A provide satisfactory agreement with the experimental data. The theory also yields reasonable cross-sectional area values of the solvent molecule water in the range between 0.12 and 0.18 nm2, which is almost independent of particle size. Also, the area per particle in a closely packed monolayer obtained from the theory is quite realistic.

Dynamic surface properties of poly(N-isopropylacrylamide) solutions.

The dynamic surface elasticity of aqueous solutions of poly(N-isopropylacrylamide) (pNIPAM) has been measured by the oscillating barrier and capillary wave methods as a function of time and concentration. While the real and imaginary parts of the surface elasticity almost did not change with the concentration, their kinetic dependencies proved to be nonmonotonic. Simultaneous measurements of the film thickness and adsorbed amount by null-ellipsometry showed that the pNIPAM adsorption can be divided into two steps corresponding to the formation of a concentrated narrow region close to the air phase and a region of tails and loops protruding into the bulk liquid. The local maximum of the elasticity can be observed in the course of the first step when the adsorbed macromolecules do not form long loops and tails. The results are in agreement with recent data on the nonequilibrium surface properties of solutions of other nonionic homopolymers and the theory of dilational surface viscoelasticity.