Theodor D. Gurkov

Interfacial layers from the protein HFBII hydrophobin: Dynamic surface tension, dilatational elasticity and relaxation times

The pendant-drop method (with drop-shape analysis) and Langmuir trough are applied to investigate the characteristic relaxation times and elasticity of interfacial layers from the protein HFBII hydrophobin. Such layers undergo a transition from fluid to elastic solid films. The transition is detected as an increase in the error of the fit of the pendant-drop profile by means of the Laplace equation of capillarity. The relaxation of surface tension after interfacial expansion follows an exponential-decay law, which indicates adsorption kinetics under barrier control. The experimental data for the relaxation time suggest that the adsorption rate is determined by the balance of two opposing factors: (i) the barrier to detachment of protein molecules from bulk aggregates and (ii) the attraction of the detached molecules by the adsorption layer due to the hydrophobic surface force. The hydrophobic attraction can explain why a greater surface coverage leads to a faster adsorption. The relaxation of surface tension after interfacial compression follows a different, square-root law. Such behavior can be attributed to surface diffusion of adsorbed protein molecules that are condensing at the periphery of interfacial protein aggregates. The surface dilatational elasticity, E, is determined in experiments on quick expansion or compression of the interfacial protein layers. At lower surface pressures (<11 mN/m) the experiments on expansion, compression and oscillations give close values of E that are increasing with the rise of surface pressure. At higher surface pressures, E exhibits the opposite tendency and the data are scattered. The latter behavior can be explained with a two-dimensional condensation of adsorbed protein molecules at the higher surface pressures. The results could be important for the understanding and control of dynamic processes in foams and emulsions stabilized by hydrophobins, as well as for the modification of solid surfaces by adsorption of such proteins.

β-Casein adsorption kinetics on air–water and oil–water interfaces studied by ellipsometry

In this work b-casein adsorption kinetics at air:water and oil:water surfaces is studied by ellipsometry. Double layer formation is observed; the process is not influenced by the type of the hydrophobic phase. A dense protein layer initially forms, and then the adsorption continues in a second more diffuse layer, which extends in the aqueous phase. The surface coverage (i.e. the adsorption, G) in the second layer approaches that in the first dense layer at long times, but the thickness and the refractive index of the second layer testify for lower volume density of protein there. The well known ellipsometric problem of correlating parameters in the case of measurement in a very thin layer (on a substrate which does not absorb light) has been overcome by using different upper phases (oil and air). This gives the opportunity to determine separately the refractive index and the thickness of the layer. The same procedure is applicable whenever a very thin layer structure with two unknown parameters is under investigation.

Surface Tension and Surface Energy of Curved Interfaces and Membranes

In this work we develop the basic idea that the mechanical properties of a curved Gibbsian dividing surface are characterized, not only by the surface tension, but also by surface moments. It is shown that an additional surface bending moment term is to be introduced in the interfacial balance of the force moments at a spherical dividing surface. The existence of surface bending moment leads to a difference between the dilational surface energy and the surface tension. These two quantities are expressed in terms of integrals over the components of the pressure tensor for an arbitrarily chosen spherical dividing surface (not necessarily the surface of tension). The relation between dilational surface energy and surface tension is generalized for interfaces with an arbitrary curvature. A similar relationship was derived for a surface shearing energy, which is important for interfaces exhibiting shearing elasticity, like biomembranes. The relation between the dilational surface energy and the interfacial density of the grand potential is discussed. The results can be useful for the analysis of data for microemulsions, interfacial waves, vesicles and biomembranes.

Nanoemulsions obtained via bubble-bursting at a compound interface

When a bubble bursts on reaching a surface, mass transfer from the liquid to the gas phase can occur—aerosol dispersion. Now, the inverse transport process is reported: submicrometre-sized oil droplets, formed during bubble-bursting, are zipped across the interface to the liquid phase.

The role of additives for the behaviour of thin emulsion films stabilized by proteins

Abstract Experimental results obtained with thin aqueous films of emulsion type stabilized by bovine serum albumin (BSA) and β-casein are presented. The film behaviour is time dependent. The contact angle increases with ageing and exhibits pronounced hysteresis. With BSA one observes slow reversible aggregation on the surface (but not in the bulk) and the protein lumps are gradually squashed by the capillary pressure as the film thins. The findings can be explained by slow surface denaturation, accompanied by developing attraction and partial entanglement of the BSA molecules. These processes are promoted by oleic acid dissolved in the oil phase. Electrostatic interactions were found to be important: without salt the films remain thick, whereas in the presence of 0.15 M NaCl one obtains Newton black films whose contact angle depends upon the molecular charge. A marked difference in the surface mobility is observed with foam and emulsion films stabilized by BSA. Lenses, containing protein aggregates and liquid, when surrounded by an area which has reached the black film stage, remain entrapped in foam films but are slowly squeezed out in emulsion films. Hydrophobization of the protein molecules may be responsible for this behaviour. With β-casein, ageing effects in films are observed only at the isoelectric point. This protein strongly aggregates in the bulk, but the lumps are readily flattened on the film interfaces. Addition of Ca 2+ ions leads to a decrease in film thickness, depending on the concentration.

Surface Pressure and Elasticity of Hydrophobin HFBII Layers on the Air–Water Interface: Rheology Versus Structure Detected by AFM Imaging

Here, we combine experiments with Langmuir trough and atomic force microscopy (AFM) to investigate the reasons for the special properties of layers from the protein HFBII hydrophobin spread on the air-water interface. The hydrophobin interfacial layers possess the highest surface dilatational and shear elastic moduli among all investigated proteins. The AFM images show that the spread HFBII layers are rather inhomogeneous, (i.e., they contain voids, monolayer and multilayer domains). A continuous compression of the layer leads to filling the voids and transformation of a part of the monolayer into a trilayer. The trilayer appears in the form of large surface domains, which can be formed by folding and subduction of parts from the initial monolayer. The trilayer appears also in the form of numerous submicrometer spots, which can be obtained by forcing protein molecules out of the monolayer and their self-assembly into adjacent pimples. Such structures are formed because not only the hydrophobic parts, but also the hydrophilic parts of the HFBII molecules can adhere to each other in the water medium. If a hydrophobin layer is subjected to oscillations, its elasticity considerably increases, up to 500 mN/m, which can be explained with compaction. The relaxation of the layers tension after expansion or compression follows the same relatively simple law, which refers to two-dimensional diffusion of protein aggregates within the layer. The characteristic diffusion time after compression is longer than after expansion, which can be explained with the impedence of diffusion in the more compact interfacial layer. The results shed light on the relation between the mesoscopic structure of hydrophobin interfacial layers and their unique mechanical properties that find applications for the production of foams and emulsions of extraordinary stability; for the immobilization of functional molecules at surfaces, and as coating agents for surface modification.

Experimental investigations on model emulsion systems stabilized with non-ionic surfactant blends

Abstract We have performed experimental research into model oil-in-water emulsion systems stabilized with non-ionic surfactant blends: thin aqueous films between oil phases and oil drops coalescing against their homophase. Xylene was chosen as the oil phase, and Tween 20 and Span 20, alone or in mixtures at different molar ratios, were used as stabilizers. The roles of the surfactant mole ratio, the electrolyte concentration and pH were studied. It was shown that there is considerable electrostatic repulsion within the aqueous films. The results obtained on thin film stability and on drop lifetime at constant electrolyte concentration indicate that Tween 20 (when present) is the emulsifier that predominantly determines the stability of the systems. This conclusion can be related to the higher surface activity and higher adsorption of Tween as deducted from interfacial tension data. Some evidence of synergism was observed only at an electrolyte concentration of 10 −3 M (NaCl). We also carried out stability tests on shaken and homogenized batch emulsion systems and the results show good correlation with the data from the model investigations.

Interarm Interaction of DNA Cruciform Forming at a Short Inverted Repeat Sequence

A novel interarm interaction of DNA cruciform forming at inverted repeat sequence was characterized using an S1 nuclease digestion, permanganate oxidation, and microscopic imaging. An inverted repeat consisting of 17 bp complementary sequences was isolated from the bluegill sunfish Lepomis macrochirus (Perciformes) and subcloned into the pUC19 plasmid, after which the supercoiled recombinant plasmid was subjected to enzymatic and chemical modification. In high salt conditions (200 mM NaCl, or 100-200 mM KCl), S1 nuclease cut supercoiled DNA at the center of palindromic symmetry, suggesting the formation of DNA cruciform. On the other hand, S1 nuclease in the presence of 150 mM NaCl or less cleaved mainly the 3-half of the repeat, thereby forming an unusual structure in which the 3-half of the inverted repeat, but not the 5-half, was retained as an unpaired strand. Permanganate oxidation profiles also supported the presence of single-stranded part in the 3-half of the inverted repeat in addition to the center of the symmetry. Both electron microscopy and atomic force microscopy have detected a thick protrusion on the supercoiled DNA harboring the inverted repeat. We hypothesize that the cruciform hairpins at conditions favoring triplex formation adopt a parallel side-by-side orientation of the arms allowing the interaction between them supposedly stabilized by hydrogen bonding of base triads.

Stokes flow caused by the motion of a rigid sphere close to a viscous interface

The subject of this work is the creeping motion of liquid caused by steady translation and rotation of a solid sphere close to an interface between two fluid phases. The case of vanishingly small Reynolds and capillary numbers is considered. We account for the intrinsic viscous properties of the liquid boundary, characterised by dilatational and shear surface viscosities, in the frame of the Boussinesq—Scriven model. Numerical computations are present- ed for the velocity and pressure distributions throughout the flow domain. The drag force and the torque exerted on the particle are obtained by means of analytical integration of the stresses over the spherical surface. At small distances of separation between the rigid sphere and the wall, the role of the surface viscosity becomes quite substantial: the drag and the toque can be several times bigger than those which correspond to motion in an unbounded fluid. For steady rotation the flow around the solid particle is restrained in a relatively narrow region, whereas with translation the velocity field extends to large distances. Consequently, the rotating sphere should be closer in order to start feeling the wall. Our results are relevant for systems which contain surfactant laden interfaces. We showed that even with low molecular weight surfactants such interfaces can behave much like solid ones. This is particularly true for fluid boundaries covered by proteins, as they turn out to be completely immobilised. ( 1998 Elsevier Science Ltd. All rights reserved.

The interfacial bending moment: Thermodynamics and contributions of the electrostatic interactions

The problem concerning the magnitude and the sign of the interfacial bending moment of the droplets in fluid disperse systems is considered on the basis of a thermodynamic approach. It is demonstrated that the total bending moment can be expressed as a superposition of contributions connected with the different components in the system. Expressions for the contributions of the electric double layer, of the interactions between adsorbed dipoles and of the electrolyte excess osmotic pressure to the value of the bending moment are derived. The effect of a possible incomplete dissociation of the adsorbed ionic surfactant monolayer is also taken into account. The results show that the van der Waals and the electric double layer interactions provide significant contributions to the bending moment, both of them of the order of 10 pN. Even at high electrolyte concentrations, the electrostatic bending moment can be important owing to the contribution of the Stern layer. The results can be applied to study the curvature dependence of the interfacial tension in microemulsions and in liquid-gas dispersions.