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.     

Adsorption kinetics under the influence of barriers at the subsurface layer

At the initial stage of surfactant adsorption (when the layer is relatively diluted), the kinetics may be dominated by factors related to the transfer of molecules through the subsurface region and onto the interface. We consider two independent physical effects: (1) diffusion through a subsurface layer with nanometer thickness, where structuring or molecular interactions can impose substantial changes on the transfer rate, as compared with the bulk diffusion and (2) hindrance to the act of adsorption itself, when the molecules hit the interface from a place directly adjacent to it. These two effects are taken into account by formulating a model which includes the balance of fluxes in the subsurface layer. This model allows one to find analytical solution for the adsorption as a function of time. Application of the theory is illustrated by analyzing experimental data for two proteins which adsorb on air/water interface. Attention is paid to the particular case when the resistance to adsorption is relatively small but is still significant as compared with the bulk diffusion. Then, the theoretical fit of the adsorption vs. time can be implemented in a specific linear scale. The overall resistance of the interfacial zone comprises additive contributions from the hindrance to the act of adsorption and from the (retarded) diffusion through the subsurface layer. They are incorporated into one physical parameter (or characteristic time), which influences the kinetics.     

Selection of surfactants for stable paraffin-in-water dispersions, undergoing solid-liquid transition of the dispersed particles

A new experimental procedure is proposed for express evaluation of the coalescence stability of dispersions, in which the dispersed particles undergo solid-liquid phase transition. The procedure includes centrifugation of the dispersion concurrently with the phase transition of the particles and allows precise quantification of dispersion stability in terms of a critical pressure, at which the coalescence between the dispersed particles/drops takes place. The method is applied for studying the effects of surfactant type and concentration on the stability of paraffin-in-water dispersions, which have potential application in energy storage and transportation systems. Several types of water-soluble surfactants (anionic, nonionic, and polymeric) are compared, whereas hexadecane or tetradecane is used as a dispersed phase. Most of the studied individual surfactants are found to be inefficient stabilizers (except for the nonionic Tween 40 and Tween 60). However, the dispersion stability increases significantly after the addition of appropriate cosurfactants, such as hexadecanol, Brij 52, or cocoamidopropyl betaine. Surfactants and cosurfactants with longer hydrophobic tails are better stabilizers than those with shorter tails. The obtained results are discussed from the viewpoint of the mechanisms of particle/drop coalescence during the solid-liquid-phase transition. The consistency and the undercooling temperatures of the studied dispersions are also discussed, because these properties are important for their practical applications. The proposed procedure for evaluation of dispersion stability and some of the conclusions could be relevant to food emulsions, in which dispersed fat particles undergo solid-liquid-phase transition of similar type.     

MECHANICS AND THERMODYNAMICS OF INTERFACES,THIN LIQUID FILMS AND MEMBRANE

First we review some recent studies devoted to the role of surface moments (torques) in the mechanics and thermodynamics of fluid interfaces of arbitrary shape. The presence of bending and torsion moments leads to a difference between the mechanical and thermodynamical surface tensions and shear stresses. Next we review recent results in the mechanics of thin liquid films and the transition region film-Plateau border. Line and transversal tensions are assigned as excesses on the contact line. The transversal tension accounts for the attractive forces in the transition zone. Experiments with floating bubbles show that the movement of the contact line can lead to non-equilibrium values of the line tension, which can be interpreted in terms of a plastic deformation.     

Formation of dimers in lipid monolayers

In this work, we analyse theoretically the hypothesis that zwitterionic lipids form dimers in adsorption monolayers on water/ hydrocarbon phase boundary. A dimer can be modelled as a couple of lipid molecules whose headgroup lateral dipole moments have antiparallel orientation. Properties including surface pressure, chemical potentials and activity coefficients are deduced from a general expression for the free energy of the monolayer. The theoretical model is in a good agreement with experimental data for surface pressure and surface potential of lipid monolayers. The results favour the hypothesis about formation of dimers in equilibrium with monomers, with the amount of the species depending on the area per molecule and temperature. The reaction of dimerisation turns out to be exothermic with a heat of about 2.5kT per dimer. The results may be applied to the molecular models of membrane structures and mechanisms.     

Electric Component of the Interfacial Bending Moment and the Curvature Elastic Moduli

The interfacial bending moment and curvature elastic moduli are related theoretically to the surface potential, ΔV, which is liable to direct measurement. The dependence of the interfacial bending properties on the position of the Gibbs dividing surface is investigated. As an application, experimental data for the surface potential of micellar surfactant solutions containing Al³⁺ions are analyzed. The changes in the bending moment, as determined from ΔVpotential, correlate with the transition from spherical to cylindrical micelles induced by the Al³⁺ions. The results can be important for interpretation of data for formation of microemulsions, flocculation in emulsions, fluctuation capillary waves at interfaces and biomembranes, interactions between inclusions in lipid bilayers, etc.