Adhesion of bubbles and drops to solid surfaces, and anisotropic surface tensions studied by capillary meniscus dynamometry.

Here, we review the principle and applications of two recently developed methods: the capillary meniscus dynamometry (CMD) for measuring the surface tension of bubbles/drops, and the capillary bridge dynamometry (CBD) for quantifying the bubble/drop adhesion to solid surfaces. Both methods are based on a new data analysis protocol, which allows one to decouple the two components of non-isotropic surface tension. For an axisymmetric non-fluid interface (e.g. bubble or drop covered by a protein adsorption layer with shear elasticity), the CMD determines the two different components of the anisotropic surface tension, σs and σφ, which are acting along the "meridians" and "parallels", and vary throughout the interface. The method uses data for the instantaneous bubble (drop) profile and capillary pressure, but the procedure for data processing is essentially different from that of the conventional drop shape analysis (DSA) method. In the case of bubble or drop pressed against a substrate, which forms a capillary bridge, the CBD method allows one to determine also the capillary-bridge force for both isotropic (fluid) and anisotropic (solidified) adsorption layers. The experiments on bubble (drop) detachment from the substrate show the existence of a maximal pulling force, Fmax, that can be resisted by an adherent fluid particle. Fmax can be used to quantify the strength of adhesion of bubbles and drops to solid surfaces. Its value is determined by a competition of attractive transversal tension and repulsive disjoining pressure forces. The greatest Fmax values have been measured for bubbles adherent to glass substrates in pea-protein solutions. The bubble/wall adhesion is lower in solutions containing the protein HFBII hydrophobin, which could be explained with the effect of sandwiched protein aggregates. The applicability of the CBD method to emulsion systems is illustrated by experiments with soybean-oil drops adherent to hydrophilic and hydrophobic substrates in egg yolk solutions. The results reveal how the interfacial rigidity, as well as the bubble/wall and drop/wall adhesion forces, can be quantified and controlled in relation to optimizing the properties of foams and emulsions.

Adsorption of Hydrophobin-Protein Mixtures at the Air-Water Interface: The Impact of pH and Electrolyte.

The adsorption of the proteins ß-casein, ß-lactoglobulin, and hydrophobin, and the protein mixtures of ß-casein/hydrophobin and ß-lactoglobulin/hydrophobin have been studied at the air-water interface by neutron reflectivity, NR. Changing the solution pH from 7 to 2.6 has relatively little impact on the adsorption of hydrophobin or ß-lactoglobulin, but results in a substantial change in the structure of the adsorbed layer of ß-casein. In ß-lactoglobulin/hydrophobin mixtures, the adsorption is dominated by the hydrophobin adsorption, and is independent of the hydrophobin or ß-lactoglobulin concentration and solution pH. At pH 2.6, the adsorption of the ß-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption over the range of ß-casein concentrations studied. At pH 4 and 7, the adsorption of ß-casein/hydrophobin mixtures is dominated by the hydrophobin adsorption at low ß-casein concentrations. At higher ß-casein concentrations, ß-casein is adsorbed onto the surface monolayer of hydrophobin, and some interpenetration between the two proteins occurs. These results illustrate the importance of pH on the intermolecular interactions between the two proteins at the interface. This is further confirmed by the impact of PBS, phosphate buffered saline, buffer and CaCl2 on the coadsorption and surface structure. The results provide an important insight into the adsorption properties of protein mixtures and their application in foam and emulsion stabilization.

Impact of the degree of ethoxylation of the ethoxylated polysorbate nonionic surfactant on the surface self-assembly of hydrophobin-ethoxylated polysorbate surfactant mixtures.

Neutron reflectivity measurements have been used to study the surface adsorption of the polyethylene sorbitan monostearate surfactant, with degrees of ethoxylation varying from 3 to 20 ethylene oxide groups, with the globular protein hydrophobin. The surface interaction between the ethoxylated polysorbate nonionic surfactants and the hydrophobin results in self-assembly at the air-solution interface in the form of a well-defined layered surface structure. The surface interaction arises from a combination of the hydrophobic interaction between the surfactant alkyl chain and the hydrophobic patch on the surface of the hydrophobin, and the hydrophilic interaction between the ethoxylated sorbitan headgroup and the hydrophilic regions on the surface of the hydrophobin. The results presented show that varying the degree of ethoxylation of the polysorbate surfactant changes the interaction between the surfactant and the hydrophobin and the packing, and hence the evolution in the resulting surface structure. The optimal degree of ethoxylation for multilayer formation is over a broad range, from of order 6 to 17 ethylene oxide groups, and for degrees of ethoxylation of 3 and 20 only monolayer adsorption of either the surfactant or the hydrophobin is observed.

Spontaneous surface self-assembly in protein-surfactant mixtures: interactions between hydrophobin and ethoxylated polysorbate surfactants.

The synergistic interactions between certain ethoxylated polysorbate nonionic surfactants and the protein hydrophobin result in spontaneous self-assembly at the air-water interface to form layered surface structures. The surface structures are characterized using neutron reflectivity. The formation of the layered surface structures is promoted by the hydrophobic interaction between the polysorbate alkyl chain and the hydrophobic patch on the surface of the globular hydrophobin and the interaction between the ethoxylated sorbitan headgroup and hydrophilic regions of the protein. The range of the ethoxylated polysorbate concentrations over which the surface ordering occurs is a maximum for the more hydrophobic surfactant polyoxyethylene(8) sorbitan monostearate. The structures at the air-water interface are accompanied by a profound change in the wetting properties of the solution on hydrophobic substrates. In the absence of the polysorbate surfactant, hydrophobin wets a hydrophobic surface, whereas the hydrophobin/ethoxylated polysorbate mixtures where multilayer formation occurs result in a significant dewetting of hydrophobic surfaces. The spontaneous surface self-assembly for hydrophobin/ethoxylated polysorbate surfactant mixtures and the changes in surface wetting properties provide a different insight into protein-surfactant interactions and potential for manipulating surface and interfacial properties and protein surface behavior.

Comparison of delayed matching-to-sample performance in monkeys and children.

Although research has consistently demonstrated that accuracy on a variety of memory tasks decreases as delay increases, relatively little research has been conducted to quantify this relationship across development in humans or directly compare rates of forgetting between humans and monkeys. This study utilized a delayed matching-to-sample (DMTS) task to compare the relative contributions of proactive interference and attention on the rate of forgetting in monkeys and children. The performance of 1125 children from four to fourteen years of age and 10 adult rhesus monkeys was compared. For this DMTS task, a shape was displayed on the center one of three press-plates. After a delay, the subjects were required to match the original shape with one of three choice shapes to receive a banana-flavored food pellet for monkeys, or a nickel for children. A modified power function provided an excellent fit for the data for monkeys and children. The forgetting rates in children decreased with age, and the forgetting rates for monkeys were most comparable to those of younger children. The data also suggest that proactive interference did not significantly contribute to the forgetting rates for monkeys or younger children. Further, the monkeys appeared to attend to the task at a level similar to that of younger children as evidenced by the similarities in response latencies. The results from this study indicate that the rate of forgetting in monkeys, as well as the mechanisms underlying this rate, appears to share more similarities with that of younger children than of older children.

Surface properties of class ii hydrophobins from Trichoderma reesei and influence on bubble stability.

We report the remarkable surface behavior of class II hydrophobin proteins HFBI and HFBII from Trichoderma reesei and the resulting effect that these proteins have on the stability of air bubbles to the process of disproportionation. The surface properties were studied using surface tensiometry and surface shear rheology. Surface tensiometry data show that hydrophobins are very surface active proteins, reducing the surface tension to approximately 30 mN m-1. The rate at which the hydrophobins adsorb at the surface may also be related to the self-assembly behavior in aqueous solution. We further show that hydrophobins form air/water surfaces with high elasticity, the magnitude of which is well in excess of that of surface layers formed by other common proteins used as foam or emulsion stabilizers. The measured surface properties translate to the stability of bubbles with adsorbed hydrophobin, and in this study, we demonstrate the ability of hydrophobin to have a dramatic effect on the rate of disproportionation in some simple bubble dissolution studies.

Coordination complexes bearing potentially tetradentate phenoxyamine ligands.

A series of metal complexes containing potentially tetradentate phenoxyamine ligands is described. The ligands are found to bind to main-group metals and first-row transition-metal centres with variable denticity depending upon the requirements of the particular metal centre. Bidentate [Al(III)], tridentate [Mg(II), Ca(II), Zn(II)] and tetradentate [K(I), Cr(III), Fe(II), Co(II)] binding modes have been established unambiguously through single-crystal X-ray structure determinations.