Adsorption and dilational rheology of mixed ß-casein/DoTAB layers formed by sequential and simultaneous adsorption at the water/hexane interface.

The interfacial behavior of ß-casein (ßCS) has been investigated in presence of the cationic surfactant dodecyl trimethyl ammonium bromide (DoTAB) at the water/hexane interface and compared to that obtained for the water/air interface. The used experimental technique is a drop profile analysis tensiometer specially equipped with a coaxial double capillary, which allows investigation of sequential adsorption of individual components besides the traditional simultaneous adsorption of two species. This method also provides the dilational rheological measurements based on low frequency harmonic drop oscillations. The tensiometric results show that the equilibrium states of the mixed ßCS/DoTAB layers built up on the two different routes do not differ significantly, that is, the general compositions of the mixed layers are similar. However, the results of dilational rheology for the two adsorption strategies are remarkably different indicating different dynamic characteristics of the adsorbed layers. These findings suggest that the respective mixed layers are more proteinlike if they are formed via sequential adsorption and more surfactant-like after simultaneous adsorption. In contrast to the W/A interface, at the W/H interface proteins remain at the interface once adsorbed and cannot be displaced just by competitive adsorption of surfactants.

Interfacial viscoelasticity of myoglobin at air/water and air/solution interfaces: role of folding and clustering.

This study describes the folding and organization of myoglobin (Mb) at the solution/air interface at different pH values of 2.5, 3.5, 5.5, 7.5, and 8.5. Dynamic surface tension and the associated dilational and shear viscoelasticity for Mb at these pH's have been studied using a sinusoidal surface compression and expansion for frequencies ranging from 0.01 to 0.4 Hz. The changes in dilational viscosity, elasticity, and fluorescence lifetime measurements have been related to the conformational changes of the protein films at the interface. It is observed that while acid-induced denaturation of the protein does not lead to large changes in dilational properties, the shear properties on the other hand are strongly influenced by it, and the protein behaves like a shear-thickening fluid. At higher pH, particularly at the isoelectric point, Mb is pseudoplastic indicating an increase in the shear viscosity. These results are strongly suggestive of formation of hydrophobic clusters at the protein-buffer interface because of the change in the overall charge distributions.

Adsorption of protein-surfactant complexes at the water/oil interface.

Interfacial tension measurements have been performed at the water/hexane interface on mixtures of the bovine milk protein ß-lactoglobulin and positively charged cationic surfactants (alkytrimethylammonium bromides). The addition of surfactants with different chain lengths leads to the formation of protein-surfactant complexes with different adsorption properties as compared to those of the single protein. In this study, the formation of complexes has been observed clearly for protein-long chain surfactant (TTAB and CTAB) mixtures, which has shown in addition to specific electrostatic interactions the relevance of hydrophobic interactions between surfactant molecules and the protein. The modeling of interfacial tension data by using a mixed adsorption model provides a quantitative understanding of the mixture behavior. Indeed, the value of the adsorption constant of the protein obtained in the presence of surfactants has strongly varied as compared to the single protein. Actually, this parameter which represents the affinity of the molecule for the interface is representative of the hydrophobic character of the compound and so of its surface activity. Even if a more hydrophobic and more surface active protein-surfactant complex has been formed, the replacement of this complex from the interface by surfactants close to their cmc was observed.

Rheology and structure formation in diluted mixed particle-surfactant systems.

In the present work, we focus on the bulk rheology of mixtures consisting of surfactant modified silica nanoparticles in water. Depending on the ratio of surfactant and nanoparticle concentrations, significant modifications in the measured rheology are evidenced. A domain where the dispersions behave like viscoelastic media is observed. Outside this domain, the dispersions exhibit viscous properties. The changes in the bulk rheology characteristics are discussed in terms of interaction effects between the surfactant and the particles. The results obtained are seen in the context of diluted emulsions' properties and characteristics.

Interfacial properties of mixed beta-lactoglobulin-SDS layers at the water/air and water/oil interface.

The adsorption behavior of the beta-lactoglobuline has been studied in the presence of the anionic surfactant sodium dodecylsulfate (SDS) and compared for two different interfaces, water/air and water/hexane. The fitting of experimental data (adsorption isotherms) by a mixed adsorption model and the determination of structural parameters such as the molecular area occupied by the protein-surfactant complex and the surfactant molecules at the interface allowed to have a better understanding of the composition and as a consequence the behavior of the mixed interfacial layer. The parameters obtained for the mixtures are similar to those obtained separately for the single components, but the comparison of the both interfaces has shown significant differences. Much higher concentration of complex is found at the water/hexane interface, which is the result of a better affinity of the protein for this interface. A higher penetration of the protein into the oil phase and the presence of interactions between protein-surfactant complexes and free surfactant molecules stabilize the interface preventing its replacement by the SDS molecules. Rheological experiments show a decrease of the visco-elastic modulus at both interfaces with increasing SDS concentration. But at the water/oil interface, contrary to the water/air interface at which the replacement of the protein has been clearly observed, this decrease is attributed to changes of complex properties. At high SDS concentrations, an increase of the hydrophilic character due to hydrophobic interactions with the surfactant molecules leads to an increase in the mobility of the complex, which favors its desorption upon increased competition by the surfactant.

Competition between lipases and monoglycerides at interfaces.

Tensiometry (the pendant drop technique), interfacial shear rheology, and ellipsometry have been used to study the effect of polar lipids that are generated during fat digestion on the behavior of lipases at the oil-water interface. Both Sn-1,3 regiospecific and nonregiospecific lipases have been used, and a noncatalytically active protein, beta-lacloglobulin, has been used as reference in the interfacial shear rheology experiments. The results from the pendant drop measurements and the interfacial rheology studies were in agreement with each other and demonstrated that the Sn-2 monoglyceride, which is one of the lipolysis products generated when a Sn-1,3 regiospecific lipase catalyzes triglyceride hydrolysis, is very interfacially active and efficiently expels the enzyme from the interface. Ellipsometry conducted at the liquid-liquid interface showed that the lipase forms a sublayer in the aqueous phase, just beneath the monoglyceride-covered interface. Sn-1/3 monoglycerides do not behave this way because they are rapidly degraded to fatty acid and glycerol and the fatty acid (or the fatty acid salt) does not have enough interfacial activity to expel the lipase from the interface. Since the lipases present in the gastrointestinal tract are highly Sn-1,3 regiospecific, we believe that the results obtained can be transferred to the in vivo situation. The formation of stable and amphiphilic Sn-2 monoglycerides can be seen as a self-regulatory process for fat digestion.

Surface shear rheological studies of marine phytoplankton cultures-Nitzschia closterium, Thalassiosira rotula, Thalassiosira punctigera and Phaeocystis sp.

The study of interfacial properties in the marine environment is important for the understanding of air-sea gas exchange processes, especially with respect to the behaviour of entrained air bubbles. Seawater contains surfactant material, much of which is thought to origin from the exudation of dissolved organic material (DOM) by phytoplankton. This study aims at investigating the influence of different phytoplankton species on the surface shear viscosity of an air-water interface. Measurements of surface shear viscosity were carried out with the ISR1 interfacial shear rheometer. Surface shear viscosities of stock cultures of Phaeocystis sp., Thalassiosira rotula, Thalassiosira punctigera and Nitzschia closterium as well as of F/2 nutrient medium and seawater were measured. The surface shear viscosity of N. closterium was investigated during different stages of its growth as well as for an unfiltered stock culture sample and its filtrate. Results reveal that the influence of phytoplankton on the surface shear viscosity is species specific. An increase in surface shear viscosity occurred for the N. closterium stock culture only. The remaining cultures showed similar behaviour to F/2 nutrient medium. The increase of surface shear viscosity during the growth of N. closterium occurred mainly during the exponential growth phase. The increases in surface shear viscosity depend on the presence of phytoplankton cells in the sample. The formation of compact mechanical structures at the air-water interface originating from the aggregation of DOM released by N. closterium as a cause for the observed increases in surface shear viscosity is discussed.