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The influence of hexane vapor in the air atmosphere on the surface tension of water and solutions of C 10 EO 8 , C n TAB and proteins are presented. For dry air, a fast and strong decrease of surface tension of water was observed. In humid air, the process is slower and the surface tension higher. There are differences between the results obtained by the maximum bubble pressure, pendant drop and emerging bubble methods, which are discussed in terms of depletion and initial surface load. The surface tension of aqueous solutions of β-сasein (BCS), β-lactoglobulin (BLG) and human serum albumin (HSA) at the interfaces with air and air-saturated hexane vapor were measured. The results indicate that the equilibrium surface tension in the hexane vapor atmosphere is considerably lower (at 13-20mN/m) as compared to the values at the interface with pure air. A reorientation model is proposed assuming several states of adsorbed molecules with different molar area values. The newly developed theoretical model is used to describe the effect of alkane vapor in the gas phase on the surface tension. This model assumes that the first layer is composed of surfactant (or protein) molecules mixed with alkane, and the second layer is formed by alkane molecules only. The processing of the experimental data for the equilibrium surface tension for the C 10 EO 8 and BCS solutions results in a perfect agreement between the observed and calculated values. The co-adsorption mechanism of dipalmitoyl phosphatidyl choline (DPPC) and the fluorocarbon molecules leads to remarkable differences in the surface pressure term of cohesion Π coh . This in turn leads to a very efficient fluidization of the monolayer. It was found that the adsorption equilibrium constant for dioctanoyl phosphatidyl choline is increased in the presence of perfluorohexane, and the intermolecular interaction of the components is strong., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The equations of state, adsorption isotherms and functions of the distribution of protein molecules in liquid interfacial layers with respect to molar area and the equations for their viscoelastic behavior are presented. This theory was used to determine the adsorption characteristics of β-casein and β-lactoglobulin at water/oil interfaces. The experimental results are shown to be describable quite adequately by the proposed theory with consistent model parameters. The data analysis demonstrated that the β-casein molecule adsorbed at equilibrium conditions is more unfolded as compared with dynamic conditions, and this fact causes the significant increase of the adsorption equilibrium constant. The theory assumes the adsorption of protein molecules from the aqueous solution and a competitive adsorption of alkane molecules from the alkane phase. The comparison of the experimental equilibrium interfacial tension isotherms for β-lactoglobulin at the solution/hexane interface with data calculated using the proposed theoretical model demonstrates that the assumption of a competitive adsorption is essential, and the influence of the hexane molecules on the shape of the adsorption isotherm does in fact exist., (Copyright © 2015 Elsevier B.V. All rights reserved.)

The influence of the addition of the nonionic surfactants C12DMPO, C14DMPO, C10OH, and C10EO5 at concentrations between 10(-5) and 10(-1) mmol/L to solutions of β-casein (BCS) and β-lactoglobulin (BLG) at a fixed concentration of 10(-5) mmol/L on the dilational surface rheology is studied. A maximum in the viscoelasticity modulus |E| occurs at very low surfactant concentrations (10(-4) to 10(-3) mmol/L) for mixtures of BCS with C12DMPO and C14DMPO and for mixtures of BLG with C10EO5, while for mixture of BCS with C10EO5 the value of |E| only slightly increased. The |E| values calculated with a recently developed model, which assumes changes in the interfacial molar area of the protein molecules due to the interaction with the surfactants, are in satisfactory agreement with experimental data. A linear dependence exists between the ratio of the maximum modulus for the mixture to the modulus of the single protein solution and the coefficient reflecting the influence of the surfactants on the adsorption activity of the protein.

The influence of the addition of the nonionic surfactants dodecyl dimethyl phosphine oxide (C12DMPO), tetradecyl dimethyl phosphine oxide (C14DMPO), decyl alcohol (C10OH), and C10EO5 at concentrations between 10(-5) and 10(-1) mmol/L to solutions of β-casein (BCS) and β-lactoglobulin (BLG) at a fixed concentration of 10(-5) mmol/L on the surface tension is studied. It is shown that a significant decrease of the water/air surface tension occurs for all the surfactants studied at very low concentrations (10(-5)-10(-3) mmol/L). All measurements were performed with the buoyant bubble profile method. The dynamics of the surface tension was simulated using the Fick and Ward-Tordai equations. The calculation results agree well with the experimental data, indicating that the equilibration times in the system studied do not exceed 30 000 s, while the time required to attain the equilibrium on a plane surface is by one order of magnitude higher. To achieve agreement between theory and experiment for the mixtures, a supposition was made about the influence of the concentration of nonionic surfactant on the adsorption activity of the protein. The adsorption isotherm equation of the protein was modified accordingly, and this corrected model agrees well with all experimental data.

Understanding the effects of digestion conditions on the structure of interfacial protein networks is important in order to rationally design food emulsions which can moderate lipid digestion. This study compares the effect of gastric conditions (pH, temperature, and ionic strength) on β-lactoglobulin films at different fluid interfaces: air-water, tetradecane-water, and olive oil-water. The experiments have been designed to simulate the passage into the stomach media. Hence, preformed interfacial protein (β-lactoglobulin) networks have been exposed to gastric conditions in order to establish generic aspects of the digestion process. The results show that the presence of an oil phase affects both the unfolding of the protein at the interface on adsorption and the subsequent interprotein associations responsible for network formation at the interface. Furthermore, the effects of the physiological conditions characteristic of the stomach also altered differently the preformed protein layer at different fluid interfaces. Initially, the effects of temperature, acid pH, and ionic strength on the dilatational modulus of β-lactoglobulin adsorbed layers at tetradecane-water and olive oil-water interfaces were studied in isolation. The presence of salt was found to have a major effect on the dilatational response at the oil-water interface in contrast to the observations at the air-water interface: it enhanced intermolecular association, hence increasing the packing at the interface causing it to become more elastic. Exposure to acid pH (2.5) also increased the elasticity of the interface, possibly due to the fact that strong electrostatic interactions acting at the interface compensated for the reduced level of intermolecular association. However, the increase in dilatational modulus at the oil-water interface was less noticeable upon exposure to combined changes in acid pH and ionic strength, as would occur in the stomach. This is consistent with previously reported observations at the air-water interface. The quantitative differences in the response of the protein networks to gastric media at different fluid interfaces are discussed in terms of the conformation of β-lactoglobulin within the networks formed at each interface based on detailed theoretical modeling of adsorption data.

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.

The general theoretical model by Garrett and Joos proposed in 1976 for the estimation of the dilational elasticity of mixed surfactant solutions, and also the theoretical model proposed by Joos for the limiting elasticity of such mixtures, demonstrate quite satisfactory agreement with experimental results obtained from the oscillating bubble shape method for mixtures of a nonionic surfactant and a protein, that is, beta-lactoglobuline and decyl dimethyl phosphine oxide, C10DMPO.