Interfacial rheology of natural silk fibroin at air/water and oil/water interfaces

The interfacial viscoelastic behavior of natural silk fibroin at both the air/water and oil/water interfaces is reported. This natural multiblock copolymer is found to be strongly amphiphilic and forms stable films at these interfaces. The result is an interfacial layer that is rheologically complex with strong surface elastic moduli that are only slightly frequency-dependent. The kinetics of surface viscoelastic evolution are reported as functions of time for various concentrations of the spread films. Films deposited by Langmuir-Blodgett deposition were studied by scanning electron microscopy (SEM) to reveal a fibrous structure at the interface. The production of stable O/W emulsions by silk fibroin further confirms the generation of the elastic films at the oil/water interfaces.     

Molecular dynamics study of surfactant monolayers adsorbed at the oil/water and air/water interfaces

Atomistic molecular dynamics (MD) simulations have been carried out to investigate the physical properties of monolayers of monododecyl diethylene glycol (C(12)E(2)) surfactants adsorbed at the oil/water and air/water interfaces. The study shows that the surfactant molecules exhibit more extended conformations with a consequent increase of the thickness of the monolayer in the presence of the oil medium. It is noticed that the hydrocarbon tails of the surfactants are more vertically oriented at the oil/water interface. Interestingly, we notice that the presence of the oil medium has a strong influence in restricting both the translational and reorientational motions of the water molecules present in the hydration layer close to the surfactant headgroups.     

Computer simulations of catanionic surfactants adsorbed at air/water interfaces

Structural properties pertaining to the solvation of mixtures of dodecytrimethylammonium/dodecylsulfate adsorbed at water/air interfaces were studied using molecular dynamics techniques. Two different surfactant coverages, both in the submonolayer regime, were considered: an infinite-diluted catanionic pair and an equimolar mixture, at a surface concentration of 78.7 A2/headgroup. The most stable solvated structures for the single surfactant pair correspond to contact-head-ion-pairs (CHIP) at a distance close to 5 A. In addition, marginally stable solvent-separated-head-ion-pairs (SSHIP) at distances approximately 7 A were also observed. The mean free energy for the dissociation of CHIP was estimated to be approximately 1 kcal/mol. At finite surfactant concentrations, one observes a considerable degree of clustering between the amphiphiles, due to the strong Coulomb coupling between headgroups. The resulting spatial domains show asymmetric structures with linear dimensions comparable to the simulation box, suggesting the onset of percolative structures. The connectivity pattern of these domains was interpreted in terms of a simplified model consisting of two-dimensional charged Lennard-Jones spheres.     

Steric stabilization of polyvinyl alcohol adsorbed on silica/water and water/oil interfaces

The steric hindrance of various polyvinyl alcohol samples was investigated by measuring the force-distance curves between crossed quartz filaments with adsorbed layers of PVA. Results were obtained by changing the concentration of macromolecules, the average macromolecular weight, the electrolyte concentration, the kind of electrolytes, the temperature and by adding organic solvents. The results were compared with calculations using the Hesselink-Vrij-Overbeek Theory.It could be shown that only a small amount of “longer” tails is responsible for the stabilization but the segment distribution functions are different for different samples.     

Computer simulations of catanionic surfactants adsorbed at air/water interfaces. II. Full coverage

We extend our previous molecular dynamics experiments [Rodriguez et al., J. Phys. Chem. B 109, 24427 (2005)] to the analysis of the adsorption of catanionic surfactants at water/air interfaces, at a surfactant coverage close to that of the saturated monolayer: 30.3 A(2) per headgroup. The mixture of surfactants investigated corresponds to equal amounts of dodecytrimethylammonium (DTA) and dodecylsulfate (DS). The structure of the interface is analyzed in terms of the local densities and orientational correlations of all relevant interfacial species. In accordance with experimental evidence, the DTA headgroups penetrate deeper into the aqueous substrate than the DS ones, although the average positions of all headgroups, with respect to the interface, lie in positions somewhat more external than the ones observed at lower coverages. Average tail tilts are close to 45 degrees. The characteristics of the headgroup-water substrate correlations are also analyzed using a tessellation procedure of the interface. The density and polarization responses of the interfacial domains closest to the DS headgroups are enhanced, compared to those adjacent to the DTA detergents. Dynamical aspects related to the diffusion and to the orientational correlations of different water layers in close contact with the surfactant are also investigated.     

The adsorbed conformation of globular proteins at the air/water interface

External reflection FTIR spectroscopy and surface pressure measurements were used to compare conformational changes in the adsorbed structures of three globular proteins at the air/water interface. Of the three proteins studied, lysozyme, bovine serum albumin and beta-lactoglobulin, lysozyme was unique in its behaviour. Lysozyme adsorption was slow, taking approximately 2.5 h to reach a surface pressure plateau (from a 0.07 mM solution), and led to significant structural change. The FTIR spectra revealed that lysozyme formed a highly networked adsorbed layer of unfolded protein with high antiparallel beta-sheet content and that these changes occurred rapidly (within 10 min). This non-native secondary structure is analogous to that of a 3D heat-set protein gel, suggesting that the adsorbed protein formed a highly networked interfacial layer. Albumin and beta-lactoglobulin adsorbed rapidly (reaching a plateau within 10 min) and with little change to their native secondary structure.     

Adsorption and surface rheology of n-dodecanol at the water/air interface

Adsorption layers of n-dodecanol at the water/air interface show phase transitions at low temperatures [Vollhardt, Fainerman, Emrich, J. Phys. Chem. B 104 (2000) 8536]. Using a drop shape technique it is shown that the dilational elasticity disappears in the coexistence region of the adsorption layer. The relaxation time between the condensed and liquid-like surface states is in the sub-second time range.     

Amphiphilic derivatives of dextran: adsorption at air/water and oil/water interfaces

Ionic amphiphilic dextran derivatives were synthesized by the attachment of sodium sulfopropyl and phenoxy groups on the native polysaccharide. A family of dextran derivatives was thus obtained with varying hydrophobic content and charge density in the polymer chains. The surface-active properties of polymers were studied at the air-water and dodecane-water interfaces using dynamic surface/interfacial tension measurements. The adsorption was shown to begin in a diffusion-limited regime at low polymer concentrations, that is to say, with the diffusion of macromolecules in the bulk solution. In contrast, at long times the interfacial adsorption is limited by interfacial phenomena: adsorption kinetics or transfer into the adsorbed layer. A semiempirical equation developed by Filippov was shown to correctly fit the experimental curves over the whole time range. The presence of ionic groups in the chains strongly lowers the adsorption kinetics. This effect can be interpreted by electrostatic interactions between the free molecules and the already adsorbed ones. The adsorption kinetics at air-water and oil-water interfaces are compared.     

Dilational rheology of protein films adsorbed at fluid interfaces

This report aims at (i) presenting a quantitative interpretation of interfacial dilational moduli (|E|) for four proteins at three different interfaces and (ii) identifying the main parameters responsible. The proteins were adsorbed from aqueous solution against air, n-tetradecane and sunflower seed oil, as a function of protein concentration and adsorption time.Experimentally, a dynamic drop tensiometer is a convenient instrument to generate the required sinusoidal oscillations for compression/expansion of interfaces (Benjamins et al., 1996 [1]).Theoretically, a simple two-dimensional solution model with a constant molecular area of the protein described the data only at fairly low pressures. Much better agreement over the entire elastic range was found with a recent extension of the model. This extension accounted for adsorbed proteins adopting smaller molecular areas with increasing surface pressure.Three factors dominated the values of the dilational modulus: (i) rigidity of protein molecules, (ii) degree of interfacial non-ideality and (iii) tension of the clean interface (Benjamins et al., 2006 [2]). The last factor is clearly of great relevance to food emulsions.For each protein at different interfaces, the elasticity increased with the enthalpy parameter (Η^S) of the equation of state. Elasticity and Η^S both increased with the clean-interface tension, γ⁰, i.e., with decreasing polarity of the interface (Benjamins et al., 2006 [2]; Fainerman et al., 2003 [3]). The elasticity of the different proteins also increased with increasing rigidity of the molecules, indicating a lower compressibility of the molecular area at the interface.Pure viscosities were never observed in our experience. However, viscoelastic behaviour was found at high pressures, i.e., in densely packed surfaces. The measured viscous phase angles strongly decreased at still higher pressures, indicating that the active relaxation mechanism slowed down with increasing molecular packing density. Specific kinetic models are yet to be developed for such mechanisms.     

Dilatational rheology of beta-casein adsorbed layers at liquid-fluid interfaces

The rheological behavior of beta-casein adsorption layers formed at the air-water and tetradecane-water interfaces is studied in detail by means of pendant drop tensiometry. First, its adsorption behavior is briefly summarized at both interfaces, experimentally and also theoretically. Subsequently, the experimental dilatational results obtained for a wide range of frequencies are presented for both interfaces. An interesting dependence with the oscillation frequency is observed via the comparative analysis of the interfacial elasticity (storage part) and the interfacial viscosity (loss part) for the two interfaces. The analysis of the interfacial elasticities provides information on the conformational transitions undergone by the protein upon adsorption at both interfaces. The air-water interface shows a complex behavior in which two maxima merge into one as the frequency increases, whereas only a single maximum is found at the tetradecane interface within the range of frequencies studied. This is interpreted in terms of a decisive interaction between the oil and the protein molecules. Furthermore, the analysis of the interfacial viscosities provides information on the relaxation processes occurring at both interfaces. Similarly, substantial differences arise between the gaseous and liquid interfaces and various possible relaxation mechanisms are discussed. Finally, the experimental elasticities obtained for frequencies higher than 0.1 Hz are further analyzed on the basis of a thermodynamic model. Accordingly, the nature of the conformational transition given by the maximum at these frequencies is discussed in terms of different theoretical considerations. The formation of a protein bilayer at the interface or the limited compressibility of the protein in the adsorbed state are regarded as possible explanations of the maximum.     

Spreading of oil from protein stabilised emulsions at air/water interfaces

Spreading of a drop of an emulsion made with milk proteins on air/water interfaces was studied. From an unheated emulsion, all oil molecules could spread onto the air/water interface, indicating that the protein layers around the oil globules in the emulsion droplet were not coherent enough to withstand the forces involved in spreading. Heat treatment (90 °C) of emulsions made with whey protein concentrate (WPC) or skim milk powder reduced the spreadability, probably because polymerisation of whey protein at the oil/water interface increased the coherence of the protein layer. Heat treatment of emulsions made with WPC and monoglycerides did not reduce spreadability, presumably because the presence of the monoglycerides at the oil/water interface prevented a substantial increase of coherence of the protein layer. Heat treatment of caseinate-stabilised emulsions had no effect on the spreadability. If proteins were already present at the air/water interface, oil did not spread if the surface tension (γ) was <60 mN/m. We introduced a new method to measure the rate at which oil molecules spread from the oil globules in the emulsion droplet by monitoring changes in γ at various positions in a ‘trough’. The spreading rates observed for the various systems agree very well with the values predicted by the theory. Spreading from oil globules in a drop of emulsion was faster than spreading from a single oil drop, possibly due to the greater surface tension gradient between the oil globule and the air/water interface or to the increased oil surface area. Heat treatment of an emulsion made with WPC did not affect the spreading rate. The method was not suitable for measuring the spreading rate at interfaces where surface active material is already present, because changes in γ then were caused by compression of the interfacial layer rather than by the spreading oil.     

Structure of mixed beta-lactoglobulin/pectin adsorbed layers at air/water interfaces; a spectroscopy study

Based on earlier reported surface rheological behaviour two factors appeared to be important for the functional behaviour of mixed protein/polysaccharide adsorbed layers at air/water interfaces: (1) protein/polysaccharide mixing ratio and (2) formation history of the layers. In this study complexes of beta-lactoglobulin (positively charged at pH 4.5) and low methoxyl pectin (negatively charged) were formed at two mixing ratios, resulting in negatively charged and nearly neutral complexes. Neutron reflection showed that adsorption of negative complexes leads to more diffuse layers at the air/water interface than adsorption of neutral complexes. Besides (simultaneous) adsorption of protein/polysaccharide complexes, a mixed layer can also be formed by adsorption of (protein/)polysaccharide (complexes) to a pre-formed protein layer (sequential adsorption). Despite similar bulk concentrations, adsorbed layer density profiles of simultaneously and sequentially formed layers were persistently different, as illustrated by neutron reflection analysis. Time resolved fluorescence anisotropy showed that the mobility of protein molecules at an air/water interface is hampered by the presence of pectin. This hampered mobility of protein through a complex layer could account for differences observed in density profiles of simultaneously and sequentially formed layers. These insights substantiated the previously proposed organisations of the different adsorbed layers based on surface rheological data.     

Interfacial properties of mixtures of lecithin with a block copolymer surfactant at the water/air and water/oil interfaces

Surface pressure-area isotherms have been determined for both a pure lecithin (L, -dipalmitoyl phosphatidyl choline) and an impure lecithin (soya bean lecithin) at the water/air and water/oil interfaces. Equations of state have been applied and an equation of Gaines was found to be particularly successful in describing the isotherms. Mixed monolayers with an ABA nonionic block copolymer surfactant (A is poly(12-hydroxystearic) acid and B is poly(ethylene oxide)) were also investigated. The additivity rule was obeyed only at high surface pressures; inefficient packing was observed at low surface pressures. The polymer may promote a horizontal headgroup orientation in the lecithin, which gives rise to this effect. The presence of electrolyte up to very high concentrations in the aqueous phase (8.75 mol dm^–3 NH₄NO₃) was shown to expand the lecithin monolayer.Glossary of symbols W/A Water-air interface - W/O Water-oil interface - E/A Electrolyte-air interface - L-C Liquid-condensed - A _c Area per molecule obtained by conventional extrapolation of the -A isotherm at close-packing - A _e Experimentally determined area per molecule - A _t Theoretically predicted area per molecule - A _v Area per molecule obtained by vertical extrapolation of the -A isotherm at close-packing - A ₀ Head group area term - f _i Activity coefficient of water in surface region - i Constant - x _i Mol fraction of componenti - Z Compressibility factor=A/kT - Interfacial tension - Surface pressure - _i Partial molar area of component i     

The electrocapillary adsorption of surface active agents at oil/water interfaces

Summary The adsorption of surface active agents, and the builder effect, were investigated by using the electro-capillarity at oil/water interfaces. The oil phase was the solution of tetrabutylammonium chloride in methylisobutylketone, and the aqueous phase contained the surface active agent in addition to the inorganic electrolyte. The interfacial tension decreased over the anodic (or cathodic) polarization range, when an anionic (or cationic) surface active agent was added to the aqueous phase, thus indicating the adsorption of this material at the interface. It was found that, in agreement with the theoretical deduction, linear relations held between the interfacial excess of surface active agent and the cubic root of its concentration, as well as of ionic strength, of the aqueous phase. For alkylsulphate anions of various chain lengths, the free energy of desorption was estimated from the above linear relation to be ca. 750 cal mole⁻¹ for each methylene group.

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Zusammenfassung Die Adsorption von grenzfl?chenaktiven Stoffen wurde mit Hilfe der Elektrokapillarit?t an ?l-Wasser-Grenzfl?chen untersucht. Die ?lphase ist eine L?sung Tetrabutylammoniumchlorids in Methylisobutylketon, w?hrend die Wasserphase einen grenzfl?chenaktiven Stoff und einen anorganischen Elektrolyten enth?lt. Die Grenzfl?chenspannung wird durch anodische bzw. kathodische Polarisation erniedrigt, je nachdem, ob der grenzfl?chenaktive Stoff in der Wasserphase anionisch oder kationisch ist. Diese Ergebnisse zeigen eine Adsorption an den Grenzfl?chen an. Im Einklang mit der Theorie tritt eine lineare Beziehung zwischen der Oberfl?chenkonzentration des oberfl?chenaktiven Stoffes und der Kubikwurzel der Konzentration und Ionenst?rke auf. Im Falle der Alkylsulfat-Ionen mit verschiedenen Kettenl?ngen wird die freie Energie der Desorption aus der Linearit?t zu ca. 750 cal per Mol CH₂-Gruppe gesch?tzt.

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The authors' gratitude is due to Mr. S. Ishida, Nippon Yushi Company, for the supply of cationic surface active agents. Thanks are also to the Ministry of Education for the financial support of a part of the present study.     

Dynamics of mixed protein–surfactant layers adsorbed at the water/air and water/oil interface

Drop and bubble shape tensiometry experiments are performed at the water/air and water/hexane interfaces in order to get more information about the differences in the adsorption layer structure of mixed protein/surfactant systems. For mixtures of β-lactoglobulin and sodium dodecyl sulphate the adsorption at the water/air interface is essentially a competitive process between protein/surfactant complexes and free surfactant molecules, while the water/oil interface is essentially covered by the complexes.     

Molecular orientation of monododecyl pentaethylene glycol at water/air and water/oil interfaces. A molecular dynamics computer simulation study

With a molecular dynamics computer simulation we investigated the dynamic properties of a monododecyl pentaethylene glycol (C₁₂E₅) molecule adsorbed at air/water and oil/water interfaces. In these simulations we investigated the molecular orientation of the surfactant molecules in detail. At the air/water interface the maximum of the C₁₂ chain tilt angle distribution measured with respect to the water surface is about 50°. This result is in fairly good agreement with neutron reflection experiments of monododecyl glycol ethers at the air/water interface. At the oil/water interface no significant changes were detected in the molecular orientation. We found that at equilibrium oil molecules penetrate into the hydrophobic monododecyl layer, this was also found by neutron reflection studies of the interactions between C₁₂E₅ and dodecane. The observed oil penetration results in an increase in the surface area per surfactant molecule. Received: 16 July 1999/Accepted in revised form: 28 August 1999     

The standard free energy of surfactant adsorption at air/water and oil/water interfaces: Theoretical vs. empirical approaches

The standard free energy of surfactant adsorption represents the work of transfer of a surfactant molecule from the bulk of solution to an infinitely diluted adsorption layer. This quantity can be determined by non-linear fits of surface-tension isotherms with the help of a theoretical model of adsorption. Here, the models of Frumkin, van der Waals and Helfand-Frisch-Lebowitz are applied, and the results are compared. Irrespective of the differences between these models, they give close values for the standard free energy. The results from the theoretical approach are compared with those from the most popular empirical approach. The latter gives values of the standard free energy, which are considerably different from the respective true values, with c.a. 10 kJ/mol for nonionic surfactants, and with c.a. 20 kJ/mol for ionic surfactants. These differences are due to contributions from interactions between the molecules in dense adsorption layers. It is concluded that the true values of the standard free energy can be determined with the help of an appropriate theoretical model. For the processed sets of data, the van der Waals model gives the best results, especially for the determination of the standard adsorption enthalpy and entropy from the temperature dependence of surface tension. The results can be useful for the development of a unified approach to the thermodynamic characterization of surfactants.     

Molecular dynamics studies of the stability of water/n-heptane interfaces with adsorbed naphthenic acids

The mechanism by which naphthenic acids stabilize water/oil interfaces has received extensive attention because of its industrial relevancy. In this work, we employed a molecular dynamics simulation to study its molecular origin. Two models were adopted, wherein naphthenic acid coverage of water/n-heptane interfaces, both spherical and flat, was hypothesized, respectively. It was found that the coalescence of two water clusters is entirely attributed to the diffusional motion of the components involved which requires the initial departure of the naphthenic acid molecules from the interface so that a water bridge can form. The naphthenic acids not only act as a steric barrier but also reduce the mobility of the water and n-heptane molecules making the formation of the water bridge rather difficult. In fact, our results show that the coalescence of two water clusters fully covered by naphthenic acid molecules is a low-probability event even at evaluated temperatures. In addition, the results from the flat interface models suggest that the emulsion stability is weakly dependent on the molecular weight of the naphthenic acids utilized. Order parameter calculations reveal liquid-crystal-like ordering of naphthenic acids at the water/n-heptane interfaces. All these observations are consistent with the corresponding experimental observations. The present work also suggests that the mobility of naphthenic acids is considerably enhanced with more n-heptane molecules present outside a water droplet. However, in such a case, coalescence could not occur as the water clusters are far apart from each other.     

Bovine serum albumin unfolding at the air/water interface as studied by dilational surface rheology

Measurements of the surface dilational elasticity close to equilibrium did not indicate significant distinctions in the surface conformation of different forms of bovine serum albumin (BSA) in a broad pH range. At the same time, the protein denaturation in the surface layer under the influence of guanidine hydrochloride led to strong changes in the kinetic dependencies of the dynamic surface elasticity if the denaturant concentration exceeded a critical value. It was shown that the BSA unfolding at the solution surface occurred at lower denaturant concentrations than in the bulk phase. In the former case, the unfolding resulted in the formation of loops and tails at surface pressures above 12 mN/m. The maximal values of the dynamic surface elasticity almost coincided with the corresponding data for the recently investigated solutions of β-lactoglobulin, thereby indicating a similar unfolding mechanism.     

Silver nanoplates formed at the air/water and solid/water interfaces

Silver nanoparticles and nanoplates were prepared at the air/AgNO₃ aqueous solution interfaces under poly(9-vinylcarbazole) (PVK) monolayers when illuminated by UV-light at room temperature and elevated temperatures, respectively. When the illuminated films at the air/water interfaces were covered by carbon-coated copper grids, nanoplates were formed even at room temperature, and the size of the nanoplates was much larger than those formed at the air/water interface under the same experimental conditions, indicating that copper took part in the formation of Ag nanoplates through the galvanic displacement reaction between Cu and Ag⁺ ions with the help of carbon layer to conduct electrons. It was found that the basal plane of these nanoplates is the (1 1 1) face of a face-centered cubic (fcc) Ag crystal. Although platelike structure can be formed at the carbon-coated copper grid/AgNO₃ aqueous solution interface without PVK film, it shows different features from those with PVK films, indicating that PVK film plays an important role in the formation of regular large nanoplates. Further observations indicate that special restrained microenvironment, adsorption of PVK molecules on a specific crystal face, anisotropic growth and attachment of the nanoparticles are responsible for the formation of the nanoplates.