Competitive adsorption of fibrinogen and dipalmitoylphosphatidylcholine at the air/aqueous interface

The competitive adsorption of fibrinogen (FB) and DPPC at the air/aqueous interface, in phosphate buffer saline at 25 degrees C, was studied with tensiometry, infrared reflection absorption spectroscopy (IRRAS), and ellipsometry. For FB/DPPC mixtures with 750 ppm (0.075 wt%) FB and 1000 ppm (0.10 wt%) DPPC, the tension behavior was found to be similar to that of FB when alone, even with DPPC and FB being at the interface. Thus, FB interferes with adsorption of DPPC and inhibits its surface tension lowering ability. When FB protein is introduced in the solution after a DPPC monolayer has formed, the adsorption of FB is inhibited by the DPPC monolayer. When a DPPC monolayer is spread onto a solution with a preadsorbed FB layer, the DPPC monolayer excludes FB from the surface and controls the tension behavior with little inhibition by FB. When a DPPC dispersion is introduced with the Trurnit method, or sprayed dropwise, onto an aqueous FB/DPPC surfaces, the DPPC layer formed on the surface prevents the adsorption of FB and dominates the surface tension behavior. These results have implications in controlling the inhibition of lung surfactant tension behavior by serum proteins, when they leak at the alveolar lining layer, and in developing surfactant replacement therapies for alveolar respiratory diseases.     

Expulsion of bovine serum albumin from the air/water interface by a sparingly soluble lecithin lipid

Dynamic surface tensiometry, ellipsometry, and infrared reflection-absorption spectroscopy (IRRAS) were used to study the dynamic adsorption and surface tensions of dilauroylphosphatidylcholine (DLPC) in the presence of bovine serum albumin (BSA). Results show that the equilibrium adsorbed layers consist mostly of DLPC, which can produce dynamic surface tensions (1 mN/m) as low as the more successful lung surfactant replacement formulations. When the aqueous surface expands and contracts sinusoidally, BSA can coadsorb and lead to slightly higher dynamic surface tensions than when DLPC is alone. Similar results were obtained with BSA and sodium myristate [McClellan and Franses, Colloids Surf. B 30 (2003) 1]. Expulsion of the BSA in the layer by DLPC can take from 5 to 15 min, depending on relative concentrations and history of solute addition. This is shown by tensiometry measurements on mixtures, and also by injecting aqueous DLPC underneath adsorbed BSA layers and probing the surface layer with ellipsometry and IRRAS. Albumin layers from buffer solutions aged up to 30 h can be expelled by DLPC. In pure water, there is an initial enhancement in protein adsorption after the DLPC is injected. This can be explained by the hypothesis that DLPC molecules bind with BSA molecules to form a hydrophobic lipoprotein complex, which is more hydrophobic than the protein itself. Since DLPC produces lower surface energy than BSA and--being slightly soluble--adsorbs to the surface by a molecular mechanism, it fulfills the thermodynamic and dynamic requirements for expelling the BSA from the surface. The results have implications for minimizing lung surfactant inhibition by serum proteins, as it occurs in the cases of adult or acute respiratory distress syndrome.     

Equilibrium surface properties of lipid mixtures of retinal,phosphatidylcholine and fatty acid derivatives at the air/water interface

The miscibilities of phosphatidylcholine, retinal and saturated fatty acid derivatives in surface phases at the air/water interface are investigated on the basis of the thermodynamic two-dimensional phase rule. The latter is applied to the ‘collapse’ pressure and the equilibrium surface pressure characteristics of binary lipid monolayers or spread amphiphilic mixtures, respectively. The equilibrium surface pressures (ESPs), at which insoluble lipid monolayers are in equilibrium with three-dimensional lipid phases, are determined by spreading of single-component or binary solutions of lipids in organic solvent up to supersaturation at the air/water interface. The kinetics of establishment of steady surface pressure values at supersaturation is followed depending on the nature of the lipid samples. ESPs and ‘collapse’ pressures of mixtures of dilaur-oylphosphatidylcholine (DLPC), all-trans retinal (t-R) and lauric acid (LA) are studied at various lipid molar ratios. The compositional phase diagrams of the ESPs and ‘collapse’ pressures, obtained at a constant temperature, indicate that the interfacial miscibilities of both DLPC and t-R and DLPC and LA are non-ideal. Owing to its ‘bulky’ molecular structure and the tendency towards self-aggregation, dominated by intermolecular π-π interactions, the t-R component could be accommodated in the hydrophobic portion of the phospholipid membrane at mole fractions less than 0.5. The accommodation of the other neutral, rod-like fatty acid component in the DLPC matrix is probably favoured by the formation of intermolecular hydrogen bonding. Phase separation between DLPC and LA is evident from the thermodynamic results at high LA mole fractions (> 0.75) in the surface mixtures.     

New protocols for preparing dipalmitoylphosphatidylcholine dispersions and controlling surface tension and competitive adsorption with albumin at the air/aqueous interface

The adsorption behavior of dipalmitoylphosphatidylcholine (DPPC), which is the major component of lung surfactant, at the air/aqueous interface and the competitive adsorption with bovine serum albumin (BSA) were studied with tensiometry, infrared reflection absorption spectroscopy (IRRAS), and ellipsometry. Dynamic surface tensions lower than 1 mN/m were observed for DPPC dispersions, with mostly vesicles, prepared with new protocols, involving extensive sonication above 50 °C. The lipid adsorbs faster and more extensively for DPPC dispersions with vesicles than with liposomes. For DPPC dispersions by a certain preparation procedure at T > T_c, when lipid particles were observed on the surface, dynamic surface tensions as low as 1 mN/m were measured. Moreover, IRRAS intensities and ellipsometric δΔ values were found to be much higher than the values for other DPPC dispersions or spread DPPC monolayers, suggesting that a larger amount of liposomes or vesicles adsorb on the surface. For DPPC/BSA mixtures, the tension behavior is controlled primarily by BSA, which prevents the formation of a dense DPPC monolayer. When BSA is injected into the subphase with a spread DPPC monolayer or into a DPPC dispersion with preadsorbed layers, little or no BSA adsorbs and the DPPC layer remains on the surface. When a DPPC monolayer is spread on a BSA solution at 0.1 wt% at 25 °C, then DPPC lipid can displace the adsorbed BSA molecules. The lack of BSA adsorption, and the expulsion of BSA by DPPC monolayer is probably due to the strong hydrophilicity of the lipid polar headgroup. When a DPPC dispersion is introduced with Trurnit's method or when dispersion drops are sprayed onto the surface of a DPPC/BSA mixture, the surface tension becomes lower and is controlled by DPPC, which can prevent the adsorption of BSA. The results may be important in understanding inhibition of lung surfactants by serum proteins and in designing efficient protocols of surfactant preparation and administration.     

Ice growth from supercooled aqueous solutions of benzene, naphthalene, and phenanthrene

Classical molecular dynamics (MD) were performed to investigate the growth of ice from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene. The main objective of this study is to explore the fate of those aromatic molecules after freezing of the supercooled aqueous solutions, i.e., if these molecules become trapped inside the ice lattice or if they are displaced to the QLL or to the interface with air. Ice growth from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene result in the formation of quasi-liquid layers (QLLs) at the air/ice interface that are thicker than those observed when pure supercooled water freezes. Naphthalene and phenanthrene molecules in the supercooled aqueous solutions are displaced to the air/ice interface during the freezing process at both 270 and 260 K; no incorporation of these aromatics into the ice lattice is observed throughout the freezing process. Similar trends were observed during freezing of supercooled aqueous solutions of benzene at 270 K. In contrast, a fraction of the benzene molecules become trapped inside the ice lattice during the freezing process at 260 K, with the rest of the benzene molecules being displaced to the air/ice interface. These results suggest that the size of the aromatic molecule in the supercooled aqueous solution is an important parameter in determining whether these molecules become trapped inside the ice crystals. Finally, we also report potential of mean force (PMF) calculations aimed at studying the adsorption of gas-phase benzene and phenanthrene on atmospheric air/ice interfaces. Our PMF calculations indicate the presence of deep free energy minima for both benzene and phenanthrene at the air/ice interface, with these molecules adopting a flat orientation at the air/ice interface.     

Insertion of tear proteins into a meibomian lipids film

The eyelid meibomian gland secretions form the outer layer of the tear film. That layer functions as a lubricant during a blink, and as a barrier against intrusion of foreign bodies. The lipid film is also exposed to proteins present in the aqueous phase that may adsorb there, and thus form an integral part of the surface of the tear film, or possibly, cause disruption to the outermost layer. Therefore, the adsorption of tear proteins to the meibomian lipid layer was object of the present investigation. A model tear was set up coating a pendant drop of saline with a film of meibomian lipids and measuring variations of the interfacial pressure after the injection of tear proteins into the aqueous subphase at their physiological concentration. All tear proteins adsorbed at the interface causing the initial surface pressure to increase. For each protein, a limiting surface pressure at which a given protein was no longer able to insert into the lipid layer was found. Among the proteins tested, lipocalin was the most surface active one and inserted into the lipid layer in the whole range of surface pressure exerted by the meibomian lipid mixture. Lactoferrin, lysozyme and IgA also interacted with the lipids whereas albumin interacted more weakly. The timescale of the protein insertion into the lipid layer was of the order of 10(2) s. It was hypothesized that protein adsorption at the interface could be associated with structural changes. Indeed, the enzymatic activity of lysozyme was maintained in the presence of an outermost meibomian lipid layer that prevented its denaturation while exposure at the air/aqueous interface induced significant lysozime degradation. meibomian lipid composition is therefore functional to maintain tear proteins activity.     

Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface

The adsorption behaviour of proteins and systems mixed with surfactants of different nature is described. In the absence of surfactants the proteins mainly adsorb in a diffusion controlled manner. Due to lack of quantitative models the experimental results are discussed partly qualitatively. There are different types of interaction between proteins and surfactant molecules. These interactions lead to protein/surfactant complexes the surface activity and conformation of which are different from those of the pure protein. Complexes formed with ionic surfactants via electrostatic interaction have usually a higher surface activity, which becomes evident from the more than additive surface pressure increase. The presence of only small amounts of ionic surfactants can significantly modify the structure of adsorbed proteins. With increasing amounts of ionic surfactants, however, an opposite effect is reached as due to hydrophobic interaction and the complexes become less surface active and can be displaced from the interface due to competitive adsorption. In the presence of non-ionic surfactants the adsorption layer is mainly formed by competitive adsorption between the compounds and the only interaction is of hydrophobic nature. Such complexes are typically less surface active than the pure protein. From a certain surfactant concentration of the interface is covered almost exclusively by the non-ionic surfactant. Mixed layers of proteins and lipids formed by penetration at the water/air or by competitive adsorption at the water/chloroform interface are formed such that at a certain pressure the components start to separate. Using Brewster angle microscopy in penetration experiments of proteins into lipid monolayers this interfacial separation can be visualised. A brief comparison of the protein adsorption at the water/air and water/n-tetradecane shows that the adsorbed amount at the water/oil interface is much stronger and the change in interfacial tension much larger than at the water/air interface. Also some experimental data on the dilational elasticity of proteins at both interfaces measured by a transient relaxation technique are discussed on the basis of the derived thermodynamic model. As a fast developing field of application the use of surface tensiometry and rheometry of mixed protein/surfactant mixed layers is demonstrated as a new tool in the diagnostics of various diseases and for monitoring the progress of therapies.     

Adsorption and direct probing of fibrinogen and sodium myristate at the air/water interface

Fibrinogen (FB), a serum protein, is considered a major inhibitor of lung surfactant function at the lining layer of the alveoli. In this study, the adsorption of aqueous bovine FB at the air/water interface was investigated with tensiometry and directly probed for the first time with ellipsometry and infrared reflection adsorption spectroscopy (IRRAS). The tension results show that FB has moderate surface activity. The surface densities of FB were calculated by using two different ellipsometry models to range from 3±0.2 to 17±2 mg/m², for 7.5 to 750 ppm of FB in water at 25°C. Although FB at concentrations from 75 to 750 ppm reached about the same steady surface tension value, the surface densities at 750 ppm FB were substantially larger. The same techniques were used for studying aqueous mixtures of 7.5 to 750 ppm FB with 2 mM of sodium myristate (SM) to investigate a possible interaction of the SM with the protein. The behavior of the FB/SM mixtures was found to be close to that of SM alone. The surface tension of the FB/SM mixtures reached values less than 10 mN/m under surface area oscillation at 20 or 80 rpm. These results and the ellipsometry and the IRRAS results indicate that at a concentration of 2 mM SM, FB, up to 750 ppm, does not inhibit the surfactant surface-tension-lowering function. In certain cases the results demonstrate that FB and SM may act cooperatively in lowering the surface tension.     

Adsorption of CETP on monolayers formed from HDL extracted lipids

To obtain information on the interactions between CETP and HDL3 lipoproteins, we have studied (by surface tension measurements) the adsorption of the CETP at the air–water interface and at the interface between the water and monolayers formed by spreading of lipids extracted from HDL3. We have compared the interfacial behavior of CETP and ApoA-1 (the constitutive protein of HDL3); and the influence of monolayers composition and pressure on the kinetics of the CETP adsorption. The results obtained show that CETP was more expanded than the ApoA-1 which adsorbed more strongly at the air–water interface. CETP adsorbs more and quickly at the lipid interface that at the air–interface, specially for 20% fraction of cholesterol in the monolayer. Our results show that the adsorption of the CETP at the HDL3 surface lipids are strongly dependent of the composition of the monolayer and that the exclusion pressure of CETP varied from 31 to 33.7 mN m⁻¹ with the addition of cholesterol. Finally, the kinetics of the adsorption at water–lipid interface exhibited two steps (quick increase followed by slow decrease of the excess surface pressure) which should indicate a penetration into monolayer followed by a partial desorption of phospholipids with or without cholesterol corresponding to a proteolipid association.     

Adsorption of amyloid beta (1-40) peptide to phosphatidylethanolamine monolayers.

The aggregation of soluble, nontoxic amyloid beta (Abeta) peptide to beta-sheet containing fibrils is assumed to be a major step in the development of Alzheimer's disease. Interactions of Abeta with neuronal membranes could play a key role in the pathogenesis of the disease. Herein, we study the adsorption of synthetic Abeta peptide to DPPE and DMPE monolayers (dipalmitoyl- and dimyristoylphosphatidylethanolamine). Both lipids exhibit a condensed monolayer state at 20 degrees C and form a similar lattice. However, at low packing densities (at large area per molecule), the length of the acyl chains determines the phase behavior, therefore DPPE is fully condensed whereas DMPE exhibits a liquid-expanded state with a phase transition at approximately 5-6 mNm(-1). Adsorption of Abeta to DPPE and DMPE monolayers at low surface pressure leads to an increase of the surface pressure to approximately 17 mNm(-1). The same was observed during adsorption of the peptide to a pure air-water interface. Grazing incidence X-ray diffraction (GIXD) experiments show no influence of Abeta on the lipid structure. The adsorption kinetics of Abeta to a DMPE monolayer followed by IRRAS (infrared reflection absorption spectroscopy) reveals the phase transition of DMPE molecules from liquid-expanded to condensed states at the same surface pressure as for DMPE on pure water. These facts indicate no specific interactions of the peptide with either lipid. In addition, no adsorption or penetration of the peptide into the lipid monolayers was observed at surface pressures above 30 mNm(-1). IRRAS allows the measurement of the conformation and orientation of the peptide adsorbed to the air-water interface and to a lipid monolayer. In both cases, with lipids at surface pressures below 20 mNm(-1) and at the air-water interface, adsorbed Abeta has a beta-sheet conformation and these beta-sheets are oriented parallel to the interface.     

Stability and state of aggregation of aqueous fibrinogen and dipalmitoylphosphatidylcholine lipid vesicles

The stability and state of aggregation of aqueous fibrinogen (FB) and dipalmitoylphosphatidylcholine (DPPC) vesicles in water or buffer at 25 degrees C were studied with dynamic light scattering (DLS), UV-vis spectroturbidimetry (ST), and cryo-transmission electron microscopy (cryo-TEM). In water, when 1000 ppm (0.10 wt %) DPPC dispersions were prepared with a protocol including extensive sonication, they contained mostly vesicles and were quite clear, transparent, and stable for at least 30 days. FB mixtures with water (0.075 wt %) were quite unstable and biphasic. They formed large aggregates which eventually precipitated. The addition of DPPC vesicles into these unstable FB dispersions reversed FB aggregation and precipitation and produced stable translucent microdispersions. The inferred lipid/protein aggregates were limited in size, with average diameters ranging from 200 to 300 nm. In buffer, DPPC dispersions were also clear and quite stable, with average dispersed particles diameter of ca. 90 nm. FB dissolved in aqueous buffer and formed transparent and stable solutions. Adding salt to an aggregated FB dispersion in water reversed the aggregation. FB aggregated and redissolved in the presence of the citrate and after the citrate was removed. There was no effect of citrate (present in FB initially) in the FB aggregation or redissolution. FB molecules in buffer form dimers or higher aggregates. Their average aggregation number is 2, determined with Rayleigh scattering analysis of turbidity data. The average hydrodynamic diameter of FB solutions from DLS was 30 nm. Mixing a stable FB solution in buffer and a stable DPPC dispersion in buffer produced highly unstable mixtures, in which large aggregates precipitated. These results have implications in understanding the interactions of lipids and proteins in many biological applications and food processing applications.     

Heterogeneous molecular distribution in supported multicomponent lipid bilayers

Membrane domains contribute important structural and functional attributes to biological membranes. We describe the heterogeneous nanoscale distribution of lipid molecules within microscale membrane domains in multicomponent lipid bilayers composed of dipalmitoylphosphatidylcholine (DPPC), dilauroylphosphatidylcholine (DLPC), and cholesterol (chol). The lipids were labeled with the fluorescent lipid analogues Bodipy-PC and DiI-C20:0 to identify the distribution of individual membrane components. We used a near-field scanning optical microscope (NSOM) at room temperature to identify the nanoscale structures in the membrane. Simultaneous multicolor NSOM imaging at the emission maxima of the fluorescent analogues revealed a patchy distribution of Bodipy-PC and DiI-C20:0 indicative of phase separations in the bilayer. In a cholesterol-free system (DPPC/DLPC = 1:1), NSOM images proved that the two phosphatidylcholine molecules can coexist in domains at the micrometer level but form nanoscopic patches within the domains; DPPC occurs at the edge of the domains, whereas DLPC is present throughout the domains. In the presence of cholesterol (DPPC/DLPC = 7:3, chol = 18.9%), the two lipid molecules were more miscible but incomplete phase separations also occurred. The average domain sizes were 140-200 nm, well below the resolution capabilities of diffraction-limited light microscopy techniques; the domains were unresolvable by confocal microscopy. Our high-resolution NSOM studies of membrane domain behavior provide a better understanding of complex membrane phase phenomena in multicomponent biological membranes.     

Effect of sucrose on the properties of caffeine adsorption layers at the air/solution interface

Sweet and bitter tastes are known to be mediated by G-protein-coupled receptors. The relationship between the chemical structure of gustable molecules and their molecular organization in saliva (aqueous solution) near the surface of the tongue provides a useful tool for elucidating the mechanism of chemoreception. The interactions between stimulus and membrane receptors occur in an anisotropic system. They might be influenced by the molecular packing of gustable molecules within an aqueous solvent (saliva) close to the receptor protein. To investigate the molecular organization of a sweet molecule (sucrose), a bitter molecule (caffeine), and their mixture in an aqueous phase near a "wall", a hydrophobic phase, we modeled this using an air/liquid interface as an anisotropic system. The experimental (tensiometry and ellipsometry) data unambiguously show that caffeine molecules form an adsorption layer, whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine molecules at the air/water interface gradually increases with the volume concentration and is delayed when sucrose is added to the solution. Spectroscopic ellipsometry data show that caffeine in the adsorption layer has optical properties practically identical to those of the molecule in solution. The results are interpreted in terms of molecular association of caffeine with itself at the interface with and without sucrose in the subphase, using the theory of ideal gases.     

Ultrafast vibrational dynamics of interfacial water

We report investigations of the vibrational dynamics of water molecules at the water–air and at the water–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the O–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the water–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems.     

Synergism in the spreading of hydrocarbon-chain surfactants on polyethylene film-anionic and cationic mixtures by a two-step procedure

A study has been made of the adsorption, interaction, and spreading of mixtures of anionic and cationic surfactants at the aqueous solution/polyethylene (PE) interface. When a drop of an aqueous solution of an anionic or cationic hydrocarbon-chain surfactant is placed on a highly hydrophobic PE film (contact angle of water > 90 degrees ), it spreads to an area very little larger than that of a drop of water of the same volume. If the anionic and cationic hydrocarbon-chain surfactant solutions are mixed prior to being applied to PE film, synergism is small, if any, and the reproducibility of the experimental results is poor. However, when the cationic and anionic aqueous solutions are applied on the PE film in a sequential manner, a remarkable synergism in spreading is observed and the results are very reproducible. The area spread by an aqueous solution of the anionic-cationic mixture may be more than 400 times that of aqueous solutions of the same volume and surfactant concentration of the individual surfactant components. Previous work in this laboratory on surfactant systems showing synergism in spreading on PE film, but only weak interaction at the aqueous solution/air interface, showed that the synergy was due to changes at the aqueous solution/PE interface and not to the changes at the aqueous solution/air or PE/air interface. Investigation of the adsorption behavior at the aqueous solution/solid interface of two of the anionic-cationic mixtures studied here indicates the reason for differences in spreading behavior observed with different anionic-cationic mixtures. The more similar the adsorption tendencies at the solid/aqueous solution interface of the anionic and cationic surfactants, and the closer their adsorption to an equimolar monolayer there, the stronger their interaction there and the greater their enhancement of the spreading. A mechanism is proposed for the synergy in spreading observed, based upon the difference between the surface tension in the precursor film at the spreading interface and that at the top of the spreading drop.     

Roles of gamma-Globulin in the Dynamic Interfacial Behavior of Mixed Dipalmitoyl Phosphatidylcholine/gamma-Globulin Monolayers at Air/Liquid Interfaces

This study investigated the roles of gamma-globulin in the dynamic interfacial behavior of dipalmitoyl phosphatidylcholine (DPPC)/gamma-globulin monolayers at air/liquid interfaces at 25 degrees C. The surface tension behavior demonstrated that gamma-globulin had a large adsorption time scale. Moreover, the surface pressure-area hysteresis behavior of adsorbed gamma-globulin monolayers suggested that no significant desorption occurred during the compression stage, and the respreading of gamma-globulin molecules at the interface during the expansion stage was slow. From the hysteresis behavior of adsorbed gamma-globulin monolayers with spread DPPC molecules, it was found that gamma-globulin molecules were expelled from the interface as DPPC molecules were in a condensed state. The squeeze-out of gamma-globulin molecules seemed to induce the loss of DPPC molecules at the interface with the extent depending on the initial gamma-globulin surface concentration. Furthermore, the expelled gamma-globulin molecules re-entered the monolayer and participated in the surface pressure increase during the following expansion stage. The exclusion of gamma-globulin associated with the removal of DPPC during monolayer compression and the re-entry of gamma-globulin during subsequent monolayer expansion represented a mechanism for DPPC depletion and gamma-globulin enrichment at the interface, which may explain the inhibitory effect of certain proteins on the surface activity of DPPC. Copyright 2000 Academic Press.     

Foam formation and stabilisation by pre-denatured ovalbumin

Various mild heat-treatments of ovalbumin solutions were applied to produce molecular species with different conformational states, and having different kinetics of adsorption to the air/water interface and different foaming properties. Molecular species with a higher degree of shear-induced deformation and a low degree of thermal conformational stability showed a slight enhancement of the rate of decrease of surface tension, 5 min after the creation of the fresh interface, and decreasing long-term values of surface tension. Solutions of ovalbumin molecular species exhibiting such initial structural patterns were shown to have enhanced foam capacity and stability against liquid drainage. Ovalbumin molecules with some degree of secondary and tertiary structural changes and increased viscosity, before adsorption at the air/water interface, were shown to be relevant to produce more or less hydrated foams with more or less stability against liquid drainage.     

Heat-stabilized glycosphingolipid films for biosensing applications

We have investigated a means of producing thin, oriented lipid monolayers which are stable under repeated washing and which may be useful in biosensing or surface-coating applications. Phosphatidylcholine and the glycosphingolipid GM1 were used as representative lipids for this work. Initially, a mixed self-assembled monolayer of octanethiol and hexadecanethiol was produced on a gold surface. This hydrophobic monolayer was then brought into contact with a thin lipid film that had been assembled at the liquid/air interface of a solution, allowing the lipid to deposit on the gold surface through hydrophobic interactions. The lipid layer was then heated to cause intermingling of the fatty acid and alkanethiol chains and cooled to form a highly stable film which withstood repeated rinsing and solution exposure. Presence and stability of the film were confirmed via ellipsometry, Fourier transform infrared spectroscopy, and quartz crystal microbalance (QCM), with an average overall film thickness of approximately 3.5 nm. This method was then utilized to produce GM1 layers on gold-coated QCM crystals for affinity sensing trials with cholera toxin. For these sensing elements, the lower detection limit of cholera toxin was found to be approximately 0.5 microg/mL, with a logarithmic relationship between toxin concentration and frequency response spanning over several orders of magnitude. Potential sites for nonspecific adsorption were blocked using serum albumin without sacrificing toxin specificity.     

Adsorption and Aggregation Properties of Some Polysorbates at Different Temperatures

Measurements of the surface tension of aqueous solutions of polysorbates (Tween 20, Tween 60 and Tween 80) at 293, 303 and 313 K were made. On the basis of the obtained results the Gibbs surface excess concentration of the Tweens at the water–air interface and critical micelle concentrations were determined. Knowing the Gibbs surface excess concentration and taking into account the difference between the limiting area occupied by water and Tween molecules at the water–air interface, the fraction occupied by Tween molecules was established. The limiting area occupied by the Tween molecule was calculated by applying the Joos equation. The area determined in such a way was confirmed by the calculations of cross section of Tween molecules based on the bond lengths and the angles between them as well as the average distance between the molecules, taking into account their different conformations. This area was used for calculation of the standard Gibbs energy of adsorption using the Langmuir equation. The standard Gibbs energy of Tweens adsorption at the water–air interface was also calculated from the hydrophobic part of Tween molecule–water interface tension and that of hydrophobic part. Using the determined values of standard Gibbs energy of adsorption at different temperatures, the standard enthalpy and entropy values were deduced. The standard thermodynamic functions of micellization were also determined and compared to the Gibbs energy of Tween molecules interactions through the water phase.     

Preparation of microscopic and planar oil-water interfaces that are decorated with prescribed densities of insoluble amphiphiles

Langmuir monolayers (monolayers of insoluble molecules formed at the surface of water), and associated Langmuir-Blodgett/Schaefer monolayers prepared by transfer of Langmuir films to the surfaces of solids, are widely used in studies aimed at understanding the physicochemical properties of biological and synthetic molecules at interfaces. In this article, we report a general and facile procedure that permits transfer of Langmuir monolayers from the surface of water onto microscopic and planar interfaces between oil and aqueous phases. In these experiments, a metallic grid supported on a hydrophobic solid is used to form oil films with thicknesses of 20 mum and interfacial areas of 280 mum x 280 mum. Passage of the supported oil films through a Langmuir monolayer is shown to lead to quantitative transfer of insoluble amphiphiles onto the oil-water interfaces. The amphiphile-decorated oil-water interfaces hosted within the metallic grids (i) are approximately planar, (ii) are sufficiently robust mechanically so as to permit further characterization of the interfaces outside of the Langmuir trough, (iii) can be prepared with prescribed and well-defined densities of amphiphiles, and (iv) require only approximately 200 nL of oil to prepare. The utility of this method is illustrated for the case of the liquid crystalline oil 4-pentyl-4'-cyanobiphenyl (5CB). Transfer of monolayers of either dilauroyl- or dipalmitoylphosphatidylcholine (DLPC and DPPC, respectively) to the nematic 5CB-aqueous interface is demonstrated by epifluorescence imaging of fluorescently labeled lipid and polarized light imaging of the orientational order within the thin film of nematic 5CB. Interfaces prepared in this manner are used to reveal key differences between the density-dependent phase properties of DLPC and DPPC monolayers formed at air-water as compared to that of nematic 5CB-aqueous interfaces. The methodology described in this article should be broadly useful in advancing studies of the interfacial behavior of synthetic and biological molecules at liquid-liquid interfaces.