Consistency of surface mechanical properties of spread protein layers at the liquid–air interface at different spreading conditions

Π/A isotherms of spread β-lactoglobulin and β-casein at the air–water interface are measured under different spreading conditions. While the isotherms do not show drastic effects of the spreading concentration and the compression rate the interfacial shear rheological behaviour is significantly influenced. In particular, the shear viscosity of β-lactoglobulin layers depend directly on the spreading concentration. Significant viscosity increase is obtained at larger surface pressures when the spreading concentration is increased. In contrast the shear rheology of the spread β-casein layers can be normalised by plotting the viscosities as a function of the surface pressure Π. The different behaviour is discussed in terms of denaturation of the β-lactoglobulin during the monolayer formation process by adsorption from the spread thin protein solution layer.     

Interfacial characteristics of β-casein spread films at the air–water interface

Casein is well known to be a good protein emulsifier and β-casein is the major component of casein and commercial sodium caseinate. This work studies the behaviour of β-casein at the interface. The interfacial characteristics (structure and stability) of β-casein spread films have been examined at the air–water interface in a Langmuir-type film balance, as a function of temperature (5–40°C) and aqueous phase pH (pH 5 and 7). From surface pressure–area isotherms (π–A isotherms) as a function of temperature we can draw a phase diagram. β-Casein spread films present two structures and the collapse phase. That is, there is a critical surface pressure and a surface concentration at which the film properties change significantly. This transition depends on the temperature and the aqueous phase pH. The film structure was observed to be more condensed and β-casein interfacial density was higher at pH 5. β-Casein films were stable at surface pressures lower than equilibrium surface pressure. In fact, no hysteresis was observed in π–A isotherms after continuous compression-expansion cycles or over time. The relative area relaxation at constant surface pressure (10 or 20 mN m⁻¹) and the surface pressure relaxation at constant area near the monolayer collapse, can be fitted by two exponential equations. The characteristic relaxation times in β-casein films can be associated with conformation–organization changes, hydrophilic group hydration and/or surface rheology, as a function of pH.     

Stress relaxation behaviour of dipalmitoyl phosphatidylcholine monolayers spread on the surface of a pendant drop

Dipalmitoyl phosphatidylcholine (DPPC) monolayers were characterised by surface pressure/area isotherms (π/A) and surface dilational rheological parameters at temperatures 20–40°C. The methods used were the Langmuir trough and the pendant drop micro-film balance. The latter allows accurate measurements at higher temperatures and transient drop deformation. Stable DPPC monolayers were found only for low surface pressures, π<15 mN m⁻¹. At higher monolayer compression π decreases over a long time, mainly caused by molecular rearrangement processes in the monolayer starting in the coexisting region. At π>25 mN m⁻¹ and 20°C relaxation experiments give evident of rupturing, brittle monolayer structures. At higher temperatures the monolayers became more fluid-like. π/A-isotherms determined by using both methods principally agree with each other, but show also remarkable differences, which cannot be explained so far satisfactory. Transient drop relaxation experiments were analysed for the short time range (600 s). At 20°C the dilational modulus (_r) and the surface dilational viscosity (ξ_r) passes a stationary maximum at 0.54 nm² molecule⁻¹ and increase strongly at higher surface coverage, thus indicating crystalline monolayer structure. Increasing temperature from 20 to 30°C causes a rapid decrease of _r and ξ_r and a shift of the stationary maximum to lower surface coverage. No evidence for crystalline structure is found. Further increase of temperature causes _r and ξ_r increase again. This increase is caused by a rising relaxation time, while the elasticity does not change in the same manner. Such intermediate decrease of _r and ξ_r in the range 30–40°C appears to be unusual and can be interpreted as a consequence of strong DPPC interactions and strongly pronounced retardation of monolayer deformation. The study is discussed in connection to the physiology of breathing. For pulmonary surfactants the observed behaviour seems to be understandable. It is however interesting that such complex behaviour is observed for monolayers consisting of DPPC only.     

Adsorption and desorption kinetics of CmE8 on impulsively expanded or compressed air–water interfaces

The adsorption kinetics of C_mE₈ (m=10, 12, and 14) at an air–water interface are investigated. A pendant bubble is formed in aqueous surfactant solution and allowed to attain equilibrium. The bubble is then impulsively expanded or compressed with some change of area large enough to appreciably deplete or enrich the surface concentration and change the surface tension. The surfactant is then allowed to re-equilibrate. The surface tension evolution during this process is measured using video images of the pendant drop. The surface tension evolution is compared to mass transfer arguments. First, the re-equilibration of interfaces laden with C₁₄E₈ are studied. For compressed interfaces, surfactant must desorb to restore equilibrium. The surface tension rises more slowly than predicted by a diffusion-controlled evolution, implying that the re-equilibration is mixed diffusive-kinetic controlled. By analyzing the surface tension evolution in terms of a mixed kinetic-diffusive model, values for the kinetic constants for adsorption and desorption are found. These results are compared to those obtained previously for C_mE₈ (m=10 and 12). For all of these molecules, the adsorption rate constant is similar (β₁=5.6±1.0×10⁻⁶ cm³ (mol s)⁻¹). However, the desorption rate constant (₁) varies strongly. Increasing m by 2 lowers the desorption rate constant ₁ by nearly a factor of 15. This is consistent with an increased resistance to re-immersion into water with the length of a hydrocarbon chain.     

Dilatational properties and morphology of surface films spread from clinically used lung surfactants

The dilatational properties, structure, and morphology of the surface films spread at the air–water interface from complex lipid/protein systems were studied by measuring the surface pressure–area and surface potential–area isotherms, the surface rheological properties, and AFM images. The commercially available lung surfactants Alveofact, Curosurf, Survanta, and Exosurf were used as examples.The isotherms of the studied lung surfactant surface films are compared with model lipid and protein monolayers spread from bulk solutions. On the basis of a simple rheological model, the values for the elasticity and the specific time of relaxation related to the reorganization processes occurring in the monolayers were calculated. The spread films of natural surfactants Curosurf and Alveofact show a high effectiveness of spreading and respreading under the conditions of this study. These observations were confirmed by AFM imaging.     

A method for correcting the contact angle from the θ/2 method

The θ/2 method, a widely used technique on measuring the contact angle of a sessile drop, assumes that the drop profile is part of a sphere. However, the shape profile of a sessile drop is governed by the Young–Laplace equation and is different from a sphere, especially for drops with a large bound number (e.g. large volume or small surface tension). The spherical assumption, therefore, causes errors on evaluating the contact angles. The deviation of contact angle from the θ/2 method is evaluated from a theoretical calculation in this work. A simple means is given for correcting the measurement error. The corrected angle results from the drop volume, surface tension, liquid density and the contact angle from θ/2 method. An algorithm for finding the correct contact angle without knowing the density and surface tension is also given. At the end, two examples of pendant drops are given for the illustration.     

Difference in enzyme activity and conformation of glucose oxidase before and after purification

Enzyme activity of commercial glucose oxidase was enhanced after purification through a strong anionic exchange resin. In order to get a better insight into this phenomenon, surface pressure–area (π–A) isotherms and surface pressure–time (π–t) isotherms was used to study the interaction and the absorption at different pH values of the subphases between octadecylamine and glucose oxidase purified by a styrene system quaternary ammonium type strongly basic anionic exchange resin. Circular dichroism (CD), electrophoresis and enzyme activity measurements were conducted to study these phenomena. A preliminary hypothesis has been suggested to explain why the enzyme activity of purified glucose oxidase was higher than that of the commercial one.     

Effect of film ageing on the surface properties of lactoglobulin and lactoglobulin+sucrose stearate monolayers

The change in β-lactoglobulin and β-lactoglobulin+sucrose stearate spread monolayers over time (ageing effect) at room temperature was studied. Measurements of the surface pressure π-mean area per molecule A isotherms of the monolayers were used to probe any time dependent changes. The increase of the protein film area with time was attributed to protein unfolding. At certain ratios of surfactant to protein, a higher film area increase was observed. This correlated well with the excess film areas of 45 h aged films. The excess film areas for newly spread films of the same composition seemed to follow the opposite trend indicating that the protein–surfactant interactions change as the conformational changes of the protein molecules progress.     

Surface activity and thermodynamic of micellization and adsorption for isooctylphenol ethoxylates, phosphate esters and their mixtures with N-diethoxylated perfluorooctanamide

Three nonionic surfactants; p-isooctylphenol ethoxylates p-[i-OPE10], p-[i-OPE15], and p-[i-OPE20], were phosphorylated to produce three anionic phosphate ester surfactants. In addition, N-diethoxylated perfluorooctanamide (N-DEFOA) was also prepared. The surface and thermodynamic properties of the three types of surfactants and mixtures of the fluorocarbon surfactant (FC) with the hydrocarbon surfactants (HC) have been investigated. Surface tension as a function of concentration of the surfactant in aqueous solution was measured at 30, 40, 50 and 60°C, using the spinning drop technique. From these measurements the critical micelle concentration (CMC), the surface tension at the CMC (γ_CMC), the maximum surface excess concentration (Γ_max), the minimum area per molecule at the aqueous solution/air interface (A_min), and the effectiveness of surface tension reduction (π_CMC), were calculated. The thermodynamic parameters of micellization (ΔG_mic, ΔH_mic, ΔS_mic) and of adsorption (ΔG_ad, ΔH_ad, ΔS_ad) for these surfactants and their mixtures were also calculated. Structural effects on micellization, adsorption and effectiveness of surface tension reduction are discussed in terms of these parameters. The results show that the FC surfactant and its mixtures with HC surfactants enhance the efficiency in surface tension reduction and adsorption in the mixed monolayer at the aqueous solution/air interface, and also, reduce γ_CMC and the tendency towards micellization.     

Study of molecular interactions between lipids and proteins using dynamic surface tension measurements: a review

Molecular interactions between small molecules and proteins, such as binding of lipids to proteins, are of fundamental importance in various biological processes. A recently-developed method based on dynamic surface tension measurement is efficient and versatile in detecting such molecular interactions: Axisymmetric Drop Shape Analysis (ADSA) provides a tool for measuring the surface tension (γ) response to surface area changes. Through the analysis of the γ response pattern, surface competitive adsorption between small organic molecules and protein molecules can be detected. Surface squeeze-out of small molecules by proteins can also be observed. Molecular binding of lipids to proteins manifests itself in a modification of the γ response which is not compatible with a simple superposition of the two individual patterns. The specific binding can be studied in terms of dose effects and specificity.     

Evaluation of the interactions between lipids and γ-globulin protein at the air–liquid interface

The interactions between lipids (cholesterol, distearoylphosphatidylcholine, distearoylphosphatidylethanolamine and sphingomyelin) and the γ-globulin protein have been analyzed using the monolayer technique at the air–liquid interface. The analysis has been carried out using both state equations and an adequate thermodynamic formulation for the surface pressure (π)–molecular area (a) isotherms. Different parameters as the virial coefficients, have been estimated. For the uncharged lipid monolayers, the interactions between the molecules are of an attractive nature, at medium and long distance, and of a steric repulsive nature at short distance. At low surface pressures the lipid molecules form small domains. The net force between γ-Globulin molecules in the monolayers has been found to be attractive. Finally, it can be concluded that when the lipid monolayers are uncharged, there is practically no interaction between the protein and lipid molecules at the mentioned interface.     

全氟辛酸铵与全氟壬酸铵混合物的表面活性

测定了全氟辛酸铵和全氟壬酸铵及其不同比例混合物的0.1mol/L氯化铵水溶液表面张力曲线。讨论了它们的胶团化作用、吸附作用和降低水表面张力的能力。改进了Ingram-Luckhurst自单一表面活性剂活性张力曲线得到混合溶液表面张力曲线的方法。     

Langmuir monolayer properties of the fluorinated-hydrogenated hybrid amphiphiles with dipalmitoylphosphatidylcholine (DPPC)

Surface pressure–area (π–A), surface potential–area (ΔV–A), and dipole moment–area (μ–A) isotherms were obtained for the Langmuir monolayer of two fluorinated-hydrogenated hybrid amphiphiles (sodium phenyl 1-[(4-perfluorohexyl)-phenyl]-1-hexylphosphate (F6PH5PPhNa) and (sodium phenyl 1-[(4-perfluorooctyl)-phenyl]-1-hexylphosphate (F8PH5PPhNa)), DPPC and their two-component systems at the air/water interface. Monolayers spread on 0.02 M Tris buffer solution (pH 7.4) with 0.13 M NaCl at 298.2 K were investigated by the Wilhelmy method, ionizing electrode method and fluorescence microscopy. Moreover, the miscibility of two components was examined by plotting the variation of the molecular area and the surface potential as a function of the molar fraction for the fluorinated-hydrogenated hybrid amphiphiles on the basis of the additivity rule. The miscibility of the monlayers was also examined by construction of two-dimensional phase diagrams. Furthermore, assuming the regular surface mixture, the Joos equation for analysis of the collapse pressure of two-component monolayers allowed calculation of the interaction parameter (ξ) and the interaction energy (−Δ) between the fluorinated-hydrogenated hybrid amphiphiles and DPPC. The observations by a fluorescence microscopy also supported our interpretation as for the miscibility in the monolayer state. Comparing the monolayer behavior between the two binary systems, no remarkable difference was found among various aspects. Among the two combinations, the mole fraction dependence in monlayer properties was commonly classified into two ranges: 0 ≤ X ≤ 0.3 and 0.3 < X ≤ 1. Dependence of the chain length of fluorinated part was reflected for the molecular packing and surface potential.     

The melt elastic behavior of polypropylene/glass bead composites in capillary flow

The melt extrudate swell and entry pressure losses are important characteristics of elastic properties during die extrusion of polymeric fluids. They are usually expressed with die-swell ratio (B) and entry pressure drop (ΔP_o). In the present paper, the die-swell behavior and entrance pressure drop of a polypropylene (PP) filled with A-glass beads were investigated by using a Rosand capillary rheometer to identify the effects of the filler contents and extrusion rate on the elastic behavior of the sample melts. The experiments were carried out under the conditions with an apparent shear rate range of 50–10⁴ s⁻¹ and a temperature of 190 °C. The results showed that B increased nonlinearly with increasing shear rate at the wall (γ_w), and increased linearly with the increase of shear stress at the wall (τ_w). With the increase of the volume fraction of the fillers B decreased nonlinearly. Similarly, the entry pressure drop increased linearly with the increase of τ_w, whereas the influence of the filler concentration on ΔP_o was insignificant in this case. Furthermore, B increased as a linear function of ΔP_o, and extension stress (σ_e) increased nonlinearly with increasing γ_w.     

Mimic oil recovery with a SDBS–dodecane–silica gel system

The wettability of the solid powder of silica gel was determined via a modified Washburn equation expressed as contact angles. The interfacial tension (γ) between the dodecane and the dilute sodium dodecyl benzene sulfonate (SDBS) aqueous solution was obtained using the spinning drop (γ<10 mN m⁻¹) or drop volume methods (γ>10 mN m⁻¹). Contact angle changes for SDBS aqueous solutions on the surface of a silica gel powder were studied. The average aggregation number of SDBS micelles in aqueous solution was determined using the fluorescence quenching method. The relationship between the wettability of the powder surface, the critical micelle concentration (CMC) of SDBS and the mimic oil recovery of the resident oil on the powder surface has been explored. It has been found that good residual oil recovery was achieved by surface wettability changes at the interfacial tensions around 4–5 mN m⁻¹, which is far from the ‘ultra low’ condition (≤10⁻³ mN m⁻¹).     

Surface free energy and bacterial retention to saliva-coated dental implant materials--an in vitro study

The aim of the present investigation was to compare the in vitro bacterial retention on saliva-coated implant materials (pure titanium grade 2 (cp-Ti) and a titanium alloy (Ti–6Al–4V) surfaces), presenting similar surface roughness, and to assess the influence of physico-chemical surface properties of bacterial strain and implant materials on in vitro bacterial adherence. Two bacterial strains (one hydrophilic strain and one hydrophobic strain) were used and the following were evaluated: bacterial cell adherence, SFE values as well as the Lifshitz-van-der Waals, the Lewis acid base components of SFE, the interfacial free energy and the non-dispersive interactions according to two complementary contact angle measurement methods: the sessile drop method and the captive bubble method.

Our results showed similar patterns of adherent bacterial cells on saliva-coated cp-Ti and saliva-coated Ti–6Al–4V. These findings could suggest that bacterial colonization (i.e. plaque formation) is similar on saliva-coated cp-Ti and Ti–6Al–4V surfaces and indicate that both materials could be suitable for use as transgingival abutment or healing implant components. The same physico-chemical properties exhibited by saliva-coated cp-Ti and TA6V, as shown by the sessile drop method and the captive bubble method, could explain this similar bacterial colonisation. Therefore, higher values of total surface free energy of saliva-coated cp-Ti and saliva-coated TA6V samples (γ_SV ≈65 mJ/m²) were reported using the captive bubble method indicating a less hydrophobic character of these surfaces than with the sessile drop method (γ_S ≈44.50 mJ/m²) and consequently possible differences in oral bacterial retention according the theory described by Absolom et al.

The number of adherent hydrophobic S. sanguinis cells was two-fold higher than that of hydrophilic S. constellatus cells. Our results confirm that physico-chemical surface properties of oral bacterial strains play a role in bacterial retention to implant materials in the presence of adsorbed salivary proteins.     

Emulsification using micro porous glass (MPG): surface behaviour of milk proteins

To investigate the emulsifying properties and adsorption behaviour of high molecular amphiphilic substances such as proteins, it is important to maintain the native status of the used samples. The new method of micro porous glass (MPG) emulsification could offer an opportunity to do this because of the low shear forces. The oil-in-water emulsions were produced by dispersing the hydrophobic phase (liquid butter fat or sunflower oil) through the MPG of different average pore diameters (d_p=0.2 or 0.5 μm) into the flowing continuous phase containing the milk proteins (from reconstituted skim milk and buttermilk). The emulsions were characterised by particle size distribution, creaming behaviour and protein adsorption at the hydrophobic phase. The particle size distribution of protein-stabilised MPG emulsions is determined by the pore size of MPG, the velocity of continuous phase (or wall shear stress σ_w) and the transmembrane pressure. A high velocity of =2 m s⁻¹ (σ_w=13.4 Pa) and low pressure (pressure of disperse phase slightly exceeded the critical pressure Δp_TM=4.5 bar of 0.2 μm-MPG) led to the smallest droplet diameter. As a consequence of average droplet diameters of d₄₃>3.5 μm creaming was observed without centrifugation in all MPG emulsions after 24 h, but no coalescence of the oil droplets occurred. The study of protein adsorption showed that the MPG emulsification at low shear forces resulted in lower protein load values (2.5±0.5 mg m⁻²) than pressure emulsification (11.5±1.0 mg m⁻²). In addition, the various emulsification conditions (MPG or pressure homogenization) led to differences in the relative proportions of casein fractions, whey proteins and milk fat globule membranes (MFGM) at the fat globule surfaces.     

Structure and mechanical properties of a polyglycerol ester at the air-water surface

We studied the structure and mechanical properties of surface films resulting from the adsorption of a dispersed L beta phase at the air-water interface. This L beta phase corresponds to multilamellar vesicles and is formed by a commercial polyglycerol fatty acid ester (PGE) in aqueous solution at temperatures below the main chain-melting temperature (Tm=58 degrees C). We measured the adsorption kinetics using the pendant drop technique and mechanical properties of PGE films using oscillatory surface shear and dilatational rheometric methods. Though the adsorption kinetics are very slow, we show that the L beta phase of PGE is surface-active and forms viscoelastic films at the air-water surface after sufficiently long adsorption times. The rheological response functions to shear and dilatational deformation are reminiscent of those of temporary networks, indicating an intermolecular connectivity at the surface. This temporary network is probably created by hydrophobic interactions of alkyl chains. We obtained more detailed information about the properties of this network by comparing the rheological signature of an adsorbed PGE film (unknown structure) with a solvent-spread monolayer (known structure). We characterized the structural features of spread PGE films by recording the Langmuir isotherm and Brewster angle micrographs (BAM).We show that the rheological responses of the adsorbed film and the solvent-spread monolayer are very close to each other, indicating a structural similarity. From this study, we conclude that a dispersed L beta phase of PGE is able to adsorb at the air-water surface at T相似文献   

Electrokinetic characterization of polystyrene–non-ionic surfactant complexes

An experimental study on the electrophoretic mobility (μ_e) of polystyrene particles after the adsorption of non-ionic surfactants with different chain lengths is described. Two sulphate latexes with relatively low surface charge densities (3.2 and 4.8 μC cm⁻²) were used as solid substrate for the adsorption of four non-ionic surfactants, Triton X-100, Triton X-165, Triton X-305 and Triton X-405, each one with 9–10, 16, 30 and 40 molecules of ethylene oxide (EO), respectively. The electrophoretic mobility of the polystyrene–non-ionic surfactant complexes was studied versus the amount of adsorbed surfactant (Γ). The presence of non-ionic surfactant onto particles surface seems to produce a slight shifting of the slipping plane because the mobilities of the different complexes display a very small decreasing. The increase in the number of EO chains in the surfactant molecule seems to operate as a steric impediment which decreases the number of adsorbed large surfactant molecules. The electrophoretic mobilities of the latex–surfactant complexes with maximum adsorption were measured versus the pH and ionic strength of the dispersion. While the different complexes showed a similar qualitative behaviour compared with that of the bare latex against the pH, the adsorption of the surfactant reduces the typical maximum in the μ_e−log[electrolyte].