Surface shear rheology of WPI-monoglyceride mixed films spread at the air-water interface

Surface shear viscosity of food emulsifiers may contribute appreciably to the long-term stability of food dispersions (emulsions and foams). In this work we have analyzed the structural, topographical, and shear characteristics of a whey protein isolate (WPI) and monoglyceride (monopalmitin and monoolein) mixed films spread on the air-water interface at pH 7 and at 20 degrees C. The surface shear viscosity (etas) depend on the surface pressure and on the composition of the mixed film. The surface shear viscosity varies greatly with the surface pressure. In general, the greater the surface pressure, the greater are the values of etas. The values of etas for the mixed WPI-monoolein monolayer were more than one order of magnitude lower than those for a WPI-monopalmitin mixed film, especially at the higher surface pressures. At higher surface pressures, collapsed WPI residues may be displaced from the interface by monoglyceride molecules with important repercussions on the shear characteristics of the mixed films. A shear-induced change in the topography and a segregation between domains of the film forming components were also observed. The displacement of the WPI by the monoglycerides is facilitates under shear conditions, especially for WPI-monoolein mixed films.     

Protein displacement by monoglyceride at the air-water interface evaluated by surface shear rheology combined with Brewster angle microscopy

In this work we have used different and complementary interfacial techniques (surface film balance, Brewster angle microscopy, and interfacial shear rheology), to analyze the static (structure, topography, reflectivity, miscibility, and interactions) and flow characteristics (surface shear characteristics) of milk protein (beta-casein, caseinate, and beta-lactoglobulin) and monoglyceride (monopalmitin and monoolein) mixed films spread and adsorbed on the air-water interface. The structural, topographical, and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. The surface shear viscosity (eta(s)) varies greatly with the surface pressure (pi). In general, the greater the pi values, the greater were the values of eta(s). Moreover, the eta(s) value is also sensitive to the miscibility and/or displacement of film-forming components at the interface. At surface pressures lower than that for protein collapse, protein and monoglyceride coexist at the air-water interface. At surface pressures higher than that for the protein collapse, a squeezing of collapsed protein domains by monoglycerides was deduced. Near to the collapse point, the mixed film is dominated by the presence of the monoglyceride. Different proteins and monoglycerides show different interfacial structure, topography, and shear viscosity values, confirming the importance of protein and monoglyceride structure in determining the interfacial characteristics (interactions) of mixed films. The values of eta(s) are lower for disordered (beta-casein or caseinate) than for globular (beta-lactoglobulin) proteins and for unsaturated (monoolein) than for saturated (monopalmitin) monoglycerides in the mixed film. The displacement of the protein by the monoglycerides is facilitated under shear conditions.     

Monoglycerides and beta-lactoglobulin adsorbed films at the air-water interface. structure, microscopic imaging, and shear characteristics

In this work we have analyzed the structural, topographical, and shear characteristics of mixed monolayers formed by adsorbed beta-lactoglobulin (beta-lg) and spread monoglyceride (monopalmitin or monoolein) on a previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm, Brewster angle microscopy (BAM), and surface shear characteristics were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The pi-A isotherm and BAM images deduced for adsorbed beta-lactoglobulin-monoglyceride mixed films at pi lower than the equilibrium surface pressure of beta-lactoglobulin (pi(e)(beta-lg)) indicate that beta-lactoglobulin and monoglyceride coexist at the interface. However, the interactions between protein and monoglyceride are somewhat weak. At higher surface pressures (at pi > or = pi(e)(beta-lg)) a protein displacement by the monoglyceride from the interface takes place. The surface shear viscosity (eta(s)) of mixed films is very sensitive to protein-monoglyceride interactions and displacement as a function of monolayer composition (protein/monoglyceride fraction) and surface pressure. Shear can induce change in the morphology of monoglyceride and beta-lactoglobulin domains, on the one hand, and segregation between domains of the film-forming components on the other hand. In addition, the displacement of beta-lactoglobulin by the monoglycerides is facilitated under shear conditions.     

Structural, topographical, and shear characteristics of milk protein and monoglyceride monolayers spread at the air-water interface

In this contribution we are concerned with the study of structure, topography, and surface rheological characteristics under shear conditions of monoglyceride (monopalmitin and monoolein) and milk protein (beta-casein, kappa-casein, caseinate, and WPI) spread monolayers at the air-water interface. Combined surface chemistry (surface film balance and surface shear rheometry) and microscopy (Brewster angle microscopy: BAM) techniques have been applied in this study to pure emulsifiers (proteins and monoglycerides) spread at the air-water interface. To study the shear characteristics of spread films, a homemade canal viscometer was used. The experiments have demonstrated the sensitivity of the surface shear viscosity (eta(s)) of protein and monoglyceride films at the air-water interface, as a function of surface pressure (or surface density). The surface shear viscosity was higher for proteins than for monoglycerides. In addition, eta(s) was higher for the globular WPI than for disordered beta-casein and caseinate due to the strong forces acting on spread globular proteins. This technique makes it possible to distinguish between beta-casein and caseinate spread films, with the higher eta(s) values for the later due to the presence of kappa-casein. The eta(s) value varies greatly with the surface pressure (or surface density). In general, the greater the surface pressure, the greater the values of eta(s). Finally, the eta(s) value is also sensitive to the monolayer structure, as was observed for monoglycerides with a rich structural polymorphism (i.e., monopalmitin).     

Structural and shear characteristics of adsorbed sodium caseinate and monoglyceride mixed monolayers at the air-water interface

The structural and shear characteristics of mixed monolayers formed by an adsorbed Na-caseinate film and a spread monoglyceride (monopalmitin or monoolein) on the previously adsorbed protein film have been analyzed. Measurements of the surface pressure (pi)-area (A) isotherm and surface shear viscosity (eta(s)) were obtained at 20 degrees C and at pH 7 in a modified Wilhelmy-type film balance. The structural and shear characteristics of the mixed films depend on the surface pressure and on the composition of the mixed film. At surface pressures lower than the equilibrium surface pressure of Na-caseinate (at pipi(e)(CS) have important repercussions on the shear characteristics of the mixed films.     

Thermodynamic and dynamic characteristics of monoglyceride monolayers penetrated by beta-casein

In this work, we have analyzed the dynamics of the penetration of beta-casein into monoglyceride monolayers (monopalmitin and monoolein) and the structural, dilatational, and topographical characteristics of mixed films formed by monoglyceride penetrated by beta-casein. Different complementary experimental techniques [dynamic tensiometry, surface film balance, Brewster angle microscopy (BAM), and surface dilatational rheology] have been used, maintaining the temperature constant at 20 degrees C and the pH at 7. The surface pressure of the monoglyceride monolayer at the beginning of the penetration process (at pi(i)MP and pi(i)MO for monopalmitin and monoolein, respectively) was the variable studied. beta-Casein can penetrate into a spread monoglyceride monolayer at every surface pressure. The penetration of beta-casein into the monoglyceride monolayer with a more condensed structure, at the collapse point of the monoglyceride, is a complex process that is facilitated by monoglyceride molecular loss by collapse and/or desorption. However, the structural, topographical, and dilatational characteristics of the monoglyceride penetrated by beta-casein mixed monolayers are essentially dominated by the presence of the monoglyceride (either monopalmitin or monoolein) in the mixed film.     

Structural and topographical characteristics of adsorbed WPI and monoglyceride mixed monolayers at the air-water interface

In this work we have analyzed the structural and topographical characteristics of mixed monolayers formed by an adsorbed whey protein isolate (WPI) and a spread monoglyceride monolayer (monopalmitin or monoolein) on the previously adsorbed protein film. Measurements of the surface pressure (pi)-area (A) isotherm were obtained at 20 degrees C and at pH 7 for protein-adsorbed films from water in a Wilhelmy-type film balance. Since the surface concentration (1/A) is actually unknown for the adsorbed monolayer, the values were derived by assuming that the A values for adsorbed and spread monolayers were equal at the collapse point of the mixed film. The pi-A isotherm deduced for adsorbed WPI monolayer in this work is practically the same as that obtained directly by spreading. For WPI-monoglyceride mixed films, the pi-A isotherms for adsorbed and spread monolayers at pi higher than the equilibrium surface pressure of WPI are practically coincident, a phenomenon which may be attributed to the protein displacement by the monoglyceride from the interface. At lower surface pressures, WPI and monoglyceride coexist at the interface and the adsorbed and spread pi-A isotherms (i.e., the monolayer structure of the mixed films) are different. Monopalmitin has a higher capacity than monoolein for the displacement of protein from the air-water interface. However, some degree of interactions exists between proteins and monoglycerides and these interactions are higher for adsorbed than for spread films. The topography of the monolayer corroborates these conclusions.     

Penetration of beta-lactoglobulin into monoglyceride monolayers. dynamics, interactions, and topography of mixed films

In this work we have analyzed the penetration of betalactoglobulin into a monoglyceride monolayer (monopalmitin or monoolein) spread at the air-water interface and its effects on the structural, dilatational, and topographical characteristics of mixed films. Dynamic tensiometry, surface film balance, Brewster angle microscopy (BAM), and surface dilatational rheology have been used, maintaining the temperature constant at 20 degrees C and the pH and ionic strength at 7 and 0.05 M, respectively. The initial surface pressure (mN/m) of the spread monoglyceride monolayer (pii(MONOGLYCERIDE)) at 10, 20, and the collapse point is the variable studied. Beta-lactoglobulin can penetrate into a spread monoglyceride monolayer at every surface pressure. The penetration of beta-lactoglobulin into the monoglyceride monolayer with a more condensed structure, at the collapse point of the monoglyceride, requires monoglyceride molecular loss by collapse and/or desorption. However, the structural, topographical, and dilatational characteristics of monoglyceride penetrated by beta-lactoglobulin mixed monolayers are essentially dominated by the presence of monoglyceride (either monopalmitin or monoolein) in the mixed film. In fact, monoglyceride molecules have the capacity to re-enter the monolayer after expansion and recompression of the mixed monolayer. Thus, monoglyceride molecular loss by collapse and/or desorption is reversible. The topography of the monolayer under dynamic conditions corroborates these conclusions.     

Structural characteristics of dipalmitoylphosphatidylcholine and hydrolysates from sunflower protein isolate mixed monolayers spread at the air-water interface

In this work, surface film balance and Brewster angle microscopy techniques have been used to analyze the structural characteristics (structure, topography, reflectivity, thickness, miscibility, and interactions) of hydrolysates from sunflower protein isolate (SPI) and dipalmitoylphosphatidylcholine (DPPC) mixed monolayers spread on the air-water interface. The degree of hydrolysis (DH) of SPI, low (5.62%), medium (23.5%), and high (46.3%), and the protein/DPPC mass fraction were analyzed as variables. The structural characteristics of the mixed monolayers deduced from the surface pressure (pi)-area (A) isotherms depend on the interfacial composition and degree of hydrolysis. At surface pressures lower than the equilibrium surface pressure of SPI hydrolysate (pi(e)(SPI hydrolysate)), both DPPC and protein are present in the mixed monolayer. At higher surface pressures (at pi > pi(e)(SPI hydrolysate)), collapsed protein residues may be displaced from the interface by DPPC molecules. The differences observed between pure SPI hydrolysates and DPPC in reflectivity (I) and monolayer thickness during monolayer compression have been used to analyze the topographical characteristics of SPI hydrolysates and DPPC mixed monolayers at the air-water interface. The topography, reflectivity, and thickness of mixed monolayers confirm at microscopic and nanoscopic levels the structural characteristics deduced from the pi-A isotherms.     

Preparation and characterisation of monolayers and Langmuir–Blodgett films of liquid crystal mixed with cubic silsesquioxanes

Polyhedral oligomeric silesquioxanes (POSS) with eight polyether substituents were mixed with the liquid crystal (LC) 4-octyloxy-4′-cyanobiphenyl and spread at the air/water interface. The surface pressure-area and surface potential-area isotherms were recorded for different weight ratios of both components. The obtained results showed that POSS molecules had beneficial influence on LC monolayer improving its stability and rigidity. Moreover, it was found that some LC–POSS mixtures collapse reversibly and form multilayer films on the top of LC monolayer. On the other hand, interfacial dilatational and shear rheology indicated decrease of elasticity of the films after mixing. Brewster angle microscopy revealed multilayer structure of the condensed film and formation of net-like structures in the expanded film. These films were successfully transferred on solid substrates using the Langmuir–Blodgett technique. The scanning electron microscopy images confirmed the film deposition and formation of networks by POSS–LC mixtures. These findings may be useful in the fabrication of electronic devices based on LCs.     

Stability of thin stagnant film on a solid surface with a viscoelastic air-liquid interface

Linear stability analysis for a film on a solid surface with a viscoelastic air-liquid interface is presented. The interfacial dilatational and shear viscoelastic properties were described by Maxwell models. Dilatational and shear interfacial elasticity and viscosity were shown to improve film stability. When the interfacial rheological properties are extremely large or small, the maximum perturbation growth coefficient is shown to reduce to those for immobile and mobile interfaces respectively. Calculated values of maximum growth coefficient for thin film stabilized by 0.5% beta-lactoglobulin approached those of mobile films for thick (>2000 nm) and those for immobile films for thin (<100 nm) films respectively with the values lying between the two limits for intermediate film thicknesses.     

Interactions of Meibomian gland secretion with polar lipids in Langmuir monolayers

The surface interactions of Meibomian gland secretion (MGS) with polar lipid (PL), Egg Sphingomyelin (SM) or Dipalmitoylphosphatidylcholine (DPPC), are studied in mixed pseudo-binary films formed at the air/water interface of Langmuir surface balance. The behavior of the mixed films during slow quasi-equilibrium compression and during fast dynamic compression–decompression is registered by measurements of surface pressure and surface potential, and by monitoring film morphology with Brewster Angle Microscopy (BAM). Quasi-equilibrium compression isotherms are used to calculate the excess Gibbs and Helmholtz energy of mixing between MGS and PLs and thus to evaluate the interactions between the lipid compounds at the interface. The effects of PLs on the mixed film's elastic moduli of area compressibility, morphology and capability to attain high surface pressures are also examined.PLs interact with MGS with different strength and in different manner: MGS–SM interaction is weak and might lead to interfacial disaggregation of the thick meibium domains when SM is in excess, while MGS–DPPC interaction is strong and results in the formation of thick lipid aggregates. Both PLs increase the mixed films reciprocal compressibility and capability to achieve higher surface pressures. The results demonstrate that in vitro studies of the surface interactions between MGS and PLs might be beneficial in the selection of PLs for artificial tear formulations and for examination on molecular scale of the possible role of PLs at the ocular surface.     

The brittle–ductile transition induced by thermal ageing of toluene cast poly(2,6‐dimethyl‐1,4‐phenylene oxide) as observed by interfacial friction

The interfacial shear stress of toluene cast poly(2,6‐dimethyl‐1,4‐phenylene oxide) films has been studied as a function of annealing temperature. The surface topography of these films was studied by scanning probe microscopy following a single sliding pass. Casting from toluene results in a semicrystalline film with a rigid amorphous phase and containing a small amount of residual solvent that exhibits a higher interfacial shear stress than a high temperature annealed solvent‐free amorphous film. Films containing small amounts of toluene exhibit a wear pattern consisting of ripples oriented perpendicular to the sliding direction following a single sliding pass. These results support the notion that the interfacial shear stress is a function of the shear yield stress, and, that during sliding friction tensile stresses must form at the polymer surface. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1637–1643, 2009     

DNA分子在气液界面的组装相变特性及其LB膜结构研究

对十八胺与DNA在气液界面上组装及其相变过程进行了研究，利用AFM观察了不 同压力下转移的DNA复合LB膜结构。发现在低表面压时，DNA复合单分子膜表现为技 术发散的分形结构；随着压力的升高，DNA复合膜逐渐由紧密的网状排布结构变为 团聚的块状和团簇结构。表明通过调节膜压，可使膜内DNA分子的构象发生大的变 化，从而生成具有特定形态的二维纳米图案。这种具有特殊形态和结构的DNA LB膜 可望为合成新型生物纳米结构有序功能体系提供模板。     

Interfacial assembly of an achiral zinc phthalocyanine at the air/water interface: a surface pressure dependent aggregation and supramolecular chirality

The aggregation and supramolecular chirality of the interfacial assemblies of an achiral phthalcyanine derivative, zinc 2,3,9,10,16,17,23,24-octakis(octyloxy)-29 H,31 H-phthaloxyanine (ZnPc), were investigated, and a surface pressure dependent behavior was observed. It was found that ZnPc could be spread as a Langmuir film on water surface and transferred onto solid substrates by the horizontal lifting method. The compound formed mixed J- and H-aggregates in the transferred Langmuir-Blodgett (LB) films. Deconvolution of the broaden Q-band revealed three main components of the spectra, which corresponded to H- and J-aggregates and medium transition aggregates, whose relative contents could be modulated by the surface pressure at which the films were transferred. On the other hand, the transferred LB films composed of these aggregates showed Cotton effects in circular dichroism spectra when the floating film was compressed over a certain surface pressure although the compound itself was achiral. The cooperative arrangement of the macrocylic ring in a helical manner through the interfacial organization was suggested to be responsible for such optical activity in the LB films. A possible explanation based on the cooperative arrangement of the ZnPc building blocks in a helical sense stacking in the aggregates was proposed.     

Protein-lipid interactions at the oil-water interface

The distribution of proteins and lipids in food emulsions and foams is determined by competitive and cooperative adsorption between the two types of emulsifiers at the fluid-fluid interfaces, and by the nature of protein-lipid interactions, both at the interface and in the bulk phase. The existence of protein-lipid interactions can have a pronounced impact on the surface rheological properties of these systems. Therefore, these results are of practical importance for food emulsion formulation, texture, and stability. In this study, the existence of protein-lipid interactions at the interface was determined by surface dynamic properties (interfacial tension and surface dilational modulus). Systematic experimental data on surface dynamic properties, as a function of time and at long-term adsorption, for protein (whey protein isolate (WPI)), lipids (monoglycerides), and protein-lipid mixed films at the oil-water interface were measured in an automated drop tensiometer. The dynamic behaviour of protein+lipid mixed films depends on the adsorption time, the lipid and the protein/lipid ratio in a rather complicated manner. The protein determined the interfacial characteristics of the mixed film as the protein at WPI>/=10(-2)% wt/wt saturated the film, no matter what the concentration of the lipid. However, there exists a competitive or cooperative adsorption of the emulsifier (WPI and monoglycerides), as the concentration of protein in the bulk phase is far lower than that for interfacial saturation.     

Topography of dipalmitoyl-phosphatidyl-choline monolayers penetrated by β-casein

In this work we have analyzed the topography by atomic force microscopy (AFM) of dipalmitoyl-phosphatidyl-choline (DPPC) monolayers previously spread at the air–water interface and penetrated by β-casein. AFM images of β-casein–DPPC monolayers were taken from Langmuir–Blodgett films deposited onto hydrophilic mica substrates at different initial surface pressures (π_i) and after the compression of the mixed films. The monolayer topography depends on the initial structure of the phospholipid:liquid expanded (LE) at 3 mN/m, coexistence between LE and liquid condensed (LC) structures at 7 mN/m, at the end of the LE–LC transition at 10 mN/m, and with a LC structure at 15 mN/m. The area occupied by DPPC domains in the mixed film increases with the π_i value, especially for DPPC with a LC structure at 15 mN/m. At this surface pressure the thickness of the film is at a maximum. After the film compression at 25 mN/m, which is above the equilibrium spreading pressure of β-casein (), this protein is displaced from the interface by DPPC and the topography of the mixed monolayer depends on the initial structure of the DPPC monolayer. A notable feature of the topography of these mixed monolayers is the presence of multilayers of β-casein and DPPC of high thickness (50–70 nm) at the lower π_i values. Although the film is dominated by DPPC at the highest surface pressures (at 25 mN/m), β-casein is not displaced totally from the interface and coexists as β-casein collapsed domains within the network of the DPPC structure.     

Interfacial rheology of protein–surfactant mixtures

The distribution of proteins and surfactants at fluid interfaces (air–water and oil–water) is determined by the competitive adsorption between the two types of emulsifiers and by the nature of the protein–surfactant interactions, both at the interface and in the bulk phase, with a pronounced impact on the interfacial rheological properties of these systems. Therefore, the interfacial rheology is of practical importance for food dispersion (emulsion or foam) formulation, texture, and stability. In this review, the existence of protein–surfactant interactions, the mechanical behaviour and/or the composition of emulsifiers at the interface are indirectly determined by interfacial rheology of the mixed films. The effect on the interfacial rheology of protein–surfactant mixed films of the protein, the surfactant, the interface and bulk compositions, the method of formation of the interfacial film, the interactions between film forming components, and the displacement of protein by surfactant have been analysed. The last section tries to understand the role of interfacial rheology of protein–surfactant mixed films on food dispersion formation and stability. The emphasis of the present review is on the interfacial dilatational rheology.     

Ceramide N-acyl chain length: a determinant of bidimensional transitions, condensed domain morphology, and interfacial thickness

Several lipids of biological interest are able to form monomolecular surfaces with a rich variety of thickness and lateral topography that can be precisely controlled by defined variations of the film composition. Ceramide is one of the simplest sphingolipids, consisting of a sphingosine base N-linked to a fatty acid, and is a membrane mediator for cell-signaling events. In this work, films of ceramides N-acylated with the saturated fatty acids C10, C12, C14, and C16 were studied at the air-aqueous interface. The dipole moment contribution (from surface potential measurements) and the surface topography and thickness (as revealed by Brewster angle microscopy) were measured simultaneously with the surface pressure at different molecular areas. Several surface features were observed depending on the asymmetry between the sphingosine and the N-linked acyl chains. At 21 °C, the C16:0 and C14:0 ceramides showed condensed isotherms and the film topography revealed solid film patches (17.3-15.7 ? thick) that coalesced into a homogeneous surface by further compression. On the other hand, in the more asymmetric C12:0 and C10:0 ceramides, liquid expanded states and liquid expanded-condensed transitions occurred. In the phase coexistence region, the condensed state of these compounds formed flowerlike domains (11.1-13.3 ? thick). C12:0 ceramide domains were larger and more densely branched than those of C10:0 ceramide. Both the film thickness and the surface dipole moment of the condensed state increased with ceramide N-acyl chain length. Bending of the sphingosine chain over the N-linked acyl chain in the more asymmetric ceramides can account for the variation of the surface electrostatics, topography, and thickness of the films with the acyl chain mismatch.     

Interfacial structure in thin water layers formed by forced dewetting on self-assembled monolayers of omega-terminated alkanethiols on Ag

A method for the spectroscopic characterization of interfacial fluid molecular structure near solid substrates is reported. The thickness and interfacial molecular structure of residual ultrathin D20 films remaining after forced dewetting on alkanethiolate self-assembled monolayers (SAMs) of 11 1-mercaptoundecanoic acid (11-MUA), 11-mercaptoundecanol (11-MUD), and undecanethiol (UDT) on Ag are investigated using ellipsometry and surface Raman spectroscopy. The residual film thickness left after withdrawal is greater on hydrophilic SAMs than on hydrophobic SAMs. This behavior is rationalized on the basis of differing degrees of fluid slip within the interfacial region due to different interfacial molecular structure. The v(O-D) regions of surface Raman spectra clearly indicate unique interfacial molecular properties within these films that differ from bulk D20. Although the residual films are created by shear forces and Marangoni flow at the three-phase line during the forced dewetting process, the nature of the films sampled optically must also be considered from the standpoint of thin film stability after dewetting. Thus, the resulting D20 films exist in vastly different morphologies depending on the nature of the water-SAM interactions. Residual D20 is proposed to exist as small nanodroplets on UDT surfaces due tospontaneous rupture of the film after dewetting. In contrast, on 11-MUD and 11-MUA surfaces, these films exist in a metastable state that retains their conformal nature on the underlying modified surface. Analysis of the peak intensity ratios of the so-called "ice-like" to "liquid-like" v(O-D) modes suggests more ice-like D20 character near 11-MUD surfaces, but more liquid-like character near 11-MUA and UDT surfaces. The creation of residual ultrathin films by forced dewetting is thus demonstrated to be a powerful method for characterizing interfacial molecular structure of fluids near a solid substrate under ambient conditions of temperature and pressure.