Equilibrium Surface Pressure Research Articles

Effect of β-casein on surface activity of Quillaja bark saponin at fluid/fluid interfaces

Effect of a model bovine milk protein, β-casein, on surface activity of Quillaja bark saponin (QBS) from Sigma was studied at three fluid/fluid interfaces: air/water, tetradecane/water and olive oil/water. In all cases, the protein concentration was fixed at 10−6 mol L−1, and QBS concentration was varied between 5·10−7 and 1·10−3 mol L−1. Dynamic interfacial tension on the timescale 5 s–3600 s was measured using a drop shape analysis technique. For the air/water system, they were complemented with short-term (50 ms–5 s) measurements using a maximum bubble pressure technique. The dynamic results together with the extrapolated equilibrium surface pressures are discussed from the point of view of a complexation between β-casein and QBS, with the surface activity of the complex changing with its stoichiometry. At low biosurfactant/protein ratios, the interfacial tension at all three interfaces passes through a maximum, corresponding to a transient decrease of both foam and emulsion formation ability. In addition, the effect of QBS on deterioration of β-casein's surface activity upon ageing at room temperature is discussed.

Pulmonary Surfactant Reduces Surface Tension to Low Values near Zero through a Modified Squeeze-Out Mechanism

Pulmonary surfactant stabilizes the lung by reducing surface tension (ST) to low values near zero at end expiration. Atomic force microscopy studies revealed that, as surface pressure increases, spread surfactant films initially form solid micro- and nanodomains and then generate 3-5 multilayers which apparently remain associated with the surface monolayer. Time-of-flight Secondary Ion Mass Spectroscopy analyses revealed selective squeeze-out must occur because the multilayers are enriched in unsaturated fluid phospholipids (PL) while the remaining monolayer becomes enriched in disaturated PL. Taken together, these results are consistent with a modified squeeze-out model where surfactant vesicles interact with the air-water interface through surfactant proteins B- and C-containing adsorption/fusion pores. PL migration onto the surface results in vesicle instability generating a monolayer at equilibrium surface pressure, which remains functionally associated with excess bilayer material. Initially, the monolayer and associated reservoir have similar composition, but film compression causes fluid PL to migrate through the fusion pores into the multilayers. Gel phase PL are restricted because they are sequestered in micro- or nanodomains. This process results in monolayers highly enriched in disaturated PL which reduce ST to near zero. Film expansion allows fluid PL to regain the surface through the fusion pores. This mechanism explains both the rapid reincorporation of surfactant PL into the monolayer during adsorption and during film expansion and the progressive improvement in surface activity during repeated compression.

Collapse in Binary Phospholipid Monolayers at the Air/Water Interface

The behavior in the stochastic collapse of lipid monolayers of a 7:3 mixture of DPPC (dipalmitoylphosphatidylcholine) and POPG (palmitoyloleoylphosphatidylglycerol), a model system of pulmonary surfactant assembly, was studied using Langmuir isotherms, fluorescence microscopy, and atomic force microscopy. The monolayers at the air-water interface appeared to be phase separated under compression and retained the continuous liquid-expanded phase network surrounding islands of condensed phase even at a surface pressure approaching 70 mN/m. When the two-dimensional monolayers were compressed beyond the equilibrium surface pressure, they collapsed and assumed a three-dimensional formation. Collapse events involved folding of the monolayer on a micron scale, and each event produced a macroscopic jerk of the layer. The distribution of waiting times between events was estimated to be an exponential function, indicating that the events were independent. Folded regions coexisted with the flat monolayer, remained attached to the interface, and reversibly reincorporated into the monolayer upon expansion.

Insertion Mechanism of a Poly(ethylene oxide)-poly(butylene oxide) Block Copolymer into a DPPC Monolayer

Interactions between amphiphilic block copolymers and lipids are of medical interest for applications such as drug delivery and the restoration of damaged cell membranes. A series of monodisperse poly(ethylene oxide)-poly(butylene oxide) (EOBO) block copolymers were obtained with two ratios of hydrophilic/hydrophobic block lengths. We have explored the surface activity of EOBO at a clean interface and under 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) monolayers as a simple cell membrane model. At the same subphase concentration, EOBO achieved higher equilibrium surface pressures under DPPC compared to a bare interface, and the surface activity was improved with longer poly(butylene oxide) blocks. Further investigation of the DPPC/EOBO monolayers showed that combined films exhibited similar surface rheology compared to pure DPPC at the same surface pressures. DPPC/EOBO phase separation was observed in fluorescently doped monolayers, and within the liquid-expanded liquid-condensed coexistence region for DPPC, EOBO did not drastically alter the liquid-condensed domain shapes. Grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XRR) quantitatively confirmed that the lattice spacings and tilt of DPPC in lipid-rich regions of the monolayer were nearly equivalent to those of a pure DPPC monolayer at the same surface pressures.

Surface active benzodiazepine-bromo-alkyl conjugate for potential GABAA-receptor purification

A conjugable analogue of the benzodiazepine 5-(2-hydroxyphenyl)-7-nitro-benzo[e][1,4]diazepin-2(3H)-one containing a bromide C(12)-aliphatic chain (BDC) at nitrogen N1 was synthesized. One-pot preparation of this benzodiazepine derivative was achieved using microwave irradiation giving 49% yield of the desired product. BDC inhibited FNZ binding to GABA(A)-R with an inhibition binding constant K(i) = 0.89 μM and expanded a model membrane packed up to 35 mN m(-1) when penetrating in it from the aqueous phase. BDC exhibited surface activity, with a collapse pressure π = 9.8 mN m(-1) and minimal molecular area A(min) = 52 Å(2)/molecule at the closest molecular packing, resulted fully and non-ideally mixed with a phospholipid in a monolayer up to a molar fraction x≅ 0.1. A geometrical-thermodynamic analysis along the π-A phase diagram predicted that at low x(BDC) (<0.1) and at all π, including the equilibrium surface pressures of bilayers, dpPC-BDC mixtures dispersed in water were compatible with the formation of planar-like structures. These findings suggest that, in a potential surface grafted BDC, this compound could be stabilize though London-type interactions within a phospholipidic coating layer and/or through halogen bonding with an electron-donor surface via its terminal bromine atom while GABA(A)-R might be recognized through the CNZ moiety.

Equilibrium and surface rheology of two polyoxyethylene surfactants (C iEO j) differing in the number of oxyethylene groups

A study of equilibrium adsorption properties and surface dilational rheology of two polyoxyethylene surfactants of technical grade have been carried out. It has been shown, that equilibrium adsorption and surface rheology are greatly influenced by the oxyethylene chain. To explore the equilibrium properties, a pendant drop profile tensiometer was used. The equilibrium surface pressure isotherms can be explained in the framework of the reorientation model. The increase in the number of oxyethylene units leads to higher surface activity at low concentration when the surfactants adsorb in the state of maximum molar area. However, at higher concentration the adsorbed surfactant molecules reorient to a state of minimum molar area. In this state the surface activity is, on contrary, lower for C 10EO 14 due to higher hydrophilicity. Dilational rheology was studied by means of the oscillating drop method. The surfactant with higher EO units showed an enhanced elasticity. Moreover, whereas C 10EO 6 experimental data can be interpreted on the basis of the diffusional theory and a diffusion coefficient of 4 × 10 −10 m 2 s −1 was estimated, C 10EO 14 behavior is more complex maybe due to the multitude of possible adsorption conformations and is closer to that expected for polymer surfactants.

Reductions of phospholipase A2 inhibition of pulmonary surfactant with hyaluronan

ABSTRACTIn acute lung injuries, secretory phospholipase A2 (sPLA2) inhibits surfactants by hydrolyzing phospholipids. Because hyaluronan (HA) reduces hydrolysis of phospholipids by sPLA2, and because sPLA2 inhibits surfactant in vitro, the authors hypothesized HA would reduce sPLA2 inhibition. Surfactants were used alone or mixed with HA and/or sPLA2 then tested for surface activity in 2 separate assays, or for sPLA2 activity. Equilibrium surface pressures were identical for surfactant with or without HA. sPLA2 inhibited surface activity but this inhibitory effect was reduced with HA by 14% in the spreading trough and by 63% in a modified bubble surfactometer. Hyaluronan caused a modest reduction (39%) of sPLA2 breakdown of labeled phospholipid. Therefore hyaluronan reduces inhibition of surfactants by sPLA2 in vitro, and reduces the activity of the enzyme.

Semifluorinated thiols in Langmuir monolayers

A series of semifluorinated thiols of the general formula CF 3(CF 2) m −1(CH 2) n SH (abbreviated to FmHnSH) have been synthesized and the Langmuir monolayers thoroughly characterized using surface pressure ( π) and electric surface potential (Δ V) measurements. These data have been complemented with Brewster angle microscopy (BAM) visualization of the monolayers structure and IR spectroscopy (PM-IRRAS) analysis of the alkyl chain conformation. The investigated thiols (namely F4H10SH, F8H6SH, F6H10SH, F10H6SH, F6H11SH, F8H10SH and F10H10SH) differ in the length of the hydrophobic chain as well as in the degree of fluorination. Although the –SH group cannot form strong hydrogen bonds with the water subphase, thereby causing only weak anchoring, it has been found that all the investigated thiols, except for F4H10SH, are capable of stable Langmuir monolayer formation. The investigated thiols can be divided into two categories – thiols with longer alkyl chain (F10H10SH, F10H6SH, F8H10SH) and compounds with shorter hydrophobic part (F6H10SH, F6H11SH, F8H6SH). The monolayers of the latter thiols have their equilibrium surface pressure (ESP) comparable with the collapse pressure ( π C) of their monolayers; thus these films are stable within the whole range of compression. On the contrary, thiols with longer perfluorinated fragments have considerably lower ESP than the π C of their monolayers; therefore, both stable and metastable regions can be distinguished in their π– A isotherms. Interestingly, for F8H6SH, a smectic ordering of molecules can be observed in monolayers as visualized with BAM. By comparing film-forming properties of the studied semifluorinated thiols with previously studied semifluorinated alkanes it has been found that the presence of –SH group facilitates spreading at the free water surface. Semifluorinated thiols have been found to maintain their monomolecular ordering in the presence of heavy metal cations in the aqueous subphase, contrary to their hydrogenated analogues, which have been reported to crystallize under the same conditions.

Overcoming rapid inactivation of lung surfactant: analogies between competitive adsorption and colloid stability.

Lung surfactant (LS) is a mixture of lipids and proteins that line the alveolar air-liquid interface, lowering the interfacial tension to levels that make breathing possible. In acute respiratory distress syndrome (ARDS), inactivation of LS is believed to play an important role in the development and severity of the disease. This review examines the competitive adsorption of LS and surface-active contaminants, such as serum proteins, present in the alveolar fluids of ARDS patients, and how this competitive adsorption can cause normal amounts of otherwise normal LS to be ineffective in lowering the interfacial tension. LS and serum proteins compete for the air-water interface when both are present in solution either in the alveolar fluids or in a Langmuir trough. Equilibrium favors LS as it has the lower equilibrium surface pressure, but the smaller proteins are kinetically favored over multi-micron LS bilayer aggregates by faster diffusion. If albumin reaches the interface, it creates an energy barrier to subsequent LS adsorption that slows or prevents the adsorption of the necessary amounts of LS required to lower surface tension. This process can be understood in terms of classic colloid stability theory in which an energy barrier to diffusion stabilizes colloidal suspensions against aggregation. This analogy provides qualitative and quantitative predictions regarding the origin of surfactant inactivation. An important corollary is that any additive that promotes colloid coagulation, such as increased electrolyte concentration, multivalent ions, hydrophilic non-adsorbing polymers such as PEG, dextran, etc. added to LS, or polyelectrolytes such as chitosan, also promotes LS adsorption in the presence of serum proteins and helps reverse surfactant inactivation. The theory provides quantitative tools to determine the optimal concentration of these additives and suggests that multiple additives may have a synergistic effect. A variety of physical and chemical techniques including isotherms, fluorescence microscopy, electron microscopy and X-ray diffraction show that LS adsorption is enhanced by this mechanism without substantially altering the structure or properties of the LS monolayer.

X-Ray Diffraction and Reflectivity Validation of the Depletion Attraction in the Competitive Adsorption of Lung Surfactant and Albumin

Lung surfactant (LS) and albumin compete for the air-water interface when both are present in solution. Equilibrium favors LS because it has a lower equilibrium surface pressure, but the smaller albumin is kinetically favored by faster diffusion. Albumin at the interface creates an energy barrier to subsequent LS adsorption that can be overcome by the depletion attraction induced by polyethylene glycol (PEG) in solution. A combination of grazing incidence x-ray diffraction (GIXD), x-ray reflectivity (XR), and pressure-area isotherms provides molecular-resolution information on the location and configuration of LS, albumin, and polymer. XR shows an average electron density similar to that of albumin at low surface pressures, whereas GIXD shows a heterogeneous interface with coexisting LS and albumin domains at higher surface pressures. Albumin induces a slightly larger lattice spacing and greater molecular tilt, similar in effect to a small decrease in the surface pressure. XR shows that adding PEG to the LS-albumin subphase restores the characteristic LS electron density profile at the interface, and confirms that PEG is depleted near the interface. GIXD shows the same LS Bragg peaks and Bragg rods as on a pristine interface, but with a more compact lattice corresponding to a small increase in the surface pressure. These results confirm that albumin adsorption creates a physical barrier that inhibits LS adsorption, and that PEG in the subphase generates a depletion attraction between the LS aggregates and the interface that enhances LS adsorption without substantially altering the structure or properties of the LS monolayer.

Long-chain alkyl thiols in Langmuir monolayers

A series of homologous alkyl thiols C n H 2 n+1 SH ( n = 14–18, 20, 22) have been synthesized and Langmuir monolayers have been thoroughly characterized using surface pressure, electric surface potential measurements and Brewster angle microscopy. Although the –SH group cannot form strong hydrogen bonding with the water subphase, thereby causing weak anchoring, it has been found that alkyl thiols having more than 15 carbon atoms are capable of Langmuir monolayer formation at room temperature. The stability of the investigated films is not very high, however, but quite sufficient for LB transfer at low surface pressures. The increase in aliphatic chain length does not lead to any considerable improvement of the monolayer stability. The influence of subphase temperature and the presence of various cations in the aqueous subphase have also been studied. Contrary to literature, it has been found that the presence of alkaline earth metals cations does not improve the monolayer stability, and although the collapse pressure is higher, the equilibrium surface pressure remains the same as on pure water.

Atomic Force Microscopy Studies of Functional and Dysfunctional Pulmonary Surfactant Films, II: Albumin-Inhibited Pulmonary Surfactant Films and the Effect of SP-A

Pulmonary surfactant (PS) dysfunction because of the leakage of serum proteins into the alveolar space could be an operative pathogenesis in acute respiratory distress syndrome. Albumin-inhibited PS is a commonly used in vitro model for studying surfactant abnormality in acute respiratory distress syndrome. However, the mechanism by which PS is inhibited by albumin remains controversial. This study investigated the film organization of albumin-inhibited bovine lipid extract surfactant (BLES) with and without surfactant protein A (SP-A), using atomic force microscopy. The BLES and albumin (1:4w/w) were cospread at an air-water interface from aqueous media. Cospreading minimized the adsorption barrier for phospholipid vesicles imposed by preadsorbed albumin molecules, i.e., inhibition because of competitive adsorption. Atomic force microscopy revealed distinct variations in film organization, persisting up to 40mN/m, compared with pure BLES monolayers. Fluorescence confocal microscopy confirmed that albumin remained within the liquid-expanded phase of the monolayer at surface pressures higher than the equilibrium surface pressure of albumin. The remaining albumin mixed with the BLES monolayer so as to increase film compressibility. Such an inhibitory effect could not be relieved by repeated compression-expansion cycles or by adding surfactant protein A. These experimental data indicate a new mechanism of surfactant inhibition by serum proteins, complementing the traditional competitive adsorption mechanism.

Oil/water surface rheological properties of hydroxypropyl cellulose (HPC) alone and mixed with lecithin: Contribution to emulsion stability

Hydroxypropyl cellulose (HPC) is a neutral branched polysaccharide derivatized from a wood substrate. Widely used in whipping vegetable creams, HPC is a thickening biopolymer having surface active properties both at air/water and oil/water interfaces. Myglyol/water interfacial rheology was used to characterize the behavior of HPC at this interface in the presence or the absence of lecithin, an emulsifier classically used in combination with HPC. Addition of HPC in the aqueous phase leads to an increase of the surface pressure at the oil/water interface depending on the HPC concentration. The higher the HPC concentration, the shorter is the lag time before the surface pressure increase. The equilibrium surface pressure is about 13–16 mN m −1 whatever the HPC concentration in the studied range. In order to understand the way the HPC positions itself at the interface, a viscoelastic characterization of the interface was performed during adsorption, in the presence and in the absence of lecithin. A comparison to the stability of emulsions containing HPC or HPC/lecithin is made and discussed using droplet size evolution and droplet displacement rate to an upper layer.

Influence of concentration and ionic strength on the adsorption kinetics of gelatin at the air/water interface

The adsorption kinetics of gelatin to the air/water interface is still not fully understood. We investigated two samples of gelatin having different gel strength (65 Bloom and 280 Bloom), named, respectively gelatin 65B and 280B. The gelatin samples in solution were characterised by viscosimetric methods. We evidence a chain expansion of gelatin upon dilution due to the polyelectrolyte effect. The measurement of the intrinsic viscosity in water gave values of 22 mL/g for the sample 65B and 70 mL/g for the sample 280B. Although gelatin is a highly polymolecular polymer containing branched chains, the normalisation of the concentration by the intrinsic viscosity allowed the construction of a master curve of viscosity, giving the reduced critical overlap concentrations c* ∼ 0.6 and c** ∼ 5. All concentrations of the gelatin solutions used for the determination of the adsorption behaviour were within the range of the dilute regime. The analysis of the adsorption behaviour leads us to the hypothesis that there are two different phenomena: first, a step governed by diffusion to the interface which lasts until full coverage of the interfacial film. The length of this induction period seemed to be dependent on the gelatin chain length, with larger chains taking more time. Second, the adsorption of the chains to the air/water interface leads to the formation of an encumbered subsurface layer which opposes steric hindrance to newly arriving chains. This causes a continuous slow down of the rate of surface pressure increase, which was linear on the logarithmic time scale and showed a slope of −1. The adsorption kinetics was independent of the salt concentration of the solvent and of the gelatin chain size measured by the intrinsic viscosity. The gelatin sample 65B shows furthermore higher equilibrium surface pressures at high bulk concentrations (>0.01 wt%) than sample 280B, while the contrary was observed at small concentrations.

Molecular weight dependence of the depletion attraction and its effects on the competitive adsorption of lung surfactant

Albumin competes with lung surfactant for the air–water interface, resulting in decreased surfactant adsorption and increased surface tension. Polyethylene glycol (PEG) and other hydrophilic polymers restore the normal rate of surfactant adsorption to the interface, which re-establishes low surface tensions on compression. PEG does so by generating an entropic depletion attraction between the surfactant aggregates and interface, reducing the energy barrier to adsorption imposed by the albumin. For a fixed composition of 10 g/L (1% wt.), surfactant adsorption increases with the 0.1 power of PEG molecular weight from 6 kDa–35 kDa as predicted by simple excluded volume models of the depletion attraction. The range of the depletion attraction for PEG with a molecular weight below 6 kDa is less than the dimensions of albumin and there is no effect on surfactant adsorption. PEG greater than 35 kDa reaches the overlap concentration at 1% wt. resulting in both decreased depletion attraction and decreased surfactant adsorption. Fluorescence images reveal that the depletion attraction causes the surfactant to break through the albumin film at the air–water interface to spread as a monolayer. During this transition, there is a coexistence of immiscible albumin and surfactant domains. Surface pressures well above the normal equilibrium surface pressure of albumin are necessary to force the albumin from the interface during film compression.

Per-6-O-(tert-butyldimethylsilyl)cyclodextrins (TBDMS-CDs) in Langmuir Monolayers: The Importance of a Spreading Solvent in the Preparation of LB Layers Suitable for Sensor Application

Commercially available amphiphilic cyclodextrins, namely per-6-O-(tert-butyldimethylsilyl) alpha, beta and gamma cyclodextrins (TBDMS-alpha-, -beta-, and -gamma-CDs) were subjected to a thorough Langmuir monolayer characterization, using both traditional methods of surface manometry (pi/A isotherms, stability experiments) and modern micrometer/nanometer resolution (BAM, AFM) surface techniques. It has been found that inconsistent behavior regarding the isotherms reproducibility obtained upon compression of TBDMS-beta-CDs is due to the aggregation of the investigated molecules in chloroform and hexane, while good reproducibility ensured a mixed spreading solvent system of hexane/isopropanol 7:3 (v/v). Although the stability of films dropped from chloroform and hexane/isopropanol solvents below the equilibrium surface pressure (ESP) was comparable, pronounced differences were observed at pressures above ESP. The investigated TBDMS-CDs were successfully transferred onto cadmium stearate covered mica substrates. AFM images confirmed the presence of discontinuous multilayered films (10 nm heights) spread from chloroform versus monomolecular dispersion achieved in hexane/isopropanol.

Puroindoline-b Mutations Control the Lipid Binding Interactions in Mixed Puroindoline-a:Puroindoline-b Systems

The interactions have been investigated of puroindoline-a (Pin-a) and mixed protein systems of Pin-a and wild-type puroindoline-b (Pin-b+) or puroindoline-b mutants (G46S mutation (Pin-bH) or W44R mutation (Pin-bS)) with condensed phase monolayers of an anionic phospholipid (L-alpha-dipalmitoylphosphatidyl-dl-glycerol (DPPG)) at the air/water interface. The interactions of the mixed systems were studied at three different concentration ratios of Pin-a:Pin-b, namely 3:1, 1:1 and 1:3 in order to establish any synergism in relation to lipid binding properties. Surface pressure measurements revealed that Pin-a interaction with DPPG monolayers led to an equilibrium surface pressure increase of 8.7 +/- 0.6 mN m-1. This was less than was measured for Pin-a:Pin-b+ (9.6 to 13.4 mN m-1), but was significantly more than was measured for Pin-a:Pin-bH (4.0 to 6.2 mN m-1) or Pin-a:Pin-bS (3.8 to 6.3 mN m-1) over the complete range of concentration ratio. Consequently, surface pressure increases were shown to correlate to endosperm hardness phenotype, with puroindolines present in hard-textured wheat varieties yielding lower equilibrium surface pressure changes. Integrated amide I peak areas from corresponding external reflectance Fourier-transform infrared (ER-FTIR) spectra, used to indicate levels of protein adsorption to the lipid monolayers, showed that differences in adsorbed amount were less significant. The data therefore suggest that Pin-b mutants having single residue substitutions within their tryptophan-rich loop that are expressed in some hard-textured wheat varieties influence the degree of penetration of Pin-a and Pin-b into anionic phospholipid films. These findings highlight the key role of the tryptophan-rich loop in puroindoline-lipid interactions.

Monoglycerides and β-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 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 ( π)–area ( A) isotherm and surface shear viscosity ( η s ) were obtained at 20 °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 π < π e CS ), both Na-caseinate and monoglyceride coexist at the interface, with a structural polymorphism or a liquid expanded structure due to the presence of monopalmitin or monoolein in the mixture, respectively. At higher surface pressures, collapsed Na-caseinate residues may be displaced from the interface by monoglyceride molecules. For a Na-caseinate–monopalmitin mixed film the η s value varies greatly with the surface pressure (or surface density) of the mixed monolayer at the interface. In general, the greater the surface pressure, the greater are the values of η s . However, the values of η s for a Na-caseinate–monoolein mixed monolayer are very low and practically do not depend on the surface pressure. The collapsed Na-caseinate residues displaced from the interface by monoglyceride molecules at π > π e CS have important repercussions on the shear characteristics of the mixed films.

Interfacial Properties of Diglycerol Esters and Caseinate Mixed Films at the Air−Water Interface

The surface pressure (π) at equilibrium (surface pressure isotherm) and surface dynamic properties (dynamic surface pressure and surface dilatational characteristics) of diglycerol ester (diglycerol-monocaprinate, diglycerol-monolaurate, diglycerol-monostearate, and diglycerol-monooleate) and protein (sodium caseinate) as emulsifiers, at different concentrations in the aqueous phase, were measured using tensiometry and a dynamic drop tensiometer, respectively. We have observed that (1) at equilibrium the value of critical micelle concentration (CMC) decreases and the maximum surface excess (Γmax) increases as the hydrocarbon chain increases because the hydrophobic character of the lipid also increases. The presence of a double bond in the hydrocarbon chain also increases the value of CMC and decreases those of Γmax. Caseinate presents higher adsorption efficiency but the surface activity is between those for lipids. The surface pressure isotherm of mixed systems is dependent on the emulsifier concentration and the protein/lipid ratio in the mixture. (2) The adsorption of pure emulsifiers at the air−water interface increases with emulsifier concentration in the aqueous phase via diffusion and penetration of the emulsifier at the interface. For mixed films, the rate of adsorption depends on the concentration and composition of the mixture. Competitive or cooperative phenomena were observed during the adsorption of both emulsifiers at the interface. (3) The surface dilatational characteristics of mixed films are viscoelastic. The surface dilatational modulus reflects the amount of emulsifier adsorbed at the interface and confirms the idea that the protein−lipid interactions at the air−water interface are somewhat weak, there even being the possibility of phase separation.