Foam, emulsion and wetting films stabilized by polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants.

This review explores three (A, B, C) polyoxyalkylated diethylenetriamine (DETA) polymeric surfactants belonging to the group of star-like polymers. They have a similar structure, differing only in the number of polymeric branches (4, 6 and 9 in the mentioned order). The differences in these surfactants' ability to stabilize foam, o/w/o and w/o/w emulsion and wetting films are evaluated by a number of methods summarized in Section 2. Results from the studies indicate that differences in polymeric surfactants' molecular structure affect the properties exhibited at air/water, oil/water and water/solid interfaces, such as the value of surface tension, interfacial tension, critical micelle concentration, degree of hydrophobicity of solid surface, etc. Foam, emulsion and wetting films stabilized by such surfactants also show different behavior regarding some specific parameters, such as critical electrolyte concentration, surfactant concentration for obtaining a stable film, film thickness value, etc. These observations give reasons to believe that model studies can support a comprehensive understanding of how the change in polymeric surfactant structure can impact thin liquid films properties. This may enable a targeted design of the macromolecular architecture depending on the polymeric surfactants application purpose.

Electrostatic and steric interactions in oil-in-water emulsion films from Pluronic surfactants.

Stabilization of oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers is described in terms of interaction surface forces. Results on emulsion films from four Pluronic surfactants, namely F108, F68, P104 and P65 obtained with the Thin Film Pressure Balance Technique are summarized. It is found that film stabilization is due to DLVO (electrostatic) and non-DLVO (steric in origin) repulsive forces. The charging of the oil/water film interfaces is related to preferential adsorption of OH(-) ions. This is confirmed by pH-dependent measurements of the equivalent film thickness (h(w)) at both constant capillary pressure and ionic strength. With reducing pH in the acidic region, a critical value (pH(cr,st)) corresponding to an isoelectric state of the oil/water film surfaces is found where the electrostatic interaction in the films is eliminated. At pH≤pH(cr,st), the emulsion films are stabilized only by steric forces due to interaction between the polymer adsorption layers. Disjoining pressure (Π) isotherms measured for emulsion films from all the four Pluronic surfactants used at pH

Effect of rhamnolipids on pulmonary surfactant foam films.

The effect of a rhamnolipid biosurfactant on pulmonary surfactant is studied employing the black foam film method. Pulmonary surfactant is modeled by a commercially available lung surfactant preparation (LSP). The effect of rhamnolipid concentration on the formation and stability of films formed from mixtures of LSP and rhamnolipids is experimentally studied by measurements of the probability W of formation of black foam films as a function of both LSP and rhamnolipid concentrations at the physiologically relevant electrolyte concentration C(el) = 0.15 mol dm(-3) NaCl. The obtained curves show that addition of rhamnolipid at a concentration C(RhL) = C(c) (critical concentration of black foam film formation) to LSP suspensions causes destabilization of the foam films. In this case, additional quantities of lung surfactant preparation are needed to obtain black films with probability W = 100%. Rhamnolipid adsorption and formation of mixed adsorbed layers at the solution/air interfaces of foam films formed from mixtures of lung surfactant and rhamnolipids are experimentally studied by monitoring the effect of electrolyte and rhamnolipid concentrations on the thickness h of the foam films. The incorporation of rhamnolipid ions in the adsorbed layers at the film interfaces is evidenced also by direct measurements of the disjoining pressure Pi in the films. The Pi(h) isotherms demonstrate that the added rhamnolipids change the surface electric parameters of the films and their thickness and stability at higher pressures. The obtained results show that the different molecular components in the mixture and the increased surface charge at the film interfaces originating from the rhamnolipid ions have a significant effect on the surface forces operative in the studied films.

Surface forces and properties of foam films from rhamnolipid biosurfactants.

Foam films are considered as a convenient model to study the interaction behaviour and surface properties of microbial rhamnolipid type biosurfactants. The Scheludko-Exerowa microinterferometric methodology of film thickness measurements is employed for experimental studies of microscopic foam films formed from aqueous solutions of a single rhamnolipid Rh1 (with one rhamnosyl head group) and of mixtures of rhamnolipid surfactants Rh1 and Rh2 (with two rhamnosyl head groups) at ratios Rh2/Rh1=1.2 and Rh2/Rh1=0.69. The measurements of the equilibrium thickness (h) of the obtained films as a function of surfactant concentration (Cs) and electrolyte (NaCl) concentration (C el) determine the conditions for obtaining common, common black and Newton black films. The saturation values of the diffuse electric layer potential phi 0 approximately 60 mV for the Rh1.2 and phi 0 approximately 94 mV for the Rh0.69 common films conform the ionic character of the rhamnolipids. The h(C el) curves of the rhamnolipid foam films and the directly measured disjoining pressure (Pi(h)) isotherms indicate the ranges of action of the DLVO and non-DLVO surface forces. The obtained foam film parameters allow their practical use in ecology and in various technological processes where rhamnolipid surfactants are used. Experiments with model lung surfactant (Infasurf) foam films with rhamnolipid added outline a perspective for the potential application of the foam film for investigating the effect of rhamnolipids on human alveoli.

Interaction forces in thin liquid films stabilized by hydrophobically modified inulin polymeric surfactant. 3. Influence of electrolyte type on emulsion films.

The interaction forces in emulsion films stabilized using hydrophobically modified inulin (INUTEC SP1) were investigated as a function of concentrations of electrolytes of different types (NaCl, Na2SO4, and MgSO4). At a constant disjoining pressure of 36 kPa, a constant temperature of 22 degrees C, and a film radius of 100 microm, the film thickness, hw, decreased with an increase in electrolyte concentration until a critical value, Cel,cr, was reached above which hw remained constant. Cel,cr decreased with an increase in electrolyte valency (Cel,cr = 5 x 10(-2) mol.dm(-3) for NaCl and 1 x 10(-2) mol.dm(-3) for Na2SO4 and MgSO4). The reduction in film thickness below Cel,cr could be accounted for by the compression of the electrical double layer. The Pi-hw isotherms below Cel,cr could be fitted using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (constant charge and constant potential cases were considered). At a certain pressure, the film jumped to a Newton black film. The pressure at the jump decreased with an increase in electrolyte valency as a result of the reduction of the electrostatic barrier. At electrolyte (NaCl, Na2SO4, or MgSO4) concentrations higher than Cel,cr, the jump occurred at a low pressure that was independent of the electrolyte type. The thickness of the Newton black film was independent of both the concentration and nature of the electrolytes studied. The results show clearly that the polyfructose loops and tails remain strongly hydrated both in water and in high concentrations of electrolytes of different types, and these results explain the high INUTEC SP1 emulsion stability against coalescence of emulsions prepared under such conditions.

Interaction forces in thin liquid films stabilized by hydrophobically modified inulin polymeric surfactant. 2. Emulsion films.

The interaction forces in emulsion films stabilized using hydrophobically modified inulin (INUTEC SP1) were investigated by measuring the disjoining pressure of a microscopic horizontal film between two macroscopic emulsion drops of isoparaffinic oil (Isopar M). A special measuring cell was used for this purpose whereby the disjoining pressure Pi was measured as a function of the equivalent film thickness hw. The latter was determined using an interferometric method. In this way Pi-hw curves were established at a constant INUTEC SP1 concentration of 2x10(-5) mol.dm-3 and at various NaCl concentrations. At a constant disjoining pressure of 36 Pa, a constant temperature of 22 degrees C, and a film radius of 100 microm, hw decreased with an increase in the NaCl concentration, Cel, and reached a constant value of 11 nm at Cel=5x10(-2) mol.dm-3. This reduction in film thickness is due to the compression of the electrical double layer, and at the above critical NaCl concentration any electrostatic repulsion is removed and only steric interaction remains. This critical electrolyte concentration represents the transition from electrostatic to steric interaction. At a NaCl concentration of 2x10(-4) mol.dm-3 the Pi-hw isotherms showed a gradual decrease in hw with an increase in capillary pressure, after which there was a jump in hw from approximately 30 to approximately 7.2 nm when Pi reached a high value of 2-5.5 kPa. This jump is due to the formation of a Newton black film (NBF), giving a layer thickness of the polyfructose loops of approximately 3.6 nm. The film thickness did not change further when the pressure reached 45 kPa, indicating the high stability of the film. Pi-hw isotherms were obtained at various NaCl concentrations, namely, 5x10(-2), 5x10(-1), 1.0, and 2.0 mol.dm-3. The initial thicknesses are within the range 9-11 nm, after which a transition zone starts, corresponding to a pressure of about 0.5 kPa. In this zone all films transform to an NBF with a jump, after which the thickness remains constant with a further increase in the disjoining pressure up to 45 kPa, with no film rupture. This indicates the very high stability of the NBF in the presence of high electrolyte concentrations. The high emulsion film stability (due to strong steric repulsions between the strongly hydrated loops of polyfructose) is correlated with the bulk emulsion stability.

Interaction forces in thin liquid films stabilized by hydrophobically modified inulin polymeric surfactant. 1. Foam films.

Using the interferometric method of Scheludko-Exerowa for investigation of foam films, we have obtained results using a hydrophobically modified inulin polymeric surfactant (INUTEC SP1). Measurements were carried out at constant INUTEC SP1 concentration of 2 x 10(-)(5) mol.dm(-)(3) and at various NaCl concentrations (in the range 1 x 10(-)(4) to 2 mol.dm(-)(3)). At constant capillary pressure of 50 Pa, the film thickness decreased gradually with an increase in NaCl concentration up to 10(-)(2) mol.dm(-)(3) NaCl above which the film thickness remains virtually constant at about 16 nm. This reduction in film thickness with an increase in NaCl concentration is due to the compression of the double layer and at the critical electrolyte concentration (C(el,cr) = 10(-)(2) mol.dm(-)(3)) the electrostatic component of the disjoining pressure is completely screened and the remaining pressure is due to the steric interaction between the adsorbed polymer layers. Disjoining pressure-thickness (Pi-h) isotherms were obtained at C(el) < C(el,cr) (10(-)(4) - 10(-)(3) mol.dm(-)(3)) and C(el) > C(el,cr) (0.5, 1, and 2 mol.dm(-)(3)). In the first case, the disjoining pressure isotherms could be fitted using the classical DLVO theory, Pi = Pi(el) + Pi(vw), and using the constant charge model. At C(el) > C(el,cr), the main repulsion is due to the steric interaction between the polyfructose loops that exist at the air-water interface, i.e., Pi = Pi(st) + Pi(vw). Under these conditions, there is a sharp transition from DLVO to non-DLVO forces. In the latter case, the interaction could be described using the de Gennes' scaling theory. This gave an adsorbed layer thickness of 6.5 nm which is in reasonable agreement with the values obtained at the solid-solution interface. The Pi-h isotherms showed that these foam films are not very stable and they tend to collapse above a critical capillary pressure (of about 1 x 10(3) Pa), and these results could be used to predict the foam stability.

Foam and wetting films: electrostatic and steric stabilization.

Foam films and wetting films on quartz, obtained from aqueous solutions of two different surfactants [cetyltrimethylammonium bromide (CTAB) and PEO-PPO-PEO triblock copolymer (F108)] with NaCl as a background electrolyte, are considered as convenient models to compare the properties of symmetric (free thin liquid films) and asymmetric (thin liquid films on solid substrate) films with the same air/solution interface. Microinterferometric methodology of assessment of foam and wetting films is used to allow precise determination of the film thickness. In the case of CTAB films, experimental data for the potential phi(0) of the diffuse electric layer at the solution/air interface and the potential phi(1) at the solution/quartz interface are used to analyze the stability of the films studied. A conclusion drawn is that electrostatic interaction forces stabilize both kinds of films studied. It is shown that with increasing CTAB concentration a charge reversal occurs at both the solution/air and solution/quartz interfaces that determines the stability/instability of the foam and wetting films. Concentration ranges where both types of films produce stable (equilibrium) films are found. There are also concentration ranges where the films either rupture or are metastable (quasi-equilibrium). The CTAB concentration ranges, which provide formation of unstable (rupturing and metastable) and stable films, are different for symmetric (foam) and asymmetric (wetting) thin liquid films. It is only at high CTAB concentrations (>2 x 10(-4) mol dm(-3)) that both cases render formation of stable equilibrium films. In the case of F108 films, the comparison of foam films and wetting films on quartz indicates film stability that is either electrostatic or steric in origin. On the basis of the effect of electrolyte concentration on film thickness, the transition from electrostatic to steric stabilization is demonstrated for both kinds of films. The critical electrolyte concentration at which this transition occurs is determined. Foam films are found to be always stable (equilibrium). Formation of either unstable (rupturing and metastable) or stable (equilibrium) wetting films on quartz is established depending on the solution composition. The effects are similar for both hydrophilic and hydrophobic quartz surfaces. The results obtained show certain similarity between foam and wetting films. In both cases, electrostatic forces below the critical electrolyte concentration, and above it steric forces govern film stability. Some specific properties of the wetting films are induced by the asymmetric boundary conditions as distinct from symmetric foam films.

Structure and surface energy of the surfactant layer on the alveolar surface.

The surface energy of the alveolar surfactant layer is determined in the scope of a modification of the structural model of Larsson et al. [(1999) J Disp Sci Technol 20:1-12], according to which this layer is built up of a lipid monolayer adsorbed at the hypophase/air interface and supported by a network of lipid bilayers immersed into the hypophase, i.e., the alveolar liquid. Formulae are derived for the dependence of the specific surface energy of the surfactant layer on the distance between the bilayers constituting the layer. It is shown that at equilibrium this energy can have values comparable with or less than 1 mJ/m2 needed for normal functioning of the alveolus during the respiration cycle. The specific surface energy of the surfactant layer with monolayer-bilayer structure can have such low values only if the layer is of optimal thickness and if the specific line energy of the monolayer-bilayer contact lines is negative and that of the bilayer-bilayer contact lines is positive. It is found that in dynamic regime the change in the specific surface energy of the alveolar surfactant layer with bilayer-monolayer structure is in qualitative agreement with that determined experimentally during lung inflation and deflation.

Thermal transitions in dimyristoylphosphatidylcholine foam bilayers.

Thermal transitions in the system dimyristoylphosphatidylcholine/water/ethanol/sodium chloride were studied in the temperature range 10-31 degrees C. The water-ethanol dispersions were investigated by differential scanning calorimetry and the foam bilayers by the microinterferometric method for investigation of thin liquid films. Calorimetry showed that an increase in ethanol content (up to 47.5 vol.%-the concentration used in the experiments with foam bilayers) did not significantly influence the temperature of the main phase transition and led to the disappearance of the pretransition. The microinterferometric study of the foam bilayer thickness showed that there were two thermal transitions-at 13 and 23 degrees C. An Arrhenius type dependence was obtained for the critical concentration of dimyristoylphosphatidyl-choline (DMPC) in the solution, which was necessary for the formation of the foam bilayer. A steep change in the slope of the linearized Arrhenius dependence was found at 23 degrees C. Values of the binding energy of a DMPC molecule in the foam bilayers were calculated using the hole-nucleation theory of stability and permeability of bilayers. It was proved that the phase transition at 23 degrees C was due to melting of the hydrocarbon tails of phospholipid molecules. The low-temperature phase transition was assumed to be due to a change in the tilt of the hydrocarbon tails. These experiments demonstrate for the first time the occurrence of phase transitions in foam bilayers.

Stability and permeability of amphiphile bilayers.

In this review the rupture and permeability of bilayers are considered on the basis of a mechanism of the formation of microscopic holes as fluctuations in the bilayers. The hole formation is treated as a nucleation process of a new phase in a two-dimensional system with short-range intermolecular forces. Free rupture and deliberate rupture (by alpha-particles) of foam bilayers (Newtonian black films) are discussed. A comparison is made between the rupture of foam and emulsion bilayers. Experimental methods for obtaining foam and emulsion bilayers from thin liquid films are considered. Methods for investigating the stability and permeability of foam bilayers, which are based on a microscopic model allowing the use of amphiphile solutions with very low concentrations, are described. Experimental dependences of the lifetime of bilayers, the probability of observing the foam bilayer in a foam film, the gas permeability of bilayers, etc. on the concentration of amphiphile molecules in the solution are reported. The influence of temperature and external impact (e.g. alpha-particle irradiation) have also been experimentally studied. A good agreement between theory and experiment is established, allowing determination of several characteristics of foam and emulsion bilayers obtained from ionics or non-ionics: the specific edge energy of bilayer holes, equilibrium surfactant concentration below which the bilayer is thermodynamically metastable, work for the formation of a nucleus hole, number of vacancies in the nucleus hole, coefficient of gas diffusion through the bilayer, etc. On the basis of the effect of temperature on the rupture of foam bilayers the binding energy of a surfactant molecule in the bilayer is determined. The adsorption isotherm of surfactant vacancies in the foam bilayer is obtained which shows a first-order phase transition. Some applications to scientific, technological and medical problems are considered. The foam bilayer is used as a model for investigating short-range forces in biological structures, the interaction between membranes and cell fusion. It is also shown that the foam bilayer is a suitable model for studying the alveolar surface and stability. On that basis a clinical diagnostic method is developed for assessment of the human foetal lung maturity.

Pure and mixed lipid black foam films as models of membrane fusion.

Black foam films (BFF) from water solutions of the phospholipid dilauroyl lecithin (DLL) with admixtures of palmitoyl lysolecithin (Lyso) were formed. Microscopic BFF were studied by the method of Scheludko and Exerowa. The formation probability for BFF and the BFF lifetime in a black state before film rupture were measured as functions of the film composition. At a fixed overal lipid concentration it was shown that an increased percentage of Lyso exponentially increased the lifetime of the film up to the CMC of Lyso. This stabilizing Lyso effect nicely corresponds with its stabilizing action on the waiting time for fusion of two contacting black lipid membranes (BLM), as found by Chernomordik et al. In contrast, Lyso is known to destabilize a single BLM. In this way we have found experimental proof of our earlier prediction that Lyso should have opposite effects on the lifetimes of BLM and BFF. In addition, we have shown for the first time that foam films made of lipids are a convenient model for monolayer membrane fusion studies. This model is characterized by its simplicity and experimental reliability and provides a means for quick screening of the fusogenic capacity of various amphiphilic and hydrophilic admixtures.

Bilayer lipid membrane permeation and rupture due to hole formation.

A theory is developed for the permeation and rupture of bilayer lipid membranes due to fluctuation formation of holes (or pores) in them. The two monolayers of the bilayer lipid membrane are considered as mutually adsorbed on each other and the bilayer lipid membrane equilibrium is described by an adsorption isotherm in mean field approximation. The theory of nucleation is used for determination of the work for hole formation and the hole equilibrium size distribution as functions of the concentration C of monomer lipid in the solution. The bilayer lipid membrane permeation and rupture are analyzed from a unified point of view and expressions are derived for the dependence of the bilayer lipid membrane diffusion permeability coefficient and lifetime on C. The effect of foreign bodies (e.g., proteins) on the bilayer lipid membrane permeation and rupture is considered and a possible experimental application of the theory is discussed. The results obtained are directly applicable to dense monolayer films on liquid surfaces.

Foam separation of DNA and proteins from solutions.

Foam separation on BSA-DNA (bovine serum albumine/deoxyribonucleic acid) and Lysozyme-DNA systems is performed. The separation of the total protein from DNA is evaluated for dissociated chromatin solution. Foam separation for the same systems is done also by a new method of creating a pressure gradient in the Plateau-Gibbs borders in the foam and obtaining a "dry" foam. It is shown that the effectiveness of the foam separation can be improved significantly by the application of the latter method. Some factors (pH, initial concentration of the solution, expansion factor of the foam) influencing the separation of proteins from DNA in the foam and in the residual solution are studied as well.