D. Exerowa

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.

Dependence of the equilibrium thickness and double layer potential of foam films on the surfactant concentration

SummaryThe effect of the surfactant concentrationcs on the equilibrium thicknesshr, of foam films and the potentialϕ0 of the diffuse electric layer is studied in a wide concentration range (including cmc). The cell for investigation of thin liquid films is improved and a good reproducibility of the measuredhr is obtained. The possibilities for determination of the capillary pressurePσ in.the cell for investigation of microscopic thin liquid films are discussed. The values ofPσ can be measured with sufficient accuracy by the capillary-rise method, using a tube, identical with the tube of the cell for thin liquid films. The dependencehr(cs) is studied for nonionic surfactants (decylmethyl sulphoxide, nonyl-phenol 20-glycol ether and dodecyl 11-ethylene oxide) and the ionic sodium dodecyl sulphate. After reaching a steady value (a plateau in thehr(cs) curve) at high concentrations (about cmc) a decrease in the equilibrium thickness of nonionic surfactants is found. The possible reasons for this decrease are discussed. It is shown that this effect probably is due to the electrostatic disjoining pressure. The values of theϕo-potential at the plateau of thehr(cs) curves are determined by the DLVO-theory using thehr, values. The so determinedϕ0-values are more correct than the obtained earlier by this method because of the more precise measurement of the equilibrium thickness and the direct experimental determining of the capillary pressure. A qualitative interpreting of theϕ0(cs) dependence is made on the basis of the surfactant adsorption at the solution/air interface and it is found that for nonionic surfactants the surface charge saturation is very near to the saturation in the adsorption layer, and for the ionic surfactants the adsorption layer saturation is at higher concentration.ZusammenfassungEs wurde der Einfluß der Tensidkonzentrationcs auf die Gleichgewichtsdickebr von Schaumfilmen und das Potential ϖ0 der diffusen elektrischen Doppelschicht in einem weiten Konzentrationsbereich (einschließlich cmc) für nichtionische Tenside (Decylmethylsulfoxide, Nonylphenol-20-Glycolether und Dodecyl-11-Ethylenoxid) und ionogene Tenside (Natriumlaurylsulfat) studiert. Nach dem Erreichen eines konstanten Wertes (= Plateau in derbr(cs)-Kurve) bei höheren Konzentrationen (etwa crnc) wird eine Abnahme in der Gleichgewichtsdicke der nichtionogenen Tenside gefunden. Dies hängt wahrscheinlich mit dem elektrostatischen “disjoining pressure” zusammen. Aus den Überlegungen über das ϖ0-Potential wird gefolgert, daß für nichtionogene Tenside die Oberflächen-ladungssättigung nahe bei der Sättigung in der Adsorptionsschicht liegt. Bei ionogenen Tensiden liegt die Sättigung in der Adsorptionsschicht bei höheren Konzentrationen.

Direct measurement of disjoining pressure in black foam films II. Films from nonionic surfactants

Abstract Experiments are reported with foam films from two different polyoxyethylenic nonionic surfactants of relatively short and long polyoxyethylene chains. The direct measurement of the film disjoining pressure was combined with measurements of the film thickness dependence on the electrolyte concentration. The results obtained are discussed in connection with: (i) the type of black films observed, (ii) the influence of the POE chain length on the film properties, (iii) the agreement between the disjoining pressure isotherms obtained and the theoretical ones, (iv) the relationship between the charge and potential of the interacting diffuse double layers.

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.

Nucleation mechanism of rupture of newtonian black films. I. Theory

Abstract A theory is developed for the rupture of Newtonian black films by a mechanism of nucleation of holes in these bimolecular foam films. The two film monolayers are considered as mutually adsorbed on each other and the film thermodynamic equilibrium is described by an adsorption isotherm in mean field approximation. Expressions are derived for the nucleation work, the steady-state nucleation rate, and the nucleation time lag of mono- and bilayer circular holes of critical size. A formula is obtained for the mean time for film rupture as a function of the activity (or the concentration) of surfactant in the bulk of the solution, and a critical activity (or concentration) for obtaining a Newtonian black film is defined. The present theory is directly applicable to the case of bilayer lipid membranes (BLM) which are a suitable model for studying the stability of biological membranes.

Effect of surfactant concentration on the critical thicknesses of liquid films

SummaryIt is shown that an increase in surfactant concentration at first causes the critical thicknesses of rupture on microscopic films to decrease but gradually a nearly constant value is reached. With films of aqueous solutions of fatty acids (valeric, caproic, caprylic and capric acids), this dependence correlates well with the effect of the surfactant concentration on the damping of capillary waves. With surfactants of the detergent type (OPE-7, OPE-20) in addition to the change of the critical thickness, the transition from rupture to formation of black spots is described, as the surfactant concentration exceedsCbl. The remarkable fact in the latter case is the independence of the critical thickness of the final state, be it rupture or formation of first or second black films.On the basis of the experimental data the assumption is put forward that the critical thickness of rupture or the critical thickness of formation of black spots is substantially affected by macroscopic non-uniformities in the film thickness. Thus the conclusion is reached that the critical thickness of an ideally plane parallel film which is the object of the theory, must be obtained by extrapolation of the measured value toward extremely small radii.ZusammenfassungEs wird gezeigt, daß die kritischen Zerreißdicken mikroskopischer Schaumfilme, die mit der Zunahme der Tensidkonzentration zunächst abnehmen, dann aber ungefähr konstante Werte erreichen. Diese Abhängigkeit ist in guter Übereinstimmungmitdem Einfluß des Tensides auf die Kapillarwellendämpfung in wäßrigen Lösungen von vier Fettsäuren. Bei Tensiden, die gute Schäumer sind (OPE-7, OPE-2) zerreißt der Film mit der Zunahme der Konzentration überCbl nicht, sondern es bilden sich schwarze Flecke. Es ist sehr interessant, daß die kritische Dicke unabhängig von dem Endzustand (Zerreißen oder Bildung schwarzer Flecke) ist.Aufgrund dieser Experimente wird angenommen, daß die kritische Dicke bei dem Zerreißen oder bei der schwarzen Fleckebildung mikroskopischer Schaumfilme wesentlich von den makroskopischen Ungleichheiten der Filmdicke beeinflußt wird. Daraus wird gefolgert, daß die kritische Dicke eines idealen planparallelen Films durcheine Extrapolation der Meßwerte aufeinen unendlich dünnen Film ermittelt werden kann.

Hole-mediated stability and permeability of bilayers

Abstract Membrane, foam and emulsion bilayers are considered from a unified point of view as two-dimensional phases and their stability with respect to rupture and permeability with regard to molecular species is analysed with the help of the fluctuation theory of nucleation of holes in them. Molecular models of bilayers are described and both the thermodynamics and the kinetics of the ‘condensation’ of the two-dimensional ‘gas’ of vacancies in a bilayer are outlined. It is shown how the bilayer mean lifetime τ, the probability W of observing a foam bilayer in a thicker foam film and the bilayer diffusion permeability coefficient P d depend on the concentration C of monomer amphiphiles in the surrounding solution. The effect of various factors (particularly α-particle irradiation and an externally applied electric field) on τ and P d is also discussed. Experimental techniques for investigating foam films are described and experimental data on τ(C), W(C) and Pd(C) are presented for bilayers of lipids and s...

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.

Common black and Newton film formation

Abstract By model investigations of microscopic foam films (stabilized with sodium octyl sulfate, NaOS, and sodium dodecyl sulfate, NaDoS, in the presence of electrolyte) and by study of the properties of the adsorbed layers of the solutions it is shown that the adsorbed layers at the interface are essential for the formation of common black and Newton black spots and films. The adsorption isotherms are obtained using a precise spherotensiometric method. The model microscopic film with a radius of 10−2 cm that has been used makes it possible to work at very low surfactant concentrations and thus the lowest surfactant concentration at which a black spot appears has been found. The concentrations of formation of common black spots and Newton black spots are distinguished. A correlation between the probability of the formation of black spots (common and Newton) and the state of the adsorbed layer is established. For instance, the formation of common black spots and the deviation from the ideal state in the adsorbed layer are related to the formation of associates (premicelles) in the bulk and their adsorption at the interface. The formation of common black spots with a probability of one occurs at a concentration close to the saturation concentration Γ∞ of the adsorbed substance. The close packing of the molecules at the interface is a necessary condition for the formation of Newton black spots. The formation of Newton black spots with a probability of one occurs at a somewhat higher concentration (two to three times higher than the concentration corresponding to the closest packing of molecules at the interface). The dependence of the lifetime (stability) of the two types of black films (common and Newton) on the surfactant concentration is also studied. It is found that at a definite concentration (different for the common black and Newton films) the film persistence increases steeply. It is shown that the proposed surface model is not decisive for the understanding of this stability (unlike the formation of the two types of black films).

Effect of adsorption, ionic strength and pH on the potential of the diffuse electric layer

ZusammenfassungDas Potentialϕ0 der diffusen elektrischen Doppelschicht wurde nach der Methode des flüssigen Gleichgewichtfilms in Schäumen (1) bestimmt. Die Dicke der Filme wurde interferometrisch (7, 8) gemessen.Es wurde die Abhängigkeit desϕ0-Potentials von der KonzentrationCS des grenzflächenaktiven Stoffes (Schaumfilmstabilisator) untersucht. Dieϕ0 (CS)-Kurve hat am Anfang ein kleines Plateau, danach folgt ein Anstieg bis zu einem Sättigungswert. Dieser Sättigungswertϕ0∞ ist für die verschiedenen Tenside unterschiedlich. Nichtionogene Tenside besitzen ein geringesϕ0∞, die ionogenen Tenside geben höhere Werte.Es wurde weiterhin der Einfluß der Elektrolytkonzentration auf dasϕ0-Potential untersucht. Eine spezifische Wirkung der Elektrolyte wurde nicht gefunden. Dagegen wurde ein starker Einfluß des pH-Wertes auf dasϕ0-Potential festgestellt. Bei einem bestimmten pH-Wert fälltϕ0 bis zu Null ab, d. h. ein isoelektrischer Punkt tritt auf. Die an Schaumfilmen gemessenenϕ0-Werte wurden mit denϕ0-Werten an der Oberfläche derselben Lösung verglichen.SummaryThe potential,ϕ0, of the diffuse electric layer was determined by the equilibrium liquid film method (1). The method is based on calculating theϕ0-potential by means of the DLVO theory of the electrostatic disjoining pressure at known concentration,Cel of the solution forming the microscopic liquid film and at known thickness,hr of the film. The thickness of the film was measured by the microinterferometric method (7, 8). The calculation of the potential was carrried out by means of the more general eqs. [2] and [3], valid for a wide range ofCel andϕ0. The dependence of the potential of the diffuse electric layer on the concentration of surface activ agent (surfactant) stabilizing the liquid film was found. It was established that ionogenic SAS show a higher potential than the non-ionogenic ones. The potential of SAS-free solutions was determined — ca. 30 mV.The dependence of the potential of the diffuse electric layer on concentration and kind of electrolyte was also found.A strong effect of pH onϕ0-potential was observed and the isoelectric points were found. An analysis is made of theϕ0-potential values obtained by the method of equilibrium liquid film with respect to their similarity to those on the boundary of the bulk liquid.