T. Kolarov

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.

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.

The transition region between aqueous wetting films on quartz and the adjacent meniscus

Abstract Aqueous wetting films on a plane quartz plate are studied experimentally to elucidate some of the details of the transition region between the film and

Thickness transitions in lysolecithin foam films

Abstract Microscopic foam films have been obtained from aqueous palmitoyl lysophosphatidylcholine (lyso PC) solutions and the effect of the monovalent Na − and divalent Ca 2− on their equilibrium thickness has been studied. At low NaCl concentrations, silver-coloured films have been obtained. A thickness transition zone is found with increase in NaCl concentration. leading to the formation of Newtonian black films (NBF). These results are compared with previous results obtained for microscopic films of non-ionic surfactant solutions. A remarkable effect of divalent Ca 2− on the properties of the films is found. In the interval 0.0001–0.0008 mol dm −3 CaCl 2 , NBF are obtained. When the electrolyte concentration ( C cl ) is increased to 0.001 mol dm −3 CaCl 2 , a transition from NBF to silver films is observed. The thickness of these films decreases with further increase in C cl , until transitions to common black films, and subsequently again to NBF, are observed. This result is interpreted as due to specific interaction ofCa 2− ions with the intrinsically uncharged phospholipid head groups. In the case of lyso PC foam films with CaCl 2 added, direct measurements of the disjoining pressure vs thickness ( II ( h )) isotherms have been performed and a barrier-like transition to NBF is found. The results are discussed in connection with the predictions of the DLVO theory. In the case of NaCl, the diffuse electric layer potential (φ 0 ) and charge density (σ 0 ) are low, similar to the results obtained for non-ionic surfactant films. In the case of CaCl 2 , an increase in C cl , leads to an increase in φ 0 , and σ 0 , which is interpreted as due to increased divalent ion binding. The comparison between the experimental II ( h ) isotherms and the theoretical predictions shows that in the cases of low φ 0 and σ 0 studied, at constant C cl , electrostatic repulsion under the conditions of constant charge is operative. The results are considered in connection with the possibilities of using the microscopic foam film model for biological studies.

Transition from electrostatic to steric stabilization in foam films from ABA triblock copolymers of poly(ethylene oxide) and poly(propylene oxide)

The equilibrium thickness of microscopic foam films obtained from aqueous solutions of two different poly(ethylene oxide)/poly(propylene oxide)/poly(ethylene oxide) triblock copolymers (Synperonic PE P85 and F108) is experimentally determined. The amphiphilic triblock copolymers adsorb at the air/solution interface as a brush layer. The foam film may be stabilized by electrostatic and/or steric repulsion. At a given copolymer concentration (slightly lower than the CMC) increasing the electrolyte concentration induces a transition from electrostatic to steric stabilization. The thicker foam films are electrostatically stabilized. The thinnest foam films are sterically stabilized, but their thickness is unexpectedly larger than twice the brush layer thickness. While the thickness h of these films strongly depends on the degree of polymerizationN, its scaling structure is essentially the same: two brush layers (h1 ∼ N) and an aqueous core (h2 ∼ N3/5). The microscopic foam film is a suitable tool for exploring the transition from electrostatic to steric stabilization.

Surface forces in foam films from ABA block copolymer: a dynamic method study

The thinning of foam films from aqueous solutions of an ABA triblock copolymer of polyethylene oxide and polypropylene oxide (average molecular weight 14,000 g/mol) is studied experimentally. The dependence of the surface forces on film thickness is obtained by the dynamic method of Scheludko and Exerowa.The total surface force measured in foam films (radius ≈ 60–70 μm) from 10−5 M (0.014 wt%) polymer solution with 0.1 M NaCl is positive at thicknesses from about 800 Å down to 460 Å. The electrostatic repulsion is negligible while the contribution of van der Waals attraction is small (within 15%). Therefore a positive surface force component predominates. Most probably it arises from steric interactions between the hydrophilic polyethylene oxide tails of the polymer. The dynamic method appears to be a suitable technique for exploring the stabilization of foam films from ABA copolymers.

Profile of the transition region between aqueous wetting films on quartz and the adjacent meniscus

Abstract Between an equilibrium wetting film and the adjacent meniscus a transition region exists, its shape being determined by both capillary and surface forces. A detailed experimental study of the profile of the transition region has been performed. Using a special device, an aqueous meniscus is pressed through a tube against a quartz glass plate. The resulting circular microscopic wetting film as well as the interference pattern of the surrounding meniscus are photographed through a microscope in reflected light. The local thicknesses are evaluated from the corresponding optical densities in the photographs and used to construct the profiles of both the transition region and the meniscus. The profiles obtained for aqueous solutions of 0.1, 0.5 and 1 mmol dm −3 KCl and those theoretically calculated on the basis of a suitable disjoining pressure isotherm are found to be in good agreement.

Charge reversal at the air/water interface as inferred from the thickness of foam films

Experiments are reported with foam films from aqueous solutions with increasing concentration of a cationic surfactant. A correlation is established between the foam film thickness and the possible variation of diffuse electric layer potential at the air/water interface from a negative value in absence of surfactant to positive values at higher surfactant concentrations. It is concluded that a charge reversal at the air/water interface is expected to occur under increasing concentration of cationic surfactants in aqueous solutions.

Foam and Wetting Films from Aqueous Cetyltrimethylammonium Bromide Solutions: Electrostatic Stability1

Foam films and wetting films on quartz formed from aqueous solutions of cetyltrimethylammonium bromide (CTAB) are investigated in a wide range of surfactant concentrations in the presence of background electrolyte (5 × 10–4 mol dm–3 NaCl). Foam and wetting films are convenient models for the study of symmetric (free thin liquid films) and asymmetric (thin liquid films on solid substrate) films with the same air/solution interface. Microinterferometric methods of assessment of foam and wetting films are used which allow precise determination of the film thickness. Determined are the values of the potential ϕ0 of the diffuse electrical layer at the solution/air interface (applying the method of “equilibrium foam films”) and the potential ϕ1 at the solution/quartz interface (applying the method of capillary electrokinetics). These values are used to analyze the stability of the films studied in terms of the DLVO theory. A conclusion drawn is that both kinds of films studied are stabilized by electrostatic interaction forces. It is shown that with increasing CTAB concentration, a charge reversal occurs at both the solution/air and solution/quartz interfaces which determines the stability/instability conditions of the foam and wetting films. Concentration ranges where both kinds 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 the 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 (higher that > 2 × 10–4 mol dm–3) that both cases render formation of stable equilibrium films. These studies give direct experimental indications that the electrostatic interactions between identical or different interfaces can differ when the surfactant concentration is varied.

Foam and wetting films from amphiphilic block copolymers: Isoelectric state and stability

Foam and wetting films from PEO-PPO-PEO triblock copolymers Synperonic P85 and F108 are studied under the identical conditions, using microinterference method. The range of background electrolyte concentration, where DLVO (electrostatic and van der Waals) forces and non-DLVO (steric) forces act in the films, is determined. From the dependence of the film thickness on pH, it is unambiguously shown that electrostatic interactions (i.e., the potential and surface charge) in the foam and wetting films caused by the presence of nonionic polymer surfactants arise due to the preferential adsorption of OH− ions at the solution-air interface. The films obtained below the critical pH values are sterically stabilized; i.e., a decrease in pH induces a transition from electrostatic to steric stabilization. Three-layer models are designed for both types of films that allow to calculate electrostatic disjoining pressure Πel. The values of ϕ0 potential of the foam film are used to calculate Πel in wetting films. A relation between the isoelectric state of foam and wetting films and their stability is found to exist in the range of pH corresponding to electrostatic stabilization. Metastable films, film rupture, or the transition to sterically stabilized films were also found.