Stability Of Foam Films Research Articles

Isoelectric state and stability of foam films, bubbles and foams from PEO–PPO–PEO triblock copolymer (P85)

Influence of pH of P85 copolymer solutions on stability of microscopic foam films (static conditions), lifetime of single bubbles at solution surface (quasi-static conditions), volume of the foam formed (dynamic conditions) and time of rupture of the foam column was investigated. Variations of the film equilibrium thickness as a function of pH were determined for different ionic strengths of the solutions using microinferometric method, while the combined pneumatic–mechanical method was applied in experiments on foams. It was found that lowering the pH reduced stability of the foam films, and at lower ionic strength the films ruptured at pH 2.9 (isoelectric point). Simultaneously, the lifetime of single bubbles was much shorter at lower pH of the P85 solutions. The average life, tav, was 11.1 s at pH 5.8, while at pH 3.0, only 3.1 s. Under dynamic conditions the pH lowering did not significantly influence the solution foamability.

Bounding the Stability and Rupture Condition of Emulsion and Foam Films

A scaling law is presented that provides a complete solution to the equations bounding the stability and rupture of thin films. The scaling law depends on the fundamental physicochemical properties of the film and interface to calculate bounds for the critical thickness and other key film thicknesses, the relevant waveforms associated with instability and rupture, and film lifetimes. Critical thicknesses calculated from the scaling law are shown to bound the values reported in the literature for numerous emulsion and foam films. The majority of critical thickness values are between 15 to 40% lower than the upper bound critical thickness provided by the scaling law.

Foams and foam films stabilized by C nTAB: Influence of the chain length and of impurities

Quantitative comparison of foam films and the corresponding foams is very demanding. One problem is the fact that investigations of foam films are usually performed at constant capillary pressures P, whereas in foams P is a function of the height of the foam column. A way out of this dilemma is to examine films and foams at the same P. The method of choice for the foam films is the thin film pressure balance (TFPB), whereas the corresponding investigation of foams is based on the foam pressure drop technique (FPDT). An extensive TFPB study on foam films stabilized by the cationic alkyltrimethylammonium bromides C n TAB with n = 10 , 12, 14, and 16 was performed by Bergeron. For this series a steep increase of the foam film stability was observed when the chain length was increased from n = 12 to n = 14 . Moreover, the influence of impurities was found to be limited to the films stabilized by C 12TAB. In order to study the correlation between the properties of films and foams, the present study deals with the respective foam properties investigated with the FPDT. It was found that both the steep increase in the film stability and the influence of impurities are also reflected in the properties of the foam.

What Do a Foam Film and a Real Gas Have In Common?

The stability of well-drained quasistatic foam films (thickness <100 nm) is usually discussed in terms of surface forces, which create an excess pressure normal to the film interfaces, called the disjoining pressure pi The disjoining pressure is the sum of repulsive electrostatic (pi(elec)), attractive van der Waals (pi(vdW)), and repulsive steric (pi(sr)) forces on the assumption that structural forces can be neglected. On the basis of these forces two different types of thin foam films are distinguished, namely common black films (CBF), which are mainly stabilized by pi(elec), and Newton black films (NBF), the stability of which is determined by pi(sr),With a thin-film pressure balance (TFPB) the thickness h of a foam film can be measured as a function of the applied pressure from which the disjoining pressure pi can be calculated. A thorough analysis of the results published so far reveals that the pi-h curves of nonionic surfactants measured at different surfactant concentrations resemble p-V(m) isotherms of a real gas measured at different temperatures. On the basis of these observations the van der Waals description of a real gas can be applied to foam films and a phase diagram for a foam film was constructed using the Maxwell construction.

Mixed Solutions of Anionic and Zwitterionic Surfactant (Betaine): Surface-Tension Isotherms, Adsorption, and Relaxation Kinetics

Here, we present experimental surface-tension isotherms of mixed solutions of two surfactants, sodium dodecyl sulfate (SDS) and cocoamidopropyl betaine (Betaine), measured by means of the Wilhelmy plate method. The kinetics of surface-tension relaxation exhibits two characteristic time scales, which have been distinguished to determine correctly the equilibrium surface tension. The transition from the zwitterionic to the cationic form of Betaine is detected by surface-tension measurements. Synergistic dependence of the critical micellization concentration on the composition of the surfactant blend is established. The experimental surface-tension isotherms are fitted by means of the two-component van der Waals model, and an excellent agreement between theory and experiment was achieved. Having determined the parameters of the model, we calculated different properties of the mixed surfactant adsorption layer at various concentrations ofSDS, Betaine, and salt. Such properties are the adsorptions ofthe two surfactants, the surface dilatational elasticity, the occupancy of the Stern layer by bound counterions, the surface electric potential, and so forth. In particular, the addition of a small amount of Betaine to SDS significantly increases the surface elasticity. The results could be further applied to predict the thickness and stability of foam films or the size of the rodlike micelles in the mixed solutions of SDS and Betaine.

Stability of Foam Films and Surface Rheology: An Oscillating Bubble Study at Low Frequencies

The dilational surface elasticity epsilon and the dilational surface viscosity eta of the two nonionic surfactants n-dodecyl-beta-d-maltoside (beta-C12G2) and tetraethyleneglycol-monodecyl ether (C10E4) were studied using the oscillating drop method. The experiments were carried out at different concentrations and frequencies with an accessible frequency range of 0.005-0.2 Hz. The results are discussed in the light of previous disjoining pressure measurements that demonstrated that the stability of thin liquid films cannot be explained solely by the magnitude of the surface forces. Indeed, a comparison of the results obtained for beta-C12G2 with those obtained for C10E4 reveals a correlation between the stability of the films and the surface dilational elasticity of the respective monolayers.

Stabilization of Foam Films with Non- Ionic Surfactants: Alkyl Polyglycol Ethers Compared with Alkyl Polyglucosides

Stabilization of Foam Films with Non- Ionic Surfactants: Alkyl Polyglycol Ethers Compared with Alkyl Polyglucosides

Film stability control

The factors, which determine the stability of foam films and emulsion films are considered. Many of the recent works in this field deal with complex systems containing macromolecules or solid particles beside the surfactant. The correlation between the stabilizing factors is less understood than in binary surfactant films due to a diversity of interactions between the different compounds.

Direct Method for Measuring Adsorption at the Aqueous Solution–Gas Interface Based on the Stabilization of Foam Films in Hydrophobic Tubes. The Adsorption Isotherm of Sodium Dodecyl Sulfate

It was found that the hydrophobization of the inner surface of a tube offers the stabilization of films of nonfoaming aqueous solutions closing the cross section of the tube. Based on this phenomenon, a modification of the known method of determining adsorption at the solution surface based on the separation of foam films from the solution was proposed for extending the potentialities of this method to nonfoaming solutions. Adsorption of sodium dodecyl sulfate on the aqueous solution–air interface was investigated in the range of low concentrations, which earlier was not easily accessible to measurements by known methods.

Relationship between surface dilational properties and foam stability

The knowledge of the stability of foams and emulsions is very important for the control of a large number of technological processes. However, in spite of much intense research, the mechanism of foam stability is still not completely clear. Foam stability depends on many parameters but the type of the added surfactant and the surface rheological properties of the adsorption layers play the most important role. Because of the lack of reliable surface rheological data a comprehensive investigation of this problem was not yet possible. However, we can now present measurements of surface dilational properties of soluble adsorption layers in a frequency range of 1–500 Hz using a new version of the oscillating bubble method. The results are compared with measurements of foam stability. This indicates that the surface dilational viscosity plays an important role in the stability of foam films. A direct relation between surface dilational elasticity values and foam stability could not be detected for the examined systems. Pure elastic adsorption layers were not able to stabilize foam lamellas.

Influence of the polymer backbone rigidity on polyelectrolyte–surfactant complexes at the air/water interface

We report dynamic surface tension and X-ray reflectivity studies on mixed polyelectrolyte–surfactant solutions. We have studied two different polyelectrolytes, polyacrylamide sulfonate, with a flexible backbone and xanthan, with a rigid backbone. We compare the results with the surface rheology and foaming behaviour of the same solutions. The static and dynamic properties of the mixed surface layers are similar for the solutions with the two polymers, only the behaviour upon large compressions is different. This might be at the origin of the differences in foamability and foam film stability observed with the two systems.

Rupture of common black films: Experimental study

The dependence of the mean lifetime of microscopic horizontal common black foam films from sodium dodecyl sulphate (SDS) on the surfactant concentration C is studied at various electrolyte concentrations C el by the thin liquid film microinterferometric technique. These data are used for determination of the surfactant concentration C τ at which the common black films have a constant mean lifetime. Thus the parameter C τ is proposed as a new characteristic of the black foam film stability at different electrolyte concentrations. The C τ ( C el) dependence is obtained within the range 0.1–0.5 mol dm −3 NaCl. The results for the stability of the common black films are compared with those for the Newton black films (foam bilayers). It is shown that the two parameters τ and C τ characterising the stability of the black films are constant above 0.2 mol dm −3 NaCl. This fact allows us to hypothesise that the thinner common black films (thickness below 8.4 nm) rupture by the hole nucleation mechanism analogously to the Newton black films. The statistical analysis of the data for the lifetimes of the SDS black foam films shows that this hypothesis can be adopted for the rupture mechanism of all common black films studied.

The isoelectric state at the solution/air interface — effect on the stability of foams and foam films from nonionic surfactants

Model studies of foams and single foam films under close and defined conditions are employed to study the effect of the isoelectric state (pH ∗) at the solution/air inteface on the stability (lifetime) of foams from tetraethyleneglycol monodecyl ether C 10(EO) 4 and eicosaoxyethylene nonylphenol ether, NP20. The parameter pH ∗ and the potential of the diffuse electric layer Φ o are found by the method of the equilibrium foam film. A metastable zone in which films are both equilibrium and rupturing, is established to appear pH values higher than pH ∗. Isotherms of disjoining pressure II( h) are measured for the first time at different pH and its influence on the interaction forces in the foam film is revealed. At constant solution ionic strength the pH change affects strongly the pressure at which films rupture. However, at equal ionic strength and surfactant concentration, the stability of foams dramatically decreases when pH ∗ is reached.

Interaction between carboxymethylchitin and tetradecyltrimethylammonium bromide: phase separation, surface tension and stability of microscopic foam films and foams

The bulk and interfacial properties of water soluble nonstoichiometric surfactant-polyelectrolyte complexes (SPEC) between oppositely charged tetradecyltrimethylammonium bromide (TDAB) and carboxymethylchitin (CMCh) were investigated by tensiometry, monolayer technique, viscosimetry and by the method of contact interactions, allowing us to measure the stability to rupture of microscopic foam films (MFF) formed between gaseous bubbles in liquid media. The calculated standard free energies of adsorption Δ adG o 298 and aggregation Δ aggG o 298 per one mol of CH 2-groups of alkyl radicals of cationic surfactant molecules bound to anionic macro-ions were found to be equal to −2.4 kJ mol −1, that is much greater than for free ionic surfactants in the solution (−1.8 kJ mol −1). The adsorption of SPEC is highly cooperative, and the area per one surfactant molecule belonging to SPEC at the air-water interface does not depend on the surfactant polyelectrolyte composition of the solution, Z=[TDAB]/[CMCh], in the range 0.001–1. The effect of the concentrations of TDAB and CMCh on the life-time before rupture of MFF was found to be highly synergistic. The reversible transfer from expanded to more compacted (aggregated) conformations of nonstoichiometric water soluble SPEC occurs at low CMCh concentration (less than 10 −4 base-mol dm −3) for a SPEC critical composition (cc) equal to φ ∗≈0.1 . The bulk and interfacial properties (viscosity, adsorption activity, stabilizing ability for MFF) of SPEC undergo a sharp variation in the course of this transfer.

Stability Factors of Foam Film in Contrast to Fluctuation Induced by Humidity Reduction

We have examined the relationship between dynamic surface properties of aqueous SDS solutions and foam film stability by observing the dynamic surface tension (γt), elongation of the lamellae (Llamellae), and the thickness of the lamellae measured by FT–IR. Three foam film models were used by controlling such factors as the Marangoni effect by changing the SDS concentration, the disjoining pressure with the addition of electrolytes, and the surface viscosity with the addition of glycerin. In order to obtain information about the most effective factor in foam film stability, the lifetime of these films was measured under the circumstances of low relative humidity, at which vigorous fluctuation occured in the foam film. Foam film stability was found to be remarkably enhanced by the Marangoni effect and surface shear viscosity in a thin aqueous film at a relative humidity between 60 and 75%. Under these conditions, mild fluctuation would occur in the films. On the other hand, foam film stability was found to be maintained by the disjoining pressure in a thin aqueous film at the humidity below 60%. Under these conditions, vigorous fluctuation would occur in the foam film. It is concluded that foam film stability of an ionic surfactant solution mainly depends on the disjoining pressure which generates a repulsion force between the two surfaces in contrast to the vigorous fluctuation induced by humidity reduction.

Disjoining Pressures and Film Stability of Alkyltrimethylammonium Bromide Foam Films

The alkytrimethylammonium bromide surfactant series, CnTAB, n = 10, 12, 14, 16, is used to perform a systematic study of the forces and stability of foam films produced from soluble cationic surfactants. Both surface tension and disjoining pressure isotherms are measured for each surfactant. This information is then combined with neutron reflectivity and dynamic surface tension results found in the literature to provide an understanding of how the surfactant chain length can effect the forces and stability in thin-liquid films. For stable films, we find good quantitative agreement between the interaction forces measured in foam films and those reported from surface force studies on similar systems. We also find that the surfactant's hydrocarbon chain length and packing can strongly influence film stability. For highly purified surfactants an abrupt increase in film stability is seen when extending the chain length from C12TAB to C14TAB. When an uncharged cosurfactant is present, this stability transition ...

Surfactant liquid and black foam film formation and stability in vitro and correlative conditions in vivo

We studied foam film formation by three preparations that are used as surfactant replacement therapy by injection into the lungs of neonatal infants with surfactant insufficiency (“respiratory distress syndrome”). The preparations contain either all putative hydrophobic components of the normal surfactant system (prepared from bovine lung lavage, Infasurf (IN); prepared from minced bovine lung tissue, Survanta (SU)) or a single component of the system, dipalmitoylphosphatidylcholine (Exosurf Neonatal (EX)). Foam film formation is relevant directly to normal surfactant structure and function in vivo in the neonatal period (Pediatr. Res. 12 (1978) 1070–1076). We assessed morphology and stability of liquid to black film formation and its dependence on substrate concentration C, adsorption time, temperature T, “capillary pressure” P c, film drainage time τ 0–1 and black film formation times τ 1–2. We found that (1) in contrast with the other preparations, IN regularly formed stable black films at the lowest C (“threshold C” C t) under all conditions of T (22 and 37°C) and P c (0.3, 0.4, 1.2 and 2.4 × 10 3 dyn cm −2), (2) EX also formed black films but differed from IN in that C t was higher, film integrity required a longer adsorption time and black film formation was blocked (instability) at P c values of 1.2 and 2.4 × 10 3 dyn cm −2 and (3) SU did not form black films but instead formed slowly (long τ 0–1 and τ 1–2), at relatively high C t, viscosity-dependent inhomogeneous atypical (“rheological”) films containing aggregates of material in the preparation. Extrapolation to in-vivo conditions of C, T and P c indicates that both IN and SU (but not EX) may form stable foam films in situ, that film formation by IN is the more efficient process and that stable surfactant foam films, formed naturally in vivo, are consistent with both the in-vitro characteristics described here and correlative lung function in vivo.

Effect of the surfactant concentration on the kinetic stability of thin foam and emulsion films

K. P. Velikov, O. D. Velev, K. G. Marinova and G. N. Constantinides, J. Chem. Soc., Faraday Trans., 1997, 93, 2069 DOI: 10.1039/A608305F

The Surface Evolver and the stability of liquid surfaces

The Surface Evolver is an interactive program for studying the shapes of liquid surfaces. Recently added features permit the calculation of the Hessian matrix of second derivatives of the energy. The Hessian can be used for fast convergence to an equilibrium, and eigenvalue analysis of the stability of that equilibrium. This paper describes the use of the Hessian by the Surface Evolver, presents some sample stability analyses, and gives some numerical results on the accuracy and convergence of the methods. It is also shown how one can evolve unstable surfaces.

Rapid in vitro tests of surfactant film formation: advantages of the Exerowa black film method.

Based on our research, the natural configuration of surfactant from birth through adulthood takes the form of intraalveolar bubbles. Thus, bubble film analysis would seem to be the specific in vitro testing method for lung surfactant. In the present study we report a battery of five in vitro tests for assessing structural and functional properties of surfactant bubbles and bubble films from hydrophobic extracts, namely, the therapeutic surfactants Survanta (SU) and Infasurf (IN) (full strength and diluted to 3 mg phospholipid/mL) and from aqueous extracts from rabbit lung lavage at 3 mg phospholipid/mL (SAM). Each substrate was assessed by: 1) Shake test: stable bubbles from SU, IN, and SAM (50/50, v/v in 95% ethanol) covered the peripheral surfaces, indicating positive response; bubble production by IN and SAM always exceeded SU; 2) Click test: bubble clicking began immediately in all preparations except for undiluted SU, in which the onset of clicking was delayed more than 40 sec; 3) Pattle's stability test: diameters of SU, IN, and SAM bubbles were unchanged for more than 20 min in aerated solution, indicating stable very low surface tension; 4) bubble generation by gas dispersion from a single capillary: full-strength concentration of SU and IN produced relatively large bubbles-bubble rate (number/min) and size were comparable; all SU bubbles rupture in < 25 min, whereas IN bubbles were stable for > 30 min; and 5) Exerowa black film method: in contrast with each of the preceding methods for studying intact bubbles, the Exerowa method focuses on the contact between bubble films and permits direct observation of film formation and determination of film structure. Stable black films were formed spontaneously by both IN (full strength and diluted) and SAM. Conversely, SU (full strength) formed no black films but stable rheological films. Diluted SU films ruptured in 50% of trials. Since methods 1, 2, and 3 were nondiscriminatory and method 4 produced unphysiologically large bubbles for most mammals, we concluded that the black film method of Exerowa is the most discriminating of the tests studied here. It provides a unique visual record of foam film formation and stability and clearly defines differences relative to both the nature and concentration of the preparations.