Foaming and antifoaming in non-aqueous liquids

Investigation into the physics of foaming has traditionally been focused on aqueous systems. Non-aqueous foams, by contrast, are not well understood, but have been the subject of a recent surge in interest motivated by the need to manage foaming across industrial applications. In this review, we provide a comprehensive discussion of the current state-of-the-art methods for characterizing non-aqueous foams, with a critical evaluation of the advantages and limitations of each. Subsequently, we present a concise overview of the current understanding of the mechanisms and methods used for stabilizing and destabilizing non-aqueous foams. We conclude the review by discussing open questions to guide future investigations.     

Foaming activity of foam compounds based on hexadecylammonium chloride and sodium carboxylates

Formation of R₁R₂ associates between hexadecylammonium chloride and sodium carboxylates (octanoate, laurate, and palmitate) was studied by IR spectroscopy and chemical methods, and their foaming activity was characterized.     

Amphiphilic nanostructures in foam films

The revised articles outline the potential of microscopic foam film instrumentation as an investigation tool in studying the amphiphilic nanostructures in aqueous surfactant solutions. The impact of amphiphilic nanostructures on the drainage behaviour and stability of foam films is traced for surfactant solutions of concentrations orders of magnitude above CMC (micellar solutions) to about two orders of magnitude lower than CMC (premicellar solutions). It is found that in the high-concentration domain the micellar entities affect mainly the stability of the films. In the low-concentration domain, the presence of smaller crumbly aggregates (premicelles), plays a significant role for the kinetic stability of the films. Through the mechanism of Marangoni effect, an enhanced coupling of the specific film hydrodynamics and the mass transfer of the surfactant is obtained. The result is a sharp rise in the kinetic stability of the foam films. The importance of this trend of research is related to providing better insight into the self-assembling phenomena and into the factors that determine the drainage and the stability of thin liquid films. The results have potential and actual applications in food, cosmetic and pharmaceutical industries, as well as in biology and medicine.     

X-ray scattering by black foam films

The x-ray scattering by the three types of black foam films (common black, Newton black, and stratified black films) was experimentally studied. A special device in which flat black films with an area of ca. 2 cm² can be produced was developed and x-ray diffraction patterns were obtained by a vertical diffractometer. The three types of films differ significantly in their x-ray reflections, which proves that they have different structure. For common black films, the comparison of observed and calculated intensities lead to a model, which corresponds to the three-layer model. The Newton black films exhibit diffraction trace with only one highly asymetric peak and there is, as of yet, no unambigous interpretation. The patterns of the stratified black films have several pronounced sharp peaks corresponding to the areas of different films with a given thickness.     

Interactions in lipid stabilised foam films

The interaction between lipid bilayers in water has been intensively studied over the last decades. Osmotic stress was applied to evaluate the forces between two approaching lipid bilayers in aqueous solution. The force–distance relation between lipid mono- or bilayers deposited on mica sheets using a surface force apparatus (SFA) was also measured. Lipid stabilised foam films offer another possibility to study the interactions between lipid monolayers. These films can be prepared comparatively easy with very good reproducibility. Foam films consist usually of two adsorbed surfactant monolayers separated by a layer of the aqueous solution from which the film is created. Their thickness can be conveniently measured using microinterferometric techniques. Studies with foam films deliver valuable information on the interactions between lipid membranes and especially their stability and permeability. Presenting inverse black lipid membrane (BLM) foam films supply information about the properties of the lipid self-organisation in bilayers. The present paper summarises results on microscopic lipid stabilised foam films by measuring their thickness and contact angle. Most of the presented results concern foam films prepared from dispersions of the zwitterionic lipid 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) and some of its mixtures with the anionic lipid — 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DMPG).     

A pH-study of n-dodecyl-beta-D-maltoside foam films

The influence the pH has on the properties of foam films stabilized by the nonionic surfactant n-dodecyl-beta-d-maltoside (beta-C12G2) was studied. Foam film measurements were carried out with the thin film pressure balance (TFPB) technique using two different film holders, namely, the Scheludko-Exerowa cell and the porous plate. With the former, the equilibrium film thickness h at a given capillary pressure Pc and, with the latter, complete disjoining pressure versus thickness curves (Pi-h curves) were measured. Most of the results were obtained for 10(-4) and 10(-5) M beta-C12G2 solutions that contained 10(-3) M electrolyte. Measurements were carried out in a pH range from 3 to 9. The major results are the following: (1) For a given pH, a pronounced effect of the surfactant concentration cs is seen only if cs approximately cmc. This holds true for both low and high pH values. (2) For a given cs, at least one pronounced effect is seen if the pH is changed, namely a drop of the surface charge density down to zero when the isoelectric point (pH* and pHcr) is reached. (3) The pH of the isoelectric point increases with increasing surfactant concentration. (4) The q0-pH curve of beta-C12G2 shows two pH ranges (3-5.5 and 7-10) in which the surface charge density q0 is pH-insensitive, while a significant change of q0 was observed between pH=5.5 and 7.0. A possible explanation is given.     

Critical capillary pressure for destruction of single foam films and foam: effect of foam film size

Foams and single foam films stabilised by ionic and amphiphile polymer surfactants are studied with foam pressure drop technique (FPDT) and thin liquid film-pressure balance technique (TLF-PBT). A pressure is reached at which the single foam films rupture and the foams destruct very fast (avalanche-like). For film rupture we named this pressure—critical capillary pressure of film rupture, P_cr,film while for foam destruction, we introduced a new parameter—critical capillary pressure of foam destruction, P_cr,foam. The surfactant kind and foam film type (common thin, common black and Newton black) affect the values of both parameters. When below 20 kPa, P_cr,film and P_cr,foam are close by value, when over 20 kPa, there is a significant difference between them. The P_cr,film versus film size and P_cr,foam versus foam dispersity dependences, indicate that the film size and foam dispersity strongly affects the critical capillary pressure values. Film size distribution histograms reveal that a foam always contains films that are of a larger than the most probable size. They rupture at lower pressures, does initiating the destruction of the whole foam, which can be an explanation why higher than 20 kPa there is a difference between P_cr,film and P_cr,foam values. This parameter, P_cr,foam is considered of significant with respect to foam stability and could find use in industry.     

Foams and foam films stabilised by solid particles

Recent developments in the field of particle-stabilised aqueous foams and foam films are reviewed. Reports on ultrastable foams stabilised by solid particles are highlighted and factors responsible for the extraordinary foam stability are discussed in view of the recent experimental and theoretical results. Mechanisms of foam film stabilisation by solid particles and the role of different factors in the film stability are considered. Link between the film stability and that of particle-stabilised foams is discussed.     

Aqueous foam films stabilized by sodium naphthenates

Stratification of a foam liquid film drawn from aqueous solutions of sodium naphthenate at relatively high concentration is likely due to a lamellar liquid crystal-like structure within the film. Film stratification, resulting in stepwise thinning, has been observed in foam films formed from systems containing either moderate to high concentrations of surfactant or in films formed from solutions containing solid particles. At moderate surfactant concentrations, film stratification is likely due to layers of ordered spherical micelles as postulated in Wasan and Nikolov's model of film stratification. At high surfactant concentrations, stepwise thinning of the films and occurrence of domains of uniform color within the film suggest a lamellar liquid crystal-like structure within the film, potentially up to hundred or more oriented layers. The LLC-like structure inside the film can occur at concentrations below the lower limit of the LLC existence as a bulk phase.     

Motion of foam films in diverging-converging channels

Existing theories of the motion of foam films in capillaries often assimilate the pressure drop over the foam films to the static capillary pressure obtained from the Young-Laplace equation. Hence, they ignore the contribution of dynamic effects associated with the rapid stretching and contraction of the foam films to the overall viscous dissipation. This paper reports an investigation of the motion of foam films in axisymmetric diverging-converging channels, taking into account surface viscosity and elasticity. First, a phenomenological theory for the motion of the foam films is developed using simple physical arguments. We show that the displacement of the film obeys a nonlinear second-order differential equation, which can be solved numerically for the (dimensionless) distance from the inlet and the pressure drop as a function of time. Experiments with foam film motion, conducted using glass diverging-converging channels (minimum radius = 3.00 +/- 0,01 mm, maximum diameter = 7,98 +/- 0,01 mm) and nitrogen foam stabilized with sodium dodecyl sulfate (SDS) in brine, are discussed. For a single film motion in the diverging channel, we find that (a) the static pressure drop is a concave-upward function of distance and decreases from 1.0 to about 0.3, whereas (b) the dynamic pressure drop is concave downward and increases from 1 to a maximum of 1.3 and then decreases to 0.7. In the converging channel both the static and dynamic pressure drops are concave-downward functions, but the dynamic pressure drop values are always higher than the static ones. For two films the motions were found to be rather sensitive to the initial arrangement in the channel. The experiments are found to be in excellent agreement with the theoretical predictions. These observations imply that the large flow resistance obtained during foam flow in granular porous media, where converging-diverging channels are abundant, is largely due to the surface elasticity and viscosity of the films.     

Effect of foam films on gas diffusion

We report an experimental investigation of the permeability to gas of systems of one or several soap films freely standing in a straight tube, using either reactive gas (NH(3)) or inert gas (argon). The series of soap films appears to be the simplest paradigm of successive lamellae arrangements encountered in foams confined in a porous medium. To conduct the experiments, we devised two novel methods for the determination of gas diffusion fluxes: one based on reactive changes of pH by NH(3) and the other on mass spectrometry. The permeability of a single film, stabilized by sodium dodecyl sulfate solution, was found to be 3.50+/-0.04 10(-2) cm/s for argon and 3.18+/-0.07 10(-4) cm/s for NH(3). The permeability value for the inert gas is in good agreement with data obtained by the diminishing-bubble method. When the number of films increases, the permeability decreases considerably as a result of cumulative film resistance effects. We also developed a simple phenomenological model based upon a combination of gas kinetic and energy barrier concepts to interpret our data. This model takes into account gas solubility and the effects of salinity, which have seemingly been ignored in previous models. The predicted film permeability decreases sharply with increase surfactant concentration, indicating the occurrence of higher adsorption and increasingly compact surfactant layers.     

Foaming ability and stability of silica nanoparticle-based triple-phase foam for oil fire extinguishing: experimental

The triple-phase foam has been widely used in oil fire extinguishing, and its two key parameters for application are the foaming ability and stability. We present a comprehensive study on the foam expansion ratio (FER) and drainage time, where factors such as the foam morphology, zeta potential of particles, foam mixing homogeneity, surfactant concentration, particle mass percentage, and specific surface area of the particle are investigated in detail. The dependence relationship curves of FER and drainage time with respect to the latter four variables are given through experiments, and optimal parameter values are selected. Moreover, the scaling laws correlating these variables are in agreement with the experimental results, and some necessary parameters are obtained by data fitting. These analyses are beneficial to better understand the foaming ability and stability mechanism of the triple-phase foam and to prepare materials of high performances for oil fire extinguishing.     

Influence of the type of foam films and the type of surfactant on foam stability

The behaviour and the life time ( _p) of different types of foam films (thin liquid films, for which DLVO-theory is valid; common black films, Newton black films) have been studied as a function of external pressure (P), applied in the Plateau-Gibbs-borders of the foam. The foam stability and the course of thep/P-dependence are determined mainly by the type of the foam films. A criterion for estimation of foam stability is proposed on the base of the obtained experimental results.     

Foaming and foam stability for mixed polymer-surfactant solutions: effects of surfactant type and polymer charge

Solutions of surfactant-polymer mixtures often exhibit different foaming properties, compared to the solutions of the individual components, due to the strong tendency for formation of polymer-surfactant complexes in the bulk and on the surface of the mixed solutions. A generally shared view in the literature is that electrostatic interactions govern the formation of these complexes, for example between anionic surfactants and cationic polymers. In this study we combine foam tests with model experiments to evaluate and explain the effect of several polymer-surfactant mixtures on the foaminess and foam stability of the respective solutions. Anionic, cationic, and nonionic surfactants (SDS, C(12)TAB, and C(12)EO(23)) were studied to clarify the role of surfactant charge. Highly hydrophilic cationic and nonionic polymers (polyvinylamine and polyvinylformamide, respectivey) were chosen to eliminate the (more trivial) effect of direct hydrophobic interactions between the surfactant tails and the hydrophobic regions on the polymer chains. Our experiments showed clearly that the presence of opposite charges is not a necessary condition for boosting the foaminess and foam stability in the surfactant-polymer mixtures studied. Clear foam boosting (synergistic) effects were observed in the mixtures of cationic surfactant and cationic polymer, cationic surfactant and nonionic polymer, and anionic surfactant and nonionic polymer. The mixtures of anionic surfactant and cationic polymer showed improved foam stability, however, the foaminess was strongly reduced, as compared to the surfactant solutions without polymer. No significant synergistic or antagonistic effects were observed for the mixture of nonionic surfactant (with low critical micelle concentration) and nonionic polymer. The results from the model experiments allowed us to explain the observed trends by the different adsorption dynamics and complex formation pattern in the systems studied.     

Thinning of foam films of micellar surfactant solutions

Drainage in microscopic circular foam films depends significantly on the radial (tangential) mobility of the film surfaces and is accelerated as compared to the limiting case of tangentially immobile surfaces, where velocity of thinning is described by the classical Reynolds’ equation (outflow of viscous fluid from a cylindrical gap between two solid plates). The structure and composition of the adsorption layer and the interfacial mass transfer determine the tangential mobility of the film surfaces and, hence, the measured velocity of film thinning. Experiments with soluble surfactants below the critical micelle concentrations (CMC) have exhibited the effect of dynamic interfacial elasticity. At relatively low bulk concentrations, the interfacial mass transfer is governed by surface diffusion; close to CMC (saturated adsorption layer), the limiting case of tangentially immobile surfaces can be reached and at concentrations above the CMC the film thinning is accelerated again. Here, we report freshly established data on the kinetic behavior of foam films from micellar solutions of soluble nonionic surfactants (decyl-octaoxyethylene alcohol and dodecyl-octaoxyethylene alcohol) in a wide range of concentrations above the CMC aiming to investigate the effect of partially disintegrated micelles acting as sources of surfactant molecules at the surface.     

Permeability of Newtonian black foam films to gas

The gas permeability of Newtonian black foam films, formed on the top of a small bubble at the solution surface, was studied experimentally. The aqueous solutions contained sodium dodecylsulphate with concentrations in the range 1.5×10^–4 to 3×10^–3 mol/dm³ and sodium chloride (constant concentration of 0.5 mol/dm³). A dependence of the gas permeability coefficient on the surfactant concentration was obtained. The experimental results are discussed on the basis of a theory assuming the presence of clusters of molecule vacancies (holes) in the bilayer foam film, their number and size depending on the surfactant concentration. The experimental results are in agreement with this film structure and confirm the existence of flow through both the hole-free bilayer film and the holes. It was found that the holes of three molecule vacancies make the main contribution to gas permeability at low surfactant concentration. The diffusion coefficients through the hole-free film and through the three-vacancy holes are calculated.Dedicated to Professor Dr. Armin Weiss on the occassion of his 60th birthday.     

Phospholipid black foam films: dynamic film tension of DMPC bilayer films

The film tension of bilayer Newton black films (NBF) from aqueous dispersions of dimyristoylphosphatidylcholine (DMPC) has been studied in dynamic conditions. The dynamic film tension values γ have been measured using the capillary method for direct measurement of the film tension. Two different solutions have been used: DMPC vesicle suspension in water obtained through sonication, denoted as ‘DMPC(Son)’ (the DMPC adsorption layers are insoluble monolayers) and DMPC dissolved in ethanol plus water mixed solvent, denoted as ‘DMPC(EthW)’ (the DMPC adsorption layers are soluble). Both solutions contain 0.1 M NaCl. The behavior of the dynamic film tension is different for NBF from the two types of solutions. In the case DMPC(Son) γ strongly depends on the film area, while in the case DMPC(EthW) this dependence is less pronounced but still exists. The dependence of the film tension on the film area in case DMPC(Son) is well described by the Frumkin equation modified for bilayer films. Reasonable values of the parameters of Frumkin equation are determined from its fit to the experimental data.