Stability of aqueous foam films and foams containing polymers: Discrepancies between different length scales

Polymer-stabilized foams and foam films have received considerable attention during the past years. This review paper gives an overview of recent studies dealing with polyelectrolyte/surfactant mixtures, proteins, and microgels adsorbed at single air/water interfaces, in foam films and in macroscopic foams. These polymeric systems have in common that their structure or shape changes when adsorbing at an air/water interface. These structural changes in comparison to their bulk behavior greatly influence the properties of foam films and foams. Regarding the foam stability, formation of adsorbed layers or aggregates plays an important role. The discrepancy between stabilization of macroscopic foams and destabilization of single foam films might be attributed to the blockage of Plateau borders and, therefore, slowed down drainage. Another important parameter is the interfacial viscoelasticity.     

Structure of mixed beta-lactoglobulin/pectin adsorbed layers at air/water interfaces; a spectroscopy study

Based on earlier reported surface rheological behaviour two factors appeared to be important for the functional behaviour of mixed protein/polysaccharide adsorbed layers at air/water interfaces: (1) protein/polysaccharide mixing ratio and (2) formation history of the layers. In this study complexes of beta-lactoglobulin (positively charged at pH 4.5) and low methoxyl pectin (negatively charged) were formed at two mixing ratios, resulting in negatively charged and nearly neutral complexes. Neutron reflection showed that adsorption of negative complexes leads to more diffuse layers at the air/water interface than adsorption of neutral complexes. Besides (simultaneous) adsorption of protein/polysaccharide complexes, a mixed layer can also be formed by adsorption of (protein/)polysaccharide (complexes) to a pre-formed protein layer (sequential adsorption). Despite similar bulk concentrations, adsorbed layer density profiles of simultaneously and sequentially formed layers were persistently different, as illustrated by neutron reflection analysis. Time resolved fluorescence anisotropy showed that the mobility of protein molecules at an air/water interface is hampered by the presence of pectin. This hampered mobility of protein through a complex layer could account for differences observed in density profiles of simultaneously and sequentially formed layers. These insights substantiated the previously proposed organisations of the different adsorbed layers based on surface rheological data.     

Modulating surface rheology by electrostatic protein/polysaccharide interactions

There is a large interest in mixed protein/polysaccharide layers at air-water and oil-water interfaces because of their ability to stabilize foams and emulsions. Mixed protein/polysaccharide adsorbed layers at air-water interfaces can be prepared either by adsorption of soluble protein/polysaccharide complexes or by sequential adsorption of complexes or polysaccharides to a previously formed protein layer. Even though the final protein and polysaccharide bulk concentrations are the same, the behavior of the adsorbed layers can be very different, depending on the method of preparation. The surface shear modulus of a sequentially formed beta-lactoglobulin/pectin layer can be up to a factor of 6 higher than that of a layer made by simultaneous adsorption. Furthermore, the surface dilatational modulus and surface shear modulus strongly (up to factors of 2 and 7, respectively) depend on the bulk -lactoglobulin/pectin mixing ratio. On the basis of the surface rheological behavior, a mechanistic understanding of how the structure of the adsorbed layers depends on the protein/polysaccharide interaction in bulk solution, mixing ratio, ionic strength, and order of adsorption to the interface (simultaneous or sequential) is derived. Insight into the effect of protein/polysaccharide interactions on the properties of adsorbed layers provides a solid basis to modulate surface rheological behavior.     

Electrochemically active films of negatively charged molecules,surfactant and synthetic clays

The electrochemical behavior of anionic molecules such as hexacyanoferrate and vitamin B₁₂ hexacarboxilate intercalated into clay–surfactant films is investigated. The results indicate that composites of surfactant cetyltrymethylammonium bromide (CTAB) and synthetic clay facilitate the permeation of the charged molecules through the film. By heating the modified electrode at 120°C for 1 h, the charged molecules, depending on hydrophobic and coulombic forces, are able to accommodate inside the film. Coulombic forces retain the anionic molecules in the clay–CTAB composite. The films are stable with good electroactivity.     

Influence of Foam Apparent Viscosity and Viscoelasticity of Liquid Films on Foam Stability

The objective of this research work was to study the relationship among the apparent viscosity of bulk foam, the viscoelasticity of liquid films, and foam stability. Bulk foam tests showed that the drainage half-life of AOS foam was higher than that of sodium dodecyl sulfate (SDS) and hexadecyltrimethyl ammonium bromide (CTAB) foams. The results of foam apparent viscosity revealed that the foam apparent viscosity was related to foam quality rather than foam stability. Higher film viscoelasticity modulus could be assigned for α -olefin sulfonate (AOS) films than those for SDS and CTAB ones. The film conductivity tests indicated that AOS liquid films, compared with SDS and CTAB liquid films, could delay the liquid drainage speed under dynamic conditions. Compared with foam apparent viscosity, the viscoelasticity of liquid films appeared to be a key factor in foam stability.     

Foams stabilized by mixed cationic gemini/anionic conventional surfactants

The mixed adsorption of a cationic gemini surfactant, ethanediyl-1,2-bis(dodecyldimethylammonium bromide) (abbreviated as 12-2-12), and an anionic conventional surfactant, sodium dodecyl sulfate (SDS), was examined using surface tension measurements. The viscoelastic properties of the mixed films were investigated by dilational interfacial rheology technique. The results showed that the addition of SDS promoted the close packing of adsorbed molecules at the interface, which increased the dilational elasticity of the mixed films. The stability of the foams was determined by the half-life of foam height collapse. The foams generated by 12-2-12/SDS mixtures were more stable than that formed by pure 12-2-12. In the presence of sodium bromide, the foam stability was further enhanced and the surfactant concentration required to attain the maximum effect in stabilizing foams was greatly reduced. The high foam stability could well relate to the high elasticity of the film.     

Decay of standing foams: drainage,coalescence and collapse

A summary of recent theoretical work on the decay of foams is presented. In a series of papers, we have proposed models for the drainage, coalescence and collapse of foams with time. Each of our papers dealt with a different aspect of foam decay and involved several assumptions. The fundamental equations, the assumptions involved and the results obtained are discussed in detail and presented within a unified framework.Film drainage is modeled using the Reynolds equation for flow between parallel circular disks and film rupture is assumed to occur when the film thickness falls below a certain critical thickness which corresponds to the maximum disjoining pressure. Fluid flow in the Plateau border channels is modeled using a Hagen-Poiseuille type flow in ducts with triangular cross-section.The foam is assumed to be composed of pentagonal dodecahedral bubbles and global conservation equations for the liquid, the gas and the surfactant are solved to obtain information about the state of the decaying foam as a function of time. Homogeneous foams produced by mixing and foams produced by bubbling (pneumatic foams) are considered. It is shown that a draining foam eventually arrives at a mechanical equilibrium when the opposing forces due to gravity and the Plateau-border suction gradient balance each other. The properties of the foam in this equilibrium state can be predicted from the surfactant and salt concentration in the foaming solution, the density of the liquid and the bubble radius.For homogeneous foams, it is possible to have conditions under which there is no drainage of liquid from the foam. There are three possible scenarios at equilibrium: separation of a single phase (separation of the continuous phase liquid by drainage or separation of the dispersed phase gas via collapse), separation of both phases (drainage and collapse occurs) or no phase separation (neither drainage nor collapse occurs). It is shown that the phase behavior depends on a single dimensionless group which is a measure of the relative magnitudes of the gravitational and capillary forces. A generalized phase diagram is presented which can be used to determine the phase behavior.For pneumatic foams, the effects of various system parameters such as the superficial gas velocity, the bubble size and the surfactant and salt concentrations on the rate of foam collapse and the evolution of liquid fraction profile are discussed. The steady state height attained by pneumatic foams when collapse occurs during generation is also evaluated.Bubble coalescence is assumed to occur due to the non-uniformity in the sizes of the films which constitute the faces of the polyhedral bubbles. This leads to a non-uniformity of film-drainage rates and hence of film thicknesses within any volume element in the foam. Smaller films drain faster and rupture earlier, causing the bubbles containing them to coalesce. This leads to a bubble size distribution in the foam, with the bubbles being larger in regions where greater coalescence has occurred.The formation of very stable Newton black films at high salt and surfactant concentrations is also explained.     

Interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 and the origin of surface charges at the air/water interface

The interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 were investigated using two techniques and three different types of surfaces. As nonpolar surfaces, the air/water interface, silanated negatively charged glass, and thiolated uncharged gold surfaces were chosen. The interactions between the air/water interfaces were measured with a thin film pressure balance in terms of disjoining pressure as a function of film thickness. The interactions between the solid/liquid interfaces were determined using a bimorph surface force apparatus. The influence of the nature of the surface on the interaction forces was investigated at surfactant concentrations below and above the cmc. The adsorption of the nonionic surfactant on the uncharged thiolated surface does not, as expected, lead to any buildup of a surface charge. On the other hand, adsorption of C12E6 on the charged silanated glass and the charged air/water interface results in a lowering of the surface charge density. The reduction of the surface charge density on the silanated glass surfaces is rationalized by changes in the dielectric permittivity around the charged silanol groups. The reason for the surface charge observed at the air/water interface as well as its decrease with increasing surfactant concentration is discussed and a new mechanism for generation of OH- ions at this particular interface is proposed.     

Gemini表面活性剂乙烷基-α,ω-双十四烷基二甲基溴化铵产生的高稳定泡沫

报道了由gemini表面活性剂乙烷基-α,ω-双十四烷基二甲基溴化铵(14-2-14)产生的高稳定泡沫体系.泡沫塌陷到初始高度一半所对应的时间(t1/2)用来表征泡沫的稳定性.测得14-2-14体系的t1/2高达961min,远大于乙烷基-α,ω-双十二烷基二甲基溴化铵(12-2-12)产生泡沫的t1/2(754min),表明带有一根短联接链和两条长尾链的gemini表面活性剂是高效的泡沫稳定剂.为了揭示界面弹性与泡沫稳定性之间的关联,测量了表面活性剂吸附膜的扩张流变行为.在指定的表面过剩量下,吸附膜的高频极限弹性再一次被发现与泡沫稳定性相关,较大的极限弹性很好地对应更加稳定的泡沫.     

Change of Surface Structure upon Foam Film Formation

The charge distribution and coverage with surfactant molecules at foam film surfaces plays an important role in determining foam film structure and stability. This work uses the concentration depth profiling technique neutral impact collision ion scattering spectroscopy to experimentally observe the charge distribution in a foam film for the first time. The charge distribution at the surface of a foam film and the surface of the corresponding bulk liquid were measured for a cationic surfactant solution and the surface excess as well as the electric potential were determined. Describing the internal pressure of foam films by using the electrochemical potential is introduced as a new concept. The foam film can be seen to have a more negative surface charge compared to the bulk liquid surface due to re‐arranging of the surfactant molecules. It is discussed how the change in surface excess and electric potential change the electrochemical potential and the stability of the foam film.     

In situ investigations on organic foam films using neutron and synchrotron radiation

We report on small-angle neutron scattering (SANS) and X-ray scattering (SAXS) investigations of foam films stabilized by sodium dodecyl sulfate. Previous measurements on dry foams (Axelos, M. A. V.; Boue, B. Langmuir 2003, 19, 6598) have shown the presence of spikes in the two-dimensional scattering data which suggest that the incident beam is reflected on some film surfaces. The latter interpretation is confirmed by new neutron studies performed on ordered ("bamboo") foams which allow selection of single films. In the first case, we show that the spikes of the scattered intensity can be obtained by reflection on two parts of the foam, namely, the films and the Plateau borders. With synchrotron radiation, first observations of distinct interference fringes have allowed an accurate determination of the film thickness. A comparison with X-ray and neutron data is made, opening a general discussion about the capabilities of small-angle scattering techniques for studying the microscopic properties of foam films.     

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 ₀ of the diffuse electrical layer at the solution/air interface (applying the method of equilibrium foam films) and the potential ₁ 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.     

带相反电荷高分子和表面活性剂复合物膜在空气/水界面上的特性

利用原子力显微镜技术、表面压松弛法对带相反电荷高分子和表面活性剂在气/水界面形成的界面复合物膜进行了特性研究.原子力显微镜研究结果表明,部分水解聚丙烯酰胺(HPAM)/十六烷基三甲基溴化铵(CTAB)所形成的界面复合物膜呈现出“团”状聚集体形貌.两性聚丙烯酰胺与CTAB形成的界面复合物膜呈现出“纤维丝”的聚集体形貌.亚相中盐的存在对界面复合物的形貌有很大影响.表面压松弛实验则进一步表明界面复合物膜形貌结构的变化对其膜稳定性有直接的影响.     

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.     

两亲性阴离子HBPN与阳离子表面活性剂CTAB络合机理的研究

用动态光散射研究了两亲性阴离子HBPN(高度枝化的聚酯纳米微粒)和阳离子表面活性剂CTAB(十六烷基三甲基溴化铵)在缓冲液中的相互作用及HBPN/CTAB络合物的形成.结果表明:在静电作用下,于碱性溶剂中,HBPN与由CTAB分子所构成的胶束形成了核壳结构络合物HBPN/CTAB.络合物粒径的大小和稳定性可以通过溶液的pH值和CTAB浓度来控制.     

Control of Ostwald ripening by using surfactants with high surface modulus

We describe results from systematic measurements of the rate of bubble Ostwald ripening in foams with air volume fraction of 90%. Several surfactant systems, with high and low surface modulus, were used to clarify the effect of the surfactant adsorption layer on the gas permeability across the foam films. In one series of experiments, glycerol was added to the foaming solutions to clarify how changes in the composition of the aqueous phase affect the rate of bubble coarsening. The experimental results are interpreted by a new theoretical model, which allowed us to determine the overall gas permeability of the foam films in the systems studied, and to decompose the film permeability into contributions coming from the surfactant adsorption layers and from the aqueous core of the films. For verification of the theoretical model, the gas permeability determined from the experiments with bulk foams are compared with values, determined in an independent set of measurements with the diminishing bubble method (single bubble attached at large air-water interface) and reasonably good agreement between the results obtained by the two methods is found. The analysis of the experimental data showed that the rate of bubble Ostwald ripening in the studied foams depends on (1) type of used surfactant-surfactants with high surface modulus lead to much slower rate of Ostwald ripening, which is explained by the reduced gas permeability of the adsorption layers in these systems; (2) presence of glycerol which reduces the gas solubility and diffusivity in the aqueous core of the foam film (without affecting the permeability of the adsorption layers), thus also leading to slower Ostwald ripening. Direct measurements showed that the foam films in the studied systems had very similar thicknesses, thus ruling out the possible explanation that the observed differences in the Ostwald ripening are due to different film thicknesses. Experiments with the Langmuir trough were used to demonstrate that the possible differences in the surface tensions of the shrinking and expanding bubbles in a given foam are too small to strongly affect the rate of Ostwald ripening in the specific systems studied here, despite the fact that some of the surfactant solutions have rather high surface modulus. The main reason for the latter observation is that the rate of surface deformation of the coarsening bubbles is extremely low, on the order of 10(-4) s(-1), so that the relaxation of the surface tension (though also slow for the high surface modulus systems) is still able to reduce the surface tension variations down to several mN/m. Thus, we conclude that the main reason for the reduced rate of bubble Ostwald ripening in the systems with high surface modulus is the low solubility and diffusivity of the gas molecules in the respective condensed adsorption layers (which have solid rather than fluid molecular packing).     

Mechanistic study of nanoparticles–surfactant foam flow in etched glass micro-models

This study was conducted in order to identify the pore-level mechanisms controlling the nanoparticles–surfactant foams flow process and residual oil mobilization in etched glass micro-models. The dominant mechanism of foam propagation and residual oil mobilization in water-wet system was identified as lamellae division and emulsification of oil, respectively. There was inter-bubble trapping of oil and water, lamellae detaching and collapsing of SDS-foam in the presence of oil in water-wet system and in oil-wet system. The dominant mechanisms of nanoparticles–surfactant foam flow and residual oil mobilization in oil-wet system were the generation of pore spanning continuous gas foam. The identified mechanisms were independent of pore geometry. The SiO₂-SDS and Al₂O₃-SDS foams propagate successfully in water-wet and oil-wet systems; foam coalescence was prevented during film stretching due to the adsorption and accumulation of the nanoparticles at the gas–liquid interface of the foam, which increased the films’ interfacial viscoelasticity.     

The effect of silica nanoparticles on the stability of aqueous foams

An experimental study was performed on aqueous foams stabilized by a mixture of hexadecyltrimethylammonium bromide (HTAB) and negatively-charged silica nanoparticles. The effects of the nanoparticles on the stability of foams at different HTAB concentrations were investigated. The foams were characterized by measuring their foamability and stability. Rheological behavior of the foams was also studied. Furthermore, rheology of the air–water interfaces was studied in the linear and nonlinear deformation ranges. The thickness of the monolayer at the interface was measured. The actual size of the silica nanoparticles at the air–water interface was measured by transmission electron microscopy. Based on these measurements, the interaction between the monolayers across the foam film containing HTAB and nanoparticles was investigated. Smaller silica nanoparticles (i.e. diameter less than 10?nm) adsorbed at the air–water interface whereas the larger particles remained in the sub-phase or in the bulk liquid phase. It was found that these nanoparticles strongly influenced the foaming behavior at the low HTAB concentrations (i.e. below the CMC). A Langmuir-type monolayer was formed. The presence of the nanoparticles at the air–water interface provided mechanical strength to the foam films and prevented their rupture. This hindered coalescence of the bubbles, which resulted in a stable foam.     

Effect of Molecular Weight on the Interfacial Dilational Viscoelasticity of Anionic Polyelectrolyte/Surfactant Systems

In this article, the effect of molecular weight on the interfacial tension and interfacial dilational viscoelasticity of polystyrene sulfonate/surfactant adsorption films at the water-octane interface have been studied by spinning drop method and oscillating barriers method respectively. The experimental results show that different interfacial behaviors can be observed in different type of polyelectrolyte/surfactant systems. PSS/cationic surfactant CTAB systems show the classical behavior of oppositely charged polyelectrolyte/surfactant systems and can be well explained by electrostatic interaction. Molecular weight of PSS plays a crucial role in the nature of adsorption film. The complex formed by CTAB and higher molecular weight PSS, which has larger dimension and stronger interaction, results in higher dilational modulus at lower surfactant bulk concentration. In the case of PSS/anionic surfactant SDS systems, the co-adsorption of PSS at interface through hydrophobic interaction with alkyl chain of SDS leads to the increase of interfacial tension and the decrease of dilational modulus at lower surfactant bulk concentration. For PSS/nonionic surfactant T × 100 systems, PSS may form a sublayer contiguous to the aqueous phase, which has little effect on interfacial tension but slightly decreases dilational modulus.