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 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.     

Foaming in aqueous solutions of zwitterionic surfactant: Effects of oil and salts

This paper reports a study on the stability of foams generated from the aqueous solutions of the zwitterionic surfactant, N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, in presence of NaCl, CaCl₂ and AlCl₃. The effect of oil (i.e. n-hexane) on foam was also studied. The surface and interfacial tensions were measured. These tensions and the CMC decreased upon salt addition, signifying an increased adsorption of the surfactant molecules at the interface. The quantity of salt required for reducing the surface tension and CMC was in the sequence: NaCl > CaCl₂ > AlCl₃. The salts had a pronounced effect on the foaming characteristics, i.e. they reduced the initial foam volume. The effectiveness of salts in reducing the foam stability followed the sequence: AlCl₃ > CaCl₂ > NaCl. However, the foam collapse rate was reduced in the presence of salt. The presence of oil decreased the foam volume and reduced its stability. The entering, bridging, and spreading coefficients were calculated, which explained the stability of foams in presence of oil.     

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.     

Dilational surface rheology of polymer and polymer/surfactant solutions

Recent studies show strong influence of the dilational surface rheological properties on the stability and dynamics of foam and emulsions. On the other hand, the dilational dynamic surface elasticity proved to be highly sensitive to conformational transitions of macromolecules at fluid–fluid interfaces and can be used to investigate the adsorption mechanism and aggregate formation in the surface layer. The intention of this review consists in the discussion of recent progress in the dialtional surface rheology of solutions of non-ionic homopolymers, block copolymers, polyelectrolytes, polyelectrolyte/surfactant and protein/surfactant complexes.     

A correlation of foam stability with surface shear viscosity and area per molecule in mixed surfactant systems

Summary Aqueous solutions of sodium dodecyl sulfate yield very unstable foam with a very high rate of drainage because they exhibit a relatively low surface shear viscosity. When a solubilizate, such as dodecanol is present in the system, the rate of drainage and thus foam stability prove to be a function of surface shear viscosity. In itself surface shear viscosity appears to be a function of the state of the film as well as the relative amount of the surfactants that is adsorbed at the surface. Systems of fatty acid — fatty alcohol (decanoic acid — decanol, octanoid acid — octanol) exhibit maximum foam stability at molar ratios of 1:3 and 9:1, respectively. At the same molar ratios in these systems, the rate of drainage is minimum and surface shear viscosity is maximum. Studies on mixed monolayers of stearic acid — stearyl alcohol showed minima in the average area per molecule at the 9:1 and 1:3 molar ratios.It is proposed that the molecular interaction which causes the reduction in the average area per molecule in the mixed monolayer also causes the maximum in surface shear viscosity, the minimum in the rate of drainage, and the maximum in foam stability.

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Zusammenfassung Wäßrige Lösungen von Natriumdodecylsulfat haben sehr instabile Schäume mit einer hohen Draingeschwindigkeit, die mit der relativ niedrigen Oberflächengeschwindigkeit zusammenhängt. Bei Gegenwart eines Solubilisators, wie z. B. Dodecanol, wird die Draingeschwindigkeit und die Schaumstabilität eine Funktion der Oberflächenscherviskosität. Die Oberflächenscherviskosität selbst scheint eine Funktion des Filmzustandes und der relativen Tensidmenge zu sein, die an der Oberfläche adsorbiert ist. In den Systemen Fettsäure-Fettalkohol treten Maxima der Schaumstabilität bei Molverhältnissen 1:3 bzw. 9:1 auf. Bei den gleichen Molverhältnissen hat die Draingeschwindigkeit ein Minimum und die. Oberflächenscherviskosität ein Maximum. In Monoschichten von Stearinsäure und Stearylalkohol erreicht der Platzbedarf pro Molekül Minima bei Molverhältnissen von 9:1 und 1:3.Es wird angenommen, daß die gleichen Wechselwirkungen, welche zur Reduktion des Platzbedarfs in den gemischten Monoschichten führen, auch die Maxima in der Oberflächenscherviskosität und die Minima in der Draingeschwindigkeit und damit die Maxima in der Schaumstabilität bedingen.

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With 7 figures     

Net charge and electrophoretic mobility of lysozyme charge ladders in solutions of nonionic surfactant

We report on the electrophoretic mobility and on the thermal diffusion of lysozyme proteins dissolved in aqueous solutions of a nonionic surfactant (C12E6) at a wide range of concentrations of the surfactant (0-20% by weight). We want to estimate the influence of a dense network of elongated micelles of C12E6 on the effective charge of the proteins as observed in the capillary electrophoresis experiments. The possible mechanism leading to the change in the effective charge of protein could involve the deformation of the cloud of counterions around the protein when it squeezes through the narrow (of the order of a protein diameter) aqueous channels formed in the solution of elongated micelles. The combination of independent measurements of the electrophoretic mobility of a family of modified proteins (lysozyme charge ladder [Colton et al. J. Am. Chem. Soc. 1997, 119, 12701]), of the microviscosity of the solutions of surfactant (obtained via fluorescence correlation spectroscopy), and of the hydrodynamic radius of the proteins (photon correlation spectroscopy) allow us to conclude that the effective charge of the proteins is not affected by the presence of surfactant, even at high concentrations.     

Dynamic surface and interfacial tensions of surfactant and polymer solutions

Dynamic surface and interfacial tensions are the most frequently measured non-equilibrium properties of adsorption layers at liquid interfaces. The review presents the theoretical basis of adsorption kinetics, taking into consideration different adsorption mechanisms, and specific experimental conditions, such as liquid flow and interfacial area changes. Analytical solutions, if available, approximations as well as numerical procedures for direct solution of the physical models are presented.Several experimental techniques are discussed frequently used in studies of the dynamic adsorption behaviour of surfactants and polymers at liquid interfaces: drop volume, maximum bubble pressure, and pendent drop technique, drop pressure tensiometry, pulsating bubble and elastic ring method. Experimental results, most of all obtained with different technique on one and the same surfactant system, are then discussed on the basis of current theories.Finally, the role of dynamic interfacial properties in several practical applications is discussed: foam and emulsion film formation and stabilisation, rising of bubbles and drops in a surfactant solution.     

Black foam and emulsion films from nonionic surfactant solutions: rupture by hole formation

The concept that rupture of bilayer films is caused by nucleation of holes was checked for foam and emulsion films stabilized by the same nonionic surfactant. Newtonian black (NB) films were formed in a ring-cell from a biconcave drop with air or nonane, respectively, as the ambient phase. The lifetime of the foam and emulsion films increases with surfactant concentration. This relation is analysed on the basis of above mentioned theory.Dedicated to Prof. Dr. A. Weiss on the occasion of his 60th birthday.     

Manipulating interfacial polymer structures through mixed surfactant adsorption and complexation

The effects of a nonionic alcohol ethoxylate surfactant, C(13)E(7), on the interactions between PVP and SDS both in the bulk and at the silica nanoparticle interface are studied by photon correlation spectroscopy, solvent relaxation NMR, SANS, and optical reflectometry. Our results confirmed that, in the absence of SDS, C(13)E(7) and PVP are noninteracting, while SDS interacts strongly both with PVP and C(13)E(7) . Studying interfacial interactions showed that the interfacial interactions of PVP with silica can be manipulated by varying the amounts of SDS and C(13)E(7) present. Upon SDS addition, the adsorbed layer thickness of PVP on silica increases due to Coulombic repulsion between micelles in the polymer layer. When C(13)E(7) is progressively added to the system, it forms mixed micelles with the complexed SDS, reducing the total charge per micelle and thus reducing the repulsion between micelle and the silica surface that would otherwise cause the PVP to desorb. This causes the amount of adsorbed polymer to increase with C(13)E(7) addition for the systems containing SDS, demonstrating that addition of C(13)E(7) hinders the SDS-mediated desorption of an adsorbed PVP layer.     

Surface properties of mixed anionic/cationic surfactant solutions

Summary Published experimental surface pressures for mixtures of anionic and cationic surfactants are compared to what can be predicted from the surface behaviour of the separate constituents. Simple theory correctly predicts, for soluble surfactants, a large increase of surface pressure upon mixing an anionic and a cationic solution having the same surface pressure, and, for insoluble surfactants, a decrease of surface pressure upon mixing at a given area per long chain ion. These effects are due to the different adsorption characteristics of the four electroneutral combinations involved, and will occur even in the absence of specific surface interactions.

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Zusammenfassung Es wurden bekannte Oberflächendrucke für Mischfilme von Aniontensiden und Kationtensiden mit theoretischen Voraussagen, begründet auf dem Grenzflächenverhalten der einzelnen Komponenten, verglichen. Die einfache Theorie sagt richtig voraus, daß die Mischung zweier wäßriger Lösungen von Aniontensid bzw. Kationtensid mit gleichem Oberflächendruck, eine starke Erhöhung des Oberflächendruckes zur Folge haben muß, während die Mischung zweier unlöslicher Filme bei gegebener Tensidadsorption den Oberflächendruck herabsetzt. Dieses Verhalten wird aufgrund der verschiedenen Adsorptionseigenschaften der vier betreffenden Salze gedeutet, und ist von spezifischen Oberflächenwechselwirkungen unabhängig.

     

Effects of polymer solutions on colloid stability

Some theoretical properties of a semidilute polymer solution (in a good solvent) near a repulsive wall (no adsorption) are discussed. (i) Near a single wall, the existence of a depletion layer (where the concentration is strongly reduced) with thickness equal to the correlation length ξ(c) is predicted. Scaling laws are obtained for the concentration c_s at the first layer as a function of the bulk concentration c, and for the concentration profile c(z) at distance z from the wall. (ii) When two parallel walls are separated by a slab of polymer solution (thickness d), there is an attractive interaction between the plates. The range of the attraction is ca. 3ξ(c) and can be varied by suitable choice of concentration. It may lead to destabilization of certain colloidal systems.     

Controlling synthesis of silver nanowires and dendrites in mixed surfactant solutions

Using a simple wet chemical route, high-yield silver nanowires with an average diameter of 25+/-5 nm and length up to several microm and dendrites with a long central backbone and symmetrically ramified secondary branches have been successfully obtained by reducing AgNO(3) with L-ascorbic acid (AsA) in the mixed surfactant solutions of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). It was found that the architecture of silver nanocrystals was drastically influenced by the concentrations of ascorbic acid. At a given high concentration, a nonequilibrium system was easily built, which favored the formation of fractals. When the concentration was lowered, one-dimensional silver nanowires were successfully obtained. In addition, the presence of electrolyte (NaCl) plays an important role in the preparation of silver nanowires, influencing the silver crystallization process in surprising ways.     

Interaction of cationic surfactant and anionic polyelectrolytes in mixed aqueous solutions

Surfactant–polymer interactions in aqueous solutions have been studied using dynamic surface tension, polyelectrolyte titration, nephelometric turbidity, and dynamic light scattering. For the preparation of complexes, a technical cationic surfactant was used in combination with two poly(maleic acid-co-polymers) of similar structure but different hydrophobicity. The dynamic surface tensions of mixed solutions as functions of surfactant concentration at constant polyelectrolyte content, as well as changes in the surface activity due to the influence of polyanion at constant surfactant concentration are discussed in terms of a complex or aggregate formation in the bulk phase. The interaction of the surfactant with poly(maleic acid-alt-propene) (P-MS-P) and poly(maleic acid-alt--methylstyrene) (P-MS-MeSty), respectively, is strong in both cases and results in the formation of nanoparticles with properties depending on the composition of the corresponding mixture.     

Phase diagrams and microstructure of aggregates in mixed ionic surfactant/foam booster systems

The phase behavior and microstructure of surfactant systems containing a new alkanolamide-type foam booster, dodecanoyl N-methyl ethanolamide (NMEA-12), were investigated by means of phase study and small angle X-ray scattering. Different from other similar alkanolamides, NMEA-12 possesses a low melting point and forms a lyotropic liquid-crystalline phase (L(alpha) phase) at room temperature. This is attributed to the attached methyl group, which increases the fluidity of the molecule. In the SDS/NMEA-12/water system, hexagonal and lamellar (L(alpha)) liquid-crystalline phases are obtained at significantly low surfactant concentrations. The stability of these phases decreases when SDS is replaced with a nonionic surfactant (C12EO8). However, for both ionic and nonionic surfactants, the effective area per surfactant molecule at the interface shrinks upon addition of NMEA-12, indicating that the surfactant layer is getting more compact. The possible implications of these results on the potential applications of NMEA-12 as foam stabilizer are discussed.     

Mixed solutions of an associating polymer with a cleavable surfactant

Mixtures of hydrophobically modified hydroxyethyl cellulose (HMHEC) and alkali-sensitive cleavable betaine ester surfactants have been studied by viscometry, 1H NMR, absorbance measurements, and birefringence determinations. Before the hydrolysis, the surfactants behaved as conventional nondegradable surfactants in terms of the effect on the viscosity of increasing surfactant concentration. As the surfactants were hydrolyzed, systems with time-dependent viscosity were obtained. The viscosity either decreased monotonically or went through a maximum as a function of time, depending on the initial surfactant concentration. Different surfactant chain lengths gave rise to different viscosity profiles. The rate of hydrolysis, and thus the time-dependency of the surfactant concentration, could be controlled by changing the pH of the solution.     

Investigation of aqueous foam stability containing pigment colorant using polyoxyethylene nonionic surfactant

To improve foam stability in pigment foaming dispersions, a series of fatty alcohol polyoxyethylene ether (CmEOn) with different alkyl chain and ethylene oxide (EO) chain length (m = 12, 14, 16 and n = 5, 7, 9) were used as foam stabilizer to select the most superior structure for stabilizing foam. The effects of CmEOn on surface tension were investigated which revealed that the CMC of CmEOn in the pigment foaming dispersion decreased with the increase of surfactant hydrophobicity or the decrease of hydrophilicity. Compared to the alkyl chain, EO chain length influenced foam more significantly. C₁₄EO₅ in the pigment foaming dispersion showed lowest CMC and equilibrium surface tension. The presence of wormlike micelles and lamellar liquid crystal of SDS and C₁₄EO₅ endowed the C₁₄EO₅ pigment foaming dispersion with highest viscosity which was distinctly different with other CmEOn. C₁₄EO₅ showed the most superior stabilization effects with foam half-life of 172.9 min at 9 wt%. To further analyze the stabilization mechanism of C₁₄EO₅, the foam volume and bubble diameter change were observed with a digital microscope. The results demonstrated that the superior stabilization effects of C₁₄EO₅ were closely related to its high viscosity, which mainly resulted in the decrease of foam drainage and gas permeability. C₁₄EO₅, the optimal CmEOn structure for stabilizing foam, shows excellent foam stabilization capability in pigment foaming dispersions, which is a promising tool to realize pigment foam coloring.     

Adsorptive modification of hydrophobic solid surfaces by mixed surfactant solutions

We studied the reciprocal influence of a nonionic surfactant (triton X-305) and a cationic surfactant (tetradecyltrimethylammonium bromide; TTAB) on their adsorption from aqueous solution on hydrophobic glass, interfacial tension at the solution/solid interface, composition of the mixed adsorption layer, and interaction parameters between surfactant molecules in mixed adsorption layers.     

Nanoparticle stability in semidilute and concentrated polymer solutions

The wetting of PDMS-grafted silica spheres (PDMS- g-silica) is connected to their depletion restabilization in semidilute and concentrated PDMS/cyohexane polymer solutions. Specifically, we found that a wetting diagram of chemically identical graft and free homopolymers predicts stability of hard, semisoft, and soft spheres as a function of the bulk free polymer volume fraction, graft density, and the graft and free polymer chain lengths. The transition between stable and aggregated regions is determined optically and with dynamic light scattering. The point of demarcation between the regions occurs when the graft and free polymer chains are equal in length. When graft chains are longer than free chains, the particles are stable; in contrast, the particles are unstable when the opposite is true. The regions of particle stability and instability are corroborated with theoretical self-consistent mean-field calculations, which not only show that the grafted brush is responsible for particle dispersion in the complete wetting region but also aggregation in the incomplete wetting region. Ultimately, our results indicate that depletion restabilization depends on the interfacial properties of the nanoparticles in semidilute and concentrated polymer solutions.