The authors have generated two-dimensional foams by imposing an intermittent drainage in a Hele-Shaw cell partially filled with a detergent/water mixture. The foam generation associated with this process is reproducible and depends on the surfactant molecules composing the solution. A kinetic model can be proposed for the foam evolution. The structure of the foam is also investigated: the average bubble side number and correlation functions are measured. Distinguishable behaviors are observed for different surfactant molecules. This way of producing a foam is thus adequate for applied foam structure characterizations and fundamental studies.

We have studied foaming dynamics in Hele-Shaw cells partially filled with a soap and water mixture. A series of upside-down flips produces an intermittent wetting of the cell and leads to foam formation. As a function of the number of flips, an increasing number of bubbles composes the foam, until saturation is observed. Statistical analysis shows that the bubble size follows a Gamma distribution. Contrary to common belief, this foaming dynamics by "shaking" creates homogeneous foam, even though the system may pass through transient heterogeneous configurations. A mechanistic interpretation is proposed and included into a theoretical model.

A series of sodium methacrylate and poly(ethylene glycol) (PEG) comb copolymers (MAA/PEG) with approximate PEG chain lengths of 7, 11, and 22 ethylene oxide units were synthesized by free radical polymerization. Their weight-average molecular mass was found to be approximately 66 000. A commercial sample of a PEG comb polymer with an acrylic backbone was also used in the studies (Sokalan HP 80). The interaction of the MAA/PEG comb polymers and pure sodium methacrylate (SPMA) with sodium dodecyl sulfate (SDS) was studied by ESR spectroscopy using 5-doxyl stearic acid (5-DSA) spin probe and by conductivity measurements. Surfactant aggregation in water occurred at SDS concentrations lower than the surfactant critical micelle concentration (cmc) and depended on the polymer concentration. The observations have been attributed to changes in the effective ionic strength of the systems due to the polymer itself, and it has been concluded that there is no interaction between the MAA/PEG comb copolymers or SPMA and SDS. This has been confirmed by the fact that the decrease in surfactant aggregation concentration is similar in magnitude to the decrease observed on adding NaCl when counterion ion condensation effects are taken into account. It is apparent that the electrostatic repulsions between the surfactant molecules and the methacrylate backbone of the MAA/PEG comb copolymers inhibit association of SDS with the PEG side chains.

We carried out experiments on detachment of oil drops from glass substrates in solutions of an anionic surfactant. The three-phase contact line shrinks spontaneously, and eventually the oil drop detaches from the substrate. Consecutive video frames of such drops are digitized, and the time dependencies of the contact radius and angle are determined. Three stages of detachment of a drop, situated above a horizontal substrate, can be distinguished. They correspond to three different driving factors: (1) the interfacial tension decrease because of surfactant adsorption, (2) the aqueous meniscus spontaneously advances owing to the penetration of water between the oil and solid phases, and (3) at sufficiently small contact radius the shape of the oil-water interface becomes unstable and the drop detaches under the action of buoyancy. Analyzing the experimental data, we identified two important characteristics of the drop-detachment process: the velocity of spontaneous advance of the contact line and the line drag coefficient. In the case of moving contact line, a dynamic Young equation must be used, which takes into account the line drag force. The latter is proportional to the velocity of contact-line motion. The experimental data agree with the latter dependence, from whose slope the line drag coefficient is determined.

Foams produced from surfactant solutions containing micelles of the anionic surfactant sodium polyoxyethylene-2 sulfate and counterions of different valence (aluminium, calcium or sodium) are investigated. For this purpose an experimental setup consisting of a glass column and units for detection of pressure, flow and frequency is constructed. Blowing gas bubbles in the surfactant solution at a constant gas pressure produces the foam. Simultaneous monitoring of the bubble volume and frequency relates the foam growth rate to the dynamic surface tension of the surfactant solution. The foam growth rate plotted versus the gas flow rate exhibits a break point at about 80 mL/min, attributed to the transition from regime of bubbles (at lower flow rates – monodisperse foam) to jet regime (at higher flow rates – polydisperse foam). Due to the high surfactant concentration, the foam is stable and its height is linearly increasing with the time. Two types of experiments are carried out. (i) At a constant counterion concentration and variable surfactant concentration, the rate of foam growth increases initially with increasing of the surfactant concentration reaching a plateau at higher concentrations. The foams of pure surfactant grow always slower than the foams with added aluminium ions. (ii) At a constant surfactant concentration and variable counterion concentration, the rate of foam growth exhibits a maximum. It corresponds to number of aggregated surfactant monomers nearly equal to the number of charges provided by the counterions, for example when one aluminium ion binds three surfactant monomers in a micelle. The point of maximum coincides with the transition from small spherical micelles to large cylindrical ones. This transition affects also the micelle lifetime, which is related to the ability of releasing monomers by a micelle in order to supply the bubble surface with surfactant. In support to this hypothesis, the maximum foam growth is found corresponding to lower dynamic surface tension allowing the generation of a large number smaller in size bubbles. The results for the foam growth agree in some extent with the data from independent measurements on the liquid drainage from wet foams.

The main target of this study is to develop a theoretical method for determining small contents of dodecanol in samples of sodium dodecyl sulfate (SDS) by a detailed analysis of surface-tension isotherms. As a tool for our analysis, we employ the van der Waals model. Its application to data for alkanols and anionic surfactants gives an excluded area per adsorbed molecule equal to the geometrical area of the molecular cross section and adsorption energies consonant with Traube's rule. Because the dodecanol and SDS have different excluded areas, we extended the van der Waals model for the case of a two-component adsorption layer, with account for the counterion binding in the Stem layer. General-expressions for the surface free energy, two-dimensional equation of state, surface chemical potentials, adsorption isotherms, and surface dilatational elasticity are derived. The experimental surface-tension isotherms are fitted by varying only one adjustable parameter. The model was successfully tested against data for solutions of SDS with a known content of dodecanol. Knowing the parameters of the model, we computed various properties of the surfactant adsorption layer. The results show that the presence of a small amount of dodecanol leads to a considerable increase of the total adsorption and surface elasticity. Even a relatively small (0.2 mol %) fraction of dodecanol in SDS may lead to a predominant content (up to 86 mol %) of dodecanol in the mixed adsorption layer. We applied the model for determining unknown contents of dodecanol in SDS samples at different stages of purification. The addition of NaCl may lead to a significant reduction in the mole fraction of dodecanol in the adsorption layer. The developed theoretical model and computational procedure are also appropriate for a quantitative analysis and computer modeling of the adsorption from other mixed ionic-nonionic surfactant solutions, at both air-water and oil-water interfaces.

Equilibrium surface tension isotherms of sodium dodecylbenzene sulfonate (DDBS) are obtained at various fixed concentrations of NaCl. The contents of unsulfonated dodecylbenzene (DDB) in the used surfactant sample is determined by processing the surface-tension data. Having determined the parameters of the best fit, we computed the adsorption of surfactants (anionic DDBS and nonionic DDB), the binding of counterions in the Stern layer, the surface electric potential, the surface elasticity, etc., each of them for various surfactant and salt concentrations. The results show that for the solutions without added NaCl, the adsorption layer consists mostly of the nonionic DDB, irrespective of its small mole fraction in the surfactant blend. The admixture of DDB in the sample of DDBS leads to a significant increase of the surface elasticity. Moreover, even minimal added amounts of CrCl3 or Fe2(SO4)3 cause a considerable reduction in the surface tension, which is due to the greater binding energies of some of the counterions released by the latter electrolytes. The paper gives a quantitative analysis and description of the adsorption from aqueous solutions of a technical ionic surfactant. The followed strategy, which was to determine the contents of the admixtures and to account for their presence in the theoretical model, rather than to purify the surfactant, may find applications to other mixed surfactant systems.

By NMR and static and dynamic light scattering we investigated how the micelle size, shape. and aggregation number vary during the solubilization of triolein in mixed aqueous solutions of the nonionic surfactant C12En (n = 5 or 6) and the nonionic triblock copolymer Synperonic L61 (SL61). The latter was found to strongly accelerate the solubilization, although the copolymer alone is unable to solubilize triglycerides. A series of solutions containing different concentrations of surfactant and copolymer has been studied. The light scattering shows that we are dealing with giant micellar aggregates, each of them containing hundreds to thousands of surfactant molecules and dozens of copolymer molecules. Assuming hat the micelles have the shape of prolate ellipsoids, we calculated their length and width. The results indicate that the initial, strongly elongated micelles transform into spherical or slightly ellipsoidal ones at the end of solubilization, The micelle aggregation number and hydrodynamic radius decrease by factors of 2-6 during the solubilization; i.e., one empty micelle splits into several smaller swollen micelles of radius 4-5 nm. The NMR measurements reveal that the equilibrium solubilization capacity (solubilized oil/mol of surfactant) is virtually not affected by the addition of SL61. Hence, the copolymer accelerates the process without influencing its final result; that is, SL61 acts as a promoter of solubilization. The results are used in the subsequent two papers of this series to develop a kinetic model of triglyceride solubilization and to verify it against experimental data.

We observed the diminishing of single microscopic oil drops to study the kinetics of solubilization of n-decane and benzene by micellar solutions of sodium dodecyl sulfate (SDS). Each drop is located in a horizontal glass capillary of inner diameter 0.06 cm filled with a thermostated surfactant solution; the small vertical dimension of the cell prevents the appearance of uncontrollable thermal convections. The experiments show that the radius of an n-decane drop decreases linearly with time, whereas for benzene this dependence is nonlinear. To interpret the data, a kinetic model of solubilization is developed. It accounts for the diffusion and capturing of dissolved oil molecules by the surfactant micelles, as well as for the finite rate of oil dissolution at the oil-water interface. By processing the data, we determined the rate constant of solubilization for a given oil and surfactant. It turns out that the elementary act of catching a dissolved oil molecule by a surfactant micelle occurs under a barrier (rather than diffusion) control. The effective rate of solubilization is greater for the oil, which exhibits a higher equilibrium solubility in pure water (benzene), despite the lower value of the solubilization rate constant for this oil.

ADVERTISEMENT RETURN TO ISSUEPREVNoteNEXTComparison of the van der Waals and Frumkin Adsorption Isotherms for Sodium Dodecyl Sulfate at Various Salt ConcentrationsV. L. Kolev, K. D. Danov, P. A. Kralchevsky, G. Broze, and A. MehreteabView Author Information Laboratory of Chemical Physics & Engineering, Faculty of Chemistry, University of Sofia, 1164 Sofia, Bulgaria, Colgate-Palmolive R&D, Incorporated, Avenue du Parc Industriel, B-4041 Milmort (Herstal), Belgium, and Colgate-Palmolive Technology Center, Piscataway, New Jersey 08854-5596 Cite this: Langmuir 2002, 18, 23, 9106–9109Publication Date (Web):October 12, 2002Publication History Received24 May 2002Revised20 August 2002Published online12 October 2002Published inissue 1 November 2002https://doi.org/10.1021/la0259858Copyright © 2002 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views2336Altmetric-Citations78LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (101 KB) Get e-AlertsSUBJECTS:Adsorption,Isotherms,Layers,Sodium dodecyl sulfate,Surfactants Get e-Alerts

A method for determining the equilibrium adsorption of an ionic surfactant on oppositely charged solid particles is proposed. Data from ζ-potential measurements are processed to obtain the adsorption constants of surfactant ions and counterions. The method can be applied to particles of arbitrary size and shape, but only if their characteristic dimension is much larger than the thickness of the electric double layer. The adsorption is quantified solely by zeta-potential experiments; additional measurements of concentration depletion and specific particle-surface area are not needed. The approach is suitable for concentrations below the threshold of hemimicelle formation. We carried out experiments on adsorption of dodecyl-trimethyl-ammonium-bromide on glass particles in the presence of 1 mM NaBr. The particles were obtained by crushing a glass plate to powder and subsequent screening to remove particles larger than 20 μm. Control experiments with spherical particles from the same material showed that the method is insensitive to particle shape. Since the surfactant (DTA+) and Na+ bear the same electric charge, they compete with each other in the adsorption at the negatively charged glass. This is accounted for in the theoretical model. The binding of Na+ counterions is found to play an essential role in the overall adsorption process.

We investigated the kinetics of solubilization of triglycerides (triolein and soybean oil) by observing the diminishing of individual oil drops (radius less than or equal to 50 mum) in a micellar surfactant solution. We used two solubilization cells. Cell no. 1 is a centimeter-sized thermostated vessel. containing the investigated micellar solution and a few oil drops. Cell no. 2 represents a set of horizontal glass capillaries filled with the solution in each of them an oil drop was injected by a syringe. Cell no. I is more easy to operate, whereas cell no. 2 allows a quantitative interpretation of the results, We carried out experiments with solutions of two nonionic surfactants and investigated the effect of various additives on the solubilization rate, The addition of an anionic surfactant is found to suppress the solubilization due to an enhanced electrostatic repulsion between the micelles and the oil-water interface. This inhibitory action can be partially removed by addition of an amphoteric surfactant. Highest solubilization rates have been achieved by Addition of E-n-P-m-E-n triblock copolymers (Synperonics) to the nonionic surfactant solutions. The experimental data for diminishing drops agree very well with the theoretical time dependence of the drop radius, From the Pits we determined the solubilization rate, the compound kinetic constant of solubilization, and the number of oil molecules/swollen micelle, n(s). The kinetic values of n(s), determined with diminishing oil drops, are compared with the equilibrium values of n(s), independently obtained by NAIR. The data confirm that, in the considered case, the time-limiting step is the adsorption of empty micelles at the oil-water interface. In particular, each swollen micelle desorbs from the triolein-water interface after taking 5-20 triolein molecules, depending on the solution's composition. The reported results can be helpful for the analysis and control of the solubilization kinetics of triglycerides and other water-insoluble oils.

A theoretical model of oil solubilization in micellar surfactant solutions is developed. We consider oils that are practically insoluble in pure water, like triolein and other triglycerides. The nonionic micelles, which are capable to solubilize such oils, are usually rodlike aggregates, composed of surfactant molecules and poly(oxyethylene)-poly(oxypropylene) triblock copolymers, like Synperonic L61 (SL61). The swollen micelles, formed after solubilization, are smaller than the empty ones. The model describes the elementary act of solubilization as a sequence of three steps: (a) adsorption of an empty micelle at the oil-water interface; (b) uptake of oil by the adsorbed empty micelle which then splits into several swollen micelles; (c) desorption of the swollen micelles. Theoretical expressions are derived, which describe the diminishing of an oil drop in the course of solubilization. The parameter values, determined from the best fit of experimental data, imply that the rate-controlling step is the step of micellar adsorption. From the determined rate constant of adsorption we estimate the respective kinetic barrier to micelle adsorption, taking into account the action of surface forces and the hydrodynamic resistance. Our analysis indicates that the triblock copolymer SL61 promotes the solubilization of triglycerides by decreasing the length of the mixed surfactant-copolymer rodlike micelles, which leads to a lowering of the kinetic barrier to their adsorption at the oil-water interface.

The interaction of polyvinylpyrrolidone (PVP) with an anionic surfactant (sodium dodecyl sulfate, SDS), a nonionic surfactant (pentaethylene glycol monodecyl ether, C(10)E(5)), and a zwitterionic surfactant (lauryl amido propyl betaine, LAPB) has been investigated by means of pulsed gradient spin-echo NMR (FT-PGSE NMR), allowing self-diffusion coefficients to be determined. The results confirm the strong interaction prevailing in the PVP/SDS system, whereas no association has been observed in the PVP/C(10)E(5) and PVP/LAPB systems. Mixing PVP with two surfactants, namely SDS and C(10)E(5) or SDS and LAPB, results in the formation of ternary aggregates between the polymer and the mixed micelles. Copyright 2001 Academic Press.

In the preceding paper of this series we studied the effect of several oils of different chemical structure on the foaming properties of sodium dodecylbenzenesulfonate solutions. A straightforward correlation was found between the foam stability and the so-called "entry barrier", which prevents the emergence of pre-emulsified oil drops on the solution surface. In the present article we perform a systematic experimental study of the entry barriers for several oils by means of the recently developed film trapping technique. The latter consists of trapping oil drops in wetting films on a solid substrate, followed by a controlled increase of the capillary pressure of the meniscus that compresses the drops against the substrate. At a certain critical capillary pressure, P-C(CR), the asymmetric oil-water-air films rupture and the drops enter the water-air interface. This event is observed microscopically, and P-C(CR) is determined as a function of various parameters (type of oil, surfactant concentration, drop size, and others). The entry barrier increases with the surfactant concentration, especially in the range where the surfactant micelles are expected to stabilize the asymmetric films. The results obtained with a series of alkanes (from octane to hexadecane) show that the entry barrier increases with the alkane chain length. Furthermore, it is shown that the presence of a spread oil (even as an ultrathin, molecular layer) on the surface of the foam film might lead to a significant change of the magnitude of the entry barrier. For decane and dodecane, the layer of spread oil reduces the entry barrier, whereas for hexadecane the effect is the opposite. As far as we know, such a role of oil spreading in the antifoaming action of oils has not been reported so far. Since the stability of thin liquid films is usually discussed in the literature in terms of the disjoining pressure, we estimate from the experimental data the critical disjoining pressure, IIASCR, at which the asymmetric oil-water-air film ruptures and the drop entry occurs. The estimates show that the curvature of the asymmetric film is very important in the overall consideration of the mechanical equilibrium in the system and there is a big difference between the numerical values of P-C(CR) and IIASCR, unlike the case of planar films where P-C(CR) = IIASCR. Additionally, we find that P-C(CR) is a weak function of the oil drop size and of the asymmetric film radius, while IIASCR scales as (film radius)(-1) for all of the studied systems. These results are discussed with respect to the possible mechanisms of film rupture. Concerning the foam stability, P-C(CR) is a more convenient quantity for description of the entry barriers, because its magnitude correlates with the foam height, whereas the magnitude of IIASCR does not.

Foam tests and model experiments with sodium dodecylbenzenesulfonate solutions are performed to clarify how the foam stability and the foaminess are affected by several oils of different chemical structure. The foam tests show that 2-butyloctanol (2BO, branched alkanol) and isohexyl-neopentanoate (IHNP, branched ester) exhibit a significant antifoam. activity at concentrations as low as 0.005 wt %. n-Heptanol also acts as an antifoam, but at concentrations above 0.15 wt % due to its higher solubility in the surfactant solution. The model experiments prove that the antifoam activity of pre-emulsified oils is determined primarily by the barrier to drop entry, which controls the drop emergence on the solution surface. If the entry barrier is high (e.g., n-dodecanol and silicone oil), the oil drops remain arrested in the Plateau borders during the process of foam drainage, without being able to destroy the foam. Thus branched long-chain alkanols (like 2BO) and esters (IHNP) behave as active antifoams, because they combine the advantages of long-chain and medium-chain n-alkanols-low solubility and low entry barrier, respectively. No direct correlation between the spreading behavior of the oils and their foam breaking activity is observed. The effect of these oils on the foamability of the solutions is far more complex. At low concentrations (below and around their solubility limit) the oils reduce the dynamic surface tension of the solutions, facilitating in this way the formation of fresh surface and enhancing the foamability. At higher oil concentrations, however, the emulsified oil drops induce a coalescence of the foam bubbles during foaming and, as a result, the foamability of the solutions decreases. That is why the foamability is a nonmonotonic function of the oil concentration.

This theoretical study is devoted to the relaxation of surface tension of an ionic surfactant solution for submicellar concentrations. The effects of added nonamphiphilic electrolyte and counterion binding are taken into account. We consider a large initial deviation from equilibrium, which is defined as the formation of a new interface: there is no adsorbed surfactant and electric double layer at the initial moment. Next, the surfactant solution and its interface are allowed to relax without any subsequent perturbation. The electrodiffusion equations, which describe the adsorption kinetics, are nonlinear, and it is impossible to find a general analytical solution, especially in the case of large initial deviations. Nevertheless, the problem can be linearized in the asymptotic case of long times. The derived theoretical expressions show that the relaxation times in the cases of large and small initial perturbations are numerically close to each other. For that reason the relaxation time can be considered as a general kinetic property of the adsorption monolayer. The theory predicts also the slope of the experimental plot of dynamic surface tension vs inverse square root of time; this makes the theory useful for interpretation of experimental data. The theoretical expressions involve the surface (Gibbs) elasticity, whose definition for adsorption monolayers of soluble ionic surfactants is discussed in detail. The Gibbs elasticity of such monolayers is found to increase strongly with the rise of salt concentration. The derived asymptotic expressions are verified against an exact computer solution of the electrodiffusion problem, and excellent agreement is found.

Instability driven by diffusion flux across an asymmetric liquid film is theoretically investigated. To specify the system, we consider an oil-water-air film; the transported species are oil molecules. The latter are dissolved (solubilized) at the oil-water interface, then they are transferred by diffusion across the aqueous film to the water-air interface, where they penetrate between the tails of adsorbed surfactant; molecules. Fluctuational capillary waves at the two film surfaces are considered. The set of theoretical equations is solved to derive a dispersion relation between the wavenumber and the exponent of wave growth. The results reveal that if a local decrease in the film thickness appears, the water-air interface enters a zone enriched in dissolved oil. The newly adsorbed oil creates a surface tension gradient, which carries water away and causes a further decrease of the local film thickness in the concave zone, until eventually the film ruptures. The numerical results show that the film becomes less stable when its thickness decreases, its radius increases, and the diffusion flux across it is more intensive. Even very small decrements of the water-air surface tension, caused by the adsorbed oil, are sufficient to trigger the instability. Its appearance is not so sensitive to the degree of mobility of the oil-water interface. If the water-air surface is preequilibrated with the transported component (there is prespread oil), then the surface tension decrement and the instability disappear. The rupture of an asymmetric oil-water-air film is a precondition for entry of an oil drop at the water-air interface and effectuation of its foam-destructive action. Our results about a specific source of film instability could be helpful for a deeper understanding of the mechanisms of antifoaming.

The micellar composition of two binary surfactant systems, that is, sodium dodecyl sulfate (SDS)/pentaethylene glycol monodecyl ether (C10E5) and SDS/lauryl amido propyl betaine (LAPB), has been analyzed by pulsed gradient spin−echo NMR (FT-PGSE NMR). The experimental data have been compared to theoretical predictions based on the regular solution theory that takes into account the nonideal mixing of the surfactant pairs. Although good agreement between experiment and theory is observed for the SDS/C10E5 pair, some discrepancy is noted for the LAPB/SDS system, particularly at a high molar fraction of SDS.