Ultrafiltration of sodium dodecylsulfate solutions

The present paper is devoted to ultrafiltration (UF) of aqueous solutions containing micellized sodium dodecylsulfate (NaDS) through zircon membranes, following permeation data of cetyltrimethylammonium bromide (CTABr), which we have reported recently.The experimental results are analyzed using the same model of diffusion in the membrane matrix based on the thermodynamics of irreversible processes (TIP). A comparison with the classical model of resistances in series leads to a new analysis of the measured resistances in terms of membrane–permeant and permeant–permeant interactions. NaDS micelles being able to cross the membrane as permeants, in contrast with CTABr micelles, the comparison of the behaviors of anionic and cationic surfactants is worthwhile. The resistance of the membrane strongly depends on the composition of the feed. The TIP approach allows us to relate the membrane resistance to the concentrations of all the species present in the feed, namely free ions, spherical micelles and rod-like aggregates. It is shown that the resistance is highly influenced by free ions (dodecylsulfate (DS⁻) and sodium (Na⁺)), very slightly by negatively charged spherical micelles, and weakly by rod-like aggregates, also negatively charged.     

Ultrafiltration of cetyltrimethylammonium bromide solutions

Permeation data of cetyltrimethylammonium bromide (CTABr) in water by ultrafiltration (UF) through zircon membranes are reported as a first step in the knowledge of micellar enhanced ultrafiltration of heavy metals by cationic surfactants [1]. The experimental results are analysed using a model of diffusion in the membrane matrix based on the thermodynamics of irreversible processes (TIP). A comparison with the classical model of resistances in series indicates that these two approaches are consistent and that the TIP method leads to new insights into the observed resistances. The major insight relates to the understanding of the membrane resistance in terms of interactions membrane-permeants and permeants-permeants.     

Diffusion in surfactant solutions

The diffusion coefficients in water of Triton X-100 and sodium dodecyl sulfate were measured as a function of concentration using the Taylor dispersion technique. For Triton X-100, a nonionic surfactant, the diffusion coefficient drops from 7.4 × 10^-7 cm²/sec at 0.45 g/liter to 6.45 x 10^-7 cm²/sec at 5 g/liter. The diffusion coefficient of methyl yellow solubilized in Triton X-100 is close to that of the surfactant. This behavior is quantitatively consistent with a chemical equilibrium between monomer and micelle. For sodium dodecyl sulfate, an anionic surfactant, the diffusion coefficient increases from 1.76 x 10^-6 cm²/sec at 0.01 M to 4.53 x 10^-6 cm²/sec at 0.125 M. The increase is less when 0.1 M NaCl is added. The diffusion coefficient of the methyl yellow solubilized by the SDS is significantly less than that of the surfactant, particularly at low ionic strength. This behavior can be quantitatively explained by including electrostatic coupling between monomer, micelle, and counterion.     

Computer simulation of surfactant solutions

Major advances have been made at several levels of computer simulation of surfactant solutions. Atomistic level studies of preassembled surfactant structures have become fairly routine. The development of structure in surfactant solutions has now been studied using atomistic, coarse grain and mesoscopic models. Coarse grain and mesoscopic simulations have been used to determine phase diagrams. The challenges involved in treating complex surfactant solutions will continue to drive this field forward.     

Thermodynamics of aqueous carbohydrate surfactant solutions

The paper deals with the application of the micelle formation theory, developed by Nagarajan and Ruckenstein [R. Nagarajan, E. Ruckenstein, Langmuir 7 (1991) 2934–2969] and Nagarajan [R. Nagarajan, in: K. Esumi (Ed.), Structure–Performance Relationships in Surfactants, Dekker, New York, 1997, pp. 1–81; R. Nagarajan, Adv. Colloid Interface Sci. 26 (1986) 205-264] to various n-alkyl-β-d-glucopyranoside surfactants, differing in the surfactant tail length (n-octyl-β-d-glucopyranoside C₈G₁, n-decyl-β-d-glucopyranoside C₁₀G₁ and dodecyl-β-d-glucopyranoside C₁₂G₁). The model predicts that the carbohydrate surfactant molecules assemble for energetic reasons in spherical bilayer vesicles. The critical micellar concentration as function of the temperature shows a minimum value. The formed micellar aggregates exhibit a broad distribution of sizes. It is demonstrated in this study that the thermodynamic theory in combination with phase separation thermodynamics can be used successfully to described the phase separation, which occurs for the system C₁₀G₁+water and C₁₂G₁+water at low surfactant concentrations.     

Unbinding and preunbinding in surfactant solutions

We propose models for the first-order unbinding transition of lyotropic lamellae in surfactant solutions. The coupling between the surfactant volume fraction and the elastic degree of freedom is considered so that the net attractive interaction between the surfactant molecules is enhanced. The elastic degree of freedom can be either (i) a membrane elastic degree of freedom or (ii) a bulk elastic degree of freedom. The phase behaviors of these two models are analyzed. For both cases, the unbinding transition becomes first order when the coupling is strong enough. We determine the associated preunbinding line which separates two lamellar phases having different repeat distances.     

Salting-out surfactant extraction of porphyrins and metalloporphyrin from aqueous non-ionic surfactant solutions

A number of cloud point temperature-depressing electrolytes have been investigated for the separation of a non-ionic surfactant (Triton X-100) from aqueous solutions and the corresponding extraction of the organic solutes into the smaller volume surfactant-rich phase using the salting-out method. High extraction efficiencies and preconcentration factors were obtained at room temperature for the extraction of several hydrophilic and hydrophobic metal-free porphyrins (uroporphyrin, coproporphyrin, protoporphyrin and hematoporphyrin) and one metalloporphyrin (iron-protoporphyrin) that were dissolved in the aqueous non-ionic surfactant solutions. Possible mechanisms responsible for the efficient extraction of these important biological molecules into the surfactant-rich layer are discussed.     

Rheological studies of aqueous alkylpolyglucoside surfactant solutions

Alkylpolyglucosides (C _Y G _X ) are industrial products of mixtures consisting of a hydrocarbon chain with Y carbon atoms linked to X sugar residues. Based on detailed analytical investigation of technical grade alkylpolyglucosides (C_8–10G _X , C_12–14G _X and C_8–16G _X )/water systems using high-performance liquid chromatography in combination with a special kind of mass spectroscopy their rheological behaviour is discussed and compared to the rheological behaviour of pure alkyl monoglycosides (C₈G₁ and C₁₀G₁) in water. The rheological properties that exhibit a dependence on the alkyl chain length, Y, and the degree of polymerisation, X, are investigated by rotation and oscillation experiments over an extended concentration range. The Maxwell model fits the frequency dependence of the dynamic functions fairly well. The viscosity shows an Arrhenius-like dependence on temperature. A comparison is drawn between the monoglucosides and the polyglucosides, which shows that the rheological properties are more sensitive to the a change in chain length than in the degree of polymerisation. Phase transitions, especially liquid-crystalline to isotropic solutions, phase split into two coexisting liquid phases, and could be followed using visual observation and rheology. Depending on the difference in the rheological properties of the corresponding phases, viscoelastic measurements showed these transitions clearly. Additionally, the changes in viscosities were measured after addition of a second surfactant. Received: 4 January 1999 Accepted in revised form: 12 April 1999     

Penetration of surfactant solutions into hydrophobic capillaries

The initial rise velocity of surfactant solutions in hydrophobic capillaries is independent of time (F. Tiberg, B. Zhmud, K. Hallstensson and M. von Bahr, Phys. Chem. Chem. Phys., 2000, 2, 5189). By analogy with the hydrodynamics of an overflowing cylinder, we present a steady-state solution for capillary penetration in which the velocity is determined by the adsorption kinetics at the air-water interface. Good agreement between the model predictions and experimental data of Tiberg and coworkers is obtained for the non-ionic surfactant C10E6 under the assumption of diffusion-controlled adsorption. The longer chain homologue, C14E6, shows evidence of kinetic barriers to adsorption.     

The behaviour of drag-reducing cationic surfactant solutions

The behaviour of two types of drag reducing surfactant solutions was studied in turbulent flows in pipes of different diameters. Our surfactant systems contained rod-like micelles; they consisted of equimolar mixtures ofn-tetradecyltrimethylammonium bromide,n-hexadecyltrimethylammonium bromide, and sodium salicylate. The structure of the turbulence was studied using a laser-Doppler anemometer in a 50 mm pipe. In the turbulent flow regime both surfactant solutions exhibited characteristic flow regimes. These flow regimes can be influenced by changing the amount of excess salt, the surfactant concentration, or the temperature. Shear viscosity measurements in laminar pipe and Couette flows show the occurrence of the so-called shear-induced state, where the viscosity increases and the surfactant solution becomes viscoelastic. The shape of the turbulent velocity profile depends on the flow regime. In the turbulent flow regime at low Reynolds numbers, velocity profiles similar to those observed for dilute polymer solutions are found, whereas at maximum drag reduction conditions more S-shaped profiles that show deviations from a logarithmic profile occur. An attempt is made to explain the drag reduction by rod-like micelles by combining the results of the rheological and the turbulence structure measurements.     

Equilibrium surface tension of aqueous surfactant solutions

Summary The applicability of the drop weight method for determining time dependent surface tension of surfactant solutions was experimentally proved.The presence of CO₂ and traces of lauryl alcohol and long chain homologs lower the surface tension of sodium dodecyl sulfate solutions in a measurable extent. The chemical purification of materials and recrystallizations are insatisfactory to obtain sufficient purity; the cleaning of the surface itself is needed by e.g. foaming. The equilibrium surface tension of pure surfactant solutions that is often required to interpret interfacial phenomena and to calculate theGibbs adsorption excess can be determined with reliable accuracy by linear extrapolation to zero time from measurements made in function of time up from 1 minute on moderately contaminated solutions.As a criterion of surface purity the time dependence of surface tension and the average life-time of thin liquid film are suggested.

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Zusammenfassung Die Tropfengewichtsmethode eignet sich auch zur Bestimmung der zeitabhängigen Oberflächenspannung wäßriger Tensidlösungen. Zur Entfernung von Verunreinigungen, die die Oberflächenspannung von Natriumdodecylsulfatlösungen herabsetzen, wie Spuren von Laurylalkohol und höheren Homologen, reichen weder chemische Methoden noch wiederholtes Umkristallisieren aus, sondern die Oberfläche selbst ist zu reinigen, z. B. durch Ausschäumen. Der Gleichgewichtswert der Oberflächenspannung läß sich mit hinreichender Genauigkeit berechnen, in dem man die an mäßig konzentrierten Lösungen über einer Minute Meßzeit gewonnenen Daten auft = 0 linear extrapoliert. Als praktisches Kriterium für die Reinheit der Oberfläche kann man sowohl die Zeitabhängigkeit der Oberflächenspannung, wie auch die mittlere Lebensdauer von dünnen Lösungsfilmen heranziehen.

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With 6 figures     

Fingering phenomena during spreading of surfactant solutions

Fingering instabilities are observed at the contact line of drops of surfactant solutions spreading spontaneously on solid surfaces coated by a film of solvent. The occurrences of instabilities, and the characteristics of the instability pattern, are controlled by the surfactant concentration and the thickness of the film adsorbed or deposited on the substrate. This work provides experimental data as a basis for forthcoming theoretical analyses.     

On the properties of surfactant aqueous solutions

Summary From studies of aqueous solutions of dodecylammoniumnitrate an association mechanism has been proposed involving multiequilibrium. In the concentration range considered we can differentiate between two aspects: ranges of marked qualitative and quantitative changes and formation of differently structured surfactant species.

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Zusammenfassung Aus der Untersuchung wäßriger Lösungen von Dodecylammoniumnitrat würde ein Assoziationsmechanismus abgeleitet, welcher ein Multiequilibrium beinhaltet. Im untersuchten Konzentrationsbereich können zwei Bereiche unterschieden werden: Bereiche mit ausgeprägten qualitativen und quantitativen Änderungen und Bereiche mit der Bildung von verschieden strukturierten oberflächenaktiven Spezies.

     

Wetting behavior of pulmonary surfactant aqueous solutions

The wetting properties of pulmonary surfactant aqueous solutions with respect to solid surfaces with different degree of hydrophobicity have been studied. The contact angles θ of drops from a pulmonary surfactant solution onto SiO₂-glass surfaces have been measured as a function of their degree of hydrophobicity θ _w. The completely hydrophilic SiO₂-glass surface is essentially hydrophobized by the animal-derived pulmonary surfactant Curosurf. The hydrophobization depends on the surfactant concentration—the contact angles increase with increasing the Curosurf concentration C _s in the low concentration range, but they remain almost constant in a wide range of C _s >90 μg/ml. Additions like NaCl and bovine serum albumin influence the θ-values. The contact angles θ naturally increase with increasing θ _w but this dependence is not linear—the curve steepens at larger θ _w values. The thickness h of the wetting thin liquid films from Curosurf aqueous solutions depends on the hydrophobicity θ _w of the solid surface and the h(θ _w) curves always pass a minimum. The h-values, as well as the h(θ _w) curve, are mainly determined by the steric and hydrophobic disjoining pressures, which depend on the orientations and conformations of the molecules adsorbed on the solid surface from the very complicated multi-component aqueous solutions.     

Wetting of surfactant solutions by alkanes.

Ellipsometry, surface tensiometry, and contact-angle measurement have been used to study the transition between partial wetting and pseudo-partial wetting of surfactant solutions by alkanes. In the partial wetting regime, the air-water surface tension is the same with and without alkane. In the pseudo-partial wetting regime, the air-water surface tension is lowered by the presence of alkane, showing that oil is solubilised into the surfactant monolayer. A discontinuous change in the coefficient of ellipticity with increasing surfactant concentration provides unequivocal evidence for the first-order nature of the wetting transitions. Ellipsometry has been used to explore the generality of wetting transitions of alkanes (dodecane, hexadecane, and squalane) on surfactant solutions [dodecyltrimethylammonium bromide, tetredecyltrimethylammonium bromide, dibucaine hydrochloride, and Aerosol OT (AOT)]. Of the systems studied, only hexadecane on AOT solutions did not show a wetting transition. Excess alkane remains as a lens on the surface of the surfactant solutions at all concentrations, but the contact angle is a minimum at the wetting transition. A semiquantitative model for the variation of the contact angle with surfactant concentration is provided.     

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.     

Drop impact on surfactant films and solutions

Drops impacting on horizontal aqueous surfactant films have been analyzed using a high-speed camera. Drops of either water or aqueous surfactant solutions had a diameter of 2.4?±?0.4 mm and impacted with a velocity of 0.1 to 1.3 m/s. As surfactants, anionic sodium dodecyl sulfate and cationic cetyltrimethyl ammonium bromide were used. Pure water drops impacting on freestanding surfactant films showed coalescence, bouncing, partial bouncing, passing, and partial passing. For bouncing, the concentration of surfactant in the surfactant film must exceed the critical micelle concentration. When surfactant was added to the drop, coalescence and partial passing were suppressed. We attribute the different behavior to different hydrodynamic boundary conditions at the surface of pure water and surfactant solution, leading to different repulsive hydrodynamic forces arising when the air has to flow out of the closing gap between the two liquid surfaces. The boundary condition changes as a function of surfactant concentration from a slip to no-slip, leading to stronger hydrodynamic repulsion. In addition, estimates of the characteristic velocities show that diffusion of air into the water is slow and can only account for the very last thinning of the air gap before coalescence.