Lamellar Droplets Research Articles

Effects of Hydrophobic Counterions on the Phase Behavior of Tetradecyldimethylhydroxylammonium Chloride in Aqueous Solutions

We investigated the phase behavior of cationic tetradecyldimethylhydroxylammonium chloride (C14DMAOH·Cl) (a fixed surfactant concentration, 20 mM) in the presence of sodium 2-naphthalenesulfonate (SNphS) as a function of the molar ratios β (= SNphS/C14DMAOH·Cl) by turbidity, viscosity, dye solubilization, and polarizing microscopy. The C14DMAOH·Cl/SNphS system formed aggregates with a lower curvature than cetyltrimethylammonium bromide (C16TAB)/aromatic counterion systems, despite the shorter alkyl chain. Addition of SNphS into the C14DMAOH·Cl solution at 25 °C produced precipitates of solid crystals above β ∼ 0.5, in contrast with long fibrous micelles of many other cationic micelle−aromatic counterion pairs. The crystalline solids (tetradecyldimethylhydroxylammonium naphthalenesulfonate, C14DMAOH·NphS) at 20 mM concentration were transformed into the lamellar liquid crystalline phase dispersion (lamellar droplets) above 54 °C. The lamellar phase was not spontaneously converted into vesicles upon dilutio...

Estimation of the Interfacial Tension Between the Lamellar and the Sponge Phases of a Lyotropic System

In lyotropic systems, the sponge and the lamellar phases possess the same local structure: a membrane made of a bilayer of surfactants. In the quasiternary lyotropic system CPCl/brine/hexanol, the bilayer is continuous through the interface between the lamellar and the sponge phases. A model based on this phenomenon has predicted a very low value of the interfacial free energy. The study of hydrodynamic relaxation time of distorted spherical lamellar droplets gives an estimation of the interfacial tension value. Results confirm the validity of the model and the dependence on membrane volume fraction is explained by a simple scaling law.

On the Shapes of Lamellar Droplets in Sponge Phase

We present a qualitative study of the macroscopic properties of the interface between the lamellar and the sponge phase and a brief account of the shape of droplets of lamellar phase in sponge phase. Similar to G. Friedel's “bǎtonnets”, but much simpler, those droplets are the result of a competition between interfacial tension, smectic elasticity and growth mechanisms.

Rheology of Concentrated Dispersions of Sterically Stabilized Polydisperse Lamellar Droplets

The rheological behavior of concentrated dispersions of sterically stabilized lamellar droplets has been studied as a function of the molecular weight and of the amount of adsorbed hydrophobically endcapped poly(sodium acrylate)s. The chemical compositions of the samples are identical to those described before. Despite the polydispersity of the sample, scaling laws and equations that are well established in the rheology of monodisperse colloidal suspensions can be successfully applied. Although the amount of added stabilizing polymer at constant molecular weight hardly influences the elastic modulus (G') as a function of(core) volume fraction of lamellar droplets (o lam ), increasing the polymer molecular weight at constant grafting density results in a pronounced increase of the elastic modulus. The ratio of particle radius to adsorbed layer thickness (R/Δ) decreases with molecular weight, thereby increasing the effective volume fraction. A peculiar effect occurs if the polymer molecular weight drops below 1000. Polymer molecules penetrate into the lamellar droplets, and o lam is increased (at constant surfactant concentration). This so-called building-in effect can be used to thicken lamellar dispersions. Thickening can also be induced by addition of bridging polymers, which carry multiple hydrophobic anchors, linking several droplets. Concomitant adverse effects of bridging flocculation can be counteracted by admixing of hydrophobically endcapped polymers; the resulting dispersions are characterized by enhanced shear-thinning behavior and good physical stability.

Microstructure of Dispersions of Lamellar Droplets Carrying Anchoring Hydrophobically Endcapped Poly(sodium acrylate)s as Novel Steric Stabilizers

We have studied the influence of anchoring hydrophobically single-endcapped poly(sodium acrylate)s on the microstructure and colloidal stabilization of self-assembled lamellar droplets formed from a mixture of anionic and nonionic surfactants in concentrated aqueous electrolyte solutions. A fluorescently labeled hydrophobically endcapped poly(sodium acrylate) has been synthesized and characterized using time-resolved fluorescence spectroscopic techniques; it appears that the fluorophore has considerable freedom of internal rotation. Using this labeled poly(sodium acrylate), the presence of an adsorbed polymer layer bound to the surface of the droplets was imaged by confocal scanning laser microscopy, providing visual evidence that the droplets are sterically stabilized. Laser diffraction and refractive index measurements were employed to determine average particle sizes of the colloidal particles, and it was established that increasing the molecular weight of the hydrophilic (pendant) backbone at a constant (hydrophobic) anchor density, or increasing the concentration of polymer in the dispersion at constant molecular weight, results in a decrease of the average droplet size. This is in agreement with theoretical predictions that an increased lateral pressure in the adsorbed layer, due to a higher polymer segment density near the surface, is relieved by increasing the curvature of the lamellar droplets. Finally, the adsorption of hydrophobically endcapped polymers to lamellar droplets has been described in terms of a Freundlich isotherm, reflecting the degressive increase of the amount of polymer adsorbed onto the surface of the droplets with increasing polymer concentration. Again, an increase of lateral pressure with surface coverage is held responsible for this effect.

Localisation of anchored polymers in lamellar liquid–crystalline dispersions by confocal microscopy

The localisation of an anchored polymer in a lamellar liquid–crystalline dispersion has been investigated by confocal microscopy. For that purpose, the polymer is tagged with a fluorescent label and the microscopic technique used is fluorescent confocal scanning light microscopy. The continuous phase of the lamellar dispersion, a model liquid detergent system, is a concentrated electrolyte solution. The polymer, a copolymer of sodium acrylate (NaA) and laurylmethacrylate (LM), also known as decoupling polymer, can be labelled successfully with fluorescein, although this labelling is not restricted to the NaA/LM molecules exclusively. Also, a fraction of this pure polymer, sodium polyacrylate homo polymer molecules, is labelled and shows up in the microscopic images as continuous background. Localisation studies of fluorescently labelled NaA/LM polymers result in positive evidence that the polymer is present in highly increased concentration at the interface between the lamellar droplet and the continuous electrolyte phase. Inside the lamellar droplets a faint fluorescence signal and, often, an intense fluorescent small “nucleus” in the centre of the dispersed droplets has been observed. A special fraction of the decoupling polymer is suggested to be responsible for these fluorescent signals. This fraction, also known as the “scum” in pure decoupling polymer solution/dispersions, has a low NaA/LM ratio, is poorly solvable in the lamellar dispersion and therefore remains present as a separate, dispersed polymer-rich liquid phase, located inside the lamellar droplets or as separate entities in the continuous phase. Confocal scanning light microscopy has demonstrated, by direct visualisation, the localisation of NaA/LM-type decoupling polymers in the very outer region of the lamellar droplets. Due to restrictions in the resolution of the light microscope, measurement of the thickness of this outer region is not possible.

Microstructure and Rheology of Lamellar Liquid Crystalline Phases

We have investigated the microstructure and rheological properties of ternary surfactant mixtures in a salt solution. The surfactants were 6% sodium 4-dodecylbenzenesulfonate, 3% C13-15 ethoxylated alcohol with seven ethylene oxide (EO) units and 1% C13-15 ethoxylated alcohol with 2, 4, 7, 9, 11, 14, 20, or 25 EO units. The salt solution was 10% nitrilotriacetate·H2O. Microstructural investigations (electron microscopy, light microscopy, confocal laser microscopy, conductivity measurements, and centrifugation) show that at rest the samples containing the surfactant with 2 EO to 9 EO units are dispersions of lamellar droplets (curved surfactant bilayers). The samples containing the surfactant with 11 EO to 25 EO units show a continuous lamellar structure (sheets of surfactant bilayers) with a small amount of lamellar droplets present. The change in several rheological parameters reflects this change in microstructure. The power law index from flow experiments at low shear rates changes from 0.1 for the lamellar dispersions to 0.4 for the continuous lamellar phases. Similar changes are observed in shear modulus and in the limiting strain for linear viscoelastic behavior. The continuous lamellar phase is converted to droplets by shearing at rates above 1 s-1. The continuous lamellar structures will recover in about a week when the samples are allowed to relax. The nature of the droplets is highly dynamic. Confocal laser microscopy shows small fluctuations in droplet shape on a time scale of about 100 s. This time coincides with a characteristic time of around 100 s pertaining to a (shallow) peak in G‘‘.

Phase transitions in cationic and anionic surfactant mixtures

The phase behavior of cationic and anionic surfactant mixtures strongly depends on the molar ratio and actual concentration of surfactants. The phase diagram contains large areas of coexisting solid crystalline catanionic surfactant and liquid crystalline phases. Intermediate phases from pure solid crystalline catanionic surfactant to mixed micelles were examined by light microscopy under crossed polarizers and potentiometric and electrophoretic measurements. The mixed liquid crystalline phase is composed of both cationic and anionic surfactants. Its appearance as lamellar droplets may be explained by the increase in electrostatic repulsion at the bilayer surface.

Alkyl Chain Symmetry Effects in Mixed Cationic–Anionic Surfactant Systems

We study symmetry effects of hydrophobic tails in mixed oppositely charged surfactant systems. Equal hydrophobicity of the participating surfactants is obtained either (1) by equal hydrocarbon surfactants chain length or (2) by unequal surfactants chain length when one chain is perfluorinated. Three systems were studied: symmetric system of type (1) or (2), and an asymmetric system. Some unusual structural changes are observed in a hydrocarbon–fluorocarbon mixed single-chain oppositely charged surfactant system in the dilute region. Both the cationic hydrocarbon surfactant, dodecytrimethylammoniumchloride (DoTAC), and the anionic fluorocarbon surfactant, sodium perfluorononanoate (SPFN), are separately soluble in water, forming micellar solutions at high water concentration followed by the formation of liquid crystal phases at low water content. Upon mixing the two surfactants in an equimolar ratio at low total surfactant concentration (C< 5 wt%), a stable, dense, flow-birefringent phase is formed at the bottom of the test tube, and a clear phase is formed at the top. Cryo-transmission electron microscopy (cryo-TEM) micrographs of the dense phase show dispersed lamellar droplets having a very smalldspacing, and fairly monodispersed ∼100 nm single-walled vesicles in the supernatant. The latter is supported by NMR self-diffusion measurements. This structural behavior is studied at different molar ratios of the two surfactants. A micelle–vesicle transformation is indicated by NMR self-diffusion measurements. The same behavior is found for symmetry of type (1). The asymmetric system DoTAC–sodium nonanoate (SN) shows an isotropic phase in all molar ratios up to 40 wt% of surfactant, where both imaging and rheological measurements indicate the existence of elongated micelles in the equimolar solution. It was found that the micellar size distribution is coupled with the molar ratio only in the SN-rich part.

Relation between the Size of Lamellar Droplets in Onion Phases and Their Effective Surface Tension

A closed packed system (“onion phase”) of so-called lamellar droplets, consisting of concentric surfactant bilayers which are separated from one another by water, may be obtained by subjecting a continuous lamellar phase to shear. We propose a model for the size of these lamellar droplets under shear as a function of their effective surface tension. This model fits the available experimental data quantitatively and explains the dependence of the size on surfactant volume fraction.

Lyotropic Phases of Dodecylbenzenesulfonates with Different Counterions in Water

The lyotropic phase behavior of (technical grade) dodecylbenzenesulfonates (DoBS) is strongly influenced by the type of counterion and the relative amount of water. Phase diagrams are composed for the following systems: HDoBS/water, NaDoBS/water, (HDoBS + NaDoBS 1:1)/water, LiDoBS/water, KDoBS/water, CsDoBS/water, Ca(DoBS)2/water, NaDoBS/water/NaCl, and NaDoBS/water/CsCl. The phases were characterized by light microscopy, freeze-fracture electron microscopy, X-ray diffraction, and macroscopic appearance. The phase diagrams all contain large areas of lamellar phases. The appearance of the lamellar phases differs along this series, in particular regarding swelling behavior, either with or without a micellar phase next to the lamellar phase, and formation of large, rather irregular lamellar units versus smaller, perfectly spherical lamellar topologies (so-called lamellar droplets). LiDoBS shows, in addition, a hexagonal phase between 25 and 50 wt %. Explanations for the occurrence of the different phases are given in molecular terms and in terms of interactions between bilayers and between aggregates.

Dynamics of multi-lamellar vesicles

The dynamics of a lamellar droplet is studied. The inertial moments and friction coefficients are found as a function of the deformation mode. The relaxation time is smaller than the one of a corresponding single-layered vesicle. The relaxation frequency (10 s −1) is of the same order of magnitude as the shear rate at which onions are being formed when a continuous lamellar phase is being sheared at this rate.

Colloidal effects of anchored polymers in lamellar liquid-crystalline dispersions

The effects of polymers on the physical properties of lamellar liquid-crystalline dispersions, composed of an anionic surfactant, a poly(oxyethylated) nonionic surfactant, water and electrolyte have been investigated. The electrolyte concentration is so high that the solvent conditions for the headgroup of the nonionic surfactant are poor. Under these conditions and in the absence of polymer, an osmotic attraction takes place between, on the one hand, the lamellar layers inside the lamellar droplets and, on the other hand, between adjacent lamellar droplets. Consequently, thin lamellar water layers and flocculated lamellar droplets are obtained. This is a result of the similarity in structure between the layers inside one lamellar droplet and between the outer lamellae of two adjacent droplets. These coupled interaction forces can be decoupled by using polymers which (i) are anchored in the layers of the lamellar droplets, (ii) have hydrophilic parts which are well hydrated in the concentrated salt solution and (iii) are predominantly anchored in the outer layer of the lamellar droplet. This is achieved by using, e.g. poly(sodium acrylate-co-lauryl methacrylate), a hydrophilic polymer backbone with one or more hydrophobic side chains (the anchors for the lamellar bilayers). The sodium acrylate backbone is strongly hydrophilic and fulfils requirement (ii). The molecular mass of, and monomer ratio in, the copolymer are important in satisfying requirements (i) and (iii). Using polymers which fulfil all three requirements, so-called decoupling polymers, colloidally and physically stable lamellar dispersions can be obtained which are substantially concentrated in electrolyte and surfactants and which have a low viscosity. However, when polymers are used which are also able to anchor in the interior of the lamellar droplets or in more than one lamellar droplet, this may result in an increase in the volume fraction of the lamellar phase or connections between lamellar droplets by polymer bridges. In either case this may cause a strong increase in the viscosity and decreased pourability.

Deformability of lamellar droplets

The deformability has been calculated of a lamellar droplet, consisting of concentric surfactant bilayers, separated by water. The deformation energy of the droplet is expressed in terms of the bending modulus k of each bilayer and the bulk compressibility modulus, B, which is directly correlated to the interaction between the layers. The expression for the deformation energy has subsequently been used to derive an effective surface tension of the droplet. For ellipsoidal deformations, this surface tension is of the same order of magnitude as that found by De Gennes for a planar stack of bilayers. Even if the bilayers themselves do not exhibit surface tension, there still is an effective surface tension, φeff, due to the bulk properties of the lamellar phase, in the order of 0.01–1 mN/m.

The effects of free polymers on osmotic compression, depletion flocculation and fusion of lamellar liquid-crystalline droplets

The effects of free polymers on the physical properties of lamellar liquid-crystalline dispersions as model systems for active structured liquid detergents have been investigated. Addition of free polymers to lamellar dispersions may lead to osmotic compression (decrease in the volume fraction or lamellar phase), depletion flocculation and fusion of lamellar droplets. Osmotic compression will occur if the size of the polymer coil (e.g. molecular mass) is too large with respect to the lamellar water layer thickness. Depletion flocculation will occur, as found for normal dispersions, at lower polymer concentrations if the molecular mass of the polymer increases. Fusion of lamellar droplets, which results in an increase in lamellar droplet size, occurs at polymer concentrations lower than the polymer concentration at which depletion flocculation is observed. Extensive fusion is observed well above the critical polymer concentration for depletion flocculation. Osmotic compression and fusion result in a decrease in the viscosity of the lamellar dispersions and allow for concentration of the products based on lamellar dispersions while retaining pourability. Depletion flocculation results in physical instability and increased viscosity of the liquid detergents and has to be avoided.