Rheological behaviour of a lamellar liquid crystalline surfactant–water system

Bijel rheology reveals a 2D colloidal glass wrapped in 3D

We present rheological evidence demonstrating the glass-like nature of bicontinuous interfacially jammed emulsion gels (bijels). Under small amplitude oscillatory shear, bijels exhibited rheological signatures akin to α and β relaxation that are also invariable to interfacial tension changes, behaviors which are reminiscent of caged particle dynamics found in colloidal glasses, and well described by a previously reported adaptation of mode-coupling theory for colloidal glass rheology. Guided by their rheological signatures and supported by particle detachment and attraction energy approximations, we rationalize that bijels can be represented as 2-dimensional (2D) colloidal glasses that percolate in 3-dimensional (3D) space, and attractive interactions are not required for their stability. To provide further support for this conjecture, we qualitatively compare the rheology of bijels and a capillary suspension that is stabilized by strong, rigid capillary bridges between the particles, beyond their limit of linear viscoelasticity. Our results demonstrate that the strong adsorption of particles to the continuous interface and the lack of strong attractive interparticle forces enable recovery by interfacial tension into new jammed configurations after shear deformation. These behaviors are qualitatively different from those in the capillary suspension, where the breaking of attractive interparticle bonds results in dramatic changes to the microstructure and rheology over a narrow range of shear amplitudes. Our findings unveil bijels as 2D colloidal glasses weaving in 3D space and establish that interparticle attractions are not required for stability in bijels, and interfacial jamming alone is sufficient to impart viscoelasticity and gel-like rheology to these materials.

The preparation of hydroxyapatite nanowires and nanorods via aliphatic micelles as soft templates

Hydroxyapatite nanoparticles were tunably synthesized via the use of an aliphatic–ethanol–water three-phase mixture system using micelles as soft templates via an emulsion–hydrothermal synergistic method.

Feasibility of a Natural Surfactant as a Stabilizer for Cosmetics with Liposome-Encapsulated Plant Stem Cells: Pre-Formulation and Formulation Through Stability Studies

During the formulation of liposome-containing products different problems can occur and the selection of a suitable carrier remains the greatest challenge. To estimate feasibility of a novel alkyl polyglucoside surfactant (hydroxystearyl alcohol and hydroxystearyl glucoside) as an emulsifier for cosmetics with liposome-encapsulated plant stem cells, we performed a two-phase study. In the first, the pre-formulation phase, the emulsifier's critical micelle concentration (CMC) and liposome-encapsulated active-emulsifier interactions were determined. The second phase was carried out to develop and characterize a cosmetic emulsion suitable to serve as a carrier for liposomes. The investigated emulsifier, with the obtained CMC value of 0.0085 wt.%, could be classified as liposome-friendly and can be used to develop stable and aesthetically acceptable cosmetics or even prospective pharmaceutical liposome-containing emulsions.

The Effect of Electroporation of a Lyotroic Liquid Crystal Genistein-Based Formulation in the Recovery of Murine Melanoma Lesions

A lamellar lyotropic liquid crystal genistein-based formulation (LLC-Gen) was prepared in order to increase the aqueous solubility of the lipophilic phytocompound genistein. The formulation was applied locally, in a murine model of melanoma, with or without electroporation. The results demonstrated that, when the formulation was applied by electroporation, the tumors appeared later. During the 21 days of the experiment, the LLC-Gen formulation decreased the tumor volume, the amount of melanin and the degree of erythema, but when electroporation was applied, all these parameters indicated a better prognosis even (lower tumor volume, amount of melanin and degree of erythema). Although hematoxylin-eosin (HE) staining confirmed the above events, application of the LLC-Gen formulation by electroporation did not lead to a significant effect in terms of the serum concentrations of the protein S100B and serum neuron specific enolase (NSE), or the tissue expression of the platelet-derived growth factor receptor β (PDGFRβ) antibody.

Formation of Multilamellar Vesicles in the Systems of Decyltrimethylammonium hydroxides/2-hydroxy 1-carboxy-naphthoate and Octyltrimethylammonium hydroxides/2-hydroxy 1-carboxy-naphthoate

We have investigated the phase behavior and characterized the microstructure of the phases that result from mixing of alkyltrimethylammonium hydroxides CxTAOH (x = 10.8) with 2-hydroxy 1-carboxy-naphthoate. We compare the results of this work with the published one of alkyltrimethylammonium 2-hydroxy-1-carboxy-naphthoate (CxTA 2,1 HCN, x = 16.14 or 12) [2]. With increasing counter-ion surfactant ratio r we observed transition from the isotropic L1-phase to multilamellar vesicles (MLV) Lα-phase. The single phases are separated by a two phase L1/Lα region. The phases were characterized with FF-TEM, rheological and SANS measurements. For 100 mM solution of either C8TAOH/2,1 HNC or C10TAOH/2,1 HNC, both MLV phases behave like a weak gel and have a yield stress value. It is thus shown that it is possible to prepare viscoelastic surfactant solutions with a small yield stress value from single chain surfactants with decyl or octylchain.

Phase behavior and rheological analysis of reverse liquid crystals and W/I2 and W/H2 gel emulsions using an amphiphilic block copolymer

This article reports the phase behavior determi-nation of a system forming reverse liquid crystals and the formation of novel disperse systems in the two-phase region. The studied system is formed by water, cyclohexane, and Pluronic L-121, an amphiphilic block copolymer considered of special interest due to its aggregation and structural properties. This system forms reverse cubic (I2) and reverse hexagonal (H2) phases at high polymer concentrations. These reverse phases are of particular interest since in the two-phase region, stable high internal phase reverse emulsions can be formed. The characterization of the I2 and H2 phases and of the derived gel emulsions was performed with small-angle X-ray scattering (SAXS) and rheometry, and the influence of temperature and water content was studied. The H2 phase experimented a thermal transition to an I2 phase when temperature was increased, which presented an Fd3m structure. All samples showed a strong shear thinning behavior from low shear rates. The elastic modulus (G') in the I2 phase was around 1 order of magnitude higher than in the H2 phase. G' was predominantly higher than the viscous modulus (G''). In the gel emulsions, G' was nearly frequency-independent, indicating their gel type nature. Contrarily to water-in-oil (W/O) normal emulsions, in W/I2 and W/H2 gel emulsions, G', the complex viscosity (|η*|), and the yield stress (τ0) decreased with increasing water content, since the highly viscous microstructure of the continuous phase was responsible for the high viscosity and elastic behavior of the emulsions, instead of the volume fraction of dispersed phase and droplet size. A rheological analysis, in which the cooperative flow theory, the soft glass rheology model, and the slip plane model were analyzed and compared, was performed to obtain one single model that could describe the non-Maxwellian behavior of both reverse phases and highly concentrated emulsions and to characterize their microstructure with the rheological properties.

Shear-induced defect formation in a nonionic lamellar phase

(2)H NMR experiments on a nonionic oriented lamellar phase demonstrate that shear flow induces structural defects in the lamellar structure. These substantial structural changes give rise to a transition from a viscous to a solidlike behavior; the elastic modulus of presheared samples was found to increase, reversibly, with the applied preshear rate. A similar behavior was found when step-cycling the temperature toward the layer-to-multilamellar-vesicle transition and back at constant shear rate. However, while shear rate controls the defect density, the temperature is found to control the defect rigidity. The lamellar phase exhibits a shear-thinning behavior under steady shear conditions, following the power law eta approximately gamma(n), with n approximately -0.4. Both the shear thinning and the elastic behavior are in agreement with the available theoretical models. The observed shear-induced structural defects are reversible and can be regarded as a pretransition prior to the shear-induced formation of multilamellar vesicles.

The influence of hydrophobic counterions on micellar growth of ionic surfactants

This review covers the effects of hydrophobic counterions on the phase behavior of ionic surfactants and the properties of the phases. Mixing hydrophobic counterions with ionic surfactant micellar solutions may initiate the micellar growth and transform the micellar microstructure into different morphologies. This behavior may also be achieved by mixing ionic surfactants with hydrophilic counterions, although higher counterionic concentrations are then required. First, the role of hydrophilic and hydrophobic counterions in regards to micelle growth is discussed. Second, the effect of the hydrophobic counterion on the self-assembly of cationic and anionic surfactants and their viscoelastic behavior are presented. Third, the relationships between geometry, hydrophobicity and their consequences on micellar growth for different hydrophobic counterions are reviewed. Forth, the influence of hydrophobic counterion substituents (substitution pattern) on the phase behavior is discussed. Some results we previously obtained for different isomers of hydroxy naphthaoic acids and the cationic surfactant cetyltrimethylammonium hydroxide are included. With these systems the effect that the hydrophobic counterion microenvironment has on the phase behavior, rheological behavior and the micellar microstructure is discussed. The results from other research groups are also discussed.

Shear-induced topology changes in liquid crystals of the soybean lecithin/DDAB/water system

The viscoelastic behavior of the two different liquid crystalline lamellar phases and the liquid crystalline cubic phase of the mixed soybean lecithin/DDAB system in water was studied through rheology, with mechanical parameters studied as a function of composition. The swollen or diluted lamellar region is formed by vesicles, and its characteristic flow curve presents two-power law regions separated by a region where viscosity passes through a maximum. Yield stress and shear-dependent flow behavior were also observed. The microstructure suffers transformation under shear stress, and rheological response shifts from thixotropic to antithixotropic loops. Similar rheological behavior has been observed for samples in the collapsed or concentrated lamellar region, at the water-rich corner of the phase diagram. Vesicle formation may therefore occur by shearing the initial stacked and open bilayers. However, concentrated lamellar samples in the water-poor part of the phase diagram are less sensitive to shear effects and show plastic behavior and thixotropy. All lamellar samples manifest high elasticity. The dynamic responses of both lamellar topologies, i.e., vesicles and open bilayers, are comparable and exhibit an infinite relation time. The bicontinuous cubic, liquid crystalline phase is highly viscous. Its dynamic response cannot be modeled by a Maxwell model.

Novel viscoelastic systems from cationic surfactants and hydrophobic counter-ions: Influence of surfactant chain length

In continuation of our previous investigations on the aqueous phase behavior of cetyltrimethylammonium 2-hydroxy-1-carboxy-naphthoate (CTA-2,1-HCN) (see J. Colloid Interface Sci. 288 (2005) 570), we have studied the phase behavior and the properties of the phases of the two shorter homologues, C(14)TA-2,1-HCN and C(12)TA-2,1-HCN. The phases were prepared from the alkyltrimethylammonium hydroxides RTAOH and the naphtholcarboxylic acid. By preparing the systems in this way the surfactant solutions contain no excess salt. With increasing counter-ion-surfactant ratio r we observed the same sequence of phases as for the previously studied C(16)-system, namely a L(1)-phase and a L(alpha)-phase with multilamellar vesicles (MLV). The single phases are separated by a two-phase L(1)/L(alpha) region. The phases were characterized with FF-TEM, rheological and SANS measurements. For both systems the viscosity of the L(1)-phases passes with increasing counter-ion/surfactant ratio over a maximum. The height of the maximum decreases strongly with decreasing chain length while the complex viscosity of the MLV-phase depends little on the chain length. For 100 mM solution both MLV phases behave like a weak gel and have a yield stress value. It is thus shown that it is possible to prepare viscoelastic surfactant solutions with a yield stress value from single chain surfactants with a dodecyl chain.

Viscoelasticity of a nonionic lamellar phase

The linear viscoelastic properties of a nonionic lamellar phase in C-orientation were studied as a function of temperature by small-amplitude oscillatory measurements in the frequency range 0.5-5 Hz. An almost solid-like elastic response was observed at all studied temperatures, from 42 to 20 degrees C. In this range, the elastic modulus was found to increase strongly with decreasing temperature. The elasticity is attributed to screw dislocations connecting layers in the stack, and the data thus suggest that the density of screw dislocations decreases with increasing temperature. The lamellar phase forms an "onion" texture when continuously sheared at lower temperatures. It is argued that a possible origin for the shear-induced "onion" texture is the instability of the screw dislocations in shear flow. By 2H NMR experimentation, we also find the formation of a random mesh phase at lower temperatures. The presence of equilibrium bilayer perforations, however, does not correlate with the "onion" stability.

An alkylpolyglucoside surfactant as a prospective pharmaceutical excipient for topical formulations: The influence of oil polarity on the colloidal structure and hydrocortisone in vitro/in vivo permeation

There is a growing need for research into new skin- and environment-friendly surfactants. This paper focuses on a natural surfactant of an alkylpolyglucoside type, which can form both thermotropic and lyotropic liquid-crystalline phases. The aim of this study was to relate some physicochemical properties (characterised by polarisation and transmission electron microscopy, thermal analysis and rheology) of the three formulations based on cetearyl glucoside and cetearyl alcohol, to the results of in vitro and in vivo bioavailability of hydrocortisone (HC). The three formulations contained oils of different polarity (medium chain triglycerides: MG, isopropyl myristate: IPM and light liquid paraffin: LP), respectively. In vitro permeation was followed through the artificial skin constructs (ASC), while the parameters measured in vivo were erythema index: EI (using instrumental human skin blanching assay), transepidermal water loss (TEWL) and stratum corneum hydration (SCH). The vehicles based on cetearyl glucoside and cetearyl alcohol showed a complex colloidal structure of lamellar liquid-crystalline and lamellar gel-crystalline type, depending on oil polarity. Rheological profile of the vehicle was directly related to the in vitro profile of the HC permeation. In vivo results suggested that the vehicle with MG retarded the HC permeation, whereas less polar IPM and non-polar LP enhanced it. It is suggested that the enhancement is achieved either by a direct interaction with lipid lamellae of the SC or indirectly by improving skin hydration. There were no adverse effects during in vivo study, which indicates a good safety profile of this alkylpolyglucoside surfactant.

A novel viscoelastic system from a cationic surfactant and a hydrophobic counterion

The phase behavior of 2-hydroxy-1-naphthoic acid (2,1-HNC) mixed with cetyltrimethylammonium hydroxide (CTAOH) is reported. This novel system is compared with the published one of 3-hydroxy-2-naphthoic acid (3,2-HNC) mixed with CTAOH. We investigated the phase behavior and properties of the phases in aqueous solutions of 100 mM CTAOH with 2,1-HNC. In both systems a multilamellar vesicle phase is formed when the naphthoate/surfactant ratio (r) reaches unity. When an increasing amount of 2,1-HNC is mixed with a micellar solution of 100 mM CTAOH, an isotropic low-viscous micellar solution, a viscoelastic gel (consisting of rodlike micelles), a turbid region (two-phase region), and a viscoelastic liquid crystalline gel (consisting of multilamellar vesicles, MLV) were formed. The vesicular phase is highly viscoelastic and has a yield stress value. The transition from the micellar to the vesicle phase occurs for CTAOH/2,1-HNC over a two-phase region, where micelles and vesicles coexist. Also it was noticed that 2,1-HNC is dissolved in 100 mM CTAOH until the naphthoate/surfactant ratio reaches approximately 1.5, and the liquid crystalline phases were found to change their color systematically when they were viewed between two crossed polarizers. The vesicles have been characterized by differential interference contrast microscopy, freeze-fracture electron microscopy, and cryo-electron microscopy (cryo-TEM). The vesicles were polydisperse and their diameter ranged from 100 to 1000 nm. The interlamellar spacing between the bilayers was determined with small angle neutron scattering and agrees with the results from different microscopical methods. The complex viscosity rises by six orders of magnitude when rodlike micelles are formed. The complex viscosity decreases again in the turbid region, and then rises approximately six orders of magnitude above the water viscosity. This second rising is due to the formation of the liquid crystalline MLV phase.

In situ shear investigation of the Synperonic A7-water system by small-angle X-ray scattering and freeze fracture

A shear cell was constructed in two variants for simultaneous small-angle X-ray scattering and freeze fracture. Using this cell, the changes in the layer structure and the domain formation of the Synperonic A7-water system were investigated under shear. The reconstructions of both the layer and domain structures were not observed in the steady state after 1 h (following a 2 h long shear period). Destruction of the lamellar arrangement and formation of aggregates occurred during the stress. It was concluded that the tixotropic behavior originates rather from the change of the domain structure with a typical size range of microm than from the changes of the inner structure of domains with lamellar arrangement.

Structure and drug release of lamellar liquid crystals containing glycerol

Lamellar lyotropic liquid crystalline (LLC) systems are thermodynamically stable, optically isotropic systems, which are formed with low energy input. New possibilities for the development of controlled drug delivery systems are inherent in these systems due to their stability and special skin-similarly structure. The present aim was to formulate multicomponent LLC systems with a relatively low surfactant content, composed of materials official in the European Pharmacopoeia 4th. Polarizing light microscopic examination of the samples was carried out, together with TEM observation of replicas produced by freeze-fractured technique for the purpose of demonstrating the presence of lamellar LC domains. Our LLC samples contained: Brij 96 (poly-oxyethylene-10-oleyl ether) with water, liquid petrolatum (LP) and glycerol in a given concentration range. The interlamellar repeated distance (d(L)) confirming the existence of a regular structure was determined by means of X-ray diffraction. The d(L) and G'values of the samples changed according to a maximum curve with increasing glycerol concentration up to 40% (w/w). A prolonged drug release was observed in case of the very water-soluble ephedrine hydrochloride and the same phenomena was observed in the case of tenoxicam, which is practically insoluble in water.