Shear-induced ordering of lamellar and gyroid structures observed in a nonionic surfactant/water system

The shear-induced ordering of lamellar and gyroid structures of a nonionic surfactant C₁₆E₇/D₂O system in a Couette shear cell ( 0.001 < < 10 s^-1, : shear rate) has been investigated by using a small angle neutron scattering technique. In the lamellar phase, the steady shear flow having > 0.01 s^-1 suppresses undulation fluctuations of lamellae (Maxwell effect). This suppression of fluctuations brings two effects; 1) shear-induced lamellae ordering toward a parallel orientation and 2) obstruction of a lamellar↦gyroid transition. It is quite interesting to note that there is a characteristic shear rate range ( 0.01 < < 0.3 s^-1), where both effects take place. We have also investigated the shear effects on the gyroid phase. Below the characteristic shear rate range, the gyroid structure keeps three-dimensional network lattice, while above the characteristic shear rate range, the gyroid structure transforms to the parallel orientation lamellae (shear-induced gyroid-lamellar transition). Thus the shear flow having the characteristic shear rate plays very important roles in shear ordering phenomena. Received 26 June 2000 and Received in final form 12 January 2001     

Coupling between flow and structure for a lamellar surfactant phase

The flow-structure relation of lamellar phases is studied using rheometry and cross-polarized microscopy under flow. The equilibrium phases show different defects. Low salinities lead to very viscous, "onion" phases, whereas at high salinity, a low viscosity plane lamellar phase is found. Under shear, the latter shows a sudden transition to a viscoelastic gel, with a texture and viscosity very similar to that of the onions. Gelation occurs after a certain delay time, increasing rapidly with salinity, by the nucleation of onions. This allows one to relate the delay time to the defect energy.     

Insertion of small anisotropic clay particles in swollen lamellar or sponge phases of nonionic surfactant

we have established experimentally the region of existence of smectic and sponge lyotropic microstructures in the presence of synthetic hydrophilic clay particles. Taking into account the adsorption isotherm, we determined the extension of the single-phase regions by visual inspection and looking at low-q behaviour of the small-angle neutron scattering. Conditions of stability such as geometric constraints associated with fluctuation quenching are discussed. Received 11 December 2000 and Received in final form 11 April 2001     

Diffusion coefficients in a lamellar lyotropic phase: evidence for defects connecting the surfactant structure

We measure diffusion coefficients in the lamellar phase of the nonionic binary system C(12)EO(6)/H(2)O using fluorescence recovery after photobleaching. The diffusion coefficient across the lamellae shows an abrupt increase upon approaching the lamellar-isotropic phase transition. We interpret this feature in terms of defects connecting the surfactant structure. An estimation of the defect density and of the variation in defect energy close to the transition is given in terms of a simple model.     

Softening of the interactions between surfactant bilayers in a lamellar phase due to the presence of a polymer

The compressibility modulus of a lamellar phase containing a neutral polymer guest molecule was measured directly using a surface force apparatus. The system studied consisted of sodium dodecyl sulphate (SDS), pentanol, water and polyethylene glycol (PEG) . The lamellar phase was induced from a micellar phase in situ via a confinement induced isotropic to lamellar phase transition. This avoided problems resulting from the viscosity and turbidity normally characteristic of these lamellar phase samples. Increasing the amount of PEG resulted in a marked decrease in the layer compressibility modulus indicating a decrease in the repulsive forces between the lamellae. The origin of such a phenomenon is discussed in terms of different mechanisms including depletion interactions, bridging interactions and modification of the electrostatic interaction between the lamellae by the polymer. Received 2 February 1998     

Interbilayer repulsion in nonionic surfactant-water lamellar phases.

We report a simulation study using the RIB modification of the Gibbs' ensemble, for a model membrane system in which the surfaces have embedded dipoles, and are separated by a fluid of discrete TIPS2 water molecules. The separation between the layers is allowed to vary, as is the number of water molecules in the membrane which is in equilibrium with a vapour phase, the whole system being kept at constant (N, V, T). The simulations were performed at 298 K and two systems were studied: in the first, the surface dipoles were modelled as OH groups attached to hydrocarbon chains; in the second, the charge on the O and H atoms was arbitrarily doubled to 0·734 e. The structure of the water around the surface dipoles was examined by collecting density and orientational distributions for different regions in the plane of the surface and for molecularly thick layers of water. Although local water structure is modified by the surface charges, this did not result in a stable bilayer for either of the two dipole strengths. The study has relevance to the stability of nonionic surfactants and phospholipids, and suggests that a factor additional to surface charge is needed to explain the stable structures observed experimentally.     

The lamellar and sponge phases of dilute surfactant systems: Structures and defects at equilibrium and under shear

We report on the physical properties of swollen solutions of the amphiphilic molecules of cetylpyridinium chloride and hexanol in brine. A remarkable characteristic of this system is the existence of a crossover between dilute and less dilute solutions, in the lamellar phase and the sponge phase, with some interesting consequences for the theory of membranes.     

Shear-induced collapse in a lyotropic lamellar phase

An entropically stabilized cetylpyridinium chloride, hexanol, and heavy brine lyotropic lamellar phase subjected to shear flow has been observed here by small angle neutron scattering to undergo collapse of smectic order above a threshold shear rate. The results are compared with theories predicting that such a lamellar phase sheared above a critical rate should lose its stability by a loss of resistance to compression due to the suppression of membrane fluctuations.     

Swelling kinetics of a compressed lamellar phase

We investigate how multilamellar vesicles prepared in a compressed state under flow return to equilibrium. The kinetics is studied by following the temporal evolution of the viscoelasticity after the shear is stopped. It exhibits a two-step relaxation whose slower stage is strongly affected by temperature. According to a simple model, the temperature-dependent permeability of the lamellar phase is deduced from the measurements. We propose to attribute the permeability to handle-like defects, and its temperature dependence to an increase of the defect density when the lamellar-to-sponge phase transition is approached. Received 25 May 2000 and Received in final form 4 August 2000     

Optical manipulation of defects in a lyotropic lamellar phase

Here we study the line defect in a hyperswollen lamellar phase of lyotropic liquid crystal by applying a laser trapping method. We have succeeded in directly measuring the tension of a single isolated line defect and the adhesion energy between two defects. We demonstrate a new possibility of intentional patterning of various defects by direct optical manipulation. Furthermore, local rheological measurements provide information on the membrane organization around a particle and also evidence suggesting that flow in a lamellar phase has a two-dimensional nature.     

SANS structural determination of a nonionic surfactant layer adsorbed on clay particles

We study adsorption of two nonionic surfactants (C₁₂E₅ and C₁₂E₈) on a dispersed suspension of negatively charged laponite particles. First, we quantify adsorption, i.e. the amount of adsorbed molecules per gram of dried solid. Then, we show that contrast variation experiments under controlled conditions along the adsorption isotherm of the surfactant on dispersed laponite particles allow to determine the average thickness of a nonionic surfactant layer adsorbed on a solid anisotropic particle. Received 4 August 1998     

Twist elasticity and anchoring in a lamellar nematic phase

Electro-optic measurements were performed on a lamellar nematic phase in which the mesogenic moieties lie in lamellae that are separated by partially perfluorinated side groups. The twist elastic constant K22, viscosity gamma(1), and the quadratic and quartic anchoring strength coefficients are reported. K22 and gamma(1) are found to be considerably smaller than that of typical three-dimensional nematics. The small K22 is due to the greatly weakened interactions between the spatially separated lamellae.     

Kinetics of layer hopping in a diblock copolymer lamellar phase

In the ordered state, symmetric diblock copolymers self-assemble into an anisotropic lamellar morphology. The equilibrium thickness of the lamellae is the result of a delicate balance between enthalpic and entropic energies, which can be tuned by controlling the temperature. Here we devise a simple yet powerful method of detecting tiny changes in the lamellar thickness using optical microscopy. From such measurements we characterize the enthalpic interaction as well as the kinetics of molecules as they hop from one layer to the next in order to adjust the lamellar thickness in response to a temperature jump. The resolution of the measurements facilitate a direct comparison to predictions from self-consistent field theory.     

Structure factor of a lamellar smectic phase with inclusions

Motivated by numerous X-ray scattering studies of lamellar phases with membrane proteins, amphiphilic peptides, polymers, or other inclusions, we have determined the modifications of the classical Caillé law for a smectic phase as a function of the nature and concentration of inclusions added to it. Besides a fundamental interest on the behavior of fluctuating systems with inclusions, a precise characterization of the action of a given protein on a lipid membrane (anchoring, swelling, stiffening ...) is of direct biological interest and could be probed by way of X-ray measurements. As a first step we consider three different couplings involving local pinching (or swelling), stiffening or curvature of the membrane. In the first two cases we predict that independent inclusions induce a simple renormalization of the bending and compression moduli of the smectic phase. The X-ray experiments may also be used to probe correlations between inclusions. Finally we show that asymmetric coupling (such as a local curvature of the membrane) results in a modification of the usual Caillé law. Received 10 March 2000 and Received in final form 30 August 2000