Arwen I. I. Tyler

A 3-D Hexagonal Inverse Micellar Lyotropic Phase

Lipids that are found in cell membranes form a variety of self-assembled phases in the presence of water. Many of these structures are liquid-crystalline with structural motifs mirrored in cells and organelles and can be exploited in the delivery of drugs and genes. We report the discovery of a lyotropic liquid crystalline phase based on a 3-D hexagonal close-packed arrangement of inverse micelles, of space group P6(3)/mmc. This is the first new inverse lyotropic liquid-crystalline phase to be reported for two decades and is the only known lyotropic phase whose structure consists of a close packing of identical inverse micelles.

Ordered micellar and inverse micellar lyotropic phases

In this article we review the ordered micellar and inverse micellar lyotropic liquid-crystalline phases that can be formed by amphiphilic molecules such as lipids and surfactants. We focus first on the self-assembly of amphiphiles into aggregates, and then consider the interfacial curvature and the role of curvature elasticity and packing constraints in determining the allowed structures. We then review the range of ordered micellar and inverse micellar phases that have so far been observed in a variety of surfactant and lipid systems. Finally, we describe certain characteristic properties, such as the epitaxy between phases, and the self-diffusion and electrical conductivity within such ordered micellar phases.

Self-Assembly and Applications of Ultraconcentrated Nanoparticle Solutions

We demonstrate a highly efficient method for concentrating, purifying and separating gold nanoparticles. The method relies on localized density gradients that can be formed at an aqueous | organic phase interface. We show that this method is able to concentrate aqueous gold nanoparticles to the point where confinement leads to variable interparticle separations. Furthermore, the physical properties of the resulting solution are drastically altered when compared to water. For example, densities higher than 4.5 g/cm(3) could be generated without nanoparticle aggregation. As far as we are aware, this is one of the highest reported densities of an aqueous solution at room temperature. Finally, the compositions of the solutions generated are highly dependent on parameters such as particle size and background analyte making this technique highly advantageous for the separation of multimodal NP populations and chemical purification, with 99.5% and >99.9% efficiency, respectively.

Solid State NMR of Lipid Model Membranes

In this chapter we describe the use of solid state nuclear magnetic spectroscopy to study the structure of lyotropic phases and lipid model membranes and show its ability to probe, site specifically, at a sub-Ångstrom resolution. Here, we demonstrate the immense versatility of the technique and its ability to provide information on the different liquid crystalline phases present. A multinuclear for example (31)P, (1)H, and (13)C approach is able to elucidate both the structure and dynamics over a wide variety of timescales. This coupled with a non-perturbing label (2)H is able to provide information such as the order parameters for a wide variety of different liquid phases.

X-Ray Diffraction of Lipid Model Membranes

In this chapter the use of X-ray diffraction to study the structure of lyotropic phases and lipid model membranes is described. Determination of the phase symmetry and lattice parameters from small-angle X-ray scattering (SAXS), and of the nature of the hydrocarbon chain packing from wide-angle X-ray scattering (WAXS), are discussed. Methods by which the sign of the interfacial curvature of non-lamellar phases may be determined are then presented. Finally, the calculation of electron density profiles from the intensities of the observed Bragg peaks is described, for the lamellar phase and for the inverse hexagonal phase.

Mechanical Characterization of Ultralow Interfacial Tension Oil-in- Water Droplets by Thermal Capillary Wave Analysis in a Microfluidic Device

Measurements of the ultralow interfacial tension and surfactant film bending rigidity for micron-sized heptane droplets in bis(2-ethylhexyl) sodium sulfosuccinate-NaCl aqueous solutions were performed in a microfluidic device through the analysis of thermally driven droplet interface fluctuations. The Fourier spectrum of the stochastic droplet interface displacement was measured through bright-field video microscopy and a contour analysis technique. The droplet interfacial tension, together with the surfactant film bending rigidity, was obtained by fitting the experimental results to the prediction of a capillary wave model. Compared to existing methods for ultralow interfacial tension measurements, this contactless, nondestructive, all-optical approach has several advantages, such as fast measurement, easy implementation, cost-effectiveness, reduced amount of liquids, and integration into lab-on-a-chip devices.

Spontaneous charged lipid transfer between lipid vesicles

An assay to study the spontaneous charged lipid transfer between lipid vesicles is described. A donor/acceptor vesicle system is employed, where neutrally charged acceptor vesicles are fluorescently labelled with the electrostatic membrane probe Fluoresceinphosphatidylethanolamine (FPE). Upon addition of charged donor vesicles, transfer of negatively charged lipid occurs, resulting in a fluorescently detectable change in the membrane potential of the acceptor vesicles. Using this approach we have studied the transfer properties of a range of lipids, varying both the headgroup and the chain length. At the low vesicle concentrations chosen, the transfer follows a first-order process where lipid monomers are transferred presumably through the aqueous solution phase from donor to acceptor vesicle. The rate of transfer decreases with increasing chain length which is consistent with energy models previously reported for lipid monomer vesicle interactions. Our assay improves on existing methods allowing the study of a range of unmodified lipids, continuous monitoring of transfer and simplified experimental procedures.