We have prepared spontaneous, single-walled, equilibrium vesicles of controlled size and surface charge from aqueous mixtures of simple, commercially available, single-tailed cationic and anionic surfactants. We believe vesicle formation results from the production of anion-cation surfactant pairs which then act as double-tailed zwitterionic surfactants. Although unilamellar vesicles have been created by numerous physical and chemical techniques from multilamellar dispersions, all such vesicle systems revert to the equilibrium, multilamellar phase over time. These catanionic vesicles are stable for periods as long as several years and appear to be the equilibrium form of aggregation. Here we review the phase behavior and structural studies of several such mixtures, with particular focus on the effect of surfactant tail lengths on size and location of the vesicle phase in the appropriate phase diagram. The approach to equilibrium is also discussed.

While ideal fluid membranes are characterized solely by curvature elasticity, many structural (“external”) properties of real membranes are strongly influenced by “internal degrees of freedom”. In particular, recent experimental reports of stable vesicles in surfactant mixtures seem to illustrate the important interplay between composition and curvature in bilayer membranes. Such stable vesicles are unexpected in single-surfactant bilayers. We show theoretically how the energetic stabilization of mixed vesicles can occur in mixed-surfactant systems. This is illustrated by a microscopic model of mixed-surfactant bilayers, which is treated within both the random mixing and strongly interacting limits. The predictions of the ranges of stability of the various phases as a function of the three concentrations (solvent - e.g., water - and the two amphiphiles) is in qualitative agreement with recent experiments.

In addition, we consider the effect of phase separation in a mixed-surfactant system. In contrast with a bulk system, we find that for a binary mixture in a two dimensional fluid bilayer membrane, phase separation determines a length scale for the formation of stable vesicles. This results in a stable, one phase vesicle region near the critical composition, with a simple dependence of the vesicle size on composition. We also find regions of coexistence of vesicles with lamellae.

Large scale Monte-Carlo simulations have been performed on a lattice model for a three component system of water, oil, and surfactants to obtain the phase equilibria and scattering behavior for a wide range of temperatures and chemical potentials. We observed that this model has a rich phase behavior, namely a water-oil phase coexistence, a microemulsion phase, a lamellar phase, and a square phase. This phase diagram is consistent with experiments, and is in qualitative agreement with a model of Gompper and Schick [ Phys. Rev. Lett. 62, 1647 (1989)].

Recent fluorescence-microscopy experiments have observed two types of defects—star defects and striped textures-in the liquid—condensed phases of Langmuir monolayers. These defects can be understood through a continuum elastic theory. This theory predicts both splay stripes, which are driven by the asymmetry between molecular heads and tails, and bend stripes, which are driven by a spontaneous breaking of chiral symmetry in the monolayer. These predictions are compared with experimental observations.

Arrays of two-dimensional magnetic bubbles in thin garnet films undergo a hexatic-toliquid transition as a function of bubble density controlled by an applied spatially uniform dc bias magnetic field that opposes the magnetization in the bubbles. The phase transition is driven by topological point defects. The bubbles are observed directly using optical microscopy and digital imaging techniques. In the presence of a linear gradient in the dc bias magnetic field the hexatic-to-liquid transition occurs spatially in the direction of the gradient. As the system goes from hexatic to liquid, a continuous decrease in bubble density accompanied by a continuous disordering of the array is observed along the gradient direction. This continuous disordering persists even after the system is allowed to equilibrate for very long periods of time, indicating that the hexatic-to-liquid transition is continuous at equilibrium. Dynamics of topological defects observed in the gradient field correspond to those observed in the uniform field.

Interlayer diffusion in Langmuir-Blodgett multilayers containing Cadmium Arachidate (CdA), polyglutamate (PG) copolymers, or both, was investigated using neutron and x-ray reflectometry. Measurements from multilayers comprised only of CdA or PG revealed that little interdiffusion occurred upon annealing at 70°C or 84°C. However, some relaxation of the intralayer microstructure in the PG samples was observed at both temperatures. In contrast, the neutron reflectivity of a multilayer sandwich containing a bilayer of PG between two multilayers of CdA showed gradual and progressive change with annealing at both temperatures for annealing times of up to several hours. This change is surprising as PG is expected to act as a barrier to interdiffusion of the CdA molecules at these temperatures.

Membranes such as lipid bilayers are highly flexible surfaces which determine the architecture of biological systems and provide a basic structural element for the mesophases of complex fluids1. Two aspects of their conformational behavior will be considered. First, the morphology of vesicles and membranes is briefly reviewed. Then, recent theoretical work on adhesion (or cohesion) phenomena which involve whole bunches of membranes will be discussed.

We have elaborated a system -that we call a ferrosmectic- which contains magnetic particles included in a lamellar phase. Because of the presence of the confined particles, this phase exhibits specific features that are revealed by submitting it to a magnetic field as well as by neutrons or quasi-elastic light scattering experiments. We present here a review of the different properties of this ferrosmectic phase.

We studied the conformation of the membrane skeleton of human red blood cells (RBC) after detergent extraction of RBC ghosts, using video microscopy, light scattering, and synchrotronbased small angle X-ray scattering (SAXS). RBC membrane skeletons are two-dimensionally connected, triangulated networks of flexible, polyionic proteins. Immediately after extraction, the skeletons exhibited large-scale thermal undulations and deformed strongly in weak shear flow. Screening of electrostatic repulsion by immersion in high ionic strength buffer led to shrinkage, while the shell-like conformations and the flexibility of the skeletons were preserved. Under high ionic strength conditions (1 M monovalent salt), the static structure factor, S(q), showed two power law regimes S(q) ∝ q −α, with α <≈ 2.0 in the range of wave vectors 4×10−4 Å−1 < g < 8×10−4 Å−1, and α = 2.3 ± 0.1 in the range of wave vectors 8×10−4 Å−1 < q < l×10−1 Å−1. The same power law behavior was observed in low ionic strength buffer (25 mM salt) for q < 2×10−3 Å−1. This result is not consistent with the occurence of a crumpling transition during skeleton shrinkage. The observed form of the static structure factor, with a transition between two regimes with different power law exponents, presents evidence for the theoretically predicted flat phase of 2D-polymers.

Entropic elastic constants of lamellar tethered membrane phases are considered both in the vicinity of and at temperatures well below the membrane crumpling transition critical point. We calculate entropic forces acting on boundaries of these systems. Various predictions of the statistical physics of tethered membranes, such as the breakdown of the classical membrane elasticity theory at low temperatures or the existence of the crumpling transition, can be tested experimentally by using our results.

We present results of an x-ray scattering study on the rotator phases of normal alkanes: CH2-(CH3)n-CH2 (20 ≤ ≤ 33). We have characterized a new tilted rotator phase and measured the distortion and tilt order parameters in all 5 rotator phases. We have shown that there is no strong even-odd chain effect within the rotator phases.

The diacetylenic phosphocholine lipid 1,2-bis(10, 12-tricosadiyonol)-sn-glycerol- 3-phosphocholine forms, upon cooling below the chain melting temperature, hollow cylindrical structures known as ‘tubules’. These tubules are approximately one micron in diameter and may range from tens to hundreds of microns in length. We have carried out x-ray diffraction studies of tubules (1) in 75/25 ethanol/water by volume and (2) in a dessicated state. We present results which show marked differences in the nature of both the in-plane and inter-layer correlations associated with the wrapped multilayer structure of the tubules in the two states.

Transmitted polarized light microscopy of various natural silk secretions reveals their ability to form nematic liquid crystalline phases. Observations of microstructure, together with a simple secondary structure analysis of known amino acid sequences in silk proteins, suggest that the rodlike structures forming the nematic phase are supramolecular aggregates, rather than individual rigid molecular segments. The optical birefringence of dragline fiber produced by controlled silking depends on the linear haul-off velocity, and can exceed the birefringence of naturally spun fibers; this suggests the possibility of in-vitro spinning of silk to obtain values of strength and stiffness even greater than those achieved in vivo.

The open-ended aggregation of amphiphilic molecules in aqueous solution generates a broadly polydisperse population of elongated particles that form a variety of partially ordered phases. Herzfeld and coworkers have shown that the phase behavior of these binary systems is well described by self-consistently combining scaled-particle theory for the effects of excluded volume in fluid dimensions, a simple cell model for the effects of excluded volume in positionally ordered dimensions, a mean-field treatment of soft-interactions, and a phenomenological model of aggregate formation. We have now extended this model to ternary systems. We find that the addition of spherical particles to a solution of rod-forming particles induces a very wide isotropic-nematic coexistence region in which a relatively dilute isotropic solution with little aggregation separates from a rather concentrated nematic solution that almost completely excludes the spherical solutes. The magnitude of this effect depends on the relative diameters of the two solutes.

Time resolved X-ray diffraction from thermotropic liquid crystalline (LC) materials has been studied in conjunction with standard LC characterization techniques to investigate phase transitions and alignment due to applied electric fields. A more detailed understanding of the kinetics of the interaction between LC systems and applied fields is being sought. Both the effect of electric fields on the inter-molecular structure and the orientation parameter have been evaluated as a function of temperature for LC model compounds. These results are compared to observations of these mesogens in applied electric fields using optical microscopy. Materials of interest include low molar mass distyrylbenzene mesogens which exhibit both nematic and smectic mesophases. Results of investigations of thermotropic polymers will be discussed.

Predictions of static and dynamic properties from a unified molecular statistical mechanics theory for complex fluids in static electric fields are summarized. Included are thermodynamic, molecular ordering, and transport properties. In this new theory, the effects of an electric field Ee on the dipole moments and bond polarizabilities of tile molecules have been added to an eaxlier successful theory for complex organic molecules in condensed phases. Included are liquid crystalline (LC) and polymeric systems (including the first interlocking polymers).

Diffusion of model main-chain and side-chain liquid crystalline polymers was studied in the nematic phase. The concentration of deuterium labeled polymer vs. depth in a thick matrix layer was profiled with Forward Recoil Spectrometry to determine diffusion coefficients. In each system, the tracer diffusion coefficient was measured as a function of temperature, and correlated both with phase transitions determined by thermal analysis and with melt viscosity data.

Ordered structures of the multiblock copolyesters composed of p-quarterphenyl (DHQ, rigid segments) and poly(ethylene adipate) (flexible segments) were investigated by means of WAXD and SAXS. It was found that only the DHQ segments crystallize but the poly(ethylene adipate) segments hardly crystallize which make this copolyester a good thermoplastic elastomer. The copolyesters exhibit microphase separation above the melting temperature of the DHQ segments.

We present elastic light scattering measurements of a semidilute polymer solution under shear flow. The basic symmetries observed in the shear-enhanced structure factor agree with the predictions of recently proposed theoretical models. The enhancement appears to be due to a coupling between the polymer concentration and shear flow through the concentration-dependent viscosity and normal stress coefficients. We also found an unexpected long wavelength peak in the shear-enhanced structure factor. Transient scattering measurements upon the cessation of shear have revealed the origin of this peak. The polymer solution demonstrates two modes of relaxation for concentration fluctuations in equilibrium due to a coupling between collective diffusion and stress relaxation. The transient scattering measurements indicate that the mechanism that couples the concentration fluctuations to the shear flow, and thereby enhances concentration fluctuations, selectively enhances only the slower of the two equilibrium modes. This selective enhancement generates the long wavelength peak in the shear-enhanced structure.

The application of techniques in optical rheometry for the study of multicomponent systems is reviewed. Small angle light scattering (SALS) patterns are related to the structure of concentration fluctuations with length scales of the order of the wavelength of light. Scattering techniques such as SALS and scattering dichroism have been applied to monitor the transient evolution of anisotropic concentration fluctuation enhancement during simple shear induced phase separation in a semi-dilute solution of polystyrene (PS) in dioctyl phthalate(DOP). Furthermore, the Onuki- Doi theory relating scattering dichroism and structure factor has been used to verify the consistency between scattering dichroism and anisotropy in structure factor. Infrared polarimetry is a useful technique in probing the transient microstructural orientation of individual chemical species in multicomponent systems. The simultaneous measurement of intrinsic infrared dichroism and birefringence is particularly effective and has been employed to monitor component relaxation dynamics in miscible blends of poly(ethylene oxide) and poly(methyl methacrylate). Polarization Modulated Laser Raman Scattering (PMLRS) has been successfully employed to study the orientation dynamics of a polymer melt subjected to transient uniaxial extension. PMLRS provides quantitative information about the time evolution of both the second and fourth moments of the orientation distribution function of molecular segments.