Understanding foods as soft materials

Fibrillation of β-lactoglobulin at low pH in the presence of a complexing anionic polysaccharide

The influence of electrostatic complexation with κ-carrageenan was tested on the fibrillation process of β-lactoglobulin at pH 2.0. Morphology and structural development were monitored through cross correlation dynamic light scattering, transmission electron microscopy, and atomic force microscopy. Scattering indicated that noncomplexed β-lactoglobulin monomers aggregated to form fibrils after 15-90 min of heating at 90 °C. However, electrostatic protein-carrageenan complexes found in the unheated system were unchanged by the thermal process. Images and scattering results showed that carrageenan complexes slowed fibrillation kinetics, possibly through reduction in available monomer concentration. Complexes adhered to fibrils at ends and junctions in TEM images, indicating interactive affinity with the fibers, presumably as heterogeneous nucleation sites.

Nonlinear rheology of colloidal dispersions

Colloidal dispersions are commonly encountered in everyday life and represent an important class of complex fluid. Of particular significance for many commercial products and industrial processes is the ability to control and manipulate the macroscopic flow response of a dispersion by tuning the microscopic interactions between the constituents. An important step towards attaining this goal is the development of robust theoretical methods for predicting from first-principles the rheology and nonequilibrium microstructure of well defined model systems subject to external flow. In this review we give an overview of some promising theoretical approaches and the phenomena they seek to describe, focusing, for simplicity, on systems for which the colloidal particles interact via strongly repulsive, spherically symmetric interactions. In presenting the various theories, we will consider first low volume fraction systems, for which a number of exact results may be derived, before moving on to consider the intermediate and high volume fraction states which present both the most interesting physics and the most demanding technical challenges. In the high volume fraction regime particular emphasis will be given to the rheology of dynamically arrested states.

Free fatty acids electronically bridge the self-assembly of a three-component nanocomplex consisting of amylose, protein, and free fatty acids

The self-assembly of a ternary complex, which is formed through heating and cooling of a mixture of amylose (1.0 mg/mL), whey protein isolate (50 μg/mL), and free fatty acids (FFAs, 250 μg/mL) was investigated. High-performance size-exclusion chromatography-multi-angle laser light scattering (HPSEC-MALLS) analysis showed that the complex is a water-soluble supramolecule (Mw = 6-7 × 10(6)), with a radius of gyration of 20-100 nm, indicating a nanoscale complex. Experimental results using 1-monostearyl-rac-glycerol (MSG) or cetyl alcohol that is similar to FFA in structure (except the headgroup) indicate that FFAs are the bridge between thermodynamically incompatible amylose and protein molecules and their functional carboxyl group is essential to the formation of the complex. Additionally, the effects of pH and salt treatments suggest that electrostatic interactions between negatively charged carboxyl groups of FFAs and polyionic protein are the foundation for the self-assembly of the complex. The fact that FFA is one important component in the self-assembled complex with an estimated molar ratio of 6:1:192 (amylose/protein/FFA, ∼4-5% FFA) demonstrates that it might be used as a nanocarrier for the controlled release of lipophilic functional materials to maintain their stability, bioactivity, and more importantly water solubility.

Material transfer in cubosome-emulsion mixtures: effect of alkane chain length

We present here results on the transfer kinetics of monoglyceride and n-alkanes in water. Transfer kinetics between cubosomes and emulsion droplets were followed using time-resolved small-angle X-ray scattering measurements, while dynamic light scattering was used to study the changes in the particle radii. The effect of the initial size of cubosomes and emulsion droplets on the final droplet size of the mixed components was investigated. Decane was transferred into the cubosomes with the disappearance of all emulsion droplets, with no detectable transfer of monoglyceride. Cubosomes were even found to absorb bulk decane into the solution from a surface layer, which raises the possibility of using cubosomes to bind hydrophobic molecules from the bulk phase. In mixtures with longer alkanes, transfer of both monoglyceride and oil was observed; while with octadecane the transfer of monoglyceride into emulsion droplets dominated. Calculating the point at which the transfer of monoglyceride becomes dominant over that of oil as the alkane chain length is increased shows that, in compositional ripening, monoglyceride behaves as an alkane with 15.3 carbons. These results further our understanding of the interactions of internally self-assembled particles in various media and suggest possible ways of controlling the size of the particles.

Key role of hydrodynamic interactions in colloidal gelation

Colloidal gelation is caused by the formation of a percolated network of colloidal particles suspended in a liquid. Thus far the major transport process leading to gelation has been believed to be the brownian diffusion of particles. Contrary to this common belief, we reveal by numerical simulations that many-body hydrodynamic interactions between colloidal particles also play an essential role in gelation: They significantly promote gelation, or lower the colloid volume fraction threshold for percolation, as compared to their absence. We find that the incompressible nature of a liquid component and the resulting self-organization of hydrodynamic flow with a transverse (rotational) character are responsible for this enhancement of network-forming ability.

Understanding amyloid aggregation by statistical analysis of atomic force microscopy images

The aggregation of proteins is central to many aspects of daily life, including food processing, blood coagulation, eye cataract formation disease and prion-related neurodegenerative infections. However, the physical mechanisms responsible for amyloidosis-the irreversible fibril formation of various proteins that is linked to disorders such as Alzheimer's, Creutzfeldt-Jakob and Huntington's diseases-have not yet been fully elucidated. Here, we show that different stages of amyloid aggregation can be examined by performing a statistical polymer physics analysis of single-molecule atomic force microscopy images of heat-denatured beta-lactoglobulin fibrils. The atomic force microscopy analysis, supported by theoretical arguments, reveals that the fibrils have a multistranded helical shape with twisted ribbon-like structures. Our results also indicate a possible general model for amyloid fibril assembly and illustrate the potential of this approach for investigating fibrillar systems.

Internally self-assembled submicrometer emulsions stabilized by spherical nanocolloids: finding the free nanoparticles in the aqueous continuous phase

This article reports on the behavior of colloids during the stabilization of internally liquid-crystalline droplets. The ability and limitations of spherical silica colloids to act as stabilizers of liquid-crystalline bulk phases have been demonstrated for monoglyceride/tetradecane/water and phytantriol/tetradecane/water systems using small-angle X-ray scattering and dynamic light scattering as probes. It has been demonstrated that these nanoparticles are very good stabilizers of phytantriol-based phases. In particular, these data showed that they are nondisruptive stabilizers for these bulk phases at ambient temperature. Interestingly, it was shown that the spherical nanoparticles were not able to stabilize the monoglyceride-based bicontinuous cubic phase (Pn3m symmetry group), in contrast with their phytantriol-based counterparts. They were, however, good stabilizers of the monoglyceride-based emulsified microemulsions and inverse micellar cubic phases (Fd3m symmetry group). We then further examined the influence of the concentration of stabilizer on the phytantriol-based dispersed particles. We showed that the scattering signals of the liquid crystal and the nanoparticles are uncorrelated, whatever the identity of the liquid-crystalline phase. A careful analysis method coupled with dynamic light scattering measurements allowed us to determine and discriminate qualitatively the cases with or without free nanoparticles in the continuous phase. The results indicate for the first time fundamental differences in the stabilization by solidlike nanoparticles of emulsified microemulsions and of bicontinuous cubic phases.

Structure, diffusion, and permeability of protein-stabilized monodispersed oil in water emulsions and their gels: a self-diffusion NMR study

Self-diffusion NMR is used to investigate monodispersed oil in water emulsions and the subsequent gel formed by removing the water through evaporation. The radius of the oil droplets in the emulsions is measured using a number of diffusion methods based on the measurement of the mean squared displacement of the oil, water, and tracer molecules. The results are consistent with the known size of the emulsions. Bragg-like reflections due to the restricted diffusion of the water around the oil droplets are observed due to the low polydispersity of the emulsions and the dense packing. The resulting data are fitted to a pore glass model to give the diameter of both the pools of interstitial water and the oil droplets. In the gel, information on the residual three-dimensional structure is obtained using the short time behavior of the effective diffusion coefficient to give the surface to volume ratio of the residual protein network structure. The values for the surface to volume ratio are found to be consistent with the expected increase of the surface area of monodisperse droplets forming a gel network. At long diffusion observation times, the permeability of the network structure is investigated by diffusion NMR to give a complete picture of the colloidal system considered.

Natural Food Nanostructures

The development in the early 1980s of new nanoscience tools such as probe microscopy and, in particular, atomic force microscopy, has provided new methods for probing food structures at the molecular level, under near native conditions. The development and use of microscopic techniques in food science has always led to new scientific understanding of food structure and has spawned new technological applications. The availability of probe microscopes has allowed the investigation and solution of previously intractable problems in food science. Such understanding provides a basis for selecting or manipulating the natural nanostructures formed by food molecules, but through rational, rather than empirical selection of new raw materials, or the improvement and new design of food materials through conventional processing methods. Nanoscience thus enables the improvement of natural nanostructures, through the use of standard and accepted selection and processing methods. This approach is illustrated through studies on starch and protein-stabilised foams and emulsions. It is shown how improved understanding of food structure at the molecular scale can be used to select, modify, or design food structures to meet current challenges in regard to nutrition and health. The use of nanoscience to enable the selection of new improved raw materials, and to modify conventional processing methods, provides a basis for designing new functional foods. The status of such products is discussed in the light of the wider debate on nanotechnologies and food.

A spherical model with directional interactions: II. Dynamics and landscape properties

We study a binary non-additive hard-sphere mixture with square well interactions only between dissimilar particles. An appropriate choice of the inter-particle potential parameters favors the formation of equilibrium structures with tetrahedral ordering (Zaccarelli et al 2007 J. Chem. Phys. 127 174501). By performing extensive event-driven molecular dynamics simulations, we monitor the dynamics of the system, locating the iso-diffusivity lines in the phase diagram, and discuss their location with respect to the gas-liquid phase separation. We observe the formation of an ideal gel which continuously crosses towards an attractive glass upon increasing the density. Moreover, we evaluate the statistical properties of the potential energy landscape for this model. We find that the configurational entropy, for densities within the optimal network-forming region, is finite even in the ground state and obeys a logarithmic dependence on the energy.

Synthesis of photo-cross-linkable microgel colloids for cluster formation studies

In many physical processes involving colloidal particles, transient structures (e.g., in transient particle gels, phase-separating suspensions) are created. Freezing these structures by chemically inter-cross-linking the particles could be an intriguing route to create network structures with special (e.g., mechanical) properties. Alternatively, photochemically fixing transient structures, thereby making them accessible to detailed analysis, could provide new insight in processes such as colloidal aggregation or crystal nucleation, which are of fundamental importance for soft matter physics as well as for applications. For this purpose we synthesized microgel particles, which carry photoreactive groups on the surface, and explored the possibility of creating chemical interparticle cross-links by UV illumination. Via seeded growth emulsion polymerization, we synthesized monodisperse polystyrene microgels with a radius of about 210 nm. In organic solvents, the particles swell to a hydrodynamic radius of 350 nm. These polystyrene microgels were functionalized by using a polymer analogous Friedel-Crafts benzoylation to introduce benzophenone groups onto the surface. The colloidal particles were characterized by static and dynamic light scattering, optical microscopy, and transmission electron microscopy. The interparticle cross-linking behavior of the modified microgels in concentrated dispersions was examined by dynamic light scattering and optical and scanning electron microscopy. To study the effect of short-range attractive forces between the particles, linear polystyrene chains acting as depletion agents were added. In time-dependent irradiation studies, we observe a coexistence of individual particles and clusters with a size range around 2 mum in radius already at shortest irradiation times upon addition of linear polymer. These (transient) clusters-introduced by depletion attraction-are permanently fixed by photo-cross-linking and, thus, on dilution of the system made amenable to analysis by light scattering.

Liquid crystalline phase behavior of protein fibers in water: experiments versus theory

We have developed a new method allowing the study of the thermodynamic phase behavior of mesoscopic colloidal systems consisting of amyloid protein fibers in water, obtained by heat denaturation and aggregation of beta-lactoglobulin, a dairy protein. The fibers have a cross section of about 5.2 nm and two groups of polydisperse contour lengths: (i) long fibers of 1-20 microm, showing semiflexible behavior, and (ii) short rods of 100-200 nm long, obtained by cutting the long fibers via high-pressure homogenization. At pH 2 without salt, these fibers are highly charged and stable in water. We have studied the isotropic-nematic phase transition for both systems and compared our results with the theoretical values predicted by Onsager's theory. The experimentally measured isotropic-nematic phase transition was found to occur at 0.4% and at 3% for the long and short fibers, respectively. For both systems, this phase transition occurs at concentrations more than 1 order of magnitude lower than what is expected based on Onsager's theory. Moreover, at low enough pH, no intermediate biphasic region was observed between the isotropic phase and the nematic phase. The phase diagrams of both systems (pH vs concentration) showed similar, yet complex and rich, phase behavior. We discuss the possible physical fundamentals ruling the phase diagram as well as the discrepancy we observe for the isotropic-nematic phase transition between our experimental results and the predicted theoretical results. Our work highlights that systems formed by water-amyloid protein fibers are way too complex to be understood based solely on Onsager's theories. Experimental results are revisited in terms of the Flory's theory (1956) for suspensions of rods, which allows accounting for rod-solvent hydrophobic interactions. This theoretical approach allows explaining, on a semiquantitative basis, most of the discrepancies observed between the experimental results and Onsager's predictions. The sources of protein fibers complex colloidal behavior are analyzed and discussed at length.

Microscopic structural relaxation in a sheared supercooled colloidal liquid

The rheology of dense amorphous materials under large shear strain is not fully understood, partly due to the difficulty of directly viewing the microscopic details of such materials. We use a colloidal suspension to simulate amorphous materials and study the shear-induced structural relaxation with fast confocal microscopy. We quantify the plastic rearrangements of the particles in several ways. Each of these measures of plasticity reveals spatially heterogeneous dynamics, with localized regions where many particles are strongly rearranging by these measures. We examine the shapes of these regions and find them to be essentially isotropic, with no alignment in any particular direction. Furthermore, individual particles are equally likely to move in any direction other than the overall bias imposed by the strain.

Two new colloidal crystal phases of lipid A-monophosphate: order-to-order transition in colloidal crystals

A study of the structure of stable regular-shaped nanocrystals of hexa-acylated (C(14)) lipid A-monophosphate from Escherichia coli was carried out using dilute electrostatically stabilized aqueous dispersions at low ionic strength (I=1.0x10(-5)M NaCl). An order-to-order transition of colloidal clusters of lipid A-monophosphate was found at two volume fractions: phi=5.9x10(-4) and phi=11.5x10(-4). The clusters belonged to the cubic space groups Pm3n and Ia3d with unit-cell dimensions of a=4.55 nm and a=6.35 nm, respectively, as revealed by small-angle x-ray diffraction and electron-diffraction results of thin nanocrystals of lipid A-monophosphate. When viewed in the scanning electron microscope these fragile clusters displayed a number of shapes: cubic, cylindrical, and sometimes-rounded hexagons, which were extremely sensitive when exposed to an electron beam. The smallest and most numerous of the clusters appeared as approximately 7 nm cubes. Crystalline cluster formation occurred over a wide volume-fraction range, between 1.5x10(-4) and 40.0x10(-4), and at temperatures of 20 and 35 degrees C. The crystalline networks of the lipid A-monophosphate clusters may be represented by space-filling models of two pentagonal dodecahedra with six tetrakaidecahedra arrangements of lipid A-"micelles" in the cubic space group Pm3n. The simulated electron density profiles are in accord with spherical clusters of lipid A-monophosphate at the corners and at the body centers of the cubic Pm3n unit cell. The profiles are rounded tetrahedrally at distances of 1/4 and 3/4 along one of the bisectors of each face of the cubic unit cell. These nanocrystalline systems provide examples of "cellular" crystalline networks, which rearrange themselves spontaneously into three-dimensional polyhedral structures. It appears that a closely related analogy exists between the tetrahedrally close-packed networks as revealed for the lipid A-mono- and diphosphates [C. A. Faunce, H. Reichelt, H. H. Paradies, et al., J. Chem. Phys. 122, 214727 (2005); C. A. Faunce, H. Reichelt, P. Quitschau, et al., J. Chem. Phys. 127, 115103 (2007)]. However, the cubic Ia3d phase consists of two three-dimensional networks of rods, mutually intertwined but not connected. For this cubic Ia3d phase each junction involves three coplanar rods at an angle of 120 degrees, showing an interwoven labyrinth of lipid A-monophosphate rods which are connected three by three. The rod diameter is approximately 2.2 nm, which is similar in diameter to the disk-shaped aliphatic chiral core of lipid A-monophosphate (2.14 nm) with an ellipticity of 0.62 seen for the "c" position of the tetrakaidecahedra in the Pm3n cubic unit cell. An epitaxial relationship appears to exist between the {211} planes of the cubic Ia3d phase and the (001) planes of the lamellar phase as well as with the {10} planes of the hexagonal phase. The transformation of the cubic into the hexagonal phase can be reconciled by the growth of a cylinderlike assembly of lipid A-monophosphate molecules of the hexagonal phase parallel to the 111 directions of the cubic Ia3d phase. Upon cooling from 35 to 20 degrees C the cubic Ia3d lipid A-monophosphate phase unexpectedly transforms and gives rise to an intermediate R3m structure (a=3.90+/-0.12 nm, c=7.82+/-0.05 nm, and gamma=120 degrees). Both cubic Ia3d and hexagonal R3m phases originate from similar rodlike units of lipid A-monophosphate clusters. However, the overall shapes of the assemblies are different because of their spatial distribution. Both assemblies morphologically bridge the lipid A-monophosphate hexagonal and and lamellar phases. The structural path followed during the phase transitions is governed by topological similarities between the phase which forms and the one from which it originates. Although the two phases, Ia3d and R3m, have similar curvature energies on cooling, the topology is more than likely to be the initial factor determining the overall phase transition path.

Metastable and unstable cellular solidification of colloidal suspensions

Colloidal particles are often seen as big atoms that can be directly observed in real space. They are therefore becoming increasingly important as model systems to study processes of interest in condensed-matter physics such as melting, freezing and glass transitions. The solidification of colloidal suspensions has long been a puzzling phenomenon with many unexplained features. Here, we demonstrate and rationalize the existence of instability and metastability domains in cellular solidification of colloidal suspensions, by direct in situ high-resolution X-ray radiography and tomography observations. We explain such interface instabilities by a partial Brownian diffusion of the particles leading to constitutional supercooling situations. Processing under unstable conditions leads to localized and global kinetic instabilities of the solid/liquid interface, affecting the crystal morphology and particle redistribution behaviour.

Phase behavior of Phytantriol/water bicontinuous cubic Pn3m cubosomes stabilized by Laponite disc-like particles

The present article reports on the specific effects of temperature on Phytantriol-based cubosomes stabilized by inorganic stabilizers as opposed to organic stabilizers. The ability of Laponite to stabilize Phytantriol-based parent bulk phase is first demonstrated. The sub-micron-sized Laponite-stabilized particles were found to be both physically and chemically stable over time. The temperature-induced behavior, both in heating and cooling directions, of these lipid-based cubosomes has been investigated and compared with their polymer-stabilized counterparts (Pluronic F127). This allows us to extract the particular influence of each stabilizer. Whereas an increased hydration of the cubic structure was evidenced at high pH values, this effect was eliminated to compare the specific influence of both stabilizers on these Phytantriol-based cubosomes. Evidence of differences in the relaxation rates of the internal structures with temperature was found for the two stabilizers, in particular in the cooling direction whilst in the heating direction the two stabilizers could be considered as undisruptive. The origin of this difference is discussed.

Multiple particle tracking investigations of acid milk gels using tracer particles with designed surface chemistries and comparison with diffusing wave spectroscopy studies

Multiple particle tracking (MPT) has been used in an attempt to probe the heterogeneity of acid milk gels, made with and without added pectin, by following the distribution of the displacements of added tracer beads during and after gelation using the Van Hove distribution. Furthermore, the surface chemistry of the latex probe particles was modified in an attempt to control their location in the system and probe the microrheological properties of the protein network and aqueous-phase voids independently. In addition, the mean square displacement (MSD) of the casein micelles/casein aggregates themselves, obtained by diffusing wave spectroscopy (DWS), has been compared to the ensemble-averaged MSD calculated from the data obtained by tracking the movement of the added tracers, with and without a kappa-casein coating. For the kappa-casein-coated tracer particles, upon acidification and subsequent gel formation, the MSDs obtained by MPT superimpose remarkably well with the MSDs obtained by DWS, despite the fact that one is obtained by tracking the movement of the particle network elements themselves and the other is obtained from directly tracking added tracers. This result has important implications: (i) it demonstrates that, although the DWS measurement is intrinsically ensemble-averaged, it really gives insight into the dynamics of the colloidal gel network; (ii) it confirms that the kappa-casein-coated probes used in this MPT experiment are well incorporated into the gel network; and hence (iii) that at least in gelled systems kappa-casein-coated latex probes are interesting probes that reveal the dynamics of the casein network.

Characterization of lipid-templated silica and hybrid thin film mesophases by grazing incidence small-angle X-ray scattering

The nanostructure of silica and hybrid thin film mesophases templated by phospholipids via an evaporation-induced self-assembly (EISA) process was investigated by grazing-incidence small-angle X-ray scattering (GISAXS). Diacyl phosphatidylcholines with two tails of 6 or 8 carbons were found to template 2D hexagonal mesophases, with the removal of lipid from these lipid/silica films by thermal or UV/O3 processing resulting in a complete collapse of the pore volume. Monoacyl phosphatidylcholines with single tails of 10-14 carbons formed 3D micellular mesophases; the lipid was found to be extractable from these 3D materials, yielding a porous material. In contrast to pure lipid/silica thin film mesophases, films formed from the hybrid bridged silsesquioxane precursor bis(triethoxysilyl)ethane exhibited greater stability toward (both diacyl and monoacyl) lipid removal. Ellipsometric, FTIR, and NMR studies show that the presence of phospholipid suppresses siloxane network formation, while actually promoting condensation reactions in the hybrid material. 1D X-ray scattering and FTIR data were found to be consistent with strong interactions between lipid headgroups and the silica framework.

A closer look at arrested spinodal decomposition in protein solutions

Concentrated aqueous solutions of the protein lysozyme undergo a liquid-solid transition upon a temperature quench into the unstable spinodal region below a characteristic arrest temperature of T(f) = 15 °C. We use video microscopy and ultra-small angle light scattering in order to investigate the arrested structures as a function of initial concentration, quench temperature and rate of the temperature quench. We find that the solid-like samples show all the features of a bicontinuous network that is formed through an arrested spinodal decomposition process. We determine the correlation length ξ and demonstrate that ξ exhibits a temperature dependence that closely follows the critical scaling expected for density fluctuations during the early stages of spinodal decomposition. These findings are in agreement with an arrest scenario based on a state diagram where the arrest or gel line extends far into the unstable region below the spinodal line. Arrest then occurs when during the early stage of spinodal decomposition the volume fraction φ(2) of the dense phase intersects the dynamical arrest threshold φ(2,Glass), upon which phase separation gets pinned into a space-spanning gel network with a characteristic length ξ.

Long-living intermediates during a lamellar to a diamond-cubic lipid phase transition: a small-angle X-ray scattering investigation

To generate nanostructured vehicles with tunable internal organization, the structural phase behavior of a self-assembled amphiphilic mixture involving poly(ethylene glycol) monooleate (MO-PEG) and glycerol monooleate (MO) is studied in excess aqueous medium by time-resolved small-angle X-ray scattering (SAXS) in the temperature range from 1 to 68 degrees C. The SAXS data indicate miscibility of the two components in lamellar and nonlamellar soft-matter nanostructures. The functionalization of the MO assemblies by a MO-PEG amphiphile, which has a flexible large hydrophilic moiety, appears to hinder the epitaxial growth of a double diamond (D) cubic lattice from the lamellar (L) bilayer structure during the thermal phase transition. The incorporated MO-PEG additive is found to facilitate the formation of structural intermediates. They exhibit greater characteristic spacings and large diffusive scattering in broad temperature and time intervals. Their features are compared with those of swollen long-living intermediates in MO/octylglucoside assemblies. A conclusion can be drawn that long-living intermediate states can be equilibrium stabilized in two- or multicomponent amphiphilic systems. Their role as cubic phase precursors is to smooth the structural distortions arising from curvature mismatch between flat and curved regions. The considered MO-PEG functionalized assemblies may be useful for preparation of sterically stabilized liquid-crystalline nanovehicles for confinement of therapeutic biomolecules.

Rheological response and dynamics of the amphiphilic diamond phase from kinetic lattice–Boltzmann simulations

The purpose of the present paper is to report on the first computational study of the dynamical and rheological response of a self-assembled diamond mesophase under Couette flow in a ternary mixture composed of oil, water and an amphiphilic species. The amphiphilic diamond mesophase arises in a wide range of chemical and biological systems, and a knowledge of its rheological response has important implications in materials science and biotechnological applications. The simulations reported here are performed using a kinetic lattice–Boltzmann method. Lyotropic liquid crystals exhibit characteristic rheological responses in experiments that include shear-banding and a non-Newtonian flow curve as well as viscoelasticity under oscillatory shear. Their behaviour under steady and oscillatory shear is correctly reproduced in our simulations. On cessation of shear, as the morphology returns to the diamond phase, the relaxation of the stress response follows a stretched-exponential form for low initial strain rates.

Oleoylethanolamide-based lyotropic liquid crystals as vehicles for delivery of amino acids in aqueous environment

We have investigated the phase behavior of self-assembled lyotropic liquid crystals (LC) formed by ternary mixtures of oleoylethanolamide (OEA), water and arginine. OEA, a natural analog of the endogenous cannabinoid anandamide involved in the peripheral regulation of feeding, was selected as a main component due to its capacity to induce efficient decreases in food intake and gains in body mass. Arginine was selected as representative hydrophilic amino acid and added to the OEA-water mixture at different concentrations. The phase diagrams were determined by combining cross-polarized optical microscopy and small angle x-ray scattering. First, the phase diagram for the OEA-water system was determined. It was shown that these two compounds give rise to reverse Ia3d double gyroid and reverse Pn3m double diamond cubic phases existing in bulk over a large window of temperature and composition, and that for water content beyond 25% Pn3m coexisted with excess water. Successively, the influence of arginine as guest molecule in the water channels of the reverse LC was investigated. For the sake of comparison, results for the OEA-water-arginine system were compared with analog series of OEA-water-glucose. The results showed that, at a fixed water content and temperature, the phase behavior of the liquid crystalline phases is strongly dependent on arginine concentration. In more detail, arginine could be encapsulated in the bulk OEA-water LC up to 2.0% wt, whereas transitions from Ia3d to Pn3m cubic phase were observed with increasing arginine concentration. Interestingly, upon an increase of water concentration beyond 20-25%, Pn3m phase started to coexist with excess water releasing the arginine in external water solution. Quantitative measurements of arginine content inside the LC water channels and in the excess external water solution revealed a complete release of the amino acid, demonstrating that the investigated lyotropic liquid crystalline systems can be used as ideal vehicles for the delivery of functional hydrophilic active molecules in aqueous environment.

Characterization and potential applications of nanostructured aqueous dispersions

The present article highlights recent advances and current status in the characterization and the utilization of nanostructured aqueous dispersions in which the submicron-sized dispersed particles envelope a distinctive well-defined self-assembled interior. The scope of this review covers dispersions of both inverted-type liquid-crystalline particles (cubosomes, hexosomes, micellar cubosomes, and sponge phases), and microemulsion droplets (emulsified microemulsions, EMEs). Recent investigations that have attempted to shed light on the characterization and the control of confined nanostructures of aqueous dispersions are surveyed, as these nanoobjects are attractive for various pharmaceutical and food applications. The focus has been placed on three main subjects: (1) our findings on the formation of EMEs and the modulation of the internal nanostructure, exploring how variations in temperature, oil content, and lipid composition significantly affect the confined nanostructures; (2) recent developments in the field of electron microscopy: using the tilt-angle cryo-TEM method or cryo-field emission scanning electron microscopy (cryo-FESEM) for observing the three dimensional (3D) morphology of non-lamellar liquid-crystalline nanostructured particles (cubosome and hexosome particles); and (3) recent studies on the utilization of nanostructured dispersions as drug nanocarriers.

Size segregation in a fluid-like or gel-like suspension settling under gravity or in a centrifuge

We investigate size segregation effects in a bidisperse concentrated suspension when slowly settling under gravity or when submitted to a centrifugal field. Experiments are carried out with PMMA spheres of two different mean diameters (190 and 25 microm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labeled PMMA spheres and a particle-image-velocimetry method. Large particles behave as hard spheres in purely hydrodynamic interactions, while small spheres interact through weakly attractive forces. For a small amount of small spheres among large ones, the suspension remains fluid during settling and the organization of the velocity field of particles into finite-sized structures also called "blobs" promotes size segregation. A larger proportion of weakly attractive small spheres in the bidisperse suspension causes a considerable slowdown of the settling process under gravity and the occurrence of a large-scale collective behavior together with a loss of size segregation. When centrifuging the gel-like bidisperse suspension, a shear-induced melting of the particle network induces a spectacular segregation of species. As a consequence, aging tests of soft yielding materials using centrifugation methods are not representative of the shelf-life stability of the products. A tentative model based on the competition between viscous stresses acting upon particles and adhesive stresses gives a correct estimate of the critical stationary acceleration for the destabilization of the particle network and the onset of size segregation in a gel-like suspension.

Self-assembly in monoelaidin aqueous dispersions: direct vesicles to cubosomes transition

BACKGROUND

In the present study, synchrotron small-angle X-ray scattering (SAXS) and Cryo-TEM were used to characterize the temperature-induced structural transitions of monoelaidin (ME) aqueous dispersion in the presence of the polymeric stabilizer F127. We prove that the direct transition from vesicles to cubosomes by heating this dispersion is possible. The obtained results were compared with the fully hydrated bulk ME phase.

METHODOLOGY/PRINCIPAL FINDINGS

Our results indicate the formation of ME dispersion, which is less stable than that based on the congener monoolein (MO). In addition, the temperature-dependence behavior significantly differs from the fully hydrated bulk phase. SAXS findings indicate a direct L(alpha)-V(2) internal transition in the dispersion. While the transition temperature is conserved in the dispersion, the formed cubosomes with internal Im3m symmetry clearly contain more water and this ordered interior is retained over a wider temperature range as compared to its fully hydrated bulk system. At 25 degrees C, Cryo-TEM observations reveal the formation of most likely closely packed onion-like vesicles. Above the lamellar to non-lamellar phase transition at 65 degrees C, flattened cubosomes with an internal nanostructure are observed. However, they have only arbitrary shapes and thus, their morphology is significantly different from that of the well-shaped analogous MO cubosome and hexosome particles.

CONCLUSIONS/SIGNIFICANCE

Our study reveals a direct liposomes-cubosomes transition in ME dispersion. The obtained results suggest that the polymeric stabilizer F127 especially plays a significant role in the membrane fusion processes. F127 incorporates in considerable amount into the internal nanostructure and leads to the formation of a highly swollen Im3m phase.