Bicontinuous Microstructure Research Articles

Formation and Characterization of Ordered Bicontinuous Microemulsions

Ordered bicontinuous microstructures formed in a fully water-dilutable, pseudoternary unique nonionic microemulsion were obtained and characterized. The concentrate contained a mixture of triacetin/d-alpha-tocopherol acetate/ethanol/Tween 60. Upon dilution, the concentrate was transformed from a reversed micellar system to oil-in-water microemulsion droplets. The transformation occurred through an intermediate phase of ordered bicontinuous structures. The factors that governed the construction of this unique phase, and its physical and structural properties, were characterized in detail. The techniques used included small angle X-ray scattering (SAXS), self-diffusion and quantum filtered NMR, differential scanning calorimetry, rheology measurements, electrical conductivity, and dynamic light scattering. This mesophase displays microemulsion properties along with some characteristics of lyotropic liquid crystals (but is not a mixture of the two). Similar to microemulsions, the structures were transparent and spontaneously formed and exhibited thermodynamic stability. Yet, unlike microemulsions, they showed short-range order at room temperature. Additionally, the microstructures exhibited non-Newtonian flow behavior, characteristic of lamellar structures. The bicontinuous ordered microemulsions were obtained upon heating (to 25 degrees C) from the lamellar phase existing at low temperatures (5 degrees C). The main feature governing the bicontinuous mesophase formation was the amphiphilic nature of oil blends composed of d-alpha-tocopherol acetate and triacetin. The oils functioned as cosurfactants, altering the packing parameter of the surfactant and leading to the construction of bicontinuous structures with short-range order. These unique structures might have drug or nutraceutical delivery advantages.

Interfacial Structuring in a Phase-Separating Mixed Biopolymer Solution Containing Colloidal Particles

We report confocal microscopy observations of the spatial distribution of monodisperse charge-stabilized colloidal particles (amphoteric polystyrene latex) incorporated within a spinodal-type phase-separating system of mixed biopolymers (gelatin + oxidized starch). Images from samples aged at 40 degrees C demonstrate a strong tendency for the added particles to accumulate at the liquid-liquid interface and to influence the rate of coarsening of the complex bicontinuous microstructure. Large variations in the local curvature of particle-rich interfacial regions are suggestive of a liquid-liquid boundary that is substantially viscoelastic.

Phase field simulation of monotectic transformation for liquid Ni-Cu-Pb alloys

Based on the subregular solution model, the liquid phase separation of ternary (NixCu100−x)50Pb50 monotectic alloys is simulated by the phase field method. It is found that if the surface segregation potential is not incorporated, the dynamic morphologies of alloy melt show a transition from disperse microstructure into bicontinuous microstructure with the increase of fluidity parameter. When the surface segregation potential is coupled, Pb-rich phase migrates preferentially to the surface of the liquid alloy, and the Ni-rich phase depends on the Pb-rich phase to nucleate. With the extension of the phase separation time, the surface layer is formed through coagulation and growth, and its thickness gradually increases. The Ni-rich phase migrates to the central part, and finally a two-layer core-shell microstructure is produced. The concentration in the surface layer fluctuates more conspicuously than that inside the bulk phase, which subsequently transfers from the surface to the interior by a wave. The fluid field near the liquid-liquid interface is strong at the beginning of phase separation, and reduces later on. The surface segregation is essential to the formation of the surface layer, concentration profile variation, fluid field distribution and phase separation morphology.

Flexibility, persistence length and bicontinuous microstructures in microemulsions

Abstract Three length scales have to be considered to describe microemulsions: persistence length, spontaneous radius of curvature of the surfactant film as intensive variables and finally the characteristic size imposed by the ratio of volume fraction to available specific area surface. The specific area per unit volume is an intensive variable linked to sizes and topologies that can be built without tearing the surfactant film. We show here that at least four types of bicontinuous microstructures have been detected so far, and that they can be distinguished by a simple experimental determination of the evolution of scattering peak position versus dilution. Besides the classical microstructures dominated by entropy or a priori considered as droplets, the other microstructures identified so far can be approximated as connected cylinders or randomly connected swollen bilayers. All these microstructures can be considered as “molten” precursors of the lyotropic liquid crystalline phases that are adjacent to microemulsions in phase diagrams.

Thermostable Biocatalytic Films of Enzymes and Polylysine on Electrodes and Nanoparticles in Microemulsions

Microemulsions of oil, water and surfactant were evaluated as media for biocatalysis at high temperatures employing films of polylysine (PLL) and the enzymes horseradish peroxidase (HRP), soybean peroxidase (SBP) and the protein myoglobin (Mb). PLL was covalently linked to oxidized pyrolytic graphite electrodes or carboxylated 500 nm diameter silica nanoparticles, then cross-linked by amidization to HRP, SBP and Mb. The resulting film systems were stable at 90 degrees C for >12 h in microemulsions. Characterization of the microemulsions by conductivity, viscosity and probe diffusion coefficients suggested that these media have bicontinuous microstructures from 25 to 90 degrees C. UV circular dichroism and visible spectroscopy confirmed that the enzymes retained near-native conformation in the films at temperatures as high as 90 degrees C. Oxidation of o-methoxyphenol to 3,3'-dimethoxy-4,4'-biphenoquinone by enzyme-PLL films on silica nanoparticles gave yields 3-5-fold larger in microemulsions at 90 degrees C compared to the same reaction at 25 degrees C. The best yields were in CTAB microemulsions and were 3-fold larger than in buffers at 90 degrees C.

Investigation of particle‐functionalized tissue engineering scaffolds using X‐ray tomographic microscopy

A low-density, porous chitosan/poly-(dl-lactide-co-glycolide) (PLGA) microparticle composite scaffold was produced by thermally induced phase separation followed by lyophilization, to provide a bicontinuous microstructure potentially suitable for tissue engineering and locally controlled drug release. PLGA particles were mixed into the chitosan solution and subsequent phase separation during chitosan solidification forced PLGA particles into chitosan phase (Plateau borders). The distributions of volume, surface area, and elongation of 15,422 inclusions of agglomerated PLGA particles were calculated and approximated with log-normal distribution functions from nanotomography reconstructions. Cluster analysis revealed a homogenous inclusion distribution throughout the scaffold. The spatial location and orientation of individual inclusions within the Plateau borders of the scaffold were determined and from these the nearest-neighbor inter-inclusion distance distribution calculated, showing a mean of 2.5 microm. The depth of the inclusions in Plateau borders peaks at 700 or 125 nm, respectively, indicating a step-wise drug release from inclusions successively exposed during scaffold decomposition. Particle diameter ranged from 400 nm to 3 microm and inclusion Feret lengths ranged from 800 nm to 12 microm. These findings on composite morphology and distribution of inclusions are fundamental for predicting scaffold deterioration and particle-mediated drug release during ex vivo and in vivo cell cultivation.

Scattering form factors for self-assembled network junctions

The equilibrium microstructures in microemulsions and other self-assembled systems show complex, connected shapes such as symmetric bicontinuous spongelike structures and asymmetric bicontinuous networks formed by cylinders interconnected at junctions. In microemulsions, these cylinder network microstructures may mediate the structural transition from a spherical or globular phase to the bicontinuous microstructure. To understand the structural and statistical properties of such cylinder network microstructures as measured by scattering experiments, models are needed to extract the real-space structure from the scattering data. In this paper, we calculate the scattering functions appropriate for cylinder network microstructures. We focus on such networks that contain a high density of network junctions that connect the cylindrical elements. In this limit, the network microstructure can be regarded as an assembly of randomly oriented, closed packed network junctions (i.e., the cylinder scattering contributions are neglected). Accordingly, the scattering spectrum of the network microstructure can be calculated as the product of the junction number density, the junction form factor, which describes the scattering from the surface of a single junction, and a structure factor, which describes the local correlations of different junctions due to junction interactions (including their excluded volume). This approach is applied to analyze the scattering data from a bicontinuous microemulsion with equal volumes of water and oil. In a second approach, we included the cylinder scattering contribution in the junction form factor by calculating the scattering intensity of Y junctions to which three rods with spherical cross section are attached. The respective theoretical predictions are compared with results of neutron scattering measurements on a water-in-oil microemulsion with a connected microstructure.

The influence of cosurfactants and oils on the formation of pharmaceutical microemulsions based on PEG-8 caprylic/capric glycerides

In the present study the effect of type and concentration of a cosurfactant and oil on the ability of nonionic surfactant PEG-8 caprylic/capric glycerides (Labrasol ®) to solubilize both oil and water phases was evaluated. Seven different cosurfactants (polyglyceryl-6 dioleate (Plurol Oleique ®) (PO), polyglyceryl-6 isostearate (Plurol Isostearique ®) (PI), polyglyceryl-4 isostearate (Isolan ® GI 34) (IGI34), octoxynol-12 (and) polysorbate 20 (Solubilisant gamma ® 2421) (SG2421), octoxynol-12 (and) polysorbate 20 (and) PEG-40 hydrogenated castor oil (Solubilisant gamma ® 2429) (SG2429), PEG-40 hydrogenated castor oil (Cremophor ® RH 40) (CRH40) and diethyleneglycol monoethyl ether (Transcutol ®)) and six oils (isopropyl myristate, ethyl oleate, decyl oleate, medium chain triglycerides, mineral oil and olive oil) were used in phase behaviour studies of a quaternary system Labrasol ®/cosurfactant/oil/water. The amount of surfactant required to completely homogenize equal masses of oil and water to form a single phase microemulsion (termed as balanced microemulsion) (Smin, %w/w), the minimal concentration of the surfactant/cosurfactant blend required to produce a balanced microemulsion (SCoSmin, %w/w) as well as the maximum concentration of water solubilized in investigated surfactant/oil and surfactant/cosurfactant/oil mixtures ( W max, %w/w) were determined. The obtained results indicated that Labrasol ® showed a good efficiency in the presence of lower molecular volume fatty acid esters with a preferred chemical structure such as isopropyl myristate (Smin 56.14% (w/w); W max 12.28% (w/w)). Oils with high molecular volume (olive oil and mineral oil) do not result in microemulsion formation. Transcutol ® decreased the capacity of Labrasol ® for solubilization of oil and water phases. The tendency of Labrasol ® to solubilize both, water and oil phases, was favoured by polyglycerol-6 ester type of cosurfactants (PO and PI) while the influence of the polyglycerol-4 ester (IGI34) as well as of polyoxyethylene type of cosurfactants (CRH40, SG2421 and SG2429) on the surfactant efficiency, was not significant. Furthermore, the results revealed the significant influence of the surfactant/cosurfactant mass ratio ( K m) on synergistic effect between polyglyceryl-6 esters and Labrasol ® in the formation of microemulsions using isopropyl myristate as oil phase. Optimized microemulsion systems were stabilized with Labrasol ®/polyglyceryl-6 esters blend at K m 5:5 (SCoSmin, 27.5% (w/w) and W max, 71.43% (w/w) for PI; SCoSmin, 29.18% (w/w) and W max, 65.00% (w/w) for PO) and the electrical conductivity measurement results for optimized balanced microemulsions showed that their structures were highly conductive indicating a bicontinuous microstructure.

Microstructure studies on biosurfactant-rhamnolipid/ n-butanol/water/ n-heptane microemulsion system

A microemulsion system, containing a natural biosurfactant, is characterized in a rhamnolipid/ n-butanol/water/ n-heptane system. The surface tension of rhamnolipid solution has been studied in detail, and an especial post-CMC has been found. Based on the surface tension measurements, it can be concluded that the phase behavior and microstructure of this microemulsion are rational to the conformational changes of rhamnolipid molecules at the interface of oil/water. For a given biosurfactant-to-cosurfactant-to-oil ratio, an aqueous dilution line is studied. At 298 K, along this dilution path, a structural transition sequence: water drops → bicontinuous microstructure → oil drops is inferred by conductance method, which is also confirmed by other two methods as dynamic light-scattering (DLS) and freeze-fracture transmission electron (FFTE). The droplet dimension, the polydispersity, and the diffusion coefficient are found to be distinctly different between the w/o and o/w samples. Droplets in w/o microemulsion are smaller and more uniform than those in o/w one.

In vitro release of diclofenac diethylamine from caprylocaproyl macrogolglycerides based microemulsions

The purpose of the present study was to determine the influence of both formulation parameters and vehicle structure on in vitro release rate of amphiphilic drug diclofenac diethylamine (DDA) from microemulsion vehicles containing PEG-8 caprylic/capric glycerides (surfactant), polyglyceryl-6 dioleate (cosurfactant), isopropyl myristate and water. From the constructed pseudo-ternary phase diagram at surfactant–cosurfactant mass ratio ( K m 1:1), the optimum oil-to-surfactant–cosurfactant mass ratio values (O/SC 0.67–1.64) for formulation of microemulsions with similar concentrations of hydrophilic, lipophilic and amphiphilic phases (balanced microemulsions) were found. The results of characterization experiments indicated bicontinuous or nonspherical water-continuous internal structure of the selected microemulsion vehicles. Low water/isopropyl myristate apparent partition coefficient for DDA as well as elevated electrical conductivity and apparent viscosity values for the investigated microemulsion formulations containing 1.16% (w/w) of DDA, suggested that the drug molecules was predominantly partitioned in the water phase and most likely selfaggregate and interact with interfacial film. Release of DDA from the selected water-continuous (W/O), oil-continuous (O/W) and balanced microemulsions was investigated using rotating paddle dissolution apparatus modified by addition of enhancer cell. A linear diffusion of DDA through regenerated cellulose membrane was observed for the W/O and O/W formulations with the low content of dispersed phase. Non-linearity of the drug release profile in the case of bicontinuous formulations was related to the more complex distribution of DDA including interactions between the drug and vehicle. The membrane flux value increases from 25.02 μg cm −2 h −1 (W/O microemulsion) to 117.94 μg cm −2 h −1 (O/W microemulsion) as the water phase concentration increases. Moreover, the obtained flux values for balanced microemulsions (29.38–63.70 μg cm −2 h −1) suggested that bicontinuous microstructure hampers the release of the amphiphilic drug.

Bicontinuous microemulsions revisited: a new approach to freeze fracture electron microscopy (FFEM)

New images by freeze fracture electron microscopy (FFEM) are obtained for microemulsions in the ternary system water–n-octane–C12E5 as function of varying water-to-oil ratio. The surfactant concentration is chosen such that the composition of the sample is close to the X̃-point, i.e. the point of highest efficiency, where the three- and the one-phase regions meet. As the phase behavior and thus the microstructure of these particular nonionic microemulsions are extremely sensitive to temperature, a precise temperature control of the sample was necessary prior to rapid freezing in liquid ethane. The images presented are the first FFEM images along the trajectory of the middle-phase in phase space. Of particular interest is the structure of the microemulsion on the water-rich side not visualized so far. The images give strong evidence that even at the highest water and oil contents, respectively, elements of a bicontinuous microstructure are visible, which confirms results obtained by means of various other techniques.

Computer simulation of structure and microphase separation in model A-B-A triblock copolymers.

A set of computer simulations for three symmetric A-B-A triblock copolymer microarchitectures at varying temperatures is reported. By using the cooperative motion algorithm we obtain energy, specific heat, end-to-end distance, and bridging fraction as a function of the reduced temperature. The order-disorder transition temperatures are determined, an outline of a symmetric A-B-A triblock copolymer phase diagram is presented, and the visualization of different microstructures is given. A bicontinuous microstructure is reported at 67% fraction of A component.

Determination of temperature dependent structure evolution by fast-Fourier transform at late stage spinodal decomposition in bicontinuous biopolymer mixtures

The evolutions of the bicontinuous microstructures of aqueous phase separating gelatin/maltodextrin mixtures quenched to different end temperatures were determined by confocal laser scanning microscopy (CLSM). The growth of the bicontinuous microstructures was quantified by Fourier image analysis. Weighted least squares were applied in order to be able to use all the spectral information. The results of Fourier image analysis and weighted least squares were related to existing theories on coarsening. The mixtures were quenched from 60 °C to different end temperatures ranging between 10 °C and 37 °C and the concentration was held constant at 4.2 w/w % gelatin and 7.9 w/w % maltodextrin. The results showed that the mixture phase separated through spinodal decomposition at all temperatures. A crossover was found from structure growth governed by diffusion to structure growth governed by hydrodynamic flow. The results showed that the structure evolution at the beginning of the phase separation was temperature independent with a growth proportional to the time raised to one-third. After the crossover, the growth of the characteristic distance between the maltodextrin domains was temperature dependent with a growth proportional to the time raised to an exponent that varied from 0.75 to 1.58. It was found that the growth exponent increases with decreasing end temperature, i.e., increasing quench depth. The maximum intensity of the circularly averaged two-dimensional fast-Fourier transform of the CLSM micrographs was found to grow exponentially with time. The increases in the maximum intensity were proportional to the time raised to an exponent that varied from 1.98 to 4.97. It was found that this exponent increases with decreasing end temperature. Before the crossover, the relation between the growth exponent of the microstructure and the growth exponent of the maximum intensity, as compared with existing theories on coarsening, showed that the phase separation was in the intermediate or transitional stages of spinodal decomposition. Similarly, after the crossover, it was found that phase separation was in the late stages of spinodal decomposition. Furukawa master plots showed that the structure growth obeyed dynamical scaling and that the dimensionality of the growth was three, given off-critical conditions.

A bending elasticity approach to the three-phase coexistence of microemulsions

In mixtures of water, oil, and surfactant, a microemulsion phase with a bicontinuous microstructure may coexist with water and oil excess phases. A common theoretical description of microemulsion properties is based on the energy of the curved bending elastic interface. The aim of the article in hand is to model the microstructures and phase behavior of microemulsions in the vicinity of the three-phase region. To this end, we compare the energy of several microstructures as function of volumetric constraints and preferred curvature parameters. Two major developments were necessary to do so: (i) A new bending energy function is introduced capable of explaining the coexistence of the bicontinuous microemulsion with both water and oil. (ii) An analytical geometric model of bicontinuous structures with variable volume ratios of the two domains is presented suitable for computing the local curvatures over the whole interface.

Copolymerization of vinyl acetate with acrylic monomers in microemulsion

Composition domains corresponding to “one phase microemulsions” have been studied for the monomer mixtures consisting in vinyl acetate (VAc)–2-ethyl-hexyl acrylate (EHA) and VAc–ethyl acrylate (EtA). Maleic monoester with nonyl phenol ethoxylated with 25 mol ethylene oxide has been used as surfactant; n-propanol ( n-PrOH), t-butanol ( t-BuOH) and 2-ethyl-hexanol have been employed as cosurfactants. The number of microemulsions formed in the frame of the existing systems (as shown in ternary diagrams) is higher for polar monomers and cosurfactants: EtA>EHA; n-PrOH> t-BuOH. Refractometric and conductometric studies have proved the reality of three types of microemulsions: O/ W, bicontinuous and W/ O respectively. The homogeneous domains lessen after radical copolymerization. However, for the zone with a bicontinuous microstructure the conversion for the comonomer mixture reaches a minimum; a degradative chain transfer of the growing radicals with the cosurfactant might explain this phenomenon. It has been proved by GPC, that the concentration of unreacted surfactant increases when the concentration of the acrylic monomer increases as well.

Homopolymer and small‐molecule tracer diffusion in a gyroid matrix

Forced Rayleigh scattering was used to measure the tracer diffusion coefficients of the photochromic dye tetrathioindigo (TTI) and a 1,4-polyisoprene (PI) homopolymer (8000 g/mol) in a poly(styrene-b-isoprene) (SI) diblock copolymer matrix that formed a bicontinuous gyroid microstructure. The diblock copolymer contained 63% polystyrene (PS) by volume and had a total molecular weight of 21,300 g/mol. Rheology and small-angle X-ray scattering confirmed that the diblock copolymer microphase-separated into the bicontinuous gyroid over the temperature range 60–230 °C, where the sample disordered. For both the TTI and PI tracers, two distinct modes of transport were observed. The faster mode displayed a temperature dependence consistent with diffusion within a PI matrix, whereas the slower mode had a temperature dependence more similar to diffusion within PS. The fast diffusivities were both over an order of magnitude lower than in a corresponding PI homopolymer matrix. For TTI, this was attributed to the preferential selectivity of the dye for PS and, therefore, an averaging of the mobility between the PS and PI domains. The slow mode was consistent with a small fraction of the TTI dye molecules becoming trapped within the much slower PS domains. For the PI tracer, the reduction in the diffusion coefficient for the fast mode was attributed to a combination of the tortuosity of the struts, the suppression of constraint release within the diblock matrix, and additional friction due to the presence of some styrene segments within the PI domains. The inevitable presence of grain boundaries or defects within the matrix interrupted the percolation of the PI struts, thereby forcing some of the PI tracers to diffuse through PS. Consequently, the slow mode was attributed to the diffusion through these defects, where the PI diffusion was retarded by both the increased segmental friction and the thermodynamic barrier to entering the PS domains. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 843–859, 2001

The Influence of Monomers upon Microemulsions with Short Chain Cosurfactant

Several monomers (i.e., acrylonitrile (ACN), ethyl acrylate (EtA), acrylate of 2 ethylhexyl (EHA), butyl acrylate (BuA), vinyl acetate (VAc), methyl methacrylate (MMA) and styrene (St)) were investigated in order to study their capacities to form microemulsions. The surfactant was nonyl phenol ethoxylate with 25 moles ethylene oxide and ethanol was used as the cosurfactant. The phase diagrams prove that the capacity for microemulsion formation varies in the following sequence: ACN = EtA > VAc > BuA > MMA > St > EHA. Conductometric and refractometric studies allowed us to evidence the formation of microemulsions with W/O, bicontinuous or O/W structure. Fluorescence studies give interesting informations on the stabilization capacity of the monomers dispersed by interaction with hydrophobic chain of the cosurfactant. H-NMR studies suggest that the monomers in microemulsions lie in regions with various polarities as function of their chemical nature. In polymerization initiated with benzoyl peroxide the minimum conversions were obtained in zones of composition where the initial microemulsions possess a bicontinuous microstructure. The reactions of degradative transfer of the increasing macroradicals account for these conversion modifications. For initiation with ammonium persulphate, its rate of decomposition depends on its ability to displace the proton bonded to the substituted vinylic carbon of the monomer. The productivity of this initiator varies within the sequence MMA > EtA > BuA > St > ACN > VAc. The consumption of the initiator, as induced by the monomer, depends on its chemical nature and on the solubilization site of the microemulsions.

Homopolymer and small‐molecule tracer diffusion in a gyroid matrix

AbstractForced Rayleigh scattering was used to measure the tracer diffusion coefficients of the photochromic dye tetrathioindigo (TTI) and a 1,4‐polyisoprene (PI) homopolymer (8000 g/mol) in a poly(styrene‐b‐isoprene) (SI) diblock copolymer matrix that formed a bicontinuous gyroid microstructure. The diblock copolymer contained 63% polystyrene (PS) by volume and had a total molecular weight of 21,300 g/mol. Rheology and small‐angle X‐ray scattering confirmed that the diblock copolymer microphase‐separated into the bicontinuous gyroid over the temperature range 60–230 °C, where the sample disordered. For both the TTI and PI tracers, two distinct modes of transport were observed. The faster mode displayed a temperature dependence consistent with diffusion within a PI matrix, whereas the slower mode had a temperature dependence more similar to diffusion within PS. The fast diffusivities were both over an order of magnitude lower than in a corresponding PI homopolymer matrix. For TTI, this was attributed to the preferential selectivity of the dye for PS and, therefore, an averaging of the mobility between the PS and PI domains. The slow mode was consistent with a small fraction of the TTI dye molecules becoming trapped within the much slower PS domains. For the PI tracer, the reduction in the diffusion coefficient for the fast mode was attributed to a combination of the tortuosity of the struts, the suppression of constraint release within the diblock matrix, and additional friction due to the presence of some styrene segments within the PI domains. The inevitable presence of grain boundaries or defects within the matrix interrupted the percolation of the PI struts, thereby forcing some of the PI tracers to diffuse through PS. Consequently, the slow mode was attributed to the diffusion through these defects, where the PI diffusion was retarded by both the increased segmental friction and the thermodynamic barrier to entering the PS domains. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 843–859, 2001

Mechanical alloying of copper-nickel-iron powders : Y.Tanaka et al. (Sanyo Special Steels, Japan.) Mater. Trans. JIM, vol 37, no 5, 1996, 1073–1077

The elastic and physical properties of a tungsten-copper (W-Cu) bi-continuous composite were predicted from microstructural data captured using TriBeam serial sectioning. The reconstructed 3D volume was converted into a Finite Element (FE) mesh. The minimum representative volume elements (RVEs) required for calculating phase volume fraction, Young's modulus and Poisson’s ratio were determined. The predicted volume fraction of Cu and Young's modulus were found to be within 2.6%, and 3.6% of the respective experimentally determined values. The minimum RVE size is found to be dependent on the material property. The variability in the required RVE size must be considered for material properties of W-Cu and similar bi-continuous composite microstructures.

On the bicontinuous microstructure induced by a guest protein in a typical AOT microemulsion

The ternary microemulsion AOT/water/isoctane was investigated in the presence of the human serum albumin (HSA) protein as solubilized guest molecule by NMR relaxation and self-diffusion measurements. The analysis of the 23Na NMR relaxation rates of the counterions and the interpretation of the 1H relaxation and self-diffusion data of water in terms of the usual spherical water droplet model gave a strong and direct evidence for important modifications of the water-in-oil droplet microstructure in the presence of HSA. With decreasing the water/surfactant ( W S ) molar ratio, which implies a linear decrease of the water core, the system containing the protein shows a transition to a bicontinuous microstructure. Some protein residues are likely to contribute to the total interfacial area to retain the favored curvature of the AOT molecules located at the polar-apolar interface. At W S larger than 25, which corresponds to an average radius of the water core larger than 2.8 nm, the relaxation parameters and the self-diffusion coefficients are almost unaffected by the presence of HSA. Consequently, it can be suggested that HSA molecules are hosted in closed water domains, which are likely to assume an average spherical shape as well as the unfilled, AOT stabilised, water droplets.