Sérgio S. Funari

Liquid–crystalline aqueous clay suspensions

This article demonstrates the occurrence of a true isotropic/nematic transition in colloidal Brownian aqueous suspensions of natural nontronite clay. The liquid–crystalline character is further evidenced by polarized light microscopy and small-angle x-ray scattering experiments in the presence and absence of modest external magnetic fields. The complete phase diagram ionic strength/volume fraction then exhibits a clear biphasic domain in the sol region just before the gel transition in contrast with the situation observed for other swelling clays in which the sol/gel transition hinders the isotropic/nematic transition. Small-angle x-ray scattering measurements of gel samples reveal strong positional and orientational orders of the particles, proving unambiguously the nematic character of the gel and, thus, clearly refuting the still prevalent “house of cards” model, which explains the gel structure by means of attractive interactions between clay platelets. Such order also is observed in various other swelling clay minerals; therefore, this very general behavior must be taken into account to reach a better understanding of the rheological properties and phase behavior of these systems.

Aqueous suspensions of natural swelling clay minerals. 1. Structure and electrostatic interactions.

In this article, we present a general overview of the organization of colloidal charged clay particles in aqueous suspension by studying different natural samples with different structural charges and charge locations. Small-angle X-ray scattering experiments (SAXS) are first used to derive swelling laws that demonstrate the almost perfect exfoliation of clay sheets in suspension. Using a simple approach based on geometrical constraints, we show that these swelling laws can be fully modeled on the basis of morphological parameters only. The validity of this approach was further extended to other clay data from the literature, in particular, synthetic Laponite. For all of the investigated samples, experimental osmotic pressures can be properly described by a Poisson-Boltzmann approach for ionic strength up to 10(-3) M, which reveals that these systems are dominated by repulsive electrostatic interactions. However, a detailed analysis of the Poisson-Boltzmann treatment shows differences in the repulsive potential strength that are not directly linked to the structural charge of the minerals but rather to the charge location in the structure for tetrahedrally charged clays (beidellite and nontronites) undergoing stronger electrostatic repulsions than octahedrally charged samples (montmorillonites, laponite). Only minerals subjected to the strongest electrostatic repulsions present a true isotropic to nematic phase transition in their phase diagrams. The influence of ionic repulsions on the local order of clay platelets was then analyzed through a detailed investigation of the structure factors of the various clay samples. It appears that stronger electrostatic repulsions improve the liquidlike positional local order.

Multi- to unilamellar transitions in catanionic vesicles.

Sodium dodecylsulfate (SDS) and cetyltrimethylammonium bromide (CTAB) dispersed in aqueous solution form catanionic vesicles. Depending on composition, such vesicles show different net charge, stability, and interaction capability, indicative of the strong impact that catanionic systems may have in gene therapy and drug delivery technologies. To reveal the interplay among composition, net charge, sensitivity to temperature changes, vesicle size, and inner structure, a series of experiments on catanionic vesicles prepared at different SDS/CTAB mole ratios was performed. Dynamic light scattering, small-angle X-ray scattering, and zeta-potential experiments allow one to characterize an unexpected critical phenomenon at the nanoscale level. On heating, vesicles increase in size, but at a critical temperature an abrupt vesicle size reduction has been observed, together with a transition from multi- to a unilamellar state. The critical temperature regularly depends on the SDS/CTAB mole ratio. The unilamellar state obtained upon heating is retained for weeks. These phenomena suggest a new way to produce stable unilamellar vesicles with tunable size and charge.

Inhomogeneous fibril stretching in antler starts after macroscopic yielding: indication for a nanoscale toughening mechanism.

Antler is a unique mineralized tissue, with extraordinary toughness as well as an ability to annually regenerate itself in its entirety. The high fracture resistance enables it to fulfill its biological function as a weapon and defensive guard during combats between deer stags in the rutting season. However, very little is quantitatively understood about the structural origin of the antlers high toughness. We used a unique combination of time-resolved synchrotron small angle X-ray diffraction together with tensile testing of antler cortical tissue under physiologically wet conditions. We measured the deformation at the nanoscale from changes in the meridional diffraction pattern during macroscopic stretch-to-failure tests. Our results show that on average fibrils are strained only half as much as the whole tissue and the fibril strain increases linearly with tissue strain, both during elastic and inelastic deformation. Most remarkably, following macroscopic yielding we observe a straining of some fibrils equal to the macroscopic tissue strain while others are hardly stretched at all, indicating an inhomogeneous fibrillar strain pattern at the nanoscale. This behavior is unlike what occurs in plexiform bovine bone and may explain the extreme toughness of antler compared to normal bone.

Intrafibrillar plasticity through mineral/collagen sliding is the dominant mechanism for the extreme toughness of antler bone

The inelastic deformability of the mineralised matrix in bones is critical to their high toughness, but the nanoscale mechanisms are incompletely understood. Antler is a tough bone type, with a nanostructure composed of mineralised collagen fibrils ∼100nm diameter. We track the fibrillar deformation of antler tissue during cyclic loading using in situ synchrotron small-angle X-ray diffraction (SAXD), finding that residual strain remains in the fibrils after the load was removed. During repeated unloading/reloading cycles, the fibril strain shows minimal hysteresis when plotted as a function of tissue strain, indicating that permanent plastic strain accumulates inside the fibril. We model the tensile response of the mineralised collagen fibril by a two - level staggered model - including both elastic - and inelastic regimes - with debonding between mineral and collagen within fibrils triggering macroscopic inelasticity. In the model, the subsequent frictional sliding at intrafibrillar mineral/collagen interfaces accounts for subsequent inelastic deformation of the tissue in tension. The model is compared to experimental measurements of fibrillar and mineral platelet strain during tensile deformation, measured by in situ synchrotron SAXD and wide-angle X-ray diffraction (WAXD) respectively, as well as macroscopic tissue stress and strain. By fitting the model predictions to experimentally observed parameters like the yield point, elastic modulus and post-yield slope, extremely good agreement is found between the model and experimental data at both the macro- and at the nanoscale. Our results provide strong evidence that intrafibrillar sliding between mineral and collagen leads to permanent plastic strain at both the fibril and the tissue level, and that the energy thus dissipated is a significant factor behind the high toughness of antler bone.

The hypotensive drug 2-hydroxyoleic acid modifies the structural properties of model membranes

We studied the interactions of the hypotensive drug, 2-hydroxyoleic acid (2OHOA), with model membranes using the techniques of DSC, 31P NMR and X-ray diffraction. We demonstrate that 2OHOA alters the thermotropic behaviour of 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine (DEPE), thereby promoting the formation of hexagonal phases (HII), despite stabilizing the lamellar phase (Lα). The lattice parameters of lamellar and non-lamellar structures were not altered by the presence of 2OHOA. The molecular bases underlying the alterations in membrane structure provoked by 2OHOA were analysed by comparing the effects produced by 2OHOA with the closely related fatty acids (FAs), oleic acid (OA) and elaidic acid (EA). The capacity of C-18 FAs to induce HII-phase formation followed the order OA>2OHOA>EA. Furthermore, while 2OHOA stabilized the Lα phase, OA destabilized it. The net negative charge of 2OHOA at physiological pH (∼7.4) influenced its effect on membrane structure. By analysing the molecular architecture of 2OHOA in DEPE monolayers, interactions between the carboxylate groups of 2OHOA and the amine groups of DEPE were observed, as well as between the 2-hydroxyl group of the FA and the carbonyl oxygen of the phospholipid acyl chain. These structural characteristics provoked an increase in the P-to-N and P-to-P distances of neighbouring phospholipid headgroups in the presence of 2OHOA, with respect to those observed with OA and EA. The higher headgroup area at the lipid–water interface in presence of 2OHOA could account for the differential effect of this drug on the phase behaviour of DEPE membranes.

Nanostructure of Nafion membrane material as a function of mechanical load studied by SAXS

The nanostructure of a perfluorinated membrane material (Nafion 117 by DuPont) is investigated as a function of strain and load by smallangle X-ray scattering (SAXS) at a synchrotron source. Two-dimensional SAXS patterns are evaluated utilizing the multi-dimensional chord distribution function (CDF). Anisotropy of the extruded material is considered. Both the ionomer domain and matrix polymer nanostructure are studied. For the neat material the classical ionomer domain model (domains as inverted micellae interconnected by channels) is confirmed and refined. Matching the plastic deformation behavior of the material, the domain structure in the relaxed and in the elongated state are found to be very similar. During elongation, ionomer channels open to form hollow ionomer layers(‘slits’) that are oriented parallel to the strain with a thickness of 1.9 nm and a long period of 3.8 nm. The slit height increases from 3 nm at elongation 1 ¼ 0:5 to 6 nm at 1 ¼ 1:25; whereas the slit width decreases to 1.5 nm. The ultimate structure is characterized by ensembles of not more than three slits that are in good lateral register. In the polymer matrix during elongation, cylindrical crystallites with a thickness of 2.5 nm and a most probable height of 7 nm are disrupted and parallelized with respect to the straining direction. The ultimate structure before sample failure is characterized by a broad domain height distribution ranging from a most probable domain height of 4 nm with a corresponding ultimate inclination of 408 to some perfectly parallelized domains of 20 nm height. q 2003 Elsevier Science Ltd. All rights reserved.

Mechanical properties of cellulose fibres and wood. Orientational aspects in situ investigated with synchrotron radiation

Highly oriented native cellulose fibres (flax) and softwood (pine) have been investigated by means of X-ray diffraction. Local structural information was obtained by using X-ray microbeams. Tensile tests were performed in situ, revealing a change of orientation of cellulose microfibrils in materials with tensile strain. In flax fibres, the microfibrils rotate during the first percent of stretching, into a more parallel orientation with respect to the fibre axis. For wood, a decrease of orientation with the onset of strain hardening is found for the first time.

Mediterranean-Style Diet Effect on the Structural Properties of the Erythrocyte Cell Membrane of Hypertensive Patients: The Prevencion con Dieta Mediterranea Study

A currently ongoing randomized trial has revealed that the Mediterranean diet, rich in virgin olive oil or nuts, reduces systolic blood pressure in high-risk cardiovascular patients. Here, we present a structural substudy to assess the effect of a Mediterranean-style diet supplemented with nuts or virgin olive oil on erythrocyte membrane properties in 36 hypertensive participants after 1 year of intervention. Erythrocyte membrane lipid composition, structural properties of reconstituted erythrocyte membranes, and serum concentrations of inflammatory markers are reported. After the intervention, the membrane cholesterol content decreased, whereas that of phospholipids increased in all of the dietary groups; the diminishing cholesterol:phospholipid ratio could be associated with an increase in the membrane fluidity. Moreover, reconstituted membranes from the nuts and virgin olive oil groups showed a higher propensity to form a nonlamellar inverted hexagonal phase structure that was related to an increase in phosphatidylethanolamine lipid class. These data suggest that the Mediterranean-style diet affects the lipid metabolism that is altered in hypertensive patients, influencing the structural membrane properties. The erythrocyte membrane modulation described provides insight in the structural bases underlying the beneficial effect of a Mediterranean-style diet in hypertensive subjects.

Structure of free Thermus flavus 5 S rRNA at 1.3 nm resolution from synchrotron X-ray solution scattering.

The shape of free Thermus flavus 5 S rRNA in solution at 1.3 nm resolution is restored from synchrotron x-ray scattering data using an ab initio simulated annealing algorithm. The free 5 S rRNA is a bent elongated molecule displaying a compact central region and two projecting arms, similar to those of the tRNA. The atomic models of the 5 S rRNA domains A-D-E and B-C in the form of elongated helices can be well accommodated within the shape, yielding a tentative model of the structure of the free 5 S rRNA in solution. Its comparison with the recent protein-RNA map in the ribosome (Svergun, D. I., and Nierhaus, K. H. (2000) J. Biol. Chem. 275, 14432–14439) indicates that the 5 S rRNA becomes essentially more compact upon complex formation with specific ribosomal proteins. A conceivable conformational change involves rotation of the B-C domain toward the A-D-E domain. The model of free 5 S rRNA displays no interactions between domains E and C, but such interactions are possible in the bound molecule.