One-shot synthesis of a poly(N-isopropylacrylamide)/silica hybrid gel.

Class I hybrid poly(N-isopropylacrylamide)/silica hydrogels, PNIPAM/SiO2, were prepared by a new one shot synthesis. In this approach, the free-radical polymerization of vinyl groups of N-isopropylacrylamide (NIPAM) and the hydrolysis-condensation of alkoxy groups of tetramethoxysilane (TMOS) are performed concomitantly using sodium persulfate and 3-(dimethylamino)-propionitrile, a well-known couple to initiate the organic polymerization. The cross-linker is N,N-methylenebisacrylamide. The kinetic study of mechanical properties from the sol-to-gel state for different ratios of TMOS/NIPAM was investigated by rheological ultrasonic measurements. The thermoresponse of hybrid materials was investigated by differential scanning calorimetry and the measurements showed that hybrid gels present a lower critical solution temperature, which is similar with one of single organic hydrogel.

Unfolding and refolding of bovine serum albumin at acid pH: ultrasound and structural studies.

Serum albumin is the most abundant protein in the circulatory system. The ability of albumins to undergo a reversible conformational transition, observed with changes in pH, is conserved in distantly related species, suggesting for it a major physiological role possibly related to the transport of small molecules including drugs. We have followed changes of bovine serum albumin (BSA) in volume by densimetry and in adiabatic compressibility during its conformational transition from pH 7-2, using ultrasound measurements. In parallel, circular dichroism was measured. The volume and adiabatic compressibility decrease from pH 4 to 2. The change in ellipticity shows a decrease over the same pH range from 70% to 40% of its alpha-helix content. Sorbitol, at concentrations from 0 to 2 M, led to the progressive restoration of BSA volume and compressibility values, as well as a substantial recovery of its original alpha-helix content. This finding implies that the compressibility variation observed reflects the conformational changes during the transition. The mutual interactions of the mechanical properties and structural features of BSA reported here are important in biotechnology for research in material sciences and for the design and the development of new, tailor-made drug carriers.

Lamb mode reflections at the end of a plate loaded by a viscoelastic material.

This paper deals with a study of the conversion of a Lamb wave at the loaded edge of a plate. An experimental study is performed to know the interaction of surface waves with a viscoelastic material. A Lamb wave is generated by a wedge. At the end of the plate, this wave gives rise to several reflected waves. A first study makes it possible qualitatively to know the Lamb modes present in reflection. Secondly, conversion phenomena are studied to know if surface waves are sensitive to evolution in the time of the viscoelastic material structure. The selected material is a sol-gel containing silica.

Ultrasonic monitoring of yoghurt formation by using AT-cut quartz: lighting of casein micelles interactions process during the acidification.

The behavior of weak gels during their formation singularly attracts attention of dairy products factories. In our study we investigate acidified pre-heated milk gels formation that are fairly often used to product yoghurts. The gel formation requires a tight control of the first step of micelles modification process and the kinetics reaction parameters. The most current rheological parameters used to achieve the monitoring are the storage G' and the loss G'' shear moduli and the gelation time. The study of these parameters is commonly performed at very low frequencies (1 Hz). Our technique uses a 6 MHz AT-cut quartz crystal immersed in an acidified milk solution kept at a constant temperature. This method is singularly effective to ensure a complete and a reliable follow-up of the viscoelastic parameters of casein gels. A suitable new model enables a complete follow-up of the micelles evolution from the viscoelastic properties. The experimental results of the G' and G'' moduli versus temperature and versus glucono-delta-lactone (GDL) added to milk are analyzed. In order to understand the micelles modifications, an analysis of the viscoelastic evolution try to explain the validity of the various models of micelles modification. In addition a new accurate kinetics characteristic time is proposed. This time corresponds to the moment for which the elastic effect of material becomes significant. From the kinetics study of casein gels at various temperatures, the Arrhenius relationship and a modified Flory-Stockmayer relationship give us access to the activation energy. By using the proposed technique and the suitable models developed, the structure thus quality of dairy products may be better controlled.

Kinetic study of silica gels by a new rheological ultrasonic investigation.

The last decades have seen the development of sol-gel (SG) process currently used to develop new materials in a wide range of scientific applications. The SG process leads to an oxide macromolecular network through a sol (liquid phase) to gel transition. To optimize this process, the control of the kinetic of the chemical reaction is required. This kinetic can be deduced from the temporal evolution of the viscoelastic parameters. Upto date no complete investigation during the SG formation can be achieved by a unique non-destructive technique. In this paper, we present an ultrasonic technique to measure the viscoelastic parameters (storage G' and loss G'' shear moduli) of the gel material during its formation. By using a suitable model which takes into account the mass loading on the surface, the viscoelastic parameters of these materials are accurately deduced. In order to study the efficiency of this technique, silica gels transition is monitored at various formation temperatures and for different initial hydrolysis molar ratio (h). In addition, the monitoring is performed at different oscillatory shear measurements in the 6-54 MHz frequency range to determine a new characteristic time t(vs) corresponding to the moment when the material is no more a newtonian liquid. This characteristic time is then compared to the gelation time t(g) determined by rheological or acoustic audible range methods. Thus the new characteristic time is also a good criterion to characterize earlier the SG matrix transition. Our AT-cut quartz technique using our model can also be used as a high frequency rheometer for the sol-gel materials.

Ultrasonic studies of alcohol-induced transconformation in beta-lactoglobulin: the intermediate state.

In mixed alcohol-water solvents, bovine beta-lactoglobulin undergoes a cooperative transition from beta-sheet to a high alpha-helix content conformer. We report here the characterization of beta-lactoglobulin by compressibility and spectroscopy measurements during this transconformation. Both the volume and compressibility increase as a function of alcohol concentration, up to maximal values which depend on the chemical nature of the three alcohols used: hexafluoroisopropanol, trifluoroethanol, and isopropanol. The order of effectiveness of alcohols in inducing the compressibility transition is identical to that previously reported for circular dichroism and thus independent of the observation technique. The highly cooperative sigmoidal curves found by compressibility determination match closely those obtained by circular dichroism at 222 nm, indicating a correlation between the two phenomena measured by the two different techniques. The presence of an equilibrium intermediate form was shown by the interaction of beta-lactoglobulin with 8-anilino-1-naphthalene sulfonic acid, a probe widely used to detect molten-globule states of proteins. It was correlated with the plateau region of the volume curves and with the inflexion points of the sigmoidal compressibility curves. Ultrasound characterization of proteins can be carried out in optically transparent or nontransparent media.

Model and measurements of the electrical input impedance of a plate-liquid-plate acoustic resonator.

A plate-liquid-plate (PLP) acoustic resonator is used to measure the acoustic velocity of liquids. An analytical model of the electrical input impedance of the resonator is proposed. Theoretical results are compared with experimental measurements using reference liquids. Accuracy of the sound velocity calculation is found to be better than 1%. Finally, factors limiting the achievable accuracy are discussed.

Hydration and protein folding in water and in reverse micelles: compressibility and volume changes.

The partial specific volume and adiabatic compressibility of proteins reflect the hydration properties of the solvent-exposed protein surface, as well as changes in conformational states. Reverse micelles, or water-in-oil microemulsions, are protein-sized, optically-clear microassemblies in which hydration can be experimentally controlled. We explore, by densimetry and ultrasound velocimetry, three basic proteins: cytochrome c, lysozyme, and myelin basic protein in reverse micelles made of sodium bis (2-ethylhexyl) sulfosuccinate, water, and isooctane and in aqueous solvents. For comparison, we use beta-lactoglobulin (pI = 5.1) as a reference protein. We examine the partial specific volume and adiabatic compressibility of the proteins at increasing levels of micellar hydration. For the lowest water content compatible with complete solubilization, all proteins display their highest compressibility values, independent of their amino acid sequence and charge. These values lie within the range of empirical intrinsic protein compressibility estimates. In addition, we obtain volumetric data for the transition of myelin basic protein from its initially unfolded state in water free of denaturants, to a folded, compact conformation within the water-controlled microenvironment of reverse micelles. These results disclose yet another aspect of the protein structural properties observed in membrane-mimetic molecular assemblies.

Adiabatic compressibility of AOT

The self-assembly of amphiphilic molecules into supramolecular aggregates involves a number of complex phenomena and forces. Recent developments of highly sensitive, densimetric and acoustic methods on small volume samples have provided novel sensitive probes to explore the physical properties of these complex fluids. We have investigated, by high precision densimetry and ultrasound velocimetry, reverse micelles of [sodium bis(2-ethylhexyl)sulfosuccinate] in oil (isooctane and decane), at increasing water concentration and at variable micellar volume fractions. The size of these spherical micelles has been determined by small angle x-ray scattering. Using these results, in the framework of the effective medium theory, we have developed a simple model of micellar compressibility, allowing the calculation of physical parameters (aggregation number, volume, and compressibility) of the surfactant monomolecular film as well as that of the micellar waters. In particular, we show that the central aqueous core designated as "free" water, located at a distance from the oil-water interacting interface, is twice as compressible as "bulk" water. One notable feature of this work is the influence of the nature of the oil on the above parameters.