Hard and soft proteins at fluid interfaces

Challenges of public health and sustainable development require replacing in food products animal proteins by plant proteins. In this optics, it is crucial to understand the structure and kinetic of formation of a film of plant proteins in order to improve the control of emulsions and foams stabilized by these proteins.In this talk we will present experimental results on the behaviour interfacial properties of wheat gluten, sunflower and rapeseed proteins at liquid interfaces. Thanks to a combination of tensiometry, dilatational rheology and ellipsometry, rational physical pictures of the dynamics of the interfacial properties are achieved, for the various proteins and at both air/water and oil/water interfaces. For gluten proteins, a time-concentration superposition of the data is evidenced whatever the subphase concentration, which reveals that the kinetics of protein adsorption at the interface is dominated by bulk diffusion. We propose a consistent physical picture of the multistep diffusion-controlled irreversible adsorption of the gliadin proteins at an air/water interface, and evidence surface-induced conformational changes of the proteins followed by film gelation [1]. Sunflower and rapeseed proteins by contrast do not reorganize once adsorbed at an interface and display a simpler dynamics of film formation. In addition the failure at high concentration of the time-concentration superposition of the tensiometry and viscoelastic data strongly suggest a surface-induced aggregation process, which we confirm with turbidity measurements. By quantitatively comparing the surface pressure dependence viscoelasticity of the various interfaces, we hightlight the crucial role on the behavior of plant proteins at liquid interfaces of the solvent quality and of the protein softness, that is discussed in regard to the protein structure.

Ordered mesoporous silica materials were prepared under different pH conditions by using a silicon alkoxide as a silica source and polyion complex (PIC) micelles as the structure-directing agents. PIC micelles were formed by complexation between a weak poly-acid-containing double-hydrophilic block copolymer, poly(ethylene oxide)-b-poly(acrylic acid) (PEO-b-PAA), and a weak poly-base, oligochitosan-type polyamine. As both the micellization process and the rate of silica condensation are highly dependent on pH, the properties of silica mesostructures can be modulated by changing the pH of the reaction medium. Varying the materials synthesis pH from 4.5 to 7.9 led to 2D-hexagonal, wormlike or lamellar mesostructures, with a varying degree of order. The chemical composition of the as-synthesized hybrid organic/inorganic materials was also found to vary with pH. The structure variations were discussed based on the extent of electrostatic complexing bonds between acrylate and amino functions and on the silica condensation rate as a function of pH.

A versatile and eco-compatible preparation route of functional ordered mesoporous materials will be described. It relies on the use of assemblies of hydrophilic diblock copolymers, in place of classical amphiphilic templates, as new structuring and functionalizing agents for the direct synthesis, at room temperature and in water, of polymer-functionalized mesoporous silica.The new structuring agents are dynamic PolyIon electrostatic Complex (PIC) micelles formed between double-hydrophilic-block-copolymers (DHBC) (constituted of a neutral block and an ionizable one) and an oppositely charged homopolyelectrolyte, which can be considered as an auxiliary of micellization of DHBC polymers.The different advantages of using PIC micelles are the following: due to their sensitivity to physicochemical parameters, PIC micelles allowed obtaining various hybrid mesostructures (cubic, 2D-hexagonal, lamellar...) by simply adjusting the material synthesis conditions using a unique DHBC/homopolyelectrolyte system. This is a consequence of the adjustable composition and water content of the polyion complex phase. The micellization process, which can be controlled by pH or ionic strength, is reversible in water and this allowed the recovery and recycling of the porogen polymers.Very interestingly, the use of PIC micelles permitted the direct preparation of polymer-functionalized mesopores. This was possible due to the easy dissociation of the polyion complex within the pores, selectively releasing the auxiliary of micellization while the DHBC remained for functionalizing the pores. This new method ensures an homogeneous functionalization of the mesopores, a control of the functional polymer chain lengths and a sufficient mesoporous volume. Moreover, the properties of the functional chains could be varied (acid, basic, metal-complexing...). Finally, this preparation route was extended to bio-functional hybrid mesophases useful as drug delivery systems [3] The direct formation of drug-loaded polymer-functionalized mesoporous materials was developed by using drug-polymer complex micelles as structuring agents ; this was shown with antibiotics exhibiting a pH-triggered release. Furthermore, studies are currently under way to estimate the catalytic performances of the new functional materials.

Double hydrophilic block copolymers (DHBC) in the presence of multivalent oppositely charged ions form polyion complexes (PIC) in the micellar form. We use PIC micelles as an ecofriendly structuring agents for templating ordered mesoporous materials [1,2] : after concomittant micellization and silica condensation, the porosity of the final material is revealed by simple elution in water and the polyions can be recycled in successive synthesis batches. Various mesoporous silica materials have been obtained from cubic to lamellar, although lyotropic behavior of PIC micelles on their own has seldom been reported if ever. Moreover, the control over the structure of the final material is not solely determined by the asymmetry ratio of the diblock copolymer : a single PIC system yields different mesostructured silica materials depending on the physico-chemical conditions. In an attempt to address the relation between the mesoporous material structure and the lyotropic behavior of these new templating agents, our attention was brought to the role of water which is actually a prominent constituent of PIC micelles.PIC micelles of PEO-b-PAA/oligoamines have been studied. The water content of the core of the micelles has been addressed by establishing the complete phase diagram of the polyelectrolyte complex PAA/oligoamine in its coacervate form. It amounts to 50%, varies non-monotonously with concentration and can be tuned by ionic strength, pH. Shape transition and phase transition are revealed by swelling laws of the correlation peak position obtained from X-ray scattering. From light scattering and osmometry we establish the equation of state  of a PEO-b-PAA/oligoamine electrostatic complex. Its lyotropic behavior is characterized by a transition of shape of the micelles at =0.16 and a transition to lamellar phase at =0.30 corresponding to various levels of deswelling of the corona and of the core.This study evidences the high compressibility of the core of the micelles as opposed to the core of amphiphilic copolymer micelles. This feature probably explains why lyotropic behavior of PIC micelles is less rich than the one of amphiphilic micelles. But it also brings versatility in the use of PIC micelles as templating agents for mesoporous materials.

Upon pH stimulus, double hydrophilic block copolymers form polyion complexes (PIC) in the micellar form. We used PIC micelles as ecofriendly templates for the synthesis of ordered mesoporous materials. The porosity of the final material is revealed by simple inversion of the pH stimulus. Various mesoporous silica materials have been obtained from cubic to lamellar.The control over the structure of the final material is expected to relate to the lyotropic behavior just as observed when micelles of amphiphilic molecules are used as template. However such lyotropic behavior has never been reported for PIC micelles. Moreover the same PIC leads to different structured silica materials depending on the physico-chemical conditions.Using X-rays and light scattering, optical microscopy and osmometry, we establish the equation of state (osmotic pressure vs volume fraction) of a PEO-b-PAA/oligoamine electrostatic complex. Its lyotropic behavior is characterized by a transition of shape at 0.16 w/w and a transition to a lamellar phase at 0.30w/w. The water content of the core of the micelles is addressed from the complete phase diagram of the polyelectrolyte complex in its coacervate form and explains why a single complex leads to porous material of different structure.