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The first example of complex coacervation between a biopolymer and polyoxometalate clusters is identified in the gelatin-decavanadate system.

This chapter is an introduction to polymeric gels. The chemical and physical characteristics of this colloidal state of matter are outlined. The specific properties of the different types of gels are briefly reported, and the derived potential applications are displayed at the end of the chapter. The most promising ones are linked to biomedical applications.

This paper explores structural, interfacial and thermal properties of two types of Pickering emulsions containing alpha-cyclodextrin inclusion complexes: on one hand, emulsions were obtained between aqueous solutions of alpha-cyclodextrin and different oils (fatty acids, olive oil, silicone oil) and on the other hand, emulsions were obtained between these oils, water and micro or nano-platelet suspensions with inclusion complexes of hydrophobically-modified polysaccharides. The emulsions exhibit versatile properties according to the molecular architecture of the oils. Experiments were performed by microcalorimetry, X-ray diffraction and confocal microscopy. The aptitude of oil molecules to be threaded in alpha-cyclodextrin cavity is a determining parameter in emulsification and thermal stability. The heat flow traces and images showed dissolution, cooperative melting and de- threading of inclusion complexes which take place progressively, ending at high temperatures, close or above 100{\deg}C. Another important feature observed in the emulsions with micro-platelets is the partial substitution of the guest molecules occurring at room temperature at the oil/water interfaces without dissolution, possibly by a diffusion mechanism of the oil. Accordingly, the dissolution and the cooperative melting temperatures of the inclusion crystals changed, showing marked differences upon the type of guest molecules. The enthalpies of dissolution of crystals were measured and compared with soluble inclusions.

This work reports on water desorption from cassava starch in relation with the structure and conditioning of granules in suspensions or after equilibration in desiccators. The experimental work is performed by thermogravimetric analysis with isothermal and non-isothermal protocols and interpreted to derive the activation energies and desorption frequencies according to the humidity range with no adjustable parameter. The analysis points out the different types of water interacting with the starch granules and relates the drying coefficients to their microscopic structure. The work helps clarifying contradictory and partial results from the literature.

The effect of Laponite (a synthetic clay) on the structure, thermal and water vapor barrier properties of nanocomposite gelatin films produced by casting with 0, 4.5 and 15% w Lap/w gelatin, was studied. X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and Fourier transform infrared spectroscopy measurements were reported. The X-ray diffraction patterns showed dual crystalline structure of the films with collagen-type bundles of triple helices, intercalated inside clay platelets, increasing interlayer distances. Depending on the renaturation of triple-helices and Laponite content, the glass transition temperatures substantially decreased. The amount of helices decreased with Lap concentration, affecting the enthalpy of melting. The nanocomposite gelatin films showed improved thermal stability. Changes of water vapor permeability could be related to the presence of larger free volume of the coils and intercalated structures, facilitating water transfer through the film., 43 pages, 5 figures, 4 tables, supplementary information

Cyclodextrins (CD) are a family of oligosaccharides with a toroid shape, which exhibit a remarkable ability to include guest molecules in their internal cavity providing a hydrophobic environment for poorly soluble molecules. Recently new types of inclusions of alpha CD with alkyl grafted polysaccharide chains (pullulan, chitosan, dextran, amylopectin, chondroitin sulfate) have been prepared which are auto assembled into micro and nano platelets. We report in this paper an extensive investigation of platelets with different compositions, including their reversible hydration (Thermo Gravimetric Analysis), crystalline structure (Powder X Rays Diffraction), dimensions and shapes, (Scanning Electron Microscopy Field Emission Gun), thermal properties, solubility and melting (Micro Differential Scanning Calorimetry). The crystalline platelets exhibit layered structures intercalating the polysaccharide backbones and CD complexes hosting the grafted alkyl chains. The monoclinic symmetry of columnar type crystals suggests a head to tail arrangement of the CDs. The platelets have a preferentially hexagonal shape with sharp edges, variable sizes, and thicknesses and sometimes show incomplete layers (terraces). The crystal parameters change upon dehydration. Melting temperatures of platelets in aqueous solutions exceeds 100 degree. Finally, we discuss the potential relation between the platelet structure and applications for mucoadhesive devices.

This paper investigates water sorption and dehydration of a model synthetic clay (Laponite) and highlights the large differences with natural clays (montmorillonite, hectorite) belonging to smectite family of structure and composition. Measurements were done by combining for the first time thermogravimetric analysis and X Ray diffraction methods on Laponite powders stored in atmospheres with controlled relative humidity. Laponite exhibits a very good reproducibility of sorption diagram and appears as the most hygroscopic among smectites. The interlayer space increases from 11 to 20 Angstrom when r = n H2O /n Na increases from 2 to 22 in agreement with recent numeric simulations by Morrow et al. We show that the desorption process, for 2 < r < 22, in isothermal conditions follow a first order kinetic identifying two regimes around r* = 7 which is close to the coordination number of Na in concentrated NaCl solution. Activation energies and frequency factors are derived for the two regimes of water desorption and discussed in relation with stability of the cationic complex.

This paper analyzes the drying rates of humid porous grains of poly(vinyl)chloride PVC by thermogravimetry. Grains have variable volume fractions of pores, representing between 16 and 33% g water/g PVC, with pore sizes varying between 0.6 and 1 μm. It is shown that the humid cakes exhibit three different drying rate regimes at constant temperature, characterizing evaporation of free water, of interstitial water and of water inside the pores. The constant rate period (CRP) and the falling rate period (FRP) of drying are clearly identified. The drying rates and drying times are presented in adimensional units by comparison with evaporation of pure water. The thermogravimetric analysis identifies a fraction of water which dries very slowly inside the pores. A method of quantifying the strongly bound water is presented. Numerical simulations of water evaporation were performed on a 2D array of channels and illustrate the contribution of geometrical effects (diameter of channels, tortuosity, etc.) in pore drying. Visual observations of the drying of droplets of solutions containing dispersants used in PVC synthesis show interesting patterns. The phase separated solutions of dispersants are analyzed and their role in drying is highlighted.

Concentration of aqueous suspensions of solid particles (called slurries in industrial processes) is achieved by centrifugation at high accelerations and it is an important step in dry powder productions; dehydration precedes drying. In aqueous suspensions of porous particles, water is both the interstitial fluid, which disperses the particles and the imbibition fluid, which fills the pores inside particles. This is the case in the pastes of poly(vinyl)chloride (PVC) polymerized in suspensions after centrifugation. PVC grains are non-colloidal particles with diameters close to 150 μm and variable inner porosity, which are synthetized in aqueous solutions using dispersants such as various poly(vinyl)alcohols (PVA). In this paper we determine the dehydration by centrifugation of different grades PVC suspensions with laboratory scale experiments. It is shown that the humid pastes reach a pendular state at high accelerations and that the compaction of the grains and their surface properties determine the final retention of the water by capillary forces. In such conditions, internal water, inside the pores of the grains can be eliminated only by evaporation. Drying was investigated in controlled relative humidity atmospheres ( RH ) in desiccators by measuring the equilibrium moisture content of the grains and the evaporation rates. The evaporation rate of the superficial water is similar to pure water and can be interpreted using Stefan's equation, whereas substantial differences exist between the total drying times of grades.

The use of nonstandard geometries like the vane is essential to measure the rheological characteristics of complex fluids such as non-Newtonian fluids or particle dispersions. For this geometry which is of Couette type, there is no analytical simple model defining the relation between the shear stress and the torque or relating the angular velocity to the shear rate. This study consists on calibrating a nonstandard vane geometry using a finite volume method with the Ansys Fluent software. The influence of geometrical parameters and rheological characteristics of the complex fluids are considered. First, the Newtonian fluid flow in a rotative vane geometry was simulated and a parametric model is derived therefrom. The results show an excellent agreement between the calculated torque and the measured one. They provide the possibility to define equivalent dimensions by reference to a standard geometry with concentric cylinders where the relationships between shear stress (resp. shear rate) and the torque (resp. the angular rotation) are classical. Non-Newtonian fluid flows obeying a power law rheology with different indices were then simulated. The results of these numerical simulations are in very good agreement with the preceding Newtonian-based model in some ranges of indices. The absolute difference still under 5 % provided the index is below 0.45. Finally, this study provides a calibration protocol in order to use nonstandard vane geometries with various heights, gaps, and distance to the cup bottom for measuring the rheology of complex fluids like shear thinning fluids and concentrated suspensions.

Micellization and gelation of binary mixtures of EO99PO69EO99 (pluronic® F127) and EO80PO27EO80 (pluronic® F68) in aqueous solutions were investigated by means of micro-differential scanning calorimetry and rheology and for a total copolymer concentration fixed at 20 wt%. The aim of this investigation is to determine the interplay between micellization and macroscopic gelation of the mixed solutions. Micro-DSC reveals the formation of two distinct populations in F127/F68 mixtures during heating and subsequent cooling of the solutions. The enthalpies of micellization of each copolymer and the respective onset temperatures remained constant after mixing indicating the predominance of two independent processes of micellization in the mixtures. The F127 exhibits a crystallization transition, at a distinct temperature which persists, but increases in the mixtures with concentrations higher than 10 wt%. Rheological measurements were performed during heating ramps or after maturation periods versus frequency. They showed two types of gelation transitions: either a steep increase of the storage and the loss moduli, which corresponds to the crystallization temperature of the F127 micelles or a progressive jamming transition when no crystal can form. Maturation process has a major effect on the rheological properties of the mixed gels, possibly related to local rearrangements of the two micellar phases. This investigation highlights the unique features of the binary pluronic® mixtures, compared to dilution effects of single component aqueous solutions.

The aim of the present work is to design a new formulation containing clotrimazole (CTZ) loaded into nanostructured lipid carriers (NLC) for the treatment of fungal vaginal infections. In order to obtain formulations with suitable viscosity for mucosal application, NLC containing CTZ produced by the ultrasonication method were viscosized by the addition of poloxamer P407 in the NLC dispersion (CTZ-NLC-gel). These systems exhibit well-known thermogelling properties. The rheological characterization of the CTZ-NLC hydrogel using a controlled stress rheometer evidenced that the presence of NLC or CTZ did not affect gelling temperature (Tgel). Dilution with simulated vaginal fluid (SVF) increased the Tgel from 17.4 to 29.6°C. For these thermogelling systems, micro-calorimetric assays conducted by a Micro-DSC III confirmed that the hydrogel-containing CTZ-NLC was able to change its structure with a rapid passage from non-crystalline (liquid) to crystalline (semi-solid) form. Furthermore, when a local application is considered, no drug should pass through the vaginal mucosa, limiting thus the systemic diffusion and toxicity. For this purpose, Franz cell has been employed to investigate the ex vivo permeation of CTZ through pig vaginal mucosa. The results showed no CTZ diffusion. The toxicological experiments performed on HeLa cells after a 24h incubation time confirmed that CTZ-NLC-gel at a concentration of 1mg/mL showed a low toxicity profile resulting in a cell vitality of 77.2%. Interestingly, anti-candida activity studies demonstrated that CTZ-NLC gel was 4-fold more active than Fungizone(®) against Candida albicans. These encouraging results suggest that the hydrogel containing CTZ-NLC could be proposed as an innovative system to administer CTZ to treat vaginal infections.

Primary cold production for the extra-heavy oils (4–10°API) of La Faja Petrolifera del Orinoco (FPO), Venezuela, is currently a low percentage (The objectives of this experimental study were to identify some mechanisms in play when surfactant and polymer solutions are injected in cores to displace extra-heavy oil and to assess for the potential of SP flooding for one of the FPO's reservoirs. The tests reported were performed with a dead crude oil of 9°API and 4500 cP, and injection water salinity of 6.4 g/L with low hardness and at a temperature of 50°C. The SP formulation consisted of a standard high molecular weight HPAM at rather high concentration to achieve high viscosity and an alkaline-free surfactant formulation providing both low interfacial tension (IFT) and good compatibility with polymer even at high polymer concentration. When possible, oil saturation profiles were determined by CT-scan at the main steps of the experiments.Conditions and methodologies to determine the relevant experimental parameters for high viscosity oil have firstly been developed. Then, a set of surfactant and polymer injection tests have been performed on Bentheimer outcrop cores. These tests demonstrated that injection of the SP formulation after a secondary polymer flood was able to achieve a significant reduction of the residual oil (ASo = 80% ROIP). Results of secondary injections of water (final oil saturation, Sofinal = 63%), surfactant solution (Sofinal = 39%) and SP formulation (Sofinal = 5%) have also shown that mobility control is of tremendous importance to achieve high recovery, even at the core-scale. The potential of the SP formulation has also been validated on unconsolidated reservoir rock material from the FPO (Sofinal = 8%). Relative permeabilities have also been determined to investigate the feasibility of an effective modeling of the impact of the surfactant on oil recovery without making any assumption of the local mechanisms in play. Future work will involve 3D reservoir simulation with physico-chemical parameters generated at the lab.

Primary cold production for the extra-heavy oils (4-10°API) of La Faja Petrolifera del Orinoco (FPO), Venezuela, is currently a low percentage ( The objectives of this experimental study were to identify the mechanisms in play when surfactant and polymer solutions are injected in cores to displace extra-heavy oil and to assess for the potential of SP flooding for one of the FPO's reservoir. The tests reported were performed with a dead crude oil of 9°API and 4500 cP, an injection water salinity of 6.4 g/L with low hardness and at a temperature of 50°C. The SP formulation consisted of a standard HPAM at rather high concentration to achieve high viscosity and an alkaline-free surfactant formulation providing both low interfacial tension (IFT) and good compatibility with polymer even at high polymer concentration. A set of surfactant and polymer injection tests has been performed on Bentheimer outcrop cores. These tests allowed demonstrate that injection of the SP formulation after a secondary polymer flood was able to achieve a significant reduction of the residual oil (ΔSo = 80% ROIP). Results of secondary injections of either water, surfactant solution or SP formulation have also shown that mobility control is of tremendous importance to achieve high recovery, even at the core-scale. Oil production data have been confirmed by determination of the CT-scan saturation profiles carried out at each step of the experiments. The results of this study stand as first insights demonstrating at core scale the potential of SP formulation to increase recovery of Venezuelan extra-heavy oils.

The initial step in mucosal infection by the herpes simplex virus type 2 (HSV-2) requires its binding to certain glycosaminoglycans naturally present on host cell membranes. We took advantage of this interaction to design biomimetic supramolecular hexagonal-shaped nanoassemblies composed of chondroitin sulfate having exalted anti-HSV-2 activity in comparison with native chondroitin sulfate. Nanoassemblies were formed by mixing hydrophobically-modified chondroitin sulfate with α-cyclodextrin in water. Optimization of alkyl chain length grafted on chondroitin sulfate and the ratio between hydrophobically-modified chondroitin sulfate and α-cyclodextrin showed that more cohesive and well-structured nanoassemblies were obtained using higher α-cyclodextrin concentration and longer alkyl chain lengths. A structure-activity relationship was found between anti-HSV-2 activity and the amphiphilic nature of hydrophobically-modified chondroitin sulfate. Also, antiviral activity of hexagonal nanoassemblies against HSV-2 was further improved in comparison with hydrophobically-modified chondroitin sulfate. This work suggests a new biomimetic formulation approach that can be extended to other heparan-sulfate-dependent viruses.

Plasticizing effects of water and glycerol are widely used in pharmaceutical applications of gelatin films. We investigated gelatin films containing water or water and glycerol equilibrated under a range of relative humidity. Films were characterized by differential scanning calorimetry running in classical and modulated modes. Glass and melting transition temperatures, relaxation and melting enthalpies and heat capacity changes were determined versus film composition and maturation time. This investigation quantifies the amount of collagen type structure through the melting enthalpy of triple helices. A unified phase diagram for glass and melting transition temperatures is presented as a universal curve for any proportion of water and glycerol in films versus a single parameter, which is the ratio of hydroxyl groups belonging to water or glycerol per amino-acid group and highlights the role played by the hydrogen bonding. The molecular mechanisms involved in the plasticizing effects are discussed.

To evaluate of the effect of size and surface characteristics of poly(isobutylcyanoacrylate) nanoparticles coated with pluronic F68 and thiolated chitosan on mucoadhesion. Nanoparticles were obtained by radical emulsion polymerization in presence of different amounts of F68 (0–4%w/v). Mucoadhesion was ex vivo evaluated by applying nanoparticle suspension on rat intestinal mucosa and quantifying the amount of attached nanoparticles after incubation. F68 unimers added in the polymerization medium allowed decreasing nanoparticle size from 251 to 83 nm, but resulted in nanoparticle surface modification. The amount of thiolated chitosan onto nanoparticle surface was decreased resulting in lower thiol groups and zeta potential. Consequently, the decrease of nanoparticle hydrodynamic diameter resulted in eight-fold-increase of the number of nanoparticles attached to the mucosa but a significant decrease of the weight of attached nanoparticles was observed. This unexpected result was due to a decrease of the amount of chitosan and thiolated chitosan available to interact with mucus upon addition of F68 in the polymerization medium. Addition of F68 should not be recommended to improve the amount of mucoadherent nanoparticles. Further studies could allow understanding if the low amount of small size nanoparticles could be able to improve oral bioavailability.

The aim of the present review is to give a concise analysis of the thermodynamic parameters obtained from isothermal titration microcalorimetry (ITC) experiments for the characterization of the self-organization of surfactants into micelles. This review is also focused on works describing some methods allowing to overcome ITC limitation and to extract accurate thermodynamic values from ITC data. Copyright © 2009 John Wiley & Sons, Ltd.

Micelle formation was followed by micro-DSC and rheology for aqueous solutions of two copolymers of PEO–PPO–PEO, the Pluronic F127 (from BASF) and the EG56 (from PolymerExpert), a branched copolymer built with three chains of F127 type. It is shown that micellization is endothermic and that, for both polymers, the enthalpy of formation/melting is proportional to total concentration. The rheology of the solutions was carefully analyzed, before gelation for F127, and it reveals firstly the progressive changes of solubility of the unimers (decease of relative solution viscosity), followed by micelle formation over a 10 °C range. In this range, the micelle concentration dependence on temperature was deduced from enthalpy measurements and the corresponding volume fractions were derived. Viscosity was interpreted within the framework of well-known theories for hard sphere suspensions (Krieger–Dougherty or Quemada) based on an analogy between micelles and nanosized hairy grain suspensions. The gel state is achieved due to formation of the colloidal crystal. For EG56, the rheology is quite different; as the aggregation increases with temperature, a progression is observed from Newtonian to visco-elastic liquid. The characteristic frequency, defined by the relation G ′ = G ″ , for EG56 varies with temperature and the corresponding times increase by two orders of magnitude according to an Arrhenius law. The frequency dependence of G ′ and G ″ at different temperatures can be superposed with a horizontal shift factor and a small amplitude adjustment. There is no elastic solid formation in this case. The “gelation” of these two copolymers is compared to the physical gelation of cold-set gels (gelatin).

Gelatin is widely used in capsules manufacturing. Most of the capsules in pharmaceutical applications are hard capsules made out of concentrated solutions of gelatin, where water has been progressively removed during the drying process. More recently soft capsules found an increasing interest in pharmaceutical and cosmetic applications where they are filled and sealed with a liquid substance. In order to keep the shells of capsules flexible after drying at room temperature, plasticizer is added to the gelatin aqueous solutions. We present in this paper a systematic investigation of gelatin films, equilibrated under a range of relative humidity (RH). The films contain glycerol as plasticizer P or only water and gelatin, (G). In order to analyze the role of the plasticizer, we fixed various P/G ratios and measured the water retention versus RH. Films were characterized by DSC (Mettler Toledo DSC823). Glass transition temperature T g , melting temperature T m and enthalpy associated with helix-coil transition were determined. The role of water and glycerol was examined in relation with the large variations of these transition temperatures with film composition. Non equilibrium effects are also discussed, in particular concerning the glass transition temperature, the relaxation effects and the water repartition between amorphous coils and helical structure. In conclusion, we propose a unique phase diagram of the gelatin films with any proportion of water and glycerol.

The influence of polyoxovanadate clusters ([H(2)V(10)O(28)](4-)) on the thermo-reversible gelation of porcine skin gelatin solution (type A, M w approximately 40 000 g.mol (-1), pH = 3.4isoelectric point (IEP) approximately 8) has been investigated as a function of temperature and vanadate concentration by combining rheology and microcalorimetry. This work shows that the rheological properties of the system depend on electrostatic interactions between [H(2)V(10)O(28)](4-) and positively charged gelatin chains. In a first stage, we describe the renaturation of the gelatin triple helices in the presence of decavanadate clusters. We reveal that, when gelatin chains are in coil conformation (30 degrees CT50 degrees C), the inorganic clusters act as physical cross-linkers that govern the visco-elastic properties of the mixture with an exponential dependence of the (G', G'') modulus with the vanadate concentration. Below 30 degrees C, we show that gelatin triple helix nucleation is slightly favored by the presence of vanadate, but above a helix concentration of 0.012 g.cm (-3), G' is fully governed by the helix concentration. During the melting process, we reveal the non-fully reversible behavior of the vanadate/gelatin rheological properties and the stabilization of gelatin triple helices due to vanadate species until 50 degrees C. This non-reversible character has also been observed in the same experimental conditions with collagen/vanadate solutions. This is the first time that such a stabilization of triple helices has been reported in the case of gelatin hydrogels chemically cross-linked or not. We propose to analyze these results by considering that triple helix aggregates should persist because of decavanadate bridging, that the nucleation of an extended triple helix network may induce a strong modification of the vanadate cross-linker distribution in the system, or both, thus promoting the formation of thermally stable vanadate/gelatin micro-gels in the dangling end of the triple helices.

Summary: This paper deals with the gelation properties of gelatins with various molecular weights. The samples are 4 different extracts from limed bones. We investigated firstly the structural properties: the helix amounts by optical rotation measurements and the enthalpy of helix formation and melting, with the same thermal protocols and for various concentrations. Comparison between the two methods allowed deriving the enthalpy of helix-coil transition and melting. Rheology was performed under the same conditions with a stress controlled rheometer working with a fixed, very small, strain during gel formation, maturation and melting. Correlation between all the measurements confirms the existence of the master curve for storage modulus versus gelatin concentration for all samples investigated.

The aim of this investigation is to combine the advantages of pluronic(®) F127 hydrogels and nanoparticles composed of poly(isobutylcyanoacrylate) (PIBCA) core coated with a mixture of chitosan and thiolated chitosan to design novel multifunctional formulation for mucosal application. Nanoparticles offer the advantage of being mucoadhesive while pluronic(®) F127 hydrogel allowed prolonged contact time onto mucosal surfaces. This work highlights an unprecedented comprehensive study on the effect of nanoparticles on gelation and micellization behaviors of pluronic(®) F127 using rheology and micro-calorimetry experiments. Results showed that presence of nanoparticles induced (i) smaller crystal peak of F127, (ii) a decrease of the enthalpy of F127 micellization and (iii) a non-reversibility of micelle formation (during heating ramp) and micelle melting (during cooling ramp). Together, these findings suggest that a part of F127 was not able to associate into micelles and the formation of mixed micelles containing F127 unimers and PIBCA/(chitosan/thiolated chitosan) copolymer and/or PIBCA homopolymer was suspected. The interaction of F127 unimers with nanoparticles resulted from their physical de-structuration as revealed by nanoparticle size measurement. In addition, we found that short polymerization duration of one hour induced more pronounced nanoparticle de-structuration. Twenty-four hour-polymerization of isobutylcyanoacrylate in the presence of chitosan and thiolated chitosan led to more stable nanoparticles when mixed with pluronic(®) F127.

Cyclodextrins are a family of oligosaccharides with a toroid shape which exhibit a unique ability of entrapping guest molecules in their internal cavity. Water is the primary guest molecule and is omnipresent in the crystalline phases stabilizing the overall architecture. Despite the presence of water molecules inside the cavity, cyclodextrins provide a hydrophobic environment where poorly soluble molecules can easily fit. In this investigation we put in evidence different types of water in the hydrated {\alpha} , \b{eta} and {\gamma} cyclodextrin crystals. Thermogravimetric measurements identify various binding sites of water and highlight the difference between the crystals equilibrated under various humid atmospheres. We establish by microcalorimetry the limit of solubility versus temperature and measure for the first time the melting temperatures of the hydrated crystals. Dissolution and melting enthalpies are derived and the solubility curves are compared to existing literature. The specific features of each cyclodextrin are underlined.

In this paper, we examine the rheological and the structural properties of different types of gelatin networks, physical, chemical and both. The physical gel is due to the formation of collagen type triple helices when cooling the solutions. Chemically crosslinked gels are obtained with a reagent, in our case the bis(vinylsulfonyl)methane (BVSM), kindly provided by Kodak Industrie (France). Each BVSM molecule provides two covalent bonds. The chemical reaction was followed by microcalorimetry (MicroDSC III from Setaram, Caluire, France). The relation between shear moduli and crosslinking for the three types of gels is discussed, in relation with theoretical models of randomly crosslinked systems.

The relative influence of physical and chemical bonds to overall gel properties are explored in gelatin gels. Physical, chemical, chemical-physical, and physical-chemical gels are obtained by cooling the protein solution and/or by transglutaminase reaction. Each type of network is characterized by rheology and polarimetry. It is shown that the overall properties as well as the dynamics inside the gels are dependent upon the order of formation and on the relative amount of triple helices and covalent bonds. Enzyme hydrolysis of covalent gels is slower than that of physical gels, as confirmed by the kinetics of helix release and degradation. A scheme is proposed to explain the results at both the physicochemical and the molecular levels.

The rheological properties of gelatin gels from various sources (fish and mammalian) were followed in the course of the sol−gel transition and during gel maturation, by performing dynamic measurements under very small deformations. Different concentrations and molecular weights were investigated. The solvent was mainly water, but mixed solvents containing water and glycerol were also considered. Blends of gelatins from fish and mammalian in aqueous solutions were prepared and rheologically characterized. A systematic comparison was established between these measurements and those presented in paper 1 of the series. The comparison between the storage modulus and the concentration of helices leads to a master curve valid for all the samples investigated. This master curve is analyzed in terms of the percolation regime, near the threshold and in terms of a homogeneous network far from the threshold. In the percolation regime, a critical exponent of 2 is found for the storage modulus versus the distance to th...

Gelatin gels are well-known for their ability to form nonpermanent, physical gels at room temperature and for their numerous applications in photographic and food industries. The difficulties in un...

Primary cold production in extra-heavy oil reservoirs (7-12° API) is currently a low percent of the OOIP, as for some oil fields in La Faja Petrolífera del Orinoco (FPO), Venezuela. EOR studies are being conducted in order to increase recovery factors primarily in those thin bedded reservoirs which host up to 35% of the OOIP. Thermal EOR is usually the first option implemented to increase recovery. However, thermal methods are not suitable for thin pay zones and involve high water consumption which can create environmental issues. For these reasons, chemical EOR is becoming a feasible tool for mobility control and mobilization of residual oil. Polymer flooding applied to extra-heavy oil may require rather high concentration polymer solutions. The objective of this study is to contribute to a better understanding of the specific mechanisms involved in this context. Transport properties of polymer in porous media are investigated through a set of corefloods tests performed on model sandpacks, the high permeability (4 Darcy) and the temperature (50°C) being representative of the conditions of the FPO. Polymer transport is analyzed versus injection concentration in terms of the key petrophysical parameters controlling the performance of a polymer flooding operation, namely: retention and inaccessible pore volume, in-depth propagation and injectivity. Results show that all parameters strongly depend on polymer concentration. Specifically, the concentration dependence of polymer retention, inaccessible pore volume and in-depth propagation behavior cannot be interpreted by conventional models. Injectivity is also strongly affected by concentration: both rheo-thinning and rheo-thickening effects become more marked when concentration increases. This set of experimental data are the key for modeling the transport of high concentration polymer solutions in porous media. They also provide direct information needed, through reservoir simulation, to sanction the technical and economical feasibility of polymer flooding for extra-heavy oil.

Physical gelation is the process of crosslinking which reversibly transforms a solution of polymers into a gel. The crosslinks of the network have a physical origin (hydrogen bonding, Van der Waals forces... ) and therefore are sensitive to variations of temperature, pH, ionic content, etc. (non-permanent crosslinks). Physical and chemical gelation have been extensively studied in quiescent conditions, where rheology experiments have been performed to follow the network formation without disturbing the process. In this study we consider gelation of a well known physical, thermoreversible, gel (gelatin gel), which proceeds under flowing conditions. The gelling solution is submitted to a shearing, with imposed, permanent shear stresses or imposed, permanent, shear rates. Under flow, a competition arises between the formation of clusters by physical crosslinking and their disruption by the shear forces. This investigation defines the flowing conditions which either allow or impede gel formation. In particular, a critical shear rate \(\dot \gamma *\), related to the gelation temperature and gelatin concentration, is identified which separates the two regimes. A microscopic model is proposed, based on the analysis of flow curves and dynamic measurements, which describes the structure of the gelling solution: microgel particles grow to a maximum size which depends on the flow. When the volume fraction of particles is high enough, percolation between particles occurs suddenly and a yield stress fluid is formed (particulate gel). The differences between gels made in quiescent conditions and gels made under flow are underlined.

The miniCD4 M48U1 was formulated into thermosensitive and mucoadhesive pluronic(®) hydrogels as anti-HIV-1 microbicide. The release kinetics of M48U1 from F127/HPMC (20/1 wt%) and F127/F68/HPMC (22.5/2.5/1 wt%) studied during 24h by using Franz diffusion cells showed that HEC hydrogel (1.5 wt%) used as control released 93% of the peptide, while about 25% of M48U1 remained in pluronic(®) hydrogels. The formulation of M48U1 in pluronic(®) hydrogels ensures a local delivery because no diffusion of the peptide was detected through vaginal Cynomolgus macaque mucosa using Ussing chamber. Finally, toxicological studies showed no significant difference in the HeLa cell viability of the pluronic(®) hydrogels in comparison with HEC and phosphate buffer saline.

Presenting a unique perspective on state-of-the-art physical gels, this interdisciplinary guide provides a complete, critical analysis of the field and highlights recent developments. It shows the interconnections between the key aspects of gels, from molecules and structure through to rheological and functional properties, with each chapter focusing on a different class of gel. There is also a final chapter covering innovative systems and applications, providing the information needed to understand current and future practical applications of gels in the pharmaceutical, agricultural, cosmetic, chemical and food industries. Many research teams are involved in the field of gels, including theoreticians, experimentalists and chemical engineers, but this interdisciplinary book collates and rationalises the many different points of view to provide a clear understanding of these complex systems for researchers and graduate students.

International audience; In this paper we report the influence of biopolymer folding on nanoparticle spatial distribution in two typical bio-nanocomposite hydrogels. These systems consist of negatively charged nanosized fillers (polyoxotungstate clusters and silica particles, 2.2 nm and 23.0 nm in diameter, respectively) dispersed at low volume fractions in a positively charged gelatin hydrogel. The filler state of dispersion is investigated during triple helix folding by combining small-angle neutron scattering (SANS) and polarimetry experiments. Neutron contrast matching/polarimetry correlations indicate that the nanoparticle spatial distribution is clearly modified during triple helix folding for the two systems. In the first case, polyoxotungstate clusters are initially arranged in small finite size aggregates that grow with increasing triple helix rate: ΔRG ≈ +150% and ΔI(q → 0) ≈ +250% for Δ[helix] ≈ +40%. In the second case, silica particles initially form a connected network that undergoes a significant densification through gelatin conformational transition. In the two cases, the kinetics of triple helix folding is only slightly affected by the presence of the nanoparticles and their state of dispersion. In our experimental conditions, these two processes are almost thermo-reversible following triple helix unfolding.

This paper undertakes a parallel analysis of the gelation mechanisms, structure and rheological properties of gelatin and collagen gels. Although the molecular compositions of collagen and gelatin are almost identical, gelation proceeds from distinct mechanisms and leads to different types of molecular assemblies. First are presented the properties of the solutions, based on their structural and rheological characterization; then the mechanisms of gelation in the networks, observed by Transmission Electron Microscopy, of three types of gels: gelatin gels, Type I collagen gels and gels made of cuticle collagen extracted from annelid worms. The rheological investigation of the sol-gel transition of gelatin is described within the context of the theories of percolation and scaling laws. Different experimental approaches to the kinetics of gelation are presented, combining dynamic light scattering and rheology in respect to gelatin gels.

The main objective of this work was to design thermosensitive and mucoadhesive vaginal hydrogels able to keep their rheological and mucoadhesive properties after dilution with vaginal fluids. Formulations were composed of pluronic F127 or a mix of two pluronics F127 and F68. Both formulations contained hydroxypropylmethyl cellulose (HPMC) as a mucoadhesive polymer. The determination of gelling temperature (T(gel)) after dilution with simulated vaginal fluid (SVF) demonstrated that hydrogels were resistant to dilution and T(gel) values were close to 30°C. Ex vivo mucoadhesion experiments conducted on porcine vaginal mucosa founded on the technique of traction of the adhesive/adherent joint allowed the characterization of mucoadhesive properties of hydrogels by measuring work of adhesion (W) and maximum force of detachment (F(max)). In the case of F127-based hydrogels, W and F(max) were lowered after dilution with SVF. However, in the case of F127/F68-based hydrogels, W, F(max) and mucoadhesion profiles were weakly affected by dilution. These differences could be attributed to the higher elasticity of F127/F68/HPMC (22.5/2.5/1% w/w) hydrogel in comparison with F127/HPMC one (20/1% w/w). Indeed, rheological analyses of the formulations showed that both elastic (G') and viscous moduli (G'') were higher for F127/F68/HPMC (22.5/2.5/1% w/w) than for F127/HPMC hydrogel (20/1% w/w). However, we demonstrated that the higher elasticity of the hydrogel was due to the higher total pluronic concentration and not due to the presence of F68 in the formulation.

This paper reviews the modern ideas on the process of gelation which have arisen from the analogy between gelation and percolation. The basic features which are common to all types of gels, colloidal or polymeric systems are first recalled; then after a simplified presentation of the percolation model, a few examples illustrating these concepts are chosen (for chemical gelation, a copolymerisation reaction and for physical gelation, the gelatin sol-gel transition). In conclusion, electron micrographs of gel networks (gelatin gel and an inorganic gel of thorium phosphate) are shown, which reveal the great diversity and complexity of the structures.

Gelatin solutions in dilute and semi-dilute regimes were characterized by using light and small-angle neutron scattering techniques. Absolute intensity measurements allowed us to evaluate the molecular parameters of these protein chains (radius of gyration, persistence length, cross-section, mass per unit length) and to determine the quality of the solvent (0.1 M NaCl solutions in H2O or D2O at pH = 7). The classical model of worm-like chains was adopted for the theoretical interpretation of chain conformation in the sol state (T = 50°C) with a persistence length of about 20 A. In semi-dilute solutions the correlation length was measured for different concentrations. Agreement was found with the scaling laws, although the description is not totally satisfactory: non-trivial scattering effects are detected in both static and dynamic light scattering experiments, which can be attributed to the presence of inhomogeneities having a different local density. Their apparent radius of gyration is of the order of several hundred angstroms. The nature and extension of these inhomogeneities are discussed.