Madeleine Djabourov

Architecture of gelatin gels

Abstract The aim of this paper is to make a statement of the present knowledge of the gelation mechanisms of a polymeric, physical gel: the gelatin gel. The description of the system includes both the microscopic and the supramolecular scales. It is shown that the use of different experimental approaches (polarimetry, electron microscopy, rheology) provides complementary views of the phenomenon. The gelation process is presented within the theoretical context of the scaling laws, which establish an analogy with a second-order phase transition. This analogy is illustrated in the case of the gelatin gel, and its implications, with a unified description of gelation in view, are underlined.

Conformation of gelatin chains in aqueous solutions: 1. A light and small-angle neutron scattering study

Abstract 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.

Structure and rheology of gelatin and collagen gels

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.

Morphology of paraffin crystals in waxy crude oils cooled in quiescent conditions and under flow

Abstract The gel formation in waxy crude oils is a major concern for oil production and transportation. When oil is extracted, the decrease in temperature induces a partial crystallisation of paraffins by a supersaturation mechanism, involving in the first place the longest alkane molecules. Gel formation appears for very low amounts of crystals (around 1–2%) when the oil is cooled in quiescent conditions. However, during normal extraction conditions (pumping or well pressure gradient) the oil remains fluid, although crystallisation still takes place. This investigation deals with the morphology of paraffin crystals in crude oils, cooled either in quiescent conditions or under extensive shearing, controlled by different shear rates. The technique of observation is transmission electron microscopy after cryofixation, cryofracture and replica preparation. The images of gels made under quiescent conditions, distinctly show a lamellar structure: the lamella thickness of the order of the size of a paraffin molecule, the inter-lamella distance decreases with the total amount of crystals. Individual disc-like particles also appear occasionally, which probably constitute the nuclei of crystallisation. Under flowing conditions, only disc-like particles are observed, either isolated or assembled into clusters whose size depends on the shear rate. The discussion deals with the crystallisation mechanism in quiescent conditions and under shear and with the consequences upon rheology of the suspensions.

Influence of thermal treatments on the structure and stability of gelatin gels

Abstract The influence of thermal history on the structure of gelatin gels has been investigated by measuring the specific optical rotation [ α ] λ at λ = 436 nm. The helix content χ in the gels was derived by reference to the native collagen. Two types of thermal treatments have been applied: (a) cooling and heating at constant rates, and (b) quenching and annealing for long periods. Our results support the idea that the gels are non-equilibrium systems and exhibit thermal hysteresis. The kinetics of helix formation were analysed by the Avrami theory and compared to other techniques. The stability of the structures formed suggests the existence of several mechanisms of hydrogen bonding of the helices: disordered aggregation or partial collagen renaturation.

State of water in gelatin solutions and gels: an 1H n.m.r. investigation

Abstract According to a now well established interpretation, the network growth in gelatin gels results from a conformational coil-helix transition leading to partial renaturing of native collagen. Proton magnetic resonance has been used in order to elucidate the role of water in this process. Proton spin-lattice T1 and spin-spin T2 relaxation times, have been measured at various concentrations and quenching temperatures. The results have been interpreted within the framework of a multiphase model involving three populations of water protons in rapid exchange which are affected differently by the macromolecular network growth in the course of gelation. In particular, the model is adequate to explain the time dependence of T2 after quenching. Our results concerning the spin-lattice relaxation of the bound water protons are in good agreement with those measured in hydrated native collagen or in agarose gels.

Conformation of gelatin chains in aqueous solutions: 2. A quasi-elastic light scattering study

We have characterized dilute and semi-dilute aqueous gelatin solutions by quasi-elastic light scattering. The concentration dependence of the scattering spectra has been explored. In semi-dilute solutions, two diffusive modes were determined: the fast mode, with a diffusion coefficient Df, is attributed to the cooperative movements of the entangled transient network of chains and it increases with the concentration as Df ∼ c0.6; the slow mode, whose diffusion coefficient Ds is inversely proportional to the Newtonian viscosity of the solution, is interpreted as the self-diffusion of clusters of chains that have a constant hydrodynamic radius Rc ≈ 750 A. The effects on the slow modes of the salt content of the solutions, the molecular weight of the chains and temperature are examined. Besides, it is shown that the addition of a surfactant (sodium dodecylsulphate) hinders the formation of the clusters, and it is concluded that the latter are probably stabilized by hydrophobic interactions between apolar lateral groups of the protein.

A technique for labelling the sample surface for quick-freeze, deep-etch, rotary replication electron microscopy: application to the study of geletin gel structure

A method using magnesium oxide crystals to label the surface of physical gels, such as gelatin gel before quick‐freezing is described and discussed. The quick‐freeze, deep‐etch, rotary replication technique is most adapted to 3‐D visualization of physical gel structure. However, it is known that the depth which ultrarapid freezing may reach is limited by the growth of ice crystals as the distance from the surface of the specimen (rapidly cooled by smashing against a cooled metal plate) increases. Consequently, intact preservation of structures occurs only in superficial zones of the specimen.

Percolation and gelation of complex systems

This paper reviews the major results concerning the sol‐gel transition of a large variety of systems, including polymeric and colloidal solutions, whose process of gelation has been analyzed in the frame of the percolation model. I first recall the basic ideas of the model and the predictions regarding the main, characteristic and experimentally accessible, parameters. Among them, emphasis is placed on the rheological measurements, but I also discuss the possibilities of other techniques (like dynamic light scattering or electron microscopy) in order, either to investigate the kinetics of network growth, or the three‐dimensional supramolecular structure of fully matured gels.