Jacques Leblond

Gelation and Transitions in Biopolymers

The appearance and the very particular consistency of solid gelatin are well known since it is very widely used in the food industry. Gelatin is a solid phase with specific properties; it is easily deformed under weak pressure, but the deformation ceases when the pressure stops, and it is homogeneous: it is a gel. We can say that it is a disordered phase or a soft substance. There are a great number of examples of gel phases more or less similar to gelatin. One finds gelled milks, in the food industry, and paints and cosmetics which are gels. There are numerous applications of gels. For example, gelatin is used to lay down the light-sensitive emulsion on the photographic films.

Dynamics of Phase Transitions

The thermodynamic description of phase transitions given in the preceding chapter says nothing about the evolution of a phase in time nor on the kinetics of the transition phenomenon. For example, the phase diagrams (Sect. 1.2.2) only give information on the conditions of the existence of phases (as a function of pressure, temperature, concentrations of constituents, etc.) and not on the time required for passing from one phase to another when the thermodynamic parameters of a system are changed (the temperature, for example). We know that a phase change is not instantaneous and that it is a dynamic phenomenon; each one has its own kinetics.

Thermodynamics and Statistical Mechanics of Phase Transitions

Any substance of fixed chemical composition, water H2O, for example, can exist in homogeneous forms whose properties can be distinguished, called states. Water exists as a gas, a liquid, or a solid, ice. These three states of matter (solid, liquid, and gas) differ in density, heat capacity, etc. The optical and mechanical properties of a liquid and a solid are also very different. By applying high pressures to a sample of ice (several kilobars), several varieties of ice corresponding to distinct crystalline forms can be obtained (Fig. 1.1). In general, for the same solid or liquid substance, several distinct arrangements of the atoms, molecules, or particles associated with them can be observed and will correspond to different properties of the solid or liquid, constituting phases. There are thus several phases of ice corresponding to distinct crystalline and amorphous varieties of solid water. Either an isotropic phase or a liquid crystal phase can be obtained for some liquids, they can be distinguished by their optical properties and differ in the orientation of their molecules (Fig. 1.2). Experiments thus demonstrate phase transitions or changes of state. For example: a substance passes from the liquid state to the solid state (solidification); the molecular arrangements in a crystal are modified by application of pressure and it passes from one crystalline phase to another. Phase transitions are physical events that have been known for a very long time. They are encountered in nature (for example, condensation of drops of water in clouds) or daily life; they are also used in numerous technical systems or industrial processes; evaporation of water in the steam generator of a nuclear power plant is the physical process for activating the turbines in electric generators, and melting and then solidification of metals are important stages of metallurgical operations, etc.

Microstructures, Nanostructures and Phase Transitions

With a few exceptions, polyphasic fluids in particular, we have always assumed up to now that materials were constituted of a single phase, that is, a domain, solid or liquid, whose properties were uniform and independent of the structure. This macroscopic vision of the structure of materials does not hold up on more detailed observation of the systems; it is only an approximation which does not allow accounting for their properties.

The Glass Transition

Most solid mineral compounds and elements form liquids of low viscosity (several centipoises) when they melt, and when the temperature is reduced, they solidify again to form a crystalline solid. Alternatively, there are materials which become liquids with a very high viscosity (105–107 P) when melted. When they are cooled below their melting point, these liquids do not solidify instantaneously but remain in a supercooled state, the viscosity of the liquid increases significantly when the temperature is reduced, and they then “freeze” in the form of a glass, which is a noncrystalline solid state. We say that the liquid has undergone a glass transition and that a glassy or vitreous state has formed.

Transitions and Collective Phenomena in Solids. New Properties

Melting and sublimation of a solid are two important phase transitions, but they are not the only changes of state in solids. Indeed, the appearance of a magnetic phase in a nonmagnetic solid when its temperature is reduced below a certain point (called the Curie temperature) is another type of important phase transition (paramagnetic/ferromagnetic transition), investigated in particular by P. Curie and P. Weiss. The transition from a conducting to a superconducting state (superconductivity) and ferromagnetism reflect the appearance of new properties of the solid state, implying electron coupling.

Phase Transitions in Liquids and Solids: Solidification and Melting

Liquid-solid (solidification) and solid-liquid (melting) phase transitions are certainly among the most widespread in nature and many of them also have very important implications. For example, think of the formation of ice crystals due to solidification of liquid water and the inverse phase transition involved in meteorological phenomena, as well as melting or solidification of metals or metal alloys. We indicated in the preceding chapter that the properties of a solid (in particular, its mechanical and thermal properties) are very significantly a function of the kinetics of solidification and more specifically the size of the microstructures formed during the phase transition.

Phase Transitions in Fluids

In the early history of physics, it was assumed for a very long time that there were three phases, or states, of matter: gas, liquid, and solid. The transition from one state to another occurs at a given temperature and pressure corresponding to the physical state with the lowest free energy F.

Phase Transitions under Extreme Conditions and in Large Natural and Technical Systems

In almost all phase transitions that we have studied up to now, extreme conditions (in particular, very high pressures and very high temperatures) in whsich new states of matter such as plasmas, or phase transitions such as the transition between a conducting metallic state and an insulating state, have been excluded. It is thus useful to consider the behaviors of matter on exposure to the very high pressures or temperatures that can be created in the laboratory today. It is also necessary to emphasize that these “extreme” conditions are found in natural systems, for example, in the earth’s core where materials are exposed to high pressures (3.5 Mbar) and temperatures of the order of 4000 K.

Transitions in Thin Films

Flow of oil on the surface of water has been observed since the 19th century, but it was only at the beginning of the 20th century that Lord Rayleigh suggested that films are composed of a monolayer of molecules. It was necessary to wait for another 20 years for Langmuir and Harkins to independently demonstrate the formation of films composed of a monolayer of molecules on the surface of water and reveal that the formation of such layers is only possible if the molecules constituting the film are amphiphilic (Sect. 9.3.1): insoluble in water due to the presence of a hydrophobic chain (paraffin chains, for example); having a hydrophilic polar “head” at one end (such as, a -COOH carboxyl or -OH hydroxyl or ionic termination).