Application des techniques de diffusion de la lumière des rayons X et des neutrons à l'étude des systèmes colloïdaux. Première partie : Présentation théorique des trois techniques Application of Light, X-Ray and Neutron Diffusion Techniques to the Study of Colloidal Systems. Part One : Theoretical Description of Three Techniques

Abstract

Les techniques de diffusion, des rayons X, des neutrons et de la lumière, jouent un rôle très important pour la compréhension des milieux colloïdaux. Peu d'articles de la littérature s'attachent à présenter conjointement les trois méthodes. Nous avons, dans la première partie de cet article, détaillé les principes théoriques en insistent tout particulièrement sur les spécificités de chacune. Après les rappels concernant la diffusion par les systèmes dilués, nous nous sommes intéressés aux systèmes concentrés pour lesquels les entités diffusantes sont en interaction. Les milieux dispersés montrent souvent une certaine polydispersité que l'on cherche à mesurer; les techniques de diffusion permettent cette mesure. Nous terminerons cette revue par une description des appareillages utilisés. La deuxième partie de cet article concernera une large illustration des possibilités de ces méthodes d'analyse à l'étude des systèmes colloïdaux, sur la base de travaux effectués à I'IFP (Institut Français du Pétrole) ou dans de nombreux laboratoires de recherche extérieurs. This article aims to describe X-ray, neutron and light scattering techniques with emphasis on their specific nature and their scope of application. Indeed, whereas light diffusion has been used for a long time in research laboratories, in particular for characterizing polymers in solution, small angle X-ray scattering has been the subject of renewed interest in recent years. Neutron scattering, which is obviously more difficultly accessible, has proven to be extremely useful for studying various systems for which light and X-ray scattering remain relatively powerless. Whereas there is an abundant literature concerning various applications of the three methods, it should be noted that only a few articles have attempted to describe all three techniques at the same time. In this article we have tried to make up for this lack, and as such it was indispensable for us to review the basic theoretical principles, and especially radiation-matter interaction. This should clearly highlight the fields of application and the specific features of each method together with the information that can be obtained about colloidal systems. This article is divided into two general parts : (1) a description of the theoretical principles, including a joint description of the specific features of the three types of radiation - light, X rays and neutrons, and (2) a bibliographic review, not an exhaustive one, based on the extensive work done at Institut Français du Pétrole (IFP) or in outside research laboratories, and concerning the characterizing of colloidal systems. Part One is divided into several chapters. First of all we review the physical laws governing the interaction of radiation with matter. X-ray or light photons are electromagnetic waves characterized by a very different wavelength, which is close to one angstrom for X rays and to about 6000 angstroms for light. Neutrons are moving particles having a wavelength of several angstroms. X rays interact solely with electrons from atoms, while neutrons interact with the nuclei of these same atoms. When a light wave passes through a diffusing medium, it creates a dipole that will radiate an electromagnetic field proportional to the polarizability of the medium. The diffused intensity or scatter cross-section appears as the Fourier transform of the autocorrelation function gamma(x). This intensity contains all information about the nature, geometry and size of the diffusing entities contained in the medium. All these characteristic magnitudes of the colloid can be deduced from the experiment when the particles are homogeneous, identical and uncorrelated. This last qualification reveals the presence of particles that are very similar to one another, possibly in the form of more or less compact aggregates. We also refer to interactions among particles, which will be negligible if the diffusing entities are very far away, i. e. when a dilute system is present. The magnitudes that characterize such interactions define the thermodynamics of the system. Some colloids are made up of inhomogeneous entities formed by regions having a different chemical nature. For examples, this is true of microemulsions where oil-rich and water-rich regions exist with an interface made up of a surfactant. Such systems are very complex, and the structural data specific to each region are mixed in the diffusion spectrum. Contrast variation techniques that are specially suited for neutron diffusion enable each region to be observed successively. Few colloids have particles with a strictly identical form. This is true in particular for polymers in solution, for which the chain length varies considerably. The diffusion signalis then strongly modified and greatly influenced by large masses. Fractionating techniques are usually used, such as gel permeation chromatography or ultracentifuging, so as to obtain almost monodispers entities in solution. This first part ends with a description of some types of equipment most commonly used for performing diffusion experiments.