Basic Principles of Dispersions

Book Preface

A colloidal dispersion is a two-phase system in which one phase (the disperse phase) is dispersed in a second continuous phase (the dispersion medium). The disperse phase can be solid, liquid or gas and the same applies for the dispersion medium. On this basis one can distinguish several classes of colloidal dispersions: solid/liquid (suspension), liquid/liquid (emulsion), gas/liquid (foam), liquid/gas (aerosol), liquid/ solid (gel), solid/gas (smoke) and solid/solid (composite). Another important class of colloids are produced by self-assembly of surfactants that form micelles with dimensions in the colloid range and these were described in Vol. 1. In all disperse systems such as suspensions, emulsions, foams, etc., the structure of the interfacial region determines its colloidal properties. For convenience, I will list the topics of colloid and interface science under two main headings: disperse systems and interfacial phenomena. This subdivision does not imply any separation for the following reasons. All disperse systems involve an interface. Many interfacial phenomena are precursors for formation of disperse systems, e.g. nucleation and growth, emulsification, etc. The main objective of the present handbook is to cover the following topics: the basic principles that are involved in the formation of colloidal dispersions and their stabilization.

The field of colloid and interface science has no boundary since chemists, physicists, engineers, biologists, and mathematicians can all be engaged in the field. For successful applications in industry, multidisciplinary teams are required.Understanding the basic principles of colloid and interface science will enable industry to develop many complex systems in a shorter period of time. Most colloidal systems used in industry are multiphase and complex formulations. They may contain more than one disperse phase, e.g. suspension/emulsion systems (suspoemulsions). Chapter 1 describes the flow characteristics (rheology) of colloidal dispersions. It starts with a section on the basic principles of rheology that includes steady state (shear stress-shear rate measurements), constant stress (creep measurements), constant strain (stress relaxation measurements) and dynamic (oscillatory) techniques. The various rheological models that are used to describe each technique are described. This is followed by a section on very dilute and moderately concentrated colloidal dispersions (which takes hydrodynamic interaction into account). The rheology of concentrated colloidal dispersions is then described by considering four different systems. The first is represented by hard sphere interactions where both repulsion and attraction are screened. The semi-empirical model that can be applied to describe th rheology of hard sphere dispersions is described. The second system is that of “soft”or electrostatic interaction whereby the rheology of the system is determined by double layer repulsion. The third system is that of sterically stabilized dispersions. The importance of the ratio of adsorbed layer thickness to particle radius is described. The fourth system is that of flocculated dispersions and a distinction can be made between weak and strong flocculation.

Chapter 2 describes the various processes of wetting, spreading and adhesion. It starts with the equilibrium thermodynamic treatment and Young’s equation. The calculation of surface tension and contact angle is described. This is followed by a description of the process of spreading of liquids on surfaces and the definition of the Harkins spreading coefficient. The process of contact angle hysteresis and effect of roughness and surface heterogeneity is described. This is followed by a section on the critical surface tension ofwetting and the effect of surfactant adsorption. The dynamic process of adsorption and wetting is described. Chapter 3 deals with solid/liquid dispersions