The behaviour of colloidal systems

In Part Four (Chapter eight) we focus on the interactions of mixed systems of surface-active biopolymers (proteins and polysaccharides) and surface-active lipids (surfactants/emulsifiers) at oil-water and air-water interfaces. We describe how these interactions affect mechanisms controlling the behaviour of colloidal systems containing mixed ingredients. We show how the properties of biopolymer-based adsorption layers are affected by an interplay of phenomena which include selfassociation, complexation, phase separation, and competitive displacement. 

The factors which contribute most to the behaviour of colloidal systems are the dimensions, the shape and the properties of the surfaces of the particles, but also the medium in which they are dispersed is of influence. The large to extremely large ratios between the surfaces and the volumes of the particles are of importance for all of these properties. 

Real systems seldom or never satisfy aU these conditions and it has indeed been found that Wiener s formulae often reproduce the behaviour of colloid systems of the type in question qt itatively or sometimes semi-qtiantitatively but that great caution is necessary in drawing any quantitative conclusions No examples are known of a good quantitative agreement In particular we would also point out that the optical behaviour of a system of parallel directed macromolecules, which are embedded in an imbibition medium, cannot at all be reproduced by the theory of Wiener ... 

The remainder of this contribution is organized as follows. In section C2.6.2, some well studied colloidal model systems are introduced. Methods for characterizing colloidal suspensions are presented in section C2.6.3. An essential starting point for understanding the behaviour of colloids is a description of the interactions between particles. Various factors contributing to these are discussed in section C2.6.4. Following on from this, theories of colloid stability and of the kinetics of aggregation are presented in section C2.6.5. Finally, section C2.6.6 is devoted to the phase behaviour of concentrated suspensions. 

Among the purely physical properties of their materials, to which the chemist and the biologist have been compelled to pay an increasing amount of attention during recent years, surface tension undoubtedly occupies the first place. In a great measure this is due to the development of colloidal chemistry, which deals with matter in a state of extreme sub-division, and therefore with a great development of surface for a given mass, so that the properties of surfaces become important, and sometimes decisive, factors in the behaviour of such systems. 

In this presentation I have given a brief review of the factors controlling the stability and instability of polymer latices. There is little doubt that as a consequence of the ready availability of dispersions of spherical, monodisperse particles, polymer latices, our knowledge of the behaviour of colloidal dispersions has progressed rapidly over the past fifteen years. However, many phenomena remain to be investigated in quantitative detail and we must remember that the small energy changes involved in these systems, by comparison with molecular reactions, make many of the phenomena very subtle. 

We may draw a close analogy here between the behaviour of colloidal dispersions and molecular systems. Thus the first case discussed above is analogous to the presence of clusters of molecules in a vapour approaching its condensation point, or in a solution close to saturation. The limiting concentration at which flocculation occurs corresponds to the saturation vapour pressure, or to the solubility of a solid in solution. More complex colloidal systems often exhibit phase behaviour which is paralleled by various phase separation phenomena in molecular systems. Detailed discussion ofthese matters is outside the scope of this book. However, pursuit of these analogies and their interpretation is a currently active area of research. 

The same would be true for equivalent non-polymeric colloidal or microemulsion systems. Another practical consequence, which seems to become practical very important in further approaches, would be that with appropriate experimental or production conditions, in particular, with sufficiently long equilibration time, the behaviour of these systems would be accurately predictable. But from some experiences, made during the research into these systems, it can be seen that most of the polymeric and non-polymeric colloidal systems shows unpredictable behaviour, sudden changes in properties, and even totally unexpected phenomena. 

In recent years other colloid systems—such as microemulsions—have been found to exhibit a wide range of structures [81,82]. We can observe spontaneous phase separation, flocculation and formation of complex bicontinuous structures after the formation of these colloidal systems. It is not possible to form a colloidal system, whether in a polymeric matrix, in water, or in an organic solvent, without a supercritical input of energy, which is provided by turbulent flow conditions during the formation of microemulsions or melt fracture conditions [86] during the formation of colloidal systems in polymers. It seems that a general principle for the behaviour of multiphase systems has been found. 

The measurement of forces between surfaces at small separation is of great importance in gaining a fundamental understanding of the complex behaviour of colloidal systems. Interactions of solid surfaces (coated or uncoated) across a fluid medium have been made through development of both the surface forces apparatus (SFA) and the atomic force microscope. Interactions between liquid surfaces are generally discussed in terms of the variation of the disjoining pressure with surface separation, and the majority of studies relate to foam (i.e. vapour-liquid-vapour) Investigations of various aspects of emulsion (i.e. liquid-liquid-... 

In practice, e.g., in nature or in fonnulated products, colloidal suspensions (also denoted sols or dispersions) tend to be complex systems, consisting of many components that are often not very well defined, in tenns of particle size for instance. Much progress has been made in the understanding of colloidal suspensions by studying well defined model systems, which allow for a quantitative modelling of their behaviour. Such systems will be discussed here. 

The essential differences between the properties of matter when in bulk and in the colloidal state were first described by Thomas Graham. The study of colloid chemistry involves a consideration of the form and behaviour of a new phase, the interfacial phase, possessiug unique properties. In many systems reactions both physical and chemical are observed which may be attributed to both bulk and interfacial phases. Thus for a proper understanding of colloidal behaviour a knowledge of the properties of surfaces and reactions at interfaces is evidently desirable. 

Until the last few decades colloid science stood more or less on its own as an almost entirely descriptive subject which did not appear to fit within the general framework of physics and chemistry. The use of materials of doubtful composition, which put considerable strain on the questions of reproducibility and interpretation, was partly responsible for this state of affairs. Nowadays, the tendency is to work whenever possible with well-defined systems (e.g. monodispersed dispersions, pure surface-active agents, well-defined polymeric material) which act as models, both in their own right and for real life systems under consideration. Despite the large number of variables which are often involved, research of this nature coupled with advances in the understanding of the fundamental principles of physics and chemistry has made it possible to formulate coherent, if not always comprehensive, theories relating to many of the aspects of colloidal behaviour. Since it is important that colloid science be understood at both descriptive and theoretical levels, the study of this subject can range widely from relatively simple descriptive material to extremely complex theory. 

The natural laws of physics and chemistry which describe the behaviour of matter in the massive and molecular states also, of course, apply to the colloidal state. The characteristic feature of colloid science lies in the relative importance which is attached to the various physicochemical properties of the systems being studied. As we shall see, the factors which contribute most to the overall nature of a colloidal system are ... 

This review concentrates on John Albery s work in the field of colloidal semiconductor photoelectrochemistry. John s major contributions to this area, as in so many others, have been through his astounding facility for generating useful asymptotic solutions for highly complex kinetic models of electrochemical systems. So as to put John s work in colloidal photoelectrochemistry into context. Sections 9.1-9.3 of this chapter provide a review of the more salient kinetic models of semiconductor photocatalysis developed over the last 20 years or so. Section 9.4 then concentrates on the Alberian view and presents, for the first time, John s model of the chronoamperometric behaviour of colloidal CdS. 

Stability of foams and other disperse systems holds a central position in colloid chemistry. Nevertheless, many aspects of this problem are not completely clear. There is no theory as yet that can offer a thorough quantitative explanation of the behaviour of foams with time. It is hardly possible to create such a unified theory, since the dominant thermodynamic and kinetic factors of stability can be different for the different foams. 

A variety of interaction behaviours can be observed between liquid/liquid interfaces based on the types of colloidal forces present. In general, they can be separated into static and dynamic forces. Static forces include electrostatic, steric, van der Waals and hydrophobic forces, relevant to stable shelf life and coalescence of emulsions or dispersions. Dynamic forces arise ftom flow in the system, for instance during shear of an emulsion or dispersion. EHrect force measurements tend to center on static force measurements, and while there is a large body of work on the study of film drainage between both liquid or solid interfaces, there are very few direct force measurements in the dynamic range between liquid interfaces. Below are general descriptions of some of the types of force observed and brief discussions of their origins. 

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