What are Colloids?

To some the word 'colloidal' conjures up visions of things indefinite in shape, indefinite in chemical composition and physical properties, fickle in chemical deportment, things infilterable and generally unmanageable.

Hedges, 1931

INTRODUCTION

The above remarks reflect the impression created by many textbooks of physical chemistry – if they deign to mention colloids at all. In fact, in both its experimental and theoretical aspects, and no less important in its technological applications and in the appreciation of its biological implications, colloid science has made impressive progress in the last few decades. In the following chapters an attempt is made to summarise the basic concepts of colloid science and to dispel some of the doubts expressed in the above quotation.

A full understanding of the properties of colloids calls upon a wide range of physical and chemical ideas, while the multitude of colloidal systems presented to us in nature, and familiar in modern society, exhibit a daunting complexity. It is this that has delayed the development of colloid science, since a detailed and fundamental theoretical understanding of colloidal behaviour is possible only through a thorough knowledge of broad areas of physics, chemistry, and mathematical physics, together in many instances with an understanding of biological structures and processes. On the experimental side there is an ever-increasing emphasis on the application of modern physical techniques to colloidal problems. Colloid science is thus a truly interdisciplinary subject.

Nevertheless, despite the sophistication needed for the development of a complete quantitative theory of colloids, the basic principles that underlie many colloid problems can be seen as extensions to such systems of the fundamental concepts of physical chemistry. One important objective of this book is to emphasise the close link between colloid science and physical chemistry and to show how a broad understanding can be built up on a few relatively simple physico-chemical ideas. We shall not only seek common features revealed by experimental study but also, of much greater significance, try to identify the fundamental concepts that link together many apparently unconnected aspects of the subject.

DEFINITION OF COLLOIDS

In setting out to define the scope of colloid science, it should first be said that any attempt to lay down too rigid a scheme of definitions and nomenclature is likely to be unnecessarily restrictive. Rather than try at the outset to develop a formal definition, it is preferable to describe examples of systems to which the term 'colloidal' is now applied.

An essential part of any study of physics and chemistry involves first the recognition of three states of matter – solid, liquid, and gas – and a general discussion of the transformations – melting, sublimation, and evaporation – between them. Pure substances are considered, and then attention passes to solutions which are homogeneous mixtures of chemical species dispersed on a molecular scale. What remained largely unrecognised until about a century and a half ago was that there is an intermediate class of materials lying between bulk and molecularly dispersed systems, in which, although one component is finely dispersed in another, the degree of subdivision docs not approach that in simple molecular mixtures. Systems of this kind, colloids, have special properties which arc of great practical importance, and they were appropriately described by Ostwald as lying in the World of Neglected Dimensions. They consist of a dispersed phase (or discontinuous phase) distributed uniformly in a finely divided state in a dispersion medium (or continuous phase).

As familiar examples of colloidal systems we cite the following: fogs, mists, and smokes (dispersions of fine liquid droplets or solid particles in a gas – aerosols); milk (a dispersion of fine droplets of fat in an aqueous phase – emulsions); paints, muds, and slurries (dispersions of fine solid particles in a liquid medium – sols or colloidal suspensions); jellies (dispersions of macromolecules in liquid – gels); opal and ruby stained glass (dispersions, respectively, of solid silica particles in a solid matrix or of gold particles in glass – solid dispersions). So-called (and miscalled) photographic emulsions are dispersions of finely divided silver halide crystallites in a gel – in a sense they are a colloid within a colloid. In association colloids molecules of soap or other surface-active substances arc associated together to form small aggregates (micelles) in water. The aggregates formed by certain substances may adopt an ordered structure and form liquid crystals. Many biological structures are colloidal in nature. For example, blood is a dispersion of corpuscles in serum, and bone is essentially a dispersion of a calcium phosphate embedded in collagen.

In the above examples, which may be called simple colloids, a clear distinction can be made between the disperse phase and the dispersion medium. However, in network colloids this is hardly possible since both phases consist of interpenetrating networks, the elements of each being of colloidal dimensions. Porous solids, in which gas and solid networks interpenetrate, two-phase glasses (opal glasses), and many gels are examples of this category.

Furthermore, there are other instances (multiple colloids) that may involve the co-existence of three phases of which two (and sometimes three) phases are finely divided. One example is a porous solid partially filled with condensed vapour, when both the liquid and vapour phases within the pores are present in a finely divided form; a similar situation arises when oil and water co-exist in the pores of an oil-bearing rock, also in frost heaving when water and ice co-exist in a porous medium. Multiple emulsions consist for example of finely divided droplets of an aqueous phase contained within oil droplets, which themselves are dispersed in an aqueous medium.

Some of the more important types of colloidal systems outlined above are summarised in Table 1.1. For simplicity we shall limit ourselves in this book to a discussion of simple colloids, although the ideas developed can be extended and applied to more complex systems.

The fundamental question which has to be answered is 'What do we mean by "finely divided"?' It turns out, for reasons which will soon be apparent, that systems usually exhibit properties of a specifically 'colloidal character' (which we shall explain in more detail in later chapters) when the dimensions of the dispersed phase lie in the range 1 — 1000 nm, i.e. between 10 Å and 1 µm.

These limits are not rigid, for in some special cases (e.g. emulsions and some slurries) particles of larger size are present. Moreover, it is not necessary for all three dimensions to lie below 1 µm, since colloidal behaviour is observed in systems containing fibres in which only two dimensions are in the colloid range. In other systems, such as clays and thin films, only one dimension is in the colloid range. This is illustrated schematically in Figures 1.1 and 1.2, while Figure 1.3 shows electron microscope photographs of colloidal particles of several types.

Colloids in which the particle size is below about 10 nm often require special consideration. One example of such particles are the nuclei which initiate bulk phase changes, while the justification for including macromolecular solutions and association colloids within this classification arises from the fact that the particles within them are either macromolecules of considerable length, which even when coiled up have diameters of well over 1 nm, or aggregates of smaller molecules forming micelles of a size falling within the colloid size range. Biocolloids again have their individual characteristics, but once more the presence of structures of colloidal dimensions justifies their inclusion as examples of colloids. The limit below which colloid behaviour merges into that of molecular solutions is usually presumed to be around 1 nm (10 Å).

An alternative subdivision of colloids which has been widely used in the past is into lyophobic (or hydrophobic, if the dispersion medium is water) and lyophilic (hydrophilic, in water) colloids, depending on whether the particles can be described in the former case as 'solvent hating' or in the latter case as 'solvent loving'. These characteristics are deduced from the conditions required to produce these colloids and from the means available for their redispersion after flocculation or coagulation. It will become apparent later that, while this subdivision has many useful aspects, it is neither entirely logical nor sufficiently all-embracing, and we shall make only limited use of it.

COLLOIDS AND SURFACE CHEMISTRY

Because of the range of dimensions involved in colloidal structures, the surface-to-volume ratio is high and a significant proportion of the molecules in such systems lie within or close to the region of inhomogeneity associated with particle/medium interfaces. These molecules will have properties (e.g. energy, molecular conformation) different from those in the bulk phases more distant from the interface. It is then no longer possible (as we do in bulk thermodynamics) to describe the whole system simply in terms of the sum of the contributions from the molecules in the bulk phases, calculated as though both phases had the same properties as they have in the bulk state. A significant and often dominating contribution comes from the molecules in the inter-facial region. This is why surface chemistry plays such an important part in colloid science and why colloidal properties begin to become evident when the particle size falls below 1 µm. We can see this in the following way.

The surface area associated with a given mass of material subdivided into equal-size particles increases in inverse proportion to the linear dimensions of the particles. Thus the area exposed by unit mass (the specific surface area, as) is given by 6/ρd, where ρ is the density of the material and d is the edge length in the case of cubic particles or the diameter in the ease of spheres. If the material is made up of molecules of linear dimension h and molecular volume ~h3, then the fraction of molecules in the surface layer is given approximately by 6(h/d). Thus for a substance of molar volume 30 cm3 mol-1 or of molecular volume 0.05 nm3 (e.g. silver bromide) h = 0.37 nm. For a 1 cm cube only two or three molecules in ten million are surface molecules, and these have a negligible influence on its properties. However, when divided into 1012 particles of 1 µm, one molecule in four hundred and fifty is a surface molecule, and the properties of the system begin to be affected. At 10 nm the ratio rises to nearly one in four and surface effects dominate. Beyond this it is hardly possible to decide what we mean by a surface molecule, and, as indicated above, special considerations apply to the size range 1 — 10 nm. To illustrate this point, Figure 1.4 shows the variation of the percentage of surface molecules with particle size for the typical case of silver bromide.

This approach to colloids, emphasising the importance of surface or interfacial properties, suggests a more meaningful description of colloids as microheterogeneous systems, the microheterogeneity being characterised by lengths in the range 1 — 1000 nm.