The colloidal state

The colloidal state of matter is distinguished by a certain range of particle size, as a consequence of which certain characteristic properties become apparent. Colloidal properties are in general exhibited by substances of particle size ranging between 0 2 /an and 5 nm (2 x 10 7 and 5 x 10 9 m). Ordinary filter paper will retain particles up to a diameter of 10-20/an (1-2 x 10 5 m), so that colloidal solutions, just like true solutions, pass through an ordinary filter paper (the size of ions is of the order of 0-1 nm = 10 10 m). The limit of vision under the microscope is about 5-10 nm (5-10 x 10 9 m). Colloidal solutions are therefore not true solutions. Close examination shows that they are not homogeneous, but consist of suspension of solid or liquid particles in a liquid. Such a mixture is known as a disperse system the liquid (usually water in qualitative analysis) is called the dispersion medium and the colloid the disperse phase. 

An important consequence of the smallness of the size of the particles in a colloidal solution is that the ratio of the surface to the volume is extremely large. Phenomena which depend upon the size of the surface, such as adsorption, will therefore play an important part. The effect of particle size upon the area of the surface will be apparent from the following example. The total surface area of 1 ml of material in the form of a cube of 1 cm side is 6 cm2. When it is divided into cubes of 10 6 cm (10 8m) size (which approximates to many colloidal systems), the total surface area of the same volume of material is 6 x 106 cm2. 

Colloidal solutions may be divided roughly into two main groups, designated as lyophobic (Greek solvent hating) and lyophilic (Greek solvent loving) when water is the dispersion medium, the terms hydrophobic and hydrophilic are employed. The chief properties of each class are summarized in Table 1.14, but it must be emphasized that the distinction is not an absolute one since some, particularly sols of metallic hydroxides, exhibit intermediate properties. 

The viscosity of the sols is similar to that of the medium. Examples sols of the metals, silver halides, metallic hydroxides, and barium sulphate. 

A comparatively minute quantity of an electrolyte results in flocculation. The change is, in general, irreversible water has no effect upon the flocculant. 

Colloid science concerns systems in which one or more of the components has at least one dimension within the nanometre (10 9m) to micrometre (10-6m) range, i.e. it concerns, in the main, systems containing large molecules and/or small particles. The adjective microheterogeneous provides an appropriate description of most colloidal systems. There is, however, no sharp distinction between colloidal and non-colloidal systems. 

The range of colloidal systems of practical importance is vast, as is the range of processes where colloid/surface chemical phenomena are involved. 

Examples of systems which are colloidal (at least in some respects) are  

Examples of processes which rely heavily on the application of colloid/surface phenomena are  

Electrophoretic deposition Emulsion polymerisation Food processing Grinding 

Problems which arise with certain precipitates include the coagulation or flocculation of a colloidal dispersion of a finely divided solid to permit its filtration and to prevent its re-peptisation upon washing the precipitate. It is therefore desirable to understand the basic principles of the colloid chemistry of precipitates, for which an appropriate textbook should be consulted (see the Bibliography, Section 11.80). However, some aspects of the colloidal state relevant to quantitative analysis are indicated below. 

The colloidal state of matter is distinguished by a certain range of particle size, as a consequence of which certain characteristic properties become apparent. 

Before discussing these, mention must be made of the various units which are employed in expressing small dimensions. The most important of these are  

An important consequence of the smallness of the size of colloidal particles is that the ratio of surface area to weight is extremely large. Phenomena, such as adsorption, which depend upon the size of the surface will therefore play an important part with substances in the colloidal state. 

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As it is now possible by choice of suitable conditions to prepare most compounds in this form, the colloid state should be considered as a physical state in which all substances can be made to exist. Many ma terials such as proteins, vegetable fibres, rubber, etc. are most stable or occur naturally in the colloidal slate. In the colloidal stale the properties of surface are all-important. 

Many attempts have been made to characterize the stabiUty of the colloidal state of asphalt at ordinary temperature on the basis of chemical analysis in generic groups. For example, a colloidal instabiUty index has been defined as the ratio of the sum of the amounts in asphaltenes and flocculants (saturated oils) to the sum of the amounts in peptizers (resins) and solvents (aromatic oils) (66) ... 

The weathering process which eventually reduces the rock of the parent material to the inorganic constituents of soil comprises both physical and chemical changes. Size reduction from rocks to the colloidal state depends not only upon the mechanical action of natural forces but also on chemical solubilisation of certain minerals, action of plant roots, and the effects of organic substances formed by biological activity. 

Whichever method is followed, a protective agent able to induce a repulsive force opposed to the van der Waals forces is generally necessary to prevent agglomeration of the formed particles and their coalescence into bulk material. Since aggregation leads to the loss of the properties associated with the colloidal state, stabilization of metallic colloids - and therefore the means to preserve their finely dispersed state - is a cmcial aspect for consideration during their synthesis. 

In support of the association theory, colloid chemists cited non-reproduceable cryoscopic molecular weight determinations (which were eventually shown to be caused by errors in technique) and claimed that the ordinary laws of chemistry were not applicable to matter in the colloid state. The latter claim was based, not completely without merit, on the ascerta-tion that the colloid particles are large aggregates of molecules, and thus not accessible to chemical reactants. After all many natural colloids were shown to form double electrical layers and adsorb ions, thus they were "autoregulative" by action of their "surface field" (29). Furthermore, colloidal solutions were known to have abnormally high solution viscosities and abnormally low osmotic pressures. 

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. 

Sols and Gels. The essence of the behavior characteristic of the colloidal state is that double-layer interactions are as significant as bulk interactions. In other words, surface interactions are on a par with volume interactions. This condition can therefore be realized in all systems where the surface-to-volume ratios are high, i.e., at submicroscopic dimensions. 

Kiispert [106] drew attention to the fact that cuprous acetylide may form a colloidal solution. The colloidal state is favoured by the use of diluted ammonia solutions of cuprous salts. 

Due to the high temperature of the water the nitrocellulose partly coagulates from the colloidal state particularly on the surface. 

The object in undertaking such a series of preparations is twofold (1) the acquisition of a certain degree of skill in dealing with the difficulties that are encountered in the process of making pure compounds and (2) the extension and classification of information regarding types of compounds, acquaintance with the methods available in their preparation, and acquaintance with their chief reactions. This should include familiarity with the colloidal state of matter which any material may assume. 

By Processes of Reduction.—Many insoluble bodies are formed by chemical reduction and under the proper conditions may be produced in the colloidal state. 

The colloidal state is important to life. It is the way in which we get most of our food, the way we digest it, and the way the blood carries nourishment throughout our bodies. 

Colloidal arsenious oxide may be obtained in a highly dispersed condition by the vaporisation of arsenic in the electric arc and oxidation of the fume in a current of air.11 The size of the particles thus obtainable corresponds with the upper limit of the colloidal state (100 ftp.). 

As can be seen from the second of these lists, the existence of matter in the colloidal state may be a desirable or an undesirable state of affairs, and so it is important to know both how to make and how to destroy colloidal systems. 

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 ... 

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