COLLOIDS. — The colloidal state of matter is of particular interest both because it is the condition of many industrially valuable products (rubbers, textiles, various kinds of plastic materials) and, above all, because it is the condition of many organic substances and of living matter (cf. PROTOPlessis).
It was the merit of Francesco Selmi (1864) to be the first to highlight certain properties of certain solutions which he distinguished as pseudo-solutions from true solutions. Only fifteen years later, Graham gave the name of colloids to the former and of crystalloids to the latter, defining by these terms the generally viscous, gluey state of the former and the property of crystallizing of the latter. This distinction is no longer universally valid today, since many colloids are known that crystallize and others that are not viscous.
A property that distinguishes colloids from crystalloids is that the former cannot be filtered through porous septa, i.e., they do not dialyze. This reveals the particulate nature of the colloidal state, in which the particles do not pass through the pores of the membrane. It is clear, however, that this possibility depends on the size of the pores. By using filters with pores of different and known sizes, it is possible to measure the dimensions of the particles. Ultrafiltration shortens the time required for such an operation.
Colloidal particles have been regarded as aggregates of several molecules, roughly spherical in shape, and thus the colloidal state is merely a degree of subdivision of matter between that of free molecules and that of a coarse disintegration representing a condition of the solid state. According to Ostwald, matter exhibits colloidal properties when it is subdivided into particles of the order of magnitude of 0.1–0.001 μ in diameter (μ = one thousandth of a millimeter); larger fragments fall within the coarse dispersions of the solid state, while smaller ones are of the order of molecules, ions, or free atoms in the liquid and gaseous states.
Colloidal particles are suspended, dispersed, in a dispersive medium that may be gaseous or liquid. Mists and smokes are gaseous colloids, but by far the most important are those with a liquid dispersive medium. The liquid may be of the most varied kinds and may be either pure or represent a solution; from a biological point of view, particular interest attaches to colloids in which the dispersive medium is water or, better, an aqueous solution of salts.
A colloid thus represents a two-phase system, in its simplest constitution, with one phase represented by the dispersive medium, which is continuous, and the other by the dispersed substance, which is discontinuous; the dispersed particles, consisting of molecular aggregates, are called micelles. The micelles may be of various kinds: metallic, mineral, organic;
number of bonds in gels: a) stiff colloid; b) linear colloid.
They normally carry an electric charge: a coloured colloid subjected to an electric field shows a migration of the colour to one or other pole according as the micelles (which are coloured) are positively or negatively charged. By this means (electrophoresis) it is possible to distinguish electropositive from electronegative colloids.
The colloidal condition so far described is that known as the sol. In a sol the particles are subject to an incessant movement in all directions, a kind of quivering known as Brownian movement. This movement is due to the impacts which the particles receive from the molecules of the dispersion medium in kinetic motion. As the temperature rises, this motion increases and the Brownian movement of the micelles becomes more lively. The observation of colloidal particles is possible for certain colloids by means of the ultramicroscope (Siedentopf and Zsigmondy), which is based on diagonal illumination of the object under observation in the microscope so that it is illuminated but the rays do not enter the microscope tube (fig. 1); it consists of an optical system which is applied in place of the microscope condenser (paraboloidal, cardioid, etc.).
The particles of a sol tend to sediment, and this tendency increases when the particles lose their electric charge; that is, when the colloid is in the isoelectric state. From the rate of sedimentation the size of the particles can be calculated. The time of sedimentation can be shortened if the colloid is subjected to a gravitational force greater than that of the earth, as, for example, with the ultracentrifuge of Svedberg, which generates a gravitational force 750,000 times that of the earth.
When the particles have settled to the bottom or have precipitated, the colloid passes into the condition of a gel.
There are colloids in which the particles have an affinity for water; they can thus take up and retain at their surface envelopes of water dipoles attracted by the end with a charge opposite to that of the particle (fig. 2); such colloids are said to be hydrophilic, and the uptake of water by the particles is called solvation. Even hydrophilic colloids, on loss of water, pass into the gel state, characterized by the union of the particles into a continuous phase, while the liquid remains trapped and dispersed among the condensed micelles (fig. 3). Whereas in lyophobic or hydrophobic colloids the gel does not revert to the sol state except under renewed mechanical disintegration, the hydrophilic colloid reverts to the sol condition on taking up water. The reversibility of the colloids or their thixotropy is here in question. In gelation there may be a contraction of the mass and expulsion of liquid (syneresis) and the formation of a coacervate.
Many properties of lyophilic colloids, such as the high viscosity of the sol, the great uptake of water by the gel, their swelling and their easy thixotropy, have been explained by the observation of the linear or filamentous condition of the dispersed particles.
In many colloids such colloidal particles consist of a single giant molecule (macromolecule) obtained by polymerization of a simple molecule.
Of the substances which constitute living matter, carbohydrates, lipids and proteins consist of macromolecular colloids. These substances are therefore at one and the same time true solutions, in that they are dispersed in individual molecules, and colloids, in that the molecules, by reason of their size, behave like micelles.
Hence, with the introduction of the concept of filamentous colloids, it has no longer been possible to maintain the classification of Ostwald based on the diameters of the particles: Staudinger has introduced a new criterion for delimiting the dimensions of particles in the colloidal state, based not on the diameter of the particles but on the number of their atoms. Thus, according to this author, particles are in the colloidal state if the number of atoms composing them lies between 10^3 and 10^9.
The reason for the great swelling of filamentous colloids is illustrated in fig. 4. Whereas in a gel of spherocolloid a mixture, once solvated, breaks away from the mass and passes into the sol state, in a filamentous gel, which may be pictured as a compressed felt, much liquid must penetrate between the filaments to allow them free movement. Thus it is explained why the passage to the sol state is easier for a swollen filamentous gel than for a spherical gel: the points of cohesion between the particles are far more numerous in the spherocolloid gel than in the linear colloid gel (fig. 5).
According to the degree of polymerization, colloids with more or less long fibres are obtained. Consider, for example, the polymerization of amino-acids to form a di- and then a polypeptide (fig. 6): the longer the fibres, that is, the higher the degree of polymerization, the more do the characteristic properties of linear colloids appear. There are distinguished hemicolloids with filaments formed of 50-500 units, little viscous, little swellable; mesocolloids (500-5000 units), more viscous and with gels more swellable; eucolloids (over 5000 units), very viscous, fibrous, and with gels very swellable.
The filamentous condition of the colloidal particles has made it possible to explain many conditions of anisotropy revealed by polarized light, anisotropy which indicates an orientation of the particles, an orientation which in spherocolloids is difficult to conceive. Indeed, simple phenomena of movement, of flow, orient the particles, on the basis of the internal friction which manifests itself, with the major axis parallel to the direction of flow. Thus, in addition to the isotropic condition, which in linear colloids indicates a complete disorder of the particles, there is added a condition of anisotropy due to an orientation of the particles. This may exist in moving sols and can be established in gels. Conditions of stretching or contraction may increase the orientation. The conditions of orientation are: the nematic, in which the particles are all oriented parallel to one another but disposed at various heights, as, for example, may be taken as a model a thread of wool, considering as particles the hairs of which it is composed; the smectic, in which to the parallel orientation is added a regular stratification, as occurs in many colloidal membranes; the cubic, in which the particles assume definite positions according to a three-dimensional lattice, thus assuming the true value of a crystalline lattice. In this case we are dealing with pseudocrystalline gels; an example is given by cellulose. Examination of this substance with X-rays reveals indeed a typical Lane lattice which indicates the crystalline orientation of the particles.
A classification of colloids may be made as follows:
A) Inorganic colloids (by dispersion, lyophobic, irreversible gels): a) spherocolloids (metallic, crystalline micelles); b) linear (colloidal silica).
B) Organic colloids. — I. Lyophobic (by dispersion, spherocolloids, irreversible gels): a) micellar (e.g., oil in water); b) molecular (e.g., latices). — II. Lyophilic: a) by dissolution of micellar (reversible): a) spherocolloids (sols little viscous); b) linear (viscous sols, swellable gels), homopolar, heteropolar (e.g., soaps). — B) by solution or macromolecular: a) spherocolloids (sols little viscous, gels little swellable, homopolar (e.g., glycogen acetate), heteropolar (e.g., ovalbumin, haemoglobin); b) linear (very viscous sols, very swellable gels). — 1) hemicolloids (macromolecules of 50-500 units: e.g., hemicelluloses); 2) mesocolloids (chains of 500-5000 units: e.g., lichenin); 3) eucolloids (chains with over 5000 units: e.g., cellulose).