Colloids

COLLOIDS. - The colloidal state of matter is of particular interest both because it is the condition of many industrial products of very great practical value (rubbers, yarns, plastic products of various kinds) and, above all, because it is the condition of many organic substances and of living matter (v. PROTOPLASM).

It was the merit of Francesco Selmi (1864) to bring out for the first time certain properties of certain solutions which he distinguished as pseudo-solutions from true solutions. Only 15 years later, Graham gave the name of c. to the former and of crystalloids to the latter, defining with these terms the colloidal, viscous (generally) state of the former and the property of crystallizing of the latter. This distinction is no longer general today, since many c. are known that crystallize and others that are not viscous.

A property that distinguishes c. from crystalloids is that the former cannot be filtered through porous septa, that is, they do not dialyze. This reveals the corpuscular condition of the colloidal state, whereby the particles do not pass through the pores of the membrane. It is clear, however, that such a possibility depends on the size of the pores. By means of filters with pores of different and known size, the dimensions of the particles can thus be measured. Ultrafiltration shortens the time required for such an operation.

Colloidal particles have been considered as aggregates of several molecules, of nearly spherical shape, and therefore the colloidal state is nothing but a degree of subdivision of matter between that of free molecules and that of a coarse disintegration which represents 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. (μ = 1 thousandth of a millimeter) in diameter; larger fragments fall among the coarse dispersions of the solid state and smaller ones are of the order of molecules, ions or free atoms of the liquid and gaseous state.

Colloidal particles are suspended, dispersed, in a dispersing medium which may be gaseous or liquid. Mists and fumes are gaseous c., but by far more important are those with a liquid dispersing medium. The liquid may be of the most varied kinds and may either be pure or constitute a solution; from a biological point of view, particular interest attaches to c. in which the dispersing medium is water or, better, an aqueous solution of salts.

A c. thus represents a two-phase system, in its simplest constitution, one phase being represented by the dispersing medium, continuous, and the other by the dispersed substance, discontinuous; the dispersed particles made up of molecular aggregates are called micelles. The micelles may be of various kinds: metallic, mineral, organic;

and they normally carry an electric charge: a colored colloid subjected to an electric field reveals a migration of the color toward one pole or the other depending on whether the (colored) micelles are positively or negatively charged. By this means (electrophoresis) one can distinguish electropositive colloids from electronegative ones.

The colloidal condition described thus far is what is called a sol. In a sol the particles are subject to an incessant movement in the most disparate directions, a kind of quivering known as Brownian motion. This movement is due to the impacts that the particles receive from the molecules of the dispersing medium in kinetic motion. As the temperature rises, this motion increases and the Brownian motion of the micelles also becomes livelier. The observation of colloidal particles is possible for certain colloids by means of the ultramicroscope (Siedentopf and Zsigmondy), based on a diagonal illumination of the object observed under the microscope in such a way that it is illuminated but the rays do not penetrate into the tube of the microscope (fig. 1); it consists of an optical system that is applied in place of the microscope's condenser (paraboloid, cardioid, etc.).

The particles of a sol tend to settle, 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 one can calculate the size of the particles. The sedimentation time can be shortened if the colloid is subjected to a gravitation greater than that of the earth, as, for example, with the ultracentrifuge of Svedberg, which generates a gravitation 750,000 times that of the earth.

When the particles have settled to the bottom or have precipitated, the colloid comes to be in the condition of a gel.

There are colloids in which the particles have an affinity for water, that is, they can attract and retain on their surface clusters of water dipoles attracted by the end with a charge opposite to that of the particle (fig. 2); such colloids are called hydrophilic and the uptake of water by the particles is called solvation. Hydrophilic colloids too, through 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 densified micelles (fig. 3). Whereas in hydrophobic or lyophobic colloids the gel does not return to a sol except through a new mechanical action of disintegration, the hydrophilic colloid returns to the sol condition through the uptake of water. One speaks of the reversibility of colloids or of thixotropy. In gelation there may be a contraction of the mass and expulsion of liquid (coacervation) 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 gels, 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 gigantic molecule (macromolecule) obtained by processes of polymerization of a simple molecule.

Of the substances that constitute living matter, the glucides, the lipids and the protides are made up of macromolecular colloids. These substances are therefore at the same time both true solutions, insofar as they are dispersed in individual molecules, and colloids, insofar as the molecules, because of their dimensions, behave as micelles.

Consequently, with the introduction of the concept of filamentous colloids, Ostwald's classification based on the diameters of the particles could no longer be maintained: Staudinger introduced a new criterion for delimiting the sizes of material particles in the colloidal state, based no longer 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 atoms composing them number between 10² and 10³.

The reason for the great swelling of filamentous colloids is illustrated by fig. 4. Whereas in a gel of a spherocolloid a mixture, once solvated, detaches from the mass and passes into the sol state, in a filamentous gel, which one may picture as a compressed felt, a great deal of liquid must penetrate between the filaments to allow them free movement. Thus it is explained why the passage to a sol 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 gel of a spherocolloid than in the gel of a linear colloid (fig. 5).

From the degree of polymerization one obtains colloids with fibers more or less long. Consider, for example, the polymerization of amino acids to form a dipeptide and then a polypeptide (fig. 6): the longer the fibers are, that is, the higher the polymerization, the more the characteristic properties of linear colloids manifest themselves. One distinguishes hemicolloids with filaments formed of 50-500 units, slightly viscous, slightly swellable; mesocolloids (500-5000 units), more viscous and with gels, more swellable; eucolloids (over 5000 units), very viscous, capable of being drawn into threads, and with highly swellable gels.

The linear condition of colloidal particles has made it possible to explain many conditions of anisotropy on examination with polarized light, an anisotropy that indicates an orientation of the particles, an orientation that in spherocolloids is difficult to conceive. Indeed, simple phenomena of movement, of flow, orient the particles, on the basis of the internal frictions that manifest themselves, with the major axis parallel to the direction of flow. Thus, in addition to the isotropic condition, which in linear colloids indicates a full disorder of the particles, there is added a condition of anisotropy due to an orientation of the particles. This may be present in moving sols and can become established in gels. Conditions of stretching or contraction can increase the orientation. The conditions of orientation are: the nematic one, in which the particles are all oriented parallel to one another but arranged at various heights, as, for example, a strand of wool may be taken as a model, considering the hairs composing it as the particles; the smectic one, in which to the parallel orientation there is added a regular stratification, as occurs in many colloidal membranes; the cubotactic one, in which the particles assume definite positions according to a three-dimensional lattice, thus taking on the true value of a crystal lattice. In this case we are dealing with pseudocrystalline gels; an example is given us by cellulose. Examination of this substance with X-rays in fact reveals a typical Lane lattice that indicates the crystalline orientation of the particles.

A classification of colloids can be made in the following way:

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., lattices); II. Lyophilic: A) by dissolution or micellar (reversible): a) spherocolloids (slightly viscous sols); b) linear (viscous sols, swellable gels), homopolar, heteropolar (e.g., soaps). B) by solution or macromolecular: a) spherocolloids (slightly viscous sols, slightly swellable gels), homopolar (e.g., glycogen acetate), heteropolar (e.g., ovalbumin, hemoglobin); b) linear (very viscous sols, highly 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., celluloses).

Alberto Stefanelli