The Chemistry of Plant Life

by Roscoe Wilfred Thatcher

CATALYSIS AFFECTED BY THE COLLOIDAL CONDITION

The velocity of a chemical reaction is the net result of opposing influences. It is directly proportional to the chemical affinity of the reacting bodies and inversely proportional to the so-called "chemical resistance." The first factor, chemical affinity, is not easily measured, as it depends upon both the mass of the reacting molecules, atoms, or ions, and their attraction for each other. But if, as the result of chemical affinity, a reaction takes place, it is evident that the time required for its completion (which measures the velocity of the reaction) is made up of two separate periods. The first is the time required for the reacting molecules to come into contact; and the second is that required for the molecular rearrangement which constitutes the reaction. Clearly, the time required for the substances to come into molecular contact will be greatly diminished if they are mutually adsorbed in large quantities on the extended surface area of some colloidal catalyst which is present in the mixture rather than scattered throughout its entire volume. The application of this principle to the catalysis of hydrolytic reactions is not apparent, if it is considered that the H₂O molecules which cause the hydrolysis are those of the solvent itself; but is clear on the assumption (which is discussed in the following chapter) that the water which enters into a colloidal complex is in multimolecular form, represented by the formula (H₂O)_n, in which the oxygen atoms are quadrivalent and, hence, much more active chemically than as illustrated in the simple solvent action of water.

Hence, the surface adsorption of reacting bodies by a colloidal catalyst may have a very important influence in decreasing the time required to bring the reacting molecules into intimate contact, and so increasing the velocity of the reaction.

But the colloidal condition of the catalyst may also aid in decreasing the "chemical resistance" which tends to slow up the reaction. Chemical resistance may be understood to be the internal molecular friction of the densely packed atoms within the reacting molecule, which tends to prevent the molecular rearrangement and so to prolong the second period of the reaction time. To overcome this friction and so decrease the reaction time, some form of energy is necessary. If there be present in the solution in which the reaction is taking place some colloidal catalyst, and if the reacting bodies are concentrated at the surface boundaries between the two phases of the colloidal system, they may be conceived to be within the sphere of influence of the surface energy of the dispersed particles of the catalyst, so that this may furnish the energy necessary to overcome the chemical resistance of the reacting bodies, and so to speed up the second portion of the reaction time.

From these considerations, it would appear that the colloidal condition of such catalysts as enzymes, etc., has much to do with their ability to increase reaction velocities, both by reducing the time necessary for the reacting bodies to come into molecular contact and by furnishing the energy to overcome the chemical resistance to the molecular rearrangement which constitutes the reaction itself. Evidence in favor of the accuracy of this view of the nature of the catalytic action of colloidal substances is afforded by the facts that catalysts accelerate the velocity of reversible reactions in either direction and that they do not change the point of final equilibrium, in any case; that is, they do not affect the nature or direction of the reaction, but only accelerate a chemical change which would otherwise take place more slowly because of the stability (or chemical resistance) of the molecules involved, or their inability to come quickly into intimate molecular contact.

These facts and principles have been clearly established in many studies of the nature of enzyme action (enzymes are typical colloidal catalysts) and probably apply equally well to the action of other types of colloidal catalysts. On the other hand, the catalytic action of certain inorganic and non-colloidal substances, such as the action of acids in accelerating the hydrolysis of carbohydrates, etc., may be conceived to be due to chemical influences upon the internal molecular resistance, which are similar in their effects, but entirely different in their mechanism, from the physical effects of the surface boundary phenomena of the colloidal catalysts.

INDUSTRIAL APPLICATIONS OF COLLOIDAL PHENOMENA

Large numbers of industrial processes are based upon colloidal phenomena. Many of these processes were known and practiced long before the nature of the phenomenon itself was understood. But with the coming of the knowledge of the nature, causes, and possibilities of the control, of the colloidal condition of the materials involved, immense improvements in the economy of the process, or the quality of the end-products, have been worked out, in many cases. Many volumes of treatises concerning the industrial applications of colloidal phenomena have been written. Any discussion of these would be out of place here; but the following list of examples will serve to illustrate the immense importance of these matters both in industry and to the needs of everyday life: the tanning of leather; the dyeing of fabrics; vulcanizing rubber; mercerizing cotton; sizing textile fabrics; manufacture of mucilages and glues; manufacture of hardened casein goods; manufacture of celluloid; production of colloidal graphite for lubrication; the prevention of the smoke nuisance by electric deposition; the purification of sewage; the manufacture of soaps; the manufacture of butter, cheese, and ice cream; fruit jellies, salad dressings, etc. This list could be extended to a great length, but is already long enough to emphasize the very great importance and practical value of colloidal phenomena in daily life.

NATURAL COLLOIDAL PHENOMENA

Many of the phenomena of nature are colloidal in character. These may be observed in the mineral, the animal, and the vegetable kingdoms. Here, again, a lengthy discussion of the nature of these phenomena would be out of place in this connection, and a few typical examples will serve to illustrate the general importance in nature of this property of matter.