To understand the difference between a crystalloidal and a colloidal solution, one should remember that the forces which keep the solute particles in solution are roughly as follows:
(a) Inherent movement of the solute particles, i.e., their diffusibility.
(b) Inherent movement of the solvent molecules which continuously dash against the solute particles and thus help to keep them in solution.
(c) Electric charge—positive or negative—carried by the particles, which by constant attraction and repulsion also help uniform solution.
(d) Hydration (carrying water molecules) of the solute particles, and so on.
If the solute particles be very small (below 1 mµ—amicrons) all these forces will be acting to their maximum and the result will be a permanent true solution. Now, if the solute particles be gradually made larger and larger, (1-200 mµ—submicrons) their own movement (diffusibility) will gradually be reduced and ultimately will be almost nil.
The first factor, mentioned above, will therefore, be completely out of the field. The other forces, such as the dashing by the solvent molecules, electric charge, etc., will then try to keep the large solute particles somehow in solution and will be able to do so up to a certain extent. Such a solution will be unstable and heterogeneous—a sort of pseudosolution. This is a colloid.
If then, the solute particles be made still larger (over 200 mµ—microns) all the forces will completely fail and the solute particles will not go into solution at all. Hence, if the solute-solvent relations be studied as a series of phenomena, it will be found that, at one extreme there is complete solubility (true solution), at the other extreme—complete insolubility, while in the intermediate stages there will be a phenomenon of semisolubility. This is colloidal solution.
Thus other factors remaining constant, the real difference between a true solution and colloidal solution lies in the size of the solute particles, and not upon their chemical nature.
Thus a colloid may be defined as a substance, (e.g., gelatin or cell cytoplasm), which by reason of the size of its molecules, is slowly diffusible rather than soluble in water (its hydrates being gelatinous in consistence), and is incapable of passing through an animal membrane (a semipermeable membrane). In this substance the solute particles are proportionally larger than the solvent molecules.