It is a common observation that when a little sugar or salt is dissolved in water, a homogeneous and stable mixture of the two components is obtained which is called as solution. Of these two components, the one (here sugar or the salt) which is present in small quantity is called as solute while the other (here water) present in larger quantity is called as solvent.
The solution is homogeneous because the molecules or the ions of the solute become evenly distributed throughout the solvent. It is a stable system because the molecules or the ions do not settle down. Sometimes more than one solutes may be dissolved in a solvent to form a stable and homogeneous mixture. Thus, a true solution may be defined as a homogeneous and stable mixture of two or more chemical substances. In a true solution the particles are not visible even under highest power of the microscope.
It is usually implied from the term solution that some solid is dissolved in water or other liquid. But, the solvent may be either a gas or solid too, and in strict physical sense there may be 9 different kinds of solutions as follows.
When certain solid is dissolved in water (which is a very common phenomenon), the solution can pass through parchment membrane or collodion. All the substances having this property were called as crystalloids by Thomas Graham.
If some fine sand is mixed with water in a beaker, the sand particles become dispersed in water. They do not break into molecules or ions but after a very short period settle down leaving almost a clear water above. Such an unstable system of a solid and liquid is called as suspension. Due to their large size, the suspended particles are visible under microscope and in coarse suspension can be seen even with naked eyes.
If in place of sand, a little fine clay is mixed with water, the clay particles remain dispersed in it. They neither break into molecules or ions nor they settle down even after long period of time. Such a heterogeneous and stable system is called as colloidal suspension or colloidal system.
In the above colloidal system the clay particles form a dispersed phase or discontinuous phase while the water forms dispersion medium or continuous phase.
(Originally, the term ‘colloids’ was used by Thomas Graham (1861) to include such substances as starch, gum gelatin etc. whose glue-like solutions have little or no tendency to pass through parchment membrane or collodion. But it is now possible to obtain all the substances in colloidal condition which may not be glue like in nature.)
Although there are 8 different kinds of colloidal systems, only one of them (solid + liquid type) is mainly discussed here. This type of colloidal system in which the solid forms the dispersed phase and the liquid dispersion medium, has a fluid like consistency and is called as colloidal solution or simply as sol.
The particles of the dispersed phase in colloidal solution are called as colloidal particles or sol particles or micelles. The size of the colloidal particles is in between the size of particles of true solutions and suspensions (Fig. 2.2).
The colloidal particles are not visible under microscope but can be observed under an ultra-microscope.
Colloidal solutions may be of two types:
(i) Lyophilic (solvent loving) and
(ii) Lyophobic (solvent hating).
The former are also called as emulsoids and the latter suspensoids. In lyophilic sols there is affinity between the particles of dispersed phase and dispersion medium. In lyophobic sols there is no affinity between the particles of the two phases. When water is the dispersion medium the lyolyophobicphilic and sols are called as hydrophilic (water loving) and hydrophobic (water hating) sols respectively. The colloidal particles of a hydrophilic sol are hydrated i.e., water molecules are adsorbed tightly around their surfaces.
Under certain conditions, the sol may change into another form having a semi-solid consistency which is called as gel, the process being called as gelation. During this process the two phases of the sol become reversed in gel (Fig. 2.3). When a gel is converted into sol the process is called as solation.
When the change from sol into gel is reversible, the colloid is called as reversible colloid. For example, if a little agar-agar is boiled with water, a sol is obtained which on cooling changes into a gel. This gel on heating again forms the sol. On the other hand, the conversion of sol into gel may not be reversible and such colloids are called as irreversible colloids. For example, the egg albumen is sol at ordinary temperature and on heating changes into a gel. But this gel cannot be converted into a sol on cooling.
The colloidal particles are unable to pass through parchment membrane or collodion but true solutions can pass through them. Thus, a true solution if mixed with a colloidal solution can be separated from the latter by filtration through such membranes. This method of purifying a colloidal solution or separating a colloid from a crystalloid by filtration through a membrane was called as dialysis by Graham and the apparatus used as dialyser.
Graham’s dialyser is very simple and consists of a vessel open at both the ends whose lower end is tied with a parchment membrane. Impure colloidal solution, for example a mixture of starch and NaCl solutions is taken in the vessel which is then suspended in a larger container filled with water (Fig. 2.8).
After sometime, only the crystalloid i.e., NaCl solution will diffuse out into the outer container. This can be confirmed by testing the water in the outer container with silver nitrate and iodine solution. Silver nitrate test for NaCl will be positive while iodine test for starch solution negative.
Dialysis by the above method can be accelerated by maintaining a continuous flow of water in the outer container as shown in the figure.
Colloidal Nature of Protoplasm:
Although a large number of chemical substances are found in the protoplasm with water constituting the major portion, the protoplasm is not a true solution. Most of the particulate phase of the protoplasm is colloidal in nature. Again, it is not a simple colloidal system but is considered as complex colloidal system of many phases and shows many properties of the colloidal systems. The consistency of the protoplasm is both of a sol and a gel type. The cell membranes seem to be more gel-like in nature. However, both these forms are not static but constantly changing.
The colloidal nature of the protoplasm is chiefly due to the presence of gigantic protein molecules which often reach colloidal dimensions and are distributed throughout it. In fact, these macro-molecules of proteins constitute next major category of chemical substances after water in protoplasm. Moreover, all the enzymes are essentially proteins which provide large surfaces due to their large and often colloidal dimensions to catalyse most of the biochemical reactions in the protoplasm — so important for life to exist.