COLLOIDS
As we have already seen, living organisms are formed essentially of
liquids. These liquids are solutions of crystallizable substances or
crystalloids, and non-crystallizable substances or colloids--a
classification which we owe to Graham.
The liquids are the most important constituents of a living organism, since
they are the seat of all the chemical and physical phenomena of life. The
junction of two liquids of different concentration is the arena in which
takes place both the chemical transformation of matter and the correlative
transformation of energy. In a former chapter we have passed in review the
class of crystalloids, we will now turn our attention to the characteristic
properties of colloids.
_Colloids._--Colloids differ from crystalloids in that they do not form
crystals from solution, being completely amorphous when in the solid state.
The solution of a colloid solidifies in the same form which it possessed in
the liquid state, the solvent being enclosed in the meshes of a sort of
network formed by the solute. This form is approximately retained even
after the water has evaporated by drying, the passage from the liquid state
of solution to the solid state being effected through a series of
intermediary states, such as a clot, coagulum, or jelly. This passage from
the state of solution into a state of jelly is called coagulation. Some
colloids, such as gelatine, coagulate with cold; while others, such as
egg-albumin, coagulate with heat. Some, like the caseine of milk, require
the addition of certain chemical substances to set up coagulation; while
still others, such as the fibrin of blood, appear to coagulate
spontaneously. The physical phenomena of {37} coagulation are still but
little understood. In some cases it is a reversible phenomenon, thus
gelatine coagulated by cold is redissolved by heat; whereas with other
colloids the process is irreversible, albumin coagulated by heat is not
redissolved on cooling.
Colloids in a state of coagulation have a vacuolar or sponge-like
structure. The solvent is imprisoned in the vacuoles of the clot, and is
expelled little by little by its retraction. Colloids diffused in water are
usually called colloidal solutions, but they are not true solutions. Such a
pseudo-solution of a colloid is called a "sol," while a colloid in a state
of coagulation is called a "gel." Colloidal solutions spread but little,
diffuse very slowly in the liquids of the body, and cannot penetrate
organic membranes.
Colloidal solutions diffuse light, unlike crystalloid solutions, which are
transparent. We all know how the trajectory of a beam of sunlight through a
darkened room is rendered visible by the particles of dust. In the same way
if a colloidal solution is illuminated by a transverse ray of light, the
light is diffused by the molecules of the colloid in semi-solution, and the
liquid appears faintly illuminated on a dark background. The light diffused
by a colloidal solution is polarized, which shows that it is reflected
light,
Siedentopf and Sigmondy have applied this principle of lateral illumination
on a dark background to the construction of the ultra-microscope. With the
aid of this instrument we may not only see, but count the particles in a
colloidal solution, which is in reality merely a pseudo-solution or
suspension, in contradistinction to the true solution of a crystalloid.
Colloidal solutions possess only a very feeble osmotic pressure. The
lowering of the freezing point and the other corresponding constants are
also quite insignificant. This arises from the fact that the molecules of a
colloid are extremely large when compared with those of a crystalloid. For
example let us take colloidal substance whose molecular weight is 2000. A
solution containing 40 grammes per litre would have an osmotic pressure
only one-fiftieth of that of a {38} solution of similar strength of a
crystalloid whose molecular weight was 40.
Not only so, but on measuring the molecular concentration, the osmotic
pressure, and the other constants of a colloidal solution, we find values
even lower than those which we should expect from a consideration of its
molecular weight. This is probably due to the tendency of a colloid to
polymerization, i.e. to form groups or associations of molecules. Suppose,
for instance, that the molecules of a colloidal solution are aggregated
into groups of ten. Since each group plays the part of a simple molecule,
the osmotic pressure will be ten times less than that corresponding to the
quantity of the solute present. Such a group of molecules is called by
Naegeli a "micella."
Similar phenomena of aggregation may be observed in the molecules of many
inorganic substances. The molecule of iodine, for example, is monatomic at
1200deg C., but becomes diatomic at the ordinary temperature. Sulphur at
860deg C. is a gas with a vapour density of 2.2, while at 500deg C. its
vapour density rises to 6.6. In both of these cases two or more molecules
of the element have been condensed into one as a result of the fall of
temperature.
We frequently find that two successive cryoscopic observations on the
freezing point of the same colloidal solution will vary. This is due to the
extreme sensitiveness of the micellae, which absorb or abandon their extra
molecules under the slightest influence. This mobility in the constitution
of the micellae appears to be one of the principal causes of the peculiar
properties of colloidal solutions.
The phenomenon of polymerization appears to be reversible. The micellae are
formed under certain conditions, and are disintegrated when these
conditions are removed. The osmotic pressure varies in the same manner,
diminishing with polymerization and augmenting with the disintegration of
the micellae. One may easily understand what an important role is played by
this alternate polymerization and disintegration in the phenomena of life.
Most colloidal substances are precipitated from their solutions by the
addition of very small quantities of electrolytic {39} solutions.
Non-electrolytic solutions do not appear to provoke this precipitation.
This is not a chemical action, for an exceedingly small quantity of an
electrolyte is able to precipitate an indefinite quantity of the colloid.
The precipitation is probably due to the electric charges carried by the
dissociated ions of the electrolytes.
When an electric current is passed through a colloid solution, the course
of the molecules of the colloid is sometimes towards the cathode and
sometimes towards the anode, according to the nature of the colloid and of
the solvent. This displacement would appear to indicate a difference of
electric potential between the molecules of the colloid and those of the
solvent. Hardy has shown that in an alkaline solution the molecules of
albumin travel towards the anode, while in an acid solution they travel
towards the cathode.
_Metallic Colloids._--Carey Lea and afterwards Crede succeeded in obtaining
silver in colloidal solution by ordinary chemical means. Professor Bredig
has introduced a more general method of obtaining a number of metals in
colloidal solutions in a state of great purity. He causes an electric arc
to pass between two rods of the metal immersed in distilled water. The
cathode is thus pulverized into a very fine powder which rests in
suspension in the liquid, constituting a colloidal solution. Bredig has in
this way prepared sols of platinum, palladium, iridium, silver, and
cadmium.
_Catalytic Properties of Colloids._--Catalysis is the property possessed by
certain bodies of initiating chemical reaction. The mass of the catalyzing
body has no definite proportion to that of the substances entering into the
reaction, and the appearance of the catalyzer is in no way altered by the
reaction.
Ostwald has shown that catalysis consists essentially in the acceleration
or retardation of chemical reactions which would take place without the
action of the catalyzer, but more slowly.
Catalytic reactions are very numerous in chemistry. The inversion of sugar
by acids, the etherization of alcohol by sulphuric acid, the decomposition
of hydrogen peroxide by {40} platinum black are all instances of catalysis.
Fermentation by means of a soluble ferment or diastase, a phenomenon which
may almost be called vital, is also a catalytic action. The action of
pepsin, of the pancreatic ferment, of zymase, and of other similar ferments
has a great analogy with the purely physical phenomenon of catalysis. The
diastases are all colloids, and so are many other catalyzers.
A catalyzer is a stimulus which excites a transformation of energy. The
catalyzer plays the same role in a chemical transformation as does the
minimal exciting force which sets free the accumulation of potential energy
previous to its transformation into kinetic energy. A catalyzer is the
friction of the match which sets free the chemical energy of the powder
magazine.
Bredig has studied the catalytic decomposition of hydrogen peroxide by
metallic colloids prepared by his electric method. He found that 1
atom-gramme of colloidal platinum gives a sensible catalytic effect when
diluted with 70 million litres of water. Caustic soda and other chemical
substances inhibit the catalytic action of colloidal platinum in the same
way as they inhibit the fermenting action of diastase. The curve of
decomposition of hydrogen peroxide by colloidal platinum may be compared
with the curve of fermentation by emulsin. Both are equally affected by the
addition of an alkali. Many other chemical and physical agents have a
similar inhibitory action on the catalysis of colloidal metals and on
diastasic fermentation. Thus a mere trace of sulphuretted hydrogen or
hydrocyanic acid will paralyse the action of a colloidal metal, just as it
does that of a ferment. This is what Bredig calls the poisoning of metallic
ferments.
We may hope that the further study of catalysis, a purely physico-chemical
phenomenon, may throw more light on the mechanism of diastasic
fermentation, which is essentially a vital reaction.
It must not be forgotten that all classification is artificial and
arbitrary, and only to be used as long as it facilitates study. This
observation is particularly applicable to the classification of substances
into crystalloids and colloids. {41} There is no sharp line between the two
groups, the passage is gradual, and it is impossible to say where one group
ends and the other begins. Many colloids such as haemoglobin are
crystallizable, and many crystallizable substances are coagulable. Many
substances appear at one time in the crystalloid state and at another time
in the colloidal state, so that instead of dividing substances into
colloids and crystalloids, we should rather consider these expressions as
denoting different phases assumed by the same substance.
In order to define clearly our various classes and divisions, we are apt to
exaggerate slight differences of properties or composition. We say that
colloids have no osmotic pressure, whereas in fact the osmotic pressure of
the colloids though feeble plays a very important part in the phenomena of
life.
So in other departments of science--a factor which is almost infinitesimal
may yet exercise a vast influence on the results. It is by infinitesimal
variations of pressure, a thousandth of a millimetre or less, that we
obtain the various degrees of penetration in the Roentgen rays.
The division into solutions and pseudo-solutions or suspensions is also an
arbitrary one. A true solution is also a suspension of the molecules of the
solute. There is no essential difference between a solution and a
suspension, but only a difference in the size of the molecules, or
agglomerations of molecules, in one case so small as to be transparent, and
in the other case just big enough to diffuse light. There are moreover many
properties common to colloidal solutions and suspensions of fine powders,
such as kaolin, mastic, charcoal, or Indian ink. These particles in
suspension are precipitated by solutions of electrolytes in a manner
similar to the coagulation of colloids.
The surface of every liquid is covered by a very thin layer, a sort of
membrane slightly differentiated from the rest of the liquid. This membrane
may be a chemical one, a pellicular precipitate like that which is formed
by the contact of two membranogenous liquids. On the other hand, the
membrane may not differ from the subjacent liquid in chemical composition,
but only in physical properties. If we {42} consider the molecules in the
middle of a liquid, each molecule is subjected to the cohesive attraction
of molecules on every side, attractions which neutralize one another. At
the surface of the liquid, however, there are quite other conditions of
equilibrium. There each molecule is drawn downwards towards the centre of
the liquid, and there is no compensating attraction in an opposite
direction. The resultant pressure is normal to the surface of the liquid,
and is mechanically equivalent to an elastic membrane which tends to
diminish the surface, and hence the volume of the liquid. We may therefore
regard this surface tension as acting the part of a veritable physical
membrane.
There is a still further differentiation of the surface of a liquid. When
the liquid is not a simple one, but complex as in a solution, we find that
the concentration of the solute is greater at the surface than in the
interior. This is the so-called phenomenon of "adsorption," which is
another cause for the production of a physical membrane covering the
surface of a liquid.
Substances in a colloidal state have a great tendency to form these
chemical or physical membranes at the point of contact between the
colloidal solute and the solvent. This is probably the reason why the
coagulum of a colloidal liquid usually presents a vacuolar or spongy
structure.
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