In this article we will discuss about Colloids:- 1. Introduction to Colloids 2. Definition of Colloids 3. Terminology 4. Precipitation 5. Protective Colloids 6. Stability 7. General Properties 8. Types 9. Physiological Application.
Contents:
- Introduction to Colloids
- Definition of Colloids
- Colloid Terminology
- Precipitation of Colloidal Particles
- Protective Colloids
- Stability of Colloidal System
- General Properties of Colloidal Solution
- Types of Colloidal Solution
- Physiological Application of Colloids
1. Introduction to Colloids:
Graham (1861) first distinguished two types of solutions-crystalloidal which diffused through a parchment or animal membrane, and colloidal which did not.
The difference is only one of the size of the solute particles and is unrelated to their chemical nature.
The particles are of the size of small molecules (like sugar or urea) and they form a crystalloidal or a true solution.
In such a solution, the difference in size of solute and solvent molecules is relatively small, so that the solution can be called homogeneous. But if the particles are large compared to the solvent molecules, the solution can be called heterogeneous, and if the particles do not separate when the mixture is allowed to stand, this is called a colloidal solution.
About 90% of the organic matter of living tissues is present in the colloidal state.
The colloids depend much on surface area. A physiological example of the importance of surface area may be pointed out in case of the R.B.C. which contains the oxygen-carrying pigment hemoglobin.
As blood circulates through the capillaries of the lungs, oxygen diffuses the surface of the corpuscles and unite with the hemoglobin to form oxy-hemoglobin. This is performed by a quick process. This is facilitated by the great surface area for taking up oxygen.
2. Definition of Colloids:
Certain substances such as proteins, polysaccharides do not diffuse through parchment or animal membrane although they form homogeneous or heterogeneous solution. These substances are called colloids.
3. Colloid Terminology:
All colloid systems are composed of two phases of matter, one of which, represented by the colloidal particles, is called the “dispersed phase”.
The second phase is the “dispersions medium” which is also designated the external phase. The term “colloid sol” or “simply sol” is synonymous with “colloidal solution”.
A hydrosol presents a colloidal system in which water is the dispersion medium while alcohol is the dispersed medium in an alcosol.
A lyophobic colloid system “suspensoid” is one in which there is little attraction between the colloid particles and the dispersion medium.
A hydrophobic colloid system is a lyophobic system in which the dispersion medium is water.
A lyophilic colloid system “emulsoid” is one in which the colloidal particles have a high affinity for the dispersion medium and are combined with some of the medium.
A Gel is a lyophilic colloid system that is more or less rigid. Gels generally are made up of “brush heap” fibrillar structures surrounded by dispersions medium.
Gels are freely permeable to non-colloid ions and the molecules. The larger aggregates of colloidal particles formed in the process of gel formation are called “miscelles”.
Size of the Colloidal Particles:
Generally the size of each particle is 10-100 nm (submicrons). Only visible under ultra-microscope.
Determination of the Size of Colloidal Particles:
i. By Ultrafiltration.
ii. By Diffusion Rate.
iii. By the Ultracentrifuge.
iv. By Light Scattering.
v. By X-ray Analysis.
vi. By Osmotic Pressure.
i. By Ultrafiltration:
(a) Colloidal particles readily pass through ordinary filter paper and porcelain filters. A series of collodion membranes with graded pore-sizes may be prepared and used for the determination of colloidal particle size.
(b) The colloidal solution tested is filtered under pressure through the series of filters.
(c) The size of the colloidal particle is between the pore size of the first filter through which it will not pass and the next larger pore size through which it will pass.
ii. By Diffusion Rate:
(a) The rate at which particles diffuse in solution has been used for the estimation of particle size and of molecular weights.
(b) The method is applicable to particles of colloidal and sub-colloidal size such as those of the sugars and amino acids.
(c) The rate of diffusion is best measured by optical methods, such as refractive index, light absorption and Tyndall effect, in which the solution is not disturbed.
(d) The diffusion coefficient represents the number of moles of solute diffusing across unit area per unit time under a concentration gradient of unity. In colloids with spherical particle, the diffusion coefficient, D, is related to the molecular weight, M, by the equation
where, R = gas constant, N = Avogadro’s number, η = Viscosity of the medium in poises.
V = Partial specific volume, M = Molecular weight, T = Absolute temperature.
iii. By the Ultracentrifuge:
(a) Ultracentrifuges have been constructed in which the rotor revolves 60,000 times per minute giving centrifugal forces of the order of 500,000 times gravity.
(b) When solution of proteins is placed in glass in the rotor of the ultracentrifuge, they are subjected to a force depending upon the angular velocity of the centrifuge and the distance of the colloidal particles from the axis of rotation.
(c) The rate at which the particles move is dependent upon this force and also upon the shape, size, and density of the particles, and upon the density and viscosity of the suspending medium.
(d) In case of homogeneous substances where the particles are all alike the particles move under the centrifugal force as a sharp boundary in the medium.
Since the particles absorb more light than the medium, their rate of movement may be determined by photographing the position of the particle boundary in the cells at various time intervals.
(e) Ultracentrifugation is used in the separation and purification of virus proteins mixed with tissue proteins.
The virus proteins of very large molecular size are centrifuged to the bottom of cell at a centrifuge speed at which these smaller tissue proteins are left suspended in the media.
Centrifugation is used in the separation of mitochondrial, microsomal, and nuclear fractions from disrupted tissue cells.
iv. By light Scattering:
(a) The scattering by light by colloidal particles affords a valuable method for determination of particle size and it has specific application in estimating the molecular weights of proteins.
(b) When a beam of light is passed through a colloidal solution, a part of the light is transmitted and a part is scattered.
(c) Determination of molecular weight from light scattering is useful over a much wider range than determination from osmotic pressure.
(d) The lower limit is about 12,000 but for very high molecular weights light scattering is greatly superior since the intensity of scattering goes up with increasing molecular weights, osmotic pressure goes down.
v. By X-ray Analysis:
From the diffraction patterns of X-rays that have passed through particles of matter, it is possible to calculate particle size.
vi. By Osmotic Pressure:
This method is successful as some of the smaller molecules leak through the membrane used, while the very large molecules give a very low osmotic pressure relative to the weight concentration and a large probable error of measurement.
Electric Charge of Colloidal Particles:
The colloidal particles are electrically charged. This charge is of much importance in stabilizing colloidal solution.
The particles repel each other and remain in suspension. The neutralization of the charges causes precipitation or flocculation of colloids.
Streaming Potential or Flow Potential:
When water is present in a glass capillary, the walls of the capillary are negatively charged and the water positively charged. If water is forced to flow through the capillary from left to right, a potential develops between the ends of the capillary. This potential is referred to as the streaming or flow potential.
It varies with the nature of the liquid and the capillary. The glass capillary probably becomes negative by absorbing OH ions from water, leaving the H+ ions to charge water molecules. This positively charged water molecules are mobile.
As water flows through the capillaries, the positive charges accumulate in excess at one end leaving an excess of negative charges at the other. This causes potential differences which opposes the flow.
The flow of fluids through the membranes of tissues develops streaming potentials which contribute to the potential differences across such membranes.
4. Precipitation or Flocculation of Colloidal Particles:
In order to precipitate emulsoids (hydrated colloids) by salts, sufficient salts must be added to dehydrate the emulsoid. At the same time the positive or negative ion of the salt neutralizes the charge upon the particles leading to precipitation.
The colloidal protein particles of blood serum may be precipitated by adding a large amount of (NH4)2SO4. The precipitations of emulsoids by adding large amount of soluble salts is referred to as “salting out”. Lyophilic colloids are precipitated by addition of colloids of the opposite charge, then neutralization of colloids occurs.
In case of lyophobic colloids they can be precipitated by addition of electrolytes. It neutralizes the charge. This precipitation of lyophobic colloid by the addition of electrolytes prevented by the addition of lyophilic colloids.
This is called protective action of lyophilic colloids on lyophobic colloids, this is expressed by gold number (least weight in mg of lyophilic colloid which can prevent the conversion of 10 cc of standard gold solution from red to violet by the addition of 10 per cent NaCl solution).
5. Protective Colloids:
i. When a gelatin solution is added to a gold sol, the particles of the emulsoids (gelatin) are absorbed by the particles of the suspensiods (gold) and the gold particles become much more resistant to precipitation.
ii. Protective colloids play an important role physiologically. Some of the calcium phosphate of blood is held in colloidal suspension by the protective action of the proteins present.
iii. The bile salts also act as protective colloids to keep sparingly soluble cholesterol and the calcium salt of bilirubin in colloidal suspension. Gallstones may result from the precipitation of such substances in the absence of sufficient protective colloids.
iv. Protective colloids in urine may prevent bladder stone formation.
6. Stability of Colloidal System:
i. Hydration shell in case of lyophilic protein.
ii. Charge of each colloidal particle in the colloidal system.
iii. Brownian movement.
7. General Properties of Colloidal Solution:
A. Brownian Movement:
i. The continuous motion of the particles is known as Brownian Movement.
ii. The particles are kept in movement by continuous buffeting by the solvent molecules which are themselves always in motion.
iii. The rate of Brownian movement depends on the size of the particles, the smaller particles are more easily moved than the big ones.
iv. If the particles are too large, they gradually sink under the influence of gravity.
v. Brownian Movement is quite haphazard.
vi. The particles move in a straight line with sudden irregular changes of direction.
B. Osmotic pressure of colloidal solutions:
i. Since the size of the colloidal particles are bigger than that of solvent molecules the number of colloidal particles are less relative to solvent molecules in a solution.
ii. The osmotic pressure of a solution is directly proportional to the number but not the size of the dissolved particles. Therefore, the colloidal solutions have a low osmotic pressure.
iii. The serum proteins which are present to the extent of 7% to 8% exert only an osmotic pressure of about 30 mm Hg.; whereas the crystalloids of serum (0.9% NaCl) have an osmotic pressure of about 5,200 mm Hg. and 6% sucrose has an osmotic pressure of about 3,000 mm Hg.
iv. Soap solutions have small osmotic pressure because the soap molecules aggregate to form micelles of colloidal dimensions.
v. Although the osmotic pressure of colloidal solutions is very small, it has immense biological importance in providing the driving force for the passage of water and other substances through cell membranes.
C. Dialysis:
i. The process of separation of crystalloids from colloids by diffusion through a membrane by osmotic force is called dialysis.
ii. The most usual membranes used today for dialysis are the various grades of collodion (made from solutions of cellulose- nitrates or acetates—in solvents such as alcohol, ether or acetic acid) or cellophane parchment.
iii. Dialysis is particularly needed for removing salt from proteins after precipitation by “salting out”.
iv. The precipitate with a little water is placed in a collodion bag and immersed in water.
v. The passage of salt into water is accelerated by keeping salt content of the water low, i.e. by running water.
vi. Arrangement must be made for a big volume increase inside the bag in the early stages of dialysis, since it is a strongly hypertonic solution and water molecules will pass in more quickly than salt molecules pass out.
vii. The pressure will only be transient if the water outside is frequently altered and the final pressure inside the bag may be very slightly more than the original.
viii. Dialysis is applied in medicine in the “artificial kidney”.
ix. This mechanism is inserted into the patient’s circulation and urea passes out from the blood, substituting for the action of the faulty kidneys.
x. Dialysis of electrolytes can be done by passing an electric current through the solution. A cell consisting of three compartments separated by membranes is used; the colloidal solution is placed in the centre compartment and the electrodes in the outer ones. Positive ions are attached to the cathode and negative ions to the anode. This process is called electro-dialysis.
xi. Haemodialysis — Blood from arterial side is allowed to flow through the inner of the two concentric cellophane tubes with dialyser fluid passing through the annular space. The non-colloidal impurities in the blood are dialysed out and the purified blood is returned to venous side.
xii. Peritoneal dialysis — Dialysing fluid is run into peritoneal cavity and after allowing time for exchange across the peritoneal’ membrane, the fluid is run off.
8. Types of Colloidal Solution:
There are two types of colloidal solutions-Emulsoids and Suspensoids.
A. Suspensoids:
i. The surface tension and viscosity of suspensoids are nearly the same as those of the solvent.
ii. The suspensoid particles carry a definite electric charge which determines the stability of the suspensoid.
iii. They are very easily precipitated if the charge is neutralized.
iv. Once they are precipitated, they are not brought back into colloidal solution again.
v. Suspensoids are not hydrated, hence they are said to be hydrophobic or lyophobic colloids (water-fearing colloids).
B. Emulsoids:
i. They are very stable and not easily precipitated by salts.
ii. If they are precipitated, they are easily re-dissolved to form a colloidal solution.
iii. They have a lower surface tension and much higher viscosity than the solvent.
iv. The particles carry electric charges; some carry positive and negative charges simultaneously, e.g., proteins.
v. The nature of the charge on a protein particle can be changed by altering the pH of the solution.
vi. Practically, all the colloids of the living cell exist as emulsoids. They have a great affinity for water, hence they are called hydrophilic.
vii. The emulsoid particles are molecules surrounded by shells of adsorbed water.
viii. An emulsoid may be changed into a suspensoid by dehydration and then is precipitated.
An emulsoid can exist in two forms-Sol and Gel.
Sol:
i. Sol can be converted into gel by changes of temperature, hydrogen ion concentration or salt concentration.
ii. The continuous phase in sol is water (or a dilute solution), and the disperse phase is a concentrated solution.
iii. Addition of CaCl2 to an alkaline solution of caseinogen gives a sol.
Gel:
i. Solutions stronger than about 1% form gels on cooling.
ii. Gels are quasi-solids.
iii. They do not assume the shape of the vessel in which they are placed.
iv. Great pressure is required to squeeze water out of a gel.
v. In a gel, the concentrated solution forms the continuous phase and water the disperse phase.
vi. Formaldehyde can remove water from the gel and hence it is used in the histological process of “fixing” tissues.
vii. Many tissue structures are essentially gels. Cytoplasm may undergo sol <=> gel transformation.
viii. Tissues, especially the skin, are not dehydrated owing to the existence of gels.
Lyophobic colloids (Suspensoids):
i. Mostly inorganic materials e.g., gold sol.
ii. Well defined under microscope or ultra-microscope.
iii. Easy coagulation by electrolytes.
iv. Undergo irreversible coagulation.
v. Viscosity same as solvent (usually water)
vi. Do not gelatinize readily.
vii. Stabilize mainly by surface charge.
Lyophilic conoids (emulsoids):
i. Mostly organic e.g., proteins, starch.
ii. Invisible under microscope or ultra-microscope.
iii. Not easily coagulated by addition of substance or by cooling.
iv. Coagulation is reversible.
v. Much more viscous than water.
vi. Gelatinise readily.
vii. Stabilize by force of solvation.
Imbibition:
i. Dried emulsoids take up (imbibe) water and swell considerably; this process is called imbibition.
ii. The process of germination of seeds is the taking up of water by imbibition.
iii. Heat is liberated during imbibition.
iv. Most dried animal and vegetable tissues show imbibition.
v. In imbibition, water is not expelled by squeezing.
9. Physiological Application of Colloids:
i. Enzyme Action:
Lyophilic colloids such as enzyme proteins are big particles and intern consists of aggregation of thousands of smaller particles. This again increases of its area, the substance it acts upon, it is absorbed (e.g., enzyme catalysis change this action).
ii. Maintenance of Blood pH:
Lyophilic colloids such as proteins (e.g., haemoglobin) are independent of blood pH.
iii. Protective Action of Lyophilic Colloids (Such as Proteins):
Lyophilic colloids afford protection to lyophobic colloids such as Cabilirubinate and cholesterol in bile and Ca3(PO4)2 in blood and milk from being precipitated by electrolytes such as Na and K and hence prevent formation of stone.
iv. Water Transport and Urine Formation:
Colloidal osmotic pressure of plasma proteins in blood is known as oncotic pressure which is responsible for the maintenance of blood volume, hence, water transport and urine formation.
v. Glandular secretion:
Because of taking up of water by lyophilic colloids and excreting water in the form of glandular secretion.
vi. Gibbs-Donnan Membrane Equilibrium:
Consider a semipermeable membrane separating solution of non-diffusible ions from another solution with diffusible ions. Non diffusible ions will enhance the diffusion of oppositely charged diffusible ions towards their side. At the same time they will reduce the diffusion of identically charged ions to their side.
Only passive forces are involved in the movement of ions across semi-permeable membrane. So, oppositely charged diffusible ions are more concentrated on the side of non-diffusible ions while diffusible ions with like charges are concentrated on the opposite side. This is called Gibbs-Donnan effect.
On both sides of semi-permeable membrane, since total cation concentration equals total anion concentration on either side at equilibrium, so total charges are being contributed both by diffusible and non-diffusible ions.
If any NaPr solution is separated by semi-permeable membrane from solution of Na+ and CI– each having molar concentration of C1 and C2, respectively, then at equilibrium Gibbs-Donnan effect becomes
Only diffusible ions Na+ and CI can cross the membrane.
According to Gibbs-Donnan membrane equilibrium, product of molar concentration of diffusible ions on one side equals on the other where non-diffusible ion is retained by membrane on one side:
