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In this article we will discuss about the Colloidal solution. After reading this article you will learn about: 1. Introduction to Colloidal Solutions 2. Basic Principles of Colloidal Solutions 3. Properties.
Contents:
- Introduction to Colloidal Solutions
- Basic Principles of Colloidal Solutions
- Properties of a Colloidal Solutions
1. Introduction to Colloidal Solutions:
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On the basis of researches on liquid diffusion, Thomas Graham (1861) divided some substances as crystalloids, e.g., all inorganic acids, bases and salts and many organic compounds, which could diffuse very rapidly in solution and could readily pass through animal and vegetable membranes; and some substances as colloids, e.g., starch, protoplasm, gelatine, proteins, etc., which failed to do so.
The colloidal solution is a stable two-phase heterogeneous system, one of which (usually a solid), called dispersed phase, remains dispersed in a finely divided state throughout the other phase (tailed the dispersion phase (generally a liquid).
Among the colloidal systems, the liquid-in-liquid and the solid-in-liquid types are called colloidal solutions. The liquid-in-liquid type of colloidal solution is also called an emulsoid solution. A comparison of the size of the colloidal particles with ordinary molecules from the order of values is given in the following table.
Table 1: A comparison of the size of particles in true solutions, colloidal solutions and suspensions:
Colloidal system exhibiting properties of a fluid is known as sol. Sols arc sometimes almost as thick as jelly, when they are called gels.
Sols are subdivided into:
(a) Lyophobic (solvent hating) and
(b) Lyophilic (solvent loving) colloids.
Lyophobic sols (irreversible sols) are simple suspensions so sometimes called suspensoids and here, there is no perceptible affinity between the dispersed phase and the dispersion phase (e.g., gold sol, silver sol, etc.).
Lyophilic colloids (reversible) have, on the other hand, a marked affinity for solvent. This type of colloid is also called emulsoids. Most of the properties of a hydrophilic colloid may be explained by assuming that it is a hydrated suspensoid. This relationship is shown in Figure 1.
The protoplasm of plants consists of colloidal systems varying from emulsions (sols) to moderately stiff gels. The solid substances of protoplasm, proteins, dextrin’s, gums, etc., are characterized by the readiness with which they form colloidal systems in water.
It is to these colloidal systems that protoplasm owes most of its characteristic properties, i.e., variation in viscosity from values not greater than that of pure water to those of jellies, pronounced capacity for imbibition of water, though itself high in water content, sometimes behaving as if immiscible with water; properties of irreversible coagulation in very high and low temperatures or to high concentration of salts, etc.
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The experiments presented below are intended only to illustrate a few of the basic principles of colloidal behaviour and to point out their possible relationships to the life processes of the plant.
2. Basic Principles of Colloidal Solutions:
i. Preparation of Suspensions:
(a) Experiment:
A few ml of dil. H2SO4 is poured into a few ml of 5% BaCl2 solution in a test tube. It is shaken well.
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Observation:
Precipitate of BaSO4 formed gradually settles down forming suspensions.
Inference:
Particles of BaSO4 being of larger dimensions (greater than 200 mil) settle out gradually.
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(b) Experiment:
A few pieces of charcoal are ground in a clean mortar. Three samples (0.2gm each) of powdered charcoal are taken in three test tubes in which distilled water, 1% NH4OH and 0.55% acetic acid solution are separately added, shaken and allowed to stand.
Observation:
The rate of settling of charcoal powder is noted in each case. If they settle slowly, the effect of centrifuging is observed (at 500 r.p.m.).
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Inference:
As in Expt. (i).
(c) Experiment:
250 ml of distilled water is heated to boiling and 1 ml of 30% FeCl2 solution is added slowly with constant stirring. An iron sol is thus formed.
Observation:
The stability of a portion of iron sol is observed in concentrated HCL and NaOH solution.
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Inference:
When FeCl3 falls in water, it suffers hydrolysis, FeCl3+ 3 H2O = Fe(OH)3+ 3 HCl. Fe(OH)3 sol (it is actually a sol of hydrated ferric oxide, Fe2O3 (H2O)2) is positively charged. Now, HCl being proton donor, Fe (OH)3 remains in solution, whereas NaOH being proton acceptor, Fe(OH)3 sol settles out.
ii. Preparation of Emulsions:
(a) Experiment:
A few drops of olive oil (or any other oil) are taken in a test tube. The tube is half-filled with absolute alcohol and shaken well. This is poured in a beaker of water.
Observation:
A fine white emulsion of oil-in-water is formed.
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Inference:
The particles of oil (dispersed phase) will not separate because the oil remains in such small droplets that a degree of stability is achieved. Here alcohol-Acts as a stabilizer or emulsifier.
(b) Experiment:
Equal quantities (1 ml) of olive oil are taken in two test tubes, and equal quantity of water is added to each. To one tube a few drops of 10% NaOH are added and shaken well.
Observation:
An emulsion is formed in both the tubes. The oil separates out in the tube in which- alkali has not been added while in the other tube the emulsion is stabilized.
Inference:
The shaking of oil with water does not necessarily make an emulsion, for, the oil and water soon separate into two layers when allowed to settle. For the stability of an emulsion the addition of an emulsifier is necessary. Here NaOH acts as an emulsifier and renders the emulsion permanent.
(c) Experiment:
A drop of oil-in-water emulsion is placed on a glass slide and a drop of oil is added at one side and gently mixed together.
On another slide a drop of the oil-in-water emulsion and a drop of water are taken and mixed together. These are observed under the microscope. A drop of Sudan-III (0.1% in 50% ethanol) is added to each slide and again observed under the microscope.
Observation:
In the first slide oil drop is not mixed with the emulsion while in the second slide water is mixed readily. In the first, red-stained larger oil globules are seen while in the second, fine red-stained particles are seen.
Inference:
In the first case water is in continuous phase, hence oil will not mix readily with this emulsion. In the second case water mixes immediately with the continuous phase (dispersion phase), i.e., water. This is clearly observed when stained with Sudan-Ill.
iii. Preparation of Colloids:
(a) Experiment:
3 gm of soluble starch is taken in a beaker and dissolved in 100 ml water.
Observation:
A colloidal solution of starch-in-water is formed.
Inference:
Here starch forms the dispersed phase of the colloidal system (being of dimension less than 200 ml) in water which is the dispersion phase.
(b) Experiment:
A strong solution of sodium thiosulphate (Na2S2O3) of approximately 3N strength is added drop by drop to conc. H2SO4. (Dissolve 372gm Na2S2O3, 5H2O in 500 ml water in a volumetric flask to prepare 3N Na2S2O3 Soln.). This is cooled and mixed with water.
Observation:
A colloidal solution of sulphur is formed.
Inference:
The solution on cooling deposits sulphur which is washed with water. This mass of sulphur dissolves completely in water to give a clear colloidal solution which may be purified by dialysis (the process of separating crystalloid from a colloid by means of diffusion through a membrane).
iv. Preparation of Suspensoid Sols (lyophobic) and Their Precipitation:
(a) Experiment:
5 ml of 1% solution of AgNO3 is taken in a 250 ml beaker and dil. NH4OH is added until precipitate formed due to its addition, disappears. This is diluted with 100 ml of water. This solution is then mixed with equal volume of 0.5% tannic acid solution. (It is saved for the next experiment).
Observation:
A colloidal solution of silver is formed.
Inference:
With excess NH4OH, Ag2O is produced which gets reduced to metallic silver on addition of tannic acid containing catechol and pyrogallol. Here metallic silver remains in dispersed phase.
(b) Experiment:
A few drops of BaCl2 solution are added to the above colloidal solution.
Observation:
The silver particles are precipitated.
Inference:
The particles of metallic silver Ag- carry negative charge, hence they are readily precipitated by divalent cation Ba++ (or trivalent cation Al+ + +, when AICI3 is added).
N.B. Very little electrolyte is necessary for this “salting out” of suspensoid sols, as these sols are very sensitive to traces of electrolytes.
v. Preparation of Emulsoid Sols (lyophilic) and Their Precipitation:
(a) Experiment:
2gm of dry starch is mixed well with distilled water and a paste is prepared. This paste is poured in 100 ml of boiling water in a beaker and boiled further for a few minutes with constant stirring. (The solution is saved for the next experiment).
Observation:
An emulsoid sol of starch-in-water is obtained which remains unchanged when cooled.
Inference:
Starch being a lyophilic colloid absorbs large amount of water and, therefore, both the phases have the properties of liquids. The concentrated solution of starch is a dispersed phase while the weak solution of it is the dispersion phase. Thus, though starch is a solid, its colloidal solution may be regarded as an emulsoid sol.
(b) Experiment:
The emulsoid sol as prepared in the above experiment, is saturated with solid (NH4)2SO4.
Observation:
A white gelatinous precipitate is formed.
Inference:
Protein and starch are precipitated by salts because these form larger aggregates in concentrated salt solution. This is called “salting out” of protein or starch.
vi. Preparation of Gels and their Precipitation:
(a) Experiment:
1gm of agar-agar is dissolved in 50 ml of water by boiling for half an hour and then cooled.
(The gel is saved for the next experiment).
Observation:
The agar forms a thick opalescent solution (sol) which sets to a gel on cooling. This change is reversible.
Inference:
The sol of agar on cooling sets to a coherent jelly-like mass. This transparent or opalescent mass contains a large percentage of liquid which retains their shape and offers some resistance to mechanical deformation.
(b) Experiment:
The gel obtained from the previous experiment is warmed to form a sol which is taken in two test tubes and to one absolute alcohol and to the other saturated solution of (NH4)2SO4 are added.
Observation:
Agar mucilage is precipitated in each case.
Inference:
Precipitate consists simply of the aggregation of the smaller particles into larger ones which are big enough to settle down.
vii. Preparation of Reversible and Irreversible Colloids:
(a) Experiment:
To get a reversible colloid, Expt. 6 (i) is to be performed.
(b) Experiment:
The egg albumen is liquid at ordinary temperature but on heating it becomes solid.
Observation:
The solid gel of egg albumen on cooling does not form a sol again.
Inference:
Egg albumen is an irreversible colloid. The solidification of the albumen on heating is due to coagulation of protein.
3. Properties of a Colloidal Solutions:
i. Brownian movement of colloids:
(a) Experiment:
A drop of sulphur sol from Expt. 3 (ii) is taken on a clean slide. It is then covered with a cover glass and observed with a beam of strong light under the high power of microscope.
Observation:
The sulphur particles dispersed in water execute a ceaseless random motion.
Inference:
The random motion is due to the unbalanced impacts of the liquid molecules on the colloidal particles. The colloidal units are constantly hit from all sides by the surrounding solvent molecules and are tossed up. This erratic movement is called Brownian movement.
(b) Experiment:
A drop or two of latex from plants (species of Apocynaceae, Euphorbiaceae, Asclepiadaceae, etc.) are taken, mounted on a slide and observed under the microscope.
Observation:
The erratic zigzag movements of the colloidal particles (proteins, gums, etc.) are observed.
Inference:
As in Expt. 8 (i.a).
N.B. This experiment may also be performed with oil-water emulsion.
(ii) Flocculation of colloids
(a) Flocculation of hydrophobic sols:
(c) Experiment:
5 ml of arsenious sulphide (AS2S3) sol is taken into each of three test tubes. A drop of 0.01 ml NaCL (0.585gm/litre) is added to the first tube, a drop of 0-01 M CaCl2 (1.11gm/litre) to the second, and a drop of 0-01 M AICI3 (1-335gm/litre) to the third and observed immediately.
Observation:
The sol is no longer stable and the particles are flocculated. The rate of flocculation is maximum with ALCL3 and minimum with NaCL.
Inference:
The colloidal solution of arsenious sulphide has a negatively changed surface. Thus if NaCl, CaCl2 and AICI3 solutions are added, Na+, Ca++, and Al+++ cations are preferentially adsorbed by the particles, the charge distribution on the surface of micelles of sol is disturbed resulting eventually in zero potential difference, when agglomeration of smaller particles into larger flakes takes place and the particles flocculate.
Increasing valency has got increasing power of flocculation (valencies of Al, Ga and Na are 3, 2 and 1 respectively). CaCl2 is about 88 times more efficient in flocculating As2S3 sol than NaCl and AICI3 is about 7 times more efficient in this respect.
ii. Flocculation of hydrophilic sols:
(a) Experiment:
2 gm of powdered gum acacia (gum arabic) is added slowly to 100 ml of boiling distilled water, stirring constantly until homogeneous liquid results. 3 ml of gum acacia sol from the stock thus prepared is taken into 3 test tubes separately.
To the first test tube 7 ml of 95% ethyl alcohol is added, shaken vigorously and observed. About half of the liquid in the tube is, poured out and to the remainder about three times their volumes of distilled water are added.
To the second test tube 7 ml of 95% ethyl alcohol is added and shaken as before. Then about 0-2gm of solid AIGI3 is added, shaken until it dissolves and observed. To the third test tube 0.2gm of solid AICI3 is first added, shaken and observed as in the preceding tubes. Then 7 ml of 95 % ethyl alcohol is added and observed.
Observation:
Lyophilic colloids are flocculated in all the test tubes. Precipitate of the first test tube is soluble in water.
Inference:
The gum arabic solution has negatively charged surface. AICI3 is a positively charged electrolyte and when these two oppositely charged colloidal particles are mixed, coagulation takes place which are hydrophobic. In the first test tube, the precipitated protein is a hydrophilic colloid which dissolves in water.
iii. Isoelectric point of colloids:
Determination of isoelectric point of gelatin:
(a) Experiment:
1 litre each of normal (approximately) solution of HCl (1 ml conc. HCL+11 ml water) and NaOH (40gm/litre) is prepared and diluted to produce 500 ml each of 0.1 N, 0.01 N, 0.001 N and 0.0001 N.
(Prepare the lower concentrations by dilution method as follows: If X be the desired strength an4 Y be the strength of the stock soln., take X ml of stock soln. and make the volume up to Y ml, i.e., add Y —X ml of water.). pH of each is determined using universal pH indicator paper. 2gm of sheet gelatin is weighed into each of 11 beakers or wide-mouthed flasks and covered with 200 ml of solutions as follows:
The distilled water should be freshly boiled and cooled and this should be protected from atmospheric CO2 in a stoppered wide-mouthed flask. The gelatin is allowed to soak 2 to 5hr until the most hydrated sample begins to soften too much to handle. This is then filtered on folded cheese cloth or glass wool and the gelatin is weighed.
The filtrates are saved and changes in pH are recorded. The gain in weight of the gelatin is plotted against the final pH of the medium and the approximate isoelectric point of the colloid is determined from the curve which corresponds to the pH at which minimum gain in weight takes place.
Observation:
The change in weight of gelatin in each ease is carefully noted.
Inference:
The pH at which the minimum change in weight of gelatin due to absorption takes place gives the isoelectric point (round about 4-7 pH). The dispersion of many colloids in water (hydration) is affected by the hydrogen ion concentration or pH of the medium and is at minimum at a point called the isoelectric point.
At isoelectric point the molecule is electrically balanced, i.e., in an electric field it will move neither to the cathode nor to the anode.
iv. Isoelectric point of plant tissue:
(a) Experiment:
5 to 10gm of potato discs is soaked for 24 hr. in the solutions prepared for Expt. 8 (iii.a). The changes in weight of the potato discs and the pH changes of the media are determined according to the previous experiment.
Observation and Inference: As in Expt. 8 (iii. a.).
N.B. Isoelectric point of egg albumen may also be determined by adding 0.1 N NH4OH and 0.1 N acetic acid separately and noting the pH at which maximum opalescence is produced.
v. Imbibition of colloids:
(a) Experiment:
Three lots of 10 or more grains of rice are weighed. First lot is covered with water, second with 75% NaCl and the third with 30% NaCl solutions for 24 to 72 hr. These lots are separately weighed. The volume of each lot is measured before and after treatments by displacement of water.
Observation:
The changes in weights and volumes of grains of each lot are noted, and plotted on a graph paper.
Inference:
The colloids (protoplasm) of the seeds imbibe water osmotically and non-osmotically. In a solution of an electrolyte like NaCl, the non-osmotic active absorption forces are eliminated and water is drawn into the cells only osmotically and the absorption of water and solutes thus comes under general metabolic control of the tissues. Hence, imbibition of water is proportionally less in higher concentrations of NaCl.
vi. Surface tension:
(a) Experiment:
The surface tension of the following liquids is measured arbitrarily by counting the time required by known volume (10 ml) of each liquid to fall completely from a burette 10 ml each of the following liquids—ethanol, acetone, chloroform, ether, toluene, xylene, mercury and water are taken in a dry burette one by one and the time required, by each liquid to fall completely through the burette is recorded with the help of a stop watch. Reading with each liquid, may be taken at least three times.
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Results:
The average time taken is noted in each case and entered in a table.
Discussion:
The attractive force between like molecules causing them to cling together is called cohesion while the similar attractive force between unlike molecules is called adhesion. The surface tension depends upon the force of adhesion between two unlike molecules.
The greater the force of adhesion, the greater is the surface tension.
In the present experiment it is found that the time taken by different liquids is different and if put in order on the basis of decreasing requirement of time, the liquids can be arranged as follows:
(1) Mercury,
(2) Water,
(3) Xylene,
(4) Toluene,
(5) Chloroform,
(6) Ether,
(7) Acetone and
(8) Ethanol.
(The surface tension determines to some extent the shapes and distribution of protoplasmic structures and the more liquid parts of plant cells).
