AC2615
Unit I: Colloids and Colloidal Stability (12L)
Colloids: Thomas Graham (1861) studied the ability of dissolved substances to diffuse into water across a permeable membrane. He observed that crystalline substances such as sugar, urea, and sodium chloride passed through the membrane, while others like glue, gelatin and gum arabic did not. The former he called crystalloids and the latter colloids (Greek, kolla = glue ; eidos = like). Graham thought that the difference in the behavior of ‘crystalloids’ and ‘colloids’ was due to the particle size. Later it was realised that any substance, regardless of its nature, could be converted into a colloid by subdividing it into particles of colloidal size.
In a true solution as sugar or salt in water, the solute particles are dispersed in the solvent as single molecules or ions. Thus the diameter of the dispersed particles ranges from 1Å (0.1 nm) to 10 Å (1.0 nm). On the other hand, in a suspension as sand stirred into water, the dispersed particles are aggregates of millions of molecules. The diameter of these particles is of the order 2,000 Å (200 nm) or more. The colloidal solutions or colloidal dispersions are intermediate between true solutions and suspensions. In other words, the diameter of the dispersed particles in a colloidal dispersion is more than that of the solute particles in a true solution and smaller than that of a suspension.
When the diameter of the particles of a substance dispersed in a solvent ranges from about 10 Å to 2,000 Å, the system is termed a colloidal solution, colloidal dispersion, or simply a colloid. The material with particle size in the colloidal range is said to be in the colloidal state. The colloidal particles are not necessarily in spherical shape. In
fact, these may be rod-like, disc-like, thin films, or long filaments. For matter in the form of spherical shape, the diameter gives a measure of the particle size. However, in other cases one of the dimensions (length, width and thickness) has to be in the colloidal range for the material to be classed as colloidal. Thus a system with at least one dimension (length, width, or thickness) of the dispersed particles in the range 10 Å to 2,000 Å, is classed as a colloidal dispersion.
Common examples of colloids: Soap bubbles, aerosols, fog, whipped cream, butter, ice-cream, etc.
Dispersed phase & Dispersion medium: A colloidal system is made of two phases.
The substance distributed as the colloidal particles is called the Dispersed phase. The second continuous pha
se in which the colloidal particles are dispersed is called the Dispersion medium. For example, for a colloidal solution of copper in water, copper particles constitute the dispersed phase and water the dispersion medium.
Types of Colloidal Systems: Since either the dispersed phase or the dispersion medium can be a gas, liquid or solid, there are eight types of colloidal systems possible. A colloidal dispersion of one gas in another is not possible since the two gases would give a homogeneous molecular mixture. The various types of colloidal systems are listed in Table:
The colloidal systems which consist of a solid substance dispersed in a liquid.
are frequently referred to as Sols or Colloidal solution. The colloidal solutions in water as the dispersion medium are termed Hydrosols or Aquasols. When the
dispersions medium is alcohol or benzene, the sols are referred to as Alcosols and Benzosols, respectively.
Lyophilic and Lyophobic Sols or Colloids: Sols are of two types –
(a) Lyophilic sols (solvent-loving) (b) Lyophobic sols (solvent-hating)
Lyophilic sols have affinity for the medium or the solvent. The examples of lyophilic sols are dispersions of starch, gum, and protein in water.
Lyophobic sols have no attraction for the medium or the solvent. The examples of lyophobic sols are dispersion of gold, iron (III) hydroxide and sulphur in water.
Characteristics of Lyophilic and Lyophobic Sols: Some features of lyophilic and lyophobic sols are listed below.
(1) Ease of preparation: Lyophilic sols can be obtained by mixing the material (starch, protein) with a suitable solvent. Lyophobic sols are not obtained by simply mixing the solid material with the solvent.
(2) Charge on particles: Particles of a hydrophilic sol may have little or no charge at all.
Particles of a hydrophobic sol carry positive or negative charge which gives them stability.
(3) Solvation: Hydrophilic sol particles are generally solvated. That is, they are surrounded by an adsorbed layer of the dispersion medium which does not permit them to come together and coagulate. Hydration of gelatin is an example.
There is no solvation of the hydrophobic sol particles for want of interaction with the medium.
(4) Viscosity: Lyophilic sols are viscous as the particle size increases due to solvation, and the proportion of free medium decreases. Warm solutions of the dispersed phase on cooling set to a gel e.g., preparation of table jelly.
Viscosity of hydrophobic sol is almost the same as of the dispersion medium itself.
(5) Precipitation: Lyophilic sols are precipitated (or coagulated) only by high concentration of the electrolytes when the sol particles are dissolved.
Lyophobic sols are precipitated even by low concentration of electrolytes, the protective layer being absent.
(6) Reversibility: The dispersed phase of lyophilic sols when separated by coagulation or by evaporation of the medium, can be reconverted into the colloidal form just on mixing with the dispersion medium. Therefore this type of sols are designated as Reversible sols.
On the other hand, the lyophobic sols once precipitated cannot be reformed merely by mixing with dispersion medium. These are, therefore, called Irreversible sols.
(7) Tyndall effect: On account of relatively small particle size, lyophilic sols do not scatter light and show no Tyndall effect. Lyophobic sol particles are large enough to exhibit Tyndall effect.
(8) Migration in electronic field: Lyophilic sol particles (proteins) migrate to anode or cathode, or not at all, when placed in electric field.
Lyophobic sol particles move either to anode or cathode, according as they carry negative or positive charge.
Comparison of Lyophilic and Lyophobic Sols
Lyophilic Sols
1. Prepared by direct mixing with dispersion medium.
2. Little or no charge on particles.
3. Particles generally solvated.
4. Viscosity higher than dispersion medium; set to a gel.
5. Precipitated by high concentration of electrolytes.
6. Reversible.
7. Do not exhibit Tyndall effect.
8. Particles migrate to anode or cathode, or not at all.
Lyophobic Sols
1. Not prepared by direct mixing with the medium.
2. Particles carry positive or negative charge.
3. No solvation of particles.
4. Viscosity almost the same as of medium; do not set to a gel.
5. Precipitated by low concentration of electrolytes.
6. Irreversible.
7. Exhibit Tyndall effect.
8. Particles migrate to either anode or cathode.
Preparation of Lyophobic Sols: Lyophilic sols may be prepared by simply warming the solid with the liquid dispersion medium e.g., starch with water. On the other hand, lyophobic sols have to be prepared by dispersion or aggregation methods.
(a) Dispersion Methods in which larger macro-sized particles are broken down to colloidal size.
(b) Aggregation Methods in which colloidal size particles are built up by aggregating single ions or molecules.
Dispersion Methods: In these methods, material in bulk is dispersed in another medium.
(1) Mechanical dispersion using Colloid mill: This method is based on the mechanical grinding of the coarse solid particles. The solid along with the liquid dispersion medium is fed into a Colloid mill. The mill consists of two steel plates nearly touching each other
and rotating in opposite directions with high speed (of the order of 7000 rpm). The solid particles are ground down to colloidal size and are then dispersed in the liquid to give the sol. ‘Colloidal graphite’ (a lubricant) and printing inks are made by this method.
(2) Bredig’s Arc Method (Electrical Dispersion Method): It is used for preparing hydrosols of metals like silver, gold and platinum. An arc is arranged between the two metal electrodes held close together in de-ionized water in traces of alkali (KOH). The water is kept cold by immersing the container in ice/water bath. The metal first changes in to vapour phase (molecular state) due to heat of the spark and then the metal present in the vapour aggregate to form colloidal particles in water.
Non-metal sols can be made by suspending coarse particles of the substance in the dispersion medium and striking an arc between iron electrodes.
(3) Peptization Method: Peptization (or Deflocculation) is the process used for the formation of stable dispersion of colloidal particles in dispersion medium. It may be defined as a process of converting a precipitate into colloidal sol by shaking it with dispersion medium in the presence of small amount of electrolyte. The electrolyte used in this process is called as peptizing agent. (Flocculation is the process by which individual particles aggregate into clot like masses or precipitate into small lumps)
In this method, the freshly precipitated ionic solids such as, AgCl, Fe(OH)3, Al(OH)3 etc. are dispersed into colloidal solution in water by the addition of small quantities of electrolytes containing a common ion. The precipitate adsorbs the common ions and electrically charged particles then split from the precipitate as colloidal particles. For example, sol of ferric hydroxide may be obtained by stirring fresh precipitate of ferric hydroxide with a small amount of FeCl3.
Similarly, silver chloride (Ag+Cl–) can be converted into a sol by adding hydrochloric acid (Cl– being common ion).
The dispersal of a precipitated material into colloidal solution by the action of an electrolyte in solution, is termed peptization. The electrolyte used is called a peptizing agent. Peptization is the reverse of coagulation of a sol.
Aggregation Methods: These methods consist of chemical reactions or change of solvent in such a way that the atoms or molecules of the dispersed phase aggregate to form colloidal particles. The experimental conditions like temperature and concentration are maintained so as to favour the formation of sol but prevent the particles becoming too large and forming precipitate. The unwanted ions present in the sol are removed by dialysis as these ions may eventually coagulate the sol.
Some common methods are –
(1) Double Decomposition: (Double decomposition is a reaction in which two compounds exchange ions, typically with precipitation of an insoluble product).
Arsenic sulphide (As2S3) sol is usually prepared by this method. It is prepared by passing a slow stream of hydrogen sulphide gas through a cold solution of arsenious
oxide (As2O3). This is continued till the yellow colour of the sol attains maximum intensity.
As2O3 + 3H2S → As2S3 (sol) + 3H2O
Excess hydrogen sulphide (electrolyte) is removed by passing in a stream of hydrogen.
In a similar manner, colloidal silica, stannic oxide and tungstic oxide may be prepared by adding HCl to solutions of sodium silicate, stannate, and tungstate respectively e.g.
Na2SiO3 + 2HCl → SiO2 (silica sol) + 2NaCl + H2O
These sols can also be prepared by adding ammonia solution to their respective chlorides, e.g.
SiCl2 + 2NH4OH + O2 → SiO2 (silica sol) + 2NH4Cl + 2H2O
Organosol of HgS is prepared by the reaction of H2S with Hg(CN)2 in methanol, ethanol, n-propanol or acetone.
Hg(CN)2 + H2S → HgS (mercuric sulphide sol) + 2HCN
(2) Reduction: Silver sols and gold sols can be obtained by treating dilute solutions of silver nitrate or gold chloride with organic reducing agents like tannic acid or methanal (HCHO)
AgNO3 + tannic acid → Ag sol AuCl3 + tannic acid → Au sol
(3) Oxidation: A sol of sulphur is produced by passing hydrogen sulphide into a solution of sulphur dioxide.
2H2S + SO2 → 2H2O + 3S (sulphur sol)
Sols of halides of silver, lead and mercury are obtained by oxidation of the corresponding metal sols with chlorine, bromine or iodine until the characteristic colour of the metal sol is destroyed. To stabilize the sol, some stabilizing agent is also added.
(4) Hydrolysis: Sols of the hydroxides of iron, chromium and aluminium can be easily prepared by the hydrolysis of salts of the respective metals. For example, red sol of ferric hydroxide can be obtained by adding a few drops 30% ferric chloride solution to a large volume of almost boiling water followed by stirring with a glass rod.
FeCl3 + 3H2O → Fe(OH)3 (red sol) + 3HCl
(5) Change of Solvent: In this method, a substance is dissolved in a solvent and then the solution is added to another solvent in which it is less soluble. For example if an alcoholic solution (true solution) of sulphur is added in excess of water, a colloidal solution of sulphur is formed. Sulphur is insoluble in water.
Purification of Colloidal Solution (Sol): The prepared sol generally contains considerable amount of impurities. The impurities and particularly electrolytes make the sols to be unstable. Thus the electrolytes have to be removed from the sols. This purification of sols can be accomplished by three methods, namely, (a) Dialysis, (b) Electrodialysis, and (c) Ultrafiltration
Dialysis: The process of separating the particles of colloids from impurities by means of diffusion through a suitable membrane is called dialysis. Animal membranes (bladder) or those made of parchment paper, have very fine pores. These pores permit ions (or small molecules) to pass through but not the large colloidal particles. When a sol containing dissolved ions (electrolyte) or molecules is placed in a bag of permeable membrane dipping in pure water, the ions diffuse through the membrane.
By using a continuous flow of fresh water, the concentration of the electrolyte present outside the membrane tends to be zero. The impurities diffuse out leaving pure coloidal solution in the bag. In this way, practically all the electrolyte present in the sol can be removed easily.
Example: A ferric hydroxide sol (red) made by the hydrolysis of ferric chloride is mixed with some hydrochloric acid. When the impure sol is placed in the dialysis bag for some time, the outside water give a white precipitate with silver nitrate (due to presence of chloride). After a long time, almost the whole of hydrochloric acid will be removed and the pure red sol will be left in the dialyser bag.
Electrodialysis: The dialysis process is slow and to speed up its rate, it is carried out in the presence of an electrical field. The dialysis carried out in the presence of electric field is known as electrodialysis. When the electric field is applied through the electrodes, the ions of the electrolyte present as impurity diffuse towards oppositely charged electrodes at a fast rate.
The most important use of electrodialysis is the purification of blood in the artificial kidney machine. The dialysis membrane allows the small particles (ions etc.) to pass, whereas large size particles like haemoglobin do not pass through the membrane.
Limitation: Electrodialysis is not suitable for the removal of non-electrolyte impurities such as sugar and urea.
Ultrafiltration: Ordinary filter papers have pores larger than one micron (10−6 m, 1000 nm, 10,000 Å). Thus, the colloidal particles can pass through these filter papers along with ions or molecules. The pores of the filter paper can be made smaller by soaking it in a solution of gelatin or a colloidal solution followed by hardening by soaking in formaldehyde. As a result the pores of the filter paper become smaller and the colloidal particles cannot pass through them. Filter papers thus prepared are known as ultra-filters. The process of separating colloids from crystalloids by using
ultrafilters is called ultra-filtration. By soaking filter papers in solutions of colloid of different concentrations, a series of graded filter papers can be prepared. Ultra-filters can also be used for the separation of colloidal particles of different sizes from one another.
Mechanism of Colloid Formation: Since colloids represent a range of particle sizes intermediate between molecules and macroscopic bulk phases, it can be prepared via two approaches – Dispersion (breaking down large piece to the size required) and Aggregation (by aggregation/condensation of molecular dispersion to the size range of a colloid).
Dispersion Method: When a bulk material is divided into two pieces and separated to infinite distance there is a characteristic (theoretical) energy change per unit area which is termed the ‘‘specific surface free energy.’’ As the subdivision of the sample is continued, the total free energy change will be the product of the surface energy (or surface tension) of the material and the total new surface area produced by the process. Theoretically, work required is equal to the energy of evaporation, sublimation, or dissolution, depending on the situation. However, in practice, more energy is required due to the various irreversible and heat-generating processes.
In fact the work required to reduce a given material to colloidal size varies directly with the surface energy of the material (higher surface-energy materials require more work input). Further, the natural tendency of subdivided particles is to reduce the total surface area by some aggregation process. The attractive interaction between particles can be reduced by the introducing an intervening medium, usually a
liquid. The liquid medium has two positive effects on the process: (1) it reduces the surface energy of the system by adsorption on the new surface and (2) it reduces the van der Waals attraction between the particles. For these reasons the dispersion processes are carried out in the presence of a liquid.
While the presence of the liquid normally favours the process, the dispersion still may not exhibit the stability necessary to make the process feasible. Thus the dispersed particles may begin to flocculate or coagulate rapidly once the dispersion process is stopped (Figure). To overcome this problem, usually a surfactant or polymeric stabilizer is added to inhibit the rapid flocculation of the newly formed particles. The addition of new components (surfactant, polymer, small particles, etc.) adsorbs at the solid–liquid interface and provide an electrostatic or steric barrier that prevents ‘‘sticky’’ collisions between particles, thereby making the dispersion more stable.
Condensation Method: Approaching the formation of colloids by this method involves one of several growth mechanisms. Such processes are commonly employed for the production of aerosols, emulsions etc. Some common examples are the fog formation (both water and chemical), crystallization processes, colloidal silica, latex polymers, silver halide emulsions’’ for use in photographic products, etc.
In emulsion polymerization, a monomer or mixture of monomers is emulsified in a liquid phase (usually water) in the presence (except under very special circumstances) of a surfactant, polymer or other stabilizer and a soluble free radical initiator. Particle formation and growth occur initially in the solution phase and then continues in precipitated particles. Under proper conditions, particle size and dispersity can be closely controlled to meet specific needs.
In the process of emulsion polymerization, the initial latex particle begins as a free- radical-initiated dimer or oligomer in solution (a). As polymerization proceeds, the growing chain precipitates and continues to grow, fed by a new monomer taken from the reservoir of emulsified material (b). Polymerization continues until all available monomer is consumed (c).
Stability of Colloidal System: The stability of a colloidal system is the capability of the system to remain as it is. Stability is hindered by aggregation and sedimentation phenomena. A stable colloidal system is one in which the particles resist flocculation or aggregation and has a long shelf-life. The stability depends upon the balance of the repulsive and attractive forces that exist between particles as they approach one
another. If all the particles have a mutual repulsion, then the dispersion remains stable.
However, if the particles have little or no repulsive force, then the system may be unstable and flocculation may occur.
Aggregation is due to the sum of the interaction forces between particles. When attractive forces (such as van der Waals forces) dominate over the repulsive ones (such as the electrostatic forces of repulsion) particles aggregate.
The stabilization against aggregation can be explained by two mechanisms, namely electrostatic stabilization and steric stabilization.
Electrical Properties of Colloidal Systems:
(a) Charge on colloidal particles: The most important property of colloidal dispersions is that all the suspended particles have either a positive or a negative charge. The mutual forces of repulsion between similarly charged particles prevent them from aggregating and settling under the action of gravity. This gives stability to the sol. All the particles in the hydrophobic colloidal system have the same charge and
the dispersion medium has an opposite and equal charge so that the system as a whole being electrically neutral.
The sol particles acquire positive or negative charge by preferential adsorption of positive or negative ions from the dispersion medium. The surface tends to adsorb the ions, which are common to the dispersed particle and dispersion medium. If the particles have the preference to adsorb positive ions, they acquire a positive charge and if they prefer to adsorb negative ions, they acquire a negative charge.
Let us consider another example. Ferric hydroxide sol particles are positively charged because these adsorb Fe3+ ions from ferric chloride (FeCl3) used in the preparation of the sol.
Fe(OH)3 + Fe3+ → Fe(OH)3/Fe3+ (Positive ferric hydroxide sol particle)
Similarly, the negative charge on arsenic sulphide sol is due to the preferential adsorption of sulphide ions on the surface of arsenic sulphide particles.
As2S3 + S2– → As2S3/S2– (Negative arsenic sulphide sol particle)
Likewise, the negative charge on metal sols prepared by Bredig’s arc method is due to the adsorption of hydroxyl ions furnished by traces of alkali added.
Thus the hydrophobic sols are stabilized by the presence of like charges on colloidal particles. Due to the presence of like charges, colloidal particles repel each other and move away from each other and thus do not form bigger aggregates.
However, the coagulation begins immediately on the removal of the charge.
The stability of lyophilic colloids is for two reasons. Their particles possess a charge and in addition, they are strongly solvated, and the solvent layer surrounding them prevents them to come into intimate contact with each other. Hence their coagulation
is prevented. For example, when sodium chloride is added to a gold sol (lyophobic sol), it gets precipitated. However, when sodium chloride is added to a colloial solution of gelatin (lyophilic sol), its particles are not precipitated. This is because of the fact that the water layer around gelatin particles does not allow the Na+ ions to penetrate and neutralize the charge on gelatin particles.
Thus gelatin sol is not precipitated. This indicates that lyophilic sols are more stable than lyophobic sols. To coagulate lyophilic sols, it is necessary to remove charge as well as the solvent layer.
(b) Electrical Double layer: Consider the formation of silver iodide sol from the double decomposition reaction:
AgNO3 (aq) + NaI (aq) → AgI (s) + NaNO3 (aq)
Let us assume that the electrolyte is in excess of AgI. This causes the preferential adsorption of I- ions and ultimately AgI sol becomes negatively charged.
The ions which preferentially adsorbed on the surface of a particle of a colloidal system are known as potential-determining ions.
The negatively charged surface of AgI particles attract positive ions (Na+) and repel the negative ions (NO3-). As a result, the positive Na+ ions tend to form a compact layer around (in the vicinity of) the potential-determining I- ions layer. This is called as Stern layer. The ions present in the stern layer are called as the counter ions.
The influence of surface charge decreases with distance and therefore at a certain distance from the surface of the particle, the concentration of Na+ ions equals to the concentration of NO3- ions and a state of electroneutrality prevail. In fact, the system as a whole is electrically neutral even though there exist regions of unequal distribution of anions and cations. The diffuse layer b/w the stern layer and the electrically neutral part of the system is called as the Gouy-Chapman layer.
The presence of charge gives rise to the potential at the surface of the particle.
The potential drops to zero at some distance away from the surface depending upon the concentration of the counter ions in the bulk phase. The region in which the influence of the charge is appreciable is known as the electrical double layer. The double-layer consists of two parts – (i) the stern layer, the thickness of which is of the order of the ionic dimensions and (ii) the Gouy-Chapman diffuse layer, the thickness of which is given by: rD = (εr RT
2ρF2I)2
where, ρ, εr, I and F are density, dielectric constant, ionic strength of the solution and Faraday constant, respectively. The value of rD is in the order of 1 – 100 nm. It decreases as the ionic strength of the solution increases. Further, it decreases more rapidly for counter ions of high valency.
DLVO Theory of the stability of Lyophobic colloids: This theory was developed by D. Derjaguin, L.D. Landau, E. Verwey and J.T.G. Overbeek in 1940. This thery is based on the following assumptions –
(i) Dispersion is dilute
(ii) Only two forces act on the dispersed particles: van der Waals force and electrostatic force
(iii) The electric charge and other properties are uniformly distributed over the solid surface
(iv) The distribution of the ions is determined by the electrostatic force, Brownian motion and entropic dispersion
The theory states that the colloidal stability is determined by the potential energy of the particles (VT) consisting of two parts – potential energy of the attractive interaction due to van der Waals force (VA) and potential energy of the repulsive electrostatic interaction (VR):
VT = VA + VR
For the spherical particles, VA is given as VA = - Ar /(12x)
A - Ham aker constant; r - radius of the particles; x - distance between the surfaces.
Electric repulsive potential energy (VR) is given as:
VR = 2π ε ε0 r Z 2e-kx
where: ε - dielectric constant of the solvent; ε0 - vacuum permittivity; k - a function of the ionic concentration (k-1 is the characteristic length of the Electric Double Layer); Z (ζ) - zeta potential. The value of Z is given as:
Z = 4 π η u/ε
where, η = viscosity of the medium, u = mobility of the colloidal particles
The graphs describing the potential energy of the interaction between two particles are presented in the following figure:
According to this theory for a sol to be stable, there should be a balance between repulsive interactions and attractive interactions. The minimum of the potential energy determines the distance between two particles corresponding to their stable equilibrium. The two particles form a loose aggregate, which can be easily re- dispersed. The strong aggregate may be formed at a shorter distance corresponding to the primary minimum of the potential energy. In order to approach to the distance of the primary minimum the particle should overcome the potential barrier.
For water as the dispersion medium, the Zeta potential (Z) is in the range of 0.03 and 0.06 V. It decreases when an ion of opposite charge to that of the colloidal particle is adsorbed. This reduces the mutual repulsion b/w the charged colloidal particles. As a result, the colloidal particles easily come closer to one another and and form bigger aggregates which lie outside the colloidal range. This phenomenon changing colloidal state to suspension state is known as coagulation, flocculation or precipitation of colloidal solution.
In case of lyophobic colloids, the stability is due to the electrical charge present on the colloidal particles whereas the stability of the lyophilic colloids depends on both electrical charge and solvation. Since in lyophobic colloids, charge on all particles is of the same sign, the repulsive force prevents the particles to close one another. The magnitude of repulsive forces depends upon the magnitude of the surface charge and the thickness of electrical double layer. These factors also determine the value of zeta potential which actually governs the stability of colloidal system. If Z is small, the resultant potential energy is negative so that the van der Waals attraction predominates over the electrostatic repulsion and sol coagulates rapidly.
On the other hand, in case of lyophilic colloids, solvation plays a very important role. In this case adsorbed layer of solvent serves as barrier and prevent the particles to aggregate. Thus in case of lyophobic sols, the removal of charge may easily bring coagulation while in case of lyophilic sols, the charge removal may not necessarily bring coagulation though it may decrease the stability of the sols.
Critical Coagulation Concentration (CCC): It is the minimum concentration
