COLLOIDS

A colloid is a substance microscopically dispersed throughout another substance. The dispersed-phase particles have a diameter of between approximately 2 and 500 nanometers. Such particles are normally invisible in an optical microscope, though their presence can be confirmed with the use of an ultramicroscope or an electron microscope. Homogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions, or hydrosols. The dispersed-phase particles or droplets are affected largely by the surface chemistry present in the colloid. Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color. Colloidal solutions (also called colloidal suspensions) are the subject of interface and colloid science. This field of study was introduced in 1861 by Scottish scientist Thomas Graham.

Classification Because the size of the dispersed phase may be difficult to measure, and because colloids have the appearance of solutions, colloids are sometimes identified and characterized by their physico-chemical and transport properties. For example, if a colloid consists of a solid phase dispersed in a liquid, the solid particles will not diffuse through a membrane, whereas with a true solution the dissolved ions or molecules will diffuse through a membrane. Because of the size exclusion, the colloidal particles are unable to pass through the pores of an ultrafiltration membrane with a size smaller than their own dimension. The smaller the size of the pore of the ultrafiltration membrane, the lower the concentration of the dispersed colloidal particules remaining in the ultrafiltered liquid. The exact value of the concentration of a truly dissolved species will thus depend on the experimental conditions applied to separate it from the colloidal particles also dispersed in the liquid. This is particularly important for solubility studies of readily hydrolysed species such as Al, Eu, Am, Cm, or organic matter complexing these species. Colloids can be classified as follows:

Medium / Phases		Dispersed phase
Medium / Phases		Gas	Liquid	Solid
Continuous medium	Gas	NONE (All gases are mutually miscible)	Liquid aerosol Examples: fog, hair sprays	Solid aerosol Examples: smoke, ice cloud, air particulates
	Liquid	Foam Example: whipped cream, shaving cream	Emulsion Examples: milk, mayonnaise, hand cream	Sol Examples: pigmented ink, blood
	Solid	Solid foam Examples: aerogel, styrofoam, pumice	Gel Examples: agar, gelatin, jelly	Solid sol Example: cranberry glass

Based on the nature of interaction between the dispersed phase and the dispersion medium, colloids can be classified as: Hydrophilic colloids: These are water-loving colloids.The colloid particles are attracted toward water. They are also called reversible sols. Hydrophobic colloids: These are opposite in nature to hydrophilic colloids. The colloid particles are repelled by water. They are also called irreversible sols. In some cases, a colloid can be considered a homogeneous mixture. This is because the distinction between "dissolved" and "particulate" matter can be sometimes a matter of approach, which affects whether or not it is homogeneous or heterogeneous.

Properties of Colloidal Solutions

All the properties of colloidal solutions can be grouped under three heads:

1. General Properties: General properties of colloidal solutions are given in difference between colloid and true solution.

2. Optical Properties:

(i) Brownian movement: Robert Brown (1927) an English Botanist, observed that the pollen grains in aqueous suspensions were in constant motion. Similar phenomenon was, later on, found in case of colloidal solution, when observed ultra-microscopically.

Brownian movement

This continuous and rapid zig-zag motion of the colloidal particles is called Brownian movement. This motion is independent of the nature of the colloidal particles. It is more rapid when the size of the particles is small and the solution is less viscous.

Cause of Phenomenon:

The Brownian movement is due to the bombardment of colloidal particles by molecules of dispersion medium. The intensity of motion depends upon the size of the particles and the viscosity of the dispersion medium. The smaller the particles and the less viscous the dispersion medium, the more vigorous is the Brownian movement and vice versa.

Importance of Phenomenon

(a) Confirmation of Kinetic Theory of Gases: It offers a visible proof of a rapid random kinetic motion of molecules in liquid as postulated by kinetic theory of gases.

(b) Determination of Avogadro’s number: With the help of ultra-microscope the number of particles in a given mass of the colloidal solution can be counted and the Avogadro’s number being the number of molecules in one mole of any substance; can be calculated.

(c) Stability of Colloidal Solution: The constant rapid zig-zag motion of the particles prevents them from setting due to gravity and thus helps in stabilising colloidal solution to some extent.

(d) Size of Colloidal Particles: The colloidal particles remain under the influence of Brownian movement and gravitation force. Thus both these effects act on colloidal particles and these particles distribute themselves in a vertical column in accordance with the following equation:

$\dfrac{RT}{N_0} ln \dfrac{n_1}{n_2}$ $= \dfrac{4}{3} \pi r^3 ( h_2 n - h_1 ) (\rho - \rho')$

where $N_0$ = Avogadro number, R = Gas constant, T = Temperature (K), $n_1 \text{and} n_2$ number of particles at depths $h_1 \text{and} h_2$ respectively, $\rho \text{and} \rho'$ densities of particles and of the liquid medium respectively. The radius r, of the particle can be determined easily.

(ii) Tyndall Effect: Tyndall (1869) observed that when a strong beam of light is focused on a colloidal solution the path of the beam becomes visible and when viewed through microscope placed at right angle to the path of light (ultramicroscopically), the colloidal particles appear as pin points of light moving against a dark background in a colloidal solution. This phenomenon is known as Tyndall effect and the illuminated path is called Tyndall cone. This phenomenon is not observed in case of true solution.

Cause of Phenomenon:

This phenomenon is due to scattering of light by colloidal particles. This scattering of light can be due to simple reflection because the size of the particles is smaller than wavelength of visible light which are, therefore, unable to reflect light waves. The colloidal particles become self luminous due to absorption of light energy which is then scattered from their surface. The maximum scattered intensity in the plane is at right angle to the path of the light and thus the path becomes visible when observed from the sides.

The intensity of scattered light the difference between the refractive indices of the dispersed phase and that of the medium

In case of lyophobic the difference is more so Tyndall effect is well observed; while in Lyophilic it is less so Tyndall effect is very poor.

Tyndall Effect

Importance of Phenomenon:

1. This phenomenon has been employed as the basic principle for the construction of ultra-microscope. It has been used to detect solid suspended impurities in solution.

2. On the basis of Tyndall effect we can explain that sky is blue in day light. The reason is that dust particles along with water are in the atmosphere. These dust particles scatter blue light and other colours are absorbed therefore the colour of the sky is blue. Since in night scattering of light is not taking place hence sky is black in night.

3. Tail of comets.

4. Blue colour of sea water.

5. Blue tinge of smoke.

3. Electrical Properties:

(i) Electrophoresis (or Cataphoresis): Since the colloidal particles are electrically charged (+ or -)with respect to the dispersion medium, hence on passing electric current through colloidal solution the charged particles move towards oppositely charged electrodes and get discharged to give precipitate. So, this migration of colloidal particles under the influence of electric field is called electrophoresis.

This phenomenon can be studied by a simple apparatus. It consists a U-tube fitted with a funnel shaped reservoir and a stop cock. An $As_2S_3$ sol is taken in the tube and two electrodes dipped in the solution. On passing electric current the As 25 3 particles move towards anode (positive electrodes) indicating that $As_2S_3$ particles are negatively charged particles and lose their charge and coagulate into coarse particles.

Applications:

(a) Determination of charge: The nature of the charge of a colloidal particle can be ascertained by its migration in an electric field.

(b) Electrodeposition of rubber: The negatively charged particles of rubber suspended in the latex of rubber plant can be deposited on another articles making them anode only as a result of electrophoresis.

(c) Removal of carbon particles from smoke: The removal of negatively charged carbon particles from smoke can be done by passing through a chamber provided with highly positive charged metallic knob.

Purification of water

(d) Purification of water: The sewage contains negatively charged particles suspended in water. They may be removed by coagulating them on anode as a result of electrophoresis.

(ii) Electro-osmosis: When electrophoresis of dispersed particles in a colloidal system is prevented by some suitable means, the dispersion medium itself begins to move in an electric field. This phenomenon is known as electro-osmosis.

(iii) Coagulation: We know that the stability of the colloidal solution is due to mutual repulsion between similarly charged colloidal particles. When the charge on the colloidal particles is neutralized by the addition of an electrolyte or oppositely charged sol, the precipitation takes place. Thus “the process of precipitating a colloidal solution is known as coagulation or flocculation”.

For example, the negatively charged $As_2S_3$ sol is readily coagulated on addition of a solution of $BaCl_2$ (due to $Ba^{2+}$ ions). The positively charged $Fe(OH)_3$ sol is readily coagulated on addition of a solution of NaOH (due to $OH^-$ ions).

Factors governing coagulation

(a) A little amount of suitable electrolyte may bring coagulation.

(b) Coagulation is brought about by oppositely charged ions of the electrolyte.

(d) Coagulation of a sol is more pronounced at high temperature.

(e) The coagulating power of different electrolytes is not equal, but depends on:

The valence of the effective ion: The coagulating power increases with the increase of the valency of the active cation i.e.

$Sn^{+4} > Al^{3+} > Ba^{2+} > Na^+$ and active anion I.e $[Fe(CN)_ 6]^{- 4} > PO^{3-}_4 > SO^{2-}_4 > Cl^-$ (Hardy Schulze’s Law).

For example the coagulating values of $Na^+ , Ba^{++} \text{and} La^{++}$ ion for the silver sol are 30, 0.5 and 0.003 milli-mole per litre respectively, hence their coagulating power may be expressed as the reciprocal of $\dfrac{1}{30}, \dfrac{1}{0.5}, \dfrac{1}{0.003}$ or 0.003: 2.0: 333.3 or 1:60:1000 respectively for $Na^+, Ba^{+ +} \text{and} La^{+++}$ ions.

In the same way the coagulating power of $Cl^-, SO^{2-}_4, \text{and} Po^{3-}_4$ ions for $Fe(OH)_3$ Sol is 1:40:90.

The type of the colloidal solution: The lyophobic colloids are easily coagulated while lyophilic colloids require more amount of electrolyte.

When air saturated with water vapours, reaches a cool region, due to condensation bigger drops of water are formed which fall due to gravity in the form of rain. The other reason is that clouds carry positive and negative charges. When these opposite charge clouds mix, rain falls, due to coagulation of oppositely charged colloids. Thus by throwing electrified sand particles from an aero plane coagulation of mist hanging in the air takes place and comes down due to gravitational force. This is called artificial rain.

Peptization: If a freshly precipitated ferric hydroxide is treated with a small amount of ferric chloride solution, a reddish brown coloured sol of ferric hydroxide is obtained. “Thus the process of transferring precipitate back into colloidal from is called peptization”. The $FeCl_3$ , which has caused this dispersion, is called peptizing agent. It is evident that peptization is just reverse of coagulation.

Cause of Phenomenon:

Peptization is due to adsorption of common ion in colloidal solution and electrolyte. The common ion gives the colloidal particle a positive or negative charge according to the charge on the absorbed ion. It results again on mutual repulsion between similar charged particles and they are separated apart.

Peptization

In the above example, the $Fe^{3+}$ ions are absorbed on the precipitate of $Fe(OH)_3$ whereby the positive charge comes on their surface and repulsion takes place due to similar charge. Thus these particles are represented as $[Fe(OH)_3]: Fe^{3+}$ . Similarly, a yellow precipitate of $As_2S_3$ obtained by passing $H_2S$ gas through a solution of $As_2 S_3$ peptizes easily with excess of $H_2S$ gas and is represented as $[As_2S_3]: s^{2-}$

Gold Number: Lyophobic colloids are readily coagulated by electrolyte, but it is difficult to coagulate lyophilic colloid. It has been seen that if a lyophilic colloid is added to the lyophobic one, the later is not coagulated easily by electrolytes and attains stability. The lyophobic colloid is supposed to be enveloped by the lyophilic colloid. Therefore the former remains protected against the action of electrolytes. The extent to which this protective action is exerted by lyophilic colloids differs from substance to substance and is measured quantitatively in terms of Gold number, an expression originated by Zigmondy (1901).

“Gold number is the number of milligrams of protective colloid which must be added to 10 mL of gold sol to prevent coagulation solution of sodium chloride is added to the gold sol”.

It is detected by a colour change from red to blue. it is obvious that the smaller the gold number the greater will be the protective action of the given hydrophilic colloid.

$\text{Gold No.} \propto \dfrac{1}{\text{Protective power}}$

The gold numbers of some of the colloids are given in the bracket with their names : gelatin (0.006 – 0.01), haemoglobin (0.03 — 0.97), gum arabic (0.15 – 0.25), albumin (0.19 – 0.20), dextrin (6.0 — 20.0) and potato starch (10.0 – 25.0).

Purification of Colloidal Solution

Various techniques for purification of colloidal solution are as follows:

1. By Dialysis: It is done by dialysis. Dialysis is the process which is used for the separation of crystalloids from a colloid by filtration or diffusion through parchment paper or animal membrane. The apparatus employed for this purpose is called dialyser. This process is based on the fact that membranes contain very small pores through which only dissolved molecules and ions of the true solution can pass readily.

The Graham dialyser consists of a hollow cylinder open at both ends, over one end of which a membrane is tied. The colloidal solution to be dialysed is placed in this cylinder which is then suspended in a large vessel containing flowing distilled water. After several hours all the molecules or ions of the crystalloid pass though the membrane leaving behind pure colloidal solution.

This process is very slow, but it can be quickened some what by using hot water or by applying an electric field if the substance in true solution is an electrolyte and removed under electric field then it is known as electro-dialysis. In this case, the ions of electrolyte readily pass through the membrane due to electrostatic pull of oppositely charged electrodes.

Hemi dialysis: Here blood of such patients is cleaned by artificial machines whose kidneys are damaged or mal functioning.

2. Ultra filtration: Here colloids are separated from crystalloids with the help of ultra filter papers. Which are obtained by treating simple filter paper with gelatin or collodion so, that the pores become less or narrow in size through which only crystalloid particles can pass.

3. Centrifugation: Here impurities are removed from colloids with the help of a machine of high speed called centrifugal machine. Here colloids settle down while impurities remain in centrifuge.

SCIENCEGEEK: COLLOIDS AND SOLUTIONS

Tuesday, August 6, 2013

Purification of Colloidal Solution

No comments:

Post a Comment