Characteristics Of Colloidal Solutions

Characteristics Of Colloidal Solutions

Colloidal solution possess some distinct characteristics. Some of them are important which are discussed as follows :

1. Hetrogeneous nature

Colloidals solution are heterogeneous in nature consisting of two distinct phases, i.e, the dispersed phase and the dispersion medium.

2. Filtrability

Colloidal particles can readily pass through ordinary filter papers because the pore−size of filter paper is much larger than that of colloidal particles.

3. Colour

The color of a colloidal sol is not always the same as that of the depressed phase, as the dispersed particles affect the color of the sol. For example, spherical gold particles impart a red color to gold sol, while flat particles a blue color,

4. Adsorption

The colloidal particles due to the presence of unbalanced forces on their surfaces attach a variety of suspended particles (molecules etc.) to their surfaces. This property has been utilized in the dyeing of cotton fabrics when the cloth is first mordanted (i.e., treated with colloidal suspension), which adsorbs dye, and in the froth flotation process of concentrating ores, etc.

5. Colligative properties

Colloidal particles have a very high molecular weight so the number of moles present in a solution is extremely low. Since colligative properties depend only on the number of effective mole present per liter, so the value of any of the colligative properties (i.e., osmotic pressure, depression of freezing point, the elevation of boiling point, or lowering of vapor pressure) of a specific substance will be quite low compared to its value, when it forms a true solution. However colloidal solutions have measurable low osmotic pressure, So osmotic pressure measurement have been utilized (using osmometer) to determine the average molecular weight of colloidal particles with reasonable accuracy e.g., proteins cellulose, starch, gelatin, high polymers, etc..

6. Mechanical properties

The Brownian movement, diffusion, and sedimentation are the important mechanical properties of colloidal solutions. Let us take these separately

(i) Brownian movement: When a colloidal solution is examined under an ultra−microscope, the colloidal particles are seen to be in a state of continuous rapid random motion. This phenomenon was first noticed by Brown, an English Botanist in 1827 and hence is called the “ Brownian movement” This motion is rapid in the case of particles of smaller sizes and the less viscous dispersion medium.

(ii) Diffusion: Similar to solute particles in true solutions the particles in a colloidal solution diffuse (or migrate) from a region of higher concentration to a region of lower concentration, until uniform concentration is achieved. Because of their high mass, they move slowly, and thus the rate of their diffusion is quite slow. The diffusion process has been used to separate colloidal particles of different sizes and also to determine their size.

(iii) Sedimentation: Colloidal solutions are quite stable and the dispersed (suspended) particles remain suspended indefinitely and move constantly. The Brownian movement efficiently explains the dynamic nature of colloidal particles. However, colloidal particles of large sizes tend to settle down very slowly under the influence of gravity and the rate of sedimentation (or settling) can be effectively increased by employing a high-speed centrifuge called ultracentrifuge

7. Tyndall Effect

Although a colloid is homogenous because the dispersed particles are quite small, it can be distinguished from a true solution by its ability to scatter light. The scattering of light by colloidal− size particles is known as the Tyndall effect. For example, the atmosphere appears to be clear gas, but a ray of sunshine against a dark background shows up many fine dust particles by light scattering. Similarly, when a beam of light is directed through clear gelatin (a colloid not a true solution), the beam becomes visible by the scattering of light from colloidal gelatin particles. The beam appears as a ray passing through the solution. When the same experiment is performed with a true solution, such as an aqueous solution of sodium chloride, the beam of light is not visible.

8. Electrical properties

(i) Electrophoresis: Colloidal particles (both lyophilic and lyophobic) are electrically charged either positive or negative. Such as

When a high potential gradient is applied between a U−tube filled partly with a colloidal solution and rest with distilled water the colloidal particles move towards oppositely charged electrode. On reaching the electrode, they lose their charge and get coagulated or precipitated.

The movement of the colloidal particles under the influence of an electric field is known as electrophoresis. If the movement of colloidal particles is towards cathode, the phenomenon is called cataphoresis as in case of negatively charged sols like As₂S₃.

(ii) Electro−osmosis: When an electric current is passed through a colloidal solution in such a way that the dispersed particles are prevented from movement, it is observed that the dispersion medium moves. This phenomenon of movement of the dispersion medium of a colloidal solution under the influence of an electric field while the dispersed particles are prevented from moving is called electro osmosis.

9. Coagulation Of Sols

The colloidal particles carry similar type of charge and due to electrical repulsions they are kept apart. However they can be precipitated or coagulated by boiling or by adding an electrolyte or by adding an oppositely charged sol. The phenomenion of precipitation of a colloidal solution is called “coagulation” and the solid which separated out under such conditions is called the “coagulum”.

(i) Coagulation by heating: Certain colloidal sol on boiling coagulates to yield precipitate. For example, egg material, a colloidal sol, coagulates on boiling.

(ii) By adding oppositely charged sol: The mutual coagulation of the two sols can be done by mixing two oppositely charged sols. For example, coagulation of positively charged Fe(OH)₃ sol can be effected by mixing it with negatively charged As₂S₃ sol in proper proportion . The charged on both the sols are mutually neutralized and hence both get coagulated simultaneously.

(iii) By electrophoresis: The coagulation of a sol may be achieved by electrophoresis, i.e., passing electric current through the sol (similar to electrolysis of true solution). The electrically charged colloidal particles move towards the oppositely charged electrode, under the influence of applied electric field. On reaching the electrode they get discharged and coagulated. This technique of coagulation has been utilized to electro−paint, i.e., the article to be painted is made electrode in a bath of colloidal paint. Colloidal particles dispersed in drinking water are also removed by this method.

(iv) Coagulation by adding electrolyte: The charge on the colloidal particles is neutralized by the oppositely charged ions furnished by added electrolyte. On being robbed of their charge, these particles coalesce together to form bigger particles, which finally settle down as precipitate under the influence of gravity. Such as on adding of a little BaCl₂ to negatively charged As₂S₃ sol the latter gets precipitated. The positively charged Ba²⁺ ions furnished by BaCl₂ are attracted by the −vely charged sol particles with the result that the negative charge of the latter gets neutralized an these particles without any charge then settle down to form precipitate.

Coagulation of a colloidal by the addition of an electrolyte is due to the neutralization of the charge on the colloidal particle by the oppositely charged ions, furnished by the added electrolyte. Since the charge on the oppositely charged ions is responsible for precipitation of the colloidal solution so the power of the ion affecting coagulation must depend on its valency. Thus polyvalent ions carrying larger amounts of charge, must be more efficient coagulating agent then monovalent ions. Schulze and Hardy stated law says, “ The coagulating of electroyte depends predominantly on the valency of the bearing charge opposite to that of the colloidal particles and higher the valency of the active ion, the greater is its precipitating action”. Thus the precipitating action of the cations Na+, Ba2+  and Al3+ on a negatively charged sol like that of As2S3 is in the order :

Al³⁺ > Ba²⁺ > Na⁺

Similarly the coagulating power of trivalent ions is grater than that of divalent ions and the coagulating power of divalent ions is much more than that of monovalent Cl− or NO₃− ions for positively charged sol like that of Fe(OH)₃ i.e., coagulating power of Cl−, and ions on a positively charged sol is in the order.

 > > Cl^-

AgCl sol is positively charged when prepared in presence of a slight excess of AgNO₃ and it is negatively charged when prepared in slight excess of NaCl. In the former case common excess Ag+ ions rare preferentially adsorbed on the AgCl colloidal particles. On the other hand in the second case the common excess chloride ions are adsorbed on AgCl colloidal particles.

NaCl + AgNO₃ → NaNO₃ + AgCl↓

AgCl + AgNO₃ → [AgCl] Ag⁺:

(Slight excess)+ve solIn diffused layer

AgCl + NaCl →  [AgCl]. Cl−: Na⁺

(Slight excess)−ve solIn diffused layer

(v) Proteins readily yield sols: The charge on a protein colloidal particle is due to the presence of acidic and basic groups in its molecule. A protein has −COOH (carboxyl group) as well as −NH₂ (amino group) and it may be represented as :

H₂N ⎯X⎯COOH

(a) In acidic solution, the protein becomes positively charged due to the ionization of the amino group.

(b) In a alkaline solution, the protein molecule is negatively charged, due to the ionization of the carboxyl group.

Other Resourceful Topics

• Preparation of Colloidal Solutions
• Purification of Colloidal Solutions