|Some important electrical properties of colloidal solutions are as follows:
(i) Presence of electrical charge on colloidal particles and stability of sols:One of the most important properties of colloidal solutions is that colloidal particles posses a definite type of electrical charge. In a particular colloidal solution, all the colloidal particles carry the same type of charge, while the dispersion medium has an equal but opposite charge. Thus, the charge on colloidal particles is balanced by that of the dispersion medium and the colloidal solution as a whole is electrically neutral. For example, in a ferric hydroxide sol, the colloidal ferric hydroxide particles are positively charged, while the dispersion medium carries an equal and opposite negative charge.
The stability of a colloidal solution is mainly due to the presence a particular type of charge on all the colloidal present in it. Due to the presence of similar and equal charges, the colloidal particles repel one another and are thus unable to combine together to form larger particles. This keeps them dispersed in the medium and the colloidal remains stable. This is why sol particles do not settle down even on standing for a long time.
Based on the nature of charge, the colloidal sols may be classified as positively charged and negatively charged sols. Some common examples of these sols are given below.
Positively charged sols:
Metallic hydroxide sols e.g., Fe(OH)3, Al(OH)3, Cr(OH)3, etc., TiO2 sol, haemoglobin, sols of basic dyes such as methylene blue etc.
Negatively charged sols:
Metal sols e.g., Au, Ag, Cu, Pt etc. sols, metal sulphide sols e.g., As2S3, CdS etc. sols; starch sol, sols of acid dyes such as Congo red etc.
Origin of charge on colloidal particles:
There are several views regarding the origin of charge on colloidal particles. According to these views, colloidal particles acquire charge due to the following reasons.
(a) Due to dissociation of the adsorbed molecular electrolytes: Colloidal particles have a strong tendency to adsorb reactant or product molecules. The molecules thus adsorbed on the surface of colloidal particles may undergo dissociation/ionization and may impart charge to them. For example, during the preparation of sulphide sols (e.g., As2S3 sol), H2S molecules get adsorbed on colloidal particles. H2S molecules thus adsorbed undergo ionization and release H+ions into the medium. Consequently, colloidal particles are left with negative charge.
(b) Due to the dissociation of molecules forming colloidal aggregates: The molecules responsible for the formation of aggregates of colloidal dimensions may themselves undergo dissociation/ionisation resulting in the development of charge on the colloidal particles formed by their aggregation. For example, the soap molecules (RCOONa) dissociate to give RCOO- and Na+ ions. RCOO- ions aggregate together to form micelles which carry negative charge as explained earlier.
c) Due to preferential adsorption of ions from solutions: The colloidal particles have a tendency to preferentially adsorb a particular type of ions from the solution. A colloidal particle usually adsorbs those ions which are in excess and are common to its own lattice. This preferential adsorption of a particular type of ions imparts a particular type of charge to colloidal particles.
For example, when a ferric hydroxide sol is prepared by the hydrolysis of ferric chloride in warm water, the colloidal particles of Fe(OH)3 formed have a tendency to adsorb preferentially the Fe3+ ions present in the solution. This is because Fe3+ ions are common to the lattice of Fe(OH)3 particle. The Fe3+ ions thus adsorbed impart positive charge to the colloidal particles present in the sol.
Fe(OH)3 + Fe3+ --> Fe(OH)3 : Fe3+
(colloidal (ions common preferential adsorption of Fe3+ ions
particle) to the lattice of (colloidal particle acquires
colloidal particle) positive charge)