The Colloidal particles are electrically charged as is indictated by their migration towards cathode or anode under the applied electric field. In a particular colloidal system, all particles carry either positive or negative charge. 
The electric charge on colloidal particles originate in several ways. According to preferential adsorption theory, the freshly obtained precipitate particles adsorb ions from the dispersion medium, whichare common to their lattice and acquire the charge of adsorbed ions.
In some cases the colloidal particles are aggregates of cations or anions having ampiphilic character. When the ions possess hydrophobic part (hydrocarbon end) as well as hydrophilic part (polar end group), they undergo association in aqueous solution to form particles having colloidal size. The formation of such particles, called micelles, plays a very important role in the solubilization of water insoluble substances, (hydrocarbons, oils, fats, grease etc.). In micelles, the polar end groups are directed towards water and the hydrocarbon ends into the centre. The charge on sol particles of proteins depends on the pH. At low pH, the basic group of protein molecule is ionized (protonated) and at
higher pH (alkaline medium), the acidic group is ionized. At isoelectric pH, characteristic to the protein, both basic and acidic groups are equally ionized.