Common methods of preparation of the colloids

Lyophilic and lyophobic colloidal solutions (or sols) are commonly prepared by different types of methods. Some of the common methods are given as follows.

(1) Preparation of Lyophilic colloids

(i) The lyophilic colloids posses strong affinity between particles of dispersed phase and dispersion medium.

(ii) Simply mixing both dispersed phase and the dispersion medium under the ordinary conditions readily forms these colloidal solutions.

(iii) For example, the substance such as gum, starch, egg, gum, albumin gum, etc. passes readily into water to provide colloidal solution.

(iv) Their nature is reversible and thus these can be precipitated and directly converted into colloidal state.

(2) Preparation of Lyophobic colloids : Lyophobic colloids can be prepared by mainly two types of methods.

(i) Condensation method : In this type of method, smaller particles of the dispersed phase are condensed suitably to be of colloidal size. This is done by the the below stated methods.

(a) By oxidation : A colloidal solution of the sulphur can be attained by bubbling oxygen (or any other oxidising agent such as HNO₃, Br₂ etc.)  through a solution of the hydrogen sulphide in water.

(b) By reduction : A number of metals like gold, silver,  and platinum, have been obtained in colloidal state by treating the aqueous solution of their salts, with a suitable reducing agent such as hydrogen peroxide, stannous chloride, phenyl hydrazine,  formaldehyde, etc.

          The gold solution so prepared has a purple colour and is called as purple of cassius.

(c) By hydrolysis : Many salt solutions are rapidly hydrolysed by boiling dilute solutions of their salts. For instance, ferric hydroxide and aluminium hydroxide sols are obtained by boiling solutions of the corresponding chlorides. 

Likewise silicic acid solution is obtained by the hydrolysis of sodium silicate.

(d) By double decomposition : A sol of arsenic sulphide is obtained by passing hydrogen sulphide through a cold solution of arsenious oxide in water.

(e) By excessive cooling : A colloidal solution of ice in an organic solvent like ether or chloroform can be prepared by freezing a solution of water in the solvent. Molecules of the water which can no longer be held in the solution separately merge to form particles of colloidal size.

(f) By exchange of solvent : Colloidal solution of certain substances like sulphur and phosphorus, which are soluble in the alcohol but insoluble in water can be prepared by pouring their alcoholic solution in the excess of water. For instance, alcoholic solution of sulphur on pouring into water provides milky colloidal solution of sulphur.

(g) By change of physical state : Sols of substances like mercury and sulphur are prepared by passing their vapour's through a cold water containing a suitable stabilizer such as ammonium salt or citrate.

(ii) Dispersion methods : In these methods, larger particles of a substance (suspensions) are broken into smaller particles. The following are the methods employed in it.

(a) Mechanical dispersion

• In this process, the substance is first ground to coarse particles.
• Then it is mixed with the dispersion medium to get a suspension.
• Suspension is then grinded in the colloidal mill.
• It comprises of two metallic discs nearly touching each other and rotating in opposite directions at a very high speed about 7000 revolution per minute.
• Space among the discs of the mill is so adjusted that coarse suspension is subjected to great shearing force giving rise to particles of colloidal size.
• Colloidal solutions of paints, varnishes, black ink, dyes etc. are obtained by this process.

(b) By the electrical dispersion process or Bredig's arc method

• This method is taken in use to prepare sols of platinum, silver, copper or gold.
• The metal whose solution is to be prepared is made as two electrodes which immerged in dispersion medium such as water etc.
• The dispersion medium is made cool by the help of ice.
• The electric arc is struck among the electrodes.
• Tremendous heat obtained by this method and provides colloidal solution.
• The colloidal solution prepared is stabilized by adding a small amount of KOH to it.

(c) By peptisation

• The method of converting a freshly prepared precipitate into the colloidal form by the addition of suitable electrolyte is called peptisation.
• The electrolyte is taken in use for this purpose is called peptizing agent or stabilizing agent.
• Due to the peptisation is the adsorption of the ions of the electrolyte by the particles of the precipitate.
• Some of the important peptizing agents are gum, sugar, gelatin and electrolytes.
• The freshly prepared ferric hydroxide can be converted into the colloidal state by shaking it with water containing Fe³⁺ or OH^- ions, which are FeCl₃ or NH₄OH respectively.
• 

• The stable solution of stannic oxide is obtained by adding a small amount of dilute HCl to stannic oxide precipitates.
• Similarly, a colloidal solution of Al(OH)₃ and AgCl are obtained by treating the corresponding freshly prepared precipitate with very dilute solution of HCland AgNO₃ or KClrespectively.

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