Properties of Colloidal Solutions

Colloidal solutions show a distinctive set of properties.

(i) Colligative properties

Because colloidal particles are large aggregates, their number in solution is small, so colligative properties — osmotic pressure, lowering of vapour pressure, depression of freezing point, elevation of boiling point — are much smaller than for a true solution of the same concentration.

(ii) Tyndall effect

A true solution viewed at right angles to a light beam looks dark, but a colloid scatters the light and its path glows with a bluish light — the Tyndall effect (first seen by Faraday, studied by Tyndall). The bright illuminated cone is the Tyndall cone (Fig. 5.11). It appears only when (i) the particle diameter is not much smaller than the wavelength of light, and (ii) the refractive indices of dispersed phase and medium differ greatly. The effect distinguishes a colloid from a true solution and underlies Zsigmondy's ultramicroscope (1903), in which particles appear as bright stars against a dark background — though it reveals only scattered light, not the particles' true size or shape.

(iii) Colour

Colour depends on the wavelength of light scattered, which in turn depends on particle size and nature, and on how the observer views the light. A mixture of milk and water looks blue by reflected light and red by transmitted light; the finest gold sol is red and turns purple, then blue, then golden as particle size grows.

(iv) Brownian movement

Under an ultramicroscope, colloidal particles show a continuous zig-zag motion (Fig. 5.12), first noted by Robert Brown. It is faster for smaller particles and less viscous media and arises from the unbalanced bombardment of each particle by medium molecules. Its stirring action keeps particles from settling, helping stabilise the sol.

(v) Charge on colloidal particles

Every colloidal particle carries an electric charge, the same sign on all particles in a given sol:

• Positive sols: hydrated metal oxides ($Al_2O_3{\cdot}xH_2O$, $Fe_2O_3{\cdot}xH_2O$), basic dyes (methylene blue), haemoglobin, $TiO_2$.
• Negative sols: metal sols (Cu, Ag, Au), metal sulphides ($As_2S_3$, $Sb_2S_3$, CdS), acid dyes (eosin, congo red), starch, gum, gelatin, clay, charcoal.

Like charges repel, preventing particles from coalescing — the main source of sol stability. Charge arises from electron capture during electro-dispersion, from preferential adsorption of ions, and/or from an electrical double layer. In preferential adsorption a particle adsorbs the ion common to itself:

• Add dilute $AgNO_3$ to excess KI and the AgI adsorbs $I^-$ → negative sol ($AgI/I^-$); add KI to excess $AgNO_3$ and it adsorbs $Ag^+$ → positive sol ($AgI/Ag^+$).
• $FeCl_3$ in excess hot water gives a positive sol ($Fe_2O_3{\cdot}xH_2O/Fe^{3+}$); with NaOH it gives a negative sol adsorbing $OH^-$.

The charged surface then attracts counter-ions into a second layer (e.g. $AgI/I^-\ K^+$). The two oppositely charged layers form the Helmholtz electrical double layer — a firmly held fixed layer plus a mobile diffused layer (Fig. 5.13). The potential difference between them is the electrokinetic (zeta) potential. This double layer makes particles repel at a distance, keeping them dispersed. Adding more electrolyte compresses the diffused layer, lowers the zeta potential, cuts the repulsion, and the colloid precipitates — which is why colloids are so sensitive to oppositely charged ions.

(vi) Electrophoresis

Apply a potential across two platinum electrodes in a sol and the charged particles move — positive to the cathode, negative to the anode (Fig. 5.14). This electrophoresis confirms the particles' charge. If the particles are held still, the medium itself moves in the field — a phenomenon called electroosmosis.

(vii) Coagulation (precipitation)

Removing the charge lets particles clump and settle — coagulation — achieved by:

• Electrophoresis — particles reach the electrode, discharge and precipitate.
• Mixing oppositely charged sols — e.g. hydrated ferric oxide (+) with arsenious sulphide (−), giving mutual coagulation.
• Boiling — increased collisions disturb the adsorbed layer.
• Persistent dialysis — removing stabilising electrolyte makes the sol unstable.
• Adding electrolytes — the ion of charge opposite to the sol (the coagulating ion) neutralises it. …