Properties of Colloids

Colloidal solutions show a distinctive combination of physical, optical, kinetic and electrical properties that set them apart both from true (molecular) solutions and from ordinary coarse suspensions.

1) Colour. A sol's colour is not necessarily the same as the colour of the bulk substance — dilute milk, for instance, shows a bluish tinge in reflected light but a reddish tinge in transmitted light. A sol's observed colour depends on several factors together: the method used to prepare it, the wavelength of the light source, the size and shape of the colloidal particles, and whether the observer is viewing reflected light or transmitted light.

2) Size. Colloidal particles range in size from about 1 nm ($10^{-9}$ m) up to about 1000 nm ($10^{-6}$ m) in diameter — larger than the particles of a true solution but smaller than the particles of a coarse (mechanical) suspension.

3) Heterogeneous, border-line nature. Colloidal solutions are genuinely heterogeneous, containing two distinct phases (dispersed phase and dispersion medium), and this heterogeneity is demonstrable experimentally via dialysis, ultrafiltration and ultracentrifuging. That said, because a colloid's particles are so much smaller than an ordinary suspension's, colloidal solutions in modern usage are considered a genuine border-line case between true (homogeneous) solutions and coarse (heterogeneous) suspensions.

4) Filtrability. Because the pores of an ordinary filter paper are relatively large, colloidal particles pass through ordinary filter paper easily, which is exactly why ordinary filtration cannot purify or separate a colloidal solution (special ultrafilters, covered in the purification section, are needed instead).

5) Non-settling (stability) nature. Colloidal solutions are reasonably stable and are not noticeably affected by gravity over ordinary timescales — the colloidal particles do not simply settle out under their own weight the way a coarse suspension would.

6) Concentration and density. A dilute colloidal solution is generally stable; if the volume of the dispersion medium is decreased (the sol is concentrated further), coagulation tends to occur. In general, the density of a sol decreases as its concentration decreases.

7) Diffusability. Unlike the particles of a true solution, colloidal particles diffuse only sluggishly through membranes, on account of their much larger size.

8) Colligative properties. Colloidal solutions genuinely show colligative properties — elevation of boiling point, depression of freezing point, and osmotic pressure — exactly as true solutions do (though the effects are smaller for a given mass concentration, since colloidal particles are far fewer in number than the same mass of small molecules would be). Measuring osmotic pressure is, in fact, a standard way of estimating the molecular weight of a colloidal particle.

9) Shape of colloidal particles. Different colloidal systems adopt genuinely different particle shapes: $\text{As}_2\text{S}_3$ sol particles are spherical, $\text{Fe(OH)}_3$ sol (blue gold sol) particles are disc- or plate-like, and $\text{WO}_3$ sol (tungstic acid sol) particles are rod-like.

10) Optical property — the Tyndall effect. When a beam of light passes through an ordinary (homogeneous) solution, it looks clear when viewed along the beam's direction but essentially dark when viewed from the side (perpendicular to the beam), since the dissolved particles are too small to scatter visible light appreciably. When the same beam instead passes through a colloidal solution, it is scattered strongly in all directions by the larger colloidal particles, making the path of the light through the solution clearly visible from the side. Faraday first observed this scattering, but Tyndall investigated it thoroughly, so the effect is named the Tyndall effect; the colloidal particles absorb part of the light and scatter the rest from their surface, and it is exactly this scattered light that makes the beam's path visible.

11) Kinetic property — Brownian movement. Robert Brown observed that pollen grains suspended in water, viewed under an ultra-microscope, showed a continuous, random, zig-zag motion — now called Brownian movement of colloidal particles. It arises because the colloidal particles are continuously bombarded on all sides by the smaller, faster-moving molecules of the dispersion medium; because the bombardment from opposite sides is never perfectly balanced at any given instant, the particle receives a small net random kick, and the accumulation of many such kicks over time produces the observed zig-zag trajectory. Brownian movement enables three important things: (I) it provides a route to calculate Avogadro's number; (II) it offers direct experimental confirmation of the kinetic theory's prediction that molecular motion is ceaseless and speeds up with rising temperature; and (III) it helps explain colloidal stability itself — because the particles are constantly in rapid, random motion, they cannot approach closely enough, for long enough, to be pulled together by gravity and coalesce, so Brownian movement effectively counteracts gravitational settling.

12) Electrical property. (i) Helmholtz double layer: a colloidal particle's surface preferentially adsorbs one type of ion from the surrounding medium; this adsorbed layer then attracts a surrounding layer of oppositely charged ions from the medium, and the boundary separating these two layers sets up what is called the Helmholtz electrical double layer. Because every particle in a given sol carries the same surface charge, neighbouring particles repel each other and cannot approach closely enough to condense together — which helps explain the sol's overall stability. (ii) Electrophoresis (cataphoresis): when an electric potential is applied across two platinum electrodes dipped into a sol, the dispersed particles migrate toward one electrode or the other. If the particles migrate toward the cathode, they carry a positive charge (examples: ferric hydroxide, aluminium hydroxide, basic dyes, haemoglobin); if they migrate toward the anode, they carry a negative charge (examples: gold, silver and platinum sols, arsenic sulphide, clay, starch). Because the migration direction directly reveals the sign of the sol's charge, electrophoresis is used specifically to DETECT the presence and sign of charge on colloidal particles. (iii) Electro-osmosis: a sol overall is electrically NEUTRAL, since its dispersion medium carries a charge equal and opposite to that of the dispersed particles. If the dispersed particles are experimentally prevented from moving (for instance held back by a membrane) while an electric field is applied, the dispersion MEDIUM itself instead moves, in a direction opposite to that in which the particles would otherwise have migrated — this movement of the medium under an applied electric potential is called electro-osmosis. …