Protective Colloids & Gold Number

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Surface Chemistry • Colloid Stability

Coagulation, Protection & The Gold Number

Mastering Lyophilic Shields, Dehydration Traps, and Zsigmondy's Metric.

By chemca Team • Updated Sep 2026

While lyophobic sols (like gold or $As_2S_3$) are notoriously unstable and coagulate at the slightest addition of an electrolyte (Hardy-Schulze rule), lyophilic sols (like gelatin, starch, and proteins) are incredibly robust. This massive difference in stability allows lyophilic sols to be used as protective shields, a concept mathematically quantified by the famous Gold Number.

1. Coagulation of Lyophilic Sols (The Dual Factor)

Why are lyophilic sols so stable compared to lyophobic sols? It comes down to a dual-defense system. Lyophilic particles are protected by two independent factors:

• 1. Charge: Like all colloids, they carry an electrical charge causing mutual repulsion.
• 2. Extensive Solvation (Hydration): Because they are "liquid-loving", the particles are surrounded by a thick, tightly held layer of the dispersion medium (water) molecules. This acts as a physical bumper against collisions.

How to Force Coagulation (The Trap):

Adding an electrolyte alone is often not enough to coagulate a lyophilic sol because the hydration shell protects the particles. You must destroy BOTH defense systems simultaneously.

The Two-Step Coagulation Requirement:

To coagulate a lyophilic sol, you must add:
1. An Electrolyte (to neutralize the electrical charge).
2. A Dehydrating Agent like Acetone or Alcohol (to strip away the protective solvation/hydration shell).

Only when both the charge and the water shell are removed will the particles aggregate and precipitate.

2. Protective Action of Lyophilic Colloids

Because lyophilic sols are highly stable and extensively solvated, they can be used to rescue unstable lyophobic sols. When a lyophilic sol is added to a lyophobic sol, the lyophobic sol becomes highly resistant to coagulation by electrolytes. This is called Protection.

Figure 1: Mechanism of Protective Action against electrolyte attack.

The Mechanism:

The highly solvated lyophilic particles form a protective layer or sheath around the lyophobic particles. This physical barrier prevents the coagulating ions of the added electrolyte from reaching and neutralizing the charge on the lyophobic particles.

3. Zsigmondy's Gold Number (The Ultimate Metric)

Different lyophilic colloids have different protective powers. To quantify and compare their effectiveness, Richard Zsigmondy introduced a strictly defined metric known as the Gold Number.

The Exact Definition (Memorize This):

The Gold Number is the minimum mass in MILLIGRAMS of the protective (lyophilic) colloid that must be added to exactly 10 mL of a standard Red Gold Sol to just prevent its coagulation when exactly 1 mL of 10% NaCl solution is rapidly added.

The Color Change Trap:

A stable gold sol is bright Red. When it coagulates (particles clump together and grow in size), the color changes from Red to Blue. Therefore, the Gold Number is the amount of milligrams needed to prevent the Red $\rightarrow$ Blue color shift.

Figure 2: The standard experimental definition of the Gold Number.

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