Properties of Colloids — Core Principles

Core Principles

Colloids are heterogeneous mixtures with dispersed particles ranging from $1,\text{nm}$ to $1000,\text{nm}$. This intermediate size gives them unique properties. The Tyndall effect is the scattering of light by colloidal particles, making the light path visible, and is used to distinguish colloids from true solutions.

Brownian movement is the continuous, random zig-zag motion of colloidal particles, caused by unbalanced bombardment from dispersion medium molecules, which prevents sedimentation and ensures stability.

Colloidal particles typically carry an electric charge, usually acquired by preferential adsorption of ions from the medium. This charge leads to mutual repulsion, further stabilizing the colloid.

The potential difference across the electrical double layer is called zeta potential. Electrophoresis is the movement of charged colloidal particles in an electric field. Coagulation is the process of settling colloidal particles by neutralizing their charge, often by adding electrolytes.

The Schulze-Hardy rule states that the coagulating power of an ion increases with its valency and is effective for ions with charge opposite to the colloid. These properties are fundamental to understanding colloidal behavior and their widespread applications.

Important Differences

vs True Solutions and Suspensions

Aspect	This Topic	True Solutions and Suspensions
Particle Size	True Solution (< $1, ext{nm}$)	Colloid ($1, ext{nm}$ to $1000, ext{nm}$)
Homogeneity	Homogeneous	Heterogeneous (appears homogeneous)
Visibility	Invisible (even with ultramicroscope)	Visible with ultramicroscope, invisible to naked eye
Tyndall Effect	Does not show	Shows
Brownian Movement	Does not show (molecules in random motion, but not visible particles)	Shows
Sedimentation	Does not settle	Does not settle under gravity (settles by ultracentrifugation)
Diffusion	Rapid	Slow
Filtration	Passes through filter paper and animal membrane	Passes through filter paper, but not animal membrane (dialysis)

The fundamental distinction among true solutions, colloids, and suspensions lies in the size of their dispersed particles, which in turn dictates their observable properties. True solutions are molecularly dispersed, homogeneous, and do not scatter light or settle. Suspensions have large, visible particles that settle readily. Colloids, with intermediate particle sizes, exhibit unique optical (Tyndall effect), kinetic (Brownian movement), and electrical properties (charge, electrophoresis) that are absent or negligible in the other two categories. Understanding these differences is crucial for classifying mixtures and predicting their behavior in various applications.