A solution in which particles measuring 10-9〜10-7 m are dispersed is called a colloidal solution. Colloids are common in milk, India ink, blood, and many other familiar substances.
Basic Knowledge
Colloidal particles lie between true solutions (<10-9 m) and suspensions (>10-7 m). Their properties include: ① the Tyndall effect (light scattering), ② Brownian motion (collisions with solvent molecules), ③ electrophoresis (particle charge), ④ dialysis (removal of small molecules using a semipermeable membrane), and ⑤ coagulation and salting out. Hydrophobic colloids (Fe(OH)3, clay) coagulate with a small amount of electrolyte, while hydrophilic colloids (proteins, starch) undergo salting out with a large amount of electrolyte.
Hydrophobic colloid（coagulates with a small amount of electrolyte）
Hydrophilic colloid（undergoes salting out with a large amount of electrolyte）
Protective colloid（a hydrophilic colloid protects a hydrophobic colloid. Gelatin in India ink）
Molecular colloid（one polymer molecule forms one particle. Protein）
Association colloid（micelles of a surfactant）
Gel and sol（a fluid sol ⇔ a solidified gel）
Further Study (Principles and Applications)
Hydrophobic colloids are stabilized by the electrostatic repulsion of surface charges. When electrolytes neutralize the charges, the particles aggregate (Schulze–Hardy rule: among oppositely charged ions, ions with greater valence have greater coagulating power). Hydrophilic colloids are also stabilized by hydration water, so a large amount of electrolyte is required for salting out. Tofu production is a classic application in which nigari (MgCl2) salts out soybean proteins. The formation of river deltas is a natural phenomenon in which clay colloids coagulate because of ions in seawater.
- Colloid particle size: 10-9〜10-7 m
- Tyndall effect = light scattering
- Brownian motion = collisions with solvent molecules
- Hydrophobic → coagulation (small amount)
- Hydrophilic → salting out (large amount)
- Schulze–Hardy rule: greater valence means stronger action
- A protective colloid is hydrophilic and protects a hydrophobic colloid
Points of Caution (Common Confusions and Frequent Mistakes)
① Distinguish coagulation (hydrophobic colloids) from salting out (hydrophilic colloids). ② Dialysis removes only small molecules through a semipermeable membrane; colloids remain. ③ A semipermeable membrane allows only solvent to pass. ④ Glue and gelatin are protective colloids.
Practice
- Explain the cause of the Tyndall effect.
- Explain why nigari (MgCl2) is added in tofu production from the viewpoint of colloid chemistry.
- Which has the greatest coagulating power for a negatively charged hydrophobic colloid: Na+, Mg2+, or Al3+?