Surface chemistry is the study of chemical and physical processes occurring at surfaces and interfaces. It encompasses four major sub-domains that are all united by the common theme of interfacial phenomena:

1. Adsorption Molecules of a gas or liquid (adsorbate) accumulate at the surface of a solid or liquid (adsorbent). This is distinct from absorption, where the substance penetrates into the bulk. When both occur simultaneously, the process is called sorption.

2. Catalysis The use of surface adsorption (chemisorption) to provide an alternative lower-activation-energy pathway for chemical reactions. Includes heterogeneous catalysis (solid catalyst, gaseous reactants), homogeneous catalysis (same phase), and enzyme catalysis (biological systems).

3. Colloids Dispersions with particle sizes of 1–1000 nm that exhibit unique properties due to their large surface-to-volume ratio. Key phenomena: Tyndall effect (light scattering), Brownian motion (random movement), electrophoresis (charge-based migration), coagulation (charge neutralization and settling).

4. Emulsions Special colloidal systems where both dispersed phase and dispersion medium are liquids (immiscible). Two types: O/W (oil-in-water, e.g., milk) and W/O (water-in-oil, e.g., butter). Stabilized by emulsifiers.

Connecting Theme: All these phenomena occur at surfaces/interfaces and depend critically on surface area, surface charge, and surface energy.