Because an emulsifier has two opposite parts; hydrophilic part and lipophilic part, its solution doesn’t turn to a simple aqueous solution but a colloidal solution, whose properties greatly vary depending on its concentration. In an extremely diluted solution, there is no special change, but the emulsifiers gather on the interface and the surface tension is reduced as its concentration gets greater. As the concentration gets higher, a uniform mono molecular layer is made on the surface and the surface tension drops to the minimum. Formation of micelle is caused by assembling of excess molecules, within which lipophilic parts face to the center, and surface tension won't show any change.
The point of concentration at which micelle start to form is called critical micelle concentration (cmc). The properties of the solution change greatly when it reaches cmc.
Similar changes happen on the interface of oil and water when their interfacial tension reaches cmc.
When the concentration exceeds cmc, spherical micelles appear and disperse into water. With increasing of concentration rod-shape micelle will be formed. Finally, lamellar micelles with higher structures called liquid crystal are produced.