Relationship of Surface Tension and Concentration
|Percent Concentration required to reduce the surface tension of water to indicated values|
|Surface tension, dynes per cm||73||50||40||30||22|
This internal group of surfactant molecules is referred to as a micelle (m).
Because of this characteristic behaviour of surfactants to orient at surfaces and to form micelles, all surfactants perform certain basic functions. However, each surfactant excels in certain functions and has others in which it is deficient.
Foaming agents, emulsifiers, and dispersants are surfactants which suspend respectively, a gas, an immiscible liquid, or a solid in water or some other liquid. Although there is similarity in these functions, in practice the surfactants required to perform these functions differ widely. In emulsification, as an example - the selection of surfactant or surfactant system will depend on the materials to be used and the properties desired in the end product. An emulsion can be either oil droplets suspended in water, an oil in water (O/W) emulsion, water suspended in a continuous oil phase, water in oil (W/O) emulsion, or a mixed emulsion. Selection of surfactants, orders of addition and relative amounts of the two phases determine the class of emulsion.
Each of these three functions is related to the surfactant absorbing at a surface, either gas, liquid or solid with the hydrophilic ends of the molecules oriented to the water phase. The surfactants form what amounts to a protective coating around the suspended material, and these hydrophilic ends associate with the neighbouring water molecules. In addition to surfactant effects the stability of these suspensions is related to the particle size and density of the suspended material.
Solubilisation is a function closely related to emulsification. As the size of the emulsified droplet becomes smaller, a condition is reached where this droplet and the surfactant micelle are the same size.
At this stage, an oil droplet can be imagined as being in solution in the hydrophobic tails of the surfactant and the term solubilisation is used. Emulsions are milky in appearance and solubilised oils, for example - are clear to the eye.