ABSTRACT
Surface active agents (usually referred to as surfactants) are amphipathic molecules consisting of a nonpolar hydrophobic portion, usually a straight or branched hydrocarbon or fluorocarbon chain containing 8–18 carbon atoms, which is attached to a polar or ionic portion (hydrophilic). In general, nonionic surfactants have lower critical micelle concentration (cmc) values than their corresponding ionic surfactants of the same alkyl chain length. One of the characteristic features of solutions of surfactants is their solubility-temperature relationship, which is strikingly different for ionic and nonionic surfactants. With ionic surfactants, the solubility first increases gradually with increase of temperature; then, above a certain temperature, there is a very sudden increase of solubility with further increase in temperature. The solubility-temperature relationship for nonionic surfactants is different from that for ionic surfactants. The Krafft point is viewed as the temperature at which solid hydrated surfactant, micelles and a solution saturated with undissociated surfactant molecules are in equilibrium at a given pressure.
