Clouding phenomenon in amphiphilic systems: A review of five decades.

Phase separation in amphiphilic systems is an important phenomenon. The temperature at which an amphiphilic solution phase separates is known as Cloud Point (CP). This article reviews in detail the process of phase separation in various amphiphiles (surfactants, polymers and drugs) and effect of different classes of additives on the CP of these amphiphilic systems. Ions affect the CP of drugs in a different way: kosmotropes and hard bases decrease while chaotropes and soft bases increase the CP of nonionic and cationic surfactants. Anionic surfactants show CP in presence of quaternary salts only. Thus, depending upon the nature and concentration of additive, the CP of an amphiphilic system gets increased or decreased and, hence, properties of the system may be tuned as per the need and use. A system with CP at high concentration can be made to phase separate at lower concentration by simply introducing an appropriate additive in it. This makes the system cost effective. On the other hand, if not required, a low CP can be enhanced with the help of another type of a suitable additive.