Abstract
Krafft point is well known as the temperature above which the solubility of a surfactant increases dramatically in aqueous systems. Two kinds of model have so far been proposed to explain this anomalous solution behavior of surfactant. Some new experimental evidences in support of the melting-point model for the Krafft point, one of the above two models, were given recently by the present author and his collaborators. The Krafft point vs. composition curves of the binary surfactant mixtures have been studied and found to be classified into three groups. In the case of group I, there exists a minimum in the Krafft point at a certain composition, whereas the Krafft point for the group II varies monotonously with the composition change. The every diagram of the group III, on the other hand, has a maximum Krafft point. These three types of phase diagrams correspond to the eutectic, solid-solution forming, and addition-compound forming systems observed in the melting point vs. composition curves of the binary mixtures of ordinary substances. A novel Krafft-point depression phenomenon of some zwitterionic surfactants caused by inorganic salts is also argued thermodynamically based on the melting-point model for the Krafft point.
