SOLUTION PROPERTIES OF ZWITTERIONIC PYRIDINIO SURFACTANTS

Two isomers of single chain zwitterionic surfactants with a rigid zwit terionic polar head have been studied as aqueous solutions. These surf actants are N-alkylpyridiniocarboxylates with their carboxylate group at positions 3 or 4 on the pyridinium ring. Krafft temperatures, criti cal micellar concentrations (cmc), and molecular area at the air-water interface (a(0)) were measured. Mesophases are formed above 60% weigh t fraction. The phase diagrams are quite unusual as transitions betwee n mesophases occur by changing temperature: a cubic Q(m) (Pm3n) mesoph ase turns into an hexagonal H-alpha one upon heating in a large concen tration domain. This transition occurs or not through a micellar L1 in termediate according to the concentration domain. For such surfactant- water binary mixtures, a temperature sensitive behavior is thus observ ed instead of the classical lyotropic one which is observed with most ionic surfactants. Differences between the two isomers are discussed a t the molecular level. Thus, influence of the dipole moment of the zwi tterionic polar head on the cmc and a(0) is established. The origin of mesophase structures and phase diagrams which are quite complex are d iscussed.