SOLUTION PROPERTIES OF ZWITTERIONIC PYRIDINIO SURFACTANTS

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