TERNARY WATER IN OIL MICROEMULSIONS MADE OF CATIONIC SURFACTANTS, WATER, AND AROMATIC SOLVENTS .1. WATER SOLUBILITY STUDIES

Two series of cationic surfactants [(I) n-CmH2m+1(n-CnH2n+1)N +(CH3)2X- with m = 12 or 16, n = 1-8, and X- = Cl- or Br-; (II) n-CmH2m+1(C6H5(CH2) p)N+(CH3)2X- with m = 10, 12, 14, 16, and 18, p = 0-2, and X- = Cl- or Br-] have been synthesized and used to prepare water in oil microemulsions, with various aromatic solvents. The water solubility in these microemulsions and the phase behavior of the systems when the water content slightly exceeded the water solubility have been investigated as a function of surfactant chain length (variation of m), size of the surfactant head group (variation of n or p), nature of the counterion (substitution of Br- by Cl-), the molar volume, and the type of aromatic solvent. Mixtures of surfactant homologues have also been investigated: for instance, C16H33(C6H5)N+(CH 3)2Cl- and C16H33-(C6H5CH2CH 2)N+(CH3)2Cl- or C16H33(C6H5CH2)N +(CH3)2Cl- and Br-. The results concerning the effect of the surfactant chain length and the nature of the counterion and of the solvent have been interpreted in terms of the Hou and Shah model (Langmuir, 1987, 3, 1086) for the solubility of water in water in oil microemulsions. This model is based on the two main effects that determine the stability of these systems, namely, curvature of the surfactant film separating the oil and water and interactions between water droplets. The effect of the surfactant head-group size was found to be opposite to the prediction of this model. This is probably due to the fact that part of the head group of the synthesized surfactants passed from the water side to the oil side as its size was increased and, thus, acted as a cosurfactant. Our results become consistent with the Hou and Shah model if this cosurfactant effect of the head group is assumed to dominate the behavior of the systems. © 1990 American Chemical Society.