QUATERNARY WATER-IN-OIL MICROEMULSIONS .2. EFFECT OF CARBOXYLIC-ACID CHAIN-LENGTH ON DROPLET SIZE AND EXCHANGE OF MATERIAL BETWEEN DROPLETS

Water solubility, electrical conductivity, and time-resolved fluorescence quenching measurements have been performed in water-in-oil (w/o) microemulsions made of water, chlorobenzene, cationic surfactant, and n-carboxylic acid (used as cosurfactant) in order to investigate the effect of the n-carboxylic acid chain length on the size and dynamics of the w/o droplets. The results obtained in this study are similar to those obtained with the same w/o microemulsions systems, using n-alcohols as cosurfactants (Lang, J.; Lalem, N.; Zana, R. J. Phys. Chem. 1991, 95, 9533). Microemulsions with a short-chain acid showed a percolation of electrical conductivity upon increasing the [water]/[surfactant] molar concentration ratio omega. The variation of the water solubility in the microemulsion with the acid chain length suggested that the droplet size was limited by the intensity of attractive interdroplet interactions. At the percolation threshold the rate constant k(e) for interdroplet exchange of material via droplet collision was (1-2) x 10(9) M-1 s-1, as for all other percolating microemulsions investigated thus far. Microemulsions with a long-chain acid showed a monotonous decrease of the electrical conductivity upon increasing omega, the water solubility in the microemulsion seemed to be limited by the natural curvature of the droplets and the rate constant k(e) remained below 10(9) M-1 s-1. The changes of droplet size and interdroplet interactions with the length of the carboxylic acid are in qualitative agreement with the trends obtained from theoretical predictions concerning the effect of the cosurfactant chain length.