Progr Colloid Polym Sci (1998) 1ll:113 116 © Steinkopff Verlag 1998 T. Wolff D. Nees Influence of solubilized stilbazolium salts in trans and cis configuration on the percolation equilibrium in AOT- isooctane-water microemulsions: Light induced switching of conductivity Prof. Dr. T. Wolff(l~) Technische Universit~it Dresden Institut ffir Physikalische Chemie und Elektrochemie D-01062 Dresden Germany Dr. D. Nees* Universit~it Siegen Physikalische Chemie Siegen Germany *Present address Surfactant Science Group School of Chemistry University of Hull Hull HU6 7LS United Kingdom Abstract Solubilized trans and cis isomers of N-methyl and N-hexadecyl hydroxystilbazolium bromide had specific influences on the percolation temperature (as revealed by the onset of conductivity) in microemulsions composed of AOT, isooctane, and water: in the methyl derivative both cis- and trans-HSB induce shifts of the temperature at which conductivity sets on, the cis form being more efficient. In the hexadecyl compound a conductivity onset at increased temperature induced by the trans form was partially reverted when the cis isomer was present. Since trans and cis stilbazolium salts can be interconverted photochemically, conductivity may be switched isothermally by exposing samples to light. Some competition between trans --, cis photoisomerization and photodimerization was found depending on concentration ratios in the systems. Key words Conductivity microemulsion - percolation - photoisomerization Introduction Microemulsions composed of surfactant, water and oil are thermodynamically stable systems which are macroscopi- cally monophasic [1]. They exist in two types: in water rich systems, the so-called oil-in-water o/w microemul- sions form consisting of oil droplets in a continuous water phase; oil rich systems exhibit water droplets in a continu- ous oil phase (water-in-oil - w/o microemulsion), cf. Fig. 5 in [2] or more generally Figs. 9 15 in [1]. In both cases, surfactants are accumulated in the interfacial regions me- diating the dispersed state. When starting with a w/o microemulsion containing an ionic surfactant the water- to-oil ratio is gradually increased (at an appropriate tem- perature) the system may convert to an o/w microemulsion passing through a bicontinuous region with an average interracial surface curvature equal to zero. At a certain ratio called percolation point the system changes from almost non-conductive (in the w/o case) to conductive since water becomes a continuous phase. As percolation points depend on temperature, systems at water-to-oil ratios close to the percolation point can be switched from conductive to non-conductive (and vice versa) upon vary- ing the temperature. isothermally percolation can be induced by applying electric fields [3] or by the addition of certain solubilizates that affect percolation temperatures specifically [4]. Recently we reported photochemical switching of conduct- ivity in microemulsions brought about by solubilized acri- dizinium bromide and its photodimerization [-5]: since monomer and dimer induce different percolation temper- atures the conductivity could be varied isothermally upon exposure of samples to light. As one of the possible origins of the effect it was discussed that the divalent cations as formed upon photodimerization of acridizinium monoca- tions may vary the ionic strength in the system. We will show below that this is not a necessary prerequisite for photochemical switching of conductivity in microemul- sions, since monomolecular reactions such as the trans-cis