Relaxation phenomena in AOT-water-decane critical and dense microemulsions

We report on extensive measurements of the low and high frequencies sound v elocity and sound absorption in AOT-water-decane microemulsions deduced fro m ultrasonic and, for the first time as far as the absorption is concerned, from Brillouin scattering experiments. New experimental results on dielect ric relaxation are also reported. Our results, which include data taken for critical as well as dense microemulsions, show new interesting relaxation phenomena. The relaxation frequencies deduced from very high frequency acou stical measurements are in good agreement with new high frequency dielectri c relaxation measurements. We show that along the critical isochore, sound dispersion, relaxation frequency, and static dielectric permittivity can be accurately fitted to power laws. The absolute values of the new exponents we derived from experimental data are nearly equal, and they are very close to beta = 0.33 characterising the shape of the coexistence curve. The expo nent characterising the infinite frequency permittivity is very close to 0. 04 relevant to the diverging shear viscosity. For dense microemulsions, two well defined relaxation domains have been identified and the temperature v ariations of the sound absorption and the zero frequency dielectric permitt ivity bear striking similarities. We also show that the relaxation frequenc y of the slow relaxation process is almost independent of temperature and v olume fraction and so cannot be attributed to percolation phenomena, wherea s it can more likely be attributed to an intrinsic relaxation process proba bly connected to membrane fluctuations. (C) 2001 Elsevier Science B.V. All rights reserved.