GENERATION OF THE PHOTOACOUSTIC EFFECT THROUGH HEAT DIFFUSION - TRANSIENT GRATING MEASUREMENTS IN REVERSE MICELLAR SOLUTIONS

We report the application of the transient grating method to measureme nt of radii of inverse micelles. A theoretical model is used to calcul ate the time evolution of the grating as a function of the solution th ermal properties and the radii of the micelles. The method is based on the large difference in the thermal expansion coefficients between wa ter and organic solvents: energy deposited in the aqueous interior of the micelles by the optical exciting beams does not result in sizeable photoacoustic effect until heat diffuses into the surrounding organic solvent. The time profile of the transient grating signal is shown to be a function of the micellar radius, and the thermal conductivity an d thermal diffusivity of the aqueous and organic subphases. For water- in-alkane reverse micelles, heat deposition is limited to the aqueous interior of the micelles through the use of ionic dyes. Experiments ar e reported using 1-3 GHz acoustic waves in solutions containing micell es with diameters in the range 10-30 nm. The correspondence between ra dii determined by the thermal diffusion method and those from dynamic light scattering measurements is discussed.