ELECTRIC BIREFRINGENCE DYNAMICS OF CRYSTALLITES AND REVERSE MICELLES OF SODIUM BIS(2-ETHYLHEXYL)PHOSPHATE IN BENZENE

Results of single square pulse and reversing pulse electric birefringence experiments on the ternary system of the surfactant sodium bis(2-ethylhexyl)phosphate (or NaDEHP) in benzene/H2O are in agreement with the existence of ca. up to 307 nm long, rod-shaped, dipolar crystallites in solutions below the critical water content (w(0,c) approximate to 3.0) and with the formation of nondipolar and nonspherical reverse micelles above the w(0,c). In the range of water content (w(0) = 0.75-4.5), [NaDEHP] (0.08-0.35 M), and temperature (15-37 degrees C) investigated, the homogeneous, isotropic solutions exhibit negative birefringence, with the Kerr constants (B) of the order of 1 x 10(-12) V-2 m. It is concluded that the plane of the terminal O-P-O- of the phosphate is perpendicular to the crystallite's major axis. In the range 1.5 < w(0) < 4.5 (where dissolution of the crystallites and the nucleation of the micelles overlap), the relaxations times and amplitudes are too small to be evaluated. Outside that range, the rise and decay signals (in the 0.2-28 mu s range) are double exponential; they are asymmetric for the crystallites but symmetric for the reverse micelles. The magnitudes of the rotational relaxation times indicate that the crystallites break into shorter segments in the initial phase of their forced reorientation by the applied field. Mechanisms are proposed involving polarization, structural changes, and linear clustering (not for the micelles) for the relaxations observed.