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In sodium bis(Zethylhexy1) phosphate (or NaDEHP)/benzene/H20 temary systems, the apparent hydrodynamic diameter Dh of the surfactant aggregates and the mean aggregation number (n), monomer concentration C , , electrical conductivity K , and absolute viscosity 9 of the solutions are investigated by quasi-elastic light scattering, controlled partial pressure-vapor pressure osmometry (CPP-VPO), conductance, and viscosity measurements, generally as a function of surfactant concentration C, temperature, and water content WO. As a function of water content, the apparent size of the aggregates and the viscosity exhibit a minimum at wo % 3.0; Dh, q, and (n) decrease with increasing temperature while C , shows the opposite trend. The conductivity (in homogeneous electric field) increases nonmonotonically with WO, exhibiting a plateau region for 1.5 5 wo I 4.0. Conductance measurements in inhomogeneous electric field indicate the aggregates present at wo < 3.0 have a permanent dipole moment that diminishes with increasing water content. The results suggest that below the "critical water content" ( W O ,~ x 3.0) the aggregates are rod-shaped, dipolar crystallites (with the same structure as the constituent rods of the hexagonal liquid crystalline solid state of NaDEHP) which successively dissolve with increasing water content. At wo > 3.0, nondipolar, proper reverse micelles exist.The role of water in the formation of reverse micelles is discussed. IntroductionDue to their complexing ability toward metal ions,'-4 dialkylphosphoric acids and their alkaline salts are widely used in extraction processes by the industry. One of the representatives, sodium bis(2-ethylhexyl) phosphate or NaDEHP, has drawn particular attention for its ability to form reverse micelles in nonpolar solvents5 which is believed to be a key for its utility in metal extraction.6As a surfactant, the NaDEHP molecule is structurally related to the classical Aerosol-OT (or AOT, sodium bis(Zethylhexy1) sulfosuccinate), which has identical hydrophobic parts and differs only in its polar head group. The present study was undertaken to investigate the effect of the difference in head group on aggregation and on the properties of reverse micelles formed. Results of quasi-elastic light scattering, controlled partial pressure-vapor pressure osmometry (CPP-VPO),7.8 conductance, and viscosity measurements on the NaDEHP/ benzene/H20 system are reported for a wide range of composition and temperature. It is shown that with increasing water content wo[H20]/[NaDEHP] of the solution structural changes occur in the aggregates. At low water content, the structures are crystalline dipoles. We also comment on recent suggestions by Neuman and co-workers that water acts as an antimicellization agent9.I0 rather than a "gluing" agent"*'2 in the formation of NaDEHP reverse micelle. Experimental SectionMaterials. Bis(2-ethylhexy1)phosphoric acid (HDEHP; ICN Biomedicals) was purified by the Cu(I1) salt precipitation methodI3 as modified by McDowell et a1.14 The purity of the resulti...

Results of single square pulse and reversing pulse electric birefringence experiments on the ternary system of the surfactant sodium bis(2-ethylhexy1)phosphate (or NaDEHP) in benzeneM20 are in agreement with the existence of ca. up to 307 nm long, rod-shaped, dipolar crystallites in solutions below the critical water content (~0 ,~ e 3.0) and with the formation of nondipolar and nonspherical reverse micelles above the WQ. In the range of water content (WO = 0.75-4.5), [NaDEHP] (0.08-0.35 M), and temperature (15-37 "C) investigated, the homogeneous, isotropic solutions exhibit negative birefringence, with the Kerr constants (B) of the order of 1 x V2 m. It is concluded that the plane of the terminal 0-P-0-of the phosphate is perpendicular to the crystallite's major axis. In the range 1.5 < wo 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 ,us 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.