EQUILIBRIUM PROPERTIES OF CRYSTALLITES AND REVERSE MICELLES OF SODIUM BIS(2-ETHYLHEXYL) PHOSPHATE IN BENZENE

In sodium bis(2-ethylhexyl) phosphate (or NaDEHP)/benzene/H2O ternary systems, the apparent hydrodynamic diameter Dh of the surfactant aggregates and the mean aggregation number [n], monomer concentration C-m, electrical conductivity kappa, and absolute viscosity eta 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 w(0). As a function of water content, the apparent size of the aggregates and the viscosity exhibit a minimum at w(0) approximate to 3.0; D-h, eta, and [n] decrease with increasing temperature while C-m shows the opposite trend. The conductivity (in homogeneous electric field) increases nonmonotonically with w(0), exhibiting a plateau region for 1.5 less than or equal to w(0) less than or equal to 4.0. Conductance measurements in inhomogeneous electric field indicate the aggregates present at w(0) < 3.0 have a permanent dipole moment that diminishes with increasing water content. The results suggest that below the ''critical water content'' (w(0,c) approximate to 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 w(0) > 3.0, nondipolar, proper reverse micelles exist. The role of water in the formation of reverse micelles is discussed.