Structure and properties of bis(2-ethyhexyl)phospheric acid microemulsionswith a network structure - Effect of counter-ions

Several physiocchemical properties were experimentally examined for calcium bis(2-ethylhexyl)phosphate (Ca(DEHP)(2))/water/n-hexane/calcium chloride s ystems and compared with sodium bis(2-ethylhexyl)phosphate (NaDEHP)/water/n -hexane/sodium chloride systems in the Winsor II regime to elucidate the ef fect of changing counter-ion species on the properties of a microemulsion i n the amphiphile-concentrated region characterized by the presence of a net work-like structure formed from worm-like molecular assemblies. The measure ments of small angle X-ray scattering, shear viscosity, electroconductivity were consistently interpreted in terms of the strong tendency of molecular assemblies to become more elongated for Ca2+ than for Na+. The dependence of the dynamical correlation length on the volume fraction of molecular ass embly obtained from the dynamic light scattering implies that the freely mo bile part in the Ca(DEHP)(2) microemulsion network would be determined by t he individual micromechanical property of the worm-like molecular assembly, being different from that in NaDEHP systems determined by the network mesh which becomes more confined with the increase in the volume fraction of th e molecular assembly. (C) 1999 Elsevier Science B.V. All rights reserved.