The state of aggregation of metal salts, M n + (DEHP - ) n, of the organophosphorous acid extractant bis(2-ethylhexyl) phosphoric acid (HDEHP) has been examined by small-angle neutron scattering (SANS) and viscosity measurements in cyclohexane. A range of di- and trivalent counterions (M n + ) were studied as a function of concentration and water content, where M n + = Ca 2+, Co 2+, Ni 2+, Cu 2+, Mn 2+, Al 3+, and Cr 3+ . The extent of water uptake by di- and trivalent salts as defined by ω = [H 2 O]/[M n + (DEHP - ) n ] was found to be low such that only hydrated reversed micelles were formed. The results support formation of rod-shaped reversed micelles by all divalent metal ion salts with the SANS I ( Q ) well-represented by a rod form-factor with radii in the range 7.5−9 Å. The length of the rod-shaped reversed micelles formed was strongly dependent upon the nature of the counterion with fitted values between ∼80 Å (M n + = Ni 2+ ) and ∼300 Å (Mn 2+ ). Although the shape and size of reversed micelles were found to be independent of surfactant concentration in the range c = 0.04−0.08 g cm - 3, Cu(DEHP) 2 was found to undergo aggregation resembling critical micelle concentration like behavior into rod-shaped micelles at c ∼ 0.02 g cm - 3 . With the exception of Ni and Cu the state of aggregation of all salts examined was unaffected by addition of water. In contrast, the trivalent-metal-ion salt, Al(DEHP) 3 is present in solution either as monomer or as small spherical aggregates of low aggregation number.
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Steytler et al. (1996) studied this question.