Abstract
The present work shows for the first time that tributylphosphate (TBP), the major ion extractant used in thereprocessing of spent nuclear fuel, acts efficiently as a cosurfactant in the formation of three-phase microemulsions.The system is composed of water, dodecane, TBP, and an extremely hydrophilic sugar surfactant, n-octyl-β-glucoside.The investigation of the three-phase region (Winsor III), the so-called “fish-cut” diagrams, revealed that TBP exhibitscosurfactant behavior comparable to that of classical cosurfactants n-pentanol and n-hexanol. Upon increasing thecosurfactant/surfactant molar ratio, TBP appears to be more efficient than single-chain alcohols in raising thespontaneous curvature of the adsorbed surfactant film toward oil. This is a direct consequence of the different lateralpacking of TBP and n-pentanol or n-hexanol in the mixed surfactant film, with TBP having three alkyl chains and so ahigher hydrophobic volume than those n-alcohols. This property is underlined by the interfacial film composition, whichis determined by the chemical analysis of the excess phases. It gives a surfactant to cosurfactant molar ratio of 1:1 forTBP and 1:3 for n-hexanol. Moreover, the local microstructure of the microemulsion becomes dependent on theaddition of salt when n-alcohol is replaced by TBP. A specific salt effect is also observed and rationalized in terms of thecomplexing property of TBP and Hofmeister’s effects. Treatment of the small-angle neutron scattering (SANS) datagives access to (i) the length scales characterizing the microemulsions (i.e., the persistence length, ξ, and aqueous ororganic domain sizes, D*) and (ii) the specific surface, Σ. It results that a subtle change is highlighted in the TBPmicroemulsion structure, in terms of connectivity, according to the type of salt added.