Aggregation and Protonation Phenomena in Third Phase Formation: An NMR Study of the Quaternary Malonamide/Dodecane/Nitric Acid/Water System

The phase behavior of the liquid−liquid solvent extraction system DMDBTDMA (see the text) in n -dodecane/aqueous HNO 3 has been studied by NMR spectroscopy. Increasing HNO 3 concentration in the aqueous phase beyond a critical value of 3.2 mol dm -3 results in the splitting of the organic phase into two layers, the so-called third phase formation. NMR investigations used three indications: (i) the variations of the chemical shifts of carbonyl groups (δ co ) in DMDBTDMA and of (ii) attached water molecules (δ AC ), induced by protonation of DMDBTDMA, and (iii) the Z, E isomerism of amide units in DMDBTDMA, characterized by the molar ratio (1-F̄) (or F̄) of Z (or E ) units. The analytical HNO 3 to DMDBTDMA ratio proved to be appropriate to represent the variations of the above NMR parameters characterizing the organic phase(s) throughout biphasic and triphasic domains, without discontinuity. Considering the organic phase(s) to contain mixtures of species AM i, resulting from the association of one DMDBTDMA (A) with i HNO 3 molecules (M), allowed us to calculate the intrinsic values δ i co, δ i AC and F i of NMR parameters relative to each species AM i . The main results from this analysis are (i) the presence of only one associated species, AM throughout the biphasic domain, and of a mixture of all associated species AM i ( i = 1 to 3) in triphasic systems; (ii) the progressive protonation of DMDBTDMA molecules on increasing the number of associated HNO 3 molecules, approaching completion when i = 2; (iii) a stepwise behavior of the conformational fraction F i, decreasing from 0.35 ( i = 0 and 1) to 0.22 ( i = 2 and 3). The above information suggests a phenomenon other than amide protonation to accompany third phase formation, namely a structural change in DMDBTDMA aggregates, tentatively described as the passage from a reversed micelle-like closed structure to an open “bicontinuous” structure, in analogy with the pseudophases mentioned in ternary water/surfactant/oil systems.