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 HNO3 has been studied by NMR spectros copy. Increasing HNO3 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 th ree indications: (i) the variations of the chemical shifts of carbonyl grou ps (delta (co)) in DMDBTDMA and of (ii) attached water molecules (delta (AC )), induced by protonation of DMDBTDMA, and (iii) the Z,E isomerism of amid e units in DMDBTDMA, characterized by the molar ratio (1-(F) over bar) (or (F) over bar) of Z (or E) units. The analytical HNO3 to DMDBTDMA ratio prov ed to be appropriate to represent the variations of the above NMR parameter s characterizing the organic phase(s) throughout biphasic and triphasic dom ains, without discontinuity. Considering the organic phase(s) to contain mi xtures of species AM(i), resulting from the association of one DMDBTDMA (A) with i HNO3 molecules (M), allowed us to calculate the intrinsic values de lta (i)(co), delta (i)(AC) and Fi of NMR parameters relative to each specie s AMi. 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 p rogressive protonation of DMDBTDMA molecules on increasing the number of as sociated HNO3 molecules, approaching completion when i = 2; (iii) a stepwis e behavior of the conformational fraction Fi, decreasing from 0.35 (i = 0 a nd 1) to 0.22 (i = 2 and 3). The above information suggests a phenomenon ot her than amide protonation to accompany third phase formation, namely a str uctural change in DMDBTDMA aggregates, tentatively described as the passage from a reversed micelle-like closed structure to an open "bicontinuous" st ructure, in analogy with the pseudophases mentioned in ternary water/surfac tant/oil systems.