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

摘要

The phase behavior of the liquid-Iiquid 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 chemicld shifts of carbonyl groups (#delta#_AC) in DMDBTDMA and of (ii) attached water molecules (#delta#_AC), induced by protonation of DMDBTDMA, and (iii) the Z,E isomerism of amide units in DMDBTDMA, characterized by the molar ratio (I-F) (or F) of Z (or E) units. The analytical HNO3 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 AMi, resulting from the association of one DMDBTDMA (A) witl:t i HNO3 molecules (M), allowed us to calculate the intrinsic values #delta#_co~i, #delta#_AC~i and Fi of NMR parameters relative to each species AMi. The main results" from this analysis are (i) the presence of only one associated species, AM throughout the biphasjc domain, and of a mixture of all associated species AMi (i = 1 to 3) in triphasic systems; (ii) the progressive protonation of DMDBTDMA molecules on increasing the number of associated HNO3 molecules, approaching completion when i= 2; (iii) a stepwise behavior of the conformational fraction Fi, decreasing from 0.35 (i = Oand 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.