Relation between the hydrophile/hydrophobe ratio of malonamide extractants and the stability of the organic phase: investigation at high extractant concentrations

Abstract

In the present paper, it is shown that in malonamide extractant/dodecane mixtures, change in the shape of the extractant aggregate may promote phase demixion. This phenomenon is known as the 3rd phase formation in extractant systems. The shape of the aggregates is dictated by the length of the alkyl chains stabilizing the reverse microemulsion by steric repulsion. Small angle scattering experiments are used to investigate the aggregation states of diamide extractant molecules, namely N,N'dimethyl-N,N'dibutyl-tetradecyl malonamide (DMDBTDMA) and N,N'dimethyl-N,N'dibutyl-pentyl malonamide (DMDBPMA), in dodecane. The results are described in a consistent way by the packing parameter concept and by expressing steric repulsion versus Van der Waals inter-aggregate interactions, both curvature dependants. Non-monotonous change in the extractant Critical Micellar Concentration (CMC) as a function of the extractant alkyl chain length is rationalized by comparing Van der Waals attraction between the alkyl chains of both the extractant and the solvent.

Depletion of water-in-oil aggregates from poor solvents: Transition from weak aggregates towards reverse micelles

“…For this class of triple chain extractants, lengths of the alkyl chains have important effects on the organic phase structuration, and can lead to unwanted "third phase" formation in the extraction process. Small angle scattering experiments have been used to investigate the aggregation states of the different extractant molecules in dodecane [33]. Longer alkyl chains favour the reverse microemulsion "pseudo-phase", and hence reduce the amount of extractant monomers present in the solvent "pseudo-phase".…”

Influence of the extracted solute on the aggregation of malonamide extractant in organic phases: Consequences for phase stability

“…Malonamide extractants diluted in organic solvent self-organise in reverse micelles with low aggregation numbers (from 3 to 10). More recently, several supramolecular structures of organic phases have been identified and studied [20][21][22]25,26]. The presence of reverse micelles with a polar core is the keystone to understanding phase separation.…”

“…He indicated that the occurrence of phase splitting was due to the formation of reverse micelles, or microemulsion, in the organic diluent by the aggregation of TBP complexes. The concept of a colloidal approach enabled a better understanding of the organic solution of extractants [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. The first direct characterization of reverse micelles with interaction potential came from the modeling of small angle scattering intensities on absolute scale by Madic et al [17][18][19].…”

“…It is now well established that the organic phases involved in the DIAMEX process are not molecular solutions [20][21][22][25][26][27][28][29]. Malonamide extractants diluted in organic solvent self-organise in reverse micelles with low aggregation numbers (from 3 to 10).…”

Solute‐Induced Microstructural Transition from Weak Aggregates towards a Curved Film of Surface‐Active Extractants

“…Diamide extractant aggregates have been accurately characterized experimentally using combined SAXS/ WAXS/osmometry analytical chemistry in "theta" solvents, that is, for intense quenching of liquid-liquid phase separation induced by the penetration of the outer layer of reverse micelles by solvent chains. [11,[16][17][18][19][20] We show here, using molecular dynamics (MD) simulations, that weak aggregation of oil-soluble surfactants, as detected by coupled SANS/SAXS experiments, [16,17] may already exist in the absence of complexed ionic species. The MD simulations herein have been performed in the gas phase in order to test [16,17] All the MD simulations-performed typically for a time of tens of nanoseconds-have been performed taking into account explicit polarization in order to be able to represent proper structures resulting both from 1) weak aliphatic chains interactions, or 2) strong interactions with the highly charged Eu 3 + cation.…”