Dynamic Light Scattering and FTIR Spectroscopic Investigations on the Reverse Micelles Produced During the Extraction of Nd(III) and Nitric Acid in Tetra Ethylhexyl Diglycolamide

Abstract

The aggregation behaviour of reverse micelles, formed during the extraction of neodymium nitrate and nitric acid, in an organic phase composed of tetra‐bis(2‐ethylhexyl) diglycolamide (TEHDGA) in n‐dodecane (n‐DD) was studied, at various temperatures. The organic phase, 0.2 M TEHDGA/n‐DD was equilibrated with aqueous solutions of nitric acid in the presence and absence of neodymium nitrate. At certain nitric acid and Nd(III) concentration in aqueous phase, the organic phase underwent splitting into two phases, with top phase known as “diluent rich phase”, the other one known as “third phase” or “metal ion rich phase”. Dynamic light scattering, attenuated total reflectance (ATR) ‐Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopic methods were employed for probing the size and distribution of aggregates present in the organic phase before and after splitting. The investigations revealed that the aggregates, formed in organic phase, interact efficiently with the extracted metal ions and these interactions increased with increase in the concentration of nitric acid and neodymium nitrate in organic phase. In addition, the interaction among the reverse micelles in third phase appear to be very strong as compared to that observed in the organic phase before third phase formation, and such strong interactions resulted in a merger and narrow distribution of reverse micelles in third phase.

Aggregation Behavior of Alkyldiglycolamides in n-Dodecane Medium during the Extraction of Nd(III) and Nitric Acid

“…The nitric acid concentration was varied from 1 to 5 M. The aggregate distribution was compared with the self-aggregate (without contacting with nitric acid) distribution of 0.2 M TODGA in n -DD. It can be seen that the average aggregate size and aggregate distribution increases with increase in the concentration of nitric acid from 1 to 4 M. ,− Increasing the concentration of nitric acid in aqueous phase increases the concentration of protonated-solvates in organic phase and facilitates the formation of newer and facilitates merger of aggregates. Pathak et al studied the aggregation behavior of 0.1 M TODGA in n -DD and 1-octanol.…”

“…In alkyl diglycolamides, the etheric and amidic functional groups act as polar coordinating sites and the alkyl group attached to this polar moiety serve as a nonpolar organophilic group. When such amphiphilic DGAs are dissolved in a nonpolar diluent namely n- dodecane, DGAs tend to undergo self-aggregation in the n -dodecane medium due to polar–polar interactions. ,,, In contrast to this, the alkyl group attached to DGA interacts with n -dodecane through van der Waals forces and such interactions facilitate the dispersion of DGAs in the n -DD medium. Therefore, the presence and absence of the third phase is essentially determined by two opposing interactions, one favoring third phase formation and the other favoring dispersion and dissolution of DGAs in n- DD. − …”

“…When 0.2 M alkyl DGA/ n -DD was contacted with nitric acid, the DGA present in the organic phase undergoes a protonation reaction leading to the formation of protonated–solvates (DGA···(HNO 3 ) x ) in organic phase. It should be noted that the nitric acid is extracted into the core of reverse micelles and therefore, the acid extracted aggregates are more polar than the aggregates before extraction . As a result, the interaggregate interaction among the aggregates present in n -DD phase increases and such interaction leads to the formation of bigger and swollen aggregates. , The degree of aggregation and swelling depends upon the quantity of nitric acid extracted into the organic phase and polarity of protonated solvates in n -DD.…”

“…Extraction of trivalent metal ion (M­(III)) from the nitric acid medium in DGA/ n -DD phase results in the protonation and coordination of DGAs by nitric acid and trivalent metal ion, respectively. They are, known as protonated-solvates (DGA···(HNO 3 ) x ) and metal-solvates ((DGA) y ...M­(NO 3 ) 3 ), extracted into the core of the reverse micelles. − The presence of extracted species in organic phase imparts more polarity to the organic phase and facilitates the formation of newer aggregates. In addition, the aggregates present in organic phase undergo interaggregate interaction that leads to the merger of aggregates. − In view of this, there is an increase in the concentration of aggregates and their size upon extraction of nitric acid and trivalent metal ion.…”

“…They are, known as protonated-solvates (DGA···(HNO 3 ) x ) and metal-solvates ((DGA) y ...M­(NO 3 ) 3 ), extracted into the core of the reverse micelles. − The presence of extracted species in organic phase imparts more polarity to the organic phase and facilitates the formation of newer aggregates. In addition, the aggregates present in organic phase undergo interaggregate interaction that leads to the merger of aggregates. − In view of this, there is an increase in the concentration of aggregates and their size upon extraction of nitric acid and trivalent metal ion. This trend continues until the aggregates are dispersed and distributed homogeneously in n -DD. , However, at certain conditions of metal loading in organic phase at a specified acidity the polar–polar interactions among the aggregates are significantly large as compared to their dispersive interactions with n -DD .…”