## 3 Results and Discussion
It turns out that this compound extracts acids from aqueous solutions well, but does not extract REE in any quantities. Thus, the sole presence of functional fragments of the P=O type does not determine the extraction capacity towards REE in acidic conditions. The calculation of the extractant and REE concentrations ratios studied in the organic phase after extraction (Table 3) showed a higher selectivity of EIPA-1: heavier elements are more easily transferred to the organic phase. The reason for this is the higher steric availability of large-diameter lanthanide atoms (Figure 7a, shows a complex with a tetra-substituted TCDP derivative). The proposed structure is generally recognized for the complexes of this type of compounds (with two phosphonyl groups) with a light group of REE [5; 23]. The aforementioned extractant to REE concentrations ratio in the organic phase very accurately confirm the formation of complexes of precisely 3 EIPA molecules and 1 metal ion. A similar structure with phenyl substituents at phosphorus atoms was described by [PERSON] et al. [44]. The small size of the lanthanide atom presumably can lead to the formation of complexes containing up to 6 fragments of extractant molecules (Figure 7b), in which the formation of hydrogen bonds between P=O and H-N occurs. This is confirmed by the higher ratios of C\({}_{\text{EIPA-1}}\): C\({}_{\text{org}}\) with values of 5.03 for La and 5.08 for Ce (Table 3). [PERSON] et al. previously described the analogous structure of the REE complex with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester [28]. Figure 6: The dependence of REE concentration transferred to the organic phase during extraction by EIPA-1, EIPA-2 and PAPNA solutions (0.3 mol/L) on the atomic number of the element. EIPA-2 compounds form complexes with metal salts through the -P(O)(OR)-NH- functional groups, the average number of which is 2-3 per 1 molecule (Table 1). Ratios C\({}_{\text{EIPA-2}}\): C\({}_{\text{org}}\) for different REE ions slightly differ and vary from 3 to 3.5 with the exception of Ho (ratio C\({}_{\text{EIPA-2}}\): C\({}_{\text{Ho}}\) = 4.00). Taking into account that CEIPA-2 corresponds to the concentration of H\({}^{+}\) ions, 3-4 EIPA-2 molecules can be combined with 1 REE ion depending on the number of phosphorus atoms in their chain. On average, only one functional group per molecule can participate in complex formation, while other functional groups tend to coordinate with each other in the organic phase. The extraction of REE by EIPA-2 is a typical cation exchange process, expressed (with the exception of La and Ho) as follows: Ln\({}^{3+}\) + 3 HA = [Ln(A)\({}_{3}\)] + 3H\({}^{+}\). The remaining functional -P(O)(OR)-NH-fragments not directly interacting with the REE ion and non-functional -P(O)(OR)-NR- fragments of these molecules are located in the outer sphere of the metal-containing complex. Thus, the properties of the complex will depend to a greater extent on fragments located in the outer sphere of the complex and to a lesser extent will be defined by the nature of the REE ion itself. As a result, the extraction selectivity of EIPA-2 is very weakly expressed with respect to certain REE ions. The organic phase is equally saturated with all metals. Metal concentration C\({}_{\text{org}}\) after extraction is close to 0.1.