Abstract
Structure analyses for hydrate models of ethyleneimine oligomer (5-mer as model of PEI) were investigated by quantum chemical calculations. Conformation energies and structures optimized for hydrate models of (ttt)5 and (tgt)5 conformers were examined. Hydrate ratio, h [h = H2O/N (mol)], was set from 0.5 to 2. In anhydrates, (tg+t)5 conformer was more stable (?1.8 kcal/m.u.) than (ttt)5. In hydrates, (ttt)5 conformers were more stable (?0.7 - ?4.3) than (tg+t)5. These results corresponded to experimental results that anhydrous linear PEI crystal changes from double helical to single planar chain in hydration process. Structures calculated for (ttt)5 agreed in those observed for hydrates of PEI. In all (tg+t)5 conformers, O???H bonds between waters were found with the decreases of N???H bonds between imino group and water. The O???H bonds in (tg+t)5 conformer resulted in its high chain torsion, and strongly related with instability and structure change (large swelling).
Highlights
Linear poly(ethyleneimine) (PEI, (-CH2CH2NH-)n) exhibits various kinds of crystalline phases
For the hydrate models with various conformations ((ttt)x,x,x,x,x, and (g+g+g+)x, x: monomer units number; x = 1 – 8) of EI oligomers, we reported [13] that thex andx conformers are the most stable in anhydrate and hydrate (hydrate ratio: h (H2O/N = 1), respectively, and the stabilities of conformers seemed to be related with hydrogen bonding between water molecules
In5 conformers with h = 0.5, the Number of water (Nw)(h’) and Nw(h) values are 3 and 2, respectively.Å The conformation energy (Ec) of hydrate conformer was estimated as the difference between the gross energy of hydrated conformer with waters (Eh) and the total energy of water molecules (∑Ew) using Equations (1) and (2) as mentioned in previous section
Summary
Linear poly(ethyleneimine) (PEI, (-CH2CH2NH-)n) exhibits various kinds of crystalline phases. The anhydrate changes to the hemi-hydrate (H2O/EI = 0.5/1 mol), subsequently the sesqui-hydrate (H2O/EI = 1.5/1), and the di-hydrate (H2O/EI = 2/1) with increasing water contents The mechanism of such characteristic transitions, is not yet clear. For the hydrate models with various conformations ((ttt)x, (ttg+)x, (tg+t)x, (tg+g+)x, (tg+g-)x, and (g+g+g+)x, x: monomer units number; x = 1 – 8) of EI oligomers, we reported [13] that the (tg+t)x and (ttt)x conformers are the most stable in anhydrate and hydrate (hydrate ratio: h (H2O/N (mol) = 1), respectively, and the stabilities of conformers seemed to be related with hydrogen bonding between water molecules. In order to deepen an understanding to the mechanism of such transfer of PEI in hydration process, the structure analyses for hydrate models of EI oligomer were investigated by QCC in more detail. The conformational characteristics of hydrates were discussed, and were compared with the experimental results observed for PEI’s crystals in hydration process
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