1. based on quantum mechanics approach
Nanofiltration and molecularly imprinted membranes: a theoretical study
based on quantum mechanics approach
G. De Luca1, L. Donato1, F. Tasselli1, S.G. Del Blanco1, F. Bisignano1 and E. Drioli1
1Research Institute on Membrane Technology (ITM-CNR), c/o UNICAL Via P. Bucci 17/c I Rende (CS), Italy
AIM OF THE WORK and COMPUTATIONAL DETAILS
The aim of this contribution is to calculate accurately binding energies and effective molecular sizes using a quantum mechanics approach in the frame of DFT. The binding energies were evaluated considering a genotoxin (4,4’-methylendianiline, MDA) as template and fragments of a P(AN-co-AA) copolymer (acrylonitrile (AN) and acrylic acid (AA)) used for MIMs preparation or considering these monomers separately used for the preparation of the pre-polymerization complexes. The the H-bonbinding energies, evaluated by using B3LYP and X3LYP functional and 6-311G** basis sets, were calculated as difference between the energy of the adducts (supramolecular complexes) and the energies of the isolated subsystems.
The effect of the solvents on the binding energies was considered using COSMO (COnductor-like Screening MOdel).
INTRODUCTION
NanoFiltration (NF) and Molecularly Imprinted Membranes (MIMs) can be considered a reliable solution for impurity removal as well as molecular recovery with low molecular weight. Furthermore, different Molecularly Imprinted Polymers (MIP) are increasingly used for the MIMs preparation. Their solubility and permeability are important proprieties as well as the MIPs affinity for the template molecule. Then, an investigation on the binding energies of the H-bonds involved both in the preparation of MIMs and MIPs can be useful.
RESULTS and DISCUSSION
The binding energy associated to the interactions between co-polymer fragments and template are reported in Table 1 and Table 2 along with the energies of H-bonds formed by two co-polymer fragments. The average cross-sections of the rose Bengal, dianiline, and aniline were also calculated and reported in Table 3. The quantum mechanical calculations show that both in dimethylformamide (DMF) and in vacuo the polymer…polymer and template…polymer binding energies are comparable. This suggests that the template molecule in the casting solution binds effectively to the carboxylic groups on the copolymer chains. This would cause a greater availability of the –COOH groups once the genotoxin is removed from the imprinted membrane and as result a greater specific binding capacity of the MIM membrane, as shown in Fig. 1 (6.6 μmol/gmem). The average cross-sections show that less than two molecules of dianiline can pass through the membrane pores, whereas approximately four molecules of aniline can go through the membrane. Assuming that template should come into contact with the free and more available –COOH groups, as a result, the imprinting effect should be less pronounced for the aniline because the average size of the membrane pores is markedly larger than its effective size.
Concerning the MIP preparation used in a mixed MIM, the competition among all the interactions occurring during the formation of the pre-polymerization complexes (PPC) in the presence of ethylene glycol dimethacrylate (EGDMA) was also analyzed. The hydrogen bonding energies formed by the template molecule in water and in vacuo are reported in Table 4 and 5. The highest energy is provided by the OH of the AA acid group (Fig 2).
Monomer
Binding Energy (kcal/mol) AN
-5.15
AA(N…HO)
-16.70
AA (NH…OC)
-5.07
EGDMA
-5,66
Monomer
Binding Energy (kcal/mol) AN
-3,97
AA(N…HO)
-11,72
AA (NH…OC)
-5,72
EGDMA
-4,38
Table 4: In Water
Table 5: In Vacuo
Figure 2: H-bond in the MDA…AA(N…HO) complex.
Figure X: Selectivity of different hybrid systems.
In addition, the H-bonds formed by AA with AN and EGDMA as well as between two AA monomers are shown in Table 6 and Table 7. These values show that the cross-linker and AA monomers form interactions comparable (reported in Fig. 3and 4) with the highest interactions formed by the template molecule. This competition would decrease the concentration of the pre-polymerization complex in solution. However, even if the energy of H-bonds between different AA monomers or between EDGA and monomers are comparable to those between template and monomers used, prepared MIP showed a MDA retention when mixed with PVDF polymer as show in Fig 5.
Table 1: Bond Energies (kilocalories per mole) in DMF
Computed at B3LYP/6-311* Level of Theory
Table 2: Bond Energies (kilocalories per mole) in Vacuo
Computed at B3LYP/6-311* Level of Theory
AA monomer
Binding Energy (kcal/mol) AN
-9,89
EGDMA
-11,87
AA (CO…HO)
-12,43
AA monomer
Binding Energy (kcal/mol) AN
-7,77
EGDMA
-11,28
AA (CO…HO)
-9,51
Table 6: In Water
Table 7: In Vacuo
Table 3: average cross-sections
Figure 1: Retained MDA and specific binding capacity of blank and MDA-imprinted membranes.
Figure 3: H-bond in the EGDMA…AA complex
Figure 4: H-bond between the two acid monomers
CONCLUTIONS
This work concluded that a quantum mechanics study can be useful :
(1) in order to give information on the cause determining the increase of the affinity by the MIMs to a template molecule.
(2) to select suitable monomers and solvent in order to optimize the formation of the pre-polymerization complexes.
REFERENCES
[1] G. De Luca, L. Donato, S. García Del Blanco, F. Tasselli, and E. Drioli, J. Phys. Chem. B 2011, 115, 9345–9351 dx.doi.org/ /jp
[2] L. Donato, F. Tasselli, G. De Luca, S. Garcia Del Blanco, E. Drioli, Novel Hybrid Molecularly Imprinted Membranes for Targeted 4,4’-Methylendianiline. To be Submitted
ACKNOWLEDGMENT
This research is funded by European Community's Seventh Framework Programme under grant agreement n (NEMOPURE). In addition, we are grateful to CASPUR for the use of High Performance Computers.