The rejection of charged and neutral molecules with low Molecular Weight, as well as the membrane fouling are fundamental aspects that should be considered during the waste water treatment. The material choice, used in the membrane preparation, is of great importance (material design) as well the optimization of parameters related to the integrated separation processes (process design). The carbon nanotubes (CNTs) have shown amazing hydrodynamic properties. Thus, CNTs-composite membranes can be considered promising solutions for the waste water treatment. The special fluiddynamics of CNTs has been thoroughly studied using experimental and modelling approaches1,2. Although the CNT embedding in the correct orientation constitutes largest drawback to the development of CNTs-composite membranes, the ability of CNTs to reject low MW solutes remains a relevant issue. A computational study on the rejection of interesting solutes by CNTs is presented here. After the optimization of the CNT diameters as a function of their rejection capability, the analysis of the water flow in the identified nanotubes was carried out. Thus, rejection and water flow were connected. The originality of this study lies in the suitably combination of different computational methods: Quantum Mechanics (QM), Monte Carlo (MC) and Molecular Dynamics (MD). Whilst QM allows a sub-nano scale investigation, MC and MD simulations permit analysis in the nano-scale. QM gives accurately description of the noncovalent interactions among solutes, water and CNTs, regardless of use of ad-hoc Force Fields3,4 while MC and MD simulations allow to analyse the solute dynamics in the CNTs. Thus, the combination of these different approaches provides an overview on the CNTs selectivity. Thirteen charged and neutral solutes of large, medium and small molecular weight were considered, such as tyrosol, vanillic acid, EDTA, octylphenol ethoxylate and etc. Their geometries were optimized at quantum mechanics level in the frame of Density Functional Theory. As regards the large molecules, conformer research was performed before QM optimizations. The ab-initio geometries permit to be released from experimental parameters without losing any generality. The calculated geometries were then used to evaluate the effective diameter, Deff, and cross-sections of each single molecule including the vdW radii of the solute atoms. The Deff calculation was performed using an home-made algorithm, which also enables to give the maximum and minimum projections of the solute atoms on the CNTs opening (Dproi-max and Dproi-min). Dproi-max was used to draw the solute arrangement into a CNT according to the conformation shown in Figure 1. The CNTs with internal diameter ranging between 1.1 nm and 10 nm were used in this study. It was assumed that the conformation, shown in Figure 1, may be indicative about the molecular packing in CNTs. Aware that this simple consideration is reductive, MC and MD simulations were carried out to investigate more accurately the solute packaging in the CNTs with diameter smaller than 2.77 nm. These simulations were carried out in the grand canonical ensemble with a very efficient Monte Carlo algorithm and they allowed to study the sorption of small molecules inside smooth single-wall nanotubes. The obtained results reveal highly-ordered structures, their degree of ordering depending strongly on the CNT diameter. Representative configurations form Figure 1. Arrangement of tyrosol molecule with the phenyl parallel to the main axis of (8,8) CNT the MC studies were then subjected to MD simulations in the isothermal-isobaric statistical ensemble at temperature T=298K and P=1atm with the LAMMPS code, using the DREIDING forcefield5. The outcome, here, was the calculation of the mean residence time of the solutes inside the CNTs as a function of their diameter and axial length. Strikingly, the above atomistic simulations yield a molecular packaging, reported in Figure 2, quite similar to what expected by using Dproi-max. Nevertheless, quantum mechanics trapping energies should be taken into account in the analysis of the solute rejection. These energies are accurately described with quantum mechanics while the molecular kinetic energy, as determined by temperature and pressure, has been taken into account by the above MC and MD simulations. The total energy, associated to a specific molecular packaging in the CNT, can be greater than sum of the isolated solute energies in water. Therefore, the energy difference yields the CNT capability to accept or reject a molecule2. The energy differences, referred to three low MW compounds, have been calculated by the quantum mechanics. The results show that for molecules, such as tyrosol, in CNTs with internal diameters smaller than 1.66 nm, the energy differences are remarkably positive. These solutes can be trapped in the CNTs according the disposition shown in Figure 1, however this arrangement gives an unfavourable molecular free energy. Thus, energy must be supplied to push these molecules inside the CNTs. Once determined the optimum CNT diameters to achieve an efficient separation of target molecules, the water flow in the selected CNTs was evaluated to optimise the MWCNTs number, which must be trapped to have water flow equal to the flow of unmodified membrane. If MWCNTs with internal and external diameters of 1.66 nm and 35 nm, respectively, are used, the required MWCNTs have a total area greater than unmodified membrane surface. Thus, to obtain a total MWCNT area smaller than unmodified membrane surface, it would be necessary to use carbon nanotubes with internal diameter larger than 2 nm. However, in this case, the rejection of low MW solutes is lacking. Thus, carbon nanotubes with outer and internal diameter smaller than 35 nm and 1.66 nm, respectively, would be recommended to attain at the same time high rejections and water flux. This work is part of the BioNexGen project which is generally dealing with the development of functionalized novel membranes for Membrane Bio Reactors, which should meet high water flux, low fouling, high rejection of low MW compounds.
Rejection of low molecular weight solutes by mean of cnts: A quantum mechanics and atomistic study
De Luca G;Figoli A
2012
Abstract
The rejection of charged and neutral molecules with low Molecular Weight, as well as the membrane fouling are fundamental aspects that should be considered during the waste water treatment. The material choice, used in the membrane preparation, is of great importance (material design) as well the optimization of parameters related to the integrated separation processes (process design). The carbon nanotubes (CNTs) have shown amazing hydrodynamic properties. Thus, CNTs-composite membranes can be considered promising solutions for the waste water treatment. The special fluiddynamics of CNTs has been thoroughly studied using experimental and modelling approaches1,2. Although the CNT embedding in the correct orientation constitutes largest drawback to the development of CNTs-composite membranes, the ability of CNTs to reject low MW solutes remains a relevant issue. A computational study on the rejection of interesting solutes by CNTs is presented here. After the optimization of the CNT diameters as a function of their rejection capability, the analysis of the water flow in the identified nanotubes was carried out. Thus, rejection and water flow were connected. The originality of this study lies in the suitably combination of different computational methods: Quantum Mechanics (QM), Monte Carlo (MC) and Molecular Dynamics (MD). Whilst QM allows a sub-nano scale investigation, MC and MD simulations permit analysis in the nano-scale. QM gives accurately description of the noncovalent interactions among solutes, water and CNTs, regardless of use of ad-hoc Force Fields3,4 while MC and MD simulations allow to analyse the solute dynamics in the CNTs. Thus, the combination of these different approaches provides an overview on the CNTs selectivity. Thirteen charged and neutral solutes of large, medium and small molecular weight were considered, such as tyrosol, vanillic acid, EDTA, octylphenol ethoxylate and etc. Their geometries were optimized at quantum mechanics level in the frame of Density Functional Theory. As regards the large molecules, conformer research was performed before QM optimizations. The ab-initio geometries permit to be released from experimental parameters without losing any generality. The calculated geometries were then used to evaluate the effective diameter, Deff, and cross-sections of each single molecule including the vdW radii of the solute atoms. The Deff calculation was performed using an home-made algorithm, which also enables to give the maximum and minimum projections of the solute atoms on the CNTs opening (Dproi-max and Dproi-min). Dproi-max was used to draw the solute arrangement into a CNT according to the conformation shown in Figure 1. The CNTs with internal diameter ranging between 1.1 nm and 10 nm were used in this study. It was assumed that the conformation, shown in Figure 1, may be indicative about the molecular packing in CNTs. Aware that this simple consideration is reductive, MC and MD simulations were carried out to investigate more accurately the solute packaging in the CNTs with diameter smaller than 2.77 nm. These simulations were carried out in the grand canonical ensemble with a very efficient Monte Carlo algorithm and they allowed to study the sorption of small molecules inside smooth single-wall nanotubes. The obtained results reveal highly-ordered structures, their degree of ordering depending strongly on the CNT diameter. Representative configurations form Figure 1. Arrangement of tyrosol molecule with the phenyl parallel to the main axis of (8,8) CNT the MC studies were then subjected to MD simulations in the isothermal-isobaric statistical ensemble at temperature T=298K and P=1atm with the LAMMPS code, using the DREIDING forcefield5. The outcome, here, was the calculation of the mean residence time of the solutes inside the CNTs as a function of their diameter and axial length. Strikingly, the above atomistic simulations yield a molecular packaging, reported in Figure 2, quite similar to what expected by using Dproi-max. Nevertheless, quantum mechanics trapping energies should be taken into account in the analysis of the solute rejection. These energies are accurately described with quantum mechanics while the molecular kinetic energy, as determined by temperature and pressure, has been taken into account by the above MC and MD simulations. The total energy, associated to a specific molecular packaging in the CNT, can be greater than sum of the isolated solute energies in water. Therefore, the energy difference yields the CNT capability to accept or reject a molecule2. The energy differences, referred to three low MW compounds, have been calculated by the quantum mechanics. The results show that for molecules, such as tyrosol, in CNTs with internal diameters smaller than 1.66 nm, the energy differences are remarkably positive. These solutes can be trapped in the CNTs according the disposition shown in Figure 1, however this arrangement gives an unfavourable molecular free energy. Thus, energy must be supplied to push these molecules inside the CNTs. Once determined the optimum CNT diameters to achieve an efficient separation of target molecules, the water flow in the selected CNTs was evaluated to optimise the MWCNTs number, which must be trapped to have water flow equal to the flow of unmodified membrane. If MWCNTs with internal and external diameters of 1.66 nm and 35 nm, respectively, are used, the required MWCNTs have a total area greater than unmodified membrane surface. Thus, to obtain a total MWCNT area smaller than unmodified membrane surface, it would be necessary to use carbon nanotubes with internal diameter larger than 2 nm. However, in this case, the rejection of low MW solutes is lacking. Thus, carbon nanotubes with outer and internal diameter smaller than 35 nm and 1.66 nm, respectively, would be recommended to attain at the same time high rejections and water flux. This work is part of the BioNexGen project which is generally dealing with the development of functionalized novel membranes for Membrane Bio Reactors, which should meet high water flux, low fouling, high rejection of low MW compounds.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
