Molecular dynamics simulation of carbon peapod-like nanomaterials in desalination process
Graphical abstract
The presence of fullerene into nanotube increases salt rejection. N-doped fullerenes also increases the salt rejection more than the pure fullerenes.
Introduction
Recent studies explored and developed nano-scale structures for purification and desalination processes of water and wastewater [[1], [2], [3], [4]]. These nanostructures represented very good ion selectivity and salt rejection. Moreover, these nanomaterials exhibited excellent water permeability at the nano-dimensions [[5], [6], [7]]. Recently, many nanomaterials have been studied as effective membranes for water desalination including graphene and carbon nanotubes (CNTs) [[8], [9], [10], [11], [12]]. However, using each of these nanomaterials has certain challenges. For instance, CNTs with their nano-scale structures are very good candidates for the desalination process [1,13]. The CNTs are also very good materials for ion selection processes [[14], [15], [16], [17], [18]]. These nanostructures also enable us to understand the transport process of water molecules at the nano-scale dimensions which is important to find the biological functions of cells [19,20]. However, preparation of highly dense, long, and perfectly aligned CNTs is difficult at the industrial scale [21,22]. Graphene is also one of the significant candidates for the desalination membranes which its properties can be improved by changing the size of the nanopores, or by addition of functional groups on the pore's edges [23,24]. Previous simulations also indicated that single-layer nanoporous graphene (NPG) membranes can effectively separate NaCl from water [2]. It is also found that the water permeability of the NPG membranes is about five times more than the other membranes [25,26]. However, it is difficult to fabricate NPG membranes in large scale values [27]. Layered grapheme oxide (GO) membranes are another significant membranes used in the desalination process which can be prepared by relatively simple techniques [[28], [29], [30]]. Recently, the lamellar GO membranes exhibited very good ions rejection by tuning the distance between the GO layers [8,31,32] which was about 4 to10 times more than the other nano-membranes. Previous investigations showed that the epoxy and hydroxyl groups prefer to aggregate on graphene surfaces [33] and therefore, it is experimentally difficult to prepare the GO surfaces with only one kind of oxide group.
Here, we want to use new nano-materials based on CNTs in the desalination process. In the recent years, many scientists investigated desalination process using CNTs by molecular dynamics (MD) simulations and tried to examine different factors which were significant on the process. For example, Corry [17] reported that the size and uniformity of nanotubes is significant to reach a favorite salt rejection. In another search, Corry [34] showed that the use of electric charges at the initial position of a nanotube hinders the entrance of ions. Corry also indicated that the CNT functionalization decreases the water flow into the nanotube pore. Chan et al. [35] showed that using of zwitterion group at the CNT ends results in rapid water flow and also ion rejection. Goldsmith and Martens [36] examined size and structure dependent salt rejection using armchair CNTs and declared some evidence of rectification of ions for the asymmetric charge distributions in the (12,12) system. Nasrabadi and Foroutan [37] indicated that the ions separation is dependent on the amount of applied electric charge. Khataee et al. [38] reported that the (7, 7) and (8,8) SiC tubes selectively separated Na+ and Cl−ions, respectively. Azamat et al. [39] reported much ion rejection by (5,5) BN tube. Razmkhah et al. [40] showed that strong electric and magnetic field increases and decreases water flow rate through CNT, respectively.
One of the efficient method to increase the salt rejection rate in the CNTs is to reduce the inside accessible volume [41]. In order to do this, fullerene (C60) molecules can be located into the CNTs without changing the homogeneity of chemical interior. The Carbon peapods are synthesized during the CNT preparation by pulsed laser vaporization [42]. The fullerene (C60) can also enter the CNT through defects or vapor-phase diffusion [43]. These new nanostructures, which are called peapods, have completely different thermal and electrical properties than the normal CNTs [[44], [45], [46]] and have attracted much interests and applications in the several investigations in the recent years [[47], [48], [49]].
In this research, we want to examine performance of the carbon peapods consisting one and three fullerene molecules into armchair nanotubes in desalination process using MD simulations. We have also examined different charge effects on the fullerene, nanotube, and graphene surfaces in this work. Previous studies showed that the N-doping of carbon nanostructures is an important method to improve their properties [[50], [51], [52]]. Therefore, we want to investigate using of N-doped fullerene into the CNTs to examine its effects on the desalination process.
Section snippets
Simulation details
Our model has been built from two graphene plates with the size of 44.5 Å × 50.5 Å comprising a membrane spanned by a (13,13) CNT with the length of 36 Å (almost similar to the MD simulation of Goldsmith and Martens [36]). One and three C60 fullerene have been also located into the CNT in different simulations. We have also located ±0.1C electric charge on different materials (such as fullerene, nanotube, or graphene surfaces) in the different simulations. We do not intend to investigate the
Water flux
The water flux (the number of water molecules transmitted from nanotube per nanosecond) has been represented for the different simulated systems in Fig. 1.
The water flux trend is as follows: Pure CNT>1 fullerene peapod > Reverse charged plates > Charged plates > Charged CNT> 3 fullerenes peapod ≈ 3N-doped fullerenes peapod > Reverse charged CNT>1 fullerene charged >1 fullerene reverse charged. The snapshot of the three N-doped fullerenes system has been presented after 2 ns of simulation time
Concluding remarks
The new nanostructures, which are called peapods, have completely different thermal and electrical properties than the normal CNTs and have attracted much interests and applications in the several investigations in the recent years. In this research, we have examined the performance of peapods consisting 1 and 3 fullerene molecules into armchair nanotubes in desalination process using MD simulations. Our results represents the acceptable performance of the peapod-like nanomaterials in the
Authorship statement
All persons who meet authorship criteria are listed as authors, and all authors certify that they have participated sufficiently in the work to take public responsibility for the content, including participation in the concept, design, analysis, writing, or revision of the manuscript. Furthermore, each author certifies that this material or similar material has not been and will not be submitted to or published in any other publication before its appearance in the Desalination.
Authorship contributions
Category 1
Conception and design of study: M. Abbaspour.
Acquisition of data: M. Abbaspour, N. Ahmadi.
Analysis and/or interpretation of data: M. Abbaspour, N. Ahmadi.
Category 2
Drafting the manuscript: M. Abbaspour.
Revising the manuscript critically for important intellectual content: M. Abbaspour, M. Namayandeh Jorabchi, H. Akbarzadeh, and N. Ahmadi.
Category 3
Approval of the version of the manuscript to be published (the names of all authors must be listed):
M. Abbaspour, M. Namayandeh Jorabchi,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to express their sincere thanks to Prof. Jafar Azamat for helpful guidance for the simulation of desalination process.
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