A nanocapsule for safe and effective methane storage is investigated by the method of molecular dynamics. The mass content of methane in the nanocapsule reaches the value of 14.5 mass%. The nanocapsule consists of two parts: a locking chamber and a storage area. The locking chamber is the nanotube (10.10), open at one end, with a K@C601+endohedral complex inside it. The storage area is a nanotube (20.20). The locking chamber and the storage area are joined with each other and form T-junction. The locking chamber is opened at the methane filling and the discharge stages, and it is closed at the storage stage. Thanks to the locking chamber, methane molecules are stored in the nanocapsules under normal external conditions. Opening and closing of the locking chamber are carried out by the K@C601+endohedral complex displacement, which is done by the electric field action. The specific structure of the nanocapsule allows two aims to be reached: a high methane mass content and significant level of safety.
KeywordsMolecular dynamics Nanocapsules Methane storage
Synthesizing a bucky shuttle or a nanopeapod [1, 2], consisting of a nanotube  and fullerene  has opened new horizons of investigations into the properties of nanostructures for researchers. At present time it is supposed, that bucky shuttles can be used, for example, as nanosized memory elements  or to serve as a basis for creating devices for target drug delivery . One of the most promising directions of nanocapsule usage is its exploitation as a nanoscale container or a capsule for different gases storage [7–11], because conventional nanotubes cannot meet the requirements of industrial use, in spite of having numerous special improvements in their structure [12–14]. Using nanostructured carbon engineering [15–17], opens new prospects for creating nanocapsules of complex structural forms.
In the present work, the nanocapsule for methane storage by the method of molecular dynamics is investigated. The nanocapsule consists of two nanotubes of different diameters joined with each other, forming T-junction. The bigger nanotube—(20.20) presents the area of methane storage. The smaller one—(10.10) has a hole for a methane molecule penetration and the K@C601+endohedral complex and also plays the role of “a locking chamber”. The position of the K@C601+ion in the locking chamber is ruled by the electric field action and defines the state of the chamber—opened or closed access for methane molecules into the nanocapsule.
The performed calculations show that the usage of nanocapsules of complex structural forms is a new way of effective and safe storage of methane.
Computational Model and Details
Molecular dynamics simulation of filling, storage, discharge processes and the K@C601+ ion displacement under the action of the electric field were performed by NAMD program . The visualization of the calculated results was performed with VMD program . The value of the time step for simulation is 1 fs. The values of the atomic charges in the methane molecule are as follows: a carbon atom is −0.628203 Mulliken and a hydrogen atom is +0.157051 Mulliken . The charge of +1e of the K@C601+ endohedral complex is uniformly distributed over the C60 shell.
The position of the K@C601+ion in the locking chamber is defined by the action of the electric field. The electric field vector is parallel to the locking chamber walls. The value of the electric field intensity needed for the K@C601+ion displacement is 5.655 × 109V/M. The K@C601+ion position defines the ability of the methane molecules to penetrate into the nanocapsule.
Results and Discussion
We demonstrated the operation of the nanocapsule of a complex structural form suitable for effective methane storage. The nanocapsule under normal conditions retains 14.5 mass% of methane molecules, which were filled up underT = 300 K andP = 20 MPa. The nanocapsule opening and closing processes are performed by the displacement of the K@C601+endohedral complex under the action of the electric field. The calculations showed that the nanocapsules of such structure and principles of operation present a new and effective way of methane storage and can be used to store other gases.