Water molecule encapsulated in carbon nanotube model systems: effect of confinement and curvature

Abstract

Water present in the nanoscale confined medium like the hydrophobic interior of the carbon nanotubes (CNT) is known to extol unique properties that depart largely from their behavior in the bulk form. Considering suitable model systems that structurally resemble single unit of the CNT, we demonstrate that two unique parameters namely the local nanoscale curvature and the confinement length are of cardinal importance in governing the structural and electronic properties of water molecule present inside CNT in a general manner. Water molecule encapsulated between the model systems that offer both the above two effects exhibits dramatic trend in the interaction energy with respect to the variation of these parameters. In relation to the curvature of the model system, we propose three different regimes where water molecule experiences a distinct trend in the stabilization energy. Similarly, a confinement distance of 6 Å also marks as a borderline for the distinct manifestations in the stabilization energy of the water molecule. These two parameters also play a key role in governing the significant variations in the structural parameters, Mulliken charges, and red and blue shift in the O-H vibrational frequencies of the encapsulated water molecule. There seems to be interplay between curvature and confinement in deciding the electronic properties of water in the nanoscale confinement.

Incremental DF-LCCSD(T) Calculations for a Water Molecule Inside and Outside Armchair Carbon Nanotubes

“…Experimental results [12] confirmed the transformation at low temperature from liquid-like water into ice-like water which was predicted previously by MD simulations [13] and demonstrated enhanced water flow through CNTs [14]. Theoretical studies employed classical molecular dynamics (MD) [15,16] or density functional theory (DFT) methods [17] to study, e. g., how the confinement of water reduces the fluctuation of bonds in the hydrogen-bond network and the enhancement of water transport inside narrow carbon nanotubes [13,14]. By applying axial pressures, the formation of ice nanotubes inside CNTs was observed and their phase transitions explored [13].…”

Microscopic Study of Proton Kinetic Energy Anomaly for Nanoconfined Water

“…One intriguing aspect of water physics is the unusual behavior of water with respect to its ordinary bulk phases, when in the vicinity of surfaces and in confined geometries . A typical example is the shift in phase boundary, which is reflected in a corresponding, pore size dependent, strong change in freezing and melting temperatures. − One such fascinating aspect is the solid like ordering of water inside (6,6) SWCNT even at room temperature . Since the behavior of water is predominantly governed by the hydrogen bonds (HBs) network, − various studies are aimed at exploring proton dynamics in H 2 O containing systems.…”

Ground-state energy for confined H2: a variational approach

“…In the study of confined molecules, special attention has been devoted to the ionic hydrogen molecule (H 2 + ) [10][11][12][13][14][15] and the neutral hydrogen molecule (H 2 ) [16][17][18][19]. Small polyatomic molecules have also been analyzed [20][21][22][23]. In particular, the eigenvalues of energy for hydrogen molecule inside an impenetrable prolate spheroidal box, with the Born-Oppenheimer approximation, have been discussed in the literature [16,17,19], where the eigenvalues are obtained using the variational method and Quantum Monte Carlo technique.…”