Finite-Size Orientational Crossover in Water–Argon Chains with Optional Carbon Nanotube Confinement

Abstract

Nanoscale confinement can strongly alter the orientational behaviour of water. This thesis investigates whether local axial orientation persists when neighbouring water molecules are separated by fixed argon atoms, and how optional confinement within a (6, 5) carbon nanotube modifies that response. Finite water–argon chains were modelled as fixed-centre asymmetric-top water rotors with stationary argon spacers and nearest-neighbour water–water interactions. Ground states were calculated using the density-matrix renormalization group in a matrix product state representation as the water–water separation, R, was varied. The analysis combined energy, central von Neumann entropy, signed and absolute axial orientation, site-resolved profiles, angular distributions, and numerical convergence diagnostics. The principal result is a finite-size orientational crossover obtained for the ordinary open-boundary finite-chain Hamiltonian. The sampled central-entropy maximum moves to larger R with increasing chain length and approaches the 9–10 Å region for the larger chains. In the same main crossover window, the signed mean axial orientation is strongly reduced while the mean absolute local orientation remains appreciable. Site-resolved results show that differently biased parts of the chain make cancelling contributions while local axial orientation remains. The ground-state energy per water varies smoothly and has no corresponding feature. CNT confinement shifts and modestly suppresses the sampled entropy response, while the matched orientation curves remain similar over most of the main crossover window. CNT OFF and CNT ON are therefore consistent with the same general response, although an identical microscopic mechanism is not established. The separate short-R boundary-sensitive feature near R ≈ 7 Å was examined over R = 6.80–7.30 Å using sine-square deformation. Its entropy and branch-selection patterns change under SSD, showing sensitivity to boundary weighting without isolating an individual boundary contribution. This dataset does not test the boundary robustness of the finite-size orientational crossover. The conclusions apply to ground states of the finite, fixed-centre, nearest-neighbour Hamiltonian and do not establish a thermodynamic phase transition or physical excitation gaps.