Freezing of a Lennard-Jones system in an open-ended and finite-length nanopore: A molecular-dynamics study

Abstract

The purpose of the present paper is to study the freezing procedures of a rare-gas system confined in a nanometer-scale pore as well as the atomic structure of a confined rare-gas solid. To this end, we carry out molecular-dynamics (MD) simulations for Lennard-Jones argon (LJ Ar) confined in an open-ended finite-length pore. Our simulation cell consists of two sections. One section contains a cubic solid with a cylindrical pore, the solid being composed of LJ atoms with interaction parameters appropriate for silica glass. The diameter and length of the cylindrical pore are $10{\ensuremath{\sigma}}_{\mathrm{ArAr}}$ (3.4 nm) and $20{\ensuremath{\sigma}}_{\mathrm{ArAr}}$ (6.8 nm), respectively. The other section is a free space into which we insert Ar atoms. The adsorption of Ar atoms into the nanopore proceeds by diffusive mass transfer from the free space. We simulate usual experimental procedures, that is, the pore is first filled with a liquid via capillary condensation, and thereafter the system is cooled. Our results show that, as the temperature is decreased, the freezing of Ar starts from the vicinity of the pore wall. The portion of solidlike Ar atoms increases gradually near the pore wall, while the Ar atoms in the central part of the pore change suddenly from liquidlike to solidlike all at the same time. We find that the structure factor of Ar confined in the pore does not change qualitatively during the freezing process, showing simple-liquid-like behaviors for both the liquid and the solid. This result agrees qualitatively with recently reported x-ray diffraction patterns of confined rare-gas solids. In a bulk LJ Ar system, on the other hand, the atomic structure changes qualitatively during the freezing process. One possible explanation for this difference is that the Ar solid in the pore consists of several different local atomic configurations, each giving distinct contributions to the structure factor of the confined Ar solid. Therefore, analyses beyond the averaged structure are helpful for extending our knowledge in the microscopic structure of confined materials. In order to further investigate this point, we evaluate the local radial distribution function (LRDF) of the confined Ar solid. The calculated LRDF for the solid is qualitatively different from that of the liquid and indicates that the atomic structure of the confined LJ Ar solid in the central part of the pore is similar to that of a bulk LJ Ar glass.