# High Interfacial Adsorption of Light Gases on Nano-Thin Molten Polyethylene Films

## TL;DR

This study shows that ethane gas accumulates more at the surfaces of thin molten polyethylene films than in their core, with adsorption increasing under higher pressure and varying with temperature.

## Contribution

The paper reveals a solubility transition temperature and quantifies the preferential interfacial adsorption of ethane in nano-thin polyethylene films.

## Key findings

- Ethane accumulates 4.8–8.6 times more at the interfaces than in the bulk region of the film at 373.15 K.
- Interfacial tension decreases exponentially with increasing ethane pressure due to inter-chain interactions.
- Ethane solubility in the film's core reverses with temperature, indicating a solubility transition between 298.15 K and 373.15 K.

## Abstract

Classical Molecular Dynamics simulations were used to investigate the interfacial adsorption of supercritical ethane on ultrathin molten polyethylene films at various temperatures (298.15–448.15 K) and pressures (0.28–13.17 MPa). Ethane was found to accumulate preferentially at the film’s interfaces rather than dissolving into the film’s core. The ultra-thin, metastable films, studied at their mechanical stability limit, are composed of two overlapping interfaces. The films show some fractions of interfacial chains transiently desorbing from the film surface and entering the gas phase, which facilitates the accumulation of ethane at the interfaces. At 373.15 K and pressures between 0.29 MPa and 9.65 MPa, the combined film interfaces adsorb between 4.8 and 8.6 times more ethane than the amount solubilized in the central, bulk region of the film. Interfacial tension of the film decreases exponentially with increasing gas pressure of ethane and is primarily governed by inter-chain interactions at the interface. Minor contributions arise from the vibrational dynamics of polyethylene chain fractions that transiently desorb from the film surface. Furthermore, the solubility of ethane in the film’s bulk region exhibits a temperature-dependent inversion: at 298.15 K, the ethane density in the film’s center slightly exceeds that of the bulk gas, but this trend reverses at 373.15 K and becomes more pronounced as the temperature increases. This indicates a potential solubility transition temperature between 298.15 K and 373.15 K.