Effect of Humidity on the Permeability of Alcohols in Hydroxypropyl Xylan Films

Abstract

Inverse gas chromatography (IGC) was used to study the effect of humidity on the permeability of selected low molecular weight alcohols (methanol, ethanol, propanol, and butanol) through films made up of a hydrophilic polymer hydroxypropyl xylan (HPX). The thin films used consisted of a neat HPX film and a HPX film with sorbitol, a commonly used food grade plasticizer, (HPX/Sorbitol) and a HPX film with sorbitol and cellulose nanocrystal (CNC) (HPX/Sorbitol/CNC). Diffusivity and solubility of the alcohols at infinite dilution were measured at relative humidity (RH) values of 30%, 50%, and 70% of the carrier gas (helium). Permeability was then calculated using the measured diffusivity and solubility. Solubility coefficients of the alcohols were more or less insensitive to the RH. Increasing humidity from dry to 50% RH increased diffusion coefficients of the alcohols due to plasticization and swelling of the films whereas at higher RH, diffusion coefficient of the alcohols decreased. Based on the water contents of the films estimated from the weight-fraction-based Henry’s constant, at 70% RH, water clustering was the reason for the reduced diffusion coefficient in the neat HPX films while alcohol–water clustering was suggested for HPX/sorbitol and HPX/Sorbitol/CNC films. Permeability coefficients of the alcohols exhibited a maximum at around 50% RH as well suggesting that diffusion, not dissolution, dominated the permeability behavior. The findings suggest that due to sensitivity of the films to humidity, films should be used along with water barrier films to control humidity of the environment for applications used as barrier to alcohols.