Inverse Gas Chromatography Study of Small Molecules Transport in Hydroxypropyl Xylan

  • Author / Creator
    Bayati, Fatemeh
  • Synthetic polymers are still widely used in today’s life despite all environmental issues attributed to them. Replacing synthetic polymers with natural biodegradable polymers has been considered as a way of reducing these environmental impacts partially, if not completely. Hydroxyporpyl xylan (HPX) a water soluble natural polymer from xylan has been considered a potential candidate for applications in food packaging. In order to achieve this goal, certain properties of HPX such as film forming and barrier properties should be studied. HPX forms transparent but relatively rigid films. Sorbitol, a commonly used food grade plasticizer was added to improve the film forming properties of HPX. Cellulose nanocrystal (CNC) was added to improve barrier properties of the plasticized films. Inverse gas chromatography (IGC) was used to study the permeability of water through neat HPX film and HPX films containing sorbitol and CNC over the temperature range of 120 – 160 °C. Rotary evaporator was used to coat HPX with and/or without additives on inert support (glass beads). The IGC column was filled with the coated beads for the experiments (the details of the method are presented in Chapter 4). Using IGC, diffusion and solubility coefficients at infinite dilution were measured and the corresponding permeability coefficients were calculated. It was found that almost 40 wt% of sorbitol would yield a HPX film with desirable flexibility, as quantified by its glass transition temperature (Tg). Also, addition of 1–5 wt% CNC to the plasticized films significantly decreased the diffusivity of water. In particular, the infinite dilution diffusion coefficient of water for the HPX film with 35 wt% of sorbitol and 5 wt% CNC was comparable to that of the bare HPX film. The diffusion data suggest that effects of sorbitol and CNC on the water diffusivity follow the free volume theory. Based on the results, neat HPX films, films containing 40 wt% of sorbitol (as the best film forming concentration) and films with 35 wt% sorbitol and 5 wt% CNC (as the films least diffuse to water ) were selected for the next steps of the experiments. The selected films (from now on defined as HPX, HPX/Sorbitl and HPX/Sorbitl/CNC) were used for studying permeability of primary alcohols vapour (C1-C4) representing aroma. It was observed that the alcohols exhibited lower solubility (three orders of magnitude) and diffusivity (two to three orders of magnitude) than that of water in the neat HPX film. Solubility coefficient showed a minimum over the range of the molecular weight of the selected alcohols. However, the diffusion coefficient decreased monotonically with increasing alcohol’s molecular weight. Presence of sorbitol decreased solubility coefficients slightly but increased diffusion coefficients by about one order of magnitude. However, adding CNC to the plasticized film did not alter the solubility coefficients much but decreased diffusion coefficients by approximately half of the values of the HPX/Sorbitol film. Permeability was observed to decrease with increasing alcohols’ molecular weight. This implies that diffusivity controls the permeability of the alcohols at high molecular weights. As expected, sorbitol increases while CNC decreases permeability. However, effects of additives are stronger for methanol and ethanol than for propanol and butanol. Temperature also exerts stronger effects for methanol and ethanol than for propanol and butanol. The above observation is likely due to the differences in the abilities of the low and high molecular weight alcohols to form hydrogen bonds with HPX, sorbitol and CNC. Further, effect of relative humidity (RH) on transport properties of the alcohols was studied at (RH) values of 30%, 50% and 70% of the carrier gas (helium). Diffusion coefficients of the alcohols showed a maximum at about 50% RH in all films. And solubility coefficients of the alcohols were more or less insensitive to the RH. Permeability coefficients of the alcohols exhibited a maximum at around 50% RH as well suggesting that diffusion, not dissolution, dominated the permeability behaviour. The RH dependence of diffusion, thereby permeability, was attributed to the combined effects of plasticizing, swelling, water clustering and alcohol-water clustering. The measured weight-fraction-based Henry’s constants suggested that water contents in the films were relatively low; suggesting that water clustering might only take place in the neat HPX film at 70% RH.

  • Subjects / Keywords
  • Graduation date
    Fall 2016
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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