Monte Carlo simulations of systems of light alcohols + water + n-dodecane and water solubility and structures in polytetrafluoroethylene.

Abstract

Polytetrafluoroethylene (Teflon®) is encountered in many environments – frying pans, clothing, osmotic distillation membranes, to name a few – yet the solubility and clustering behaviour of water with this material was not found in the open literature. This information may be useful in applications where an absence of water is desired, such as in clothing and textiles. Previous work on polyethylene + water has shown that small water clusters form in the amorphous portion of the polymer. This work investigated this phenomenon for the case of polytetrafluoroethylene + water. Initially, a test system of light alcohols + water + n-dodecane was investigated using Gibbs Ensemble Monte Carlo simulations and compared to previous laboratory experiments. This test system was investigated in order to gain expertise in the methodologies and theory behind Monte Carlo simulation, as well as to gain experience with using the necessary software. For this test system, it was found that the TraPPE parameters representing the interactions between the alcohols and the n-dodecane were not adequate and lead to increasing deviations with increasing carbon number in the alcohol. To replicate the conditions of the amorphous polymer matrix, liquid-liquid equilibrium between water and the polymer was investigated. Gibbs Ensemble Monte Carlo simulations have been performed for systems of perfluoroalkanes and water to determine the influence of temperature and carbon number on the solubility and clustering behaviour of water within the perfluoroalkanes. The temperature range in this study was from 450 K to 600 K, and the perfluoroalkane carbon number range was from 8 to 300 carbon atoms. With increasing carbon number, it was found that there was an asymptotic value of 98.0 mole percent water in the polymer phase. With increasing temperatures it was found that there were exponential increases in solubility of water into the polymer matrix. Previous work on clustering and supramolecular structure of perfluoroalkanes described the rigidity of the perfluoroalkane chains in comparison to alkane chains, thus explaining the large increases in free volume with increasing temperature in the polymer matrix observed in this work. A discontinuity with regard to both solubility and clustering behavior was observed for a polymer carbon number of 10 to 12 carbon atoms. Prior work on the energy contributions towards the helical structure of perfluoroalkanes showed a shift in the energy contribution regime for carbon numbers larger than ~10 carbon atoms, which may explain this discontinuity. It was found that linear water clusters accounted for up to ~90 percent of the water clusters, concurring with previous work on water clustering in polyethylene.