The influence of nitrogen on the liquid—liquid phase behaviour of the system n-hexane—polyethylene: experimental results and predictions with the mean-field lattice-gas model

Abstract

The phase diagram of the system n-hexane+linear high density polyethylene shows a region of limted miscibility in the liquid phase, which is of the lower critical solutions temperature type (LCST behaviour). The influence of nitrogen on the location of this region of limited miscibility is investigated experimentally at pressures up to 7.5 MPa in the temperature range 393–453 K for weight fractions of nitrogen in the liquid phase between 0 and 0.013. Nitrogen induces a shift of the region of limited miscibility to lower temperatures and higher pressures of about 22 K at constant pressure or 2m.Pa at constant temperature per weight percent nitrogen. Model calculations have been performed for the ternary system nitrogen + n-hexane + polyethylene using the mean-field lattice-gas model of Kleintjens and Koningsveld. Parameters used in these calculations were obtained from pure component and binary data only. The results prove that this model is able to predict the observed peculiar behaviour in the ternay system.