Viscosity of carbon dioxide in the liquid phase

Abstract

The viscosity coefficient of liquid carbon dioxide has been measured along isotherms from the triple-point temperature (217 K) up to the critical temperature (304 K) at temperatures of 220, 230, 240, 260 and 280 K by means of a vibrating-wire viscometer. The measurements extended beyond both phase-transition lines into the coexistence region (superheated liquid) and into the solid range (undercooled liquid). The accuracy of the measurements is estimated to be 1%. The agreement with data of other authors is rather good. For the most part, the results show a linear pressure dependence in three neighbouring pressure ranges for the various isotherms with a common intersection with the negative pressure axis p i. The fluidity, the reciprocal of the viscosity, shows a linear dependence of the molar volume in adjacent density ranges. After reduction of the molar volume with the volumes of close packing, three sets of linear functions result with common intersections of the axis V B. The aim of the present investigation was to produce a new set of viscosity data on request of the Subcommittee on Transport Properties of the International Union of Pure and Applied Chemistry in order to provide supplementary data for the improvement of representative equations for the viscosity of carbon dioxide. The representation of the fluidity as a linear function of the reduced molar volume in various density ranges provides us with the possibility of presenting a correlation protocol which produces the viscosity of liquid carbon dioxide within about 1%. For this purpose, a consistent set of “volumes of close packing” is obtained by means of a comparison with the results of computer simulations. The combination of the two types of linear relations gives reason to interpret p i as the internal pressure and V B as the excluded volume. This interpretation enables us to state that, at constant temperature, the viscosity coefficient is directly proportional to the thermal pressure, i.e. the sum of the measured pressure and the internal pressure, and thereby proportional to the number of collisions of the molecules. The viscosity coefficient is, thus, determined by the number of collisions per unit time and volume, and by the efficiency of the exchange of momentum during a collision. From the linearity of the fluidity as a function of the molar volume, we conclude that the number of collisions is inversely proportional to the free volume V − V B. Thus our measurements of the viscosity of liquid carbon dioxide lead to simple views on this phenomenon.