Diffusion of krypton in liquid argon at infinite dilution K. Srinivasan Instrumentation and Services Unit and Department of Mechanical Engineering, Indian Institute of Science, Bangalore 560 012, India Received 2 August 1988 Recently reported experimental data on tracer diffusion coefficients of krypton-85 in liquid argon in the temperature range 85-103 K are compared with the results of computer simulation for the same system. At 115.7 K the extrapolated value of the tracer diffusion coefficient (DKr) from the experimental data is in good agreement with the value of DK, calculated from computer simulation results. Our experimental data along with other predictions of molecular dynamics calculations can be used to predict self-diffusion coefficients of liquid argon, which are also found to be in good agreement with previously reported experimental results. Keyword~: thermodynamics; properties of materials; rare gas mixtures; diffusion The purpose of this Paper is to present an interpretation of the experimental data on diffusion of krypton-85 in liquid argon at infinite dilution in the light of results from computer simulation using molecular dynamics reported in the literature. Experimental tracer diffusion coefficients for the above system have been measured in the tem- perature range 85-103 K using the diaphragm cell tech- niqueL A brief description of the apparatus, raw data and a fit for the temperature dependence of the diffusion coefficient have been reported 2. The following Arrhenius relation was found to fit the data from 14 measurements (seven sets with two experiments at each temperature) in the above temperature range DK, = 55.9 exp (--305/7") x 10 -5 cm2s -1 (1) The above form of expression was chosen for two reasons. First, traditionally the Arrhenius relation has been used to describe the temperature dependence of diffusion coefficients. Second, it provides a convenient equation for correlating the experimental data in the temperature range below the thermodynamic critical point for several liquids and for liquefied rare gases in particular 3- 6 Theoretical correlations or computer simulation re- suits do not exist for a direct interpretation of the above results for diffusion of krypton in argon at infinite dilution. However, Jacucci and McDonald 7 have ob- tained concentration dependence of self and mutual diffusion coefficients for the Ar-Kr system (namely, DAr, DKr and DAr- K~)at a temperature of 115.7 K and a molar volume of 33.31 cm3moi -x. This thermodynamic state corresponds to the saturated liquid condition. Their results cover a concentration range (C1) of 0.1 - 0.9 for argon as shown in Figure 1. In binary solutions three diffusion coefficients are observed, the two self-diffusion coefficients (D^r and DJ and the mutual diffusion coefficient (DAr-K~). To avoid ambiguity, D~, is referred to as the tracer diffusion coefficient instead of as the self-diffusion coefficient. The experimental data presented here corresponds to infinite dilution ie., Cx almost 1. Thus, it is appropriate to //s ~E u =o x o 2 0 0 0.2 0.z, 0.6 0.8 1.0 Mote fraction of argon (c~) Figure 1 Variation of diffusion coefficients in argon-krypton mixtures: x, Ar7; A, Ar-Kr 7, O, Kr7; , best fit; - - - , extrapola- tion; V=33.31 cm3mo1-1, T=115.7 K 0011-2275/89/090935-02 $03.00 © 1989 Butterworth & Co (Publishers)Ltd Cryogenics 1989 Vol 29 September 935