The self-diffusion coefficient of methane has been measured from 150" to 350°K. and from 200 to 5,000 tb./sq. in. abs. A t constant temperature, the density-diffusivity product is constant up to neighborhood of critical density and decreases sharply above that density. The temperature dependence of tke low density data agrees with Chapman-Enskog theory. The LennardJones (6-12) parameters determined from the low density data ( e / k = 130"K., u = 3.85A.) are in good agreement with those determined by other methods. A correlation for the sewdiffusion coefficient for methane has been developed which may provide predictions of other spherical nonpolar gores.The coefficient of self-diffusion was considered (9) to be the limiting case of the binary diffusion coefficient until Carr and Purcell (2) developed the pulsed (or spin echo) nuclear magnetic resonance (NMR) technique to measure self-diffusion. Directly, the study of this significant transport property, self-diffusion, has led to interesting results.The self-diff usion coefficient has considerable theoretical interest. I t is the transport coefficient most easily calculated by kinetic theory, and it provides a good check for the vnrious theories of the dense gas state. The possibility exists ( I ) that binary (mutual) diffusion coefficients may be calculated from the self-diffusion coefficients of the two pure components. The measurement of the self-diffusion coefficient of methane in the dilute and dense gas regions is reported here. T H E O R Y Theory o f Self-DiffusionThe molecules in a fluid are continuously in motion. In a uniform homogeneous fluid, this motion is random and represents the thermal energy present in all fluids. This random motion causes any particular molecule of the fluid to move from place to place in the fluid; this movement is called setf-digusion.The coefficient of self-diffusion is defined aswhere is the mean square net distance a molecule moves in time t. This definition is consistent with the Ficks law definition of a binary diffusion coefficient, 0 1 2 :On this basis D is defined as the limit of D12 as the molecules of 1 and 2 become identical.In the past, the closest approach to the limit of identical molecules was obtained by observing diffusion of isotopes.For example, C14 methane may be observed diffusing in C12 methane (20). The radioactivity of the former and the nuclear stability of the latter serves to distinguish the two and allows the motion of particular methane molecules to be observed. The change in the diffusion coefficient, caused by the difference in molecular weight of the two species, is small but not negligible. Spin echo NMR fol- Rapier Dawson is with Esso Production Research Company, Houston, Texas.lows the molecules by observing their nuclear spin states; this is a much more subtle "tag." Spin Echo N M RThe basic method of generating nuclear spin echos was discovered by Hahn ( 7 ) , who also suggested their use for measuring self-diffusion coefficients. His suggestions were clarified and the experimental me...

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