Diffusion of nanoparticles and macromolecules in dense gases and liquids

The method of molecular dynamics is used to study the diffusion of large molecules or nanoparticles in a dense molecular medium (liquid or gas) in a wide range of densities. Particles and molecules are simulated by hard, absolutely elastic spheres. The ratio of the particle and molecule diameters of the medium varies from unity to four, and the mass ratio, from unity to $300$. The density of the carrier medium is characterized by the parameter $V/V_0$ ($V_0$ is the volume of close-packed structure of molecules, and $V$ is the volume of the calculation cell), which is varied from $2$ to $75.3$. The dependences of the diffusion coefficient of a particle on its mass and on the density of carrier gas are investigated. It is found that the relaxation of the autocorrelation function of the velocity of a particle is described well by the superposition of two exponential functions with different relaxation times. The obtained data are compared with known theoretical models.