COMPUTER-SIMULATION STUDIES OF DIFFUSION IN GELS - MODEL STRUCTURES

We have investigated particle diffusion through different obstacle geo metries by computer simulations. The model structures used in this wor k - randomly placed point obstacles and cage-like structures - were ch osen with the aim of represent a broad range of geometrical structures similar to gels and in order to be compared with our previous simulat ions of particle diffusion through polyacrylamide gels. The diffusion behavior was studied as a function of tracer size and obstacle concent ration. The isomorphism between the diffusion of finite-sized tracers and the diffusion of point tracers in the presence of expanded obstacl es was applied. Only hard-sphere interactions of the tracer with the i mmobile obstacles were considered and the theoretical description was made in terms of theory of the obstruction effect. In the case of rand omly placed point obstacles an analytical expression for the dependenc e of the diffusion coefficient on tracer radius and obstacle concentra tion, applying the model of spherical cells, could be deduced. The sam e description was applied numerically to the other model systems. Up t o moderatly high fractions of excluded volume this description was fou nd to be successful. For very high fractions of excluded volume - high er concentrations or larger tracers - the validity of Fick's second eq uation for describing diffusion breaks down and anomalous diffusion wa s found. The anomalous diffusion exponent diverges as the tracer size becomes comparable to the size of the pores. Analysis of the trajector y of tracers in the cases where an anomalous diffusion takes place sho ws a Levy-flight-like characteristic. (C) 1997 American Institute of P hysics.