Effective encounter distance, definition

When the distance between each A reactant is very large compared with that between each pair of B reactants, at a point about midway between a pair of A reactants, the concentration of B reactants is unlikely to be significantly affected by the presence of the A reactants. Smoluchowski suggested that such B reactants are effectively an infinite distance from the A reactants under discussion. By effectively an infinite distance is meant perhaps 1000 times the molecular diameter or encounter distance R. In this region, the concentration of B reactants at any time during the reaction is very close to the initial concentration, i.e. [B](1000iZ) [B]0 for all time (t > 0). From the definition of the density distribution, eqn. (2), this boundary condition as r - °° is... 

---