Many chemical reactions occur at the surface of interstellar dust grains, producing a large diversity of molecules more or less complex. Most current astrochemical models include only a single size of grains (0.1 micron representing most of the mass of silicate grains) to study the formation and destruction of molecules on the dust surface. We have studied the effect of considering a distribution of grain sizes on the chemical evolution of various complex molecules in cold clouds in the ISM.
We used two different types of grain size distributions, MRN and WD. Each grain has its own grain number density which comes from either MRN or WD distribution. Other important parameters such as grain surface temperature or cosmic ray induced desorption also vary with grain sizes. We present abundance of various molecules including some complex molecules in gas phase and also on the surface of dust grains at different time interval during the simulation. We also compare our results with observed abundances in TMC-1 and L134N. We show how a multi grain model can be a better tool in explaining the observed abundances in these cold dense clouds.