  Mantles of iced water, mixed with CO, H2CO, and CH3OH are formed during the
so called prestellar core phase. In addition, radicals are also thought to be
formed on the grain surfaces, and to react to form complex organic molecules
later on, during the warm-up phase of the protostellar evolution. We aim to
study the formation of the grain mantles during the prestellar core phase and
the abundance of H2CO, CH3OH, and radicals trapped in them. We have developed a
macrosopic statistic multilayer model that follows the formation of grain
mantles with time and that includes two effects that may increase the number of
radicals trapped in the mantles: i) at each time of the mantle formation, only
the surface layer is chemically active rather than the entire bulk, and ii) the
porous structure of grains allows to trap reactive particles. The model
considers a network of H, O and CO forming neutral species such as water, CO,
formaldehyde, and methanol, plus several radicals. We run a large grid of
models to study the impact of the mantle multilayer nature and grain porous
structure. In addition, we explored the influence of the uncertainty of other
key parameters on the mantle composition. Our model predicts relatively large
abundances of radicals. In addition, the multilayer approach makes it possible
to follow the chemical differentiation within the grain mantle, showing that
the mantles are far from being uniform. For example, methanol is mostly present
in the outer layers of the mantles whereas CO and other reactive species are
trapped in the inner layers. The overall mantle composition depends on the
density and age of the prestellar core, and on some microscopic parameters.
Comparison with observations allows us to constrain the value of few parameters
and provide some indications on the physical conditions during the formation of
the ices.
