  A model is proposed for the formation of water ice mantles on grains in
interstellar clouds. This occurs by direct accretion of monomers from the gas,
be they formed by gas or surface reactions. The model predicts the existence of
a threshold in interstellar light extinction, A(v), which is mainly determined
by the adsorption energy of water molecules on the grain material; for
hydrocarbon material, chemical simulation places this energy between 0.5 and 2
kcal/mole, which sets the visible exctinction threshold at a few magnitudes, as
observed. Once the threshold is crossed, all available water molecules in the
gas are quickly adsorbed, forming an ice mantle, because the grain cools down
and the adsorption energy on ice is higher than on bare grain. The model also
predicts that the thickness of the mantle, and, hence, the optical thickness at
3 mu, grow linearly with A(v), as observed, with a slope which depends upon the
total amount of water in the gas. Chemical simulation was also used to
determine the adsorption sites and energies of O and OH on hydrocarbons, and
study the dynamics of formation of water molecules by surface reactions with
gaseous H atoms, as well as their chances of sticking in situ.
