  Weak reactions are critical for the neutron richness of the matter
dynamically ejected after the merger of two neutron stars. The neutron
richness, defined by the electron fraction (Ye), determines which heavy
elements are produced by the r-process and thus directly impacts the kilonova
light curve. In this work, we have performed a systematic and detailed
post-processing study of the impact of weak reactions on the distribution of
the electron fraction and of the entropy on the dynamic ejecta obtained from an
equal mass neutron star binary merger simulated in full general relativity and
with microscopic equation of state. Previous investigations indicated that
shocks increase Ye, however our results show that shocks can also decrease Ye,
depending on their thermodynamical conditions. Moreover, we have found that
neutrino absorption are key and need to be considered in future simulations. We
also demonstrated that the angular dependence of the neutrino luminosity and
the spatial distribution of the ejecta can lead to significant difference in
the electron fraction distribution. In addition to the detailed study of the Ye
evolution and its dependences, we have performed nucleosynthesis calculations.
They clearly point to the necessity of improving the neutrino treatment in
current simulations to be able to predict the contribution of neutron star
mergers to the chemical history of the universe and to reliable calculate their
kilonova light curves.
