  Neutrino-matter interactions play a key role in binary neutron star mergers.
Thermodynamics conditions at the surfaces where neutrinos decouple from matter
influence neutrino spectra, ultimately affecting the evolution of the remnant
and the properties of the ejecta. In this work, we post-process results of
general relativistic merger simulations employing microphysical equations of
state and approximate neutrino transport to investigate the thermodynamics
conditions at which weak and thermal equilibrium freezes out (equilibrium
surfaces), as well as conditions at which the transition between diffusion and
free-streaming regime occurs (diffusion surfaces). We find that the rest mass
density and the neutrino energy are the most relevant quantities in determining
the location of the decoupling surfaces. For mean energy neutrinos ($\langle
E_{{\nu}_e} \rangle \approx 9~{\rm MeV}$, $\langle E_{{\bar{\nu}}_e} \rangle
\approx 15~{\rm MeV}$, $\langle E_{{\nu}_{\mu,\tau}} \rangle \approx 25~{\rm
MeV}$), diffusion surfaces are located around $10^{11}{\rm g~cm^{-3}}$ for all
neutrino species, while equilibrium surfaces for heavy flavor neutrinos are
significantly deeper (several $10^{12}{\rm g~cm^{-3}}$) than the ones of
$\bar{\nu}_e$ and $\nu_e$ ($\gtrsim 10^{11}{\rm g~cm^{-3}}$). The resulting
decoupling temperatures are in good agreement with the average neutrino
energies ($\langle E_{\nu} \rangle \sim 3.15~T$), with the softer equation of
state characterized by systematically larger decoupling temperatures ($\Delta T
\lesssim 1~{\rm MeV}$). Neutrinos streaming at infinity with different energies
come from very different regions of the remnant. The presence of a massive NS
or of a BH in the remnant influences the neutrino thermalization process.
