  Binary mixtures are known to phase separate via both nucleation and spinodal
decomposition, depending on the initial composition and extent of the quench.
Here we develop an energy landscape view of phase separation and non-ideality
in binary mixtures by exploring their potential energy landscape (PEL) as
functions of temperature and composition, by employing a molecular model that
promotes structure breaking abilities of the solute-solvent parent binary
liquid at low temperatures. PEL that provides the inherent structure (IS) of a
system is obtained by removing the kinetic energy (including that of
intermolecular vibrations). Broad Distribution of inherent structure energy
demonstrates the larger role of entropy in stabilizing the parent liquid of the
structure breaking type of binary mixtures. At high temperature, although the
parent structure is homogenous, the corresponding inherent structure is found
to be always phase separated, with a density pattern that exhibits marked
correlation with the energy of inherent structure. Over broad range of
intermediate inherent structure energy, bicontinuous phase separation prevails
with interpenetrating stripes as signatures of spinodal decomposition. At low
inherent structure energy, the structure is largely phase separated with one
interface where as at high inherent structure energy, we find nucleation type
growth. Interestingly, at low temperature, the average inherent structure
energy (<EIS>) exhibits a drop with temperature which signals onset of
crystallization in one of the phases while the other remains in the liquid
state. The non-ideal composition dependence of viscosity is anti-correlated
with average inherent structure energy.
