208 research outputs found
Thermodynamic stability of fluid-fluid phase separation in binary athermal mixtures: The role of nonadditivity
We study the thermodynamic stability of fluid-fluid phase separation in
binary nonadditive mixtures of hard-spheres for moderate size ratios. We are
interested in elucidating the role played by small amounts of nonadditivity in
determining the stability of fluid-fluid phase separation with respect to the
fluid-solid phase transition. The demixing curves are built in the framework of
the modified-hypernetted chain and of the Rogers-Young integral equation
theories through the calculation of the Gibbs free energy. We also evaluate
fluid-fluid phase equilibria within a first-order thermodynamic perturbation
theory applied to an effective one-component potential obtained by integrating
out the degrees of freedom of the small spheres. A qualitative agreement
emerges between the two different approaches. We also address the determination
of the freezing line by applying the first-order thermodynamic perturbation
theory to the effective interaction between large spheres. Our results suggest
that for intermediate size ratios a modest amount of nonadditivity, smaller
than earlier thought, can be sufficient to drive the fluid-fluid critical point
into the thermodinamically stable region of the phase diagram. These findings
could be significant for rare-gas mixtures in extreme pressure and temperature
conditions, where nonadditivity is expected to be rather small.Comment: 17 pages, 7 figures, to appear in J. Phys. Chem.
Phase diagram of softly repulsive systems: The Gaussian and inverse-power-law potentials
We redraw, using state-of-the-art methods for free-energy calculations, the
phase diagrams of two reference models for the liquid state: the Gaussian and
inverse-power-law repulsive potentials. Notwithstanding the different behavior
of the two potentials for vanishing interparticle distances, their
thermodynamic properties are similar in a range of densities and temperatures,
being ruled by the competition between the body-centered-cubic (BCC) and
face-centered-cubic (FCC) crystalline structures and the fluid phase. We
confirm the existence of a reentrant BCC phase in the phase diagram of the
Gaussian-core model, just above the triple point. We also trace the BCC-FCC
coexistence line of the inverse-power-law model as a function of the power
exponent and relate the common features in the phase diagrams of such
systems to the softness degree of the interaction.Comment: 22 pages, 8 figure
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