| |
 |
UNIVERSITY OF BUCHAREST
FACULTY OF PHYSICS Guest
2024-11-22 2:01 |
 |
|
|
|
Conference: Bucharest University Faculty of Physics 2006 Meeting
Section: Quantum Mechanics and Statistical Physics Seminar
Title:
Three-body and bonding effects on phase separation in a model binary
solution
Authors:
Radu P. Lungu
Affiliation:
University of Bucharest, Faculty of Physics, P.O.Box MG-11, Bucharest, Romania
E-mail
RaduPLungu@yahoo.com
Keywords:
phase transition, Ising models
Abstract:
A microscopic statistical mechanical model for a binary solution is presented, in which the links of a honeycomb lattice are covered by rod-like molecules of types AA and BB; the molecular ends near a common site have three-body
interactions, and also there are many internal states (bonding and non-bonding)for each molecular end. The model is equivalent to a spin-1/2 Ising model in a field, with two-spin and three-spin interactions on an associated
lattice that has sites at the positions of the molecular ends; then, using transformation methods, this Ising model is itself equivalent to a simple spin-1/2 Ising model
on the original lattice with a magnetic field and pure two-spin interactions.The generalized model contains 20 parameters, so a simplified version of this generalized model, that has only 6 adjustable parameters with physical
meaning,is considered. Exact equations for the mole fractions of the components and exact two-phase coexistence diagrams, corresponding to the separation into AA-rich phase and BB-rich phase, are obtained in the temperature-composition space. Using the condition for the occurrence of phase transition, the possibilities for different types of phase diagrams, as function of the parameters of the simplified model (such as the asymmetry of the three-body interactions, and the strengths of single and double bonding) are studied. The results show that for small values of bonding energies, there are
the following possibilities: i. the phase coexistence occurs in the zero-temperature limit, so the phase
diagrams are open curves, for weak asymmetry; ii. the phase coexistence occurs only at intermediate temperatures
(there are a lower and a upper critical temperatures), so the phase diagrams are closed loops, for moderate asymmetry; iii. no phase transition, when strong asymmetry is considered. For stronger bonding energies, no phase coexistence occurs at low temperatures,so only two cases are obtained: closed-loop diagrams for small asymmetry, and no phase transition for strong asymmetry.
|
|
|
|