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Modello di calcolo per l'analisi termica del sistema di termoregolazione del corpo umano
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Properties of some mean-field-like approximations for the triangular Ising antiferromagnet
No full textPolymer solution model with anisotropic phase
No full textWe investigate a lattice self-avoiding walk model with directive (hydrogen bond like) interactions between chain segments and isotropic interactions between monomers and solvent molecules. The model, meant to describe a polymer in the dilute solution regime, is investigated in the framework of the Bethe approximation. Directive interactions produce, in addition to the ordinary collapse, a first order transition to an anisotropic (structured) phase. The interplay with solvent interactions modifies the nature of the isotropic phase, which can be either swollen or collapsed. Such an interplay may be an important ingredient for the formation of ordered structures in biopolymers. The relationship with previously investigated self-avoiding walk models is also discussed
Thermodynamic anomalies in a lattice model of water
No full textWe investigate a lattice-fluid model of water, defined on a three-dimensional body centered cubic lattice. Model molecules possess a tetrahedral symmetry, with four equivalent bonding arms, aiming to mimic the formation of hydrogen bonds. The model is similar to the one proposed by Roberts and Debenedetti [J. Chem. Phys. 105, 658 (1996)], simplified in that no distinction between bond "donors" and "acceptors" is imposed. Bond formation depends both on orientation and local density. In the ground state, we show that two different ordered (ice) phases are allowed. At finite temperature, we analyze homogeneous phases only, working out phase diagram, response functions, the temperature of maximum density locus, and the Kauzmann line. We make use of a generalized first-order approximation on a tetrahedral cluster. In the liquid phase, the model exhibits several anomalous properties observed in real water. In the low temperature region (supercooled liquid), there are evidences of a second critical point and, for some range of parameter values, this scenario is compatible with the existence of a reentrant spinodal
Herringbone ordering and lattice distortions in a planar-molecule model for Langmuir monolayers
Full text linkA model of planar molecules, made up of "atoms" interacting by Lennard-Jones potentials and arranged to mimic the cross section of alkyl chains, is used to study the problem of backbone plane ordering in Langmuir monolayers. It is shown that two minima of the interaction energy are reached if molecules lie on the sites of a centered rectangular lattice in a herringbone configuration with two different dihedral angles. These orientationally ordered phases can be related to the so-called herringbone and pseudoherringbone structures, whose lattice distortions qualitatively agree with those determined by means of grazing incidence x-ray diffraction experiments on Langmuir monolayers. A third energy minimum is obtained for a configuration of parallel molecules on an oblique lattice, which has also been observed in some experiments. The competition between the three phases is investigated, upon varying geometric parameters of the model molecules and surface pressure. The effect of temperature is analyzed in a mean field approximation, by taking into account the orientational entropy contribution on a lattice system with variable unit cell parameters. In this framework the transition to an orientationally disordered phase is also pointed out
Cluster variation approach to the Ising square lattice with two- and four-spin interactions
No full textPHYS. REV.
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