The penetrable-sphere model: A fluid of ghost

Andres Santos

Departamento de Fisica, Universidad de Extremadura, E-06071 Badajoz, Spain

In the last few years, interaction potentials either bounded or diverging very weakly for short distances have received much attention as models of the effective two-body interaction in some colloidal systems, such as micelles in a solvent, star copolymer suspensions or polymer chains in good solvents. The simplest bounded potential is that of so-called penetrable spheres (PS), which takes a positive finite value if the two spheres are overlapped, being 0 otherwise. In the first part of my talk, I will consider the exact structural and thermodynamic properties of the d-dimensional PS model in the combined limit where the reduced temperature and density go to infinity, its ratio being kept finite. The PS fluid exhibits a spinodal instability with a divergent correlation length at a certain maximum value of the density/temperature parameter, even in the one-dimensional case. A simple free-volume approximate theory for the solid phase allows one to get estimates of the freezing and melting densities. In the second part of the talk a simple analytic theory for the structure of the low-temperature one-dimensional PS fluid will be constructed. This theory reduces in the zero-temperature limit to the exact solution for hard rods. Finally, the approximate low-temperature theory and the asymptotically exact high-temperature correlation function are compared with Monte Carlo simulation results for finite temperatures.