The compressibility equation for soft-matter liquids

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Effective interactions in soft-matter physics result from a formal contraction of an initial multicomponent system, composed of mesoscopic and small particles, into an effective one-component description. By tracing out in the partition function the degrees of freedom of the small particles, a one-component system of mesoscopic particles interacting with a state-dependent Hamiltonian is found. Although the effective Hamiltonian is not in general pairwise additive, it is usually approximated by a volume term and a pair-potential contribution. In this paper the relation between the structure, for which the volume term plays no role, and the thermodynamics of a fluid of particles interacting with a density-dependent pair potential is analysed. It is shown that the compressibility equation differs from that of atomic fluids. An important consequence is that the infinite-compressibility line derived from the thermodynamics does not coincide with the spinodal line stemming from the divergence of correlations.
© 2003 IOP Publishing Ltd. Liquid Matter Conference (5. 2002. Constance, Germany). The author acknowledges financial support from the Ministerio de Ciencia y Tecnología (Spain), reference: BFM2001-1017-C03-03. I wish to thank M Baus, G Ruiz, E Lomba and N G Almarza for many useful discussions and A A Louis for communicating with me prior to publication.
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