Callen Postulate II

Question

In his classic text, Callen gives the key postulate (Postulate II) as

There exists a function (called the entropy $S$) of the extensive parameters of any composite system, defined for all equilibrium states and having the following property: The values assumed by the extensive parameters in the absence of an internal constraint are those that maximize the entropy over the manifold of constrained equilibrium states.

My question is, should this postulate be tacitly understood to apply only to closed (isolated) systems?

Answer 1

You are right. Callen's postulate II applies to closed systems. Even though Callen does not explicitly mention this requirement in its postulate, it should be clear by reading previous pages.

Indeed, Postulate II is in section 1.10 (I am referring to the second edition of his book). The starting paragraph clearly states that the postulates provide the simplest formal solution to the basic problem of thermodynamics. The basic problem explicitly defined in the previous section is the determination of the equilibrium state that eventually results after the removal of internal constraints in a closed, composite system (bold type is mine).