Characteristics of Pure Substances

Vapor pressure p is a function of temperature T only. The vapor pressure curve provides information about the behavior of pure substances. In Fig. 2.1-1, the vapor pressure curves of water, benzene, and naphthalene are depicted. Vapor pressure curves show a bend at the triple point TP and end at the critical point CP. 

The pressure that is reached at a given temperature of a closed, equilibrated two-phase system (L/G) of a pure substance is called vapor pressure p. The irreversible exchange processes within the system come to rest if temperature and pressure of liquid and gas are in equilibrium. Then the entropy of the entire system reaches its maximum. 

At equilibrium, aity small changes in the state variables T and p are reversible and therefore 

The pressure reached in equilibrium is denoted vapor pressure p (of the pure component). From this and with Gibbs fimdamental equation, which is valid for each of the phases, it follows that 

Sufficiently far from the critical point (T T, and p° ) it follows that 

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In some polymer-nonpolar solvent systems, % has been calculated as a function of concentration on the basis of the statistical-thermodynamical theory called the equation of state theory [13,14]. This semiempirical theory takes into account not only the interaction between solute and solvent, but also the characteristics of pure substances through the equations of state of each component. At present, however, we cannot apply this approach to such a complex case as the NIPA-water system. Thus, at the present stage, we must regard % as an empirical parameter to be determined through a comparison between calculated and experimental results. The empirical estimation of % for the NIPA-water system will be described in the next section. 

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