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The Behavior of a Pair of Partially Miscible Liquids
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Abstract
In this paper, the fundamental reasons for liquids not mixing are carefully examined, along with the question of the compositions of the co-existent phases. This is developed using only the general criterion that, at equilibrium, the Gibbs energy of the system should be minimised, and assuming that the liquid mixtures behave as "regular solutions".
On this basis, it is first shown that, where two phases are formed, each contains a finite if small amount of the other component. This results from the fact that, at mole fractions very close to zero, the entropy of mixing term will always outweigh that from the enthalpy of mixing. Also, it is shown that some of the known effects of temperature on a two-phase system can be replicated simply by evaluating DeltaG = DeltaH - T DeltaS for the mixing process. As the temperature is increased, the compositions of the two phases present are shown to become closer until they eventually coalesce at the upper critical solution temperature. The occurrence of a lower critical solution temperature can not be rationalised on this basis, but it is known to occur only where there are more specific attractive interactions between the molecules of the two components, so that the "regular solution" assumptions are not valid.
Keywords (Audience):
Upper-Division UndergraduateKeywords (Domain):
Physical ChemistryKeywords (Subject):
Phases / Phase Transitions / DiagramsTools
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- Received: August 03, 2009
ACS
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