Fugacity

Introduction

The fugacity was introduced by G. N. Lewis in 1901, and became widely used after the appearance of Thermodynamics, a very influential textbook by Lewis and Randall in 1923. Lewis describes the need for such a function in terms of an analogy with temperature in the attainment of equilibrium between phases. Just as equilibrium requires that heat must flow such that temperature is the same in all parts of the system, so matter must flow such that chemical potentials are also equalized. He referred to the flow of matter from one phase to another as an “escaping tendency,” such as a liquid escaping to the gas form to achieve an equilibrium vapor pressure. He pointed out that in fact vapor pressure is equilibrated between phases under many conditions (and in fact is the basis for the isopiestic method of activity determinations, §5.8.4), and could serve as a good measure of escaping tendency if it behaved always as an ideal gas.

The chemical potential is of course another measure of “escaping tendency,” but Lewis pointed out that there are “certain respects in which this function is awkward.” This refers to the fact that µ → −∞ as a → 0, because activity is defined as µ − µ° = RT ln a. Lewis defined a function which would be much like a vapor pressure, which would be equilibrated between phases at equilibrium, even in nonideal cases, and even if no vapor phase actually existed.