Molecular basis for mixture behavior

The relations between excess properties and property changes of mixing (Sec. 12.3) facilitate discnssionof the molecular phenomena which give rise to observed excess-property behavior. An essential coimectionis provided by Eq. (12.33), which asserts the identity of and AH. Tims we focus on the mixing process (and hence on for explainingthe behavior of.  

In Secs. 16.1 and 16.5 we identify intemiolecular attractive interactions of four kinds dispersion, induction, direct electrostatic, and quasicheniical. A sununaiy list of important points follows  

With these notions in mind, we offer some rationalizations of the observed signs and magnitudes of AH) for binary liquid mixtures of the kinds discussed in Sec. 16.6. 

For tliis class of mixtures, interactions between molecules of like species are different in kind for the two species. In particular, two molecules of the polar species experience a direct-electrostatic interaction and a (usually weak) induction interaction, in addition to the usual dispersion interaction here, the attractive forces are stronger than would be observed for a nonpolar species of similar size and geometry. Interactionbetween uiihke species, on the other liand, involves only the dispersion and (weak) induction forces. One therefore expects to be positive, only more so than for otlierwise similar NPNP mixtures. Experiment bears tliis out, on average (Fig. 16.5). 

As for NPNP and NA/NP mixtures, one expects positive H tliis is wliat Fig. 16.5 shows. However, is often observed to be only modest in magnitude, frequently less than for 

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