Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.3 Physical properties of mixed solvents 519
without chemical interaction between components. The latter isotherms are calculated from
equations [9.24] - [9.26], (Figure 9.2).
Type I isotherm corresponds to the experimental isotherm with isotherms [9.24] or
[9.26] of the liquid systems without interaction. The additive ε increase is compensated by ε
decrease, due to homomolecular association process. The system diethyl ether - m-cresol illustrates
this type of isotherm.
Type II is determined by the negative deviations from isotherm [9.26]. This type corresponds
to systems with weak heteromolecular interactions between components, but with
strong homomolecular association of one of the components. The system formic acidanisole
is an example of this kind of isotherm. Furthermore, these isotherms are characteristic
when non-associating components in pure state form heteromolecular associates with
lower dipole moment, DM, then DM of both components. The average DM for such kind of
interaction in mixed systems is lower than correspondent additive value for non-interacting
system. The system 1,2-dichloroethane - n-butylbromide can be referenced as an example
of this kind of mixed binary solvent.
Type III combines εisotherms, which are above the isotherm calculated from equation
[9.26]. This kind of isotherm suggests interaction between components of mixed solvent.
The variety of such systems allows us to distinguish between five isotherm subtypes.
Subtype IIIa is represented by isotherms lying between the curve obtained from eq.
[9.26] and the additive line. This isotherm subtype occurs in systems with low value of
heteromolecular association constant. The system n-butyric acid - water can be mentioned
as a typical example of the subtype.
Subtype IIIb combines S-shaped ε isotherms. This shape is the result of coexistence of
homo- and heteromolecular association processes. System pyridine-water is a typical example
of this subtype.
Subtype IIIc combines most common case of ε isotherms - curves monotonically con-
2 2
vex from the composition axis (∂ε / ∂V≠ 0, ∂ ε / ∂V
0).
Chemical interaction influences the shape of viscosity isotherm
that is typical when experimental isotherm is situated above the curve calculated under
assumption of absence of any interaction (i.e., from equation [9.31a]). The means of
increasing heteromolecular association and determination of stoichiometry of associates for
this type of isotherms was discussed elsewhere. 3 Piperidine-aniline system is an example of
this kind of interacting system.