Handbook of Solvents - George Wypych - ChemTech - Ventech!

9.3 Physical properties of mixed solvents 521
nents. System trifluoroacetic acid-acetic acid is common example. The true stoichiometry
determination method for this case was described elsewhere. 3
Subtype IIIc - isotherms with maximum at rational stoichiometry, which corresponds
to composition of heteromolecular associate. This case can be exhibited by the system
pyrosulfuric acid-monochloracetic acid.
Occasionally, isotherms with singular maximum can also occur. This behavior is characteristic
of the system mustard oil-amine.
This classification covers all basic types of viscosity isotherms for binary mixed systems.
Although the classification is based on geometrical properties of isotherm,
heteromolecular associations determine specific isotherm shape and its extent. The relative
level of interactions in binary mixed systems increases from systems with isotherms of type
I to systems with type III isotherms.
9.3.2.3 Density, molar volume
Unlike the dependencies of density deviation (from additive values) on system composition,
correspondent dependencies of specific (molar) volume deviation from the additive
values:
[ ( ) ]
Δθ = θ − θ − θ x + θ
[9.41]
exp 1 2 2
where:
Δθ departure from of molar volume
θexp experimental value of molar volume
θi molar volumes of components
x molar fraction of the second component
allow us to make a meaningful assessment of the heteromolecular association
stoichiometry. 3 Classification of volume-dependent properties of binary mixed solvents are
based on dependencies of Δθ value (often called “excessive molar volume”, θ E ).
It is evident both from “ideal system” definition and from equation [9.36], that for
non-interacting systems Δθ ≈ 0.
Interaction is accompanied by formation of the heteromolecular associates. It can be
demonstrated by analysis of volumetric equations for the liquid mixed systems, data on volume
compression, i.e., positive density deviation from additivity rule, and hence negative
deviations of experimental specific molar volume from partial molar volume additivity
rule.
There are three basic geometrical types of Δθ isotherms for binary liquid systems with
chemically interacting components 12 (Figure 9.4.).
Type I - rational Δθisotherms indicating that only one association product is formed in
the system (or that stability constants for other complexes in system are substantially lower
than for main adduct). Most often, adduct of equimolecular composition is formed, and the
maximum on Δθ isotherm occurs at x = 0.5. Fluorosulfonic acid-trifluoroacetic acid system
is an example of the type.
Type II - irrational Δθ isotherms with maximum not corresponding to the
stoichiometry of the forming associate. This irrationality of Δθ isotherms indicates formation
of more than one adduct in the system. In the case of formation of two stable adducts in
the system, the Δθ maximum position is between two compositions which are
stoichiometric for each associate. If in the system, two adducts AB and AB2 are formed,