Preferential Solvation in Mixed Solvents - Fluctuation Theory of Solutions - page 83

termed solvents . KBIs have been obtained, for instance, for mixtures of argon and
krypton (Matteoli 1997), from which slight self-preference was deduced for both
components, but this is not further elaborated here.
In the following, mixtures of methanol, ethanol, acetone, and triethylamine,
marked as solvent A , each with many cosolvents marked as B (Marcus 1991) are
described. Mixtures of 1,4-dioxane (A) (Marcus 2006b) and of tetrahydrofuran (A)
(Marcus 2006a) with cosolvents (B) are also presented as well as a few studies by
others. It is, of course, arbitrary which solvent is marked as A and which as B . The
numerical values of the preferential solvation parameter extrema quoted below were
read from figures and have an uncertainty of ±10%.
Methanol and cosolvents. Preferential solvation curves δ x AA for the self-
association of methanol, A, and δ x AB for preferential solvation of the cosolvent B
by methanol (neither being volume-corrected) were obtained (Marcus 1991) from
thermodynamic data. For B = benzene and toluene, δ x AA (max) ~ 0.50 in the first
solvation shell (for toluene, even in the second shell δ x AA (max) = 0.35), for B = nitro-
methane, δ x AA (max) = 0.40, and smaller values were obtained for B = sulfolane,
δ x AA (max) = 0.20, for B = chloroform, δ x AA (max) = 0.16, and for B = acetonitrile,
δ x AA (max) = 0.08. Correspondingly, the solvation of the cosolvent by methanol is
disfavored. For B = benzene, δ x AB (min) = -0.25, for B = toluene and nitromethane,
δ x AB (min) = -0.30, for B = sulfolane, δ x AB (min) = -0.12, and for B = chloroform and
acetonitrile, δ x AB (min) = -0.09. Smaller values are obtained for B = tetrahydrofuran:
δ x AB (min) = -0.06 whereas for the self-association of methanol, δ x AA (max) = 0.01
only. For B = acetone, the δ x AA curve is S-shaped but with very small values, >0 in
methanol-rich mixtures and <0 in acetone-rich ones, and δ x AB (min) = -0.042. For
some other systems, results were reported qualitatively, in that mixtures of metha-
nol with B = chlorobenzene resemble those with B = acetonitrile, those with B =
pyridine do not have appreciable preferential solvation, those with B = formamide
resemble those with tetrahydrofuran mentioned above, whereas those with B = N , N -
dimethyl- and N , N -diethylformamide have positive mutual solvation, δ x AB > 0 and
disfavored self- association, δ x AA < 0, see also Matteoli (1997). Mixtures of methanol
and B = 1-decanol (Matteoli 1997) showed the expected effect of the large differ-
ence in the molar volumes of the components (their ratio is ~5). The Δ G BB integrals
are positive, contrary to the negative G BB integrals, hence some self-association of
the decanol in the presence of the methanol, due to dispersion forces between the
alkyl chains, is manifested.
Ethanol and cosolvents . Preferential solvation curves δ x AA and δ x AB (not volume-
corrected) for A = ethanol and several cosolvents, B, were obtained (Marcus 1991). For
all the mixtures, self-association of the ethanol is favored whereas mutual solvation
is disfavored. For instance, when B = n- heptane, δ x AA (max) = 0.40; for B = sulfolane,
δ x AA (max) = 0.32; for B = nitromethane, δ x AA (max) = 0.22; for B = benzene, δ x AA (max)
= 0.16; for B = acetonitrile, δ x AA (max) = 0.10; and, for B = chloroform, δ x AA (max)
= 0.06, in descending order. The mutual solvation parameter extrema are: for B =
n- heptane δ x AM (min) = -0.12; for B = sulfolane δ x AM (min) = <-0.30; for B = nitro-
methane δ x AB (min) = -0.28; for B = benzene, δ x AB (min) = -0.25; for B = acetonitrile,
δ x AB (min) = -0.12; and, for B = chloroform, δ x AB (min) = -0.08. For some other systems,