Spectrofluorometric Probe Method for Examining Preferential Solvation in Binary Solvent Mixtures Page: 1,175
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p. B
A "
00
360 410 460z
wWAVELENGTH (nm)
FIG. 2. Fluorescence emission behavior of 3,4-dibenzo [ghi] perylene
dissolved in neat n-heptane [(A) (-)], in neat 1,4-dioxane [(B)
(- @ -)], and in a binary n-heptane-dioxane mixture [(C) (- @ -)]
having a stoichiometric volume fraction composition of <pehptn.e = 0.50.
Spectrum D (-) represents the emission spectrum calculated with
the use of Eqs. 6 and 7, with a preferential solvation mole fraction of
Yhept.n. = 0.74. After normalization to a common band intensity, spectra
C and D are superimposable.
manipulation software. The fluorescence emission spec-
tra are recorded for the PAH probe dissolved in both
pure solvents, and the measured I and II band intensities
are substituted into the numerator and denominator,
respectively. "Trial" values of YA and 1 - YA are com-
puted from the measured I/II ratio for each binary sol-
vent composition studied. These trial values are then
used in Eq. 6 to generate the calculated fluorescence
emission spectra, which are compared to the observed
data. Careful attention is given to ensure that the entire
detailed emission fine structure (wavelengths and all in-
tensity ratios) is correctly produced, rather than just the
experimental I/II ratio. If necessary, the trial values can
be adjusted repetitively until the "best possible" agree-
ment between observed and calculated spectra is
achieved, as shown in Fig. 2. Spectra A and B in this
figure represent the emission intensities of 3,4-dihydro-
benzo[ghi]perylene dissolved in neat n-heptane and 1,4-
dioxane, respectively, scanned from 360-460 nm. A trial
value of Yheptane = 0.74 reproduced very accurately the
observed dihydrobenzo[ghi]perylene emission spectra at
the binary solvent volume fraction composition of kheptane
= 0.50 after normalization to a common band intensity
(spectra C vs. spectra D in Fig. 2). Normalization corrects
for small differences in experimental conditions, such as
fluorophore concentration, ambient room temperature,
and excitation source intensity, which may occur during
any given series of fluorescence measurements. In most
cases there was less than a 1 % difference between the
observed and calculated spectra after normalization.
Figure 3 summarizes results of our preferential sol-
vation computations for 3,4-dihydrobenzo[ghi]perylene
dissolved in binary n-heptane + 1,4-dioxane and dibutyl
ether + acetonitrile mixtures. The nine stoichiometric
binary compositions are represented as both mole (@)C
'l "
D
360 410 4601001
1.0
0.8
0.6
0.4-
0.2
0.01
1.0
0.8
0.6
0.4
0.2
6.6
0.0
0..0
0.2 0.4 0.6 0.8 1.0
4) Dibutyl ether
or XDibutyl ether
FIG. 3. Preferential salvation of 3,4-dihydrobenzo[ghilperylene dis-
solved in n-heptane + 1,4-dioxane (upper set of curves) and dibutyl
ether + acetonitrile (lower set of curves) mixtures. Numerical values
of Yheptane and Ydihutyrethe, were calculated from Eq. 7 with the use of
observed I (at - 380)/Il (at - 390) emission intensity ratios. The x-axis
denotes the stoichiometric mole (@) and volume (U) fraction compo-
sitions of the binary solvent.
and volume (U) fractions. Uncertainties assigned to the
various Yheptane and Ydibutyl ether values, indicated by the
error bars, were based upon the reproducibility of the
emission intensity ratios (circa 0.02 or so) and the range
of possible "trial" values that predicted identical I/II
ratios. For example, if numerical values of Iheptane = 48.3,
Idioxane = 365.2, Iheptane = 43.2, and Idioxane = 207.6 for 3,4-
dihydrobenzo[ghi]perylene are substituted into Eq. 6,
then weighted fractions ranging from Yheptane = 0.17 to
Yheptane = 0.21 give exactly the same intensity ratio of I/II
= 1.73 when rounded to the second decimal place.
Examination of Fig. 3 reveals that the computational
method returned fairly smooth curves for how the degree
of presumed preferential salvation varies with solvent
composition. In the ideal case, where the solvational
sphere micro-environment is governed exclusively by the
relative mole numbers of both solvent components, the
local composition around the solute probe should equal
the solution's stoichiometric mole fraction. For binary
n-heptane + 1,4-dioxane mixtures there is an unex-
pectedly large local composition of the nonpolar n-hep-
tane cosolvent around the polycyclic aromatic hydrocar-
bon probes. Naively, one would have expected that dipole-
induced dipole interactions involving the oxygen lone
electron pairs on the ether and the PAH's polarizableAPPLIED SPECTROSCOPY 1173
4Heptane or XHeptane
SII
rS I I
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0
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TC
V-
0)
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Acree, William E. (William Eugene); Wilkins, Denise C. & Tucker, Sheryl A. (Sheryl Ann). Spectrofluorometric Probe Method for Examining Preferential Solvation in Binary Solvent Mixtures, article, August 1, 1993; [Frederick, Maryland]. (https://digital.library.unt.edu/ark:/67531/metadc699832/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.