1076
C. T. MARTINS, M. S. LIMA AND O. A. EL SEOUD
(Eqn (6)) was carried as given elsewhere.8e The solvent
and excess n-butyl iodide were removed and the product
(light amber liquid, free of 4-methylquinoline) was used
without further purification. Condensation of this iodide
with 3,5-dibromo-4-hydroxybenzaldehyde in the pre-
sence of piperidine (Eqn (7)), followed by treatment
with KOH8c and recrystallization from ethanol–acetone
(1:1), gave BuQMBr2 as green–violet crystals, yield 80%,
m.p. ¼ 235–237 ꢁC. Calculated for C21H19Br2NO (%): C,
54.29; H, 4.15; N, 3.04. Analyzed (%): C, 54.29; H, 4.22;
N, 3.01. IR (KBr, cmꢀ1): 3069, 2958 (ꢁC—H); 1594
aliquot of this solution was agitated with 4 ml of n-octanol-
saturated buffer at room temperature for 2 h. After phase
separation at 25 ꢁC, the absorbance (Aequilibrium) of the
n-octanol phase was measured and the partition coefficient
was calculated from: log P ¼ log (Aequilibrium ꢂ 4/
(Ainitial ꢀ Aequilibrium)ꢂ 0.7); log PBuQMBr was found to
2
be 2.51 ꢃ 0.05.
Spectrometric determination of pKa of BuQMBr2
1
—
(ꢁC—
Table 1.
C
); 1201 (ꢁC—N), 1041 (ꢁC—Br). For H NMR, see
The pKa was calculated from the Henderson–Hasselbach
equation. Solutions of the probe (final con-
centration ¼ 5 ꢂ 10ꢀ4 mol lꢀ1) were prepared in a potas-
sium hydrogen phthalate buffer (0.05 mol lꢀ1) and the
concentrations of the zwitterionic form were measured at
490 nm at 25 ꢁC. The pKa of BuQMBr2 at this ionic
strength was found to be 4.89 ꢃ 0.02.
Sample preparation and spectrometric
determination of ET
Binary mixtures (16 per set) were prepared by weight at
25 ꢁC. Probe solution in acetone was pipetted into 1-ml
volumetric tubes, followed by solvent evaporation under
reduced pressure over P4O10. Pure solvents and/or binary
solvent mixtures were added, and the probe (final con-
centration 2–5 ꢂ 10ꢀ4 mol lꢀ1, was dissolved. The UV–
Vis spectra of probe solutions showed no changes in ꢀmax
and/or spectrum shape as a function of probe concentra-
tion in the range 10ꢀ4–10ꢀ3 mol lꢀ1. This was taken to
indicate that no intermolecular probe interactions occur
under our experimental conditions. A Beckman DU-70
UV–Vis spectrophotometer was used. The temperature
inside the thermostatted cell-holder was controlled to
within ꢃ 0.05 ꢁC with a digital thermometer (model
4000A, Yellow Springs Instrument, Ohio, USA). Each
Determination of the polarity of interfacial
water of SDS micelles
The probe solution in acetone (0.1 ml, 5 ꢂ 10ꢀ3 mol lꢀ1
)
was pipetted into 1-ml volumetric tubes, followed by
solvent evaporation under reduced pressure. The volumes
then were made up to the mark with aqueous SDS
solutions. Values of ꢀmax were found to be practically
constant at 542.5 ꢃ 0.5 nm as a function of [SDS] in the
concentration range of 0.016–0.204 mol lꢀ1. The zwitter-
ionic form of BuQMBr2 was present in SDS solutions
without the addition of base, whereas the corresponding
form of MePM appeared at a (bulk) solution pH of 12.8.
The polarity of interfacial water was found to be
spectrum was recorded twice at a rate of 120 nm minꢀ1
;
the values of ꢀmax were determined from the first deri-
vative of the absorption spectra. The uncertainties in
ET(BuPM), ET(BuPMNO2) and ET(BuQMBr2) are
0.1 kcal molꢀ1. The temperature range investigated was
dictated either by the b.p. of the solvent (MeOH, 64.5 ꢁC)
or its m.p. (2-Me-2-PrOH, 25.5 ꢁC). Stable absorbance
readings were observed for probe solutions in the latter
alcohol at 25 ꢁC, probably because its m.p. is depressed
by the solute and by the low atmospheric pressure in the
city of Sa˜o Paulo.
52.7 kcal molꢀ1
.
RESULTS AND DISCUSSION
Comment on the structure of the probe
The BuPM probe was synthesized to evaluate the effect
of increasing the hydrophobic character of the merocya-
nine structure on its solubility in organic solvents. Indeed,
this probe was found to be soluble in THF, 1,4-dioxane
and chloroform, i.e. in solvents where MePM is not
soluble. In order to decrease the probe pKa, a strong
electron-attracting group (NO2) was introduced into
the phenolate moiety. The expected pKa of BuPMNO2
is 5.6, based on the pKa of MePM (8.37) and those of
4-hydroxybenzaldehyde (7.66) and 3-nitro-4-hydroxy-
benzaldehyde (4.9). The new probe was found to be
soluble in the same solvents as BuPM, although its
solvatochromism was much less, as shown by values of
ꢀmax, pyridine ꢀ ꢀmax, ethanol ¼ 31 and 93 nm for BuPMNO2
and BuPM, respectively. This decreased solvatochro-
mism is due to the competition of the nitro group and
Spectrometric determination of log P, the
partition coefficient of the probe between
water and n-octanol
The definition of this coefficient is: log P ¼ [probe]n-octanol
/
14
[probe]water.
The aqueous phase was a phosphate buffer
solution (0.05 mol lꢀ1, pH 7.50). Equal volumes of this
buffer and n-octanol were agitated for 1 h (tube rotator) and
the phases were separated.
A
probe solution
(5 ꢂ 10ꢀ4 mol lꢀ1) in buffer-saturated n-octanol was pre-
pared and its absorbance (Ainitial) was measured. A 0.7 ml
Copyright # 2005 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2005; 18: 1072–1085
Martins, Clarissa T.
