MacInnis et al.
341
Fig. 1. Structural formula of alkyl sulfosuccinate surfactants
(R = alkyl group).
The luminescence probing experiments to obtain the
pyrene I1/I3 ratios in the sulfosuccinate micellar interior
were carried out as follows. Pyrene (Aldrich, 99%) was puri-
fied through sublimation and recrystallization (twice) with
ethanol. The pyrene probe was held at a constant concentra-
tion of 2 × 10–5 M. A small amount of a pyrene–ethanol so-
lution was placed in a small flask, and the solvent was
allowed to evaporate, depositing the pyrene as a monolayer
at the bottom of the flask.
The stock aqueous solution of the surfactant was added to
the flask containing the pyrene; this solution was stirred
overnight to ensure complete dissolution of the pyrene in the
surfactant solution. Steady-state pyrene emission spectra
were recorded on a Perkin–Elmer MPF-66 spectrophotometer,
using an excitation wavelength of 338 nm and scanning the
emission spectra from 350 to 500 nm. The intensities of the
first and third peaks of the emission spectra were recorded
and used to determine the I1/I3 ratio and hence, an estimate
of the micropolarity of the micellar interior.
O
-
+
Na
C
O
CH
2
+
Na
SO
-
CH
3
C
OR
O
spectroscopy, ultra-violet spectroscopy, and acid–base titra-
tions of surface solubilized indicator 7-hydroxycoumarin. As
a first step in determining the micellar properties of the
sulfosuccinate surfactants, as a function of pH, we have also
determined the pKa values of both of the anionic head
groups of the alkyl sulfosuccinates, employing acid–base ti-
trations. Pyrene I1/I3 ratios and the luminescent intensities of
ANS were used to determine the polarity of the microenvir-
onment of the micelle. The measurement of the pKa of
hydrophobic indicator, 7-hydroxycoumarin, at the sulfosuc-
cinate micellar surface, was used to determine the micellar
surface potentials. All of these properties are discussed in
terms of the influence of having two negatively charged
head groups on the surfactant chain. Nuclear magnetic reso-
nance spectroscopy was also used to investigate the location
of a typical aromatic probe (benzene) in the surfactant mi-
celles.
The ANS polarity studies were carried out as follows: 8-
anilino-1-naphthalenesulfonic acid ammonium salt was pur-
chased from Aldrich and was used as received. A stock solu-
tion of the probe (10–5 M) was prepared in absolute ethanol.
Micellar solutions of ANS were prepared by taking an
aliquot of the stock solution and adding the surfactant solu-
tion. The resulting solutions were sonicated for 5 min. Fluo-
rescence measurements were made using an excitation
wavelength of 389 nm and the emission spectra was ob-
tained from 400 to 525 nm.
The nmr spectra (1H and proton decoupled 13C) were ob-
tained on a Bruker AC-200. D2O was used as a solvent for
1
all of the sulfosuccinate experiments. The H spectra were
referenced to the HOD peak (δ = 4.81 ppm), while the 13C
chemical shift signals were referenced to the deuterium lock
signal. Two sets of nmr experiments were carried out to de-
termine the location of a typical aromatic probe (benzene) in
the surfactant micellar interior.
(a) Benzene solubilization studies: The alkyl sulfosucc-
inate solutions were prepared, at a concentration of twice the
critical micelle concentration. To 1 mL of each of these solu-
tions was added 10 or 20 µL of benzene (Aldrich), and the
mixture was stirred for 30 min.
(b) Surfactant Studies: At least five solutions, up to twice
the critical micelle concentration were prepared in D2O. To
1 mL of each of these solutions was added 10 mL of ben-
zene.
Surface potential measurements were carried out as fol-
lows. Sodium dodecyl sulfate (Aldrich, 99%) was
recrystallized twice from absolute ethanol. The nonionic
surfactant octaethylene glycol decyl ether (C10E8) was sup-
plied by Fluka and was used without further purification.
The acid–base indicator 7-hydroxycoumarin was obtained
from Aldrich Chemical Co. and was used without further
purification. The pH was adjusted by NaOH (A.C.S. Re-
agent Fisher Scientific) or HCl (A.C.S. Reagent Fisher Sci-
entific). NaCl (A.C.S. Fisher Scientific) was used to adjust
the ionic strength of the surfactant solutions. Solutions were
titrated from high pH to low pH by the addition of small
amounts of NaOH or HCl solutions (0.10 M). After the addi-
tion of either NaOH or HCl the pH of the solution was re-
corded, and the degree of ionization of the indicator was
The alkyl sulfosuccinates were synthesized using the fol-
lowing general procedure. Equimolar amounts of maleic an-
hydride and an alcohol were heated for 20 min, with
constant stirring, in a 500 mL round bottom flask. An
equimolar amount of an aqueous sodium metabisulfite solu-
tion was added, and the mixture was neutralized to pH 7–
9. The mixture was then allowed to reflux for 20–24 h. The
progress of the reaction was monitored by running a UV
spectrum on the reaction mixture to confirm the absence of
the maleic anhydride double bond. The resulting surfactants
were purified through Soxhlet extraction with diethyl ether
for at least 72 h. The structures of these surfactants were
confirmed by nuclear magnetic resonance and FT-IR spec-
troscopy.
The pKa values of the respective anionic head groups
were determined as follows. Aqueous surfactant solutions,
with concentrations twice the critical micelle concentration,
were titrated with standardized sodium hydroxide (0.10 M,
Fisher Scientific A.C.S.) and were used to obtain the titra-
tion curve data. The pH of the solution was measured using
a glass electrode and a Cole–Palmer Digi-Sens pH/mv me-
ter. The electrode was calibrated using pH 4 and 10 buffer
solutions (Fisher Scientific). The pH values were reproduc-
ible to ±0.15 pH units.
© 1999 NRC Canada