BHATTAMISHRA & PADHY: ESTIMATION OF IBUPROFEN SOLUBILIZATION IN SURFACTANT MEDIA
solutions of ibuprofen in water and in surfactant
minor change in the cmc value9 (9.2×10-4 mol dm-3).
solutions of different concentrations were prepared by
The change in the extents of solubilization with
adding excess of the drug to the desired medium. To
change in the concentrations of both SDS and CTAB
ensure saturation solubility, each solution was
is shown in Figs 3 and 4. In either case at pre-micellar
subjected to shaking and this was accomplished in
concentrations, there is an initial rise in solubility with
three different ways; (i) by mechanical means, (ii)
increase in surfactant concentration. The solubility
using a magnetic stirrer or (iii) centrifuging at
reaches a maximum then decreases and rises again at
different rpm to a maximum of 4500 rpm at 27ºC.
about the cmc value of the surfactant. With SDS as
These saturated solutions were left for 24 h to
the surfactant, in the region of [surfactant] > cmc, the
equilibrate before being taken for spectroscopic
solubility increases almost linearly with the increase
estimation. Remi C-24BL Cooling Centrifuge was
in concentration of the surfactant. However, with
used for centrifuging the sample solutions.
CTAB, a second maximum is obtained in the plot of
The electronic spectrum of ibuprofen recorded by
solubility versus [surfactant] in the region [surfactant]
Shimadzu UV-visible spectrophotometer 1700, showed
two peaks at 264 and 272 nm with extinction
The relationship for association of a substrate
coefficients as 299 and 257 mol-1 dm3 cm-1 respec-
molecule in micellar medium is given by,
tively. The concentration of ibuprofen in each
saturated solution, measured (24 h) after mixing, was
determined from its absorbance value at 264 nm. The
conductance values of the saturated solutions of
The equilibrium constant KM, also known as
ibuprofen in water and in surfactant solutions of
Systronics model 306 conductometer with extension conductivity cell, type CD-30 at 27ºC. The pKa values of ibuprofen in aqueous and aqueous-SDS media were determined pH metrically using Systronics µ 361 pH meter with extension glass, calomel electrode and temperature probe.
Results and Discussion 
INDIAN J. CHEM. TECHNOL., SEPTEMBER 2009
Fig. 4 –Solubilization of ibuprofen as a function of [CTAB]
Fig. 5 – Plot of P versus (Dt – cmc) for ibuprofen solubilized in
both [SDS] and [CTAB] media are included in
where [SW] and [SM] stand respectively for the
The presence of maxima in the plots of solubility
solubility of the substrate (ibuprofen) in water and in
versus [surfactant] in the region where [surfactant]
the surfactant medium; [DM] is equal to [D]– cmc, [D]
<cmc, may be explained on the basis of complex
being the molar surfactant concentration in the
formation between the substrate (ibuprofen) and the
medium. This is because below cmc, the surfactant
surfactant monomer11. As a result, the solubility
molecules are largely in the monomeric form but
increases. However, as the micelles start forming, the
above cmc, these take up mostly the micellar form,
substrate-surfactant complex breaks and so the
the monomer concentration being approximately
solubility decreases12,13. The electronic spectrum of
equal to the cmc. The micelle-water partition
ibuprofen in water and in 1.4×10-2 mol dm-3 SDS
coefficient10, which is the ratio of substrate
solution is shown in Fig. 6. There is no shift in
concentration in micellar medium to that in water
absorption maxima and the spectra are by and large
the same. Resemblance of the absorption spectra of a
solubilizate in the micellar medium to that in polar
solvent implies a polar environment of the substrate in
the micelle5. Obviously in an anionic surfactant like
SDS, the linear increase in solubility of ibuprofen
with increase in [SDS] in the post-micellar region
P = K {[D ]−cm }
may be ascribed to incorporation of the former in the
so that the plot of partition coefficient against
The following experiments further establish the site
[Dt]-cmc will be a straight line of slope KM (Fig. 5).
of adsorption of iburprofen in SDS micellar medium.
Since in a micellar medium, the substrate is
When SDS is added to ibuprofen solution an increase
considered to be partitioned between two phases, the
in pH is recorded. The pH change is smaller, in the
micellar phase and the aqueous phase, the standard
pre-micellar region but increases rapidly above the
cmc to reach a saturation value at [SDS] concen-
tration of about 5×10-2 mol dm-3. This implies that the
acid dissociation of ibuprofen is hindered by the
ibuprofen in a number of SDS solution with SDS
where Pm is the molar partition coefficient between
>cmc have been measured pH-metrically. There is a
steady, almost linear increase in pKa values with
BHATTAMISHRA & PADHY: ESTIMATION OF IBUPROFEN SOLUBILIZATION IN SURFACTANT MEDIA
Table 1—Solubility of ibuprofen in aqueous solutions of SDS and CTAB at 27ºC.
a: Either hand shaken or magnetically stirred b: Centrifuged at 4500 rpm.
Table 2—Variation in the pKa value of ibuprofen with
carboxylate anion of ibuprofen may compete with the anionic micelle for the H+ ion to form the neutral polar ibuprofen molecule, which then easily gets attracted to the micellar surface. This in turn explains
both the observed phenomenon i.e. the increase of pKa
value and solubility of ibuprofen with increasing SDS
Fig. 6 – Electronic spectra of ibuprofen in water and in SDS solution (1.4×10-2 mol dm-3).
The solubility pattern of ibuprofen in CTAB up to
increasing [SDS] (Table 2), which further supports
the cmc is explained exactly in a similar manner as in
the above view that SDS micelles hinder deproto-
nation. Several factors may be attributed to this
concentration increases, the concentration of the
behaviour14. Firstly, there may be a concentration
ibuprofen increases in the micellar phase and so the
gradient of protons between the bulk and interface.
solubility increases. The cationic micellar surface
Again the dielectric constant on the micellar surface is
attracts the carboxylate anion of ibuprofen whose
different from that of the bulk; the same on the
formation is facilitated by formation of HBr, a
micellar interface of SDS has been estimated14,15 as
stronger acid compared to ibuprofen. Here the
32. In a medium of low dielectric constant compared
electrostatic force is more effective compared to that
to bulk water, acid dissociation is likely to be
in SDS medium. This is evident from the decrease in
hindered. Further, an anionic micellar surface like that
pKa value of ibuprofen with increase in [CTAB]. But
of SDS is expected to attract positively charged
after micelles incorporate all the reactant molecules,
species or polar neutral compounds. In such a
further increase in micellar concentration only
case rather protonation will be encouraged. The
produces dilution effect and so the solubility
INDIAN J. CHEM. TECHNOL., SEPTEMBER 2009
decreases forming the second maximum in the
at concentration of about 5×10-3 mol dm-3 and
5×10-4 mol dm-3 for SDS and CTAB surfactants
Table 1 shows some interesting results. When
respectively. As a result, the solubility of ibuprofen
centrifuged, solubility of ibuprofen in water increases
decreases. The release of dissolved ibuprofen at a
greatly, but its solubility in SDS solution of any
particular concentration of the surfactant is extremely
concentration is greatly suppressed compared to hand
useful in drug delivery. However, the temperature and
shaken or magnetically stirred solubility. The
pH effects on solubility of ibuprofen in surfactant
decrease in the rate of increase of solubility on
media need to be conducted for better understanding
centrifuging, is found to be directly proportional to
the rpm value. It is felt that churning of the surfactant
solution, especially at high rpm, most likely prevents
References Source: https://communities.acs.org/servlet/JiveServlet/previewBody/5517-102-1-6446/IJCT.pdf
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