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Br micelles with log
K
s
values of 3.3-3.9, and greater than 80% of the applied
sulfometuron (11) and sulfosulfuron (12) was incorporated (Mishael et al. 2002,
2003). The effect of electrostatic interactions on solubilization was examined for
neutral and protonated fuberidazole (14), and the protonated species was found to
be markedly incorporated into anionic micelles (Lopes et al. 1992). In the case of
the ionizable 2,4-D (26), electrostatic attraction operates in its adsorption to micel-
lar flocculates of SDS-Al
3+
(Porras-Rodriguez and Talens-Alesson 1999). Enhanced
solubilization has been reported for some pharmaceuticals (Chowdary and Rao
2001; de Castro et al. 1999; Gerakis et al. 1993; Sheng et al. 2006).
B Dissociation
When a chemical is incorporated into micelles, it experiences a different surround-
ing environment from the bulk water phase, with an influence of electrostatic
micellar surface potential, especially for ionic surfactants. This different environ-
ment would cause a change of dissociation profiles of an ionizable chemical, which
has been investigated by potentiometric and spectrophotometric analyses. Through
spectrophotometric analysis of p
K
a
for dyes in micelles of quaternary alkylammo-
nium surfactants, nonelectrostatic and electrostatic effects were found to be impor-
tant (Mukerjee and Banerjee 1964; Pourreza and Rastegarzadeh 2005). The former
effect by micelles can be evaluated by an effective dielectric constant (
e
eff
) obtained
through comparison of the dissociation constant in a water-organic solvent mixed
at various proportions with that in an aqueous micellar system (Drummond et al.
1989). The
e
eff
values were estimated to be approximately 35 for cationic DDTMA
Br, nonionic Brij 35, and
n
-dodecyloctaoxyethylene glycol monoether micelles,
which are in good agreement with those estimated by the solvatochromic UV
absorption study. In the case of the nonionic surfactant, the
e
eff
value shows that the
polyethoxy moiety is associated with 0.5-1 water molecules. The latter effect is
important for the micelles consisting of ionic surfactants, and the observed dissoci-
ation constant (p
K
a
obs
) can be described (see below) by using the p
K
a
value without
an electrostatic effect (p
K
a
o
), surface potential (
j
), and Boltzmann's constant (k
B
)
(Drummond et al. 1989; Saha et al. 1994).
p
K
a
obs
= p
K
a
o
−
e
j
/2.303k
B
T
There have been many investigations on the dissociation process effected by
micelles for simple organic acids and amines together with some pharmaceuticals.
The typical effects on the dissociation are summarized in Table
4. The p
K
a
value of
benzoic acid decreased by 0.56 in cationic HDTMA Br micelles but increased by
0.4 in the nonionic Tween 80 as compared with that in a bulk water phase (Gerakis
et al. 1993). The decrease of p
K
a
in the cationic micelles means that the equilibrium
is shifted to ionization by stabilization of the carboxylic anion by the (CH
3
)
3
N
+
-alkyl
cation, as demonstrated by
1
H-NMR study (Bunton and Minch 1974). As the
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