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with the ethoxy units of surfactant. The importance of a core volume was
thermodynamically shown for alkyl sulfate surfactant micelles (Abu-Hamdiyyah
and Rahman 1987). Similar estimation of their solubilization sites has been reported
by using the changes in 1 H-NMR chemical shifts of HDTMA Br micelles (Eriksson
and Gillberg 1966).
In contrast, polar molecules are considered to be mainly solubilized in the Stern
layer or the micelle-water interface. Comparison of an triplet decay in phosphores-
cence between homogeneous organic solution and micelles has shown that dipolar
4-nitro-4'-methoxystilbene experiences a microenvironment of moderate polarity
similar to alcohol and glycerol which is the likely environment of a micelle-water
interface (Schanze et al. 1985). The characteristic n
p * absorption with a shielding
effect in 1 H-NMR has been conveniently used to estimate the solubilization site of
a benzoyl moiety in micelles. Both the dependency of the n
p * wavenumber of the
benzoyl moiety on the solvatochromic E T (30) value describing a medium polarity,
and the changes in proton chemical shifts of polar head, methylene, and methyl
groups of surfactant, have clearly shown that the aromatic moiety is solubilized from
the core to Stern layer with the carbonyl group oriented toward the micellar surface
(Fendler et al. 1975). The similar orientation of a molecule in the micellar solubili-
zation by HDTMA Br was reported for phenyl alkanoate derivatives where the long
alkyl chain protrudes to the core with the ester moiety located near the micelle
surface (Al-Awadi and Williams 1990; Tee and Fedortchenko 1997).
Amides, carbamates, and ureas are considered to behave similarly to esters in
solubilization to the Stern layer (Broxton and Chung 1986; Broxton and Sango
1983; Mollett and O'Connor 1976), and this type of association was found to cause
the change of a micelle structure with a decrease of cmc via interaction of the sur-
factant head group (Abu-Hamdiyyah and Rahman 1985). Introduction of a carboxyl
group at the o -position of phenyl alkanoate derivatives caused more restricted orien-
tation of the phenoxycarbonyl moiety in the surface of HDTMA Br micelles
(Broxton et al. 1987). Alcohols and phenols have a very polar hydroxyl group, which
would make them locate in the micellar surface. 1 H-NMR studies using SDS and
HDTMA Br micelles together with other spectroscopy have shown that the
molecules having a hydroxyl group are solubilized in the Stern layer of micelles,
concomitant with modification on a micelle structure and microviscosity (Bertolotti
et al. 1989a; Fendler and Fendler 1975; Mata et al. 2006; Suratkar and Mahapatra
2000; Wolfe and von Bünau 1986). The protection of a hydroxyl group by a methyl
resulted in deeper penetration of anisole toward the core but it mainly resided in the
neighborhood of
-carbons from the surfactant head group. The ionizable
functional groups such as COOH and NH 2 make a solubilizate located in the
micelle-water interface. Benzoic acid and aniline respectively solubilized in
HDTMA Br and SDS micelles are demonstrated by 1 H-NMR so that the ionized
functional group is associated with each head group of the surfactant with the aro-
matic moiety orienting toward the Stern layer (Bunton and Minch 1974; Kim et al.
2001; Manohar et al. 1986).
The accumulated evidence on solubilization of small chemicals having a variety
of structure in micelles show that either hydrophobic or electrostatic interactions
α
- and
β
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