Micellar formation study of crown ether surfactants

Article abstract of DOI:10.1002/jccs.200200092

Fluorescence probe and nuclear magnetic resonance (NMR) methods were employed to investigate the micellation of prepared crown ether surfactants, e.g. decyl 15-crown-5 and decyl 18-crown-6. Pyrene was employed as the fluorescence probe to evaluate the critical micellar concentration (CMC) of these surfactants in aqueous solutions while spin lattice relaxation times (T₁) and chemical shifts of H-1 NMR were applied in non-aqueous solutions. Decyl 15-crown-5 with lower CMC forms micelles much easier than decyl 18-crown-6 with higher CMC in aqueous solutions, whereas decyl 18-crown-6 forms micelles easier than decyl 15-crown-5 in nonaqueous solutions. Comparison of the CMC of crown ether surfactants and other polyoxyethylene surfactants such as decylhexaethylene glycol was made. Effects of salts and solvents on the micellar formation were also investigated. In general, additions of both alkali metal salts and polar organic solvents into the aqueous surfactant solutions increased in the CMC of these surfactants. The formation of micelles in organic solvents such as methanol and acetonitrile was successfully observed by the NMR method while it was difficult to study these surfactants in organic solutions by the pyrene fluorescence probe method. The NMR study revealed that the formation of micelles resulted in the decrease in all H-1 spin lattice relaxation times (T₁) of hydrophobic groups, e.g. CH₃ and CH₂, and hydrophilic group OCH₂ of these surfactants. However, upon the micellar formation, the H-1 chemical shifts (δ) of these surfactant hydrophobic groups were found to shift to downfield (increased δ) while the chemical shift of the hydrophilic group OCH₂ moved to up-field. Comparison of the spin lattice relaxation time and H-1 chemical shift methods was also made and discussed.

Full text of DOI:10.1002/jccs.200200092

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Journal of the Chinese Chemical Society, 2002, 49, 599-605
599
Micellar Formation Study of Crown Ether Surfactants
Fang-Fei Chou (
) and Jeng-Shong Shih* (
)
Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan 116, R.O. C.
Fluorescence probe and nuclear magnetic resonance (NMR) methods were employed to investigate the
micellation of prepared crown ether surfactants, e.g. decyl 15-crown-5 and decyl 18-crown-6. Pyrene was em-
ployed as the fluorescence probe to evaluate the critical micellar concentration (CMC) of these surfactants in
aqueous solutions while spin lattice relaxation times (T1) and chemical shifts of H-1 NMR were applied in
non-aqueous solutions. Decyl 15-crown-5 with lower CMC forms micelles much easier than decyl 18-
crown-6 with higher CMC in aqueous solutions, whereas decyl 18-crown-6 forms micelles easier than decyl
15-crown-5 in nonaqueous solutions. Comparison of the CMC of crown ether surfactants and other polyoxy-
ethylene surfactants such as decylhexaethylene glycol was made. Effects of salts and solvents on the micellar
formation were also investigated. In general, additions of both alkali metal salts and polar organic solvents
into the aqueous surfactant solutions increased in the CMC of these surfactants. The format ion of micelles in
organic solvents such as methanol and acetonitrile was successfully observed by the NMR method while it
was difficult to study these surfactants in organic solutions by the pyrene fluorescence probe method. The
NMR study revealed that the formation of micelles resulted in the decrease in all H-1 spin lattice relaxation
times (T1) of hydrophobic groups, e.g. CH3 and CH2, and hydrophilic group OCH2 of these surfactants. How-
ever, upon the micellar formation, the H-1 chemical shifts ( ) of these surfactant hydrophobic groups were
found to shift to downfield (increased ) while the chemical shift of the hydrophilic group OCH2 moved to up-
field. Comparison of the spin lattice relaxation time and H-1 chemical shift methods was also made and dis-
cussed.
INTRODUCTION
Artificial macrocyclic polyethers such as crown ethers
demonstrated as a good tool for the determination of the
CMC of various surfactants in aqueous solutions; thus, in this
study, the pyrene fluorescence probe method was applied to
determine the CMC of these crown ether surfactants in aque-
ous solutions. However, the pyrene fluorescence probe
method theoretically can’t work for the surfactant in organic
solvents or organic/water mixed solvents, because the pyrene
molecules place in organic solvent whether the micelle of
surfactant forms or not. In other words, the pyrene can’t
move into the surfactant micelles in organic solvents which
leaves the intensity ratio (I1/I3) of pyrene fluorescence un-
changed. It means that it is difficult to use the pyrene fluores-
cence probe to determine the CMC of surfactants in organic
solvents. Therefore, in this study, we have developed the
NMR method for the determination of CMC of these crown
ether surfactants in organic solutions. Spin lattice relaxation
times (T1) and chemical shifts of H-1 NMR have been suc-
cessfully applied to determine the CMC of crown ether sur-
factants in non-aqueous solutions.
and cryptands have a dual functionality, with a lipophilic seg-
ment on the outer surface of the ring and a polar segment on
the inner surface that can form stable complexes with metal
ions1-4 and polar organic species.5-7 Attaching a lipophilic
long-chain alkyl group to a crown ether or cryptand can con-
vert it into a surfactant, which is capable of forming micelles
in a manner analogous to classical non-ionic surfactants.8-10
In this study the crown ethers, decyl 15-crown-5 and decyl
18-crown-6, were prepared.
Like common surfactants, the macrocyclic polyether
surfactant can produce foams and can be applied in the sepa-
ration of ions by flotation.10 Furthermore, the long-chain
macrocyclic polyether surfactant can be employed as an
ionophore to carry various alkali metal ions through liquid
membranes.11-13 In general, a pyrene fluorescence probe is
employed to measure the critical micellar concentration
(CMC) of surfactants including the macrocyclic polyether
surfactant in the aqueous solutions. Upon formation of mi-
celles, pyrene will move into the inside of the micelles from
the aqueous phase which results in an alteration in the inten-
sity ratio (I1/I3) of pyrene fluorescence bands I (373 nm) and
III (383 nm).14,15 The pyrene fluorescence probe has been
EXPERIMENTAL
Synthesis of Crown Ether Surfactants
Crown ether surfactant, decyl 15-crown-5, was synthe-

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600 J. Chin. Chem. Soc., Vol. 49, No. 4, 2002
Chou and Shih
sized by the reaction of 1,2-epoxydodecane with tetraethyl-
ene glycol to get decyl pentaethylene glycol as an intermedi-
ate followed by the cyclization of decyl pentaethylene glycol
as shown in Fig. 1. Firstly, tetraethylene glycol (0.5 mole)
was added in drops into a 150 mL solution of sodium hydride
(0.25 mole) in 1,4-dioxane and mixed with 1,2-epoxydode-
cane (0.24 mole). The intermediate decyl pentaethylene gly-
col was obtained from the reaction of 1,2-epoxydodecane and
tetraethylene glycol by reflux for 24 hr and acidification with
6 N HCl followed by vacuum evaporation at 200 C/2 mm
Hg. The product of decyl 15-crown-5 was obtained by addi-
tion of p-toluenesulphony chloride (0.12 mole) in drops into a
mixture of decyl pentaethylene glycol (0.12 mole) with
NaOH (0.48 mole) in 1,4-dioxane (600 mL) and vacuum
evaporation at 140 C/2 mm Hg. The final product of decyl
15-crown-5 identified with mass spectrometry (M+ = 361)
and H-1 NMR spectrum with (ppm), 0.90 (t, CH3, 3H), 1.26
(s, CH2, 18H), 3.20-3.78 (m, OCH2CH2O, OCHCH2O, 19H).
The synthetic procedure for decyl 18-crown-6 is also de-
picted in Fig. 1. The final product of decyl 18-crown-6 was
obtained by vacuum evaporation at 160 C/2 mm Hg and also
identified with mass spectrometry (M+ = 405) and H-1 NMR
spectrum with (ppm), 0.86 (t, CH3, 3H), 1.28 (s, CH2, 18H),
3.28-3.92 (m, OCH2CH2O, OCHCH2O, 23H).
significant slope change in the plot of I1 (373 nm)/I3 (383 nm)
intensity ratio against surfactant concentration are denoted as
CMC. In the NMR method, the CMC of the crown ether sur-
factant was determined as the concentration at a point where
a significant change in the plot of spin lattice relaxation times
(T1) or chemical shifts of H-1 NMR against surfactant con-
centration. The spin lattice relaxation time T1 was measured
by the inversion recovery method16 with [180 - (Delay
time)-90 ]n pulse sequences as shown in Fig. 2.
Apparatus
A JASCO FP770 fluorimeter was used to measure the
fluorescence of pryrene, whereas a JEOL JNM-FX100 Fou-
rier transform NMR was employed to measure spin lattice re-
laxation times (T1) and chemical shifts of H-1 NMR. The
JEOL Fourier transform NMR and JEOL JMS-D300 mass
spectrometers were applied to identify the intermediates and
final products of the crown ether surfactants.
Chemicals
Pentaethylene glycol and 1,2-epoxydodecane were ob-
tained from Aldrich Chemical Co. and tetraethylene glycol
was purchased from Janssen Chemical Co. All other chemi-
cals used were of analytical grade.
Critical Micellar Concentration (CMC) Measurements
In this study, pyrene fluorescence probe and NMR
methods were employed to measure the CMC of crown ether
surfactant in aqueous and non-aqueous solutions, respec-
tively. In the pyrene fluorescence method, the CMC of sur-
factants was measured at various aqueous concentrations
with 3 10-7 M pyrene added. The concentrations where a
RESULTS AND DISCUSSION
Fluorescence Study on CMC of Crown Ether Surfactant
Pyrene fluorescence probe method was applied to study
the formation of crown ether surfactants, e.g. decyl 15-
crown-5 and decyl 18-crown-6 in aqueous solutions. The
Fig. 2. H-1 Spin lattice relaxation time (T1) measure-
ment for decyl 15-crown-5 in acetonitrile.
Fig. 1. Schematic synthesis diagram of decyl 15-
crown-5 and decyl 18-crown-6.

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Micellar Formation Study of Crown Ether Surfactants
J. Chin. Chem. Soc., Vol. 49, No. 4, 2002 601
CMC value of the crown ether surfactants was obtained by
measuring pyrene fluorescence and plotting I1 (373 nm)/I3
(383 nm) intensity ratio vs. surfactant concentration which
shows a dramatic change in slope as shown in Fig. 3. The
CMC value is generally estimated as the surfactant concen-
tration at the second inflection point (Points P1 and P2 in Fig.
3) followed by the observation of a level-off curve. As shown
in Fig. 3, the CMC (1.25 10-4 M) of decyl 15-crown-5 is
smaller than that of decyl 18-crown-6 (2.00 10-4 M) which
means that decyl 15-crown-5 forms micelles easier than
decyl 18-crown-6 in aqueous solutions. It is well known that
nonpolar or less polar surfactants form micelles easier in
aqueous solutions. A decyl 15-crown-5 molecule with five
oxygen donor atoms can be expected to be less polar than
decyl 18-crown-6 with six oxygen donor atoms resulting in
the smaller CMC of decyl 15-crown-5. Comparison of the
CMC of these crown ether surfactants and decylhexaethylene
glycol was also made. Even though a decylhexaethylene gly-
col molecule has the same number of carbons and one more
oxygen atom compared to decyl 18-crown-6, decylhexae-
thylene glycol exhibited greater CMC with 9.0 10-4 M than
either decyl 18-crown-6 (2.00 10-4 M) or decyl 15-crown-5
(1.25 10-4 M), which may be attributed to the hydrogen
bonding between the OH group of glycol and solvent H2O re-
sulting in greater CMC of the glycol.
crown ether surfactant in aqueous solution which results in
the increase in CMC of the crown ether surfactant. As ex-
pected, the CMC value of decyl 15-crown-5 surfactant in the
presence of 0.1 M NaCl in aqueous solutions is estimated
from pyrene fluorescence measurements to be 2.50 10-4
M
(Curve B) which is greater than 1.25 10-4 M (Curve A) as
shown in Fig. 4 and Table 1. However, as shown in Fig. 4 and
Table 1, the CMC value of the decyl 15-crown-5 surfactant in
aqueous solution decreases at high salt concentrations 1.0
M. This decrease in CMC occurs primarily due to the “salting
out” effect on addition of electrolyte.17 The decreased CMC
due to the salt effect was also reported for other crown ether
surfactants, e.g. cryptand surfactants.18 The CMC of the
decyl 15-crown-5 surfactant in the presence of KCl is also
shown in Table 1. The plot of the CMC value against the con-
centrations of salts (Fig. 5) reveals that both decyl 15-
crown-5 and decyl 18-crown-6 show an increased CMC at
low salt concentrations 0.1 M while the CMC at high con-
The effect of addition of salts, e.g. NaCl and KCl, on
the micellar formation of the crown ether surfactants was in-
vestigated. The crown ether surfactant is considered as a
non-ionic surfactant, but becomes a “pseudo cationic sur-
factant” after complexing with metal ions of salts. Owing to
the organic character of the crown ether surfactant, the com-
plexation between metal ion and crown ether surfactant can
be expected to lead to an increase in the solubility of the
Fig. 4. Plot of I1/I3 ratio of pyrene fluorescence vs con-
centration of decyl 15-crown-5 in aqueous solu-
tions with (A) no salt, (B) 0.1 M NaCl, (C) 1.0
M NaCl and (D) 1.6 M NaCl.
Fig. 3. Pyrene fluorescence micellar studies for (A)
decyl 15-crown-5 and (B) decyl 18-crown-6 in
aqueous solutions.
Fig. 5. Concentration effect of salts on CMC of decyl
15-crown-5 in aqueous solutions.

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602 J. Chin. Chem. Soc., Vol. 49, No. 4, 2002
Chou and Shih
Table 1. CMC and I1/I3 of Pyrene Fluorescence for Decyl 15-
Crown-5 in Aqueous Solutions with NaCl and KCl Salts
Table 2. CMC and I1/I3 of Pyrene Fluorescence for Decyl 15-
Crown-5 in MeOH/H2O and MeCN/H2O Mixed
Solvents
[NaCl]
CMC (M)
I1/I3
[KCl]
0
CMC (M)
I1/I3
MeOH
CMC (M)
I1/I3
MeCN CMC (M)
(v/v
I1/I3
0
1.25 10-4 0.932
2.50 10-4 0.939
1.10 10-4 0.842
0.70 10-4 0.769
1.25 10-4 0.932
(v/v
)
)
0.1 M
1.0 M
1.6 M
0.1 M 1.75 10-4 0.919
1.0 M 1.20 10-4 0.881
1.6 M 1.05 10-4 0.839
0
1.25 10-4 0.932
6.40 10-4 0.978
1.00 10-3 1.016
2.60 10-3 1.023
0
1.25 10-4 0.932
3.30 10-4 1.162
5.20 10-4 1.311
6.00 10-4 1.360
10
17
25
10
17
20
33
5.80 10-3
1.035
25
8.00 10-4 1.568
centrations ( 1.0 M) of salts decreased.
Effect of mixed solvents on the formation of micelles of
these crown ether surfactants was also investigated. The
composition of continuum phase (solvent) of surfactant solu-
tion always shows a significant effect on the formation of mi-
celles. For instance, the increased concentration of methanol
leads to an increment in CMC of decyl 15-crown-5 as shown
in Fig. 6. In pure water, the O/W type (dispersion phase: or-
ganic, continuum phase: water) micelle could be expected
(Fig. 7). In other words, the O/W type micelle is stable and
easily formed in water, but unstable in organic or lipophilic
solvents. In contrast, the W/O type micelle can be easily
formed in organic or lipophilic solutions (Fig. 7). Due to the
low solubility of the crown ether surfactant in pure water, the
mixed MeOH/H2O solvent was used in this CMC study. By
adding methanol into water, the solution becomes more hy-
drophobic resulting in the instability of O/W type micelles
and greater CMC of the crown ether surfactant. The hydro-
phobic effect of the solvent on the CMC of the decyl 15-
crown-5 surfactant can also be observed by adding acetoni-
trile (Table 2). A similar organic solvent effect was also re-
ported for other surfactants.19-21 In both methanol and aceto-
nitrile mixed solvents, the CMC of decyl 15-crown-5 showed
a nearly linear relationship to the volume % of the organic
solvents as shown in Fig. 8.
The micellar formation of these crown ether surfactants
in pure organic solvents, e.g. methanol and acetonitrile, was
also investigated with the pyrene fluorescence method. As
shown in Fig. 9, although the I1 (373 nm)/I3 (383 nm) inten-
sity ratio of pyrene changes with the increased concentrations
of both decyl 15-crown-5 and decyl 18-crown-6 surfactants,
no second inflection point is observed in each case which in-
Fig. 7. Micellar type crown ether surfactant decyl 15-
crown-5 with pyrene (py).
Fig. 8. Solvent effect on CMC of decyl 15-crown-5 in
mixed solvents. (A) pure H2O, (B) MeOH/H2O
and (C) MeCN/H2O.
Fig. 6. Concentration effect of methanol (MeOH) on
the micellar formation of decyl 15-crown-5 in
MeOH/H2O (V/V %)mixed solvents.