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Cyclodextrins Increase Surface Tension and Critical Micelle Concentrations of Detergent Solutions.

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[3] K. 0. Christe, J. Fluorine Chem. 35 (1987) 621.
[4] E H.Appelman. A. W. Jache, J . Am. Chem. Sor. 109 (1987) 1754.
[ 5 ] Crystallographic data: a=4.075(1), b=5.040(1), c=5.939(2) A; P 2 , 2 , 2 , ;
Z = 4 : 329 observed unique reflections; R=0.027. Coordinates (atom, x,
y, z ) : F. 0.10128(9), 0.24300(9), 0.34039(9): 0, 0.36012(12), 0.28006( I I ) ,
0.17792(1 I ) : H, 0.460(3), 0.149(4), 0.194(2). Further details of the crystal
structure investigation may be obtained from the Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG). on quoting the depository number CSD-52727, the
names of the authors. and the journal citation.
[6] H Kim. E. F. Dearson, E. H.Appelman, J . Chem. Phys. 56 (1972) I : E. F.
Pearson. H. Kim, rhrd 57 (1972) 4230.
Cyclodextrins Increase Surface Tension and Critical
Micelle Concentrations of Detergent Solutions
By Wolfram Saenger* and Anke Miiller-Fahrnow
The amphiphilic detergent molecules have a characteristic tendency to self-aggregate in aqueous solution such that
their hydrophobic parts are removed from and their hydrophilic parts are exposed to the solvent. At low concentrations, the detergents form a variety of low-molecularweight aggregates in solution, and monolayers are formed
at the surface of the solvent which reduces the surface tension. Above the "critical micelle concentration" (crnc), micelles coexist in solution. It should in principle be possible
to destroy the surface monolayers and the micelles, and to
change the properties of the detergent solutions, by transferring the amphiphilic molecules to bulk solvent, i.e., by
making them better soluble.
Based on this assumption, we have added cyclodextrins
to detergent solutions, because these cyclically closed oligosaccharides with six, seven, or eight a-D-glUCOSeS per
macrocycle (a-, b-, y-cyclodextrins) are able to form inclusion complexes".*] and have been used to enhance the solubility of organic molecules in ~ a t e r . ~ ~ . ~ ~
The central cavity of the cyclodextrins is of hydrophobic
character and has a diameter of 5 A (a), 6-7 A (p), and 78 A (y),1'.21which is ideally suited to accommodate the hydrophobic, aliphatic part of the- chosen detergent molecules with a diameter of about 5 A (n-dodecylmaltoside (nDM), N,N-dimethyl-N-lauryl-N-amine
oxide (LDAO), decanoyl-N-methylglucamide (MEGA-lo), b-octylglucoside
([3-OG),Triton X-100). Moreover, the rims of the cyclodextrins are lined with OH groups, which should be able to
form hydrogen bonds with the hydrophilic parts of the enclosed amphiphilic molecules and thus add to the stability
of the formed complexes.
Experiments were carried out by the ring removal
method with an interfacial tensiometer based on the concept of Lecomte du Noiiy. The detergent solution (20 mL)
was contained in a quartz beaker thermostated at 20°C, in
which a platinum ring was immersed. The ring was attached to a balance and by lowering the beaker, the surface tension (a) could directly be measured in m N m - '
when the surface film broke. As calibration, pure water
was used ( 0 ~ 7 2m N m-I).
Solutions of the detergents used were prepared with
concentrations about 10 times the literature cmc values,
and then diluted in series to afford solutions with concentration ratios of 1 : 1/2 : 1/4 : 1/8, etc. The surface tensions
of these solutions were first measured and then a lo-, loo-,
or 300-pL aliquot of a l o - ' M a-cyclodextrin solution was
added to each of the 20-mL solutions and the surface tension again measured. The obtained values were plotted
versus the logarithms of the detergent concentrations to
derive the cmc (see the results for n-DM in Fig. la). For
n-DM, we have also added 8-and y-cyclodextrin (Fig. lb,
c) and the permethylated tri-0-methyl-8-cyclodextrin (Fig.
2) to study the dependence of surface tension and cmc of
the detergent solutions on ring size and hydrophilic character of the cyclodextrin.
clMlFig. I. Dependence of surface tension u o n the concentration c of the detergent n-DM without (*) and with added cyclodextrin. a) m 10 ' M, A
M, o 21 x
M a-cyclodextrin. b) m
M , A 5.7 x lo-' M , o
~ 0 - 4 0-cyclodextrin.
c ) m 6 x 1 0 - 4 ~A , 20x
M, o 6 6 . 3 ~
y-cyclodextrin. The concentration at the vertical dotted line corresponds to
the cmr of n-DM in water.
[*] Prof. I)r W. Saenger, Dipl. Biochem. A. Muller-Fahrnow
lnstitut fur Kristallographie
Takustrasse 6. D-I000 Berlin 33
We are grateful to Barbara Saenger for carrying out the measurements,
and to the lnstitut fur Physikalische Chemie, Freie Universitat Berlin,
for providing the Interfacial Tensiometer. This work was supported in
part by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 312, Teilprojekt DI) and by the Fonds der Chemischen Industrie.
Anyen. Chrm. Int. E d . Engl. 27 (1988) No. 3
Fig. 2. Dependence of surface tension (5 on the coiiceiitrdtioii (' 0 1 the detergent n-DM without (*) cyclodextrin and in the presence of 5.66 x I0 ' M
(m) and 17.44 x lo-" M (A) tri-0-methyl-B-cyclodextrin.Additional measurements were carried out without detergent (17) and with varying concentration
c of tri-0-methyl-P-cyclodextrin(top abscissa). The dotted line corresponds
to the cmc of n-DM in water.
0 VCH Verlagsgesellschaft mbH. 0-6940 Weinheim. 1988
0570-0833/88/0303-0393 $ 02.50/0
For all the investigated detergents, the addition of a-cyclodextrin shifts the cmc to higher concentrations by u p to
one order of magnitude, depending on the detergent. A
similar effect is observed with the surface tension, as indicated by the dotted line in Figure 1 and by the data in Table 1. For n-DM and LDAO, the addition of 2.1 x
and 6.5 x lo-' M a-cyclodextrin, respectively, shifts the
surface tension from that measured at the cmc (ca. 36 m N
m - ') to that of pure water.
The change of surface tension induced by 6.5 x
a-cyclodextrin at cmc condition is medium for MEGA-I0
(56 mN m - ' ) but only modest for the other detergents
used, Triton X-100 and 0-OG (see Table I).
Table 1. Dependence of cmc and surface tension of detergents on the concentration of added a-, p-, and y-cyclodextrin (a-CD, p-CD, y-CD).
Detergent and
added cyclodextrin
cmc (M)
1.7 x
(1.6 x lo-")
lo-' M a - C D
x 10-4
6.2 x
6 x 10 " w a - C D
18 x
21 x lo-' M a - C D
10-4 M ~ C D 2.4 x
5.7 x lo-' M B-CD
2 x10-3
2.3 x lo-'
6 x
M y-CD
20 x 10-4 M Y-CD
1.4 x lo-'
66.3 x
M y-CD
2.35 x lo-'
(2.1 x l 0 - j )
3.2 x I O - ~
5 x
M a-CD
6.4 x 10-4
20 x
M a-CD
12.5 x
65 x
M a-CD
2.6 x lo-*
x 10-2)
2.8 x lo-'
5 x 10-4 M ~ - C D
3.2 x lo-*
20 x
M a-CD
4.6 x
65 x 10-4 M ~ - C D
x I O - ~
( 2 4 . 5 ~lo-')
5 x I O - ~M ~ - C D 23 x x lo-'
M a-CD
x lo-* M a - C D
1.75 x
Triton X-100
~ i o 3.6 x I O - ~
5.3 x lo-' M a - C D
5.8 x
17.1 x
M a-CD
at cmc of
pure detergent [mN m -'I
(Literature values
in parentheses)
The effects of the cyclodextrins on the surface tension
and on the crnc of the different detergents studied here can
be explained with the formation of inclusion complexes.
This interpretation is suggested by the complex-forming
ability of the cyclodextrins for n-DM which decreases in
the series a- > 0- > y-cyclodextrin, probably because the fit
of the aliphatic part of the detergent is tightest with acyclodextrin with a cavity diameter of ca. 5
We have
tried to determine the equilibrium constants assuming a
simple 1 : 1 complex formation. Since we did not obtain a
consistent value from the measured data, we have to infer
that the complex formation between the detergents and the
cyclodextrins is of a more complicated nature.
The detergent molecules become more water soluble by
inclusion complexation, and are transported from the micelles or from the surface of the solution into bulk aqueous
solvent. This obviously interferes with the aggregation of
the detergent molecules into micelles, thereby shifting the
crnc to higher concentration, and, as is indicated by the
large rise in surface tension, it also removes the amphiphilic molecules from the surface monolayer.
The addition of cyclodextrins to detergent solutions can
be used to advantage in all cases where micelles have to be
destroyed or where the surface tension should be increased, e.g., in order to avoid foaming. The detergents
chosen in this study were taken from our work with membrane proteins, but we expect that the effects also apply to
other detergents as long as they are able to form inclusion
complexes with the cyclodextrins.
Received: August 13, 1987 [Z 2399 IE]
German version: Angew. Chem 100 (1988) 429
[ I ] J. Szejtli: Cvclodextrms and their Inclusion Complexes. Akademiai Kiado,
Budapest 1982.
[2] W. Saenger, Angew. Chem. 92 (1980) 343: Anyew. Chem Int. E d . Engl. 19
(1980) 344.
[3] J. L. Lach, W. A. Pauli, J. Pliorm. Sci. 55 (1966) 32.
[4] K. H. Fromming, 1. Weyermann, Arch. Pharm. /Weinhermi 305 (1972)
~ j
Methylrhenium Oxides: Synthesis from R z 0 7 and
Catalytic Activity in Olefin Metathesis**
By WolfSang A . Herrrnann,* Josef G . Kuchler,
Josef K . Felixberger, Eberhardt Herdtweck, and
Werner Wagner
For the larger analogues !3- and y-cyclodextrin and for
the permethylated P-cyclodextrin, a systematic study was
only performed with n-DM. The effects are smaller with
P-cyclodextrin and even more reduced with y-cyclodextrin
(Table 1). As to the permethylated B-cyclodextrin, it gives
rise to a large increase in cmc, which is, however, smaller
compared to unmethylated P-cyclodextrin (cf. Figs. Ib and
2). The change in surface tension is much less with permethylated B-cyclodextrin and, in the range of measurement,
does not exceed 50 m N m-'. At a detergent concentration
M the data points describing surface tension in
of 5 x
the presence of the permethylated P-cyclodextrin even
cross that found for pure detergent, suggesting that permethylated P-cyclodextrin itself reduces the surface tension.
In order to test this finding, the surface tension of aqueous
solutions of permethylated a-cyclodextrin was measured
(see Fig. 2). It is in fact surface active, but the concentration dependence is low.
0 V C H Verlagsge.~ell~chaJt
mbH, 0-6940 Wemheim, 1988
The system ReZO7/AI2O3is an effective heterogeneous
catalyst for carrying out olefin metatheses under mild conditions. Its activity can be further increased by the addition
of alkyltin compounds Sn&, and tin-containing cocatalysts are essential for the metathesis of functionalized olefins (e.g., unsaturated carboxylic esters and nitriles), which
is of special industrial importance."] Since the first synthesis of hexamethylrhenium(v1) and several alkylrhenium
oxides was reported by Wilkinson et al., the general accessibility of alkyl complexes of rhenium is no longer in question.12]However, there has been little subsequent investiga[*] Prof. Dr. W. A. Herrmann, Dipl.-Chem. J. G. Kuchler, Dipl.-Chem. J. K.
Felixberger, Dr. E. Herdtweck, DipLChem. W. Wagner
Anorganisch-chernisches lnstitut der Technischen Universitat Munchen
Lichtenbergstrasse 4, D-8046 Garching (FRG)
Multiple Bonds between Main-Group Elements and Transition Metals,
Part 50.-Part 49: W. A. Herrmann, R. A. Fischer, E. Herdtweck, J . Organomet. Chem. 329 ( 1987) C I.
0570-0833/88/0303-0394 $ 02.50/0
Angew. Chem. Int E d . Engl. 27 (1988) No. 3
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solutions, increase, concentrations, critical, surface, micelle, tension, cyclodextrin, detergent
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