Langmuir 1995,11, 191-196
191
Amphiphilic Monolayers of Insoluble Cyclodextrins at the Water/Air Interface. Surface Pressure and Surface Potential Studies P. C. Tchoreloff,' M. M. Boissonnade,t A. W. Coleman,$ and A. Baszkin*lt Physico-Chimie des Surfaces, URA CNRS 1218,and Cyclodextrines Amphiphiles, CNRS ERS45, Universite Paris Sud, 92296 Chbtenay-Malabry, France Received April 28,1993. I n Final Form: September 19, 1994@ The monolayer properties of amphiphiliccyclodextrinsesterified at positions 2 and 3 have been assessed by surface pressure and surface potential measurements at a constant area. The cyclodextrins (CDS) possessing six (a),seven (P), and eight ( y ) linked glucopyranose units with c14 hydrocarbon chains have been examined. In a second experiment, the modified cyclodextrinswith the chain length varying from 2 to 14 carbons (PC2, Bc6, PCs, PClo, PC12, and Pc14)have also been characterized. The PC2 system is best considered apart from the other molecules, belonging to the class of amphiphilic cyclodextrins in which the organizationalproperties are related to the presence of a short impermeablehydrophobic layer parallel to the aqueous surface. The surface pressure (n)-surface density ( 6 ) relationships reveal that Pc6 is surprisingly the most surface active molecule as compared to other studied CDs, and yet this molecule appears to display the lowest value of the maximum effective surface potential. The sharp differences between this molecule and PCs arise from the different packing at the interface, with Pc6 in a close packed arrangement (A = 1.8 times the effectivediameter of the PCD area itself); in contrast, PCs packs in an open arrangement (A = 2.8 times the effective diameter of the PCD area itself). The observed surface properties of modified PCDs are analyzed in terms of hydrocarbon chain orientation, coordination of water between the molecules,and the interaction of this surface layer with the strata ofwater moleculesextending into the bulk phase. The effective dipole moment of each studied CD was calculated as well as the dipole moment corresponding to one aliphatic chain and one glucopyranose unit. The role of the symmetry of the cyclodextrin molecules upon their surface properties is discussed for the a&, Pc14, and $14 series, in which it is observed that the 7-fold geometry ofPC14 is unfavorable for packing at the waterlair interface.
Introduction The cyclodextrins are cyclic oligosaccharides possessing six (a), seven (p),or eight ( y ) linked glucopyranose units and forming a cylindrical molecule, (Figure 1). Due to their relatively nonpolar cavity, they are capable of including a wide variety of organic molecules and find therefore wide applications in alimentary, pharmaceutical, or separation technologies. The cyclodextrins may be modified by grafting to one of their hydrophilic faces (the primary or secondary hydroxyl groups) hydrophobic moieties. Thus, the obtained amphiphilic cyclodextrinsmay form self-assembling systems such as micelles, vesicles, and monomolecular layers.lS2 These self-organized systems possess a cavity having a rigid cylindrical geometry and may be obtained both in polar organic solvents as well as at the waterlair interface. There is also evidence of their multimolecular organization within supramolecular systems.3 The systematic studies of cyclodextrin monolayers may provide important information on their arrangements at the waterlair interface. While the surface pressure measurements enable us to determine the type of organization of molecules under lateral pressure and their ability to build up a close-packed monolayer, the knowledge of local dipole strength on the surface should allow a better understanding of the nature of the organization of amphiphilic cyclodextrins molecules a t the water/air
* Author to whom correspondence should be addressed. +
Physico-Chimie des Surfaces.
* Cyclodextrines Amphiphiles.
@Abstractpublished in Advance A C S Abstracts, December 1, 1994. (1)Gennis, R. B. Biomembranes. Molecular Structureand Function, Springer, Berlin 1989. (2) Bonar-Law, R. P.; Davis, A. P.; Murray, B. A. Angew. Chem. Int. Ed. Engl. 1990,29, 1407. (3) Lehn, J. M. Angew. Chem. Int. Ed. Engl., 1988,27,89.
Figure 1. Cylindrical cyclodextrin molecule formed of cyclic oligodextrins possessing six (a), seven (P), or eight ( y ) linked glucopyranose units. interface and also facilitate their use in the preparation of other organized assemblies. In the present work we report the surface pressure and surface potential studies of amphiphilic a, p, and y cyclodextrin monolayers. The surface potential is directly proportional to the change in the normal component of the dipole moment with respect to the pure water surface, and results from the complex contribution of hydrophobic and hydrophilic parts of the molecules forming monolayers. Although the surface properties of some amphiphilic cyclodextrin monolayers as characterized by surface pressure measurements have already been reported by others and 0urselves,4-~the systematic study ofthe surface (4)Kawabata, Y.; Matsumoto, M.; Tanaka, M.; Takahashi, H.; Irinatsu, Y.; Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 19M,1933. (5)Tanaka, M.; Ishizuka, Y.; Matsumoto,M.; Nakamura, T.; Yaba, A,; Nakanhiaha, H.; Kawabeta, Y.; Takahashi,H.; Tamura, S.;Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 1987,1307. (6)Taneva, S.;Ariga, K.; Okahata, Y.; Tagaki, W. Langmuir, 1989, 5,111. (7) Taneva, S.;Ariga, IC;Tagaki, W.; Okahata,Y. J . Colloid Interface Sci. 1989,131,561. (8)Parrot-Lopez, H.; Ling, C. C.; Zhang, P.; Baszkin, A.; Albrecht, G.; de Rango, C.; Coleman, A. W. J . Am. Chem. SOC.1992,114,5480. (9)Zhang, P.; Parrot-Lopez,M.; Tchoreloff,P.; Baszkin, A.; Ling, C. C.; de Rango, C.; Coleman, A. W. J . Phys. Org. Chem. 1992,5, 518.
0743-746319512411-0191$09.00/0 0 1995 American Chemical Society
Tchoreloff et al.
192 Langmuir, Vol. 11, No. 1, 1995
potential and surface pressure of monolayers of a n amphiphilic cyclodextrin family has, to our knowledge, never been undertaken. The electric potential data are not only valuable for controlled deposition of monolayers of cyclodextrins on solid substrates with a view to their technical applications but are also extremely relevant to the characterization of membrane and vesicle systems. Materials and Methods Synthesis. The synthetic route to the CD diesters has been previously described9 and to simply recapitulate involves a protection,substitution, deprotectionstrategy, as shown in Figure 2 in which the schematic formulas of pCz, ~ C S~ ,C S~ ,C I O ~ ,C I Z , and pC14 amphiphilic cyclodextrins are presented. Due to major technical problems, pC4 CD could not be synthetized. Monolayers Spread at the WaterIAir Interface. Amphiphilic cyclodextrins were spread from a solution of pure chloroform by means of a micropipet (Microman Gilson 25 pL) at the surface of triply distilled water subphase. The measurements have been performed 15min after spreading. The addition of the spreading solvent alone to a clear surface produced no measurable change either in surface tension or in surface potential of water after that time. The surface tension of water was typically 2 7 1 mN/m. Aliquots of cyclodextrin were thus successively deposited both for the surface tension and surface potential measurements a t constant area (11.7 cm2). The measurements were performed a t 23 4Z 0.5 "C in thermostated enclosed chambers, to reduce airborne contamination. Prior to the surface pressure and surface potential measurements of deposited monolayers, the water interface was cleaned by suction through a narrow pipet. Surface Pressure Measurements. The Wilhelmy plate method, as previously describedlOJ1was used for all surface tension measurements. The surface pressures of cyclodextrin monolayers, II = yo - y , were deduced from the values of pure water surface tension (yo = 72 mN/m a t 20 "C) and that obtained in the presence of a spread cyclodextrin monolayer ( y ) . The Wilhelmy plate (sand-blasted, 1cm Pt),was attached to a force transducer (Hewlett-Packard,model 140604). The signal from the transducer was amplified by a transducer amplifier (Hewlett(10)Baszkin, A.; Deyme, M.; Couvreur, P.; Albrecht, G. J. Bioactiue Compatible Polym. 1989, 4 , 110. (11) Casas, M.; Baszkin, A. Colloids Surf. 1992, 63, 301.
Packard, model 8805B). By taking these measurements without detaching the platinum plate from the interface, the data could be continuously plotted as a function oftime on a calibrated strip chart recorder. The stationary values for each measured surface density of a cyclodextrin monolayer obtained about 30 min after deposition were considered to correspond to equilibrium. The accuracy of measurements was estimated to be 4ZO.l mN/m. All reported surface tension values are mean values of three experiments. Their reproducibility was within 4Z0.5 mN/m. As no hysteresis was observed on drawing the plate through the interface from either side in all experiments, it was assumed that the contact angle was zero. Surface Potential Measurements. The surface potential of spread cyclodextrin monolayers was measured using the differential method in which a Keithley Instruments electrometer (model 610C) was connected to two identical 241Amair-ionizing a-emitting electrodes suspended at about 2-3 mm above the reference (left cell) and the measuring cell (right cell) as previously described.1°J2J3The two cells were connected by a liquid bridge for electrical continuity. The measured difference in surface potential (AV) is that between the surface potential of the measuring cell (VM)and the monolayer free reference cell (VR). In the presence of a cyclodextrinmonolayer, the surface potential of the system is given by the following equation: AV= V, - V, The potential jump at the water-air interface, AV, induced by formation of a monolayer, is defined by the Helmholz equation aa
1
AV = -np, €0
where n is the number of spread molecules and EO the permittivity in a vacuum. The quantity is the effective dipole moment perpendicular to the surface. The formation of monolayer brings about a change in surface potential which is proportional to the change of the vertical component of the dipole density of the spread molecule with respect to the pure water surface. In the case of a cyclodextrin (12) Plaisance, M.; Ter-Minassian-Saraga,L. C.R.Acad. Sci. Ser. 1970, C 270,1269.
(13) Ter-Minassian-Saraga,L. Langmuir, 1986,2, 24.
Monolayers of Cyclodextrins at the Water/Air Interface
-
i
'"
Langmuir, Vol. 11, No. I, 1995 193
400
w2
300
s E
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E
1
4
E
200
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i
e
100 10
0 T
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1
'
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I
x 0
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2
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x IO' 3 m o ~ e ~ ~ ~ e s / ~ m 2 Figure 3. Surfacepressure (n)-surface density (6)isotherms for BCz, BC6, BCs, BClo, #Xlz,andPC14amphiphiliccyclodextrins.
monolayer, the local contributions of the hydrophobic and hydrophilic parts of the CDs will both influence the total surface potential. Hence, surface potential measurements can be used to predict the magnitude and the direction of potential changes which occur as a consequence of changes in the packing density of the cyclodextrins. The sensitivities of measurements were 1mV in the 0-100 mvrangeand 1OmVinthe100-400mVrange. Itwasconsidered that equilibrium was established when the value of AV did not change after 30 min. All reported surface potential values are mean values of at least three measurements. The standard deviation of the mean never exceeded &5 mV.
'
2
d
'
1
'
4
5
10' ' m o ~ e c u ~ e s ~ c m ~
Figure 4. Surfacepotential( A m dependence on surface density ( 6 ) for BCZ,BC,, BCS,BCIO,PC12, and Bc14 amphiphilic cyclodextrins. Symbols as in Figure 3.
E 1
E
10
c _o_
2
Results Figure 3 illustrates the surface pressure (TI)-surface density (6) relationships for a series of amphiphilic P cyclodextrins; the secondary hydroxyl groups of these molecules are substituted with fatty acids containing varying amounts of CHZgroups in the CZ-c14 range. Differentiating from the start, it is immediately apparent that in the presence of an extremely short fatty acid chain the resultant amphiphilic cyclodextrin exhibits rather a low surface pressure (PC2 = 10 mN/m) relative to other CDs a t small molecular areas. For the other systems, surprisingly, the surface properties of these amphiphilic cyclodextrins do not correlate directly with the hydrocarbon chain length as reported, for example, for alkanethiol systems14and one may remark that the maximum surface pressure is displayed by the PC, CD molecule (33 mN/m). The PC, CD shows a surface pressure similar to that ofPCloCD(25.8 mN/m) and there is a subsequent decrease with increasing chain length: PCloCD (25.8 mN/m) > PClzCD (24.2 mN/m) > @C14CD (18.2 mN/m). The surface potential (AV)-surface density (6) relationships, a s shown in Figure 4,clearly indicate that the trend observed in Figure 3 is reversed in the case of surface potential properties of these cyclodextrins. The PCzCD molecule has the highest surface potential (405 mV) while PC&D is characterized by the lowest one (298 mV). The trend in AV-6 relationships for the CDs with a higher chain length [PCBCD(355 mV) > PCloCD (340 mV) > PClzCD (325 mV) > PC&D (318 mV)] parallels that observed in TI-6 relationships. Figures 5 and 6 represent the TI-6 and AV-6 isotherms for a,p, and y cyclodextrins (CD14). For this series of CDs (14)Evans, S. D.;Ulman, A. Chem. Phys. Lett. 1990, 170, 462.
1
a c14
P
z
c14
1 Y c14 0
FI
0
I
T
x
I
3
2
1
I 5
4
~ ~ ~ ~ m o ~ e c u ~ e s / c m ~
Figure 5. Surface pressure (n)-surface density ( 6 )relationships for aC14, Bc14, and $14 amphiphilic cyclodextrins.
-1 Po
5 1
P loo
p-0-u-o-M
I
2
I
-a
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l
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1
' 1
l 2
'
l 3
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c14
c14
Y c14
l 4
'
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x IO' 3 m o ~ e c u ~ e r / c m 2 Figure 6. Surface potential (Am-surface density (6)relationships for aCl4, Bc14, and yC14 cyclodextrins. Symbols as in Figure 5.
the hydrophilic moiety contains increasing amounts of glucopyranose self-assembled units, while the hydrophobic moiety is identical and contains, for all studied CDs, 14 carbons. One may note that the maximum TI and AV
Tchoreloff et al.
194 Langmuir, Vol. 11, No. 1, 1995 values obtained with aClcare higher than those exhibited by pC14 and $14. It is worthwhile to point out that no significant differences were observed betweenb and y CDs, for the maximum values with regard to either Il or AV,
Discussion It is a well-established fact that at least eight CH2 groups are necessary in order to obtain a stable monolayer in the case of fatty acids.g In the case of amphiphilic CDs, such monolayers may be obtained with a very short hydrophobic chain since two CH3 groups allow the formation of a stable monomolecular film (Figure 3). This arises from the presence of a large two-dimensional head group bound to a quasi-continuous lipophilic layer which exists parallel to the aqueous surface and which is effectively impermeable to water. Since cyclodextrins which are not substituted with hydrophobic moieties are soluble in water,15 one would expect that, if short chains were attached to glucopyranose units, the hydrophilic-hydrophobic balance (HLB)of such molecules would be higher than those containing long aliphatic chains. This hypothesis does not hold, however, first because the amphiphilic CDs are insoluble in water and second because the highest efficiencyand effectiveness in reducing surface tension of water is observed with Pc6CD. Evidently, other factors are involved which determine the specific properties of the amphilic cyclodextrins. In order to have a better insight on how the hydrophilic and hydrophobic moieties of CDs influence the surface potential values, we have calculated the effective dipole moments using the classical equation
where 6 = 1/A is the surface density expressed in molecules/cm2, A the molecular area in A2, AV the maximum surface potential in mV, and p l the vertical component of the total dipole moment expressed in mD.16 The above equation is the Helmholtz equation expressed in international units considering €0 equal to the unity. The linear character of the plots AV versus 6 a t low surface densities (Figure 4) and the fact that these plots extrapolate to AV= 0 with essentially no scatter indicates homogeneous surfaces with no reorientation in the films up to surface densities of about 2 x 1013molecules/cm2. Within the linear portion of AV-6 relationships AV is proportional to the number of spread molecules ( n )and their dipole moment in a direction perpendicular to the surface (AV- npJ. It seems very unlikely that variations i n n andpl would exactly balance each other, which would have to happen if all the film-forming molecules remained a t the interface and AV became independent of 6. For this reason it appears preferable to relate AV values to a constant surface density value on the initial linear portion ofthe graphs. The data a t 6 = 1.4 x 1013molecules/ cm2 were taken to calculate the p l for all CDs. They are summarized in Table 1. It is significant to note that, if the AVvalues corresponding to other surface densities in the (0.8-1.9) x 1013molecules/cm2range were used, the error on calculated pl never exceeded 2% relative to the tabulated data. Using the calculated pL values, we have represented in Figure 7a their dependence on the chain length, which when extrapolated to the zero carbon chain length yielded the p l corresponding to the contribution of hydrophilic (15)Coleman, A. W.; Nicolis, I.; Keller, N.; Dalbiez, J. P. J.Inclusion Phenomena Molecular Recognition Chem. 1992,13,139. (16) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces,
Wiley: New York, 1966.
6ooo
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1
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L
1 -
-
2
l
-4
J
eo
8
IO
12
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ChrJnbnglh
Figure 7. Dependence of the effective dipole moment (data from Table 1) (a) and of the ratio observed surface area/CD head group area (b) as a function of chain length for the PCD series. The observed CD surface areas were taken at the intersection of the lines of linear portions of AV-6 curves and of those extrapolated from the portions of these curves when the leveling occurred. Table 1. Effect of Aliphatic Chain Length upon AV Values (Taken at d = 1.4 x l O l S molecules/cm2)and upon the Total Effective Dipole Moment (111)and Its Fraction per One Hydrocarbon Chain chain length AV (mV) ,ULtotal (mD) pL/chain (mD) 2 6 8 10 12 14
167 162 263 234 210 215
3166 3071 5364 4436 4151 4075
-7.0 -13.8 150.0 83.7 63.3 57.9
polar groups ofPCD to itspl total. A similar partial dipole compensation approach enabling one to discern between the dipole moments due to hydrocarbon chains in contact with air and those of polar head groups in the water phase was used by others.17J8 Thus the contribution of hydrophilic polar groups of PCD to its p l total was found to be equal to 3264 f 58 mD. The high value of this dipole moment may be easily understood if one takes into account the presence of seven OH groups forming the hydrophilic moiety. From Figure 7a it is clear that except for PC&D and PC&D all values ofpl for other CDs are higher than 3264 mD and that a drastic increase in the dipole moment for PC&D relative to PC&D is observed. While the negative values ofpl/chain calculated for PC2CD andPC&D would indicate a preponderant contribution of the effective dipole moment of hydrated OH groups directed from the monolayer (-1 to the water (+I," the increase between PC&D and PC&D might be predominantly ascribed to the variation or hydrocarbon chain orientation resulting in a higher area (lower 6 ) a t a given AV (Figure 4) that PC&D molecules display compared to PC6CD. The PCD head group has a constant area of 190 A2 and thus any observed surface areas having larger values must contain voids between the head groups. The shape of the curve represented in Figure 7b, where the observed areas a t film collapse (from Figure 3) were divided by the occupied surface area of the primary face (190 Az) as a function of the number of CH2 groups, mimics that in (17) Vogel, V.; Mobius, D. J. Colloid Interface Sci. 1988,126, 408. (18)Demchak, R. J.; Fort, T.,Jr. J.Colloid Interface Sci. 1974,46, 191.
Langmuir, Vol.11, No.1, 1995 195
Monolayers of Cyclodextrins a t the Water lAir Interface Figure 7a. These data confirm those from X-ray reflectivity experiments which have clearly shown the existence of voids in the case of PC14CD. Conversely, for a and Y C ~ ~ C Dwhich S , have a very good lateral packing due to the 6- and 8-fold symmetries, the voids were not observed. At high surface pressures, the 7-fold symmetry of BCDs induces disorder which results in formation of “corolles” of cone-shaped aliphatic chain distribution which are more or less opened.lg The difference in surface properties between Pc6 and PC, CD may thus come about from the differences in orientation of the hydrocarbon chains leading to the higher molecular area in the case of PC8CD. In general, monolayers of complex compounds with incorporated hydrophobicchains in various positions along a hydrophilic moiety display important orientation effects, resulting in various possible configurations that such molecules may form at the airlwater interface. Their molecular areas to a large extent would depend on the chain length and the intramolecular interactions. Thus for diacetylene alkyl phthalates comprising benzoic acid a s a n aromatic head group and a diacetylene moiety in the hydrocarbon chain, it has been shown that the chain length influences both the monolayer and multilayer behavior and that the shorter dodecyl chain did not provide sufficient lateral interaction energy to stabilize the high area per molecule head group.2o For discotic molecules such as benzene hexa-n-alkanoates, the aliphatic chains were shown laying almost flat a t the interface, giving molecular areas in agreement with those obtained from monolayers experiments.21 It seems quite likely therefore that for PCDs with longer chain a diminution in the observed molecular areas results from the stronger intramolecular van der Waals attractive interactions operating between the aliphatic chains offilm forming molecules giving rise to a geometry in which the 0-3 chains are oriented only slightly outward from the molecular a x k g Although the factors contributing to monolayer surface potential are complex, surface potential is known to be largely dependent of the orientation of the intrinsic molecular dipoles (p) of the film-forming molecules and of the orientation of the subphase water molecules. The effect of the Orientation of spread molecules on their dipole moments and surface pressure as reported by Kuchhal et al. for long chain n-alkoxyethanol monomolecular film seems to be very significant in the case of spread molecules.22 In general p is the vector sum of all the bond moments arising due to the hydrated polar group and that of the hydrocarbon chain (p = p, pch). While the dipole moment of the hydrocarbon chain is directed from the monolayer (-) to the air (+), that due to the hydrated group is directed from the monolayer (-1 to the water phase (+).17 In the case of a large cyclodextrin molecule, the dipole moment arising due to the hydrated group may be considered as constant and independent of molecule orientation. However pch is oriented at a n angle 8 with respect to the vertical so that the effective vertical component of the total dipole moment of the molecule is given b y p l = p , p c h cos 8. As the C-H bond moments are relatively small and cancel each other due to the trans configuration of the hydrocarbon chains, the Fch results
+
+
~~~
~~
~
(19) Schalchli, A.; Benattar, J. J.;Tchoreloff, P.;Zhang, P.;Coleman, A. W. Langmuir 1993,9, 1968. (20) Scoberg, D. J.; Furlong, D. N. Drummond, C. J.; Grieser, F.; D a y , J.; Prager, R. H. Colloids Surf. 1991,58,409. (21)Rondelez, F.; Baret, J. F.;Bois, A. G. J.Phys. 1987,48,1225. (22)Kuchhal, Y. K.; Katti, S. S.; Biswas, A. B. J. Colloid Interface Sci. 1973,45,529.
Table 2. Surface Potential (Taken at d = 1.4 x 1013 molecules/cm2)and Effective Dipole Moment @l) and Its Fraction per One Hydrocarbon Chain for a,B, and y Amphiphilic C14 Cyclodextrins molecule
AV (mV)
a
153 215 235
P Y
ICL total
(mD)
2900 4075 4455
pllglucopyranose unit (mD) 483 582 557
mainly from ester groups at the 0-2 and 0-3 positions. The increase inpLwould thus occur when the hydrocarbon chains of the CD molecules become more horizontally oriented, e.g. when 6’ increases. Another contribution to the observed AV values comes from a consideration of the hydration of the hydrophilic primary face of cyclodextrins. The voids between the head groups are filled by water molecules, and when a cyclodextrin packs to a more compact structure the amount of the bound “interstitial” water will be reduced. Hence, for PC6CD exhibiting a more compact structure at the water1 air interface than PCsCD, the dipole moment originating from bound water molecules would appear to be less appreciable in controlling the overall effective dipole moment. The invoked hydration effect which directly affects surface properties of cyclodextrins should account for the possible existence at the interface of three zones of water: water directly bound by hydrogen bonds to the CDs, water in which there has been significant restructuring and bulk water. Given that the surface pressure is a mesure of surface tension and thus of the forces structuring a liquid at the interfacial boundary, all changes in the first two zones will greatly influence the observed ll values. For BC&D the length of the hydrophilic portion of the molecule is about 6.5 A and is not very much different from that of its hydrophobic part which, as estimated from molecular graphics ofP amphiphilic cyclodextrins: is equal to 7.8 A. This relative geometrical equilibrium of hydrophibic and hydrophilic moieties of the PC&D molecule is a possible factor of its differences with the longer chain analogs. The zone ofbound water for the molecule is small in-plane, but the effect of the 7-fold symmetry on the dynamic 6-fold structure of water23will penetrate deeply into the aqueous subphase. In contrast, for PC&D there should be a large in-plane effect, but the depth of restructuring will be masked by the presence of the structured water a t the interface. Apparently the effect of restructuring is more important and hence there is a higher surface pressure for PC&D. Now for BCloCD, the diminution in area leads to a reorientation in the Hbonding network and the water depth of restructuring and compensates for the loss in bound water. For longer chain lengths the smaller decreases in area possibly do not generate such reorientations and only the loss ofbound water causes a decrease in the surface pressure. Analysis of the surface pressure and surface potential data concerning aC14,PCl4,and yC14 cyclodextrins (Figures 5 and 6 )indicates a n important influence of the hydrophilic moiety of these molecules upon their surface properties. Thus aC14 CD displays the highest efficiency both in lowering the surface tension of water as in increasing the surface potential. However, the effectiveness in lowering of the surface tension of water and that of the surface potential change is for pC14 CD lower than for aCll and yc14 CDs. The calculated pl (Table 2) increases with the number of glucopyranose units and is logically the highest for yC14 (23) Franks, F. In Water a Comprehensive Treatise; F. Franks Ed.; Plenum Press: New York and London, 1982.
Tchoreloffet al.
196 Langmuir, Vol. 11,No. 1, 1995 CD. It is remarkable thatpLper one molecule unit appears to be the highest for thepCD molecule. This gives further support for the role of the symmetry order which for aand yCDs are respectively 6 and 8, while for /ED this order is 7. Evidently, the spacial distribution and interfacial ordering of the hydrophobic chains of these CDs as well as their interaction with the water subphase determines the observed differences in pul data and the highest value of ,ulper one glucopyranose unit observed for the PCD.
Conclusion We have shown that the amphiphilic cyclodextrins having two hydrophobic ester groups a t the secondary face of each glucopyranose unit are capable of forming stable monolayers characterized by varying surface prop-
erties a t the water/air interface. The PC, molecule has been found to exhibit exceptionally high surface pressure and the lowest surface potential which was attributed to its specific geometry. This specificity of the surface properties may have a direct influence on the application of this molecule for molecular recognition and inclusion systems as well as on its self-organizing structures in micelles, vesicules, and emulsions. We have also demonstrated that the electrical properties, as reflected by the surface potential measurements, are in a direct relation with axial molecular symmetries of a,p, and y amphiphilic cyclodextrins. These data have also to be taken into consideration for the formulation of self-organizing systems. LA930248K
