Langmuir 1993,9, 1968-1970
1968
Structure of a Monomolecular Layer of Amphiphilic Cyclodextrins A. Schalchli and J. J. Benattar’ Service de Physique de I’Etat Condensk, CEA-Saclay, F-91191 Gif-sur- Yvette Cedex, France
P. Tchoreloff CNRS URA 1218, Laboratoire de Physico-Chimie des Surfaces, Centre Pharmaceutique, Universitk Paris Sud, 92290 Chbtenay-Malabry, France
P. Zhang and A. W. Coleman CNRS ER 45, Groupe Cyclodextrines Amphiphiles, Centre Pharmaceutique, Universitk de Paris Sud, 92290 Chatenay-Malabry, France Received April 13,1993. In Final Form: June 17,1993 The cyclodextrins are known to present solubility properties which strongly depend on the symmetry of their macrocycle. In the present paper, we investigate by X-ray reflectivity monolayers of amphiphilic a-,j3-,and y-cyclodextrins deposited on silicon wafers. We show the relationships between the in-plane packing of the three kinds of cyclodextrin molecules and the measured structural parameters such as the interfacial roughnesses, densities, and thicknesses.
The cyclodextrins are a class of cyclic oligosaccharides possessing 6(a),7(8), or 8(y) 1-4-linked glucopyranose units; their geometry is that of a truncated cone with two hydrophilic facessurrounding a hydrophobic cavity. They have been widely used for their ability to include and transport a wide variety of organic molecules.’ The solubility of 8-CD (20 g L-9 is anomalously low compared to a-CD (145 g L-I) or 7-CD (220 g L-9, and it has been possible recently2 to correlate this low solubility to unfavorable interactions between the 7-fold symmetry of the macrocycle and the dynamic 6-fold symmetry of water. Amphiphilic cyclodextrins may be synthesized by the selective modification of one face with hydrophobic groups, and a wide range of such molecules are now available.” We have recently shown, via measurement of the dipole constants at the air-water interface of cyclodextrins having hydrophobic ester groups attached to the secondary face, that there exists a clear difference in the hydration of the polar saccharide headgroups between a-or y- and 8-CD. Such differences should lead to a differencein the structure of the monolayer and also in the absorption of monolayers onto solid supports. In the present paper, we will investigate by means of X-ray reflectivity the three kinds of CD monolayers deposited on silicon wafers at low and high surface pressures. This X-ray optical technique provides very rich, accurate, and direct information on electron density along the normal to the layer. Thus, it is possible to obtain separate information on these Langmuir monolayers: their total thicknesses, their roughnesses, and their densities.
* To whom corresmndence should be addressed.
(1)Coleman, A. W.f Keller, N.; Dalbiez, J. P.; Nicolis, I. J. Inclusion Phenom. 1992,13,139. (2)Szetjili, J. Cyclodertrin Technology; ~. Kluwer Academic Publishem Dordiecht, 1988. (3) Parrot-Lopez,H.; Ling, C. C.; Zhang, P.; Baezkin, A.; Albrecht, G.; de Ftango, C.; Coleman, A. W. J. Am. Chem. SOC.1992,113,6479. (4)Zhang, P.; Ling,C. C.; Coleman, A. W.; ParrotLopez, H.; Galons, H. Tetrahedron Lett. 1991,32,2769-2770. (5) Zhang, P.; Parrot-Lopez,H.;Tcherloff, P.;Ling,C. C.;Baszkin,A.; de Rango, C.; Coleman, A. W. J. Phys. Org. Chem. 1992,5,507. (6)Coleman, A. W.;Kaeeelouri, A. Supramol. Chem. 1993,1,161. (7)Taneva, S.;Ariga, K.; Okahata,Y.;Tag&, W. Langmuir 1989,5, 111.
X-ray Reflectivity Method and Apparatus An X-ray reflectivity experimentconsists in the measurement of the ratio R(8) = Z(O)/ZO,where ZOis the intensityof the incident beam, and Z(8) that of the reflected beam at an angle 8. The wave vector transfer q is perpendicular to the plane of the substrate (q = 4u sin O/Xz), thus giving information on the projection of the electron density p(z) onto the normal to the surface of the sample. The refractiveindex of matter n for X-ray wavelengths is n = 1- 6 - ib, where 6 is proportional to the electron density p (6 = (X2rd2u)p where Xis the X-ray wavelength, r. the classical radius of an electron and p the electron density) and @ is proportionalto the linear absorption coefficient. Sincethe index is less than one, total external reflectivity occurs for angles of incidence smaller than a critical angle 0, = (26)’12 (typically 3.9 mrad for silicon). The reflectivity averaged over the coherence of the beam is given by
wherep, is the electrondensityof the substrate$&), the Fresnel reflectivity of the surface of the substrate considered as a perfect diopter, falls as (8J28)4. Obviously in (l),the shape of the reflectivity curves results from interferences between the beams reflected by the different interfaces. As in many similar cases, the present system can be described as being composed of chemicallyhomogeneous layers and a model can be constructed by considering these layers as slabs of constant density. In this paper, we shall use only one stratum for the monolayer of amphiphilic cyclodextrins and a semiinfiiite stratum for the silicon substrate. We have to take into account the roughness of the sample (i.e., the standard deviation of the interface height u = ( z 2 ) W by smearing the model through a convolution by a Gaussian function. Practically reflectivity calculations are performed using the matrix formalism for stratified media.* Experimenta were carried out using a special four-circle diffractometerg designed in the laboratory and constructed in collaboration with Micro-Contr8le (Evry, France). A conventional fine-focus (40pm X 8 mm) copper tube is used as the X-ray source. A Si(ll1) monochromator is used to select the Cu K a l line (A = 0.154 05 nm). A low divergence (fwhm = 0.3 mrad) (8)Born, M.;Wolf,E. Principles of0ptics;Pergamon Press: London, 1969. (9)Daillant, J.; Benattar, J. J.; LBger, L. Phys. Rev. 1990,A41,19631977.
0743-7463/93/2409-1968$04.00/0 0 1993 American Chemical Society
Langmuir, Vol. 9, No. 8,1993 1969 I on
,E z
60.
E
...........
Y
o-CD14
0-C.14
and
7-CD14
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2
- IO+ -
VI VI
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al
s
o 8-CD14
lo-'
v
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v)
20
10-6
'
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0' I
.
100
200
300
400
Molecular area (A2) Figure 1. Surfacepressure isotherms for the diesters a-,8-, and y-CD14. Arrows indicate the surface pressures of the different transfers on silicon wafers.
0
15 30 incidence angle h a d )
45
Figure 2. X-ray reflectivity curves of the a-,8-, and y-CD14 recorded at low surface pressure (-9 mN/m). The best fit is given by a solid line.
is obtained by means of a small slit (SD= 100 pm width) placed at a distance of 40 cm from the source. For each incident angle, the reflected beam is scanned around the specular position in order to measure the background which ia then subtracted.
Io-' lo-2
t\
Synthesis and Deposition on Silicon Wafers The synthesis of the C W 1 4 ester has been previously described,' as have their aggregation behavior in nonaqueous solvents and the formation of monolayers by these systems! Chloroform solutions of the CD were spread by means of a micropipet, 16 min before compression. Since longer times appeared to have no effect on the shape of the surface-pressure isotherms (*/A curves), it was considered that total solvent evaporation took place during this 16 min. The LangmuirBlodgett technique was wed to transfer the spread monomolecular f i i from the water surfaceto the solid surfaceof silicon wafers. The mobile barrier of the Langmuir trough was moved at a constant rate of 8.33 X 1 V ma/,, and the vertical drawing was adjusted so that the mnplingswere performed at a constant surface preasure of firstly 9 mN/m and secondly 40 mN/m, just before the collapse pressure, for a-,8-, and y-CD14. All the experiments were carried out at 22 OC. The surface of the Langmuir trough was in every case constituted of triply distillatad water. The surface of the water was cleaned with a capillary pipet connected to a vacuum pump before each experiment. The surface-pressure isotherms of the a-,8-, and yCD14 are represented in Figure 1, along with the schematic representation of the general CD14 diester molecule.
Results and Discussion The experimental reflectivity curves of the CY-,,&,and y-CD systems have been recorded both at low surface pressure in aliquid phase a t -9 mN/m and a t high surface pressure in a solid phase a t 40 mN/m. The set of curves recorded a t low and high surface pressures are reported, respectively, in Figures 2 and 3. In these figures all the observed reflectivity profiles exhibit a well-defined interference fringe correspondingto the interference between beams reflected a t the two interfaces9 aidfilm and film/ substrate. The observation of such a fringe in a reflectivity curve demonstrates that all the monolayers were well deposited on the substrate. The experimental reflectivity curves were well fitted using a one-slab model, for each CD film. It should be pointed out that the electron density gradients between the chemical parts of the molecule are too small to use (10)Mlorgey, 0.;Tchoreloff,P.; Benattar, J. J.; Proust, J. F. J.Colloid Interface Sci. 1991,146,373-382.
10-8
'
0
1
15 30 incidence angle h a d )
45
Figure 3. X-ray reflectivity curves of the a-,8-, and yCD14 recorded at high surface pressure (40 mN/m). The best fit is given by a solid line.
Table I. Surface Pressure ( v ) , Electron Density (a), Thickness (e), Tilat Angle, Film/substrate and &/film roughneesee (Sps and om), and Hydration (water moleculee/CD molecule) of the Different Samples r bX e tilt UM CD (mN/m) lV (A) (deg) (A) (A) hydration a-and./-CD -9 3.9 22 48 5 6.4 ( ~ , 6 . 3
8-CD
-40
4
23
44
5
5
-9 -40
2.5 4.2
20 24.3
39
5 5
4.6 8
7, 6.6
a,2.4 B15
6.75
At low surface pressure due to the presence of holes, the tilt angle and hydration cannot be estimated for 8-CD. a
a multislab model. The calculated reflectivity curves are represented in Figures 2 and 3 by solid lines, and the corresponding parameters are summarized in Table I. The most striking feature is that the curves of a-CD14 and y-CD14 are identical a t both low and high surface pressures and differ from that of 8-CD14, indicating a very similar organization of the a-and y-CD14 layers. The other results which can be extracted from Table I are the following: The influence of the surface pressure on the layer thickness seems to be very weak for the a-and y-CD14 whereas in the case of the 8-CD14 the thickness strongly depends on the surface pressure. The density of the a- and y-CD14 increases slightly with the surface pressure. For 8-CD14, the density is very low at low surface pressure (much less than the density of a homogeneous layer) and increases strongly a t high surface pressure. The last and main feature is that the
1970 Langmuir, Vol. 9, No. 8, 1993 external (film/air interface) and internal (substrate/film interface) roughnesses are equal for the a- and y-CD14 (5-6 A) whereas for the O-CD14 the external roughness is higher (8 A) than the internal one (4.6 A). In another respect, it should be pointed out that in Table I the degrees of hydration of the different molecules are directly derived from the densities assuming that the molecular areas (measured on the isotherms) remain unchanged after deposition. The hydration of the a-CD14 decreases from 6.3 to 2.4 (water molecules/a-CD14) between low and high pressures. This effect can be simply explained by the expulsion of the water located between CD moleculesduring the compression,leavingthe 2.4 water molecules in the cavity. In the case of the y-CD14, the hydration decay is weaker (6.6-5) as the number of water molecules inside the CD cavity at the level of the primary OH and pyranosidic and glycosidic oxygens is greater due to the increased diameter of y-CD14. Concerning the O-CD14 the hydration is higher than expected at high pressure (6.75 water molecules/~-CD14)because water molecules can be trapped between molecules whose packing is not fully compact, in contrast to the packing observed for a- and y-CD14. All the observations mentioned above seem to be consistent with the following description. The a- and y-CD14 have a very good lateral packing due to the 6- and %fold molecular symmetries. Moreover, the roughness being identical and small for both interfaces, the CD-film packing must closely fit the substrate surface structure. This feature indicatesthat the f i i is made of homogeneous molecular units and that the tilt angle a of the aliphatic chains is well defined. The mean tilt angle values (Table I) are estimated from the ratio cos a = (e-ecD)/lwhere e is the total film thickness, ecD the depth of the CD molecules (-8 A), and 1 the length of the fully extended aliphatic chains. Note that we suppose that the tilt only concerns the aliphatic chains. In the case of O-CD14 the behavior sppears to be very different. At low surface pressure, the density is far less than that of a close-packed arrangement. The film has holes (-25% of the total area without molecules) whose origin during the deposition
Letters
a-
CD14
8% y-
Us:extra roughness
mm CD14
p-
CD14
Figure4. Schematicdrawingof the possible molecular packings.
could be related to the 7-fold symmetry. At high surface pressure nevertheless, although the density corresponds to a close-packed arrangement, the 7-fold symmetry induces a disorder. This disorder is evidenced by the increase of the external roughness which becomes higher than that of the substrate (Figure 4). This feature could be due to a modulation of the tilt angle of the molecules. This modulation is induced by the rather poor in-plane molecular packing related to the 7-fold symmetry. The ‘corollas” formed by the cone-shaped aliphatic chain distribution (Figure 4) are more or less opened. This implies a modulation of the film thickness. Thus, this length modulation could be directly responsible for the observed extra external roughness of the p-CD14. In conclusion we have shown that the X-ray reflectivity is an excellent method for the investigationof the packing of cyclodextrin amphiphiles at the solid interface. The differences among a-,y-,and O-CD14 in their mode of packing and oganization are clearly evident. These differences would seem to arise from an incompatibility between the 7-fold symmetry of O-CD14 and a good inplane ordering.
