Langmuir 1996,l I, 3997-4000
3997
Monolayer and Multilayer Films of Cyclodextrins Substituted with Two and Three Alkyl Chains M. H. Greenhall,tl$JiP. Lukestl$J, R. Kataky,t N. E. Agbor,t>§J. P. S. Badyal,# J. Yanvood,§ D. Parker,# and M. C. Petty*!+ School of Engineering and Centre for Molecular Electronics and Department of Chemistry, University of Durham, South Road, Durham D H l 3LE, U.K., and Materials Research Institute, Shefield Hallam University, City Campus, Pond Street, Shefield S I 1WB, U.K. Received December 2, 1994. I n Final Form: June 15, 1995@ An investigation is reported on the behavior of amphiphilic cyclodextrins at the aidwater interface. Stable surface pressure vs area curves were recorded for many ofthe materials investigated. The addition of N(CH&Cl o r KCl to the subphase w a s found to result in a small b u t reproducible contraction in the isotherms. Preliminary work also showed that Langmuir-Blodgett films of some materials could be built u p on solid surfaces, although the resulting layers were of poor quality.
Introduction Cyclodextrins (CDs) are cyclic oligosaccharides forming a macrocyclic cavity that c a n form inclusion complexes with c e r t a i n cations and neutral Figure 1 s h o w s the basic glucose m o n o m e r unit of a C D ring. A different number of repeating units lead t o a (6 units), ,b (71, o r y (8)CDs; the dimensions of p-CD are s h o w n in Figure 2a. The complexation behavior of the C D s c a n be modified b y s u b s t i t u t i o n s at the 2, 3, and 6 oxygen positions, Figure 2b. F o r example, if R2 and R6 are alkyl chains, the r e s u l t i n g molecules s h o w selectivity for a l k y l a m m o n i u m ions and d ~ p a m i n e . ~S-u~c h substitutions c a u s e the hydrophilic hydroxyl faces of the CD molecule t o become hydrophobic. The material can then form an insoluble layer at the aidwater interface and m a y be deposited onto solid s u p p o r t s using the L a n g m u i r Blodgett (LB) technique. There has recently been s o m e interest in the properties of s u c h ultrathin layers. The motivation for this w o r k is 2-fold; first, t o s h o w specific recognition (on the water surface o r in the solid s t a t e ) at the molecular level;6 and second, in the longer term t o exploit s u c h interactions in electronic (or optoelectronic) molecular devices. Previous studies have involved the a t t a c h m e n t of an alkyl c h a i n at the 0-6 position (via a S o r SO link)',* and the addition of amphiphilic ester groups at the 0 - 2 and 0-3 position^.^ Here, we report o n the monolayer formaSchool of Engineering and Centre for Molecular Electronics, University of Durham. Department of Chemistry, University of Durham. 5 Sheffield Hallam University. Current address: Department of Chemistry & Biological Sciences, University of Huddersfield, Huddersfield HD13DH, U.K. Current. address: Dipartimento di Chimica, Universita la Sapienza, Piazzale Aldo Moro 5, 00185 Roma, Italy. Abstract published in Advance ACS Abstracts, September 1, 1995. (1)Szjelti, J. Cyclodertrin Technology; Kluwer-Dordrecht: The Netherlands, 1989. +
*
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(2) Duchene, D., Ed. Cyclodextrins and their Industrial Uses;Editions de Sante: Pans, 1987. (3)Bates, P. S.;Kataky, R.; Parker, D. J.Chem. Soc., Chem. Commun.
1993,691. Chem. Commun. (4)Bates, P. S.;Kataky, R.; Parker, D. J.Chem. SOC., 1993,693. (5)Bates, P. S.; Kataky, R.; Parker, D. Analyst 1994,119, 181. (6)Schierbaum, K. D.;Weiss, T.; Thoden van Velzen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Gopel, W. Science 1994,265, 1413. (7)Tanaka, M.; Azumi, R.; Tachibana, H.; Nakamura, T.; Kawabata, Y . ;Matsumoto, M.; Miyasaka, T.; Tagaki, W.; Nakahara, H.; Fukuda, K. Thin Solid Films 1994,244, 832.
0743-746319512411-3997$09.00/0
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I O %1
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Figure 1. Monomer unit of cyclodextrins used. Details of the substitutions at the positions indicated are in Table 1.
I
fi
0.79 m nQ
p. cydodemin
Pi
(a)
(b)
10.62 nm 1.54 nm
Figure 2. (a) Dimensions of P-cyclodextrin. (b) Structure of substituted cyclodextrins. tion and LB film deposition of a range of C D materials substituted with alkyl groups at the 0-2, 0-3, and 0-6 positions.
Experimental Section The CDs used in this work are shown in Table 1. The compounds were synthesized in-house using methods that have been described p r e v i o ~ s l y . ~Most J ~ of the CDs studied were p type, but there were also two a-CDs and one I/-CD. The materials were dissolved in chloroform to a concentration of approximately 1 g L-' and spread onto the surface of carefully purified water (produced by reverse osmosis, deionization, and W sterilization) at a temperature of 20 f 2 "C and pH 5.8 k 0.1 in a constant perimeter barrier LB trough. l1 Following evaporation of the solvent (10 min), the floating layer was compressed at a rate of (8)Matsumoto, M.; Tanaka, M.; Azumi, R.; Tachibana, H.; Nakamura, T.; Kawabata, Y.; Miyasaka, T.; Tagaki, W.; Nakahara, H.; Fukuda, K.
Thin Solid Films 1992,2101211, 803. (9) Coleman, A. W.; Zhang, P.; Parrot-Lopez, H.; Tchoreloff, P.; Baszkin, A,; Ling, C. C.; de Rango, C. J.Phys. Org. Chem. 1992,5,518. (10)Bates, P. S.;Parker, D.; Patti, F. J. Chem. SOC.,Perkin Trans. 2 1994,657.
0 1995 American Chemical Society
Greenhall et al.
3998 Langmuir, Vol. 11, No. 10, 1995 Table 1. Substituted Cyclodextrins Used in This Worka compound no. 2,6-di-O-octyl a-CD 1 2,6-di-O-octylp-CD 2 2,6-di-O-octyl y-CD 3 2,3,6-tri-O-octyl p-CD 4 2,6-di-O-dodecylp-CD 5 3-O-methyl-2,6-di-O-dodecyl 6 B-CDb
n Rz R3 6 C8H17 H 7 C8H17 H 8 C8Hi7 H 7 %Hi7 C8Hi7 7 C12H25 H 7 C12H25 CH3
Rs C8H17 C8H17 C8H17 CsHi7 C12H25 C12H25
The number of the compound is referred to in the text. n = number of glucose units in ring. The position of the R,groups is given in Figure 1. Compound 7 has an average of 17.4 octylgroups per CD, and the remaining OH groups (3.6)are capped with methyls. In contrast, compound 6 has 7 methyls per CD and 14 Clz chains and is close to being monodisperse. about nm2 molecule-' s-1. The same floating film was compressed and expanded several times to investigate any hysteresis in the surface pressure vs area curves. The effect of deliberately adding ions to the subphase was studied by successive compressions of the same floating CD layer over subphases of increasing ionic strength. Gentle agitation of the water outside the enclosed area aided mixing ofthe solution. The trough was left a t least 10 min after stirring before the floating layer was recompressed. The effect of leaving for a longer time (up to 2 h) was investigation but found not to affect the results. A control experiment with each monolayer was also undertaken. Here, the water was stirred without the addition of solute; this was found to have no effect on the measured isotherms. LB film deposition was undertaken using either goldcoated glass microscope slides or silicon crystals. Reflection absorption infrared spectra (RAIRS)(for gold-coated glass substrates) were measured. The angle of incidence was 85". Fourier transform infrared (FTIR) spectra were recorded at 4-cm-l resolution with a Mattson Sirius 100 spectrometer equipped with a liquid-nitrogen-cooled mercury-cadmium telluride detector. Each spectrum was the accumulation of 1024 scans, taking approximately 10 min to record. Atomic force microscopy offers structural characterization of surfaces in the 10-4-10-10-m range without the prerequisite of special sample preparation (e.g., metallization). A Digital Instruments Nanoscope I11 atomic force microscope was used to examine the topographical nature of the LB films. All of the AFM images were acquired in air using the tapping mode. This technique employs a stiff silicon cantilever oscillating a large amplitude near its resonant frequency (several hundred kilohertz). The rms amplitude is detected by an optical beam system. Alarge rms amplitude is used to overcome the capillary attraction of the surface layer, while the high oscillation frequency allows the cantilever to strike the surface many times before being displaced laterally by one tip diameter. These features offer the advantage of low contact forces and no shear forces.
Results and Discussion MonolayerExperiments. Figure 3 shows the surface pressure vs area isotherms over pure water for the CDs listed in Table 1. Numerical data obtained from these curves are given in Table 2. Most of the molecules with OH groups (1, 2, and 5 ) exhibited a distinct constantpressure plateau in their isotherms, followed b y a f u r t h e r rise in pressure as the molecular area decreased. The exception was 3,the y-CD, which only showed a change in slope in the surface pressure vs area plot. A small, but reproducible, secondary pressure rise w a s also evident for molecule 8 (Figure 3b). The area per molecule obtained by extrapolating the steeply rising section of the initial pressure rise t o zero pressure clearly increases with the number ofglucose units (11)Petty, M. C. In Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum: New York, 1990.
0.0
0.5
1.5
1.0
2.0
2.5
3.0
3.5
4.0
4.5
h a per molecule [nm2 1
25
b
O
0.0
'
I
0.5
'
I
'
1.0
I
'
1.5
I
2.0
'
I
2.5
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I
3.0
'
3.5
4.0
4.5
k e a per molecule [run2 1
Figure 3. Pressure vs area isotherms for CDs recorded on a pure water subphase: pH 5.8 f 0.1; temperature 20 "C, compression rate nm2 molecule-' s-l. (a)CDs without a substitution at the 0 - 3 position. (b) CDs with an alkyl substitution at the 0-3 position. The numbers on the plots correspond to the materials given in Table 1.
in the CD ring (compare the isotherms for 1, 2, and 3). However, the v a l u e s are significantly larger than those calculated for the cross-sectional areas of unsubstituted CDs, Table 3.12J3 For example, compound 1, the 2,6-di0-octyl a-CD, exhibits an area per molecule of 2.67 nm2 compared t o a figure of 1.67 nm2calculated for the parent a-CD. However, 0-alkylation is known t o increase the external diameter of the CD molecule^.'^ The areas per molecule are also significantly greater than those reported by Kawabata et al. for C D s substituted with one chain15 but similar t o those of Ahang et al. for the diester compound^.^ Increasing the length of the alkyl g r o u p produces a slight increase in the area per molecule and a decrease in the constant-pressure plateau (compounds 2 and 6). This is analogous t o the effect of molecular length o n the expanded-to-condensed phase transition in simple long-chain fatty acids.16 A further observation from Figure (12) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980,19, 344. (13)Schurig, V.;Nowotny, H.-P.Angew. Chem.,Int. Ed. Engl. 1990, 29., 939. -.
(14) Harata, K.;Uekama, F.; Otagiri, M.; Hirayama, F. Bull. Chem.
SOC.Jpn. 1987,60,497.
(15) Kawabata, Y.; Matsumoto, M.; Nakamura, T.; Tanaka, M.; Manda, E.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Thin Solid Films 1988,159, 353. (16)Gaines, G.L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley-Interscience: New York, 1966.
Langmuir, Vol. 11, No. 10,1995 3999
Monolayer and Multilayer Films of Cyclodextrins
Table 2. Measurements from the Surface Pressure vs Area Isotherms of Amphiphilic CDs initial pressure rise second pressure rise
(CD)
area per molecule, nm extrapolated to at onset zero pressure of plateau
1 2
3 4 5 6 7 8 a
2.67 3.69 4.41n 4.55 4.07 3.93 4.10 3.69
plateau pressure, mN m-l 12.1 10.1
2.35 3.19 3.51a 4.28 3.73 3.34 3.50 2.96
area per molecule, nm extrapolated to zero pressure at collapse
collapse pressure, mN m-l
1.39 1.69
0.95 1.38
15.8 17.0 34.3b
1.86
C
30.6b
1.41
19.7
11.9
11.5 7.3 5.5 11.3 17.5
Point of inflection. Maximum pressure attained. No distinct collapse. Errors: area 4~0.05nm2 molecule-’; surface pressure f 0 . 2
mN m-l.
Table 3. Properties of Unsubstituted Cyclodextrins11J2 no. of glucose units molec mass ext diam, nm
largest cross-sectional area, nm2 int diam, nm height, nm cavity vol, nm3
P
Y
a 6 973 1.37- 1.46 1.67
2.24
1297 1.69- 1.75 2.41
0.47-0.52 0.78 0.176
0.60-0.65 0.78 0.346
0.75-0.85 0.78 0.510
7 1135 1.53- 1.69
8
3 is that CDs with three alkyl substitutions (i.e., 4,6, and 8 ) possess larger areas per molecule than those with a simple hydrogen in the R3 position (1,2, 3,and 5). It is likely that our CDs will orient on the subphase surface with their hydrophobic alkyl groups uppermost, directed away from the water. The only way in which this can happen is if one o r more of the alkyl groups are folded back around the “bucket-shaped”CD molecule. For molecules with two chains (Rz and Re), it is plausible that these are oriented with their polar OH groups (R3) in contact with the subphase. The alkyl chain at the 0-6 position will then be pointed directly away from the water surface, while that at Rz will be folded around the molecule. This, of course, will prevent close packing ofthe CD moiety. The addition of another alkyl substituent at the Rz position will produce a further increase in the average area per molecule. The surface pressure plateau and secondary pressure rise noted in some CDs (1,2, and 5) may be associated with an expanded-to-condensed (first-order) phase transformation of the alkyl regions between the CD units. However, the extrapolated areas per molecule in these regions of the isotherms are less than expected from the unsubstituted CDs. It is therefore possible that the constant-pressure region is associated with bilayer formation. The recompression isotherms for all eight molecules were within 0.01 nm2 of those measured initially if the surface pressures corresponding to the plateau region were not exceeded. Subsequent isotherms, measured up to 2 h later, were identical within experimental error (again, ifthe plateau pressures were not exceeded). Ifthe plateau pressures were exceeded, subsequent recompression isotherms were all shifted to smaller areas per molecule (by 0.03-0.12 nm2). Addition of N(CH&Cl to the subphase beneath CDs 2 and 7 caused no change in the isotherm for concentrations below M. At a subphase concentration of M, a shift to smaller areas was noted for both compounds: 0.02 nm2for 2 and 0.04 nm2 for 6. This contraction increased on further increasing the concentration of N(CH&Cl. The isotherms measured for compound 7 are shown in Figure 4. Experiments were also conducted using KC1 added to
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lo4,
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2
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i
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Figure 4. Effect on surface pressure vs area curve of methyl per octylp-CD(7) on the addition ofN(CH&Cl to the subphase. 0.025
-
3500
3000
2500
2000
1500
1000
Wawnumbers [cm” ]
Figure 5. FTIR M R S spectrum for LB layers (six deposition cycles) of 2,6-di-O-dodecylp-CD (5) deposited onto gold-coated glass at 20 mN m-l.
the subphase. Similar effects were noted for compounds 1 and 7. A different phenomenon was noted for the y-CD 3. Here the pressure vs area curve moved to larger areas on the addition of KC1: up to 0.04 nm2 for 1 x M solution; above this, no further expansion was noted. The CDs used in this study do not have a strong affinity for KC1 as a guest.3 Therefore, the changes in the isotherms with increasing concentration of both KC1 and N(CH3)&1 are most likely to be associated with electrostatic effects occurring at the aidwater interface rather than to specific guest-host interactions. LB Film Deposition. Preliminary attempts were made to deposit layers of the CDs onto solid supports using the LB technique. In general, the floating layers were transferred on the upstroke of the substrate but came off as the substrate was subsequently lowered through the aidwater interface. Further work showed that this effect could be reduced by increasing the speed of the substrate on its downstroke. For compound 5,Z-typeLB films could be built up at a surface pressure of 20 mN m-l (.i.e., above
Greenhall et al.
4000 Langmuir, Vol. 11, No. 10, 1995
2.000 Uu/diu 2 2.000 uw/div X
Figure 6. AFM image of eight LB layers of 2,6-di-O-dodecyl p-CD (5) deposited onto a silicon crystal at 20 mN m-l.
the plateau in the isotherm in Figure 3a) using a substrate upstroke speed of 60 mm min-l and downstroke speed of 20 mm min-l. However, the deposition was poor, with transfer ratios on the upstroke varying from 0.2 to 0.8. Similar results were found for deposition pressures below the plateau. Figure 5 shows the RAIRS spectrum of compound 5 (six dipping cycles) deposited a t a surface pressure of 20 mN m-l. The RAIRS technique effectively couples the IR radiation to transition dipole moments that are aligned perpendicular to the substrate surface. The v(CH2) stretching region shows bands at 2927 cm-l (the antisymmetric CH2 stretch) and 2955 cm-l (the symmetric CH2 stretch), showing that the hydrocarbon chains are largely disordered, with a high population of gauche conformers.l7*l8Support for this conclusion is provided by a very weak v,(CH3) band intensity at 2875 cm-l. Our (17) Synder, R. G.;Strauss, H. L.; Ellinger, C . A. J . Phys. Chem. 1982,86,5154.
IR data are, in fact, similar to those measured in LB films of fatty acids using the attenuated total reflection (ATR) method where there are active electric fields in all directions. Unfiltered AFM data are shown in Figure 6 for eight LB layers of compound 5 deposited onto single-crystal silicon. AFM photographs of the uncoated substrate revealed a completely flat surface under the same magnification. It is evident that the CD molecules have formed aggregates of diameters from approximately 0.1 to 2 pm. This accounts for the poor LB deposition characteristics. On the water surface, a well-ordered monolayer film is formed. However, on transfer to the solid substrate, the CD molecules coalesce to form aggregates that are more than 1 molecule in thickness. Further work is clearly needed to improve the quality of the transferred layers.
Conclusions The monolayer characteristics of a range of cyclodextrin compounds substituted with alkyl groups at the 0-2,O-3, and 0-6 positions have been investigated. Stable pressure vs area characteristics were noted for several materials studied. The addition of N(CH&Cl or KC1to the subphase was found to result in small but reproducible shifts in the surface pressure vs area curves. Preliminary work showed that Langmuir-Blodgett film depositionwas possible with some materials. However, atomic force microscopy and infrared spectroscopy revealed that the resulting films were disordered, consisting of aggregates of the cyclodextrin molecules. Acknowledgment. We thank the Molecular Electronics Committee of EPSRC for supporting this work. Tony Patti and Paul Bates are thanked for their work in synthesizing the cyclodextrins used in this study. LA940959Y (18)McPhail, R. A.; Strauss, H. L.; Synder, R. G.; Ellinger, C. A. J . Phys. Chem. 1984,88,334.
