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Complexes between β-Cyclodextrin and Aliphatic Guests as New Noncovalent Amphiphiles: Formation and Physicochemical Studies Tzvetana Bojinova,† Yannick Coppel,‡ Nancy Lauth-de Viguerie,*,† Alain Milius,§ Isabelle Rico-Lattes,† and Armand Lattes† Laboratoire IMRCP, UMR CNRS 5623, Universite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 04, France, Laboratoire de Chimie de Coordination, UPR CNRS 8241, 205 route de Narbonne, 31077 Toulouse Cedex 04, France, and SEPPIC, 75 Quai d’Orsay, 75321 Paris Cedex 07, France Received January 27, 2003. In Final Form: March 31, 2003 The aim of this work was to develop new sugar nonionic surfactants from raw materials of plant origin. These nonionic surfactants are based on the noncovalent association between cyclodextrins (CDs) and fatty alcohols or fatty acids. We describe here (i) the preparation of various complexes between β-cyclodextrin (β-CD) and fatty alcohols or fatty acids, (ii) the evaluation of the surfactive properties of the complexes, and (iii) the structure of one of them which presents interesting surfactive properties. Among the prepared complexes, the β-CD/alcohol complexes present high surface tension efficiencies compared with β-CD/acid complexes. The surfactive properties of the β-CD/alcohol complexes are influenced by the length of the hydrocarbon chain and reach an excellent efficiency for guests having 10-12 carbon atoms. For the complex between β-cyclodextrin and undec-10-en-1-ol, the inclusion mode in aqueous solution was described by nuclear magnetic resonance techniques. The results showed a 1:1 stoichiometry and a dynamic process exchanging the two possible orientations of the guest relative to the β-CD.
Introduction Nonionic surfactants based on sugars have many prospects and actual applications in both chemistry and biochemistry.1-4 In addition to performance, there is a growing demand for surfactants which are also environmentally friendly and produced from renewable resources. In particular, alkylpolyglucosides (APGs) are being increasingly developed,5-9 as in addition they have good dermatological compatibility and biodegradability. Although effective at low temperatures, their development is nevertheless limited by their hydrophilic/lipophilic balance (HLB) which does not reach values as high as those obtained with ethylene oxide derivatives. The aim of this work was to develop new sugar-based nonionic surfactant derivatives from cyclodextrins (CDs). We used the ability of CDs to form inclusion complexes10,11 with a variety of guest molecules to obtain new noncovalent surfactants in which the nonionic polar-head is constituted by the cyclodextrin (Figure 1). * To whom correspondence should be addressed. Phone: 00.33. 5.61.55.61.35. Fax: 00.33.5.61.55.81.55. E-mail: viguerie@chimie. ups-tlse.fr. † Laboratoire IMRCP, UMR CNRS 5623, Universite ´ Paul Sabatier. ‡ Laboratoire de Chimie de Coordination, UPR CNRS 8241. § SEPPIC. (1) Rico-Lattes, I.; Lattes, A. Colloids Surf., A 1997, 123-124, 3718. (2) Blanzat, M.; Massip, S.; Speziale, V.; Perez, E.; Rico-Lattes, I. Langmuir 2001, 17, 3512-3514. (3) Houlmont, J.-P.; Vercruysse, K.; Perez, E.; Rico-Lattes, I.; Bordat, P.; Treilhou, M. Int. J. Cosmet. Sci. 2001, 23, 363-368. (4) Blanzat, M.; Perez, E.; Rico, I.; Lattes, A.; Gulik, A. Chem. Commun. 2003, 244-245 and references cited therein. (5) Andree, H.; Middelhauve, B. Tenside, Surfactants, Deterg. 1991, 28, 413-418. (6) Andree, H.; Tesmann, H. SOFW J. 1995, 121, 598-611. (7) Busch, P.; Hensen, H.; Tesmann, H. Tenside, Surfactants, Deterg. 1993, 30, 116-121. (8) Perez, E.; Rico-Lattes, I.; Lattes, A.; Godefroy, L. French Patent No. 9708461, June 20, 1997, STEPAN. (9) Hill, K.; Von Rybinski, W.; Stoll, G. Alkyl polyglucosides: technology, properties and applications; VCH: New York, 1997.
Figure 1. Covalent and noncovalent amphiphiles.
For the preparation of complexes,12 fatty alcohols and fatty carboxylic acids with different lengths were retained as guests and different cyclodextrins, three native (R-, β-, and γ-) and two modified cyclodextrins (HP-CD and MeCD), as hosts. We present here the results concerning the β-CD complexes: (i) their preparation and the proof of their existence, (ii) their surfactant activity, and (iii) a complete study of the structure of one of these new noncovalent amphiphilic compounds by the techniques of NMR titration13,14 and rotating-frame Overhauser effect spectroscopy (ROESY) experiments.15,16 (10) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803-822. (11) Rekharsky, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 1875-1917. (12) Milius, A.; Trouve´, G.; Boiteux, J.-P.; Bojinova, T.; de Viguerie, N.; Poinsot, V.; Rico-Lattes, I. French Patent No. 0107499, June 8, 2001; extended PCT/FR02/01876, June 4, 2002, Air-Liquide-SEPPIC. (13) Connors, K. Binding Constants. The Measurement of Molecular Complex Stability; John Wiley & Sons: New York, 1987. (14) Fielding, L. Tetrahedron 2000, 56, 6151-6170. (15) Botsi, A.; Yannakopoulou, K.; Perly, B.; Hadjoudis, E. J. Org. Chem. 1995, 60, 4017-4023. (16) Botsi, A.; Yannakopoulou, K.; Hadjoudis, E. Carbohydr. Res. 1993, 241, 37-46.
10.1021/la030030q CCC: $25.00 © 2003 American Chemical Society Published on Web 05/20/2003
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Experimental Section β-Cyclodextrin (β-CD) (purity, 99%) was purchased from Roquette, France, and was used as received. The water content, determined by thermogravimetric analysis and the Karl Fischer techniques, was β-CD‚9.7H2O. Alcohols and acids (octan-1-ol, decan-1-ol, undec-10-en-1-ol, dodecan-1-ol, hexadecan-1-ol, docosan-1-ol, and dodecan-1-oic, tetradecan-1-oic, hexadecan1-oic, octadecan-1-oic, and docosan-1-oic acids) (98% or 99%) were purchased from Aldrich Chemical Co. and were used without further purification. Deuterium oxide (99.6% D2O) was manufactured by Euriso-top (CEA Saclay, France). The water used was ultrapure, obtained with a Pur1te Select Analyst HP (SELI, France) apparatus working at a resistivity of 16 MΩ‚cm. Complex Preparation. The inclusion complexes were prepared using the suspension method. The general method is indicated below. General Method. An aqueous equimolecular suspension of β-CD (∼600-700 g‚L-1) and guest was heated at 55 °C under mechanical stirring (300 t‚min-1) for 1-18 h. The resulting suspension was freeze-dried, leading to the complex as a white powder. Differential Scanning Calorimetry (DSC) Measurements. The thermograms of the various products were recorded on a differential scanning calorimeter (model PYRIS 1DSC, Perkin-Elmer). About 15 mg of sample was placed in a pinholed aluminum sample pan (50 µL, Perkin-Elmer B 0143017 model) and heated at a rate of 10 °C/min in the range of -30 to 200 °C under a stream of nitrogen. For undec-10-en-1-ol, which is liquid at ambient temperature, the sample pan was stored at -18 °C for 3 days before analysis. Determination of the Free Guest Quantity in the Prepared Samples. For the calibration curve, physical mixtures (total weight, 1 g) were prepared by simple homogenization of the weighed guest and β-CD with various mass percentages of guest of 1-100% [(mguest/mmixture) × 100]. Then about 15 mg of sample was analyzed. The melting of free guest present in the sample was associated with an endothermal effect characterized by a Tonset (°C) and a fusion enthalpy in joules per gram of sample. From the measured value of ∆Hf of the guest and from the calibration curve, the weight of noncomplexed guest in each sample was calculated. Tensiometry. The measurements were made with a drop tensiometer (KRUSS GmbH) model DSA 10-Mk2 at 25 °C and repeated on a Prolabo (tensiomat no. 3) apparatus using the stirrup detachment method. Minimum Surface Tensions (γmin). Aqueous solutions of each complex at its saturated concentration were prepared at 25 °C using the solid quantitative complexes prepared as described above. Saturated aqueous solutions of guest and β-CD were also prepared at 25 °C. The surface tensions (γmin) of these saturated solutions were measured at 25 °C. 1H NMR Analysis. NMR measurements were recorded at 318 K in D2O on a Bruker AMX400 spectrometer. All chemical shifts for 1H are relative to TMS using 1H (residual) chemical shifts of the solvent as a secondary standard. Relaxation periods of 30 s were used to obtain reliable integration data in 1H spectra. 1H NMR Titration Experiments. A stock solution of β-CD (10-2 mol‚L-1) in D2O was prepared. It was distributed into several aliquots, and increasing quantities of alcohol were added to give host/guest molar ratios of 1:0.2, 1:0.4, 1:0.6, 1:0.8, 1:1, 1:2, 1:3, and 1:5. The suspensions obtained were vigorously stirred at room temperature for 1 min and stored at 318 K for 2 days. 1H 2D-ROESY. Two-dimensional 1H-1H T-ROESY experiments were acquired with mixing times of 200, 300, and 500 ms. Twenty-four scans for each of the 256 t1 values were collected with 4096 points. The spectra were obtained from suspensions of CD/undec-10-en-1-ol in a 1:2 ratio in D2O. A 1D selective GROESY experiment17 with a mixing time of 300 ms was performed with 1600 transients. A 1D selective total correlation spectroscopy (TOCSY)-ROESY experiment18 with TOCSY and ROESY mixing times of 60 and 300 ms was obtained with 6400 transients. (17) Dalvit, C. J. Magn. Reson., Ser. A 1995, 113, 120-123. (18) Gradwell, M. J.; Kogelberg, H.; Frenkiel, T. A. J. Magn. Reson. 1997, 124, 267-270.
Bojinova et al. Table 1. Fusion Temperatures (Tf in °C) and Fusion Enthalpies (∆Hf) of the Different Guests guest molecule
Tf (°C)
∆H (J/g)
CH3(CH2)11OH CH3(CH2)15OH CH3(CH2)17OH CH3(CH2)21OH CH2dCH(CH2)9OH CH3(CH2)10COOH CH3(CH2)12COOH CH3(CH2)14COOH CH3(CH2)16COOH
26.2 51.4 61.9 73.0 -3.4 47.0 56.4 64.1 71.0
203 ( 0.09 236.0 ( 0.03 253.0 ( 0.15 259.5 ( 0.08 178.0 ( 0.05 175.7 ( 0.09 206.2 ( 0.09 205.4 ( 0.05 211.2 ( 0.09
Selective pulses used were Gaussian truncated at the 1% level with durations as short as possible. Gradients were sine shaped in all experiments with durations of 1 ms.
Results and Discussion Complex Preparation. The inclusion complexes were prepared from equimolecular aqueous suspensions of β-CD and alcohol or carboxylic acids using the usual suspension method.12-19 The alcohols chosen were octan-1-ol, decan1-ol, undec-10-en-1-ol, dodecan-1-ol, hexadecan-1-ol, and docosan-1-ol, and the carboxylic acids were dodecan-1oic, tetradecan-1-oic, hexadecan-1-oic, octadecan-1-oic, and docosan-1-oic acids. To verify if complexation takes place, DSC was used. DSC allows comparison of the thermal behavior for the complexes and for the pure components (host and guest) and enables the formation of complex to be checked by visualizing the disappearance of the characteristic peak of the guest.20 The DSC traces of each pure guest show only the melting peak of the substance characterized by a Tonset (°C) and a fusion enthalpy in joules per gram of sample (Table 1). Pure β-CD has no defined melting peaks but a thermal rise near 300 °C attributed to its decomposition and a broad thermal rise between 120 and 160 °C corresponding to its dehydration.21 The DSC curves (not recorded up to 200 °C) of β-CD-guest physical mixtures show two endothermic peaks, one corresponding to the fusion of the guest and the other corresponding to dehydration from β-CD. Figure 2 depicts the thermograms of β-CD (1), of pure undec-10-en-1-ol (2), the physical mixture undec-10-en-1-ol/β-CD 1:6 (3), partially complexed undec-10en-1-ol (4), and a 1:1 complex (5). The disappearance of the DSC peak corresponding to the fusion of the guest indicates the formation of new species which is evidence for complexation leading to a solid compound in which the guest is totally complexed (trace 5). In trace 4, the area of the fusion peak of the guest (peak A) corresponds to the fusion enthalpy in joules per gram of sample. From this value, which is proportional to the mass of guest present in the physical mixture (Figure 3), the amount of free guest can be determined from the area ∆Hf of this peak compared with that of the melting peak of the pure guest using the relation above:
%MInot-complexed )
∆HMIfree ∆HMIpure
× 100
The different inclusion complexes prepared were obtained with 0-5% (w/w) of free guest.12 The stirring time (19) Szente, L.; Szejtli, J.; Szeman, J.; Kato, L. J. Inclusion Phenom. Mol. Recognit. Chem. 1993, 16, 339. (20) Preiss, A.; Mehnert, W.; Fro¨mming, K.-H. J. Inclusion Phenom. Mol. Recognit. Chem. 1994, 18, 331. (21) Szejtli, J. Cyclodextrins. In Comprehensive Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D.; Vo¨gtle, F., Eds.; Pergamon: Oxford, 1996; Vol. 3, pp 189-205.
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Figure 2. Schematic typical DSC curves recorded between -30 and 200 °C. (1) Pure β-CD; dehydration of β-CD (peak B). (2) Pure undec-10-en-1-ol (peak A: Tf ) -3.4 °C and ∆H ) 169.3 J/g). (3) Physical mixture of 50% undec-10-en-1-ol and 50% β-CD (w/w) (peak A: Tf guest ) -3.4 °C and ∆H ) 85 J/g). (4) Undec-10-en-1-ol partially complexed (peak A: Tf guest ) -3.4 °C and ∆H ) 5.2 J/g). (5) Complex; disappearance of peak A. Table 2. Surface Tensions of Aqueous Saturated Solutions of CDs and Guests at 25 °C guest molecule
Figure 3. Fusion enthalpy (joules/gram) corresponding to the melting peak of the guest (undec-10-en-1-ol) versus the weight percentage of guest in various β-CD-guest physical mixtures.
necessary for the quantitative complexation increased with the length of the hydrocarbon chain. Surfactant Properties of the Complexes. To evaluate the surfactive properties of the complexes prepared, the minimum surface tensions (γmin) of aqueous solutions of each complex were measured at 25 °C and compared with those determined for aqueous solutions of the corresponding guest alone (Figure 4). The surface tensions
β-CD octan-1-ol decan-1-ol undec-10-en-1-ol dodecan-1-ol octadecan-1-ol hexadecan-1-ol dodecanoic acid tetradecanoic acid hexadecanoic acid octadecanoic acid
saturated concentration in water (mol‚L-1) at 25 °C 10-2
1.4 × 3.20 × 10-3 2.5 × 10-4 7.2 × 10-5 9.1 × 10-6
