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Formation of a Supramolecular Gel between r-Cyclodextrin and Free and Adsorbed PEO on the Surface of Colloidal Silica: Effect of Temperature, Solvent, and Particle Size Ce´cile A. Dreiss,* Terence Cosgrove, Francisco N. Newby, and Edvaldo Sabadini† School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, United Kingdom, and Instituto de Quimica, Universidade Estadual de Campinas, Caixa Postal 6154, CEP 13084-862 Campinas, Brazil Received February 26, 2004. In Final Form: July 29, 2004 Aqueous solutions of R-cyclodextrin (R-CD) complex spontaneously with poly(ethylene oxide) (PEO), forming a supramolecular structure known as pseudopolyrotaxane. We have studied the formation of the complex obtained from the threading of R-CD onto PEO, both free in solution and adsorbed on colloidal silica. The kinetics of the reaction were studied by gravimetric methods and determined as a function of temperature and solvent composition for the PEO free in solution. PEO was then adsorbed on the surface of colloidal silica particles, and the monomers were displaced by systematically varying the degree of complexation, the concentration of particles, and the molecular weight of the polymer. The effect of the size of the silica particles on the yield of the reaction was also studied. With the adsorbed PEO, the complexation was found to be partial and to take place from the tails of the polymer. The formation of a gel network containing silica at high degrees of complexation was observed. Small-angle X-ray and neutron scattering experiments were performed to study the configuration of the polymeric chains and confirmed the partial desorption of the polymer from the surface of the silica upon complexation.
Introduction R-Cyclodextrin (R-CD) is a cyclic oligosaccharide consisting of six glucose units linked by 1,4-R-glucosidic bonds. It has a toruslike conformation, with an outer surface that is hydrophilic in nature, with primary (small end) and secondary (large end) hydroxyl groups on the rims of the molecules. The cavity, which is lined with alkyl groups and glycosidic oxygen atoms, is hydrophobic1-3 and can act as a host for a great variety of molecular guests.4 Aqueous solutions of R-CD spontaneously form a complex with poly(ethylene oxide) (PEO), known as a pseudopolyrotaxane. These complexes are the starting point of further interesting structures, such as molecular necklaces, molecular trains, and molecular tubes.5-8 The formation of the complex involves the threading of the R-CD along the polymer chain, into a “necklacelike” structure. This process is driven by noncovalent attractive forces, therefore allowing the R-CD to slide along the polymer backbone. The formation of the supramolecular adduct is entropically unfavorable for the guest polymer, as the linear polymer chain must fit into several host units to produce the final complex. The pseudopolyrotaxane is promoted by strong hydrophobic interactions between the cavity of the R-CD * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Universidade Estadual de Campinas. (1) Szejtli, J. In Comprehensive Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D, Vogtle, F., Eds.; Pergamon: Exeter, 1996; Vol. 3, p 12. (2) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (3) Rusa, C. C.; Luca, C.; Tonelli, A. E. Macromolecules 2001, 34, 1318. (4) Rusa, C. C.; Bullions, T. A.; Fox, J.; Porbeni, F. E.; Wang, X.; Tonelli, A. E. Langmuir 2002, 18, 10016. (5) Harada, A.; Li, J.; Kamachi, M. Nature 1993, 364, 516. (6) Lo Nostro, P.; Lopes, J. R.; Ninham, B. W.; Baglioni, P. J. Phys. Chem. B. 2002, 106, 2166. (7) Fujita, H.; Ooya, T.; Yui, N. Macromolecules 1999, 32, 2534. (8) Harada, A. Coord. Chem. Rev. 1996, 148, 115.
and the PEO repeating units and by the formation of hydrogen bonds between the hydroxyl groups of the threaded R-CD.16 The solubility of the pseudopolyrotaxane is dependent on the ability of the threaded CDs to form hydrogen bonds with the solvent molecules.9 As this interaction is inhibited, a phase separation can be observed. Depending on the PEO MW and the relative concentrations of the polymer and R-CD, a gel is formed.10,11 In this case, hydrogen bonds are established between adjacent R-CD molecules on the same chain and critically between R-CD molecules on neighboring polymer chains, which act as physical crosslinks in the gel network. The free PEO units can hold a large amount of water in the gel structure. The complexation of R-CD and PEO adsorbed on the surface of colloidal particles offers interesting perspectives, which have, as yet, received little attention.12 Studies on the mechanism of the gel formation when PEO is adsorbed on colloidal silica12 indicate that R-CDs thread onto the PEO chains via the tails of the polymer. The process is entropically unfavorable, but the overriding enthalpic effect resulting from the strong interactions established between the polymer units and the hydrophobic cavity of the R-CD lead to the partial desorption of the polymer from the surface of the silica and the subsequent formation of a (9) Szejtli, J. Chem. Rev. 1998, 98, 1743. (10) Huh, K. M.; Ooya, T.; Lee, W. K.; Sasaki, S.; Kwon, I. C.; Jeong, S. Y.; Yui, N. Macromolecules 2001, 34, 8657. (11) Li, J.; Harada, A.; Kamachi, M. Polym. J. 1994, 26, 1019. (12) Sabadini, E.; Cosgrove, T.; Taweepreda, W. Langmuir 2003, 19, 4812. (13) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polym. Interfaces; Chapman and Hall: London, 1993. (14) Joseph, J. PhD Thesis, University of Bristol, 2001. (15) King, S. M. Small-angle Neutron Scattering. In Modern techniques for Polymer Characterisation; Dawkins, E. R. A. P. a. J. V., Ed.; John Wiley and Sons, Ltd: New York, 1999; p 171. (16) Lo Nostro, P.; Lopes, J. R.; Cardelli, C. Langmuir 2001, 17, 4610.
10.1021/la049495m CCC: $27.50 © 2004 American Chemical Society Published on Web 09/09/2004
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Table 1. Polydispersity and Specific Surface of the Three Colloidal Silica Samples Used in the Experimentsa particle diameter (nm) (as provided) particle diameter (nm) from SAXS and SANS data polydispersity specific surface (m2/g)
9.0
12.0
25.0
8.4 ( 0.6
11.9 ( 0.7
24.7 ( 0.7
25% 360
25% 200
30% 130
a Size and polydispersity were determined from SAXS and SANS measurements.
gel. The supramolecular gel consists of cross-linked polymer chains complexed with the R-CD molecules and silica particles cross-linked by part of the ethylene oxide (EO) units of the polymer that have not been included by the R-CDs. The behavior of a polymer adsorbed at a surface has been extensively investigated13 and is commonly described by the loop-train-tail model. A wide range of properties can be expected from the competition between the processes of adsorption and desorption of the PEO (due to the inclusion of the polymer units into the cavity of the R-CD). In particular, small-angle neutron scattering studies carried out on PEO grafted on the surface of a polystyrene latex suggest that the layer thickness increases with the amount of R-CD threading onto the polymeric chains.14 In this paper, we report a kinetic study of the complexation of PEO and R-CD. We investigate separately the influence of two fundamental parameters on the rate of gel formation: the temperature and the strength of the hydrogen-bonding of the solvent. We then study complexation when PEO is physically adsorbed on the surface of colloidal silica and where the ratio of R-CD/PEO and the adsorbed amount are systematically varied. The effect of PEO molecular weight is also investigated. Finally, small-angle neutron and X-ray scattering (SANS/SAXS) experiments were performed to study the effect of the complexation on the configuration of the silica particles and the adsorbed polymer layer. Materials and methods R-CD (Wacker) and PEO of MW 4000 (Polymer Laboratories), 6000 and 20 000 g mol-1 (BDH) were used as received. Urea was obtained from BDH. Samples of silica Klebosol were supplied by Clariant. Three sizes of particles were used: 9, 12, and 25 nm diameter. Before use, the stock solutions of silica were extensively dialyzed against milli-Q Plus water. The diameter and polydispersity of the particles were determined from TEM images and SANS/SAXS. These values are summarized in Table 1, together with the specific surface area of the dispersion provided by Clariant. All the solutions were prepared using milli-Q Plus water and D2O of 99% purity (Goss Scientific Instruments). The mixtures of the components were prepared by weight (within ( 0.1 mg) from aqueous solutions of PEO (3%), silica (17%), and R-CD (10% in H2O, 8% in D2O) and urea (8 M in H2O). All experiments were performed in 1.5 mL Eppendorf tubes. Samples with adsorbed polymer were prepared by adding a known quantity of silica dispersion to a polymer solution with an equilibration time of 48 h before the addition of the R-CD. For the experiments involving the determination of the final yield of the reaction, the samples were left for 7 days in order for the system to reach final equilibrium (after this time, no more changes in the amount of gel phase are observed). For the kinetic study, samples were prepared in a set of Eppendorf tubes (approximately 20) and sampled at regular intervals of time. To determine the yield of the reaction, the samples were centrifuged for 3 min (at 3200 rpm), the supernatant was removed and the solid phase was dried in an oven at 80 °C for 48 h. It is still possible that after this process some residual free R-CD and PEO are incorporated into the gel but their mass is not significant compared to that of the solid sample and this does not affect our results. The yield (Y) of the gel was calculated by the following equation:
Y)
(
)
WC × 100 WPEO + WR-CD
where WC, WPEO, and WR-CD are the dry masses of the complex obtained, PEO, and R-CD, respectively. Most reactions were performed at room temperature. In the experiments involving the variation of the temperature, a water bath was used at temperatures of 35 and 40 °C. The SAXS measurements were performed on a NanoSTAR instrument (Bruker AXS). The SANS experiments were carried out on LOQ at ISIS (Rutherford Appleton Laboratory, Didcot, UK). The LOQ instrument at ISIS uses incident wavelengths between 2.2 and 10 Å, sorted by time-of-flight, with a sample detector distance of 4.1 m. This gives a Q-range between 7 × 10-3 and 0.285 Å-1. The samples were kept in stoppered quartz cells (Hellma, Germany) with a path length of 1 mm. The raw spectra were corrected for incoherent background from the solvent, sample cell, and electronic noise by standard procedures.15 The two-dimensional isotropic scattering spectra were azimuthally averaged, converted to an absolute scale, and corrected for detector efficiency.
Results and discussion The complexation between R-CD and PEO can be described as a multistep reaction, which has been summarized by Lo Nostro et al.:16 1. Diffusion of the PEO and R-CD through the solvent. 2. Initial threading of the R-CD on the polymer chain. 3. Sliding of the R-CD along the polymer chain and inclusion of successive EO units. 4. De-threading of the R-CD from the polymer chain. 5. Formation of hydrogen bonds between adjacent R-CD molecules on the same chain and between chains leading to the precipitation of the final adduct. The kinetics of the gel formation are affected by many factors, including the polymer molecular weight, the concentration of the reactants, and the end group structure. The main driving forces are the temperature and the ability to form hydrogen bonds. Although R-CD has been reported to form columnar structures in aqueous solutions,4 in our case, the majority of R-CD is in monomeric form, as shown by SANS measurements performed on aqueous solutions of CD.24 Therefore, among other possible mechanisms, the Lo Nostro model is the most appropriate to describe our system. 1. Effect of the Temperature. We have studied the reaction of gel formation for the R-CD/PEO (6000) system at three temperatures: room temperature (23 °C), 35, and 40 °C. The reactions were performed in aqueous solution with a ratio of R-CD/PEO ≈ 5.5. The gel yields obtained for the reactions as a function of time are presented in Figure 1. The evolution of the yield with time follows a similar behavior at all temperatures. Three regions can be distinguished. The first region shows little or no variation with time and corresponds to the formation of the soluble pseudopolyrotaxanes. In the second region, the yield is seen to increase sharply with time and at a (17) Chou, S. I.; Shah, D. O. J. Colloid Interface Sci. 1981, 80, 49. (18) Van der Beek, G. P.; Cohen Stuart, M. A.; Cosgrove, T. Langmuir 1991, 7, 327. (19) Lafuna, F.; Wong, K.; Cabane, B. J. Colloid Interface Sci. 1991, 143, 9. (20) Killmann, E.; Maier, H.; Kaniut, P.; Gutling, N. Colloids Surf. 1985, 15, 261. (21) Hone, J. H. E.; Cosgrove, T.; Saphiannikova, M.; Obey, T. M.; Marshall, J. C.; Crowley, T. L. Langmuir 2002, 18, 855. (22) Amu, T. C. Polymer, 1982, 23, 1775. Candia, F.; Vittoria, V. Bianchi, U.; Paltrone, E. Macromolecules 1972, 5, 493. Dormidontova, E. E. Macromolecules 2002, 35, 987. (23) Rusa, C. C.; Tonelli, A. E. Macromolecules 2000, 33, 1813. (24) Taweepreda, W., Ph.D. Thesis, University of Bristol, in preparation. Similar results for β-CD have also been published recently by Maccarrone et al. (Physica B 2004, in press).
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Figure 1. Gel yield for the R-CD/PEO complexation reaction as a function of time for three different temperatures: room temperature (b), 35 °C (O), and 40 °C (1). The reactions were performed in aqueous solution with PEO 6000 and a ratio of R-CD/PEO of ∼5.5. The lines are guidelines to the eye. The standard deviation for the complexation is 3%.
Figure 2. Gel yield for the R-CD/PEO complexation reaction as a function of time for three different solvents: H2O (b), D2O (O), and 0.1M urea in H2O (1). The reactions were performed at room temperature (∼23 °C) with a ratio of R-CD/PEO of ∼5.5 and PEO 6000. The lines are guidelines to the eye. The standard deviation for the complexation is 3%.
Table 2. Time Constant tc (in minutes) for the Reaction of Complexation between r-CD and PEO 6000 at Three Different Temperatures: 23, 35, and 40 °C
Table 3. Time Constant tc (in minutes) for the Reaction of Complexation between r-CD and PEO in Three Different Solvent: D2O, H2O, and 1M Urea in H2Oa
temperature tc (min)
23 °C 141
35 °C 1118
40 °C 2082
constant rate, as the large particles start aggregating and the gel phase is formed. Finally, a plateau region is reached. A comparison of the yield as a function of time at the three temperatures studied shows that the rate of gelation decreases as the temperature increases. To compare the kinetics of the gelation at different temperatures, each reaction is characterized by a time constant, tc, defined as the time when the yield of the reaction has reached 50%. The results are shown in Table 2. Following the model of reaction described above,16 the effect of temperature on the kinetics of the gel formation affects positively steps 1-4. However, an increase in temperature also reduces the radius of gyration of the PEO22 and consequently reduces the threading and dethreading of CD onto the PEO chain. This effect is not considered in this simple model. 2. Effect of the Solvent. The effect of the solvent on the rate of the gel formation was also investigated. Figure 2 shows the gel yield as a function of time for the complex R-CD/PEO 6000 in three different solvents: D2O, H2O, and a solution of 0.1M urea in H2O. The ratio of R-CD/ PEO in all reactions is approximately 5.5. The rate of gel formation in the three solvents studied follows the same general trend. The relative rates of gel formation clearly decrease in the order: D2O > H2O > urea. Table 3 reports the values of tc for the three solvents studied and two molecular weight polymers (6000 and 20 000). A fundamental property of the solvent to consider in the gelformation process is the capacity to form hydrogen bonds. The solvents can be characterized by their hydrogen-bond strength, which, in decreasing order, is as follows: D2O > H2O > aqueous urea solution. For both molecular weight polymers, the time constant for the gel formation decreases with the hydrogen-bond strength, and this trend is in agreement with that obtained by Lo Nostro et al. for the initial formation of pseudopolyrotaxane. Urea has the ability to break the hydrogen bonds between the water molecules,17 thus decreasing the rate of gel precipitation, and this is shown clearly by the results reported in Figure
PEO 6000 PEO 20 000
D2O
H2O
1M urea in H2O
90 319
142 1027
182 1290
a The reactions were performed at room temperature, with a ratio R-CD/PEO of ∼5.5 and PEO of molecular weight 6000 and 20 000.
2. D2O has a higher hydrogen bonding strength than H2O; as a result, the gel is formed more quickly. The values of tc displayed in Table 3 indicate also a lower rate of gel formation in the case of the 20 000 PEO in all solvents studied, and this is associated with the lower number of polymer end groups in comparison with PEO 6000. Although the complexes formed by the inclusion of PEO by R-CD molecules have been shown to be thermodynamically more stable with longer polymer chain guests,23 the complexation is clearly faster for PEO of low molecular weight, in agreement with Harada’s findings.8 3. Effect of Adsorption onto Colloidal Silica. The effect of silica on the pseudopolyrotaxane formation is presented in Figures 3 and 4. Samples were prepared by first adsorbing PEO on the silica in various proportions and then adding the R-CD.25 The yield of the reaction is shown as a function of the ratio of silica and PEO, with silica of 9 nm diameter (Figure 3) and silica of 25 nm (Figure 4) and for two different ratios of R-CD/PEO, namely, ≈5.5 and ≈11. As the mass of silica is not used in the calculation of the yield, yields may be greater than 1. The visual appearance of the samples after having reached equilibrium is shown next to the corresponding data points on the graph. From the pictures, it can be seen that the amount of gel formed at low concentrations of silica is significantly higher than that formed at higher concentrations of silica. However, the incorporation of silica in the gels is also associated with an increase in their density.12 Similar trends are observed for the two (25) An alternative experiment, in which silica and R-CD are brought into contact before addition of PEO, is also possible. However, the adsorption of PEO onto silica is a much slower process than the onset of phase separation caused by complexation. This would mean that little adsorption would occur and that the initial conditions, rate of mixing, etc. would be very difficult to control reproducibly.
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Figure 3. Gel yield as a function of the ratio of silica 9 nm and PEO 6000 for two R-CD/PEO ratios: ∼5.5 (b) and ∼11 (O). For each ratio of silica/PEO studied, the visual appearance of the sample is displayed next to the corresponding data point. The standard deviation for the complexation is 3%.
Figure 4. Gel yield as a function of silica (25 nm)/PEO 6000 ratio for two R-CD/PEO ratios: ∼5.5 (b) and ∼11 (O). For each ratio of silica/PEO studied, the visual appearance of the sample is displayed next to the corresponding data point. The standard deviation for the complexation is 3%.
sizes of particles. In Figure 3 (9 nm silica), for the lower ratio of R-CD/PEO (≈5.5) the yield decreases linearly in the region silica/PEO ) 1-8. Beyond this point, the yield is approximately zero. For the higher R-CD/PEO ratio (≈11), the yield remains approximately equal to 1 in the region silica/PEO ) 1-6, and then decreases linearly until it reaches zero for a ratio of silica/PEO ≈ 12. In Figure 4 (25 nm silica), similar trends are observed, but the curves are shifted to higher proportions of silica. For the ratio of R-CD/PEO ≈ 5.5, the yield of the gel decreases linearly for ratios of silica and PEO between 1 and 20 and reaches approximately zero for ratios above 20. For R-CD/PEO ≈ 11, the yield of the gel initially increases at low silica concentrations (up-to-a ratio of silica/PEO ≈ 12). It then decreases and is still above zero for the highest ratio shown here (silica/PEO ) 25). The shifts are attributed to the lower specific surface for silica 25 nm (as shown in Table 1); hence, for the same ratio of silica/PEO, less polymer is adsorbed on the silica particles of size 25 nm. This is verified quantitatively: if the yield plots are scaled by the specific surface corresponding to each particle size, they
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Figure 5. Gel yield as a function of the silica/PEO ratio for the two sizes of particles (9 and 25 nm) and the two R-CD/PEO ratios shown in Figures 3 and 4. The silica concentration is expressed in units of surface, as it has been scaled by the specific surface area: 360 m2/g and 130 m2/g, respectively, for the 9 and 25 nm particle size (cf. Table 1). For the 9 nm particles, the symbols used are full triangles (2) for the R-CD/PEO ratio of 5.5 and empty triangles (4) for the 11 ratio. For the 25 nm particles, full circles (b) correspond to the R-CD/PEO ratio of 5.5 and empty circles (O) to the ratio of 11.
fall onto the same master curve. This appears clearly in Figure 5, which presents the data shown in Figures 3 and 4 scaled by the specific surface of each particle size. For each R-CD/PEO ratio studied (5.5 and 11), the data points from the 9 nm and the 25 nm particles overlap. The threading of R-CD onto the PEO chains establishes an equilibrium between the adsorption and the desorption processes of the PEO onto the surface of the silica. As the R-CD threads onto the tail of the polymer, part of the noncomplexed segments of the PEO chains remain in contact with the surface, but the complexed segments are displaced.12 With a high degree of complexation, silica particles may be withheld in the matrix of the gel, as the noncomplexed EO units remain physisorbed onto the silica. This explains the higher yields observed with the highest ratio of R-CD/PEO (≈11) for both sizes of particles. For the lowest ratio of R-CD/PEO (≈5.5) and at low concentrations of silica, complexation occurs primarily with the free PEO in solution. Therefore, at very low concentrations of silica (silica/PEO e 2), the yield is similar to that obtained with a higher ratio of R-CD/PEO (≈11). With increasing concentration of silica, more PEO chains are adsorbed onto the surface of the colloidal particles; as a result, the complexation occurs mainly with the tails of the polymer. Therefore, the yield of the gel decreases. The presence of silica inhibits the formation of cross-links between the R-CD/PEO complex. When PEO is adsorbed on the silica surface, only the tails of the polymer are free in solution and these are not long enough to allow the formation of the microcrystals.12 Beyond a threshold value for the ratio of silica/PEO, the yield drops to approximately zero. However, this does not mean that no complex is formed, as the yield is calculated only from the mass of complex that is not soluble. It is likely that there is a dispersion of the complex in equilibrium with R-CD, but the degree of complexation is not high enough to form a gel.12 For the highest ratio of R-CD/PEO (≈11), a slight increase of the yield is observed as the proportion of silica increases, in the region silica/PEO ) 1-6. This reflects the inclusion of the silica particles into the gel network,
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Figure 6. Gel yield as a function of time for the complexation between R-CD and PEO 6000 and PEO 20 000 in aqueous solutions and adsorbed on colloidal silica. The ratio R-CD/PEO was ∼13 and silica/PEO for the samples containing silica is ∼10. In this case, the calculated yield takes into account the total amount of R-CD, PEO, and water (therefore, the values shown are significantly smaller than in the other graphs shown). The full symbols correspond to PEO 6000 (2 for free PEO and 9 for PEO adsorbed on silica) and the empty symbols correspond to PEO 20 000 (4 for free PEO and 0 for PEO adsorbed on silica). The lines are guidelines to the eyes. The standard deviation for the complexation is 3%.
which is made possible by the high degree of complexation. The yield reaches a maximum at a value which corresponds to the formation of a monolayer of adsorbed PEO on the silica surface (corresponding to 0.6 mg m-2, cf. refs 1820), namely, silica/PEO ≈ 6 for the 9 nm silica and silica/ PEO ≈ 12 for the 25 nm silica. Beyond these values, the yield decreases as the coverage of PEO on the surface decreases, and the degree of complexation is not sufficient for the precipitation of the final aggregate. The comparative rates of gel formation for PEO 6000 and PEO 20 000 in aqueous solutions and when adsorbed on colloidal silica (12 nm diameter) are shown in Figure 6. The ratio R-CD/PEO is ≈ 13, and for the samples containing silica, the ratio silica/PEO is ∼10. Two general trends are immediately clear from these results: the rate of gel formation and the amount of gel formed both increase with lower-molecular-weight polymer and when PEO is adsorbed on the colloidal silica. The effect of the polymer molecular weight is very obvious. It is associated with the higher number of extremities present in the PEO 6000 system than in the 20 000 system, favoring the steps 1-3 of Lo Nostro model.16 The kinetics are faster for the PEO adsorbed on silica because, in contrast with the free PEO, the distribution of end groups is not homogeneous but concentrated around the silica particles. In this case, the probability of interactions between end groups (tails) and R-CD and the possibility of cross-linking are higher. It is worth noting here that we are not considering the fullinclusion complex, as part of the chains is adsorbed onto the silica surface. The onset of the cloud point is visually detected earlier for the gel-containing silica. At the high ratio of R-CD/PEO used in these experiments, the effect of silica incorporation in the mass of the complex is clearly observed. 4. Scattering Experiments. Using SAXS, we have studied the effect of gelation on the conformation of the silica particles in the R-CD/PEO/silica system. The ratio of silica/PEO was ≈ 2 (thus ensuring conditions of full
Dreiss et al.
Figure 7. Small-angle X-ray scattering curves obtained for the R-CD/PEO 6000/silica 9 nm system at time t ) 0 of the reaction (corresponding to the plot with the full circles b) and after gelation has occurred (O), showing the effect of the gelation on the conformation of the silica particles in the gel matrix. The ratio of R-CD/PEO is ∼5.5 and silica/PEO ∼2.
coverage), and the ratio of R-CD/PEO ≈ 5.5. The reaction was performed at room temperature in aqueous solution, using PEO 6000 and silica of 9 nm diameter. The scattering patterns were collected for 3 h and are shown in Figure 7. The plot with the full circles corresponds to the time t ) 0 of the reaction when the R-CD has just been mixed with PEO previously adsorbed on silica and, therefore, no gelation has yet occurred. The plot with the empty circles gives the scattering pattern obtained after 48 h, when the reaction has been completed and the gel has formed due to the complexation of R-CD and PEO (as assessed by the gravimetric measurements presented above). Due to the very low contrast between PEO, R-CD, and water with X-rays, the scattering originates mainly from the silica particles, therefore, the scattering pattern is essentially a measure of the variation in the silica particles arrangement caused by the gelation of the system. The difference observed between the two scattering patterns is slight, indicating that the arrangement of the silica particles is not significantly affected by the complexation. However, after the gel has formed, a higher scattering is detected at low-Q. This could suggest some aggregation of the silica particles, which is consistent with the model proposed by Sabadini et al.12 As the gel is formed, the silica particles are incorporated into the gel phase, therefore, coming closer together than when dispersed in solution. We also performed a SANS experiment on a similar system: 0.2% w/w PEO 4000 was adsorbed on silica (2% w/w), with and without R-CD. A very attractive feature of neutrons comes from the significant difference in neutron scattering lengths between hydrogen and deuterium, making it possible to contrast-match a given component in the system by mixing the protonated and deuterated forms of the solvent in the appropriate ratios. In this particular system, silica was contrast-matched by mixing H2O and D2O in a 2:3 volumetric ratio, making only the adsorbed PEO layer visible. It is then possible to observe the effect of the complexation of R-CD on the configuration of the chains. However, the experiment is limited to a narrow range of R-CD concentrations, as at high concentrations, gelation occurs, giving rise to a very high scattering arising from the two-phase system. Hence, the concentration of R-CD used was quite low: 1.5% w/w. The scattering curves are shown in Figure 8. As expected,
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fit the scattering from the PEO with a model21 that assumes an exponential profile for the adsorbed layer. The volume fraction profiles obtained from the fits (Figure 9) clearly show that, as the R-CD molecules thread onto the chains, the volume fraction of the polymer at the surface decreases and the chains extend further into the solution. Although these effects are relatively weak due to the conditions employed, this is clear evidence that the complexation of PEO with R-CD modifies the configuration of the adsorbed chains.
Figure 8. Small-angle neutron scattering curves obtained for the R-CD/PEO 4000/silica 12 nm system. The concentrations of PEO and silica are 0.2 and 2% w/w, respectively. The three scattered plots correspond to PEO 4000 adsorbed on silica, where the silica is contrast-matched (b) and PEO 4000 adsorbed on silica with 1.5% w/w of R-CD (O). The dashed line is the fit to the scattering curve from the adsorbed PEO layer on its own (no R-CD) and the dotted line is the fit to the scattering curve from the system containing 1.5% w/w R-CD.
Figure 9. Volume fraction profiles obtained from the fits to the scattering curves shown in Figure 8. φ(z) is the volume fraction of adsorbed polymer, where z is the distance from the surface of the silica particle (given in angstroms). The dashed line corresponds to PEO 4000 adsorbed on silica without R-CD and the dotted line to PEO adsorbed on silica with 1.5% w/w of R-CD.
the changes in the patterns are very subtle due to the very low number of R-CD molecules threaded onto the chains. Despite the poor statistics of the data, we could
Conclusions We have studied the formation of a supramolecular complex obtained from the threading of R-CD onto PEO free in solution and adsorbed on colloidal silica. For PEO MW above 2000 and a sufficient R-CD/PEO mixing ratio, a gel is formed. The rate of complexation and gel yield both decrease with increasing temperature. The rate of the reaction also depends significantly on the nature of the solvent, as the strength of the hydrogen-bonding network favors the formation of the complex. When PEO is adsorbed onto silica particles, the complexation still takes place and the inclusion process starts from the tails of the PEO, leading to a displacement of the chains from the surface. The kinetics are faster in the presence of silica, as the polymer end groups are concentrated around the silica particles. At high degrees of complexation, the silica particles are incorporated into the gel. The same trends are observed for different sizes of silica particles, and the results were shown to scale with the specific surface; hence, there is no particle size effect. At excessively high silica concentration, the surface coverage of PEO decreases and the degree of complexation is not sufficient to enable the precipitation of the final aggregate. The effect of polymer molecular weight is also very clear, as the reduced number of tails associated with higher molecular weight leads to a decrease of the gel yield and rate of complex formation. The configuration of the adsorbed PEO chains has been characterized by SANS. The results confirm that some desorption takes place as the R-CD thread onto the polymer backbone and the chains extend further from the surface. Acknowledgment. The authors are very grateful to Clariant (Division Functional Chemicals) for the samples of silica, Wacker-Chemical AG for the samples of R-CD, Dr. Stephen M. King, Dr. Richard Heenan, and Dr. Andrew J. R. Nelson for help on LOQ, and ISIS for providing the beam time. E.S. would like to thank CNPq-Brazil for funding of this work via a Senior Research Fellowship. F.N.N acknowledges the Nuffield Science Bursary Scheme. C.D. would like to acknowledge the EPSRC Impact Faraday Partnership and ACORN for the provision of a PDRA fellowship. LA049495M
