Influence of the Ring Size on the Behavior of Polymeric Inclusion

Degussa AG Werk Wesseling, Postfach 1164, D-50375 Wesseling, Germany. Received November 17, 1999. In Final Form: March 10, 2000. With the aim to ...
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Langmuir 2000, 16, 5311-5316

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Influence of the Ring Size on the Behavior of Polymeric Inclusion Compounds at Mica Surfaces Steffen Kelch, Walter R. Caseri,* Ronald A. Shelden, and Ulrich W. Suter Department of Materials, ETH, Institute of Polymers, CH-8092 Zu¨ rich, Switzerland

Gerhard Wenz Polymer-Institut der Universita¨ t Karlsruhe, Herzstrasse 16, D-76187 Karlsruhe, Germany

Bruno Keller Degussa AG Werk Wesseling, Postfach 1164, D-50375 Wesseling, Germany Received November 17, 1999. In Final Form: March 10, 2000 With the aim to investitgate the influence of the ring size of cyclodextrin molecules in polymeric inclusion compounds (polypseudorotaxanes, pseudopolyrotaxanes) on the behavior of the corresponding supramolecular structures at surfaces, solutions of poly(decamethylenedimethylammonium) (PDDA) alone and in the presence of R-cyclodextrin (R-CD), β-cyclodextrin (β-CD), or both R-CD and β-CD were allowed to stand for various times during which polymeric inclusion compounds formed. The so obtained solutions were subsequently exposed to delaminated muscovite mica, the surface of which was saturated either with lithium or, occasionally, potassium ions (called Li-mica or K-mica, respectively). Li-mica was better suited for quantitative evaluations. Basically, muscovite surfaces shift the equilibrium of the inclusion complexes previously established in solution entirely or almost entirely to the side of the free components upon adsorption. However, the equlibrium was not installed after adsorption in the system PDDA/R-CD where the release of cylcodextrin was virtually prevented in the presence of R-CD for kinetic reasons. Most successful was the anchoring of cyclodextrins when the release of β-CD was prevented by R-CD molecules that occupied the ends of the PDDA chains.

Introduction A decade ago, supramolecular structures consisting of cyclic molecules which are threaded on a polymer chain like beads on a string were described.1-3 Such polymeric inclusion compounds4-6 have been reported, e.g., with protonated poly(imino oligomethylene)s and related compounds containing methylammonium groups,3,4,7 poly(viologen)s,4 poly(glycols),5,8 or poly(ester)s.9-11 R- and β-cyclodextrin (R-CD and β-CD; for structures see Figure 1) are usually employed as the cyclic component of polymeric inclusion compounds. The inner diameter of these rings increases with increasing number of amylose units and amounts to 4.9 and 6.2 Å for R- and β-CD, respectively.12,13 The (reversible) formation of polymeric inclusion compounds is often carried out in aqueous solutions which (1) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821. (2) Harada, A.; Kamachi, M. J. Chem. Soc., Chem. Commun. 1990, 19, 1322. (3) Wenz, G.; Keller, B. Angew. Chem., Int. Ed. Engl. 1992, 31, 197. (4) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (5) Harada, A. Acta Polym. 1998, 49, 3. (6) Raymo, F. M.; Fraser Stoddart J. Chem. Rev. 1999, 99, 1643. (7) Herrmann, W.; Keller, B.; Wenz, G. Macromolecules 1997, 30, 4967. (8) Fujita, H.; Ooya, T.; Yui, N. Macromol. Chem. Phys. 1999, 200, 706. (9) Harada, A.; Kawaguchi, Y.; Nishiyama, T.; Kamachi, M. Macromol. Rapid Commun. 1997, 18, 535. (10) Weickenmeier, M.; Wenz, G. Macromol. Rapid Commun. 1997, 18, 1109. (11) Harada, A.; Nishiyama, T.; Kawaguchi, Y.; Okada, M.; Kamachi, M. Macromolecules 1997, 30, 7115. (12) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (13) Saenger, W. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic Press: London, 1984; p 231.

Figure 1. Chemical structure of cyclodextrins and PDDA.

contain both polymer and rings. The related inclusion processes are driven mainly by hydrophobic interactions.14 Depending on the system, the equilibrium may be established already within 1 h or within months.3,7 It seems that the equilibrium may be shifted toward the side of the free components not only by an increase of temperature or a decrease of concentration but also upon adsorption.15 For example, a polymeric inclusion compound of R-CD and a cationic polyelectrolyte, poly(N-4,4′-bipyridiniumN-decamethylene dibromide) (PV-10), readily adsorbed on muscovite mica by ion exchange.15 It was, however, surmised that the rings threaded over the PV-10 were (14) Schneider, H.-J. Angew. Chem., Int. Ed. Engl. 1991, 30, 1417. (15) Meier, L. P.; Heule, M.; Caseri, W. R.; Shelden, R. A.; Suter, U. W.; Wenz, G.; Keller, B. Macromolecules 1996, 29, 718.

10.1021/la991510d CCC: $19.00 © 2000 American Chemical Society Published on Web 05/19/2000

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trapped at the surface only for kinetic reasons and that a major part or even all of the rings might be released in the equilibrium state, which, however, was assumed to be established only over extended periods (months or years).15 Here, we investigate the influence of kinetic factors on the anchoring of cyclodextrins via polymeric inclusion compounds on muscovite surfaces with the example of poly(N-dimethyldecamethyleneammonium) (PDDA, Figure 1) and cyclodextrins of two different ring sizes (R- and β-CD). In homogeneous solutions, the equilibrium of such systems is established by far slower with R-CD than with β-CD.16 The well-defined, flat surface of muscovite has rendered this mineral a common substrate for surface studies. In contrast to many other layered silicates, the interlayer cations of muscovite do not exchange at ordinary conditions.17-22 The following investigations were performed with a muscovite of ultrahigh specific surface area.23 Such materials are obtained by chemical delamination of muscovite particles with hot LiNO3 solutions. As a consequence, the mica contains lithium ions on the surface which readily exchange with organic cations in aqueous suspensions;24-27 i.e., the polyelectrolyte PDDA is expected to adsorb strongly on such mica surfaces. Experimental Section Chemical Reagents. R- and β-cyclodextrin (99% purity, molecular weights 937 and 1135 g/mol, respectively) were obtained from Wacker-Chemie GmbH (Mu¨nchen, Germany). PDDA was prepared according to the literature,28 and low molecular weight fractions were subsequently removed by dialysis (cellulose hollow-fiber module Alwall GFE 8 supplied by Gambro, Hechingen, Germany). The weight and number average molecular weights (Mn and Mw, respectively) of PDDA were 63 000 and 95 000 g/mol, respectively, corresponding to a polydispersity index of 1.5. An isolated PDDA/R-CD/β-CD inclusion compound was prepared by dissolution of 1.17 g of PDDA (as bromide salt) in 100 mL of water at room temperature. A 5 g amount of β-CD was added, and the reaction mixture was stirred overnight. Then 0.43 g of R-CD was added. After dissolution of the R-CD, the solution was divided into 10 parts, which were allowed to stand for different periods ranging from 1 to 16 days. After specified periods, free cyclodextrin was removed by dialysis (cellulose hollow-fiber module Alwall GFE 8 supplied by Gambro, Hechingen, Germany), where 10 mL of PDDA/R-CD/β-CD solution was diluted to 50 mL, and the molecules passing the dialysis membrane were removed during 24 h at a water flux of 650 mL/h. Finally, the product was isolated by freeze-drying. The cyclodextrin content in the polymeric inclusion compound was determined by 1H NMR spectroscopy. Mica Delamination. Muscovite fines, designated with Mica 21, were supplied by vonRoll Isola, Breitenbach, Switzerland. The mica was delaminated with hot lithium nitrate solution as (16) Wenz, G.; Keller, B. Polym. Prepr. 1993, 34 (1), 62. (17) Theng, B. K. G. Formation and Properties of Clay-Polymer Complexes; Elsevier: Amsterdam, 1979. (18) Lagaly, G. In Clay Minerals and Clays; Jasmund, K., Lagaly, G., Eds.; Steinkopff Verlag: Darmstadt, Germany, 1993. (19) Gaines, G. L., Jr. J. Phys. Chem. 1957, 61, 1408. (20) Barshad, I. Soil Sci. 1954, 78, 57. (21) Reichenbach, H. G.; Rich, C. I. Clays Clay Miner. 1969, 17, 23. (22) Reichenbach, H. G. Clay Miner. 1968, 7, 331. (23) Caseri, W. R.; Shelden, R. A.; Suter, U. W. Colloid Polym. Sci. 1992, 270, 392. (24) Shelden, R. A.; Caseri, W. R.; Suter, U. W. J. Colloid Interface Sci. 1993, 157, 318. (25) Wa¨lti, M.; Kelch, S.; Geke, M. O.; Shelden, R. A.; Caseri, W. R.; Suter, U. W.; Rehahn, M.; Knapp, R. J. Colloid Interface Sci. 1997, 189, 305. (26) Geke, M. O.; Shelden, R. A.; Caseri, W. R.; Suter, U. W. J. Colloid Interface Sci. 1997, 189, 283. (27) Osman, M. A.; Moor, C.; Caseri, W. R.; Suter, U. W. J. Colloid Interface Sci. 1999, 209, 232. (28) Kern, W.; Brenneisen, E. J. Prakt. Chem. 1941, 14, 159.

Kelch et al. described previously29 and thereafter stirred in water (ca. 10 g of muscovite in 200 mL of water) for 1 h, filtered out, washed in the funnel with 2-propanol (200 mL), and dried at 60 °C and ca. 100 mbar for 2 d. The specific surface areas of the resulting muscovites were 101 and 104 m2/g, respectively, for the two batches used here (determined by methylene blue adsorption).24,30 The ion-exchange capacity, measured by N-dodecylpyridinium adsorption,24 resulted in a value of 4.5 µmol/m2. Mica with potassium surface ions was prepared according to the literature by exchange of the surface lithium ions with potassium chloride.31 Adsorption of PDDA and of Polymeric Inclusion Compounds. All the adsorption experiments were performed with bidistilled water (pH 5.8) in plastic tubes (Semadeni; Ostermundigen, Switzerland) to avoid contamination with ions released from the glass walls. For the transformation of solutions, plastic syringes were used. For adsorption experiments with PDDA and R- or β-CD, 0.5 g of 40 mM solutions (the PDDA concentrations referring to constitutional repeat units; note that the β-CD dissolves completely only upon formation of the inclusion compound within ca. 0.5 h under stirring) was kept at the periods indicated in the text and then diluted with 4 g of water immediately before additon of typically 250 mg of muscovite (for the determination of adsorption isotherms, the mass range of muscovite was between 22 and 313 mg). The adsorption time was 2-3 h. For experiments with PDDA and both R- and β-CD, a 40 mM solution with PDDA and β-CD was first allowed to stand for 1 d before the R-CD was added. Investigations with PDDA alone (i.e., in absence of cyclodextrins) were performed with 4 g of a 4.75 mM PDDA solution which was treated with various amounts of muscovite (22-313 mg, according to the ratio PDDA/mica indicated in the text). The solutions were worked up for analyses as described below. The isolated inclusion compound comprised of PDDA and both R- and β-CD was adsorbed from 4 g of a 2 mM solution (the concentration refers to the constitutional repeat units of PDDA) onto 100 mg of mica for various periods (30 min, 1 h, 2 h, and 4 h). Polarimetry. The optical rotation was measured at 546 nm on a Perkin-Elmer 241 polarimeter in 10 cm quartz glass cuvettes at 20 °C. To obtain particle-free solutions for the analysis with polarimetry, the muscovite suspensions were subjected to centrifugation in a Hettich Universal centrifuge at 3500 rpm during 10 min. The supernatant solution was thereafter decanted and filtered through a PTFE filter (pore diameter 0.2 µm) put on top of a syringe. Thermogravimetric Analysis (TGA). TGA measurements were performed with a Perkin-Elmer TGA 7 device under nitrogen atmosphere with heating rates of 20 °C/min. For thermogravimetric analyses, the mica was filtered out, washed with ca. 100 mL of water in the funnel, and dried at 60 °C and ca. 100 mbar for 16 h.

Results and Discussion Three types of polymeric inclusion compounds were prepared in situ, comprised of (a) PDDA and R-CD, (b) PDDA and β-CD, and (c) PDDA and β-CD followed by addition of R-CD. These solutions were exposed to delaminated muscovite mica of a specific surface area of ca. 100 m2/g and an ion-exchange capacity of 4.5 µmol/m2. The delaminated mica originally contained lithium ions at the surface and is designated in the following as “Limica”. Further, mica saturated with potassium ions at the surface (“K-mica”), prepared by ion exchange of Limica with potassium chloride,31 was used occasionally. Rand β-CD alone did not adsorb in detectable amounts under the experimental conditions applied here. The adsorption of PDDA on Li-mica was followed with thermogravimetric analysis (TGA). The mass of adsorbed (29) Meier, L. P.; Shelden, R. A.; Caseri, W. R.; Suter, U. W. Macromolecules 1994, 27, 1637. (30) Ha¨hner, G.; Marti, A.; Spencer, N. D.; Caseri, W. R. J. Chem. Phys. 1996, 104, 7749. (31) Herzog, E.; Caseri, W.; Suter, U. W. Colloid Polym. Sci. 1994, 272, 986.

Polymeric Inclusion Compounds

Figure 2. Amount of PDDA adsorbed on Li-mica, Γ, as a function of the total amount PDDA per surface area mica in the system, a.

PDDA was determined by the weight loss of the surfacemodified mica between 250 and 450 °C, a temperature range in which pure PDDA (as bromide salt) loses virtually 100% of its mass (the mass loss of pure mica in this temperature range, 0.50%, probably due to the release of water, was subtracted for the determination of the adsorbed quantity of PDDA). A case with complete adsorption of PDDA was confirmed with a control experiment in D2O using 1H NMR spectroscopy. After adsorption, the mica solids were separated by centrifugation, a fraction of the supernatant solution was taken, and a defined amount of sodium benzenesulfonate was added as an internal standard for the quantitative determination of the nonadsorbed PDDA with 1H NMR spectroscopy, where the integrals of the signals of PDDA can be compared with those of benzenesulfonate. However, signals of PDDA were completely missing in the spectrum, demonstrating the complete adsorption of PDDA. As expected for complete adsorption, the negligible free PDDA molecules in the supernatant cannot be determined even using highly sensitive techniques. From experiments with known amount of adsorbed PDDA (i.e., complete adsorption), we estimate the precision of the adsorbed PDDA masses to 5%. The Li-mica was exposed to PDDA solutions for 3 h (preliminary experiments showed that the adsorption equlibrium was established already after 2 h, in agreement with earlier observations showing that polyelectrolytes adsorb rapidly on layered silicates17). The adsorbed mass of PDDA was determined for various PDDA/mica ratios using TGA and polarimetry according to the procedure described in the Experimental Section. Analogous to other adsorption studies with delaminated muscovite,15,24-27 a, the amount of PDDA in the system per surface area of mica in the system (in µmol/m2, referred to constitutional repeat units or charges of the polymer, respectively) was plotted against Γ, the amount of adsorbed polymer per surface area of mica, and the corresponding results are presented in Figure 2. Under the experimental conditions applied here, the ion exchange is not influenced by factors such as pH value (5-6) or adventitious salts.27 The PDDA adsorbs initially quantitatively, and the adsorbed mass steeply increases as the a value increases to ca. 4 µmol/m2 where Γ is 2.7 µmol/m2. This value does not differ significantly for an a value of 8 µmol/m2. Obviously, the muscovite surface is saturated with PDDA molecules at a surface concentration around 2.7 µmol/m2. This value lies below the ion-exchange capacity of 4.5 µmol/m2; i.e., the surface lithium ions probably cannot be exchanged

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completely due to steric effects: the space available for an ionic site is 37 Å2 (calculated from the ion-exchange capacity) while we estimate the space requirement of an extended constitutional repeat unit of PDDA to 54 Å2, considering standard bond lengths, bond angles and van der Waals radii.32,33 At 2.7 µmol/m2, the space available for a constitutional repeat unit in a closely packed monolayer is 62 Å2, i.e., sufficient for the adsorption of PDDA in an extended chain conformation which is expected to be the favored state for polyelectrolytes on mica.34,35 Hence, the PDDA molecules form most likely a relatively dense but not compact monolayer. The ionic segments in cationic polyelectrolytes adsorbed on layered silicates such as clay minerals17 and Li-mica36 are typically in contact with the surface; i.e., the cations are as close to the negative charges in the silicate layers as possible which maximizes the electrostatic attraction. On the other hand, the repulsion of the cationic segments in PDDA extends the polymer molecules but for entropic reasons the polymers are hardly present as perfect rods which could pack with highest density at the surface. For all the adsorption experiments with polymeric inclusion compounds reported below, sufficient mica was added to allow a complete adsorption of the polymer (a ) 0.2 µmol/m2), unless otherwise indicated. In absence of PDDA, R-CD and β-CD did not adsorb significantly on muscovite. Under the experimental conditions used here, PDDA and β-CD were present in equimolar amounts but the employed amount of β-CD exceeded 2.4 times the maximum solubility. However, upon formation of the polymeric inclusion compounds, the residual β-CD body dissolved, which took roughly 0.5 h, and in agreement with the literature16 it is assumed that the inclusion equilibrium is established within a few hours. Under the experimental conditions applied here, we may expect that around 60% of the constitutional repeat units of PDDA are occupied by β-CD in homogeneous solution.4 After periods varying from 2 to 14 d, the solutions were diluted (ca. 9 times) immediately before exposure to Li-mica, to keep the solution homogeneous if β-CD was released markedly upon adsorption, i.e., to avoid precipitation of β-CD in such a case. An adsorption time of 2 h was selected to prevent changes in the inclusion equilibrium of the adsorbed polymeric inclusion compound which may occur over extended adsorption periods.15 As cyclodextrins are optically active and the optical rotation depends linearly on the concentration of the optically active molecules in solution, the amount of surface bound β-CD was calculated from the change in the optical rotation of the supernatant solution. It was considered that the optical rotation of β-CD decreased by 5% upon inclusion of PDDA (in the final solution used for the adsorption experiments), although in principle this quantity was of negligible influence on the results. The evaluation of the optical rotation showed that only 2-4% of the initially present β-CD molecules were enclosed at the surface after 2-14 d equilibration time of the inclusion compound in homogeneous solution, while on mica the amount of PDDA expected for quantitative adsorption was found with TGA (32) Lide, D. R., Ed. Handbook of Chemistry and Phsics; CRC Press: Boca Raton, FL, 1995. (33) Pauling, L. Basics of Chemistry; Verlag Chemie: Weinheim, Germany, 1973. (34) Davies, R. J.; Dix, L. R.; Toprakcioglu, C. J. Colloid Interface Sci. 1989, 129, 145. (35) Van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984, 88, 6661. (36) Caseri, W. R.; Shelden, R. A.; Suter, U. W. In Polymer-Solid Interfaces; Pireaux, J. J., Bertrand, P., Bre´das, J. L., Eds.; IOP Publishing: Bristol, U.K., 1992; p 121.

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Figure 4. Fraction of cyclodextrin molecules per constitutional repeat units of PDDA, f, for various equilibration times, t. Filled squares: PDDA/R-CD adsorbed on Li-mica. Open squares: PDDA/R-CD in homogeneous solution. Filled circles: PDDA/ R-CD/β-CD adsorbed on Li-mica. Open circles: PDDA/R-CD/ β-CD in homogeneous solution.

Figure 3. Schematic representation of the formation and adsorption behavior of polymeric inclusion compounds on Limica: (a) PDDA/R-CD; (b) PDDA/β-CD; (c) PDDA/R-CD/β-CD.

(the mass of coadsorbed β-CD, which also decomposes in the temperature range used for PDDA analysis, is known from the measurements of the optical rotation and was considered not to be relevant for the low CD coverages found here). It seems, therefore, that β-CD rapidly leaves the polymer chains under the conditions used here (Figure 3b), in agreement with the previous suggestion that a release of cyclodextrin rings upon adsorption might be thermodynamically favored for steric or entropic reasons.15 Exposure of mica to solutions containing R-CD and PDDA was perfomed analogously to the experiments with

β-CD described above, but R-CD is better soluble than β-CD and was already dissolved completely from the beginning. Due to the smaller ring size of R-CD, the inclusion equilibrium of PDDA and R-CD is established considerably slower than that of PDDA and β-CD.4,16 The optical rotation of R-CD decreased by 3% when this cyclodextrin was kept in solution together with PDDA for 39 d; i.e., also this change cannot affect the calculations of the surface-trapped R-CD molecules from optical rotation measurements significantly. The amount of anchored R-CD per constitutional repeat unit PDDA, f, calculated from TGA and measurement of the optical rotation of the outstanding solutions, slowly increased over periods of days for which the PDDA-R-CD solutions had been allowed to stand prior to adsorption (Figure 4). After 40 d, f is only 10%, and in fact, the formation of PDDA and R-CD in homogeneous solution is not yet in equilibrium after 3 months.37 The fraction of constitutional repeat units of PDDA occupied by R-CD in homogeneous solution reported4 after 3-19 d agrees well with the values of f found here at the respective times. Hence, the slow increase in surface-bound rings with the contact time of R-CD and PDDA reflects the slow threading and release processes of R-CD with PDDA. The inner diameter of the R-CD rings is close to the sterical requirements of the dimethylammonium groups of PDDA; i.e., it is more difficult for an R-CD molecule to pass over the dimethylammonium groups than for the larger β-CD molecules. As a consequence, the kinetics of R-CD release from PDDA becomes extremely sluggish and the rings coadsorb with PDDA. Therefore, unlike in the case of β-CD, R-CD rings are not released in significant quantities during the adsorption period of 2 h but are kinetically trapped on the surface (Figure 3a). In another series of experiments, an aqueous solution of an equimolar amount of PDDA and β-CD was first allowed to stand for 1 d and then treated with an equimolar amount of R-CD for various periods. It is expected that R-CD rings begin to occupy the end positions of the PDDA molecules and herewith hinder the threaded β-CD rings to leave the PDDA coil (Figure 3c).4,16 The PDDA/R-CD/ β-CD solutions were subsequently exposed to Li-mica as described above for the other systems. The optical rotation of the solution containing both cyclodextrins decreased in (37) Wenz, G.; Keller, B. Macromol. Symp. 1994, 87, 11.

Polymeric Inclusion Compounds

Figure 5. Circles: Amount of PDDA adsorbed on Li-mica, Γ, as a function of the total amount PDDA per surface area mica in the system, a, in the presence of R-CD and β-CD (equilibration time 20 d). Squares: Corresponding amount of coadsorbed cyclodextrin rings.

the presence of PDDA by 5% after 48 d. For the calculation of the cyclodextrin concentrations in the supernatant solutions, it has to be considered that the contribution of R-CD and β-CD to the optical rotation is not equal. Since in such systems predominantly β-CD is threaded on PDDA and the R-CD rings probably occupy mainly the ends of the polymer molecules,3 it was assumed for the calculation of coadsorbed cyclodextrins from the supernatant solutions with polarimetry that exclusively β-CD was coadsorbed. This simplification leads to an error in the calculated cyclodextrin concentration below 5%. Figure 4 shows a slow increase in coadsorbed cyclodextrins up to a PDDA/R-CD/β-CD equilibration time of ca. 40 d where ca. 60% of the available positions in PDDA were occupied with cyclodextrin molecules. Corresponding experiments were also performed in homogeneous solution at various equilibration times (see Experimental Section). The coverage of PDDA with CD molecules of PDDA/RCD/β-CD inclusion compounds in homogeneous solution was calculated by integration of the signals in 1H NMR spectra and is also given in Figure 4. The rate of cyclodextrin trapping at the surface follows a similar kinetics as the threading of cyclodextrin in homogeneous solution. It seems, therefore, that similar to the system with PDDA/R-CD but in contrast to that with PDDA/βCD, the supramolecular structure in the PDDA/R-CD/βCD solution can be retained at the surface. This suggestion is supported by the adsorption of a sample of an isolated PDDA/R-CD/β-CD inclusion compound in which 55% of the decamethylene groups were occupied with cyclodextrin rings, as determined with 1H NMR spectroscopy. No significant release of CD rings was measured upon adsorption on Li-mica within an adsorption time of 4 h. Most likely the cyclodextrin rings in the PDDA/R-CD/βCD systems are retained at the surface for kinetic reasons since the R-CD molecules at the polymer chain ends are slowly released and therefore the β-CD rings are enclosed between R-CD molecules (Figure 3c). As a consequence, for equilibration times between 4 and 40 d in the system with PDDA and both cyclodextrins, 6-7 times more cyclodextrin molecules were coadsorbed per adsorbed PDDA molecule than with the PDDA/R-CD and at least 10-30 times more than with the PDDA/β-CD system. For PDDA/R-CD/β-CD at an equilibration time of 20 d, the adsorption was followed at various ratios of the total amount of PDDA per surface area of mica, a, in the range of 0.5-9 µmol/m2 (Figure 5). The PDDA present in the

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system adsorbs quantitatively or almost quantitatively up to an a value of ca. 2 and remains virtually constant at higher a values. The cyclodextrin rings are not adsorbed quantitatively, but similarly to PDDA, their amount increases linearly up to an a value of ca. 2 and reaches a plateau at higher a values. The fraction of decamethylene units enclosed in cyclodextrin rings was found to be constant at 57% for all a values. The adsorption behavior resembles that of PDDA alone. The plateau value, i.e., the maximum coverage with PDDA, was ca. 20% lower for the PDDA/R-CD/β-CD system, probably due to the additional space requirement of the coadsorbed cyclodextrin rings. To study the influence of the initially present surface ions, K-mica was treated with PDDA similarly to the experiments described above with Li-mica, the only exception being a longer adsorption time (24 h) in the case of K-mica as it has been reported that the adsorption equilibria on K-mica are established after considerably longer periods than on Li-mica;15,38 most likely the equilibrium of PDDA adsorption on K-mica is established only after weeks.15 Indeed, in the a range of 1-10 µmol/ m2, only 0.2-0.3 µmol/m2 PDDA was measured on K-mica, i.e., a substantially lower amount than on Limica. Thermodynamic effects might also contribute to a lower coverage of PDDA on K-mica: the affinity of potassium ions for mica is much higher than that of lithium ions;24 i.e., lithium ions are easier to replace than potassium ions due to the higher hydratation energy of Li+. Solutions of PDDA in the presence of both R-CD and β-CD, prepared as described above for the adsorption experiments with Li-mica, were also exposed to K-mica for 24 h. Using an equilibration time of 50 d for the PDDA/ R-CD/β-CD system, only 0.1-0.2 µmol/m2 PDDA was measured on K-mica for a values between 1 and 10 µmol/ m2. The amount of coadsorbed rings per decamethyleneammonium units corresponds roughly to 50% referred to adsorbed decamethyleneammonium units; the adsorbed amounts were too low for a precise determination. The results clearly demonstrate that Li-mica is better suited for adsorption experiments with PDDA and cyclodextrins than K-mica. Conclusions The adsorption of poly(N-decamethylenedimethylammonium) (PDDA) alone and of solutions containing PDDA and cyclodextrin rings was studied with delaminated muscovite whose surface was saturated with lithium and, in a few experiments, potassium ions (called Li-mica and K-mica, respectively). Under the conditions applied here, Li-mica was better suited for adsorption studies since larger amounts of organic molecules adsorbed on Li-mica. This is probably mainly due to a markedly slower adsorption kinetics, although it cannot be excluded that the adsorption equilibrium also plays a role as the affinity of potassium ions for muscovite is much higher than that of lithium ions. Upon adsorption on Li-mica, the equilibrium between polymeric inclusion compounds comprised of PDDA and β-cyclodextrin (β-CD) and the free components is strongly shifted to the side of the free components, maybe for entropic reasons or because the threaded rings are bulky and therefore elevate the ammonium groups from the surface thus lowering the electrostatic attraction of PDDA and mica. The fast kinetics of β-CD release allows the system to establish (38) Osman, M. A.; Caseri, W. R.; Suter, U. W. J. Colloid Interface Sci. 1998, 198, 157.

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the thermodynamically stable state or to come close to it. In contrast, R-CD can be bound to muscovite surfaces via PDDA. When solutions containing R-CD and PDDA are kept in solution over longer periods before exposure to muscovite, R-CD is trapped at the surface, reflecting the rate of formation of the inclusion compound in homogeneous solution. For example, ca. 10% of the decamethylene units are occupied at Li-mica surfaces with R-CD after 30 d of PDDA/R-CD equilibration time. The release of the adsorbed rings is thought to be prevented by the slow kinetics. However, β-CD can be anchored at the surfaces when solutions of β-CD and PDDA are, after equilibration, treated with R-CD for extended periods before addition of muscovite. Here, β-CD rings are caught between R-CD

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molecules that slowly enter the ends of the PDDA chains. As threaded R-CD is released very slowly at muscovite surfaces, a larger quantity of rings can be “caught” at Li-mica surfaces via a polymeric inclusion compound. Around 60% of the decamethylene units can be covered with rings after 30 d PDDA/R-CD/β-CD equilibration. The threaded rings do not prevent the PDDA molecules from adsorption; differences in the maximum amount of adsorbed PDDA with and without threaded rings (ca. 20%) are probably due to the additional space requirement of the cyclodextrin rings. LA991510D