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Langmuir 1996, 12, 5614-5619
Monolayers of Cellulose Ethers at the Air-Water Interface Patricia Basque, Ane´mone de Gunzbourg, Philippe Rondeau, and Anna M. Ritcey* De´ partement de chimie and CERSIM, Universite´ Laval, Que´ bec, Que´ bec, Canada G1K 7P4 Received August 28, 1995. In Final Form: July 25, 1996X Aliphatic ethers of cellulose are found to form stable monolayers when spread from dilute chloroform solution at the air-water interface. Monolayer pressure-area isotherms exhibit a liquid analogous phase followed by a transition region of relatively high compression at constant pressure. Limiting molecular areas are found to depend on side-chain length, indicating that the hydrocarbon substituents do not adopt an orientation strictly perpendicular to the water surface. Monolayers of mixed ethers containing, on average, a single long chain substituent per repeat unit, form liquid analogous phases at molecular areas equivalent to the area of the anhydroglucose ring. It is thus concluded that the cellulose backbone lies flat on the water surface. The transition region observed upon compression beyond the liquid analogous phase is attributed to the formation of bi- or multilayers, with molecules leaving the water surface. This interpretation is consistent with the observed decrease in plateau pressure with increasing temperature.
Introduction Attempts to prepare ultrathin polymer films by the Langmuir-Blodgett (LB) technique are motivated by the desire to combine the favorable material properties of polymers with the control of molecular organization available with this method of multilayer assembly. Initial studies of polymeric LB films involved the introduction of polymerizable functional groups in traditional amphiphilic molecules and the subsequent polymerization after transfer to a solid substrate.1 This method proved rather unsatisfactory, however, as density changes which accompany the chemical reaction create stresses and film defects. Alternatively, the polymerization can be carried out directly on the water surface.2 Monolayers have also been successfully spread from dilute solutions of a large variety of preformed polymers,3 including many amphiphilic copolymers. In general, strongly hydrophilic groups are required in order to overcome the tendency of a flexible macromolecule to adopt a coiled conformation and instead lie flat on the water surface. Another class of polymers that has received significant attention is that comprised of rigid rodlike molecules.4 Substituted phthalocyaninatopolysiloxanes, polyglutamates, and cellulosics have been found to form stable monolayers, which can be subsequently transferred to solid substrates to give multilayers of well-defined thickness. These studies suggest that under conditions of sufficient natural chain extension, a strong amphiphilic nature may not be required. In the present article we report the formation of spread monolayers by a series of aliphatic cellulose ethers. Monolayers of cellulose derivatives were first investigated more than 60 years ago.5 The limiting area evaluated from the pressure-area isotherm of a spread film of unsubstituted cellulose6 was, however, found to be significantly smaller than the 60 Å2 calculated for the area of anhydroglucose ring, thus suggesting that the intramolecular attractive forces in this polymer are too strong to allow for complete molecular dispersion. Similar obserX
Abstract published in Advance ACS Abstracts, October 1, 1996.
(1) Breton, M. J. Macromol. Sci. Rev. Macromol. Chem. 1981, C21, 61 and references therein. (2) Bader, H.; Dorn, K.; Hupfer, B.; Ringsdorf, H. Adv. Polym. Sci. 1985, 64, 1. (3) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Toronto, 1991; pp 191-203 and references therein. (4) Wegner, G. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1326. (5) Adams, N. K. Trans. Faraday Soc. 1933, 29, 90. (6) Giles, C. H.; Agnihortri, V. G. Chem. Ind. 1967, 4, 1874.
S0743-7463(95)00718-9 CCC: $12.00
vations were made for cellulose acetate,7 although possible tilting of the anhydroglucose rings was offered as an alternative explanation for the small limiting area. Monolayers of ethyl cellulose, on the other hand, were found to have limiting areas which agree well with the theoretical ring size.8 More recently, monolayers of a series of long chain cellulose esters have been investigated.9,10 In the case of cellulose tridodecanoate, satisfactory multilayers could be prepared, but only by the horizontal lifting method and at surface pressures within the condensed phase. Electron microscopy examinations of cellulose tridecanoate samples transferred under these conditions showed inhomogeneous films.9 The first investigation of LangmuirBlodgett film formation by long chain cellulose ethers was carried out by Wegner et al.11,12 Rigid rod polymers represent an important class of materials for the preparation of LB films. The introduction of side-chain chromophores13-15 leads to multilayer assemblies that possess unique properties of interest for applications in the fields of nonlinear optics and microelectronics. The general architecture of these side-chain polymers allows for the systematic variation of molecular parameters such as side-chain length and the degree of substitution. The influence of these features on the properties of spread monolayers is examined in this paper. Experimental Section Cellulose ethers, of the general structure shown in Figure 1, were prepared from cellulose acetate (Aldrich, Mw 30 000, acetyl content 39.8%) and alkyl bromides in NaOH/DMSO. Finely ground NaOH (10-fold molar excess) was added to a 2% solution (7) Hittmeier, H.; Sandell, L. S.; Luner, P. J. Polym. Sci.: Part C 1971, 36, 267. (8) Borgin, K.; Johnson, P. Trans. Faraday Soc. 1953, 49, 956. (9) Matsumoto, M.; Itoh, T.; Miyamoto, T. In Cellulosics Utilization; Inagaki, H., Phillips, G. O., Eds. Elsevier: London, 1989; pp 151-160. (10) Kawaguchi, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 29. (11) Ritcey, A.; Wenx, G.; Wegner, G. Presented at the IUPAC International Symposium on Macromolecules, Montreal, Canada, 1990. (12) Schaub, M.; Fakirov, C.; Schmidt, A.; Lieser, G.; Wenz, G.; Wegner, G.; Albouy, P.-A.; Wu, H.; Foster, M. D.; Majrkzak, C.; Satija, S. Macromolecules 1995, 28, 1221. (13) Tsujii, Y.; Itoh, T.; Ito, S.; Miyamoto, T. In Cellulosics: Chemical, biochemical and material aspects; Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; Ellis Horwood: New York, 1993; pp 483-488. (14) Basque, P.; Ritcey, A. M. Proc. Am. Chem. Soc., Div. Polym. Mater.: Sci. Eng. 1994, 71, 488. (15) Mao, L.; Ritcey, A. M. Thin Solid Films 1994, 242, 263.
© 1996 American Chemical Society
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Langmuir, Vol. 12, No. 23, 1996 5615
Figure 1. Chemical structure of the cellulose derivatives investigated. In the case of mixed ethers, all R groups are not identical.
Figure 3. 300 MHz 1H NMR of hexyl cellulose recorded in CDCl3.
Figure 2. FT-IR spectrum of hexyl cellulose recorded as a cast film on a KBr pellet. of cellulose acetate in dry DMSO under nitrogen. The appropriate alkyl bromide (10-fold molar excess) was introduced and the reaction allowed to proceed with stirring at room temperature for 3-5 days. The reaction mixture was poured into a excess of water and the product polymer extracted in dichloromethane. In the case of octyl, hexyl, and butyl cellulose, the ethers thus obtained were further reacted with 3 equiv of alkyl bromide in the presence of 1 equiv of potassium tert-butoxide in THF. Mixed ethers were prepared from methyl cellulose (Aldrich Mw 40 000, methoxy content 30%) and alkyl bromides in NaOH/DMSO, following the method outlined above. Final purification of all polymers was achieved by gel permeation chromatography (GPC) on Lipophilic Sephadex (Sigma LH 20-100) in CHCl3. Samples were analyzed by FTIR (Mattson Surius 100) and 1H NMR (Bruker AC F-300). Molecular weights relative to polystyrene standards were determined by GPC (Wyatt) in chloroform. Monolayers were spread at the air-water interface from dilute chloroform solutions containing approximately 1 mg/mL. Pressure-area diagrams were recorded with a KSV model 3000 film balance.
Results and Discussion 1. Characterization of Cellulose Ethers. The cellulose ethers were characterized by NMR and IR spectroscopy and, in all cases, found to be completely substituted. Typical results are illustrated by the spectra shown in Figures 2 and 3. The absence of the characteristic hydroxyl and carbonyl vibrations, at 3400 and 1760 cm-1, respectively, in the IR spectrum indicates the efficient removal of acetate groups and complete etherification. The successful introduction of alkyl side chains is evident from the methylene stretches near 2900 cm-1. Charac-
teristic methyl and methylene resonances, situated below 2.0 ppm, are also apparent in the 1H NMR spectrum of Figure 3. The ring protons, as well as the side-chain methylene hydrogens R to the ether linkage, appear between 2.9 and 4.3 ppm. The comparison of the integrated intensities of these two regions allows for the calculation of the average number of side chains per anhydroglucose ring as being equal to 3. This value corresponds to the complete substitution indicated by the IR spectra. The average degree of polymerization of octyl cellulose was determined by to be 140 by GPC measurements. This compares well with the value found for the cellulose acetate starting material and it can be concluded that no significant degradation occurs during the etherification reactions. 2. Liquid Analogous Phase and Limiting Areas. The surface pressure-area isotherms recorded for spread monolayers of dodecyl, octyl, hexyl, and butyl cellulose are presented in Figure 4. The isotherms for the first three ethers in this series all exhibit similar general features. The case of butyl cellulose will be discussed separately below. Upon compression from high molecular areas, a sharp increase in surface pressure is evident in all of the isotherms of Figure 4. The limiting molecular areas, obtained by extrapolation to zero surface pressure, are summarized in Table 1 for the various ethers. This area decreases systematically with decreasing length of the alkyl side chains. Since each polymer repeat unit contains three hydrocarbon substituents, these limiting molecular areas correspond to 38, 32, and 28 Å2 per side chain for dodecyl, octyl, and hexyl cellulose, respectively. These areas are intermediate between the 25 Å2 required for a highly ordered hydrocarbon chain in an all-trans conformation perpendicular to the water surface16 and the values of 40-70 Å2 per molecule17 typically observed in the liquid expanded state of surfactants. On the basis (16) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966; p 162. (17) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley and Sons: Toronto, 1990; p 137.
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Basque et al.
Figure 5. Sketch illustrating the difference in surface area required by two alkyl chains of differing length (L1 and L2). The tilt angle, θ, represents the mean of an orientational distribution symmetric about the surface normal.
Figure 4. Surface pressure-area isotherms recorded at 25 °C for straight chain alkyl ethers of cellulose: (a) dodecyl cellulose, (d) octyl cellulose, (c) hexyl cellulose, and (d) butyl cellulose. Table 1. Limiting Molecular Areas and Plateau Pressures Determined from Surface Pressure-Area Isotherms of Spread Monolayers at 25 °C
polymer
limiting molecular area plateau pressure (Å2/repeat unit) ((2) (mN‚m-1) ((0.5)
dodecyl cellulose octyl cellulose hexyl cellulose butyl cellulose methyl pentyl cellulose dodecyl methyl cellulose
113 94 86 60 75 75
8.8 11.8 13.6 15.5 28.8 28.1
of these comparisons, the state of the cellulose ether monolayers at molecular areas corresponding to this first increase in surface pressure can be classified as a liquid analogous phase. This analogy is also supported by the monolayer compressibility, calculated from eq 1, where A is the molecular area and Π is the surface pressure.
Cs )
1 ∂A A ∂Π
( )
T
(1)
Values of the order of 0.02 m N-1 are obtained from all of the cellulose ethers at the midpoint of the liquid analogous phase. These values are similar to those found for the liquid phase of small molecule amphiphile monolayers.17 The surface pressure of a spread monolayer is an indication of the interaction between neighboring molecules. It is therefore possible to deduce information about molecular organization from a comparison of the surface area at which the liquid analogous phase occurs and the physical size of the molecules. As noted above, molecular areas larger than 25 Å2 per hydrocarbon substituent indicate that the side chains are not aligned in an orientation strictly perpendicular to the water surface. Furthermore, limiting areas are found to depend on substituent length. This dependence provides additional evidence that interactions between neighboring molecules, leading to a nonzero pressure, begin with contact between
side chains that are somewhat extended out from the polymer backbone. The variation in limiting molecular area with side-chain length appears to be quite systematic. Comparison of the isotherms for dodecyl, octyl, and hexyl cellulose indicates that the limiting molecular area decreases by 5 Å2 per repeat unit for each carbon atom removed from the alkyl side chain. The methylene group can be modeled as a sphere of radius 1.8 Å,18 and the addition of this group in a side chain flat on the water surface would thus require an increase in area of 10 Å2 per chain. It is therefore clear that the side chains are neither flat on the water surface nor strictly perpendicular to it. Using three different theoretical approaches, Rice et al.19-21 predict significant hydrocarbon chain extension away from the water surface in fatty acid monolayers at molecular areas of the order of 50 Å2. Furthermore, monolayer densities at these relatively high surface areas are found to be typically half those of the corresponding close-packed phase. The existence of a low density layer is possible because the gain in entropy, resulting from the additional conformational freedom of the aliphatic chain, is sufficient to compensate for the enthalpy loss due to fewer chain-chain interactions. It is reasonable to apply a similar model to the monolayers studied here. Each side chain can be thought of as being confined to a cylindrical volume element of cross-sectional area equal to the observed molecular area and height equivalent to the film thickness. Within this volume, the side chain can sample numerous conformations, and gauche conformers are surely present. Molecular dynamics simulations22 predict a mole fraction of trans isomers equal to 0.8 at 30 Å2 per molecule for a surfactant containing 15 methylene units. In this case, it is impossible to calculate directly an average tilt angle, since a single chain axis does not exist. It is possible, however, to determine the average tilt angle, θ, of a hypothetical all-trans chain that would occupy the same volume element. This is illustrated by the simple geometrical model sketched in Figure 5, where it is assumed that θ is described by a probability distribution function symmetric about the normal. If it is further supposed that the average angle is the same for the various ethers, this model is consistent with the systematic increase in surface area with side-chain length. The difference in the surface area, ∆A, required by two chains of lengths (18) Karaborni, S. Langmuir 1993, 9, 1334. (19) Popielawski, J.; Rice, S. A. J. Chem. Phys. 1988, 88 (2), 1279. (20) Wang, Z.-G.; Rice, S. A. J. Chem. Phys. 1988, 88 (2), 1290. (21) Harris, J.; Rice, S. A. J. Chem. Phys. 1988, 88 (2), 1298. (22) Karaborni, S.; Toxvaerd, S. J. Chem. Phys. 1992, 96 (7), 5505.
Monolayers of Cellulose Ethers
L1 and L2 is given by ∆A ) π cos2 θ(L22 - L12). From the experimental values of ∆A, and chain lengths calculated from 1.25 Å per methylene unit, an average angle of 10° is obtained. A number of cellulose derivatives have been shown to adopt a helical conformation in their crystalline form.23 Molecular mechanics calculations24 indicate that, in the absence of the intermolecular interactions and packing constraints present in the crystal, cellulose adopts a highly extended conformation in which neighboring anhydroglucose rings do not lie within the same plane. The twisting of successive monomer units about the chain axis suggests that the free cellulose backbone is better modeled as a cylinder than a ribbon. The long chain cellulose ethers can thus be considered as “hairy rods”.25 In a spread monolayer, the hydrophobic side chains can be expected to turn away from the polar subphase, but because of their random placement over the surface of the central rod, no specific angle would be expected to exist between the side chains and the water surface. In this case, the average tilt angle corresponds rather to the mean of an orientational distribution about the normal. In this respect, these monolayers are significantly different from classic amphiphiles. The isotherm recorded for butyl cellulose does not follow the systematic variations observed for the other three ethers. This polymer exhibits a liquid analogous phase at molecular areas from 60 to 32 Å2. Since it is impossible to accommodate the spread molecules as a monolayer at areas below 60 Å2 per repeat unit, the isotherm suggests that this polymer does not exist as a molecularly dispersed layer. Intermolecular aggregation becomes increasingly probable for shorter side chains as strong interactions between cellulose backbones become less shielded. It must be noted that Wegner et al.26 have reported an isotherm for butyl cellulose that differs significantly from the one obtained here. These authors find a limiting area for the liquid analogous phase of 80 Å2 per repeat unit. Characterization of our sample by NMR and IR confirms that the butyl cellulose studied here is fully substituted. The reason for the observation of different molecular areas remains unclear. One final characteristic of the isotherm recorded for butyl cellulose that should be mentioned is the discontinuous change in slope evident at molecular areas of 45 and 50 Å2 per repeat unit. This same feature appears in the isotherm reported by Wegner et al.,26 although at slightly different molecular areas. Although the origin of these kinks is unknown, it is of interest to note that they are present in the two isotherms recorded for butyl cellulose but not apparent in the isotherms of the other cellulose ethers. 3. Transition to Bilayers or Multilayers. The second important feature of the isotherms of Figure 4 is the transition region, characterized by a constant pressure upon compression beyond the liquid analogous phase. Further reduction in surface area beyond this plateau results in a second rise in surface pressure. In all cases the transition region extends, at the high compression end, to molecular areas on the order of 30-45 Å2 per residue. This value is significantly smaller than the area required by three vertical hydrocarbon chains (i.e., 60 Å2). Since this area is too small to accommodate the molecules in a single layer, the plateau region can be attributed to the formation of bi- or multilayers. The surface pressure (23) Zugenmaier, P. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1983, 37, 277. (24) Burton, B. R.; Brant, D. A. Biopolymers 1983, 22, 1769. (25) Seufert, M.; Fakirov, C.; Wegner, G. Adv. Mater. 1995, 7 (1), 52. (26) Schweigk, S.; Vahlenkamp. T.; Xu, Y.; Wegner, G. Macromolecules 1992, 25 (9), 2513.
Langmuir, Vol. 12, No. 23, 1996 5617 Table 2. Molecular Areas at the Beginning (A1) and End (A2) of the Transition Plateau for the Various Cellulose Ethers Investigateda molecular areas delimiting the transition region (Å2/repeat unit) ((2) polymer
A1
A2
dG J/10-23 repeat unit ((10%)
dodecyl cellulose octyl cellulose hexyl cellulose butyl cellulose methyl pentyl cellulose dodecyl methyl cellulose
97 80 70 47 48 48
44 40 38 27 28 28
460 480 440 310 560 560
a The free energy chance associated with the transition is also given as calculated from dG ) π dA.
at the plateau is reported in Table 1 for the various polymers studied and is found to increase with decreasing side-chain length. The free energy change associated with the removal of a monolayer from the water surface can be calculated from the differences between the free energy of the spread monolayer and that of the clean water surface, as given by eq 2
dG ) -γ dA + γ0 dA,
(2)
where γ and γ0 are the surface tensions of the monolayercovered and clean water surfaces, respectively, and dA is the area of monolayer removed. Since the surface pressure Π is defined as (γ0-γ), this change in free energy is given by dG ) Π dA, or simply, the area under the pressure area isotherm over the transition region. Values of dG calculated for the various cellulose ethers are summarized in Table 2. This free energy change corresponds to the work of removing a monolayer from the water surface. The values reported in Table 2 are determined from molecular areas and are thus expressed in terms of energy per repeat unit. However, since only half of the total number of spread molecules are removed from the water surface during the formation of a bilayer, the work required to remove a single monomer is, in fact twice that given in the table. The values obtained for dodecyl, octyl, and hexyl cellulose are not significantly different and correspond to 5-6 kJ‚mol-1. Since the major interaction between the monolayer and the water surface most likely involves the cellulose backbone, it is reasonable that the work necessary to remove a monolayer from the surface should be similar in all cases. The value calculated for butyl cellulose is somewhat smaller than that obtained for the other ethers, but since this polymer does not appear to form a truly molecularly dispersed monolayer, the nature of the transition is unclear. Surface pressure-area isotherms exhibiting pronounced plateau regions have also been observed for spread monolayers of cellulose esters. Matsumoto et al.9 attribute the corresponding transition to the formation of bilayers, partly on the basis of film thicknesses measured by electron microscopy. Kawaguchi et al.,10 however, surmised that the transition region corresponds to an ordering of the alkyl side chains. It should be pointed out that these authors present pressure-area diagrams in which the plateau region extends only to 60 Å2. The origin of the large discrepancy in molecular areas between the cellulose ester isotherms reported by these two groups remains unknown. A similar transition region is also evident in the isotherms of synthetic polypeptides. In this case, however,
5618 Langmuir, Vol. 12, No. 23, 1996
Figure 6. Surface pressure as a function of molecular area recorded for spread monolayer of hexylcellulose during film compression (s) and expansion (- - -) at barrier speeds of 25 mm/min. Subsequent compressions yield identical curves.
ellipsometry measurements27 indicate that the plateau region corresponds to the piling up of undefined films near the barriers rather than the formation of a discrete bilayer. This interpretation does not offer a ready explanation for the presence of a second rise in surface pressure at the end of the plateau. As summarized in Table 2, the molecular area at which this occurs is very close to half that at the beginning of the transition region. It is thus somewhat tempting to hypothesize that discrete bilayers are indeed formed. This is supported by the results of a recent study of spread monolayers of dodecyl cellulose by infrared spectroscopy,28 which show that the density of molecules on the surface, far from the barriers, increases regularly with compression through this transition area. 4. Isotherm Reversibility. Surface pressure-area isotherms for dodecyl, octyl, and hexyl cellulose are reversible. This is illustrated in Figure 6 for hexyl cellulose. Upon decompression, some hysteresis is observed, but as the liquid analogous phase is reached, surface pressures identical to those recorded on compression are obtained. Subsequent compressions yield isotherms identical to the original one. This important observation indicates that these derivatives spread spontaneously at the air-water interface, and the monolayers represent equilibrium states and not metastable states created by dispersion of the dilute chloroform solution. The corresponding pressure-area isotherm for butyl cellulose is not reversible, as shown in Figure 7. Upon decompression, the surface pressure does not rejoin the compression curve, and a nonzero pressure persists at large surface areas. The exact form of the curve recorded during decompression, as well as the isotherm recorded upon subsequent compressions, are not reproducible. This observation could explain the difficulties noted above in preparing molecularly dispersed monolayers from this polymer. Spontaneous spreading at the air-water in(27) Motschmann, H.; Reiter, R.; Lawall, R.; Duda, G.; Stamm, M.; Wegner, G.; Knoll, W. Langmuir 1991, 7, 2743. (28) Desbat, B.; Mao, L.; Ritcey, A. M. Langmuir 1996, 12, 0000.
Basque et al.
Figure 7. Surface pressure as a function of molecular area recorded for spread monolayer of butyl cellulose during film compression (s) and expansion (- - -) at barrier speeds of 25 mm/min. Isotherms recorded during subsequent compressions are not reproducible.
Figure 8. Surface pressure-area isotherms recorded at 25 °C for mixed alkyl ethers of cellulose: dodecyl methyl cellulose (s) and (methyl pentyl cellulose (- - -).
terface is favored for substances that have both low surface tensions and low interfacial tensions with water. It is not surprising that derivatives with shorter side chains will not meet these criteria. 5. Mixed Ethers. The pressure-area isotherms for two mixed ethers prepared from methyl cellulose are presented in Figure 8. These two polymers, denoted methyl pentyl cellulose and dodecyl methyl cellulose, contain, on average, two methyl substituents and one
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Langmuir, Vol. 12, No. 23, 1996 5619
Figure 9. Surface pressure-area isotherms recorded for dodecyl cellulose as a function of temperature.
Figure 10. Surface pressure-area isotherms recorded for dodecyl cellulose as a function of temperature.
pentyl or dodecyl chain per repeat unit. The two curves of Figure 8 are essentially identical and different from the isotherms reported in Figure 4. Since the mixed ethers contain only one long alkyl substituent per residue, the limiting area is determined by interactions between neighboring anhydroglucose rings, rather than by the lateral packing of side chains. The molecular area at which the liquid analogous phase occurs corresponds very well to the 60 Å2 estimated as the size of the anhydroglucose ring.6 This implies that the cellulose backbone lies flat on the water surface. The isotherms for the mixed ethers differ from those in Figure 4, not only in the limiting molecular areas but also in the surface pressure at the plateau. This pressure is significantly higher in the case of the mixed ethers and can be attributed to the presence of the relatively hydrophilic methoxy substituents. The resulting increased attraction for the water surface permits the formation of a denser film before the critical pressure for bilayer formation is reached. This is also reflected in the larger values of the free energy change associated with the transition, as reported in Table 2. 6. Temperature Dependence of Isotherms. Pressure-area isotherms were recorded as a function of temperature from 10 to 40 °C. With increasing temperature, similar systematic changes are observed for hexyl, octyl, and dodecyl cellulose. This is illustrated in Figure 9 for dodecyl cellulose. Neither curve shape nor the molecular areas characteristic of the observed transitions are found to vary significantly with temperature. The pressure at the plateau, however, does decrease by approximately 0.2 mN‚m-1 per °C in the temperature range specified above. This decrease in pressure can be interpreted as resulting from the lowering of the free energy of bilayer formation at higher temperatures, as the attraction between the monolayer and the water surface is reduced. In the case of the mixed ethers a much smaller temperature dependence is observed, as shown in Figure 10. The stronger interactions with the water
surface are less attenuated by the modest increases in temperature investigated here. 7. Monolayer Transfer to Solid Substrates. All of the above monolayers can be successfully transferred to solid hydrophobic substrates at surface pressures just below the plateau pressure. Conclusions Alkyl ethers of cellulose can be spread as monolayers at the air-water interface. Pressure-area isotherms recorded for these polymers exhibit liquid analogous phases. In the case of completely substituted ethers, the molecular area at which this phase exists depends on sidechain length. It is thus concluded that the hydrocarbon substituents do not adopt an orientation strictly perpendicular to the water surface. The observed limiting molecular areas do, however, suggest that the side-chain orientation is not completely random. In the case of mixed ethers containing both short and long alkyl substituents, limiting areas correspond to the size of the anhydroglucose ring, indicating that the cellulose backbone lies flat on the water surface. A transition region is also evident in the isotherms of these monolayers. This region is characterized by a relatively constant surface pressure over a large change in surface area. This transition region is attributed to the formation of a bi- or multilayer, with molecules leaving the water surface. This interpretation is consistent with the observed decrease in plateau pressure with increasing temperature. In the case of dodecyl, octyl, and hexyl cellulose, successive compressions of the same monolayer yield identical isotherms, suggesting that these polymers spontaneously spread at the air-water interface. Acknowledgment. The financial support of NSERC (Canada) and FCAR (Que´bec) is gratefully acknowledged. LA9507186
