- Email: [email protected]
Preparation and characterization of composite LB films of lignin and cadmium stearate
Thin Solid Films 327–329 (1998) 47–51
Preparation and characterization of composite LB films of lignin and cadmium stearate C.J.L. Constantino a, A. Dhanabalan a, A.A.S. Curvelo b, O.N. Oliveira Jr. a ,* a
Instituto de Fı´sica de Sa˜o Carlos, USP, CP 369, 13560-970, Sa˜o Carlos, SP, Brazil Instituto de Quı´mica de Sa˜o Carlos, USP, CP 369, 13560-970, Sa˜o Carlos, SP, Brazil
b
Abstract We report on studies of composite Langmuir films containing lignin (extracted from sugar cane bagasse) and cadmium stearate and their subsequent transfer as Langmuir–Blodgett (LB) films. Pure lignin was found to form non-monomolecular films at the air-water interface. Composite monolayers of lignin and cadmium stearate with varying compositions have been obtained and characterized by surface pressure and surface potential isotherm studies which essentially indicated the formation of a meta-stable composite monolayer at the air-water interface. Nevertheless, these composite monolayers could be uniformly transferred onto solid substrates under optimized deposition conditions. Transferred multilayer LB films were characterized by UV-vis and Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and surface potential measurements. FTIR studies confirmed the transfer of lignin along with cadmium stearate. UV-vis results revealed a layer-by-layer transfer of the composite monolayer. In the composite film, both cadmium stearate and lignin were present as separate domains and the stacking order in the cadmium stearate domains was found to be influenced by the incorporation of lignin, as evidenced from XRD. Surface potential measurements indicated a good film uniformity. 1998 Elsevier Science S.A. All rights reserved Keywords: Lignin; Cadmium stearate; Langmuir–Blodgett films
1. Introduction The Langmuir–Blodgett (LB) technique is normally being employed with typical amphiphilic molecules (e.g. fatty acids) which possess distinct polar and nonpolar parts [1]. However, in recent times, the usefulness of this technique has been extended to obtain uniform ultrathin films of non-amphiphilic macromolecules such as polymers [2], various bio-molecules [3], fullerenes [4] and other macro heterocyclic compounds [5]. Though the forces responsible for holding these molecules on the water surface are not clearly understood, it has now been well recognized that these macromolecules may not form a true monomolecular Langmuir film at the air–water interface. Instead, a stable multilayer structure is more likely to be formed [6]. Studies on Langmuir films of bio-molecules are of special interest as they can be extended for understanding the organization of these molecules in biological systems in nature. This has been the main motivation for our project on Langmuir films of lignins extracted from different plant sources * Corresponding author. Tel.: +55 16 2715365; fax: +55 16 2713616; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0585-9
and their transfer as multilayer LB films [7,8]. Lignins are especially interesting materials in this context, because their molecular motions are still a matter of debate [9]. Our earlier studies indicated that, as it has been observed with other polymeric molecules, lignins may form multilayer stacks when spread and compressed on the water surface. Stability experiments revealed a continuous decrease of mean molecular area on holding the monolayer in a compressed state, possibly indicating the poor stability of the lignin monolayer. The composite LB film approach in which the nonamphiphilic molecules are co-spread with typical amphiphilic molecules has often resulted in stable and uniformly transferable monolayers, and this has proven particularly true for conducting polymers [10]. It is to note that in the above approach the long chain amphiphilic molecules have been used just as builder materials and they are not covalently attached to the macromolecules, in contrast to the long chain derivatives of macromolecules. Hence, these builder molecules can be selectively removed in later stages through post-deposition treatments as it has been demonstrated recently with composite LB films made from conducting polymers [11]. In the present study, we report on the preparation of composite monolayers containing various
1998 Elsevier Science S.A. All rights reserved
48
C.J.L. Constantino et al. / Thin Solid Films 327–329 (1998) 47–51
compositions of lignin and cadmium stearate. This approach has led to a reasonable enhancement in monolayer stability and transferability. The uniformly transferred composite monolayers were characterized by UV-vis, FTIR, XRD and surface potential measurements.
2. Experimental The lignin extraction from sugar cane bagasse fibers has already been described in [8]. Various compositions of lignin and stearic acid were dissolved in a mixture of tetrahydrofuran (THF) (Merck, 99%) and chloroform (Merck, HPLC grade) (1:99 by volume). The typical total concentration was about 1 mg/ml. Ultrapure water supplied by a RO60 Millipore filter connected to a Milli-Q system was used to prepare sub-phase solutions. 4 × 10−4 M cadmium chloride was added to the sub-phase whose pH was maintained at 6.0 with the addition of 5 × 10−5 M sodium bicarbonate. In surface pressure and surface potential isotherms, the mean molecular area axis is solely based on the number of stearic acid molecules spread on the water surface and hence the isotherm results presented in this paper will essentially indicate the effect of the addition of lignin on the pure cadmium stearate isotherm. Monolayer studies and multilayer LB deposition were carried out with a KSV-5000 LB system placed in a class-10 000 clean room. After a waiting period of 30 min after the spreading, the monolayer was compressed with different barrier speeds. The compressed mixed monolayers were transferred at 31 mN/m by the vertical dipping method with dipping speeds ranging from 1–3 mm/min. BK7 glass, thin gold evaporated glass and calcium fluoride plates were used as substrates which were cleaned thoroughly prior to use. UV-vis and FTIR spectral measurements were carried out with a Hitachi U2000 spectrophotometer and a BOMEM Michelson series instrument, respectively. X-ray diffraction measurements were carried out with a Rigaku Rotaflex (model RU-200B) X-ray diffractometer in the 2v range from 3–20° using a Cu target. Surface potential measurements of the LB films on glass coated with a thin gold layer were conducted with a Trek 320B electrostatic voltmeter.
the above mentioned problems may be eliminated. In order to infer about the possible molecular organization of the composite monolayer, isotherms were obtained with different compression speeds. Fig. 1 presents the isotherms of monolayers with 50:50 (by weight) of lignin and cadmium stearate on the surface of water containing cadmium ions. In a control experiment for the pure cadmium stearate monolayer, a condensed isotherm with a limiting mean molecular ˚ 2 and a collapse pressure of about 63 area of about 19.5 A mN/m was observed which was similar to that reported in the literature [1]. As shown in Fig. 1, though there is no significant change with the decrease in barrier speed from 100 to 30 mm/min, isotherm curves are found to shift towards lower mean molecular areas with the further decrease in compression speed. The change of mean molecular area with the compression speed may usually be related to the compression of a meta-stable monolayer under non-equilibrium conditions [12]. Under such conditions, the molecules still either undergo reorganization on the water surface or dissolve into the sub-phase water during the compression process. The decrease of the effective mean molecular area with the decrease of barrier speeds essentially reflects the increase of time duration of the compression process. Also worth noting are the exceptionally very high collapse pressures (about 75–80 mN/m) for these composite monolayers, which obviously result from experimental artifacts. This may probably be also associated with the meta-stable characteristics of the composite monolayer. Fig. 2 shows isotherms of composite monolayers containing different ratios of lignin and cadmium stearate during the first compression. For the sake of comparison, all experiments were performed with the same compression speed. For a typical film forming material, one would anticipate an increase of mean molecular area with increasing lignin content in the composite, as the mean molecular area is calculated on the basis of the number of stearic acid molecules
3. Results and discussion In contrast to our earlier work [8] on pure lignin monolayers where THF was employed as the spreading solvent, in the present study we have used a mixture of THF and chloroform (1:99). With the use of a water soluble solvent like THF one would anticipate a material dissolution into the aqueous sub-phase or an inefficient spreading on the water surface which may lead to the formation of micro aggregates immediately after the solvent dissolution into the water. With the use of a mixture of solvents which contain more of a water immiscible solvent like chloroform,
Fig. 1. p–A isotherms of a composite monolayer containing lignin and cadmium stearate (50:50 by weight) at different compression speeds. Area per molecule is calculated based on the stearic acid molecules.
C.J.L. Constantino et al. / Thin Solid Films 327–329 (1998) 47–51
spread on the water surface. As shown in Fig. 2 the limiting mean molecular area for the initial compression was found to increase proportionally to the amount of lignin in the composite. Therefore, it is likely that one can exercise a finite control over the molecular organization of the composite monolayer despite its metastable characteristics as discussed above. In order to further investigate the stability of the composite monolayer, hysteresis experiments were carried out by holding the monolayer in the compressed state for different time durations. After the initial compression, the monolayer was held at 31 mN/m for 2 h and the changes in the mean molecular area were monitored. As shown in Fig. 3, the isotherm curves are found to shift towards lower mean molecular areas as time evolves, implying the metastable characteristics of the monolayer under study. It seems that even in the presence of a builder material like cadmium stearate, the lignin molecules still undergo reorganization as it was observed with pure lignin monolayers. The surface potential-mean molecular area isotherm of a composite monolayer (50:50) obtained during the initial compression with a compression speed of 10 mm/min. is shown in Fig. 4 (experimental conditions are similar to those employed in Fig. 3). It must be stressed that in comparison with typical amphiphilic molecules, the interpretation of the absolute value of the surface potential is not trivial for complex systems as in the present case [13]. However, the relative changes with monolayer composition have often been employed to investigate the possible packing arrangements at the air-water interface. As shown in Fig. 4, after an initial plateau region, the surface potential was found to increase at a critical area which is almost twice the limiting mean molecular area. This feature is typical of amphiphilic molecules and it is related to the coming together of molecules even when the surface pressure is still at zero [13]. The decrease ˚ 2 where the surface presin potential occurred at about 40 A sure begins to rise and is probably related to molecular
Fig. 2. p–A isotherms of composite monolayers containing different weight percentages of lignin and cadmium stearate. Area per molecule is calculated based on the stearic acid molecules.
49
Fig. 3. Hysteresis isotherm of a composite monolayer containing lignin and cadmium stearate (50:50 by weight).
reorientation upon compression. The observed non-zero surface potential at larger mean molecular areas is related to the formation of larger domains immediately after the spreading. Even though monolayer stability was not as good as in traditional amphiphilic materials, uniform transfer of the monolayer could be performed by the vertical dipping method with a dipper speed ranging from 1 to 3 mm/min at a constant surface pressure of 31 mN/m. The composite monolayer was usually transferred to form Y-type LB films. A poor Z-type deposition was noticed when we attempted to transfer the monolayer at a lower surface pressure (15 mN/ m). Films were also transferred after holding the monolayer for different time durations. The transferred LB films were characterized by UV-vis, FTIR, XRD and surface potential measurements. Fig. 5 shows the UV-vis spectra of composite LB films of lignin
Fig. 4. DV–A isotherm of a composite monolayer containing lignin and cadmium stearate (50:50 by weight). Area per molecule is calculated based on the stearic acid molecules.
50
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Fig. 5. UV-vis spectra of different odd numbers of layers of composite LB films containing lignin and cadmium stearate (50:50 by weight).
and cadmium stearate containing different odd numbers of layers. The cast film of sugar cane bagasse lignin exhibits absorption starting from 500 nm with an absorption maximum at about 280 nm [14]. As we have employed glass substrates, we could not observe the lignin absorption below 360 nm. The observed increase of the absorption in 360– 500 nm region with an increasing number of layers may indicate that nearly equal amounts of lignin have been transferred during each deposition process. The non-linear increase of absorption corresponding to lignin may indicate the relatively poor transfer of the lignin at the initial stages of the transfer process during which period the monolayer was comparatively less stable. Fig. 6 shows the FTIR spectrum of the composite monolayer containing the 50:50 ratio of lignin and cadmium stearate. The presence of strong absorption at 1546 cm−1 corresponding to the C–O stretching vibration of the carboxylate group indicates the complete ionization of the monolayer. Also seen are the strong absorption peaks at 2918 and 2849 cm−1 corresponding to asymmetric and symmetric C–H stretching vibrations and the weak absorption peaks in the region of 1400 cm−1 for C– H bending vibrations [15]. A relatively small absorption at 1700 cm−1 and 1300–1200 cm−1 region are attributed to the C–O absorptions of carbonyl groups and C–O–C absorptions of ether groups present in the lignin molecule [14]. The possibility of a 1700 cm−1 peak associated with the unionized stearic acid molecules may be ruled out as the pure cadmium stearate LB film deposited under similar subphase conditions did not show any absorption at 1700 cm−1. The possible packing arrangement in the composite LB films was investigated by X-ray diffraction. Fig. 7 presents the XRD patterns obtained with composite LB films containing different compositions of lignin and cadmium stearate (curve-a to curve-c). All these films were deposited after holding the monolayer at the deposition pressure for 1 h. Also shown in Fig. 7 is the LB film containing 50:50 lignin and cadmium stearate in which case the transfer was started after 15 min (curve-d). The composite LB film con-
taining 25% of lignin exhibited a set of intense diffraction ˚ (curve-a), peaks with a bilayer distance of 50.0 ± 0.2 A which is close to the bilayer distance found with pure cadmium stearate LB films [1]. These results may indicate that the cadmium stearate molecules are present in separate domains within the composite LB films. It should be noted that with the increasing amount of lignin in the composite, the intensities of the diffraction peaks are found to decrease (curve-b). The composite LB film with 75% of lignin exhibited practically no diffraction peaks (curve-c). Despite this decrease in intensity which is due to the influence of lignin on the organization of cadmium stearate domains, the bilayer distance is found to be very similar in all three cases. However, for the films transferred with a waiting period of only 15 min, the bilayer distance was ˚ (curve-d). This result may indiincreased to 56.0 ± 1.0 A cate that some lignin molecules are possibly incorporated between the cadmium stearate molecules within the domain structure. Such molecular reorganization might have possibly originated from the meta-stable characteristics of the composite monolayer as inferred from the monolayer studies. Surface potential measurements were carried out with 7 and 13-layer composite LB films. The variation of surface potential while scanning the whole surface of the film was about 10 mV, indicating the excellent film uniformity, at least within the resolution of the surface potential probe. The observed surface potential was in the range 130–150 mV for different numbers of layers which is very close to that observed with composite monolayers at the air-water interface. Previous surface potential measurements on pure lignin LB films led to values that were lower than the corresponding monolayer potentials [8]. This is a general feature owing to the negative contribution from the film/ substrate interface. In the results presented here, however, the monolayer potential contained a considerable negative contribution from the double layer because the stearic acid molecules were completely ionized under the sub-phase
Fig. 6. FTIR spectrum of a composite LB film (21 layers) containing lignin and cadmium stearate (50:50 by weight).
C.J.L. Constantino et al. / Thin Solid Films 327–329 (1998) 47–51
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domains of cadmium stearate in the composite LB films, while surface potential measurements demonstrated excellent film uniformity at the macroscopic level.
Acknowledgements The authors are pleased to acknowledge FAPESP and CNPQ for their financial support.
References
Fig. 7. XRD pattern of composite LB films containing different weight percentages of lignin and cadmium stearate; (a) 25:75, (b) 50:50, (c) 75:25, (In all these cases, the deposition was started after holding the monolayer at the deposition pressure for 1 h). (d) 50:50 monolayer after holding for 15 min.
conditions employed [16]. Upon transfer onto a solid substrate, this negative contribution appears to be either lost or minimized. As a result, the LB film surface potential is no longer less than the monolayer potential.
4. Conclusions Meta-stable composite monolayers from lignin and cadmium stearate were characterized by surface pressure and surface potential isotherms. Despite their meta-stable characteristics, the lignin molecules could be transferred along with cadmium stearate as evidenced from FTIR and UV-vis results. XRD results indicated the presence of separate
[1] G. Roberts, (ed.) Langmuir–Blodgett Films, Plenum Press, New York, 1990. [2] Y.K. Kim, M.H. Sohn, B.C. Sohn, E. Kim, S.D. Jung, Thin Solid Films 284–285 (1996) 53. [3] A. Tronin, T. Dubrovsky, C. Nicolini, Thin Solid Films 284–285 (1996) 894. [4] S. Ravaine, C. Mingotaud, P. Delhae`s, Thin Solid Films 284–285 (1996) 76. [5] C. Gu, L. Sun, T. Zhang, T. Li, Thin Solid Films 284–285 (1996) 863. [6] J.H. Cheung, M.F. Rubner, Thin Solid Films 244 (1994) 990. [7] O.N. Oliveira Jr., , C.J.L. Constantino, D.T. Balogh, A.A.S. Curvelo, Cellulose Chem. Technol. 28 (1994) 541. [8] C.J.L. Constantino, L.P. Juliani, V.R. Botaro, D.T. Balogh, M.R. Pereira, E.A. Ticianelli, A.A.S. Curvelo, O.N. Oliveira Jr., Thin Solid Films 284–285 (1996) 191. [9] T. Elder, Internal motions of lignin, Viscoelasticity of Biomaterials, Chap. 25, 1992 pp. 370. [10] I. Watanabe, K. Hong, M.F. Rubner, Langmuir 6 (1990) 1164. [11] T. Suwa, M. Kakimoto, Y. Imai, I. Araki, K. Iriyama, Mol. Cryst. Liq. Cryst. 255 (1994) 45. [12] A. Dhanabalan, S.S. Talwar, S. Major, Mater. Sci. Eng. C3 (1995) 235. [13] H. Morgan, D.M. Taylor, O.N. Oliveira Jr., Biochim. Biophys. Acta 1062 (1991) 149. [14] S.Y. Lin, C.W. Dence (eds.), Methods in Lignin Chemistry, Springer–Verlag, Berlin, 1992. [15] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, 5th ed., Wiley, New York, 1991. [16] A. Dhanabalan, A. Riul Jr. O.N. Oliveira Jr., Supramolecular Sci. in press.
Preparation and characterization of composite LB films of lignin and cadmium stearate C.J.L. Constantino a, A. Dhanabalan a, A.A.S. Curvelo b, O.N. Oliveira Jr. a ,* a
Instituto de Fı´sica de Sa˜o Carlos, USP, CP 369, 13560-970, Sa˜o Carlos, SP, Brazil Instituto de Quı´mica de Sa˜o Carlos, USP, CP 369, 13560-970, Sa˜o Carlos, SP, Brazil
b
Abstract We report on studies of composite Langmuir films containing lignin (extracted from sugar cane bagasse) and cadmium stearate and their subsequent transfer as Langmuir–Blodgett (LB) films. Pure lignin was found to form non-monomolecular films at the air-water interface. Composite monolayers of lignin and cadmium stearate with varying compositions have been obtained and characterized by surface pressure and surface potential isotherm studies which essentially indicated the formation of a meta-stable composite monolayer at the air-water interface. Nevertheless, these composite monolayers could be uniformly transferred onto solid substrates under optimized deposition conditions. Transferred multilayer LB films were characterized by UV-vis and Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and surface potential measurements. FTIR studies confirmed the transfer of lignin along with cadmium stearate. UV-vis results revealed a layer-by-layer transfer of the composite monolayer. In the composite film, both cadmium stearate and lignin were present as separate domains and the stacking order in the cadmium stearate domains was found to be influenced by the incorporation of lignin, as evidenced from XRD. Surface potential measurements indicated a good film uniformity. 1998 Elsevier Science S.A. All rights reserved Keywords: Lignin; Cadmium stearate; Langmuir–Blodgett films
1. Introduction The Langmuir–Blodgett (LB) technique is normally being employed with typical amphiphilic molecules (e.g. fatty acids) which possess distinct polar and nonpolar parts [1]. However, in recent times, the usefulness of this technique has been extended to obtain uniform ultrathin films of non-amphiphilic macromolecules such as polymers [2], various bio-molecules [3], fullerenes [4] and other macro heterocyclic compounds [5]. Though the forces responsible for holding these molecules on the water surface are not clearly understood, it has now been well recognized that these macromolecules may not form a true monomolecular Langmuir film at the air–water interface. Instead, a stable multilayer structure is more likely to be formed [6]. Studies on Langmuir films of bio-molecules are of special interest as they can be extended for understanding the organization of these molecules in biological systems in nature. This has been the main motivation for our project on Langmuir films of lignins extracted from different plant sources * Corresponding author. Tel.: +55 16 2715365; fax: +55 16 2713616; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0585-9
and their transfer as multilayer LB films [7,8]. Lignins are especially interesting materials in this context, because their molecular motions are still a matter of debate [9]. Our earlier studies indicated that, as it has been observed with other polymeric molecules, lignins may form multilayer stacks when spread and compressed on the water surface. Stability experiments revealed a continuous decrease of mean molecular area on holding the monolayer in a compressed state, possibly indicating the poor stability of the lignin monolayer. The composite LB film approach in which the nonamphiphilic molecules are co-spread with typical amphiphilic molecules has often resulted in stable and uniformly transferable monolayers, and this has proven particularly true for conducting polymers [10]. It is to note that in the above approach the long chain amphiphilic molecules have been used just as builder materials and they are not covalently attached to the macromolecules, in contrast to the long chain derivatives of macromolecules. Hence, these builder molecules can be selectively removed in later stages through post-deposition treatments as it has been demonstrated recently with composite LB films made from conducting polymers [11]. In the present study, we report on the preparation of composite monolayers containing various
1998 Elsevier Science S.A. All rights reserved
48
C.J.L. Constantino et al. / Thin Solid Films 327–329 (1998) 47–51
compositions of lignin and cadmium stearate. This approach has led to a reasonable enhancement in monolayer stability and transferability. The uniformly transferred composite monolayers were characterized by UV-vis, FTIR, XRD and surface potential measurements.
2. Experimental The lignin extraction from sugar cane bagasse fibers has already been described in [8]. Various compositions of lignin and stearic acid were dissolved in a mixture of tetrahydrofuran (THF) (Merck, 99%) and chloroform (Merck, HPLC grade) (1:99 by volume). The typical total concentration was about 1 mg/ml. Ultrapure water supplied by a RO60 Millipore filter connected to a Milli-Q system was used to prepare sub-phase solutions. 4 × 10−4 M cadmium chloride was added to the sub-phase whose pH was maintained at 6.0 with the addition of 5 × 10−5 M sodium bicarbonate. In surface pressure and surface potential isotherms, the mean molecular area axis is solely based on the number of stearic acid molecules spread on the water surface and hence the isotherm results presented in this paper will essentially indicate the effect of the addition of lignin on the pure cadmium stearate isotherm. Monolayer studies and multilayer LB deposition were carried out with a KSV-5000 LB system placed in a class-10 000 clean room. After a waiting period of 30 min after the spreading, the monolayer was compressed with different barrier speeds. The compressed mixed monolayers were transferred at 31 mN/m by the vertical dipping method with dipping speeds ranging from 1–3 mm/min. BK7 glass, thin gold evaporated glass and calcium fluoride plates were used as substrates which were cleaned thoroughly prior to use. UV-vis and FTIR spectral measurements were carried out with a Hitachi U2000 spectrophotometer and a BOMEM Michelson series instrument, respectively. X-ray diffraction measurements were carried out with a Rigaku Rotaflex (model RU-200B) X-ray diffractometer in the 2v range from 3–20° using a Cu target. Surface potential measurements of the LB films on glass coated with a thin gold layer were conducted with a Trek 320B electrostatic voltmeter.
the above mentioned problems may be eliminated. In order to infer about the possible molecular organization of the composite monolayer, isotherms were obtained with different compression speeds. Fig. 1 presents the isotherms of monolayers with 50:50 (by weight) of lignin and cadmium stearate on the surface of water containing cadmium ions. In a control experiment for the pure cadmium stearate monolayer, a condensed isotherm with a limiting mean molecular ˚ 2 and a collapse pressure of about 63 area of about 19.5 A mN/m was observed which was similar to that reported in the literature [1]. As shown in Fig. 1, though there is no significant change with the decrease in barrier speed from 100 to 30 mm/min, isotherm curves are found to shift towards lower mean molecular areas with the further decrease in compression speed. The change of mean molecular area with the compression speed may usually be related to the compression of a meta-stable monolayer under non-equilibrium conditions [12]. Under such conditions, the molecules still either undergo reorganization on the water surface or dissolve into the sub-phase water during the compression process. The decrease of the effective mean molecular area with the decrease of barrier speeds essentially reflects the increase of time duration of the compression process. Also worth noting are the exceptionally very high collapse pressures (about 75–80 mN/m) for these composite monolayers, which obviously result from experimental artifacts. This may probably be also associated with the meta-stable characteristics of the composite monolayer. Fig. 2 shows isotherms of composite monolayers containing different ratios of lignin and cadmium stearate during the first compression. For the sake of comparison, all experiments were performed with the same compression speed. For a typical film forming material, one would anticipate an increase of mean molecular area with increasing lignin content in the composite, as the mean molecular area is calculated on the basis of the number of stearic acid molecules
3. Results and discussion In contrast to our earlier work [8] on pure lignin monolayers where THF was employed as the spreading solvent, in the present study we have used a mixture of THF and chloroform (1:99). With the use of a water soluble solvent like THF one would anticipate a material dissolution into the aqueous sub-phase or an inefficient spreading on the water surface which may lead to the formation of micro aggregates immediately after the solvent dissolution into the water. With the use of a mixture of solvents which contain more of a water immiscible solvent like chloroform,
Fig. 1. p–A isotherms of a composite monolayer containing lignin and cadmium stearate (50:50 by weight) at different compression speeds. Area per molecule is calculated based on the stearic acid molecules.
C.J.L. Constantino et al. / Thin Solid Films 327–329 (1998) 47–51
spread on the water surface. As shown in Fig. 2 the limiting mean molecular area for the initial compression was found to increase proportionally to the amount of lignin in the composite. Therefore, it is likely that one can exercise a finite control over the molecular organization of the composite monolayer despite its metastable characteristics as discussed above. In order to further investigate the stability of the composite monolayer, hysteresis experiments were carried out by holding the monolayer in the compressed state for different time durations. After the initial compression, the monolayer was held at 31 mN/m for 2 h and the changes in the mean molecular area were monitored. As shown in Fig. 3, the isotherm curves are found to shift towards lower mean molecular areas as time evolves, implying the metastable characteristics of the monolayer under study. It seems that even in the presence of a builder material like cadmium stearate, the lignin molecules still undergo reorganization as it was observed with pure lignin monolayers. The surface potential-mean molecular area isotherm of a composite monolayer (50:50) obtained during the initial compression with a compression speed of 10 mm/min. is shown in Fig. 4 (experimental conditions are similar to those employed in Fig. 3). It must be stressed that in comparison with typical amphiphilic molecules, the interpretation of the absolute value of the surface potential is not trivial for complex systems as in the present case [13]. However, the relative changes with monolayer composition have often been employed to investigate the possible packing arrangements at the air-water interface. As shown in Fig. 4, after an initial plateau region, the surface potential was found to increase at a critical area which is almost twice the limiting mean molecular area. This feature is typical of amphiphilic molecules and it is related to the coming together of molecules even when the surface pressure is still at zero [13]. The decrease ˚ 2 where the surface presin potential occurred at about 40 A sure begins to rise and is probably related to molecular
Fig. 2. p–A isotherms of composite monolayers containing different weight percentages of lignin and cadmium stearate. Area per molecule is calculated based on the stearic acid molecules.
49
Fig. 3. Hysteresis isotherm of a composite monolayer containing lignin and cadmium stearate (50:50 by weight).
reorientation upon compression. The observed non-zero surface potential at larger mean molecular areas is related to the formation of larger domains immediately after the spreading. Even though monolayer stability was not as good as in traditional amphiphilic materials, uniform transfer of the monolayer could be performed by the vertical dipping method with a dipper speed ranging from 1 to 3 mm/min at a constant surface pressure of 31 mN/m. The composite monolayer was usually transferred to form Y-type LB films. A poor Z-type deposition was noticed when we attempted to transfer the monolayer at a lower surface pressure (15 mN/ m). Films were also transferred after holding the monolayer for different time durations. The transferred LB films were characterized by UV-vis, FTIR, XRD and surface potential measurements. Fig. 5 shows the UV-vis spectra of composite LB films of lignin
Fig. 4. DV–A isotherm of a composite monolayer containing lignin and cadmium stearate (50:50 by weight). Area per molecule is calculated based on the stearic acid molecules.
50
C.J.L. Constantino et al. / Thin Solid Films 327–329 (1998) 47–51
Fig. 5. UV-vis spectra of different odd numbers of layers of composite LB films containing lignin and cadmium stearate (50:50 by weight).
and cadmium stearate containing different odd numbers of layers. The cast film of sugar cane bagasse lignin exhibits absorption starting from 500 nm with an absorption maximum at about 280 nm [14]. As we have employed glass substrates, we could not observe the lignin absorption below 360 nm. The observed increase of the absorption in 360– 500 nm region with an increasing number of layers may indicate that nearly equal amounts of lignin have been transferred during each deposition process. The non-linear increase of absorption corresponding to lignin may indicate the relatively poor transfer of the lignin at the initial stages of the transfer process during which period the monolayer was comparatively less stable. Fig. 6 shows the FTIR spectrum of the composite monolayer containing the 50:50 ratio of lignin and cadmium stearate. The presence of strong absorption at 1546 cm−1 corresponding to the C–O stretching vibration of the carboxylate group indicates the complete ionization of the monolayer. Also seen are the strong absorption peaks at 2918 and 2849 cm−1 corresponding to asymmetric and symmetric C–H stretching vibrations and the weak absorption peaks in the region of 1400 cm−1 for C– H bending vibrations [15]. A relatively small absorption at 1700 cm−1 and 1300–1200 cm−1 region are attributed to the C–O absorptions of carbonyl groups and C–O–C absorptions of ether groups present in the lignin molecule [14]. The possibility of a 1700 cm−1 peak associated with the unionized stearic acid molecules may be ruled out as the pure cadmium stearate LB film deposited under similar subphase conditions did not show any absorption at 1700 cm−1. The possible packing arrangement in the composite LB films was investigated by X-ray diffraction. Fig. 7 presents the XRD patterns obtained with composite LB films containing different compositions of lignin and cadmium stearate (curve-a to curve-c). All these films were deposited after holding the monolayer at the deposition pressure for 1 h. Also shown in Fig. 7 is the LB film containing 50:50 lignin and cadmium stearate in which case the transfer was started after 15 min (curve-d). The composite LB film con-
taining 25% of lignin exhibited a set of intense diffraction ˚ (curve-a), peaks with a bilayer distance of 50.0 ± 0.2 A which is close to the bilayer distance found with pure cadmium stearate LB films [1]. These results may indicate that the cadmium stearate molecules are present in separate domains within the composite LB films. It should be noted that with the increasing amount of lignin in the composite, the intensities of the diffraction peaks are found to decrease (curve-b). The composite LB film with 75% of lignin exhibited practically no diffraction peaks (curve-c). Despite this decrease in intensity which is due to the influence of lignin on the organization of cadmium stearate domains, the bilayer distance is found to be very similar in all three cases. However, for the films transferred with a waiting period of only 15 min, the bilayer distance was ˚ (curve-d). This result may indiincreased to 56.0 ± 1.0 A cate that some lignin molecules are possibly incorporated between the cadmium stearate molecules within the domain structure. Such molecular reorganization might have possibly originated from the meta-stable characteristics of the composite monolayer as inferred from the monolayer studies. Surface potential measurements were carried out with 7 and 13-layer composite LB films. The variation of surface potential while scanning the whole surface of the film was about 10 mV, indicating the excellent film uniformity, at least within the resolution of the surface potential probe. The observed surface potential was in the range 130–150 mV for different numbers of layers which is very close to that observed with composite monolayers at the air-water interface. Previous surface potential measurements on pure lignin LB films led to values that were lower than the corresponding monolayer potentials [8]. This is a general feature owing to the negative contribution from the film/ substrate interface. In the results presented here, however, the monolayer potential contained a considerable negative contribution from the double layer because the stearic acid molecules were completely ionized under the sub-phase
Fig. 6. FTIR spectrum of a composite LB film (21 layers) containing lignin and cadmium stearate (50:50 by weight).
C.J.L. Constantino et al. / Thin Solid Films 327–329 (1998) 47–51
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domains of cadmium stearate in the composite LB films, while surface potential measurements demonstrated excellent film uniformity at the macroscopic level.
Acknowledgements The authors are pleased to acknowledge FAPESP and CNPQ for their financial support.
References
Fig. 7. XRD pattern of composite LB films containing different weight percentages of lignin and cadmium stearate; (a) 25:75, (b) 50:50, (c) 75:25, (In all these cases, the deposition was started after holding the monolayer at the deposition pressure for 1 h). (d) 50:50 monolayer after holding for 15 min.
conditions employed [16]. Upon transfer onto a solid substrate, this negative contribution appears to be either lost or minimized. As a result, the LB film surface potential is no longer less than the monolayer potential.
4. Conclusions Meta-stable composite monolayers from lignin and cadmium stearate were characterized by surface pressure and surface potential isotherms. Despite their meta-stable characteristics, the lignin molecules could be transferred along with cadmium stearate as evidenced from FTIR and UV-vis results. XRD results indicated the presence of separate
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