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Investigation of Aggregation and Assembly of Alkali Lignin Using Iodine as a Probe Yonghong Deng,† Xinjia Feng,† Mingsong Zhou,† Yong Qian,† Haifeng Yu,‡ and Xueqing Qiu*,† †
State Key Lab of Pulp and Paper Engineering, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, People's Republic of China ‡ Top Runner Incubation Center for Academia-Industry Fusion, and Department of Materials Science and Technology, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka 940-2188, Japan ABSTRACT: Molecular iodine has been introduced into the alkali lignin (AL) solutions to adjust the π-π aggregation, and the effect of lignin-iodine complexes on the aggregation and assembly characteristics of AL have been investigated by using fluorescence, UV-vis spectroscopy, light scattering, and viscometric techniques. Results show that AL form π-π aggregates (i.e., J-aggregates) in THF driven by the π-π interaction of the aromatic groups in AL, and the π-π aggregates undergo disaggregation in THF-I2 media because of the formation of lignin-iodine charge-transfer complexes. By using iodine as a probe to investigate the aggregation behaviors and assembly characteristics, it is estimated that about 18 mol % aromatic groups of AL form π-π aggregates in AL molecular aggregates. When molecular iodine is introduced into the AL solutions, lignin-iodine complexes occur with charge-transfer transition from HOMO of the aromatic groups of AL to the LUMO of iodine. The formation of lignin-iodine complexes reduces the affinity of the aromatic groups approaching each other due to the electrostatic repulsion and then eliminates the π-π interaction of the aromatic groups. The disaggregation of the π-π aggregates brings a dissociation behavior of AL chains and a pronounced molecular expansion. This dissociation behavior and molecular expansion of AL in the dipping solutions induce a decrease in the adsorbed amount and an increase in the adsorption rate, when AL is transferred from the dipping solution to the self-assembled adsorbed films. Consequently, the adsorption behavior of AL can be controlled by adjusting the π-π aggregation. Above observations give insight into the occurrence of J-aggregation of the aromatic groups in the AL molecular aggregates and the disaggregation mechanism of AL aggregates induced by the lignin-iodine complexes for the first time. The understanding can provide an academic instruction in the efficient utilization of the alkali lignin from the waste liquor and also leads to further development in expanding functionalities of the aromatic compounds through manipulation of the π-π aggregation.
’ INTRODUCTION Lignin occupies about 20-30% of the wood as a natural polymer, but it is mostly present as a byproduct in spent liquor from the paper and pulping industry.1 With the growing crisis of oil resource and increasing deterioration of ecological environment, the recovery of lignin from spent liquor, especially the most abundant alkali lignin (AL), has attracted worldwide attention. How to utilize AL with high efficiency is still a great challenge; a better understanding of AL basic properties is required to improve its value-added application. The soda pulping process is widely used in Chinese paper mills presently, and the AL obtained from the waste liquor can be chemically modified to prepare lignin-based biopolymers for various application.2-4 The exact chemical structure of AL is hard to determine because of the composition complexity of the natural lignin according to plant species and the random cleavage of the lignin main chain during the soda pulping process. It is widely accepted that lignin consists of three phenyl-propanoid units (C9 unit), such as para-hydroxybenzene (H), guaiacyl (G), and syringyl (S).5 The randomness of linkage of r 2011 American Chemical Society
the three phenyl-propanoid units and split of lignin network during the soda pulping process lead to a wide polydisperse AL fragments. An important physicochemical property of AL is the prominent aggregation tendency in solutions, which has a significant influence on the delignification, the biodegradation processes, and in the course of preparation of lignin-based biopolymers.6-8 Accordingly, a fundamental investigation of AL aggregation has both theoretical significance and application value. AL aggregation was first noted by Benko in 1964.9 AL aggregation causes a change of the apparent molecular weight when measured in the different solvents and at different temperatures.10 The aggregation phenomenon of AL has been broadly detected by light scattering,6 turbidity,8 cryogenic transmission electron microscopy,8 spectrophotometry,11 gel permeation chromatography,12 and viscosity measurements.13 For the driving forces of these aggregation processes, there are two points of view. Received: December 1, 2010 Revised: December 29, 2010 Published: March 02, 2011 1116
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Biomacromolecules Lindstrom found that the carboxylic groups played an important role in the association of alkali lignin in aqueous solutions and the thermally irreversible nature of the association, and then suggested that the AL aggregates were formed by hydrogen bonding between the carboxylic groups and various ether oxygens and hydroxylic groups.13 In contrast, Sarkanen et al. observed that the aggregation process of AL was stoichiometrically constrained, and each AL aggregate had a locus that was respectively complementary to only one type of component. Therefore, they proposed that AL aggregation was governed by the π-π interaction among the benzene groups.14 In their experiments, complete acetylation of lignin samples had no significant effect on the relative proportions of AL aggregates observed in nonaqueous solvents, indicating that hydrogen bonding could not provide the driving force for association. Actually, the AL aggregation can be classified into two levels of aggregation: one is the molecular aggregation of polymer chains because of van der Waals attraction, and the other is π-π aggregation of the aromatic groups in lignins because of nonbonded orbital interaction (π-π interaction).15-18 In those early stage investigations, the π-π aggregation has been named according to the relative position of the aggregate absorption band to the molecular absorption band (M-band). H-aggregation represents the aggregation exhibiting a blue-shifted band to the M-band, while J-aggregation corresponds to the aggregation displaying a red-shifted band.15-17 Molecular aggregation does not go with energy transfer, while π-π aggregation is generally accompanied by energy splitting. Therefore, π-π aggregation of the aromatic compounds does not necessary result in molecular aggregation, and vice versa17,18 To be exact, the AL aggregation driven by hydrogen bonding as suggested by Lindstrom is a kind of molecular aggregation of AL chains that is not necessarily accompanied by π-π aggregation of the aromatic rings. However, the AL aggregation governed by the π-π interaction as proposed by Sarkanen et al. is a kind of π-π aggregation of the aromatic groups that induces molecular aggregation, so complete acetylation of lignin samples had no obvious effect on π-π aggregation of the aromatic groups. Argyropoulos et al. reported that the presence of iodine in the AL samples caused a significant decrease in the apparent molecular weight based on an assumption that addition of iodine should eliminate the π-π interaction of the aromatic groups of AL.19 They had provided valuable insight into the π-π interaction mechanism of AL aggregation in solutions, but the evidence pointing to the interaction of iodine and AL is still lacking. It is well-known that iodine can form charge-transfer complexes with solvents, such as benzene and tetrahydrofuran.20-27 The charge-transfer transition phenomenon from benzene (electron donor) to iodine (electron acceptor) was first reported by Benesi and Hildebrand in 1949.20 The conformations for the iodine-benzene charge-transfer complex have been widely reported.21-24 Mulliken originally stated that a resting structure was the most likely conformation.21 Nelander et al. claimed that the iodine-benzene complex is axial although the other halogen complexes have an unsymmetric structure.22 Lenderink et al. supported that oblique structure was the most stable conformation.23 Grozema et al. predicted an unsymmetric ground-state structure, such as the above-bond and the above-carbon conformations.24 Excitation energies of the charge-transfer band are related to the conformations of iodine-benzene complexes and the media.24,25 It was reported that only the above-bond, the above-carbon, and the oblique conformations show considerable oscillator strengths for the charge-transfer transition, while the
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other conformations show nearly no absorption.24 Cheng et al. reported that when the media of iodine-benzene complexes was changed from gas phase to condensed phase, the charge-transfer absorption band had a red shift due to the symmetry breaking, polarizability, and polarity of the solvent.25 Cataldo has reported that iodine formed a charge-transfer complex with THF when iodine was dissolved in THF.26 The electronic spectra of iodine in THF showed that a charge-transfer band developed at 250 nm with a shoulder at 290 nm, resulting from the formation of iodinated oxonium cation, and the absorption bands appeared at 366 and at 420 nm due to I3- and I5-, respectively. The UV-vis spectrum of iodine solutions changed with solvents. The absorption band of iodine in the gas phase was detected to be located at 530 nm.27 In nonpolar solvents like hexane and CCl4, the absorption bands of iodine were observed to be at 523 and 515 nm, respectively.26 In polar solvents like H2O and THF, the absorption bands of I- CTTS (charge-transfer-tosolvent) were reported to be at 225 and 254 nm, respectively.28 It is possible that there exist lignin-iodine charge-transfer complexes in the AL solutions with addition of iodine. The lignin-iodine complex might reduce the affinity of the aromatic groups approaching each other, and prohibit the π-π interactions of AL. However, to our knowledge, there is still no report pointing to these iodine-lignin charge-transfer complexes. In this work, we studied the iodine-lignin charge-transfer complexes existing in the AL/I2 solutions, and then using iodine as a probe investigated the aggregation behaviors and assembly characteristics of AL. AL solutions with the addition of iodine were used as dipping solutions for preparing AL self-assembled films. It was found that π-π aggregation and molecular aggregation coexisted in the AL dipping solutions, and disaggregation occurred when iodine was introduced. The AL self-assembled films were prepared by immersing the bare quartz slides into the AL dipping solutions, and the aggregation behaviors of the dipping solutions had a great influence on the adsorbed amount and the adsorption rate during the self-assembly process. In the earlier studies, there are some important reports about the ion-ion and cation-π interactions that relevant to lignin adsorption at solidliquid interfaces.29,30 These studies focused on the interaction between lignin and substrates during the adsorption process but paid little attention to the effect of lignin aggregation on the adsorption. Although there are a lot of interactions that may be relevant to the lignin dipping solutions and the lignin assembled films, this work only focuses on the π-π interaction. Iodine was proved to be a useful probe to detect π-π interaction of the aromatic groups and monitor the effect of π-π aggregation on the adsorption characteristics of AL. Potential industrial applications of AL may benefit from a better understanding of the π-π interaction of AL in both solutions and adsorbed films.
’ EXPERIMENTAL SECTION Materials. The natural polymer AL separated from wheat pulping black liquor was supplied by the Quanlin paper mill in Shandong province, China. The AL sample was purified carefully by acidification, filtration, and washing.33 The weight average molecular weight (Mw) of AL was measured to be 3801 with the polydispersity index of 2.15 (by GPC). The contents of the carboxyl, phenolic, and methoxyl group of AL were measured to be 6.51, 2.86, and 12.38 wt %, respectively, according to the methods described in ref 33. By elemental analysis, the contents of elemental carbon, hydrogen, and oxygen of AL were measured to be 60.35, 5.72, and 27.81 wt %, respectively. Based on 1117
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Biomacromolecules the results of the elemental and functional group analysis, the average phenyl-propanoid unit (C9 unit) of AL was calculated to be C9H8.88O2.69(OCH3)0.73, with a monomer molecular weight of 183. Poly(diallyldimethylammonium chloride) (PDAC, Mw 200000350000, 20% solution, Aldrich) was used as a polycation and was diluted to a concentration of 0.1 mmol/L (repeated unit). Tetrahydrofuran (THF) used as solvent of AL is HPLC grade from commercial source. Milli-Q water (resistivity > 18 MΩ 3 cm) was obtained from a Millipore water purification system. Iodine and the other reagents were purchased commercially as analytical grade products and used directly without further purification unless otherwise indicated. Characterization. The molecular weights and their distributions of AL were determined by using gel permeation chromatography (GPC) with Ultrahydragel 120 and Ultrahydragel 250 columns. The 0.10 mol/L NaNO3 aqueous solution with pH 8 was used as the eluent at a flow rate of 0.5 mL/min. The effluent was monitored at 280 nm with a Waters 2487 UV Absorbance Detector (Waters Corp., U.S.A.) at a flow rate of 1.0 mL/min. Polystyrene sulfonate (PSS) was employed as the standard substance to calibrate the instrument. The UV-vis absorption measurements were performed with a UV-vis spectrophotometer (UV-2450, Shimadzu Corp., Japan). For the sample solutions with THF as the solvent, THF was scanned at the same wavelength as a baseline. For the measurements of the AL films, air was scanned at the same wavelength as a baseline. The fluorescence measurements were performed on an Fluorosens System (Gilden Photonics Ltd., England) equipped with a 150 W xenon Arc Lamp at 298 K. The excitation spectra of AL solutions at different concentrations were recorded in a range of 200-500 nm using an emission wavelength of 530 nm, the slit width for both emission and excitation was 2.5 nm, and integration time was 100 ms. Before measurement, the solution was ultrasonicated for 10 min and then kept undisturbed. Laser light scattering (LLS) experiments were performed on a commercial light scattering instrument (ALV/CGS-3) equipped with a multi-τ digital time correlator (ALV-7004) and a solid-state He-Ne laser (JDS-Uniphase, output power = 22 mW at 632.8 nm). A high performance laser-line bandpass filter (NT47-494) was placed between the sample solution and the photomultiplier to filter the scattered light so as to avoid an overestimation of the molecular weight of lignin due to fluorescence.19 All the experiments were performed at 25 °C. The details of LLS theory can be found elsewhere.31,32 To ensure that light scattering measurements were not affected by dust, the stock solutions were filtered through Whatman filters with normal pore size of 0.45 μm, transferred to dust-free glassware. Viscosity measurements were performed with an Ubbelohde-type capillary viscometer, located in a thermostatted bath (25 ( 0.1 °C). The time for the sample to flow from one level indicator to another, known as flow time, as well as the density of samples were measured. All of the experiments were repeated three times and the average values were taken. The reduced viscosity (ηsp/C) was calculated according to eq 1. ! Ft ηsp =C ¼ -1 =C ð1Þ F0 t0 where ηsp is the specific viscosity, C is the concentration of AL, F and F0 are the density of AL solution and THF at 25 °C, and t and t0 are the flow time of AL solutions and THF. Self-Assembly Film. AL self-assembled films were prepared by immersing bare quartz slides into the AL dipping solutions and rinsed with solvent. AL solutions with addition of iodine were prepared as dipping solutions to investigate the effect of iodine on the adsorption characteristics of AL. For the adsorption kinetics of an AL layer on the quartz slide, a bare quartz slide was dipped into the AL dipping solution for a given time and then thoroughly rinsed with THF and blown dry with air. This process was repeated until the adsorbed equilibrium of AL
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Figure 1. Schematic representation of the arrangement for H and J aggregation and their different energy levels. θ is defined as the tilt angle between the molecular axis connecting the center of chromophores in the aggregate and a transition dipole moment of each chromophore. was reached. The bare quartz slides are treated as follows: the quartz substrates were sonicated in a 98% H2SO4/30%H2O2 solution for 1 h, and sonicated in a H2O/H2O2/NH4OH (5:1:1) solution for 1 h, which was then followed with a thorough rinse and drying with an air stream.
’ RESULTS AND DISCUSSION AL possesses polydispersity both in the molecular weight and in the loading density of the functional groups, including carboxylic, phenolic hydroxyl, and methoxyl groups. The Mw of the AL used here was estimated to be 3801 by GPC with the polydispersity index of 2.15. The contents of the carboxyl, phenolic, and methoxyl group of AL were measured to be 6.51, 2.86, and 12.38 wt %, respectively. By elemental analysis, the contents of elemental carbon, hydrogen, and oxygen of AL were measured to be 60.35, 5.72, and 27.81 wt %, respectively. The average phenylpropanoid unit (C9 unit) of AL was calculated to be C9H8.88O2.69(OCH3)0.73, based on the data of the elemental analysis and the functional group contents. For convenience, the structure of C9H8.88O2.69(OCH3)0.73 is recorded as the “AL unit” in the following sections. π-π Aggregates of AL. It is well-known that alkali lignin trends to form molecular aggregates,6-13 and the π-π interaction of the aromatic rings has been reported to be responsible for lignin association phenomena in organic solvents.14 The π-π interaction of the aromatic rings results in two distinct aggregation modes by which AL can be assembled, such as a sandwich-type arrangement and a head-to-tail arrangement, analogous to the well-known H and J aggregation in organic chromophores.15-18 Previous studies have proposed similar molecular aggregation structures of AL but did not use the term “H or J aggregates.14,19 Figure 1 illustrates a basic scheme of H and J aggregation by which AL may be assembled. According to one-dimensional model of molecular exciton by McRae and Kasha,15 the H-aggregate is formed in case that a tilt angle between the molecular axis connecting the center of chromophores in the aggregate and a transition dipole moment of each chromophore, θ, is larger than 54.7°. On the contrary, the J-aggregate is formed when θ is less than 54.7°. Here, the chromophores are the aromatic groups of AL molecules, that is, the C9 unit. The orientation of the transition dipole moment of each chromophore is parallel to the longitudinal direction of the C9 unit. A sandwich-type arrangement and a head-to-tail arrangement are two opposite extremes for H-aggregation (θ is 90°) and J-aggregation (θ is 0°) types. According to the molecular exciton coupling theory, when the aromatic rings of AL form the π-π aggregates, such as H- or 1118
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Figure 2. (a) Fluorescence spectra of AL in THF with different concentrations (molar concentration of AL unit); λexc = 450 nm. (b) Normalized excitation spectra of AL in THF, monitored at 520 nm.
Figure 3. (a) Normalized excitation spectra of AL in THF with low concentrations, monitored at 530 nm. (b) UV-vis absorption spectra of AL in THF with low concentrations (molar concentration of AL unit).
Figure 4. UV-vis absorption spectra of I2 in THF with different concentrations.
J-aggregates, they will exhibit perturbed absorption and excitation spectra. Figure 2a shows the emission spectra of AL in THF
Figure 5. UV-vis absorption spectra of AL/I2 in THF with different molar ratios of I2 to AL unit. The concentration of iodine in all the samples is fixed at 0.32 mM.
with different concentrations, excited at wavelength of 450 nm. The maximum emission band is located at 520 nm, without 1119
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Figure 6. (a) Normalized UV-vis absorption spectra of AL-I2 (0.13 mM AL unit, 0.32 mM I2), AL (0.13 mM), and I2 (0.32 mM) in THF. (b) Multiple peak fitting of the UV-vis spectrum of the AL-I2 (0.13 mM AL unit, 0.32 mM I2) in THF. The spectrum of THF is also shown for comparison. For the sample solutions with THF as the solvent, THF was scanned at the same wavelength as a baseline.
change with concentration. Figure 2b shows the excitation spectra of AL in THF with different concentrations, monitored at an emission wavelength of 520 nm. For convenience of comparison, the excitation spectra are normalized at wavelength of 450 nm. The excitation spectra have a significant red-shift with increasing concentration, which give a strong evidence for the occurrence of π-π aggregates in the AL aggregates. According to the molecular exciton coupling theory, this spectral red-shift indicates that the aromatic rings of AL form J-aggregates with a head-to-tail arrangement.15 As shown in Figure 2b, this degree of π-π aggregation depends on the AL concentration, indicating the presence of intermolecular π-π aggregates formed from the aromatic groups of AL. However, in the low concentration range (i.e.,
