Lignin - American Chemical Society

Chapter 13

Structural Investigation of Dehydrogenation Polymer (models of lignin) Films at the Air-Water Interface by Neutron Reflectivity

Downloaded by FUDAN UNIV on December 13, 2016 | http://pubs.acs.org Publication Date: November 30, 1999 | doi: 10.1021/bk-2000-0742.ch013

B . Cathala, N. Puff, V . Aguié-Béghin, R. Douillard, and B . Monties I N R A , Equipe de biochemie des macromolécules végétales, U P B P , C R A , 2 Esp. R. Garros, 51686 Reims Cedex 2, France ([email protected])

Two DHPs (Dehydrogenation Polymers, models of lignins) were spread at the air/water interface at several surface concentrations. The structure of the interfacial layer was investigated by neutron reflectivity. At low surface concentrations DHPs form an uniform layer containing a high percentage of water (70-80%). The thickness of this layer increases with the amount of DHP deposited. At high surface concentrations a two-layer structure is observed with a water content of 50-60% on the air side and 8090% on the bulk side. These films exhibit long relaxation times after compression (determined by dynamic surface pressure measurements) indicating that the DHP molecules are probably involved in a network.

Plant cell walls are heterogeneous structures mainly composed of three types of polymers: cellulose, hemicelluloses and lignins. Cellulose and hemicelluloses are polysaccharides and they are hydrophilic (1). On the contrary lignins are thought to be more hydrophobic because they are composed of phenylpropane units containing fewer hydroxyls groups (2). In the plant cell walls these polymers are associated with each others and the contact zone (the interface) between such hydrophilic and hydrophobic polymers should be very important for the molecular architecture of the cell walls. In this study, we investigate the structure and the properties of dehydrogenation polymers, models of lignins, at the air/water interface modeling an hydrophobic / hydrophilic interface in order to evaluate some features of the macromolecular organization and properties of DHPs in an heterogeneous medium. Interfacial adsorption layers have a thickness which is of the order of magnitude of the size of the adsorbed macromolecule (3). Thus they provide a means to study, at the molecular level, the properties and the organization of the

278

© 2000 American Chemical Society Glasser et al.; Lignin: Historical, Biological, and Materials Perspectives ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


279 DHPs in an anisotropic environment. These studies are complementary to those made in a solution which is a homogeneous and isotropic medium. It is usually accepted that lignins are amorphous and have a low degree of organization, however some general orders can be detected. For example, Agarwall and Atalla (4) using Raman spectroscopy have demonstrated that the aromatic ring of the phenyl propane units are parallel to the plane of the plant cell wall surface. This was also demonstrated by molecular modeling by the same authors and also by Jurasek (5) who moreover pointed out that lignins can be porous. The same conclusion was also drawn by studying air/water interfacial films of lignin by ellipsometry (6) and in our team by neutron reflectivity (7). Luner and Kempf (8) have also found that lignin at the air/water interface can form a gel-like structure indicating that lignin is involved in a network. DHPs are not lignins, however they can mimic lignin properties and structures (9). They can be obtained in a reproducible way and free of any type of contamination and particularly free of polysaccharides which can have a significant effect on the surface properties. They allow also large and easy changes in the chemical composition. As a consequence DHPs are useful to support hypotheses on structure-properties relations in lignins. In the present work, we have used neutron reflectivity to determine, at several concentrations, the structure of DHP interfacial layers and their surface concentrations which are relevant parameters for the interfacial properties. These structural results are compared with the compressibility behavior of the films which is determined by dynamic surface pressure measurements. Experimental D H P synthesis. DHPs were prepared according to Higuchi's procedure (10) from coniferyl alcohol (11). A l l the parameters of the synthesis were identical for the Zutropfverfahren DHP (ZT) and Zulaufverfahren DHP (ZL) except the addition rate of the precursors. In the ZT, the precursors (coniferyl alcohol and hydrogen peroxide) were added in a slow and continuous way on a solution of peroxidase, whereas for the Z L , all the reagents were added at the same time. Some characteristics of DHPs are listed in Table I, and they are published in details elsewhere (12). They were dissolved and used in a 2 g/L 9/1 dioxan/water solution (v/v). ZT DHPs have a higher content of β-Ο-4 linkages and a lower content of cinnamyl alcohol end groups compared to Z L DHPs. In the latter case, at the beginning of the reaction, the concentration of monomelic radicals is high (due to the fast addition) leading to the formation of dimeric products. These dimers polymerize rapidly to larger molecules. On the contrary, in the Z T method, the concentration of radicals is low (due to the slow addition) and as a consequence the coupling between two monomers is less frequent. Here, the monomelic radicals react with polymeric radicals leading mainly to linkages of the β-Ο-4 type involving a decrease of phenolic hydroxyl groups. This way of addition results also in higher molecular weight products.

Glasser et al.; Lignin: Historical, Biological, and Materials Perspectives ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

280 Table I : Main characteristics of the ZT and ZL

DHPs

ZTDHP

ZLDHP

%C : 58.9 %H : 5.64 %0:35.5

%C : 63.8 %H : 5.85 %0:30.3

1800 1400 1.3

1400 1050 1.312

630

527

ratio phenolic OH groups versus aliphatic OH groups

0.26

0.32

Scattering length density ΝΜΙΟ^Α- )"

2.75

2.8

Elemental composition GPC determination" Mw Mn polydispersity Thioacidolysis yield (μπιοΐ/g of DHP)


b

0

2

1

a. Weight average molecular weight (M ) = Σ^ι C fin (Ci /Mi); Number average molecular weight (H,) = Σ (C Md^i ton Q (C = concentration; M = molecular weightfromcalibration curve), polydispersity = Mw/Mn. GPC analysis in THF, flow rate :lml/mn, T° :40°C, injection volume : 100μ1, detection UV 280nm. (12) b. Thioacidolysis is an acid catalysed degradation in dioxan-ethanethiol with boron trifluoride which results in the cleavage of aryl-glycerol paryl ether (21) These values reflect the content of β04 linkages which is the most abundant intermonomeric linkage in lignins and in DHP c. Based on the intensity of carbonyl infrared bands of acetylated DHPs according to Faix's procedure (22). The contents of phenolic and aliphatic OH change according to the distribution of intermonomeric linkages. ZL DHPs have a higher content of phenolic OH than ZT DHPs (9) d. Calculated for 1.5 OH group per C9 unit (13) and a density of 1.41 g/cm (8) (see text) w

1 ο η

toll

Μ t o n

3

Neutron reflectivity. Theoretical background. Neutrons interact with matter in the same manner as light, when their electromagnetic properties are concerned and so neutron reflectivity is linked with toe neutron index o f refraction, n, of the medium crossed by the neutron beam. The index of refraction of the medium is : η = 1-λ *Λ>/2π where λ is the neutron wavelength, Ν is the atomic number density, and b is the coherent length density, thus, the product Ν b is the scattering length density. The b parameter is characteristic of the nuclei of atoms and is very different when considering hydrogen or deuterium. Thus in order to increase the contrast between the layers and the subphase, all the neutron reflectivity experiments were performed using D 0 instead of hydrogenated 2

2


281 water. In these cases the replacement of the hydroxyl hydrogen by deuterium has to be considered for DHP Nb calculation. It can be assumed that the average number of O H groups per C9 unit is 1.5 for Z L and ZT DHP based on the results obtained by Saake (13). The DHP density has also to be taken in account for the Nb calculation. The density value used in this study is 1.41 g/cm according to the value obtained on Milled Wood Lignin (MWL) of spruce by Luner et al (8). The use of these two approximations is not crucial in the context of this work. It may only change the absolute values but not the main tendencies observed here (variation on DHP volume fraction should be less than +/- 0.1). 3

Experimental. The reflectivity spectra were determined with a polychromatic beam of neutrons at a fixed incident angle θ using the « time of flight » method. Reflectivity is the ratio of the intensity of the specularly reflected beam to the intensity of the incident beam. When an adsorption layer occurs at the interface, the value of n(z) in the direction perpendicular to the interface plane is not constant The deviation of the reflectivity from the Fresnel reflectivity (n(z) constant) provides information on the variation of the refractive index n(z). Because the refractive index is a function of the scattering length density of the atoms, one can therefore deduce the solvent and DHP composition of the interfacial layer. The method used to calculate the variation of refractive index is based on the replacement of the continuous variation by a series of discrete homogeneous layers and the application of the standard optical methods. In each layer, the DHP volume fraction Φ is linked to the scattering length density of the DHP ( N b ) and of water (Nb ) by (14) : Ν b( ) = Φ(Ζ) N b + [1- Φ(Ζ)] Nb^ter


0

DHP

Z

water

D H P

Experiments have been performed at the « time of flight » reflectometer DESIR (15) in the Orphée reactor (Léon Brillouin Laboratory, Saclay) at the grazing angle θ of 1.265°. The useful neutron wavelength ranges from 3 to 15.5 Â. The reflectivity measurements were performed using a teflon trough 6.5 χ 13.5 χ 0.3 cm in a gas tight cell thermostated at 20°C enclosed in a second cell which helps to maintain constant temperature of the air surrounding the first cell. The trough was filled up with 9 mL of water forming a meniscus 2-3 mm over its edge. DHPs were spread on the air / water interface from a 9/1 dioxan/D 0 solution (2g/L). The reflectivity spectrum was recorded during eight hours and a flat background was subtracted before reflectivity calculation 0

2

Dynamic surface pressure experiments : DHP layers were formed by droplet deposition of a 2 g/L (dioxan/water, 9/1, v/v) solution on the surface of ultra pure water filling the trough (LB 105, Atemeta, Paris; maximum area 286 cm ). After deposition, the DHP layer was allowed to settle during 30 min, then the barrier was moved with a rate proportional to the area : dA/dt = -Α/τ where A is the area of the film and τ is a constant whose value is obtained by integration : τ =-At / In (Ae/Ao), where A o and Ae are the values of A at the beginning and at the end of the compression (Ao-Ae =10 cm here) and At its duration (presently 30 min). 2


282 When the compression was completed, relaxation was allowed to take place during 180 min. The limit concentration was determined by deposition of low amounts of DHP. It is the surface concentration where the film starts to develop a significant pressure. The results presented here are the average of four independent determinations. Results and discussion : Neutron reflectivity. Figure 1 shows the experimental and calculated reflectivity curves of DHPs spread on deuterium oxide (24 mg/m deposited). Spreading DHPs on deuterium oxide yields a favorable situation for neutron reflectivity measurements because the Nb of DHPs (2.8 ÎO^Â" ) is practically half-way between those of D2O (6.37 10" Â" ) and air (« 0 Â" ). Experimental neutron reflectivity spectra were fitted with calculated models and the best mathematical result was retained. Tables Π and III report the volume fractions and the thicknesses found for Z L and Z T DHPs at several deposited surface concentrations ranging from 3 to 48 mg/m . For the low concentration regime (less than 12 mg/m deposited for Z T and 18 mg/m deposited for ZL) a good fit is obtained using a one layer model. Two layer or more complicated models do not improve the % of the fits. At higher concentrations (high concentration regime), an improvement of the results is achieved with a two layer model.


2

2

6

2

2

2

2

2

Volume fraction. The layer formed at low concentration has a constant DHP volume fraction of around 0.2-0.25 for all the concentrations and for Z T and Z L DHP (Tables Π and ΙΠ). A similar stability of the volume fraction is observed in the case of high concentration deposits: the air side layer has a DHP volume fraction of around 0.4 and the water side layer has a lower content of DHP (φΟΗΡ=0.15,

Lignin - American Chemical Society

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