Investigation of OH bond energy for chemically treated alfa fibers

Highlights

• This work focuses on the structure analysis of treated alfa fiber using FTIR-ATR spectroscopy.

• An in-depth study of the changes on hydrogen bond energy of treated alfa fibers was conducted.

• The region of interest was the OH bond ranging from 2800 to 3800 cm⁻¹.

• Inter and intramolecular hydrogen bonds and free OH groups were fully analyzed.

Abstract

This work aims to study the hydrogen bond energy and distance for different samples of alfa fibers treated with thymol. The treatment duration and thymol concentration were varied and seem to have a great influence on infrared band intensities and positions. The number of hydrogen bonds is related to the infrared band intensity, whereas their energy and distance depend on the infrared band position.

It was proven that the free hydroxyl groups are weakened and tend to disappear with fiber treatment. It is the same for intermolecular hydrogen bands between cellulosic chains that present a decrease in both intensity and frequency.

The two intramolecular hydrogen bands increase in intensity but exhibit different behaviors regarding the calculated energy: while the band at 3268 cm⁻¹ is weakened and shifted to higher wavenumbers, that at 3338 cm⁻¹ keeps the same peak position and energy.

Introduction

Cellulose molecules are biosynthesized by a complex process involving enzymes located on the cell surface. To form the cellobiose repeating unit, anhydroglucose residues alternate in an inverted orientation resulting on β(1 → 4) glucosidic linkages. Cellulose molecule is straight (and never branched), and its hydroxyl groups are free to hydrogen–bond with the hydroxyls of other cellulose molecules lying parallel to it. In plant cell walls, cellulose chains at angstrom scale are organized into microfibrils, each measuring about 3 to 6 nm in diameter and containing up to 36 glucan chains having thousands of glucose residues. The cell wall’s rigidity is due mainly to the microfibril scaffold.

In a glucan unit, the three hydroxyl groups (O2-H, O3-H, and O6-H) lie parallel to the ring plane (equatorial), in contrast to the hydrophobic C(1…5)-H groups and the methylene groups (C6-H₂) which end up perpendicular to the mean plane (axial). Consequently, in-plane hydrogen bonds may be formed between two adjacent glucan units (intermolecular) or within the same chain (intramolecular). This results in a compilation of cellulosic chains into flat sheets stacked by weak van der Waals interactions and forming microfibrils.

It is worth noting that the formation of intrachain and interchain hydrogen bonds is a key factor for the stability of the three-dimensional structure. Consequently, the understanding of OH bonds network of the supramolecular structure is o;1;f great importance since it dictates the way in which cellulose chains are packed, and one can then interpret in larger scale the mechanical, physical and chemical behavior of cellulosic materials, especially when these materials are submitted to a chemical treatment (Bansal, Kumar, Latha, Ray, & Chatterjee, 2015; Panwar, Bansal, Raya, & Jaina, 2016; Verma, Jain, & Sain, 2011a; Verma, Jain, & Sain, 2011b).

In this context, our work aims to investigate hydrogen bond network of cellulosic materials. In particular, we focused our analysis on a detailed study of hydrogen bond energy carried on thymol treated alfa fibers.

An exhaustive bibliographic study was carried out in the aim of determining infrared bands assignment relating to intermolecular, intramolecular and free OH bonds. No conflicts were identified regarding band positions. However, diverging interpretation for some band assignments was found. This results from cellulose chains orientation in the three-dimensional structure which is not so far well identified. For this reason, Infrared data interpretation is hampered by the misunderstanding of cellulose structure.

To revise and reevaluate band assignments, recent works were based on new technologies and improvements in both spectrophotometric techniques and computational chemistry methods.

• Bands at 2850 cm⁻¹ and 2917 cm⁻¹ are assigned for cellulose Iβ to symmetric and asymmetric CH stretching in aromatic methoxyl groups and in methyl and methylene groups of side chains (Carillo, Colom, Sunol, & Saurina, 2004; El Oudiani, Msahli, & Sakli, 2017; El Oudiani, Turki, Msahli, & Sakli, 2017 Pandey & Pitman, 2003; Schwanninger, Rodriguez, Pereira, & Hinterstoisser, 2004).

• The peak at 3273 cm⁻¹ can be assigned to the stretch vibration mode containing the intramolecular hydrogen bonds with a major contribution of O6H⋯O2. As shown in Fig. 1a, this hydrogen bond occurs in cellulose Iβ between O2 and O6 of successive glucosidic units in the same chain (Guo & Wu, 2008; Hinterstoisser & Salmean, 2000; Poletto, Zattera, & Santana, 2012; Watanbe, Morita, & Ozaki, 2006). Band at 3273 cm⁻¹ is specific to large cellulose Iβ crystallites, and it appears only if hydroxymethyl groups adopt the trans–gauche conformation. In fact, unlike carbons C1 to C5 which are rigid, exocyclic carbon C6 has some freedom to rotate between three different conformers (trans–gauche, gauche–trans, and gauche–gauche) depending on a gauche stereoelectronic effect (Perez & Mazeau, 2004) (Fig. 1b).

• With regard to the band positioned at 3337 cm⁻¹, there is some conflict in its assignment. The majority of authors consider that this band is attributed to O3H⋯O5 intramolecular hydrogen bonding of cellulose Iβ. This bond corresponds to the hydrogen linkage between O3 and the ring ether O5 of another residue (Fig. 1a) (Fan, Dai, & Huang, 2012; Guo & Wu, 2008; Hinterstoisser & Salmean, 2000; Kolpak & Blackwell, 1976; Kondo, 2005; Lennart, Margaretha, & Kerholm, 2004; Poletto et al., 2012; Watanbe et al., 2006), but some other researchers assign this band to the contribution from both intramolecular hydrogen bonds O3H⋯O5 and O2H⋯O6 (Lee, Mohamed, Watts, Kubicki, & Kim, 2013; Lee et al., 2015; Marechal & Chanzy, 2000; Nishiyama, Johnson, French, Forsyth, & Langan, 2008).

• The peak at 3375 cm⁻¹ is observed in Halocynthia mantle, Glaucocystis, algal and bacterial cell walls (Lee, Kafle, Park, & Kim, 2014; Lee et al., 2015). Thus, it was suggested that the 3370 cm⁻¹ band is typical to samples rich in cellulose Iα and ascribed to intramolecular hydrogen bonds O3H⋯O5.

• While the massif at 3200–3300 cm⁻¹ involves intramolecular hydrogen bonds of cellulose Iβ, that between 3400 and 3500 cm⁻¹ is rather attributed to intermolecular hydrogen bonds of cellulose Iβ (Hatakeyama & Nagasaki, 1969; Lee et al., 2015; Liang & Marchessault, 1959; Marechal & Chanzy, 2000; Nishiyama, Isogai, Okano, Müller, & Chanzy, 1999; Tashiro & Kobayashhi, 1991). This assumption is grounded by evidence that intermolecular H bonds are generally weaker than intramolecular ones. However, this band assignment is not definitive since, in some works, bands around 3400 cm⁻¹ were also assigned to intramolecular H bonds (Ivanova, Korolenko, Korolik, & Zbankov, 1989; Kondo, 2005).

• Peaks in the range of 3500–3600 cm⁻¹ are generally attributed to stretching vibration of the free OH groups (Guo & Wu, 2008; Hinterstoisser & Salmean, 2000; Kondo, 1997; Kondo, 2005; Poletto et al., 2012; Popescu et al., 2007; Schwanninger et al., 2004; Silverstein, Bassler, & Morrill, 1981; Watanbe et al., 2006). These bands appear at higher wavenumbers compared to those for interchains and intrachains hydrogen bonds.

• High wavenumbers in the range of 3700–3800 cm⁻¹ (bands at 3720, 3750 and 3780 cm⁻¹) are free hydroxyl groups of lower acidity that occur at defect sites (hydroxyl nests). These free OH groups are much stronger than those appearing around 3500 cm⁻¹. Hydroxyl nests are a consequence of a disordered cluster of molecules, and there number is inversely related to the fiber degree of crystallinity. Bands at 3720, 3750 and 3780 cm⁻¹ due to hydroxyl nests are also observed in aluminosilicate and zeolite materials (Cornell & Schwertmann, 2006; Karge, 2001).

In the “OH⋯O” linkage, there is a trade-off between the covalent bond “OH” and the hydrogen bond “H⋯O” strengths. When “OH” covalent bond length is shortened (increase in frequency or blue-shift), that of hydrogen bond “H⋯O” increases. This leads to the weakening of hydrogen bond and a reduction of its energy. On the opposite side, the increase of “OH” covalent bond length induces a decrease in frequency (red-shift), which implies a strengthening of hydrogen bond and a rise in its energy.

In Table 1 are summarized the characteristics of different categories of hydrogen bonds and the relation between hydrogen bond energy and the position of “OH” covalent band in IR spectrum.

To interpret the variation in infrared band intensities and positions, four different cases presented in Fig. 2 may arise.

• Case 1 in Fig. 4a shows a decrease in OH band intensity as well as in frequency (red shift). This may be explained by the reduction of OH bond number at this frequency, but some strong ones with higher energy persist and are maintained even after treatment.

• In case 2 (Fig. 4b), we notice an increase in OH band intensity as well as in frequency (blue shift). This may be explained by the formation of new OH bonds at this frequency, but these bonds are not as strong as the initial ones due to the lack of stability and to the packing effect that makes the length of OH covalent bonds shorter.

• Case 3 in Fig. 4c shows a lowering of OH band intensity accompanied by an increase in frequency (blue shift). This may be due to the weakening of hydrogen bonds that disappear at this frequency under treatment.

• Case 4 in Fig. 4d presents an increase in OH band intensity, and a decrease in frequency (red shift). This may be explained by formation of strong and stabilized hydrogen bonds under treatment corresponding to this infrared frequency.