Crystal structure of cellulose-iodine complex

Highlights

• Formation of cellulose-iodine complex has been established.

• X-ray crystal structure analysis of cellulose-iodine complex has been performed successfully.

• Formation mechanism of the complex has been studied by the density functional theoretical calculation.

Abstract

Different from starch and amylose [(1 → 4)-α-D-glucopyranan], cellulose [(1 → 4)-β-D-glucopyranan] or wood fiber (natural cellulose or cellulose I) and paper (re-generated cellulose or cellulose II) have been believed for long years in many papers and high-school textbooks that they do not form an iodine complex at all. This legend originated from the characteristic conformation of cellulose chain, which is fully extended and had been believed to be impossible to entrap the long iodine ions likely the case of amylose helical chain. However, people often observe such a phenomenon that a filter paper is dyed easily when it is immersed into the iodine aqueous solution. On the basis of the X-ray diffraction measurements of the various kinds of cellulose materials (a piece of wood, bacteria cellulose, paper, cellophane, and so on), the present study has confirmed that cellulose forms the crystalline complex with iodine, as known from the change of X-ray diffraction pattern before and after the complexation. The crystal structure of cellulose-iodine complex has been proposed through the X-ray data analysis and the complex formation mechanism was discussed quantum-chemically by comparing the crystal structure between the original cellulose and its iodine complex. After the completion of the structural analysis of this complex, we happened to notice the paper by Hess et al. (Kolloid Z. (153) 1956, 128) reported about 60 years ago, who measured the X-ray diffraction patterns of cellulose immersed into iodine solution and assigned one reflection of 9.0 Å spacing to the characteristic to the complex. But no detailed analysis was made, and their paper did not get much attention in these 60 years, resulting in the above-mentioned wrong and long legend. The present paper has succeeded for the first time to break through such a common sense and to establish the formation of crystalline cellulose-iodine complex by the detailed X-ray structure analysis.

Introduction

It is well known that iodine forms a complex with the various kinds of substance including polymers [1]. Some of the most typical iodine complexes of polymers are starch-iodine complex [[2], [3], [4], [5], [6]] and poly(vinyl alcohol) (PVA)-iodine complex [[7], [8], [9], [10]]. In the latter case, the immersion of an oriented PVA film into a dilute KI/I₂ aqueous solution gives the iodine complex formed in the amorphous region, which is utilized industrially as an optical polarizer. When the oriented PVA film is immersed into an iodine solution of higher concentration, it creates the crystalline complex [9,10]. The 3 kinds of complex (forms I, II and III) are found out depending on the iodine concentration and the kind of counter cation (H⁺, Li⁺, Na⁺ and K⁺) [[11], [12], [13]]. The driving force of the complex formation is the hydrogen-bond-like interactions between I₃⁻ ions and OH groups of PVA, which include the so-called charge transfer interaction from iodine to OH group [14].

The similar type of interactions between iodine and hydroxyl group may be predictable for polysaccharides [1]. Starch is one of the most well-known iodine complexes since 1 century ago [[2], [3], [4], [5], [6]]. The typical method to check the inclusion of starch is named the iodine test. The linear iodine ions are entrapped inside the helix. The helical conformation is formed because of the α-1,4-linkage of the neighboring glucose rings of starch. Amylose (1,4-α-D-glucan) exhibits the similar helical conformation as starch and shows also the iodine complex [15]. The similar structure is seen also for the iodine complex of cyclodextrine, where the linear iodine ions are trapped through the holes of the rings [16,17].

The opposite case is cellulose composed of the β-1,4-linkage of glucose rings, which takes a linearly extended chain conformation in the solid state [[18], [19], [20], [21], [22], [23], [24], [25]]. Because of such an extended conformation of the chain, cellulose had been believed for a long time not to form the complex with iodine ions. In fact, we can visit many homepages in which the cellulose is said to have no color change with iodine [1,[26], [27], [28]]. However, why does a complex need to have a structure with the iodine ion rods trapped inside the helical chains with the round hole ? In the case of PVA complex, the iodine molecules form the columnar structure, and they are surrounded by the plural PVA chains and stabilized through the OH⋯I bonds [9,[11], [12], [13], [14]]. This is a good suggestion for the extended cellulose chain to form the iodine complex in the solid state. In fact, many phenomena are observed, suggesting the cellulose-iodine complex formation. For example, a filter paper or a cotton sheet is dyed in dark blue color to dark red color when the iodine aqueous solution is dropped on the sheet. This phenomenon is quite popular, and is introduced in the textbook of an elementary school. Abe measured the UV–Visible spectral pattern change in the zinc chloride aqueous solution of a filter paper and interpreted the color change by the charge transfer complex formation between cellulose and iodine species [29]. In spite of these clear indications of the cellulose-iodine complex formation, however, people say simply that this phenomenon is observed because the filter paper contains a small amount of starch component, not due to the formation of complex with the main cellulose component. But, even the purified cellulose substance without any starch component shows the dying phenomenon by the immersion into the iodine solution, as will be mentioned later. We need to check the possibility of cellulose-iodine complex in the solid state, in particular, the formation of the complex in the crystalline part.

As will be reported here, we have measured the 2-dimensional X-ray diffraction patterns for the various types of cellulose materials after immersion into the KI/I₂ aqueous solution of relatively high iodine concentration, and obtained the clear X-ray diffraction pattern which is quite different from that of the original cellulose. This experimental result has confirmed the formation of crystalline complex between cellulose and iodine ions. On the basis of the quantitative X-ray data analysis a model of cellulose-iodine complex has been proposed and the interaction between these two chemical species was discussed from the quantum chemical point of view.

It must be mentioned here that, after finishing the structure analysis of cellulose-iodine complex, we have happened to notice one paper by Hess et al. published about 60 years ago [10]. They reported the X-ray diffraction patterns of the cellulose materials immersed into iodine solutions and observed one reflection of 9.0 Å spacing, which was assigned to the characteristic reflection of the cellulose-iodine complex. Unfortunately, however, no concrete and detailed analysis of the observed X-ray diffraction data was made by them. Besides, it may be sad to say, but their paper had not gotten much attention up to now, and as a result the above-mentioned wrong legend or a wrong common sense has been believed and described in many papers and textbooks repeatedly. In such a sense, the present paper has resuscitated the report of Hess et al. by presenting the clear and compelling X-ray diffraction data and by analyzing the concrete crystal structure of cellulose-iodine complex for the first time.