Structure of Cellulose−Soda Solutions at Low Temperatures

The addition of cellulose does not change the amount of the free water. ..... If this should not be true (if a cellulose + soda + water complex should...
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Biomacromolecules 2001, 2, 687-693

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Structure of Cellulose-Soda Solutions at Low Temperatures Ce´ dric Roy,†,‡ Tatiana Budtova,†,§ Patrick Navard,*,† and Olivier Bedue‡ Ecole des Mines de Paris, Centre de Mise en Forme des Mate´ riaux, UMR CNRS/Ecole des Mines 7635, BP 207, 06904 Sophia-Antipolis, France, Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoi prosp. 31, 199004 St.-Petersburg, Russia, and Spontex, Direction de la Recherche, 74, rue de St-Just-des-Marais, 60026 Beauvais, France Received January 2, 2001; Revised Manuscript Received April 23, 2001

Calorimetry, small-angle X-ray scattering, and viscometry were used to study the structure of NaOHwater and cellulose-NaOH-water solutions in the range of 0-20% NaOH and 0-5% cellulose concentrations in the low-temperature region of -60 to 0 °C. Pure NaOH-water solutions show a pseudoeutectic behavior with three phases: free water that crystallizes and melts at a certain melting temperature which decreases with the increase of NaOH concentration; a NaOH hydrate that melts at -35 °C; water bound to hydrates that does not crystallize. The addition of cellulose does not change the amount of the free water. The cellulose chains are located in the hydrate region, one to two hydroxyl groups of the glucopyranose unit being bound to a soda hydrate. Introduction To process cellulose requires either to chemically modify it or to find proper solvents. The search of solvents has attracted an enormous amount of work and has led to the discovery of a series of substances where cellulose can swell or dissolve. Among them are alkali solutions. It has been known for a long time that cellulose and alkali solutions interact strongly: swelling of cellulose fibers takes place in alkali solutions and the swelling degree depends on cation atomic number and concentration, temperature, cellulose molecular weight, and structure.1 Dissolution can also take place in a narrow range of the phase diagram, leading to the so-called cellulose Q. Comprehensive work on the cellulose behavior in aqueous sodium hydroxide solutions showed that the maximal solubility of low-to-moderate DP cellulose occurs in 8-10% soda solutions (cellulose Q).2-5 The structural parameters of cellulose Q determined by X-rays were obtained.2 A plausible mechanism of cellulose dissolution in a 9% NaOH solution was proposed,4 starting from the penetration of hydrated sodium and hydroxyl ions into the amorphous regions of cellulose, followed by the solvatation of sodium hydrates and hydroxyl ions to cellulose and then the destruction of neighboring crystalline regions and creation of new amorphous ones. By X-ray scattering2 and NMR,6 it was also shown that soda hydrates composed of NaOH and several water molecules are bound to cellulose. What still remains unclear is how cellulose-soda solutions are organized in the cellulose Q region: What is the role of * To whom the correspondence should be addressed (Patrick.Navard@ cemef.cma.fr). † Ecole des Mines de Paris, Centre de Mise en Forme des Mate ´ riaux, UMR CNRS/Ecole des Mines 7635. ‡ Spontex, Direction de la Recherche,. § Institute of Macromolecular Compounds, Russian Academy of Sciences.

free water and soda hydrates, are all soda hydrates bound to cellulose and, if not, what is the proportion between boundto-cellulose and free soda hydrates, and finally, how do these parameters depend on soda and cellulose concentration and way of preparation? To our knowledge there is no publication discussing cellulose + NaOH + H2O structure at low temperatures (-60 to 0 °C) and or providing a similar model as reported for temperatures above 0 °C. To answer these questions is the objective of this paper. The paper is composed of two parts. In the first part we study the structure and properties of pure soda solutions as a function of NaOH concentration and temperature. The phase diagram of aqueous sodium hydroxide solutions has been known since the end of the 19th century.7 However, recent detailed analyses have been performed mostly for concentrated solutions, starting from 20% NaOH.8,9 As far as the best swelling and dissolution of cellulose occurs in 8-10% soda solutions,2-5 it was necessary to understand the NaOH + H2O structure in this dilute region, from 0 to 20%. In the second part we investigate the properties of microcrystalline cellulose + NaOH + H2O solutions at several soda and cellulose concentrations and we propose a model of cellulose + NaOH + H2O structure. Experimental Section Materials. The cellulose sample used in our study was Avicel PH-101 microcrystalline cellulose, called “cellulose” in the following, from FMC Corporation. It is a purified, partially depolymerized R-cellulose derived from purified special grades of wood, with a mean degree of polymerization of 230, as given by the manufacturer. NaOH was supplied by Prolabo. NaOH + H2O solutions were prepared by direct mixing of soda and demineralized water. NaOH concentration was varied from 0 to 20%, and

10.1021/bm010002r CCC: $20.00 © 2001 American Chemical Society Published on Web 06/29/2001

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cellulose concentration varied from 0 to 5%. All concentrations were calculated in weight percent. Preparation of Cellulose + NaOH + H2O Solutions. Cellulose + NaOH + H2O solutions were prepared in two different ways depending on which concentration was varied, NaOH or cellulose. In the set of experiments where the influence of NaOH concentration was studied, the cellulose concentration was kept constant, and cellulose was directly mixed with soda solutions at -6 °C and stirred at 100 revolutions min-1 for 1.5 h. When the influence of cellulose concentration was studied, the NaOH concentration was kept constant at CNaOH ) 9%. In this case, to get a better mixture homogenization and to decrease the amount of undissolved cellulose, a two-stage process was used. First, a certain amount of dry cellulose was immersed in soda solution of 6% at -6 °C and the mixture was stirred for about half an hour at 100 revolutions min-1. Then a soda solution of 15% was added to this mixture to reach a total soda concentration of 9% and a cellulose concentration at the desired value (Ccell from 1 to 5%). The final mixture was stirred again for about 1 h at 100 revolutions min-1 at -6 °C. The influence of the preparation procedure, i.e., the rate of stirring, was also studied for the CNaOH ) 9% solutions. We used an Ultra Turrax T25 mixer made by IKA Labortechnik. As shown in ref 5, the increase in the rate of stirring leads to an increase in cellulose solubility. Cellulose + NaOH + H2O solutions with CNaOH ) 9% were prepared in the same two-step way, as described in the previous paragraph, but instead of stirring at 100 revolutions min-1 first for half an hour and then for an hour, the mixture was stirred at 11 000 revolutions min-1 each step for 3 min. Methods. DSC experiments were performed on a Perkin Elmer DSC-7 calorimeter in stainless steel cells (Perkin Elmer BO18-2901). The heating and cooling rates used in all experiments were 1 °C min-1, because of the stainless steel cell high thermal capacity as compared with typical aluminum cells. Stainless steel had to be used because of aluminum erosion caused by NaOH. The experimental error in temperature and enthalpy determination was lower than 10%. Wide-angle X-ray scattering experiments were performed on a Enraf Nonius DELFT diffractometer, type Diffractis 581, in the Laboratoire de Cristallochimie des Solides, Universite´ de Paris VI, France. A special cooling system was used: First, air was pumped out from the diffraction chamber that was then filled with helium gas. The sample temperature was controlled by nitrogen circulating inside the sample holder. In our experiments we kept temperature constant at -43 °C. The structure of ice in soda solutions was examined by optical microscopy at -18 °C in the Laboratoire de Glaciologie, Universite´ de Grenoble, France. The sample preparation and microscopic observations were performed at room with temperature at -18 °C. First, frozen at -40 °C soda solutions were cut by a microtome in slices of about 2 mm thickness. Then, a transmission polarized optical microscope was used.

Roy et al.

Figure 1. DSC thermogram of NaOH + H2O solution melting, CNaOH ) 4% (1), 9% (2), 15% (3), and 20% (4). The solution was cooled to -60 °C; heating rate was 1 °C/min. Table 1. NaOH + H2O End of Melting (Tm,end) and Crystallization (Tcryst) Temperatures as a Function of Soda Concentration

a

CNaOH, %

Tm,end

Tcryst

1 3 5 7 9 11 15 18 20

-1.2 -2.7 -7.1 -8.2 -12.5 -14.1 -14.1 -23 b

-19 -20.7 -23.8 -23.1 -31.2 -32.4 a a b

Not measured. b No peak.

The viscosity of aqueous soda solutions was measured in a Ubbelohde capillary viscometer at 15 and 25 °C. The experimental error in relative viscosity determination was lower than 2%. Results and Discussions Structure and Properties of Aqueous NaOH Solutions of CNaOH ) 0-20%. To understand how cellulose + soda + water mixtures are organized, we first examined in detail pure NaOH + H2O solutions on micro and macro levels and built a model of the soda-water structure. To do this, we used differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and viscometric methods. The examples of DSC melting diagrams of NaOH + H2O solution of CNaOH ) 4, 9, 15, and 20% are presented in Figure 1. Two peaks can be clearly distinguished. As shown in the following, the first one, at higher temperature, corresponds to the melting of free water, and the second one, at -35 °C, corresponds to melting of a soda hydrate. We varied NaOH concentration and studied its influence on the melting temperature and enthalpy of both peaks. The peak at high temperature is broad and is due to the gradual melting of crystallized water. Low-temperature optical microscopy experiments showed that ice is in the hexagonal form Ih, which correlates well with the ice phase diagram.10 At about 20% of NaOH there is no more free water able to crystallize. Table 1 gives the temperature of the maximum of the high-temperature melting peak and the crystallization temperature. The temperature of the end of

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Table 2. Fractions of Bound Water Fbound and Number of Water Molecules X per 1 NaOH in the Soda Hydrate

CNaOH, %

Fbound - fraction of bound water, (0.02

X ) (NH2O/NNaOH)

3 5 7 9 11 15 18 20

0.32 0.45 0.51 0.62 0.73 0.91 0.96 1

23 19 15 14 13 11 10 9

of NaOH concentration in Table 2. It was calculated as Figure 2. NaOH + H2O solution enthalpies ∆H of the higher temperature (1) and the lower temperature (2) melting peaks as a function of soda concentration. The dashed lines are given to guide the eye.

melting peak, which corresponds to the liquidus curve of the phase diagram, correlates well with the old data of ref 7. The crystallization temperatures are very low, below -20 °C for the solutions above 1%. The behavior of the hightemperature peak as a function of soda concentration associated with the fact that the low-temperature peak does not change its melting temperature with soda concentration (Tm = -35 °C) is in full agreement with published phase diagrams showing a pseudo eutectic point around 20% of NaOH. The peak at low temperatures is due to the melting of the crystallized NaOH hydrate. The change in the melting enthalpy as a function of soda concentration for both first and second peaks is shown in Figure 2, curves 1 and 2, respectively. The enthalpy of the peak at higher temperatures decreases down to zero (curve 1), and the enthalpy of the second peak increases with the increase of NaOH concentration (curve 2). The formation of stable soda hydrates that consist of several water molecules bound to NaOH with the number of bound H2O molecules depending on NaOH concentration has already been reported in several publications.6,7-9 The fact that the low-temperature peak is at a single temperature shows that there is a single hydrate involved. The increase in NaOH concentration leads to the decrease in the amount of free water (i.e., able to crystallize) up to zero at CNaOH ) 20%. When there is no free water (above CNaOH ) 18%), only one peak that corresponds to the melting of this soda hydrate remains. This is a pseudo eutectic point. The fraction of bound water, Fbound, located in NaOH + H2O hydrate is presented in Table 2. It was calculated as (1 - Ffree) where Ffree is a fraction of free water (i.e., melting at high temperatures) calculated as a ratio between the melting enthalpy of the first peak at a given soda concentration (see Figure 2, curve 1) and the melting enthalpy of pure water (365 J/g). The fraction of bound water increases with the increase of soda concentration up to 100% for CNaOH ) 20%. The number of H2O molecules not participating to the high-temperature melting peak, and thus supposed to be bound to NaOH, X ) (NH2O/NNaOH), is given as a function

X)

NH2O NNaOH

) Fbound

(100 - CNaOH) MNaOH MH2O CNaOH

(1)

where MNaOH ) 40 g/mol and MH2O ) 18 g/mol are the molar masses of soda and water, respectively. The number of water molecules per NaOH in soda hydrate decreases down to a certain saturation value of 9H2O per NaOH, leading to a 9H2O × NaOH at CNaOH ) 20% when there is no free water in soda solution. This can be considered as the composition of the stable, able to crystallize hydrate that is at the eutectic point. The last column of Table 2 shows that nevertheless the number of water molecules bound to NaOH increases with the decrease of NaOH content. If all these water molecules should participate to the NaOH + water crystalline hydrates, these hydrates would have different structures and their melting temperatures would differ for different soda concentrations. The existence of a single melting temperature shows that the pseudo eutectic model is valid and that at all NaOH concentrations, it is the same hydrate that is melting. The soda hydrates (that can be considered as inclusions in crystallized water) were seen by low-temperature WAXS experiments performed at -43 °C for two NaOH concentrations (4 and 12%). For these concentrations the scattering patterns are similar to that of a 7H2O × NaOH hydrate. They also showed hexagonal ice crystals. As well as results obtained by the NMR method 6 (8H2O × NaOH for CNaOH ) 5% and 6H2O × NaOH for CNaOH ) 15%, at 4 °C), the number of water molecules in soda hydrate seen by WAXS (7) is lower than that calculated from DSC experiments (9). This is because NMR detects bound water molecules and WAXS detects crystal entities, while DSC shows water that does not crystallize, which is not the same. Such a discrepancy means that soda hydrate must consist of a “core” of bound-to-NaOH water molecules that crystallizes at -35 °C and a “shell” of “amorphous” water that does not crystallize. A possibility of a formation of a “multiple-layer” soda hydrate has been already discussed, and a model of 9% NaOH + H2O solution with a soda hydrate containing eight water molecules bound to NaOH and 0.2 water molecules forming a second solvatation area was proposed.6 So our DSC results are counting all the water bound to the NaOH, crystallizing or not. This allows a better understanding of why at -43 °C WAXS shows only one soda hydrate consisting of 7H2O × NaOH and why the amount of “bound”

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water molecules calculated from DSC increases with the decrease of soda concentration (see Table 2). We can then conclude that it is in the amorphous shell where the amount of water molecules change as a function of NaOH concentration and not in the crystalline part. If this is true, there should be a certain specific enthalpy corresponding to only boundto-NaOH crystallizable water that should not depend on soda concentration and should be equal to the enthalpy value of 200 J/g which corresponds to the pure hydrate when CNaOH ) 20%. Such a specific enthalpy value ∆Hsp was calculated for each soda concentration assuming that soda hydrate consists of 9 bound H2O molecules and (X - 9) amorphous water molecules ∆Hsp ) ∆H2(CNaOH)

msample m9H2O×NaOH

(2)

where ∆H2(CNaOH) is the experimental enthalpy value of the second peak at a given NaOH concentration (see Figure 2), msample is the total sample mass (in grams), m9H2O×NaOH is the mass (in grams) of all 9H2O × NaOH in the sample. The latter can be calculated as m9H2O×NaOH ) mtotal hydrate(CNaOH)

M9H2O×NaOH Mtotal hydrate(CNaOH)

(3)

where mtotal hydrate(CNaOH) ) msample - mice ) msample msample(∆H1(CNaOH)/∆H(CNaOH ) 0)) is the mass (in grams) of all hydrates (with bound and amorphous water) in the sample, mice is the mass of free water that can be calculated from the total mass of the sample taking into account the ratio between the melting enthalpy of the first peak ∆H1(CNaOH) at a given NaOH concentration and the enthalpy of water ∆H(CNaOH ) 0) ) 365 J/g, M9H2O×NaOH ) 202 g/mol is the molar mass of 9H2O × NaOH, and Mtotal hydrate(CNaOH) ) M9H2O×NaOH + (X - 9)18 is the molar mass of total soda hydrate (with bound and amorphous water, see X values in Table 2). For each soda concentration the value of 9H2O × NaOH specific enthalpy was calculated. Within the experimental error of 10% it is close to the melting enthalpy of the lowtemperature peak (hydrate) obtained for the case where there is no free water (200 J/g), the mean value being 205 J/g. This means that the hypothesis that soda hydrate consists of a “core” of 9H2O × NaOH where 9 water molecules are bound to NaOH and a “shell” of “amorphous” water molecules which amounts (X - 9) decreases with the increase of soda concentration (see Table 2) is correct. Another proof that NaOH + H2O solutions can be seen as a “suspension” of soda hydrates in free water at room temperatures is given by a simple viscometric test. Supposing in a very rough approximation that a soda hydrate can be presented as a “spherical particle” (such a model was also proposed in ref 6), we should get a power law dependence of the “suspension” viscosity on soda concentration. The dependence of the NaOH + H2O relative viscosity ηrel measured at two temperatures of 15 and 25 °C as a function of NaOH concentration is presented in Figure 3. Since ηrel(CNaOH)0) ) 1, a linear approximation can fit only the first

Figure 3. Dependence of the NaOH + H2O solution relative viscosity ηrel on NaOH concentration at 15 °C (dark symbols) and 25 °C (opened symbols). The straight line indicates the linear viscosityconcentration regime.

three experimental points, up to CNaOH ) 2.5%. As far as the viscosity-concentration dependence is nonlinear, one can conclude that NaOH + H2O solutions behave as a “suspension” of spherical-like soda hydrates in free water, in agreement with what was found by DSC and WAXS methods. In this case, the increase of water that does not crystallize with the decrease of NaOH concentration suggests that a model more complex than a simple hydrate in free water must be considered. As a conclusion, we propose a soda + water structural model in the 0-20% NaOH concentration range that consists of three phases: free water that able to crystallize; a NaOHwater hydrate that is composed of 9 water molecules per 1 NaOH and crystallizes at -35 °C; a shell of uncrystallizable water around the crystallizable hydrate. Structure and Properties of Cellulose + NaOH + H2O Mixtures, CNaOH ) 0-11%, Ccell ) 0-5%. The goal of this section is to understand how solutions of cellulose + NaOH + H2O are organized, what is the role of soda hydrates and of the water amorphous layer around it, and what is bound and how is it bound to cellulose and if there is an influence of solution preparation on the solution structure. First, we shall discuss the influence of cellulose addition to NaOH + H2O solutions on the melting temperature and enthalpy of crystallizable entities. For this, cellulose concentration will be kept constant (Ccell ) 5%) and only soda concentration will be varied. Then, both cellulose concentration and preparation conditions (two stirring rates were used, 100 and 11 000 min-1) will be varied. In these cases soda concentration will be of CNaOH ) 9%, which is known to induce the maximal cellulose swelling and dissolution.4,11 As well as for NaOH + H2O solutions, two melting peaks on DSC thermograms were obtained. An example is shown in Figure 4 for Ccell ) 5% and CNaOH ) 9%. First, we keep Ccell ) 5% and vary NaOH concentration. The results on cellulose + NaOH + H2O melting and crystallization temperatures of the high-temperature peak (i.e., corresponding to ice melting in a pure soda solutions) as a function of NaOH concentration are shown in Table 3. Here for comparison we also show the corresponding values for pure

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Structure of Cellulose-Soda Solutions

Figure 4. DSC thermogram upon heating for cellulose + NaOH + H2O solution of CNaOH ) 9%, Ccell ) 5%. The solution was first cooled to -60 °C; the heating rate was 1 °C/min.

soda solutions. Surprisingly, there is no influence of the cellulose presence in NaOH solutions neither on melting and crystallization temperatures nor on the enthalpy for NaOH concentrations higher than 3%. The melting enthalpy of both pure NaOH + H2O and cellulose + NaOH + H2O solutions decreases with the increase of NaOH concentration. The melting temperature of the second peak (at lower temperatures) does not depend on soda concentration and was always at -35 °C. This shows that the cellulose chains are not perturbing the areas where the free water is, and thus cellulose must be selectively located in the NaOH-water hydrate regions. The presence of cellulose does not change the amount of free water in the NaOH + water solutions. The presence of cellulose in the NaOH hydrate should change its structure. The first result is that there is still a NaOH hydrate peak, located exactly at the same melting temperature as for pure NaOH + water solutions. This implies that the cellulose chains are not perturbing all the hydrate. A certain fraction of the hydrate stays free of cellulose influence, and this hydrate fraction crystallizes and melts as in the pure NaOH + water system. The influence of the cellulose presence on the thermal properties of cellulose + NaOH + H2O solutions was detected when varying cellulose concentration and way of mixing and analyzing both melting peaks on DSC diagrams. In this set of experiments, soda concentration was kept constant at 9%. The results are presented in Table 4. Here again, one can see that there is no influence of cellulose on the melting temperature and enthalpy of the first peak (at higher temperatures). The melting temperature of the peak at lower temperatures also does not change with the increase of cellulose concentration, but its enthalpy decreases. As far as for pure soda solution this peak corresponds to the melting of the soda hydrate, the decrease in the enthalpy values with the increase of cellulose concentration means that the amount of free soda hydrates decreases. Thus a certain fraction of soda hydrates is bound to cellulose and is prevented from crystallizing. If this should not be true (if a cellulose + soda + water complex should crystallize), there should be no reason the melting temperature would stay 35 °C. The peak at -35 °C corresponds to the same NaOH hydrate found in the NaOH + water solutions.

Using the values of enthalpies obtained for the melting peak at -35 °C (see Table 4), we calculated the fraction of free soda hydrates F that melts at -35 °C and of bound-tocellulose soda hydrates B (B ) 100% - F (%)) that do not crystallize in the temperature interval from 0 to -60 °C. The results are presented in Table 5 for both low and high stirring rates. By soda hydrate at CNaOH ) 9%, we mean nine H2O bound to NaOH plus five amorphous water molecules (result obtained in the previous section, see Table 2). We assume that the number of water molecules in the amorphous shell is the same in NaOH + H2O and in cellulose + NaOH + H2O solutions. The expression for calculating the fraction of free soda hydrates F is F ) mfree/mhydrate where mfree is the mass (in grams) of free soda hydrates in the sample and mhydrate is the mass (in grams) of all soda hydrates in cellulose + NaOH + H2O solution. We assumed that the number of water molecules in the amorphous shell around 9H2O × NaOH is the same in pure soda solution and in cellulose + NaOH + H2O solution (5H2O), the calculations made considering soda hydrate composed either of only the “core” 9H2O × NaOH or of (9H2O × NaOH + 5H2O) are identical, they differ by the factor of (M9H2O×NaOH/ M9H2O×NaOH+5H2O) ) 0.7, where M9H2O×NaOH ) 202 and M9H2O×NaOH+5H2O ) 292 are the molar masses of the “core” and the total hydrate, respectively. The mass of free soda hydrates mfree can be calculated through the hydrate-specific enthalpy value obtained in the previous section (expression 2): mfree ) msample

∆H2(Ccell) ∆Hsp

where msample is the total sample mass (in grams), ∆H2(Ccell) is the enthalpy experimental value for the peak at -35 °C for cellulose + NaOH + H2O solution of a certain cellulose concentration (see Table 4), and ∆Hsp ) 205 J/g is the soda hydrate (9H2O × NaOH) specific enthalpy. The mass of all “core” soda hydrates (9H2O × NaOH)mhydrate in cellulose + NaOH + H2O mixtures is mhydrate )

M9H2O×NaOH M9H2O×NaOH+5H2O

mtotal hydrate )

M9H2O×NaOH M9H2O×NaOH+5H2O

(msample - mice - mcell)

mice being the mass of free water (in grams) in cellulose + NaOH + H2O solution, calculated as a ratio between the enthalpy of the first peak ∆H1(Ccell) (at high temperatures, see Table 4) in each experiment at a corresponding cellulose concentration and the enthalpy of pure water ∆H(CNaOH,Ccell)0) ) 365 J/g (mice ) msample(∆H1(Ccell)/∆H(CNaOH,Ccell)0)) and mcell being the mass (in grams) of cellulose in the sample. The results obtained for the fraction of bound-to-cellulose soda hydrates B for both high and low stirring rates (Table

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Table 3. Melting and Crystallization Temperatures and Melting Enthalpies of Cellulose+NaOH+H2O (CCell ) 5 %) and NaOH+H2O Solutions for the Higher Temperature Peak as a Function of NaOH Concentration

CNaOH, %

Tm,cell+NaOH+H2O, °C

Tm,NaOH+H2O, °C

Tcryst,celll+NaOH+H2O, °C

Tcryst,NaOH+H2O, °C

cellulose + NaOH + H2O ∆H1(CNaOH), J/g

NaOH + H2O ∆H1(CNaOH), J/g

3 5 7 9 11

-7.4 -10.7 -11.3 -11.5 -15.9

-2.7 -7.1 -8.2 -12.5 -14.1

-23.5 -26.4 -27.6 -26.2 -31.4

-20.7 -23.8 -23.1 -31.2 -32.4

250 180 165 140 110

240 200 180 140 100

Table 4. Melting Temperatures and Enthalpies of Cellulose + NaOH + H2O Solutions as a Function of Cellulose Concentration, CNaOH ) 9% high-temperature peak (ice melting)

low-temperature (eutectic) peak (hydrate melting)

Ccell, Tm,cell+NaOH+H2O, ∆H1 (Ccell), % °C J/g 0 1 3 5

-12.5 -12 -12 -11.5

Tm,cell+NaOH+H2O, °C

∆H2 (Ccell), J/g

-35 -35 -35 -35

85 80 56 53

140 150 150 150

Table 5. Fraction of Bound-to-Cellulose Soda Hydrates in Cellulose + NaOH + H2O Mixtures Calculated from DSC Measurements (B) and from Preparation Conditions (B*) no. of OH groups on anhydroglucose linked to soda hydrate

B for low stirring rate, %

B for high stirring rate, %

B*, %

Ccell ) 1% 1 2 3

1

5

3 6 9

32

9 17 26

25

15 30 45

Ccell ) 3% 1 2 3

30

groups per glucopyranose unit may be coordinated with soda hydrates. A similar result was obtained by NMR analysis,12 where a site-preferential interaction of NaOH with hydroxyl groups at C-2 and C-3 was recorded. Within the experimental error, there is no influence of the stirring rate on the amount of bound-to-cellulose soda hydrates. Thus we can conclude that the calculations made according to our model that cellulose + NaOH + H2O solution is composed of soda hydrates bound to cellulose (not melting), “core” of free soda hydrates 9H2O × NaOH (melting at -35 °C), and free water (ice melting in the interval of -1 to -14 °C) correlate well with the assumption that from one to two hydroxyl groups are linked to soda hydrates. The model proposed provides more details in the structural organization of cellulose + NaOH + water solutions as compared with the one discussed in ref 6. We could speculate that our model is valid for any type of cellulose. According to the structure of NaOH hydrate, cellulose may be dissolved at any soda concentration lower than 20% and we did not make any special assumptions for cellulose dissolution exactly in 9% NaOH. Depending on the cellulose degree of crystallinity, crystallite size, and microfibril width, the best NaOH concentration for dissolution may be either 8-10% (ramie cellulose) or, for example, 15-17% (cotton cellulose).13 Our model is in the agreement with both results.

Ccell ) 5% 1 2 3

30

5) were compared with three hypothetical cases where the fraction of bound-to-cellulose soda hydrates was calculated supposing that either one, two, or three OH groups of one glucopyranose unit are occupied by soda hydrates. In this case the fraction of bound-to-cellulose soda hydrates B* is calculated knowing the mixture preparation conditions simply as B* ) Ngn/NNaOH where Ng is the number of glucopyranose moles in the cellulose + NaOH + H2O mixture, n is number of OH groups linked to soda hydrate per glucopyranose unit, and NNaOH is the total number of NaOH moles in the mixture. These results are shown in Table 5. Certainly, in such general calculations it is not possible to get any dependence on the way of cellulose treatment (rate of stirring). The comparison between B (for both low and high stirring rates) and B* shows that, on average, one to two hydroxyl

Conclusions The study of cellulose + NaOH + H2O solutions at soda concentrations of CNaOH ) 0-20% and Ccell ) 0-5% shows that they are composed of three parts: (i) free water that freezes much below zero and melts at a temperature of -1 to -14 °C, depending on NaOH concentration, (ii) soda hydrates that are composed of a “core” of 9H2O bound to NaOH that can crystallize and melt at -35 °C with a specific enthalpy of 205 J/g of hydrate and a “shell” of amorphous water molecules, that do not melt at -35 °C, the amount of amorphous water molecules depending on soda concentration, and (iii) soda hydrates bound to cellulose that do not freeze up to -60 °C, assuming that cellulose does not interact with amorphous water. From one to two hydroxyl groups per glucopyranose unit seem to be coordinated to a soda hydroxyde. The amount of free water is the same in both NaOH + H2O and cellulose + NaOH + H2O solutions. Acknowledgment. Authors are grateful to Professor M. Quarton and Mr. J. P. Souron from the Laboratoire de Cristallochimie des Solides, Universite´ de Paris VI, France, for their help in performing and analyzing WAXS experi-

Structure of Cellulose-Soda Solutions

ments and Dr. Paul Duval, Laboratoire de Glaciologie, Universite´ de Grenoble, France, for his help in carrying out optical microscopy experiments at low temperatures. We thank Dr. H. Chanzy (CERMAV, Grenoble, France) for his valuable comments. This work was sponsored by Spontex. References and Notes (1) Klemm, D.; Philipp, B.; Heinze, T.; Heinze, U.; Wagenknecht, W. ComprehensiVe Cellulose Chemistry, V.2. Functionalization of Cellulose; Wiley-VCN: New York, 1998; pp 31-66. (2) Sobue, H.; Kiessig, H.; Hess, K. Z. Phys. Chem. B 1939, 43, 309. (3) Kamide, K.; Okajama, K.; Matsui, T.; Kowsaka, K., Polym. J. 1984, 12, 857 (4) Kamide, K.; Okajima, K.; Kowsaka, K., Polym. J. 1992, 24, 71. (5) Yamane, C.; Saito, M.; Okajima, K. Sen’i Gakkaishi 1996, 52, 310. (6) Yamashiki, T.; Kamide, K.; Okajima, K.; Kowsaka K.; Matsui, T.; Fukase, H., Polym. J. 1988, 20, 447.

Biomacromolecules, Vol. 2, No. 3, 2001 693 (7) Pickering, S. U. J. Chem. Soc. 1893, 63, 890. (8) Wunderlich, J. A. The`se de doctorat; Contribution a l’e´tude cristallochimique des hydrates de la soude; Universite´ de Paris, 1958. (9) Cohen-Adad, R.; Tranquard, A.; Peronne, R.; Negri, P.; Rollet, A.P. C. R. Acad. Sci. Paris 1960, 251, 2035-2037. (10) Lobban, C.; Finney, J. L.; Kuhs, W. F. Nature 1998, 391, 268. (11) Kamide, K.; Kowasaka, K.; Okajima, K. Polym. J. 1985, 17, 707. (12) Fink, H.-P.; Walenta, E.; Kunze, J.; Mann, G. In Cellulose and Cellulose DeriVatiVes, Physico-chemical Aspects and Industrial Applications; Kennedy, J. F., Phillips, G. O., Williams, P. A., Piculell, L., Eds.; Cambridge: Woodhead Publ. Ltd., 1995; pp 523-528. (13) Warwicker, J. O.; Jeffries, R.; Colbran, R. L.; Robinson, R. N. A review of the literature on the effect of caustic soda and other swelling agents on the fine structure of cotton; Shirley Institute Pamphlet No. 93; The Cotton Silk and Man-made Fibres Research Association, Shirley Institute: Didsbury, Manchester, 1966.

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