LA SEDE DEL TAS Y LA CELEBRACIÓN DE AUDIENCIAS EXTRA-MUROS:

The dissolution temperature of cellulose / aqueous alkali hydroxide has a crucial effect on the solubility of cellulose. It has been shown that the low temperature (from 4 ºC to -20 ºC) facilitates the solubility of cellulose (Davidson 1936; Davidson 1937; Yamashiki

et al. 1988; Laszkiewicz and Cuculo 1993; Isogai and Atalla 1998; Cai and Zhang 2005). Moreover, the low temperature of the solution improves its stability towards gelation (Roy et al 2003).

Yamashiki et al. (1988) studied the effect of temperature on the structure of aqueous sodium hydroxide without and with cellobiose. They found many features which showed that the properties of aqueous sodium hydroxide were unique in the concentration range 9 – 12 % compared to the lower or higher concentrations, and at temperatures below 4 ºC compared to the higher temperatures. The findings included plateau in electrical conductivity, which indicated the presence of a metastable state that restricts to some extent the ionic transportations. The changes in proton chemical shift suggested rapid interaction between NaOH and cellulose, as well as the high number of water molecules dissolved in NaOH at the specific concentration range (9 – 12 %) and at low temperature. Furthermore, based on the ²³Na chemical shift it was evident that Na⁺ ion does not have crucial effect on the dissolution of cellulose, although it was suggested the cellulose interacts strongly with both cationic and anionic species in the solution. Raman spectroscopy showed that the strength of hydrogen bonds was the weakest in the range of 9 - 12% NaOH, and the sudden change in specific rotatory angle of the cellobiose/aqueous alkali solutions suggested that cellobiose might take a specific conformation when the alkali concentration was 9 – 12 % ( Yamashiki et al. 1988).

Later on it has been suggested (Lindman and Karlström 2009; Medronho et al. 2012;

Kihlman et al. 2013) that the reason for the beneficial effect of lowered temperature in cellulose dissolution is due to the conformational change in molecular structure of cellulose. The hypothesis is based on the bi-functional behaviour of oxyethylene oxide based surfactants which exhibit strongly decreased solubility in water with increasing temperature while at the same time their solubility in hydrocarbons increases. The behaviour is due to the conformational changes of ethylene oxide chains in non-ionic surfactant. Low temperature favours such conformation of O-CH₂-CH₂-O segments around the C-C bond that makes the ethylene oxide more polar and thus solubility in polar solvent is enhanced. Lindman’s group suggest that similar kind of conformational change due to temperature could apply to cellulose also, thus explaining the improved solubility at low temperature

Isobe et al. (2013) studied the dissolution of cellulose in static system containing cellulose/LiOH/urea. They found that cellulose started to dissolve rapidly when the temperature decreased below 10 ºC, i.e. four times more cellulose dissolved at 0 ºC compared to the amount dissolved at 10 ºC. The result clearly shows that the dissolution was driven only by the decreased temperature as no stirring was used.

The structure of alkaline cellulose solutions at low temperature was studied with DSC based on the principle of eutectic mixture (Roy et al. 2001; Egal et al. 2007; Wang et al.

2015). Roy et al. (2001) showed that 20 % aqueous NaOH produced only one melting peak at -35 ºC, whereas the solutions with lower NaOH concentrations (1-15 %) produced two peaks; one at the same temperature as 20 % NaOH (i.e. melting of eutectic mixture) and another one at higher temperature (from -1.2 to -14.1 ºC, melting of ice). This confirmed that the pseudo eutectic model was valid for the NaOH hydrates.

The DSC curves of cellulose/NaOH/water mixtures with 5 % cellulose and various NaOH (3-11 %) contents were identical with the corresponding curves of aqueous NaOH without cellulose (Roy et al. 2001). Thus, undissolved cellulose did not have any effect on the melting behaviour of the cellulose/NaOH/water mixture. Instead, when the cellulose/NaOH/water solutions with various cellulose contents (0.5-7.5 %) were studied, it was found that the enthalpy of the eutectic peak decreased with increasing cellulose content (Roy et al. 2001; Egal et al. 2007). Thus, there was a clear interaction between the dissolved cellulose and sodium hydroxide. Based on the decreased enthalpy the minimum amount of NaOH molecules per one anhydroglucose (AGU) that was required to dissolve cellulose was concluded to be four (Egal et al. 2007). The same relationship was found when cellulose/NaOH/urea solutions were studied with the same experimental setup (Egal et al. 2008). However, in both studies unusual correlation between the amount of bound NaOH and the concentration of cellulose was found. It was apparent that much higher amount of NaOH was bound to cellulose at low cellulose concentration compared to the high concentration. The authors (Egal et al.

2007) recalculated the results of Kuo and Hong (2005) and found those to coincidence with their own results. Egal et al. (2007) speculated that the behaviour might be due to the steric hindrance at higher cellulose concentration or due to the complex interaction between the bound and unbound NaOH hydrates in the presence of cellulose which prevents NaOH to participate in the crystallisation of the eutectic mixture. One possible reason for the contradictive NaOH/AGU ratio which the authors did not discuss is the amount of undissolved cellulose in the sample. It is plausible, when considering the reported sample preparation procedure, that the solutions have contained undissolved cellulose which share had increased with increased cellulose concentration. As the undissolved cellulose did not make any change in the enthalpy of eutectic peak as shown by Roy et al. (2001), the recalculation may change the NaOH/AGU ratio of the cellulose/NaOH/water solutions to be more consistent.

The resent results of Wang et al. (2015) with low-substituted hydroxyethyl cellulose (HEC) showed that it was not possible to derive the NaOH/AGU solubility limit from the changes in eutectic enthalpy peaks. They found that the enthalpy of eutectic peak decreased to zero already with 1 % HEC concentration even though the highest

concentration dissolved was 7 %. The authors suggested that the phase diagram might be more complicated than considered up to now, since a third peak below the temperature of eutectic peak has occasionally been found, but not included in the studies.

Nevertheless the ratio between NaOH/AGU needs some further research, the thermal studies of cellulose/NaOH/water samples confirmed that there is a clear interaction between cellulose and NaOH. Moreover, the interaction involves NaOH hydrates at eutectic mixture as active compounds to penetrate into the cell wall layers. This is the reason why the solubility of cellulose in aqueous NaOH is favoured by the lowered temperature.