REVIEW

Rationalizing cellulose (in)solubility: reviewing basicphysicochemical aspects and role of hydrophobicinteractions

Bruno Medronho • Anabela Romano •

Maria Graca Miguel • Lars Stigsson •

Bjorn Lindman

Received: 2 December 2011 / Accepted: 26 December 2011 / Published online: 6 January 2012

� Springer Science+Business Media B.V. 2012

Abstract Despite being the world’s most abundant

natural polymer and one of the most studied, cellulose

is still challenging researchers. Cellulose is known to

be insoluble in water and in many organic solvents, but

can be dissolved in a number of solvents of interme-

diate properties, like N-methylmorpholine N-oxide

and ionic liquids which, apparently, are not related. It

can also be dissolved in water at extreme pHs, in

particular if a cosolute of intermediate polarity is

added. The insolubility in water is often referred to

strong intermolecular hydrogen bonding between

cellulose molecules. Revisiting some fundamental

polymer physicochemical aspects (i.e. intermolecular

interactions) a different picture is now revealed:

cellulose is significantly amphiphilic and hydrophobic

interactions are important to understand its solubility

pattern. In this paper we try to provide a basis for

developing novel solvents for cellulose based on a

critical analysis of the intermolecular interactions

involved and mechanisms of dissolution.

Keywords Cellulose � Solubility � Hydrophobic

interactions � Amphiphiles � Hydrogen bonding

Dissolving simple and complex saccharides

Cellulose is the world’s most abundant renewable

material. It is estimated that nearly 700 billion tons are

produced every year. However, only a tiny fraction of

it is used for further processing (Kihlman et al. 2011).

Cellulose is already used often in our daily lives and

has numerous applications. Its potential is enormous;

it is believed that forest based raw materials can play a

major role in replacing fossil oil based fibers and

cotton by new ecological man-made fibers in both

woven and nonwoven end applications. However,

when working with cellulose some important appli-

cations involve its dissolution and, for different

reasons, this task is normally complicated. Traditional

dissolution methods have important limitations espe-

cially allied to costs and environmental issues and thus

there is a growing need to replace these severe

B. Medronho � M. G. Miguel � B. Lindman

Department of Chemistry, University of Coimbra,

Coimbra, Portugal

B. Medronho (&) � A. Romano

Faculty of Sciences and Technology,

Institute for Biotechnology and Bioengineering,

Centre of Genomics and Biotechnology (IBB/CGB),

University of Algarve, Campus de Gambelas, Ed. 8,

8005-139 Faro, Portugal

e-mail: bfmedronho@portugalmail.pt

L. Stigsson

KIRAM AB, Norra Villavagen 17, 23734 Bjarred,

Sweden

B. Lindman

Division of Physical Chemistry, Center of Chemistry and

Chemical Engineering, Lund University, Lund, Sweden

123

Cellulose (2012) 19:581–587

DOI 10.1007/s10570-011-9644-6

processes. The development of cheaper and environ-

mentally ‘‘friendly’’ alternatives to the solvents used is

thus of great interest to the industry. Cellulose is

difficult to dissolve. It is insoluble in water and in

typical organic solvents, but soluble in a few classes of

solvents, which, according to current understanding,

have no apparent common properties. On the other

hand, glucose is highly soluble in water, but insoluble

in nonpolar solvents. Also glucose derivatives, like

alkylpolyglucosides (APGs), made less polar by

introduction of alkyl chains, can be highly soluble in

water. Furthermore, the aqueous insolubility of cellu-

lose contrasts to some other nonionic polysaccharides,

like dextran, another polyglucose. Cellulose is a quite

polar molecule, with several hydroxyl groups, and thus

has a good hydrogen bonding ability. The insolubility

in nonpolar organic solvents, therefore, poses no

problems of understanding. On the other hand, the

aqueous insolubility is more difficult to understand

and has created a lot of interest, and concomitant

research. There are many and scattered opinions.

Nevertheless, there seems to be a consensus among

leaders in the field that the insolubility of cellulose is

due to its ability to form intra- and intermolecular

hydrogen bonds; some authors emphasize in addition

the role of crystallinity of cellulose, a matter ques-

tioned by others.

After a literature survey, supported by common

knowledge of intermolecular interactions and solution

physical chemistry, we found that the current mech-

anistic basis, that leading groups in academia and

industry use as a basis for their development, must be

seriously questioned (Lindman et al. 2010). It is

essential to look deeper into the underlying mecha-

nisms and thus a rather general (but somehow

neglected in the cellulose scientific community)

perspective, with key ideas, is reviewed in the next

sections.

Polymer dissolution: kinetics and thermodynamics

issues

The dissolution of compounds in a solvent and the

formation of a homogeneous solution take place only

if the mixed state corresponds to a lower free energy

than two separate phases. However, dissolution may

not be achieved even if favorable free energy condi-

tions prevail. The process may be too slow on the time-

scale of the observation. Standard techniques to

enhance the rate of dissolution involve heating and

stirring, as this speeds up and increases contacts

between solvent and solute. Kinetic control of disso-

lution is much more important for macromolecules

than for low molecular weight solutes; in the latter

case mixing is easier and molecular diffusion faster.

Scientific research on the kinetics of polymer disso-

lution is still rather undeveloped. Once a polymer

sample is placed in a solvent, the solvent molecules

will contact the polymer relatively fast and penetrate

into the surface. Often this results in a gel-like

consistency of the outermost part. Polymer molecule

diffusion is intrinsically much slower than solvent

diffusion and for concentrated polymer solutions (as

would apply for the surface layer) severely retarded by

entanglement and association. Therefore, polymer

dissolution is slow in general and further slowed

down for semi-crystalline polymers. A thorough study

of polymer dissolution was done recently (Axelsson

et al. 2006; Korner 2006; Korner et al. 2005a, b;

Larsson et al. 2009). In the literature, particularly such

with a more applied focus, a distinction between

kinetic and thermodynamic aspects is commonly not

done and this applies also for many publications on

cellulose dissolution. It is often stated that cellulose

dissolution depends on the way of handling, temper-

ature and time of heating, as well as the molecular

weight. It has also been pointed out by several authors

that higher concentrations are reached using micro-

wave heating. This clearly indicates that kinetic rather

than thermodynamic control is decisive. In any case,

literature is somewhat confused, as discussions about

mechanisms are generally based on thermodynamic

arguments (interactions, etc.), even if authors state the

importance of time effects.

Charged and non charged polymers: entropy

of mixing as the driving force for dissolution

In general, dissolution and miscibility are almost

always driven by the entropy of mixing, and not, as it is

sometimes assumed, by favorable interactions. For

dissolution, the change in free energy needs to be

negative. The free energy change is the sum of two

terms, one normally positive, enthalpy, which depends

on intermolecular interactions, and one entropic,

which is negative. It is more difficult to dissolve

582 Cellulose (2012) 19:581–587

123

polymers than low molecular weight compounds. The

reason for this is that the number of molecules, and

thus the entropic term, becomes smaller (less nega-

tive). The higher the molecular weight the weaker is

the entropic driving force contribution for dissolution;

it is thus more difficult to dissolve high molecular

weight macromolecules than low molecular weight

ones (Holmberg et al. 2002; Korner 2006). These

considerations refer to the translational entropy of

mixing. There is also another term for polymers,

related to the conformational freedom. For a polymer,

which increases its conformational freedom on going

into solution, dissolution is more favorable than for a

polymer that cannot change conformation. Flexible

polymers are, therefore, more soluble than stiff ones.

Applying this to cellulose, the fully equatorial con-

formation of b-linked glucopyranose residues stabi-

lizes the chair structure, minimizing its flexibility (for

example, relative to the slightly more flexible a-linked

glucopyranose residues in amylose). The stiffness of

cellulose thus, directly, due to low configurational

entropy in solution, contributes to a lowering of the

solubility. There is also another effect: A polymer with

hydrophilic and hydrophobic parts will in water adjust

its conformation to reduce the contacts between

hydrophobic parts and water. For a stiff polymer such

conformational changes are hindered. This effect will,

as we shall see, contribute to a low solubility of

cellulose in water. The discussion above is valid for

nonionic polymers. For ionic polymers, polyelectro-

lytes, the behavior is different. Solubility is due to the

large number of small counterions, which contribute

strongly to entropy. Additionally, the electrostatic

repulsion between charged polymer backbones may

also contribute to dissolution but Coulombic interac-

tions are much less significant and can also have an

opposite effect (Schneider and Linse 2002). Therefore,

polyelectrolytes are normally more soluble in water

than nonionic ones. Charging up a polymer is always

expected to help solubility. Clearly, this is the reason

why cellulose tends to be more soluble/be more

penetrated by water at either high or low pH. However,

the pK values are such that rather extreme conditions

are needed for either deprotonation of the hydroxyls

or protonation. For instance, Staric and Schofield,

assuming that only one hydroxyl group per anhydro-

glucose unit dissociates, found a pKa of 13.3 at room

temperature (Saric and Schofield 1946). On the other

extreme, too low pH will, however, have adverse

effects on the chain length of the cellulose polymer

and lead to a breaking of the glycosidic bonds in the

anhydroglucose polymer. The charging up principle is

used in the modification of cellulose, by either cationic

(i.e. cationic trimethylammonium derivative of

hydroxyethyl cellulose, such as ‘‘polymer JR’’) or

anionic groups (i.e.: carboxymethyl cellulose, CMC).

One could also foresee the same effect by association

of cellulose with some ionic cosolute, like a surfactant

or another polymer. This approach appears not to have

been tried to an important extent for cellulose,

although it is well known for nonionic cellulose

derivatives. The charging mechanism is expected to

play a role for some of the systems developed for

cellulose dissolution like phosphoric acid (Boerstoel

et al. 2001; Northolt et al. 2001), Na/LiOH in

combination with, for example, urea (Cai and Zhang

2005) and polyethylene glycol (PEG) (Yan and Gao

2008).

How cellulose solubility is currently understood?

The insolubility of cellulose in water is in many

publications considered as a result of the hydrogen

bond systems. A typical citation from recent literature

is ‘‘cellulose itself is insoluble in water due to the many

and strong hydrogen bonds’’ (Bodvik et al. 2010)

which suggests the dissolution of the polymer by

breaking the cellulose–cellulose hydrogen bonds and

‘‘the key to the solution of this problem of cellulose

solubility is to search for a solvent that can destroy

effectively the interchain hydrogen bonding in cellu-

lose’’ (Zhang et al. 2002). These quotations corre-

spond very well with what is expressed in most current

papers in the field. Thus, the general view of cellulose

insolubility in water is that it forms intra- and

intermolecular hydrogen bonds. The reason for cellu-

lose solubility in ionic liquids and other solvents is

then that they ‘‘break’’ these hydrogen bonds. Some

authors also refer, in a somewhat imprecise way, to the

crystallinity of cellulose as a contributing cause for its

insolubility. Cellulose is known to form extended

crystalline regions and, from a thermodynamic point

of view, the crystalline state has always a lower free

energy than the amorphous one. Therefore, the

crystalline state of cellulose should be more difficult

to dissolve than the amorphous one (Cao et al. 1994;

Isogai and Atalla 1998). Nevertheless, this is currently

Cellulose (2012) 19:581–587 583

123

disputed and no systematic work has been performed

(Ying 2008; Pinkert et al. 2010). On the other hand,

there are good arguments to make us believe in the

important role of crystallinity. For instance, essen-

tially any chemical modification done on cellulose

tends to make it more soluble in water. Modifications

would, of course, affect adversely the good packing of

cellulose chains in crystals. Notable examples are the

mentioned methyl cellulose (MC) and hydroxyethyl-

cellulose (HEC). MC is highly soluble in water, even if

from a polarity point of view it would be expected to

be less soluble. Regarding HEC, also highly soluble,

substitution would not change the number of hydrogen

bonds so, from a hydrogen-bonding perspective,

solubility is difficult to understand.

Hydrogen bonds, water solubility

and ‘‘anomalous’’ temperature behavior

Water is a very strongly hydrogen-bonded liquid.

Breaking these hydrogen bonds increases the total

energy of the system. Therefore, solutes which are

unable to form hydrogen bonds but decrease the

number of hydrogen bonds of water tend to have low

solubility. Examples are many nonpolar compounds,

including hydrocarbons. For that reason, the common

teaching is that compounds capable of significant

hydrogen bonding should be soluble in water. For

example, we explain the solubility of alcohols, like

ethanol and glycerol, on this basis. Another example is

glucose, which has a very high solubility in water. This

lesson seems to be forgotten in the cellulose field; we

expect the interactions (but not the entropy) to be the

same when glucose is polymerized. The understanding

is different in other fields dealing with polysaccha-

rides. In a textbook on food chemistry (BeMiller and

Whistler 1996), the solubility of polysaccharides in

water is entirely discussed in terms of hydrogen

bonding with water as a promoter of solubility. The

behavior of a complex system is due to a balance

between different intermolecular interactions, in this

case hydrogen bonding, van der Waals and hydropho-

bic interactions, which all need to be considered. In the

cellulose field it is striking that the discussion focuses

on hydrogen bonding as driving cellulose association

and insolubility in water. How can this hydrogen

bonding mechanism of cellulose insolubility be

disproved? There are many examples. For instance

dextran, which should have a similar capacity for

hydrogen bonding as cellulose, is soluble in water.

Furthermore, cellulose derivatives, like MC and HEC,

may be highly soluble in water even if they have a

high, often as high as cellulose itself, capacity for

intermolecular hydrogen bonding. Another example is

glucose, quoted above. If intermolecular hydrogen

bonding were very important, glucose should show a

strong tendency for self-association and phase sepa-

ration, which is not observed. As said, solubility has to

be considered in the light of the balance between

different interactions, and focusing on hydrogen

bonding, for aqueous solubility of carbohydrates we

have to take into account not only water–carbohydrate

interactions, but also water–water and carbohydrate–

carbohydrate hydrogen bonding. Regarding a hydro-

gen-bonding mechanism, it is striking that all these

interactions are very similar in magnitude; ca. 5 kcal/

mol. Quantum chemical calculations provide a rigor-

ous way of determining the strength of hydrogen

bonds. From the literature we find that hydrogen bonds

between standard ethers, alcohols and water typically

are in the range 5.0–5.7 kcal (Astrand et al. 1995;

Canuto et al. 2004). The larger values occur between

alcohols and ethers, but they also contain some long

range attractive dispersive interactions between atoms

not directly involved in the hydrogen bond. The

remaining difference, when correcting for this error is,

at most, a few tenth of 1 kcal. There is yet another

effect that is not normally considered that may

influence the solubility of cellulose in water. In

cellulose the number of hydroxyl groups with protons

capable of forming hydrogen bonds is actually less

than the number of oxygen atoms that are capable to

form hydrogen bonds. Additionally, each oxygen atom

is capable to form two hydrogen bonds. This means

that there is a large possibility for a liquid as water to

form extra hydrogen bonds with the cellulose mole-

cule. In the presence of excess water then clearly

cellulose should be highly soluble if hydrogen bonding

is the sole interaction. Insolubility due to hydrogen

bonding would only occur if the carbohydrate–carbo-

hydrate hydrogen bonding would be very much

stronger, which is clearly not the case.

Another important issue is the not obvious thermal

behavior of cellulose. In several studies it is observed

that the cellulose dissolution is favored by a decrease

in temperature which, from a kinetic and thermody-

namic perspective, is not at all expected (Cai et al.

584 Cellulose (2012) 19:581–587

123

2007; Isogai and Atalla 1998; Kamide et al. 1992;

Sobue et al. 1939; Zhang et al. 2002). Such thermal

behavior is normally found in ethylene oxide based

polymers, different classes of nonionic polymers (as

well as for most nonionic surfactants) and in cellulose

derivatives like, for instance, MC and HEC (Holmberg

et al. 2002). The mechanisms behind such an unusual

behavior are still controversial but we have provided

strong evidence for it to be related with temperature-

induced conformational changes. Briefly, the O–CH2–

CH2–O segments with conformational freedom

around the C–C bond can change their conformation

as a function of temperature from a less polar state

(higher temperatures) around the C–C bond to more

polar states at lower temperatures (Lindman and

Karlstrom 2009). Thus, as the temperature is

decreased and more polar states are being populated,

attractive interactions with the polar solvent are

favored facilitating cellulose dissolution (Lindman

and Karlstrom 2009). Such a mechanism has been

supported by spectroscopic studies of conformational

changes as well as studies on other solvents than water.

Cellulose is an amphiphilic molecule

Many polymers are amphiphilic, i.e. contain both

polar and nonpolar groups/segments/sides. Amphi-

philic self-assembly is well known in the surfactant

and lipid field, as well as for block and graft

copolymers, but seems to be rather neglected for

homopolymers. It should be noted that for high

molecular weight polymers, even a slight amphiphi-

licity may have a significant impact on properties like

solubility. An example is poly-(ethylene glycol),

PEG, which can induce surfactant self-assembly;

another is ethylene oxide-propylene oxide block

copolymers showing a strong self-assembly even at

molecular weights of a few thousand. What is the

evidence for amphiphilicity of cellulose or of its

constituent glucose rings? An interesting example is

cyclodextrins, which have a high aqueous solubility at

the same time as they can incorporate in their interior

very non polar molecules (Del Valle 2004). This

demonstrates that a chain of glucose rings can have

sides of very different polarity. Another demonstra-

tion is single helix amylose, which behaves similarly

to cyclodextrins by possessing a relatively hydropho-

bic inner surface. Therefore, hydrophobic molecules

like hydrophobic lipids and aroma molecules can be

found in these hydrophobic ‘‘pockets’’. A similar

conclusion has been drawn from the structure of

cellulose crystals (Chaplin 2011; Yamane et al. 2006)

and computer simulations (Biermann et al. 2001;

Yamane et al. 2006). The equatorial direction of a

glucopyranose ring has a hydrophilic character

because all three hydroxyl groups are located on the

equatorial positions of the ring. On the other hand,

the axial direction of the ring is hydrophobic since the

hydrogen atoms of C–H bonds are located on the axial

positions of the ring. Thus, cellulose molecules have

intrinsically structural anisotropy and due to intra- and

intermolecular hydrogen bonding, there is a formation

of rather flat ribbons, with sides that differ markedly

in their polarity (Biermann et al. 2001; Yamane et al.

2006, 2009). Recently, Bergenstrahle et al. (2010)

have used molecular dynamic simulations to calculate

the potentials of mean force for separating short

cellulose oligomers in aqueous solution. Both the

contributions of hydrogen bonding and hydrophobic

interactions to the crystalline cellulose insolubility

were estimated and the free energy simulations

showed a significant hydrophobic pairing energy

favoring the stacking association of cellulose oligo-

mers chains in a comparable way to that found in the

various proposed crystal structures for cellulose. In

addition, the magnitude of this pairing energy was

estimated to be approximately 2.0 kcal/mol/residue

(much higher than the hydrogen bond contribution).

Indeed, the hydrophobic association does favor a

crystal-like structure over solution state.

Dissolution strategy: weakening hydrophobic

interactions

If amphiphilicity is significant, and if hydrophobic

interactions are important for the low solubility in

water, some straightforward predictions can be made:

1. Solubility would be facilitated in solvents that

also are amphiphilic, i.e. have both polar and

nonpolar parts. Ionic liquids (all cations used that

are known to work are amphiphilic!) and NMMO

clearly belong to this group.

2. Cosolutes, which have the tendency to weaken

hydrophobic interactions, would be expected to

promote aqueous solubility.

Cellulose (2012) 19:581–587 585

123

We note that with such an understanding we are

looking for very different systems than if we use the

currently adopted hydrogen bonding idea; according

to this we should use systems that break hydrogen

bonds to induce/increase solubility.

Yan et al. (2007) have used a mixture of NaOH and

thiourea to dissolve cellulose. Thiourea, like urea, is

known to weaken hydrophobic interactions, thus

enhancing solubility. In a similar way, Yan and Gao

(2008) used a mixed solution of PEG (1%) and NaOH

(9%) to promote cellulose dissolution using a freez-

ing–thawing process. PEG was found to work better

than urea. The authors claimed that the role of PEG

molecules was rather related to its ability to work as a

hydrogen-bonding acceptor preventing the re-associ-

ation of hydroxyl groups of cellulose and consequent

gel formation. However, in our opinion what was

observed was rather a manifestation of a weakening of

hydrophobic interactions. The stability of a hypothetic

cellulose solution is another key feature in order to

evaluate a solvent system. Normally, a cellulose

solution is unstable and the self-association of cellu-

lose chains results in gelation of the system. In Fig. 1

we show an alkaline solvent system where the

presence of an amphiphilic cosolute (betaine deriva-

tive) increases the temperature needed to start gelling

the solution. Gelation is believed to be due to self-

aggregation of the cellulose chains in the solution with

time and/or at elevated temperatures.

The progressively increased number of more

hydrophobic junction zones between the cellulose

chains in the solution is prevented by the presence of

surfactant. Therefore, the amphiphilic cosolutes

reduce the hydrophobic interactions responsible for

aggregation, resulting in an increase of the transition

or gelation temperature. We also stress the findings

mentioned above that systems like urea, PEG and

other amphiphilic molecules facilitate aqueous disso-

lution of cellulose by reducing viscosity, increasing

the amount of cellulose in the dope and improving the

thermal stability of the system (delaying gelation), as

these compounds would weaken hydrophobic interac-

tions and thus lend support to our notion that the

amphiphilic properties of cellulose are crucial.

Conclusions

Work in developing new solvents for cellulose has

been following a ‘‘trial and error’’ empirical character.

It is clear that a better understanding of the dissolution

of cellulose has deep implications, not least for

industrial developments. The underlying hypothesis

of most current work is that cellulose insolubility is

due to the fact that there are strong intermolecular

hydrogen bonds between cellulose molecules and that

bringing cellulose into solution is dependent on

breaking these hydrogen bonds. We have argued in

this brief analysis that this hydrogen bonding mech-

anism alone cannot explain the low aqueous solubility.

Indeed we have presented strong evidence for cellu-

lose being significantly amphiphilic and that the low

aqueous solubility must have a marked contribution

from hydrophobic interactions.

Fig. 1 Elastic molulus, G’, and viscous modulus, G’’, versus

temperature for 3.5% microcrystalline cellulose sample dis-

solved in 10% NaOH/H2O solvent system; left) without betaine

derivative and right) with betaine derivative. Constant heating

rate of 1 �C/min. The temperature of gelation (G’ = G’’) is

increased 10 �C in the presence of the amphiphilic additive

586 Cellulose (2012) 19:581–587

123

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