ORIGINAL PAPER

AKD-Modification of bacterial cellulose aerogelsin supercritical CO2

Axel Russler • Marcel Wieland •

Markus Bacher • Ute Henniges • Peter Miethe •

Falk Liebner • Antje Potthast • Thomas Rosenau

Received: 8 March 2012 / Accepted: 15 May 2012 / Published online: 29 May 2012

� Springer Science+Business Media B.V. 2012

Abstract Different approaches towards hydropho-

bic modification of bacterial cellulose aerogels with

the alkyl ketene dimer (AKD) reagent are presented. If

AKD modification was performed in supercritical

CO2, an unexpectedly high degree of loading was

observed. About 15 % of the AKD was bound

covalently to the cellulose matrix, while the other part

consisted of re-extractable AKD-carbonate oligomers,

which are novel chemical structures described for the

first time. These oligomers contain up to six AKD and

CO2 moieties linked by enolcarbonate structures. The

humidity uptake from environments with different

relative humidity by samples equipped with up to

30 % AKD is strongly reduced, as expected due to the

hydrophobization effect. Samples above 30 % AKD,

and especially at very high loading between 100 and

250 %, showed the peculiar effect of increased

humidity uptake which even exceeded the value of

unmodified bacterial cellulose aerogels.

Keywords Cellulose aerogel � Bacterial cellulose �Surface modification � Hydrophobization �Supercritical carbon dioxide � Alkyl ketene dimer

(AKD) � AKD-CO2-oligomers � Humidity uptake

Introduction

Aerogels from bacterial cellulose (BCAG) are fasci-

nating ultra-lightweight structures, exhibiting a hier-

archical, three-dimensional porous morphology made

of pure cellulose, combined with extraordinarily high

surface area and toughness (Liebner et al. 2010).

However, for some application fields, the character-

istics of pure bacterial cellulose do not translate into

the desired material properties: The natural hydrophi-

licity may be a drawback for some applications where

an interaction with hydrophobic structures is neces-

sary or hydrophilic substances have to be prevented

from interaction with the BCAG structure. Modifica-

tion of bacterial cellulose generally can follow the

rules of ordinary cellulose modification when the

morphology of the product is of no further interest.

However, modification of BCAG under simultaneous

preservation of the inherent structural features

requires special approaches. In principle, both deriv-

atization (covalent reagent bonding) and coating

(reagents are just physically attached) are suitable

strategies for BCAG modification. Because of the

fragile nature of BCAGs, techniques based on

A. Russler � M. Wieland � M. Bacher �U. Henniges � F. Liebner � A. Potthast � T. Rosenau (&)

Christian Doppler Laboratory for Advanced Cellulose

Chemistry and Analytics, University of Natural Resources

and Life Sciences Vienna, Muthgasse 18, 1190 Vienna,

Austria

e-mail: thomas.rosenau@boku.ac.at

P. Miethe

FZMB GmbH Forschungszentrum fur Medizintechnik

und Biotechnologie, Bad Langensalza, Germany

123

Cellulose (2012) 19:1337–1349

DOI 10.1007/s10570-012-9728-y

supercritical carbon dioxide (scCO2) have to be

applied for appropriate drying to largely avoid

changes of the natural BC structure (Liebner et al.

2010; Haimer et al. 2008; Liebner et al. 2009, 2007,

2008; Gavillon and Budtova 2008). Due to its

intrinsically low viscosity and special dissolution

properties, scCO2 can be used as multi-functional

medium for processing of (bacterial) cellulose aero-

gels: drying, chemical reactions, and solvent for

impregnation or fixation of various substances within

the porous material without altering the natural

morphology (Liebner et al. 2010; Russler et al.

2011). Combining supercritical drying with derivati-

zation or surface coating in one step would represent a

highly efficient and advantageous approach.

Alkyl ketene dimer (AKD, 1) is a well known and

widely used sizing agent in paper production that is

used to hydrophobize the cellulose surface to increase

water repellency and thus to enable paper to be printed

and written on in a way that applied inks do not leach

into the paper material and the print remains sharp and

detailed (Roberts 1996; Ek et al. 2009). It is added in

the head box of the paper machine in rather small

amount of 0.1–0.5 %. It is prepared e. g. from natural

fatty acids and stearic acid. Conventional AKD is a

waxy material with a melting point of about 50 �C,

nevertheless also formulations exist which are liquid at

room temperature. Having alkyl chains of normally

16–18 C-atoms, AKD wax is not soluble in water and,

when added to the headbox in paper production, is used

in the form of an aqueous dispersion in cationically

modified starch. Application is possible within a pH

range of about 7–10, so AKD became quite important

for neutral and alkali paper formation systems, which

compare favorably to acidic systems with regard to

long-term stability of the produced paper.

AKD is proposed to react with the hydroxyl groups

of cellulose to form a b-keto ester moiety (2). The

reaction with ethanol to the corresponding ethyl ester

2a is of limited predictive power as a model for the

reaction with cellulose: the esterification is in fact

similar, but the accessibility of the cellulose surface is

of course largely different from that of the low-

molecular weight alcohol. AKD will slowly react with

water to a b-keto acid (3) that readily decarboxylates

to produce a ketone (4) (see Fig. 1). Why the reaction

with the OH-groups of cellulose is favored over the

reaction with excess water under normal application

conditions is not clear until now (Odermatt et al. 2003;

Seo et al. 2008a, b; Lindfors et al. 2007). Nevertheless

it is known that the sizing effect is not an instant one,

but needs some time to develop (Shen and Parker

2001; Shen et al. 2005; Zhang et al. 2007). It is

assumed that in this time the AKD is spreading over

the cellulosic surface.

AKD is a well-known, well-established and cheap

agent for hydrophobization of cellulose in papermak-

ing—we were interested in its applicability in mod-

ifying BCAG by means of a novel application system

with scCO2. Novel cellulosic aerogels with more

hydrophobic surfaces will be the result. These mate-

rials will be better compatible with synthetic-organic

matrices than non-modified cellulose aerogels, which

would open up new fields of applications. Different

processes and process step sequences are possible for

the modification of BCAG with AKD. They differ in

complexity, limitations in reagent (AKD) loading,

effectiveness and homogeneity of the modification.

Naturally, it should be the aim to reduce the technical

complexity and work input while maintaining the

BCAGs0 above-discussed structural features. The

different processes are given in Fig. 2 schematically.

All of these methods contain at least one process

step involving scCO2, which we have recently shown

to be a promising medium for production of aerogels

also from bacterial celluloses (Liebner et al. 2010). As

AKD is soluble in scCO2, it is possible to perform also

the reagent-loading itself in scCO2 as the solvent, not

only the drying step. Some modifications of cellulose

with AKD applying scCO2 are known in literature, but

the techniques applied and the cellulose types used

were different from our approach (Hutton and Parker

2009; Quan et al. 2009).

OO

R

R

OCell

O

R

O

OHR'

O

R

O O

R

OEt

R

R

R

O

R

O

Cell-OH

H2O

+ CO2

1

2

3 4

2a

Et-OH

Fig. 1 Reaction of AKD with cellulose (Cell-OH) or ethanol,

and the hydrolytic reaction with water, R and R0 being C14-alkyl

or C16-alkyl

1338 Cellulose (2012) 19:1337–1349

123

Experimental

General

All chemicals were available from commercial sup-

pliers. Ethanol (absolute, Merck) and n-hexane

(Merck) of reagent grade were used as received.

Distilled water was used throughout. TLC was

performed using Merck silica gel 60 F254 pre-coated

plates. Flash chromatography was performed on Baker

silica gel (40 lm particle size). All products were

purified to homogeneity (TLC/GC analysis). Bacterial

cellulose was obtained from FZMB GmbH, Bad

Langensalza, Germany (see Liebner et al. 2010;

Russler et al. 2011 for preparation and purification).

AKD starting material

Alkyl ketene dimer (AKD, technical grade, Herkules)

was further purified by flash chromatography using n-

hexane as the solvent. GCMS analysis confirmed the

remainder to be pure AKD of different chain lengths,

without any impurities of AKD ketone (4) or esters.

NMR and GCMS

1H NMR spectra were recorded at 400 MHz for 1H

and at 100 MHz for 13C NMR in CDCl3. Chemical

shifts, relative to TMS as internal standard, are given

in d values, coupling constants in Hz. 13C peaks were

assigned by means of APT, H,H-COSY, HMQC and

HMBC spectra. As AKD is a mixture of several

compounds (due to the different chain lengths of the

used fatty acids), resonances are sometimes superim-

posed, and multiplicities are only given when clearly

discernible. GCMS analysis was carried out on an

Agilent 6890 N/5975B in the ESI (70 eV) ionization

mode.

Compound characterization by NMR

AKD (1, starting material). 1H NMR: d 0.87 (t, 6H,

CH3), 1.22–1.40 (m, br, CH2–), 1.39 (m, 2H, CH2–

CH2–CH–COO), 1.77 (q, 2H, CH2–CH2–CH–COO),

2.12 (q, 2H, CH2–CH=C), 3.94 (t, 1H, CH2–CH–

COO), 4.68 (CH2–CH=C). 13C NMR: d 14.1 (CH3),

22.7 (CH3–CH2), 24.6 (CH2–CH2–CH–COO), 26.3

(CH2–CH2–CH–COO), 27.5 (CH2–CH=C), 29.1–29.7

(CH2), 31.9 (CH3–CH2–CH2), 53.7 (=C–CH–COO),

101.7 (CH2–CH = C), 145.6 (CH=C), 169.7 (COO).

AKD ethyl ester (2a). 1H NMR: d 0.87 (t, 6H, CH3

in alkyl), 1.20–1.36 (m, br, CH2–), 1.28 (3H, super-

imposed by m, O–CH2–CH3), 1.51–1.64 (m, 2H,

CH2–CH2–CO), 1.78–1.84 (q, 2H, CH2–CH2–CH–

COO), 2.38 (t, 2H, CH2–CH2–CO), 3.40 (t, 1H, CO–

CH–COO), 4.18 (q, 2H, O–CH2–CH3). 13C NMR: d14.1 (CH3 in alkyl), 14.2 (O–CH2–CH3), 22.7 (CH3–

CH2–CH2), 23.8 (CH2–CH2–CO), 28.2 (CH2–CH–

COO), 29.1–29.9 (CH2), 32.0 (CH3–CH2–CH2), 43.7

(CH2–CH2–CO), 59.3 (CO–CH–COO), 169.8 (COO),

205.7 (CO).

AKD-ketone (4). 1H NMR: d 0.88 (t, 6H, CH3 in

alkyl), 1.22-1.34 (m, br, CH2–), 1.55 (pent, 2H, CO–

CH2–CH2), 2.38 (t, 2H, CO–CH2–CH2). 13C NMR: d14.2 (CH3 in alkyl), 22.8 (CH3–CH2–CH2), 24.0

BCG BCAGscCO2

Drying mBCAG

AKD

ExtractionscCO2

DryingscCO2

Loading

BCG Loading Bath mBCAG

AKD

Solvent ExchangescCO2

Drying

BCG BCAGscCO2

Drying mBCAG

AKD

scCO2

DryingLoading Bath Solvent Exchange

1

3

2

Fig. 2 Overview of the

different process approaches

for AKD modification of

BCAGs (see the

experimental section for a

detailed description), BCG:

bacterial cellulose gel,

BCAG, bacterial cellulose

aerogel, mBCAG: AKD-

modified bacterial cellulose

aerogel)

Cellulose (2012) 19:1337–1349 1339

123

(CH2–CH2–CO), 29.4–29.8 (CH2), 32.0 (CH3–CH2–

CH2), 42.9 (CH2–CH2–CO), 211.9 (CO).

AKD-CO2 hexamer (5). ‘‘Terminal unit’’ denotes

the AKD unit with the ketone moiety, ‘‘proximal unit’’

the one with the free acid moiety. 1H NMR: d 0.87

(36H, CH3), 1.20–1.42 (CH2–), 1.39 (m, 10H, CH2–

CH2–CH–COO), 1.51–1.59 (2H, CH2–CH2–CO in

terminal unit), 1.78 (q, 10H, CH2–CH2–CH–COO),

1.78–1.82 (q, 2H, CH2–CH–COO in terminal unit),

2.12 (q, 10H, CH2–CH=C), 2.38 (t, 2H, CH2–CH2–CO

in terminal unit), 3.40 (t, 1H, CO–CH–COO in

terminal unit), 3.95 (t, 5H, CH2–CH–COO), 4.72 (t,

5H, CH2–CH=C). 13C NMR: d 14.1 (CH3), 22.7

(CH3–CH2), 23.8 (CH2–CH2–CO in terminal unit),

24.4–24.6 (CH2–CH2–CH–COO), 26.3–26.4 (CH2–

CH2–CH–COO), 27.5 (CH2–CH=C), 28.2 (CH2–CH–

COO in terminal unit), 28.9–30.4 (CH2), 31.9 (CH3–

CH2–CH2), 43.7 (CH2–CH2–CO in terminal unit),

53.7-53.8 (=C–CH–COO), 59.3 (CO–CH–COO in

terminal unit), 101.7–101.9 (CH2–CH=C), 145.5–

145.7 (CH=C), 169.3–169.6 (COO), 169.9 (COOH

in proximal unit), 205.7 (CO).

scCO2 systems

For the drying and AKD-modification of BCAG, a

high-pressure system was used. CO2 from a gas bottle

was pressurized in a high pressure pump and charged

into a 500 cm3 reactor. Temperature was set to 40 �C

and was controlled by a thermostat. The reactor was

equipped with a stirrer and a grid to store the aerogels

separated from the stirrer. The stirrer was not neces-

sary for the drying of the aerogels alone, but is

required for AKD-modification to insure uniform

distribution of the reagent. The system was equipped

with a two-stage separator unit and with the possibility

to recycle the gas.

Before supercritical drying and AKD modification,

bacterial cellulose aquogels have to undergo solvent

exchange. A solvent exchange against absolute etha-

nol was performed three times for at least 24 h. The

subsequent supercritical drying of the BCGs was

performed in the pressure vessel at 100 bar, 40 �C and

a CO2 flow of 3.5 kg/h with no stirring. Treatment

times were 1 h per centimeter of aerogel diameter. For

properties and morphology of the BC aerogels see:

Liebner et al. 2010; Russler et al. 2011.

In the case of AKD-modification, the same proce-

dure as for the supercritical drying was used, with the

following modifications: pressure and time were

varied (see text), no CO2 flow was applied, and

stirring was applied at 350 min-1.

The simple setup of the pressure system guaranteed

proper cleanability and good repeatability. Neverthe-

less, since precipitated AKD can block valves and

tubes, it is advisable to flush the apparatus with

suitable organic solvents (chloroform, petrol ether)

from time to time, and to release the pressure by a

valve directly, without separators (Odermatt et al.

2003; Seo et al. 2008; Hutton and Parker 2009; Quan

et al. 2009).

SEM

Scanning electron microscopy (SEM) was performed

on a Phillips XL 30 ESEM (Environmental Scanning

Electron Microscope, ESEM) at an acceleration

voltage of 10 kV with different magnifications.

FTIR

Fourier transform infrared spectroscopy (FTIR) was

performed on a Bruker Vertex 70 HTS-XT in the

Attenuated Total Reflectance (ATR) mode with 32

scans per measurement between 4000 and 400 cm-1.

Humidity uptake

The humidity uptake of the samples was measured

gravimetrically after storage in desiccators of con-

trolled humidity for at least 72 h. The desiccators were

filled with phosphorous pentoxide for 0 % relative

humidity, with saturated solutions of calcium chloride

for 30 %, of ammonium nitrate for 65 % and of

potassium sulfate for 98 % relative humidity, and

were stored at room temperature.

Modification approaches according to Fig. 2

In the case of direct supercritical loading, the AKD

wax was charged directly into the pressure vessel

together with BCAG. The BCAG samples were put

into the internal wire caskets to prevent them from free

floating in the supercritical fluid. The system was then

pressurized at different pressure levels for 60 min.

The stirring was started when supercritical conditions

were reached, and was stopped during pressure

release before dropping below supercritical conditions.

1340 Cellulose (2012) 19:1337–1349

123

The AKD-modified BCAG was immediately subject to

analytical characterization, extracted after some curing

time (1 or 2 weeks at room temperature) or further

stored at ambient conditions. The extraction of AKD

from loaded samples (to test for physical adsorption vs.

covalent binding) was performed batchwise with neat

n-hexane (20-fold sample volume) for 24 h in a shaker,

and this extraction was repeated twice. As indicated by

the dotted line in Fig. 2, a combination of BCG drying

with AKD loading is possible if the scCO2 apparatus is

equipped with separate pressure chambers connected

with valves, so that after drying scCO2-dissolved AKD

can be introduced.

According to the second approach of Fig. 2, the

BCG was pretreated by solvent exchange to ethanol

and then subject to a loading bath, containing different

amounts of AKD dissolved in n-hexane. The modified

BCG was directly dried supercritically in the high

pressure system. Alternatively, another solvent

exchange step to ethanol, a non-solvent for AKD,

was done beforehand. The conditioning in the loading

bath should last long enough, depending on the sample

dimensions, to ensure homogeneous distribution of the

loading medium throughout the sample, at least 1 h

per centimeter of aerogel.

The third way in Fig. 2 for the loading with AKD

was starting from already dried BCAG. Modification

of this aerogel with AKD was performed by AKD

solutions in n-hexane of set concentrations, so that a

defined amount of AKD was homogeneously distrib-

uted throughout the BC matrix. To obtain aerogels

from the solvogels, it was necessary to apply another

scCO2-drying step, either with or without preliminary

solvent exchange to ethanol.

Results and discussion

BCAG modification with AKD (‘‘AKD loading’’)

The supercritical loading of AKD onto BCAG (path 1

in Fig. 2) was found to be a very flexible approach.

The degrees of loading, i.e. the ratio between mass of

AKD to mass of original, non-modified BCAG, was

varied in a wide range from about 30 % to over 250 %.

When the pressure in the vessel is reduced below

supercritical conditions, the dissolved AKD would be

expected to precipitate homogeneously within the

whole pressurized volume. If, for example, the density

of BCAG is 10 mg/cm3 and the density of the agent

to be precipitated into the system is 2 mg/cm3, a

maximum degree of loading of about 20 % is

expected. Such a result coming close to the ‘‘theoret-

ical’’ value is obtained, for instance, for inert long-

chain aliphatics and triglycerides as the substances to

be deposited within the aerogel matrix. However, in

the case of AKD, we found in all cases a higher

loading of the BCAG than calculated from the mass of

AKD put into the pressure system, with values ranging

up to 250 %.

Three mechanisms can be summoned to account for

this peculiar behavior. The first is the reaction of AKD

with the cellulose. The contribution of this mechanism

to the enrichment effect, however, must be rather

small since 85 ± 3 % of the deposited AKD can be re-

extracted with n-hexane from the cellulosic bodies

(independent of the degree of loading), so that a

maximum of 15 ± 3 % of the initially deposited AKD

can be covalently bound to cellulose, thus being non-

extractable. The second mechanism of the enrichment

as described might be a preferred precipitation within

the nano-scale pore system of the BCAG due to

capillary effects. However, it is not clear why such

effects would be operative for AKD, but not for

paraffin waxes or triglycerides (which do not show the

enrichment effect). Hence, this mechanism is rather

unlikely. The third mechanism is an oligomerization

of AKD. By polymer–polymer (polymer-oligomer)

interactions, these bigger molecules would be prefer-

ably adsorbed on the cellulose surface, so that as the

net effect the aerogel is enriched with AKD moieties at

the same time depleting the supercritical solvent of

AKD. The initial trigger for assuming that such a

reaction could have occurred was the observation that

re-extracted AKD was no longer monomeric, but

oligomeric. This prompted us to look into the mech-

anism more closely.

Mass spectrometric analysis of the n-hexane

extracts of the deposited AKD, showed the presence

of larger AKD-derived molecules (5), which however

were not just AKD-oligomers, but appeared to contain

additional CO2-moieties, readily identifiable by a

mass difference of 44 and its multiples. Apparently,

the supercritical medium did not behave as inert

solvent for AKD, but participated in a reaction,

promoting an oligomerization. Such oligomerization

is not observed in aqueous suspension or in organic

solutions in common solvents, so that this effect must

Cellulose (2012) 19:1337–1349 1341

123

be a peculiarity of the scCO2 medium. NMR analysis

showed the absence of non-reacted AKD. However,

the expected a-alkyl-substituted b-keto acid, as the

usual product of AKD ring opening (see Fig. 1), was

not present. Instead, a b-enolate structure was found,

which is linked to the carboxylic acid motif via a

carbonate moiety. By two-dimensional experiments

(HSQC and HMBC), the structure of the oligomers as

enolcarbonate anhydrides of b-keto-carboxylic acids

were unambiguously confirmed (Fig. 3) (Rosenau and

Russler 2012). No direct linkages between the enolate

of one AKD molecule and the carboxylic acid of the

next one are contained—the two molecules are always

connected via one CO2 as ‘‘spacer’’, i.e. in the form of

an enolcarbonate. The substances can thus be per-

ceived as ‘‘co-oligomers of AKD and CO2’’ in a 1:1

ratio.

The formation mechanism can reasonably be

assumed to be a CO2-induced opening of the AKD0sfour-membered oxetan ring. A transient structure with

negatively charged enol carbonate oxygen is attacking

the next AKD molecule under ring opening to give a

dimer, of which the formed enolate motif is once more

attacking another CO2 molecule. This results in a new

enol carbonate that reacts with the next AKD, and so

on. Finally the observed oligomers are built up (see

Fig. 3). Whether ionic structures indeed occur as

intermediates or just as transition state cannot be

answered conclusively at present. The observation of

only minor amounts (\0.5 % of re-extracted material)

of an addition product of AKD and CO2, a cyclic

organic carbonate with a stable six-membered ring

structure (6), disfavors the presence of the zwitterionic

intermediate as independent species. If indeed exist-

ing—even as highly transient species—the hypothetic

zwitterion would preferably react by intramolecular

recombination to the cyclic carbonate (6), rather than

by attacking another AKD molecule to form linear

oligomers (5). The near-absence of this cyclic car-

bonate by contrast to the larger amount of oligomers

formed argue against formation of a free zwitterionic

intermediate.

The presence of a cellulosic surface with small

amount of water present appeared to be imperative for

OO

R

R

OO O

O

R

O

O

O

R

O

OO

O

R

O

O

O

R

OOH O

O

R

O

O

O

R

O

O

OO

R

R

OHCell

CO2

H2O

OO

O

R

OOH O

O

R

O

O

O

R

R RR RR R

RR RO

n

O O

R

O

R

O

O OH

R

O

R

O

O_

Cellulosesurface AKD, scCO2

oligomers detected for 0 < n < 4

1

5 (hexamer)

5 (general formula)

putative ionicintermediate

AKD, scCO2

Fig. 3 Formation of AKD-CO2 ‘‘co-oligomers’’ as found in the

n-hexane extracts of AKD-modified bacterial cellulose aerogels.

Middle row: Grey shades show the CO2-derived units, while the

dotted boxes denote ring-opened AKD motifs. The oligomers

contain an acid (left) and a ketone (right) terminus. Lower row:

general formula of the AKD-CO2-compounds emphasizing the

‘‘oligomeric’’ character by showing the repeating AKD-CO2

‘‘repeating unit’’

1342 Cellulose (2012) 19:1337–1349

123

the oligomer formation: no such products were

observed when working with cellulosic model com-

pounds that were dry and non-hydrated, whereas

addition of small amounts of water (one equivalent per

hydroxyl group) induced the oligomer formation once

more. By contrast, larger amounts of water, in turn,

prevent oligomer formation from the beginning.

Polyhydroxylic compounds, such as polyvinyl alcohol

and even silica gel appeared to favor the oligomeri-

zation, and always required small amounts of water for

this reaction to proceed. Non-hydroxylic surfaces did

not at all induce AKD-oligomerization, independent

of the presence of water traces. The exact formation

mechanism of the oligomeric AKD compounds and

the peculiar water and surface effects are currently

under study, and an account will be given as soon as a

conclusive mechanism is available.

The AKD loading values of 30–250 % did not

represent a possible minimum or maximum, respec-

tively. Values beyond these limits were just not tested.

These variations were performed by changing the

amount of AKD charged into the pressure vessel. Also

variation of the pressure (100 bar to 300 bar) had

some influence on the degree of loading, see Figs. 4, 5.

For AKD concentrations below 5 mg/cm3, the solu-

bility of AKD in the supercritical fluid increased with

increasing pressure until about 150 bar and then

moderately decreased. Above 5 mg/cm3 AKD con-

centration, the solubility increased over the whole

tested pressure range, although above 200 bar the

increase was less pronounced than at lower pressures.

Thus, the actual degree of loading of the gel is

determined by both the starting AKD concentration

and the working pressure.

The application of the two other techniques for the

loading of BCG/BCAG with AKD (path 2 and 3 in

Fig. 2) resulted in much lower degrees of loading.

This is especially due to the fact that for the production

of aerogels—either by loading via loading bath or by

filling the BCAG with an AKD-containing solution—

a subsequent supercritical drying step is required.

During this drying excessive AKD was dissolved in

the supercritical fluid and removed.

Homogeneity of modification according

to pressure vessel geometry and sample geometry

The degree of loading of BCAG under supercritical

conditions was influenced also by the geometry of the

setup, i.e. the vessel containing the supercritical

medium. BCAG located in the upper part of the

pressure reactor showed somewhat lower degrees of

loading than samples located near the bottom. This

might be due to a limited solubility of AKD in the

supercritical CO2 fluid: when the pressure was

increased to 250 bar, the solubility increased and the

inhomogeneity of deposition got smaller (see Figs. 4,

5). At conditions of limited solubility (lower pres-

sures) the samples nearer to the bottom face an

environment with a higher concentration of AKD due

Fig. 4 Dependency of

AKD loading on pressure

and sample location in the

pressure vessel, AKD

density in the supercritical

medium: 2 mg/cm3

Cellulose (2012) 19:1337–1349 1343

123

to the presence of a suspension of non-dissolved AKD

particles that adhere to the BCAG and remain attached

beyond pressure reduction to subcritical conditions.

At a pressure of 250 bar, no inhomogeneities of the

material within a batch are present any longer.

However, differences in loading among samples of

different batches—despite similar AKD concentra-

tions in the supercritical medium—are still noticeable

(see Figs. 4, 5). Altogether, the samples with higher

AKD concentration (e.g. 10 mg/cm3) showed a more

homogeneous and better reproducible modification

than samples loaded at lower pressures.

These findings show that the control of the degree

of loading is not as easy to control as was expected. In

an upright reaction vessel, the circulation of the

supercritical fluid was not sufficient, even when

stirring was applied. Tumbling of the supercritical

vessel solved the problem of inhomogeneities within a

batch, but the experimental setup and the procedure

became more complicated.

Another solution to the problem of batch inhomo-

geneity is the preparation according to the other two

methods proposed, i.e. paths 2 or 3 in Fig. 2. In these

cases, no differences were observed according to the

location of the samples within the pressure system.

Some differences can also be seen, if homogeneity

is not considered with regard to the sample position in

the pressure vessel, but with respect to the degree of

modification within or across an aerogel body. As

weight analyses of different regions of individual

samples are tedious to perform for larger sample

numbers, we used FTIR measurements for a semi-

quantitative analysis. Figure 6 shows the FTIR spectra

of pure BCAG (bottom) and pure AKD (top), as well

as of BCAG loaded with 15 % AKD and 70 % AKD

loading. The sample with a content of 15 % AKD was

produced by extraction of the higher loaded sample

with n-hexane after initial AKD loading. Some

characteristic IR bands of AKD are indicated by

arrows in Fig. 6, the most prominent ones are the

intense aliphatic stretching bands at 2,916 and

2,848 cm-1, which are also prominently found in the

AKD-modified BCAG. The bands at 1,848, 1,720 and

1,467 cm-1 originate from AKD0s oxetanone struc-

ture. Some of its structural motifs, such as the enol

ether structure and the C(=O)O motif, are retained

after covalent attachment to cellulose or formation of

the CO2-containing oligomers (see Fig. 3), while

others are canceled, such as the four-membered ring.

In the case of 70 % loading, both AKD oligomers (5)

and cellulose-bound AKD (2) contribute to the

spectrum, while only the latter adds to the spectrum

in the case of 15 % loading (the oligomers are

removed by extraction). The prominent bands at

2,916 and 2,848 cm-1 are most suited for quantifica-

tion of AKD on the BCAG matrix.

Figure 7 (left) shows the characteristic part of the

FTIR spectra of different regions (surface, outer part,

core) of a BCAG sample supercritically loaded with

70 % AKD. The high surface coverage with AKD

Fig. 5 Dependency of

AKD loading on pressure

and sample location in the

pressure vessel, AKD

density in the supercritical

medium: 10 mg/cm3

1344 Cellulose (2012) 19:1337–1349

123

(in the form of oligomers) is evident. After extraction

with n-hexane (Fig. 7, right), this surface coverage is

removed, but a loading of 15 % remained persistently.

This portion corresponds to AKD covalently bound to

the cellulose surface, while the major, extractable part

was just physically adsorbed. After extraction, the

loading differences between surface and core vanish,

proving that covalent modification was uniform over

the cellulose matrix, while deposition of the physi-

sorbed portion occurred only on the surface, but not in

the interior. As mentioned above, the amount of

covalently bound AKD was nearly constant at about

15 ± 3 %, independent of the respective conditions of

supercritical loading. By contrast, physically adsorbed

AKD and its CO2-containing oligomers, which are just

deposited on the surface in a re-extractable manner,

can be enriched up to 250 % (and likely beyond) in the

loaded BCAG sample.

In SEM pictures of the samples with 70 % loading

(see Fig. 8) AKD-oligomer agglomerates can be seen.

In the sample subsequently extracted with n-hexane,

which contained 15 % covalently bound AKD, no

such agglomerates were still visible, but a fibrillar

structure with smooth surface coating was observed

(Fig. 9).

Just from weight determination, it could evidently

not be clear whether the amount of covalently

bound AKD (approx. 15 %) consisted of ‘‘proper’’

monomeric AKD in the form of the expected b-

ketoester or whether it was made up of covalently

bound CO2-AKD oligomers. We therefore used per-

acetylation of the sample with subsequent 1H NMR

analysis in CDCl3, which showed a DSAKD of 0.07

(± 0.03). A mass percentage of 15 % monomeric

AKD (C16, M = 494 g mol-1) would mean a theo-

retical DSAKD of 0.05, while substitution with AKD-

hexamers (5) translates into a theoretical DS(AKD)6 of

0.008. The experimental value of 0.07 clearly indi-

cates that only monomeric AKD is bound to the

cellulose matrix—or at least that the amount of

covalently linked oligomeric AKD is negligibly small.

Thus, the loading of the cellulose matrix did occur

5001000150020002500300035004000

Wavenumber cm-1

Fig. 6 FTIR spectrum of AKD-loaded modifications of bacte-

rial cellulose aerogels with 70 % AKD and 15 % AKD. Spectra

of pure AKD (top) and pure bacterial cellulose (bottom) are

given for comparison. y-axis: relative absorbance

2700280029003000

Wavenumber cm-1

2700280029003000

Wavenumber cm-1

Fig. 7 Left: characteristic section of an FT-IR spectrum of

surface, outer part and core of a BCAG loaded with an average

of 70 % AKD, indicating different AKD contents in different

aerogel parts. Right: extraction of the aerogel with n-hexane

leaves only covalently bound AKD behind, which is uniformly

distributed throughout the material, eliminating the observed

loading variations between different aerogel parts. y-axes:

relative absorbencies

Cellulose (2012) 19:1337–1349 1345

123

with monomeric AKD, but not with AKD oligomers—

in contrast to the physically absorbed, extractable

residue which consists only of such oligomers.

It should be noted that hydrolytic cleavage of the

bound AKD molecules and subsequent analysis—

which at a first glance appears to be the method of

choice to characterize the covalently bound AKD

fraction—is not a feasible approach here: the oligo-

meric AKD—if present—will fragment under these

conditions into CO2 and the equivalent amount of

monomeric AKD residues, rendering monomeric and

oligomeric AKD forms indistinguishable by this

method. Under aqueous hydrolysis conditions, the

monomeric AKD-derived ketone (3) is formed from

CO2-AKD oligomers such as (5), under non-aqueous

conditions (e.g. gaseous HCl in ethanol) the corre-

sponding AKD-derived ethyl ester (2a) is produced.

Hydrolysis thus offers no way to distinguish between

bound monomers and bound oligomers.

All these findings led us to the conclusion that only

a relatively minor (about 15 %), non-extractable part

of AKD monomers was covalently bound to the

cellulose. The starting content of AKD, after loading

but before extraction, is itself variable in wide ranges,

but does not, however, influence the eventual amount

of covalently bound AKD, i.e. the degree of loading

after the extraction. If an AKD-loading in percentages

higher than 15 % was found, the reason was always a

physical deposition of AKD oligomers in addition to

the chemically bound part.

Fig. 8 ESEM pictures (left 5.000x, right 20.000x) of a BCAG sample scCO2-loaded with 70 % AKD, exhibiting agglomerates of

AKD-oligomers within the BCAG matrix, besides the coating of the BC fibrils

Fig. 9 ESEM pictures (left 5.000x, right 20.000x) of a BCAG sample scCO2-loaded with 15 % AKD (70 % initial loading and

subsequent extraction with n-hexane), exhibiting smooth fibrillar coating and no visible agglomerates

1346 Cellulose (2012) 19:1337–1349

123

Samples prepared according to the other methods

described in Fig. 2 (paths 2 and 3) showed only

negligible differences between different parts of the

samples, provided the curing time in case of method

No. 2 was long enough ([1 h) to allow a homogeneous

distribution of AKD in the solvogels by diffusion.

Humidity uptake

As AKD is providing hydrophobic properties if

applied to cellulosic material in the form of paper, it

was reasonable to assume that is has the same effect

also on cellulosic aerogels. We tested the humidity

uptake of different modified BCAG samples by

monitoring the weight gain during storage in con-

trolled atmospheres with different relative humidities

(rH). After storage of the prepared samples for some

time at ambient conditions, they were put into a

desiccator over phosphorous pentoxide to completely

dry the samples to weight constancy. This condition

was set as the starting point. The weight re-gain of the

samples when placed in different humidity environ-

ment (rH 30, 65, 98 %) is displayed for selected

specimens in Fig. 10. Pure AKD powder which was

placed in the humidity controlled desiccators as a

reference showed no weight gain at all. Samples

extracted with n-hexane having an effective AKD

loading of just 15 % showed results similar to the

sample with 29 % AKD loading. If dried again by

storing at 0 % relative humidity for several days, all

samples returned to their initial, constant weight.

At relative humidities of 30 %, none of the samples

tested showed significant humidity uptake. The high-

est weight gain observed was just 0.6 %, compared to

a weight gain of 2.5 % for non-modified BCAG

samples. Also at a rH of 65 %, all modified samples

stayed below 1 % weight gain; the non-modified

sample showed a mass increase of 3.4 % here. In an

atmosphere saturated with water vapor (98 % rH),

non-altered BCAG samples showed a weight gain of

20 %. Modified samples with loadings up to 70 %

stayed below a weight gain of 8 %. The weight gain of

a sample with 29 % AKD was 5 %, that of a specimen

with 15 % AKD only 2 %. All these samples showed

thus a pronouncedly decreased humidity uptake. So

far, the outcomes reflect the expectations. AKD-

modified BCAG samples with a loading higher than

70 % showed a rather unexpected behavior: the

humidity uptake increased quite drastically; samples

with very high loading (102 and 212 %) even signif-

icantly exceeded the level of unmodified BCAG. The

weight gain of all samples was completely reversible.

This ‘‘humidity enrichment’’ effect was readily repro-

ducible. It generally occurs only at near-saturated

relative humidities (rH [ 95 %), only at AKD-load-

ings above 70 %, and it increases further with

increasing AKD amounts in the modified BCAG.

The production of samples with such very high

AKD loadings is not possible according to the

Fig. 10 Humidity uptake

by BCAG samples with

different degrees of AKD

loading at room temperature

in atmospheres of different

relative humidities (0, 30, 65

and 98 % rH)

Cellulose (2012) 19:1337–1349 1347

123

alternative modification methods (paths 2 and 3 in

Fig. 2). Samples with the effect of increased humidity

uptake at very high AKD-loads could thus only be

obtained by the supercritical deposition method (path

1 in Fig. 2) just discussed.

The peculiar effect that the humidity uptake

increases despite (or better: due to) surface-deposited

AKD agglomerates can be assumed to be caused by

some special morphological features created upon

loading under supercritical conditions, which boost

a kind of water condensation already beneath the

saturation point. As shown above, the high AKD loads

can be explained with globular agglomerates of AKD-

oligomers which appear to cause the condensation

effect, whereas simply a thicker surface coverage does

not cause this effect. High AKD-percentages do not

necessarily translate into increased surface hydro-

phobization, but rather into increased concentrations

of strongly hydrophobic spots. We are currently

following routes to visualize water droplet formation

in the material and to account for the water enrichment

in cooperation with material physicists.

Conclusions

AKD, a well known sizing agent for paper, can also be

used for the modification of BCAG. There were three

main routes followed to perform the modification, all

are based on super-critical CO2 which is able to

preserve the delicate porous network structure of

cellulosic aerogels without morphological changes.

The direct loading of BCAG with AKD dissolved in

the supercritical fluid is the most elegant and most

flexible variant, which also allows especially high

degrees of loading.

About 15 % of the deposited AKD—this number

being largely independent of starting AKD concen-

tration and system pressure—was bound to the

cellulose surface in a covalent way—this part of the

AKD loading remains non-extractable. Covalently

bound AKD is predominantly monomeric as seen by

derivatization/NMR studies, and is homogeneously

distributed throughout the BCAG. With the scCO2

approach, degrees of loading up to 250 % can be

reached. The additional AKD beyond the limit of

15 % is bound by physisorption only and thus is

extractable. The main part of the extractable portion is

composed of an CO2-AKD oligomer (dimer to

hexamer), which forms spot-like agglomerates on

the BCAG surface, but does not penetrate into the bulk

material by contrast to the covalently fixed AKD part.

The BCAG samples loaded with up to 70 % AKD

exhibited hydrophobic behavior with reduced humid-

ity uptake at all relative humidity levels. The uptake

was reduced to one tenth up to one quarter of non-

modified BCAG. An interesting reverse effect, which

is currently being scrutinized, was observed for BCAG

with very high AKD loads of above 70 % up to 250 %.

The humidity uptake at nearly 100 % rH increased

significantly, by far surpassing that of genuine

BCAGs. This peculiarity might be due to superhy-

drophobicity effects of the AKD-oligomer deposits

inducing water condensation.

The AKD-modification of bacterial cellulose aero-

gels in scCO2 media is a convenient and flexible way

to alter the BCAGs0 properties towards increased

hydrophobicity and compatibility with hydrophobic

polymers. The finding of AKD-CO2-oligomers

formed under conditions of supercritical processing

is novel. At present, it cannot yet be decided whether

this behavior is positive or negative with regard to the

general processability and applicability of AKD in

scCO2. The chemical nature and the properties of the

AKD oligomers are topics of current studies in our

group.

Acknowledgments We would like to thank the Austrian

Christian-Doppler-Society, Vienna and the FZMB GmbH, Bad

Langensalza, Germany, for financial support within the

‘‘Christian Doppler Laboratory for Advanced Cellulose

Chemistry and Analytics’’ and Walter Klug for recording the

SEM pictures.

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