PAPER www.rsc.org/softmatter | Soft Matter

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

AView Online / Journal Homepage / Table of Contents for this issue

Aerogels from nanofibrillated cellulose with tunable oleophobicity

Christian Aulin,ab Julia Netrval,b Lars W�agberg*b and Tom Lindstr€omc

Received 28th January 2010, Accepted 19th April 2010

First published as an Advance Article on the web 14th May 2010

DOI: 10.1039/c001939a

The formation of structured porous aerogels of nanofibrillated cellulose (NFC) by freeze-drying has

been demonstrated. The aerogels have a high porosity, as shown by FE-SEM and nitrogen adsorption/

desorption measurements, and a very low density (<0.03 g cm�3). The density and surface texture of the

aerogels can be tuned by selecting the concentration of the NFC dispersions before freeze-drying.

Chemical vapor deposition (CVD) of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFOTS) was used

to uniformly coat the aerogel to tune their wetting properties towards non-polar liquids. An XPS

analysis of the chemical composition of the PFOTS-modified aerogels demonstrated the reproducibility

of the PFOTS-coating and the high atomic fluorine concentration (ca. 51%) in the surfaces. The

modified aerogels formed a robust composite interface with high apparent contact angles (q* [ 90�)

for castor oil (glv ¼ 35.8 mN m�1) and hexadecane (glv ¼ 27.5 mN m�1).

Introduction

Aerogels are materials prepared by replacing the liquid solvent in

a gel by air without substantially altering the network structure or

the volume of the gel body.1 The first aerogels were reported by

Kistler in 1931–1932,2,3 but active research in this area did not

start until about 40 years later. Some of the unique properties of

aerogels are their low density, high specific surface area, low

thermal conductivity and low dielectric permittivity. The prepa-

ration, physical properties and applications of aerogels are

described elsewhere.4,5 The liquid in a wet gel is usually replaced

with air by using supercritical drying, but ambient-pressure

drying has also been attempted.4,6 From a practical point of view,

the challenge has been to prepare aerogels without supercritical

drying in order to reduce the cost. An alternative can be freeze-

drying7–9 where the solvent in the gel is first frozen and then

sublimated without entering the liquid state. The most common

aerogels are inorganic, prepared by sol–gel polymerization of

inorganic metal oxides.10–14 They are usually very brittle, but have

a high compressive strength. Various types of organic aerogels

have also been presented.15–17 Resorcinol/formaldehyde (RF) and

melamine/formaldehyde (MF) are two of the most common

precursor mixtures for forming the organic network.17–19

In the context of a sustainable society, there is strong moti-

vation to replace petroleum-based polymers with polymers from

renewable resources. Porous materials with nano- and micro-

sized pores made from natural polymers are of special interest for

medical, cosmetic, pharmaceutical, and other applications where

biocompatibility and biodegradability are required.20–22 Three-

dimensional scaffolds for tissue engineering, delivery matrices,

‘‘green’’ packaging, and environment-friendly insulating mate-

rials are examples of such applications. Among the poly-

saccharides, cellulose has a special potential as one of the most

aBIM Kemi AB, Box 3102, SE-443 03 Stenkullen, SwedenbDepartment of Fibre and Polymer Technology, School of ChemicalScience and Engineering, The Royal Institute of Technology, SE-100 44Stockholm, Sweden. E-mail: wagberg@kth.secInnventia AB, Box 5604, SE-114 86 Stockholm, Sweden

3298 | Soft Matter, 2010, 6, 3298–3305

abundant renewable natural polymers, widely used in industry.

In plant cellulose, the polysaccharide chains with b-(1–4)-D-glu-

copyranose repeating units pack into long fibrils with cross-

sectional dimension of ca. 5–30 nm, depending on the plant

source.23 The high modulus and high strength of native cellulose

I are results of the organization of the cellulose chains in a crystal

structure in which the long parallel polysaccharide chains are

physically bonded together by a large number of hydrogen bonds

and are organized in sheets packed in a ‘‘parallel-up’’ fashion.24 It

is interesting and challenging to consider the long nanofibers as

construction units for nanoscale material engineering.

The term ‘‘nanofibrillated cellulose’’ (NFC) refers to cellulosic

I fibrils disintegrated from the plant cell walls. The preparation of

NFC derived from wood, and specification of the term, were first

described by Turbak et al.25 and Herrick et al.26 more than two

decades ago. Through a homogenization process, wood pulp is

disintegrated, to give a material in which the fibres are degraded

and opened into their sub-structural units. New methods for the

manufacture of smaller and more homogeneous nanofibrillated

cellulose (NFC) have recently been developed and offer new

attractive concepts for material science and significantly enhance

the applicability of cellulose in novel applications.27–30 Recently,

NFC was prepared by a combination of mechanical and enzy-

matic pretreatment followed by high-pressure homogenization31

or by high-pressure homogenization of carboxymethylated

cellulose fibers followed by ultrasonication and centrifugation.32

The latter carboxymethylation pretreatment makes the fibrils

highly charged and easier to liberate, and this results in slightly

smaller and more uniform fibril dimensions than in the enzyme-

treated NFC.33 Since the fibrils are 5–20 nm thick and have

a length of up to several mm, they can be regarded as nanofibres.

Cellulose nanofibrils show very interesting properties as rein-

forcement elements in polymer nanocomposites,34–36 but

a surface modification of the nanofibrils might further widen the

applications since they can be incorporated into new bio-based

materials with a tuned interaction potential. In this respect,

a hydrophobation of the aerogels is a very interesting research

area.

This journal is ª The Royal Society of Chemistry 2010

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

A

View Online

The need for surface hydrophobicity has become a hot

research topic which has stimulated a rich variety of studies

describing different approaches for material engineering.37–41

Several studies have also been conducted on the chemical

modification/hydrophobation of nanocellulose.30,42,43 The most

effective treatments use chemical or physical means to append

fluorinated moieties which induce both hydrophobic and oleo-

phobic properties to the resulting low-energy surfaces. The

presence of these moieties also tends to impart a high thermal

stability and reduce chemical and biological fragility.

It is highly desirable for superhydrophobic surfaces also to be

oil-repellent in order to maintain their superhydrophobicity. For

instance, in an industrial or household environment, a super-

hydrophobic surface with poor oil repellency can easily be

contaminated by oily substances, and this will compromise the

superhydrophobicity of the surface. Therefore, superoleophobic

surfaces combining both superhydrophobic and superoleophobic

properties are desirable for many practical applications.44,45

Modifying the cellulose aerogel network with fluorinated

organic compounds could widen their applications towards self-

cleaning surfaces in a vast array of products, including green

constructions, packaging materials, protection against environ-

mental fouling, sports and outdoor clothing, and microfluidic

systems. The use of cellulose-based materials could thus be

extended to new areas by the introduction of functional moieties

onto the fiber surface.

Despite extensive investigations on superhydrophobic surfaces,

studies on superoleophobic surfaces with high repellency against

liquids with low surface tension (<35 mN m�1) have so far been

rather limited. Oil repellency has been examined on various per-

fluorinated, superhydrophobic surfaces.46–48 Very recently, truly

superoleophobic surfaces have been achieved on the basis of

etched silicon surfaces45 with nanonail49 structures, exemplified by

low contact angle hysteresis for probe liquids of low surface

tension (<30 mN m�1), such as octane. In both cases, the key to

obtaining true superoleophobicity is the re-entrant or overhang

surface structure, which ensures the entrapment of air beneath the

top solid surface and prevents the transition from the Cassie–

Baxter state to the Wenzel state.45,49,50 However, the fabrication of

most superoleophobic surfaces involves lithography and etching

steps, and this may limit their practical applications.

In the present work, a novel route for the production of

superoleophobic NFC aerogels is used based on aerogel skele-

tons formed by long and entangled cellulose I containing nano-

scopic fibrils. It gives an aerogel with sufficient strength to

oppose its tendency to collapse during the solvent extraction. The

aerogel network was modified with the aid of chemical vapor

deposition of a fluorinated silane. By adjusting the concentration

of the NFC dispersion before freeze-drying, the surface texture of

the aerogels was altered, making it possible to optimize the

degree of oleophobicity.

Experimental

Preparation of NFC aerogels and films

The anionic NFC used in this study was prepared by a proce-

dure similar to that previously described31 but using a carboxy-

methylation32 pretreatment of the fibers. In brief, the

This journal is ª The Royal Society of Chemistry 2010

dissolving pulp (Domsj€o dissolving plus) was first dispersed in

deionized water at 10 000 revolutions in an ordinary laboratory

reslusher. The fibers were then solvent-exchanged to ethanol by

washing the fibers in ethanol four times with an intermediate

filtration step. The fibers were then impregnated for 30 min with

a solution of 10 grams of monochloroacetic acid in 500 mL of

isopropanol. This carboxymethylation reaction was allowed to

continue for 1 h. Following the carboxymethylation step, the

fibers were filtered and washed in three steps: first with deion-

ized water, then with acetic acid (0.1 M), and finally with

deionized water. The fibers were then impregnated with

a NaHCO3 solution (4 wt% solution) in order to convert the

carboxyl groups to their sodium form to further enhance the

delamination of the fibers into nanofibrils. Finally, the fibers

were washed with deionized water and drained on a B€uchner

funnel. After this treatment, the fibers were passed through

a high-pressure homogenizer (Microfluidizer M-110EH, Mir-

cofluidics Corp). Cellulose slurries containing a 2 wt% pulp fibre

suspension in deionized water were processed through the

homogenizer. Such a procedure lead to the liberation of cellu-

lose I nanofibers, mostly with cross-sectional diameters of 5–20

nm and lengths of a few micrometres, although some larger

entities were formed. The so prepared NFC is hence a disper-

sion of nanofibers. NFC dispersions with concentrations of ca.

0.003–2.6 wt% were prepared by diluting the 3.13 wt% NFC

dispersion with deionized water followed by mixing (8000 rpm)

using an Ultra Turrax mixer (IKA D125 Basic, Germany) for

5 min. The total charge density of the highly carboxymethylated

NFC dispersions was measured to be 627 meq. g�1 (ref. 32) by

conductometric titration.51 The degree of substitution was

measured to be 0.1. The surface charge density, measured by

polyelectrolyte titration52 using poly-DADMAC (Ciba, York-

shire, UK, Mw ¼ 440 000 g mol�1 and 3 ¼ 6.19 meq. g�1), was

measured to be 426 meq. g�1.

Cylindrical PDMS cups (48 mm in diameter, 16 mm in height)

were used as moulds for the preparation of the aerogels. The

aqueous gel was placed in the mould and the mould was plunged

into liquid nitrogen. Thereafter, the frozen sample in the mould

was transferred to a vacuum oven at �52 �C (Labconco Free-

Zone 6, US), and the sample was kept frozen during the drying at

a pressure of ca. 0.016 mbar. The drying was typically finished

within 24 h. The density of the aerogels was obtained as the mass

divided by the volume (4.9 cm3) of the samples.

A free-standing film of NFC was prepared by pouring a 0.1

wt% NFC dispersion onto polystyrene Petri dishes with a diam-

eter of 14 cm. The film was allowed to form upon drying at

a temperature of 23 �C and a relative humidity (RH) of 50%. The

dried free-standing film was stored in a desiccator prior to

analysis.

Modification by perfluoroalkylsilane

The silane treatment was carried out by chemical vapor deposi-

tion of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFOTS)

(97%, Sigma Aldrich). Samples were placed on a copper grid

located 5 cm above a beaker containing the fluorinated silane,

which was heated at 140 �C for one hour. The samples were

stored in a desiccator prior to analysis.

Soft Matter, 2010, 6, 3298–3305 | 3299

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

A

View Online

Scanning electron microscopy

To study the micro-structure of the NFC aerogels, the specimens

were studied with a Hitachi S-4800 field emission scanning

electron microscope (FE-SEM) to obtain secondary electron

images. The specimens were fixed on a metal stub with colloidal

graphite paint and coated with a 6 nm thick gold/palladium layer

using a Cressington 208HR High Resolution Sputter Coater.

Fig. 1 Example of cylindrically shaped aerogels obtained from freeze-

drying of (a) 0.7 and (b) 1.1 wt% NFC dispersions.

Nitrogen adsorption/desorption measurements

The specific surface areas were determined by N2 adsorption/

desorption measurements at the temperature of liquid nitrogen

(ASAP 2020, Micromeritics, US). Before measurement, the

samples were dried at a temperature of 117 �C until a vacuum of

<10�5 mmHg was reached. Both adsorption and desorption

isotherms were measured and the surface area was determined

from the adsorption results using the Brunauer–Emmet–Teller

(BET) method.

Contact angle measurements

A CAM 200 (KSV Instruments Ltd, Helsinki, Finland) contact

angle goniometer was used for advancing contact angle

measurements. The software delivered by the instrument manu-

facturer calculates the contact angle on the basis of a numerical

solution of the full Young–Laplace equation. Measurements

were performed at room temperature with two non-polar, low

surface tension probe liquids: castor oil (Sigma Aldrich) and

hexadecane (>99%, anhydrous, Sigma Aldrich). The contact

angle was determined at three different positions on each sample.

The values reported were taken after the contact angle had

reached a stable value, typically less than 10 s after deposition of

the droplet. Typical uncertainties in the experiments were �4�.

X-Ray photoelectron spectroscopy (XPS)

The XPS spectra were collected with a Kratos Axis Ultra DLD

electron spectrometer (UK) using a monochromated Al Ka

source operated at 150 W, with a pass energy of 160 eV for wide

spectra and a pass energy of 20 eV for individual photoelectron

lines. The surface potential was stabilized by the spectrometer

charge neutralization system. Photoelectrons were collected at

a take-off angle of 90� relative to the sample surface and the

depth of analysis was ca. 10 nm. The binding energy (BE) scale

was referenced to the C 1s line of aliphatic carbon, set at 285.0

eV. The spectra were processed with the Kratos software and the

CasaXPS program package including experimental values for the

atomic sensitivity factors, and peak intensities were determined

by integrating the areas under the peaks.

Fig. 2 Aerogel density (g cm�3) as a function of NFC dispersion

concentrations during freeze-drying (g l�1).

Results

Structural characteristics of NFC aerogels

Aerogels from NFC dispersions of various concentrations

(ranging from 0.0031 wt% to 3.13 wt%) were prepared and their

structural properties were compared. After complete water

removal through freeze-drying, lightweight sponge-like aerogel

was produced which had not significantly collapsed. Fig. 1

3300 | Soft Matter, 2010, 6, 3298–3305

demonstrates the macroscopic integrity achieved, as specimens

with well-defined shapes were prepared.

The densities of the samples were very low. Fig. 2 shows the

density of the aerogels as a function of the initial NFC dispersion

concentration. The aerogel density was almost linearly propor-

tional to the dispersion concentrations. For example, aerogels

prepared from 0.5 and 3.13 wt% dispersions resulted in material

densities of 0.0053 and 0.030 g cm�3, respectively. The aerogel

prepared from the 0.5 wt% dispersion had a very high porosity,

ca. 99.7%, where the porosity F is defined as F ¼ 1 � (r/rs),

where r and rs (1.63 g cm�3)53 are the densities of the aerogel and

the crystalline Ib cellulose fibril, respectively. For comparison,

the aerogel prepared from the 3.13 wt% dispersion had a porosity

of ca. 98.2%.

The specific surface area was analyzed by N2 adsorption/

desorption at 77 K. Aerogels with densities of 0.030 and 0.020 g

cm�3 showed BET specific surface areas of 11 and 15 m2 g�1,

respectively. Evidently, the BET-area increased with decreasing

density of the aerogels as expected. These values were lower than

in a previous study by P€a€akk€o et al.,20 who reported a value of 66

m2 g�1 for low-charged NFC aerogels prepared from a 2 wt%

NFC-dispersion.

As previously shown in detail under aqueous conditions, the

initial ‘‘wet’’ gel, i.e. NFC in water, consists of long and entan-

gled fibrils with diameters of ca. 5 nm with occasional thicker

fibril bundles.31 Fig. 3 presents FE-SEM micrographs showing

the surface texture of aerogels with densities of 0.00027, 0.0018,

0.0053, 0.0070, 0.011 and 0.030 g cm�3, respectively. As the

This journal is ª The Royal Society of Chemistry 2010

Fig. 3 Low-magnification (�100) FE-SEM micrographs of aerogels

fabricated by the freeze-drying of aqueous NFC dispersions. The densi-

ties of the aerogels are (a) 0.00027 (b) 0.0018 (c) 0.0053 (d) 0.0070 (e)

0.011 and (f) 0.030 g cm�3. All images are top-view images and the scale

bars are 500 mm.

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

A

View Online

density of the aerogels increased, the amount of pores and

protruding sheets decreased. This was accompanied by a gradual

closure of the surface texture. At densities <0.0070 g cm�3, the

aerogel typically consisted of an open network of extended thin

sheets forming macroscopic ‘‘open channels’’ and large, several

micrometre wide pores that connect the different cells and

thin sheets of the aerogel. When the density was increased to

Fig. 4 FE-SEM micrographs showing (a) a typical cross-section of

a sheet, (b and c) fibril-aggregates protruding from the cell walls forming

network structures and (d) the fibrillar aerogel skeleton. Scale bars are (a)

40 mm (b) 10 mm (c) 50 mm and (d) 10 mm.

This journal is ª The Royal Society of Chemistry 2010

0.011 g cm�3, the number of pores and the pore size decreased

significantly accompanied with the formation of more sheet-like

structures. At a density of 0.030 g cm�3, no macroscopic pores

were observed (Fig. 3f). Instead, the aerogel had a much more

closed surface texture with thin sheets exhibiting random

formations of ‘‘wave-like’’ roughness.

The pore and sheet structures of the 0.0070 g cm�3 aerogel were

further studied by FE-SEM at higher magnifications, and some

micrographs are presented in Fig. 4. A typical sheet thickness of

about 4–8 mm formed from aggregated nanofibrils is revealed in

Fig. 4a. Nanofibril-aggregates or nanofibril bundles with diam-

eters in the order of 500 nm, are protruding from the surface of

the sheets (Fig. 4b), some of them forming very open networks

(Fig. 4c). These surface features were especially apparent in the

aerogels with densities <0.01 g cm�3. Fig. 4d shows an inter-

connected fibrillar structure of long and entangled nanofibril

aggregates with diameters of 100 nm with occasional thicker

bundles. The image reveals that the fibril network has a random-

in-plane orientation.

Surface wettability

It is well established that in order to obtain a thin film coverage

of trichlorosilanes on any substrates, reactive OH-groups on the

surface are generally required.47,54 These OH-groups can react

with functionalized silanes (usually trichlorosilane) in the

gaseous or solvent phase to yield preferably monolayers with

the desired functionality depending on the chemical composi-

tion of the selected silane. In the work, this type of coating was

applied to the structured and flat cellulose surfaces (aerogels

and solvent-cast film, respectively) by simply reacting the

hydroxylic groups of the cellulose with trichlorosilane. The

unique structure of the coating was found to significantly

decrease the wetting properties evaluated by contact angle

measurements using two probe liquids: castor oil and hex-

adecane with surface tensions of 35.8 and 27.5 mN m�1,

respectively. However, pure non-modified NFC aerogels and

films were completely wettable (q z 0�) by castor oil. When

1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS) was

applied in the refunctionalization step, the structured coatings

displayed clear oleophobic properties (q [ 90�). Fig. 5 shows

the advancing contact angle for castor oil as a function of the

density of the PFOTS-coated aerogels.

Fig. 5 Advancing contact angle for castor oil (glv ¼ 35.8 mN m�1) as

a function of the density of the PFOTS-coated aerogels.

Soft Matter, 2010, 6, 3298–3305 | 3301

Fig. 6 Droplets of (a) castor oil and (b) hexadecane on top of a PFOTS-

coated aerogel with a density r ¼ 0.0070 g cm�3.

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

A

View Online

The most oleophobic aerogel (r ¼ 0.0070 g cm�3) exhibited

contact angles of roughly 166� and 144� for castor oil and hex-

adecane respectively (Fig. 6), which, to the knowledge of the

authors, are among the highest contact angles reported in the

literature for any substrate against liquids or liquids with similar

surface tensions.45,47,50,55 At densities >0.020 g cm�3, the

advancing contact angle gradually decreased down to ca. 100�.

At densities below 0.0003 g cm�3, the advancing contact angle

decreased down to 0�. Between the aerogels with densities of

0.00035 and 0.00027 g cm�3 there is a rapid transition from

a perfectly non-wetting (q [ 90�) to a completely wetting

surface (q z 0�). A contact angle denoted �0� indicates that the

surface was not sufficiently robust to support a 10 ml droplet of

castor oil, and that the liquid droplet was rapidly (<10 s) imbibed

into the surface. Transitions from perfectly non-wetting (q [

90�) to completely wetting surfaces (q z 0�) between aerogels

with densities of 0.00035 and 0.00027 g cm�3 were also observed

for hexadecane and water. As expected, there was a large

difference between the contact angle values on the structured

aerogels and on the smooth solvent-casted NFC film. For

comparison, the contact angles for castor oil and hexadecane on

PFOTS-modified smooth NFC films were 96� and 71�, respec-

tively, demonstrating the necessity of having a highly textured

surface to achieve the high contact angles against low surface

tension liquids.

The XPS analysis revealed the surface chemical composition of

the NFC aerogels and film before and after PFOTS-coating. The

atomic concentrations of the surfaces of pure NFC aerogel and

film and their modified analogues are given in Table 1. Carbon,

oxygen and very small amounts of Na as counterions of the

carboxymethylated cellulose fibrils were detected on the surface

of the cellulose samples, whereas the modified samples clearly

showed also the presence of fluorine and silicon, which supported

that PFOTS had reacted with the cellulose. The total oxygen

concentration decreased from 38.2% to 6.6%, and the fluorine

concentration was found to be 51%. The atomic surface

concentrations of C, O, Si and F were similar for all the

Table 1 Atomic surface concentration on PFOTS-coated and non-coated NFC aerogels and films

Surface concentration (at%)

C O Na Si F

Pure NFC aerogels/film 61.3 38.2 0.6PFOTS-coated NFC aerogels/film 37.6 6.6 4.8 51.0

3302 | Soft Matter, 2010, 6, 3298–3305

structured PFOTS-coated aerogels and films. As in the case of

the fluorinated NFC aerogels, the fluorine signal from the

PFOTS-coated NFC film was determined to be 51%, indicating

reproducible results from the PFOTS-coating. The pure

NFC aerogel and film showed a lower oxygen to carbon ratio

(O/C-ratio), 0.62, compared with the theoretical value of pure

cellulose, 0.83. Hydrocarbons tend to be the most common

surface contaminant, and it is quite common to see C–H peaks in

cellulose samples.56 Although the aerogels and films were

handled carefully, it is possible that some surface contamination

occurred. The high-surface area of the cellulose nanofibrils

makes it even more difficult to avoid contamination during

e.g. film formation and storage. The uncertainty of the XPS

determinations was within �1%.

Discussion

Preparation of NFC aerogels by freeze-drying

In the present work, highly porous NFC aerogels were prepared

by direct water removal by freeze-drying. The primary aim has

been to clarify the factors controlling the surface properties of

these aerogels and to further modify and tune the oil-wetting/

resistance properties of these materials. The surface texture

rather then the bulk properties of the aerogels was the aspect of

major interest in this study. Nevertheless, as indicated by the

FE-SEM and nitrogen adsorption/desorption-measurements, the

porosity, density and surface morphology can be tuned by

switchable adjustments of the concentration of the NFC

dispersions before freeze-drying. A higher NFC concentration

leads to a lower specific surface area of the aerogel. The same

phenomenon was reported for freeze-dried cellulose/calcium

thiocyanate solutions of different cellulose concentrations57 and

for low-charged NFC aerogels.20 The influence of freeze-drying

conditions on the morphology and porosity of aerogels has been

studied before, e.g. for NFC-based foams,20,22 regenerated

cellulose II57 and for starch-based foams reinforced with NFC.36

The observed specific surface areas of the present aerogels were

lower than those reported for freeze-dried fibrillar cellulose aer-

ogels.20,21 This is probably due to a slower cooling of the NFC

dispersions by the liquid nitrogen than the cooling by liquid

propane used in the work of P€a€ak€o et al.20 Rapid cooling is

known to be accompanied by the formation of amorphous ice

which may lead to a more homogeneous fibrillar structure of the

aerogel with smaller pores and less pronounced sheet-like

structure of the aerogel.58 In contrast, cooling by liquid nitrogen

enhances the formation of non-amorphous ice (crystals) which

contributes to the sheet-formation. The aerogel structure is

therefore directly related to the size and distribution of the ice

crystals in the frozen system. In addition, the thickness of the

aerogels plays an important role for the cooling rate. Thicker

samples cool more slowly. Samples with a thickness of 10 mm

were used in this study, whereas samples with thicknesses

between 1 and 5 mm were prepared by P€a€ak€o et al.20

The fact that the aerogels have a structured porosity and

morphology can be an advantage in various applications, for

example, as in the present work, for modifying the surface

wetting properties of the aerogel. The native cellulose nanofibers

are composed of cellulose I crystal domains as well as less

This journal is ª The Royal Society of Chemistry 2010

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

A

View Online

ordered fractions,33 and this promotes high length, flexibility,

entanglements, and robust networks of the so prepared aerogels,

even from very low NFC dispersion concentrations.20 As a result,

the present nanofibers make it possible to produce sponge-like

aerogels with a lower density than previous cellulose aerogels.57

This means that lower gel concentrations are needed for the

formation of robust aerogels that oppose collapse.20 In order to

decrease the density and to tune the porosity by creating large

pores, a surfactant has previously been used.21 In this study, no

additional surfactant has been used, but, as previously discussed,

a low density is due to fibrillar entanglement, and the porosity is

tuned by adjusting the concentration of the NFC dispersions.

Influence of the aerogel surface chemistry and surface texture on

the wettability by oils

The apparent contact angle q* for a composite interface beneath

a strongly non-wetting droplet is typically computed using the

Cassie–Baxter relation:59

cos q* ¼ f1cos q � f2 (1)

where f1 is the surface area of the liquid in contact with the solid

divided by the projected area and f2 is the surface area of the

liquid in contact with air trapped in the pores of the rough

surface divided by the projected area, i.e.:

f1 ¼area in contact with liquid

projected area

and

f2 ¼area in contact with air

projected area(2)

The Lotus leaf is an example of a natural surface that is

forming a composite interface where water (glv ¼ 72.1 mN m�1)

droplets form beads on the surface. This natural surface

comprises randomly distributed almost hemispherically topped

papillae with sizes 5–10 mm.60 The formation of the Cassie–

Baxter state enhances water super-repellency by promoting

a high apparent contact angle (q*) when f1� 1.59

On the other hand, if the liquid fully penetrates into the surface

texture, the apparent contact angle q* is determined by the

Wenzel relation:61

cos q* ¼ rcos q (3)

where r is the surface roughness, defined as the ratio between the

actual surface area and the projected area. Since r is necessarily

greater than unity, roughness amplifies both the wetting and

non-wetting behavior of materials in the Wenzel regime, i.e., cos

q* [ 0� if cos q > 0� and cos q* � 0� if cos q < 0�. A conse-

quence of this dependence on the roughness of the texture is that,

once initiated, the imbibition of a liquid drop into a roughened

texture can rapidly lead to super-wetting, because the apparent

contact angle q* / 0� when r [ 1 and q < 90�. Eqn (1) has

recently been rewritten as follows:62

f1 ¼ rff (4)

f2 ¼ 1 � f (5)

This journal is ª The Royal Society of Chemistry 2010

cos q* ¼ rffcos q + f � 1 (6)

where f is the fraction of the projected area of the solid surface in

contact with the liquid and rf is the roughness of the portion of

the solid in contact with liquid. When f ¼ 1, rf ¼ r in the Wenzel

model. It is important to note that rf in eqn (6) is not the

roughness ratio of the total surface, but only of that in contact

with the liquid. In this form of the Cassie–Baxter equation, the

contributions of surface roughness and of entrapped air are

clearer than in the other forms of the equation.62 The conditions

for highly non-wettability (q* [ 90�) can be realized only in the

case of a composite interface where the solid–liquid contact area

is low. However, for low surface tension liquids with q < 90�, the

fully wetted or Wenzel state represents the thermodynamic

equilibrium state, whereas the composite interface or the Cassie–

Baxter state is metastable,45,50,63–65 representing a local minimum

in the overall Gibbs free energy. Thus, for low surface tension

liquids, the transition from a composite interface to a fully wetted

interface is irreversible, and typically this transition leads to

a loss of non-wettability. Therefore the ability to preserve this

metastable composite interface is crucial for engineering non-

wettable surfaces.66

In our previous work, we demonstrated how the incorporation

of a re-entrant surface texture (i.e., a multiscale surface topog-

raphy) in conjunction with surface chemistry can be used to

fabricate highly oleophobic surfaces, i.e., surfaces that can

support a robust composite (solid–liquid–air) interface and

display contact angles greater than 150� with various low-

surface-tension liquids such as castor oil and hexadecane.67 In the

present work, the development of NFC aerogels with different

surface textures has made it possible to fine-tune the surface

wettability, including the capacity to switch the surface wetting

properties between super-repellent and super-wetting against i.e.

castor oil.

The use of 1H,1H,2H,2H-perfluorodecyltrichlorosilane

(PFOTS) to generate oleophobic cellulose surfaces has been

discussed earlier.67 The high concentration of perfluorinated

carbon atoms in the alkyl chains leads to a very low solid-surface

energy for these molecules in the form of a thin layer structure

(gsv z 13.5 mN m�1).67 As a comparison, the surface energy of

Teflon is gsv¼ 18 mN m�1.68 To provide a conformal and flexible

coating of PFOTS molecules on the NFC aerogels and films

possessing re-entrant and flat surface textures, respectively,

a simple chemical vapor deposition (CVD) procedure was used.

After the CVD, the equilibrium contact angle for castor oil on

a smooth NFC film increased to q z 96� compared to q z 0� on

an uncoated film. As previously discussed, a PFOTS-modified

aerogel is able to support a composite interface even with hex-

adecane (glv ¼ 27.5 mN m�1), as shown in Fig. 6b. FE-SEM

micrographs showing the surface textures of aerogels with

various densities are shown in Fig. 3. A comparison with the

surface morphology of the PFOTS-coated aerogels (not shown)

shows that all the surface details, even features in the sub-

micrometre range, are preserved after modification.

As previously discussed, for a given surface texture, the non-

wetting properties can be most readily enhanced by markedly

lowering the surface energy of the solid, leading to increased

values of the equilibrium contact angle q (based on the Young’s

equation69). According to XPS measurements, the fluorine

Soft Matter, 2010, 6, 3298–3305 | 3303

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

A

View Online

concentration of the PFOTS-coated aerogels was 51 � 1%. The

variation in contact angle for castor oil on the aerogels is

therefore attributed to the variation in surface texture and should

be taken as the primary cause of the large difference in oleo-

phobicity. The tunable oleophobic properties of the PFOTS-

coated aerogels result from the rough and porous surface

textures and can be interpreted in terms of the Wenzel and

Cassie–Baxter models. It is clear in Fig. 5 that, for very low-

density aerogels (<0.0003 g cm�3), the liquid is in contact with the

entire solid surface and completely penetrates the surface texture

generated by the multiple scale of roughness and pores, i.e.

Wenzel mode of wetting. The number of pores and the pore size

are most probably too large to enable these surfaces to support

a composite interface (Fig. 3a). More robust surfaces are

obtained on higher density aerogels with a maximum in oleo-

phobicity (q z 166�) at a density of 0.0070 g cm�3 (Fig. 3d).

These properties are probably related to a fine balance between

the number of pores, the pore size and the extending sheet-like

structures protruding from the surface. Multiple scales of

roughness generated by protruding threads of fibril aggregates

probably also enhance the oleophobic properties. It is assumed

that there is no penetration of oil into the gaps and that the liquid

rests on the rough features of the protruding solid material. The

air bridging these features then acts as further support for the oil

droplet. The oil droplet can thus be considered to rest on only

a part of the solid surface exposing a large fraction of its surface

towards air and this situation can hence be modeled by the

Cassie–Baxter equation. However, as the number of pores

decreases for higher-density aerogels (>0.02 g cm�3), a much

more closed and non-porous surface structure is obtained

(Fig. 3e and f). This simply results in a decreased surface area

which greatly resembles that of a smooth NFC film. A conse-

quence of this dependence on roughness is a transition from the

highly non-wetting state to the Wenzel state with q* z 96�.

The development of highly oil-repellent surfaces requires the

design of substrates that promote the formation of a composite

interface with almost any liquid. The two important character-

istics for arriving at a Cassie–Baxter state of wetting on

a textured surface with a given liquid are: (i) the magnitude of the

apparent contact angle q* on the composite interface and (ii) the

robustness of the composite interface against external pertur-

bation.66 Tuteja et al.45 developed a dimensionless design

parameter A* to predict the robustness of a composite interface.

This robustness factor represents the ratio of the breakthrough

pressure, Pbreakthrough, required to cause sufficient sagging and

disruption of the liquid–vapor interface, to a characteristic

reference pressure Pref, given as Pref ¼ 2glv/lcap where lcap ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiglv=rg

p(here r is the fluid density and g is the acceleration due

to gravity). If the sagging becomes severe enough for the inter-

face to touch the underlying level of the solid texture, then the

composite interface collapses, and the liquid droplet rapidly

switches to a fully wetted state. The threshold pressure difference

that triggers the transition, i.e. the breakthrough pressure,

Pbreakthrough, can be computed as Pbreakthrough z A*� Pref. Thus,

large values of the robustness factor (A* [ 1) are associated

with the formation of a robust composite interface with a very

high breakthrough pressure.70

As previously discussed, the aerogels exhibit a random 3D

structural feature consisting of fibrils sheets and protruding fibril

3304 | Soft Matter, 2010, 6, 3298–3305

bundles, and it is difficult to systematically relate or model the

surface texture of the aerogels to the corresponding wetting

properties. Nevertheless, attempts have been made to systemi-

cally relate certain surface geometries to a robustness factor and

resulting wetting properties. Choi et al.66 designed geometrical

parameters for a texture dominated by periodical cylindrical

features; duck feathers, while Tuteja and co-workers45 analyzed

structural parameters governing the properties of super-oleo-

phobic electrospun fibers. They showed that for a given liquid

(given lcap), the robustness factor (A*) can be varied systemati-

cally, either by tuning the geometrical parameters describing the

surface (such as fiber radius and inter-fiber gap) or by changing

the equilibrium contact angle (q) through modification of the

surface chemical composition. It should, however, be stressed

that the authors used surfaces with well-defined periodically

arranged structures and fiber dimensions and that a similar

treatment of the interfaces in the present work demands a new

surface characterization procedure that is left for future studies

on these materials.

Conclusions

Native nanofibrillar cellulose aerogels have been prepared by

vacuum freeze-drying of aqueous dispersions of carboxymethy-

lated cellulose I nanofibers. The morphology and porosity of the

aerogels, as indicated by FE-SEM microscopy, can be tuned

simply by adjusting the concentration of the NFC dispersions.

A simple chemical vapor deposition process was developed to

achieve a conformal coating of low-surface-energy PFOTS

molecules on the aerogel surfaces. An XPS study of the chemical

composition of the PFOTS-modified aerogels confirmed the

reproducibility of the PFOTS-coating and the high atomic fluo-

rine concentration (ca. 51%) in the surface. The synergistic effect

of roughness, re-entrant topography of the aerogels, and the low

surface energy of the PFOTS molecules enables the CVD-coated

surfaces to support a composite interface with low-surface-

tension liquids such as castor oil and hexadecane. Advancing

contact angle measurements on the PFOTS-coated aerogels were

made, using castor oil as probe liquid. The results demonstrated

the very high oleophobic nature (q* [ 90�) of the aerogels

compared to the corresponding contact angle (q z 90�) for

a smooth NFC film. Aerogels with suitable surface textures were

developed allowing the systematical adjustment of their surface-

wettability characteristics. By combining this understanding with

a CVD process that provides a conformal fluorinated coating, we

can switch the wettability behavior of the cellulose surfaces

between super-wetting and super-repellent, using different scales

of roughness and porosity created by the freeze-drying technique

and change of concentration of the NFC dispersion.

Acknowledgements

The authors thank BIM Kemi Sweden AB and the Knowledge

Foundation through its graduate school YPK for financial

support. Profs. Lars €Odberg and Lars Berglund are greatly

acknowledged for valuable discussions. M. Sc. Mikael Ankerfors

is gratefully acknowledged for supplying the NFC dispersions.

Dr Andrei Shchukarev at Ume�a University is acknowledged for

performing the XPS experiments.

This journal is ª The Royal Society of Chemistry 2010

Dow

nloa

ded

by N

anya

ng T

echn

olog

ical

Uni

vers

ity o

n 31

May

201

2Pu

blis

hed

on 1

4 M

ay 2

010

on h

ttp://

pubs

.rsc

.org

| do

i:10.

1039

/C00

1939

A

View Online

References

1 C. Tan, B. M. Fung, J. K. Newman and C. Vu, Adv. Mater., 2001, 13,644–646.

2 S. S. Kistler, J. Phys. Chem., 1932, 36, 52–64.3 S. S. Kistler, Nature, 1931, 127, 741.4 N. Husing and U. Schubert, Angew. Chem., Int. Ed., 1998, 37, 22–45.5 H. D. Gesser and P. C. Goswami, Chem. Rev., 1989, 89, 765–788.6 S. Dai, Y. H. Ju, H. J. Gao, J. S. Lin, S. J. Pennycook and

C. E. Barnes, Chem. Commun., 2000, 243–244.7 H. Tamon, H. Ishizaka, T. Yamamoto and T. Suzuki, Carbon, 2000,

38, 1099–1105.8 H. Tamon, H. Ishizaka, T. Yamamoto and T. Suzuki, Carbon, 1999,

37, 2049–2055.9 S. Kuga, D.-Y. Kim, Y. Nishiyama and R. M. Brown, Mol. Cryst.

Liq. Cryst. Sci. Technol., Sect. A, 2002, 387, 13–19.10 Z. Zhu, L. Y. Tsung and M. Tomkiewicz, J. Phys. Chem., 1995, 99,

15945–15949.11 D. J. Suh, T.-J. Park, J.-H. Kim and K.-L. Kim, Chem. Mater., 1997,

9, 1903–1905.12 A. Pierre, R. Begag and G. Pajonk, J. Mater. Sci., 1999, 34, 4937–

4944.13 F. Meng, M. R. Schlup and L. T. Fan, Chem. Mater., 1997, 9, 2459–

2463.14 M. R. Ayers, X. Y. Song and A. J. Hunt, J. Mater. Sci., 1996, 31,

6251–6257.15 R. W. Pekala and D. W. Schaefer, Macromolecules, 1993, 26, 5487–

5493.16 R. W. Pekala, J. Mater. Sci., 1989, 24, 3221–3227.17 X. Lu, M. C. Arduini-Schuster, J. Kuhn, O. Nilsson, J. Fricke and

R. W. Pekala, Science, 1992, 255, 971–972.18 J. Gross and J. Fricke, J. Non-Cryst. Solids, 1992, 145, 217–222.19 M. H. Nguyen and L. H. Dao, J. Non-Cryst. Solids, 1998, 225, 51–57.20 M. P€a€akk€o, J. Vapaavuori, R. Silvennoinen, H. Kosonen,

M. Ankerfors, T. Lindstr€om, L. A. Berglund and O. Ikkala, SoftMatter, 2008, 4, 2492–2499.

21 R. Gavillon and T. Budtova, Biomacromolecules, 2008, 9, 269–277.22 H. Sehaqui, M. Salajkov�a, Q. Zhou and L. A. Berglund, Soft Matter,

2010, 6, 1824–1832.23 D. Klemm, B. Heublein, H.-P. Fink and A. Bohn, Angew. Chem., Int.

Ed., 2005, 44, 3358–3393.24 Y. Nishiyama, P. Langan and H. Chanzy, J. Am. Chem. Soc., 2002,

124, 9074–9082.25 A. F. Turbak, F. W. Snyder and K. R. Sandberg, J. Appl. Polym. Sci.,

1983, 37, 815–827.26 F. W. Herrick, R. L. Casebier, J. K. Hamilton and K. R. Sandberg,

J. Appl. Polym. Sci., 1983, 37, 797–813.27 T. Saito, S. Kimura, Y. Nishiyama and A. Isogai, Biomacromolecules,

2007, 8, 2485–2491.28 H. Yano, J. Sugiyama, A. N. Nakagaito, M. Nogi, T. Matsuura,

M. Hikita and K. Handa, Adv. Mater., 2005, 17, 153–155.29 A. N. Nakagaito and H. Yano, Appl. Phys. A: Mater. Sci. Process.,

2004, 78, 547–552.30 G. Siqueira, J. Bras and A. Dufresne, Biomacromolecules, 2009, 10,

425–432.31 M. P€a€akk€o, M. Ankerfors, H. Kosonen, A. Nyk€anen, S. Ahola,

M. €Osterberg, J. Ruokolainen, J. Laine, P. T. Larsson, O. Ikkalaand T. Lindstr€om, Biomacromolecules, 2007, 8, 1934–1941.

32 L. W�agberg, G. Decher, M. Norgren, T. Lindstr€om, M. Ankerforsand K. Axn€as, Langmuir, 2008, 24, 784–795.

33 C. Aulin, S. Ahola, P. Josefsson, T. Nishino, Y. Hirose, M. €Osterbergand L. W�agberg, Langmuir, 2009, 25, 7675–7685.

34 A. Dufresne, Can. J. Chem., 2008, 86, 484–494.

This journal is ª The Royal Society of Chemistry 2010

35 A. N. Nakagaito and H. Yano, Appl. Phys. A: Mater. Sci. Process.,2004, 80, 155–159.

36 A. J. Svagan, M. A. S. A. Samir and L. A. Berglund, Adv. Mater.,2008, 20, 1263–1269.

37 B. Bhushan, Y. C. Jung and K. Koch, Langmuir, 2009, 25, 3240–3248.38 L. Gao and T. J. McCarthy, J. Am. Chem. Soc., 2006, 128, 9052–9053.39 H. Y. Erbil, A. L. Demirel, Y. Avci and O. Mert, Science, 2003, 299,

1377–1380.40 L. Zhang and D. E. Resasco, Langmuir, 2009, 25, 4792–4798.41 L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu,

L. Jiang and D. Zhu, Adv. Mater., 2002, 14, 1857–1860.42 S. Utsel, E. Malmstr€om, A. Carlmark and L. W�agberg, Soft Matter,

2010, 6, 342–352.43 H. L€onnberg, L. Fogelstr€om, M. A. S. A. Samir, L. Berglund,

E. Malmstr€om and A. Hult, Eur. Polym. J., 2008, 44, 2991–2997.44 A. Marmur, Langmuir, 2008, 24, 7573–7579.45 A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S. A. Mazzella,

G. C. Rutledge, G. H. McKinley and R. E. Cohen, Science, 2007,318, 1618–1622.

46 A. Nakajima, M. Hoshino, J.-H. Song, Y. Kameshima and K. Okada,Chem. Lett., 2005, 908–909.

47 J. Zimmermann, M. Rabe, G. R. J. Artus and S. Seeger, Soft Matter,2008, 4, 450–452.

48 H. Yabu, M. Takebayashi, M. Tanaka and M. Shimomura,Langmuir, 2005, 21, 3235–3237.

49 A. Ahuja, J. A. Taylor, V. Lifton, A. A. Sidorenko, T. R. Salamon,E. J. Lobaton, P. Kolodner and T. N. Krupenkin, Langmuir, 2008,24, 9–14.

50 L. Cao, T. P. Price, M. Weiss and D. Gao, Langmuir, 2008, 24, 1640–1643.

51 S. Katz, R. P. Beatson and A. M. Scallan, Sven. Papperstidn., 1984,87, 48–53.

52 L. Winter, L. W�agberg, L. €Odberg and T. Lindstr€om, J. ColloidInterface Sci., 1986, 111, 537–543.

53 Y. Nishiyama, J. Sugiyama, H. Chanzy and P. Langan, J. Am. Chem.Soc., 2003, 125, 14300–14306.

54 D. C. Duffy, J. C. McDonald, O. J. A. Schueller andG. M. Whitesides, Anal. Chem., 1998, 70, 4974–4984.

55 T. Onda, S. Shibuichi, N. Satoh and K. Tsujii, Langmuir, 1996, 12,2125–2127.

56 C. D. Edgar and D. G. Gray, Cellulose, 2003, 10, 299–306.57 H. Jin, Y. Nishiyama, M. Wada and S. Kuga, Colloids Surf., A, 2004,

240, 63–67.58 T. A. Jennings, Lyophilization: Introduction and Basic Principles,

Interpharm Press, USA, 1999.59 A. B. D. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546–551.60 V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis,

S. H. Anastasiadis and C. Fotakis, Adv. Mater., 2008, 20, 4049–4054.61 R. N. Wenzel, Ind. Eng. Chem., 1936, 28, 988–994.62 A. Marmur, Langmuir, 2003, 19, 8343–8348.63 O. Werner, L. W�agberg and T. Lindstr€om, Langmuir, 2005, 21,

12235–12243.64 L. Cao, H.-H. Hu and D. Gao, Langmuir, 2007, 23, 4310–4314.65 M. Nosonovsky, Langmuir, 2007, 23, 3157–3161.66 W. Choi, A. Tuteja, S. Chhatre, J. M. Mabry, R. E. Cohen and

G. H. McKinley, Adv. Mater., 2009, 21, 2190–2195.67 C. Aulin, S. H. Yun, L. W�agberg and T. Lindstr€om, ACS Appl.

Mater. Interfaces, 2009, 1, 2443–2452.68 E. Kissa, Fluorinated Surfactants and Repellents, Revised and

Expanded, Marcel Dekker, New York, 2nd edn, 2001.69 T. Young, Philos. Trans. R. Soc. London, 1805, 95, 65.70 S. S. Chhatre, A. Tuteja, W. Choi, A. Revaux, D. Smith, J. M. Mabry,

G. H. McKinley and R. E. Cohen, Langmuir, 2009, 25, 13625–13632.

Soft Matter, 2010, 6, 3298–3305 | 3305