Biochemical Engineering Journal 42 (2008) 20–27

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Biochemical Engineering Journal

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Cellular structure in an N-acetyl-chitosan membrane regulate water permeability

Tomoki Takahashi1, Masanao Imai ∗, Isao SuzukiBioresource Utilization Sciences, Graduate School of Bioresource Sciences, Nihon University, 1866 Kameino, Fujisawa, Kanagawa Prefecture 252-8510, Japan

a r t i c l e i n f o

Article history:Received 15 October 2007Received in revised form 30 April 2008Accepted 21 May 2008

Keywords:N-Acetyl-chitosanMembraneDeacetylation degreeCellular structureWater permeabilityNovel permeation mechanism

a b s t r a c t

A novel model of the water permeation mechanism in an N-acetyl-chitosan membrane with a cellularstructure is proposed. Although the entire membrane structure has a hydrophilic character, the cellularstructure incorporates junction zones that practically prevent water permeation. Chitosan membraneswith a controlled degree of deacetylation (DD) were prepared using a casting method. Changes in the waterflux and total water content of the membrane were observed with a change in DD. The membrane proper-ties were analyzed and evaluated using water permeability measurements, scanning electron microscopy(SEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). SEM observations indicatedthat the membrane structure was an individual cellular structure and that this cellular structure grewwith decreasing DD. XRD measurements indicated the crystal structure of the membrane was amorphousregardless of the DD in the experimental range. The free water content (Wf), the freezable bound water(Wfb), and the bound water not able to freeze (Wb) were evaluated by DSC. The free water mainly containedinside the cellular structure, and resulted in swelling the chitosan membrane. Water flux was measuredusing ultrafiltration apparatus; it was dependent on the operational pressure, membrane thickness, andthe feed solution viscosity, and obeyed the Hagen–Poiseuille flow. At a higher DD, water permeationproceeds due to degradation of the cellular structure; the amount of water in permeation channels was

greater than that for lower DD membranes even though the total water content in the membrane was less.

osand no

1

epptt

utop

fm

DT

cldlasdb

1d

The water flux of the chitthrough the membrane an

. Introduction

Membrane fouling, especially “biofouling” [1], negatively influ-nces the permeation flux and the performance of some membraneroperties (e.g., reduced salt rejection and elevated operationalressure) [2–5]. Chitosan is known as a biopolymer material (Fig. 1)hat inhibits the growth of mold and bacteria [6], and is expectedo provide long-life hygienic membrane processing.

Control of water flux is very important for separation processessing a chitosan membrane. Therefore, a detailed investigation of

he influence of the structure and water content of the membranen the water flux is necessary for desirable control of waterermeation.

Abbreviations: CPS, counts per second; DD, deacetylation degree; DSC, dif-erential scanning calorimetry; MW, molecular weight; SEM, scanning electron

icroscope; XRD, X-ray diffraction.∗ Corresponding author. Tel.: +81 466 84 3978; fax: +81 466 84 3978.

E-mail address: XLT05104@nifty.ne.jp (M. Imai).1 Present address: Department of Industrial Chemistry, Faculty of Engineeringivision 1, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku,okyo 162-8601, Japan.

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369-703X/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.bej.2008.05.013

membrane was determined by the water content constructing channelst on the total water content in the membrane.

© 2008 Elsevier B.V. All rights reserved.

The degree of deacetylation (DD) of a chitosan membrane is stoi-hiometrically controlled to a designed level based on the observedinear relationship between the DD and the amount of acetic anhy-ride additive. Although a chitosan membrane produced under

ower DD conditions has more hydrophilic character and becomespparently swollen with water, the permeated water flux throughuch membranes is dramatically reduced [7]. This observed trendid not agree with the general known behavior of hydrophilic mem-ranes.

This paper demonstrates the preparation of a chitosan mem-rane by a casting method in combination with N-acetylation toontrol molecular properties (i.e., DD). The effects of the DD of chi-osan on permeability and hydrophilic character are discussed andnovel model of the water permeation mechanism in an N-acetyl-

hitosan membrane with a cellular structure is proposed.

. Materials and methods

.1. Preparation of an N-acetyl-chitosan membrane

6 g of chitosan (low molecular weight, Sigma–Aldrich) and 3 gf polyethylene glycol (MW: 7500, Wako) were dissolved in 111 gf 10 (v/v)% acetic acid. The chitosan solution was diluted to 2

T. Takahashi et al. / Biochemical Engineering Journal 42 (2008) 20–27 21

Nomenclature

A additive volume of acetic anhydride (�l (50 g chi-tosan solution of 2 wt%)-1)

Am apparent surface area of the membrane (m2)Ap total cross-sectional area of pore of the membrane

(m2)DD deacetylation degree (%) previously defined in our

paper [7]Dp diameter of pore in membrane (m)Jv volumetric water flux (m3 m−2 s−1)Lm thickness of membrane (m)Lp length of pore membrane (m)n total number of poreNp numerical density of pores (m−2)Qendo endothermic heat per unit mass of swollen mem-

brane (J (g of we)−1)Qf heat of fusion of ice; (333.9 J (g of water)−1)Rt volumetric ratio of total contained water of mem-

braneR1 volumetric ratio of contained water in the channelsR2 volumetric ratio of immobilized water content

inside cellsu flow rate of water in pore Dp (m s−1)wb mass of bound water (g)wd mass of dried membrane (g)we mass of swollen membrane at equilibrium (= wt +

wd) (g)wf mass of free water (g)wfb mass of freezable bound water (g)wt total mass of contained water in membrane (= wf +

wfb + wb) (g)Wb bound water content ratio defined as wbw−1

d (g g−1)Wf free water content ratio defined as wfw

−1d (g g−1)

Wfb freezable bound water content ratio defined aswfbw−1

d (g g−1)Wt total water content ratio defined as wtw

−1d (= Wf +

Wfb + Wb) (g g−1)

Greek symbols�P operational pressure (Pa)

(ostogt

Faa

Fba

a(

igTufit

u(

D

Irtcf

2

uTvs

� angle of X-ray diffraction (degree)�L viscosity of feed solution (Pa s)

w/w)% with methanol and then vacuum-filtered. Desired amountf acetic anhydride (97.0 wt%, Wako) was added to the chitosan

olution after vacuum filtration. A desired amount of cast solu-ion (5–15 g) was introduced into a petri dish with a diameterf 75 mm and then dried for 12 h at 333 K, and subsequentlyelled by immersion into a sufficient amount of 4% NaOH solu-ion. The resultant membrane was washed with distilled water,

ig. 1. Chemical structure of chitosan and N-acetyl-chitosan. Chitosan with an addedcetyl group as the acyl group becomes N-acetyl-chitosan, and exhibits propertiespproximately equal to those of chitin.

d

2

Cco

2

df(

w

ig. 2. Control of the DD in a chitosan membrane using a casting method in com-ination with N-acetylation (at 298 K). This figure was previously reported by theuthors [7].

nd a membrane was finally formed by placing it into hot water353 K).

Colloidal titration is a method for measuring free amino groupsn a chitosan solution, and the DD indicates molar percentages oflucosamine to the sum of glucosamine and acetylglucosamine.he DD of the chitosan membranes produced above were analyzedsing the colloidal titration method, with potassium polyvinyl sul-

ate solution (2.5 × 10−3N) as the titrant and toluidine blue as thendicator. The terminal point of titration was clearly confirmed byhe change of color from blue to claret.

The relationship between the quantity of acetic anhydride (A)sed in the preparation and the DD percentage of resultant chitosanFig. 2) [7] is given by

D = −0.1045A + 92.22 (1)

n the preparation of a membrane by the casting method, a DDange of over 50% is preferable to avoid solution gelation, even ifhe relationship is effective over the range of 0–100%. N-Acetyl-hitosans of DD 71.3%, 76.5%, 81.8%, 87.0%, and 92.2% were preparedor analysis.

.2. Scanning electron microscopy (SEM)

The morphology of the chitosan membranes was observedsing a scanning electron microscope (S-3500N, Hitachi High-echnologies Corporation, Tokyo) operated at an acceleratingoltage of 10 kV. As a pretreatment for clear observation of the innertructure, the chitosan membrane was vacuum freeze-dried. Theried membrane samples were then coated with zinc.

.3. X-ray diffraction

X-ray diffraction (XRD) patterns were recorded on a Rigaku RAD-diffractometer (monochromatic Cu K� radiation) with the X-ray

athode operated at 20 mA and 30 kV. The samples were supportedn a glass frame and were determined at 2� angles from 10◦ to 50◦.

.4. Water content

The total mass of water contained in the membrane (wt) was

efined to be the sum of three categories of water – free water (wf),reezable bound water (wfb), and bound water not able to freezewb) – as shown in the following equation:

t = wf + wfb + wb (2)

2 Engineering Journal 42 (2008) 20–27

TcaEpf

W

2

mEasatmr

c

w

Tf

W

2

fmTbwsEr

fp

FUma

aa

mtmfio

Fm

2 T. Takahashi et al. / Biochemical

he total water content ratio (Wt) was composed of the free waterontent ratio (Wf), the freezable bound water content ratio (Wfb),nd the bound (but not able to freeze) water content ratio (Wb).ach water content ratio was defined to be the ratio of water masser unit mass of the dried membrane (wd), as presented by theollowing equation:

t ≡ Wf + Wfb + Wb = wf + wfb + wb

wd(3)

.4.1. Measurement of total water contentGravimetric methods were used to determine Wt. The prepared

embranes had already achieved the equilibrium water content.xcess water attached on the membrane surface was removed usingfilter paper. The mass of the swollen samples (we) was then mea-

ured, before the wetted membrane was dried at 353 K for 24 h,nd the mass of dried membrane (wd) was measured. Experimen-al mean values of we and wd were determined by repeating the

easurements at least thrice for satisfactory reproducibility andeliability.

The difference between we and wd indicates the mass of totalontained water (wt):

t = we − wd (4)

he total water content ratio (Wt) was then calculated using theollowing equation:

t = we − wd

wd= wt

wd(5)

.4.2. Measurement of free water and freezing bound waterUsing differential scanning calorimetry (DSC) [8], Wf and Wfb

or the N-acetyl-chitosan membrane were determined. The DSCeasurement was conducted using a calorimeter (TAS300, Rigaku,

okyo) under a N2 flow. The sealed sample pan in the DSC cham-er was quickly frozen to 243 K using liquid N2, and several minutesere allowed for the system to reach thermostatic equilibrium. The

ample holder assembly was then heated at a rate of 5 K min−1.

ach calorimetric scan was measured with three repetitions foreproducibility and reliability, and the mean value was employed.

An endothermic peak at 273 K is regarded as the sum of theusion of free water (wf) and the freezable bound water (wfb), sincehase transition phenomena for chitin/chitosan are not reported

2

dt

ig. 4. SEM micrographs of N-acetyl-chitosan membrane cross-sections. (a) and (b) DD 7easure in (a), (c) and (e) indicates 10 �m. The full length of reference scaling measure in

ig. 3. Apparatus for water permeation experiment. (a) Ultrafiltration apparatusHP-62K (Advantec), (b) feed solution, (c) stirring bar, (d) sample membrane, (e)agnetic stirrer, (f) regulator valve, (g) N2 gas, (h) manometer, (i) electric balance,

nd (j) permeated solution.

t this temperature. Some bound water (wb) is not frozen at 243 K,nd is considered to have no effect in this thermometry.

The DSC measurement (Qendo) therefore indicates the endother-ic heat per unit mass of swollen membrane (we). Consequently,

he net endothermic heat per total mass of contained water inembrane is given by Qendo(we/wt). The mass ratio of the sum of

ree water and freezable bound water to the total contained watern membrane can be obtained by dividing Qendo(we/wt) by the heatf fusion of ice, Qf (333.9 J (g of (wf + wfb))−1) [8,9]:

Qendo(we/wt)Qf

= wf + wfb

wt= (Wf + Wfb)

wd

wt(6)

Thus, the sum of Wf and Wfb can be evaluated:

Qendo(we/wd)Qf

= Wf + Wfb (7)

.5. Permeability

The water permeability of the N-acetyl-chitosan membrane wasetermined from the water mass flux throughput using ultrafil-ration apparatus (Fig. 3, UHP-62K, Advantec, Tokyo). The initial

1.3%; (c) and (d) DD 81.8%; (e) and (f) DD 92.2%. The full length of reference scaling(b), (d) and (f) indicated 5 �m.

T. Takahashi et al. / Biochemical Engineering Journal 42 (2008) 20–27 23

Fig. 5. X-ray spectra of N-acetyl-chitosan membranes with different DD. (a) Originalpurchased reagent of chitin (DD 1.1%), (b) original purchased reagent of chitosan(ai

vsimicd

3

3

tTctoasd

a

F(

NmucH0Dca

3

bro2t

Fc

DD 92.2%), (c) DD 71.3%, (d) DD 81.8%, (e) DD 87.0%, and (f) DD 92.2%. The spectrare arbitrarily displaced to clarify the diffraction patterns. The intensity numberndicated on the ordinate was temporary, not corresponded to each illustrated data.

olume of water was 100 ml, and the apparatus was stirred con-tantly. The operational pressure was adapted and maintained byntroducing N2 gas at room temperature (298 K). The mass of per-

eated water throughput was accurately measured based on thendication of an electric balance (Mettler PJ3600, Zurich), and wasonverted to a volumetric water flux by recalculation using theensity of the permeated water (998 kg m−3).

. Results and discussion

.1. Structure of the N-acetyl-chitosan membrane

Fig. 4 presents SEM micrographs of cross-sections for three ofhe N-acetyl-chitosan membranes (DD of 71.3%, 81.8% and 92.2%).he membranes were structured with individual cells separated byellular walls. Assuming the cellular structure to be the key fac-or in determining water permeability, the mean inside diameterf the cells was 1.7 �m for a DD of 92.2%, 2.6 �m for DD 81.8%,

nd 4.7 �m for DD 71.3%. In other words, as the DD decreased theize of the cells increased, and the thickness of the cellular wallsecreased.

The mechanism of the cellular structure formation was the sames that reported for chitin gel by Hirano et al. [10–13]. Molecules of

mttaD

ig. 7. Effect of operational pressure on water flux (298 K, �L = 0.901 mPa s). (�) DD 92.2onvenient, expanded view of the lower level water flux for DD 87.0%, DD 81.8% and DD 7

ig. 6. Change of water composition in an N-acetyl-chitosan membrane with DD.©) Wt , (�) Wf + Wfb, and (�) Wb.

-acetyl-chitosan formed partial junction zones or regions whereolecular strands gathered closely together, due to the intermolec-

lar association of acetamide groups with the N-acetylation ofhitosan, leading to the formation of cellular walls or a honeycomb.irano et al. [10–13] have described the structure of chitin gel (DD%), but not the relationship between the cellular structure and theD. The junction zone and honeycomb structure were formed by aonformational conversion often observed in polysaccharides suchs carrageenan gel [14].

.2. Crystallinity of the N-acetyl-chitosan membrane

Fig. 5 presents the XRD patterns of four N-acetyl-chitosan mem-ranes (DD of 71.3%, 81.8%, 87.0%, and 92.2%) and the purchasedeagents (chitin (DD 1.1%), chitosan (DD 92.2%)). Specific peaksf chitin/chitosan were observed at diffraction angles of 19◦ and2◦, respectively, for all samples. In addition, broad intensity inhe range from 10◦ to 20◦ indicated that the crystallinity of the

embrane was amorphous. It is generally known that the crys-allinity of chitin is higher than that of chitosan. However, inhese membrane samples, no dominant intensity was observed,nd the microcrystal domain did not alter with change in theD.

%, (�) DD 87.0%, (�) DD 81.8%, and (�) DD 76.5%. The left of the figure represents a6.5%.

24 T. Takahashi et al. / Biochemical Engineering Journal 42 (2008) 20–27

F ectionf aged

3

cfwwTlDi

ifs

3

3

tsflpboar

tte

Fo

9ramc

3p

ogwoiSb

titi(

flonthe volume of cast solution placed in the petri dish before drying(described in Section 2.1); the other membrane preparing condi-

ig. 8. SEM micrographs of the N-acetyl-chitosan membrane (DD 76.5%). (a) Cross-sull length of reference scaling measure indicates 10 �m. (b) Detailed picture of dam

.3. Swelling of the N-acetyl-chitosan membrane

Fig. 6 presents the water composition of the prepared N-acetyl-hitosan membranes. The total water content ratio (Wt) was highestor the chitosan with lowest DD, and then dramatically decreasedith increasing DD. The sum of the free water and freezable boundater content ratios (Wf + Wfb) also decreased with increasing DD.

his sum dominated the total water content ratio for chitosans withower DD levels but this dominance also decreased with increasingD. Consequently, the bound water content ratio (Wb) gradually

ncreased with increasing DD.The cellular structure for chitosans of lower DD (Fig. 4(a and b))

s remarkable, particularly in regard to cellular size. Free water andreezing bound water are thus mainly contained inside the cellulartructure, resulting in a swelling of the chitosan membrane.

.4. Permeability of the N-acetyl-chitosan membrane

.4.1. Effect of operational pressureFig. 7 illustrates the effect of operational pressure on water flux

hrough the membranes. The water flux increased linearly at pres-ures less than 100 kPa. In contrast, at pressure over 100 kPa, theux was reduced for all membranes except that of highest DD. Fig. 8resents SEM photographs of one of the N-acetyl-chitosan mem-ranes (DD 76.5%) used in the permeation experiment at pressuresver 100 kPa. As can be clearly seen, the cellular structure was dam-ged, and a layer-like structure was formed by compaction, therebyeducing water permeation.

Fig. 9 depicts the effect of the DD on water flux and water con-ent. The water flux was calculated based on 50 kPa, thus avoidinghe impact of membrane compaction. The water flux increasedxponentially with the increase in the DD ranging from 80% to

ig. 9. Change in water flux (50 kPa, 298 K, �L = 0.901 mPa s) and total water contentf membrane with regulated DD. (�) Water flux, Jv and (©) total water content, Wt .

tcb

F9r

damaged due to the operational pressure during the permeation experiment. Thecross-section. The full length of reference scaling measure indicated 5 �m.

0%. In contrast, the water content decreased in the same DDegion. This result is opposite to the general trend of water perme-tion in hydrophilic membranes. As previously seen for celluloseembranes, a higher water flux usually results from higher water

ontent in the membranes.

.4.2. Effect of membrane thickness and viscosity of theermeating solution

Fig. 10 presents the effect of the permeating solution’s viscosityn the water flux of the N-acetyl-chitosan membrane. An aqueouslucose solution was employed as a feed solution and its viscosityas changed by altering the glucose concentration. Glucose is rec-gnized as an inert solute with respect to the membrane because its non-electrolyte character and hydrophilic low molecular weight.hrinking and/or swelling of membrane by glucose did not appeareforehand.

The flux of aqueous glucose solution was regarded as essen-ially equivalent to the flux of water with an inert solute. Byncorporating the density of the respective aqueous glucose solu-ions into the calculations, the resulting volumetric water fluxesncreased linearly with increase in the reciprocal of the viscosity�−1

L ).Fig. 11 presents the effect of membrane thickness on the water

ux of the N-acetyl-chitosan membrane. Increasing the reciprocalf the thickness (L−1

m ) linearly increased the water flux. The thick-ess of the membrane was determined only as a result of altering

ions, the drying temperature, the drying time and the washingondition were constant throughout the course of testing the mem-ranes.

ig. 10. Effect of the feed solution viscosity on the water flux (100 kPa, 298 K, DD2.2%). The ordinate Jv in this figure indicates the flux of an aqueous glucose solutionecalculated for use as the flux of water containing an inert solute.

T. Takahashi et al. / Biochemical Engineering Journal 42 (2008) 20–27 25

F0

3b

i

u

wdtw

J

wA

A

N

Sflo

J

Eotsto

R

Tmiviv

J

R

Ts

rpl

L

Su

J

Tttto(

fldttmi

3m

mto

R

wvtFimssn

ig. 11. Effect of membrane thickness on the water flux (50 kPa, 298 K, �L =.901 mPa s, DD 92.2%).

.4.3. Water permeation model and comparison with observedehavior of water flux of chitosan membrane

Typical flow rate of water, u (m s−1), in a single cylindrical pores determined using the Hagen–Poiseuille equation:

= �PD2p

32�LLp(8)

here �P is the operational pressure (Pa), Dp is the mean poreiameter (m), Lp is the pore length (m), and �L is the viscosity ofhe feed solution (Pa s). This can be adapted to obtain a volumetricater flux, Jv (m3 m−2 s−1):

V = uAp

Am= �PD2

p

32�LLp

Ap

Am(9)

here Am is the apparent surface area (m2) of the membrane andp is the total cross-sectional area (m2) of pores in the membrane.

p = n�D2

p

4(10)

p = n

Am(11)

ubstituting Eqs. (10) and (11) into Eq. (9), the volumetric waterux can be expressed in terms of the number of pores per unit areaf the membrane, Np:

V = �P�D4pNp

128�LLp(12)

q. (12) is conventionally employed to examine the water flux basedn the number of pores per unit area, Np. However, accurate, prac-ical determination of the number of pores is very difficult. In thistudy, a parameter Rt representing the volumetric ratio of total con-ained water in the membrane was introduced instead of the ratiof Ap/Am.

t = ApLm

AmLm(13)

he numerator of Rt corresponds to the total pore volume of theembrane, and is equivalent to the total volume of contained water

n the pores at equilibrium. The denominator of Rt is the entireolume of wetted membrane at equilibrium. Substituting Eq. (13)nto Eq. (9) therefore gives the volumetric water flux in terms of the

olumetric ratio of total contained water in the membrane:

V = �PD2pRt

32�LLp(14)

t ∝ Wt (15)

w

wuw

Fig. 12. Schematic illustration of water “classes” in the membrane.

his suggests that JV is linearly proportional to Rt. In a practicalense, Rt corresponds to Wt as a first approximation.

As a further complication, the pore length (Lp) is not yet accu-ately known and its determination, involving the tortuosity of theore, is very difficult. It is assumed here for simplicity that Lp is

inearly proportional to the membrane thickness, Lm.

p ∝ Lm (16)

ubstituting these additional approximations into Eq. (14), the vol-metric water flux becomes

V ∝ �PD2pWt

32�LLm(17)

his derivative equation shows the water flux to be proportionalo operational pressure �P, the reciprocal of viscosity (�−1

L ), andhe reciprocal of membrane thickness (L−1

m ), as a first approxima-ion. This agrees well with the observed data of the dependencef pressure (Fig. 7), viscosity (Fig. 10), and membrane thicknessFig. 11).

However, the correlation suggested by Eq. (17) between waterux Jv and the water content Wt is not consistent with the observedata. As presented in Fig. 9, a higher water content (Wt) in the chi-osan membrane resulted in a lower water flux. Therefore, in ordero understand the permeation mechanism in an N-acetyl-chitosan

embrane with a cellular structure, a further novel considerations necessary.

.5. Mechanism of water permeation in an N-acetyl-chitosanembrane with a cellular structure

To understand water permeation in a hydrophilic biopolymerembrane, two “classes” of water should be considered: water in

he channels, and the immobilized water inside cells. The volumef total contained water Rt is thus

t = R1 + R2 (18)

here R1 is volume of water in the channels, and R2 is theolume of water immobilized inside the cells. A schematic illus-ration of this for the N-acetyl-chitosan membranes is presented inig. 12. According to the experimental results, Rt decreased with anncrease in DD. This is because the total water content Wt in the

embrane decreased with an increase in DD, as shown in Fig. 6. R2hould correspond to the experimental data of (Wf + Wfb). R1 is notimply corresponded to Wb in our data, the Wb is minor compo-ent. The change of Wb could not explain the dramatical change of

ater flux. The detail of R1 should be investigated further.

For the membranes with a lower DD, the volumetric fraction ofater contained in cells (i.e., R2, is very large). As a result, the vol-metric fraction of water in the channels (R1), or in other wordsater permeating through the membrane, is very small. This is a

26 T. Takahashi et al. / Biochemical Engineering Journal 42 (2008) 20–27

F l-chitm for los the ju

ctt

bzgtma

Nsddtmwlmtw

oaodtv

J

TwDs

4

hbzww

mteihwgltp

dt(

ig. 13. Schematic illustration of the water permeation mechanism in an N-acetyembrane, (b) the structure of low DD chitosan membrane, and (c) detailed image

tructure illustrated in (b) is composed of immobilized water, the cellular wall and

onsequence of the larger size of the cellular structure, in combina-ion with the presence of junction zones, which limits the channelshat could form.

For the membranes with a higher DD, R2 should be lowerecause the size of the cells is smaller, the presence of junctionones is reduced, and the resultant cellular structure permits areater formation of channels. Thus, R1 should dominate Rt evenhough the total water content in the membrane is less than for

embranes of lower DD. Consequently, there is a greater perme-tion of water through membranes with a higher DD.

This novel model of the water permeation mechanism in an-acetyl-chitosan membrane with a cellular structure is shownchematically in Fig. 13. A chitosan membrane has a greater ten-ency to have a cellular structure containing free water with aecrease in DD. As seen from the enlarged cells in Fig. 4 (a and b),he cells in the chitosans of lower DD are not conducive to the for-

ation of water channels necessary for permeation. The availableater channels are decreased with the enlargement of the cellu-

ar structure, reducing the possible permeation. Even if the entireembrane structure has hydrophilic character, the cellular struc-

ure in combination with the junction zones practically preventsater permeation.

At a higher DD, water permeation proceeds due to degradationf the cellular structure as a result of fewer junction zones. Themount of water forcing channels is thus larger than for chitosansf lower DD. The water flux of the higher DD chitosan membraneepends on the channel constructing water content (R1) not onotal water content (Rt). Therefore experimental relationship for the

olumetric water flux, previously approximated in Eq. (8), becomes

V ∝ �PD2pR1

32�LLm(19)

nsitc

osan membrane with a cellular structure. (a) The structure of high DD chitosanw DD chitosan membrane with cellular structure and water channels. The cellularnction zone. The structure prevents water flux.

he role of R1 on the water flux will be similarly considered forater permeation in other hydrophilic biopolymer membranes.etailed consideration of Dp in cellular structured membranes

hould be a focus of future investigations.

. Conclusion

The effects of the DD of a chitosan membrane on structure,ydrophilic character, and permeability were discussed. The mem-rane body has an individual cellular structure involving junctionones, and the cellular size increases with a decrease in the DD. Freeater mainly contained inside the cellular structure of chitosansith a lower DD, and resulted in swelling of the membrane.

Water flux was dependent on the operational pressure, theembrane thickness, and the viscosity of feed solution, a situa-

ion that strongly suggests the aptness of the Hagen–Poiseuille flowquation. The water flux decreased exponentially with decreas-ng DD, despite the entire membrane structure having moreydrophilic character. The contrasting trend of water flux and totalater content of membrane with changing DD is opposite to the

enerally known behavior of hydrophilic membranes, e.g., cellu-ose membrane. It is found that the cellular structure incorporatinghe presence of junction zones in N-acetyl-chitosan membranesrevents ready permeation of water.

The special trend of water flux through the chitosan membraneepended on the channel constructing water content (R1) not onhe total water content (Rt) in the membrane. Immobilized waterR2), which corresponds to the experimental data of (Wf + Wfb), did

ot contribute to the formation of water permeation channels, ando is unrelated to the flux. The concept of the water flux depend-ng on the hydrophilic cellular structure is similarly applicableo hydrophilic membranes composed of hydro-gels (i.e., polysac-haride gels). Future quantitative evaluation of the relationship

Engine

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A

Asfom

R

[

[

[

T. Takahashi et al. / Biochemical

etween cellular structure and permeability in hydrophilic mem-ranes will be necessary for their application in industrial fields.

cknowledgements

The authors sincerely thank Professor Katsuto Otake and Dr.tsushi Shono of the Tokyo University of Science for useful discus-ions. Dr. Tao Kei of the Nihon University provided technical adviceor the DSC measurements and SEM operation. Dr. Tsutomu Imaif the Ueda Lime Co., Ltd. offered references for the XRD measure-ents.

eferences

[1] H.-C. Flemming, G. Schaule, Biofouling on membranes—a microbiological

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