effect of membrane preparation conditions on solute permeability in chitosan membrane

Effect of Membrane Preparation Conditions on SolutePermeability in Chitosan Membrane

HIDETO MATSUYAMA, HIDETOSHI SHIRAISHI, YOSHIRO KITAMURA

Department of Environmental Chemistry and Materials, Okayama University,2-1-1 Tsushima-naka, Okayama 700-8530, Japan

Received 15 July 1998; accepted 31 January 1999

ABSTRACT: Chitosan membrane was prepared in various conditions and diffusive per-meabilities of theophylline and vitamin B12 were investigated. The membrane prepa-ration procedure consists of dissolving chitosan in acid solution, cast on the glass plate,drying the dope solution, and immersion of the plate in the gelating agent. Effects of thekinds of acids to dissolve chitosan, chitosan concentration, drying time of the dopesolution, and the kinds of the gelating agent on the membrane structure and perfor-mance were studied in detail. With increasing the chitosan concentration, the solutepermeability decreased, while the selectivity of theophylline to vitamin B12 increased.The membrane changed from the wholly porous structure to the asymmetric structureby the increase of the chitosan concentration. Furthermore, the use of ethanol as thegelating agent brought about the wholly porous structure with the high permeabilityand the low selectivity. The asymmetric structure and the wholly dense structure wereobtained in the cases of the gelating agents, such as aqueous NaOH solution anddimethyl sulfoxide (DMSO), respectively. Thus, the membrane structure can be con-trolled by the kinds of gelating agent. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73:2715–2725, 1999

Key words: chitosan membrane; membrane preparation condition; asymmetricstructure; theophylline; vitamin B12

INTRODUCTION

Chitosan (b-(1-4)-2-amino-2-deoxy-D-glucose), de-rived from chitin by deacetylation, is a naturalpolycationic polymer, that possesses valuableproperties as a biomaterial. This unique basicpolysaccharide is attractive as biopolymer formany biomedical applications. In addition, thebiocompatibility and biodegradability of chitosanhave been well established.1,2

The film-forming property of chitosan offersmany applications in various membrane separa-tion fields. A number of studies have been carried

out on pervaporation separation of water–alcoholmixtures.3–8 Mochizuki et al. reported that thepermselectivity of chitosan membrane increasedand reached 450 in the separation of water andethanol with increasing the degree of the neutral-ization of chitosan with H2SO4.5 Lee and Shininvestigated the pervaporation performance ofnovel phosphorylated chitosan membranes to sep-arate water from aqueous ethanol solution.7

Chemical modification of the chitosan membranecontributed to an improved pervaporation perfor-mance.

In addition, application to ultrafiltration9,10

and diffusive permeation11,12 have been reported.Beppu et al. prepared the chitosan membrane forultrafiltration by the phase inversion method.9

The effects on various organic solvents used as

Correspondence to: H. Matsuyama.Journal of Applied Polymer Science, Vol. 73, 2715–2725 (1999)© 1999 John Wiley & Sons, Inc. CCC 0021-8995/99/132715-11

2715

the gelating agents on the membrane perfor-mance were investigated in detail. Nakatsukaand Andrady studied the diffusive permeation ofvitamin B12 in the crosslinked chitosan mem-brane.11 The equilibrium swelling ratios, diffu-sion coefficients, and partition coefficients weremeasured; and they concluded that the transportmechanism in chitosan–vitamin B12 system waspredominantly of the pore mechanism.

In the previous works on separations by thesechitosan membranes, the membranes were usu-ally prepared in the fixed procedure except forchanging the degree of crosslinking. Thus, therelations between the membrane preparationconditions, and the membrane structure and per-formance have not yet been investigated system-atically. The purpose of this work is to clarify howeach step in the membrane preparation procedureinfluences the membrane characteristics. As themembrane performance, the diffusive permeabili-ties of theophylline and vitamin B12 and the se-lectivity of theophylline to vitamin B12 were stud-ied. The permeabilities and selectivities obtainedwill be discussed with relation to the membranepreparation conditions, such as kinds of acid inthe dope solution, chitosan concentration, dryingtime after casting, and kinds of gelating agents.

EXPERIMENTAL

Materials

Chitosan was kindly supplied by Katokichi Co.,Ltd. (Japan). Its deacetylation degree was about100 mol %. The solutes used for the transportexperiment were theophylline (MW, 180; Stokesradius, 3.7 Å13) and vitamin B12 (MW, 1355;Stokes radius, 8.4 Å13). These were purchasedfrom Nakalai Tesque Inc.

Membrane Preparation

Chitosan solution was prepared by dissolving chi-tosan in aqueous acid solution at ambient temper-ature with stirring overnight. The solution wasallowed to stand for about half a day to removethe air bubbles. Several types of acids, such asacetic acid, nitric acid, formic acid, and hydrochlo-ric acid, were used. The concentration of chitosanin acid solution was changed as variable. Thedope solution was then cast onto a glass platewith desired thickness (1000 mm) and placed in adrying air oven (usually 60°C). The drying time

was also changed as variable. The dry film wasimmersed in several gelating agents, such as 1Naqueous sodium hydroxide, ethanol, and dimethylsulfoxide (DMSO) for 3 h. The chitosan mem-brane was repeatedly washed with water andstored in water until use.

Characterization of Chitosan Membrane

Infrared (IR) spectra of freeze-dried chitosanmembranes were measured with a Shimadzu IR-460 spectrometer. For scanning electron micros-copy (SEM) analysis, the freeze-dried membranewas sputtered with Au–Pd in vacuum. The struc-tures of both air-facing and glass-facing surfaceswere observed by a scanning electron microscope(Hitachi Co., Ltd., S-2150) under an acceleratingvoltage of 15 kV.

The water content of the membrane was calcu-lated from the following equation by using theweight of membrane swollen and equilibrated inthe buffer solution (Ws) and the dry membraneweight (Wd):

Water content 5 ~Ws 2 Wd!/Ws (1)

The water content was evaluated as the weightfraction in this work instead of the volume frac-tion.

Permeation Experiments

The procedure and the apparatus used in thepermeation experiments were the same as de-scribed in previous work.14 The diffusion cell con-sisted of two cylindrical half cells of volume of 20cm3 made of Pyrex glass. The chitosan membranewas sandwiched between two cells. The mem-brane area was 5.3 cm2. The solutions in the twocells were stirred by magnetic stirring bars at 250rpm. The diffusion cell was placed in a water bathmaintained at 25°C.

The feed solution was prepared by dissolvingthe solute in a phosphate buffered saline solution(0.0027M KCl; 0.137M NaCl; pH 5 7.4). Soluteconcentration was 5 g/dm3 for theophylline and0.5 g/dm3 for vitamin B12. The receiving solutionwas the buffer solution alone. Samples (1 cm3) ofthe receiving solution were taken at various timeintervals, and solute concentrations were ana-lyzed by spectrophotometer (Hitachi Co., Ltd.,U-2000; wavelength, 271 nm for theophylline; 361nm for vitamin B12). After taking out a sample of1 cm3, 1 cm3 of the fresh buffer solution was

2716 MATSUYAMA, SHIRAISHI, AND KITAMURA

always added to the receiving solution to main-tain the constant volume. The change in concen-tration due to the addition of fresh buffer solutionwas taken into account in the calculation of thesolute amounts transported into the receiving so-lution.

Total overall resistance for the solute transportconsists of mass transfer resistances in the stag-nant layers of feed and receiving phases and theresistance of the membrane phase. In our previ-ous study,14 it was confirmed that even in a mem-brane with the higher permeability than the high-est permeability obtained in this work, the resis-tance of the membrane phase was predominant.Therefore, the flux N is given by

N 5 ~V/A!~dCb1/dt!

5 ~P/L!~Cb2 2 Cb1! (2)

where V is the receiving solution volume, A is themembrane area, and Cb2 and Cb1 are the bulkconcentrations in feed and receiving solutions,respectively. The membrane permeability P wascalculated from eq. (2) by using the membranethickness L measured by a micrometer (Mitsu-toyo Co., Ltd., MDC-25M)

RESULTS AND DISCUSSION

Effect of Kinds of Acids to Dissolve Chitosan

The membrane thickness, water content and the-ophylline permeability are summarized in Table Ifor various acid cases. In the case of acetic acid,remarkably higher membrane thickness and the-ophylline permeability were obtained, comparedwith those in other acid cases. The gel membrane

with high water content can increase its thick-ness remarkably due to the increase of the mem-brane swelling brought about by the looser mem-brane structure. Therefore, it is not surprisingthat the thickness of the membrane prepared us-ing chitosan solution dissolved with acetic acidwas about three times higher than that dissolvedwith other acids. The water content is also thehighest in the acetic acid case, although the dif-ference is not so distinguished. These experimen-tal results suggests that the looser membranestructure is formed in the case of acetic acid.

To confirm the above expectation, IR spectrawere measured for all membranes. The result isshown in Figure 1. Bands at 3455 and 665 cm21

are reported to be related to crystallization of

Figure 1 IR spectra of chitosan membranes. Bands of(a) and (b) are related to the crystallization of chitosan.

Table I Membrane Thickness, Water Content, and TheophillinePermeability for Various Acid Cases

Acid

MembraneThickness

(mm)

WaterContent(wt %)

TheophyllinePermeability

(cm2/s)

Acetic acid 0.0645 52 10.6 3 1027

Nitric acid 0.0175 45 3.79 3 1027

Formic acid 0.0155 49 3.59 3 1027

Hydrochloric acid 0.0208 46 4.31 3 1027

Chitosan concentration: 2 wt %. Acid concentration: 10 wt %. Drying time: 20 h. Gelating agent:1N NaOH solution.

SOLUTE PERMEABILITY IN CHITOSAN MEMBRANE 2717

chitosan.15,16 The band intensities at about 3455and 665 cm21 were clearly smaller in the case ofacetic acid than those in the cases of hydrochloricacid and nitric acid. This lower crystallinity prob-ably leads to the higher permeability and watercontent shown in Table I. Because acetic acid isbulky compared with other acids, the aggregationof chitosan in the gelating process may be inhib-ited by the existence of acetic acid near chitosanand may result in the lower crystallinity. Thechitosan membrane prepared with formic acidhas the low band intensities at about 3455 and665 cm21. However, the resultant membrane per-meability is not high, as shown in Table I. Thereason for this result is not yet clear.

Effect of Acetic Acid Concentration

Figure 2 shows the effect of acetic acid concentra-tion on theophylline permeability and the mem-brane thickness. The dotted lines in this figureshow the critical amount of acetic acid necessaryfor carboxyl groups of acetic acid to correspond toamino groups of chitosan in the ratio of 1 : 1.When the acetic acid concentration was lowerthan this critical value, the dope solution becameopaque and, therefore, had insoluble chitosan. Inthe high concentration region, the acetic acid con-centration hardly influenced the permeability andthe membrane thickness.

Effect of Drying Time

Figures 3 and 4 shows the effects of the dryingtime on the permeability and the selectivity andthe water content and the membrane thickness,respectively. The selectivity was defined as theratio of theophylline permeability to vitamin B12permeability. In all cases, theophylline with thesmaller Stokes radius showed the higher perme-ability than vitamin B12. Thus, all selectivitiesobtained were more than unity. In the shorterdrying time, the permeabilities were higher withthe lower selectivity. Between 1.5 and 2.0 h dry-ing time, the membrane performances drasticallychanged. The obtained lower permeabilities andthe higher selectivity means that the dense mem-brane structure was formed. For the drying timemore than 2.0 h, it hardly influenced the mem-brane performances. As can be seen in Figure 4,both the water content and the membrane thick-ness decreased with the drying time, and after thedrastic decrease between 1.5 and 2.0 h, thesevalues became nearly constant. From these exper-

imental results, the membrane formation is pre-sumed as follows. In the shorter drying time, themembrane dope solution contains still largeamount of water, which inhibits the aggregationof chitosan and formation of crystallization in thegelation process. This leads to the looser and po-rous membrane structure. Consequently, thehigher permeabilities, the lower selectivity, andthe higher water content and membrane thick-ness were obtained. Between 1.5 and 2.0 h dryingtime, evaporation of water is assumed to benearly completed because the membrane perfor-mances changed drastically. Even for the longerdrying time, the similar dense and nonporousstructure will be formed. This is probably thereason that the nearly constant membrane per-formances were obtained after 2.0 h drying time.

Figure 5 shows the membrane structures of theair-facing and the glass-facing surfaces in various

Figure 2 Effect of acetic acid concentration on the-ophylline permeability and the membrane thickness.Chitosan concentration: 2 wt %. Drying time: 20 h.Gelating agent: 1N NaOH solution. Dotted lines showthe critical amount of acetic acid necessary for carboxylgroups of acetic acid to correspond to amino groups ofchitosan in the ratio of 1 : 1.


drying times. It should be noted that the magni-fication of SEM observation are different in eachcases. When the drying times are 0 and 1.0 h,porous structures were formed at both surfaces.On the other hand, dense and nonporous struc-tures were obtained in the cases of the dryingtimes of 5.0 and 20 h. These observations are inagreement with our expectation in the membraneformation described above.

Effect of Chitosan Concentration

The effects of chitosan concentration on the per-meability and the selectivity, and the water con-tent and the membrane thickness are shown inFigures 6 and 7, respectively. Two kinds of exper-iments, such as the drying times of 1.5 and 20 h,were carried out. The experimental results werequite different in two cases. When the drying timeis 20 h, water in the membrane dope solution isevaporated nearly completely, which probably

leads to the similar dense structures regardless ofthe chitosan concentrations. Therefore, nearlyconstant permeability, selectivity, and water con-tent were obtained, as shown in Figures 6 and 7,while the membrane thickness increased monot-onously with the increase of the chitosan concen-tration because the membrane thickness is di-rectly related to the chitosan concentration, evenin the case of the similar membrane structure. Onthe other hand, when the drying time is 1.5 h, thepermeability, the water content, and the mem-brane thickness decreased accompanying the in-crease of the selectivity as the chitosan concen-tration increased. It should be noted that the ten-dency of the change in the membrane thickness,in this case, is opposite to that in the drying timeof 20 h. Because all of water in the dope solutioncannot be evaporated in the drying of 1.5 h, thehigher the initial chitosan concentration, thehigher the chitosan concentration after the dryingbecomes. The higher chitosan concentration in

Figure 4 Effect of drying time on the water contentand the membrane thickness. Membrane preparationconditions are the same as those described in the cap-tion of Figure 3.

Figure 3 Effect of drying time on the permeabilityand the selectivity. Dope solution: 2 wt % chitosan in 10wt % acetic acid solution. Gelating agent: 1N NaOHsolution.


Figure 5 Membrane structures of the air-facing and the glass-facing surfaces invarious drying times.


dope solution leads to easy aggregation of eachchitosan molecule in the gelation process and theresultant denser structure. This is the reasonthat the higher selectivity, lower permeability,and lower membrane thickness were obtained inthe membrane prepared by the chitosan solutionwith the higher initial concentration.

Figure 8 shows the membrane surface struc-tures in various cases with the different chitosanconcentrations. When the concentration is 2.0 wt%, both surfaces had the porous structures. Thestructure at the air-facing surface approached tothe dense structure, but some pores are still ob-served in the case of 3.0 wt % concentration.When the concentration increased to 4.0 wt %, theair-facing surface had the dense homogeneousstructure, while the glass-facing surface had theporous structure. This means that the clear asym-metric structure was formed in this case. Sincewater evaporates only the air-facing surface ofthe dope solution in this membrane preparation

condition, the chitosan concentration gradient,that is, the higher concentration at the air-facingsurface and the lower concentration at the glass-facing surface, may be formed in certain specialcases. The high and low chitosan concentrationlead to the dense and porous structures, respec-tively. Thus, the asymmetric structure is proba-bly attributable to such the chitosan concentra-tion gradient in the membrane dope solution. Theasymmetric structure is usually favorable be-cause the higher permeability can be obtainedwith keeping the high selectivity. Although anattempt to produce the asymmetric structure forpoly(vinyl alcohol) membrane had already beendone,13 the studies on the asymmetric structureof chitosan membrane were hardly reported. Itshould be noted that the asymmetric structurecan be formed in chitosan membrane by the sim-ple procedure in this work.

Figure 6 Effects of chitosan concentration on perme-ability and the selectivity. Dope solution: chitosan in 10wt % acetic acid solution. Drying time: 20 h. Gelatingagent: 1N NaOH solution.

Figure 7 Effect of chitosan concentration on the wa-ter content and the membrane thickness. Membranepreparation conditions are the same as those describedin the caption of Figure 6.


Effect of Kinds of Gelating Agents

Effects of kinds of gelating agents were investi-gated, and the obtained results are summarizedin Table II. Aqueous NaOH solution, ethanol, andDMSO were used as the gelating agents. Thepermeabilities, water contents, and membranethicknesses decreased in the order of ethanol,

NaOH solution, and DMSO. In the case of etha-nol, it is expected based on the extremely lowselectivity (2.3) that the porous structure areformed at both surfaces. When NaOH solutionwas used, the high selectivity was obtained aswell as the relatively high permeabilities. Thus,the formation of the asymmetric structure is ex-

Figure 8 Membrane surface structures in various cases with the different chitosanconcentrations. Dope solution: chitosan in 10 wt % acetic acid solution. Drying time:1.5 h. Gelating agent: 1N NaOH solution.


pected in this case. The lower permeabilities andthe lower membrane thickness in the case ofDMSO suggests that the dense nonporous struc-ture forms at both surfaces.

Figure 9 shows the membrane surface struc-tures when various solutions were used as thegelating agents. As it is expected above, whollyporous, asymmetric, and wholly dense structureswere confirmed from these SEM observationswhen ethanol, NaOH solution, and DMSO wereused, respectively. The asymmetric structure inthe case of NaOH solution is probably due to thechitosan concentration gradient in the dope solu-tion formed in the drying process. When othersolvents were used, such concentration profilesare also formed because the drying process is thesame regardless of the gelating agents. However,if the diffusion of ethanol into the dope solution isvery fast, the chitosan concentrations at both sur-faces similarly become very low. In this situation,the wholly porous structures are formed, asshown in Figure 9(a). Certainly, in the prepara-tion of reverse osmosis membrane from celluloseacetate, it was reported that when the diffusion ofthe gelating agent into the membrane dope solu-tion is faster than that of solvent in the dopesolution into the gelating solution, the loose struc-ture is formed; whereas the dense structure isformed in the opposite situation.10,17 Contrary tothe above, if the diffusion of DMSO into the dopesolution is very slow, the chitosan concentrationsat both surfaces become high because of the fastleakage of water in the dope solution. This leadsto the wholly dense structure, as shown in Figure9(c). The diffusion rate is related to the thermo-dynamic properties, such as the chemical poten-tial difference, as well as the kinetic properties,such as the diffusion coefficient. Further investi-gation is necessary to estimate the diffusion ratesof water and gelating agents in this phase inver-sion process.

CONCLUSION

1. The kinds of acids to dissolve chitosan inthe membrane dope solution influenced themembrane performance. The membraneprepared with acetic acid showed the high-est theophylline permeability. From IRanalysis, the membrane was found to havethe low crystallinity.

2. With an increase in the drying time of thedope solution, the permeability, the watercontent, and the membrane thickness de-creased, while the selectivity increased. Ina drying time of more than 2.0 h, it hardlyinfluenced the membrane structure andperformance. The SEM observation re-vealed the wholly porous structure in theshorter drying time and the wholly densestructure in the longer drying time.

3. The effect of chitosan concentration wasinvestigated in two drying times, such as20 and 1.5 h. In the case of the drying timeof 20 h, the chitosan concentration did notinfluence the membrane performance. Onthe other hand, the increase of the concen-tration in the drying time of 1.5 h broughtabout the decrease of the permeability andthe increase of the selectivity. The mem-brane structure changed from the whollyporous to the asymmetric with the increaseof the chitosan concentration.

4. The kinds of the gelating agents greatlyinfluenced the resultant membrane struc-ture and performance. The use of ethanolleaded to the wholly porous structure withthe high permeability and low selectivity.Asymmetric and wholly dense structurewere obtained in the cases of NaOH solutionand DMSO, respectively. This indicates thatthe membrane structure can be controlled bythe gelating agent.

Table II Permeabilities, Selectivity, Water Content, and Membrane Thickness for Various Cases ofGelating Agents

Permeability(Theophylline)

(cm2/s)

Permeability(Vitamin B12)

(cm2/s)Selectivity

(2)

WaterContent(wt %)

MembraneThickness

(mm)

Ethanol 3.9 3 1026 1.7 3 1026 2.3 86 0.3301N NaOH 1.7 3 1026 9.1 3 1028 18.7 66 0.105DMSO 7.2 3 1027 5.3 3 1028 13.6 50 0.046

Dope solution: 2 wt % chitosan in 10 wt % acetic acid solution. Drying time: 1 h at 80°C.


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Figure 9 Membrane surface structures when various solutions were used as thegelating agents. Dope solution: 2 wt % chitosan in 10 wt % acetic acid solution. Dryingtime: 1 h at 80°C.


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effect of membrane preparation conditions on solute permeability in chitosan membrane

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