water vapor adsorption isotherms of agar-based nanocomposite films

5
N: Nanoscale Food Science Water Vapor Adsorption Isotherms of Agar-Based Nanocomposite Films Jong-Whan Rhim Abstract: Adsorption isotherms of agar and agar/clay nanocomposite films prepared with different types of nanoclays, that is, a natural montmorillonite (Cloisite Na + ) and 2 organically modified montmorillonites (Cloisite 30B and Cloisite 20A), were determined at 3 different temperatures (10, 25, and 40 C). The water vapor adsorption behavior of the nanocomposite films was found to be greatly influenced with the type of clay. The Guggenheim–Anderson–de Boer (GAB) isotherm model parameters were estimated by using both polynomial regression and nonlinear regression methods and it was found that the GAB model fitted adequately for describing experimental adsorption isotherm data for the film samples. The monolayer moisture content (m o ) of the film samples was also greatly affected by the type of nanoclay used, that is, m o of nanocomposite films was significantly lower than that of the neat agar film. Nanocomposite films prepared with hydrophobic nanoclays (Cloisite 30B and Cloisite 20A) exhibited lower m o values than those prepared with hydrophilic nanoclay (Cloisite Na + ). Keywords: adsorption, agar, clay, isotherm nanocomposite film Introduction Bionanocomposites have been emerging for the development of new packaging materials with properties that are superior to those of not only the parent materials but also of conventional micro- composites (Pandey and others 2005; Sinha Ray and Bousmina 2005; Rhim and Ng 2007; Pavlidou and Papaspyrides 2008; Boredes and others 2009). Bionanocomposites are hybrid ma- terials of biopolymer with inorganic fillers which have at least one dimension in the nanometer scale. The inorganic fillers gener- ally used for such purpose are layered aluminosilicates, and most commonly montmorillonite (MMT). It is generally known that nanoscale dispersion of the filler phase in the polymer matrix leads to property enhancements such as decreased permeability to gases (O 2 , CO 2 , and water vapor) and liquids, better resistance to solvents, increased thermal stability, and improved mechanical properties (Pavlidou and Papaspyrides 2008; Zhao and others 2008; Arora and Padua 2009). Various natural biopolymers including carbohydrates such as cellulose (Park and others 2004), starch (Avella and others 2005), chitosan (Rhim and others 2006), and agar (Rhim and others 2011), and proteins such as soy protein (Kumar and others 2010), gelatin (Rao 2007), wheat gluten (Tunc and others 2007; Mauricio-Iglesias and others 2010), and whey protein (Hedenqvist and others 2006; Sothornvit and others 2010) have been tested to improve one property or another, with varying degrees of success. Since most of natural biopolymers are hydrophilic in nature, the resulting biopolymer-based nanocomposite packaging mate- rials are expected to readily absorb moisture when placed at high humidity environments or come in contact with high moisture foods. Generally, the absorbed moisture affects the physical and MS 20110784 Submitted 6/29/2011, Accepted 7/29/2011. Author is with Dept. of Food Engineering, Mokpo National Univ., 61 Dorimri, Chungkyemyon, Muangun, Jeonnam 534-729, Republic of Korea. Direct inquiries to author Rhim (E-mail: [email protected]). mechanical properties of the packaging materials, which leads to a substantial degradation in performance above certain level of moisture content (Rhim and Lee 2009). The water vapor barrier properties of hydrophilic bioploymer-based films are significantly influenced by the presence of moisture. It is therefore important to understand the water sorption properties of the biopolymer-based films, such as how the moisture content varies under different environmental conditions (that is, relative humidity and tempera- ture), which is usually represented as the water sorption isotherm. The water vapor sorption isotherms provide information on film performance under different RH conditions. The water vapor sorption properties of various biopolymer films have been eval- uated by determining the water vapor sorption isotherms (Peng and others 2007; Carvalho and others 2008; Zhang and Han 2008; uller and others 2009, 2011; Perdomo and others 2009; Fabra and others 2010). The incorporation of nanoclays in biopolymer films is expected to affect the equilibrium moisture content (EMC)/RH relation- ships due to the change in the overall hydrophilicity of the polymer matrix as a consequence of the presence of hydrophobic organic modifier of nanoclays and the potential interactions between poly- mer matrix and nanoclays that can influence the active sites for water vapor adsorption. However, only a few works on water vapor adsorption isotherms for biopolymer-based nanocomposite films are available in the literature (Zeppa and others 2009; M¨ uller and others 2011). The objective of this study was therefore to test the mathematical fit to the water vapor adsorption behavior of agar- based nanocomposite films using the Guggenheim–Anderson–de Boer (GAB) isotherm model. Materials and Methods Materials Food grade agar was obtained from Fine Agar Agar Co., Ltd. (Damyang, Jeonnam, Korea). A natural MMT (Cloisite Na + ) and 2 organoclays (Cloisite 20A and Cloisite 30B) were purchased C 2011 Institute of Food Technologists R N68 Journal of Food Science Vol. 76, Nr. 8, 2011 doi: 10.1111/j.1750-3841.2011.02378.x Further reproduction without permission is prohibited

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N:NanoscaleFoodScience

Water Vapor Adsorption Isotherms of Agar-BasedNanocomposite FilmsJong-Whan Rhim

Abstract: Adsorption isotherms of agar and agar/clay nanocomposite films prepared with different types of nanoclays,that is, a natural montmorillonite (Cloisite Na+) and 2 organically modified montmorillonites (Cloisite 30B and Cloisite20A), were determined at 3 different temperatures (10, 25, and 40 ◦C). The water vapor adsorption behavior of thenanocomposite films was found to be greatly influenced with the type of clay. The Guggenheim–Anderson–de Boer(GAB) isotherm model parameters were estimated by using both polynomial regression and nonlinear regression methodsand it was found that the GAB model fitted adequately for describing experimental adsorption isotherm data for thefilm samples. The monolayer moisture content (mo) of the film samples was also greatly affected by the type of nanoclayused, that is, mo of nanocomposite films was significantly lower than that of the neat agar film. Nanocomposite filmsprepared with hydrophobic nanoclays (Cloisite 30B and Cloisite 20A) exhibited lower mo values than those prepared withhydrophilic nanoclay (Cloisite Na+).

Keywords: adsorption, agar, clay, isotherm nanocomposite film

IntroductionBionanocomposites have been emerging for the development of

new packaging materials with properties that are superior to thoseof not only the parent materials but also of conventional micro-composites (Pandey and others 2005; Sinha Ray and Bousmina2005; Rhim and Ng 2007; Pavlidou and Papaspyrides 2008;Boredes and others 2009). Bionanocomposites are hybrid ma-terials of biopolymer with inorganic fillers which have at least onedimension in the nanometer scale. The inorganic fillers gener-ally used for such purpose are layered aluminosilicates, and mostcommonly montmorillonite (MMT).

It is generally known that nanoscale dispersion of the fillerphase in the polymer matrix leads to property enhancements suchas decreased permeability to gases (O2, CO2, and water vapor) andliquids, better resistance to solvents, increased thermal stability, andimproved mechanical properties (Pavlidou and Papaspyrides 2008;Zhao and others 2008; Arora and Padua 2009). Various naturalbiopolymers including carbohydrates such as cellulose (Park andothers 2004), starch (Avella and others 2005), chitosan (Rhim andothers 2006), and agar (Rhim and others 2011), and proteins suchas soy protein (Kumar and others 2010), gelatin (Rao 2007), wheatgluten (Tunc and others 2007; Mauricio-Iglesias and others 2010),and whey protein (Hedenqvist and others 2006; Sothornvit andothers 2010) have been tested to improve one property or another,with varying degrees of success.

Since most of natural biopolymers are hydrophilic in nature,the resulting biopolymer-based nanocomposite packaging mate-rials are expected to readily absorb moisture when placed at highhumidity environments or come in contact with high moisturefoods. Generally, the absorbed moisture affects the physical and

MS 20110784 Submitted 6/29/2011, Accepted 7/29/2011. Author is with Dept.of Food Engineering, Mokpo National Univ., 61 Dorimri, Chungkyemyon, Muangun,Jeonnam 534-729, Republic of Korea. Direct inquiries to author Rhim (E-mail:[email protected]).

mechanical properties of the packaging materials, which leads toa substantial degradation in performance above certain level ofmoisture content (Rhim and Lee 2009). The water vapor barrierproperties of hydrophilic bioploymer-based films are significantlyinfluenced by the presence of moisture. It is therefore important tounderstand the water sorption properties of the biopolymer-basedfilms, such as how the moisture content varies under differentenvironmental conditions (that is, relative humidity and tempera-ture), which is usually represented as the water sorption isotherm.The water vapor sorption isotherms provide information on filmperformance under different RH conditions. The water vaporsorption properties of various biopolymer films have been eval-uated by determining the water vapor sorption isotherms (Pengand others 2007; Carvalho and others 2008; Zhang and Han 2008;Muller and others 2009, 2011; Perdomo and others 2009; Fabraand others 2010).

The incorporation of nanoclays in biopolymer films is expectedto affect the equilibrium moisture content (EMC)/RH relation-ships due to the change in the overall hydrophilicity of the polymermatrix as a consequence of the presence of hydrophobic organicmodifier of nanoclays and the potential interactions between poly-mer matrix and nanoclays that can influence the active sites forwater vapor adsorption. However, only a few works on watervapor adsorption isotherms for biopolymer-based nanocompositefilms are available in the literature (Zeppa and others 2009; Mullerand others 2011).

The objective of this study was therefore to test themathematical fit to the water vapor adsorption behavior of agar-based nanocomposite films using the Guggenheim–Anderson–deBoer (GAB) isotherm model.

Materials and Methods

MaterialsFood grade agar was obtained from Fine Agar Agar Co., Ltd.

(Damyang, Jeonnam, Korea). A natural MMT (Cloisite Na+) and2 organoclays (Cloisite 20A and Cloisite 30B) were purchased

C© 2011 Institute of Food Technologists R©N68 Journal of Food Science � Vol. 76, Nr. 8, 2011 doi: 10.1111/j.1750-3841.2011.02378.x

Further reproduction without permission is prohibited

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Isotherms of agar-based nanocomposite films . . .

from Southern Clay Co. (Gonzales, Tex., U.S.A.). The substi-tuted cations of Cloisite 20A and Cloisite 30B are known to bedimethyl di(hydrogenated tallowalkyl) quaternary ammonium andbis-(2-hydroxyethyl)methyl (hydrogenated tallowalkyl) quaternaryammonium, respectively. Glycerol was purchased from DaejungChemicals & Metals Co., Ltd. (Siheung, Gyonggido, Korea). Nineanalytical reagent grade salts (LiCl, CH3COOK, MgCl2, K2CO3,Mg(NO3)2, KI, NaCl, KCl, and KNO3) were used for the prepa-ration of saturated salt solutions.

Preparation of filmsAgar and agar-based nanocomposite films were prepared using

a solvent casting method used by Rhim and others (2011). Fourgrams of agar powder was dissolved in 150 mL of distilled waterwith 2 g of glycerol as plasticizer while mixing vigorously for 30min at 95 ◦C using a magnetic stirrer and cast evenly onto a leveledTeflon film (Cole-Parmer Instrument Co., Chicago, Ill., U.S.A.)coated glass plate (24 × 30 cm), and then dried for about 24 hat room temperature. Agar nanocomposite films with 3 differenttypes of nanoclays (Cloisite Na+, Cloisite 30B, and Cloisite 20A)with contents of 5 wt% (5 part clay per 100 part agar) were preparedusing a solution intercalation method (Rhim and others 2011). Tohelp intercalation of clays, a series of pretreatments were applied tonanoclays. First, precisely weighed nanoclays were dispersed intodistilled water (150 mL) and stirred using a magnetic stirrer for 24h. The fully hydrated nanoclay solutions were homogenized usinga high shear mixer (T25 basic, Ika Labotechnik, Janke & KunkelGmbh & Co., KG, Staufen, Germany) at 20500 rpm for 10 minfollowed by sonication for 10 min using a high intensity ultrasonicprocessor (Model VCX 750, Sonics & Materials Inc., Newtown,Conn., U.S.A.) with a medium size tip (2 cm dia). Then, 4 g ofagar with 2 g of glycerol was dissolved into the nanoclay solutionfollowed the same film preparation method as described for thepreparation of neat agar films.

Test films were preconditioned in a constant temperature hu-midity chamber set at 25 ◦C and 50% RH for at least 48 h.

Sorption isothermsThe water vapor adsorption isotherms for the agar and

agar/clay nanocomposite film samples were determined using thestatic gravimetric method. Nine saturated salt solutions (LiCl,CH3COOK, MgCl2, K2CO3, Mg(NO3)2, KI, NaCl, KCl, andKNO3) were used to maintain constant water activities rangingfrom 0.11 to 0.96 at 3 temperatures, 10, 25, and 40 ◦C (Bell andLabuza 2000). Agar and agar/clay nanocomposite films were cutinto the size of 2.54 cm × 5 cm (about 0.08 to 0.10 g) by usinga precision double blade cutter (model LB.02/A, Metrotec, S.A.,San Sebastian, Spain) and then were dried in a vacuum dryer at60 ◦C and 720 mmHg vacuum for 24 h and kept in a desiccatorsover calcium sulfate (DrieriteTM, Sigma-Aldrich, St. Louis, Mo.,U.S.A.) before the test. In each of 9 hygrostats (sealed glassbottle), the film samples were placed at different constant wateractivities established using the saturated salt solutions, and EMCwas measured after 20 d at a constant temperature (Rhim and Lee2009). Three replications were made at each water activity, andaverage values of moisture content were used for constructing theisotherm curves. The moisture content of each sample was basedon the dry basis (g water/g dry solids), which was determinedafter each adsorption experiment by using the oven dryingmethod at 105 ◦C for 24 h drying. Weight of film samples wasdetermined using a digital balance (MC1 Analytic 210S, SatoriusAG, Gottingen, Germany) with an accuracy of 0.1 mg.

Analysis of isotherm dataThe GAB model was used to fit the water vapor adsorption data

of the agar and agar/clay nanocomposite film samples. The GABmodel contains 3 parameters as follows:

m = mo Ckaw

(1 − kaw) (1 − kaw + Ckaw), (1)

where m is the EMC (on dry weight basis) at water activity, aw; mo isthe moisture content of monolayer corresponding to formation ofa monomolecular layer on the internal surface of the film samples;C is the Guggenheim constant; and k is a factor correspondingproperties of the multilayer molecules with respect to the bulkliquid.

The GAB model parameters (mo, C, and k) were estimatedby both a polynomial regression method (Bell and Labuza 2000;Rhim and Lee 2009; Fabra and others 2010) and a nonlinearregression method (Zhang and Han 2008; Muller and others 2009,2011). For the application of the polynomial regression method,the GAB equation (Eq. 1) was transformed to a quadratic form:

aw/m = α a 2w + β aw + γ, (2)

where α = k/mo (1/C – 1); β = 1/ mo (1 − 2/C); γ = 1/(moCk).And the values of α, β, and γ were determined by least-squaresregression analysis (Bell and Labuza 2000).

The parameters of the GAB model were also estimated by anonlinear regression method employing the Marquardt–Levenbergalgorithm using the Solver function of Excel R© (Billo 2007).

The goodness of fit of the GAB model to the experimentalEMC and water activity data was evaluated based on the coefficientof determination (r2) and the mean relative percent errors (MRPE)for the polynomial regression method, and by the MRPE for thenonlinear regression method,

MRPE = 100n

n∑

i=1

∣∣mi − m pi

∣∣mi

, (3)

where mi is the experimental moisture content; mpi is the predictedmoisture content; and n is the number of experimental points. TheMRPE has been widely adopted through the literature to evaluatethe goodness of fit of sorption models, and an MRPE value lessthan 10% indicates a good fit for practical purposes (Zeppa andothers 2009).

Results and Discussion

Adsorption isothermsAdsorption isotherms for the agar and agar/clay nanocomposite

film samples were obtained by plotting the EMC of the film sam-ples against aw for 3 different temperatures as shown in Figure 1.The adsorption isotherms of all the film samples exhibit typicalsigmoid-shaped curves which belong to Type II isotherm as clas-sified by Brunauer (Bell and Labuza 2000), which have been fre-quently observed with hydrophilic biopolymer-based films (Maliand others 2005; Muller and others 2009, 2011; Perdomo andothers 2009; Zeppa and others 2009). In all the film samples,the EMC increased slowly with increasing aw up to 0.7, beyondwhich a steep rise in the EMC in film samples was observed prob-ably due to condensation of water in capillaries and pores of thefilm matrix as well as to probable solubilization of water vapor inthe polymer matrix. This behavior is characteristic of hydrophilic

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Isotherms of agar-based nanocomposite films . . .

biopolymer-based films, which has been frequently observed withdifferent biopolymer films (Zhang and Han 2008; Muller andothers 2009, 2011; Perdomo and others 2009; Zeppa and others2009; Chowdhury and Das 2010; Fabra and others 2010). Figure 1also shows a considerable deviation between the predicted values ofthe GAB model and the experimental values especially at aw > 0.9.Kristo and Biliaderis (2006) explained that the observed deviationis a consequence of unreliable accuracy of moisture determina-tion at high aw, because the values obtained could be affected byosmotic, swelling, and capillary phenomena.

The EMC of the film samples increased with the rise in aw whentemperature was kept constant, and decreased with the increase intemperature at constant aw. This may be attributed to a changein the total number of active sites for water binding caused bytemperature-induced physical changes in the film (Moreira andothers 2008). The mobility of water molecules also increased atincreased temperatures caused them to become less stable and tobreak away from water binding sites of the sorbed films, thusdecreased the EMC of the films at the same aw (Rhim and Lee2009).

The EMC of the film samples varied in the range of 0.14 to 1.54,0.10 to 1.46, 0.07 to 1.77, and 0.06 to 1.55 g water/g solid for theagar, agar/Cloisite Na+, agar/Cloisite 30B, and agar/Cloisite 20Ananocomposite films, respectively, depending on aw and temper-ature. Clearly, the EMC of nanocomposite films was lower thanthat of the neat agar films at the same aw and temperature con-dition, and those of nanocomposite films decreased in the orderof agar/Cloisite Na+, agar/Cloisite 30B, and agar/Cloisite 20Ananocomposite films. As reported works on starch/clay nanocom-

posites (Huang and others 2004; Dean and others 2007; Mullerand others 2011), the hydroxyl group of the polymer matrix (agar)and plasticizer (glycerol) could interact directly with the clay ionsor with the hydroxyl edge groups of clay thus reducing the poten-tial for the hydroxyl interaction with water molecules, which mayresult in the lower EMC of the agar/clay nanocomposite films.Rhim and others (2011) also reported that nanocomposite filmsprepared with more hydrophobic nanoclays such as Cloisite 30Band Cloisite 20A were more water resistant than that preparedwith hydrophilic nanoclay (Cloisite Na+). This explains why theEMC values of agar/Cloisite 30B and 20A nanocomposite filmswere lower than that of agar/Cloisite Na+ nanocomposite film.

GAB parametersFigure 1 also shows that the GAB model fitted well to the

experimental isotherm data of the film samples. A typical testresult for the neat agar film samples plotted using the 2nd-order polynomial regression method is shown in Figure 2. Theother experimental isotherm data for the agar/clay nanocom-posite film samples were also analyzed using the same method(data not shown). The 2nd-order polynomial regression modelfitted well to the experimental isotherm data with high r2 values(> 0.96). In addition, the nonlinear regression analysis was per-formed for the same experimental data for comparison. The GABparameters, mo, k, and C, estimated by the two analysis meth-ods are listed in Table 1 and 2, respectively, together with thecorresponding goodness of fit. Since for each film the regressionwas done with the same number of data points, the MRPE canserve as a comparative measure of the goodness of fit (Moreira

Water activity

0.0 0.2 0.4 0.6 0.8 1.0

EM

C (

g w

ater

/g d

ry m

atte

r)

0.0

0.2

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10oC25oC40oC

A agar film

Water activity

0.0 0.2 0.4 0.6 0.8 1.0

EM

C (

g w

ater

/g d

ry m

atte

r)

0.0

0.2

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0.6

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10oC25oC40oC

B agar/Cloisite Na+ nanocomposite film

Water activity

0.0 0.2 0.4 0.6 0.8 1.0

EM

C (

g w

ater

/g d

ry m

atte

r)

0.0

0.2

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Cagar/Cloisite 30B nanocomposite film

Water activity

0.0 0.2 0.4 0.6 0.8 1.0

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C (

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ater

/g d

ry m

atte

r)

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Dagar/Cloisite 20A nanocomposite film

Figure 1–Experimental and predicted (from the GAB model) EMC and water activity relationship for the agar and agar/clay nanocomposite films atdifferent temperatures.

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and others 2008). Practically, a good fit of an isotherm is as-sumed when the MRPE value is less than 10% (Zeppa and others2009). The results obtained by both analysis methods showedhigh coefficient of determination (r2 > 0.96) and low MRPE(< 9%) indicating that the GAB model fits well to the experimen-tal isotherm data for the agar and agar/clay nanocomposite films.

The results on the goodness of fit also indicate that the poly-nomial regression method exhibits, in general, a slightly better fitthan the nonlinear regression method. It is probably due to that itis sometimes hard to convergeto a unique solution caused by ex-perimental errors when applying the nonlinear regression method.Schuchmann and others (1990) reported that unique solutions arenot always guaranteed in nonlinear regressions, especially when3 or more parameters are involved in the model function likethe GAB model. For example, the Guggenheim constant, C, forthe agar film at 40 ◦C estimated by the method was extraordi-narily high indicating some unexpected errors involved with theexperimental data. In order to avoid such problems encounteredwhen applying the nonlinear regression method, Schuchmann andothers (1990) recommended repeating the regression procedurewith several different initial values of the parameters and differentupper and lower limits on their magnitude, until stable and repro-ducible values are reached. Except some points, most C values ofagar and agar/clay nanocomposite films were in the range of usual

Table 1– The GAB model constants (mo, k, and C) for agar andagar/clay nanocomposite films at different temperatures esti-mated by using the polynomial regression method.

GAB constants

Film Temp. (◦C) moa k C r2 MRPE (%)

Agar 10 0.15 0.94 68.79 0.98 3.5025 0.13 0.91 103.13 0.98 3.3740 0.12 0.97 1170.34 0.96 4.53

Agar/Cloisite Na+ 10 0.14 0.95 46.35 0.98 4.5925 0.12 0.940 76.06 0.99 2.8140 0.12 0.96 20.05 0.97 3.61

Agar/Cloisite 30B 10 0.09 0.99 14.52 0.97 5.4525 0.09 0.94 14.51 0.96 3.1240 0.09 0.97 17.45 0.97 3.70

Agar/Cloisite 20A 10 0.11 0.98 16.96 0.98 5.6725 0.09 0.96 54.36 0.96 6.1940 0.10 0.96 12.29 0.97 3.16

aunit: g water/g solid.

aw

0.0 0.2 0.4 0.6 0.8 1.0

a w/ m

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

10oC25oC40oC

Figure 2–A typical plot of moisture adsorption for the agar films at differenttemperatures analyzed by the polynomial regression method.

biomaterials, in which ranges 4 and 40 (Blahovec 2004). On thecontrary, the ranged 4 and 40 values of the k parameter (a factorrelated to interaction energies between multilayer molecules withrespect to bulk liquid) are within the reported limits for food andbiopolymer products, where the value of k varied from 0.7 to 1(Bell and Labuza 2000; Labuza and Altunakar 2007).

The monolayer moisture content (mo), the minimum mois-ture content covering hydrophilic sites on the film surface, ofthe agar film samples was in the range of 0.09 to 0.16 gwater/g solid depending on temperature and type of film sam-ples. The mo values of agar and agar/clay nanocomposite filmsare comparable to those of other biopolymer-based films such aspotato starch films (0.218 g water/g solid at 25 ◦C; Talja andothers 2007); corn starch film (0.164 g water/g solid at 25 ◦C;Chowdhury and Das 2010); gelatin films (0.1252, 0.1218, and0.1170 g water/g solid at 15, 25, and 35 ◦C, respectively; Carvalhoand others 2008); corn zein, wheat gluten, and wheat gluten/soyprotein isolate composite films (0.1241, 0.1052, and 0.1386 gwater/g solid, respectively; Gennadios and Weller, 1994); soy pro-tein films (0.085 g water/g solid at 25 ◦C; Cho and Rhee 2002);and cassava starch/Na+-bentonite nanocomposite films (0.085 and0.087 g water/g solid; Muller and others 2011). The higher mo

values of agar and agar/clay nanocomposite films compared withcassava starch and cassava starch/Na+-bentonite nanocompositefilms are presumably due to the high water holding capacity ofagar polymer. Rhim and others (2011) demonstrated that the wa-ter holding capacity of agar-based films was about 300%.

Generally, the mo of all the film samples determined by boththe polynomial regression method (Table 1) and the nonlinearregression method (Table 2) decreased with the increase in tem-perature. Such a decrease in mo with temperature has also beenobserved in the isotherm tests with other biopolymer films such ascellulose and its derivative films (Velazquez de la Cruz and others2001), cassava starch films (Perdomo and others 2009), corn starchfilms (Peng and others 2007), and gelatin-based films (Carvalhoand others 2008). This may also be attributed to the fact that thebinding of water molecules become less stable at increased tem-perature and break away from water binding sites of the film, thusdecreasing the mo values.

The results also exhibit clearly that the mo values of agar filmsdecreased significantly after formation of nanocomposite withnanoclays and the degree of reduction in the mo values wasaffected by the type of clay used. It may be mainly due to thedecrease in the number of active sites for water binding in the

Table 2–The GAB model constants (mo, k, C) for agar and agar/claynanocomposite films at different temperatures estimated by using thenonliner regression method.

GAB constants

Film Temp. (◦C) moa k C MRPE (%)

Agar 10 0.16 0.93 37.66 3.8325 0.14 0.89 65.02 3.0440 0.12 0.97 996767 8.98

Agar/Cloisite Na+ 10 0.16 0.93 20.50 5.6425 0.12 0.92 40.73 3.1640 0.12 0.94 17.42 3.92

Agar/Cloisite 30B 10 0.09 0.99 14.78 4.7225 0.10 0.94 12.05 3.0240 0.10 0.95 11.76 4.14

Agar/Cloisite 20A 10 0.12 0.96 8.77 5.8625 0.10 0.93 25.18 7.0940 0.10 0.94 9.56 3.23

a unit: g water/g solid.

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Isotherms of agar-based nanocomposite films . . .

polymer matrix which was preoccupied by the nanoclays, and alsodue to the hydrohpobicity of clay materials. Actually, the morehydrophobic nanoclays (Cloisite 30B and Cloisite 20A) resulted inthe more reduction in the mo values than the hydrophilic nanoclay(Cloisite Na+). Muller and others (2011) also reported that themo values of starch/clay nanocomposite films decreased comparedwith neat starch films. They explained the lower mo values ofstarch/clay nanocomposite films was due to lower affinity forwater than the neat starch films as indicated with s reducedsolubility coefficient of the composite films. They also explainedthat a reduction in the solubility coefficient of the composite filmsseems to be caused by the reduction of available hydroxyl groups,as a consequence of interactions between the nanoclay and thepolymer matrix (Zeppa and others 2009; Muller and others2011). Among the clay types tested, the organically modifiednanoclays (Cloisite 30B and Cloisiet 20A) exhibited lower mo

values than the natural MMT (Cloisite Na+). It is not only dueto the difference in hydrophilic/hydrophobic properties of clays,but also due to the number of water binding sites of the clay. Theinteraction of nanoclays with water molecules through ion–dipoleinteractions may help to retain moisture in the nanocompositefilms (Dean and others 2007). Dean and others (2007) postulatedthat binding water molecules into the matrix may be related tothe cation exchange capacity (CEC) of the clay. The CEC ofCloisite Na+ is 110 meq/100 g, whereas the CEC of Cloisite30B and Cloisite 20A are 90 and 95 meq/100 g, respectively(Hong and Rhim 2008). Accordingly water adsorption and waterresistance properties of agar/clay nanocomposite films may bechanged by the type of clays. Rhim and others (2011) reportedthat the water vapor permeability and water resistance propertiessuch as water solubility, water contact angle, swelling ratio, andwater vapor uptake ratio of agar-based nanocomposite films werevaried depending on the type of nanoclays.

ConclusionsMoisture adsorption behavior of agar-based films was greatly

influenced by the formation of nanocomposite with nanoclay, andit was also affected by both temperature and the type of nanoclaystested. The GAB isotherm model parameters (mo, k, and C) for thewater vapor adsorption of agar and agar/clay nanocomposite filmswere estimated adequately by using both the polynomial regres-sion and nonlinear regression methods. The simple polynomialregression method was found to be comparable to or better thanthe nonlinear regression method to estimate the GAB param-eters. The monolayer moisture content (mo) of nanocompositefilms was significantly lower than that of neat agar film. Amongthe nanoclays tested, more hydrophobic nanoclays (Cloisite 30Band Cloisite 20A) exhibited lower mo values than the hydrophilicnanoclay (Cloisite Na+). Consequently, bionanocomposite filmswith desirable properties can be formed and their water vapor ad-sorption behavior can be adjusted by proper choice of nanoclays.

AcknowledgmentSupport from the Mokpo National Univ. (the Research Grant

2009) is acknowledged.

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