ISSN 0023�1584, Kinetics and Catalysis, 2010, Vol. 51, No. 5, pp. 754–761. © Pleiades Publishing, Ltd., 2010.

754

1 Silica gels are widely used in dehumidification pro�cesses because their good moisture sorption capacities[1] which mainly depend on its larger surface area andpore distribution. It is well known that the pore struc�ture and surface chemistry of silica gels also playimportant roles in the sorption of water vapor besidestheir developed surface area.

The sorption performance of silica gels wasimproved by different methods such as irradiation byneutron flux [2], manipulation of preparation param�eters [3], and modification by plasma treatment [4].Another way to increase moisture sorption capacity isthe impregnation of silica gels with hygroscopic salts[5–11] such as CaCl2, LiBr, LiCl, Ca(NO3)2, andMgSO4, which can adsorb water vapor as the solidcrystalline and/or salt solution. All of these methodsare good for the improvement of sorption performanceof adsorbents through improving the pore volumeand/or surface chemistry. However, the effects of dif�ferent salts impregnation and porous structure of thehost matrix on the water sorption properties of initialand modified silica gels have not been investigated sys�tematically.

In this paper, the sorption dynamics and desorp�tion activation energy of water vapor on modified sil�ica gels were investigated. The object of study was tosearch for the influencing factors that can greatly

1 The article is published in the original.

improve the sorption capacity, sorption rates and theactivation energy of water vapor desorption on the sil�ica gel in order to develop highly effective dehumidifi�cation materials with lower energy consumed in theregeneration process for industrial and residentialapplications.

EXPERIMENTAL

Materials

The three commercial silica gels used in this studywere purchased from Qingdao Haiyang Chemical Co.,Ltd and Spegial Silica Gel Factory (Qingdao, China).Their particle sizes ranged from 3 to 4 mm. They willbe referred in the text as Asg, Bsg, and Csg, respectively(Table 1).

Dynamics and Isotherms of Water Vapor Sorptionon Mesoporous Silica Gels Modified by Different Salts1

Li Xina,b, Li Huilinga, Huo Siqia, and Li Zhongb

aInstitute of Biomaterial, College of Science, South China Agricultural University, Guangzhou, ChinabThe Guangdong Provincial Laboratory of Green Chemical Technology, School of Chemistry and Chemical Engineering,

South China University of Technology, Guangzhou, Chinae�mail: cezhli@scut.edu.cn

Received October 9, 2009

Abstract—The mesoporous silica gels impregnated with different metal salts were prepared and studied. Thepore structure and specific surface area of adsorbents were evaluated using nitrogen adsorption. Then, thesorption isotherms and dynamics of water vapor were carried out at 303 K and different relative humidity(RH). The temperature programmed desorption experiments were conducted to estimate the activationenergy (Ed) of water desorption on the silica gels. The results showed that the sorption capacity for waterdecreased with the increase of the ionic radius (except the calcium ion) and that CaCl2 and LiCl were partic�ularly suitable for use in modification of the mesoporous silica gel to improve their sorption rates and capac�ities for water vapor at the lower and medium RH (RH < 80%). The larger the average pore diameter and porevolume of the initial silica gels, higher the accrual rates of the water vapor sorption rate and capacity were aftermodification with hygroscopic salts. The activation energy of the water desorption on the mesoporous silicagel modified by CaCl2 were much higher than that on the silica gel modified by LiCl, because the polarizabil�ity of the Ca2+ was higher than that of Li+.

DOI: 10.1134/S0023158410050186

Table 1. Porous structure parameters of silica gels

Silica gel

BET surface area, m2/g

Volumeof pores,

cm3/g

Average pore diameter, nm

Saltcontent,

wt %

Asg 690.5 0.36 2.0 –

Bsg 591.4 0.82 5.3 –

Csg 349.2 0.96 10.7 –

Li/Csg 242.5 0.84 13.8 3.9

Ca/Csg 212.1 0.84 15.3 9.5

KINETICS AND CATALYSIS Vol. 51 No. 5 2010

DYNAMICS AND ISOTHERMS OF WATER VAPOR SORPTION 755

Prior to the synthesis of the composites, silica gelswere dried at 413 K in a vacuum for 4 h. A 10 g of hostsilica gel was immersed and kept in the 20 ml of 1Msolutions of different salts for 8 h. Then the sampleswere taken out of the solution and laid at 140°C inoven for 4 h. At last, the dry composite adsorbents wereplaced in a vacuum desiccators for use. The compositeadsorbents of Csg treated by 1 M solutions of CaCl2,ZnCl2, MgCl2, LiCl, KCl, or NaCl will be referred inthe text as Ca/Csg, Zn/Csg, Mg/Csg, Li/Csg, K/Csg, andNa/Csg, respectively, and Asg or Bsg treated by 1 M solu�tion of CaCl2 will also be referred as Ca/Asg or Ca/Bsg.

Methods

Sorption of nitrogen. The specific surface area, thepore volume, and the average pore diameter of thesamples were measured by nitrogen sorption at the liq�uid nitrogen temperature 77 K in a “Micromeritics”gas sorption analyzer ASAP 2010. The silica gel sam�ple was degassed at 573 K for 3 h in the vacuum priorto nitrogen sorption measurements. The surface areawas calculated from the sorption isotherms using thestandard Brunauer–Emmett–Teller (BET) equation.The pore size distributions were determined usingDensity Functional Theory (DFT) based on statisticalmechanics. The average pore diameter Dp = 4Vp/SBET(assuming a cylindrical shape of pores) was calculatedfrom the BET surface area SBET and pore volume Vp.

Sorption of water. The adsorption isotherms andkinetic curves of the water vapour on the silica gels at303 K were separately determined by means of thehomemade apparatus with the function of keepingconstant temperature and humidity (Fig. 1). Theapparatus consisted of an adiabatic sorption chamberand the temperature and humidity controlling system.A microelectronic balance was located within theadsorption chamber. Its accuracy was of 0.0001 g. The

temperature and humidity of the sorption chambercan be adjusted and maintained constant by using thecycle of gas whose temperature and humidity can beadjusted. Relative humidity (RH) values were con�trolled by means of a dehumidifier and a humidifier,with accuracy of ±3%, and temperature can be con�trolled with accuracy of ±0.5°C.

Determination of Adsorption Kinetic Curveof the Water Vapor on the Silica Gels

Adsorption kinetic experiments were performed atatmospheric pressure by using the experimental appa�ratus shown in Fig. 1. Firstly, 2 g of fresh silica gel wasintroduced in airtight flask with a cover, and then theflask was placed on the microelectronic balancelocated in the sorption chamber. Secondly, after thetemperature and RH within the sorption chamber wasmaintained onto some designed values by means oftemperature and humidity controllers, the flask wasopened and thus the adsorption experiment was start,i.e., the weight of the sample silica gel would be peri�odically recorded with the help of the electronicmicro�balance as the adsorption occurred, and aadsorption kinetic curve giving the sorbed amounts ofthe water vapor on the silica gel as a function of timewas measured (Fig. 2). The adsorption kinetic experi�ment ended when the weight of the sample got unvar�ied, meaning that equilibrium had been achieved (thesilica gel was saturated with the water vapor). Theadsorption kinetic curves were carried out at differentrelative humidity and on different silica gels.

�� ��

ControllerSamplingaperture

Electronicbalance

Box Electronicheater

Dehumidifier

Vacuum pump

HumidifierElectrovalve

Fig. 1. Dynamic adsorption apparatus.

40003000200010000Time, min

0.20

0.16

0.12

0.08

0.04

0

−0.04

1

2

3

4

5

6

7

Sorption of water, g/g

Fig. 2. The sorption kinetics of water vapor on Csg andmodified silica gels at 303 K and RH = 30%: 1—Ca/Csg,2—Li/Csg, 3—Mg/Csg, 4—K/Csg, 5—Zn/Csg, 6—Csg,7—Na/Csg.

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LI XIN et al.

Determination of Isotherm of the Water Vaporon the Silica Gels

Adsorption equilibrium experiments were carriedout at atmospheric pressure in the similar way statedabove. Firstly, 2 g of fresh silica gel was introduced inairtight flask with a cover, and then the flask was placedon electronic microbalance located in the sorptionchamber. Secondly, after the temperature and relativehumidity within the sorption chamber was maintainedonto some designed values by means of temperatureand humidity controllers, the flask was opened usinginternal mechanism and thus the adsorption of thewater vapor on the silica gel began. The weight of thesample increased uninterruptedly as the adsorptioncarried out. When the time of the adsorption was longenough (generally, about 200 h), the weight of thesample hardly varied, which indicated the sample sil�ica gel had been saturated with the water vapor, andhence the equilibrium sorption amount of the watervapor on the unit silica gel corresponding to some rel�ative humidity was obtained. After finishing of adsorp�tion equilibrium experiments under the condition ofdifferent RH, the corresponding equilibrium data ofthe water vapor adsorption on the unit silica gel can beobtained, and thus the plot of the equilibriumadsorbed amounts of the water vapor on silica gel ver�sus corresponding relative humidity yielded an iso�therm of the water vapor adsorption on the silica gel.

Thermal analysis. Thermal analysis was carried outusing by NETZSCH STA 449C Simultaneous Ther�mal Analyzer. The heating rate was 20 K/min in air ornitrogen atmosphere with 25 ml/min flow rate and thetemperature ranged from 303 to 523 K.

The temperature programmed desorption (TPD)experiments. The TPD experiments were conducted atdifferent heating rates from 4 to 8 K/min. In eachexperiment, the sample that had saturated with thewater vapour was packed in a stainless reaction tubewhose inner diameter was 0.3 cm and whose packedlength was 0.5 cm. Subsequently, the stainless tube wasplaced in a reaction furnace and then heated in thehigh�purity N2 flow at a constant rate of 46.9 ml/min.The desorbed water vapour was measured by using theGC�9501 gas chromatograph with a thermal conduc�tivity detector at the outlet of the stainless reactiontube, and effluent curves were recorded, which werecalled the TPD curves. According to the experimentalTPD curves, the activation energy for desorption ofwater from three kinds of silica gels was estimated.

RESULTS AND DISCUSSION

Textural Properties of Silica Gels

The structure parameters of the silica gels derivedfrom the basis of the nitrogen sorption data are sum�marized in Table 1. The table clearly indicated that Asg,Bsg, and Csg were all mesoporous silica gel. Addition�ally, surface modification with CaCl2 and LiCl

increased the average pore diameter and decreasedsignificantly the pore volume. This was because theformation of the uniform salt layer on the silica surfaceprobably resulted, on the one hand, in the completefilling of the small pores, and enlarging of the averagepore diameter and, on the other hand, in partial fillingand, hence, narrowing the large pores. It seemed thatthe first effect hold dominant position, and thisresulted in an increase in the average pore size [10].

The Effect of Different Impregnating Saltson the Hygroscopic Properties of Silica Gels

Figure 2 showed the sorption kinetics of watervapor on Csg and modified samples at 303 K and RH =30%. It is seen that the hygroscopic abilities of the sil�ica gels modified by CaCl2, LiCl, MgCl2, and ZnCl2were improved. The sorption capacity of the silica gelmodified by CaCl2 was the highest among the modi�fied mesoporous silica gels and was three times higherthan that of the unmodified silica gel. But the hygro�scopic abilities of the silica gel modified by KCl andNaCl decreased slightly. However, their sorption ratesand sorbed amounts were both lower compared withthe reported for CaCl2/silica gel [9] and LiBr/silica gel[6], due to the lower salt content in our work. Also oth�ers modified samples were compared with those in the[10, 11], and the similar results were obtained. Fromthese results, it is seen that the hygroscopic abilities ofCaCl2 and LiCl are much stronger than those of oth�ers. Therefore, CaCl2 and LiCl particularly suitable foruse in modification of the mesoporous silica gels toimprove their sorption capacity.

The effect of ionic radius on the water sorption at303 K and RH = 30% is shown in Fig. 3. A tendencycan be seen that the sorption capacity decreased withthe increase of the ionic radius except the calcium ion.This was because that the acting forces between waterand the ion with smaller ionic radius were strongerthan those between water and the ion with larger ionicradius. But for the calcium ion, it was different fromthe other ions because the CaCl2 loaded in the pas�sageway of the silica gel more easily formed the CaCl2 ·6H2O complex, and Ca/Csg had larger average porediameter and salt content.

The Effect of CaCl2 and LiCl on Sorption Isothermsand Dynamics of Water Vapor Sorption

The kinetics of water vapor sorption on modifiedsilica gels at 303 K and RH = 10, 30, 45, and 65% areshown in Figs. 4, 2, 5, and 6, respectively. The sorptioncapacity and the sorption rate on Csg were bothimproved greatly through modification by CaCl2 andLiCl, which were even larger than those on Asg or Bsgwith smaller pore diameter. Especially, the sorptioncapacity of Csg modified by CaCl2 at different RH hadbeen up to or exceeded that of Asg and its sorption ratewas larger than that of Asg significantly. So CaCl2 and

KINETICS AND CATALYSIS Vol. 51 No. 5 2010

DYNAMICS AND ISOTHERMS OF WATER VAPOR SORPTION 757

LiCl are good modification agents for improving ofsorption capacity and sorption rate of mesoporous sil�ica gel at RH < 80%.

Termogravimetric (TG) curves of Li/Csg andCa/Csg saturated with moisture at 303 K and RH =30% are shown in Fig. 7. The weight loss of Li/Csg andCa/Csg is equal to 15.99 and 12.70%, respectively.Then, the sorption capacity for water on the Li/Csgand Ca/Csg at 303 K and RH = 30% calculated fromthe weight loss was 19.03 and 14.94%, respectively,

which were very close to the experimental values 18.69and 14.55%. This indicated that the Li/Csg and Ca/Csgsaturated with moisture can be regenerated completelyat 413 K for 4 h.

Figure 8 shows the water vapour sorption isothermson Csg, Li/Csg, and Ca/Csg at 303 K. The sorptioncapacities of Li/Csg and Ca/Csg are larger than those ofCsg when the RH is below 80%. These results also indi�cated that the hygroscopic abilities of CaCl2 and LiClwere great and Li/Csg and Ca/Csg can be used at thelower and medium RH as dehumidification materialswith better sorption capacity for water vapor.

The Effect of CaCl2 and LiCl on the Activation Energies of Water Vapor Desorption

TPD is a technique of surface analysis [12]. It isusually used to estimate binding energy between theadsorbate and the adsorbent, and desorption activa�tion energy [13–16], which can be used to choose theadsorbents and estimate adsorption isotherms [15].Basically, some kind of adsorbate is adsorbed onto theadsorbent, and then the adsorbent is heated in condi�tions of an inert gas flowing when the adsorbent tem�perature varies linearly with time. As the temperaturebecomes high enough, the adsorbate will desorb grad�ually. In a TPD experiment, a gas chromatograph(or a mass spectrometer) is used to measure the rate atwhich the adsorbate desorbs from the adsorbent.Using the TPD, one can know how strongly moleculesare bound to the surface of the adsorbent. The TPDspectrum was a plot of desorption the rate of the adsor�

0.140.120.100.080.06Ionic radius, nm

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

Saturated sorption of water, g/g

Li+

Mg2+

Zn2+

Na+

Ca2+

K+

Fig. 3. The effect of ionic radius on the saturated sorptionwater at 303 K and RH = 30%.

120001000080006000400020000Time, min

0.12

0.10

0.08

0.06

0.04

0.02

Sorption of water, g/g

1

2

3

45

Fig. 4. The kinetics of water vapor sorption on the initial and modified silica gels at 303 K and RH = 10%: 1—Ca/Csg, 2—Li/Csg,3—Asg, 4—Bsg, 5—Csg.

758

KINETICS AND CATALYSIS Vol. 51 No. 5 2010

LI XIN et al.

bate as a function of the sample temperature [13–16].Desorption from a surface in a TPD experiment isusually described by Polanyi–Wigner equation:

(1)

The desorption reaction is assumed to follow firstorder kinetics, the activation energy for desorption

θθ= − = −

Ad A A/0 exp( ).ndr k E T

dt

can be calculated by equation

, (2)

where n is the order of the desorption reaction, θ—thefractional surface coverage, TP—the peak desorptiontemperature, βH—the heating rate, Ed—the activa�tion energy for desorption, R—gas constant, and K0—a constant that depends on the desorption kinetics.

TPD spectra of water desorption from the Li/Csg,Ca/Csg, and Csg at different heating rates are shown inFigs. 9–11. It can be seen that there is an obvious peakin each TPD spectrum due to desorption of the watervapor from the silica gels. A gradual increase in theheating rate βH led to an increase in the peak temper�ature TP. TP can be obtained from the TPD spectradepicted in Figs. 9–11. Knowing the values of TP fordifferent heating rates, it was possible to employEq. (2) to estimate the activation energy of water des�orption from the silica gels studied. Figure 12 depictedthe corresponding linear dependencies of 1/TP versus

ln(βH/ ) for these silica gels. From the slop of theselines, activation energy Ed can be found out, and thenk0 can be also obtained from the intercept of these lineswith y axis. The calculation results of the activationenergy of water desorption from Li/Csg, Ca/Csg, andCsg are listed in Table 2. The activation energies ofwater desorption from Li/Csg, Ca/Csg, and Csg were36.4, 43.1, and 26.2 kJ/mol, respectively. It means that

d dH

pp2

0

ln lnE ERT KRT

⎛ ⎞ ⎛ ⎞ ⎛ ⎞β = − −⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎝ ⎠⎝ ⎠⎝ ⎠

2PRT

14000120001000080006000400020000

0.25

0.20

0.15

0.10

0.05

Sorption of water, g/g

1 2

3

4

5

Time, min

Fig. 5. The kinetics of water vapor sorption on the initialand modified silica gels at 303 K and RH = 45%: 1—Ca/Csg, 2—Asg, 3—Li/Csg, 4—Bsg, 5—Csg.

200001600012000800040000Time, min

0.5

0.4

0.3

0.2

0.1

Sorption of water, g/g

1

2

3

4

5

Fig. 6. The kinetics of water vapor sorption on the initial and modified silica gels at 303 K and RH = 65%: 1—Ca/Csg; 2—Li/Csg,3—Asg, 4—Bsg, 5—Csg.

KINETICS AND CATALYSIS Vol. 51 No. 5 2010

DYNAMICS AND ISOTHERMS OF WATER VAPOR SORPTION 759

Ed on Li/Csg and Ca/Csg is more than that on the Csg.In other words, the modification by CaCl2 and LiClnot only increased the hygroscopic abilities of Csg, butalso increased the activation energy of water desorp�tion from Csg. This was because acting forces betweenwater molecules and the surface of adsorbents becamestronger if CaCl2 and LiCl were inserted into the poreof Csg. It was also seen that the Ed value on the meso�porous silica gel modified by CaCl2 was much higherthan that on the silica gel modified by LiCl, which wasbecause the water molecule was polar, and the staticelectricity action between adsorbate and absorbent

increased with the increase in the polarizabilities ofthe atoms on the surface of absorbent, and the polariz�ability of the Ca2+ ( ) was higher than that

of Li+ ( ) [16]. That is to say, the higher thepolarizability of the ion, the stronger is the attractionforces between the ion and water. Hence the activationenergy of water desorption from the silica gel modifiedby the ion with higher polarizability was increased.Therefore, comparing the dehumidification amountsof unit energy consumption, Li/Csg was superior toCa/Csg for the practical application.

α + = 0.4712Ca

α + = 0.029Li

500480460440420400380360340320Temperature, K

100

98

96

94

92

90

88

86

84

82

Loss of weight, %

12

Fig. 7. TG curves of Ca/Csg (1) and Li/Csg (2).

10080604020RH, %

0.8

0.6

0.4

0.2

0

Saturated sorption of water, g/g

1

23

Fig. 8. The water vapour adsorption isotherms on Csg (1),Li/Csg (2), and Ca/Csg (3) at 303 K.

480460440420400380360340320300Temperature, K

7

6

5

4

3

2

1

01

2

3

4

5

Desorption rate, arb. units

Fig. 9. TPD spectra of water on the LiCl/Csg at heatingrates, K/min: 1—4, 2—5, 3—6, 4—7, 5—8. Tp = 358.2,362.2, 365.8, 369.0 and 371.8, respectively.

480460440420400380360340320300Temperature, K

8

6

5

4

3

2

1 1

2

3

4

5

Desorption rate, arb. units

7

Fig. 10. TPD spectra of water on the CaCl2/Csg at heatingrates, K/min: 1—4, 2—5, 3—6, 4—7, 5—8. Tp = 360.8,366.0, 370.2, 373.9, and 376.6, respectively.

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LI XIN et al.

The Combination Effect of Pore Structure and Impregnating Salt on the Hygroscopic Properties

of Silica Gel

Figure 13 showed the kinetics of water vapor sorp�tion on the initial and modified silica gels with differ�ent surface areas, pore size distributions and pore vol�ume at 303 K and RH = 30%. It was found that thesorption capacity of Ca/Bsg and Ca/Csg was improvedsignificantly. A comparison of saturated water sorptionbetween initial and modified silica gels is shown inFig. 14. The content of the saturated hygroscopicmoisture in Ca/Asg, Ca/Bsg, and Ca/Csg was 0.989,146.0, and 291.4% in comparison with those in initialsilica gels, respectively. It is seen that saturated hygro�scopic moisture contents after modification becomemuch larger with increasing in the average pore diam�eter and pore volume of initial silica gels. So the silicagels with larger average pore diameter and pore vol�ume were mostly suitable for the modification in orderto improve their hygroscopic abilities.

Thus, the effect of the modification of silica gels byimpregnating with different salts on sorption of watervapor was studied through the isotherms and dynamicsof water sorption and the activation energy of waterdesorption. It was found that the surface modificationwith hygroscopic salts increased the average porediameter and decreased significantly the pore volume.The hygroscopic abilities of the silica gels wereimproved through the modification by CaCl2, LiCl,MgCl2, and ZnCl2. CaCl2 and LiCl were better tomodify the mesoporous silica gels. The sorptioncapacity and the sorption rate of Csg were bothimproved greatly through modification by CaCl2 andLiCl, which were larger than those of Asg or Bsg withsmaller pore diameter, a tendency can be seen that thesorption capacity decreased with the increase of theionic radius except of calcium ion. The mesoporoussilica gels should be applied for the medium and higherhumidity control because their latent sorption capac�ity was largest but the rate of water uptake need to be

400350300Temperature, K

7

6

5

4

3

2

1

0

1

2

3

4

5

Desorption rate, arb. units

450

Fig. 11. TPD spectra of water on the Csg at heating rates:1—4, 2—5, 3—6, 4—7, 5—8. Tp = 337.8, 343.7, 349.0,355.2 and 359.3, respectively.

Table 2. Water desorption peak temperatures at different heating rates and activation energies of water desorption from modifiedsilica gels

Silica gelDesorption peak temperature (Tp, K) at heating rates, K/min

Activation energyof desorption Ed, kJ/mol

4 5 6 7 8

Ca/Csg 360.8 366.0 370.2 373.9 376.6 43.1

Li/Csg 358.2 362.3 365.8 369.0 371.8 36.4

Csg 337.8 343.7 349.0 355.2 359.3 26.2

−2.64−2.70−2.76−2.82−2.88−2.94−3.001000/Tp

−11.4

−11.6

−11.8

−12.0

−12.2

−12.4

ln(βH/RTp2)

12 3

Fig. 12. Linear dependence between ln(βH/ ) and1/TP for TPD of water from the silica gels: 1—Csg (Ed =26.2 kJ/mol), 2—LiCl/Csg (Ed = 36.4 kJ/mol), 3—CaCl2/Csg (Ed = 43.1 kJ/mol).

2PRT

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DYNAMICS AND ISOTHERMS OF WATER VAPOR SORPTION 761

improved. The larger the average pore diameter andpore volume of the initial silica gels, the higher theaccrual water vapor sorption rate and capacity aftermodification. And it was also found that the activationenergy of the water desorption from the mesoporous

silica gel modified by CaCl2 were much higher thanthat from the silica gel modified by LiCl, because thepolarizability of the Ca2+ was higher than that of Li+.

ACKNOWLEDGMENTS

The authors would like to thank the National NaturalScience Foundation of China (no. 20936001 andno. 20906034) and the key discipline construct project(211 projects, no. 2009B010100001) for financial sup�ports.

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500040003000200010000Time, min

0.20

0.15

0.10

0.05

Sorption of water, g/g

1

2

3

4

5

6

Fig. 13. The kinetics of water vapor sorption on the initialand modified silica gels at 303 K and RH = 30%: 1—Ca/Csg, 2—Ca/Bsg, 3—Ca/Asg, 4—Asg, 5—Bsg, 6—Csg.

0.28

0.24

0.20

0.16

0.12

0.08

0.04

0

Saturated sorption, g/g

Asg Bsg Csg

Initial silica gel

Modified silica gel

Fig. 14. A comparison of the saturated sorption waterbetween initial and modified by 1 M CaCl2 silica gels.