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Synthesis of hydrophilic and hydrophobic xerogels with superior propertiesusing sodium silicate

Pradip B. Sarawade a, Jong-Kil Kim b, Askwar Hilonga a, Dang Viet Quang a, Hee Taik Kim a,⇑a Department of Chemical Engineering, Hanyang University, 1271 Sa 3-dong, Sangnok-gu, Ansan-si, Gyeonggi-do 426-791, Republic of Koreab E&B Nanotech. Co., Ltd., Ansan-si, Gyeonggi-do, Republic of Korea

a r t i c l e i n f o

Article history:Received 2 August 2010Received in revised form 20 September2010Accepted 19 October 2010Available online 23 October 2010

Keywords:Silica xerogelsAmbient pressure dryingInexpensiveHydrophobic

a b s t r a c t

Highly porous hydrophilic and hydrophobic silica xerogels were synthesized by surface modification ofsilica hydrogels at ambient pressure drying. The silica hydrogels were prepared by a sol–gel polymeriza-tion of an inexpensive silica precursor (sodium silicate) under atmospheric conditions. In order to min-imize shrinkage due to drying, the hydrogel surface was modified using trimethylchlorosilane (TMCS) inthe presence of ethanol/n-hexane solution before ambient pressure drying (APD). Properties of the finalproduct were investigated using Field-Emission Scanning Electron Microscopy (FE-SEM), TransmissionElectron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Dif-ferential Analysis (TG–DTA), and nitrogen physisorption studies. The final product was observed to havean extremely high specific surface area (783 m2/g) and a large cumulative pore volume (2.74 cm3/g).Highly porous hydrophilic xerogels were obtained after heat-treating the modified xerogels. At temper-atures above 450 �C the surface alkyl groups (ACH3) were significantly oxidized and, consequently, theproperties of the resulting xerogels were altered. Products obtained via the proposed inexpensiveapproach have superior properties and the method exploits an inexpensive silica source (sodium silicate).Thus it is feasible for large-scale economic industrial production.

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1. Introduction

Aerogels are the most highly porous nanostructured materials.They exhibit large surface area (�1200 m2/g), high porosity(80–98%), low bulk density (�0.03 g/cm3), extremely low thermalconductivities (0.005 W/mk), and unique acoustic properties(sound velocities as low as 100 m/s) [1,2]. Because of these proper-ties, aerogels are utilized as thermal super-insulators in solarenergy systems, refrigerators, and thermal flasks [3]. Despite theseapplications, the high production costs have thus far preventedtheir commercial use. Meanwhile, applications for porous silicaxerogels continuously expand as their production costs decreaseand their properties improve. Hydrophobic and hydrophilic silicaxerogels with superior physical properties such as high surfacearea and large pore volume have potential applications in fieldssuch as adsorbents, separations, biomedicine, sensors, drug deliv-ery systems, catalyst carriers, thermal insulation, glazing, paints,and oil spill clean-up [4–8].

Conventional silica xerogels have relatively high density, lowsurface area, and small pore volume, restricting their applications.Recent observations suggest that the properties of porous materi-

als improve following modification with silica gels (alcogel orhydrogel) during synthesis before the ambient pressure drying(APD) [9–13]. Moreover, silylating hydrogels and drying at ambientpressure can give less-dense silica xerogels. During the drying pro-cess, non-polar alky groups (which repel each other) replace sur-face OH groups, resulting in the ‘‘spring back-effect’’, whichpreserves the silica gel network and, hence, the porosity [14]. Sur-face modification of silica hydrogels by alkyl groups has been re-ported to preserve the porous network even after drying atambient pressure [15]. Prakash et al. [16] have synthesized silicaaerogel films at ambient pressure via solvent exchange and surfacemodification processes. Solvent exchange is a lengthy and tediousprocess because it simply depends on diffusion of the solutionwithin the gel. Hence, its take several days to produce silica aero-gels at ambient pressure. Schwertfeger et al. [17] developed a newsynthesis for sodium silicate-based silica aerogel at ambient pres-sure. Since then, many researchers have focused on synthesizingsodium silicate-based silica aerogels at ambient pressure. Never-theless, the solvent exchange process, which is required for silicaaerogel synthesis at ambient pressure, makes it a tedious process.Recently, Shi et al. [18] reported a new method, called one-stepsolvent exchange and surface modification process. This methodis based on combining different solvents (trimethylchlorosilane(TMCS), n-hexane, and ethanol) for surface modification. ThoughShi et al. used ethanol to reduce the reaction between the silylating

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⇑ Corresponding author. Tel.: +82 31 400 5493; fax: +82 31 500 3579.E-mail address: [email protected] (H.T. Kim).

Microporous and Mesoporous Materials 139 (2011) 138–147

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agent and water, the ethanol consumes large amount of the silylat-ing agent in the surface modification process. We found that thisprocess may be more suitable for surface modification of high con-tent silica hydrogels, as there is less water present in the pore com-pared to low content silica hydrogels, leaving less silylating agentconsumed. Here we observed that very little silylating reagent isrequired for surface modification of water silica gels with lowpore content than those with higher pore content. The amount ofsilylating agent consumed for surface modification of silica hydro-gels depends upon the amount of pore water, since the silylatingreagent can directly react with pore water. Also, modifying silicahydrogels (high pore water content) can take a long time to replacethe pore fluids. Therefore, a one-step solvent exchange and surfacemodification process is more suitable to produce hydrophobicsilica xerogel, reducing both the time and the amount of silylatingagent required. This study is not intended to compare silica aerogeland xerogel properties. Our intention was to produce high poroussilica xerogels (hydrophobic and hydrophilic) with extreme physi-cal properties, such as high surface area and large pore volume,using one-step solvent exchange and surface modification of silicahydrogels and drying at ambient pressure.

In the present study, we synthesized low density, relativelytransparent, very high specific surface area, and high pore volumehydrophobic silica xerogels by surface modification of hydrogelfollowed by APD. The silica precursor utilized in this study is rela-tively inexpensive (sodium silicate) and the reaction proceduresemployed are considerably versatile. We employed a simultaneoussolvent exchange and surface modification process (one-step sol-vent exchange and surface modification) to reduce the synthesisduration and the drying shrinkage of silica xerogels (at an ambientpressure) from 7 to �2 days. The results for the modified silicaxerogel were compared with those for the unmodified silica xero-gel. Moreover, hydrophilic silica xerogels resulted from heat-treating modified silica xerogel. The specific surface area, porevolume and pore diameter of TMCS modified hydrophobic silicagel increase slightly with increase in heating temperature from150 to 500 �C. This paper reports, in detail, the results obtained.

2. Materials and methods

2.1. Preparation of silica hydrogel using sodium silicate (water-glass)

Silica hydrogel was prepared by sol–gel polymerization usingsodium silicate as a silica precursor (molar ratio SiO2:Na2O = 3.4),purchased from Shinwoo Materials Co. Ltd., South Korea. Trimeth-ylchlorosilane (TMCS) (silylating agent) and sulfuric acid (acid cat-alyst) were purchased from Duksan chemical. Scheme 1 shows themethod used to prepare the mesoporous hydrophobic and hydro-philic silica xerogels at an ambient pressure via simultaneoussolvent exchange and surface modification. In order to preparethe silica sol, the aqueous sodium silicate solution (molar ratioSiO2:Na2O = 3.4) was mixed with purified water and sulfuric acid(40%) through a line mixer and finally through a nozzle as shownin Scheme 1. The flow rate of sodium silicate solution (200g/min), purified water (150 g/min), and sulfuric acid (200 g/min)was controlled by using chemical feed pump and air chamber.Then the acidic (pH �1) silica sol was collected in a gelation unitprior to gelation. The gelation time was about 4 min, since the sil-ica concentration in the silica sol solution was considerably highand the pH was strongly acidic. After gelation, the resultant silicahydrogels were aged for two different aging conditions i.e. (i) acidiclow temperature (ALT) and (ii) basic high temperature (BHT) in or-der to see the effect of aging time and temperature on the texturalproperties of the silica xerogels. For this, 100 g of silica xerogel wasaged in acidic condition (pH �4) and the temperature of 40 �C for

various durations (0–70 h). For the second aging condition, thesame amount of silica xerogel was aged in basic condition (pH �9)and the temperature of 70 �C for various durations (0–70 h).

2.2. Washing (removal of byproducts), solvent exchange/surfacemodification and drying silylated wet gels

The prepared silica gel was thoroughly washed with a continu-ous flow of water, for 12 h to remove trapped sodium ions (Na+)from the wet silica gel. The removal of Na+ from the washed gelwas confirmed by a sodium ion detector (NeoMet, ISTEK, pH/ISEmeter). Furthermore, the washed gel was chemically modified withtrimethylchlorosilane in the presence of n-hexane/ethanol solu-tion. In this case, the gel (100 g) was mixed with n-hexane(100 ml) solution, TMCS, and ethanol for 12 h at room temperature(�25 �C). In all cases, the molar ratio of ethanol/TMCS (M1) wasfixed at 1. Ethanol was used to slow down the reaction betweenpore water and TMCS. The molar ratio of n-hexane/TMCS (M2)was fixed at 21.85 to minimize shrinkage, as reported elsewhere[19]. After surface modification, the gel was removed and washedwith fresh n-hexane to remove unreacted TMCS. The resultantmaterial was dried at an ambient pressure (room temperature,�25 �C) for 12 h, followed by further drying at ambient air (80 �Cfor 2 h). These drying procedures reduced shrinkage. To completelyevaporate the pore liquid, the product was finally dried at 150 �Cfor an hour at ambient pressure. The hydrophobic silica gels werecooled to room temperature and characterized using various tech-niques. Furthermore, to obtain hydrophilic silica xerogel, thehydrophobic xerogel samples were heated in air at various temper-atures: 150, 200, 250, 300, 350, 400, 450, and 500 �C for an hour.

2.3. Characterization methods

Silica xerogel hydrophobicity and hydrophilicity were exam-ined by measuring the percentage of absorbed water after puttingthe samples directly on a water surface. The weight change overtime was examined using an electronic microbalance (ModelOHAUS EPG214C, USA) at 10�5 g accuracy. Oil absorption studieswere carried out on silica xerogel powder. Hydrophobic and hydro-philic silica xerogels were crushed into refined powder, and oilabsorption was measured as follows. Ten grams of the samplewas placed on a polyethylene plate (which does not absorb oil).Di-n-butyl phthalate (DOP) was dropped little by little on the cen-ter of each sample from a burette and was thoroughly kneadedwith a spatula after each drop. The dropping and kneading were re-peated until the entire mixture no longer became a solid puttylump. The amount of DOP used was determined, and oil absorptionwas expressed as ml of oil absorbed/g of the sample.

The tapping densities of the silica xerogel were calculated fromthe mass to volume ratios. The volume was calculated by placingthe silica xerogel in a graduated cylinder. An electronic microbal-ance (Model OHAUS EPG214C USA) measured the mass. The per-centage porosity of the prepared silica xerogel was determinedas follows:

% Porosity ¼ ð1� qb=qsÞ � 100 ð1Þ

where, qs and qb are the skeletal and bulk densities (of silica xero-gel), respectively.

The specific surface area and pore size distributions (PSDs) ofxerogel were analyzed using Brunauer Emmet and Teller (BET)and BJH nitrogen gas adsorption and desorption methods (ASAP2020, Micromeritics, USA). The BJH nitrogen gas absorption meth-od was used to obtain the average pore diameter for the silica gel.BET analysis from the N2 gas adsorbed at various partial pressures(10 points 0.05 < p/po < 0.3, nitrogen molecular cross sectionalarea = 0.162 nm2) was employed to determine the surface area,

P.B. Sarawade et al. / Microporous and Mesoporous Materials 139 (2011) 138–147 139


and a single condensation point (p/po = 0.99) was used to find thecumulative pore volume. Before N2 adsorption, the sample was de-gassed at 150 �C. Pore size distributions were calculated from thedesorption isotherms [20–22]. To study the thermal stability ofthe xerogel, in terms of hydrophobicity retention, samples wereexamined by Thermo Gravimetric and Differential Thermal Analy-sis (TG–DTA). Ten milligrams of hydrophobic xerogel was heat-treated in air from 25 to 1000 �C at a rate of 1.5 �C min�1 using amicroprocessor based Parr temperature controller (Model 4846)connected to a muffle furnace (A.H. JEON Industrial Co. Ltd., Korea).The thermal stability refers to the temperature at which the silicaxerogel retains its hydrophobicity [23].

Surface modification was confirmed using Infrared (IR) spec-troscopy, Perkin-Elmer (Model No. 783). For this purpose, the silicaxerogel was ground into a refined powder, mixed with KBr, andpressed to form a sample pellet for FTIR measurements. Micro-structure studies of xerogel samples were carried out byField-Emission Scanning Electron Microscopy (FE-SEM) and trans-mission electron microscopy (TEM, JSM 6700 F, JEOL).

3. Results and discussion

3.1. Effect of excess H2SO4 in silica sol on gelation time

Sodium silicate (Na2SiO3) has been, and probably always been,the cheapest source of relatively pure silicic acid to make silicagel. Sodium silicate reacts with water and acid (sulfuric acid) togive silicic acid as shown in the following chemical equation:

Na2SiO3 þH2OþH2SO4 ! SiðOHÞ4 þ Na2SO4 ð2Þ

The silicic acid condenses to form small silica particles, chains, andconsequently a silica network (silica gel/slurry) as shown below:

SiðOHÞ4 þ ðOHÞ4Si! ðOHÞ3BSiAOASiBðOHÞ3 þH2O ð3Þ

These reaction mechanisms have been suggested to take placewhen using other sources of silica [24–26]. The silica gel prepa-ration involves a sol-to-gel transition. This transition is referredas a gelation, where the sol becomes highly viscous and ceasesto move [27]. The gelation time depends strongly on the amountof catalyst added to the sol (Fig. 1). The degree of gelation was

calculated by the concentration of excess sulfuric acid (H2SO4)in the silica sol. These data can be explained using the generaltheory of silica polymerization. The condensation polymerizationshows that silica gelation in aqueous solution involves an ionicmechanism. As the amount of sulfuric acid in the sol increasesfrom 0.1 to 1 N, the gelation time decreases from 60 to�4 min (Fig. 1). The decrease arises from the increase in acidin the sol, increasing the condensation reaction rate. Therefore,silica clusters aggregate faster, forming a three-dimensional, por-ous silica network in a short time [28]. At relatively lower sulfu-ric acid concentrations silica particles have very little ioniccharge. Consequently, they first aggregate into chains and finallyinto a three-dimensional gel network. Below 0.5 N sulfuric acidthe silica particles are positively charged; hence, they repel eachother. Sols with the longest gelation time have a maximum tem-porary stability at about 1 N because they are positively chargedbetween 0.5 N and 1 N sulfuric acid; hence the gelation time ofsilica sols increases significantly because they attract each other.At around 1 N the silica monomers rapidly convert into particlesthat simultaneously aggregate into a gel network.

Flow line

Flow line

Flow line

Line Mixer

Hydrosol (Silica sol)

(Gelation Unit)

Nozzle

Chemical feed pump

Chemical feed pump

Chemical feed pump

Valve

SiO2:Na2O =3.4

Purified Water

H2SO4

(40%)

Air Chamber

Air Chamber

Air Chamber

Purified Hydrogel Surface

Modification Drying &

Heat-treatment

Aging & Washing

Scheme 1. A flow chart showing experimental procedures for the synthesis of hydrophobic and hydrophilic mesoporous silica gels at an ambient pressure.

Normality (N) of excess H2SO4 in silica sol

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6 0.8 1

Gel

atio

n tim

e (m

inut

e)

Fig. 1. Effect of normality (N) of excess H2SO4 in silica sol on gelation time.

140 P.B. Sarawade et al. / Microporous and Mesoporous Materials 139 (2011) 138–147


3.2. Variation of textural properties with aging conditions (pH,temperature and time)

Aging conditions (temperature, time, and pH) have a strong im-pact on the textural properties of dried silica xerogels, includingBET surface area, pore volume, and pore size. In order to observethe effects of pH and aging temperature with respect to aging timeon textural properties of dried silica xerogel, the gels were treatedwith two different conditions: (i) acidic low temperature and (ii)basic high temperature for various durations (time periods), asshown in Figs. 2–4. It is evident that, at basic high temperature,the surface area gradually decreases from 588 to 190 m2/g overtime. On the other hand, the BET surface area for acidic low tem-perature samples decreased slightly from 588 to 445 m2/g. A de-crease in BET surface area at higher temperatures is likely due tohydrothermal effects on the silica gel [29–31]. In Figs. 3 and 4the pore volume and pore size gradually increase over time for ba-sic high temperature samples. In addition, pore size and pore vol-ume at above 20 h increased suddenly (uptake) at basic hightemperature than at acidic low temperature aging conditions. Thiscan be attributed to the precipitation rate of silica monomers intothe pores as well as into the surface of the gel network. The precip-itation rate is higher at above 20 h at basic high temperature thanat acidic low temperature. If the precipitation occurs at the porewalls, there is a chance to decrease the pore size which willcontribute to the microporosity. This increase in microporositycontributes to the increase in internal surface area and therebythe total pore volume [32,33]. Moreover, this difference is due tothe fact that the condensation in silica gels continues long after

gelation [33,34]. Moreover, aging silica hydrogels may modify theirtexture by continued condensation reactions leading to a morebranched gel network. A more branched gel network leads to largerinterconnected particles resulting in a larger pore size [35,36]. Inaddition, aging at higher temperature allowed reorganization ofthe pore structure by several processes, such as dissolution, con-densation, de- and re-polymerization, and syneresis, resulting inincreased pore size and pore volume [37]. The data obtained forthe samples aged at acidic low temperature and basic high temper-ature are complied in Table 1.

3.3. Simultaneous solvent exchange and surface modification of silicawet gel

Generally, colloidal silica polymerization forms a weak net-work. During the drying process, liquid and vapor coexist withinthe gel pores. As the liquid begins to evaporate, a meniscus formsat the liquid–vapor interface. The formation of a liquid–vaporinterface within the gel results in surface tension and creates con-cave menisci in the gel pores. With the progressive evaporation,the menisci recede into the gel body and build a compressive forcethat acts on the pore walls. This force causes considerable shrink-age due to partial collapse of the gel network [38]. In addition, the

0

300

350

400

450

500

550

600

0 10 20 30 40 50 60 70Time (hrs)

BE

T s

peci

fic s

urfa

ce a

rea

(m2 /g

)

: pH 4.0, Temp. 40 oC (ALT) : pH 9.0, Temp. 70 oC (BHT)

Fig. 2. Effect of two aging conditions on the BET surface area of the silica gels: (i)acidic low temperature (ALT) and (ii) basic high temperature (BHT).

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

0 10 20 30 40 50 60 70 Time (hrs)


P

ore

volu

me

(cm

3 /g)

Fig. 3. Effect of two aging conditions on the pore volume of the silica gels: (i) acidiclow temperature (ALT) and (ii) basic high temperature (BHT).

0

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70

Time (hrs)

BE

T a

vera

ge p

ore

diam

eter

(nm

)


Fig. 4. Effect of two aging conditions on the pore size of the silica gels: (i) acidic lowtemperature (ALT) and (ii) basic high temperature (BHT).

Table 1Effects of washing pH and aging temperature on the physical and textural propertiesof the unmodified water-glass-based silica xerogels (hydrophilic) dried at ambientpressure (APD).

No. Agingtime(h)

Bulk density(g/cm3)

Porosity(%)

Oilabsorption(ml/g)

Waterabsorption(ml/g)

Acidic low temperature (ALT) (pH 4; temp, 40 �C)(a1) 10 0.591 73.13 2.11 2.23(b1) 20 0.633 71.22 2.05 2.14(c1) 30 0.647 70.59 2.04 2.12(d1) 40 0.655 70.22 2.02 2.10(e1) 50 0.664 69.81 1.99 2.08(f1) 60 0.681 69.04 1.97 2.04(g1) 70 0.694 68.45 1.96 2.01

Basic high temperature (BHT) (pH 9; temp, 70 �C)(a2) 10 0.570 74.09 2.15 2.27(b2) 20 0.582 73.54 2.12 2.24(c2) 30 0.605 72.49 2.08 2.21(d2) 40 0.610 72.27 2.07 2.19(e2) 50 0.623 71.68 2.04 2.16(f2) 60 0.668 69.63 1.95 2.07(g2) 70 0.681 69.04 1.93 2.04



terminal silanol groups (SiAOH) present on the silica surface un-dergo a condensation reaction forming new siloxane bonds, whichultimately result in irreversible shrinkage [39]. Attaching the non-polar alkyl group to the silica surface replaces OH with H, reducingirreversible shrinkage. Also, gel collapse ceases when the gel struc-ture is strong enough to withstand the tensile strength of the liquid[40]. Therefore, to reduce irreversible shrinkage during drying, thegel surface was organically modified with tri-methyl groups (foundin TMCS). The modification was carried out via a simultaneous sol-vent exchange and surface modification process using ethanol/TMCS/n-hexane solution. The expected chemical reactions for thesimultaneous solvent exchange/surface modification of the silicahydrogel are presented as follows:

C2H5OHþðCH3Þ3ASiACl!ðCH3Þ3ASiAOACH2CH3½ETMS�þHClð4Þ

2ðCH3Þ3SiACl½TMCS�þH2O!ðCH3Þ3ASiAOASiAðCH3Þ3½HMDSO�þ2HCl ð5Þ

2ðCH3Þ3ASiAOACH2CH3þH2O!ðCH3Þ3ASiAOASiAðCH3Þ3þ2C2H5OH ð6ÞðCH3Þ3ASiAClþBSiAOH!BSiAOASiAðCH3Þ3þHCl ð7Þ

Thus, TMCS reacts with ethanol (Eq. (4)), pore water (Eq. (5)),and the OH group on the silica surface (Eq. (7)), to form ethyltri-methoxysilane [ETMS] and hexamethyldisiloxane (HMDSO); con-sequently, the hydrophilic surface of the silica network becomeshydrophobic. As the reaction proceeds, transferable yellowish liq-uids (HCl/residual ethanol) spontaneously come out of the wetgel [41]. During the simultaneous solvent exchange/surface modi-fication process, the reaction between ethanol and TMCS can slowdown the reaction rate of TMCS with the OH group on the silicasurface. The latter process spontaneously replaces pore water withn-hexane. Chemical surface modification of the hydrogel by non-polar alkyl/aryl groups is an indispensable step before APD, as itprohibits the formation of new siloxane bonds between the adja-cent silica cluster and thereby irreversible gel shrinkage [42,43].In the present work, tri-methyl groups present in the trimethyl-chlorosilane organically modified the surface of the water-glassbased hydrogel by simultaneous solvent exchange and surfacemodification, as explained above.

3.4. Hydrophobic properties of sodium silicate-based xerogel

Surface modification of all samples was carried out for 12 h atroom temperature (�25 �C). As shown in Fig. 5, the hydrophobicityof silica xerogel granules was measured by putting samples

directly on a water surface and measuring the increase in weightafter 3 months. The results are tabulated in Table 2. The data forthe surface modified silica gel (xerogel granules) have been com-pared with those for the unmodified silica gel (xerogel). Althoughthe wettability of materials depends upon their surface chemistryis well known, however, water absorption studies show that theunmodified silica xerogel (hydrophilic xerogel) absorbed morewater than the TMCS surface modified silica xerogel (hydrophobicxerogel). This discrepancy arises from the non-polar methyl groups(ACH3) attached to the silica gel surface, which repels water mol-ecules, consequently reducing water absorption. The mesoporoussilica xerogel granules exhibited hydrophobic behavior as a resultof the surface modification. Surface modification of silica xerogelgranules was also confirmed by using Fourier Transform InfraredSpectroscopy (FTIR). The FTIR spectra of modified (hydrophobic)xerogel with TMCS, unmodified xerogel (hydrophilic), and heat-treated xerogels are provided in Fig. 6. The strong absorption peakat 1260 cm�1 corresponds to the terminal ACH3 group, and thepeak at 840 cm�1 is due to SiAC bonding. These absorption peaksare attributed to the surface modification by TMCS [36,44]. Theabsorption peak centered at 976 cm�1 corresponds to the OHgroups absorbed on the silica surface, the bending of HAOAHbonds, and the stretching of the SiAOH bonds [45]. The peaks cen-tered at 1070 and 495 cm�1 correspond to the SiAOASi bonds andare the most informative of the silica network structure [46,47].The two weak peaks at 976 and 960 cm�1 for the unmodified silicagel represent the SiAOH stretch vibrations, indicating that there isno modification of the silica gel surface. Moreover, there are noCAH peaks in the FTIR spectra of the unmodified silica gel. SiAOHand SiAH2O peaks are present in the FTIR spectra and show thehydrophilic nature of the unmodified gel [48]. A strong peak at1256 cm�1 for the modified samples indicates significant surfacemodification by TMCS. The OH and SiAOH peaks are conspicuousfor the unmodified xerogel. Fig. 6 also shows the FTIR spectra forheat-treated, TMCS modified xerogel. The peak at 1256 cm�1,which was present in modified xerogel, clearly disappears afterheat treatment at 450 �C. This change indicates that the attachedgroup (ACH3) gets oxidized at that temperature, converting thehydrophobic gel into its hydrophilic counterpart. The transitiontemperature of the xerogel (hydrophobic to hydrophilic) wasdetermined using TG/DTA and water absorption measurements.

3.5. Oil adsorption studies

Oil absorption studies were carried out on unmodified, modi-fied, and heat-treated silica xerogel powders. The mesoporoushydrophobic (TMCS modified) and hydrophilic (heat-treated at450 �C) silica xerogels had maximum oil absorptions of more than3.96 and 3.88 ml/g, respectively, measured using dioctyl phthalate.The unmodified xerogel powder has a very low oil absorption com-pared to the others. The latter retains its maximum porosity be-cause it does not shrink. The method used to measure oilabsorption was explained in detail in the characterization section,and the results are summarized in Tables 1–3.

3.6. TG–DTA studies and the effect of heat-treatment on xerogelhydrophobicity

In order to investigate the thermal stability/transition tempera-ture (hydrophobic to hydrophilic transformation), 25 mg of xerogelwas heated at various temperatures (150–500 �C). Also, waterabsorption in heat-treated xerogel was tested as shown inFig. 5(a and b). Xerogels heat-treated at temperatures below250 �C do not absorb water and float on the surface (Fig. 5(a)).Xerogels heat-treated at 350 �C absorb some water, partially sink-ing below the surface but still floating. Xerogels heat-treated at

Fig. 5. Photograph showing (a) hydrophobicity of the modified mesoporous silicagel on water surface and (b) its hydrophilicity when heat-treated at 450 �C.



450 �C completely sink, settling at the bottom (Fig. 5(b)). Furtherincreases in heating temperature make the xerogel more hydro-philic. The maximum temperature at which the xerogel retainsits hydrophobic character (before becomes hydrophilic) was440 �C. At this temperature the surface methyl groups (ACH3) be-come oxidized, forming a hydrophilic xerogel. The thermal stabilityof the xerogel and the oxidation temperature for the ACH3 groupswere estimated by thermogravimetric (TG) and differential ther-mal (DT) analyses, respectively. The TG–DTA results for hydropho-bic xerogels were compared with those for unmodified xerogel.Fig. 7(a and b) shows the TG–DT analyses of the mesoporous TMCSmodified and unmodified xerogels heat-treated in an oxygenatmosphere up to 1000 �C. The curves clearly show that the mod-ified xerogel exhibited a negligible weight loss up to 440 �C, be-yond which the xerogel underwent a significant weight loss. Thisdecrease in weight is due to oxidation of surface methyl groups,which can be clearly seen by two sharp exothermic peaks in theDTA curve when the temperature is raised above 440 �C [49]. Onthe other hand, the unmodified silica gel showed a continuousweight loss with increased temperature (Fig. 7(b)).

3.7. Nitrogen physisorption studies

The impact of surface modification by TMCS on textural proper-ties of water-glass based xerogels was investigated by BET analysis.Tables 1 and 2 show the physical and textural properties of xero-gels synthesized without (unmodified) and with (modified) TMCS.Table 3 shows the effect of heat-treatment on the physical and tex-tural properties of the hydrophobic xerogel with respect to the twoaging conditions (acidic low temperature and basic high tempera-ture) and heating temperature (450 �C). TMCS has a significant ef-fect on the physical and textural properties of xerogel samples.TMCS altered specific surface area, average pore size, cumulativepore volume, tapping density, and percentage porosity. Thetapping density of the unmodified xerogel is much higher(0.57 g/cm3) than that of its modified counterpart (0.098 g/cm3)and nearly equal to that of the ambient pressure-dried silicaaerogel (�0.05 g/cm3) [50]. As illustrated in Table 2, the modifiedxerogel possesses high surface area (783 m2/g), large pore size(9.21 nm), and high pore volume (2.74 cm3/g). On the other hand,the unmodified xerogel shows low surface area (588 m2/g), low

Table 2Surface modification effects (TMCS) with respect to aging conditions on the physical and textural properties of the water-glass-based silica xerogels (hydrophobic) dried atambient pressure (APD).

No. Aging time(h)

Bulk density(g/cm3)

BET surface area(m2/g)

Pore diameter(nm)

Pore volume(cm3/g)

Porosity(%)

Oil absorption(ml/g)

Water absorption(after 3 month) (ml/g)

Acidic low temperature (ALT) (pH 4; temp, 40 �C)(a11) 10 0.110 765 9.21 2.71 95.00 3.93 0.131(b11) 20 0.142 761 9.40 2.75 93.54 3.87 0.128(c11) 30 0.156 757 9.73 2.82 92.90 3.84 0.126(d11) 40 0.164 751 10.24 2.93 92.54 3.82 0.125(e11) 50 0.173 747 10.82 3.02 92.13 3.81 0.124(f11) 60 0.184 740 11.35 3.07 91.63 3.79 0.123(g11) 70 0.192 735 12.13 3.10 91.27 3.77 0.121

Basic high temperature (BHT) (pH 9; temp, 70 �C)(a22) 10 0.098 783 9.41 2.74 95.54 3.96 0.132(b22) 20 0.120 779 9.83 2.77 94.56 3.91 0.130(c22) 30 0.133 772 11.42 2.84 93.95 3.89 0.129(d22) 40 0.138 763 12.03 2.95 93.72 3.88 0.128(e22) 50 0.151 754 12.64 3.03 93.13 3.85 0.127(f22) 60 0.164 748 13.20 3.09 92.54 3.83 0.126(g22) 70 0.170 742 13.35 3.14 92.27 3.82 0.125

90014001900240029003400

Series4

Series7Series10

Series13

400900140019002400290034003900

(a) Unmodified

(b) 150 oC (modified)

(c) 250 oC

(d) 350 oC

(e) 450 oC

OH Si-CH3 OH

Si-CH3

Si-O-Si

Si-C

Si-CH3

Wave number (cm-1)

Rel

ativ

e in

tens

ity (

a. u

.)

Fig. 6. FTIR spectra of (a) unmodified silica gel together with those modified with (b) TMCS and heat-treated at (c) 250 �C, (d) 350 �C and (e) 450 �C.



pore volume (0.78 cm3/g), and small pore size (5.10 nm). Thesedata indicate that the hydrophobic surface of the pore networkreduces the capillary pressure by lowering the surface tension,consequently reducing shrinkage during the aging and dryingstages. Previous studies [13] also reported the same phenomenon,wherein surface modification of the silica gel increases its BETsurface area. Furthermore, the repulsion between ACH3 groupson the surface during the final drying stages also tends to expandthe surface and affects partial recovery of the wet gel structure[51]. The percentage porosity depends on gel shrinkage duringdrying. As reported in Tables 1 and 2, the percentage porosity forthe xerogels prepared by surface modification was considerablyhigher (95%) than that for the unmodified xerogels (69.04%). Thisdifference in percentage porosity can be attributed to the non-polar methyl groups attached to the silica surface. The methylgroups are responsible for the ‘‘spring back effect’’ [52,53], whichpreserves the highly porous silica network and reduces collapse.Table 3 illustrates the changes in physical properties of TMCS sur-face-modified xerogel after heating at 450 �C for 1 h. The specificsurface area, cumulative pore volume, and average pore diameterincrease slightly with increase in heating temperature from 150to 500 �C (Fig. 8, Table 3). Generally, when the xerogel is heated

Table 3Effect of heat treatment (450 �C) on the physical and textural properties of the TMCS surface modified water-glass-based silica xerogel.

No. Heating temp.(�C)

Bulk density(g/cm3)

Pore diameter(nm)

Pore volume(cm3/g)

Porosity(%)

Oil absorption(ml/g)

Water absorption(after 3 month) (ml/g)

Acidic low temperature (ALT) (pH 4; temp, 40 �C)(a111) 150 0.110 9.21 2.71 95.00 3.93 3.51(a111) 200 0.115 9.25 2.72 94.77 3.92 3.50(a111) 250 0.120 10.29 2.99 94.54 3.91 3.49(a111) 300 0.129 11.34 3.27 94.12 3.90 3.47(a111) 350 0.132 12.40 3.56 94.00 3.89 3.46(a111) 400 0.138 12.80 3.67 93.72 3.88 3.45(a111) 450 0.140 13.15 3.71 93.63 3.87 3.44(a111) 500 0.141 13.41 3.73 93.59 3.86 3.44

Basic high temperature (BHT) (pH 9; temp, 70 �C)(a222) 150 0.098 9.41 2.74 95.54 3.96 3.56(a222) 200 0.113 11.23 2.93 94.86 3.94 3.53(a222) 250 0.118 11.64 3.02 94.63 3.93 3.52(a222) 300 0.127 11.91 3.09 94.22 3.92 3.51(a222) 350 0.131 12.46 3.46 94.04 3.91 3.50(a222) 400 0.134 13.87 3.56 93.90 3.90 3.49(a222) 450 0.137 14.49 3.75 93.77 3.88 3.48(a222) 500 0.139 14.73 3.76 93.68 3.88 3.47

Wei

ght (

%)

DT

A (

uV/m

g) E

xo u

p

440oC

DTA

TGA

Temperature (oC)

0

20

40

60

80

100

120

0 150 300 450 600 750 900

TGA

DTA

(a) Modified (b) Unmodified

Fig. 7. TG/DTA curves of (a) hydrophilic (unmodified), and (b) hydrophobic(modified) mesoporous silica gel.

600

650

700

750

800

850

900

100 150 200 250 300 350 400 450 500

BE

T s

peci

fic s

urfa

ce a

rea

(m2 /g

)

Heating temperature (oC)

Fig. 8. Variation of BET surface area with the increase in heating temperature from150 to 500 �C.

100

200

300

400

500

600

700

800

900

0

Volu

me

adso

rbed

(cm

3 /g) S

TP

0 0.2 0.4 0.6 0.8 1Relative Pressure (p/po)

(a) Unmodified(b) Modified (c) Heat-treated

Desorption

Adsorption

Fig. 9. N2 adsorption/desorption isotherms of (a) unmodified silica gel togetherwith (b) those modified with TMCS and (c) heat-treated at 450 �C.



at high temperature its particle size (particle diameter) decreaseswhile the pore size increases. Hence increase in pore size, internalpore volume, and pore surface may increase. Consequently, thetotal pore volume and the total specific surface area increased. Thisobservation is consistent with another report in which it was con-cluded that the specific surface area increases with the decrease inparticle diameter and increase in pore diameter, after heating thegel above 150 �C [54–56]. In addition, the pore size and the porevolume increased with increase in heating temperature (Table 3).The increase in total pore volume with increase in heating temper-ature can be attributed to the extraction of residual solvent (water)which may create more micropore surface area and hence the totalpore volume increased. Hence, there was a significant increase intotal pore volume, total specific surface area, and total microporesurface area. Also, after heating the modified silica gel at high tem-

perature (above 450 �C), the alkyl groups (ACH3) that were at-tached on the silica surface were oxidized as shown in TG–DTAgraph (Fig. 7). This oxidation of surface methyl groups may createmore micro and mesopores; hence, the total pore volume and, con-sequently, the total specific surface area may increase with in-crease in heating temperature (Fig. 8). The xerogel porosities(unmodified, modified, and heat-treated) were further studied byexamining nitrogen adsorption–desorption isotherms and poresize distributions. Fig. 9 shows the N2 adsorption/desorption iso-therms of unmodified, modified (TMCS), and heat-treated xerogels.The unmodified samples show an adsorption/desorption curvewith hysteresis behavior typical of xerogels, which indicates ink-bottle shaped pores (H4 type) [48]. On the contrary, the isothermcurves of the modified and heat-treated xerogels are very similarto those of the supercritical/ambient pressure-dried silica aerogels(H1 type) [37], although with a relatively low surface area and poresize. H1 type hysteresis loops indicate a cylindrical-like pore [11].The limited N2 uptake at low relative pressures (<0.1) indicates theexistence of mesoporous structures [40,57]. The isotherm of theheat-treated xerogel decreased slightly compared to that of itsheat-treated counterpart. The increase in pore volume is associatedwith the removal of residual organic components and ACH3 groupsfrom the gel. The physisorption isotherms obtained for all xerogelsexhibit hysteresis loops which correspond to the characteristic fea-tures of mesoporous materials (Type IV isotherms) [58,59].

Fig. 10 shows the pore size distribution (PSDs) curves ofunmodified, modified, and heat-treated xerogels. The unmodifiedxerogel shows a narrow pore size distribution with an averagediameter of 5.10 nm, while the modified xerogel shows a broaddistribution with an average diameter of 9.21 nm. The heat-treatedxerogel shows a broader pore size distribution with an averagediameter of 14.49 nm. After heating the modified samples at450 �C the peak pore diameter shifted to the higher value. This shiftarises from the removal of residual organic compounds and ACH3

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

1 10 100

(a) Unmodified

(b) Modified

(c) Heat-treated

Pore diameter (nm)

d

v/dl

og(d

) (c

m3 /g

.nm

)

Fig. 10. Pore size distributions of (a) unmodified silica gel together with (b) thosemodified with TMCS and (c) heat-treated at 450 �C.

Fig. 11. FE-SEM micrograph of (a) unmodified silica gel together with those modified with (b) TMCS and (c) heat-treated at 450 �C.



groups from the gel. As per the IUPAC classification of pores [60],the modified and heat-treated samples show a pronounced peakin the mesopore region (2–50 nm), indicating that the xerogelsmaintain mesoporosity even after high temperature heat-treatment.

3.8. FE-SEM and TEM studies

The xerogel mesoporosity was confirmed using Field-EmissionScanning Electron Microscopy (FE-SEM) and Transmission ElectronMicroscopy (TEM). Fig. 11 shows FE-SEM micrographs of (a)unmodified, (b) TMCS modified, and (c) heat-treated TMCS modi-fied xerogels. Generally, unmodified xerogels have a less porousstructure with dense aggregated spheres. The unmodified wet sil-ica gel shrinks more during APD, leading to a dense microstructureof dried xerogel and a loss of mesopores [61]. The modified xerogelshows highly porous structures (Fig. 11(b)) due to reversibleshrinkage (spring back effect) that takes place during drying, a con-sequence of organic modification of tri-methyl groups on the silicasurface. The modified xerogel has a mesopore in the structure withan average diameter of 9.21 nm. Fig. 11(c) shows an FE-SEM micro-graph of heat-treated modified xerogel. The TMCS modified xerogelthat was heat-treated at 450 �C appears to have more regularspheres that are small and uniform in size. The pore size for theheat-treated xerogel is wider than that for the unheated counter-part. Fig. 12 shows representative TEM images of unmodified,modified (TMCS), and heat-treated TMCS modified xerogels. Theunmodified xerogel shows more dense aggregated silica particlesas expected. Samples modified with TMCS and heated at 450 �Cexhibited a highly porous sponge-like silica network microstruc-ture with a uniform pore size distribution in the range of9–15 nm. The surface morphology observed by TEM for heat-treatedxerogel is more uniform and has a smaller particle size than thatfor the unheated xerogel.

4. Conclusions

Hydrophobic and hydrophilic mesoporous sodium silicate-based silica xerogels were obtained by simultaneous solventexchange and surface modification of wet silica gel with trimethyl-chlorosilane (TMCS) followed by ambient pressure drying. Ahydrophilic xerogel with better physicochemical properties wasobtained by heating the TMCS modified silica gel at 450 �C. The sil-ica wet gel was obtained by a novel fast gelation of colloidal silicasol. The surface modifying agent (TMCS) as well as the heat-treatment process has a strong effect on the properties of the finalproduct. The properties examined were surface area, pore volume,pore diameter, hydrophobicity, and morphology. The TMCS sur-face-modified xerogel has high surface area (783 cm2/g), low den-sity (0.098 g/cm3), large pore diameter (9.21 nm), and extremelylarge pore volume (2.74 cm3/g). Heat-treatment had a great impacton the xerogel, mainly on hydrophobicity, surface area, pore size,and pore volume. Hydrophilic mesoporous xerogel with superiorsurface area (788 cm2/g), pore diameter (14.49 nm), and density(0.137 g/cm3) was obtained when modified with TMCS and heat-treated at 450 �C. This product has properties desirable for variousapplications and sanctions the proposed synthesis, which exploitsa low-cost silica source (sodium silicate), for the large-scale com-mercial production of hydrophobic and hydrophilic silica xerogelsat an ambient pressure.

Acknowledgement

This research is supported by the collaborative researchProgram among industry, academia, and research institutesthrough Korea Industrial Technology Association (KOITA) fundedby the Ministry of Education Science and Technology (KOITA-2010).

Fig. 12. TEM micrograph (low and high magnification) of (a) unmodified silica gel together with (b) those modified with TMCS and (c) heat-treated at 450 �C.



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