effect of tio2 on the pore structure of sio2-pdms ormosils

Journal of Sol-Gel Science and Technology 18, 105–113, 2000c© 2000 Kluwer Academic Publishers. Manufactured in The Netherlands.

Effect of TiO2 on the Pore Structure of SiO2-PDMS Ormosils

F. RUBIO∗, J. RUBIO AND J.L. OTEOInstituto de Ceramica y Vidrio (C.S.I.C.), 28500 Arganda del Rey, Madrid, Spain

[email protected]

Received June 16, 1999; Accepted February 28, 2000

Abstract. In this work, pore structure evolution of Ormosils containing TBT (Tetrabutyl titanate) has been char-acterized by means of mercury porosimetry, nitrogen adsorption and helium pycnometry. These ormosils havebeen prepared by the sol-gel method by the reaction of TEOS (tetraethoxysilane), PDMS (polydimethylsiloxane,silanol terminated) and TBT under acid-catalyzed conditions. The addition of TiO2 increases the volume and spe-cific surface of secondary micropores and at the same time decreases the corresponding volumes of mesoporesand macropores. The presence of TiO2 gives also a continuous decreasing in the pore connectivity being 9.7 forthe ormosil without TiO2 and 4.4 for that of the higher concentration of TiO2. However, the pore length showsa significant decrease with the first addition of TiO2 changing from 9.1 to 2.2 at the higher TiO2 concentration.Pore volumes show a decrease as the TiO2 concentration is increased in the ormosil. On the other hand, densityincreases and porosity decreases with the TiO2 concentration. These results are in accordance with the presence ofTiO2 nanoparticles in the ormosil and the size of such nanoparticles increases with the TiO2 concentration. Fractalconstant has a low value, close to 2, for all different samples meaning that these ormosils can be considered as lowsurface roughness materials.

Keywords: mercury porosimetry, titanium ormosils, pore structure, interconnection

1. Introduction

Hybrid organic-inorganic materials have been widelystudied during the last decade. This has been due to theirrelatively easy synthesis utilizing the sol-gel process.These hybrid materials are a mixture of organic andinorganic groups at a molecular level, their propertiesbeing dependent on both organic and inorganic parts.ORganically MOdified SILicates (ORMOSILS) arenew hybrid materials with interesting properties.Ormosil materials prepared by the reaction oftetraethoxysilane (TEOS) and polydimethylsiloxane(PDMS) have been extensively studied by Mackenzieand co-workers [1]. These ormosils may be consideredas “ceramic rubbers” depending on the TEOS/PDMSmolar ratio. When the PDMS concentration isincreased, the ormosil material presents rubbery

∗To whom correspondence should be addressed.

properties. However, for high TEOS concentrationsvery hard ormosils can also be obtained. These rub-bery properties are also dependent on different reac-tion parameters, such as temperature, acid concentra-tion, reaction time, etc. [2]. Mackenzie et al. [3] haveshown that different pore structures can be obtained inthe ormosil materials.

SiO2-TiO2-PDMS ormosils may be used as passivewaveguides [4] and their homogeneity level caused bythe inorganic content may influence the optical prop-erties. Most of the time, preparation of ormosils bythe sol-gel method leads to phase-separated materi-als in which SiO2 and TiO2 particles are dispersedin an organic polymer matrix [5]. It is the aim ofthis work to study the influence of the incorpora-tion of TiO2 in the pore structure of TEOS-PDMSormosils.

106 Rubio, Rubio and Oteo

2. Experimental

2.1. Material Preparation

Ormosils were prepared by using TEOS (Merck foranalysis) as source of SiO2, PDMS OH terminated(Gelest, Germany) with an average molecular weightof 1750 was used as an organic component, and tetra-butyl titanate, TBT, (Aldrich for analysis) as a sourceof TiO2. Isopropyl alcohol (iPrOH) was the solventand hydrochloric acid (HCl) the catalyst. Firstly threesolutions were prepared, A, B and C. The A solutioncontains the total volume of TEOS, PDMS and 1/3 ofthe total volume of iPrOH. The B solution contains 1/3of iPrOH and the total volume of HCl and H2O and theC solution contains the remaining iPrOH and the totalvolume of TBT. All solutions are stirred for 5 minutesat room temperature for homogenization. The A andB solutions are mixed in a flask container and intro-duced into a thermostatic bath at 80◦C under refluxingand stirring. The C solution (TBT solution) was added,using a pipette, each 5 minutes in order to avoid precip-itation of Ti(OH)4. The reaction time was 25 minutesfor all studied samples. After that time, the solution wascast in a plastic container and sealed until gelation.

In order to know the influence of TBT in the or-mosils, samples with different TBT content were pre-pared. In our experiments, we kept constant the massratio Inorganic/Organic to 70/30 (where Inorganicis the sum of TEOS and TBT and Organic is thePDMS) and the molar ratios of H2O/Inorganic= 3,HCl/Inorganic= 0.3 and iPrOH/Inorganic= 4.5. Theratios TEOS/TBT were 70/0, 69/1, 67/3, 65/5 and60/10 (T00, T01, T03, T05 and T10 samples respec-tively). The total volume of solutions was always 75 mlin order to avoid volume influence in the final materialproperties.

Gelation time of all these samples was about 3 hours.Samples are kept in closed containers for 1 week pour-ing out, each day, the syneresis liquid. After that time,containers are opened to evaporate solvents at roomtemperature for 5 days followed by a drying in anoven at 50◦C until no weight loss was observed. Allof the obtained ormosils were transparent showing thatno apparent phase separation was produced during thereaction.

In accordance with the high acid concentration, itis assumed that TEOS and TBT are fully hydrolysedand condensed to yield SiO2 and TiO2 respectively,and therefore they can react with PDMS to form SiO2-TiO2-PDMS ormosils.

2.2. Material Characterization

Material pore structures were characterized by Hgporosimetry, nitrogen adsorption and He pycnometry.For Hg porosimetry we have used an Autopore II 9215(Micromeritics Corp.) in the range of pressures be-tween 0 and 4.081 MPa. All samples were outgassedbelow 6.67 Pa at room temperature before measure-ments.

Nitrogen adsorption-desorption isotherms were ob-tained with an Accusorb 2100 E (Micromeritics Corp.)in the whole partial pressure range. The temperatureof measurement was 77 K. All samples were alsodegassed for 18 hours at 120◦C.

Density results have been obtained by He pycnome-try and Hg porosimetry. We have assigned as real den-sity to that measured with He assuming that such gascan penetrate into the smallest pores. Bulk density isassigned to that measured with Hg when no pressure isgiven and therefore any pores are filled with it. Finally,apparent density is defined as that measured with Hgwhen pores with sizes higher than 50 nm are filled withHg. Real density values were performed in a micro-pycnometer (Multipycnometer, Quantachrome, USA).

3. Results and Discussion

Mackenzie et al. [3, 6] and Babonneau et al. [7, 8]have mainly studied by29Si MAS-NMR and SEM themolecular structures and microscopic structures of dif-ferent ormosils obtained by using TEOS and PDMS.Different structures have been proposed which explainthe influence of the addition of a titanium alkoxide tothe TEOS-PDMS ormosil reaction. The TEOS plays arole of a cross-linking agent. On the other hand, tita-nium acts as both cross-linking agent between PDMSmolecules and also catalyses the condensation reac-tions to form long PDMS chains [7, 8]. The finalormosil may be described as a nanocomposite madeof PDMS chains and oxide-based particles. Therefore,the ormosil pore structure must be formed by differenttype of pores corresponding to the above mentionednanocomposite material.

We have studied the changes produced in the porestructure when TiO2 is added to SiO2-PDMS ormosilsby means of nitrogen adsorption, mercury porosimetryand helium pycnometry.

Nitrogen adsorption-desorption isotherms for thestudied materials are presented in Fig. 1. In this figure,isotherms were vertically shifted in order to avoid

Effect of TiO2 on the Pore Structure of SiO2-PDMS Ormosils 107

Figure 1. Nitrogen adsorption-desorption isotherms at 77 K for ormosil samples with different TiO2 content.

overlapping. These isotherms show well-defined hys-teresis loops corresponding to a porous materials. Suchhysteresis loops are of type H2 according to the IUPACclassification [9], and they were in the past attributedto a difference in mechanism between condensationand evaporation processes occurring in pores with nar-row necks and wide bodies (“ink-bottle” pores). How-ever, as it will be discussed below, it is now recognizedthat such hysteresis loops correspond to interconnectedpore structures in which the pore size distribution canbe determined from the nitrogen adsorption isotherm,and the desorption isotherm is determined by the dis-tribution of neck sizes [10].

Mercury intrusion-extrusion curves obtained for thestudied ormosils are shown in Fig. 2. These curves cor-respond to the Class I according to Day et al. [11]which are characterized by one-step intrusion curve anda similar extrusion curve. These curves correspond tounimodal pore distributions. All of these curves arevery close to each other showing close pore volumesfor all samples.

Using both the nitrogen adsorption-desorption me-thod and the mercury porosimetry technique, a com-plete picture of the whole pore size distribution canbe obtained. It is well-known that nitrogen adsorptionis used for studying pores of diameter between 0 and25 nm. On the other hand, mercury porosimetry permitsto study pores of diameter between 2 nm and 0.15 mmdepending on the intrusion pressure. However, becausehigh intrusion pressures can compress the sample, themeasurements are finished at about 25 nm, i.e. at theend of the nitrogen method. Pore size distributionshave been determined by the BJH method [12] in therange of mesopores (pores of radius between 1 and25 nm). We have used the adsorption branch of the ni-trogen isotherm for such analysis because, as has beenmentioned before, now it is recognized that the step ofthe desorption branch corresponds to interconnectedpore structures where larger pores are connected tosmall pores. Such small pores avoid nitrogen desorp-tion until the equilibrium pressure reaches a value thatcorresponds to the evaporation of these small pores.


Figure 2. Hg intrusion-extrusion curves for ormosil samples with different TiO2 content.

Cylindrical geometry is assumed for all pores for bothnitrogen isotherms and mercury porosimetry. Pore sizedistributions of the studied ormosil samples are shownin Fig. 3. These curves are very close except in the lowpore radius region where an increase of the volume ofpores is observed as the concentration of TiO2 in theormosil is increased. The observed increase near thepore radius limit of 1 nm, indicates the presence of mi-cropores (pores of radius below 1 nm). The continuousincrease in the pore volume as the pore radius decreasesis in accordance with a homogeneous distribution ofpores in the samples. Here, in the SiO2-TiO2-PDMSormosils, the pore size distribution is homogeneous,and the presence of TiO2 does not seem to change theform of the distribution. However, these pore size dis-tributions do not take into account the respective micro-porosity. The analysis of microporosity can be carriedout by thev-α or v-t methods [13].

Thev-α or v-t methods can be used for assessmentof microporosity in any kind of porous materials. Bothmethods consist on representing the experimental ad-sorbed volume versus a reference isotherm obtained for

a non-porous material. If the obtained representationis a straight line then the studied sample is also non-porous. However, if an upward or downward deviationis obtained, then the studied sample will have meso-pores or micropores respectively. Figure 4 shows thev-α plot for the studied ormosils. It can be observedthat a more gradual downward deviation appears as theTiO2 concentration is increased in the ormosil. There-fore, the addition of TiO2 to TEOS-PDMS ormosils in-creases the amount of micropores. All curves of Fig. 4show also a linear region that can be fitted to a straightline passing through the origin axes. This result showsthat all samples only have secondary micropores, i.e.micropores of size dimensions between 0.6 and 2 nm[14].

In the downward deviation of thev-α plot a fit to astraight line can also be made, and the correspondingintercept on the adsorption axis gives the microporecontribution, i.e., adsorbed volume of nitrogen in themicropores. Then, if this micropore volume is added tothe mesopore one obtained by the BJH method andalso to the macropore volume obtained by mercury


Figure 3. Pore size distributions of ormosil samples containing TiO2. The measurements above 10−2 µm are carried out by Hg porosimetryand below 10−1 µm by the nitrogen adsorption-desorption method.

porosimetry, the total pore volume of the studied or-mosil samples can be calculated. These values aregiven in Table 1. In accordance with the results ofTable 1, it is observed that the addition of TiO2 to the or-mosil samples increases the volume of micropores anddecreases the volumes of meso and macropores. Thefinal porosity of the ormosil also decreases with theTiO2 content. The ormosil sample T00, sample with-out TiO2, has a lower micropore volume than thoseof meso or macropores, being that micropores abouthalf of macropores. However, the ormosil sample with

Table 1. Pore volumes for the studied ormosil samples.

Pore volume (cm3/g) T00 T01 T03 T05 T10

Micropores (A) 0.153± 0.007 0.171± 0.007 0.202± 0.008 0.209± 0.008 0.222± 0.009

Mesopores (B) 0.247± 0.009 0.228± 0.008 0.204± 0.008 0.178± 0.007 0.169± 0.007

Macropores (C) 0.282± 0.009 0.281± 0.009 0.271± 0.009 0.270± 0.009 0.244± 0.008

Sum (A+B) 0.400 0.399 0.406 0.387 0.391

Sum (A+B+C) 0.682 0.680 0.679 0.657 0.635

higher concentration of TiO2 (T10 sample) the micro-pore volume is very close to that of macropores.

These results are in accordance with the above de-scription of these ormosils as nanocomposite materials.The increase in the TiO2 concentration in the ormosilforms nanometric particles in size where microporesare present. This result is in accordance with the struc-tural model proposed by Babonneau [15].

In accordance with results of Table 1, it can be ob-served that as the TiO2 concentration is increased inthe ormosil samples, a continuous increasing in the


Figure 4. α plots for the studied ormosil samples corresponding to the isotherms presented in Fig. 1.

micropore volume and a continuous decreasing in themesopore volume is obtained. From T00 sample toT10 sample the increase in the micropore volume isabout 31% and this value is similar to the decreasing inthe mesopore volume (31.6%). On the other hand, ifwe add the volume of micro and mesopores (A+B ofTable 1), such volume is about 0.4 cm3/g for all sam-ples. It means that the incorporation of TiO2 createsnew microporosity inside the mesopores. However, adifferent behaviour is observed for macropores. In thiscase, the volume of macropores does not change withadditions of TiO2 lower than 3%, and slightly decreaseswith additions up to 10%. This result shows that TiO2 iswell-incorporated in the ormosil network. The additionof titanium alkoxide in part catalyzes the siloxane net-work formation and, in parallel, titano-siloxane bondsare also developed. The particulate heterogeneities oftitania and silica are the results of somewhat pref-erential formation of pure titanoxane and siloxanebonds. These particulate heterogeneities are then theresponsible for the increase in the observed microporevolume.

In the characterization of a porous solid it is not atall sufficient to consider a pore size distribution, butthe more delicate aspect it is to know the “morphol-ogy” or how these pores are topologically distributedthroughout the system. Morphology plays an equal oreven more an important role than the mere pore sizedistribution. Several works are related with the mor-phology of pore materials [16, 17]. One important as-pect of pore morphology is the connectivity betweenpores. The nitrogen isotherms of Fig. 1 correspond tothe interconnected pore structures. In other work, wehave shown that pore interconnection can be used forcharacterizing the pore structure of silica xerogels [18].Seaton [19] has presented an analysis method, basedon the percolation theory, that allows a measure of theconnectivity and the mean coordination number of thepore network. These data can be easily determinedfrom nitrogen adsorption measurements. This methodis applicable to hysteresis loops of the IUPAC types H1and H2. In these types of pores, the adsorption of ni-trogen occurs according to a multilayer-capillary con-densation mechanism where the condensation pressure


is a function of pore size, with nitrogen condensing insmaller pores at lower pressures. However, during thedesorption process, nitrogen vaporises from a liquid-filled pore when two conditions are satisfied: (i) theapplied pressure is below the condensation pressurefor a pore of that size, and (ii) the pore has access tothe vapour phase. Figure 1 shows nitrogen isothermswhich have type H2 hysteresis loops and then percola-tion theory can be applied. These hysteresis loops showa different mechanism of adsorption-desorption. Theadsorption of nitrogen increases with pressure. How-ever, the desorption at high pressures is practically pre-vented when decreases the pressure until reaching agiven pressure from which desorption takes place veryrapidly. In the high pressure region, the desorption oc-curs in some pores which are on the surface of the sam-ple. However, most part of these pores located on thesurface have a pore size which condensation pressureis lower than the experimental applied pressure and,therefore, these pores prevent the evaporation of nitro-gen of other inner pores of higher pore size. When theexperimentally applied pressure is lower than the con-densation pressure, those surface pores release nitrogenand permits the evaporation of nitrogen condensed inthe inner pores. This evaporation occurs through a per-colation mechanism. At the end of the hysteresis loopall pores have access to the vapour phase and the des-orption and adsorption isotherms coincide. Therefore,desorption isotherms show a sharp knee while adsorp-tion isotherms show an increasing trend, as can be seenin Fig. 1.

Seaton [19] has proposed a method from which theconnectivity of the pore structure is defined by the meancoordination number,Z, and the mean linear dimensionof pores,L. This linear dimension is defined as a hypo-thetical number of mean pore lengths. Both parameterscan be determined from adsorption-desorption nitro-gen isotherms. This method is based on the percolationtheory of porous materials. In the application of thepercolation theory to the nitrogen desorption process,the pore occupation probability,X, is defined as theratio of the number of pores in which nitrogen is belowits condensation pressure to the total number of poresin the network, and the accessibility,XA, is defined asthe ratio of the number of pores from which nitrogenhas vaporised to the total number of pores. Experi-mental values ofX andXA determined from nitrogenisotherms are used in an universal function from whichZ andL can be obtained [20]. Figure 5 shows the resultsfor the accessibility as a function of pore occupation

Table 2. Interconnection (Z) and mean pore length (L) values forthe ormosil samples.

Ormosil sample

T00 T01 T03 T05 T10

Z 9.7± 0.1 7.9± 0.1 6.7± 0.1 5.8± 0.1 4.4± 0.1

L 9.1± 0.1 2.5± 0.1 2.3± 0.1 2.3± 0.1 2.2± 0.1

probability for the studied ormosils, and Table 2 givesthe obtained values ofZ and L when such curvesare fitted to the universal function. This fitting proce-dure was carried out using a non-linear least-squaresalgorithm.

Values of Table 2 show that important changes areproduced in the pore structure of the ormosils whenTiO2 is incorporated. It must be taken into account thatthese results only refer to mesopores because the calcu-lation procedure is applied to nitrogen isotherms. Simi-larly, such procedure could be applied to the mercuryporosimetry curves. However, curves of Fig. 2 do notshow any sharp knee characteristic of a percolation pro-cess as nitrogen isotherms do. Seaton has shown thatthe more highly connected the network the righter devi-ation of the knee is observed [19]. In the limit of infinitemean coordination number, the adsorption and desorp-tion isotherms should coincide over the whole pressurerange. Intrusion and extrusion Hg curves of Fig. 2 co-incide and we can assume that a high connectivity existsbetween macropores in the studied ormosils and suchconnectivity does not change with TiO2 addition. Onthe other hand, there is not any method for obtainingthe connectivity between micropores.

The ormosil without TiO2 shows the highest val-ues of Z and L showing both a high connectivitybetween mesopores and high mean pore length. Theaddition of TiO2 continuously decreases the connec-tivity. This result is in accordance with the formationof particulate heterogeneities of titania due to the re-sult of preferential formation of pure titanoxane bonds.These nanoparticles of TiO2 are located so as to dis-turb the connectivity of mesopores and, therefore, thepore interconnection decreases. These nanoparticlesincrease also the micropore volume of the ormosil,as it is presented in Table 1. However, the mean porelength shows a drastic change with the first additionof TiO2 to the ormosil structure, remaining practicallyconstant the mean pore length as the TiO2 concentra-tion in the ormosil is increased. These results show thatfor the lowest addition of TiO2 (1%) a homogeneous


Figure 5. Accessibility (XA) as a function of pore occupation probability (X) for the studied ormosil samples.

distribution of TiO2 is achieved where nanoparticlesof TiO2 are formed suppressing both the growth ofthe mesopore length and their interconnection. Thesize of these TiO2 nanoparticles increases with theTiO2 concentration in the ormosil and new nanopar-ticles of TiO2 are formed where mesopores are con-nected. That results in an increment of the microp-ore volume, remaining constant the mesopore meanlength. Therefore the incorporation of TiO2 to TEOS-PDMS ormosils forms nanocomposite materials in ac-cordance with the description of Babonneau et al.[7, 8].

Table 3. Density, porosity and fractal constant for the studied ormosil samples.

Ormosil sample

T00 T01 T03 T05 T10

Real density (cm3/g) 1.30± 0.02 1.30± 0.02 1.32± 0.03 1.33± 0.03 1.34± 0.03

Apparent density (cm3/g) 1.29± 0.02 1.29± 0.02 1.30± 0.03 1.31± 0.03 1.33± 0.03

Bulk density (cm3/g) 0.90± 0.02 0.91± 0.02 0.93± 0.02 0.96± 0.02 0.98± 0.03

% Porosity 30± 2 30± 2 29± 2 27± 2 26± 2

Ds 2.00± 0.09 2.00± 0.09 2.04± 0.08 2.07± 0.07 2.16± 0.09

Finally, we have studied the evolution of the densityand the surface fractal constant (Ds) as a function ofthe TiO2 concentration in the TEOS- PDMS ormosils.Table 3 gives density values obtained for the studiedsamples. It can be observed that the addition of TiO2

increases the density of ormosils and decreases theirporosity. Such increase is associated to both the higherdensity of TiO2 than SiO2 and the above mentionedelimination of mesopores and macropores (Table 1).The observed decrease in porosity is also consistentwith the elimination of mesopores and macropores andalso with the loss of pore connectivity.


Fractal characterization of the ormosil samples hasbeen carried out by the method proposed by Ismailet al. [21] using the adsorption-desorption nitrogenisotherms. Such method is based on plotting the ad-sorption isotherm according to the Frenkel-Halsey-Hillequation [21] and fitting to a straight line in the rangeof 1 to 2 monolayers. The exponent of this equationis related to the surface fractal dimension,Ds. Dataof this analysis is also shown in Table 3. Fractal con-stant (Ds) is close to 2 (smooth surface) for all stud-ied samples meaning that the incorporation of TiO2

does not increase the surface roughness. These resultsare in accordance with those obtained by Mostakefet al. [22] where SiO2-TiO2-PDMS ormosils can beconsidered as high volume homogeneity and low sur-face roughness materials. However it can be observedhow Ds shows a gradual increase as the titania con-centration in the ormosil is increased. It indicates thatthere is an average increase of the roughness of poresurfaces, suggesting that the effect of the added tita-nium alkoxide on the overall structure of the silica-PDMS ormosil network is not limited only to boththe local particle formation and the change in the porestructure. The increase of TiO2 in the ormosil networkproduces also an increase in the surface roughness ofsuch materials.

4. Conclusions

We have studied the influence of TiO2 in the porestructure of TEOS-PDMS ormosils by means of ni-trogen adsorption, mercury porosimetry and He pyc-nometry. The addition of TiO2 increases the volumeof micropores and decreases the volume of mesoporesand macropores. On the other hand, the connectivitybetween adjacent pores decreases continuously whenTiO2 is added to a TEOS-PDMS ormosil, and at thesame time the main pore length is drastically decreased.The density of such ormosils increases with the in-corporation of TiO2 due to the removal of porosity.Due to the low value of the fractal constant this kindof ormosils can be considered as high volume homo-geneity and low surface roughness. The observedincrease of the fractal constant with the TiO2 con-centration in the ormosil shows that the pore struc-ture, density and surface roughness are affected by theaddition of a titanium alkoxide to the TEOS-PDMScomposition.

Acknowledgments

We are grateful to the Comisi´on Interministerial deCiencia y Tecnologia (CICYT) of Spain for supportof this work under project MAT96-0564.

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effect of tio2 on the pore structure of sio2-pdms ormosils

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