Journal of Colloid and Interface Science 317 (2008) 206–213www.elsevier.com/locate/jcis

Characterization of the hydrophobicity of mesoporous silicas and clayswith silica pillars by water adsorption and DRIFT

João Pires a,∗, Moisés Pinto a, Juncal Estella b, Jesús C. Echeverría b

a Departamento de Química e Bioquímica da Faculdade de Ciências de Lisboa, Centro de Química e Bioquímica, Edifício C8, Campo Grande, Lisboa, Portugalb Departamento de Química Aplicada, Universidad Pública de Navarra, Campus Arrosadía, 31006 Pamplona, Spain

Received 21 June 2007; accepted 14 September 2007

Available online 18 September 2007

Abstract

The hydrophobic–hydrophilic properties of a solid are related to the material chemistry and, often, these properties are relevant to the applica-tions of a particular material. Contrarily to what happens with other properties, such as specific surface areas or pore volumes, the methodologiesto ascertain on the hydrophilicity of a porous material are not well defined. In this work, we discuss and relate the information on the hydrophobic-ity degree obtained from water adsorption isotherms and from diffuse reflectance infrared Fourier transform (DRIFT), in a set of porous materials.The studied materials were mainly mesoporous solids, namely of MCM-41 and SBA-15 types, two xerogels and also different porous clays het-erostructures. Both techniques were informative on the hydrophobic–hydrophilic properties of the studied samples, but the correlation betweenthe information obtained by each technique was not straightforward. Water adsorption isotherms are much more sensitive to the differences of thestudied materials than the DRIFT spectra. For silica-based mesoporous materials with similar surface chemistry, the water adsorption process andhence, the hydrophobic–hydrophilic properties, is mainly dependent on the pore diameters. However, water adsorption is much more sensitive tochanges in the nature of the adsorbent surface than to changes in the pore diameter.© 2007 Elsevier Inc. All rights reserved.

Keywords: Water adsorption; DRIFT; Mesoporous materials; Silicas; Hydrophilicity; Hydrophobicity

1. Introduction

Mesostructured silica and silica-like materials, either un-modified or surface modified, have a wide range of applicationsin adsorption and catalysis [1–3]. Textural properties such asspecific surface area, or pore size distributions and pore vol-umes are the most relevant properties of these materials in re-lation with their potential uses. However, for some applicationsas, for instance, in the abatement of volatile organic compounds(VOCs) [4], in the controlled drug release [5], or even for thestability of some silica structures [6,7], the hydrophobic natureof the material is also a relevant property.

There are in the literature various proposal to access the hy-drophobicity degree of porous materials. Some of these propos-als are based on the water loss at different temperatures using

* Corresponding author. Fax: +351 217 5000 088.E-mail address: jpsilva@fc.ul.pt (J. Pires).

0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2007.09.035

thermogravimetry [8], and the use of data from thermogravime-try and nitrogen adsorption [9] or heats of immersion [10] werealso suggested. In the case of zeolites, the use of competitive ad-sorption of water and hydrocarbons was also considered [11].

The degree of hydrophobicity of the surface reflects the ma-terial chemistry and, by its very nature, can be related with theinteraction with water molecules and, therefore, it is expectedthat water adsorption isotherms can be highly informative inthis context. A considerable number of studies exist in the lit-erature related to the adsorption of water in mesoporous silicaor related materials [7,12–15]. However, these studies normallyconcern more about the characterization of a given surface, orthe structural stability toward water, and less the hydrophobicitydegree of a given sequence of materials. One of the few studiesthat proposed the assessment of the hydrophobic–hydrophilicproperties of a series porous solids [16], with different mate-rial chemistry, from the water adsorption isotherms used theenergetic parameter of the Dubinin–Asthakov equation [17].More recently, the use of the diffuse reflectance infrared Fourier

J. Pires et al. / Journal of Colloid and Interface Science 317 (2008) 206–213 207

transform (DRIFT) spectroscopy, was proposed for the quantifi-cation of the degree of hydrophobicity [6]. Nevertheless, to ourknowledge, the information on the hydrophobicity degree ob-tained for a given set of materials from the water adsorptionisotherms and DRIFT experiments was not yet compared anddiscussed.

In the present work we intend to discuss the informationon the hydrophobicity degree obtained, from water adsorptionisotherms and from DRIFT spectra, in a set of porous solidswith different material chemistry. The studied solids are mainlymesoporous (pore widths between 2 and 50 nm) [18], and twoof them, the MCM-41 [19] and the SBA-15 materials, presenta well defined mesoporous structure [14,20]. Additionally, twoamorphous xerogels prepared by sol–gel process in acid me-dia and aged in NH3(aq), as well as various porous clays het-erostructures (PCHs) were also studied. PCH solids are rela-tively new and are obtained by surfactant direct assembly ofopen framework silica in the galleries of different types ofclays [21–23]. PCHs have pore sizes in the transition betweenmicro to small mesopores, a particularity that make them po-tential catalysts for transformations that involve molecules withsizes larger than the pores of conventional zeolites [24].

2. Experimental

2.1. Materials

MCM-41 was synthesized according to procedures de-scribed in the literature [25]. Briefly, 5.28 g of cetyltrimethy-lammonium bromide—CTAB (Aldrich), was dissolved in264 cm3 of deionized water and aqueous ammonia was added.The silica source, tetraethoxysilane—TEOS (Aldrich, 98%)was then added (22.4 cm3) under stirring. The solid was driedat 90 ◦C and heated at 550 ◦C for five hours, after a ramp of1 ◦C min−1. The preparation of SBA-15 was adapted from orig-inal procedures [20,26] by adding 126 cm3 of a 1.6 M HClsolution to 4.0 g of poly(ethylene glycol)-block-poly(propyleneglycol)-block-poly(ethylene glycol) (from Aldrich, Mn ∼5.800). The mixture was stirred until it became colourless andTEOS (9.1 cm3) was then added also under stirring. The mix-ture was sealed and kept in a drying oven at 35 ◦C for 24 h andat 100 ◦C for an additional period of 24 h. The solid was fil-tered, dried, and calcined at 550 ◦C for five hours, after a rampof 1 ◦C min−1.

Xerogels were synthesized using the sol–gel process asdetailed described elsewhere [27]. TEOS (Fluka, 98%) andethanol (Merck, GR absolute) were mixed in a 1:4.75 molar ra-tio with a magnetic stirrer. While stirring, the amount of water(Milli-Q quality) needed to obtain a TEOS:water molar ratioof 1:5.5 was added drop by drop, and the pH of the solutionwas adjusted at 4.5 by adding 0.05 M HCl (Merck) with an au-tomated burette (Titrino 702 SM, Metrohm, Herisau, Switzer-land). Sample containers were closed and kept until gelation inan orbital shaker. Alcogels were covered with 5 ml of the agingsolvent, which was 0.5 or 2.0 M aqueous ammonia for XG 0.5and XG 2 samples, respectively, and allowed to age for 7 days.Then the alcogels were dried at atmospheric pressure and room

temperature to obtain xerogels. Glass containers were coveredwith parafilm with several needle holes to allow for the evapo-ration of the aging solvent.

The PCHs were prepared from a Portuguese clay (struc-tural formula: (Si3.70Al0.30)IV(Al1.16Fe0.51Mg0.45)VI(Ca/2, K,Na)0.39) previously characterised [28]. To the fraction <63 µm,the carbonates were removed and the sample was then washedin a dialysis tube, until a conductivity lower than 1 mS m−1.In this work, four different porous clays heterostructureswere studied, which will be labelled from PCH-1 to PCH-4.The preparation of these samples was based in the literature[21–23,29–31]. For the samples PCH-1 and PCH-2, a suspen-sion of the clay (1 g in 100 cm3 of water) was firstly equilibratedwith a 0.5 M solution of an ionic surfactant, the cetyltrimethy-lammonium bromide—CTAB (Aldrich)—under stirring duringone night at 50 ◦C. The solid was then separated from solu-tion by centrifugation and washed with demineralised wateruntil pH ≈ 7 and air-dried. A given amount of neutral aminewas then added: decylamine (Aldrich, 95%) or dodecylamine(Aldrich, 98%) for PCH-1 and PCH-2, respectively. The aminewas added under stirring, after which, TEOS was added as silicasource, in a molar proportion amine:TEOS of 20:150 (PCH-1)or 20:120 (PCH-2). After 3 h of mixing the solid was air-driedand calcined at 650 ◦C for 5 h with a ramp of 1 ◦C min−1. ForPCH-3 and PCH-4, 2.5 or 25% (molar in relation to TEOS), re-spectively, of 3-aminopropyltriethoxysilane—APTES (Aldrich,99%) was added. After addition of dodecylamine and TEOS, asalready described for PCH-1 and PCH-2, the air-dried solidswere solvent extracted with a solution of 1 M HCl in ethanol inreflux during 24 h.

2.2. Methods

Nitrogen adsorption isotherms at −196 ◦C were determinedin an automatic apparatus (Micromeritics, mod. ASAP 2010).Water adsorption isotherms were also measured in a auto-matic apparatus (Coulter, mod. Omnisorp 100Cx), using afixed vapour dosing method as described in more detail else-where [16]. The adsorption temperature (30 ± 0.1 ◦C) wasmaintained with a recycle bath equipped with a temperaturecontroller (Eurotherm, mod. 2216L). Water was bi-distilled,de-ionised, and purified by freeze-vacuum-thaw cycles. The re-producibility of the water adsorption isotherms was, as moredetailed discussed in a previous work [16], checked againstdata obtained in a manual installation, and was better than 5%.A similar value was obtained for the nitrogen adsorptionisotherms also.

X-ray diffractograms were determined in a Philips PX 1710instrument using the CuKα radiation. In the case of the PCHssamples, oriented and non-oriented mounts [32] were used butno diffraction peaks were detected. This experimental fact wasalready reported in the literature [23], and can be related to thepoor long-range order presented by some types of porous claysheterostructures [23,24].

The diffuse reflectance infrared Fourier transform (DRIFT)spectra of the samples were collected on a Nicolet 6700 at2 cm−1 resolution using the Smart Diffuse Reflectance acces-

208 J. Pires et al. / Journal of Colloid and Interface Science 317 (2008) 206–213

sory, at room temperature with a DTGS TEC detector. Thesamples were prepared by mixing with KBr in 1% weight andpowdered in an agate mortar. Each collected spectrum was anaverage of 128 scans of the sample subtracted by the averageof 64 background scans using only KBr in the sample con-tainer. These conditions allowed obtaining spectral absorbancein the range for the application of the Kubelka–Munk transfor-mation [33]. Within each sample, the most intense absorptionband, observed at ∼1090 cm−1 was used for the normalizationof the band intensities. Only for the observation of the APTESabsorption bands the spectra (not shown) of PCH-3 and PCH-4 were obtained without the dilution with KBr. The bands ofAPTES were observed at 2927–2857 cm−1 due to the C–Hstretching modes, at 1650 cm−1 due to N–H bending, and thebands between 1500 and 1400 cm−1 due to the aliphatic C–Hbends [34,35].

3. Results and discussion

3.1. Textural properties

The textural properties of the samples were evaluated fromthe N2 adsorption isotherms at −196 ◦C (Fig. 1) and the re-spective specific surface areas, micro, meso, and total pore vol-umes are given in Table 1. The curves obtained for MCM-41,SBA-15, and both xerogels are of type IV according to the IU-PAC classification [18], although for MCM-41, due to the lackof hysteresis, the isotherm should be classified type IVc [36].These isotherms are characteristic of mesoporous solids andagree with literature results [7,19,37]. In the cases of the PCHsthe total adsorbed amounts are lower than those found for thematerials already mentioned and the isotherms in Fig. 1 presentmixed characteristics, of types I and IV, as a most probable con-sequence of the pore sizes being in the transition range frommicro to mesopores, as already discussed by several authors[23,24,31].

From the low temperature nitrogen adsorption data, the poresize distributions were obtained by the Broekhoff–de Boermethod in a version that also uses Frenkel–Halsey–Hill equa-tion (BdB–FHH) [38]. This methodology gives more reliablepore size distributions than the more currently used Barret–Joyner–Halenda (BJH) method [39] and, as discussed else-where [13], the results obtained by the BdB–FHH method cangive mesopore size distributions that match well those obtainedby more elaborated methods as those based on the density func-tional theory (DFT). The pore size distributions so obtainedare given in Fig. 2 and the maxima for the more well-knownMCM-41 (3.2 nm) and SBA-15 (6.4 nm) are within the values

published by other authors [25,37]. In the case of the xero-gels, the samples presented pore size distributions which werebroader than for the remaining materials and the maximum ap-peared around 4.5 nm for xerogel aged in 0.5 M NH3(aq) and6.0 nm for xerogel aged in 2.0 M NH3(aq). For the PCHs, themaxima in the mesopore size distributions are near 3.0 nm, butthe distributions are broader for samples PCH-3 and PCH-4 inwhich 3-aminopropyltriethoxysilane was added in the synthe-sis.

3.2. Water adsorption isotherms and DRIFT spectroscopy

The water adsorption isotherms at 30 ◦C, with the adsorbedamounts in each material per unit of surface area (mmol/m2),in the studied samples are given in Fig. 3. In the case of theMCM-41 material, the shape of the curve can be considered oftype V according to the IUPAC recommendations [18,40], andit is indicative of an hydrophobic character. In some way this

Fig. 1. Nitrogen adsorption isotherms at −196 ◦C in the indicated materials.

Table 1Specific surface area (ABET), microporous (Vmicro) and total pore volumes (Vtotal) obtained from the N2 adsorption isotherms at −196 ◦C

PCH-1 PCH-2 PCH-3 PCH-4 MCM-41 SBA-15 XG 0.5 XG 2

ABET (m2/g) 665 908 677 916 1087 728 666 517Vmicro

a (cm3/g) 0.29 0.38 0.27 0.37 0 0.06 0.22 0.17Vtotal

b (cm3/g) 0.43 0.57 0.39 0.50 0.86 0.87 0.45 0.55

a From the t -method [36].b From the amount adsorbed at the relative pressure of 0.97 [36].

J. Pires et al. / Journal of Colloid and Interface Science 317 (2008) 206–213 209

Fig. 2. Pore size distributions obtained by the BdB–FHH method for the studiedsamples from the nitrogen adsorption isotherms at −196 ◦C.

Fig. 3. Water adsorption isotherms at 30 ◦C expressed by surface unit of eachadsorbent material.

curve approaches the sigmoid shape of the isotherms obtainedin highly hydrophobic materials such as activated carbons [41].In the case of SBA-15, and also for both studied xerogels, theconvexity toward the relative pressure axis is extended, in rela-tion to what happened to the MCM-41. For PCH-1 and PCH-2

samples, where only TEOS was used, the sigmoid character(type V isotherms) can still be observed. But for PCH-3 andPCH-4 samples, where TEOS and APTES were used, the shapeof the curves reveals an enhanced interaction between the ma-terial and the adsorbed molecules at low relative pressures.

According to shape of water adsorption surface concentra-tion, the material can be divided into three groups. PCH-3 andPCH-4 presented a sharp initial rise, increasing the pressurethe curves reach a plateau at p/p0 < 0.60. Samples MCM-41,XG 0.5, PCH-1, and PCH-2 present sigmoid curves. SBA-15and XG 2 samples have reduced isotherms that are convex to-ward the relative pressure axis in the highest range of pressures.SBA-15 presented values very close to the MCM-41 until p/p0

near 0.5, but from this point onwards the curves diverge.The water adsorption process seems to be dependent on the

surface chemistry of the sample, surface coverage and pore size.For PCH samples that have similar pore size, the different be-haviour of PCH-3 and PCH-4 at low pressures, in relation toPCH-1 and PCH-2, can be explained by the presence of aminegroups on the surface. For materials with pores similar to thoseof MCM-41 or smaller, the water adsorption proceeds by aprocess of surface coverage, being completely covered at valuesabout 0.02 to 0.035 mmol/m2. It is interesting to note that thesevalues are comparable with those reported by Dubinin et al. inactivated carbons [42] and by us in a previous work with vari-ous types of materials [43]. In the cases of materials with widerpores like xerogels, the water capillary condensation at high rel-ative pressures can also play an important role [42]. In fact, inthe cases of materials with wider pores the water adsorptionprocess seems to be dependent not only on the surface coveragebut also on a superimposed process of capillary condensation,as reported by other authors [42].

The above mentioned observations denote, in a qualitativeway, the different behaviours of the various samples towardthe adsorption of water, that is, their different hydrophobic–hydrophilic properties. One approach to quantify the hydropho-bic–hydrophilic properties of the materials from the water ad-sorption isotherms could be the analysis of the Henry con-stants (K). These values, determined from the initial slopes ofthe isotherms (as p/p0 → 0) are given in Table 2. The PCH-1and PCH-2 presented low values of K , close to the value ob-tained for the MCM-41 sample, but the SBA-15 material pre-sented a somewhat high value. The high values obtained for thexerogels, when comparing with MCM-41 and SBA-15 could

Table 2Hydrophobicity parameters obtained from the water adsorption isotherms(K and Amax) and the DRIFT spectra (A(Si–Od))

Water adsorption DRIFT

K (µmol/m2) Amax (kJ/mol) A(Si–Od) (%)

MCM-41 9.2 1.5 4.7SBA-15 19.2 0.4 2.7XG 0.5 39.0 1.0 6.6XG 2 40.6 0.6 5.2PCH-1 7.5 1.7 3.8PCH-2 9.9 1.5 7.0PCH-3 63.5 3.4 8.4PCH-4 45.8 3.7 6.8

210 J. Pires et al. / Journal of Colloid and Interface Science 317 (2008) 206–213

eventually be related with a highest number of silanols in thelatter materials. Nevertheless, what is more important to ob-serve in the context of the present work is the fact that the valuesof K would lead to the observation that the xerogels would bemore hydrophilic than the MCM-41 or the SBA-15 samples. Inthe low pressure region (p/p0 below 0.1) the adsorbed amountsare very sensitive to the presence of hydrophilic groups, evenif the number of these groups is small and does not change theproperties of the materials surface. Therefore the analysis basedon the K values could be misleading since it does not reflect themain behaviour of the material toward water adsorption.

A different approach to quantify these observations is ap-plying the well known concept of adsorption potential A (A =RT ln(p0/p)) to determine the value of A where the adsorp-tion isotherm changes the curvature (from convex to concave)in relation to the pressure axis. Mathematically this point cor-responds to the inflexion point of the adsorption curve. It canbe more easily determined from the maximum of the numeri-cal derivative of the adsorbed amounts with respect to A (theadsorption potential distribution curves). This particular pointAmax depends on the given adsorbent material and is asso-ciated with the position of the sharp rise in the adsorptionisotherm, since mathematically it also corresponds to the pointof the curve where the maximum slope is observed. Althoughin the present work the determination of Amax is independent ofany adsorption model assumption, this parameter is analogousto the energetic parameter of the Dubinin–Asthakov equation[17,44]. In fact, it was previously shown [16] that this parame-ter can coherently express the evolution of the hydrophobic–hydrophilic properties of various materials although, the exten-sion of this treatment to mesoporous materials was not, to ourknowledge, previously made.

This methodology can, in principle, be applied to severaltypes of materials and water adsorption isotherms. In theory,Amax can vary between ∞ and 0. A high value of Amax willcorrespond to a material with a high affinity to water, whereas alow value of Amax is characteristic of a hydrophobic material. Interms of the shape of the water adsorption isotherms, this meansthat only the isotherms that are highly concave to the pressureaxis (type I isotherms [18]) will have a high value of Amax.Conversely, the isotherms that are convex (type III [18]) or thatchange the curvature from convex to concave (type V [18]) willpresent low Amax values.

The Amax values obtained for the materials studied in thiswork are given in Table 2. Amax values for PCH-3 and PCH-4materials are 3.4 and 3.7 kJ/mol, respectively, being in this waythe more hydrophilic samples of the series. The materials thatpresent sigmoid isotherms (MCM-41, PCH-1, and PCH-2) havesimilar Amax values in the range 1.5–1.7 kJ/mol. The SBA-15,XG 0.5 and XG 2 samples, for which the isotherm is convextoward the relative pressure axis in the highest range of pres-sures for the series of studied materials, present Amax values�1.0 kJ/mol.

The enhanced hydrophilicity of PCH-3 and PCH-4, in rela-tion to the other studied materials, is related to the presence ofthe amine groups, due to the use of APTES, as discussed moredetailed below. In fact, in a first approximation the Amax val-

Fig. 4. Relation between the Amax values and the pore diameters of the indi-cated materials.

ues for these samples could be compared with the Gibbs energyfor the hydration (�hG

0) of the propylamine. In this way, usingthe values of Plyasunov et al. [45], and making the correctionfor the same standard state (298.15 K and 0.1 MPa), the �hG

0

value for propylamine is −10.4 kJ/mol. This can be comparedwith the negative Amax values, corrected for the same standardstate, for PCH-3 and PCH-4 which are about −11 kJ/mol. Thesimilarity of these values is in favour that in the PCH-3 andPCH-4 cases the relative pressure at which the inflexion pointin the isotherms occurs is related with the specific interactionbetween the amine groups and the water. These interactions,which seem to be comparable to the hydration interactions, de-termine the hydrophilic properties of these materials.

It is highly informative if we consider at this point the datain Fig. 4 where the Amax values are plotted against the porediameters corresponding to the maxima in the pore size distrib-utions (Fig. 2). As can be seen, Amax values are linearly relatedto pore diameters, except for the samples PCH-3 and PCH-4,which have the amine groups. For a series of materials with asimilar chemical nature of the surface, Amax values are linearlyrelated to the pore diameters. Fig. 4 also shows that the Amaxparameter is much more sensitive to changes in the nature ofthe adsorbent surface than to changes in the pore diameter. Infact, the Amax values vary from 0.6 kJ/mol at 6.5 nm for theSBA-15 to 1.7 kJ/mol at 3 nm for PCH-1, in contrast with thechange from about 1.5 to 3.5 kJ/mol due to the presence of theamine groups in solids with similar pore sizes.

As already mentioned, a different approach to characterizethe hydrophilic–hydrophobic properties of silica based mate-rials by using DRIFT spectra has been proposed [6]. Beingeventually more available than the water adsorption isotherms,this methodology could have some practical advantages. TheDRIFT spectra for the various studied materials in the region1500–700 cm−1 are given in Fig. 5. These spectra agree withother previously obtained, in the cases where literature dataexists, namely for the MCM-41 [46] and the SBA-15 [47]materials. In Fig. 5 the strong and broad absorbance band inthe 1300–1000 cm−1 region is assigned to the asymmetricstretching modes of Si–O–Si vibrational moiety—νas(Si–O–Si)[6,46–49]. The band near 800 cm−1 is assigned to the symmet-ric stretching mode νs(Si–O–Si) [37,47–49], and the band near950 cm−1 is assigned to the dangling ν(Si–Od) due to Si–OHand Si–O− groups [6,50]. This latter band is particularly rel-evant in the context of the present work since the Si–OH and

J. Pires et al. / Journal of Colloid and Interface Science 317 (2008) 206–213 211

Fig. 5. DRIFT spectra between 1500 and 700 cm−1 for the indicated samples.

Si–O− are both hydrophilic and it was proposed [6] that the ra-tio of these groups to the total silica could be an indication ofthe hydrophilicity degree of the material (the A(Si–Od) parame-ter in Table 2). This could be then calculated (in %) by the ratiobetween the area of the ν(Si–Od) band and the sum of the areasof the νas(Si–O–Si) and ν(Si–Od) bands [6].

To calculate the areas of the νas(Si–O–Si) and ν(Si–Od)bands, a deconvolution of the normalized DRIFT spectra wasmade by fitting Gaussian functions, using the nonlinear leastsquares method. This deconvolution took into account that theasymmetric mode (band around 1300–1000 cm−1) composesof two major components [6,48], due to the in-phase and theout-of-phase motion of two adjacent oxygen atoms in relationto the central silicon atom and, moreover, each of these compo-nents is associated with a longitudinal-optic and a transverse-optic vibrational modes [48]. Therefore, the band around 1300–1000 cm−1 was fitted with four Gaussians, the best fittingbeing obtained when maxima around 1220, 1160, 1090, and1060 cm−1 were used. For the band at 950–900 cm−1 twoGaussians were used with maxima around 960 and 940 cm−1.The band around 800 cm−1 was fitted with one Gaussian withthe purpose of improving the global fit of the spectral region.As an example, Fig. 6 shows the deconvolution fittings for(a) MCM-41 and (b) PCH-4. The absorption maxima (ν̃) andrelative areas (%A) of the respective bands are given in Table 3.The hydrophilicity degrees—A(Si–Od)—(in %) are given in Ta-ble 2 where these values can be compared with the K and Amaxvalues.

Concerning the ν(Si–Od) vibration mode, and accordingto the assignments described in the literature [6,50], the ab-sorption maxima listed in Table 3 due to the Si–OH speciesappear between 966 (PCH-3 and PCH-4) and 974 (XG 0.5)and those due to Si–O− between 902 (PCH-2) and 951 cm−1

Fig. 6. Deconvolution of spectral region between 1300 and 700 cm−1 for(a) MCM-41 and (b) PCH-4.

(SBA-15 and XG 0.5). The contribution to the total area of theν(Si–Od) from the Si–O− species in PCHs samples, is 7.3% forPCH-3 and 5.4% for PCH-4, while for the other PCHs is al-ways lower than 2.5%. This aspect, that has consequences onthe hydrophobic–hydrophilic properties of PCH-3 and PCH-4,as discussed below, may be due the presence of APTESin these samples. In fact, the terminal amine groups of theAPTES molecule can form an inter-chain structure of type Si–O−· · ·H· · ·NH+

2 –CH2–CH2–CH2–Si [51] thus enhancing therelative amount of the Si–O− type species. It can be noticed thatthe organo-functionalization of various types of silica-like ma-terials with aminoalkoxysilanes is important either for reinforc-ing the surfaces [51] or as catalysts and catalysts supports [52].

At this point, the information on the hydrophobic–hydrophi-lic nature of the various materials, accessed from the wateradsorption isotherms and from the DRIFT spectra, can be dis-cussed. For this, the A(Si–Od) values (in %) can be comparedwith those found for the Amax parameter in each solid (Table 2).As denoted in this table the relation between the informationobtained by the two methodologies used is not clear. Severalreasons can justify this situation. For instance, in cases such asthe PCH-2 sample the contribution to the total area of the ν(Si–Od) band is predominantly due to species type Si–OH (band at967 cm−1) and less from species type Si–O− (Table 3). Theformer species, which always contribute to the infrared absorp-tion, may not be strong adsorption sites for the water molecule.Therefore, the DRIFT analysis can overestimate the hydrophiliccharacter of the solid. Another reason is that the DRIFT spec-tra account for all the Si–OH groups but part of these groups

212 J. Pires et al. / Journal of Colloid and Interface Science 317 (2008) 206–213

Table 3Maximum absorption wavenumber ν̃ (cm−1) and relative area (%A) obtained by deconvolution of the bands observed in the spectral region 1300–700 cm−1

PCH-1 PCH-2 PCH-3 PCH-4 MCM-41 SBA-15 XG 0.5 XG 2

νas(Si–O–Si) ν̃ 1225 1219 1221 1219 1234 1216 1224 1221%A 4.6 5.6 4.2 4.0 5.5 7.3 5.8 6.2

ν̃ 1165 1143 1169 1166 1162 1139 1162 1162%A 28.1 36.9 26.6 24.0 31.4 39.1 36.8 36.6

ν̃ 1094 1082 1090 1089 1090 1085 1087 1091%A 3.9 32.8 2.9 3.9 6.2 31.3 44.8 46.1

ν̃ 1081 1041 1080 1080 1076 1045 1045 1048%A 58.0 16.4 56.1 60.3 50.5 17.3 3.3 3.7

ν(Si–Od) (Si–OH) ν̃ 967 967 966 966 968 973 974 973%A 1.6 6.6 1.0 1.2 2.1 0.7 1.0 0.7

(Si–O−) ν̃ 935 902 946 946 938 951 951 950%A 2.1 0.3 7.3 5.4 2.5 1.9 5.5 4.4

νs(Si–O–Si) ν̃ 805 806 801 800 804 809 802 803%A 1.6 1.4 1.8 1.1 1.7 2.4 2.7 2.4

can be inaccessible to the water molecules and hence, in prac-tice, they do not contribute to the hydrophilicity of the samplevia adsorbent–adsorbate interactions. In this situation the Amaxvalues would be lower than expected on the basis of the DRIFTanalysis. The possibility that this situation also arises when thecontribution for the hydrophilicity comes more from the Si–O−species is lowest since the formation of these species impliessome type of opening of the silica rings and, therefore, rendingthese groups less hindered to the water molecules. In the sameline to what was discussed for PCH-2, in the xerogel samplesthe hydrophilicity accounted by DRIFT may be overestimatedin relation to the information from water adsorption. A possi-ble justification for this is that the Ostwald ripening may occurto some extent. In this situation the probability of the existenceof larger particles is higher and, therefore, a significant numberof terminal groups would not be able to interact with the wa-ter molecules. Moreover, the different amorphous or crystallinecharacter of the various studied samples, which can be for in-stance accounted by the band near 1100 cm−1 [53] may difficultthe comparison of the information obtained by the DRIFT andthe water adsorption.

An important additional reason to justify the lack of relationbetween the information obtained from water adsorption andDRIFT is related to the relatively high standard deviation asso-ciated with the determination of the A(Si–Od) parameter, whencompared for instance with the standard deviation for the Amaxvalues. In fact, in the MCM-41 and SBA-15 cases the stan-dard deviation for A(Si–Od) is very high, reaching 40% of theestimated value. This is due to the relatively low area of theν(Si–Od) band that makes the determination of the area of thisabsorption band very sensitive to the goodness of the fit of thetwo Gaussians and also to the experimental noise in the data.

4. Conclusions

Both techniques used in this work, that is, the water ad-sorption isotherms and the DRIFT, were informative on thehydrophobic–hydrophilic properties of the studied samples but

the correlation between the information obtained by each tech-nique, was not straightforward. Water adsorption isotherms aremuch more sensitive to the differences of the studied materialsthan the DRIFT spectra. For silica based mesoporous materi-als with similar surface chemistry, the water adsorption processand hence, the hydrophobic–hydrophilic properties, is mainlydependent on the pore diameters. The lack of a more direct re-lation between the hydrophilicity degrees measured by DRIFTand by water adsorption can be partially related to the existenceof terminal groups that are accounted by DRIFT but may notbe accessible to interact with the water molecules and that, infact, do not contribute to the hydrophobic–hydrophilic proper-ties of the material. Additionally the standard deviation asso-ciated with the determination of the area of the ν(Si–Od) bandcan be very high for some materials.

As a consequence of reasons discussed above, the DRIFTanalysis, due to their experimental simplicity when comparedwith the determination of water adsorption isotherms, couldeventually be used as a first approach, particularly in materi-als that are structurally closed related but differ in the natureof the surface chemistry. However, to access the hydrophilic–hydrophobic properties of the materials and to relate these prop-erties with the structure and the surface chemistry, in a way thatdifferent materials can be compared, the water adsorption, inparticular through the analysis of the Amax parameter, shouldbe preferred.

Acknowledgments

Thanks are due to FCT for the plurianual funding to CQBand to the project POCTI/CTM/56192/2004. M. Pinto thanksFCT for a post-doctoral grant.

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