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Vibrational Spectroscopy 68 (2013) 51–60

Contents lists available at SciVerse ScienceDirect

Vibrational Spectroscopy

jou r n al hom ep age: www.elsev ier .com/ locate /v ibspec

ynchronous ATR infrared and NIR-spectroscopy investigation ofepiolite upon drying

anessa Jane Bukas1, Maria Tsampodimou, Vassilis Gionis, Georgios D. Chryssikos ∗

heoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48, Vass Constantinou Avenue, Athens 11635, Greece

r t i c l e i n f o

rticle history:eceived 30 October 2012eceived in revised form 14 May 2013ccepted 14 May 2013vailable online 30 May 2013

eywords:alygorskite/sepiolite groupalcilanolTR spectroscopyIR spectroscopyGA

a b s t r a c t

A new environmental cell allowing for the independent synchronous collection of the near- and mid-infrared spectra (12,000–600 cm−1) in the diffuse reflection and attenuated total reflection (ATR) modes,respectively, is reported. The cell is employed to study in real time the dehydration of the phyllosilicatemineral sepiolite, Mg8Si12O30(OH)4(OH2)4·wH2O, in both its natural form and after in situ deuterationat ambient. The spectra are obtained under dynamic purging with dry N2 and compared to those of thesame material conditioned over saturated salt solutions. Sepiolite is an important industrial mineral witha modulated structure of alternating tunnels and ribbons. Its mild drying is associated with pronouncedvibrational spectral changes due to the removal of surface and zeolitic H2O and the concomitant struc-tural relaxation of the ribbons. Detailed assignments are provided for the fundamental, combinationand overtone spectrum of H2O confined in the tunnels of sepiolite, SiOH groups on the external sur-face of the particles, and Mg3OH groups in the 2:1 ribbons. The spectra are discussed in comparison tothose of palygorskite (modulated phyllosilicate with narrower ribbons and tunnels), talc (trioctahedralmagnesian phyllosilicate without modulation) and high-surface area silica. It is demonstrated that sepi-

olite exhibits three discrete states of zeolitic hydration at ambient temperature: Besides the previouslyknown hydrated (w = 7–8) and dry (w = 0–1) states which dominate the spectra above 30% and below 3%relative humidity, respectively, a hitherto unknown intermediate (w = 4–5) is found in the 3–10% range.The new state is most conveniently identified in the near-infrared by a �02 Mg3O-H stretching modeat 7205 cm−1 (�01 = 3686 cm−1, X = 83.5 cm−1) and a characteristic H2O combination band at 5271 cm−1

(D2O: 3908 cm−1).

. Introduction

Sepiolite is a hydrous 2:1 phyllosilicate clay mineral with aodulated structure, first determined by Brauner and Preisinger

1] and later refined by others [2–4]. It is a member of thealygorskite/sepiolite group ([5] and references therein) which,ontrary to common smectites, is characterized by lath-like par-icles with typical cross section in the order of 150 A × 300 A andength up to several �m. Due to its unique structure, morphologynd inertness, sepiolite shares with palygorskite a growing num-er of applications, traditionally as a specialty absorbent, carrier,ller or viscosity modifier and, more recently, as a template or host

atrix in functionalized hybrid nanomaterials [6,7].The ideal formula of sepiolite per half unit cell (phuc),

g8Si12O30(OH)4(OH2)4·wH2O, is that of a Mg-trioctahedral

∗ Corresponding author. Tel.: +30 2107273819.E-mail address: gdchryss@eie.gr (G.D. Chryssikos).

1 Current address: Department of Theoretical Chemistry, Technische Universitätünchen, Lichtenbergstr. 4, Garching 85747, Germany.

924-2031/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.vibspec.2013.05.009

© 2013 Elsevier B.V. All rights reserved.

mineral with no heterovalent cation substitutions and, hence, nolayer charge [8]. The tetrahedral sheets are inverted every threepyroxene chains causing the discontinuity of the octahedral sheetand the formation of alternating ribbons (modules) and tunnelsrunning along the fiber axis (Fig. 1). This is different to palygorskite,ideally Mg2Al2Si8O20(OH)2(OH2)4·4H2O phuc, where the modula-tion of the tetrahedral sheet occurs every two pyroxene chains [9].Due to this modulation, sepiolite exhibits high internal and externalsurface areas [10,11], decorated by various types of H2O: There areOH2 species coordinated to the outer Mg cations of the discontinu-ous octahedral sheet, zeolitic H2O inside the tunnels, H2O adsorbedon the external surface and physisorbed water. Apart from theMg3OH structural groups of the trioctahedral sheet, the numerousterminations of the tetrahedral sheet result in the formation ofabundant surface SiOH groups. As a result, sepiolite has a complexdehydration profile [12–14]: Various low-temperature H2O des-orption processes are activated below ca. 150 ◦C. These leave the

external and internal surfaces of sepiolite exposed, leading to thesignificant increase of the specific surface area of the material (from≈200 m2/g to >300 m2/g, [15]) and rendering the tunnels open andaccessible to small molecules [16]. This is unlike common smectites

52 V.J. Bukas et al. / Vibrational Spec

Fig. 1. Structure of sepiolite at ambient showing the periodic inversion of the sili-cate tetrahedral sheet and the discontinuity of the octahedral sheet. This modulationresults in the formation of surface steps and tunnels running along the particlelength. Two kinds of OH species are shown: Mg3OH units of the trioctahedralsheet (green in color version) and abundant surface SiOH terminations (red). OH2

molecules in the coordination sphere of the outer octahedral ions line the side wallsof the tunnels (blue). The latter contain zeolitic H2O (gray). (For interpretation of thert

A

wt(t

icwaacctmm

tottevttsfititetoisvth[

eferences to color in this figure legend, the reader is referred to the web version ofhis article.)

dapted from Post et al. [2].

hich, under similar drying conditions, undergo a step-wise reduc-ion of their interlayer distance [17,18]. At higher temperaturesca. 300 ◦C) the partial desorption of the OH2 species of sepioliteriggers folding of the ribbons and collapse of the tunnels [2,13,19].

The octahedral sheet of sepiolite occasionally exhibits substitut-ons by Al or Fe cations which may induce a partial dioctahedralharacter [20]. An opposing trend is observed for palygorskite,hich is primarily dioctahedral but is often encountered with vari-

ble trioctahedral character due to excess Mg. Although sepiolitend palygorskite are easily distinguished by XRD, recent analyti-al data of García-Romero and Suárez [21] have in fact indicated noompositional gap between them. Instead, a smooth compositionalransition (trioctahedral to dioctahedral) is observed and there are

embers of the series which cannot fit satisfactorily into any endember.Infrared spectroscopy had a critical and early role in the struc-

ural understanding of sepiolite. Detailed studies were performedn self supported clay films by transmission mid-infrared spec-roscopy. These studies focused on the fundamental spectrum ofhe O-H moieties (H2O, OH2, and OH) as a function of heat orvacuation treatment and/or deuteration [19,22–25]. They pro-ided evidence for surface SiOH groups, and reported changes inhe vibrational signature of both the H2O species and the struc-ural Mg3OH hydroxyls upon heating. A later infrared emissiontudy by Frost et al. [26] is in qualitative agreement with thesendings. Apart from the spectroscopic signature of the folded struc-ure (which exists above 300 ◦C), lower-temperature events weredentified and associated with the removal of zeolitic H2O fromhe tunnels of sepiolite. Similar conclusions were reached by Weirt al. [27] using 29Si NMR spectroscopy. The effect of these low-emperature events on the unit cell dimensions of sepiolite was notbserved until recently [2]. Interestingly, the early infrared stud-es addressed (though inconclusively) the issue of the heterovalentubstitutions in the octahedral sheet and their influence on the

ibrational spectra [24]. Much less was known about the vibra-ional spectrum of the modulated silicate sheet in sepiolite whichas been reported by Frost et al. [28] and studied by McKeown et al.29] with no reference however to the effect of dehydration.

troscopy 68 (2013) 51–60

The near-infrared (NIR) spectrum of sepiolite has been includedin the compilation of Clark et al. [30] and compared to those ofother trioctahedral minerals. The first systematic mid- and nearinfrared study of eight sepiolite samples from different localities[28] revealed differences which could be attributed to the (pre-sumably) varying composition of the samples. Post and Crawford[31] reported the OH-combination spectra of several sepiolites withknown composition and identified weak features which could berelated to the presence of octahedral Fe. Both studies were per-formed on ambient samples and possible hydration effects werenot addressed. Interestingly, Frost et al. [28] interpreted the spec-tra in the OH stretching region as suggestive of both trioctahedraland dioctahedral types of MgOH species, an assignment adoptedalso by the recent study of a Spanish sepiolite by Mora et al. [32].On the contrary, a preliminary NIR investigation of sepiolite SepSp-1 as a function of mild dehydration by Stathopoulou et al. [33]found no evidence for dioctahedral-like MgOH and classified thissample as the trioctahedral end-member of a palygorskite-sepiolitepolysomatic series.

The present paper is an extension to the aforementioned stud-ies and reports on the mid- and near infrared spectra of a typicalsepiolite which are recorded simultaneously at ambient condi-tions under dynamically varying humidity. The effect of relativehumidity on the structure and water uptake of sepiolite is quanti-fied from experiments on samples pre-conditioned over saturatedsalt solutions. The central theme of this work is that H2O and OHcan be used as very sensitive proxies for the structure of sepio-lite, its composition and its interactions with other species. Such adetailed structural investigation of the sepiolite–water interactionsis needed to support future efforts in a) understanding the adsorp-tion of chemicals on sepiolite [34–36], b) designing sepiolite-basedhybrid nanomaterials (e.g. [4,37,38]), c) addressing the structuralfootprint of the heterovalent octahedral cation substitutions insepiolite [3] in a manner similar to that achieved for palygorskite[39,40] and d) scrutinizing the palygorskite/sepiolite polysomatichypothesis [33,41].

2. Materials and methods

This work is based on sepiolite SepSp-1, a Clay Minerals Societyspecial clay from Valdemore, Spain, ground to less than 250 �m ina Retsch hammer mill. SepSp-1 has a composition SiO2 (52.9), MgO(23.6), Al2O3 (2.56), Fe2O3 (1.22) in % of the main oxides.

Attenuated Total Reflectance (ATR) spectra in the mid infrared(4000–550 cm−1) were measured on a Fourier transform instru-ment (Equinox 55 by Bruker Optics) equipped with a singlereflection diamond ATR accessory (Durasampl IR II by SensIR) anda DLATGS detector. Typical measurements were at 4 cm−1 spectralresolution, (�� = 2 cm−1 by interferogram zerofill), with a scannervelocity of 5 kHz. Reference (steady state) and process monitoringspectra were averaged over 100 and 25 scans, respectively. Sam-ples were in a film form (8–10 mm in diameter and 100–150 �min thickness) made by slowly drying a few drops of a sonicatedaqueous dispersion of clay placed on the ATR element.

NIR spectra were measured on a Fourier transform instrument(Vector 22N by Bruker Optics) equipped with a 1.5 m-long fiberoptic bundle probe for powders and an integrating sphere acces-sory. Similarly to ATR, all spectra were at 4 cm−1 spectral resolutionand �� = 2 cm−1 with a scanner velocity of 10 kHz. Reference 200-scan spectra were measured with the integrating sphere modulein the 12,000–3600 cm−1 range against a gold reference frompowder samples placed in capped flat-bottom clear-glass vials.

Process monitoring spectra were measured under the same con-ditions with fewer scans or by the powder probe fiber accessory(12,000–4000 cm−1). In the latter case, a Spectralon® reference wasemployed.

V.J. Bukas et al. / Vibrational Spectroscopy 68 (2013) 51–60 53

Fig. 2. Cross section of the environmental chamber (a) designed to couple the singlereflection diamond ATR cell with the NIR fiber optic probe. A metal ring with radialperforations (b) distributes evenly the inlet gas of choice around the sample. Beforeassembly, the sample (100–150 �m thick film, not shown) is deposited on the sur-fc

shtotowbt(act(

pgphftrsfit

tpstsTtsmro

Fig. 3. Typical drying and deuteration experiment of sepiolite SepSp-1, monitoredby infrared spectroscopy. Dashed lines mark the switching between drying and wet-

the OPUS software (Bruker Optics) is employed, with a 2k + 1 point

ace of the diamond element by drying a few drops of the slurry with purging N2. Alamp securing the cell on the ATR plate is not shown.

Synchronous ATR and NIR spectral acquisition was made pos-ible by coupling the mid-and near-infrared spectrometers via aome-made environmental chamber (Fig. 2). The cell is clamped onhe ATR plate and allows for positioning the NIR optical fiber probepposite to the internal reflection element which is coated withhe sample. This arrangement enables the simultaneous acquisitionf the mid-infrared spectrum by internal reflection (evanescentave with a few �m penetration depth) and the NIR spectrum

y diffuse reflectance (penetration depth >100 �m). In additiono allowing for optimum acquisition in terms of instrumentationsource, detector, etc.), the different sampling depths of the twoccessories compensate for the differences in the extinction coeffi-ient between the fundamental and higher order modes and lead tohe collection of high quality data over a broad wavenumber range12,000–550 cm−1).

The ATR-NIR coupling chamber is fitted with gas inlet and outletorts and a radially perforated ring which distributes the incomingas evenly around the sample (Fig. 2). Dry N2 gas is employed forurge-drying, and then switched to pass through H2O bubblers forydration (or D2O for deuteration) of the sample. These conditions

or H/D exchange are considerably milder than those needed forhe replacement of the structural OH groups by OD, which typicallyequires temperatures of 300–400 ◦C or higher [24,42]. The sameetup can be easily adapted to a variable temperature ATR plate, ortted with a controlled humidity gas generator, or used to studyhe interactions of the sample with organic vapors.

In a typical experiment, the sample film is deposited first onhe ATR element. The cell is subsequently assembled and the NIRrobe is secured without touching the sample. A few trial NIRpectra are needed to fine-tune the distance between the tip ofhe fiber and the plate. A representative measurement sequencetarts by dry-purging a sample pre-saturated with H2O vapors.he two spectrometers are activated simultaneously and triggeredo initiate measurements every 60 s. As both ATR and NIR mea-urements are adjusted to last about 55 s each, the two acquisition

acros remain synchronized during the whole experiment. A rep-

esentative data series from an experiment involving the dryingf the H2O-saturated sample and two cycles of D2O-flushing and

ting steps. The contour plot of the absorbance in the range of OH/OD stretchingfundamentals is depicted with a time resolution of 1 min. The full dataset covers the12,000–600 cm−1 range. For details, see text.

purge-drying is shown in Fig. 3. During each stage, the flow of N2is occasionally adjusted (in the 50–1000 ml/min range) to ensurethe time resolution of the observed spectral changes.

The spectrometers need to be well stabilized, purged and/orcharged with fresh desiccants in order to ensure that the back-ground spectra collected prior to sample deposition and assemblyof the cell remain stable during the whole duration of the exper-iment (2–3 h). Nevertheless, small (positive or negative) changesof instrumental H2O vapor are possible. These have a strong effecton the 2nd derivative spectra due to the sharpness of their spec-tral lines. To compensate for this unwanted effect, a second set ofbackground spectra is obtained after completion of the experimentand used to subtract any residual H2O vapor contribution from themeasured absorption spectra.

In addition to the dynamic ATR and NIR study of sepiolite upondrying, the equilibrium structure of SepSp-1 at 20 ± 2 ◦C and ambi-ent pressure was investigated on 500 mg samples pre-conditionedfor a period of at least 30 days over P2O5 and eleven saturated saltsolutions covering the 4–95% range of relative humidity [43]. Thesesamples were measured by both thermogravimetric analysis (TGAQ500 by T&A Instruments) at a heating rate of 5 ◦C/min and nearinfrared spectroscopy.

The spectra are presented and discussed in the 2nd derivativeformalism which enhances the resolution of weak but sharp com-ponent bands overlapping with broad features or with a sloppingbaseline. The Savitzky–Golay numerical differentiation routine of

smoothing window (±k points around the wavenumber of inter-est). For a given value of k, the smoothing window in wavenumbersdepends on the �� spacing, which is set during acquisition but

54 V.J. Bukas et al. / Vibrational Spectroscopy 68 (2013) 51–60

7500 700 0 650 0 600 0 550 0 500 0 450 0 400 0

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5175

5030

3867

4188

4324

5064

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H2O

dry

4182

5394

4964

4327

4570

5220

7030

7192

7271

3885

wavenu mbers , cm-1

4000 350 0 300 0 250 0 200 0 150 0 100 0 50 0

1240

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3526

85

3250

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A

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3627

3680

1620 1212

1423

261526

94

x5

wavenu mbers, cm-1

Fig. 4. Comparison of the NIR integrating sphere (left) and ATR (right) spectra of sepiolite in the ambient (top) and purge-dried (bottom) states. The spectra correspond tothe starting and end-points of the first H2O (thin line, blue in the color version) and last D2O (thick line, red in the color version) drying stages of Fig. 3. Intensities in the4 −1

interpw

cw�ia�

3

dSaiiaa

3

3mbptali

000–2000 cm range of the ATR spectra have been amplified for clarity (5×). (Foreb version of this article.)

an be further manipulated by post zero-filling. Depending on theidth and intensity of the bands of interest, different settings for� and k can be applied to different regions of the same dataset

n order to achieve the optimum compromise between peak sep-ration and signal-to-noise ratio. Typical settings range from k = 4,� = 1 cm−1 to k = 10, �� = 2 cm−1.

. Results and discussion

The complete spectral sequences collected during the purge-rying of H2O- and D2O-saturated sepiolite (Fig. S1, Electronicupplement) allow for a quick allocation of the main trends butre not practical for a more detailed presentation. A better start-ng point is the spectral comparison of H2O- and D2O-sepioliten their ambient and dry states (Figs. 4 and 5). This comparisonllows for the identification of modes sensitive to H/D exchangend highlights the differences between the two states.

.1. The vibrational spectrum of H2O in sepiolite

The stretching modes of H2O in sepiolite dominate the700–3000 cm−1 range of the absorbance spectra (Fig. 4). In agree-ent with the literature [23,25], hydrated sepiolite exhibits two

road absorption bands at 3370 and 3250 cm−1 and two sharp com-onents at ∼3630 and 3565 cm−1, all responding to deuteration in

he expected manner (Fig. 4). This dependence on deuteration isgainst the assignment of the ca. 3630 cm−1 mode to dioctahedral-ike “MgOH groups” [28,32]. The two broad bands are eliminatedn the 2nd derivative (Fig. 5), but the two sharp H2O modes are

retation of the references to color in this figure legend, the reader is referred to the

clearly observed at 3630, 3564 cm−1 and shift to 2685, 2635 cm−1

upon deuteration (H/D = 1.352). The ı H2O bending region exhibitstwo sharp components at 1665, 1623 cm−1 (1241, 1212 cm−1 inthe D2O-saturated sample, Fig. 5). The highest energy bendingcomponent couples with the two sharp stretching modes to pro-duce well-defined (� + ı) H2O combinations at 5255, 5208 cm−1,(D2O: 3890, 3858 cm−1), Fig. 5. Overall, the H2O spectrum of sepi-olite is similar to that of palygorskite [39,44] but different fromsmectites, which exhibit single sharp H2O stretching, bending andcombination modes, attributed to H2O in the coordination sphereof the exchangeable cations [30,45–49]. As ideal sepiolite lacksexchangeable cations, the sharp bending, stretching and combi-nation doublets in its infrared spectrum are tentatively linked tothe presence of the Mg-(OH2)2 species which decorate the edges ofthe discontinuous octahedral sheet inside the tunnels. In the caseof hydrated sepiolite, such OH2 species interact with zeolitic H2O[2]. A note is made here of a weak component at 5318 cm−1 (D2O:3932 cm−1, H/D = 1.352) which is attributed to the combination ofH2O molecules associated with surface silanol groups (see below).

In dry sepiolite, a broader H2O stretching fundamental isobserved at ca. 3540 cm−1 (D2O: 2613 cm−1) and a sharp ı H2Omode appears at 1618 cm−1 (ı HDO: 1423 cm−1, ı D2O: 1212 cm−1,Figs. 4 and 5), in agreement with previous studies [25]. At highresolution, the 1618 cm−1 ı H2O mode can be resolved in twocomponents of equal intensity at 1622 and 1616 cm−1 (spectra

not shown). Finally, the (� + ı) OH2 doublet of dry sepiolite isobserved at 5225, 5160 cm−1 and is accompanied by a third com-ponent at 5271 cm−1 (Fig. 5), in agreement with Stathopoulouet al. [33].

V.J. Bukas et al. / Vibrational Spectroscopy 68 (2013) 51–60 55

Fig. 5. Spectra of H2O- and D2O-saturated sepiolite in the ambient (top) and purge-dried (bottom) states in the 2nd derivative formalism with �� = 2 cm−1 and k = 6 ork pectiv4 band

3

1olatos9Ha

ratbatt(

66twatcva

ttti6om

= 4 smoothing in the left (7400–1250 cm−1) and right (1200–600 cm−1) panels, res100–3800 cm−1 range which are from a NIR integrating sphere experiment. Some

.2. Si-O stretching and lower energy modes (1250–600 cm−1)

The strongest infrared absorption of sepiolite is observed around000 cm−1 and attributed to the antisymmetric stretching modesf the silicate tetrahedra [25,29,50]. By analogy to montmoril-onite, in-plane Si-O modes are expected around 1050–1000 andt ∼1120 cm−1, and out-of-plane modes at ∼1080 cm−1 [51]. Inhe 2nd derivative spectrum of ambient sepiolite these modes arebserved at 1116, 1082, 1051, 1018, 1003 and 975 cm−1 (Fig. 5)hifting and splitting to 1132, 1108, 1077, 1054, 1023, 1013 sh, and77 cm−1 upon drying (c.f. [25]). None of these modes is affected by/D exchange, thus suggesting that there are no H-bonding inter-ctions between the tetrahedral sheets and tunnel H2O (c.f. [2]).

In addition to the aforementioned bands in the 1150–950 cm−1

ange, which are similar to those of smectites, sepiolite shows doublet at 1212 and 1197 cm−1 which red-shifts upon dryingo 1194 and 1178 cm−1 (Fig. 5). By analogy to pyrosilicates [52],ands in this region have been associated with the Si-O-Si link-ges between the ribbons of the modulated 2:1 layer [50]. Inhe corresponding wavenumber range of the deuterated samples,hese Si-O-Si modes overlap with the strong D2O bending modes1241 cm−1 and shifting to 1212 cm−1 upon drying).

Below 950 cm−1, a number of weak sharp bands at 786, 765,92, 665, and 644 cm−1 (ambient sepiolite) and 784, 694 and54 cm−1 (dry sepiolite) do not shift upon deuteration. In addi-ion, dry H2O-sepiolite shows two peaks at 879 and 860 cm−1,hereas the corresponding D2O-sample exhibits a single feature

t 873 cm−1. Several weak features in the 900–700 cm−1 range ofalc are attributed to various Si-O-Si or O-Si-O modes [29,53]. In thease of sepiolite, assignments are complicated by the presence ofibrational modes related to SiOH species and will be discussed in

subsequent section.Infrared bands in the 700–640 cm−1 region are typical of trioc-

ahedral phyllosilicates [50]. Magnesian talc exhibits two bands inhis range, at 688 and 669 cm−1. The 688 cm−1 band is attributedo an out-of-plane Si-O mode with contribution from the stretch-

ng of the apical bonds [54] and is tentatively related here to the94–692 cm−1 band of sepiolite (Fig. 5). The 669 cm−1 mode of talc,bserved at ∼ 655 cm−1 in saponite, hectorite and hydrated ver-iculites, has been assigned to a rocking vibration (libration) of

ely. The infrared spectra were measured with the cell of Fig. 2, except those in theassignments are included. For details, see text.

the structural Mg3OH species [42,54–56]. In hydrated sepiolite thislibration mode is observed at 644 cm−1 and shifts by +10 cm−1 upondrying [24,25,57] (Fig. 5). In addition to these bands, talc exhibitsabsorption at lower wavenumbers (∼535 and 465 cm−1) which isalso related to the Mg3OH species [42]. Similar modes have beenobserved in trioctahedral smectites and sepiolite (530, 472 cm−1,[25,50]), but are outside the transparency window of the ATR cellemployed in this study.

3.3. Mg3OH stretching fundamental, combination and overtonemodes

In agreement with the early infrared studies of sepiolite [22–25],the sharp stretching fundamental modes of the Mg3OH species inthe hydrated and zeolitically dry states are observed at 3690 and3680 cm−1, respectively, and remain unaffected by the deuterationconditions employed (Figs. 4 and 5). The corresponding �02 and�03 overtones are observed at 7214, 7192 cm−1 (Fig. 5) and 10,576,10,548 cm−1 (spectra not shown) yielding anharmonicity values(X ≈ 83–84 cm−1) which are identical to those reported for magne-sian talc and other trioctahedral clays [58]. However, the positionand sharpness of the Mg3OH stretching modes as well as theirdependence on drying (see below), make sepiolite easily distin-guishable from talc (�01 = 3676 cm−1, �02 = 7184 cm−1) or saponite(�01 = 3688–3678 cm−1), see e.g. [30,58,59]. It is emphasized thatthe variability in the energy of the Mg3OH stretch corresponds toextremely subtle changes in bonding. For example, the observed10 cm−1 difference in the position of the Mg3OH stretching funda-mental in hydrated and dry sepiolite corresponds to a change of theO-H bond length by less than 10−3 A (e.g. [60]).

Three peaks in the 4400–4100 cm−1 range of sepiolite withapparent maxima at ca. 4370, 4325 and 4180 cm−1 (Fig. 5) exhibita very weak dependence on hydration and no response to D2Oexchange. Post and Crawford [31] observed similar bands in thespectra of several sepiolites from various localities. Talc exhibits a

remarkably similar spectrum (peaks at 4369, 4323 and 4182 cm−1)which has been assigned to combinations of its Mg3OH stretchingfundamental (3676 cm−1) with modes at 781, 670 and 538 cm−1

[56].

5 l Spectroscopy 68 (2013) 51–60

tfucdrb34oe∼[tupa∼

3

stSf(at

aaci�ct�Ve(t(

s(ttOsstı(stba4∼pima

Table 1Characteristic fundamental and higher order vibrations of exposed surface SiOHgroups in dry sepiolite, SepSp-1. Peak positions and anharmonicity values (X) are incm−1. Diagnostic vibrations of H2O (D2O) associated with the SiOH (SiOD) species inthe ambient state of sepiolite are also included. For a discussion of the assignments,see text.

SiOH SiOD

860 “ı”, Fermi component –3719 �01 2742 �01, H/D = 1.3564570 �01 + ı –7271 �02, X = 83.5 5394 �02, X = 45, H/D = 1.3488116 �02 + ı –10,660 �03 7957 �03, H/D = 1.340

6 V.J. Bukas et al. / Vibrationa

Given the relative insensitivity of the Mg3OH combination spec-rum of sepiolite to drying, the assignments of the combinationeatures must be compatible with the shift of the fundamentalspon drying. Thus, the mode at ∼4325 cm−1 can be assigned to theombination between the Mg3OH stretching and the previouslyiscussed libration modes at ∼ 650 cm−1. The two fundamentalsespond to dehydration by shifting in opposite directions and com-ine to a position which is independent of hydration (hydrated:690 + 644 cm−1, dry: 3680 + 654 cm−1, calc.: 4334 cm−1, obs.:327–4324 cm−1, Fig. 5). Another OH mode below 600 cm−1 (i.e.utside the range of our measurements) with similar depend-nce on drying is assumed to account for the combination at4182 cm−1. Finally, following the assignment of Zhang et al.

56], the 4370 cm−1 component ought to involve the fundamen-als at ∼780 cm−1. However, this assignment would result innusually high anharmonicity (X ≈ 90 cm−1). As an alternative, weropose the 4370 cm−1 combination band to involve a Ramanctive and infrared inactive fundamental Mg3OH libration mode at680 cm−1, which shifts by approximately +10 cm−1 upon drying.

.4. Surface SiOH groups

A strong sharp mode which emerges at 3730–3720 cm−1 in thepectra of sepiolite (or palygorskite) upon drying and respondso mild deuteration has been attributed to the abundant terminaliOH defects on the exposed edges of the 2:1 ribbons [24]. Thiseature should not be confused with a band at similar position3720 cm−1) which has been observed in K+-exchanged saponitend attributed to structural OH groups perturbed by K+ in the di-rigonal cavity [59].

The SiO–H stretching modes of sepiolite are observed heret �01 = 3719 cm−1, �02 = 7271 cm−1 (X = 83.5 cm−1, Figs. 4 and 5)nd �03 = 10,660 cm−1 (not shown). Deuteration results in theomplete H/D exchange, as manifested by the SiO–D stretch-ng modes at �01 = 2742 cm−1, �02 = 5394 cm−1 (X = 45 cm−1) and03 = 7957 cm−1 (H/D = 1.356, 1.348 and 1.340, respectively). Inomparison, the SiOH modes of dioctahedral palygorskite areypically observed at lower energies [33] (e.g. �01 = 3711 cm−1,02 = 7252 cm−1, X = 85 cm−1 for palygorskite PFl-1, [44]). Driedycor glass and porous silica heated in vacuum at 750 ◦C alsoxhibit bands of similar origin at �01 = 3747 cm−1, �02 = 7330 cm−1

X = 82 cm−1) and �03 = 10,750 cm−1, which shift upon deutera-ion to 2762 (H/D = 1.357), 5348 (H/D = 1.348) and 8026 cm−1

H/D = 1.339) [61,62].The SiOH stretching modes of sepiolite are accompanied by

eries of well-defined combination modes at (�01 + ı) = 4570 cm−1

Fig. 5) and (�02 + ı) = 8116 cm−1 (not shown). These peaks suggesthe coupling of the SiOH stretching modes with another fundamen-al, related to the same species and expected just above 850 cm−1.n this basis and in agreement with similar assignments in high-

urface area silica [63], one could assign the 860 cm−1 peak of dryepiolite (H2O-form, Fig. 5) to a ı(SiOH) mode. However, despitehe quantitative H/D exchange of the silanol group, neither the(SiOD) component nor the corresponding (�01 + ı) SiOD vibrationexpected at ∼630 cm−1 [61] and ∼3370 cm−1) are observed in thepectra of deuterated dry sepiolite (Fig. 5). This leads to the sugges-ion that the 860 cm−1 peak of the dry H-form is not a ı(SiOH) mode,ut instead a component of a Fermi resonance doublet involving

low-energy Si–OH libration mode (at ∼430 cm−1, similar to the26 cm−1 hydroxyl libration mode of talc, [64]) and a Si–O mode at885 cm−1. This assignment would explain why resonance is only

ossible in the H2O-form (doublet at 879, 860 cm−1, Fig. 5), but not

n the D2O-form (singlet at 873 cm−1). The positions of the funda-ental and higher order vibrations of the silanol group in sepiolite

re summarized in Table 1.

H2O···HOSi D2O···DOSi5318 (� + ı) H2O 3932 (� + ı) D2O, H/D = 1.352

It is noted that many types of silica with isolated silanol groupshave been reported to exhibit two SiOH deformation modes at767 ± 10 cm−1 and 834 ± 17 cm−1 [61,65]. The need for assigningtwo deformation modes stems from the observation of two (�01 + ı)SiOH combination components in the spectra of these samples,instead of the single band observed in sepiolite (4570 cm−1, Fig. 5)or palygorskite (4570–4578 cm−1 depending on composition, [33]).The possible origin of two SiOH deformation and combinationmodes in silica remains an open issue [61,63,65,66]. Interestingly,one of the proposed models involves a Fermi resonance betweenı(OH) and the overtone of a lower wavenumber fundamental [63].

Remarkably, the rich vibrational signature of SiOH groups isabsent from the spectra of ambient (hydrated) sepiolite. Thisimplies that mild drying by heating, purging or evacuation is nec-essary in order to study the silanol-related aspects of the sepiolite(and palygorskite) structure. The only clear sign of silanols in thespectra of the ambient sample is the weak feature at 5318 cm−1

(D2O-form: 3932 cm−1, H/D = 1.352, Fig. 5) which vanishes upondrying. A band which is similar in both position and dependenceon drying has been identified in the 2nd derivative NIR spectra ofpalygorskite [33,44] and attributed to the combination mode of sur-face H2O associated with the silanol groups in the hydrated state.Independently, Christy assigned a similar 5314 cm−1 peak of silicato a combination mode of H2O molecules which interact throughhydrogen bonds with the free SiOH groups [67]. The latter assign-ment has also been adopted by Pentrák et al. [68] for a 5317 cm−1

band of acid-treated montmorillonite.

3.5. Spectral changes on drying

Apart from the aforementioned spectrum of the surface silanolgroups which simply emerges upon drying, nearly all other vibra-tional features of sepiolite show an evolution regarding both theirposition and intensity (Figs. 5 and 6). A similar vibrational study ofpalygorskite [44] has indicated that a two-state transition betweenthe ambient and dry structures is sufficient to describe the observedtrends, whereas Stathopoulou et al. [33] have presented prelimi-nary evidence toward the more complicated zeolitic dehydrationof sepiolite which involves a well-defined intermediate.

When studied at high resolution, the 2nd derivative real timeNIR spectra measured during the drying of SepSp-1 (Fig. 6) reveala complicated pattern of changes in the 7230–7170 cm−1 range,which contrasts the simple phenomenology of the SiOH bandat 7271 cm−1. In addition to the �02 Mg3OH peaks at 7214 and7194 cm−1 which characterize ambient and dry sepiolite, respec-tively, a new intermediate spectrum is recorded with a medium

−1

intensity (broader) peak at 7205 cm . This peak is not affectedby mild deuteration (Fig. 6) and corresponds to a fundamentalat 3686 cm−1 (X = 83.5 cm−1, Fig. 7). On this basis, the 7205 and3686 cm−1 bands are assigned to a new Mg3OH species, favored

V.J. Bukas et al. / Vibrational Spectroscopy 68 (2013) 51–60 57

Table 2Positions of fundamental and higher order Mg3OH modes and anharmonicity values (X) of sepiolite SepSp-1 (in cm−1) at room temperature as a function of zeolitic H2Ocontent. Corresponding data of talc are provided for comparison.

ı �01 �01 + ı (X) �02 (X) �03 Comments

644 3690 4324 (10) 7214 (83) 10,576 Ambient647 3686 4325 (8) 7205 (83.5) 10,565 Intermediate654 3680 4327 (7) 7194 (83) 10,548 Dry

* *

amfa

tbotset

FSsttidt

665 3675666 3676 4323 (19)

* Below detection limit.

t intermediate levels of zeolitic hydration. Although this inter-ediate phase of sepiolite is best seen in the �02 Mg3OH range,

undamentals and combination modes are also identified at 647nd 4325 cm−1, respectively (Fig. 7 and Table 2).

In addition to the aforementioned intermediate, weak and rela-ively sharp components at 665, 3675 and 7183 cm−1, visible in allut the driest samples, indicate trace amounts of a different typef Mg3OH species (Figs. 5–7). These peak positions are very similaro those of talc (Table 2). It is unknown whether this faint talc-like

ignature corresponds to a spike of talc in SepSp-1, signals the pres-nce of a talc-like intergrowth, or indicates a distorted ribbon withwo non-degenerate types of Mg3OH.

7300 7250 7200 5350 5250 5150

7300 7250 7200 3950 3900 3850 3800

7271

7194

7214 72

05 7183

H2O-Sepioli te

02 SiOH 02 Mg3OH ( + ) OH2

5157

5213

52265254

5271

5317

Abso

rban

ce, 2

nd d

eriv

ativ

e

D2O-Sepioli te

7271

7194

7214 72

05 7183

Abso

rban

ce, 2

nd d

eriv

ativ

e

3892

3853

3885

382439

3139

08

( + ) O D2

wavenu mbe rs, cm-1

ig. 6. (Upper) Details of the 2nd derivative NIR spectra of H2O-sepiolite in the �02

iOH, �02 Mg3OH and (� + ı) OH2 spectral ranges upon drying. (Lower) The corre-ponding spectra of D2O-sepiolite. The ambient and dry end-members are shown inhin (blue in color version) and thick (red) lines, respectively. The peak positions ofhe intermediate state are marked in frames. Second derivatives are by k = 4 smooth-ng, �� = 1 cm−1 (overtones) and k = 6 smoothing, �� = 2 cm−1 (combinations). Foretails, see text. (For interpretation of the references to color in this figure legend,he reader is referred to the web version of this article.)

7183 (83.5) Talc-like?7185 (83.5) 10,535 Talc

In agreement with earlier findings [33], the appearance of the7205 cm−1 Mg3OH overtone on drying is paralleled by an interme-diate signature of OH2 combination modes (Fig. 6). This signatureis mostly evident by a new strong and sharp band at 5271 cm−1

which is clearly distinguished from the corresponding doubletsof the hydrated and dry sepiolite spectra at ca. 5255, 5210 cm−1

and 5225, 5160 cm−1, respectively (Fig. 6). Upon deuteration, the5271 cm−1 band shifts to 3908 cm−1 (Fig. 5, H/D = 1.349). Changesover the same range of drying are evident also in the envelope ofthe Si–O stretching fundamentals (Fig. 7).

Based on the above, it is evident that the zeolitic dehydrationof sepiolite is accompanied by structural changes which involve astate of intermediate structure. These changes, which are reversibleat ambient temperature, must be specific to the modulated struc-

ture of sepiolite. The overall effect of H2O nano-confinement in thetunnels of sepiolite appears significantly more complex than ini-tially believed [69]. Clearly, when comparing sepiolites of differentorigin by infrared spectroscopy (e.g. [28,31]) one must carefully

1250 115 0 105 0 95 0

3720 370 0 368 0

880 84 0 80 0 76 0 72 0 68 0 64 0

4600 450 0 440 0 430 0 420 0

Si-O

1212 1130

1107 10

8310

7410

53

1178

1023 10

1110

0397

5

Mg3OH

3686

3675

SiOH

369037

19

3680

647

Mg3OH

786 76

5

665

644

781

654

860

879

694

4325

4324

( + ) SiOH ( + ) Mg3OH

Abso

rban

ce, 2

nd d

eriv

ativ

e

wavenu mbers, cm-1

H2O-Sepioli te

4304

4183

4570 43

7143

27

Fig. 7. Details of the 2nd derivative spectra of H2O-sepiolite in the (� + ı) O-H, �01

O-H, � Si-O and ı Mg3OH spectral ranges upon drying. Line thickness and color areas in Fig. 6. Second derivatives are by k = 4 smoothing and �� = 1 cm−1.

58 V.J. Bukas et al. / Vibrational Spectroscopy 68 (2013) 51–60

0 20 40 60 80 10 00

4

8

12

16

20

rel ative hu midity, % (30 da ys at 20 oC)

Mg8Si 12

O30

(OH)4(OH

2)

4· w H

2O

w H

2O

w = 8

Fig. 8. Number of H2O molecules (w) per half unit cell of SepSp-1(Mg8Si12O30(OH)4(OH2)4·wH2O) as a function of relative humidity at ambi-ent, based on TGA weight loss data (Tmax = 120 ◦C). The shaded area corresponds toz

cai

lcaecwsttroHbmpOmtlawi

ciwspda

mapasfa

7180

7190

7200

7210

7220

0 20 40 60 80 10 05130

5150

5170

5190

5210

5230

5250

5270

5290

7265

7275

wav

enum

ber,

cm-1

02 Mg3OH

( + ) O H2

relative hu midity, % (30 da ys at 20 oC)

02 SiOH

Fig. 9. Dependence of selected overtone and combination modes of sepiolite SepSp-1 on relative humidity. The shading intensity scales with the amplitude of the 2ndderivative signal (as in Figs. 6 and 7) and the size of the points is proportional to thewidth between zero-crossings. Lines correspond to the apparent peak positions at

insensitive to changes in humidity, thus suggesting a fixed arrange-

eolitic H2O. For details, see text.

onsider the zeolitic H2O content. Failure to do so may result inssigning compositional effects to the complicated, but otherwiserrelevant, H2O-induced changes.

In search of a plausible structural description of drying sepio-ite, we note that the response of the spectra to decreasing H2Oontent cannot be attributed to the removal of H-bonding inter-ctions between zeolitic H2O and the Mg3OH species. Should theyxist in the ambient state, such Mg3OH···OH2 interactions wouldause a blue-shift of the Mg3O-H stretching modes upon drying,hich is opposite to the observed red-shift. Instead, the trends

hown in Figs. 6 and 7 must reflect the changing interactions ofhe Mg-coordinated OH2 with the remaining zeolitic H2O in theunnels, as well as the concomitant structural relaxation of the TOTibbon upon drying. Post et al. found that the evacuation of sepi-lite at ambient temperature causes the release of ∼7/8 zeolitic2O and the shortening of the height of the tunnels (representedy the a dimension of the orthorhombic unit cell) by approxi-ately 1.2% [2]. The last remaining zeolitic H2O species occupy

ositions which allow for H-bonding interactions with coordinatedH2. Therefore, the intermediate structure reported in this workust correspond to a state characterized by a partial occupancy of

he zeolitic H2O sites. Presumably, this state exhibits well-definedocalized interactions between the remaining H2O in the tunnelsnd the Mg-coordinated OH2 species, but lacks the extended net-ork of H-bonding interactions between the various H2O species

n fully hydrated tunnels.The dynamic nature of purge-drying does not facilitate a direct

orrelation between the vibrational spectra of drying sepiolite andts zeolitic H2O content. For this reason, several sepiolite samples

ere independently pre-conditioned at 20 ◦C in vials over P2O5 oraturated salt solutions (4–95% relative humidity) for long timeeriods. Small portions of each pre-conditioned sample were with-rawn for TGA and the remaining quantity was immediately sealednd measured non-invasively by NIR spectroscopy.

The total weight loss recorded in the 25–120 ◦C range and nor-alized to the weight of the sample at 120 ◦C can be expressed

s w per half-unit cell content, Mg8Si12O30(OH)4(OH2)4·wH2O, andlotted as a function of % relative humidity (Fig. 8). The data, whichre similar to those reported earlier by Caturla et al. [70], show a

teep increase of w up to ca. 10% relative humidity (w ≈ 5–6 H2O),ollowed by a slower increase which tends to extrapolate to w = 8t the saturation limit. This extrapolated value of w is in agreement

lower resolution (as in Fig. 4). For the corresponding H2O content, see Fig. 8.

with the original formula of Bailey [8]. At relative humidity in excessof 60%, w increases again rapidly, well above the w = 8 threshold,indicating the adsorption of physisorbed H2O. The driest sample ofthe measured series (pre-conditioned over P2O5) is found to main-tain a small amount of H2O (w ≈ 1). At ambient indoor conditions,w is in the range of 6.5–7 and relatively independent of relativehumidity (Fig. 8). A similar analysis of published TGA data on (pre-sumably ambient) sepiolite gives values of w ranging between 5.6and 9.6 (w = 9.6 [27], w = 7.4–8 [13], w = 7.8 [71], w = 7.6 [25], w = 7.6[72], w = 7–7.1 [14], w = 6.7 [3], w = 5.8 [23], w = 5.6 [73]). Calcula-tions based on the refined occupancy of the zeolitic H2O sites (z) ofa very pure sepiolite from Durango, Mexico, indicate z≈7.8 at ambi-ent, which drops to z≈1 under vacuum [2], in excellent agreementwith the data in Fig. 8. Similarly, two more recent synchrotron stud-ies of sepiolite at ambient are consistent with z = 7.6 [4] and z = 6.9[3].

All NIR spectra of the pre-conditioned samples match with snap-shots of the real-time monitoring infrared experiments. Spectralevolution as a function of relative humidity is shown for selectedwavenumber regions in Fig. 9. Several remarks stem from thesedata:

As it might have been anticipated from the weight loss measure-ments (Fig. 8), all the spectral changes which accompany zeoliticdrying are confined within a narrow range of relative humiditybelow ca. 20–25%. Above this level, the 2nd derivative spectra are

ment of zeolitic H2O in the tunnels. The excess H2O phuc which isdetermined by TGA in samples pre-conditioned at relative humid-ity exceeding 60% (w > 8) is not visible in the second derivative NIR

l Spec

sT7vsreAstswudIhsat

4

cicHasjTsdsof

daacoa7ripttfMns(iaa

A

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[

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[[[[[[[[

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V.J. Bukas et al. / Vibrationa

pectra due to its broad, liquid-like bands which are filtered out.he intermediate drying state, most evident by the �02 Mg3OH at205 cm−1 and the (� + ı) OH2 at 5271 cm−1, is maximized over aery narrow range around 5% relative humidity (Fig. 9) and corre-ponds to w ≈ 4 H2O molecules phuc (Fig. 8). In the 5–20% and 0–5%elative humidity ranges, SepSp-1 is distributed between the ambi-nt/intermediate and intermediate/dry states, respectively (Fig. 9).s a result, the spectroscopic identification of the intermediatetate requires a very fine tuning of H2O content. It is noted thathe intensity of the 5271 cm−1 combination mode persists in thepectrum of the driest sample of the series (Fig. 9), in agreementith the non-zero w value of this sample (Fig. 8). We have beennable to eliminate this band by prolonged N2 purging (Fig. 5) orrying over P2O5 at ambient temperature and pressure (Fig. 8).

nstead, we found that elimination of the 5271 cm−1 peak requireseating in the range of 140 ◦C. Such heating yields the unperturbedpectrum of zeolitically dry sepiolite (e.g. �02 Mg3OH at 7194 cm−1

nd � + ı OH2 at 5226, 5157 cm−1) which persists up to the foldingemperature range (>250 ◦C, data not shown).

. Conclusions

The vibrational spectrum of sepiolite SepSp-1, a clay mineralonsisting of alternating phyllosilicate ribbons and zeolitic tunnels,s studied upon drying at ambient temperature and the structuralhanges that accompany the gradual removal of surface and zeolitic2O are reported. To this purpose, a special cell was designed tollow for the simultaneous acquisition of NIR and ATR mid-infraredpectra (12,000–600 cm−1) from the same sample which was sub-ected to in situ hydration, deuteration or N2-purging treatment.he fundamental and higher order spectra of Mg3OH, H2O and SiOHpecies of sepiolite are presented in the 2nd derivative mode andiscussed as structural proxies of the ribbons, tunnels and externalurfaces, respectively. Measurements on dynamically drying sepi-lite are combined with and connected to equilibrium studies as aunction of relative humidity by NIR and TGA.

The overall effect of drying is a convolution of two indepen-ent, yet parallel, processes: The first involves the removal of H2Odsorbed on the external surface of the sepiolite particles, and isnalogous to drying high surface area silica. This process is mostonveniently observed via the monotonically decreasing intensityf a H2O (� + ı) combination mode at 5317 cm−1 (D2O: 3931 cm−1)nd the concomitant increase of the �02 SiO–H stretching at271 cm−1 (SiOD: 5394 cm−1). The second process involves theemoval of zeolitic H2O from the tunnels which affects the bond-ng of coordinated OH2 species and leads to the distortion of thehyllosilicate ribbons. Upon mild dehydration, sepiolite is foundo consecutively occupy three distinctive structural phases charac-erized by specific amounts of zeolitic H2O: The ambient (almostully hydrated) state, with 7–8 H2O molecules phuc, exhibits �02

g3OH at 7214 cm−1 (�01 at 3690 cm−1) and (� + ı) OH2 combi-ations at 5254 and 5213 cm−1. A newly discovered intermediatetate with 4–5 H2O phuc is identified by �02 Mg3OH at 7205 cm−1

�01 at 3686 cm−1) and (� + ı) OH2 at 5271 cm−1. Finally, the zeolit-cally dry state (0–1 H2O phuc) is identified by a sharp �02 Mg3OHt 7194 cm−1 (�01 at 3680 cm−1) and a (� + ı) OH2 doublet at 5226nd 5157 cm−1.

cknowledgments

Many helpful discussions with Elizabeth Stathopoulou (Univ.f Athens), George H. Kacandes (Geohellas S.A.), Mercedes SuárezUniv. of Salamanca), Marisa García-Romero (Complutense Univ.

adrid) and Manuel Sánchez del Río (ESRF) are gratefully

[[[[

troscopy 68 (2013) 51–60 59

acknowledged. VJB and MT acknowledge financial support fromthe Applied Spectroscopy Laboratory of TPCI/NHRF.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.vibspec.2013.05.009.

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