Journal of Nanoparticle Research2: 293–298, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

Synthesis and properties of layered organic–inorganic hybrid material:Zn–Al layered double hydroxide–dioctyl sulfosuccinate nanocomposite

Mohd Zobir bin Hussein and Tan Kim HwaMultifunctional Materials for Industrial Application (MULIA) Research Group, Department of Chemistry,Universiti Putra Malaysia, 43400UPM, Serdang, Selangor, Malaysia (Tel.: 603-8948-6101-10;Fax: 603-8943-2508; E-mail: mzobir@sfsas.upm.edu.my)

Received 26 April 2000; accepted in revised form 17 September 2000

Key words:nanocomposite, layered double hydroxide, hydrotalcite, surfactant, dioctyl sulphosuccinate,organic–inorganic hybrid

Abstract

A layered organic–inorganic hybrid nanocomposite was prepared by using a surfactant, dioctyl sulphosuccinate(DSS) as a guest in Zn–Al layered double hydroxide (LDH) inorganic host by a self-assembly technique. The Zn–Al ratio of the mother liquor was kept constant at 4 at the beginning of the synthesis. Powder X-ray diffractogramshows that the basal spacing of the Zn–Al LDH with sulphate as the intergallery anion (ZASUL) expanded from11.0 to 26.3 Å to accommodate the DSS surfactant anion for the formation of the Zn–Al LDH–DSS layered organic–inorganic hybrid nanocomposite (ZADON). It was also found that the BET surface area reduced by about 90%,from 22.5 to 2.2 m2/g, for ZASUL and ZADON, respectively if 0.1 M DSS was used for the synthesis of the latter.

Introduction

Over the past 30 years or so, tremendous efforts havebeen made to learn to control materials properties atthe atomic scale using wet chemistry methods, espe-cially in the area of organic–inorganic hybrid materials.Efforts have been targeted in the syntheses of molec-ular and nanocomposite hybrid materials, with lengthscale of 1–2 nm and 1–1000 nm, respectively, as wellas processing and characterization of porous materi-als produced by hybrid processing methods. This isbecause the hybrid materials exhibited optical, elec-tronic, catalytic or physical properties different fromtheir counterparts and the properties can be tailor-madeby choosing the right combination of the guest–hostspecies [1–5].

Nanocomposite materials [6] of layered structurecan be prepared by inserting guest molecules to alayered host. This type of material is also called

pillared layered structure (PLS), and can be synthe-sized directly or indirectly. The former, or sometimescalled spontaneous self-assembly method [7] and canbe synthesized by co-precipitation technique in whichthe guest species is included in the reaction solution,followed by aging process to form nanocomposite.The latter can be prepared by preparing the host fol-lowed by modification or further treatment of the hostand finally insertion of the guest molecule inside thelayers [8,9].

Attempts have been made to intercalate variousmolecules inside layered solid structures, especiallyanionic clays for various purposes and applications.For example, photoisomerization of indolenespiroben-zopyran in anionic clay matrices of Zn–Al layered dou-ble hydroxide (LDH) was exploited to better control thephotochromic reaction via nanocomposite formation.The co-intercalation agent, toluene-p-sulphonic acidwas used in this study. The resulting material exhibited

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reversible photoisomerization with a high stability ofthe merocyanine form [10].

High-saturation and high-resolution color imagescomparable to that of silver salt photographic disper-sion can also be formed on printing papers. This isbased on intercalation principle. The intercalated com-pound, capable of fixing water soluble dyes to a printingpaper (due to the intercalation based on ion-exchangebetween them) is incorporated into the dye-receivingsecond layer of the paper, on which color images areformed by ink jet recording using an aqueous ink com-position containing water soluble dye. The intercalatedcompound took place in hydrotalcite-group mineralwhen water-soluble anionic dyes were used [11].

Intercalation of linear chain anionic surfactantsbetween LDH layers was done by inserting alkylsulphate and dodecyl glycerol ether sulphate anionicsurfactant into the Zn–Cr LDH [12]. A single or bilay-ered long chain alkanols was inserted into the interlayerregion of the layered Zn–Cr LDH, resulting in variousdegrees of exchange and basal spacing. It was shownthat the latter depended on the carbon length of thesurfactant.

In the present study, intercalation of anionic surfac-tant of branched chain, dioctyl sulphosuccinate (DSS)into Zn–Al LDH to form nanocomposite material(ZADON) is investigated. In this study, Zn–Al LDHwas chosen as a host and DSS as a guest or intergalleryanion. Spontaneous self-assembly of organic and inor-ganic component molecules was adopted in this study.The XRD diffractograms, FTIR spectra, surface prop-erties and morphology of the resulting nanocompositewill be discussed here.

Due to hygroscopic nature of DSS, formation ofZADON is expected to inherit the same property. Thisproperty is useful for drying agents. In addition, if theloading percentage of the guest is high, then such aproperty can be exploited as a route for the formationof carbon in a layered structure. With this possibilityin mind, therefore we think this material is interestingand worth further investigating.

Materials and methods

All the chemicals used in this synthesis were obtainedfrom Sigma and used without further purification.

The synthesis of Zn–Al-DSS nanocomposite(ZADON) was done by the spontaneous self-assemblymethod. A solution containing Zn–Al (R = 4) andDSS was prepared and the pH was adjusted to about

pH = 7.5. The concentration of DSS was adjustedfrom 0.00 to 0.1 M. Dropwise addition of DSS to themother liquor in the reaction vessel was done undernitrogen atmosphere. The solution was aged for 5 daysin an oil bath shaker at 70◦C. The resulting precipitatewas centrifuged, thoroughly washed and dried in anoven at 120◦C, overnight and kept in a sample bottlefor further use.

A similar method was adopted for the preparationof Zn–Al-LDH with sulphate as the intergallery anion(ZASUL). However, the addition of DSS solution wasomitted in the preparation.

Powder X-ray diffraction (PXRD) patterns of thesamples were obtained by using filtered CuKα radiationin a Siemens Diffractometer D-500. FTIR spectra wererecorded by a Perkin-Elmer 1750 spectrophotometer.KBr pallet of 1% sample was used to obtain the FTIRspectra. The surface morphology and bulk structureof the samples were observed by a scanning electronmicroscope (SEM), model JOEL JSM-6400.

Surface characterization of the materials was car-ried out by nitrogen gas adsorption–desorption at 77 Kusing a Micromeretics ASAP 2000. Prior to the adsorp-tion of nitrogen gas, the samples were out-gassed to atleast 5×10−3 mmHg in an evacuation-heated chamberat 120◦C, overnight.

Results and discussion

The molecular structure of the surfactant, DSS sodiumsalt, also known as bis(2-ethylhexyl) sodium sulfos-uccinate, C20H37O4SO3Na is given in Figure 1. It is awhite, wax-like solid with a characteristic odor and hasa boiling point of 235◦C, melting point of 155◦C andspecific gravity of 1.1 g/cm3. It decomposes at around235◦C and the solubility in water is 15%. DSS is widelyused as wetting agent in food industry, and is usuallyadded to powdered gelatin, drink mixes and cocoas to

Figure 1. Molecular structure of DSS sodium salt.

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Figure 2. PXRD patterns for ZASUL (A) and ZADON (B).

make them dissolve more quickly and completely inliquids. It is also used as a stabilizer in chewing gumsand canned milks.

Powder X-ray diffraction

Figures 2A and B show PXRD patterns for ZASUL andits surfactant nanocomposite, ZADON, respectively.As shown in Figure 2A, the basal spacing for ZASULis 11.0 Å.

Propping apart the Zn–Al double layer and insert-ing the DSS species was successfully done by using0.1 M DSS in a mother liquor containing zinc and alu-minium sulphate salts with Zn–Al molar ratio of 4 at pH7.5. The PXRD pattern is shown in Figure 2B, show-ing an expansion of the basal spacing to about 26.3 Å.However, attempt has been made to intercalate DSSat lower concentration, but no expansion of the basalspacing could be observed if< 0.05 M DSS was used.This indicates that the expansion of basal spacing is due

to the inclusion of DSS, and can only be achieved if asuitable concentration of DSS is available in the motherliquor, under the experimental conditions stated earlier.

As a result of successful intercalation of DSS, wesubsequently used 0.1 M DSS to repeat the synthesis ofZADON. It was found that the formation of ZADON isreproducible with similar basal spacing. We used thisnanocomposite for further characterizations.

FTIR spectroscopy

Figure 3A shows the FTIR spectrum of DSS, showingseveral main important absorption bands. The molec-ular structure is shown in Figure 1. A broad band at3450 cm−1 is attributed to the OH stretching. Althoughthe presence of this band is unexpected in DSS, butthis must be due to the absorption of the moisture dur-ing the preparation of the KBr pellet. As expected thepresence of methyl groups of CH stretching vibrationscan be implied by the observation of triplet bands at2800–3000 cm−1. The band at 1734 cm−1indicates thepresence of carbonyl groups. Another band at around1463 cm−1 is due to CH2 scissoring and O–CO–C

Figure 3. FTIR spectra for DSS (A), ZASUL (B) and ZADON(C).

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stretching vibration can be observed at 1226 cm−1.Bands at around 1050 cm−1 is attributed to sulphonatesalts [13].

Figure 3B shows the FTIR spectrum for ZASULprepared without using any DSS, or simply the Zn–Al-LDH itself. The absorption band at around 3450 cm−1,is attributed to OH stretching due to the presence ofhydroxyl group of LDH and/or physically adsorbedwater molecule, as in DSS. The appearance of a weakband at 982 cm−1, and another strong absorption bandat around 1116 cm−1 can be attributed to the presenceof sulphate group, the intergallery anion in the Zn–Al LDH. The band at 1634 cm−1 is due toνH–O–H bend

vibration. As shown in the figure, ZASUL formed isfree from carbonate anion, due to the absence of anysharp absorption band at around 1357 cm−1, which canbe attributed to the carbonateν3 vibration [14]. Anothertwo bands at 618 and 428 cm−1 can be attributed tothe Al–OH and Zn–Al–OH bending vibration [15],respectively.

Also shown in Figure 3C is the FTIR spectrumof ZADON, synthesized by using 0.1 M DSS. Asexpected, the spectrum resembles a mixture of eachspectrum of DSS and ZASUL. This indicates thatboth functional groups of DSS and ZASUL is simul-taneously present in ZADON. In addition the H–O–Hbending vibration (νH–O–H bend) and O–H symmetricstretching vibration (νO–H sym) in these nanocompositesappear in the ranges of 1610–1630 and 3440–3470 cm−1, respectively. The lower values ofνO–H sym

in these nanocomposites compared to that of free OHgroups (> 3650 cm−1) indicates that all the OH groupsare involved in hydrogen bonding with DSS and theZn–Al double metal hydroxide interlayer [15].

Surface properties

Adsorption–desorption isotherm and surface areaIn order to study the effect of surface properties of theresulting material upon successful intercalation of DSSinside the Zn–Al LDH for the formation of a nanocom-posite, we measured the surface area and pore sizedistribution using nitrogen gas adsorption–desorptiontechnique at 77 K.

Figure 4 shows the adsorption–desorption isothermsfor ZASUL and ZADON. As shown in the figure,the adsorption–desorption isotherm for ZASUL is ofType II, by BDDT classification [16] with adsorptionincreased fairly rapidly at low relative pressure in arange at 0.0–0.05, followed by a slow uptake of theadsorbent at a relative pressure of 0.05–0.8. Further

Figure 4. Adsorption–desorption isotherms for ZASUL (opened-square) and ZADON (closed-square).

increase of the relative pressure to> 0.8 resulting inthe rapid adsorption of the absorbent, and reached anoptimum at more than 80 cm3/g at STP.

The general aspect of the isotherm for ZADON hasnot changed very much from that of ZASUL, i.e. theType II isotherm is still remained. However, as shownin Figure 4, the adsorbate uptake is slow, in the relativepressure range of 0.0–0.9, after which rapid adsorptioncan be observed. An optimum uptake was only about10 cm3/g at STP, indicating of low capacity uptake ofthe nitrogen gas. In addition, the slope of the adsorptionisotherm plateau becomes less pronounced in ZADONcompared to that of ZASUL.

Noteworthy also that the desorption branch of thehysteresis loop for ZADON is much wider compared toZASUL, indicating different pore texture of the result-ing materials. This can be related to the expansion of thebasal spacing together with the formation of interstitialpores between the crystallites during the formation ofthe nanocomposite.

As a result of nitrogen adsorption, we obtained theBET surface area of the materials. This is shown inTable 1, in which the surface area of the resulting

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Table 1. Surface properties of Zn–Al-sulphate LDH (ZASUL) and its surfactantnanocomposite (ZADON)

Material Basal BET BJH BET BJHspacing (Å) surface area desorption average pore average pore

(m2 g−1) pore volume diameter (Å) diameter (Å)(cm3 g−1)

ZASUL 11.0 22.5 0.126 178.1 174.8ZADON 26.3 2.2 0.017 301.6 133.8

Figure 5. BJH pore size distribution for ZASUL (opened-square)and ZADON (closed-square).

nanocomposite decreased by about 90% compared tothe host, from 22.5 to 2.2 m2/g.

Pore size distributionThe BJH pore size distribution for ZASUL andZADON is presented in Figure 5. As shown in thefigure, both materials show mesoporous property, inagreement with the Type II adsorption isotherm.

BJH pore size distribution for ZASUL shows anintense peak centered at around 400 Å and another one,a small very weak peak at around 40 Å. However, thispeak converged to a peak centered at around 100 Åregion with low intensity, when DSS was intercalatedinside the Zn–Al LDH interlayer, indicating the poretexture was modified, in agreement with the formationof new nanocomposite phase with a basal spacing of

26.3 Å. As was mentioned earlier, the pore texture waschanged not only due to the basal spacing, but alsodue to the formation of interstitial pores during theformation of the nanocomposite crystallite.

The average pore diameter by BET or BJH des-orption is given in Table 1. The BET average porediameter increased from 178 to 301 cm3/g for ZASULand ZADON, respectively. On the other hand, theBJH desorption pore volume decreased from 0.126 to0.017 cm3/g from ZASUL to ZADON.

Surface morphologyFigures 6A–C show the morphology of DSS, ZASULand ZADON obtained by an SEM. DSS shows wax-likesolid. On the other hand, ZASUL and ZADON showingtypical morphology of LDH and its nanocomposites,which shows agglomerates of compact and non-porousgranule structure.

As shown in the figures, there is no significantdifference in the morphology of the two samples.They are also very similar to the morphology ofother nanocomposites, such as Zn–Al-poly(acrylicacid)-LDH, Zn–Al-poly(vinyl sulphonate)-LDH andZn–Al-poly(styrene sulphonate)-LDH [17].

Conclusions

Layered inorganic–organic hybrid nanocomposite canbe prepared by using a surfactant, DSS as a guest in Zn–Al LDH inorganic host by a self-assembly technique,with the Zn/Al ratio of the mother liquor of 4. PXRDshows that the basal spacing of the Zn–Al LDH withsulphate as the intergallery anion expanded from 11.0to 26.3 Å to accommodate the DSS surfactant anionfor the formation of the Zn–Al LDH–DSS layeredinorganic–organic hybrid nanocomposite. It was alsofound that the BET surface area reduced by about 90%,from 22.5 to 2.2 m2/g, for the host (ZASUL) and thenanocomposite (ZADON), respectively if 0.1 M DSSwas used for the synthesis of the latter. The pore tex-ture of the resulting materials was also changed. This

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Figure 6. Electron micrographs for DSS (A), ZASUL (B) andZADON (C).

is a result of the intercalation and the expansion of thebasal spacing together with pore formation betweenthe crystallite during the formation of the resulting lay-ered nanocomposite. Generally, it is not desirable tohave a nanocomposite that had a small surface area, andthus poorer adsorption/desorption activity. However, ifthe lower surface area is due to high loading of guestmolecules inside the layered host, then this is useful,for example for controlled release purposes.

Acknowledgements

The support of this research by Ministry of Science,Technology and Environment, Malaysia under IRPAmechanism, Grant No. 09-02-04-0027 is gratefullyacknowledged.

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