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Detection and quantification of montmorillonitenanoclay in water-ethanol solutions by graphitefurnace atomic absorption spectrometryXia Yininga, Maria Rubinoa & Rafael Aurasa

a School of Packaging, Michigan State University, East Lansing, MI, USAAccepted author version posted online: 18 Sep 2013.Published online: 12 Nov 2013.

To cite this article: Xia Yining, Maria Rubino & Rafael Auras (2013) Detection and quantification of montmorillonite nanoclayin water-ethanol solutions by graphite furnace atomic absorption spectrometry, Food Additives & Contaminants: Part A,30:12, 2177-2183, DOI: 10.1080/19440049.2013.845800

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Detection and quantification of montmorillonite nanoclay in water-ethanol solutions by graphitefurnace atomic absorption spectrometry

Xia Yining, Maria Rubino* and Rafael Auras*

School of Packaging, Michigan State University, East Lansing, MI, USA

(Received 8 July 2013; accepted 12 September 2013)

Organo-modified montmorillonite (O-MMT) was detected in various water-ethanol solutions by using graphite furnaceatomic absorption spectrometry, based on the simultaneous elemental analysis of silicon (Si) and aluminium (Al). Thequantification limits of the method were 0.03 mg L–1 for Si and 0.01 mg L–1 for Al, but can be improved to 8 μg L–1 for Siand 3 μg L–1 by using the concentration function of the instrument. A stability test of O-MMT in suspension was conductedby considering the solvent type (i.e., water and/or ethanol) and a surfactant. A good dispersion of O-MMT was achieved insuspensions with water and ethanol at 1:2 (v/v) or with pure ethanol, while the addition of a surfactant improved thedispersion of O-MMT in water. The Si and Al concentrations in the suspensions were correlated with O-MMT concentra-tions. The Si and Al contents in the O-MMT were determined by using graphite furnace atomic absorption spectrometry tobe 22% ± 1.1% and 9.3% ± 0.5% wt/wt, respectively, which was in agreement with values obtained by X-ray fluorescencespectrometry method performed in this study.

Keywords: nanoclay; montmorillonite; atomic absorption spectrometry

Introduction

In the past decades, engineered nanomaterials (ENMs) withat least one dimension within the nanoscale (1–100 nm)have been widely used in the manufacture of nanocom-posites, providing improved performance and properties(Polyakova & Hubert 2001; Han & Yu 2006; Bal &Samal 2007; Pereira de Abreu et al. 2007; Duncan 2011).The market share for nanocomposites was US $920 millionin 2011, and it is estimated to grow to more than US$2.4 billion by 2016 (BCC Research 2012). As the use ofnanocomposites has expanded, there are increasing con-cerns regarding the transport and fate of ENMs and theassociated environmental impacts and health risks due toexposure (Colvin 2003; Farre et al. 2011; Yokel &MacPhail 2011; Badireddy et al. 2012; Lowry et al.2012). However, information about the exposure to ENMsis not sufficient and the effects of ENMs on biologicalsystems and environments are not well understood(Thomas et al. 2006; Savolainen et al. 2010; EuropeanFood Safety Authority 2011). The U.S. National ResearchCouncil has recently addressed the urgency of understand-ing the risks associated with ENMs, with emphasis on thetransport and fate of nanoparticles within different biologi-cal systems and environments (National Research Council2012).

Nanoclays, such as organo-modified montmorillonite(O-MMT), are extensively used because of their efficiencyand low cost, and account for about half of the nanocom-posite market (BCC Research 2012). The market for

polymer-clay nanocomposites reached more than 450 mil-lion kg (1 billion lb) in 2009, with applications in differentfields such as packaging, automotive, coatings and con-struction (Patel et al. 2006). MMT belongs to the smectitefamily, also known as 2:1 phillosilicates. The crystalstructure of MMT consists of two silica tetrahedral sheetsfused to an edge-shared alumina octahedral sheet (SinhaRay & Okamoto 2003). MMT is usually in the form oftactoids, which are the stacks of parallel clay platelets withabout 1 nm interlayer space. The interlayer space containsexchangeable cations (e.g., Na+ or K+) that can bereplaced by organic cationic surfactants (e.g., alkylammo-nium or alkylphosphonium cations) to improve the com-patibility of the nanoclay with the polymer (De A. Pradoet al. 2005).

The addition of O-MMT to polymers has beenreported to improve mechanical and barrier properties(Sinha Ray & Okamoto 2003; Choudalakis & Gotsis2009), which enable the potential use of polymer-claynanocomposites in food packaging applications, such asbottles for beer and carbonated drinks and wrap films forvarious foods (Akbari et al. 2006; Chaudhry et al. 2008;Silvestre et al. 2011). Recently, more attention has beenplaced on the transport of nanoclays within different sys-tems, especially in food packaging systems, because of thepotential release of nanoclays into the packaged foods,which may adversely affect human health (Chaudhryet al. 2008; Mauricio-Iglesias et al. 2010; Diaz et al.2013). A major research challenge in this area is the lack

*Corresponding author. Emails: mariar@msu.edu (Maria Rubino); aurasraf@anr.msu.edu (Rafael Auras)

Food Additives & Contaminants: Part A, 2013Vol. 30, No. 12, 2177–2183, http://dx.doi.org/10.1080/19440049.2013.845800

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of methodologies for tracking and detecting nanoparticlesin different environments (Tiede et al. 2008; EuropeanFood Safety Authority 2009; Stamm et al. 2012).Current approaches for tracking and detecting nanoclayparticles in liquid systems are focused on elemental ana-lysis by mainly using atomic absorption spectrometry andICP-MS (Avella et al. 2005; Schmidt et al. 2009, 2011;Mauricio-Iglesias et al. 2010). Acid digestion procedureson nanoclay particles are applied to obtain homogenoussolutions, as required by these techniques (EnvironmentalProtection Agency 1996, 2007). However, the digestionprocedures are time consuming and inconvenient andintroduce issues with the clay concentration, which affectsthe suitability for real-time transport studies. Also, in sometransport studies of nanoclays, only the elemental concen-trations have been reported. Little attention has beenfocused on nanoclay quantification and characteristics indifferent mediums, although the understanding of thesefactors are essential to evaluate the potential risks ofnanoclays (Chau et al. 2007; Linsinger et al. 2013).Thus, methodologies for the rapid and reliable detectionof nanoclay in different mediums are highly sought toproperly understand the transport of nanoclays withindifferent systems and to understand their behaviour insolution.

The overall aim of this work was to develop a meth-odology for the rapid and reliable measurement ofO-MMT concentration in solution by correlation with theSi and Al concentrations of the nanoclay. Water and etha-nol were selected because both solvents and their combi-nations are commonly used to simulate various foodsystems (Food and Drug Administration 2007; Castle &Bradley 2010). Firstly, a graphite furnace atomic absorp-tion spectrometry (GFAAS) method was adopted for thedirect multi-elemental analysis of O-MMT in suspensionswithout prior acid digestion. Secondly, the stability of O-MMT dispersed in water and/or ethanol as a function oftime was evaluated. Finally, a correlation was establishedbetween the amounts of Si and Al and the concentration ofO-MMT in solution. The correlation was validated withelemental composition results for the O-MMT obtained byusing X-ray fluorescence (XRF) spectrometry method.

Materials and methods

Characterisation of O-MMT

The organo-modified nanoclay (Nanomer® I.44P) wasobtained from Nanocor (Hoffman Estates, IL, USA): itcontains about 65% MMT and 35% surfactant – dimethyldialkyl (C14–C18) amine – and is herein referred to as O-MMT. The particle size of O-MMT, as demonstrated by thesupplier, was mainly below 10 μm. The specific surfacearea (SSA) of the O-MMT was measured (in duplicate)with an ASAP 2020 accelerated surface area and porosityanalyser (Micromeritics Instrument Corporation, Atlanta,

GA, USA). Before analysis, the O-MMT powder (~0.3 g)was degassed at 160°C under a vacuum of 100 mTorr(13.33 Pa) for 16 h to remove any absorbed water. TheSSAwas obtained by applying the Brunauer-Emmett-Tellersorption isotherm equation (Allen 2004) to the measuredadsorption of nitrogen gas (N2) at 77 K (–196°C).

The zeta potential of O-MMT in various water-ethanolsolutions, as an indication of the surface charge, wasmeasured by using a Malvern Zetasizer (model Nano-ZS,Malvern Instruments, Inc., Houston, TX, USA) andSmoluchowski’s model (Marras et al. 2007). Nanoclaysuspensions of 200 mg L−1 were prepared in three differ-ent solvent systems (water, ethanol, and water/ethanol[1:1]) and stirred for 24 h at 23°C. All the measurementswere conducted at 25°C ± 0.1°C with at least 10 runs oneach sample suspension.

The elemental composition of O-MMT was measuredby using an XRF spectrometry method. The test was doneaccording to a previously established procedure (Deeringet al. 2008). Briefly, 1 g of O-MMT powder (pre-dried)was combined with 9 g of lithium tetraborate (Li2B4O7)and 0.5 g of ammonium nitrate (NH4NO3, used as oxidi-ser) in a platinum crucible and placed on an orbital mixingstage and fused at 1000°C for 20–30 min. The melt waspoured into a platinum mould to form a glass disk and thedisk was analysed by using the XRF spectrometry methodwith a Bruker S-4 system (Bruker Co., Billerica, MA,USA). The XRF major-element analysis was performedby using a fundamental parameter data reduction methodand Bruker Spectra Plus software; O-MMT samples weretested in triplicate.

Preparation of O-MMT suspensions

A stock suspension of 200 mg L−1 was prepared by disper-sing 40 mg of O-MMT in 200 mL of ethanol (200 proof)and sonicating (VWR ultrasonic cleaner water bath, Model75HT, 35 kHz, VWR International LLC, Radnor, PA, USA)the mixture in a 250 mL beaker for 30 min before furtherdilution with water or ethanol. Sonication helps to breakdown O-MMT clusters and achieve a better dispersion(Herrera-Alonso et al. 2009; Santos et al. 2011).

GFAAS

Elemental analysis of O-MMT was conducted with aHitachi Z-9000 simultaneous multi-element atomicabsorption spectrometer (Hitachi High-Technologies Co.,Tokyo, Japan) equipped with a HGA-700 atomiser and anautosampler system. A graphite tube-type cuvette wasused as the atomisation furnace with a temperature pro-gram as follows: (a) drying at 120°C for 30 s, (b) ashingfrom 710°C to 990°C for 30 s, (c) atomising at 3000°C for10 s and (d) cleaning at 3000°C for 3 s. Detection of Siand Al was performed by injecting 20 μL of sample

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solution into the graphite furnace and recording the absor-bance at 251.6 and 309.3 nm, respectively, with hollowcathode lamps (Hamamatsu Photonics Corp., Japan) set at15 mA.

Si and Al standard solutions of 1000 mg L−1

(PerkinElmer, Inc., MA, USA), made with (NH4)2SiF6and Al(NO3)3 as solutes, were diluted with deionisedwater in the 50-mL PP centrifuge tube and used to estab-lish external calibration curves. A good linear range wasachieved for the Si standard solution between 0.03 and0.5 mg L−1 (R2 = 0.999) and for the Al standard solutionbetween 0.012 and 0.2 mg L−1 (R2 = 0.998). The LOQs ofthe method (based on a signal-to-noise ratio of 10) were0.03 mg L−1 for Si and 0.01 mg L−1 for Al. The calibra-tion curves generated from the Si and Al standards wereused to determine the amount of Si and Al in nanoclaysuspensions. The LOQ can be further improved by usingthe concentration function of the instrument. This wascarried out by repeat injections (up to 25 times) of thesample via the autosampler and execution via the dryingstage. For instance, LOQs of 8 μg L−1 for Si and 3 μg L−1

for Al were achieved by injecting the sample six times togive one absorbance data point.

Stability of the dispersion of O-MMT in solution

To study the stability of O-MMT dispersed in differentwater/ethanol solvent systems, the stock suspension ofO-MMT was diluted to 5 mg L−1 with water/ethanol atfive different vol/vol ratios (1:0, 2:1, 1:1, 1:2 and 0:1). Thefinal suspensions were mechanically agitated with a vortexmixer (Scientific Industries, Inc., NY, USA) for 30 sbefore GFAAS measurements to maintain the homoge-neous dispersion of the nanoclay. To indicate the disper-sion stability of O-MMT, the Si and Al absorbance ofO-MMT suspensions (conducted in triplicate) wasrecorded every 2 min up to 36 min.

The effect of the surfactant on O-MMT dispersion wasalso evaluated. The dimethyl dialkyl (C14–C18) amine(Sigma-Aldrich, St. Louis, MO, USA) was added to thestock suspension of O-MMT, which was further diluted to5 mg L−1 with deionised water to achieve final suspen-sions with surfactant:O-MMT ratios of 1:1 and 5:1. Si andAl absorbance of the suspensions (conducted in triplicate)was recorded every 2 min up to 36 min.

Results and discussion

Properties of O-MMT

The zeta potential of O-MMT was 38.2 ± 1.8 mV in water(pH = 7), 11.6 ± 0.8 mV in the 1:1 water/ethanol mixtureand −23.6 ± 0.8 mV in ethanol. The measured O-MMTsurface area was 12.4 m2 g–1. The theoretical SSA of fullyexfoliated O-MMT nanoclay has been estimated as 750m2 g−1 (Nikolaidis et al. 2011). A measured SSA may be

affected by the aggregation state of the clay particles andthe type of methods used. The BET sorption isothermequation gives the external surface area on the basis ofN2 adsorption. Therefore, the measured SSA value may befar less than the theoretical. Si and Al contents within thenanoclay, based on the XRF spectrometry method analy-sis, were 20.14% ± 0.06% and 7.70% ± 0.02% wt/wt,respectively, with a Si/Al ratio of 2.58 ± 0.01.

Dispersion of O-MMT in different solvent systems

Measurement of O-MMT concentration in a suspended sol-vent system requires a homogenous and stable dispersion ofthe O-MMTwithin the time frame the sample is analysed toguarantee that the real concentration of the O-MMT in solu-tion is measured. Dispersion of the O-MMT can be affectedby the interaction between the O-MMT and the solventsystem. Because organo-modification of MMT changes thesurface from hydrophilic to organophilic, a poor dispersionof O-MMT nanoparticles would be expected in water. Theaggregation behaviour of MMT in water after organo-mod-ification along with an increase in zeta potential from nega-tive towards positive has been reported (Marras et al. 2007,2011). However, a better dispersion of O-MMT particles inethanol would be expected because of the relatively goodaffinity between the organic solvent and the organic surfac-tant, as previously reported by the adsorption of ethanol intothe interlayer space and the swelling of nanoclay particles(Burgentzlé et al. 2004).

Si and Al absorbance of 5 mg L−1 nanoclay suspen-sions made with different water/ethanol ratios wasrecorded from up to 18 continuous injections (over about36 min) and data are shown in Figure 1. Maximum absor-bance values were obtained for both Si and Al in suspen-sions with pure ethanol and with a water/ethanol ratio of1:2, and no obvious decreasing trend was observed for theabsorbance with time (slope β = 0; p > 0.05). As watercontent in the suspension increased (water/ethanol ratiosof 1:1 and 2:1), a decreasing trend for absorbance wasobserved, indicating an ongoing precipitation of O-MMTparticles as a function of time. In pure water, the Si and Alabsorbance reached minimum values, which were aboutone fifth of those in pure ethanol. Generally, precipitationof nanoclay particles happens both in ethanol and in water,which could be explained by the solubility parameter ofthe solvent (Ho & Glinka 2003). However, a more stabledispersion of O-MMT particles was achieved in ethanolthan in water within the short period of time studied asindicated in Figure 1. In addition, sonication and mechan-ical agitation helped to stabilise the dispersion of O-MMTparticles and slow down their precipitation.

The dispersion of O-MMT in water could be improvedby adding a surfactant (dimethyl dialkyl amine) that has bi-affinity to water and the nanoclay. Figure 2 shows the Siand Al absorbance of nanoclay suspensions (5 mg L−1)

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with the added surfactant (5 and 25 mg L−1 surfactant) andwithout the surfactant (0 mg L−1, control) in water.Compared with the control, the absorbance was muchhigher for the sample with a surfactant/nanoclay ratio of1:1, indicating that a large number of O-MMT particleswere dispersed. The absorbance was even higher at thesurfactant/nanoclay ratio of 5:1, and the slope β wasreduced. However, the absorbance of the suspension inthe 5:1 mixture was lower than that in pure ethanol (about10% less) and the decreasing trend of absorbance was moreobvious compared with that in pure ethanol (Figure 1) asindicated by the slope β for the two solutions. The additionof the surfactant significantly improved the dispersion of O-MMT in water although not to the extent as achieved forthe dispersion of O-MMT in ethanol.

Dissolution of clay minerals usually happens in aqu-eous systems. The preferential release of Si compared withAl from montmorillonite occurs because of the abundanceof Si at the clay surface and the higher solubility of Si inwater (Huang & Keller 1971; Rozalen et al. 2008; Sondiet al. 2008). Figure 3 shows the change in Si/Al ratiosobserved over time in the various water/ethanol systems.The Si/Al ratio increased with an increase in the watercontent of the system, which may be attributed to theuneven release of Si and Al from the O-MMT into wateralong with the precipitation of the O-MMT. Therefore, O-MMT quantification in water on the basis of elementalanalysis may not give reliable results due to the largedispersion of values and the preferential release of Si.Figure 3 confirms that a water/ethanol ratio of 1:2 and

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β = –0.0017 ± 0.0004, p < 0.001

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Figure 2. Absorbance of Si (a) and Al (b) as a function of time in nanoclay suspension (5 mg L−1 in water) with added surfactant of5, 25 and 0 mg L−1, corresponding to surfactant/nanoclay ratios of 1:1, 5:1 and 0:1 (control). Linear regression (solid line) wasperformed with the 95% confidence interval (CI) band (dashed line). The slope of each regression line was also given with the 95% CIand p value.

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β = –0.0003 ± 0.0003, p < 0.001

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β = –0.0044 ± 0.0010, p < 0.001

Figure 1. Absorbance of Si (a) and Al (b) as a function of time in nanoclay suspension (5 mg L−1) at water/ethanol ratios of 1:0, 2:1,1:1, 1:2 and 0:1. Linear regression (solid line) was performed with the 95% confidence interval (CI) band (dashed line). Slopes of eachregression line are also reported with the 95% CI and p value.

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pure ethanol can be used to quantify O-MMT in solution,which would be valuable for measuring O-MMT duringmass transport experiments such as migration from poly-mer nanocomposites.

Determination of Si and Al content

Although O-MMT concentration cannot be directly mea-sured by using the GFAAS equipment or other instruments,the elements of this clay can be identified and quantified inethanol. Therefore, the elemental (Si and Al) concentrationscould be correlated with the O-MMT concentration in solu-tion. A series of O-MMT suspensions with a concentrationrange of 0.5–2 mg L−1 were prepared, and linear regres-sions of Si and Al concentrations versus O-MMT concen-trations were obtained (Figure 4). The slope of each

regression line gives the Si and Al contents within the O-MMT, which are 22% ± 1.1% and 9.3% ± 0.5% wt/wt,respectively, with an Si/Al ratio of 2.4 ± 0.1. These valuesare aligned with the values obtained by using the XRFspectrometry method (Si = 20.14% ± 0.06%,Al = 7.80% ± 0.02% wt/wt and an Si/Al ratio of2.58 ± 0.01). The deviation between the results from thetwo methods might be due to the difference in samplepreparation (liquid vs. solid samples), the sample analysismethod and/or the variation in surfactant content.

Conclusions

A GFAAS method was developed for a rapid measure-ment of O-MMT concentration in water-ethanol solutionswithout the need for acid digestion of the O-MMT sample.

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Figure 3. Change in Si/Al ratio over time in nanoclay suspension at water/ethanol ratios of 1:0, 2:1, 1:1, 1:2 and 0:1. Solid lines areexperimental linear regressions to demonstrate the trends.

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1.2 1.4 1.6 1.8 2.0 2.2O-MMT concentration (mg L–1)

Figure 4. Correlations between Si and Al concentrations and O-MMT concentration. Linear regression was applied on Si and Alconcentrations versus O-MMT concentration. Si and Al contents in the O-MMT, given by the slope of the regression line, were22% ± 1.1% and 9.3% ± 0.5% wt/wt, respectively, at p < 0.001.

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Because no digestion was applied, special precautionswere taken to ensure the stability of the O-MMT inwater-ethanol solutions to perform a reliable measurementon the nanoclay particles. The stabilised dispersion wasaffected by the interaction between the O-MMT and thewater-ethanol solutions. A solution with an ethanol con-centration of higher than 70% (v/v) was preferred toobtain a good dispersion of the O-MMT in the sonicatedsolutions because of the good affinity between the solutionand the organophilic O-MMT. The dispersion in water wasimproved by the addition of an organic surfactant. Thecorrelation between Si and Al concentrations and O-MMTconcentrations in solution and the Si/Al ratio gave resultsin good agreement with the expected composition of theO-MMT in solution, which was further validated byresults obtained with an XRF spectrometry method.Therefore, GFAAS can be used as a tool for determiningthe amount (e.g., mass concentration) of O-MMT in sus-pension by measuring the concentration of Si and Al. Thismethodology can be applied to measure the migration ofO-MMT from nanocomposites in contact with food orfood simulants. Further work is needed to understand thebehaviour of O-MMT and other nanoclays in differentsolvent systems, so that the application of this instrumen-tal methodology can be expanded.

AcknowledgementsThe authors thank Kathy Severin from the Department ofChemistry at MSU for assistance and allowing access to theGFAAS equipment and Wei Zhang from the Department ofCrop and Soil Sciences at the MSU for the use of the Malvernunit and the measurement of the zeta potential. The authors alsoappreciate the assistance of Carlos Diaz for the GFAAS experi-ments and Ajay Kathuria for the measurement of SSA.

FundingThis work was financially supported by the Center for PackagingInnovation and Sustainability at the School of Packaging, MichiganState University.

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