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Na-montmorillonite modified with ammonium salts and azobenzene as a photoactive nanomaterial

Koteja A., Matusik J.

Geology, Geophysics and Environment

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2016

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Vol. 42, no. 1

87--88

EN

Modification of inorganic solid structures (e.g. minerals) with organic molecules is a constantly developed topic in material sciences. The organic functionalization leads to the production of new materials with integrated properties of both the organic and inorganic component. In the presented study we have modified a Na-montmorillonite with alkylammonium surfactants and subsequently azobenzene, in order to obtain a nanomaterial that shows response to UV radiation. Azobenzene is a photoswitchable organic molecule capable to change its conformation upon UV radiation from the trans- to cis-azobenzene isomer. This reaction is coupled with a change of the molecules shape and dimensions (Klajn 2010). The montmorillonite is a layered aluminosilicate that serves as an excellent host structure for organic guest species. Due to the net negative layer charge it shows the ability to swell and to exchange the originally present interlayer cations. These properties allow the intercalation of bulk organic molecules and to control their arrangement. Much attention has been paid to the possibility of transferring the photoswitching ability of organic molecule into the motion of the whole organo-mineral structure (Heinz et al. 2008). Such nanoswitch is particularly appealing as it is controlled with radiation – remotely and at a precise location. The efficiency of a synthesized nanoswitch depends on an accurate selection of the host and guest component. The target of this study to test a series of organic surfactants and to establish a modification pathway that leads to obtaining a material most promising in the view of its photoresponsive behavior. The montmorillonite modification was performed in a two-step procedure, as the direct intercalation of a nonionic azobenzene is not possible. First, the Na-montmorillonite (denoted SWy) was ion-exchanged with trimethylalkylammonium cations abbreviated C n and benzyldimethylalkylammonium cations – BC n , where n refers to the number of carbon atoms in the alkyl chain and is equal to 12, 14 or 16. In the second step the organo-montmorillonites were reacted with azobenzene (AzBz) for 24 h at 120°C in a hermetically closed teflon vessel. The yellowish products were characterized with the X-Ray diffraction (XRD), the infrared spectroscopy (FTIR) and CHN elemental analysis. In all cases the intercalation of the ammonium cation caused an increase of the montmorillonites basal spacing ( d 001 ). The d 001 values were equal to 16.4 Å, 18.2 Å and 20.5 Å for SWy-C 12 , SWy-C 14 and SWy-C 16 , respectively. The samples modified with the BC n cations showed ~1.5 Å larger basal spacing, due to the presence of the benzyl group in the intercalated molecule. A linear relationship was observed between the d 001 value and the alkyl chain length of the introduced salts. This suggests that the organic cations formed paraffin-type aggregates in the interlayer (Ogawa et al. 1999) where the molecules are inclined to the layer surface. The FTIR spectra of modified SWy sample showed intense bands corresponding to CH 2 vibration modes. Along with the increasing alkyl chain length the CH 2 stretching bands shifted towards lower energies. This is an effect of growing packing density of alkylammonium molecules in the interlayer (He et al. 2004) and it is coupled with straightening of the alkyl chains due to transformation of disordered gauche conformer to the ordered all-trans conformer (Vaia et al. 1994). It can be concluded that the longer alkyl chains (C 16 and BC 16 ) form more ordered, solid-like aggregates in the interlayer space. The molar content of organic molecules was calculated basing on the CHN elemental analysis. The amount of intercalated alkylammonium cations was nearly equal to the cation exchange capacity (CEC) of montmorillonite – 88.9 meq/100 g. The reaction with azobenzene was most effective for montmorillonite modified with the alkylammonium cations having the longest chains as confirmed by the XRD patterns. The d 001 values of SWy-C 16 and SWy-BC 16 samples after reaction with AzBz increased to 36.9 Å and 35.9 Å, respecively. Well resolved and intense (001) peaks as well as the presence of the 2 nd and 3 rd order reflections indicated a highly ordered structure of these intercalates. On the contrary, diffraction peaks were less resolved and broadened for samples prepared with the shorter C 12 , C 14 , BC 12 and BC 14 molecules after reaction with AzBz. Based on these results, it is assumed that the long chain alkylammonium ions are more effective surfactants for the further intercalation of azobenzene into the montmorillonites interlayer space. The obtained highly ordered structures are promising materials for application as photo-actuated nanoswitches.

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Preparation and characterization of azobenzene-smectite photoactive mineral nanomaterials

Koteja A., Matusik J.

Geology, Geophysics and Environment

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2015

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Vol. 41, no. 1

99--100

EN

Smectites are 2:1 layered minerals built of one octahedral sheet located between two tetrahedral sheets. The layer charge derived from the isomorphic substitutions in the mineral structure is compensated by the interlayer cations. The capability to exchange the interlayer cations is an important property of smectites as it enables to design and produce new nanomaterials through their modification with organic compounds. Such hybrid materials are highly desirable in industry and environmental protection due to their specific properties that may be designed in nanoscale. Preparation of photoactive materials using intercalation of layered minerals, mainly synthetic micas, with azobenzene and other azocompounds was proposed previously (Fujita et al. 2003, Ogawa et al. 2003, Heinz et al. 2008). Azobenzene molecules show a change in their shape and dimensions upon the UV irradiation, what may affect the structure of host mineral. The photoactive materials may find application in nanotechnology as molecular nanoswitches and nanosensors controlled by UV radiation (Klajn 2010). The objective of this study was to prepare azobenzene-smectite intercalation compounds. The results of structural and chemical characterization of obtained materials are crucial for further improvement of their photoresponsive properties.The Na-montmorillonite (SWy), Camontmorillonite (STx), beidellite (BId) and synthetic laponite (SynL) were used in the experiments. The modification procedure involved (1) the intercalation of smectites with hexadecyltrimethylammonium bromide (C16), and (2) insertion of azobenzene into the interlayer space. The reaction with C16, in amount equal to 1.0 CEC (cation exchange capacity) of the smectite, was performed in an aqueous suspension (20 g/L) for 2 h in 60°C. The obtained organosmectites were prepared as thin films on the glass plates and reacted with azobenzene in a teflon vessel at ~100°C for 24 h. In such conditions the azobenzene vaporizes and penetrates the interlayer space of the organomineral. The azobenzene/smectite weight ratio was equal to 0.2. The chemical and structural analyses of all obtained samples were carried out using X-ray diffraction (XRD), infrared spectroscopy (FTIR), and CHN (carbon-hydrogen-nitrogen) elemental analysis. The increased amount of nitrogen and carbon in modified samples confirmed the occurrence of intercalation process of both the ammonium salt and the azobenzene. Moreover, new bands appeared in the infrared spectra of the C16-smectites at ~2924 cm−1 and ~2851 cm−1 due to the C-H stretching vibrations in the C16 molecules. The spectra of azobenzene intercalation compounds showed add it ionally a series of bands corresponding to the vibrations characteristic for the azobenzene 2015, vol. 41 (1): 99–100100molecule at ~3061 cm−1, ~1581 cm−1, ~1455 cm−1, and ~1302 cm−1. The basal spacing of tested minerals increased after the C16 intercalation, as confirmed by XRD analysis. The increase was equal to 6.1 Å, 3.3 Å, 4.1 Å and 3.5 Å for SWy, STx, BId and SynL samples, respectively. This suggests nearly horizontal arrangement of the C16 molecules and formation of a monolayer in the smectite’s interlayer space. Introduction of azobenzene lead to a further increase of d001. The increase was visibly different for all the samples and it was equal to 7.0 Å, 15.0 Å, 21.7 Å and 23.5 Å for SWy, STx, Bid, and SynL samples, respectively. The arrangement of organic molecules in the interlayer space is influenced by a number of factors including (1) type of the mineral, (2) layer charge and its location in the layer, and (3) the amount and arrangement of the cationic surfactant (Klapyta et al. 2001, Lagaly et al. 1976). A correlation between azobenzene location in the interlayer space and the photo-response behaviour of tested materials will be the subject of further studies.

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Improved copper sorption on grafted kaolinites of different structural order

Koteja A., Matusik J.

Geology, Geophysics and Environment

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2014

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Vol. 40, no. 1

96--97

EN

Kaolin group minerals exhibit relatively low sorption capacity as the migration of ions and their sorption in the interlayer space is not possible. The ions attraction is limited to the particles faces and edges through surface complexation and ion-exchange mechanisms. The ongoing research on functionalized kaolinites enabled to synthesize new nanomaterials with possible applications in industry and environmental protection. The modification procedures mainly involve structure alteration by intercalation and/or grafting processes. It is worth to underline that kaolinite 1:1 layer has exposed inner surface hydroxyls which are susceptible for reactions with selected organic molecules and as a results new materials with interesting properties may be obtained. Heavy metals in excessive amount are toxic and may lead to serious health problems. Thus, the purification of heavy metal contaminated aqueous solutions is of environmental importance. The aim of the study was to examine the sorption properties of kaolinites grafted with selected aminoalcohols towards Cu(II). For the experiments, two types of Polish kaolinites were chosen: well ordered type from Maria III deposit (M) (located about 20 km SW from Bolesławiec) and poorly ordered type from Jaroszów deposit (J) (located about 10 km NE from Strzegom). The modification consisted of (i) a preparation of kaolinite-dimethyl sulfoxide intercalate (MDMSO and JDMSO) and (ii) its further grafting with diethanolamine (DEA) or triethanolamine (TEA) (Letaief & Detellier 2007). The synthesized MDEA, MTEA, JDEA and JTEA samples were examined using XRD, IR and CHNS methods. The Cu(II) sorption equilibrium experiments were performed for the 0.005-10.0 mmol/L concentration range at room temperature (initial pH 5). The suspensions (20 g/L) were shaken for 24 hours. Afterwards, the Cu(II) concentration was measured using AAS method. The d001 reflections for the MDEA, MTEA, JDEA and JTEA increased from 7.2 Å (M and J samples) to 10.2 Å, 10.8 Å, 10.1 Å and 11.0 Å, respectively which confirmed the formation of grafted compounds. The presence of organic molecules resulted in an appearance of C-H stretching bands (2800-3000 cm-1) in the IR spectra. Moreover, changes in the O-H stretching region (3600-3700 cm-1) were also noticed due to interaction of aminoalcohols with kaolinites hydroxyls. An assumption was made that the sorption of cations will take place by the nitrogen lone electron pair of the grafted DEA or TEA. Thus, the theoretical sorption capacity associated with nitrogen was calculated on the basis of CHNS analysis and was the following: 184 mmol/kg (MDEA), 223 mmol/kg (MTEA), 122 mmol/kg (JDEA) and 323 mmol/kg (JTEA). The highest Cu(II) sorption was observed for the JTEA sample: 119 mmol/kg, while for the J and JDEA samples it reached 65 mmol/kg and 88 mmol/kg, respectively. The sorption improvement was less pronounced for materials based on the M sample. Both for the pure M sample and the MTEA sample, the sorption capacity was equal to ∼62 mmol/kg, while for the MDEA sample it was higher and reached 72 mmol/kg. It can be concluded that the performed modifications have improved the kaolinites sorption capacity. The improvement was due to cations attraction by the nitrogen lone electron pairs after their migration into the interlayer space. Worth emphasizing is that the Cu(II) ions readily form Cu-aminoalcohol complexes in aqueous solutions (Karadag et al. 2001). As a result of such mechanism, the pH value has increased, which is attributed to competitive protons sorption. The adsorption energy, estimated on the basis of Dubinin-Radushkevich equation, for all the reactions was in the 10-14 kJ/mol range. This indicates that the adsorption energy corresponds to energies characteristic for the ion exchange reactions (Debnath & Ghosh 2007). In most cases the sorption isotherms followed the Langmuir model.