Characteristics of Hybrid Pigments Made from Alizarin Dye on a Mixed Oxide Host

materials

Article Characteristics of Hybrid Pigments Made from Alizarin Dye on a Mixed Oxide Host

Anna Marzec 1,* , Bolesław Szadkowski 1 , Jacek Rogowski 2, Waldemar Maniukiewicz 2 , Małgorzata Iwona Szynkowska 2 and Marian Zaborski 1

1 Institute of Polymer and Dye Technology, Faculty of Chemistry, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland; [email protected] (B.S.); [email protected] (M.Z.) 2 Institute of General and Ecological Chemistry, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland; [email protected] (J.R.); [email protected] (W.M.); [email protected] (M.I.S.) * Correspondence: [email protected]

Received: 30 December 2018; Accepted: 22 January 2019; Published: 24 January 2019

Abstract: This paper describes the fabrication of a new hybrid pigment made from 1,2-dihydroxyanthraquinone (alizarin) on a mixed oxide host (aluminum-magnesium hydroxycarbonate, LH). Various tools were applied to better understand the interactions between the organic (alizarin) and inorganic (LH) components, including ion mass spectroscopy (TOF-SIMS), 27-Aluminm solid-state nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), and thermogravimetric analysis (TGA). TOF-SIMS showed that modiﬁcation of the LH had been successful and revealed + − the presence of characteristic ions C14H7O4Mg and C14H6O5Al , suggesting interactions between the organic chromophore and both metal ions present in the mixed oxide host. Interactions were also observed between Al3+ ions and Alizarin molecules in 27Al NMR spectra, with a chemical shift detected in the case of the modiﬁed LH matrix. Any changes in color following reactions with Mg2+ and Al3+ ions were observed. Some of the physicochemical properties of alizarin, such as resistance to dissolution and color stability at elevated temperatures, were improved in comparison to the pure dye. This effect can be attributed to strong dye-LH interactions and the effective transformation of alizarin into an insoluble form. Moreover, the pigments exhibited higher thermal resistance and greater color stability in comparison to commercially available alizarin lakes (Alizarin Crimson).

Keywords: Alizarin; hybrid pigment; aluminum-magnesium hydroxycarbonate; mixed oxide; color stability

1. Introduction Natural and synthetic dyes are used in a wide range of applications, including in the optics, cosmetics, and food packaging industries [1,2]. However, their uses are limited by both their high solubility and low chemical, thermal, and photo resistance. Natural dyes and their identical synthetic analogues can be transformed into insoluble pigments with enhanced chemical and thermal stability [3]. Natural organic dyes have been used since antiquity to produce organic-inorganic pigments, known as lakes [4]. These can be prepared by the complexation of dye molecules with metallic cations present in an inorganic support material, such as alumina, silica, talc, calcium carbonate, or barium sulfate. One of the oldest known compound dyes is alizarin (1,2-dihydroxyanthraquinone), which is extracted from the roots of the common madder plant (Rubia tinctorium L.) [5,6]. From a chemical point of view, alizarin belongs to the anthraquinone class of dyes, with condensed aromatic rings, two carbonyls (at positions 9 and 10), and two hydroxyl groups.

Materials 2019, 12, 360; doi:10.3390/ma12030360 www.mdpi.com/journal/materials Materials 2019, 12, 360 2 of 14

The most common method of obtaining alizarin lakes is by the complexation of 1,2-dihydroxyanthraquinone with alumina. This reaction is based on the formation of a six-membered cyclic chelate, with a hydroxyl group (–OH) substituent at position 1 and the nearest oxygen atom from the carbonyl (C=O) [7,8]. The optical properties and stability (thermal and chemical resistance) of alizarin lakes depend on the types of metal salts applied and on the structure of the metal-dye complex [9]. Numerous methodologies are now available for studying the interactions of organic chromophores with the metal ions present in inorganic carriers, including infrared (IR) [10,11] and Raman [12,13] spectroscopies, spectrophotometry, and fluorimetry [14,15]. Studies based on NMR spectroscopy, most often of 13C and 27Al MAS NMR spectra, suggest that organic dyes form complexes with clay minerals [16,17]. However, the 27Al MAS NMR method is restricted to the analysis of dye interactions involving a limited number of ions and is unsuitable for the study of interactions involving ions such as Mg2+ or Zn2+. Alizarin can be immobilized and stabilized on various inorganic bases [18,19]. Perez et al. [20] stabilized several dyes, including alizarin, on an acid matrix, α gamma alumina. The stability of the pigment was greatly enhanced by the presence of coordinative unsaturated sites on the surface of the gamma-alumina. Interactions between the alumina and the organic chromophore were found to determine the color and hue of the pigment. The metal-dye complexes were studied using FT-IR and 27Al MAS NMR spectroscopy, as well as colorimetry. Trigueiro et al. [21] synthesized Ti- and Al-pillared montmorillonite, which they modified with carminic acid (CA) and alizarin dye. Interactions were observed between the organic guest and the inorganic host using FT-IR, 13C, and 27Al solid state magnetic nuclear resonance and time resolved fluorescence spectroscopies. The organic-inorganic pigments based on Al-pillared montmorillonite showed a higher stability under UV irradiation than the Ti-based hybrid. Ghannam et al. [22] synthesized sensitive colored hybrid inorganic/organic pigments based on polymer-coated mica particles. The color variations were attributed to the superimposition of different layers of adsorbed alizarin molecules. When closer to the surface, alizarin is more influenced by oxanions and is pinkish in color. When it is further from the surface, its electronic state is affected less and it has a yellowish hue. The global color may therefore be ascribed to the combined effect of the different electronic states of alizarin, resulting in an orange pigment. In this study, a novel hybrid pigment was produced by the complexation of 1,2-dihydroxyanthraquinone dye with aluminum-magnesium hydroxycarbonate (LH). In a previous work [23], LH with different Mg:Al ratios had been employed to produce new color-tunable hybrid pigments. In the present study, we used an Al rich mixed oxide (Al:Mg ratio 80:20) with a large surface area to stabilize the alizarin chromophore. Interactions between the alizarin and metal ions in the LH host (Mg2+ and Al3+) were confirmed by mass spectroscopy (TOF-SIMS). A common method for investigating interactions between organic chromophores and Al ions, 27Al solid state magnetic nuclear resonance, was also used to confirm the formation of an alizarin-Al complex. The morphology of the hybrid pigment was characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The organic-inorganic hybrid was evaluated for thermal stability (TGA) and resistance to dissolution. The physical and chemical properties of the new alizarin lake were compared to those of a commercially available alizarin-based lake pigment (Alizarin Crimson).

2. Materials and Methods

2.1. Raw Materials The inorganic host, magnesium-aluminum hydroxycarbonate with an Mg/Al weight ratio of 20/80, was purchased from Sasol GmbH (Schnelldorf, Germany). Toluene, ethanol, acetone, and cyclohexane were purchased from Sigma-Aldrich (Schnelldorf, Germany). The color agents Alizarin Crimson (madder lake) and alizarin were kindly supplied by Kremer Pigmente GmbH & Co. KG (Munich, Germany) and Sigma-Aldrich (Schnelldorf, Germany), respectively. The reagents were analytical grade and did not require further puriﬁcation prior to use. Materials 2019, 12, 360 3 of 14

2.2. Synthesis of Hybrid Pigments Pigments modified with alizarin (LH/Alizarin) were prepared in an aquatic environment with high purity deionized water. For LH/Alizarin (15%), the method proceeded as follows: 8.5 g of LH was added dropwise with vigorous stirring to a solution of 1.5 g of alizarin in 200 cm3 water and 50 cm3 ethanol. During modification, the pH value changed from 9.4 to 8.1. The mixture was then heated to 80 ◦C under mechanical agitation. The reaction was continued under the same conditions for a further 3 h. Finally, the reaction mixture was filtered, washed with water, and dried at 70 ◦C until a dry hybrid pigment powder was obtained.

2.3. Characterization of Hybrid Pigments Secondary ion mass spectra of positive and negative ions were obtained using a TOF-SIMS IV mass spectrometer (IONTOF GmbH, Muenster, Germany) equipped with a 25 keV bismuth primary ion gun and a high mass resolution time of flight mass analyzer. The primary ion beam current was set to 0.4 pA. The structural changes in the examined samples were observed using powder X-ray diffraction (PXRD) analysis, with a PANalytical X’Pert Pro MPD diffractometer (Malvern Panalytical Ltd., Royston, UK) in Bragg-Brentano reflection geometry and (CuKα) radiation from a sealed tube in the range of 2θ = 3◦–70◦ at a step length of 0.0167◦. The XRD curves were interpreted based on the positions of the basal reflections. Solid state Nuclear Magnetic Resonance (MAS NMR) measurements were performed in a Bruker Avance III 400 WB spectrometer (Rheinstetten, Germany) operating 27 at a resonance frequency of 104.26 MHz. Al chemical shifts were compared to AlCl3·6H2O in 1M solution as an external reference (0 ppm). The thermal stability of the hybrid pigments was determined using a Q500 Thermogravimetric Analyzer, (TA Instruments, Greifensee, Switzerand). Thermogravimetric analysis (TGA) was performed with a heating rate of 10 ◦C/min under a nitrogen atmosphere, across a temperature range of 25–600 ◦C. The specific surface area was measured based on nitrogen adsorption at 77 K over a relative pressure (P/P0) range of 10−6–1 using a Gemini 2360 V2.01 porosimeter (Micromeritics, Norcross, GA, USA). Measurements were performed according to the Brunauer-Emmett-Teller (BET) nitrogen adsorption method. The samples were degassed for 20 h at 100 ◦C under a vacuum. The solvent resistance of the hybrid pigments was evaluated based on the PN-C-04406 standard. The pigment samples were immersed in cylinders with solvent (acetone, toluene, or ethyl alcohol) or water for 24 h, at room temperature. Their solvent resistance was estimated (on a scale of 1/5) based on the degree of decolorization. The UV-VIS spectra were recorded using an Evolution 201/220 UV-Visible Spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA). The experiment was conducted in a spectral window from 1100 to 200 nm at room temperature. The measurement specimens were in the form of solid state powders. They were put in the special powder cell holder. Before the measurements, correction of the baseline with a special calibration adapter was made. The accuracy of the apparatus was ±0.8 nm and the repeatability was ≤0.05 nm. The morphology of the materials was determined by scanning electron microscopy (SEM) using a LEO 1530 Gemini scanning electron microscope (Zeiss/LEO, Oberkochen, Germany). Prior to the analysis, the pigment powders were coated with a carbon target using a Cressington 208 HR system.

3. Results and Discussion

3.1. Secondary Ion Mass Spectrometry (TOF-SIMS) and 27Al NMR Analysis The TOF-SIMS technique was employed to investigate the interactions between the matrix and the alizarin molecules. It was expected that alizarin would be stabilized in the LH matrix by ionic interactions with Mg2+ and/or Al3+ ions, and that such interactions would be revealed by the emission + of characteristic ions from the LH/alizarin. This was conﬁrmed by the presence of a C14H7O4Mg ion peak at m/z 263 in the TOF-SIMS spectrum of LH/Alizarin (Figure1a). TOF-SIMS analysis was also used to investigate the formation of possible Al-alizarin complex ions. The presence of − C14H7O5Al ions (m/z 281) may be ascribed to the fragmentation of complexes of alizarin with Materials 2019, 12, 360 4 of 14 Materials 2019, 12, x FOR PEER REVIEW 4 of 14 aluminumNMR that alizarin (Figure 1formedb). This a iscomplex supported with by Al the3+ in results both aqueous of a study and by methanolic Soubayrol etsolutions al. [ 24], of in sodium which 3+ ithydroxide. was found Complexes using NMR of thatalizarin alizarin have formed been reported a complex with with metals Al suchin both as Ca aqueous or Mg and [25]. methanolic However, solutionsthe formation of sodium of complexes hydroxide. of organic Complexes chromophores of alizarin with have mixed been reported oxides in with LH metalsmay be such hindered as Ca by or Mgsteric [25 limitations.]. However, This the formation is due to ofthe complexes fact that ofaluminum-magnesium organic chromophores hydroxycarbonate with mixed oxides inconsists LH may of beirregular hindered plate by stericlayers. limitations. Alizarin molecules This is due therefor to the facte have that to aluminum-magnesium be properly oriented hydroxycarbonate towards the LH consistslayers, in of order irregular to interact plate layers. effectively Alizarin with molecules the Mg therefore2+ and/or have Al3+ toions. be properly In traditional oriented alizarin towards lakes, the 2+ 3+ LHaluminum layers, informs order part to interactof a chelate effectively ring with with the the 9-carboxyl Mg and/or and Al 1-hydroxylions. In groups traditional of alizarin, alizarin while lakes, aluminuma divalent formscation part (usually of a chelate calcium) ring is with bound the 9-carboxylas an ion and[8]. 1-hydroxylHowever, groupsin the current of alizarin, study, while the a divalentformation cation of an (usually alizarin calcium) lake with is boundsimilar as structur an ion [e8 ].was However, impossible in the due current to steric study, limitations, the formation as the of anmetal alizarin ions lakewere with trapped similar in structure the LH wasinorganic impossible layers. due It tomay steric be limitations, concluded asthat the the metal color ions of were the trappedLH/Alizarin in the pigments LH inorganic was the layers. result It of may the beformation concluded of individual that the color alizarin of the complexes LH/Alizarin with pigmentsMg2+ and 2+ 3+ wasAl3+ theions. result of the formation of individual alizarin complexes with Mg and Al ions.

FigureFigure 1.1. PositivePositive ((aa)) andand negativenegative ((bb)) TOF-SIMSTOF-SIMS spectraspectra ofof LHLH andand LH/AlizarinLH/Alizarin (10%) samples.

27 Further analysisanalysis ofof thethe structurestructure ofof thethe organic-inorganicorganic-inorganic frameworkframework waswas conductedconducted usingusing 27AlAl 27 MAS NMR.NMR. 27Al resonance line positions are very sensitivesensitive to the coordination number and tend toto occupy thethe −−55 to to 15 15 ppm ppm range range for for AlO AlO6 sitessites [26]. [26]. Figure Figure 2 shows thethe spectrumspectrum ofof LHLH recordedrecorded before before and after the adsorption of the anthraquinone dye. The resonance correspondingcorresponding to hexacoordinated 27 aluminum isis clearlyclearly visible.visible. TheThe 27Al MAS spectrum of the LH sample before complexation shows a relativelyrelatively narrownarrow resonanceresonance peakpeak atat aa chemicalchemical shift,shift, δδ,, ofof ~3.6~3.6 ppm.ppm. TheThe NMRNMR signalsignal ofof thethe hybridhybrid pigmentpigment was different from that that of of the the unmodified unmodiﬁed LH LH sample. sample. Adding Adding the the organic organic chromophore chromophore to tothe the LH LH host host contributed contributed to a to chemical a chemical shift shift of the of theresonance resonance peak peak from from 3.6 to 3.6 3.9 to ppm, 3.9 ppm, which which is very is veryclose closeto the to chemical the chemical shift shiftmeasured measured (3.4 to (3.4 3.2 to ppm 3.2 ppm)) for montmorillonite for montmorillonite modified modiﬁed with with alizarin alizarin in a inreaction a reaction carried carried out outunder under similar similar pH pH conditions conditions (8.4) (8.4) [21]. [21 ].The The results results of of NMR NMR and TOF-SIMSTOF-SIMS 3+ providedprovided complementarycomplementary evidenceevidence ofof thethe interactioninteraction betweenbetween thethe AlAl3+ ions and dye molecules.

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27 FigureFigure 2. 27 2.Al Al MAS MAS NMR NMR spectra spectra ofof alizarinalizarin lake lake before before and and after after adsorption adsorption of alizarin of alizarin dye. dye. Figure 2. 27Al MAS NMR spectra of alizarin lake before and after adsorption of alizarin dye. TheThe modiﬁcation modification of of the the mixedmixed oxide with with alizarin alizarin was was also alsoaccompanied accompanied by a reduction by a reduction in the in emissionThe modification of CO2− ions− of(Figure the mixed 3). This oxide reduction with wasalizarin most was probably also accompanied related to the partialby a reduction replacement in the the emission of CO2 ions (Figure3). This reduction was most probably related to the partial in the LH host of CO32− ions by the organic chromophore, as a result of the complexation of C14H7O4− replacementemission of in CO the2− LHions host (Figure of CO 3). This2− ions reduction by the was organic most chromophore, probably related as ato result the partial of the replacement complexation ions with Mg2+ and Al3+ ions. This3 observation is in line with the results of further studies conducted in the LH −host of CO32− ions2+ by the organic3+ chromophore, as a result of the complexation of C14H7O4− of C14usingH7O thermogravimetric4 ions with Mg analysisand Al(TG).ions. The mechanism This observation for the formation is in line of with lake the with results aluminum- of further ions with Mg2+ and Al3+ ions. This observation is in line with the results of further studies conducted studiesmagnesium conducted hydroxycarbonate using thermogravimetric seems similar analysis to that (TG). for lake The mechanismpigments obtained for the on formation traditional of lake using thermogravimetric analysis (TG). The mechanism for the formation of lake with aluminum- withinorganic aluminum-magnesium supports, involving hydroxycarbonate interactions between seems the dye similar and relative to that insoluble for lake pigmentsalkaline earth obtained salts on magnesium hydroxycarbonate seems similar to that for lake pigments obtained on traditional traditional[3,27]. It inorganic appears that supports, CO32− ions involving from the interactions weak carbonate between acid are the displaced dye and from relative the edge insoluble of the LH alkaline inorganic supports, involving interactions2− between the dye and relative insoluble alkaline earth salts earthinterlayer salts [3, 27by]. acidic It appears dye molecules, that CO forming3 ions insoluble from the alizarin weak pigments. carbonate A acid similar are mechanism displaced was from the [3,27]. It appears that CO32− ions from the weak carbonate acid are displaced from the edge of the LH edgeposited of the in LH our interlayer previous work by acidic for alizarin dye molecules, lakes based formingon Mg-rich insoluble hosts [23]. alizarin pigments. A similar interlayer by acidic dye molecules, forming insoluble alizarin pigments. A similar mechanism was mechanismposited in wasour positedprevious in work our previousfor alizarin work lakes for based alizarin on Mg-rich lakes based hosts on[23]. Mg-rich hosts [23].

Figure 3. TOF-SIMS spectra of negative secondary CO2− ions in LH (a) and LH/Alizarin (10%) (b).

−− FigureFigure 3. TOF-SIMS 3. TOF-SIMS spectra spectra of of negative negative secondary secondary CO CO22 ionsions in in LH LH (a (a) )and and LH/Alizarin LH/Alizarin (10%) (10%) (b). ( b).

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Materials 2019, 12, x FOR PEER REVIEW 6 of 14 3.2. Powder X-ray Diffraction (PXRD) 3.2. Powder X-ray Diffraction (PXRD) Further structural analysis revealed that no changes had taken place in the basal spacing of the LH, followingFurther structural the arrangements analysis revealed of thethat alizarinno changes dye had in taken the mineral place in the structure. basal spacing The diffractionof the patternLH, offollowing LH shown the arrangements in Figure4a of consists the alizarin of threedye in sharp the mineral peaks structure. at a low The 2 θdiffractionangle, equivalent pattern to of LH shown in Figure 4a consists of three sharp peaks at a low 2θ angle, equivalent to diffraction by diffraction by planes (003), (006), and (009), respectively. This indicates the good crystallinity of the LH. planes (003), (006), and (009), respectively. This indicates the good crystallinity of the LH. The The interlayer distances of (d003) and (d006), corresponding to 0.753 nm and 0.378 nm, are due to basal interlayer distances of (d003) and (d006), corresponding to 0.753 nm and 0.378 nm, are due to basal reflectionsreflections indexed indexed to ato hexagonala hexagonal crystal crystal latticelattice with rhombohedral rhombohedral 3R 3Rsymmetry symmetry [28]. [ 28Broadened]. Broadened peakspeaks are observedare observed on theon diffractionthe diffraction pattern pattern of theof the LH, LH, which which could could be be due due to to unaccounted unaccounted for, for, poorly crystallinepoorly impurities.crystalline impurities. After the After addition the ofaddition alizarin, of thealizarin, basal the reflection basal reflection (003) of the(003) LH/Alizarin of the pigmentsLH/Alizarin (Figure pigments4b,c) shifted (Figure slightly 4b,c) shifted to lower slightly 2 toθ angles,lower 2θwhich angles, forwhich LH20, for LH20, was was 11.733 11.733°◦ and for LH/Alizarineand for LH/Alizarine (20%), was (20%), 11.504 ◦was, although 11.504°, thealthou interlayergh the interlayer distances distances remained remained similar, regardlesssimilar, of the contentregardless of dyeof the molecules. content of dye The molecules. presence The of a presence characteristic of a characteristic non-basal non-basal (110) reflection (110) reflection at 2θ ~60.90 ◦ confirmsat 2θ that~60.90° the confirms LH structure that the had LH been structure retained. had Thebeen PXRD retained. pattern The PXRD for LH/Alizarin pattern for LH/Alizarin (20%) (Figure 4c) (20%) (Figure 4c) also shows very small reflections, characteristic of the crystalline structure of also shows very small reflections, characteristic of the crystalline structure of alizarin. alizarin.

Figure 4. Powder XRD patterns for: LH (a), LH/Alizarin (15%) (b), LH/Alizarin (20%) (c), and alizarin (d). Figure 4. Powder XRD patterns for: LH (a), LH/Alizarin (15%) (b), LH/Alizarin (20%) (c), and alizarin (d).

3.3. Thermogravimetric3.3. Thermogravimetric Analysis Analysis (TG) (TG) Thermogravimetric analysis (TG/DTG) was used to determine the thermal stability of the Thermogravimetric analysis (TG/DTG) was used to determine the thermal stability of the pigments. As shown in Figure 5, the thermal stability of the hybrid pigments improved significantly pigments. As shown in Figure5, the thermal stability of the hybrid pigments improved significantly in comparison to the commercial alizarin lake and raw LH. The DTG curves of the hybrid pigments in comparisondisplay multistep to the commercialdecomposition alizarin behavior lake typical and rawof mixed LH. Theoxides. DTG The curves first stage of the below hybrid 100 pigments °C ◦ displaycorresponds multistep partly decomposition to the desorption behavior of water typical physisorbed of mixed on the oxides. external The surface first of stage LH, and below partly 100 C correspondsto the removal partly of to water the desorption molecules from of water the interlay physisorbeder galleries. on the The external second surfaceweight loss of LH, on the and DTG partly to the removalcurve with of water a strong molecules peak centered from the interlayerat 213 °C galleries.can be attributed The second to weightthe water loss produced on the DTG by curve withdehydroxylation a strong peak centeredof the LH layers. at 213 The◦C canfinal beweight attributed loss step to occurs the water as the producedtemperature by increases, dehydroxylation with of thetwo LH peaks layers. at around The final 313 °C weight and 393 loss °C. The step peaks occurs can as bethe attributed temperature to the loss increases, of interlayer with carbonates two peaks at aroundand 313to the◦C complete and 393 decomposition◦C. The peaks of can metal be attributedhydroxide layers, to the respectively. loss of interlayer It was carbonatesduring this stage and to the that metal oxide structures formed [29]. The absence of a TG-DTG peak for the dye chromophore can complete decomposition of metal hydroxide layers, respectively. It was during this stage that metal be explained by the similar position of the dye degradation peak (326 °C) and the carbonate oxide structures formed [29]. The absence of a TG-DTG peak for the dye chromophore can be explained decomposition peak of the inorganic LH. However, the◦ TG-DTG curve for Alizarin Crimson was by theshaped similar differently. position Weight of the dyeloss degradationwas observedpeak at low (326 temperaturesC) and the (<200 carbonate °C), probably decomposition as a result peakof of the inorganicwater loss. LH. Above However, 200 °C, slow the TG-DTGdegradation curve was forobse Alizarinrved. The Crimson weight loss was in shaped the temperature differently. range Weight ◦ ◦ loss wasof 200–500 observed °C may at lowbe attributed temperatures to the degradation (<200 C), probablyof the organic as amoiety. result of water loss. Above 200 C, slow degradation was observed. The weight loss in the temperature range of 200–500 ◦C may be attributed to the degradation of the organic moiety.

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FigureFigure 5. 5.TG/DTG TG/DTG curves for alizarin lake with 10% andand 20%20% alizarinalizarin contentcontent andand AlizarinAlizarin Crimson.Crimson.

AsAs thethe concentrationconcentration ofof alizarinalizarin increased,increased, thethe thermalthermal stabilitystability ofof thethe hybridhybrid pigmentpigment improvedimproved ◦ gradually.gradually. AfterAfter modification,modification, thethe 10%10% weight weight loss loss temperature temperature (T (T10%10%)) ofof LHLH (199(199 °C)C) waswas increasedincreased toto 227227 ◦°CC forfor LH/AlizarinLH/Alizarin (15%) (15%) and and to to 235 235 °C◦C for for LH/Alizarin LH/Alizarin (20%) (20%) (Table (Table 1).1). In In comparison comparison to toAlizarin Alizarin Crimson Crimson and and unmodified unmodified LH, LH, the the hybrid hybrid pigments pigments exhibited exhibited noticeably noticeably lower lower weight weight loss lossabove above 300 °C, 300 due◦C, to due the to liberation the liberation of carbonate of carbonate and hydroxyl and hydroxyl groups groups from the from layered the layered mineral. mineral. Costa Costaet al. et[30] al. [suggest30] suggest that thatanthraquinone-like anthraquinone-like compounds compounds can can improve improve the the thermal thermal stability stability of LHLH systems,systems, whichwhich maymay explainexplain ourour results.results. OnOn thethe otherother hand,hand, thethe increasedincreased thermalthermal resistanceresistance ofof thethe hybridhybrid pigmentspigments maymay bebe thethe resultresult ofof thethe strongstrong incorporationincorporation ofof alizarinalizarin onon thethe LHLH structurestructure andand thethe decreasedecrease inin thethe concentrationconcentration ofof carbonatecarbonate ions,ions, asas confirmedconfirmed by by TOF-SIMS TOF-SIMS experiments. experiments.

TableTable 1. 1.Thermogravimetric Thermogravimetric analysis analysis of of LH/Alizarin LH/Alizarin pigments.pigments.

1 ◦1 ◦ ◦ ◦ Sample Sample T05% ( TC)05% (°C)T 10% T10%( C) (°C) TT20%20% (°C)( C) T30% T(°C)30% ( C) LHLH 93 93 199 199 342342 493 493 AlizarinAlizarin Crimson Crimson 72 72 120 120 301301 427 427 LH/AlizarinLH/Alizarin (15%) (15%) 133 133 227 227 393393 616 616 LH/AlizarinLH/Alizarin (20%) (20%) 132 132 235 235 406406 620 620 1 T05,T10,T20,T30—Temperature of 5%, 10%, 20%, and 30% weight loss, respectively. 1T05, T10, T20, T30—Temperature of 5%, 10%, 20%, and 30% weight loss, respectively.

3.4.3.4. UV-VISUV-VIS SpectroscopySpectroscopy andand ColorColor StabilityStability FigureFigure6 6shows shows the the absorption absorption spectra spectra for for different different LH, LH, alizarin, alizarin, Alizarin Alizarin Crimson, Crimson, and and LH-based LH-based alizarinalizarin lake powders. powders. The The absorption absorption spectrum spectrum of the of the alizarin alizarin chromophore chromophore shows shows a structured a structured band band with a maximum at 459 nm. The UV-Vis spectrum of the alizarin lake has a λ at 571 nm, with a maximum at 459 nm. The UV–Vis spectrum of the alizarin lake has a λmax at max571 nm, with a significant bathochromic shift in comparison to free dye, due to complexation of the magnesium (II) and aluminum (III) ions into the LH layers. The variations in the color and hue of alizarin are pH-

Materials 2019, 12, 360 8 of 14 with a signiﬁcant bathochromic shift in comparison to free dye, due to complexation of the magnesium Materials(II) and 2019 aluminum, 12, x FOR (III)PEER ions REVIEW into the LH layers. The variations in the color and hue of alizarin8 of are14 pH-dependent (Figure6)[ 21]. Neutral free alizarin can exist in several tautomeric forms. In solution, dependent (Figure 6) [21]. Neutral free alizarin can exist in several tautomeric forms. In solution, alizarin occurs in the form of partially dissociated yellow molecules below pH 5.2. At pH 6–10, it is alizarin occurs in the form of partially dissociated yellow molecules below pH 5.2. At pH 6–10, it is deprotonated and occurs in red monovalent cations. Finally, at around pH 12, it occurs in violet deprotonated and occurs in red monovalent cations. Finally, at around pH 12, it occurs in violet di- di-anionic form. At low pH, the interactions between the organic dyes and the clay surface are anionic form. At low pH, the interactions between the organic dyes and the clay surface are generally generally driven by electrostatic forces. At a higher pH (such as in our study), interactions can occur driven by electrostatic forces. At a higher pH (such as in our study), interactions can occur via the via the complexation of chromophore structures with the metal cations present in the LH layers. complexation of chromophore structures with the metal cations present in the LH layers.

FigureFigure 6. 6. UV–VisUV–Vis spectra of LH, alizarin, LH/Alizarin (10%), (10%), and and Alizarin Alizarin Crimson Crimson powders. powders.

TheThe pH pH of of the the reaction reaction solution solution is isnot not the the only only po possiblessible factor factor responsible responsible for forthe thevariation variation in the in opticalthe optical properties properties of alizarin. of alizarin. The The types types of metals of metals used used can can also also have have an an effect effect on on the the color color of of the the complexescomplexes they they form form with with alizarin, alizarin, as as well well as as on on their their thermal thermal and and chemical chemical stability. stability. Complexes Complexes of of alizarinalizarin with with Al(III), Al(III), Cr(III) Cr(III),, Ni(II), Ni(II), Cu(II), Cu(II), Zn(II), Zn(II), Cd Cd(II),(II), and and Fe(III)) Fe(III)) are are associated associated with with a a red-shift red-shift of of thethe visible visible band band with with respect respect to to the the isolated isolated dye dye [9 [9,31,32].,31,32]. Both Both environmental environmental effects effects are are present present and and influenceinfluence madder madder lake color changes. The absorption band of the LH/Alizarin pigments pigments in in the the visible visible regionregion showed showed a a maxim maxim at at higher higher wavelengths wavelengths in in comparison comparison to to a a commercial commercial lake, lake, producing producing a a dark dark redred color color with with a a violet violet hue. hue. This This result result was was in in line line with with previous previous studies, studies, in in which which alizarin alizarin complexes complexes withwith metals such asas MgMg oror MnMn were were associated associated with with blue blue shades shades [9 ,[9,23].23]. Therefore, Therefore, the the final final color color of theof thealizarin alizarin lake lake may may beconsidered be considered to be to the be resultthe result of the of formationthe formation of individual of individual alizarin alizarin complexes complexes with withboth both ions presentions present in the in LH the structure LH structure (Mg2+ (Mgand2+ Aland3+ ).Al3+). FiguresFigures 7 and 8 present the the digital digital photos photos and and result resultss of ofCIE CIE Lab Lab studies studies of alizarin, of alizarin, alizarin alizarin lake, ◦ andlake, Alizarin and Alizarin Crimson Crimson powders, powders, heated in heated an oven in at an 200, oven 250, at and 200, 300 250, °C, and respectively, 300 C, respectively,for 30 min. ◦ Therefor 30 min.were There significant were significant color differences color differences (ΔE) in (∆theE) insamples the samples thermally thermally aged aged at at200 200 °C.C. As As the the temperaturetemperature was was increased, increased, the the changes changes in in the the total total color color difference difference parameter parameter ( (Δ∆E)E) became became much much moremore marked, indicating thatthat thethe alizarinalizarin chromophorechromophore and and Alizarin Alizarin Crimson Crimson began began to to decompose decompose at at 200 °C. In the case of the alizarin pigments, no significant variation in color was observed after heating at 200 °C.

Materials 2019, 12, 360 9 of 14

◦ Materials200 C. 2019 In the, 12,case x FOR of PEER the alizarinREVIEW pigments, no signiﬁcant variation in color was observed after heating9 of 14 Materials 2019, 12, x FOR PEER REVIEW 9 of 14 at 200 ◦C.

Figure 7. Color changes in alizarin, Alizarin Crimson, and alizarin pigments after thermal aging at FigureFigure 7. 7. ColorColor changes changes in in alizarin, alizarin, Alizarin Alizarin Crimson, Crimson, and and alizarin alizarin pigments pigments after after thermal thermal aging aging at at different temperatures. differentdifferent temperatures. temperatures.

FigureFigure 8. Color changes in alizarin, Alizarin Crimson, and alizarin pigments after thermal aging at Figure 8. ColorColor changes changes in in alizarin, alizarin, Alizarin Alizarin Crimson, Crimson, and and alizarin alizarin pigments pigments after after thermal thermal aging aging at at differentdifferent temperatures. different temperatures. SomeSome slight slight change change in in the the color color of of the lake pi pigmentsgments was observed after after 250 250 °C,◦C, resulting resulting from from Some slight change in the color of the lake pigments was observed after 250 °C, resulting from thethe dehydroxylation dehydroxylation of of the the LH LH layers. layers. These These results results show show that that the the alizarin alizarin lake lake can can tolerate tolerate higher higher the dehydroxylation of the LH layers. These results show that the alizarin lake can tolerate higher temperaturestemperatures than than pure pure chromophore chromophore or or the the commercial commercial lake lake (Alizarin (Alizarin Crimson). Crimson). The The ΔE values∆E values for temperatures than pure chromophore or the commercial lake (Alizarin Crimson). The ΔE values for alizarinfor alizarin and andAlizarin Alizarin Crimson Crimson were werelarger larger than th thanose thoseof the ofLH-based the LH-based lake after lake thermal after thermal aging above aging alizarin and Alizarin Crimson were larger than those of the LH-based lake after thermal aging above 200above °C, 200confirming◦C, conﬁrming that the thatLH/Alizarin the LH/Alizarin pigments pigments had a higher had thermal a higher stability thermal than stability the reference than the 200 °C, confirming that the LH/Alizarin pigments had a higher thermal stability than the reference samples.reference It samples. can be concluded It can be that concluded the incorporation that the incorporation of alizarin dye of alizarininto the LH dye structure into the LHsignificantly structure samples. It can be concluded that the incorporation of alizarin dye into the LH structure significantly improvedsigniﬁcantly the improvedthermal stability the thermal of the stabilityorganic chromophore. of the organic This chromophore. observation Thiswas confirmed observation by wasthe improved the thermal stability of the organic chromophore. This observation was confirmed by the diffuse reflectance UV-vis spectra of the studied pigments (Figure 9). The spectra of the dye and diffuse reflectance UV-vis spectra of the studied pigments (Figure 9). The spectra of the dye and commercial lake showed marked changes after heating at 300 °C. However, in the case of alizarin commercial lake showed marked changes after heating at 300 °C. However, in the case of alizarin pigments, there were no significant changes in the spectra after exposition at any of the considered pigments, there were no significant changes in the spectra after exposition at any of the considered temperatures. temperatures.

Materials 2019, 12, 360 10 of 14 confirmed by the diffuse reflectance UV-vis spectra of the studied pigments (Figure9). The spectra of the dye and commercial lake showed marked changes after heating at 300 ◦C. However, in the case of alizarin pigments, there were no significant changes in the spectra after exposition at any of the considered temperatures. Materials 2019, 12, x FOR PEER REVIEW 10 of 14

(a) (b)

(c)

FigureFigure 9. UV-VISUV-VIS spectra spectra of of alizarin, alizarin, Alizarin Alizarin Crimso Crimson,n, and and alizarin alizarin lake lake exposed exposed to to different temperatures:temperatures: ( (aa)) alizarin; alizarin; ( (bb)) Alizarin Alizarin Crimson; Crimson; ( (c)) LH/Alizarin. LH/Alizarin.

3.5.3.5. Scanning Scanning Electron Electron Microscopy Microscopy (SEM) ScanningScanning electron electron microscopy microscopy (SEM) (SEM) experiment experimentss were were performed performed to tostudy study the the morphology morphology of theof thehybrid hybrid pigments. pigments. The micrographs The micrographs were wereobtained obtained using using50,000× 50,000 magnifications.× magnifications. As can be As seen can inbe Figure seen in 10, Figure the LH 10 ,carrier the LH was carrier mainly was composed mainly composed of plate-like of plate-like particles particleswith thicknesses with thicknesses of a few hundredsof a few hundredsof nanometers of nanometers and a lateral and dimension a lateral in dimensionthe range ofin 400 the nm range to 1–2 of µm. 400 The nm particles to 1–2 hadµm. moreThe particles or less hadregular more structures or less regular and different structures degrees and different of sharpness degrees around of sharpness their edges. around After their modification,edges. After modification, the structures the of structuresthe hybrid of pigments the hybrid were pigments quite similar were quite to the similar structure to the of structure the parent of LH,the parentirrespective LH, irrespective of the concentration of the concentration of alizarin. of However, alizarin. However,the shapes the of shapes the particles of the particleswere slightly were moreslightly irregular more irregular than in the than case in of the raw case LH. of Moreover raw LH. Moreover,, the new alizarin the new crystals alizarin appeared crystals appearedon the outer on surfacethe outer of surface the inorganic of the inorganic host, while host, whilethe layere the layeredd structure structure of the of the LH LH remained remained unchanged. unchanged. Interestingly,Interestingly, the the structure structure of of the the new new crystals crystals on onthe theLH LHdiffered differed significantly significantly in comparison in comparison to their to initialtheir initial states. states. Originally, Originally, the alizarin the alizarin took the took form the of form oblong of oblong block/needle-like block/needle-like shape crystals, shape crystals, while afterwhile modification, after modification, some irregular some irregular crystals appeared crystals appeared.. In the case In ofthe the casecommercial of the commerciallake, completely lake, differentcompletely morphological different morphological features were features observed. were Agglomerations observed. Agglomerations of particles of with particles more with irregular more structures formed. Scanning electron microscopy thus enabled the study of both the morphology and the aggregation of the particles. In general, HT-like compounds are known for their relatively low specific surface areas. Due to their lack of layer ordering and irregular shapes, the particles of LH exhibited a higher surface area value of 201.4 ± 17.2 m2/g in comparison to LH with a higher concentration of magnesium ions, as measured in our previous work (107.4 ± 9.5 m2/g) [23]. As anticipated, after modification with 15% and 20% alizarin, the surface area of the LH decreased to 157.2 ± 13.3 m2/g and 125.4 ± 11.7 m2/g in each respective case.

Materials 2019, 12, 360 11 of 14 irregular structures formed. Scanning electron microscopy thus enabled the study of both the morphology and the aggregation of the particles. In general, HT-like compounds are known for their relatively low speciﬁc surface areas. Due to their lack of layer ordering and irregular shapes, the particles of LH exhibited a higher surface area value of 201.4 ± 17.2 m2/g in comparison to LH with a higher concentration of magnesium ions, as measured in our previous work (107.4 ± 9.5 m2/g) [23]. As anticipated, after modiﬁcation with 15% and 20% alizarin, the surface area of the LH decreased to 157.2 ± 13.3 m2/g and 125.4 ± 11.7 m2/g in each respective case. Materials 2019, 12, x FOR PEER REVIEW 11 of 14

(a) (b)

(e) (f)

Figure 10. SEMSEM images images of LH ( a); LH/Alizarin (15%) (15%) ( (bb);); LH/Alizarin LH/Alizarin (20%) (20%) (c (c);); alizarin alizarin crystals crystals ( (dd,,ee);); and Alizarin Crimson ( f).

Materials 2019, 12, 360 12 of 14

3.6. Solvent Resistance The stability of the prepared hybrid pigments was evaluated based on their resistance to selected organic solvents and water. Since anthraquinone dyes are highly soluble, sustained washing with water and/or organic solvents should remove any excess or weakly physisorbed dye from the LH surface. The degree of discoloration after 24 h of exposure to solvents was therefore assessed, on a scale of 1/5 (where 5 means total insolubility). Based on the experiment, it can be concluded that both Alizarin Crimson and the alizarin pigments exhibited excellent solvent resistance, as the established values for all media were 5. Moreover, from Figure 11, it can be seen that the solvents containing the unmodiﬁed alizarin chromophore had a veryMaterials intense 2019, 12 yellow, x FOR PEER color, REVIEW whereas the solutions with the hybrid pigment and Alizarin Crimson12 of 14 were colorless. This conﬁrms that the alizarin molecules had been effectively stabilized onto the aluminum-magnesiumstabilized onto the aluminum-magnesium hydroxycarbonate surface, hydroxycar transformingbonate surface, the soluble transforming dye into a solvent-resistantthe soluble dye hybridinto a solvent-resistant pigment. It is also hybrid worth notingpigment. that It theis also hybrid worth pigments noting with that both the 15%hybrid and pigments 20% concentrations with both 15% and 20% concentrations of the chromophore were characterized by an equally good resistance of the chromophore were characterized by an equally good resistance to organic solvents. The LH to organic solvents. The LH host produced alizarin pigments with a higher solvent resistance than host produced alizarin pigments with a higher solvent resistance than matrices with a higher matrices with a higher concentration of magnesium, as had been used in a previous study [23]. This concentration of magnesium, as had been used in a previous study [23]. This is most likely related to is most likely related to the aforementioned difference in the surface area values of the inorganic the aforementioned difference in the surface area values of the inorganic matrices. matrices.

Figure 11. Digital images of alizarin, Alizarin Crimson, and alizarin pigments after 24 h of immersion Figure 11. Digital images of alizarin, Alizarin Crimson, and alizarin pigments after 24 h of immersion in toluene (a), acetone (b), and ethyl alcohol (c). in toluene (a), acetone (b), and ethyl alcohol (c).

4. Conclusions In this this study, study, we we investigated investigated the the possibility possibility of stabilizing of stabilizing alizarin alizarin on solid on state solid mixed state oxides. mixed oxides.A new type A newof hybrid type pigment of hybrid was pigment created was by the created complexation by the complexation of alizarin dye of with alizarin an aluminum- dye with anmagnesium aluminum-magnesium hydroxycarbonate hydroxycarbonate inorganic host. inorganic27Al solid-state host. NMR27Al solid-state and TOF-SIMS NMR were and TOF-SIMSapplied to werestudy appliedthe local to structures study the of local pristine structures LH and of th pristinee chemically LH and modified the chemically LH matrix. modified The dye LH molecule matrix. Theinteracted dye molecule with different interacted ions with simultaneously different ions and simultaneously the final color and was the a finalresult color of the was combination a result of the of combinationdifferent alizarin of different complexes. alizarin The complexes. stability of The the stability hybrid ofpigments the hybrid was pigments tested under was testedsolvent under and solventtemperature and temperature treatment. At treatment. both 15% At and both 20% 15% dye and concentrations, 20% dye concentrations, the metal-dye the metal-dye complexes complexes showed showedexcellent excellent resistance resistance to dissolution to dissolution in solvents: in solvents: acetone, acetone, ethyl ethyl alcohol alcohol, ,and and toluene. toluene. A A further advantage was found to be their considerably improvedimproved thermal and color stability in comparison to the commercial lake. These enhanced properties can be explained by the strong affinityaffinity of alizarin to LH, which leads to the formation of of stable stable complexes complexes with with Al Al3+3+ andand Mg Mg2+ 2+ions.ions. TOF-SIMS TOF-SIMS proved proved to be a powerful method for studying the interactions in solid state materials. TOF-SIMS can be considered as a complementary technique alongside NMR, providing insight into the mechanisms by which anthraquinone chromophores are stabilized by a mixed oxide host.

Author Contributions: A.M. and B.S. designed the concept, processed the modification and characterization of samples, and wrote the manuscript; J.R. and W.M. performed the experiments; M.I.S. and M.Z. supervised the work; all authors discussed, edited, and reviewed the manuscript.

Funding: This research received no external funding.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Velho, S.R.K.; Brum, L.F.W.; Petter, C.O.; dos Santos, J.H.Z.; Simunić, S.; Kappa, W.H. Development of structured natural dyes for use into plastics. Dyes Pigm. 2017, 136, 248–254, doi:10.1016/j.dyepig.2016.08.021.

Materials 2019, 12, 360 13 of 14 to be a powerful method for studying the interactions in solid state materials. TOF-SIMS can be considered as a complementary technique alongside NMR, providing insight into the mechanisms by which anthraquinone chromophores are stabilized by a mixed oxide host.

Author Contributions: A.M. and B.S. designed the concept, processed the modification and characterization of samples, and wrote the manuscript; J.R. and W.M. performed the experiments; M.I.S. and M.Z. supervised the work; all authors discussed, edited, and reviewed the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

References

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