Characterization and structure–property relationships of organic–inorganic hybrid composites based on aluminum–magnesium hydroxycarbonate and azo chromophore

Article abstract of DOI:10.3390/molecules24050880

In this study, novel organic–inorganic composites were prepared by the complexation of dicarboxylic azo dye (AD) with aluminum–magnesium hydroxycarbonate (AlMg–LH). This procedure provides an effective method for the stabilization of dicarboxylic organic

Full text of DOI:10.3390/molecules24050880

molecules
Article
Characterization and Structure–Property
Relationships of Organic–Inorganic Hybrid
Composites Based on Aluminum–Magnesium
Hydroxycarbonate and Azo Chromophore
Anna Marzec 1,
*
, Bolesław Szadkowski 1 , Jacek Rogowski 2, Waldemar Maniukiewicz 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; boleslaw.szadkowski@edu.p.lodz.pl (B.S.);
marian.zaborski@p.lodz.pl (M.Z.)
Institute of General and Ecological Chemistry, Lodz University of Technology, Zeromskiego 116,
2
90-924 Lodz, Poland; jacek.rogowski@p.lodz.pl (J.R.); waldemar.maniukiewicz@p.lodz.pl (W.M.)
*
Correspondence: marzec.anna@hotmail.com
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 26 December 2018; Accepted: 25 February 2019; Published: 1 March 2019
Abstract: In this study, novel organic–inorganic composites were prepared by the complexation
of dicarboxylic azo dye (AD) with aluminum–magnesium hydroxycarbonate (AlMg–LH). This
procedure provides an effective method for the stabilization of dicarboxylic organic chromophores
on an AlMg LH host. The structures of the hybrid composites were examined by X-ray
−
diffraction (XRD), secondary ion mass spectrometry (TOF-SIMS), 27-Al solid-state nuclear magnetic
resonance (NMR) spectroscopy, thermogravimetric analysis (TGA) and scanning transmission
electron microscopy (STEM). The TOF-SIMS method was applied to investigate the metal–dye
interactions and to monitor the thermal stability of the organic–inorganic complexes. Secondary ion
mass spectrometry confirmed the presence of a characteristic peak for C18H10O5N2Mg22+, indicating
that both carboxylic groups interacted with AlMg
Modification with hybrid pigments affected the crystal structure of the AlMg
by the appearance of new peaks on the X-ray diffraction patterns. Adsorption of the dicarboxylic
chromophore not only led to significantly enhanced solvent resistance but also improved the thermal
and photostability of the hybrid pigments. We propose a possible arrangement of the azo dye in the
inorganic matrix, as well as the presumed mechanism of stabilization.
−
LH by forming complexes with two Mg2+ ions.
LH mineral, as shown
−
Keywords: hybrid pigment; azo dye; aluminum–magnesium hydroxycarbonate; organic–inorganic
composite; pigment stability
1. Introduction
Azo dyes are an important class of modern dyes and are extensively used in textiles, leather,
printing, food, drugs, and optical devices. However, some azo dyes have a limited range of applications,
for example, due to their low melting points, high tendency to migrate, and poor thermo- or/and
photostability, some dyes cannot be used in polymer materials. Most of these disadvantages can be
overcome by complexation of the organic chromophores with inorganic hosts [1,2]. Recently, there
has been great interest in multifunctional hybrid materials, which combine the advantages of both
organic materials (light weight, versatility, intense color) and inorganic materials (high thermal and
chemical resistance) [3,4]. Such hybrid composites may also exhibit special physicochemical properties,
which are not present in the separate components [5,6]. Various procedures have been proposed for
Molecules 2019, 24, 880; doi:10.3390/molecules24050880
www.mdpi.com/journal/molecules

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obtaining chemically stable pigments. Numerous studies have focused on sol-gel methods, and on the
adsorption of organic chromophores onto an inorganic host [ ]. Organic–inorganic materials can
7,8
also be produced by the complexation of natural and synthetic dye molecules with solid host matrices.
Inorganic host matrixes such as gamma alumina, silica, zeolites, and layered double salts can improve
the thermal-, photo-, and chemical stability of dyes [9–18].
Guillermin et al. prepared hybrid materials based on montmorillonite, a cationic polymer,
and carminic acid [19]. They suggest that the adsorption mechanism may be related to the OH
groups of the carminic acid and in both surface groups present on the edge faces of montmorillonite
bonding with adsorbed cationic polymers molecules on the basal faces. These authors propose different
methods for studying the interactions between the organic and inorganic parts, including infrared
(IR) and nuclear magnetic resonance (NMR) spectroscopies. Ramirez et al. describe the synthesis
of hybrid pigments by the combination of natural chromophores, such as carminic acid, alizarin,
purpurin, curcumin, fluorescein, and betacyanins with a gamma alumina matrix (γ-Al2O3) [20].
They also used spectroscopic techniques to confirm the formation of aluminum complexes between
coordinative unsaturated sites in the aluminum and some organic groups (carboxylic acid, quaternary
ammonium, and β-keto enol) present in the dye chromophores. Girdthep et al. produced natural
lake pigments based on Sappan and kaolinite mineral [21]. The enhanced thermal stability of the
hybrid pigment was attributed to electrostatic attraction between the Al(III) ion, the kaolinite surface,
and brazilein molecules. Recently, several works have attempted the intercalation of various azo
dyes into layered aluminum–magnesium (or zinc–aluminum) hydroxycarbonate structures [22
However, the intercalation of dye into the interlayer space of aluminum–magnesium hydroxycarbonate
is time-consuming and requires an inert atmosphere [28 30]. Therefore, the precipitation method still
–27].
–
represents an important method of stabilization of organic–inorganic hybrids.
The purpose of this study was to investigate the synthesis, characterization, and application of a
red pigment obtained by the precipitation of azo dye onto the AlMg
method has been shown to lead to the formation of metal–dye complexes characterized by high
insolubility, good color strength, and improved technical performance in applications [31 32]. Due to
the presence of two carboxylic groups, the selected azo chromophore was expected to have an affinity
for the inorganic component, which is basic in nature. The morphology of the MgAl LH before and
−
LH matrix. The precipitation
,
−
after the incorporation of the guest dye was characterized by X-ray powder diffraction (XRD) and
scanning electron microscopy (SEM/STEM). The organic–inorganic hybrid was also examined for
thermal stability (TGA), solvent resistance, and photostability. TOF-SIMS and 27Al NMR spectroscopy
were used to investigate the interactions between the azo dye and particular ions present in the
AlMg−LH structure.
2. Results and Discussion
2.1. Secondary Ion Mass Spectrometry (TOF-SIMS)
To better understand the stabilization mechanism of the organic–inorganic hybrid, the possible
interactions between the dicarboxylic azo dye (AD) and particular ions present in the AlMg LH layers
−
were investigated using the TOF-SIMS method. In our previous work, we studied the interactions
between alizarin and aluminum–magnesium hydroxycarbonates with different Mg/Al ratios [33].
The results of TOF-SIMS experiments confirmed the formation of alizarin complexes with both Mg2+
and Al3+ ions in the inorganic hosts. In the current study, it was found that the addition of AD dye to
AlMg LH resulted in the appearance of a characteristic peak at m/z 191 in the TOF-SIMS spectrum,
−
which was not observed in the spectrum of the AlMg
−
LH sample (Figure 1a). This peak was assigned
2+
to the C18H10O5N2Mg2 ion, indicating interaction between the azo dye and the two magnesium
ions in the AlMg
not present in the AlMg
Figure 1a,b demonstrates that the intensity of the C18H10O5N2Mg22+ peak grew as the concentration of
−
LH/AD 10% matrix, whereas C18H11O5N2Mg+ containing only one Mg atoms was
LH/AD 10% spectrum. Moreover, comparison of the TOF-SIMS spectra in
−

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azo dye in the inorganic matrix increased. We concluded that the azo-based colorants were stabilized by
interactions with Mg2+ ions. In contrast to anthraquinone-based colorants [33], the TOF-SIMS spectra
of the AlMg
−
LH/AD 10% and AlMg LH/AD 20% samples did not contain peaks corresponding
−
to complex of dye with Al3+ ions. These observations may indicate that the complex formed by the
dye and Al was not stable enough to be emitted from the sample as a non-fragmented ion, but this
does not exclude the possibility of interaction between Al3+ and dye in the LH matrix. The presence of
some slight interactions was observed by NMR studies.
Figure 1. Positive secondary ion mass spectrometry TOF-SIMS spectra for aluminum–magnesium
hydroxycarbonate AlMg
line) and AlMg−LH /AD 20% (red line) (b) samples.
−
LH (black line) and AlMg
−
LH /AD 10% (red line) (
a
) and AlMg LH (black
−
It is worth noting that the modification of AlMg
−
LH with AD was accompanied by a decrease in
−
2−
the emission of CO2 ions, indicating the presence of carbonate ions (CO3 ) in the sample (Figure 2).
−
This reduction in the emission of CO2 ions from the AlMg
−
LH/AD pigment was most likely
2−
related to the fact that CO3 ions were pa−rtially removed from the AlMg
−
LH/AD, as a result of
LH structure. This
the competitive interaction of C18H10O5N2 ions with Mg2+ ions in the AlMg
−
observation is in line with the results of further TGA studies.
Numerous reports based on NMR spectroscopy, most often of 13C and 27Al MAS NMR
spectra, have suggested that organic dyes form complexes with clay minerals [20,34]. However,
this method is restricted to the analysis of dye interactions involving only a limited number
of ions, and does not involve ions such Mg2+ or Zn2+. TOF-SIMS may be considered as a
complementary technique, providing insight into the mechanism of stabilization by metal mixed
oxides of carboxylic chromophores.

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−
Figure 2. TOF-SIMS spectra of CO2 ions emitted from (
10% samples.
a) AlMg
−
LH and (
b
) AlMg LH/AD
−
2.2. Solid State Nuclear Magnetic Resonance of 27Al (MAS NMR)
27Al MAS NMR spectra of AlMg
LH before and after the adsorption of the azo dye are depicted
in Figure 3. 27Al resonance line positions are very sensitive to coordination number and expected
to occupy the 5 to 15 ppm range for AlO6 sites [35]. Spectrum of the AlMg LH sample before
complexation shows a relatively narrow resonance peak at a chemical shift, , of ~3.9 ppm, which
represents the octahedral coordination of Al. The NMR signal of the hybrid pigment was very close to
that of the unmodified AlMg LH sample. The addition of the azo chromophore to the AlMg LH
host decreased the relative intensity of the resonance peak. We also noted a chemical shift (from 3.9 to
3.8 ppm) in both the case of AlMg LH/AD 10% and AlMg LH/AD 20% (Figure 3). A similar shift
−
−
−
δ
−
−
−
−
has been reported for Al in montmorillonite modified with organic dyes [36]. Therefore, we conclude
that the changes we observed in the NMR spectra may suggest the presence of some interactions
between the Al ions and the dye in the samples.
Figure 3. Solid state nuclear magnetic resonance 27Al MAS NMR spectra for AlMg
AlMg−LH/AD 10%, and AlMg−LH/AD 20%.
−
LH,

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2.3. X-ray Diffraction Study (XRD)
Figure 4 shows X-ray powder diffraction patterns for AlMg
−
LH, AlMg
−
LH/AD 10%,
AlMg
of three sharp peaks at a low 2
indicates good AlMg
−
LH/AD 20%, and AD. The diffraction pattern of pure AlMg
−
LH in Figure 4a consisted
θ
angle, equivalent to diffraction by planes (003), (006), and (009). This
−
LH crystallinity [37]. The interlayer distances of (d003) and (d006), corresponding
to 0.758 nm and 0.381 nm, were due to basal reflections indexed to a hexagonal crystal lattice with
rhombohedral 3R symmetry. These values, together with other non-basal spacing at 0.152 nm (d110) and
0.149 nm (d113), were consistent with the typical XRD pattern of LDH minerals. Small broadened peaks
were seen at at 2
boehmite (CDD, PDF-2, 21-1307). The addition of AD to the AlMg
of the peak (003) towards the lower angles of 2 theta. As a result, the interlayer distances of (d003
increased from 0.758 to 0.766 nm for AlMg LH/AD 20%. The reason for this could be the strong
affinity of carboxylic acid groups (
COO−) to magnesium ions (Mg2+), which were located in the
inner sheets of the AlMg−LH matrix.
θ
= 14.10◦ and 28.18◦, indicating the presence of co-precipitated poorly-crystalline
−
LH host caused a slight shifting
)
−
−
Figure 4. Diffraction patterns for AlMg
and azo dye (AD) (d).
−
LH (a), AlMg
−
LH/AD 10% (
b
), AlMg LH/AD 20% (c),
−
On more detailed inspection, the diffractogram of the AlMg
reveals a series of new peaks that did not match either pure AlMg
of this diffraction pattern, a small well-defined peak at low 2 could be observed, corresponding to
−
LH/AD 20% sample (Figure 4c)
−
LH or the AD dye. At the start
θ
an expanded basal spacing of ~1.88 nm. This suggests the appearance of a new crystalline phase in
the system. This phase may be related to the partial incorporation of AD into the layered structure of
AlMg
−
LH, and to the formation of a new organic–inorganic crystalline structure. The mechanism
LH pigments seems similar to that for lake pigments, which are usually
for the formation of AlMg
obtained by the precipitation of dye acid structures with salts of alkaline earth [32]. The possible
−
arrangement of the azo dye molecules in the AlMg−LH host is shown in Figure 5.
Figure 5. Possible structural arrangement of azo chromophore in AlMg−LH structure.

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2.4. Morphology of Hybrid Pigment (SEM/STEM)
The SEM micrographs of AlMg
of 300–700 nm (Figure 6a–c), approximately 10–30 nm in thickness. After modification, the brick-like
crystals of the azo chromophore seemed to be bound to the surface of the AlMg LH. Based on the
micrographs, the sizes of the new crystallites varied from 200 to 300 nm, and occasionally extended to
m. Modification changed both the size and shape of the dye crystals (Figure 6d). This observation
−
LH show irregular layered structures with lateral dimensions
−
1
µ
correlates with XRD results and can be explained by the formation of new organic–inorganic crystal
structures. The STEM image in Figure 7 provides insight into the structure and agglomeration of
crystals within the dye-modified AlMg LH adsorbent. The dark lines in Figure 7a,c show the
−
LH-layered structure (marked by an arrow). As can be seen, large numbers of dye crystals were
stacked on the AlMg−LH surface after stabilization.
Figure 6. SEM micrographs of AlMg
−
LH (
a
), AlMg
−
LH/AD 10% (
b
), AlMg LH/AD 20% (c), and AD (d).
−
Figure 7. Cont.

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Figure 7. Scanning transmission electron microscope STEM micrographs of AlMg LH (a,c) and
−
AlMg−LH/AD 10% (b,d).
2.5. Solvent Resistance
To evaluate their resistance to dissolution, the organic–inorganic pigments were immersed in
five different solvents. The degree of discoloration was estimated after 24 h on a scale of 1–5, where
5 refers to colorless solvent (total insolubility) and 1 to intense solvent colorization (high solubility).
The results in Table 1 show that the hybrid pigments exhibited excellent resistance to dissolution, while
the azo dye turned the tested solvents an intense red color after immersion (Figure 8). The significantly
improved solvent resistance of the azo chromophore is most likely related to fact that both carboxyl
groups were involved in the formation of the organic–inorganic pigments.
Table 1. Solvent resistance of hybrid pigments.
Solvent Resistance
Hybrid Pigment
Water
Acetone Toluene Ethanol
Butyl Acetate
AlMg−LH/AD 10%
AlMg−LH/AD 20%
5
5
5
5
5
5
5
5
5
5
Figure 8. Digital images of AD dye, AlMg
immersion in acetone (a) and ethyl alcohol (b).
−
LH/AD10%, and AlMg
−
LH/AD 20% after 24 h of
2.6. Thermal Analysis (TGA) and Photostability of Hybrid Pigments
General regions of mass loss can be observed in the TGA curves and three mass loss peaks in
the DTG thermograms of the modified and unmodified samples. The first characteristic weight loss

Molecules 2019, 24, 880
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◦
peak below 100 C corresponded to the loss of physically absorbed water. The second mass loss
◦
region with a peak at 215 C corresponded to the removal of water molecules from the interlayer
galleries. The last weight loss stage visible at around 300–500 ◦C was related to the dehydration of
hydroxyl groups in the brucite-like layer, as well as to the complete removal of the interlayer carbonate
anions (decarbonization) [38]. The complexation of the AlMg
−
LH host with organic chromophore
contributed to improve the thermal stability of the AlMg LH hybrids (Figure 9). This effect was
−
also dependent on the content of the azo dye, as can be seen by comparing the TGA curves of the
different samples.
Figure 9. Thermogravimetric analysisTGA (a) and difference thermogravimetric analysis DTG(b) for
hybrid pigments with 10% and 20% dye content.
Table 2 provides a more detailed overview of the results for thermal stability. Compared to neat
AlMg LH, the T05% values of the hybrid pigments rose with increases in the quantity of AD dye.
The T05% values for AlMg LH/AD 10% and AlMg
LH/AD 20% increased from 119 ◦C, for pure
AlMg
LH, to 131 and 145 ◦C, respectively. Two main weight loss peaks, at 221 and 280 ◦C, on the
DTG curve of the AD dye were located at the same level as the dehydroxylation and carbonate
decomposition peaks of the AlMg LH matrix (Figure 9). For that reason, these peaks were absent on
−
−
−
−
−
the TGA-DTG thermograms fo◦r the hybrid pigments (Figure 9). The weight loss temperatures of the
AlMg
−
LH hybrids, at over 300 C, were significantly higher than in the case of neat AlMg LH. Weight
−
loss temperatures in the range of 300–500 ◦C were related to the liberation of hydroxyl groups and
carbonate ions from the layered mineral. It was assumed that carbonate ions from the weak carbonate
acid were displaced from the edge of the AlMg LH interlayer by the acidic dye. These results were
−
consistent with TOF-SIMS measurements, in which a decrease was also noticed in the concentration of
carbonate ions after modification (Figure 2). The TGA data demonstrate that incorporating the azo
chromophore into the AlMg LH structure improved the thermal stability of the organic–inorganic
−
pigments. This was most likely due to the high thermal stability of the dye, as well as to strong
dye–metal interactions between the AlMg−LH and the AD chromophore.

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Table 2. Thermogravimetric analysis of AlMg−LH/AD pigments.
1
◦
1
◦
1
◦
1
◦
Sample
T05 ( C)
T10 ( C)
T20 ( C)
T30 ( C)
AlMg−LH
AlMg−LH/AD 10%
AlMg−LH/AD 20%
1
119
131
145
205
214
217
330
353
370
441
466
488
T05, T10, T20, T30—temperature of 5%, 10%, 20%, and 30% weight loss.
Samples of the AD dye, AlMg
100, 150, 200, 250, 300, 325, 350, and 375 C for 30 min. The treated pigments were measured using the
−
LH/AD 10% and AlMg
−
LH/AD 20% were heated in an oven at
◦
CIE 1976 L*a*b* method. The results were expressed in terms of the total color difference
and differences in the color parameters L*, a*, b* (Table S1, Supplementary data). The
the pure dye rose from 5 (200 ◦C) to 20 after treatment at 250 ◦C, indicating that the non-modified
dye underwent thermal decomposition at this temperature. The color variation of the AlMg LH/AD
∆
E (Figure 10)
∆
E value of
−
20% pigment was significantly lower than that of the pure dye after temperature treatment for the
same length of time (Figure 11). This observation was confirmed by the diffuse reflectance UV–VIS
spectra of the resulting pigments (Figure 12). The spectra of the dye underwent marked variation after
heating at 300 ◦C, especially in the range of 400–700 nm. However, in the case of AlMg
LH/AD 20%
−
there were no significant changes in the spectra after heating at any of the considered temperatures.
When the samples were heated at 300 ◦C, the azo dye became carbonized and turned black, as shown
in Figure 11. In contrast, the AlMg
much more limited color change, indicating better thermostability. Decomposition was observed
at 375 ◦C. The
E parameter was slightly higher for AlMg LH/AD 10% pigment (Figure 10) and
−
LH/AD 10% and AlMg
−
LH/AD 20% pigments showed a
∆
−
more pronounced changes were observed in the absorbance spectra (Figure S1), most likely due to
a lower concentration of azo dye being incorporated into the inorganic host. This demonstrates,
conclusively, that the enhanced thermostability of the dye in the layered mineral was due mainly to
strong host–guest interactions (AlMg−LH/AD).
Figure 10. Color difference (
during temperature treatment.
∆
E) values for AD, AlMg
−
LH/AD 10%, and AlMg LH/AD 20% powders
−
The TOF-SIMS technique was also employed to monitor the thermal resistance of the azo dye
and its complexes in the AlMg LH/AD 10% and AlMg LH/AD 20% samples. Figure 13a shows
the complete disappearance of the peak characteristic for AD after annealing of the sample at 300 ◦C.
−
−
The characteristic peak of the C18H10O5N2Mg22+ ion in the TOF-SIMS spectrum of the AlMg
20% sample annealed at 300 ◦C indicates that interaction between the AD and the AlMg
−
LH/AD
LH host
−
resulted in a hybrid with improved color stability (Figure 13b). Hybrid pigment modified with 10%
2+
dye showed a less intense characteristic peak for the C18H10O5N2Mg2 ion after annealing at 300 ◦C.

Molecules 2019, 24, 880
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However, the decomposition trend was similar to that of the AlMg
These results are in line with spectrophotometric studies (CIEL*a*b* and UV–VIS), demonstrating that
TOF-SIMS can be a complementary method for monitoring the stability of such complexes.
−
LH/AD 20% sample (Figure S2).
Figure 11. Color changes of azo dye, AlMg
thermal aging at different temperatures.
−
LH/AD 10%, and AlMg LH/AD 20% powders after
−
The AlMg
ethylene–norbornene copolymer and exposed to UV light irradiation (Figure 14 and Figure S3).
The color difference ( E) values of the irradiated samples were measured every 50 h, up to a total
exposure time of 500 h. It was noted that composites containing organic–inorganic pigments exhibited
higher photostability to UV light than the neat chromophore. The E value of the dye increased
up to 8 when the aging time reached 200 h, indicating that the non-modified dye had undergone
photooxidation. The color changes of the AlMg LH/AD 20% pigment were significantly weaker
than those of the pure dye after irradiation for the same length of time. For the AlMg LH/AD 10%
−
LH/pigments (10% and 20% AD) and azo dye were used as colorants for the
∆
∆
−
−
pigment, the total value for color difference was half that of AD after irradiation for 300 h. These
results indicate that, as well as increasing the heat resistance of the metal–dye hybrid, modification
with AD also contributed to improve the light resistance of the studied chromophore.
Figure 12. UV–VIS spectra of azo dye and hybrid pigment exposed to different temperatures: (
a) AD;
and (b) AlMg−LH/AD 20%.

Molecules 2019, 24, 880
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−
2+
Figure 13. TOF-SIMS spectra of C18H11O5N2
(a
) and C18H10O5N2Mg2 ions (
b) from the
◦
AlMg−LH/AD 20% sample before and after heating at 300 C.
Figure 14. Color difference (
∆E) values for ethylene–norbornene (EN) composites filled with AD and
AlMg−LH/AD pigments during UV aging.
3. Experimental
3.1. Materials
Aluminum–magnesium hydroxycarbonate (AlMg
−
LH, Mg:Al weight ratio 30:70) with the
empirical formula Mg0.33Al0.67(OH)2(CO3)0.33 nH2O was supplied by Sasol (Germany). Anthranilic
acid (98%), 3-hydroxy-2-naphthoic acid, hydrochloric acid (37%), acetic acid (99.8%), ethyl
alcohol (95%), acetone (99.9%), n-butyl acetate (99.9%), and toluene (99.9%) were purchased from
Sigma-Aldrich (Germany). Ethylene–norbornene random copolymer (EN) was employed as the
polymer matrix, supplied by TOPAS Advanced Polymers (Germany). The synthesis of carboxylic
azo dye (4-[2-(2-Carboxyphenyl)diazenyl]-3-hydroxy-2-naphthalenecarboxylic acid) was carried out
according to the procedure described in the next section.

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3.1.1. Azo Dye Synthesis
Dicarboxylic azo dye was synthesized following the typical diazotization procedure with some
minor changes (Figure 15) [39]. First, 14 g of anthranilic acid (0.1 mol) was dissolved in 100 ml of
deionized H2O and 10 cm3 of 30% NaOH solution. Next, 40 cm3 of 30% hydrochloric acid was added
and the mixture was cooled to below 10 ◦C. Finally, ice cold aqueous NaNO2 solution (25 cm3) was
◦
used for diazotization. The reaction mixture was stirred for 1 h at 0–5 C. Completion of diazotization
was verified using starch-iodide paper.
Figure 15. Structural formula of studied azo dye (AD).
In the coupling step, a solution of 3-hydroxy-2-naphthoic acid (0.1 mol, 18.8 grams) was prepared
in 150 cm3 of deionized water and 30 cm3 of 30% NaOH. The reaction mass was cooled to 0–5 ◦C,
and a previously-prepared diazonium salt solution was added dropwise with continuous stirring
◦
and the temperature maintained at 0–5 C. A color reaction with H-acid was used to confirm the
reaction endpoint. The product of the coupling reaction was acidified with 20 cm3 of 30% hydrochloric
acid. Finally, the separated dye was filtered, washed several times with water, and dried at 60–65 ◦C
.
The crude product was recrystallized from acetic acid (yield: 68%). 1H-NMR, FT-IR, and UV–VIS
spectra of the prepared azo dye are presented in Figures S4 and S5.
1H-NMR (DMSO-d6, 250 MHz) δH: 8.37 (d, 1H, (A) H-1), 7.75 (m, 2H, (A) H-2), 7.68 (m, 2H, (A)
H-3), 8.45 (d, 1H, (A) H-4), 8.62 (s, 1H, (B) H-1), 7.92 (d, 1H, (B) H-2), 7.36 (t, 2H, (B) H-3), 7.−51 (m, 2H,
(B) H-4), 8.04 (d, 1H, (B) H-5), 13.53 (COOH), 3.29 (OH). TOF-SIMS Calcd for C18H11O5N2 [M-H]−,
m/z = 335.075; Found, m/z = 335.074.
3.1.2. Hybrid Pigment Preparation
Modification was carried out for samples with 10% and 20% AD content. In this section,
the AlMg
−
LH/AD 10% sample is taken as an example. First, 1 g (for AlMg LH/AD 20% sample 2 g)
−
of synthetic azo dye was dissolved in an aqueous solution (200 cm3 of deionized water) with the
addition of 10 cm3 of ethanol. The sample was subjected to ultrasonication for 30 min. The solution
was then heated and 9 ◦g (for AlMg
−
LH/AD 20% sample 8 g) of AlMg LH was added. The reaction
−
mixture was kept at 80 C for 3 h with stirring, after which the precipitated pigment slurry was filtered
under a vacuum and washed repeatedly with deionized water until a colorless solution was observed.
Finally, the hybrid pigment powder was dried in an oven at 75 ◦C for 24 h.
The amount of the organic chromophore stabilized by the AlMg LH matrix was measured using
−
N elemental analysis (Table S2, Supplementary Material). The largest increase in N concentration was
observed for the sample modified with 10% azo chromophore, where the found (0.84) and calculated
(0.84) nitrogen were at the same level. The modification with 20% of dye resulted in a lower N
concentration of approximately 1.58, while expected concentration was around 1.67.
3.1.3. Preparation of Ethylene-Norbornene (EN) Composites
The ethylene–norbornene composites were prepared using a Brabender measuring mixer N50
(Duisburg, Germany) at 120–130 ◦C. Homogenization of the EN/hybrid pigments was continued for

Molecules 2019, 24, 880
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10 min. After the mixing process, the compounds were sheeted between two steel plates at 120 ◦C
for 10 min. Composites were prepared using the following formulation (in parts per hundred parts of
rubber—phr): ethylene–norbornene copolymer (100 phr) and hybrid pigment (3 phr).
3.2. Characterization
Nuclear magnetic resonance (NMR) was performed to confirm the structures of the synthesized
carboxylic dyes. The 1H-NMR spectra were recorded on a Bruker Avance DPX (Rheinstetten, Germany)
250 MHz instrument, using DMSO-d6 as solvent.
Solid state Nuclear Magnetic Resonance (MAS NMR) measurements were performed in a Bruker
Avance III 400 WB (Rheinstetten, Germany) spectrometer operating at a resonance frequency of 104.26
MHz. 27Al chemical shifts were referenced using AlCl3
(0 ppm).
·
6H2O in 1M solution as an external reference
Secondary ion mass spectra were recorded using a TOF-SIMS IV mass spectrometer (ION-TOF
GmbH, Muenster, Germany). This instrument is equipped with a Bi liquid metal ion gun as well as a
high mass resolution time of flight mass analyzer. The measuring area covered 100
×
100 µm of the
sample surface and the analysis time was 30 s. During analysis, the area of the tested sample was
+
irradiated with pulses of 25 keV Bi3 ions at a 10 kHz repetition rate and with an average ion current
of 0.4 pA.
X-ray powder diffraction patterns were collected using a PANalytical X’Pert Pro MPD
diffractometer in the Bragg–Brentano reflection geometry with (CuKα) radiation from a sealed tube
◦
(Malvern Panalytical Ltd., Royston, UK). The apparatus operates in the range of 2
size of 0.0167◦.
θ
= 2–70 with a step
UV–Visible absorption measurement were obtained using an Evolution 201/220 UV–Visible
Spectrophotometer (Thermo Scientific, Waltham, MA, USA), with a spectral window from 1100 to
200 nm. The measurement specimens were in the form of solid-state powders. They were placed in
a special powder cell holder. Before the measurements, the baseline was corrected using a special
calibration adapter. The accuracy of the apparatus was
±
0.8 nm and the repeatability was
≤
0.05 nm.
The UV–Visible absorption of the pure dye was also studied in chloroform (dye concentration
1 × 10−4
M) using an Evolution 201/220 UV–Visible Spectrophotometer (Thermo Scientific, Waltham,
MA, USA) and standard quartz cuvettes. The absorption spectra were recorded across a wavelength
range of 1100–200 nm.
Thermal stability (annealing) of hybrid pigments and AD dye was performed in an oven under a
static air atmosphere (Binder, Tuttlingen, Germany). The samples were heated in an oven at 100, 150,
200, 250, 300, 325, 350, and 375 ◦C for 30 min.
The FT-IR spectra were recorded using a Thermo Scientific Nicolet 6700 FT-IR spectrometer
(Waltham, MA, USA) with a resolution of 4 cm−1. The measurements were performed at room
temperature across a wave number range 600–4000 cm−1 (64 scans).
Elemental analyses of carbon, hydrogen, and nitrogen in the hybrid pigment powders were
carried out using a Vario EL III analyzer equipped with special adsorption columns and a thermal
conductivity detector (TCD).
Thermogravimetry and differential thermal analysis (TGA-DTG) were performed with a
TA Instruments Q500 Thermogravimetric Analyzer (TA Instruments, Greifensee, Switzerand).
Measurements were conducted in the presence of argon, in a temperature range of 25–600 ◦C, with a
heating rate of 10 ◦C/min.
The resistance of the hybrid pigments to organic solvents was determined based on the
PN-C-04406/1998 standard. The powders (about 0.05 g) were immersed in various solvents (water,
toluene, acetone, n-butyl acetate, and ethyl alcohol) for 24 h at room temperature. After this period,
their degree of decolorization was assessed.
Accelerated weathering of the pigments was performed using a UV2000 Atlas solar simulation
chamber (Atlas, Linsengericht, Germany). The measuring setup included day conditions (radiation

Molecules 2019, 24, 880
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intensity 0.7 W/m2, temperature 60 ◦C, duration 8 h) and night conditions (no UV radiation,
temperature 50 ◦C, duration 4 h). In total, the simulation lasted 500 h. The samples were inspected
every 50 h.
The color change (∆E) in the materials aged under UV light was determined in terms of the
CIE L* a* b* color space system using a CM-3600d spectrophotometer (Konica Minolata Sensing Inc.,
Osaka, Japan) with a spectral range of 360–740 nm. The color measurements were carried out on a
white calibration plate. The total color difference was calculated using the equation:
q
∆E = (∆L)2 + (∆a)2 + (∆b)2
(1)
where
∆
L—level of lightness or darkness,
∆
a—relationship between redness and greenness,
and ∆b—relationship between blueness and yellowness.
Scanning electron microscopy (SEM) analysis was performed on a Leo 1530 Gemini scanning
electron microscope (Zeiss/Leo, Oberkochen, Germany). The pigment powders were coated with a
carbon target using a Cressington 208 h system. The structures of the pigments were also analyzed
using a high-resolution scanning transmission electron microscope (STEM) (FEI, NovaNanoSEM 450;
accelerating voltage 30 kV). Samples for STEM measurements were prepared by depositing the colloids
onto carbon-coated copper grids.
4. Conclusions
In this study, a new type of hybrid pigment was created by the complexation of dicarboxylic
azo dye with an aluminum–magnesium hydroxycarbonate inorganic host. The presence of
2+
C18H10O5N2Mg2 ions in TOF-SIMS spectra showed that the dye can be stabilized by two Mg
ions due to the presence of two COO− groups in dye molecule. Moreover, the 27Al MAS spectra of
the hybrid pigments revealed possible interactions of the studied dye with Al ions. Formation of the
new organic–inorganic structure was confirmed by XRD analysis, as the diffraction patterns contained
a set of reflections which did not belong to either AlMg LH or the AD chromophore. The new
−
organic–inorganic pigment showed excellent resistance to dissolution in the solvents: acetone, ethyl
alcohol, and toluene. A further advantage is their considerably improved thermal and photostability,
which are suitable for a wider range of polymer applications. Secondary ion mass spectrometry was
found to be a valuable technique for investigating the interactions between the organic chromophore
and particular components in the inorganic matrix, including Mg2+ ions. Such observations are not
possible using more commonly-used methods, such as NMR spectroscopy. The results of TOF-SIMS
studies proved that this technique can also be considered a useful tool for analyzing the thermal
stability of hybrid colorants.
Supplementary Materials: The following are available online. Figure S1. UV–VIS spectra of hybrid pigment
(AlMg
ion from the AlMg
ethylene–norbornene (E–N) composites: E–N, E–N/AlMg
after 500 h of UV aging. Figure S4. 1H-NMR spectrum of AD dye. Figure S5. Absorption spectra of AD dye
in chloroform (c = 1 10 4 M) (a), and FT-IR spectra of AD dye (b). Table S1. Color parameters of AD dye
LH/AD 20% during temperature treatment. Table S2. Elemental composition of the
−
LH/AD 10%) exposed to different temperatures. Figure S2. TOF-SIMS spectra of the C18H10O5N2Mg22+
−
LH/AD 10% sample before and after heating at 300 ◦C. Figure S3. Digital photographs of
LH/AD 10% and AlMg LH/AD 20% before and
−
−
×
−
and hybrid pigment AlMg
hybrid pigments.
−
Author Contributions: Author contributions were as follows: Conceptualization, investigation, methodology,
writing—original draft, and writing—review and editing, A.M.; investigation, methodology, and writing—original
draft preparation, B.S.; investigation, writing—review and editing, J.R. and W.M.; supervision, writing—review
and editing, M.Z.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflicts of interest.

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©
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).