An easy and effective method for the intercalation of hydrophobic natural dye into organo-montmorillonite for improved photostability
Graphical abstract
Introduction
Naturally occurring dyes are more environmentally friendly and suitable for humans, compared with their synthetic counterparts. These dyes find extensive use in cosmetics and food industries. β-carotene (BC) is one such dye, a well-known constituent of the carotenoid group dyes. The structure of the BC is shown in Scheme 1. BC shows strong hydrophobic and lipophilic properties due to its long unsaturated carbon chain, as well as the hydrocarbon groups not containing oxygen at both ends of the chain [1]. From a human health viewpoint, BC is known to be an antioxidant, protecting cells against reactive oxygen species and free radicals [[2], [3], [4]]. Unfortunately, natural dyes such as BC are generally quite reactive and easily lose their color by light or heat [5,6]. Extensive practical application of BC as colorants are therefore substantially hampered. If the stability of BC could be improved, its broadened use as a safe and environmentally friendly coloring material could be realized [7,8].
Incorporation of natural and synthetic dyes into inorganic host materials is known as a promising method to improve the stability. For instance, the incorporation of dyes into clays [[9], [10], [11]], zeolites [[12], [13], [14], [15]], and mesoporous silicates [16,17] has been reported to enhance the stability of the guest dye molecules. In a previous report, the dye perylenediimide was intercalated into layered double hydroxide and montmorillonite. The intercalated dye was shown to have improved photostability [18,19]. We have also investigated the improved stability of various natural dyes by producing dye composites with nontoxic inorganic host materials [[20], [21], [22]], including montmorillonite [23]. Montmorillonite is a clay mineral with a layered structure composed of an octahedral Al2O3 or MgO sheet between two tetrahedral SiO2 sheets. There are exchangeable cations between the layers of montmorillonite to compensate for the net negative charge of the octahedral sheets [24,25]. In a previous study, we proved that the cationic anthocyanin dye could be successfully intercalated and stabilized in the interlayer space of the montmorillonite [23]. Such clays were, however, found to be incapable of intercalating carotenoid dyes because they are generally hydrophobic and nonionic [26].
Modifying the interlayer space of the clays, including montmorillonite, with cationic surfactants such as alkyl ammonium ions via an ion exchange reaction can transform the hydrophilic interlayer space into a hydrophobic one [[27], [28], [29], [30]]. The surfactant-modified montmorillonite is named organo-montmorillonite and is known to exhibit different adsorption properties compared with the original clay mineral [31,32]. Based on this technique, BC has been reported to be successfully intercalated in the interlayer space of the organo-montmorillonite [33]. The intercalation of a hydrophobic carotenoid dye (annatto dye) into organo-montmorillonites, exhibiting improved its stability, has also been achieved by our group [34].
Organic solvents have been used to prepare the composites thus far. This is due to the notion that the dye molecules had to be fully dispersed in the organic solution in order to be more easily adsorbed onto the host materials. This method requires solvent waste treatment facilities and composite drying processes at elevated temperature, both of which are practically disadvantageous and environmentally unfriendly. In a previous report, it was found that BC could be successfully incorporated into the pores of the organo-modified mesoporous silicates by solventless physical mixing due to the hydrophobic interaction. This resulted in the stabilization of BC [22]. It has also been reported that carotenoids can be successfully intercalated into arabinogalactan by a mechano-chemical method, rendering aqueous solubility [35]. It can therefore be expected, similar to the organo-modified mesoporous silicates and arabinogalactan, that the hydrophobic interaction could also promote the intercalation of BC between the layers of the organo-modified clay by the simple physical mixing, just like other organic compounds reported earlier [36].
In this study, we sought to prepare BC/organo-montmorillonite composite with limited solvent use. Two methods were used to achieve this end. The first is through solventless physical mixing, where the powdery organo-montmorillonite and BC are mixed together directly. The second method is through the mixing of powdery organo-montmorillonite and a small amount of a concentrated BC/hexane solution. Because the amount of the hexane solution used is minimal and the solvent can be easily evaporated during mixing, this procedure was named the “semi-dry mixing method”. These methods could eliminate solvent waste and additional drying processes, resulting in a reduction of cost and energy consumption. The efficiency of the semi-dry mixing method for the preparation of intercalated composites was first evaluated. The enhanced stability of BC in the composite with organo-montmorillonite prepared by the semi-dry mixing method was then investigated. The influence of the length and number of alkyl chains of the surfactant on the stability of the incorporated BC was also of interest. This was done to evaluate the effect that the intercalation had on the stability of the composite. Through these methods, we were able to investigate whether the semi-dry mixing method was effective for the intercalation of BC into organo-montmorillonite.
Section snippets
Sample preparation
BC was supplied by Wako Chemical Co. As the montmorillonite, Kunipia F (denoted as KF) was supplied by Kunimine Industries. The cation exchange capacity (CEC) of KF was 115 meq/100 g. Dodecyltrimethylammonium bromide (DTAB) and hexadecyltrimethylammonium bromide (HTAB) were purchased from Wako Chemicals. Octyltrimethylammonium bromide (OTAB), dioctyldimethylammonium bromide (DODAB), didodecyldimethylammonium bromide (DDDAB), and dihexadecyldimethylammonium bromide (DHDAB) were purchased from
Structural analyses of the composite samples by XRD
The XRD patterns of each organo-modified KF sample are illustrated in Fig. 1. We guess that the exchangeable cations of the KF were exchanged by more than 100% CEC of the surfactant, and that the excess surfactant was removed by washing. Therefore, it can be assumed that an approximate equimolar amount of surfactant was retained in all of the organo-modified KF samples. However, it was not confirmed that all of the exchangeable cations were completely exchanged by the surfactant molecules, and
Conclusion
As a hydrophobic natural dye, BC was intercalated into the organo-modified montmorillonite hydrophobic interlayer space by semi-dry mixing, using a concentrated BC/hexane solution and an organo-modified montmorillonite powder. Highly dispersed BC was obtained by the addition of a small amount of hexane at the physical mixing step, because even a minute amount of solvent is sufficient in enhancing the mobility of the BC molecules. The BC molecules, dissolved in the solvent, have higher
Acknowledgment
This work was financially supported by JSPS KAKENHI Grant number 15K00724, 17H01894 and Grant in Aid from Amano Institute of Technology.
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