The aim of this study is the improvement of the photostability of several natural anionic dyes, carmine (CM), carthamus yellow (CY), and annatto dye (ANA), by complexation with hydrotalcite. The composite of the dyes and hydrotalcite is prepared by the coprecipitation method. CM is successfully intercalated in the hydrotalcite layer when the amount of introduced CM is large. The photostability of CM in CM/HT composites is superior to the CM adsorbed on silica surface. The effect of the stability enhancement is larger when the amount of introduced CM exceeds 0.23 g/g-host, or when the layer charge density of the hydrotalcite is larger. CY is also stabilized by complexation with hydrotalcite, whereas ANA is not stabilized by complexation with hydrotalcite. The photostability of an anionic natural dye can be improved by intercalation into the hydrotalcite layer, if the dye has a hydrophilic nature and a rather planar structure. The intercalated dye is stabilized by the protection from the attack of the atmospheric oxygen. In addition, contribution of the electrostatic interaction between the positively charged hydrotalcite layer and the intercalated anionic dye is also proposed.
19The stability of the naturally occurring annatto dye against irradiation with visible light 20 was enhanced by intercalation into octyl trimethylammonium and dodecyl 21 trimethylammonium montmorillonite. The intercalated dye exhibited improved light fastness. 22
The hydrogel made by adding poly(acry1ic acid) (PAA) to iron(m) chloride solution has been investigated as a function of the water content of the gel by means of EPR, visible and infrared spectroscopies. The red-brown PAA-Fe3+ complex gel formed is homogeneous and transparent as is the solution, while the xerogel obtained by drying the gel was yellow. Iron(m) ions in the gel aggregate [as iron(m) ion clusters] and disperse (as mononuclear species) reversibly with repeated wetting with water and drying of the gel. In the hydrated gel PAA carboxyl groups bond strongly to the cluster iron(m) ions, thus networks of intermolecularly cross-linked PAA cause gelation, whereas in the dry gel the networks are loosened.
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