Intercalation behavior of l-ascorbic acid into layered double hydroxides

Abstract

The intercalation of l-ascorbic acid (ASA) into three kinds of layered double hydroxides, such as Mg–Al, Mg–Fe and Zn–Al LDHs, has quantitatively been investigated by the calcination-rehydration (reconstruction) and coprecipitation methods. The amount of ASA intercalated was considerably different by the LDH systems that was estimated to be ca. 0.40 mol/mol-M³⁺ for the Mg–Al and Mg–Fe systems by the reconstruction method. The Zn–Al system was hardly restored to the LDH structure by the rehydration reaction with the intercalation of ASA. ASA was also intercalated into the Mg–Al and Zn–Al LDHs by the coprecipitation method, while the intercalation of NO₃⁻ was observed in the Mg–Fe LDH. The basal spacing of the solid products were expanded to d₀₀₃ = 0.84 (Mg–Al) and 0.86 (Mg–Fe) nm by the reconstruction method, 0.97 (Mg–Al) and 1.06 (Zn–Al) nm by the coprecipitation method, respectively, suggesting that ASA was intercalated into the LDHs. A large fraction of ASA was intercalated as reduced form after the light and heat resistance tests of the ASA/LDHs. This result confirmed that ASA was stabilized by the intercalation of the LDHs interlayer space. Furthermore, the intercalated ASA was easily deintercalated from the LDH interlayer space by the ion exchange method using CO₃²⁻. It is expected that LDHs will be good host materials for safe storage of vitamins.

Introduction

Clay minerals are focused on the synthesis of new organic–inorganic nanohybrid materials in recent years. A kind of clay minerals, layered double hydroxides (LDHs) are widely known as hydrotalcite-like compound and often called anionic clay comparing with the more conventional cationic clay. Hydrotalcite, Mg₆Al₂(OH)₁₆CO₃·4H₂O, is the most frequently investigated anionic clay and is rarely found in nature. The chemical composition of LDHs is represented by general formula [M²⁺_1−xM³⁺_x(OH)₂][Aⁿ⁻_x/n·yH₂O], where M²⁺ is a divalent cation such as Mg²⁺, Zn²⁺, Co²⁺, Mn²⁺ and Cu²⁺, M³⁺ is a trivalent cation such as Al³⁺, Cr³⁺, Co³⁺ and Fe³⁺. Aⁿ⁻ is an ion exchangeable anion such as OH⁻, Cl⁻, NO₃⁻, CO₃²⁻, SO₄²⁻ and various organic anions. The x value is equal to the ratio M³⁺/(M²⁺M³⁺), generally ranging between 0.20 and 0.33. This value is attributed to the charge density of the hydroxide basal layer, namely, anion exchange capacity (AEC). LDHs basal layer possesses a positive charge due to the trivalent cation substituted for the divalent cation, and the interlayer space is neutralized by the intercalation of anions with water molecules. The intercalation of various anions into LDHs has been attained by the following methods: coprecipitation, ion exchange, calcination-rehydration (reconstruction), thermal reaction and hydrothermal reactions (Miyata, 1980, Rives, 2001). Recently, LDHs are employed as the host material to synthesize a new organic–inorganic nanohybrid material and have received considerable attentions. The organic/LDHs nanohybrid materials have been investigated because the resulting intercalation compounds are expected to possess a novel nanostructure and new function (Ambrogi et al., 2001, de Melo et al., 2002, Choy et al., 2004, Kwak et al., 2004, del Arco et al., 2004a, del Arco et al., 2004b, Khan and O'Hare, 2002). A synthesis of biomolecule/LDH nanohybrid materials in particular has become of great interests. For a fact, the intercalation of the biomolecule such as nucleotide (Lotsch et al., 2001, Aisawa et al., 2005), deoxyribonucleic acid (Choy et al., 1999), amino acid (Whilton et al., 1997, Fudala et al., 1999, Aisawa et al., 2001, Aisawa et al., 2004) and polypeptide (Nakayama et al., 2004) into LDHs was described in order to prepare the biomolecule/LDH nanohybrid materials. Hydrotalcite is known to be biocompatible materials and has found pharmaceutical applications as antacid. In addition, anionic drug molecules have been intercalated into various LDHs, with an aim to determine the study of using these intercalation compounds as materials for storage, transport and ultimately controlled release of drug (He et al., 2004, del Arco et al., 2004a, del Arco et al., 2004b, Dupin et al., 2004, Li et al., 2004).

l-ascorbic acid (ASA) is known as vitamin C and helps some of our most important body systems. ASA assists the immune system to fight off foreign invaders and tumor cells and also supports the cardiovascular system by facilitating fat metabolism and protecting tissues from free radical damage. The solution of ASA is unsettled for natural light, thermal and alkaline conditions. Choy et al. reported the intercalation of vitamins such as vitamin A, C and E by the ion exchange and coprecipitation methods, moreover, the controlled release of their vitamins from vitamin/LDH was examined by ion exchange method (Hwang et al., 2001, Choy and Son, 2004). However, the durability, such as light and heat, of the intercalated ASA was not mentioned.

In the present study, the intercalation behavior of ASA into three kinds of LDHs, Mg−Al, Mg−Fe and Zn−Al systems, by the reconstruction and coprecipitation methods was investigated with the aim of synthesizing the ASA/LDHs. The quantitative determination of the intercalated ASA form, reduced and oxidized, after the light and heat resistance tests of the ASA/LDHs was studied with the intention of storing stability for ASA. In addition, the deintercalation of ASA from the ASA/LDHs, namely, release behavior of ASA, was also examined by the ion exchange method with CO₃²⁻.