Studies Of Assembly, Supramolecular Structure And Properties Of Sorbic Acid, Lactic Acid And Glyphosate Intercalated Layered Double Hydroxides

Using the concept of intercalation chemistry, with layered double hydroxides (LDHs) chosen as host material and organic active agents as guest species, food preservative sorbic acid (SA)-intercalated LDHs, food additive lactic acid (LA)-intercalated LDHs, and herbicide glyphosate (GLY)-intercalated LDHs have been successfully synthesized by using coprecipitation self-assembly techniques. The supramolecular structure, thermal behavior and relations between structure and release properties of these organic active agent-intercalated LDHs have been studied in detail. The results suggest potential applications of these intercalated materials in slow/controlled release of the organic active agents and as template reservoirs with enhanced thermal stability of organic active agents.Characterization using XRD, FT-IR, UV-Vis, BET, TEM, SEM, TG/DTA/MS, in situ HT-XRD, MAS NMR and XPS techniques confirm the supramolecular structures of the three organic active agent-intercalated LDHs, and corresponding structural models are presented. The interlayer anions of SA-intercalated ZnAl-LDHs are positioned in a vertical interdigitated monolayer mode based on a host-guest interaction involving both electrostatic attraction and hydrogen bonding and a guest-guest interaction involving intermolecular n-n functions of SA anions within the two-dimensional interlayer galleries. The interlayer anions of LA-intercalated ZnAI-LDHs are oriented in a tilted bilayer mode with two conformations involving intramolecular and intermolecular hydrogen bonding, respectively. The interlayer anions of GLY-intercalated ZnAl-LDHs are strongly stabilized with a verticalinterdigitated monolayer mode based on host-guest and guest-guest interactions involving both electrostatic attraction and hydrogen bonding.The thermal decomposition process of the three organic active agent-intercalated LDHs exhibits four steps involving the removal of physisorbed and cointercalated water, dehydroxyiation and collapse of the host layer, decomposition and combustion of interlayer guest anions, and the formation of oxide, respectively. Furthermore, the decomposition and combustion temperature of interlayer intercalated guest anions of the three organic active agent- intercalated LDHs is increased by 60~200°C compared with that of the pure organic active agents.As a consequence of the supramolecular intercalation structures, all three organic active agent-intercalates present obvious differences in release behavior compared to the corresponding physical mixtures with rapid release in the initial step, followed by a more sustained release of the remaining guest anions. The release mechanism in neutral and basic aqueous solution is an ion-exchange process between the interlayer guest anions and inorganic anions in the release media, and the diffusion process of guest anions in the interstices and interlayers of organic active agent-intercalates is the rate-limiting step. On the other hand, the guest release of organic active agent-intercalates in acidic aqueous solution is essentially controlled by the dissolution process of the host layers.The relationship between the structural parameters and the release behavior for the organic active agent-intercalated LDHs has been studied systemically. The interlayer conformation of GLY-intercalated LDHs strongly influences its release behavior. The release rate of GLY intercalates markedly decreases according to the sequence: interlayer guest conformation in the tilted monolayer mode > vertical monolayer mode > vertical interdigitated monolayer mode. This is due to the increase in host-guest and guest-guest interactions and interlayer guest packing density of GLY-intercalated LDHs. An increase in layer charge density of intercalates also causes a decrease of release rate for GLY-intercalated LDHs due to the increase in the host-guest and guest-guest interactions and the interlayer guest packing density.