The Electrical Properties Of Layered Double Hydroxides Colloidal Aggregation Behavior, As Well As Drug Carrier Applications

Layered double hydroxide (LDH for short), a kind of new material possessing permanently positive charges, have important application values in the industry fields such as catalysts, antacids, the treatment of the waste water, antisettling agents, medicine, anionic exchangers, and rheology modifiers. Recently, the studies on the synthesis and properties of LDH compounds have been paid lots attention to. The electrical properties and aggregation kinetics of dispersions with LDH and water are the most important properties to direct applications. Lack of knowledge in these properties limits the development of industrial applications of LDH. Hence, researches on electrical properties and aggregation kinetics have very high theoretical and practical value. LDH with special electrical properties, layered structure and ionic exchangeable capacity can play important role in medicine carrier systems. In the dissertation, we systematically studied the electrical properties, aggregation kinetics, fractal structure of agglomerates of LDH, and the application as safe drug delivery in naproxen/LDHs nanocomposites. As the author known, few relevant studies have been reported in the former literature.Scetion 1. Studies on isoelectric point (pH_IEP), zero point of charge (pH_ZPNC) and permanent charge density (σ_p) of LDH compoundsSeries of Mg-Al LDH, Zn-Al LDH and Mn-Al LDH have been synthesized by coprecipitation method and the chemical composition, crystal structure and appearance of particles of LDH have been studied in this section. It has been found that LDH particle has the hexagon form and the similar, not same, molar ratio of metal ion to that in raw material. The crystal lattice parameters c of Mg-Al LDH, Zn-Al LDH and Mn-Al LDH is as three times long as the basal spacing, indicating that every crystal lattice in c aspect has three layers of hydrotalcite-like compounds. The basal spacing of Mg-Al LDH (NO₃) can been controlled by regulating the molar ratio of Mg/Al in raw material and reach the biggest value, 0.895 nm, while the molar ratio of Mg/Al is 2:1.Electrochemical parameters of Mg-Al-NC^LDH particles in different electrolytic solutions(LiCl, NaCl and KCI), including zeta potential, isoelectric point (pHiep), permanent charge density(o-p), and zero point of charge (pHzpnc) have been measured by electrophoresis method and potentiometric titration. Effects of electrolyte, pH, and chemical composition on electric property of the positive sol are investigated. It has been found that the zero point of charge (pHzpnc) and permanent charge density (o>) increase with the increase of the content of Al in LDH chemical composition, which is agreed with the isomorphic substitution of Mg2+ by trivalent metal ion Al3+ to form positive charged layers. However, the isoelectric point (pHiep) decreased with the increase of the content of Al in LDH chemical composition, indicating that pHiep is significantly affected by the affinity of Al3+ for electrons. Another phenomenon has been found that the pHiep value of Mg-Al-N03LDH decrease with the thinning of the hydration layer of Li+, Na" and K+, indicating that monovalent cations have effect on the electric property of LDH surface in some degree. Under the fixed values of pHzpnc and op, pHiep decreases with the increase of affinity strength of cations for anions. The op obtained from the PT method is clearly lower than apj theoretically calculated because in the titration process OH" ions in the bulk solution can only exchange part of the gallery anions. The aP is attributed to the amount of the exchanged OH ions and should be an apparent value.Scetion 2. Studies on aggregation kinetics and fractal structure of LDH AgglomeratesThe aggregation phenomenon has widely existed in the application processes of LDH in the catalyzer, antacid, medicine, anionic exchangeable material and many other fields. Aggregation behavior is the basement of kinetics in many physical and chemical systems. However, the limited knowledge on these complicated, anomalous agglomerates hinders the further study of aggregation kinetics and the further application of LDH in industrial field. In this paper, colloidal stability and aggregation kinetics of the layered double hydroxide dispersion with structural charges are investigated at various electrolyte concentrations by dynamic lightscattering (DLS). It has been found that the Mg-Al LDH dispersions go through mostly stable, slow aggregating (Reaction Limited aggregation) and fast aggregating stages (Diffusion Limited aggregation) in turn with the increase of the inertial electrolyte concentration (NaCl, NaNCb). The analysis by DLVO theory shows that the repulsive force between particles dominates and reduces the collision probability between particles while the LDH dispersion with NaCl solution is in stable state. When the LDH dispersion turns into reaction limited aggregation range, there is a second least value in interaction energy curves for two plate-like particles, indicating the reversible aggregation under the second least value may exist. In the diffusion limited aggregation range, the static barrier in interaction energy curves has disappeared (Debye screening length, k'x = 0.95 nm) and the attractive van der Waals is weakly affected by the intervening electrolyte, so the increase of electrolyte concentration has little effect on the aggregation velocity. These phenomena do not occur in the dispersions with various NaOH concentrations (0-4 mM) in our studies. A short "quasi-stable phase" has appeared in the initial aggregating process with NaOH. The double layer is compressed by NaCl or NaNOs resulting in aggregation. However, in the dispersion containing NaOH the adsorption of OH" ions on the particle surface reduces the total net charge and %, which leads to aggregation. The critical electrolyte concentrations of NaCl and NaNO3 for this Mg-Al LDH dispersion are found to be 127 and 180 mM respectively. The relations between the stability ratio and the concentration of NaCl, NaNO3 and NaOH in the slow aggregating stage have been measured to be logJF=10.18 — 4.441ogC, logfF== 11.98 —4.79IogC, log W= 1.92 — 0.741ogC, respectively. Here, W is the stability ratio in aggregation and C is electrolyte concentrations in the dispersion.The aggregation behavior and fractal structure of LDH agglomerates with increasing ionic strength have been studied by SEM image analyses. The experiments indicate that the LDH agglomerates have two different structures in RLA regime and DLA regime, growing from one compact grape-like structure to another loose structure with flakes with the increase of the KC1 concentration, which corresponding to different modes of particle association. The research also gives theproof that the transition region between RLA and DLA may occur. By SEM image analyses, the fractal dimension of agglomerates decreases gradually from 2.26 to 1.76 with the increase of electrolyte concentration. The evolvement of fractal dimension also approves that the agglomerate structures grow from compact and fine agglomerates to loose-flake agglomerates.Scetion 3. Synthesis and Characterization of naproxen/LDHs nanocompositesRecently, LDH are biocompatible, nontoxic, week alkaline, decomposable as drug carrier and have larger ionic exchangeable content to carry the large molecule, such as DNA, which has been paid more attention to. But the studies on synthesis and characterization of drug (biological material) /LDH with the controllable release ability has been little reported. In this paper, naproxen has been intercalated into Zn-Al layered double hydroxide (LDHs) by the method of ion exchange. The effects of the reaction time, various metal molar ratios of LDHs, the naproxen initial concentration and pH value of dispersion on the naproxen-adsorption process and the naproxen-release process of naproxen/LDHs composite in buffer solution have been studied. It's found that LDHs adsorb naproxen by monolayer in accord with Langmuir adsorption isotherm. The steric effect and the electrostatic attraction are the main factors to affect the adsorption rate and the release rate. The XRD data and the Langmuir adsorption isotherm suggest that a naproxen monolayer is adsorbed on the surface of the LDHs hydroxide basal layer with the carboxyl of individual anions of naproxen attaching to the hydroxide layer, and the two neighbouring naproxen monolayers adsorbed on two opposite surfaces of LDHs hydroxide basal layer have an overlapping part. The naproxen ions strongly interact with each other via n-n interactions of naphthalene rings, and the mean height of the overlapping part is about 5.62 A. It has been found that the Am value in mmol-g"1 increases with decreasing the molar ration of Al/(A1+Zn) (i.e. x value), which is because the S value of the LDHs(Cl') samples increases with decreasing the x value. If the Am values are expressed in units of mmol-m'2, it can be found that the Am value increases with increasing the x value. This is because the (Tsj value of the LDHs(Cl") samples increases with increasing the x value, and the higher the asj value is, the larger theintercalation trend of naproxen ions into the gallery space of the LDHs(Cl') is, which can be proved by the increase of the absolute value of the adsorption enthalpy AG with increasing the o(rel) of naproxen/LDHs nanocomposite is much higher than that of naproxen troche (about 30min).This may be attributed to the restricted motion of naproxen anions by narrow channels of LDHs and the attractive interaction with positive charged layer.In this paper, naproxen/Mg-Al LDH, naproxen/Mn-Al LDH, naproxen/Zn-Al LDH nanocomposites have been successfully prepared by the method of coprecipitation. The effects of pH value on the structure of products, the drug capacity carried, the controllable release ability and the thermal stability have been studied. The research shows that the inserted quantities of naproxen and the highest extent of coprecipitation have occurred in the sequence of Mg-Al > Mn-Al > Zn-Al (pH = 10, 9.5, 9). The suitable pH value is 9.5 for the synthesis of the three types of naproxen/LDH nanocomposites, and the products are pure and well crystalline, especially for naproxen/Mn-Al LDH. The naproxen capacity of naproxen/Mn-Al LDH is highest and the spacing distance of naproxen/Mn-Al LDH is lowest. In thepH range of 9.00-10.00, the products of naproxen/Mg-Al, Mn-Al, Zn-Al LDH are pure and well crystalline, especially for naproxen/ Mn-Al, which has the wide pH range of 8.30-10.50. The thermal stabilities of the three types of naproxen/LDH composites are similar, i.e. the LDH layer is easily decomposed by the escape of water, and the thermal stability of LDH decrease after the intercalation of naproxen. However, the weight loss temperature for naproxen increases. Comparing the release experiments and the TEM telegraph of composites in the release processes, it has been found that the differentia of the controllable release function between various types of naproxen/LDH composites was ca used by the ionic exchange process and the corrupted process of LDH by H+. The naproxen/Zn-Al LDH nanocomposites have the best controllable release ability in pH 7 and 4. The /9o(rel) of various types of naproxen/LDHs nanocomposite in the buffer solution is much higher than that of naproxen troche. The research has showed that the naproxen/LDH composites made by the ionic exchange method and coprecipitation method have better controllable release function than that of naproxen troche.