Layered solid particles as self-assembled films

Effects of ultrasonic irradiation on the synthesis, crystallization, thermal and dissolution behaviour of chloride-intercalated, co-precipitated CaFe-layered double hydroxide

The output power (30-150 W) and the periodicity (20-100%) of ultrasound emission were varied in a wide range to regulate and improve the crystallization process in the commonly used co-precipitation technique of chloride-intercalated CaFe-layered double hydroxides. The influence of ultrasound irradiation on the as-prepared materials was studied by X-ray diffractometry, dynamic light scattering, UV-Vis-NIR diffuse reflectance spectroscopy, specific surface area measurement, pore size analysis, ion-selective electrode potentiometric investigations and thermogravimetry. Additionally, structural alterations due to heat treatment at various temperatures were followed in detail by Fourier-transform infrared and X-ray absorption spectroscopies as well as scanning electron microscopy. The ultrasonic treatment was capable of controlling the sizes of primarily formed (from 19 nm to 30 nm) as well as the aggregated (secondary) particles (between 450 nm and 700 nm), and thus modifying their textural parameters and enhancing the incorporation of chloride anions into the interlamellar space. For the first time, the optical energy gap of CaFe-LDH was reported here depending on the nature of applied stirring (4.18-4.34 eV). The heat-treatment investigations revealed that the layered structure was stabile until 200 °C, even at the atomic level.

Synthesis and stabilization of Prussian blue nanoparticles and application for sensors

Prussian blue (PB) nanoparticles were synthesized by two methods from FeCl2 and K3Fe(CN)6 and from FeCl3 and K3Fe(CN)6 based on the method published by Fiorito et al., and stabilized by different polymers like polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), polyallylamine hydrochloride (PAH), polydiallyl-dimethyldiammonium chloride (PDDA) and polystyrene sulfonate (PSS). The effect of the monomer/Fe3+ ratio was studied regarding the average particle size and zeta-potential. The forming PB structure was checked by X-ray diffraction. The stabilization was successful for every applied polymer, but the average particle size significantly differs. Particle size distributions were determined by Malvern type nanosizer equipment and by transmission electron microscope (TEM) and zeta potential values were determined for the obtained stabile samples. The results revealed that by using FeCl2 and K3Fe(CN)6 for PB preparation particles with narrow size distribution and average diameter of 1.7 nm occurred but stabilization was necessary. By the other method the dispersion was stabile with 182 nm particles but the particle size exponentially decreased to 18 nm with increasing PVP concentration. Ultrathin nanofilms were prepared on glass support by the alternating layer-by-layer (LbL) method from PB particles and PAH. The morphology of the prepared films was investigated also by AFM. The films were immobilized on interdigitated microsensor electrodes (IME) and tested in sensing hydrogen peroxide and different acids like acetic acid, hydrochloric acid vapors.

Mechanism of smectic arrangement of montmorillonite and bentonite clay platelets incorporated in gels of poly(acrylamide) induced by the interaction with cationic surfactants

Structure transitions, induced by the interaction with the cationic surfactant cetylpyridinium chloride in nanocomposite gels of poly(acrylamide) with incorporated suspensions of the two closely related layered clays bentonite and montmorillonite, were studied. Unexpectedly, different behaviors were revealed. X-ray diffraction measurements confirm that, due to the interaction with the surfactant, initially disordered bentonite platelets arrange into highly ordered structures incorporating alternating clay platelets and surfactant bilayers. The formation of these smectic structures also in the cross-linked polymer gels, upon addition of the surfactant, is explained by the existence of preformed, poorly ordered aggregates of the clay platelets in the suspensions before the gel formation. In the case of montmorillonite, smectic ordering of the disordered platelets in the presence of the surfactant is observed only after drying the suspensions and the clay-gel composites. Rheology studies of aqueous suspensions of the two clays, in the absence of both surfactant and gel, evidence a much higher viscosity for bentonite than for montmorillonite, suggesting smaller clay-aggregate size in the latter case. Qualitatively consistent results are obtained from optical micrographs.

Formation of Langmuir-Blodgett films of a clay and a water-soluble alkylammonium cation

We have studied the formation of ultrathin hybrid films composed of a water-soluble alkylammonium cation (trimethyloctadecylammonium caiton: TMODAH+) and clay nanosheets at an air-water interface. When a chloroform solution of the ammonium salt of TMODAH+Cl- was spread onto a surface of an aqueous clay suspension in a Langmuir trough, a stable monolayer of the water-soluble ammonium cation was formed by hybridization with the clay nanosheets at the air-water interface. Surface pressure--molecular area (pi-A) isotherm curve of the hybrid monolayer shifted to the smaller molecular area side with the increase in the concentration of the ammonium salt in the solution or with the decrease in that of the clay in the suspension. In some cases, the lift-off areas in the pi-A isotherm curves were less than the cross-sectional area of an alkyl chain (ca. 0.19 nm2). These results indicated that some ammonium cations spread onto the clay suspension were dissolved into the aqueous subphase before the hybridization with the clay nanosheets. The hybrid monolayers were transferred onto a glass plate in a layer-by-layer way by horizontal dipping to form a hybrid multilayerd film. Interestingly, the densities of TMODAH+ determined by the infrared spectral analysis were constant in the hybrid multilayers prepared from the clay suspensions at the same concentration, regardless of the concentrations of the ammonium salt solutions. XRD patterns of the films showed that the cations of TMODAH+ would lie down on the clay layer in the hybrid film.

Dodecyl sulfate–hydrotalcite nanocomposites for trapping chlorinated organic pollutants in water

A series of hybrid organic-inorganic nanocomposite materials was synthesized by three different procedures using sodium dodecyl sulfate (DDS) and magnesium-aluminum layered double hydroxide (Mg/Al LDH with a Mg/Al molar ratio of 2 to 5). Both the pH of the exchange medium (6.5 to 10) and the Mg/Al molar ratio of the LDH affected the basal spacing, the content of DDS retained and the orientation of the DDS chains within the interlamellar space. For LDH with higher charge density (Mg/Al=2 and 3), DDS molecules likely formed a perpendicular monolayer within the LDH interlayer and the solution pH had little effect on the basal spacing, with a mean and standard deviation of 25.5+/-0.4 A. However, for LDH with lower charge density (Mg/Al=4 and 5), DDS molecules more likely formed an interpenetrating bilayer, and the basal spacing significantly increased with increasing pH, with a mean and standard deviation of 32.7+/-5.2 A. Sorption of trichloroethylene and tetrachloroethylene by DDS-LDH varied with synthesis conditions, LDH type and DDS configuration in the interlayer. DDS-Mg(3)Al-LDH had the highest affinity for both trichloroethylene and tetrachloroethylene in water, either comparable to or as much as four times higher than other clay-derived sorbents, followed by DDS-Mg(4)Al-LDH and DDS-Mg(5)Al-LDH. DDS-Mg(2)Al-LDH had the lowest sorption affinity although the highest amount of DDS. The pH of the exchange solution also affected the amount of DDS retained by the LDH as well as the sorption efficiency. Mg(3)Al-LDH has a charge equivalent area of 32.2 A(2)/charge, which allows the formation of optimal DDS configuration within its interlayer, thus resulting in the highest affinity for the chlorinated compounds. The DDS-Mg/Al-LDHs can be easily synthesized either ex situ or in situ at low temperature, indicating the feasibility of practical applications. The results obtained by controlling the synthesis procedure suggest that different arrangements of DDS molecules in the LDH interlayers can be obtained and optimized for the sorption of specific sorbates.

A potentiometric study of the adsorption of a cationic surfactant onto laponite in water-methanol and water-dimethylsulfoxide mixtures

We studied the adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (DTABr), onto laponite in water-methanol and water-DMSO mixtures at 298 K. Adsorption isotherms were determined by potentiometric methods using DTA(+) and Br(-)-selective electrodes. All of the isotherm shapes in the mixture were classical. Activity coefficients were determined for the transfer between water and hydroorganic mixtures. Comparison of the adsorption isotherms obtained for different surfactant chemical potentials showed that at higher concentrations of free surfactant, the aggregation state does not depend on the nature of the surrounding media. A study of the influence of the ionic strength showed that there is a common intersection point in each hydroorganic mixture. This point corresponds to the charge compensation point and depends on the nature of the surrounding medium.