Layer Charge Density Research Articles

Acidic Porous Clay Heterostructures (PCH): Intragallery Assembly of Mesoporous Silica in Synthetic Saponite Clays

Mesostructured intercalates belonging to the class of solid acids known as porous clay heterostructures (PCH) have been prepared through the surfactant-directed assembly of mesoporous silica within the galleries of synthetic saponite clays with targeted layer charge densities in the range x = 1.2−1.7 e- units per Q+x[Mg6](Si8-xAlx)O20(OH)4 unit cell. The removal of the intragallery mixture of neutral alkylamine and quaternary ammonium ion surfactant (Q+) by calcination afforded PCH intercalates with basal spacings of 33−35 A, BET specific surface areas of 800−920 m2 g-1, and pore volumes of 0.38−0.44 cm3 g-1. The framework pore sizes were in the supermicropore to small mesopore region ∼15−23 A. Temperature-programmed desorption of chemisorbed cyclohexylamine (CHA) indicated the presence of both weak and strong acid sites, corresponding to desorption temperatures near 220 and 410 °C, respectively. The total acidity (0.64−0.77 mmol CHA g-1) increased with the saponite layer charge density (x), indicating th...

Self-assembly of Tiled Perovskite Monolayer and Multilayer Thin Films

Dion−Jacobson phases are layer perovskites of the general formula A‘[An-1BnO3n+1], where A‘ is typically an exhangeable alkali or alkali earth ion. Because of their low layer charge density, acid-exchanged Dion−Jacobson phases are easily exfoliated to make single-sheet colloids. When the conditions temperature and pH were controlled, densely tiled monolayers of anionic sheets derived from HCa2Nb3O10, HSr2Nb3O10, HCaLaNb2TiO10, and HLaNb2O7 were grown on cationic substrates. Multilayer films were also assembled by alternately adsorbing these sheets and organic polycations.

Interfacial properties of polycrystalline gold electrode in tetramethylammonium hydroxide solutions

Electrochemical properties of polycrystalline gold electrode/tetramethylammonium hydroxide solution interface have been studied as function of concentration and temperature by cycling voltammetry and impedance spectroscopy. Interfacial impedance has constant phase element (CPE) character with parameters depending on potential and concentration. In wide concentration range, from 0.005 to 4 mol dm −3, (the highest concentration corresponds to stoichiornetric clathrate hydrate, (CH 3) 4NOH 10H 2O) CPE is identical with double layer capacity for potentials more negative than −0.28 V versus SCE up to hydrogen evolution. In this potential range the electrode is ideally polarizable and the determination of double layer capacity and charge density is possible. At far negative potentials double layer capacity depends on kind of cation and is independent of its concentration. In wide concentration and potential range strong adsorption of hydroxyl anions is observed on polycrystalline gold electrode. It is also shown that in (CH 3) 4NOH 10H 2O electrochemical measurements can be done in wide temperature range, including liquid and solid state of electrolyte.

A physical model for hole direct tunneling current in p/sup +/ poly-gate pMOSFETs with ultrathin gate oxides

A model of the hole direct tunneling gate current accounting for heavy and light hole's subbands in the quantized inversion layer is built explicitly. This model comprises four key physical parameters: inversion layer charge density, hole impact frequency on SiO/sub 2/-Si interface, WKB transmission probability, and reflection correction factor. With the effective hole mass m/sub oxh/=0.51 M/sub o/ for the parabolic dispersion relationship in the oxide, experimental reproduction without any parameter adjustment is consistently achieved in p/sup +/ poly-gate pMOSFETs with 1.23, 1.85, and 2.16 nm gate oxide thicknesses. The proposed model can thereby serve as a promising characterization means of direct tunnel oxides. In particular, it is calculated that the secondary subbands and beyond, although occupying few holes, indeed contribute substantially to the direct tunneling conduction due to effective lower barrier heights, and are prevailing over the first subbands for reducing the oxide field down below 1 MV/cm.

Preparation, thermal stability and decomposition routes of clay/Triton-X100 composites

The adsorption isotherms for the octylphenol ethoxylate, TX100, onto a range of bentonites and a saponite have been determined and the resulting thermal stability of composites has been studied. In general TX100 exhibited a high affinity for the different clay surfaces although the maximum amount adsorbed varied with both clay type, layer charge density and resident exchange cation. XRD and variable temperature XRD showed that TX100 was adsorbed into the interlamellar region, and that the thermal stability of the clay/TX100 composite was dependent upon the exchangeable cation present. For monovalent cation exchanged clays (M+-clay/TX100) the TX100 decomposed between 220 and 300 °C, and the d001 spacing decreased from 15.2 to 9.6 A. In contrast, divalent cation exchanged clays imparted a greater thermal stability to the complex (M2+-clay/TX100) owing to coordination of adsorbed TX100 to the exchange cation, wherein weakly coordinated TX100 was stable to 270 °C and strongly coordinated TX100 was held to temperatures in excess of 300 °C. FTIR analysis of the gases evolved evolved during thermogravimetry confirmed that the octylphenol unit was desorbed from M2+-clay at temperatures below 400 °C and that portions of the ethoxylate chain were held to 500 °C.

Comparative Study of the Synthesis and Properties of Polyoxometalate Pillared Layered Double Hydroxides (POM-LDHs)

The pillaring of (NO3)-ZnAl-LDHs with the polyoxometalates (POMs) [PV2W10O40]5−, [Mo7O24]6−, [V10O28]6− and [H2W12O40]6−, using large organic anions like terephthalate for pre-swelling the LDH structure forms a promising method for the controlled creation of small micropores. The use of the terephthalate precursor ((T)-ZnAl-LDH) avoided almost completely the formation of undesired side phases during pillaring, although anion exchange with the large POM complexes proceeded with more difficulty than in the case where (NO3)-LDHs were used as a starting material. Direct pillaring via the (NO3)-LDHs resulted in multiphased materials, and no correlation was found between the M(II)/M(III) ratios in the starting LDHs and the created porosity. For the [POM]-ZnAl-LDHs pillared via the terephthalate precursor, the layer charge density arising from the amount of isomorphically substituted Al3+ in the LDH layers forms the crucial parameter with regard to the created microporosity. Improving the surface area (SA) and micropore volume (μPV) values was accomplished by lowering the charge density on the LDH layers (increasing the Zn2+/Al3+ ratio). In this way, a [PV2W10O40]-ZnAl-LDH (Zn2+/Al3+ = 4.26) with a SA (BET) of 166 m2/g and a μPV of 0.047 cm3/g was formed. For the different types of pillars, small micropores were formed due to the pillaring process. In the case of the smaller POM complexes [Mo7O24]6+ and [V10O28]6+, an increase in μPV and SA was not accompanied by a detectable shift in average pore size, which was the case for the second group of complexes, [PV2W10O40]5− and [H2W12O40]6−. Due to their larger dimensions, mainly micropores between 0.71 and 1.06 nm were created at high Zn2+/Al3+ ratios, together with a substantial amount of pores smaller than 0.71 nm.

EFFECT OF HyA-INTERLAYERING ON NH4/Ca SELECTIVITY OF EXPANDABLE LAYER SILICATES

Hydroxy-interlayering in vermiculites (Vt) and montmorillonites (Mt) is very common in acid soils and sediments which causes significant changes in many physical, chemical, electrochemical and mineralogical properties of the host clays. Cation exchange capacity and selectivity are essential parameters that influence cation distribution between adsorbed and solution phases, and their mobility in the soil profile. Adsorption and exchange isotherms for NH 4 -Ca system were derived using Vt and Mt clays interlayered with hydroxyaluminum (HyA) ions of initial basicities of 1.0 and 2.5. The principal objective of this study was to clarify the effect of hydroxy-interlayering on the NH 4 /Ca selectivity of the host clays. Vermiculite in contrast to Mt, showed more preferential adsorption and exchange of NH 4 over Ca. The HyA-interlayering greatly increased NH 4 /Ca selectivity of Vt, whereas that of Mt was little affected. The greater NH 4 /Ca selectivity of Vt in comparison to Mt might be a result of higher layer charge density in the former predominantly of tetrahedral origin. The HyA-interlayering might have increased the NH 4 /Ca selectivity through making the clay interlayers propped open, occupying the Ca adsorbing sites and/or exerting steric hindrance to the entry of more hydrated, larger Ca ions. Larger amount of HyA adsorption at higher initial basicity of the HyA ionic solution appeared to increase NH 4 /Ca selectivity, especially of Vt. The greater NH 4 /Ca selectivity of Vt, Mt, and HyA-Vt/Mt as well as the effect of HyA-interlayering on this parameter were limited to low NH 4 -saturation.

Theoretical evaluation of the microporosity of pillared layered double hydroxides

Over the last ten years, the concept of pillaring has frequently been applied on layered double hydroxides (LDHs). Due to the variety of possible anionic pillaring species and the adjustable layer charge density, LDHs offer good perspectives with regard to the creation of porous adsorbents and catalysts. But despite these possibilities, their porosity properties can still not compete with those of industrially applicable materials like zeolites. In this study, theoretical calculations based on geometrical models and performed on both Fe(CN)6-MgAl-LDHs (A) and [PV2W10 O40]-ZnAl-LDHs (B) were reported. Properties such as the micropore volume and the interpillar distance were calculated, and compared to experimental data. For a M(II)/M(III) ratio in the layers of 3, the theoretical maximum micropore volumes were 0.3843 cm3/g (A) and 0.1497 cm3/g (B), respectively. By implementing parameters like the stack size, pillars on the outside of the stacks and the possibility of collapse, the model was adjusted in order to create a realistic picture of the microstructure of pillared LDHs. This led to a better understanding of the limiting factors, and gave an explanation for the relatively low micropore volumes of pillared LDHs. For the Fe(CN)6-MgAl-LDHs, small interpillar distances were responsible for the partial inaccessibility of the interlayer regions by N2. This effect was the most pronounced for high charge density LDHs. The situation for the [PV2W10O40]-ZnAl-LDHs is more complex. Probably due to an incomplete pillaring process, the theoretical maximum values are not reached.

Polydiacetylene-inorganic clay nanocomposites

Two new diacetylenes, 14-amino-10,12-tetradiynoic acid (a diacetylenic aminoacid) and 10,12-docosadiyndiamine (diacetylenic diamine), were successfully synthesized via Eglinton and Cadiot–Chodkiewicz coupling. These two diacetylenes and 10,12-pentacosadiynyl amine (monoamine) were intercalated in mica-type layered silicates, montmorillonite and vermiculite, via a cationic exchange reaction. The intercalation process was confirmed by X-ray diffraction and Fourier transform infrared spectroscopy. The interlayer spacing and polymerizability of the intercalated diacetylene were found to depend upon the length of the diacetylene molecule, the layer charge density of the clay and the solvent treatment. The X-ray diffraction pattern of vermiculite–diacetylene showed evidence of the in-plane packing of the diacetylene. The polymerization of the intercalated diacetylene was confirmed by Raman spectra. © 1998 John Wiley & Sons, Ltd.

Interlayer Arrangement of Hydrated MgAl Layered Double Hydroxides Containing Guest Terephthalate Anions: Comparison of Simulation and Measurement

The hydration of MgAl layered double hydroxides (LDHs) with interlayer terephthalate anions and controlled layer charge is studied. A combination of powder X-ray diffraction (PXRD), thermal gravimmetry (TG), and computer simulation is used to study in detail the effect of layer charge density and interlayer water content on the interlayer arrangement of the organo-LDH. For high water content and layer charge, an interlayer separation of approximately 14.0 A is favored, which corresponds to a vertical orientation of the terephthalate anion with respect to the hydroxide layers. For low water content and layer charge, an interlayer separation of approximately 8.4 A is favored, which corresponds to a horizontal terephthalate orientation. During cycles of dehydration−rehydration, PXRD indicates that the 14.0 and 8.4 A units coexist in varying proportions depending on the layer charge and water content; in certain cases, a 22.4 A interstratified phase consisting of a regular alternation of the 14.0 and 8.4 A co...

Polydiacetylene–inorganic clay nanocomposites

Two new diacetylenes, 14-amino-10,12-tetradiynoic acid (a diacetylenic aminoacid) and 10,12-docosadiyndiamine (diacetylenic diamine), were successfully synthesized via Eglinton and Cadiot–Chodkiewicz coupling. These two diacetylenes and 10,12-pentacosadiynyl amine (monoamine) were intercalated in mica-type layered silicates, montmorillonite and vermiculite, via a cationic exchange reaction. The intercalation process was confirmed by X-ray diffraction and Fourier transform infrared spectroscopy. The interlayer spacing and polymerizability of the intercalated diacetylene were found to depend upon the length of the diacetylene molecule, the layer charge density of the clay and the solvent treatment. The X-ray diffraction pattern of vermiculite–diacetylene showed evidence of the in-plane packing of the diacetylene. The polymerization of the intercalated diacetylene was confirmed by Raman spectra. © 1998 John Wiley & Sons, Ltd.

Removing prometrone from water by clays and organic clays

The removing power of natural or inorganic clays and organoclays (OCls) for prometone from water was studied as related to the clay layer charge density, organic cation type and organic cation saturation degree of the clay. Two different natural smectites (high charge SAz and low charge SWy) and those treated with two alkylammonium cations (octadecylammonium and hexadecyltrimethylammonium) were assayed by measuring the adsorption isotherms and fitting the data to the Freundlich equation. The Freundlich parameter Kf, the distribution coefficients Kd 0.3 and, that on organic carbon basis, Koc were calculated and used as relative measures of the sorption capacity. The inorganic original smectites show low and medium prometrone adsorption (Kf SAz = 29 and Kf SWy = 138 mmol/kg) inversely proportional to their layer charge density, although the relative sorption capacity depends on the prometrone concentration level. The OCls generally adsorb more prometrone (Kf from 61 to 1031 mmol/kg) than the inorganic samples. The organoclay characteristics favouring the prometrone sorption are high charge density, quaternary alkylammonium in the interlayer and organic cation saturation slightly below CEC. However, for low charge smectite primary alkylammonium and organic cation saturation above CEC promoted herbicide adsorption.

Analysis of the MOS transistor based on the self-consistent solution to the Schrodinger and Poisson equations and on the local mobility model

The effects of carrier energy quantization in the semiconductor surface region on performance of the metal-oxide-semiconductor (MOS) transistor are theoretically considered by comparison of results of a self-consistent solution to the Schrodinger and Poisson equations and the results of the classical description. The gate voltage dependencies of the surface potential and inversion layer charge density are compared. Using the local mobility model the relations between the electron effective mobility and the electric effective field obtained from the both descriptions are for the first time compared. The accuracy of the commonly used triangular well approximation is examined. This approximation is used for calculation of the transistor current-voltage (I-V) characteristics. Simulations are performed for MOS transistors with ultrathin oxides and highly doped substrates, in accordance with the state of the art of today's VLSI/ULSI technology.

Influence of the layer charge density of smectites on the interaction with methylene blue

Six Na-saturated smectites were isolated from bentonites of various deposits. Their H-forms were prepared using H–OH–H resins. The interaction of methylene blue (MB) (0.05 mmol g-1) with Na-saturated untreated and H-autotransformed clay samples was investigated by the VIS spectroscopy of their dispersions. The spectra provided information about the formation of protonated dye cations, dimers, and higher agglomerates on the clay surface. The amounts of various dye forms were compared with the distributions of negative layer charge of the used clays, obtained by the alkylammonium method. Formation of MB dimers and higher agglomerates reflected sensitively the layer charge density of the smectites. MB cations formed predominantly higher agglomerates at the surface of those smectites with high charge density. Suppressed dye agglomeration was observed for the samples of lower layer charge density and was caused by greater distances between the adsorbed MB cations. Due to the high sensitivity of MB agglomeration on the layer charge and the simplicity of the method, MB–clay interaction can potentially be utilized for the layer charge characterization of smectites.

Hysteretic Depinning of Anisotropic Charge Density Waves

We investigate the depinning transition in a dirty periodic medium considering a model of layered charge density waves as a prototype system. We find that depinning from strong disorder occurs via a two stage process, where, first, the pinned system experiences a continuous transition into a plastically sliding state and undergoes a second sharp hysteretic transition into a coherently moving 3D state at higher drives. In the weakly disordered system the depinning into a coherently sliding state remains continuous.

Interaction of Methylene Blue with Reduced Charge Montmorillonite

Interaction of methylene blue (MB) with reduced charge Li−montmorillonites (RCM) in the aqueous suspensions was investigated using visible absorption spectroscopy. Dye cation agglomeration and protonation at the clay surface depended very sensitively on layer charge density. With increasing layer charge reduction, the content of agglomerates of MB cations decreased in favor of monomers and the protonated form of MB. Lower negative charge density on the clay basal surface induces a greater distance between neighboring MB cations sorbed at the clay surfaces, which suppresses dye agglomeration. Since each form of MB absorbs visible light at a different wavelength, different layer charge densities induce different colors of the resulting clay−dye suspension. Therefore, visible spectra of MB−smectite suspensions may be a simple but extremely sensitive method for the detection of layer charge density of smectites.

Effects of pH and Mg:Ga Ratio on the Synthesis of Gallium-Containing Layered Double Hydroxides and Their Polyoxometalate Anion Exchanged Products

We have synthesized a series of [Mg2+(1-x)M3+x(OH)2][TA2-(x/2)]·zH2O (M3+= Ga3+ or Al3+ and TA = C6H4(COO-)2) from mixed-metal chloride solutions with initial M3+ mole fractions of 0.20, 0.25, and 0.33, at constant pH values of 6.8, 7.7, 8.6, and 10.0. Terephthalate-containing products were obtained only under conditions that resulted in a Mg:M3+ ratio of 2:1 in the mixed metal hydroxide layers, indicating that a high layer charge density is necessary to incorporate terephthalate anions between the inorganic layers. Anion-exchange reactions were subsequently performed to produce LDHs intercalated with PW11O397- and H2W12O406- anions, yielding products that displayed sharp powder diffraction patterns. In each case, the surface area of the POM-intercalated product was found to be approximately 40−45 m2/g, indicating that the micropore system is totally blocked and that the materials are virtually nonporous; this may result from the high layer charge density of the LDH−terephthalate precursors.

Foam Films Stabilized with Lysophosphatidylcholine: A Comparison of Microinterferometric and Fourier Transform Infrared Spectroscopy Thickness Measurements

Previous studies of foam films stabilized with the soluble phospholipid palmitoyl lysophosphatidylcholine (lyso PC) have demonstrated the effect of Ca2+ ion binding by the phospholipid head group. Two different optical methods have been used in these studiesthe microinterferometric method and Fourier transform infrared (FT-IR) spectroscopy. In both studies some additional treatment specific for the given technique is needed to convert the measured data into real film properties. In the case of the microinterferometric method this is the three-layer sandwich model of the film structure. In the FT-IR spectroscopy method, the Lambert−Beer law with a molar absorption coefficient ε = 150 has been used to derive the aqueous core thickness from the film water content. In the present study thicknesses of silver and common black films determined by both optical methods are compared taking into consideration the assumed film structure models. It is shown that the results are essentially similar, but some small discrepancies are observed in the values of the obtained aqueous core thicknesses which can be explained by the necessity of assuming different optical models for the interpretation of the obtained experimental data. It is demonstrated that the use of suitably selected optical parameters can lead to similar equilibrium film thicknesses. The obtained results are used in the analysis of the values and change of the potential of the diffuse electric layer φo and charge density σo with CaCl2 concentration (Cel). The importance of the model assumptions necessary for the determination of parameters characterizing interaction forces in the film is demonstrated. It is shown that some of the previously observed differences in the φo(Cel) and σo(Cel) curves are due to the different models for estimation of aqueous core thicknesses used in microinterferometric and FT-IR spectroscopy investigations.

Stable InP MIS device using silicon nitride/anodic oxide double-layer dielectric

MIS devices are fabricated on InP using a double-layer dielectric consisting of anodic oxide and PECVD silicon nitride. Two different sets of experiments are conducted using tartaric acid and oxalic acid based AGW electrolyte solutions respectively for growing the anodic oxide layer. Devices having the oxalic acid grown anodic oxide layer exhibit capacitance-voltage (C-V) characteristics close to the ideal and are stable for 7200 s under applied bias conditions. The stability of MIS devices is evaluated by determining the accumulation layer charge density versus time.

Dielectric properties of smectite clays

The dielectric properties of a series of smectite clays, namely, Na +, Li + and Cu ++-fluorohectorites, Na + montmorillonite and Na + laponite have been measured as a function of water content over the frequency range 40−10 5 Hz. These clays span a wide range of particle sizes, surface areas, charge densities and intercalant exchange cations. The objective of this study was to correlate the dielectric properties with these variables. In general the dielectric constant changes slowly as the first monolayer of absorbed water builds up in the galleries. Water loadings beyond the first monolayer cause the dielectric constant to rapidly rise to very high values. The relative permittivity and the conductivity increase with increasing layer charge density and with decreasing charge to radius ratio of the gallery cation. This suggests that the cations rather than the protons are the main current carriers. The frequency dependence is measured and discussed.