R. Leboda

Aqueous suspension of fumed oxides: particle size distribution and zeta potential

Abstract Comparative analysis of the results of the investigations, which were performed by means of photon correlation spectroscopy (PCS) and electrophoresis, of aqueous suspensions of individual fumed oxides (SiO2, TiO2, Al2O3), binary X/SiO2 (X=TiO2, Al2O3, GeO2, C) and ternary Al2O3/SiO2/TiO2, fumed oxides modified by organic and organosilicon compounds or interacting with dissolved polymers, surfactants, etc., allows us to deeper understand the dependencies of structural and other characteristics of the dispersions on features of primary particles, their morphology, the nature of the surfaces of unmodified and modified individual and mixed oxides varied due to the synthetic techniques and the concentrations of the oxide phases or modifiers, concentrations of oxides in the liquid media, adsorption of soluble polymers, influence of surfactants, polyelectrolytes, inert electrolytes, etc. Some results related to chemically similar oxides but prepared by using other synthetic methods or data obtained by means of other experimental methods, e.g. DRS, TSD, NMR, etc., were used only in minor amounts in order to elucidate some aspects of the aqueous suspensions of fumed oxides studied by using PCS and electrophoresis methods. The main reasons of non-linear dependencies of the particle size distributions (PSD) and ζ potential on the concentration of fumed oxides in the aqueous suspensions, on the amounts of the guest X phases in X/SiO2 or the modifiers, on the pH values, etc., can be (a) a large polydispersity of PSD of both unmodified and modified fumed oxides linked with a four-stepwise structural hierarchy of fumed oxide particles, which can keep (or rearrange) in the suspensions despite different pretreatments of initial silica, X/SiO2 or their dispersions; and (b) a significant non-uniformity of the surfaces of mixed oxides due to features of the grafted or dopant phase distribution on/in the matrix depending on the guest oxide concentrations and the synthetic techniques.

Application of 1H NMR spectroscopy method for determination of characteristics of thin layers of water adsorbed on the surface of dispersed and porous adsorbents

The paper presents 1H NMR spectroscopy as a perspective method of the studies of the characteristics of water boundary layers in the hydrated powders and aqueous dispergated suspensions of the adsorbents. The method involves measurements of temperature dependence proton signals intensity in the adsorbed water at temperatures lower than 273 K. Free energy of water molecules at the adsorbent/water interface is diminished due to the adsorption interactions causing the water dosed to the adsorbent surface freezes at T < 273 K. Thickness of a non-freezing layer of water can be determined from the intensity of the water signal of 1H NMR during the freezing-thawing process. Due to a disturbing action of the adsorbent surface, water occurs in the quasi-liquid state. As a result, it is observed in the 1H NMR spectra as a relatively narrow signal. The signal of ice is not registered due to great differences in the transverse relaxation times of the adsorbed water and ice. The method of measuring the free surface energy of the adsorbents from the temperature dependence of the signal intensity of non-freezing water is based on the fact that the temperature of water freezing decreases by the quantity which depends on the surface energy and the distance of the adsorbed molecules from the solid surface. The water at the interface freezes when the free energies of the adsorbed water and ice are equal. To illustrate the applicability of the method under consideration the series of adsorption systems in which the absorbents used differed in the surface chemistry and porous structure. In particular, the behaviour of water on the surface of the following adsorbents is discussed: non-porous and porous silica (aerosils, silica gels); chemically and physically modified non-porous and porous silica (silanization, carbonization, biopolymer deposition); and pyrogeneous Al2O3 and aluminasilicas. The effect of preliminary treatment of the adsorbent (thermal, high pressure, wetting with polar and non-polar solvents) on the characteristics of the structurized water layers was discussed. The influence of the adsorbent porous structure on the free energy of the adsorbed water was also studied. The discussion of the obtained results was made.

Carbon-mineral adsorbents — new type of sorbents? Part I. The methods of preparation

Abstract The methods of preparation of carbon-mineral adsorbents produced on the basis of typical sorbents (e.g. silica gels, alumina, titanum oxide, Celite 545) and other sorbents important in industry (mainly aluminosilicates and natural silicates) are described. Among the methods of preparation have been distinguished mechanical mixing, incorporation, carburization of organic substances bonded chemically or physically with the surface of mineral adsorbent, as well as deposition of the carbon on catalyst and adsorbent surfaces during different adsorption and catalysis processes. The source of starting materials for preparation of complex adsorbents has been shown. The thermodynamic bases of the process of formation of carbon deposits are also discussed.

Photon correlation spectroscopy investigations of proteins.

Physical principles of photon correlation spectroscopy (PCS), mathematical treatment of the PCS data (converting autocorrelation functions to distribution functions or average characteristics), and PCS applications to study proteins and other biomacromolecules in aqueous media are described and analysed. The PCS investigations of conformational changes in protein molecules, their aggregation itself or in consequence of interaction with other molecules or organic (polymers) and inorganic (e.g. fumed silica) fine particles as well as the influence of low molecular compounds (surfactants, drugs, salts, metal ions, etc.) reveal unique capability of the PCS techniques for elucidation of important native functions of proteins and other biomacromolecules (DNA, RNA, etc.) or microorganisms (Escherichia coli, Pseudomonas putida, Dunaliella viridis, etc.). Special attention is paid to the interaction of proteins with fumed oxides and the impact of polymers and fine oxide particles on the motion of living flagellar microorganisms analysed by means of PCS.

Driving forces of conformational changes in single-layer graphene oxide

The extensive oxygen-group functionality of single-layer graphene oxide proffers useful anchor sites for chemical functionalization in the controlled formation of graphene architecture and composites. However, the physicochemical environment of graphene oxide and its single-atom thickness facilitate its ability to undergo conformational changes due to responses to its environment, whether pH, salinity, or temperature. Here, we report experimental and molecular simulations confirming the conformational changes of single-layer graphene oxide sheets from the wet or dry state. MD, PM6, and ab initio simulations of dry SLG and dry and wetted SLGO and electron microscopy imaging show marked differences in the properties of the materials that can explain variations in previously observed results for the pH dependent behavior of SLGO and electrical conductivity of chemically modified graphene-polymer composites. Understanding the physicochemical responses of graphene and graphene oxide architecture and performing selected chemistry will ultimately facilitate greater tunability of their performance.

Estimation of the pore-size distribution function from the nitrogen adsorption isotherm. Comparison of density functional theory and the method of Do and co-workers

A comparative analysis of the results for the estimation of the pore-size distribution based on the methods of Do and co-workers (ND) and density functional theory (DFT) is given. A new algorithm (ASA, Adsorption Stochastic Algorithm) is adopted and it is shown that this algorithm can be successfully applied for the determination of the PSD curve from the ND method. The obtained results show that, generally, the ND and DFT methods lead to almost the same PSD curves and this similarity is observed for carbons of different origin and possessing different pore structures. However, if the contribution of micropores to the PSD increases, the differences in the fit of ND and DFT to the experimental data are more pronounced.

Carbon-mineral adsorbents — new type of sorbents part II. Surface properties and methods of their modification

Abstract Chemical structure and morphology of carbon deposits contained in complex carbon-mineral adsorbents prepared by pyrolysis of different substances on the surface of mineral adsorbents such as silica gels, aluminium oxide, aluminosilicates, porous glasses, etc. have been discussed. The morphology of the carbon deposit depends on the course of the pyrolysis reaction and determines the surface properties of complex adsorbents and the possibilities of their practical utilization as sorbents and catalysts. The methods of modification of adsorption properties of such adsorbents prepared on technical and laboratory scales were also presented. Thermal properties of carbon-mineral adsorbents in atmospheres of different gases were also discussed.

pH-driven physicochemical conformational changes of single-layer graphene oxide

Single-layer graphene oxides (SLGOs) undergo morphological changes depending on the pH of the system and may account for restricted chemical reactivity. Herein, SLGO may also capture nanoparticles through layering and enveloping when the pH is changed, demonstrating potential usefulness in drug delivery or waste material capture.

Effect of calcium catalyst loading procedure on the porous structure of active carbon from plum stones modified in the steam gasification process

Abstract The effect of mode of calcium catalyst deposition on the active carbon (produced from plum stones) surface on the formation of micro- and mesoporous structures during gasification of the carbon with steam as well as on the kinetics of this process was investigated. The catalyst (1.44%) was deposited by conventional impregnation or ion exchange. Partial gasification of the modified carbon in the absence of catalyst causes significant increase of micropore volume and small increase of mesopore surface area with the increase of burn-off degree. On the contrary, gasification of the samples in the presence of calcium catalyst causes the decrease of sorption capacity of micropores and the significant increase of mesopore surface. The above changes are more effective for the samples containing the catalyst deposited by ion-exchange. Micropore capacity and surface area of mesopores were determined by means of the αs-method. The adsorption process on the heterogeneous solids was described by means of integral equation including various local isotherms. This equation has been solved by the regularization method. On the basis of this method changes in the structural parameters were estimated. The activation energies of individual reactions were determined. The mechanism of gasification process as well as the selection of proper concentration of steam optimum from the kinetic viewpoint have been discussed.

Investigations of adsorbent heterogeneity by gas chromatography : II. Evaluation of the energy distribution function

Abstract The second part is devoted to the application of gas chromatographic data in evaluating the differential distribution of adsorption energies on heterogeneous surfaces. To this purpose we have chosen Hobsons method for evaluating the energy distribution function. It is shown that the function VNVN(p), obtained directly in gas chromatography, is extremely useful in evaluating the energy distribution function.