Showing papers in "Colloid Journal in 2012"

Determination of nanoparticle sizes by X-ray diffraction

Abstract: Different procedures for analysis of particle sizes by the X-ray diffraction method are compared by the example of nanoparticles of nickel and iron(3+) oxide (Fe2O3). A modified Warren-Averbach method is proposed for the analysis of the X-ray diffraction line profile based on the approximation by the Voigt function, which yields stable solutions, and the efficiency of the method is shown. The analysis within the frame-work of the Warren-Averbach method makes it possible to restore the distribution function of nanoparticles (crystallites) over true diameters, which satisfactorily correlates with electron microscopy data. The applicability of the Warren-Averbach method to the estimation of crystallite sizes by the analysis of a single diffraction line is substantiated. The range of the applicability of the Scherrer, Williamson-Hall, Warren-Averbach, and modified Warren-Averbach methods to the substructure analysis by the X-ray diffraction is determined as depending on the method of nanostructure formation.

The standard free energy of surfactant adsorption at air/water and oil/water interfaces: Theoretical vs. empirical approaches

Abstract: The standard free energy of surfactant adsorption represents the work of transfer of a surfactant molecule from the bulk of solution to an infinitely diluted adsorption layer. This quantity can be determined by non-linear fits of surface-tension isotherms with the help of a theoretical model of adsorption. Here, the models of Frumkin, van der Waals and Helfand-Frisch-Lebowitz are applied, and the results are compared. Irrespective of the differences between these models, they give close values for the standard free energy. The results from the theoretical approach are compared with those from the most popular empirical approach. The latter gives values of the standard free energy, which are considerably different from the respective true values, with c.a. 10 kJ/mol for nonionic surfactants, and with c.a. 20 kJ/mol for ionic surfactants. These differences are due to contributions from interactions between the molecules in dense adsorption layers. It is concluded that the true values of the standard free energy can be determined with the help of an appropriate theoretical model. For the processed sets of data, the van der Waals model gives the best results, especially for the determination of the standard adsorption enthalpy and entropy from the temperature dependence of surface tension. The results can be useful for the development of a unified approach to the thermodynamic characterization of surfactants.

Regularities and mechanisms of the Rehbinder’s effect

Abstract: The fundamental regularities and contemporary concepts of the nature of Rehbinder’s effect, i.e., adsorption-induced reduction of strength, are reviewed. Experimental data on the effect of adsorption-active environment on the fracture of solids, the strength of which is controlled by different interatomic bonds, are generalized. The reasons for the chemical selectivity and the influence of solid supramolecular structures and test conditions on the character of the effect are discussed.

Study of polyol synthesis reaction parameters controlling high yield of silver nanocubes

Abstract: The influence of the parameters and conditions of sodium sulfide-induced reaction of polyol synthesis of silver nanoparticles on the yield of cube-shaped particles and the optical properties of colloids is studied. The protocol proposed by Skrabalak et al. for the synthesis of nanocubes in small volumes (Nature Protocols, 2007, vol. 2, p. 2182) is taken as an initial variant for optimization. The effects of the reagent concentrations, degree of ethylene glycol oxygenation, the presence of impurities, reaction time, and temperature are studied. Suspensions containing nanoparticles with different shapes and sizes, including polydisperse particles of irregular shapes, silver nanocubes with a yield of 0 to 97%, nanoprisms, and nanorods, can be produced by varying the synthesis parameters. The key parameters controlling the yield of nanocubes are the degree of ethylene glycol oxygenation and the presence of trace amounts of ions of other metals (not silver). It is established that variations in the reaction time make it possible to vary the sizes of nanocubes in the range of 30–60 nm. Suspensions with high contents of cube-shaped particles are shown to exhibit three maxima in the plasmon extinction resonance spectrum at wavelengths of 350, 390, and, depending on the particle size, 435–470 nm.

Dynamic light scattering study of cetyltrimethylammonium bromide aqueous solutions

Abstract: Cetyltrimethylammonium bromide (CTAB) aqueous solutions are studied by dynamic light scattering method in a wide concentration range covering the first and second critical micelle concentrations (CMC1 and CMC2, respectively). Nonmonotonic and ambiguous behavior of diffusion coefficients D with an increase in concentrations above CMC1 is revealed. An increase in the D values in the first decade of CTAB concentration above CMC1 agrees with known published data for aqueous solutions of ionic surfactants. It is shown that an increase in the ionic strength of solution with the addition of KBr leads to a decrease in the positive slope of the dependence of diffusion coefficients on CTAB concentration up to zero at 0.05 M KBr. Two relaxation processes corresponding to large and small D values are simultaneously observed in micellar solutions, beginning with 0.03 M CTAB concentration. The data obtained are compared with published data, as well as with the results of viscosity measurements. The performed analysis indicates that the observed relaxation processes are explained by the coexistence of spherical and nonspherical micelles. It is established that micelles acquire a cylindrical shape at CTAB concentrations ranging from 0.2 to 0.25 M. Hydrodynamic radii and lengths of micelles are calculated.

Micellization in solutions of ionic liquids

Abstract: This review is devoted to the problem of aggregation in solutions of ionic liquids (ILs) and the results of relevant studies published in recent years To a great extent, this problem remains urgent, because the ability to self-organization extends the possibilities of the practical application of ILs The fields of IL application in which their amphiphilicity is of importance in connection with widening the spectrum of objects under investigation are discussed The results of studying the aggregation behavior of different systems are briefly considered, including IL solutions in water, water-organic, and organic solvents; aqueous solutions of IL-classical surfactant mixtures; and solutions in which one IL plays the role of a solvent, while another IL or a classical surfactant serves as an amphiphilic solute Some experimental results are analyzed, and thermodynamic aspects of micellization and problems of molecular-thermodynamic simulation are discussed

Computer simulations of micellar systems

Abstract: Works published during the last decade devoted to simulation of micellar solutions of different surfactants are reviewed. The main attention is focused on studying the local structure of spherical and cylindrical micelles and their surface layers in solutions, as well as problems relevant to self-aggregation kinetics.

Study of Cryostructuring of Polymer Systems. 33. Effect of Rate of Chilling Aqueous Poly(vinyl alcohol) Solutions during Their Freezing on Physicochemical Properties and Porous Structure of Resulting Cryogels

Abstract: Poly(vinyl alcohol) cryogels are obtained and investigated. The cryogels are prepared by freezing an initial polymer solution (100 g/L) at chilling rates of 1.85, 0.3, 0.03, or 0.003°C/min followed by defrosting the frozen (−20°C/12 h) preparations at heating rates of 0.3, 0.03, or 0.003°C/min. It is shown that a noticeable influence of the chilling rate on the rigidity and heat endurance of the gel matrix is observed at very slow regimes of chilling (on the order of thousandths of degree centigrade per minute). One of the reasons for a reduction in the rigidity of resulting cryogels and the fusion enthalpy of the nodes in their supramolecular networks is the formation of a homo-phase hydrogel during the long-term exposure of the concentrated polymer solution to low temperatures prior to the onset of ice crystallization. The effect of the regime of chilling the polymer solution being frozen on the porous structure of cryogels resulting from the thawing of the preparations is ambiguous due to the multifactor character of this effect; therefore, in many cases, well-defined structure-property correlations are not observed.

Micellization in sodium deoxycholate solutions

Abstract: NMR self-diffusion, tensiometry, and measurement of solubilization capacity are employed to comparatively study micellization in aqueous solutions of a facial amphiphilic compound, sodium deoxycholate (NaDC), and a conventional micelle-forming sodium dodecyl sulfate. Based on the two-state model, which is commonly used to analyze the data of NMR diffusometry, a method is proposed for determining variable sizes of NaDC micelles. It is shown that, in the concentration range from the critical micelle concentration to 0.1 M, the sizes of NaDC micelles monotonically increase. At comparable sizes of molecules of the examined surfactants, NaDC micelles are characterized by noticeably smaller aggregation numbers and solubilization capacity than sodium dodecyl sulfate due to the rigid structure of NaDC molecules, their facial amphiphilicity, and a low value of hydrophilic-lipophilic balance.

Effect of the magnetic field on the hydrodynamic permeability of a membrane

Abstract: The present paper concerns the influence of the magnetic field on the permeability of a membrane of solid cylindrical particles covered with porous layer. Here, we have considered the flow along the axis of cylinder and the alignment of uniform magnetic field is assumed to be perpendicular to the axis. The Brinkman equation is used for flow through porous region and Stokes equation is used for flow through clear fluid region. To model flow through assemblage of particles, cell model technique has been used i.e. the porous cylindrical shell is assumed to be confined within a hypothetical cell of same geometry. The stress jump condition has been employed at the fluid-porous interface and all four alternative conditions Happel, Kuwabara, Kvashnin and Mehta-Morse/Cunningham are used at the hypothetical cell. Effect of the Hartmann number on the hydrodynamic permeability of the membrane is discussed.

Rheological peculiarities of concentrated suspensions

Abstract: The rheological properties of concentrated suspensions of metal oxides dispersed in transformer oil, which are used as electrorheological fluids, are systematically studied. Colloidal particles have intermediate sizes between nano- and microsized scales. Low-amplitude dynamic measurements show that the storage moduli of the examined suspensions are independent of frequency and these materials should be considered as solidlike elastic media. The storage modulus is proportional to the five-powdered particle volume concentration. At the same time, a transition through an apparent yield stress with a reduction in the viscosity by approximately six orders of magnitude is distinctly seen upon shear deformation. The character of the rheological behavior depends on the regime of suspension deformation. At very low shear rates, a steady flow is possible; however, upon an increase in the rate, an unsteady regime is realized with development of self-oscillations. When constant shear stresses are preset, in some range of stresses, thickening of the medium takes place, which can also be accompanied by self-oscillations. In order to gain insight into the nature of this effect, measurements are performed for samples with different volume/surface ratios, which show that, in some deformation regimes, suspension is separated into layers and slipping occurs along a low-viscosity layer with a thickness of several dozen microns. Direct observations show a distinct structural inhomogeneity of the flow. The separation and motion of layers with different compositions explain the transition to the flow with the lowest apparent Newtonian viscosity. Thus, the deformation of concentrated suspensions is associated with self-oscillations of stresses and slipping along a low-viscosity interlayer.

Adsorption of diazinon and fenitrothion on nanocrystalline alumina from non-polar solvent

Abstract: Nanocrystalline γ-alumina was prepared by the sol-gel method from aluminum sec-butoxide and, after characterization by X-ray diffraction and transmission electron microscopy, was used as an adsorbent for two common organophosphorus pesticides, diethoxy-[(2-isopropyl-6-methyl-4-pyrimidinyl)oxy]-thioxophosphorane (diazinon) and dimethoxy-(3-methyl-4-nitrophenoxy)-thioxophosphorane (fenitrothion). Absorption of these pesticides from hexane was monitored by UV-vis and 31P NMR spectroscopies. Adsorption of the aforementioned pesticides by nanocrystalline alumina prepared by the sol-gel method and calcined at 500°C and 800°C was compared with their adsorption by micron-size commercial alumina. Sol-gel derived nanocrystalline alumina showed significantly higher activity than commercial alumina in the destructive adsorption of diazinon and fenitrothion. The bonding mode of pesticides over nanocrystalline and commercial γ-aluminas was investigated by infrared spectroscopy.

Gemini surfactant-nonionic polymer mixed micellar systems

Abstract: Tensiometry, dynamic light scattering, and optical spectrophotometry are employed to quantitatively characterize the aggregation behavior of a gemini (dicationic) surfactant in aqueous solutions in the presence of poly(ethylene glycol) with a molecular mass of 20000 Da or a Pluronic F127 block copolymer with a molecular mass of 12600 Da. It is shown that the formation of surfactant-polymer mixed aggregates decreases the positive surface potential of gemini surfactant micelles and is one of the main reasons for weakening of the micellar catalytic effect in hydrolysis of carboxylic acid esters.

Collective interactions in the mechanism of adhesion of condensed phase nuclei to a crystal surface. 1. Spatial organization

Abstract: Self-organization into domains with spontaneous polarization is revealed for an ensemble of water molecules occurring in a contact layer on a defectless polarizable crystal surface. These domains are the sources for specific heteropolarization interactions of condensed phase nuclei with a substrate. The formation of a spatially nonuniform structure is energetically advantageous due to a nonlinear dependence of polarization energy on field strength. Domain sizes equal to several nanometers are governed by the competition between the energy advantage resulting from the coalescence of the domains and the entropy gain caused by their disintegration into smaller fragments. The forces of spontaneous mutual polarization between an adsorbed film divided into domains and a polarized substrate enhance the adhesion to the surface and markedly affect the adsorption mechanism. Computer simulation of the domain formation in a film of water molecules adsorbed on the surface of crystalline silver iodide particles is implemented by the Monte Carlo method with the summation of the long-range electrostatic interactions by means of the Fourier expansion of the field potential. Comparative analysis of the collective behavior of molecules, which underlies the layer-by-layer nucleation, and the indirect signs of domain formation is performed on the basis of experimental data on polarized surfaces with a hexagonal crystalline structure.

Self-organization in the chitosan-clay nanoparticles system regulated through polysaccharide macromolecule charging. 2. films

Abstract: Scanning and transmission electron microscopy and dynamic rheology are used to thoroughly characterize the morphology and mechanical properties of bionanocomposite hydrogel prepared by mixing cationic chitosan with negatively charged particles of a synthetic clay (saponite) followed by gradual increasing of the charge of chitosan macromolecules by decreasing the pH of a medium. Gelation of the system is found to be due to the formation of a fibrillar three-dimensional network structure. It is shown that the size fibrils, the density of the network structure, and its mechanical properties are determined by the concentration ratio between the polysaccharide and saponite particles.

Supramolecular systems based on cationic surfactants and amphiphilic macrocycles

Abstract: The effect of the structure of mono- and dicationic surfactants and amphiphilic calixarenes on their aggregation properties, aggregate morphology, and catalytic activity in reactions of nucleophilic substitution in esters is analyzed.

Orientation-induced redox transformations in Langmuir monolayers of double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] and multistability of its Langmuir-Blodgett films

Abstract: The spectral, electrochemical, and optical properties of Langmuir-Blodgett films (LBFs) and cast films from a solution of new double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] (Ce(R4Pc)2) are studied. Based on analysis of compression isotherms and quantum-chemical calculations, schemes of the organization of Ce(R4Pc)2 molecules at different states of its monolayers are proposed. Correlation dependences are determined in order to relate the optical and electrochemical characteristics of monolayers and LBFs of sandwich-type lanthanide phthalocyaninates to the ionic radii of their metal centers. The valent state of Ce ions in a monolayer-forming complex is determined, and a sequence of redox transformations occurring in LBF uppon appliance of a potential is proposed, one of the transformations being associated with the Ce3+/Ce4+ redox transition. Orientation-induced intramolecular electron transfer is revealed in the planar supramolecular system. It is shown that, during the formation of a monolayer from a Ce(R4Pc)2 solution, a tetravalent metal center passes to a trivalent state. Monolayer compression to a high surface pressure reverts the complex to the electronic state typical of the solution. The reversible transformations observed upon the monolayer compression result from intramolecular electron transfer from the 4f-orbital of Ce to the phthalocyanine ring and backwards. The high operation rate and the reversibility of switching between the stable states, which are determined by means of the surface plasmon resonance technique, upon a stepwise change in the electrode potential within the range of 200–850 mV may underlie the development of optoelectronic systems. With a large number of molecules in a stacking aggregate, changes in the distance between the decks of the complex that occur with changes in the oxidation level of the metal center can substantially modulate the sizes of molecular ensembles. A supramolecular device capable of performing mechanical work can be developed based on this effect.

Collective interactions in the mechanism of adhesion of condensed phase nuclei to a crystal surface. 2. Thermodynamic stability

Abstract: The Monte Carlo method is used to calculate, at the molecular level, the free energy, entropy, and the work of formation at an initial stage of nucleation of a condensed phase from water vapor on the surface of a solid crystalline silver iodide substrate. The pattern of the obtained dependences confirms the pronounced layer-by-layer character of the growth of nuclei and the thermodynamic stability of a molecular film formed at the contact with the substrate. An increased hydrophilicity of the substrate surface with respect to the first monomolecular layer is enhanced by the formation of regions of spontaneous polarization in the latter. The reasons for the thermodynamic advantage of the separation of the nucleus contact layer on the substrate into domains with different types of polarization are analyzed in terms of a lattice model. Computer simulation within the framework of the lattice model demonstrates that a rise in the polarizability of the substrate is accompanied by a continuous increase in the equilibrium sizes of the domains; moreover, the model predicts their strongly nonlinear dependence on both temperature and the polarizability of the substrate.

Copper Nanoparticles Synthesized by the Polyol Method and Their Oxidation in Polar Dispersion Media. The Influence of Chloride and Acetate Ions

Abstract: Copper nanoparticles have been synthesized from Cu(2+) acetate in the medium of dimethylformamide by the polyol method using hydrazine as a reductant and poly(vinylpyrrolidone) as a stabilizer. Optical spectroscopy has been employed to study the influence of chloride and acetate ions on the oxidation of copper nanoparticles by atmospheric oxygen in aqueous and dimethylformamide media, respectively. It has been established that the presence of ions of one of the aforementioned types in a colloidal solution significantly accelerates the oxidation of metal nanoparticles. In the case of acetate ions, copper oxide particles rapidly dissolve to form the corresponding salt. It has been shown that Cu nanoparticles can be repeatedly formed in such a solution by the addition of hydrazine.

A study of cryostructuring of polymer systems. 34. Poly(vinyl alcohol) composite cryogels filled with microparticles of polymer dispersion

Abstract: Macroporous filled and unfilled poly(vinyl alcohol) (PVA) cryogels are produced by cryogenic treatment (freezing at −20°C for 12 h followed by thawing at a rate of 0.03°C/min) of mixtures of an aqueous PVA solution and a full-component poly(vinyl acetate) (PVAc) dispersion or its individual components. The values of the elasticity modulus and fusion temperature are determined for obtained samples; their microstructure is studied by light microscopy of thin sections. It is shown that the effects that are induced by the incorporation of PVAc dispersion into the macroporous matrix of the PVA cryogel are due to the presence of both a discrete phase, i.e., solid PVAc microparticles, and ingredients of the liquid phase of the PVAc dispersion, mainly, urea. Therewith, the dispersed particles themselves serve as a reinforcing filler, i.e., increase the rigidity and (to a lesser extent) heat endurance of the cryogel, while urea, which possesses chaotropic properties and hinders the intermolecular hydrogen bonding of PVA chains, reduces the rigidity and heat endurance of the composites. As a result, the total effect is determined by the competition of differently directed influences of these components of PVAc dispersion and depends on its concentration in the resulting filled cryogel. It is also shown that PVAc microparticles are mainly entrapped in the gel phase of the macroporous matrix and form necklacelike aggregates, the cross-sectional areas and lengths of which depend on the degree of composite filling.

Colloidal catalysts based on iron(III) oxides. 1. Decomposition of hydrogen peroxide

Abstract: The catalytic activity of a colloidal catalyst based on iron(III) oxides in decomposition of H2O2 is studied. The catalyst is obtained by hydrolysis followed by peptization of FeCl3 · 6H2O salt in water in the presence of 1% ethanol. The structure, composition, and size of colloidal particles of the catalyst are studied by the methods of Mossbauer spectroscopy, X-ray fluorescence, X-ray diffraction, and transmission electron microscopy. The obtained catalyst is based on α-Fe2O3 crystals with an admixture of other crystalline structures of iron oxides and carbon-containing compounds. The activity of the catalyst with respect to H2O2 decomposition undergoes nonlinear and nonmonotonic variations and its particle size enlarges beginning from 1 to 3 nm with increasing initial concentration of FeCl3 · 6H2O. The catalyst obtained under optimal conditions exhibits high activity corresponding to the most efficient agents of H2O2 decomposition.

The development of the fundamental concepts of surface thermodynamics

Abstract: Author’s results concerning the most fundamental problems of the thermodynamics of surface phenomena are reviewed. The generalized Laplace-Young-Kelvin equation, phase rules, and Gibbs adsorption equations are presented. Analogs of Konovalov’s laws are describes as applied to surface phenomena. The surface tension dualism, the Gibbs equation for adsorption on solid surfaces, and the phase equilibrium condition for a soluble nanoparticle are explained. The general mechanochemical approach, chemical affinity tensor, and the discovery of the mechanochemical dissolution effect are characterized. A novel approach to the monolayer state equation is formulated based on an excluded area. The problems of nucleation and the theory of surface separation are reperted.

On the Theory of Birefringence in Magnetic Fluids

Abstract: The influence of homogeneous correlations between the positions and orientations of ferroparticles on the effect of light birefringence in magnetic fluids has been studied. It has been demonstrated that, for typical magnetic fluids, the optical effects associated with the homogeneous correlations can be stronger than the effects caused by elongated primary aggregates formed at the stage of ferrofluid synthesis, as well as heterogeneous chains resulting from magnetic attraction between the largest particles of a magnetic fluid.

A self-similar regime of droplet growth with allowance for the Stefan flux and dependence of diffusion coefficient on vapor-gas medium composition

Abstract: An analytical self-similar description is formulated for the problem of nonstationary diffusion droplet growth in a vapor-gas medium with allowance for the Stefan flux of the medium, themotion of the surface of a growing droplet, and an arbitrary concentration dependence of the diffusion coefficient of a condensing vapor in the medium. Analytical expressions are found for the diffusion profile of condensing vapor concentration and for the droplet growth rate at a constant diffusion coefficient of the vapor and at a coefficient linearly dependent on vapor concentration. The combined allowance for the Stefan flux of the medium, the motion of the surface of the growing droplet, and the dependence of the vapor diffusion coefficient in the medium on the vapor concentration is shown to result in renormalization of the rate of droplet growth related to the stationary diffusion regime. At small deviations from the stationarity, the Stefan flux, nonstationary diffusion, and dependence of the diffusion coefficient on the vapor-gas mixture composition lead to corrections of the same order of magnitude.

Asymmetric Transmembrane Transfer Caused by a Difference in Adsorption Characteristics at Interfaces

Abstract: A surface adsorption mechanism is proposed for the effect of asymmetric diffusion permeability of a modified membrane. A model is developed to describe diffusion transfer through the modified membrane with regard to the finiteness of the rate of adsorption on external membrane surfaces. Expressions are derived for diffusion fluxes as functions of transfer direction, and asymmetry coefficient is calculated for the diffusion permeability of the membrane. It is shown that the difference in the kinetic properties of the external surfaces of the membrane, which results from the modification of one of the surfaces, may induce the effect of asymmetric diffusion permeability.

The effect of gas surface diffusion on the asymmetric permeability of two-layer porous membranes

Abstract: The permeability is calculated for two-layer membranes composed of porous layers with nano- and microsized pores. It is found that, in nanosized pores, the gas transfer occurs in the free-molecular regime in the pore volume and via diffusion along the adsorption layer. The degree of adsorption layer filling is determined from the Langmuir isotherm. The dependence of the diffusion coefficient in the adsorption layer on the degree of surface coverage is taken into account. The transfer in the microsized pored is described in a hydrodynamic regime. The values of the membrane permeability are determined at different orientations with respect to the direction of a gas flow. It is shown that the difference between the permeability values may be as large as 60%.

The problem of critical damping in nanofriction

Abstract: The progress that has been made in describing atomic friction in terms of mechanical instabilities of the stick-slip type masks the role of damping and makes researchers ignore the true mechanisms of dissipation. This study shows that there is a necessary condition for the realization of regular stick-slips occurring with atomic periodicity, which are observed in friction force microscopy (a variant of atomic force microscopy) experiments. This condition lies in the existence of a direct relation between the rate of dissipation and the characteristic frequency of a measuring system, i.e., its stiffness and mass (“critical damping”). The conclusion seems to be paradoxical; it indicates a nontrivial dissipation mechanism, which, according to the proposed explanation, essentially depends on memory effects.

Equilibrium sorption of hexavalent chromium from aqueous solution using synthetic hematite

Abstract: Hematite sample prepared by transformation of ferrihydrite was used to remove Cr(VI) from aqueous solution. Effects of initial concentration, contact time, pH, ionic strength, and temperature were investigated. The data were fitted to Langmuir, Freundlich, Dubinin-Kaganer-Radushkevich and Temkin models of adsorption. The results showed the best fit to the Langmuir isotherm. The increase of pH as well as that of increasing ionic strength resulted in the decrease of adsoiption of Cr(VI). Two kinetic models namely pseudo-first order and pseudo-second order were used to fit the experimental data. The thermodynamic parameters were found, and the results indicated spontaneous and exothermic nature of the process.

Kinetics of nucleation and growth of metal nanoparticles in the presence of surfactants

Abstract: Numerical simulation has been performed for the process of nucleation and growth of nanoparticles in the presence of surfactants. Surfactant molecules are adsorbed on the surface of nanoparticles and decelerate their growth in supersaturated solutions. It has been assumed that nanoparticles are completely blocked after a certain degree of surface coverage is achieved, and they cease to grow. It has been demonstrated that, at low concentrations, surfactants influence the average size and the width of the size distribution of nanoparticles; i.e., the average particle size decreases and the distribution becomes narrower with the growth of surfactant concentration. At high concentrations, the effect of surfactants is more dramatic; namely, the particle size distribution becomes bimodal. At high surfactant concentrations, the periodic supply of a precursor, which serves as a source of monomers (metal atoms), may result in the formation of polymodal ensembles of the nanoparticles.

Comprehensive study of electrosurface properties of detonation nanodiamond particle agglomerates in aqueous KCl solutions

Abstract: The electrosurface properties of nanoporous agglomerates of detonation nanodiamond (DND) particles purified from acidic impurities by dialysis are comprehensively investigated. Acid-base potentiometric titration, laser Doppler electrophoresis, and conductometry are employed to measure the adsorption isotherms $$\Gamma _{H^ + } (pH)$$ and $$\Gamma _{OH^ - } (pH)$$ of potential-determining ions, as well as the dependences of surface charge density σ0, electrophoretic mobility u e, and specific conductivity K p of the agglomerates on the pH = 3.5–10.5 of aqueous 0.0001–0.1 M KCl solutions. The obtained adsorption isotherms indicate heterogeneity of the DND surface, i.e., the presence of different proton-donor and proton-acceptor surface functional groups. Computer simulation of the adsorption isotherms is carried out for a DND surface containing two types of functional groups, namely, acidic carboxyl (-COOH) and amphoteric hydroxyl (-COH) groups, the predominant content of which is confirmed by FTIR spectroscopy data. The optimal values are determined for the reaction constants of ionization of these groups. It is revealed that the effective conductivity of the porous agglomerates is one or two orders of magnitude higher than the conductivity of equilibrium solutions. Corresponding values of electrokinetic potential ξ are calculated as functions of pH and KCl concentration from the electrophoretic mobility of the agglomerates using different equations of electrophoresis theory. It is shown that use of the Miller formula, which takes into account the electromigration fluxes of ions and electroosmotic flows of solutions in pores of dispersed particles, yields more correct ξ potential values for DND agglomerates.