Effect of equilibrium fluctuations on microcrystalline particles on adsorption isotherms and reaction rates

The effect of the finiteness of face areas and equilibrium fluctuations on the adsorption isotherms of binary molecular mixtures and the rate of surface processes occurring on nanosized particles is studied. The adsorption process is considered in the grand canonical ensemble; the rates of elementary stages are calculated in the kinetic mode. The surface region on the face is found to contain a number of adsorption centers ranging from 10 to 10⁵. The effect of density fluctuations of adsorbed molecules on the partial adsorption isotherms and rate fluctuations is discussed. A calculation procedure for density fluctuations in heterogeneous microparticles with different faces and different binding energies of molecules with the surface is considered. It is shown that the greatest effect of density fluctuations is manifested at a low occupancy of each face of the particle.     

Equilibrium density fluctuations and the rate of a Langmuir-Hinshelwood type reaction on microcrystal particles

We investigate the effect of restricting the area of planes of microcrystals and equilibrium density fluctuations in components of binary mixtures on partial isotherms of the adsorption of binary mixtures of molecules and the rate of a surface reaction of the Langmuir-Hinshelwood type. Adsorption of components of mixture is considered in a large canonical assembly, and the rate of an elementary step is calculated in kinetic regime. The value of a section of the surface on a plane contains a number of adsorption centers in the range of 10 to 10⁵. The effect of the structure of a heterogeneous surface on the rate of the considered reaction is studied. The effect of the density fluctuations of adsorbed molecules on partial adsorption isotherms and fluctuations in the rate of reaction on heterogeneous surfaces is discussed. It is shown that the greatest effect of density fluctuations on the rate of a step is observed at low fillings of each plane of a particle and at the almost complete filling of a plane.     

Influence of size and functionality of polymeric nanoparticles on the adsorption behavior of sodium dodecyl sulfate as detected by isothermal titration calorimetry

Isothermal titration calorimetry was used to monitor the adsorption of the surfactant sodium dodecylsulfate (SDS) on different sized pure and carboxy functionalized polystyrene nanoparticles prepared by the mini-emulsion process. The ITC experiment gives, additionally to the CMC values, information about the interaction of the surfactant molecules to the particle’s surface due to the particle surface properties. The adsorption heat depends on the chemical composition of the polymer as well on the particle size. It also provides information about the surface coverage with surfactant and the number of additional adsorbed molecules per particle until full coverage by surfactant is obtained. The surfactant adsorption increases from 0.3 molecules per nm² for 50 nm to 8.5 molecules per nm² for carboxy functionalized particles with diameters larger than 160 nm. The area A _Surf-dens after the adsorption process gives information about the packing density of surfactant molecules on the particles in dependence of carboxy groups: an increasing number of carboxylic groups decreases the area occupied per SDS molecule. The adsorption process was also monitored by zeta potential measurements, where an increasing potential during the adsorption was detected.     

Density functional theory modeling of the adsorption of small analyte and indicator dye 9-(diphenylamino)acridine molecules on the surface of amorphous silica nanoparticles

The adsorption of small analyte molecules (H(2)O, NH(3), C(2)H(5)OH, and (CH(3))(2)CO) and an indicator dye, 9-(diphenylamino)acridine (DPAA), on the surface of amorphous silica particles is studied using electronic structure calculations at the DFT-D level of theory taking into account explicit corrections for van der Waals forces. Cluster models of three different types are used; two of them have been constructed using classical MD methods. The effect of particle size, local environment, and the choice of the exchange-correlation functional and basis set on the adsorption energies is studied, and adsorption energies are extrapolated to nanosized clusters. It is shown that the dye is more strongly bound to amorphous silica particles than the studied analyte molecules and that the energy of DPAA adsorption increases with the particle size, being at least twice as high as the energy of analyte adsorption for nanosized clusters. Electrostatic interactions play an important role in the adsorption of acridine dyes on the surface of silica nanoparticles.     

Structure of the CO bond on supported Pd particles: influence of size and surface state

We have investigated the surface of supported palladium particles by static secondary ion mass spectrometry (SSIMS). Pd particles were grown in situ on alumi na (oxide layer and sapphire surfaces) and stabilized by heating treatment. The particle size, density and crystallographic structure were determined in previous studies by transmission electron microscopy and diffraction (TEM and TED). Various ionic species are detected by SSIMS analysis which makes it possible to characterize the CO absorbed layer: Pd _n CO⁺ (n=1, 2) for molecular adsorption and Pd _n C⁺ for CO dissociation. The size dependence of the bonding state of CO (linear, bridge, ...) was monitored by: PdCO⁺/σ _n Pd _n CO⁺ signal ratio over various size particles (mean diameter in the 2–9 nm range). Investigations were performed as a function of CO coverage (adsorption at room temperature) and also under CO dissociation conditions: heating under CO atmosphere or CO+O₂ (catalysis). The data analysis shows that on clean Pd particles smaller than 3 nm the CO molecules give rise mainly to PdCO⁺ species. We have interpreted this result by the adsorption of CO on two palladium atoms, the carbon end being tightly bonded to a low coordination Pd atom and the oxygen end weakly bonded to a neighbour Pd atom. These couples of Pd atoms form the specific sites for CO dissociation, the density of which depends on the roughness of the particle surface.     

An XPS and STM study of the size effect in NO adsorption on gold nanoparticles

The effect of the gold particle size, temperature of the model gold catalyst, and NO pressure on the composition of the adsorption layer was studied by in situ XPS and STM methods. Adsorption of nitric oxide was carried out on gold nanoparticles with a mean size of 2?C7 nm prepared on the thin film surface of alumina. In high-vacuum conditions (P _NO ?? 10^?5 Pa), only atomically adsorbed nitrogen is formed on the surface of gold nanoparticles. At about 1 Pa pressure of NO and in the temperature range from 325 to 475 K, atomically adsorbed nitrogen coexists with the N₂O adsorption complex. The surface concentration of the adsorbed species changes with a change in both the mean gold particle size and adsorption temperature. The saturation coverage of the surface with the nitrogen-containing complexes is observed for the sample with a mean size of gold particles of 4 nm. The surface of these samples is mainly covered with atomically adsorbed nitrogen, the saturation coverage of adsorbed nitrogen of about ??0.6 monolayer is attained at T = 473 K. The change in the composition of the adsorption layer with temperature of the catalysts agrees with the literature data on the corresponding temperature dependence of the selectivity of N₂ formation observed in the catalytic reduction of NO with carbon monoxide on the Au/Al₂O₃ catalyst. The dependences of the composition of the adsorption layer on the mean size of Au nanoparticles (size effect) and temperature of the catalyst are explained by the sensitivity of NO adsorption to specific features of the gold surface.     

Equilibrium fluctuations in the theory of surface processes on microparticles

The question of the role of equilibrium fluctuations in the adsorption theory and kinetics of surface processes occurring on the particles of the nanometer size range is discussed. Differences are put forward that need to be introduced to the fluctuation theory of surface processes on microparticles and that generalize Hill’s approach to describing the thermodynamic properties of small systems. We show the importance of allowing for the discrete character of adsorption centers on the surfaces and their heterogeneity when describing adsorption isotherms and the rates of adsorption processes.     

Adsorption of anionic surfactants on positively charged polystyrene particles II

In the case of cationic polystyrene latex, the adsorption of anionic surfactants involves a strong electrostatic interaction between both the particle and the surfactant, which may affect the conformation of the surfactant molecules adsorbed onto the latex-particle surface. The adsorption isotherms showed that adsorption takes place according to two different mechanisms. First, the initial adsorption of the anionic surfactant molecules on cationic polystyrene surface would be due to the attractive electrostatic interaction between both ionic groups, laying the alkyl-chains of surfactant molecules flat on the surface as a consequence of the hydrophobic interaction between these chains and the polystyrene particle surface, which is predominantly hydrophobic. Second, at higher surface coverage the adsorbed surfactant molecules may move into a partly vertical orientation with some head groups facing the solution. According to this second mechanism the hydrophobic interactions of hydrocarbon chains play an important role in the adsorption of surfactant molecules at high surface coverage. This would account for the very high negative mobilities obtained at surfactant concentration higher than 5×10^–7 M. Under high surface-coverage conditions, some electrophoretic mobility measurements were performed at different ionic strength. The appearance of a maximum in the mobility-ionic strength curves seems to depend upon alkyl-chain length. Also the effects of temperature and pH on mobilities of anionic surfactant-cationic latex particles have been studied. The mobility of the particles covered by alkyl-sulphonate surfactants varied with the pH in a similar manner as it does with negatively charged sulphated latex particles, which indicates that the surfactant now controls the surface charge and the hydrophobic-hydrophilic character of the surface.Dedicated to the memory of Dr. Safwan Al-Khouri IbrahimPresented at the Euchem Workshop on Adsorption of Surfactants and Macromolecules from Solution, Åbo (Turku), Finland, June 1989     

Adsorption of carboxylate-modified gold nanoparticles on an octadecylamine monolayer at the air/liquid interface

Gold nanoparticles (Au NPs) were prepared and surface-modified by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups (MSA-Au). Octadecylamine (ODA) was used as a template monolayer to adsorb the Au NPs dispersed in the subphase. The effect of MSA concentration on the incorporation of Au NPs on the ODA monolayer and the relevant behavior of the mixed monolayer were studied using the pressure-area (pi-A) isotherm and transmission electron microscopy (TEM) observations. The experimental results showed that the adsorbed density of Au NPs is low without the surface modification by MSA. When MSA was added into the Au NP-containing subphase, the incorporation amount of Au NPs increased with increasing MSA concentration up to approximately 1 x 10-5 M for the particle density of 1.3 x 1011 particles/mL. With a further increase in the MSA concentration, the adsorbed particle density decreases due to competitive adsorption between the free MSA molecules and the MSA-Au NPs. It is inferred that free MSA molecules adsorb more easily than the MSA-Au NPs on the ODA monolayer. Therefore, an excess amount of MSA present in the subphase is detrimental to the incorporation of gold particles. The study on the monolayer behavior also shows that the pi-A isotherm of the ODA monolayer shifts right when small amounts of Au NPs or free MSA molecules are incorporated. However, when larger amounts of particles are adsorbed at the air/liquid interface, a left shift of the pi-A isotherm appears, probably due to the adsorption of ODA molecules onto the particle surface and the transferring of the particles from beneath the ODA monolayer to the air/water interface. According to the present method, it is possible to prepare uniform particulate films of controlled densities by controlling the particle concentration in the subphase, the MSA concentration, and the surface pressure of a mixed monolayer.     

Influence of Polydispersity on Adsorption of Nanoparticles

The adsorption of polydisperse, interacting nanoparticles is studied experimentally and discussed in terms of the random sequential adsorption model. Two kinds of polystyrene particles with different size variation (41+/-6 and 107+/-5 nm) were used in adsorption experiments at or close to saturation. The dried monolayer particle films were analyzed with scanning electron microscopy. Selective adsorption of smaller particles resulted in altered size distributions on the surface compared to that in solution. Varying the ionic strength was seen to influence the effective polydispersity of the particles. With increasing salt concentration there was a relative increase in the adsorption of smaller particles, resulting in a large shift toward smaller particle sizes in the size distribution on the surface. Polydispersity gave a slight increase in coverage at high salt concentrations and a decrease in the ordering of the particles on the surface. Copyright 2001 Academic Press.     

BSA adsorption on differently charged polystyrene nanoparticles using isothermal titration calorimetry and the influence on cellular uptake

BSA adsorption onto negatively and positively charged polystyrene nanoparticles was investigated. The nanoparticles were characterized in terms of particle size, zeta potential, surface group density, and morphology. The adsorption behavior of BSA on the particle surface, as a function of pH and overall charge of the particle, was studied using ITC. Different thermodynamic data such as enthalpy changes upon binding and stoichiometry of the systems were determined and discussed. The degree of surface coverage with BSA was calculated using the thermodynamic data. The cellular uptake of particles before and after BSA adsorption was studied using HeLa cells in the presence and absence of supplemented FCS in the cell culture medium.     

Specific features of luminescence of Q-CdS colloids with different sizes

The specific features of luminescence of colloidal solutions of Q-CdS with particles of different size and the regularities of luminescence quenching by quenchers of various nature were studied. The luminescence spectra of Q-CdS consist of several bands, which are shifted to the long-ware region as the particle size increases. The dependence of the integral quantum yield of luminescence on the particle size has a sharp maximum at a particle diameter of ?23Å. A Stem—Volmer-type equation including the adsorption isotherm of the quencher molecules on the surface of the Q-CdS colloidal particles was used to describe the regularities of luminescence quenching of Q-CdS colloidal solutions. The CdS particle size was found to affect the efficiency of luminescence quenching. The regularities of luminescence quenching depend both on the rate constant of electron transfer to the quencher molecules and on the ability of the quencher molecules to be adsorbed on the surface of the CdS colloidal particle.     

Effect of polyethylene oxide on the viscosity of dispersions of charged silica particles: interplay between rheology, adsorption, and surface charge

In this study a systematic investigation on the adsorption of polyethylene oxide (PEO) onto the surface of silica particles and the viscosity behavior of concentrated dispersions of silica particles with adsorbed PEO has been performed. The variation of shear viscosity with the adsorbed layer density, concentration of free polymer in the solution (depletion forces), polymer molecular weight, and adsorbed layer thickness at different salt concentrations (range of the electrostatic repulsion between particles) is presented and discussed. Adsorption and rheological studies were performed on suspensions of silica particles dispersed in solutions of 10⁻² M and 10⁻⁴ M NaNO₃ containing PEO of molecular weights 7,500 and 18,500 of different concentrations. Adsorption measurements gave evidence of a primary plateau in the adsorption density of 7,500 MW PEO at an electrolyte concentration of 10⁻² M NaNO₃. Results indicate that the range of the electrostatic repulsion between the suspended particles affects both adsorption density of the polymer onto the surface of the particles and the viscosity behavior of the system. The adsorbed layer thickness was estimated from the values of zeta potential in the presence and absence of the polymer and was found to decrease with decreasing the range of the electrostatic repulsive forces between the particles. Experimental results show that even though there is a direct relation between the viscosity of the suspension and the adsorption density of the polymer onto the surface of the particles, variation of viscosity with adsorption density, equilibrium concentration of the polymer, and range of the electrostatic repulsion cannot be explained just in term of the effective volume fraction of the particles and needs to be further investigated. Received: 15 February 2000/Accepted: 26 June 2000     

The Effect of Mechanical Treatment on Colloidal and Chemical Properties and Reactivity of Powdered Cellulose

The effect of mechanical activation on colloidal and chemical properties of aqueous dispersions of powdered cellulose (particle size, electrokinetic parameters, surface electrical properties, adsorption, etc.) and its solubility in a methylmorpholine N-oxide monohydrate–dimethyl sulfoxide mixture is studied. A rise in the number of mechanical treatment cycles increases the surface charge density, adsorbability, and solubility of cellulose, but decreases particle size and electrophoretic mobility, as well as the viscosity of cellulose solutions due to a reduction in the degree of cellulose polymerization, development of microporosity, and a rise in the internal surface area of polymer particles as a result of their disintegration.     

Influence of the layer charge and clay particle size on the interactions between the cationic dye methylene blue and clays in an aqueous suspension

The spectroscopic behavior of the dye MB in suspensions of different clays have been used for evaluating layer charge density influence on the adsorption properties of the particles. The clays with higher charge density, like SAz-1 and SCa-3, promote a higher aggregation and do not show deaggregation at longer times, so that practically only the aggregate peak at approximately 570 nm is observed, without any change with time. This is due to, on one side, the larger particle size that decreases the surface area available for adsorption. Additionally, the clay layers will be held together more tightly, avoiding the migration of the dye to the interlamellar region. On the other hand, SWy-1, having a lower charge density, shows a completely different behavior. The dye molecules, initially adsorbed as aggregates on the outer surface of the clay, deaggregate to form monomers that migrate to the interlamellar spaces, giving rise to absorption bands at 670 and 760 nm. Experiments using Ca-exchanged SWy-1, variation of the ionic strength by addition of salt, and the use of different size fractions of the clays confirm the finding that the main factor ruling the adsorption behavior of the probe is the size of the clay particles.     

Insights into the effect of combustion-generated carbon nanoparticles on biological membranes: a computer simulation study

Classical molecular dynamics simulations of atomistic models of combustion-generated carbon nanoparticles and lipid bilayers have been performed to explore their possible structural, dynamical, and thermodynamic effects on biological membranes. The DREIDING generic force field is used for the carbonaceous nanoparticles of different morphologies, as produced from combustion sources, and the united atom model was employed for the dimyristoylphosphatidylcholine (DMPC) bilayer. It is observed that particle shape and structure have significant effects on solvation, mobility, adsorption, and permeation behavior of the particles. While combustion-generated carbon nanoparticles with an aspect ratio close to unity prefer to stay near the membrane center, precursors with other shapes mostly reside within the hydrocarbon tail region of the membrane. Carbon nanoparticles are not trapped in a local region even inside the membranes but move freely with a speed depending on their molecular weight. The adsorption of the particles on the surface of the biological membrane is comparable to thermal fluctuations because the weak segregation effect by water molecules is the main driving force to the adsorption behavior. The bigger the precursors are, the stronger they are bound to the membrane surface. The presence of combustion-generated nanoparticles inside the membrane perturbs local lipid density by pushing the neighboring lipid molecules away from the nanoparticles. This, coupled with thermal fluctuations, can induce an instantaneous membrane pore to allow water protrusion. From the umbrella sampling method, the potential of mean force for the permeation of carbona nanoparticles into the bilayer was also obtained. Surprisingly, elongated particles have a free energy barrier an order of magnitude smaller compared with more round ones. In addition, the round carbon nanoparticles showed strong hysteresis due to the local trapping of water molecules. Although the carbon soot precursors studied in this work are not the well-known carbon nanoparticles such as fullerenes or carbon nanotubes, the qualitative features of this study may be applicable to them as well.     

Control of the PEO chain conformation on nanoparticles by adsorption of PEO-block-poly(L-lysine) copolymers and its significance on colloidal stability and protein repellency

The physical adsorption of PEO(n)-b-PLL(m) copolymers onto silica nanoparticles and the related properties of poly(ethylene oxide) (PEO)-coated particles were studied as a function of the block copolymer composition. Copolymers adopt an anchor-buoy conformation at the particle surface owing to a preferential affinity of poly(L-lysine) (PLL) blocks with the silica surface over PEO blocks when a large excess of copolymer is used. The interdistance between PEO chains at particle surface is highly dependent on the size of PLL segments; a dense brush of PEO is obtained for short PLL blocks (DP = 10), whereas PEO chains adopt a so-called interacting "mushroom" conformation for large PLL blocks (DP = 270). The size of the PEO blocks does not really influence the copolymer surface density, but it has a strong effect on the PEO layer thickness as expected. Salt and protein stability studies led to similar conclusions about the effectiveness of a PEO layer with a dense brush conformation to prevent colloidal aggregation and protein adsorption. Besides, a minimal PEO length is required to get full stabilization properties; as a matter of fact, both PEO(45)-b-PLL(10) and PEO(113)-b-PLL(10) give rise to a PEO brush conformation but only the latter copolymer efficiently stabilizes the particles in the presence of salt or proteins.     

Spectra of the liquid cluster surface thermal fluctuations

Classification of cluster particles is proposed that introduces three particle types: the internal particles, surface particles, and virtual chains of particles. Thermal fluctuations of a surface passing through the surface particles of a Lennard-Jones liquid cluster are studied using a molecular dynamics simulation. It is shown that for large clusters, the Fourier spectral amplitude of these fluctuations decays faster than 1q, where q is the wave number. The frequency Fourier spectrum shows an overdamped system behavior, which is the evidence for the absence of thermal capillary waves for clusters comprising less than 10(5) particles. The time-averaged cluster density profile is given by an error function with the width parameter diverging as the logarithm of the cluster size.     

Interactions and stability of silver nanoparticles in the aqueous phase: Influence of natural organic matter (NOM) and ionic strength

The rapid development of nanotechnology and the related production and application of nanosized materials such as engineered nanoparticles (ENP) inevitably lead to the emission of these products into environmental systems. So far, little is known about the occurrence and the behaviour of ENP in environmental aquatic systems. In this contribution, the influence of natural organic matter (NOM) and ionic strength on the stability and the interactions of silver nanoparticles (n-Ag) in aqueous suspensions was investigated using UV–vis spectroscopy and asymmetrical flow field-flow fractionation (AF⁴) coupled with UV–vis detection and mass spectrometry (ICP-MS). n-Ag particles were synthesized by chemical reduction of AgNO₃ with NaBH₄ in the liquid phase at different NOM concentrations. It could be observed that the destabilization effect of increasing ionic strength on n-Ag suspensions was significantly decreased in the presence of NOM, leading to a more stable n-Ag particle suspension. The results indicate that this behaviour is due to the adsorption of NOM molecules onto the surface of n-Ag particles (“coating”) and the resulting steric stabilization of the particle suspension. The application of AF⁴ coupled with highly sensitive detectors turned out to be a powerful method to follow the aggregation of n-Ag particle suspensions at different physical–chemical conditions and to get meaningful information on their chemical composition and particle size distributions. The method described will also open the door to obtain reliable data on the occurrence and the behaviour of other ENP in environmental aquatic systems.     

A continuous DC-insulator dielectrophoretic sorter of microparticles

A lab-on-a-chip device is described for continuous sorting of fluorescent polystyrene microparticles utilizing direct current insulating dielectrophoresis (DC-iDEP) at lower voltages than previously reported. Particles were sorted by combining electrokinetics and dielectrophoresis in a 250 μm wide PDMS microchannel containing a rectangular insulating obstacle and four outlet channels. The DC-iDEP particle flow behaviors were investigated with 3.18, 6.20 and 10 μm fluorescent polystyrene particles which experience negative DEP forces depending on particle size, DC electric field magnitude and medium conductivity. Due to negative DEP effects, particles are deflected into different outlet streams as they pass the region of high electric field density around the obstacle. Particles suspended in dextrose added phosphate buffer saline (PBS) at conductivities ranging from 0.50 to 8.50 mS/cm at pH 7.0 were compared at 6.85 and 17.1 V/cm. Simulations of electrokinetic and dielectrophoretic forces were conducted with COMSOL Multiphysics^® to predict particle pathlines. Experimental and simulation results show the effect of medium and voltage operating conditions on particle sorting. Further, smaller particles experience smaller iDEP forces and are more susceptible to competing nonlinear electrostatic effects, whereas larger particles experience greater iDEP forces and prefer channels 1 and 2. This work demonstrates that 6.20 and 10 μm particles can be independently sorted into specific outlet streams by tuning medium conductivity even at low operating voltages. This work is an essential step forward in employing DC-iDEP for multiparticle sorting in a continuous flow, multiple outlet lab-on-a-chip device.