Retention behavior of metal particle dispersions in aqueous and nonaqueous carriers in thermal field-flow fractionation

Until quite recently, theories on thermophoresis of particles predicted very low thermophoretic velocities of metal particles in liquids. This prediction was based on the understanding that the very high thermal conductivities of metals relative to most liquid media resulted in quite low temperature gradients across the metal particle thereby leading to low net force on the particle. In this paper, we report the retention behavior of submicrometer size metal particles of silver (Ag), gold (Au), palladium (Pd) and platinum (Pt) suspended in both aqueous and organic (specifically, acetonitrile and tetrahydrofuran) carrier liquids in thermal field-flow fractionation (ThFFF). The dependence of the metal particle retention on various factors such as particle composition, amount of added electrolyte, carrier liquid composition, field strength, channel thickness, and carrier flow-rate is evaluated and discussed. A comparison in particle retention behavior among equal-sized metal, latex and silica particles is also provided.     

Magnitude and direction of thermal diffusion of colloidal particles measured by thermal field-flow fractionation

In this paper we provide experimental evidence showing that various types of submicrometer-sized particles (latexes, inorganic, and metallic), suspended in either aqueous or nonaqueous carrier liquids to which a temperature gradient dT/dx is applied, experience a force in the direction opposite to that of dT/dx. This behavior is similar to that of small particles such as soot, aerosols, and small bubbles suspended in stagnant gases across which temperature gradients are applied, a phenomenon known as "thermophoresis in gases." We report the use of a thermal field-flow fractionation (ThFFF) apparatus in two different configurations to establish the direction of particle motion subject to a temperature gradient. The first approach employed the conventional horizontal ThFFF channel orientation. In this case, small electrical potentials were applied across the narrow channel thickness either to augment or to act in opposition to the applied thermal gradient, depending on whether the accumulation wall was maintained at a positive or negative potential relative to the depletion wall. Thus, by observing the changes in the retention behavior of surface-charged latices or silica particles with changes in potential difference across the channel thickness, we were able to ascertain the direction of migration of the particles in the thermal gradient. The second approach involved the use of a ThFFF column oriented vertically in an implementation of a technique known as thermogravitational FFF. In this approach, the convective flow along the channel length (due to density gradients associated with the temperature gradient) couples with the thermal diffusion effect across the channel thickness to result in a combined particle retention mechanism. A retarded upward migration rate is indicative of accumulation of particles at the cold wall, while enhanced upward migration would indicate a hot-wall accumulation. From the results of our investigations, we conclude that submicrometer-sized particles suspended in either aqueous or nonaqueous carrier liquids and subjected to a temperature gradient migrate from the hot wall toward the cold wall of a ThFFF channel.     

Influence of the carrier composition on thermal field-flow fractionation for the characterisation of sub-micron polystyrene latex particles

A study on the influence of the carrier composition in a ThFFF system on the retention and thermal diffusion of sub-micron polystyrene latex particles has been carried out. Various factors that may influence retention were studied. These include: the type of electrolyte and surfactant, their respective concentrations, and the addition of an organic modifier. Particle retention is highly sensitive to small changes in the carrier composition. It is demonstrated that under the conditions applied, secondary effects, such as particle-wall and particle-particle interactions, are negligible. Addition of surfactants is required to minimise particle-wall interactions. Generally, retention increases at higher electrolyte concentration. Furthermore, the addition of acetonitrile (ACN) to an aqueous carrier leads also to an increased retention. The type of surfactant as well as its concentration is of influence on the retention time. The three surfactants that were studied, i.e., sodium dodecyl sulfate, Brij 35 and cetyltrimethylammonium bromide, showed significant differences in particle retention behaviour. The observed differences in retention in the carriers can be attributed to actual changes in thermal diffusion. D(T) appears to be mainly determined by the interaction between the particle's surface and the carrier liquid, and is therefore highly sensitive to changes in the chemical composition of the particle surface and the carrier. Strong differences in size selectivity were found for different carrier compositions. This allows a relatively easy optimisation of the separation. On the other hand, it complicates the size and composition analysis of particles.     

重力场流分离系统中载液及流速条件对分离的影响

重力场流分离作为最简单的一种场流分离技术,常用于分离微米级颗粒。选择两种不同粒径(20 μ m和6 μ m)的聚苯乙烯(PS)颗粒作为样品,通过改变载液中叠氮化钠浓度、混合表面活性剂的比例及载液流速,利用自行设计生产的重力场流分离(gravitational flow field-flow fractionation, GrFFF)仪器,对颗粒混合样品进行分离,得到了相关谱图与数据,考察了这3种因素对分离效果(保留比(R)、塔板高度(H))的影响。结果表明:20 μ m PS颗粒的R值均大于6 μ m PS颗粒的R值,H值均小于6 μ m颗粒的H值;PS颗粒的R值与H值均随着载液中叠氮化钠浓度的增加而增加;但随着载液流速的增加,R值增加,H值减小。该研究为GrFFF系统的开发及应用提供了重要的参考价值。     

Asymmetric flow field‐flow fractionation of liposomes: optimization of fractionation variables

The purpose of this study was to investigate the influence of ionic strength of the carrier liquid, cross flow rate, focus flow rate, and sample load on the retention behavior of liposomes in asymmetric flow field‐flow fractionation (AF4). Two differently prepared samples of large unilamellar vesicles (LUV) were used. Experiments were performed varying the factors systematically and evaluating their effect on both retention behavior of the liposomes and on particle size as obtained from online coupled multi‐angle light scattering (MALS) analysis. The results showed that the focus flow rate had the least influence on the elution of liposomes. Elution of LUV is mainly governed by the chosen cross flow condition and ionic strength of the carrier liquid as well as its sample load. Optimal fractionation and size analysis were achieved using a sample load of about 10 μg, a cross flow gradient from 1.0 to 0.1 mL/min over 35 min and a carrier solution of NaNO₃ with a concentration of 10 mM.     

Lowering the molecular mass limit of thermal field-flow fractionation for polymer separations

Thermal field-flow fractionation (ThFFF) is capable of separating a wide molecular mass range of polymers by their molecular mass (Mr) and chemical composition. However, retention and resolution decrease significantly for polymers with Mr<20 kDa. Various approaches for increasing the retention of low Mr (<15 kDa) polymers were investigated. Our results showed that temperature conditions and single-component solvents had a limited effect on polymer retention and that certain binary solvent mixtures caused a dramatic increase in retention. The binary solvents approach has enabled the use of a ThFFF system and temperature conditions to separate 2.6 kDa PS from 4.4 kDa PS, thereby extending the applicability of ThFFF to lower molecular masses. The effect of binary solvent mixtures on polymer retention is correlated with the mixture viscosity.     

On the effect of thermophoretic force,gravitational force,and particle–particle interactions in field-flow fractionation

The theoretical calculations confirmed that the gravitational force cannot be neglected in all field-flow fractionation techniques separating nanometer-sized colloidal particles whenever particle diameter is approximately 200?nm and larger. Particle–particle repulsive interactions, mostly electrostatic repulsions, influence substantially concentration distribution established by any effective field acting across the fractionation channel, as confirmed explicitly for thermophoretic force generated by temperature gradient in microthermal field-flow fractionation. The ionic strength of the carrier liquid causes the screening of the electrostatic double layer around the dispersed particles and thus influences the retention. The attractive particle–particle forces occur when the zeta potential of the particles approaches to 0?mV, the electrostatic repulsions are screened, and the aggregation of the particles is observed. The pH influences differently the size and zeta potential of the plain polystyrene latex particles and of the particles modified on the surface by the groups –COOH and –NH₂. The role of a detergent in carrier liquid is non-negligible, as demonstrated by its presence or absence in carrier liquid.     

Flow field-flow fractionation of poly(ethylene oxide): effect of carrier ionic strength and composition

The effects of carrier ionic strength and electrolyte composition on the retention of poly(ethylene oxide) in aqueous flow field-flow fractionation have been investigated in this work. The study shows retention to be particularly sensitive to the presence of salts, as well as to the nature of the cation. Specifically, retention effects due to sample load are found to be very different in solutions containing potassium salts compared to those observed in solutions of the corresponding sodium salts. In a potassium-containing medium, the dependence of retention on sample mass is similar to that found previously for polyelectrolytes. This effect, which is particularly prominent for samples of low molecular mass, can be attributed to specific interactions between cation and polymer.     

Determination of calibration function in thermal field flow fractionation under thermal field programming

A new procedure for determining the calibration function able to relate retention and operative parameters to molecular weight of the species in thermal field flow (ThFFF) under thermal field programming (TFP) conditions is presented. The procedure involves determining the average values of retention parameters under TFP and determining a numerical function related to the temperature variations that occur during TFP. The calibration parameters are obtained by a procedure fitting the retention and operative parameters that hold true at the beginning of the TFP. The procedure is closely related to the one previously developed to calibrate the retention time axis under TFP ThFFF and, together, they constitute a full calibration procedure. Experimental validation was performed with reference to polystyrene (PS)-decalin and PS-THF systems. The calibration functions here obtained were compared to those derived by the classical procedure at constant thermal field ThFFF to obtain the calibration function at variable cold wall temperatures. Excellent agreement was found in all cases proving "universality" of the ThFFF calibration concept, i.e. it is independent of the particular system on which it was determined and can thus be extended to ThFFF operating under TFP. The new procedure is simpler than the classical one since it requires less precision in setting the instrumentation and can be obtained with fewer experiments. The potential applications for the method are discussed.     

Impedance‐based viscoelastic flow cytometry

Elastic nature of the viscoelastic fluids induces lateral migration of particles into a single streamline and can be used by microfluidic based flow cytometry devices. In this study, we investigated focusing efficiency of polyethylene oxide based viscoelastic solutions at varying ionic concentration to demonstrate their use in impedimetric particle characterization systems. Rheological properties of the viscoelastic fluid and particle focusing performance are not affected by ionic concentration. We investigated the viscoelastic focusing dynamics using polystyrene (PS) beads and human red blood cells (RBCs) suspended in the viscoelastic fluid. Elasto‐inertial focusing of PS beads was achieved with the combination of inertial and viscoelastic effects. RBCs were aligned along the channel centerline in parachute shape which yielded consistent impedimetric signals. We compared our impedance‐based microfluidic flow cytometry results for RBCs and PS beads by analyzing particle transit time and peak amplitude at varying viscoelastic focusing conditions obtained at different flow rates. We showed that single orientation, single train focusing of nonspherical RBCs can be achieved with polyethylene oxide based viscoelastic solution that has been shown to be a good candidate as a carrier fluid for impedance cytometry.     

Contact conductivity detection of polymerase chain reaction products analyzed by reverse-phase ion pair microcapillary electrochromatography

We describe the development of an integrated microelectrophoretic system consisting of a contact conductivity detector mounted on-chip for monitoring the separation of double-stranded (ds) DNA fragments produced via the polymerase chain reaction (PCR) using microcapillary electrochromatography as the separation mode. The separation was carried out in a polymer-based microfluidic device, hot-embossed into poly(methylmethacrylate) (PMMA), whose walls were functionalized to produce a C(18)-terminated surface to act as the stationary phase (open channel format). The carrier electrolyte contained the ion-pairing agent, triethylammonium acetate (TEAA) to allow the separation to be carried out using reverse-phase ion-pair capillary electrochromatography (RP-IPCEC). The microelectrophoretic separations were investigated utilizing various solvent strengths (acetonitrile/water) with 25 mM TEAA to observe the effects on the separation efficiency as well as the chromatographic development time and detector performance. The field strength significantly affected the quality of the separation, with no separation observed at 333 V/cm for a low mass dsDNA sizing ladder, but baseline separation achieved using a field strength of 67 V/cm. It was observed that the solvent strength affected the retention behavior of the polyanionic molecules as well as the electroosmotic mobility. Higher acetonitrile compositions in the run buffer resulted in reduced plate numbers, which produced lower chromatographic resolution. The use of conductivity detection allowed mass detection sensitivities in the range of 10(-21) mol with a separation efficiency of 10(4) plates and the performance of the detector independent of the acetonitrile content used in the carrier electrolyte.     

Field-flow fractionation of magnetic particles in a cyclic magnetic field

Although magnetic field-flow fractionation (MgFFF) is emerging as a promising technique for characterizing magnetic particles, it still suffers from limitations such as low separation efficiency due to irreversible adsorption of magnetic particles on separation channel. Here we report a novel approach based on the use of a cyclic magnetic field to overcome the particle entrapment in MgFFF. This cyclic field is generated by rotating a magnet on the top of the spiral separation channel so that magnetic and opposing gravitational forces alternately act on the magnetic particles suspended in the fluid flow. As a result, the particles migrate transversely between the channel walls and their adsorption at internal channel surface is prevented due to short residence time which is controlled by the rotation frequency. With recycling of the catch-release process, the particles follow saw-tooth-like downstream migration trajectories and exit the separation channel at velocities corresponding to their sedimentation coefficients. A retention model has been developed on the basis of the combined effects of magnetic, gravitational fields and hydrodynamic flow on particle migration. Two types of core-shell structured magnetic microspheres with diameters of 6.04- and 9.40-μm were synthesized and used as standard particles to test the proposed retention theory under varying conditions. The retention ratios of these two types of particles were measured as a function of magnet rotation frequency, the gap between the magnet and separation channel, carrier flow rate, and sample loading. The data obtained confirm that optimum separation of magnetic particles with improved separation efficiency can be achieved by tuning rotation frequency, magnetic field gradient, and carrier flow rate. In view of the widespread applications of magnetic microspheres in separation of biological molecules, virus, and cells, this new method might be extended to separate magnetically labeled proteins or organisms for multiplex analyte identification and purification.     

Effects of ampholyte concentration on protein behavior in on-chip isoelectric focusing

The effects of mobility corrections on carrier ampholytes are studied at various ampholyte concentrations to understand protein behavior during IEF. IEF simulations are conducted in the presence of 25 biprotic carrier ampholytes within a pH range of 6-9 after applying the Onsager-Debye-Hückel correction to the carrier ampholytes. Two model proteins with ten charge states but without ionic strength corrections are allowed to focus under an electric field of 300 V/cm in a 1 cm long channel. The IEF simulation results show that higher ionic strengths (50 - 100 mM) cause significant changes in the transient movement as well as the final focused profiles of both ampholytes and proteins. The time required for a single, well-defined peak to form increases with ionic strength when Onsager corrections are applied to the carrier ampholytes. For a particular ampholyte concentration, the space-averaged conductivity does not change during the final focusing stage, but the magnitude of space averaged conductivity is different for different ampholyte concentration. The simulation results also reveal that at steady-state ionic strength profiles remain flat throughout the channel except at the locations of proteins where a significant change in ampholyte concentration is obtained.     

Synthesis of monodisperse anionic submicron polystyrene particles by stabilizer-free dispersion polymerization in alcoholic media

In this work, monodisperse polystyrene (PS) particles were synthesized in ethanol/water medium using sodium salt of styrene sulfonic acid and 2,2′-azobis(isobutyronitrile) as ionic comonomer and nonionic initiator, respectively. The polymerization was carried out at low agitation speed, and no stabilizer (or surfactant) was added to the polymerization medium. This polymerization system (stabilizer-free dispersion polymerization) was initiated as a homogeneous solution of monomer, comonomer, medium, and initiator. With the production of free radicals, polymerization developed into a heterogeneous system. The effect of various polymerization conditions on the size and size distribution of the obtained particles was evaluated. The experimental results showed that with an increase in ethanol content, the size of the particles increased while no significant change was observed in particle size distribution. Furthermore, with increasing ionic comonomer content, the size of the particles decreased and their size distribution became broader. Moreover, it was observed that addition of an electrolyte to the polymerization medium also increased the particles’ size and broadened their size distribution. It is noteworthy to point out that the coagulation occurred in higher amounts of electrolyte. Finally, it is concluded that the polar component of Hansen solubility parameter of the polymerization medium affects the particle size and particle size distribution greatly.     

Optimization of cyclical electrical field flow fractionation

Cyclical electrical field flow fractionation (CyElFFF) is a variation on electrical field flow fractionation (ElFFF) where cyclical electrical fields are used instead of steady DC fields to increase the effective field experienced by particles in the flow channel. Even though the effective field increases more than 20-fold compared to normal ElFFF, the retention and resolution in CyElFFF has not been shown to be better than in ElFFF. In this paper we report how one can optimize operational parameters in CyElFFF to obtain good retention and resolution in CyElFFF. The effects of offset voltage, frequency, flowrate, concentration of particles and sample size on retention, resolution and retained peak/void peak ratio have been observed. The results obtained from these experiments were analyzed and suggestions have been made to improve both retention and resolution. A 4-fold improvement in retention without a significant increase in band broadening is reported.     

Coupled Influence of Colloidal and Hydrodynamic Interactions on the RSA Dynamic Blocking Function for Particle Deposition onto Packed Spherical Collectors

The influence of electrostatic double-layer and hydrodynamic interactions on random sequential adsorption (RSA) of colloidal particles onto packed spherical collectors was investigated using inverse analysis of colloid breakthrough data obtained from well-controlled particle deposition experiments. Deposition experiments were carried out using monodisperse aqueous suspensions of positively charged latex colloids and packed columns of negatively charged uniform glass beads for different combinations of ionic strength, particle size, and approach velocity. From the experimental particle breakthrough data, the initial particle deposition rates and the virial coefficients of the dynamic blocking function based on RSA mechanics were determined. The magnitudes of the virial coefficients were observed to increase from the hard sphere values with increasing flow rates and decreasing ionic strengths of the background electrolyte. Particle size also plays a significant role in governing the deposition dynamics. The deviation from the hard sphere RSA behavior becomes more prominent for larger particles. Copyright 2000 Academic Press.     

SANS and SAXS analysis of charged nanoparticle adsorption at oil-water interfaces

A systematic study of the adsorption of charged nanoparticles at dispersed oil-in-water emulsion interfaces is presented. The interaction potentials for negatively charged hexadecane droplets with anionic polystyrene latex particles or cationic gold particles are calculated using DLVO theory. Calculations demonstrate that increased ionic strength decreases the decay length of the electrostatic repulsion leading to enhanced particle adsorption. For the case of anionic PS latex particles, the energy barrier for particle adsorption is also reduced when the surface charge is neutralized through changes in pH. Complementary small-angle scattering experiments show that the highest particle adsorption for PS latex occurs at moderate ionic strength and low pH. For cationic gold particles, simple DLVO calculations also explain scattering results showing that the highest particle adsorption occurs at neutral pH due to the electrostatic attraction between oppositely charged surfaces. This work demonstrates that surface charges of particles and oil droplets are critical parameters to consider when engineering particle-stabilized emulsions.     

Different elution modes and field programming in gravitational field-flow fractionation. 2. Experimental verification of the range of conditions for flow-rate and carrier liquid density programming

Gravitational field-flow fractionation utilises the Earth's gravitational field as an external force that causes the settlement of particles towards the channel accumulation wall. Hydrodynamic lift forces oppose this action by elevating of particles from the channel accumulation wall. Therefore there are several possibilities to modulate the resulting force field acting on particles in gravitational field-flow fractionation. Regarding the force field programming in gravitational field-flow fractionation, this work focused on two topics: changes of the difference between particle density and carrier liquid density in Brownian and focusing elution modes and influencing of lift forces achieved by changing the flow-rate in focusing elution mode. We have found and described the experimental conditions applicable to force field programming in the case of separations of silica gel particles by gravitational field-flow fractionation. It was shown that the effect of carrier liquid viscosity in the water-methanol system is implemented as an additional factor enhancing the desired effect of carrier liquid density. Some other forces influencing the retention behaviour of the model particles are discussed.     

Transition from dilute to concentrated electrokinetic behavior in the dielectric spectra of a colloidal suspension

Dielectric spectroscopy is used to measure the complex permittivity of 200 and 100 nm diameter polystyrene latex suspended in potassium chloride (KCl) solutions over the frequency range 10(4)-10(7) Hz as a function of particle volume fraction (?) and ionic strength. Dilute suspension dielectric spectra are in excellent agreement with electrokinetic theory. A volume fraction dependence of the dielectric increment is observed for low electrolyte concentrations (0.01, 0.05, and 0.1 mM) above ? ≈ 0.02. This deviation from the dilute theory occurs at a critical frequency ω* that is a function of volume fraction, particle size, and ionic strength. The dielectric increment of suspensions at the highest salt concentration (1 mM) shows no volume fraction dependence up to ? = 0.09. Values of ω* are collapsed onto a master curve that accounts for the length and time scales of ion migration between neighboring particles. The measured conductivity increment is independent of volume fraction and agrees with theory after accounting for added counterions and nonspecific adsorption.     

Thermodynamic Characteristics of Acid-Base Reactions of Aromatic Derivatives of α-Aminopropionic Acid in Aqueous Solution

The thermal effects of acid-base reactions of phenylalanine and tryptophan in aqueous solutions (ionic strength 0.5, 1.0, and 1.5; supporting electrolyte KNO₃) at 298.15 K were determined calorimetrically. The thermodynamic characteristics of stepwise dissociation of the amino acids in aqueous solution were calculated. The influence of the structure of the amino acids on the thermodynamic parameters of the protolytic equilibria was examined.