Objective testing for the dependence of electrophoretic mobilities upon size in capillary zone electrophoresis

Summary Eighteen peptides have been modeled. From the volumetric data derived, and published mobilities, the relationship between electrophoretic mobility (μ_ep) and the hydrodynamic radius(r) has been examined. Objective testing with respect to size has been achieved by the log-log version of generalized relationship. (1) From the gradient of the plot versus log r(2.02) there is good support for the inverse square law (μ_ep α 1/r²). Equivalent calculations using molecular weight (M_r) and the number of amino acid residues (n) similarly lead to μ_ep α 1/M _r ^2/3 and μ_ep α 1/n^2/3, respectively. However, the strength of the correlation is diminished as the precision of the representation of size is degraded. (2) An examination of the effect of size at fixed charge and a statistical analysis of the charge distribution on the peptides leads to the conclusion that deviations from the averaged behaviour arise from a charge-induced volumetric effect. Taken together, (1) and (2) indicate that whilst net charge and total size can describe average electrophoretic behaviour well, these parameters are inadequate to describe the specific mobilities of individual analytes. Objective analysis of alkylpyridine data indicates μ_ep α 1/r^x where x=2.6–2.8 (depending upon the nature of the r values utilized), but is certainly ≠1 as may have been presumed. A very small range of values may be responsible for this surprising result.     

Unexpected slow near wall dynamics of spherical colloids in a suspension of rods

In this paper, we will show the influence of an additional rodlike component, that is, fd-virus, on the diffusion of spherical polystyrene colloids close to a wall. The sphere diffusivity normal to the wall, D perpendicular, is strongly affected by the presence of the rods, while the effect on the parallel diffusivity, D||, is less pronounced except in the immediate vicinity of the wall. We show that this observation cannot be explained by describing the effect of the rods as a simple mean field depletion potential alone.     

Aqueous suspensions of charged spherical colloids: dependence of the surface charge on ionic strength, acidity, and colloid concentration

We theoretically investigate the dependence of the surface charge developed on charged spherical colloids upon several environmental parameters: the ionic strength of the monovalent added electrolyte, acidity (stabilized by a pH buffer solution), and colloid concentration. In the framework of the mean-field Poisson-Boltzmann spherical cell model, we include the charged colloid-microion correlations into the buffer equation, and we allow for the specific binding of ions to the ionizable groups on the colloid surface. Theoretical predictions are compared to the results obtained under the planar-symmetry Gouy-Chapman approximation and analyzed for the experimental conditions of an aqueous dispersion of the phospholipid dimyristoyl phosphatidylglycerol (DMPG). Experimental measurements of the partition ratio of an aqueous soluble cationic spin label on buffered dispersions of polyanionic unilamellar vesicles of DMPG in the presence of added monovalent salt are theoretically interpreted in terms of ion partition due to electrostatic interactions. We show that the specific binding of the probe must be admitted to explain the experimental results.     

A method for determining electrophoretic and electroosmotic mobilities using AC and DC electric field particle displacements

We have developed a method for measuring the electrophoretic mobility of submicrometer, fluorescently labeled particles and the electroosmotic mobility of a microchannel. We derive explicit expressions for the unknown electrophoretic and the electroosmotic mobilities as a function of particle displacements resulting from alternating current (AC) and direct current (DC) applied electric fields. Images of particle displacements are captured using an epifluorescent microscope and a CCD camera. A custom image-processing code was developed to determine image streak lengths associated with AC measurements, and a custom particle tracking velocimetry (PTV) code was devised to determine DC particle displacements. Statistical analysis was applied to relate mobility estimates to measured particle displacement distributions.     

Capillary electrophoresis separation of vinpocetine and related compounds: prediction of electrophoretic mobilities in partly aqueous media

Offord's equation, a relationship between electrophoretic mobility and charge, size and shape of peptides, has been extended to quantitate the electrophoretic mobility of vinca alkaloids. Partly aqueous protonation constants and the derived theoretical mobilities have been proven to be able to predict experimental electrophoretic mobilities. In practice, seven vincamine derivatives of very low water-solubility were separated by capillary electrophoresis. Buffer total concentration, apparent pH and methanol content, the three most important parameters of the running buffer, were used in triangular resolution mapping to characterize separation. Even though electrophoresis is well known to slow down in partly aqueous media, under our optimized circumstances a baseline separation was achieved within 8 min in each case.     

A simpler expression for Henry’s function describing the electrophoretic mobility of spherical colloids

An approximate expression for Henry’s function, describing the electrophoretic mobility of a spherical colloidal particle in the limit of low surface potentials, is developed through a physical analogy to a colloidal particle with a linearly slipping surface (i.e. satisfies the Navier slip condition). The resulting expression reproduces Henry’s function with a relative error of no more than 0.1%. This approach is generalized for the electrophoretic mobility of a particle regardless of surface potential though necessary data for rigorous testing is lacking.     

Numerical calculation of the electrophoretic mobility of a spherical particle in a salt-free medium

By extending an approximate theory of the electrophoretic mobility of dilute spherical colloidal particles in a salt-free medium containing only counterions (H. Ohshima, J. Colloid Interface Sci. 248 (2002) 499--503), a systematic numerical method is given for the calculation of the electrophoretic mobility, which is based on an iteration method. We assume that each sphere is surrounded by a spherical free volume, within which counterions are distributed so that electro-neutrality is satisfied. The electrophoretic mobility is found to be determined mainly by the pressure due to the counterions at the outer surface of the free volume. It is shown how the mobility values deviate from those expected from Hückel's formula for high particle charges or zeta potentials because of the counterion condensation effect.     

3-D transient electrophoretic motion of a spherical particle in a T-shaped rectangular microchannel

This paper considers the electrophoretic motion of a spherical particle in an aqueous electrolyte solution in a T-shaped rectangular microchannel, where the size of the channel is close to that of the particle. This is a complicated transient process where the electric field, the flow field, and the particle motion are coupled together. A theoretical model was developed to investigate the influences of the applied electric potentials, the zeta potentials of the channel and the particle, and the size of the particle on the particle motion. A direct numerical simulation method using the finite element method is employed. This method employs a generalized Galerkin finite element formulation that incorporates both equations of the fluid flow and equations of the particle motion into a single variational equation where the hydrodynamic interactions are eliminated. The ALE method is used to track the surface of the particle at each time step. The numerical results show that the electric field in the T-shaped microchannel is influenced by the presence of the particle, and that the particle motion is influenced by the applied electric potentials and the zeta potentials of the channel and the particle. The path of the particle motion is dominated by the local electric field and the ratio of the zeta potential of the channel to that of the particle. The particle's velocity is also dependent on its size in a small channel.     

Viscosity and electrophoretic mobility of cesium fullerenehexamalonate in aqueous solutions--comparing experiments and theories on nanometer-sized spherical polyelectrolyte

The viscosity of aqueous solutions of cesium fullerenehexamalonate T h -C 66(COOCs) 12, a rigid spherical nanometer-sized polyvalent salt, was measured by the Ubbelohde-type viscometer. The measurements were performed without added salt at 25 degrees C in the concentration range between 7 and 320 g/dm (3). THe concentration dependence of the obtained reduced viscosity was compared with the theoretical prediction, taking into account contributions stemming from the intrinsic viscosity, hydrodynamic perturbations of the hypothetically bare fullerenehexamalonate macroion, the primary electroviscous effect, and the secondary electroviscous effect. Using the geometric radius of the bare macroion from the previous measurements of the estimated effective charge of the macroion and from the small-angle X-ray scattering data of the estimated thickness of the compact shell of counterions electrostatically bound to the macroion, a good agreement between theory and the experiment was obtained in the range of the lowest and of the highest concentrations. Electrostatic interactions are identified as the main cause of the increased reduced viscosity at the lowest measured concentrations. At the highest concentrations, electrostatic interactions are effectively screened, and the influence of binary hydrodynamic interactions and perturbations of the hypothetical bare macroion prevails over electrostatic contributions to the increased viscosity. The electrophoretic mobility of the fullerenehexamalonate ion in aqueous salt-free medium was computed with the same value for the radius of the fullerenehexamalonate macroion as that used in the calculation of viscosity. The numerical solution of Ohshima's equation agreed well with the experimental values.     

Cross-condensation and particle growth in aqueous silane mixtures at low concentration

We have observed the condensation of a mixture of gamma-glycidoxypropylmethyldiethoxysilane and 3-aminopropyltriethoxysilane in dilute aqueous solutions. NMR and IR spectroscopy have allowed to follow the condensation process in the mixture, which is noticeably enhanced and proceeds faster than for each silane on its own. Cross-condensation between the two silanes was evidenced. When the hydrophilic aminosilane is in excess, the condensation, as evidenced by dynamic light scattering, proceeds toward gel formation because the oligomers formed are essentially hydrophilic. When the more hydrophobic epoxysilane is in excess, oligomer growth proceeds slowly and results in a destabilization of the solution: due to their hydrophobic character, the oligomers formed coalesce suddenly into 200-nm-diameter aggregates. Coatings deposited from such solutions with high epoxysilane content can be used to strengthen glass. We show that the progress of condensation in solution results in a wetting transition during deposition of the silane film on glass by dip coating. The production of increasingly hydrophobic oligomers as the reaction time increases results in adsorption of more hydrophobic aggregates at the surface, which eventually leads to dewetting of the film: in the absence of film, glass strengthening disappears.     

Calculation of electrophoretic mobilities in water-organic modifier mixtures in capillary electrophoresis

In order to correlate/predict electrophoretic mobility data in the mixture of water+organic modifier four equations have been presented and examined. The experimental mobilities of five analytes were determined in a water-methanol mixture. These data have been used to assess the accuracy and predictability of the models. Also, some previously published mobility data in water-organic modifier mixtures has been employed for further evaluation of the models. The models produced accurate results and the means of percentage deviations were in the range of 0.66-1.30.     

Surface charge density on spherical silica particles in aqueous alkali chloride solutions

The results of a systematic investigation on the influence of different alkali ion species on the surface charge density, ₀, of spherical silica particles (AEROSIL 300) in the pH range between 4 and 8 and with electrolyte concentrations from 0.005 M to 0.3 M are presented. The accuracy of the data may be described by a residual deviation,s( ₀ ^m ), including at least four single measurements:s( ₀ ^m )>0.2C/cm². The alkali sequence found for the spherical particles is in agreement with data for porous silica published by other authors.     

Temperature dependence of ion mobilities: Experiment and theory

The mobilities of several positive ions plus chloride ion in a variety of polar and nonpolar gases were measured over the temperature range 170–380 K by ICR spectrometry. The experimental results were compared to the predictions of the first-order kinetic theory for the nonpolar neutral systems. For the ion-polar neutral pairs, several theories were used for comparison. In all cases, adequate and reasonable fits were not possible. This result is attributed to theoretical shortcomings. Possible sources of error in the experimental data are discussed in detail. Other mobility measurements (mostly from drift tubes) were also compared with the kinetic theory results. The only systems which could be fit were alkali ion/rare gas neutral pairs. The mobility-temperature curves of other atomic ions and of all polyatomic ions could not be duplicated by the first-order theory.     

Distribution of single DNA molecule electrophoretic mobilities in semidilute and dilute hydroxyethylcellulose solutions

The distribution of center of mass electrophoretic mobility mobilities and normalized migration time of up to 1080 lambda DNA molecules per experiment were measured in both semidilute hydroxyethylcellulose HEC/0.5 x Tris-borate-EDTA (TBE) solutions and dilute HEC/0.5 x TBE solution by high-speed video microscopy. Measurements were made microscopically over a short migration distance in homogeneous DNA HEC/0.5 x TBE solution and after electrophoretic migration of a plug of DNA through 7 cm. Video at 120 frames/s (semidilute HEC solution) and 236 frames/s (dilute HEC solution) allowed visualization with adequate resolution for single molecule mobility measurements. The electrophoretic migration times and band shapes predicted from the measurements corresponded well with those measured by conventional capillary electrophoresis (CE) in both semidilute and dilute HEC. In semidilute solution, the band width predicted by a square root of time scaling is in good agreement with the results of conventional CE. However, in dilute solution the precision of the measurements was not good enough to allow scaled estimates of band widths.     

Temperature and concentration dependences of density and refraction of aqueous duloxetine solutions

Present paper reports the measured densities (ρ) and refractive indices (n _D) of aqueous solutions of Duloxetine drug in wide range of molal concentrations (m = 0.0101–0.1031 mol kg^?1) and at different temperatures (297.15, 302.15, and 307.15 K). Apparent molar volumes (φ_v) of drug were calculated from density data and fitted to Masson’s relation \((\phi _\nu = \phi _\nu ^0 + S_\nu ^* \sqrt c )\) and partial molar volumes (φ _v ⁰ ) were evaluated at different temperatures. Concentration dependence of refractive index (n _D = Kc + n _D ⁰ ) at experimental temperature has been studied. Density and refractive index data has been used for the calculation of specific refractions (R _D). Experimental (ρ and n _D) and calculated (φ_v, φ _v ⁰ , and R _D) properties have been interpreted in terms of concentration and temperature effects on structural fittings and drug-water interactions.     

The concentration dependence of the reduced viscosity for a spherical,non-expanding macroion

Summary The variation of the reduced viscosity with solvent dilution was investigated for a non-expanding sulphonated polystyrene latex. Based on previous measurements of the primary and secondary electroviscous effects, an empirical equation was derived forη _sp/c in terms of the concentration of the latex and the ionic strength of the solvent. Validity of the equation was confirmed by quantitative agreement of the predictedη _sp/c with the experimental values at several ionic strengths. Various factors, determining the shape of the solvent dilution curve, were discussed. The results suggested that the upsweep of theη _sp/c and the occurrence of a maximum at low ionic strength can no longer be regarded as the exclusive characteristic of the molecular expansion of a linear flexible macroion.

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Zusammenfassung Es wird die Variation der reduzierten Viskosit?t mit der L?sungsmittelverdünnung für nicht expandierende sulfonierte Polystyrollatex untersucht. Basierend auf vorangehenden Messungen des prim?ren und sekund?ren elektroviskosen Effekts wird eine empirische Gleichung fürη _sp/c in Termen der Konzentration der Latex- und der Ionenst?rke des L?sungsmittels abgeleitet. Die Gültigkeit der Gleichung wird durch quantitative übereinstimmung des vorausgesagtenη _sp/c mit den experimentellen Werten bei mehreren Ionenst?rken best?tigt. Verschiedene Faktoren, die die Gestalt der Abh?ngigkeit der Kurve von der Konzentration des L?sungsmittels bestimmen, werden diskutiert. Die Ergebnisse legen nahe, da? der Anstieg derη _sp/c und das Auftreten eines Maximums bei kleinen Ionenst?rken nicht l?nger als ein ausschlie?liches Charakteristikum für Molekularexpansion von linearen flexiblen Makroionen betrachtet werden darf.

     

Temperature dependence of second critical micelle concentration of dodecyldimethylbenzylammonium bromide in aqueous solution

The specific conductivity of dodecyldimethylbenzylammonium bromide (C12BBr) in aqueous solutions, in the temperature range of 15 to 40 °C, has been measured as a function of molality. The two breaks which were found on the conductivity against molality plots were attributed to the critical micelle concentration, cmc, and second critical micelle concentration, 2^nd cmc, respectively. The ratio of the slopes, S, of the three linear fragments on the plots, S2/S1 and S3/S1, was attributed to the degree of ionization of the micelles at cmc and 2^nd cmc respectively. It was shown that the values of the 2^nd cmc estimated above 27 °C are only apparent due to thermal disintegration of the micelles. In the temperature range of 15 to 27 °C, the values of the 2^nd cmc increase gradually and the plot of the 2^nd cmc against temperature is concave. The ratio of 2^nd cmc/cmc for C12BBr at 25 °C amounts to 15 and appears to be high compared to the literature values for other surfactants. For comparative purposes the cmc and 2^nd cmc values were also estimated conductometrically for decyldimethylbenzylammonium bromide (C10BBr) at 25 °C. The 2^nd cmc value for this surfactant is higher compared to the value for the C12 homologue by a factor of 2.6.The standard Gibbs free energies of micellization at cmc and at the 2^nd cmc were estimated from the experimental data for both surfactants at 25 °C.