An Enthalpic Analysis on the Aggregation of Colloidal Particles Studied by Microcalorimetry

There are different theories concerning the stability of colloidal suspensions. Most of them arise from the well-known DLVO theory which relates colloidal stability to intermolecular forces between particles. Experimental corroboration of these theories has been obtained mainly by using different optical techniques that analyze changes in the optical properties of the solution while particles aggregate. However, no attention has been paid to studying the aggregation process thermodynamically. This is why we have focussed on studying the heat released during the agglutination of polystyrene particles. The enthalpy change in this aggregation process was detected by using a highly sensitive and modern technique called isothermal titration calorimetry. In addition, some results about repeptization, that is, reversibility in the aggregation process, are also shown. Copyright 2001 Academic Press.     

Computer controlled titration with piston burette or peristaltic pump – a comparison

The advantages and problems of the use of piston burettes and peristaltic pumps for dosage of titrant solutions in automatic titrations are shown. For comparison, only the dosing devices were exchanged and all other components and conditions remained unchanged. The results of continuous acid base titration show good agreement and comparable reproducibility. Potentiometric sensors (glass electrodes) with different equilibration behaviour influence the results. The capability of such electrodes was tested. Conductometric measurements allow a much faster detection because there is no equilibration of electrodes. Piston burettes should be used for titration with very high precision, titration with organic solvents and slow titrations. Peristaltic pumps seem to be more suitable for continuous titrations and long time operation without service.     

Application of the zeta potential measurements to explanation of colloidal Cr2O3 stability mechanism in the presence of the ionic polyamino acids

In the presented paper, the influence of the molecular weight and the type of polyamino acid functional groups on the electrokinetic properties and the stability of chromium (III) oxide suspension were examined. Analysis of the data obtained from the adsorption, potentiometric titration, zeta potential, and stability measurements allows to propose stabilization or destabilization mechanism of the studied systems. In the studies, there were used polyamino acids with different ionic characters: anionic polyaspartic acid and cationic polylysine. The measurements showed that the zeta potential depends on the concentration and molecular weight of the applied polymer. Stability of the chromium (III) oxide suspensions in the presence of ionic polyamino acids increases compared to the results obtained in the absence of polymers. The exception is LYS 4,900 at pH?=?10. Under these conditions, the decrease in stability is observed due to formation of polymer bridges between the polymer chains adsorbed on different colloidal particles. Determination of the stabilization/destabilization mechanism of the polyamino acid/chromium (III) oxide system and examination of the effects of polymer molecular weight on the stabilization properties can contribute to a wider use of this group of compounds as potential stabilizers or flocculants in many industrial suspensions.     

Surface potential of hematite in aqueous electrolyte solution: hysteresis and equilibration at the interface

Electrostatic potential at the inner plane of the hematite aqueous interface, i.e., surface potential, was measured by means of a single-crystal hematite electrode. Acidic solutions were titrated with base and then back-titrated with acid. Surface potentials were evaluated from electrode potentials by setting the zero value at the isoelectric point. In the case of fast titrations the equilibration time was approximately 10 min, and significant hysteresis was obtained, more pronounced at higher electrolyte concentrations. Hysteresis disappeared in slow titration runs when the equilibration time was extended up to 120 min, and also when ultrasound was applied. Hysteresis was observed in the pH region close to neutrality, where the concentrations of potential-determining H+ and OH- ions are low. Equilibration was fast in acidic and basic regions. These results are explained on the basis of the kinetics of surface reactions, supported by the following rate of single-crystal electrode equilibration. It is concluded that the equilibration rate at the interface is specific for a given system and is not a general phenomenon. As several systems may undergo fast equilibration, such data may be regarded as equilibrium data and interpreted by the surface complexation model. In other cases, one should perform kinetic tests and apply extended equilibration times.     

Significance of data treatment and experimental setup on the determination of copper complexing parameters by anodic stripping voltammetry

Different procedures of voltammetric peak intensities determination, as well as various experimental setups were systematically tested on simulated and real experimental data in order to identify critical points in the determination of copper complexation parameters (ligand concentration and conditional stability constant) by anodic stripping voltammetry (ASV). Varieties of titration data sets (Cu_measuredvs. Cu_total) were fitted by models encompassing discrete sites distribution of one-class and two-class of binding ligands (by PROSECE software). Examination of different procedures for peak intensities determination applied on voltammograms with known preset values revealed that tangent fit (TF) routine should be avoided, as for both simulated and experimental titration data it produced an additional class of strong ligand (actually not present). Peak intensities determination by fitting of the whole voltammogram was found to be the most appropriate, as it provided most reliable complexation parameters.Tests performed on real seawater samples under different experimental conditions revealed that in addition to importance of proper peak intensities determination, an accumulation time (control of the sensitivity) and an equilibration time needed for complete complexation of added copper during titration (control of complexation kinetics) are the keypoints to obtain reliable results free of artefacts.The consequence of overestimation and underestimation of complexing parameters is supported and illustrated by the example of free copper concentrations (the most bioavailable/toxic specie) calculated for all studied cases. Errors up to 80% of underestimation of free copper concentration and almost two orders of magnitude overestimation of conditional stability constant were registered for the simulated case with two ligands.     

Assessment of surfactant adsorption in oil-based magnetic colloids

We describe in this paper different and complementary experimental methods for assessing the adsorption of surfactants on metal particles in oil-based suspensions. Two different kinds of particles are dispersed in mineral oil: iron microparticles and CoNi nanoparticles. The adsorption of oleic acid in the Fe/oil interface in diluted suspensions can be determined by obtaining the adsorption isotherm. In addition, we present a method based on the time evolution of the optical absorbance of suspensions, from which the existence of adsorption can be inferred. For concentrated suspensions, the used of optical methods is not recommended, since they are affected by a significant inaccuracy. We present here a useful alternative based on electromagnetic induction phenomena. The results obtained allow a more comprehensive knowledge of the aggregation process in concentrated suspensions. With the same purpose, a third group of experiments, based on rheological techniques, is carried out in Fe/oil and CoNi/oil concentrated suspensions. In these series of experiments, the effect of three surfactants (oleic acid, aluminum stearate and lecithin) is tested by measuring either the viscosity, or the magnetic field-induced yield stress of the suspensions. The combination of these series of experiments gives us valuable information about the most appropriate surfactant/carrier combination capable of imparting a high stability and a strong magnetorheological response in magnetic colloids.     

Combined electrical and rheological properties of shear induced multiwall carbon nanotube agglomerates in epoxy suspensions

In this work the rheological and electrical properties of semi-dilute carbon nanotube (CNT)–epoxy suspensions have been discussed. The suspensions are produced using two types of industrially available CNTs (Nanocyl 3150 and 7000) and using two different dispersion techniques, namely 3-roll milling and sonication. In-situ optical microscopic analysis and electrical conductivity measurements have been conducted. It is shown that despite using CNTs with similar aspect ratios, the dispersability of the raw material and the time stability of the suspensions are quite different. Additionally, viscosity measurements are used to evaluate the initial dispersion quality and time stability.     

Regeneration of tetrabutylammonium ion-pairing reagent distribution in a gradient elution of reversed phase ion-pair chromatography

The regeneration of ion-pairing reagent distribution on liquid chromatography columns after gradient elution has been well recognized as the cause for long column equilibration time, a major drawback associated with gradient elution reverse phase ion-pair chromatography. To date, the majority of studies have focused on optimizing the separation conditions to shorten the equilibration time. There is limited understanding of the ion-pairing reagent distribution process between the mobile phase and stationary phase in the course of gradient elution, and subsequent column re-equilibration. The focus of this work is to gain a better understanding of this process. An ion-pair chromatographic system, equipped with a YMC ODS C(18) column and a mobile phase containing tetrabutylammonium (TBA) hydroxide as the ion-pairing reagent, was used in the study. The TBA distribution profile was established by measuring its concentration in the eluent fractions collected during the gradient cycle using different column equilibration times with an ion chromatographic method. Furthermore, the analyte retention time was evaluated as the function of the column equilibration time and TBA concentration in the mobile phase. The column equilibration and its impact on the method robustness will also be discussed.     

Electrical impedance spectroscopy for detection of bacterial cells in suspensions using interdigitated microelectrodes

In this study, we present a new, simple and rapid impedance method to detect bacterial cells by making use of the impedance properties of bacterial cell suspensions using interdigitated microelectrodes. It was found that bacterial cell suspensions in deionized (DI) water with different cell concentrations could generate different electrical impedance spectral responses, whereas cell suspensions in phosphate buffered saline (PBS) solution could not produce any significant differences in impedance spectra in response to different cell concentrations. In DI water suspensions, impedance at 1 kHz decreased with the increasing cell concentrations in the suspensions. The impedance of cell suspensions in DI water was discussed and found that it was resulted from the cell wall charges and the release of ions or other osmolytes from the cells. A linear relationship between the impedance and the logarithmic value of the cell concentration was found in the cell concentration range from 10⁶ to 10¹⁰ cfu/ml, which can be expressed by a regression equation of Z (kΩ) = −2.06 log C (cells/20 μl) + 5.23 with R² = 0.98. The detection limit was calculated to be 3.45 × 10⁶ cfu/ml, which is comparable with many label-free immunosensors for detection of pathogenic bacteria reported in the literature. To achieve the selectivity of this method, we also demonstrated the feasibility of integrating magnetic separation to this impedance method. This study has demonstrated that bacterial cell concentration can be inferred by measuring the impedance of cell suspensions in DI water. This new detection mechanism could be an alternative to current impedance methods that have been reported for the detection of bacterial cells, e.g. impedance microbiology and electrical/electrochemical impedance biosensors.     

Reduced aggregation and sedimentation of zero-valent iron nanoparticles in the presence of guar gum

Injection of nanoscale zero-valent iron (NZVI) is potentially a promising technology for remediation of contaminated groundwaters. However, the efficiency of this process is significantly hindered by the rapid aggregation of the iron nanoparticles. The aim of this study was to enhance the colloidal stability of the nanoparticles through the addition of the "green" polymer guar gum. We evaluated the properties of guar gum and its influence on the surface properties, particle size, aggregation, and sedimentation of iron nanoparticles. Commercial iron nanoparticles were dispersed in guar gum solutions, and their aggregation and sedimentation behaviors were compared to those of bare iron nanoparticles and commercial nanoparticles modified with a biodegradable polymer (polyaspartate). High performance size exclusion chromatography, charge titration, and viscosity assessment showed that guar gum is a high molecular weight polymer which is nearly neutrally charged, rendering it suitable for steric stabilization of the iron nanoparticles. Electrophoretic mobility measurements demonstrated the ability of guar gum to adsorb on the nanoparticles, forming a slightly negatively charged layer. Dynamic light scattering experiments were conducted to estimate the particle size of the different nanoparticle suspensions and to determine the aggregation behavior at different ionic strengths. Guar gum effectively reduced the hydrodynamic radius of the bare nanoparticles from 500 nm to less than 200 nm and prevented aggregation of the nanoparticles even at very high salt concentrations (0.5 M NaCl and 3 mM CaCl(2)). Sedimentation profiles of the different nanoparticle suspensions confirmed the improved stability of the iron nanoparticles in the presence of guar gum. The results strongly suggest that guar gum can be used to effectively deliver stabilized zero-valent iron nanoparticles for remediation of contaminated groundwater aquifers.     

A simple microfluidic probe of nanoparticle suspension stability

We describe a simple experimental tool that enables stability of multicomponent nanoparticle suspensions to be readily assessed by establishing a confinement-imposed chemical discontinuity at the interface between co-flowing laminar streams in a microchannel. When applied to examine Al(2)O(3) nanoparticle suspensions, this method readily reveals compositions that are susceptible to aggregation even when conventional bulk measurements (zeta potential, dynamic light scattering, bulk viscosity) suggest only subtle differences between formulations. This microfluidic stability test enables simple and rapid assessment of quality and variability in complex multicomponent mixtures for which few, if any, comparable data exist. The paradoxical ease at which localized aggregation can be triggered in suspensions that would otherwise appear stable also serves as a caution to researchers undertaking tracer-based studies of nanomaterial suspensions.     

Slow equilibration of reversed-phase columns for the separation of ionized solutes

Reversed-phase columns that have been stored in buffer-free solvents can exhibit pronounced retention-time drift when buffered, low-pH mobile phases are used with ionized solutes. Whereas non-ionized compounds exhibit constant retention times within 20 min of the beginning of mobile phase flow, the retention of ionized compounds can continue to change (by 20% or more) for several hours. If mobile phase pH is changed from low to high and back again, an even longer time may be required before the column reaches equilibration at low pH. The speed of column equilibration for ionized solutes can vary significantly among different reversed-phase columns and is not affected by flow rate.     

重新审视滴定分析

Volumetric titration analysis is an important content of chemical analysis; but it is traditionally considered to use large amount sample, take long analysis time, have low sensitivity, and involve harsh analytical conditions. This paper provides perspectives for the above issues. Examples are introduced to illustrate that the sample amount can be easily reduced with improved sample throughput by the use of automatic titration analysis. The non-traditional end-point indication method improves the sensitivity with simplified analytical condition. We hope to introduce the different titration analysis and the breadth of applications to students, and thus, to stimulate their interest.     

Determination of lysine pK values using [5-13C]lysine: application to the lyase domain of DNA Pol beta

Determination of the protonation state of titratable protein residues is of critical importance for the interpretation of active site chemistry, as well as for understanding the role of electrostatic interactions in protein folding and stability. However, protein titration studies are limited by the fact that, at extreme pH values, increasing fractions of unfolded or partially unfolded structures may be present. This problem is particularly acute for lysine residues which have high pK values. In the present study, we point out that the use of the 13C resonance of lysine C-5 as a reporter for titration of the epsilon-amino group is preferable to the use of C-6 due to the 5-fold greater titration shift, so that reasonable results can be obtained using a two parameter fit of data obtained over a more limited pH range. A new synthetic procedure for [5-13C]lysine is described, and the pK value for Lys72 in the lyase domain of DNA polymerase beta has been determined using the [5-13C]lysine-labeled enzyme. The results agree well with recent studies of the Pol lambda lyase domain, demonstrating that the pK value for this residue is not optimized for Schiff base chemistry (Gao et al., Biochemistry 2006, 45, 1785-1794). We also have re-evaluated data for the pK of Lys73 in the TEM-1 beta-lactamase.     

Intermolecular interaction studies using small volumes

We present the use of 1-mm room-temperature probe technology to perform intermolecular interaction studies using chemical shift perturbation methods and saturation transfer difference (STD) spectroscopy using small sample volumes. The use of a small sample volume (5-10 μl) allows for an alternative titration protocol where individual samples are prepared for each titration point, rather than the usual protocol used for a 5-mm probe setup where the ligand is added consecutively to the solution containing the protein or host of interest. This allows for considerable economy in the consumption and cost of the protein and ligand amounts required for interaction studies. For titration experiments, the use of the 1-mm setup consumes less than 10% of the ligand amount required using a 5-mm setup. This is especially significant when complex ligands that are only available in limited quantities, typically because they are obtained from natural sources or through elaborate synthesis efforts, need to be investigated. While the use of smaller volumes does increase the measuring time, we demonstrate that the use of commercial small volume probes allows the study of interactions that would otherwise be impossible to address by NMR.     

Colloidal stability of magnetite/poly(lactic acid) core/shell nanoparticles

In this work, we describe an experimental investigation on the colloidal stability of suspensions of three kinds of particles, including magnetite, poly(lactic acid) (PLA), and composite core/shell colloids formed by a magnetite core surrounded by a PLA shell. The experiments were performed with dilute suspensions, so that recording the optical absorbance with time gives a suitable indication of the aggregation and sedimentation of the suspensions. The method allowed us to distinguish very accurately between the different surface and magnetic forces responsible for the structures acquired by particle aggregates. Thus, the pure PLA suspensions are very sensitive to ionic strength and almost unaffected by pH changes. On the contrary, the stability of magnetite systems is mainly controlled by pH. The effect of vertical magnetic fields on the stability of magnetite and magnetite/PLA suspensions is also investigated. The PLA shell reduces the magnetic responsiveness of magnetite, but it is demonstrated that the mixed particles can also form structures induced by the field, despite their lower magnetization, and they can be considered in magnetically targeted biomedical applications.     

Study of electronic effect of Grignard reagents on their electrochemical behavior

The electrochemical behavior of three electrolyte solutions containing Grignard reagents (RMgBr) with different organic groups were investigated with regard to the potential application in rechargeable magnesium battery. It is found that the electrochemical reversibility of magnesium deposition and dissolution processes and the anodic stability of the Grignard electrolyte can be significantly improved by replacing alkyl group with more stable 4-Fluorophenyl group. In addition, the ionic conductivity of the Grignard electrolyte solution is enhanced by 1.5 times by such a replacement. The test results indicate that 4-Fluorophenyl-MgBr/THF solution could be promising for use in rechargeable magnesium battery systems.     

在纯水中高能球磨稀土氧化物制备超细纳米悬浮液

通过在纯水中对稀土氧化物进行高能球磨制备出超细纳米悬浮液,考察了球磨时间、转速等制备条件对悬浮液稳定性以及粒子结构的影响.结果表明，球磨时间和转速的增加有利于提高悬浮液的稳定性,粒子尺寸的大小，分布的均匀性以及表面电荷性质是决定悬浮液稳定性的重要因素.利用这种方法制备的悬浮液具有良好的稳定性和较高的浓度,同时不含分散剂和稳定剂,因而具有表面张力高、粘度低等特点.