Colloid Vibration Potential in a Concentrated Suspension of Spherical Colloidal Particles

A relation between the dynamic electrophoretic mobility of spherical colloidal particles in a concentrated suspension and the colloid vibration potential (CVP) generated in the suspension by a sound wave is obtained from the analogy with the corresponding Onsager relation between electrophoretic mobility and sedimentation potential in concentrated suspensions previously derived on the basis of Kuwabara's cell model. The obtained expression for CVP is applicable to the case where the particle zeta potential is low, the particle relative permittivity is very small, and the overlapping of the electrical double layers of adjacent particles is negligible. It is found that CVP shows much stronger dependence on the particle volume fraction φ than predicted from the φ dependence of the dynamic electrophoretic mobility. It is also suggested that the same relation holds between the electrokinetic sonic amplitude of a concentrated suspension of spherical colloidal particles and the dynamic electrophoretic mobility. Copyright 1999 Academic Press.     

Influence of Sedimentation on Crystallization of Charged Colloidal Particles

The method of density matching between the solid and liquid phases is often adopted to effectively eliminate the effect of sedimentation of suspensions on dynamic behavior of a colloidal system. Experiments on crystallization of charged colloidal microspheres with di-ameter of 98 nm dispersed in density-matched and -unmatched media (mixtures of H₂O and D₂O in proper proportion) are compared to examine the influence of sedimentation. Reflection spectra of colloidal suspensions were used to monitor the crystallization process. Results showed that the crystal size of the density-unmatched (namely, in the presence of sedimentation) sample grew faster than that of the density-matched (in the absence of sedi-mentation) case at the initial stage of the crystallization, and then the latter overtook and outstripped the former. To explain these observations, we assume that in the settling of crystals sedimentation facilitates result in more particles getting into the crystal structures. However, as the crystals increase to varying sizes, the settling velocities become large and hydrodynamic friction strips off some particles from the delicate crystal structures. Overall, the sedimentation appears to accelerate the crystal size growth initially and then retard the growth. In addition, the crystal structures formed under microgravity were more closely packed than that in normal gravity.     

Primary Electroviscous Effect with a Dynamic Stern Layer: Low kappaa Results

The classical treatments of the primary electroviscous effect show important discrepancies with respect to the experimental data. A possible better agreement may be found if the contribution of the ions adsorbed on the Stern layer, which can move tangentially near the particle surface, is taken into account. This contribution has been incorporated into the Watterson-White theory. A study of the influence of the Stern-layer parameters on the primary electroviscous coefficient has been made. Copyright 2000 Academic Press.     

Electrical Conductivity of a Concentrated Suspension of Spherical Colloidal Particles

A general expression for the electrical conductivity of a concentrated suspension of spherical colloidal particles is obtained for the case where the particle zeta potential is low and the overlapping of the electrical double layers of adjacent particles is negligible by using Kuwabara's cell model. It is shown how the conductivity of a concentrated suspension depends on the particle volume fraction, the zeta potential zeta, and the reduced particle radius kappaa (kappa = Debye-Hückel parameter and a = particle radius). It is also found that the obtained conductivity formula tends to Maxwell's formula for two different extreme cases: (i) when the particles are uncharged (zeta = 0) and (ii) when the electrical double layers around the particles are infinitesimally thin (kappaa --> infinity). That is, in the latter limiting case (kappaa --> infinity), the conductivity becomes independent of the zeta potential, just as in the case of dilute suspensions. Copyright 1999 Academic Press.     

Low-Shear Viscosities of Dilute Dispersions of Colloidal Rodlike Silica Particles in Cyclohexane

Low-shear viscosities have been studied for isotropic dispersions of uncharged colloidal silica rods in cyclohexane as a function of volume fraction Φ and aspect ratior_p. Intrinsic viscosities in the limit Φ → 0 agree with theoretical results [Brenner, H.,Int. J. Multiphase Flow1,195 (1974)] for colloidal cylinders. The Φ²term in the low-shear viscosity, accounting for two-particle interactions, is considerably larger than predicted by D. H. Berry and W. B. Russel [J. Fluid Mech.180,475 (1987)] for dilute hard-rod dispersions. The discrepancy is very likely due to weak interparticle attractions, which also account for the novel observation of an isotropic–isotropic phase separation in the silica rod dispersions. Our results demonstrate that, in comparison to spheres, high aspect ratios markedly increase both the shear viscosity and its sensitivity to even minor attractions.     

Effect of Electrolyte Concentration on the Stern Layer Thickness at a Charged Interface

The chemistry and physics of charged interfaces is regulated by the structure of the electrical double layer (EDL). Herein we quantify the average thickness of the Stern layer at the silica (SiO₂) nanoparticle/aqueous electrolyte interface as a function of NaCl concentration following direct measurement of the nanoparticles’ surface potential by X‐ray photoelectron spectroscopy (XPS). We find the Stern layer compresses (becomes thinner) as the electrolyte concentration is increased. This finding provides a simple and intuitive picture of the EDL that explains the concurrent increase in surface charge density, but decrease in surface and zeta potentials, as the electrolyte concentration is increased.     

The Effect of Polydispersity on the Size of Colloidal Particles Determined by the Dynamic Light Scattering

An attempt is made to allow for a sample polydispersity while measuring particle size distribution by the dynamic light scattering. The scattering ability R(r) is calculated for monodisperse samples and samples with rectangular, bell-shaped, and triangular (increasing and decreasing) particle size distributions. It is shown that, when the processing program for the data of light bearing spectroscopy assumes a linear scale of particle sizes, the scattering ability, irrespective of the distribution pattern, is proportional to the sixth power of the particle size for the Rayleigh region and to the second power, for larger particles. However, when the scale of the half-width of the Rayleigh spectrum is linear, the pattern of the curve describing the particle scattering capability substantially changes upon passing from monodisperse to polydisperse samples. This curve can be approximated by a function R r ⁷ in the Rayleigh region and by a function R r ³ in the range of large (as compared with the wavelength) particles.     

球胶态颗粒在中等电位条件下双电层相互作用能和力

根据有效表面电位的定义,使用改进的Derjaguin法,推导出在中等电位条件下等同和不等同球形胶态颗粒相互作用能和力的近似表达式.与精确的数值解相比,对等同球颗粒,表面电位≤100mV;对不等同球颗粒,表面电位≤75mV;最大相对误差均小于±10%,表明该近似式是目前较好的.     

Dynamic Mobility of Particles with Thick Double Layers in a Nondilute Suspension

The dynamic mobility of a nondilute suspension of spherical particles is investigated in the case where the thickness of the electrical double layer around each particle is comparable to the particle radius. A formula is obtained for the O(φ) correction in a random suspension of particles with volume fraction φ, involving an integral over the dynamic mobility of a pair of spheres. This formula is then evaluated using both analytical approximations and numerical results previously obtained for the pair mobilities and valid for low surface potentials. The effect of double-layer thickness on the O(φ) coefficient is most pronounced at low frequencies, and lessens once the hydrodynamic penetration depth is smaller than the particle radius. Various approximations are considered that use the O(φ) result to predict the dynamic mobility in concentrated suspensions, and at high frequencies these approximations are shown to give results qualitatively different from those of recent cell models. Copyright 2000 Academic Press.     

Investigation of Dynamic Stern Layer of Liposomes by Measurements of Conductivity and Electrophoresis

Ionic surface diffusitivity is important parameter for the electrostatic interaction between colloid particles and hence its measurement is a new important factor for the surface characterization of colloid particles. If the adsorbed ion mobility is small the approximation of the interaction at constant charge can be valid,but in opposite case the charge regulation occurs during their interaction.

The large surface diffusitivities of polyvalent ions adsorbed on negatively charged liposomes are found combining measurements of conductivity and electrophoresis. At a rather low surface charge and high adsorbability of polyvalent cations, the diffuse-layer charge can be very small in comparison with the Stern-layer charge, which can predominate in surface conductivity and provide information about the diffusitivity of adsorbed ions if it is not small. An efficient and simple method for the discrimination between small and large surface diffusitivities of adsorbed ions based on the determination of both iso-electric point and iso-conducting point is proposed and experimentally proved.     

Sedimentation Velocity and Potential in Concentrated Suspensions of Charged Spheres with Arbitrary Double-Layer Thickness

The sedimentation in a homogeneous suspension of charged spherical particles with an arbitrary thickness of the electric double layers is analytically studied. The effects of particle interactions are taken into account by employing a unit cell model. Overlap of the double layers of adjacent particles is allowed, and the polarization effect in the double layer surrounding each particle is considered. The electrokinetic equations that govern the ionic concentration distributions, the electric potential profile, and the fluid flow field in the electrolyte solution in a unit cell are linearized assuming that the system is only slightly distorted from equilibrium. Using a perturbation method, these linearized equations are solved for a symmetrically charged electrolyte with the surface charge density (or zeta potential) of the particle as the small perturbation parameter. An analytical expression for the settling velocity of the charged sphere in closed form is obtained from a balance among its gravitational, electrostatic, and hydrodynamic forces. A closed-form formula for the sedimentation potential in a suspension of identical charged spheres is also derived by using the requirement of zero net electric current. Our results demonstrate that the effects of overlapping double layers are quite significant, even for the case of thin double layers. Copyright 2000 Academic Press.     

用积分方程方法决定胶粒之间的空耗势

Binary components Ornstein-Zernike integral equation with the concentration of large particle component being set to zero was employed to study the depletion potential behavior between two large neutral colloid particles （modeled as hard spheres）immersed in a sea of small neutral solvent particles. The prediction for the depletion potential behavior compared well with simulation data and experimental data available in the literature. It is found that the Hansen-Verlet one phase criterion,based on the effective one component system with the present depletion potential,for the freezing transition is completely not suitable for the real binary components system. It is disclosed that the unsuitability is due to the volume term of the solid phase and liquid phase which can not be treated selfconsistently in the Hansen-Verlet one phase criterion.     

不同电解质体系中土壤胶体凝聚动力学的动态光散射研究

利用动态光散射技术研究在不同浓度的KNO3和Mg(NO3)2中土壤胶体颗粒的凝聚过程动力学. 通过分析凝聚过程中光强和有效粒径随时间的变化得到: (1)根据凝聚过程中光强的稳定与否, 可以判断土壤胶体凝聚过程中碰撞的发生是由布朗运动支配还是由重力作用支配; (2)在不同的电解质体系下土壤胶体凝聚表现为快速凝聚特征或不同的慢速凝聚特征, 并且在慢速凝聚中存在一个对重力敏感的电解质浓度; (3)两种电解质作用下的土壤胶体凝聚特征相似, 但对Mg(NO3)2体系浓度变化的敏感性远远大于KNO3体系. 此外, 通过分析凝聚平均速率随电解质浓度的变化, 找到慢速凝聚与快速凝聚的电解质浓度转折点, 即临界絮凝浓度(CFC), 提供了一个实验测定CFC的可能方法.     

Effect of Charge,Size and Temperature on Stability of Charged Colloidal Nano Particles

Molecular simulation of charged colloidal suspension is performed in NVT canonical ensemble using Monte Carlo method and primitive model. The well-known Derjaguin-Landau-Verwey-Overbeek theory is applied to account for effective interactions between particles. Effect of temperature, valance of micro-ions and the size of colloidal particles on the phase stability of the solution is investigated. The results indicate that the suspension is more stable at higher temperatures. On the other hand, for a more stable suspension to exist, lower micro-ion valance is favorable. For micro-ions of higher charge the number of aggregates and the number of particle in each of aggregate on average is higher. However for the best of our results larger colloidal particle are less stable. Comparing the results with theoretical formula considering the influence of surface curvature shows qualitative consistency.     

Clustering of Particles in Colloidal and Molecular Fluids

Abstract

A review is given of the nature and implications of cluster formation in colloidal and molecular systems. We consider large clusters that can be described in terms of the fractal dimension and percolation exponents. The role of computer molecular simulation is discussed as a new method for probing random clustering of particles, short range structural correlations being of diminished importance here.     

醇对均匀胶体粒子形成的影响

Alcohols extremely affect the formation of uniform colloidal particles in the hydrolysis of ferric ions for Fe(NO_3)_3-HNO_3 system. The affecting ability of the alcohols decreases by the order: 2-propanol, ethanol, t-butylalcohol, 1-propanol and n-butanol. The uniform colloidal particles could prepare in the aging solutions containing lower concentration of Fe(NO_3)_3 and higher concentration of HNO_3 in the presence of alcohols comparing with that in the absence of alcohols. The shapes and sizes of the particles formed depend not only on the concentrations of Fe(NO_3)_3 and HNO_3, but also the content of alcohols. In general, lower saturation of precipitating species favours the formation of spherical particles, and high saturation for the elliptic ones.     

The Primary Electroviscous Effect: Thin Double Layers (akappa>1) and a Stern Layer

The primary electroviscous effect due to the charge clouds surrounding spherical charged particles suspended in an electrolyte was studied by Hinch and Sherwood (J. Fluid Mech. 132, 337 (1983)) in the limit of double layers thin compared to the particle radius a. Here we introduce the effect of a dynamic Stern layer into that analysis, in order to explain the numerical results of Rubio-Hernández et al. (J. Colloid Interface Sci. 206, 334 (1998)) in terms of the ratio of the tangential ionic fluxes within the charge cloud to those within the Stern layer. The predictions of the asymptotic analysis are compared with those of numerical computations. The thickness of the charge cloud is characterized by the Debye length kappa(-1). If akappa>10 the predictions of the asymptotic analysis exhibit the same qualitative behavior as the numerical results, but akappa>1000 is required to achieve quantitative agreement to within 2.5%. Copyright 2000 Academic Press.     

电磁处理对水溶液中碳酸钙微粒沉降及附着性能的影响

通过Zeta电位、表面自由能及其分量、粘附功等热力学参数的测定与分析, 研究了电磁处理对等物质的量Na₂CO₃与CaCl₂溶液混合生成的碳酸钙微粒的沉积特性及阻垢机理. 结果显示电磁处理可以使碳酸钙微粒的Zeta电位绝对值减小约5 mV, 同时加快碳酸钙微粒的沉降速度. 经电磁处理生成的碳酸钙微粒的表面自由能为31.59 mJ•m^－2, 比未经处理时减小30%, 同时表面自由能Lifshitz-van der Waals分量从处理前的43.53 mJ•m^－2下降到25.50 mJ•m^－2, 说明成垢溶液的电磁处理可以提高碳酸钙微粒的热力学稳定性. 水溶液中碳酸钙微粒与316L不锈钢表面之间的理论粘附功计算显示, 电磁处理降低了该理论粘附功, 使碳酸钙微粒在不锈钢表面的附着能力下降.