Spectroscopic characterization of core and shell regions in monolayer protected gold nanoparticles

We report studies on the structure of the metallic core and the alkyl cap layer in monolayer protected gold nanoparticles having sizes down to 1.6 nm. These particles are obtained by laser ablating gold targets in alkane-thiol solutions at different concentrations. The electronic structure of gold core and the vibrational properties of the capping hydrocarbon chains reveal effects connected with the nanosized nature of the particles.     

Fundamental reactions in TiO2 nanocrystallite aqueous solutions studied by pulse radiolysis

Reactions of the hydrated electron, H atoms, 2-propanol, and methanol radicals with the TiO₂ nano-particles have been studied either directly or by competition kinetics. The radicals were produced by radiolysis of 2-propanol, t-butanol, or methanol aqueous solutions in acid pH's. The reactions involve electron injection to the conduction band. As expected, the t-butanol radical is inert towards TiO₂ under our conditions, while the other reducing radicals react with TiO₂. The reactivity decreases in the order: e_aq⁻>H>CH₃COHCH₃>CH₂OH. Two TiO₂ nanocrystallite sizes, with average diameters of 1.0 and 4.7 nm were compared. For equal concentrations (in terms of TiO₂ molecules), the rate of electron injection shows relatively little dependency on particle size. The rates of interfacial electron transfer and transfer coefficient are also reported.     

Colloid stability of electrostatically stabilized sols

 The dependence of the stability of SiO₂ and Al₂O₃ sols on the pH of their medium has been studied. Vapor adsorption isotherms on powders were prepared from acidic and basic sols, the immersion enthalpy (heat) of samples containing preadsorbed water have been determined and, also, the reversibility of the sol ⇌ xerogel transformation, i.e. the peptizability of the powders, has been investigated. Based on the flocculation values determined with KCl, the sols have been classified into three groups. The stability of highly hydrophilic sols (acidic SiO₂- and Al₂O₃-sols) is ensured by a thick continuous diffuse lyosphere formed around the particles, as the continuity principle by Ostwald–Buzágh suggests. In this case, no electric charge is needed for ensuring stability. These sols are thermodynamically stable (group 1). Sols with medium stability are stabilized by the electrical double layer around the particles and by 1–2 layers of adsorbed water. The flocculation value of these sols is determined by the electrostatic interaction, whereas the peptizability of the flocs is related to formation of water layers. Such sols are the basic SiO₂- and Al₂O₃-sols (group 2). Sols of low stability are of hydrophobic nature. Their flocculation value with 1:1 electrolytes is smaller than 0.1 molkg^-1. The transformation process sol ⇒ floccule ⇒ xerogel is mostly irreversible. There are a lot of such sols (group 3). The existence of a hydrosphere is proved by the almost identical value of the hydration energy for both the acidic and the basic SiO₂ sols, in spite of an order of magnitude difference in the flocculation value. The remnants of adsorbed water after drying hinders sintering of the particles and ensures the peptizability of powders. The highly hydrophilic sols (sequence of hydrophilicity: SiO₂> Al₂O₃>FeO OH ⋅ 0.5 H₂O) are all oxides which are formed in acidic media. Received: 25 May 1997 Accepted: 13 October 1997     

带电胶体离子扩散与交换动力学的比较

Taking soil colloid and hydrated silica (quartz sand) as the experimental material, the comparative study has been made on the kinetics of ion diffusion and ion exchange in charged colloid and charged coarse disperse systems. The results showed that ion exchange kinetics in the two systems conform to the kinetic law of ion diffusion. Besides, through this comparative study on the kinetics of ion exchange and ion diffusion, a method has been advanced theoretically to estimate the quantity of adsorbed ion that is located in the inner of the Helmholtz layer. As far as hydrated silica is concerned , there were about 33 per cent of the total adsorbed quantity of Mg2+that were located in the inner of the Helmholtz layer under the given experimental conditions, bu tfor soil colloid the percentage was only 7.5.     

A colloid perspective of hair cell cilia: a selective literature review

Cilia of hair cells are structurally similar to unilaminar phospholipid vesicles. The close juxtaposition of adjacent cilia is similar to the intervesicle distances found in groups of vesicles. Both cilia and vesicles operate in similar ionic environments. By comparing the cross section of cilia, which are cylinders with the cross section of vesicles, which are spherical, we can see how colloid theory can be applied to both cilia and vesicles. While vesicles have been studied as colloid particles, thus far colloid theory has not been applied to hair cell cilia. This paper presents a basic explanation of colloid theory in a simple graphic form that facilitates a colloid perspective of hair cell cilia behavior. A review of relevant hair cell cilia behavior supports the hypothesis that colloid knowledge is applicable to the problem of understanding cilia functionality in the hair cell. The electromagnetic nature of colloid forces allows for their involvement in the relatively high speed operation required of hair cells which are dealing with signals of up to 2×10⁵ Hz. A fresh look at the biophysics of the hair cell from a colloid perspective may lower the barrier to a closer understanding of active mechanical sensory transduction, amplification and adaptation, and suggest a new domain for the application of colloid theory.     

The lift force on a charged sphere that translates and rotates in an electrolyte

The distortion of the charge cloud around a uniformly charged, dielectric, rigid sphere that translates and rotates in an unbounded binary, symmetric electrolyte at zero Reynolds number is examined. The zeta potential of the particle ζ is assumed small relative to the thermal voltage scale. It is assumed that the equilibrium structure of the cloud is slightly distorted, which requires that the Péclet numbers characterizing distortion due to particle translation, , and rotation, , are small compared to unity. Here, a is radius of the particle; D is the ionic diffusion coefficient; and , where U and Ω are the rectilinear and angular velocities of the particle, respectively. Perturbation expansions for small and are employed to calculate the nonequilibrium structure of the cloud, whence the force and torque on the particle are determined. In particular, we predict that the sphere experiences a force orthogonal to its directions of translation and rotation. This “lift” force arises from the nonlinear distortion of the cloud under the combined actions of particle translation and rotation. The lift force is given by . Here, ε is the permittivity of the electrolyte; is the Debye length; and is a negative function that decreases in magnitude with increasing . The lift force implies that an unconstrained particle would follow a curved path; an electrokinetic analog of the inertial Magnus effect. Finally, the implication of the lift force on cross‐streamline migration of an electrophoretic particle in shear flow is discussed.     

Effect of Rough Nanoparticle Orientation on DLVO Energy Interaction

This study investigated the effect of the orientation of rough nanoparticles on Derjaguin and Landau, Verwey and Overbeek (DLVO) energy interaction. Rippled sphere model was used to survey van der Waals attraction energy and repulsive double-layer interaction energy between nanoparticles. The effect of particle size, asperity size, number of asperities, and concentration was studied for different orientations. Spherical coordinates were used to evaluate the effect of changes in orientation under controlled conditions. Surface element integral method was used to calculate DLVO energy interactions between rough particles at specific orientations. The DLVO energy results show that a change in orientation significantly affected the stability of the nanoparticles. The stability of dispersion varied as the contact surface between nanoparticles changed.     

Time-resolved particle image velocimetry analysis and computational modeling of transient optically induced electrothermal micro vortex

Trapping, sorting, transportation, and manipulation of synthetic microparticles and biological cells enable investigations in their behavior and properties. Microfluidic techniques like rapid electrokinetic patterning (REP) provide a non-invasive means to probe into the nature of these micro and nanoparticles. The opto-electrically induced nature of a REP micro vortex allows tuning of the trap characteristics in real-time. In this work, we studied the effects of transient optical heating on the induced electrothermal vortex using micro-particle image velocimetry (μ-PIV) and computational modeling. A near infra-red (980 nm) laser beam was focused on a colloidal suspension of 1 μm polystyrene beads sandwiched between two parallel-plate electrodes. The electrodes were subjected to an AC current. The laser spot was scanned back-and-forth in a line, at different frequencies, to create the transient vortex. This phenomenon was also studied with a computational model made using COMSOL Multiphysics. We visualize fluid flow in custom-shaped REP traps by superposing multiple axisymmetric (spot) vortices and discuss the limitations of using superposition in dynamically changing traps.     

熵驱大分子胶体粒子的柱状受限结晶

熵是物理化学的基本状态参量,在统计力学和热力学中处于核心位置.按照玻尔兹曼的微观解释,熵可以由孤立系统微观状态的数目(W)给出,即S=kBlnW,这里kB为玻尔兹曼常数[1,2].根据此公式,微观状态数越多,系统越混乱,熵越大,所以熵常被视作体系无序程度的度量.但熵增仅对应体系微观状态数的增加,与可观测的结构有序程度无关[3~5].在一些典型的软物质体系中,结构越有序熵反而越大,如胶体硬球在随机密堆积点的有序结晶[6]及描述各向异性棒状分子从各向同性相到向列相转变的Onsager原理[7].     

Principles of Surface Potential Estimation in Mixed Electrolyte Solutions:Taking into Account Dielectric Saturation

The dielectric properties between in-particle/water interface and bulk solution are significantly different, which are ignored in the theories of surface potential estimation. The analytical expressions of surface potential considering the dielectric saturation were derived in mixed electrolytes based on the nonlinear Poisson-Boltzmann equation. The surface potentials calculated from the approximate analytical and exact numerical solutions agreed with each other for a wide range of surface charge densities and ion concentrations. The effects of dielectric saturation became important for surface charge densities larger than 0.30 C/m\begin{document}$ ^2 $\end{document}. The analytical models of surface potential in different mixed electrolytes were valid based on original Poisson-Boltzmann equation for surface charge densities smaller than 0.30 C/m\begin{document}$ ^2 $\end{document}. The analytical model of surface potential considering the dielectric saturation for low surface charge density can return to the result of classical Poisson-Boltzmann theory. The obtained surface potential in this study can correctly predict the adsorption selectivity between monovalent and bivalent counterions.