Influence of Interfacial Tension on Seeded Calcium Carbonate Scale Precipitation: Effect of Adsorbed Asphaltenes

Clay particles with adsorbed asphaltenes, which are commonly found in produced water, have been used as seed particles during precipitation of calcium carbonate in order to determine whether such particles may influence the kinetics of precipitation. The results show that the presence of the adsorbed asphaltenes accelerates the precipitation, and there is also a significant difference between different types of adsorbed asphaltenes. The adsorption of asphaltenes at the seed surface leads to a significant increase in the interfacial tension between the seed surface and the aqueous solution, and calcium carbonate therefore precipitates at the seed surface in order to reduce this high interfacial tension.     

Difference in enzyme activity and conformation of glucose oxidase before and after purification

Enzyme activity of commercial glucose oxidase was enhanced after purification through a strong anionic exchange resin. In order to get a better insight into this phenomenon, surface pressure–area (π–A) isotherms and surface pressure–time (π–t) isotherms was used to study the interaction and the absorption at different pH values of the subphases between octadecylamine and glucose oxidase purified by a styrene system quaternary ammonium type strongly basic anionic exchange resin. Circular dichroism (CD), electrophoresis and enzyme activity measurements were conducted to study these phenomena. A preliminary hypothesis has been suggested to explain why the enzyme activity of purified glucose oxidase was higher than that of the commercial one.     

Surface and Subsurface Oxygen on Platinum in a Copper Perchlorate Solution

The formation of an adatom layer on polycrystalline platinum and the three-dimensional nucleation of copper in a copper perchlorate solution are studied by cyclic voltammetry at 0.1 V s^–1 while varying potential ranges and by recording potentiostatic current transients. About 0.6 monolayers of copper adatoms are deposited when cycling with anodic limit E _a = 1.35 V, the process is slower than that in an acid sulfate solution. Decreasing E _a accelerates the process (nearly one monolayer forms for E _a = 0.80–0.95 V in a cathodic scan) due to an increased number of active centers (metastable copper oxides) and, probably, to a change in the platinum surface microstructure. Oxygen for copper oxides is presumably supplied by water molecules adsorbed on a monolayer of copper adsorbed atoms and by subsurface oxygen (O_ss), which appears on the platinum surface after the destruction of complexes O_ss–Pt _n –ClO₄. Both the copper nucleation and the deposit growth accelerate at higher concentrations of copper oxides, which form at low E _a. High cathodic overvoltages decrease the number of active crystallization centers due to reduction or removal of copper oxides.     

Evaluation of the enantiomeric selectivity in the chiral ligand-exchange chromatography of amino acids by a computational model

The chromatographic resolution of enantiomeric amino acids is accomplished on a reversed phase column using aqueous mobile phase containing the chiral reagent N,N-dimethyl-S-phenylalanine-Cu(II). The separation is a result of the whole interaction between the diastereomeric complex surface and the mixed stationary phase realized by the dynamic coating of the RP-18 carbon chains layer. The elution order seems to be related to the different water coordination capability on copper ion in the formation of the mixed ternary complexes.     

Analysis of particle formation under monomer-starved conditions in emulsion polymerization reactors

An important characteristic of monomer-starved nucleation in semibatch reactors is that the rate of growth of particles is controlled by the rate of monomer addition. The reduced rate of growth of particles prolongs the nucleation interval by slowing down the rate of emulsifier micelle depletion and forms a larger number of particles (N_p). Model calculations show how N_p varies with the formulation parameters as the monomer-flooded nucleation shifts into monomer-starved one. Particle formation in the intermediate conversion of interval III of the styrene batch emulsion polymerization also showed an enhancement because of a low rate of growth of newly formed particles. However, at a higher conversion, the rate of particle formation decreased significantly. Modeling results show that the reduction in the rate of particle formation at high conversions could not be simply explained by existing theories which rely on the decrease in monomer concentration in the aqueous phase as a means to explain the decrease in the rate of radical capture.     

Atomistic in situ investigation of the morphogenesis of grains during pressure-induced phase transitions: molecular dynamics simulations of the B1-B2 transformation of RbCl

The mechanistic details of the pressure-induced B1-B2 phase transition of rubidium chloride are investigated in a series of transition path sampling molecular dynamics simulations. The B2→B1 transformation proceeds by nucleation and growth involving several, initially separated, nucleation centers. We show how independent and partially correlated nucleation events can function within a global mechanism and explore the evolution of phase domains during the transition. From this, the mechanisms of grain boundary formation are elaborated. The atomic structure of the domain-domain interfaces fully support the concept of Bernal polyhedra. Indeed, the manifold of different grain morphologies obtained from our simulations may be rationalized on the basis of essentially only two different kinds of Bernal polyhedra. The latter also play a crucial role for the B1→B2 transformation and specific grain boundary motifs are identified as preferred nucleation centers for this transition.     

Unraveling the Pressure Effect on Nucleation Processes of Amyloidogenic Proteins

The influence of pressure on the nucleation rate of insulin under fibril‐forming conditions was studied and subsequently analysed using classical nucleation theory. The aim was a better understanding and quantification of the influence of pressure on protein aggregation/fibrillation reactions. The application of pressure has a drastic accelerating effect on the nucleation and growth process of insulin fibrils. We show that this effect arises from a volume decrease upon nucleus formation, due to formation of a less hydrated and more compact transition state that can be quantified extending nucleation theory by a pressure–volume term. Conversely, the absolute values of the lag time and the critical size of the nucleus cannot be satisfactorily described by the classical nucleation theory, which might be due to the presence of secondary effects, such as parallel aggregation pathways or fragmentation processes.     

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape‐controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution‐phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape‐controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.     

MMA/BA种子半连续乳液聚合成核过程及机理研究

引入一种不溶于水的染色剂(BL-S)作示踪剂,研究甲基丙烯酸甲酯(MMA)/丙烯酸丁酯(BA)种子半连续乳液聚合中各变量对成核过程及成核机理的影响,运用最终乳胶粒中染色剂的含量(Pdye)、最终乳胶粒子数(Npc)、胶束成核和均相成核所形成的粒子数目(Nm和Nh)等参数对聚合过程中的成核情况进行定量分析.结果发现,当引发剂浓度[I]增大时,Pdye、Nh、Nm和Nh/Npc均随之增大,同时Nm/Npc相应地减小,且Nm/Npc=-0.0262[I]+0.8833,表明均相成核随[I]的增大而增加,但胶束成核的比例减小.乳化剂浓度[E]在不同范围内对成核机理的影响不同,在[E]=0.7216×10-2mol.L-1时,体系中胶束成核和均相成核比例相等,各为50%;当[E]>0.7216×10-2mol.L-1时,随[E]增大,成核时间t1,2逐渐缩短,Nm/Npc增加,Nh/Npc减小,胶束成核所占比例大于均相成核,胶束成核逐渐上升为主要成核方式;反之当[E]<0.7216×10-2mol.L-1时,体系中胶束成核所占比例小于均相成核,均相成核为主要成核方式.当MMA的摩尔分率fMMA由0增至0.6时,Nm/Npc从80.93%下降至50%,体系中以胶束成核为主,均相成核为辅;当fMMA由0.6增至1时,Nm/Npc已降至40%,而Nh/Npc增至60%,体系中已转变成均相成核为主,胶束成核为辅.在常规和种子半连续乳液聚合中,t1,2分别为12min和6min,而Pdye变化较小,表明聚合方式只影响粒子的形成过程和成核时间的长短,对成核方式影响甚微.     

用密度梯度理论研究缔合流体的汽液成核性质

在密度梯度展开的基础上,将影响参数k 表达成温度的函数,建立了一个适用于均相和非均相缔合流体的状态方程。应用流体的蒸汽压和液相密度实验数据关联分子参数。在密度梯度理论的框架下,计算了水,重水,甲醇,乙醇,正丙醇,正丁醇,正戊醇和正己醇的成核速率并与实验数据进行了对比,计算结果令人满意。结果表明,密度梯度理论与密度泛函理论一样,可研究液核的结构和性质,但通过调整影响参数k, 可获得更为准确的成核速率。