Clustering properties of partly charged superfine particles in a light medium

This paper investigated average cluster sizes （ACS） and cluster size distributions （CSD） at different shear rates by Brownian dynamics in non-, bi-, and uni-polar systems with partly charged superfine particles, The investigation indicates that clusters in non- polar systems are the weakest and easiest to be damaged by increasing shear stresses; charged particles play important and different roles： in bi-polar system, it intends to strengthen clusters to some extent provided that the sign-like ions homogeneously arranged; in uni-polar system charged particles cracked the clusters into smaller ones, but the small clusters are strong to stand with larger shear stress. The relationship between ACS and shear rates follows power law with exponents in a range 0.18-0.28, these values are in a good agreement with experiment range but at the lower limit compared with other systems of non-metallic cluster particles.     

Protein–polysaccharide associative interactions in the design of tailor-made colloidal particles

This article reviews some recent advances in the use of diverse protein–polysaccharide associative interactions in the design of colloidal particles having potential to be used for both fortification of food colloids with health-promoting bioactive compounds with better control of their physical stability and breakdown within the gastrointestinal tract. Protein–polysaccharide associative interactions are discussed in the following aspects: (i) the formation of micro- and nanoparticles for the delivery of health promoting ingredients (nutraceuticals); (ii) the controlled gastrointestinal fate of colloidal particles; (iii) the formation of biopolymer-based particles as fat replacers; and (iv) the behavior of colloidal particles as stabilizers of emulsions and foams. The first aspect concerns soluble protein–polysaccharide complex particles (electrostatic nanocomplexes, complex coacervates, covalent conjugates), mixed hydrogel particles, and nanoemulsion-based delivery systems.     

Steric interactions between physically adsorbed polymer-coated colloidal particles: soft or hard?

The steric interaction potential between colloidal particles imparted by adsorbed polymer layers is directly related to their structure. Due to the complexity of these interfacial structures, the steric potential may behave differently at different interparticle separations. In this study, we proposed a combined model of the equivalent hard-sphere model (EHS) and the Hayter-Penfold/Yukawa model (HPY) to describe the steric potential due to adsorbed homopolymers on colloidal particles. The EHS potential describes the dense train/small-loop region and the HPY potential the more diffuse tail/long-loop region. The steric potential was extracted from the structure factors measured by small-angle neutron scattering (SANS). It was found that this combined model gave better agreement with experimental data than either of its component models alone. This study also shows that the adsorbed polymer layer in a good solvent partially collapses when the layers approach one another, which is also supported by an NMR solvent relaxation study.     

Can linear micelles bridge between two surfaces?

Surfactants are extensively used as stabilizers of colloidal particles, even though the use of high surfactant concentrations can induce a loss of the stability of the dispersion. The depletion mechanism is believed to be responsible for this instability. In this paper, we show that there exists an alternative interpretation, namely that wormlike micelles can bridge between two surfaces. Such a stalk-like object connecting two adsorbed bilayers is (in first order) stable when the endcap (free) energy of the wormlike micelle (in solution) is higher than the connection (free) energy of the stalk with the surface layer. As an example, we consider an aqueous solution of nonionic C(12)E(6) surfactants and use a molecularly realistic self-consistent field approach to evaluate the free-energy of bridge formation. It appears impossible to connect linear micelles to hydrophobic surfaces onto which a monolayer of surfactants exists, and stalks only occur with an exponentially low probability for very hydrophilic surfaces. However, at a wide regime of moderately hydrophilic surfaces the stalks are thermodynamically stable. In this regime, the adsorbed bilayers are typically only marginally stable. We identify a range of parameters for which such adsorbed bilayer ruptures around the stalk and then the wormlike micelle essentially connects (head-on) to the bare surface. The strength of interaction is of the order of the endcap energy which easily exceeds 10 k(B)T. The range of interactions is expected to be large as it is set by the characteristic size of the linear micelles in solution. The regime of moderately hydrophilic surfaces is relevant experimentally, and we conclude that surfactant-induced flocculation may well be the result of stalks. The depletion mechanism is only expected for systems with extremely hydrophobic and with very hydrophilic particles.     

Approximate analytic solution of the nonlinear Poisson–Boltzmann equation for spherical colloidal particles immersed in a general electrolyte solution

By means of polynomial approximation and iteration procedure using linearization solution as initial solution, the non-linear Poisson–Boltzmann equation describing a spherical colloidal particle immersed in an arbitrary valence and mixed electrolyte solution is solved analytically, and analytical expressions for electrical potential distribution ψ(r) and surface charge density/surface potential relationship (σ/ψ ₀) are acquired. The σ/ψ ₀ expression performs very well for the entire range of a reduced colloidal radius x ₀ only if a reduced surface potential y ₀ is lower than 15.5; particularly, in the case of x ₀ >10, the σ/ψ ₀ expression applies over the entire range of y ₀, and maximum of the absolute value of percent relative error (PRE) is not larger than 2 if one does not come across a parameter combination as of an extreme case of x ₀?<?0.9 and y ₀?>?15.5. The ψ(r) expression outperforms the linearization solution greatly and performs very well for the domain of x ₀?≤?1.5 and y ₀?≤?4.5 with maximum of the absolute value of PRE not larger than 3, whereas that of the linearization solution may stand 22 for the same parameter range. It is concluded that the present σ/ψ ₀ expression is the first one valid for entire range of x ₀, given that y ₀ is not unreasonably high, and the present paper proposes, to some extent, a “universal” way for solving nonlinear differential equations.     

Electrical interaction between two rod-like particles covered by an ion-penetrable membrane in a water–oil interface

The electrical interaction between two long, parallel rod-like particles in a water–oil interface is investigated based on a Green function method, which is applicable to a system containing particles with different physical parameters. This is a highly desirable feature from practical considerations. We consider the case where each particle is covered by a membrane, and assumes a general class of shapes. This extends previous results in the literature in that they can be recovered as the special cases of the present model. We show that: (1) The higher the ionic strength in bulk water phase, the less stable the system concerned. (2) The closer the shape of the particles to a cylinder, the more stable the system under consideration. (3) The larger the fraction of particle immersed in the water phase, the greater the electrical interaction force. (4) The electrical interaction force increases with the increase in both the surface potential ratio between two particles and the fixed charge density in the membrane.     

Analytical solutions of nonlinear Poisson–Boltzmann equation for colloidal particles immersed in a general electrolyte solution by homotopy perturbation technique

We present the first application of the homotopy perturbation technique to analytically solve the nonlinear PB equation describing spherical and planar colloidal particles immersed in an arbitrary valence and mixed electrolyte solution. Analytical expressions for electrical potential distribution and surface charge density/surface potential relationship are acquired. Our analytical solutions contrast sharply with previous ones by two striking features: (1) the present ones apply irrespective of the types of electrolyte considered or whether single electrolyte or mixed electrolytes are being considered, and (2) the valid application scopes of present solutions are in small κa domain and thus are complementary with those of previous ones. Our expressions are considered to provide the constituents whose combinations with previous solutions may end up global valid expressions.     

Intramolecular Förster energy transfer in a dendritic system at the single molecule level

The photophysics of a dendrimer containing four donor chromophores and one acceptor chromophore are studied at the single-molecule level. Upon excitation of the donors exclusive acceptor emission is observed due to efficient F?rster energy transfer. For 70% of the molecules donor emission is observed after bleaching of the acceptor, leading to a reduction of the F?rster energy transfer efficiency. Furthermore, we demonstrate that in this molecular system the donor chromophores do not bleach by a triplet-sensitized photooxidation.     

How Can a Carbene be Active in an Ionic Liquid?

The solvation of the carbene 1‐ethyl‐3‐methylimidazole‐2‐ylidene in the ionic liquid 1‐ethyl‐3‐methylimidazolium acetate was investigated by ab initio molecular dynamics simulations in order to reveal the interaction between these two highly important classes of materials: N‐heterocyclic carbenes with superb catalytic activity and ionic liquids with advantageous properties as solvents and reaction media. In contrast to previously published data on analogous systems, no hydrogen bond is observed between the hypovalent carbon atom and the most acidic ring hydrogen atoms, as these interaction sites of the imidazolium ring are predominantly occupied by the acetate ions. Keeping the carbene away from the ring hydrogen atoms prevents stabilization of this reactive species, and hence any retarding effect on subsequent reactions, which explains the observed high reactivity of the carbene in acetate‐based ionic liquids. Instead, the carbene exhibits a weaker interaction with the methyl group of the imidazolium cation by forming a hitherto unprecedented kind of C???H?C hydrogen bond. This unexpected finding not only indicates a novel kind of hydrogen bond for carbenes, but also shows that such interaction sites of the imidazolium cation are not limited to the ring hydrogen atoms. Thus, the results give the solute–solvent interactions within ionic liquids a new perspective, and provide a further, albeit weak, site of interaction to tune in order to achieve the desired environment for any dissolved active ingredient.     

Slow vertical deposition of colloidal crystals: a Langmuir-Blodgett process?

For an evaporating colloidal suspension in which the evaporation velocity exceeds the sedimentation velocity, particles will accumulate at the solvent-air interface. If neither diffusion nor convection can disperse this accumulation, it is expected to grow into a colloidal multilayer several microns thick. We observe that the thickness of colloidal crystals vertically deposited from 1 mum diameter polystyrene latex suspensions of 0.002 < or = phi < or = 0.008 increases linearly with distance in the growth direction and that these thickness profiles are consistent with their growth from a horizontal colloidal layer accumulated beneath the solvent-air interface. We describe a means for performing vertical deposition at growth rates slower than the evaporation rate by adding solvent to the bottom of the colloidal suspension and observe that halving the growth rate of vertical deposition increases both the thickness and the reflectivity of the resulting colloidal crystals, effects indistinguishable from those of doubling the concentration of the colloidal suspension, data also consistent with the colloidal crystals' growth from a horizontal layer of particles beneath the interface. If sufficiently little reorganization is involved as particles move from this horizontal layer to the vertically deposited colloidal crystal, slow vertical deposition of polymer microspheres might be thought of as the Langmuir-Blodgett transfer of a horizontal colloidal crystal onto a vertical substrate. Colloidal crystals deposited using both high concentration and slowed growth can have peak IR reflectance in excess of 80%, exceeding most published values. These observations provide a conceptual framework for engineering vertically deposited colloidal crystals that combine thickness with good optical performance.     

Intramolecular directional förster resonance energy transfer at the single-molecule level in a dendritic system

We report on the directional F?rster resonance energy transfer (FRET) process taking place in single molecules of a first (T1P4) and a second (T2P8) generation of a perylenemonoimide (P)-terrylenediimide (T)-based dendrimer in which the chromophores are separated by rigid polyphenylene arms. At low excitation powers, single-molecule detection and spectroscopy of T1P4 and T2P8 dendrimers point to a highly efficient directional FRET from P donors to the central T acceptor, optical excitation at 488 nm resulting in exclusively acceptor emission in the beginning of the detected fluorescence intensity. Donor emission is seen only upon the bleaching of the acceptor. High-resolution time-resolved single-molecule fluorescence data measured with a microchannel plate photomultiplier reveal, for T2P8, a broad range of FRET rates as a result of a broad range of distances and orientations experienced by the donor-acceptor dendrimers when immobilized in a polymer matrix. Single-molecule data from T2P8 on 488 nm excitation are indicative for the presence, after terrylenediimide bleaching, of a P-P excited dimer characterized by a broad emission spectrum peaking around 600 nm and by fluctuating fluorescence decay times. At high excitation powers, single T1P4 and T2P8 molecules display simultaneous emission from both donor and acceptor chromophores. The effect, called "exciton blockade", occurs due to the presence of multiple excitations in a single molecule.     

Electrosynthesis of polyphenylpyrrole coated silver particles at a liquid–liquid interface

This communication reports the production of polyphenylpyrrole coated silver nanoparticles at the liquid/liquid interface by an EC-type mechanism. In the electrochemical step of the reaction N-phenylpyrrole facilitates the transfer of the silver ion from an aqueous to an organic phase. This step is followed by a slow homogeneous electron transfer reaction from the N-phenylpyrrole to the silver ion followed by polymerization and metal cluster growth.     

Förster resonance energy transfer in nanoclusters of InP@ZnS colloidal quantum dots with dodecylamine ligand shells

Förster resonance energy transfer between InP@ZnS hydrophobic colloidal quantum dots of two different sizes has been studied in the closely packed nanoclusters formed spontaneously in an organic solvent upon the addition of a precipitating solvent. The quantum dots had a core@shell structure and were stabilized by dodecylamine ligands.     

Putting Titania Shells in a Polymer: Which Properties Can Be Achieved?

Based on three‐dimensional morphology‐adaptive meshes, we use a finite‐element approach to predict the overall elastic and dielectric properties of a solid polymer reinforced with hollow titania shells. We demonstrate that there is an optimal wall thickness maximizing the overall specific properties. Interestingly, for both the stiffness and dielectric constants, it is the spherical titania shells with a wall thickness of about 10% of the radius that allows one to attain the maximal specific performance.     

Can a single oxidant with two spin states masquerade as two different oxidants? A study of the sulfoxidation mechanism by cytochrome p450

Density functional calculations were performed on the sulfoxidation reaction by a model compound I (Cpd I) of cytochrome P450. By contrast to previous alkane hydroxylation studies, which exhibit a dominant low-spin (LS) pathway, the sulfoxidation follows a dominant high-spin (HS) reaction. Thus, competing hydroxylation and sulfoxidation processes as observed for instance by Jones et al. (Volz, T. J.; Rock, D. A.; Jones, J. P. J. Am. Chem. Soc. 2002, 124, 9724) are the result of a two-state reactivity scenario, whereby the hydroxylation originates from the LS pathway and the sulfoxidation from the HS pathway. In this manner, two spin states of a single oxidant (Cpd I) can be disguised as two different oxidants. The calculations rule out the possibility that a second oxidant (the ferric peroxide, Cpd 0 species) interferes in the observed results of Jones et al.     

Hydroxyl functional polyester dendrimers as stabilizing agent for preparation of colloidal silver particles—a study in respect to antimicrobial properties and toxicity against human cells

The presented study concerns the preparation and investigation of silver particles in presence of hydroxylated polyester dendrimers used as stabilizing agent. Altogether a full series of water soluble and aliphatic bis-MPA dendrimers from first to fifth generation was used as to stabilize silver nanoparticles in situ. A special focus is set on the biological properties. The antibacterial properties of the dendrimer stabilized silver particles are tested against Escherichia coli and the toxicity against human cells is tested with the human epithelial cell line A549. Under the chosen testing arrangement, it was observed that the silver particles contain a significant antibacterial effect against E. coli. Silver particles stabilized in situ with dendrimers of higher generation seem to contain a stronger antibacterial property. No toxicity against human cells was observed for the silver particles even in case of the highest investigated silver concentration. Altogether the here prepared and investigated silver particles could offer a great potential for application as antibacterial agent with low human toxicity.     

Can auditing save us from a quality disaster?

Auditing of clinical laboratories is a recognized component of quality assurance practice. National regulatory standards, such as the U.S. Clinical Laboratory Improvement Amendments (CLIA), and international voluntary standards, such as those promulgated by the International Organization for Standardization (ISO), are used in the auditing. Professional organizations have long recognized the benefit of on-site, peer review. However, recent events in the U.S. question the validity of current auditing practices. To be effective, auditing must be part of a continuous quality improvement system and a key component of laboratory leadership.

Can nanofiltration be fully predicted by a model?

There is an increasing need for model-based tools to design membrane processes for new industrial applications or to optimise existing membrane installations. The advantage of such tools is that costs can be saved by reducing the number of expermiments. In this study, the requirements for a membrane filtration model, suitable for practical use, are summarised. It is investigated to what extent it is possible to set-up such a model with the current available literature and knowledge. A membrane filtration model has been set-up based on the Maxwell–Stefan transport equations. A Freundlich equation is used to describe the membrane charge by means of adsorption of ions. With the model the permeate flux and rejections of multi-component liquid feeds can be calculated as a function of membrane properties (mean pore size, porosity, thickness, surface charge characteristic) and feed pressure. With two NF-membranes (Desal 5DK and a prototype capillary type 2 membrane) rejection experiments have been carried out with glucose, single salt solutions (NaCl, CaCl₂, Na₂SO₄) and ternary ion mixtures of these salts. With the model the experimental flux-rejection curves can be fitted reasonably well. However, each salt mixture needs its own set of fitted parameters for the membrane charge isotherms. Furthermore, the fitted membrane charges are in contradiction with values from the literature obtained by electrokinetic measurements. Obviously, the membrane charge parameters have lost their physical meaning and are used to compensate for physical phenomena not included in the model. Extending the model with an electrostatic free energy term will be a step forward in development. Further research is needed to fulfil all requirements for the wide scope of industrial applications.