Synthesis of pH-responsive particles with shape anisotropy

Seeded emulsion polymerization is used to produce large quantities of shape anisotropic, amphoteric particles in a size range of about 1 μm. Copolymer dicolloids (CDCs) containing pyridine groups are synthesized by swelling spherical, lightly cross-linked polystyrene seeds with a mixture of styrene and pH-responsive monomer 2-vinyl pyridine followed by secondary polymerization to contrast with their analogue homopolymer dicolloids (HDCs) where the swelling step is carried out with styrene alone. After the particles are coated with a nonionic surfactant to minimize van der Waals attractions, surface potentials and aggregation properties of dilute suspensions are studied as functions of pH and ionic strength. Compared to HDCs, which remain stable at all pH values studied (3 < pH < 9) up to an ionic strength of 5 M, the CDC particles show amphoteric behavior with strong attractions under conditions where dipolar interactions are expected to dominate.     

Diffusing Wave Spectroscopy of aggregating and gelling systems

Diffusing Wave Spectroscopy is a relatively recent but rather under-utilized branch of light scattering. It has the advantage that it can be used in highly-scattering suspensions to measure the motions of the scattering particles. This means that it can be used to study food colloidal systems in their normal state, with no requirement for dilution of the samples. The motions of the particles in a suspension depends on their state (free or limited diffusion, gelation, phase separation, etc.). Therefore, analysis of the light scattering behaviour can give information not only on particle sizes, but on the mechanisms by which they aggregate. In addition to the dynamics of the particles, it is also possible to gather information about the interactions of the particles from the overall turbidity of the suspensions. This review describes the application of the DWS technique to the study of a number of food-related suspensions, and shows how the results can be interpreted for the different systems.     

Nanoparticle shape anisotropy dictates the collective behavior of surface-bound ligands

We report on the modification of the properties of surface-confined ligands in nanoparticle systems through the introduction of shape anisotropy. Specifically, triangular gold nanoprisms, densely functionalized with oligonucleotide ligands, hybridize to complementary particles with an affinity that is several million times higher than that of spherical nanoparticle conjugates functionalized with the same amount of DNA. In addition, they exhibit association rates that are 2 orders of magnitude greater than those of their spherical counterparts. This phenomenon stems from the ability of the flat, extended facets of nonspherical nanoparticles to (1) support more numerous ligand interactions through greater surface contact with complementary particles, (2) increase the effective local concentration of terminal DNA nucleotides that mediate hybridization, and (3) relieve the conformational stresses imposed on nanoparticle-bound ligands participating in interactions between curved surfaces. Finally, these same trends are observed for the pH-mediated association of nanoparticles functionalized with carboxylate ligands, demonstrating the generality of these findings.     

Theory of diamagnetic anisotropy for conjugated monocyclic systems

Deficiencies are pointed out in the theory based on the one-electron approximation. The anisotropy is calculated for a cyclic polyene with 4n electrons in the one-configuration and multiconfiguration approximations for complete axial symmetry and for perturbed axial symmetry. The relation of the paramagnetic term to the perturbation is derived for both approximations.     

Photofixation of carbon dioxide in semiconductor particulate and microbial systems

Photocatalytic reduction of CO₂ to HCOOH and HCHO was carried out in a Pt/CdS/RuO₂ semiconductor particulate system using [Ru^III(EDTA-H)H₂O] complex as catalyst. Upon illumination at 505 nm (band gap energy of CdS), the system produced HCOOH and HCHO at rates equal to 3.05 × l0⁻² Mh⁻¹ and 2.0 × 10⁻² M h⁻¹, respectively. Trace amounts of CH₂OH, CH₄ and CO were also detected in the reaction vessel. Photobiological conversion of CO₂ to formic acid was achieved by usingHalobacterium halobium MMT₂₂ in aqueous solution at a rate equal to 0.45 M h⁻¹. A one-and-half-fold increase in the rate of formation of formic acid was observed when the photobiological reduction of CO₂ was performed in the presence of L-ascorbic acid as electron-donating agent and [Ru^III(bipy)₃]²⁺ as photosensitizer.     

Ligand effects toward the modulation of magnetic anisotropy and design of magnetic systems with desired anisotropy characteristics

Magnetic anisotropy of a set of octahedral Cr(III) complexes is studied theoretically. The magnetic anisotropy is quantified in terms of zero-field splitting (ZFS) parameter D, which appeared sensitive toward ligand substitution. The increased π-donation capacity of the ligand enhances the magnetic anisotropy of the complexes. The axial π-donor ligand of a complex is found to produce an easy-plane type (D > 0) magnetic anisotropy, while the replacement of the axial ligands with π-acceptors entails the inversion of magnetic anisotropy into the easy-axis type (D < 0). This observation enables one to fabricate a single molecule magnet for which easy-axis type magnetic anisotropy is an indispensable criterion. The equatorial ligands are also found to play a role in tuning the magnetic anisotropy. The magnetic anisotropy property is also correlated with the nonlinear optical (NLO) response. The value of the first hyperpolarizability varies proportionately with the magnitude of the ZFS parameter. Finally, it has also been shown that a rational design of simple octahedral complexes with desired anisotropy characteristics is possible through the proper ligand selection.     

Observation of a hexagonal blue phase in systems with negative dielectric anisotropy

Abstract

The occurrence of a three dimensional hexagonal blue phase BPH (possible space groups P6₂22 and P6₄22) in materials with negative dielectric anisotropy ?_a = ?_∥ - ?_? has been detected by the investigation of Kossel diagrams. This is the first observation of a field-induced transition between different blue phase modifications for systems with ?_a < 0. BP I as well as BP II can be transformed to BPH at a sufficiently high field strength E _BP-BPH For neither transitions has any discontinuity of the wavelength of selective reflection in the field direction been observed. Investigations for field strengths E < E _BP-BPH show continuous deformation and reorientation of BP I and BP II. Decreasing (increasing) lattice constants in the field direction are found for BP I oriented with a two-fold (four-fold) axis parallel to E and for BP II oriented with either a four-fold or two-fold axis parallel to E. The preferred orientation in the field is the same as that found for ?_a > 0 (four-fold axis parallel to the field direction).     

Ambiguities in the interpretation of time-resolved fluorescence anisotropy measurements on lipid vesicle systems

Analysis of time-resolved fluorescence anisotropy measurements on DPH and TMA-DPH in POPC vesicles with and without cholesterol in terms of the rotational diffusion model shows two distinct χ_r² minima which are statistically equivalent. This is explained by the fact that the anisotropy decay function is given by a sum of three correlation functions which cannot be uniquely separated into individual contributions. The two solutions yield contradictory results for the effect of cholesterol on the probe dynamics. It is shown that the Maier-Saupe potential and the “wobble-in-cone” model do not give an adequate picture of the orientational order and the reorientational dynamics.     

Applications of the MFI-UF to measure and predict particulate fouling in RO systems

The MFI-UF was developed to include smaller colloidal particles not measured in the existing Silt Density Index (SDI) and MFI_0.45 fouling indices. This research investigates the application of the MFI-UF to measure and predict the particulate fouling potential of reverse osmosis (RO) feedwater and also to assess pretreatment efficiency. MFI-UF measurements were carried out at the IJssel Lake and River Rhine RO pilot plants of the influent feedwater, RO concentrate and after pretreatment processes. Pretreatment efficiency was compared based on MFI-UF, MFI_0.45, and SDI measurements. The MFI-UF of the influent feedwater was approximately 700–2400 times higher than the corresponding MFI_0.45 and SDI, due to the retention of smaller particles. A pretreatment efficiency of ≥80%, was found by the MFI-UF at both pilot plants. For the larger particles the MFI_0.45 gave a 90–100% reduction. Minimum predicted run times for a 15% flux decline from MFI-UF measurements were shorter than that observed at the IJssel Lake pilot plant. This may be explained by problems with the ultrafiltration (UF) pretreatment at the time and/or almost negligible particle deposition in the RO pilot systems. Moreover, it was shown that cake resistance increased with ionic strength in MFI-UF tap water experiments and therefore, a correction of the MFI-UF index is required for salinity effects in RO concentrate.     

Communication: Bulkiness versus anisotropy: the optimal shape of polarizable Brownian nanoparticles for alignment in electric fields

Self-assembly and alignment of anisotropic colloidal particles are important processes that can be influenced by external electric fields. However, dielectric nanoparticles are generally hard to align this way because of their small size and low polarizability. In this work, we employ the coupled dipole method to show that the minimum size parameter for which a particle may be aligned using an external electric field depends on the dimension ratio that defines the exact shape of the particle. We show, for rods, platelets, bowls, and dumbbells, that the optimal dimension ratio (the dimension ratio for which the size parameter that first allows alignment is minimal) depends on a nontrivial competition between particle bulkiness and anisotropy because more bulkiness implies more polarizable substance and thus higher polarizability, while more anisotropy implies a larger (relative) difference in polarizability.     

A new surfactant-fluorescence probe for detecting shape transitions in self-assembled systems

A new kind of fluorescence probe, a fluorophore-labeled anionic surfactant, sodium 12-(N-dansyl)amino-dodecanate (12-DAN-ADA), was designed and synthesized. The applications of 12-DAN-ADA as a fluorescence probe in molecular assemblies, especially in the transitions between micelles and vesicles, were investigated systematically. It was found that 12-DAN-ADA can efficiently differentiate the two different aggregate types (shapes) in mixed cationic and anionic surfactant systems and double-chain cationic surfactant systems. Experimental results showed that the fluorescence anisotropy of 12-DAN-ADA increased sharply, the emission maxima became blue-shifted, and the fluorescence lifetime rose notably when the aggregates transformed from micelles to vesicles in mixed cationic and anionic surfactant systems. The fluorescence anisotropy can also distinguish different aggregate types in single-component double-chain cationic surfactant systems. Further studies demonstrated that 12-DAN-ADA is a more useful probe of transitions between micelles and vesicles than commonly used fluorescence probes, such as pyrene and 1,6-diphenyl-1,3,5-hexatriene (DPH).     

Electric field-induced blue phases in liquid-crystalline systems of high chirality and negative dielectric anisotropy

Electric field effects on liquid-crystalline blue phases (BP) of high chirality and negative dielectric anisotropy have been studied by polarizing microscopy and reflection spectrometry. Temperature-electric field phase diagrams are presented and selective reflections of different blue phases in electric fields are shown. In systems showing only BP I and BP III but lacking a zero field BP II an electric field-induced blue phase was observed in the temperature region of BP III and was identified as a BP II.     

A further test of the shape and anisotropy of the F---H2 interaction potential

A recently proposed anisotropic potential model for the interaction of a fluorine atom with a hydrogen molecule treated as a rigid rotor analysed by carrying out exact quantum calculations of elastic and rotationally inelastic differential cross sections for comparison with previoully reported F---H₂ and newly measured F---D₂ state selected measurements. The sensitivity of the cross sections to changes of the potential anisotropy and to isotopic substitution is examined. The results provide specific indications on the features of the best potential energy surface in terms of its average ‘size’ and its most likely anisotropy responsible for inelastic rotational excitations occuring at collision energies of about 85 meV.     

Adsorption of ionic surfactants in particulate systems: flotation,stability, and interaction forces

The flotation efficiency of silica particles using the ionic surfactants, sodium dodecylbenzenesulfonate (SDbS) and cetyltrimethylammonium bromide (CPB), have been investigated. Results from adsorption, electrophoretic mobility, dispersion stability and direct interaction force measurements are used to develop an understanding of the role of ionic surfactants in particulate flotation. Adsorption and mobility data indicate that SDbS adsorbs at the silica/solution interface, though without improving the flotation efficiency. CPB was found to adsorb on the silica particles as a result of electrostatic interaction; initially to neutralize the surface charge and destabilize the suspension, and at higher surfactant concentrations, to reverse the particle charge and re-stabilize the suspension. Direct force measurements in the presence of CPB confirm that the electrostatic interactions between approaching surfaces are neutralized at low CPB concentrations. Additionally, evidence for a strong adhesive interaction after contact is seen. At higher concentrations, the surfaces begin to recharge, and the adhesive interaction decreases in magnitude. The flotation efficiency was found to correlate well with the measured particle interactions, and to be a function of the particulate electrophoretic mobility.     

Photoelectrochemistry in particulate systems. 5. Visible light-induced polymerization of 1-vinylpyrene in semiconductor suspensions

Photoelectrochemical polymerization of 1-vinylpyrene has been achieved in a semiconductor photocatalyst suspension under visible light irradiation (λ > 480 nm). Bandgap excitation of CdS, CdSe or Fe₂O₃ led to the oxidation of 1-vinylpyrene, which initiated the polymerization process. A 25–47% yield of poly(1-vinylpyrene) could be obtained upon irradiation of these semiconductor photocatalyst suspensions with 1-vinylpyrene in acetonitrile. Dependence of the polymer yield on various experimental conditions is described.