Colloids anisotropic particles

In colloidal suspensions of anisotropic particles, the static structure factor plays a prominent role in particle size analysis. We have used transient electric birefringence (TEB) and electron microscopy, in addition to laser light scattering, to correlate our analysis of particle size distributions of bentonite suspensions. The complementary nature of TEB and photon correlation spectroscopy (PCS) in particle size analysis will be discussed. 

In the case of crystals, one usually encounters structures that are anisotropic on a molecular scale, due to anisotropic interactions between the molecules. Clusters formed by colloidal-scale particles often do not exhibit anisotropy due to the isotropic character of the interactions on that colloidal scale. One may introduce anisotropy in that case by applying an external field, for example, a flow field or electric-magenetic field, and subsequently freezing the morphology by, for example, gelation. 

We have seen that it can be difficult to reach the critical concentration required to observe an isotropic-anisotropic transition because concentrated suspensions of colloids are not always stable. However, orientation of flexible polymers as well as of anisotropic particles in suspension can be induced by flow, a phenomenon that has long been observed, reported, and studied. This phenomenon is especially strong when a pretransitional effect exists, which can be easily observed by the naked eye on a sample that is shaken between crossed polarizers (see for example the section on clays). In these systems, birefringence is induced via mechanical forces, like the shear stresses in a laminar flow ( Maxwell-dy-namo-optic effect ). 

The previous sections reviewed recent advancements in sequential electrostatic assembly to form NP-shelled structures. An alternate route to NP assembly arises from interfacial activity and stabilization of NPs. Colloidal particles with partial hydrophilic and hydrophobic character are known to behave like surface-active molecules (surfactants), particularly when adsorbed to a fluid-fluid interface. The assembly of small particles at interfaces is of relevance to advance fields that traditionally feature emulsions, foams, and flotation systems. It is also of pertinence to the development of new fields such as the synthesis of novel materials that include Janus particles, colloidosomes, porous solids, and anisotropic particles, all recently prepared by particle assembly at interfaces [36,38]. 

The amount of colloidal silica present in an aqueous solution of high ratio alkali metal silicate can be determined for example by ultrafiltration. Ultrafiltration refers to the efficient selective retention of solutes by solvent flow through an anisotropic skinned membrane such as the Amicon Diaflo ultrafiltration membranes made by the Amicon Corporation of Lexington, Mass. In ultrafiltration solutes, colloids or particles of dimensions larger than the specified membrane cut-ofT are quantitatively retained in solution, while solutes smaller than the uniform minute skin pores pass unhindered with solvent through the supportive membrane substructure. 

The list could be made longer, taking the idea of electrokinetics in a wide sense (response of the colloidal system to an external field that affects differently to particles and liquid). Thus, we could include electroviscous effects (the presence of the EDL alters the viscosity of a suspension in the Newtonian range) suspension conductivity (the effect of the solid-liquid interface on the direct current (DC) conductivity of the suspension) particle electroorientation (the torque exerted by an external field on anisotropic particles will provoke their orientation this affects the refractive index of the suspension, and its variation, if it is alternating, is related to the double-layer characteristics). 

On the other hand, the method of flow birefringence, of course, allows one to study how the hydrodynamic field influences the formation of the new phase particles. Besidata interpretation using this method. The effect of flow birefringence in a system with colloidal particles, measured by the method traditional for polymers, has several components as in the case of macromolecules the proper anisotropy of particles, the effects of macro- and microforms (Tsvetkov et al., 1964), and conservative dichro-ism, i.e. the light scattering efficiency factor K of oriented anisodiametric, anisotropic particles differs in different observation planes (Onuki and Doi, 1986 Khlebtsov, 1988ab Khlebtsov and Melnikov, 1990). 

Another interesting aspect of the solvothermal synthesis of LaP04 YbJir nanocrystals includes the fact that this method allows for the synthesis of anisotropic particles. Ghosh et al. trace the anisotropic growth back to a diffusion model, whereas the surfactant cetyltriammonium bromide (CTAB) blocks certain crystallographic faces of the nuclei selectively [22]. It has to be mentioned that Ghosh et al. have calcined their particles at 900°C, which removed all ligands from the particles surfaces. However, colloidal nanoparticles can be prepared by solvothermal synthesis methods as well. 

Haseloh S, Ohm C, Smallwood F, Zentel R (2011) Nanosized shape-changing colloids from liquid crystalline elastomers. Macromol Rapid Commun 32(l) 88-93 Ohm C, Kapemaum N, Nonnenmacher D, Giesselmann F, Serra C, Zentel R (2011) Microfluidic synthesis of highly shape-anisotropic particles from liquid crystalline elastomers with defined director field configurations. J Am Chem Soc 133(14) 5305-5311. doi 10.1021/ jal095254... 

Mesocrystals are a very interesting form of colloidal crystal, as they extend the so far known colloidal crystals with spherical building units to those with non-spherical building units. This offers new possibilities of superstructme formation due to the anisotropic particle shape of the nanoparticle building units [113]. Thus, mesocrystals are colloidal crystals but with extended possibihties for their self-assembled superstructure, offering new handles for crystal morphology control. 

The intrinsic viscosity of a colloidal dispersion is always positive, even for dispersions such as foams in which the particles are less viscous than the medium. For rigid particles, intrinsic viscosity depends on particle shape (but not on particle size). For emulsions, both interfacial tension and the viscosity of the particles affect intrinsic viscosity. For deformable particles and for small anisotropic particles, the intrinsic viscosity can depend also on shear rate (non-Newtonian intrinsic viscosity). 

Martfnez-Pedrero R, Tirado-Miranda M., Schmitt A., and Callejas-Femandez J. 2006. Forming chain like filaments of magnetic colloids The role of the relative strength of isotropic and anisotropic particle interactions./. Chem. Phys. 125 084706. 

Finally a separate treatment is required for the double refraction of light in those colloids in which an orientation of anisotropic particles occurs through mechanical strain, streaming, an electric or a magnetic field By this orientation a colloid can obtain the optical properties of a uni- or biaxial anisotropic medium ... 

In a colloidal solution of anisotropic particles an electric or a magnetic field can also cause orientation and thereby double refraction. In this case the particles are oriented by the field with their longest axis parallel or perpendicular to the field direction while the BROwriian movement again disturbs this orientation. There is thus produced a Boltzmann distribution symmetrical about one axis whereby the colloid behaves in this case as a uniaxial doubly refractive body. 

The effect of surfactant on the dispersibility of Au nanoparticles depends on the sort of ionic type. When AOT was used instead of DOAC, the color of sols changed blue indicating the process of coagulation and the suspension was not stable. Within a few hours, the colloids fully precipitated. Figure 9-4.33 shows a TEM picture of this system sampled just after ultrasonic treatment to promote dispersion of sols. The particles in this picture are anisotropic and irregular in shape, and the size is larger than for the case of DOAC, as seen in the histogram. However, it is... 

Equations for calculating van der Waals interaction forces/energies between macromolecules or colloidal particles are quite well established (Israelachvili, 1992 Dickinson and McClements, 1995 McClements, 2005). (For example, see equations (3.35) and (3.36) in chapter 3). The interactions between nanoparticles are potentially more complicated, however, because the nanoparticle size and interparticle separation are comparable in magnitude, precluding the use of the asymptotic forms of the equations also nanoparticles are commonly anisotropic, and their dielectric properties are often not known (Min et al., 2008). 

Mourchid, A., Delville, A., Lambard, J., Lecolier, E. and Levitz, P. (1995) Phase Diagram of Colloidal Dispersions of Anisotropic Charged Particles Equilibrium Properties, Structure, and Rheology of Laponite Suspensions. Langmuir 11, 1942-1950... 

A dilute suspension of electrically and optically anisotropic colloidal particles becomes birefringent when subjected to an electric field. In random orientation the suspension is optically isotropic but when the grains align with a uniform electric field the suspension becomes anisotropic in particular the effective refractive index of the ordered suspension parallel to the field direction differs from its refractive index... 

Velikov, K., van DiUen, T., Poknan, A. et al., Photonic crystals of shape-anisotropic colloidal particles, Appl. Phys. Lett., 81, 838, 2002. 

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