Controlled colloid formation

E. Matijevic Controlled Colloid Formation. Current Opinion in Colloid Interface Science, 1, 176-183 (1996). 

The synthesis of metal colloids in Nanomer coatings has not been investigated systematically so far. For Au-DIAMO complexes in a MPTS/Zr02 matrix sol however, it could be shown (Mennig, 1995) that controlled colloid formation could be obtained by a combined UV-IR irradiation of the wet film in presence of a suitable photo initiator within about 1 minute. During this process the matrix was cured simultaneously. It is assumed that the photoreaction leads to reduction of Au + ions and the aminosilane simultaneously catalyzes the curing of the matrix, but further experiments are required to prove this hypothesis. 

The major disadvantages of colloidal catalysts studied so far can be attributed to problems in controlling the metal colloid formation (control of particle size, particle size distribution, structure of metal colloids) and stabilization of the prepared particles, which are not yet completely solved. But it is exactly the stability of the nanoparticles, that is decisive for long-term usage during catalytic processes. Moreover for catalytic application, it is extremely important to preserve the large surface of such colloidal systems. 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

One of the main advantage of this method is that it allows different possibilities of synthesis of metallic particles deposited on a carbon support (1) synthesis of the catalysts with a controlled atomic ratio by coreduction, which consists in mixing different metal salts before their reduction leading to colloid formation and deposition on carbon (2) synthesis of the catalysts with a controlled atomic ratio by codeposition, which consists in mixing colloids of different... 

The most successful approach to control membrane formation involves segregation of the processes of crystal nucleation and growth [24]. The so-called ex situ or secondary (seeded) growth methods, unlike the direct synthesis procedures just discussed, include a first step in which a closely packed layer of colloidal zeolite crystals, synthesized homogenously, is deposited onto... 

The participation of colloidal particles in thermal motion (the entropic factor) was taken into consideration, mostly indirectly, in earlier studies dealing with the molecular-kinetic properties of disperse systems. Volmer was the first to realize the importance of the role that the thermal motion of colloidal particles played in controlling the formation and stabilization of disperse systems. However, the attempt to compare the work of surface formation and the entropic factor directly, undertaken by March, was not successful, since it was applied only to systems with high interfacial energy. 

The water content in the medium plays a key role to control the colloid formation. The influence of the water content can be studied by using dynamic light scattering (DLS) and UV-vis spectroscopy. DLS indicates that when the water content increases above CWC, the hydrodynamic radius (i b) gradually increases as the water content increases. When the water content is above a certain value, i h starts to decrease as the water content further increases and then stabilizes at the final value. The structure evolution in the process can be better understood from the UV-vis spectroscopic investigation. It is well known that the photoisomerization rate and isomerization degree at the photostationary state are related to the free volume surrounding the azo chromophores (Kumar and Neckers, 1989). For azobenzene-type molecules, the isomerization behavior can be monitored by UV-vis spectroscopy and used as a molecular probe to detect the environmental variation in the systems. 

M.J. Geerken, R.G.H. Lammertink, M. Wessling, Tailoring siuface properties for controlling droplet formation at microsieve membranes. Colloids Surf. A, 2007, 292, 224-235. 

Much is known about colloids, their formation, properties, and applications, but considerably more surely remains unknown. In particular, the full potential to control colloids is not presently realized. There are several types of mixed colloids that are only poorly understood. For example, the properties of colloids in which more than one type of colloidal particle is dispersed may be dominated by the behavior of the minor dispersed-phase component. The nature and properties of colloids within colloids, such as suspended solids in the dispersed phase of an emulsion, or emulsified oil within the aqueous lamellae of a foam, are only beginning to be understood (13-15). 

One unique advantage of pre-formed PECs over in situ PECs is the possibility of controlling their formation and of analysing their structures and colloidal properties before addition to the pulp fibres. Since complex formation occurs in a separate environment, which is more controllable than the paper stock that can vary substantially with time, the PEC formatiOTi can also be made more reliable and robust than if it occurs in the fibre suspension. 

Again it is possible to control the formation of condensed species by varj ing the value of X. Precipitation occurs rapidly for x=0. Stable colloids are obtained for x=4. Polymeric gels are formed for intermediate values of x close to 2. 

In this chapter, special attention is given to the use of radiation for the synthesis of colloidal nano-particles and for the investigation of their chemical and electronic properties. Radiolytic preparation under strictly anaerobic conditions has been shown to be an especially useful method, which allows one to control particle formation in a reproducible manner. Most studies in the literature are concerned with particles of the noble metals, i.e. of Au, Pt, Ag and Cu. The radiolysis method enables one to prepare also colloids of the more electronegative metals, such as of cadmium and thallium. In addition, this method has also been used to initiate chemical reactions on the surface of nanoparticles in a controlled manner. 

Roosjen A, Norde W, van der Mei HC, Busscher HJ The use of positively charged or low surface free energy coatings versus polymer brushes in controlling biofilm formation. Prog Colloid Polym Sci 132 138-144, 2006. 

Since cake, mud, and sphere formations appear to be a result of colloidal behavior, effective control of the colloidal behavior of a slurry will probably control the formation of such aggregates. 

Important objectives of the later prodnction methods were to control the size and shape of the catalyst particles during precipitation of the magnesium chloride and to improve stabihty. Catalysts with better-controlled size and shape were based on the reaction of a precipitated magnesium chloride with titanium tetrachloride in a high-boiling-point hydrocarbon diluent at 80°C, with di-isobutyl phthalate added as an internal electron donor. " After separation, the sohd formed was reacted with more titaninm tetrachloride at 120°C, before being washed and dried. The catalyst contained between 2-3% titanium and the phthalates used were hmited to C4-C8 esters to avoid potential problems with colloid formation. The catalysts prodnced with phthalates as the internal donor had mnch higher snrface area and pore volume than when ethyl berrzoate was nsed. This method provided more active arrd stereospecifrc catalysts when used with the same triethyl aluminum co-calalyst and an external electron donor such as phenyl triethoxy silane. 

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