Particulate films colloidal

Finely divided solid particles that are wetted to some degree by both oil and water can also act as emulsifying agents. This results from the fact that they can form a particulate film around dispersed droplets, preventing coalescence. Powders that are wetted preferentially by water form O/W emulsions, whereas those more easily wetted by oil form W/O emulsions. The compounds most frequently used in pharmacy are colloidal clays, such as bentonite (aluminum silicate) and veegum (magnesium aluminum silicate). These compounds tend to be adsorbed at the interface and also increase the viscosity of the aqueous phase. They are frequently used in conjunction with a surfactant for external purposes, such as lotions or creams. 

Chemical solution deposition (CSD) procedures have been widely used for the production of both amorphous and crystalline thin films for more than 20 years.1 Both colloidal (particulate) and polymeric-based processes have been developed. Numerous advances have been demonstrated in understanding solution chemistry, film formation behavior, and for crystalline films, phase transformation mechanisms during thermal processing. Several excellent review articles regarding CSD have been published, and the reader is referred to Refs. 5-12 for additional information on the topic. Recently, modeling of phase transformation behavior for control of thin-film microstructure has also been considered, as manipulation of film orientation and microstructure for various applications has grown in interest.13-15... 

It is important to state the difference between particles and particulate films at the onset of this section. Particles are separate nanometer- to micron-sized colloids dispersed in solution. Physically interconnected colloidal metal particles constitute a particulate film which may be supported by a monolayer floating on an aqueous subphase or be deposited on a solid substrate. 

Annealing the particulate film (transferred to a quartz substrate) at 140 °C for 10 min led to the development of colloidal gold particles showing a characteristic plasmon absorption band with a maximum at 560 nm (Fig. 94) [110]. 

As confirmed from the spectroelectrochemical study of Ti02 particulate films [148, 149], the inherent semiconductor properties such as trapping of electrons at the defect sites is responsible for the coloration effects (Figure 19.8). Electrochemical and photoelectrochemical approaches have supported this mechanism for coloration in WO3 colloids [137] and nanostmctured films [143]. The net color change was found to be spectrally similar whether one employed direct bandgap excitation of WO3 nanopartides or subjected them to a negative electrochemical bias. 

In the particulate-sol method a metal alkoxide dissolved in alcohol is hydrolyzed by addition of excess water or acid. The precipitate that results is maintained as a hot solution for an extended period during which the precipitate forms a stable colloidal solution. This process is called peptization from the Greek pep—to cook (not a misnomer many descriptions of the sol-gel process have a strong culinary flavor). The colloidal solution is then cooled and coated onto the microporous support membrane. The layer formed must be dried carefully to avoid cracking the coating. In the final step the film is sintered at 500-800 °C. The overall process can be represented as ... 

Dechlorinated and softened water flows to the RO skid through a 5.0-nm (nominal pore size) cartridge filter. The cartridge filter removes resin fines, particles and complexed colloids necessary to protect the RO membranes from particulate fouling. The RO membranes are thin-film composite (TFC) polyamide RO membranes (20 cm diameter X 100 cm long spiral wound elements) with rejection 99%. 

An appreciation of the value of hydrous oxide colloids for wetting hydrophobic surfaces with water is shown by Kenney (593), who claims that certain aqueous particulate colloids can wet any known hydrophobic surfaces without chemical change. However, I have found that wetting by a silica sol occurs on certain hydro-phobic surfaces only at low pH and with an optimum particle size of silica. After the surface has been wetted by sol, rinsed, and dried, the area is rewettable only because there remains an adsorbed monolayer of silica particles bonded to the surface. The type of bonding depends on the type of hydrophobic surface. For example, a metal surface, hydrophobic because of an adsorbed film of fatty acids (grease) is made hydrophilic by the silica replacing some of the fatty acid and bonding to the oxide surface film until the area is well populated by silica. This occurs mainly at neutral... 

DNase, a well characterized and biologically specific enzyme with therapeutic value in the treatment of cystic fibrosis, was chosen as a model agent for studying the effects on an enzymatic bio/molecule of surface immobilization on nanocrystalline ceramic particulates. As the data show, and consistent with previous observations, DNase exhibits a marked retention of biological activity when surface immobilized on the solid phase of a colloid comprised of polyhydroxyl oligomeric films investing degradable calcium-phosphate nanoparticles. 

Surfactants and polymers used in boundary Inbrication systans adsorb on solid surfaces and form a protective film. The effectiveness of bonndary lubricants has often been attributed to the adsorption affinity and the integrity of the adsorbed film. Such adsorption is inflnenced by additives incorporated into the system to reduce thermal degradation, corrosion, sludge formation, foaming, etc. There are many interactions that can take place between the additives, the snrfactants, and the base oil, leading, in addition to adsorption effects, to a number of interfacial and colloidal phenomena such as micellization, precipitation, and solubilization as well as flocculation of particulate matter in fluid [1-3]. 

Kalyanasundaram K., Gratzel M. Applications of functionalized transition metal con5)lexes in photonic and optoelectronic devices. Coord. Chem. Rev. 1998 77 347-414 Kamat P.V. Photoelectrochemistry in particulate systems. 3. Phototransformations in the colloidal TiOa-thiocyanate system. Langmuir 1985 1 608-611 Kamat P.V., Bedja I., Hotchandani S., Patterson L.K. Photosensitization of nanocrystalline semiconductor films. Modulation of electron transfer between excited ruthenium complex and SnOa nanocrystalUne with an externally applied bias. J. Phys. Chem. 1996 100 4900-4908 Kamat P.V., Vinodgopal K. Environmental photochemistry with semiconductor nanoparticles. Mol. Supramol. Photochem. 1998 2 307-350... 

The most restrictive drawback is that the deposition should be carried out on a conducting or a doped semiconducting surface. This is why it is not yet popular for most high-tech electro-optic applications, whereas it is extremely useful for coatings in the automotive industry, for the fabrication of batteries, fuel cells, and electrochromic windows, all of which involve films on conductive supports. Electrophoretic deposition conditions involve the presence of colloidal suspensions, but methods for in situ formation of particulates by faradaic acidification or base formation were amply described as well [44]. Finally, gas evolution (Hj) may be considered a technicality, but it adversely affects the film quality and may pose a safety hazard unless appropriately handled. 

Refractive index of particulate silica films as a function of the pH of the aggregation step prior to deposition (- 2 hour aggregation times). From Brinker el al. (20). Inset Gel times for aqueous colloidal silica as a function of pH. From Her (26]. 

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