Colloids applications

Kuo PL, Chen CJ. Functional polymers for colloidal applications. V. Novel behavior of polymeric emulsifiers in emulsion polymerization. J Polym Sci, Part A Polym Chem 1993 31 99-111. 

Theoretically, polysoaps may be of interest for all the potential applications mentioned for micellar polymers in the introduction. In practice, the attempted uses of polysoaps cover a much narrower range. Besides a number of diverse suggestions, proposed uses are basically classical colloidal applications, medical or pharmaceutical applications, and catalytic systems. 

Concerning colloidal applications, the uses as dispersants or emulsifiers are the most frequent ones. But although there exist several studies on the use of polymerizable surfactants [82,91,93,219,251,314,333,337,352,452-477] - i.e. of the precursors for polysoaps - the use of polysoaps in emulsion polymerization is still the exception [376]. In both cases improved latex stabilities and improved resistance to moisture of film-formed latexes were aspired to, but at least in the former case the results were often disappointing [93, 251, 470-472]. Polysoaps are used to stabilize lattices [214, 230], The opposite use for flocculation has also been tested, [284], Polysoaps have also been studied as viscoelastic fluids [108]. 

F. Pichot, J.R. Pitts, B.A. Gregg, Low-Temperature sintering of Ti02 colloids application to flexible dye-sensitized solar cells , Langmuir, 16(13), 5626-5630, (2000). 

In order to understand the role of polymers in their various surface and colloidal applications, it is necessary to understand when, where, why, and how they adsorb at interfaces. While the interactions that control polymer adsorption at the monomer level are the same as those for any monomolecular species, the size of the polymer molecule introduces many complications of analysis that must be treated in a statistical manner, which means that we seldom really know what the situation is but must make educated guesses on the basis of the best available evidence. 

PAK LEUNG is a research scientist at Union Carbide Corporation. He earned a B.S. in chemical engineering from National Taiwan University in 1957 and M.A. (1962) and Ph.D. (1967) from Columbia University. Since he joined Union Carbide, he has been working in the polymer physics area and in surface and colloidal applications. 

The influence of electrical charges on surfaces is very important to their physical chemistry. The Coulombic interaction between charged colloids is responsible for a myriad of behaviors from the formation of opals to the stability of biological cells. Although this is a broad subject involving both practical application and fundamental physics and chemistry, we must limit our discussion to those areas having direct implications for surface science. 

A number of refinements and applications are in the literature. Corrections may be made for discreteness of charge [36] or the excluded volume of the hydrated ions [19, 37]. The effects of surface roughness on the electrical double layer have been treated by several groups [38-41] by means of perturbative expansions and numerical analysis. Several geometries have been treated, including two eccentric spheres such as found in encapsulated proteins or drugs [42], and biconcave disks with elastic membranes to model red blood cells [43]. The double-layer repulsion between two spheres has been a topic of much attention due to its importance in colloidal stability. A new numeri-... 

R. J. Hunter, Zeta Potential in Colloid Seience Principles and Applications, Academic, Orlando, FL, 1981. 

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

Among the many applications of LB films, the creation or arrangement of colloidal particles in these films is a unique one. On one hand, colloidal particles such as 10-nm silver sols stabilized by oleic acid can be spread at the air-water interface and LB deposited to create unique optical and electrooptical properties for devices [185]. 

Another approach is to use the LB film as a template to limit the size of growing colloids such as the Q-state semiconductors that have applications in nonlinear optical devices. Furlong and co-workers have successfully synthesized CdSe [186] and CdS [187] nanoparticles (<5 nm in radius) in Cd arachidate LB films. Finally, as a low-temperature ceramic process, LB films can be converted to oxide layers by UV and ozone treatment examples are polydimethylsiloxane films to make SiO [188] and Cd arachidate to make CdOjt [189]. 

Although the remainder of this contribution will discuss suspensions only, much of the theory and experimental approaches are applicable to emulsions as well (see [2] for a review). Some other colloidal systems are treated elsewhere in this volume. Polymer solutions are an important class—see section C2.1. For surfactant micelles, see section C2.3. The special properties of certain particles at the lower end of the colloidal size range are discussed in section C2.17. 

Colloidal dispersions often display non-Newtonian behaviour, where the proportionality in equation (02.6.2) does not hold. This is particularly important for concentrated dispersions, which tend to be used in practice. Equation (02.6.2) can be used to define an apparent viscosity, happ, at a given shear rate. If q pp decreases witli increasing shear rate, tire dispersion is called shear tliinning (pseudoplastic) if it increases, tliis is known as shear tliickening (dilatant). The latter behaviour is typical of concentrated suspensions. If a finite shear stress has to be applied before tire suspension begins to flow, tliis is known as tire yield stress. The apparent viscosity may also change as a function of time, upon application of a fixed shear rate, related to tire fonnation or breakup of particle networks. Thixotropic dispersions show a decrease in q, pp with time, whereas an increase witli time is called rheopexy. 

In practice, sedimentation is an important property of colloidal suspensions. In fonnulated products, sedimentation tends to be a problem and some products are shipped in the fonn of weak gels, to prevent settling. On the other hand, in applications such as water clarification, a rapid sedimentation of impurities is desirable. 

In section C2.6.4.3 it was shown how tlie addition of non-adsorbing polymer chains induces a depletion attraction between colloidal particles. If sufficient polymer is added, tliese attractions can be strong enough to induce a phase separation of tire colloidal particles. An early application of tliis was tire creaming of mbber latex [93]. 

Saunders B R and Vincent B 1999 Microgel particles as model colloids theory, properties and applications Adv. Colloid Interface Sol. 80 1 -25... 

Flough D B and White L R 1980 The calculation of Flamaker constants from Lifshitz theory with applications to wetting phenomena Adv. Colloid Interface Sc/. 14 3-41... 

For tire purjDoses of tliis review, a nanocrystal is defined as a crystalline solid, witli feature sizes less tlian 50 nm, recovered as a purified powder from a chemical syntliesis and subsequently dissolved as isolated particles in an appropriate solvent. In many ways, tliis definition shares many features witli tliat of colloids , defined broadly as a particle tliat has some linear dimension between 1 and 1000 nm [1] tire study of nanocrystals may be drought of as a new kind of colloid science [2]. Much of die early work on colloidal metal and semiconductor particles stemmed from die photophysics and applications to electrochemistry. (See, for example, die excellent review by Henglein [3].) However, the definition of a colloid does not include any specification of die internal stmcture of die particle. Therein lies die cmcial distinction in nanocrystals, die interior crystalline stmcture is of overwhelming importance. Nanocrystals must tmly be little solids (figure C2.17.1), widi internal stmctures equivalent (or nearly equivalent) to drat of bulk materials. This is a necessary condition if size-dependent studies of nanometre-sized objects are to offer any insight into die behaviour of bulk solids. 

J. N. Israelachvih, Intermolecular and Suface Forces, With Applications to Colloidal and Biological Systems, Academic Press, Inc., San Diego, 1985. 

M. S. El-Aasser, in F. Candau and R. H. OttewiU, eds.. Scientific Methodsfior the Study ofi Polymer Colloids and Their Applications Kluwer Academic Pubhshers, the Netherlands, 1990, pp. 12—15. 

M. F. De Boodt, Soil Colloids and Their dissociation indiggregates NATO ASI Sei. B 215, 1990, pp. 517—556, applications of polymeric substances as physical soil conditioners. 

R. J. Hunter, in R. H. OttewiU and R. L. RoweU, eds., Xeta Potential in Colloid Science, Principles and Applications, Academic Press, Inc., New York, 1981. 

Tyj)e of dryer Applicable with dry-product recirculation True and colloidal solutions emulsions. Examples inorganic salt solutions, extracts, milk, blood, waste liquors, rubber latex, etc. Pumpable suspensions. Examples pigment slurries, soap and detergents, calcium carbonate, bentonite, clay sbp, lead concentrates, etc. does not dust. Recirculation of product may prevent sticking Examples filter-press cakes, sedimentation sludges, centrifuged sobds, starch, etc. 

Polymeric flocculants are available in various chemical compositions and molecular weight ranges, and they may be nonionic in character or may have predominantly cationic or anionic charges. The range of application varies but, in general, nonionics are well suited to acidic suspensions, anionic flocculants work well in neutral or alkaline environments, and cationics are most effective on organic material and colloidal matter. 

Modes of Operation There is a close analogy between sedimentation of particles or macromolecules in a gravitational field and their elec trophoretic movement in an electric field. Both types of separation have proved valuable not only for analysis of colloids but also for preparative work, at least in the laboratoiy. Electrophoresis is applicable also for separating mixtures of simple cations or anions in certain cases in which other separating methods are ineffectual. 

Applications of Dielectrophoresis Over the past 20 years the use of DEP has grown rapidly to a point at which it is in use for biological, colloidal, and mineral materials studies and handfing. The effects of nonuniform elec tric fields are used for handling particulate matter far more often than is usually recognized. This includes the... 

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