Colloidal systems stability

A similar polymer-stabilized colloidal system is described by James and coworkers [66]. Rhodium colloids are obtained by reducing RhCls, 3H2O with ethanol in the presence of PVP. The monophasic hydrogenation of various substrates such as benzyl acetone and 4-propylphenol and benzene derivatives was performed under mild conditions (25 °C and 1 bar H2). The nanoparticles are poorly characterized and benzyl acetone is reduced with 50 TTO in 43 h. 

Similar surfactant-stabilized colloidal systems have been reported by Albach and Jautelat, who prepared aqueous suspensions of Ru, Rh, Pd, Ni nanoparticles and bimetallic mixtures stabilized by dodecyldimethylammonium propane-sulfonate [103]. Benzene, cumene and isopropylbenzene were reduced in biphasic conditions under various conditions at 100-150 °C and 60 bar H2, and TTO up to 250 were obtained. 

Surfactants are commonly used to kinetically stabilize colloidal systems. An alternative way to achieve long-term metastability consists of adsorbing... 

Polymers are often used to stabilize colloidal systems by grafting them on the particle surfaces to provide steric repulsion [1,2], Polymers can also induce flocculation due to either depletion or bridging interactions [3],... 

For the synthesis of the nanocomposites, additionally to the reaction principles described above, nanoscaie particles have to be generated within or added to the hetero polysiloxane composite matrix. As described elsewhere [26], the sol-gel process, is a suitable means. It can be considered as a growth process from solution where a controlled growth reaction takes place, that leads to a stabilized colloidal system and thus avoiding precipitation. 

Parenteral administration (injection), which is the immediate option for orally undeliverable drugs, has advanced greatly in recent years for systemic and local drug delivery. " The novel drug delivery system has metamorphosed from simple polymer and antibody conjugates to sterically stabilized colloidal systems. Liposomes and nanoparticles can improve pharmacokinetic-pharmacody-... 

Nonionic surfactants stabilize colloidal systems not by electrostatics but basically by osmotic forces. If two sterically stabilized particles approach each other, the soluble parts of the adsorbed chains causes a higher concentration in the interstitials when compared to the average continuous phase. This will cause a flux of continuous phase into the interstitials, which subsequently leads again to drop separation. As nonionic stabilizers are mainly polymeric in nature (for instance, poly(ethylene glycol) chains), elastic forces may contribute to the stability as well. The elastic force per... 

As a practical application of the relationship in Equation (10.28), one may characterize the stability of a charge-stabilized colloidal system by its critical coagulation concentration (ccc), the concentration of electrolyte necessary to bring the system into the regime of rapid coagulation. The process involves the extrapolation of the curve of In IT versus In Co to In IT = 0, which gives In (ccc). However, what is the practical use of the ccc and what does it mean in theoretical terms ... 

In summary High surface potentials stabilize colloidal systems. The addition of inert salt leads to stronger screening and destabilizes the system. The point at which rapid coagulation (case b) sets in is defined as the critical coagulation concentration (ccc). One key result of DLVO theory is the explanation of the Schultze-Hardy rule, which states that the ccc depends on the counterion valency z like 1/z . 

Sengupta S and Sood AK. 1991. Theory of liquid-bcc-fcc coexistence in charge-stabilized colloidal systems. Physical Review A 44 1233-1236. 

The potential importance of the double layer to adsorption in charged systems can be seen from the fact that the relationship in Eq. (10.1) predicts that potential interactions between surface charges and ions in solution will be significantly affected by the conditions in the solution phase. Double-layer theory is most important in the general field of colloidal stability, in which the concentration and valence of ions present can dramatically affect the stability and utility of an electrostatically stabilized colloidal system. 

In many colloidal systems, both in practice and in model studies, soluble polymers are used to control the particle interactions and the suspension stability. Here we distinguish tliree scenarios interactions between particles bearing a grafted polymer layer, forces due to the presence of non-adsorbing polymers in solution, and finally the interactions due to adsorbing polymer chains. Although these cases are discussed separately here, in practice more than one mechanism may be in operation for a given sample. 

The hard sphere (HS) interaction is an excellent approximation for sterically stabilized colloids. However, there are other interactions present in colloidal systems that may replace or extend the pure HS interaction. As an example let us consider soft spheres given by an inverse power law (0 = The energy scale Vq and the length scale cr can be com-... 

A study was made of the comparative stabilities at various exposures of an upset stomach remedy suspension. This product consisted of a dispersion of bismuth subsalicylate and phenylsalicylate in an aqueous system. Methyl-cellulose and magnesium aluminum silicate were selected as the suspending agents, because the presence of polyvalent metallic ions precluded the use of hydrocolloids affected by these ions. In addition, it was found that methyl-cellulose contributed a demulcent effect. The viscosity, as well as the suspension characteristics of the combination of protective colloids used, was of a synergistic nature. These colloids formed a thixotropic system. The thixotropy undoubtedly aids in stabilizing this system. In order to make this product palatable and impart elegance, color and flavor were added. Sample 1 (with protective colloids) showed no separation, while sample 2 (without... 

Colloidal systems of particles are different from molecular clusters in having a small distribution in sizes. Also, colloidal systems may be stabilized by organic molecules, resulting in chemical bonds between these and the outer monolayer of metal atoms. The Mossbauer study of such systems includes rather diverse fields ... 

This chapter describes the basic principles involved in the development of disperse systems. Emphasis is laid on systems that are of particular pharmaceutical interest, namely, suspensions, emulsions, and colloids. Theoretical concepts, preparation techniques, and methods used to characterize and stabilize disperse systems are presented. The term particle is used in its broadest sense, including gases, liquids, solids, molecules, and aggregates. The reader may find it useful to read this chapter in conjuction with Chapters 8, 12, and 14, since they include some of the most important applications of disperse systems as pharmaceutical dosage forms [1]. 

The hydrogen ion concentration at which a colloidal system is electrically neutral the addition of acidic substances to, for example, rubber latex causes the pH value to move towards the isoelectric point, which is the region of minimum stability, and coagulation may take place. 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

Bonnemann, EL, Brijoux, W., Brinkmann, R., and Richter, J., Process for producing tenside-stabilized colloids of mono- and bimetals of the group VIII and lb of the periodic system in the form of precursors for catalysts which are isolable and water soluble at high concentration, US. Pat., 849,482, 1997 (to Studiengesellschaft Kohle mbH). 

The procedure chosen for the preparation of lipid complexes of AmB was nanoprecipitation. This procedure has been developed in our laboratory for a number of years and can be applied to the formulation of a number of different colloidal systems liposomes, microemulsions, polymeric nanoparticles (nanospheres and nanocapsules), complexes, and pure drug particles (14-16). Briefly, the substances of interest are dissolved in a solvent A and this solution is poured into a nonsolvent B of the substance that is miscible with the solvent A. As the solvent diffuses, the dissolved material is stranded as small particles, typically 100 to 400 nm in diameter. The solvent is usually an alcohol, acetone, or tetrahydrofuran and the nonsolvent A is usually water or aqueous buffer, with or without a hydrophilic surfactant to improve colloid stability after formation. Solvent A can be removed by evaporation under vacuum, which can also be used to concentrate the suspension. The concentration of the substance of interest in the organic solvent and the proportions of the two solvents are the main parameters influencing the final size of the particles. For liposomes, this method is similar to the ethanol injection technique proposed by Batzii and Korn in 1973 (17), which is however limited to 40 mM of lipids in ethanol and 10% of ethanol in final aqueous suspension. 

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. 

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