Colloid stability, aqueous suspensions

Mevellec, V., Roucoux, A., Ramirez, E., Philippot, K. and Chaudret, B. (2004) Surfactant-stabilized aqueous iridium(O) colloidal suspension an effident reusable catalyst for hydrogenation of arenes in biphasic media. Advanced Synthesis and Catalysis, 346 (1), 72-76. 

Larpent and coworkers were interested in biphasic liquid-liquid hydrogenation catalysis [61], and studied catalytic systems based on aqueous suspensions of metallic rhodium particles stabilized by highly water-soluble trisulfonated molecules as protective agent. These colloidal rhodium suspensions catalyzed octene hydrogenation in liquid-liquid medium with TOF values up to 78 h-1. Moreover, it has been established that high activity and possible recycling of the catalyst could be achieved by control of the interfacial tension. 

Ammonium salts are commonly used to stabilize aqueous colloidal suspensions of nanoparticles. The first such example was reported in 1983-84 by Januszkie-wicz and Alper [96, 97], who described the hydrogenation of several benzene derivatives under 1 bar H2 and biphasic conditions starting with [RhCl(l,5-hexa-diene)]2 as the metal source and with tetraalkylammonium bromide as a stabilizing agent Some ten years later, Lemaire and coworkers investigated the cis/... 

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. 

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. 

In aqueous suspension, the stability is discussed in reference to the DLVO (Deryaguin-Landau-Verway-Overbeek) theory. Within this framework, all solid substances have a tendency to coagulate due to their large van der Waals attractive force. The coulombic repulsive force among colloidal particles more or less prevents this tendency. These two opposite tendencies determine the stability of suspensions. What kind of parameters are concerned in the present nonaqueous system, for which little is known about the stability This is an interest in this section. 

Silver iodide particles in aqueous suspension are in equilibrium with a saturated solution of which the solubility product, aAg+ai, is about 10 16 at room temperature. With excess 1 ions, the silver iodide particles are negatively charged and with sufficient excess Ag+ ions, they are positively charged. The zero point of charge is not at pAg 8 but is displaced to pAg 5.5 (pi 10.5), because the smaller and more mobile Ag+ ions are held less strongly than-the 1 ions in the silver iodide crystal lattice. The silver and iodide ions are referred to as potential-determining ions, since their concentrations determine the electric potential at the particle surface. Silver iodide sols have been used extensively for testing electric double layer and colloid stability theories. 

Stable silver sols were prepared by reduction of AgNOs (Aldrich, purity 99.998%) with excess NaBH4 (Aldrich, purity 99%), aged a week to prevent the formation of reduction products. The usual pH value of the aqueous suspension was about nine. NaCl (Aldrich purity 99.999%) was added in a small amount (10 M) to the Ag colloids to improve the SERS effect, without altering the sol stability. 

Colic, M. and Fuerstenau, D.W., The influence of surfactant impurities on colloid stability and dispersion of powders in aqueous suspensions, Potvder Technol., 97, 129, 1998. 

Liufu. S., Xiao, H., and Li, Y.. Adsorption of poly(acrylic acid) onto the surface of titanium dioxide and the colloidal stability of aqueous suspension, 7. Colloid Interf. Sci.. 281, 155, 2005. 

Eremenko, B.V. et al.. Stability of suspensions of alumina nanoparticles in aqueous solutions of electrolytes. Colloid J., 58, 436, 1996. 

Singh. B.P. et al., Stability of dispersions of colloidal alumina particles in aqueous suspensions, J. Colloid Interf. Sci., 291, 181, 2005. 

Tkachenko, N.H. et al.. Influence of polyfmethacrylic acid) on aggregative stability and electrical surface properties of aqueous suspensions of titanium dioxide. Colloids Surf. A, 279, 149, 2006. 

Interfacial phenomena at metal oxide/water interfaces are fundamental to various phenomena in ceramic suspensions, such as dispersion, coagulation, coating, and viscous flow. The behavior of suspensions depends in large part on the electrical forces acting between particles, which in turn are affected directly by surface electrochemical reactions. Therefore, this chapter first reviews fundamental concepts and knowledge pertaining to electrochemical processes at metal oxide powder (ceramic powder)/aqueous solution interfaces. Colloidal stability and powder dispersion and packing are then discussed in terms of surface electrochemical properties and the particle-particle interaction in a ceramic suspension. Finally, several recent examples of colloid interfacial methods applied to the fabrication of advanced ceramic composites are introduced. 

Figure 5 shows the calculated potential energy of interaction Vt of AI2O3 particles (t/ = 0.25 pm, A = 4.5 x 10 J, and 0.01 M ionic strength) as a function of the surface-to-surface distance of separation for various conditions of potential in an aqueous suspension. Note that the height of the potential energy barrier increases quite sharply as the potential becomes larger than a certain critical value ( 30 mV in Fig. 5). Therefore, the potential is a very good index of the magnitude of the repulsive interaction between colloid particles. Because of this, measurements of potential are most commonly used to assess the stability of a given colloidal sol.

Recently, the group of Roucoux has investigated the stabilization of Ru(0) colloids with classical methylated cyclodextrins, which are modulated by the cavity and the substitution degree (SD) [61]. The catalyticaUy active aqueous suspension of metallic Ru(0) nanoparticles was prepared by chemical reduction of ruthenium chloride with sodium borohydride in dilute aqueous solutions of methylated cyclodextrins. The TEM observations show that the average particle size is about 1.5 nm with 70% of the nanoparticles between 1 and 2.5 nm (Fig. 11.6). 

---