Stability of Binary Suspensions

The microprocesses in binary suspensions are not only affected by the particle interactions, but also by the size ratio and the mixing ratio of the particulate components. A quantification of the suspension stability needs a separate consideration of the three principal pairs of aggregating materials, which is eventually expressed by three partial stability ratios—Wu, W22 and W12  

The fonnation of dimers from particles of the components i and j obeys the following kinetic, when dominated by Brownian collisions  

If the particles of both components are approximately equal in size, an effective stability ratio for the whole suspension can be defined (Hogg et al. 1966)  

2 The Stability of Aqueous Suspensions with Two Chddic Components 

In the case of aqueous suspensions of oxide materials, the stability is mainly determined by the van-der-Waals and double layer interactions (DLI) between the particles. The former is always attractive in the event of particle collision. Whether the DLI is attractive or repulsive depends on the signs of the surface charges, the absolute values of the zeta-potential, and the regulation capacity of the double layer. When all particles are identically charged, the DLI is repulsive. The most influential factors on the stability are the pH value, the total electrolyte concentration, and the valency of the ions. The pH range in which a binary suspension remains stable is typically different to the respective stability ranges of the two particle components. 

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Yasrebi, M. et al.. Role of particle dissolution in the stability of binary yttria-silica colloidal suspensions, 7. Am. Ceram. Soc., 79, 1223, 1996. 

The stability of binary Si02-Ti02 suspensions can be, for instance, clearly affected when the suspensions are first stored at basic pH values ( 9) for at least 24 h, before the final pH in the neutral or acid range is adjusted (Paciejewska 2010, pp. 124-126). 

The main conclusion of Paciejewska s thesis is the necessity to consider the specific kinetics of interfacial phenomena when evaluating the stability of colloidal suspensions. This applies not only to binary, but to all kinds of colloidal suspensions. A major factor is the dissolution of the dispersed phase(s)—in particular if the solubility and the intrinsic dissolution rate are relatively large. Its relevance is especially pronounced for a large total surface area, which depends on the particle concentration and the specific surface of the particles and which determines the amount of substance that can be dissolved in a given period of time. For many nanoparticle (x < 100 nm) systems (e.g. additives for paints and coatings), it will not be permissible to ignore the influence of dissolution on the interfacial properties and even on suspension stability—independent from the Gibbs-Thomson effect, which becomes relevant at particle sizes below 10 nm (cf. Sect. 3.1.4). 

As shown in section C2.6.6.2, hard-sphere suspensions already show a rich phase behaviour. This is even more the case when binary mixtures of hard spheres are considered. First, we will mention tire case of moderate size ratios, around 0.6. At low concentrations tliese fonn a mixed fluid phase. On increasing tire overall concentration of mixtures, however, binary crystals of type AB2 and AB were observed (where A represents tire larger spheres), in addition to pure A or B crystals [105, 106]. An example of an AB2 stmcture is shown in figure C2.6.11. Computer simulations confinned tire tliennodynamic stability of tire stmctures tliat were observed [107, 1081. 

Reaction of the sandwich-type POM [(Fc(0H2)2)j(A-a-PW9034)2 9 with a colloidal suspension of silica/alumina nanopartides ((Si/A102)Cl) resulted in the production of a novel supported POM catalyst [146-148]. In this case, about 58 POM molecules per cationic silica/alumina nanoparticle were electrostatically stabilized on the surface. The aerobic oxidation of 2-chloroethyl ethyl sulfide (mustard simulant) to the corresponding harmless sulfoxide proceeded efficiently in the presence of the heterogeneous catalyst and the catalytic activity of the heterogeneous catalyst was much higher than that of the parent POM. In addition, this catalytic activity was much enhanced when binary cupric triflate and nitrate [Cu(OTf)2/Cu(N03)2 = 1.5] were also present [148],... 

From a knowledge of the adsorption, immersion, and wetting properties of solid particles, we have examined the influence of particle-particle and particle-liquid interactions on the stability and structure formation of suspensions of hydrophobic and hydrophilic Aerosil particles in benzene-n-heptane and methanol-benzene mixtures. For the binary mixtures, the Hamaker constants have been determined by optical dispersion measurements over the entire composition range by calculation of the characteristic frequency (Vk) from refractive index measurements [7,29,36,64], The Hamaker constant of an adsorption layer whose composition is different from that of the bulk has been calculated for several mixture compositions on the basis of the above results. Having the excess isotherms available enabled us to determine the adsorption layer thickness as a function of the mixture composition. For interparticle attractive potentials, calculations were done on the basis of the Vincent model [3-5,39]. In the case of hydrophobic particles dispersed in benzene- -heptane and methanol-benzene mixtures, it was established that the change in the attractive potential was in accordance with the interactions obtained from rheological measurements. 

Systems comprising binary or tertiary surfactant mixtures or polymers deliver greater flexibility with higher viscosity values (from 10 cps to several thousand) and a better control of the viscosity stability over longer pmod of time. Colloidal suspensions of clay or of aluminum oxide seem to be the preferred route to generate aqueous gels with more complex rheological behavior. 

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