Colloidal coalescence

Slurry particle agglomeration can take place in the slurry in which abrasive particles and colloids coalesce to form extended particles. 

Fig. 6.14. Interaction energy versus distance, (a) Surfaces repel strongly small colloidal particles remain stable, (b) Surfaces come into stable equilibrium at secondary minimum if it is deep enough colloids remain stable, (c) Surfaces come into secondary minimum colloids coagulate slowly, (d) Surfaces may remain in secondary minimum coUoids coagulate rapidly, (e) Surfaces and colloids coalesce rapidly. (After Isradachvili [26].)...

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Water and Waste Water Treatment. PAG products are used in water treatment for removal of suspended soHds (turbidity) and other contaminants such as natural organic matter from surface waters. Microorganisms and colloidal particles of silt and clay are stabilized by surface electrostatic charges preventing the particles from coalescing. Historically, alum (aluminum sulfate hydrate) was used to neutralize these charges by surface adsorption of Al cations formed upon hydrolysis of the alum. Since 1983 PAG has been sold as an alum replacement in the treatment of natural water for U.S. municipal and industrial use. 

Foam Coalescence Coalescence is of two types. The first is the growth of the larger foam bubbles at the expense of the smaller bubbles due to interbubble gas diffusion, which results from the smaller bubbles having somewhat higher internal pressures (Adamson, The Physical Chemlstiy of Suifaces, 4th ed., Wiley, New York, 1982). Small bubbles can even disappear entirely. In principle, the rate at which this type of coalescence proceeds can be estimated [Ranadive and Lemhch,y. Colloid Inteiface Sci., 70, 392 (1979)]. 

Nature of a Fog. Fog, like smoke, is a colloid. Once a fog is formed, it is very difficult to knock down. It will go right through packed columns, mist eliminators, or other such devices. Special devices are required to overcome a fog, such as an electric precipitator with charged plates. This can overcome the zeta potential of the charged particles and make them coalesce. 

Fig. 8 TEM (a) and HREM (b) micrographs of octanethiol stabilized platinum colloid showing nearby particles with no coalescence...

With foams, one is dealing with a gaseous state or phase of matter in a highly dispersed condition. There is a definite relationship between the practical application of foams and colloidal chemistry. Bancroft (4) states that adopting the very flexible definition that a phase is colloidal when it is sufficiently finely divided, colloid chemistry is the chemistry of bubbles, drops, grains, filaments, and films, because in each of these cases at least one dimension of the phase is very small. This is not a truly scientific classification because a bubble has a film round it, and a film may be considered as made up of coalescing drops or grains. ... 

V. Mishra, S. M. Kresta, J. H. Masliyah 1998, (Self-preservation of the drop size distribution function and variation in the stability ratio for rapid coalescence of a polydisperse emulsion in a simple shear field), J. Colloid Interface Sci. 197, 57. 

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. 

IB Ivanov, KB Danov, PA Kralchevsky. Flocculation and coalescence of micron-size emulsion droplets. Colloids Surfaces A Physicochem Eng Aspects 152 161-168, 1999. 

Weissenbom PK, Pugh RJ (1996) Surface tension of aqueous solutions of electrolytes relationship with hydration, oxygen solubility, and bubble coalescence. J Colloid Interface Sci 184 550-553... 

Nanoparticles can be deposited onto a substrate by spin coating or inkjet printing a colloidal solution and can be sintered into a continuous conductive film during the subsequent annealing step. Coalescence of individual... 

Whichever method is followed, a protective agent able to induce a repulsive force opposed to the van der Waals forces is generally necessary to prevent agglomeration of the formed particles and their coalescence into bulk material. Since aggregation leads to the loss of the properties associated with the colloidal state, stabilization of metallic colloids - and therefore the means to preserve their finely dispersed state - is a cmcial aspect for consideration during their synthesis. 

Many people love cool autumn mornings, with the scent of the cool air and a rich dew underfoot on the grass and paths. The dew forms when molecules of water from the air coalesce, because of the cool temperature, to form minute aggregates that subsequently nucleate to form visible drops of water. These water drops form a stable colloid (see Chapter 10). 

By using this technique only water insoluble monomers can be polymerised. In this process, the monomer is suspended as discrete droplets (0.1 to 1.0 mm diameter) in dilute aqueous solution containing protective colloids like polyvinyl alcohol and surfactants, etc. The droplets have large surface area and can readily transfer heat to water. Suspension is brought about by agitating the suspension. Protective colloids prevent coalescence of the droplets. A monomer soluble initiator is used. The product is obtained by filtration or spray drying. This process cannot be carried out yet in a continuous process hence batch processing has to be used. 

L7. Lindblad, N. R., Effects of relative humidity and electric charge on the coalescence of curved water surfaces, J. Colloid Sci. 19, 729 (1964). 

S. Mohan and G. Narsimham Coalescence of Protein-Stabilized Emulsions in a High-Pressure Homogenizer. J. Colloid Interface Sci. 192, 1 (1997). 

L. Loho and A. Svereika Coalescence During Emulsification. 2. Role of Small Molecule Surfactants. J. Colloid Interface Sci. 261, 498 (2003). 

An analogy may be drawn between the phase behavior of weakly attractive monodisperse dispersions and that of conventional molecular systems provided coalescence and Ostwald ripening do not occur. The similarity arises from the common form of the pair potential, whose dominant feature in both cases is the presence of a shallow minimum. The equilibrium statistical mechanics of such systems have been extensively explored. As previously explained, the primary difficulty in predicting equilibrium phase behavior lies in the many-body interactions intrinsic to any condensed phase. Fortunately, the synthesis of several methods (integral equation approaches, perturbation theories, virial expansions, and computer simulations) now provides accurate predictions of thermodynamic properties and phase behavior of dense molecular fluids or colloidal fluids [1]. 

T. H. Whitesides and D.S. Ross Experimental and Theoretical Analysis of the Limited Coalescence Process Stepwise Limited Coalescence. J. Colloid Interface Sci. 169, 48 (1995). 

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