Incipient flocculation

Figure 2 schematically presents a synthetic strategy for the preparation of the structured catalyst with ME-derived palladium nanoparticles. After the particles formation in a reverse ME [23], the hydrocarbon is evaporated and methanol is added to dissolve a surfactant and flocculate nanoparticles, which are subsequently isolated by centrifugation. Flocculated nanoparticles are redispersed in water by ultrasound giving macroscopically homogeneous solution. This can be used for the incipient wetness impregnation of the support. By varying a water-to-surfactant ratio in the initial ME, catalysts with size-controlled monodispersed nanoparticles may be obtained. 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

These effects have been observed for both aqueous and non-aqueous media and good correlation between the point of incipient flocculation and the 0-temperature is well established112. The transition from stability to instability usually occurs over a very narrow temperature range (1 or 2 K). Enthalpic stabilisation tends to be the more common in aqueous media and entropic stabilisation the more common in non-aqueous media. Owing to the elastic effect, aggregation into a deep primary minimum does not take place (as is possible with lyophobic sols) and redispersion takes place readily on reverting to better than 0-solvent conditions. 

The asymptotic approach has not yet been extended to interactions between stretched layers in theta and poor solvents, which control the incipient flocculation of polymerically-stabilized colloidal dispersions. Application of the mean-field theory to poor solvents produces an attractive minimum only for —nll2v/w112 > new112/l, when layers begin to interpenetrate rather than... 

The condition for incipient flocculation, i.e., the delineation between stable and unstable dispersions, generally corresponds to the theta condition for the stabilizer in solution. A large body of experimental data gathered by Napper and others (e.g., Napper, 1983) first established this correlation, and the theoretical predictions for the interaction potentials agree provided nl2u < 1. 

The flocculation of stericaUy stabihsed dispersions, including weak, incipient, and depletion flocculation. 

Incipient flocculation this occurs when the solvency of the medium is reduced to become worse than -solvent (i.e., x > 0-5). This is illustrated in Eigure 9.9, where x was increased from <0.5 (good solvent) to >0.5 (poor solvent). 

This occurs when the solvency of the medium for the chain becomes worse than a 0-solvent x > 0.5). Under these conditions, becomes negative (i.e., attractive) and a deep minimum is produced that results in catastrophic flocculation (this is referred to as incipient flocculation). This is shown schematically in Figure 10.29. With many systems a good correlation between the flocculation point and the 0-point is obtained. 

This process of incipient flocculation can be followed by measuring the turbidity of the suspension as a function of temperature. Above the CFT, the turbidity of the suspension rises very sharply. 

If incipient flocculation occurs (on reducing the solvency of the medium for the change to worse than 0-condition), a much deeper minimum is observed and this is accompanied by a much larger increase in E. ... 

The above increase in yield value indicated flocculation of the emulsion, and this was confirmed with optical microscopy. The smaller increase in yield value below 0.8 mol dm NaCl was indicative of weak flocculation, and this could be confirmed by a redispersion of the emulsion by gently shaking. Above 0.8 mol dm NaCl, the flocculation was strong and irreversible, and in this case the solvency of the medium for the PEO chains became poor, resulting in incipient flocculation. 

Weakly flocculated systems have been studied by Buscall et al. (165), Heath and Tadros (166), Goodwin et al. (167), Patel and Russel (168, 169), Otsubo (170-172), Buscall et al. (51,173), Woutersen and de Kruif (174), and Nakai et al. (175). The weak flocculation can be obtained by several means, including secondary-minimum flocculation, depletion flocculation, polymer-bridging flocculation, and incipient flocculation. Details of the various mechanisms of interparticle attraction can be found in Russel et al. (27) and Somasundaran and Yu (176). Normally, flocculated systems have a solid volume fraction of no less than 0.2. When 0 < 0.2, an attractive system will settle down quickly. When 0 > 0.2, a flocculated suspension can be maintained easily for a period of time for a steady shear measurement to be completed. 

Thermodynamically Limited Steric Stabilization The Phenomenology of Incipient Flocculation... 

The first strategy, studies of incipient instability, provides insights into the thermodynamic factors that control stability. Further, they show how optimum stability can be imparted and allow the bounds of stabilization (e.g. the upper and lower limits to temperature stability) to be determined. Necessarily, however, incipient instability studies examine dispersions in what might be described as their death throes. This could well change the very properties that we wish to study in order to understand how stability is imparted. It is therefore important to complement incipient instability studies with measurements on robustly stable dispersions. In this and the two subsequent chapters, the phenomenology of incipient flocculation will be discussed. The presentation of the properties of robustly stable dispersions will be deferred until Chapter 13. 

We conclude that provided the anchor polymer is attached sufficiently strongly to the colloidal particles, its chemical nature has no significant bearing on the observed CFPT. This conclusion is buttressed by the studies of Clarke and Vincent (1981a) who covalently grafted polystyrene chains on to silica particles and so obviated the need for an anchor polymer. The pattern of incipient flocculation observed for these systems conforms with that detailed in this chapter for stabilizing moieties anchored by polymer chains. 

In the last three chapters we dealt with the theoretical predictions of the magnitudes of steric interactions and their dependence upon distance. The incipient flocculation behaviour that was discussed in Chapters 5-9 is determined primarily by the magnitude and the sign of these steric interactions. Incipient flocculation studies, however, do not provide direct information about the distance dependence of the interaction. An additional drawback is that it is inevitable in incipient flocculation experiments that the dispersions be studied under conditions of pathological instability. This may change the very properties that we wish to study in order to determine how stability is imparted. It is therefore necessary to complement the incipient flocculation experiments with studies on dispersions that exhibit robust stability. 

Qualitative predictions of the theory Provided that L3 is not excessively small, the interpenetrational domain will determine the flocculation behaviour in heterosterically stabilized systems, just as it does in homosteric stabilization. In the limit of small L3, the interpenetrational-plus-compressional domain may well become important in predicting incipient instability. The elaboration of the general principles that govern heterosteric stabilization is then quite different. 

Generally, oilfield emulsions are most often W/O with the surface-active emulsifiers residing in the crude-oil continuous phase. According to the Bancroft rule (109) the phase for which the emulsifiers are most soluble is the continuous phase. The emulsifiers possess some degree of polarity which attracts them to the water phase. Solid emulsifiers would be very fine particles in a state of incipient flocculation (110). The emulsifiers may be one or more of the following solids whieh are partially hydrophobic with contact angle (9>90°), polar asphaltenes and resins with some partial insolubility indueed by solvents which dilute the crude oils, or metalloporphyrins integrated within the asphaltenes (24, 25). 

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