Coalescence, effect electrolyte

Craig VSJ, Ninham BW, Pashley RM (1993) Effect of electrolytes in bubble coalescence. Nature 364 317-319... 

In a study of the effect of electrolyte concentration on gas holdup, Bly and Worden (1990) found a strong effect. A salt solution resulted in twice the gas holdup that distilled water did under otherwise identical operating conditions, because the salt solution suppressed bubble coalescence. Investigation of this phenomenon is important in biofluidization, because biological media commonly have high electrolyte concentrations. 

A stabilising effect in the presence of salt was also noted by Aronson and Petko [90]. Addition of various electrolytes was shown to lower the interfacial tension of the system. Thus, there was increased adsorption of emulsifier at oil/water interface and an increased resistance to coalescence. Salt addition also increased HIPE stability during freeze-thaw cycles. Film rupture, due to expansion of the water droplets on freezing, did not occur when aqueous solutions of various electrolytes were used. The salt reduced the rate of ice formation and caused a small amount of aqueous solution to remain unfrozen. The dispersed phase droplets could therefore deform gradually, allowing expansion of the oil films to avoid rupture [114]. 

The scrub solution joins the aqueous waste feed stream to constitute the aqueous phase in the extraction section of the contactor cascade its low flow rate and low electrolyte concentration have little effect on the extraction section. The scrubbed solvent passes to the strip section of the cascade, where it is contacted with 1 mM nitric acid to transfer the cesium to the aqueous phase. This concentration of nitric acid was chosen to minimize the DCs for best stripping efficiency, while maintaining sufficient acidity to keep the amine protonated and sufficient ionic strength for adequate coalescence. 

The effect of Na+ on the stability of water-in-oil emulsions is exercised mainly through its influence on sodium caseinate. It has been shown that as the surface concentration of casein on oil droplets is increased, the oil-in-water emulsion becomes less susceptible to flocculation/coalescence in the presence of electrolyte. Added NaCl broadens the droplet size distribution at a low casein content (0.25%) but causes this effect at a high casein content (0.5%) only when CaCl2 is added (Dickinson et al., 1984). 

The parity plot for the above equation for holdup is shown in Figure 1. While the validity of the above correlation for an electrolyte solution may be questionable due to its coalescence hindering property, at present, no conclusive data illustrating this effect, particularly for dilute NaOH solutions are available. Experiments were performed at... 

As shown by Ono et al, (1974, 1975) decrease of the HLB of a mixed emulsifier by use of an increasing proportion of a nonionic emulsifier increases the stability of the latex to coagulation by electrolyte addition despite an increase in its average particle size. This is because purely eledrostatic stabilization by adsorbed ionic emulsifier is supplemented by steric stabilization by the adsorbed nonionic emulsifier which effectively decreases the van der Waals attractive force between the latex particles (which causes them to coalesce), thereby increasing their stalrility. 

A whole range of cations and anions in different combinations have been explored. The results are surprising. Measurements of coalescence rates for a range of typical electrolytes as a function of electrolyte concentration are shown in Fig. (3.5). There is a correlation between valency of the salt and transition concentration, defined as 50% bubble coalescence, with more highly charged salt effective at lower concentration. The effect is independent of gas flow rate. All the results scale with Debye length (ionic strength). Some salts and acids have no effect at all on bubble coalescence, a situation summarised in Table 3.1. 

The sugars sucrose, fructose and glucose have also been found to affect bubble coalescence. On addition to water these sugars raise the surface tension and are desorbed from the air-water interface. Thus their effect on bubble coalescence equally cannot be described in terms of surfactant-like behaviour and certainly no charge effects are involved. Hence, even if an "explanation" could be found within the confines of the primitive model of electrolytes, that explanation could not accommodate this observation. The reduction in bubble coalescence achieved with increasing concentration is shown in Fig. 3.7. 

Although earlier work had focussed on those electrolytes which exhibited coalescence inhibition, it has now been shown that some other salts and mineral acids have no effect whatsoever. For those electrolytes inhibiting coalescence there does appear to be a correlation with the ionic strength, which brings the results into a relatively narrow band. However, and to repeat, as yet there is no obvious explanation why some electrolytes produce no effect on coalescence. 

One of the most important technological parameters in molten salt chemistry is surface tension, as the majority of important reactions take place at the interface of electrolytes or molten reacting media. In aluminum electrolysis, for instance, this parameter influences the penetration of the electrolyte into the carbon lining, the separation of carbon particles from the electrolyte, the coalescence of aluminum droplets and fog in the electrolyte, the dissolution rate of aluminum oxide in the electrolyte, etc. Similar is the effect of interface in aluminum recovery. 

Keitel and Onken [266] showed, that there was a relationship between the concentration Co, at which coalescence inhibition begins, and the quotient da/dc, from which the coalescence inhibiting effect of aqueous electrolytes could be estimated. It increases in the order NaCl - NaOH - Na2SO4 - Al2(SO4)3. 

Zahradnik J. et al., The effect of electrolytes on bubble coalescence and gas holdup in bubble column reactors, Trans IChemE 73 (1995), Part A,... 

The electrostatic repulsion between dispersed particles can be diminished by increasing the concentration of background electrolyte (e.g. NaCl, CaCl2)- Polyvalent ions are more effective than monovalent. There is a critical electrolyte concentration for every system at which flocculation or coalescence takes place. These principles must be taken into account when emulsions have to form in very hard water. 

It is necessary to note that the abundance of zinc ions in the electrolyte (O.IM) only slightly affects the deposition rate. It seems possible to explain this fact by competition of the two factors zinc codeposition (positive) and mass transfer retardation due to evacuation of zinc ions from microanodes (negative). It allows us to conclude that the processes of tin and zinc reduction and dissolution are repeated many times while the tin film is growing. Such peculiarity of tin cementation in zinc has the principal effect on the nuclei growth and coalescence and, consequently, on film microstructure. 

Effect of Electrolyte, Emulsions stabilized with hydrophobically modified poly(acrylic acid) are sensitive to electrolytes. Upon contact with a brine solution, emulsion stability is immediately lost, and rapid coalescence of the oil droplets ensues (Figure 23). This instability can be understood by consideration of the Donnan equilibrium of counterions in polyelectrolytes (discussed earlier in this chapter). Addition of salt causes collapses of the polyelectrolyte microgels that are adsorbed at the oil-water interface. Shrinkage of the microgels could conceivably lead to immediate loss of stability, as depicted schematically in Figure 24. 

The effect of dissolved salts was clarified by Robinson and Wilke [32], who showed that adding any electrolyte to water inhibited bubble coalescence and that the agitation exponent in the equation for k a gradually increased from 0.4-0.9 as the ionic strength changed from 0 to 0.40. Since surfactants can also affect the bubble size, it is difficult to predict a or k a for multi-component solutions. 

Electrolytic gas evolution is a complicated and important problem in many industrial processes. The details of bubble formation and the effects of the bubbles presented in this review are but the microscopic aspects of a phenomenon that affects the macroscopic behavior of electrochemical cells. The knowledge summarized here applies in part to the even larger world of boiling liquids in which bubbles also nucleate, grow, coalesce with each other, and detach. In general, the sudden appearance of a phase much less dense than its parent phase will always be an important phenomenon for research because that phase will profoundly affect the process in which it appears. 

Drop settles and coalesces but is re-entrained] faulty location of exit nozzles for liquid phases/distance between exit nozzle and interface is < 0.2 m/overflow baffle corroded and faUure/interface level at the wrong location/faulty control of interface/liquid exit velocities too high/vortex breaker missing or faulty on underflow line/no syphon break on underflow line/liquid exit velocities too high. [Drop settles but doesn t coalesce [phase inversion] /pH far from zpc/surfactants, particulates or polymers present/electrolyte concentration in the continuous phase < expected/[coalescer pads ineffective] /[drop size decrease] /[secondary haze forms] /[stable emulsion formation] /[interfacial tension too low] /[Maran-goni effect]. ... 

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