Coalescence, effect interfacial tension

One of the main objectives of this study has been to determine the effect of interfacial properties on coalescence, emulsion stability and oil recovery efficiency for various surfactant and caustic systems. We have recently reported (6, 19) that for a petroleum sulfonate system there is no direct correlation between rates of coalescence and interfacial tension or interfacial charge. However, a qualitative correlation has been found between coalescence rates and interfacial viscosities. 

With respect to good adhesion, reduced interfacial tension, fine distribution of TLCP phase, and the use of a compatibilizer can be very effective for this purpose. Remarkably improved mechanical properties (good impact properties as well as tensile properties) can be obtained with optimum amounts of the compatibilizer. Excess amounts of the compatibilizer causes the emulsifying effect to coalesce the dispersed TLCP... 

Increasing temperature has the effect of decreasing emulsion stability this has been demonstrated by Kunieda et al. [11,14], among others, and is due to the increase of the rate of coalescence of the dispersed phase droplets with increasing thermal energy. Pons et al. [100] also noted that a temperature increase caused an increase in average droplet size due to increasing interfacial tension. 

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]. 

Setting of all relevant material properties In the preceding relevance list, only the density and the viscosity of the liquid were introduced. The material properties of the gas are of no importance as compared with the physical properties of the liquid. It was also ascertained by measurements that the interfacial tension cr does not effect the stirrer power. Furthennore, measurements [7] revealed that the coalescence behavior of the material system is not affected if aqueous glycerol or cane syrup mixtures are used to increase viscosity in model experiments. 

The use of copolymers as surfactants is widespread in macromolecular chemistry in order to compatibilize immiscible blends. These additives are sometimes named surfactants , interfacial agents or more usually compatibi-lizers . Their effect on improving different properties is observed interfacial tension and domain size decrease, while there is an increase in adhesion between the two phases and a post-mixing morphology stabilization (coalescence prevention). The aim of the addition of such copolymers is to obtain thermodynamically stable blends, but the influence of kinetic parameters has to be kept in mind as long as they have to be mastered to reach the equilibrium. Introducing a copolymer can be achieved either by addition of a pre-synthesized copolymer or by in-situ surfactant synthesis via a fitted re-... 

Sundararaj and Macosko (1995) and Beck Tan et al. (1996) observed that the addition of a block copolymer to the droplet phase before mixing it with the matrix phase had little effect on the resulting droplet size at low droplet volume fraction. Although a block copolymer should reduce the interfacial tension between the two phases, and thereby lead to smaller droplets, the diffusion time of the block copolymer may be too long for it to saturate the new interfacial area that must form rapidly if a droplet is to fragment. However, block copolymers do seem to suppress coalescence, possibly by immobilizing the interface... 

Figure 9.13 Number averaged diameter of droplets d of polypropylene (M = 60,000) in polystyrene (M = 200,000) as a function of wt% polypropylene mixed in three different mixers at a nominal shear rate of around 65 sec and T — 200°C. The viscosities of the the PP and PS under these conditions are 840 and 950 Pa-s, respectively. The interfacial tension F is 5.0 dyn/cm. The error bars represent the distribution of droplet sizes, and they encompass one standard deviation in each direction from the mean. The deviation from the Taylor limit at low concentrations is attributed to non-Newtonian effects, while the increase in droplet size at higher concentrations is attributed to droplet coalescence. Note that similar droplet sizes are obtained in all three different mixers. (Reprinted with permission from Sundararaj and Macosko, Macromolecules 28 2647. Copyright 1995, American Chemical Society.)...

The choice of chemical is usually based on trial-and-error procedures hence, demulsifier technology is more of an art than a science. In most cases a combination of chemicals is used in the demulsifier formulation to achieve both efficient flocculation and coalescence. The type of demulsifiers and their effect on interfacial area are among the important factors that influence the coalescence process. Time-dependent interfacial tensions have been shown to be sensitive to these factors, and the relation between time-dependent interfacial tensions and the adsorption of surfactants at the oil-aqueous interface was considered by a number of researchers (27, 31-36). From studies of the time-dependent tensions at the interface between organic solvents and aqueous solutions of different surfactants, Joos and coworkers (33—36) concluded that the adsorption process of the surfactants at the liquid-liquid interface was not only diffusion controlled but that adsorption barriers and surfactant molecule reorientation were important mecha-... 

In extreme cases, material can adsorb at an interface to create a film. Interfacial film formation can occur in crude-oil systems and has been reported by Blair (16), and by Reisberg and Doscher (17), Film formation is relatively common with crude oils and can effectively stabilize emulsions by preventing droplet coalescence even with high values of interfacial tension. 

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