Macroemulsions breaking

BDS process. The pore size of the filter (0.2-1.0 xm) is selected such that the liquid phase, which is miscible with the liquid that is used to wet the filter, passes through the filter, while the second liquid phase remains. Thus, an aqueous filter is wet with a liquid, which is miscible with water, but immiscible with oil. The flow rate is chosen so as to prevent solid deposition through the filter. Although, such a separation process can be applied to any oil/water emulsion, it was particularly envisioned as part of a BDS process. One may ask, whether it would be more efficient to break a macroemulsion by filtering than it is by any other means Second, in the case of microemulsions, how efficient would such a filtration process be ... 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

In spite of this progress, we still do not have good predictive methods for the formation or breaking macroemulsions. For the formation of a stable macroemulsion from two immiscible liquids, there is no reliable predictive method for selecting the emulsifier or... 

The term stability, when applied to macroemulsions used for practical applications, usually refers to the resistence of emulsions to the coalescence of their dispersed droplets. The mere rising or settling of the droplets (creaming) because of a difference in density between them and the continuous phase is usually not considered instability. Flocculation or coagulation of the dispersed particles, without coalescence of the liquid interior of the particles, although a form of instability, is not considered as serious a sign of instability as coalescence or breaking of the... 

The rate at which the droplets of a macroemulsion coalesce to form larger droplets and eventually break the emulsion has been found to depend on a number of factors (1) the physical nature of the interfacial film, (2) the existence of an electrical or steric barrier on the droplets, (3) the viscosity of the continuous phase, (4) the size distribution of the droplets, (5) the phase volume ratio, and (6) the temperature. 

Physical Nature of the Interfacial Film The droplets of dispersed liquid in an emulsion are in constant motion, and therefore there are frequent collisions between them. If, on collision, the interfacial film surrounding the two colliding droplets in a macroemulsion ruptures, the two droplets will coalesce to form a larger one, since this results in a decrease in the free energy of the system. If this process continues, the dispersed phase will separate from the emulsion, and it will break. The mechanical strength of the interfacial film is therefore one of the prime factors determining macroemulsion stability. 

Size Distribution of Droplets A factor influencing the rate of coalescence of the droplets is the size distribution. The smaller the range of sizes, the more stable the emulsion. Since larger particles have less interfacial surface per unit volume than smaller droplets, in macroemulsions they are thermodynamically more stable than the smaller droplets and tend to grow at the expense of the smaller ones. If this process continues, the emulsion eventually breaks. An emulsion with a fairly uniform size distribution is therefore more stable than one with the same average particle size having a wider distribution of sizes. 

Droplet size plays an important role in macroemulsion stability. Forming a macroemulsion requires energy input because of the increased interfacial area between the oil and water phases (Fig. 12). Conversely, as the emulsion breaks and the system returns to the original state, energy is released. In the example in Fig. 12, 31 J of work is required to form one mieron droplet of 100 mLs of heptane in 1 L of water. If smaller droplets were... 

The hydrophilic-lipophilic deviation (HLD) is a dimensionless representation of SAD, given by HLD = SAD/RT. Either SAD or HLD values can be used to determine composition regions for which macroemulsions or microemulsions are likely to be stable, break or invert. Negative SAD or HLD values refer to Winsor type 1 systems (O/W), positive SAD or HLD values refer to Winsor type II systems (W/O) and SAD = HLD = 0 refers to Winsor type III systems (most of the surfactant is in a middle phase with oil and water). Much of the use of SAD and HLD has been in developing surfactant formulations. 

Other petroleum applications such as emulsion breaking, particularly for crude oil dehydration, are of first importance. A by-product of enhanced oil recovery was a better understanding of the relationship between the phase behavior of surfactant-oil-water systems and the properties of the corresponding macroemulsions [111-118]. 

In the macroemulsion range (1-100 pm) the settling may be rather quick unless the external phase is viscous or the density difference very small. In the mintemulsion range (100-500 A) the settling is generally very slow because of the importance of the squared radius factor. Thus, it may be said that an extremely small drop size would slow down the flrst step of the breaking process and as such would slow down the whole process. This statement is true in the event... 

Stability of a macroemulsion is an important factor as this determines its extent of usability for particle preparation or various other applications. Instability is basically coalescence of the dispersed phase droplets or Ostwald ripening (growth of large droplets at the expense of much smaller ones). When this process goes on, the emulsion eventually breaks into two layers. Other processes related to stability but considered less important [3] are (a) creaming or sedimentation, the rate of which is dependent on the difference in density between the continuous and dispersed phases, droplet size, viscosity of the continuous phase and interdroplet interaction and (b) flocculation, dependent on colloidal interactions between the droplets [8, 12]. Several factors determine the stability of macroemulsions these are discussed here in brief. This discussion is largely derived from Rosen [3] and some subsequent investigations [e.g. 6, 7, 13-15]. 

Since the basic ideas in surfactant science have been introduced, one can now proceed with the main topic of this chapter macroemulsion stability. Macroemulsions are formed by mechanical mixing of oil and water in the presence of surfactants, e.g. by mixing the phases of the Winsor 1 equilibrium in each other. As a result of mixing, one of the phases breaks into macroscopic droplets, while the other stays continuous. Macroemulsions are thermodynamically unstable and gradually resolve with time into two distinct layers. However, in some cases they show a remarkable kinetic stability. Most experimental trends in macroemulsion stability were established a long time ago and will be outlined below. 

There are two more subtle features of the nonionic macroemulsion stability to be discussed. Firstly, within the Winsor III region, the stability of macroemulsions is very temperature sensitive. Although exactly in the balanced state, the macroemulsions are very unstable and break within minutes, the system becomes stable only several tenths of a degree away from the balanced point, while still being within the Winsor III region. Secondly, the macroemulsion stability pattern is not completely symmetric. W/O emulsions reach maximum stability at ca. 20 C above the balanced point, after which the stability starts to decrease. On the other... 

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