Breakdown, emulsions

Electrical Stability of Emuisions. The electrical stability test indicates the stability of emulsions of water in oil. The emulsion tester consists of a reliable circuit using a source of variable AC current (or DC current in portable units) connected to strip electrodes. The voltage imposed across the electrodes can be increased until a predetermined amount of current flows through the mud emulsion-breakdown point. Relative stability is indicated as the voltage at the breakdown point. 

Emulsifying capacity Maximum amount of oil that can be emulsified by a protein solution before emulsion breakdown Emulsifying capacity = volume of oil emulsilied/wl. of protein... 

Containers that are not properly sealed are prone to accelerated loss of continuous phase molecules due to evaporation. This in turn increases oil droplet concentration, which may subsequently accelerate emulsion breakdown because of the increase in the frequency of droplet collisions. 

Storage conditions should accurately reflect the conditions encountered in the actual production/storage process. The primary parameter to control is temperature. If the container is not properly seeded, other parameters such as relative humidity, air pressure, and air composition may affect the rate of emulsion breakdown. 

A number of techniques are used commercially to accelerate emulsion breakdown. Mechanical methods indude centrifugal separation, freezing, distillation and filtration. Another method is based on the principle of antagonistic action - i.e. the addition of O/W-promoting emulsifiers tends to break W/O emulsions, and vice-versa. Emulsions can also be broken by the application of intense electrical fields, the principal factors involved being electrophoresis in the case of O/W emulsions and droplet deformation in the case of W/O emulsions. 

Addition of powder which is agglomerated by dispersed oil and causes emulsion breakdown. 

BURCO DEFOAMER RS is a "stabilized" silicone defoamer. By altering the defoaming molecule, its foam killing efficiency makes it far more resistant to emulsion breakdown. The water dispersibility of BURCO DEFOAMER RS is also superior to that of other silicones, so it rinses freely from the goods. 

Low-fat spreads are mixed gel systems of fat, water, emulsifiers, and hydrocolloids. Because they often undergo emulsion breakdown, starch derivatives and gelatin are used to provide the aqueous phase with acceptable rheological and melting characteristics. In a recent study, creep-compliance studies were used to examine the rheological characteristics of butter, margarine, and a low-fat spread (Chronakis... 

Lecithin use in low fat spreads may decrease the emulsion stability and increase the tendency to oil off however, it also functions to slow the emulsion breakdown in the mouth. Therefore, the use of lecithin and the level of use must be evaluated for each formulation. 

Selection of a suitable chemical emulsion breaker and dosage is crucial. A particular demulsifier may be effective and efficient for one emulsion yet entirely unsatisfactory for another. Contemporary demulsifiers are formulated with polymeric chains of ethylene and propylene oxides of alcohol, alkyl phenols, amino compounds, and resinous materials that have hydroxy acceptor groups. Each of these polymers is carefully formulated to yield a molecule with a particular affinity for water. Demulsifier dosage is also important excessive demulsifier addition can inhibit the efficiency of emulsion breakdown. 

Fig. 3 Schematic representation of the various processes of emulsion breakdown.

In most cases AA/i2 T AS °, which means that AG °" is positive that is, the formation of emulsions is nonspontaneous and the system is thermodynamically unstable. In the absence of any stabilisation mechanism, the emulsion will break by flocculation, coalescence, Ostwald ripening, or a combination of all these processes. This is illustrated in Figure 10.4, which shows several pathways for emulsion breakdown processes. 

Figure 10.4 Free energy path in emulsion breakdown. The solid line indicates flocculation + coalescence the dashed line indicates flocculation +coalescence + sedimentation the dotted line indicates flocculation -F coalescence H-sedimentation -I- Ostwald ripening.

Hesp SAM, Woodhams RT. Asphalt polyolefin emulsion breakdown. Colloid Polym Sci 1991 269 825-834. 

Petrov etal. (1980) analyzed the causes for the entrapment of water between the solid substrate and the monolayer in Z-type depositions. This phenomenon has many common features with film thinning processes found during foam and emulsion breakdown and it is dependent on interfacial properties and on molecular interactions between the solid substrate and the monolayer. Petrov etal. (1980) measured the maximum speed of removal of the solid substrate before entrainment of a water layer and found it to be dependent on pH and ionic strength. There is no record in the publication of the measurement of dynamic contact angles. 

Levchenko D. N., Bergshtein V. V, Hudjakova A. D., Nikolaeva N. M., Oil-Water Emulsions and Methods of Preventing Emulsion Breakdown, Chemistry, Moscow, 1967 (in Russian). 

Sjoblom and coworkers surveyed several oils from the Norwegian continental shelf. After the interfacially active traction was removed from the oils, none would form water-in-oil emulsions (24— 26). Model emulsions could be made from the extracted interfacially active fractions. Stability was gaged by measuring the separation of water with time. Destabilization studies showed that the rigidity of the interfacial film or reaction with the film components are the principle methods of emulsion breakdown. Medium-chain alcohols and amines destabilized emulsions the most. 

BJ CarroU. The stahUity of emulsions and mechanisms of emulsion breakdown. In E Matijevic, ed. Surface and Colloid Science. Vol 9. New York John Wiley, 1976, p 1. 

Figure 18 Schematic illustration of the putative process of emulsion breakdown during the measurement of cef in W/O emulsions (see Refs 20 and 85-92 for detailed discussion of the method and phenomenon Ref. 93 describes our application).

These natural or spontaneous mechanisms of emulsion breakdown, as well as others, must be addressed in the formulation stage in order to understand and control a particular multiple emulsion system of interest. In all cases, the final stability of the system will depend on the nature of the oil phase of interest, the characteristics of the primary and secondary emulsifier systems, and the relationship between the internal and continuous phases. 

As mentioned in the introduction, the emulsion breakdown processes is far from understood at a molecular level. It is thus, necessary to develop methods of assessment of each process and to attempt to predict the long-term physical stability of emulsions. 

Various emulsion breakdown process may be identified (schematically represented in Chapter 6). These breakdown processes will be briefly summarised below, with particular attention as to how one can stabilise the emulsion against the instability described. 

Fig. 1.8 Free energy path in emulsion breakdown -, Flocc. + coal., —, Flocc. + coal. + Sed.,..., Flocc. + coal. + sed. + Ostwald ripening.

---