EMULSION PROBLEM ENCOUNTERED IN EXTRACTIONS
It may be defined as-‘a dispersed system containing at least two immiscible liquid phases’.
The effective and meaningful extraction of an analyte is rendered almost impossible when one en-counters an emulsion formation during an extraction process thereby the separation of the two phases be-comes difficult. Actually, it offers a frequent and serious problem when dealing with the extraction of drugs from biological as well as pharmaceutical formulations.
Emulsion formation enhances the area of the interface between the two immiscible solvents and as a result also enhances the ‘free energy’ of the system, which may be designated by the following expression :
Free energy = γ × ∆A
where γ = Interfacial tension, and
A = change in surface area resulting from emulsification.
Obviously the ‘lowest free energy’ is given by the most stable state for a system at constant pressure and, therefore, in due course an emulsion shall ‘break’ spontaneously to the two-layered system. However, the breaking of an emulsion could be relatively a rather slow phenomenon. There are a number of factors which may be responsible for the slow-coalescence of an emulsion, namely :
(a) Finely divided powders, albumin, gelatin and natural gums have a tendency to coat the droplets formed in an emulsion which ultimately prevent them from coalescing,
(b) Usually surfactants decrease the interfacial tension between the two immiscible liquids which help in stabilizing an emulsion, and
(c) Ionic species may get absorbed at the interface of two immiscible layers resulting in the formation of a net charge on the droplets. Because all droplets shall essentially bear the similar charge, naturally they will repel one another thereby preventing coalescence.
In fact, there are many natural and synthetic products that are profusely incorporated in the ‘formula-tion of drugs’ which are found to stabilize emulsions either by coating the droplets or by minimizing the interfacial tension, namely :
(i) Coating the droplets : e.g., starch acacia, silica, gelatin finely divided talc, and
(ii) Minimizing the interfacial tension : e.g., mono-and di-glycerides ; stearates and sorbitan monoleate.
It has been observed that once an emulsion is formed it is rather difficult to break it. Therefore, it is absolutely necessary to adhere to the following guidelines, as far as possible, in order to avoid forming emulsions in the course of an extraction process :
1) Always affect very cautious and gentle agitation besides employing a sufficiently large liquid-liquid interface to obtain a reasonably good extraction. Especially when the two-liquid layers have a large contact surface in an extraction process, vigorous or thorough shaking of the two phases is not required at all,
2) The removal of any finely divided insoluble material(s) in a liquid phase must be done by filtration before carrying out the extraction process,
3) Always prefer and use such solvent pairs that have a large density difference and a high interfacial tension, for instance : water and hexane, as they are less prone to emulsion problems. In contrast, such solvent pairs as water and benzene should not be used in the extraction process,
4) When performing extraction from water always ensure not to work at pH extremes and particu-larly at high pH ranges to avoid emulsification, and
5) In cases, of acute emulsion-problems substances like-anion exchangers alumina or silicagel are used specifically to resolve the problem by adsorption of the emulsifying agents. In fact, it would be advisable to employ the technique of column chromatography for the effective separation of the analyte as compared to an extraction process.
Following are the various techniques invariably used so as to break an emulsion or to achieve coalescence, namely :
1) Mechanical Means : Coalescence may be achieved by mechanically creating turbulence on the surfaces of the droplets either by passing the emulsion through a bed of glass-wool or by stirring with the help of a glass-rod simply,
2) Centrifugation : In cases where the densities of the two liquids are appreciably different coales-cence may be afforded by centrifugation-a physical means,
3) Addition of Monovalent and Divalent Ions : Relatively simple emulsions are broken by adding monovalent salts like sodium chloride ; whereas charge-stabilized emulsions are specifically sen-sitive to the divalent ions, such as : CaCl2 ; MgCl2 etc.
4) Ethanol or Higher Alcohol : Addition of small quantities of either ethanol or sometimes a higher homologous alcohol shall aid in coalescing an emulsion,
5) Sudden Cooling of Emulsion (Thermal Shock) : Sudden temperature drop or freezing (i.e., giving a thermal shock) of an emulsion mostly enhances the interfacial tension between the two immiscible phases thereby causing coalescence.
6) Altering the Ratio of Solvents : Coalescence of an emulsion may also be achieved either by altering the ratio of the prevailing dispersed phase or even by partial evaporation of the solvent,
7) Silicone Defoaming Agent : A few drops of the silicone-defoaming agent sometimes help in breaking an emulsion, and
8) Thin-Bed of an Adsorbent : Sometimes simply passing an emulsion through a thin-bed of an adsorbent remarkably helps in achieving coalescence taking note of the fact that the analyte will not be absorbed from either solvent.