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.
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