Liquid-liquid Extraction

PHARM 309

Liquid-liquid extraction is a versatile and dependable separation technique wherein an aqueous solution is usually brought into contact with another organic solvent, exclusively immiscible with the former, so as to affect a legitimate and actual transfer of either one or more solutes into the latter.

The separation technique is superior to others due to ease of use, faster extraction times, decreased volumes of solvent, and their superior ability to concentrate the analytes.

Introduction:

Invariably such separations may be performed by shaking the two liquids in a separatory funnel (separating funnel) for a few minutes; and may be extended either to large quantities of pharmaceutical substances or trace levels.

Liquid-liquid extractions are usually accomplished with a separatory funnel. The two liquids are placed in the separatory funnel and shaken (moderately) to increase the surface area between the phases. When the extraction is complete, the liquids are allowed to separate, with the denser phase settling to the bottom of the separatory funnel.

Fig: Separatory funnel

The solvent is chosen so that the solute in the solution has more affinity toward the added solvent.

In the case of pharmaceutical chemicals that are mostly ‘organic solutes’, the liquid-liquid extraction system may very often make use of two immiscible organic solvents (e.g., alcohol and ether) instead of the aqueous-organic type of extraction.

On the contrary, the ‘inorganic solutes’ normally encountered are invariably in aqueous solutions; therefore, it has become absolutely necessary to produce such neutral substances out of them, for instance ion-association complexes and metal-chelates (using organic-ligands) that may be extracted into an appropriate organic solvent.

Feed phase contains a component, i, which is to be extracted. Addition of a second phase (solvent phase) which is immiscible with feed phase but component i is soluble in both phases. Some of component i (solute) is transferred from the feed phase to the solvent phase.

After extraction the feed and solvent phases are called the raffinate (R) and extract (E) phases respectively.

Liquid-liquid extraction principles:

Normally one of the two phases is an organic phase while the other is an aqueous phase. Under equilibrium conditions the distribution of solute i over the two phases is determined by the distribution law. After the extraction the two phases can be separated because of their immiscibility. Component i is then separated from the extract phase by a technique such as distillation and the solvent is regenerated. Further extractions may be carried out to remove more component i. Liquid liquid extraction can also be used to remove a component from an organic phase by adding an aqueous phase.

The Nernst Distribution Law or the Partition Law states that at constant temperature, a solute distributes itself between two immiscible solvents only in a particular ratio.

Theory:

Kp= Co/Caq

Kp is the distribution constant or the partition coefficient or partition ratio (Solvent-to-Feed Ratio)Co is the concentration of the analyte in the organic phase (Solvent)Caq is the concentration of the analyte in the aqueous phase (Feed)

Extraction is an equilibrium process, and therefore a finite amount of solute might be in both phases, necessitating other processing steps or manipulation of the chemical equilibria.

The Partition Law offers the following two limitations, namely:- It is solely applicable to very dilute solutions.- It does not hold good when the distributing substances encounters association or distribution in either phases (i.e., ‘a’ and ‘b’).

Solute Feed solvent Extraction solvent Kp (wt% basis)

Acetone Water 1-Pentanol 1.14 (at 300 C) Acetone Water 2-Octanol 0.66 (at 300 C) Acetone Water Chloroform 1.83 (at 250 C)

In liquid-liquid extractions the following two aspects are very crucial and important, namely:

(a) Error due to the volume change(b) Effectiveness of an extraction

(a) Error due to the volume changeIn a situation wherein two immiscible solvents are employed in an extraction, the volumes of the two individual phases after attainment of equilibrium may be appreciably different in comparison to the initial volumes of the solvents used.

Therefore, a number of procedures have been adopted to avoid ‘error due to the volume change’ incurred thereby, namely:

(i) Measure the volume of the phase employed for the analysis and incorporate this volume in the calculations

(ii) Separate the phase quantitatively and subsequently dilute to a known volume

(iii) Separate the phase quantitatively and make use of the entire volume in the remaining steps of the ongoing analysis

(iv) Carry a marker substance through the extraction to automatically compensate for volume changes (used in chromatographic methods of analysis)

(b) Effectiveness of an extractionBased on the appropriate partition coefficient of an immiscible solvent pair it is possible to calculate the ‘effectiveness of an extraction’.

Let us assume that ‘x’ moles of solute present initially in a volume V2 of solvent ‘b’. Now, this particular sample undergoes extraction with a volume V1 of solvent ‘a’ and subsequently ‘y’ moles of compound are left in V2 at equilibrium.

Substituting these values in partition coefficient equation we have:

Kp =x-y

V1( )

y

V2after simplifying and rearranging-

Kp =x

V1( )

y

V2y-V1

x

y

V2

V1= -

V2

V1

x

y

V2

V1= -( 1 )

or KpV1

V2=x

y- 1

or KpV1

V2=x

y+1

or Kp

V1

V2=

x

y+1( ) -1 = f

where, f = fraction not extracted

From the above equation it is quite evident that the fraction extracted is absolutely independent of the initial solute concentration.

Hence, the fraction left unextracted after ‘n’ extraction may be given by the following expression-

KpV1

V2+1( ) -n=fn

Miscibility:Solvents defined as miscible if the two components can be mixed together in all proportions without forming two separate phases.

Solvents miscible with water in all proportions include acetone, acetonitrile, dimethyl acetamide, N,N-dimethylformamide, dimethyl sulfoxide, 1,4-dioxane, ethyl alcohol, glyme, isopropyl alcohol, methanol, 2-methoxyethanol, N-methylpyrrolidone, n-propyl alcohol, pyridine, tetrahydrofuran, and trifluoroacetic acid.- Solvents should be immiscible

Solvent selection criteria:

Density:Another consideration when selecting an extraction solvent is its density. Solvents that are more dense than water will form the lower layer of the pair when mixed together, while solvents that are less dense than water will form the upper layer or ‘‘float’’ on water. For example, ethyl ether has a density of 0.7133 g/mL at 20oC and would constitute the upper phase when combined with water, which has a density of 0.9982 g/mL at that temperature. On the other hand, the density of chloroform is 1.4892 g/mL at 20oC. Therefore, water would form the top layer in a water–chloroform solvent pair.

Ethyl ether (0.7133 g/mL)Water (0.9982 g/mL)

Chloroform (1.4892 g/mL)

Water (0.9982 g/mL)

- Should have high density difference

Water (0.9982 g/mL) and Hexane (0.6548 g/mL) They are less prone to emulsion problems.

Water (0.9982 g/mL) and Benzene (0.8765 g/mL) Should not be used in the extraction process.

Solubility:Although immiscible solvents may form two visibly distinct phases when mixed together, they are often somewhat soluble in each other and will, in fact, become mutually saturated when mixed with each other.

For example, 1.6% of the dichloromethane (solvent) is soluble in water. Conversely, water is 0.24% soluble in dichloromethane.

- Should be insoluble to each other.

When the phases are separated for recovery of the extracted analyte, the organic solvent layer will contain water.

Similarly, after extraction the depleted aqueous phase will be saturated with organic solvent and may pose a disposal problem.

Effect of temperatureThe effect of temperature on the partition coefficient may be estimated conveniently from its effect on the solubilities of the substance in the two respective solvents. Considerations: solvents used should be immiscible and the concentrations of solutes are fairly low in both the phases.

KP =S1

=Solubility of solute in solvent ‘a’

Solubility of solute in solvent ‘b’ S2

Factors influence solvent extraction:

Temperature should be constant throughout the process.

Effect of pH on extractionGenerally, it has been found that the organic acids and bases in solution may exist as equilibrium mixtures of their respective neutral as well as ionic forms.

Thus, these neutral and ionic forms may not have the same identical partition coefficients in a second solvent; therefore, the quantity of a substance being extracted solely depends upon the position of the acid-base equilibrium and ultimately upon the pH of the solution.

In conclusion, it may be observed that the pH for an ‘extraction system’ must be selected in such a fashion so that the maximum quantity of the analyte is present in the extractable form, that obviously suggests that the analyte should always be in the form of either a free base or a free acid.

Emulsion: Emulsion may be defined as- a dispersed system containing at least two immiscible liquid phases and the system is stabilized by emulsifying agents.

The effective and meaningful extraction of a solute is rendered almost impossible when there is an emulsion formation during an extraction process. Emulsion formation makes the separation of the two phases difficult.

Emulsion problem encountered in extraction:

Actually, emulsion formation is a frequent and serious problem when dealing with the extraction of drugs from biological as well as pharmaceutical formulations.

Fig: Emulsion formation during liquid liquid extraction

Emulsion formed during extraction

Factor causes slow-coalescence emulsionThe breaking of an emulsion (coalescence) could be a slow process. There are a number of factors which may be responsible for the slow-coalescence of an emulsion, namely:

(a) Finely divided powders of 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 (or surface tension) between the two immiscible liquids which help in stabilizing an emulsion.

(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 substances that are profusely incorporated in the formulation 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.

Prevention of emulsion formation:It has been observed that once an emulsion is formed it is difficult to break the emulsion. Therefore, it is absolutely necessary to concentrate to the following guidelines, as far as possible, in order to avoid forming emulsions in the course of an extraction process:

(1) Very cautious & gentle agitation and employing a sufficiently large liquid-liquid interface provides a reasonably good extraction. 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.

Water (0.9982 g/mL) and Hexane (0.6548 g/mL)They are less prone to emulsion problems.

Water (0.9982 g/mL) and Benzene (0.8765 g/mL) Should not be used in the extraction process.

(4) When performing extraction from water always ensure not to work at pH extremes and particularly at high pH ranges (basic) to avoid emulsification.

A major inconvenience of this method is the extreme pH (either strongly acidic, <4, or basic, >9)

(5) In cases of acute emulsion problems substances like ion exchangers, alumina or silica gel are used specifically to resolve the problem by adsorption of the emulsifying agents.

Process of breaking of an emulsion (coalescence):Following are the various techniques invariably used to break an emulsion or to achieve coalescence:

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 simply by stirring with the help of a glass-rod.

Centrifugation: In cases where the densities of the two liquids are appreciably different coalescence may be achieved by centrifugation.

Addition of monovalent and divalent ions: Relatively simple emulsions are broken by adding monovalent salts like sodium chloride; whereas charge-stabilized emulsions are specifically sensitive to the divalent ions, such as: CaCl2, MgCl2 etc.

Ethanol or higher alcohol: Addition of small quantities of either ethanol or sometimes a higher homologous alcohol aid in coalescing an emulsion.

Silicone- defoaming agent: A few drops of the silicone-defoaming agent sometimes help in breaking of an emulsion.

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.

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

Thin-bed of an adsorbent: Sometimes simply passing of an emulsion through a thin-bed of an adsorbent remarkably helps in achieving coalescence. The analyte must not be absorbed from either solvent.

Liquid-liquid extraction (washing):