Introduction

Extractions are common in the world around us. Each time we brew a cup of tea or a pot of coffee, and each time we launder our clothes, we're performing a chemical extraction process. Perhaps because of this familiarity, extraction processes in chemical laboratories are often not fully appreciated, or fully understood. Quite simply, an extraction is the process of moving one on more compounds from one phase to another. Yet behind this simple definition lies a great deal of subtlety: separations are contrary to thermodynamic intuition, because entropy is gained through mixing, not separation; extraction methods are developed based on a drive towards equilibrium, yet the kinetics of mass transfer cannot be ignored. Such a list of physical chemical nuances provides the basis for this chapter on the fundamentals of chemical extractions.

Extractions are carried out for a variety of reasons, for example when distillation is either impractical (e.g., distillations are favourable when the relative volatility of the compounds to be separated is greater than about 1.2) or is too expensive, to isolate material for characterization, to purify compounds for subsequent processing, etc. Extractions can be classified according to a number of schemes:

• analytical versus preparative (depending on the quantity of pure compound to be separated);

• batch versus continuous (depending on the mode of feeding the material to be separated into the extraction apparatus);

• based on the physical principles involved (is the extraction strictly based on partitioning, or are adsorption or other processes involved?);

• based on the types of phases involved (so called liquid-liquid extraction, gas-solid extraction, supercritical fluid extraction, etc.).

Perhaps the biggest recent advances in the field of chemical extractions have taken place in the petroleum, nuclear, and pharmaceutical industries. The understanding and practise of extraction lies at the crossroads of analytical, inorganic, organic, and physical chemistry, with theoretical and applied chemical engineering. Yet the fundamental physico-chemical principles involved are the same. Because of the author's background, this chapter presents a description of the fundamental basis for chemical extractions and an overview of extraction techniques with a slant, or emphasis, towards the analytical chemists' perspective.

In general, the extraction process occurs as a series of steps. First the extracting phase is brought into intimate contact with the sample phase, usually by a diffusion process. Then the compound of interest partitions into or is solubilized by the extracting solvent. With liquid samples this step is generally not problematic. However with solid samples, for the compound being extracted to go into the extracting solvent the energy of interaction between the compound of interest and the sample substrate must be overcome. That is, the material's affinity for the extracting solvent must be greater than its affinity

Table 1 Summary of selected extraction techniques by phases involved and the basis for separation

Extraction technique

Sample phase

Extracting phase

Basis for separation

Liquid-liquid extraction

Liquid

Liquid

Partitioning

Solid-phase extraction

Gas, liquid

Liquid or solid

Partitioning or adsorption

(and microextraction)

stationary phase

Leaching

Solid

Liquid

Partitioning

Soxhlet extraction

Solid

Liquid

Partitioning (with applied heat)

Sonication

Solid

Liquid

Partitioning (with applied ultrasound energy)

Accelerated solvent extraction

Solid

Liquid

Partitioning (with applied heat)

Microwave-assisted extraction

Solid

Liquid

Partitioning (with applied microwave irradiation)

Supercritical fluid extraction

Solid, liquid

Supercritical fluid

Partitioning (with applied heat)

Purge-and-trap

Solid, liquid

Gas

Partitioning

Thermal desorption

Solid, liquid

Gas

Partitioning (with applied heat)

for the sample. Finally the extracting phase (containing the compound of interest) must diffuse back through the sample, separate into a distinct phase, and be removed for subsequent processing. With proper selection of the extracting solvent this final step is generally not difficult, though the formation of emulsions must be avoided with liquid samples.

As previously mentioned, extractions (and other separation processes) are contrary to the principles of thermodynamics and work must be applied to overcome these thermodynamic constraints. Perhaps this has been expressed most eloquently by Giddings:

It seems enigmatic that we often struggle so hard to achieve desired separations when the basic concept of moving one component away from another is inherently so simple. Much of the difficulty arises because separation flies in the face of the second law of thermodynamics. Entropy is gained in mixing, not in separation. Therefore it is the process of mixing that occurs spontaneously. To combat this and achieve separation, one must apply and manipulate external work and allow diffusion in a thermodynamically consistent way.

This external work is often applied as heat (temperature), which results in faster kinetics, decreased solvent viscosity and surface tension, increased solute solubility and diffusivity, and aids in overcoming interactions between the solute and the sample. A general analytical chemistry textbook (Peters, Hayes, and Hieftje (1974) Chemical Separations and Measurements: Theory and Practice of Analytical Chemisty. Philadelphia: Saunders) further describes the extraction process and areas for improvement:

If two compounds are to be separated, we must, somewhere along the line, get them into two dif ferent and separable phases... At the heart of any chemical separation are the processes of (1) phase contact and equilibrium and (2) phase separation. These steps occur in all separation techniques, and a key in understanding a given method is the identification and classification of the steps according to the nature of the phases involved and the mechanism of phase contact and separation. Similarly, if a particular method of separation is to be improved, these are the only processes worth adjusting.

Using this discussion as a framework we can classify various extraction techniques according to the phases and applied work (or the basis of separation), as shown in Table 1 for several selected extraction techniques.

The progress of an extraction is graphically depicted in Figure 1, which is a plot of the extraction yield (e.g. mass extracted) versus the progress of an extraction (e.g. solvent volume, time, equilibrium stages, etc.). This plot is generally asymptotic and consists of two regions. The initial, more steeply sloped region is the equilibrium region. This is the area where the effects of solute partitioning and solubility exist.

Amount extracted

Figure 1 Plot of the relative amount (mass) extracted as a function of extraction progress (e.g. solvent volume, time, etc.). Three regions are defined: an equilibrium region dominated by solute partitioning, a diffusion region controlled by solute diffusion, and a transitional region.

Amount extracted

As the extraction progresses it transitions into a region predominated by solute diffusion as well as the necessity for the solute to overcome effects such as solute-sample matrix interactions.

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