Extraction Technologies For Medicinal And Aromatic Plants - Unido

EXTRACTION TECHNOLOGIES FOR MEDICINAL AND AROMATIC PLANTS
that ultrasound waves can produce free radicals and that many active molecules
are susceptible to such highly reactive species, this approach does
not seem feasible.
8.3 Solvent Extraction of MAPs
This process entails the extraction of solid MAPs by liquid solvents.
This is a typical solid-liquid extraction process. Two factors that affect
the extent and rate of extraction are the thermodynamics and kinetics (rate
of mass transfer) of the process.
8.3.1 Thermodynamics of Solvent Extraction and Choice
of Solvent
Relative sorption of solutes in the solvent depends on the interactions
between the solutes and the solvent. Solubility or miscibility of a
component with the solvent depends on their relative solubility parameters.
For mutual solubility of two components, their free energy of mixing, ΔGm
should be negative. ΔGm is defi ned as
ΔGm = ΔHm – T ΔSm (3.1)
Enthalpy of mixing, ΔHm can be correlated to cohesive energy
density, i.e. solubility parameter (δ) as:
ΔHm = n1 n2 V1 (δ1 – δ2) 2 (3.2)
In equation (3.2), the solubility parameter is that due to only
dispersive forces between structural units of the concerned solute and solvent,
since the original regular solution theory of Scatchard and Hildebrand
was restricted to non-polar, non-hydrogen bonding solute-solvent systems.
However, for many liquids and solutes, contributions from polar and hydrogen
bonding forces need to be considered. Accordingly, equation (3.2) becomes:
ΔHm = n1 n2 V1 [(Δδd) 2 + (Δδp) 2 + (Δδh) 2 ] (3.3)
From equations (3.2) and (3.3), it is clear that to make ΔGm
negative, the difference between δi (solvent) and δi (solute), i.e. (Δδ) for
all the three forces of interactions, should be as small as possible. It implies
that the solvent and the desired solute to be extracted should have
comparable polarity and hydrogen bonding capabilities to achieve similar
solubility parameter values. Grulke has given an exhaustive tabulation of
solubility parameters for the most common chemical compounds. The in-
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