Carnosic
acid (CA) is a hydrophobic secondary metabolite and the
main antioxidant of rosemary. Extractions of CA from whole leaves
of rosemary have been performed with aqueous solutions of 12 short-chain
alkyl polyethylene glycol ethers, abbreviated as C
i
E
j
with i = 4–8
and j = 1–4. Such compounds act as hydrotropes
which are known to enhance the solubilization of hydrophobic molecules
in water while avoiding the formation of liquid crystals like surfactants.
The extractions are compared with those carried out with alcohols,
sodium xylene sulfonate (SXS) which is an archetypical ionic hydrotrope,
and longer C
i
E
j
(i = 10 or 12 and j = 4) which
behave as true surfactants. C5E2 and C4E1 are the best candidates and provide 1.21 g/L CA in
21 h and 1.02 g/L in 8 h, respectively. They are more efficient than
SXS and alcoholic aqueous solutions. Correlations between the chemical
structure and activity have highlighted three requirements for better
performances: (i) a small molecular volume V
m (<250 Å3), (ii) a log P above 1, and (iii) a linear alkyl chain rather than a branched one.
Finally, kinetic study and optical microscopy observations of the
leaves after extraction give insight into the mode of action of the
C4E
j
compared to ethanol.