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- Results and Discussion
- Physical Extraction Results
Physical extraction studies were executed by using pure solvents alone.
Oleyl alcohol and isobutanol were the types of alcohol, isopropyl methyl
ketone and methyl ethyl ketone were the types of ketone, and dimethyl
adipate was the type of ester used in the physical extraction
experiments. The initial acid concentration was varied from 0.04 to 0.06
mole kg-1 and the results were expressed in terms of
KD and E % (see Table 2). As seen from the E % values,
ketones appeared to be the most effective solvents for HGA extraction,
followed by alcohols. About 54 % extraction efficiency was achieved by
isopropyl methyl ketone, and about 48 % extraction efficiency was
achieved by methyl ethyl ketone. Ketones here seemed to be more
successful at forming hydrogen bonds and dipole-dipole interactions with
the acid molecule. For the ketones, the weakness of the bonds between
water and acid molecules provided easy accessibility to the acid
molecules. Dimethyl adipate gave the lowest extraction efficiency of 21
%, which could be due to the higher affinity of HGA towards water than
dimethyl adipate.
The initial pH of HGA was measured to be less than pKaof HGA (pKa = 4.48), showing that the maximum number of
acid molecules was present in the undissociated form and could be
distributed to the organic phase [27–29]. The physical extraction
success is dependent on two main parameters; namely the distribution of
undissociated acid molecule into the aqueous and organic phases and the
dimerization of undissociated acid molecule in the organic phase [30,
31]. The distribution coefficient of acid was presented by Kertes and
King [32]:
KD = P +
2P2D[HA]aq (3)
where P and D represent the partition coefficient and dimerization
constant of acid, respectively. P is obtained from the slope and D is
obtained from the intercept when KD is plotted versus
[HA]aq [29]. Dimer formation between the acid
molecules occurs when acid-acid interactions dominate over the
acid-solvent interactions. Thus, lower dimerization constant is expected
for higher extraction efficiency. As seen in Table 2, P and D values
have already an inverse relationship, which supports the assumption
mentioned above. The solvents showing higher extraction efficiency are
more successful at breaking the dimer formation between acid molecules
and forming hydrogen bonds with acid molecules. As seen from the
extraction efficiency and distribution coefficient results in Table 2,
solvents alone were not capable to efficiently extract HGA. Therefore,
for further experiments, Ph3PO as a reaction-enhancing
reagent was used together with the solvents to achieve higher extraction
efficiencies and distribution coefficient values.