1. Results and Discussion
  2. 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.