364 JOURNAL OF COSMETIC SCIENCE the number of experiments, reactants, time, financial inputs, and energy. Also, experimen- tal errors are minimized. Statistical methods measure the change of controllable variables affecting the process and their interactions through experimental design (49). This study aims to consider environmentally friendly and reusable ILs as an alternative to conventional solvents used as catalysts in the esterification reaction of AA with BA and to optimize the reaction conditions. There are no published studies on the estima- tion of reaction parameters for benzyl acetate esterification reaction catalyzed by the ILs ([EMIM] [HSO 4 ], [EMIM] [BF 4 ], [OMIM] [BF 4 ], [EMIM] [NTf 2 ], and [DEIM] [NTf 2 ]) in this study. For this purpose, the effects of variables such as the structure of ILs, initial acid/alcohol molar ratio, catalyst amount, reaction temperature, and reaction time on acid conversion were investigated. Optimum conditions were studied using the Box–Behnken experimental design of the response surface methodology. EXPERIMENTAL MATERIALS The chemicals acetic acid (100%) and benzyl alcohol (99.5%) used in this work were purchased from Merck (Darmstadt, Germany) and used without any purification. For this study, the ILs (99%), 1-Ethyl-3-methylimidazolium hydrogen sulphate [EMIM] [HSO 4 ], 1-Ethyl-3-methylimidazolium tetrafluoroborate [EMIM] [BF 4 ], 1-methyl-3- octylimidazolium tetrafluoroborate [OMIM] [BF 4 ], 1-ethyl-3- methylimidazolium bis [(trifluoromethyl)sulfonyl] imide [EMIM] [NTf 2 ], and 1,3-diethylimidazolium bis [(trifluoromethyl)sulfonyl] imide [DEIM] [NTf 2 ] were supplied by IoLiTec (Heilbronn, Germany) and used without any pretreatment. Aqueous NaOH solution was prepared by 0.1 N NaOH Titrisol (Merck), which was used for the acid analysis. REACTION MECHANISM The reaction mechanism of the esterification between AA and BA catalyzed by ILs can be explained by the following equations: Firstly, the IL was divided into anion and cation (Eq. 1). IL A B- → + + (1) Then, the BA and AA, intermediate complexes with the anion and cation of the IL, respec- tively, were formed (Eq. 2–4). The alcohol formed by the strong interaction between the anion of the IL and the hydroxyl group of the alcohol can be activated by a Lewis-type complex formed with the Lewis acidic cation. + → + - C H CH OH B C H CH B OH- 6 5 2 6 5 2 (2) → + - CH COOH CH COO H+ 3 3 (3) + → - CH COO A+ CH COO A 3 3 (4)

365 SYNTHESIS OF BENZYL ACETATE At the end of the reaction, ester and water are formed as products. With the formation of the ester, the catalyst becomes free. All steps are basic and irreversible, and the stage of final product formation is the rate-determination stage (Eq. 5) (50,51). C H CH B CH COOA H OH C H CH CH COO H O IL 6 5 2 3 6 5 2 3 2 + + + → + + + - (5) PROCEDURE OF ESTERIFICATION REACTION Esterification experiments were conducted in a 100 mL three-necked round-bottom flask glass reactor. Esterification of AA and BA catalyzed by IL experiments were conducted in a 100 mL three-necked round-bottom flask glass reactor. The reaction was carried out by effec- tive mixing by adding simultaneously certain amounts of AA (20 mmol), BA (20 mmol), and IL (10 mmol) (for AA:BA:IL molar ratio of 1:1:05) to the preheated reactor. The reactor was placed in a temperature-controlled oil bath with a mixer heater with a contact ther- mometer (IKA C-MAG HS7, IKA, Staufen, Germany.), and the reaction temperature was controlled. A reflux condenser was connected to the reactor to prevent evaporation losses during the reaction. The reaction mixture was effectively stirred with the magnetic stirrer. At the end of the reaction, a certain amount of sample was titrated with phenolphthalein indicator with 0.1 N NaOH, and acid conversion (%) was calculated by determining the amount of acid remaining in the phase without reacting (52–54). Trials were repeated at least twice for each case. The experiment was performed until reproducible results were obtained. The acid value (AV) was determined by Eq. 6.    = × × L N f S T Acid Value mol (6) where N is the normality of NaOH, f is the factor of 0.1 N NaOH, S is the volume of the titrant (mL), and T is the volume of the sample (mL). The conversion (X) was calculated by comparing the AV of the initial acid (AV 0 ) to the AV of acid at time t (AV t ), using Eq. 7. (%) = - ×100 X AV AV AV t 0 0 (7) [EMIM] [HSO 4 ] was used as a catalyst in the esterification reaction biphasic layers are formed at the end of the reaction after cooling the solution to room temperature. The upper layer contains the desired ester product, while the bottom layer contains the IL, unreacted reactants, and water. The product (ester) can be easily removed from the reac- tion medium by decanting without any additional cost. Since the unreacted acid may be present in both phases, the acid analysis was carried out in both phases. [EMIM] [BF 4 ] was used as a catalyst in the reaction, the IL was precipitated as a solid, and the liquid phase consists of ester and unreacted reactants (55). The organic phase was analyzed to calculate the acid, which remained unreacted. [DEIM] [NTf 2 ], [EMIM] [NTf 2 ], and [OMIM] [BF 4 ] were used as the catalyst in the reaction, and the reaction medium was not biphasic because the ester was dissolved in the IL after cooling the solution to room temperature (56). In this case, the acid analysis was carried out in the reaction mixture.