Influence of Microwave Irradiation on Hydrolysis Reaction of Sunflower Oil in Aqueous Emulsion

Microwave irradiation (MW) has been widely applied as heating in chemical processing. It offers a clean, convenient and inexpensive method of heating which often results in higher yields and shorter reaction times. Here, we study the microwave heating influence on the hydrolysis of the triglyceride (sunflower oil) in aqueous emulsion catalyzed by using 4-dodecylbenzenesulfonic acid (DBSA). The progress of the hydrolysis reaction was determined by Fourier Transform Infrared spectroscopy (FTIR). The effects of temperature, reaction time and the catalyst nature and concentration on the hydrolysis reaction were investigated. The hydrolysis was carried out at temperatures ranging between 90 ̊C and 150 ̊C. The polarity of the reaction medium accelerated this reaction.


Introduction
The hydrolysis of triglycerides contained in vegetable oils or animal fats is produced industrially from the reaction of vegetable oils or animal fats with superheated steam [1].Different technologies have been used in industry.The most important are the batch autoclave process (the Twitchell process) [2] and the continuous process (the Colgate-Emery process) [3].Currently, the reaction is carried out by the action of a steam in a countercurrent column at 100˚C -260˚C and 20 -60 bar [4], [5].The reaction does not require the use of a catalyst, and af-ter 1 to 3 hours, the conversion of triglycerides is about 98% -99% [6].This reaction was also carried out in the subcritical water at 270˚C -340˚C and 130 bar in a relatively short time (10 min) using oil/water molar ratio of 1/5 [7].More recently, supercritical carbon dioxide (SC-CO 2 ) was used as hydrolysis medium to enhance the solubility of water in the oil phase (triglyceride) [8].A conversion of 100% of triolein was achieved after 3 hours at 250˚C.
In milder conditions, hydrolysis of tricaprylin was carried out using solid acid catalysts at 110˚C -150˚C and atmospheric pressure in a semibatch reactor with continuous addition of water at low flow rates [9].Hydrolysis in emulsion has been also studied [10].Palm oil hydrolysis was carried out using a mixture of linear alkyl benzene sulfonate as anionic surfactant combined with (H 2 SO 4 ) as acid catalyst at different oil/water proportions below 100˚C.The use of a surfactant in combination with an acid catalyst allows the acceleration of the hydrolysis reaction [11].
In the recent years, considerable researches have been devoted to the application of microwave heating in chemistry [12].This type of heating is more penetrative than the conventional one [13].It offers a clean, convenient and inexpensive method of heating which often results in higher yields and shorter reaction times [14].This process mainly depends on the specific polarity of molecules.Since water is polar it has good potential to absorb microwaves and to convert them into heat energy, consequently accelerating the reactions in an aqueous medium as compared to results obtained using conventional heating [15]- [17].
In this context, the aim of the present work is to study the hydrolysis reaction of sunflower oil carried out under microwave heating in aqueous emulsion using an anionic surfactant.

Chemicals Reagents
Commercial sunflower oil was used without further purification.4-dodecylbenzenesulfonic acid (DBSA, mixture of isomers 95%) and sulfuric acid were purchased from Sigma Aldrich Chemicals.Deionized water was used.

Materials
Reactions were performed into commercial microwave (multimode reactor Synthos 3000, Anton-Paar) equipped with two magnetrons able to provide 1400 W of continuous MW power at 2.45 GHz.The control system supplied with the instrument allows working with either controlled temperature or irradiation power.

Experimental Procedure
The emulsion was prepared by adding 7.6 mL of water into 12 g of sunflower oil containing 0.4 g of DBSA with a magnetic stirrer, so that the water-to-oil ratio was 40% (w/w).The mixture was exposed to ultrasounds during 90 s with 70% of amplitude (Branson sonifier W450 digital) at ambient temperature.The emulsion was heated under microwaves.The microwave reactor was programmed to maintain a constant temperature by adjusting the applied power.

Analysis
Infrared Spectroscopy Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were run with a Brucker Tensor 27 DTGS spectrophotometer.The spectra were recorded between 4000 and 450 cm −1 with an average of 30 consecutive scans with a resolution of 4 cm −1 .
The hydrolysis progress rate was determined by ATR-FTIR spectroscopy.The relative ester residual was calculated according to the following equation: which allows to plot the normalized curve of the triglyceride hydrolysis.Where A ester corresponding to the integration of the 1730 cm −1 band resulting from the C=O stretching vibration of ester group and A acid corresponding to the 1690 cm −1 band resulting from the C=O stretching vibration of carboxylic acid group.

Results and Discussion
The hydrolysis reaction was carried out in emulsion at different temperatures using different proportions of surfactants with water-to-oil ratio of 40% (w/w) (Table 1).
The rate of hydrolysis reaction can be expressed by using a decreasing exponential according to the following equation: where C t is the concentration of the triglycerides during the reaction and C 0 is the concentration of triglycerides at the beginning of the reaction.
The results obtained by varying the temperature of the hydrolysis are presented in Figure 1.
The curves were fitted by the linear-regression method using Origin software.Each curve represents the progress of the hydrolysis reaction over time at 110˚C, 130˚C and 150˚C temperatures.It was then observed that the increase of temperature raises the rate constant of the chemical reaction (Table 2) and the equilibrium was reached more rapidly.This suggested that higher temperatures would enhance the hydrolysis reaction.
The effect of varying catalyst level on the hydrolysis at 150˚C is shown in Figure 2. It was observed that decreasing of the catalyst concentration does not affect the kinetic of the reaction.Furthermore, no hydrolysis was observed in reactions conducted at 150˚C and 230˚C (run 9, 10) under microwave heating using anionic surfactant (DBSNa) but without using a catalyst.
The surfactant adsorbs on the droplets surface leading to large interfaces between water and oil and allowing better contact between both phases.On the other hand, water has a high dielectric loss (ε" = 19.1)and good potential to absorb microwaves and to convert them into heat energy.However, these conditions were not sufficient to lead to hydrolysis.
The other parameter we investigated was the nature of catalyst.DBSA has been replaced by another anionic surfactant (DBSNa) using stoichiometric amounts of H 2 SO 4 as catalyst.The reaction was conducted at 150˚C.The progress of the hydrolysis reaction is shown in Figure 3. Thus, when using DBSA, the reaction kinetic is slightly faster than when using DBSNa.This emphasizes the catalytic effect of DBSA.Contrary to the sulfuric acid which is soluble in aqueous phase, the DBSA is hydrophobic.So, it is either adsorbed on the surface or located in the core of the particles, consequently catalyzing more efficiently the hydrolysis reaction.
Therefore, the surfactant polar heads (dipole ionizable SO 3 − , H + ) located on the surface of droplets containing triglyceride and the presence of water owning high dielectric loss (ε" = 19.1)are favorable to the penetration of electromagnetic waves providing good conditions for optimal microwave heating.

Conclusion
Hydrolysis reaction was conducted on sunflower oil under microwaves (MW) heating in aqueous media.The influence of temperature and the catalyst nature and concentration were investigated.The obtained results high-      lighted that: 1) The increase of temperature raises the rate constant of the chemical reaction; 2) Temperature does not influence the hydrolysis reaction in the absence of catalyst; 3) The hydrolysis reaction is accelerated when using hydrophobic catalyst.From these results, we conclude that under optimum conditions of catalyst concentration and maximum water absorption, MW heating is optimum and consequently hydrolysis is accelerated.

Figure 1 .
Figure 1.Effect of temperature on the hydrolysis of sunflower oil.

Figure 2 .
Figure 2. Effect of catalyst level on the hydrolysis of sunflower oil using 40/60 (w/w) water-to-oil ratio at 150˚C.

Figure 3 .
Figure 3.Comparison between using DBSA and DBSNa/H 2 SO 4 on the hydrolysis of sunflower oil at 150˚C.

Table 1 .
Conditions of the sunflower oil hydrolysis in aqueous emulsion.
*Using 0.6 g of H 2 SO 4 added to oil, 0.5 g of DBSNa and 7.6 g of water.

Table 1 .
Reaction rate constant of sunflower oil hydrolysis in aqueous emulsion at different temperatures.