Esterification Kinetics of Glycerol with Fatty Acids in the Presence of

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Ind. Eng. Chem. Res. 2004, 43, 7744-7753

Esterification Kinetics of Glycerol with Fatty Acids in the Presence of Zinc Carboxylates: Preparation of Modified Acylglycerol Emulsifiers Adam Macierzanka and Halina Szela¸ g* Department of the Technology of Fats and Detergents, Chemical Faculty, Gdan´ sk University of Technology, Narutowicza 11/12, 80-952 Gdan´ sk, Poland

The main goal of this study is the elaboration of a one-step synthesis of acylglycerol emulsifiers with modified hydrophobic properties. The esterification kinetics of glycerol with four individual fatty acids (C12:0, C14:0, C16:0, and C18:0), in the presence of zinc carboxylates (ZnC) of these acids, was investigated. The influence of the ZnC concentration and temperature on the reaction progress was studied. It was found that, under the applied conditions, the process is, with respect to glycerol esterification, a consecutive reaction of first order, with monoacylglycerol (MAG) as an intermediate, stable product. The maximum concentration of MAG in the reaction mixture did not exceed 50%. The reaction conditions as well as the types of carboxylate and fatty acid used influenced the rate of glycerol conversion and MAG formation. Zinc salts of fatty acid were found to be effective in increasing the contact between reactants and, as a consequence, accelerating the reaction progress. The formation of microemulsions in all studied esterification processes was observed. We studied the reaction kinetics in order to prepare emulsifiers with desirable contents of MAG and zinc ions. The influence of the synthesized emulsifiers’ composition on their hydrophilic-lipophilic properties was also investigated. The fatty acid chain length as well as the concentration of ZnC influenced the HLB values of the synthesized preparations. 1. Introduction Ester derivatives of polyols and fatty acids are widely used, as emulsifiers, in the food, cosmetic, pharmaceutical, and chemical industries.1-10 The most known are monoacylglycerols (monoglycerides, MAGs), which (because of their surface activity) are mainly employed as water-in-oil (W/O) emulsion stabilizers. Either an interesterification process with glycerol and triacylglycerols (glycerolysis) or a direct esterification of glycerol with fatty acids can be used to prepare these compounds. Both reactions lead to a final product that is a mixture of mono-, di-, and triacylglycerols and some amount of unreacted substrates.5,6 The proportions depend on the presence and type of catalyst, as well as the reaction conditions, such as temperature and the molar ratio of substrates. Among synthesized acylglycerols, the monoester has the highest surface activity, and therefore, its concentration is very important for direct usage of reaction product as an emulsifier. Generally, the concentration of MAG does not exceed 50%. It can be separated from di- and triacylglycerols by molecular distillation. The total MAG content in distilled products is over 90%.11-13 Basic (e.g., sodium hydroxide) and acidic (e.g., ptoluenesulfonic acid) catalysts are commonly used in the direct esterification of glycerol. The reaction can be also carried out in the presence of porous materials, e.g., ionexchange resins14 and zeolites.15-17 The application of enzymatic methods allows for the synthesis of MAG in a low-temperature reaction.18-20 Lipase-catalyzed synthesis of glycerol esters can be realized in reverse micelles (microemulsion systems).21 * To whom correspondence should be addressed. Tel.: +48 58 347 29 27. Fax: +48 58 347 26 94. E-mail: szelag@ altis.chem.pg.gda.pl.

Other authors22,23 have reported the formation of microemulsions in the reaction of fatty acids and polyols carried out in the presence of sodium soaps. Osipow and Rosenblatt22 prepared sucrose and glycerol esters by means of the transesterification of polyols with fatty acid methyl esters or triacylglycerols using sodium stearate or oleate as the emulsifying agent. Kaufman and Garti23 demonstrated the influence of sodium stearate on glycerol-in-methyl stearate microemulsion formation. These authors reported a positive influence of the carboxylate on the consumption of the substrates. An increase of the sodium stearate concentration resulted in worse monoester selectivity because of the better contact between reactants and the faster esterification of monoester to diester. The formation of glycerol-fatty acid microemulsions, for esterification carried out in the presence of fatty acid (C12:0-C18:0) sodium and potassium salts, was reported previously by our laboratory.24 The concentration of those carboxylates was found to be an essential factor determining the conversion of the substrates and the MAG yield. The esterification kinetics of glycerol with fatty acids has been the subject of a limited number of publications. It was proposed that this reaction is of second or third order for equivalent weight concentrations of substrates, according to the type of catalyst and reaction conditions, such as temperature and the presence of solvent.25-27 Hartmann28 reported that the uncatalyzed esterification kinetics is of second order, but the reaction becomes more complicated when equimolar amounts of glycerol and fatty acid are used. The reaction rate was dependent on the miscibility of glycerol with fatty acid. Sanchez et al.15 studied the kinetics of the esterification of glycerol with oleic acid, using zeolite of the fajuasite type as the catalytic system. These authors showed that the selective synthesis of glycerol mo-

10.1021/ie040077m CCC: $27.50 © 2004 American Chemical Society Published on Web 10/27/2004

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7745

nooleate can be described by a pseudo-second-order kinetic model. The formation of mono- and diesters of glycerol was characterized as two parallel reactions. An increase in temperature and catalyst concentration caused the acceleration of the reaction rate. Optimization studies have shown that the most important factor affecting the yield of MAG is temperature.16 It has been demonstrated that the esterification of glycerol with fatty acids in the presence of sodium and potassium carboxylates is a consecutive reaction of first order with respect to glycerol, with MAG as an intermediate product.24 This result was observed for glycerol/ fatty acid molar ratios in the range of 0.8-1.2. Alkali soaps influence the increase of the interface between glycerol and fatty acid and, in that way, enhance the reaction rate. The carboxylates and synthesized MAG cause the formation of a fatty acid-glycerol microemulsion. First-order kinetics with respect to glycerol can be derived from the facts that (i) during the course of the esterification, the water produced is being removed from the reaction mixture and (ii) the reaction is realized in a microemulsion. The contradiction between this finding and the results of other authors could be due to the difference between experimental procedures. A second beneficial effect of the alkali soaps employed is that their presence in the reaction product causes an increase of the hydrophilic character of the synthesized product. The obtained preparations can be used directly as emulsifiers of O/W-type emulsions.29 In our previous work,30 we showed that the type of carboxylate cation has a significant influence on the glycerol esterification progress, as measured by the conversion of the substrates (glycerol and fatty acid) and MAG concentration changes. Under the conditions of our experiments, the catalytic influence of different soaps on the reaction progress is as follows: zinc soaps > sodium soaps > potassium soaps. Zinc oxide, fluoride, and chloride were stated as very active catalysts of the esterification.31,32 For zinc chloride, it was suggested that its initial reaction with fatty acids and glycerol leads to the formation of metal soaps and chlorohydrins and that esterification proceeds through the interaction of these two initial reaction products. Only limited information is available on the usage of metallic soaps, such as zinc salts, as catalysts for esterification reactions.31 Zinc salts of fatty acids, like other polyvalent alkyl carboxylates (e.g., calcium, magnesium and aluminum soaps), are used as emulsifiers in high-oil-content cosmetic products of the W/O type, such as night and cleansing creams.7 In this study, the kinetics of the esterification of glycerol with fatty acids in the presence of zinc carboxylates was investigated. The influence of these compounds on the consumption of the substrates and the formation of glycerol esters was studied. The kinetic investigations were focused mainly on the characterization of MAG formation. As we assumed, direct use of the obtained reaction product as an emulsifier can be limited to that with high MAG contents. Therefore, the goals of this study were (i) to gain the ability to predict the desired concentration of monoester and (ii) to prepare MAG-rich emulsifiers with modified hydrophilc-lipophilc properties in a one-step reaction. The kinetic equations were derived for the practical reason of evaluating the maximum concentration of MAG in the reaction product and calculating the time at which

Table 1. Composition of the Fatty Acids content (wt %) fatty acid C10:0 C11:0 C12:0 C14:0 C14:1 C15:0 C16:0 C16:1 C17:0 C18:0 C18:1 C18:2 C19:0 C20:0 C20:1 C22:0 C24:0

lauric acid myristic acid palmitic acid stearic acid (C12:0) (C14:0) (C16:0) (C18:0) 0.3 0.1 98.8 0.8 -

0.8 98.9 0.1 0.2 -

0.1 0.2 4.0 0.2 92.2 1.7 0.1 1.1 0.2 0.1 0.1

0.2 0.9 1.8 0.1 91.5 0.2 0.3 5.0 -

this concentration can be obtained. Hydrophile-lipophile balance (HLB) values of the synthesized preparations were also studied. 2. Experimental Section 2.1. Materials. The fatty acids: lauric, C12:0 (98.8%); myristic, C14:0 (98.9%); palmitic, C16:0 (91.5%); and stearic, C18:0 (92.2%) were obtained from Sigma-Aldrich Chemie Gesellschaft GmbH & Co. KG (Taufkirchen, Germany). Gas chromatography was applied to check their composition. Complete analysis of the composition of fatty acids is reported in Table 1. Glycerol (98.8%) and other reagents, e.g., sodium hydroxide and zinc sulfate (analytical grade), were obtained from POCH Company (Gliwice, Poland). 2.2. Preparation of the Zinc Carboxylates. Zinc carboxylates (ZnC) were prepared by the precipitation process in a three-necked flask fitted with a mechanical stirrer, a thermometer, and a water condenser. Fatty acids were heated to the melting temperature, and then an aqueous 5% NaOH solution was added. Reaction leading to sodium soap formation was carried out at a temperature of 90°C.

RCOOH + NaOH f RCOONa + H2O In the next step, 10% ZnSO4 aqueous solution was added to precipitate the zinc carboxylate.

2RCOONa + ZnSO4 f [RCOO]2Zn + Na2SO4

(2 h, 75°C)

The product was washed, filtered, and dried to the constant mass. To estimate the carboxylate purity, the sodium concentration was checked by the flame photometry method. The zinc soaps did not contain residual sodium in amounts equal to more than 0.1% of alkali soap. The melting temperature of each soap was estimated. It was found to be 123.5-127.5 °C for zinc laurate, 124-129 °C for zinc myristate, 123.5-126.5 °C for zinc palmitate, and 123.5-128 °C for zinc stearate. The obtained results are comparable to those presented by other authors.31,33 2.3. Esterification of Glycerol with Fatty Acids. The reaction was conducted in a thermostatic reactor equipped with a stirrer, a thermometer, and a nitrogen tube. The stirring rate was adjusted to 200 rpm. The

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pressure in the system was reduced (800 hPa) to eliminate water that was formed during the esterification process. Glycerol was poured into a heated mixture of fatty acid and ZnC. The reactions were carried out at temperatures of 130, 140, 150, and 160 °C ((1 °C) for 6 h. The molar ratios of glycerol (G) to fatty acid (FA) and zinc carboxylate (G/FA/ZnC) employed were 1:1:0.00625; 1:1:0.0125; 1:1:0.025, and 1:1:0.05. The progress of the reaction was investigated by analyzing the composition of the reaction mixture at 1-h intervals. In reactions carried out with an individual fatty acid, the zinc soap of this acid was used (e.g., for stearic acid, zinc stearate). Estimation of the standard error of the fatty acid, glycerol, and MAG concentration analyses, from five reactions conducted under the same conditions, revealed a maximum confidence intervals of (0.6% for fatty acid and (0.7% for glycerol and MAG. 2.4. Analytical Methods. The concentrations of glycerol and MAGs (total of 1-MAG and 2-MAG) were determined, as trimethylsilyl derivatives, by programmed GC with internal standardization, according to the method previously described.24 The fatty acid concentrations were determined by potentiometric titration method, according to the IUPAC method.34 For statistical evaluation of the analytical methods, the concentrations of fatty acid, glycerol, and MAG were checked five times in a few samples of the reaction mixture. The maximum confidence intervals were (0.4% for the fatty acid analysis and (0.5% for glycerol and MAG. The total concentration of di- and triacylglycerols (DAG + TAG) was calculated taking into account the concentrations of the other compounds in the reaction mixture. 2.5. Hydrophile-Lipophile Balance (HLB) of the Emulsifiers. HLB values were evaluated experimentally by the Griffin method.35,36 The emulsion composition during estimation of the HLB number was as follows: oil phase (paraffin oil), 40 wt %; water, 55 wt %; emulsifier, 5 wt %. Tween 60 (HLB ) 14.9) was used as a standard emulsifier. The estimated confidence interval of the HLB value was 0.1. 3. Results and Discussion For a thorough evaluation of the esterification kinetics, about 30 experiments at various temperatures (130-160 °C) and ZnC molar fractions (0.00625-0.05 mol) and in the presence of one of four individual fatty acids (C12:0, C14:0, C16:0, or C18:0) were carried out. It is obvious that, for a complete understanding of the reaction progress, changes in the concentrations of the substrates as well as the formed products should be estimated during the reaction time. As shown in Figure 1, together with the conversions of fatty acid and glycerol, the concentration of MAG tends to reach a maximum and then decreases. Simultaneously, di- and triacylglycerol are formed. Their concentration is very low during the first 30 min of the reaction, which is probably caused by preferential esterification of glycerol to MAG during this time period. The concentrations of di- and triester of glycerol continue to grow after the maximum in the MAG concentration is observed. This indicates that MAG is further esterified to DAG and TAG. 3.1. Conversion of the Substrates. The ZnC concentration, hydrocarbon chain lengths of the fatty acid

Figure 1. Concentrations of fatty acid (FA), glycerol (G), monoacylglycerol (MAG), and di- and triacylglycerol (DAG + TAG) in the reaction mixture during the esterification of glycerol with lauric acid at 150 °C. Molar ratio of G/FA/ZnC ) 1:1:0.0125.

and carboxylate, and reaction temperature each had a significant influence on the conversion of the fatty acid and glycerol, as measured by RFA (eq 1) and RG (eq 2)

RFA ) (FA0 - FA)/FA0

(1)

RG ) (G0 - G)/G0

(2)

where FA and G represent the fatty acid and glycerol concentrations (wt %), respectively, at the real time and FA0 and G0 represent the fatty acid and glycerol concentrations (wt %), respectively, at the beginning of the reaction. It was found that the presence of zinc soaps in the reaction mixture influenced the formation of transparent glycerol-fatty acid microemulsions. In the reaction conducted with the smallest amount of zinc salt (i.e., 0.00625 or 0.0125 mol) or at the lowest temperatures (i.e., 130 and 140 °C), some small amount of separated glycerol, located in the bottom of the reactor, was also observed. That glycerol layer was consumed gradually with reaction time, forming MAG. It appears that initially formed MAG molecules can act as cosurfactants in such microemulsion systems. As a result of the coadsorption of ZnC and MAG at the glycerol-fatty acid interface, the interfacial tension is reduced, and the esterification process can proceed in microemulsion. In reactions of glycerol with lauric acid, the formation of a single transparent phase was observed in the whole reaction mixture just after the beginning of the process, regardless of temperature and ZnC concentration. Such an effect was caused by the faster conversion of both substrates (glycerol and lauric acid) and the faster MAG formation in those esterification processes than in reactions of glycerol with longer fatty acids (C14:0, C16:0, C18:0; see Figure 2). For example, in the esterification of glycerol with lauric acid, RFA ) 0.5 was achieved after 1 h (Figure 2a), whereas in the reaction carried out with stearic acid, about 1.5 h was needed for the same conversion of fatty acid. For these two compared reactions, RG ) 0.5 was reached after 1.5 and 3 h, respectively (Figure 2b). This means that the longer-chain


Figure 3. Influence of the presence of ZnC in the reaction mixture on the conversions of fatty acid and glycerol (RFA, RG). Esterifications of glycerol with lauric acid (molar ratio ) 1:1) were performed without zinc laurate (O) and in the presence of 0.00625 mol (1.0 wt %) of zinc laurate (b). Temperature ) 150 °C.

Figure 2. Influence of the fatty acid chain length on the conversion of the substrates: (a) RFA, conversion of fatty acid; (b) RG, conversion of glycerol. G/FA/ZnC molar ratio ) 1:1:0.025, temperature: 150 °C.

fatty acids are less reactive and/or the type of fatty acid probably influences the structure of the formed microemulsion, which can determine the reaction rate. All transparent systems that arose were stable throughout the reaction. As alternative reaction systems, microemulsions are used for the lipase-catalyzed esterification of glycerol.18,21,37 Experiments with different saturated and unsaturated fatty acids showed that the reactivity of the substrates, as measured by the initial rate, was (among the saturated fatty acids) C14:0 ) C12:0 > C18:0 ) C16:0.37 Kaufman and Garti23 studied the influence of the type of emulsifier on the conversion of methyl stearate in NaOH-catalyzed reactions with tetraglycerol and glycerol. According to the authors, when nonionic emulsifiers were used (e.g., Span 80 or Tween 80), the transesterification of methyl stearate (at 130 °C, 8 h) was up to about 20%, as a result of low emulsion stability. Ionic emulsifiers were found to be much more effective. Sodium stearate allowed the conversion of

83.9% and 91.1% of methyl stearate, respectively, when it was treated with tetraglycerol or glycerol. Decreasing the temperature (150-95 °C) made the conversion of the substrate lower. As expected, a similar influence of temperature was found in our present experiments. The highest conversions of both substrates were achieved at 160 °C, regardless of the fatty acid used. The RFA and RG values were also influenced by the concentration of ZnC present in the reaction mixture. After 6-h esterifications of glycerol with stearic acid in the presence of 0.00625, 0.0125, 0.025, and 0.05 mol of zinc stearate, RFA and RG were equal to 0.81, 0.86, 0.92, 0.93 and 0.62, 0.66, 0.74, 0.83, respectively. Similar increases of the conversions of both substrates, together with higher molar ratios of ZnC, were also observed in the reactions of glycerol with lauric, myristic, and palmitic acids. Compared to the esterification of glycerol with lauric acid carried out without zinc laurate, 1.0 wt % addition of the carboxylate significantly enhanced the conversion of the substrates (Figure 3). After 6 h, RFA and RG were 25-30% higher than in a blank reaction. These results and observations led to the conclusion that zinc soaps act as emulsifing agents that cause an increase of the glycerol-fatty acid interface at the start of the process. In this way, the contact between reactants is facilitated, and the synthesis of MAG can proceed more rapidly. Increasing the ZnC concentration results in the more effective dispergation of glycerol in the fatty acid and, thereby, increases the rate of formation of the interfacial area. The mixed ZnC/MAG film thus formed stabilizes the microemulsion and can be penetrated by fatty acid molecules that react with glycerol. Thus, carrying out the reaction in a microemulsion overcomes the problem of the low solubility of glycerol in fatty acids. The interface is probably also an area where the esterification of MAG to higher substituted esters can proceed. 3.2. Monoacylglycerol Formation. All analyses of reaction mixtures verified that the concentration of 2-MAG did not exceed 5 wt % of the amount of 1-MAG. Experimental data show that rate of MAG formation depends on the temperature, the concentration of catalyst, and the length of the fatty acid carbon chain. The

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Figure 4. Influence of the fatty acid chain length on the concentration of MAG in the reaction mixture. G/FA/ZnC molar ratio ) 1:1:0.025, temperature: 150 °C.

process occurs with the highest rate for lauric acid (Figure 4). Upon elongation of the fatty acid chain (C12:0-C18:0), the esterification of glycerol to MAG became slower. For the applied reaction conditions, the maximum concentration of monolauroylglycerol was achieved after about 3 h, whereas the maximum concentration of monostearylglycerol was observed after about 5.5 h (Figure 4). The influence of the fatty acid chain length on the progress of the uncatalyzed esterification of glycerol was studied by Hartmann,28 for equimolar proportions of the reactants. According to the author, the percentage of esterified fatty acid decreases with increasing chain length (C8:0-C18:0). This effect was most pronounced after the first hour of the reaction, whereas after 5 h, the conversions of all studied fatty acids were similar (e.g., 90.1% and 90.0% for C8:0 and C18:0, respectively). Esterification was carried out in the stoppered test tubes, so there was no possibility for removal of the water formed in the reaction system. Hence, the reverse reaction was possible. It appears that the loss of fatty acid specificity can be explained by partial hydrolysis of the synthesized acylglycerols. Such an effect was not discussed by Hartmann.28 The results cited above as well as other authors’ reports on esterification kinetics15,25-28 give no information about the concentration of MAG in the reaction mixture. The increase of both the temperature and the ZnC concentration influenced the enhancement of the MAG formation rate (Figure 5). In the esterification of glycerol with lauric acid conducted without ZnC, the maximum monoester concentration was not achieved during the 6-h reaction (Figure 5b). For the process carried out in the presence of 0.00625 mol of zinc laurate (1.0 wt % of the reaction mixture), the maximum MAG concentration was observed during the fifth hour of the reaction. A higher concentration of carboxylate in the reaction mixture reduced the time required for the maximum content of MAG to be obtained. As shown in Figures 1-3, the concentrations of fatty acid and glycerol decreased continuously with reaction time. Thus, the observed decrease of the MAG concentration (after the maximum was reached) was caused

Figure 5. Esterification of glycerol with lauric acid in the presence of zinc laurate. Influence of the (a) reaction temperature and (b) zinc carboxylate (ZnC) concentration on the MAG content in the reaction mixture. (a) G/FA/ZnC molar ratio ) 1:1:0.025, (b) G/FA/ZnC molar ratio ) 1:1:0-0.05. Temperature ) 150 °C.

by its further esterification to DAG and TAG. Simultaneous consumption of glycerol during this time period indicated that “new” MAG molecules were also formed. In reactions with all examined fatty aids, the highest [MAG]/[G]0 ratios were noted for RG ) 0.70-0.73. The same dependence was found at various reaction temperatures and ZnC concentrations (data not shown). In our previous report,30 the catalytic activity of sodium and potassium soaps was compared with the activity of zinc salts of fatty acids. Each catalyst was used in the amount of 0.07 mol per 1 mol of glycerol. For both lauric and stearic acids used in the esterification of glycerol (molar ratio of the substrates ) 1:1), reactions conducted with zinc carboxylates exhibited faster conversion of the substrates and formation of MAG than reactions conducted with potassium or sodium soaps. However, kinetic studies were not presented. 3.3. Reaction Order. Based on analyses of fatty acid and glycerol concentrations vs reaction time, the reaction progress (p), considering both substrates (pFA and pG), was estimated (see eqs 3 and 4). It is worth mentioning that all studied esterifications were per-

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 7749 Table 2. Correlation Coefficients for Fatty Acid and Glycerol Concentration Changes vs Time (rFA2 and rG2), Calculated for First-, Second-, and Third-Order Reactionsa reaction orderb fatty acid

I

II

III

0.7916 0.9055 0.9428 0.9458

0.5661 0.7120 0.7869 0.7795

0.9415 0.9736 0.9649 0.9763

0.7957 0.8705 0.8560 0.8898

rFA2 c C12:0 C14:0 C16:0 C18:0

0.9982 0.9884 0.9918 0.9849

C12:0 C14:0 C16:0 C18:0

0.9954 0.9908 0.9970 0.9966

rG2 d

a G/FA/ZnC molar ratio ) 1:1:0.05, temperature ) 150 °C, reaction time ) 6 h. b Data for esterification reactions conducted with various fatty acids (C12:0-C18:0). c rFA2 ) correlation coefficient for fatty acid concentration changes over time. d rG2 ) correlation coefficient for glycerol concentration changes over time.

formed with equimolar amounts of glycerol and fatty acid.

pFA )

AV × MFA 56100

Gm 100MG pG ) mG MG

3.4. Reaction Rate Constants. Under the conditions applied in this work (continuous distillation), water formed during the esterification was removed, so that the reverse reaction (hydrolysis of acylglycerols) could be avoided. Thus, the esterification can be characterized as an irreversible reaction, and with all probability, the rate constants are such that kn . k-n. Fatty acids react with glycerol, forming MAG, and simultaneously, glycerol monoester is esterified to di- and then triacylglycerol. Taking into account the concentration changes of all determined compounds during the reaction time, the esterification of glycerol carried out in the presence of ZnC can be defined as a consecutive reaction. MAG is an intermediate product. The reaction kinetics can be described by the following expression with respect to glycerol esterification k1

Considering the consecutive character of the reaction, we proposed the following equations to describe the dependencies of the changes in the glycerol and MAG concentrations as a function of reaction time38-41

-d[G] ) k1[G] dt

(5)

d[MAG] ) k1[G] - k2[MAG] dt

(6)

(3)

(4)

where G is the glycerol concentration at the real time (wt %), m is the reaction mixture weight at the start of the process (g), MG is the molecular weight of glycerol (g/mol), mG is the glycerol weight in the reaction mixture at the start of the process (g), AV is the acid value at the real time (mg of KOH/g), MFA is the average fatty acid molecular weight (g/mol), and 56100 is the molar weight of KOH (mg/mol). For first-, second-, and third-order reactions, there is a linear relationship of ln p, 1/p, and 1/p2 with respect to the reaction time (t).38,39 On the basis of the analytical data, the correlation coefficients (r2) were calculated for first-, second-, and third-order reaction curves, for both fatty acid and glycerol (rFA2 and rG2). It was assumed that the reaction order was characterized by the curve for which the correlation coefficient was closest to 1. The very good linear relationships for ln pFA and ln pG vs t found for the studied conversions of fatty acids and glycerol verify that the reaction follows first-order kinetics for both substrates (Table 2). The differences between the r2 values for the compared reaction orders were more pronounced the greater the conversions of fatty acid and glycerol observed after 6 h of the reaction. The highest RFA and RG values were observed for the esterifications of glycerol with lauric acid at 160 °C (G/ FA/ZnC molar ratio ) 1:1:0.025) and at 150 °C (G/FA/ ZnC molar ratio ) 1:1:0.05). Dunlap and Heckles25 proposed a second-order rate for the esterifications of ethylene glycol with oleic acid carried out in the presence of zinc acetate and stearate. The reaction order was determined from the slope of a plot of the reciprocal of the fatty acid concentration vs time. Among the bivalent metal acetates studied as esterification catalysts, zinc acetate had the highest catalytic activity.

k2

G 98 MAG 98 DAG + TAG

For t ) 0, [G] ) [G]0, [MAG] ) 0, [DAG] ) 0, [TAG] ) 0, and k1 * k2, upon rearrangement, one obtains24,39,41

k1 [exp(-k1t) - exp(-k2t)] (7) [MAG] ) [G]0 k2 - k 1 where [G] is the glycerol concentration at the real time; [G]0 is the glycerol concentration at the beginning of the reaction; [MAG], [DAG], and [TAG] are the mono-, di-, and triacylglycerol concentrations at the real time; t is the reaction time; k1 is the rate constant of the reaction G f MAG; and k2 is the rate constant of the reaction MAG f DAG + TAG. The molar ratios [G]/[G]0 ) f(t) and [MAG]/[G]0 ) f(t) were calculated on the basis of analytical data and corresponding mass balances. Rate constants were calculated for each experiment (for a series of processes carried out at constant temperature and ZnC concentration) by a numerical method with a specially developed computer program. As presented in Figure 6, a comparison of the experimental and theoretical results shows good agreement. An average standard error of the calculated rate constants is on the order of 4%. For the kinetic curves of the MAG concentration changes vs the reaction time, the maximum concentration of monoester (MAGmax) reached in time t ) tmax can be described by eq 8. The time when such a contentration of MAG can be obtained (tmax) was calculated from eq 9.

(d[MAG] dt )

) t)tmax

k1[G]0 [k exp(-k2tmax) - k1 exp(-k1tmax )] ) 0 (8) k2 - k1 2 tmax )

ln(k2/k1) k2 - k1

(9)

7750

Ind. Eng. Chem. Res., Vol. 43, No. 24, 2004 Table 3. Influence of the Reaction Temperature on the Velocity of the Esterification of Glycerol with Fatty Acids (C12:0-C18:0)a reaction rate constantsb (×10-5 s-1) temp (°C)

k1

k2

C12:0

130

5.6

2.7

45.7

7.0

140* g

7.2

3.3

46.5

5.5

150* g

12.6

5.9

46.4

3.2

160* g

14.9

7.1

46.0

2.6

130

3.7

2.1

43.7

9.9

140

5.1

2.7

44.6

7.4

150* g

9.2

4.3

46.7

4.3

160* g

10.2

5.8

43.0

3.6

130

3.5

1.9

44.2

10.8

140

4.6

2.3

45.3

8.4

150* g

8.0

3.8

46.4

4.9

160* g

9.6

5.5

43.0

3.8

130

3.1

1.7

44.3

12.0

140

3.8

2.1

44.1

9.7

150* g

7.0

3.5

45.5

5.5

160* g

8.4

4.9

42.9

4.3

C16:0

The maximum concentration of monoester (MAGmax) for t ) tmax can be computed as follows

k1 [MAG]max ) [G]0 k2 - k1 k1 ln(k2/k1) k2 ln(k2/k1) exp - exp k2 - k1 k2 - k1

{ [

] [

]}

(10)

The reaction temperature and ZnC concentration influenced the esterification rate. For all studied processes, the MAGmax value did not exceed 50%. The time tmax depended on the reaction conditions. When the reaction temperature was increased from 130 to 160°C, k1 and k2 increased about 3-fold, regardless of the fatty acid used (Table 3). Both k1 and k2 depended strongly on the type of fatty acid. The rate constants decreased with fatty acid hydrocarbon chain length. For example, when the esterifications of glycerol with lauric and stearic acid are compared (G/FA/ZnC molar ratio ) 1:1:0.025, 150 °C), the ratios of the constants are as follows: k1(C12:0)/k1(C18:0) ) 1.8 and k2(C12:0)/k2(C18:0) ) 1.7 (Table 3). The type of fatty acid essentially does not influence the MAGmax value but has an appreciable effect on tmax. For the reactions described above, tmax increases from 3.2 to 5.5 h (Table 3). Similar dependencies were observed for glycerol esterifications carried out in the presence of sodium and potassium soaps.24 According to the presented results, elongation

MAGmaxc tmaxd MAGe tf (wt %) (h) (wt %) (h)

fatty acid

C14:0

Figure 6. Comparison between experimental (solid line) and theoretical (dashed line) kinetic curves of the esterifications of glycerol with lauric (C12:0) and stearic (C18:0) acids. Concentrations of glycerol (G) and monoacylglycerol (MAG) vs reaction time. G/FA/ ZnC molar ratio ) 1:1:0.0125, temperature ) 150 °C.

selected exptl data

C18:0

38.6 43.5 45.6 44.6 46.5 45.5 45.2 46.5 43.6 39.7 44.6 43.0 33.6 36.2 37.5 37.9 41.3 43.3 44.3 45.9 45.3 41.5 42.8 41.0 31.6 35.2 36.9 37.2 40.1 41.3 45.2 45.5 44.4 38.9 42.2 42.1 30.0 33.4 35.3 33.1 36.9 39.6 43.8 45.3 44.1 41.1 42.7 42.0

4 5 6 4 5 6 2 3 4 1 2 3 4 5 6 4 5 6 3 4 5 2 3 4 4 5 6 4 5 6 4 5 6 2 3 4 4 5 6 4 5 6 4 5 6 4 5 6

a G ) glycerol, FA ) fatty acid, ZnC ) zinc carboxylate. G/FA/ ZnC molar ratio ) 1:1:0.025, total reaction time ) 6 h. b k1 ) rate constant of the reaction G f MAG, k2 ) rate constant of the reaction MAG f DAG + TAG. c MAGmax ) maximum concentration of monoacylglycerol; calculated according to eq 10. d tmax ) time after which MAGmax is reached; calculated according to eq 9. e MAG ) concentration of monoacylglycerol in reaction mixture. f t ) reaction time. g Asterisks (*) indicate reactions for which the experimentally stated maximum in MAG concentration occurred within the 6-h reaction time.

of the fatty acid chain length (C12:0-C18:0) resulted in an increase of the tmax value. For the reaction of stearic acid with glycerol [molar ratio of G/FA/Na (or K) soap ) 1:1:0.07, temperature ) 150 °C], tmax was equal to 7.4 and 9.1 h, in the reactions catalyzed by sodium and potassium stearate, respectively. The use of the shortest fatty acid, lauric acid, resulted in a reduction tmax to 5.3 and 7.2 h, in reactions catalyzed by sodium and potassium laurate, respectively. MAGmax values calculated for the reactions cited above varied in the narrow range of 46.3-49.3 wt %. Thus, one can conclude that the type of carboxylate cation does not have a significant influ-


ence on the maximum concentration of MAG. It is difficult to compare precisely the kinetic results of esterifications catalyzed by ZnC with those carried out in the presence of Na and K soaps.24 Different concentrations of the catalysts were applied. Nevertheless, it is worth mentioning that the use of only 0.00625 mol of ZnC results in lower or similar (depending on the FA used) tmax values, as compared to the tmax values presented for the reactions performed with 0.07 mol of sodium and potassium carboxylates.24 Despite the about 11-fold increase of the ratios of the Na and K soaps compared to the ZnC content, the values of the calculated rate constants are comparable, or even higher for the processes conducted with Zn soaps. For example, when lauric acid was used, the k1 and k2 values were 8.1 × 10-5 and 4.4 × 10-5 s-1 in reactions carried out in the presence of 0.00625 mol of zinc laurate (see Table 4). The corresponding reaction rates were equal to 7.5 × 10-5 and 3.39 × 10-5 or 5.48 × 10-5 and 2.57 × 10-5 s-1, respectively, when the applied catalyst was sodium or potassium laurate (0.07 mol).24 These results demonstrate the higher catalytic activity of the zinc soaps. Such an effect is probably caused by the different structures of the fatty acid-glycerol microemulsions that can be produced in the presence of ZnC compared to those observed in reactions realized with alkali soaps. In contrast to zinc soaps, sodium and potassium salts of C12-C18 fatty acids are hydrophilic surfactants.7 Thus, one might expect different interactions of MAG with alkali soaps than with ZnC at the glycerol-fatty acid interface formed in the reaction mixtures. Neither temperature nor ZnC concentration had a pronounced influence on the maximum concentration of MAG, but the tmax values were lower in reactions carried out at higher temperature or in the presence of greater amounts of ZnC (Tables 3 and 4). For the esterification of glycerol with myristic acid, an increase in temperature from 130 to 160 °C caused a reduction of tmax from 9.9 to 3.6 h (Table 3). A decrease of the ZnC molar ratio (0.05-0.00625 mol) leads to a lowering of the rate constants (k1(0.05 ZnC)/ k1(0.00625 ZnC) ) 1.8-2.1, k2(0.05 ZnC)/k2(0.00625 ZnC) ) 1.61.8; see Table 4). Additionally, a comparison between the experimental data and the calculated MAGmax and tmax values is presented in Tables 3 and 4. Single asterisks indicate the reactions for which the experimentally stated maximum concentration of MAG occurred within 6 h. The composition of the reaction mixture was analyzed at 1-h intervals; thus, some small discrepancies between experimental and theoretical values of maximum MAG concentrations can be expected. When the highest concentration of monoester was achieved in a 6-h reaction, the following decrease of its concentration was noted. It was caused by the aforementioned further esterification of MAG to higher substituted esters of glycerol. 3.5. Activation Energy Values. The activation energy values were estimated from the Arrhenius equation for the investigated reactions: E1 (G f MAG) and E2 (MAG f DAG + TAG). No significant differences between E1 and E2 values were found (Table 5). Arrhenius plots for the reactions carried out at temperatures of 130-160 °C and in the presence of different fatty acids are shown in Figure 7 (obtained correlation coefficients show a good linear fitting). The results suggest that neither reaction toward MAG nor DAG +

Table 4. Influence of Zinc Carboxylate Concentration on the Rate of the Esterification of Glycerol with Fatty Acids (C12:0-C18:0)a reaction rate constantsb (×10-5 s-1)

selected exptl data

fatty acid

G/FA/ZnCc molar ratio

k1

k2

C12:0

1:1:0.00625* h

8.1

4.4

44.8

4.6

1:1:0.0125* h

9.6

5.1

45.0

3.9

1:1:0.025* h

12.6

5.9

46.4

3.2

1:1:0.05* h

17.0

8.0

44.4

2.3

1:1:0.00625

6.8

3.5

46.4

5.6

1:1:0.0125* h

7.2

3.6

46.4

5.4

1:1:0.025* h

9.2

4.3

46.7

4.3

1:1:0.05* h

12.4

5.7

45.2

3.2

1:1:0.00625

5.8

3.2

45.3

6.3

1:1:0.0125

6.4

3.4

45.5

5.9

1:1:0.025* h

8.0

3.8

46.4

4.9

1:1:0.05* h

11.0

5.0

45.3

3.6

1:1:0.00625

4.7

2.6

45.3

7.9

1:1:0.0125

5.4

3.0

44.6

6.7

1:1:0.025* h

7.0

3.5

45.5

5.5

1:1:0.05* h

9.2

4.8

43.1

4.1

C14:0

C16:0

C18:0

MAGmaxd tmaxe MAGf tg (wt %) (h) (wt %) (h) 44.3 44.8 42.3 44.8 44.9 42.3 45.2 46.5 43.6 36.1 42.7 41.3 43.3 45.4 45.6 44.8 45.9 44.2 44.3 45.9 45.3 43.9 45.1 42.9 40.2 44.0 45.7 41.8 44.7 45.3 45.2 45.5 44.4 42.0 44.0 43.1 37.3 41.3 43.7 38.6 42.9 45.2 43.8 45.3 44.1 40.1 42.8 41.9

4 5 6 3 4 5 2 3 4 1 2 3 4 5 6 4 5 6 3 4 5 2 3 4 4 5 6 4 5 6 4 5 6 3 4 5 4 5 6 4 5 6 4 5 6 3 4 5

a Temperature ) 150 °C, total reaction time ) 6 h. b k ) rate 1 constant of the reaction G f MAG, k2 ) rate constant of the c reaction MAG f DAG + TAG. G ) glycerol, FA ) fatty acid, ZnC ) zinc carboxylate. d MAGmax ) maximum concentration of monoacylglycerol; calculated according to eq 10. e tmax ) time after which MAGmax is reached; calculated according to eq 9. f MAG ) concentration of monoacylglycerol in reaction mixture. g t ) reaction time. h Asterisks (*) indicate reactions for which the experimentally stated maximum in MAG concentration occurred within the 6-h reaction time.

TAG formation is favored. The E1 and E2 values are comparable for all studied reactions (Table 5), which probably indicates similar mechanisms of esterification with the four examined fatty acids. The evaluated standard errors of activation energies do not exceed 0.5 kJ/mol. 3.6. Hydrophile-Lipophile Balance (HLB). On the basis of kinetic data, we synthesized products with 43-47 wt % MAG contents. Esterifications were conducted until the moment at which the desired MAG concentration was expected to be achieved. The composition of each reaction product is presented in Table 6. According to our recent studies,42,43 the synthesized preparations can be directly used as emulsifiers to

7752


Figure 7. Arrhenius plots for the esterifications of glycerol with lauric (C12:0), myristic (C14:0), palmitic (C16:0), and stearic (C18:0) acids, carried out in the presence of zinc carboxylates (ln k1, O; ln k2, 0). G/FA/ZnC molar ratio ) 1:1:0.025, temperature ) 130-160 °C. Table 5. Activation Energy Values for the Esterification of Glycerol with Individual Fatty Acids (C12:0-C18:0) activation energya,b (kJ/mol) fatty acid

E1

E2

C12:0 C14:0 C16:0 C18:0

50.8 52.5 52.3 52.2

50.4 52.3 54.2 54.3

a Calculations based on k and k values from experiments 1 2 conducted at temperatures of 130-160°C (Table 3). b E1 ) activation energy of the reaction G f MAG, E2 ) activation energy of the reaction MAG f DAG + TAG.

Table 6. Compositions and Hydrophile-Lipophile Balance (HLB) Values of the Synthesized Emulsifiers content (wt %) G/FA/ZnCa initial molar DAG + fatty acid ratio MAGa ZnCa FAa TAGa Ga HLBb C12:0

C14:0 C16:0 C18:0

1:1:0.00625 1:1:0.0125 1:1:0.025 1:1:0.05 1:1:0.025 1:1:0.025 1:1:0.00625 1:1:0.0125 1:1:0.025 1:1:0.05

44.6 45.1 46.8 44.2 45.5 45.3 44.0 43.6 44.8 43.0

1.0 2.0 3.8 7.4 3.9 4.0 1.0 2.1 4.0 7.7

10.5 10.2 9.3 9.2 8.7 9.5 12.3 11.0 9.5 9.9

35.3 33.3 31.9 31.7 33.4 33.0 34.1 34.7 33.5 32.4

8.6 9.4 8.2 7.5 8.5 8.2 8.6 8.6 8.2 7.0

6.6 6.2 5.7 5.6 5.3 4.9 4.6 4.7 4.7 4.7

a G ) glycerol, FA ) fatty acid, MAG ) monoacylglycerol, DAG ) diacylglycerol, TAG ) triacylglycerol, ZnC ) zinc carboxylate. b HLB ) hydrophile-lipophile balance.

obtain W/O emulsions. The properties of the emulsion systems stabilized with some emulsifiers (presented in Table 6) were discussed in our recent report.44 In relation to this work, we decided to examine the influence of the ZnC concentration and type of fatty acid used in the reaction on the HLB value of the synthesized preparations. The results obtained are included in Table 6. For the preparations synthesized from lauric acid, an increase of the zinc laurate concentration in the reaction mixture (1.0-7.4 wt %) resulted in a lowering of the HLB number of the obtained emulsifiers from 6.6 to 5.6. Such a dependence was not observed for the emulsifiers

based on stearic acid. It appears that the stearic acid systems exhibit such low hydrophobicity that addition of the lipophilic zinc stearate in the amount of 1.0-7.7 wt % does not effect any significant changes in the hydrophobic properties of the studied preparations. Among the reaction products containing about 4 wt % ZnC and synthesized from different fatty acids, the highest HLB value (HLB ) 5.7) was noted for lauroylglycerol emulsifier. As expected, the HLB number decreased with fatty acid chain length, and for stearylglycerol emulsifier, it was equal to 4.7. Compared to the acylglycerol emulsifiers prepared in the presence of sodium and potassium carboxylates,24 the use of Zn soaps results in lower HLB values of the synthesized preparations. The interaction of lipophilic MAG and hydrophilic Na/K soaps determines the formation of emulsifiers of O/W type,29,45 whereas, for acylglycerol emulsifiers modified with hydrophobic ZnC, one expects lipophilic character of the obtained preparations. As was mentioned above, the formation of W/O emulsions with these emulsifiers was demonstrated in our recent reports.42-44 4. Conclusions The results presented above indicate that zinc fatty acid carboxylates have an advantageous influence on the esterification progress, measured as the conversion of the substrates and the rate of MAG formation. Zinc soaps present in the reaction mixture bring about an increase in the glycerol-fatty acid interfacial area (microemulsion formation). Esterification of glycerol carried out under the applied conditions can be described as a first-order consecutive reaction. Knowledge of the reaction kinetics allowed for the preparation of emulsifiers with desirable contents of MAG and zinc ions. Both the hydrocarbon chain length of the fatty acid and carboxylate used and the concentration of the zinc soap influenced the reaction rate and hydrophilic-lipophilic properties of the synthesized product. Thus, conducting the esterifications in the presence of ZnC makes it possible to synthesize acylglycerol emulsifiers with defined HLB values in a one-step reaction. As we reported recently,42-44 the


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Received for review March 4, 2004 Revised manuscript received August 31, 2004 Accepted September 1, 2004 IE040077M

Esterification Kinetics of Glycerol with Fatty Acids in the Presence of

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