Influence of Sodium Dodecyl Sulfate Presence on Esterification of

Aug 10, 2009 - Influence of Sodium Dodecyl Sulfate Presence on Esterification of Propylene Glycol with Lauric Acid. Halina Szela̧g* and Elwira Sadeck...
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Ind. Eng. Chem. Res. 2009, 48, 8313–8319

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APPLIED CHEMISTRY Influence of Sodium Dodecyl Sulfate Presence on Esterification of Propylene Glycol with Lauric Acid Halina Szela¸g* and Elwira Sadecka Department of Fat and Detergent Technology, Chemical Faculty, Gdan˜sk UniVersity of Technology, Narutowicza 11/12, 80-952 Gdan˜sk, Poland

In this study, the influence of sodium dodecyl sulfate (SDS) on the direct esterification of 1,2-propanediol (propylene glycol, PG) with dodecanoic acid (lauric acid, LA) was investigated. It was stated that formation of a mixed interfacial film of the anionic surfactant and synthesized monoacylpropyleneglycol (PGML) emulsifier lowers the interfacial tension between phases to produce a transparent microemulsion. Therefore, SDS was found to be effective in increasing the contact between reagents and, as a consequence, accelerating the reaction progress. The obtained products were liquid and transparent substances at room temperature. The particle size distribution of the formed microemulsion systems range from about 25-45 nm, depending on the concentration of surfactant in the reaction mixture. The hydrophilic-lipophilic properties of synthesized products were also studied. The concentration of SDS in the reaction mixture influenced the hydrophile-lipophile balance (HLB) values of the obtained emulsifiers. 1. Introduction Monoesters of fatty acids (FAs) and 1,2-propanediol belong to nonionic surfactants, which are widely used as emulsifiers in the pharmaceutical, cosmetic, and food industries as waterin-oil (W/O) emulsion stabilizers.1-4 There are two main methods used to obtain monoacylpropyleneglycols: interesterification of triacylglycerols with propylene glycol and direct esterification of propylene glycol (PG) with fatty acids. The products of direct esterification are mixtures of mono- and diacylpropyleneglycols and some unreacted substrates. Interesterification leads to a product that is a mixture of mono- and diacylpropyleneglycols, mono-, di-, and triacylglycerols and unreacted PG. Both of these reactions occur very often in the presence of catalysts, such as sodium hydroxide or lime (calcium hydroxide).2 The composition of final products depends on the reaction conditions, such as the molar ratio of the substrates, temperature, and the type and concentration of surfactants used. Among the reaction products, mainly monoesters are surface active. Therefore, the concentration of monoacylpropyleneglycol in products is important in regard to their direct use as an emulsifier. Most commercial products contain 45-70 wt % propylene glycol monoesters. The higher concentration of monoesters, even 90 wt %, can be obtained by a molecular distillation of the product.5 The strong hydrophobic properties of monoacylpropyleneglycol emulsifiers limit their use in the stabilization of waterin-oil emulsion type only.1 One of the methods to modify hydrophile-lipophile balance (HLB) values is the preparing of mixtures containing compounds with different hydrophiliclipophilic properties. This method is effective because it leads to obtaining emulsifiers with the desired HLB values, but is difficult and unprofitable. Ester derivatives of propylene glycol and fatty acids with various numbers of carbon atoms (C12-C18) are widely used as * To whom correspondence should be addressed. Tel.: +48 58 347 29 27. Fax: +48 58 347 26 94. E-mail: [email protected].

stabilizers, according to European Economic Community (EEC) regulations.6 They are applied e.g. in cakes and nondiary whipped product as imitation cream toppings. The surfactants usually used in toppings accelerate the crystallization rate of coalesced fat, which seems to be important for the texture and foam stability of whipped toppings.7 In a mixture with monoand diacylglycerols, propylene glycol monoesters are utilized to obtain cake batter behavior. Monolaurylpropyleneglycols also have a major employment as emulsifiers and emollients in pharmaceutical preparations.1,5 It was found that the esterification of polyols (glycerol, ethylene glycol) with FAs in the presence of anionic surfactants (Na, K, Zn, Mg carboxylates) leads to obtain emulsifiers with defined hydrophilic-lipophilic properties.8-10 The HLB values of the emulsifiers can be controlled by the quantity of surfactants or by using for synthesis fatty acids with different hydrocarbon chain lengths. It was stated that the presence of the surfaceactive agents causes a microemulsion formation and significantly increases the reactions rate. The surfactant concentration and type also have an influence on the surface properties of the synthesized compounds.11-13 Products obtained with this proposed method can be directly used as stabilizers in dispersed systems. In this study, the influence of sodium dodecyl sulfate on progress of propylene glycol with lauric acid (LA) esterification has been investigated. The main goal of kinetic studies was characterization of propylene glycol monolaurate (PGML) formation. The hydrophilic-lipophilic properties (HLB) of synthesized products were also examined. 2. Experimental Section 2.1. Materials. The lauric acid (dodecanoic acid, C12:0, 98,8%) was obtained from Aldrich Chemie Gesellschaft GmbH & Co. KG, Germany. Propylene glycol (1,2-propanediol, 99,7%) was purchased from PPH POCh, Poland. Sodium dodecyl sulfate was obtained from Apollo Scientific Ltd., Great Britain.

10.1021/ie8019449 CCC: $40.75  2009 American Chemical Society Published on Web 08/10/2009

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2.2. Esterification of Propylene Glycol with Lauric Acid. Reaction was conducted in a thermostatic reactor equipped with a stirrer, thermometer, and nitrogen tube. The stirring rate was adjusted to 200 rpm. Esterification was carried out under reduced pressure (800 hPa), to eliminate water that was formed during the esterification process. All syntheses were carried out at the temperature 150 °C (( 1 °C) for 6 h. Propylene glycol was added to the heated mixture of lauric acid and sodium dodecyl sulfate. The molar ratio of dodecanoic acid to propylene glycols1:1.25swas constant in all of the esterification processes. The concentrations of sodium dodecyl sulfate were 0.001, 0.005, 0.01, 0.025, and 0.05 mol. The progress of the esterification reaction was investigated by analyzing the composition of the reaction mixture at 1-h intervals. The estimation of the standard error of the lauric acid, propylene glycol, and PGML concentration analyses, from three separate reactions conducted under the same conditions, revealed maximum confidence intervals of (0.7% for LA and PGML and (1.1% for PG. 2.3. Analytical Methods. The concentrations of PG and PGML in products were determined as trimethylsilyl derivatives by programmed gas chromatography (GC) with internal standardization.8 The LA concentrations were determined by the potentiometric titration method, according to the IUPAC method.14 The total amount of diesters of propylene glycol (PGDLs) was calculated taking into account the concentrations of the other compounds in the reaction mixture. For statistical evaluation of the analytical methods, the concentration of lauric acid, propylene glycol, and PGML were checked five times in a few samples of the reaction mixture. The maximum confidence intervals were (0.7% for LA and (0.9% for PGML and PG. 2.4. Particle Size Measurements. The particle size was measured at room temperature using the NIBS (non-invasive back scatter) method on a Zetasizer Nano ZS analyzer (Malvern Instruments, Great Britain). This apparatus can measure particle size ranging from 0.6 nm to 6 µm even from highly concentrated solutions. The Zetasizer Nano ZS also allows the measurement at higher particle concentrations than conventional light scattering techniques. 2.5. Transmission Measurements. The transmission of esterification of PG with LA in the presence of sodium dodecyl sulfate (SDS) products was measured by the DLS (dynamic light scattering) method on a Turbiscan Lab Expert (Formulaction, France). The products were placed into a flat-bottomed cylindrical glass cell. The sample was scanned by using two synchronous optical sensors that detected light transmitted through the sample. The reading head acquired the transmission data every 40 µm while moving along the entire height of the cell. The light source was an electroluminescent diode. The scanning was carried out at room temperature. 2.6. Hydrophile-Lipophile Balance of the Emulsifiers. The HLB values were estimated experimentally by the Griffin method.15 The composition of the emulsion during the evaluation of HLB was as follows: oil phase (paraffin oil + paraffin wax, 1:9 w/w), 40 wt %; water, 55 wt %; emulsifier, 5 wt %. As a standard emulsifier, Tween 60 (HLB ) 14.9) was used. The confidence interval of the HLB values was 0.1. 3. Results and Discussion In order to evaluate the influence of the SDS presence, the esterifications of propylene glycol with lauric acid were conducted and the role of anionic surfactant concentration on

Table 1. Compositions of Products of Esterification of Propylene Glycol with Lauric Acid in the Presence of SDS concentration [wt %]

molar ratio PG:LA:SDS

LA

PG

PGML

PGDL

SDS

1.25:1.00:0.001 1.25:1.00:0.005 1.25:1.00:0.01 1.25:1.00:0.025 1.25:1.00:0.05

19.9 4.3 3.1 10.4 7.9

24.0 5.8 6.7 5.3 3.1

50.0 45.6 50.2 39.7 23.8

6.0 43.8 39.1 42.2 60.6

0.1 0.5 0.9 2.4 4.6

PGML formation and product composition was investigated. As can be seen from data presented in Table 1, the high concentration (>50%) of PGML in the reaction product can be obtained if the SDS concentration does not exceed 0.01 mol. It was stated that the increase of anionic surfactant molar ratio in the reaction mixture (0.025-0.05 mol) caused a decrease of PGML and an increase of PGDL concentration in products. It should be mentioned that high concentrations of monoesters is particularly important due to their efficiency as dispersed systems stabilizers. 3.1. Influence of Surfactant Presence on Esterification Progress. The concentration of sodium dodecyl sulfate had a significant influence on the conversion of lauric acid and propylene glycol, as measured by RLA (eq 1) and RPG (eq 2) RLA )

(LA0 - LA) LA0

(1)

RPG )

(PG0 - PG) PG0

(2)

LA and PG represent lauric acid and propylene glycol concentrations [wt %] in the real time in the reaction mixture, LA0 and PG0 represent concentrations [wt %] of lauric acid and propylene glycol at the beginning of the reaction. It was observed that the presence of the anionic surfactant in the reaction mixture influenced the formation of microemulsions just after the beginning of the esterification regardless of the SDS concentration. In the past decade, it was stated that microemulsions are appropriate systems for the solubilization of lipophilic and hydrophilic reactants causing a local increase of reagent concentration in widely different environments in accordance with their physicochemical properties. In a discussed esterification process, formation of a mixed interfacial film of the anionic and synthesized PGML lowered the interfacial tension between phases to produce a transparent microemulsion. The formation of this microemulsion caused a faster conversion of both substrates (propylene glycol and lauric acid) and a faster PGML formation. For example, during the esterification which was carried out in the presence of 0.01 mol of SDS RLA ) 0.8 was achieved after 1 h, whereas about 6 h were needed to reach the conversion of fatty acid RLA ) 0.7 in the presence of 0.001 mol of anionic surfactant in the reaction mixture (Figure 1). It was found that RPG ) 1.0 may be reached after 1 h of esterification if the concentration of SDS was 0.05 mol and 3 h respectively during the reaction with 0.025 mol SDS in the reaction mixture (Figure 2). It is well-known that the “microemulsion catalysis” is influenced by the nature of the surfactant. Thus, an optimally formulated microemulsion accelerated the reaction on two accounts: by the large interfacial area between the hydrophobic and hydrophilic phase and by the positive charge interaction at the interface.

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Figure 1. Influence of the SDS concentration on the lauric acid conversion (RLA). PG/LA/SDS molar ratios ) 1.25:1:0.001, 0.005, 0.01, 0.025, and 0.05.

Figure 2. Influence of the SDS concentration on the propylene glycol conversion (RPG). PG/LA/SDS molar ratios ) 1.25:1:0.001, 0.005, 0.01, 0.025, and 0.05.

It was observed in some investigations that the structural transition of microemulsion from oil/water (O/W) to water/oil (W/O) occurs when mixtures of water and glycerol or propylene glycol are used instead of pure water.16 It is possible that in the mentioned system containing propylene glycol and SDS at different molar ratios different types of microemulsions may be formed, depending on the concentration of SDS in the reaction mixture. According to our observations, we can conclude that sodium dodecyl sulfate works, at the beginning of the esterification, as an emulsifier. Thus, the interface between lauric acid and propylene glycol increases and, as a result, contact between reagents is facilitated. 3.2. Formation of Monolaurylpropyleneglycol. Experimental data show that rate of PGML formation depends on the sodium dodecyl sulfate concentration in the reaction system. In our previous work, we showed that anionic surfactant (carboxylate) acts as an emulsifying agent that resulted in an increase of the polyol(glycerol)-fatty acid interface.10 Increasing anionic surfactant concentration results in the more effective dispersion of polyol in FA and increases the rate of formation of the interfacial area. During the esterification of LA with PG, the mixed SDS/PGML film forms and stabilizes the microemulsion, and the problem of the low solubility of PG in fatty acid may be overcome. As can be seen from Figure 3, PGML concentration in the reaction mixture during the esterification process increases,

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Figure 3. Effect of the sodium dodecyl sulfate concentration on the PGML formation. PG/LA/SDS molar ratios ) 1.25:1:0.001, 0.005, 0.01, 0.025, and 0.05.

reaches a maximum, and then decreases. For example, in the reaction carried out in the presence of 0.01 mol of SDS, the maximal concentration of PGML (50.5%) was achieved after 1 h and a further execution of the process resulted in a decrease of the PGML concentration. It was found also that with an increase of the SDS molar ratio, the concentration of PGML in the final product decreased. For example: in the esterifications carried out in the presence of 0.001 and 0.005 mol SDS after 6 h, the amount of PGML equaled, respectively, 50.5 and 37.2 wt %. After the same time, the concentrations of monoesters were 27.0, 5.0, and 2.3 wt % at, respectively, 0.01, 0.025, and 0.05 mol of anionic surfactant in the reaction system. For the applied reaction conditions, the maximum concentration of PGML (53.5 wt %) was achieved after about 2 h, when the esterification was carried out in the presence of 0.005 mol of SDS. 3.3. Reaction Rate Constants. Under the conditions applied in this study, water which was formed during the esterification was removed, so that the reverse reaction (hydrolysis of acylpropyleneglycol) could be avoided. Thus, the process can be characterized as an irreversible reaction. Lauric acid reacts with propylene glycol, forming PGML which are esterified to propylene glycol diesters (Figure 4). Taking into account the concentration changes of all compounds formed during the process, the esterification of propylene glycol with lauric acid in the presence of SDS can be described as a consecutive reaction and PGML is an intermediate product. The reaction kinetics can be described by the following expression with respect to propylene glycol esterification k1

k2

PG 98 PGML 98 PGDL

(3)

Taking into consideration the consecutive character of the reaction, we proposed the following equations to describe the dependences of the concentration changes of propylene glycol and monoesters as a function of reaction time -d[PG] ) k1[PG] dt

(4)

d[PGML] ) k1[PG] - k2[PGML] dt

(5)

For t ) 0, PG ) [PG]0, [PGML] ) 0, [PGDL] ) 0, and k1 * k2, upon rearrangement, one obtains8

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k2 k1 ) k2 - k1 ln

tmax

(8)

The maximum concentration of monoesters (PGMLmax) for t ) tmax can be computed as follows [PGML]max ) [PG]0

{ [

]

k1 ln k2 /k1 k1 exp k2 - k1 k2 - k1 k2 ln(k2 /k1) exp k 2 - k1

[

]}

(9)

The proposed method of PG with LA esterification leads to program the process in the way enabling one to achieve products with desirable concentration (>50%) of PGML. The obtained results show that the most optimal reaction system should contain not more than 0.01 mol of SDS. For example, when esterification was carried out in the presence of 0.005 mol of SDS, the maximal concentration of PGML (53.5 wt %) was achieved after 2 h of the reaction. When molar ratio PG/LA/SDS was 1.25:1:0.025, the maximal concentration of propylene glycol monoester decreased to 36.6 wt % and was obtained after 0.5 h of the reaction course. The further increase of SDS molar ratio caused the PGML to PGDL conversion to occur almost immediately. The maximal concentration of PGML was only 20.2 wt % and was obtained after 0.5 h of the process. After the same time, the concentration of PGDL was 61.7 wt % and increased to 92.4 wt % at the end (after 6 h) of the reaction. Figure 4. Concentrations of lauric acid (LA), propylene glycol (PG), propylene glycol monolaurate (PGML), and propylene glycol dilaurate (PGDL) in the reaction mixture during the esterification of propylene glycol with lauric acid in the presence of SDS. Molar ratio PG/LA/SDS ) 1.25:1:0.001 (A) and 1.25:1:0.005 (B).

[PGML] ) [PG]0

k1 [exp(-k1t) - exp(-k2t)] k2 - k 1

(6)

where [PG] is the propylene glycol concentration at real time, [PG]0 is the propylene glycol concentration at the beginning of the reaction, [PGML] and [PGDL] are the mono- and diesters of propylene glycol concentrations in the real time, t is the reaction time, k1 is the rate constant of the reaction PG f PGML, and k2 is the reaction constant of the reaction PGML f PGDL. The molar ratios [PG]/[PG]0 ) f(t) and [PGML]/[PG]0 ) f(t) were calculated on the basis of analytical data and corresponding mass balances. Rate constants were calculated for each experiment by a numerical method with a special computer program. As can be seen from Figure 5, a comparison of the experimental and theoretical results shows positive agreement. For the kinetic curves of the PGML concentration changes vs the reaction time, the maximum concentration of monoester (PGMLmax) reached in time t ) tmax can be described by eq 7. The time when such a concentration of PGML can be reached (tmax) was calculated from eq 8.

(

d[PGML] dt

)

) t)tmax

k1[PG]0 [k exp(-k2tmax) k2 - k 1 2 k1 exp(-k1tmax)] ) 0

(7)

Figure 5. Comparison between experimental (solid line) and theoretical (dashed line) kinetic curves of the esterification of propylene glycol with lauric acid. Concentrations of propylene glycol (PG, Figure 5A) and propylene glycol monolaurate (PGML, Figure 5B) vs reaction time. PG/ LA/SDS molar ratios of 1.25:1:0.001 and 0.005.

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Table 2. Influence of SDS Concentration on the Rate Constants of the Esterification of GP with Dodecanoic Acid rate constants LA:PG:SDS

k1 × 105

k2 × 105

PGMLmax [%]

Tmax [h]

1:1.25:0.001 1:1.25:0.005 1:1.25:0.01 1:1.25:0.025 1:1.25:0.05

7.8 12.7 19.9 78.3 87.3

3.9 6.4 9.8 46.9 142.6

52.1 53.6 54.3 50.1 29.7

5.0 3.0 1.9 0.5 0.2

Table 3. Comparison of Some Kinetic Constant of Reactions Proceeded in the Presence of Different Surface-Active Agents surfactant

concentration [mol]

k1 × 105

k2 × 105

k1/k2

t50 [h]

SDS ABSNa ZnC MgC NaC KC

0.005 0.005 0.050 0.050 0.070 0.070

12.7 6.5 19.7 9.0 4.1 4.6

6.4 3.6 8.6 5.6 2.2 2.3

1.9 1.8 2.3 1.6 1.9 2.0

2.7 5.7 2.0 3.9 9.1 8.4

Table 4. Influence of PGML, PGDL, and SDS Concentration on Hydrophile-Lipophile Balance (HLB) of the Synthesized Emulsifiers content [wt %]

molar ratio PG:LA:SDS

PGML

PGDL

SDS

HLB

1:1.25:0.001 1:1.25:0.005 1:1.25:0.01 1:1.25:0.025 1:1.25:0.05

50.0 45.6 50.2 39.7 23.8

6.0 43.8 39.1 42.2 60.6

0.1 0.5 0.9 2.4 4.6

4.1 4.0 5.0 3.0 4.0

Garti16 found that the addition of short chain alcohols and polyol (glycerol or propylene glycol) induces the formation of W/O and O/W microemulsions. When oil is the continuous phase, water swollen micelles are formed, termed also W/O microemulsions. It is possible that in reaction systems in which the molar ratio of SDS was higher than 0.01 mol, the reverse microemulsion was formed and compounds, which accumulate on the interface, might be solubilized in swollen micelles. The excess of PG might act as a cosurfactant or as a polar phase of the microemulsion. The k1 and k2, PGMLmax, and tmax significantly depended on the concentration of sodium dodecyl sulfate in the reaction mixture. The rate constants increased with an increase of the amount of SDS in the system. For example, when esterifications were carried out in the presence of 0.01 and 0.05 mol of surfactant, the rate constant were as follows: k1 ) 19.9 × 10-5 and 87.3 × 10-5 s-1; k2 ) 9.8 × 10-5 and 142.6 × 10-5 s-1 (Table 2).

Figure 6. Products of esterification of propylene glycol with lauric acid in the presence of SDS. PG/LA/SDS molar ratios ) 1.25:1:0.001, 0.005, 0.01, 0.025, and 0.05.

Figure 7. Influence of SDS concentration on the particle size distribution in microemulsions formed during esterification of propylene glycol with lauric acid. Molar ratios of SDS: 0.001 (A); 0.005 (B); 0.01 (C).

With an increase of the surface active agent concentration from 0.001 to 0.01 the maximal concentration of PG monoesters which can be obtained in the reaction mixture (PGMLmax) increased. For example, when the molar ratio of PG/LA/SDS was 1.25:1:0.001, PGMLmax was 52.1 wt %. If esterification was carried out in the presence of 0.01 mol of SDS, the PGMLmax increased to 54.3 wt %. However, the increase of anionic surfactant concentration from 0.025 to 0.05 mol resulted in a decrease of the maximal concentration of PG monoesters from 50.1 to 29.7 wt %, respectively (Table 2). The concentrations of SDS also significantly influenced tmax. According to the presented results, increasing of the surfactant molar ratio resulted in tmax decreasing. For example, for the reaction of lauric acid with PG in the presence of 0.001 and 0.01 of SDS, tmax values were 5.0 and 1.9, respectively. Increasing the SDS concentration to 0.05 mol resulted in decreasing the tmax value to 0.2 h.

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Figure 8. Dependence of transmission of PG with LA esterification products on the height of the probe. Molar ratios of PG/LA/SDS: 1.25:1:0.025 (A) and 0.05 (B).

The esterification progress can be described by means of the changes of the time when 50 wt % PGML concentration in the reaction product (t50) is reached. The comparison of some kinetic constants of the esterification which were carried out in the presence of anionic surfactants, e.g. Na (NaC), K (KC), Zn (ZnC), and Mg (MgC) carboxylates, SDS, and sodium alkylbenzene sulfonate (ABSNa), is presented in Table 3t. In comparison to the acylpropyleneglycol emulsifiers prepared in the presence of sodium and potassium carboxylates, the use of Zn soaps (ZnC) results in about a four times shorter t50. As can be seen from the obtained results, the highest PGML concentration in the reaction mixture and t50 depends on the relation of the corresponding rate constants k1/k2. The properties of the surfactant used influences the time after which 50 wt % PGML is reached. For example, when SDS was used, the k1 and k2 values were 12.7 × 10-5 and 6.4 × 10-5 s-1. The corresponding reaction rates were equal to 6.5 × 10-5 and 3.6 × 10-5 s-1, respectively, when the applied surfactant was ABSNa. These results demonstrated that the higher activity of the sodium dodecyl sulfate significantly enhanced the conversion of the substrate in comparison to sodium dodecylbenzene sulfonate. This rate enhancement can be due to the local increase of reactant concentration at the interface or the partitioning of lipophilic domains in the self-assembly structures. Thus, one might expect that formed PGML molecules can act as cosurfactants in such microemulsion systems. As a result of the coadsorption of SDS and PGML at the propylene glycol-lauric acid interface, the interfacial tension is reduced and the esterification can proceed in a microdispersed system. 3.4. Hydrophile-Lipophile Balance (HLB). According to kinetic data, we synthesized products with about 23.0-50.0 wt % PGML concentrations. The compositions of the obtained products are presented in Table 4. The properties of the emulsion systems stabilized with acylpropyleneglycol emulsifiers were described in our previous work.17,18 The influence of SDS concentration on the hydrophiliclipophilic properties (HLB value) of the synthesized preparation are included in Table 4. It was stated that the concentration of PGML mainly influenced the HLB value of the obtained product. The increase of monoester concentration in the reaction mixture from 39.7 to 45.6 wt % resulted in an increase of HLB from 3.0 to 4.0 no matter what was the decrease of the SDS concentration. As

expected, the HLB value increased from 4.1 to 5.0 with an increase of the anionic surfactant concentration from 0.1 to 0.9 wt % in preparation with a similar concentration of PGML. 3.5. Particle Size Analysis. As mentioned before, the esterification of polyols with fatty acid in the presence of surfactants occurs with microemulsion formation.8-10 In all esterification processes occurring in the presence of SDS, microemulsion was formed. The obtained products were transparent not only at esterification temperature e.g. 150 °C but also at room temperature (Figure 6) after 2 y of storage. Thus, it was found to be interesting to determine the particle size (see section 2.1) of the dispersed phase and verify that the preparations are actually microemulsion systems. The particle size was significantly influenced by SDS concentrations. When concentrations of SDS in the reaction mixture were 0.001, 0.005, and 0.01 mol, the particle size ranged from 25 to 45 nm depending on the amount of the surfactant (Figure 7). The obtained values are typical for microemulsions, so we can treat these preparates as microdispersions. For all preparations high values of transmission (more than 95%) were achieved which confirms that, in fact, the products are transparent. The examples of curves obtained from measurements using the Turbiscan Lab Expert are shown at Figure 8. Conclusions The esterification of propylene glycol with dodecanoic (lauric) acid in the presence of anionic surfactantssodium dodecyl sulfatescreates possibilities to synthesize an emulsifier in a microemulsion system. The concentration of the anionic surfactant influences the rate constant of the esterification and concentration of the surface active monoester of propylene glycol which is an intermediate product. The esterification of propylene glycol carried out under applied conditions results in preparations with desired contents of PGML and anionic surfactant in a one-step reaction. Knowledge of the reaction kinetics allowed for a preparation of products with programmed composition and properties. The interaction of nonionic, lipophilic PGML and anionic, hydrophilic SDS influences the HLB of the products, which may be used to stabilize W/O emulsions. The further aspects, particularly, the influence of temperature on kinetics of the esterification of propylene glycol with fatty

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acids in the presence of surfactants, are under investigation and will be published in the future. Acknowledgment This work was supported by Ministry of Science and Higher Education (Research Project N205 273935). Literature Cited (1) Garti, N. Food Emulsifiers: Structure-Reactivity Relationships, Design and Applications. In Physical Properties of Lipids; Marangoni, A. G., Narine, S. S., Eds.; Marcel Dekker: New York, 2002; pp 265-386. (2) Zielin˜ski, R. J. Synthesis and Composition of Food-Grade Emulsifiers. In Food Emulsifiers and Their Applications; Chapman & Hall: New York, 1999; pp 11-38. (3) Stauffer, C. E. Emulsifiers; Eagan Press: St. Paul, MN, 1999. (4) Krog, N. J.; Vang Sparso, F. Food Emulsifiers: Their Chemical and Physical Properties. In Food Emulsions; Friberg, S. E., Larsson, K., Sjo¨blom, J., Eds.; Marcel Dekker: New York, 2004; pp 45-92. (5) Krog, N. Food Emulsifiers. In Lipid Technologies and Applications; Gunstone, F. D., Padley, F. B., Eds.; Wiley-VCH: New York, 1999; pp 521-535. (6) Hassenhuettl G. L.; Hartel, R. W. Food Emulsifiers and Their Applications; Chapman & Hall: New York, 1997. (7) Barfold, N. M.; Krog, N. Destabilization and Fat Crystallization of Whippable Emulsions (Toppings) Studied by Pulsed NMR. J. Am. Oil Chem. Soc. 1987, 64, 112. (8) Szelag, H.; Zwierzykowski, W. Esterification Kinetics of Glycerol with Fatty Acids in the Presence of Sodium and Potassium Soaps. FettLipid 1998, 100, 302. (9) Szelag, H.; Macierzanka, A. Synthesis of Modified Acylglycerol Emulsifiers in the Presence of Na, K and Zn Fatty Acid Carboxylates. Tenside Surf. Det. 2001, 38, 377.

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ReceiVed for reView December 17, 2008 ReVised manuscript receiVed July 22, 2009 Accepted July 22, 2009 IE8019449