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Relationship of solubility parameters to interfacial properties of sucrose esters
Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 127–133
Relationship of solubility parameters to interfacial properties of sucrose esters Li Yanke, Zhang Shufen∗ , Yang Jinzong, Wang Qinghui State key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, People’s Republic of China Received 8 January 2004; accepted 14 July 2004
Abstract A two-dimensional model of solubility parameters was established by simplifying the Hansen’s three-dimensional model. In the model, there are three concentric quadrants and corresponding three regions—water region (W), first xylene region and second xylene region (X1 and X2 ). W, X1 and X2 indicate regions in which sucrose esters are soluble in water, insoluble in water or liquid paraffin but soluble in xylene, and completely soluble in xylene, respectively. Interfacial phenomena of different sucrose monoesters, diesters, triesters and some commercial sucrose esters at oil–water interface can be interpreted logically by means of the model. Interfacial properties of synthesized sucrose laurates, palmitates and stearates have been investigated. Their changes in interfacial tensions and CMC values with the changes in proportion of monoesters in sucrose esters conform to the principle obtained from the model. © 2004 Elsevier B.V. All rights reserved. Keywords: Solubility parameter; Two-dimensional model; Interfacial properties; Sucrose esters; Critical micelle concentration
1. Introduction Sucrose fatty acid esters have been the subject of intensive investigations for decades, since they can be produced from renewable, inexpensive, and inexhaustible natural resources. They are widely employed as biodegradable, non-toxic, skincompatible additives in cosmetics, pharmacy and foods because they proved to be non-toxic, tasteless, odorless and non-irritant. There are several classic methods for the commercial production of the sucrose esters, such as “dimethyl formamide process” [1], “transparent emulsion process” [2], “solvent-free process” [3], etc, which all lead to a complex mixture of products. The sucrose esters synthesized by these methods are often mixtures of mono-, di- and even higher esters. The change in isomer content and the chain length of fatty acid results in the wide range of HLB values (HLB 1–16). That is, properties of a sucrose ester can range from those of a water-soluble surfactant, to an oil soluble emulsi∗
Corresponding author. Tel.: +86 411 3631333 3297. E-mail address: [email protected] (Z. Shufen).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.07.032
fier. A number of studies focusing on sucrose esters in various applications have showed that they have good functional properties, i.e. emulsification [4], viscosity [5], rheology [6], and foaming and interfacial properties [7]. Among these investigations, however, data on the interfacial properties of different sucrose esters are often immethodical and reasonable interpretations are unavailable. The main objective of this study was to investigate the interfacial properties of different sucrose esters and to find the relations between solubility parameters and them.
2. Material and methods 2.1. Materials Commercial sucrose and fatty acid methyl ester with high purity were obtained from domestic companies. Anhydrous alkali metal soaps were prepared by saponification of methyl esters in methol containing a slight excess of ester; the products were vacuum dried. All solvents were of analytical grade.
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L. Yanke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 127–133
Ryoto sucrose esters were obtained from Mitsubishi-Kasei Food Corporation (Tokyo, Japan).
Table 1 Components of sucrose esters Sample number
Sucrose esters
2.2. Solvent-free synthesis of crude sucrose esters Hundred gram of powdered white granulated sucrose (particle size 4.11 m), 78.95 g of methyl palmitate, 2.68 g of anhydrous potassium carbonate and 17.9 g of potassium oleate were added to a 500 ml three-neck flask fitted for heating, stirring and vacuum distillation. The contents of the flask were stirred vigorously for 20 min to form a dispersion, and then heated with stirring for about 3 h, while at 130–140 ◦ C, 664.47 Pa pressure. A reaction mixture containing sucrose ester was obtained.
SS1 SS2 SS3 SS4 SS5 SP1 SP2 SP3 SP4 SL1 SL2 SL3
Sucrose stearate Sucrose stearate Sucrose stearate Sucrose stearate Sucrose stearate Sucrose palmitate Sucrose palmitate Sucrose palmitate Sucrose palmitate Sucrose laurate Sucrose laurate Sucrose laurate
Component (wt.%) Monoester
Diester
75.2 62.8 50.5 36.5 31.0 50.5 46.5 36.5 31.2 46.5 40.1 31.1
22.7 27.4 33.8 34.8 33.9 36.5 32.8 33.3 35.2 23.8 31.4 40.6
Polyester 2.1 9.8 15.7 28.7 35.1 13.0 20.7 30.2 33.6 29.7 28.5 28.3
2.3. Purification of sucrose esters Two hundred fifty milliliter of methyl ethyl ketone and 150 ml of water were added to 50 g of the crude reaction mixture crushed. The mixture was stirred vigorously at a temperature of 50 ◦ C. After adjusting the pH of the system at about 6.5, 1.9 g of anhydrous calcium chloride were added to cause a double decomposition reaction. A precipitation was formed in the system as the result of this reaction. After the precipitation was filtered out from the system, the remained filtration was separated into an upper organic layer and a lower water layer. Subsequent to the removal of water, the organic layer was dried under a reduced pressure to finally obtain 29 g of a solid mass of sucrose esters. Other sucrose esters were prepared from the same process. 2.4. Analysis of sucrose esters Quantitative analysis of sucrose esters was finished by high-performance liquid chromatography (HPLC) in combination with evaporative light scattering detection [8]. HPLC analysis was carried out on a Hewlett-Packard 1050 instrument. This instrument, which consists of HPLC HP series 1050, equipped with evaporative light scattering detector (Alltech 2000). The composition of synthesized sucrose esters is given in Table 1.
prior to interfacial activity measurements being carried out in duplicate. 2.6. Calculation of solubility parameters Solubility parameters are not available for sucrose esters. In this case, values can be estimated by incremental methods. The contributions of different groups to the cohesion energy and the molar volume have been published by Fedors [9]. Some data from reference [10] is collected in Table 2. Although the solubility parameters can be calculated by Hildebrand equation, we are apt to the calculation method of Hansen solubility parameter. The calculation of Hansen solubility parameters is a developed incremental method [10]. These are based on group attraction constants FDi and FPi , for dispersion and polar components, and group cohesion energies EHi . Corresponding solubility parameters can be calculated by means of Eqs. (1)–(3). FDi (1) σD = i i Vi σP =
2 i FPi
i Vi
(2)
2.5. Measurement of interfacial tension and surface tension Surface tensions and liquid paraffin–water interfacial tensions were measured according to Wilhelmy palte method at room temperature (25 ◦ C) with a kr¨uss K12 processor tensiometer. Each curve shown is the mean result of three replicate experiments. The sucrose esters as supplied do not disperse easily in water, and to ensure proper dispersal the samples were first melt. Each concentration was made up separately by weighting; the prepared sample solution was heated to the melting point of the sucrose ester, shaken vigorously, then cooled to ambient temperature for at least 1 h
σH =
EHi i i Vi
(3)
3. Result and discussion 3.1. Solubility parameters of sucrose mono-, di- and triesters In this paper, the applied model is Hansen’s threecomponent system [11], with a dispersion term δD , a polar term δP , and a hydrogen-bonding term δH , which together make up the total solubility parameter δt , and their relation-
L. Yanke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 127–133
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Table 2 Group increments for calculation of solubility parameters Group
Molar volumes (cm3 mol−1 )
FDi (J1/2 cm3/2 mol−1 )
FPi (J1/2 cm3/2 mol−1 )
EHi (J mol)
–CH3 –CH2 – >CH– >C< –O– –COO– –OH –OH (disubst. or on adjacent C) Ring closure, five or more atoms
33.5 16.1 −0.1 −19.2 3.8 18.0 10.0 13.0 16.0
420 270 80 −70 100 390 210 – 190
0 0 0 0 400 490 500 – –
– – – – 4800±1200 5200±600 19500±1700 19500±1700 –
ship can be expressed as in Eq. (4): σt2 = σD2 + σP2 + σH2
(4)
According to Hansen, the region of good solubility can be defined as a sphere in the three-dimensional space of the solubility parameters if the δD axis is expanded by a factor of 2 (Fig. 1) The corresponding interaction radius for solvent i and polymer j is therefore given by Eq. (5): 1/2
Rij = [4(σDi − σDj )2 + (σPi − σPj )2 + (σHi − σHj )2 ]
(5) This means that a polymer is probably soluble in a solvent if the Hansen parameters for the solvent lie within the solubility sphere for the polymer. In order to determine this it must be calculated whether Rij is less than the radius of interaction for the polymer-Rj . Here, we replaced polymer by sucrose ester and simplified the three-dimensional model into two-dimensional one. If Xij2 = 4(σDi − σDj )2 + (σPi − σPj )2 , Yij2 = (σHi − σHj )2 then 1/2
Rij = (Xij2 + Yij2 )
origin of the double coordinates. Two component parameters (X, Y) of some sucrose mono-, di- and triesters are plotted in Fig. 2. There are three concentric quadrants and corresponding three regions—water region (W), first xylene region and second xylene region (X1 and X2 ) in Fig. 2. The region W indicates a region in which sucrose esters are soluble in water. Sucrose monoesters with chain lengths of fatty acid less than 18 carbons lie in the region. Since sucrose monostearate (corresponding to the point C18 ME) is insoluble in water at room temperature but soluble or dispersible at higher temperatures, the point C18 ME is regarded as limit point of the water-soluble region, and therefore, Rij for sucrose monostearate and water is defined as interaction radius (R1j ) for sucrose ester soluble in water. Any sucrose ester whose Rij is less than R1j (R1j = 26.89) will be soluble in water, whereas insoluble in water. X1 is a region in which sucrose esters are insoluble in water or liquid paraffin, but soluble in xylene. Sucrose monoesters whose chain lengths of fatty acid are not shorter than stearic acid, some sucrose diesters including sucrose dioctanoate (C8 DE), dicaprate, dilaurate and dimyristate and some triesters like sucrose trioctanoate lie in the region. Sucrose dipalmitate whose corresponding point
(6)
Therefore, three-dimensional coordinates (δD , δP , δH ) are converted into two-dimensional coordinates (X, Y). Rij is the interaction radius for solvent i and sucrose ester j. The calculated values of X, Y and Rij are given in Table 3. Since we focused on the study on water–oil interface, water was considered as solvent, and its two component parameters as
Fig. 1. Spherical solubility volume of a polymer in the three-dimensional space of the solubility parameters.
Fig. 2. Two-dimensional model of solubility parameters of sucrose esters.
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Table 3 Hansen solubility parameters of sucrose esters Sucrose esters
Sucrose monooctanoate Sucrose monocaprate Sucrose monolaurate Sucrose monomyristate Sucrose monopalmitate Sucrose monostearate Sucrose dioctanoate Sucrose dicaprate Sucrose dilaurate Sucrose dimyristate Sucrose dipalmitate Sucrose distearate Sucrose trioctanoate Sucrose tricaprate Sucrose trilaurate Sucrose trimyristate Sucrose tripalmitate Sucrose tristearate Xyleneb Liquid paraffinb Waterb a b
Hansen solubility parameters δt
δD
28.76 27.77 26.93 26.22 25.61 25.14 24.46 23.74 23.03 22.46 21.97 21.57 22.70 21.90 21.28 20.80 20.42 20.10 18.0 15.3 47.8
16.50 16.52 16.55 16.56 16.58 16.59 16.36 16.42 16.46 16.50 16.52 16.54 16.73 16.74 16.74 16.74 16.75 16.75 17.8 15.6
Two-dimensional parameters δP 5.50 4.96 4.50 4.15 3.84 3.57 3.74 3.26 2.88 2.59 2.35 2.15 3.40 2.90 2.52 2.24 2.00 1.82 1.0 16.0
δH
a
22.90 21.76 20.77 19.90 19.13 18.54 18.04 16.84 15.85 15.02 14.30 13.68 14.96 13.82 12.90 12.14 11.50 10.96 3.1 42.3
X
Y
Rij
10.65 11.19 11.66 12.00 12.32 12.59 12.35 12.84 13.23 13.53 13.77 13.98 12.80 13.30 13.67 13.95 14.19 14.36 15.63
19.40 20.54 21.53 22.40 23.17 23.76 24.26 25.46 26.45 27.28 28.00 28.61 27.34 28.48 29.40 30.16 30.80 31.34 44.70
22.13 23.40 24.48 25.41 26.24 26.89 27.22 28.51 29.57 30.45 31.20 31.84 30.18 31.43 32.42 33.23 33.91 34.47 47.35
0
0
0
For all sucrose esters, calculation of δH without considering ± value. Ref. [10] values.
(C16 DE) lies on the perimeters of the second quadrants are extraordinary surfactant in the model. Since sucrose dipalmitate is partially soluble in liquid paraffin, the corresponding point C16 DE is defined as limit point of xylene-soluble region; R2j (R2j = 31.20) is defined as interaction radius for sucrose ester soluble in xylene but insoluble in liquid paraffin. Any sucrose ester whose Rij is less than R2j but greater than R1j will be soluble in xylene but not soluble in liquid paraffin. The X2 region denotes a region in which sucrose esters are partially soluble in liquid paraffin, but completely soluble in xylene. Sucrose distearate (C18 DE), sucrose tricaprate (C10 TE), sucrose trilaurate, sucrose trimyristate, sucrose tripalmitate and sucrose tristearate (C18 TS) are included in this region. After the two-dimensional model was established, the solubility and hydrophobicity of sucrose esters can be predicted according to their R values. For example, sucrose trioctanoate in the X1 region whose R-value is less than that of sucrose dipalmitate, though the solubility is not tested, can be considered soluble in xylene but insoluble in liquid paraffin. The significance of the model is that hydrophobicity and solubility of sucrose esters can be quantified by R values. If R value of a sucrose ester is greater than those of others, the surfactant transfers into the oil phase more easily. 3.2. Interfacial properties of sucrose esters at oil–water interface So far, interfacial properties of some sucrose monoesters and commercial sucrose esters at water–oil interface have
been studied. Ferrer et al. [12] have reported that the xylene–water interfacial tension of 6-O-palmitoylsucrose is four-fold lower than that of 6-O-lauroylsucrose. Soultani et al. [13] have reported the oil–water interfacial tension of S1670, SP70 and SP30 and a similar result was obtained. Likewise, Donnelly et al. [14] have reported that the values of n-hexadecane–water interfacial tensions of F160, F140, F110, F70, F50, F20 and F10 initially decrease and then increase. Our results (given in Table 4) showed that xylene–water interfacial tensions of some commercial sucrose esters initially decrease then increase. In order to explain these phenomena, some speculations were given. In their studies, hydrophobicity of sucrose esters was considered as a main reason. Certainly, hydrophobicity of sucrose esters increases with the increase in chain length or the proportion of diesters and higher esters. It is, however, not enough to interpret these phenomena. Especially, quantitative descriptions about hydrophobicity and solubility of sucrose esters Table 4 Literature and tested values of interfacial tension of some sucrose esters Sucrose esters
Concentration
Interfacial tension (mN/m)
6-O-Palmitoylsucrosea 6-O-Lauroylsucrosea S-570 S-970 S-1150 S-1570 S-1670
>28 M >250 M 0.5 g/L 0.5 g/L 0.5 g/L 0.5 g/L 0.5 g/L
1.0 3.8 9.7 8.9 7.2 6.8 8.5
a
Ref. [13] values.
L. Yanke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 127–133
were not available. In our study, the two-dimensional model can solve the problem and give clear answer. According to the model, 6-O-lauroylsucrose and 6-O-palmitoylsucrose lie in the W region and the interaction radii (R values) for water and these monoesters increase with the increase in chain length. Therefore, hydrophobicity of 6-O-palmitoylsucrose is greater than that of 6-O-lauroylsucrose while both keeping soluble in water. Since 6-O-palmitoylsucrose is more hydrophobic, it is more easily adsorbed at the interface, which leads to lower value of interfacial tension as compared with 6-O-lauroylsucrose. Commercial sucrose esters are usually composed of mono-, di- and higher esters. Sucrose monoesters lie in the W region and are soluble in water, but sucrose diesters and triesters lie in the region X1 and X2 , and are soluble or completely soluble in xylene due to their longer radii and closer distance from xylene. When sucrose esters are overwhelmingly composed of monoesters, the interface is mainly occupied by sucrose monoesters, and the diester is mainly in the bulk phase rather than at the interface [7]. The interfacial properties rely on the properties of monoesters. When the proportion of diesters and triesters in sucrose esters gradually increases, more diesters and triesters are transferred into the interface, but at the time monoesters at the interface still dominate the interfacial properties and hinder the transfer of diester and triester to the oil phase by molecular interaction. However, when the content of diesters and triesters in sucrose esters exceeds that of monoesters, a greater amount of diester and triester diffuses from the bulk phase to the interface and dominate at the interface at last. According to the model, the hydrophobicity of sucrose monoesters is poorer than that of diesters and triesters. Therefore, when the monoesters overwhelmingly dominate at the interface, the values of interfacial tensions can be reduced but not decreased further. With the increase in the proportion of diesters and triesters at the interface, the interfacial tensions would necessarily take on the characteristic of the diesters and triesters. Consequently, the values of interfacial tensions decrease further. However, with the further transfer of diesters and triesters, they would finally dominate at the interface. Since sucrose distearate and tristearate are completely soluble in xylene in term of the model, they would overcome the molecular interaction at the interface and transfer to the xylene phase. As a consequence, the values of interfacial tensions increase again.
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Fig. 3. Interfacial tension vs. concentration plots for sucrose stearates.
Fig. 4. Interfacial tension vs. concentration plots for sucrose palmitates.
3.3. Interfacial properties of sucrose esters at the liquid paraffin–water interface The interfacial tension data for these synthesized sucrose esters are shown in Figs. 3–5, respectively. Fig. 3 contains the curves for sucrose stearates. The corresponding values of and interfacial tension ␥ICAC and CACIT are shown in Table 5, and the values of surface tension ␥SCAC and CACST are also given in Table 5. With the decrease in the proportion of monoesters, the ␥ICAC values initially decrease and then increase. Accord-
Fig. 5. Interfacial tension vs. concentration plots for sucrose laurates.
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Table 5 Interfacial tensions at liquid paraffin–water interface Sample number
CACST (g/L)
CACIT (g/L)
␥SCAC (mN/m)
␥ICAC (mN/m)
SS1 SS2 SS3 SS4 SS5 SP1 SP2 SP3 SP4 SL1 SL2 SL3
0.7964 0.3097 0.3342 0.5856 0.7421 0.0742 0.0768 0.1027 0.1996 0.5479 0.4757 0.4342
0.8097 0.3232 0.4737 0.7601 0.8097 0.2089 0.2437 0.2865 0.3801 0.6162 0.5972 0.5430
34.70 34.87 35.94 39.84 40.90 34.09 38.99 39.17 40.48 34.65 36.50 38.26
8.22 6.91 8.59 9.62 10.52 7.28 7.97 10.47 12.75 9.59 10.26 12.39
␥SCAC , CACST : surface data corresponding to surface tensions. ␥SCAC , CACIT : interfacial data corresponding to interfacial tensions.
ing to the model, sucrose monostearate is more hydrophilic than diesters and triesters because of its lower R-value. As shown in Fig. 2, since the sucrose monoestearate lies on the borderline of the region W, it is insoluble in liquid paraffin and corresponding diester and triester are partially soluble in the oil phase. Therefore, when sample used contains more monostearate, the interface would be overwhelmingly dominated by the sucrose monostearate and take on their characteristic. With the increase in the proportion of sucrose diester and triester at the interface, the interfacial tensions would necessarily take on the characteristic of the diesters and triesters. As a consequence, the values of interfacial tensions initially lower. To compare the CACST values in air–water system with the CACIT values in paraffin–water system, it would find that when sucrose stearate is mainly composed of monoester, the CACIT values in paraffin–water system hardly increase. This suggests that diester and triester are bound at the interface instead of the transfer to oil phase. However, with the further increase of sucrose diester and triester in samples, they would finally dominate at the interface. Since sucrose distearate and tristearate are partially soluble in liquid paraffin in term of the model, they would overcome the molecular interaction at the interface and transfer to the oil phase as it has been suggested by Soultani et al. [14]. As a result, the values of interfacial tensions increase again and the values of CACIT in the paraffin–water system become greater than those in air–water system. Figs. 4 and 5 show the interfacial tension data for the C16 and C12 sucrose esters, with the corresponding CACI and ␥ICAC values shown in Table 5. With the decrease in the proportion of monoesters, the ␥ICAC values increase gradually. Since these samples contain the higher proportion of diesters and triesters, these results show the characteristics of dominant diesters and triesters at the interface. Compared with the CAC values in air–water system, the CAC values in liquid paraffin system increase, which suggests the diesters and triesters transfer to the oil phase. In addition, the CACIT and interfacial tension are not consistent between those sucrose esters with different chain
lengths and similar proportion of monoesters. These inconsistencies are probably due to the different proportion of diand higher esters.
4. Conclusions A two-dimensional model of solubility parameters of sucrose esters was built by the conversion of three-dimensional coordinates (δD , δP , δH ) into two-dimensional coordinates (X, Y). In the model, there are three concentric quadrants and corresponding three regions—water region (W), first xylene region and second xylene region (X1 and X2 ). W, X1 and X2 indicate regions in which sucrose esters are soluble in water, insoluble in water or liquid paraffin but soluble in xylene, and completely soluble in xylene, respectively. The model can be used for the phenomenological interpretation of oil–water interfacial properties. According to the model, sucrose monoesters lie in the W region and are soluble in water, but sucrose diesters and triesters lie in the region X1 and X2 and are insoluble, soluble or completely soluble in oil phase due to their longer radii R and closer distance from xylene. Changes in interfacial tensions with the increase in chain lengths or the decrease in the proportion of monoesters rely on changes in the solubility and hydrophobicity or hydrophilicity of monoesters, diesters and trimesters. Some phenomena reported by literatures were interpreted logically in term of the model.
References [1] L. Osipow, F.D. Snell, W.C. York, A. Finchler, Ind. Eng. Chem. 48 (1956) 1459. [2] L. Osipow, W. Rosenblatt, F.D. Snell, J. Am. Oil Chem. Soc. 44 (5) (1967) 307. [3] R.O. Feuge, H.J. Zeringue, T.J. Weiss, J. Am. Oil Chem. Soc. 47 (2) (1970) 56. [4] C.C. Akoh, J. Am. Oil Chem. Soc. 69 (1) (1992) 9. [5] M. Berjano, A. Guerrero, J. Mu˜noz, C. Gallegos, Colloid Polym. Sci. 271 (1993) 600.
L. Yanke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 127–133 [6] C. Calahorro, J. Mu˜noz, M. Berjano, J. Am. Oil Chem. Soc. 69 (7) (1992) 660. [7] F.A. Husband, D.B. Sarney, M.J. Barnard, P.J. Wilde, Food hydrocolloids 12 (1998) 237. [8] M.H. Moh, T.S. Tang, G.H. Tan, Food chem. 69 (2000) 105. [9] R.F. Fedors, Polym. Eng. Sci. 14 (1974) 472. [10] A.F.M. Barton, CRC Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press, Boca Raton, FL, 1983.
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[11] C.M. Hansen, J. Paint Techonol. 39 (1967) 505. [12] M. Ferrer, F. Comelles, F.J. Plou, M.A. Cruces, G. Fuentes, J.L. Parra, A. Ballesteros, Langmuir 18 (2002) 667. [13] S. Soultani, S. Ognier, J.M. Engasser, M. Ghoul, Colloids Surf. A: Physicochem. Eng. Asp. 227 (2003) 35. [14] M.J. Donnelly, J.D. Bu’lock, J. Am. Oil Chem. Soc. 65 (2) (1988) 284.
Relationship of solubility parameters to interfacial properties of sucrose esters Li Yanke, Zhang Shufen∗ , Yang Jinzong, Wang Qinghui State key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, People’s Republic of China Received 8 January 2004; accepted 14 July 2004
Abstract A two-dimensional model of solubility parameters was established by simplifying the Hansen’s three-dimensional model. In the model, there are three concentric quadrants and corresponding three regions—water region (W), first xylene region and second xylene region (X1 and X2 ). W, X1 and X2 indicate regions in which sucrose esters are soluble in water, insoluble in water or liquid paraffin but soluble in xylene, and completely soluble in xylene, respectively. Interfacial phenomena of different sucrose monoesters, diesters, triesters and some commercial sucrose esters at oil–water interface can be interpreted logically by means of the model. Interfacial properties of synthesized sucrose laurates, palmitates and stearates have been investigated. Their changes in interfacial tensions and CMC values with the changes in proportion of monoesters in sucrose esters conform to the principle obtained from the model. © 2004 Elsevier B.V. All rights reserved. Keywords: Solubility parameter; Two-dimensional model; Interfacial properties; Sucrose esters; Critical micelle concentration
1. Introduction Sucrose fatty acid esters have been the subject of intensive investigations for decades, since they can be produced from renewable, inexpensive, and inexhaustible natural resources. They are widely employed as biodegradable, non-toxic, skincompatible additives in cosmetics, pharmacy and foods because they proved to be non-toxic, tasteless, odorless and non-irritant. There are several classic methods for the commercial production of the sucrose esters, such as “dimethyl formamide process” [1], “transparent emulsion process” [2], “solvent-free process” [3], etc, which all lead to a complex mixture of products. The sucrose esters synthesized by these methods are often mixtures of mono-, di- and even higher esters. The change in isomer content and the chain length of fatty acid results in the wide range of HLB values (HLB 1–16). That is, properties of a sucrose ester can range from those of a water-soluble surfactant, to an oil soluble emulsi∗
Corresponding author. Tel.: +86 411 3631333 3297. E-mail address: [email protected] (Z. Shufen).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.07.032
fier. A number of studies focusing on sucrose esters in various applications have showed that they have good functional properties, i.e. emulsification [4], viscosity [5], rheology [6], and foaming and interfacial properties [7]. Among these investigations, however, data on the interfacial properties of different sucrose esters are often immethodical and reasonable interpretations are unavailable. The main objective of this study was to investigate the interfacial properties of different sucrose esters and to find the relations between solubility parameters and them.
2. Material and methods 2.1. Materials Commercial sucrose and fatty acid methyl ester with high purity were obtained from domestic companies. Anhydrous alkali metal soaps were prepared by saponification of methyl esters in methol containing a slight excess of ester; the products were vacuum dried. All solvents were of analytical grade.
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L. Yanke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 127–133
Ryoto sucrose esters were obtained from Mitsubishi-Kasei Food Corporation (Tokyo, Japan).
Table 1 Components of sucrose esters Sample number
Sucrose esters
2.2. Solvent-free synthesis of crude sucrose esters Hundred gram of powdered white granulated sucrose (particle size 4.11 m), 78.95 g of methyl palmitate, 2.68 g of anhydrous potassium carbonate and 17.9 g of potassium oleate were added to a 500 ml three-neck flask fitted for heating, stirring and vacuum distillation. The contents of the flask were stirred vigorously for 20 min to form a dispersion, and then heated with stirring for about 3 h, while at 130–140 ◦ C, 664.47 Pa pressure. A reaction mixture containing sucrose ester was obtained.
SS1 SS2 SS3 SS4 SS5 SP1 SP2 SP3 SP4 SL1 SL2 SL3
Sucrose stearate Sucrose stearate Sucrose stearate Sucrose stearate Sucrose stearate Sucrose palmitate Sucrose palmitate Sucrose palmitate Sucrose palmitate Sucrose laurate Sucrose laurate Sucrose laurate
Component (wt.%) Monoester
Diester
75.2 62.8 50.5 36.5 31.0 50.5 46.5 36.5 31.2 46.5 40.1 31.1
22.7 27.4 33.8 34.8 33.9 36.5 32.8 33.3 35.2 23.8 31.4 40.6
Polyester 2.1 9.8 15.7 28.7 35.1 13.0 20.7 30.2 33.6 29.7 28.5 28.3
2.3. Purification of sucrose esters Two hundred fifty milliliter of methyl ethyl ketone and 150 ml of water were added to 50 g of the crude reaction mixture crushed. The mixture was stirred vigorously at a temperature of 50 ◦ C. After adjusting the pH of the system at about 6.5, 1.9 g of anhydrous calcium chloride were added to cause a double decomposition reaction. A precipitation was formed in the system as the result of this reaction. After the precipitation was filtered out from the system, the remained filtration was separated into an upper organic layer and a lower water layer. Subsequent to the removal of water, the organic layer was dried under a reduced pressure to finally obtain 29 g of a solid mass of sucrose esters. Other sucrose esters were prepared from the same process. 2.4. Analysis of sucrose esters Quantitative analysis of sucrose esters was finished by high-performance liquid chromatography (HPLC) in combination with evaporative light scattering detection [8]. HPLC analysis was carried out on a Hewlett-Packard 1050 instrument. This instrument, which consists of HPLC HP series 1050, equipped with evaporative light scattering detector (Alltech 2000). The composition of synthesized sucrose esters is given in Table 1.
prior to interfacial activity measurements being carried out in duplicate. 2.6. Calculation of solubility parameters Solubility parameters are not available for sucrose esters. In this case, values can be estimated by incremental methods. The contributions of different groups to the cohesion energy and the molar volume have been published by Fedors [9]. Some data from reference [10] is collected in Table 2. Although the solubility parameters can be calculated by Hildebrand equation, we are apt to the calculation method of Hansen solubility parameter. The calculation of Hansen solubility parameters is a developed incremental method [10]. These are based on group attraction constants FDi and FPi , for dispersion and polar components, and group cohesion energies EHi . Corresponding solubility parameters can be calculated by means of Eqs. (1)–(3). FDi (1) σD = i i Vi σP =
2 i FPi
i Vi
(2)
2.5. Measurement of interfacial tension and surface tension Surface tensions and liquid paraffin–water interfacial tensions were measured according to Wilhelmy palte method at room temperature (25 ◦ C) with a kr¨uss K12 processor tensiometer. Each curve shown is the mean result of three replicate experiments. The sucrose esters as supplied do not disperse easily in water, and to ensure proper dispersal the samples were first melt. Each concentration was made up separately by weighting; the prepared sample solution was heated to the melting point of the sucrose ester, shaken vigorously, then cooled to ambient temperature for at least 1 h
σH =
EHi i i Vi
(3)
3. Result and discussion 3.1. Solubility parameters of sucrose mono-, di- and triesters In this paper, the applied model is Hansen’s threecomponent system [11], with a dispersion term δD , a polar term δP , and a hydrogen-bonding term δH , which together make up the total solubility parameter δt , and their relation-
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Table 2 Group increments for calculation of solubility parameters Group
Molar volumes (cm3 mol−1 )
FDi (J1/2 cm3/2 mol−1 )
FPi (J1/2 cm3/2 mol−1 )
EHi (J mol)
–CH3 –CH2 – >CH– >C< –O– –COO– –OH –OH (disubst. or on adjacent C) Ring closure, five or more atoms
33.5 16.1 −0.1 −19.2 3.8 18.0 10.0 13.0 16.0
420 270 80 −70 100 390 210 – 190
0 0 0 0 400 490 500 – –
– – – – 4800±1200 5200±600 19500±1700 19500±1700 –
ship can be expressed as in Eq. (4): σt2 = σD2 + σP2 + σH2
(4)
According to Hansen, the region of good solubility can be defined as a sphere in the three-dimensional space of the solubility parameters if the δD axis is expanded by a factor of 2 (Fig. 1) The corresponding interaction radius for solvent i and polymer j is therefore given by Eq. (5): 1/2
Rij = [4(σDi − σDj )2 + (σPi − σPj )2 + (σHi − σHj )2 ]
(5) This means that a polymer is probably soluble in a solvent if the Hansen parameters for the solvent lie within the solubility sphere for the polymer. In order to determine this it must be calculated whether Rij is less than the radius of interaction for the polymer-Rj . Here, we replaced polymer by sucrose ester and simplified the three-dimensional model into two-dimensional one. If Xij2 = 4(σDi − σDj )2 + (σPi − σPj )2 , Yij2 = (σHi − σHj )2 then 1/2
Rij = (Xij2 + Yij2 )
origin of the double coordinates. Two component parameters (X, Y) of some sucrose mono-, di- and triesters are plotted in Fig. 2. There are three concentric quadrants and corresponding three regions—water region (W), first xylene region and second xylene region (X1 and X2 ) in Fig. 2. The region W indicates a region in which sucrose esters are soluble in water. Sucrose monoesters with chain lengths of fatty acid less than 18 carbons lie in the region. Since sucrose monostearate (corresponding to the point C18 ME) is insoluble in water at room temperature but soluble or dispersible at higher temperatures, the point C18 ME is regarded as limit point of the water-soluble region, and therefore, Rij for sucrose monostearate and water is defined as interaction radius (R1j ) for sucrose ester soluble in water. Any sucrose ester whose Rij is less than R1j (R1j = 26.89) will be soluble in water, whereas insoluble in water. X1 is a region in which sucrose esters are insoluble in water or liquid paraffin, but soluble in xylene. Sucrose monoesters whose chain lengths of fatty acid are not shorter than stearic acid, some sucrose diesters including sucrose dioctanoate (C8 DE), dicaprate, dilaurate and dimyristate and some triesters like sucrose trioctanoate lie in the region. Sucrose dipalmitate whose corresponding point
(6)
Therefore, three-dimensional coordinates (δD , δP , δH ) are converted into two-dimensional coordinates (X, Y). Rij is the interaction radius for solvent i and sucrose ester j. The calculated values of X, Y and Rij are given in Table 3. Since we focused on the study on water–oil interface, water was considered as solvent, and its two component parameters as
Fig. 1. Spherical solubility volume of a polymer in the three-dimensional space of the solubility parameters.
Fig. 2. Two-dimensional model of solubility parameters of sucrose esters.
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Table 3 Hansen solubility parameters of sucrose esters Sucrose esters
Sucrose monooctanoate Sucrose monocaprate Sucrose monolaurate Sucrose monomyristate Sucrose monopalmitate Sucrose monostearate Sucrose dioctanoate Sucrose dicaprate Sucrose dilaurate Sucrose dimyristate Sucrose dipalmitate Sucrose distearate Sucrose trioctanoate Sucrose tricaprate Sucrose trilaurate Sucrose trimyristate Sucrose tripalmitate Sucrose tristearate Xyleneb Liquid paraffinb Waterb a b
Hansen solubility parameters δt
δD
28.76 27.77 26.93 26.22 25.61 25.14 24.46 23.74 23.03 22.46 21.97 21.57 22.70 21.90 21.28 20.80 20.42 20.10 18.0 15.3 47.8
16.50 16.52 16.55 16.56 16.58 16.59 16.36 16.42 16.46 16.50 16.52 16.54 16.73 16.74 16.74 16.74 16.75 16.75 17.8 15.6
Two-dimensional parameters δP 5.50 4.96 4.50 4.15 3.84 3.57 3.74 3.26 2.88 2.59 2.35 2.15 3.40 2.90 2.52 2.24 2.00 1.82 1.0 16.0
δH
a
22.90 21.76 20.77 19.90 19.13 18.54 18.04 16.84 15.85 15.02 14.30 13.68 14.96 13.82 12.90 12.14 11.50 10.96 3.1 42.3
X
Y
Rij
10.65 11.19 11.66 12.00 12.32 12.59 12.35 12.84 13.23 13.53 13.77 13.98 12.80 13.30 13.67 13.95 14.19 14.36 15.63
19.40 20.54 21.53 22.40 23.17 23.76 24.26 25.46 26.45 27.28 28.00 28.61 27.34 28.48 29.40 30.16 30.80 31.34 44.70
22.13 23.40 24.48 25.41 26.24 26.89 27.22 28.51 29.57 30.45 31.20 31.84 30.18 31.43 32.42 33.23 33.91 34.47 47.35
0
0
0
For all sucrose esters, calculation of δH without considering ± value. Ref. [10] values.
(C16 DE) lies on the perimeters of the second quadrants are extraordinary surfactant in the model. Since sucrose dipalmitate is partially soluble in liquid paraffin, the corresponding point C16 DE is defined as limit point of xylene-soluble region; R2j (R2j = 31.20) is defined as interaction radius for sucrose ester soluble in xylene but insoluble in liquid paraffin. Any sucrose ester whose Rij is less than R2j but greater than R1j will be soluble in xylene but not soluble in liquid paraffin. The X2 region denotes a region in which sucrose esters are partially soluble in liquid paraffin, but completely soluble in xylene. Sucrose distearate (C18 DE), sucrose tricaprate (C10 TE), sucrose trilaurate, sucrose trimyristate, sucrose tripalmitate and sucrose tristearate (C18 TS) are included in this region. After the two-dimensional model was established, the solubility and hydrophobicity of sucrose esters can be predicted according to their R values. For example, sucrose trioctanoate in the X1 region whose R-value is less than that of sucrose dipalmitate, though the solubility is not tested, can be considered soluble in xylene but insoluble in liquid paraffin. The significance of the model is that hydrophobicity and solubility of sucrose esters can be quantified by R values. If R value of a sucrose ester is greater than those of others, the surfactant transfers into the oil phase more easily. 3.2. Interfacial properties of sucrose esters at oil–water interface So far, interfacial properties of some sucrose monoesters and commercial sucrose esters at water–oil interface have
been studied. Ferrer et al. [12] have reported that the xylene–water interfacial tension of 6-O-palmitoylsucrose is four-fold lower than that of 6-O-lauroylsucrose. Soultani et al. [13] have reported the oil–water interfacial tension of S1670, SP70 and SP30 and a similar result was obtained. Likewise, Donnelly et al. [14] have reported that the values of n-hexadecane–water interfacial tensions of F160, F140, F110, F70, F50, F20 and F10 initially decrease and then increase. Our results (given in Table 4) showed that xylene–water interfacial tensions of some commercial sucrose esters initially decrease then increase. In order to explain these phenomena, some speculations were given. In their studies, hydrophobicity of sucrose esters was considered as a main reason. Certainly, hydrophobicity of sucrose esters increases with the increase in chain length or the proportion of diesters and higher esters. It is, however, not enough to interpret these phenomena. Especially, quantitative descriptions about hydrophobicity and solubility of sucrose esters Table 4 Literature and tested values of interfacial tension of some sucrose esters Sucrose esters
Concentration
Interfacial tension (mN/m)
6-O-Palmitoylsucrosea 6-O-Lauroylsucrosea S-570 S-970 S-1150 S-1570 S-1670
>28 M >250 M 0.5 g/L 0.5 g/L 0.5 g/L 0.5 g/L 0.5 g/L
1.0 3.8 9.7 8.9 7.2 6.8 8.5
a
Ref. [13] values.
L. Yanke et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 248 (2004) 127–133
were not available. In our study, the two-dimensional model can solve the problem and give clear answer. According to the model, 6-O-lauroylsucrose and 6-O-palmitoylsucrose lie in the W region and the interaction radii (R values) for water and these monoesters increase with the increase in chain length. Therefore, hydrophobicity of 6-O-palmitoylsucrose is greater than that of 6-O-lauroylsucrose while both keeping soluble in water. Since 6-O-palmitoylsucrose is more hydrophobic, it is more easily adsorbed at the interface, which leads to lower value of interfacial tension as compared with 6-O-lauroylsucrose. Commercial sucrose esters are usually composed of mono-, di- and higher esters. Sucrose monoesters lie in the W region and are soluble in water, but sucrose diesters and triesters lie in the region X1 and X2 , and are soluble or completely soluble in xylene due to their longer radii and closer distance from xylene. When sucrose esters are overwhelmingly composed of monoesters, the interface is mainly occupied by sucrose monoesters, and the diester is mainly in the bulk phase rather than at the interface [7]. The interfacial properties rely on the properties of monoesters. When the proportion of diesters and triesters in sucrose esters gradually increases, more diesters and triesters are transferred into the interface, but at the time monoesters at the interface still dominate the interfacial properties and hinder the transfer of diester and triester to the oil phase by molecular interaction. However, when the content of diesters and triesters in sucrose esters exceeds that of monoesters, a greater amount of diester and triester diffuses from the bulk phase to the interface and dominate at the interface at last. According to the model, the hydrophobicity of sucrose monoesters is poorer than that of diesters and triesters. Therefore, when the monoesters overwhelmingly dominate at the interface, the values of interfacial tensions can be reduced but not decreased further. With the increase in the proportion of diesters and triesters at the interface, the interfacial tensions would necessarily take on the characteristic of the diesters and triesters. Consequently, the values of interfacial tensions decrease further. However, with the further transfer of diesters and triesters, they would finally dominate at the interface. Since sucrose distearate and tristearate are completely soluble in xylene in term of the model, they would overcome the molecular interaction at the interface and transfer to the xylene phase. As a consequence, the values of interfacial tensions increase again.
131
Fig. 3. Interfacial tension vs. concentration plots for sucrose stearates.
Fig. 4. Interfacial tension vs. concentration plots for sucrose palmitates.
3.3. Interfacial properties of sucrose esters at the liquid paraffin–water interface The interfacial tension data for these synthesized sucrose esters are shown in Figs. 3–5, respectively. Fig. 3 contains the curves for sucrose stearates. The corresponding values of and interfacial tension ␥ICAC and CACIT are shown in Table 5, and the values of surface tension ␥SCAC and CACST are also given in Table 5. With the decrease in the proportion of monoesters, the ␥ICAC values initially decrease and then increase. Accord-
Fig. 5. Interfacial tension vs. concentration plots for sucrose laurates.
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Table 5 Interfacial tensions at liquid paraffin–water interface Sample number
CACST (g/L)
CACIT (g/L)
␥SCAC (mN/m)
␥ICAC (mN/m)
SS1 SS2 SS3 SS4 SS5 SP1 SP2 SP3 SP4 SL1 SL2 SL3
0.7964 0.3097 0.3342 0.5856 0.7421 0.0742 0.0768 0.1027 0.1996 0.5479 0.4757 0.4342
0.8097 0.3232 0.4737 0.7601 0.8097 0.2089 0.2437 0.2865 0.3801 0.6162 0.5972 0.5430
34.70 34.87 35.94 39.84 40.90 34.09 38.99 39.17 40.48 34.65 36.50 38.26
8.22 6.91 8.59 9.62 10.52 7.28 7.97 10.47 12.75 9.59 10.26 12.39
␥SCAC , CACST : surface data corresponding to surface tensions. ␥SCAC , CACIT : interfacial data corresponding to interfacial tensions.
ing to the model, sucrose monostearate is more hydrophilic than diesters and triesters because of its lower R-value. As shown in Fig. 2, since the sucrose monoestearate lies on the borderline of the region W, it is insoluble in liquid paraffin and corresponding diester and triester are partially soluble in the oil phase. Therefore, when sample used contains more monostearate, the interface would be overwhelmingly dominated by the sucrose monostearate and take on their characteristic. With the increase in the proportion of sucrose diester and triester at the interface, the interfacial tensions would necessarily take on the characteristic of the diesters and triesters. As a consequence, the values of interfacial tensions initially lower. To compare the CACST values in air–water system with the CACIT values in paraffin–water system, it would find that when sucrose stearate is mainly composed of monoester, the CACIT values in paraffin–water system hardly increase. This suggests that diester and triester are bound at the interface instead of the transfer to oil phase. However, with the further increase of sucrose diester and triester in samples, they would finally dominate at the interface. Since sucrose distearate and tristearate are partially soluble in liquid paraffin in term of the model, they would overcome the molecular interaction at the interface and transfer to the oil phase as it has been suggested by Soultani et al. [14]. As a result, the values of interfacial tensions increase again and the values of CACIT in the paraffin–water system become greater than those in air–water system. Figs. 4 and 5 show the interfacial tension data for the C16 and C12 sucrose esters, with the corresponding CACI and ␥ICAC values shown in Table 5. With the decrease in the proportion of monoesters, the ␥ICAC values increase gradually. Since these samples contain the higher proportion of diesters and triesters, these results show the characteristics of dominant diesters and triesters at the interface. Compared with the CAC values in air–water system, the CAC values in liquid paraffin system increase, which suggests the diesters and triesters transfer to the oil phase. In addition, the CACIT and interfacial tension are not consistent between those sucrose esters with different chain
lengths and similar proportion of monoesters. These inconsistencies are probably due to the different proportion of diand higher esters.
4. Conclusions A two-dimensional model of solubility parameters of sucrose esters was built by the conversion of three-dimensional coordinates (δD , δP , δH ) into two-dimensional coordinates (X, Y). In the model, there are three concentric quadrants and corresponding three regions—water region (W), first xylene region and second xylene region (X1 and X2 ). W, X1 and X2 indicate regions in which sucrose esters are soluble in water, insoluble in water or liquid paraffin but soluble in xylene, and completely soluble in xylene, respectively. The model can be used for the phenomenological interpretation of oil–water interfacial properties. According to the model, sucrose monoesters lie in the W region and are soluble in water, but sucrose diesters and triesters lie in the region X1 and X2 and are insoluble, soluble or completely soluble in oil phase due to their longer radii R and closer distance from xylene. Changes in interfacial tensions with the increase in chain lengths or the decrease in the proportion of monoesters rely on changes in the solubility and hydrophobicity or hydrophilicity of monoesters, diesters and trimesters. Some phenomena reported by literatures were interpreted logically in term of the model.
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