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Effects of Oxyethylene Chain Length and Temperature on Partitioning of Homogeneous Polyoxyethylene Nonionic Surfactants between Water and Isooctane M. Ben Ghoulam,† N. Moatadid,† A. Graciaa,‡ and J. Lachaise*,‡ Faculty of Sciences, University Moulay Ismail, Mekne` s, Morocco, and Laboratoire des Fluides Complexes, Universite´ de Pau et des Pays de l’Adour, Pau, France Received December 7, 2001. In Final Form: March 4, 2002 Partition coefficients of homogeneous poly(oxyethylene glycol n-dodecyl ether) C12H25(OCH2CH2)iOH (i ) 2-9) between water and isooctane were determined from experimental measurements in the temperature range 20-40 °C. All surfactants were found to have partition coefficients lower than unity, which indicates that their hydrophobic character is more important than their hydrophilic one. An increase in ethylene oxide number from 2 to 9 induces an exponential increase of the partition coefficient of 3 orders of magnitude, while an increase of 20 °C in the temperature decreases its magnitude only 0.5 of an order of magnitude. The hydroxyl group contribution to the free energy of transfer of the surfactant molecules from water to isooctane was found to be more important than that of an ethylene oxide group. An increase in temperature leads to an important decrease in its contribution, while the ethylene oxide contribution remains practically constant. Contributions of the functional groups of the surfactant molecules to the free energy of partitioning are evaluated in order to build up a database to predict the partitioning of nonhomogeneous nonionic surfactants of the same family within the temperature range investigated.
Introduction It was reported1 that even if some surfactants are considered to be insoluble in water or oil, most of them partition between these two phases. The determination of surfactant molecules partitioning between oil and water is usually used as the basis of the definition of the hydrophilic-lipophilic balance (HLB) of a surfactant,1-8 when measurement is performed at low concentration. Today, a large and growing number of nonionic surfactants are being used in various applications, so knowledge of the parameters accounting for their properties is needed. As most of the available nonionic surfactants are the polyoxyethylenated ones with a distribution of ethylene oxide chain lengths, their use in polyphasic systems such as oil-water mixtures leads to a partition of their molecular species between oil and water.9-12 This phenomenon may affect considerably the phase behavior of such systems. However, understanding of this parti† ‡
University Moulay Ismail. Universite´ de Pau et des Pays de l’Adour.
(1) Ravera, F.; Ferrari, M.; Liggieri, L.; Miller, R.; Passerone, A. Langmuir 1997, 13, 4817. (2) Altomare, C.; Carotti, A.; Trapani, G.; Liso, G. J. Pharm. Sci. 1997, 86, 1417. (3) Greenwald, H. L.; Kice, E. B.; Kenly, M.; Kelly, J. Anal. Chem. 1965, 33, 465. (4) Schott, H. J. Pharm. Sci. 1995, 84, 1215. (5) Cratin, P. D. Chemistry and Physics of Interfaces II Symposium; Ross, S., Ed.; American Chemical Society: Washington, DC, June 1971; p 97. (6) Harusawa, F.; Saito, T.; Nakajima, H.; Fukushima, S. J. Colloid Interface Sci. 1980, 74, 435. (7) Schott, H. J. Pharm. Sci. 1971, 60, 648. (8) Davies, J. T. Proceedings of the 2nd International Congress on Surface Activity. I. Gas-Liquid and Liquid-Liquid Interface, Butterworths Scientific Publications: London, 1957; p 426. (9) Graciaa, A.; Lachaise, J.; Sayous, J. G.; Grenier, P.; Yiv, S.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93, 474. (10) Graciaa, A.; Lachaise, J.; Bourrel, M.; Osborne-Lee, I.; Schechter, R. S.; Wade, W. H. SPE Reservoir Engineering. August 1987, 305. (11) Wormuth, K. R.; Geissler, P. R. J. Colloid Interface Sci. 1991, 146, 320. (12) Marquez, N.; Anton, R.; Graciaa, A.; Lachaise, J.; Salager, J. L. Colloid Surf. A: Physicochem. Eng. Aspects 1995, 100, 225.
tioning requires knowledge of the actual partition of each molecular species between the oil and water phases. Brooks et al13 have pointed out the relationship between surfactant partitioning and transitional emulsion phase inversion in polydisperse nonionic surfactant-oil-water systems on the basis of thermodynamic processing. Salager et al.14 have shown that retrograde transition in the phase behavior of commercial surfactant-oil-water systems due to the increase of alcohol content is ascribed to the strong partitioning of surfactants. On the other hand, the threephase body consisting of excess water, surfactant (microemulsion), and excess oil phases has been found to be largely skewed toward higher temperatures with decreasing total polydisperse nonionic surfactant concentration. This behavior was also attributed to the partitioning phenomenon.15 Most of the previous work dealing with partitioning has adopted a qualitative approach, and even when a quantitative approach is adopted, it focused only on ethoxylated alkylphenol surfactant and did not perform measurements at various temperatures.16 To extend the database of the partitioning coefficient of nonionic surfactants, this paper investigates partitioning of homogeneous poly(oxyethylene glycol n-dodecyl ether) between water and isooctane at various temperatures. Experimental Section Materials. The surfactants studied are homogeneous poly(oxyethylene glycol n-dodecyl ether) (C12Ei):
C12H25(OCH2CH2)iOH As it is well-known that the ratio of hydrocarbon to ethylene (13) Brooks, B. W.; Richmond, H. N. J. Colloid Interface Sci. 1994, 162, 67. (14) Salager, J. L.; Marquez, N.; Anton, R. E.; Graciaa, A.; Lachaise, J. Langmuir 1995, 11, 37. (15) Kunieda, H.; Yamagata, M. Langmuir 1993, 9, 3345. (16) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Colloid Sci. 1965, 20, 191.
10.1021/la0117707 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/25/2002
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Table 1. Surfactant Concentrations in the Mother Solutions and in the Tested Solutions mother solution (g %) tested solution (g %)
C12E2
C12E3
C12E4
C12E5
C12E6
C12E7
C12E8
C12E9
3 2.2
2 1.5
1 0.5
0.5 0.2
0.1 0.05
0.1 0.03
0.1 0.01
0.1 0.005
oxide chain length controls partitioning as well as interfacial tension, micellization, surface adsorption, and emulsion formation, the ethylene oxide number i was varied from 2 to 9. These surfactants were supplied by Nikko Chemical Co. (Tokyo, Japan) and their purity grade is very high. Analysis by gas chromatography reveals the existence of a single peak confirming the homogeneity in ethylene oxides of each of them. Isooctane is spectroscopic grade and water was twice distilled. Procedures. Greenwald et al.3 have pointed out that the distribution of the surfactant between water and oil remains constant within a considerable concentration range below the critical micelle concentration (cmc). So, our measurements were performed in such a way that at equilibrium the surfactant concentrations in the aqueous phases are always below cmc. Partition coefficients were determined at equilibrium of diffusion of the surfactant molecules from water to isooctane, and (or) from isooctane to water. For each surfactant, the mother solutions (aqueous and (or) organic) were first prepared separately. Dilutions from these mother solutions performed in separated bottles yielded solutions of the desired concentrations in surfactant. The surfactant concentrations in the mother solutions and in the diluted solutions are reported in Table 1. The concentrations in the diluted solutions have been chosen from the reckoned orders of magnitude of the partitioning coefficients to give surfactant concentrations of about one-tenth of the cmc in the aqueous phases after partitioning. The bottles were designed with a base area sufficiently large to increase transfer by diffusion of the surfactant molecules and thus to shorten equilibration times. Generally two types of samples were prepared. In the first type, surfactant was dissolved initially in water; in the second, it was dissolved initially in the isooctane. Only the second kind of sample was prepared for C12E2, since this surfactant is too weakly soluble in water. For each surfactant, a number of samples equal to the number of temperatures studied were prepared. First, 80 mL of the aqueous phase of surfactant was placed in a screw-cap bottle and overlaid with an equal volume of isooctane. In another bottle, 80 mL of water was overlaid with an equal volume of the oil phase of surfactant. This procedure had to be done carefully in order to prevent emulsion formation. Tightly closed with aluminum-lined caps, the bottles were placed at constant temperature in a liquid bulk. Equilibrium was then allowed to occur by diffusion. Generally, 2 weeks were sufficient to reach this equilibrium.17 For most of the surfactants used, measurements were performed at temperatures ranging from 20 ( 0.1 to 40 ( 0.1 °C. Analysis. The surfactant concentrations at equilibrium in the two superposed phases were measured with a Chrompack CP 9000 temperature-programmable gas chromatograph. Fused Silica capillary column (WCOT) of 10 m length, 0.53 mm inner diameter, and with SIL 5 CB as coating phase, were used with a temperature program ranging from 70 to 290 °C at 4 °C/min. The detector was FID mode and the injector was a Wide Bore. Analysis of the organic phase was performed without difficulty. A sample of 1 µL taken in the equilibrated upper phase was injected directly into the chromatograph; its concentration in surfactant was calculated by using a calibration performed with the same surfactant at a known concentration. Analysis of the aqueous phase was not easy to perform since on one hand the chromatograph column was unstable in the presence of water and on the other hand the surfactant concentrations in the aqueous phase were very small. This problem was overcome by slow evaporation up to dryness, at 50 °C during 1 day, of the water contained in a sample of 3 mL of aqueous phase. The subtract of surfactant obtained was redissolved in 0.1 mL of methanol, and 1 µL of this alcoholic solution of surfactant was then analyzed using the same procedure as (17) Warr, G. G.; Grieser, F.; Healy, T. W. J. Phys. Chem. 1983, 87, 4520.
Table 2. Partition Coefficients as a Function of Temperature for the Surfactants C12Ei T C12E2 C12E3 C12E4 C12E5 C12E6 C12E7 C12E8 C12E9 (°C) (×105) (×105) (×104) (×104) (×103) (×103) (×102) (×102) 20 25 30 35 40
10.9 6.0 3.7
32.9 22.2 17.3 13.5 9.8
12.2 8.9 6.4 4.4 3.7
36.6 26.8 18.1 13.3 10.0
10.0 6.6 4.9 3.6 2.4
26.0 18.0 11.9 9.0 6.0
6.0 4.9 3.0 2.2 1.4
19.8 9.5 4.0
that employed for the oil phase of surfactant. The average of three or four analyses was taken as a result. The partition coefficient Ki of the surfactant C12Ei between water and isooctane defined as
Ki )
Cw i Coi
(1)
was then calculated from the molar concentrations of this surfactant measured in water (Cw i ) and measured in isooctane (Coi ).
Results and Discussion Partition Coefficients. When surfactant can be initially dissolved in water and in isooctane, we verified that the partition coefficient obtained at equilibrium of surfactant diffusion from water to isooctane differed randomly a few percent from that obtained at equilibrium of surfactant diffusion from isooctane to water. This slight difference, included within the margin of uncertainty ((6%), confirms that equilibrium of diffusion has been actually reached in the two cases. We took the average of these two measurements as the value of the partition coefficients. The corresponding values for C12E3, C12E4, C12E5, C12E6, C12E7, C12E8, and C12E9 are reported in Table 2. In the same table are reported values of the partition coefficients of C12E2 obtained from measurements made only at equilibrium of diffusion from isooctane to water. All the partition coefficients measured are less than unity, which means that the surfactants studied have a greater affinity for isooctane than for water. The variations of the partition coefficients as a function of the ethylene oxide chain length of the surfactant molecule are given in Figure 1 for 20, 30, and 40 °C. At each temperature, the partition coefficient increases exponentially with ethylene oxide number. An increase in ethylene oxide number from 2 to 9 corresponds to an increase in the partition coefficient of 3 orders of magnitude. This behavior can be directly attributed to the increase of the hydrophilic character of the surfactant. On the other hand, the partition coefficient decreases by more than one-half an order of magnitude over a 20 °C increase in temperature. This is the result of the decrease of the solubility of the surfactant molecules in the aqueous phase. Global Free Energy of Partitioning. Since measurements were performed at concentrations much lower than cmc, oil and water phases may be regarded as sufficiently diluted solutions of surfactants to behave ideally. Therefore, the chemical potential of the surfactant
Partitioning of Homogeneous Surfactants
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Figure 1. Partition coefficient of C12Ei as a function of ethylene oxide number at different temperatures.
Figure 2. Free energy of partitioning of C12Ei as a function of ethylene oxide number at different temperatures.
Table 3. Standard Free Energy of Partitioning ∆G0t (kJ/mol) as a Function of Temperature for the Surfactants C12Ei
lization (∆G0m).18 For example ∆G0t ) -20.2 kJ/mol and ∆G0m ) -34.7 kJ/mol for C12E5 at 25 °C. This difference can be ascribed to the fact that the transfer of the surfactant molecules into micelles is an easier process than the transfer into isooctane. Indeed the environment of the hydrophilic parts of the surfactant molecules is more altered in isooctane than outside the micelle where they remain in contact with water molecules. The variation of ∆G0t vs ethylene oxide number is reported in Figure 2 for 20, 30, and 40 °C. For a given temperature, ∆G0t increases linearly with the ethylene oxide number. Thus, an increase of this number leads to a less favorable transfer from water to isooctane (∆G0t increases). On the other hand, for a given ethylene oxide number, ∆G0t decreases as the temperature increases. This behavior is similar to that of the solubility of the surfactant in water which also decreases as temperature increases, facilitating the transfer of the surfactant from water to isooctane (∆G0t decreases). As a consequence of the decrease of ∆G0t , the entropy of partitioning is positive. This positive value can be attributed to the destruction of the local order of the water molecules surrounding the surfactant monomers, when these monomers are transferred from water to isooctane. The value obtained (about 236 J/(mol‚deg) for all surfactants) is of the same order of magnitude as that reported by Crook et al for the transfer of octylphenyl nonaethylene glycol ether from water to isooctane (180 J/(mol‚deg)).16 ∆S0t is generally higher than that corresponding to the micellization process, since in the former case the surfactant molecule leaves the aqueous phase completely, thus having a greater impact on the local order of the water molecules. Contributions of Functional Groups of the Surfactant Molecules to the Free Energy of Partitioning. The global free energy of partitioning ∆G0t can be broken down into contributions from the functional groups of the surfactant molecule. This can be useful in order to
T (°C) 20 25 30 35 40
C12E3
C12E4
C12E5
C12E6
C12E7
C12E8
-27.6 -25.0 -26.3 -30.1 -27.4 -28.5 -32.4 -32.8
C12E2
-21.8 -23.0 -24.1 -25.5 -26.4
-19.1 -20.2 -21.5 -22.6 -23.8
-16.6 -17.9 -19.0 -20.1 -21.5
-14.3 -15.5 -16.8 -17.8 -19.1
-12.3 -9.4 -13.0 -14.5 -11.5 -15.4 -17.0 -14.1
C12E9
C12Ei can be written as o µoi ) µ0,o in isooctane i + RT ln Xi
(2)
0,w + RT ln Xw in water µw i ) µi i
(3)
0,w µ0,o are the standard chemical potentials, and Xoi i and µi w and Xi are the mole fractions of the surfactant. At equilibrium the chemical potentials of the surfactant in water and in isooctane are equal. As a consequence, we can write its standard free energy of partitioning as
()
0,w ) RT ln ∆G0t ) µ0,o i - µi
Xw i Xoi
( )
Vw m ) RT ln Ki o (4) Vm
o where Vw m ) 0.018 and Vm ) 0.165 are the molar volumes of water and oil, respectively. This free energy corresponds to the transfer of 1 mol of surfactant from water to isooctane. This direction of transfer was chosen in order to compare partitioning and micellization, since this latter phenomenon corresponds to the transfer of surfactant molecules from water to micelles, the cores of which are usually considered to be like an hydrocarbon liquid. The values of ∆G0t calculated from eq 4 by using the Ki values of Table 1 are reported in Table 3. All the ∆G0t values are negative, which is a consequence of the pronounced hydrophobic character of the surfactants already indicated under the conditions used. This free energy is also smaller in magnitude than that of micel-
(18) Ben Ghoulam, M.; Moatadid, N.; Graciaa, A.; Mendiboure, B.; Lachaise, J. Submitted for publication to Tenside, Surf., Deterg.
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Table 4. Contributions (in kJ/mol) to the Free Energy of Partitioning of the Two Parts of the C12E5 Molecules at Different Temperatures T (°C) ∆G0t (h) ∆G0t (w)
20 -48.8 29.7
25 -49.2 28.9
30 -49.5 28.0
35 -49.9 27.2
40 -50.2 26.4
predict the free energy of partitioning, and therefore the partition coefficient, of other surfactants belonging to the same family but having different chain lengths. Contributions of Hydrophilic and Hydrophobic parts. The standard free energy of partitioning can be written as
∆G0t ) ∆G0t (h) + ∆G0t (w)
(5)
where ∆G0t (h) is the contribution of the hydrocarbon (hydrophobic) part of the surfactant molecule and ∆G0t (w) the contribution of its hydrophilic part. The contribution of the hydrophobic part, common to the eight surfactants used, was determined by the equation
∆G0t (h) ) ∆G0t (CH3-) + 11∆G0t (-CH2-)
(6)
using the contributions of the terminal methyl group (CH3-) and of the methylene groups (-CH2-) given in the literature:19
Figure 3. Contributions of the hydrophilic and the hydrophobic parts of C12Ei to the free energy of partitioning, compared to RT, as a function of temperature.
∆Got (CH3-) 4064 ) 3.38 ln T + - 44.13 + 0.02595T RT T (7) ∆Got (-CH2-) 896 ) 5.85 ln T + - 36.15 - 0.0056T RT T (8) The values of ∆G0t (h) obtained are then reported in the first line of Table 4. Thus, for each surfactant at any temperature, the contribution of the hydrophilic part ∆G0t (w) may be evaluated from eq 5 by using the values of ∆G0t given in Table 3 and the values of ∆G0t (h) which have just been calculated. The values ∆G0t (w) obtained for C12E5 are reported in the second line of Table 4. The values of ∆G0t (h) are all negative, whereas the values of ∆G0t (w) are always positive, and smaller in magnitude than the former. As an example, the variations of ∆G0t (w)/RT, ∆G0t (h)/ RT, and ∆G0t /RT vs temperature are reported in Figure 3 for C12E5. The first term decreases with temperature due to dehydration of the hydrophilic part, leading to a decrease of the solubility of the hydrophilic part in water. At the same time the second term increases due mainly to an increase of the solubility of hydrophobic part in water. Since ∆G0t /RT decreases with temperature, as ∆G0t (w)/ RT, dehydration of the hydrophilic part may be considered as the predominant effect. Contributions of Hydroxyl and Ethylene Oxide Groups. The contribution of the hydrophilic part of ∆G0t can be broken down into those of ethylene oxide units and that of the terminal hydroxyl unit:
∆G0t (w) ) i∆G0t (-EO-) + ∆G0t (-OH) (19) Nagarajan, R.; Wang, C. C. Langmuir 1995, 11, 4673.
(9)
Figure 4. Contributions of the (-OH) and (-EO-) units to the free energy of partitioning, compared to RT, as a function of temperature. Table 5. Contributions (in kJ/mol) of the (-OH) and (-EO-) Units to the Free Energy of Partitioning at Different Temperatures T (°C) ∆G0t (-OH) ∆G0t (-EO-)
20 16.34 2.58
25 15.49 2.61
30 14.57 2.61
35 13.93 2.60
40 13.07 2.57
For each temperature, ∆G0t (-EO-) and ∆G0t (-OH) have been determined from the linear variation of ∆G0t (w) as a function of the ethylene oxide number i. The values obtained are reported in Table 5; compared to RT, their variations vs temperature are drawn in Figure 4. The contribution of an ethylene oxide unit is clearly lower than
Partitioning of Homogeneous Surfactants
that of the terminal hydoxyl group. The values obtained are comparable to those of Crook16 for the poly(oxyethylene glycol octylphenyl ether) series (∆G0t (-EO-) ) 2.52 kJ/ mol at 25 °C) and those of James et al.20 for the transfer of nonionic nitrogen-based surfactants from 0.1 M aqueous sodium hydroxide to heptane (∆G0t (-EO-) ) 2.68 kJ/mol and ∆G0t (-OH) ) 14.13 kJ/mol at 25 °C). As temperature increases, the contribution of the ethylene oxide unit remains almost constant, while that of the hydroxyl group decreases significantly. These two contributions are smaller than those corresponding to the micellization process, since in this latter case, the hydroxyl and ethylene oxide groups remain in contact with water, so their transfer into micelles needs less energy than their transfer into isooctane. (20) James, A. D.; Wates, J. M.; Wyn-Jones, E. J. Colloid Interface Sci. 1993, 160, 158.
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Conclusion The partition coefficients of homogeneous poly(oxyethylene) nonionic surfactants between water and isooctane increase exponentially by 3 orders of magnitude when the hydrophilic chain is increased by only six ethylene oxide units. When temperature increases from 20 to 40 °C the dehydration of the hydrophilic part of the surfactant molecule imposes a decrease of the global free energy of transfer. As in the micelle formation process, the hydroxyl group contribution is more important than that of an ethylene oxide group and decreases as temperature increases. The values of the contributions of the functional groups of the surfactant molecules to the free energy of partitioning determined in this work may be used as a database to predict partitioning of nonhomogeneous nonionic surfactants of the same family. LA0117707
