Langmuir 2002, 18, 6021-6024
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Partitioning of Ethoxylated Alkylphenol Surfactants in Microemulsion-Oil-Water Systems: Influence of Physicochemical Formulation Variables Nelson Ma´rquez,† Alain Graciaa,‡ Jean Lachaise,‡ and Jean-Louis Salager*,§ Laboratorio de Petroquı´mica y Surfactantes, Facultad Experimental de Ciencias, Universidad del Zulia, Maracaibo, Venezuela, Laboratoire de Fluides Complexes, Centre Universitaire de Recherche Scientifique, Universite´ de Pau et des Pays de l’Adour, Pau, France, and Laboratorio FIRP, Ingenierı´a Quı´mica, Universidad de Los Andes, Me´ rida, Venezuela Received February 22, 2002. In Final Form: May 23, 2002 The partitioning of ethoxylated alkylphenol surfactant species between oil and water depends on all physicochemical variables which are likely to alter the balance of affinity of the surfactant for the oil and water phases. Experimental data indicate how the logarithm of the partitioning coefficient varies with the oligomer characteristics (degree of ethoxylation, alkyl chain length), the phase nature (oil alkane carbon number, aqueous phase salinity), and cosurfactant (n-pentanol) content.
Introduction Commercial ethoxylated nonionic surfactants are made by a polycondensation reaction that results in a variety of oligomers, whose degree of ethoxylation, so-called ethylene oxide number (EON), is distributed according to a statistical law often of the Poisson’s type.1 As a consequence, these surfactants are always a mixture, and some annoying phenomena, such as selective fractionation or selective partitioning, can take place in a surfactant-oil-water system. In such a case, the different oligomers exhibit their individual affinities for the bulk oil and water phases and the surfactant mixture which is present at interface can be strongly different from the one which has been added in the system in the first place. This means that the actual formulation at interface can depart considerably from the overall formulation, sometimes resulting in surprising phenomena such as the retrograde transition.2,3 These uncollective behavior effects are not uncommon in surfactant-oil-water systems containing mixtures. In reality, they are often the rule as recently reported in different instances.4,5 In any case, it is necessary to predict the importance of these effects and to be able to correct them whenever a surfactant mixture is used. The partitioning of ethoxylated surfactants has been successfully interpreted using a pseudophase model6-8 in the Winsor type III phase behavior case.9 The model, which * Corresponding author. E-mail:
[email protected]. † Universidad del Zulia. ‡ Universite ´ de Pau. § Universidad de Los Andes. (1) Schick, M. Nonionic Surfactants; Marcel Dekker: New York, 1967. (2) Salager, J. L.; Ma´rquez. N.; Anto´n, R. E.; Graciaa, A.; Lachaise, J. Langmuir 1995, 11, 37. (3) Ysambertt, F.; Anto´n, R. E.; Salager, J. L. Colloids Surf., A 1997, 125, 131. (4) Graciaa, A.; Lachaise, J.; Cucuphat, C.; Bourrel, M.; Salager, J. L. Langmuir 1993, 9, 669. (5) Graciaa, A.; Lachaise, J.; Cucuphat, C.; Bourrel, M.; Salager, J. L. Langmuir 1993, 9, 1473. (6) Biais, J.; Bothorel, P.; Clin, B. J. Dispersion Sci. Technol. 1981, 2, 67. (7) Biais, J.; Clin, B. J. Colloid Interface Sci. 1984, 102, 361. (8) Sayous, J. G. Doctoral Dissertation, University of Pau P. A., Pau, France, 1983.
is essentially a phase separation model,10 allows the calculation of the composition of the interfacial oligomer mixture and thus the true or acting formulation. This is the formulation on which the properties of the system are dependent. The model has been used to interpret the apparent change in formulation with total surfactant concentration and water-to-oil ratio,11 as experimentally found.12 The determination of the interface oligomeric composition has been crucial in the finding of the lipophilic linker effect with long-chain alcohols4,13,14 as well as the interfacial segregation of the most polar component present in the oil phase.5 Partitioning is studied either with isomerically pure oligomers at low surfactant concentration, so that there are no micelles,15-17 or at high concentration in Winsor’s type III system,9 in which all structures are gathered in the microemulsion middle phase. In such a case, the partitioning is measured between the two excess phases.8,11 The partitioning coefficient K of a surfactant is defined as the ratio of its concentration in the aqueous phase Cw to its concentration in the oil phase Co. As a consequence, the term RT ln K is the molar free energy of transfer of a surfactant molecule from water to oil:
RT ln K ) RT ln Cw/Co ) µ*o - µ*w ) ∆µ*wfo (1) This change in standard chemical potential is the negative of the so-called surfactant affinity difference (SAD)18 which can be split into different terms19 to lead (9) Winsor, P. Solvent Properties of Amphiphilic Compounds; Butterworth: London, 1954. (10) Shinoda, K. Principles of Solution and Solubility; Marcel Dekker: New York, 1967; p 169. (11) Graciaa, A.; Lachaise, J.; Sayous, J. G.; Grenier, P.; Yiv, S.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93, 474. (12) Shinoda, K.; Arai, H. J. Colloid Interface Sci. 1967, 25, 429. (13) Graciaa, A.; Lachaise, J.; Cucuphat, C.; Bourrel, M.; Salager, J. L. Langmuir 1993, 9, 3371. (14) Salager, J. L.; Graciaa, A.; Lachaise, J. J. Surfactants Deterg. 1998, 1, 403. (15) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Phys. Chem. 1963, 67, 1987. (16) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Colloid Interface Sci. 1965, 20, 191. (17) Warr, G. G.; Grieser, F.; Healy, T. W. J. Phys. Chem. 1983, 87, 4520.
10.1021/la020199o CCC: $22.00 © 2002 American Chemical Society Published on Web 07/04/2002
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to an expression of the generalized formulation of the surfactant-oil-water system, as discussed elsewhere.20 Moreover, the value of the partition coefficient of each surfactant species is required to estimate the interfacial mixture composition, that is, the true formulation of the surfactant-oil-water system. The availability of the partition coefficient value or its prediction is thus quite important in practice. In our previous paper,21 the variation of the partition coefficient with temperature was reported, and it was discussed why and how the partition coefficient is expected to vary with all other formulation variables as well. The present paper combines former experimental evidence and new data, to propose an expression of the partition coefficient of ethoxylated alkylphenol oligomers as a function of all formulation variables. Experimental Procedures Surfactant-oil-water systems are studied according to the so-called unidimensional scan technique discussed in detail elsewhere.20-22 For each scan, a dozen test tubes are prepared, each containing oil, water, and surfactant in identical amounts, but with the nature of one of the components changing progressively from one test tube to the next. In the present study, the average number of ethylene oxide groups per surfactant molecule or EON is the scanned variable. Variable EON is attained by mixing commercial surfactants with different but similar EONs, for instance, 4 and 6.23,24 Test tubes typically contain 10 mL of oil, 10 mL of aqueous phase, and 0.2 g of commercial surfactant mixture. The surfactant is introduced as a water or oil solution depending on its EON. The test tubes are tightly sealed with a screw cap and are introduced into a constant-temperature bath at 25 ( 0.5 °C. They are gently shaken every 2 h during the first 12 h and then left to stand for at least 10 days to equilibrate. The tube of the scan which is selected to measure the partition coefficient of the different oligomers is the one which displays three-phase behavior with similar amounts of excess phases. It is generally at the center of the EON range in which three-phase behavior is exhibited. The oil and water excess phases are analyzed by high performance liquid chromatography (HPLC) according to general procedures that have been reported elsewhere.25 Commercial polyethoxylated alkylphenols were provided by Stepan Chemicals (Makon series) and Gaf Corp. (Igepal series). Since their alkyl chain comes from the polymerization of a short olefin, they are branched according to Markovnikov’s rule. They are referred to as C#PX, where # indicates the number of carbon atoms in their alkyl chain and X is the average EON, calculated on a mole fraction basis according to our HPLC data. N-Alkylphenol ethoxylates were provided by Hu¨ls Chemische Werke Huils. They are noted LC#PX to differentiate them from their branched counterparts.
Effect of Surfactant Characteristics Each polyethoxylkated alkylphenol oligomer is characterized by the number of ethylene oxide groups (EON or i) in the hydrophilic part and the number of carbon (18) Ma´rquez, N.; Anto´n, R. E.; Graciaa, A.; Lachaise, J.; Salager, J. L. Colloids Surf., A 1995, 100, 225. (19) Cratin, P. D. In Chemistry and Physics at Interfaces - II; Ross, S., Ed.; American Chemical Society: Washington, DC, 1971; p 97. (20) Salager, J. L. In Handbook of Detergents - Part A; Broze, G., Ed.; Marcel Dekker: New York, 1999; p 253. (21) Salager, J. L.; Ma´rquez, N.; Graciaa, A.; Lachaise, J. Langmuir 2000, 16, 5534. (22) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Marcel Dekker: New York, 1988. (23) Hayes, M.; El-Emary, M.; Bourrel, M.; Schechter, R. S.; Wade, W. H. Soc. Pet. Eng. J. 1979, 19, 349. (24) Bourrel, M.; Koukounis, C.; Schechter, R. S.; Wade, W. H. J. Dispersion Sci. Technol. 1980, 1, 13. (25) Ma´rquez, N.; Anto´n, R. E.; Usubillaga, A.; Salager, J. L. J. Liq. Chromatogr. 1994, 17, 1147.
Figure 1. Partitioning coefficient variation with the degree of ethoxylation (data from Crook et al., ref 15).
atoms in the alkyl chain or surfactant alkyl carbon number (SACN) in the lipophilic part. Data from Crook et al.15 plotted in Figure 1 indicate the variation of the partition coefficient of ethoxylated octylphenol oligomers between water and isooctane at 25 °C as a function of the degree of ethoxylation denoted as i. Figure 1 shows that the logarithm of the partition coefficient linearly increases with the degree of ethoxylation over 7 orders of magnitude. This early result15 is very close to the variation corroborated in recent works using the last HPLC techniques,18 which is expressed as the variation of the decimal logarithm of the partitioning coefficient (log K) according to
log Ki ) log K0 + 0.45i
(2)
where i is the EON of the oligomer and K0 is the partition coefficient value extrapolated at i ) 0, which depends on the surfactant lipophilic part and the oil nature. The partition coefficient of ethoxylated linear alkylphenol oligomers between water and n-heptane has been found to fit the following expression:18,26
log Ki ) -3.54 + 0.45i - 0.0425 SACN
(3)
as a function of the ethoxylation degree and surfactant alkyl chain length for the n-octyl, n-nonyl, n-decyl, and n-dodecyl alkylate species, not with branched ones as mistakenly stated in refs 18 and 26 in which the branched and linear data have been swapped. For surfactants containing branched alkylates coming from the dimerization or trimerization of a short olefin, the expression of the partition coefficient between water and n-heptane is26
log Ki ) -3.63 + 0.45i - 0.0425 SCAN
(4)
The difference with eq 3 is small but significant. It means that the branching very slightly decreases K (about 20%), probably as a consequence of two opposite effects, that is, decreased hydrophobicity because of a slightly shorter “tail” and a larger increase in oil affinity because of the branching. (26) Ma´rquez, N.; Anto´n, R. E.; Graciaa, A.; Lachaise, J.; Salager, J. L. Colloids Surf., A 1998, 131, 45.
Partitioning of Alkylphenol Surfactants
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Figure 2. Partitioning coefficient between water and n-alkanes versus ethoxylation degree.
Effect of Temperature and Other Formulation Variables The temperature is known to strongly influence the hydrophilicity of nonionic surfactants. In our previous paper,21 it was pointed out that the logarithm of the partition coefficient approximately varies with temperature in a linear way expressed by
∂ log K ) -0.02 to -0.03 (K-1 units) ∂T
(5)
This means that the partition coefficient decreases 1 order of magnitude when the temperature increases 30-50 °C. When more accuracy is required, it was shown from thermodynamics arguments21 that a Van’t Hoff expression including the degree of ethoxylation i is to be preferred:
961 + 196i -1 ∂ log K )(K units) ∂T T2
(6)
Figure 2 shows the variation of the partitioning coefficient of ethoxylated alkylphenol between water and three n-alkanes (heptane, decane, and hexadecane). Intermediate data for tetradecane are not shown for the sake of simplicity but are used to fit the numerical expressions. The left and right plots look exactly the same with a vertical shift which accounts for the fact that the surfactant in the left plot exhibits a lower partition coefficient because it is more hydrophobic (nonyl instead of octyl) and because it is branched (see the previous section). In both cases, as well as for other ethoxylated alkyl phenols, the logarithm of the partition coefficient increases 0.03 unit per carbon atom added to the n-alkane chain. The introduction of this variation in eq 3 results in the following expression for the partitioning coefficient of ethoxylated linear alkyl phenols between water and n-alkane:
log Ki ) -3.75 + 0.45i - 0.0425 SACN + 0.03 ACN (7) where ACN is the number of carbon atoms in the n-alkane molecule. The equivalent expression for branched counterparts is derived from eq 4 as
log Ki ) -3.84 + 0.45i - 0.0425 SCAN + 0.03 ACN (8)
Figure 3. Influence of ethylene oxide number, n-pentanol content, and aqueous phase salinity on the partitioning coefficient.
These equations take into account the surfactant hydrophilic and lipophilic groups and the oil nature. The effect of branching appears as a slight reduction of the constant term, which is linked to the reference value of the partition coefficient, so-called Kref in our previous paper.21 If the oil is not an n-alkane, the relationship is probably more complex, as may be deduced from the only reported case3 of aromatic-aliphatic oil mixtures in which both the Ki value and its variation with the ethoxylation degree, that is, the 0.45 coefficient in eqs 7 and 8, are affected by the oil aromaticity. Further data on this effect will be reported in a forthcoming paper. According to the SAD expression,21 other formulation variables are expected to produce an effect on the partition coefficient. An increase in the salinity of the aqueous phase would certainly produce a reduction in the affinity of the surfactant for this phase, though not as much as in the case of ionic surfactants. Figure 3 (left) shows that an increase in salinity of the aqueous phase or in alcohol content does not affect the linear relationship indicated in eq 2. It results only in a vertical shift which may be taken into account by a decrease of the constant term in eqs 7 and 8 noted as a negative variation ∆ log K in Figure 3 (right) plots. Figure 3 indicates that the addition of 8 wt % NaCl to the aqueous phase results in a decrease ∆ log K ) -0.20 for all oligomers. Since the oil phase remains unchanged, the variation in partition coefficient might be related to the change in surfactant concentration in the aqueous phase, which is essentially its critical micelle concentration (cmc). The cmc of the C9P10 oligomer was measured in pure water and in 8 wt % NaCl brine to be 60 and 38 ppm, respectively. Coincidently, the variation ∆ log cmc is exactly equal (-0.20) to ∆ log K and thus explains the change in partition coefficient. The variation due to the addition of an electrolyte depends on the electrolyte, with a stronger effect for polyvalent salts, but with no equivalence on a molar concentration basis as found for SAD.27 Figure 3 also indicates that the addition of n-pentanol reduces the partition coefficient, probably because the lipophilic alcohol preferentially partitions into the oil phase and thus increases its affinity for the surfactant. The effect is quite important at a low concentration of alcohol, but it is not linear,2,28 and as in the SAD expression, it tends (27) Anto´n, R. E.; Salager, J. L. J. Colloid Interface Sci. 1990, 140, 75.
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to weaken as more and more alcohol is added (see Figure 3, upper right plot). Conclusions
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of all formulation variables, with the exception of the alcohol effect, which is similar to the generalized formulation SAD expression.
The available experimental data allow prediction of the partitioning coefficient of ethoxylated alkylphenol oligomers between water and alkanes. The logarithm of the partitioning coefficient may be expressed as a linear form
Acknowledgment. The authors thank CONICITFONACIT for sponsoring the cooperation between their research groups (PCP and ADG programs). Financial backing from the FONACIT “Agenda Petro´leo” program, CDCHT-ULA and CONDES-LUZ is gratefully acknowledged.
(28) Ma´rquez, N.; Bravo, B.; Cha´vez, G.; Ysambertt, F.; Salager, J. L. Anal. Chim. Acta 2000, 405, 267.
LA020199O
