Langmuir 1993,9,1473-1478
1473
Interfacial Segregation of an Ethyl Oleate/Hexadecane Oil Mixture in Microemulsion Systems A. Graciaa,+J. Lachaise,? C. Cucuphat,t M. Bourre1,t and J. L. Salager'J LTEMPM, CURS, Universit4 de Pau et des Pays de l'Adour, Pau 64OO0, France, Groupement de Recherches de Lacq, Artix 641 70, France, and Laboratorio FIRP, Ingenierla Qutmica, Universidad de Los Andes, Mgrida 5101, Venezuela Received September 29,1992. In Final Form: March 8,1993 The phase behavior and optimum formulation of systems containing commercial polyethoxylated octylphenolsurfactant,water, and a mixture of ethyl oleate and hexadecaneexhibit variations which may be interpreted by two phenomena: firstthe partitioningand fractionationof the surfactantspeciesbetween the different phases, which is found to depend significantlyupon the polarity of the oil phase. By taking into account these effects, the interfacialor real formulation can be computed, and a second phenomenon is found, i.e., the segregation of the oil near the interface. The interfacial oil layer segregation has been experimentally put in evidence for the first time through the oil partitioning-fractionation phenomena which occur in low solubilizationoff-optimumsystemscontainingslightlyswollenmicellea. The experimental evidence supports a model in which the oil layer located next to the interface contains more ethyl oleate than the bulk oil phase. At and beyond 50 mol % ethyl oleate in the oil mixture with hexadecane, the interfacial oil behaves as if it were essentially composed of pure ethyl oleate, with an equivalent alkane carbon number (EACN) estimated at 6, a clear hint of its polarity. Introduction The phase behavior of surfactant-oil-water systems depends upon a score of formulation variables which are able to contribute to the balance of interactions of the surfactant (adsorbed at the interface) with the oil and aqueous phases. A lot of research work has been carried out in the past 15years becauseof the enhanced oil recovery by surfactant flooding and other applications. The state of the art1indicates that a good understanding has been attained with systems containing pure components or mixtures behaving as pseudocomponents. However, this is not the general case, and a lot of research work has been and is currently dedicated to the surfactant mixtures,2J particularly in relation to surfactant partitioning and intera~tion.~'Electrolyte anion mixing has also been dealt withms As far as the oil phase is concerned, the alkane carbon number concept (ACN)and the equivalent ACN (EACN) linear mixing rule which was first introduced 15 years agos have been taken for granted in spite of obvious discrepancies,especiallyin the presence of polar or aromaticoib.1° Polar oils are of top-notch interest in biomedical applications. However, very few papers have dealt with their incorporation in microemulsion systems,probably because + Universite de Pau et des Pays de l'Adour.
Groupement de Recherches de Law.
8 Universidad de Los Andes.
(1)Bourrel,M.; Schechter,R.5.MicroemuleionaandRehtedSystem; M. Dekker: New York, 1988. (2) Wade, W. H.; Morgan, J. C.; Jacobson,J.; Salager, J. L.; Schechter, R. 5.SOC.Petrol. Eng. J. 1978,18,242. (3)Sahger, J. L.; Bourrel, M.; Schechter, R. S.; Wade, W. H.SOC.
Petrol. Eng. J. 1979,19,271. (4)Koukounie, C.; Wade, W. H.;Schechter, R. 5.SOC.Petrol. Eng.J. 1983,23,301. ( 5 )Graciaa,A.;Lachaise, J.; Sayous,J.G.; Grenier,P.;Yiv, S.;Schechter, R. S.;Wade, W. H.J. Colloid Interface Sci. 1983,93,474. (6) Graciaa, A.; Lachaise, J.; Bourrel, M.; Osbome-Lee, I.; Schechter, R. S.; Wade, W. H.SPE Reservoir Eng. 1984,Aug, 305. (7) Graciaa, A.; Ben-Ghoulam, M.; Marion, G.; Lachaise,J. J. Phye. Chem. 1989,93,4167. (8) Antbn, R. E.; Selager, J. L. J. Colloid Interface Sci. 1990,140,75. (9)Cash, R.; Cayias, J. L.; Fournier, G.; Mac Allistar, D.; Shares,T.; Schechter, R. 5.;Wade, W. H.J. Colloid Interface Sei. 1977,59. (10)Puerto, M. C.; Gale, W. W. SOC.Petrol. Eng. J. 1977,17,193.
of their nonconventionalphase behavior and the difficulty to solubilize them in significant amount.11-14 In order to relate the phase behavior of such oils with the well-known case of alkanes, the present paper is dedicated to the phase behavior of systems containing a mixture of hexadecane and ethyl oleate, a fatty ester representative of polar oils. Special attention will be dedicated to investigate the composition of the oil located near the interface, i.e., the oil which is actually interacting with the surfactant, as key information to interpret the oil mixture behavior. Since anionic surfactant systemslJs generally require the addition of alcohol and electrolyte to exhibit a threephase behavior, ethoxylated nonionic surfactanta16J7are used instead, in order to keep the system as simple as possible. The formulation variable used is the surfactant HLB through the number of ethylene oxide groups per molecule. Products and Experimental Methods Nonionic surfactantsare polyethoxylated octylphenols manufactured by Seppic under the name Montanox. They are symbolized by OP+EON, where EON is the number of ethylene oxide groups per molecule of oligomer,or the average number for commercialproducta or mixtures. Moat integervaluescorreapond to commercial products, which were found to exhibit the conventionalPoieeon distributionof EON. Nonavailableintegers or intermediate values are obtained by mixing the two nearest substances. (11)Kunieda, H.;Miyajima, A. J. Colloid Interface Sci. 1989,129, 564.
(12)Kunieda, H.J. Colloid Interface Sci. 1989,133,237. Salager, (13)Graciaa,A.;Lachaiw, J.; Marion,G.;Antbn,R.;Girardi,S.; J. L. P h behavior of surfactanhil-water system containing oil mixtures. II International Forum Formulation Physical-Chemistry (FORMULA II), Toulouse, France,Oct 1990. (14)Salager,J.L.;Lopez-Castal,G.; Miilana-PBrez,M.;CucuphatLemercier,C.; Graciaa, A.;Lachaise, J. J. Dispersion Sci. Technol. 1991, 12,59. (15)Salager,J.L.;Morgan,J.;Schechter,R.S.; Wade, W.H.;Vasquez, E. SOC.Petrol. Eng. J. 1979,19,107. (16)Bourrel,M.; Salager,J. L.; Schechter,R. S.; Wade, W. H.J.Colloid Interface Sci. lS80, 75,451. (17)Hayes,M.;El-Emary,M.;Schechter,R.S.;Wade,W.H. J.Colloid Interface Sci. 1979,68,591.
Q743-7463/93/24Q9-1473$Q4.QQ/Q0 1993 American Chemical Society
1474 Langmuir, Vol. 9, No. 6, 1993 Hexadecane is a purum grade Fluka reagent, while ethyl oleate is a pharmaceutical grade (98%) product made by LasereonSabetay. Phase behavior experiments are carried out according to a standard procedure reported elsewhere.lJBJ8 The surfactant is introduced as an oil or aqueous solution depending on ita hydrophilicity. The proper amounts of surfactant, oil, and water are introduced in elongated graduated test t u b , which are then sealed and placed in a constant temperature enclosure or water bath. They are gently stirred twice a day during the first week, and then left to equilibrate for two months, a time long enough to ensure complete equilibration. The phase behavior is then observed, and the phase volume recorded in order to calculate the solubilization parameters, as the ratio of the amount of solubilized oil or water (mL) to the amount of surfactant (8). Experiments are generally carried out as a unidimensional scan, i.e., in a series of test tubes along which a single parameter is changed (here the surfactant EON). The parameter value which corresponds to the optimum formulation of the scan is noted with an asterisk and refered to as optimum, e.g., EON*. The optimum formulation corresponds to the point where the tension is minimum or where the microemulsion middle phase solubilizes equal amounts of oil and water;'@as a matter of fact, this is in most cases essentially the same as the middle of the three-phase range. Properties at optimum will also be noted with an asterisk. At optimum, both solubilization parameters are equal and will be symbolized by SP*. The oil-phaseconcentration of surfactants with different EON is determined by gas chromatography with satisfactory accuracy from EON = 2 to EON = 8. Ethyl oleate/hexadecane mixtures are also analyzed by gas chromatography. In both cases a 50-cm column filled with chromosorb containing 5 % SE30 is used. A flame ionizationdetector and alinear temperature gradient (150300 "C)are used. Unless otherwise stated, the systems contain the same volume of oil and water phases (WOR= 1). The micelle diameter is determined from a light scattering measurement with a Malvern correlator apparatus.
Basic Concepts on Phase Behavior and Solubilization The phase behavior is symbolizedaccording to Winsor's early work.20t21Winsor type I systems exhibit two-phase behavior in which a surfactant-rich microemulsion (aqueous) phase is in equilibrium with an excess oil phase. This phase behavior has also been referred to as 2 to indicate that the microemulsion is the lower (moredense) aqueous phase. Conversely, in Winsor type I1 phase behavior, a surfactant-rich microemulsion (oil) phase is in equilibrium with an excess aqueous phase, a situation also symbolized by 2. Winsor type I11 systems contain three phases in equilibrium: a surfactant-richmicroemulsion,the so-called middle phase because of its location, and two excess (oil and water) phases. The attainment of a three-phase behavior is associated with several other features such as ultralow interfacial tension, which is of utmost interest in enhanced oil recovery and is at the origin of the expletive optimumeZ2 Another property of three-phase systems is that they correspond to a formulation which leads to a maximum solubilizationof oil and water per unit mass of surfactant; (18) Bowel, M.;Graciaa, A.; Schechter,R. S.;Wade, W. H.J. Colloid Interface Sci. 1979,72, 161. (19) Reed, R.L.;Healy,R. N. Inlmproved OilRecovery by Surfactants and Polymer Flooding; Shah,D. O., Schechter, R. S., EMS.; Academic Press: New York, 1977. (20) Wineor, P. A. Soluent Properties of Amphiphilic Compounde, Butterworth London, 1954. (21) Wineor, P. A. Trans. Faraday Soc. 1984,44,376. (22)Shah,D. O., Schechter, R. S., Ede. Improved Oil Recovery by Surfactants and Polymer Flooding; Academic Press: New York, 1977.
Graciaa
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. I 1
MOLE FRACTION OF ETHYL OLEATE IN OIL MIXTURE
Figure 1. Variation of optimum formulation (apparent EON*) and solubilization parameters at optimum SP* versus the oilphase composition. this is the reason why three-phase systems are looked for when a high solubilization is sought. It has been shown that in a formulation scan a I I11 I1 transition of the phase behavior is observed, or vice versa. The attainment of an optimum formulation occurs when the interaction of the surfactant for the oil phase equals ita interaction for the water phase, a concept introduced almost 40 years ago by Winsor.20 The surfactant should be understood as the surfactant adsorbed at the interface, and the oil and water phases obviously refer to the moleculesof the substances contained in these phases, which are near enough to the interface to interact with the surfactant.' It must be pointed out that if the phases are not homogeneous, it is then the nature of the actually adsorbed surfactant and the interacting oil and water which must be taken into account. Winsor introduced the ratio of interactions R = ACQ/ Acw where Aco (respectivelyAcw) indicatesthe interaction between the surfactant and the oil phase (respectively water phase) per unit area of interface. The Winsor R ratio will be used here to interpret the phase behavior changes, because of its simplicity. Optimum formulation corresponds to R = 1,Winsor type I to R < 1,and Winsor type I1 to R > 1. However, it is worth noting that the interaction energies A cannot be estimated with accuracy at present; as far as the numerical aspect is concerned, semiempirical correlations involving the actual formulation variables have been proposed and are very useful in practice. An exhaustive bibliographicalreview on these concepts has been recently available,' as well as a short abstracted presentation.23
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Phase Behavior and Solubilization of Surfactant-Oil-Water Systems Containing Different Oil Mixtures of Ethyl Oleate and Hexadecane Various EON scans are carried out with systems containing different oil mixtures with roughly the same molecular volume, but very different polarity: ethyl oleate and hexadecane. Figure 1 indicates the variation of EON*, Le., the EON value of the scan at the center of the three-phase behavior zone, for different oil mixtures ranging from pure hexadecane to pure ethyl oleate. EON*,p bears the "apm (23) Salager, J. L.In Encyclopedia of Emuleion Technology;M e r , P., Ed.;M. Dekker: New York, 1988; Vol. 3, Chapter 2.
Interfacial Segregation in Microemulsion Systems subscript for "apparent", because it refers to the apparent content of the surfactant mixture at optimum, i.e., the EON of the overall amount of surfactant present in the system;the differencewith the "interfacial" or "real" value will be explained later 011.24 Figure 1indicates that EON*, first increases and then decreases with increasing amount of polar oil, at two different temperatures. The number in parentheses indicatesthe solubilizationparameter at optimum (SP*ap), which follows exactly the same trend as EON*,,. The increase of EON* with increasing content of ethyl oleate in the oil phase can be interpreted according to Winsor's R ratio. In effect, the more polar the oil phase, the higher the interaction Aco; on the other hand, the higher the EON the higher the interaction Acw. When both Aco and Acw increase in the same amount, the R ratio is kept equal to unity. Nevertheless, both the numerator and denominatorincrease,with a corresponding increase in the solubilization parameter SP*. This trend is verified up to approximately 50% ethyl oleate in the oil mixture. Beyond this amount the tendency levels off and tends to invert, a variation that cannot be explained directly by the previous straightforward arguments. It is necessary to go one step further in the description of this system, and to consider that the surfactant is actually a mixture which contains different oligomer substances. Each of them can partition between the various phases in a different way depending on its EON, according to the so-called partitioning and fractionation p h e n ~ m e n a . ~ ~ Surfactant Partitioning and Fractionation Whatever the phase behavior,the pseudophase modelzs brings a simplifiedand easy to handle, yet accurate enough, description of the surfactant-oil-water system. There are two or three pseudophases: one of them is the excess oil which contains oil and eventually dissolved surfactant species. Actually part of the excess oil may be present as solubilized oil into micelles or a microemulsion; however, all the oil is rejected into the excess oil phase. The second pseudophase is the excess water, which may contain some surfactant species, and may really be separated as solubilized water. The third, the so-called interfacial pseudophase, is supposed to gather all the surfactant adsorbed at the oil-water interface, including the microemulsion in three-phase systems. It does not contain any oil or water.26127 The fractionation comes from the fact that the different chemical species encountered in the polyethoxylated nonylphenolmixture do not partition equally between the three pseudophases. It has been found that low EON species,which are lipophilic,tend to significantlypartition into the excess oil. On the other hand, the aqueous phase does not solubilize any significant amount of this kind of surfactant. As a consequence the average surfactant mixture present in the interfacial pseudophase is more hydrophilic than the overall surfactant mixture. The symbol EON*bt will be used to specify the EON of the surfactant mixture at the interface, Le., in the interfacial pseudophase, while EON*, will refer to the overall surfactant mixture, both at optimum formulation. (24) Graciaa, A,; Lachaise, J.; Marion, G.; Schechbr, R. S. hngmuir 1989,6, 1316. (26) Shincda, K.Colloidal Surfactants; Academic Prees: New York, 1963. (26) Biah, J.; Bothorel, P.; Clin, B.; Lalenne, P. J. Dispersion Sci. Technol. 1981,2,647. (27) Bids. J.: Barthe. M.:Clin, B.: Lalanne,P. J. Colbidlnterface Sci.
Langmuir, Vol. 9, No. 6,1993 1475 ETHYLOLEATE
32 g of total OP+6 EO WOR-I, T=35DC
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ETHYLENE OXIDE NUMBER EON
Figure 2. Mass distribution of the different oligomer species presentin the overall surfactant (octylphenol+ 6EO) for various oil-phase compositions. In previous research the partition coefficients of isomerically pure surfactants have been used to calculate the composition of the different pseudophases, in very good agreement with the experimental data.686 Here we have estimated the partition coefficientsfrom the measurement of the concentration of each oligomer in the excess oil phase in Winsor I and I11 systems, i.e., when there is an excess oil phase. It is assumed that the amount of surfactant in the excess water phase is negligible. Figure 2 indicatesthe EON distribution, starting from a surfactant with an overall average EON = 6 (see Figure 11,in systems equilibrated with different oil mixtures. For each oil mixture, the mass (g) of each oligomer present in the oil phase is plotted in ordinate, while the total mass of surfactant in the excess oil phase is written under the curve maximum. The overall surfactant contained in the systems has a molar average of six ethylene oxide groups per molecule; the overall amount is 32 g. It is seen that while only 1/20of the surfactant partitions in pure hexadecane,half of it partitions in pure ethyl oleate, clear-cut evidence that the polarity of the oil phase influencesdrasticallythe partitioning. It is worth pointing out that Figure 2 represents mass distributions; molar distribution would be even more displaced toward low EON. From these data and the critical micelle concentration of each oligomer, the partition coefficients (Ki) can be calculated for each of the oil mixtures, by using a relationship discussed Ki is the ratio of molar concentrationsof oligomerEON = i in water and oil phases. Figure 3 shows that the logarithm of Ki varies linearly with EON, a result already found with other systems.Sp6ap29 Ki decreases with increasing amount of ethyl oleate, an indication than the partitioning into the oil phase is increased by the presence of the polar oil for all EON; the slope is less with ethyl oleate than with hexadecane, an indication that the partitioning difference between the two oils increases with EON. By a calculation discussed elsewhere, the distribution of oligomer in the interfacialpseudophase can be calculated for all optimum systems of Figure 1.6 From this distribution the average EON*ht is calculated and plotted in Figure 4 versus the oil mixture composition. Figure 4 is similar to Figure 1 (at 35 "C),but applies only to the (28) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Colloid Interface Sei. 1965,20, 191. (29) Crook, E. H.J. Phys. Chem. 1963,67,1987.
Craciaa et al.
1476 Langmuir, Vol. 9,No.6, 1993
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SWOLLEN MICELLE
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Figure 5. Swollen micelle representation near (left) and far away from (right)optimumformulationin Winaor type I systems, showing where the interfacial and bulk oils are located.
.
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ETHYLENE OXIDE NUMBER EON
Figure 3. Partition coefficientof the different oligomer species present in ethoxylated octylphenol (OP + 6EO) for various oilphaae compositions. " I
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SP* increases because of an interaction increase on both the oil and water "sides" of the interface is now satisfactory. Figure 4 shows that at and beyond 60 mol % ethyl oleate in the oil mixture, everything occurs as if the oil phase were pure ethyl oleate. This suggesta that the surfactant adsorbed at the interface "sees" only pure ethyl oleate. The surfactant interaction on the oil side is probably limited to the oil molecules located in the vicinity of the interface, which may be called the interfacial oil layer. If so, the results indicate that the polar oil tends to accumulate in the vicinity of the interface, a segregation feature which has been recently reported for the so-called lipophilic linkeram It is thus worth trying to find some direct experimental evidence of this segregation. Oil Segregation and Fractionation
I
MOLE FRACTION OF ETHYL OLEATE IN OIL MIXTURE
Figure 4. Variation of corrected optimum formulation (interfacial EON*)and solubilization parameters at optimum SP* versus the oil-phase composition.
surfactant adsorbed at the interface. It is seen that the correctad optimum EON tends to increase when some ethyl oleate is added to hexadecane then levels off at 50% ethyl oleate, and beyond that point remains essentially constant. The increase in EON* was fully explained when Figure 1 results were discussed. It is seen that when the interfacial value of EON is used there is no decrease, but the attainment of some plateau. The value of EON* with pure ethyl oleate allows the eetimation of ita equivalent alkane carbon number; in effect the correlation for optimum formulation of nonionic systemslBcan be written in this case as -(EON) - k(EACN) = constant where the constant accounts for the effect of the formulation variables which are unchanged in Figure 4 data. The change from hexadecane (ACN = 16)to ethyl oleate produces a change of +1.6 unite of EON*. Since k = 0.15 for this kind of surfactant, a variation of +1.5 EON unit is equivalent to a variation of -10 EACN units. In other words the EACN of ethyl oleate can be estimated at 6, i.e., the ACN of hexane. Sincethe oleoyl chain may be affected, an ACN of 18, this result indicates the drastic EACN reduction produced by the introduction of the polar group. Figure 4 indicates also the SP*htvalues. Ae in Figure 1,the solubilization increases when some ethyl oleate is added to hexadecane, but now a plateau is reached at about 50% ethyl oleate. The straightforward interpretation that
The segregation can be detected if the interfacial region of the oil phase can be separated from the bulk by any means. The fractionation between the interfacial oil and the bulk oil will thus put in evidence the segregation. To gather interfacial oil, it is necessary to look for a system in which the oil is spread over a large surface area, and a very slim depth, maybe one or two molecular sizes. Large surface areas are encountered in microemulsions and liquid crystals; however, these structures, especially near optimum formulation, are often associated with high solubilization, an indication that a large fraction of the solubilizedoil might be located deeper than what has been referred to as the interfacial oil layer. In such a case the solubilized oil might not qualify as mostly interfacial oil, and no evidence of partitioning would be found. If a segregation is to be put in evidence, a case of very low solubilization is sought, in which there are only one or two layers of oil, and practically no bulk oil, so that most solubilized oil would qualify as interfacial oil. Such a case occurs in the microemulsion of the Winsor type I system, when the formulation is far away from optimum on the 2phase behavior side. Figure 5 indicates a swollen micelle representations1of such an aqueous microemulaion both near the optimum formulation (left) and far away from it (right). Near optimum formulation, a lot of oil is solubilized in the central volume of the swollen micelle, i.e., out of the interfacial layer where segregation is expected to occur. This central volume oil will mask any segregation evidence. When the formulation is shifted away from optimum, (30)Graciaa, A.; Lachaise, J.; Cucuphat, C.; Bowel, M.;Salager, J.
L.Lungmuir lWS,9,669.
(31)Graciaa, A. C.R. Acad. Sci. 1977, W B , 343.
Interfacial Segregation in Microemulsion S y s t e m the swollen micelle size becomes smaller and smaller,3233 and can be reduced to essentially the size of a micelle with a few molecules of solubilized oil, which may be then considered aa interfacialoil. This situation will be reached far away from optimum formulation, when the micellar size ia only slightly larger than the expected from the surfactant lipophilic chain length (Figure 5, right). SinceEON* is about 6 in the studied system,EON values in the 6-10 range will be used to ensure a departure from optimum formulation. The segregation in the interfacial oil layer is expected to occur according to differences in polarity,% in molar volume>6 and eventually in rigidity. Ethyl oleate is significantly more polar than hexadecane, as may be deduced from ita molecular formula, or ita EACN; its higher polarity may be deduced also from the comparison of ita interfacial tension against water (30.7 dyn/cm) with that of hexadecane against water (41 dyn/cm). Ethyl oleate has a molecular volume (487 A3) slightly lower than that of hexadecane (600A3),and is probablymore rigid because of the double bond. The strong difference in polarity is probably the main driving force for the segregation to occur, with the two other effects as additional contributions to enhance it. Experimental Results on Interfacial Segregation SeveralWinsor type I off-optimumsystems of increasing average EON higher than EON* are prepared in the following conditions: 0.236mol/L surfactant, unit WOR, 10 mol % ethyl oleate in oil mixture with hexadecane, 36 OC. The excess oil phase is analyzed directly by gas chromatography, while the solubilized oil, labeled "interfacial" in what follows, must be first extracted from the aqueousphase according to the followingprocedure. The aqueous phase is removed and contacted with a small amount of hexane in the presence of an excess of lipophilic surfactant,so that the equilibrated system becomes of the Winsor I1type. As a consequence the micelles which were solubilizing the interfacial oil mixture disappear; the interfacial oil mixture is thus released and collected in the hexane phase, from which it is concentrated and analyzed. The ratio of the concentration of ethyl oleate in the interfacial oil mixture to the concentration of ethyl oleate in the excess or bulk oil is called the segregation parameter
SO: % ethyl oleate in interfacial oil SG = mol mol % ethyl oleate in excess oil
If So is unity, there is no segregation; if SG = 2, it means that the interfacial oil contains twice as much ethyl oleate than the bulk oil, and so forth. As a matter of fact, in the present experiment the solubilized amount is very small, so that the bulk oil content is essentially constant when EON changes. However, the oil collected as interfacial depends upon the EON as shown in Figure 6, upper plot; in effect the segregation increases as the formulation gets away from optimum, which is EON*, = 4.8 (from Figure 1). As expected from the previous discussion, Figure 6, lower plot, indicates also the concomitant decrease of oil solubilization and swollen micelle diameter, measured by light scattering. As the swollen micelles get smaller, less oil is solubilized, but this oil gets richer in ethyl oleate. (32) Graciaa, A.; Lachaise, J. J. Phys. Lett. 1977,38, 253. (33) Graciaa, A,; Lachaiee, J. J. Phys. Lett. 1978, 39, 235. (34) Nagarejan,R.,Ruckeinstein,E.InSwfoctantsinSo~tMn;Mittal, K.,Ed.; Plenum Preee: New York, 1983; Vol. 2.
(I) Oats, J. D. Ph.D. Dissertation, Univereity of Texan at Austin,
1989.
Langmuir, Vol. 9, No. 6, 1993 1477 WDR-1, T-350 0.235 movliter OPiEON
1.5 10 mol % ETHYL OLEATE OIL { 90 mol % HEXAECANE I1
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Figure 6. Variations of the segregation parameter Sa,the oil solubilization, and the micelle diameter, aa a function of the departure from optimum formulation (EON*, = 4.8). 2.5
ETHYL OLEATE o'L{HEXADECANE
I
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Figure 7. Variation of the segregation parameter SO,(far away from optimum formulation) versus the oil mixture composition. These three simultaneous effects result in strong evidence supporting the assumption that the segregation occurs in the oil layer located near the interface, where a direct interaction can take place with the surfactant lipophilic group. If this oil layer exhibits a composition pattern which depends upon the interface condition and not upon the bulk oil, the segregation would increase (at constant solubilization)when a smaller overall amount of the polar substance is present; in effect the lower the amount of polar oil, the higher the proportion of it which will be required to fill the interfacial layer. Below some overall quantity, the polar oil amount extracted from the bulk oil mixture will be high enough to significantly decrease the polar oil concentration in the bulk oil mixture. As a consequence, the segregation parameter will increase, not only because its numerator increases, but also because ita denominator decreases. Figure 7 showsthe variation of the segregation parameter with the proportion of polar oil in the overall oil mixture, and for EON = 10, a value far away from optimum formulation, and in the region where the solubilization is low and almost independent of EON (see Figure 6); the solubilization and the swollen micelle diameter are essentiallyunchanged by the variation of the oil composition. As expected from the previous discussion, the segregation increases as the proportion of polar oil decreases in the oil mixture; the effect becomes particularly significant below 20% ethyl oleate.
1478 Longmuir, Vol. 0, No.6, 1003
A Model for the Segregation of Oil Near the Interface A model may be proposed to explainthe different results presented in this paper. The oil side of the interfacial region seems to exhibit some stratified structure, in which the polar oil molecules tend to be located preferentially, i.e., in a proportion higher than their proportion in the overall oil. This segregation is probably due to some interaction of polar nature between the polar oil and the surfactant or water, and is eventually opposed by the molecular motion which tends to eliminate any concentration gradient. As a consequence the final segregated concentration profile of the polar oil might look quite similar to the ionic concentration in the diffuse part of the electrical double layer. If the polarity of the segregated oil increases, the attractive forces may be assumed to increase, and the segregated layer will be more concentrated in the polar substance. The analogy probably may be discussed further, and the layer “thickness”may be thought to be related to the difference in polarity of the two oils. However, an appropriate treatment of this analogy would have to start with the numerical estimationof the oppositeeffects,which is beyond the goal of the present study. However, it is worth pointing out that the present case may exhibit a feature with no equivalence in the electrical double layer theory, Le., the tendency of the polar oil molecules to orientate in the polarity gradient perpendicular to the interface;this orientation would favor lateral interactions of polar oil molecules with one another, and the eventualformationof more or less organized structures. This situation may be of utmost importance to produce increasedsolubilization,as shown recently in the so-called lipophilic linker role.30 In any case it may also enhance the segregation,since it reduces the freedom of motion of the molecules in the interfacial oil layer.
Graciaa et al. As an analogy with the surfactant purification by fractionation,% it must be noted that this segregation phenomenon in low solubilizationsystemsmay be thought of as the basis of a separation process for purification or extraction purposes, to be carried out evidently in a multistage fashion. It may be particularly interesting at a very low proportion of polar oil, since the segregation parameter, Le., the single stage maximum performance, might be expected to be quite high in such a case. Conclusions The oil layer located near the interface is found to exhibit a composition different from that of the bulk oil; the more polar oil species tends to accumulate in the interface vicinity. As a consequence,the adsorbed surfactant interacts with an oil mixture whose composition is different from that of the overall oil phase; this segregation phenomenon results in changes in optimum formulation and solubilization, which are interpreted. A t and beyond 50 mol % ethyl oleate in the oil mixture with hexadecane, the oil phase that interacts with the surfactant seems to be pure ethyl oleate, whose equivalent alkane carbon number is estimated at 6. The segregation increases when the solubilization decreases, and when the proportion of polar oil is reduced, a feature with potential applicationin a separation process. Acknowledgment. The research at Universit4 de Pau et Pays de l’Adour has been backed by Groupement de Recherche8de Lacq, a ELF-AQUITAINEsubsidiary.The Laboratorio FIRP research program at Universidad de Los Andes has been backed by CDCHT-ULA and CONICIT, and by the Sponsor Group of industrial companies: CORIMON, CORPOVEN, HOECHST, and INTEVEP. (36) Graciaa, A.; Lachaise, J.; Marion, G.; Bowel, M.; Rico, I.; Lath, A. Tenside, Surfactants, Deterg. 1989, 6, 38.
