Langmuir 1995,11, 37-41
37
Retrograde Transition in the Phase Behavior of Surfactant-Oil-Water Systems Produced by an Alcohol Scan J. L. Salager,*jt N. Mhrquez,* R. E. AnMn,t A. Graciaa,§ and J. Lachaise§ Lab. FIRP, Ingenieria Quimica, Universidad de Los Andes, Mirida, Venezuela, Lab. P S M , Facultad de Ciencias, Universidad del Zulia, Maracaibo, Venezuela, and LTEMPM, CURS, Universitt! de Pau PA., Pau, 64000, France Received March 15,1994. In Final Form: September 19, 1994@ The phase behavior of surfactant-oil-water systems is affected by the so-called formulation variables, i.e., by the nature of the components or their physicochemical characteristics. One of the formulation variables is the alcohol effect, which accounts for the type and concentration of alcohol. In most cases the addition of a lipophilic alcohol contributes to the increase of the amphiphile mixture lipophilicity at the interface, which results in a WI WIII WII transition, similar to the one observed when the water salinity is increased. However, in some cases the Winsor I1 phase behavior is never reached with the alcohol content increase, and a WI WIII WI so-called retrograde transition is exhibited instead. Such an anomalouscase is analyzedhere for a system containinga commercial nonionic polyethoxylatedsurfactant, n-heptane, water, and n-pentanol,the concentration of the latter playingthe role ofthe formulation variable. HPLC analysis of the different phases indicates that the surfactant oligomer partitioning between phases is affected by the alcohol content. The retrograde transition due to the increase in alcohol content is shown to come from the strong increase in the partitioning of lipophilic and balanced oligomers into the oil phase, with the remaining surfactant, in particular the interfacial mixture, becoming more hydrophilic.
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Introduction Since the phase behavior of ternary surfactant-oilwater systems was first elucidated by Winsorl 40 years ago, a lot of research has been carried out to understand more and more complex situations. In the 1970s, the oil price surge and the prospect of enhanced oil recovery drove scores of industrial and university research teams to focus on these topics. As a consequence the formulation variables were recognized and their influence was established in a quantitative fashion for many system^.^^^ The interest in phase behavior studies did not fade away with the return to cheap oil and the subsequent decline of enhanced oil recovery processes, because the generalized physicochemical formulation was linked to practical applications dealing with solubilization of oil substances3 and the properties of dispersed systems such as emulsions and foam^.^-^
Phase Behavior Transitions The phase behavior transitions are easily explained by using the Winsor R ratio'B of interaction energies between the surfactant at interface and the oil and water bulk t Universidad de Los Andes. Universidad del Zulia. 5 Universitb de Pau P.A. Abstract published in Advance ACS Abstracts, December 1,
*
@
1994. (1)Winsor, P. A.Solvent Properties o f h p h i p h i l i c Compounds;Butterworth London, 1954. (2)Shah, D. 0.; Schechter, R. S., Eds., Improved Oil Recovery by Surfactants and Polymer Flooding, Academic Press: New York, 1977. (3)Bowel, M.; Schechter, R. S.Microemulsions andReIatedSystems; M. Dekker: New York, 1988. (4)Bowel, M.; Graciaa, A.; Schechter, R. S.; Wade, W. H. J . Colloid Interface Sci. 1979,72, 161. (5)Salager, J. L.; baiza-Maldonado, I.; Mifiana-PBrez, M.; Silva, F. J . Dispersion Sci. Technol. 1982,3, 279. (6)Salager, J. L.; Mifiana-PBrez, PBrez-Sbnchez, M.; RamirezGouveia, M.; Rojas, C. J . Dispersion Sci. Technol. 1983,4 , 313. (7) Brooks, B.; Richmond, H. N. Colloids Surfaces 1991,58, 131. (8)AndBrez, J. M.; Iglesias, E.; Forgiarini, A. 68th Am. Chem. SOC. Colloid and Surface Symposium, Stanford, June 19-22, 1994. (9)Lachaise, J.; Bred, T.; Graciaa, A.; Marion, G.; Monsalve, A.; Salager, J. L. J . Dispersion Sci. Technol. 1990,11,443.
phases. For our purposes, the Winsor R ratio can be written as
R = -Aco ACW
where Aco indicates the interaction between the surfactant and the oil phase, and Acw, the interaction between the surfactant and the water phase, generally a n electrolyte solution. Accordingto Winsor, the phase behavior is linked to the R value. When R 1 (respectively R 1) the interaction of the surfactant with the water (respectively oil) phase dominates and the phase behavior in the polyphasic region corresponds to the so-called Winsor type I or WI (respectively type I1 or WII) phase behavior in which a n aqueous (respectively oily) microemulsion is in equilibrium with a n excess phase which contains essentially oil (respectively water). Whenever the R ratio is unity, a three-phase behavior is exhibited, in which a microemulsion is in equilibrium with both water and oil excess phases, Le., a three-phase behavior case which is referred to as Winsor type I11 or WIII. Such a threephase behavior situation has been called optimum formulation by people working in enhanced oil recovery, since it corresponds to a minimum in interfacial tension, and thus to the highest oil recovery. The Winsor R ratio is a n excellent tool to discuss qualitative matters, but it is not satisfactory when numerical values of the formulation variables are required, because the interaction energies can be only estimated at present. Other alternate routes have been explored. Scores of publications have been dedicated to this subject, and an extensive review in which both theR ratio and semiempirical equations are discussed extensively has been recently a ~ a i l a b l e .The ~ following is thus limited to the situations to be discussed in this paper. Any of the formulation variables which can alter the interactions ofthe surfactant with the water and oil phase can change the R value, and the corresponding phase behavior. If the change of a formulation variable can increase the Aco term or decrease the Acw term, the R
0743-7463/95/2411-0037$09.00/00 1995 American Chemical Society
38 Langmuir, Vol. 11,No. 1, 1995
Salager et al.
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ratio tends t o increase, and the observed phase behavior transition is WI WIII WII, and vice versa. The effect of the different formulation variables has been described quantitatively in the correlations for the attainment of a three-phase behaviorloJ1 for anionic systems
+ 0 - uT (2'-
1nS - KACN + f ( A ) for nonionic systems
a - EON - K ACN
25) = 0 (1)
+ bS + @(A)+ cT (T- 25) = 0 (2)
where S is the salinity in grams of NaCl per 100 mL of aqueous phase; the alkane carbon number, ACN, is a characteristic of the oil phase; u and a are characteristic parameters of anionic and nonionic surfactants, while EON refers to the number or average number of ethylene oxide groups per molecule of nonionic surfactant; flA)and @(A)are functions which render the effect of the alcohol; for lipophilic alcohols, they increase with both the alcohol chain length and the alcohol concentration; and UT and CT are positive coefficients linked with the temperature effect. Whenever the left hand term of a correlation is negative, positive, or zero, the phase behavior is WI, WII, or WIII. Thus a n increase in salinity and an increase in PA) or @(A),as produced by a n increase in alcohol concentration, result in a WI WIII WII transition. Conversely, both a n increase in the EON of a nonionic surfactant and a n increase in oil ACN result in the opposite transition, WII WIII WI.
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- -
Fractionation of Mixtures In many industrial and domestic applications, both the surfactant and the oil phase are mixtures of difTerent substances. Surfactant mixture is the consequence of either a voluntary choice,in order to attain some average property or some synergetic effect, or the result of a situation imposed by chemical or economical constraints. This is the case of most ethoxylated nonionic surfactants whose EON Poisson distribution results from the ethylene oxide polycondensation mechanism. When the mixed surfactant species are not widely different, they behave in some collective way; i.e., the composition of the mixture is essentially the same everywhere in the system, and thus the mixture may be considered as a pseudocomponent, and its property can be deduced by some linear mixing rule.12J3 When very different surfactant species are mixed, uncollective behavior becomes the rule. Each surfactant species partitions into the different phases and a t the interface, according to its own structure; as a consequence, the composition of the surfactant mixture can vary widely from one part of the system to another. This is the case of the commercial ethoxylated alkylphenol surfactants with low average ethylene oxide content. It has been shown14J5that the low EO oligomers (EO < 4) tend to partition preferentially into the oil phase, leaving the rest (10)Salager, J.L.;Morgan, J. C.; Schechter, R. S.;Wade, W. H. SOC. Petroleum Eng. J . 1979, 19,107. (11)Bourrel, M.; Salager, J. L.; Schechter, R. S.; Wade, W. H. J . Colloid Interface Sci. 1980, 75,451. (12)Salager, J. L.; Bourrel, M.; Schechter, R. S.; Wade, W. H. Soc. Petrol. Eng. J . 1979, 19,271. (13)Hayes, M.; El-Emary, M.; Schechter, R. S.;Wade, W. H. J . Colloid Interface Sci. 1979, 68, 591. (14)Graciaa, A.; Lachaise, J.; Sayous, J. G.; Grenier, P.; Yiv, S.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93,474. (15)Graciaa, A.;Lachaise, J.;Bourrel, M.; Osbome-Lee, I.; Schechter, R. S.; Wade, W. H. SPE Reservoir Eng. 1984, Aug., 305.
of the system, i.e., the water and the interface, with a higher content of high EO oligomers. Such a fractionation results in a more hydrophilic composition a t the interface.14 The effect was interpreted by using a pseudophase model and a n ideal mixing rule at interface. The partition coefficient of each surfactant species, i.e., the ratio of its concentrations in water and oil phases, respectively, Ki = Pi/C0i,is determined through the analysis of the excess phases in three-phase optimum systems. Ki is a n important parameter which is expected to depend upon the formulation variables. This paper will show how the partitioning is affected by a change in salinity and alcohol content and how this change can explain a retrograde transition in the phase behavior.
Materials and Experimental Procedures Commercial ethoxylated nonylphenol surfactants were provided by Stepan and Hoechst; they are referred to as NPXwhere X stands for the average number of ethylene oxide groups per molecule. Unless otherwise specified, the surfactant concentrationis1/30moYLofsystem,e.g., 1.5wt%forNP5.Sodiumchloride, n-pentanol, and n-heptane are reagent grade products from Merck. Each of the surfactant-oil-water systems contain 15 mL of water and 15 mL of oil. The surfactant is a mixture of NP4 and NP10, the former solubilized in the oil, the latter in the water. When alcohol is added, the oil and water amounts are reduced equally in order to maintain the total volume and the same water/ oil ratio. Sodium chloride concentration is expressed in grams of NaCl per deciliter of aqueous phase, which is symbolized as g/dL; alcohol concentrationis expressed as mL of alcohol per 100 mL of system, that is ~ 0 1 % . The 30 mL systemsare mixed in 50 mL graduated vials,tightly closed with a screw cap, and left to equilibrate for 2 weeks in a constant temperature enclosure(25"C). During the first 2 days the vials are gently shakentwice a day to speed up equilibration. m e r equilibration,the phase behavior is read, and the phase volume are measured in order to calculate the solubilization parameters. The systems are prepared according to the one-dimensional formulation scan technique, Le., only one variable changes at once. EON, salinity, and alcohol concentration scans are reported. The optimum value of a scan is taken as the point where the microemulsionmiddle phase solubilizes equalamounts of oil and water phases,a criterionwhich is knownto be equivalent to the interfacial tension minimuma3 The nonylphenol oligomer distribution and total concentration are determinedin both excess phases of all three-phasesystems. The analysis is carried out by high pressure liquid chromatography (HPLC)with a N H 2 column and a heptane-chloroformmethanol mixed solvent; typical chromatogramsand details of the optimized techniquesare reported elsewhere.18-ls The EON are calculated from the HPLC analysis data by using a mole fraction mixing rule. Normal Formulation Scan Transition First, an EON scan is carried out by mixing NP4 and NPlO surfactants in different proportions at constant molar concentration, with no salt nor alcohol in the systems. Although a NP4 NP6 mix is sufficient to reach three-phase behavior, the NP4 NPlO pair is selected so that oligomers up to 12EO are present in sizeable amount, and their partitioning can be measured. Three-phase behavior occurs from EON = 5.02 to 5.38 with the optimum formulation of the scan a t EON = 5.08, which is attained with a mixture containing 18 mol % of NP10. Table 1
+
+
(16) MArquez, N.; AnMn, R. E.;Usubillaga, A,; Salager, J. L. Separation Sci. Technol. 1993,28, 1769. (17)Marquez, N.;Ant6n, R. E.;Usubillaga, A,; Salager, J. L. Separation Sci. Technol. 1993,28, 2387. (18)Mhrquez, N.; AnMn, R. E.;Usubillaga, A.; Salager, J. L. J . Liquid Chromatogr. 1994, 17, 1147.
Retrograde Transition of Surfactant-Oil- Water Systems
Langmuir, Vol. 11,No. 1, 1995 39
Table 1. NPX Oligomer Distributions in Oil, Water, and Microemulsion Phases for the Optimum System of a NP4 + N P l O Mixture Scana mol % EON 1 2 3 4 5 6 7 8 9 10 11 12 13 14 all
in oil phase
2.70 5.4 52.4 26.3 8.5 3.3 0.9 0.4
100
in microemulsion 0.30 0.9 22.4 16.1 17.6 13.7 10.9 8.2 4.2 2.6 1.5 1.0 0.5
100
in water phase 0.06 0.2 15.1 9.2 19.7 16.1 10.5 8.6 6.6 4.7 3.7 3.0 1.6 0.7 100
Ki 3.2 x 5.3 x 10-4 4.1 x 5.0 x 10-3 3.3 x 10-2 7.0 x 1.7 x 10-l 3.1 x 10-1
in microemulsion 78.7
0
-1
3 CI,
5
-2
-3
-4
6 8 10 1E EON ( i ) Figure 1. Partition coefficient of ethoxylated alkylphenolsin two systemsas a functionofthe number of ethyleneoxide groups per molecule (EON or i).
0
mol % Proportion of Total Surfactant Oligomers (all EON) in Each Phase in oil phase 21.0
1
1 ,
in water phase
4
2
NPX, n-heptane, Water
0.3
Average EON of NPX Mixture in Each Phase in oil phase
in microemulsion
3.48
5.44
in water phase 6.34
indicates the oligomer composition of each of the phase as found from the HPLC analysis. The oil, microemulsion, and water phases contain respectively 21, 78.7, and 0.3 mol % ofthe total surfactant in the system, with respective average EON of 3.48,5.44,and 6.34. These data show clearly the fractionation of the low EON species into the oil phase and the excess of hydrophilic species in the microemulsion. The partition coefficient Ki can be readily calculated from these data. Figure 1 indicates the variation of log Ki versus “i”,i.e. the number of ethylene oxide groups per oligomer molecule, for the system water-n-heptane. Figure 1indicates also the variation of logKi for the system octylphenol oligomers-water-isooctane, which is the only one reported in the 1 i t e r a t ~ r e . Both l ~ data sets fit a linear approximation with the same slope ethoxylated octylphenol-isooctane-water log Ki = -3.93 ethoxylated nonylphenol-n-heptane-water logKi = -4.02
+ 0.45
538 EON
5.08 three-phase region
NP + 5.72 EO groups, n-heptane, NaCl Brine
I
I
oil phase
qJ-9 E
i
..................b.........
3I
; ..................... i
i
c
f
m i d d l e p h a s e v
3
8
+ 0.45Z
It is seen that by changing the alkyl group from octyl to nonyl, and the oil phase from isooctane to n-heptane, i.e., by producing changes which are both expected to slightly favor the partitioning into the oil phase, all Ki are decreased by the same factor (1.2) since the change in formulation produces a variation (-0.09)on the logarithm of the partition coefficient which is not dependent upon the number of ethylene oxide groups. The second formulation scan is carried out by changing the salinity ofthe aqueous phase, for a system ethoxylated nonylphenol-n-heptane-brine which corresponds to a borderline WI case in absence of salt, e.g. EON = 5.72. When the salinity is increased, a WI WIII WII transition is observed; the three-phase region is attained for salinities ranging from 3 to 11g/dL with the optimum formulation at 8 g/dL. Figure 2 indicates the phase volume variations for both this salinity scan and the EON scan previously discussed.
-
5x12
-
0
11
sgpa
three-phase region
Figure 2. Variationofthephase volumes along two formulation scans.
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The transition WI WIII WII occurs in either one or the other way according to the sign which determines the direction of influence of the corresponding scanned variable in eq 2. These two phase-volume maps follow the general pattern and agree with previously published report^.^,^ Figure 3 indicates the variation of log Ki versus “i”,i.e., EON, for the two optimum systems in the previous scans. I t can be seen that the addition of 8 g/dL of sodium chloride in the water phase produces a slight decrease in log Ki, for all oligomers (A log Ki = -0.20).
Salager et al.
40 Langmuir, Vol. 11, No. 1, 1995
a 0
100
-
LogKi NP n-heptane - water Log Ki = 4.0 + 0.45 i
-
NP +5.72 EO, n-heptane, water, WOR =l
(a)
-1
oil phase
8
3
Q,
-2
water phase
c Q
-3 idem with 8% NaCl in Log Ki = - 4.2 + 0.45 i
2oj
I
0
1
-4 0
2
4
6
8
10
EON (i) Figure 3. Partition coefficient of ethoxylated nonylphenolsin two systems with different water salinity as a function of the number of ethylene oxide groups per molecule (EONor i). Since the variation exhibits the same slope, the effect of the salinity can be measured by the shift of the log Ki-EON line. It is worth remarking that this decrease in the logarithm of the partition coefficientversus salinity matches the decrease of the logarithm ofthe critical micelle concentration (cmc) versus salinity with this kind of surfactant. In effect, the cmc of a commercial NPlO is found to change from 60 to 38 ppm when the aqueous phase is changed from pure water to a 8 wt % NaCl brine, a n identical shift (A log cmc = -0.20).
Retrograde Transition
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The addition of lipophilic alcohol is ~ I I O W I I ~ J ~ Jto~ result in a phase behavior transition of the WI WIII WII type because it increases the f(A)or @(A)terms in eqs 1 and 2. This is not always the case as will be shown in the following abnormal situation. An alcohol concentration scan is carried out by increasing amounts of n-pentanol to a borderline WI system containing l/30 m o m of a n ethoxylated nonylphenol with EON = 5.72, n-heptane, and water. As more and more pentanol is added, the amounts of water and heptane are reduced equally to keep the same total volume and unit water-to-oil ratio; note anyway that this is actually an unnecessary precaution, since the change would have insignificant consequences on the fractionation. WI type phase behavior occurs from 0 to 0.95 vol % of n-pentanol and three-phase behavior from 1.0to 4.7 vol%, with the optimum formulation ofthe scan, i.e., the point where the microemulsion solubilizes equal amounts of oil and water, located a t 1.5 vol % of n-pentanol. Above 4.8 vol % of n-pentanol, the transition to a two-phase system occurs, but it is of the WIII WI type, Le., the phase behavior returns to the WI case, from which the “retrograde” expletive used in labeling this transition comes. The occurrence of this retrograde transition is best understood by analyzing the phasevolume map. Figure 4a indicates that from 1.0 to 4.7 vol % of n-pentanol, the middle phase volume is shrinking from 16 to 8%of the total volume, while the oil phase is increasing from 41 to 50% of the total volume and the excess water phase remains essentially constant. It may be thought that the reduction of the middle phase volume is due to a decrease in solubilization, a common feature encountered when more and more alcohol molecules go to the interface and pull apart the surfactant molecules. However such a dilution effect is not characteristic of
-
2
3
4
5
vol % of n-pentanol 100
80
Mol % of total surfactant in oil
-
0
(b)
4 Mol Yo of total suffactant in microemulsion
1
2
3
4
5
vol % of n-pentanol Figure 4. Phase volume variations and total surfactant partitioning along an alcohol scan with retrograde transition. pentanol, but of shorter a l c o h ~ l s Figure . ~ ~ ~4b ~ ~indicates that the amount of surfactant in the middle phase decreases when the n-pentanol content increases. This change essentially follows the variation in middle phase volume, so that the concentration of surfactant in the middle phase remains roughly constant. However, the analysis of the water and oil excess phases indicates that the nature ofthe middle phase surfactant mixture changes with the alcohol concentration. As alcohol is taken up into the oil and water phases, the affinity of all surfactant species for the water phase decreases, because the water phase becomes less polar; conversely their affinity for the oil phase increases because it becomes more polar. As a consequence, P i tends to decrease while Coi tends to increase, with a resulting decrease in the partition coefficient Ki. Figure 5 indicates the variation of log Ki for different three-phase systems near optimum formulation. Line A corresponds to the absence of alcohol in the system, while lines B-D correspond to three-phase systems containing different amounts of n-pentanol. It is worth noting that the partitioning is not affected by the fact that the average EON is different in cases A-D. It is actually affected by the alcohol content. Because of the bell shape of the EON distribution, the increase of alcohol content results in an increase in the surfactant amount which partitions into the oil phase (see Figure 4b); because of the variation of log Ki when the alcohol content increases, the residual surfactant mixture (19) Graciaa, A.; Lachaise, J.;Cucuphat, C.; Bourrel, M.; Salager, J. L. Langmuir 1993,9, 669. (20) Graciaa, A.; Lachaise, J.;Cucuphat, C.; Bourrel, M.; Salager, J. L. Langmuir 1993,9, 3371.
Retrograde Transition of Surfactant-Oil- Water Systems
of a polar oil (n-pentanol) and a n apolar oil (heptane). Such a mixture is known21 to exhibit a n interfacial segregation ofthe polar oil; i.e., the interfacial oil contains more pentanol than the bulk oil. The oil interfacial or effective ACN is thus reduced, a n effect that favors a WIII WII transition and which also opposes the fractionation effect. This may explain why the retrograde transition occurrence trails so long (from 1.5 to 4.8 vol % alcohol) after the optimum formulation is reached.
Log Ki = - 4.90 + 0.45 i
0.0 % alcohol
Log Ki = 4.02 + 0.45 i
3.0 % alcohol 4.5 % alcohol
Log Ki = 5.04 + 0.45 i
-
-
n V
-1
3
Langmuir, Vol. 11,No. 1, 1995 41
-2
0
Conclusions The increase in pentanol concentration results in a socalled retrograde transition (WI WIII WI) because of two opposite effects: on the one hand, the well-known increase in interfacial lipophilicitybecause of the increased fraction of lipophilic alcohol a t interface and, on the other hand, an increase in partitioning of all surfactant oligomers into the oil phase, specially the lipophilic and balanced ones, which results in a depletion of these substances a t the interface which turns hydrophilic because of the mass balance. The first effect dominates a t low alcohol concentration and the WI WIII transition can occur; then this effect levels off and the second effect becomes important when more and more alcohol partitions into the oil and water phases; finally the second effect dominates at high alcohol concentration to produce the WIII WI retrograde transition.
- -
4
-3 -4
-5 0
2
4
6
8
10
EON (i) Figure 5. Change in partition coefficient with the n-pentanol content of three-phase systems at or near optimum formulation.
-
a t interface becomes more hydrophilic, and the WIII WI retrograde transition takes place. Actually several competitive effects are to be considered when alcoholconcentration is increased. First, the alcohol plays a cosurfactant role according to the correlation (2) which in this case, may be written as -EON
+ @(A)= constant
When the interfacial surfactant becomes more hydrophilic as its average EON increases, the lipophilic alcohol increasing concentration tends to compensate it. However, the @(A)term effect levels off and the increasing EON tendency finally dominates. As it happens, the WIII WI retrograde transition occurs. I t is worth noting that another effect may be playing a role in this situation. As the alcohol concentration increases, more and more alcohol partitions into the oil phase. As a consequence the oil phase becomes a mixture
-
-
-
Acknowledgment. The Lab. FIRP research program at Universidad de Los Andes is sponsored by the following institutions: CDCHT-ULA, Corimon, Hoechst de Venezuela, INTEVEP, and Procter and Gamble de Venezuela. The Lab. PSAS research program at Universidad del Zulia is sponsored by CONDES-LUZ. The stay in MBrida and Pau ofone ofus (N.M.)was financed by the BID-CONICIT Scholarship Program and CEPET, a subsidiary of Petroleos de Venezuela. LA940228Y (21) Graciaa, A.; Lachaise,J.;Cucuphat, C.; Bourrel, M.; Salager, J. L. Langmuir 1993,9, 1473.
