J. Phys. Chem. 1983, 87, 707-714
scription of transport properties of ternary "diluted" microemulsions. Such a model allows direct determination of micellar parameters-e.g., micellar conductivity or viscosity-and leads to an original evaluation of intermicellar attractive potential. Two points remain poorly understood at this time: (1) There is the discrepancy between the thermal conductivity extrapolated at 4 = 0 from the microemulsion experimental values, and the thermal conductivity of pure CClk It is Dossible that this difference could be fundamentally connected to the intramicellar inhomogeneous structure.
707
(2) There is the difficult problem of the "concentrated" ternary microemulsions which is outside the scope of this work. Experiments related to these points are now in progress in our laboratory.
Acknowledgment. We are indebted to Drs. J. J. Piaud and c. Vaucamps for their experimental contributions, to Drs. J. Buchert and P. Maraval for their numerical analysis Of the TLT, and to D. Roux and c. Codon for many discussions. Registry No. CC14, 56-23-5; AOT, 577-11-7.
Phase Behavior of Microemulsion Systems Studied by Positron Annihilation Techniques' Juan Serrano, Rocio Reynoso, Rafael L6pez, Oscar Olea, Belkacem Djermounl,' and Lur Alicia Fucugauchi" Odmica del Positronio, Instituto Nacional de Investigaciones Nucleares, 06 140 Mexico, 0.F. (Received: August 24, 1982; In Final Form: October 14, 1982)
The positron annihilation technique was applied to the study of phase behavior of sodium stearate (or oleate)-alcohol-oil-water microemulsion systems. The positron annihilation parameters revealed a dependence of the water/oil (w/o) ratio at which microemulsion formation occurs on the hydrocarbon chain length of both alcohol cosurfactant and solvent as well as surfactant concentration. Dynamic laser light scattering has been utilized for substantiating the phase transitions determined in the different microemulsion systems by positron annihilation. The difference in the behavior between saturated and unsaturated surfactants is the most remarkable result of the present investigation. Thus, replacing sodium stearate by sodium oleate in the surfactant-1-hexanol-isooctane-water systems obviated microemulsion formation. This behavior has been rationalized by considering packing and kink presence in microemulsion formation.
Introduction In spite of the extensive utilization of microemulsions in tertiary oil recovery, octane improvement, pollution abatement, and chemical proce~sing,"~ there has been very little systematic investigation of their properties and phase behavior. Positron annihilation has been proved to be an extremely sensitive technique for investigating aggregation mechanisms in both aqueousg10 and reversed (1) This research is in furtherance of the USA.-Mexico Cooperative Science Program through the Nationa! Science Foundation and the Consejo Nacional de Ciencia y Tecnologia (Project BCCBCEU-010569). (2) Visiting Scientist from Virginia Polytechnic Institute and State University, Blacksburg, VA. (3) Experimental assistance performed by Cristino Rodriguez. (4) For recent general reviews on microemulsions see, e.g.: (a) Shinoda, K., Frieberg, S. Adu. Colloid Interface Sci. 1975,4, 281. (b) Rosano, H. Cosmet. Chem. 1971,25,609. (c) Mittal, K., Ed. 'Micellization, L. J. SOC. Solubilization,and Microemulsions"; Plenum Press: New York, 1977. (d) Shah, D. 0.; Bansal, V. K.; Chan, K.; Hsieh, W. C .In "Improved Oil Recovery by Surfactant and Polymer Flooding"; Shah, D. O., Schechter, R. S., Eds.; Academic Press: New York, 1977; pp 291-337. (e) Winsor, P. A. Chem. Reu. 1968,68, 1. (0 Nakagawa, T.; Tokiwa, F. In "Surface and Colloid Science"; Matjevic, E., Ed.; Wiley: New York, 1976; Vol. 9, pp 69-164. (9) Prince, L. M., Ed. "Microemulsions"; Academic Press: New York, 1977. (h) Fendler, J. H. "Membrane Mimetic Chemistry"; Wiley-Interscience, in press. (5) Bansal, V. K.; Shah, D. 0. In ref 4d, pp 87-113. (6) Frieberg, S. CHEMTECH 1976, 124. (7) Letts, K.; MacKay, R. A. Inorg. Chem. 1975, 14, 2990. (8)Jean, Y. C.; Ache, H. J. J . Am. Chem. SOC.1977, 99, 7504. (9) Jean, Y. C.; Ache, H. J .J. Phys. Chem. 1978,82, 811. (10) Handel, E. D.; Ache, H. J. J . Chem. Phys. 1979, 71, 2083. (11) Jean, J. C.; Ache, H. J. J. Am. Chem. SOC.1978,100, 984.
as well as in microemulsions.20 The application of this technique is based on the very high dependence of the positronium atom (Ps) (the bound state of a positron and an electron) formation and its subsequent reactions on phase changes occurring in the aggregates. Positronium formation and positronium reactions can both be observed by measuring the positron (12) Jean, J. C.; Ache, H. J. J. Am. Chem. SOC. 1978,100,6320. (13) Fucugauchi, L. A.; Djermouni, B.; Handel, E. D.; Ache, H. J. J. Am. Chem. SOC. 1979,101, 2841. (14) Fucugauchi, L. A.; Djermouni, B.; Handel, E. D.; Ache, H. J. In 'Proceedings of the 5th International Conference on Positron Annihilation"; Hasiguti, R. R., Fujiwara, K., Eds.; The Japan Institute of Metals: Yamanaka, 1979; pp 857-60. (15) Djermouni, B.; Ache, H. J. J. Phys. Chem. 1979,83, 2476. (16) Ache, H. J. Adu. Chem. Ser. 1979,175, 1-49. (17) Jean, J. C.; Djermouni, B.; Ache, H. J. In "Solution Chemistry of Surfactants"; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. I, pp 129-52. (18)Nicholas, J. B.; Ache, H. J. J. Chem. Phys. 1972,57, 1597. (19) Madia, W. J.; Nichols, A. L.; Ache, H. J. J. Am. Chem. SOC.1975, 97, 5441. (20) Boussaha, A.; Djermouni, B.; Fucugauchi, L. A.; Ache, H. J. J. Am. Chem. SOC. 1980,102,4554. (21) For general references on positron annihilation see: (a) Green, J.; Lee, J. "Positronium Chemistry"; Academic Press: New York, 1964. (b) Goldanskii, V. I. At. Energy Reo. 1968,6, 3. (c) McGervey, J. D. In "Positron Annihilation"; Stewart, A. T., Roelling, L. O., Eds.;Academic Press: New York, 1967; p 143. (d) Merrigan, J. A.; Tao, S. J.; Green, J. H. In "Physical Methods of Chemistry"; Weissberger, D. A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. I, Part 111. (e) Ache, H. J. Angew. Chem., Int. Ed. Engl. 1972,11, 179. (fl Tao, S. J.; Green, J. H. J. Phys. Chem. 1969, 73,882. (9) Goldanskii, V. I.; Virsov, V. G. Annu. Reu. Phys. Chem. 1971,22, 209. (h) Ache, H. J. Adu. Chem. Ser. 1979,175, 1-49.
0022-3654/83/2087-0707$01.50/00 1983 American Chemical Society
700
12 v s WATER CONTENT IN SODIUM STEARATE12
Fucugauchi et al.
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983
O/o
ALCOHOL-ISOOCTANE
SOLUTIONS
23
I
CLEAR
II
TURBID
TABLE I: Water/Oil Ratios at Which Microemulsions Are Formed ( R MPoints (v/v)), the Corresponding Molar Ratios, and the Upper Limit of the Clear Range of the Solutions ( w / o ) Determined by Turbidimetry in Sodium Stearate Systems water/oil alcohol
v/v
molar ratio
clearrange upperlimit w/o
Sodium Stearate (2.5 g)-Alcohol 22
(10.0 mL)-Isooctane (25.0 mL)-Water
a) BUTANOL
1-butanol
I
1-pentanol 3-methyl-1-butanol 1-hexanol
, I t ,
2.31 1.85 1.85 1.38
0.40 0.45 0.45 0.50
Sodium Stearate (2.5 g)-Alcohol (10.0 mL)-Hexadecane (25.0 mL)-Water 1-butanol no formation 1-pentanol 0.35 5.67 0.40 1-hexanol 0.30 4.86 0.50
23
22
Sodium Stearate (2.5 g)-Alcohol (10.0 mL)-Cyclohexane (25.0 mL)-Water 1-butanol 0.20 1.20 0.70 1-pentanol 0.20 1.20 0.50 1-hexanol 0.20 1.20 0.50
b) PENTANOL 21
Sodium Stearate (5.0 g)-Alcohol (10.0 mL)-Isooctane (25.0 mL)-Water 1-pentanol 0.40 3.70 0.60 1-hexanol 0.30 3.23 0.80
,It,
I 23
emulsion formation mechanism, we replaced sodium stearate by sodium oleate in all the solutions under investigation. The observed phase changes, detected by positron annihilation, have been substantiated by dynamic laser light scattering.
22 c) ISOPENTANOL
21
Experimental Section
23
22
d ) HEXANOL
21
0.25 0.20 0.20 0.15
I
, E , I
1
I
0.4
0.8
1.2
w/o
Figure 1. I, vs. water contents in sodium stearate-alcohol-isooctane-water systems at room temperature.
The positron annihilation parameters can be correlated with changes occurring in the solutions as a function of their composition. The drastic decrease in positronium formation probability in both aqueous"'O and reversed"-" micellar systems as well as in microemulsions20has been explained by an effective trapping of (energetic) positrons or Ps atoms, thereby reducing the number of (thermal) Ps atoms formed. Effects of increasing the amount of water solubilized in reversed micelles on positronium formation probabilities have been studied.20 A more quantitative investigation is reported in the present work. Sodium stearate-alcoholoil-water microemulsions were investigated by varying the solvent (oil), the cosurfactant (alcohol), and the surfactant concentrations. Isooctane, hexadecane, and cyclohexane were used as solvents. 1-Butanol, 3-methyl-1-butanol, 1-pentanol, and 1-hexanol were the cosurfactants. In order to assess the effect of the double bond on the micro-
Materials. Sodium oleate and sodium stearate (INC) were of pharmaceutical-grade purity and were used without further purification. Solvents such as isooctane, hexadecane, cyclohexane, and alcohols (1-butanol, 3-methyl-lbutanol, 1-pentanol, and 1-hexanol) were spectroscopic grade (Merck). The hydrocarbons were further dried by distillation over metallic sodium. Preparation of Solutions for Positron Lifetime Measurements. The various solutions studied in this investigation were prepared by mixing the components in the indicated proportions, to which different amounts of triple-distilled water were added.20 Sodium stearate-alcohol-isooctane: sodium stearate (2.5 g), 1-butanol, 3-methyl-1-butanol, 1-pentanol, or 1hexanol(10 mL), isooctane (25 mL), and water (2.5 mL). Sodium stearate-alcohol-hexadecane: sodium stearate (2.5 g), 1-butanol, 1-pentanol, or 1-hexanol (10 mL), hexadecane (25 mL), and water (2.5 mL). Sodium stearate-alcohol-cyclohexane: sodium stearate (2.5 g), 1-butanol, 1-pentanol, or 1-hexanol (10 mL), cyclohexane (25 mL), and water (2.5 mL). Sodium stearate-alcohol-isooctane: sodium stearate (5.0 g), 1-pentanol or 1-hexanol(10 mL), isooctane (25 mL), and water (2.5 mL). Sodium stearate-1 -heranol-isooctane: sodium stearate (2.5 g), 1-hexanol (7 mL), isooctane (25 mL), and water (2.5 mL) . Sodium oleate-alcohol-isooctane: sodium oleate (5.0 g), 1-butanol, 1-pentanol, or 1-hexanol(10 mL), isooctane (25 mL), and water (2.5 mL). Sodium oleate-alcohol-hexadecane: sodium oleate (5.0
Phase Behavior of Microemulsion Systems
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983 709
I p v s WATER C O N T E N T I N SODIUM S T E A R A T E -
I, Yo
ALCOHOL-ISOOCTANE-WATER
I2
1, vs WATER CONTENT IN SODIUM OLEATE-
Oh
SYSTEMS
SOLUTIONS
ALCOHOL-ISOOCTANE
I
CLEAR
11 T U R B I D 24
I
A
0
SODIUM STEARATE 2.5 g
-
23
-
0)
B UTA N 0 L
22
I
I
j11
23
I"
22
0
SODIUM STEARATE 2.5 9
0
SODIUM STEARATE 5.0 p
21 23
-c)
22 I
0
1
HEXANOL
0
.
21 I
0
0.4
I
I
0.8
1.2
w/o
Figure 2. I, vs. water contents In sodium stearate (2.5 and 5.0 g)-alcohol-isooctane-water dispersions at room temperature.
g), 1-butanol, 1-pentanol,or 1-hexanol(10 mL), hexadecane (25 mL), and water (2.5 mL). Sodium oleate-alcohol-cyclohexane: sodium oleate (5.0 g), 1-butanol, 1-pentanol, or 1-hexanol(10mL) and water (2.5 mL). Sodium oleate-alcohol-isooctane: sodium oleate (2.5 g), 1-pentanol or 1-hexanol (10 mL), isooctane (25 mL), and water (2.5 mL). Sodium oleate-1-hexanol-isooctane: sodium oleate (5.0 g), 1-hexanol ( 5 mL), isooctane (25 mL), and water (2.5 mL). Positron Lifetime Measurements and Preparation of Samples. Positron lifetime measurements were carried out by a fast coincidence technique as described previously.2o The resolution of the system, measured by the prompt time distribution of a 6oCosource, was found to be 0.400 ns fwhm. Corrections for the source component, which had an intensity of less than 3%, were made in the usual way by using conventional computational methods.E20 Specially designed cylindrical sample vials (Pyrex glass 100 mm long and 10 mm id.) were filled with about 2 mL of sample solution. The positron sources consisted of 5-20 pCi 22Na,prepared by diffusing carrier-free 22NaC1into a thin, soft glass foil. The sources were placed inside the vials and completely immersed in the liquid sample. The
I
I
1
I
0.4
I
I
0.8
I
I
1.2
w/ 0
Figure 3. I, vs. water contents in sodium oleate-alcohol-isooctane-water solutions at room temperature.
vials were degassed and subsequently sealed off and counted at room temperature. General Method of Data Analysis. Positron lifetimes and distributions were obtained by standard computational techniques.a20 The lifetime spectra were resolved as previously describedam into two components, a short-lived component, which is the result of p-Ps annihilation, free positron annihilation, and epithermal Ps interactions, and the long-lived component, with a decay constant X2 and its associated intensity I, which originates from the reactions and subsequent annihilation of thermalized or nearly thermalized 0-Ps. Dynamic Laser Light Scattering Experiments. The dynamic laser light scattering measurements have been carried out by using a Malvern 2000 spectrometer in conjunction with a 80-mW Spectraphysics argon ion laser.
Results and Discussion The complexity of microemulsions became painfully apparent during the course of the present study. Many parameters affected the onset of microemulsion formation and phase diagrams described them. Effects of altering the sovlent (isooctane, hexadecane, and cyclohexane),the
710
12 v s WATER CONTENT IN SODIUM O L E A T E -
I2
Fucugauchi
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983
HEXANOL ( 5 and 10 ml)-ISOOCTANE
SOLUTIONS
et
al.
TABLE 11: Water/Oil Ratios at Which Microemulsions Are Formed (RM Points (v/v)), the Corresponding Molar Ratios, and the Upper Limit of the Clear Range of the Solutions ( w / o )Determined by Turbidimetry in Sodium Oleate Systems ______
I
CLEAR
1I
TURBID
water /oil alcohol
23
vlv
molar ratio
clearrange upperlimit w/o
Sodium Oleate (5.0 g)-Alcohol (10.0 mL)-Isooctane (25.0 mL)-Water 1-butanol 0.50 4.59 0.80 1-pentanol 0.50 4.59 0.60 1vhexanol no formation
22
Sodium Oleate (5.0 g)-Alcohol (10.0 mL)-Hexadecane (25.0 mL)-Water 1-butanol no formation 1-pentanol 0.40 6.50 0.90 1-hexanol 0.30 4.88 1.00
21
n
I
P
23
Sodium Oleate (5.0 g)-Alcohol (10.0 mL)-Cyclohexane (25.0 mL)-Water 1-butanol 0.40 2.40 0.70 1-pentanol 0.20 1.20 0.30 1.00 1-hexanol 0.40 2.4 0
22
Sodium Oleate (5.0 g)-Alcohol (5.0 mL)-Isooctane (25.0 mL)-Water 1-hexanol 0.20 1.83 0.70
21
Sodium Oleate (2.5 g)-Alcohol (10.0 mL)-Isooctane (25.0 mL)-Water 1-pentanol 0.20 1.83 0.80 1-hexanol 0.10 0.92 0.30
2c k
I 0 ‘4
0.8
12
w/o
Flgure 4. I, vs. water contents in sodium oleate-l-hexanol-isooctane-water solutions at room temperature.
type and concentration of the surfactant (sodium stearate and oleate), and the cosurfactants (1-butanol, 3-methyl1-butanol, 1-pentanol, and 1-hexanol) had to be examined systematically. Relatively small alterations of each of these parameters caused profound changes in the composition of RM points. (In order to avoid the cumbersome “water/oil ratio at which microemulsion formation occurs”, we introduce the abbreviation RM point or RM value and use it throughout this paper.) Differences in the behavior between saturated and unsaturated surfactants is the most remarkable result of the present investigation. Thus, replacing sodium stearate by sodium oleate in the surfactant-l-hexanol-isooctanewater system obviated microemulsion formation. Presentation of the results and discussion will be facilitated by the separate treatment of the properties of microemulsions prepared from saturated and unsaturated surfactants. Effects of the various components on the RM points in these two systems will be discussed sequentially. I. Sodium Stearate Systems. Effect of the Cosurfactant. Phase changes for the sodium stearate-alcoholoil-water microemulsion systems have been investigated as a function of the hydrocarbon chain length of the cosurfactant. 1-Butanol, 3-methyl-1-butanol,1-pentanol,and 1-hexanol have been used as cosurfactants at different water concentrations. All the solutions investigated in the present work contained 2.5 g of sodium stearate, 10 mL of alcohol, 25 mL of solvent, and various amounts of water. The data are shown in Figure 1. 12,the long-lived component in the positron annihilation spectra, is plotted as a function of the water contents. The plots reveal an increase of I2up to RM points of 0.25,0.20, and 0.15 in the presence of 1-butanol, 1-pentanol, and 1-hexanol, respec-
tively. Maxima are reached at the same value for 1-pentanol and 3-methyl-1-butanol (0.2; Figure 1). If the values of the RM point increase above those maxima, Iz shows a significant drop which can be assocated with the onset of microemulsion formation.20At values of the waterloil ratio higher than the R M point, I2 increases, again signaling additional structural changes. A consistent trend is seen to emerge which depends on the chain length of the alcohol. In each system the value of the RM point decremes with increasing chain length of the alcohol. This behavior can be rationalized by the lesser alcohol partitioning into the oil phase with increasing chain length of the cosurfactant. Longer chain length alcohols partition more favorably in the oil phase than shorter ones.np23 The average intermolecular distance is larger for shorter than longer chain length alcohols (compare maxima for butanol (RM = 0.25), pentanol (RM = 0.2), hexanol (RM = 0.15) in Figure 1). Results similar to those shown in Figure 1 have been obtained in hexadecane (Table I). Conversely, RM-point values have been found to be independent of the chain length of the alcohol cosurfactant in the cyclohexane solvent (Table I). Influence of the Solvent. Microemulsion formation has been studied as a function of the solvent composition in sodium stearate systems which contained 1-pentanol or 1-hexanol in the presence of isooctane or hexadecane. RM values are seen to increase with increasing chain length of the hydrocarbon (Table I). Apparently, larger average intermolecular distances are created between the surfactant m o l e c ~ l e swhich ~ ~ * ~require ~ more water to fill the available space. This, in turn, leads to increased RM values. The effect of the hydrocarbon chain length of the solvent on microemulsion formation is the opposite of that observed for the chain length of the alcohol cosurfactant on (22) Bansal, V. K.; Chinnaswamy, B. J.; Ramachandran, C.; Shah, D. 0. J . Colloid Interface Sci. 1979, 72, 524. (23) Bansal, V. K.; Shah, D. 0.; O’Connell, J. P. J . Colloid Interface Sci. 1980, 75, 462.
The Journal of Physical Chemistry, Voi. 87, No. 4, 1983 711
Phase Behavior of Microemulsion Systems
Alcohol
I2
I2 vs WATER CONTENT IN SODIUM OLEATE 70
( 2 . 5 and 5.0 g)-PENTANOL-ISOOCTANE
SOLUTIONS
I
CLEAR
II
TURBID
23
22
21
, I I
I
I
23
water t Surfactant
ai
22
h
21
I
I
Alcohol
b ) 5.0 g
II
I
I
0.4
I
0.8
I
I
w/o
1.2
octane-water systems at room temperature.
TABLE 111: Partitioning and Free Energy Transfers in Sodium Stearate or Sodium Oleate-Alcohol-Oil-Water Systems AG,
n,h alcohol Sodium Stearate (2.5 g) 1-butanol 5.60 1-pentanol 6.51 3-methyl-1-butanol 5.71 5.31 1-hexanol 1-butanol 6.08 1-pentanol 4.02 6.94 1-hexanol
n,)s
kcd mol
0.050 0.235 0.185 0.109 0.025 0.043 0.073
-2.748 -1.935 -1.998 -2.263 -3.196 -2.646 -2.655
Sodium Oleate (2.5 g) 1-butanol 3.28 1-pentanol 2.78 1-hexanol 47.49 1-pentanol 2.43 1-hexanol 2.92 1-butanol 2.71 1-pentanol 1.87 46.85 1-hexanol
0.026 0.046 0.379 0.155 0.080 0.026 0.027 0.238
-2.185 -2.391 -2.813 -1.604 -2.094 -2.694 -2.455 -2.738
(na/
solvent isooctane
cyclohexane
isooctane hexadecane cyclohexane
isooctane
(na/
Sodium Oleate (5.0 g) 1-pentan01 1.69 0.078 -1.792 1-hexanol 1.33 0.060 -1.802
the same aggregation phenomenon in sodium stearate systems. Effect of the Surfactant. Effects of changing the surfactant concentration from 2.5 to 5.0 g in 1-pentanol or 1-hexanol-isooctane-water systems are shown in Figure 2. In 1-pentanol systems, R M pointa reached values of 0.2 and 0.4 at 2.5- and 5.0-g surfactant concentrations. In 1-hexanoldispersions, R M pointa reached values of 0.15 and
Surfactant
L
I
b
4
b)
Figure 6. Partitioning determined by titration2’ and the microemulsion range studied by the positron annihilation technique are shown for sodium stearate or sodium oleate-isooctane systems (Figure 6a) and for sodium stearate or sodium oleate-cyclohexane dispersions (Figure 6b) in the presence of different alcohols. Initial compositions of the solutions were as foilows: 25 mL of solvent; 2.5 g of surfactant; 10 (0),7 (e), and 5 (X) mL of alcohol (expressed in w/w). Microemulsion range is indicated for 1-butanol (I’), 1-pentanol (II‘), 1-hexanoi (III’), and 3-methyl-1-butanol (IV’). I-IV correspond to 1‘-IV’ except that they contain 5.0 g of surfactants. Indexes o and s indicate sodium oleate and sodium stearate, respectively. The partitioning ( n a / n J , is indicated on the alcohol-water surfactant axls by the same symbols used for microemuision ranges. Compositions of the solutions (w/w) titrated with 1-butanol (A),1-pentanol (O), 1-hexanol ( O ) , and 3methyl-1-butanol (‘) are indicated by the appropriate symbols. The closed symbols correspond to sodium oleate and the open ones to sodium stearate.
+
0.3 at 2.5- and 5.0-g surfactant concentrations, respectively. These results are explicable in terms of the water-solubilizing capacity of microemulsions. At higher surfactant concentrations there are more aggregates which require more water in the microemulsion formation. The observed values of I,, the long-lived component in
712
Fucugauchi et al.
The Journal of Physical Chemistry, Vol. 87, No. 4, 1983
I2
12 vs WATER C O N T E N T IN SODIUM OLEATE-
vs WATER C O N T E N T I N SODIUM S T E A R A T E -
I2
ALCOHOL-OIL
I2 Oh
SOLUTIONS
-a 0 0
I
CLEAR
II
TURBID
ALCOHOL-OIL SOLUTIONS
I II
In
CLEAR TURBID
23
23
t
22
22
0
ISOPENTANOL-ISOOCTANE
21
0
21
r-
I
II
I ,
0
4
,II,
20
23 PENTANOL-HEXADECANE
19 22
18 21
II
I
P
0
04
0.8
I
1.2
w/o
0
I 0.4
,II 0.8
1.2
w/o
Figure 7. Microemulsion diameters measured by dynamic laser light scattering at room temperature.
the spectra of positron annihilation which is closely related to the number of positronium atoms formed, might explain this fact since Iz reaches smaller values at higher surfactant concentration (Le., 5.0 g) than at lower ones (2.5 g), which suggests that at higher surfactant concentration there might be more aggregates which trap the energetic positrons or positronium atoms, thus reducing the number of positronium atoms formedkz0 (compare I2 values in plots of Figure 2 ) . II. Sodium Oleate Systems. Effect of the Cosurfactant. As in the saturated surfactant systems (section I), microemulsion formation has been studied by varying the alcohol from l-butanol to l-pentanol and l-hexanol in sodium oleate-alcohol-isooctane-water systems. The results are shown in Figure 3, where I, is plotted as a function of the water contents. Microemulsion formation occurs at the R M point of 0.5 in 1-butanol- or 1-pentanol-containing systems. Conversely, in 1-hexanol-containing dispersion, microemulsion formation does not occur since Iz is seen to reach the first maximum outside the clear-range ratio of these systems (Figure 3).20 The R M value decreases with increasing the chain length of the alcohol in sodium oleatealcohol-hexadecane-water systems for 1-pentanol and 1-hexanol. Similar behavior has been observed in sodium stearate dispersions (vide supra). However, for isooctane-containing mixtures with 1-butanol or 1-pentanol microemulsion formation occurs at the same R M value (0.5);in contrast there is no micro-
emulsion formation in the 1-hexanol-containing system. Similarly, no microemulsion formation is seen in sodium oleate-1-butanol-hexadecane solutions. Lack of microemulsion formation cannot be attributed to unsaturation since sodium oleate and stearate systems behave similarly in the presence of 1-butanol. Combination of short-chain alcohol with long-chain solvents (or vice versa: combination of long-chain alcohols with short-chain solvents) is generally expected to preclude microemulsion formation.23 In the sodium oleate-1-hexanol-isooctane-water system microemulsions do not form. If the concentration of sodium oleate is changed from 5.0 to 2.5 g in this mixture, microemulsion formation occurs at an R M point of 0.1. The same phenomenon is observed if the amount of alcohol is changed from 10.0 to 5.0 mL in the same dispersion at 5.0-g surfactant concentration at an RM point of 0.2 (Figure 4). In sodium stearate-alcohol-cyclohexane systems there is no influence of the hydrocarbon chain length of the alcohol on microemulsion formation while in sodium oleate-cyclohexane dispersions microemulsions are formed at the same R M point for butanol- and hexanol-containing solutions (0.4) and at a lower R M point for the pentanolcontaining dispersion (0.2). Influence of the Solvent. Influence of the solvent was investigated in sodium oleate systems by substituting isooctane by hexadecane or cyclohexane. Microemulsions are not seen to form in the sodium oleate-l-butanolhexadecane dispersion (Table 11).
Phase Behavior of Microemulsion Systems
In solutions containing cyclohexane, microemulsion formation occurs at an R M point of 0.4 in the presence of 1-butanol and 1-hexanol. In the pentanol-containing system microemulsions are formed at an R M point of 0.2. It seems that, except in the case of the 1-pentanol system, when cyclohexane is present in the microemulsion solutions the hydrocarbon chain length of the alcohol has no influence on the RM value in the sodium oleate system. Data for sodium stearate dispersions have been rationalized analogously. For sodium oleate-1-butanol or l-hexanolcyclohexane systems the R M value is higher (0.4) than that in the sodium stearatel-butanol or 1-hexanol-cyclohexane systems (0.2). Conversely, if 1-pentanol is the cosurfactant, microemulsion formation is the same in the saturated and unsaturated systems (0.2; Tables I and 11). In sodium oleate-1-pentanol-isooctane or hexadecanewater systems the R M values decrease with increasing hydrocarbon chain lengths of the solvent. In l-hexanolcontaining solutions microemulsion does not form in isooctane but it is formed in hexadecane at 5.0-g surfactant concentration at an R M value of 0.3. This trend is opposite to that observed in sodium stearate systems. Effect of Altering the Concentration of the Surfactant, The effect of altering the surfactant concentration in sodium oleate systems from 2.5 to 5.0 g has been assessed. In sodium oleate-pentanol-isooctane-water systems RM points occur at 0.2 and 0.5 at 2.5 and 5.0 g, respectively (Figure 5). Surfactant concentration influences microemulsion formation more drastically. In the hexanolcontaining systems, in the presence of 2.5 g of surfactant microemulsion formation occurs at an R M point of 0.1 whereas in the presence of 5.0 g of surfactant the R M point does not occur. Similar behavior has been observed when the amount of 1-hexanol was decreased from 10.0 to 5.0 mL ( R M = 0.2; Figure 4). III. Partitioning and Free Energy Transfers. Partitioning of the cosurfactant between the continuous phase and the microemulsion has been determined by titration.24,25A typical solution contains 2.5 or 5.0 g of surfactant, 2.5 mL of oil, and 25 mL of water. The initially turbid solutions were titrated to clarity with the alcohol. A minimum amount of oil was added to cause turbidity. Clarity was reestablished by retitrating with the alcohol. This procedure was repeated several times until a sufficient number of points were obtained in the plots of moles of alcohol/mole of surfactant vs. moles of oil/mole of surfactant. Intercepts ( b ) of these plotted values of Table I11 were taken to be the number of moles of alcohol at the interphase/mole of surfactant. Slopes (m)of the plotted values of Table I11 gave the solubility of the alcohol in the continuous phase. Values are given in Table I11 and in the corresponding diagrams shown in Figure 6, a and b. From the obtained values, the free energy per mole of alcohol absorption into the interphase from the continuous phase was calculated by using the formula AG = -RT In l(na/nsh/(na/nS)sl where (na/ns)I and (n,/& are the mole fractions of the alcohol in the interphase and the continuous phase, respecti~ely.~~ Composition of the solutions (in weight percent), studied by the positron annihilation technique, is indicated by open circles in the phase diagrams in Figure 6, a and b.26127 (24)Schulman, H.J.; Stoeckenius, W.; Prince, L. M. J . Phys. Chem. 1969,11, 169. (25)Gerbacia, W.;Rosano, H. L. J. Colloid. Interface Sci. 1973,44, 242.
The Journal of Physical Chemistty, Voi. 87, No. 4, 1983 713
I2 %
I2 v s WATER CONTENT IN SURFACTANTHEXANOL-ISOOCTANE
SOLUTIONS
I CLEAR II TURBID 22
21
-
0 )
SODIUM STEARATE 5.0 g
I
23
22
21
II
I ,
4
, 04
08
12
w/o
Flgure 8. I, vs. water contents in surfactant-l-hexanol-isooctanewater systems at room temperature.
These points have the same coordinates in the alcohol-oil axis due to the similarities of molecular weights of the surfactants (sodium stearate and oleate) and those of the densities of alcohols). The sodium stearate-l-hexanolisooctane system was reinvestigated by the positron annihilation technique by changing the amount of alcohol in the initial composition of the solution (Figures 6a and 7). The lower limit of the microemulsion range in the oil-water + surfactant axis in Figure 6a corresponds to the first maximum in the curve of I 2 vs. w/o ratios in the positron annihilation experiments (Figures 1-4; Tables I and 11). The upper limit of the microemulsion range corresponds to the upper limit of the clear range of the solutions determined by turbidimetry. The partitionings in the interphase shown in the water + surfactant-alcohol axis in Figure 6, a and b, were obtained by titration (see (na/nJ1 values in Table 111). The composition of each solution, obtained by alcohol titrations, is indicated for the different alcohols used. Since the molar ratio, (na/& is different for each solution, they do not extrapolate to the origin. In sodium stearate-isooctane systems (na/ns)I values decrease with increasing chain length of the alcohol (Figure 6a); however, for the 1-pentanol system they increase (Table 111). The opposite situation exists for the same system in cyclohexane for all the alcohols studied except for 1-pentanol (Table 111). In the sodium oleakisooctane system at 2.5-g surfactant concentration the partitionings for 1-butanol- and l-pen(26)Prince, L. M.In ref 4g,pp 147-8. (27)Frieberg, S. In ref 4g,pp 131-41.
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The Journal of Physical Chemistty, Vol. 87, No. 4, 1983
tanol-containing dispersions are similar, whereas for 1hexanol the (na/rQ1value is much higher. In the sodium oleate-alcohol-cyclohexane system the partitioning increases as the hydrocarbon chain length of the alcohol increases from 1-butanol to 1-hexanol;however, in l-pentanol the (na/r&value is lower than those in 1-butanol and 1-hexanol but it is quite similar to that in 1-butanol. Apparently the behavior of sodium oleate in cyclohexane values in sodium is the same as that in isooctane. (na/ns)I oleate-isooctane systems are higher than in cyclohexane dispersions. The partitioning of the cosurfactants in sodium stearate-isooctane systems is lower than that in cyclohexane except for pentanol. Partitioning of alcohols in sodium oleate-1-butanol or 1-pentanol-isooctane or cyclohexane systems is smaller than those of sodium stearate systems containing the same solvents and cosurfactants. Conversely, for 1-hexanol dispersions the partition values are much higher in sodium oleate systems than in sodium stearate solutions in both isooctane and cyclohexane. IV. Dynamic Laser Light Scattering. Dynamic laser light scattering has been utilized for substantiating the phase transitions determined in the different microemulsion systems by positron annihilation. Diameters for microemulsions range from 500 to 2000 A (3-methyl-l-butanol-isooctane),from 1347 to 1656 A (1-pentanol-hexadecane) for sodium stearate systems, and from 780 to 550 A for the sodium oleate-l-pentanol-hexadecane solution (Figure 7). These diameters correspond to values generally attributed to microemulsions. Significantly, no diameters smaller than 10200 A were obtained in the sodium oleate-hexanol-isooctane system
(Figure 7). Positron annihilation techniques indicated, of course, no microemulsions here. This once again substantiates the power of positron annihilation techniques for determining subtle phase transitions. V. Effect of Unsaturation on Microemulsion Formation. Differences in behavior of saturated and unsaturated surfactants can be rationalized by considering packing and kink presence in microemulsion formation (compare Figures 1-4; Tables I and II).4h32s Assuming one kink formation for a surfactant with a hydrocabron chain length of 18 carbon atoms and one more for the presence of the double bond leads to volumes of 2(25-50) and 3(25-50) A3 occupied by sodium stearate and sodium oleate, respectively. Sodium oleate occupies, therefore, a larger volume than its saturated analogue. This larger volume may well prevent packing the long-chain cosurfactant 1-hexanolinto the microemulsion assembly. Inability of packing the cosurfactants precludes microemulsion formation. Conversely, shorter-chain cosurfactants butanol and pentanol pack easier and can, therefore, form microemulsions (Figure 8).
Acknowledgment. We express our gratitude to Dr. Hans J. Ache for his valuable advice and the discussion of this paper. Registry No. Sodium stearate, 822-16-2; sodium oleate, 143-19-1; isooctane, 2663564-3; hexadecane,544-76-3; cyclohexane, 110-82-7; 1-butanol, 71-36-3; 1-pentanol, 71-41-0; 1-hexanol, 111-27-3. (28) Lagaly, C. Angew. Chem., Int. E d . En&. 1976, 15, 575. (29) Fendler, J. H.In ref 4h.
