J. Phys. Chem. 1981, 85, 1693-1697
1893
Association in Anionic and Cationic Dispersion Systems Studied by Positron Annihilation Technique All Boussaha and Hans J. A c h e * Depallment of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 I (Received: November 17, 1980; In Final Form: February 25, 198 1)
Positron annihilation technique was used to determine the structure of dispersions containing anionic and cationic surfactants. The influence of electrolyte, nature of cosurfactant, and solvent on the onset of association was studied in anionic (sodium or potassium oleate) surfactant systems. The results show that within the isotropic reversed micellar region different association structures can exist. Data for cetyltrimethylammonium bromide-hexanol-water indicate, consistent with a previous study, that micelle formation occurs only in solutions containing less than 70% (w/w) of hexanol and requires a certain amount of water. No influence of the addition of aromatic solvent on the onset of association is observed in this (cationic) system in comparison to microemulsion systems containing anionic surfactant, where a strong effect was noted if the aliphatic solvent was replaced by an aromatic solvent.
Introduction Previous studies have demonstrated the potential of positron annihilation technique for the investigation of structural changes in liquid crystals,l micelles,2-10and microemulsions systems.11-13 In recent investigationll we observed that increasing amounts of water accomodated in spherical micelles lead to a drastic decrease of the positronium formation probability. This finding has been used to determine the onset of association in microemulsion systems. The extreme sensitivity of the technique to microemulsion formation when compared to other conventional methods such as electrical resistance, highresolution NMR, dielectric relaxation, etc., was demonstrated in previous paper.11J2 In this paper, we report on further studies directed toward an evaluation of the influence of some parameters such as electrolyte, nature of hydrocarbon, and cosurfactant on the formation of microemulsions containing anionic surfactants. Also, we present the first results concerning the study of microemulsions containing a cationic surfactant. Experimental Section Purity of Compounds. Sodium and potassium oleate purchased from ICN Inc. were purified by recrystallization in ethanol, and the products were washed with diethyl ether and dried in a vacuum desiccator. Cetyltrimethylammonium bromide (CTAB) from Fischer Scientific Co. was recrystallized in methanol and dehydrated in a desiccator under vacuum. Benzene, hexadecane, and alcohols (cyclopentanol, 1-hexanol, 1-pentanol, 2-pentanol, and 3-pentanol) were spectroscopic grade and appropriately (1) Nicholas, J. B.; Ache, H. J. J. Chem. Phys. 1972,57,1597. (2)Jean, Y. C.; Ache, H. J. J. Am. Chem. SOC.1978,100, 984. (3) Jean, Y. C.; Ache, H. J. J. Am. Chem. SOC.1978,100,6320. (4) Fucugauchi, L. A.; Djermouni, B.; Handel, E. D.; Ache, H. J. J. Am. Chem. SOC.1979,101,2841. (5) Djermouni, B.; Ache, H. J. J.Phys. Chem. 1979,83,2476. (6) Ache, H. J. Adu. Chem. Ser. 1979,No. 175, 1-49. (7) Jean, Y. C.; Djermouni, B.; Ache, H. J. In “Solution Chemistry of Surfactants”; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. I, pp 129-52. (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) Boussaha, A.; Djermouni, B.; Fucugauchi, L. A.; Ache, H. J. J. Am. Chem. SOC.1980,102,4654. (12) Boussaha, A.; Ache, H. J. J . Colloid Interface Sci. 1980,78,257. (13) Boussaha, A.; Ache, H. J. J . Phys. Chem. 1980,84,3249.
0022-3654/81/2085-1693$0 1.25/0
dehydrated. Triple-distilled water was used. Positron Lifetime Measurements and Preparation of Samples. Positron lifetime measurements were carried out by the usual delayed coincidence method and resolved as previously described14J5into two components: a short-lived component, which is the result of p-Ps annihilation, freepositron annihilation, and epithermal Ps interactions; and the long-lived component, with a decay constant X2, which originates from the reactions and subsequent annihilation of thermalized 0-Ps. 12,the intensity associated with h2, is related to the number of thermalized positronium atoms formed. The resolution of the system, as measured by the prompt time distribution of 6oCo source and without changing the 1.27- and 0.511-MeV bias, was found to be 0.390 nm fwhm. Corrections for the source component, which had an intensity of less than 4%,were made in the usual way by using conventional computational methods. Specially designed cylindrical sample vials (Pyrex glass 100 mm long and 10-mm i.d.) were filled with -2 mL of the sample solution. The positron sources consisted of 5-20 pCi of 22Na. They were 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 vials were degassed and subsequently sealed off and counted at room temperature.
Results and Discussion Anionic Surfactant Systems. General Aspects of Positronium Interactions in Anionic Surfactant Systems. There has been considerable discussion in the literature about the exact nature of anionic microemulsions. Schulman’s interfacial approachlG20led to the conclusion as recently presented by P r i n ~ e lthat ~ l ~water-in-oil ~ microemulsions are structurally different from reversed micelles, whereas Friberg and others21-28suggest that these (14) Williams, T. L.; Ache, H. J. J . Chem. Phys. 1969,50,4493. (15) Madia, W. J.; Nichols, A. L.; Ache, H. J. J . Am. Chem. SOC.1975, 97,5041. (16) Stockenius, W.; Schulman, J. H.; Prince, L. M. Kolloid-2. 1960, 169,170. (17) Schulman, J. H.; Montagne, J. B. Ann. N. Y. Acad. Sci. 1961,92, 3661. (18) Prince, L. M. J . Colloid Interface Sci. 1967,23,165. (19) Prince, L. M. J . Colloid Interface Sci. 1975,52,182. (20) Prince, L. M. In “Microemulsions”; Prince, L. M., Ed.; Academic Press: New York, 1977; pp 91-131.
0 1981 American Chemical Society
1694
The Journal of Physical Chemistry, Vol. 85, No. 72, 1981
Boussaha and Ache
------7
I2ws WATER IN
I-
CONTENT POTASSIUM - 0 L E A T E PENTANOL SOLUTIONS
I
12
W S . WATER CONTENT IN POTASSIUM-OLEATEI - PENTANOL- I M N a C l SOLUTIONS
E X P T L . ERROR
I
vr
19
12(%) 18
* 17 0
0.2 0.4 H20 W ) H20+C50H *KO1 ( Iw
microemulsions should be considered as reversed micellar solutions, pointing out their colloidal nature. In previous investigations,11J3we have been able to establish that the formation of (thermalized) positronium atoms in reversed micellar solutions such as AOT (sodium di-Zethylhexyl sulfosuccinate) in apolar solvents is strongly reduced by the presence of micelles and is also related to the water contents of the spherical reverse micelles; when the amount of water increases, the intensity Iz which is related to the number of thermalized positronium atoms formed decreases drastically. The reduction in the number of thermal 0-Ps atoms formed in these solutions is attributed to an efficient trapping of energetic positrons and/or energetic positronium atoms by micelles. In four-component systems (surfactant-alcohol-hydrocarbon-water) the same behavior of Iz was found to be indicative of the formation of spherical aggregates in these microemulsion systems. Before association takes place at a certain minimum amount of water, the intensity I2 increases on increasing water contents. This variation is opposite of that expected for micelle formation and is probably correlated with the cosolubilitzation state of the system at low water contents. The differences in variations of Iz before and when microemulsion is formed thus appear to be a sensitive means to determine the onset of aggregation in microemulsion systems.11J2 Micellization Process in the Potassium Oleate-1-Pentanol-Water System. In a previous study we could confirm by positron annihilation techniquell that microemulsion formation in the four-component system potassium oleate-alcohol-hexadecane-water requires a certain minimum water content. Considering this four-component system as an extension of the inverse micellar potassium oleate-alcohol-water solution with hydrocarbon added, one could expect similarities in the micellization process in the three-component (surfactant-alcohol-water) and the ____________~
~~
~~
(21) Adamson, A. W. J . Colloid Interface Sci. 1969,29,261. (22) Gillberg, G.; Lehtinen, H.; Friberg, S. J . Colloid Interface Sci. 1970,33,40. (23)Shinoda, K.; Kuneida, H. J. Colloid Interface Sci. 1973,42,381. (24) Ahmad, S. I.; Shinoda, K.; Friberg, S. J. Colloid Interface Sci. 1974,47,32. (25) Friberg, S. In “Microemulsions”; Prince, L. M., Ed.; Academic Press: New York, 1977;pp 133-46. (26) Rance, D. G.; Friberg, S. J. Colloid Interface Sci. 1977,60,207. (27) Sjoblom, E.; Friberg, S. J. Colloid Interface Sci. 1978,67, 16. (28)Friberg, S.;Burasczenska, I. Prog. Colloids Polym. 1978,63,1.
c
Clear
I
I
1
02
04
06
0
Flgure 1. I2 vs. water contents in potassium oleate-1-pentanol solutions (alcohol/soap = 4.9 (w/w); same conditlons as series I1 in ref 27).
E X P T L ERROF
H2O
+
H2O ( */W CSOH + KO1
Figure 2. I2 vs. water contents in potassium oleate-1-pentanol-1 NaCl solutions (alcohol/soap = 0.90 (w/w)).
M
four-component (surfactant-alcohol-water-hydrocarbon) systems. We, therefore, performed positron annihilation measurements on potassium oleate-1-pentanol-water systems (alcohol/soap = 4.9 (w/w)). The results (Figure 1)show that, up to 20% (w/w) of water, the intensity of the long-lived component I2 in the positron lifetime spectra which is indicative of the number of thermalized 0-Ps formed is constant and then increases till 30% (w/w) of water. In accordance with previous results,11J2we suggest that, up to 30% of water, the system is molecularly dispersed; i.e., surfactant molecules are in the form of small aggregates, presumably ion pairs. The increase of I2 between 20% and 30% (w/w) of water can be attributed to a gradual association of water molecules in the premicellar aggregates. These aggregates seem to have a polarity different from that of inverse micelles which have a high capability of trapping the precursors of Ps. Above 30% of water the decrease of I2 is indicative of the formation of spherical reversed micelles. In a recent studyll we found that, when a fourth component, hexadecane, is added, micelle formation occurred at -23.5% (w/w) of water. As the present data for the three-component system show, the presence or absence of an aliphatic hydrocarbon seems to affect only slightly the association concentration in accordance with light-scattering measurement^.^' The results clearly indicate that the formation of positronium is affected by the same parameters in (three-component) reverse micellar systems and (four-component) microemulsions of anionic surfactants. Effect o f the Presence of an Electrolyte (NaC1)on the Association Process in the Potassium Oleate-l-Pentanol-Water System. In order to assess the effect of electrolyte on the association process, we performed measurements on potassium oleate-l-pentanol-water-NaC1 systems. Phase-diagram s t u d i e ~ ~ showed & ~ ~ that the addition of electrolyte increased the amount of water content necessary to obtain solubility and that the maximum water solubility was increased. In Figure 2 we report the results of measurements in potassium oleate-1-pentanol-1 M NaCl solutions (alcohol/soap = 0.90 (w/w). The plot of I2 as a function of water contents shows a decrease of I2 from 25% to 35% (w/w) of water; beyond 35% of water (29)Friberg, S.;Burasczenska, I.; Sjoblom, E. Adu. Chem. Ser. 1979, NO.177,205-14. (30)Friberg, S.;Burasczenska, I. Micellization, Solubilization, Microemulsions, [Proc.Int. Symp.], 1976 1977,2, 791-9.
Anionic and Cationic Dispersion Systems
The Journal of Physical Chemistry, Vol. 85,No. 12, 1987 12 v s WATER CONTENT IN SODIUM-OLEATE-2-PENTANOLHYDROCARBON SOLUTIONS
I, vs WATER CONTENT IN SODIUMOLEATE - I - P E N T A N O L - HYDROCARBON SOLUTIONS
1
IS1
dn
Ll
I
Ip (%I
T EXPTL. ERROR
I
I
I
19
0 HEXADECANE
Separation
0 BENZENE
I
' O h
Clear
I I
I
0
0 2
0
U
06
05
WATER / O I L
(7")
04
1695
I O
Figure 3. I2 vs. water contents in sodium oleate-l-pentanol-hexadecane and sodium oleate- 1-pentanol-benzene solutions (composition of solution: 1 g of sodium oleate, 2 mL of 1-pentanol, and 5 mL of hydrocarbon).
I2 shows a significant increase. Contrary to the solution with pure water, solubilization cannot be achieved up to -25% (w/w) of water, and it can be seen that reversed micelles are formed with the first added water needed for solubilization. The shift in solubilization can be related to the nonformation of ion pairs in the range 0-25% (w/w) of water. The ion pairs require for their formation a minimum of water (- 10% for electrolyte-free solution) which must be strongly bound to the polar groups of the s ~ r f a c t a n t When . ~ ~ an electrolyte is added, a dehydration of surfactant polar groups and in the limiting case a disappearance of ion pairs occurs. The further increase of I2 at 35% (w/w) of water was previously observed in other systems and attributed to an elongation of the reversed micelles.llJ3 E f f e c t of the Nature of the Cosurfactant on the Association Process in t h e Sodium Oleate-CosurfactantHydrocarbon (Benzene or Hexadecane)-Water System. In another series of experiments, we investigated the effects of the nature of cosurfactant and hydrocarbon on the formation of microemulsion in four-component systems containing 1-pentanol, 2-pentanol, 3-pentanol, or cyclopentanol as cosurfactant, and benzene or hexadecane as solvent, the surfactant being sodium oleate. The composition of the system was the following in each case: 1 g of sodium oleate, 2 mL of the alcohol, and 5 mL of hydrocarbon. For the 1-pentanol system (Figure 3), the solubilization of water is greater for hexadecane than for benzene. As can be seen from Figure 3, for the hexadecane-containing system, I2plotted as a function of water contents increases from a water/oil ratio of 0.1 to 0.35 (v/v), decreases from a ratio of 0.35 to 0.6 (v/v), and then remains constant from a ratio of 0.6 to 0.9 (v/v) after which point phase separation occurs. Using the same interpretation of the positron annihilation data as discussed above, one would conclude that, in hexadecane solutions, up to a water/oil ratio of 0.35 the system is molecularly dispersed, between 0.35 and 0.6 spherical reversed micelles exist, and above a ratio of 0.6 a transition to anisotropic structure occurs. This
I
02
I
I
04
06
WATERIOIL
('&I
Flgure 4. I2 vs. water contents in sodium oleate-2-pentanol-hexadecane and sodium oleate-2-pentanol-benzene solutions (cornpositlon of solution: 1 g of sodium oleate, 2 mL of 2-pentano1, and 5 mL of hydrocarbon).
structure may be considered as water cylinder^.^' In benzene solutions (Figure 3) it can be seen that the water initially added to the system leads to a significant drop of I2followed by an increase beyond a water/oil ratio of 0.25. The data indicate that spherical micelles are formed with the added water, and elongation of the reversed micelles is detected at a water/oil ratio of -0.25, which corresponds to 15% (w/w) of water. It is important to notice that, as for the hexadecane system, the transition is observed within the solubility region L2. This observation suggests that, in an isotropic region, small aggregates, spherical reversed micelles, and elongated micelles can exist. The subdivision of isotropic regions into different association structures seems to exist in L2as well as in normal micellar L1 region^.^^,^^^^ In further experiments, the effects of 2-pentanol, 3pentanol, and cyclopentanol were investigated. The resulis of solubilization and positron annihilation measurements are reported in Figures 4-6. Compared to 1-pentanol, 2-pentanol and 3-pentanol are poor cosurfactants for both solvents, while the water solubilization capacity of microemulsions prepared with cyclopentanol is low for the hexadecane system and large for the benzene system. In hexadecane solutions containing either one of the three alcohols, the increase of I , which occurs before phase separation seems to indicate that no micelles are formed and the clear solutions are molecular dispersions containing small aggregates in which the added water is gradually associated with the polar groups of the surfactant molecules. The onset of this association occurs in all three cases at a 0.1 water/oil ratio, which corresponds to -8 water molecules per molecule of surfactant, and phase separation occurs at a 0.25-0.30 water/oil ratio, corresponding to 20-25 molecules of water per molecule of surfactant. It is important to point out that in sodium oleate-l-pentanol-hexadecane-water, 25 molecules of water per sodium
-
(31) Ekwall, P.; Danielsson, I.; Stenius, P. M T P Int. Reu. Sci. Phys. Chem., Ser. One 1972, 7, 97-145. (32) Smith, H. D.; Templeton, S. A. J. Colloid Interface Sci. 1979,68, 59. (33) Boussaha, A.; Ache, H. J. J. Phys. Chem., in press.
1698
The Journal of Physical Chemistry, Vol. 85,No. 12, 198 1
WATER CONTENT SODIUM -OLEATE - 3 -PENTANOLHYDROCARBON SOLUTIONS 12 vs
I EXPTL ERROR I Clear
II 19
Phase Separation
d 0 HEXADECANE
0
02
04
1
06
WATER / O I L (v/v)
Figure 5. 1, vs. water contents in sodium oleate-3-pentanol-hexadecane and sodium oleate-3-pentanol-benzene solutions (composition of solution: 1 g of sodium oleate, 2 mL of 3-pentanol, and 5 mL of hydrocarbon).
-
I, vs WATER CONTENT IN SODIUM OLEATE CYCLOPENTANOL-HYDROCARBON SOLUTION
-
20
19
I8
12(%)
t . ) *
I
II
*
20
0 0
HEXADECANE BENZENE
I
Clear Phose separation
II
19
18
I
0
02
04
06
08
IO
12
WATER /OIL (v/v)
Flgure 6. I , vs. water contents in sodium oleate-cyclopentanolhexadecane and sodium oleate-cyclopentanol-benzene solutions (composition of solution: 1 g of sodium oleate, 2 mL of cyclopentanol, and 5 mL of hydrocarbon).
oleate molecule are required for the formation of reversed micelles. This indicated that the properties of the solutions before formation of reversed micelles are independent of the nature of alcohol when the solvent is hexadecane and confirms the existence of ion pairs in the range of solubilization. The water solubilization capacity for benzene solutions changes in the order cyclopentanol > 1-pentanol > 2pentanol > 3-pentanol. Bowcott and Schulmana observed that for potassium oleate-alcohol-benzene-water solutions, the capacity of water solubilization varied in the order 1-pentanol > 1-hexanol > 1-heptanol. One could assume that from a practical point of view, 2-pentanol and 3pentanol produce the effects of long chain length cosur(34) Bowcott, J. E. L.; Schulman, J. H. 2.Elektrochern. 1955,59,283.
Boussaha and Ache
factants when compared to 1-pentanol. The plots of I2 as a function of the amount of added water in benzene solutions containing either l-pentanol, 2-pentanol, or cyclopentanol (Figures 3,4, and 6) show that, after the decrease in 12,corresponding to the formation of spherical reversed micelles at low water content, a pronounced increase occurs at a water/oil ratio of 0.25, 0.25 and 0.15 (v/v), respectively. This indicates that deformation of micelles appears at lower water concentration for cyclopentanol than for 1-pentanol and 2-pentanol. No deformation of micelles is detected for 3-pentanol-containing microemulsion. It has been observed for this microemulsion system that, when excess water was added, a lamellar liquid crystal appeared at a water/oil ratio of 0.35, whereas with cyclopentanol, 1-pentanol and 2-pentanol separated phases occurred. Similar phase behavior of microemulsions upon addition of excess water beyond the solubilization limit was recently found to depend on the nature of oil, surfactant, and cosurfactant by Shah and co-workers.% These authors interpreted solubilization on the basis of a chain length compatibility effect which produces a disorder of the terminal segment of the alkyl chain. Compared to 2-pentanol, 3-pentanol would sterically prevent more the growth of water droplets of reversed micelles; this would lead to less disorder at the interface and relatively to a larger solubilization range. The actual solubilization capacities of the two alcohols are certainly due to the difference of partitioning of 2-pentanol and 3-pentanol among water, benzene, and the interface considering the difference in solubility of these alcohols. The large solubilization capacity of water by benzene microemulsions prepared with cyclopentanol as cosurfactant cannot be attributed solely to the ring structure of cyclopentanol%since in the hexadecane system no micelles are formed upon addition of cyclopentanol, but to an interaction between benzene and alcohol molecules at the interface. Micelles tend to be elongated when ca. eight molecules of water per surfactant molecule are added, and the stability of the anisotropic micelles can be understood on the basis of a participation of benzene at the interface which, with its hydrophobic character, sterically prevents the growth of the microemulsion droplets. Cationic Surfactant Systems. (AssociationProcess in the Cetyltrimethylammonium Bromide-Hexanol- Water System with and without Benzene Present). The system cetyltrimethylammoniumbromide (CTAB)-hexanol-water has been investigated by viscosity and density measurements and X-ray d i f f r a ~ t i o n . ~At ~ a constant CTAB/ hexanol ratio of 0.8 (w/w), the viscosity decreased linearly up to -30% (w/w) of water and then increased. The Bragg spacing increased with the water content. The increase of the Bragg spacing at low water content was attributed to an increase of the size of reversed micelles. Lindblom et al.37studied the counterion binding by using nuclear magnetic relaxation of *lBr in the same system and concluded that micelles are not formed when the alcohol content is more than 70% (w/w). Kinetics of catalysis in the same system was studied by Friberg and Ahmad.38 These authors confirmed that, at concentrations of hexanol above 70% (w/w), no micelles were formed when water WM added and that, for a CTAB/hexanol ratio of 0.73 (w/w), (35) Bansal, V. K.; Shah, D. 0.;O’Connell,J. P. J. Colloid Interface Sei. 1980, 75, 462. (36) Ekwall.. P.:. Mandell, L.: Solvom. P. J. Colloid Interface Sci. 1971. 35, 266. (37) Lindlom, G.; Lindman, B.; Mandell, L. J. Colloid Interface Sci. 1970, 34, 262. (38) Friberg, S.; Ahmad, S. I. J. Phys. Chern. 1971, 75, 2001.
The Journal of Physical Chemistty, Vol. 85, No. 12, 198 1
Anionic and Cationic Dispersion Systems
a t least up to 27.5% (w/w). Addition of 20% (w/w) of benzene to the system does not lead to the formation of reversed micelles either, as indicated by the constant I2 value up to 15% (w/w) water (Figure 7). The addition of benzene only reduces the solubilization range of water. This is contrary to microemulsions containing anionic surfactant. The findings suggest that the interaction of aromatic solvent as proposed by FribergZ7depends on the nature of the surfactant, anionic or cationic. In another series containing less hexanol (CTAB/hexanol = 0.73 (w/w)) and corresponding to series 6 in Friberg’s paper,38Figure 7 shows that I2 decreases a t water contents higher than 32.5% (w/w), at which point micellization occurs. The increase of I2 between 25% and 32.5% (w/w) water may be due to a gradual association of water molecules in the ion pairs prior to micelle formation.
I p vs WATER CONTENT IN C T A B - I - HEXANOL - BENZENE SOLUTIONS EXPTL ERROR
I
24
A CTAB/Hemro 073(”1,) Benzene 0% e CTAB/Hexanol 0 4 3 (W/w) Benzene 0% 0 CTAB/Hemnol 0 4 3 (w/w) Benzene 20% ( w / ~ l (CTAB *Hexand BO%(W/wl
0
10
20
30
40
1697
50
% WATER (w/w)
Figure 7. I2 vs. water contents in CTAB-hexanol-benzene solutions. Composition of solutions: (A)CTAB/hexanol = 0.73 (w/w), 0% benzene, same condition as series 6 in ref 38; ( 0 )CTAB/hexanol = 0.43 (wlw), 0% benzene; (0)CTAB/hexanol = 0.43 (w/w), 20% (w/w) of benzene added.
-
the rate constant of hydrolysis of p-nitrophenyl laurate showed a sudden increase followed by a maximum a t 20% (w/w) of water and by a further increase a t 30% (w/w) of water. The sudden increase of the rate constant was attributed to the formation of micelles. Positron annihilation measurements were carried out in CTAB-hexanol-water solutions in order to determine the onset of micellization in this system and to clarify this issue. Also, the addition of benzene to the system was investigated to determine the influence of aromatic solvent on the onset of association. The results are shown in Figure 7, where I2is plotted as a function of water contents. For the solutions with a constant CTAB/hexanol ratio of 0.43 (w/w), I, is constant from 5% up to 27.5% (w/w) water and increases just before phase separation occurs a t 30% (w/w). In accordance with our previous results, we can assume that no micelles are formed for this series
Summary The results of the experiments presented in this paper and in previous communications2-13clearly demonstrate the capability of the positron annihilation technique to study the micellization process and to determine structural changes in micellar and microemulsions systems. The results accumulated so far indicate that formation of spherical reversed micelles leads to a reduction of the probability of formation of 0-Ps whatever the nature of surfactant, anionic, nonionic, or cationic. The investigation of the model micellar system AOT-901vent-water~~ showed that the variation of I2 is quite similar to the variations of the internal rotational relaxation time measured in fluorescence polarization techniques- and the rotational correlation time of water molecules determined by NMR r e l a ~ a t i o n . ~Since ~ these two parameters measure the microviscosity of the solubilized water core, we believe that trapping of the precursors of 0-Ps by the water pool is an important factor to consider in order to explain the reduction of the number of o-Ps atoms formed. (39) Boussaha, A,; Ache, H. J., unpublished results. (40) Wong, M.; Thomas, J. K.; Gratzel, M. J . Am. Chem. SOC.1976, 98, 2391. (41) Valeur, B.; Keh, E. J. Phys. Chem. 1979,83, 3307. (42) Zinsli, P. E. J. Phys. Chem. 1979, 83, 3223. (43) Eicke, H. F.; Zinsli, P. E. J. Colloid Interface Sci. 1978,65, 131. (44) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. SOC.1977,99, 4730.
