J. Phys. Chem. 1980, 84, 3249-3254 (16) R. L. Wllliams and F. S. Rowland, J. Phys. Chem., 76, 3509 (1972). (17) F. S.Rowland, F. Rust, and J. P. Frank In “FluorlneContalningFree Radicals”,J. W. Root, Ed., American Chemical Society, Washington, D.C., 1978, p 26. (18) J. M. Ferrar and Y. T. Lee, J . Chem. Phys., 65, 1414 (1976).
3249
(19) H. D. Roth, J. Am. Chem. Soc., 94, 1761 (1972). (20) 0. L. Closs in “Carbenes”, Vol. 11, R. A. Moss and M. Jones, Jr., Eds., Wlley, New York, 1975, p 159. (21) “Reaction IRate and Photochemical Data for Atmospheric Chemistry-l977”, NBS Spec. Pub/. (U.S.),No. 513 (1978).
Structural Changes in Micellar Solutions and Microemulsions and Their Relevance to Emulsiont Counting All Boussaha and Hans J. Ache* Department of Chemistry, Virginia Polytechnic Institute and State Unlverslty, Biacksburg, Virginia 2406 1 (Received: May 16, 1980)
@-countingefficiencies or the decrease of the amplitude of the signals originating from the interaction of low energetic @ particles with typical emulsion counting solutions as mieasured by a conventional liquid scintillation spectrometer reflect very sensitively structural changes occurring in these solutions upon addition of water or aqueous samples. It appears that the capability of the species present in the solutions, reverse micelles, microemulsion droplets, etc., to trap electrons determines the size of the output signal and thus the counting efficiency to a significant extent. The positronium formation process shows a similar behavior and seems to be affected by the same parameters. Both processes provide very sensitive probes for the study of structural changes in micellar solutions or microemulsions.
Introduction Emulsion counting utilizing toluene or other organic solvent based counting solutions which can accomodate substantial (quantitiesof aqueous samples has become an important technique in liquid scintillation spectroscopy.l This technique is based on the fact that in the presence of surfactants (and cosurfactants) the aqueous sample is solubilized iin reversed micelles, microemulsions, or similar species: which allows the measurement of the radioactivity under very reproducible conditions. While thiB technique has found wide practical application, still relatively little is known about the structure of these counting solutions, their structural changes with increasing amounts of water present, and the interactions of the p particles, Le., energetic or thermalized electrons with the micellar or microemulsions droplets in the solutions, and thus their effect on the observed counting efficiencies. Recently3-10we have investigated the fate of the positron, e+, which is the antiparticle of the negatron, e-, and found that not only a change in the size of the micelles but also the structure, composition, etc., of the aggregates forming microemulsions have a definite effect on the fate of the positron. One impoirtant feature of the positron interaction with matter is the formation of the positronium, which is the bound state of an electron and a positron.6 We have found that the presence of various types of surfactant aggregates, such as miselles, leads to a reduction of (thermalized) positronium formation and suggested that trapping of energetic positrons (or positronium atoms possessing excess kinetic energies) by the surfactant aggregates is responsible for the reduced formation of thermalized positronium. Because of ithe similarity of the species, positron and electron, particle and antiparticle, one could postulate that the behavior of the positron in liquid scintillator solutions would resemble very much that of the /3 particle (e-). Thus an attempt was made to correlate the phenomena which affect the liquid scintillation counting efficiency determined as a function of the composition of the solution
with structural changes occurring in the solution and thus to obtain a clearer picture of the processes responsible for changing the counting efficiency of weak /3 emitters.
Experimental Section Materials. F’otassium oleate from ICN Inc. was pharmaceutical grade. Triton X-100 purchased from Amersham Corp. was scintillation grade quality. Aerosol OT obtained (AOT) (sodiumi 2-ethyl-n-hexylsulfosuccinate) from Fischer Scientific Co., with a stated purity of loo%, was recrystallized by the procedure described in ref 11. PPO and dimethyl-POPOP (DMPOPOP) as well as [3H]H20 and [3H]toluene were obtained from Amersham Corp. Solvents such as benzene, toluene, cyclohexane, and alcohols were spectroscopic grade from Fischer Co. and appropriately dehydrated. Triple-distilled water was used. Preparation of the Micellar and Microemulsion Systems Studied. Experiments Designed to Determine the Effects of Water Additives on Counting Efficiency and Positron Lifetime Spectra. The solutions investigated were prepared Iby mixing solvent and surfactant (cosurfactant) in the indicated proportions, to which different amounts of water were added. AOT-Toluene. Stock solutions: 0.08, 0.565, and 1.08 M AOT in toluene. Potassium Oleate-Benzene. The four samples studied contained 1g of potassium oleate, 7.5 mL of benzene, and various amountti of hexanol. The molar ratios of water to surfactant were respectively 8.9, 16.0, 31.2, and 53.3. Triton-Toluene. Stock solutions: (a) Triton-toluene 3:7 (v/v), 0% alcohol. (b) Triton-toluene 3:7 (v/v), 93% (v/v); 1-propanol 7% (v/v). (c) Triton-toluene 3:7 (v/v), 80% (v/v); 1-propanol 20% (v/v). Triton-Cyclohexane-Pentanol. Stock solution of triton-pentanol41 (v/v) mixture, 20% (w/v) in cyclohexane. Experiments Designed to Determine the Effects of Triton Concentration on Counting Efficiency and Positron Lifetime Spectra. Solution of triton (3-160 mM) in toluene, 0-2% (v/v) of water was added.
0022-3654/80/2084-3249$0~i~00/0 0 1980 American Chemical Society
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No. 24,
1980
Liquid Scintillation Counting. The liquid scintillation spectrometry was performed on a Beckman LS-100 counter a t room temperature. Gain and windows were adjusted to provide maximum counting efficiency in unquenched samples. Each solution contained 7 g/L of PPO and 0.35 g/L of DMPOPOP. To each vial containing 10 mL of solution was added the desired amount of water followed by the addition of 10 pL of the radioactive material ([3H]Hz0 or [3H]toluene). Positron Lifetime Measurements. Positron lifetime measurements were carried out by the usual delayed coincidence method and resolved as previously described12J3 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, which originates from the reactions and subsequent annihilation of thermalized or nearly thermalized 0-Ps. 12,the intensity associated with X2, is related to the number of thermalized positronium atoms formed. The resolution of the system, as measured by the prompt time distribution of the 6oCosource and without changing the 1.27- and 0.511-MeV bias, was found to be 0.390 ns fwhm. Corrections for the source component, which has 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, consisting of 5-20 Ci of 22Na,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.
Boussaha and Ache
35 30 25 20 15
-s! >
y
0
k w
IO
20 15 0565M AOT
IO
5 (I
3
8
15 IO
I O 8 M AOT
5
0
30
40
Yo VOL. WATER
50
Figure 1. Tritium p counting efficiency in AOT-toluene solutions (with PPO and DMPOPOP fluor) vs. vol % H20 present and [H,O]/[AOT].
I2
VS
H20 AND RATIO IN TOLUENE SOLUTIONS % VOL
[H20] / [AOT]
Results and Discussion Correlation between Water Contents of Reversed AOT Micelles in Apolar Solvents, Liquid Scintillation Counting Efficiency, and Positron Annihilation Parameters. In a previous investigationg we were able to establish that the formation of (thermalized) positronium in reversed micellar solutions such as AOT in apolar solvents is related to the water contents of the reversed micelles. Thus in a first series of experiments the counting efficiency was determined as a function of the volume of H20 solubilized or the ratio [H20]/[AOT] in toluene solutions containing various amounts of AOT and [3H]Hz0or [3H]toluene as low-energy p sources with PPO and DMPOPOP fluors present. The results are shown in Figure 1, where the counting efficiency (in %) is plotted as a function of [HzO]/[AOT] and vol % HzO. Similar to the positron annihilation results, where Iz,the intensity of the long-lived component in the positron-lifetime spectra, is significantly reduced (for comparison see Figure 2), the counting efficiency of these solutions initially drops if the water pool in the reversed micelle is increased. It levels off at higher [H,O]/ [AOT] ratios, followed by another drastic reduction a t the point where the solution turns turbid and phase separation occurs. It should be pointed out that except in the case of very M AOT) the counting low AOT concentrations (8 X efficiency remains practically the same regardless of whether [3H]H20or [3H]tolueneprovides the source of P particles. A more detailed inspection of the decline of the counting efficiency also reveals that this efficiency drops sharply until a ratio [HzO]/[AOT] of -5-6 is reached, whereupon the efficiency levels off until, at a ratio of 11-12, another drastic decrease is observed. Five to six moles of water are needed for the hydration of the Na+ counterion. Evidence for this has previously been obtained by various
20
IO
-
AOT 20
19
"1 L 16
0
2
4 6 8 101214 % VOL. WATER
0
2
4
6
12
20
b 2 0 1 1W T I
Figure 2. I, (%) In AOT-toluene solutlon vs. vol % H,O present and [H,O] I [AOT]
.
rnethods,l4-lge.g., from the measurement of the partial specific volume of water solubilized in similar AOT solutions which remained relatively small until 5 mol of water per mole of AOT were added.16 This further indicates that this water is bound more energetically by the AOT molecules than water added beyond that concentration, or up to a ratio of 11-12, in which case it still does not show the properties of free water but is definitely less tightly bound than the first five or six molecules. The results also confirm previous findingsz0that the [H20]/[AOT]ratio, a t which phase separation occurs, is fairly independent of the AOT concentration, at least over the range 8 X 10-2-1.08 M AOT studied in these experiments. The latter findings suggest that the reversed micelles can solubilize a maximum of -11 molecules per AOT molecule present.
The‘Journal of Physical Chemistty, Vol. 84, No. 24, 1980 3251
Structural Changes in Micellar Solutions
TRITIUM BETA COUNTING EFFICIENCY IN BENZENE POTASSIUM -OLEATE- n-HEXANOL SOL-UTIONS vs THE RADIUS, rw ( A ) , OF THE SWOLLEN -MICELLES
RELATIVE COUNTRATES vs Yo VOL WATER AND [HzO] / [K-OLEATE] RATIO IN POTASSIUM-OLEATE n-HEXANOL - TOLUENE SOLUTIONS 0 3H-TOLUENE
’H - H ~ O
I
0 8
U
m
v)
w
c
CLEAR TURBID
PHASE SEPARATION
06 0:
It 3
8
04
c
5 4
8
no 20 rw 0
0 4
10
20
Yo VOL. WATER
30
40
6 12 20 32 50 60 70 [H,O] /[K-OLEA.rE]
0
Flgure 3. Relative tritium counting rates in toluene-potassium oleate-lhexanol solutions vs. vol % H20 present and [H,O]/[potassium oleate].
Another effect associated with the Eiolubilization of water is the increase of the size of the micelle with increasing water contents. Light-scattering measurements by Ekwall et al.16indicates that the particles increase in diameter with water contents, and, at least in excless of -5-6 mol per mole of AOT.,the aggregation numbers seem to increase. With these experimental facts available an attempt to explain the trends observed in the 0counting efficiencies and positronium formation will have to consider the changes caused by the presence of the water pool on the geometric factors, size or aggregation number, as well as on the physical properties of the “swollen” micelles, polarity, etc. It (appearsthat after 5-6 mol of water per mole of AOT are incorporated into the micelle, its capability for trapping positrons or electrons remains practically unchanged until at higher water concentrations phase separation occurs. Since, as discussed above, up to a [H,O]/ [AOT] ratio of 5 or 6 only very little change in the aggregation number and thus in the size of the micelle occurs while on the other hand counting efficiency and positronium formation show in this region their most dramatic change, it appears that the geometric factor at least in this range cannot be responsible for the observed behavior. We, therefore, suggest that the presence of the 5-6 mol of water per mole of AiOT changes sufficiently the polarity of the micelles to provide an efficient trap for electrons, as recently postulat,ed by Thomas et al.,19 and also for positrons. A quite similar behavior was observed in a series of experiments (Figure 3) where the 0counting efficiency was measured in reverse micellar solutions consisting of potassium oleate as surfactant, l-hexanol as cosurfactant, and toluene (PPO and DMPOPOP were the primary and secondary fluors and 13H]H20provided the 0 source) which clearly show a pronounced drop of the counting efficiency until the [H20]/[potassium oleate] ratio reaches -4-5 followed by a It?velingoff and subsequent drop when phase transitions occur. In this case the couriterion is potassium, and the results would indicate that the hydration is complete under these conditions with four or five H 2 0 molecules surrounding the potassium ion, which is slightly less
30
40
50
60
(A)
Figure 4. Tritium 0 counting efficiency in benzene-potassium oleate-1-hexanol solutions vs. the radius, r, (A), of the swollen micelles.
than in the case of sodium.21 Effect of the Size of the Micelle or Microemulsion Droplets on 0 Counting Efficiency. It is difficult to evaluate the effect of the size of the water droplet in the swollen micelles on the counting efficiency,since in most cases the change of the size, radius, etc., is accompanied by a change in the aggregation number. This means that at a given monomer concentration the number of micelles or microemulsion droplets changes accordingly; i.e., it increases if the aggregation number is getting smaller. Both factors, number of droplets as well as size of droplets, are expected to affect the counting efficiency and positronium formation. Thui3 the results obtained in a series of experiments where the 0counting efficiency was measured in reverse micellar solutions consisting of potassium oleate as surfadant, 1-hexanolas cosurfactant, and benzene (PPO and DMPOPOP were the primary and secondary fluors and [3H]H20provided the /3 source) which clearly show a correlation between the particle radiusz2and counting efficiency (Figure 4) are not quite conclusive. The same effect could have been caused by a change in the aggregation number. The drastic increase of the radius requires a simultaneous increase of monomers per micelle, which would at a given constant monomer concentration reduce the total number of micelles present in the solution. It seems, however, difficult to explain the reduced counting efficiency by the ]presenceof a smaller number of micelles in the solution, and one might want to relate the effect to the size of these species, which would mean micelles containing larger water droplets are more effective in trapping electrons. There is also a distinct difference depending on the nature of the 0source, whether the source is solubilized inside the micelle ([3H]H20])or dissolved in the bulk solvent ([3H]~luene).In the former case the resulting curve is significantly steeper than in the latter, indicating that the less energetic 0particles cannot escape the droplet and are therefore not contributing to the excitation of the solvent and ultimiately to the amplitude of the resulting signal from the photomultiplier. On the other hand the reduction of the efficiency when [3H]tolueneis the P source could be simply due to the trapping of electrons by the micelles, depending solely on the geometric cross section of the latter and their individual capability of trapping electrons. Since the trapping of these very small charged species, electrons or positrons, is probably the result of mainly electrostatic interaction of the charge of these particles
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The Journal of Physical Chemistry, Vol. 84, No. 24,
Boussaha and Ache
7980
TRITIUM BETA ( ~ H - H ~ o COUNTING ) EFFICIENCY vs 1 2 (%) IN REVERSE
MICELLAR
1
1 2 AND X p vs TRITON CONC. IN TOLUENE SOLUTIONS (NO H20 %)
SOLUTIONS
0
0.40
O%n-PROWC n-PROPANC
A 7%
U 20% n-PROPANC
12%
:
0
10
016
30
20
40
50
60
70 %TRITON
0 3 2 0 4 8 064 0 8 0 096 I12 (M1TRITON
Figure 6, I2 and X2 vs. Triton X-100 concentration in toluene solutions (no water added). n - H E X A N O L - WATER
4/
7t
6t 5
1, AND X p vs TRITON CONC. IN TOLUENE SOLUTIONS CONTAINING 2% VOL HO ,
d ) BENZENE-K.OLEATEn-HEXANOL- WATER
e ) CYCLOHEXANE-TRITON X-100- n-PENTANOL -
--
1T
I 0
I
I
16
17
18
19 20 21 12
4 030
I
WATER
I 20 4 0 60 m M TRlT'
ON
22 23 24
(04
counting efficiency vs. I2 % in reverse micellar solutions consisting of (a)toluene-Triton X-1 00-1-propanol-water, (b) toluene-AOT-water, (c) toluene-potassium oleate- I-hexanol-water, (d) benzene-potassium oleate-I-hexanol-water, and (e) cyclohexane-Triton X- 100- I-pentanol-water.
I
Figure 5. Tritium
I
I
I
I
I
I
20
30
40
50
60
70
%VOLTRlTON
016 0 3 2 0 4 8 0 6 4 0 8 0 096 I I2 (MiTRlTON
Figure 7. I, and A, vs. Triton X-100 (2 vol % H20 present).
with the micelles which possess different polarities from the bulk solvent, one might expect that the sign of the charge, positive or negative, may not greatly affect the trapping process. To test this contention we have measured the number of thermalized positronium formed in these solutions. From Figure 5a-e, where the counting efficiency is plotted as a function of the Iz values observed in a variety of reverse micellar solutions or microemulsions, it can be seen that such a correlation reflecting the similarity between positron and electron trapping indeed exists.
-In
I
[;;:$j #
concentration in toluene solutions
vs TRITON
CONC
11.0 M"
30
20
IO
/3 Counting Efficiencies and Positronium Formation in
Triton X-100-Alcohol System. In the third series of experiments we investigated the effect of the microstructure of the solution on positronium formation and /3 counting efficiency in toluene-Triton X-100 solutions, mixtures which are widely used in actual emulsion counting. The positron annihilation parameters were determined in toluene-Triton mixtures containing various amounts of water. As can be seen from Figures 6 and 7, where the parameter 12,which is correlated to the formation probability of thermal positronium, is plotted as a function of Triton (in the presence of 0 and 2% water) concentration, increasing amounts of Triton reduce I2 to a semiplateau value, while X2, the annihilation rate of the thermal positronium, changes only slightly. A more detailed plot of I2 at lower Triton concentration reveals that I 2 remains constant up to 20 or 10 mM Triton in solutions containing 0 or 2% water, respectively. In previous investigations we have been able to correlate this abrupt break in the 1,-surfactant concentration plots with the formation of micelles or surfactant aggregates.8
I
I
IO
I
>Y
A 2% H Z 0
IO
20
30
40
50
60
I
I
I
I
I
I
016 0 3 2 048 064 0 8 0 0.96
Figure 8. -In
([I2- I,m]/[I: -
(%) TRITON
(M)TRlTON
vs. Triton X-100 concentration.
An analysis of the present data in terms of the previously described assumption that energetic positrons become trapped in these aggregates, in this case reversed micelles, reducing their chance to form thermal positronium atoms, leads to the following correlation between 12mthe plateau value, Izo,the value observed in the pure solvent, I2 measured at a given surfactant concentration, and the surfactant concentration:
where K is the so-called inhibition constant (for positro-
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980
Structural Chainges in Micellar Solutions
r
TRITIUM ACTIVITY (CPM) vs TRITON CONC. IN 2000 TOLUENE SOLUTIONS
3253
TRITIUM BETA ( 3 ~ - ~ , ~ ) EFFICIENCY AND I2 v!j % VOL WATER IN (4 I ) TRITON X-100 - n-PENTANOL MIXTURES IN CYCLOHEXANE ( 20 010 W!"
f
CMC
LL
CPM 1000
{
0 2 V O L % H20
looo
3H-H20 A - 0 VOL % H20
I
idc 0 20 L
CI
100
60 l
5
140
-
I
(mM)
tU
TRITON
PIiASE
TIPP"E1T'""
Flgure 9. Tritium activity (cpm) vs. Triton X-'IOO concentration in toluene solutions.
nium formation), cmc is the critical micelle concentration, and N is the aggregation number. The corresponding plots in Figure 8 show that at the higher water contents (1-2%) the cmc is shifted from -20 to 10 mM Triton and that inhibition is more effective in the presence of a small amount of water than in its absence (K 11 M-' vs. 5 M-'). In order t o evaluate which effect the formation of reversed micelles might have on the counting efficiency of tritium p particles, we studied in a series of experiments the (tritium) counting efficiencies, of toluene solutions containing I'PO-DMPOPOP fluors and 0 or 2% water, respectively, as a function of Triton concentration. In Figure 9 the results of these investigations were plotted in terms of cpm as BL function of Triton concentration. In both systems, where the same amount of tritium activity was used, the counting efficiency in the form of [3H]H20 steeply increases as a function of Triton additive, which is known to assist in the excitation energy transfer processes to the fluors. At 20 or 10 mlM, respectively, a distinct break in the curve can be observed, coinciding with the formation of reversed micelles, followed by further increase at higher surfactant concentration. These results clearly indicate that the interaction between the @ particles (and the resulting d-rays) and the monomeric surfactant molecules leads to a more efficient energy transfer resulting in higher count rates than in a system where the surfactant aggregates are present in the form of reversed micelles and may trap thermal and epithermal electrons. To further demonstrate the correlation between the /3 counting efficiency and the yield of the thermal positronium formation process, we carried out [3H]H20 counting efficiency and positron lifetime measurements in solutions of 20% (w/v) of Triton-pentanol mixtures (4:l) in cyclohexane to which increasing amounts of water were added, It can be seen from Figure 10 that, in the first clear region, the @ counting efficiency as well as the long-lived component in the positron lifetime spectra decrease sharply. In this region the water molecules form a pool inside the spherical micelles. In the anisotropic region 11,I , increases with the amount of water added, whereas the decrease of the efficiency is less pronounced. The transition from region I to region I1 has previously been attributed to a transition from swollen spherical micelles to a lamellar struct~re.~~~~~ We also measured the effect of water additives on [3H]H20 and [3H]toluene efficiencies and I2 in a series of
II
SEPARATION
IO VOL%
CLEAR I20 ISOTdOPlC CLEAR - 19 ANISOTROPIC TRANSLUCENT - 18 MILKY
t+ IO e /'
I
I
20
30
I
VOL. WATER
Figure 10. Tritium fl counting efficiency and I , (%) vs . vol % water In (4: 1) Triton X-100-1-pentanol mixtures In cyclohexane ((20% (w/v)).
--
1
~ R I T I U M BETA COUNTING EFFICIENCY AND 1, V S % V O L WATER IN
N
i
~
I
( 73 )
28 20
12
-as
4
TOLUENE-TRITON X - I O 0 MIXTURES CONTAINING VARIOUS AMOUNTS O F n -PROPANOL
P%
0 % n-PROPANOL
> z 20 w 0 12 0
n- PROPANOL
LL
k i 4
I2
i
1
2
F 20 2
0
1.
20
(3
g
24
12
n.PROFANOL
4 'H - H 2 0 0 3H -TOLUENE A 12
J
- 20
-
19
t
LL.1
6
IO
20
PHASE TRANSITION .. PHASE SEPARATION I
I
1
30
40
50
cyo VOL.
I
WATER
Figure 11. Tritium 0counting efficiency and I , (%) vs. vol % H20 in 7:3 toluene-Triton X-100 mixtures containing various amounts of 1-propanol,
toluene-Triton (7:3) solutions with various amounts of 1-propanol (Figure 11). For all solutions, the efficiency decreases drastically even with relatively amall amounts of water present. In the solutions containing 0 and 7% 1-propanol, the number of thermal Ps (I,) formed drops as a function of increasing amount of water accomodated in the micelle up to -44% HzO or 5-8 moll of water per mole of Triton and then increases with further additions of water. This increase of I2 is indicative, as postulated for the cyclohexaneTriton-1-pentanol system, of the formation of large asymmetric aggregates, probably with cylindrical or lamellar shape. For the solution containing 20% (v/v) 1propanol, no such transition could be observed, and Iz
3254
The Journal of Pbysical Cbemistty, Vol. 84, No. 24, 1980
drops initially and then levels off at -6% water present, at which point a macroemulsion is formed. The same behavior was previously observed for the cyclohexaneTriton-1-pentanol system, where at high alcohol content, the system passed directly from the spherical-typemicellar arrangement to a rnacroem~lsion.~~ The tritium /3 counting efficiency in these solutions shows a relatively steady decline with the amount of added water if the /3 source is present in the form [3H]toluene, while in the case of [3H]Hz0a drastic drop occurs in the region of 2-6% HzO, which could be caused by the formation of a very small amount of a second phase. This second phase may be a mesophase structure in equilibrium with the reverse micellar phase. This kind of equilibrium is possible at such a relatively high surfactant concentration (0.48 M) and has been observed in many amphiphilic systems.16*26 The pronounced reduction in the counting efficiency may therefore be caused by a preferential solubilization of [3H]Hz0in this mesophase, where electrons are more easily trapped than in the main phase, in which the bulk of the [3H]toluene is located. In the presence of 20% 1-propanol, no such phase separation occurs, and the counting efficiency is the same for both sources and its overall reduction is caused by the effective trapping of the electrons in the micelle or microemulsion droplets.
Conclusions The evidence accumulated in these experiments clearly points to a distinct similarity between the phenomena responsible for the weak /3 counting efficiency and the positronium formation process. Recent results have suggested that the majority of the excited states, at least in alkanes and possibly also in aromatic solvents, are formed via the recombination of ion-electron pairs resulting from the interactions of /3 particles or electrons with the solvent molecules. Thus a reduction of the number of available electrons will decrease accordingly the number of excited states and ultimately the amplitude of the output pulse from the photomultiplier and in the case of a fixed discriminator setting the count rate which is registered. Changes in the counting efficiency of weak /3 particles in emulsion counting solutions have previously been loosely described in terms of self-absorption of the /3 particles in water droplets, etc., present in these solutions.26,2‘Water molecules are interfering with the recombination of the ion-electron pairs formed as a result of /3 energy deposition in these droplets; as a result of this no or fewer excited species are formed which can transfer their energy to the primary or secondary fluors, thus reducing the amplitude of the signal from the photomultiplier. The present investigation, however, clearly shows that a significant contribution to the reduction of the counting efficiency is made by the capability of micellar or microemulsion droplets to thermalize and trap energetic electrons,lgnot only those generated in the water pool but also those formed in the bulk solvent, although the results indicate that the former are more effectively trapped than the latter. In other words, the capability of the various species present in the solution, micelles, microemulsion droplets, etc., to trap electrons and render them unable to generate ions has a definite role in the number of successful recombinations and determines the counting efficiency. In this way variations in the counting efficency will sensitively reflect all changes occurring in the solutions which influence the trapping of electrons. A similar mechanism can be postulated for the mechanism of the positronium formation process.
Boussaha and Ache
Although the exact nature of this interaction has not been established yet, a distinct possibility would be the attachment of energetic positrons to the strongly polar headgroups or to the highly concentrated counterions on the micellar surface. Alternatively energetic positrons trapped in the core of the micelle may find only a limited number of electrons available for combination due to rapid removal of electrons from the micelle via electrostatic interaction with the charges on the micellar surface. If reactions of hot Ps atoms are responsible for the drop in Iz, one could possibly invoke fast reactions between those and the micelles. While the extreme sensitivity of this process which favorably compares with other conventional techniques can again be utilized to determine structural changes in these solutions, at the present time it does not provide the means of detecting the exact nature of these structural changes. Attempts to extend the positron annihilation technique to a method of absolute structure determination are being carried out in this laboratory.
Acknowledgment. This work was supported by the US. Department of Energy, Division of Chemical Science. References and Notes (1) (a) Greene, R. C. “Liquid Scintillation Counting”; Bransome, E. D., Ed.; Grune and Stratton: New York, 1970; p 189. (b) Kalbhen, D. A.; Rezvani, A. “Organic Scintillators and Liquid Scintillation Countlng”; Horrocks, D. L., Peng, Chin-Tzu, Eds.; Academic Press: New York, 1971; p 149. (c) Chow, P. N. P. Anal. Biochem. 1974, 60, 322. (d) Pande, S. V. Ibid. 1978, 74, 25. (e) Benson, R. H. Int. J . Appl. Radiat. Isot. 1978, 27, 667. (f) Horrocks, D. L. “Liquid Scintlllation”; Noulaim. A. A.. Ediss. C.. Welbe. L. I.. Eds.: Academic Press: New Yo&, 1976; p 117. (g) Zarybnicky, V.; Rei& M. Int. J. Appl. Radiat. mt. 1979, 30,729. (2) For general revlews on micellar systems and microemulsions see, e.g.: (a) Friberg, S. “Microemulsions”; Prince, L. M., Ed.; Academlc Press: New York, 1977; p 133. (b) Shinoda, K.; Frlberg, S. Adv. Colloidlnterface Sci. 1975, 4, 281. (c) Winsor, P. A. Chem. Rev. 1988, 68, 1. (d) Fendler, J. H. “Micellizatlon, Solubilization and Microemulsions”; Mittal, K., Ed.; Plenum Press: New York, 1977; p 695. (e) Eicke, H. F. Top. Curr. Chem. 1980, 87, 85. (3) Jean, Y. C.; Ache, H. J. J. Am. Chem. Soc. 1978, 100,984, 8320. (4) Fucugauchi, L. A.; Djermouni, 8.; Handel, E. D.; Ache, H. J. J . Am. Chem. SOC. 1979, 101, 2841. (5) Jean, Y. C.; Djermouni, B.; Ache, H. J. “Solution Chemistry of Surfactants”; M M , K. L., Ed.; Plenum Press: New York, 1979, Vol. I, p 129. (6) Ache, H. J. Adv. Cbem. Ser. 1979, No. 175, 1. (7) Jean, Y. C.; Ache, H. J. J. Phys. Cbem. 1978, 82, 811. (8) Handel, E. D.; Ache, H. J. J. Cbem. Phys. 1979, 71, 2083. (9) Boussaha, A.; Djermouni, B.; Fucugauchi, L. A.; Ache, H. J. J. Am. Chem. Soc., in press. IO) Boussaha, A.; Ache, H. J. J. Colloid Interface Scl. In press. 11) Eicke, H. F.; Christen, H. J. Colloid Interface Scl. 1974, 48, 281. 12) Williams, T. L.; Ache, H. J. J. Cbem. Phys. 1989, 50, 4493. 13) Madla, W. J.; Nichols, A. L.; Ache, H. J. J. Am. Cbem. SOC.1975, 97, 5041. 14) Kitahara, A.; Ishikawa, T.; Tasamori, S. J. ColbM Interface Scl. 1967, -23. - , -243. .- . (15) Kitahara, A,; Watanare, K.; Kon-Ko, K.;Ishikawa, T. J . Colloid Interface Sci. 1989, 29, 48. (16) Ekwall. P.: Mandell, L.; Fontell, K. J. ColloM Interface Scl. 1970, 33, 215. (17) Eicke, H. F.; Christen, 11. Helv. Chim. Acta 1978, 61, 2258. (18) Wong, 98, 2391. M.; Thomas, J. K.; Gratzel, M. J. Am. Chem. Soc. 1978, .
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