Positronium reactions in the triton X-100-p-nitro phenol system

Radiat. Phys. Chem. Vol. 50, No. 4, pp. 355-361. 1997

Pergamon PII:

S0969-806X(97)00047-9

1997 Elsevier Science ltd. All rights reserved Printed in Great Britain 0969-806X/97 $17,00 + 0,00

POSITRONIUM REACTIONS IN THE TRITON X-100-p-NITRO PHENOL SYSTEM SUBIR K U M A R DAS and B. NANDI G A N G U L Y t Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Saltlake, Calcutta. 700 064, India (Received 16 October 1996: revised 17 February 1997)

Abstract--The positronium reactions have been used as a probe to study the solubilization of p-nitro phenol within the micellar phase of a Triton X-100 solution and the corresponding changes in the molecular association phenomenon. The presence of p-nitro phenol resulted in an enhancement of micellizationwhich has been corroborated by surface tension measurements.This paper also lays emphasis on the secondary aggregation phenomenon of Triton X-100 molecules at the far post-micellar stage (~ 10-15 raM). The solubilization of p-nitro phenol at various stages of aggregation has been discussed through the interaction with positronium atoms by setting up a kinetic model and reaction equilibria, 1997 Elsevier Science Ltd

!. INTRODUCTION

Nonionic surfactants find an extensive use as solubilizing and emulsifying agents. The micellar system of these surfactants can change the solubilizing power of the bulk system remarkably (Jean et al., 1979). Triton X-100 forms one of such surfactants where the hydrophobic portion is comprised of an alkyl phenyl group while the hydrophilic portion is a long poly oxyethylene chain. Depending on the site of solubilization of the solute and the type of interactions present within the micellar system, the properties of the solute may be altered due to the "new" local microenvironment that it experiences in the micellar system (Maiti et al., 1995). In other words, the formalism of micelle entrapped solutes is substantially different from that observed in pure bulk solvent. The positron annihilation technique serves as an important diagnostic tool to elucidate the changes taking place (Jean and Ache, 1977) in the micellar system, due to molecular association and solubilization of the guest molecules within the micelles. The method is based on the formation of a positronium atom (Ps), which is the bound state of an electron and a positron, as well as the interaction of this Ps within the medium. The main features of Ps reactions are laid down by their average lifetime and decay modes which are dependent on the chemical reactivity and chemical composition (electronic) of the environment (Ache, 1979). An interaction between a Ps and an organic diamagnetic compound in solution can occur via Ps-molecule complex formation which results in significant quenching of the Ps lifetime. It is also tTo whom all correspondence should be addressed.

commented that most Ps-molecule complex formers undergo a conventional donor=acceptor relation. For such reactions to occur, the molecule must possess a low lying electronic level in the molecular orbit (Goldanskii and Shantarovich, 1974). The existence of such a vacant orbit was found by the presence of the carbonium ion !C + in the molecular structure of organic compounds and the sensitivity with respect to positronium quenching was also confirmed (Shantarovich et al., 1971). Referring to the above facts, one of the nitro phenol isomers, e.g. p-nitro phenol was chosen here as the complex former and reactant molecule with Ps in studying its solubilization within a nonionic micelle, namely Triton X-100 as compared to the bulk aqueous solution. This paper also emphasizes on the post-aggregation (Tanford et al., 1977: Ghosh et al., 1993) behaviour of Triton X-100 after CMC as probed by the positronium reactions (Das and Ganguly, 1996a), with and without the addition of guest molecules in the system, through complex formation equilibria. Such an effect has been demonstrated here for the first time with the reaction models in the Triton X-100 system. In addition to this, a comparative account of absorption spectrophotometric data is also reported to make a concise study of the interaction of p-nitro phenol as a solubilizate in a Triton X-100 micelle. 2. EXPERIMENTAL

Triton X-100 (or TX-100, Rohm and Hass Co.) is the trade name of the liquid, nonionic surfactant poly oxyethylene tert-octyl phenylether [(H~C)3CH_,C(CH3).,C6H4-(OCH2CH_,)~.~-OH]. It was obtained from Sigma Chemical Co. (St Louis, MO) and its 355

356

Subir K u m a r D a s and B. N a n d i G a n g u l y 0,56 - -

0,54 FCMC 0.54 -

0.52

I./. •

o. o 0.52

0

.,_

I

I

I

2

0.53

m•r .1. CMC

Enlarged portion close to the CMC

'o

0.50

•

T

i

--

1 0

•

T

"

T _L

Further structural reorganization

I

I

I

I

I

I

I

I

P

10 20

30

40

50

60

70

80

90

100

0.46

T

0.51

_L

0.48

In presence of p-nitro phenol

-(b)

( a )

_?_

Further structural reorganization 0.49

I

I

I

I

I

2

4

6

8

10

T r i t o n X - 1 0 0 (raM) Fig. I.

(a) 23 vs T r i t o n X-100 c o n c e n t r a t i o n in a q u e o u s solution, (b) 23 vs T r i t o n X-100 c o n c e n t r a t i o n in the presence o f p-nitro p h e n o l in a q u e o u s solution at r o o m temperature.

purity was checked spectroscopically. To ensure uniformity in the results, all our measurements were conducted by using the same batch of surfactant. p-nitro phenol (PNP) was purchased from Sd Fine Chemical Company (India) and was recrystallised and used after checking the spectroscopic purity. Millipore water was used in all the cases for preparing the solutions. The source of positrons was -'2Na ( ~ 5 laCi) deposited on a Mylar film and sealing was done by using another identical Mylar film on top of it. This source was put in a cylindrical specimen container placed between the two detectors. Positron lifetime measurements have been made using a conventional fast-slow coincidence circuit, with time to amplitude converter and a multichannel analyzer (Ganguly et al., 1991). The positron lifetime spectrometer used in these experiments had a time resolution (FWHM) of ~260 ps for the prompt 6°Co y rays at the

positron experimental window settings of the upper 50% of the Compton emission accepted in both the channels. A total of ~500,000 counts in each lifetime spectrum was accumulated with peak to background ratios of ~10,000 at room temperature. The measurements have been mainly confined to the determinations of the long lived pick-off component ~3 and its intensity 13 only. The data analysis was done with the help of the PATFIT programme (Kirkegaard et al., 1989). The errors shown in Fig. 1 and Fig. 2 are computer fitted errors which also take care of the experimental errors. The surface tension of the liquids was measured using a K8 Kruss Interfacial Tensiometer with a resolution of 0.05 dyne/cm. The spectroscopic measurements were done using a Shimadzu UV-VIS Scanning spectrophotometer, UV-2101 PC, using a window of path length 1 cm.

m

28 (a)

In presence of p-nitro phenol

(b)

27

24

--

26 .

I

25

23 T

24

~i'm .

't

.

23

•

22

--

0

•

•

~

•

22

,~T

•

Further structural reorganization

I

21

•

T

.L

Further structural reorganization

I

I

1

I

I

I

J

I

I

10 20

30

40

50

60

70

80

90

100

21 0

I

I

I

I

[

2

4

6

8

10

T r i t o n X - 1 0 0 (raM) Fig. 2. (a) lfl/o vs T r i t o n X-100 c o n c e n t r a t i o n in a q u e o u s solution, (b) 13% vs T r i t o n X-100 c o n c e n t r a t i o n in the presence o f p-nitro phenol in a q u e o u s solution at r o o m temperature.

Ps in TX-100/p-nitro phenol 3. RESULTS AND DISCUSSION

The aggregation behaviour of Triton X-100, with a long oxyethylene chain as the hydrophilic moiety and a short-chain hydrophobic group has been studied earlier with the help of the positron annihilation technique very recently (Das and Ganguly, 1996a). This has shown a CMC at a concentration of ~0.3mM and a further morphological change or higher structural order at ~ 10raM concentration. The CMC region was critically substantiated by currently existing literature records. The experimental facts, that were reminiscent of further structural reorganisation (Tanford et al., 1977; Ghosh et al.. 1993) at higher concentrations of the surfactant (~10mM) were brought into prominence. A qualitative explanation on the basis of charge segregation due to the polar head groups of the micetles would reduce the probability of an encounter of the electrons with the positrons and hence the formation of Ps (L) and also their pick-off annihilation rate (,;,0 in accordance with the "Spur" model (Mogensen, 1974; Mogensen, 1995) and the "Diffusion Recombination" model (Byakov and Stepanov, 1993). In the attempts to incorporate the guest molecule with this micellar system as a solubilizate, the basic features of the molecular assembly are significant and therefore are documented through this and in earlier studies (Das and Ganguly, 1996). Insertion of a guest molecule such as p-nitro phenol within the Triton X-100 micelle brings about certain important changes that can be directly compared with earlier results (Das and Ganguly, 1996a, and references therein) and with Fig. 1 and Fig. 2. In the presence of p-nitro phenol, the break in the ),3 pick-off annihilation rate occurs at a much lower concentration of Triton X-100 i.e. at 0.22mM which is an important effect towards any configura-

(a)

E

35

m

r~

\

•

\

357

tional changes occurring within the system. This means that the CMC is lowered considerably. At the post-CMC stage, the second transition in the ),3 and L values is also observed at a much lower concentration, i.e. at ~ 4 m M rather than around 10mM in the case of the pure Triton X-100 system (Das and Ganguly, 1996b). This means that the addition of p-nitro phenol augments the aggregation process (Luck, 1976). This effect has been supported by surface tension (Fig. 3) measurements. There has been a marked lowering of the surface tension values in the presence of p-nitro phenol. The break in the curve occurs at ~0.22mM Triton X-100, where surface saturation has been reached by the surfactant and above which the tension remains virtually unchanged, Fig. 3. All the above observations finally lead to the conclusion that the presence of p-nitro phenol acts as a water structure promotor (Luck, 1976), lowers the CMC of the system and brings about a first order phase transition (Sears and Salinger, 1993 Eicke and Christen, 1974) of the system at a concentration of 0.22mM Triton X-100 in aqueous solution. This is ascribed as an enhancement of the CMC by the additive and the consequent lowering of the surface tension also means an increasing efficiency of the surfactant, in terms of its surface occupancy (Rosen, 1978). This effect could be thermodynamically related to a decrease in the surface free energy of the system (lowering of surface tension) with the total concentration of the surfactant in the solution by the Gibbs absorption isotherm (Sears and Salinger, 1993).

dT= - ~ F ,

where d7 is the change in the surface tension of the solvent, [', is the surface excess concentration of any

In presence of p - n i t r o phenol

36 -- (b)

34 - ~

\ \

31

dp,

32 --

\ 30 --

\ \

\

l1 i t 27 0.00

i 0.2 0.16

0.32

i

i

0.48

0.64

L 28 0.0 0.80

]~CMC 0.2 0.22

]

]

0.4

0.6

.L

I 0.8

TX-100 (mM) Fig. 3. (a) Reduction in surface tension with bulk concentration of Triton X-100, (b) reduction in surface tension in the same system in the presence of p-nitro phenol.

358

Subir Kumar Das and B. Nandi Ganguly

component of the system, d#, is the change in the chemical potential of any component of the system. At equilibrium with the bulk phase and the surface

Positronium lifetime quenching with diamagnetic organic compounds could be caused by a donoracceptor interaction scheme. The carbonium ion in the molecule serves as the reactive centre (Shantarovich et al., 1971; Goldanskii and Firsov, 1971). The rate constant of the Ps interaction with p-nitro phenol in the aqueous solution was calculated according to the relation (Ache, 1979) through the following reaction as the rate determining step in Scheme A:

dFs = R T din as

where as is the activity of any component in the bulk phase. Therefore, d7 = 2 - R T ~ F s

din a,

t

i.e. with the increase in the concentration of the nonionic surfactant Triton X-100 the surface tension decreases (Fig. 3). The break in the curve occurs as the monomers aggregate and micelle formation takes place. The abrupt change in the physical property of the liquid system was found to match exactly with the change in 23 as the first transition point in the positron annihilation spectroscopic studies in Fig. l and Fig. 2 and in our earlier results (Das and Ganguly, 1996). Any additive tending to increase the F , the surface excess concentration, would affect the 7- This increase in efficiency of surface occupancy can be affected by polar organic compounds capable of forming hydrogen bonds with water in terminal polar groups such as p-nitro phenol. The hydrogen bonding between the polar groups of the additive and water molecules helps to counter balance the lateral pressure tending to push the additive into the interior of the aggregate (Rosen, 1978). Therefore, they remain in the outer core, consequently the CMC is 2~l 9

[PsPNP]aq.,

27~-

annihilation in solvent (2p) Ps + PNP KI annihilation in + H,O~--[PsPNP] ,27 " K, complex (2c)

where 2p and 2c are the annihilation decay rates in the bulk solvent water and the complex respectively, K~ and K, are the forward and backward reaction rates. From the experimentally observed decay rate 23 (Ganguly et al., 1991) one obtains: (1)

2 3 = 2p t- go°bsa[C]

where [C] is the concentration of the PNP and Ko°b~, the observed reaction rate constant, defined as K~;tc/(K2 + 2c), was found to be 0.0988 x 10 ~° M - ' s-~ which is in perfect agreement with the results of Goldanskii and Shantarovich, 1974. The solubilization of p-nitro phenol in Triton X-100 micellar solution could be understood in terms of its interaction with Ps as shown in Scheme B (Jean and Ache, 1977; Das and Ganguly, 1996a):

• Ps + [PNP]a q + [TX - 100]aq + [PNP]mic .

b [PsPNP]mi c ~

2y

1l

[PsrX-

lO0]aq

2~' reduced (Fig. 3). These changes have been clearly reflected in Fig. 1 and Fig. 2. In order to understand the role of p-nitro phenol in association with the Triton X-100 surfactant system and its solubilization within the micellar aggregate, one must first recall its molecular structure and then the interaction of Ps with this compound in aqueous solution can be followed (Goldanskii and Shantarovich, 1974).A valence s c h e m e o f p - n i t r o p h e n o l as an acceptor: O--

2~ = Km~¢(.q)[TX-100l.q+ Km,¢[PNP]m,¢ + K°bsd[PNP]~q (2) where Kmj~ and Ko°b~ are the rate constants for Ps reactions with PNP in the micellar phase and in the

O--

-O

N 0

where [PNP]~q and [PNP]m~¢ are the solute concentration in aqueous and micellar phase respectively. From this scheme, the observed ).3 is defined by:

-0

OH

~q

+

-0 c a r b o n i u m ion

dipole moment - 5.04 g[D].

OH

Ps in TX-100/p-nitro phenol homogeneous aqueous phase respectively and Kn,c4,q, is the rate constant of the Ps interaction in aqueous solvent containing the surfactant TX-100. The first term of equation (2) could be obtained from the measurement of 23 in the absence of PNP species and 2~ = K,,,~,~,[TX-100I,~

(3)

where 2~ is the decay rate in the surfactant solution in the absence of PNP. So ,;.~ - ).~ = Km,dPNP],,,~ + K°b~a[PNP]~q

(4)

OF

Kobsd - -

2, - 20 [PNPlm,c [PNP]~s [PNP], - Ko,,~ [PNPlm + K°°"~a[PNP],

(5) where [PNP], is the total PNP concentration. If one assumes that after micellization PNP molecules are completely located in the micellar phase, [PNP L is negligible, then the situation is quite simple. Ko,~ can be equated with deduced Kob~ values from equation (5). However, from the reaction scheme stated, it is also possible to visualize the microenvironment of PNP molecules. A careful examination of reaction rate constants can depict (Fig. 4) the association of surfactant TX-100 with PNP. One can divide the plot of KobsdVS TX-100 concentration in aqueous solution (Fig. 4) into three distinct zones. The first one is the premicellar zone (0-0.3mM, Zone 1), where the interaction of Ps with PNP is rapidly enhanced in the presence of the surfactant, which finally attains a maximum at C M C and then declines just after C M C over a certain range of surfactant concentration. As PNP is a quencher here,

359

according to equation (1) it is to be noticed that the 2~ value for PNP in the presence of surfactant is higher than in the presence of surfactant alone (see Fig. 1). Once engulfed within the micelles, we assume that the K,,h~d value in this range i.e. the second zone ( > 0.3mM, < 4mM, Zone II), corresponds to a relatively shielded reactivity or a reduced reactivity towards Ps due to competition with the reaction of PNP within the micelles. After CMC, the drastic reduction in Ps interaction with PNP up to a certain concentration inclines one to presume (Jean and Ache, 1977; Rosen, 1978) a strong binding of PNP within the peripheral hydrophilic moiety of TX-100 through hydrogen bonding as depicted earlier. As it appears in the plateau region (Fig. 4) the concentration of the micelles is sufficient to dissolve lmM PNP. Under these circumstances, Km,ccan be directly determined from equation (5) for this zone. The reaction equilibria of the surfactant TX-100 and PNP at the different regions along with the spectrophotometric data will subsequently clarify this situation. The third zone. >_ 4mM concentration, corresponds to a zone of increment in K,,,~,~ratios, where a reverse trend in the curve (Fig. 4) is markedly discernible. This is possible only if a part of the PNP becomes free or exposed to interact with Ps while the micellar head groups further associate together among themselves to form a further closely packed structure (Tanford et al., 1977). It is possible to determine the binding constant, K,, of p-nitro phenol with TX-100 micelles and also the fraction of p-nitro phenol present in the micellar phase (Jean and Ache, 1977) for Zone 1II. In order to determine these, let us consider the interaction of p-nitro phenol with the TX-100 micelle (Das and Ganguly, 1996a) K

(TX-100)m,~ + PNP~-(PNP) .....

16 - 14

With the assumption [TX-100]m,c>>[PNP]m~ and 1 • 1 stoichiometry is followed in the reaction, the equilibrium constant Kc can be expressed as

--

T 12

(6)

-1-

[PNPlm,. K~ = ([PNP], - [PNP]m.)[TX-100]..~

10 - u

8 m

= Fm/[M](I - Fro) (7) 6 --

where [M] = [TX-100]m~c, F,,, is the fraction of' PNP present in the TX-100 micellar medium and F0 is the fraction of PNP remaining in the aqueous medium. With this consideration, equation (5) is modified to

4-2-0 0

I

L

I

I

I

2

4

6

8

10

TX-100 (mM) Fig. 4. Koh~vs Triton X-100 concentration in the presence of p-nitro phenol in aqueous solution at room temperature, where the concentrations of TX-I00 divide into three zones. Zone I: 0-0.3mM TX-100; Zone II: > 0.3mM, < 4mM TX-100; Zone III: > 4mM TX-100.

Kobsd

K,..i/Fo,+ Kou~aF,,

(8)

where Kmf is the Ps rate constant for Ps interaction with PNP in the micellar phase at the third zone. The concentration of the micelle, [M], is given by [M] -

CD -- CMC

N

(9)

360

Subir Kumar Das and B. Nandi Ganguly 0.25

I00

--

0.20

~.

80

0.15

5 '~

60

I

.5 0.10

"~ I

40

i j-

e~

0.05

9 .~

I 0.1

o.o0 0.0

I 0.2

I 0.3

I 0.4

e~

I 0.5

1/(CD-CMC ) (mM -1) Fig. 5. - l / ( K ~ b ~ d - Koh~d) VS 1 / ( C o - C M C ) in Triton X-100 micellar solution in the presence of p-nitro phenol, where the concentration of TX-100 is > 4mM, zone Ill.

where Co is the total concentration of the surfactant TX-100, CMC is the critical micelle concentration and N is the aggregation number (Fendler and Fendler, 1970). Combination of equations (7) and (8) and rearrangements lead to Kob~d= K°b~d+ Km,~'K~[M] 1 + KdM] Combination of equations rearrangements give

(9)

and

(10) (10)

and

l/(Ko°b~ - Kob,d) = l/(Ko°b~d- Km,¢') N + 1/(K°b~a-- Km,~') K~(CD - CMC)

(l 1)

Or K o ~ - K°b~d Km,~' -- Kob,d

K¢(CD - CMC) N

(12)

In zone III ( > 3mM), if one considers that surfactant molecules reassociate themselves to form a further assembled region, the association of PNP with the micelles is now competitive. Hence, in this region PNP is relatively more available to Ps. In this region (zone III) one may expect from equations (11) and (12) a high value of K,,~c', the rate constant of the Ps reaction with PNP in the micellar phase at the third zone. It is found that equations (11) and (12) are valid for the Ps interaction with PNP. A plot (Fig. 5) of l / K ° b ~ d - Kob~d VS I / ( C D - - C M C ) allows the calculation of Km~' and K, and thus of Fo, and F0 i.e. the fractions of PNP present in the micelle and exposed to water, respectively. The rate constant values are presented in Table 1. The percentage of PNP

Premicellar region, Zone I (0--0.3 mM) K,,b~J values increase

20 I

I

I

I

4

6

8

l0

[TX-100] (raM) Fig. 6. Percentage of p-nitro phenol in the micellar phase vs Triton X-100 concentration in aqueous p-nitro phenol solution at room temperature.

associated in the micellar phase is given in Fig. 6 for Zone I I I. The magnitude of K,,,c' represents a stronger interaction of Ps with PNP which in turn signifies that the association of PNP in the secondary aggregated phase of TX-100 has undergone a substantial change and thus reflects a different kind of microenvironment for the guest molecule. An independent spectroscopic investigation was performed in the same micellar system with PNP. From the spectrophotometric data (Fig. 7) it is possible to establish the fact that PNP enters into the interaction zone of the surfactants, from the successive nature of the spectra. There is a ground state complex formation of Triton X-100 and PNP. At the micellar region, i.e. in the second zone, 2 .... of p-nitro phenol is highly blue shifted ( ~ I 1 nm) with an enhanced absorption intensity. Here PNP is gradually engulfed within the micellar zone (2max of TX-100 in aqueous solution is 278 nm). There appears to be an overlap of the TX-100 complexed PNP absorption area and the absorption edge of TX-100 alone. Finally in the case of total encapsulation of PNP at ~ 4 m M Triton X-100, it tends to appear as one broad maximum representing the micellar complexed species. This corroborates with the fact that the micelle solubilized PNP species experiences a "new" local environment in which it is totally bound to the surfactant molecules (Fendler and Fendler, 1975; Formosinho and Miguel, 1985). Here, the concentration of PNP was kept quite low, so that the progressive migration towards the micellar phase is clearly followed from Fig. 7.

Table I Micellar region, Zone II (>0.3raM, <4raM) K,,~ drops after CMC (in between 2-3 raM) Km~= 5.994 M - ~ns-

Postmicellar secondaryaggregation. Zone 111 (>4mM) K~,h.~again increases 1(,=2.1337 x I(P M -~ Km,¢'= 16.623854 M -~ ns -~

Ps in TX-100/p-nitro phenol 0.10

Q

0.05

<

0.00 200

300

400

Wavelength (nm) Fig. 7. Absorption spectrum of p-nitro phenol in Triton X-100 aqueous system. (a) 0, (b) 0.08raM, (c) 0.2mM, (d) 0.3raM, (e) 0.5raM, (f') 0.9mM, (g) 2mM, (h) 3mM, (i) 4mM of Triton X-100. Concentration of p-nitro phenol was 0.004mM.

4. CONCLUSION This p a p e r emphasizes on the use of p o s i t r o n i u m reactions as a p r o b e to study the solubilization a n d the changes in aggregation b e h a v i o u r of TX-100 molecules in aqueous solution. The secondary association p h e n o m e n o n o f the TX-100 micellar solution is expressed discernibly a n d the solubilizate P N P is exposed to different kinds o f microenvironm e n t at the various stages of aggregation.

REFERENCES

Ache, H. J. (1979) Positronium and Muonium Chemistry, Advances in Chemistry Series 175 (Edited by H. J. Ache), p. 1. American Chemical Society, Washington, DC. Byakov, V. M. and Stepanov, S. V. (1993) Positronium formation in aqueous solutions of surfactant substances. Radiat. Phys. Chem. 41, 559. Das, S. K. and Ganguly, B. N. (1996a) Study of Triton X-100 surfactant-UO~ ÷ aqueous system by positron annihilation technique, Radiat. Phys. Chem. 47, 257.

RPC 50/4--C

361

Das, S. K. and Ganguly, B. N. (1996b) A photochemical study of uranyl ion interaction with the Triton X-100 micellar system. J. Colloid Interface Sci. 180, 377. Dennis, E. A., Ribeiro, A. A., Roberts, M. F. and Robson, R. J. (1979) Solution Chemistry of Surfactants (Edited by K. L. Mittal), Vol. 1, p. 175. Plenum Press, New York. Eicke, H. F. and Christen, H. (1974) Nucleation process of micelle formation in apolar solvents. J. Colloid Interface Sci. 48, 281. Fendler, E. J. and Fendler, J. H. (1970) Advances in Physical and Organic Chemistry (Edited by V. Gold), Vol. 8, p. 271. Academic Press, New York. Fendler, J. H. and Fendler, E. J. (1975) Catalysis in Micellar and Microemulsion Systems. Academic Press, New York. Formosinho, S. J. and Miguel Maria Da Graca. M. (1985) Photophysics of the excited uranyl ion in aqueous solutions. J. Chem. Soc. Faraday Trans. 1 81, 1891. Ganguly, B. N., Subrahmanyam, V. S. and Sen, P. (1991) Positronium reactions in uranyl ion-HDEHP microemulsion system. Radiat. Phys. Chem. 38, 213. Ghosh, H. N., Palit, D. K , Sapre, A. V., Ramarao, K. V. S. and Mittal, J. P. (1993) Dual sites of solution for electrons produced by photoionisation in aqueous micellar solutions. Chem. Phys. Lett. 203, 5. Goldanskii, V. 1. and Firsov, V. G. (1971) Chemistry of new atoms. Ann. Rev. Phys. Chem. 22, 209. Goldanskii, V. I. and Shantarovich, V. P. (1974) The role of bound states in positron annihilation. Appl. Phys. 3, 335. Jean, Y. C. and Ache, H. J. (1976) Studies of molecular complex formation by positron annihilation technique. J. Phys. Chem. 80, 1693. Jean, Y. C. and Ache, H. J. (1977) Positronium reactions in micellar systems. J. Am. Chem. Soc. 99, 7504. Jean, Y. C., Djermouni, B. and Ache, H. J. (1979) Solution Chemistry of Surfactants (Edited by K. L. Mittal), Vol. 1, p. 129. Plenum Press, New York. Kirkegaard, P., Pedersen, N. J. and Eldrup, E. (1989) PATFIT-88. Riso-M-2740, Denmark. Luck, W. A. P. (1976) Topics in current chemistry (Edited by A. Davison, M. J. S. Dewar, K. Hafner, E. Heilbroner, U. Hoffmann, J. M. Lehn, K. Niedenzu, K. Schafer and G. Wittig), Vol. 64, p. 113. Springer-Verlag, Berlin, Heidelberg, New York. Maiti, N. C., Mazumdar, S. and Perrasamy, N. (1995) Dynamics of porphyrin molecules in micelles. Picosecond time resolved fluorescence anisotropy studies. J. Phys. Chem. 99, 10708. Mogensen, O. E. (1974) Spur reaction model of positronium formation. J. Chem. Phys. 60, 1998. Mogensen, O. E. (1995) Positron Annihilation in Chemisto,, Springer Series in Chemical Physics 58 (Edited by V. I. Goldanskii). Springer-Verlag, Berlin, Heidelberg, New York. Rosen, M. J. (1978) Surfactants and huerfacial Phenomena, Vol. 53, p. 93. Wiley, New York. Sears, F. W. and Salinger, G. L. (1993) Thermodynamics, Kinetic theoo' and Statistical Thermodynamics, p. 178. Addison-Wesley/Narosa, New Delhi. Shantarovich, V. P., Petersen, K. and Goldanskii, V. I. (1971) Carbonium ion and positronium quenching by diamagnetic organic molecules. Chem. Phys. Lett. II, 464. Tanford, C., Nozakk Y. and Rohde, M. F. (1977) Size and shape of globular micelles formed in aqueous solution by n-alkyl polyoxyethylene ethers. J. Phys. Chem. 81, 1555.