Positronium reactions in uranyl ion-HDEHP microemulsion system

0146-5724/91 $3 00 + 0 00 Copyright © 1991PergamonPress plc

Radzat Phys Chem Vol 38, No 2, pp 213-220, 1991 Int J Radtat Appl Instrum, Part C Pnnted m Great Britain All nghts reserved

POSITRONIUM REACTIONS IN U R A N Y L I O N - H D E H P MICROEMULSION SYSTEM B. (NANDI) GANGULY, V S. SUBRAHMANYAMand P. SEN'~ Saha Institute of Nuclear Physics, Calcutta 700009, Indm (Recewed 14 June 1990, m revtsed form 8 October 1990)

Abstract--Positron anmhdatlon technique has been apphed to the study of a mlcroemulslon system (water m o11)involving intermoleeular association of dl(2-ethyl hexyl)phosphonc aod (HDEHP) in the presence of minute quantmes of water (2%) m the bulk of the nonpolar n-heptane phase These investigations were extended to the same system Incorporating polar solubdizate i e uranyl ion (UO~-2) to determine its site of solublhzatxon. The rate constants of pos~tromum reactions wxth the substrate molecules and the uranyl 1on are discussed m aqueous as well as m the m~croemulslonphase. One of the stnking observatmns of the present study has been the detection of an extra component of -~ 0.61 ns alongwtth a marked decrease m the intensity of the pick-off component, arising only in the case of the mlcroemulsxonphase where UO~2 is solublhzed in HDEHP/n-heptane/water system. The origin of this extra component could be ascribed to the posltronlum complex formation m the restricted polar phase The Interpretation of posltronium reactmns are corroborated with an independent l r spectroscopic investigation

l. INTRODUCTION Surface active agents are profitably used in the extraction technology of metal ions from the aqueous solutions. Of these, di(2-ethyl hexyl) phosphoric acid (HDEHP) constitutes an important surfactant which can react with a positively charged metal ion through its functional group (the head group), > P - - O namely the phosphoryl moelty In addition to this, it possesses the long hydrophobic chains, which render the metal complex soluble in the apolar phase (Vandegrift and Horwitz, 1980) At a relatively high concentration of this extractant, the molecules undergo a self-association process, predominantly, through a dipole--dipole interaction. This modifies the extraction system, which is mamfested in terms of the change in the trend of the distribution ratio values (Das et al., 1984). We have earlier studied the molecular association phenomenon and the onset of the aggregation process (Ganguly and Sen, 1987) using the positron annihilation technique, which has been well recognized for ItS application in micelles (Jean and Ache, 1978). In the extraction process, when the nonpolar phase containing the surfactant is contacted with aqueous solution, a small quantity of water (2-3%) may be extracted, which in fact augments the aggregation phenomenon of the surfactant molecules (Frendler, 1982). The fact that these surfactant aggregates incorporate minute quantities of water and form reversed micelles has been proven (Bhattacharyya and tTo whom correspondence should be addressed

Ganguly, 1987). Such a system, In actuahty means that the organic phase is a microemulsion (M.E.) phase (Osseo-Asare, 1988) At a high concentration of HDEHP (concentration range 0.5-1.0 M of the extractant) it appeared that the hydrated metal ions are solubihzed by the polar core of the water in oil M E. phase (Bhattacharyya and Ganguly, 1986). Ganguly (1990) has recently probed the mechanism of uranyl ion extraction in the same system using photochemical and spectroscopic investigations. In the present work, an alternative method using positron annihilation technique is attempted, on the same extraction system A positron may combine with an electron to form an electron-positron bound state, the positronIum atom (Ps). The formation probability of Ps and the chemical reactions of the Ps with the substrate molecules can yield the desired information about the chemical or physical changes involved in the system. The chemical reactions of positronium can be studied by observing the changes in its average lifetime, decay modes and the pick-off intensity parameters which are dependent on the chemical reactivity and physical composition of its environment (Mlttal, 1979) Here, the reactions of Ps with uranyl ion in the HDEHP/n-heptane/water (water in oil) M E. system have been investigated as a potential tool for the determination of its location w~thln the M.E. phase. 2. EXPERIMENTAL

The reagent HDEHP a product of ICN pharmaceuticals, Inc., Life Science Group (Plainvlew, NY) was used as the extractant HDEHP was purified

213

214

B (NANDI) GANGULY et al. 10 6

• UO~2 m O,SM HDEHP/n-Heptone/woter system o Prompt spectrum token wtth o 6 ° C 0 source 1 channeL = 0.133ns

10 5

_.

10 4

== o c)

10 3

%...

o

10 2 o

101

I 180

I

I

I

190

200

210

220

230

240

250

260

270

280

290

300

Channel number

Fig 1 Positron hfetxme distribution spectrum of UO~ 2 in 0 5 M HDEHP in n-heptane with 2% water. The time resolution (FWHM) of the prompt spectrum taken with a 6°Co source is 260 ps

according to the method developed by Peppard (1957), as described previously. The nonpolar solvents, namely n-heptane and other chemicals used were of spectroscopy grade, procured from E. Merck. Uranyl nitrate used was from Malhn Ckrodt Chemical Works, U S.A. Triply distilled (T.D.) water was used for preparing the solutions. In all the cases calculated concentrations of triple distilled water and uranyl nitrate solutions were incorporated within the HDEHP/n-heptane system with the help of a micro pipette and the resultant volume was made up with the addition of n-heptane. The resultant fraction of water m the organic phase remained 2% and the concentration of UOJ"2 was calculated according to the resultant dilution made. That the system forms a stable M.E. was estabhshed in (Bhattacharyya and Ganguly, 1987). The pH of aqueous UO~-2 solution remained between 2 6-3. The source of positrons was 22Na ( ~ 4 # C 0 , deposited on a 0.5 mil thick nickel foil and seahng was done by using another identical nickel foil

on the top of it. This source was put in a cyhndrical specimen container placed between the two scintillation detectors. The positron lifetime distribution measurements have been made using a conventional arrangement with time-to-amplitude convertor and a multichannel analyser. Two tapered N E l l l phosphors with Pilot reflecting paints were coupled to two RCA 8575 photomultipller tubes The fast pulse was processed in Ortec 583 constant fraction discriminators before feeding to the Ortec 467 gated time-to-amplitude convertor. The MCA was a ND60 system The positron lifetime spectrometer used in these experiments had a prompt time resolution (FWHM) of 260ps for the prompt 6°Co y-rays at the positron experimental window settings with the upper 50 % of the Comptons of 1.28 and 0.511MeV accepted m both the channels, respectively. A total of 0.8 x 106 counts in each lifetime spectrum were accumulated and all the measurements were made at room temperature.

Posltromum reactions m mlcroemulsion system The measurements have been mainly confined to the determinations of the long lived pick-off components z2 and its intensity 12 only. The general trend of the lifetime distribution curve is shown in Fig. 1, for the case of UO~2 in 0.5 M H D E H P / n - h e p t a n e with 2% water, alongwith the prompt curve taken with a 6°Co source. The data analysis of the lifetime distribution spectra was done using the standard computer programs P O S I T R O N F I T and R E S O L U T I O N (Kirkegaard et al., 1981). The variance of fit in the analysis lay between 0 9 and 1.1. The errors shown in Fig 1 are experimental errors of pure statistical nature, and those stated in the tables are computer fitted errors which also take care of the experimental errors. The i.r. experiment with the extracted uranium in the H D E H P phase was done by making a KBr pallet in P e r k m - E l m e r 783 IR The other i.r experiments for binding of water molecules in the H D E H P phase as well as aqueous uranium samples were done using C a F 2 window with a Shlmadzu IR 408 instrument.

215

actually leads to the aggregated phase of H D E H P in the n-heptane system as shown below:

Aggregation behawour of HDEHP molecules RO /OR . O - - - H O - - p .-.~-O.

//"

RO~

Ro/P~

"glO~ / OR

..o/P~0R OH.--O ~ - P - - O H

/\

OR (a) Self assocmtton process m HDEHP-n heptane phase RO

RO

OR

\/

, H - - O ....H O - - P

=0

.... H - - O I

l H

RO\p// 0 / RO / ~OH•

H "HO \

H

",,I

"O--H. . . .

H

1 /

,

/OR

.0/~OR

O=P--OH. . . . O-H"

/\

RO OR (b) Assoelatmn of HDEHP molecules m the presence of water

3. RESULTS AND DISCUSSION 3 1 Nonpolar phase

A set of positron lifetime spectroscopy experiments were performed with the bulk of the nonpolar solvent, namely, n-heptane with subsequent changes in the concentration of the surfactant H D E H P and the results are listed in Table 1 The trend of the results is in agreement with our previous observations (Ganguly and Sen, 1987) The near constancy of the orthopositronium lifetime, z2, for these samples indicates that no positronium quenching mechanism is taking place in the system. However, a notable decrease in the intensity parameter, 12, was obtained. In our previous report on the system (Ganguly and Sen, 1987), the abrupt changes in the pos]tronlum formation intensity with increase in concentration of H D E H P were attributed to the aggregated form of H D E H P molecules beyond 0.2 M concentration. Thus the drop in the I 2 values corroborates to our earlier observation on molecular association in the H D E H P / n - h e p t a n e phase, where a dipole-dipole interaction between the head group could be discussed (Bhattacharyya and Ganguly, 1987), which Table 1 Positron anmhflaimn parameters for HDEHP/n-heptane system Lffetlme Intensity S No Nonpolar phase z2(ns) 12(%) 1 n-Heptane 2 30 + 0 02 23 8 _ 02 2 02M HDEHP/n-heptane 229_+002 178_+02 3 05M HDEHP/n-heptane 247_+002 163_+02 4 0 2 M HDEHP/n-heptane 2 42 _+0 02 17 3 + 0 2 and 20 water 5 05M HDEHP/n-heptane 254_+002 156_+02 and 2% water

The inhibition of the Ps formation could be related to the solvated electron involvement in the bridged H D E H P molecules, in the aggregated phase. This is in accordance with the spur model (Mogensen, 1974), where Ps formation in the spur competes with other processes such as the diffusion of rather e + and e - out of the spur, e - M ÷ ion recombination or electron attachment e - + M--+ M - by the medium molecules, M. It is thus obvious that efficiency of inhibition would depend upon electron scavengers and solvated or presolvated electrons in the systems (Lazzarim, 1986). Further, calculated fractions of water (2%) was introduced in the system since it was referred through our earlier results (Bhattacharyya and Ganguly, 1987) that specific amount of water could be taken up by the extractant H D E H P upon molecular association and thus resulting in a swelling process as shown above The positron annihilation parameters however do not show any significant change in r2 and /2 values since the same kind of hydrogen bonding interaction exists between the molecules, which does not alter the micro-vlscomty of the m e d m m and hence does not affect the posltronium formation probability (Lazzarinl, 1986). This has been shown in Table 1. Further, the Ps reaction rate upon solubilization of uranyl ion in the M.E. phase i.e the extracted phase or in the aqueous phase can be combined through setting up appropriate kinetic equations For the sake of simplicity, we shall consider the case of aqueous solution first 3 2 Reaction in the aqueous envtronment

In the aqueous phase, the positron annihilation parameters are represented as in Table 2.

216

B.

(NANDI)

GANGULYet al

Table 2 Positron anmhdatton parameters for the aqueous phase Aqueous phase

Lifetime z2(ns)

Intensity 12(%)

22(ns -~)

S No 1 2 3

TD H20 UO +2 0 001 M solutmn UO~ 2 0 0 1 M solutmn

178+002 I 80 _+ 0 02 162+002

161+02 14 8 + 0 2 136_+02

0562 0 555 0617

If we consider the complex formation of the Ps with that of the substrate molecules (Mittal, 1979; Jean and Ache, 1976) and the UO~-2, then at may be represented by the following equations:

The rate constant for Ps reaction is given by kob s = (22 - -

2p)/M

(8)

where kl 2c

2y (Annth,lat,onPs + H20 + U O f 2

- =:~kobs ( k 2 + 2¢)

an solvent H20

.

kl k2

" ( P s U O ~ 2) Complex formation

,~

,2y

Anmhtlatton in complex

where 2p and 2c are the annihllatmn decay constants in the bulk solvent water, and the complex respectively Setting up appropriate klnetm equatmns d - d t [Ps] = 2p[Ps] + k 1[PslIUOf 2] - kz[Ps UO~-21 (!) d [ p s UO~2 ]

=

--kl [Vs][UO~-2] + (k 2 + 2~)[Ps U O ; 21

(2)

Assuming that 2¢ is independent of the type of complex formation and also another approxlmatmn to invoke forward &recUonal con&tions, i.e. d/dt [Ps UO~ 2] - 0, then k, [Ps] [UO~-21 = (k2 + 2c)[Ps UO~-21.

and M is the concentration of UO~ 2 m molarlty. 2p is the annihilation rate m pure solvent which as adentical in extremely dilute solutions. By substatutlng the appropriate values in thas simplified equation one obtains a value for kobs for Ps anteractlon with uranyl ion, equal to 0 5 5 5 x 101°s-lM i for 10-2M UO~ 2 solution in aqueous medium. This is in accordance with the rate constant values obtained in aqueous solution (DuplRtre and Abb6, 1985). It as also noticed from Table 2 that the antenslty of o-Ps formation decreases gradually with the addmon of UO~ 2. In the dilute aqueous uranyl nitrate solution, the hydrated species of uranium may either be present m the UO~-2 or UOE(OH) + form (Baflar et al., 1973) and in the nitrate medium uranyl ion remains as monomeric species at the aforesaid concentratmn. Generally In the pure bulk water, the mode of bonding both as electron donor and acceptor is perfectly balanced By the introduction of the posatlvely charged uranyl species, a local electron drawing capacity is set in, which could be illustrated as below

(3)

Subsmuung (3) in (1) we obtain _ d [Ps]= ~2p + .. k, 2¢.. [UO~_21~ [Ps]. dt ( (k2 + ~.c) J

(4)

The integration of this gives the time dependent concentration of Ps k 12¢

+2

[Pst] = [Ps0]exp{-(2p + k ~ - ~ [UO 2 ])t}.

(5)

The time dependent two photon annihilation rate R2a can be represented by the following two-exponential equatmn R2~=A e x p ( - - 2 1 t ) + B e x p ( - - 2 2 t ).

(6)

Here A and B are related to number of positrons anmhflatlng at rates 21 and 2a and B as the contribution from o-Ps atoms. 2l is attributed to anmhilation rates of free pomrons, p-Ps and hot Ps reactaons. 22 under this exponential condmon is ± kl2c rlT¢~+21 2 2 ~ - - 2 p T ~ t ~ 2 J' ( k 2 --}- )

Ac

(7)

-H

,H.

\o /

"'"'o ,o ./\...""

H~,,,

/ ~0 /

.

.o---

',

~2.~

uo,

,, / H

.... ,o

',/o.

/

(a) (b) (a) Hydrogen bonded water (b) UO~2 solvated m the aqueous environment

Considering the spur model and dealing with the assumption of electron scavenging, at as expected that Ps lnhibmon would result. The efficiency of Ps mhibitmn could be related to the presolvated electrons according to Abb6 et al (1986), where an they have suggested certain empirical relatmns. The mhlbitmn would also result due to the presence of N O ; ion m the system However, the inhibiting power of N O ; is expected to be slgmficantly altered, since as it appears from the reference (Duplfitre et al., 1983) that this should arise from the change in the nature of the electron trapped on UO~ 2 (or delocahzed) with a resulting change m reactivity and availability for both NO~- and e +.

Positromum reactions in mleroemulslon system

•:::::•! ~

f

!~ ~

.....

~

where kot,s, ku E and kb are, the rate constants of the reactions of Ps with UO~ 2 in M . E , Ps m the M.E. alone and the bulk nonpolar phase respectively. The observed long lifetime component is,

Nonpolarregion Polarregionfor UO;t ,olubllizotmn ( ~ a r head

22 = kobs[UO22M E.] + kM E [M.E.] + k b [ b u l k N.P.]

- t Nonpolar

22 - 20 = k[UO~-2M E.] + kb[bulk N.P.]

hydrophobic

chain

Fig. 2 Schematic representation of U O system

+2 i n

HDEHP M E.

3.3 Reaction with UO ~ 2 in M.E. phase In the M.E. phase, which is indeed a heterogeneous phase as shown m the Fig. 2, and thus the situation is a little complex. One of the most striking features m the present investigation (see Table 3) is that o-PS formatlon process shows an ad&tlonal long lived component in the M.E. phase containing UO~-2 m the H D E H P solution, at concentrations/> 0.2 M. The long lived pick-off o-Ps component (a) of ~ 2.5 ns which was asstgned to the annthilatton process in the nonpolar phase showed a marked decrease in its intensity Le. -- 7%. This was accompamed with the extra component (b) of = 0.61 ns with an intensity of ~ 16.0%. This could mean, a fracnon of the long hved pick-off component has been affected through positronium complex formation mechamsm. Thus we make the same kind of kinetic treatment for Ps complex formation and observe the apparent rate constant. Obviously, one can write m general, for reactmn wtth U O f 2 [Ps] + [UO~ 2] ~

k2

[Ps UO~ 2] ~ , 2y

k, 2c kobs = k2 2------~ + '

217

(9)

The reaction scheme in the M.E. phase could be represented as follows:

i.e. kb [bulk N.P.]=='0. Thus we find 22 - ~o [UO~_2M.E. ]

kob~= 12.3 x 101°s -j M -l

From the values of the rate constants, /Cobsfor Ps complex formation with the polar solubtlizate namely UO~-2, it can be inferred that it ts considerably enhanced m the M.E. phase. Alternatively, if it is presumed that a part of the Ps ts involved m the strongly bonded complex formation in the restricted polar phase of the M.E. Fig. 3, m the presence of the polar solubilizate, UO~-2, then the influence of its electric field on r 2 (marked quenching effect) can be considerable (Ache, 1982) which Is reflected m the data, in Table 3 A careful examination of the magnitude of z z values of the long lived pick-off component as presented m Table 3 (a fraction of Ps that probably annihilates as the long lived component m the nonpolar phase) shows a tendency of gradual increase Also,

TA¢ kobs

" Ps + (UO~2M.E.) + M.E. +

(bulk N P.)

b

llkM~

(Ps M.E.) (Ps bulk N.P.) 27

S No 1

(1 I)

where 20 represents ku E [M.E.] when UO~-2 is absent in the M.E. phase. At concentrations of H D E H P / > 0.2 M (with the added water) encircling a mmute polar pocket, where a total reversed mlcellar structure ts formed (Bhattacharyya and Ganguly 1987; Ganguly, 1990) Fig. 2, the total UO~-2 is concentrated within the M.E. phase which means the concentration of UO~ 2 in the bulk nonpolar phase is unfeasible

27 (Ps UO~-2M.E.).

(10)

27

Table 3 Positron anmhdatlon parameters for the M E system Long hved Intensity M E Phase components (ns) 12(%) 22(ns-1) UO~-2m02M (a) 280_+004 85+02 0357 HDEHPME (b) 061+004 141_+15 1639

2

UO~"2m 05M HDEHP ME

(a) 263+003 (b) 061 +003

78±02 160± 1 7

0380 1 639

3

UO~"2 m 0 1 M HDEHP

2 18+003

70+02

0459

B (NANDI)GANGULYet al.

218

100 (O)

Aqueous

phase

80 ~i 6o-

band w ~ d t h ~ difference=12cm-I ~ / HaLf

8

~aLfband wldth difference= 50 cm -I

i

I-20--

0 4000

IJ - -

i 3000

I 2000

t

Wavenumber

100

q

(b)

I 1800

=

I 1600

( c m -~ )

M=croernuLs=on phase

80

A 8

60

#

I'-

4o

20

i I

0 4000

3ooo

2000

18o0

160o

.~,-

1 1400

I

I 1200

~ I

~

1000

t 800

Wavenurnber (cm -~ )

Fig 3 (a) I R Spectra of UO~ -2 in the aqueous environment, the dotted line shows the spectrum of T D water (b) 1 r Spectra of UO +2 extracted HDEHP (0 5 M) phase containing 2% water

the same increasing trend of z2 is observed m Table 1, upon the incorporation of water m the H D E H P / n - h e p t a n e system Such a change Is attributed to the nature of intermolecular interactions (e g., dipole--dipole interactions) which depend upon the structural order of the M.E. phase (Nicholas and Ache, 1972) which is m fact very sensitive to the Ps lifetime changes But in the case of 0.1 M H D E H P solunon where UO~2 IS solublhzed only through ion ligand interaction (Das et al., 1984) (since intermolecular aggreganon ~s absent at this concentrahon), there is no marked distinction of the zones and thus shows both quenching and &stmctly marked inhibition effects

3 4 1 R. spectroseoptc results It would be worthwhile to show the co-ordlnanon effects of mlcroenwronment by taking recourse to the Lr spectroscopic analysis (a) Aqueous envtronment" A comparative account of the spectra of pure T . D . H 2 0 and 0.01 M U O +z solution m this water shows [Fig. 3(a)] a defimte increase m the intensity of absorpnon peaks at 1640 cm -~ due to O H deformation mode and the broad O H stretching around 3400 cm-~ which could be ascribed to the local assocmnon effects since the polarity is increased due to ion-dipole type of mteracnon, as discussed m the earlier paragraphs with the

Positronium reactions m mlcroemulsxon system Ps interaction. At the same time, there is also a definite broadening of the absorption peaks, the noted band width at the half-maximum are shown in the Fig. 3(a). This can be ascribed to increase in the bond length and decrease in force constant due to the association effects. (b) M.E. phase. The i.r spectrum of the U O f 2 ion extracted H D E H P phase shows [Fig 3(b)] an intense absorption in the 895-930 cm -~ region attributable to the v3 or Vas U O f 2 mode (Ghosh and Malti, 1987). The broad intense band around 1000 cm -l can be ascribed to the superimposition of P---O(C), C - - 4 ) ( P ) - - O H , P - - O ( H ) groups. The P~----O stretchlng vibration is observed at 1220cm -~ (Bellamy, 1959) where as a strong shifted band around l l 7 0 c m -1 is ascribed to the involvement of group in association with U O ] ~ ion. Further the prominent stretching band due to co-ordinated water (to the head groups of the surfactant) around 3400 c m - I and intense structural bands near 2800-3000 era-1 of H D E H P itself is especially helpful to describe the polar aqueous environment (Bellamy, 1959) Probably, H D E H P hydrogen bonded to water molecules as well as assocmted water species contribute m the later region (Svehla, 1976; Schuster et al., 1976) The O H bending around 1620-1670cm -~ is also of assistance to detect the presence of water In the M.E phase of H D E H P / n heptane system. The observations revealed a clear indication of solubihzatlon of U O f 2 in the polar core of the M.E. phase which corroborates the inference drawn from positron annihdatlon data. 4. CONCLUSION The Ps interaction with UO~ -2 in the aqueous phase shows both inhibition and quenching effects, which could be correlated to the solvatlon effect. The association effect of the water molecules set in by the solvated ions ~s illustrated through i.r. spectra In the UO~-2 solubllized M.E. phase, Ps interaction shows two definite zones: (1) a polar zone concentrating the UO~-2 and gwing rise to a highly quenched lifetime component and high value of the rate constant for Ps interaction; (ii) a bulk nonpolar zone contributing a usual long pick-off component. The association of UO~ -2 through ) ~ moelty of H D E H P molecules and the assocmtlon of water molecules in the M.E. phase corroborates to the polar core assigned in the above observations. Acknowledgements--One of the authors (BNG) acknowledges, Mr Ajay Das for assisting m l r. spectroscopic measurements, and also the financial support recewed from CSIR, Govt of India

219 REFERENCES

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