Charge transfer between two immiscible electrolyte solutions

J. Electroanal. Chem., 145 (1983) 213--218

213

Elsevier Sequoia S.A., Lausanne- Printed in The Netherlands Preliminary note CHARGE T R A N S F E R BETWEEN TWO IMMISCIBLE ELECTROLYTE SOLUTIONS T R A N S F E R OF TRIS(2,2'-BIPYRIDINE)RUTHENIUM(II) AND A L K Y L VIOLOGEN DICATIONS ACROSS THE WATER/NITROBENZENE, WATER/DICHLOROMETHANE AND WATER/DICHLOROETHANE INTERFACES

Z. SAMEC, D. HOMOLKA, V. MARECEK and L. KAVAN J. Heyrovsk~ Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, U tovdren 254, 102 O0 Prague 10 (Czechoslovakia)

,(Received 9th December 1982)

Tris(2,2'-bipyridine)ruthenium(II) and alkyl viologen dications (Fig. 1) have attracted much attention as the photosensitizer or the electron acceptor, respectively, the redox properties of which fulfil the thermodynamic requirements 2+

a

b

Fig. 1. Structure of tris(2,2'-bipyridine)ruthenium(II) (a) and alkyl viologen (b) dications. for the photodecomposition of water [ 1--3]. In this field, the quenching of the excited state S* of a photosensitizer S by an electron donor D or acceptor A S*+D S*+A

-* S- + D ÷ -* S÷ + A -

(Ia) (Ib)

is the fundamental step [ 1 ]. Since the reverse electron transfer between the products will restore the initial state of the system without any net chemical change, the efficient energy conversion will depend on whether the separated products can be rapidly converted into other useful products or they are preserved in some way [ 1--3 ]. The latter can be accomplished by introduction o f

0022-0728/83/0000--0000/$03.00

© 1983 Elsevier Sequoia S.A.

214 an interface as a kinetic barrier for the back reaction [2,3], i.e. the charge separation (I) proceeds as the interfacial charge transfer, e.g. S*(a) + D(fl) -~ S- (a) + D+(~)

(II)

where a and ~ symbolize the phases to which the reactants or products are confined. Such an interface is generated with water-in-oil microemulsions [ 2] or in micellar solutions [3]. The distribution of the charged reactants between the phases a and ~ and the properties of the electron transfer system (II) can be investigated at the interface between two immiscible electrolyte solutions (ITIES) using the electrochemical approaches [4]. The former problem is to be considered first. In this communication we report briefly on the transfer of tris(2,2'-bipyridine)Ru(II) and t w o alkyl viologens (R = CH3-- or CH3(CH2)6--) across the water/nitrobenzene, water/1,2-dichloroethane and water/dichloromethane interfaces. EXPERIMENTAL Methyl viologen, MVC12 (1,1'-dimethyl-4,4'-bipyridinium chloride, purum) and heptyl viologen, HVBr2 (1,1'-diheptyl-4,4'-bipyridinium bromide) were purchased from Fluka AG and Aldrich, respectively. Ru(bpy)3C12 • 6 H20 (tris(2,2'-bipyridine)ruthenium(II) chloride) was synthesized according to literature [ 5,6]. The oil soluble Ru(bpy)3(TPB)2 • H20 (tris(2,2'-bipyridine)ruthenium(II) tetraphenylborate) was prepared by the precipitation of the preceding c o m p o u n d of sodium tetraphenylborate (puriss. p.a., Fluka AG) in water. The precipitate was dried in the air at 80 ° C. The electrolytic cell for the cyclic voltammetric experiments [ 7 ] and the electronic circuit [8] were described previously. In the cell an interface between the aqueous and the oil phase was formed, which had an area of 56 mm 2 . The base electrolytes were 0.01 M LiC1 in the aqueous phase and 0.01 M tetrab u t y l a m m o n i u m n(3)-l,2-dicarbollyl cobaltate(III) (TBA DCC) [9] in the oil phase. The latter c o m p o u n d was a generous gift from Dr. M. Ple~ek, Institute of Inorganic Chemistry, Czechoslovak Academy of Sciences. Nitrobenzene (p.a., Lachema), 1,2-dichloroethane (puriss. p.a., Fluka AG) and dichloromethane (puriss. p.a., Fluka AG) were used as received for the preparation of the oil phase. The potential difference E of the galvanic cell 0.01 M 0.01 M 0.01 M Ag ] AgC1 ] LiCI(H20) ]1 TBA DCC (oil) I TBACI(H20) I AgC1 ] Ag (RE 1)

(RE 2)

was controlled through the four-electrode potentiostat [8]. Assuming that the association of TBA DCC in the oil phase is negligible, the cell potential difference E is the Galvani potential difference AoW¢ = ¢(w) -- ~(o) between the aqueous and the oil phase related to the formal potential difference for tetrabutylammonium cation, A W o ¢ O TBA÷: E = Awe -- A w o ~oTBA÷ [8] However, this assumption may n o t be quite correct in the case of dichloromethane or 1,2-dichloroethane [ 10]. The determination of the association constant of TBA DCC in these two solvents is in progress.

215 RESULTS AND DISCUSSION Figure 2 shows the cyclic v o l t a m m o g r a m s o f Ru(bpy)~ ÷ ion t r a n s f e r across the w a t e r / n i t r o b e n z e n e i n t e r f a c e o b t a i n e d for Ru(bpy)~ ÷ dissolved e i t h e r in the a q u e o u s phase (full line) or in the n i t r o b e n z e n e phase (dashed line). In Fig. 3 the cyclic v o l t a m m o g r a m s o f Ru(bpy)~ ÷ and m e t h y l viologen dication transfer across t h e w a t e r / 1 , 2 - d i c h l o r o e t h a n e i n t e r f a c e are c o m p a r e d . All t h r e e cations e x h i b i t similar v o l t a m m e t r i c b e h a v i o u r at b o t h the w a t e r / n i t r o b e n z e n e and the w a t e r / 1 , 2 . . d i c h l o r o e t h a n e as well as at the w a t e r / d i c h l o r o m e t h a n e interfaces. It is characteristic for this b e h a v i o u r t h a t the peak p o t e n t i a l s are alm o s t i n d e p e n d e n t o f t h e scan rate (5 m V s-1 t o 100 m V s-1 ) and o f the dicat i o n c o n c e n t r a t i o n , with the p e a k p o t e n t i a l d i f f e r e n c e AEp being close to 30 m V . On the o t h e r h a n d , t h e p e a k c u r r e n t is p r o p o r t i o n a l t o the square r o o t o f the scan rate and t o the dication c o n c e n t r a t i o n . This points t o the simple charge t r a n s f e r r e a c t i o n M2+(w) ~_ M2+(o)

(III)

which is c o n t r o l l e d b y the diffusion o f the d i c a t i o n t o the interface. Conse~1~ were evaluated f r o m the p e a k q u e n t l y , the reversible half-wave p o t e n t i a l s wrev p o t e n t i a l values on p o l a r i z a t i o n t o w a r d s m o r e positive (E~) or m o r e negative (Ep) p o t e n t i a l s [11] Erev 1~ = E p -+ ~ 0 . 0 1 4 3 V

(1)

T h e y are s u m m a r i z e d in Table 1. 50 Ilia 40

30

20

0.2 0

/ -10 -

~

/

/

E/V

J

-20 -30

Fig. 2. Cyclic voltammograms of Ru(bpy)] ÷ ion transfer across the water/nitrobenzene interface after the subtraction of the background current. Besides the base electrolytes, the system contained either 0.5 mM Ru(bpy)3Cl2 • 6 H20 in the aqueous phase ( ) or 0.5 mM Ru(bpy)3(TPB)2 .H20 in the nitrobenzene phase (------). Scan rate 50 mV s-~ .

216 T

r

"I"

T

5pA

0.2

0.3

k

J

EIV

I20~A

J-

Fig. 3. Cyclic v o l t a m m o g r a m s o f R u ( b p y ) ~ ÷ (1) a n d m e t h y l v i o l o g e n (2) d i c a t i o n t r a n s f e r across t h e w a t e r / 1 , 2 - d i c h l o r o e t h a n e interface. C o m p o s i t i o n o f t h e a q u e o u s phase: 0.01 M LiC1 + 0.2 m M MVC12 ; t h e d i c h l o r o e t h a n e p h a s e : 0.01 M T B A DCC + 1 m M R u ( b p y ) 3 ( T P B ) : - H 2 0 . Scan r a t e 50 m V s -1 . Initial p o t e n t i a l : 0 . 2 5 0 V.

TABLE 1 Reversible half-wave p o t e n t i a l s E ~ v o f t h e t r a n s f e r o f R u ( b p y ) ~ ÷, m e t h y l viologen (MV 2÷) a n d h e p t y l v i o l o g e n ( H V 2÷) across t h e w a t e r / o i l i n t e r f a c e Ion

Ru(bpy)~ ÷

Erevl~r 1/2 / - a Water/ nitrobenzene

Water/ 1,2-dichloroethane

Water/ dichloromethane

0.116 b (0.117) c

0.121 b

0.109 b (0.130) c

M V 2÷

0.294

0.391

0.389

H V 2÷

0.13 d

0.138

0.115

a b c d

Relative t o t h e f o r m a l p o t e n t i a l o f T B A ÷ ion. F o r R u ( b p y ) ~ ÷ dissolved in t h e oil phase. F o r R u ( b p y ) ~ ÷ dissolved in t h e a q u e o u s phase. A n a p p r o x i m a t e value derived f r o m a single e x p e r i m e n t .

217 Since the s t a n d a r d Gibbs energies o f t r a n s f e r o f the reference T B A ÷ ion f r o m w a t e r t o n i t r o b e n z e n e , 1 , 2 - d i c h l o r o e t h a n e or d i c h l o r o m e t h a n e d o n o t differ significantly f r o m each o t h e r (cf. Table 2 a n d ref. 19), t h e reversible half-wave p o t e n t i a l s ~,rev reflect m a i n l y the n a t u r e o f t h e d i c a t i o n and the solvent e f f e c t o n the s t a n d a r d Gibbs e n e r g y o f d i c a t i o n transfer f r o m w a t e r t o organic solvent phase. O t h e r c o n t r i b u t i o n s t o EI~ rev , w h i c h are p r e s u m a b l y less significant, m a y arise f r o m the d i f f e r e n c e in the diffusion coefficients o f the d i c a t i o n in the a q u e o u s and the oil phase and f r o m the association b e t w e e n M 2÷ and DCC- in the oil phase. As e x p e c t e d , m e t h y l viologen appears t o be m u c h less h y d r o p h o b i c t h a n h e p t y l viologen, the latter being c o m p a r a b l e with Ru(bpy)~ ÷. The n a t u r e o f the organic solvent has a p p a r e n t l y little e f f e c t o n the s t a n d a r d Gibbs energy o f ~tr ~- - - 22 k J mo1-1 ) a n d H V 2÷ transfer o f t h e h y d r o p h o b i c Ru(bpy)~ ÷ tl ~ ^ ' ~o,w-.o ( A t'2-° - - - 21 kJ tool -1 ). This is quite in a c c o r d a n c e with the b e h a v i o u r o f "~tr~W--->O -----o t h e r b u l k y h y d r o p h o b i c ions like t e t r a b u t y l a m m o n i u m , t e t r a p e n t y l a m m o n i u m , t e t r a h e x y l a m m o n i u m a n d t e t r a p h e n y l a r s o n i u m cations or t e t r a p h e n y l b o r a t e a n i o n (cf. Table 2). On the o t h e r h a n d , the t r a n s f e r o f the h y d r o p h i l i c MV 2÷ f r o m w a t e r b e c o m e s m u c h easier w h e n less p o l a r 1 , 2 - d i c h l o r o e t h a n e (dielectric p e r m i t i v i t y e 298 = 10.23) or d i c h l o r o m e t h a n e (e 298 = 8.93) is replaced TABLE 2 Standard Gibbs energies of transfer from water to oil ~ GtrW.x° (in kJ mo1-1 ) for various monovalent cations and anions. The corresponding standar~ electrical potential differences o w-->o A ° 4~ = AGtr,X /zF (in V) are given in parentheses Ion

Water/ nitrobenzene a

Water/ 1,2-dichloroethane b

Water/ dichloromethane c

M%N÷ Et,N ÷ Pr4N÷ Bu,N ÷ Pe,N ÷ He,N ÷ Ph,As ÷

3.3 -- 5.7 --15.5 --24.0 --39.5 --45.5 --35.9

17.6 4.2 -- 8.8 --21.8 --34.7 --47.7 --35.1

(0.182) (0.044) (--0.091) (--0.225) (--0.360) (--0.494) (--0.364)

18.8 4.2 -- 8.8 --22.2 --36.4 --43.9

(0.195) (0.044) (--0.091) (--0.230) (--0.377) (--0.455)

46.4 38.5 26.4 17.2

(--0.481) (--0.399) (--0.273) (--0.178)

46.4 39.3 26.4 21.3 6.7

(--0.481) (--0.408) (--0.273) (--0.221) (--0.069)

C1BrIC10; PicratePh4B-

(0.035) (--0.059) (--t}.161) d (--0.248) (--0.408) e (--0.472) e (--0.372)

29.7 (--0.308) 18.8 8.0 -- 4.6 --35.9

(--0.195) (--0.083) (0.048) (0.372)

--35.1 ( 0 . 3 6 4 )

a Calculated [9] from partition data [12] on the basis of "Ph4AsPh4B" assumption [13]. b Calculated [14] from partition data [14] on the basis of "Ph,AsPh,B" assumption [13]. c Calculated [15 ] from partition data [16] and expressed on arbitrary energy scale. For to maintain the consistency with data for nitrobenzene and 1,2-dichloroethane, the calculated values [15 ] for cations were made more negative by 4.184 kJ tool -1 and those for anions more positive by 4.184 kJ mo1-1 . d From ref. 17. e Calculated from partition data [ 18 ] for Pe,NC1, He4NCI and Bu4NC1, taking n o O ~ B A + = --0.248 V.

218 b y m o r e p o l a r n i t r o b e n z e n e (e 29s = 34.82), cf. also the b e h a v i o u r o f t e t r a m e t h y l a m m o n i u m or t e t r a e t h y l a m m o n i u m cations and m o s t o f the anions in Table 2. I n v e s t i a t i o n o f the transfer o f o t h e r alkyl viologens is in progress. Finally, t h e s y s t e m s c o n t a i n i n g m e t h y l viologen in w a t e r and R u ( b p y ) ~ ÷ in 1 , 2 - d i c h l o r o e t h a n e or d i c h l o r o m e t h a n e o f f e r a quite appreciable range of potentials, within w h i c h the e l e c t r o n transfer (II) m i g h t be investigated. The potential can be c o n t r o l l e d either t h r o u g h the f o u r - e l e c t r o d e p o t e n t i o s t a t or b y the p a r t i t i o n equilibrium o f the suitable salt.

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