Ion transfer of tetraalkylammonium cations at an interface between frozen aqueous solution and...

ELSEVIER Journal of Electroanalytical Chemistry 424 (1997) 159-164

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Ion transfer of tetraalkylammonium cations at an interface between frozen aqueous solution and 1,2-dichloroethane

Md. Aminur Rahman, Hidekazu Doe *

Department of Chemistry, Faculty of Science, Osaka City University, Sumiyoshi-ku. Osaka 558, Japan

Received 12 April 1996; revised 19 July 1996

Abstract

Cyclic voltammetry with a frozen electrode has been used t,~ investigate the ion transfer of tetramethylammonium (TMA+), tetraethylammonium (TEA 4), and tetrapropylammonium (TPrA +) across the waterl1,2-dichloroethane (DCE) interface at - 15°C and to compare these results with the liquidlliquid system at 25°C. All the half-wave potentials of these ion transfers shift to negative potential in the frozen systena, and the ion transfers exhibit less reversibility comparing transfers in the liquid system with the same composition at 25°(=. These negative shifts indicate that the ionic solvations are unstable in the frozen phase. The degree of negative shifts increases in the order TMA÷< TEA÷< TPrA 4. This order has been discussed in terms of the aqueous solvation of these ions. The diffusion coefficients of ions in the aqueous phase have been evaluated, and a very slow transport process found in the frozen system. In both frozen and liquid systems these values decrease in the order TMA 4> TEA 4> TPrA +. The apparent standard rate constants have also been evaluated for both systems. The values in the liquid system are larger than those in the frozen system, and both increase in the order TMA+< TEA+< TPrA 4. That is, the ion transfer rate increases but the ion transport rate decreases with the Stokes radius of the ion in both systems.

Keywords: ITIES; Interfacial ion transfer; Frozen electrode; Tetraalkylammonium cations

1. Introduction

Ion tr~asfer across the interface between two immisci- ble electrolyte solutions (ITIES) has been extensively studied. Chronopotentiometry [1-6], polarography with an electrolyte dropping electrode [7-9], and cyclic voltammetry, [ 10-13] are particularly useful for the studies of ITIES. These techniques were applied to quantitative investiga- tions of tetraalkylammonium ions from tetramethytammo- nium (TMA ÷) [4,5,8,10] through tetrabutylammonium (TBA ÷) [ 1-4,10] up to tetrapentylammonium (TPA ÷) [3] cations from water to nitrobenzene and TMA + and TBA ÷ cations from water to 1,2-dichloroethane (DCE) interfaces [14]. These studies were made at a temperature in the vicinity of room temperature. As a new approach to the nature of ITIES, in order to study the ion transfer reaction in frozen solution, recently a frozen electrode was devel- oped and characterized in 1,2-dichloroethane containing various electrolytes [ 15].

* Corresponding author.

In the last 15 years, electrochemical studies have been carried out in the frozen media to get useful information about electrolyte properties, activation energies, transfer coefficients, and so on [16-21]. Voltammetric behavior of a redox couple at a platinum microdisk electrode in the frozen LiCI solution has been reported and discussed from the kinetic and thermodynamic points of view [22]. The Gibbs energy of transfer across an interface is one of the most important thermodynamic parameters in order to describe the solvation phenomenon, and the electrochemical study of ITIES is very powerful in the evaluation of this parameter. Since the dielectric constant of the solvent, the thermal motion of solvent molecules, and the number of hydrogen bonds among water molecules vary with temperature, the Gibbs transfer energy may be affected by temperature [23]. Therefore, electrochemical studies of the frozen electrode will give us very interesting information about ionic solvation in frozen aqueous electrolytes [15]. The subject of this paper is the evaluation of thermodynamic, transport, and kinetic parameters regarding the ion transfer of tetraalkylammonium cations from frozen water to 1,2-dichloroethane at - 15°C using a frozen electrode

0022-0728/97/$17.00 Copyright © 1997 Elsevier Science S.A. All rights reserved. PI! S0022-0728(96)04906-6

160 Md.A. Rahman, H. Doe/Journal of E/ectroanalyticai Chemistry 424 (1997) 159-164

and the comparative discussion of these results with those of the liquidlliquid system with the same composition at 25°C.

2. Experimental

A pasteur pipette (IK-PAS-9P, Iwaki Glass, USA) was used to make the frozen electrode. First, the pasteur pipette (1,7mm in diameter after cut-down about l l0mm from the tip) was placed on a flat surface to put some highly water absorbent polymer (obtainable from sanitary pam- pers etc., Kao Corporation, Japan) from the upper side of the pipette. It w~s then immersed in an aqueous sample solution containing 0.70M Li2SO 4 as supporting electrolyte, so that the lower part of the polymer absorbed sufficient solution to be a kind of macromolecular gel. The upper part of the polymer contained 0.70M Li2SO 4 + 0.30M LiCi, and a AglAgC! electrode (0.3 mm in diameter and 60mm long) was dipped in this part. Since it was confirmed that this gel electrode gave the same cyclic voltammograms for ion transfer as those measured with the conventional liquidlliquid system in DCE, all the ion transfers in the liquidlliquid system were measured with this electrode, Finally, this gel electrode was dipped into liquid nitrogen where it froze instantly in order to com- plete the preparation of a frozen electrode. The surface area of the electrode was 0.036cm 2, which was evaluated by referring to the published diffusion coefficient of TEA + [24]. This area differs from the geometric surface area of the tip, which indicates that the surface of the electrode is

4

3 5

Fig. !. Voltammetric cell with a water jacket for temperature control: (!) aqueous phase contained in highly water absorbent polymer; (2) organic phase; (3) reference electrode; (4) and (5) counter electrodes.

rough. This frozen electrode differs slightly from previously reported frozen electrodes [15]. Since 0.3M LiC! was used as supporting electrolyte in the previous aqueous phase, the potential window was narrower than the present one. Fig. 1 shows the scheme of the voltammetric cell. The cell volume was about 7.0cm 3 and it was thermostated at - 15.0 + 0.1°C for the frozen system and 25.0 + O.I°C for the liquidlliquid system by circulating an ethylene glycol + water mixture through the water jacket. All measurements were carried out in an air conditioned room at 25°C.

The cell structure was as follows:

A,,A,Cll07MLi2SO I 07MLi,SO 1005 THATPB 0.2 mM X (organic phase)

0.3 M LiC! (aqueous phase) 0.3M LiCl IAgCI[Ag

0.01 M NaTPB

where X, THATPB and NaTPB denote the investigated tetraalkylammonium chloride, tetraheptylammonium tetraphenylborate and sodium tetraphenylborate respectively. A computer controlled apparatus described elsewhere [25] was t,sed for the cyclic voltammetric measurements. The ohmic potential drop of the cell was compensated by a positive feedback circuit of a Fuso HECS 310B differential potentiostat. Lithium sulfate and tetraheptylammonium tetraphenylborate were used as supporting electrolytes in water and DCE phases respectively. Aqueous solutions were prepared from water purified with a miUipore filter (Miili-Q Labo, Nihon Miilipore, Kogyo). DCE (Nacalai Tesque, GR) was used as received. The preparation of THATPB was described elsewhere [25]. All other reagents were of guaranteed grade (Nacalai Tesque, GR) and were used without further purification.

3. Results and discussion

3.1. Shift in half-wave potential

Cyclic voltammograms of TMA+, TEA + and TPrA + cations in liquid lliquid systems are shown in Fig. 2. Fig. 3 shows the voltammograms of TMA + and TEA + ion transfer with background voltammogram in frozen systems. All electrode potentials of liquidlliquid systems at 25.0°C are referred to the half-wave potential of TEA + transfer, Ez/2(TEA+). Both the shapes and widths of the potential windows of the background voltammogram are very similar for the liquid[liquid and the frozen systems. From previous results [15], the half-wave potential of TEA + in the frozen system is determined as - 13 mV, assuming that the transfer of supporting CI- in the frozen system occurs

Md.A. Rahman, H. Doe / Jounml of Electroanalytical Chemistry 424 (1997) 159-164 16 i

4

c

2

o

-2 - i

I

-~00 -200 I I I I

-100 0 100 200 300

E I mV (vs. E1/2(TEA)I

Fig. 2. Cyclic voltammograms of the liquid systems at 250C: (a) TMA *; (b) TEA +; (c) TPrA +. 2.0 × I 0- 4 M quaternary ammonium salt + 0.70 M LiaSO 4 in the aqueous phase; 5.0× 10-2M THATPB in the organic phase; scan rate 50mVs -I .

at the same electrode potential as that in the liquid lliquid system. According to this assumption, all half-wave potentials of the studied ions in the frozen system are shifted to more negative potentials than those of the liquidlliquid system. This means that the transfer of ions from the frozen aqueous to the DCE phase is more thermodynami- cally favorable than that from the liquid aqueous phase; that is, the Gibbs energy of transfer in the frozen system is more negative than that in the liquid system. As the TPrA + peaks in the liquidlliquid system appear near the negative end of the potential window, its anodic peak is observed only in the frozen system (see Fig. 4). qlaus, the half-wave potential of TPrA ÷ in the frozen system was estimated by subtracting 30mV from the anodic peak potential at 50 mV s-~ scan rate. These results are summarized in Table 1.

As shown in Table 1, the E~/2 values decrease in the order T M A + > T E A + > T P r A + in both the liquid and frozen systems; that is, the Gibbs energy of ion transfer decreases with an increase in the size of ion (Wandlowski

1 . 5 - -

a

1.0 b

0.5

~ o.o

-0.5 l .... ':

-1.0

-1.5 I i i t i I -200 -100 0 100 200 30• 400

E I mV (vs. Ell2(TEA))

Fig. 3. Cyclic voltammograms of the frozen systems at -15°C with background voltmmogram (dotted line): (a) TMA+; (b) TEA +. Other conditions are the same as Fig. 2.

et al. observed the same order in a waterlnitrobenzene system at 293 K [26]). This behavior can be un&~tood in terms of the electrostatic and non-electrostatic parts of the Gibbs transfer energy; that is, the former value gradually approaches zero and the latter one becomes more negative as the ionic radius increases [27,28]. The differences in half-wave potential, A E l / 2 , b e t w e e n the liquid and frozen systems for the respective ions ate also :~ummarized in Table 1. The magnitude of the negative shift in E~/2, the value - A E,/2, increases in the order TMA + < TEA + < TPrA +. Before this fact is discussed, it is necessary to consider the temperature dependence of E~/2. Samec and coworkers studied the change in E~/2 with temperature in detail at a waterlnitrobenzene interface [24]. According to their results, E~/2 shows a slight negative shift with decreasing temperature. Although Doe et al. observed large negative shifts in Em/2 at some aqueous frozenlDCE interfaces [15], these shifts were attributed to aqueous phase freezing. In their discussion, the temperature dependence of ion associations of studied ions with TPB in DCE, which is one possibility in a series of elementary steps of

Table 1 Comparable wave parameters between liquid and frozen systems

Liquid system (25°C)

Ell 2 ~/mV A Ep/mV Ipa/lpc

TMA + 128 60 0.92 TEA + 0 60 0.90 TPrA + - 122 59 0.90

Frozen system ( - ! 5°C)

Ej/2/mV A Ep/mV ~a/~c AEI/2 b/mV 124 62 1.15 -4

- 13 c 62 1.25 - 13 - 1 7 4 d ~ ~ - - 5 2

a Values referred to the El~ 2 of TEA + transfer. b A El~ 2 = Ei/2(frozen ) - Ei/2(liquid). c Ref. [15].

See discussions.

162 Md.A. Rahman, H. Doe~Journal of Electroanalytical Chemistry 424 (1997) 159-164

1.5

1.0

0.5

0.0

4 1 . 5 -

- 1 . 0

- 1 . 5

- 2 . 0 I I , t , , -200 -100 0 100 200 300 400

E I mV (vs. E 112(TEA))

Fig. 4. Cyclic voltammograms of TPrA + in the frozen system at different scan rates: 50, 100, 200 and 300mVs- ~. The background voltammogram is shown by a dotted line. Other conditions are the same as Fig. 2.

ion transfer, is small. That is to say, the temperature dependence of these ion associations does not make a large contribution to the value of A E~/2 because the DCE phase does no~ freeze at -15°C. Thus, the large negative value of AE,/2 should be caused mainly by the difference in Gibbs energy of the elementary ion transfer step before the ion association in DCE between the liquid and frozen systems. Then the negative value of A Em/2 means that the ionic solvation is more unstable in the frozen phase than in the liquid one; that is, the Gibbs energy of ionic solvation is less negative in the frozen phase than in the liquid one.

Furthermore, the order TMA + < TEA + < TPrA + of AEt/2 means that the negativity of the Gi0bs energy increases in that order. TPrA + is a hydrt, phobic structure making ion in water and TEA + is slightly structure making, but TMA + is neither a structure breaking nor a structure making ion, considered as an intermediate [29]. Because the network structure of frozen water is much stronger than that of liquid water through hydrogen bonds, it is very difficult to produce a structure making region around the ions in the frozen system. On the contrary, it was reported in the previous work [15] that a typical structure breaking ion K + also has a large negative A E~/E; this was then attributed to the large energy required to break the strong network structure of frozen water during solvation. Considering these results, it will be concluded in general that ions which are solvated by making or breaking the network structure of water are more unstable in frozen water than in liquid water. The unstable solvations for these ions in the frozen phase should bring about such large negative AEt/2 values.

3.2. Determination of transport parameters

The transport behavior of these ions is influenced to a significant extent by tk,e enforcement of water structure about their hydrocarbon chain [30]. This influence from the enforcement of water structure will increase with increasing size of ion involved. This will also become greater in frozen water because of its strong network structure. The values of the diffusion coefficient of studied ions in the aqueous phase, D w, are listed in Table 2. The temperature dependence of the aqueous diffusion coefficient of TMA ÷ and TEA + was studied by Samec and coworkers, where the D , values of these ions were almost the same as each other, especially at 10, 20, 30°(2 [24]. An almost linear relationship is found when log D w is plotted against the reciprocal of temperature using their data.

Extrapolating the D , of those ions at - 15°C from this plot, it is estimated as 3.2 × 10 -6 cm 2 s - I , which is larger than our experimentally observed values. This is not sur- prising because the aqueous freezing was not taken into account in that estimation. However, it is very interesting that the rate of decrease in the D w values with size of ion, which decrease in the order TMA+> TEA+> TPrA ÷, is much larger at - 1 5 ° C than at .:5°C, as shown in Fig. 5. This result shows that the enforcement from the strong network structure of frozen water works very effectively against ions bigger than the size of hole in the network, and besides affects the ion transport very sensitively de- pending on ion size.

3. 3. Determination of kinetic parameters

The apparent standard rate constants of the studied ions, k s, were determined by the method of Nicholson [31], and are summarized in Table 2. A value of the transfer coefficient a equal to 0.5 was used. No effort was made to determine t~ accurately. In both systems, the value of k s increases as T M A + < TEA+< TPrA +, that is in the reverse order to D w. As shown in Fig. 6, k s increases with increasing size of ions. As ionic hydration becomes unstable with an increase in ion size (see the discussion in Section 3.1), the dehydration process in ion transfer may occur more easily with the larger ion. The values of k s for these tetraalkylammonium cations can be compared with

Table 2 Comparable transport and kinetic parameters between liquid and frozen systems

Liquid system (25°C) Frozen system ( - 15°C)

105Dw/cm 2 s- i ks/cms- I 106Dw/cm 2 s- i ks/cms- i

TMA + 1.2 0.12 2.2 0.033 TEA + 1.05 0.16 0.88 0.037 TPrA + 0.82 0.20 0.49 0.040 a

"~ See discussions.

Md.A. Rahman. H. Doe/Journal of Electroanalytical Chemistry 424 (1997) 159-164 163

-4.5

-5.0

A

to

- 5 . 5 -

O ) o

-6.0

__O

-6.5 t t t t t 2.0 2.5 3.0 3.5 4.0 4.5 5.0

107rs Icm

Fig. 5. Correlation of aqueous diffusion coefficient/9, and Stokes radius r s [33]: (A) liquid system; (B) frozen system.

previously published a.c. polarographic studies at a waterlnitrobenzene interface. Our values are similar to theirs even in a different organic phase (DCE), although the order of magnitude is not the same [32]. This may be caused by the different supporting electrolyte and organic

solvent. As already discussed, the Gibbs solvation energies of

the ions studied here are less negative in the frozen aqueous phase than in the liquid one. It may then be expected that the activation energy of ion transfer from the frozen phase is also lower than that from the liquid one;

- 0 . 6

- 0 . 8

-1.0 -

g

~ -1.2

- 1 . 4

A

- !

I I I i t - 1 . 6

2.0 2.5 3.0 3.5 4.0 4.5 5.0

107r, lem

Fig. 6. Correlation of apparent standard rate constants k s with Stokes radius r s [33]: (A) liquid system; (B) frozen system.

that is, the desolvation process is faster in the frozen phase than in the liquid one. Before the k s values are discussed from this point of view, we need to consider the temperature dependence of rate constants, because rate constants generally decrease with a decrease in temperature. Samec and coworkers also studied the temperature dependence of rate constants of some tetraalkylammonium cation transfers in a waterlnitrobenzene system and found a linear relation between log k s and the reciprocal of temperature [24]. Extrapolating the k s value at - 15°C from that relation on the assumption that the k s values in our systems show the same temperature dependence as theirs, values of 0.027 and 0.032 cm s - ! are estimated for TMA + and TEA + respectively. These values are somewhat smaller than our experimentally observed ones. Since aqueous phase freezing is not taken into account in these estima- tions, it seems that freezing makes the k s value a little high.

Fig. 6 shows a further interesting effect of the freezing of the aqueous phase. As shown in the figure, the dependence of k s values on the size of ion is very small in the frozen system compared with the liquid system. This is the opposite tendency to that of transport properties, Dw (see Fig. 5). The strong network structure of the frozen aqueous phase decreases the Dw value greatly, and it is affected by the ion size more sensitively in frozen water than in liquid water, as already mentioned. In contrast, the rate constant k s is little affected by the network structure of the frozen aqueous phase, or the influence of the network structure may be cancelled out by some unknown properties of water. Although it is difficult to explain the small dependence of k s on the ion size anyway, this phenomenon may mean that the rate constant k s is relatively free from three-dimensional effects of the network structure because of a heterogeneous reaction at the frozen surface. The fact that a remarkable increase in the k s value is not observed with aqueous freezing should also be due to the same

cause.

4 . C o n c l u s i o n s

Thermodynamic, transport and kinetic parameters of ion transfer of TMA +, TEA + ~-.:t TPrA + at an interface between liquidlliquid and liquidlfrozen phases have been evaluated. All the half-wave potentials, E~/2, shift in the negative direction in the frozen system. These negative shifts are mainly contributed by the less negative Gibbs solvation energies of those ions in frozen water compared with liquid water. The amount of negative shift increases in the order TMA+< TEA +< TP rA+, which also shows the order of the less negativity of the Gibbs energy. The diffusion coefficient in the aqueous phase/9 , is decreased by aqueous phase freezing at -15°C. Although the Dw value decreases in the order T M A + > TEA+> T PrA+ in both the liquid and frozen systems, the degree of decrease

164 Md.A. Rahman, H. Doe/Journal of Electroanalytical Chemistry 424 (1997) 159-164

is much larger in the frozen system than in the liquid one. This means that the enforcement effect of the network structure through hydrogen bonds is very sensitive to the size of the ion, since the frozen phase has a much stronger network structure than the liquid one. On the contrary, the apparent standard rate constants k, increase in the order TMA+< TEA+< TPrA + in both systems, that is in the reverse order to Dw. Although it is expected that this result also has some relation to the network structure, the k s values depend little on the size of ion in the frozen system and are not remarkably affected by aqueous phase freezing. Hence the strong network structure of the frozen phase seems to interact effectively with transport properties in a homogeneous phase, but not so much with transfer properties at a heterogeneous interface.

Acknowledgements

This study was partly supported by a Grant-in-Aid for Scientific Research (No. 07804049) from the Ministry of Education, Science and Culture, Japan. Md.A.R. also thanks the Ministry of Education, Science and Culture, Japan for support as a scholarship student in Japan.

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