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The effect of anions on the phase transition in the adsorbed layer of thiourea at the mercury-aqueous solution interface
201
J. Electroanal. Chem., 212 (1989) 207-216 Elsevier Sequoia S.A., Lausamre - Printed in The Netherlands
The effect of anions on the phase transition in the adsorbed layer of thiourea at the mercury-aqueous solution interface M. Skompska and K. Jaszczynski Laboratory of Electrochemistry, 02-093 Warszawa (Poland) (Received
Chemistry Department,
Warsaw University, ul. Pasteura I,
10 February 1989; in revised form 19 June 1989)
ABSTRACT
The condensation of thiourea (TU) molecules at a mercury electrode in concentrated aqueous solutions was investigated by using impedance measurements and cyclic voltammetric techniques. The effects of the nature and concentration of the supporting electrolyte, the temperature and the thiourea concentration on the position of the phase transition potentials are discussed. It is concluded that the hydrogen bonds between the -NH, groups of TU and water molecules from the outer part of the double layer are an immobilizing agent for the adsorbed TU molecules, which leads to a stable structure in the “pit” zone. Electrostatic interaction between the TU dipoles and anions is responsibile for the formation of a condensed structure at the positively-charged mercury electrode.
INTRODUCTION
The electrochemical behaviour of thiourea (TU) at the mercury-aqueous solution interface has been studied widely in the presence of various supporting electrolytes [l-9]. It was found that thiourea behaves more like an anion than a neutral organic molecule [2]. In 1987 Buess-Herman et al. [8] reported that in the presence of strongly hydrated ions (F-, SO:-, CO:-, CH,COO-, PO:-) thiourea formed two distinct condensed monolayers totally devoid of anions, provided that the TU concentrations was high and the temperature was low. They stated that the capacity-potential profiles obtained in these supporting electrolytes are practically unaffected by the anion concentrations. The appearance of two condensed films, which are stable in particular ranges of potentials, resulted in capacity troughs in the C-E curve called a “pit” and a “dip”, separated by a high pseudo-capacitive peak. These interfacial changes occurred within a very narrow range of potential and indicated that phase transitions were involved. OO22-0728/89/$03.50
0 1989 Hsevier
Sequoia S.A.
208
Our results show partial disagreement in comparison with those cited above. We found an influence of the concentration of all the anions studied (SO,‘-, CH,COO-, ClO; , NO;) on both phase transition processes. Moreover, dilution of the supporting electrolyte in the presence of sulphates or acetates shifts the negative boundary of the “pit” region in the opposite direction to that for perchlorates or nitrates. We also found a similarity of the C-E profiles obtained in the solutions of different anions, when the nature of the anions was taken into account. The aim of our investigations was to find the reason for these differences and similarities. EXPERIMENTAL
A conventional water-jacket cell with the usual three-electrode configuration was used. The working electrode was a hanging mercury drop electrode (HMDE) of the type described by Kemula and Kublik [lo], with a surface area of 0.037 cm*. A platinum sheet was the counter-electrode. All potentials are referred to a saturated calomel electrode (SCE). The solutions of thiourea in an appropriate supporting electrolyte (Na,SO,, NaOOCCH,, NaClO,, NaNO,) were prepared from recrystallized chemicals and triply-distilled water. All experiments were carried out in solutions acidified to pH 4 using the acids corresponding to the anions. Acidification was necessary to remove traces of S*- from the solution to avoid the formation of HgS at a potential of about -0.620 V (vs. SCE). Solutions were prepared just before the experiments to eliminate the oxidation of TU. All solutions were deaerated with purified argon. Voltammograms were recorded using an EP-20 potentiostat coupled with a programmable EG-20 generator (both by ELPAN) and a R&en Denshi X-Y recorder. Capacity data were obtained from impedance measurements carried out in an electrochemical system consisting of a transfer function analyser (type 272 UNIPAN) equipped with a Neptune 184 computer system controller. Measurements were performed potentiostatically with potentials ranging from - 1.6 to - 0.4 V. A sinewave of 5 mV,, amplitude and at a frequency of 225 Hz was used. The values of the differential capacity determined from the impedance measurements are in agreement with the results obtained using a conventional symmetrical ac bridge [ll] on a HMDE and a DME as well. RESULTS
AND
DISCUSSION
Cyclic voltammetry is the most convenient method for preliminary examination of an electrochemical process. Figure 1 shows a comparison between a typical voltammogram and a differential capacity-potential curve. As can be seen, the potentials of the sharp and very symmetrical peaks in the i-E curve correspond exactly to the potentials at which rapid changes in the capacitance values are observed. Owing to their only slight dependence on the sweep rate, the positions of the peaks were assumed to be the potentials of the phase transition in the adsorbed layer at the mercury-solution interface.
209
Fig. 1. Capacity-potential curve (f = 225 Hz) (a) and cyclic voltammogram solution of 1 M NaClO, containing 1.2 M TU at 15 o C.
(o = 20 mV/s)
(b) in a
Figure 2 presents the capacity-potential curves obtained in a solution containing 1.2 M TU in various supporting electrolytes. Comparative analysis of the results allows us to distinguish four domains in the C-E plots.
-0.6
- 0.8
-1.0
-1.2
-1.4
E/V
Fig. 2. Capacity-potential curves in solutions of 1.2 M TU in the presence of various supporting electrolytes at lS” C. (1) 1 M NaOOCCH,; (2) 1 M Na,SO,; (3) 1 M NaCLO,; (4) 1 M NaNO,. Dashed lines: in the supporting electrolytes.
Zone A. Non-condensed At a potential blank supporting layer of hydrated positive is caused
region
of - 1.6 V (vs. SCE), the C-E curves coincide with those for the electrolytes. The surface of the mercury electrode is covered with a cations. The increase of the capacity as the potential is made more by the adsorption of thiourea.
Zone B. High& organized, quasi-solid film region At the potential E,, the differential capacity-potential plots show drastic discontinuities. The capacity decreases from 32 PF cm-2 to about 15 PF cmm2 over a span of a few millivolts (Fig. 2, curves 1 and 2). This sharp decrease in capacity reflects significant changes in the inner-layer structure the adsorbed TU molecules rearrange from a diluted to a highly-organized, quasi-solid film. The region of existence of this compact structure, j-e. the width of the so-called “pit” zone in the C-E curve, is strongly dependent on the temperature and the TU concentration, as has been reported by Buess-Herman et al. [8]. The results presented in Fig. 3 show an interesting feature of this region. The walls of the trough are symmetrical with
Fig. 3. Effect of the temperature on the C-E Na,SO,, (b) 1 M NaCIO., and (c) 1 M NaNO,.
plots in solutions of 1.2 M TU containing (a) 1 M
211
I
C/pFcm-z
C/pFcrK2
,.“_J 22’C
I I 21-c 18’1:15’c
a
b
I
46
-0.8
-1.0
E/ V
-0.6
-08
-1.0
E/V
I
-0.6
-0.8
-1.0
E/V
212
respect to a potential of about -0.760 V. This value is probably close to the potential of zero charge [3,8]. As the temperature is increased, both walls shift towards this characteristic value and the bottom of the trough rises. Above a critical temperature Tc,B,condensation does not proceed. Buess-Herman et al. claimed that in the presence of hydrophilic supporting anions the capacity behaviour is practically unaffected by the electrolyte concentration. Therefore, they suggested that the anions of this group do not take part in the phase transition processes. Our results, however, show that the position of the boundary between the diluted and the condensed film (potential E,) depends strongly on the nature of the anion as well as on its concentration. The data in Table 1 indicate that the potential E, shifts from -0.955 to -0.885 V when the concentration of SO:- or CH,COO- anions is lowered from 1 to 0.1 M, respectively. Moreover, in the presence of NO; or ClO; anions this potential shifts in the opposite direction from - 0.860 to - 0.890 V when the supporting electrolyte is diluted. Other information on the stability of the compact structure of zone B may be taken from the phase transition diagrams (Fig. 4). It is of interest to note that when the concentration of the supporting electrolytes was lowered to 0.1 M, similar C-E profiles and nearly identical values of the potential E, were obtained regardless of the nature of anion and the temperature. Furthermore, the critical temperature, above which condensation does not occur, is also strongly dependent on the nature of the anion and its concentration. In our opinion, the anions may indirectly modify the molecular structure of the compact layer. The molecules of thiourea from the condensed interfacial layer interact not
TABLE 1 Effect of the nature and concentration of the supporting electrolytes on the boundaries between zones A, B and C and on the “critical” and “melting” temperatures of zone B in solutions of 1.2 M TLJ Supporting electrolyte
c/M
Na,SO,
Et /V at 15OC
Critical temp. k/OC
?,=I, / o C
1.0 0.5 0.1
- 0.954 -0.908 -0.890
22 20 19
21 19 18
0.275 0.240 0.230
- 0.680 - 0.670 - 0.660
NaOOCCH,
1.0 0.5 0.1
- 0.955 - 0.915 - 0.885
22 21.5 20
20 20 18.5
0.300 0.295 0.285
- 0.650 - 0.620 -0.600
NaClO,
1.0 0.5 0.1
-0.855 - 0.875 -0.890
16.5 18 18
14.5 16 16
0.170 0.220 0.250
- 0.660 - 0.660 - 0.640
NaNO,
1.0 0.3 0.1
- 0.860 - 0.873 - 0.885
16 18 18
10 12 13
0.135 0.200 0.240
- 0.610 - 0.615 - 0.610
Zone B width at 15’C /V
E,/V
at 15OC
213
I
-(Is
- 0.6
-a,
-0.8
-09
Fig. 4. Phase transition diagrams in solutions of 1.2 M TU containing (a) 1 M supporting electrolytes and @) 0.1 M supporting electrolytes. (. -. - .) Na2S04; ( X X) NaOOCCH,; (A -A) NaCIO,; NaN03. (0 -0)
only with mercury but also with molecules of the solvent from the outer part of the double layer. Water molecules probably form hydrogen bonds with the amino groups of thiourea. In this way, unfavourable repulsion between the adjacent TU dipoles is diminished. The stability of such a highly organized TU-water film should depend on the mobility of the water molecules. This may explain the high stability of the TU film in sulphate or acetate supporting electrolytes. They are known as structure-making anions, i.e. they decrease the flmdity of water and thus immobilize the compact layer. NO; or ClO; ions cause the oposite effect. In their vicinity, water molecules become more mobile than those in pure water. Therefore at high (1 M) ClO; or NO; concentrations the condensation may occur only at low temperature. As can be seen in Figs. 3a and 3b, the C-E profiles in solutions of 1.2 M TU + 1 M Na,SO, and 1.2 M TU + 1 M NaClO, become nearly identical when the temperature of the second solution is about 6 o C lower than that of sulphates. In this way, the same highly organized arrangement of an interfacial TU-H,O layer may be achieved despite the water structure-breaking properties of ClO; anions. Zones C and D Further polarization towards more positive potentials leads to a subsequent structural transition in the compact layer. A second trough in the C-E curve (zone
214
C), usually proceeded by a sharp capacitance peak at the potential E,, can be observed. Detailed measurements carried out for a wide range of temperatures and anion concentration show that the mechanism of the process is more complicated than was suggested by Buess-Herman et al. [S]. Returning to Fig. 3, one can observe a small hump inside the trough of zone B. When the temperature is increased, the hump becomes sharper and is probably responsibile for raising the bottom of the trough. This process facilitates the destruction of the interfacial compact structure. The hump always occurs at a slightly positive electrode charge and its position depends on both the adsorbability of the anions and the temperature. It is especially well developed in the solution of nitrate. All these observations allow us to consider the hump as the result of interaction of the anions and the positively charged electrode. With an increase of temperature, the hydrogen bonds between the -NH, groups of TU and water molecules are weakened and attracted anions may destroy the compact structure. The adsorbed TU molecules rearrange into a diluted, non-condensed film. In consequence, in the C-E curves there occurs a region of raised capacity, denoted by us as zone D, which separates zones B and C. We call the temperature above which zone D exists the “melting” temperature, and it is marked on the phase transition diagram as T, (Fig. 4b). Above the “critical” temperature Tc,B, when the condensation in zone B does not occur, the hump transforms into a sharp maximum. Further increase of the temperature as well as dilution of the supporting electrolyte causes it to shift towards more positive
I -0.6
-0.8
-1.0
Fig. 5. Effect of the concentration of TU on the C-E (1) 1.2; (2) 0.95; (3) 0.8; (4) 0.5.
-1.2
-14
E/V
curves in 1 M NaClO, solution at 12O C. [TU]/M:
215
potentials. The stronger the adsorbability of the anion, the lower is the “melting” temperature and the higher is the hump. In our opinion, the anions take part in the subsequent structural transition process. It is difficult, however, to determine definitively whether the anions intrude into the compact layer or whether they remain outside the adsorbed TU molecules. Buess - Herman et al. suggested that in the presence of Na,SO, as the supporting electrolyte the inner layer is devoid of anions. The similarity in the general shape of the C-E profiles and the phase transition graphs obtained in all the solutions studied (Fig. 4) allows us to suppose that other anions also remain outside the condensed layer of TU molecules. However, these anions may act as bridges between the adjacent adsorbed TU molecules, thus causing the relatively high stability of the compact layer formed in zone C. Obviously these considerations deal with the extreme condition - low temperature and high TU concentration. A decrease of the TU concentration causes changes in the C-E profiles in the region of the studied potentials. The rising part of the curve in zone A is shifted towards more positive potentials and region B is squeezed (Fig. 5). At a sufficiently low concentration of TU (60% of the saturated value in 1 M NaClO, at 12OC), zone B is no longer present. The condensation takes place probably only when a definite surface concentration of TU is achieved. The width of zone C is not as sensitive to the lowering of the TU concentration as the previous one. Unfortunately, the studies are limited to a potential of about -0.38 V, since beyond this value anodic dissolution of mercury occurs. CONCLUSIONS
The adsorbed molecules of thiourea may form two distinct condensed monolayers under extreme conditions of temperature and TU concentration. The difference in stability of the two structures seems to be connected mainly with the immobilizing agents of the superficial film. The compact layer formed in the range of more negative potentials (zone B) probably consists of TU molecules in the inner part of the double layer and water molecules outside the TU dipoles. Hydrogen bonds between water molecules and the amino groups of thiourea immobilize the compact layer. In the second structure, which exists in the region of high positive charge densities (zone C), the anions probably act as a stabilizing agent. In order to elucidate more completely the role of hydrogen bonds in the phase transition process, further studies in non-aqueous solvents will be undertaken. The effect of a number of anions on the interfacial process in non-aqueous solutions will be also studied. ACKNOWLEDGEMENTS
The authors would like to express their thanks to Dot. Dr. J. Jastrzebska for her inspiration and useful discussions, to Dr. M. Jurkiewicz-Herbich for helpful advice
216
and to Dr. K. Jackowska for critical comments of the manuscript. This work was supported financially%y CPBP 01.15 for which the authors are grateful. REFERENCES 1 2 3 4 5 6 7 8 9
F.N. Schapink, M. Oudeman, W. Leu and H.K. Helle, Trans. Faraday Sot., 56 (1960) 415. R. Parsons, Proc. R. Sot, London, Ser. A, 261 (1961) 79. R. Narayan and K.C. Pillai, Trans. Sot. Adv. Electrochem. Sci. Technol., 13 (1978) 315. N. Mauree and L. Gierst; Collect. Czech. Chem. Commun., 36 (1971) 389. R. Parsons and P. Symons, Trans. Faraday Sot., 64 (1968) 1077. G. Hills and F. Silva, J. Electroanal. Chem., 137 (1982) 387. K. Chandrasekara Pillai and W.E. Waghome, J. Electroanal. Chem., 125 (1981) 487. C. Buess-Herman, L. Gierst, M. Gonze and F. Silva, J. Electroanal. Chem., 226 (1987) 267. M. Skompska, M. Jurkiewicz-Herbich and J. Jasttzebska in P. Nowak (Ed.), Electrocatalysis, Materials of the IXth Symp. of the Polish Chem. Sot., Karnowice near Cracow, 1987, Cracow, 1988, p. 256. 10 W. Kemula and 2. Kubhk, Anal. Chim. Acta, 18 (1958) 104. 11 S. Mint, J. Jasttzebska and M. Jurkiewicz-Herbich, J. Electroanal. Chem., 65 (1975) 351. 12 H.S. Frank and W.Y. Wen, Discuss. Faraday Sot., 24 (1957) 133.
J. Electroanal. Chem., 212 (1989) 207-216 Elsevier Sequoia S.A., Lausamre - Printed in The Netherlands
The effect of anions on the phase transition in the adsorbed layer of thiourea at the mercury-aqueous solution interface M. Skompska and K. Jaszczynski Laboratory of Electrochemistry, 02-093 Warszawa (Poland) (Received
Chemistry Department,
Warsaw University, ul. Pasteura I,
10 February 1989; in revised form 19 June 1989)
ABSTRACT
The condensation of thiourea (TU) molecules at a mercury electrode in concentrated aqueous solutions was investigated by using impedance measurements and cyclic voltammetric techniques. The effects of the nature and concentration of the supporting electrolyte, the temperature and the thiourea concentration on the position of the phase transition potentials are discussed. It is concluded that the hydrogen bonds between the -NH, groups of TU and water molecules from the outer part of the double layer are an immobilizing agent for the adsorbed TU molecules, which leads to a stable structure in the “pit” zone. Electrostatic interaction between the TU dipoles and anions is responsibile for the formation of a condensed structure at the positively-charged mercury electrode.
INTRODUCTION
The electrochemical behaviour of thiourea (TU) at the mercury-aqueous solution interface has been studied widely in the presence of various supporting electrolytes [l-9]. It was found that thiourea behaves more like an anion than a neutral organic molecule [2]. In 1987 Buess-Herman et al. [8] reported that in the presence of strongly hydrated ions (F-, SO:-, CO:-, CH,COO-, PO:-) thiourea formed two distinct condensed monolayers totally devoid of anions, provided that the TU concentrations was high and the temperature was low. They stated that the capacity-potential profiles obtained in these supporting electrolytes are practically unaffected by the anion concentrations. The appearance of two condensed films, which are stable in particular ranges of potentials, resulted in capacity troughs in the C-E curve called a “pit” and a “dip”, separated by a high pseudo-capacitive peak. These interfacial changes occurred within a very narrow range of potential and indicated that phase transitions were involved. OO22-0728/89/$03.50
0 1989 Hsevier
Sequoia S.A.
208
Our results show partial disagreement in comparison with those cited above. We found an influence of the concentration of all the anions studied (SO,‘-, CH,COO-, ClO; , NO;) on both phase transition processes. Moreover, dilution of the supporting electrolyte in the presence of sulphates or acetates shifts the negative boundary of the “pit” region in the opposite direction to that for perchlorates or nitrates. We also found a similarity of the C-E profiles obtained in the solutions of different anions, when the nature of the anions was taken into account. The aim of our investigations was to find the reason for these differences and similarities. EXPERIMENTAL
A conventional water-jacket cell with the usual three-electrode configuration was used. The working electrode was a hanging mercury drop electrode (HMDE) of the type described by Kemula and Kublik [lo], with a surface area of 0.037 cm*. A platinum sheet was the counter-electrode. All potentials are referred to a saturated calomel electrode (SCE). The solutions of thiourea in an appropriate supporting electrolyte (Na,SO,, NaOOCCH,, NaClO,, NaNO,) were prepared from recrystallized chemicals and triply-distilled water. All experiments were carried out in solutions acidified to pH 4 using the acids corresponding to the anions. Acidification was necessary to remove traces of S*- from the solution to avoid the formation of HgS at a potential of about -0.620 V (vs. SCE). Solutions were prepared just before the experiments to eliminate the oxidation of TU. All solutions were deaerated with purified argon. Voltammograms were recorded using an EP-20 potentiostat coupled with a programmable EG-20 generator (both by ELPAN) and a R&en Denshi X-Y recorder. Capacity data were obtained from impedance measurements carried out in an electrochemical system consisting of a transfer function analyser (type 272 UNIPAN) equipped with a Neptune 184 computer system controller. Measurements were performed potentiostatically with potentials ranging from - 1.6 to - 0.4 V. A sinewave of 5 mV,, amplitude and at a frequency of 225 Hz was used. The values of the differential capacity determined from the impedance measurements are in agreement with the results obtained using a conventional symmetrical ac bridge [ll] on a HMDE and a DME as well. RESULTS
AND
DISCUSSION
Cyclic voltammetry is the most convenient method for preliminary examination of an electrochemical process. Figure 1 shows a comparison between a typical voltammogram and a differential capacity-potential curve. As can be seen, the potentials of the sharp and very symmetrical peaks in the i-E curve correspond exactly to the potentials at which rapid changes in the capacitance values are observed. Owing to their only slight dependence on the sweep rate, the positions of the peaks were assumed to be the potentials of the phase transition in the adsorbed layer at the mercury-solution interface.
209
Fig. 1. Capacity-potential curve (f = 225 Hz) (a) and cyclic voltammogram solution of 1 M NaClO, containing 1.2 M TU at 15 o C.
(o = 20 mV/s)
(b) in a
Figure 2 presents the capacity-potential curves obtained in a solution containing 1.2 M TU in various supporting electrolytes. Comparative analysis of the results allows us to distinguish four domains in the C-E plots.
-0.6
- 0.8
-1.0
-1.2
-1.4
E/V
Fig. 2. Capacity-potential curves in solutions of 1.2 M TU in the presence of various supporting electrolytes at lS” C. (1) 1 M NaOOCCH,; (2) 1 M Na,SO,; (3) 1 M NaCLO,; (4) 1 M NaNO,. Dashed lines: in the supporting electrolytes.
Zone A. Non-condensed At a potential blank supporting layer of hydrated positive is caused
region
of - 1.6 V (vs. SCE), the C-E curves coincide with those for the electrolytes. The surface of the mercury electrode is covered with a cations. The increase of the capacity as the potential is made more by the adsorption of thiourea.
Zone B. High& organized, quasi-solid film region At the potential E,, the differential capacity-potential plots show drastic discontinuities. The capacity decreases from 32 PF cm-2 to about 15 PF cmm2 over a span of a few millivolts (Fig. 2, curves 1 and 2). This sharp decrease in capacity reflects significant changes in the inner-layer structure the adsorbed TU molecules rearrange from a diluted to a highly-organized, quasi-solid film. The region of existence of this compact structure, j-e. the width of the so-called “pit” zone in the C-E curve, is strongly dependent on the temperature and the TU concentration, as has been reported by Buess-Herman et al. [8]. The results presented in Fig. 3 show an interesting feature of this region. The walls of the trough are symmetrical with
Fig. 3. Effect of the temperature on the C-E Na,SO,, (b) 1 M NaCIO., and (c) 1 M NaNO,.
plots in solutions of 1.2 M TU containing (a) 1 M
211
I
C/pFcm-z
C/pFcrK2
,.“_J 22’C
I I 21-c 18’1:15’c
a
b
I
46
-0.8
-1.0
E/ V
-0.6
-08
-1.0
E/V
I
-0.6
-0.8
-1.0
E/V
212
respect to a potential of about -0.760 V. This value is probably close to the potential of zero charge [3,8]. As the temperature is increased, both walls shift towards this characteristic value and the bottom of the trough rises. Above a critical temperature Tc,B,condensation does not proceed. Buess-Herman et al. claimed that in the presence of hydrophilic supporting anions the capacity behaviour is practically unaffected by the electrolyte concentration. Therefore, they suggested that the anions of this group do not take part in the phase transition processes. Our results, however, show that the position of the boundary between the diluted and the condensed film (potential E,) depends strongly on the nature of the anion as well as on its concentration. The data in Table 1 indicate that the potential E, shifts from -0.955 to -0.885 V when the concentration of SO:- or CH,COO- anions is lowered from 1 to 0.1 M, respectively. Moreover, in the presence of NO; or ClO; anions this potential shifts in the opposite direction from - 0.860 to - 0.890 V when the supporting electrolyte is diluted. Other information on the stability of the compact structure of zone B may be taken from the phase transition diagrams (Fig. 4). It is of interest to note that when the concentration of the supporting electrolytes was lowered to 0.1 M, similar C-E profiles and nearly identical values of the potential E, were obtained regardless of the nature of anion and the temperature. Furthermore, the critical temperature, above which condensation does not occur, is also strongly dependent on the nature of the anion and its concentration. In our opinion, the anions may indirectly modify the molecular structure of the compact layer. The molecules of thiourea from the condensed interfacial layer interact not
TABLE 1 Effect of the nature and concentration of the supporting electrolytes on the boundaries between zones A, B and C and on the “critical” and “melting” temperatures of zone B in solutions of 1.2 M TLJ Supporting electrolyte
c/M
Na,SO,
Et /V at 15OC
Critical temp. k/OC
?,=I, / o C
1.0 0.5 0.1
- 0.954 -0.908 -0.890
22 20 19
21 19 18
0.275 0.240 0.230
- 0.680 - 0.670 - 0.660
NaOOCCH,
1.0 0.5 0.1
- 0.955 - 0.915 - 0.885
22 21.5 20
20 20 18.5
0.300 0.295 0.285
- 0.650 - 0.620 -0.600
NaClO,
1.0 0.5 0.1
-0.855 - 0.875 -0.890
16.5 18 18
14.5 16 16
0.170 0.220 0.250
- 0.660 - 0.660 - 0.640
NaNO,
1.0 0.3 0.1
- 0.860 - 0.873 - 0.885
16 18 18
10 12 13
0.135 0.200 0.240
- 0.610 - 0.615 - 0.610
Zone B width at 15’C /V
E,/V
at 15OC
213
I
-(Is
- 0.6
-a,
-0.8
-09
Fig. 4. Phase transition diagrams in solutions of 1.2 M TU containing (a) 1 M supporting electrolytes and @) 0.1 M supporting electrolytes. (. -. - .) Na2S04; ( X X) NaOOCCH,; (A -A) NaCIO,; NaN03. (0 -0)
only with mercury but also with molecules of the solvent from the outer part of the double layer. Water molecules probably form hydrogen bonds with the amino groups of thiourea. In this way, unfavourable repulsion between the adjacent TU dipoles is diminished. The stability of such a highly organized TU-water film should depend on the mobility of the water molecules. This may explain the high stability of the TU film in sulphate or acetate supporting electrolytes. They are known as structure-making anions, i.e. they decrease the flmdity of water and thus immobilize the compact layer. NO; or ClO; ions cause the oposite effect. In their vicinity, water molecules become more mobile than those in pure water. Therefore at high (1 M) ClO; or NO; concentrations the condensation may occur only at low temperature. As can be seen in Figs. 3a and 3b, the C-E profiles in solutions of 1.2 M TU + 1 M Na,SO, and 1.2 M TU + 1 M NaClO, become nearly identical when the temperature of the second solution is about 6 o C lower than that of sulphates. In this way, the same highly organized arrangement of an interfacial TU-H,O layer may be achieved despite the water structure-breaking properties of ClO; anions. Zones C and D Further polarization towards more positive potentials leads to a subsequent structural transition in the compact layer. A second trough in the C-E curve (zone
214
C), usually proceeded by a sharp capacitance peak at the potential E,, can be observed. Detailed measurements carried out for a wide range of temperatures and anion concentration show that the mechanism of the process is more complicated than was suggested by Buess-Herman et al. [S]. Returning to Fig. 3, one can observe a small hump inside the trough of zone B. When the temperature is increased, the hump becomes sharper and is probably responsibile for raising the bottom of the trough. This process facilitates the destruction of the interfacial compact structure. The hump always occurs at a slightly positive electrode charge and its position depends on both the adsorbability of the anions and the temperature. It is especially well developed in the solution of nitrate. All these observations allow us to consider the hump as the result of interaction of the anions and the positively charged electrode. With an increase of temperature, the hydrogen bonds between the -NH, groups of TU and water molecules are weakened and attracted anions may destroy the compact structure. The adsorbed TU molecules rearrange into a diluted, non-condensed film. In consequence, in the C-E curves there occurs a region of raised capacity, denoted by us as zone D, which separates zones B and C. We call the temperature above which zone D exists the “melting” temperature, and it is marked on the phase transition diagram as T, (Fig. 4b). Above the “critical” temperature Tc,B, when the condensation in zone B does not occur, the hump transforms into a sharp maximum. Further increase of the temperature as well as dilution of the supporting electrolyte causes it to shift towards more positive
I -0.6
-0.8
-1.0
Fig. 5. Effect of the concentration of TU on the C-E (1) 1.2; (2) 0.95; (3) 0.8; (4) 0.5.
-1.2
-14
E/V
curves in 1 M NaClO, solution at 12O C. [TU]/M:
215
potentials. The stronger the adsorbability of the anion, the lower is the “melting” temperature and the higher is the hump. In our opinion, the anions take part in the subsequent structural transition process. It is difficult, however, to determine definitively whether the anions intrude into the compact layer or whether they remain outside the adsorbed TU molecules. Buess - Herman et al. suggested that in the presence of Na,SO, as the supporting electrolyte the inner layer is devoid of anions. The similarity in the general shape of the C-E profiles and the phase transition graphs obtained in all the solutions studied (Fig. 4) allows us to suppose that other anions also remain outside the condensed layer of TU molecules. However, these anions may act as bridges between the adjacent adsorbed TU molecules, thus causing the relatively high stability of the compact layer formed in zone C. Obviously these considerations deal with the extreme condition - low temperature and high TU concentration. A decrease of the TU concentration causes changes in the C-E profiles in the region of the studied potentials. The rising part of the curve in zone A is shifted towards more positive potentials and region B is squeezed (Fig. 5). At a sufficiently low concentration of TU (60% of the saturated value in 1 M NaClO, at 12OC), zone B is no longer present. The condensation takes place probably only when a definite surface concentration of TU is achieved. The width of zone C is not as sensitive to the lowering of the TU concentration as the previous one. Unfortunately, the studies are limited to a potential of about -0.38 V, since beyond this value anodic dissolution of mercury occurs. CONCLUSIONS
The adsorbed molecules of thiourea may form two distinct condensed monolayers under extreme conditions of temperature and TU concentration. The difference in stability of the two structures seems to be connected mainly with the immobilizing agents of the superficial film. The compact layer formed in the range of more negative potentials (zone B) probably consists of TU molecules in the inner part of the double layer and water molecules outside the TU dipoles. Hydrogen bonds between water molecules and the amino groups of thiourea immobilize the compact layer. In the second structure, which exists in the region of high positive charge densities (zone C), the anions probably act as a stabilizing agent. In order to elucidate more completely the role of hydrogen bonds in the phase transition process, further studies in non-aqueous solvents will be undertaken. The effect of a number of anions on the interfacial process in non-aqueous solutions will be also studied. ACKNOWLEDGEMENTS
The authors would like to express their thanks to Dot. Dr. J. Jastrzebska for her inspiration and useful discussions, to Dr. M. Jurkiewicz-Herbich for helpful advice
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and to Dr. K. Jackowska for critical comments of the manuscript. This work was supported financially%y CPBP 01.15 for which the authors are grateful. REFERENCES 1 2 3 4 5 6 7 8 9
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