On the effect of adsorbed substances undergoing interfacial rearrangements on the kinetics of ion and electron-transfer reactions

279

Chem., 317 (1991) 279-284 Elsevier Sequoia S.A., Lausanne

J. Electroanal.

JEC 01692

On the effect of adsorbed substances undergoing interfacial rearrangements on the kinetics of ion and electron-transfer reactions The case of n-hexadecyltributylphosphonium

bromide

A. Anastopoulos Laboratory of Physical Chemisny, 54006 7’hessaloniki (Greece)

Department

of Chemistry

333-2, Aristotle

University

of Thessaloniki,

M. Kaisheva University of Sofia, Fact@

of Chemistry, No. I A. Ivanov au., Sofia 1126 (Bulgaria)

(Received 14 January 1991; in revised form 30 May 1991)

AbStlWt

The polarography of inhibited ion-transfer and electron-transfer reactions and tensametric measurements are used for the elucidation of a phenomenon of decreasing inhibitory action with increasing concentration of n-hexadecyltributylphosphonium bromide.

INTRODUCT ION

Within the broader frame of the study of the mechanism of inhibited reactions, the use of inhibitors which undergo phase transitions has rather recently gained remarkable interest [l-4]. Investigations of this type are interesting for two reasons: (1) they enable the determination of the mechanism of inhibited reactions [1,2,5-71 and (2) to the extent that the polarographic current of inhibited reactions is a very sensitive measure of the rearrangements occurring at the interface [3,4], such studies may help the further elucidation of the development of the adsorption film. The present work is mainly oriented to the latter direction. Hexadecyltributylphosphonium bromide (CTBPB) is an extremely surface-active substance. Moreover, it is already known [8] that micelle formation is detected in the presence of CTBPB, while preliminary tensametric measurements have revealed curves of unusual form. 0022-0728/91/$03.50 0 1991 - Elsevier Sequoia S.A. All rights reserved

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For these reasons, we believe that the interest in the study of the inhibitory effect of adsorbed CTBPB, on simple ion-transfer (ITR) and outer-sphere electron-transfer reactions (ETR) is self-evident. EXPERIMENTAL

DC polarographic and tensametric (ac frequency 50Hz, phase difference 90 “) measurements were carried out using an E-261 Metrohm polarograph, modified to offer three-electrode and ac facilities. The DME had a flow rate of 1.43 mg s-l and an open-circuit free drop of 8 s, dislodged at 4 s. All potentials were measured against a KC1 saturated calomel electrode and the temperature was maintained at 25OC. NaF (Fluka, purum p.a.), NaOH (Fluka, puriss p.a.), Cd(NO,), (Merck, 99%, K,Cr,O, (Merck, puriss p.a.) and CTBPB (Fluka, pm-urn) were used without further purification. The diffusion coefficients of Cd2+ and CrOi- ions used were found polarographically to be 6.9 X 10e6 and 1.05 X lop5 cm2 s-* respectively. H,O and Hg were distilled twice. RESULTS AND DISCUSSION

The dc instantaneous polarographic current vs. potential curves of Cd’+ reduction, in the presence of various additions of CTBPB, are shown in Fig. 1. The polarographic curves of CrOi- reduction, in the presence of increasing CTBPB

Fig. 1. Instantaneous dc polarographic current vs. electrode potential curves of 1 mM Cd’+ reduction in the presence of the following CTBPB concentrations in mM: (1) 0.00, (2) 0.06, (3) 0.075, (4) 0.09, (5) 0.10, (6) 0.125, (7) 0.15, (8) 0.20, (9) 0.25, (10) 0.30. Base solution: aqueous 0.2 M NaF. Inset: Dc polarographic current-time curves of 1 mM Cd’+ in 0.2 M aqueous NaF, with the following additions of CTBPB in mM: (1) 0.00, (2) 0.05, (3) 0.06, (4) 0.075, (5) 0.10. At E = -0.6 V vs. SCE. Drop time: 8 s. Fig. 2. Instantaneous dc polarographic current vs. electrode potential curves of 1 mM CrOi- reduction in the presence of the following CTBPB concentrations in n&f: 0.00, (2) 0.01, (3) 0.02, (4) 0.04, (5) 0.06, (6) 0.08, (7) 0.10. Base solution: aqueous 0.4 M NaOH.

I

I

9

8

’

-In(+nol

I”)

Fig. 3. Numbers correspond to the following CTBPB concentrations in mM: (1) 0.00, (2) 0.10, (3) 0.125, (4) 0.135, (5) 0.15, (6) 0.20, (7) 0.25, (8) 0.30. Otherwise as in Fig. 2. Fig. 4. (a) Dependence of In kapp on In ci for 1 mM Cd 2t reduction in aqueous 0.2 M NaF in the presence of CTBPB at the following electrode potentials in V vs. SCE: (0) -0.60, (A) - 0.70, (0) - 0.80, (0) - 0.90, (A) - 1.00, (M) - 1.10. (b) Dependence of In /carp on In ci for 1 mM CrOi- reduction in aqueous 0.4 M NaOH in the presence of CTBPB at the following electrode potentials in V vs. SCE: (0) -0.75,(A) -0.80, (0) -0.85,(S) -O?%&(A) -0.95,(m) -1.00.

concentrations, up to 0.1 mM (Fig. 2) and from 0.1 mM and above (Fig. 3), are also shown. The degree of approach to adsorption equilibrium, within the drop life and for the concentrations used, is indicated by the Z-t curves of the inset of Fig. 1. There it can be seen that for t = 4 s and above 0.075 mM CTBPB, a steady state is established. The same situation occurs over the whole potential range from - 0.6 to -1.1 V vs. SCE. The Z-E and I-t curves of Fig. 1 allow the following observations. The I-E curves, even at CTBPB concentrations up to 0.09 mM, present an initial “reversible” part followed by an irreversible part. This finding can be attributed either to the non-att~nment of full coverage or to the fact that the reduction Cd’++ 2 e-+ Cd(Hg) follows two parallel reaction channels [9], even at CTBPB concentrations corresponding to relatively high electrode coverages. In our opinion, the I-t curves show that at the higher CTBPB concentrations the condition 8 = 1 is approached satisfactorily within the drop life. Hence, it may be assumed that, even at relatively high coverages, the reduction of the hydrated cadmium ion prevails and only at 8 = 1 is the discharge of Cd2+, coordinated by CTBPB, favoured. The form of the Z-t curves also shows that the heterogeneous apparent rate constant, kapp, is not constant during the drop life. This implies that the method of Koutecky, for the derivation of the rate constants, is not strictly applicable and any relative conclusions must be kept at a purely qualitative level. In the presence of adsorbed CTBPB, the reduction of Cd2+ ions appears as a typical ITR [1,2]. Over the whole ~ncentration range studied, the rate constants

282

determined by Koutecky’s method depend on the CTBPB concentration. The concentration dependence of the rate constant of Cd’+ reduction, in the presence of CTBPB, is shown more comprehensively by the plots of k,, vs. In Ci, in Fig. 4a, corresponding to concentrations above 0.09 mM and in the potential range from - 0.60 to - 1.10 V vs. SCE. Although for the reduction of Cd 2+ the situation displayed by the polarographic curves seems to be a regular one, the curves of Figs. 2 and 3, for the reduction of CrOj-, reveal that the development of the CTBPB surface film is a complex process. In Fig. 2, it is seen that for CTBPB concentrations up to 0.1 mM, the inhibitory action develops regularly. However, just as the concentration exceeds 0.1 mM, the inhibition starts to decrease until a weaker, but still remarkable, inhibition is attained at 0.25 mM. This situation is reflected by the In k,, vs. In ci plots of the pair CrOi--CTBPB provided in Fig. 4b. In the concentration range 0.1 I ci 5 0.2 mM, the In kapp vs. In ci dependence reflects the changes observed in the polarographic curves. In the potential range from -0.75 to - 1.00 V vs. SCE and for ci > 0.2 mM, In k,, tends to become independent of In Ci. The reduction of chromate ions is known [2,3] as an outer-sphere ETR. The concentration dependence of the rate constants of an outer-sphere ETR [1,2] may indicate the structural changes occurring at the interface. But what type of interfacial rearrangements would justify the decrease of the inhibition of CrOi- reduction? To obtain more information about the characteristics of the CTBPB film development, we carried out tensametric measurements for CTBPB adsorption from 0.4 M NaOH and 0.2 M NaF solutions. The measurements in NaF solutions were carried out for the sake of comparison with the polarographic curves of Fig. 1. The results of the tensametric experiments are shown in Figs. 5 and 6. In analogy to the polarographic curves of Figs. 2 and 3, it is observed that as the concentration exceeds 0.1 mM, in the potential range from - 0.50 to - 1.00 V vs. SCE, the tensametric current and consequently the corresponding double-layer capacitance values re-increase up to a limiting value, reached at ci = 0.3 mM. The tensametric curves of Figs. 5 and 6 also indicate the occurrence of a slowly progressing reorientation process. It is possible that, in passing from low to higher negative potentials, say in the vicinity of -0.5 V vs. SCE, the orientation of the adsorbed CTBPB particles changes. This reorientation possibly brings the P atom closer to the electrode and makes the long n-hexadecyl chain roughly parallel to the electrode surface, resulting in a flattening of the adsorbed surfactant molecule and consequently in a decrease of the film thickness, i.e. in an increase in the values of the double-layer capacitance. It is also known from surface tension measurements [8] that CTBPB forms micelles with cmc 0.16 mM in the absence of base electrolyte. The conditions at the DME are very different, but the introduction of electrolyte may favour the formation of micelles in the “vicinity” of 0.1 mM CTBPB. However, above the cmc the micellization would result in a constant bulk monomer concentration. There is no

283

r

1.5 r/VCSCE, Fig. 5. Tensametric curves of aqueous 0.4 M NaOH in the presence of the following CXBPB concentrations in mM: (1) 0.00, (2) 0.04, (3) 0.08, (4) 0.10, (5) 0.20, (6) 0.30, (7) 0.40. 0.5

evidence for this in the curves of Fig. 4a, which suggest that up to 0.3 mM the CTBPB bulk.concentration seems to increase regularly. On the other hand, the concentration independence of the kapp values of Fig. 4b, at the higher CTBPB concentrations, may be attributed to the attainment of a constant film structure. Nevertheless, it cannot be excluded that even a pre- micellar association in the bulk

cl5 15 -E/V(SCEI Fig. 6. Tensametric curves of aqueous 0.2 M NaF in the presence of the following CTBPB concentrations in mM: (1) 0.00, (2) 0.04, (3) 0.06, (4) 0.08, (5) 0.10, 96) 0.15, (7) 0.20, (8), 0.30, (9) 0.40, (10) 0.50.

284

would cause an increase in the permeability of the absorbed film, for reasons not adequately elucidated [lo]. From this point of view, we suggest that the reorientation is the main and most probable cause of the capacitance re-increase at potentials negative to -0.5 V vs. SCE. Moreover, this assumption allows for a reasonable interpretation of the decrease in the i~bito~ action of CTBPB and the observed re-acceleration of the CrOj- reduction, as is shown below. At a phenomenological level, the re-acceleration of chromate ion reduction can be attributed to: (a) eventual formation of complexes reduced at lower negative potentials; (b) a non-electrostatic (chemical) effect of “catalytic” character; (c) an electrostatic effect caused by the attraction between the oppositely charged reactant and the i~bitor particles; or (d) interfacial rearrangements which increase the permeability of the surface film. Although in such complex systems none of the above possibilities can be safely discarded, we believe that the re-acceleration of the chromate reduction is more likely related to the reorientation of the adsorbed surfactant. Therefore a combination of points (c) and (d) seems to offer a reasonable explanation. In this sense, the reorientation of CTBPB at potentials around -0.5 V/SCE, from a vertical orientation with the P atom towards the solution to a flat orientation with the positively charged P atom close to the electrode surface may accelerate CrOi- reduction, owing to the stronger electrostatic attraction between the reactant and the inhibitor. Moreover, the resulting decrease in film thickness creates more favourable conditions for the tunnell~g of electrons through the CTBPB film and contributes to the further increase of the rate of an outer-sphere ETR, like the reduction of chromate ions. On the contrary, Cd2+ reduction, as a typical ITR, does not enjoy such an accelerating effect as long as the work required for the formation of a hole in the CTBPB film, to accommodate the Cd2+, is expended against film pressure. Therefore it depends on the bulk ~ncentration and not on the CTBPB film structure. ACKNOWLEDGEMENT

The authors wish to thank the referee of this work for his very helpful remarks. REFERENCES 1 R. Srinivasan and R. de Levie, J. Electroanal. Chem., 201 (1986) 145. 2 J. Lipkowski, Cl. Buess-Herman, J.P. Lambert and L. Gierst, J. Electro~al. Chem., 202 (1986) 169. 3 A. Anastopoulos, A. Christodoulou and I. Pouhos, J. Electroanal. Chem., 262 (1989) 235. 4 A. Anastopoulos and A. Cbristodoulou, Collect. Czech. Chem. Commun., 53 (1988) 732. 5 J. Lipkowski and Z. Galus, J. Electroanal. Chem., 98 (1979) 91. 6 R. Guiddli, M.L. Foresti and M.R. Moncelli, J. Electroanal. Chem., 113 (1980) 171. 7 M. Goledzinowski, J. Dojhdo and J. Lipkowski, J. Phys. Chem., 89 (1985) 3506. 8 Shen Hanxi and Zang Erie, Fenxi Huaxue, 13 (1985) 736. 9 E. Moller, H. Emons, H.-D. Dorfler and J. Lipkowski, J. Electroanal. Chem., 142 (1982) 39. 10 -N. Batina, 2. Kozarac and B. Cosovic, J. Electroanal. Chem., 188 (1985) 153.