- Email: [email protected]
Structural changes at the interface between polycrystalline Ag and electrolyte solutions containing surfactants as probe molecules
Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Structural changes at the interface between polycrystalline Ag and electrolyte solutions containing surfactants as probe molecules A.G. Anastopoulos ⇑, M. Paschalidou, A. Papoutsis Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 26 October 2013 Received in revised form 29 January 2014 Accepted 7 March 2014 Available online 15 March 2014 Keywords: Structural changes Polycrystalline silver Surfactant reorientation Adsorbed layer rarefaction
a b s t r a c t Structural changes are detected at the interface between polycrystalline Ag electrodes and electrolyte solutions containing surfactants as probe molecules. Capacitance and impedance measurements indicate that in aqueous solutions water and 1-butanol molecules change their position on the electrode due to dipole-field interaction. In methanolic solutions such a change is indicated to occur only for triphenyphosphine oxide molecules. The above changes in both solvents are taking place along with the transition of the adsorbed layer from a dense liquid state to a thinner one with increasing negative potentials. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Solid metal electrodes usually provide a favorable substrate for the occurrence of structural changes within organic adlayers. As pointed out by important reviews [1,2] associative processes of the type of two-dimensional condensation of the adsorbed layer are detected more often on monocrystal Au electrodes [3–6], less often on silver monocrystals [3,7,8] and much rarely on other metals [9,10]. Other types of structural rearrangements, viz. two state adsorption due to field effected adsorbate reorientation, are also reported for Au monocrystal and polycrystalline electrodes and Ag monocrystals [11–15]. However, as far as we know there is a lack of information in respect to the occurrence of interfacial rearrangements on polycrystalline Ag electrodes. The study of interfacial rearrangements is of interest for modification purposes of the electrode surface including the development of protective layers and biological membranes, the sensor technology and systems of energy conversion, the electrode functionalization for catalysis and medical applications etc. [1,2,16]. Double layer capacitance measurements and electrochemical impedance spectroscopy occupy a central position among the methods devoted to the study of electrochemical interfaces. Capacitance measurements are based on the conventional requirement for the validity of ideal polarizability of the electrode/solution interface. ⇑ Corresponding author. Tel.: +30 2310997861. E-mail address: [email protected] (A.G. Anastopoulos). http://dx.doi.org/10.1016/j.jelechem.2014.03.009 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
Impedance spectroscopy measurements reveal possible frequency dispersion effects related to the deviation of electrochemical interfaces from the ideally polarized condition and contribute to the elucidation of their origin. The extent of deviation from the ideal capacitance behavior is a measure of the roughness of the electrode or/and the heterogeneity of the interface [17]. In the absence of faradaic processes and interfacial phase transitions the deviation of the interface from ideal capacitive behavior is necessarily associated to the roughness of the metal electrode surface. On the other side, the adsorption of surface active molecules and ions may contribute sometimes to the increase and sometimes to the decrease of interfacial heterogeneity. In the present work the characteristics of electrochemical interfaces of polycrystalline silver electrodes were investigated in the presence of 1-butanol in aqueous solutions and triphenylphosphine oxide in methanolic ones. 1-Butanol and triphenylphosphine oxide may be considered as model adsorbates as long as they are adsorbed in a wide potential range, do not react with the electrode within their adsorption region and are markedly soluble in the solvents used. The capacitive adsorption studies of alcohols at monocrystal Ag electrodes are rather infrequent [18,19], while no such literature information is found for capacitance and impedance studies at polycrystalline Ag electrodes. The adsorption of triphenylphospine oxide and other phosphororganic compounds has been thoroughly studied at mercury electrodes in a variety of pure and mixed solvents [20–25]. However in
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
53
our knowledge, no literature information is quoted for capacitance and impedance studies of triphenylphospine oxide at polycrystalline Ag electrodes. 2. Materials and methods Single compartment double walled electrochemical cells were used, kept at constant temperature of 298 K. Polycrystalline Ag disk electrode, of 2 mm diameter, embedded in a plastic cylinder of 52.5 mm length and 7 mm external diameter, was used as working electrode. The working electrode was mechanically polished by Al2O3, rinsed by distilled water in an ultrasonic bath, washed again with working solution and immediately transferred to the electrochemical cell, taking care to avoid contact with air. A Pt foil was used as auxiliary electrode, the free end of which was bended so as to be parallel to the disk face at a constant distance of 2 mm, in order to avoid nonuniform potential distribution associated with the geometry of disk Ag electrode. In all measurements a 2810 SCHOTT reference calomel electrode saturated with KCl (SCE) was used. The electrochemical activation of the working electrode was carried out by successive scan cycles at a rate of 0.5 V/s in the potential region from 0.2 to 1.2 V (vs. SCE) in aqueous solutions and 0.4 V to 1.1 V (vs. SCE) in methanolic ones, where no faradaic processes are detected, until very reproducible voltammetric curves were obtained. All the experiments were carried out by the electrochemical system Autolab PGSTAT302N connected to a PC running the Autolab software necessary for the control of capacitance, impedance and cyclic voltammetry measurements. Capacitance measurements were carried out with an ac signal of 20 Hz and 5 mVpp. Electrochemical impedance measurements were carried out within a frequency spectrum of 1 Hz to 10 kHz with ac signal of 5 mVpp at various dc potentials. The percent standard deviation between at least two successive capacitance measurements was found to be equal to 5–6% within the potential range from 0.5 to 1.2 V (vs. SCE). On account of the low reproducibility of capacitance measurements at polycrystalline electrodes [26] this result may be considered as a satisfactory one. The reproducibility of all electrochemical impedance measurements was very satisfactory, as proved by the comparison of 2–3 successive measurements at the same potential followed by the Kramers–Kronig linearity-stability test. The chemical reagents used without further purification are LiClO4 (Fluka, purity P 98%), triphenylphosphine oxide (TPO, Fluka purity 99%), 1-butanol (1-BuOH, MERCK, pro analysi) and methanol (MeOH, LAB-SCAN, purity > 99%). Water used was doubly deionized-distilled by the DD8467 Autostill Jencons system. 3. Results and discussion The experimental capacitance curves of the interface between polycrystalline Ag and aqueous solutions of 1-butanol and methanolic solutions of triphenylphosphine oxide are illustrated in Figs. 1 and 2 respectively. Capacitance values are expressed with respect to the geometrical area of silver electrodes in agreement to an acceptable practice mentioned in the literature [12,27–29]. In aqueous solutions the minimum capacitance of the base solution is equal to 43.36 102 F m2. The literature value [30–32] of the minimum capacitance of low roughness monocrystal (1 1 1) and (1 0 0) Ag electrodes in aqueous perchloric solutions is equal
Fig. 1. Capacitance curves of polycrystalline Ag in contact to aqueous 0.1 M LiClO4 containing the following % v/v 1-butanol additions: (j) 0, (I) 1, (d) 2, (r) 4, (N) 6, (H) 7, (s) 8.
Fig. 2. Capacitance curves of polycrystalline Ag in contact to methanolic solutions of 0.1 M LiClO4 containing the following mM TPO additions: (j) 0, (I) 5, (d) 10, (r) 20, (N) 30, (H) 40, (s) 50, (.) 60.
to 32.5 102 F m2. Therefore, an approximate estimation of the roughness factor of our Ag electrode equal to 1.33 can be obtained from the ratio [33] of the above capacitance values. This is a typical value for a polycrystalline Ag electrode of intermediate roughness [34]. In Fig. 1 the curve of base electrolyte presents two main features. A narrow peak at 0.6 V and a broad one at 0.4 V. The latter is also observed in Fig. 2 in methanolic solutions. However, at this potential range capacitance curves of polycrystalline Ag both in aqueous and methanolic solutions are necessarily featureless [35,37]. Thus it can be assumed that the peak at 0.4 V is related both to the specific morphological features of the electrode and to the solvent used as long as the corresponding capacitance values in H2O and methanol are markedly different. On the contrary, the peak at 0.6 V is observed only in aqueous solutions [18,29,37,38] and it seems to be related to a phase transition of the interfacial solvent as for example the reorientation of water molecules. In the presence of 1-butanol the broad peak at 0.4 V is drastically depressed and finally disappears. Moreover, at potentials slightly negative to 0.6 V a sharp peak manifests itself. The adsorption of 1-butanol mainly affects the form and not the height and the position of this peak. For this reason it may not be identified as an adsorption–desorption maximum. The addition of 1-butanol results to the decrease of capacitance values at two potential regions from both sides of the maximum at 0.6 V (vs. SCE). Those two regions may correspond to different states of the adsorbate at the electrode surface. In agreement to literature, polar aliphatic alcohols adsorbed at silver electrodes [15] change their position from low negative potentials with the OH group towards the electrode surface to higher negative ones with the carbon chain towards the electrode surface.
54
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
At potentials E 6 1.0 V (vs. SCE) a strong increase of capacitance is observed, which may be related to the desorption of butanol. In Fig. 2 the curve of base electrolyte presents a broad peak at the potential range from 0.35 to 0.55 V (vs. SCE) similar to that of aqueous solutions, which may be described in the same way as in Fig. 1. In the presence of TPO the capacitance values are uniformly depressed from 0.3 to 1.1 V (vs. SCE) and present a fluctuation at about 0.7 V (vs. SCE). It may be assumed that the form of these curves reveals a change of the structure of adsorbed layer, possibly due to the change of the position of adsorbed TPO molecules from low negative potentials with the phenyl groups in contact to the electrode to higher negative potentials with phosphorus atom close to the electrode surface. At the negative end of potential range no tendency for TPO desorption is observed. In order to elucidate the nature of changes occurring in the adsorbed layer, isotherm analysis was carried out based on capacitance measurements. This analysis is limited at potentials where the condition dhapp/dE 0 is satisfied viz close to the two minima of the C–E curves. The apparent degree of electrode coverage is calculated by the known relation [39]:
happ ¼
C0 Ci C 0 C sat
ð1Þ
where Co is the capacitance in the absence of adsorbate, Ci is the capacitance in the presence of an intermediate adsorbate concentration and Csat is the saturation capacitance corresponding to the condition happ 1. Csat is calculated by extrapolation of 1/Ci vs. 1/cA plots at infinite bulk adsorbate concentration cA. Isotherm analysis has shown that both electrode-solvent-adsorbate systems studied are in very satisfactory agreement with the theoretical Langmuir isotherm:
BcA ¼
happ 1 happ
ð2Þ
where B is the adsorption equilibrium constant, which is related to the electrode potential E by means of the relation [39]: 1 ðC 0 C sat ÞE2 þ C sat EN E 1 B ¼ exp 2 RT Cmax cS
! ð3Þ
In Eq. (3) cS is the bulk solvent concentration, EN is the displacement of the zero charge potential due to the adsorption of ionic and polar non-ionic species and Cmax is the limiting surface adsorbate concentration. According to literature [40,41] in the case of polycrystalline electrodes the condition EN 0 is satisfied. Thus setting Eq. (3) in logarithmic form we obtain:
1 ðC 0 C sat ÞE2 ln B ¼ ln cs 2RT Cmax
ð4Þ
The results of the calculation of B as a function of E, at potentials corresponding to the two regions of minimum capacitance, are shown in Fig. 3 in the form of ln B vs. E2 point diagrams. The stepwise form [42] of these diagrams seems to agree with the structural change of adsorbed layer from positive to negative potentials with respect to the observed capacitance peaks, whereas the dotted line connecting the two groups of points corresponds to the range of transition potentials. The limiting surface concentration Cmax of 1-butanol and triphenylphosphine oxide is calculated
Fig. 3. Plots of ln B vs. E2 for polycrystalline Ag in contact to 0.1 M LiClO4: (H) aqueous solutions of 1-butanol and (I) methanolic solutions of TPO.
by linear regression, applied independently to each one of the two groups of points of the ln B vs. E2 plots. The calculated values of Cmax together with the corresponding values of Csat are provided in Tables 1 and 2. The results of Tables 1 and 2 suggest that the adsorption of 1butanol and TPO is accompanied by a decrease of the compactness of the adsorbed layer following the transition from anodic to cathodic polarizations. This change is confirmed by the corresponding increase of Csat and decrease of Cmax values. The transition of the superficial layer from higher Cmax values to lower ones by increasing negative potentials on Ag electrodes is already reported in the literature, although no evidence for capacitance depression at two different potential regions is observed on the C–E curves [43,44]. Moreover, several authors report capacitance curves at Au and Ag electrodes showing more or less clearly the existence of two potential regions of capacitance depression [11,16,29,45,46] but do not apply isotherm analysis at those potential regions and thus two state adsorption and related phase transitions are not quantitatively confirmed. According to the theory of Frumkin for the capacitance hump [36,40] capacitance fluctuations in the form of humps and peaks are interpreted in terms of recession of dielectric saturation in the adsorbed layer resulting either from adsorption–desorption processes of adsorbed particles or from their reorientation, the latter seemingly being our case. However, the description of the reorientation process of a polar molecule as a 180° flip of the adsorbed dipole is strictly valid only for short dipoles and for absolutely smooth electrodes viz Hg and Ga and approximately valid for low roughness monocrystal (1 0 0) and (1 1 1) faces of solid metal electrodes. On the other hand the degree of disorder of adsorbed layers on electrodes of high roughness is significant and implies a variety of
Table 1 Saturation capacitance and limiting surface adsorbate concentration at the interface Ag/aqueous 0.1 M LiClO4 in the presence of 1-butanol. Potential region V (vs. SCE)
Csat 102 F m2
Cmax
0.40 6 E 6 0.20 1.10 6 E 6 0.80
31.27 35.25
5.45 1011 1.65 1011
mol cm2
Table 2 Saturation capacitance and limiting surface adsorbate concentration at the interface Ag/methanolic 0.1 M LiClO4 in the presence of TPO. Potential region V (vs. SCE)
Csat 102 F m2
Cmax
0.60 6 E 6 0.30 1.05 6 E 6 0.70
14.75 16.92
3.56 1011 1.14 1011
mol cm2
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
orientations of molecules adsorbed at single crystal micro and macro-facets and other defects of the electrode surface. Therefore, the reorientation of long chain bulky molecules adsorbed on rough electrodes may be understood as the change of the position of the various structural groups of molecules under charge-field interactions. As shown in Fig. 4, over all the potential range where isotherm analysis was applied, the apparent degree of coverage happ,max, corresponding to the higher bulk concentration of the adsorbate, is always less than unity. This means that the adsorbed layer may be always described as a perfect liquid mixture in agreement to the validity of Langmuir isotherm. Using Csat as a measure of the packing of adsorbed molecules and Cmax as a measure of their interfacial population, we come to the conclusion that the observed structural change is the transition of the interface from a dense liquid state to a thin liquid one, which resembles the transition from a condensed liquid to an expanded liquid state [42] except that at the condensed liquid state h 1. During this transition the decrease of packing of adsorbed particles facilitates the change of their position under the influence of electric field, which in turn leads to the decrease of dielectric saturation of the interfacial mixture and hence to the emergence of capacitance peaks.
Fig. 4. Plots of apparent electrode coverage at maximum adsorbate concentration against electrode potential in the presence of 1-butanol H and TPO I.
55
Further insight into the nature and characteristics of the changes occurring in the adsorption layer is gained by electrochemical impedance spectroscopy measurements. Spectra of electrochemical impedance are indicatively shown for the systems Ag/0.1 M LiClO4–H2O + 1-BuOH and Ag/0.1 M LiClO4–MeOH + TPO in Figs. 5 and 6 respectively. Frequency dispersion in the form of deviation from vertical direction is observed in all spectra, imposing the replacement of the conventional capacitor in the equivalent circuit of the interface by a constant phase element (CPE). Moreover, as long as a slight curvature is observed in all spectra, which may be considered as representing an incomplete semicircle, the Randles’ type equivalent circuit is adopted with overall impedance equal to:
Z ¼ RS þ
Rp Z CPE Rp þ Z CPE
ð5Þ
where RS is the series solution resistance, Rp the parallel polarization resistance and ZCPE the CPE impedance expressed by the relation:
Z CPE ¼
1 n Y 0 ðjxÞ
ð6Þ
where Y0 is the pre-exponential factor and n the CPE exponent. Figs. 5 and 6 also show that at potentials positive to 1.1 V (vs. SCE) the addition of 1-butanol and TPO is accompanied with the increase of deviation from the ideal capacitance behavior, while at E 6 1.1 V (vs. SCE) this deviation tends to be reversed. In order to set the above observations on a quantitative basis, the experimental impedance measurements were fitted to Eq. (5) by means of a complex nonlinear least squares routine included in the control software of the Autolab system. The mean fitting quality of the experimental spectra, expressed by the factor x2, is of the order of 5 103, which can be considered as reasonable [47] for a polycrystalline electrode. The results of the calculation of the CPE exponent as a function of electrode potential are provided in Table 3. The observed decrease of the CPE exponent, at E > 1.0 V (vs. SCE) in aqueous solutions and at 0.4 P E P 1.1 V (vs. SCE) in
Fig. 5. Impedance plots of polycrystalline Ag in contact to aqueous solutions of 0.1 M LiClO4 in the absence (circles) and in the presence (triangles) of 8% v/v 1-butanol at the following electrode potentials in V(SCE) (a) 0.5, (b) 0.7, (c) 0.9, and (d) 1.1.
56
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
Fig. 6. Impedance plots of polycrystalline Ag in contact to methanolic solutions of 0.1 M LiClO4 in the absence (circles) and in the presence (triangles) of 0.04 M TPO at the following electrode potentials in V(SCE) (a) 0.4, (b) 0.7, (c) 0.9, and (d) 1.1.
Table 3 Values of CPE exponent of Ag/0.1 M LiClO4 interface in aqueous solutions in the presence of 1-butanol and in methanolic solutions in the presence of TPO at various electrode potentials. E V (vs. SCE)
0% v/v 1-BuOH
8% v/v 1-BuOH
E V (vs. SCE)
0 M TPO
0.04 M TPO
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.853 0.867 0.894 0.906 0.906 0.898 0.905 0.912
0.846 0.856 0.884 0.899 0.902 0.909 0.916 0.918
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
0.826 0.836 0.857 0.878 0.882 0.884 0.850 0.828
0.795 0.835 0.855 0.862 0.865 0.862 0.843 0.805
methanolic ones, is a typical indication that the structural inhomogeneity of the interface as a whole is increased by the adsorption of 1-butanol and TPO molecules. In this respect the observed increase of the CPE exponent in aqueous solutions at E 6 1.00 V can be understood as the result of 1-butanol desorption. The structural changes of the adsorbed layer, found out by means of capacitance measurements, do not clearly manifest themselves in the form of the impedance spectra of Figs. 5 and 6. However, a careful observation of Table 3 reveals that the absolute value of the difference of CPE exponent in the absence and in the presence of adsorbate, n0 and nA respectively:
Dn ¼ jn0 nA j
Fig. 7. Potential dependence of the absolute value of the difference of CPE exponent in the absence and in the presence of adsorbate. (a) 8% v/v 1-butanol, and (b) 0.04 M TPO.
ð7Þ
is markedly increasing within the potential range from 0.6 to 0.7 V in aqueous solutions and 0.7 to 0.9 V in methanolic ones, as shown in Fig. 7 in the form of column diagram. These potential regions represent the transition potentials from one state of adsorption to the other. The increase of the absolute values of n0–nA may be used as a criterion for the occurrence of phase transitions, within the adsorption layer. However, the generalization of this conclusion may be based on the study of a greater number of metal-solvent-adsorbate systems.
Fig. 8. Potential dependence of pre-exponential CPE factor Yo at the interface between polycrystalline Ag and 0.1 M LiClO4 in water (a) and methanol (b) in the absence (I) and in the presence (H) of adsorbate.
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
According to Table 3, increase of absolute Dn values is also observed at potentials more negative than those of Fig. 7 probably due to the desorption of adsorbates. This is why it is preferable to talk about structural changes instead of phase transitions. Further insight relatively to the detected structural changes may be gained by comparing the potential dependence of the CPE pre-exponential factor Yo with the corresponding capacitance curves. On account of Figs. 1, 2 and 8 in the absence of adsorbates, Yo vs. E curves closely resemble C vs. E curves only in the case of methanolic solutions but not in the case of aqueous ones. This may be the result of field-solvent dipole interactions, which in the case of water are considerably stronger than methanol. In the presence of adsorbates Yo vs. E curves fail to approximate the corresponding C vs. E curves particularly at the transition potentials between the two states of adsorbed 1-butanol and TPO. These deviations can hardly be attributed to the increase of interfacial inhomogeneity. Under conditions of rarefaction and hydrodynamic stirring of the adsorbed layer inhomogeneity may be decreased. However, the deviation of the metal-solution contact from the ideally polarized interface may increase due to the disturbance of the spatial separation of electric charges within the Helmholtz layer by the rearrangements of adsorbed 1-butanol and TPO. This may be also the case of aqueous base solution. This can be considered as one of the issues encountered by the phenomenological interpretation of the CPE calculations. 4. Conclusions The form of capacitance vs. potential curves at polycrystalline Ag electrodes in aqueous and methanolic solutions containing 1-butanol and TPO respectively and the results of adsorption isotherm analysis suggest for the occurrence of a combination of structural changes in the adsorbed layer. A type of structural change in the systems under consideration is the change of the position of 1-butanol and TPO molecules and the reorientation of water molecules under the influence of the polarity of the electric field. Such changes manifest themselves by the appearance of capacitance humps or peaks and are related to the recession of dielectric saturation of the interface. At either side of the capacitance peaks two regions of depression of capacitance are observed corresponding to two different states of the adsorbed layer. The application of isotherm analysis reveals better agreement with Langmuir isotherm which means that the adsorbed layer may be better described as a perfect liquid mixture. The calculation of Csat and Cmax, viz the saturation capacitance and the limiting surface concentration of the adsorbates, shows that on passing from potentials positive to the observed peaks to negative ones, the adsorbed layer becomes less compact and the surface concentration of the adsorbates is markedly reduced. On account of these results we may conclude that the change of the position of the adsorbed molecules is accompanied by a decrease of packing of the adsorbed layer. The electrochemical impedance spectra show remarkable frequency dispersion in the absence of adsorbates due to the roughness of electrode surface. The adsorption of 1-butanol and TPO increases the frequency dispersion, due to the non-uniform distribution of the adsorbed molecules on the rough and energetically non-uniform electrode surface. By the use of equivalent circuits incorporating constant phase element (CPE) it is observed that the absolute value of the
57
difference of CPE exponent in the absence and in the presence of both adsorbates is increasing at potentials where the transition from one adsorption state to the other is taking place. This increase may be due to the distortion of the arrangement of the interfacial charges within the overall Helmholtz layer following the rarefaction of the adsorbed layer in combination to the change of the position of adsorbed molecules. Acknowledgement This work was financially supported in part by the Research Committee of Aristotle University of Thessaloniki, Project number 89350. References [1] Th. Wandlowski, in: M. Urbakh, E. Gileadi (Eds.), Encyclopedia of Electrochemistry, vol. 1, VCH-Wiley, Weinheim, 2002 (Chapter 3.3). [2] T. Dretschkow, Th. Wandlowski, Top. Appl. Phys. 85 (2003) 261. [3] Th. Wandlowski, J. Electroanal. Chem. 395 (1995) 83. [4] M.H. Holtze, Th. Wandlowski, D.M. Kolb, Surf. Sci. 335 (1995) 281. [5] Th. Wandlowski, B.M. Ocko, O.M. Magnussen, S. Wu, J. Lipkowski, J. Electroanal. Chem. 409 (1996) 155. [6] M. Scharfe, A. Hamelin, C. Buess-Herman, Electrochim. Acta 40 (1995) 61. [7] M.H. Höltze, D. Krznaric, D.M. Kolb, J. Electroanal. Chem. 386 (1995) 235. [8] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, J. Electroanal. Chem. 304 (1991) 195. [9] V.V. Batrakov, B.B. Damaskin, Y.B. Ipatov, Elektrokhimiya 10 (1974) 216. [10] A. Popov, R. Naneva, K. Dimitrov, T. Vitanov, V. Bostanov, R. de Levie, Electrochim. Acta 37 (1992) 2369. [11] L. Stolberg, J. Richer, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 207 (1986) 213. [12] T. Luczak, Colloids Surf. 280 (2006) 125. [13] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, J. Electroanal. Chem. 272 (1989) 241. [14] J. Lipkowski, L. Stolberg, D-F. Yang, B. Pettinger, S. Mirwald, F. Henglein, D.M. Kolb, Electrochim. Acta 39 (1994) 1045. [15] R.L. Sobocinski, J.E. Pemberton, Langmuir 8 (1992) 2049. [16] M. Brzostowska-Smolska, P. Krysinski, Colloids Surf. 131 (1998) 39. [17] U. Rammelt, G. Reinhard, Electrochim. Acta 35 (1990) 1045. [18] M. Jurkiewicz-Herbich, M. Milkowska, R. Slojkowska, Colloids Surf. A 197 (2002) 235. [19] R. Zhang, A.J. Gellman, J. Phys. Chem. 95 (1991) 7433. [20] A.G. Anastopoulos, Electrochim. Acta 41 (1996) 2537. [21] A.G. Anastopoulos, J. Phys. Chem. B 104 (2000) 5102. [22] A.G. Anastopoulos, P. Karabinas, D. Jannakoudakis, J. Electroanal. Chem. 127 (1981) 219. [23] A. Anastopoulos, A. Christodoulou, I. Moumtzis, Can. J. Chem. 66 (1988) 1053. [24] A. Anastopoulos, I. Moumtzis, J. Electroanal. Chem. 294 (1990) 143. [25] A. Anastopoulos, I. Moumtzis, Electrochim. Acta 35 (1990) 1805. [26] L. Ramaley, C.G. Enke, J. Electrochem. Soc. 112 (1965) 947. [27] R. Hölze, M. Beltowska-Brzezinska, J. Electroanal. Chem. 201 (1986) 387. [28] R.P. Janek, W.R. Fawcett, A. Ulman, J. Phys. Chem. B 101 (1997) 8550. [29] J. Bukowska, K. Jackowska, K. Jaszczynski, J. Electroanal. Chem. 260 (1989) 373. [30] A. Hamelin, T. Vitanov, E. Sevastyanov, A. Popov, J. Electroanal. Chem. 145 (1983) 225. [31] J. Clavilier, C. Nguyen van Huong, J. Electroanal. Chem. 80 (1977) 101. [32] T. Vitanov, A. Popov, E. Sevastyanov, J. Electroanal. Chem. 142 (1982) 289. [33] S. Trasatti, O.A. Petrii, Pure Appl. Chem. 63 (1991) 711. [34] L.I. Daikhin, A.A. Kornysev, M. Urbakh, Electrochim. Acta 42 (1997) 2853. [35] M. Milkowska, Electrochim. Acta 36 (1991) 965. [36] E. Shvarts, B.B. Damaskin, A.N. Frumkin, Rus. J. Phys. Chem. 36 (1962) 1311. [37] M. Milkowska, Electrochim. Acta 32 (1987) 159. [38] A.V. Vedenskii, E.V. Bobrinskaya, Russ. J. Electrochem. 38 (2002) 1178. [39] B.B. Damaskin, O. Petrii, V. Batrakov, Adsorption of Organic Compounds on Electrodes, Plenum Press, New York, 1971. [40] A.N. Frumkin, J. Res. Inst. Catalysis, Hokkaido Univ. 15 (1967) 61. [41] F. Danilov, V. Obraztsov, A. Kapitonov, J. Electroanal. Chem. 552 (2003) 69. [42] J. Lipkowski, Ion and electron transfer across monolayers of organic surfactants, in: B.E. Conway et al. (Eds.), Modern Aspects of Electrochemistry, vol. 23, Plenum Press, New York, 1992. [43] M. Szklarczyk, N. Nhu Hoa, P. Zelenay, J. Electroanal. Chem. 405 (1996) 111. [44] A. Lukomska, J. Sobkowski, J. Solid State Electrochem. 9 (2005) 277. [45] J. Richer, L.S. Stolberg, J. Lipkowski, Langmuir 2 (1986) 630. [46] M. Brzostowska-Smolska, Electrochim. Acta 33 (1988) 1013. [47] A.de J. Motheo, R.M.P. Saldanha, R.de S. Neves, E. de Robertis, A. Sadkowski, Ecl. Quim., Sao Paulo 28 (2003) 29.
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Structural changes at the interface between polycrystalline Ag and electrolyte solutions containing surfactants as probe molecules A.G. Anastopoulos ⇑, M. Paschalidou, A. Papoutsis Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 26 October 2013 Received in revised form 29 January 2014 Accepted 7 March 2014 Available online 15 March 2014 Keywords: Structural changes Polycrystalline silver Surfactant reorientation Adsorbed layer rarefaction
a b s t r a c t Structural changes are detected at the interface between polycrystalline Ag electrodes and electrolyte solutions containing surfactants as probe molecules. Capacitance and impedance measurements indicate that in aqueous solutions water and 1-butanol molecules change their position on the electrode due to dipole-field interaction. In methanolic solutions such a change is indicated to occur only for triphenyphosphine oxide molecules. The above changes in both solvents are taking place along with the transition of the adsorbed layer from a dense liquid state to a thinner one with increasing negative potentials. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Solid metal electrodes usually provide a favorable substrate for the occurrence of structural changes within organic adlayers. As pointed out by important reviews [1,2] associative processes of the type of two-dimensional condensation of the adsorbed layer are detected more often on monocrystal Au electrodes [3–6], less often on silver monocrystals [3,7,8] and much rarely on other metals [9,10]. Other types of structural rearrangements, viz. two state adsorption due to field effected adsorbate reorientation, are also reported for Au monocrystal and polycrystalline electrodes and Ag monocrystals [11–15]. However, as far as we know there is a lack of information in respect to the occurrence of interfacial rearrangements on polycrystalline Ag electrodes. The study of interfacial rearrangements is of interest for modification purposes of the electrode surface including the development of protective layers and biological membranes, the sensor technology and systems of energy conversion, the electrode functionalization for catalysis and medical applications etc. [1,2,16]. Double layer capacitance measurements and electrochemical impedance spectroscopy occupy a central position among the methods devoted to the study of electrochemical interfaces. Capacitance measurements are based on the conventional requirement for the validity of ideal polarizability of the electrode/solution interface. ⇑ Corresponding author. Tel.: +30 2310997861. E-mail address: [email protected] (A.G. Anastopoulos). http://dx.doi.org/10.1016/j.jelechem.2014.03.009 1572-6657/Ó 2014 Elsevier B.V. All rights reserved.
Impedance spectroscopy measurements reveal possible frequency dispersion effects related to the deviation of electrochemical interfaces from the ideally polarized condition and contribute to the elucidation of their origin. The extent of deviation from the ideal capacitance behavior is a measure of the roughness of the electrode or/and the heterogeneity of the interface [17]. In the absence of faradaic processes and interfacial phase transitions the deviation of the interface from ideal capacitive behavior is necessarily associated to the roughness of the metal electrode surface. On the other side, the adsorption of surface active molecules and ions may contribute sometimes to the increase and sometimes to the decrease of interfacial heterogeneity. In the present work the characteristics of electrochemical interfaces of polycrystalline silver electrodes were investigated in the presence of 1-butanol in aqueous solutions and triphenylphosphine oxide in methanolic ones. 1-Butanol and triphenylphosphine oxide may be considered as model adsorbates as long as they are adsorbed in a wide potential range, do not react with the electrode within their adsorption region and are markedly soluble in the solvents used. The capacitive adsorption studies of alcohols at monocrystal Ag electrodes are rather infrequent [18,19], while no such literature information is found for capacitance and impedance studies at polycrystalline Ag electrodes. The adsorption of triphenylphospine oxide and other phosphororganic compounds has been thoroughly studied at mercury electrodes in a variety of pure and mixed solvents [20–25]. However in
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
53
our knowledge, no literature information is quoted for capacitance and impedance studies of triphenylphospine oxide at polycrystalline Ag electrodes. 2. Materials and methods Single compartment double walled electrochemical cells were used, kept at constant temperature of 298 K. Polycrystalline Ag disk electrode, of 2 mm diameter, embedded in a plastic cylinder of 52.5 mm length and 7 mm external diameter, was used as working electrode. The working electrode was mechanically polished by Al2O3, rinsed by distilled water in an ultrasonic bath, washed again with working solution and immediately transferred to the electrochemical cell, taking care to avoid contact with air. A Pt foil was used as auxiliary electrode, the free end of which was bended so as to be parallel to the disk face at a constant distance of 2 mm, in order to avoid nonuniform potential distribution associated with the geometry of disk Ag electrode. In all measurements a 2810 SCHOTT reference calomel electrode saturated with KCl (SCE) was used. The electrochemical activation of the working electrode was carried out by successive scan cycles at a rate of 0.5 V/s in the potential region from 0.2 to 1.2 V (vs. SCE) in aqueous solutions and 0.4 V to 1.1 V (vs. SCE) in methanolic ones, where no faradaic processes are detected, until very reproducible voltammetric curves were obtained. All the experiments were carried out by the electrochemical system Autolab PGSTAT302N connected to a PC running the Autolab software necessary for the control of capacitance, impedance and cyclic voltammetry measurements. Capacitance measurements were carried out with an ac signal of 20 Hz and 5 mVpp. Electrochemical impedance measurements were carried out within a frequency spectrum of 1 Hz to 10 kHz with ac signal of 5 mVpp at various dc potentials. The percent standard deviation between at least two successive capacitance measurements was found to be equal to 5–6% within the potential range from 0.5 to 1.2 V (vs. SCE). On account of the low reproducibility of capacitance measurements at polycrystalline electrodes [26] this result may be considered as a satisfactory one. The reproducibility of all electrochemical impedance measurements was very satisfactory, as proved by the comparison of 2–3 successive measurements at the same potential followed by the Kramers–Kronig linearity-stability test. The chemical reagents used without further purification are LiClO4 (Fluka, purity P 98%), triphenylphosphine oxide (TPO, Fluka purity 99%), 1-butanol (1-BuOH, MERCK, pro analysi) and methanol (MeOH, LAB-SCAN, purity > 99%). Water used was doubly deionized-distilled by the DD8467 Autostill Jencons system. 3. Results and discussion The experimental capacitance curves of the interface between polycrystalline Ag and aqueous solutions of 1-butanol and methanolic solutions of triphenylphosphine oxide are illustrated in Figs. 1 and 2 respectively. Capacitance values are expressed with respect to the geometrical area of silver electrodes in agreement to an acceptable practice mentioned in the literature [12,27–29]. In aqueous solutions the minimum capacitance of the base solution is equal to 43.36 102 F m2. The literature value [30–32] of the minimum capacitance of low roughness monocrystal (1 1 1) and (1 0 0) Ag electrodes in aqueous perchloric solutions is equal
Fig. 1. Capacitance curves of polycrystalline Ag in contact to aqueous 0.1 M LiClO4 containing the following % v/v 1-butanol additions: (j) 0, (I) 1, (d) 2, (r) 4, (N) 6, (H) 7, (s) 8.
Fig. 2. Capacitance curves of polycrystalline Ag in contact to methanolic solutions of 0.1 M LiClO4 containing the following mM TPO additions: (j) 0, (I) 5, (d) 10, (r) 20, (N) 30, (H) 40, (s) 50, (.) 60.
to 32.5 102 F m2. Therefore, an approximate estimation of the roughness factor of our Ag electrode equal to 1.33 can be obtained from the ratio [33] of the above capacitance values. This is a typical value for a polycrystalline Ag electrode of intermediate roughness [34]. In Fig. 1 the curve of base electrolyte presents two main features. A narrow peak at 0.6 V and a broad one at 0.4 V. The latter is also observed in Fig. 2 in methanolic solutions. However, at this potential range capacitance curves of polycrystalline Ag both in aqueous and methanolic solutions are necessarily featureless [35,37]. Thus it can be assumed that the peak at 0.4 V is related both to the specific morphological features of the electrode and to the solvent used as long as the corresponding capacitance values in H2O and methanol are markedly different. On the contrary, the peak at 0.6 V is observed only in aqueous solutions [18,29,37,38] and it seems to be related to a phase transition of the interfacial solvent as for example the reorientation of water molecules. In the presence of 1-butanol the broad peak at 0.4 V is drastically depressed and finally disappears. Moreover, at potentials slightly negative to 0.6 V a sharp peak manifests itself. The adsorption of 1-butanol mainly affects the form and not the height and the position of this peak. For this reason it may not be identified as an adsorption–desorption maximum. The addition of 1-butanol results to the decrease of capacitance values at two potential regions from both sides of the maximum at 0.6 V (vs. SCE). Those two regions may correspond to different states of the adsorbate at the electrode surface. In agreement to literature, polar aliphatic alcohols adsorbed at silver electrodes [15] change their position from low negative potentials with the OH group towards the electrode surface to higher negative ones with the carbon chain towards the electrode surface.
54
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
At potentials E 6 1.0 V (vs. SCE) a strong increase of capacitance is observed, which may be related to the desorption of butanol. In Fig. 2 the curve of base electrolyte presents a broad peak at the potential range from 0.35 to 0.55 V (vs. SCE) similar to that of aqueous solutions, which may be described in the same way as in Fig. 1. In the presence of TPO the capacitance values are uniformly depressed from 0.3 to 1.1 V (vs. SCE) and present a fluctuation at about 0.7 V (vs. SCE). It may be assumed that the form of these curves reveals a change of the structure of adsorbed layer, possibly due to the change of the position of adsorbed TPO molecules from low negative potentials with the phenyl groups in contact to the electrode to higher negative potentials with phosphorus atom close to the electrode surface. At the negative end of potential range no tendency for TPO desorption is observed. In order to elucidate the nature of changes occurring in the adsorbed layer, isotherm analysis was carried out based on capacitance measurements. This analysis is limited at potentials where the condition dhapp/dE 0 is satisfied viz close to the two minima of the C–E curves. The apparent degree of electrode coverage is calculated by the known relation [39]:
happ ¼
C0 Ci C 0 C sat
ð1Þ
where Co is the capacitance in the absence of adsorbate, Ci is the capacitance in the presence of an intermediate adsorbate concentration and Csat is the saturation capacitance corresponding to the condition happ 1. Csat is calculated by extrapolation of 1/Ci vs. 1/cA plots at infinite bulk adsorbate concentration cA. Isotherm analysis has shown that both electrode-solvent-adsorbate systems studied are in very satisfactory agreement with the theoretical Langmuir isotherm:
BcA ¼
happ 1 happ
ð2Þ
where B is the adsorption equilibrium constant, which is related to the electrode potential E by means of the relation [39]: 1 ðC 0 C sat ÞE2 þ C sat EN E 1 B ¼ exp 2 RT Cmax cS
! ð3Þ
In Eq. (3) cS is the bulk solvent concentration, EN is the displacement of the zero charge potential due to the adsorption of ionic and polar non-ionic species and Cmax is the limiting surface adsorbate concentration. According to literature [40,41] in the case of polycrystalline electrodes the condition EN 0 is satisfied. Thus setting Eq. (3) in logarithmic form we obtain:
1 ðC 0 C sat ÞE2 ln B ¼ ln cs 2RT Cmax
ð4Þ
The results of the calculation of B as a function of E, at potentials corresponding to the two regions of minimum capacitance, are shown in Fig. 3 in the form of ln B vs. E2 point diagrams. The stepwise form [42] of these diagrams seems to agree with the structural change of adsorbed layer from positive to negative potentials with respect to the observed capacitance peaks, whereas the dotted line connecting the two groups of points corresponds to the range of transition potentials. The limiting surface concentration Cmax of 1-butanol and triphenylphosphine oxide is calculated
Fig. 3. Plots of ln B vs. E2 for polycrystalline Ag in contact to 0.1 M LiClO4: (H) aqueous solutions of 1-butanol and (I) methanolic solutions of TPO.
by linear regression, applied independently to each one of the two groups of points of the ln B vs. E2 plots. The calculated values of Cmax together with the corresponding values of Csat are provided in Tables 1 and 2. The results of Tables 1 and 2 suggest that the adsorption of 1butanol and TPO is accompanied by a decrease of the compactness of the adsorbed layer following the transition from anodic to cathodic polarizations. This change is confirmed by the corresponding increase of Csat and decrease of Cmax values. The transition of the superficial layer from higher Cmax values to lower ones by increasing negative potentials on Ag electrodes is already reported in the literature, although no evidence for capacitance depression at two different potential regions is observed on the C–E curves [43,44]. Moreover, several authors report capacitance curves at Au and Ag electrodes showing more or less clearly the existence of two potential regions of capacitance depression [11,16,29,45,46] but do not apply isotherm analysis at those potential regions and thus two state adsorption and related phase transitions are not quantitatively confirmed. According to the theory of Frumkin for the capacitance hump [36,40] capacitance fluctuations in the form of humps and peaks are interpreted in terms of recession of dielectric saturation in the adsorbed layer resulting either from adsorption–desorption processes of adsorbed particles or from their reorientation, the latter seemingly being our case. However, the description of the reorientation process of a polar molecule as a 180° flip of the adsorbed dipole is strictly valid only for short dipoles and for absolutely smooth electrodes viz Hg and Ga and approximately valid for low roughness monocrystal (1 0 0) and (1 1 1) faces of solid metal electrodes. On the other hand the degree of disorder of adsorbed layers on electrodes of high roughness is significant and implies a variety of
Table 1 Saturation capacitance and limiting surface adsorbate concentration at the interface Ag/aqueous 0.1 M LiClO4 in the presence of 1-butanol. Potential region V (vs. SCE)
Csat 102 F m2
Cmax
0.40 6 E 6 0.20 1.10 6 E 6 0.80
31.27 35.25
5.45 1011 1.65 1011
mol cm2
Table 2 Saturation capacitance and limiting surface adsorbate concentration at the interface Ag/methanolic 0.1 M LiClO4 in the presence of TPO. Potential region V (vs. SCE)
Csat 102 F m2
Cmax
0.60 6 E 6 0.30 1.05 6 E 6 0.70
14.75 16.92
3.56 1011 1.14 1011
mol cm2
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
orientations of molecules adsorbed at single crystal micro and macro-facets and other defects of the electrode surface. Therefore, the reorientation of long chain bulky molecules adsorbed on rough electrodes may be understood as the change of the position of the various structural groups of molecules under charge-field interactions. As shown in Fig. 4, over all the potential range where isotherm analysis was applied, the apparent degree of coverage happ,max, corresponding to the higher bulk concentration of the adsorbate, is always less than unity. This means that the adsorbed layer may be always described as a perfect liquid mixture in agreement to the validity of Langmuir isotherm. Using Csat as a measure of the packing of adsorbed molecules and Cmax as a measure of their interfacial population, we come to the conclusion that the observed structural change is the transition of the interface from a dense liquid state to a thin liquid one, which resembles the transition from a condensed liquid to an expanded liquid state [42] except that at the condensed liquid state h 1. During this transition the decrease of packing of adsorbed particles facilitates the change of their position under the influence of electric field, which in turn leads to the decrease of dielectric saturation of the interfacial mixture and hence to the emergence of capacitance peaks.
Fig. 4. Plots of apparent electrode coverage at maximum adsorbate concentration against electrode potential in the presence of 1-butanol H and TPO I.
55
Further insight into the nature and characteristics of the changes occurring in the adsorption layer is gained by electrochemical impedance spectroscopy measurements. Spectra of electrochemical impedance are indicatively shown for the systems Ag/0.1 M LiClO4–H2O + 1-BuOH and Ag/0.1 M LiClO4–MeOH + TPO in Figs. 5 and 6 respectively. Frequency dispersion in the form of deviation from vertical direction is observed in all spectra, imposing the replacement of the conventional capacitor in the equivalent circuit of the interface by a constant phase element (CPE). Moreover, as long as a slight curvature is observed in all spectra, which may be considered as representing an incomplete semicircle, the Randles’ type equivalent circuit is adopted with overall impedance equal to:
Z ¼ RS þ
Rp Z CPE Rp þ Z CPE
ð5Þ
where RS is the series solution resistance, Rp the parallel polarization resistance and ZCPE the CPE impedance expressed by the relation:
Z CPE ¼
1 n Y 0 ðjxÞ
ð6Þ
where Y0 is the pre-exponential factor and n the CPE exponent. Figs. 5 and 6 also show that at potentials positive to 1.1 V (vs. SCE) the addition of 1-butanol and TPO is accompanied with the increase of deviation from the ideal capacitance behavior, while at E 6 1.1 V (vs. SCE) this deviation tends to be reversed. In order to set the above observations on a quantitative basis, the experimental impedance measurements were fitted to Eq. (5) by means of a complex nonlinear least squares routine included in the control software of the Autolab system. The mean fitting quality of the experimental spectra, expressed by the factor x2, is of the order of 5 103, which can be considered as reasonable [47] for a polycrystalline electrode. The results of the calculation of the CPE exponent as a function of electrode potential are provided in Table 3. The observed decrease of the CPE exponent, at E > 1.0 V (vs. SCE) in aqueous solutions and at 0.4 P E P 1.1 V (vs. SCE) in
Fig. 5. Impedance plots of polycrystalline Ag in contact to aqueous solutions of 0.1 M LiClO4 in the absence (circles) and in the presence (triangles) of 8% v/v 1-butanol at the following electrode potentials in V(SCE) (a) 0.5, (b) 0.7, (c) 0.9, and (d) 1.1.
56
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
Fig. 6. Impedance plots of polycrystalline Ag in contact to methanolic solutions of 0.1 M LiClO4 in the absence (circles) and in the presence (triangles) of 0.04 M TPO at the following electrode potentials in V(SCE) (a) 0.4, (b) 0.7, (c) 0.9, and (d) 1.1.
Table 3 Values of CPE exponent of Ag/0.1 M LiClO4 interface in aqueous solutions in the presence of 1-butanol and in methanolic solutions in the presence of TPO at various electrode potentials. E V (vs. SCE)
0% v/v 1-BuOH
8% v/v 1-BuOH
E V (vs. SCE)
0 M TPO
0.04 M TPO
0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2
0.853 0.867 0.894 0.906 0.906 0.898 0.905 0.912
0.846 0.856 0.884 0.899 0.902 0.909 0.916 0.918
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
0.826 0.836 0.857 0.878 0.882 0.884 0.850 0.828
0.795 0.835 0.855 0.862 0.865 0.862 0.843 0.805
methanolic ones, is a typical indication that the structural inhomogeneity of the interface as a whole is increased by the adsorption of 1-butanol and TPO molecules. In this respect the observed increase of the CPE exponent in aqueous solutions at E 6 1.00 V can be understood as the result of 1-butanol desorption. The structural changes of the adsorbed layer, found out by means of capacitance measurements, do not clearly manifest themselves in the form of the impedance spectra of Figs. 5 and 6. However, a careful observation of Table 3 reveals that the absolute value of the difference of CPE exponent in the absence and in the presence of adsorbate, n0 and nA respectively:
Dn ¼ jn0 nA j
Fig. 7. Potential dependence of the absolute value of the difference of CPE exponent in the absence and in the presence of adsorbate. (a) 8% v/v 1-butanol, and (b) 0.04 M TPO.
ð7Þ
is markedly increasing within the potential range from 0.6 to 0.7 V in aqueous solutions and 0.7 to 0.9 V in methanolic ones, as shown in Fig. 7 in the form of column diagram. These potential regions represent the transition potentials from one state of adsorption to the other. The increase of the absolute values of n0–nA may be used as a criterion for the occurrence of phase transitions, within the adsorption layer. However, the generalization of this conclusion may be based on the study of a greater number of metal-solvent-adsorbate systems.
Fig. 8. Potential dependence of pre-exponential CPE factor Yo at the interface between polycrystalline Ag and 0.1 M LiClO4 in water (a) and methanol (b) in the absence (I) and in the presence (H) of adsorbate.
A.G. Anastopoulos et al. / Journal of Electroanalytical Chemistry 720-721 (2014) 52–57
According to Table 3, increase of absolute Dn values is also observed at potentials more negative than those of Fig. 7 probably due to the desorption of adsorbates. This is why it is preferable to talk about structural changes instead of phase transitions. Further insight relatively to the detected structural changes may be gained by comparing the potential dependence of the CPE pre-exponential factor Yo with the corresponding capacitance curves. On account of Figs. 1, 2 and 8 in the absence of adsorbates, Yo vs. E curves closely resemble C vs. E curves only in the case of methanolic solutions but not in the case of aqueous ones. This may be the result of field-solvent dipole interactions, which in the case of water are considerably stronger than methanol. In the presence of adsorbates Yo vs. E curves fail to approximate the corresponding C vs. E curves particularly at the transition potentials between the two states of adsorbed 1-butanol and TPO. These deviations can hardly be attributed to the increase of interfacial inhomogeneity. Under conditions of rarefaction and hydrodynamic stirring of the adsorbed layer inhomogeneity may be decreased. However, the deviation of the metal-solution contact from the ideally polarized interface may increase due to the disturbance of the spatial separation of electric charges within the Helmholtz layer by the rearrangements of adsorbed 1-butanol and TPO. This may be also the case of aqueous base solution. This can be considered as one of the issues encountered by the phenomenological interpretation of the CPE calculations. 4. Conclusions The form of capacitance vs. potential curves at polycrystalline Ag electrodes in aqueous and methanolic solutions containing 1-butanol and TPO respectively and the results of adsorption isotherm analysis suggest for the occurrence of a combination of structural changes in the adsorbed layer. A type of structural change in the systems under consideration is the change of the position of 1-butanol and TPO molecules and the reorientation of water molecules under the influence of the polarity of the electric field. Such changes manifest themselves by the appearance of capacitance humps or peaks and are related to the recession of dielectric saturation of the interface. At either side of the capacitance peaks two regions of depression of capacitance are observed corresponding to two different states of the adsorbed layer. The application of isotherm analysis reveals better agreement with Langmuir isotherm which means that the adsorbed layer may be better described as a perfect liquid mixture. The calculation of Csat and Cmax, viz the saturation capacitance and the limiting surface concentration of the adsorbates, shows that on passing from potentials positive to the observed peaks to negative ones, the adsorbed layer becomes less compact and the surface concentration of the adsorbates is markedly reduced. On account of these results we may conclude that the change of the position of the adsorbed molecules is accompanied by a decrease of packing of the adsorbed layer. The electrochemical impedance spectra show remarkable frequency dispersion in the absence of adsorbates due to the roughness of electrode surface. The adsorption of 1-butanol and TPO increases the frequency dispersion, due to the non-uniform distribution of the adsorbed molecules on the rough and energetically non-uniform electrode surface. By the use of equivalent circuits incorporating constant phase element (CPE) it is observed that the absolute value of the
57
difference of CPE exponent in the absence and in the presence of both adsorbates is increasing at potentials where the transition from one adsorption state to the other is taking place. This increase may be due to the distortion of the arrangement of the interfacial charges within the overall Helmholtz layer following the rarefaction of the adsorbed layer in combination to the change of the position of adsorbed molecules. Acknowledgement This work was financially supported in part by the Research Committee of Aristotle University of Thessaloniki, Project number 89350. References [1] Th. Wandlowski, in: M. Urbakh, E. Gileadi (Eds.), Encyclopedia of Electrochemistry, vol. 1, VCH-Wiley, Weinheim, 2002 (Chapter 3.3). [2] T. Dretschkow, Th. Wandlowski, Top. Appl. Phys. 85 (2003) 261. [3] Th. Wandlowski, J. Electroanal. Chem. 395 (1995) 83. [4] M.H. Holtze, Th. Wandlowski, D.M. Kolb, Surf. Sci. 335 (1995) 281. [5] Th. Wandlowski, B.M. Ocko, O.M. Magnussen, S. Wu, J. Lipkowski, J. Electroanal. Chem. 409 (1996) 155. [6] M. Scharfe, A. Hamelin, C. Buess-Herman, Electrochim. Acta 40 (1995) 61. [7] M.H. Höltze, D. Krznaric, D.M. Kolb, J. Electroanal. Chem. 386 (1995) 235. [8] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, J. Electroanal. Chem. 304 (1991) 195. [9] V.V. Batrakov, B.B. Damaskin, Y.B. Ipatov, Elektrokhimiya 10 (1974) 216. [10] A. Popov, R. Naneva, K. Dimitrov, T. Vitanov, V. Bostanov, R. de Levie, Electrochim. Acta 37 (1992) 2369. [11] L. Stolberg, J. Richer, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 207 (1986) 213. [12] T. Luczak, Colloids Surf. 280 (2006) 125. [13] A. Hamelin, S. Morin, J. Richer, J. Lipkowski, J. Electroanal. Chem. 272 (1989) 241. [14] J. Lipkowski, L. Stolberg, D-F. Yang, B. Pettinger, S. Mirwald, F. Henglein, D.M. Kolb, Electrochim. Acta 39 (1994) 1045. [15] R.L. Sobocinski, J.E. Pemberton, Langmuir 8 (1992) 2049. [16] M. Brzostowska-Smolska, P. Krysinski, Colloids Surf. 131 (1998) 39. [17] U. Rammelt, G. Reinhard, Electrochim. Acta 35 (1990) 1045. [18] M. Jurkiewicz-Herbich, M. Milkowska, R. Slojkowska, Colloids Surf. A 197 (2002) 235. [19] R. Zhang, A.J. Gellman, J. Phys. Chem. 95 (1991) 7433. [20] A.G. Anastopoulos, Electrochim. Acta 41 (1996) 2537. [21] A.G. Anastopoulos, J. Phys. Chem. B 104 (2000) 5102. [22] A.G. Anastopoulos, P. Karabinas, D. Jannakoudakis, J. Electroanal. Chem. 127 (1981) 219. [23] A. Anastopoulos, A. Christodoulou, I. Moumtzis, Can. J. Chem. 66 (1988) 1053. [24] A. Anastopoulos, I. Moumtzis, J. Electroanal. Chem. 294 (1990) 143. [25] A. Anastopoulos, I. Moumtzis, Electrochim. Acta 35 (1990) 1805. [26] L. Ramaley, C.G. Enke, J. Electrochem. Soc. 112 (1965) 947. [27] R. Hölze, M. Beltowska-Brzezinska, J. Electroanal. Chem. 201 (1986) 387. [28] R.P. Janek, W.R. Fawcett, A. Ulman, J. Phys. Chem. B 101 (1997) 8550. [29] J. Bukowska, K. Jackowska, K. Jaszczynski, J. Electroanal. Chem. 260 (1989) 373. [30] A. Hamelin, T. Vitanov, E. Sevastyanov, A. Popov, J. Electroanal. Chem. 145 (1983) 225. [31] J. Clavilier, C. Nguyen van Huong, J. Electroanal. Chem. 80 (1977) 101. [32] T. Vitanov, A. Popov, E. Sevastyanov, J. Electroanal. Chem. 142 (1982) 289. [33] S. Trasatti, O.A. Petrii, Pure Appl. Chem. 63 (1991) 711. [34] L.I. Daikhin, A.A. Kornysev, M. Urbakh, Electrochim. Acta 42 (1997) 2853. [35] M. Milkowska, Electrochim. Acta 36 (1991) 965. [36] E. Shvarts, B.B. Damaskin, A.N. Frumkin, Rus. J. Phys. Chem. 36 (1962) 1311. [37] M. Milkowska, Electrochim. Acta 32 (1987) 159. [38] A.V. Vedenskii, E.V. Bobrinskaya, Russ. J. Electrochem. 38 (2002) 1178. [39] B.B. Damaskin, O. Petrii, V. Batrakov, Adsorption of Organic Compounds on Electrodes, Plenum Press, New York, 1971. [40] A.N. Frumkin, J. Res. Inst. Catalysis, Hokkaido Univ. 15 (1967) 61. [41] F. Danilov, V. Obraztsov, A. Kapitonov, J. Electroanal. Chem. 552 (2003) 69. [42] J. Lipkowski, Ion and electron transfer across monolayers of organic surfactants, in: B.E. Conway et al. (Eds.), Modern Aspects of Electrochemistry, vol. 23, Plenum Press, New York, 1992. [43] M. Szklarczyk, N. Nhu Hoa, P. Zelenay, J. Electroanal. Chem. 405 (1996) 111. [44] A. Lukomska, J. Sobkowski, J. Solid State Electrochem. 9 (2005) 277. [45] J. Richer, L.S. Stolberg, J. Lipkowski, Langmuir 2 (1986) 630. [46] M. Brzostowska-Smolska, Electrochim. Acta 33 (1988) 1013. [47] A.de J. Motheo, R.M.P. Saldanha, R.de S. Neves, E. de Robertis, A. Sadkowski, Ecl. Quim., Sao Paulo 28 (2003) 29.