Adsorption of tert-butanol at the electrode from concentrated NaClO 4

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1 Cent. Eur. J. Chem. 11(1) DOI: /s Central European Journal of Chemistry Adsorption of tertbutanol at the electrode from concentrated NaClO 4 Research Article Jolanta Nieszporek Department of Analytical Chemistry and Instrumental Analysis, Faculty of Chemistry, Maria CurieSkłodowska University, 231 Lublin, Poland Received 22 June 212; Accepted 7 September 212 Abstract: The parameters of the double layer for tertbutanol adsorption determined in the supporting electrolytes: 2 mol dm 3, 3 mol dm 3 and 4 mol dm 3 NaClO 4, show an increase of tertbutanol adsorption on the mercury electrode together with the increase of NaClO 4 concentration. The adsorption of tertbutanol on the electrode takes place via the CH 3 group which is shown by the changes in the values of zero charge potentials, E z. On the basis of the analysis of the changes of relative surface excess values, Γ, and the parameters determining the maximum adsorption, the process of adsorption in the discussed systems can be recognized as a physical process. Besides, it can be said that a major drawback in the process of adsorption of the organic substance on the electrode is to remove water from the electrode surface. Keywords: Energy of adsorption Adsorption isotherms Tertbutanol Concentrated NaClO 4 Mercury electrode Versita Sp. z o.o. 1. Introduction The study of organic compounds adsorption at electrode/ solution interface allows description of the structure of the electrical double layer [13], the kinetics of electron transfer [47], effect of metal corrosion inhibitors [8] and the mechanism of the electrode processes [9,1]. The thermodynamics analysis of surface excess concentrations allows a more thorough examination of organics adsorption [11]. To this end the experimental results of the electrode processes are used to calculate adsorption isotherms and corresponding adsorption parameters. Adsorption of aliphatic alcohols at different electrodes was described in the literature [1217]. Adsorption should always be considered as a competitive process between the adsorbate and the solvent. In order to gain an insight into the competitive effect of the solvent, the present study of the adsorption of tertbutanol (TB) from 2 mol dm 3, 3 mol dm 3 and 4 mol dm 3 NaClO 4 as the base electrolyte was undertaken. In the literature, only a few papers can be found dealing with the influence of the electrolyte concentration on the adsorption parameters [18]. Our previous investigations [19] revealed that the NaClO 4 concentration clearly affects the determined adsorption parameters for specific adsorption of tetramethylthiourea at the mercury electrode. Therefore it is worth finding out whether or not a similar dependence takes place for the physical adsorption of tertbutanol. The molecular structure of tertbutanol having hydrophobic methyl groups allows us to expect that the orientation of its molecules at the electrode surface may change insignificantly. The choice of NaClO 4 as a supporting electrolyte results from the fact that ClO 4 ions cause the strongest disruption in the water structure [2]. Tertbutanol adsorption parameters were determined by using Frumkin, corrected FloryHuggins and virial isotherms. The adsorption isotherms were determined by using the surface tension data as a function of electrode potential and bulk concentration. 2. Experimental procedure The measuring system and the instruction of determination of tertbutanol adsorption parameters at the dropping mercury electrode have already been described [19,21]. The charge density and surface 86 * jolan@poczta.umcs.lublin.pl

2 J. Nieszporek C/F.m mol dm 3.3 mol dm 3.8 mol dm 3.1 mol dm 3.3 mol dm 3.5 mol dm E/V Figure 1. Differential capacity potential curves at the mercury electrode in contact with 4 mol dm 3 NaClO 4 containing different concentrations of tertbutanol as in figure legend. tension were derived by the integration of differential capacity potential dependences on the E z value. No corrections were made to the effects of the medium on the activity of the supporting electrolyte and the activity coefficient of the adsorbate. Analytical reagentgrade NaClO 4 and tertbutanol (SigmaAldrich) were used without any further purification. Water and mercury were double distilled before use. The were prepared to cover a range of tertbutanol concentrations from.1 mol dm 3 to.5 mol dm 3, and were deaerated using nitrogen. This gas was passed over the solution during the measurements which were carried out at 298±.1 K. 3. Results and discussion The parameters of the double layer for tertbutanol adsorption were obtained from differential capacity versus potential data for ten concentrations of tertbutanol in the supporting electrolytes: 2 mol dm 3, 3 mol dm 3 and 4 mol dm 3 NaClO 4. Fig. 1 presents the selected differential capacity potential curves in 4 mol dm 3 NaClO 4 extrapolated to zero frequency. Fig. 1 shows a strong decrease of the differential capacity at more negative potentials than the zero potential charge E. The curves in z Fig. 1 present well developed peaks of adsorption and desorption which move away from each other with an increase of tertbutanol concentration. The potential range of lowered differential capacity, which is caused by the maximum concentration of tertbutanol with respect to the basic electrolyte curve, is extended with the increase of NaClO 4 concentration and is equal to:.777 V,.822 V and.888 V for 2 mol dm 3, 3 mol dm 3 and 4 mol dm 3 NaClO 4, respectively. The minimum values of differential capacity decrease in the same direction and are equal to:.438 F m 2,.42 F m 2 and.357 F m 2. The obtained results show unequivocally that the adsorption of tertbutanol increases with the increase of NaClO 4 concentration. The capacitypotential data were numerically integrated from the E z point. The integration constants are presented in Table 1. The E z values are shifted towards less negative potentials with the increase of tertbutanol concentration. The results confirm the adsorption of tertbutanol with the positive field, i.e., the CH 3 groups placed on the mercury surface. Both the total shift of E z values and the total decrease of g z values are the highest in 4 mol dm 3 NaClO 4. The results presented in Table 1 confirm earlier observations about the strongest adsorption of tertbutanol in 4 mol dm 3 NaClO 4. From the q = f ( E ) 87

3 Adsorption of tertbutanol at the electrode from concentrated NaClO 4 Table 1. The values of the zero charge potentials E z / V vs. Ag AgCl electrode and surface tension g z /mn m 1 at E z for the studied systems. C TB / mol dm 3 2 mol dm 3 NaClO 4 3 mol dm 3 NaClO 4 4 mol dm 3 NaClO 4 E Z g z E Z g z E Z g z dependences characteristic parameters of maximum adsorption q max as well as E max were determined. Fig. 2 presents these dependences for 4 mol dm 3 NaClO 4. These parameters have increasingly more negative values with the increase of NaClO 4 concentration: q max from 1.35 µc m 2 to 2.36 µc m 2 whereas E max from.555 V to.628 V. This is, undoubtedly, the result of weak adsorption of ClO 4 ions. That is why the thermodynamic description of tertbutanol adsorption was based on the relative surface excess Γ ' and not on surface excess Γ. The surface tension obtained by integration of differential capacity curves was subsequently used to calculate the surface pressure,, where g is the surface tension for NaClO 4 and g is the surface tension for containing tertbutanol. The values of Φ were used to calculate Γ ' according to the Gibbs adsorption isotherm: where c is the bulk concentration of tertbutanol. The Γ ' values obtained for 4 mol dm 3 NaClO 4 as a function of the electrode potential and tertbutanol concentration are presented in Fig. 3. Together with the increase of NaClO 4 concentration, the Γ ' values increase, and, at the same time, the maxima on the curves are shifted towards negative potentials: from.6 V to.8 V. This is indicative of the occurrence of optimal composition of the surface layer of H 2 O:ClO 4, at which the adsorption of tertbutanol is the greatest. The obtained results show that water is a major deterent for the adsorption of tertbutanol in comparison with easily polarizable ClO 4 ions Adsorption isotherms The adsorption parameters of tertbutanol in the studied systems were calculated based on the Frumkin, the corrected FloryHuggins, and virial isotherms. The (1) Frumkin isotherm constants were determined from the equation: where x is the mole fraction of tertbutanol in the solution, is the adsorption coefficient (, D G is the standard Gibbs energy of adsorption), A is the interaction parameter, and is the coverage ( ). The above expression is based on the lattice formalism. This means that above the surface there exists a lattice of energetically identical adsorption centers and the adsorbed molecule occupies only one adsorption site the adsorbed species form a monolayer at the electrode surface with each molecule replacing one solvent molecule. It is interesting to note that many theoretical adsorption isotherms based on the lattice formalism can be written as follows: where Λ are, respectively, the entropic and thermal contributions to adsorption. In the case of Eq. 2 Λ entr has the following form: whereas thermal contribution is as follows: The surface excess at saturation, Γ s, was estimated by extrapolation of the 1 / Γ ' vs. 1 / c tertbutanol curves at different electrode charges to 1 / c = tertbutano l. The Γ s values obtained in this way were: mol m 2 in 2 mol dm 3 NaClO 4, mol m 2 in 3 mol dm 3 NaClO 4 and mol m 2 in 4 mol dm 3 NaClO 4. The obtained Γ values are greater than the s (2) (3) (4) (5) 88

4 J. Nieszporek σ/1 2 C.m mol dm 3.3 mol dm 3.8 mol dm 3.1 mol dm 3.3 mol dm 3.5 mol dm E/V Figure 2. Surface charge of the electrode vs. its potential for various concentrations of tertbutanol in 4 mol dm 3 NaClO Γ'/mol. m c TB.1 mol dm 3.3 mol dm 3.5 mol dm 3.8 mol dm 3.9 mol dm 3.1 mol dm 3.2 mol dm 3.3 mol dm 3.4 mol dm 3.5 mol dm E / V Figure 3. Relative surface excess of tertbutanol as a function of electrode potential and tertbutanol bulk concentration in 4 mol dm 3 NaClO 4. 89

5 Adsorption of tertbutanol at the electrode from concentrated NaClO 4 x(1θ) Θ ln 3 4 E=.2V E=.3V E=.4V E=.5V E=.6V E=.7V E=.8V E=.9V E=1.V Θ Figure 4. A linear test of the Frumkin isotherm for the system 3 mol dm 3 NaClO 4 + tertbutanol. theoretical ones and amount to mol m 2 this can be the result of deformation of the tertbutanol molecule. At the same time, these values point to easy tertbutanol adsorption from the more concentrated basic electrolyte. The obtained effect confirms the positive role of ClO 4 ions which pertain to the disruption of water structure hampering the process of adsorption. The surfaces occupied by one tertbutanol molecule, S ( S 1 / Γ ) were s.266 nm 2,.167 nm 2 and.83 nm 2 in the used supporting electrolytes. Fig. 4 shows a linear test of the Frumkin isotherm. The values of parameter A were calculated from the slopes of the lines in Fig. 4, and the corresponding D G F values were determined by extrapolation of the lines of the entropic term versus to =. The obtained values of the free adsorption energy decrease in terms of absolute values with the increase of NaClO 4 concentration. With the increase of NaClO 4 concentration, the maximum D G values are shifted towards less negative values (Fig. 5). The values of parameter A point to the occurrence of repulsive interactions between the adsorbed tertbutanol molecules. These interactions are more intensive when the NaClO 4 concentration increases. Their high values at the least negative potentials in 3 mol dm 3 and 4 mol dm 3 NaClO 4 may be the result of a large accumulation of the ClO 4 ions on the positively charged electrode surface. The weakest repulsive interactions between the adsorbed tertbutanol molecules occur in the area of potentials for which the relative surface excess values are the largest. The adsorption of tertbutanol was further analyzed on the basis of the constants obtained from the modified FloryHuggins isotherm [22] for longrange particleparticle interactions: where n is the ratio of the surface occupied by tertbutanol molecule adsorbed on the electrode and the surface of H 2 O molecule displaced from the electrode. In comparison with the Frumkin isotherm (Eq. 2) the above equation is different by the entropic term Λ. entr In the present case, using the projected area.123 nm 2 for water [23] and that for tertbutanol calculated from Γ s, the parameter n is equal to 2.16, 1.36 and.68 in 2 mol dm 3, 3 mol dm 3 and 4 mol dm 3 NaClO 4, respectively. As the ClO 4 ions cause the strongest disruption in water structure [2], the surface of one water molecule is used in calculations instead of water clusters. The D G values obtained from the FloryHuggins isotherm are clearly higher in comparison with those obtained from the Frumkin isotherm, whereas the values of parameter A are similar. The tendencies of variation of D G and parameter A are similar for both isotherms. (6) 9

6 J. Nieszporek DG /kj. mol mol dm 3 NaClO 4 3 mol dm 3 NaClO 4 4 mol dm 3 NaClO 4 25 A E/V Figure 5. Variation of the Gibbs energy of adsorption DG (solid lines) and the interaction parameter A (dashed lines) due to electrode potential and NaClO 4 concentration for tertbutanol derived from the Frumkin isotherm log Γ'/c E=.2V E=.3V E=.4V E=.5V E=.6V E=.7V E=.8V E=.9V E=1.V Γ'/molecule. m 2 Figure 6. A linear test of the virial isotherm for the system 3 mol dm 3 NaClO 4 + tertbutanol. 91

7 Adsorption of tertbutanol at the electrode from concentrated NaClO DG /kj. mol 1 2 mol dm 3 NaClO 4 3 mol dm 3 NaClO 4 4 mol dm 3 NaClO B/nm 2. molecule The data obtained from the Frumkin and the modified FloryHuggins isotherms were verified using the virial isotherm. The application of the virial isotherm does not need the knowledge of Γ s value. The virial isotherm equation is: E/V Figure 7. Variation of the Gibbs energy of adsorption DG (solid lines) and of the 2D second virial coefficient B (dashed lines) due to electrode potential and NaClO 4 concentration for tertbutanol, derived from the virial isotherm. (7) concentration. It should be emphasized here that in the areas of electrode potentials for which the adsorption of tertbutanol is the greatest, the parameters describing mutual interactions between the adsorbate molecules obtained from three isotherms show that the weakest repulsive interactions occur between the molecules of tertbutanol. where B is a twodimensional (2D) second virial coefficient. Fig. 6 shows a linear test of the virial isotherm for 3 mol dm 3 NaClO 4. The values of 2D second virial coefficient were calculated from the line slopes in Fig. 6. The corresponding D G values were obtained from the intercepts of those lines on the axis log( Γ '/ c) using the standard states 1 mol dm 3 in the bulk solution and 1 mol cm 2 on the surface. The obtained values of the virial isotherm constants are presented in Fig. 7. The changes of D G values depending on electrode potential and NaClO 4 concentration are similar to those in the previous isotherms. However, there are significant differences as concerns the changes in parameter B against NaClO 4 concentration. Namely, the repulsive interaction between the adsorbed molecules of tertbutanol usually decreases with the increase of the basic electrolyte concentration. Such changes of parameter B seem to be responsible for the increase of Γ ' values together with the increase of NaClO 4 4. Conclusions The obtained results point unequivocally to an increase of tertbutanol adsorption together with the increase of concentration of NaClO 4 solution from 2 mol dm 3 to 4 mol dm 3. This is shown by the curves of differential capacity, the values E z, and Γ '. The above observation confirms that the greatest problem in the process of adsorption of an organic substance on the electrode involves removing water from the electrode surface [2426]. The increase of concentration of ClO 4 ions affects the parameters determining maximum tertbutanol adsorption as well as the constants of used isotherms. The increase of tertbutanol adsorption, together with the increase of basic electrolyte concentration, is connected to a decrease of interactions between the molecules of adsorbate which results from the value of parameter B obtained from the virial isotherm. 92

8 J. Nieszporek References [1] [2] [3] [4] [5] [6] [7] [8] [9] [1] [11] [12] [13] A.J. Bard, H.D. Abruna, Ch.E. Chidsey, L.R. Faulkner, S.W. Feldberg, K. Itaya, M. Majda, O. Melroy, R.W. Murray, J. Phys. Chem. 97, 7147 (1993) C. Fontanesi, L. Benedetti, Electrochim. Acta 42, 1373 (1997) D. Jadreško, M. Lovrič, Electrochim. Acta 53, 845 (28) F. Danilov, V. Obraztsov, A. Kapitonov, J. Electroanal. Chem. 552, 69 (23) A. Baars, K. Aoki, Y. Numata, J. Electroanal. Chem. 436, 133 (1997) J. Saba, D. Gugała, J. Nieszporek, D. Sieńko, Z. Fekner, Electrochim. Acta 51, 6165 (26) J. Nieszporek, Monats. Chem. 141, 521 (21) C. Cachet, R. Wiart, Electrochim. Acta 44, 4743 (1999) W.R. Fawcett, J. Electroanal. Chem. 31, 13 (1991) J. Nieszporek, J. Electroanal. Chem. 662, 47 (211) L. Stolberg, S. Morin, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 37, 241 (1991) G. Zilberman, J. Electroanal. Chem. 52, 1 (21) S. Romanowski, K. Maksymiuk, Z. Galus, J. Electroanal. Chem. 385, 95 (1995) [14] E. Lust, A. Jänes, K. Lust, P. Miidla, J. Electroanal. Chem. 413, 175 (1996) [15] M. BrzostowskaSmolska, P. Krysiński, Colloids and Surfaces A: Physicochem. Eng. Aspects 131, 39 (1998) [16] V.V. Emets, B.B. Damaskin, J. Electroanal. Chem. 582, 97 (25) [17] M.R. Moncelli, M.L. Foresti, R. Guidelli, J. Electroanal. Chem. 295, 225 (199) [18] M. Zelič, M. Lovrič, J. Electroanal. Chem. 541, 67 (23) [19] D. Sieńko, D. GugałaFekner, J. Nieszporek, Z. Fekner, J. Saba, Collect. Czech. Chem. Commun. 74, 139 (29) [2] J. Koryta, J. Dvorak, V. Bohackova, Elektrochemia (PWN, Warszawa, 198) (in Polish) [21] D.J. Schiffrin, J. Electroanal. Chem. 23, 168 (1969) [22] S. Trasatti, J. Electroanal. Chem. Interfacial. Electrochem. 28, 257 (197) [23] J. Lawrence, R. Parsons, J. Phys. Chem. 73, 3577 (1969) [24] A. NosalWiercińska, G. Dalmata, Electroanalysis 22, 281 (21) [25] A. NosalWiercińska, M. Grochowski, Collect. Czech. Chem. Commun. 76, 265 (211) [26] A. NosalWiercińska, Cent. Eur. J. Chem. 1, 129 (212) 93

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