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Langmuir 2003, 19, 2338-2343
Chronoamperometric Study of the Films Formed by Salts of Heptyl Viologen Cation Radical on Mercury: Desorption-Nucleation and Reorientation-Nucleation Mechanisms J. I. Milla´n, J. J. Ruiz, L. Camacho, and R. Rodrı´guez-Amaro* Departamento de Quı´mica Fı´sica y Termodina´ mica Aplicada, Universidad de Co´ rdoba, Campus de Rabanales, Edificio C-3, 14071 Co´ rdoba, Spain Received September 26, 2002. In Final Form: December 17, 2002 The formation of condensed phases by salts of heptyl viologen cation radical on mercury electrode in aqueous media under potentiostatic conditions is explained in light of a new mathematical model based on a desorption-nucleation mechanism. This initially assumes the nucleation process to be governed by the desorption of molecules previously adsorbed on the electrode surface. The potential subsequent incorporation of adsorbed molecules into the condensed 2D phase via surface reorientation (a reorientationnucleation mechanism) is considered. The proposed model was successfully applied to the 2D nucleation of heptyl viologen in the presence of both strongly (bromide) and weakly (sulfate) adsorbed anions on a mercury electrode surface.
Introduction 1,1′-Disubstituted 4,4′-bipyridils (V2+), usually referred to as “viologens”, are of great electrochemical interest on account of their involvement in redox reactions which produce stable free radicals that can be used as electrochemical mediators in homogeneous1-9 and heterogeneous reactions.10-17 Electrogenerated condensed films are of special significance as electrode surfaces facilitate the study of the formation and rearrangement of adsorbed monolayers by virtue of the ease with which interfacial conditions can be controlled and altered, in a flexible manner, through the electric potential. In recent work,18 we showed a two-dimensional phase of the salts of the resulting cation radical to be formed in * Corresponding author: e-mail
[email protected]; phone 34-957 218617; Fax 34-957 218618. (1) Rodkey, F. L.; Donovan, J. A. J. Biol. Chem. 1959, 234, 677. (2) Kuwana, T.; Ito, M. J. Electroanal. Chem. 1972, 198, 415. (3) Rauwel, F. J. Electroanal. Chem. 1977, 75, 579. (4) Castner, J. F.; Hawkridge, F. M. J. Electroanal. Chem. 1983, 143, 217. (5) Hadjian, J.; Pilard, R.; Bianco, P. J. Electroanal. Chem. 1985, 184, 391. (6) Wei, J. F.; Ryan, M. D. Anal. Biochem. 1980, 106, 269. (7) Albery, W. J.; Eddowes, M. J.; Hill, H. A. O.; Hillman, A. R. J. Am. Chem. Soc. 1981, 103, 3904. (8) Taniguchi, I.; Toyosawa, K.; Yamaguchi, H.; Yasukouchi, K. J. Chem. Soc., Chem. Commun. 1982, 1032. (9) Evans, A. G.; Dodson, N. K.; Rees, N. H. J. Chem. Soc., Perkin Trans. 2 1976, 859. (10) Bard, A. J.; Abrun˜a, A. D. J. Am. Chem. Soc. 1981, 103, 6898. (11) Razuman, V. J.; Gudavicius, A. V.; Kulys, J. J. J. Electroanal. Chem. 1986, 198, 81. (12) Park, K. K.; Lee, C. W.; Oh, S. Y.; Park, J. W. J. Chem. Soc., Perkin Trans. 1 1990, 2356. (13) Park, J. W.; Choi, M. H.; Park, K. K. Tetrahedron Lett. 1995, 36, 2637. (14) Liu, F. T.; Yu, X. D.; Feng, L. B.; Li, S. B. Eur. Polym. J. 1995, 31, 819. (15) Bowden, E. F.; Hawkridge, F. M.; Blout, H. N. Bioelectrochem. Bioeneg. 1980, 447, 7. (16) Liu, F. T.; He, B. L.; Liang, L. J.; Feng, L. B. Eur. Polym. J. 1997, 33, 311. (17) Zen, J. M.; Jeng, S. H.; Chen, H. J. J. Electroanal. Chem. 1996, 408, 157. (18) Milla´n, J. I.; Sa´nchez-Maestre, M.; Camacho, L.; Ruiz, J. J.; Rodrı´guez-Amaro, R. Langmuir 1997, 13, 3860.
the electrochemical reduction of heptyl viologen (HV2+) on mercury in aqueous media; a study of the 2D phase including the influence of temperature, the reagent concentration, and the type of anion present in the medium was conducted from cyclic voltammetry and capacitance measurements. However, a preliminary chronoamperometric study had exposed a complex behavior that could not be accurately described by available models.19 In this work, we developed a new mathematical model to account for the experimental current transients observed during the film formation process under the abovedescribed conditions. At potentials preceding nucleation, heptyl viologen molecules are adsorbed on the electrode surface, probably as ion pairs of the HV2+-counterion type. HV2+ molecules must previously be desorbed and/or the ion pair must break and rearrange for the nucleation process to occur. The proposed model considers both potential mechanisms and was tested on strongly (bromide) and weakly (sulfate) adsorbed anions on a mercury electrode surface, both with satisfactory results. Experimental Section Practical grade 1,1′-diheptyl-4,4′-bipyridinium dibromide (purum grade, 97%) was purchased from Aldrich and used as received. All other chemicals were obtained in analytical reagent grade from Merck and also used as supplied. Mercury was purified in dilute nitric acid and triply distilled in vacuo. All solutions were made in bidistilled water from a Millipore Milli-Q system and deaerated by bubbling gaseous nitrogen through them. All electrochemical measurements were made by using an EG&G PAR 273 potentiostat with automatic correction for the iR drop. A static mercury drop electrode (SMDE) with a surface area of 2.2 mm2 was used as the working electrode. Ag|AgCl and Pt wire were used as the reference and auxiliary electrode, respectively. The measuring cell was thermostated to within (0.1 °C. All measurements were made in a nitrogen atmosphere. The instrumental setup was computer-controlled with the aid of M270 Research Electrochemistry Software. (19) Rodrı´guez-Amaro, R.; Ruiz, J. J. In Handbook of Surfaces and Interfaces of Materials; Nalwa, H. S., Ed.; Academic Press: New York, 2001; Vol. 1, p 660.
10.1021/la026609q CCC: $25.00 © 2003 American Chemical Society Published on Web 02/01/2003
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Figure 1. Voltammetric recording for 1 mM HV2+ in 0.1 M KBr, obtained at v ) 50 mV s-1 and T ) 25 °C.
Results and Discussion Figure 1 shows selected cyclic voltammogram obtained for 1 mM HV2+ in 0.1 M KBr over a Hg electrode at T ) 25 °C and v ) 50 mV/s, using potentials over the range -300 to -500 mV. Peaks A1 and A2 were analyzed elsewhere18 and assigned to the formation (A1) and destruction (A2) of a two-dimensional phase formed by salts of heptyl viologen cation radical with the anion present in the medium. For other anions, such as sulfate and chloride, which are less strongly adsorbed on mercury, these peaks appear at more positive potentials (for more details, see Figure 1 in ref 18). Integration with respect to time of peaks A1 or A2 allowed us to calculate the charge Q exchanged during the formation or dissolution of the 2D phase. Below 30 °C, Q was found to be ca. 21 ( 2 µC cm-2, so the area occupied by HV•+ should be about 77 ( 8 Å2 molecule-1.18 This value is consistent with a configuration where the plane containing the bipyridine group is edge-on or tilted relative to the electrode surface. We designated this condensed phase “phase R”.20 The influence of the anion present in the medium on the behavior of peaks A1 and A2 was also examined by the authors in previous work.18 In bromide media, the peak potential for process A1, EpA1, was found to shift to more negative values as the anion concentration increase. In sulfate media, however, the anion concentration was found to have no appreciable influence on the peak potential for process A1. The two-dimensional phase transition can be examined in kinetic terms by using the chronoamperometric technique.19,21,22 Figure 2 shows selected j-t curves for 0.1 M K2SO4 and 0.1 M KBr recorded under the same experimental conditions as in Figure 1, but using a temperature of 17 °C. The curves were experimentally obtained by applying an initial potential E0 that was followed by a potential pulse up to a potential immediately following that of appearance of peak A1. The final potential of the potentiostatic jump is shown in the figure. As can be seen, the curves exhibit typical maxima that allow the nucleation processes involved to be characterized. For clarity, the fast initial decay corresponding to the double-layer contribution has been excluded from the bromide solution curves. (20) Milla´n, J. I.; Rodrı´guez-Amaro, R.; Ruiz, J. J.; Camacho, L. Langmuir 1999, 15, 618. (21) Fleischmann, M.; Thirsk, H. R. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Interscience: New York, 1963; Vol. 3. (22) Harrison, J. A.; Thirst, H. R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1977; Vol. 5.
Figure 2. Chronoamperograms obtained for 1 mM HV2+ in 0.1 M K2SO4 or 0.1 M KBr at T ) 17 °C. The initial and final potential (E0 and Ef, respectively) for each curve are shown.
However, the experimental data fit no simple mathematical nucleation-growth-collision (NGC) models;19,21,23 this suggests that the process involves a complex phenomenon presumably due to the presence of adsorbed anions. It should be noted that, at potentials more positive than those of appearance of peak A1, the capacitance is somewhat higher than that for the supporting electrolyte (see zone I in Figure 7 of ref 18), which suggests an increased thickness of the interfacial structure. In this potential zone, adsorption must preferentially involve Br ions; the outer Br ions may form ion pairs with HV2+, thereby increasing the thickness of the layer of adsorbed molecules at the electrode and hence the capacitance. The results obtained with sulfate as the counterion were quite similar to those for bromide. To derive information about the nature of the nucleation process, the current density j(t) for such as process was assumed to conform to the well-known Bewick-Fleischmann-Thirsk (BFT) model,21,24 expressed as
j(t) ) nβqmtn-1 exp(-βtn) ) qm
dS dt
(1)
qm being the total charge involved in the phase transition (usually corresponding to a monolayer), β a parameter (23) Obretenov, W.; Petrov, I.; Nachev, I.; Staikov, G. J. Electroanal. Chem. 1980, 109, 195. (24) Bewick, A.; Fleischmann, M.; Thirsk, H. R. Trans. Faraday Soc. 1962, 58, 2200.
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poor results in all cases. A new mathematical model capable of accounting for this behavior had thus to be developed. The Proposed Model. The proposed mathematical model is based on the following initial assumptions: (i) Initially, a potential E0 prior to nucleation at which adsorption occurs is applied to the electrode. The initial electrode coverage is thus given by θ0, which can be nonzero. In the studied system, θ0 ≈ 1, and as noted earlier, the material initially adsorbed on the electrode surface may be formed by specific adsorption of anions from the background electrolyte or by HV2+-counterion ion pairs. Although this latter possibility seems to be the more likely, the actual situation will depend on the relative extent of the different interaction forces for each ion. (ii) A new adsorption-desorption kinetics is established upon application of the potential pulse E0 f E1 before equilibrium is restored. (iii) HV2+ molecules randomly adsorbed on electrode surface forming ion pairs are not electroactive. On the basis of these assumptions, the material on the electrode must previously be desorbed for the nucleation process to take place. Thus, if θ(t) is the time-dependent surface coverage by anions from the background electrolyte, or by HV2+-counterion ion pairs, and S(t) is the timedependent surface coverage by molecules involved in the nucleation and growth process (NG), then the corresponding adsorption-desorption kinetic will be given by29 Figure 3. Plots of n vs t obtained from the corresponding experimental j-t transients.
related to nuclear growth rate, and n a constant dependent on the nature of the nucleation process. Moreover, S is the effective surface covered by the condensed phase. The overlap between neighboring sites can be estimated by applying the Avrami theorem.25-27 Thus
S)
∫0t j dt ) 1 - exp(-SX) ) 1 - exp(-βtn)
1 qm
(2)
where SX is the expanded surface area (i.e., the surface area in the absence of overlap). SX can be calculated integrating experimental j-t curves at each t value during the experiment. If we write eq 2 in the form
ln(β) + n ln(t) ) ln[-ln(1 - S)]
(3)
we obtain
dθ ) kd (1 - θ - S) - kiθ dt
where kd and ki are the rate constants for the adsorption and desorption process, respectively. If the overlap between neighboring centers is taken into account by applying the Avrami theorem,25-27 and so is the surface coverage by randomly adsorbed molecules as defined by Noel et al.,31 then
S ) (1 - θ)[1 - exp(-SX)]
∂ ln[-ln(1 - S)] ∂ ln(t)
(4)
Figure 3 shows lots of n vs t obtained from the corresponding experimental j-t transients with bromide and sulfate counterions. As can be seen, n ≈ 2 at short times and tends to unity at longer times. The experimental j-t curves were fitted to reported models for nucleation and adsorption mixed processes such as those of Rangarajan et al.28,29 and Guidelli et al.,30 with (25) Avrami, M. J. Chem. Phys. 1939, 7, 1103. (26) Avrami, M. J. Chem. Phys. 1940, 8, 212. (27) Avrami, M. J. Chem. Phys. 1941, 9, 177. (28) Bosco, E.; Rangarajan, S. K. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1673. (29) Bhattacharjee, B.; Rangarajan, S. K. J. Electroanal. Chem. 1991, 302, 207. (30) Guidelli, R.; Foresti, M. L.; Innocenti, M. J. Phys. Chem. 1996, 100, 18491.
(6)
The usual mathematical expression for describing the time dependence of the surface coverage, SX, has the form of a convolution integral:25,32 t dr dz ∫0t(dN dt )u [∫u ( dt )z ]
SX ) π
2
du
(7)
where N is the number of nuclei that are formed from active sites an follows an exponential law:33
N ) N0[1 - exp(-At)] n)
(5)
dN/dt ) N0A exp(-At) (8)
N0 is the maximum possible number of nuclei in the absence of subsequent growth processes, and A is the frequency of nucleus formation on a separate active site. Hence, dN/dt is the rate of nucleation. The current density, j2D(t), can be obtained from
(dS dt )
j2D(t) ) qm
(9)
This equation is different from that used by Rangarajan et al.,29 as it has been obtained under the assumption that the randomly adsorbed material (RA) is not electroactive either because it is an inert material or because the (31) Noel, M.; Chandrasekaran, S.; Ahmed Basha, C. J. Electroanal. Chem. 1987, 225, 93. (32) Jacobs, P.; Tomkins, F. In The Chemistry of the Solid State; Garner, W., Ed.; Butterworth: London, 1955; Chapter 7. (33) Kaischev, R.; Mutafchiev, B. Electrochim. Acta 1965, 10, 643.
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Langmuir, Vol. 19, No. 6, 2003 2341
Figure 4. Simulations (solid lines) of selected experimental results shown in Figure 2 using the model based on eq 5.
overpotential needed to reduce it is not reached; in other words, at the working overpotentials, only material undergoing nucleationsnot the adsorbed materialsis reduced. This is the same phenomenon observed in the UPD of metal ions.34 The experimental C-E curves (see Figure 7 in ref 18) show that, prior to nucleation (potential E0 preceding the jump ∆E), the adsorption is high (θ0 ≈ 1), so kd . ki. We shall henceforward assume that this relationship holds after the potential jump, i.e, that desorption does not occur unless the nuclei formed displace the adsorbed material. Accordingly, S(t) + θ(t) ≈ 1, so
dθ i dS ) ≈q dt dt
(10)
i.e., the nucleation process is governed by desorption. Note that, at very short times, the rate-determining step in the formation of the condensed phase is the nucleation process, so eq 10 is not applicable under these conditions. For simplicity, we shall adopt the BFT model for the nucleation process.21,24 This model assumes that the rate of radial growth of a nucleus v(r) ) dr/dt, considered to be a circular disk, does not vary with time. The general equation of this model35 can be simplified for two limiting cases designated “instantaneous nucleation” (A f ∞) and “progressive nucleation” (A f 0). If we assume the instantaneous nucleation mechanism to prevail, then eq 7 remains24,35 (34) Budevski, E.; Staikov, G.; Lorenz, W. J. In Electrochemical Phase Formation and Growth; VCH: Weinheim, 1996. (35) Obretenov, W.; Petrov, I.; Nachev, I.; Staikov, G. J. Electroanal. Chem. 1980, 109, 195.
SX ) βt2
(11)
where, as stated above and can be seen in Figure 3, the quadratic dependence on time is consistent with the experimental result for n at short times. This approximation was previously used by Rangarajan et al.29 under competitive nucleation-adsorption conditions. Although, in real systems, the nucleation kinetics must changeover time as suggested by Guidelli et al.,31 in our case this will have little effect on the final shape of the j-t curve as eq 10 will be obeyed during the most of the process. The differential equation (5) can be readily solved numerically using the fourth-order Runge-Kutta method, in conjunction with eqs 6, 9, and 11. Figure 4 shows selected simulated curves obtained from Figure 2 using this model. As can be seen, the simulated (solid lines) and experimental curves (dotted lines) are consistent at both short and long times. At intermediate times, however, marked deviations are observed. These discrepancies between the model and the experimental results led us to introduce new considerations. Thus, the proposed model assumes that the HV2+ molecules must desorb from the electrode surface prior before nucleation take place; a direct contribution of the adsorbed material (basically HV2+ forming the ion pair) to condensed phase can also be considered. This potential contribution would be preceded by a slow step (possibly a reorientation of the adsorbed material) and would occur only on the periphery of the nuclei already formed. In our mathematical model, we can consider this potential contribution of the adsorbed material to the condensed phase by including a term accounting for the rearrangement in eq 5. Hence, we have
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dθ ) kd(1 - S - θ) - kiθ - k′rθPSP dt
(12)
where θP and SP are the θ and S values on the contact perimeter between nuclei and adsorbed material and k′r is the rate constant for the reorientation process. Moreover, if the electrode surface is fully covered, then θ + S ≈ 1 and, in contact area, θP ≈ SP. From the above assumption that the nucleus is a circular disk it follows that
SP ∝
dSX dS ) (1 - S) dr dr
(13)
and SX ) πr2, so
SP ∝ (1 - S)xSX ) (1 - S)x - ln(1 - S)
(14)
θPSP ∝ - (1 - S)2 ln(1 - S) ≈ -θ2 ln(θ)
(15)
and
Substitution of eq 15 into the differential eq 12 yields an also differential expression that is difficult to solve numerically with θ f 0. The former expression can be simplified by expanding ln(θ). Thus, for θ f 0 we obtain
θPSP ∝ -θ2 ln(θ) ≈
[
-θ2 -(1 - θ) -
]
(1 - θ)2 (1 - θ)3 - ... ≈ 2 3 θ2(1 - θ) (16)
Moreover, if one assumes θ g 0.5, then
θPSP ∝ -θ2 ln(θ) )
[
2
-θ2 -
3
]
(1 - θ) 1 - θ (1 - θ) + + ... ≈ 2 θ 3θ 2θ θ(1 - θ) (17)
Equations 16 and 17 are approximate expressions for the reorientation term that are applicable at low and high θ values, respectively. Both expressions can be used to simulate the experimental potentiostatic transients. As noted below, we obtained the bests fits for heptyl viologen by using eq 17, i.e., when θPSP ≈ θ(1-θ). On the basis of these results, eq 5 can be substituted by
dθ ) kd(1 - S - θ) - kiθ - krθ(1 - θ) dt
(18)
kr now being the actual rate constant for the reorientation process. The differential equation (18) can be readily solved numerically using the fourth-order Runge-Kutta method in cojunction with eqs 6, 9, and 11. In the particular case where eq 10 (S + θ ≈ 1) is obeyed by a full monolayer over the whole time interval, then eq 18 has an analytical solution. Under these conditions, we obtain
i≈
qki exp(-(ki + kr)t) kr 1[1 - exp(-(ki + kr)t)]2 (ki + kr)
(19)
In practice, eq 19 is not valid for t f 0 as the condition S(t) + θ(t) ≈ 1 is not correct. However, the experimental
Figure 5. Simulations (solid lines) of selected experimental results shown in Figure 2 for bromide as counterion using the model based on eq 18. Table 1. Results Obtained in the Simulation of Experimental Curves Shown in Figure 3 Using the Proposed Model Based on Eq 18 (kd Was Fixed at 3000 s-1) Ef (mV)
ki (s-1)
kr (s-1)
β × 10-5/s-2
-430 -431 -432 -433 -434 -435 -436
25.5 30.5 42 48.5 68 78 94
22.5 31.5 40 49.5 64 82 99
0.9 1.6 2.4 3.5 4.5 5.8 8.0
curves closely fit eq 19 at t values above the current maximum time. Such fits can be used to obtain rough estimates of ki and kr. Application of the Model to a Bromide Medium. Figure 5 shows the simulation of selected curves in Figure 2 obtained in the presence of bromide ion by using the proposed model, described mathematically by eq 18. As can be seen, the simulated (solid lines) and experimental curves (dots) are quite consistent. Table 1 compiles the parameter values obtained in the simulations. All fulfill the condition kd . ki, so initially the electrode is covered virtually throughout its surface (i.e., θ0 ≈ 1). Good results can be obtained provided kd . ki, irrespective of the absolute value of kd. Thus, the method used involves fixing a constant value of kd for all transients in each set of experiments. Likewise, the parameter β depends on the kd value (the higher kd is, the higher will be β). Therefore, the absolute value of β is not significant either in these simulations. The initial values of ki and kr employed in the simulation were obtained by applying eq 19 to the last interval (long times) of the experimental transients. The qm values thus obtained were 23 ( 0.5 µC cm-2 and thus very similar to those found by integrating the voltammetric peak. Of special interest in regard to the contribution of the reorientation term is λ, which is defined as
λ)
kr ki + kr
(20)
As can be seen in Table 1, the reorientation rate constant is similar to the desorption rate constant, so λ ≈ 0.5 (i.e, the reorientation term contributes significantly to the formation of the condensed phase). The variation of constant kr with the final potential of the potentiostatic pulse was also examined. Figure 6 shows
Heptyl Viologen Cation Radical on Hg
Langmuir, Vol. 19, No. 6, 2003 2343 Table 2. Results Obtained in the Simulation of Experimental Curves Shown in Figure 7 Using the Proposed Model Based on Eq 18 (kd Was Fixed at 2500 s-1)
Figure 6. Plots of kr vs final potential of the potentiostatic jump obtained from simulated curves for bromide and sulfate as counterions.
Figure 7. Simulations (solid lines) of selected experimental results shown in Figure 2 for sulfate as counterion using the model based on eq 18.
a plot of kr vs Ef, the extrapolation of which to kr ) 0 provides the standard potential of the process, E°. As can be seen from the figure, E° ≈ -425.5 mV. Application of the Model to a Sulfate Medium. Figure 7 shows selected simulated curves from Figure 2 obtained in the presence of sulfate ion (only the nucleation
Ef (mV)
ki (s-1)
kr (s-1)
λ
β × 10-5 (s-2)
-410 -411 -412 -413 -415
95 118 158 140 179
20 25 55 63 76
0.17 0.175 0.26 0.31 0.30
1.3 2.3 4.4 4.25 4.25
contribution). The values obtained from eq 18 (solid lines) are also highly consistent with the experimental ones (dotted lines). Table 2 shows the parameter values obtained. The simulations were made in the same manner for bromide ion. The qm values thus obtained were 20.5 ( 0.5 µC cm-2 and thus very similar to that obtained by integrating the corresponding voltammetric peak and to the values for the bromide medium. Figure 6 shows a plot of kr vs Ef. For the sulfate anion, the standard potential obtained by extrapolation is E° ≈ -409 mV. Taking into account that the data in Tables 1 and 2 correspond to similar overpotential (Ef-E°) jumps, and based on the previous E° values, the λ values obtained for sulfate ion (see Table 2) are smaller than those for bromide ion. However, these smaller values of λ arise from differences in ki values since kr is similar in both media. The fact ki is greater for sulfate ion is consistent with the fact that this ion is less strongly adsorbed on the electrode surface than is bromide; this may reflect a lower energy of desorption and hence an increased contribution of desorbing and nucleating molecules. (As can be seen from Figure 4, simulated curves based on desorption-nucleation alone, eq 5, exhibit less marked deviations at intermediates times with sulfate than with bromide as the counterion.) Conclusions The chronoamperometric behavior of the condensed phases, formed by salts of heptyl viologen cation radical on mercury electrodes in aqueous media, can be explained in light of a mathematical model that considers a desorption-nucleation mechanism. The model assumes that randomly adsorbed molecules can only contribute to the reduction current if they experiment a prior, slow reorientation (a reorientation-nucleation mechanism). The results are quite consistent with the expected significance of the adsorption-desorption term regarded to anion-electrode interaction. Thus, the proportion (>λ) of condensed phase formed through reorientation of previously adsorbed molecules will increase with increase in the strength with which the anion is adsorbed (
