Langmuir 1986,2, 630-638
630
parts of the chains. With increase in volume fraction and consequently less free space in the system it appears either that the PHS chains compact as a consequence of a change in solvency of the PHS layer or that the greater frequency of collisions between the particles causes the solvent to be squeezed out from between the chains so that the surface layer becomes much more compact. This suggestion of changing interaction with volume fraction could not be explained with the system used either by desorption of the stabilizing molecules or by extensive lateral mobility since
the PHS chains were chemically bonded to the surface of the core PMMA particle. Acknowledgment. We express our thanks to SERC and IC1 PLC for support of this work. We also gratefully acknowledge the use of neutron beam facilities at the Institut Laue-Langevin, Grenoble. Registry No. PMMA, 9011-14-7;12-hydroxystearicacid, copolymer with glycidyl methacrylate and methyl methacrylate, 103817-71-6.
Quantitative Investigations of Adsorption of tert -Amyl Alcohol at the Gold(110)-Aqueous Solution Interface Jocelyn Richer, Lorne Stolberg, and Jacek Lipkowski* Guelph- Waterloo Centre for Graduate Work i n Chemistry, Guelph Campus, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N l G 2W1, Canada Received March 10, 1986. I n Final Form: July 7, 1986 Chronocoulometry was used in the quantitative investigation of the physical adsorption of tert-amyl alcohol on the (110) face of a gold single crystal. The amount of surfactant adsorbed was determined from the charge density at the electrode surface. A computerized system was used to obtain the charge density and perform the data treatment. The film pressure, Gibbs surface excess, and free energy of adsorption were determined as functions of the electrode potential, the charge density, and the potential drop across the inner layer. The congruency of the adsorption data with respect to charge and potential is discussed. Introduction
Modern spectroscopic techniques such as surface-enhanced Raman scattering (SERS), electromodulated infrared spectroscopy (ELMIRS), and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) offer new possibilities to probe molecular properties of organic surfactants adsorbed at the solidsolution interface.' In principle, information about the orientation of the adsorbed molecules, absence or presence of a partial charge transfer, interactions between adsorbates, solvent, and surface, etc. could be obtained by these techniques. However, for the proper interpretation of the spectroscopic experiments as well as for the better understanding of electrocatalytic processes, macroscopic data such as the surface concentration, free energy of adsorption, and electrosorption valency are needed. Unfortunately, in contrast with the fast development of the microscopic techniques, little progress has been made in the acquisition of macroscopic data on surfactant adsorption on solid electrodes. However, quantitative information about the adsorption of organic molecules has already been obtained on platinum by Hubbard et al.2-5 Recently, we demonstrated that adsorption of organic surfactants a t the metal-solution interface can be determined quantitatively from the charge density at the electrode surface and that the charge density corresponding to the adsorption equilibrium can be determined by chronocoulometry.6--8 (1)Pons, S.J.Electrounul. Chem. 1983,150, 495. (2)Soriaga, M.P.; Hubbard, A. T. J. Am. Chem. Soc. 1982,104,2735. (3)Soriaga, M.P.; Stickney, J. L.; Hubbard, A. T. J.Mol. Cutal. 1983, 21, 211. (4)Soriaga, M.P.;White, J. H.; Hubbard, A. T. J . Phys. Chem. 1983, 87,3048. (5)Soriaga, M.P.; Hubbard, A. T. J.Phys. Chem. 1984,88,1089,1758.
In the present paper, chronocoulometry is used to investigate the adsorption of tert-amyl alcohol (t-AA)on the gold(ll0) single-crystal plane. The objective of this work is to show how the Gibbs surface excess, the free energy of adsorption, and the potential drop across the inner layer can be obtained for adsorption of an organic surfactant on a solid electrode. The surfactant chosen was t - A A because (1)it does not oxidize on the gold electrode and (2) its adsorption on mercury is well described, allowing for comparison with the data obtained on gold. The Au(ll0) surface is energetically homogeneous, it is ideally polarizable over a large potential range, and has known electrocatalytic p r o p e r t i e ~ . ~ J ~ The technique of data acquisition and analysis developed on this model system could be used later to obtain results needed for the interpretation of the spectroscopic experiments or to explain the mechanisms of electrocatalyzed reactions. Experimental Section (i) Solutions. Solutions were prepared with Milli-Q water (Waters) with a resistivity higher than 16 MQ cm. The KCIOl (ACS Certified from Fisher) was calcinated at 300 "C, twice recrystallized, and dried before use. tert-Amyl alcohol (BDH Chemicals Ltd. Gold label 99+%) was used without further purification. The supporting electrolyte consisted of a 0.05 M KCIOl solution. The t-AA solutions were prepared by spiking a known volume of the supporting electrolyte solution with a small amount of a concentrated solution of alcohol. The experiments were (6) Richer, J.; Lipkowski, J. J. Electrochem. SOC.1986,133,121. (7)Stolberg, L.; Richer, J.; Lipkowski, J.; Irish, D. E. J.Electrounul. Chem. 1986,207,213. (8) Lipkowski, J.; Van Huong, C. N.; Hinnen, C.; Dalbera, J. P.; Parsons, R. J . Electround. Chem. 1983,143,375. (9)Schwank, J. Gold Bull. 1983,16, 103. (10) Wachs, I. E. Gold Bull. 1983,16,98.
0743-7463/86/2402-0630$01.50/0 0 1986 American Chemical Society
Langmuir, Vol. 2, No. 5, 1986 631
Investigations of tert- Amyl Alcohol Adsorption conducted in the concentration range extending from 0 to 0.56 M of t-AA. Solutions were degassed with argon for about 20 min before each experiment, and the solution was protected against ambient atmosphere by allowing argon to flow above the solution during the experiment. The temperature was 25 1 "C. (ii) Electrodes. The working electrodeconsisted of a 4 N gold (Engelhard)rod prepared according to the Bridgeman technique." The single crystal was then oriented by using the back Laue diffraction technique. The electrode was polished on felt and nylon cloths with successively smaller grades of alumina suspensions (5-0.05 pm) up to a mirror finish. The electrode was then annealed 12 h at 700 "C in a muffle furnace. The counter electrode consisted of a gold coil. An external saturated calomel electrode (SCE) was used as the reference electrode. (iii) Instrumentation. The experimental setup consisted of a homemade potential ramp generator, a PAR Model 173 potentiostat, and a PAR Model 5204 two-phase lock-in amplifier equipped with a built-in oscillator. Potential step experiments were performed with the help of a computerized data acquisition system which consisted of an Apple I1 microcomputer equipped with two Computerscope boards (RC Electronics, CA) interfaced to the potentiostat. Each board has its own buffer and can acquire 2048 point at an AID conversion rate of 500 kHz and with a resolution of 14 bits. (iv) Surface Cleaning. Before each experiment, the working electrode was cleaned by the flaming technique.12 The hanging electrolyte method13was used to make contact between the surface and the solution. This method ensured that only the crystallographic plane of interest was in contact with the investigated solution and that the surface area of the electrode in contact with the solution could be reproduced easily. (v) Current Transient Acquisition. The first step involved in the quantitative determination of the relative charge density data involved the acquisition of chronoamperometric curves. These could subsequently be integrated to yield the charge transients. The current-to-voltage follower was interfaced to the two Computerscope channels of the microcomputer. These were configurated-through software-to acquire simultaneously current transients in a short (10 ms) and longer (100 to 200 ms) time windows. As soon as the transients were acquired,they were stored on floppy disks for further data processing. The time sequence of the different steps involved in the data acquisition and processing has been described previously! In this work, the desorption of t-AA was investigated. Hence, the experiments were performed as follows: the potential was held at Ea (at which adsorption of t-AA takes place) for 60 s to allow adsorption to reach equilibrium and was then forced to step from Ea to Eo (at which t-AA is totally desorbed) while the chronoamperometric curves were recorded. The same experiment was repeated sequentially for values of Earanging from -0.75 to 0.60 V in increments of 50 mV, Eo was always equal to -0.80 V. The chronoamperometric curves were then digitally integrated to obtain the charge transients. The linear part of the chronocoulometric curves (corresponding to the steady-state current) was extrapolatedby linear regression to t = 0. This procedure ensured that the extrapolated charge was equal to the charge difference between the electrode charge densities at Eo and at a given value of Ea:
*
A~M(EB) =
- EO)
(1)
Data Treatment and Experimental Strategy The data treatment on which the technique relies was already presented in a previous paper.6 However, for the sake of clarity, it is given again here. The experimental strategy involved the following steps: (i) Measurement of the relative charge density AUM between Eo and E6. (11) Faure, R. Thesis, Institut National Polytechnique de Grenoble. (12) Clavilier, J. J.EZectroanaL Chem. 1980, 107, 211. (13) Dickertmann, D.; Shultze, J. W.; Koppitz, F. D. Electrochim. Acta 1976, 21, 967.
(ii) Conversion of the relative charge density to the absolute charge density UM From independent measurement of the potential of zero charge and from eq 1,~ ~ ( - 0 . 8V)0 was calculated: AUM(PZC)= UM(PZC)- UM(-0.80 V) = -~,(-0.80
V) (2) From ~ ~ ( - 0 . 8V) 0 and eq 1,values of uM(E0) were determined over the whole potential range. (iii) Calculation of the film pressure. Integration of uM with respect to E gives
The lower integration constant, ~(-0.80V), is not known but its value is independent of the concentration of t-AA (since there is no adsorption at -0.80 V). Hence, a is given as follows:
(4)
(iv) Calculation of the relative Gibbs excess by differentiation of a with respect to In c. From the electrocapillary equation and eq 4, I' can be calculated:
(5) The maximum bulk concentration of t-AA corresponded to a mole fraction if 0.004. This is sufficiently small for the solvent mixture to be assumed to follow Henry's law.14J5 Hence, concentrations instead of activities could be used to determine I?. (v) Determination of the free energy of adsorption from a fit of the I' vs. In c data to a Frumkin isotherm:16
In this case, In (c(1- 8)/8) is plotted against 8, the slope of the curve is equal to A, and the intercept allows the calculation of the free energy. The AGA corresponds to unit mole fraction of the solute in the solution and unit coverage in the monolayer as the standard state. The above treatment applies to a constant-potential analysis. The analysis can also be carried out under conditions of constant charge as outlined by parson^.^' First, the function 6 = y + EuM has to be evaluated for every vdue of uM Next, the surface pressure CP = &=o - 5 6 must be calculated. Finally, the relative Gibbs excess of t-AA at a constant value of charge density could be determined by differentiation of @ with respect to In c:
.=-(-E--) 1 RT a In c
(7)
The surface concentration data can then be fitted to an adsorption isotherm in order to determine the free energy of adsorption as a function of charge.
Results and Discussion (i) Cyclic Voltammetry. Cyclic voltammograms (CV) were recorded in order to test the purity of the solutions and the surface. Assessment of the purity relied on how well the results agreed with those found in the literature. (14) DeBattiGi, A.; Trasatti, S. J. ElectroanaL Chem. 1974, 54, 1. (15) Mohilner, D.; Nakadorami, N. J . Electroanal. Chem. 1975, 65, 843. (16) Damaskin, B. B.; Petrii, 0. A.; Batrakov, V. V. Adsorption of Organic Compounds on Electrodes; Plenum: New York, 1971. (17) Parsons, R. Trans. Faraday SOC.1955, 51, 1518.
632 Langmuir, Vol. 2, No. 5, 1986
Richer et al.
Voltammograms were also very helpful in detecting traces of oxygen and creeping of the solution on the walls of the electrode. Figure l a shows a CV recorded in a solution of the supporting electrolyte. This curve is similar to those reported in the literature.'8 The double-layer region extends from -0.80 to 0.50 V. The presence of a small current at E = 0.25 V can be correlated to the large increase in the equilibrium values of differential capacity at this potential (see Figure 6). It is believed that incipient oxidation of gold is responsible for this phen0men0n.l~ CVs were also recorded in the presence of t-AA and the curve for a 0.10 M solution of t-AA is shown in Figure lb. The double-layer region spans over the same potential domain whether t-AA is present or not in solution. Only a very small increase in the anodic current density can be observed at the positive end of the potential scale as the concentration of t-AA increases indicating that tert-amyl alcohol does not oxidize significantly. At any rate, Figure 1indicates that up to the potential of 0.6 V no oxidation of t-AA takes place. Figure ICshows a CV recorded in a 0.56 M solution of t-AA. The perturbations visible in the double-layer region can be associated with two phenomena: (1)creeping of the solution along the walls of the electrode; ( 2 ) condensation of vapors visible on the emerging part of the electrode. These phenomena seriously affected the precision of the adsorption measurements in solutions of concentration above 0.20 M. Therefore, the quantitative data treatment was restricted to the concentration range between 0 and 0.20 M of t-AA. It will be possible to study higher concentrations once these technical problems have been solved. (ii) Differential Capacity. The differential capacity curves (C-E) were determined at 25 Hz of ac modulation by using a two-phase lock in amplifier and assuming a simple series RC equivalent circuit. They are presented in Figure 2 for the supporting electrolyte and four representative concentrations of t-AA in the range investigated. The curve for the supporting electrolyte shows a diffuse layer minimum a t -0.05 V which corresponds to the potential of zero charge; this value is in agreement with the pzc reported in the literature.20 The curves show a typical decrease of the capacity near the pzc with increasing Concentration of the alcohol; the capacity decreases from 27 pF cm-2 at 0 M to 17 pF cm-' at 0.10 M t-AA. The C-E curves for every concentration of t-AA merge with the curve for the supporting electrolyte at Eo = -0.80 V which indicates that no adsorption of t-AA occurs a t this potential. Unlike most cases involving the adsorption of neutral organics on soft or liquid metals, the differential capacity recorded in the present case does not display the characteristic adsorption/desorption peaks.'I Instead, the capacity decreases with increasing bulk concentration of alcohol. This behavior was also observed by Holze and BeMowska-Brzezinska for a number of primary and secondary alcohols adsorbed on polycrystalline gold.22 Actually, the adsorption/desorption maxima disappear from the capacity curves if the rate of surfactant adsorption/
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(18) Hamelin, A. C. R. Acad. Sci. Paris, Serie C 1976,282, 1013. (19) Nguyen Van Huong, G.; Hinnen, C.; Lecoeur, J. J. EZectroanaZ. Chem. 1980, 106,186.
(20) Lecoeur, J. These de Doctorat d'Etat, Universite Pierre et Marie Curie, Paris. (21) Mohilner, D. In Electroanalytical Chemistry; Bard, A. J., Ed.; Dekker: New York, 1966; Vol. 1. (22) Holze, R.; Beltowska-Brzezinska, J. Electround. Chem. 1986, 201, 387. Beltowska-Brzezinska, M.;Dutkiewicz, E.; Skoluda, P. J. EZectroanaZ. Chem. 1984, 181, 235.
Figure 1. Cyclic voltammograms recorded in a 0.05 M KC10, solution containing (a) no surfactant and (b) 0.10 M and ( c ) 0.56 M tert-amyl alcohol.
Langmuir, Vol. 2, No. 5, 1986 633
Investigations of tert-Amyl Alcohol Adsorption I
I
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1
-08
-0A
I 0
A
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Figure 2. Differential capacity as function of electrode potential determined by impedance measurements in a 0.05 M KC104 solution containing different concentrations of tert-amyl alcohol. Capacity calculated assuming a single series RC equivalent circuit.
El
0
t/mr
9
--
9 Figure 3. Three-dimensional plots of the current transients 0
t/ma
obtained over the whole potential range investigated for a 0.05 M KC104 solution containing (a) no surfactant and (b) 0.10 M tert-amyl alcohol. desorption is slow enough so that the surface concentration of surfactant is unable to follow the modulation of the
Figure 4. Three-dimensional plots of the charge transients obtained over the whole potential range investigated for a 0.05 M KC104 solution containing (a) no surfactant and (b) 0.10 M tert-amyl alcohol.
electrode potential introduced by the ac signal. In the present experiments, the frequency of the ac modulation was relatively low (25 Hz), hence the absence of the maxima may indicate that adsorption/desorption of t-AA is indeed slow. (iii) Current Transients. The current transients for solutions of the supporting electrolyte and t-AA were first recorded in a 10 ms time window. The three-dimensional i-t-E plot for the solution of the supporting electrolyte is presented in Figure 3a. It is constructed in such a way that the time and current axes are in the plane of the figure while the potential axis is normal to this plane. The current decreases monotonically with time at potentials more negative than -0.20 V, but above that value, a hump appears in the transients. This potential coincides with an increase in the capacity observed in Figure 2. This hump can be associated with the dependence of the time constant (RC)of the cell on time as explained in ref 6. Figure 3b presents the three-dimensional i-t-E plot for a 0.10 M solution of t-AA. The decrease in the amplitude of the adsorption/desorption peak with increasing concentration of t-AA observed in Figure 2 can be directly related to the disappearance of the hump on the i-t-E plot. These observations suggest that the processes are now limited by slow kinetics of adsorption and desorption. (iv) Charge Transients. The current transients were also recorded in a 100-ms time window for the solution of the supporting electrolyte and integrated to yield the charge transients. The corresponding three-dimensional Aa-t-E plot is presented in Figure 4a. The plot is constructed in the same way as those shown in Figure 3; i.e., the charge and time axes are in the plane of the figure and the potential axis is normal to this plane. The transients for potential are relatively horizontal after the initial rise due to the charging of the double layer. This indicates that
634
Langmuir, Vol. 2, No. 5 . 1986 I
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* Richer et al.
I
r
70
// - a8
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I
-a4
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I
L
0
Figure 5. Charge density as a function of the electrode potential determined by chronocoulometry for 0.05 M KC104 solution containing different concentrations of tcrt-amyl alcohol.
no charge-transfer reaction takes place at the surface. Hence, the transients arise only from the charging of the double layer. Notice the large increase in charge a t potentials around 0.20 V which coincides with an increase in capacity in Figure 2 and current density in Figure la. These phenomena are probably caused by the incipient oxidation of the gold surface.Ig The charge transients were obtained in a 200-ms time window for solutions containing t-AA. The three-dimensional ila-t-E plot for a 0.10 M solution of t-AA is presented in Figure 4b. The time window had to be increased in the presence of t-AA because a longer time is required for the charge to reach a time-independent value. This again suggests that the process of charging the double layer is now controlled by a slow adsorption/desorption step whose relaxation time is longer than RC (with R being the solution resistance and C the differential capacity of the interface). A t potentials more positive than 0.15 V, the charge has not reached equilibrium even after 200 ms and therefore cannot be extrapolated to t = 0 to yield UM. Hence, only results obtained at potentials more negative than 0.15 V were considered in the further data treatment. (v) Charge Density. Thz charge density was plotted as a function of the logarithm of the bulk concentration of t-AA at constant potential in order to estimate the precision of the uM measurements. A smooth curve was drawn through the points at a given potential. The precision of the measurements was estimated by calculating the standard deviation of aMa t constant t-AA concentration by using all electrode potentials up to 0.15 V. It was found that the standard deviation was smaller than 2 % at t-AA concentrations lower than 0.10 M and was between 2% and 5% at concentrations between 0.10 and 0.20 M. The decrease in precision at concentrations higher than 0.10 M is due probably to creeping and condensation along the walls of the electrode as demonstrated by the poor quality of the CV presented in Figure IC. The quantitative analysis was therefore carried out for t-AA concentrations up to 0.10 M; the points corresponding to higher concentrations presented in certain figures were not
-0.8
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Figure 6. Differential capacity as a function of electrode potential obtained by differentiation of the a w E curves of Figure 5.
considered when conclusions were drawn. The smoothed values of the charge density were then plotted with respect to E for potentials up to 0.15 V and the curves for a few representative concentrations of t-AA are presented in Figure 5. As is expected in the case of adsorption of neutral organics, the charge density at constant potential increases monotonically with concentration of surfactant below the potential of maximum adsorption. The uwE curves were differentiated and the resulting capacity curves are shown in Figure 6. Since the equilibrium charge density data were used to calculate the capacity, these values correspond to the equilibrium or the “zero-frequency” differential capacity. These curves display the typical shape observed for the organic surfactants adsorbed on mercury;21 they display the characteristic adsorption/desorption peaks which were absent on the curves determined by impedance measurements. As expected, the capacity at the cathodic desorption peak increases with concentration of t-AA and the potential at the peak maximum shifts toward more negative values as the amount of surfactant adsorbed increases. (vi) Film Pressure. The r-E curves for concentrations of t-AA up to 0.20 M are presented in Figure 7. They were obtained by integration of the charge density data as a function of potential. They display a characteristic bell shape as expected in the case of adsorption of neutral organics on mercury from aqueous solutions.21 The adsorption could not be investigated up to the potential of maximum adsorption which lies beyond 0.15 V, hence the film pressure at E,, could not be determined. However, the trend of the curves indicates that the maximum in K should occur around 45 to 50 mN m-l for the 0.20 M solution of t-AA. These values are comparable with r found for t-AA adsorbed on mercury for the corresponding bulk concentrations of t-AA.23 (23) Lorenz, W.; Mockel,
F.2.Elektrochem. 1956,60, 507.
Langmuir, Vol. 2, No. 5, 1986 635
Investigations of tert-Amyl Alcohol Adsorption
r-7-I
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Il/mNm'l
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i
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Figure 7. Film pressure as a function of electrode potential for different concentrations of tert-amyl alcohol.
The film pressure was also plotted as a function of the logarithm of the concentration of t-AA a t constant potential for E up to 0.15 V. The r-ln c curves were then shifted along the In c axis in order to superimpose them onto the curve corresponding to E = 0.15 V. This procedure produced the so-called composite curve24 and is presented in Figure 8a. One can see that there is very little scatter of the points around the curve. The film pressure was also calculated as a function of the charge density and was plotted as a function of the logarithm of the t-AA concentration at constant charge for uM between -25 and 11p C cm-2. The composite curve at constant UM shown in Figure 8b was obtained following the same procedure as in the constant-potential analysis. The points are much more scattered around the mean curve in the analysis a t constant charge than in the analysis a t constant potential. The extent of scatter of the points around the composite curve gives an indication of the level of congruency of the adsorption isotherm with the given electrical ~ a r i a b l e . ~Hence, ~ , ~ ~ the film pressure results suggest that potential rather than the charge seems to be the appropriate variable in the investigation of adsorption of t-AA on gold. (vii) Relative Gibbs Surface Excess. The relative Gibbs surface excess, which is approximately equal to the surface concentration, was calculated from the differentiation of the ?r or vs. In c curves a t constant E and UM. (The relative Gibbs excess for t-AA with respect to water, r A , W , is equal to F A , & ? = - r w ( x A / x w ) , where and rware the surface concentration of t-AA and water, respectively, and X A and Xw their mole fractions in the bulk. Under the present experimental conditions X A I 0.004, hence r A , W N r A . To simplify the notation the subscripts a t r are omitted in the text.) The electrode potential E depends on the magnitude of the potential drop across the double layer and on a constant which depends on the nature of the reference electrode. However, adsorption of an organic surfactant is influenced only by the potential (24) Trasatti, S. J.Electroanal. Chem. 1974, 53, 335. (25) Goledzinowski, M.; Dojlido, J.; Lipkowski, J. J. Electroanal. Chem. 1985,185, 131. (26) Payne, R. J.Electroanal. Chem. 1973, 41, 277.
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(27) DeBattisti, A.; Trasatti, S. J.Electroanal. Chem. 1973, 48, 213. (28) Delahay, P. Double Layer and Electrode Kinetics; Wiley-Interscience: New York, 1965.
636 Langmuir, Vol. 2, No. 5, 1986
Richer et al. 1
I
5.4
in c
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Figure 9. Three-dimensionalplots representing the isotherms for adsorption of tert-amyl alcohol calculated with respect to (a) charge density and (b) potential drop across the inner layer. The electrical variable varies along the axis "normal" to the plane of the figure. ther @M-2 or uM are plotted in the three-dimensional plots of Figure 9. The vertical and horizontal axes correspond to the surface concentration and the logarithm of the concentration of t-AA, respectively; the axis going into the plane of the figure represents the electrical variable. In the case of the analysis a t constant UM (Figure 9a), the isotherms show a plateau at higher concentrations even a t charges far from maximum adsorption. This plateau corresponds to the limiting surface concentration, r-. It is clear that at negatively charged surfaces, rmax depends strongly on uM. No physical model can explain this variation because t-AA is a nearly spherical, rigid molecule and is unlikely to change its configuration to account for the dependence of rmax on charge. Hence, as was pointed out by Damaskin and Dyatkina,29the dependence of on uM may be an artifact due to the noncongruency of adsorption with respect to that electrical variable. On the other hand, the isotherms calculated at constant @-' (Figure 9b) show a smooth increase of r with bulk concentration of t-AA and the surface concentration reaches a maximum value of 6.5 X 10-lomol cm-2 for 4M-2more positive than -0.05 V. In conclusion, the dependence of rmax on charge suggests that adsorption is not congruent with respect to charge and that 4-'2 might be the better electrical variable. (viii) Free Energy of Adsorption. AGA was obtained by fitting the r data to the Frumkin isotherm given in eq 6. In this case, In (c(1 - @)/e)was plotted against 6 for different values of uMor @M-2 (where 6 x I'/rmax and rmar = 6.5 X mol cm-I). The curves obtained were relatively linear. The slope gives the value of A and the intercept allows the calculation of AGA. The free energy of adsorption a t constant UM was also calculated from the composite curve presented in Figure 8b. First, the r data determined by the differentiation of the composite curve were fitted to the Frumkin isotherm to determine AGA at uM = 10 pC cm-2. Then, the AGA data at more negative charges were obtained by evaluating the shift along the In c axis required to superimpose the curve for a given uM (29) Damaskin, B. B.; Dyatkina, S. L. Elektrokhiniya 1978, 14,152.
11
-A
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-15
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Figure 10. Free energy of adsorption of tert-amyl alcohol as a function of (a) charge density and (b) potential drop across the inner layer. Curves represented by filled circles were obtained from the shift of the @-uM curves along the x-axis and those represented by open circles were obtained from a fit of the data t o the Frumkin isotherm. The interaction parameter A is also plotted as a function of the electrical variable and is represented by x . onto the composite curve.** The precision in the determination of AGA attainable a t present is of the order of 10%; this figure should decrease substantially once the
Langmuir, Vol. 2, No. 5, 1986 637
Investigations of tert-Amyl Alcohol Adsorption technique has been improved. The plots of AGA against the potential drop across the inner layer and against the charge density are shown in Figure 10. The plot of AGA vs. uM (Figure loa) contains data obtained by the two methods mentioned above. However, the differences between the two curves are within the domain of error and are therefore not significant. The plots show a nearly parabolic dependence of the free energy of adsorption on the electrical variable a t UM < 0 pC cm-2 and @-*< -0.1V. However, the plots are asymmetric with respect to UM = 0 pC cm-2 or 4M-2= -0.1 V. A t more positive charges or potentials, the free energy still decreases slowly. This type of asymmetry in the dependence of AGA on the electrical variable indicates the presence of specific interactions between adsorbed molecules and the metal ~urface.~'A possibility for such interactions is provided by the presence of nonbonding molecular orbitals on the hydroxyl group of t-AA molecule. The interactions may have donor-acceptor character involving the nonbonding orbital and empty electronic states in the metal. This in turn suggests that the t-AA molecules may be oriented at the interface with their hydroxyl group facing the electrode surface. The dependence of the parameter A on the electrical variable is also presented in Figure 10. The lateral interaction constant A can be expressed in terms of different particle-particle interactions a t the interface:16 where wij are the particleparticle interaction energies and W stands for water, A for surfactant. The negative values of A mean that 2wWA> wM + wwW;i.e., the surfactantsurfactant and water-water interactions are much more attractive than the surfactant-water interactions. (ix) Potential Drop across the Inner Layer. Further information about the structure of the interface in the presence of adsorbed t-AA molecules may be obtained from an analysis of dependence of uMon r at constant 4M-zand the dependence of 4M-2on r a t constant uM. In the presence of neutral tensioactive species, the structure of the inner part of the double layer is frequently represented by models of two capacitors either in parallel or in series.26 If adsorption conforms to the model of two parallel capacitors, a t any given 4M-2,the following relation is satisfied: UM
= uM,@=o(l - e)
+ 'JM,s=1(8)
(9)
Conversely, if adsorption is well described by a model of series capacitors, then the potential across the inner layer a t a constant charge density is a linear function of the surface concentration: 4 M - 2 = 4s=oM-2(i - e) + 4s=1M-2(e) (10) The uMvs. r plots constructed a t different 4M-2 values are presented in Figure l l a . The 4M-zvs. r plots a t constant charge density are given in Figure l l b . Unfortunately, the curves for 4M-2vs. F a t u < -5 pC cm-2 and uM vs. I' a t 4M-2< -0.10 V covered a range of surface concentrations too small to allow one to draw unambiguous conclusions about their linearity. Only a t uM and 4'-2 more positive than the above limits could the curves be calculated in the whole range of r investigated. Apparently, the uMvs. r plots are not linear which may indicate that the adsorption does not conform to the parallel capacitor model. However, the deviations from linearity are small and within the domain of uncertainty. In contrast, the c # J ~ - against ~ l? diagrams are strictly linear. It means that in the vicinity of the pzc and a t positively charged surfaces, the adsorption of t-AA definitely conforms to the
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I
I
I
I
'
-15
0
@"YV -0.1 -0.2
r/xio-%ot I I 4
5
c"2
I 6
Figure 11. (a) Charge density as a function of surface concentration for different values of q5M-z and (b) potential drop across the inner layer as a function of the surface concentration for different values of uM.
series capacitor model. This in turn implies that the adsorption should be congruent with respect to charge.2e However, we observed earlier that a t negatively charged surfaces the results obtained were much more consistent when potential was taken as the electrical variable. Hence, neither one of the electrical variables can be used to describe adsorption of t-AA on gold over the whole range of potentials or charges investigated. This result confirms once again that thermodynamic analysis of the data should always be performed for the two variables independentlyem The shift of the pzc from zero to full coverage of the surface by t - A A can be determined from the bMU2 vs. r curve corresponding to u = 0. EN is equal to -0.1 V and, according to a simple electrostatic m0de1,~~g~l can be expressed as 4?rp
E ~ --rmax = c
where
6=p
-
L ~ r ~npiw
(12)
The negative value of ENindicates that the pzc is shifted toward negative potentials as t - A A molecules adsorb. Incidentally, EN for t - A A adsorption on mercury is posit i ~ e .According ~~ to eq 12, EN could be negative if either (a) Inplwl> lployl and piw > 0 or (b) lplo71 > (ncllWland plorg < 0. The first possibility may be rejected because a positive value of piw would indicate that water molecules are oriented with the hydrogen atoms facing the metal side ~~
~
(30) MohilneiD.; Karolczak, M. J . Phys. Chem. 1982.86, 2838. (31) Damaskin, B.;Frumkin, A.; Chizhov, A. J. Electroanal. Chem. 1970,28, 93. (32) Damaskin, B.B.;Grigoriev, N. B. Dokl. Akad. Nauk SSSR 1962, 147, 135.
638 Langmuir, Vol. 2, No. 5, 1986
Richer et al.
Table I. Adsorption Parameters for t-AA Adsorbed on A u ( l l 0 ) and Ha
r,,/xio-lo
mol cm-2 AGAo/kJ mol-'
Au(ll0)
Hk?
6.5 -18.5
4.1 -18.0 0.3
-0.1 -2
E N P
A
-3
a From ref 32. Standard state unit mole fraction of the solute in the solution and unit coverage in the monolayer.
of the interface. On the basis of independent arguments: it is known that the Au(ll0) plane is quite hydrophilic and the gold-water interactions orient water molecules preferentially with the oxygen atom facing the metal side of the interface. The second possibility (b) indicates that the effective dipole moment of the organic molecule must be negative. This shows that the t-AA molecule is oriented with its polar head facing the metal and that the hydrocarbon tail faces the solution side of the interface. The same conclusion was reached earlier from the asymmetry of the free energy of adsorption vs. electrical variable plots.
Summary Data have been presented for the adsorption of t-AA on the Au( 110) surface. The quality of the data has allowed a detailed analysis of the adsorption process to be carried out. To summarize, it is convenient to compare adsorption of this compound on the Au(ll0) electrode with adsorption on the mercury electrode. The adsorption parameters for t-AA are compared in Table I. The maximum surface concentration of t-AA is higher at Au(ll0) than at the Hg surface. However, the difference in rmax may be explained by the roughness of the solid electrode surface. The surface concentration of t-AA on the Au(ll0) electrode was calculated with respect to the geometric area; the actual surface area of the solid electrode may be easily higher by a factor of 20% to 50%.33,34 The lateral interaction constant is negative and comparable for the two surfaces. It shows that the adsorbed t-AA molecules interact attractively at the two interfaces.16 ENhas opposite sign for the two metals, suggesting that the t-AA molecules assume opposite orientations at the gold and at the mercury surfaces. The positive value of E N shows that t-AA is oriented a t the Hg electrode with the polar head facing the solution and the hydrocarbon tail directed toward the metal. In contrast, a negative value of EN as well as the asymmetry of the free energy of adsorption vs. electrical variable plots indicate that at the Au surface, t-AA molecules are oriented with the functional group toward the metal. Clearly, the orientation of the adsorbate at the metal-solution interface is a result of competition between interactions of the functional group (33) Hamelin, A.; Vitanov, T.;Sevastyanov, E.;Popov, A. J . Electroanal. Chem. 1983,145, 225. (34) Clavilier, J.; Van Huong, C. N. J.Electround. C h e n . 1977, 80, 101.
with the metal and with the aqueous phase. However, in view of the fact that the free energy of adsorption on Au(ll0) and on Hg have comparable magnitudes, we can conclude that the differences between the metal-adsorbate and the adsorbate-solvent interactions are not large.
Acknowledgment. The financial support of the Natural Science and Engineering Research Council of Canada is gratefully acknowledged. Definitions tert-amyl alcohol time absolute temperature gas constant pressure bulk concentration of surfactant differential capacity current density surface coverage (r/rmax) electrode potential potential at which the surfactant is totally desorbed potential at which adsorption takes place potential drop across the inner layer potential drop across the diffuse layer relative charge density on the metal side of the interface absolute charge density of the metal side of the interface charge density when the solution contains no surfactant charge density when the solution contains surfactant potential of zero charge PZCP n / N m-l film pressure at constant electrode potential @ / N m-l film pressure at constant charge density specific surface work YlJ specific surface work when the solution conY e 0 1J tains no surfactant specific surface work when the solution conYslJ tains surfactant r/mol cm-2 relative Gibbs surface excess or surface concentration r,,/mol cm-2 limiting surface concentration AGA/J mol-' free energy of adsorption of the surfactant on the electrode Parsons' function shift of the pzc from zero to full coverage of the surface by the surfactant dielectric constant of the inner layer effective dipole component of the dipole moment of the water molecules normal to the surface component of the dipole moment of the organic molecules normal to the surface n number of water molecules replaced by an organic molecule upon adsorption Registry No. Au, 7440-57-5; t-AA, 75-85-4.
