466
Langmuir 1989, 5 , 466-473
sation). For a given thin-film pore size application, each of the two methods has particular advantages and disadvantages. The SAW approach has a higher sensitivity (i.e., can be used with thin films with very low porosity) and is based upon a generally accepted technique (gas/vapor condensation). Disadvantages include those normally associated with condensation pore sizing (network/percolation effects, pore shape assumptions, limited pore size) as well as the experimental difficulties associated with making the SAW measurements (i.e., no commercial instruments are available). NMR has advantages such as no pore shape assumption, fast analysis time, a much wider range of sample configurations (i.e., does not need to be deposited on a planar substrate) and no network effects. It is also easier to implement experimentally. The major disadvantage is the lower sensitivity as compared to SAW analysis.
Acknowledgment. The work performed a t the University of New Mexico has been funded by Sandia National Laboratories under contract 55-6778. The work performed at Sandia National Laboratories was supported by the U.S. Department of Energy under contract DE-AC0476DP00789. P. Davis of UNM performed some of the 20-MHz NMR measurements, and C. Gasparovic of the
UNM Center for Non-Invasive Diagnosis performed the 300-MHz experiments. C. S. Ashley of Sandia prepared the film and bulk samples used in this study as well as performed the ellipsometer analysis.
Nomenclature mass of solid/volume of fluid (g/cm3) mass sensitivity of SAW device (cm2.s/g) unperturbed SAW oscillator frequency (s-l) fraction of pore fluid with bulk properties fraction of pore fluid in surface-affected phase magnetization at time 7 equilibrium magnetization adsorbate mass/area (g/cm2) adsorbate partial pressure (Torr) adsorbate saturation pressure (Torr) pore volume (cm3/g) mean pore radius (nm) hydraulic pore radius, 2PV/SA (nm) particle size (nm) surface area (m2/g) spin-lattice relaxation time (s) T1 of bulk phase (s) minimum expected T, (s) maximum expected TI (s) Tl of surface-affected phase (s)
Adsorption of Pyrazine at the Polycrystalline Gold-Solution Interface Anna Iannelli, Jocelyn Richer, and Jacek Lipkowski" Guelph- Waterloo Center for Graduate Work i n Chemistry, Guelph Campus, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N l G 2 W l Received July 20, 1988. In Final Form: December 16, 1988 The thermodynamics of pyrazine adsorption onto a polycrystalline gold electrode was investigated quantitatively by employing chronocoulometry. The measurement of the charge density on the metal side of the metal-solution interface allowed the determination of the adsorption parameters. The film and surface pressures, the relative Gibbs surface excesses, the free energies of adsorption, and the electrosorption valency as functions of both electrode potential and surface charge density were calculated. The maximum mol cm-2. The Gibbs free energy of adsorption Gibbs surface excess found for pyrazine is 5.8 X determined at the potential and charge of maximum adsorption (0.25 V (SCE) and 17.5 NCcm-2, respectively) is equal to -32.8 kJ mol-'. The adsorption of pyrazine on Au and Hg is compared, and the differences in the orientation of pyrazine molecules at the two interfaces are also discussed. In addition, the similarities between the adsorption of pyrazine and pyridine on a polycrystalline gold electrode are described.
Introduction This work is part of a project devoted to studying the influence of the crystallographic orientation of gold electrodes on the adsorption of neutral organic molecules from aqueous electrolyte solutions. The adsorption of pyrazine on a gold polycrystalline electrode is described in this paper. Pyrazine is particularly well suited to probe the molecular properties of the metal-solution interface. The pyrazine molecule is symmetric and rigid, so it is easy to determine its orientation at the metal surface. The isolated molecule has no permanent dipole moment. Therefore Conway et al.1-3 have suggested that the adsorption of (1) Conway, B. E.; Dhar, H. P. Croat. Chem. Acta 1973, 45, 109. (2) Conway, B. E.; Dhar, H. P.; Gottesfeld, S. Colloid Interface Sci. 1973, 43, 303.
pyrazine at the mercury-solution interface does not contribute to the surface potential. In consequence, the changes in potential drop across the interface, caused by the replacement of a solvent monolayer by a monolayer of pyrazine, can be taken as a measure of the surface potential generated by the preferentially oriented solvent molecules. Pyrazine is structurally related to pyridine, which has a large permanent dipole moment. Comparison of pyrazine and pyridine adsorption can therefore be useful to assess the effect of the permanent dipole moment on molecular orientation at the metal-solution interface. The adsorption of pyridine has been recently investigated in our laborat ~ r y . ~Thus ? ~ the present study constitutes a valuable (3) Conway, B. E.; Angerstein-Kozlowska, H.; Dhar,H. P. Electrochim. Acta 1974, 19, 455.
0743-7463/89/2405-0466~01.50/0 0 1989 American Chemical Society
Langmuir, Vol. 5, No. 2, 1989 467
Pyrazine Adsorption a t Au-Solution Interface
extension to these investigations. Within this paper, the energetics of pyrazine adsorption at the polycrystalline Au electrode are described. Quantitative data such as the Gibbs surface excesses, the Gibbs free energies of adsorption, and electrosorption valencies are determined. The differences between the adsorption of pyrazine at the Au and Hg surface are discussed. A brief comparison with pyridine adsorption at the polycrystalline Au-solution interface is presented.
Experimental Section Solutions. All solutions were prepared from Milli-Q water (Waters) with a resistivity higher than 17 M a cm. The supporting electrolyte was 0.1 M KCIOl (ACS Certified from Fisher). The KC104 was calcinated a t 300 "C, recrystallized twice, and dried before use. The pyrazine (Aldrich Chemical Company, Gold Label 99+ %) was utilized without further purification. The pyrazine concentrations investigated ranged from loF5to M. Before each experiment, the freshly made solutions were degassed with argon for approximately 30 min. Argon was circulated over the solution throughout the experiment as a protection against the ambient atmosphere. The flow of argon was temporarily interrupted during the recording of the chronoamperometric transients to avoid vibration of the meniscus between the solution and the working electrode. The temperature was 25 k 1 "C. Electrodes. The working electrode was a 99.99% pure polycrystalline gold rod formed by inductively heating gold nuggets (Engelhard) under vacuum. The electrode WBS polished to a mirror finish with progressively finer grades of alumina powder down to a 0.05-pm grade. The electrode was then annealed for 12 h a t 700 OC in a muffle furnace. The counter electrode was a coil of gold wire. The reference electrode was an external saturated calomel electrode (SCE). Before each experiment, the working electrode was cleaned by flaming and quenching with ultrapure water.6 The hanging electrolyte method was then used to make contact between the gold and the solution.' Instrumentation. The instrumentation employed within these experiments has been described in previous paper^.^^^^**^ The experiments were performed with a PAR Model 173 potentiostat connected to a homemade programmable voltage generator. A PAR Model 5204 two-phase lock-in amplifier was interfaced to measure the in-phase and out-of-phase components of the ac current. An Apple I1 computer equipped with two Computerscope boards (RC Electronics) was used for fast data acquisition. Each board had its own memory buffer and allowed the acquisition of 2048 points at an A/D conversion frequency of 500 kHz and a resolution of 14 bits.
Results and Discussion The experimental procedure followed in this work involved three main steps. First, the properties of the interface were characterized qualitatively by cyclic voltammetry and differential capacity measurements. Second, potential step experiments were performed, and the current transients were acquired. Last, the data were analyzed, yielding the relative Gibbs excess, the free energy of adsorption, and the electrosorption valency. Each step is described below. Characterization of the Interface 1. Cyclic Voltammetry. The cyclic voltammograms were recorded in order to determine the potential range (4) Stolberg,L.; Irish, D. E.; Lipkowski, J. J.Electroanul. Chem. 1987, 238,333. (5)Stolberg, L.; Richer, J.; Irish, D. E.; Lipkowski, J. J.ElectroanaL Chem. 1986,207,213. (6)Clavilier, J. J.Electroanal. Chem. 1980,107,211. (7) Dickertmann, D.; Shultze, J. W.; Koppitz, F. D. Electrochim. Acta 1976,21,967. (8) Richer, J.; Lipkowski, J. J. Electrochem. SOC.1986,133,121. (9)Richer, J.;Stolberg, L.; Lipkowski, J. Langmuir 1986,2,630.
-1404 -0800
-49
-0.400
o.Oo0
0.400
0.800
1.200
-0.400
0.m
0.400
0.m
1.200
4
-0.800
Figure 1. Cyclic voltammograms recorded in a 0.10 M aqueous solution of KC104 (a) free from organics and (b) containing 0.001 M of pyrazine. A 10 mV s-l voltage sweep rate was used.
in which the electrode behaved as an ideally polarizable electrode. The voltammograms also allowed the detection of traces of oxygen and other electrolyte impurities, in addition to creeping of the solution along the electrode walls. All cyclic voltammograms were recorded between -0.80 and 1.20 V (SCE) a t a sweep rate of 10 mV s-l. Cyclic voltammograms were obtained for the polycrystalline gold electrode in both the absence and presence of pyrazine, as shown in parts a and b of Figure 1, respectively. The voltammogram presented for the supporting electrolyte alone shows small values of current density between -0.80 and 0.70 V (SCE), commonly termed the double-layer region. The cyclic voltammetry curve is symmetric with respect to i = 0 in the double-layer region, and this feature indicates that the solution was oxygen-free and that creeping along the electrode walls was eliminated. The creeping effect, and the presence of oxygen, result in a characteristic distortion of the double-layer region of the cyclic voltamm~gram.~,~ The presence of peaks at potentials more positive than 0.70 V (SCE) is the result of surface oxide formation and is characteristic for the polycrystalline Au surface, as reported in literature.lOJ1 The voltammogram presented in Figure l b was recorded in a lov3M solution of pyrazine with the same supporting electrolyte concentration as above. There are a few subtle differences between the cyclic voltammograms obtained for polycrystalline gold in the absence and presence of pyrazine. As Figure l b shows, there are symmetric pyraz h e adsorption-desorption peaks a t a potential of -0.50 V (SCE). Pyrazine apparently affects the oxidation of gold and reduction of gold oxide. The gold oxidation peaks are clearly distorted in the voltammograms recorded in the presence of pyrazine. The surface area under the anodic branch was found to be 1.15 times larger than the area under the cathodic branch. The two areas were equal in (10)Clavilier, J. J.Electroanal. Chem. 1977,80,101. Conway, B. E.; Barnett, B.; Mozota, (11)Angerstein-Kozlowska, H.; I. J. Electroanal. Chem. 1979,100, 417.
Iannelli et al.
468 Langmuir, Val. 5, No.2, 1989
55 1
45
I
p
-
30
v
k
15 c\i 1
E
0
0 0 3
\
,
2
b
-15
’5 i
-\ I
I
I
I 1
-850 -600 -350 -100 150 E,”\’
(SCE) Figure 2. Differential capacity curves recorded as a function of the electrode potential in 0.10 M KCIOl solutions of varying pyrazine concentration. The concentrations are given in the figure. The curves correspond to a potential sweep in the positive direction. The capacity was determined by using a 5-mV rms sine wave modulated at 25 Hz and a 5 mV s-l voltage sweep rate. A series RC equivalent circuit was assumed in the calculations. the case of the supporting electrolyte alone. This suggests that pyrazine is being slightly oxidized and that this process is irreversible. In summary, the voltammograms indicate that the polycrystalline Au electrode behaves as an ideally polarizable electrode in the potential range between -0.80 and 0.60 V (SCE) in both the absence and presence of pyrazine. The adsorption of pyrazine was investigated in this patential domain. 2. Differential Capacity. The differential capacity curves were determined as a function of applied potential by ac impedance measurements. An ac signal of 25 Hz and 5 mV rms was superimposed on the 5 mV s-l voltage ramp. The differential capacity was calculated assuming a simple RC equivalent circuit for the cell. The differential capacity curves were used to determine the potential of zero charge (pzc) and to indicate the potential at which pyrazine was totally desorbed (E,,). These curves could not, however, be used in the quantitative study of pyrazine adsorption since the capacity was found to be strongly dependent on frequency, and thus the values measured at 25 Hz did not reflect the equilibrium state. The pzc was determined from the position of the diffuse layer minimum on the capacity curve for a 0.01 M KC104 solution. This value is equal to -0.04 V (SCE) and is in excellent agreement with earlier literature results.l0 The differential capacity curves measured for both the supporting electrolyte alone and the electrolyte containing varying concentrations of pyrazine are presented in Figure 2. The value of capacity at 4.80 V (SCE) is independent of the bulk pyrazine concentration as long as the pyrazine concentration is lower than M. Thus pyrazine can be considered totally desorbed from the surface at this potential (the potential of total desorption, Eo). Further measurements were therefore performed for the pyrazine concentrations ranging from to M and for electrode potentials ranging between -0.80 and 0.60 V (SCE).
--30
800
-
400
0
400
E,/mV (SCE) Figure 3. Plots of the absolute charge density as a function of the electrode potential determined by chronocoulometry for 0.10 M KC104 solutions of varying pyrazine concentration; the concentrations of pyrazine are given in the figure.
Data Acquisition and Treatment 1. Electrode Charge Density. The potential step experiments were performed to determine the charge density on the metal side of the interface (uM). The technique and the data processing have been described in previous paper^.^^^^^ The step experiments involved the following: (1)the electrode was held at an initial potential Eo > Eo,for 60 s, a period within which pyrazine adsorption reached equilibrium; (2) the potential was stepped to E,,, a potential at which total pyrazine desorption occurred, and the chronoamperometric curves (current transients) were simultaneously recorded; (3) the transients were digitally stored on the microcomputer. The potential steps were applied sequentially for E8 ranging from -0.75 to 0.60 V (SCE) in increments of 50 mV, with Eo remaining constant at -0.80 V (SCE). The chronoamperometric curves were digitally integrated to obtain the relative charge density, which is given by A~M(Eo) = UM(EO) - OM(&) (1) Knowing the pzc, obtained independently from differential capacity experiments, we converted AuM values to absolute charge densities ( uM)with the aid of the following formula: The absolute charge densities were calculated over the entire potential range, and the procedure was repeated for all pyrazine concentrations investigated. A plot of the charge density versus electrode potential for five pyrazine concentrations is presented in Figure 3. The shape of these curves is typical of the behavior observed for adsorption of neutral organic molecules on metals.12 All the curves merge at Eo.They also intersect (12) Mohilner, D.; In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1966; Vol 1, p 241.
Pyratine Adsorption at Au-Solution Interface
Langmuir, Vol. 5, No. 2, 1989 469
50
40
40 7
E
I
30
r
t
Z
30
z
E
S
\ 20
10
'0
0 -600 -400 -200
0
200
0 -25 -15
400
E //mV(SCE)
-5
5
/(uCcm-
25
15
2
Figure 4. Film pressure curves determined with respect to the electrode potential in 0.1 M KC104 solutions of varying yrazine concentration: (a) 3.4 X lo+; (b) 7 X lo*; (c) 2.0 X 10-P, (d) 4.5 x (e) 6.0 x (f) 1.0 X (9) 1.5 X lod4;(h) 2.5 X (i) 4.1 X (j) 6.1 X (k) 9.9 X M.
Figure 5. Surface pressure curves versus charge density determined in 0.1 M KC104 containing different amounts of pyrazine: (d) 4.5 X (a) 3.4 X lo4; (b) 7.5 X lo4; (c) 2.0 X (e) 6.0 x (f) 1.0 X (g) 1.5 X (h) 2.5 X (i) 4.1 X lo4; (j) 6.1 X (k)9.9 X M.
within a narrow range of potential, approximately 0.22-0.25 V (SCE), corresponding to charges between 16.8 and 18.0 yC cm-2, The potential and charge of maximum adsorption (E,, and UM,,~, respectively) are found within these ranges. The charge densities obtained experimentally were also plotted as a function of the logarithm of pyrazine concentration at constant electrode potential. From these curves, charge densities for additional pyrazine concentrations were interpolated. These results were used in the film pressure calculations and in further data treatment. 2. Film Pressure. The film pressure ( T ) is determined by the back-integration of the relative charge density data according to the equation
pressures are calculated from a difference between relative values of charge. Thus the uncertainty in the determination of the pzc will be incorporated into the surface pressure results, making 9 less precise than T . Figure 5 presents the calculated surface pressure plotted as a function of charge density. Qualitatively, the results in Figures 4 and 5 are very similar. For a given solution, the valuesof H and 0 at E,, and omaxare approximately equal. 3. Adsorption Isotherms. The relative Gibbs surface excess (I?) can be calculated as a function of E and UM for all concentrations of pyrazine in the following manner. Both film pressure and surface pressure values are plotted as functions of the logarithm of the bulk concentration of pyrazine (In c) at constant values of potential and charge density, respectively. The resultant curves are graphically differentiated with respect to In c, according to the formulas below where R is the gas constant, T is the temperature, and P is pressure.
where yc=oand yc are the interfacial tensions in the absence and in the presence of pyrazine and AuM,~=,and AuM,~are the corresponding relative charge densities. The calculated film pressures are plotted as a function of potential for varying pyrazine concentrations in Figure 4. The film pressure curves are bell-shaped, and their maximum corresponds to E., The most concentrated pyrazine solution M) yields a film pressure at E,, of 47.1 mN m-l. This is a fairly high value of the film pressure and indicates that pyrazine is strongly adsorbed at the gold surface. The above analysis treats potential as the independent electrical variable; however, the data analysis may also be carried out using charge as the independent electrical variable. In this case, the Parson's function, given as = y + uME,has to be evaluated, and the surface pressure (a) of the adsorbed pyrazine molecules can then be determined from =
tc=o
-tc
r = (l/RT)(dn/d r = (1/RT)(da/d
In c
)
~
In C ~ S , , , ~
~(5a) ~ (5b)
The maximum bulk concentration of pyrazine corresponded to a mole fraction of 1.8 X This value is sufficiently small as to assume that the mixture follows Henry's law. Thus concentrations instead of activities were used in the determination of surface e x c e ~ s e s . ~ , ~ ~ , ~ ~ The adsorption isotherms calculated with respect to electrode potential are illustrated in Figure 6a, and the isotherms determined with respect to charge density are presented in Figure 6b. The plots are three dimensional. At constant positive potentials, the adsorption isotherms display a characteristic sigmoidal shape with a well-defined
(4)
Note that the determination of the Parson's function requires the absolute charge density, while in contrast, film
(13) DeBattisti, A.; Trasatti, S. J. ElectroanaL Chem. 1974, 54, 1. (14) Mohilner, D.; Nakadomari, N. J. Electroanal. Chem. 1975, 65, 843.
410 Langmuir, Vol. 5, No. 2, 1989
Iannelli et al.
-
E
30
-3
> Y
-0 00
c3
28
a 1
26
, + & 4 2 -500
-300
E
-100
300
100
/mV(SCE)
’‘
____
I b
Figure 6. Three-dimensional graphs representing the isotherms for the adsorption of pyrazine calculated with respect to (a) the electrode potential and (b) the charge density. The electrical variable varies along the axis “normal” to the plane of the figure. The maximum surface excess corresponding to the plateau on both graphs averages out to a value of 5.8 x mol cm-2.
plateau observed at high bulk pyrazine concentrations. The limiting surface excess (r,) was estimated to be 5.6 X mol cm-2. In addition, the isotherms plotted a t constant charge density also exhibit a plateau at high bulk pyrazine concentrations. The maximum value of the surface excess determined from this plot is 5.9 X mol cm-*. This value is in good agreement with that of rm, found for the plateau in Figure 6a. 4. Free Energy of Adsorption. The free energy of adsorption (AGO,&) is usually determined from a fit of the experimental surface excess data to an equation of a particular adsorption i ~ 0 t h e r m . IThe ~ choice of the isotherm used in the fit is arbitrary; however, in the limit of zero coverage, all isotherms simplify to‘ the Henry iso- 4 therm:15
= RTl?,,,P(c/55.5)
(64
@ = RTr,,P(c/55.5)
(6b)
K
where P is the adsorption equilibrium constant and is related to the standard Gibbs energy of adsorption through the equation AGoads = -RT In Pa The standard state corresponds to unit mole fraction of the organic species in the bulk of the solution and monolayer coverage (0 = 1) of the surface by noninteracting a d ~ o r b a t e s . ~ J The ~J~ equilibrium constant P can be determined from the initial slopes of the film and surface pressure versus the bulk concentration plots according to the expressions = (an/aXb)x,+/RTr,,
(74
P = t a@/dXb)X,-o/R TI’,,
(7b)
where Xb is the mole fraction of pyrazine in the bulk equal to c155.5. (15) Parsons, R.h o c . R. SOC.London 1961,A261,79. (16)Conway, B. E.;Joshi, J.; Dhar, H.P.Electrochim. Acta 1973,18, 789.
2
t -20
-10
0
20
10
o-h/l / pcclV,-2
Figure 7. (a, Top) Free energy of adsorption of pyrazine calculated as a function of electrode potential. The data for the free energy of adsorption were fitted to a second-order polynomial of the form of eq 8a, and the result of the fit is represented by the solid line. (b, Bottom) Free energy of adsorption of pyrazine calculated with respect to the charge density on the metal surface.
The values of AGOad, determined from the initial slope of the x and @ versus Xb plots are presented in parts a and b of Figure 7. The data of x and CP must be very precise to obtain meaningful values of AGoads by using this method. The free energy of adsorption depends in a parabolic way on the electrical variable, and the data can be described well by AGoada = AGO,,
+ b(E - E,)’
AGoada = AGo,,,
+ b’(aM )-, ,a
(84 2
(8b) where AGO,, is the standard free energy of adsorption a t E,, or . , ,a The magnitude of b and b’determines the potential and charge ranges in which the adsorption takes place, respectively.
Pyrazine Adsorption at Au-Solution Interface
Langmuir, Vol. 5, No. 2, 1989 471 0.000
Table I. Summary of Adsorption Parameters
parameter
electrode surface polycrystalline Au mercury pyridine pyrazine pvrazine ref 5 ref 2 -40 0 -290 -250
units mV (SCE) mV
PZC
E,,-E,
em-
pC
-150 -32.8 5.8
mV
AGO,,
kJ mol-' X mol cm-2 kJ mol-' V-2 21 kJ mol-' (& cm-z)-2 6.1 X
b'
-
-2.0
17.5
cm-2
EN
--500 -38.0
~150 -21
4.5
7
1
-+ >; -0.100 0 K a, 0
>
K
.o -0.200 + 0
cn
P
+.
G------150 mV
N
1
E
2
0
a, -
-0.300
w
,
w:
-0.400 -600 -400 -200
-50 mV
0
200
E/ mV (SCE) Figure 9. Dependence of the electrosorption valency on the electrode potential. The dots correspond to data that were obtained from the slope of the plots in Figure 8. The straight line corresponds to the first derivative of a second-order polynomial used to fit the free energy data given in Figure 7a.
0
0 Y
'-6
I
b
4-350 mV -14 -450mV
0
1
2
150
3
4
5
6
r /XI 0-I 'molc-m-2 Figure 8. Plots of the charge density as a function of the surface excess of pyrazine for different values of the electrode potential. The values of E are indicated in the figure; some intermediate curves were omitted for the sake of clarity. The straight lines were drawn through the points corresponding to low values of the surface excess.
In the case of the Henry isotherm, the standard maximum free energy of adsorption was -32.6 kJ mol-' at a, u of 17.5 pC cm-2 and was -33.0 k J mol-' a t an E , of 0.25 V (SCE). A summary of the thermodynamic adsorption parameters calculated is presented in Table I. It is interesting to note that the free energies of adsorption determined for the Au electrode are higher than those free energies obtained for pyrazine adsorption on Hg.'s2 This suggests that pyrazine adsorption on gold is stronger than that on Hg. 5. Electrosorption Valency. Electrosorption valency can be calculated in two ways: (i) from the first derivative of the free energy versus potential plot and (ii) from the slope of the uM vs r plots a t constant electrode potential. This can be described by the expression"J8 Y = ( ~ / F ) ( ~ G o / w= ,
-(l/n(duM/ar)E (9)
where F is the Faraday constant. Equation 9 can be used to test the consistency of the experimental results. Figure 8 presents a series of charge density versus surface excess plots obtained at different electrode potentials. (17)Vetter, K. J.; Schulze, J. W. Ber. Bunsen-Ges. Phys. Chem. 1972,
76, 920, 927.
(18)Schulze, J. W.; Vetter, K. J. J . Electroanal. Chem. 1973,44, 63.
s o 0 0
v
> -150 E
\
0
1
2
3
r / X I0- O
4
5
6
mo1~m-2
Figure 10. Plots of the electrode potential as a function of the
surface excess of pyrazine for different values of the charge density. The values of UM are given in the figure. For the sake of clarity, several intermediate curves were omitted. The straight lines were drawn through the points corresponding to low values of the surface excess.
-
The plots are nonlinear; however, the limiting slopes of the curves taken at r 0 progressively decrease with increasing potentials. The initial slopes of these curves were compared to the result of the numerical differentiation of the AGoads versus E curve from Figure 7a. The plots of the electrosorption valency obtained by both methods are shown in Figure 9. The agreement between these two sets of data is good. The observed linear dependence of electrosorption valency on potential is a simple consequence of the quadratic dependence of the free energy of adsorption on the electrical variable. It can also be shown that the slope of the electrode potential versus the surface excess taken at constant charge
Iannelli et al.
472 Langmuir, Vol. 5, No. 2, 1989
a 3 0 T-
=:
b
n
i
a
-2 a --L
DM
1P.CCt-n
Figure 11. Dependence of d E / d r on the charge density. The points were calculated from the initial slopes of the plots in Figure 10. The solid line corresponds to the first derivative of a second-order polynomial used to fit the free energy data given in
Figure 7b. density is equal to the first derivative of the Gibbs energy of adsorption as a function of the charge density,lg that is
(dE/dr),, = (dAGo/daM)r
(10)
A plot of E versus r at constant charge densities is shown in Figure 10. The data appear to be linear at low values of the surface excess, but they deviate from linearity as r increases, especially at negative charges. The initial slopes of the E versus r plots are plotted as a function of charge density in Figure 11 (black points). The solid line in Figure 11 corresponds to the dAGo/daM obtained by differentiation of AG" versus UM given in Figure 7b. The agreement between the two sets of data is fully satisfactory. 6. Molecular Model of Adsorption. Throughout the paper so far, the thermodynamic parameters have been calculated without making any assumption about the structure of the interface. A simple electrostatic model can now be applied to gain further insight into the molecular structure of the interface.'pi20 The potential drop across the inner region of the double layer, 4M-2,can be expressed as
where E is the dielectric constant, x 2 is the thickness of the inner layer, and ,iiis the effective dipole moment equal to p = p g - npw (12) where porg and pw are the components of the dipole moments normal to the surface of the organic molecule and water, respectively; n is the number of water molecules displaced by one adsorbed organic molecule. (19)Parsons, R. Trans. Faraday SOC.1969,55, 999. (20) Parsons, R. J . Electroanal. Chem. 1964, 7, 136. (21) Hamelin, A.; Stoicoviciu, L.; Silva, F. J.Electround. Chem. 1987, 229, 107. (22) Bockris, J. OM.; Devanathan, M. A. V.; Muller, K. Proc. R . SOC. London 1963, A274, 55.
The potential drop across the inner layer can be determined by 4M-2 = E - PZC - 42 (13) where 42 is the potential drop across the diffuse part of the double layer. If the charge density is kept constant, 42 is independent of the surface excess and q5M-2 = E + constant. Therefore, at constant aM,the dependence of E upon r is caused exclusively by the changes in the potential drop across the inner part of the double layer. A plot of E vs r at constant charge densities is shown in Figure 10. According to the molecular model, the nonlinearity of these plots may be due either to reorientation of the pyrazine molecule (if in the adsorbed state, its dipole moment is not equal to zero) or to a change in the orientation of the water molecules. Alternatively, the curvature may be caused by an increase in the thickness of the inner layer, x 2 , due to pyrazine adsorption. The magnitude of the effective dipole moment of the adsorbed pyrazine molecule can be estimated from the shift of the pzc from zero coverage to maximum coverage of the surface by pyrazine, EN, which is given by the equation EN = ( 4 i r ~ / 4 ~ , ~ ~ (14) The value of EN found by linear extrapolation of the initial segment of the curve to rmax is approximately equal to -120 mV. The negative shift of pzc indicates that either InpwI > IF."?! and pw > 0 or InpwI < Ip""I with 1.1"- < 0. The first condition would mean that pw is positive and that the water molecules are preferentially oriented with their hydrogen atoms facing the metal side of the interface at UM = 0. A previous studyz3has shown that water molecules are adsorbed preferentially with their oxygen atoms facing the metal at UM = 0; thus this first possibility may be rejected. The second condition indicates that at zero charge the effective dipole moment for pyrazine is negative, and its absolute value is larger than the absolute value of the effective dipole moment for water. The dipole moment for the isolated pyrazine molecule is equal to zero. However, at the metal-solution interface, the pyrazine molecule can acquire a dipole moment induced by asymmetric interactions with the solvent and the metal phases. In the vertical orientation the lone pair of electrons from one nitrogen atom can overlap with the empty states from the metal, while the lone pair from the second nitrogen atom can form hydrogen bonds with water molecules. For the flat orientation, asymmetric interactions will also occur between the ir electrons of the pyrazine ring and the two phases, which can easily induce a dipole moment in the pyrazine molecule. According to eq 11 and 13, the change in the electrode potential with varying r is given by
At low coverages, the variation of the thickness of the inner layer with changing is sufficiently small as to permit the use of the initial slopes of the plots presented in Figure 10 as an estimate of the magnitude of the effective dipole moment. Therefore, the dependence of the initial slopes of the E versus 'I plots on charge shown in Figure 11 illustrates the influence of the electrical variable on the effective dipole moment. The relationship is fairly linear. A linear variation of dE f d r with charge was also observed for pyrazine adsorption on Hg by Conway et al.293 If the pyrazine molecules do not change orientation with charge, (23) Bockris, J. O'M.; Gileadi, E.; Muller, K. Electrochim. Acta 1967, 12, 1301.M.
Langmuir 1989,5, 473-478 then the variation of the effective dipole moment shown in Figure 11 can be solely due to changes in the average dipole moment of the solvent. The quasi-linear dependence of pw is predicted by the two-state and multistate models for solvent adsorption as presented by Bockris et al.22,23and by G ~ i d e l l i respectively. ,~~ However, in the present case, the molecular interpretation should be considered with caution. The charge density which is measured and used in the data processing is an averaged quantity. In fact, the excess charge is not uniformly distributed across the polycrystalline metal surface, which is composed of a variety of single crystals microfacets each with its own specific electron charge density. The charge density fluctuates a t the surface, and the extent of these fluctuations depends upon the density and dimensions of the microfacets.
Conclusion To conclude this paper, we will compare the present results for pyrazine adsorption at the Au surface with (a) literature data for pyrazine adsorption at Hg2 and (b) our earlier results for pyridine adsorption at the polycrystalline Au e l e ~ t r o d e .In ~ this way, we will assess the effect of the nature of the metal and the molecular structure of the adsorbate on adsorption of organic molecules a t the metal-solution interface. The thermodynamic parameters for pyrazine adsorption onto polycrystalline Au and Hg surfaces are compared in Table I. The data show significant differences in adsorption of this molecule a t the two metal surfaces. The pyrazine molecule is adsorbed flat at the Hg surface. The (24)Guidelli, R.;In Trends in Interfacial Electrochemistry; Silva, A. F., Ed.; NATO AS1 Series C, 1986; Vol. 179,p 387.
473
maximum relative surface excess for pyrazine adsorption on gold is consistent with the vertical orientation of the molecule. In addition, the AGoad, values for gold are approximately 50% larger than the AGOads values for Hg, which indicates significantly stronger interactions of pyrazine molecules with gold than with mercury. The strong interactions between pyrazine and gold generate a dipole moment in the direction normal to the surface. In contrast, the pyrazine molecule behaves as a nonpolar species a t the mercury electrode surface. As a result, E- > pzc and u> 0 for the case of pyrazine adsorption a t the gold electrode, while E,, C pzc and um, C 0 for mercury. The comparison of the present results for pyrazine with our earlier studies of pyridine adsorption at Au surfaces4i5 shows many similarities. The free energies of adsorption are high and of comparable magnitude. Additionally, both molecules display a tendency toward adsorption in a vertical orientation at high coverages and positive charge densities. However, the adsorbed pyridine molecules have a much higher effective dipole moment than the adsorbed pyrazine molecules. The shift of pzc, due to adsorption of a monolayer of pyridine a t the Au electrode, is equal to -0.50 V, and its absolute value is much higher than the magnitude of E N determined for pyrazine. These results indicate that at the gold-solution interface the orientation of the adsorbed pyridine or pyrazine molecules is primarily determined by the magnitude of the overlap between nonbonding orbitals from the nitrogen heteroatom with the electronic states in the metal rather than by the strength of the field-dipole interactions.
Acknowledgment. We gratefully acknowledge the American Electroplaters and Surface Finishers Society for their financial support. Registry No. Au, 7440-57-5; pyrazine, 290-37-9.
Effects of Triton X-100 on Sonicated Lecithin Vesicles Katarina Edwards,* Mats Almgren, Jayesh Bellare,? and Wyn Brown Institute of Physical Chemistry, Uppsala University, Box 532, S - 751 21 Uppsala, Sweden Received May 13, 1988. In Final Form: September 27, 1988 The effect of Triton X-100on small unilamellar lecithin vesicles has been studied by means of quasi-elastic light scattering and cryogenic transmission electron microscopy. Low concentrations of Triton have little effect on the vesicle size, and very high concentrations give rise to the formation of mixed micelles. Addition of intermediate concentrations of surfactant results in the formation of very large vesicles with radii of up to 1000 A. The two methods give aggregate sizes in good agreement. Time-resolved cryo-TEM experiments showed that the time for the formation of the large vesicles is about 2 min. The use of a controlled environment vitrification system (CEVS) for the cryo-TEM sample preparation made it possible to visualize the aggregates formed without introducing artifacts due to changes in temperature or saturation.
Introduction Because of their similarities to biological membranes and their use as potential drug carriers, vesicles have rapidly come into widespread use. As the possible areas of application have grown, the need for basic physicochemical knowledge about vesicles has increased. a number of papers have been published on vesicle formation, stability, and interaction with other mo1ecules.l4 Much research has been focused on the interaction between vesicles and t Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455-0132.
0743-7463/89/2405-0473$01.50/0
small surface-active agents. It is known that some surfactants have a profound effect on the membrane permeability and structure of the vesicle, and it has been shown that surface-active agents may greatly enhance the (1)Silver, B. L. The Physical Chemistry of Membranes; Allen and Unwin: Winchester, 1985. (2) Mitchell, D. J.; Ninham, B. W. J . Chem. Soc., Faraday Trans. I , 1981,77,601. ( 3 ) Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1986,90,226. (4) Suezaki, Y.; Tsuji, N. J. Colloid Interface Sci. 1986, 114, 131. (5)Schurtenberger, P.;Mazer, N.; Kanzig, W. J. Phys. Chem. 1985, 89,1042. (6)Hepatology; Arias, I. M., Ed.; 1984;Vol. 4,No. 5.
0 1989 American Chemical Society
