Langmuir 1994,lO,2403-2408
2403
Double Layer Effects at Molecular Films Containing Acid/Base Groups W. Ronald Fawcett,*Milan Fedurco, and Zuzana KovaEova Department of Chemistry, University of California, Davis, California 95616 Received October 7, 1993@ The effect of the double layer on the ionization of acidic head groups in a self-assembled monolayer on a gold electrode is considered. A model is developed which assumes that the head group is located in a region of low dielectric constant adjacent to the oillike monolayer. Expressions are developed for the potential distribution in the interface and its differential capacity. It is emphasized that discretenessof-charge plays an important role in determining the local potential at the ionizable head group. The results obtained are discussed with respect to other models and experimental data presented recently in the literature.
Introduction Double layer effects on interfacial phenomenahave been studied for some time, especially with respect to anion adsorption and electrode kinetics at polarizable electrodes.1*2 More recently, they have been discussed for processes occurring a t self-assembled monolayers (SAM) a t gold electrode^.^-^ When the molecule in the S A M contains an acidic head group, its degree of ionization depends on electrode potential reflecting the fact that the local potential at the head group also changes. Smith and White4recently discussed the double layer properties of such a system and pointed out that, since a proton can dissociate from the head group, this process gives a contribution to the differential capacity of the polarizable interface. In their model, they assumed that the electrostatic potential a t the ionizable head group is equal to that a t the outer Helmholtz plane (0.H.p.) located at the distance of closest approach of the counterions to the S A M . As a result a large fraction of the potential drop between the electrode and bulk of the solution is attributed to the diffuse part of the double layer, and simulation of observed differential capacity data6 requires that the electrode charge density be changed over a rather large range. In the traditional model of the double layer both at the metallaqueous solution and oillaqueous solution interfaces,’ it is assumed that there is a region in which water molecules are sufficiently perturbed from their structure in the bulk of the pure liquid that the local dielectric permittivity is significantly lower. Such a model, originally due to Stern,8 was elaborated by Grahameg and successfully applied to the analysis of data for anion adsorption at the mercurylaqueous solution interface. Since the permittivity of the Stern layer, or inner layer, is lower than that in the bulk, a larger fraction of the total potential drop occurs in the region of the adsorbed ion or ionized head group, and the potential drop across the diffuse layer is significantly reduced. In the present paper, the model of Smith and Whitell is extended to include a Stern layer. Discreteness-ofAbstract published in Advance ACS Abstracts, J u n e 1, 1994. (1)Delahay, P. Double Layer and Electrode Kinetics; WileyInterscience: New York, 1965,Chapters 4 and 9. (2)Frumkin, A. N. Potentsialy Nulevogo Zariada; Nauka: Moscow, 1979,Chapters 4 and 9. (3)Smith, C. P.;White, H. S. Anal. Chem. 1992,64, 2398. (4) Smith, C. P.; White, H. S.Langmuir 1993,9,1. (5)Becka, A. M.; Miller, C. J. J.Phys. Chem. 1993,97,6233. (6)Bryant, M. A.;Crooks, R. M. Langmuir 1993,9,385. (7)Levine, S.;Mingins, J.; Bell, G. M. J.Electroanal. Chem. 1967, 13, 280. (8)Stern, 0.2.Elektrochem. 1924,30, 508. (9)Grahame, D. C. Chem. Rev. 1947,41,441. @
charge effects are also included in the model. These effects are especially important at a S A M containing charged head groups because of the fact that the charge is located in a region where the dielectric permittivity is not uniform. Stated otherwise, estimation of the local electrostatic potential at an ionizable head group in the S A M requires that one consider the variation in potential both parallel to the interface as well as perpendicular to it. These effects are described in more detail in the following section of the paper.
The Model It is assumed that a metal electrode is covered by a S A M irreversibly adsorbed on the metal through an anchoring group (e.g. thiol). Some of the alkyl chains terminate with an ionizable head group HA, whereas others terminate with no polar head group (Figure 1). Thus, the surface excess of ionizable head groups, rT, is determined by the composition of the S A M when it is formed. The important feature of this monolayer for the present discussion is that the degree of dissociation of the head group HA to give the anionic terminating group Adepends on solution pH and electrode potential. Following Smith and White,4 this is assessed by writing down the thermodynamic condition for the equilibrium
HA--+++-
(1)
which is
ji: is the electrochemical potential of species i at location 1. The undissociated acid, HA, and ionized form, A-, are located only at the head group which is assumed to be at the inner Helmholtz plane (i.H.p.1designated as location “a”.199These groups are also assumed to be surrounded by water molecules whose local structure is determined by the hydrophobic interactions with the oillike materials in the remainder of the monolayer. The dielectric permittivity of the region between the metallsolution interface and the i.H.p. is assumed t o be uniform and have a value 61. Thus, any difference between the permittivity of the oillike monolayer and the water molecules between the S A M and i.H.p. is ignored. This approximation is reasonable because the dielectric permittivity of the water molecules at the oil/water interface is much less than that in the bulk of the solution. The average permittivity in the region between the i.H.p. and
0 1994 American Chemical Society 0743-7463/94/2410-2403$04.50/0
2404 Langmuir,
Vol.10,No. 7,1994
Fawcett et al. ln[O/(l - ON = 2.3 pH
- 2.3 pK, + pa
(8)
where pH and pKa have their usual meanings and f = F/RT. This expression was derived by Smith and White* who assumed that Ya,is equal to the potential drop across the diffuse layer. In the present treatment, Ya,is estimated on the basis of the dielectric properties of the interface including a Stern region in which the water molecules have a much different structure from those in the b ~ l k . ' J ~ - ' ~ The average potential drop from the i.H.p. to the 0.H.p. is given by
I
I
I
IHP OHP Figure 1. Schematic representation of a metal electrode covered with a self-assembled monolayer. Some ofthemolecules in the monolayer terminate in an ionizable head group, HA, whose deprotonated form is A-. The unlabaled circles represent water molecules located between the plane going through the charge centers of the head groups (inner Helmholtz plane or IHP) and the plane going through the charge centers of the nearest counterions in the diffise layer (outerHelmholtz plane or OHP).
o.H.p.,€2, is assumed to be somewhat higher. This region is free of charge and has a thickness corresponding to one to two water molecules. The 0.H.p. defines the boundary of the diffise layer, and its electrostatic properties are assumed to depend on the charges on the ions in the electrolyte and their concentrations as described in the Guy-Chapman model.'gg The proton Hf associated with an ionizable head group is in equilibrium with the protons in the bulk of the solution, whose location is designated as "5". Writing out the expressions for each of the electrochemical potentials, one obtains
and
p i - = p i - + RT In r A - - FYa
(5)
where rm is the surface excess of undissociated head groups, r A - , that of dissociated head groups, and Yaythe local or micropotential at the charged head group measured with respect to the average potential in the bulk of the solution, @ (@ 0). The condition for equilibrium thus can be written as
RT ln(T,/TA_)
= -RT In K,
+ RT In aH+- FY"
where qP is the average potential on the i.H.p., 4d,that on the o.H.p., ,a, the charge density on the electrode a,, that due to ionized head groups on the i.H.p., and Kz,the integral capacity ofthe region between the i.H.p. and 0.H.p. (Figure 1). The average potential, @, overestimates the repulsive effect of one ionized head group by the others in the same plane. Because of the discrete nature of these charges which are located in a medium of low permittivity adjacent to the diffise layer which has a higher permittivity and the metal electrode whose permittivity is effectivelyinfinite, one must correct the average potential, by the discreteness-of-charge potential 5" which accounts for the stabilizing effect of the surrounding medium.10-12The discreteness-of-chargepotential at the interface can be considered to be a two-dimensionalanalog of the Debye-Huckel self-atmosphere potential experienced by an ion in an electrolyte solution. For the present system it has two components, specifically, that due to the stabilizing effect on a given ionized head group of its images in the electrode and diffise layer and that due to the images of other ionized head groups. The first contribution is given bylo
(10) where eo is the h d a m e n t a l electronic charge, /3, the distance between the metal surface and the i.H.p., and KI,the integral capacity of this region. When K1 and B are independent ofelectrode potential, as one would expect to a good approximation for the present system is a constant and represents the fixed part of the stabilizing influence of the dielectric inhomogeneitynear the charged head groups. The second contribution due to other ionized head groups is estimated here using the cut-off disk model1°-13 in which the charge density on the i.H.p. is assumed to be uniform outside of a disk surrounding a given ionized head group on the i.H.p. The radius of this disk depends on the net charge density on the i.H.p. and is given by -eo
(6) ro
where
RT In K, = ,ii& - pk+ - fi&
(7)
and K,is the acid dissociation constant for HA. Defining the fraction 8 of the ionizable head groups which are dissociated ( 8 = r*-/(rA- rd), one may write the expression
+
= (E)
(11)
The cut-off disk potential is given by (10)Levine, S.;Bell, G. M.; Calvert, D.Can. J . Chem. 1962,40,518. (11)Levine, S.;Mingine, J.; Bell, G. M. J. Phys. Chem. 1963,67, 2095. (12)Levine, S.J. Colloid Interface Sci. 1971,37,619. (13)Levine, S.;Robinson, IC;Bell, G. M.; Mingins,J. J. Electroaml. Chem. 1972,38,253.
Langmuir, Vol. 10,No.7,1994 2405
Double Layer Effects (12) where K is the integral capacity of the whole region between the metal electrode and the 0.H.p. and g is a dimensionless parameter which depends on imaging conditions in the metal electrode and diffise layer. The relationship between K and KI and KZas capacitors in series is (13) The parameter, g, depends on the thickness of the Stern region, the relative permittivities in this region, the cutoff disk radius ro,and the imaging conditions in the diffise layer.13 For the present system it is close to unity and is set equal to 1, independent of potential in the following development of the model. The consequences of this assumption are discussed further below. Now, one can relate the local or micropotential at an ionized head group to the electrostatic parameters for the system. This potential is given by
Ya = 4a
+ c; + 5;
pendent of the electrical field. Although this is a reasonable assumption for the S A M ,there may be some change in both K1 and K2 due to reorientation of solvent molecules, which is ignored in the present treatment. The two derivatives on the right-hand side of eq 19 may be related by differentiating eq 17 with respect to am.This gives the result that 1
dua = fA ~ 0 ~ h C f 4 ~dqbd d4d +/ 2 )= C dum dum ddom
(20)
and c d is the diffise layer capacity. From eq 8, at constant PH (21) and from eq 16 (22) Combining eqs 18 and 19, one may write
(14)
dYa -- - -c -
Combining eqs 9,10, and 12 and settingg = 1, one obtains
dum
edum
(23)
where CS= fO(1 is the differential capacity due to the dissociation process at the head groups.14 Finally, from eq 15, one obtains Several other relationships are required to complete the electrostatic description of the interface. Firstly, the charge density due to ionized head groups is given by Ua =
-FTA- = -FB(TA-
+ r,)
= -Fer,
(16)
where r T is the total surface coverage by ionizable head groups in the S A M . On the basis of the Gouy-Chapman model, the potential drop across the diffuse layer is given by
+
om ua = 2~ s i n h ~ f 4 ~ / 2 )
(17)
where A = (2RT E ~ C ~ ~ cs ) ~being ' ~ , the dielectric constant of the pure solvent, eo, the permittivity of free space, and cs, the ionic strength of the solution which contains only 1/1 electrolytes. For water at 25 "C,A is equal to 5.86 pC cm+ when cs is 1 M. Finally, the potential drop across the interfacial region from the metal electrode to the 0.H.p. is given by (18) q5m is the electrode potential on the rational scaleg and is
given t o a good approximation by the electrode potential against a reference electrode in an electrolyte whose ions do not adsorb on the electrode minus the potential at the point of zero charge (P.z.c.)against the same reference. In order to derive an expression for the differential capacity of the interface C , one differentiates the expression for 4mwith respect to am. From eq 18, the result is 1 -d4"d4d C dum do,
1 1 doa +-+-K K2dom
(19)
In the present treatment, the integral capacities in the inner part of the double layer are assumed to be inde-
dYa- d4d ---+-+ dum
1 ----1 "a' K doa dum K2 K2dum K1K2dom
(24)
Using eqs 20-23, one easily obtains the expression
Thus, the reciprocal of the interfacial differential capacity is given by
It is clear that the first two terms on the right-hand side of eq 25 give the reciprocal of the interfacial capacity on the basis of a simple model of two capacitors in series, namely, the inner layer capacity, K, and the diffuse layer capacity, c d . The third term accounts for the contribution from the ionization process at the head groups of the S A M and is related to the local field experienced by this part of the S A M . In the following section of the paper, capacity-electrode potential curves are plotted for typical values of the inner layer properties including those ofthe SAM. These results are compared with those from the model proposed by Smith and White.4
Results Capacity against electrode potential curves were calculated for typical conditions observed at a S A M . The integral capacity of the region between the electrode and i.H.p., Kl, was assumed to be 15 pF cm-2. Assuming a (14) The corresponding equationwritten by Smith and White4differs by a factor of U2.3. Since their equation is derived from eq 4 in their
paper, it should agree with the definition of COgiven here.
Fawcett et al.
2406 Langmuir, Vol. 10, No. 7, 1994 I(
pH-53
E,
2
I
1
LL
3 .
\ 0
.-d V
P
I -1.2
-0.4
0.4
I
-t .Z
-0.4
Electrode Potential, Om/ V
0.4
Electrode Potential, Om/ V
Figure 2. Differential capacity of the electroddsolution interface, C,plotted against electrode potential on the rational scale,p ,at three Merent pH values in the bulk of the solution for an electrode covered by a self-assembled monolayer with ionizable head groups. The surface coverage by these groups moles cm-2 and the pKa of the head group is 2. The is 8 x or 3 (A) as indicated. pH of the solution is 1 (01, 2 (01, thickness of 1nm for this region, the corresponding relative permittivity is 16.9. The region between the i.H.p. and 0.H.p. was assigned a capacity of 40 p F cm-2 which corresponds to a relative permittivity of 22.6for a thickness of 0.5 nm. The total capacity of the inner layer is thus 10.9pF cm-2. The pK.for the acidic head group was set equal to 2. Calculations were carried out to cover a wide range of values of 8 and assuming a fixed pH. The local potential at the head group Yawas then determined using eq 8. The value of the charge density comes directly from eq 13. Equations 14 and 15 were then solved by an iterative technique to obtain the electrode charge density amand the potential drop across the diffise layer bd.The capacities CQand c d were then estimated and used in eqs 25 and 26 to calculate dYa/damand the differential capacity C. Capacity curves estimated for a surface coverage of 8 x 10-lo mol cm-2 are shown as a fwnction of pH in Figure 2. Since the electrolyte is assumed to be a strong acid, for example, perchloric acid, the ionic strength changes with pH. As the pH increases, the capacity maximum shifta in the negative direction due to the decrease in the bulk activity of the hydrogen ion. However, the shiR is not large, corresponding to less than 200 mV for a change in pH from 1 to 3. This shift is reduced in magnitude due to a corresponding increase in the double layer effect. As the ionic strength increases, the potential drop across the diffise layer becomes larger and reduces the tendency of the head group to dissociate when the field due to the electrode charge density is negative. The capacity maximum is large and of the order of magnitude seen for a system studied in this laboratory (see below). Values of the local potential at the head group Yaand potential drop across the diffise layer #d for a system at pH = 2 (c, = 0.01M) are shown in Figure 3. In addition, the fraction 0 of the head groups which are ionized is also shown as a function of electrode potential. It should be noted that the capacity maximum (Figure 1)occurs close to but not exactly at the potential where the acidic head groups in the S A M are halfionized. The capacity features are due to the rather complex way in which the local potential Ya at an ionizable head group changes with potential. In general, Ya becomes more positive as the electrode potential moves in this direction, but the relationship between these quantities is complex. The potential drop across the diffise layer ddis negative for small values of 8, and starts to increase, However, when
Figure 3. Micropotential on the i.H.p. Ya ( 0 )and average plotted against electrodepotential potential on the 0.H.p.#d (0) for a S A M with a coverage by ionizable head groups of 8 x mol cm-2 at a pH of 2. The solid curve gives the fraction of the ionizable groups in the anionic form (right-hand ordinate).
.-d 0 0.
s
1
-
-1.2
-0.4
0.4
Electrode Potential, O m / V
Figure4. Differentialcapacity,C,plotted against the electrode potential on the rational scale, p ,for various concentrations in the self-assembled monolayer of the ionizable head group. The solution pH is 2 and the ionic strength 0.01 M. The surface mol cm-2 as concentration varies from 0 to 8 in units of indicated.
the degree of ionization becomes significant, rjd decreases again and reaches quite negative values. This result reflects the fact that the charge associated with the ionized head groups is quite large and negative, reaching a maximum value of -77.2 pC cm-2. As a result, the sum am-ta. is always negative for the chosen conditions and #d below zero. I t should also be noted that Yaand #d are significantly different so that interfacial capacity estimated on the basis of a model which assumes that the head groups are located on the 0.H.p. would clearly not be the same. Plots of interfacial capacity against electrode potential for varying surface concentrations of the ionizable head group in the S A M are shown in Figure 4 for a solution pH of 2 and ionic strength equal to 0.01 M. Variation in r T could be achieved experimentally by coveringthe electrode with two molecules with the same anchoring group and of the same length, but with one of the molecules terminatingin an inert nonionizable group. The potential a t which the film is half ionized moves in the positive direction with increase in rT as indicated by the positive shift in the capacity maximum. This also indicates that the field required to ionize this fraction of the head groups becomes increasingly positive. The capacity against potential data for the case that there are no ionizable head groups (rT = 0) is also shown. In this system a small minimum is seen on the capacity curve corresponding to the minimum in the diffuse layer at the P.Z.C. It is also interesting to examine the dependence of the electrode charge density on electrode potential for the same set of
Lungmuir, Vol.10,No. 7,1994 2407
Double Layer Effects n
I
a
5 50
.
30
-
N
6 LL
a
\
v
10 -0.6 iij
-0.4
-1.2
-0.2
0.2
0.6
0.4
E/V
Electrode Potential, 9”/ V
Figure 5. Surface charge density, am,plotted against electrode potential p for the systemsconsidered in Figure 4. The central “0” on the plot at zero concentration of the ionizable head groups indicates the point of zero charge. I
I)
I
1 .o
E LL
4
\ U
.-5U a
0
0.5
28
0
0
8 -1.5
-0.5
Electrode Potential,
0.5
1.5
em/ V
Figure 6. Differential capacity, C (left hand ordinate), and fraction of the head groups ionized 0 (right hand ordinate) plotted against the electrode potential 4” according to the and the model of Smith and White4(0). The present model (0) mol cm-2, coverage due t o the ionizable head group is 8 x the pK, of these groups, 2, and the pH 2. The capacity of the organic layer K1 is assumed t o be 15 pF cm-2 in both models. conditions. From these data, which are presented in Figure 5, there are two important observations. First of all, the P.Z.C.shifts in the negative direction as the surface concentration of ionizable groups increases. This is precisely what one would expect since ionization of the head group produces a negative charge which is anchored on the i.H.p. The second observation is that the electrode charge density is independent of surface coverage r T a t sufficiently negative potentials. Thus, if one integrates the capacity data obtained in the absence of ionizable head groups to the very negative range of potentials, one obtains a value of the charge density which can be used to back integrate all other capacity curves. This provides a means of further analyzing real experimental data for which the P.Z.C. is not known. Capacity against electrode potential data were also calculated using the model proposed by Smith and White.4 In these calculations, the integral capacity of the S A M was set equal to 15pF cm-2 and the 0.H.p. assumed to be coincident with the plane through the centers of the head groups. Results from the Smith and White model4 are compared with those from the present model in Figure 6 for the case that r T is 8 x mol cm-2, the pH 2 (c, = 0.01 MI, and pKa, 2. The predicted capacity is low and almost constant over the potential range where large changes in capacity are found by the present treatment. At the same time, the degree of ionization of the S A M is much less for the same electrode potential according to
Figure 7. Differential capacity as a function of electrode potential measured against a calomel electrode (0.05 M KCI) for a single crystal gold electrode (Au(100))in 0.01 M HClOl (O), and for the same electrodein this solutionafter modification by a monolayer of 2-mercaptoethanesulfonic acid (A). the model by Smith and Whitee4As a result extremely high charge densities, beyond those normally found experimentally, are required to induce complete ionization when this process is associated with diffuse double layer effects only. Thus, it is concluded that their model is not capable of predicting capacity changes of the magnitude found by the present model on the basis of a reasonable set of interfacial parameters.
Discussion The important contribution ofthe earlier work by Smith and White4is the recognition that the potential-dependent ionization process at the S A M contributes to the differential capacity of the electrode/solution interface. The present calculations have shown that the important feature of this contribution is a maximum in the capacity whose height and shape depend on the dielectric properties of the S A M and the region between this monolayer and the diffuse layer. Other important features of the system are the pH and ionic strength of the electrolyte solution and the pK, of the ionizable head group in the SAM. On the basis of earlier work a t the mercury/aqueous solut i ~ n , ~ JoiVaqueous O solution interfaceqll and the present study, it is important to include a Stern layer or region where the water molecules are significantly perturbed from their bulk structure in developing a model for the interfacial region. Very few experimental data with which a comparison can be made have been reported in the literature. Bryant and Crooks6 recently reported capacity data at gold electrodes modified with 4-mercaptopyridine and 4-aminothiophenol as a function of electrode potential and solution pH. The changes in capacity observed for these systems were much smaller than those estimated here. This may be due to the fact that the S A M consisted of a pyridine ring or a phenyl ring rather than hydrocarbon chains which have been used in other ~ t u d i e s .Capacity ~ data obtained recently in this laboratory for a single crystal gold electrode (Au(100))before and after a modification by a S A M containing an ionizable head group are shown in Figure 7. The data were obtained in 0.01 M HC104 and the S A M formed by 2-mercaptoethanesulfonic acid (MESA). The features of the capacity curve for the unmodified electrode reflect mainly the variation of the inner layer capacity due to reorientation of the water dipoles with change in the charge density on the electrode. The deep minimum at -0.05 V is due to a minimum in the diffuse layer capacity and gives an approximate indicate of the P.Z.C. if ionic adsorption is absent. When
2408 Langmuir, Vol. 10, No. 7, 1994
Fawcett et al.
the electrode is modified by the MESA monolayer, a dramatic change is observed in the interfacial capacity. The capacity is generally lower in most of the potential range but a high capacity maximum is observed at a potential close to 0.15 V. The capacity maximum is much sharper than those estimated by the model here, but the magnitude of the maximum is approximately the same. Although the present model is reasonably successful, it must be kept in mind that there are important shortcomings in it as it is applied here. "he most important defect is the fact that the integral capacity of the layer of water molecules in the Stern region (Kz)is assumed to be constant, and rather low (40 pF cm-2). Although this may be Satisfactorywhen the charge density on the i.H.p. is low, one definitely expects the integral capacity to increase at high charge densities. This phenomenon is well-known from studies of the adsorption of anions1and can be attributed to the effect of the charged head groups on the orientation of water molecules in the Stern layer. Although one could modify the model to include a varying value ofK2, it does not seem worthwhile without some experimental indication of the range over which this quantity changes. The second feature of the cut-off disk model used here which needs to be examined more carefully relates to the relative values of the cut-off disk radius ro and the thickness ,4 of the SAM. Equation 12 used above was derived for the condition that ro >> ,4 so that its use at higher values of a, can be questioned. However, the most important feature of the imaging conditions which lead to eq 12 is that the charged head group be between two conducting media in a region of low dielectric constant. Under these conditions the parameter g discussed above is equal to 1. Because the model does not include variation in the integral capacity KZwith a,, it was considered not worthwhile to use the more detailed models for g which are a ~ a i l a b l e . ' ~ JThe ~ molecule MESA used in our experimental work is not large, so that for this S A M is -0.5 nm. However, other systems used to form SAMs have usually involved much longer alkyl chains4 so that the question of the role of the distances ro and ,4 in determining the discreteness-of-charge potential needs to be considered in more detail. The values of K1 and K2 chosen to generate the capacity curves are those that give a result which is similar to that
c:
observed experimentally. Variation in these parameters would change the value of the estimated capacity as can be seen from eq 26. "he value of K can be estimated from the properties of the S A M without ionizable head groups in a potential region far from the p.2.c. Separation of K into its component parts KI and KZ is more difficult especially when one expects KZto depend on the degree of ionization of the head groups. Other recent discussions of double layer effectsat SAMs have included the case of a molecular film terminating in an electroactive redox coupl$ and redox reactions involving bulk solution speciesa4 On the basis of the present analysis, these effects will be quite different for the two systems. When the redox couple is anchored at the end of the S A M ,it is expected to be in a region of low dielectric permittivity so that the local potential is affected by the properties of the Stern layer at the SAM. This means that discreteness-of-charge effects should be important for these rea~ti0ns.l~ On the other hand, when the redox couple is only present in the bulk of the solution, the reaction site for most systems is expected to be further away and closer to the 0.H.p. The case of the anchored redox couple is especially interesting because the concentration of the reactant in the experiment is a chosen independent variable. Thus, the study of double layer effects for these systems should help to elucidate some of the details of these effects which have been considered earlier in the 1iterat~re.l~ In conclusion, the study of the double layer effects at SAMs containing ionizable head groups provides an interesting new way of examining phenomena which have been discussed in the literature some time This is expecially true because techniques for examining the structure of these monolayers at the molecular level are now available. Further studies in this area should help elucidate the role of the dielectric properties of these systems in determining processes which occur at their boundary.
Acknowledgment. Helpful discussionswith Professor Henry White are gratefully acknowledged. This work was supported by the Office of Naval Research, Washington, (16)Fawcett, W.R. Can.J. Chem. 1861, 69, 1844.
