mercury interface

311

J. Electroanal. Chem, 249 (1988) 311-319 Elsevier Sequoia S.A., Lausanne - Printed

in The Netherlands

THE TWO-DIMENSIONAL CONDENSATION OF 2-METHYL+%DIHYDROXY-PYRIMIDINE AT THE WATER/MERCURY INTERFACE

CHRISTOS

KONTOYANNIS

and ROBERT

DE LEVIE

Chemistry Department,

Georgetown University,

(Received

1988; in revised form 4th March

7th January

Washington, DC 20057 (U.S.A.) 1988)

ABSTRACT The named compound exhibits two-dimensional differ in capacitance, charge density and inhibiting potentials.

condensation leading to two condensed properties, yet can be made and studied

films which at the same

INTRODUCTION

As first reported by Vetterl [l-3], a number of purines and pyrimidines form condensed films at the water/mercury interface; for a recent review including extensive references, see ref. 4. In this paper we report the formation of two distinct condensed phases by 2-methyl-4,6-dihydroxy-pyrimidine (MDHP), as illustrated in Fig. 1. 0.3 C/Fmm2 0.2 ..

0 -0.2

-0.4

-0.6

-1.0

-0.8 E

/ V vs Ag/AgCl

Fig. 1. The interfacial capacitance of a hanging mercury drop in aqueous 0.1 M NaCl buffered with 0.1 M sodium acetate and 0.1 M acetic acid (pH 4.65), containing 7 mM 4,6-dihydroxy-2-methyl-pyrimidine. The curve was measured at a single hanging mercury drop, starting from -0.2 V (vs. an internal Ag/AgCl electrode) at a scan rate of - 6 mV s-‘, and returning at + 6 mV s-‘. Temperature 20 o C; ac signal used: 1.19 kHz at 1.5 mV amplitude; drop area 1.05 mm’.

0022-0728/88/$03.50

0 1988 Elsevier Sequoia

S.A.

312 EXPERIMENTAL

The water used was pyrodistilled. All other chemicals were used as received: MDHP (Aldrich) and hexaminocobalt(II1) chloride (Sigma), NaCl (Suprapur, MCB), sodium acetate and acetic acid (Mallinckrodt), KH,PO, and Na,HPO, (Baker Analyzed). All capacitance measurements were made on a Princeton Applied Research 303 hanging mercury drop electrode, in a thermostated three-electrode cell with a Ag/AgCl reference electrode and a Pt wire as the auxiliary electrode. The potential was applied with an ac polarograph of conventional design, using a commercial oscillator and lock-in amplifier. The ac polarograph was used either as a stand-alone instrument or in conjunction with a Digital Equipment Corp. PDP 11-20 minicomputer. All solutions were deaerated with presaturated nitrogen. EFFECTS

OF TEMPERATURE

AND CONCENTRATION

On scanning the potential, the transitions leading into and out of the pit regions exhibit hysteresis. The potentials of the transitions from one phase to another. depend strongly on scan rate only when the capacitance decreases during the transition, i.e., A-to-B, B-to-C and D-to-C, see Fig. 1. Apparently, these transitions involve nucleation. In contrast, the potentials of the transitions in the reverse direction, B-to-A, C-to-B and C-to-D, are virtually independent of scan rate, as one would expect when they involve growth only, and will be called Etl, Et2 and Et3 respectively. The values for Etl, Eti and E,, were determined as the extreme potentials at which, coming from the lower capacitance, one can still go back to the lower capacitance value without hysteresis. The dependence of the transition potentials E,, and Et3 on temperature T is shown in Fig. 2. Clearly, the capacitance pits become progressively narrower as the temperature increases, and E,, and Et3 fit a common quadratic dependence on potential, with a maximum at E = E *. ‘IYhe value of E * is virtually independent of ~ncentration, while the maximum temperature T* for a given MDHP concentration appears to be a linear function of In c. The corresponding dependence of the transition potentials Et2 on temperature is linear over the limiting temperature range over which they can be observed, see Fig. 3. Similarly, there is a quadratic dependence of the transition potentials Et, and E,, on In c. Here, E * is virtually independent of temperature T, while In c* appears to be a linear function of T. All the above data were obtained on mercury in contact with an aqueous solution of 0.1 M NaCl + 0.1 M NaAc + 0.1 M HAc containing from 1.25 to 7 mM MDHP, at temperatures between 4 and 2O*C. A recent two-dimensional two-state Ising model IS] predicts that (A Et)* - BT - C should be a linear function of kTln c, where the pit width is defined as AE, = E,, E t3, B and C are adjustable parameters, and k, T and c are the Boltzmann constant, absolute temperature, and the adsorbate concentration (in M), respectively. An unweighted linear least-squares fit for all 66 experimental AE, values is shown in Fig. 4. The good fit shows this simple description of the phase transitions

313 320 7 ..

t PC

T/K

E / V vs Ag/RgCi

Fig. 2. The dependence of the transition potentials E,, and Et3 on temperature, at constant MDHP concentration c as indicated in mA4 with each curve. All data were obtained on mercury in contact with aqueous solutions containing 0.1 M NaCl +O.l M HAc+O.l M NaAc plus 1.25 to 7 mM MDHP.The parabolas were drawn using unweighted quadratic least-squares fits to the data for each concentration.

tl and t3 to be fully adequate. The inside transition potentials Et2 between two condensed films, shown in Fig. 3, cannot be expected to fit that model, because the Ising model used [S] does not consider transitions between two condensed phases. From the slope A of the line in Fig. 4 one can estimate the change in polarizability upon replacing solvent molecules in the interface by adsorbent. We obtain a value of - 3 X 10w41 E: F m* or, in c.g.s. units, of -0.3~: A3, which is of a

280

-d.2

-0.4

-0.6

-0.8

-1.0

_I 7 ..L

E / V vs Ag/AgCl

Fig. 3. The dependence of the transition potentials Et2 on temperature, at constant MDHP concentration c as indicated in mM. All data were obtained on mercury in contact with an aqueous solution containing 0.1 M NatI3 +O.l M NaAc+O.l M HAc and 2.5 to 7 mM MDHP.The parabolas repeating those of Fig. 2 at the same MDHP concentrations, help to form an equilibrium phase diagram.

I

-2.2 -2.6 AE:-BT- C /V*

-3.0

Fig. 4. Plot of ( Et3 - E,t)2 - BT - C vs. kT In c. The line shown was calculated as ( Et3 - E,1)2 - BT C = AkT In c, with A = 5.49X10” F-‘, B = -2.29~10~~ V2 K-l, and C = 9.85 V2.

plausible order of magnitude, since the value of the relative dielectric permittivity Er for the Helmholtz layer is usually assumed to be of the order of 10. THE DEPENDENCE OF PIT WIDTH ON pH

The capacitance-potential curves of a number of 7 mM MDHP solutions buffered at different pH values using phosphate buffers at constant ionic strength [6] plus 0.1 A4 NaCl are shown in Fig. 5. These results suggest that neutral MDHP rather than its anion is responsible for the pit formation. Potentiometric titration of MDHP shows its pK to be 6.25. Consequently, at pH 4.65 as used in the earlier-described experiments, only about 2.5% of MDHP is in the anionic form, 0.3 C /Fmee 0.2

0 -0.2

-0.4

-0.6

-0.8

-1.0

E/V

vs Ag/AgCl

-1.2

Fig. 5. Capacitance vs. potential curves for buffered solutions containing 7 mM MDHP and 0.1 M NaCl plus constant ionic strength phosphate buffers of different pH values: (a) 4.40, (b) 4.85, (c) 5.75, (d) 6.15, (e) 6.40, (f) 6.60 and (g) 7.45. All traces were obtained by scarming at +3 or -3 mV s-’ away from - 0.80 V (vs. an internal Ag/AgCl electrode), in order to avoid the hysteresis loops. All other conditions as listed with Fig. 1.

315

10-6[H’]-? M Fig. 6. A plot of c,@IA] - 1 vs. l/[H+ 1, where e = [HA] + [A- J is the analytical concentration of MDHP, [HA] is the concentration of neutral MDHP estimated From the pit width in Fig. 5, and [H” ] is the proton concentration calculated from the pH. The slope of the least-squares line yields the dissociation constant K = 5.7 X lo-’ M.

and its effect on a logarithmic concentration scale is virtually negligible. We can therefore use the quadratic dependence of pit width AEc on In c to check whether the observed decrease in pit width with pH indeed reflects the deprotonation of MDHP. By combining the pit widths shown in Fig. 5 with those of Fig. 2 we estimate the concentration [HA] of the neutral MDHP molecules. Since the pH and the total analytical concentration c = [A-] + [HA] of MDHP are known, and assuming that K= [Hf][A-]/[HA], a plot of [A-]/[HA] = c/[HA] - 1 vs. l/[H+] should yield a straight line with slope K. Figure 6 shows the resulting plot, which yields a value of K= 5.7 x lo-? M or pK = 6.23, in close agreement with the result (6.25) of the potentiomet~c titration. This provides quantitative confi~ation that the pit is due to the neutral form of MDHP. MAPPING THE CAPACITANCE

PITS OF AN AQUEOUS MDHP SOLUTION

In Fig. 1 one can notice clearly the two capacitance pits, and the wide hysteresis loop in the region between B and C. One can map the capacitance in that region by voltage step experiments, in which one jumps the applied voltage from a value outside this hysteresis region to one inside it, and mo~tors the resulting change in capacitance. Upon jumping into the hysteresis region, the capacitance changes instantaneously (given the data rate used, 100 points per second) to a value which depends on whether the jump originated in region A or B, as illustrated in Fig. 7. For a potential step starting from region A, the mean capacitance value for the last 4 s of the experiment, 0.069 F m-‘, is lower than the corresponding capacitance of 0.087 F me2 for a potential step originating from region B. The latter is metastable and will, eventually, show a transition to the lower capacitance.

316

0.31

. +.%-J

C /Fmm2

t

b a

o.i.i.j.,.5

i

i/S

Fig. 7. Capacitance transients following potential steps (a) from -0.2 to -0.7 V and (b) from -0.3 to -0.7 V, versus an internal Ag/AgCl electrode for 7 mM MDHP in aqueous 0.1 M NaCl buffered with 0.1 M sodium acetate and 0.1 M acetic acid at pH 4.65. Other conditions as in Fig. 1. The mean capacitance values over the last 4 s of the experimental traces shown are (a) 6.88 X lo-* F m-* and (b) 8.74 x lo-* F m-*.

CHARGE

DENSITY

MEASUREMENTS

The existence of two distinct pit regions is confirmed by charge density measurements, based on the analysis of the time-dependence of the polarographic background current [7,8]. The resulting interfacial charge density for a 5 mM solution of MDHP is shown in Fig. 8. The two, clearly distinct data sets in regions B and C corroborate that, indeed, two different pits exist. The two curves are, within experimental accuracy, linear. Their slopes should yield the capacitance of the two

-0.1

..... ‘.

WC mm2

I

0.1 -0.25

J -0.50

-075

-1.00

E / V vs Ag/AgCl Fig. 8. The interfacial charge density for mercury in contact in 0.1 M NaCl+O.l M sodium acetate+ 0.1 M acetic polarograpbic baseline current [7,8].

-1.25

with an aqueous solution of 5 mM acid at 20 o C, based on analysis

MDHP of the

317

regions. The slope of the data in region B, from -0.34 to -0.46 V, yields 0.09 F m-*, while the slope of the second line segment, between - 0.475 and - 1.09 V, is 0.064 F rnv2, in approximate agreement with the direct measurements of pit capacitance. KINETICS OF FILM CONDENSATION

Avrami plots for single- and double-pulse experiments (the latter with a first pulse to - 350 mV for 10 ms) for potential steps from region A to B gave a slope of 3 for the former (“progressive” nucleation) and a slope of 2 for the latter (“instantaneous” nucleation), while the Avrami plots for potential steps from region B to C gave increasing slopes (from 2 to 4) as the final potential of the pulse experiment was made more negative, somewhat similar to the behavior reported for pit-to-pit transitions in concentrated solutions of thymine 191. EFFECTS

ON ELECTRODE

KINETICS

The eIectro-reduction of several compounds was studied by dc polarography, in the presence and absence of MDHP. In a number of cases, such as in the reduction of Cd2+, Tl+ and Ru(NH&+, the presence of either film causes no perceptible change in the polarographic wave while, in others, such as V3+ and Cr3+, the polarographic wave was suppressed totally in the pit region. Only two of the exhibited a partial in~bition of their compounds studied, 4 and Co(NH,)z+, polarographic waves. In the case of Co(NH,)z+, aqueous solutions of 1 mM Co(NH,)z+ containing 0, 4, 5.5 and 7 mM of MDHP buffered at pH 4.65 were used. Potential steps were applied from 0.0 V (region A) or -0.3 V (region B) to potentials in the hysteresis region. The initial potentials of 0.0 and -0.3 V were selected because, at these potentials, no appreciable faradaic current would flow before application of the potential step. The current in a potential-step transient was sampled at 1 ms intervals and analyzed, after omitting the first 10 points immediately following the potential step (which still contained a significant contribution of the charging current) by simple linear regression of the current i versus the square root of time t, to yield the extrapolated initial faradaic current i,=, at the moment of application of the pulse [lo]. Note that, under these conditions, the phase transitions are instantaneous at the time-resolution of our experiments and, therefore, do not distort the faradaic current-time transients. Over a range of about 0.2 V, one can form and maint~n either film, merely by selecting the appropriate initial potential, even though the higher-capacitance film is metastable and will eventually give way to the lower-capacitance state. This opens the possibility of comparing the inhibiting properties of the two films for the same faradaic process and at the same potential. The results are shown in Fig. 9. Note that, for the data shown by filled circles, there is a monotonic decrease of reduction rate with increasing MDHP concentration, while the data shown as open circles are

318 -54 -5.6..

W

i,,,lA)

‘0

-5.8

0 G

+

.-

(

+

$ 5;;o

-6.0.. -6.2 -0.3 Fig. 9. The logarithm of the NaCl +O.l M NaAc+ 0.1 M obtained after a potential step and the same starting from Temperature 20 o C.

-0.4

-0.5

.

.’

‘.

l

.

l7

$5 l

l

.

l

.

-0.6 E/VvsAg/AgCI

-0.7

initial reduction current ~,,a, in A, of 1 mM Co(NH&+ in 0.1 M HAc, as a function of electrode potential (in V vs. internal Ag/AgCl), from - 0.3 V to the potentials shown, for 4, 5.5 and 7 mM of MDHP (0) 0.0 V (0). The data shown as (+) are for the absence of MDHP.

rather insensitive to a change of the adsorbate concentration. This observation in the presence of the film with the suggests that the reduction of Co(NH,)i+ higher capacitance involves direct contact with the mercury, as it apparently requires that work be done to make a “hole” in the film [ll]. The same does not in the presence of the other film. seem to apply to the reduction of Co(NHj)z+ Results obtained for the reduction 0, showed the same pattern of behavior. One can estimate the approximate areas of these holes by plotting the logarithm of the initial current i,,, versus the logarithm of the MDHP concentration [ll]. This yields areas of 1.0 and 0.1 nm2 ion-‘, respectively, for the data shown as filled and open circles respectively in Fig. 9. Since the minimum cross-sectional area of a cobalthexammine ion is of the order of 0.6 nm2, its reduction through the film formed with potential steps starting from region A either proceeds via electron tunneling through the film or involves preexisting pinholes. The above data establish that two condensed films, one stable and the other metastable, can be studied over the same interval of applied potentials. These two films differ in their interfacial capacitance and charge density and, also, in their inhibiting properties. ACKNOWLEDGEMENT

This study was supported by NSF grant CHE-6807361. REFERENCES 1 2 3 4 5 6

V. Vetterl, Experientia, 21 (1965) 9. V. Vetterl, Collect. Czech. Chem. Commun., 31 (1966) 2105. V. Vetterl, J. Eleetroanal. Chem., 19 (1968) 169. R. de Levie, Chem. Rev., 88 (1988) 599. R. Sridharan, R. de Levie and S.K. Rangarajan, Phys. Chem. Lett., 142 (1987) 43. G.D. Christian, Analytical Chemistry, 2nd ed., Wiley, New York, 1977, p. 222.

319 7 8 9 10 11

J.N. Butler and M.L. Meehan, J. Chem. Phys., 69 (1965) 4051; 70 (1966) 3582. R.J. Atwell, Jr., R. Sridharan and R. de Levie, J. Electroanal. Chem., 194 (1985) 143. R. Sridharan and R. de Levie, J. Electroanal. Chem., 230 (1987) 241. H. Gerischer and W. Vielstich, Z. Phys. Chem. N.F., 3 (1955) 16. R. Srinivasan and R. de Levie, J. Electroanal. Chem., 201 (1986) 145.