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Adsorption of Ortho-, Meta- and Para-toluidines from KI solution
Electroanalytical Chemistry and Interfacial Electrochemistry, 54 (1974) 371~78 ~-? Elsevier Sequoia S.A., Lausanne
371
Printed in The Netherlands
A D S O R P T I O N OF ORTHO-, M E T A - AND P A R A - T O L U I D I N E S F R O M KI SOLUTION
K. M. JOSHI, S. I. MAHAJAN and M. R. BAPAT
Department of Physical Chemistry, Institute of Science, Bombay-400032 ( lndia) (Received 18th February 1974; in revised form 2nd April 1974)
INTRODUCTION
The adsorption of aniline at the mercury-solution interface has been investigated in detail by several workers 1~. Interesting observations like contribution of dipole~tipole interaction energy and influence of base electrolyte 2 have been made. Comparatively the toluidines do not appear to have been investigated. Capacitance and electrocapillary data for o-, and p-toluidines in sodium sulphate and H2SO 4 solutions have been reported by Damaskin et al. 5. They observed a change in orientation of adsorbed molecule as charge on the mercury changed sign. The present investigation deals with the adsorption of o-, m- and ptoluidines from potassium iodide solution using the capacitance method. The influence of substitution and base electrolyte is also shown. EXPERIMENTAL
A symmetrical a.c. bridge as described elsewhere 6 was used employing a dropping mercury electrode. The capillary used was according to a design by Randles 7, that is, it was constricted in the middle so that the rate of flow was controlled and ended in a small bulb which was drawn out into a fine capillary with thin walls. This minimized the screening effect due to the glass of the capillary. The D M E was coupled to a cylindrical platinum gauze electrode and the impedance of this system was determined using superposed a.c. of 10 mV amplitude. An electronic timing device detached the drop at a fixed time from its birth and the bridge null was obtained at the end of this period. Frequencies from 50 Hz to 4000 Hz were employed and the capacitance reported is the extrapolated capacitance at zero frequency. A 5-stage transistorized amplifier with an oscilloscope was used in the detecting arm. The capacities reported are correct to + 1°/. Interracial tensions at electrocapillary maximum (e.c.m.) were obtained on a Lippmann capillary electrometer and the potentials of e.c.m, were obtained using a streaming electrode as described by Grahame 8. A saturated calomel electrode was used as a reference electrode during capacitance measurements. All measurements were carried out using a conventional three compartment cell thermostatted at 34_+ 0.2"C. The solutions were deaerated using purified nitrogen.
372
K. M, JOSHI, S. I. MAHAJAN, M. R. BAPAT
Chemicals Ortho-, meta- and para-toluidines were of B.D.H. Analar grade. These were purified by distillation under low pressure and only middle fractions were collected. The purity was checked by measuring their boiling points. RESULTS AND DISCUSSION
Differential capacitance of the mercury-solution interface was measured for various concentrations of o-, m- and p-toluidines using 0.1 M KI as base electrolyte. Differential capacities for various concentrations of KI solution containing a fixed concentration of organic amine were also determined for a potential range of -500 to -1500 mV against saturated calomel electrode. The equipment was calibrated by comparing the various values obtained with those reported by Grahame 9. After the necessary correction for the potential of the reference electrode and temperature, it was observed that there was an excellent correspondence between the two sets of values. Capacitance-potential curves for six concentrations of o-, m- and p-toluidines in 0.1 M KI are shown in Figs. 1-3, respectively. The chief feature is the lowering of differential capacity from its value in the base electrolyte in the vicinity of maximum adsorption. The differential capacity curves for aniline and three substituted anilines show a peak at the negative end of l
I
1oo
IOO
80
,~ •
I
,O,IM KI
6o
~4
"
7~
/ ~Y,4
5 ta.
~o (o
20
- 400
-
lo00
E/mV
--
160o
I
I
I
-500
-1000 E/mY
-~500
Fig. 1. Differential capacity as a ftmction of electrode potential (with respect to saturated calomel electrode) at mercury-solution interface for 0.1 M KI plus various concentrations of o-toluidine at 34°C. (1) 0.005 M, (2) 0.0075 M, (3) 0.01 M, (4) 0.015 M, (5) 0.02 M and (6) 0.025 M o-toluidine. Fig. 2. Differential capacity a s a function of electrode potential (with respect to saturated calomel electrode) at mercury-solution interface for 0.l M KI plus various concentrations of m-toluidine at 34'C. (1) 0.0075 M; (2) 0.01 M; (3) 0.015 M; (4) 0.02 M; (5) 0.025 M and (6) 0.03 M m-toluidine.
A D S O R P T I O N O F o-, m-, p-TOLUIDINES F R O M KI I
373
I
100
80 ),IM
KI
6O (2 LL ca
40
20
[
I
-1000
-1500
0
-500
E/rnV
Fig. 3. Differential capacity as a function of electrode potential (with respect to saturated calomel electrode) at mercury-solution interface for 0.1 M KI plus various concentrations of p-toluidine at
34°C. (1) 0.0075 M, (2) 0.01 M, (3) 0.015 M, (4) 0.02 M, (5) 0.025 m and (6) 0.03 m p-toluidine.
the curve which is frequency dependent. The small kink at - 5 0 0 mV is also frequency dependent. The region between the peaks shows little effect of frequency of superposed a.c. signal. The capacity curves were integrated using the interfacial tension at e.c.m. and the value of the potential at e.c.m., determined separately as described. Thus the charge on the metal (qu) and interfacial tension (7) are given as E
qM
-
=
C dE_ ~e.c.m.
and ME 7 =
7 ......
-
fE
-
_
Ee.c.m.
CdE
z-
ge.c.m.
A typical set of interfacial tension/potential curves for one single system is brought out in Fig. 4. The curves converge into one another at extreme negative potentials showing desorption at these potentials while adsorption of organic appears to continue right to the far positive region investigated. The potential of e.c.m, shifts to negative potentials in conformity with the behaviour of aromatic compounds 1°. The correspondence of these twice integrated capacity curves with the ones obtained from direct electrocapillary measurements was checked and it was observed that in the region of e.c.m., the correspondence was better than 0.2 dyn cm-1 while at extreme positive potentials, the electrocapillary curves were lower than the twice integrated differential capacitance curves by about one dyn cm-1.
374
K. M. JOSHI, S. I. MAHAJAN, M. R. BAPAT
The relative surface excess of the organic was obtained at different values of surface charge. Figure 5 represents one such set of curves. It is pertinent to note that in this case, the F / q ~ curves show a very near plateau beyond qM> 0, showing that at positive charge, the amount adsorbed is nearly independent of the charge but depends on the base solution concentration. This figure brings out clearly the influence of the interaction of the delocalized g-electrons with the charge on the metal. 4- 20
I
I
1 2 I
I
r
I
0
+4.
8 390
360
330
i i
-- ,500
~ --
1000
E/mV
- 8 --1500
-4
c~M/#C crfi2
Fig. 4. Interfacial tension against potential (with respect to saturated calomel electrode) for mercurysolution interface for 0.1 M KI plus various concentrations of p-toluidine at 34°C. (1) 0.1 M KI, (2) 0.0075 M, (3) 0.01 M, (4) 0.015 M, (5) 0.02 M, (6) 0.025 M and (7) 0.03 M p-toluidine. Fig. 5. Relative surface excess as a function of charge density for 0.1 M KI plus various concentrations of p-toluidine at 34°C.(V) 0.0075 M, (V) 0.01 M, ( 0 ) 0.015 M, (Q)) 0.02 M, ( 0 ) 0.025 M, and ([~) 0.03 M p-toluidine.
The influence of base electrolyte on F/cI~ curves can be seen by comparing a similar set of results obtained using 0.1 M KC104 as base electrolyte 1. A larger degree of adsorption for the same concentration of the adsorbate and at the same q value is observed for KC104 than for KI (see Table 1). The behaviour of aniline which is similar to the toluidines in these two base electrolytes has been reported elsewhere 2. Because of the greater adsorption of I compared to ClOg, a larger free surface is available for organic adsorption in the latter case. However, the influence of base electrolyte appears to be specific with the system investigated. Thus Trasatti 11 found no influence on the adsorption of ethylene glycol from NaF, KC1, KBr or KI solutions while a base electrolyte effect was observed by Joshi and co-workers 12 in the adsorption of lower homologues of aliphatic acids.
+8
ADSORPTION OF o-, m-, p-TOLUIDINES FROM KI
375
TABLE 1 COMPARISON OF THE INFLUENCE OF BASE ELECTROLYTE IN THE FORM OF KCIO4 AND KI ON F/q M DEPENDENCE Charge density q/IzC cm 2
+8 +6 +4 +2 0 - 2 - 4 -6
F /mol cm- 2 0.01 M o-toluidine in
0.01 M m-toluidine in
0.01 M p-toluidine in
K CIO 4
KI
K CI0 4
KI
K CIO 4
KI
3.55 3.60 3.65 3.65 3.60 3.40 2.88 --
3.00 3.05 3.08 3.10 3.08 2.88 2.22
3.70 3.85 4.00 4.10 4.10 3.55 2.30 1.75
3.08 3.20 3.30 3.40 3.35 3.05 --
3.15 3.25 3.50 4.00 3.70 2.90 2.05 1.55
2.18 2.25 2.325 2.375 2.375 2.05 1.45 0.70
-8
l
I
I
200
o []
oo o o
o m
o D
o o
>
..~E-200 w
-4oo
I
I
I
2
4
6
-600
o
r~,
Fig. 6. Esin a n d
8
,o'°/.oL c~ z
Markov
plots for 0.1 M
KI
plus various concentrations of p-toluidine at 34"C.
(V) - 6 # C c m 2, (0) - 4 # C c m z,(m) - 2 p C c m - 2 , ( x ) 0 , u C c m -2,(r7) +2 /~Ccm -2,(C)) +4 /~Ccrn 2and(V) +6/~Ccm -2.
Esin and Markov
plots
I n Fig. 6 is shown the shift in the p o t e n t i a l across the d o u b l e layer due to the a d s o r p t i o n of the organic molecules at c o n s t a n t charge (calculated as [ E - E o ] at c o n s t a n t q, Eo being the p o t e n t i a l in base solution at that charge) with the a m o u n t of organic adsorbed. These plots indicate the c o n t r i b u t i o n to the p o t e n t i a l in the d o u b l e layer due to the a c c u m u l a t i o n of a d s o r b e d dipoles at the interface, the dipoles m a y be p e r m a n e n t or i n d u c e d ones. A salient feature of these plots is that the slopes increase r e m a r k a b l y at large negative values of qu
376
K.M. JOSHI, S. I. MAHAJAN, M. R. BAPAT
while at positive qM values, there is no shift in the potential even when the amount of adsorbed organic increases at the interface. This contrasts with the behaviour of other organic molecules like aliphatic acids13. Pyrazine 14 shows a symmetric change of slope about the charge for maximum adsorption while pyridine shows change in the sign of the slope at all values of qM. This behaviour is associated with the orientation of the adsorbed molecule which (in the present case due to the hydrophilic character of the - N H 2 group) is adsorbed in an upright position with the -NHz group on the solution side. At large positive charge, the molecule presumably is drawn towards the electrode due to the negative charge cloud interaction of the benzene ring with the metal charge. The dipole now acts in a direction perpendicular to the field and hence the contribution to the field due to adsorption is virtually nil in this position resulting in an insignificant (AE)q with F.
Standard free energy of adsorption (AG °) Standard free energy of adsorption of the organic compound was evaluated using the Bockris-Swinkels 15 isotherm. The total free energy of adsorption can be considered to be made up of a "chemical" free energy term independent of q, a free energy of interaction of adsorbed molecule and free energy of replacement of oriented solvent molecules. At constant value of charge, the contribution to the free energy of adsorption due to the solvent orientation term is given by exp
{(nR*/kT)(tZwX-R*EC*)}
where n refers to the number of molecules displaced by a single organic molecule, R*=(NT-NJ,)/NT, X is the field strength, E is the dipole~tipole interaction energy, C is the coordination number and k and T have the usual significance. Bockris and co-workers observed that maximum adsorption for aliphatic compounds occurs at - 2 pC cm-Z from which it was concluded that even at q=0, water is preferentially oriented. At q = - 2 #C cm-2, the net orientation R* becomes zero. It e3/2 0 i
0.2 ]-
0.4
0.6
i
5 "7
q=-2
-6 E
_~3
IO< 3
o
L
t
o:2
I
G
0:4
i
o:6
Fig. 7. Standard free energy of adsorption at q= - 2 pC cm -2 as a function of 0~ for (O) o-toluidine, (V) p-toluidine and (11) m-toluidine and as a function of 0 for (O) o-toluidine, ( 5 ) m-toluidine and (V) p-toluidine.
ADSORPTION OF o-, m-, p-TOLUIDINES FROM KI
377
would be therefore interesting to study the variation of ~G ° with 0 at q= - 2 #C c m - 2 , so that the variation in ~ o will be due to mutual interaction effects only. In order to evaluate this, 0, the fraction of surface covered, was evaluated a s /"//"max, /"max corresponding to saturation coverage obtained from an estimate of cross-sectional area of the molecules obtained from the knowledge of bond angles and bond lengths 16-a8. /"~x for o-, m-, and p-toluidines were obtained as 9.4 × 10 -1°, 9.4 × 10 - l ° and 9.7 × 10 -1° mol c m -2, respectively. The dipole~lipole interaction in the case of rigid monotayer following the Blomgren-Bockris model 19 is proportional to 0~ at not too high coverage, while following the Frumkin 2° model, the proportionality appears to be to the first power of 0 where the slope of A-G°/0 curve represents an average interaction energy. Figure 7 shows the plots of ~ o against 0~ and 0. It is obvious from the plots that although a straight line relationship is obtained, these plots cannot distinguish between the two isotherms 21. Both the plots, however, give the same value of A-~o at 0= 0. ACKNOWLEDGEMENT
One of us (S.I.M.) wishes t o thank the C.S.I.R. (India) for the award of Junior Fellowship during the tenure of which this work was carried out. SUMMARY
Adsorption of o-, m - and p-toluidines from O.1 M potassium iodide solution at mercury-solution interface is reported using capacitance method. Surface excess and free energy of adsorption using Bockris-Swinkels isotherm are evaluated. Adsorption energy is shown to depend on the dipole~:tipole interaction energy. Esin-Markov plots are discussed on the basis of change of orientation at positive charges. REFERENCES 1 S. I. Mahajan, Ph.D. Thesis, University of Bombay, 1969. 2 K. M. Joshi, S. I. Mahajan and S. Rajagopalan, Indian J. Chem., 10 (1972) 619. 3 B. B. Damaskin, V. M. Gerovich, I. P. Gladkikh and R. I. Kaganovich, Zh. Fiz. Khim., 38 (1964) 2495. 4 S. L. Diatkina and B. B. Damaskin, Elektrokhimiya, 4 (1968) 1000. 5 B. B. Damaskin, S. L. Diatkina and V. K. Venkatesan, Elektrokhimiya, 5 (1969) 524. 6 G. J. Hills and R. Payne, Trans. Faraday Soc., 61 (1965) 316. 7 J. E. B. Randles, Discuss. Faraday Soc., 1 (1947) 11. • 8 D. C. Grahame, E. M. Coffin, J. I. Cummings and M. A. Poth, J. Amer. Chem. Soc., 74 (1952) 1207. 9 D. C. Grahame, J. Amer. Chem. Soc., 80 (1958) 4203. 10 G. Gouy, Ann. Chim. Phys., Ser. 7, 29 (1903) 145; Ser. 8, 8 (1906) 291; Ser. 8, 9 (1906) 75; Ann. Phys. (Paris), Set. 9, 6 (1916) 5; Ser. 9, 7 (1917) 129. 11 S. Trasatti, J. Electroanal. Chem., 28 (1970) 257. 12 K. M. Joshi, P. K. Jadhav, S. W. Dhawale and S. Rajagopalan, Indian J. Chem., 10 (1972) 279. 13 K. M. Joshi and M. R. Bapat, 14th Seminar on Electrochemistry, Karaikudi, India, 1973. 14 H. P. Dhar0 Ph.D. Thesis, University of Ottawa, 1972. 15 J. O'M. Bockris and D. A. J. Swinkels, J. Electrochem. Soc., 111 (1964) 736. 16 L. Pauling, The Nature o f Chemical Bond and the Structure of Molecules and Crystals, Oxford and IBH Publishing Co., Bombay, 1963.
378
K. M. JOSHI, S. I. MAHAJAN, M. R. BAPAT
17 Fergusen, The Modern Structure and Theory of Organic Chemistry, Prentice-Hall of India Private Limited, New Delhi, 1969. 18 Hand Book of Chemistry and Physics, The Chemical Rubber Co., Cleveland, Ohio, 48th edn., 1967. 19 E, Blomgren and J. O'M. Bockris, J. Phys. Chem., 63 (1959) 1475. 20 A. N. Frumkin, Z. Phys. Chem., 116 (1926) 466. 21 R. Parsons, Private Communication.
371
Printed in The Netherlands
A D S O R P T I O N OF ORTHO-, M E T A - AND P A R A - T O L U I D I N E S F R O M KI SOLUTION
K. M. JOSHI, S. I. MAHAJAN and M. R. BAPAT
Department of Physical Chemistry, Institute of Science, Bombay-400032 ( lndia) (Received 18th February 1974; in revised form 2nd April 1974)
INTRODUCTION
The adsorption of aniline at the mercury-solution interface has been investigated in detail by several workers 1~. Interesting observations like contribution of dipole~tipole interaction energy and influence of base electrolyte 2 have been made. Comparatively the toluidines do not appear to have been investigated. Capacitance and electrocapillary data for o-, and p-toluidines in sodium sulphate and H2SO 4 solutions have been reported by Damaskin et al. 5. They observed a change in orientation of adsorbed molecule as charge on the mercury changed sign. The present investigation deals with the adsorption of o-, m- and ptoluidines from potassium iodide solution using the capacitance method. The influence of substitution and base electrolyte is also shown. EXPERIMENTAL
A symmetrical a.c. bridge as described elsewhere 6 was used employing a dropping mercury electrode. The capillary used was according to a design by Randles 7, that is, it was constricted in the middle so that the rate of flow was controlled and ended in a small bulb which was drawn out into a fine capillary with thin walls. This minimized the screening effect due to the glass of the capillary. The D M E was coupled to a cylindrical platinum gauze electrode and the impedance of this system was determined using superposed a.c. of 10 mV amplitude. An electronic timing device detached the drop at a fixed time from its birth and the bridge null was obtained at the end of this period. Frequencies from 50 Hz to 4000 Hz were employed and the capacitance reported is the extrapolated capacitance at zero frequency. A 5-stage transistorized amplifier with an oscilloscope was used in the detecting arm. The capacities reported are correct to + 1°/. Interracial tensions at electrocapillary maximum (e.c.m.) were obtained on a Lippmann capillary electrometer and the potentials of e.c.m, were obtained using a streaming electrode as described by Grahame 8. A saturated calomel electrode was used as a reference electrode during capacitance measurements. All measurements were carried out using a conventional three compartment cell thermostatted at 34_+ 0.2"C. The solutions were deaerated using purified nitrogen.
372
K. M, JOSHI, S. I. MAHAJAN, M. R. BAPAT
Chemicals Ortho-, meta- and para-toluidines were of B.D.H. Analar grade. These were purified by distillation under low pressure and only middle fractions were collected. The purity was checked by measuring their boiling points. RESULTS AND DISCUSSION
Differential capacitance of the mercury-solution interface was measured for various concentrations of o-, m- and p-toluidines using 0.1 M KI as base electrolyte. Differential capacities for various concentrations of KI solution containing a fixed concentration of organic amine were also determined for a potential range of -500 to -1500 mV against saturated calomel electrode. The equipment was calibrated by comparing the various values obtained with those reported by Grahame 9. After the necessary correction for the potential of the reference electrode and temperature, it was observed that there was an excellent correspondence between the two sets of values. Capacitance-potential curves for six concentrations of o-, m- and p-toluidines in 0.1 M KI are shown in Figs. 1-3, respectively. The chief feature is the lowering of differential capacity from its value in the base electrolyte in the vicinity of maximum adsorption. The differential capacity curves for aniline and three substituted anilines show a peak at the negative end of l
I
1oo
IOO
80
,~ •
I
,O,IM KI
6o
~4
"
7~
/ ~Y,4
5 ta.
~o (o
20
- 400
-
lo00
E/mV
--
160o
I
I
I
-500
-1000 E/mY
-~500
Fig. 1. Differential capacity as a ftmction of electrode potential (with respect to saturated calomel electrode) at mercury-solution interface for 0.1 M KI plus various concentrations of o-toluidine at 34°C. (1) 0.005 M, (2) 0.0075 M, (3) 0.01 M, (4) 0.015 M, (5) 0.02 M and (6) 0.025 M o-toluidine. Fig. 2. Differential capacity a s a function of electrode potential (with respect to saturated calomel electrode) at mercury-solution interface for 0.l M KI plus various concentrations of m-toluidine at 34'C. (1) 0.0075 M; (2) 0.01 M; (3) 0.015 M; (4) 0.02 M; (5) 0.025 M and (6) 0.03 M m-toluidine.
A D S O R P T I O N O F o-, m-, p-TOLUIDINES F R O M KI I
373
I
100
80 ),IM
KI
6O (2 LL ca
40
20
[
I
-1000
-1500
0
-500
E/rnV
Fig. 3. Differential capacity as a function of electrode potential (with respect to saturated calomel electrode) at mercury-solution interface for 0.1 M KI plus various concentrations of p-toluidine at
34°C. (1) 0.0075 M, (2) 0.01 M, (3) 0.015 M, (4) 0.02 M, (5) 0.025 m and (6) 0.03 m p-toluidine.
the curve which is frequency dependent. The small kink at - 5 0 0 mV is also frequency dependent. The region between the peaks shows little effect of frequency of superposed a.c. signal. The capacity curves were integrated using the interfacial tension at e.c.m. and the value of the potential at e.c.m., determined separately as described. Thus the charge on the metal (qu) and interfacial tension (7) are given as E
qM
-
=
C dE_ ~e.c.m.
and ME 7 =
7 ......
-
fE
-
_
Ee.c.m.
CdE
z-
ge.c.m.
A typical set of interfacial tension/potential curves for one single system is brought out in Fig. 4. The curves converge into one another at extreme negative potentials showing desorption at these potentials while adsorption of organic appears to continue right to the far positive region investigated. The potential of e.c.m, shifts to negative potentials in conformity with the behaviour of aromatic compounds 1°. The correspondence of these twice integrated capacity curves with the ones obtained from direct electrocapillary measurements was checked and it was observed that in the region of e.c.m., the correspondence was better than 0.2 dyn cm-1 while at extreme positive potentials, the electrocapillary curves were lower than the twice integrated differential capacitance curves by about one dyn cm-1.
374
K. M. JOSHI, S. I. MAHAJAN, M. R. BAPAT
The relative surface excess of the organic was obtained at different values of surface charge. Figure 5 represents one such set of curves. It is pertinent to note that in this case, the F / q ~ curves show a very near plateau beyond qM> 0, showing that at positive charge, the amount adsorbed is nearly independent of the charge but depends on the base solution concentration. This figure brings out clearly the influence of the interaction of the delocalized g-electrons with the charge on the metal. 4- 20
I
I
1 2 I
I
r
I
0
+4.
8 390
360
330
i i
-- ,500
~ --
1000
E/mV
- 8 --1500
-4
c~M/#C crfi2
Fig. 4. Interfacial tension against potential (with respect to saturated calomel electrode) for mercurysolution interface for 0.1 M KI plus various concentrations of p-toluidine at 34°C. (1) 0.1 M KI, (2) 0.0075 M, (3) 0.01 M, (4) 0.015 M, (5) 0.02 M, (6) 0.025 M and (7) 0.03 M p-toluidine. Fig. 5. Relative surface excess as a function of charge density for 0.1 M KI plus various concentrations of p-toluidine at 34°C.(V) 0.0075 M, (V) 0.01 M, ( 0 ) 0.015 M, (Q)) 0.02 M, ( 0 ) 0.025 M, and ([~) 0.03 M p-toluidine.
The influence of base electrolyte on F/cI~ curves can be seen by comparing a similar set of results obtained using 0.1 M KC104 as base electrolyte 1. A larger degree of adsorption for the same concentration of the adsorbate and at the same q value is observed for KC104 than for KI (see Table 1). The behaviour of aniline which is similar to the toluidines in these two base electrolytes has been reported elsewhere 2. Because of the greater adsorption of I compared to ClOg, a larger free surface is available for organic adsorption in the latter case. However, the influence of base electrolyte appears to be specific with the system investigated. Thus Trasatti 11 found no influence on the adsorption of ethylene glycol from NaF, KC1, KBr or KI solutions while a base electrolyte effect was observed by Joshi and co-workers 12 in the adsorption of lower homologues of aliphatic acids.
+8
ADSORPTION OF o-, m-, p-TOLUIDINES FROM KI
375
TABLE 1 COMPARISON OF THE INFLUENCE OF BASE ELECTROLYTE IN THE FORM OF KCIO4 AND KI ON F/q M DEPENDENCE Charge density q/IzC cm 2
+8 +6 +4 +2 0 - 2 - 4 -6
F /mol cm- 2 0.01 M o-toluidine in
0.01 M m-toluidine in
0.01 M p-toluidine in
K CIO 4
KI
K CI0 4
KI
K CIO 4
KI
3.55 3.60 3.65 3.65 3.60 3.40 2.88 --
3.00 3.05 3.08 3.10 3.08 2.88 2.22
3.70 3.85 4.00 4.10 4.10 3.55 2.30 1.75
3.08 3.20 3.30 3.40 3.35 3.05 --
3.15 3.25 3.50 4.00 3.70 2.90 2.05 1.55
2.18 2.25 2.325 2.375 2.375 2.05 1.45 0.70
-8
l
I
I
200
o []
oo o o
o m
o D
o o
>
..~E-200 w
-4oo
I
I
I
2
4
6
-600
o
r~,
Fig. 6. Esin a n d
8
,o'°/.oL c~ z
Markov
plots for 0.1 M
KI
plus various concentrations of p-toluidine at 34"C.
(V) - 6 # C c m 2, (0) - 4 # C c m z,(m) - 2 p C c m - 2 , ( x ) 0 , u C c m -2,(r7) +2 /~Ccm -2,(C)) +4 /~Ccrn 2and(V) +6/~Ccm -2.
Esin and Markov
plots
I n Fig. 6 is shown the shift in the p o t e n t i a l across the d o u b l e layer due to the a d s o r p t i o n of the organic molecules at c o n s t a n t charge (calculated as [ E - E o ] at c o n s t a n t q, Eo being the p o t e n t i a l in base solution at that charge) with the a m o u n t of organic adsorbed. These plots indicate the c o n t r i b u t i o n to the p o t e n t i a l in the d o u b l e layer due to the a c c u m u l a t i o n of a d s o r b e d dipoles at the interface, the dipoles m a y be p e r m a n e n t or i n d u c e d ones. A salient feature of these plots is that the slopes increase r e m a r k a b l y at large negative values of qu
376
K.M. JOSHI, S. I. MAHAJAN, M. R. BAPAT
while at positive qM values, there is no shift in the potential even when the amount of adsorbed organic increases at the interface. This contrasts with the behaviour of other organic molecules like aliphatic acids13. Pyrazine 14 shows a symmetric change of slope about the charge for maximum adsorption while pyridine shows change in the sign of the slope at all values of qM. This behaviour is associated with the orientation of the adsorbed molecule which (in the present case due to the hydrophilic character of the - N H 2 group) is adsorbed in an upright position with the -NHz group on the solution side. At large positive charge, the molecule presumably is drawn towards the electrode due to the negative charge cloud interaction of the benzene ring with the metal charge. The dipole now acts in a direction perpendicular to the field and hence the contribution to the field due to adsorption is virtually nil in this position resulting in an insignificant (AE)q with F.
Standard free energy of adsorption (AG °) Standard free energy of adsorption of the organic compound was evaluated using the Bockris-Swinkels 15 isotherm. The total free energy of adsorption can be considered to be made up of a "chemical" free energy term independent of q, a free energy of interaction of adsorbed molecule and free energy of replacement of oriented solvent molecules. At constant value of charge, the contribution to the free energy of adsorption due to the solvent orientation term is given by exp
{(nR*/kT)(tZwX-R*EC*)}
where n refers to the number of molecules displaced by a single organic molecule, R*=(NT-NJ,)/NT, X is the field strength, E is the dipole~tipole interaction energy, C is the coordination number and k and T have the usual significance. Bockris and co-workers observed that maximum adsorption for aliphatic compounds occurs at - 2 pC cm-Z from which it was concluded that even at q=0, water is preferentially oriented. At q = - 2 #C cm-2, the net orientation R* becomes zero. It e3/2 0 i
0.2 ]-
0.4
0.6
i
5 "7
q=-2
-6 E
_~3
IO< 3
o
L
t
o:2
I
G
0:4
i
o:6
Fig. 7. Standard free energy of adsorption at q= - 2 pC cm -2 as a function of 0~ for (O) o-toluidine, (V) p-toluidine and (11) m-toluidine and as a function of 0 for (O) o-toluidine, ( 5 ) m-toluidine and (V) p-toluidine.
ADSORPTION OF o-, m-, p-TOLUIDINES FROM KI
377
would be therefore interesting to study the variation of ~G ° with 0 at q= - 2 #C c m - 2 , so that the variation in ~ o will be due to mutual interaction effects only. In order to evaluate this, 0, the fraction of surface covered, was evaluated a s /"//"max, /"max corresponding to saturation coverage obtained from an estimate of cross-sectional area of the molecules obtained from the knowledge of bond angles and bond lengths 16-a8. /"~x for o-, m-, and p-toluidines were obtained as 9.4 × 10 -1°, 9.4 × 10 - l ° and 9.7 × 10 -1° mol c m -2, respectively. The dipole~lipole interaction in the case of rigid monotayer following the Blomgren-Bockris model 19 is proportional to 0~ at not too high coverage, while following the Frumkin 2° model, the proportionality appears to be to the first power of 0 where the slope of A-G°/0 curve represents an average interaction energy. Figure 7 shows the plots of ~ o against 0~ and 0. It is obvious from the plots that although a straight line relationship is obtained, these plots cannot distinguish between the two isotherms 21. Both the plots, however, give the same value of A-~o at 0= 0. ACKNOWLEDGEMENT
One of us (S.I.M.) wishes t o thank the C.S.I.R. (India) for the award of Junior Fellowship during the tenure of which this work was carried out. SUMMARY
Adsorption of o-, m - and p-toluidines from O.1 M potassium iodide solution at mercury-solution interface is reported using capacitance method. Surface excess and free energy of adsorption using Bockris-Swinkels isotherm are evaluated. Adsorption energy is shown to depend on the dipole~:tipole interaction energy. Esin-Markov plots are discussed on the basis of change of orientation at positive charges. REFERENCES 1 S. I. Mahajan, Ph.D. Thesis, University of Bombay, 1969. 2 K. M. Joshi, S. I. Mahajan and S. Rajagopalan, Indian J. Chem., 10 (1972) 619. 3 B. B. Damaskin, V. M. Gerovich, I. P. Gladkikh and R. I. Kaganovich, Zh. Fiz. Khim., 38 (1964) 2495. 4 S. L. Diatkina and B. B. Damaskin, Elektrokhimiya, 4 (1968) 1000. 5 B. B. Damaskin, S. L. Diatkina and V. K. Venkatesan, Elektrokhimiya, 5 (1969) 524. 6 G. J. Hills and R. Payne, Trans. Faraday Soc., 61 (1965) 316. 7 J. E. B. Randles, Discuss. Faraday Soc., 1 (1947) 11. • 8 D. C. Grahame, E. M. Coffin, J. I. Cummings and M. A. Poth, J. Amer. Chem. Soc., 74 (1952) 1207. 9 D. C. Grahame, J. Amer. Chem. Soc., 80 (1958) 4203. 10 G. Gouy, Ann. Chim. Phys., Ser. 7, 29 (1903) 145; Ser. 8, 8 (1906) 291; Ser. 8, 9 (1906) 75; Ann. Phys. (Paris), Set. 9, 6 (1916) 5; Ser. 9, 7 (1917) 129. 11 S. Trasatti, J. Electroanal. Chem., 28 (1970) 257. 12 K. M. Joshi, P. K. Jadhav, S. W. Dhawale and S. Rajagopalan, Indian J. Chem., 10 (1972) 279. 13 K. M. Joshi and M. R. Bapat, 14th Seminar on Electrochemistry, Karaikudi, India, 1973. 14 H. P. Dhar0 Ph.D. Thesis, University of Ottawa, 1972. 15 J. O'M. Bockris and D. A. J. Swinkels, J. Electrochem. Soc., 111 (1964) 736. 16 L. Pauling, The Nature o f Chemical Bond and the Structure of Molecules and Crystals, Oxford and IBH Publishing Co., Bombay, 1963.
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17 Fergusen, The Modern Structure and Theory of Organic Chemistry, Prentice-Hall of India Private Limited, New Delhi, 1969. 18 Hand Book of Chemistry and Physics, The Chemical Rubber Co., Cleveland, Ohio, 48th edn., 1967. 19 E, Blomgren and J. O'M. Bockris, J. Phys. Chem., 63 (1959) 1475. 20 A. N. Frumkin, Z. Phys. Chem., 116 (1926) 466. 21 R. Parsons, Private Communication.