Langmuir 1995,11, 2231-2236
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Adsorption of Maleimide and Its Derivatives on an Iron Electrode in Acidic and Neutral Media Emilia Lazarova,*>+ Temenushka Yankova,+and Boris Aleksiev$ Departments of Physical Chemistry and Organic Chemistry, Higher Institute of Chemical Technology, 1756 Sofia, Bulgaria Received December 6, 1993. I n Final Form: February 8, 1 9 9 P The impedance method was applied to the study of the adsorption of maleimide, N-(p-aminopheny1)on a polycrystallineiron electrodein neutral and acidic media. maleimide, and N-(p-nitropheny1)maleimide It was found that the addition of maleimide and its derivatives to a sodium sulfate or a sulfuric acid solution, respectively, leads to the decrease of the iron double layer capacitance. It was shown that the adsorption of the substances investigated was described by Frumkin's isotherm. A correlation is found between the adsorption parameters. determined electrochemically,and the data obtained by quantumchemical calculations.
Introduction
It was shown in a series of papers1-12that the impedance method can be applied to the study of the adsorption of organic substances, containing different h c t i o n a l groups, on a polycrystalline iron electrode. A correlation between the adsorption parameters and the structure ofthe organic substances has been attempted. The aim of the present paper is to study the effect of the functional groups on the adsorption behavior of maleimide and its derivatives: N-(p-aminopheny1)maleimideand N-(p-nitropheny1)maleimide. With the application of MNDO method13 quantumchemical calculations have been carried out with the aim of determining the electron charges of the atoms in the molecule, as well as the bond lengths, the valency, and the dihedral angles. The data provided have enabled the calculation of the partial charges in the five-atom imide ring, of the phenyl radical, of the amino and the nitro group. The geometric area of the different groups and of the entire molecules have been determined as well. In the calculations of the geometric area of the molecules of N-(p-aminopheny1)maleimide and N-(p-nitrophenyllmaleimide it has been taken into account that the phenyl residue is positioned in a preferred conformational state, perpendicular to the plane of the five-atom imide ring. ~~
~
Department of Physical Chemistry. t Department of Organic Chemistry. Abstract published in Advance A C S Abstracts, April 15,1995. (1)Petkova, G.; Sokolova, E.; Raicheva, S.; Lazarova, E. Elektro T
@
khimia 1989,25,1555. (2)Petkova,G.; Sokolova,E.; Raicheva, S.;Lazarova, E. Proceedings of the UK Corrosion Conference, Brighton,Oct 3-5,1988;Vol. 2,p 165. (3)Raicheff, R.; Valtcheva, E.; Lazarova, E. Proceedings of the 7th European Symp. Corros. Inhibitors, Ferrara (Italy),Sept 17-20,1990; Vol. 1,p 407. (4)Betova, I.; Raicheff, R.; Lazarova, E. Elektrokhimia 1992,28, 265. (5)Lazarova, E.;Betova,I.; Raicheff,R.; Nikolova, L. Symposiumon Electrochemical and Inhibition Corrosion Control, Albena (Bulgaria), 1989;Ext. Abstr., p 195. (6)Bozhinov,M.; Betova, I.; Lazarova,E.; Raicheff,R. J . Electroanal. Chem. 1992,325,333. (7)Raicheff,R.;Betova, I.;Lazarova, E.; Bozhinov, M. NATO Advance Research Workshop, Plymouth U.K., Sept 6-9, 1993. (8)Betova, I.; Raicheff, R.; Lazarova, E. J . Electroanal. Chem., submitted for a publication. (9)Betova, I.;Neykov, G.; Raicheff,R.; Lazarova, E. Langmuir 1993, 9,3452. (10)Yankova, T.; Lazarova, E.; Nikolov, Tz. Chem. Ind. (Bulgaria) 1990,62,16. (11)Betova, I.;Bozhinov, M.; Raicheff, R.;Lazarova, E. Compt.Rend. Bulg. Acad. Sei., in press. (12)Betova, I.; Raicheff,R.; Lazarova, E. Zastita metallou 1993,29, 4 (Russian). (13)Dewar, M.; Thiel, W. J . A m . Chem. SOC.1977,99,4899;4907.
It is known that in an acidic medium maleimide and its derivatives protonize at the nitrogen atom of the fiveatom ring where bonding with H+from the solution takes place. This state is the preferred one for the three substances mentioned above. The differencejust outlined presumes different adsorption behavior of the substances investigated in both media. The comparative studies on the adsorption behavior of maleimide and its derivatives on a polycrystalline iron electrode are conducted by measuring the differential double-layer capacitance vs the potential. The analysis of the experimental data obtained allows the calculation of the corresponding adsorption parameters. Experimental Section Cylindrical electrodes of spectroscopicallypure iron (15 ppm impurity level) were used as working electrodes. In order to obtain a reproducible electrodesurface areal4the electrodeswere polished both mechanically and chemically and then subjected to a cathodic polarization for 1 h in a 0.015 M under continuous hydrogen bubbling. After several rinses with doubly distilled water the electrode was immersed in the cell being already under current. The roughness factor cf= 1.66)ofthe polycrystalline electrode thus treated was defined by the method described in refs 1and 15. The capacitance was evaluated with respect to the real electrode surface area. The impedanceexperimentswere carried out in a conventional three-electrode cell. The counter electrode was a Pt mesh positioned symmetrically around the working electrode. A saturated calomel electrode was used as a reference electrode, but all potential values cited in the present paper are referred to that of a standard hydrogen electrode. The solutions were prepared from analytical grade &So4 (Merck)and doubly distilled water. The compounds studied were synthesized in our laboratory. Their purity was controlled by TLC and IR spectroscopy. The concentrationrange investigated was 0.05-5 pmol L-l. Purified hydrogen was bubbled through the solution studied. The differential capacitance of the Fe electrode was recorded using an AC P-568 bridge at a frequency of 870 Hz. Preliminary investigation^^-^ showed that the differential capacitance on the iron electrode is practically independent of the perturbation frequency for frequencies above 800 Hz. The potential range studied was rather narrow which was determined by the fact that at potential values, E , more negative than -1.05 (for neutral media) and -0.5 V (for acidic media) hydrogen was evolved while at potentials more positive than -0.5 (for neutral media) and -0.25 V(for acidicmedia)Fe started t o passivate.14 (14)Ribalka, L.; Leikis, D. Elektrokhimia 1975,11, 1619. (15)Batrakov, B.; Naumova, N. Elektrokhimia 1979,15, 551.
0743-746319512411-2231$09.00/0 0 1995 American Chemical Society
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2232 Langmuir, Vol. 11, No. 6, 1995
The capacitance decreases with the increase of the concentration of the organic additives. The change of the capacitance with the organic substance studied results from the adsorption behavior of the latter. The values of the relative decreases of the capacitance (AC) are shown in Table 1. They are calculated according to
- 0.12 o
-8.-
-- 0,16
u
-
Y
-0.28
-0.105
fr I 1 -I
Figure 1. Molecular structure and distribution of partial charges in the molecules of (a) maleimide, (b)N-(p-aminophenyl)maleimide, (c) N-(p-nitrophenyl)maleimide,(d)protonized form of maleimide; (e) protonized form of N-(paminopheny1)maleimide, and (d) protonized form of N-(p-nitropheny1)maleimide.
AC=C"-C' where c" is the capacitance minimum value for iron in the blank solution (0.005 M NazS04 and 0.1 M HzSO4) while C' is the capacitance for 0 = 1and its corresponding potential EN were estimated from the cross point of the anodic and the cathodic linear branches of the 1/C - 1/E dependence for the maximum additive concentration. From the data in Table 1 it can be concluded that the capacitance decrease is almost one and the same for maleimide and its derivatives in neutral medium. In acidic medium the values of AC are from 2 to 3 times greater than those obtained in neutral medium. The different adsorbabilities of maleimide and its derivatives in acidic medium are revealed in Figure 4 where the capacitance curves presented are recorded a t maximum concentration of the corresponding additive (5 pmol L-I). It is seen from the figure that the adsorbability order is as follows: N-(p-aminopheny1)maleimide> maleimide > N-(pnitropheny1)maleimide. The degree of the electrode's surface coverage was evaluated by the use of Frumkin's method:16
?
$
- 05
'Ob,4
08
d,8
I
i0
62
-E /v Figure 2. Capacitancevs potential curves recorded on an iron electrode in 0.005 M NazS04 in the presence of maleimide: curve 1,0; curve 2, 0.1; curve 3, 2.5; curve 4,5 @mol L-I).
The quantum-chemical calculations were carried out using
where C" and C' were already defined while C was the capacitance minimum for a definite concentration of the surfactant. The degree of coverage vs concentration dependences for both media studied are shown in Figures 5 and 6, respectively. It can be concluded that (1)the degree of surface coverage by the amino derivative is greater (curve 1 in Figures 5 and 6) than that by the other compounds in neutral as well as in acidic medium; (2) the dependence of 8 on CORG for maleimide and the nitro derivative differ insignificantly in neutral medium; and (3) a considerable change in the curves 0 vs CORG obtained in the case of all compounds studied is observed in acidic medium. A criterion offerred by Damaskin17is used to determine the validity of the isotherm describing the adsorption process studied. For this purpose subsidiary curves a In c o ~ J a 0vs 8 are obtained by graphic differentiation of 8 - CORG dependences (Figures 5 and Figure 6). The position of the minimum and the width of these curves verify the validity of Frumkin's isotherm:
software developed on the basis of the optimization procedure of Davidov-Fletcher-Powell.
Results The data on the partial electron charges distribution, obtained by the quantum-chemical calculations, is presented in Figure 1. The differential capacitance versus potential curves of a n Fe electrode in 0.005 M NazSO4in the absence and the presence of maleimide and its derivatives are similar in form. The corrsponding curve recorded in the presence of maleimide is shown in Figure 2. The corresponding data obtained in 0.1 M HzS04 is shown in Figure 3.
B in eq 2 is the adsorption equilibrium constant and a is the attraction constant characterizing the forces of interaction between the species adsorbed. The linear character of the dependence in coordinates ln[O/(l - 0)CORGI vs 0 (Figure 7) confirms the validity of Frumkin's isotherm. With the aim of attaining a more accurate determination of the attraction constant, an algorithm offerred by (16) Frumkin, A,; Damaskin, B. In Modern Aspects ofElectrochemistry; Bockris, J. O'M., Conway, B. E., Eds.; Butterworths: London, 1964; p 170. (17) Damaskin, B. Dokl. Akad. Nauk USSR 1964, 156, 128.
The Adsorption Behavior of Maleimide
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a
C
Figure 3. Capacitance vs potential curves recorded on an iron electrode in 0.1 M HzS04 in the presence of (a) maleimide, (b) N-(p-aminophenyl)maleimide, and (c) N-(pnitropheny1)maleimide. Concentrations of a-c @molL-l) are as follows: curve 1, 0; curve 2, 0.1; curve 3, 0.5; curve 4, 1;curve 5, +5. Table 1. Summary of the Electrochemical and Adsorption Parameters for Maleimide and Its Derivatives on Iron Electrode in 0.005 M NazSO4 and 0.1 M HzSOp neutral media acidic media substances Co - C', F cm-2 a Co - C', F cm+ a r,.io10, mol cm-2 S, A 2 N-@-aminopheny1)maleimide 8.7 -0.4 25.05 0.20 2.13b 786 1.15c 144c maleimide 8.1 -1.1 22.35 -0.30 1.98 83 N-@-nitropheny1)-maleimide
7.25
-0.9
15.9
-0.60
2.72
61
Co - C, relative decrease of the differential capacitance; a , attraction constant in Frumkin's isotherm at EN;Tm,maximum surface excess; S, area of a molecule adsorbed. I state. e I1 state. a
Damaskin et al.ls is applied. The latter requires a n introduction of a relative concentration y = c/ce=~.a providing the elimination of the equilibrium constant B . Thus combining eqs 2 and 3
Bc,,,,,
= exp(-a)
(3)
the simplified expression y = C/C~,,.~ =
Leu - e11 exp[a(l - 2e)i
(4)
is obtained. The values of the attraction constant are determined by solving eq 4 which gives
a = Ml
- 2011 lnly(1 - 6)/01
repulsion forces between the species adsorbed and (2)the repulsion forces decrease in a n acidic medium. The values of the attraction constant are less negative for maleimide and its nitro derivative while for the corresponding amino derivative the forces observed indicate in fact an attraction (a = 0.2) in the adsorbed layer. With the application of the method described in ref 12 it has been attempted to calculate the values of the maximum surface excess (r,) and of the area (SI of the species adsorbed. For this purpose, a dependence of ln[ M l - e)] vs q2is looked for. 11, is the potential referred to that of maximum adsorption and is connected to the adsorption equilibrium constant through
B = Bo exp(-bq2)
(6)
(5)
The y - 8 dependences calculated for the amino and the nitro derivative of maleimide in acidic medium are presented by solid straight lines in Figure 8. The corresponding experimental results are marked by points in the same figure. The values of a obtained are presented in Table 1. Summarizing them it can be concluded that (1) the negative values of the attraction constant obtained in a neutral medium are a n indication for the presence of (18)Damaskin, B.; Saffonov, B.; Daitkina, S.Elektrokhzmia 1986, 22, 308.
The constant b in eq 6 is the slope of the linear dependence ln[8/(1- €91vs q2,whose value is used for the calculation of rmwith the application of
T,=(C,
- C') x lO-'/RTb
(7)
while S is obtained using the relation
s= (l/rm) N, with N , being Avogadro's number. The relationship ln[8/(1- e)]vs q2is obtained upon the analysis of the data on the variation of the differential
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2234 Langmuir, Vol. 11, No. 6, 1995
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E
' I
F
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-.. 2' I
0.8
2%6 0
3
0.6 0.L
50
0.2 LO
-
1
30
20
2 4
70 0,3 0.4
0.2
0,s 0,6
- EIV
Figure 4. Capacitance vs potential curves of iron electrode in 0.1 M HzS04in the presence of 5 pmol L-' of (1)blank solution, (2) N-@-nitrophenyl)maleimide, (3) maleimide, (4) N-@-aminopheny1)maleimide.
15.5
& 15.0 14.5
Figure 7. Dependence of In [ O m - @CORGI vs O according to Frumkin's isotherm describing the adsorption in 0.005 M Nazso4of(1)N-@-aminophenyl)maleimide,(2)maleimide,(3)N-@nitropheny1)maleimide.
1
2
3
L
5
c/,u
mol./"
Figure 5. Surface coverage degree vs additive's concentration at the potential value O f EN in 0.005 M Na2S04in case of (1) N-@-aminophenyl)maleimide, (2) maleimide, and (3) N-@nitropheny1)maleimide.
capacitance with the electrode potentialE (Figure 3). These dependences are recorded in the case of the maximum concentration of addition of maleimide and its derivatives in an acid medium. (The application of this dependence for the determination of rmis correct for the cases when a is equal to 0 or tends to 0. The value thus obtained for the nitro derivative should be considered as a n approximate one.) These dependences are presented in Figure 9. It can be concluded from the latter that the adsorption process of maleimide (straight line 3) and of N-(pnitropheny1)maleimide (straight line 4) is characterized by a single adsorption state while that of N-(p-aminopheny1)maleimide is characterized by two adsorption states, the first one (straight line 1)corresponding to a vertical and the second one to a horizontal orientation resulting from the z-electron interaction of the phenyl radical with the electrode surface. The values of rmand S calculated in the case of adsorption ofmaleimide and its derivatives on Fe in acidic medium are presented in Table 1.
i
2
3
Y
Figure 8. Dependence of O vs y in the adsorption study of (1) N-@-aminopheny1)maleimide and (2)N-@-nitropheny1)maleimide in 0.1 M HzS04.
The adsorption parameters rm, S, and a for maleimide and its nitro derivative in a n acidic medium are determined numerically as well. This has been achieved with the application of software written on the basis of Frumkin's model of two parallel capacitors.16 The advantage of the numerical method introduced is connected with the direct application of the data derived from the experimental C - E curves.
Discussion
A qualitative, as well, as a quantitative interpretation of the adsorption behavior of maleimide and its derivatives in neutral and acidic medium can be presented on the basis of the analysis of the experimental data obtained
The Adsorption Behavior of Maleimide
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Figure 9. Dependence of In [W(l - e)] vs qz obtained in 0.1 M HzS04solutions of (1 and 2) N-(p-aminophenyl)maleimide, (3) maleimide, and (4)N-(p-nitropheny1)maleimide. -0.29
-0,lO
0
-0.29
0
- 0,lO
Figure 10. Molecular structure and distribution of partial charges in the dimers’molecules of (a)maleimide in a neutral medium; (b)N-(p-aminophenyllmaleimidein a neutral medium; (c) maleimide in an acidic medium; and (d)N-(p-nitropheny1)maleimidein an acidic medium.
from the capacitance curves and from the values calculated of the adsorption parameters Tm,S, and a. On the basis of this data and on that from the quantum-chemical calculations of the molecule’s geometry and of the distribution ofthe partial charges over the adsorption centers, a hypothesis can be advanced for the possible orientation of the molecules adsorbed. On the basis of the data on the distribution of the partial electron charges of the atoms and the functional groups in the maleimide molecule and those of its derivatives it can be concluded that the electron charge of the five-atom ring varies insignificantly with the introduction of a phenyl radical, whereas the protonization of the substances in acidic medium leads to a substantial change in the partial positive and negative charges typical for the various atoms and groups in the molecules of the organic substances studied. It should be taken into account in the interpretation of the resutls that maleimide and N-(p-aminopheny1)maleimide form dimers in aqueous solutions, i.e. the hydrogen by the nitrogen atom in the first molecule bonds the oxygen atom in the carbonyl group of the second
molecule (Figure 10). The dimer in case of N-(p-aminophenyllmaleimide is characterized by a n antiparallel disposition of both molecules. Antiparallel structure for the maleimide’s dimer is highly unprobable because of conformational reasons. (1)The commensurate adsorbability of maleimide and its derivatives registered in neutral medium can be explained by the fact that the introduction to the molecule of maleimide of a phenyl residue with an amino or a nitro group varies insignificantly the partial positive charge of the five-atom ring. The latter can be accepted as a n adsorption center in the case of the substances’ adsorption on an iron electrode a t potential values more negative than EN. (2) The greater adsorbability of maleimide and its derivatives in acidic medium when compared to that in neutral one is due to the essential variation of the partial charges resulting from the protonization (Figure 1). The partial charge of the five-atom ring which is positive grows in acidic medium for the three compounds from 2.5 to 3 times. The protonization brings about a great increase ofthe partial positive charge ofthe amino group (15times)
Lazarova et al.
2236 Langmuir, Vol. 11, No. 6, 1995 and to a decrease of the partial negative charge of the nitro group and of the oxygen atom in the carbonyl group. The relative increase of the partial positive charge favors the adsorption of maleimide and its derivatives a t a potential value more negative than that of the minimum (EN).At potential values more positive than EN the adsorption of the amino and the nitro derivative is determined by the partial negative charge of the phenyl radical (while the differential capacitance of Fe in maleimide solution starts to increase with the potential variation to more positive values (Figure 4, curve 3). (3)N-(pAminopheny1)maleimide is characterized by a greater adsorbability on Fe in acidic medium (Figure 4). This behavior of the amino derivative is explained with the presence of an extra, comparatively high, positive partial charge of the amino group attained upon the protonization of the compound. The negative values of the attraction constant obtained in the case of the adsorption of maleimide and its derivatives in a neutral medium can be explained with the appearance of repulsion forces in the compound’s molecule. (a) The highest negative value (a = -1.1) is calculated in the case of maleimide’s adsorption. This can be explained in the following way. In the process of maleimide dimers formation (Figure loa) the charge a t one of the oxygen atoms is partially compensated while those at the other three oxygen atoms bring about repulsion forces between the dimers adsorbed. (b)The partial charges of the same sign in the molecule of N-(pnitropheny1)maleimide (Figure IC)are also three in number, but the partial charge of the nitro group is comparatively smaller than that of the oxygen atom. That is why repulsion forces (a = -0.9) appear as well, in the case of nitro derivative adsorption on the iron electrode surface, but they are weaker compared to those arising in the maleimide’s adsorption. (c)The smallest negative value ofthe attraction constant (a = -0.4) has been determined in the case of the adsorption of N-(paminopheny1)maleimide in neutral medium. The dimers’ formation through a n antiparallel disposition of both molecules brings about a partial compensation of the negative charge a t two of the oxygen atoms (Figure lob) resulting in weaker repulsion forces between the dimers in the adsorption layer when compared to those of the other two compounds investigated. The less negative value of the attraction constant of the amino derivative could have as well been explained with the appearance of attraction forces between partial charges of opposite signs. (4) The adsorption process of all compoundsinvestigated in acid medium is characterized by a more positive value of the attraction constant which becomes even positive in sign for the amino derivative. The decrease of the repulsion forces between the molecules adsorbed in acidic medium can be explained with the decrease (about three times) of the negative partial charges (at the oxygen atoms) resulting from the protonization of the compounds. On the other hand, in the process of maleimide dimers formation, one of the partial charges is completely compensated. In the case of the amino derivative antiparallel dimers formation both negative partial charges
are compensated and the arising attraction forces start to dominate the adsorption process (a = 0.2). An evaluation of the probable orientation of the organic compounds adsorbed on the iron electrode surface is attempted. This is done on the basis of the adsorption parameters rm and S obtained from the experiments in acidic medium and the geometric areas determined by quantum-chemical calculations a s follows: (1)The areaocalculated of one adsorbed particle of maleimide is 83A2(Table 1). This providesthe assumption that a planar adsorption of its dimers is proceeding. The geometric area of the dimgrs obtained by quantumchemical calculations is 78 A2. (2) The amino derivative is characterized by two adsorption states: the first one is determined by a vertical orientation of the dimer while the second one by a planar orientation. At potential values more negative than the potential of the minimum, the dimer ofN-(p-aminophenyllmaleimide is adsorbed through the five-atom ring of a molecule bonded to the amino group of the phenyl residue of another molecule. The geometric area of this part of the dimer is 80 A2while the area of one adsorbed particle, calculated by us in the case of a vertical orientation, is 78 A2, i.e. a comparatively good coincidence is reached. At potential values close to that ofEN the dimer ofN-(paminophenyllmaleimide is reoriented and adsorbs planarly. This results from the n-electron interaction. The area of this a4sorbed species, determined electrochemically, is 144 A2 while the geometric area of the dimoer obtained by the quantum-chemical caluclations is 127A2. We suppose that the difference of 17 k is in fact the area between both moleculesincluded in the antiparallel dimer. N-(p-Nitrophenyl)maleimide,as already mentioned, does not form dimers and adsorbs planarly in the whole potential range investigated. We accept a planar adsorption because of the coincidence of the gFometric area of the molecule in a planar state ( S = 58 A2) and the area (S = 61 Az)obtained electrochemically.
Conclusions The study by the impedance method of the adsorption of maleimide, N-(p-aminophenyl)maleimide,and N-O,nitropheny1)maleimide on iron electrode in acidic and neutral media provides the drawing of the following conclusions: (1)The adsorption of maleimide and its derivatives on iron electrode in acidic medium is considerably greater than that in neutral medium. (2)The adsorption process of the substances investigated in both media is described by Frumkin’s isotherm. (3) The two parallel capacitors model is valid for the adsorption of maleimide and N-O,-nitrophenyllmaleimide while the three parallel capacitors model (the adsorption process is characterized by two adsorption states) is valid for the adsorption of N-(paminopheny1)maleimide. (4) A suggestion of different probable orientations of the adsorbate on the iron electrode surface in acidic medium is advanced on the basis of the areas calculated of the adsorbed species. LA930708+
