Langmuir 1993,9, 3452-3456
3452
Adsorption of a Quaternary Salt of Indenone on Polycrystalline Iron in Acidic and Neutral Media I. Betova,*'+G. Neykov,l R. Raicheff,s and E. Lazarovag Department of Electrochemical Engineering and Corrosion Protection, Sofia University of Technology, 1156 Sofia, Bulgaria, Central Laboratory of Electrochemical Power Sources, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria, and Institute of Foreign Students, 1111 Sofia, Bulgaria Received May 11, 1993. I n Final Form: September 13,199P The adsorption of a quaternary ammonium cation from a 3-trimethyl[4-(l-oxo-2-phenylinden-3-yl)phenyllammoniumiodide on polycrystalline iron in acidic and neutral media is studied by electrochemical techniques. Assuming the validity of the second-order approach of the biparallel capacitor model, the dependence of the electrode charge on potential is evaluated for the maximum additive concentration using capacitance vs potential data as a starting point. According to the model predictions, a nonlinear least-squares fit of this dependence provides estimates for the adsorption parameters. The orientation of the adsorbed specieson the electrode surface is discussed. From dc polarization and ac impedance data, the inhibition efficiency of trimethylamino-2-phenyl-indenonylammoniumiodide with respect to iron corrosion in 1 M HzSO4 is evaluated.
Introduction In a series of papers'" the adsorption behavior of indenone derivatives on polycrystalline iron in the acidic and neutral media was investigated. The ultimate aim of these studies was to find a correlation between the structure of the organic molecules and their electrosorption capability. Combining quantum chemical calculations with electrochemical (ac impedance and dc polarization) measurements, a mechanism of the adsorption was advanced for a range of molecular a,&diphenylindenone derivatives in 1M HzS04.3 The orientation of the adsorbed species was deduced from a comparison between the occupied surface area, calculated from the maximum surface excess (determined from the analysis of capacitance vs potential curves) and the areas of the active parts of the molecules derived from quantum chemical calculations. The aim of the present paper is to present a study of the adsorption of trimethy1[4-(1-0~0-2-phenylinden-3-y1)phenyllammonium iodide on polycrystalline iron in acidic and neutral media. The method of calculation used throughout this work is based on the analysis of experimental data for the influence of the bulk concentration of trimethyl[4-( l-oxo-2-phenylinden-3-yl)phenyll ammonium iodide on the differential capacitance vs potential dependence of the iron electrode. The influence of trimethyl[4-( l-oxo-2-phenylinden-3-yl)phenyl]a"onium iodide on the electrode processes occurring during iron corrosion in 1 M HzS04 will also be discussed. Experimental Section Electrodes and Electrolytes. Cylindrical electrodes of spectroscopically pure iron (15 ppm impurity level) were used (working surface 0.36 cm2). In order to obtain a reproducible surface, the working electrode was polished mechanically and electrochemicallyand washed thoroughly with bidistilledwater. 8 Sofia University of Technology.
Bulgarian Academy of Sciences. Institute of Foreign Students. Abstractpublishedin AduanceACSAbatmcts, October 16,1993. (1)Betova, I.; Raicheff, R.; Lazarova, E. Sou. Electrochem. 1992,28,
t
266. ~~
(2) Bozhinov, M.;Betova, 1.; Lazarova,E.; Raicheff, R. J.Electroanul. Chem. 1992,326,333. (3) Betova, 1.; Bojinov, M.; Raicheff, R.; Lazarova, E. J. Electrochem. Soc., submitted for publication.
The native oxides formed on the electrodesurfacewere removed by 1h of cathodic polarization at -0.5 V (SHE) in 1M Ha04 and -1.0 V (SHE) in 0.005 M Nad04. This procedure results in a mean roughness factor of 1.66. A large platinum mesh situated symmetrically around the iron cylinder was employed as a counter electrode. A saturated calomel electrodewaa used as a reference;all the potentialscitedin the paper are recalculated on the hydrogen scale. Working electrolyteswere prepared from pa H 8 O 4 and N a 8 0 4 (Merck) and bidistilled water. The solutionswere deaerated by purifiedAr bubbling. The indenone derivative studied was synthesized by the recommended laboratory procedure, and ita purity was controlled using IR spectroscopy. The investigatedrange of additive concentrationswae 0.05-10 pmol-L-1. Apparatus and Procedure. Polarization curves in the potential range -0.5 to -0.1 V were obtained at a sweep rate of 0.1 m V 4 using a Solartron ECI 1286 potentiostat. The differential capacitance of the iron electrode wae registered simultaneously during the potentiodynamicscan by a Solartron FRA 1260 frequency response analyzer at a frequency of 870 Hz and ac signal amplitudeof 2 mV (ma).Preli"ryinvwtigatiom showed that the differential capacitanceof the iron electrode is practically independent of the perturbation frequency for frequencies above800 Hz." The currentdensity and the differentied capacitanceare recalculated for the true surface area using the above-mentioned roughness factor. The ac impedance spectra were registered in galvanostaticmode (i = 0) after a stable rest potential was reached (the potential drift was lese than 1%/h). The ac signal amplitude was 10pA, and the frequency range was 0.05-65 kHz. Results Determination of the Adsorption Parametere of Trimethyl[ 4 4 l-oxo-2-phenylinden-3-yl)phenyl]ammonium Iodide on Iron in Acidic and Neutral Media. The differential capacitance vs potential dependences for the iron electrode in 1 M Has04 with and without the addition of 0.05-10 pmol-L-1 of trimethyl[&( 1-oxo-2phenylinden-3-y1)phenylla"onium iodide are presented in Figure la, and the corresponding dependences for iron in 0.005 M NazSO4 are plotted in Figure lb. For the sake of simplicity, it is assumed that, at the sufficiently low scan rate used, the capacitance vs potential curves reflect the adsorptionldesorption processes at the part of the metal surface covered with organic surfactant. It is postulated that this area is completely blocked for faradaic reactions, the latter proceeding only at the unoccupied part of the surface.
0743-7463/93/2409-3452$04.00/00 1993 American Chemical Society
Adsorption of a Quaternary Salt of Indenone
Langmuir, Vol. 9, No. 12,1993 3463 I
1.01
9080cb
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’,I
I
1
W“r
.
or0
50-
L V
1
2
I
I
4
6
8
1
I
1
3
I
1
2
I
C/pmd.l-’
40-
-
30 -
20-
Figure 2. Degree of coverage vs concentration dependence at
the potential of capacitanceminimum for the following systems: (a) Fe/l M HzSO, + trimethyl[4-(1-0~0-2-phenylinden-3-y1)phenyllammonium iodide; (b) Fe/0.005 M NazS04+ trimethyl[4-(l-oxo-2-phenylinden-3-yl)phenyl] ammonium iodide.
10 -
I
Fe/O.OOSM Na,SO, +additive
I
Figure 3. Test of Frumkin’s isothermfor the following system: (a) Fe/l M HzSOd + trimethyl[4-(1-0~0-2-phenylinden-3-y1)-
phenyllammonium iodide; (b)Fe/0.005 M NaaSO4 + trimethyl[4-(l-oxo-2-phenylinden-3-yl)phenyll ammonium iodide.
ai, 05 0.6 a7 0.8 0.9 ID
- E/V
Figure 1. Capacitance vs potential curves for polycrystalline iron in the following media: (a, top) 1 M H&O, + trimethyl[4-(l-oxo-2-phenylinden-3-yl)phenyl] ammonium iodide at concentrations (pmol.L-’) of 1,O; 2, 0.05; 3, 0.25; 4,l.O; 5, 5.0; and 6, 10.0; (b, bottom) 0.005 M Na&3Od + trimethyl[4-(l-oxo-2phenylinden-3-yl)phenyl]ammonium iodide at concentrations (ccmol.L-’) of 1,0; 2, 0.25; 3, 1.0;4,5.0; 5, 7.5; and 6, 10.0.
A considerably greater decrease of the differential capacitance of the iron electrode when adding trimethyl[4-(l-oxo-2-phenylinden-3-yl)phenyl]ammonium iodide is observed in 1M HzS04 at the potential of the capacitance minimum (6C = 33 pF.cm-2 compared to 12 pF-cm-2 for the Fe10.005 M Na~S04system). Moreover, a capacitance plateau of ca. 0.24 V of width is discerned for the maximum additive concentration in 1 M HzS04; no corresponding plateau is detected for the Fe/0.005 M Na~S04system. A qualitative conclusion arises from the data presented in Figure 1 that trimethyl[4-(1-0~0-2-phenylinden-3-y1)phenyllammonium iodide is more strongly adsorbed on iron in 1 M HzS04 than in 0.005 M NazSO4. As a first step of the experimental data analysis, the dependence of the degree of coverage of the iron surface with adsorbed species 8 (at the potential of the capacitance minimum) on the bulk concentration of trimethyl[4-(1oxo-2-phenylinden-3-yl)phenyll ammonium iodide was estimated using the well-known formula
e = (co- c)/(co - c’)
(1) where Cois the capacitance minimum for iron in the blank
solution (1M HzSO4 and 0.005 M NazSO4) and C is the capacitance minimum for a definite bulk concentration of the surfactant. C’-the capacitance for 6 = 1-and ita corresponding potential ENwere estimated from the cross point of the anodic and cathodic linear branches of the 1IC-lIE dependence for the maximum additive concentration. The degree of coverage vs bulk concentration dependences for the two investigated media are shown in Figure 2. The two dependences support the conclusion derived from Figure 1that trimethyl[4-(1-0~0-2-phenylinden-3y1)phenyllammonium iodide is adsorbed more strongly in the acidic medium than in the neutral one: saturation coverage is reached by lower concentrations. On the other hand, the values of the saturation coverage for both media are close to each other (0.8 f 0.05). The validity of Frumkin’s isotherm was then tested for the e vs c data: Bc/55.5 = (e/(i- 8 ) ) exp(-2a8) (2) where B is the equilibrium constant of the adsorption process and a is the attraction constant, proportional to the energy of adsorbateladsorbate interactions in the adsorption layer. The h(55.58/(1- 0)c) vs e dependences for the adsorption of trimethyl[4-(l-oxo-2-phenylinden3-y1)phenyllammonium iodide on iron in both media are presented in Figure 3. The linear shape of the dependences confirms the validity of Frumkin’s model. In order to determine the parameters B and a in eq 2, it is assumed that the maximum surface excess and the capacitance of the electrode/electrolyte interface for 8 = 1(C’)do not depend on potential in the investigated interval. Subject to these assumptions, the model of the
Betoua et al.
3454 Langmuir, Vol. 9, No. 12,1993
-1
Fe/lM H,SOL +additive ai mvs-'
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0.2
a3 -EIV
OL
05
-4
-45
-0.3
-0.L
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-01
E/V
Figure 5. Polarization curves for the iron electrode (scan rate 0.1 mV-8-1) in 1 M H2SO4 trimethy1[4-(1-0~0-2-phenylinden3-yl)phenyl] ammonium iodide at concentrations(pmol-L-') of 1, 0; 2, 1.0; 3, 5.0; and 4, 10.0.
LT
2
I 0.4
0.6
0.8
+
between a and the potential is expected4
1
-E/V
Figure 4. Electrode charge vs potential dependence at the maximum additive concentration(10pmo1.L-1) for the following systems: (a)Fe/l M HzSO4 + trimethyl[4-(l-oxo-2-phenylinden3-y1)phenylla"onium iodide; (b) Fe/0.005 M Na2SOd + trimethyl[4-(l-oxo-2-phenylinden-3-yl)phenyl] ammonium iodide.
biparallel capacitor is valid.4 According to this model,4 the charge of the electrode surface as depending on the potential is given by the expression +C'(E-EN)dRm,,(dalWW - 8 ) (3) where q(E) is the charge vs potential dependence in the solution with maximum additive concentration and qo(E) is the corresponding dependence in the blank solution. The q(E) and qo(E)dependences obtained by numerical integration of the C(E) dependence for the maximum additive concentration and the C" (E) dependence, respectively (Figure 11, are presented in Figure 4. Accordingto the assumptions of the biparallel capacitor model? the potential dependence of the equilibrium constant of adsorption can be expressed by the simplified equation
a
a,
+ a,(E-EN)
(8')
Using nonlinear least-squares analysis by the algorithm of Levenberg-Marquardt, correlation was searched for between eq 3 and the experimentally obtained q(E) dependences (Figure 4). The resulting optimum values for the adsorption parameters-maximum surface excess rm, and attraction constant ao-are listed in Table I together with the values of the surface area of the adsorbed species, calculated from ,,?I using the relationship
q(E)'q0(E)(1-8)
(4) where CY
(C" - C')/R",
For the attraction constant a, being proportional to the interaction energy between adsorbed species,the following dependence analogous to the one proposed for the adsorption of organic cations4 can be advanced: u = a0 + u,(E - EN) + u ~ ( EEN)2 (8) For molecular-type compounds, a linear relationship (4)Frumkin, A. N.; Damaskin, B. B. In Modern Aspects
of
Electro-
chemistry; Bockrie, J. O., Conway, B.,Eds.;Mir: Moscow, 1967;Chapter 3, p 202 (in Russian).
The followingconclusions can be drawn from the results collected in Table I: (1)The maximum surface excess I'max is greater for the adsorption of trimethyl[4-(l-oxo2-phenylinden-3-y1)phenyll ammonium iodide on iron in 1M HzS04 than in 0.005 M Na2S04. (2) The attraction constant a0 exhibits negative values for both media, indicating significant repulsive adsorbate/adsorbate interactions in the adsorption layer. A squared dependence of the attraction constant is obtained for the Fe/l M HzSO4 system, whereas a linear dependence is valid for the system Fe/0.005 M Na2S04. The value of a0 is lower for the system Fe/0.005 M NaaSO4; Le., the repulsion forces are weaker. (3) The surface area of the adsorbed species is smaller for the adsorption of trimethyl[4-(l-oxo-2phenylinden-3-y1)phenylJa"onium iodide on iron in 1 M &Sod. Influence of the Trimethyl[l-(l-oxo-t-phenylinden3-yl)phenyl]ammoniumIodide Additive on the Faradaic Processes during Corrosion of Fe in 1 M H2SO4. The polarization curves of the iron electrode in 1 M HzSO4 with and without the addition of trimethyl[4-(l-oxo2-phenylinden-3-y1)phenyllammoniumiodide (scan rate 0.1 m V d ) are presented in Figure 5. The results indicate that the addition of trimethyl[4-( l-oxod-phenylinden3-y1)phenylla"onium iodide slows the rate of both partial reactions of the iron corrosion procesa,this influence being more strongly exerted on the h.e.r. (Le.,this additive can be considered as a predominantly cathodic-type inhibitor). The corrosion current density i, and the inhibition efficiency 2 = (iocom- icom)/iommX 100% as depending on the concentration of the surfactant are collected in Table 11. The drop in the corrosion current density is significant, and quite high inhibiting efficiencies are reached for sufficiently low concentrations.
Adsorption of a Quaternary Salt of Indenone
Langmuir, Vol. 9, No. 12,1993 3455
Table I. Summary of the Adsorption Parameters of Trimethyl[ 44l-oxo-2-phenylinden-3-yl)phenyl]ammonium Iodide on Polycrystalline Iron in Acidic and Neutral Media:,-'l Maximum Surface Excess; S, Area of the Adsorbed Swcies: a.Frumkin's Attraction Constant medium rmux 1010 (mol-cm-2) S (nm2) a0 1M 4.99 0.30 -2.65 0.005 M N82SO4 4.61 0.36 -2.05 Table 11. Corrosion Current Densities and Inhibiting Efficiencies Calculated from Polarization Curves for the System Fe/l M HaSol + Different Concentrations of Trimethyl[ 44l-oxo-2-phenylinden-3-yl)phenyl]ammonium Iodide concn Lwrr (rmo1.L-1) (mA-cm-2) Z (% ) 0.0 1.00 0.05 0.18 82
concn karr (rcmo1.L-1) (mAacm-2) 2 ( % ) 5.00 0.07 93 10.00 0.06 94
Fe/l M H2%, +additive impedance spectra ot 1 = 0
Figure 6. Ac impedance spectra for the iron electrode at the rest potential in 1 M H2SO4 + trimethyl[4-(l-oxo-2-phenylinden3-y1)phenyllammoniumiodide at concentrations (Irmol-L-') of 1, 0; 2,0.25;3,l.O; 4,5.0; and 5,lO.O. The parameter is frequency in hertz. +0.212
CH3
Table 111. Charge Transfer Resistances and Inhibiting Efficiencies Calculated from Ac Impedance Spectra for the System Fe/l M HISO, + Trimethyl[4-(l-oxo-2-phenylinden-3-y1)phenyllammoniumIodide concn (umo1.L-9 0.0 0.25
Rt (Q-cm2) 2 ( % )
29 180
81
concn (umo1.L-1) 5.00 10.00
1
+0.210
CH,3
N
1+0.10L
Rt
(n.cm2) 211 220
Z (% )
~~
85 86
In order to investigate directly the corrosion process of iron in 1M H2SO4 at the open circuit potential and the corresponding influence of trimethyl[4-(l-oxo-2-phenylinden-3-y1)phenyllammonium iodide, ac impedance spectra of the system Fe/l M H2S04 with and without additive are registered. The obtained spectra are presented in Figure 6. The addition of trimethyl[4-( l-oxo-2-phenylinden-3-yl)phenyllammonium iodide slows the rate of the corrosion process considerably at a concentration of 0.25 pmo1.L-l. Some saturation is reached by higher concentrations (for a ca. 2 order of magnitude increase of the bulk surfactant concentration the charge transfer resistance Rt increases only by 20%). The values of Rt and the corresponding inhibiting efficiency ZR= ((Rt'1-l- (Rt)-l)/(Rto)-l are also listed in Table 11. It has to be pointed out that the inhibition efficiency calculated from impedance data is only slightly lower (810%) than the one determined from the polarization curves. This fact is an indication that stable adsorption layers are formed by trimethyl[4-(1-0~0-2-phenylinden3-yl) phenyl] ammonium iodide on iron, both under cathodic polarization and at the rest potential.
9
92
0 Figure 7. Geometrical structure and distribution of partial charges for the trimethyl[4-(l-oxo-2-phenylinden-3-yl)phenyl]ammonium cation as deduced from quantum chemical calculations using the MNDO method.
mind that the latter are most probably chemisorbed on the metal surface and transferred at the "metal side" of the double layer.6 This process results in an increase of the region of the negative electrode charge contributing to the adsorption of organic on the iron surface, modified by iodide ions. From the results presented in Figure 7 it can be assumed that the energetically preferable orientation for the adsorption of the studied cation is by the active -N(CH3)3 pyramid. The area of the adsorbed species in 1M HzS04, determined by the regression analysis of the q(E) dependence according to eq 9, is 0.30 nm2. This value is quite close to the projected area of the -N(CH3)3 pyramid (0.28 Discussion nm2). This fact seems to confirm the hypothesis for the orientation of the adsorbate in an acidic medium. The The geometrical structure and the distribution of partial charges in the trimethyl[4-(1-0~0-2-phenylinden-3-y1)- significant negative value of the attraction constant a indicating the presence of considerable repulsion forces phenyllammonium cation are presented in Figure 7. The in the adsorption layer is in agreement with the fact that nitrogen atom from the quaternary amino group bears a the layer consists of adsorbed cations. Furthermore, the partial positive charge (0.104). From general organic observed squared dependence of the attraction constant chemistry5 it is known that the nitrogen atom occupies on potential seems to confirm the suggestion of cation the top of a pyramid, the base of which is constituted from adsorption. the three -CH3 radicals. Each of these radicals bears a The substantially different capacitancepotential curves, significant partial positive charge (0.21); i.e., the positive obtained in a neutral medium, point to a different nature charge of the cation is delocalized in the -N(CH3)3 group. of the adsorbed species. A further confirmation of this When considering the adsorption mechanism of cationactive surfactants on iron in acidic media containing (6) Frumkin,A. N. Potentials of Zero Charge; Nauka: Moscow, 1979; halogen (and especially iodide) ions, one has to bear in Chapter 6, p 170 (in Russian). (5) March,J. Advanced Organic Chemistry- Reactions,Mechanisms and Structure;J. Wiley and Sons: New York, 1985;Vol. 1, Chapter 1, p 31.
(7) Iofa, Z. A.; Batrakov, V. V.; Ba, C. N. Electrochim. Acta 1964,9, 1645. (8) Djelali, V. V.; Hanin, A. M. Sou. Electrochem. 1982, 18, 836. (9)Schweinsberg, D.P.;Ashworth, V. Corros. Sci. 1988,28, 539.
3456 Langmuir, Vol. 9, No. 12, 1993
suggestion seems to be the obtained linear dependence of the attraction constant a, characteristic for the adsorption of molecular species on electrode^.^ The greater area of the adsorbed species calculated from the maximum surface excess using eq 9-0.36 nm2 (cf. Table 1)-is also an indication of a change in the nature of the adsorbed species. A possible explanation could be the hydrolysis of the quaternary ammonium cation resulting in a hydroxide formation. This possibility was checked out by measuring the pH of the 10 pmo1.L-l trimethyl[4-( l-oxo-2-phenylinden-3-y1)phenyllammonium iodide solution in 0.005 M Na2SO4 and comparing it to the pH of the blank 0.005 M NazSO4 solution. A substantial difference of ca. 1.1pH units was reproducibly obtained, thus confirming the suggestion for the hydrolysis of the ammonium cation in a neutral medium. The advanced hypothesis of hydrolysis could be of help in determining the position of the adsorbed species in a neutral medium. Assuming an adsorption with the -N(CH&OH radical (estimated surface area of 0.36 nm2), a fair coincidence with the calculated area of the adsorbed species is found (Table I). Moreover, the value of the attraction constant a0 is significantly lower than the one obtained in 1M HzSO4, and a linear dependence of this parameter on the potential is observed. This fact seems to confirm the suggestion of a change in the nature of the adsorbate.
Conclusions The adsorption of a quaternary ammonium cation-the ammonitrimethyl[C( l-oxo-2-phenylinden-3-yl)phenyll um cation-on polycrystalline iron in acidic and neutral
Betova et al.
media is studied by electrochemical (ac impedanceand dc polarization) techniques. The following conclusions can be drawn from the results. (1) Using capacitance vs potential data as a starting point and assuming the validity of the biparallel capacitor model, the dependenceof the electrode charge on potential a t the maximum additive concentration can be evaluated. From a nonlinear least-squares fit of this dependence to the model predictions, estimates can be obtained for the maximum surface excess (adsorbed species area) and the attraction constant a characterizing adsorbate/adsorbate interactions. (2) The results point to a preferred orientation of the adsorbed cation in an acidic medium by the active -N(CH3)3 group, which is confirmed by a coincidence between the area of this group and the area of the adsorbed species (derived from r-). In a neutral medium, a change in the nature of the adsorbed species is proposed due to hydrolysis of the quaternary ammonium cation; this change could explain the different adsorption behavior in this medium. (3) The stability of the adsorption layer formed in 1M H2S04 is further confirmed'by the influence of the trimethyl[C( l-oxo-2-phenylinden-3-yl)phenylla"onium iodide additive on the electrode processes during corrosion of iron. Both polarization curves and ac impedance spectra at the rest potential indicate relatively high inhibition efficiencies. A stronger impact of the additive on the h.e.r. is observed i.e., the investigated surfactant can be regarded as a predominantly cathodictype inhibitor.
