Langmuir 1991, 7, 389-393
389
Investigation of Cetylpyridinium Bromide Adsorption at a Glassy Carbon Electrode Surface by Spectroelectrochemistry with a Long Optical Path Length Thin-Layer Cell Shaojun Dong,* Yongchun Zhu, and Guangjin Cheng Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin 130022, People’s Republic of China Received August 16, 1988.I n Final Form: July 10, 1990 The adsorption of cationic surfactant cetylpyridinium bromide (CPB) on a glassy carbon (GC) electrode surface has been studied by spectroelectrochemistry with a long optical path length thin-layer cell (LOPTLC) for the first time. A fine adsorption isotherm of CPB molecules from an aqueous solution containing 0.10 M KBr has been obtained over the range of (1.00-8.00) x M. From theoretical calculation and experimental data, adsorption of CPB on the GC electrode surface shows four distinct orientations and three large orientation transitions. Compared with the ordinary isotherm, the differential isotherm is more characteristic and would be suitable for the study of orientation transitions of organic compounds. With a theoretical treatment of the adsorption isotherm, four orientations of adsorbed CPB on a GC electrode surface coincide with the Frumkin-Langmuir type. From adsorption parameters in the FrumkinLangmuir equations, the adsorption free energy and, therefore, the equilibrium constants of orientation transitions of the CPB molecule can be obtained.
It is a fundamental interest in electroanalytical chemistry that the adsorption of compounds on an electrode surface shows different electrochemical behavior from the bare one. Recently, Hubbard e t al. studied the behavior of adsorption of aromatic compounds a t Pt electrode surfaces by using a thin-layer electrochemical method.’-3 Kuwana et al. developed a long optical path length thinlayer cell (LOPTLC) and used it to study the adsorption of organic compounds on a Pt electrode The former is suitable for studying irreversible adsorption of electrochemically active compounds, and the latter is applicable to study adsorption of compounds having an absorptive spectrum but does not require compounds to be electrochemically active and adsorption to be irreversible. Since the 1950’s, the adsorption of surfactants on electrode surfaces had attracted electrochemist’s attention, and a number of papers have been p u b l i ~ h e d .Cationic ~ surfactant cetylpyridinium bromide (CPB), for example, had come into use in spectrophotometry with micelle solubilization8 and catalysis and in electroanalysisgJO sensitized by surfactant. CPB has a strong absorbance in the UV region, which corresponds to the charge transfer comp1ex;ll therefore, it is suitable for studying adsorption
* To whom correspondence should be addressed. (1)White, J. H.; Soriaga, M. P.; Hubbard, A. T. J.Electroanal. Chem. Interfacial Electrochem. 1985, 185, 331. (2)White, J. H.; Soriaga, M. P.; Hubbard, A. T. J. Phys. Chem. 1985, 89,3221. (3)Soriaga, M.P.; Stukney, A. J. L.; Hubbard, A. T. J. Electroanal. Chem. Interfacial Electrochem. 1983,144, 207. (4) Jerzy, Zak; Porter, M. D.; Kuwana, T. Anal. Chem. 1983,55,2219. ( 5 ) Gui, Y.-P.; Porter, M. D.; Kuwana, T. Anal. Chem. 1985,57,1474. (6)Gui, Y.-P.; Kuwana, T. Langmuir 1986,2,471. (7)Heyrovsky, J.; Kuta, J. Principles of polarography; Publishing House of the Czechoslovak Academy of Sciences: Prague, 1965. (8) Qi, Wenbin Surfactants and Analytical Chemistry, Metrological Press: Beijing, 1986. (9)Mcintire, G. L. Am. Lab. 1986, 173. (10)Feredler, J. H.; Fendler Catalysis in miceller and macromolecular systems; Academic Press: New York, 1975. (11)Ashoka, Ray; Pasupatic, Muker Jee J. Colloid Sci. 1958,13,208.
0743-7463/91/2407-0389$02.50/0
by LOPTLC. The adsorption of CPB molecules on a glassy carbon (GC) electrode surface had not yet been reported. In this paper, we describe the adsorption behavior of CPB on a GC electrode surface by spectroelectrochemistry with LOPTLC. A fine adsorption isotherm of CPB has been obtained, from which four stable orientations of CPR on a GC electrode surface have been suggested, and the thermodynamic treatments of the adsorption isotherm have been considered. If a compound has an absorption spectrum that obeys the Lambert-Beer law in the range of concentrations studied, the quantity and potential dependence of its adsorption on electrode can be studied by spectroelectrochemistry. By use of LOPTLC in a spectroelectrochemistry study, the optical axis is parallel to the electrode surface through the solution. Therefore, comparing with the absorbance of solution specis, the absorbance due to adsorbed layer is very small and negligible. The amount of species adsorbed on the electrode surface in each filling can be calculated by the equation’
where V is the cell volume, a is the effective surface area of the electrode, Ai is the absorbance in the ith solution injection, and A,, is the absorbance in nth injection, which is generally the absorbance of the injected solution concentration (CO).The total surface coverage for a given concentration (rT)is the sum ri
rT = Cri
(2)
The area occupied by a molecule on the electrode surface (a) can be obtained from rT (3)
where NOis Avogadro’s constant. 0 1991 American Chemical Society
Dong et al.
390 Langmuir, Vol. 7, No. 2, 1991
35.0
30.0
25.0
l
0.60
l
0.40
l
l
l
l
0.20
20.0
0.00
POTENTIAL (V.VS.SCE)
F i g u r e 1. Determination of parameters of LOPTLC with 1.00 x M KaF,(CN)s in 0.10 M KCl aqueous solution: cyclic voltammogram, E (V vs SCE) from 0.60 to -0.10; scan rate, 1mV/s.
15.0
10.0
5.00
0.300 0.00
0.00
0.200
2 .tin
4.00
8.00
6.00
~ o n c e n r r a f i m( X l o - 6 )
mal
dm-3
Figure 3. Adsorption curves for adsorbed CPB on GC electrode surface: (a) adsorption isotherm; (b) differential adsorption isotherm.
0.100
220
300
500
400
WAVELENGTll (A)nm
Figure 2. CPB spectra in LOPTLC: (1) vacuum cell; (2) electrolyte only (0.10 M KBr aqueous solution); (3) electrolyte M CPB; (4) electrolyte containing 8.00 X containing 4.00 X 10-5M CPB. Table I. Adsomtion Data of CPB on GC Electrode Surface surface coverage occupied area of CPB on GC of CPB concn of CPB surface r, lo-" mean relative molecule c, 10-5 M mol cm-2 derivation, % 6,A 2 1.oo 2.00 2.80 3.00 3.20 3.36 3.60 3.76 4.00 4.16 4.40 4.60 4.80 5.00 5.60 6.00 7.00 8.00
1.00 2.49 3.88 4.81 6.92 8.25 8.64 9.16 10.2 11.4 13.3 13.9 14.5 16.3 26.4 36.9 37.9 38.8
3.0 2.8 3.3 0.10 2.0 1.8 2.3 1.0 1.6 0.1 0.1 4.3 2.0 0.1 2.5 0.3 0.1 4.8
1661 667 428 345 240 202 192 181 162.9 145.7 125 119.6 114.6 102 62.9 45.0 43.8 42.8
Experimental Section Reagents a n d Instrumentation. CPB (chemical pure) was recrystallized 3 times with acetone and then dried. KBr and other reagents used in the experiments were all analytical grade, and doubly distilled water was used where applicaple. Spectroelectrochemical measurements were conducted with a DMS-90 UV-vis spectrophotometer (Varian Instrument Co, Palo Alto, CA) and PAR Model Electrochemistry System 370 and 1286 Electrochemical Interface (Solatron, England). LOPTLC5 is used in the experiment with specifications: cell volume was 5.25 p L measured from electrolysis charge integrated from a cyclic voltammogram (1.00 X lO-3M K3FJCN)Gcontaining 1.0M KC1 aqueous solution), Figure 1;effective area of the GC electrode surface is 55.9 mm2 determined by the adsorption stripping cyclic voltammogram of Alizarin Red S which occupied
a electrode surface of A2 with flat orientation a t saturated adsorption state;l2 solution layer thickness was 94 pm approximately. A three-electrode system was used in the experiment, in which GC was used as a working electrode, Pt wire as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode, to which all potentials were measured and reported. A vacuum solution degassing/filling system and experimental procedures are used as in ref 5. T r e a t m e n t of GC Electrode Surface. A GC electrode surface was polished successively with 1-, 0.3-, and 0.05-pm a-Al203respectively and then washed each time in an ultrasonic water bath for 1 min, rinsed with water, and gradually dried in clean surroundings. After the adsorptive experiment, the GC electrode surface was cleaned with lens paper, soaked in base aqueous solution containing KMn04 for 10 min, and rinsed with water thoroughly, and the MnOz produced can be dissolved with concentrated H2SO4. After that, the procedure of polishing and washing described above was repeated. With this treatment, a good reproducibility of the electrode surface can be obtained.
Results
Under the e x p e r i m e n t a l conditions, an absorbance spectrum of CPB in 200-500-nm regions has been obtained with m a x i m u m absorbance at 260 n m as shown i n Figure 2. Besides the GC surface, CPB can b e adsorbed on quartz windows of the cell, which m a y have finite absorbance but c a n be corrected with the absorbance of the vacuum cell measured before injection of the solution. The wavelength selected for optical monitoring was 260 nm, and zero a d j u s t m e n t was m a d e at 500 nm where no absorption occurred. According to the principle described above, the surface coverage of CPB at the electrode surface over the concentration range of (1.00-8.00) X M aqueous solution containing 0.10 M KBr has been measured. The absorbance measuring accuracy of the spectrophotometer is 0.001 au. Each experimental p o i n t has been measured at least t w o times with a mean relative deviation of 5 5%. The measured adsorption data a r e listed i n Table I, from which the isotherm (I'-C) and differential isotherm (AI'/AC C) of C P B has been m a d e as shown i n Figure 3. It has been seen that with the increase of concentration of CPB,
-
~~~
~~
~~
~~
(12) (a) Brown, A. P.; Anson! F. C. Anal. Chem. 1977,49,1057. (b)Li, Nanjiang; et al. Acta Chim. Sin. 1984, 42, 1057.
CPB Adsorption at a GC Electrode
Langmuir, Vol. 7, No. 2, 1991 391
curve a shows four plateaus and three distinct inflection points, which correspond to three peaks in curve b. It is considered that the adsorption of CPB on the GC surface has four stable orientations corresponding to four plateaus in curve a and three orientation transitions correlated with three inflection points in curve a or three peaks in curve b. According to the calculation method proposed by Hubbard et al.13and parameters of molecular structure of CPB, bond lengths are dc=c = dC=N = 1.4 A in the pyridinium ring, Dc-H = 1.1 A, dC-N = 1.47 A, and dc-c = 1.54 A, and bond angles are LCCC = LHCH = 109.5'. With a van der Waal's radius of H at 1.2 A and the width of aromatic ring a t 3.4 A,14 we can estimate that the occupied area (a) of a pyridinium ring adsorbed with the flat orientation is a,+ = 42.1 A2,with the edge a2,3.,,2 = 22.1 A2 and with the edge U3,4.$ = 25.2 A2. The occupied area of each carbon with two hydrogens in a long carbon chain is 16.9 A2, estimated from sodium dodecyl~ulfonate.~~ The occupied area of methyl is 12.5 A2, with radius of 2.0 A, and that of Bris 12.1 A2, with radius of 1.96 A. The shaded area of a vertical carbon chain is 21.5 A2, with a radius of 2.62 A, and the shaded area of a carbon next to the adsorbed carbon chain with its two hydrogens is 3.9 A2. From the theoretical calculation and experimental data, it can be considered that when the concentration of CPB is below 3.10 X low5M, CPB molecules adsorb on the GC surface in a flat orientation with coadsorption of Br-proven with a differential electric capacity curve, which shows the adsorption and desorption peaks of Br-. The experimentally determined molecular area, UE, is 313.4 A2,while the theoretical molecular area (UT) is 314.0 A2 (Figure 41). When the concentration of CPB is 2.60 X M, seven carbons in the tail of carbon chain rise. This flat orientation with a partially risen tail has CTTof 192 A2 and UE of 189 A2 (Figure 411). When the concentration is 4.50 X 10-5 M, the pyridinium ring has an orientation transition from flat to 2,3-q2edgewise, only seven carbons adsorb on the surface, and Br- is not coadsorbed. The Q and UT for this 2,3-q2 edgewise orientation are 121.3 and 123.2 A2 (Figure 4111). Above the concentration of 6.00 X 10-5 M, orientation of pyridinium ring changes from 2,3-q2edgewise to 3,4-q2 edgewise and the carbon chain rises completely. This orientation has UE = 45.0 A2 and UT = 44.6 A2 (Figure 4IV).
I
w j IV
Figure 4. Orientations of CPB adsorbed on GC electrode surface at different concentration ranges: I, flat orientation; 11, flatpartially rising tail orientation; 111, edge 2,3-7* partially rising tail orientation; IV, edge 3,4-v2vertical orientation.
2.0
t
1 %
I
0.0
0.W
0.20
0.10
0.60
9.80
,
Thermodynamic Treatment of the Adsorption Isotherm According to the Frumkin-Langmuir adsorption isotherm16
--O - PC exp(b0) 1-0
1.03
0.W
0.20
0.40
0.M
0.10
0.m
0.80 R
1.W
1.W)
(4)
where 0 is the fraction of an interface covered by adsorbed species as defined by 0 = F/rm,r is the surface coverage representing the amount of species adsorbed per unit area (mol cm-2) and rm is the maximum surface coverage that can be obtained from the plateaus on the isotherm, C is the adsorbate concentration, and is the adsorptive activity constant of adsorption containing intrinsic equilibrium constant for adsorption and conversion factor from mole fraction to Concentration. b is the interaction parameter controlled by lattice interaction. (13) Soriaga, M. P.; Hubbard, A. T. J. Am. Chem. SOC.1982,104,3937. (14) Pauling, L. C. The nature of the chemical bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1960. (15) Chi,Yunxiang;Zhou, Zemin Coordinationcompound in analytical chemistry; Beijing University Press: Beijing, 1986; pp 199, 372. (16) Frumkin, A. Z. Phys. 1926, 35, 792.
0.w
0.20
0.a
0.60
0.80 0
I.W
o.w
0.x
o x R
1.w
Figure 5. Plots of log 6 / ( l - 0)C vs 8. Parts I, 11, 111, and IV represent the four orientations I, 11, 111, and IV, respectively.
Taking the logarithm of eq 4 0 b o log (1 - 0)C = log p + 2.303
The plot of log O / ( l - 0)C vs 0, a straight line, can be obtained with an intercept of p and a slope of bl2.303. The four kinds of plots related to the four orientations of CPB adsorbed on GC surfaces have been made as shown in Figure 5, in which the experimental points show two straight lines, described by two Frumkin equations. The
392 Langmuir, Vol. 7, No. 2, 1991
Dong et al.
Table 11. Adsorption Parameters for CPB at Different Orientations adsorption orientation
I
I1 111 IV
max surface coverage, lo-" mol cm-2
bl
log 81
5.30 8.64 13.9 37.9
2.38 4.55 3.84 3.37
4.18 3.50 3.64 3.43
bz
log PZ
6.55 2.89 23.0 -3.5 41.5 -6.33 67.8 -21.0
Ocrn
0.68 0.905 0.927 0.945
a O,, is the critical coverage obtained from the cross point of two isotherm equations that is near the maximum point in the differental adsorption isotherm.
parameters (P, b) of adsorption from the plots are listed in Table 11. Each of the CPB orientations can be described with two equations, which belong to Frumkin-Langmuir type. From Table 11,PI > p 2 and bl< bz. The former can be considered an increase of coverage, the adsorption process tends to the equilibrium state, so P 0, and the latter relates to the different hydrophobic lattice interactions. Cantor and Dill17 dealt with the interaction between molecules with long carbon chain theoretically. The lattice interaction between carbon chains includes two parts. The chain configurational (repulsive) and volume-dependent (attractive) contributions to the lateral pressure, both depend exponentially on area and nearly linearly on chain length and are of similarly large magnitude. According to this result, the lattice interaction can be written as
-
b 0: V e x p S (6) where V ,the average volume of adsorbed molecules in one orientation, can be described as
V = LvnLa (7) where n is the number of raising carbons in adsorbed CPB molecules, cr is the occupied area of adsorbed CPB molecule on electrode surface in one orientation state, and L is the effective bond length of C-C. S is the area of interaction between raising carbon chains of adsorbed CPB molecules, which is approximate to the surface area of raising carbon chains of adsorbed molecules and can be described as
S = y2ar'n (8) here r is the radius of carbon chain, y is the correction coefficient, and n ' = [ n / 2 ]( n is defined as before), which represents the effective height of raising carbon chain of adsorbed CPB molecules. Substituting eqs 7 and 8 into 6 b
0:
in the low coverage, S
a n u exp(y2ar2)
b
(9)
blcr
nacr 0: na
(10) (11)
in the height coverage, take the logarithm of eq 9 In b 0: In ( m u ) + 2 ~ y 'n (12) in this case, In b a n '. Plot b / o against n and In b against n ' with the data from Table I1 (Figure 3) show very good linear relationships. From Figure 6, it can be seen that each orientational adsorption can be described with two Frumkin-Langmuir equations, they are different in lattice attractive force, the one with low 8 is volume-dependent and the other (17) Cantor, R. S.; Dill, K. A. Langmuir 1986,2, 331.
4
8
12
16
a
6
4
2
0
n n'
Figure 6. Plots of bl/a vs n (a) and In bz vs n ' (b). Table 111. Free Energy of Each Orientation orientation O,, AGads 1, J mol-l AGads 2, J mol -l I 0.68 -2.39 x 104 -1.65 x 104 2.00 x 104 I1 0.905 -2.00 x 104 111 0.927 -2.08 x 104 3.61 x 104 IV 0.945 1.20 x 105 -1.95 x 104 Table IV. Parameters of Orientation Transitions transition process
change of free energy, J mol-'
equilibrium constant
I1 I11 111 IV
AGP - A G ~ I -3.48 x 103 A G - A~ G -4.08 ~ x 104 AWV - ~ ~ ~ -5.57 1 1 1x 104
4.07 1.39 x 107 5.70 x 109
--
I I1
with height 8 depends exponentially on surface area of raising carbon chain of adsorbed CPB molecules. The Gibb's free energy for each orientation can be calculated from and b by18 AG,,, = -RT In P - b8
(13)
and listed in Table 111,where Ocr is obtained from maximum points in the differential adsorption isotherm related to the orientation which are close to the cross points of two Frumkin-Langmuir isotherms for each orientation. The change of free energy in the adsorption process is from a large negative value to a positive one, which implies a tendency from a spontaneous process to an equilibrium state, then a new orientation begins with a new negative free energy until a final stable orientation is formed. The change of free energy in orientation transition can be obtained from the difference between the free energy of the former orientation established by saturated adsorption and that of the adsorption orientation of newly formed, which is a driving force for the orientation transition. From the relationship between change of free energy and equilibrium constant in orientation transition AGO = -RT In K
0, eq 9 becomes 0:
0
(14)
The equilibrium constants of orientation transition are obtained and listed in Table IV. From the free energy AG and equilibrium constant of orientation transition, we can observe the driving force of orientation transitions and the tendency of equilibrium, which are dominated by the stability of saturated adsorption for both orientations, resistance of orientation transition and the space supplied in the orientation transition. In the I I1 transition process, seven carbons raise from the electrode surface, the change of free energy and K are small because of the weak interaction between the tail of the carbon chain and the electrode surface resulting in the facility of tail warping. In the transition
-
(18)Koopal, L. K. In Fundamentals of adsorption; Myers, A. L., Belfort, G., Eds.; Engineering Foundation: New York, 1984; p 283.
CPB Adsorption at a GC Electrode
-
of I1 111, orientation transition of the pyridinium ring is shown with large K , because of the stable orientation states of I1 and 111 and large space supplied and small resistance in the orientation transition. By the same reasons, the large K can be obtained in the orientation transition from 111 to IV.
Langmuir, Vol. 7, No. 2, 1991 393 From theoretical treatment of the adsorption isotherm, we can obtain the thermodynamic evidence of four orientationsand three large orientation transitions for CPB on a GC electrode surface. The same treatment was also proposed to be used for other compounds adsorbed on an electrode surface.
