Langmuir 1992,8, 2804-2809
2804
Adsorption of Surfactants: Differential Capacitance Studies Using Phase-Sensitive Alternating Current Voltammetry Antonis Avranas and Nikos Papadopoulos’ Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54006, Thessaloniki, Greece Received April 3,1992. In Final Form: July 21, 1992
The interfacial behavior of cetylpyridinium chloride adsorbed on a hanging mercury drop electrode has been studied using three-dimensional phase sensitive ac voltammetry (3D/AC/V). This methodology, which has recently been developed in our laboratory, possesses a very wide time window that permits a detailed study of adsorption phenomena. It is based on the reconstruction of the differential capacitance (C)versus applied potential (E) curves, sampled after many phase-sensitive ac chronoamperometric experiments. Cetylpyridiniumchloride molecules, which aggregateto form a range of association structures in solution,strongly bond onto the electrode surface. They also associateinto a variety of surfaceaggregates on mercury depending on their concentration and the applied potential. A capacitance oscillation appears at a certain potential for a wide range of concentrations. Introduction Adsorption of surfactants from aqueous solutions onto particulate solids has been the subject of important fundamental research for many years1+ because of potential applications in many fields. The adsorption of surfactants on electrode surfaces has also attracted the attention of electrochemists. Research on their adsorption is important for understanding mechanisms of electrode reactions on synthesis and electrocataly~is.~*~ Numerous studies have been carried out at variouselectrodesurfaces, such as Pt,7+ glassy carbon,1° or mercury,11J2 to find structure-function relationships on electrode surfaces. It is well-known that surfactants in dilute aqueous solutions and dispersions assemble into a variety of microstructuressuch as micelles, microemulsions,vesicles, and liquid crystalline structures of biological and technological importance.13 The cationic surfactant cetylpyridinium chloride also forms aggregatesby self-assembly of the molecules in s01ution’~J~ when its concentration exceeds the critical micellar concentration (cmc). The evolution of the morphology of the various types of aggregates formed with or without additives is a subject of current interest.16J7 In aqueous solutions the cetylpy-
* To whom correspondence should be addressed.
(1) Chandran, P.;Somasundaran, P.;Turro, N. J. Colloid Interface Sei. 1987,117,31. (2)Meguro, K.; Tomioka, S.;Kawashima, N.; Esumi, N.B o g . Colloid Polym.Sei. 1983,68,97. (3)Viaene, K.; Caigui, J.; Schoonheydt, R. A.; De Schryrer, F. C. Langmuir 1987,3,107. (4)Hasaan, M. H.; Zollars, L. R. J. Colloid Interface Sci. 1991,146, 299. (5)Rusling, J. F.;Shi, C.; Gosser D. K.; Shukla, S. J. Electroanal. Chem. 1988,240,201. (6) Rusling, J. F.; Couture, E. Langmuir 1990,6,425. (7)White, J. H.; Soriaga,M. P.;Hubbard,A. T.J. Electroanal. Chem. 1986,185,331. (8) Hubbard, A. T. Langmuir 1990,6,97. (9)Whitesides, G.M.; Laibinis, P.E. Langmuir l990,6,87. (10)ShaojunDong;Yongchun Zhun; Guangjin Cheng Langmuir 1991, 7.389. (11)DBrfler,H.-D.;Bergk,K.H.;MClller,K.;Miiller,E. TensideDeterg. 1984,21,226. (12)Miiller, E.; Miiller, K.; DBrfler, H.-D. J . Colloid Interface Sci. 1987,116,334. (13)Vinson, P.K.; Bellare, J. R.; Davis, H. T.; Miller, W. G.; Scriven, L. E. J . Colloid Interface Sci. 1991,142,74. (14)Porte,G.; Appell, J.; Poggi,Y. J. Phys.Chem. 1980,84,3105. (15)Porte,G.;Appell, J. J. Phys.Chem. 1981,85,2511. (16)Tiddy, G. J. T. Phys.Rep. 1980,57, 1. (17)Kohler, H.-H.;Stmad, J. J . Phys.Chem. 1990,94,7628.
0743-7463/92/2408-2804$03.00/0
ridinium chloridemolecules form spherical micelles. The addition of an amount of salt changes a number of static properties of a micellar solution of cetylpyridinium chloride,i.e. decreasethe cmc and increase the aggregation number. With an increase of the salt concentration a sphere-to-rod transition is induced that has been thoroughly i n ~ e s t i g a t e d . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ The adsorption of surfactant ions on mercury causes a change of the electrode double-layerdifferential capacity (C)which is the result of an exchange between the counterions and water molecules on the electrode surface with surfactant ions from the solution. Phase sensitiveac polarography has been used as a convenient and rapid method for surfactant concentration determination.11J2 Adsorption phenomena take considerable time to complete. A technique is required for the elucidation of the adsorption mechanism with a time window giving the opportunity to investigate adsorption processes at both short and long times. Three-dimensional phase sensitive ac voltammetry (BDIACIV) is a fully automated methodologythat has been recentlydeveloped in our laboratory. It has been proved to be extremely useful in studying the kineticsof complex adsorptionmechanisms onto a hanging drop mercury electrode (HDME) with respect to both electrode potential and time under strictly potentioetatic conditions. The kinetics of adsorption processes are studied by reconstruction of the differential capacitance (C)versus applied potential (E) curves for selected time values. This is done by sampling the capacitance current after a potential step has been applied to the electrode and then plotting isochronous C data against the final potential values of the correspondingpulse experimente. Three-dimensional phase sensitive ac voltammetry can be considered as a kind of ac polarography with a fineresolved time window. The aim of this paper is to study the adsorption of cetylpyridinium chloride on HDME at concentrations below and above its cmc using three-dimensional phase sensitive ac voltammetry. The extended time window of this method can give both short and long time results to achieve a better understanding of the adsorption mechanism. (18)Myignan, J.; Appell, J.; Baeeereau, P.;Porte,G.;May, R. P.J. Phys.(Pane)1989,50,3553. (19)Gomati, R.; Appell, J.; Bassereau, P.;Marignan, J.; Porte,G. J. Phys.Chem. 1987,91,6203.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 11, 1992 2805
Adsorption of Surfactants
C
r
- a2
Figure 1. Three-dimensional representation of the C-E-t surface for (a) 1 X aqueous solutions in 0.1 M NaCl.
Experimental Section Cetylpyridinium chloride was obtained from Fluka and was further purified by two recrystallizationsin water and one in wet acetone (2 g of water in 100 cm3 of acetone).19 The absence of minimum in the surface tension versus log concentration curve indicated the absence of surface active impurities. The surface tension of solutions of cetylpyridinium chloride was measured with a Kriisselectronictensiometer employingthe plate method. The cmc was found 9.0 X lo4 M in water and 4 X M in the presence of 0.1 M NaCl in good agreement with Heckmann et aL20 NaCl from Merck (p.a. 99.5%), which served as the supporting electrolyte, was used as received. The experiments were carried out at 25 "C using surfactant concentrationsranging from 5 X lo* to 5 X lo-' M, in the presence of 0.1 M NaCl. The solutions were deareated with nitrogen before each experiment while during the measurement nitrogen was kept flowing continuously over the solution. The capacitive current (I,) of the double layer formed between a HMDE and aqueous 0.1 M NaCl solutions of cetylpyridinium chloride was recorded by means of phase-sensitive ac chronoamperometry using a PAR Model 170 electrochemistrysystem interfaced to an AT compatible (VIP 200 12 MHz) via a 14-bit AD/DA card. A standard three electrode cell arrangement was used in this study. As reference electrode a saturated calomel electrodewas used and a HMDE served as the working electrode. Each mercury drop was formed within a 3-s period at -0.15 V vs SCE (to which all potentials in this paper are referred), where preliminary DME experiments for short drop times showed a limited adsorption. After a 0.5-s rest time, a potential step with a 5-mV peak-to-peak modulation at 740 Hz was applied to the electrode and the capacitive current (90"out of phase signal) was recorded as a function of time. The surface area of the working electrode was 0.002 615 cm2. The drops were produced by an isochronous motor (5 rotations/min) which was adjusted on top of Metrohmtype E410 HDME. The rotation of this motor was controlled by the computer via a relay giving reproducible drops. At the end of each experiment the mercury was dislodged by intense deaeration,which was controlledby an electromagnetic valve. These phase sensitiveac chronoamperometricexperimenta were performed successively in 10-mV intervals for the whole potential range studied (i.e. -0.15 to -1.65 V vs SCE) and the capacitive current was sampled every 5 ms. The reconstruction of I, vs E or C vs E curves for various times, from isochronous I,-E or C-E data obtained after chronoamperometricor phase-sensitive ac chronoamperometric experiments, that has both the advantages of chronoamperometric and voltammetric techniques,2' was performed with an (20) Heckmann, K.; Schwarz, R.; Stmad, J. J. Colloid Interface Sci. 1987,120, 114. (21) Bard,A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications; Jon Wiley & Sons: New York, 1980.
M and (b) 5 X
v
lo-" M cetylpyridinium chloride
on-line computer. A computer program, written in Basic creates from the experimental data ASCII files that are used as imput to graphic programs that create the I,-E-t surfaces and the (IcE)t plots. The three-dimensionalsurfaceshave been createdwith the help of Perspective and the two-dimensionalplots have been created with the help of Harvard Graphics computer programs.
Results and Discussion and 5 X loa M The Ic-E-t surfaces for 1 X cetylpyridinium chloride are given in Figure 1, which provide an overall picture of the whole adsorption phenomenon. In these surfaces is embodied all kinetic mechanistic informationresultingfrom capacitance change during the adsorption process in the double layer. The shortest time useful information can be collected depends on the settling time of the lock-in amplifier (300ms in our case). The longest time of exposure of the interface to the solution (70 s) ensures a satisfactory establishment of the adsorption equilibrium. Longer time experiments were avoided because in these experiments it is indispensable to reduce as much as possible the level of absorbable impurities in the bulk.22 Convection will also become important at long times. In some previous works the shape! of such three-dimensional plots had been displayed but no attempt had been made to extract useful information from these curve^.^^-^^. In Figures 2, 3, 5, 6, and 7 are given the differential capacitance vs applied potential curvesof the double layer, formed between the polarized HDME and aqueous surfactant solutions in 0.1 M NaC1. Selected time values ranging from 0.5 to 70 s give a picture of the adsorption phenomena. These figures clearly display that cetylpyridinium chloride is strongly adsorbed onto mercury and different phase transitions take place, depending on ita concentration,the applied potential, and time elapsed from the instant of formation of a clean mercury solution interface. These curves both have the advantages of a chronoamperometric technique, i.e. they are taken under strictly potentiostatic conditions, and also give the possibility of measuring for long time .periods. Adsorption of surface-active compounds on electrodes is a heterogeneous process by its very nature and mass (22) Vetterl, V.; de Levie, R. J. Electroanal. Chem. 1991, 310, 305. (23) PospiiEl, L. J. Electroanal. Chem. 1986,206, 269. (24) Tallman, D. E.; Shepherd, G.; Mackellar, W. J. J. Electroaal. Chem. 1990,280,327. (25) Sun, S. G.; N d l , J.; Lipkowski, J.; Altounian, Z. J. J.Electroanal. Chem. 1990,278,205.
Aurancrs and Papadopoulos c=
0
0.5
1
5 x 1c6
1.5
C( P/")
I
2
- E (Volt vs SCE)
Figure 2. Differential capecitance va applied potential curves for selected time periods, of the interface formed between a HDME and 0.1 M NaCl aqueous solution of 6 X 10-6 M cetylpyridinium chloride. The concentration of the surfactant is well below ita cmc and the adsorption kinetics in the time scale of our experiment are mainly controlled by mass transport.
0
0.5
-E
1 1.5 (Volt v s SCE)
0
15
30
45 t
60
(8)
I
2
Figure 3. Differential capacitance va applied potential curves for selected time periods, of the interface formed between a HDME and 0.1 M NaCl aqueous solution of 1 X 1od M cetylpyridinium chloride. The concentration of the surfactant is well below ita cmc and the adsorptionis controlledby diffusion for accumulation time as long as approximately 20 s. A combination of diffusion and phase change kinetics increases the capacitance for longer times and at more negative potentiale.
transport always plays an important and frequently dominant role. For low concentrations of the adsorbate and in the case of very strong adsorption on the electrode, The difthe adsorption is controlled by diffusionsm ferential capacitance vs applied potential curves of the double layer formed between the polarized hanging mercury electrode and 5 X 10" and 1 X 106 M cetylpyridinium chloride are presented in Figures 2 and 3. For the diffusion-controlled adsorption of species at the mercury/solution interface a plot of the capacitance C versus the square root of time t should yield a straight line.n*31*32In Figure 4a are shown some capacitance transients followingpotential steps from 4.14 to 4.63 V (outaide to inside the pit region) for 1 X 106 M cetylpyridinium chloride. The aame data are plotted vereue t1I2 in Figure 4b. A very good linearity is observed for accumulationtime as long as approximately 20 a, meaning that for thie concentration and for this time period the adsorption kinetics are mainly controlled by maes transport. The aame linearity is also observed for the lower concentration for the entire transient range. In thie low concentration range below cmc (Figures 2 and 31, monomers of cetylpyridinium chloride exbt in the bulk. These monomere are adsorbed on the electrode and are surface orientated. The electrode potential controls (26)Jehring, H.J. Electrwml. Chem. 1969,21,77. (27)Delahny, P.;"rachtenberg, I. J. Am. Chem. Soc. 1967,79,2365. (28)Delahay, P.;Trachtenberg, I. J. Am. Chem. Soc. 1968,80,2094. (29) DeLahay, P.;F i b , C . J. Am. Chem. SOC.1968,80,2628. (30)Nikitee, P.; Papoutah, A. Electrochim. Acto 1988,39,683. (31)Koryta, J. Collect. Czech. Chem. Commun. 1963,18,206. (32)Shidharan,R.;de Levie, R. J. Electroanal. Chem. lS86,206,303.
t
lh
Figure 4. (a) Capacitance-time dependence measured in 0.1 M NaCl aqueous solution of 1 x 1V M cetylpyridinium chloride following a potentialstep from 4 . 1 4 to 4.63V. (b)Capacitance plotted as a function of t1I2. The very good linearity indicatea that the adsorption is diffusion controlled for approximately 20 8.
the surface coverage. The existence of different adsorbed states is likely to be connected with different orientations of the adsorbed species at the interface and coverage and hence with various degrees of close packing. The double layer capacitancesubstantiallydrop in the potential range from 4.5 to -1.1 V (centered around 4.8 V). The point of zero charge (pzc) on mercury is approximately 4.5 V. Cetylpyridinium chloride has a nitrogen-containingheterocyclic head that can interact with mercury through ita conjugate T orbital and or with C1- anions situated on the outer Helmholtz plane where the positively charged pyridine group is able to form an ion pair with an adsorbed Cl- anion,ss at potentials positive to the pzc. At the left edge of the pit, and for this low concentration range, it is very probable that the monomers are adsorbed with the alkyl chain lying flat on mercury. With an increase of the potential to more negative values, the positively pyridine group may be attached to the surface, leavingthe long tail away from the surface. In the m e of cetylpyridinium bromide adsorbed at a glasey carbon electrode surface,1o depending on ita concentration, four dinstict orientations have been reported, with the pyridine head groupadjacent to the electrodeand most or part of the alkyl chain directed toward the solution. Some surface-enhanced raman spectroscopy dataw have been used to tell whether the head group is adsorbed in a flat or end-on orientation. According to these data cetylpyridinium chloride is in a standing up confiiation at the electrode surface. According to a detailed surface-enhanchedraman spectroscopy of adsorption of cetylpyridinium chloride for con(33)Bunding, K.A.;Bell, M. I.; Duret, R. A. Chem. Phys. Lett. 1982, 89,M. (34)Mcekovita, M.;Suh,J. S. J. Phy8. Chem. 1988, Se, 6327.
Langmuir, Vol. 8, No. 11, 1982 2807
Adsorption of Surfactants
0
0.5
1
1.5
2
1.5
2
-E (Volt v s SCE)
0
0.5
1
-E (Volt vs SCE)
Figure 6. Differential capacitance vs applied potential CWBB for (a) short and (b) long times of the interface formed between a HDME and 0.1 M NaCl aqueous solution of 2 X 1od M cetylpyridiniumchloride. The surfactantconcentration is below but cloeeto ita cmc;howeversurface aggregationand micellization are observed in a few seconds. The electrified interface helps the formation of different types of micelles.
centrations above ita cmc on Ag,36 the pyridinium head group is attached to the surface with the alkyl chain directed toward the solution at 4 . 2 V, potential which is positive to the pzc on Ag, while on increasing the potential positiveto the pzc a quite possible change in the orientation of the surfactant is observed. From our data it is difficult to clarify the orientation of the molecules for all the potential range studied. It is very interesting to note here that for surfactant concentration equal to 1 X M and at longer times of accumulation (70a, Figure 3)an increasein the capacitance with time is observed (at potentials more negative than 4.7 V),while the left edge of the pit is moving toward less negative potential values. The same phenomenon is observed within less than 5 s at 2 X lo"M cetylpyridinium chloride (FigureSa),concentrationwhich is below the cmc. As more adsorbate is being transferred from the solution to the electrodesurface,the surface concentration increases and the surface aggregation is accelerated. The time dependence of the differential capacitance YB potential is due here not only to diffusion but to a combination of diffusion and phase change kinetics. The association of surfactant molecules occurs in a stepwise manner with one monomer added to the aggregate that is formed at a time. When the concentration is 2 X 10-6 M,only a few secondsis needed for surface aggregationand micellization. Severalmodelsknown as hemimicellea and admicelleshave been proposed to account for the structural characteristics and the formation mechanism of the adsorbed surfactant layers on solids.*a The picture of the molecules on the electrode surface, when micelles are formed, looks very (35)Sun,S.;Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1990,94, 2005.
(36)Somaeundaran, P.; Fueretenau, D. W. J. Phya. Chem. 1966,70, 90. (37)Schamehorn,J.F.;Schecter,R. S.;Wade, W. H. J. Colloid Interface Sci. 1982,86,463. (38)Dick, S. G.;Fueretenau,D. W.; Healy, T. W. J. Colloid Interface Sci. 1971,97,595.
much like the situation in the bulk, but it is more likely micelles on the surface to be two-dimensional in the beginning of their life. The capacitance continuously decreases until the complete coverageof the surface of the electrode with cetylpyridinium ions. In the presence of excesssurfactant, a two-dimensionalphase transition from a gaseous to a liquid-expanded state of the adsorbed layer of cetylpyridinium chloride takes place. This phase transition is accompanied with an increase in the differential capacitan~e.~~ In Figure 6 the concentration of cetylpyridiniumchloride in the bulk is less than, but very close, to the cmc. The electrode accumulates the adsorbate and after some time the concentration of the adsorbate inaide the double layer will be greater than the cmc. It is quite reasonable to expect surface aggregation phenomena to appear on the electrode at a lower concentration than aggregation phenomena in the bulk. The appearanceof deformedor even split up capacitance peak at cathodic polarizations on Hg is well-known to be a common feature of the adsorption of many micelleforming ~ u r f a c t a n t a . ~This ~ * ~was first attributed to the fact that at concentrationsat or around cmc, both micellar and monomeric entities are adsorbed on Hg at different potential values and a different peak should be expected for the adsorptionldesorption of each surfactant form. According to an alternative treatment of the micelleformingsurfactant on Hg,&the existenceof micelleawithin the charged interface is treated as a phase change: the transition of a saturated surface solution of monomers into a surface micellar phase. For surface concentrations around or above cmc, surface micellization is likely to extend severalmolecular diameters within the adsorption layer. Since each micellization process leads to a capacitance peak,'g the micellization within successive layers should lead to deformed or split up peaks. In surfactant solutions at concentrationsexceedingtheir cmc, micelles coexist with monomers. Recent evidencein the literature involving the use of the surfactant selective electrodeto monitor monomer concentrationof surfactants indicates that the concentration of monomers is not constant above cmc but decreases.up4 It is well-known that cetylpyridiniumchloride moleculesat concentrations above the cmc form Spherical micelle^.'^ The addition of high salt concentrationsmay changethe sphericalmicelles of cetylpyridinium chloride to large elongated micelles (a sphere-to-rod transition) which are pictured as longflexible cylinders expecting to display polymer-like behavior.18It has been showna that the sphere-to-rod transition is one of the possible evolutions of the shape and the size of micelles. These micelles have the characteristic that they exist with a large distribution of lengths in dynamic ~ ~ another sequence equilibrium. It is also k n 0 w n l ~ 1that for the local structure of cetylpyridinium chloride aggregates due to self-assembly of the surfactant is the planar structure, i.e. the formation of a thin bilayer of very small (39)Knhheva, M.;Saraivanov, G.; Anantopodon, A. Longmuir 1991, 7,2580. (40)Vollhardt, D.Colloid Polym. Sci. 1976,264,64. (41)MiLller, E.;Dbfler, H. D. Temide Deterg. 1977,14, 75. (42)Nikitar, P.;Sotiropodon,9.; Papadopodon, N. J. Phyu. Chem., in prec38. (43) Buwa-Hennan, C. J. Electroanal. Chem. lS.S6,186,27. (44)W,T.;Hattori, M.;Sasalri, J.; Nulrina, K. Bull. Chem. Soc. Jpn. 1976,48,1397. (45)Cutler, S. G.; Meares, P.; Hall,D. G. J. Chem. Soc., Faraday Tram. 1 1978,74,1768. (46)haelachvili, J. N.;Mitchell, D. J.; Ninham, B.W .J. Chem. Soc., Faraday Tram.2 1976, 72,1525. (47)&,I. J.;Zimm&,Y. LnSolutionBehouiorofSurf~tantu;Mittrl, K. L., Fendler, E. J., U.; Plenum Prm: New York, 1982;Vol. 1,p. 464.
2808 Langmuir, Vol. 8, No. 11, 1992
Avranaa and Papadopouloe a
0.5s 0
-
1s
5-i
5s
2s -
._
,
c=5 x 10-1
,
0.5
0
lo)
A
V
5
0.5
1
1.5
0.5
0
-E (Volt vs SCE)
Figure 6. Differential capacitance vs applied potential curves of the interfaceformedbetween a HDMEand 0.1 M NaCl aqueous solution of 1 X lo-' M cetylpyridinium chloride: (a) for short times where diffusion is completed in lese than a second and the f i i grows in the third dimension; (b) for long times where a formation of a polylayer is observed at potentials near to -1 and -1.3 V. The concentration of the surfactant is above ita cmc.
1
70s -
0 2
2
10s -
L,
,Ot
:: I
-
1 15 -E (Volt vs SCE)
5
70s -
0 0
,
0
1 1.5 -E (Volt v s SCE)
2
Figure 7. Differential capacitance vs applied potential curvea for selected time periods, of the interface formed between a HDME and 0.1 M NaCl aqueous solution of 5 X lo-' M cetylpyridinium chloride. The polylayer formation ie obeerved at shortertimes on increasingsurfactant concentration (compare with Figure 6b). 2o N / c m
2
I
thickness comparedto ita extension. Both types of micelles (rod and planar) are unrestricted in their growth in length and thus are able to accommodate any number of cetylpyridinium chloride molecules. There is no a priori reason to preclude the possible coexistence of micelles of various shapes in solution or on the mercury surface. It is very probable that the existence of the electrified interfacehelps the formationof rodlike and planar micelles that are directed toward the interface. This polydispersity of micelles in the bulk, predicted by Mukerjeea and Israelachvilli,a explains the splitting of the differential capacitance peaks and the deformations observed. At higher concentrations (Figures 6 and 7) our instrumentation is not able to follow the diffusion phenomena. They are generally expected to be completed in less than a and diffusion as a rate-determining step can be neglected.s0 In Figure 6a the C-E curves do not change appreciably within 5 a. It is very interesting to observe here the largedifferencesthat are observed between curves recorded at short and at high adsorption times. The situation which is observed especially in Figure 6a may be attributed to an adsorption mechanism characterized by fractal dimension which evolves with time. An analogous mechanism, i.e. the growth of the f i i in the thir,d dimension, has been proposed by Wandlowski and PospiEii123*61952 in order to explain the capacitance maxima they recorded a t long periods in the case of uracil and some imine complex adsorption on Hg. The low rate of film formation even in the center pit potential leaves enough time for deposition on top of monolayers formed. The growth in the third dimension forms a structured layer which impedes the access, by bulk species, to still uncovered areas of the electrode.61.62
At longer times of adsorption (Figures 6b and 7b) a formation of a polylayer is observed at potentials near -1 and -1.3 V. The capacitance vs time transients are S shaped (Figure8). This type of transient is characteristic of a nucleation and growth mechanismP2 During the fmt nearly horizontalpart, nuclei are formed. In the gradually descending part, which is an induction period in the experimentallymeasured capacitance,that has a sigmoidal shape, the nuclei grow and spread. Finally, in the second horizontal part of the capacitance transient a halting of further film growth takes place, because the film fragments have coalesced to cover the entire electrode. Very similar S-shaped capacitance transient have also been obtained during the adsorption of other substanceson mercury such as adenine,2l cetyldimethylbenzylamonium cMoride,m thymine,&uracil,&thiouracil.68 In the case of adsorption of cetylpyridinium chloride on W bat very negative potentials where the interface becomesmore hydrophobic,
(48)Mulrerjee, P.J . Phys. Chem. 1972, 76,565. (49) Guidelli,R.;Moncelli, M. R.J. E l e c t r o a d . Chem. 1978,89,261. (50) Retter, U.J. Electroanal. Chem. 1980,106, 371. (51) Wandlowski,T.;Poe.phil,L.J.Electroanu1. Chem. 1989,268,179. (52) Wandlowski, T.;Posphil,L. J. Electroanal. Chem. 1989,270,319.
(53) Papadopouloe,N.;Sotiropoulos,S.;Nikitan,P.J. Colloid Interface Sei. 1992,161, 523. (54) Sridharan, R.;de Levie, R.J. Electroanul. Chem. 1986,201,133. (55) Wandlowski, T.J . Electroanal. Chem. 1991,302, 233. (56) Wandlowski, T.J. Eiectroanal. Chem. 1991,312,246.
-0.98 Volts 0
0
20
40
60
Time (seconds)
Figure 8. Capacitancetranaienta,S shaped, following potential steps for 1 X lo-'M cetylpyridinium chloride in 0.1 M aqueous NaCl for -0.98 and -1.31 V where the formation of a polylayer is observed. These transients are characteristic of a nucleation and growth mechanism.
Langmuir, Vol. 8, NO.11,1992 2809
Adsorption of Surfactants
the appearance of two intensity maxima of the C-H stretching modes in a surface enhanced raman spectroscopy has also been attributed to a new highly organized structure at the interface. This qualitative interpetation of the nucleation and growth mechanism, provided there is a statistically large number of nuclei, by using the simple mathematical formula of Canac et al.67-'3" ln (-ln (1- 6 ) ) = mln t + l n b can be put on a quantitative basis. 0 is the fraction of the surface covered by the condensed film,m is the nucleation order, t is the time, and b is a parameter that depends on the rate constants of growth process and nucleous formation. This is known as the Avrami equation. The analysis of C,t data a t potentials closeto the pit edge yields a linear dependenceof In (-ln (1- 0 ) )with time, supporting the nucleation and growth mechanism. This analysis yields the slope m = 4 for both -0.98 and -1.31 V for 1 X 10-4 M cetylpyridinium chloride of Figure 8. Finally a very interesting phenomenon is observed at potentials equal to -1.31 f 0.02 V. This is the appearance of capacitance oscillations in the C-t curves (Figure 9) These oscillations appear at concentrations higher than 1 X lo+ M and are similar to those found for the adsorption of adenine at the mercury-water interface in the initial times (up to 10 A quantitative interpretation of the appearance of these oscillations cannot be given. At this potential, the desorption peak of the monomers is observed at low concentrations and the formation of a highly ordered polylayer is observed at higher concentrations. The desorption of the monomers has as a consequence an increase in the capacitance of the double layer while the (57) Canac, C . C. R. Acad. Sci. 1933, 196, 51. (68) Kolmogoroff, A. A. Bull. Acad. Sci. USSRISci. Math. Nat. 1937, 3, 356. (59)Avrami, M.J. Chem. Phys. 1940,8, 212. (60)Evans, U.R. Trans. Faraday SOC.1946,41,365.
111 0
10
20
25
Time (seconds)
Figure 9. Capacitanceoecilhtionsfollowingsinglevoltage etap at -1.31 V, in 0.1 M NaCl aqueous solution of 1 X 1od M cetylpyridinium chloride. Similar oscillations also appear at higher concentrationsand at this potential region.
formation of a polylayer leads to a decrease in the capacitance. It is probable that even a t very low concentrations, nuclei of this polylayer, which is stably adsorbed on the electrode surface, are formed. The competition of these two effects seems to produce these oscillations.
Conclusions Strong kinetic effectsare observed duringthe a b r p t i o n of cetylpyridinium chloride onto mercury. The major experimental features of the adsorption process, which seemsto be very complex, are given. The lack of a suitable model system and of a quantitative theory explaining all the adsorption phenomena for both short and long times makes them sometimes difficult to understand and does not permit a more quantitative data analysis. The kinetic effectsare mainly due to diffusion at lower concentrations and to phase transitions at the interface at higher concentrations. Three-dimensional phase sensitive ac voltammetry which has an extended time window provides a convenientway to study a slowly proceeding adsorption. Registry No. Hg, 7439-97-6;cetylpyridiniumchloride, 123-
03-5.
