Langmuir 1992,8, 571-576
571
Ellipsometric and Reflectance Study of Electroadsorption from a Water-Based Metalworking Fluid onto Gold Surfaces Robert B. Bjorklund' and Hans Arwin Laboratory of Applied Physics, Linkiiping University, S-581 83 LinkGping, Sweden
Ingeglird Johansson and Rolf Skold Metalworking Chemistry Laboratory, Berol Nobel AB, S-444 85 Stenungsund, Sweden Received June 17, 1991.In Final Form: September 16, 1991 Thin films were formed on gold electrodes in contact with a model metalworking fluid by applying positive potentials. The fluid was composed of nonanoic acid, diethylene glycol monobutyl ether, and N-butyl-N,N-bis(2-hydroxyethy1)aminedissolved in water. Concentrationsof the anionic surfactant were above the critical micelle concentration (cmc). Threshold voltages for f i i formation were strongly dependent on water to ether and acid to amine ratios. Kinetic studies by ellipsometry indicated that the film growth was initiated by a voltage-induced disruption or precipitation of micelles at the electrode1 solution interface. Reflectance measurements detected the effect of an added cosurfactant, 1-dodecanol, on the film desorption. Introduction Adsorption of surfactants on solid surfaces plays an important role in several industrial processes. A large market exists for metalworking chemicals used as lubricating, degreasing, cooling, and anticorrosion fluids. Environmental concerns have increased interest in the development of water-based metalworking fluids aimed at replacing the neat oils currently used in many applications. Although the advantages gained by using waterbased metalworking fluids are significant, there are also problems associated with their use. Metalworking of active metals, such as aluminum, exhibiting high reactivity with water poses serious problems where complete coverage of the metal surface by the surfactant is necessary. We have studied electroadsorption from a model waterbased metalworking fluid onto gold surfaces in order to evaluate if adsorption of surfactants can be enhanced by applying an electric potential between the fluid and the metal surface. The active component in the fluid was an anionic surfactant, nonanoic acid. Applying positive potentials to the gold electrodes resulted in reversible film deposition from the fluid. Film formation was also observed on Cr, Ni, Co, and Fe surfaces, but oxide growth made interpretation of these results difficult. Nonanoic acid is only slightly soluble in water, and it was neutralized in the fluid by N-butyl-Nfl-bis(2-hydroxyethy1)amine. In addition, diethylene glycol monobutyl ether was also included as cosolvent and some work was done with 1dodecanol as nonionic cosurfactant. The model fluid was thus a complicated solution chosen for its similarity to compositions used in industry. The interplay between the different components was also important for obtaining electroadsorption of the fluid at applied potentials below that for gas evolution. Reflectance measurements, ellipsometry, and cyclic voltammetry were used to follow the adsorption of surfactant on the surfaces. Reflection of light from the surface was a convenient way to follow total surfactant adsorption and desorption from the electrodes. Ellipsometry is based on reflection of polarized light from a surface and is sensitive to optical changes at the interface between the
electrode surface and solution.' Ellipsometry has been applied to a variety of problems in surface chemistry. A description of the technique and an overview can be found in ref 2. Ellipsometry has also been used previously to study the formation of surface micelles by applying potentials on platinum electrodes in contact with sodium dodecyl sulfate (SDS)solutions3 and to investigate the cleaning of hard surface^.^ Surfactants containing more than eight carbon atoms in the chain spontaneously form micelles when the solution concentration is higher than a certain critical value. This behavior is mimicked on solid surfaces where surface aggregates called hemimicelles begin to form a t a concentration (chmc) of approximately 0.1 cmc5-* (critical micelle concentration). The previous work describing the adsorption of SDS by applying a potential at the electrode was done using concentrations at or below c ~ c Our . ~ work has focused on concentrations of nonanoic acid much above cmc where the action of the applied potential was not to induce local monomer concentrations above chmc but rather to disturb the solubility balance between components in the model fluid leading to film formation on the electrode. Experimental Section Nonanoic acid, diethyleneglycol monobutylether,and l-dodecanol, all of for synthesis quality, were obtained from Merck. N-Butyl-N,IV-bis(2-hydroxyethyl)amineof practicalgrade, assay >98% was obtained from Fluka. All solutions were prepared fresh before use in water from a Milli-Qwater purificationsystem (conductance0.10 pS). Conductance measurements were performed with a KonduktometerE527 from Metrohm Herisau and (1) Azzam,R. M.A.;Basha~a,N.M.EllipsometryandPolarizedLight; North-Holland New York, 1977. (2) Gottesfeld, S. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; Vol. 15, p 143. (3) Gregory, J. B.; Prud'homme, R. K.; Benziger, J. B. Langmuir 1988, 4, 140. (4) Malmsten, M.; Lindman, B. Langmuir 1989,5, 1105. (5) Nunn, C. C.: Schecter, R. S.;Wade, W. H. J. Colloid Interface Sci. 1981, 80, 598. (6) Scamehom, J. F.;Schecter, R. S.;Wade, W. H. J. Colloid Interfoce Sci. 1982, 85,463. (7) Harwell, J. H.;Hoekins,J. C.; Schechter, R. S.;Wade, W. H . h n g muir 1985, 1, 251. (8) Chandar, P.; Somasundaran, P.; Turro, N. J. J . Colloid Interfoce Sci. 1987, 117, 31.
0743-7463/92/2408-0571$03.00/00 1992 American Chemical Society
572 Langmuir, Vol. 8, No. 2, 1992 pH measurements with an Orion Research Model 701A digital ion analyzer. The weight ratio between nonanoicacid and amine was 3:2 for all solutions. Three dilution series were investigated. One in which the weight ratio between water and ether was a constant 15:l and two in which base solutions with an initial water to ether ratio of 7 5 1 were diluted with only water. Surface tension measurements to determine cmc for these solutions was done on a Du Nouyring apparatus. Concentrationsare expressed as molal nonanoic acid. Electrochemical experimentswere done using a three-electrode system controlled by a Wenkel351potentiostat. The reference electrode was a KCl saturated calomel electrode (SCE). The auxiliary electrode was a platinum wire. Gold film electrodes were prepared by vacuum depositing in sequence2 nm of Cr and 200 nm of Au onto glass microscope slides. The working area was 0.5 cm2. Prior to use the electrodeswere cleanedin a mixture of HzO, HzOz, and NH40H (ratio 20:l:l) at 80 "C for 5 min. Ellipsometric measurements were performed at room temperature in a quartz cell of about 8-mL volume. Solutions were stirred by a Teflon-coated magnetic bar at about 130 rpm. Two instruments were used, a single-wavelength (546 nm, Rudolph Research, model 436) apparatus and a rotating analyzer spectroscopic ellipsometerg(275-825 nm) constructed in our laboratory. The angle of incidencefor both was 68O, and the shortest time between points for kinetic measurements on the spectroscopic instrument was 0.2 s. The single-wavelengthinstrument was also used in the reflectance studies by measuring the light reflected from surfaces at constant polarization parallel to the plane of incidence. Visual observation of the electrode surface through a microscope was also possible. We have determined the ellipsometric reflectance ratio p = tan4 exp(iA),where 4 and A are the so-called ellipsometricangles' and i = 4-1. We have made no attempt to analyze these data due to the complexityof the optical model necessary to describe the microstructureat the interfaceregion. Therefore,we present our results in terms of the time variation in 4 and A. For a simple one-layer model, a decrease in A corresponds to formation of a surface film (refractive index 1.5) with a sensitivity of approximately 3.2 nm/deg for changes in A of less than a few degrees. Since applyingpotentials to the electrodesresulted in thick films, ellipsometrywas used to study the initiation of the films and also in a comparative sense to the reflectance results since ellipsometry i s sensitiveto the interface region whereas reflectancedetects the total film on the electrode.
Results and Discussion For convenience, our presentation of the electroadsorption phenomenon will begin with a general description of film formation, including the effect of concentration. This is followed by results and discussion of the kinetic studies and the effect of added cosurfactant. Mechanisms for the electroadsorption are proposed throughout the discussion. Since a thin oxide is formed on the Au surfaces under anodic potential, there are in fact two films formed on the electrodes during the electroadsorption process. Although the oxide film is less than 1% of the total film thickness, its contribution to the data will be addressed several times during the discussion. Electroadsorption. Since films clearly visible by lowpower microscopy were formed by applying a potential to electrodes, it was convenient to use reflectance measurementa as a method to follow the film formation on the gold surfaces. No attempt was made to calibrate the quantity of film deposited, but the limits of reflectance before adsorption (gold surface reflectance RA") and no reflection (beam blocked) are defined in the figures as points of reference. The reflectance from an electrode in contact with a 0.52 m solution of nonanoic acid during a voltage sweep from 0 to 1.55 V vs SCE is shown in Figure 1. A reflectance decrease corresponding to film formation was observed at (9) Aspnes, D. E.; Studna, A. A. Appl. O p t . 1975,14, 220.
Bjorklund et al.
0.9,
0
L Potential, V vs SCE
Figure 1. Reflectance and current curves during voltage sweep for a gold electrodein contactwith a 0.52 m nonanoic acid solution having a water to ether weight ratio of 15. Sweep rate was 0.5 mV/s. Reflectance limits were RA"at no applied voltage and 0
for light beam blocked.
about 1.1V. Film formation was reversible as indicated by the return of the reflectance level to its original value duringthe cathodic sweep. The I-Vcurve recorded during the sweep is also shown in Figure 1. A leveling off of the current curve during the initiation of film formation was observed. We assign the inflection shown for the current curve to the initial coverage of the electrode when the film began to deposit. However, since the film was porous and never completely covered the electrode, the current continued ita rapid increase at about 1.4 V. A further decrease of the reflectance curve was also observed at 1.4 V corresponding to an increase in the film deposition at the higher potential. Numerous studies have been made on the formation of an oxide layer on gold surfaces under anodic potential.lel2 Adiit. and Markovit. have used optical reflectance to follow oxide growth during potential sweeps in acidic solutions.11 They measured decreases of reflectivity on the order of 10% in their experiments. In addition, reduction peaks of the same magnitude as the oxidation peaks were observed during cathodic sweeps. For our work the film formation resulting from the anodic voltage sweep caused a 90% decrease in the reflectance from the Au surface, as shown in Figure 1. No reduction peak was observed during the cathodic sweep for the current scale shown in the figure. However, by expanding the scale, we were able to observe a reduction peak at about 0.5 V during the cathodic sweep. We conclude from this information that a thin oxide film formed on the electrode during voltage sweeps but that the contribution of the oxide to the reflectance and current curves shown in Figure 1 was small. Three series of solutions were studied to determine the effect of nonanoic acid concentration on the voltage required for film formation. This threshold voltage was defined as the potential at which the reflectance decrease was 10%. It was determined by increasing the applied voltage with0.05-V incrementa at 30-min intervals. Visual inspections were also made. (10)Kim, Y-T.; Collins, R. W.; Vedam, K. Surf.Sci. 1990, 233, 341. (11)AdZiC, R.R.;MarkoviC, N. M. Electrochim. Acta 1986,30,1473. (12)Arvia, A. J.; Salvarezza, R. C.; Triaca, W. E. Electrochim. Acta 1989,34, 1057.
Langmuir, Vol. 8, No. 2, 1992 573
Electroadsorption from Water-Based Metalworking Fluid
t
1.4
ir--
l
-
I
von
w
I
I
Voff
-
r
1.0
0.81
0
I
I
0.5 1.0 Conc. of nonanoic acid (m)
-
Figure 2. Threshold voltages for film formation determined as described in text at differentnonanoic acid concentrations. Series A was diluted with a water-ether mixture of weight ratio 15. Series B and C were diluted with only water. Series C contained 1-dodecanol at an acid to alcohol ratio of 10.
Preliminary experiments had indicated that film formation a t potentials well below that for gas evolution were obtained only for an acid to amine weight ratio of approximately 3:2. The threshold voltages for the three solution series versus nonanoic acid concentrations are shown in Figure 2. In series A, a base solution was diluted with a water-ether mixture having a constant weight ratio of water to ether of 15:l. This series exhibited a gradual decrease in the threshold voltage with increasing nonanoic acid concentration. In series B the base solution was diluted only with water, and the water to ether ratio varied from 7.5 to 30. The threshold voltage increased sharply with increasing acid concentration. In the third series (C) dilution was also made with water which contained 1dodecanol as cosurfactant. The threshold voltage curve was similar to that for series B but shifted to lower voltages. The phenomena contributing to the curves shown in Figure 2 are quite complex. However, we can identify some points which are in agreement with previous studies of the adsorption of this metalworking fluid on y-aluminum oxide.13 I t was there observed that the cosolvent ether suppressed adsorption of the acid by a solubilizing action. Series B clearly shows that film formation at the lower water to ether ratios required higher applied potentials. This was in contrast to series A where the water to ether ratio was constant. In ref 13, the cosurfactant l-dodecan01was observed to lower the mixed cmc due to ita higher hydrophobicity relative to nonanoic acid and to increase adsorption by providing dielectric shielding between adsorbed ionic molecules. In this study, the cosurfactant’s effect on the electroadsorption phenomenon resulted in a lowering of the threshold voltage curve for series C relative to series B. This may be due to the changes in both the bulk solution and the adsorbed film caused by the addition of the nonionic surfactant. The concentrations of the different components in the solutions had been chosen to correspond to those typically employed in industrial metalworking fluids.13 As a point of reference the cmc for series A and B dilutions were determined by surface tension measurements. For series A the cmc was at 0.0068 m nonanoic acid and 0.0076 m for series B. Thus, the concentration range studied was far above the cmc for the solutions. Both the conductance and pH of the solutions increased with increasing nonanoic (13)Tunius, M.; SkBld, R. Colloids Surf.1990, 46, 297.
a
> J
2 I
0
I
8 t (min)
12
Figure 3. Reflectance and rest potential curves during +l.O-V voltage-on and voltage-off (open circuit) periods for an electrode in contact with a 0.26 m nonanoic acid solution from series B.
acid concentration. With regard to pH, the observed concentration dependence was due to the fact that the protonated acid form is favored in the micellar aggregates and the removal of both forms of the acid into micelles leaves a relative excess of amine in solution.13J4 The role of solution pH for electroadsorption on gold was not as pronounced as for adsorption of an anionic surfactant on an oxide surface. We in fact observed with ellipsometry no adsorption onto the gold surfaces of ether, amine, acid, or dodecanol without an applied potential. Our preliminary conclusion was that the electroadsorption effect resulted from the disturbance of the micellar aggregates by the electric field near the electrode surface. Kinetic Studies. Measurements were made on the kinetics of film formation by applying different positive potentials on the electrode for 4 min followed by a time with either no potential (open circuit) or 0.0 V vs SCE. The result for reflectance measurement of film adsorption and desorption for the most dilute solution in series B is shown in Figure 3. The film formed during the 4-min voltage-on period and desorbed when the potential was disconnected (open circuit). The rest potential of the electrode vs SCE was followed before and after film formation. The rest potential curve in Figure 3 shows that the electrode retained a charge during film desorption and returned to the original rest potential after the reflectance curve indicated that no film remained. It was observed that if the electrode was not properly cleaned before the measurements, the adsorption/desorption curves were quite different from those shown in Figure 3. For such an unclean surface, the film formation was much faster when the potential was first applied than for a clean surface, and the film remained on the surface for several hours after the voltage was removed. Cleaning the electrode by cathodic cycling of the potentia13J5 resulted in a reflectance curve similar to that shown in Figure 3. Although dirt on a surface is difficult to characterize, it seems to illustrate the effect of the solubilizing action of the ether. The solution in Figure 3 had (14) Wennerstrom, H.; Lindman, B. Phys. Rep. 1979,52, 1. (15) Benziger, J. B.; Pascal, F. A.; Bernasik, S. L.; Soriga, M. P.; Hubbard, A. T. J.ElectroanaL Chem. InterfacialElectrochem. 1986,198,65.
Bjorklund et al. 42.5
----15. '4
-
I
I
I
$ ( t ) curves was found to be very different for experiments
I
lk
90 90
-
C-
E
eo
t
-:;.7-i' A
a 85-
80
n
B
I
I
I
I
I
8
4 t (mid
0
Figure4. A and$ recorded during +1.O-V voltage-onand voltageoff (0.0V) applied) periods for an electrodein contact with a 0.26 m nonanoic acid solutionfrom seriea B. Labels A-H are explained in the text.
41.0
y
:c
- 85 -A , -=5 80m
J D
r---,]
I U 75 .-
I
0
under different conditions. This was due to the fact that we dealt with several critical parameters, such as potential, concentration, and solution composition, which all influenced the film formation phenomena. The two experiments in Figures 4 and 5 contain most of the features observed. These features are labeled in the figures and will be discussed below. Generally, the curves showed a rapid decrease (marked A) in both A and $ when the potential was turned on. Then A began to increase and rC, decrease (B). After a while, therate of change in Aincreasedrapidly(C),followed by a decrease in both A and $ (D). During the D phase, we observed an oscillatory behavior in several experiments as can be seen in Figure 4 (E). When the potential waa reduced to 0 V (after 4 min), transients in A and $ (F)were observedfrequently. Thereturnof Aandqtotheir original values then followed essentially the same traces as during the potential-on period, although with different kinetics. Thus, we can identify phase G, correspondingto phase D, phase H corresponding to phase B, and the final rapid changes (I) corresponding to phase A. Before we begin to discuss the different features, we review which parameters influence the ellipsometric parameters A and $. Ellipsometry is sensitive to the detailed microstructure of the interface region and the optical properties of the different media present in this region, An electrochemically induced concentration change in the double layer may give rise to changes in the local refractive index close to the metal surface, and applied potentials can also generate electron redistributions in the electrode. These effects are however rather small, but could possibly explain some of the transients. Of more importance are the microstructural induced changes. The formation of a surface film is the most important, but also reorientation leading to anisotropy in an already adsorbed film can cause changes. Swelling or density changes can play a role, and unknown optical polarization phenomena can probably arise if micelles and/or hemimicelles form, disrupt, adsorb, or desorb in the interface region. In our case the system is too complicated to allow a quantitative evaluation of the ellipsometric results at present. We therefore only discuss the results qualitatively below. Electric Double-Layer Charging. The magnitude of the initial changes (A) when the potential was turned on depended on the potential. Below the film formation potentials, an almost linear dependance was observed. Reversing the potential also reversed the sign of the changes. We conclude that phenomena A is an effect of "charging the interface region" due to rearrangements in the electric double layer. Similar results have also been observed on platinum a t potentials below where oxide formation was ~ b s e r v e d . ~ However, at higher potentials there is the possibility that part of the initial decrease in A observed was the result of oxide formation. A control experiment was done by applying positive potentials to an electrode in a phosphate buffer solution at pH 7. When 1.1 V was applied, A decreased rapidly from 87' to 86' and was constant. Using the optical data of Kim et al.,l0 we calculate that a '1 decrease in A correspondsto formation of a 4-A-thick oxide layer in good agreement with their own measurements in acid solution. However, a comparison of this result with the curves shown in Figure 6 for potentials applied in the presence of the metalworking fluid clearly shows that the initial decreases in A were less than for the control experiments in the phosphate buffer. This would seem t o indicate that a thinner oxide layer was
4
I
8 t [min)
12
I
Figure 5. Aand$recorded during+1.2-Vvoltage-onandvoltageoff (0.0V applied) periods for an electrode in contact with 0.52 m nonanoic acid solutionfrom seriesB. Labels A-I are explained in the text.
the highest water to ether ratio used, and thus the solubilizing effect to counteract the attraction between the anionic surfactant and dirt was weakest for this system. This phenomenon was not observed for the solutions with the lower water to ether ratios. Ellipsometric measurements were made in order to follow film formation at the electrode/solution interface. The ellipsometric angles A and $ for two representative experiments are plotted in Figures 4 and 5 as a function of time during film formation and desorption. In the experiment in Figure 4 , l V was applied to the electrode in the most diluted solution in series B (0.26m), and in Figure 5, the conditions were 1.2 V and a 0.52 m solution. It should here be pointed out that the shape of A(t) and
Electroadsorption from Water-Based Metalworking Fluid
88t
11 a 87
I
I
86' 0
1
2 t (mln)
3
I
Figure 6. A recorded during applied voltages (as shown at each curve) for an electrode in contact with a 0.52 m nonanoic acid solution from series B. formed during the electroadsorption process. In addition, the passivating effect of the organic film prevented any surface roughness from prolonged electrolysis as would be expected from the growth of thicker oxides on the surface.1°J2 No evidence for roughening of the electrode surface was observed during our experiments on the basis of the excellent reproducibility in both A and # after several cyclesof the electroadsorption process. Surface roughness, even on the angstrom level, would be easily detectable by ellipsometry. Prefilm Phase. During phase B, A increased and # decreased. Thus, we can have no film formation during this phase as this should give a decrease in A and an increase in The length of phase B depended on the potential, and phase B was present also at potentials below the filmforming potential as can be seen in Figure 6. Figure 6 also shows that the interface region can be in equilibrium in phase B a t suitable concentrations and potentials. One possible explanation could be that initial adsorption of nonanoic acid replaces other species already present on the surface, such as dirt, ether, or amine. However, by control experiments we have ruled out replacement phenomena. Dirt is very unlikely to be present, due to the extremely high reproducibility of A and # when repeated measurements were done on the same electrode. The initial A observed was also unchanged by all attempts to clean the electrode by cyclic ~ o l t a m m e t r y . ~Adding J~ components in the order ether, amine, and nonanoic acid to water in contact with the electrode resulted in no change in A which could be related to molecular adsorption, but could fully be explained by changes in the refractive index of the solution. We believe that the increase in A during phase B can be explained by the behavior of micelles in an electric field. Since we are over the cmc for all solutions studied, the electric field near the electrode encounters a solution composed of micelles in equilibrium with monomers. Electroadsorption of monomers resulted in only decreases in A and increases in # for solutions below the cmc for I t has also been reported that micelles formed by surfactants with ferrocenyl moiety can be broken up into monomers when the surfactants were oxidized.16 Nonanoic acid is not electroactive, but the positive electric field near the electrode surface could still result in micelle disruption or precipitation. The driving away of amine cations could trigger such events, for example. In addition, it should not be overlooked that the thin oxide formed on the surface #.193
(16) Saji, T.; Hoshino, K.; Aoyagui, S. J. Am. Chem. SOC.1986,107, 6865.
Langmuir, Vol. 8, No. 2, 1992 575 upon application of the positive potential may influence the events occurring at the solidlliquid interface. While none of the components of the metalworking fluid were observed to adsorb on the Au electrode with no applied potential, it has been shown that nonanoic acid and the amine readily adsorb on oxide surfaces.13 Thus, the rapid formation of a thin oxide layer may play a role in initiating the growth of the much thicker organic film. Film Formation. The film formation starts with a rapid increase in A (C).This may result from some phase transition or some rapid disruption of micelles in the surface region. During phase D a film was formed on the electrode. In Figure 6, the dependences of phases B-D on potential around the critical film formation potential can be seen. Self-passivation eventually occurred if thick films were formed by the higher potentials (see Figure 5). However, we have not evaluated this as we have focused on film formation. From the total decrease in A, it should be possible to estimate the thickness of the film. By assuming a refractive index of 1.5, the total change in A from 8 7 O to 75O in the experiment in Figure 5 corresponds to a film thickness of approximately 400 A if an isotropic single-layer optical model is used. However, the change in # is then neglected and the real microstructure is certainly muchmore complicated, so the value on thickness is only a very rough estimate. In many experiments we also observed an oscillatory behavior in the ellipsometric data during phase D as is clearly seen in Figure 4 (E).We believe that the calculated thickness of 400 A is far too low. The oscillations may be an effect of the formation of microscopic films. For such films with complicated microstructure, ellipsometry cannot be used to follow film growth. The f i i observed can therefore have thicknesses on the order of thousands of angstroms or even more. This is also supported by the fact that the films are visible by eye. Film Desorption. The film formation was fully reversible, and the original values on A and # could be obtained with remarkable repeatability (less than O.O2O), and the same electrode could be used in many filmformation experiments. The desorption data we present here were all obtained when 0 V was applied. We have also done some experiments under open circuit conditions, which exhibited faster kinetics. As mentioned above, A and # essentially traced back to their original values, but with different kinetics. The main film desorption took place during phase G. We observed transients in A and # (F), which we attribute to electrochemical charging phenomena. Postfilm Phase. The desorption phase was followed by a relatively long period (phase H), which we believe corresponds to rearrangements of molecules and micelles to reconstruct the original state of the system. Phase H would then be related to phase B during the voltage-on period. At the end of phase H, a surprisingly rapid change took place (I, Figure 5 ) and the ellipsometric parameters suddenly returned to their original values. This could be either some phase transition and disintegration of some ordered surface layer or a discharge of the surface region correspondingto the chargingdiscussed above (A). It could also be related to the disappearance of the thin oxide layer formed during the electroadsorption. Gold oxides are only stable under anodic potentials,1° and the organic film present on the surface may have delayed the removal of the oxide layer. For other electrodes such as Cr where stable oxides were formed, the final A and $ values after the electroadsorptionldesorptionwere different from the initial values.
Bjorklund et al.
576 Langmuir, Vol. 8, No. 2, 1992
I
I
0
I
I
8 t (min)
I
I
12
*
Figure 7. Reflectance and rest potential curves (solidline) during +1.15-V voltage-on and voltage-off (open circuit) periods for an electrode in contact with a 0.64 m nonanoic acid solution containing dodecanol. Broken line is for +1.4-V voltage-on and voltage-off periods for a 0.64 m nonanoic acid solution without dodecanol.
We observed adsorptionldesorption curves for other solutions in series A and B shown in Figure 2 similar to those described in Figures 4 and 5. In summary we conclude that the film formation could be followed ellipsometrically, but that the system was too complicated to allow a quantitative interpretation. Effect of Cosurfactant. Measurements similar to those described above were done with solutions from series C containing 1-dodecanolas cosurfactant. Applying +1.15 V to an electrode in contact with aseries C solution resulted in the reflectance curve shown in Figure 7. A decrease and then increase in reflectance was observed during the voltage-on period. Stopping the voltage resulted in two additional minima in the reflectance curve before the original level was restored. Visual observation through the microscope confirmed these observations. During the voltage-on period, a film was observed to deposit on the electrode. This film became more transparent with time. When the voltage was stopped, parts of the film appeared to stand up which darkened the electrode (first minimum in desorption curve) and subsequently desorbed, followed by a second weaker darkening effect. The rest potential curve also returned to the original value at the time when
the reflectance curve indicated that no film remained on the electrode. Solutions not containing 1-dodecanol did not exhibit this double minima in the desorption curve (see Figure 3 and broken line in Figure 7). I t is generally agreed that a neutral cosurfactant can influence the adsorption of ionic surfactants both by influencing the mixed cmc and by providing a dielectric shielding between charged adsorbed species.17J8 Previous studies of the model metalworking fluid had shown that 1-dodecanol had the highest relative adsorption on aluminum oxide of all the components, despite its neutral character.13 Its presence in the electroadsorption studies was manifested by a loweringof the f i i threshold voltages, shown in Figure 2, possibly a result of mixed micelle formation, and by the reflectance desorption curves. Ellipsometric measurements showed no qualitative differences in A and $ for adsorptionldesorption between solutions with and without 1-dodecanol. Thus, the effect is in the bulk film and not at the electrodelsolution interface. The effect may be related to the stepwise ordering observed in thin films formed in a capillary from anionic surfactant solutions.lg.20 Summary Ellipsometry and reflectometry were used to study electroadsorption from a model metalworking fluid onto gold electrodes. The threshold voltage required to form films on the surface was strongly dependent on the water to cosolvent and the anionic surfactant to amine ratios. It was less dependent on the surfactant concentration. In the presence of a cosurfactant, the threshold voltage was lower. Kinetic studies by ellipsometry showed that electroadsorption was induced by micelle disruption or precipitation at the electrodelsolution interface and was not the result of monomer adsorption. The porous nature of the films formed seems to indicate that the electroadsorption phenomenon has limited utility in protecting metal surfaces during operations in aqueous media. Acknowledgment. R.B.B. thanks the Carl Tryggers Scientific Research Foundation for supporting this work. This work was also supported by the Swedish National Board for Technical Development. Registry No. Au, 7440-57-5;nonanoic acid, 112-05-0;diethylene glycol, 111-46-6;N-butyl-N,N-bis(2-hydroxyethyl)amine, 102-79-4; 1-dodecanol, 112-53-8. (17) Harwell, J. H.; Roberta, B. L.; Scamehorn, J. F. Colloids Surf. 1988, 32, 1.
(18)Lindman, B.; Wennerstrom, H. In Topics in Current Chemistry; Springer Verlag: Berlin, Heidelberg, New York, 1980;p 63. (19) Bruil, H. G.; Lyklema, J. Nature 1971, 232, 19. (20) Nikolov, A. D.; Wasan, D. T. J.Colloid Interface Sci. 1989,133, 1.
