Langmuir 1994,10,4339-4343
4339
Voltammetry of Cadmium Dodecyl Sulfate Micelles R. Andriamanampisoa and R. A. Mackay* Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699-5665 Received June 17, 1994. In Final Form: August 18, 1994@ The voltammetric behavior of cadmium dodecyl sulfate micelles i n aqueous NaCl and NaN03 solutions
has been examined. In conjunction with potentiometric and conductance measurements, the critical micelle concentrations (CMC's) and apparent cadmium diffusion coefficients have been determined. The CMC's are not greatly affected by salt concentration up to 0.1 M. The apparent diffusion coefficients are satisfactorily accounted for by a two-layer diffusion model involving both micellar-bound and free cadmium(I1) transport.
Introduction Cadmium ion in aqueous solution undergoes a twoelectron reduction at about -0.6 V vs a saturated calomel electrode (SCE) at a mercury electrode. The process is reversible, o r nearly so, at moderate scan rates ( ~ 2 0 0 mV/s). Since this potential is approximately in the midrange of those accessiblet o a mercury electrode, cadmium ion has been employed as an electrochemical probe in micellar and microemulsion media.1-3 Recently, cadmium ions solubilized in reverse micelles have been employed to synthesize nanoparticles of cadmium ~ u l f i d e .In ~ the course of electrochemical studies of CdS colloids, we h a v e synthesizedthese particles in the 2-3 nm size range using micelles of cadmium dodecyl sulfate, Cd(DS)Z, according t o a modified method of Pileni.5 As an adjunct t o these studies, it became necessary to examine t h e electrochemical behavior of Cd(DS)z itself. Although studies of an electroactive micelle have been reported, it w a s the surfactant head group itself which was oxidizable.6 To t h e best of our knowledge, this represents the first voltammetric study of a micelle in which the counterion is electroactive.
Experimental Section Reagents. All of the following materials were used without further purification: chloroform (Baker Analyzed Reagent); dodecyl alcohol (Eastman Kodak);sodium dodecyl sulfate (Sigma 99%);cadmium carbonate, cadmium sulfate, cadmium nitrate, and chlorosulfonic acid (Aldrich); sodium chloride and sodium nitrate (Fisher A.R.); and ion exchange resins Amberlite IR-120 and IRC-76 (Fluka). Analyses. Elemental analysis were performed by Galbraith Laboratories, Inc., Knoxville, TN. Synthesis of Cd(DS)Z. Two methods were employed to prepare the Cd(DS)Z, an ion exchange method and a chemical reaction method. Zon Exchange Method. This preparation was based on the procedure of Petit et aL4 Sodium dodecyl sulfate (SDS)was first converted to dodecyl sulfuric acid by the use of a strongly acidic resin (Amberlite IR-120). Subsequently, the acid form was
* Author to whom correspondence should be addressed. Abstract published inAduance ACSAbstracts, October 1,1994. (1)Novodoff,J.;Rosano, M. L.; Hoyer, H. W. J.Colloid Interface Sci. 1972,38,424. (2)Mackay, R. A. In Microemulsions; Robb, I. D., Eds.; Plenum Publishing Corp.: New York, 1982;p 207. (3)Mackay, R. A.;Dixit, N. S.; Agarwal, R. Inorganic Reactions in Organized Media;Holt, S . L., Ed.; ACS Symposium Series 177;American Chemical Society: Washington, DC, 1982;Chapter 11, p 179. (4)Petit, C.; Lixon, P.; Pileni, M. P. J.Phys. Chem. 1990,94,1598. ( 5 )(a)Pileni, M. P. Private communication. (b) Petit, C.; Jam, T. IC; Billondet, F.; Pileni, M. P. Submitted for publication. ( 6 )Saji, T.; Goto, M.; Takeo, F.; Sugimoto, T.; Ohnuma, T. In Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH Publishers: New York, 1992; Chapter 7, p 89 and @
references therein.
treated with a weakly acidic resin (Amberlite IRC-76)previously transformed into the cadmium form. The resulting aqueous solution was freeze-dried to give a waxy material, which was then decolorized with activated charcoal in methanol. After filtration, the solventwas stripped from the filtrate using a rotary evaporator. The final product is obtained as a white amorphous powder of cadmium dodecyl sulfate. Anal. Calcd44.83% C, 7.80% H, 9.97% S. Found: 44.93% C, 8.16% H, 10.48% S. Chemical Reaction Method. Dodecyl alcohol (0.10 mol) was reacted with chlorosulfonicacid (0.11mol) in 200 mL of chloroform a t 0 "C. The acid was added over a period of 30 min. After addition of 100mL ofethanol, the solutionwas slowly neutralized with cadmium carbonate. Water (15 mL) was added at the end to ensure complete reaction. The excess of CdC03 remained insoluble. After filtration, the filtrate was taken to dryness on a rotary evaporator to yield a white powder which was recrystallized twice from ethanol. Found: 44.00% C, 7.54% H, 10.01% S. Voltammetry. Polarographic measurements in the sampled DC mode (Tast polarography) were performed on a Princeton Applied Research (PAR) Model 174A polarographic analyzer. A PAR Model 303A static mercury drop electrode was employed as the working electrode, AglAgCl in 3 M NaCl as the reference electrode, and Pt as the counter electrode. The drop time, drop mass, and scan rate were maintained at 5 s, 5.7 mg, and 2 mV/s, respectively. Cyclic voltammetry measurements employed a hanging Hg drop a t scan rates of 10-200 mV/s. All measurements were taken at 22 f 1 "C. Potentiometry. Critical micelle concentrations (CMC's) of Cd(DS)Za t 22 & 1"C in the presence of various electrolytes (NaC1 or NaN03) in aqueous solution were determined using a combination cadmiumion selective electrode(Cole-Pmer, Model G27502-07). The potential was measured with a Keithley Model 2001 multimeter. The slopes of the emfvs (log)cadmium dodecyl sulfate concentration below the CMC were generally 26-29 mV. Electrode performance was checked with cadmium nitrate solutions. Conductance Measurements. For Cd(DS)Z in pure water, the CMC was also determined from conductance measurements a t 22 "C using a YSI Model 32 conductance meter and a dip cell with a cell constant of 1.0 cm-l.
Results and Discussion Critical Micelle Concentrations. Some representative plots of t h e cadmium specific ion data are given in Figure 1. The values of CMC of Cd(DF& obtained from s u c h plots in water and in the presence of added salt a r e given in Table 1. The value in w a t e r has been reported as 0.85' and 0.80 mM,5in contrast t o o u r value of 1.1mM. The reason for this discrepancy of about 30% is not clear, since the s a m e value w a s obtained for the Cd(DS)Z prepared by both methods. In both cases, the deviation of the percentage cadmium from the theoretical w a s only (7)Treiner, C.; Makayssi,A. J. Colloid Interface Sci. 1993,150,314.
0743-7463/94/2410-4339$04.50/0 0 1994 American Chemical Society
Andriamanampisoa and Mackay
Langmuir, Vol. 10, No. 11, 1994
-
0.2
e
\
E
6
h . Y 3
>
.3
Y
0 V E
0.1 . U 3
. CI 3
V v1 a
0
P I
potentia1 (-mVI
Figure 1. Cadmium ion selective electrode measurements for Cd(DS)2in 0.01M NaCl (circles)and 0.10M NaN03 (squares). Table 1. Cadmium Specific Ion Electrode Data for Cd(DS)ain Various Electrolytes none NaCl Nd03
NaC104
0.00 0.01 0.05 0.01 0.05 0.10 0.01
l.ld 1.3 1.4 1.2 1.1 0.7 1.3
2
C d ( I 1 ) Concentration
21 27 26 33 27 30 26
-1.0 .6-.7 .2-.5 .6-.4 .4-.5 .6-.4
-0.9
a &lo%. 3!, = (C - C,,f)/(C - CMC), where Cis the total Cd(I1) concentration and C,f is determined from the extrapolated linear section below the CMC (see Figure 1). From 2-4 mM Cd(DS12, except for 0.1M N d & , where C = 1-2 mM. A CMC of 1.2iz 0.1 mM was obtained from the conductivity measurements. The value of 3f, is estimated as > 0.8. 3%. It may be noted that the value of 0.85 mM was obtained by the cadmium ion selective electrode, and the value of 0.8 mM from surface tension measurements. Our values as per the Experimental Section were obtained by the cadmium ion selective electrode. In pure water, the slope of the potential vs log concentration plot is only 21 mV (Table 11, considerably less than the expected Nernstian value of 27 mV. In all of the other cases, the slopes varied from 26-33 mV, with a n average of 28 mV. Some difficulty with the use of a cadmium ion selective electrode was noted in the earlier s t ~ d yand , ~ in our hands the value ofthe CMC was reproducible within &lo%,but the values of the fraction of bound cadmium counterion, ,8 were much less so. Also shown in Table 1 are values of p determined over a small concentration range above the CMC. In view of the experimental uncertainties noted above, these values should only be employed qualitatively. In pure water, it was also possible to employ conductance measurements (Figure2),which yield a CMC of 1.2 f0.1mM, in agreement with the specific ion electrode value. It is also possible to estimate a value ofp from the conductance
4
(mM)
Figure 2. Conductivity of Cd(DS)2 in water.
data. Ignoring the concentration dependence of the equivalent conductance above the CMC (up to 3 mM), a s well as the contribution of the micelles to the conductivity, allows a@value of 0.82to be obtained. It can be estimated that this value may be low by about 10% by ignoring the micellar conductivity. AB value of 0.82f .03is reported by Treiner et al.,7while a value of 0.8 may be calculated from the data of Pileni et al.5 There have been a number of studies of the interaction of dodecyl sulfate micelles with divalent metal ions,a and the general result is that the CMC is lower and /3 higher than for SDS. For Cu(DS)z the CMC increased from 1.2 to 2.5 mM, and j3 decreased from 0.87 to 0.57 with increasing NaN03 concentration up to 0.02 M. This was ascribed to the replacement of some divalent metal ion by sodium ion. At higher salt concentrations, the CMC decreased again, and p remained constant a t 0.57.8cThe same general trends are observed here with NaN03. However, NaCl seems to cause a more substantial reduction in /3 a t higher concentrations, likely due to complexation of cadmium ion by chloride. In a .01M NaCl solution, Cd(I1) is complexed ~ignificantly.~ If CaCl2 is examined in .01M NaCl using the cadmium ion electrode, a deviation of the potential vs log [Cd(II)I from linear behavior with a slope of 25 mV/decade begins at about 1.5 mM CdC12. At total concentrations of 3 and 6 mM, the effective concentration is reduced by 10% and 22%, respectively. While some of this may be due to changes in activity coefficient, most of it is not since Cd(N03)~in NaN03 does not exhibit any significant deviation over the same concentration range. Complexation may have the (8) (a)Baumueller, W.; Hoffman,H.; Ulbricht, W.; Tondre, C.; Zana, R. J.Colloid Interface Sci. 1978,64,418. (b)Newbery, J. E. Zbid. 1980, 74,483. (c) Treiner, C.; Nguyen, D. J.Phys. Chem. 1990,94,2021 and
references therein. (9)Lange's Handbook of Chemistry, 12th ed.; Dean, J. S., Ed.; McGraw-Hill Book Co.; New York, 1979.
Voltammetry of Cadmium Dodecyl Sulfate Micelles
Langmuir, Vol. 10, No. 11, 1994 4341
I
1
-.6
-.5 YOltS
VS.
I -.7
A.g/1\9Cl
Figure 4. Cyclic voltammogram (a) and sampled DC polarogram (b) for Cd(DS12 in aqueous 0.05 M NaN03. For the CV, the Cd(DS)z concentration is 5 mM, and the scan rate is 50
mV/s.
~
2
cd(Ds)2
4
6
Concentration ( m M )
Figure 3. Polarographic limiting current vs Cd(DS)Zconcentration in aqueous 0.01 M NaCl. The arrow denotes the CMC from potentiometric data (Table 1).
effect of increasing dissociation of cadmium ion from the micelle, but it will also decrease the apparent value since the dissociated but complexed cadmium will be counted as bound. This may explain the reason for a@value around .29 for .01 M NaC104, 0.5for .01 M NaNoa, but 0.7for .01 M NaC1. It may also be noted that the lowering of the CMC a t 0.1 M NaNO3, presumably due to the (screening) salt effect, is reversed again since no micelles are present in either 3 mM Cd(DS)2in 0.2 M NaN03 or a t 5 mM Cd(DS)2in 0.5 M NaN03. While it was not the intent ofthis investigation to study in detail the effect of salts on the behavior of divalent metal dodecyl sulfate micelles, it is clear that additional studies will be required to acquire a complete understanding. Diffusion Coefficients. Studies were conducted in 0.01 M NaCl, a s well a s in 0.01,0.05, and 0.10 M NaN03. Although it was recognized that the micelles themselves might make a aon-negligible contribution to the migration current a t the lower supporting electrolyte concentrations, it was still of interest to examine this range because of the uncertainty in @ and their use in other applications. The basic results are illustrated in Figure 3, where the dc polarographic limiting current (id) in 0.01 M NaCl is plotted vs the Cd(DSI2 concentration. At a number of concentrations, particularly at higher values, 2-3 different samples were run a t a given concentration. “he values shown are averages. One standard deviation is fa%, and two illustrative error bars are shown. If the limiting current is diffision controlled, then id = id, where id is given by the Ilkovic equation:
id = 708nD’12Cm2/3t116
(1)
Here n is the number of electrons transferred, D the diffusion coefficient of the electroactive species (cm2/s), C its concentration (mol/cm3), m the mercury drop rate (mg/s), and t the drop time(s). This predicts that a plot of id vs C should be linear. At lower concentrations, below the CMC, it is indeed linear. At higher concentrations, i/ is lower due to the lower diffusion coefficient of the cadmium ion bound to the micelle compared with free Cd(I1). The CMC as obtained from the potentiometric measurements is also shown in Figure 3. It may be noted that the concentration a t which there is a noticeable deviation of the current from the linear portion is higher than the CMC. This is true for all of the systems examined in this study. In the treatments to follow, only data a t concentrations 2 2 mM will be used for the micellar range. The constant (4~15%) value of the diffusion coefficient obtained by applying eq 1to the linear portion of the i/vs C plot is denoted as DOand is taken to be the diffusion coefficient of “free”Cd(I1)both below and above the CMC. While this is of course not necessarily true, the errors inherent in the data treatment (vide infra) are likely of much greater magnitude. The polarographic results have been calibrated by employing CdClz in 0.1 M KC1, for which a value ofD = 7.0f0.1 x lod6cm2/shas been employed.’O As an additional check, cyclic voltammetry (CV)has also been applied to both the calibration system and to Cd(DS)2below the CMC. A representative polarogram and cyclic voltammogram are shown in Figure 4. The electrochemical process is reversible, or nearly so, over the range of scan rates examined. Under these conditions, eq 2 may be employed. i, = (2.69x 105In 312AD 112v 112C (2) Here i, is the linear sweep peak current (amperes),A the electrode area (in cm2,obtained from the drop mass), and v the scan rate (V/s). The CV data were treated in two (10) (a) Meites, M. Polarographic Techniques, 2nd ed.; Interscience Publishers: John Wiley and Sons, New York, 1965. (b) Macero, D. J.; Rulfs, C. L. J . Electroanal. Chem. 1964, 7, 328.
Andriamanampisoa and Mackay
4342 Langmuir, Vol.10,No. 11, 1994 Table 2. Comparison of Diffusion Coefficients Obtained from Polarography and Cyclic Voltammetry
~~~
method i, vs ~ 1 ’ i, vs Cb polarography
~
Cd(DScin .01 M NaCl 4.3 f 0.6 4.9 f 0.5 4.4 f 0.2
I
I
-
I
CdClz in 0.1 M KC1 6.1 f 0.4c 6.5 f OBd 7.0 f O . l e
a c = 0.6, 1.0, and 1.2 mM. v = 10-200 mV/s. v = 10,50, and 200mV/s. C=0.1-1.2mM.Cc=0.1,0.4,1.0,and5.0mM. v= 10-200 mV/s. v = 10, 50, and 200 mV/s. c = 0.1-5.0 mM. e Calibration value (ref lob). Through the use of a dropping Hg electrode, at 11concentrations, an average value ofD = (8.6 f 0.4) x cm2/s is obtained. With the Koutecky correction,1oathis becomes 7.4 x cm2/s.
0
distance
Table 3. Values of Do (below the CMC) and Dppp(above the CMC) for Cd(DS)2in Various Salt Solutions salt (conc) Cd(DS)z (mM) D,,, (cm2/s x lo6) 3.80 NaCl(O.01 M) 2.0 3.40 D, = 4.4 x cm2/s 2.5 3.0 2.88 3.5 2.50 4.0 2.18 4.5 1.92 1.70 5.0 1.52 5.5 5.5 1.52 6.0 1.36 6.44 N f l 0 3 (0.05 M) 2.0 5.96 D, = 7.2 x cm2/s 2.2 6.08 2.5 3.0 5.86 5.20 3.5 5.14 4.0 4.76 5.0 3.98 N f l 0 3 (0.10 M) 2.0 3.24 Do= 4.6 x cm% 3.0 3.34 4.0 3.06 5.0
ways. First, i, vs C was plotted for a fmed value of v. Second, i, vs v1l2 was plotted for a given value of C . In each case, D was obtained from the slope of the straight line, according to eq 2. A comparison of the results is given in Table 2. In one instance (4 mM cadmium dodecyl sulfate in .01 M NaCl), the value of m was varied from 0.28 to 5.70 mg/s by varying the drop mass (1.4-5.7 mg) and time (1-5 s). The polarographic value of D was randomly distributed and constant (one standard deviation from the mean was f20%). These results indicate that the polarographic values of D are reliable within the given experimental error. Above the CMC, or more specifically at those Cd(DS)Z concentrations where the i, vs C plot deviates from linearity, a n apparent value of the diffusion coefficient (Dapp) can be calculated from eq 1. These values are given in Table 3. Measurements in 0.01 M NaN03 were not performed above the CMC since the results in 0.01 M NaCl indicated that a higher supporting electrolyte concentration was needed (vide infra). However, measurements below the CMC gave a value of Do = 5.2 x cm2/s. It is worth emphasizing once more that the “free” (unmicellized) Cd(I1) is not simply the aquocadium ion. In 0.01 M NaCl, much of the cadmium is in the form of CdCl+,CdC12, and CdC1-3.1° Since the dodecyl sulfate ion is also present, species such as Cd(DS)+ and Cd(DS)Cl may exist. Even nitrate ion complexes Cd(I1) to some extent. In addition, the fraction of these species will change somewhat as the bulk Cd(DS)2 concentration is varied. However, all ofthese are takenin the model below to be a single “species” in a given electrolyte with a n effective diffusion coefficientDO. This is justified, as noted above, by the constant value of Do within the relatively
f r o m electrode s u r f a c e
Figure 5. Schematic of the theoretical model. The diffusion layer extends from dz at C = CB(bulk concentration) to dl at C = CO(CMC). At dz,the total free Cd(I1) concentration is CZ = CO+ C1, where C1= (1 - /~)(CB - CO).The micellized (bound) where @ is the Cd(I1) concentration at dzis given by @(CB- CO), fraction of bound (divalent) counterion. The free Cd(I1) has a diffusion coefficient Do,while the bound Cd(I1) has the micellar diffusion coefficient D,.
large uncertainty associated with its polarography measured value (f10-20%). Theoretical Model. For the case of a n electroactive micelle in which the surfactant is the redox moiety, a model was proposed which was shown to semiquantitatively account for the data.ll The diffusion layer consists of a n outer zone where the total Cd(DS12 concentration drops from its bulk value CBto the CMC, and a second zone where only monomer exists from the CMC to zero concentration a t the electrode surface when i = i d . For this case, the model needs to be modified to account for additional “free” Cd(II), above that equal to the CMC, resulting from that which dissociates from the micelle. The model is shown schematically in Figure 5. If it is assumed that (i)the diffusion coefficient of unmicellized cadmium(I1) is constant a t DOboth above and below the CMC (here, designated as CO),(ii) the concentration of unmicellized Cd(I1) above the CMC is given by CO a(CB - CO),where a = 1- p i s the fraction of Cd(I1)dissociated from the micelle, where Codoes not change with CB,and (iii) ,L? is constant, then the result is given by eq 3.
+
Dapp
= 11 - p - P(CdCB)’ + 2p(cdcB)lDO
+
p D m ( 1 - CdCB12 (3) Here, D m is the diffusion coefficient of the micelle, equal to the diffusion coefficient of micellized Cd(I1). For p = 1, eq 3 reduces to the equation obtained by Saji et al. for electroactive surfactant.ll Following their treatment, eq 3 may be rearranged to give eq 4.
(Do- Dapp)Y’= -p1/2co(Do- D,) 1/2c,-1
+
p1/2(Do-
(4)
A plot of (DO - Dapp)1/2 vs (7’; should yield a straight line. Such a plot is shown in Figure 6 for 0.01 M NaC1. At least-squares fit of the data yields the results given in Table 4. Reasonably good fits to a straight line are which are equal to obtained, with values of the CMC (CO) or slightly larger than those in Table 1. With D m = 1.0 x cm2/s for Cd(DS)2 micelles: reasonable values of /3 are obtained for NaN03 in qualitative agreement with Table 1. For 0.01 M NaC1, a negative value of D, results even for p = 1. This is most certainly a result of the contribution ofthe micelles to the migration current, with (11)Saji, T.; Hoshino,K.; Aoyagui, S.J.Am. Chem. SOC.1985,107,
6865.
Voltammetry of Cadmium Dodecyl Sulfate Micelles
Langmuir, Vol. 10, No. 11, 1994 4343
4.0
-
3.c
(o
0 i
X \ 10
0 E
a
d 2.c
Figure 6. Plot of eq 4 for Cd(DS)2 in 0.01 M NaC1.
1.c
Table 4. Parameters Calculated from Application of Eq
( l-cO/cB ) 2
4
salt(conc) NaCl (.01 M) N a 0 3 (0.5M) N d 0 3 (.loM)
-slopen
4.88 3.00 2.02
intercept?
-CCb
Coa
3.16 2.76 2.20
1.00
1.5
0.96 0.94
1.1
0.9
Figure 7. Plot of eq 5 for Cd(DS)2 in aqueous 0.01 M NaC1. .73 .92
The values ofD and C are in cm2/s x lo6and mM, respectively. cm2/s b Correlation coefficient. Value of B for D, = 1.0 x (vide text), usingvaluesofD, from Table 3. Negative values ofD, for all values of ,!I (51).
the result that iJ* i d . The transport number of the micelles under the conditions employed here can be estimated to bein thevicinityof0.02-.05. Becauseofthelargenumber of Cd(I1)ions carried by each micelle (80-901, the negative contribution of the micelles will outweigh the positive contribution of any positively charged “free” cadmium species. Therefore, i d > ihand the corrected value of the apparent diffusion coefficientDipp=. Dapp.An increase in Dappby 10% will produce positive values of p. I t is also possible to rearrange eq 3 to give eq 5.
(5) Dapp= pco, - Do)(1 - co/cB)2 + Do A plot of Dapp vs (1- C ~ C Bshould ) ~ again yield a straight line with slope ,&Dm - DO)and intercept DO. Such a plot for .01 M NaCl is shown in Figure 7. Similar plots are obtained for NaN03. Employing the values of CO(CMC) from Table 1and Dappfrom Table 3 yields the parameters in Table 5. It may be noted that the values of DOare in good agreement with those obtained from the i/vs C plots below the CMC (Table 3). The values of p are again in qualitative agreement, and somewhat better quantitative
Table 5. Parameters Calculated from Application of Eq 5
salt (conc)
-slopen
NaCl(.Ol M) NaN03 (0.5M) NaN03 (.lo M)
5.08 3.78 2.80
intercept? (Do) -Wb 4.5 7.1 5.2
1.00 0.96 0.91
B d
0.62 0.67
a The values ofD and C are in cm2/s x lo6 and mM, respectively. Correlation coefficient. Value of ,!I for D, = 1.0 x cm2/s, using values of Do in this table. Negative values of D, for all /3
5 1.
agreement, with those in Table 1. This might be expected since the independently determined values of COhave a higher precision (&lo%)than those of DO(f20%).
Conclusion The voltammetric behavior of a micelle with an electroactive counterion is well accounted for by the two-layer diffision model. The apparent partial failure of the model at low supporting electrolyte concentration is due to the significant contribution of the micelles to the migration current. Acknowledgment. The authors wish to thank Professor M. P. Pileni for assistance with the preparation of cadmium dodecyl sulfate and for consultation on the properties of the micelles. The support of the New York State Science and Technology Foundation, through the Clarkson University Center for Advanced Materials Processing, is gratefully acknowledged.
