Langmuir 1989,5, 283-286
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Evidences for Noncondensed Counterions in the Nonionic-Rich Composition Domain of Mixed Anionic/Nonionic Micelles C. Treiner,* M. Fromon, and M. H. Mannebach Laboratoire d’Electrochimie, UA 430 CNRS, Uniuersite Pierre et Marie Curie, Bat.F., 4 Place Jussieu, Paris 75005, France Received July 12, 1988 Counterion condensation data have been obtained from potentiometric measurements for mixed micelles of copper dodecyl sulfate with two nonionic surfactants, Triton X-100 (system I) and Brij 35 (s stem 11), by using a highly sensitive cupric ion selective electrode. A critical micellar composition X E value is determined beyond which cupric counterions do not condensate on the mixed micelles. XM values are equal to 0.98 (system I) and 0.95 (system 11). These results are discussed in terms of Bjerrum’s ion-ion association model. The relevance of these findings to the recently observed increase of cloud point phenomena of nonionic surfactants upon addition of small quantities of an anionic surfactant is outlined.
Introduction It has been shown in recent that the addition of very small quantities of an ionic surfactant to a nonionic
cmc was equal to 1.24 X mol/kg, in excellent agreement with literature value^.^^^ The Krafft point is 24 “C. The following electrochemical cell was used:
surfactant solution in water increases considerably the cloud point of the nonionic surfactant. This observation is of fundamental and practical interest. The mechanism which has been proposed to interpret this phenomenon is that when mixed micelles are formed the micellar surface becomes negatively charged so that a repulsive interaction between these mixed micelles predominates. Bjerrum’s classical6 model of ionic association may help explain the above phenomena by suggesting a limiting charge density value above which the counterions of the ionic component will start to condensate. This limiting value corresponds to a given composition of the mixed micelle. Thus, there would be a composition interval between the pure nonionic micelle and a mixed micelle critical composition for which a weak repulsive interaction controls the properties of the micellar solutions. We have recently shown in the case of mixed sodium dodecyl sulfate/dodecylpoly(oxyethylene) micelles that Bjerrum’s hypothesis was compatible with experimental ion condensation values deduced from an electromotive force study using a sodium ion selective electrode.’ Unfortunately, this electrode is not sensitive enough at low sodium activity, so that direct evidence could not be provided to Bjerrum’s hypothesis in the micelle composition range close to the pure aqueous nonionic solutions. This report presents ion condensation data obtained with the cupric ion solid-state sensitive electrode, which was found much more sensitive than the sodium electrode t o small ion activity changes. More convincing evidence could then be presented on the validity of Bjerrum’s concept to the ion condensation phenomenon a t very low ionic surfactant composition of mixed anionic/nonionic micelles.
Hg /Hg,Cl,/KCl(sat) / /NH4N03(3M) / /solution/ /cupric-sensitive electrode
Experimental Section Two systems have been investigated: the copper dodecyl sulfate (C~(DS)~)/dodecylpoly(oxyethylene)(23)(C12E23) and (Cu-
(DS)2)/Triton X-100 binaries. Triton X-100 is an isooctylphenoxypoly(oxyethano1) with an average of 9.5 ether groups (Packard) with a critical micelle concentration (crnc) of 2 X lo4 mol/kg. The cmc of Cl2E23 (EGA Chemie) was 6 X 15 X 10” mol/kg. CU(DS)~ was prepareds by adding an excess of copper chloride to a sodium dodecyl sulfate (SDS) solution (Merck). The precipitate was thoroughly washed in cold water and dried. Its *Author to whom all correspondence should be addressed.
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The cupric selective electrode from Orion (Model 94-29)was calibrated with copper nitrate and showed an excellent Nernstian behavior. The electrode reached equilibrium within 2-5 min even for the most nonionic micellar compositions and showed good stability. The reproducibility of the measurements was of the order of 10.1 mV. The calibration was performed in the presence of both nonionic surfactants used, and a slight but measurable variation of potential was observed as a function of nonionic surfactant concentration. The cell potential was measured by using an electrometer (Model 614) from Keithley. The ion condensation values B were calculated from the simple expression @ = (CM - cc)/(cM- c m )
(1)
where CMis the total concentration of ionic surfactant, Cc is the cupric ion concentration at the same potential on the calibration curve, and C, is the total monomer concentration. This procedure was preferred to the classical one, which apparently takes into account the activity coefficient of the metal ion by applying the Debye-Huckel law, but fails to take into account (for lack of a suitable theory) the effect of the mixed ionic micelles to the solution ionic force. As the concentration domain is the same for the surfactant and the calibration solution, the corresponding activity coefficients largely cancel out in eq 1. The monomer concentration C, is equal to the cmc for the pure surfactant solutions but shows a slight concentration dependence with total surfactant concentration. This effect could be neglected in the present case because of the very small cmc of the mixed surfactant used. The cmc of mixtures of CU(DS)~ with various nonionic surfactants of the series C12En(with n = 6,29,49)have been studied by Nishikid~.~ They were recalculated from an enlarged graph and interpolated for the present case with n = 23. The correction was proven to be negligibly small, and the same (1)Nilsson, P.-G.; Lindman, B. J.Phys. Chem. 1984,88, 5391. (2) Valaunikar, B. S.; Manohar, C. J. Colloid Interf. Sci. 1985, 108, 403. (3) De Salvo Souza, L.; Corti, M.; Cantu, L.; Degiorgio, V. Chem. Phys. Lett. 1986, 131,160. (4) Marszall, L. Langmuir 1988,4, 90. (5) Jansson, M.; Rymden, R. J. Colloid Znterace Sci. 1987, 119, 185. (6)Bjerrum, N. K. Danske Vidensk. Sekrk. 1926, 7, 9. (7) Treiner, C.; Khodia, A. A.; Fromon, M. J. Colloid Interface Sci.,
in press. (8) Mjyamoto, S. Bull. Chem. Soc. Jpn. 1960, 33,372. (9) Nishikido, N. J. Colloid Interface Sci. 1977, 60, 242.
0 1989 American Chemical Society
284 Langmuir, Vol. 5, No. 1, 1989
Letters A
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I TI
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> - - - L _ -
'
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-4
I
-3
4 -
-2
-3 log cu++
log cu++ - 2
-1
Figure 2. Variation of emf potential with Cu(DS), molality at various nonionic surfactant mole fractions x : ( 0 )Cl2EZ3;(0) Triton X-100. oi
Figure 1. Variation of emf potential with copper dodecyl sulfate
molality at constant CI2EBmole fractions x : (C) calibration curve; ( 0 )pure Cu(DS),; (0) x = 0.510; * x = 0.95. conclusion was applied to the mixed system with Triton X-100. Note also that at the total surfactant concentrations studied the micellar composition was equal to the stoechiometric solution composition. Two types of experiments were performed (i) at constant molar ratio x of the mixed micelles and (ii) at variable molar ratios, starting the experiments from a pure nonionic solution and adding small amounts of the ionic surfactant. Thus, with the first procedure, the /3 values were the average of a number of experiments for various CU(DS)~ concentrations, whereas with the second procedure single @ values were obtained. The solutions were thermostated in a electrochemical cell with a circulating jacket at 25 i 0.02 O C . The total CU(DS)~ concentration was in the interval 10-4-10-2 mol/kg.
Results and Discussion Figure 1 presents some of the results obtained by using procedure i for pure Cu(DS), ( x = 0) and for two different molar ratios of the mixed C U ( D S ) ~ / C system ~ ~ E ~ ( x = 0.51 and x = 0.95). An average value of p = 0.91 f 0.01 is found for the pure anionic surfactant, which is (as expected) higher than the accepted value for SDS obtained by using the same emf method of investigation (p = 0.80 f 0.05).1° At x = 0.95, the mixed micelles are fully dissociated. This conclusion is confirmed in Figure 2, where procedure ii was applied. Very clearly, the deviation from the calibration curve starts at a given nonionic/anionic mole fraction which corresponds for C12E23 to x = 0.95. The complete set of ion condensation data for this mixed system is displayed in Figure 3. The results from the two experimental procedures overlap very well. It must be reminded ~~
(10) (a) Botre, C.; Creacenzi, V. L.; Me1G-A. J. Phys. Chem. 1959,63, i50. (b) Shedlovsky, L.; Jakob, C. W.; Epstein,M. B. J . Phys. Chem. 1963, 67,2075. (c) Tokiwa, F.; Moriyama, N. J. Colloid Interface Sci. .969,30,338. (d) Kale, K. M.; Cuasler, E. L.; Evans, D. F. J. Phys. Chem. 980,84,593. (e) Hall, D.G.; Huddleston, R. W.Colloids Surf. 1986,13, !09. (0 Georges, J.; Chen, J. W. J.Colloid Interface Sci. 1986,113, 143. ( 9 ) Reference 7.
i 45:
1
0
a5
X
10
Figure 3. Variation of the degree of counterion condensation with nonionic surfactant mole fraction x : (a) Cl2EZs;(b) Triton X-100. ( 0 )procedure i; (0) procedure ii; ( a )micellar critical compositions XM. that the C12Eza/watersystem forms viscoelastic gels a t high surfactant concentrations. The total surfactant concentrations studied were therefore below 0.05 mol/kg for the two binary systems investigated in order to ensure that only direct classical micelles are formed. Experiments performed at higher surfactant concentrations showed indeed ion condensation changes above 0.1 mol/kg of nonionic surfactant a t a constant mole fraction x . Figures 3 and 4 present the results obtained with the second mixed system investigated. The general profile is the same with Triton X-100 as the nonionic component as with C12E23 in the whole mole fraction range with the particularity that with the former nonionic component the degree of cupric ion condensation is systematically higher than with the latter. The results of Figure 2 show that the deviation from the calibration curve starts closer to the pure nonionic micelle with Triton X-100. This corresponds
Letters
Langmuir, Vol. 5, No. 1, 1989 285 for ion condensation is thus
R < IZilN(1 - X’)/3.57 (4) For CU(DS)~, N is equal to 9513 and Zi= 2. Taking the experimentally determined XMvalues for the two nonionic systems, we get the R values of 34 and 18 A, respectively, for C12E23 and Triton X-100. The values of XMare close
-2
-3 log cu**
Figure 4. Variation of e m f potentials with CU(DS)~ concentration at various Triton X-100 mole fractions r: ( 0 )0.99; (*) 0.985; (*) 0.98; (*) 0.97; (0) 0.96.
to a minimum XMvalue of 0.98; thus, 2% of C U ( D S )is ~ necessary to start the cupric ion condensation. Figure 4 shows a few additional experiments which were performed at high constant x values: x = 0.96,0.97,0.98,0.985, and 0.99. For the first two mixed micelle compositions, average B values of 0.76 f 0.02 and 0.84 f 0.03 were respectively obtained. The three last series of experiments cannot be distinguished from the calibration curve. They confirm that above zM= 0.975 the mixed micelles are completely dissociated; i.e., there is a cut-off composition below which ion condensation occurs. The composition interval between the pure nonionic micelle and this cut-off distance might correspond to the micellar composition for which the increase of cloud point has been ~ b s e r v e d . ~ - ~ The question which may be asked is the following: to what extent may an ion condensation model be used to interpret these data? There have been controversies in the literature on ion condensation models for polyelectrolytes.” Belloni et d.12have shown that, in that case, the various definitions used in order to distinguish between free and bound counterions (two-state versus cell model) lead all to essentially the same fraction of bound counterions. It was therefore decided to apply B j e r m ’ s model, which can easily be adapted to the case of a spherical micelle considered as having the electrical properties of a polyelectrolyte. Bjerrum’s chemical model states that two ions separated by a distance R, such as
should be considered as associated (they do not contribute to the solution ionic force). Zi and Zj may be considered respectively as the charge of the counterion and that of the mixed micelle; R is the radius of the micelle, the counterion being taken as a point charge. The other symbols have their usual meaning. The charge of the mixed micelle will be taken simply as where N is the number of ionic charges of the pure micelle in water (its aggregation number) and XM the mixed micelle composition. In water a t 298.15 K the condition (11)Westra, S.W. T.; Leyte, J. C. Ber. Bumenges. Phys. Chem. 1979, 83,672. (12)Belloni, L.; Drifford, M.; Turq P. Chem. Phys. 1984,83, 147.
enough to the pure nonionic micelle so that the above numbers may be considered as the radii of the pure micelles. Discussions on the shape of the Triton x-100 micelle, which is considered as spherical or of a more complex structure according to different a u t h ~ r s , l is ~-~~ prematurate a t this stage of the discussion because the model used is too crude for such refinements. Note, however, that the R values obtained for the two nonionic surfactants are compatible with their respective structural size. If the above analysis is correct, we may now turn to the mixed SDS/nonionic solutions which are of more practical interest than the systems presently investigated. As the xM compositions for ion condensation are very close to 1, the value for the pure nonionic micelle, the assumption can be made that the R values deduced above are still valid in the presence of other ionic surfactants. In other words, the structural changes induced by the addition of either anionic surfactant to the nonionic micelle are small or similar. Then, using for SDS Zj = 6217 and Zi = 1, we obtain for the two systems studied xM = 0.85 for C12E23 and xM = 0.92 for Triton X-100. It is clear that small differences in XMlead to large variations of R. Thus, further refinements in the Bjerrum model in terms of a more realistic cut-off distance are fruitless in the present case. The main conclusion which can be drawn from this investigation is that the experimental data on ion condensation in mixed micelles are compatible with the concept of a minimum charge density for ion condensation and therefore of a mixed micelle composition interval within which ion condensation does not yet occur. The idea of a minimum charge density for ion condensation in the case of mixed ionic/nonionic micelles was investigated or suggested bef0re,5”~J~ but the techniques used were not able to detect a discontinuity in the binding data although the experimental results were not in opposition to it. It was one of the main advantages of the cupric ion selective electrode to provide some evidences for this hypothesis. Furthermore, the present approach quantifies the micellar composition range where the phenomenon occurs and offers a plausible model for the interpretation of the cloud point increase with the addition of small amounts of an ionic surfactant to a nonionic micellar solution. As shown above according to Bjerrum’s model, addition of up to 8% of SDS to Triton X-100 leads to the formation of mixed micelles without counterion condensation. Most a u t h o r P have worked with ionic to nonionic surfactant ratios of around 2% SDS. (Note that the smaller the nonionic micelle size, the larger the XMcritical composition.) The negatively charged micelles then experience repulsive interactions. The amplitude of the repulsive potential must change when the counterion condensation starts to occur. Most authors have noted that the increased cloud point (13)Satake, I.; Iwamatse, I.; Hasakawa, S.; Matuura, R. B u r C h e m . Soc. Jpn. 1963,36,205.
(14)Paradies, H.H. J. Phys. Chem. 1980,84,599. (15) Robson, R. J.; Dennis, E. A. J . Phys. Chem. 1977,81,1075. (16)Corti, M.; Degiorgio, V. Opt. Commun. 1975,14,358. (17)Mysels, K. J.; Princen, L. H. J. Phys. Chem. 1959,63, 1699. (18)Meyer, M.; Sepulveda, L. J. Colloid Interface Sci. 1984,99,536. (19)Rathman, J. F.;Scamehorn, J. F. J. Phys. Chem. 1984,88,5807.
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effect diminishes by further addition of the ionic surfactant; this seems to correspond to the counterion condensation threshold. As noted above, the cut-off distances should not be taken too literally. Even within Bjerrum's model, this distance is only defined as a minimum of a painvise potential function. Thus the above analysis only points out a possible origin of the cloud point increase
phenomenon discussed. Further investigation is necessary in order to provide evidence of the change of micellar interactions around the micelle composition for which counterions start to condensate on the mixed micelle. Registry No. CU(DS)~, 7016-47-9;Triton X-100, 9002-93-1; Brij 35, 9002-92-0.
Notes Resonant Nonlinear Surface Spectroscopy: Range and Limitations Frank W. Gordon, Stephanie A. Cresswell, and Jack K. Steehler* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received August 2, 1988. I n Final Form: September 30, 1988
Introduction As has been amply demonstrated in recent years, x@) spectroscopy is a unique tool for probing surface structures with monolayer selectivity.' Signal generation has been observed from both the surface of the bulk substrate and from molecular adsorbates. Recently the emphases in this area have been on finding detailed explanations for observed signals2" (e.g., symmetry properties of signals from single-crystal surfaces) and defining unique applications for this surface probe. Applications emphasized by different groups include time-resolved monitoring of surface processes6 and selective monitoring of single species in mixture environment^.'-'^ The widest range of application of x@) spectroscopy results from the selective monitoring of adsorbates. Surface systems of practical importance including lubricants, corrosion inhibitors, surfactants, and electrochemical electrodes are potential users of this methodology. The initial demonstration efforts in this area have concentrated on the use of dye molecules as surface ad~orbates,'J'-'~ primarily for the convenient spectral resonances which they provide. A notable exception was the work of Van Wyck et al.,14 where a ni..mber of nondye molecules were considered. In that work it was noted that a number of these molecules did not give observable x ( ~signals ) at the low laser intensities required to prevent loss of the adsorbate layer. This paper will report data for a range of molecular species on both the experimentally convenient fused silica surface and on metal surfaces, which are chemically more interesting. The nondye molecules chosen for these studies included systems of interest to electrochemical and corrosion inhibitor applications. The first question being asked is whether noncentrosymmetric surface layers are forming for all sample species. In general it will be seen that such layers do form for almost all cases. The second question is whether the intensities of adsorbate specific *Author to whom correspondence should be addressed. Current address: Department of Chemistry, Roanoke College, Salem, VA 24153.
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signals are high enough to be conveniently useful in practical analysis of mixed surface systems. This desired condition will be seen to exist for only a subset of cases, including resonant experiments with molecules possessing strong optical transitions. If the selectivity of optical resonances is not required and high laser intensities can be tolerated, nonresonant experiments provide useful signals for a wider range of molecules.
Experimental Section Our experimental system has been described previously." Briefly, the system includes two dye lasers pumped by the same XeCl excimer laser, appropriate optics to generate and collect the s u m of the two laser frequencies, a 0.22-m double monochromator, gated detection, and photon-counting software. Laser wavelengths varied from 425 nm to 1.06 pm, depending on the experiment. Nonresonant experiments were performed with 840-nmor 1.06-pm light (using a DCR 11NdYAG laser), where neither the incident laser frequencies nor the sum (or second harmonic) frequency was resonant with sample energy levels. Doubly resonant experiments were performed for rhodamine 590, nile blue 690, and cresyl violet 670. All other resonant experimenta were singly resonant, generally at the sum frequency, and sometimes used second harmonic generation from a single incident laser. The 1-in.-diameter metal disks were polished with alumina pastes down to 0.05 pm. All samples were spin coated onto the substrate from MeOH solutions M). Fused silica samples were used in a (typically 1 X transmission geometry while metal surface samples were used in a 45O reflection geometry. Molecules of electrochemical interest were also studied directly in solution environments with Ag, Cu, and Pt electrodes. Survey results are presented here with in-depth discussion of solution results to be reported separately. Similarly, (1) Richmond, G. L.; Rojhantalab, H. M.; Robinson, J. M.; Shannon, V. L. J. Opt. SOC.Am. B 1987,4,228. (2)Shannon, V. L.; Koos, D. A.: Richmond, G. L. J.Phvs. Chem. 1987, 91, 5548. (3)Epperlein, D.; Dick, B.; Marowsky, G.; Reider, G. A. Appl. Phys. B 1987,44,5. (4)Sipe, J . E.; Moss, D. J.; van Driel, H. M. Phys. Rev. B 1987,35, 1129. (5)Guyot-Sionnest, P.;Chen, W.; Shen, Y. R. Phys. Reu. B 1986,33, 8254. (6) Shannon, V. L.; Koos, D. A.; Robinson, J. M.; Richmond, G. L. Chem. Phys. Lett. 1987,142,323. (7) Cresswell, S. A.; Steehler, J. K. Appl. Spectrosc. 1987,41,1444. (8)Harris, A. L.; Chidsey, C. E. D.; Levinos, N. J.; Loiacono, D. N. Chem. Phys. Lett. 1987,141,350. (9)Zhu, X. D.; Suhr, H.; Shen, Y. R. Phys. Rev. B 1987,35,3047. (10)Guyot-Sionnest,P.;Hsiung, H.; Shen, Y. R. Phys. Reu. Lett. 1986, 57,2963. (11)Cresswell, S. A.;Steehler, J. K. Appl. Sepctrosc. 1987,41,1329. (12)Marowsky, G.; Gierulski,A.; Dick, B. Opt. Commun. 1985,52,339. (13)Nguyen, D. C.;Muenchausen, R. E.; Keller, R. A.; Nogar, N. S. Opt. Commun. 1986,60,111. (14)Van Wyck, N. E.; Koenig, E. W.; Byers, J. D.; Hetherington, W. M . 111. Chem. Phys. Lett. 1985,122,153.
0 1989 American Chemical Society
