766
Langmuir 1989,5, 766-776
region I1 are dramatic, suggesting a change in the mechanism of the process. This behavior can be directly correlated to past work in which it was found that the adsorption process changed once an "FeO-like" layer was completely formed: oxygen uptake slowed, and a significant increase in either temperature or oxygen exposure was needed to further oxidize the surface, ultimately forming a "spinel-like" structure.'O Other data further support the suggested stoichiometry change from FeO (region I) to Fe304(region 11). First, the ratio of the amount of oxygen adsorbed in region I (45.0 kmol/g) to that in region I1 (17.5 Mmol/g) is very close to 3.0, exactly what it should be for a change in stoichiometry from FeO to Fe301. Second, the measured hyperfine splitting of the oxide structure at 8 K from MES (Table III) is consistent with previous investigati~nsw*~~ in which similar MES spectra were assigned to thin layers of Fe301. The third aspect of the model (III), that two layers of iron are oxidized, is the most speculative of the points raised above but is the simplest explanation of the data. The average particle diameter and dispersion, as determined by XRD and TEM (Figures 7 and 8, Table 4), can be used to determine the total amount of exposed iron surface. If a hemispherical particle shape is assumed (on the basis of the Miissbauer results), then two monolayers are necessary to explain the measured 10% oxidation of the total metal. The conclusion that amroximately two layers of iron are oxidized is consistent &th earlie; thin film studiess3' and is qualitatively supported by the MBssbauer data, which show that a Signal from the oxide surfacelayer can Only be Obtained at very low temperatures (8 K). In contrast, in previous Mossbauer studies of the oxidation of small iron particles23-28and iron films,4s*47v62 hyperfine splitting is clearly visible at 77 K. This suggests
that the passivating layer formed in the present work is thinner than those reported previously. Thinner layers might be expected to form in the present caae as a result of a slower, controlled rate of oxygen exposure. Finally, point I11 is consistent with XRD results, which showed very clear a-Fe lines, with no evidence of iron oxide.
Summary The results of this work can be summared as follows: Differential microcalorimetric data from the adsorption of oxygen on Grafoil-supported iron particles have shown that the chemisorption process proceeds through two distinct stages, initially a rapid, high-heat process, followed by a significantly slower, falling-heat process. Throughout the adsorption, an equilibrium surface structure is maintained by the reordering of the adsorbed oxygen atoms. The data can be explained by assuming that the initial stage of the proteas represents the formation of a layer with FeO stoichiometry, while the second stage is a restructuring to a layer with Fe304stoichiometry. The calorimetric data, along with the evidence from XRD, MES, and TEM, indicate that only about two layers of surface iron are oxidized during the differential oxidation. Further exposure to oxygen after this passivation layer has formed does not measurably increase the extent of oxide formation. Acknowledgment. We gratefully acknowledge the support of the National Science Foundation (Grant CBT~ 8315046) and The pennsylvania state university ~ neering School Equipment Program (Ben Franklin Partnership and Advanced Technology). Registry No. 02,1782-44-1; Fe, 7439-89-6.
Characterization and Stabilization of Inverse Micelles A. Verbeeck, G. Voortmans, C. Jackers, and F. C. De Schryver* KU Leuven, Department of Chemistry, Laboratory for Molecular Dynamics and Spectroscopy, Celestijnenlaan 200F,B-3030 Heverlee-Leuven, Belgium Received February 4, 1988. I n Final Form: January 9, 1989 The aggregation behavior of the cationic surfactant didodecyldimethylammonium chloride (DDAC) and its derivatives is studied in apolar solvents. The characteristics of the reverse micellar aggregates are correlated with the structure of the detergent monomers. Factors responsible for the stability of the w/o aggregates and the w/o microemulsions are estimated. The role of the electric double layer in the stability of inverse micellar aggregates is evaluated by comparing the characteristics of both solutions. Evaluation of the luminescence properties of chromophores solubilized in the micellar aggregates provides information on the aggregation characteristics. UV absorbance and fluorescence decay measurements of sodium 1-pyrenesulfonate (PSA) support an aggregation process corresponding to the E-model while fluorescence quenching of PSA with I- allows the determination of the average aggregation number (N,) and the rate of intermicellar exchange (ke). Introduction The influence of surfactant stNcture on critical concentration (cmc), size, and water solub&&ion capacity of reverse micellar aggregates is studied. Controlling these characteristics of reverse micelles is important in many of their potential technological applications. The aggregation behavior of the cationic surfactant didodecyldimethylammonium chloride (DDAC) and some of its derivatives is studied in apolar solvents. Two main 0743-7463/89/2405-0766$01.50/0
reasons justify the choice of DDAC as surfactant. It forms spontaneous three-component water in oil (w/o) microemulsions at room temperature, without addition of alcohol.'" Knowledge obtained from the study of three(1) (a) Angel, L.R.; Evans,D.F.;Ninham, B.W. J.Phys. Chem. 1983, 87,538. (b)Ninhem, B.W.; Chen, S. J.; Evans, D. F. J. PhW. C h m . 19&1, 88,5855. (c)Evans, D. F.; Ninham,B.W . J. Phys. Chem. 1986,90,226. (2) K.unieda, H.; Shinoda, K. J. Colloid Interface Sci. 1979, 70, 577. (3) Lindman, B. R o c . Int. Sci. Phys. 1985, 90,7.
0 1989 American Chemical Society
~
i
Langmuir, Vol. 5, No. 3,1989 767
Characterization of Inverse Micelles CH3
HC-2 -R
X'
CODE
................................. -CH3 -CH3 -CH2CH20H -CH2CM)H -CH~CHZ-O-C-C=C II I 0 CH3
C1Brc1c1C1-
DDAC DDAB D m c DMMC DMCAC
E
CH3(CH2)3-CH-CH2-O-1 y 2 0 CH3
2
CODE
---_____---____
S O ~ - N ~ + AOT NH3'Clc.AOT
Figure 1. Structure of the cationic and anionic Surfactants
studied.
Table I. NMR Data on Surfactant Monomers in CDCln surfactant NMR absorptions (6) DDAC 0.98 (t),1.39 (81, 1.9 (m), 3.75 (s), 3-86(m) DMHAC 0.9 (t),1.27 (s),1.7 (m), 3.3 ( 8 ) , 3.45 (m), 3.7 (t), 4.1 (t), 5.78 (t) DMAAC 0.89 (t),1.27 (s), 1.8 (m),2.78 (s), 2.97 (m) DMAcrAC 0.9 (t),1.28 (81, 2.69 (81, 2.95 (s), 3.4 (e), 3.5 (m),4.12 (m),4.65 (m), 5.6 (81, 6.05 (8)
salts KI (U.C.B., p.a.) and CoClzwere used without further purifications. All solutions of the surfactant monomers in apolar solvent after addition of water were sonicated for 15 min and degassed before fluorescenedecay and fluorescenceintensity measurementswere performed. The aggregation mechanism of these cationic surfactant molecules was studied by UV absorbanceand fluorescencedecay measurementsof sodium 1-pyrenylsulfonicacid (FS ' A) as a probe.'O The solubilization of water in the micellar systemsin toluene was determined, and the maximal hydration number is obtained with the method proposed by Thomas." The average aggregation number (N ) and the stability were determined by fluorescence quenching.v On the basis of a previous study, PSA/KI was chosen as a suitable probe/quencher system for this study.13 Fluorescence decays were determined by using a picosecond single-photon timing apparatus, with a mode-locked, cavitydumped, synchronouspumped dye laser as an excitation source, coupled to a PDP 11/23 computer for data handling.14 To judge the goodness of fit, several statistical criteria were used. Fluorescence intensities were measured with a Spex Fluorolog 212/datamate, connected to a Digital Equipment Corp. PDP11/23 computer for data analyses.16
Results and Discussion I. DDAC. 1. Aggregation Mechanism. The aggregation of DDAC in apolar solvents in the absence and component microemulsions can simplify the interpretation presence of added water has been reported.'O Measureof analogous measurements on four- or five-component ments of the UV absorbance and the fluorescence decay microemulsions systems. of PSA in the surfactant solutions proved that the model The second reason that influenced our choice of DDAC of EickelB adequately describes the aggregation process of as a prototype was the fact that easy synthetic pathways DDAC in an apolar medium. Aggregation starts at very applicable to functionalize its polar head group e x i ~ t . ~ * ~ low surfactant concentrations (lob M) with the formation This was an important consideration since it is thought of "open" aggregates. They grow in a stepwise manner, that mutual interactions of the polar head groups of the as a function of the concentration of DDAC. Around 2 X surfactant as well as with added water are dominant factors M, these "open" aggregates close to form cyclic reverse controlling the formation of reverse micelles.s Figure 1 micelles. The cyclic structures grow stepwise upon inlists the derivatives of DDAC that were synthesized. In creasing the surfactant concentration until they reach an this paper, the effect of alteration of the surfactant aggregation number (N,) with a minimal free energy.12 structure on the stability of the reversed micelle is disThis average aggregation number is a characteristic for the cussed. It is hoped that an enhanced interaction with surfactant system studied. water and/or the polymerizability of the surfactant moThe concentration at which cyclization occurs could be nomers will lead to a better stability of the reverse micelles, called "the operational cmc" and is situated for DDAC a topic of strong current interest.' M. around 2 X 2. Solubilization of Water. The solubilization caExperimental Section pacity for water in 0.08 M DDAC in toluene was studied. The preparation of didodecyldimethylammonium chloride For R I 12, the solutions were clear after sonication and (DDAC) from didodecylmethylammonium bromide (DDAB) remained homogeneous upon standing for days. At water (Kodak) on an ion-exchangeresin (Merck 4767) was similar to that reported for cetyltrimethylammonium chloride (DTAC).* concentrations of R = 12-40, the solutions were turbid, and The functionalized surfactants (Figure 1) were synthesized starting phase separation occurred. from dodecylamine. Preparation of didodecylamine and didoThe maximal hydration number for DDAC was deterdecylmethylamine was described by Ralston et al.' For the mined by using the absorption spectra of Co2+solubilized quaternatization of the tertiary amines, 1 equiv of didodecylin the waterpool. In Cl- solutions, Co2+exists in two forms: methylamine and 2 equiv of the corresponding chloride (Figure [ C O C ~ ~and ] ~ -CO[H~O],~+ with an absorption maximum 1) were stirred in dimethylformamide (DMF) at 50 "C for at least respectively at 694 and 625 nm. As the concentration of 24 h. All surfactants were purified by several recrystallizations in ethyl acetate and then dried for 24 h under vacuum. Structural characterization was based on NMR spectra (Table I). (IO) (a) Gelad6, E.; Verbeeck, A.; De Schryver, F. C. Surfactants in The sodium salt of 1-pyrenesulfonicacid was synthesized by Solution; Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1987;Vol. a modification of the method of Tietze and B a ~ e r .The ~ purity 5, pp 565-579. (b) Verbeeck, A.; GeladC, E.; De Schryver, F. C. Langmuir of the probe was checked by measuring the fluorescence decay 1986,2, 448. time of a 10" M methanol solution. The decay was single ex(11) Mc Neil, R.; Thomas, J. K. J. Colloid Interface Sci. 1981,83,57. (12)GeladC, E.;De Schryver, F. C. Reverse Micelles: Biological and ponential with a lifetime (q,) of 110 ns at room temperature. The (4)Ralston, R.W.;Eggenberger, D. N.; Du Brow, P. L. J. Am. Chem. SOC.1948,70, 977. (5) Kunitake, T.; Okahata, Y. Chem. Lett. 1977,1337. (6)Zundel, G. Hydration and Intermolecular Interaction; Mittal, K. L., Ed.;Academic Press: New York, 1969. (7)Voortmans, G.; Verbeeck, A.; Jackers, C.; De Schryver, F. C. Macromolecules 1988,21,1977. (8) Roelants, E.; Gelad6, E.; Van der Auweraer, M.; Croonen, Y.; De Schryver, F. C. J. Colloid Interface Sci. 1983,96,288. (9)Tietze, E.;Bayer, 0. Ann. Chem. 1939,540,189.
technological relevance of Amphiphilic Structures in apolar Media; Luisi, P. L., Straub, B. E., Eds.;Plenum: New York, 1984,pp 143-164. (13)Verbeeck, A.; De Schryver, F. C. Langmuir 1987,3,494. (14)(a) Bcens, N.; Van den Zegel, M.; De Schryver, F. C. Chem. Phys. Lett. 1984,111,340.(b) Van den Zegel, M.; Boens, N.; Daems, D.; De Schryver, F. C. Chem. Phys. 1986,101,311. (15)Desie, G.; De Schryver, F. C. Instruments and Computers 1985,
3,44. (16)(a) Eicke, H. F. Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.;Plenum: New York, 1977;Vol. 1. (b) Eicke, H. F.; Hopmann, R. F. W.; Christen, H. Ber. Bunsen-Ges. Phys. Chem.1975, 79, 667.
768 Langmuir, Vol. 5, No. 3, 1989
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R 0
2
I
4
6
9
Figure 2. Absorbance of the complex [CoCl#- at 694 nm as a function of the water solubilized in the micellar aggregates of DDAC in toluene.
water in the reverse micelle increases, the concentration of [CO(C~,)]~decreases. At the concentration of water where the absorbance of [CoCl,]" has disappeared, the quaternary ammonium chloride head groups are totally hydrated. A plot of the absorbance intensity at A = 694 nm versus the concentration of water in the solution is presented in Figure 2. From this figure, it is concluded that the maximal hydration number of DDAC is situated between R = 5 and R = 6. Between R = 6 and R = 12, free water is present in the waterpool of the reverse micellar aggregates. In this region of R values, three-component water in oil microemulsions are formed. 3. Fluorescence Quenching. It was proved that fluorescence quenching is an adequate method for the characterization of reverse micelles and w/o microemulsions, provided that for the probe/quencher system chosen the intramicellar quenching process is faster than the intermicellar exchange process (kq, > k,[M], with k,, the intramicellar quenching rate constant, k, the interncellar exchange rate constant, the [MI the concentration of micelles in solution)." This indicates that both probe and quencher must be solubilized in the same region of the micellar interior. Since for small micellar aggregates the exchange process is rather slow, the fluorescence decay time must be sufficiently long to measure this dynamic process. On account of both factors, the combination PSA/KI was selected. Both are solubilized in the micellar and T~ for PSA is situated around 130-150 ns, depending on the size of the waterpool. The following characteristic parameters of the micellar aggregates can be obtained from fluorescence quenching measurements: the size of the aggregate as expressed by the average aggregation number (Nw), the stability of the aggregate as expressed by k,, the intermicellar exchange rate constant for quenchers and probes between two mi(17) Eicke, H. F.; Christen, H. J. Colloid Interface Sci. 1974,48,281. (18) (a) Eicke, H. F.; &hake, J. Helu. Chim. Acta 1976,59,2883. (b)
Day, R. A.; Robinson, B. H.; Clarcke, J. H. R.; Doherty, J. V. J. Chem. SOC.,Faraday Trans 1979,1,132. (19) Zulauf,M.; Eicke, H. F. J. Phys. Chem. 1979,83,480. (20) (a) Eicke, H. F.; Shepherd, J. C. W.; Steinemann, A. J. Colloid Interface Sci. 1976,J6,168. (b) Fletcher, P. D. I.; Robinson, B. H. Ber. Bunsen-Ges Phys. Chem. 1981,85,863. (c) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H., submitted to J. Chem. SOC.,Faraday Trans 1. (21) Johnson, K. A,; Shah, D. A. J. Colloid Interface Sci. 1985, 107, 269. (22) (a) Hunter, T. F.; Younis, A. 1. J. Chem. SOC.,Faraday Trans. 1 1979, 76,650. (b) Klein, U. K. A.; Miller, D. J.; Hauser, M. Spectrochim. Acta 1976, 32, 379.
Figure 3. Variation of the average aggregationnumber with the concentration of water for the micellar solution of DDAC ( e) and DMHAC ( 0 )in toluene.
0
Figure 4. Temperature dependence of the average aggregation number of DDAC in toluene at different water concentrations: R = 1 (e);R = 2.5 (0);R = 4 ( A ) ; R= 5.5 (m);R = 7 (VI;R = 8.5 (*).
celles via dimerization, and the fluidity of the micellar interior related to kqm,the intramicellar quenching rate constant. a. N , The variation of the aggregation number with the concentration of solubilized water in the micelle is presented in Figure 3. As R increases, Naesincreases, first slowly, but after full hydration of the head group is achieved, the Nwg increases more rapidly. Whereas the respective N for the well-known system sodium bis(2ethylhexy1)szauccinate (AOT) (Figure 1)is much higher, a similar trend as a function of R was noticed.18 A study of Nwgas a function of the temperature gives information concerning the stability of the reverse micellar aggregates. Since N is a characteristic for a micellar system and is associaa with a minimum in the free energy and since aggregation in an apolar medium is enthalpy controlled, the minimum of AGO is associated with a minimum in AHo.17The mutual interactions of the head groups determine to a large extent the value of AH', while the van der Waals interactions of the apolar tails contribute to a lesser extent. It is accepted that both AHo (23) Israelachvili, J. N.; Marcelja, S.; Horn, R. G. Q. Reo. Biophys. 1980, 13, 121. (24) Norne, J. E.; Hjalmarsson, S. G.; Lindman, B.; Zeppezauer, M. Biochemistry 1975, 14, 3401.
Langmuir, Vol. 5, No. 3, 1989 769
Characterization of Inverse Micelles
00 .+
:-e;
ken
a?: 0 encounter pair
dimer
kfus
0
symbols the waterpool of a reverse micellar aggregate; ken and
k-en a r e the r a t e contants respectively f o r the formation and the
destruction of the encounter p a i r ; kfus and k-fus a r e the r a t e constants f o r respectively the dimerisation and t h e f i s s i o n of t h e dimer. Figure 5. Schematic presentation of the dimerization upon collision of two reverse micellar aggregates, proposed by Robinson.20c
i al
2
2
0 0
,
I 2
.
I 4
R.
I 8
.
I 8
.
I
.
Figure 6. Rate constant of intermicellar exchange as a function of the water concentration for the aggregates of DDAC (+) and DMHAC ( 0 )in toluene. and ASo remain constant in the temperature range 20-60 "C. A decrease of AGO, measured as a change of N increasing the temperature is due to an increase o f i s ? Changes of T A S O in this temperature range are noticeable only for aggregates where AHo is small. Figure 4 illustrates the variation of NW with temperature for all R values measured. Only for the lowest concentration of water is N a constant at increasing temperatures. The higher $e value of R, the stronger the variation of N with temperature. For the mice% system AOT, (for R 5 15 ( N , 5 200)), NW remains constant over the same temperature range.lg Therefore, it can be concluded that the reverse micelles of this anionic surfactant are more stable than those of the cationic DDAC. b. K,.The intermicellar exchange of quenchers and/or probes is a measure for the stability of reverse micellar aggregates provided that the rate-limiting step that determines k, is the dimerization process and not the intermicellar quenching process (Figure 5). The validity of this supposition will be discussed in section III.2.b. In Figure 6, the exchange rate constants of a 0.08 M solution of DDAC in toluene at different concenirations of water are given. For water concentrations below the total hydration of the cationic surfactants, k, has a constant value of lo8 M-' s-l. In these solutions, 1%of all collisions between the micelles results in the formation of a dimer. Adding free water to the aggregates reduces the barrier for dimerization. Study of the exchange process as a function of temperature allows determination of the activation enthalpy (AH,*) and the activation entropy (AS,'). As shown in
0
2
4
8
8
Figure 7. Activation enthalpy (a) and entropy (b) for the intermicellar exchange of the aggregates of DDAC in toluene for increasing water concentrations. Figure 7, both AHe' and AS,* are functions of the concentration of water in the micelle. While for all concentrations of water measured AH,' is positive, AS,*on the contrary is positive at low R values but becomes negative for the totally hydrated reverse micellar aggregates. Since at the fusion of two micelles the surface to volume ratio diminishes, the activation barrier is due to the Coulombic repulsion between the charged head groups and to the steric hindrance, which depends on the geometry of the surfactant monomer. The activation enthalpy decreases with increasing concentration of water, and the effect of water is especially marked before total hydration of the head groups occurs. This could be due to the fact that the increasing hydration layer increases the distance between the charged head groups and therefore decreases the Coulombic repulsion. The activation entropy (AS,') is related to the dimerization of the micelles and changes with the molecular
770 Langmuir, Vol. 5, No. 3, 1989
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Table 11. Activation Enthalpy (AH,'), Entropy (AS,'), and Free Energy (AG,') for the Intermicellar Exchange of DDAC in Toluenea and of AOT in n -Hexaneb ASe*, J surfactant AH.', kJ mol-' mol-' K-' AG.', kJ mol-' DDAC AOT
15 70
-2 1 +93
E
1500
I
I
1
21 43
"R = 8.5. * R = 10. AOT
Figure 9. Intramicellar quenching rate constant of PSA with I- in the micellar aggregates of DDAC in toluene, before (+) and after ( 0 )correction for the increase of the reaction layer at increasing concentrations of solubilized water. Na+
Figure 8. Orientation of the monomers of AOT at an oil/water interphase.*l
packing of the detergent monomers in the aggregates by fusion. Robinson et a1.20b*c measured a positive AS,* for the fusion of AOT reverse micellar aggregates and s u g gested as an explanation the release of surfactant monomers from the aggregates into the bulk solution (increasing the entropy of mixture) as a consequence of the shrinking of the interphase. On the other hand, the pushing together of the monomers by fusion of two micelles increases the order in the micellar interphase, resulting in a negative ASe*. Probably both processes contribute to the value of AS,* but to a different extent depending on the concentration of water solubilized in the reverse micelles. A t low concentration of water, before the total hydration of the polar head groups, the release of monomers in the bulk apolar solvent is favored above the closer packing of the detergent molecules in the aggregates. The higher the concentration of water, the lower ASe*, emphasizing the importance of water molecules in the aggregation of detergent monomers and the stability of their aggregates. Beyond the total hydration of the detergent molecules, the interactions of the water molecules with the polar head groups of the monomers prevent a strong release into the bulk; Le., ASe* is negative as a result of the increased ordering of the interphase upon compressing the interphase. Figure 7 indicates that comparing the order of magnitude of AHe* and ASe* for DDAC with AOT can only be meaningful for comparable water concentrations. Since the literature contains no information of AH * and AS,* for the exchange process on AOT for R < 10,m:c the values for DDAC at R = 8.5 are compared with those of AOT at R = 10. Table I1 provides data that indicate that the barrier for dimerization is much higher for the aggregates of AOT than for DDAC. The higher activation barrier for AOT micelles is thought to be due to the more rigid geometry of an AOT surfactant that strongly hinders the pushing together of the monomers in a micelle. Figure 8 shows the assumed orientations of AOT at an oil/water interphase.21 The carboxyl groups force the apolar tails in a more defined position while the branching of the tails provides a good filling up of the interphase. For a DDAC monomer, with
two long apolar tails on the same nitrogen atom, the fluidity in an oil/water interphase is much greater. Nevertheless, in DDAC the packing at the interphase is better than that of reverse micelles based on single-tailed monomers, as dodecylammonium propionate (DAP). For DAP, k, is 1 order of magnitude larger compared to that for DDAC.12 Since the polar head group of a DAP monomer can be compared to that of DMAAC (vide infra 11.3), the higher ke is due to the difference in structure of the apolar tail and is therefore related to a lower rigidity at the interphase. The large positive activation entropy for the fusion of two AOT micelles reported by Robinson indicates a strong release of AOT monomers to the bulk solvent. This can be an additional indication of the optimal order of AOT monomers at a micellar interphase, in agreement with the high activation enthalpy observed for this system. c. k ,. The intramicellar quenching rate constant, determined by quenching of PSA with I-, is a measure for the number of collisions (that result in quenching) between one probe and one quencher in the micelle. In view of the fact that probe and quencher both are localized a t the interface, it is clear that k, is a function of the size of the aggregate (Figure 9). To correct for this size dependence of the value on kqm,it is necessary to use the expression k,,S, where S is the surface area of the interface of the reverse micellar aggregates and is considered as the reaction layer for quenching of PSA with I-.2s Knowing the aggregation number of the micellar aggregate, S can be calculated from the concentration of water added to the solution according to eq 1 and 2:
V = );$ = NJ3u S=P
(2)
where V is the volume of the micellar waterpool and u is the volume of one water molecule. The temperature dependence of kqmSprovides information on the activation enthalpy of quenching (AHq*). In Figure 9, the values of k,S for different water concentrations are presented. An increase of kqmSis observed (25) Van der Auweraer, M.; Dederen,J. C.; GeladB, E.; De Schryver, F. C.J. Chen. Phys. 1981, 74, 1140.
Langmuir, Vol. 5, No. 3, 1989 771
Characterization of Inverse Micelles Table 111. Activation Enthalpy of Quenching of PSA with I- in the Micellar Aggregates of DDAC in Toluene upon Increasing- the Water Concentration R AHq*,kJ mol-’ R AHq’, kJ mol-’ 5.5 7.0 1 7.8 1 8.3 2.5 1.9 4
1.3
8.5
200
L a
9.1
Table IV. cmc of the Cationic Detergent DDAC and Its Derivatives in Cyclohexane surfactant operational cmc, M DMHAC 2 x 10-4 DDAcrAC 3 x 10-4 DMAAC 9 x 10-4 DDAC 2 x 10-3 DDAB 8 x 10-3 Table V. Solubilization of Water in Cationic Micellar Solutions with Dry Toluene as Apolar Solventa surfactant Rmin R, DDAC 0 12 DMHAC 0 9 DHAAC 0 2 DDAcrAC 0 4 DDAB 0 2 OAt room temperature.
upon increasing the water concentration. A study of k,, as a function of temperature indicates that AHq*remains constant over the range of water concentrations examined (Table 111). Since I- and PSA are localized in the interphase, regardless of the concentration of water solubilized, it is not unreasonable that AH * is relatively insensitive to the R values. The increase 0fJ-S as a function of R is explained by the effect of the hydration of the polar head groups, allowing a higher mobility of the oppositely charged quencher ions. 11. Derivatives of DDAC. Functionalization of DDAC was limited to modifications of the polar head group, since the interactions of the polar head group of the monomers mutual and/or with water are thought to be of prime importance in micelle aggregation. 1. Aggregation Mechanism. In analogy with the study of the aggregation behavior of DDAC, UV absorbance and fluorescence decay measurements of PSA were used to establish the aggregation process of the four DDAC derivatives (Figure la). In Figures 10-13, the results are reported. These results are all similar to those obtained for DDAC. The absorbance measurements show that aggregation starts at very low concentrations and progresses in a stepwise manner. Fluorescence decay measurements are indicative of a structural transformation from “open” to cyclic aggregates. Therefore, the model of Eicke can be generalized to the aggregation of the cationic surfactants with a quaternary ammonium head group in apolar solvents. The “operational cmc” is a characteristic of the surfactant (Table IV)and is situated in the concentration range 10-4-10-2 M. 2. Solubilization of Water. The maximal amount of water solubilized in the aggregates in toluene is a characteristic of the surfactant (Table V) and is very small for all systems considered. A low solubilization capacity for water is a general characteristic of cationic surfactant systems and probably originates from the nature of the polar head group. The cationic analogue of AOT (c.AOT) (Figure lb) was synthesized. While large amounts of water
0.0
-6
....
,...-3
l o p [ O M H A C I (MI
I..,.
-5
l....L....l..&
-4
-2
-i
l o p I O M H A C l IM)
4
I
-6
-I
-4
-3
-2
-1
Figure 10. Influence of the concentration of DMHAC in cyclohexane at R = 2 on the absorption (A) and the fluorescence decay (% T L and TL) of PSA.
can be solubilized in micelles at AOT (R = 35-70, dependent on the apolar solvent used), the solubilization capacity of the cationic analogue is very poor (R < 1). 3. Fluorescence Quenching. To obtain information concerning the value of Nag, k,, and k,, of the abovementioned surfactant systems, the fluorescence quenching of PSA/KI, the same probe/quencher combination as in the study of DDAC, was evaluated. All surfactant solutions were 0.08 M in toluene, for the different water concentrations. Since relatively large amounts of water can be solubilized in the micellar aggregates of DMHAC, this system was examined fist, and its characteristics were compared with those obtained for DDAC. For the three other surfactant systems, the solubilization capacity of water is strongly restricted, and measurements can be performed at one (or two) R value(s) only. These results are presented at the end of each of the following subsections. a. Nag. The variation of Nag of DMHAC with R is similar to that of DDAC in an apolar medium (Figure 3).
772 Langmuir, Vol. 5, No. 3, 1989
Verbeeck et al.
200 a
I
.
1
1
0.i
0.0
Iy
-6
Figure 12. Influence of the concentration of DDMAcrAC in cyclohexaneat R = 2 on the absorption (A) and the fluorescence decay (% 71.and 7L) of PSA.
I
-6
-5
-4
-3
'
-2
-i
0
Figure 11. Influence of the concentration of DMAAC in cyclohexane at R = 2 on the absorption (A) and the fluorescencedecay (% 7~ and 7 ~ of) PSA. Table VI. Average Aggregation Number for the Micellar Solution in Toluene R = l R = 2.5
surfactant DDAB DDAC DMHAC
N.89 7 8 17
surfactant DDAC DMHAC DDAcrAC
N.89 12 20 33
However, the value of NWfor the micellar system based on DMHAC is larger than the value of NWfor DDAC, at equivalent concentrations of water. In Table VI, the values of NBgsfor the other surfactant systems at R = 1 or R = 2.5 are given and compared to those of DDAC and DMHAC at corresponding R values. From these data, a correlation is observed between the aggregation number and vllq, (the ratio of the cross section of the apolar tails (vll) to the cross section of the polar head group (ao)): the smaller u/Zao, the larger NWg.
In Figure 14, the results of a temperature study of the value of NWgfor reverse micelles based on DMHAC are given. In the temperature range examined, NWremains constant for R < 4. These measurements suggest that the enthalpy of micelle formation (AH") for DMHAC is larger compared to that for DDAC. Probably, the stronger interaction possibility of the surfactant monomers of DMHAC with the water, solubilized in the micelle, is responsible for the enhanced stability. While the head group of DDAcrAC also possesses functions that strongly interact with water 'molecules, the aggregation number decreases with increasing temperature at R = 2.5 (Figure 15). These measurements prove not only the possibility of the monomer head groups interacting with water but alsb that the geometry of the surfactant monomer influences the enthalpy of micelle formation. Since v/Zao(DMHAC) > u/lao(DDAcrAC), the geometry of the surfactant DMHAC is better suited for reverse micelle formation than D D A C ~ A C . ~ ~ b. k,. In Figure 6, the value of k, for reverse micelles based on DMHAC at different concentrations of water is compared with that of DDAC aggregates. For the system DMHAC, the exchange rate constant slightly decreases as R increases beyond the maximal hydration of the surfactant monomers, while k, of DDAC aggregates remains constant in the same region. Above complete hydration, iz, strongly increases, in agreement with the DDAC data. In Figure 16, the activation enthalpy and entropy of dimerization of DMHAC are given. Compared to DDAC, both AH,*and AS,* are lower. The lower activation entropy could indicate that fewer surfactant monomers are released in the bulk solution by the dimerization of two
Langmuir, VoE. 5, No. 3, 1989 113
Characterization of Inverse Micelles 1
1
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!
I
40
t
0 , 300
, , , I , , , , [
310
320
350
340
330
Figure 15. Temperature dependenceof the average aggregation number of DDMAcrAC at R = 2.5 (v),of DMAAC at R = 1 (O), and of DDAB at R = 1 (0) in toluene.
)-\ 1
t Figure 13. Influence of the concentration of DDAB in cyclohexane at R = 2 on the absorption (A) and the fluorescence decay (% TL and TL) of PSA.
0 0
I
R
.
I
2
.
I
6
4
a b
-6
1
0 O
,
.
I
.
( ~ 1 0 ~ I )
I
.
_
Figure 14. Temperature dependenceof the average aggregation number of DMHAC in toluene for different concentrations of water: R = 1 (+); R = 1.75 (e);R = 2.5 (A);R = 3.25 (e);R = 4 (A);R = 6.6 (*).
micelles. The mutual interaction of the OH functions on the polar head groups of DMHAC monomers as well as the interaction with the water molecules is thought to be responsible for the stronger binding of a monomer to a micelle. Since AHe* is associated with the electric repulsion between the surfactant monomers when compressing them by dimerization, a small AHe* points to a smaller interaction of the charges of the ammonium groups for DMHAC than for DDAC. Probably the substitution of a methyl group by a larger hydroxyethyl group, which also interacts better with the water molecules, is responsible
.
R
2
l
.
I
6
4
Figure 16. Activation enthalpy (a) and entropy (b) for the intermicellar exchangeof the aggregates of DMHAC in toluene at increasing concentrations of water. Table VII. Values of k,,AHe*,and ASB*for the Detergent Systems Studied" surfactant R DMHAC 1 1.5 DMAAC 1 3.0 DDAcrAC 2.5 2.3 DDAC 1 1.2 DDAB 1 1.9 OIn toluene at R = 1 or R = 2.5.
m*,
kJ mol-' 17 18 19 33 27
-23 -11 -10 +30 +15
for this effect. Further evidence for this decreased repulsion due to the ethanol group, compared to the methyl group, is given later. Since the ethanol group already strongly shields the positive charges of the ammonium head group, the effect
774 Langmuir, V d . 5 , No. 3, 1989
Verbeeck et al. 70
60
0 0
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R
I
2
4
6
1 8
Figure 17. Frequency for quenching of PSA with I- on the micellar aggregates of DDAC (A)and DMHAC (0)in toluene, corrected for the change of the reaction layer at increasing concentrations of water. Table VIII. Activation Enthalpy of Quenching of PSA with I- in Reverse Micelles of DMHAC in Toluene for Increasing- Concentrations of Water R AHq', kJ mol-' R AHq*,kJ mol-' 1 1.75 2.5
16.9 16.6 16.4
3.25 4 5
16.1 17.0 17.8
of water on AHe* of the micellar system DMHAC is smaller than the effect of the micellar aggregates of DDAC (Figure 19). For the other surfactants examined, the values of k,, AHe*, and AS,* are presented in Table VII. Since k,, AHe*,and AS,* are about constant in the water concentration range R = 0-2.5, the values for the different surfactant systems can be compared. The results reported in Table VI1 further support the suggestion that the strength of the electric repulsion between the polar head groups can be associated with the value of AHe*. Because of the more efficient screening of a positive charge by Br- compared to C1-,% AH,' for DDAB is lower than AHe* for DDAC. Moreover, for both surfactants, the activation enthalpy is considerably higher than for the three others, where the larger substituents on the polar head group provide an even better screening of the electric charges. The values of AS,* obtained for DDAC and DDAB are positive, while ASe* for the surfactants possessing functional groups that interact more strongly with the water molecules solubilized in the core of the reverse micelle are negative. At a given concentration of water, the exchange rate constant for all surfactants examined is of the same order of magnitude. c. k,. In Figure 17, k,S (the intramicellar quenching rate constant corrected for the size of the aggregate) is plotted against the concentration of water. Compared to DDAC, kq&3 for DMHAC is larger at all water concentrations measured. The activation enthalpy of quenching, AH *,remains constant in the concentration range of water stuhed (Table VIII). For DMHAC, AHq* is larger than for DDAC.
Figure 18. Average aggregation number of DMHAC in toluene as a function of both the concentration of water ( 0 )and the concentration of glycerol (A). The fact that AHq*is a constant at all concentrations of water measured suggests that probe and quencher are localized in the micelle at the interphase, regardless of the total amount of water solubilized in the micelle. The difference of AHq* between DDAC and DMHAC reflects a difference of P and/or Q localization in the interphase of a DDAC micelle compared to a DMHAC micelle. Furthermore, k ,S at comparable values of R for the aggregates of DMdAC is larger than for micelles of DDAC. This can be explained by the decrease in repulsion due to the ethanol group. This allows a higher mobility of the quencher ions at the interphase. 111. Glycerol as Polar Phase. The effect of substituting another polar solvent for water in the micellar core was examined by using DMHAC as surfactant in toluene. Two important factors led to the choice of glycerol as polar phase. First, glycerol is insoluble in toluene, and all glycerol added is localized in the aggregates. Second, the strong H-bond-forming capacity and the higher viscosity of glycerol led to the expectation that this might increase the stability of the reverse micelles. 1. Solubilizationof Glycerol in DMHAC Micelles. The maximal concentration of glycerol that could be solubilized in 0.08 M of DMHAC in toluene was R' = 2.6 (R' = [glycerol]/[DMHAC]). The interphase of aggregates that stabilize drops of a polar organic solvent in an apolar bulk medium is very different from that of a w/o microemulsion. At the glycerol core, the polar head groups behave as ion pairs; hence no electric double layer surrounds the polar drops in solution, as is the case for w/o microemulsions. This totally different nature of the interphase does influence the characteristics of the aggregates formed. 2. Fluorescence Quenching. For the characterization of the reverse micellar system DMHAC/toluene/glycerol, the combination of PSA with I- was used as a probe/ quencher system. a. N, The average aggregation numbers obtained for the system DMHAC/toluene/glycerol are almost identical with those obtained for DMHAC/toluene/H20 for comparable R and R'values (Figure 18). For R' d 2.6, N is a constant between 20 and 60 OC, in analogy with %MHAC/toluene/H,O (Figure 19). Therefore, it can be concluded that glycerol does not destabilize the aggregates. Because of the limited solubility of glycerol in reverse micelles of DMHAC, no evaluation of AHo a t higher R' values could be obtained. Whether glycerol increased the stability of the reverse micelles,
Langmuir, Vol. 5, No. 3, 1989 775
Characterization of Inverse Micelles
I
A
A A
0
300
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310
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l / IT I X 1 I0 5 1
320
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340
.
350
Figure 19. For R'= 0.9 (A),R = 1.7 (A),and R = 2.6 (*),Nw
of the micellar system DMHAC/toluene/glycerol as a constant function of the temperature. Table IX. Activation Enthalpy (AHe*)and Entropy (ASe') for Intermicellar Exchange and Activation Enthalpy (AH,*) for Intramicellar Quenching of the Surfactant System DMHAC/Toluene/Glycerol R AH.*,kJ mol-' ASa*. J mol-' K-' A",', kJ mol-' 0.9 13 -28 22 1.7 13 -30 20 2.6 13 -23 15
compared to those with a water core, could not be evaluated. b. k,. Figure 20 shows that k, for DMHAC reverse micelles with a glycerol pool is larger than for analogous aggregates with a waterpool and increases steeply for increasing concentrations of glycerol, even a t low values of R'. A temperature study yielded values for AH,*and ASe' (Table IX). Both are constant for the concentration range of glycerol measured. While AS,' is comparable to that for micelles with a waterpool, AH,' is smaller and equal to AH,*for micelles with a waterpool at the maximal hydration of the polar head groups. The results obtained for the system DMHAC/ toluene/glycerol are a further affirmation of the process of intermicellar exchange proposed in Figure 5. The AH,' can then be associated with the Coulombic repulsion of the polar head groups, resulting from the compression of the monomers in a micelle with the dimerization of two aggregates. Since in glycerol the polar head groups are ion pairs, the repulsion is much smaller, and therefore AHe' is lower. This lower AH,' leads to a higher value for k, in these aggregates. The experiments also prove that the dimerization and not the diffusion of molecules in the aggregates is the rate-controlling step in the exchange of molecules between two micelles. Otherwise, k, for the system DMHAC/ toluene/glycerol had to be twice as low as its value in the system DMHAC/toluene/H20, since the viscosity of glycerol is twice that of water. In fact, the absence of an electric double layer at the interphase results in a higher value of k, for DMHAC/toluene/glycerol. For both systems, AS,' is almost the same, as expected since hydrogen bridge interactions of monomers with the polar phase are responsible for the retention of these monomers in an aggregate. Such interactions are possible both with water molecules and with glycerol molecules in the polar core.
f Figure 20. Rate constant for the intermicellar exchange of the aggregates of DMHAC in toluene with a waterpool (A)and a glycerol pool (e)as a function of, respectively, R and R'. c. k , . Because of the higher viscceity of glycerol, A",' for this system is markedly higher than for the aggregates of DMHAC in toluene with a waterpool (Table IX).
Conclusion Cationic reverse micellar aggregates of DDAC and its derivatives, with modified polar head groups, were examined. The aggregation process of these surfactant monomers in apolar solvents can be described by the model of Eicke. Aggregation starts at low surfactant concentration (lov5M). "Open" aggregates are formed that grow with increase of the concentration in a stepwise manner. At the "operational cmc" these open aggregates cyclize to form reverse micelles. The "operational cmc" is a characteristic for the surfactant and i s situated between lo4 and M monomer concentration. In reverse micelles, water can be solubilized. The solubility for water is a function of the bulk apolar solvent and the surfactant but in general is small for the systems studied. This low solubilization capacity for water is probably due to the low hydrophilicity of the quaternary ammonium groups compared to the head groups of anionic surfactants, whose aggregates usually can solubilize larger amounts of water. Because of its high sensitivity, fluorescence quenching is an excellent method for the characterization of small organized structures. This method allows one to obtain information on those factors that control the stability of reverse micelles and are related to the structure and characteristics of the micellar interior. It was concluded that two important factors control the stability of reverse cationic micellar aggregates. Through modification of the polar head group of the surfactant with functions that increase their water affinity, it appeared that the interaction of the monomer head groups mutual and/or with the polar phase favors the enthalpy of micelle formation. Since micelle formation in apolar medium is enthalpy controlled, the stability is thus increased. An improved interaction possibility of the surfactant head groups with the polar phase improves the retention of a monomer in a micelle and diminishes the possibility for escape. Differences observed for the surfactants studied when the value of R and the nature of the polar phase are varied revealed a second factor affecting the stability of the micellar aggregates. The presence of an electric double layer at the interphase creates a barrier for the dimerization of micelles. The less the electric charges are screened, the larger the Coulombic repulsion between the monomers
776
Langmuir 1989,5, 776-782
positive curvature
Figure 21. Local positive and negative curvatures of the interface of the transition state for the dimerization of two reverse micelles.Me
at the compression of the interphase by dimerization and the higher the activation enthalpy (AHe*). An aspect that was not studied in this contribution but was studied previously by the elegant work of Robinson et al. relates to the influence of the geometry of a surfactant on the kinetic stability of a reverse micelle. They kept
the volume of the polar head group of the surfactant constant but varied the volume of the apolar tails changing the bulk apolar solvent and thus changing the degree of penetration of the solvent between the apolar tails. The role of the geometry was associated with the ease of transition at the interphase between a negative curvature and a positive curvatute, as required in the transition state of the dimerization process (Figure 21). In view of the different factors that influence the properties of reverse micellar aggregates, further systematic study is necessary to fully understand the structureproperty relation of these detergents. Acknowledgment. Financial support to the laboratory by Agfa, the FKFO, the FGWO, and the Ministry of Scientific Programmation and ERO is gratefully acknowledged. Registry No. DDAC, 3401-74-9; PSA, 59323-54-5;DDAB, 3282-73-3; DMHAC, 25725-51-3;DMAAC, 119747-42-1;DDAmAC, 114199-09-6;[CoClJ", 14337-087;I-, 20461-54-5;toluene, 108-88-3; glycerol, 56-81-5;cyclohexane, 110-82-7.
Spatial Resolution of Redox Processes within Nickel Oxide Films on Gold Using Real-Time Surface-EnhancedRaman Spectroscopy David Gosztola and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received August 16, 1988. In Final Form: January 13, 1989 Surface-enhanced Raman s ectroscopy (SERS) has been utilized to explore the spatial compositional changes within thin (30-500 Ni(OH)2 films on gold in 1 M KOH during their electrooxidation and subsequent reduction. This involved monitoring the integrated intensity of the 480/560-cm-l doublet, Ism, arising from Ni-0 vibrations for the oxidized form as a function of the faradaic charge passed, qr. Real-time sequences of SER spectra were obtained under both anodic-cathodic galvanostatic and cyclic voltammetric conditions. The Is x profile obtained for a series of fully oxidized f i i of varying thickness, x , exhibits a sharp peak at x = 50%-(qf = 2 mC cm-2),with an extended tail toward larger 2. This observation is consistent with the occurrence of SERS predominantly for sites within ca. 20-30 A of the gold surface, the Raman scattering intensity diminishing progressively for thicker layers as a result of increasing absorption of the incident (and scattered) light. These results are consistent with the expectationsof electromagnetic enhancement models but inconsistent with "chemical" enhancement mechanisms requiring adsorption of the Raman scatterer on the metal surface. Since only the oxidized nickel oxide form absorbs light strongly under the conditions chosen (647.1-nm irradiation),the Ism-qf profiles obtained during oxidation of a given film are sensitive to the spatial reaction mechanism. Such ISER-qf curves exhibit a pronounced peak at an oxidation charge, qf 2 mC cm-2,similar to the Ism-qf profiie for fully oxidized films of varying thickness, although the Im values for the former curves are uniformly smaller than for the latter pmfiie and increasingly so as the film thickness is increased. Similar ISm-qf curves are also obtained during subsequent film reduction, although some hysteresis is observed. These results are compared with simulated ISm-qf curves obtained for idealized models involving oxidation proceeding either from the inner to the outer fiim edge or in a spatially homogeneous manner. While the observations are not entirely consistent with either model, they suggest that a hybrid mechanism is likely.
H)
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We have recently been utilizing surface-enhanced Raman spectroscopy (SERS) to probe the potential-dependent structure of thin nickel' and manganese oxide2 films electrodeposited onto gold, as well as related studies involving the oxidation of gold, platinum, and other transition-metal over layer^.^ These studies exploit the chemical inertness and stability of electrochemically roughened (i.e., SERS-active) gold substrates? enabling SER spectra to be obtained for the films them~elvesl-~ and (1) (a) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J.Phys. Chem. 1986, 90,6408. (b) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J. Electrochem. SOC.1988, 135, 885. (c) Desilvestro, J.; Weaver, M. J. J. Electroanal. Chem. 1987, 234, 237. (2) Gosztola, D.; Weaver, M. J. J. Electroanal. Chem., in press.
0743-7463/89/2405-0776$01.50/0
for adsorbates bound to the overlayer material,3a*b,6 provided that the species involved are located in sufficiently close proximity (ca. 10-30 A, vide infra) to the SERS-active substrate. One specific application involves the elucidation of structural changes incurred by the oxidation/reduction of (3) (a) Leung, L.-W. H.; Weaver, M. J. J . Am. Chem. SOC.1987,109, 5113. (b) Leung, L.-W. H.; Weaver, M. J. Langmuir, in press. (c) Desilvestro, J.; Weaver, M. J. J. Electroanol. Chem. 1986, 209, 377. (4) (a) Gao, P.; Patterson, M. L.; Tadayyoni, M. A,; Weaver, M. J. Langmuir 1985, 1, 173. (b) Gao, P.; Goaztola, D.; Leung, L.-W. H.; Weaver, M. J. J. Electroanal. Chem. 1987,233,211. (5) (a) Leung, L.-W. H.; Weaver, M. J. J. Electroanol. Chem. 1987, 217, 367. (b) Leung, L.-W. H.; Gosztola, D.; Weaver, M. J. Langmuir 1987, 3, 45.
0 1989 American Chemical Society
