8216
J. Phys. Chem. B 1999, 103, 8216-8220
ARTICLES Effect of Counterion Substitution on AOT-Based Micellar Systems: Dielectric Study of Cu(AOT)2 Reverse Micelles in CCl4 Daniele Fioretto,† Mariangela Freda,‡ Giuseppe Onori,*,† and Aldo Santucci† Istituto per la Fisica della Materia, Unita` di Perugia and Dipartimento di Fisica, UniVersita` di Perugia, Via Pascoli, I-06100 Perugia, Italy, and Dipartimento di Fisica and Istituto per la Fisica della Materia, Unita` di Tor Vergata, Via della Ricerca Scientifica, 1 I-00133 Roma, Italy ReceiVed: June 3, 1999; In Final Form: July 27, 1999
The effect of substituting Na+ counterion of Aerosol-OT [sodium bis(2 ethylhexyl) sulfosuccinate] with doubly charged counterion Cu2+ has been studied for the Cu(AOT)2/H2O/CCl4 system by infrared and dielectric spectroscopy as a function of surfactant concentration and water content. A relaxation phenomenon has been observed whose behavior depends strongly on the degree of surfactant hydration W (W ) water-to-surfactant molar ratio). The hydration of Cu(AOT)2 reverse micelles has also been investigated by means of IR spectroscopy. IR spectra of surfactant-entrapped water show a notable complexity, which could be ascribed to strong interaction between the Cu2+ counterion and water inside the micellar core. The results of IR and dielectric spectroscopy studies have been compared and discussed with those previously obtained for the corresponding NaAOT and Ca(AOT)2 reverse micelles.
1. Introduction Recently, several authors have studied the effect of changing the nature of AOT surfactant counterion on the properties and structure of AOT reverse micelles.1-4 These aggregates of nanometer size and being almost spherical can solubilize large amounts of water, forming a water pool whose properties have been determined with different techniques. The surfactant counterion is responsible for surface electric interactions at the micellar interface. These interactions can have a strong influence both on the equilibrium shape and size of the micellar aggregates and on their phase diagrams. The ability to change the nature of the counterion in an AOT reverse micelle is a useful and flexible tool for understanding the structural properties of these aggregates, i.e., shape and size. We expect that the variation of electrical properties of the micellar interface can also influence the hydration properties of the micellar system and, as previously found5-9 for NaAOT and Ca(AOT)2, their intrinsic dynamical properties. In fact, dielectric spectroscopy in the 0.02-3 GHz region on Ca(AOT)2 and Na(AOT) micelles revealed the presence of a relaxation phenomenon whose characteristics strongly depend on the hydration degree of the micelle and that it is connected, at the highest hydration values, to the motion of single AOT headgroups. A quantitative analysis of dielectric6,7,9 data has shown that for each value of W the fraction of AOT reorienting itself in the applied electric field superimposes on the fraction of AOT headgroups with at least three hydration water molecules (AOT totally hydrated), as determined from IR data.10 To complete * To whom correspondence should be addressed: E-mail: symbio@ pg.infn.it. † Universita ` di Perugia. ‡ Unita ` di Tor Vergata.
this analysis and verify further this model, we investigate here the effect of substituting the Na+ counterion with the divalent, transition metal cation Cu2+ on the hydration and dynamics of AOT reverse micelles. The choice of Cu2+ counterion is principally due to the fact that copper is a transition metal, so it is expected that its cation can complex water molecules,11,12 showing hydration properties very different from those of Ca2+, Na+, etc. In the present work, Cu(AOT)2 reverse micelles in CCl4 have been studied by means of IR and microwave dielectric spectroscopy in the range 0.02-3 GHz as a function of surfactant concentration and water content. A relaxation phenomenon has been observed in the microwave frequency region, similar to that previously found for both Ca(AOT)2 and NaAOT micelles, whose behavior strongly depends on the surfactant hydration. The hydration of reverse micelles has also been studied by means of IR spectroscopy. IR spectra of surfactant-entrapped water show a notable complexity, which could be ascribed to the strong interaction between the Cu2+ counterion and water inside the micellar core. All results obtained for Cu(AOT)2 reverse micelles are compared with analogous results previously found for NaAOT and Ca(AOT)2 reverse micelles systems. The results show significant differences in dynamical and hydration properties of these systems. 2. Experimental Setup The Cu(AOT)2 surfactant has been prepared as described in ref 1. NaAOT 99% (Aldrich product) purified by recrystallization from methanol and dried in a vacuum was stored in a vacuum over P2O5. Residual water [W0 ) 2.3 for Cu(AOT)2, 0.2 for NaAOT] was revealed by a Karl Fisher titrator and considered
10.1021/jp9918048 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/10/1999
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Figure 1. OH stretching absorption band for pure water (‚‚‚) and water in Cu(AOT)2/H2O/CCl4 system at selected values of W: (a) (-) W ) 2.3, (- - -) W ) 7.4; (b) (-) W ) 9.1, (- - -) W ) 17.1.
as a part of total water in the mixture. IR spectra were recorded by means of a Shimadzu model 470 IR spectrophotometer, equipped with a variable path length cell and CaF2 windows. A typical path length employed was 50 µm for AOT/H2O/CCl4 mixtures. Pure water spectra were taken with a shorter path length. The complex dielectric function of Cu(AOT)2 samples was measured in the frequency domain by using an open coaxial cell consisting of a section of a transmission line with its center abruptly terminated. A detailed description of the experimental procedure is reported in ref 13. Measurements were performed in the 0.02-3 GHz frequency range at 20 °C. The mixtures under study were prepared by weighing and keeping the molar ratio W in the range 0.2-10 and 2.3-17.1 for NaAOT and Cu(AOT)2 samples, respectively. 3. Results and Discussions 3.1. IR measurements. Parts a and b of Figure 1 show the molar extinction coefficient of the OH stretching absorption band for pure water and water in Cu(AOT)2/H2O/CCl4 system at selected values of W. It is evident from Figure 1 that the molar absorption coefficient varies with W; however, the total peak area of the OH stretching band increases linearly with the water content according to the Beer’s law
int )
A ) [H2O]d
∫ (νj) dνj ) 37 × 103 L mol-1 cm-1
(1)
The IR spectrum of surfactant-entrapped water is significantly different from that of bulk water, indicating that the water solubilized in the reverse micelle lacks the normal hydrogenbonded structure of bulk water. The most significant difference in the spectrum of water confined in the Cu(AOT)2 reverse micelle with respect to that of bulk water consists of higher
values of the molar extinction coefficient (νj) at lower frequencies, which is particularly evident at the lowest W (see Figure 1a). There was an increase in the intensity of the low-frequency part of the O-H absorption band as the water content was reduced. Since the stretching frequency reflected the strength in the interaction between neighboring molecules, the observed behavior could be ascribed to the strong interactions of water molecules with the Cu2+ counterions. This behavior is qualitatively different from those found from IR spectra of water confined in NaAOT10 and Ca(AOT)29 reverse micelles, for which a progressive decrease in intensity of the low-frequency part of the OH absorption band has been observed as the water content is reduced. Furthermore, the spectra of water in Cu(AOT)2 reverse micelles exhibit an isosbestic point for W > 9 (see Figure 1b), while for NaAOT and Ca(AOT)2 micellar systems, it is found for W g 2;9,10 a progressive departure of the spectra from the isosbestic point is observed with decreasing W. The presence of an isosbestic point is a strong argument in favor of the existence of an equilibrium between two optically absorbing species having concentration independent of band shapes and extinction coefficients. It was previously shown that the IR spectra for W > 2 in NaAOT and Ca(AOT)2 can be expressed as the sum of two distinct contributions associated with “bound” and “bulk” water, respectively. With regard to Cu(AOT)2, a description of O-H bands in terms of two contributions appears possible for W > 9. At lower values of W, (W < 9) a “bound” region of water appears to form, with spectroscopic properties depending on W. This bound region consists of both the “interfacial water” that hydrates the polar heads of the surfactant and the Cu2+ hydration water.12,14 In contrast, for NaAOT and Ca(AOT)2 samples the Na+ and Ca2+ counterion hydration water is spectroscopically indistinguishable from the bulklike water. In this case, the “bound” water revealed by IR spectroscopy is only water hydrating the anionic group of AOT. The notable complexity of OH stretching band shapes for water in Cu(AOT)2 reverse micelles accounts for the difficulty in calculating from these spectra the fractions of “bound” and “bulk” water as a function of W, as previously seen for NaAOT and Ca(AOT)2 reverse micelles. 3.2. Dielectric Measurements. Information on the change of structure and dynamics of both the AOT shell and water confined in the reverse micelles can be obtained from dielectric measurements. A relaxation phenomenon in the microwave region has been observed in Cu(AOT)2/H2O/CCl4 diluted systems as shown in Figure 2, for the dielectric spectrum of a Cu(AOT)2/H2O/CCl4 sample at W ) 9 and at a value φ ) 0.1 of the volume fraction of dispersed matter (water + AOT). A very similar behavior is found both for Na(AOT)/H2O/CCl4 and for Ca(AOT)2/H2O/CCl4 micellar systems.5-9 Two distinct relaxation phenomena are present in all dielectric spectra for these samples, and the frequency dependence of dielectric function *(ω) ) ′(ω) - i′′(ω) can be well described in terms of a sum of a Cole-Cole and a Debye-type relaxation function, according to the equation
*(ω) ) ∞ +
∆1
∆2 + 1 + (iωτ1)1-R 1 + (iωτ2)
(2)
where ∞ is the high-frequency (unrelaxed) dielectric constant, ω the angular frequency of the applied field, ∆1 and ∆2 the low- and high-frequency dielectric increments, respectively, τ1
8218 J. Phys. Chem. B, Vol. 103, No. 39, 1999
Figure 2. Real (′) and imaginary (′′) parts of the dielectric function of Cu(AOT)2/H2O/CCl4 system vs frequency for W ) 9, φ ) 0.1: (O) experimental points; (-) best-fit curve according to eq 2. Cole-Cole and Debye-type contributions are also shown (- - -).
Fioretto et al. Cu(AOT)2 and Ca(AOT)2 are systematically higher than the corresponding ones for NaAOT and practically coincident except for the fact that the solubility limit for water in Cu(AOT)2/ CCl4 micelles is higher than that of Ca(AOT)2. However, both NaAOT and Cu(AOT)2 systems show the same asymptotic behavior for τ1 vs W; i.e., the experimental values of τ1 tend to a value of ca. 0.2 ns at the solubility limit of water (larger W). Furthermore, the experimental values of τ1 do not depend on the dilution of the samples (not shown here). The microscopic origin of the above-described relaxation process has been extensively discussed at the highest values of W in ref 5, where dielectric data have been analyzed in terms of different models, such as the Maxwell-Wagner interfacial polarization, the counterion diffusion polarization, and the diffusion of dipolar headgroups. Only this last mechanism has been found to adequately represent both relaxation time τ1 and strenght ∆1 of the relaxation experimentally observed. In the whole range of W, the dielectric data for Cu(AOT)2 system have been analyzed with the model previously reported5-9 for Ca(AOT)2 and NaAOT reverse micelles. In this model, one supposes that at the lowest water content the almost dehydrated reverse micelles exhibit a nearly rigid structure, and the experimental relaxation time can be described according to the reorientation of the whole micellar aggregate in the applied electric field. At the intermediate values of W, the observed relaxation process was interpreted in terms of two coexisting mechanisms whose relative weights depend on the hydration degree of reverse micelles. The first mechanism is the reorientation of the whole micelle, while the second one is the “free” diffusion of completely hydrated AOT headgroups. Concerning the lowest W region, if an approximately spherical shape is assumed for the micellar aggregates, the rotational diffusion time can be calculated according to the Debye-Stokes formula:
τ) Figure 3. Relaxation time (τ1) vs W for Surf/H2O/CCl4 systems (φ ) 0.1): (O) Surf ) NaAOT; (9) Surf ) Ca(AOT)2; (4) Surf ) Cu(AOT)2.
and τ2 the relaxation times of the two processes, and R a parameter characterizing the width of the relaxation time distribution around τ1. The best-fit curves of the experimental spectra are reported in Figure 2 (solid lines) together with the Cole-Cole and the Debye type contributions to the best fit of ′′(ω) (dashed lines). The Debye dispersion is located at higher frequencies in the region of relaxation of bulk water; in our frequency range it makes only a small contribution that increases with water content in the sample, as previously found for NaAOT and Ca(AOT)2.9 This result confirms the hypothesis for which the high-frequency relaxation process can be attributed to the reorientation of water inside the micellar core. A deeper discussion of this process is beyond the scope of this work, which instead refers to the observed low-frequency relaxation process characterized by the dielectric parameters τ1, ∆1, and R. Typical values of R between 0.2 and 0.4 have been obtained for Cu(AOT)2 samples. Figure 3 shows the trend of τ1 as a function of W for Cu(AOT)2 samples at φ ) 0.1. For the sake of comparison, in the same figure the trends of τ1 vs W are reported for Ca(AOT)2 and NaAOT micellar systems. For each of the above series reported, the experimental values of τ1 exhibit similar trends as a function of W, i.e., τ1 shows an initial decrease followed by a region in which it is nearly constant. The values of τ1 for
4πηR3 KBT
(3)
where η is the viscosity of solution, R the micellar radius, and KBT the thermal energy. With this simple model, a value of R ) (11.7 ( 0.7) Å has been obtained for the NaAOT6,7 reverse micelle at W0 ) 0.2 (the residual water content in the NaAOT surfactant), while for Ca(AOT)2 a value of R ) (15.3 ( 0.7) Å has been found9 for W ) 1 (residual water in Ca(AOT)2). Using eq 3 for the Cu(AOT)2 sample at W ) 2.3 [the residual water content in Cu(AOT)2], we found a value of R ) (13.9 ( 0.4) Å. These three values for R are consistent with each other and agree with the data reported in the literature for AOT-based micelles.15 Furthermore, the Debye extension of the Clausius-Mossotti equation6,8 has been used to estimate the apparent dipole moment µapp of a single AOT group inside a reverse micelle. We obtained for µapp a value of approximately 0.7 D for an AOT group embedded in a Cu(AOT)2 reverse micelle; the same value has been obtained for AOT in a Ca(AOT)2 aggregate.9 This value is found to be consistent with the corresponding one for the AOT group8,16 in a NaAOT micellar aggregate, and it is lower than that of the same isolated group by at least a factor of 10. These results are consistent with the model of almost spherical micelles of NaAOT, Cu(AOT)2, and Ca(AOT)2 in CCl4 at the lowest values of W, corresponding to the residual water of each surfactant. In fact, an almost spherical symmetry in the distribution of dipoles causes a compensation effect that reduces the apparent dipole moment of the single AOT group with respect to that of the same isolated group.
Cu(AOT)2 Reverse Micelles
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Figure 4. Plot of the free AOT headgroups fraction X(W) calculated according to eq 4: (O) NaAOT/H2O/CCl4; (3) NaAOT/H2O/n-heptane; (9): Ca(AOT)2/H2O/CCl4; (4) Cu(AOT)2/H2O/CCl4.
With increasing water content inside the micelle, the micellar radius R increases almost linearly with W so that τ1 is expected to increase approximately with W3, according to the DebyeStokes law [see eq 3]. A different behavior is found for all micellar systems: τ1 initially decreases, and then it is nearly constant as a function of W (see Figure 3). The model previously reported for Ca(AOT)2 and NaAOT reverse micelles5-9 states that for each value of W there exists a fraction [1 - X(W)] of AOT headgroups, reorienting rigidly with the whole micelle with the Debye diffusion time τD(W) calculated from eq 3, and a fraction [X(W)] of “free” AOT headgroups, reorienting in the applied electric field independently with respect to the micellar aggregate. This reorientation is characterized by a relaxation time τ0 , τD(W). The configuration depicted above has an intrinsic dynamical nature because each AOT group experiences fluctuations in its local aqueous environment whose characteristic time is comparable to the mean residence time of a water molecule in the hydration shell of each ion.17 Since it is found experimentally that this fluctuation rate is higher with respect to the intrinsic rates of the observed process17 (τD and τ0) we applied the socalled fast-exchange condition18 to express the experimental relaxation time τ1 in terms of the τ0 and τD:
1 X(W) 1 - X(W) ) + τ1 τ0 τD
(4)
Starting from eq 4 the fraction X(W) of the free AOT headgroups for NaAOT, Ca(AOT)2, and Cu(AOT)2 reverse micelles has been calculated by inserting into eq 4 the experimental values of τ1, the values of τD calculated according to the Debye formula [eq 3], and finally, a value of τ0 of 0.19 ns (the same for the three systems). The values of X(W) obtained from eq 4 are reported in Figure 4 for NaAOT, Ca(AOT)2, and Cu(AOT)2 (the first in CCl4 and n-heptane). In all samples examined, X(W) ) 0 at the lowest values of W and it tends to increase above a threshold value of W, which depends on the nature of counterion (W ) 2 for NaAOT, W ) 3 for Ca(AOT)2 and Cu(AOT)2). As the water content in the micellar core grows, an increasing fraction of AOT headgroups achieves enough mobility to give separate contribution to the observed relaxation process. It is evident (Figure 4) that at corresponding values of W, the fractions X(W) of AOT headgroups reorienting independently in the applied electric field are lower for Ca(AOT)2 and Cu(AOT)2 than that calculated for NaAOT, and the values of X(W)
for Ca(AOT)2 and Cu(AOT)2 are practically coincident for W < 9.0. Furthermore, for W > 9, the X(W) values calculated for Cu(AOT)2 tend asymptotically to the limiting value of X(W) for NaAOT in n-heptane. These results indicate that the process through which a growing number of AOT headgroups achieves sufficient mobility to reorient itself in the applied electric field is slower in Ca(AOT)2 and Cu(AOT)2 with respect to the NaAOT case. However, in both the Cu(AOT)2 and NaAOT systems, the limiting values of X(W) are the same (ca. 90% of the headgroups achieve independent mobility). This suggests5 that the faster dynamics predominating at the highest W is a reorientation of headgroups that is not affected by the nature and charge of the counterion, but it is probably a property of the anionic part of the surfactant. In previous studies,6,7,9 a comparison of IR and dielectric data has been made for NaAOT and Ca(AOT)2 reverse micelles. From IR data, the fraction of AOT headgroups hydrated with at least three water molecules (full hydration) has been calculated as a function of W. This fraction has been found to superimpose on the fraction X(W) of AOT calculated from dielectric spectroscopy. This coincidence indicates that AOT headgroups with sufficient mobility to contribute separately to the observed relaxation process can be identified with the totally hydrated headgroups. From dielectric data we see that the dynamical behavior shown by AOT headgroups in Cu(AOT)2 reverse micelles is similar to that found for Ca(AOT)2 and NaAOT aggregates; however, owing to the complexity of IR spectra, we are not able to calculate in this case the fraction of AOT headgroups with a defined number of water molecules. In summary, the results presented above show that hydration can dilute the interactions between charged groups enhancing their individual mobility. This might have a relevance that goes beyond the particular case of dynamics-hydration connections in reverse micelles. In fact, it is well-known that water plays an essential role in the starting and maintainence of protein functionality, but this occurrence is not at present completely explained, especially at the molecular level. Experimental evidence that water affects protein functionality arises, for example, from measurements of enzyme activity of the hydrated lysozyme powder.19 In this is shown that the enzyme activity is negligible at the lowest hydration degree, (0 < h < 0.2, where h ) grams of water/grams of lysozyme); the activity increases by increasing the water content (0.2 < h < 0.4-0.5), and for h > 0.5 it increases again, approaching the value corresponding to dilute lysozyme-water solution. It is noteworthy that this behavior of lysozyme activity vs h closely resembles that observed for the fraction X(W) of AOT headgroups reorienting independently in an external electric field (see Figure 4). It should be noted that a value of h ≈ 0.4 corresponds to the formation of a complete hydration monolayer around the protein surface. These analogies suggest that the presence of water in protein systems may have a major effect on the protein dynamical flexibility, which is in turn essential for their functionality. This intrinsic dynamics activates only when the hydration process is completed, and it can explain the starting of lysozyme activity with a threshold hydration degree. 4. Conclusions From the results reported above on the Cu(AOT)2 reverse micelles, a close connection appears, as in the case of NaAOT and Ca(AOT)2 systems, between the behavior of the relaxation time τ1 vs W and the progressive hydration of AOT headgroups.
8220 J. Phys. Chem. B, Vol. 103, No. 39, 1999 This confirms the hydration model for which only totally hydrated AOT headgroups can reorient itself in the applied electric field, independently with respect to the micellar aggregate. This model accounts for the faster relaxation process in the interior of the aggregate starting above a threshold in the water content. This could be an important result because on large scale a growth in the mobility of single groups with increasing hydration could explain, for example, the start of protein functionality with a defined degree of hydration of the same macromolecule.18 Acknowledgment. This work was supported in part by a contribution from the Consiglio Nazionale delle Ricerche (Rome). References and Notes (1) Eastoe, J.; Fragneto, G.; Robinson, B. H.; Towey, T. F.; Heenan, R. K.; Lang, F. J. J. Chem. Soc., Faraday Trans. 1 1992, 88, 461. Giordano, R.; Migliardo, P.; Wanderling, U.; Bardez, E. J. Mol. Struct. 1993, 296, 265. (2) Giordano, R.; Migliardo, P.; Wanderling, U.; Bardez, E. J. Mol. Struct. 1993, 296, 265. (3) Eastoe, J.; Towey, T. F.; Fragneto, G.; Robinson, B. H.; Williams, J.; Heenan, R. K. J. Phys. Chem. 1993, 97, 1459.
Fioretto et al. (4) Dunn, C. M.; Robinson, B. H.; Lang, F. J. Spectrochim. Acta 1990, 46A, 1017. (5) D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A. Colloid Polym. Sci. 1995, 273, 899. (6) D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A. Phys. ReV. E 1995, 52, R4620. (7) D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A. Phys. ReV. E 1996, 54 (1), 993. (8) Camardo, M.; D’Angelo, M.; Fioretto, D.; Onori, G.; Palmieri, L.; Santucci, A.; Prog. Colloid Polym. Sci. 1996, 100, 177. (9) Fioretto, D.; Freda, M.; Onori, G.; Santucci, A. J. Phys. Chem. B 1999, 103 (14), 2631-2635. (10) Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430. (11) Towey, T. F.; Baglioni, P.; Martini, G.; Ristori, S. J. Phys. Chem. 1995, 99, 3939. (12) Camardo, M.; D’Angelo, M.; Mannaioli, S.; Onori, G.; Santucci, A. Colloid Surf. A 1996, 119, 183. (13) Fioretto, D.; Marini, A.; Massarotti, M.; Onori, G.; Palmieri, L.; Santucci, A.; Socino, G. J. Chem. Phys. 1993, 99, 8115. (14) Aliotta, F.; Migliardo, P.; Donato, D. I.; Turco-Liveri, V.; Bardez, E.; Larrey, B. Prog. Colloid. Polym. Sci. 1992, 89, 258. (15) Maitra, A. J. Phys. Chem. 1984, 88, 5122. (16) Eicke, H. F.; Christen, H. J. Colloid Interface Sci. 1974, 48, 281. (17) Giese, K. Ber. Bunsen-Ges. Phys. Chem. 1972, 76, 495. (18) Anderson, J. E. J. Chem. Phys. 1967, 47, 4879. (19) Rupley, J. A.; Yang, P. H.; Tollin, G. In Water in Polymers: Thermodynamics and Related Studies of Water Interactions with Proteins, Rowland, S. P., Ed.; ACS Symposium Series 127; American Chemical Society: Washington, DC, 1980; pp 111-132.
