J. Phys. Chem. B 1998, 102, 3335-3340
3335
ARTICLES Fourier Transform Infrared Investigation of Water States in Aerosol-OT Reverse Micelles as a Function of Counterionic Nature M. Bey Temsamani,* M. Maeck, I. El Hassani, and H. D. Hurwitz Laboratoire de Thermodynamique Electrochimique, UniVersite´ Libre de Bruxelles, Campus Plaine, CP 256, BouleVard du Triomphe, 1050 Brussels, Belgium ReceiVed: June 5, 1997; In Final Form: December 12, 1997
A Fourier transform infrared study of the states of water included in reverse micelles made from Aerosol-OT (AOT) in isooctane is carried out for the whole sequence of alkali metal counterions as a function of the water content. The decomposition of the aqueous pool of these micelles into “bound” and “free” water is assumed. Two independent methods are applied for the purpose of interpreting the shape and the intensity of the water OH stretching vibration band. The modelization of the OH stretching band by three Gaussian components is retained as the most efficient decomposition and as an adequate smoothing method. To determine the number of water molecules bound per AOT polar head, three water adsorption models are tested for their ability to fit of the experimental data, to reproduce the effect of the AOT concentration, and to indicate the influence the counterion forming the ion pair with AOT. A Langmuir adsorption isotherm model satisfies best these criteria. The results suggest that the hydration process concerns exclusively the AOT molecule without any marked interference depending on the counterion. The maximum hydration number of AOT determined by this treatment is between 2 and 3. The estimated standard free energy of the AOT polar head hydration is about 2.5 kJ/mol.
I. Introduction The progressive hydration of a solution of amphiphilic molecules in an apolar solvent can lead to the formation of threedimensional structures known as reverse micelles.1-3 The extensive use of these micelles as microreactors for aqueous phase reactions3-5 requires precise knowledge of the states of the enclosed water. According to accepted views,6-9 the water pool can be divided into at least two populations: polar head hydration water (“bound” water) and, near the center, bulklike water (“free” water). For bound water, unusual properties are expected (high microviscosity, no freezing point, etc.), whereas free water should exhibit normal bulk water characteristics. The main goal of most studies in this field is to obtain the amount of bound water as a function of the total water content. The experimental parameter is
W ) [H2O]/[S] ) Wb + Wf
(1)
where [H2O] and [S] are respectively the concentration of water and surfactant molecules in the solution (either a micellar solution or a microemulsion); the subscripts b and f stand for bound and free water. The experimental plot Wb ) f(W) generally increases monotonically to a plateau with a slope almost equal to 1 at low W values.10 The maximum value Wmax so defined is related to b the number of hydration sites associated with one polar head. This parameter is of importance in order to speculate over the structure of the polar heads hydration shell. A literature review suggests values between 3 and more than 10 hydration sites per polar head in the case of Aerosol-OT
(AOT, (sodium bis(2-ethylhexyl) sulfosuccinate).7,10-16 Most studies use a mixture of AOT stereoisomers. In fact the value of Wmax deduced from the evaluation of a specific property of b water as a function of W depends largely on the selected investigation method (NMR spectroscopy, Fourier transform infrared (FTIR) spectroscopy, calorimetry, density, or microviscosity measurements, etc.). With respect to FTIR spectroscopy, many detailed studies of the water OH stretching band in reverse micelles can be found in the literature10,15-20 which rely on different models for the OH stretching band that are used to fit the Wb ) f(W) plot. They also lead to significantly different conclusions on the value of Wmax and the intensity of the solvation interactions. Furb thermore these conclusions refer only to the sodium AOT system and therefore lack some of the crucial proof with regard to the contribution of the counterion hydration. To discriminate between the different possible models resulting from the FTIR water vibration band decomposition, we present in this work some studies with systems of surfactants derived from AOT (the sodium salt of the diester) by varying the counterion. Thus, the entire sequence of alkali metal ions, Li+, Na+, K+, Rb+, and Cs+, will be investigated. Both analytical procedures used by Giammona et al.15 and Onori and Santucci10 will be applied to decompose the FTIR water OH vibration band measured with these salts in isooctane used as the solvent of the inverted micelles. For the interpretation of these spectra we will consider several models of the equilibrium distribution of bound and free water and provide arguments for selecting from among these models the most satisfactory one.
S1089-5647(97)01844-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/14/1998
3336 J. Phys. Chem. B, Vol. 102, No. 18, 1998
Figure 1. Infrared spectrum of a hydrated micellar solution in isooctane ([(AOT)Na] ) 0.1 M; W ) 6).
II. Experimental Section 2.1. Materials. The procedure used to exchange the counterion is that of Eastoe et al.,21 extended in order to obtain anhydrous surfactant solutions. A 10 g sample of high-purity grade AOT (>99% from Fluka) was dissolved in 20 mL of a 1:1 (v/v) mixture of water and ethanol. The solution was passed (2 drops/s) through a column (40 cm × 2 cm) of a strong ion exchanger in the H+ form (Amberlite IR-120, 16-45 mesh, from Fluka). The free sulfonic acid was immediately neutralized with an aqueous solution of the hydroxide of the desired counterion (Li+, K+, Rb+, or Cs+). The solvent was removed under vacuum (bath temperature, 40 °C), and the waxy solid was dried in a vacuum over P2O5. This material contained residual water which was removed by the action of phosphoric oxide (p.a. from Merck) on a solution of the surfactant in isooctane (p.a. from Fluka). An essentially complete drying was obtained by magnetic stirring in less than 1 h, and the anhydrous solution was either used in the FTIR studies or evaporated when dry AOT was desired. No modification of the surfactant (ester group splitting or ion exchange) could be observed (FTIR spectroscopy), even after the prolonged (24 h) contact of P2O5 with the solution. The Na+/H+ ion exchange step was optimized by a check of the H+ content of the AOT-H+ solution by titration with NaOH, and the overall yield of cation exchange (Li+, K+, Rb+, and Cs+ case) was found in the 99-100% range by atomic absorption spectroscopy. 2.2. Methods. The IR spectra were obtained from a 20DXB NICOLET FTIR spectrophotometer (100 scans, resolution set to 1 cm-1, ZnS windows, with an optical path adjusted in order to ensure the best signal-to-noise ratio). When the absorption of the solvent is subtracted, the residual spectrum of dissolved hydrated AOT (Figure 1) shows two sulfonyl stretching vibration bands (1045 and ∼1220 cm-1), a carbonyl band (1710-1750 cm-1), a O-H stretching band (3100-3800 cm-1), and a H-O-H bending (∼1640 cm-1) vibration band appearing as a shoulder of the carbonyl band. No perturbative effects on the peak shapes of OH stertching modes due to the solvent absorption subtraction were observed. However, for high water contents, and also almost anhydrous material, even examined with a long optical path, the residual noise at the 2800-3000 cm-1 region resulting from this subtraction overlapped the OH stretching band; this led to some scattering of the experimental points, and it was then decided to fit a three Gaussian profile to the OH band as a smoothing procedure.
Bey Temsamani et al.
Figure 2. Integrated intensity of the OH stretching band as a function of the total water content for the entire series of counterions. Except for Li+, an offset has been applied in order to ensure clarity.
The integrated intensity of the OH stretching band was found to obey the same linear function of W for the entire series of counterions (Figure 2). Since the main state of water is expected to shift from an almost “bound” to an almost “free” population, it is inferred from Figure 2 that the specific absorbance of both populations must be the same. The fittings was performed with the application of the Marquardt algorithm using the usual, nonweighted, least-squares objective function. III. Results 3.1. Analyses of the OH Vibration Band. 3.1.a. Linear Combination of Bound and Free Water Absorbance. With this method,15 the water spectrum in an almost anhydrous reverse micelle is taken as a good approximation of that of bound water and the pure water spectrum is taken as a good approximation of that of free water. The analysis of a given spectrum is classical: after numerical normalization, i.e., reduction to a common arbitrary surface in order to compensate for different total amounts of water in the micelles, each spectrum is obtained as a linear combination (weighted mean) of the normalized spectra of bound and free water
A(calculated) ) RA(bound) + (1 - R)A(free)
(2)
where A(bound) and A(free) are the absorbance values as a function of wavenumber of bound and free water, respectively, and R represents the fraction of bound water (0 e R e 1). The fitting procedure is carried out by minimizing the function
∑[A(smoothed) - A(calculated)]2 3.1.b. Water Absorption Band DeconVolution in Gaussian Profiles. In this method,10 each of the three Gaussian OH stretching band fits will be given separate consideration (Figure 3). The validity of this fitting has first to be proved for pure water. Figure 4 shows that our recorded FTIR spectrum of pure water measured with a very short optical path, obtained with one water drop clamped between two ZnS windows, is nicely well-described by three Gaussians. The contributions to the total integrated absorbance are respectively 62, 28, and 10% following increasing energy. As shown by comparison of Figures 3 and 4, the largest contribution in pure water becomes the smallest in the case of water included in almost anhydrous reverse micelles. The decreasing sequence from low to high
FTIR Investigation of Water States in AOT Micelles
J. Phys. Chem. B, Vol. 102, No. 18, 1998 3337 To calculate Wb as a function of W, we chose in this work the following procedure. We assume that S ) S1 + S2 + S3, where S1, S2, and S3 refer respectively to the integrated absorbance of the three Gaussian bands in increasing energy. In the case of the OH spectrum of micelles S1 ) Sf1, S2 ) Sf2 + Sb2, and S3 ) Sf3 + Sb3, where Sf and Sb refer respectively to free and bound water. It is further supposed that the ratios β21 ) Sf2/Sf1 and β31 ) Sf3/Sf1 are the same in water hydrating the micelles and in pure water. Thus, the determination of β21 and β31 is obtained from the spectrum of pure water (Figure 4), and the ratio Wb/W becomes
S1 Wb (S2 - Sf2) + (S3 - Sf3) ) ) 1 - (β21 + β31 + 1) (3) W S1 + S2 + S3 S The solution of eq 3 yields the experimental values of Wb. 3.2. Models for Bound and Free Water Equilibrium Distribution. 3.2.a. Binding Coefficient Model. In a model suggested by Giammona et al.,15 the fraction of the total watermicelle interface area occupied by bound water molecules (Wb/ Wmax b ) is set equal to the volume fraction occupied by free water in the micelle (Wf /W). It follows that
Wmax b W+
Figure 3. O-H stretching band as fitted with a set of three Gaussians for two values of W. Contributions in percent of each Gaussian to the total integrated absorbance are recorded in the figures.
Wmax b
)
Wb )R W
(4)
Note that an initial slope of 1 for Wb ) f(W) is implied by the function. This is consistent with the assumption made in the linear combination method expressed by eq 2 but not with the deconvolution method since S1 remains finite in eq 3 even in almost anhydrous micelles, as shown in Figure 3a (top). According to eq 4, a linear relationship was found between which we obtain from the slope 1/R and W.15 Values of Wmax b of such lines are reported in Table 1 (system A). 3.2.b. Chemical Equilibrium Model. The experimental function Wb ) f(W) was interpreted by Onori and Santucci10 according to a model based on the assumption of the existence of a chemical equilibrium constant involving the free water concentration [H2O], bound water (N‚H2O), and free sites on polar heads (N). Hence, they wrote the following relations:
N + H2O a N‚H2O K)
Figure 4. Pure water spectrum as fitted by a set of three Gaussians.
energy is progressively restored as hydration increases, but the pattern observed for bulk water in hydrated micelles remains still altered, even for high water contents (W > 15). Each Gaussian band is believed to be associated with a particular type of hydrogen bond.10,16 The low-energy band (3314 cm-1) is assumed to originate from linear, fully developed-hydrogen bonds, whereas the medium component (3463 cm-1) reflects distorted structures. Free O-H groups contribute to the highenergy Gaussian (3595 cm-1). The critical hypothesis in the method of Onori and Santucci10 consists of a guess about the spectrum of bound water. This is based on the assumption that the low-energy component, thus the contribution of linear H bonds, is absent in bound water, because no chainlike polymeric conformation of water molecules linked with hydrogen bonds is formed.
(5a)
[N‚H2O]
(5b)
[N][H2O]
leading to
Wb )
[(
)
1 1 W + Wmax + b 2 K[AOT] 2 1 W + Wmax + - 4WWmax b b K[AOT]
[(
)
]] 1/2
(6)
As shown in Figure 5a, our experimental points for (AOT)Na can be quite satisfactorily fitted with eq 6 (r2 ) 0.988), and the calculated values of K ) 2.53 ( 0.26 M-1 and Wmax ) 2.77 ( b 0.11 (isooctane, 25 ( 1 °C) are in reasonable agreement with values found in the literature (K ) 6.4 ( 0.7 M-1; Wmax ) 3.5 b ( 0.1; AOT in carbon tetrachloride) (Table 1, system B). 3.2.c. Langmuir Adsorption Model. An additional criterion of the validity of the Onori and Santucci model has to be found in the predicted dependence of Wb on the total surfactant concentration [AOT] as expressed in eq 6. Because this
3338 J. Phys. Chem. B, Vol. 102, No. 18, 1998
Bey Temsamani et al.
TABLE 1: Experimental Results Corresponding to the Methods and Models Described in the Texta value per given counterion system A B C D a
Li+
Na+
K+
Rb+
Cs+
11.69 (0.12)
5.70 (0.05)
5.40 (0.08)
4.21 (0.05)
3.50 (0.04)
2.57 (0.08) 2.56 (0.16) 5.28 (0.18) 1.42 (0.14) 2.77 (0.11) 0.39 (0.04)
2.77 (0.11) 2.53 (0.26) 4.57 (0.18) 0.61 (0.08) 2.57 (0.08) 0.39 (0.02)
2.31 (0.07) 2.73 (0.24) 3.78 (0.06) 1.16 (0.10) 2.29 (0.06) 0.42 (0.03)
2.94 (0.24) 2.50 (0.51) 3.20 (0.17) 1.03 (0.24) 2.94 (0.23) 0.38 (0.07)
2.48 (0.12) 3.17 (0.53) 2.81 (0.06) 1.26 (0.16) 2.48 (0.12) 0.49 (0.08)
property method 3.1.a model 3.2.a method 3.1.b model 3.2.b method 3.1.a model 3.2.c method 3.1.b model 3.2.c
Wmax b Wmax b K/M-1 Wmax b K Wmax b K
The values in parentheses are the standard deviations of the mean.
Wb ) 2 max 2 max 1/2 1 + K(W + Wmax b ) - [[1 + K(W + Wb )] - 4K WWb ] 2K (8)
Figure 5. Number of bound water molecules as a function of (AOT)Na micelle hydration. (a) Application of eq 6 to the treatment of experimental data resulting from the progressive hydration of a 0.1 M AOT solution in isooctane: (b) experimental points; (s) fitting according to eq 6. (b) The predicted lines for solutions of lower surfactant concentration (- -, 0.05 M and ‚‚‚ 0.01 M). (b), (O) are respectively the experimental points for [AOT] ) 0.05 M and [AOT] ) 0.01 M.
dependence was not checked before in ref 10, we decided to apply eq 6 to the data resulting from our measurements on two more systems of lower surfactant concentration. As shown in Figure 5b, the model as proposed by Onori and Santucci10 predicts an incorrect dependence of Wb on the total concentration of surfactant. It is reasonable to admit that this fact is merely the consequence of the model (eq 5b) and that the method used to evaluate Wb remains entirely valid. Therefore we associated each of these analytic methods represented respectively by eqs 2 and 3 to a new model, including a parametric function of W and Wmax b , now independent of the total surfactant concentration. We considered still the progressive hydration of an inverted micelle as a process at equilibrium between the water localized on the AOT adsorption sites and the free bulk water in the micelle core. Then it follows from a simple Langmuir adsorption isotherm model that
θ/(1 - θ) ) KWf ) K(W - Wb) where θ ) Wb/Wmax b . Solving this equation leads to
(7)
If we use the spectral analysis corresponding to eq 3, owing to the similarity between eqs 6 and 8 we may expect the fitting for (AOT)Na obtained by using eq 8 to be of equal quality as the fitting with eq 6 (r2 ) 0.988). Actually, we determine virtually the same maximum hydration number (Wmax ) 2.57 b ( 0.08). The value of the equilibrium constant is (K ) 0.39 ( 0.02). (Table 1, system D). If, on the other hand, we use the spectral analysis corresponding to eq 2, we obtain from eq 8 the curve depicted in Figure 6, which also fits well the experimental points. This time, however, the maximum hydration number Wmax and b constant K are found respectively equal to 4.57 ( 0.18 and 0.61 ( 0.08 (Table 1, system C). 3.3. Influence of Counterionic Nature. Values of Wmax and, b if relevant, of K were determined in AOT systems neutralized respectively with Li+, K+, Rb+, and Cs+ ions and compared to those values already presented in the case of (AOT)Na. These data are recorded in Table 1 in relation to the method and model used for their evaluation. In processes that involve hydration of a salt with a common anion in the presence of different cations, the hydration number and free energy of solvation are, in an important way and essentially for cations belonging to the alkaline group, expressed as a function of their respective ion sizes. Consequently, Figures 7 and 8 indicate the relation between the ionic radius, Wmax b , and ln K, as computed from our data using the linear combination (eq 2) and deconvolution in Gaussians (eq 3) methods and the binding coefficient and Langmuir models, which both remained acceptable on the grounds of our previous analysis of the (AOT)Na system. IV. Discussion Whatever the spectral analysis and model used, the overall dependence of Wb as a function of W appears to be quite similar, and the fitting of the experimental points is always excellent. However these treatments, as defined by systems A-D in Table 1, give for 0.1 M (AOT)Na different values of Wmax b , respectively 5.7, 2.7, 4.6, and 2.6. Thus, additional arguments have to be found for the interpretation of the spectroscopic data. In predicting the dependence of Wb with the concentration of AOT, the chemical equilibrium model (system B) was proved to be incorrect and must be rejected. To appreciate the reliability of the other methods and models, it is necessary to introduce new criteria. These are obtained by investigating the behavior of Wb as a function of W in the case of different salts of AOT, (AOT)Li, (AOT)K, (AOT)Rb, and (AOT)Cs. A decrease in
FTIR Investigation of Water States in AOT Micelles
J. Phys. Chem. B, Vol. 102, No. 18, 1998 3339
Figure 6. Number of bound water molecules as a function of micelle hydration. Application of eq 8 to the spectral analysis corresponding to eq 2.
Figure 8. Variation of ln K as a function of the ionic radius of (a) system C and (b) system D.
Figure 7. Variation of the maximum amount of bound water molecules in AOT reverse micelles as a function of the counterion radius: (4) system A; (b) system C; (0) system D.
Wmax with the atomic number down a column of the periodic b table can be expected from studies of aqueous solutions of electrolytes if the hydration relates specifically to the counterion.22 However, in this case, the principal factor that determines the free energy of solvation should be the ionic size. On the contrary, if the solvation arises from water molecules linked by hydrogen bonds to the sulfonate and eventually carbonyl sites of the AOT polar heads, one expects that values of Wmax and RT ln K should be independent of the counterion b radius. In system A the nature of this evolution is unclear because, following the authors of ref 23, eq 4 relies on geometrical considerations. These, by nature, cannot fit the idea of a close interaction of water with the counterions which involves some hydration free energy contribution dependent on the ionic nature. With such a model in mind, system C seems more adequate because it contains a constant K, which is function of the free energy of solvation. Indeed, as shown in Figure 7, a linear decrease of the maximum hydration number is observed along the counterion ionic radius sequence. Taking into account the fact that we are using in this case a model based on two parameters, we infer that such a neat dependence of one
parameter is meaningful only if it is confirmed by a significant influence of the ionic size on the second parameter, ln K. This is far from the case, as depicted by the evolution of ln K in Figure 8 which discloses some erratic behavior. Thus, if we and ln K are complemenaccept the idea that values of Wmax b tary criteria for the choice of a realistic model, we ought to conclude that the linear function of the ionic radius found for is just fortuitous or an artifact. Wmax b Finally, we are left with the application of system D. If we examine the predictions of the Langmuir adsorption model, we observe that the Wmax values remain rather well constant b within the error margin as a function of the ionic size and that ln K exhibits a similar constant behavior. This is tantamount to saying that the hydration process leading to Wmax concerns b exclusively the AOT molecule without any marked interference reveal that the maxiof the counterion. The values of Wmax b mum average number of water molecules hydrogen bound to the AOT polar head is low and ranges from 2 to 3. The constant K may now be used to estimate ∆rG, the solvation free energy of AOT. According to eq 7 the definition of ∆rG results from the following condition
[ [1 -θ θ] - ln(W - W )]
RT ln
b
std st
) [∆rG + RT ln K]std st ) ∆rG° + RT ln K ) 0 (9)
where std st indicates that the evaluation is made under standard state conditions. Thus, the value of ∆rG depends on the arbitrary standard states definitions for bound water at the micelle shell inner surface and for free water in the micelle core. Actually, both standard states refer to 1 mol of AOT. It turns out, as implied by the formalism used in eq 7, that the standard bound water state is obtained when, at adsorption equilibrium, Wb ) Wmax b /(2 mol of water) are bound to 1 mol of AOT. The definition of the standard free water state condition requires
3340 J. Phys. Chem. B, Vol. 102, No. 18, 1998 more caution because the activity of free water, although considered as pure bulk water, depends on the presence of AOT. In other words a given number of moles of free water are occupying a free volume available in the micelle cores fixed by the number of moles of AOT forming the micelles. Actually we selected implicitly in eq 7 a standard free water state condition, realizing an ideal state. This state results from the extrapolation from large Wf values to Wf equal to 1 mol of free water per free intramicellar volume corresponding to 1 mol of AOT, thereby keeping the water in its bulk equilibrium state. Such a definition differs from that used in eq 5b, where the free water standard state is linked to an arbitrary value of [H2O] unrelated to the number of AOT molecules in the system. If the equilibrium adsorption process is isothermic, the standard free energy as given in Figure 8 represents differential free energy of bound water at a coverage of Wb ) 1.3 with respect to the free energy of 1 mol of free water in the presence of 1 mol of AOT. This hydration free energy, as calculated from ln K, is about 2.5 kJ/mol. Such a small positive quantity is most probably due to the contribution of a large negative standard hydration entropy arising from the decrease of molecular freedom between bulk water in the micelle core and water bound to the AOT sulfonate and carbonyl groups. V. Conclusion FTIR spectroscopy is the method of choice for the study of water in reverse micelles, but unfortunately information about water conformation has to be gained from OH stretching bands, which are lacking any fine structure. The decomposition of the water OH stretching band by means of three Gaussians is now fairly well accepted. The assignment of each Gaussian to individual vibrations of water OH groups involved in particular types of hydrogen bonding enables one to account for water states in reverse micelles. The intensity changes of these components as a function of the total amount of water allows us to determine the proportions of free and bound water in the micelle. It is shown that the choice of the Langmuir isotherm as a model for water leads to consistent Wmax values. The number b of hydration sites associated with one polar head is independent of the counterion nature and therefore is only related to the hydration sites binding water molecules to the AOT molecule.
Bey Temsamani et al. The striking low and positive value of the free energy of hydration could reflect a compensation between enthalpic and entropic terms. To check this point, it is important to extend the study to AOT systems with divalent and trivalent counterions and eventually to investigate the water bending absorption band as a function of these counterions and W. Acknowledgment. The authors gratefully acknowledge the support by a Ph.D. fellowship from the Alice and van Buuren Foundation. References and Notes (1) Martin, C. A.; and Magid, L. J. J. Phys. Chem. 1981, 85, 3938. (2) Eicke, H. F.; Christen, H. HelV. Chim. Acta 1978, 61, 2258. (3) Casado, J.; Izquierdo, C.; Fuentes, S.; Moya´, M. L. J. Chem. 1994, 71, 446, and references cited therein. (4) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (5) Pileni, M. P. AdV. Colloid Interface Sci. 1993, 46, 139. (6) D’Aprano, A.; Lizzio, A.; Turco Liveri, V.; Aliotta, F.; Vasi, C.; Migliardo, P. J. Phys. Chem. 1988, 92, 4436. (7) Jain, T. K.; Varshney, M.; Maitra, A. J. Phys. Chem. 1989, 93, 7409. (8) Haandrikman, G.; Daan, G. J. R.; Kerkhof, F. J. M.; Van Os, N. M.; Rupert, L. A. M. J. Phys. Chem. 1992, 96, 9061. (9) D’Angelo, M.; Onori, G.; Santucci, A. J. Phys. Chem. 1993, 98, 3189. (10) Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430. (11) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L. J. Phys. Chem. 1989, 93, 7869. (12) Bertolini, D.; Cassetari, M.; Salvetti, G.; Tombari, E.; Veronesi, S.; Squadrito, G. Prog. Colloid Polym. Sci. 1992, 89, 278. (13) Goto, A.; Harada, S.; Fujita, T.; Miwa, Y.; Yoshioka, H.; Kishimoto, H. Langmuir 1993, 9, 86. (14) Hasegawa, M.; Sugimura, T.; Suzaki, Y.; Shindo, Y.; Kitahara, A. J. Phys. Chem. 1994, 98, 2120. (15) Giammona, G.; Goffredi, F.; Turco Liveri, V.; Vassallo, G. J. Colloid Interface Sci. 1992, 154, 411. (16) Amico, P.; D’angelo, M.; Onori, G.; Santucci, A. NuoVo Cimento Soc. Ital. Fis. 1995, 17 (9, Sep), 1053. (17) Camardo, M.; D’angelo, M.; Mannailo, S.; Onori, G.; Santucci, A. Colloids Surf. A 1996, 119 (2-3, Dec 16), 183. (18) Gonzalez-blanco, C.; Rodriguez, J.; Velasquez, M. M. Langmuir 1997, 13, 1938. (19) MacDonald, H.; Bedwell, B.; Gulari, E. Langmuir 1986, 2, 704. (20) Moran, D.; Bowmaker, A.; Cooney, P. Langmuir 1995, 11, 738. (21) Eastoe, J.; Robinson, B. H.; Heenan, R. K. Langmuir 1993, 9, 2820. (22) Bockris, J. M.; Reddy, A. K. N. Modern Electrchemistry: An Introduction to an Interdisciplinary Area; Plenum Press: New York, 19711970. (23) D’Aprano, A.; Lizzio, A.; Turco Liveri, A. J. Phys. Chem. 1988, 92, 1985.
