Micellar Behavior of the Aqueous Solutions of ... - ACS Publications

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Langmuir 1997, 13, 219-224

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Micellar Behavior of the Aqueous Solutions of Dodecylethyldimethylammonium Bromide. A Characterization Study in the Presence and Absence of Hydroxypropyl-β-cyclodextrin Elena Junquera, Lourdes Pen˜a, and Emilio Aicart* Departamento de Quı´mica Fı´sica I, Facultad de Ciencias Quı´micas, Universidad Complutense de Madrid, 28040 Madrid, Spain Received July 29, 1996. In Final Form: October 18, 1996X Conductivity and static fluorescence measurements have been carried out to study the micellar behavior of the aqueous solutions of dodecylethyldimethylammonium bromide (D12EDMAB) in the absence and in the presence of hydroxypropyl-β-cyclodextrin (HPBCD) at 25 °C. Several micellar parameters, as the aggregation number (n), the critical micelle concentration (cmc), and the dissociation degree of the micelle (β) have been determined for the aqueous surfactant solution. In the presence of HPBCD, the D12EDMAB forms an inclusion complex with a 1:1 stoichiometry and a binding constant K ) 3200 M-1. The effect of this complex in the micellar parameters, such as the cmc*, the β, and the free [D12EDMAB] available to the micellization process have been analyzed. The ionic molar conductivities λoi of the D12EDMA+ ion, both in the nonassociated and the associated forms, have been calculated as well. The results have been discussed and compared with those previously reported for other cyclodextrin/dodecyltrimethylammonium bromide/water systems.

Introduction During the last decades, the study of the behavior of ionic surfactants has received much attention.1-5 Studies involving physicochemical properties have proved powerful for elucidating the mechanism of the micellization process. Increased aqueous solubility of otherwise slightly soluble organic substances brought about by the presence of micelles is known as solubilization, a phenomenon that plays a relevant role in industrial and biological processes. Actually, the removal of organic residues from an aqueous medium has been known for a long time, but it is still used and improved nowdays with the arrival of modern high-yield detergents of increasing interest in the treatment of wastewaters and industrial residues. This phenomenon permits as well the solubilization of extremely hydrophobic drugs, fact of great importance in the pharmacological industry.6 In the last 2 decades, the interest for better knowledge of polymer-surfactants systems has grown enormously for their widespread applications and for their inherently interesting properties.7,8 Polymer-surfactant complexes are used, for example, in laundry detergents, in tertiary oil recovery, in cosmetic products, in antiredeposition agents, and also in paints and coatings. Another fact which contributes to increase the interest in these surfactant molecules is their suitability to be used as model systems to partially * To whom correspondence should be addressed: fax, 34-13944135; e-mail, [email protected]. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Mukerjee, P.; Mysels, K. J. Natl. Stand. Ref. Data Ser. 1971, 36. (2) Zana, R. Surfactant Solutions: New Methods of Investigation; Marcel Dekker Inc.: New York, 1987. (3) Fendler, J. K.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (4) Tanford, Ch. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley & Sons: New York, 1980. (5) Attwood, D.; Florence, A. T. Surfactant Systems: Their Chemistry, Pharmacy and Biology: Chapman and Hall: London, 1983. (6) Florence, A. T. Techniques of Solubilization of Drugs; Yalkolwsky, S., Ed.; Marcel Dekker: New York, 1982, and references therein. (7) Shah, D. O. Surface Phenomena in Enhanced Oil Recovery; Plenum Press: New York, 1981. (8) Lucassen-Reynders, E. H. Anionic Surfactants, Physical Chemistry of Surfactants Action; Marcel Dekker: New York, 1981.

mimic the behavior of biological systems, such as cellular membranes, vesicles, liposomes, etc.9 It can be concluded for all the above mentioned that a good physicochemical characterization of micellar aggregates in aqueous solutions could contribute to shed light in a wide variety of researching and applied fields. This characterization involves the knowledge of a series of micelle parameters, such as, the cmc, the aggregation number (ns), the polidispersity (σ), the counterion dissociation degree of ionic surfactants (β), the kinetic constants for the entrance (k+) and the exit (k-) of the monomer into the micelle, the thermodynamic parameters such as apparent molar volumes (ΦoV,m), apparent molar isoentropic compressibilities (ΦoΚs,m), apparent molar heat capacities (ΦoCp,m), and standard free energy, enthalpy, and entropy changes (∆Gom, ∆Hom, ∆Som) for the micellization process, and the influence of external factors (temperature, pressure, additives) in all the mentioned parameters. A vaste number of different studies can be found in the literature, all involved in the characterization of micellar systems from the most varied experimental techniques (see ref 2 and references therein). More recently, several groups10-16 are involved in the analysis of the behavior of these micellar aggregates in the presence of a third component, a cyclodextrin, whose apolar cavity is capable of encapsulating the hydrophobic tail of the surfactant, competing then with the micelle formation. Cyclodextrins are a type of host molecules, among others, consisting of a different number of R-D-glucopyranose units bonded by (9) Fendler, J. H. Membrane Mimetic Chemistry; John Wiley & Sons: New York, 1982. (10) Okubo, T. J.; Maeda, Y.; Kitano, H. J. Phys. Chem. 1989, 93, 3721. (11) Satake, Y.; Ikenoue, T.; Takeshita, T.; Hayakawa, K.; Maeda, T. Bull. Chem. Soc. Jpn. 1985, 58, 2746. (12) Palepu, R.; Richardson, J. E.; Reinsborough, V. C. Langmuir 1989, 5, 218. (13) Mwakibete, H.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1994, 10, 3328. (14) Junquera, E.; Tardajos, G.; Aicart, E. Langmuir 1993, 9, 1213. (15) Jobe, D. J.; Verrall, R. E.; Junquera, E.; Aicart, E. J. Phys. Chem. 1994, 98, 10814. (16) Park, J. W.; Song, H. S. J. Phys. Chem. 1989, 93, 6454.

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R(1f4) glycosidic linkages.17,18 As a result of this disposition, they are truncated cone-shaped molecules, with a hydrophilic outer surface and a hydrophobic cavity, where small organic molecules can shelter their most apolar parts, with the formation of an inclusion complex. As previously mentioned, an effect of the presence of such supramolecular entities on the micellization parameters could be expected. Actually, the critical micelle concentration has been found to shift to higher values in the presence of CD and/or complex (cmc*),10-16 while the concentration of free monomer ([S]f) remains basically the same,14,19 slightly increases,20 or slightly decreases20,21 depending on the surfactant. In all the studied cases, since the association constant of the complex CD/surfactant is always higher than that of the micelle formation from the monomers, the micelles break up when the inclusion complex formation occurs.11,12,14 The micellization of the cationic surfactants of the family of the alkylammonium bromides has been widely studied both in the absence and in the presence of cyclodextrin from different experimental techniques.13,14,22-24 Although the effects of the alkyl chain length and of the type of cyclodextrin have been analyzed,19-24 little attention has been paid to the influence of the polar head in the micelle parameters, without changing the charge.20 In the present investigation we have tried to elucidate the polar head’s role in the micelle formation by characterizing the micelles of dodecylethyldimethylammonium bromide (D12EDMAB) in aqueous solution in the absence and in the presence of hydroxypropyl-β-cyclodextrin (HPBCD), a hydroxypropylated derivative of β-cyclodextrin, 20 times more soluble. Comparing the results of this work with those of a previous study21 where the micelles of dodecyltrimethylammonium bromide (D12TAB) were analyzed in the presence and absence of CD’s as well, we expected to find conclusions about the effect of changing a methyl to an ethyl group (D12TAB f D12EDMAB) in the polar head of the surfactant, keeping the tail length (12 carbon atoms in both cases), the counterion, and the charge unchanged. The critical micelle concentration both in the absence (cmc) and in the presence of cyclodextrin (cmc*), the concentration of free monomer ([S]f), and the micelle dissociation degree (β), as well as the stoichiometry (A), the binding constant (K) and the ionic molar conductivity (λoCDS+) of the CD/surfactant complex have been determined from highly precise conductometric measurements at 25 °C. In addition, the aggregation number of the D12EDMAB micelles as well as information about the lipophilic character of the micellar microenviroment has been obtained by using a static fluorescence technique, also at 25 °C. Experimental Section Materials. Dodecylethyldimethylammonium bromide (D12EDMAB), pyrene, and hexadecylpyridinium chloride (CePyCl) were from Aldrich Co., while hydroxypropyl-β-cyclodextrin (HPBCD), containing an average of 0.4 hydroxypropyl groups (17) Szjetli, J. The Cyclodextrin and Their Inclusion Complexes; Akademiai Kiado: Budapest, 1982. (18) Saenger, W. Inclusion Compounds; Academic Press: London, 1984. (19) Junquera, E.; Tardajos, G.; Aicart, E. J. Colloid. Interface Sci. 1993, 158, 388. (20) Pen˜a, L.; Junquera, E.; Aicart, E. J. Solution Chem. 1995, 24, 1075. (21) Junquera, E.; Pen˜a, L.; Aicart, E. Langmuir 1995, 11, 4685. (22) Gharibi, H.; Palepu, R.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. Langmuir 1992, 8, 782. (23) Jobe, D. J.; Verrall, R. E.; Junquera, E.; Aicart, E. J. Phys. Chem. 1993, 97, 1243. (24) Gormally, J.; Sharma, S. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2497.

Junquera per glucopyranose unit was from Jansen Biotech. CePyCl and HPBCD consist of 4.8 and 3.4% mass of water content, respectively, which was considered to calculate solute concentrations. Double-distilled water was deionized using a Super Q Millipore system and finally was also degassed with a vacuum pump prior to the preparation of the solutions. Conductivity Measurements. Conductivity data were collected at 25.000 ( 0.001 °C with a Hewlett-Packard 4263A LCR Meter, using a Metrohm electrode with a cell constant of 0.8129 cm-1. Mixtures were prepared from a Metrohm digital buret, whose cylinder was kept at the same constant temperature of the measuring cell. The conductometer and the buret were controlled via IEEE-488 Bus and RS-232C interfaces, respectively. The experimental procedure was widely described previously.25 The accuracy on the specific conductivity κ, obtained as an average of 2400 measurements for each concentration, is believed to be better than 0.03%. The following conductivity measurements were made: (i) as a function of D12EDMAB concentration for different constant values of HPBCD concentrations, the range of [D12EDMAB] going from the pre- to postmicellar region; (ii) as a function of cyclodextrin concentration for a nonmicellar D12EDMAB solution; and (iii) as a function of [HPBCD] and [D12EDMAB] with their stoichiometric ratio [CD]/ [surfactant] constant and equal to 1. Fluorescence Measurements. Steady state fluorescence experiments were carried out with a Perkin-Elmer LS-50B luminiscence spectrometer. The equipment is connected to a PC-386 SX computer via a RS-232C standard. Data acquisition and analysis of the fluorescence spectra were performed with the Fluorescence Data Manager Software supported by PerkinElmer. A 10 nm stoppered rectangular silica cell was placed in a stirred cuvette holder whose temperature was kept constant at 25.00 ( 0.01 °C with a recirculating water circuit. Both excitation and emision band slits were fixed at 2.5 nm, and the scan rate was selected at 240 nm/min. Pyrene was used as luminiscence probe and hexadecylpyridinium chloride was chosen as static quencher. The excitation wavelength was selected at 340 nm, while the emission spectra were collected from 360 to 500 nm. The first and third vibronic peaks of the pyrene appear at 373 and 384 nm, respectively. The pair pyrene/CePyCl assure that the residence time of the quencher into the micelle is much longer than the fluorescence lifetime of the probe.26,27 Solutions were prepared following the Infelta and Gra¨tzel28 procedure. An aliquot of a stock solution of pyrene in ethanol (for fluorescence spectroscopy) was transferred into a flask and the solvent evaporated with nitrogen. The surfactant solution was added, the probe being solubilized into the micelles after stirring the solution 24 h. Pyrene and D12EDMAB concentrations were kept constant at 10-6 M and 20 mM, respectively, and the quencher concentration was varied from 0 to 9 × 10-5 M. These values give [pyrene]/[micelles] and [quencher]/[micelles] ratios about 0.01 and less than 0.8, respectively, assuring a Poisson distribution.2,28-30

Results and Discussion D12EDMAB/Water System. Aqueous solutions of dodecylethyldimethylammonium bromide have been characterized through conductivity and static fluorescence measurements. The lower curve of Figure 1 shows the behavior of specific conductivity κ vs D12EDMAB concentration. The concentration at which there is a break in the property is assigned to the critical micelle concentration, cmc, being the obtained value (14.4 mM) in agreement with that obtained for this system from speed of sound studies20 and close to the cmc determined for dodecyltrimethylammonium bromide D12TAB in water.21,31 (25) Junquera, E.; Aicart, E. Rev. Sci. Instrum. 1994, 65, 2672. (26) Atik, S. S.; Thomas, J. K. J. Am. Chem. Soc. 1982, 104, 5868. (27) Malliaris, A.; Lang, J.; Zana, R. J. Chem. Soc., Faraday Trans. 1 1986, 82, 109. (28) Infelta, P. P.; Gra¨tzel, M. J. Chem. Phys. 1979, 70, 179. (29) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (30) Infelta, P. P. Chem. Phys. Lett. 1979, 61, 88. (31) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208.

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Figure 1. Specific conductivity κ at 25 °C for the ternary system HPBCD/ D12EDMAB/H2O as a function of [D12EDMAB] at various [HPBCD].

Figure 2. Emission fluorescence spectra of a 10-6 M solution of pyrene in aqueous micellar solutions of D12EDMAB at 25 °C: curve 1, in the absence of CePyCl; curves 2-10, in the presence of CePyCl, with a concentration ranging from 1 × 10-5 to 9 × 10-5 M.

The dissociation degree of the micelle, β, can be estimated from the ratio between the slopes above and below the cmc. The obtained value (β ) 0.28) is slightly higher than that obtained for D12TAB in water21 (β ) 0.25), revealing that the bigger polar head of D12EDMA+ stabilizes the micelle with a slightly lower counterion concentration. The aggregation number of the monomers in the micelles, n, can be determined from the steady-state fluorescence data, if a Poisson distribution is assumed to be valid for the equilibrium of solubilizates between the aqueous and micellar phases. The equation to be applied is2,29

ln I ) ln I0 - [Q]/Cm ) ln I0 - n[Q]/(Ct - cmc)

(1)

where [Q], Cm, and Ct are the concentrations of quencher, micelles, and total surfactant, respectively, while I0 and I are the fluorescence intensities in the absence and in the presence of quencher. Figure 2 reports the pyrene emission spectra for micellar solutions in the presence of several quencher concentrations. Figure 3 shows a plot of ln I vs [Q], the ratio [Q]/Cm always being less than 0.8. From the slope in this plot and the cmc previously determined from conductivity data, a value of n ) 48 ( 4 has been obtained for the aggregation number of D12EDMAB in water, slightly lower than that obtained for D12TAB (n ) 5532 ). Assuming that both D12TAB and D12EDMAB form spheric micelles, and according with the model proposed by Tanford4 to estimate the maximun aggregation number of an hydrocarbon tail of 12 carbon atoms, 55 monomers would be obtained. This value is in agreement with the aggregation number of D12TAB, while in the case of D12EDMAB a lower value would be expected, due to the slight increase of the polar head with respect to D12TAB. This estimation is confirmed by the aggregation number determined from the static fluorescence measurements reported in this work. The relative intensities of the first and third vibronic peaks of pyrene (Figure 2) are directly related with the apparent dielectric constant of the medium where the probe is housed. For pyrene, characteristic values of II/ IIII are 1.04, 1.33, and 1.84 in toluene, methanol, and water (32) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981, 84, 100.

Figure 3. Plot of the logarithm of the fluorescence intensity (ln I) as a function of the CePyCl concentration for a 10-6 M solution of pyrene in aqueous micellar solutions of D12EDMAB.

respectively.33 The average value obtained in this work for II/IIII among all the curves in Figure 2 is 1.38 ( 0.01, revealing that the medium where the probe is inside the D12EDMAB micelles has an apparent dielectric constant similar to, although slightly higher than, that of methanol ( ) 32.6). Thus, although the polarity of the probe enviroment is much lower than that of water, the pyrene is expected to be not far away from the polar heads of the surfactant. In conclusion, the picture which emerges from conductivity and fluorescence measurements for the aqueous solutions of D12EDMAB shows that the substitution of an ethyl group for a methyl group in the polar head of the surfactant has little effect on the cmc. This feature is in contrast with the decrease of the cmc, approximately by a factor of 2, when the hydocarbon tail of ionic surfactants (33) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.

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Table 1. Values of Apparent Critical Micellar Concentration (cmc*) and [D12EDMAB]f Determined from Conductivity vs [D12EDMAB] at Constant [HPBCD] [HPBCD] (mM)

cmc* (M)

[D12EDMAB]f (M)

0.00 4.91 9.85 14.75

14.7 18.3 22.5 26.9

14.4 13.4 12.9 12.1

Figure 5. Values of ∆Λ ()Λf - Λ) at 25 °C as a function of [HPBCD] at various [D12EDMAB].

Figure 4. Molar conductivity Λ at 25 °C for the ternary system HPBCD/ D12EDMAB/H2O as a function of [D12EDMAB] at various [HPBCD].

is lengthed by a CH2 group.5 However, the bigger size of the polar head makes the aggregation number of D12EDMAB micelles decrease by 10%, while the micellization degree increases to the same extent. HPBCD/D12EDMAB/Water System. Figure 1 shows the variation of the specific conductivity κ vs [D12EDMAB] for aqueous solutions of D12EDMAB in the presence of different constant HPBCD concentrations. The change in the slope of the property is assigned to the apparent critical micelle concentration in the presence of cyclodextrin (cmc*). Table 1 reports the values of cmc* obtained at all the studied HPBCD concentrations. The cmc* of D12EDMAB increases linearly with CD concentration from the cmc of the pure surfactant (cmc* ) cmc + 0.85[HPBCD]). The intercept of this line is consistent with the experimental cmc of pure D12EDMAB previously obtained. This fact is due to the formation of the inclusion complex HPBCD/D12EDMAB. Since the association between these host and guest species is tighter than that between the monomers to form the micelles,12,14 the addition of surfactant to the cyclodextrin solution results in the inclusion of the surfactant apolar tail into the cyclodextrin cavity. Only when the cyclodextrin is in the complexed form does the addition of more surfactant result in the formation of micelles. The complex formation is, however, better detected in Figure 4, where the molar conductivity Λ ()κ/c) (instead of the specific one) is plotted as a function of surfactant concentration. As can be seen, this figure shows a double change in the property for all the curves with the exception of the curve corresponding to the pure surfactant in the absence of cyclodextrin, where only one change is observed. These two changes are related to the complex and micelles (cmc*) formation, and consequently, the curve of the pure surfactant shows only the second change (cmc). From the first change, the stoichiometry of the complex (A) can

be determined as the ratio between [HPBCD] and [D12EDMAB], [HPBCD] being the initial cyclodextrin concentration which is kept constant and [D12EDMAB] the surfactant concentration at which the first change in the property is observed. This stoichiometry can be obtained as well from the study of the variation of the conductivity with cyclodextrin concentration of nonmicellar solutions of D12EDMAB. Figure 5 shows the plot of the difference between molar conductivities in the absence (Λf) and in the presence (Λ) of cyclodetrin (∆Λ ) Λf - Λ) as a function of [HPBCD] for two different nonmicellar solutions of D12EDMAB ([D12EDMAB] < cmc). The increase in ∆Λ when the cyclodextrin concentration is increased is due to the inclusion of the monomers of D12EDMAB into the cavity of CD. This inclusion process implies a decrease in Λ, since the ionic molar conductivity of the associated dodecylethyldimethylammonium ion, D12EDMA+, is expected to be less than that of the free ion. From the [HPBCD] at which the change due to the complex formation is observed in Figure 5 and following the same procedure previously described in Figure 4, the stoichiometry A of the complex has been calculated. The average value obtained from the results of both studies is A ) 1.0 ( 0.05, indicating that the complex HPBCD/D12EDMAB is statistically formed by the association of a molecule of cyclodextrin per each molecule of surfactant. This value is similar to that obtained for the complexes β-CD/D12EDMAB, DIMEB/D12EDMAB, β-CD/D12TAB, and HPBCD/D12TAB in aqueous solutions.20,21 It can be deduced, consequently, that for host molecules with similar cavities, as is the case of β-CD and their methylated (DIMEB) and hydroxypropylated (HPBCD) derivatives, the stoichiometry of the complexes formed with the hydrophobic tail of a 12 carbon atom surfactant is 1:1, being unaffected by the size of the polar trimethylammonium or ethyldimethylammonium head of the monomer. The values of [S]f of Table 1 indicate that the concentration of D12EDMAB available for the micellization process in the presence of HPBCD and/or complex decreases linearly with [HPBCD] ([S]f ) cmc - 0.15[HPBCD]). This decrease is 50% lower than that observed for D12TAB in the presence of the same cyclodextrin.21 The other parameter of great importance when characterizing the inclusion complex is the binding constant, K. It has been calculated from conductivity measurements

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Figure 6. Values of molar conductivity Λ at 25 °C as a function of [CD] and [S] at stoichiometric ratio for various CD/surfactant/ H2O ternary systems. Table 2. Values of the Ionic Molar Conductivities at Infinite Dilution (λio) and Size Ionic Parameters (an) ion

λio (Ω-1‚cm2‚mol-1)

an (Å)

BrD12EDMA+ HPBCD/D12EDMA+

78.1434 26 13 ( 2

3 8 17 ( 2

of Figure 1 (premicellar range), Figure 5 and also Figure 6, which is a plot of the molar conductivity Λ vs [HPBCD] and [D12EDMAB] varying simultaneously and keeping their concentration ratio ([HPBCD]/[D12EDMAB] ) A ) 1). This Figure 6 also reports the results obtained in previous studies for the complexes β-CD/D12TAB and HPBCD/D12TAB. The model used to determine the association constants has been widely described in a previous work21 and takes into account not only the association of the surfactant ion to the cyclodextrin (KCDS+), but also the slight association of the counterion to the complex (KCDSBr). This model allows the simultaneous determination of these binding constants together with the ionic molar conductivity of the complex, λoCDS+. The specific conductivity, κ, of the solution is related to the ionic molar conductivities λi and the concentration of the ionic species through the equation:

κ ) λBr- [Br ] + λS+ [S ] + λCDS+ [CDS ] -

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(2)

being all the species in the solution related through the above mentioned binding constants and the mass balance for both the cyclodextrin and the surfactant. The values of λoi for the ions Br- and D12EDMA+, related with the ionic molar conductivities λi through the Onsager relation, have been taken from the literature34 and obtained from the conductivity data of the pure D12EDMAB in water, respectively (Table 2). The nonlinear fitting of the experimental data according with this model gives the following values for the studied parameters: λoCDS+ ) 13 ( 2 Ω-1‚cm2‚mol-1, KCDSBr ) 1.5 ( 1 M-1, and KCDS+ ) 3200 ( 800 M-1. In all the cases, the saturation degree ranges from 0.15 to 0.85, as is advisible.35,36 (34) Tinner, U. Electrodes in Potentiometry; Metrohm Herisau A.G.: Herisau, 1989. (35) Derenlau, D. A. J. Am. Chem. Soc. 1969, 91, 4044. (36) Schneider, H. J.; Kramer, R.; Simova, S.; Schneider, U. J. Am. Chem. Soc. 1988, 110, 6442.

The values of λοD12EDMA+ and λοCDS+ of Table 2 confirm the assertions previously made in Figures 1 and 4-6, in the sense that the ionic conductivity of the associated surfactant decreases around 50% with respect to the free surfactant. The association of the counterion Br- to the complex HPBCD/D12EDMA+ is very low as is indicated in the value of KCDSBr, but not negligible (∼1%), and is consistent with the value found for other CD/surfactant systems from conductivity21,37 and/or potentiometric measurements.38,39 Comparing the λοi of Table 2 with those previously obtained21 for D12TA+, it can be concluded that both surfactants, in the free or complexed forms, have similar mobilities in aqueous solution. The fact that the binding constant of the complex HPBCD/D12EDMA+ is 35 and 10% higher than those of the complexes β-CD/D12TA+ and HPBCD/D12TA+, respectively, indicates (i) the substituted CD’s associate surfactant molecules tighter than the parent β-CD, even though the cavities are very similar in all the cases and (ii) the substitution of a methyl by an ethyl group in the polar head of the surfactant has a small effect in the affinity of the inclusion process, and even smaller than that created by the lengthening of the surfactant tail by a methylene group.19-24 Finally, another interesting feature is the effect of the CD and/or the complex in the micelle. The parallelism on the curves of κ or Λ in Figures 1 and 4 respectively, above the cmc, reveals that these properties are not affected by the presence of both CD and the complex. It can be deduced that these species neither participate in the micelle nor affect the micellar parameters. Moreover, the average value of the micelle dissociation degree in the presence of CD has been estimated (Figure 1) to be β ) 0.29 ( 0.02, analogous to that of D12EDMAB in water in the absence of CD. This fact, which has been already pointed out by our group from speed of sound experiments first40 and from conductivity data later,21 has been recently confirmed by Jobe et al.41 from fluorescence measurements of aqueous solutions of sodium dodecyl sulfate in the presence of different cyclodextrins. These authors conclude that the aggregation number of the SDS micelles is constant and not affected by the presence of the cyclodextrins, and this conclusion seems to be valid not only for anionic but also for cationic micelles. Conclusions The conductivity and static fluorescence measurements of the aqueous solutions of D12EDMAB indicate that the substitution of a methyl group by an ethyl group in the polar head of a 12 carbon atom cationic surfactant (D12TAB f D12EDMAB) has a minimum effect in the cmc, while makes the aggregation number and the dissociation degree of the D12EDMAB micelles decrease and increase, respectively, in a 10% extent. It has been also concluded that, in the presence of a host molecule as HPBCD, D12EDMAB forms an inclusion complex with a 1:1 stoichiometry and a binding constant of K ) 3200 M-1, with the counterion Br- associated to the complex in a very low extent (around 1%). The ionic (37) Junquera, E.; Pen˜a, L.; Aicart, E. J. Inclusion Phenom. Mol. Recognit. Chem. 1996, 24, 233. (38) Palepu, P.; Reinsborough, V. C. Can. J. Chem. 1989, 67, 1550. (39) Mc Pherson, Y.; Palepu, P.; Reinsborough, V. C. J. Inclusion Phenom. Mol. Recognit. Chem. 1990, 9, 137. (40) Junquera, E.; Aicart, E.; Tardajos, G. J. Phys. Chem. 1992, 96, 4533. (41) Jobe, D. J.; Reinsborough, V. C.; Wetmore, S. D. Langmuir 1995, 11, 2476.

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molar conductivities of both the free and associated surfactant ion (λoD12EDMA+ and λoCDS+) have been calculated, and the results indicate that the associated species has a mobility around 50% lower than the free ion. The conductivity data have also confirmed that the association process between the HPBCD and D12EDMAB has a higher affinity than the micellization process, since only when the CD in the solution is complexed, the posterior addition of D12EDMAB results on the formation of micelles. This fact implies an increase of the cmc (cmc*, in the presence of CD), while the surfactant concentration available for the micellization process ([D12EDMAB]f) shows a slight linear decrease as [HPBCD] increases. In any case, the presence of the cyclodextrin and/or the complex does not affect either the aggregation number or to the dissociation degree of the micelles.

Junquera

In comparison with other CD/D12TAB/water systems previously reported, the conductivity measurements show that (a) for a given surfactant, the derived cyclodextrins (HPBCD or DIMEB) associate with the hydrocarbon tail of the surfactant tighter than their parent β-CD and (b) the substitution of a methyl group by an ethyl group in the polar head of the surfactant has no effect on the stoichiometry of the complex. The effect on the binding constant is small and much lower than that when the surfactant tail is lengthening by a methylene group. Acknowledgment. The authors are grateful to Ministerio de Educacio´n y Ciencia of Spain through a DGICYT Project No. PB92-0229 for financial support. LA960750F