Inclusion Complexes between Î²-Cyclodextrin and a Gemini Surfactant

J. Phys. Chem. B 2006, 110, 13819-13828

13819

Inclusion Complexes between β-Cyclodextrin and a Gemini Surfactant in Aqueous Solution: An NMR Study Andre´ s Guerrero-Martıńez,† Gustavo Gonza´ lez-Gaitano,‡ Montserrat H. Vin˜ as,§ and Gloria Tardajos*,† Departamento de Quı´mica-Fı´sica I, Facultad de Ciencias Quı´micas, UniVersidad Complutense de Madrid, 28040, Madrid, Spain, Departamento de Quı´mica y Edafologıá, UniVersidad de NaVarra, 31080 Pamplona, NaVarra, Spain, and EUIT Informa´ tica, UniVersidad Polite´ cnica de Madrid, 28031 Madrid, Spain ReceiVed: March 14, 2006; In Final Form: May 22, 2006

1

H NMR spectra, diffusion-ordered NMR (DOSY), and 2D rotating-frame Overhauser enhancement spectroscopy (ROESY) experiments for aqueous solutions at 298 K containing the gemini surfactant, bis (dodecyl dimethylammonium)diethyl ether dibromide (12-EO1-12), in the absence and presence of β-cyclodextrin (β-CD) were used to characterize the surfactant and to determine the effects of the complexation in the micellization. For the binary system, the critical micelle concentration (cmc), the aggregation number, the stepwise micellization constant, and the size of the monomer have been obtained by studying the dependence of the chemical shifts and the self-diffusion coefficients with the concentration of surfactant. For the ternary system, the analysis of the 1H NMR spectra and the self-diffusion coefficients reveal the formation of complexes of 1:1 and 2:1 stoichiometry (β-CD:gemini), with a calculated stability constant for the second binding step higher than that of the first. The values of the hydrodynamic radii of the complexes were obtained from the calculated diffusion coefficients. The presence of β-CD modifies the cmc in an extension that indicates mainly the formation of a 2:1 complex. The analysis of the chemical shifts of the surfactant indicates the nonparticipation of the complexes into the micelles. ROE enhancements depend substantially on the amount of the macrocycle added and therefore on the stoichiometry; at low concentrations of β-CD, one of the hydrocarbon chains binds favorably with the cavity whereas the other interacts with the outer face. By contrast, at higher concentrations of β-CD, the two hydrocarbon tails are included in two different macrocycles.

Introduction The microencapsulating effects of organic hosts on the inherent properties of guest molecules as well as their interactions have been studied extensively over the last few decades.1 Cyclodextrins (CDs) have often been used for such studies because of their marked ability to form inclusion complexes with a wide variety of guests in solution.2 These macrocycles are cyclic oligosaccharides formed by units (six, seven, or eight) of R-D-(+)-glucopyranose named R-, β-, or γ-CD, respectively. They form a doughnut-shaped structure, with a hydrophobic cavity and two hydrophilic rims in which the primary and secondary OH groups are inserted (Figure 1). Thus, a CD constitutes a singular microenvironment where molecules with suitable size and hydrophobic character can be housed. This property renders their study of great interest and calls for a detailed investigation of their inclusion process, with all the potential applications that this fact implies.3 Of particular interest is the effect of CDs on the selforganization process of surfactants in which the presence of these macrocycles introduces a new equilibrium which competes with the aggregation.4 These amphiphilic molecules are used as probes to understand the phenomenon of complexation, since their molecular structures (e.g., polar nature of the head, charge, * To whom correspondence should be addressed. E-mail: tardajos@ quim.ucm.es. † Universidad Complutense. ‡ Universidad de Navarra. § Universidad Polite ´ cnica de Madrid.

Figure 1. Chemical structures of β-cyclodextrin (β-CD) and bis (dodecyl dimethylammonium)diethyl ether (12-EO1-12) dibromide.

length of the hydrocarbon tail, etc.) affect the binding process. Recently, a new class of double-chained surfactants, called gemini surfactants, has been introduced.5 In contrast to their more traditional single-chain counterpart analogues, these novel surfactants are made of two hydrophobic tails and two hydrophilic headgroups linked by a spacer chain. In colloid science, these molecules have gained recent attention because of their exceptional properties,6 such as low critical micelle concentrations and a stronger efficacy in decreasing the surface tension of water.7 The possibility of synthesizing a whole variety of geminitype architectures,7 by selecting the number, hydrophobic nature,

10.1021/jp0615813 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/28/2006

13820 J. Phys. Chem. B, Vol. 110, No. 28, 2006 and flexibility of the hydrocarbon tails and spacers, and also the ionic character of the headgroups, offers new ways to explore the intermolecular forces responsible for the inclusion processes with CDs. A battery of experimental techniques has been used to investigate the micellization of gemini surfactants in water, including, among others, surface tension,8 electrical conductivity,9 time-resolved fluorescence,10 SANS,11 microcalorimetry,12 NMR,13 and volumetric measurements.14 However, there are few references in the literature dealing with the encapsulating properties of CDs with these amphiphilic molecules.15 In this study, we have chosen a gemini surfactant, bis (dodecyl dimethylammonium)diethyl ether dibromide (12-EO1-12), to investigate host-guest interactions with β-CD in conditions above and below the critical micellar concentration (cmc). This surfactant presents two quaternary ammonium headgroups as well as a flexible hydrophilic oxyethylene spacer (Figure 1). In particular, we have studied the effects of the complexation with β-CD in the micellization of the gemini surfactant by different NMR experiments, such as 1H NMR spectra, diffusion-ordered NMR (DOSY), and rotating-frame Overhauser enhancement spectroscopy (ROESY).16 DOSY experiments have been previously used for the analysis of a wide variety of complex processes, providing basic information on the diffusion characteristics of molecules.17 With DOSY, it is possible to obtain the self-diffusion coefficients, D, of individual compounds from a mixture according to the differences in their effective sizes.18 In addition, the ROESY technique can provide detailed information on the structure of the complexes, always keeping in mind the dynamic character of these systems.19 These studies, combined with those obtained via 1H NMR titrations, give us information at a molecular level of the nature and structure of the complexes, their stoichiometries, and the effect that the macrocycle has on the micellization process, and represent the first investigation of the interaction between ionic gemini surfactants and β-CD. Experimental Methods Chemicals. β-CD was purchased from Aldrich, having a water content of 13.5%, as determined by thermal analysis. All the NMR samples were prepared in D2O (Aldrich Chemical Co., 99.9% minimum in D). Bis (dodecyl dimethylammonium)diethyl ether dibromide was synthesized according to the procedure described by Bhattacharya et al.11 Phosphorus tribromide, n-dodecylbromide, diethylene glycol, and dimethylamine were purchased from Aldrich Chemical Co. Common reagents and solvents used in the synthesis were obtained from commercial suppliers, without further purifications. 1H NMR Spectra. In the study of the gemini system in water, a stock solution of 12-EO1-12 was added to vials containing different volumes of water. In the case of the 1H NMR and DOSY experiments of the ternary systems, different CD/gemini molar ratios were prepared from stock solutions of surfactant, by weighing increasing amounts of CD in suitable vials. The concentration of surfactant in these experiments was kept constant at 0.6 mM (under this condition, the concentration of surfactant is always below the cmc). The studies on the shifting of the cmc were performed using virtually the same procedure, but in this case, the concentration of macrocycle was kept constant at 6.0 mM (above the cmc) and the concentration of surfactant was changed. All the mixtures were sonicated and transferred to NMR tubes (final volume 0.5 mL). The proton spectra were recorded at 298 K in a Bruker Avance AV-500 spectrometer (11.7 T) by averaging 32 scans, with a digital resolution of 0.30 Hz. The HDO signal was used

Guerrero-Martıńez et al. as the reference.20 The signal assignment of the surfactant was established by conventional NMR methods (COSY, TOCSY). DOSY NMR Spectra. For the binary and ternary systems, the samples were prepared in the same conditions as those described in the previous section. A stimulated echo sequence incorporating bipolar gradients (BPPLED),21 with a longitudinal eddy current delay of 5 ms, was used for acquiring DOSY spectra. The duration of the magnetic field pulse gradients and the diffusion times were optimized for each sample to observe the complete signal decay with the maximum gradient strength. Gradient strengths of 2.0-2.5 ms were incremented in 32 steps ranging between 2% and 95% of the total gradient using a linear ramp. An exponential window function with 1 Hz line broadening was applied before Fourier transformation. All the measurements were carried out at 298 K. ROESY Experiments. A Bruker Avance DPX-300 spectrometer (7.05 T) was used, by applying the pulse sequence defined in the literature.22 Different spin-lock mixing times (ranging between 200 and 800 ms) were applied to ensure the validity of the linear approximation for the ROE cross-peaks and to obtain the best signal-to-noise ratio, which was achieved with 600 ms. Thirty-two scans were collected in each spectrum. Before the subsequent Fourier transformation and 2D phase tuning, linear prediction in F1 and cosine square apodization in both dimensions were applied to the free induction decays (FIDs). The temperature was kept constant in these experiments at 298 K. Results and Discussion System Gemini Surfactant + Water. Chemical Shifts. The micellization process of 12-EO1-12 has been investigated by NMR spectroscopy. The cmc of a surfactant may be determined through any property that changes sharply around the critical concentration, for example, from the plots of chemical shift or diffusion coefficient vs the surfactant concentration. Two sets of signals can be distinguished in the 1H NMR spectra of the gemini according to its structure (Figure 2). Regarding the hydrophobic tail region,23 it is noted that, for the surfactant in monomeric form, a broad signal composed of two superimposing resonances at δHλ ) 1.268 (br, 28H, (CH2)14) and δHγ ) 1.336 (br, 8H, (CH2)4) can be unambiguously identified as the corresponding nine nonequivalent methylene groups of the alkyl chains. As for the oxyethylene region, the two broad multiplets at δHR′ ) 3.572 (br, 4H, (N(CH2))2) and δHβ′ ) 3.941 (br, 4H, ((CH2)O)2) are assigned to the spacer. At low concentrations, the chemical shifts change smoothly with the gemini concentration, and the extrapolations at infinite dilution provide the property corresponding to the monomer. The relative values of the chemical shifts with respect to the extrapolations vs the surfactant concentration have been plotted in Figure 3. The cmc of the pure surfactant is observed at 1.0 mM, in good agreement with the literature values.24 At concentrations above the cmc, the chemical shifts increase as a result of micelle formation, up to reaching a constant value. All the resonances undergo remarkable downfield shifts in which we can observe two different trends. The relative changes in δ show that protons located close to the head ionic groups undergo larger changes, in a maximum extent of +0.150 ppm for HR at 18.0 mM. In contrast, protons of the alkyl chains change scarcely, in a minimum extent of +0.034 ppm for Hλ. Under fast exchange in the NMR time scale,25 the measured chemical shifts for the corresponding proton i, δiexp, can be expressed as the sum of the chemical shift of the free surfactant, δSi F, and the aggregated form, δSi N, each one averaged with its

β-Cyclodextrin and a Gemini Surfactant

J. Phys. Chem. B, Vol. 110, No. 28, 2006 13821

Figure 2. 1H NMR spectra and proton assignment for 12-EO1-12 in D2O at different concentrations of surfactant.

TABLE 1: Micellization Parameters of 12-EO1-12 Obtained by Non-linear Fitting Analysis of the Chemical Shifts and Self-diffusion Coefficients

respective molar fractions, that is

δiexp ) χSFδSi F + NχSNδSi N

(1)

with N being the aggregation number of the micelles. If a mass action law model, NSF T SN, and the corresponding selfassociation equilibrium, K ) |SN|/|SF|N, are assumed, then the concentration of free monomers and micelles can be expressed as

|SF| ) |ST| - NK|SF|N

Figure 3. Increment in the chemical shifts vs concentration of surfactant for selected protons of 12-EO1-12. Solid lines are the best fit to eq 1. The experimental error of the chemical shifts is (1 × 10-3 ppm.

(2)

with |ST| being the total concentration of gemini. Thus, it is possible to calculate using an iterative nonlinear fitting the chemical shifts of the surfactant in both forms and get values of N and log K, provided these properties remain constant within the fitting interval. The properties thus calculated for the different protons of the surfactant are collected in Table 1. The result for the aggregation number, N ) 34, agrees well with data of Verrall et al.,24 obtained by light scattering measurements (N ) 33). In the case of micelles formation that involves an aggregation number smaller than 100,26 we can obtain, from N and log K ) 98.0, the corresponding stepwise association constant through Kn ) K 1/N, resulting in 763 M-1. This value is intermediate between those obtained for ionic surfactants with higher alkyl chains, tetradecyltrimethylammonium bromide (TTAB, Kn ) 263 M-1) and hexadecyltrimethylammonium bromide (CTAB, Kn ) 1087 M-1),27 which indicates the higher hydrophobicity of the gemini surfactants with respect to its single-chain counterpart analogues. NMR Diffusion Measurements. Further information about the aggregation process can be attained by using data from DOSY experiments through the analysis of the self-diffusion coef-

Hω Hλ Hγ Hβ Hn HR HR′ Hβ′

D

N

log K

δSi F/ppm

δSi N/ppm

δSi N - δSi F/ppm

34 32 33 34 35 34 34 34

98.4 90.0 91.0 98.0 99.9 97.0 97.3 97.7

0.832 1.268 1.336 1.747 3.092 3.328 3.572 3.941

0.880 1.302 1.382 1.787 3.193 3.490 3.675 4.034

0.048 0.034 0.046 0.040 0.101 0.162 0.103 0.093

N

log K

DSF 10-10/m2 s-1

DSN10-10/m2 s-1

34

99.1

3.40

0.51

ficients. These measurements provide qualitative and quantitative information about the molecular association since the micellization involves changes in the hydrodynamic radii of the different surfactant states. These changes can be monitored if the NMR time scale of the diffusion and aggregation process match.28 The expansions of the DOSY spectra at different concentrations of surfactant in water, within the chemical shift range of the Hλ proton, are shown in Figure 4a. One of the features of the DOSY spectra of 12-EO1-12 is that all the spins that belong to the molecule have the same diffusion coefficient at different concentrations. The simple inspection of these plots reveals that the surfactant diffuses more slowly than in the monomer region above the cmc, as a result of self-aggregation, with implications on its effective size. The experimental selfdiffusion coefficients for the surfactant in water solution as a function of the concentration are plotted in Figure 4b. The shape of the curve resembles that of chemical shifts giving the same cmc, and the extrapolation at infinite dilution for the surfactant

13822 J. Phys. Chem. B, Vol. 110, No. 28, 2006

Guerrero-Martıńez et al.

Figure 4. (a) Expansions of the 1H DOSY spectra at the chemical shift range of the Hλ, at different concentrations of surfactant. (b) Self-diffusion coefficients of the binary systems, 12-EO1-12 (b) and β-CD (O). Solid lines are the best fits to eqs 3 and 5. The experimental error of the diffusion coefficients is (5.0 × 10-12 m2 s-1.

in the monomer region yields the diffusion coefficient of the monomer. Due to the fact that aggregation is a dynamic process faster than the NMR time scale, in the limit of the fast exchange, the experimental diffusion coefficient is a weighed average of the coefficient in the monomer and micelle form. The measured value at each concentration can be expressed as

Dexp ) χSFDSF + NχSNDSN

(3)

that is, the sum of the contributions due to the unimer, DSF, and to the micelle, DSN, weighed each one by its mole fraction. To estimate the change in the diffusion coefficient of the unimers upon micellization, we have used the same iterative procedure employed with the chemical shifts. The values of the aggregation number, N, and the micellization constant, K, are in good agreement with those obtained by the former method (Table 1). In the well-known approach of a small spherical particle under no-slip boundary conditions, the hydrodynamic relation between the self-diffusion coefficient, D, and the viscosity, η, is given by the Stokes-Einstein equation

D)

kBT 6πηRh

(4)

where Rh is the hydrodynamic radius of the diffusing particle. The hydrodynamic radius of the monomer, Rh ) 5.7 Å, has been obtained combining the diffusion coefficient of the free monomer, DSF ) 3.4 × 10-10 m2 s-1, and the viscosity of D2O at 25 °C, η ) 1.132 cP, in the preceding equation.29 The change in the diffusion coefficient of 12-EO 1-12 with the concentration has contributions, mainly from the fact that the monomer is incorporated into the micelles but also from the overall change in viscosity.30 We have not detected significant changes in the diffusion coefficient of the solvent in the range of concentrations studied,31 which proves that the contribution of the viscosity in the measured diffusion coefficients of the surfactant is small compared with the increase in its effective size. System β-Cyclodextrin + Water. Diffusion Measurements. The diffusion coefficients for β-CD solutions in D2O as a function of the molarity are plotted in Figure 4b. The experimental points were fitted to the following equation

Dexp ) DCD0 + BD|CD| ) 2.71 × 10-10 6.7 × 10-10|CD| (m2 s-1) (5) where the zero superindex stands for the property at infinite dilution and the BD coefficient accounts for the solute-solute interactions. A negative BD is indicative of a rather slight global increment of the viscosity and/or the formation of low concen-



Figure 6. Molar ratio plots for selected protons of 12-EO1-12 in the presence of β-CD, with [12-EO1-12] ) 0.6 mM. Solid lines are the multivariable fit to a three-site model (eqs 6 and 7). The experimental error of the chemical shifts is (1 × 10-3 ppm.

Figure 5. Expansion of the 1H NMR spectra of 12-EO1-12 + β-CD mixtures at different molar ratios r ) [β-CD]/[12-EO1-12], with [12EO1-12] ) 0.6 mM.

tration of labile aggregates.32 The small value of the slope is in accordance with the nonsignificant changes observed in the diffusion coefficient of water. The value for D0CD is in good agreement with previous results,33 yielding a hydrodynamic radius of the β-CD, Rh ) 7.1 Å, that is reasonable in the light of the known dimensions of CDs from X-ray diffraction and light scattering measurements.34 System Gemini Surfactant + β-CD + Water. Estimation of the Binding Constants: Chemical Shifts. The effect of micellization in the chemical shift of gemini resonances has been avoided using a fixed concentration of surfactant below the cmc (0.6 mM). The concentration of β-CD was varied to obtain different molar ratios, r ) [β-CD]/[12-EO1-12], ranging from pure CD to pure surfactant in water. Representative results of 1H NMR spectra for mixtures at different r values are shown in Figure 5. Upon addition of variable amounts of β-CD, the resonances of all the guest protons undergo shifting (see Figure 1 for notation). HR′ cannot be resolved due to the overlap with the H4 proton of β-CD. Whereas protons far from the methyl group of the alkyl chains Hγ, Hβ, Hn, and Hβ′ move upfield, the resonances of protons located close to the edges, Hω and some of the Hλ, move upfield up to r ) 2 and then conversely shift downfield when r > 2. Since chemical shifts reflect the surrounding chemical environment, the direction and displacement of the signals suggest that the interaction between the surfactant and the CD undergoes a substantial dependence in the stoichiometry upon addition of macrocycle. A similar behavior between other surfactants and R-cyclodextrin has been reported previously,35 in which the deshielding of these nuclei is attributed to the formation of 2:1 channel type complexes in different single-chain surfactants. At the same time, the protons H3 and H5 of β-CD, located in the cavity of the macrocycle,

move upfield, and protons H2 and H4, placed in the external part of the CD, shift downfield. Also, H6, at the rim made up of the primary hydroxyls, undergoes a slight shift to higher fields. The maximum changes registered in some of these resonances reach -0.166 ppm in the case of H5 and -0.075 ppm for Hγ. Assuming stoichiometries 1:1 + 2:1 and a fast exchange on the NMR time scale,36 the measured chemical shifts, δiexp, for the host (CD) or the guest (S) can be expressed as the sum of the contributions of the chemical shifts due to the free molecule, δiF, to the 1:1 complex, δi1:1, and to the 2:1 complex, δi2:1, each one weighted by its mole fraction, that is

δiexp ) χFδiF + χ1:1δi1:1 + nχ2:1δi2:1

(6)

where χF, χ1:1, and χ2:1 refer to the molar fraction of the molecule whose chemical shifts are being observed in the different situations and n is a stoichiometric factor equal to 2 for CD and 1 for the gemini. The concentrations of all the components in solution are connected by the corresponding mass balance and mass action law. The development in terms of the concentration of free host, |CD|, leads to a cubic polynomial equation in the form

(

|CD|F3 + 2|S|T - |CD|T +

(

)

1 |CD|F2 + K2

)

|CD|T 1 1 |S|T - |CD|T + |CD|F ) 0 (7) K2 K1 K1K2

where the T subindex refers to the corresponding total concentration and K1 and K2 are the association constants for the first and second binding step, respectively. The binding constants can be estimated by using a multivariable analysis, fitting the whole set of resonances of the host and guest molecules, imposing the condition that the binding constants must be the same for each proton.37 The chemical shifts for the protons of 12-EO1-12 have been plotted vs the molar ratio in Figure 6. This plot suggests a 2:1 stoichiometry, that is, two molecules of β-CD per one of surfactant, as deduced from the intercept between the extrapolations from high and low molar ratios. We have excluded from the regression analysis the protons Hλ of 12-EO1-12, which are


Guerrero-Martıńez et al.

TABLE 2: Multivariable Non-linear Fitting Analysis of the Chemical Shifts and Self-Diffusion Coefficients for the Complex i,0 β-CD:12-EO1-12 and Chemical Shifts at Infinite Dilution, δ2:1 , in the cmc* Calculations Hγ Hβ Hn HR Hβ′

δiF/ppm

δi1:1/ppm

δi2:1/ppm

δi2:1 - δiF/ ppm

δi,0 2:1/ppm

1.337 1.748 3.088 3.328 3.940

1.377 1.777 3.092 3.342 3.944

1.414 1.803 3.108 3.364 3.954

0.077 0.055 0.020 0.036 0.014

1.409 1.799 3.105 3.360 3.948

K1 ) (8 ( 2) × 103 L mol-1 K2 ) (2.8 ( 0.9) × 104 L mol-1

D12-EO1-12 Dβ-CD

DF/ppm

D1:1/ppm

D1:2/ppm

3.40 2.62

2.45

2.01

K1 ) (1.0 ( 0.5) × 103 L mol-1 K2 ) (5 ( 3) × 104 L mol-1

poorly resolved, and the protons Hω, whose chemical shifts do not converge along the binding isotherm due to a change in the stoichiometry. In addition, the whole proton set of CD has not been included because more data at high r ) [β-CD]/ [12-EO112] should be necessary for a reasonable fit. The results of the regressions are collected in Table 2, yielding binding constants K1 ) (8 ( 5) × 103 L mol-1 and K2 ) (2.8 ( 0.9) × 104 L mol-1. When dealing with multiple equilibria, the standard deviations of the parameters exhibit a comparatively higher value with respect to those obtained for systems with a 1:1 stoichiometry, because of the increased number of variables.36 The higher value of K2 reveals that this complexation process does not follow a simple statistic model.38 From the fitted chemical shifts of the complexes (Table 2), we observe that protons located in the hydrocarbon chains, Hγ and Hβ, have comparatively higher relative changes of the chemical shifts i - δij, than those from the monomer to the 2:1 complex, δ2:1,j registered for protons located close to the headgroups, Hn, HR, and Hβ′. If one compares these results with the calculated changes upon micellization, then we come to the conclusion that, in the formation of complexes with different stoichiometries, the surrounding chemical environment of the headgroups is slightly influenced by complexation. Thus, the CDs must be found remote from the charged quaternary ammonium groups. Estimation of the Binding Constants and Molecular Sizes: Diffusion Measurements. Determination of self-diffusion coefficients in a multicomponent system is a very helpful tool for extracting information about the complexation process.32 Just like chemical shifts, a fixed concentration of surfactant below the cmc (0.6 mM) was used and the concentration of β-CD was varied. In a similar analysis as that described above, the measured diffusion coefficient, Dexp, can be expressed as

Dexp ) χDF + χ1:1D1:1 + nχ2:1D2:1

its effective size due to the inclusion. The diffusion coefficients of the surfactant and the macrocycle vs the surfactant concentration have been plotted in Figure 8. The addition of β-CD slows down the movement of the surfactant. In the case of β-CD, the effect of the complexation on the diffusion coefficients is less marked. A concomitant decrease of the diffusion occurs up to 1.2 mM pointing to a change in the stoichiometry, in accordance with the 1H NMR experiments. We observe largely a 1:1 stoichiometry at low concentrations of CD, which diffuses faster, and a 2:1 stoichiometry when the concentration of β-CD increases, with a slower diffusion. At higher CD concentrations, the diffusion coefficient of the macrocycle becomes higher as a result of an increase in the CD concentration in free form, and the curve in the absence (binary mixture) and presence of surfactant coalesces, indicating that, when the complex forms, it does not interact with other complexes or free macrocycles. The analysis of the self-diffusion coefficient of water signal confirms that there are not significant changes in the global viscosity of the solutions. The stoichiometry, binding constants, and diffusion coefficients of the different complexes can also be determined applying the multivariable analysis previously described to the diffusion coefficient of the surfactant and CD vs the concentration of the macrocycle, given by eqs 7 and 8.36 The fitted binding constants and diffusion coefficients of the different complexes

(8)

where DF refers to the diffusion coefficient of the free molecule (CD or S) and D1:1 and D2:1 stand for the diffusion coefficient of the 1:1 and 2:1 complexes, respectively. Figure 7 shows the DOSY plot of a mixture 0.6 mM 12-EO1-12 and 3.0 mM β-CD obtained according to the procedure described in the NMR Experiments section. Both groups of resonances that correspond to the host or guest molecule have the same diffusion coefficient. The simple inspection of the DOSY spectrum reveals some features of the system under study by comparison with the diffusion coefficient of the pure gemini and CD (Figure 4b). Upon addition of the CD, the surfactant diffuses more slowly than in its free form, which can be attributed to an increase in

Figure 7. 1H DOSY plot for the ternary system: [β-CD] ) 3.0 mM (CD) + [12-EO1-12] ) 0.6 mM (S) + water (W).



Figure 8. Self-diffusion coefficients for the ternary system, 12-EO112 (b) and β-CD (O) ([12-EO1-12] ) 0.6 mM). Solid lines are the best fit to eqs 7 and 8. Dotted line corresponds to the linear fit of the CD binary system. The experimental error of the diffusion coefficients is (5.0 × 10-12 m2 s-1.

are compiled in Table 2, yielding K1 ) (1.0 ( 0.5) × 103 L mol-1 and K2 ) (5 ( 3) × 104 L mol-1. These values follow the same trend as those obtained from the chemical shifts but with higher relative errors. For the rest of the paper, we will use the calculated constants of the previous section due to the much higher experimental points of the fit. The results of the diffusion coefficient of complexes, D1:1 ) 2.45 × 10-10 m2 s-1 and D2:1 ) 1.90 × 10-10 m2 s-1, reveal a plausible decrease in size for the complex, owing to the change in the stoichiometry, that correspond to hydrodynamic radii of 7.9 and 10.1 Å, respectively. Cabaleiro-Lago et al.32 have examined the complexation of β-CD and alkyltrimethylammonium bromide surfactants by diffusion measurements using pulse-gradient spinecho proton NMR. These authors found, in the case of dodecyltrimethylammonium bromide (DoTAB), a 1:1 equilibrium with a constant value and diffusion coefficients of the surfactant and complex of K1 ) (1.9 ( 0.4) × 104 L mol-1, DDoTAB ) 4.96 × 10-10 m2 s-1, and D1:1 ) 2.52 × 10-10 m2 s-1, respectively. These results are in good agreement with our values, taking into account the higher size of the gemini (D12-EO1-12 ) 3.40 × 10-10 m2 s-1). It should be mentioned that the diffusion coefficient of our 1:1 complex, D1:1, is slightly lower than that obtained for DoTAB. This observation could be explained in terms of a partial folding of the free hydrocarbon chain in the external part of the macrocycle, which would lead to a diminution of the expected hydrodynamic radii. cmc* Calculations: Chemical Shifts. A different experiment has been carried out to study the effect of CD on the aggregation of the surfactant. The ternary system has been studied keeping the β-CD concentration constant in 6.0 mM and increasing the concentration of surfactant. As a comparison, chemical shifts of representative protons of the surfactant in these ternary mixtures, together with the corresponding values in the binary system, are plotted in Figure 9. Since at infinite dilution of surfactant the interactions between 12-EO1-12 molecules are negligible, the chemical shift of the different resonances in this limit, δi,0 2:1, corresponds to the interaction between the CD and the surfactant in the complex of the highest stoichiometry (Table 2). These values can be compared with those obtained by the multivariable analysis of the experiments in which the concentration of surfactant was kept constant. An inspection of these

Figure 9. Selected chemical shifts for 12-EO1-12 in water and mixtures with β-CD vs 12-EO1-12 molarity: [β-CD] ) 0 mM (open symbols); [β-CD] ) 6.0 mM (close symbols). Dotted line indicates the calculated cmc*. The experimental error of the chemical shifts is (1 × 10-3 ppm.

results confirms the coherence of δi,0 2:1 with the values of the complex of 2:1 stoichiometry, δi2:1, and shows the goodness of the applied model. The interpretation of Figure 9 in the case of HR and Hn is quite evident. At low concentrations of surfactant, a complex of dominant 2:1 stoichiometry is observed, in agreement with the higher value of K2. The new cmc value, cmc*, is reached at concentrations above that for the pure surfactant. This value can be quantitatively obtained when the concentration of free surfactant in the presence of β-CD reaches the value of 1.0 mM.39 For this purpose, K1 and K2 from the chemical shifts have been used, and the resulting cmc* is 4.4 mM. This shift indicates a mixture of 1:1 + 2:1 stoichiometries and confirms that the competitive equilibrium due to the affinity of the monomer for the micelle or for the β-CD is resolved in favor of the latter. The analysis of the Hβ proton shows an analogous result, in which the chemical shift of the surfactant in the micelle form is between that of the complex and monomer. This resonance is evidence that the curves of the chemical shift of the micellization process in the presence and absence of CD coalesce, providing that the complex does not have any influence on the micelles. It should be mentioned that higher surfactant concentrations would be necessary to detect this behavior in the case of HR and Hn. Structure of the Complexes. For the 2D ROESY experiments on the ternary mixtures, the concentration of gemini was fixed


Guerrero-Martıńez et al. TABLE 3: Normalized Cross-Peaks Intensities for Intermolecular Homonuclear ROE between 12-EO1-12 and β-CD Protons (600 ms mixing time) H3+6

H5

H4

r ) 0.2 Hω Hλ Hγ Hβ Hω Hλ+γ Hβ Hω Hλ+γ Hβ a

1.64 2.61

1.00a r ) 1.6 0.67

2.70

1.00b

0.27 3.30

r ) 5.5 0.06 1.00b

0.28

Signal overlap of H2, H5, and Hλ. b Signal overlap of H5 and Hλ.

Figure 10. Partial view of the 2D ROESY spectra for the [β-CD]/ [12-EO1-12] system ([β-CD] ) 0.6 mM): (a) r ) [β-CD]/[12-EO112] ) 0.2 and (b) r ) 5.5.

below the cmc (0.6 mM) and those of β-CD were 0.12, 0.96, and 3.3 mM (molar ratios CD/surfactant, r ) 0.2, 1.6, and 5.5, respectively). At 300 MHz, the 1H NMR of the different mixtures is crowded and not all the signals can be used in a bidimensional integration without overlap. Figure 10 shows an expanded region of the 2D ROESY spectra that correspond to r ) [β-CD]/ [12-EO1-12] of 0.2 and 5.5, respectively. The inspection of these bidimensional spectra reveals intense crosspeaks between the inner protons of β-CD and some of the gemini surfactant, where a concomitant increment of the CDgemini contacts occurs with the increase of the macrocycle concentration. These results are consistent with the observed changes in the chemical shifts and diffusion coefficients and confirm that the gemini binds favorably into the cavity. Representative numerical values of the relative volume integrals with respect to the ROE cross-peak Hλ{H5} are given in Table 3. From inspection of Figure 10a at r ) 0.2, the resonance corresponding to the methyl protons, Hω, only exhibits a ROE effect with the external proton H4. Also, Hλ shows a strong interaction with H3 and especially with H6. In the spectral region of H2 and H5, there is a signal correlating with Hλ, which renders a quantitative assessment more difficult. However, from the above-mentioned results, the presence of ROE enhancements for the pairs Hω{H4} and Hλ{H6} suggests that these Hλ methylene protons can bind favorably with both H2 and H5.

Figure 11. Proposed structures of the complexes of (a) 1:1 and (b) 2:1 stoichiometries.

Moreover, these results and the simultaneous absence of ROE values for Hγ, Hβ, HR, Hn, HR′, and Hβ′, as expected by the induced chemical shifts, indicate a preference of the cavity for the methylene moiety far away from the charged headgroups. On the basis that the calculated concentrations of complexes at r ) 0.2 are C1:1 ) 0.28 mM and C2:1 ) 0.15 mM, the measured ROEs suggest a possible geometry for the inclusion complex with a main 1:1 host/guest stoichiometry. The lack of intermolecular ROEs between the terminal methyl group and the inner of the macrocycle, together with the interaction that this group undergoes with the external part of the CD, can only be explained in terms of a complete inclusion of one of the surfactant chains, whereas the other tail must be interacting with the exterior of the macrocycle. The cross-peaks point to a complex structure in which the terminal Hλ methylene groups of one chain must be found at the narrower rim, whereas the Hλ remaining methylenes are directly interacting with the wider rim (Figure 11a). The trend of the relative volume integrals indicate that the relationship of CD and surfactant has a remarkable effect on the ROE transfer. The experiment shows that the relative intensities of the ROEs differ with each other to some extent at

β-Cyclodextrin and a Gemini Surfactant different concentrations of β-CD, and this might reflect a change in the site-specific interactions as a result of a modification of the stoichiometry. The situation at higher concentrations of CD, r ) 5.5, exhibits some differences with respect to r ) 0.2, showing ROE effects between the cavity protons H3+6 and the resonances Hγ and Hω (Figure 10b) and an important decrease of the interaction between the proton Hω and the external part of β-CD. On the basis of these results and the calculated concentrations of complexes at r ) 5.5, C1:1 ) 0.001 mM, and C2:1 ) 0.021 mM, we can associate this finding with the complexation of the free chain at low r values by a second CD, which would lead to a close contact of the first complexed CD with Hγ. These evidences are compatible with two possible geometries for the inclusion complex without any preference in the orientation of the second macrocycle (Figure 11b). It must be noticed that these structures account for the remarkable changes in the chemical shifts of Hγ (more than 0.07 ppm) and with the fact that Hω first moves upfield to r ) 2 and then conversely shifts downfield when r > 2. Conclusions Different NMR experiments have been conducted to study the micellization process of the gemini 12-EO1-12 and its inclusion complex formed with β-cyclodextrin, as well as the pure β-CD in solution. The aggregation number, stepwise micellization constant, self-diffusion coefficients, and the chemical shifts for different resonances of the free monomer and the aggregated form were determined by using a nonlinear fitting. The structure and binding of the inclusion complexes between gemini and β-CD have been studied by 1D proton NMR, DOSY, and 2D ROESY experiments. When the surfactant concentration is lower than the cmc, the system is a mixture of two complexes, CD:S and CD2:S. From the calculated diffusion coefficients of these complexes, it is concluded that the 2:1 complex is significantly larger than the 1:1 complex. The two binding constants obtained by a multivariable nonlinear regression analysis of the chemical shifts are K1 ) (8 ( 5) × 103 L mol-1 and K2 ) (2.8 ( 0.9) × 104 L mol-1. The fact that K2 > K1 means that the complexation of both CDs are not independent processes and that a pure statistical model is not followed. ROESY experiments reveal an evolution of the ROE contacts with the stoichiometry of the complexes. This is also evidenced by the changes of the chemical shifts. At low r values (r ) [β-CD]/[12-EO1-12]), the ROESY spectrum shows the formation of a 1:1 complex in which one tail of the surfactant enters the cavity through the wide rim, with the other chain interacting with the external surface of the macrocycle. All the spectra display no contacts between the protons close to the charged headgroups with the β-CD and a strong interaction between the internal cavity protons and the Hλ and Hω. When r increases, a new cross-peak with Hγ rises. In addition, the diminution of the interaction between the aliphatic chain and the external part of the macrocycle is in good agreement with the increment of the concentration of the 2:1 complex. These experimental evidences indicate that the second included CD induces a movement to a deeper position of the first CD in the gemini hydrophobic moiety. This result shows a cooperative effect and an interaction between both CDs resulting in a higher value of K2. The presence of the macrocycle shifts the aggregation of the gemini to higher concentrations of surfactant. The calculated value of the cmc* using the obtained binding constants is in agreement with the experimental results confirming that the inclusion of both hydrophobic tails is not independent. Further

J. Phys. Chem. B, Vol. 110, No. 28, 2006 13827 thermodynamic studies to elucidate the changes in the hydration state of the surfactant hydrocarbon chains and β-CD are currently in progress. Acknowledgment. The authors acknowledge financial support from the U.C.M. (PR1/06-14507) and Ministerio de Ciencia y Tecnologıá (BQU2001-1426-CO2-01). References and Notes (1) (a) Atwood, J. L.; Davies, J. E. D.; Macnicol, D. D.; Vo¨gtle, F. C. ComprehensiVe Supramolecular Chemistry (Volume 2: Molecular Recognition: Receptors for Molecular Guests); Pergamon: Tarritown, NY, 1996. (b) Rivera, J. M.; Martin, T.; Rebek, J. J. Am. Chem. Soc. 2001, 123, 5213. (c) Cheetham, A. G.; Hutchings, M. G.; Claridge, T. D. W.; Anderson, H. L. Angew. Chem., Int. Ed. 2006, 45, 1596. (2) (a) Atwood, J. L.; Davies, J. E. D.; Macnicol, D. D.; Vo¨gtle, F. C. ComprehensiVe Supramolecular Chemistry (Volume 3: Cyclodextrins); Szejtli, J., Osa, T., Eds.; Pergamon: Tarritown, NY, 1996. (b) Saenger, W. Angew. Chem. 1980, 92, 343; Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (3) (a) Szejtli, J. Cyclodextrins and Their Inclusion Complexes; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1988. (b) Duchene, D. Cyclodextrins and Their Industrial Uses; Editions de Sante´: Paris, 1987. (c) Duchene, D. New Trends in Cyclodextrins and DeriVatiVes; Editions de Sante´: Paris, 1991. (d) Hedges, A. R. Chem. ReV. 1998, 98, 20352044. (4) Nicolle, G. M.; Merbach, A. E. Chem. Commun. 2004, 854. (5) (a) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (b) Zana, R.; Berraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (6) (a) Zana, R.; Talmon, Y. Nature 1993, 362, 229. (b) Rosen, M. J. CHEMTECH 1993, 23, 30. (7) (a) Renouf, P.; Mioskowski, C.; Lebeau, L. Tetrahedron Lett. 1998, 39, 1357. (b) Sumida, Y.; Masuyama, A.; Maekawa, H.; Takasu, M.; Kida, T.; Nakatsuji, Y.; Ikeda, I.; Nojima, M. Chem. Commun. 1998, 2385. (c) Menger, F. M.; Migulin, V. A. J. Org. Chem. 1999, 64, 8916. (8) (a) Rosen, M. J.; Song, L. D. J. Colloid Interface Sci. 1996, 179, 261. (b) Zana, R. J. Colloid Interface Sci. 2002, 248, 203. (9) Wang, X.; Wang, J.; Wang, Y.; Ye, J.; Yan, H.; Thomas, R. K. J. Phys. Chem. B 2003, 107, 11428. (10) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Kwetkat, K.; Zana, R. J. Colloid Interface Sci. 2001, 235, 119. (11) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. 1998, 100, 11664. (12) Bai, G.; Wang, Y.; Yan, H.; Thomas, R. K.; Kwak, J. C. T. J. Phys. Chem. B 2002, 106, 2153. (13) Huc, I.; Oda, R. Chem. Commun. 1999, 2025. (14) Wetting, S. D.; Nowak, P.; Verrall, R. E. Langmuir 2002, 18, 5354. (15) (a) Abrahmsen-Alami, S.; Alami, E.; Eastoe, J.; Cosgrove, T. J. Colloid Interface Sci. 2002, 1, 191. (b) Alami, E.; Abrahmsen-Alami, S.; Eastoe, J.; Grillo, I.; Heenan, R. K. J. Colloid Interface Sci. 2002, 15, 403. (c) Qui, X. M.; Li, L.; Wei, X. L.; Yin, B. L.; Sun, D. Z. Acta Phys.Chim. Sin. 2005, 21, 1415. (16) Berger, S.; Braun, S. 200 and More Basic NMR Experiments, A Practical Course; Wiley-VCH: Weinheim, Germany, 2004. (17) Morris, K. F.; Johnson, C. S. J. Am. Chem. Soc. 1993, 115, 4291. Cabrita, E. J.; Berger, S. Magn. Reson. Chem. 2001, 39, 142. Cameron, K. S.; Fielding, L. Magn. Reson. Chem. 2002, 40, 106. (18) Cohen, Y.; Avram, L.; Frish, L. Angew. Chem., Int. Ed. 2005, 44, 520. (19) (a) Neuhaus, D.; Williamson, M. P. The nuclear OVerhauser effect in structural and conformatinal analysis, 2nd ed.; Wiley-VCH: Chichester, U.K., 2000. (b) Noggle, J. H.; Schirmer, R. E. The Nuclear OVerhauser Effect: Chemical Applications; Academic Press: New York, 1971. (20) Gottlieb, H. E.; Kotlyar, V.; Nudelmicen, A. J. Org. Chem. 1997, 62, 7512. (21) (a) Johnson, C. S. Prog. NMR Spectrosc. 1999, 34, 203. (b) Gibbs, S.; Johnson, C. S. J. Magn. Reson. 1999, 93, 395. (22) Bax, A.; Davis, D. G. J. Magn. Reson. 1985, 63, 207. (23) Funasaki, N.; Ishikawa, S.; Neya, S. J. Phys. Chem. B 2004, 108, 9593. (24) Wetting, S. D.; Li, X. F.; Verrall, R. E. Langmuir 2003, 19, 3666. (25) (a) Persson, B. O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124. (b) Persson, B. O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1979, 83, 3011. (26) Evans, D. F.; Wennersto¨m, H. The Colloid Domain. Where Physics, Chemistry, and Biology Meet; John Wiley & Sons: New York, 1999; pp 157-160.

13828 J. Phys. Chem. B, Vol. 110, No. 28, 2006 (27) Gonza´lez-Gaitano, G.; Crespo, A.; Tardajos G. J. Phys. Chem. B 2000, 104, 1869. (28) Berger, S.; Cabrita, E. J. Magn. Reson. 2002, 40, 122. (29) Kestin, J.; Imaishi, N.; Nieuwoudt, J. C.; Sengers, J. V.; Nott, S. H. Physica A 1985, 38. (30) (a) Borse, M. S.; Devi, S.; Colloids Surf., A 2004, 245, 1. (b) Han, L. J.; Chen, H. Surf. Sci. 2004, 564, 141. (31) Avram, L.; Cohen, Y. Org. Lett. 2002, 4, 4365. (32) (a) Gonza´lez-Gaitano, G.; Rodrı´guez, P.; Isasi, J. R.; Fuentes, M.; Tardajos, G.; Sańchez, M. J. Inclusion Phenom. Macrocyclic Chem. 2002, 44, 101. (b) Bonini, M.; Rossi, S.; Karlsson, G.; Lo Nostro, P.; Baglioni, P. Langmuir 2006, 22, 1478. (33) Cabaleiro-Lago, C.; Nilsson, M.; So¨derman, O. Langmuir 2005, 21, 11637.

Guerrero-Martıńez et al. (34) (a) Gonza´lez-Gaitano, G.; Tardajos, G.; Brown, W. J. Phys. Chem. B 1997, 101, 710. (b) McMullan, R. K.; Saenger, W.; Fayos, J.; Mootz, D. Carbohydr. Res. 1973, 31, 37. (35) Wilson, L. D.; Verrall, R. E. Can. J. Chem. 1998, 76, 25. (36) Connors, K. A. Binding Constants. The Measurement of Molecular Complex Stability; John Wiley & Sons: New York, 1987; pp 189-192. (37) Gonza´lez-Gaitano, G.; Sainz-Rozas, P. R.; Isasi, J. R.; GuerreroMartıńez, A.; Tardajos, G. J. Phys. Chem. B 2004, 108, 14154. (38) (a) Connors, K. A.; Paulson, A.; Toledo-Velasquez, D. J. Org. Chem. 1988, 53, 2023. (b) Funasaki, N.; Ishikawa, S.; Hirota, S. Anal. Chim. Acta 2006, 555, 278. (39) Funasaki, N.; Yodo, H.; Hada, S.; Neya, S. Bull. Chem. Soc. Jpn. 1992, 65, 1323.

Inclusion Complexes between Î²-Cyclodextrin and a Gemini Surfactant

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