Micellar behavior of aqueous solutions of dodecyldimethylethylammonium bromide, dodecyltrimethylammonium chloride and tetradecyltrimethylammonium chloride in the presence of α-, β-, HPβ- and γ-cyclodextrins

Journal of Colloid and Interface Science 321 (2008) 442–451 www.elsevier.com/locate/jcis

Micellar behavior of aqueous solutions of dodecyldimethylethylammonium bromide, dodecyltrimethylammonium chloride and tetradecyltrimethylammonium chloride in the presence of α-, β-, HPβ- and γ -cyclodextrins S.K. Mehta a,∗ , K.K. Bhasin a , Shilpee Dham a , M.L. Singla b a Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University, Chandigarh-160 014, India b Central Scientific and Instrumentation Organisation, Sector 30, Chandigarh, India

Received 12 December 2007; accepted 15 February 2008 Available online 4 March 2008

Abstract Conductivity, static fluorescence and 1 H NMR measurements have been carried out to study the micellar behavior of aqueous solutions of dodecyldimethylethylammonium bromide (DDAB), dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TDAC) in absence and presence of α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), hydroxypropyl-β-cyclodextrin (HPβ-CD) and γ -cyclodextrin (γ -CD). The conductivity measurements were carried out at 298.15 K. The influence of cyclodextrins on the micellar parameters, such as cmc∗ (apparent critical micellar concentration), β (degree of ionization) have been analyzed. Thermodynamics of the systems was discussed in terms of the change in standard free energy of micellization, G0m . Micellization was found to be less spontaneous in presence of cyclodextrins. The fluorescence intensity of the surfactant solutions is enhanced by the addition of cyclodextrins. The association constants obtained from conductivity and fluorescence data suggest the binding of γ -CD with the surfactants to be strongest among all the cyclodextrins used. 1 H NMR chemical shift changes provide powerful means for probing the cyclodextrin–micellar interactions and inclusion of surfactant is shown by the change in the chemical shift of some of the guest and host protons in comparison with the chemical shifts of the same protons in the free compounds. © 2008 Elsevier Inc. All rights reserved. Keywords: DDAB; DTAC; TDAC; Cyclodextrin; Conductivity; Fluorescence; 1 H NMR

1. Introduction Cyclodextrins (CDs) are cyclic oligoglycosides made up of six to eight (α = 6, β = 7, γ = 8) D-glucose monomers linked covalently at 1 and 4 carbon atoms. The internal cavities are relatively hydrophobic which gives CDs ability to form inclusion complexes with a variety of organic and inorganic molecules in aqueous solution. Accommodation of the guest molecule depends on its size and polarity and on the size of the particular cyclodextrin. CDs, as a result of their complexation ability and other versatile characteristics, are continuing to have different * Corresponding author. Fax: +91 172 2545074.

E-mail address: [email protected] (S.K. Mehta). 0021-9797/$ – see front matter © 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.02.030

applications in different areas of drug delivery and pharmaceutical industry [1–13]. Surfactant molecules, [S], which have an ionic head group as well as a large hydrocarbon chain of varying hydrophobicity, are expected to form complex with cyclodextrins by the inclusion of the hydrophobic chain of the surfactant into the apolar cavity of the cyclodextrin, affecting the micellization process of the surfactant itself [14–20]. Inclusion complexes, formed between surfactants and cyclodextrins have been a subject of interest for further exploration by scientists since these systems can be used to mimic the effect of cyclodextrins on phospholipids (a major constituent of cell membrane) [21]. The importance of these inclusion complexes has been well appreciated and established from biophysical point of view, therefore,

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efforts are required to study the physicochemical properties and behavior of surfactants and cyclodextrins in solution and at interfaces to further improve our understanding of the CD/S interactions. A number of studies are focused on interaction between surfactant with β-CD and its derivatives only. However, much work has not been done on interactions of other forms of cyclodextrins (α- and γ -CD) with surfactants. Therefore, association between surfactants and cyclodextrins requires further investigations and explorations using all the different forms of cyclodextrins. The objective of the present study is to study the encapsulation processes of cationic dodecyldimethylethylammonium bromide, dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride (quaternary ammonium compounds) into the cavity of α-, β-, HPβ- and γ -cyclodextrins and its effect in the micellization process of the surfactant itself. Conductometric measurements and spectroscopic studies were done to predict the thermodynamics and to understand the type of interactions existing in the system respectively. In all, the influence of the presence of the inclusion complex on the micellization process of the surfactant has been focused in detail.

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2.2.2. Fluorescence spectroscopy Fluorescence measurements were carried out with VARIAN spectrophotometer. A 1 cm rectangular silica cell was placed in a multicell holder whose temperature was kept constant at 298.15 K with a recirculating water circuit. Pyrene was used as luminescence probe and its concentration was kept constant at 10−3 M. Both excitation and emission band slits were fixed at 5 nm, and the scan rate was selected at 500 nm/min. The excitation wavelength was selected at 340 nm, while the emission spectra were collected from 350 to 450 nm. In the determination of aggregation numbers, the concentration of the quencher, hexadecylpyridinium chloride, was held low enough so as not to interfere with the assembly of the micelle. The ratio of the fluorescence intensity of the highest energy vibrational band (II ) to the fluorescence intensity of the third highest energy vibrational band (IIII ) has been utilized to investigate the formation of surfactant micelle.

2. Materials and methods

2.2.3. 1 H NMR spectroscopy 1 H NMR for the binary and ternary systems of the surfactant was recorded in deuterated water (D2 O) using BRUKER AVANCE II (400 MHz) spectrometer. Chemical shifts are represented in ppm relative to tetramethylsilane as an internal standard (δ = 0 ppm).

2.1. Materials

3. Results and discussion

Dodecyldimethylethylammonium bromide (DDAB) (purity >98%), dodecyltrimethylammonium chloride (DTAC) (purity >99%), tetradecyltrimethylammonium chloride (TDAC) (purity >98%), β-cyclodextrin (purity >99%), HPβ-cyclodextrin (purity >98%), γ -cyclodextrin (purity >98%) and pyrene (purity 97%) were purchased from Fluka while α-cyclodextrin (purity >98%) and hexadecylpyridinium chloride (purity 97%) were from HIMEDIA. Potassium chloride (purity >99%) and D2 O (99.9% atom D) was purchased from Merck and Aldrich, respectively. All the chemicals were used without further purification. Water used for the preparation of samples was deionized and triply distilled (conductivity lower than 3 µS). 2.2. Methods 2.2.1. Conductivity studies The conductivity of mixtures was measured in a thermostatic glass cell with two platinum electrodes and Pico conductivity meter from Lab India. The conductivity meter was calibrated by measuring the conductivity of the solutions of potassium chloride of different concentrations (0.001, 0.01 and 0.1 M). Electrodes were inserted in a double walled glass cell containing the solution. The glass cell was connected to the thermostat controlled to better than ±0.01 K temperature variation. The cell constant of the cell used was 1.01 cm−1 . The measurement of conductivity was carried out with an absolute accuracy up to ±3%. The solutions were prepared by weight using an electronic balance with an accuracy of ±1 × 10−4 g.

Aqueous solutions of the ternary systems of dodecyldimethylethylammonium bromide, dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride and various cyclodextrins (see Fig. I in Supplementary material) have been characterized through conductivity, fluorescence and NMR spectroscopy measurements. 3.1. Conductivity measurements Figs. 1–3 show the plots of the specific conductivity κ vs [S] for solutions of DDAB, DTAC and TDAC, containing various constant concentrations of α-, β-, HPβ-, and γ -cyclodextrins. The inflection observed in all curves at a certain concentration of surfactant is considered to be the cmc of the micelles. In the presence of different cyclodextrin at different concentrations, here it is termed as apparent cmc or cmc∗ . The lower curve in all the above graphs (Figs. 1–3) shows the behavior of binary S/W system. Before the cmc∗ is reached, another phenomenon occurs when the surfactant is added to the CD solution, i.e., the formation of the inclusion complex: CD + S ↔ CDS. The association between the CD and the surfactant is stronger in comparison with the micelle formation and thus the addition of surfactant shifts the above equilibrium toward the complex formation. When all the CD molecules present in the solution are complexed, the addition of more surfactant leads to the formation of micelles: nS ↔ Sn .

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Fig. 1. Conductivity plots for S/CD/W ternary system in comparison to binary S/W system at 298.15 K for different cyclodextrins at constant concentration of 0.001 M.

Fig. 2. Conductivity plots for S/CD/W ternary system in comparison to binary S/W system at 298.15 K for different cyclodextrins at constant concentration of 0.005 M.

The dynamics of the system can be visualized in the way that, initially the system is comprised of constant CD molecules only. Most of the added surfactant participates in formation of complex with cyclodextrin and thus there is little increase in the surfactant free monomer concentration. With more increase in surfactant concentration, the cyclodextrin free monomer concentration decreases and the surfactant free monomer concentration increases. Incorporation of surfactant monomers into the CD cavity leads to a decrease in the amount of “free” surfactant

monomers available in solution, making that higher concentrations of surfactant are needed to form micelles and thus the surfactant cmc bears considerable shift in the presence of cyclodextrin. The cmc∗ of DDAB increases linearly with CD concentration (Fig. 4) from the cmc of the pure surfactant (cmc∗ ) = (cmc + 0.62[α-CD]), (cmc + 0.79[β-CD]), (cmc + 0.79[HPβCD]), (cmc + 1.2[γ -CD]). The intercept of the plot is consistent with the experimental cmc of pure DDAB previously

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Fig. 3. Conductivity plots for S/CD/W ternary system in comparison to binary S/W system at 298.15 K for different cyclodextrins at constant concentration of 0.01 M.

Fig. 5. Molar conductivity plots at 298.15 K for the ternary system DDAB/HPβ-CD/W in comparison to binary DDAB/W system as a function of [DDAB] at 0.01 M [HPβ-CD]. Fig. 4. Variation of cmc for DDAB in presence of different cyclodextrins.

obtained [22]. This fact is due to the formation of the inclusion complex DDAB:CD. Similar linear dependence was observed for DTAC and TDAC systems. Since the association between these host and guest species is tighter than that between the monomers to form the micelles [18,23], 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.

However, complex formation, which occurs also in the first part of the curves of Figs. 1–3 (before the cmc∗ ), is detected clearly in the plots of the molar conductance, Λ, vs [surfactant] as shown in Fig. 5. As can be seen, the plot shows a double change in the property for the ternary system, which is different from the curve corresponding to the pure surfactant in the absence of cyclodextrin, where only one change is observed. The two changes observed for the ternary S/CD/W system are related to the complex and micelles (cmc∗ ) formation. How-

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Table 1 Values of cmc∗ (M) for DDAB, DTAC and TDAC at 298.15 K in presence of different cyclodextrins at various constant concentrations CD

Concentration (M) 0

0.001

0.005

0.01

Table 2 Values of the free energy of micellization, G0m (kJ mol−1 ), for DDAB, DTAC and TDAC at 298.15 K for different cyclodextrins at various constant concentrations CD

0.001

α-CD β-CD HPβ-CD γ -CD

−35.75 −35.75 −35.75 −35.75

−35.16 −35.36 −34.95 −35.09

α-CD β-CD HPβ-CD γ -CD

−31.53 −31.53 −31.53 −31.53

−30.30 −31.28 −30.70 −31.02

α-CD β-CD HPβ-CD γ -CD

−37.68 −37.68 −37.68 −37.68

−37.56 −36.89 −37.34 −34.10

DDAB α-CD β-CD HPβ-CD γ -CD

0.014 0.014 0.014 0.014

0.016 0.016 0.016 0.017

α-CD β-CD HPβ-CD γ -CD

0.021 0.021 0.021 0.021

0.022 0.023 0.023 0.027

α-CD β-CD HPβ-CD γ -CD

0.006 0.006 0.006 0.006

0.006 0.006 0.006 0.007

0.017 0.018 0.018 0.019

0.021 0.022 0.022 0.027

0.025 0.026 0.027 0.028

0.028 0.030 0.030 0.034

0.008 0.009 0.009 0.012

0.011 0.012 0.012 0.013

DTAC

TDAC

ever, the curve of the pure surfactant shows only the second change (cmc). Moreover, Λ decreases more sharply in the presence than in the absence of cyclodextrin because the surfactant mobility is decreased upon complexation. Similar behavior was observed for other systems. From the first change (Fig. 5), the stoichiometry (A) of the complex can be determined as the ratio between [HPβ-CD] and [DDAB], [HPβ-CD] being the initial cyclodextrin concentration which is kept constant and [DDAB] the surfactant concentration at which the first change in the property is observed. The average value obtained for all the systems is A = 1 ± 0.06, indicating that the complex 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 βCD/D12 EDMAB, DIMEB/D12 EDMAB, β-CD/D12 TAB, and HPβ-CD/D12 TAB in aqueous solutions [20,24]. Thus, it can be deduced, that for host molecules, the stoichiometry of the complexes formed with the hydrophobic tail of a surfactant is 1:1 for the surfactants belonging to same class, e.g., quaternary ammonium salts (in present study), being unaffected by the size of the polar head and increase in tail of the monomer. The cmc values for all the systems as a function of cyclodextrin concentration were estimated from Philips method [25] and are summarized in Table 1. The cmc values for all the threesurfactant system are higher in the presence of cyclodextrin than those in pure water, and the values increase with the increase in the amount of cyclodextrin. It can be also observed that in the postmicellar region (Figs. 1–3) all the curves present the same slopes, Sm (DDAB: Sm  20.0), (DTAC: Sm  29.0) and (TDAC: Sm  30.0), independently of different concentrations of each type of cyclodextrin. Since the concentration of the complex is basically constant in this postmicellar zone, all the changes observed in the conductivity in this region can be assigned to the micelles. This feature implies that the micelles are the same independently of the cyclodextrin concentration, thus the cyclodextrin does not participate in the micelle. In that case, it is possible to say that

Concentration (M) 0

0.005

0.01

−34.89 −34.28 −34.48 −34.62

−33.59 −33.38 −33.38 −32.91

−30.19 −30.99 −29.50 −30.10

−29.16 −29.83 −28.89 −28.80

−35.76 −35.06 −35.27 −33.89

−35.31 −34.94 −34.52 −33.77

[DDAB]

[DTAC]

[TDAC]

the aggregation number is constant, whether the surfactant is in the presence or in the absence of the cyclodextrin which implies that addition of cyclodextrin do not hinder the structure of micelle rather it only delays the process of micellization as evident from the conductivity curves for ternary systems. Another interesting feature is the effect of the CD and/or the complex on the micellar parameters, e.g., degree of ionization, β. The degree of ionization of the micelles estimated from the ratio of the slopes of the two intersecting lines below and above cmc. The parallelism on the curves of κ or Λ in Figs. 1–3, 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 (Figs. 1–3) to be nearly same for DDAB, DTAC and TDAC analogous to that of the respective surfactants in water in the absence of CD (see Table I in Supplementary material). In accordance with the charged pseudo-phase separation model, the standard free energy of micellization, G0m , for cationic surfactant was calculated from the relation G0m = (2 − β)RT ln Xcmc ,

(1)

where Xcmc is the cmc value expressed in terms of mole fraction. The estimated values of G0m are summarized in Table 2. The free energy values for all DDAB, DTAC and TDAC ternary systems fall within the range of values for amphoteric and ionic surfactants, i.e. between −23 and −42 kJ mol−1 at 298.15 K [26]. The trend in G0m values for the ternary S/CD/W system of DDAB, DTAC and TDAC in comparison to the binary S/W system shows that the micellization is less favored in presence of increasing concentration of cyclodextrins. Also the results show that the micellization become less spontaneous as we move from α-CD to γ -CD for most of the systems. The negative value

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Table 3 Values of the association constant, K (M−1 ), for S/CD/W at 298.15 K calculated from conductometric studies S

CD α-CD

β-CD

HPβ-CD

γ -CD

DDAB DTAC TDAC

132 ± 0.79 102 ± 0.08 202 ± 0.02

210 ± 0.05 219 ± 0.06 281 ± 0.02

211 ± 0.07 313 ± 0.07 314 ± 0.04

403 ± 0.16 727 ± 0.27 819 ± 0.15

of G0m in general indicates the onset of hydrophobic interactions between the hydrocarbon chain of surfactant molecule and the interior of cyclodextrin cavity. Association constants As discussed above, the apparent cmc values of DDAB (Fig. 4), DTAC and TDAC are linearly correlated with the CD concentration, which indicates that more surfactant monomers form inclusion complexes with CD and lead to a delay in the micelle formation. From these data, we are able to evaluate association constant (K) between surfactant and CD in the submicellar concentration by assuming a 1:1 association stoichiometry [27] as given by CD + S ↔ CD:S, [CD:S] K= or [CD:S] = K[CD][S], [CD][S] [CD]t = [CD] + [CD:S],

(2) (3) (4)

[CD]t = [CD] + K[CD][S],   [CD]t = [CD] 1 + K[S] ,

(5)

[S]t = [S] + [CD:S].

(7)

(6)

By substituting Eq. (3) into (7), we get [S]t = [S] + K[CD][S].

(8)

By substituting Eq. (6) into (8), we get [S]t = [S] +

K[S][CDt ] , 1 + K[S]

(9)

where K, CD, S, CD:S, CDt and St are the association constant, free CD, free surfactant, inclusion complex, total CD, and total surfactant concentrations at cmc, respectively. Equation (9) predicts a linear correlation between St and CDt which we also observe in Fig. 4. This leads to determination of association constants of CD:S systems under study and the values obtained are collectively presented in Table 3. The higher values in case of γ -CD:S systems indicate that surfactant binds more strongly with γ -CD than other cyclodextrins. 3.2. Fluorescence studies Fluorescence probe analysis constitutes a simple yet a very versatile technique for the study of static and dynamic properties of aggregated systems such as micelles. When a hydrophobic probe such as pyrene is transferred from the aqueous bulk to the micellar phase, the change in the microenvironment experienced by the probe is often reflected in the relative intensity of

Fig. 6. Emission fluorescence spectra of a 10−3 M solution of pyrene in aqueous micellar solutions of DTAC at 298.15 K: curve 1, in the absence of Q (CePyCl); curves 2–10, in the presence of CePyCl, with a concentration ranging from 1 × 10−5 to 18 × 10−5 M.

the vibronic fine structures of the monomer fluorescence. The study can be used in the calculation of aggregation number of the micelles and also in understanding the host-guest inclusion processes. 3.2.1. Aggregation number The aggregation number of the monomers in the micelles, n, can be determined from the steady-state fluorescence data [14,21,28,29], 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 is ln I = ln I0 − [Q]/Cm = ln I0 − n[Q]/(Ct − cmc),

(10)

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. Fig. 6 reports the pyrene emission spectra for micellar solution of DTAC in the presence of several quencher concentrations. Similar types of graphs were obtained for DDAB and TDAC systems. Fig. 7 shows a plot of ln I vs [Q]. From the slope in this plot and the cmc previously determined from conductivity data, a value of n = 47 has been obtained for the aggregation number of DDAB and DTAC in water, which is slightly lower than that obtained for TDAC (n = 50). 3.2.2. Host–guest inclusion process The investigation on the cyclodextrin (CD) molecule with fluorophore used as a sensor to detect foreign species via identifying the changes in fluorescence intensity shows a profound meaning in the molecular recognition field [30–33]. The obtained results were very interesting and a significant enhancement of the guest fluorescence with increasing cyclodextrin concentration was observed in all the cases, which means that the appended moiety i.e., the hydrocarbon chain of surfactant is moving more deeply in to the CD cavity. Fig. 8 depicts the representative spectra of DTAC:γ -CD system. This phenomenon may attribute to the hydrophobic interactions between the CD cavity and the hydrocarbon chain. When the hydrocarbon chain

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Fig. 7. Plot of the logarithm of the fluorescence intensity (ln I ) as a function of the CePyCl concentration for a 10−3 M solution of pyrene in aqueous micellar solutions of DDAB, DTAC and TDAC.

Fig. 8. Emission fluorescence spectra of an aqueous solution of DTAC at constant concentration (0.01 M) at 298.15 K, in the absence and presence of different concentrations of γ -CD (0.0–0.01 M). Table 4 Effects of various cyclodextrins (10 mM) on the maximum emission wavelength (λF, max ) of 393 nm and fluorescence enhancement (F /F0 , relative to no CD) of different surfactants in aqueous media CD

[DDAB] (0.007 M)

[DTAC] (0.01 M)

[TDAC] (0.002 M)

α-CD β-CD HPβ-CD γ -CD

2.15 6.51 4.28 6.76

2.19 6.02 3.66 7.19

2.84 6.31 5.11 6.87

is located in the cavity of CD, it can emit stronger fluorescence than that in the aqueous solution owing to better protection from quenching and other processes occurring in the bulk solvent. Table 4 lists the enhancement, F /F0 , for all of the cyclodextrins studied, at 10 mM concentration, where F and F0 are the fluorescence intensity in the presence and absence of CD. For all the CDs, γ -CD gave a larger enhancement than did α-, β- and HPβ-CD, suggesting a better size match of surfactant

Fig. 9. Benesi–Hildebrand plot for 1:1 TDAC:CD inclusion complex.

monomers with γ -CD and so on. Also, indicating that with increase in cavity size surfactant moiety is experiencing an increasingly less polar cavity upon inclusion. As can be seen from Table 4, γ -CD gives the strongest enhancement in all the systems due to the less polar local environment of the guest molecules in the cavity of the hosts as compared to aqueous solution. 3.2.3. Association constants for the S:CD complexes The enhancement in the fluorescence observed in all systems can be used in estimating the stoichiometry and association constants involved in the complex formation process [34]. For

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Table 5 Values of the association constant, K (M−1 ), for S/CD/W at 298.15 K calculated from fluorescence studies S DDAB DTAC TDAC

CD α-CD

β-CD

HPβ-CD

γ -CD

707 ± 35 887 ± 50 1116 ± 78

13272 ± 155 13391 ± 175 13806 ± 200

5248 ± 250 5544 ± 288 9099 ± 312

14007 ± 345 20032 ± 350 36922 ± 427 Fig. 10. Schematic representation of relative positions of cyclodextrin protons.

1:1 complexes, the enhancement is related to the added host concentration ([CD]) according to the equation [CD]K F = 1 + (F∞ /F0 − 1) F0 1 + [CD]K

3.3. Mode of host–guest interaction: 1 H NMR studies 1H

(11)

where F and F0 are the fluorescence intensities in the presence and absence of added host (cyclodextrin) and F∞ /F0 is the fluorescence enhancement at infinite host concentration i.e. when all of the guests have been included into the host cavity. Also, the double reciprocal plot or Benesi–Hildebrand plot [34,35] of 1/(F /F0 − 1) vs 1/[CD] will be linear if only 1:1 complexes form. Equation (11) provides a good fit to the experimental data and linearity of the plots clearly depicts the existence of 1:1 complexes (Fig. 9). The association constants obtained are tabulated in Table 5. The results in Table 5 reveals that, for a given “guest” molecule (S), the γ -cyclodextrin form stronger associated inclusion complexes than the other cyclodextrins. Also, the association constants are highest for TDAC, which means that the values vary with the chain length of the surfactant and are in good agreement with the findings of Lu and co-workers [36]. The present K value for CD:S is different with those evaluated by conductometric methods which clearly indicate that the association constant between host–guest molecules significantly depends upon the technique/method used for their evaluation [14,20,37]. Maeso et al. [28] used X-ray crystallography and calculated the diameter and depth of α-CD, β-CD, HPβ-CD and γ -CD as 5.3, 7.8 Å; 6.5, 7.8 Å; 6.5, 7.8 Å and 8.3, 7.8 Å, respectively (see Fig. II in Supplementary material). The following equations were used by Rafati et al. [17] to calculate the length, l, and diameter, d, of the structural alkyl group with formula Cn H2n+1 : l (Å) ≈ 1.5 + 1.265(n − 1), (12)   1/2 . d (Å) ≈ 34.89 + 34.25(n − 1) 1.5 + 1.265(n − 1) (13) The length and diameter of the chain for DDAB, DTAC and TDAC were estimated from Eqs. (12) and (13) and values obtained are 15.42, 5.17 Å; 15.42, 5.17 Å and 17.95, 5.17 Å, respectively. Comparison of these values with the structural characteristics of cyclodextrins shows that there is a good match fitting between cyclodextrin molecules in the length of surfactant chain in all the cases.

NMR spectroscopy helps in studying the interactional behavior of the components [5,38–41]. To gain more insight into the interaction between the two moieties, 1 H NMR for all the three-surfactant systems in the absence and presence of different cyclodextrins were recorded (see Figs. III–V in Supplementary material). The changes in chemical shifts observed for all the three surfactants are analyzed (see Tables II–VII in Supplementary material). Cyclodextrins have an overall shape reminiscent of a truncated cone with an internal cavity, which is predominantly hydrophobic. The internal cavity is open at the two ends, which have different diameters and are therefore referred to as the wide and narrow rings. Only protons H3 and H5 from each sugar unit face the internal cavity and are located near the wide and narrow ring, respectively (Fig. 10). These can be used, therefore, to probe the internal cavity of cyclodextrins for the presence of a guest molecule and to understand the nature of interaction. Preliminary indications of the solution structure of the complex were derived by a proton chemical shift analysis. For instance, the observed largest chemical shift changes upon complex formation (see Tables II, IV, VI in Supplementary material) were those exhibited by H3 and H5. Since H3 and H5 face the cavity of the cyclodextrin, their chemical shift changes are most likely due to the presence of hydrocarbon chain of surfactant present in the cavity of the cyclodextrin. Similarly, the largest chemical shift variation observed in the whole surfactant molecule occurs for hydrocarbon chain protons (see Tables III, V, VII in Supplementary material). To summarize, although conductivity and fluorescence results indicated the existence of complexation between S and CD in solution, a deeper insight into the complexation mechanism was obtained from 1 H NMR. All these features points toward S:CD complex formation where the hydrocarbon chain of the surfactant monomer occupies the cavity of cyclodextrin due to which delay in micellization is observed. The increase in cmc∗ of surfactants or delay in micellization may be interpreted as follows: Firstly, the solution contains only CD molecules and when S is added, surfactant monomers were encapsulated into CD cavities, assuming the interaction between CDs and surfactants is stronger than that of between surfactants in micelle formation. Secondly, owing to inclusion of surfactant in CD, more surfactant monomers must be provided in order to form micelles; this leads to the increase in cmc∗ of the surfactants. Scheme 1 shows the dynamics of all the above processes occurring in the solution phase.

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The 1 H NMR results allowed a thorough understanding of the nature of the inclusion complex. Acknowledgment S.D. is grateful to CSIR for Senior Research Fellowship. Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2008.02.030. References

Scheme 1. Scheme showing micellization of surfactant in aqueous media and proposed packing pattern in ternary S/CD/W system.

4. Summary The interest for better knowledge of cyclodextrin–surfactant systems has grown enormously for their widespread applications in chemical industries and for their inherently interesting properties and thus, it is necessary to find out any possible interaction between these agents because the interaction can adversely affects the performance of both. In this study we have been able to examine the behavior observed when CDs are added to micellar solutions of cationic surfactants and to show that CDs delay the micelle formation by siphoning away surfactant monomers from the bulk solution into their cavities. Conductivity measurements have been coupled with the results of the static fluorescence and 1 H NMR investigations to obtain a more detailed picture of the positioning of the surfactant within the CD cavity. Looking at the results reported in the literature we have a feeling that potential work has been done on β-CD/surfactant interactions, however, not much is known about the complexes of other forms of cyclodextrins or how their presence may affect the surfactant micellization process. We here present the complete thermodynamic and spectroscopic analysis of association of α-CD, β-CD, HPβ-CD and γ -CD with cationic surfactants system (DDAB, DTAC and TDAC). The thermodynamic study has shown that the micellization process becomes less spontaneous in the presence of increasing cyclodextrin concentration. The changes in the spectral intensities confirmed the occurrence of inclusion phenomenon between the surfactant monomers and cyclodextrins. The results demonstrate that the association constants vary with the chain length of the surfactant and values are highest for TDAC systems. For the same surfactant, the association constant with γ -CD is greatest in comparison to other CDs. This shows that the interaction between γ -CD and the surfactants is strongest.

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