Cyclodextrin–Surfactant Coassembly Depends on the Cyclodextrin

Article pubs.acs.org/Langmuir

Cyclodextrin−Surfactant Coassembly Depends on the Cyclodextrin Ability To Crystallize Jonas Carlstedt,*,† Azat Bilalov,*,†,§ Elena Krivtsova,†,§ Ulf Olsson,† and Björn Lindman†,‡ †

Physical Chemistry, Center of Chemistry and Chemical Engineering, Lund University, POB 124, 221 00 Lund, Sweden Department of Chemistry, University of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal

‡

S Supporting Information *

ABSTRACT: Full equilibrium phase diagrams are presented for two ternary systems composed of the cationic surfactant dodecyltrimethylammonium bromide (DTAB), water (D2O), and a cyclodextrin, either β-cyclodextrin (β-CD) or (2-hydroypropyl)-β-cyclodextrin (2HPβCD). 2H NMR, SAXS, WAXS, and visual examination were used to determine the phase boundaries and characterize the nature of the phases formed. Additionally, diffusion 1H NMR was used to investigate parts of the diagrams. The water solubility of 2HPβCD is 80% (w/w), whereas it is only 1.85% (w/w) for β-CD. Solubility increases for both species upon complexation with DTAB; while the increase is minute for 2HPβCD, it is dramatic for β-CD. Both systems displayed an isotropic liquid solution (L1) one-phase region, the extension of which differs extensively between the two systems. Additionally, the DTAB:2HPβCD:water system also comprised a normal hexagonal (H1) area, which was not found for the DTAB:β-CD:water system. In the DTAB:β-CD:water system, on the other hand, we found cocrystallization of DTAB and β-CD. From this work we conclude that DTAB and CD molecules form 1:1 inclusion complexes with high affinities. Moreover, we observed indications of an association of 2HPβCD to DTAB micelles in the isotropic solution phase, which was not the case for β-CD and DTAB micelles. This is, to our knowledge, the first complete phase diagrams of surfactant−CD mixtures; as a novel feature it includes the observation of cocrystallization at high concentrations.

1. INTRODUCTION A cyclodextrin (CD) is a cyclic oligomer of glucose, with glucose units connected through α-(1,4)-glucosidic bonds. The three-dimensional structure resembles a truncated cone which has an apolar inner cavity and a polar exterior.1 Consequently, cyclodextrins can be used to form water-soluble inclusion complexes, sometimes referred to as host−guest complexes, by encapsulating small apolar molecules or parts thereof. The three main cyclodextrins, which are all formed by enzymatic degradation of starch,2 contain 6 (α-cyclodextrin), 7 (β-cyclodextrin), or 8 (γ-cyclodextrin) glucose units, which results in different sizes of the apolar cavity. Additionally, a number of cyclodextrins with substituted hydroxyl groups, which gives the cyclodextrins new properties, are industrially produced. Because of the ability of complex formation of cyclodextrins and since they have a low toxicity and do not produce immune responses in mammals,3 they are used in numerous industrial applications such as in the food, pharmaceutical, and cosmetic industries.4 The inclusion complex formation between cyclodextrins and surfactants has been studied by means of, for example, NMR methods,5,6 surface tension,7,8 calorimetry,9,10 conductivity,5,11 and recently also capillary electrophoresis.12 Among the main findings are that the association constant is very high, so that the formation of surfactant−cyclodextrin inclusion complexes competes with surfactant micellization. The binding constant increases with increasing hydrocarbon chain length of the surfactant, since hydrophobic interactions are of importance in the inclusion complex formation. It is also observed that the presence of cyclodextrin shifts micelle formation to higher surfactant © 2012 American Chemical Society

concentrations. The apparent critical micelle concentration, cmc*, is generally found to be linearly dependent on the concentration of cyclodextrin.11,13−15 In a recent study, the more concentrated surfactant−CD concentration regime was also addressed.16 The solubility in water of the three cyclodextrins mentioned differs remarkably. While α-CD has a solubility of 0.121 M and γ-CD of 0.168 M, the solubility of β-CD is only 0.0163 M.17 The low solubility of β-CD in water is referred to a particularly stable crystalline state, in turn due to a combination of hydrophobic interactions and matching hydrogen bonds, which is not the case in α-CD and γ-CD.1 In addition, there have been some observations of β-CD self-assembly18,19 already at very low concentrations. Upon inclusion complex formation, the water solubility of the guest molecule is normally substantially increased while the host molecule (CD) obtains a lower solubility.1 However, ionic species that form inclusion complexes with CD molecules1 are generally increasing the CD water solubility. Other additives give different effects. Thus, addition of urea to water solutions increases the solubility of β- and γ-CD (the increase is particularly noticeable for β-CD), whereas it decreases the α-CD solubility.20 We have previously shown that cyclodextrins can be used to decompact DNA that was compacted by cationic surfactant.21,22 In the present work we look further into the Received: September 20, 2011 Revised: December 1, 2011 Published: January 4, 2012 2387

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subsequently immediately flame-sealed. The tubes were then vortexed, followed by centrifugation at 4000 rpm and 40 °C, and turned end over end every 15 min. The samples were equilibrated at 25 °C for at least 2 months prior to phase characterization. The number of phases in each sample was determined by visual observation, including cross-polarizer, and 2H NMR at 25 °C. By looking at samples under cross-polarized light, it is possible to distinguish isotropic phases from anisotropic ones. The 2H NMR method27 makes use of the fact that interfacial water molecules, i.e., water in the first hydration layers, have a preferred orientation. In an isotropic sample, the orientational orders are averaged to zero, which results in a sharp peak (singlet) in the 2H spectrum of heavy water. However, in anisotropic phases a peak splitting will be observed. The presence of more than one phase in a sample will be observed as superimposed spectra. All 2H NMR experiments were performed on a Bruker 200 MHz spectrometer, and the temperature was controlled by an air-flow through the sample holder. Diffusion 1H NMR, small-angle X-ray scattering (SAXS), and wideangle X-ray scattering (WAXS) as well as light microscopy were used to further characterize the nature of the phases. A Bruker 200 MHz spectrometer operating at 25 °C (with temperature controlled by an air-flow through the sample holder) was used for the diffusion measurements, employing either a spin echo or stimulated spin echo technique.28 The duration time of the gradient pulse was kept constant (δ = 0.5 ms), and the diffusion time (Δ) was 20 ms. SAXS spectra were recorded at 25 ± 0.1 °C (controlled by a Peltier element) on an in-house Kratky compact small-angle system. The wavelength of the beam was 1.54 Å, and the sample−detector distance was 277 mm. Spectra were also recorded using synchrotron radiation at MAX-lab at 25 °C at beamline 711, which is further described elsewhere.29 An inhouse SAXSess instrument (wavelength was 1.54 Å) with a TCU 50 temperature control unit (both from Anton Paar) was used for WAXS measurements. Image plates were used for detection. An Axioplan microscope equipped with an Axiocam camera (both from Carl Zeiss, Germany) was used. Axiovision (Carl Zeiss) software was used for image collection. Chemical analysis of crystals from the DTAB:β-CD:water system was made after separation (filtration) of the two phases (sedimented crystals and clear, isotropic supernatant). The crystals were collected and washed with cold water (>1 L), which was repeated up to four times. After each washing step, crystals were collected and dissolved in D2O at 25 °C to a concentration of 1% w/w and analyzed by 1H NMR (Bruker Avance 500 MHz spectrometer).

surfactant−cyclodextrin interactions and present phase studies of ternary systems composed of the cationic surfactant DTAB, CD (either β-CD or (2-hydroxypropyl)-β-CD, 2HPβCD), and water (D2O). To the best of our knowledge, these are the first complete ternary phase diagrams of surfactant:CD:water systems to be published. The cyclodextrins used in this work, shown schematically in Figure 1, differ significantly in their

Figure 1. Schematic representation of cyclodextrin. R is hydrogen in the β-CD structure, whereas R is either hydrogen or 2-hydroxypropyl in the 2HPβCD structure. There are, on average, seven 2-hydroxypropyl groups per molecule in 2HPβCD 1.0, i.e., one per glucose unit.

aqueous solubility, which has been referred to a disruption of the stabilized crystalline state by insertion of a hydroxypropyl substituent.23 This solubility paradox, i.e., increased water solubility by nonpolar substitution, is also found in cellulose.24 The binary phase diagram of DTAB and water comprises three one-phase regions at 25 °C: isotropic solution, normal hexagonal liquid crystalline phase, and solid crystalline phases.25 Since cyclodextrins form inclusion complexes with DTAB, the phase behavior is expected to be different from that of DTAB in water. With the phase diagrams as the point of departure, a strategy which has been shown to be a successful way to understand surfactant self-assembly,26 we aim to link the macroscopic behavior to the intermolecular interactions in these two ternary systems.

3. RESULTS AND DISCUSSION 3.1. General Outline of the Phase Diagrams. The equilibrium phase diagrams of the two ternary systems at 25 °C are presented in Figures 2 and 3. DTAB is a well-characterized, typical single-chained cationic surfactant, and its phase behavior in water was described by McGrath.25 It forms micelles from a well-defined concentration (critical micelle concentration, cmc), which for DTAB is 1.35 × 10−2 M, and at higher concentrations, it forms a hexagonal liquid crystalline mesophase. Increased concentration (at the temperature that we performed our studies, 25 °C) leads to a crystalline phase. The one-phase regions in the binary system are separated by two-phase areas. These features are captured on the binary DTAB−water axis of the present phase diagrams. The binary systems of β-CD:water and 2HPβCD:water are comprised of one region of CD dissolved in water and one region of CD crystals with in between a region of phase coexistence. As noted in the Introduction, there is a significant difference in water solubility between the two studied cyclodextrins, which will be further discussed below. Both systems display an isotropic solution (L1) one-phase region. Additionally, the DTAB:2HPβCD:water system also comprises a normal hexagonal liquid crystalline phase (H1). For the other system, the hexagonal phase does not accommodate

2. EXPERIMENTAL SECTION 2.1. Materials. Dodecyltrimethylammonium bromide (>99.0%), DTAB, was from TCI Europe, Zwijndrecht, Belgium. β-Cyclodextrin (98%), β-CD, and (2-hydroxypropyl)-β-cyclodextrin 1.0 (where 1.0 indicates the average molar substitution degree, i.e., one 2-hydroxypropyl group is, on average, substituted for one hydroxyl group on each glucose unit, Mw = 1540 g mol−1), 2HPβCD, were both from Sigma-Aldrich, St. Louis, MO. Heavy water (99.8%), D 2O, was from Dr. Glaser AG, Basel, Switzerland. All chemicals were used as received. 2.2. Sample Preparation and Characterization. Desired amounts of heavy water, D2O (which was used instead of water for experimental reasons), cyclodextrin (either β-CD or 2HPβCD), and DTAB were added to 8 mm (i.d.) glass tubes, which were 2388

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phase composed of DTAB and β-CD, which will be further discussed below. 3.2. Characterization of the Solution Regions. The samples from the L1 region are transparent and optically isotropic with a viscosity that increases as the amount of water is reduced and the amount of CD is increased. At high CD concentrations the samples appeared slightly bluish when observed through cross-polarizers. The extensions of the one-phase regions are significantly different between the two systems: L1 occupies a much smaller area in the DTAB:β-CD:water system. This is a consequence of a much higher water solubility of the chemically modified CD. Quite clearly, the presence of DTAB dramatically increases the solubility of β-CD in water (from ∼2% w/w in water to nearly 35% w/w at a DTAB concentration of 9% w/w), a finding which would be of importance in various applications of similar systems. Furthermore, in the same system, the 1:1 molar ratio line coincides with one of the phase boundaries of L1, which is a strong indication that 1:1 surfactant:cyclodextrin inclusion complexes are formed. At a β-CD:DTAB ratio greater than unity, all additionally added cyclodextrin precipitates out of solution. The increased solubility of cyclodextrin can also be observed in the case of 2HPβCD (from approximately 80% w/w to 88% w/w); however, the relative effect is not as remarkable in this system since the solubility of 2HPβCD itself in water is high. Ionic guest molecules that form inclusion complexes with CD molecules can, as mentioned in the Introduction, increase the CD solubility in water, so even if the effect should be expected, it is very striking. Actually, ionic surfactants have been shown to increase water solubility of many other substances, including nonionic polymers, like ethyl(hydroxyethyl)cellulose, EHEC,30 and the surfactant−CD inclusion complex formation can be regarded as a special case of polymer−surfactant association. Confining counterions to a separate phase is accompanied by a large entropic penalty, which is the main reason why this charging-up effect results in increased “polymer” solubility. Diffusion 1H NMR was used for further characterization of the L1 regions, and a representative diffusion 1H NMR experiment is shown in Figure 4. Diffusion data from the two investigated ternary systems, at a water concentration of 80% (w/w), are shown in Figure 5 together with calculated values from a theoretical model for the DTAB diffusion. The latter were obtained from eq 1:

Figure 2. Phase diagram of DTAB and β-CD in water (D2O), where compositions are given by wt %. The 1:1 CD−DTAB molar ratio line runs straight from a β-CD content of ∼79% to the water corner. We propose the position of the crystalline phase, denoted X, at 0% water and a DTAB:β-CD molar ratio of unity, i.e., at ∼79% w/w of β-CD. L1 is the isotropic solution phase while the normal hexagonal liquid crystalline phase, H1, only exists as a one-phase region between DTAB concentrations of 51−76% w/w on the DTAB:water axis. White areas (others than L1) are two-phase regions, while shaded areas are threephase regions of the diagram.

c − cCD c Dobs = DTAB DDTAB,micelle + CD DCD cDTAB cDTAB

(1)

Dobs is the experimentally observed diffusion coefficient of DTAB, cDTAB is the total concentration of DTAB, cCD is the total concentration of CD, DDTAB,micelle = 4 × 10−11 m2 s−1 (from experiment) and DCD = 1.1 × 10−10 m2 s−1 (from experiment) for the β-CD system, while the observed diffusion, which varied significantly with sample composition, of CD for the actual composition of each sample was used for the 2HPβCD system. The model assumes two sites for the surfactant: either in an aggregate (micelle) or complexed with a CD molecule; i.e., no free surfactant unimers were taken into account into the model. The association constant for DTAB−CD inclusion complex formation was assumed to be infinite, which means that all CD molecules form complexes when surfactant is in excess. Cepeda et al.31 found that 3.2% of β-CD existed as free molecules in equilibrium with sodium decylsulfonate above the cmc. The amount

Figure 3. Phase diagram of DTAB and 2HPβCD in water (D2O), where the compositions are given by wt %. The dashed line corresponds to a 1:1 CD−DTAB molar ratio. L1 indicates an isotropic solution phase, and H1 indicates a normal hexagonal liquid crystalline phase. White areas (others than L1 and H1) are two-phase regions, while shaded areas are three-phase regions of the diagram.

any appreciable amount of cyclodextrin, which is due to the formation of a very stable crystalline state of DTAB and β-CD. There are two three-phase regions in the DTAB:2HPβCD:water diagram, while there are three three-phase regions in the DTAB:β-CD:water system. These are shaded for clarity. Furthermore, there are four two-phase regions in the DTAB:2HPβCD:water system and three two-phase areas in the DTAB:β-CD:water system. Additionally, the DTAB:β-CD:water system has a crystalline 2389

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Figure 4. A typical spin echo self-diffusion 1H NMR experiment from the L1 region of the DTAB:β-CD:water system at a composition of 10:10:80 (DTAB:CD:water, w/w), showing (a) intensity as a function of gradient and chemical shift, and (b) the echo decays for the two components with corresponding fits (solid lines). k is defined as γ2δ2G2(Δ − δ/3), where γ is the gyromagnetic ratio, G is gradient strength, δ is the duration time of the gradient pulse, and Δ is the diffusion time.

Different systems and various techniques have resulted in diverse conclusions regarding CD−micelle interactions. From studies on β-CD and 2HPβCD systems with dodecyltrimethylammoium chloride, DTAC (which are very similar systems to ours), the authors did not infer any attractive micelle−CD interactions;5 however, in an investigation of CTAB:β-CD, the authors interpreted their data as β-CD being incorporated into spherical micelles but not into rodlike.32 De Lisi et al.10,33 inferred from their studies that different CD molecules, both free and complexed with surfactant, could associate to micelles. Despite previous contradictory interpretations, we believe that our approach clearly shows that there are attractive interactions between 2HPβCD and DTAB micelles but not between β-CD and DTAB micelles. A plausible explanation for our observation is that whereas β-CD does not display any surface activity, 2HPβCD is known to lower the surface tension in aqueous solution.34 This property is ascribed to the 2-hydroxypropyl groups of 2HPβCD being less polar than the hydroxyl groups of β-CD, and it is, furthermore, well-known that alcohols interact with micelles.35 These facts support our conclusions that it is only the nonpolarly substituted, and slightly amphiphilic, CD molecules, which associate with the micelles. In Figure 7, we present additional diffusion data from the DTAB:β-CD:water system at a water content of 65% w/w. At this water content we also observe micelle disintegration indicated by an increased diffusion of DTAB but also a significantly decreased CD diffusion at increased CD:DTAB ratio. The decrease in cyclodextrin diffusion is expected, because as the volume fraction, ϕ, of dispersed particles increases (in this case the particles are represented by CD molecules), the diffusion rate of hard spheres decreases according to eq 3:36

of uncomplexed CD showed, however, a strong decrease at increasing hydrocarbon chain length. We use C12 chains in this work, and it is, therefore, reasonable to assume that the amount of uncomplexed CD is negligible. It was further assumed that the diffusion of CD was unaffected by the CD− DTAB inclusion complex formation; i.e., the difference in hydrodynamic radius between free CD and the CD:DTAB complex is assumed to be negligible, which is expected if the surfactant is mainly located in the interior of the CD. This behavior is consistent with the literature.6 The increase in the DTAB diffusion coefficient that is inferred from Figure 5 can, from the good fit to the model, be interpreted as micelle disintegration upon increased CD:DTAB ratio. This result is expected13 and is ascribed to surfactant−CD inclusion complex formation, and it further strengthens our conclusion that 1:1 surfactant−cyclodextrin complexes are formed with very high association constants. The diffusion of CD differs for the two systems: while the diffusion coefficient for β-CD is slightly decreasing upon increased CD:DTAB ratio, the diffusion coefficient is increased for the 2HPβCD. The former observation is due to increased viscosity at increasing CD concentration, as will be further discussed below. We suggest that the latter observation is the result of an attractive CD−micelle interaction. To assess the fractions of free and micellarly bound 2HPβCD, we used eq 2, which is a two-site model:

Dobs = PCD,freeDCD,free + (1 − PCD,free)Dmicelle

(2)

where Dobs is the observed diffusion coefficient of 2HPβCD, PCD,free is the fraction of CD molecules that are not associated with the micelles (but exist as inclusion complexes with single surfactant molecules), DCD,free = 1 × 10−10 m2 s−1, and Dmicelle = 4 × 10−11 m2 s−1. PCD,free is plotted versus molar compositions in Figure 6. As can be observed, the fraction of free CD increases sharply initially with increased CD concentration to reach a point where all CD molecules occur as complexes with single surfactant molecules.

D = D0(1 − 2ϕ)

(3)

where D is the diffusion at volume fraction ϕ and D0 is the diffusion coefficient at infinite dilution. Consequently, the decreasing diffusion rate of β-CD at a water content of 65% w/w (and to a much lesser extent at a water content of 2390

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Figure 6. Calculated fraction, Pfree, of 2HPβCD which is not micellarly bound (but rather inclusion-complexed with single surfactant molecules) versus CD:DTAB molar ratio. A Pfree value of unity indicates that all cyclodextrin molecules are complexed with single surfactant molecules and that none are associated with micelles.

Figure 5. Diffusion of (a) DTAB and β-CD and (b) DTAB and 2HPβCD at a water content of 80% w/w, presented as a function of both CD:DTAB mass and molar ratios. Both experimental and calculated values for DTAB diffusion are shown for both systems. Figure 7. Diffusion of DTAB (experimental value and calculated diffusion coefficient for surfactants in micelles) and β-CD at a water content of 65% w/w. Note that the diffusion of the micelles, as inferred from our calculations, is unchanged in the investigated concentration interval, while the diffusion of β-CD is considerably reduced.

80% w/w, cf. Figure 5) should not be ascribed to CD aggregation but rather to a larger obstruction effect. The calculated diffusion of DTAB in micelles is obtained from eq 1, solved for DDTAB,micelle (as opposed to DObs that we present in Figure 5). This is found, to a good approximation, to be constant within the range of concentrations investigated. The reason for this behavior is that there are two oppositely acting tendencies present: at low CD:DTAB ratios the viscosity of the solution is low, while the micelle density is large and there are non-negligible micelle−micelle interactions. At increased CD:DTAB ratio, the viscosity increases, thus reducing the diffusion; however, simultaneously the micelle density is reduced which means that micelle−micelle interactions are reduced. 3.3. Characterization of the Hexagonal Liquid Crystalline Region of the DTAB:2HPβCD:Water System. Liquid crystalline samples are characterized by long-range order

and short-range disorder. The samples from this area are soft but very viscous, and moreover, they are transparent but optically anisotropic. The liquid crystalline hexagonal (H1) mesophase of the binary DTAB:water system was found to exist between approximately 51% and 76% w/w of DTAB. In the ternary system, the H1 region of the DTAB:2HPβCD:water system is not very extended, and the surfactant is always in large molar excess compared to CD. The H1 region was further investigated by SAXS. The lattice parameter, d, which indicates the distance between the 2391

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gave identical lattice parameters for the two samples at 70 and 75% w/w of DTAB, respectively. However, the sample at the lowest DTAB concentration (i.e., highest CD concentration) gave the smallest lattice parameter. This indicates that the main mode of interaction is CD molecules, or DTAB−CD inclusion complexes, adsorbing to the cylindrical aggregates, since an extraction of surfactant to the interaggregate space would result in the opposite result. It should be noted, however, that the hexagonal phase is very narrow in this system, and it is, consequently, not possible to obtain very reliable quantitative information. 3.4. Cocrystallization in the DTAB:β-CD:Water System. Crystals were found in a large part of the ternary DTAB:βCD:water system, and in order to determine their compositions, WAXS studies were performed on samples from different parts of the phase diagram. It is beyond the scope of this investigation to give detailed information on this crystalline phase, and we will, therefore, just briefly discuss our main findings. WAXS spectra with given global sample compositions are shown in Figure 9, from which we can infer that cocrystallization of DTAB and β-CD occurs: the characteristic peaks from the individual components, i.e., the hydrated crystals of DTAB and β-CD, disappear and new peaks appear. The characteristic distances in the cocrystals are 21.5 Å (a) and 11.6 Å (b, c), leading to a unit volume of ∼2900 Å3, which can accommodate two DTAB:β-CD inclusion complexes. The normal hexagonal liquid crystalline mesophase that is found in the binary DTAB:water system cannot accommodate any appreciable amounts of β-CD but exists in equilibrium with crystals, in line with the high stability of the crystalline state of β-CD. Results from SAXS, WAXS, and 2H NMR indicated that the compositions of the hexagonal phase varied (i.e., differences in lattice parameter and magnitude of splitting, respectively) in this area, so in accordance with Gibbs phase rule this should be a two-phase area (i.e., normal hexagonal phase and one crystalline structure) rather than a three-phase area, given that a thermodynamic equilibrium was reached. This crystalline structure is not composed of either pure hydrated β-CD or DTAB molecules, as can be inferred from Figure 9. A microscope image from this area, showing a single crystal, at a global sample composition of 40:5:55 (DTAB:β-CD:water, w/w) is shown in Figure 10. Close to the lower L1 phase boundary, we found a crystal structure that was very sensitive to temperature. Crystals were present at 25 °C; however, already at ∼30 °C the crystals were soluble (i.e., the L1 region expanded). In Figure 11, we show how the phase border of L1 changes as a function of temperature. To determine if the crystals were composed of either pure DTAB or β-CD, or a mixture of both, we recorded 1H NMR spectra from the separated and washed redissolved crystals. From these spectra, which are shown in the Supporting Information, it is clear that DTAB and β-CD cocrystallize. Global and crystal compositions after separation and repeated washing from three different samples are presented in Table 2. Here we see that the crystal composition is close to a 1:1 DTAB:β-CD molar ratio, even if, when DTAB is in excess in the samples, some surfactant is not possible to remove by the employed washing procedure. The striking qualitative difference between the phase behaviors of the two systems can, as discussed, be referred to a difference in the crystallization behavior between the two cyclodextrins. It is remarkable that the strong tendency of β-CD to crystallize also persists when it has associated with the

hexagonally arranged surfactant rods, in a SAXS spectrum from a hexagonal phase corresponds to the Bragg reflections according to eq 4:

2π(h2 + k2 + hk)1/2 (4) d where qhk is the scattering vector in Å−1 and the first three reflections correspond to (h,k) = (1,0), (1,1), and (2,0). This gives a pattern that is characteristic for the hexagonal phase, 1:√3:2, which can be seen in Figure 8. qhk =

Figure 8. Characteristic SAXS spectrum (showing the logarithm of the intensity, lg Int., as a function of the scattering vector, q) from the hexagonal phase of DTAB:2HPβCD:water at a composition of 70:5:25 (DTAB:CD:water, w/w), showing the typical Bragg reflections found in a hexagonally arranged liquid crystalline phase.

The lattice parameters were subsequently calculated from the first Bragg reflections for the hexagonal phase samples and are presented in Table 1. Table 1. Composition, Scattering Vector, and Calculated Lattice Parameter for the Hexagonal Phase of the DTAB:2HPβCD:Water System

a

composition/wt %

q10a/Å−1

d/Å

60:5:35 60:10:30 66:14:20 70:5:25 70:10:20 75:5:20

0.1861 0.1829 0.1900 0.1869 0.1877 0.1877

33.77 34.35 33.06 33.62 33.48 33.48

q10 is the scattering vector for the first reflection in the profile.

Three compositions at constant CD concentration (60:5:35, 70:5:25, and 75:5:20) from the DTAB:2HPβCD:water system were analyzed. Here we observed a (slight) swelling (i.e., increased lattice parameter, d) of the hexagonal phase at increasing water content (and consequently at decreasing surfactant content). This is the excepted result, since a lower volume fraction of surfactant is equal to a less dense (in terms of cylindrical aggregates) system. The three compositions at constant water concentration (66:14:20, 70:10:20, and 75:5:20) 2392

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Figure 9. WAXS spectra (showing the logarithm of the intensity, lg Int., as a function of the scattering vector, q) with Miller indices for various compositions from the DTAB:β-CD:water system, including spectra for the pure components, i.e., DTAB and β-CD, in their hydrated crystalline state. Miller indices with no star correspond to the a dimension in the normal monoclinic symmetry, while one and two stars correspond to the b and c dimensions, respectively. The peaks observed in some spectra to the left of the [100] peak are artifacts of the slit collimation system when there are only a few crystallites in the scattering volume.

Figure 10. Microscope image of a single DTAB:β-CD:water crystal at a global sample composition of 40:5:55 w/w, where the hexagonal and L1 phases coexist with the solid crystalline DTAB:β-CD phase.

surfactant. The large tendency of β-CD to crystallize is referred to a crucial packing between molecules in the solid state, leading to strong hydrophobic attraction as well as an optimal hydrogen-bond network. Any substitution of β-CD will inhibit this organization, leading to a higher energy of the solid state and thus to a higher solubility. In our opinion the situation is very similar to that of cellulose. Cellulose has a very low solubility in water due to a low energy of the solid state, reflecting strongly attractive hydrophobic interactions as well as matching orientation of cellulose molecules for optimized hydrogen bonding. Essentially any substitution of cellulose strongly decreases the stability of the solid state leading to a higher solubility; it is striking that even making cellulose more

Figure 11. Temperature dependence of the L1 area in the DTAB: β-CD:water system. As the temperature is increased, the crystalline phase dissolves; i.e., the isotropic solution region expands.

hydrophobic like in methyl cellulose leads to high solubility. An analogous behavior also for other poly-(glucoses), including cyclodextrins, can be expected and seems to be supported by our findings. 2393

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Table 2. Global Sample Compositions (DTAB:β-CD:Water) by wt % and DTAB:β-CD Molar Ratio for Cocrystallization Analysis Are Given in the First Two Columns from Lefta global molar ratio

1st wash

2nd wash

3rd wash

4th wash

25:25:50 15:35:50 10:40:50

3.68 1.58 0.92

1.56 1.15 1.01

1.30 1.05 1.00

1.33 1.05 0.96

1.44 1.09

a

DTAB:β-CD molar ratio for the redissolved crystals after 1−4 washing steps follow.

4. CONCLUSION We studied the phase behavior of two systems consisting of DTAB and cyclodextrin (either β-CD or 2HPβCD) in water by producing equilibrium phase diagrams. An analysis of the isotropic solution phase by diffusion 1H NMR leads us to the conclusion that 1:1 surfactant−cyclodextrin complexes form, and furthermore, the association constant of the inclusion complex formation is found to be very high. An increased solubility of cyclodextrin (which was particularly striking for β-CD) by formation of surfactant−CD inclusion complexes was found and attributed to the formation of charged complexes; an analogous effect is commonly observed in mixed systems of a nonionic polymer and ionic surfactant. In the isotropic solution phase, we also found indications of attractive CD−micelle interactions in the case of 2HPβCD but not in the case of β-CD, which we argued to be a consequence of differences in surface activity between the two CD species. The hexagonal liquid crystalline phase that is formed by DTAB could accommodate a small amount of 2HPβCD but not any appreciable amounts of β-CD, owing to the facts that 2HPβCD has a higher aqueous solubility than β-CD and that it interacts attractively with micelles. In the DTAB:β-CD:water system we found cocrystallization of DTAB and β-CD as a result of their strong mutual attraction combined with the low solubility of β-CD. Our results clearly show that the ability of the CD to crystallize has an important role in its coassembly behavior with surfactants.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of the isotropic solution phase and the redissolved solid crystalline phase of a two-phase sample from the DTAB:β-CD:water system. This material is available free of charge via the Internet at http://pubs.acs.org.

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molar ratio after global composition/ wt %

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.C.), azat.bilalov@ fkem1.lu.se (A.B.); Fax: +46-46-222 44 13. Notes §

On leave from Physical and Colloid Chemistry, Kazan State Technological University, Russia.

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ACKNOWLEDGMENTS Financial support from the Swedish Research Council (VR) through the Linnaeus center of excellence grant Organizing Molecular Matter (OMM) (239-2009-6794) is gratefully acknowledged. Agnieszka Nowacka is gratefully acknowledged for assistance with NMR measurements. 2394

dx.doi.org/10.1021/la203673w | Langmuir 2012, 28, 2387−2394