A Hexanol-Induced Sphere-to-Flexible Cylinder Transition in Aqueous

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A Hexanol-Induced Sphere-to-Flexible Cylinder Transition in Aqueous Alkyl Polyglucoside Solutions Anna Stradner,†,§ Otto Glatter,† and Peter Schurtenberger*,‡ Institut fu¨ r Physikalische Chemie, Universita¨ t Graz, A-8010 Graz, Austria, and Department of Physics, University of Fribourg, CH-1700 Fribourg, Switzerland Received December 23, 1999. In Final Form: March 7, 2000 Sugar surfactants have recently attracted considerable interest. Here we report a systematic light and neutron scattering investigation of aqueous solutions of the alkyl polyglucoside C8/10G1.5. The system exhibits a transition from globular to giant wormlike micelles upon the addition of hexanol. We can finely tune the extent of micellar growth and in particular reach conditions where solutions of relatively short cylinders exist, which dramatically grow into giant wormlike micelles upon the addition of small amounts of hexanol. While considerable progress has been made in the understanding of interaction effects in solutions of spheres or large polymer-like objects, limited information exists for short flexible cylinders. An important reason for this is the lack of good model systems. Here we now demonstrate that the APG system provides us with an ideal set of data for an analysis of micellar growth and interactions in solutions with cylindrical particles of moderate axial ratios. We apply theoretical expressions for the static structure factor derived from Monte Carlo computer simulations of semiflexible polymers with excluded volume interactions, which allow us to self-consistently incorporate micellar interactions and growth in a quantitative analysis of the light and neutron scattering data.

Introduction Alkyl glucosides (AG)sor alkyl polyglucosides (APG) as they are often called due to the fact that they have frequently more than one glucose unitsare nonionic surfactants produced from renewable raw materials such as starch and fats and their derivatives. The hydrophilic headgroup consists of one or more glucose molecules and is linked to the hydrophobic tail, which is a hydrocarbon chain, via a so-called glucoside bond. This linkage can be of either R- or β-type, giving rise to different physicochemical properties of the surfactant such as the critical micelle concentration, packing parameters, Krafft points, and solubilities.1 Notations such as AG, APG, sugar surfactants, or CiGj are often used, where i is the number of carbon atoms in the alkyl chain and j is the average number of glucose units linked to it, the degree of polymerization.2 In the industrial processes not pure alkyl monoglucosides, but a complex mixture of alkyl mono-, di-, tri-, and oligoglucosides, as well as R- and β-anomers are produced. At present there is a large interest in these sugar-based surfactants due to many valuable properties. They are extremely mild, display dermatological safety, and excellent ecological and toxicological properties.3,4 They are readily biodegradable which can be mainly attributed to the fact that the formation and breakdown of the glucoside bond are enzymatically controlled by different glucosidases present in nature. In addition they * To whom correspondence should be addressed. Telephone: +41-26-300 9115. Fax: +41-26-300 9747. E-mail: [email protected]. † Universita ¨ t Graz. § ‡University of Fribourg. § Current address: Department of Physics, University of Fribourg, CH-1700 Fribourg, Switzerland. (1) Nilsson, F.; So¨derman, O.; Johansson, I. Langmuir 1996, 12, 902. (2) Eskuchen, R.; Nitsche, M. In Alkyl Polyglycosides; Hill, K., von Rybinski, W.; Stoll, G., Eds.; VCH: Weinheim, 1997. (3) Aulmann, W.; Sterzel, W. In Alkyl Polyglycosides; Hill, K., von Rybinski, W., Stoll, G., Eds.; VCH: Weinheim, 1997. (4) Matthies, W.; Jackwerth, B.; Kra¨chter, H.-U. In Alkyl Polyglycosides; Hill, K., von Rybinski, W., Stoll, G., Eds.; VCH: Weinheim, 1997.

show good foaming behavior and high stability toward oxidation and hydrolysis, especially in strongly alkaline media. Therefore, these surfactants become increasingly important for use in detergents and personal care products.5,6 In contrast to the other big group of nonionic surfactants, alkyl polyglycol ethers CiEj, the phase boundaries in aqueous APG systems and the size and structure of the aggregates are little influenced by temperature.7-10 This temperature insensitivity arises from the strength of the hydrogen bonds between the hydroxy groups of the highly hydrophilic sugar unit and the water molecules, whereas the hydrate shell of the ethoxylate headgroup is very sensitive to temperature. The phase behavior of binary APG-water systems is influenced to a far greater extent by i and j than in the case of CiEj surfactants. Electrolytes do not markedly affect their physicochemical properties. The different interaction of the sugar residue with water also affects the formation of microemulsions in wateroil-surfactant systems.11 Due to the hydrophilic nature of the sugar headgroup, which makes sugar surfactants extremely insoluble in many oils, the addition of appropriate cosurfactants or cosolvents12-17 is required for the production of microemulsions in these mixtures. This (5) Tesmann, H.; Kahre, J.; Hensen, H.; Salka, B. A. In Alkyl Polyglycosides; Hill, K., von Rybinski, W., Stoll, G., Eds.; VCH: Weinheim, 1997. (6) Andree, H.; Hessel, J. F.; Krings, P.; Meine, G.; Middelhauve, B.; Schmid, K. In Alkyl Polyglycosides; Hill, K., von Rybinski, W., Stoll, G., Eds.; VCH: Weinheim, 1997. (7) Nickel, D.; Nitsch, C.; Kurzendo¨rfer, P.; von Rybinski, W. Prog. Colloid Polym. Sci. 1992, 89, 249. (8) Fukuda, K.; So¨derman, O.; Lindman, B.; Shinoda, K. Langmuir 1993, 9, 2921. (9) Platz, G.; Po¨like, J.; Thunig, C.; Hofmann, R.; Nickel, D.; von Rybinski, W. Langmuir 1995, 11, 4250. (10) Balzer, D. Langmuir 1993, 9, 3375. (11) Fo¨rster T.; Guckenbiehl B.; Hensen H.; von Rybinski, W. Prog. Colloid Polym. Sci. 1996, 101, 105. (12) Ryan, D. L.; Kaler, E. W. J. Colloid Interface Sci. 1999, 210, 251. (13) Ryan, D. L.; Kaler, E. W. J. Phys. Chem. B 1998, 102, 7549. (14) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 3382. (15) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1996, 12, 862.

10.1021/la991679r CCC: $19.00 © 2000 American Chemical Society Published on Web 05/17/2000

Aqueous Alkyl Polyglucoside Solutions

leads to interesting applications sincesin contrast to alkyl polyglycol ethersstemperature-stable sugar surfactantbased microemulsions can be formulated. In a previous paper, it has been shown that micellar solutions of the alkyl polyglucoside C8/10G1.5 (technical product from Henkel KGaA, Du¨sseldorf), containing small amounts of hexanol can serve as excellent host systems for the determination of lipase activities by means of fluorescence methods.18 The structure and overall size of the micellar sugar surfactant aggregates can be tuned by the amount of added hexanol. Hexanol acts as a “cosurfactant” that dissolves mainly in the interfacial layer, giving rise to a structural evolution from small globular APG micelles to short cylindrical and finally giant, flexible cylindrical structures, also referred to as polymer-like or wormlike structures. This transformation occurs on the addition of only very small amounts of hexanol. While the overall size of these aggregates strongly depends on the hexanol-to-APG ratio, the local structure (e.g., crosssectional dimension) remains unchanged. It has been found that the extent of micellar growth was correlated with the enzyme activity in the guest/host system. This opened up interesting possibilities to investigate enzyme kinetics and control the performance in complex nanostructured systems, but also posed questions with respect to the underlying mechanisms responsible for this phenomenon. However, the structural investigations were of rather limited and preliminary nature, and several questions related to micellar flexibility and intermicellar interaction effects remained open. Here we now report a detailed structural characterization of the effect of hexanol content on the micellar size, shape, and flexibility, where we take advantage of the recent progress made in the area of equilibrium polymers. We focus on obtaining quantitative information on micellar growth, flexibility, and structure as a function of total concentration and hexanol content using a combination of static light scattering (SLS) and small-angle neutron scattering (SANS). We have performed measurements on several concentration series with varying hexanol content. The growth of globular micelles to giant polymer-like aggregates has been observed in many aqueous surfactant systems upon a reduction of the spontaneous curvature by means of varying ionic strength (in ionic surfactant systems), temperature (in CiEj surfactant systems), or cosurfactant concentration as well as in organic solvents [see ref 19 and references therein]. These aggregates can then entangle and form a transient network above a crossover concentration c* with properties analogous to semidilute polymers. Therefore, wormlike micelles can serve as a prime example of equilibrium polymers, whereby the term “equilibrium polymer” is used for linear macromolecules that can break and recombine. Those supramolecular surfactant aggregates show novel static and dynamic properties due to their transient nature. As a consequence of the structural analogy between these structures and classical polymers theoretical concepts from polymer physics are applied to achieve a better understanding of polymer-like aggregates. The main difference between classical polymers and wormlike micelles arises from the concentration-induced micellar growth of polymerlike micelles, where the size distribution is an equilibrium (16) Ryan, D. L.; Schubert, K.-V.; Kaler, E. W. Langmuir 1997, 13, 1510. (17) Stubenrauch, C.; Paeplow, B.; Findenegg, G. H. Langmuir 1997, 13, 3652. (18) Stradner, A.; Mayer, B.; Sottmann, T.; Hermetter, A.; Glatter, O. J. Phys. Chem. B 1999, 103, 6680. (19) Jerke, G.; Pedersen, J. S.; Egelhaaf, S. U.; Schurtenberger, P. Phys. Rev. E 1997, 56, 5772.

Langmuir, Vol. 16, No. 12, 2000 5355 Table 1. Nomenclature and Relative Composition of Buffer-APG-Hexanol Samples sample A L M H

wt % C6E0 per wt % APG

pure APG low hexanol medium hexanol high hexanol

0.08 0.12 0.15

quantity that strongly depends on a variety of different parameters such as concentration and temperature. As it is very difficult to distinguish between the contributions of concentration-induced growth and intermicellar interactions to the scattering data the analysis of the detailed structure and size distribution of anisotropic micelles is a complex task. It is only recently that it had been possible to self-consistently incorporate micellar growth and intermicellar interaction in a quantitative interpretation of scattering experiments using polymer renormalization group theory and Monte Carlo simulations. Materials and Methods Materials and Preparation. The technical grade alkyl polyglucoside C8/10G1.5 (Glucopon 215 CSUP) was kindly provided by Henkel, Germany (see also ref 18). This commercial alkyl polyglucoside (APG) was prepared by Fischer glycosidation20 with technical grade fatty alcohols and represents a mixture of R- and β-glucosides. It is supplied as a 62-65 wt % solution in water with high pH value to avoid microbial attack. The surfactant was used without further purification. 1-Hexanol (purity >99%) was purchased from Fluka (Buchs, Switzerland) and used without further purification. D2O with isotopic purity >99.75% was obtained from Merck (Darmstadt, Germany). Buffer (0.1 M Tris/ HCl in D2O, pH 7.4) was used as solvent. A highly concentrated APG stock solution was prepared by weighing the surfactant into a glass bottle and diluting it to the desired concentration with buffer by taking into account that the APG is a 62-65 wt % solution in water. Next, hexanol was weighed into glass bottles and diluted to the appropriate hexanol weight percent with the APG solution. Stock solutions with four different hexanol-to-APG ratios were prepared, namely: A (pure APG-solution), L ([C6E0]/[APG] ) 0.08), M ([C6E0]/[APG] ) 0.12), and H ([C6E0]/[APG] ) 0.15) (see Table 1). The concentration series were prepared by adding appropriate amounts of buffer to the stock solutions. SANS and SLS experiments were performed at 30 °C. All solutions investigated are located in the L1 phase of the pseudoternary phase diagram (see Figure 1 of ref 18). Methods. Small-Angle Neutron Scattering (SANS). All samples were measured at a temperature of 30 °C. SANS measurements have mainly been performed at the SINQ small-angle scattering facilities of the Paul Scherrer Institut (PSI), Switzerland. The range of scattering vectors 2.1 × 10-3 Å-1 e q e 0.3 Å-1 was covered by three different sample-to-detector distances (d ) 1.55, 8, and 18 m) [for two detector distances: 5.5 × 10-3 Å-1 e q e 0.3 Å-1]. The length of the scattering vector q is given by q ) (4π/λ) sin(θ/2) where λ is the wavelength and θ is the scattering angle. The neutron wavelength was 8 Å for all experiments, and the wavelength resolution was 10% (full width at half-maximum value). The samples were kept in stoppered quartz cells (Hellma, Germany) with a path length of 2 mm. The neutron spectra of water used for calibration was measured with a 1 mm path length quartz cell. The raw spectra were corrected for background from the solvent, sample cell, and electronic noise by conventional procedures. After correction for detector efficiency by dividing with the incoherent scattering spectra of pure water the twodimensional scattering spectra were azimuthally averaged and converted to absolute scale. Some additional SANS measurements have also been performed at the D22 instrument of the Institut Laue Langevin (ILL), France. The range of scattering vectors 3.0 × 10-3 Å-1 e q e 0.47 Å-1 was covered by two different sample-to-detector (20) Fischer, E. Ber. Chem. Ges. 1895, 28, 1145.

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Stradner et al. 220 °C) and increased temperature stability (better than (0.01 °C for several hours). Two different cylindrical scattering cells (8 and 4.2 mm inner diameter, respectively) were used to demonstrate the absence of contributions from multiple scattering. All experiments were made at a temperature of 30 °C. Approximately 1 mL of the sample was transferred into the cylindrical scattering cell, which was then stoppered and centrifuged for 40 min at 30 °C and 5000g to remove dust particles from the scattering volume. The experiments were performed at 67 different angles (15° e θ e 150°), and 10 individual measurements were made and averaged for each angle. The data were then corrected for background (cell and solvent scattering) and converted into absolute scattered intensities dσ/dΩ using toluene as a reference standard covering the q range of 3.3 × 10-4 Å-1 e q e 2.5 × 10-3 Å-1. Data Analysis. From the normalized scattering intensity an apparent molar mass Mapp and a static correlation length ξs has been determined from the intercept and slope of the plot cK/ (dσ/dΩ) versus q2 using a Lorentzian scattering law of the form

1 cK ) (1 + q2ξs2) dσ Mapp dΩ

(1)

where K is the contrast term. For the light-scattering data, KSLS is defined as

KSLS )

Figure 1. (a) Apparent molar mass Mapp (obtained from SLS data; full circles mark molar masses obtained from SANS data) and (b) apparent Radius of gyration Rg,app (obtained from SANS data; gray symbols mark Rg,app obtained from SLS data) versus total concentration c for aqueous APG solutions. Samples A (O) contain pure APG micelles, whereas samples L (3, [hex]/[APG] ) 0.08), M (0, [hex]/[APG] ) 0.12), and H (), [hex]/[APG] ) 0.15) additionally contain low, medium, and relatively high amounts of hexanol. Mapp and Rg are dominated by the concentration-induced micellar growth at low concentration and by intermicellar interactions at high concentration. distances (d ) 2 and 18 m). The neutron wavelength was 6 Å for all experiments. The wavelength resolution was 10% (full width at half-maximum value). All experiments were done with a 39 cm detector offset. The samples and the water used for calibration were kept in stoppered quartz cells (Hellma, Germany) with a path length of 1 mm. The same procedure was used to bring the raw scattering data on absolute scale. The smearing induced by the different instrumental setups is included in the data analysis discussed below. For each instrumental setting the ideal model scattering curves were smeared by the appropriate resolution function when the model scattering intensity was compared to the measured one by means of leastsquares methods.21,22 The parameters in the models were optimized by conventional least-squares analysis, and the errors of the parameters were calculated by conventional methods.23 Static Light Scattering (SLS). Static light scattering measurements were performed with a commercial goniometer system (ALV/DLS/SLS-5000F monomode fiber compact goniometer system with ALV-5000 fast correlator) and an argon ion laser (Coherent Innova 300, λ ) 4880 Å). The instrument had been modified to allow for a much larger temperature range (-6° to (21) Pedersen, J. S.; Posselt, D.; Mortensen, K. J. Appl. Crystallogr. 1990, 23, 321. (22) Barker, J. P.; Pedersen, J. S. J. Appl. Crystallogr. 1995, 28, 105. (23) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969.

4π2n2 dn NAλ04 dc

( )

(2)

where λ0 ) 4880 Å is the wavelength of the incident beam in a vacuum, c is the total concentration, NA is the Avogadro number, and dn/dc is the refractive index increment (with dn/dc ) 0.1318 mL/g (A samples), 0.130 mL/g (L samples), 0.129 mL/g (M samples), and 0.128 mL/g (H samples) determined with a Wyatt Optilab refractometer at a temperature of 30 °C and a wavelength of 4880 Å). For the SANS data, KSANS is defined as KSANS ) (∆Fm)2/ NA where ∆Fm is the average excess scattering length density per unit mass.19

Results and Discussion Hexanol-Induced Micellar Growth. In a first step of our data evaluation we concentrate on the dependence of the micellar size and shape on the hexanol content and overall concentration. Figure 1a summarizes the results from a systematic study of the apparent molar mass Mapp as a function of surfactant concentration and hexanol content as obtained from SLS experiments on absolute scale. Figure 1 clearly demonstrates that hexanol has a major influence on the resulting micellar size and shape. At low surfactant concentrations, where the influence of intermicellar interactions is less important, we directly observe the considerable hexanol-induced micellar growth. However, the addition of hexanol not only leads to the formation of larger micelles, it also has a dramatic effect on the concentration dependence of the micellar size distribution. While in the absence of hexanol the APG micelles remain quite small and the apparent molar mass Mapp decreases due to the effect of (presumably) repulsive excluded volume effects, at higher hexanol content the micelles exhibit significant concentration-induced micellar growth which initially leads to an enormous increase of Mapp. At higher surfactant concentrations, intermicellar interaction effects start to dominate, and Mapp now decreases for c > c*, where the individual micelles overlap. The effect of hexanol on the micellar size and shape becomes even more obvious when looking at the SANS data. The SANS results are summarized in Figures 2-5, where the complete q dependence of the normalized intensity (dσ/dΩ)/c is given for different surfactant concentrations and four different hexanol contents. With


Figure 2. q dependence of the normalized scattering intensity (dσ/dΩ)/c for the pure APG samples (A) as a function of the surfactant concentration c.

Figure 3. q dependence of the normalized scattering intensity (dσ/dΩ)/c for the APG samples with low hexanol content (L; [hex]/[APG] ) 0.08) as a function of the total concentration c.

no added hexanol, the scattering intensity qualitatively exhibits the typical appearance of relatively small micelles. With increasing surfactant concentration, the forward intensity becomes suppressed, and the intermicellar interactions eventually lead to the formation of a weak peak at intermediate values of q at the highest concentrations investigated. However, the appearance of the SANS data completely changes at higher hexanol content. While the high-q data remain almost unaltered, at low and intermediate values of q we now observe all the typical features of giant polymer-like micelles. At very low values of q (i.e., 1/q > Rg), the scattered intensity I(q) is insensitive to structural details and is dominated by the finite overall size of the particles, and one can determine the apparent radius of gyration Rg,app and the apparent molar mass. This has been done using eq 1 and the relation ξs ) Rg,app/x3.19 The forward intensity is now much higher when compared to the samples of the A series without hexanol, and for the highest hexanol content (H series, Figure 5) we observe the enormous initial concentration-induced micellar growth already found with SLS. The resulting values of Rg,app are summarized in Figure 1b, and we see that they reflect the same hexanol and surfactant concentration dependence

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Figure 4. q dependence of the normalized scattering intensity (dσ/dΩ)/c for the APG samples with medium hexanol content (M; [hex]/[APG] ) 0.12) as a function of the total concentration c.

Figure 5. q dependence of the normalized scattering intensity (dσ/dΩ)/c for the APG samples with high hexanol content (H; [hex]/[APG] ) 0.15) as a function of the total concentration c.

as the SLS data in Figure 1a. Moreover, for those samples where the micellar sizes are large enough so that the corresponding Rg,app can be determined by light scattering, the two sets of data completely overlap. The micellar growth induced by the addition of hexanol is not only visible from Mapp and Rg,app, but the resulting micelles are now large enough that we can obtain additional structural information and in particular observe the clear signs of the formation of locally cylindrical and highly flexible polymer-like aggregates previously postulated from the SANS data at intermediate and high values of q. This is most obvious for the H series, where the micelles formed are very large, and where we clearly observe the crossover to the asymptotic power law dependence of dσ/dΩ typical for large flexible polymers with excluded volume effects (i.e., an exponent -1.66 for a self-avoiding walk structure). At large values of q, the scattered intensity is controlled by distances over which polymers are rodlike rather than flexible which shows up as a crossover to a q-1 behavior characteristic for locally cylindrical structures. However, the APG micelles possess a very high flexibility, which causes this crossover to occur at relatively large q values, where the local cross-section

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Figure 6. The effect of hexanol on the q dependence of the normalized scattering intensity (dσ/dΩ)NA/c(∆Fm)2 for four different ratios of hexanol to APG. (A (O), pure APG; L (3), [hex]/[APG] ) 0.08; M (0), [hex]/[APG] ) 0.12; H ()), [hex]/ [APG] ) 0.15). The theoretical curve based upon a least-squares fitting procedure of a single-chain scattering function with excluded-volume effects is shown as a solid line. (a) Low concentrated samples (2 wt % APG) and (b) high concentrated samples (10 wt % APG).

structure of the chains give rise to a cross-section Guinier behavior and a strong decrease in the scattered intensity at still larger q values. Therefore, the typical cylinder scattering pattern is hardly visible in these samples, which has important consequences for a determination of the micellar persistence (lp) or Kuhn length (b, with b ) 2lp). While this is often done using approximate relationships for the crossover from one asymptotic regime to the other, i.e., from the locally stiff chain with q-1 to the flexible coil with q-1.66, which should occur at qb ≈ 3.8, this is not possible with sufficient accuracy for the system under investigation. Therefore, we have to rely on numerical expressions for the full scattering function of semiflexible chains with excluded volume interactions, as will be described in detail in the next section. The evolution of the scattering data upon the addition of hexanol is further demonstrated in Figure 6 for two

Stradner et al.

different surfactant concentrations. We clearly see that the forward scattering increases dramatically for increasing hexanol content, while the high-q part of the data completely superimposes on absolute scale, thus demonstrating that the local micellar structure does not change neither with hexanol nor with surfactant concentration. This is due to the fact that micellar growth occurs in a one-dimensional fashion along the micellar contour and leaves the cylindrical cross-section unaltered, and due to the strong screening of intermicellar interactions on local length scales. It is in agreement with previous detailed investigations of the local micellar cross-section using SAXS and SANS, where the local structure had been determined using the IFT method (indirect Fourier transformation).18 This procedure uses the assumption that the cross-sectional contribution to the total scattering can be decoupled from the rest. This has the advantage that it does not rely on the low-q part of the data and that no specific model assumptions have to be made regarding the structure of the micelles except for the locally cylindrical symmetry.24 The thus obtained dimension of the local cross section of the micelles is now used (a twoshell model with core radius R1 ) 8 Å and outer cylinder radius R2 ) 17 Å)18 to investigate the intermediate q range present in the SANS data. The addition of hexanol not only induces micellar growth and results in a transition from short to long and highly flexible cylindrical micelles, it also considerably modifies the effects of intermicellar interactions on the scattering data. This becomes apparent when looking at Figures 2-5. While for samples without added hexanol the intermicellar interactions lead to a significant decrease of the forward intensity and the formation of a weak interaction peak at high concentrations, such a peak is not present for the systems with added hexanol. Here the scattered intensity is qualitatively in agreement with previous findings for semidilute polymer-like micelles, where the surfactant aggregates form an entanglement network with the corresponding typical scattering pattern that is qualitatively described by a simple blob model for polymers. At low q values, the intensity follows an Ornstein Zernicke scattering law, from which the osmotic compressibility and static correlation length as a measure of the mesh size of the transient network can be deduced. Provided that 1/ξs , q , 1/b, one then still sees the internal blob structure given by a self-avoiding walk and the subsequent crossover to the stiff cylinder regime. It is only for very high concentrations, i.e., in the concentrated solution regime, where the strong repulsive interactions between coil segments could lead to a weak interaction peak in dσ/dΩ that would show up at q ≈ 2π/d, where d is the cross-section diameter of the micelles. Micellar Flexibility. Despite the considerable experimental and theoretical attention given to the characterization and understanding of polymer-like micelles, we still lack detailed and quantitative information on the flexibility of the micelles as a function of composition, ionic strength or temperature. This is even more important as the Kuhn length b or bending modulus κ (which are related for one-dimensional objects via the thermal energy kBT through b ) 2κ/(kBT))25 are key parameters in a more fundamental description of fluid membrane phases provided by the flexible surface model. One of the major problems in the past had been the quantitative determination of b due to the possible influence of the intrinsically (24) Schurtenberger, P.; Jerke, G.; Cavaco, C.; Pedersen, J. S. Langmuir 1996, 12, 2344. (25) Landau, L. D.; Lifshitz, E. M. Statistical Physics; AddisonWesley: Reading, MA, 1958.


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high polydispersity of the micelles and the intermicellar interaction effects that could easily interfere with the interpretation of various experimental data.26 While the scattering data on local length scales are not significantly influenced by interaction effects, the situation is more complex on intermediate length scales. It had been demonstrated quite recently that the analysis of SANS data using newly developed numerical expressions for the scattering function of wormlike chains with excluded volume effects over an extended range of q values provides a method for obtaining b with high precision, that is independent of polydispersity, and where intermicellar interaction effects can be accounted for.19,27 In a preceding paper it had been demonstrated that the addition of hexanol to micellar solutions of APG induces a sphere-to-wormlike micelle transition, where the wormlike structures exhibit extremely high flexibility when compared to previously published data of polymer-like aggregates.18,19,26-28 However, the reported high flexibility was obtained from a limited set of data only, and no detailed investigation of the dependence of the Kuhn length upon overall surfactant concentration and hexanol content had been performed. Here we now present a systematic characterization of the micellar flexibility as a function of hexanol content, where we in particular carefully monitor the possible effects of intermicellar interactions on the value of the apparent Kuhn length. This has been achieved using the same procedure as described in ref 19. We use a scattering function of the form

dσ (q) ) cMappKSANS〈Swc(q)〉zScs(q) dΩ

Figure 7. Kuhn lengths b of the samples with low, medium, and high hexanol content as a function of the total concentration c (L (3, [hex]/[APG] ) 0.08; M (0), [hex]/[APG] ) 0.12; H ()), [hex]/[APG] ) 0.15). See text for details.

(2)

where 〈Swc(q)〉z is the z-average scattering function of the infinitely thin wormlike chain and Scs(q) the cross-section scattering function. The contrast term KSANS is given by KSANS ) (∆Fm)2/NA, where the average excess scattering length density per unit mass ∆Fm needs to be calculated for every sample. This is due to the fact that the surfactant contains 35 wt % H2O. The final values of ∆Fm have thus been calculated using the corresponding values of the scattering length densities of APG, H2O, D2O, and hexanol, their relative weight concentrations, and the fact that some of the hydrogen atoms of the hydroxyl groups in the surfactant headgroups and the hexanol exchange with deuterium. The fit requires the optimization of seven parameters, that are the inner and outer radii (R1 and R2) of the cylindrical cross-section where R1 ) 8 Å and R2 ) 17 Å were held constant in accordance with the findings in ref 18, the mass per length (ML), the ratio of the scattering length densities of the inner core and the outer shell, the background, the Kuhn length b, and the contour length Lc. In these calculations we have assumed a Schulz-Zimm distribution of the micellar size distribution and fixed the polydispersity to 〈M〉w/〈M〉n ) 2.19 The values of b obtained from a fit of the SANS data using the scattering function of wormlike chains with excluded volume effects are summarized in Figure 7. We indeed observe very low values b ≈ 100 Å for the Kuhn length, and there appears to be no measurable dependence of b on the hexanol content. Moreover, while there is a small increase of the apparent Kuhn length with surfac(26) Schurtenberger, P. In Light Scattering: Principles and Development; Brown, W., Ed.; Clarendon Press: Oxford, 1996. (27) Jerke, G.; Pedersen, J. S.; Egelhaaf, S. U.; Schurtenberger, P. Langmuir 1998, 14, 6013. (28) Magid, L. In Dynamic Light Scattering: the Method and Some Applications; Brown, W., Ed.; Clarendon Press: Oxford, 1993.

Figure 8. Comparison of experimental data of the sugar surfactant-based solutions (A (O), pure APG; L (3), [hex]/[APG] ) 0.08; M (0), [hex]/[APG] ) 0.12; H ()), [hex]/[APG] ) 0.15) with theoretical curves based upon a least-squares fitting procedure of a single-chain scattering function with excluded volume effects in a Holtzer representation. The APG concentration is 8 wt % (A, L, and M) and 10 wt % (H), respectively.

tant concentration, this concentration dependence is considerably weaker than previously found for other systems. We believe that this is mainly due to the fact that b is much smaller than for these other surfactant systems, and thus the crossover from the locally stiff to the flexible coil regime occurs at much higher values of q. Therefore, the influence of intermicellar interactions is weaker and does not influence dσ/dΩ measurably on this length scale. This is consistent with the finding that for the L series we were only able to obtain b values for the two lowest concentrations. At higher values of c, b increased dramatically and was confounded with a very high statistical inaccuracy. We believe that this is due to the fact that the micelles with low hexanol content do have contour lengths Lc of the same order of magnitude as b. Therefore, there is only a limited part of dσ/dΩ that contains the information required to extract b, and a small suppression of the forward scattering has a considerable influence on the b values obtained in the analysis of the data. This is illustrated in Figure 8, where the SANS data for a fixed concentration and different hexanol content have

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been plotted in a so-called Holtzer or bending rod plot given by qI(q) versus q. This representation provides a sensitive way to test the agreement between theoretical and experimental scattered intensity for semiflexible chains as it significantly amplifies all deviations between data and theoretical curve in the crossover region around bq ≈ 1.19 The crossover from polymer-like to rodlike behavior shows up as a plateau due to the characteristic q-1-scattering pattern of an infinitely thin cylinder, where the plateau height corresponds to the mass-per-length ML. In the intermediate q range of the data in Figure 8 one can see an indication of such a plateau, although the high flexibility together with the finite cross-section diameter, which gives rise to an exponential decay in the cross-section Guinier regime at higher q values, makes the plateau region extremely narrow. For large coils with Lc . b, one should observe a distinct upturn at lower values of q, followed by a decrease to zero for q < 1/Lc due to the finite length of the micelles, and with a maximum that is related to the number of Kuhn length per contour length Lc/b. Figure 8 shows the extremely good agreement between the experimental data and the fitted scattering function at all q values. It also demonstrates why b values can only be determined with sufficient accuracy for the L, M, and H series. For the A series the overall length is comparable or smaller than b, which results in a direct crossover from the overall Guinier regime to the local cross-section Guinier regime that makes it impossible to determine the Kuhn length. We know from systematic Monte Carlo simulations that we can determine both Lc and b using least-squares methods for Lc/b g 2.5, which immediately sets the limits for the system under investigation. Micellar Growth and Interaction Effects. There has been a considerable effort devoted to the experimental and theoretical characterization of the concentration dependence of the micellar size distribution in particular under conditions, where a sphere-to-rod transition occurs. However, a decisive test of the various micellar growth models using scattering methods is by no means an easy task. An analysis of the detailed structure and size distribution of anisotropic micelles is severely hampered by the difficult task of distinguishing between the contributions from intermicellar interactions and concentration dependent micellar growth to the scattering data. Because of the recently postulated analogy between micelles and polymers, we are now confronted with a situation which at first seems contradictory to our intuition: While an analysis of micellar properties (size, shape) for solutions with limited micellar growth based on (light) scattering data is extremely difficult because no rigorous theory exists for interactions between polydisperse anisotropic and charged particles, the problem becomes manageable for systems with very pronounced growth, despite the fact that the micelles are so large that they experience strong intermicellar (excluded volume) interactions even close to the cmc. This is based on the fact that for polymers, enormous progress has been made in the understanding of the results from scattering experiments, and it has been possible to demonstrate that various properties such as the osmotic compressibility, the correlation length or the single chain radius of gyration exhibit a concentration dependence which follow universal scaling functions of a single variable, the reduced concentration X ≈ c/c*. For micelles the situation is of course more complicated due to the fact that the concentration dependence of the experimental results reflects contributions from the equilibrium size distribution, i.e., micellar growth, as well

Stradner et al.

as from intermicellar interactions. It is only recently that it had been possible to combine both micellar growth and interaction effects and apply polymer renormalization group theory and Monte Carlo (MC) simulations to quantitatively analyze the scattering results from micellar solutions.19,29-32 We can now apply the same theoretical framework and see if we can successfully incorporate micellar growth and interactions in the analysis of the scattering data. It has previously been suggested that pure APG micelles should possess a roughly globular shape, and we can thus first try to model the micelles as hard spheres with limited or no growth. In such an analysis we use the same approach as successfully applied for example to oil-in-water microemulsions formed in the system C12E5-decanewater.33,34 However, an analysis of the SANS data at low concentrations obtained with the A series immediately reveals that the q dependence of the intensity is inconsistent with a model of core-shell particles of limited polydispersity and interacting via a hard sphere potential. Even when assuming unreasonably large values of the polydispersity this does not result in a satisfactory fit of the SANS data as demonstrated in Figure 9a. Here the data obtained with a pure APG sample at a concentration of 2 wt % are compared with the best fit result (dotted line) for a calculation using the full numerical solution of the Percus-Yevick closure relation for polydisperse coreshell particles.35 We clearly see from Figure 9a that even for a very high value of the polydispersity σ ) 0.75, which is completely unrealistic due to the geometrical packing constraints for spherical or globular micelles, there are large systematic deviations between data and calculated scattering function. This finding is furthermore supported by a full analysis of the concentration dependence of the apparent molar mass Mapp as obtained from SLS (Figure 1a), where we use an expression of the form Mapp ) MwS(0) together with an expression for S(0) for hard spheres. Here we use the semiempirical extension of the Percus-Yevick theory derived by Carnahan and Starling,36 which provides us with a very accurate analytical expression for the osmotic pressure of a monodisperse hard sphere suspension, which leads to

S(0) )

(1 - φhs)4 1 + 4φhs + 4φhs2 + 4φhs3 + φhs4

(4)

where φhs is the hard sphere volume fraction. Equation 4 has been applied successfully for example to solutions of spherical oil-in-water microemulsion droplets formed by nonionic surfactants.34 Using eq 4 and converting the weight concentrations into volume fractions, we can then immediately calculate the expected concentration dependence of Mapp and compare it with the SLS data from Figure 1a. The result of this comparison is shown in Figure 10a (dashed line), and we immediately see that eq 4 is not capable of reproducing the measured concentration dependence of Mapp. (29) Schurtenberger, P.; Cavaco, C. J. Phys. II (France) 1993, 3, 1279. (30) Schurtenberger, P.; Cavaco, C. Langmuir 1994, 10, 100. (31) Schurtenberger, P.; Cavaco, C. J. Phys. II (France) 1994, 4, 305. (32) Schurtenberger, P.; Cavaco, C.; Tiberg, F.; Regev, O. Langmuir 1996, 12, 2894. (33) Olsson, U.; Bagger-Jo¨rgensen, H.; Leaver, M.; Morris, J.; Mortensen, K.; Strey, R.; Schurtenberger, P.; Wennerstro¨m, H. Prog. Colloid Polym. Sci. 1997, 106, 6. (34) Olsson, U.; Schurtenberger, P. Langmuir 1993, 9, 3389. (35) Pedersen, J. S. Adv. Colloid Interface Sci. 1997, 70, 17. (36) Carnahan, N. F.; Starling, K. E. J. Chem. Phys. 1969, 51, 635.


Langmuir, Vol. 16, No. 12, 2000 5361

Figure 9. A comparison between calculated and measured q dependence of the SANS intensity using eq 7. The fits to the data are shown as the solid line: (a) A sample (2 wt % APG, no added hexanol); the dotted line marks the best fit result using PercusYevick for polydisperse core-shell particles (polydispersity σ ) 0.75) and the dashed line results from the GIFT calculation for the micelles; (b) A sample (15 wt % APG, no added hexanol); (c) L sample (10 wt % APG, [hex]/[APG] ) 0.08); and (d) H sample (7 wt % APG, [hex]/[APG] ) 0.15). Inset: Holtzer representation.

The above findings clearly indicate that the data from the 2 wt % APG sample (Figure 9a) cannot be satisfactorily interpreted by strictly spherical particles. The recently developed generalized indirect Fourier transformation technique (GIFT) relaxes the condition of spherical shape for the form factor, i.e., allowing for arbitrary shape, while still describing the particle interactions with the PercusYevick closure relation for polydisperse spheres.37,38 This approach is far from perfect to describe interacting rodlike micelles; however, it has been successfully used to find sphere to rod transitions for moderate concentrations38 without any a priori assumptions about the shape of the particles. This technique gives an excellent fit to the data (see dashed line in Figure 9a). The corresponding pair distance distribution function p(r) is very similar to the results shown in our previous contribution,18 giving clear indication of that the micelles are rather short rods with a certain length distribution. In a next step we thus have to allow for anisotropic particle shape and try to model the system as a polydisperse solution of short cylindrical particles which interact via excluded volume interactions. As soon as we consider an anisotropic micellar shape, we can of course relax the restricted polydispersity and allow for a concentration(37) Brunner-Popela, J.; Glatter, O. J. Appl. Crystallogr. 1997, 30, 431. (38) Brunner-Popela, J.; Mittelbach, R.; Strey, R.; Schubert, K.-V.; Kaler, E. W.; Glatter, O. J. Chem. Phys. 1999, 21, 10623.

induced micellar growth. However, we are now confronted with the problem that no solid theoretical basis for the structure factor of short cylindrical particles in the limit of low axial ratios Lc/d exists. We have thus resorted to a purely phenomenological approach and used the same expressions as recently established for semiflexible chains with excluded volume effects.39 These were based on previous works using renormalization group theory (RGT) and a detailed study of the full structure factor of wormlike polymers and micelles using a combination of Monte Carlo simulations of many chain systems and SANS experiments on well-defined model systems (polystyrene in deuterated toluene). MC simulations were used to determine accurate static structure factors S(q) for semiflexible chains with excluded volume effects over a large range of contour lengths Lc and volume fractions φ. The correlation length ξ and osmotic compressibility κT have been determined for the simulations as well as experimentally for polystyrene in d-toluene by small-angle neutron scattering, and excellent agreement was found between the experimental data and the simulations as well as with the corresponding renormalization group calculations. Interpolations of the MC scattering functions fitted the SANS data in the full measured range of scattering vectors, demonstrating agreement almost down to atomic level. Since the simula(39) Pedersen, J. S.; Schurtenberger, P. Europhys. Lett. 1999, 45, 666.

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Stradner et al.

where the small amounts of hexanol added already induced considerable micellar growth. It is noteworthy that the corresponding values of the growth exponent R are R ) 0.1 for the A series and R ) 0.8 for the L series, i.e., we find a dramatically enhanced concentration-dependent micellar growth under conditions where larger micelles form. This finding is in agreement with previous reports on micellar growth for different systems.19,42 In a next step we can then try to calculate the full normalized scattering intensity for the samples from the A and L series. The starting point for this calculation is the so-called PRISM theory for polymers, which is an analogy to integral equation theory of liquids.43 S(q) is given by

S(q) ) Ssc(q)/[1 + Ac(q)Ssc(q)]

Figure 10. Concentration dependence of the apparent molar mass Mapp as determined by SLS. Also shown are the fits using a combination of eqs 5 and 6 as solid lines and using eq 4 as a dashed line: (a) A series (O, pure APG) and L series (3, [hex]/ [APG] ) 0.08) and (b) M series (0, [hex]/[APG] ) 0.12) and H series (), [hex]/[APG] ) 0.15).

tions were carried out over a relatively large range of Lc values, we therefore expect that the same expressions could be used even for relatively short particles with Lc ≈ b. In a first step, we thus use the same approach for analyzing the concentration dependence of Mapp as described in detail previously in refs 29-31, and 40, i.e., we assume an expression of the form

Mapp ) MwS(0) ) BcRS(0)

(5)

Here we use the well-established fact that for micelles exhibiting one-dimensional growth the molar mass depends with a power law on the surfactant concentration. In the analysis of the experimental data we applied an expression for the structure factor S(0) derived from renormalization group theory for polymers:19,32,40,41

S(0)-1 ) 1 +

(

) {[ ) ]}

2 ln(1 + X) 1 1 1 + 9X - 2 + exp 8 X 4 X 1 1 - 2 ln(1 + X) (6) X

(

where X ≈ c/c* is a reduced concentration. Figure 10a clearly demonstrates that the combination of eqs 5 and 6 reproduces the experimental data over the entire range of concentration both for the solutions with pure APG as well as for the L series, i.e., under conditions, (40) Oono, Y. Adv. Chem. Phys. 1985, 61, 301. (41) Ohta, T.; Oono, Y. Phys. Lett. 1982, 89A, 460.

(7)

where S(q) and Ssc(q) are the full (many chain) and single chain static structure factor and c(q) is the Fourier transform of the direct correlation function of the “monomers”. In a systematic MC simulation study, S(q), Ssc(q), and c(q) have been determined previously, and a parametrized numerical version has been successfully applied to polystyrene with different molar masses in deuterated toluene.39 While this numerical expression is relatively complicated, an important result of this simulation study has been the finding that if the appropriate concentration dependence of S(0) and of Rg are taken into account, and c(q)Ssc(q) in eq 7 is replaced by the scattering function of Gaussian chains PD(q2Rg2), this gives an estimate of S(q) which is valid to within about 5% in the entire concentration range. This makes it relatively easy to perform a calculation of S(q) which can be used in the analysis of experimental data. It is in particular possible to incorporate the known concentration dependence of the micellar size, the previously determined intrinsic flexibility, and the known cross-section structure of the micelles and calculate the full structure factor of polymer-like micellar solutions in the dilute and semidilute regime on absolute scale.19,39 Here we use the relation A ) S(0)-1 - 1, where S(0) is calculated using eq 6. This is now done in Figure 9a-c, where we demonstrate how well the experimental SANS data is reproduced by this theoretical approach. The calculations shown in this figure have been done for two samples from the A (c ) 2 wt %, Figure 9a, and c ) 15 wt %, Figure 9b) and one from the L (c ) 10 wt %, Figure 9c) series. The deviation between calculated curve and experimental data is generally well below 10% over the entire range of q values, which spans 2 orders of magnitude. It is important to point out that the solid curves in Figure 9 have not been fitted, but represent the result of a calculation where integral parameters (Mw, S(0) from SLS and b, mass-per-length ML, cross-section excess scattering length density profile ∆Fcs(r) from SANS) obtained from a combined analysis of a large set of measurements have been used only. In these calculations, we have used the same ad-hoc formalism to include polydispersity as previously described in Jerke et al. (ref 19), where we assumed a Schulz-Zimm distribution with 〈M〉w/〈M〉n ) 2. The close correspondence between calculation and experimental data shown in Figure 9 is quite remarkable as it is performed on absolute scale and relies on the perfect agreement between the light scattering and the SANS data. It also demonstrates that the phenomenological expression for the full static structure factor of semiflexible (42) Kato, T.; Kanada, M.; Seimiya, T. Langmuir 1995, 11, 1867. (43) Schweizer, K. S.; Curro, J. G. Adv. Polym. Sci. 1994, 116, 319.


Langmuir, Vol. 16, No. 12, 2000 5363

Table 2. A Summary of the Resulting Values of the Weight-Averaged Contour Length (〈Lsc〉w) and Kuhn Length (bsc) from a Fit of the Single Chain Scattering Function Ssc(q) to the Data Shown in Figure 9a-d, and a Comparison with the Corresponding Values (〈LMC〉w, bMC) Used in the Calculation of the Scattering Curves Applying the Many Chain Scattering Function Given by Eq 7 [hex]/[APG]

c [g/cm3]

〈Lsc〉w [Å]

bsc [Å]

〈LMC〉w [Å]

bMC [Å]

S(0)

0 0 0.08 0.15

0.022 0.169 0.12 0.089

72 20 53 3200

290 30 500 112

86 120 1000 7040

100 100 100 100

0.83 0.32 0.13 0.45

polymers with excluded volume effects derived from systematic Monte Carlo simulations works even in the limit of very short chains with Lc ≈ b, where we previously lacked an appropriate form that could reproduce the effect of interparticle interactions on the scattering data. It is important to point out that the calculated curves in Figure 9 were obtained with the correct values for the micellar size and flexibility, whereas a direct fit of the experimental data using the appropriate single particle scattering functions would either have failed completely or resulted in completely wrong values of Lc and b. This can be demonstrated with the corresponding values obtained in a fit of the single wormlike chain scattering function Ssc(q) to the samples shown in Figure 9. The results of these fits are summarized in Table 2. If we, however, include interaction effects through eq 7, we use a constant value of b ) 100 Å, and the corresponding values for Lc and S(0) obtained from the analysis of the SLS data given in Table 2. It should be mentioned that the GIFT technique in its present version cannot be used to interpret the data of the higher concentrated samples shown in Figure 9. This is due to the fact that there exist elongated rods but only a simple averaged structure factor for polydisperse spheres is used. However, the situation changes completely for the two series with higher hexanol content, where we find much more extended micellar growth and the formation of giant polymer-like micelles. At low concentrations c < c* the scattering data are fully consistent with the presence of polymer-like micelles, and the local structural properties up to length scales of order b remain constant in the entire range of concentrations investigated. This is also demonstrated in Figure 9d, where we calculated the full normalized scattering intensity for a sample with high hexanol content using eqs 5-7 and the same procedure as for the A and L series. The Holtzer representation shown as an inset underlines the perfect agreement between experimental data and calculation. However, the theoretical framework obtained from the polymer analogy and described above is not capable of reproducing the data at concentrations c > c*. This is already clearly visible when looking at the concentration dependence of the apparent molar mass shown in Figure 10b. While the initial concentration dependence of Mapp can be reproduced by eqs 5 and 6 using values for the growth exponent R ) 1.1 (M series) and R ) 1.2 (H series), at c > c* we find an extremely strong decay of Mapp with increasing c that is completely inconsistent with polymer theory for semiflexible chains in a good solvent. Under these conditions, the light-scattering experiments are not sensitive to individual micellar properties any longer, but probe collective features of the entanglement network which are independent of the micellar size distribution or local

chemistry.44,45 Mapp should become completely independent of the micellar size and decrease with a power law of the form Mapp ≈ c-1.31.44,46 Such a power law has indeed been observed for various micellar systems in the semidilute regime.30 While we expect slight modifications of the power law decay of Mapp with concentration due to the fact that for equilibrium polymers c* is not a constant as a result of the concentration-dependent size distribution,19 this can never account for the extremely steep decay of Mapp with c observed both for the M and H series. The same inconsistency with the polymer analogy can also be observed for the concentration dependence of the apparent radius of gyration Rg,app shown in Figure 1b. In the semidilute regime, Rg,app is not related to the micellar size and shape any longer, but corresponds to the static correlation length ξs ) Rg,app/x3, i.e., a screening length for excluded volume interactions in the network which can be linked to the “mesh size” of the network. According to scaling theory for semidilute polymer solutions, ξs should be independent of M and be related to the concentration according to a power law of the form ξs ≈ c-0.77. Such a power law behavior has indeed been found for polymers and polymer-like micelles, with values of approximately -0.72 for the exponent (see ref 30 and references therein). It is clear from Figure 1b that the concentration dependence of Rg,app is much too pronounced to be accounted for by entanglement effects for linear chains only. However, at present it is difficult to understand these findings on the basis of the available data only. There remain several different possibilities, which require an additional experimental and theoretical effort. It is for example conceivable that the unusually strong concentration dependence of Mapp and Rg,app could be the result of micellar branching, where the micelles would start to resemble star polymers rather than linear polymers, with a subsequently modified effective intermicellar interaction behavior. It is known from other micellar systems that micellar branching can occur under conditions where the value of the spontaneous curvature favors extensive micellar growth.47 Another possibility which we cannot disregard at present could be a more complicated aggregation behavior due to the effect of a concentrationdependent hexanol partitioning that could be superimposed on the interaction effects. Finally, due to the incorporation of the hexanol the modified surfactant film could give rise to a deviation of the effective intermicellar interaction potential from the purely hard sphere or excluded volume behavior taken into account in our interpretation. Conclusion The present study of the structural properties of sugarbased surfactants as a function of surfactant concentration and added hexanol has demonstrated the enormous effect of hexanol on the resulting micellar size and shape. We believe that this system is particularly interesting as it allows for a finely tunable micellar growth with a controllable transition from short to giant and highly flexible micellar aggregates that can be used to test the currently existing theoretical approaches for analyzing the effects of interparticle interactions on scattering data. While we have achieved very promising agreement (44) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (45) Brown, W.; Nicolai, T. Colloid Polym. Sci. 1990, 268, 977. (46) Daoud, M.; Cotton, J. P.; Farnoux, B.; Jannik, G.; Sarma, G.; Benoit, H.; Dupplessix, R.; Picot, C.; de Gennes, P. G. Macromolecules 1975, 8, 804. (47) Lequeux, F. Curr. Opin. Colloid Interface Sci. 1996, 1, 341.

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between the theoretical predictions and the experimental findings for the solutions where shorter micelles exist, we have found a rather surprising behavior under conditions where extensive micellar growth occurs. Moreover, the micelles appear to be extremely flexible when compared to most other known systems, and it will be interesting to further investigate possible links between the interfacial properties and composition, the resulting bending moduli, and the self-association behavior of different surfactant systems in an attempt to better understand micellar flexibility as one of the most important parameters in a theoretical description of surfactants in two and multicomponent systems. Finally, we believe that this structural study, combined with the previously demonstrated possibility to use these sugar surfactant solutions as interesting guest/host systems for lipases, where the enzyme activity appears to strongly depend on the aggregation properties of the micellar host, has provided a basis for a systematic investigation of the use of micelles and microemulsion as “microreactors” with tunable chemical

Stradner et al.

properties. This system could become a prime candidate for a good model system to strive for a quantitative understanding of the link between aggregate microstructure and enzyme activity in surfactant-based nanostructured materials. Acknowledgment. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland, and the Institut LaueLangevin, Grenoble, France, for providing the neutron research facilities, and we gratefully acknowledge the expert help of our local contacts, Dr. Joachim Kohlbrecher at the PSI and Dr. Roland May at the instrument D22. We thank Cornelia Sommer for measuring dn/dc. In addition, we are grateful to Dr. Wolfgang von Rybinski from the Henkel company, who supplied us with the alkyl polyglucoside C8/10G1.5 used in this study. This work was supported by the O ¨ sterreichischer Fonds zur Fo¨rderung der wissenschaftlichen Forschung under Grant P11778CHE. LA991679R

A Hexanol-Induced Sphere-to-Flexible Cylinder Transition in Aqueous

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