From Crab Shells to Smart Systems: ChitosanâAlkylethoxy

Article pubs.acs.org/Langmuir

From Crab Shells to Smart Systems: Chitosan−Alkylethoxy Carboxylate Complexes Leonardo Chiappisi,*,† Sylvain Prévost,†,‡,§ Isabelle Grillo,∥ and Michael Gradzielski*,† †

Stranski Laboratorium für Physikalische Chemie und Theoretische Chemie, Institut für Chemie, Technische Universität Berlin, Straße des 17, Juni 124, Sekretariat TC 7, D-10623 Berlin, Germany ‡ Helmholtz-Zentrum Berlin, Lise-Meitner Campus, Hahn-Meitner-Platz, 1 D-14109 Berlin, Germany ∥ LSS Group, Institut Laue-Langevin, 6 rue Jules Horowitz BP 156, F-38042 Grenoble, Cedex 9, France S Supporting Information *

ABSTRACT: In this work, self-assembly of alkyl ethylene oxide carboxylates and the biopolymer chitosan into supramolecular structures with various shapes is presented. Our investigations were done at pH 4.0, where the chitosan is almost fully charged and the surfactants are partially deprotonated. By changing the alkyl chain length and the number of ethylenoxide units very different water-soluble complexes can be obtained, ranging from globular micelles incorporated in a chitosan network to formation of ordered multiwalled vesicles. The structural characteristics of these complexes can be finely controlled by the mixing ratio of chitosan and surfactant, i.e., simply by the solutions composition. For instance, the vesicle wall thickness can be varied between 5 and 50 nm just by varying the mixing ratio. Accordingly, we expect this system to be an outstanding carrier for hydrophilic compounds with tunable release time option. Moreover, an easy route for preparation of chitosan-based complexes in the solid state with controlled mesoscopic order is presented. This work opens the way to prepare biofriendly materials on the basis of chitosan and mild anionic surfactants which are rather versatile with respect to their structure and properties, allowing for preparation of complexes with highly variable structures in both aqueous and solid phase. Formation of such different structures can be exploited for preparation of carriers, which are able to transport hydrophilic as well as hydrophobic molecules. Furthermore, as chitosan is well known to exhibit antibacterial and anti-inflammatory properties, different applications of these complexes can be indicated, i.e., as drug delivery systems or as coatings for medical implants.

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INTRODUCTION Mixtures of polymers and surfactants play a fundamental role in the modern world, as they are present in a number of different formulations, e.g., in cosmetics, paints, and food products or for drug delivery purposes.1,2 Particularly interesting are mixtures of polyelectrolytes (PE) and oppositely charged surfactant, as they represent an easy approach for preparation of highly complex, mesostructured materials.3,4 Such mixtures of anionic surfactant and polyelectrolytes are, for instance, conventionally employed in detergency, where the surfactant has the role of solubilizing and cleaning and the polyelectrolyte to treat the fibers and inhibit soil redeposition.5 In recent years, intensive research was performed on mixtures of charged polysaccharides and surfactants.5−10 Charged polysaccharides are particularly interesting materials. On one hand, they are available in large quantities from renewable resources and are mostly nontoxic and biodegradable. On the other hand, the presence of different functional groups on their backbone allows for synthesis of tailormade derivatives.11−13 In complexes formed by polysaccharides and oppositely charged surfactants often one-dimensionally ordered © 2014 American Chemical Society

structures are found, which are particularly relevant for rheological control of the desired formulation.14 For further particular properties of such mixtures we address the reader to some recent reviews.15,16 Within the class of polysaccharides, chitosan plays a leading role. It is obtained from the deacetylation of chitin, with one major source being crustacean shells. Due to its antibacterial, antimicotic, and anti-inflammatory properties, low cost, and high binding affinity to a variety of compounds it is used for pollutant recovery17 and attracted particular attention in the medical18,19 and food industry.20,21 Chitosan was shown to strongly interact with oppositely charged surfactants.8,22−24 In particular, highly ordered complexes are formed with sodium dodecyl sulfate, which however exhibit low water solubility.22,24 The problem of low solubility can be circumvented when surfactants with enhanced hydrophilicity and less ionic character are used, e.g., alkyl ethylenoxide carboxylates.8 For Received: June 30, 2014 Revised: August 13, 2014 Published: August 13, 2014 10608

dx.doi.org/10.1021/la502569p | Langmuir 2014, 30, 10608−10616

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Table 1. Summary of Properties of the Surfactants Used According to the Producera short name C18:1E9Ac C18:1E5Ac C18:1E2Ac C12E10Ac C12E4.5Ac C12E2.5Ac C8E5Ac a

chemical formula

Mw/g·mol−1

active matter

CH3(CH2)15/17:1EO9CH2COOH CH3(CH2)15/17:1EO5CH2COOH CH3(CH2)15/17:1EO2CH2COOH CH3(CH2)11/13EO10CH2COOH CH3(CH2)11/13EO4.5CH2COOH CH3(CH2)11/13EO2.5CH2COOH CH3(CH2)7EO5CH2COOH

711 544 411 686 444 356 408

91.8% 91% 94.0% 89% 92% 92.5% 88.5%

commercial name AKYPO® AKYPO® AKYPO® AKYPO® AKYPO® AKYPO® AKYPO®

RO 90 VG RO 50 VG RO 20 VG RLM 100 RLM 45 CA RLM 25 LF1

Where EO = −(CH2CH2O)−. centrifuged at 1500 rpm for 10 min and kept at rest for at least 2 days before measurements. Solutions were prepared using Milli-Q water. Small-angle neutron scattering experiments were performed using D2O (D content >99.8%) from Eurisotop (Gif-sur-Yvette, France). Methods. Surfactant Characterization. The critical micelle concentration of the alkyl ether carboxylic acids at 25 °C and pH 4.0 was determined by surface tension and fluorescence experiments. Surface tension was measured with a Krüss K11 tensiometer (Krüss GmbH, Hamburg, Germany) at 25.0(1) °C using a du Nouÿ’s ring. Experimental data were corrected according to Huh and Manson’s procedure.29 The cmc is determined from the abrupt change in the slope of the surface tension. The headgroup area at the air−water interphase Aaw h at the cmc was obtained applying the well-known Gibbs adsorption isotherm

instance, mixtures of chitosan and nonaoxyethylene oleylether carboxylate C18:1EO9CH2COOH (C18:1E9Ac) show a surprisingly structural response to pH in addition to their high water solubility. In general, alkyl ethylenoxide carboxylic acids are interesting amphiphilic compounds whose properties can be tuned either by changes in pH or by exploiting the temperature dependence of the EO chain.25−27 Furthermore, many physicochemical properties such as critical micelle concentration (cmc), hydrophilic−lipophilic balance (HLB), or curvature of the surfactant aggregate can be strongly varied by changing the length of the alkyl chain or the number of EO units. A remarkable advantage of these compounds is given by their high biodegradability.28 Accordingly, mixtures of alkyl ether carboxylates and chitosan represent interesting systems for both applicative perspectives and fundamental science. The goal of this work is to explore further the structural variety of complexes formed by chitosan/alkyl ether carboxylates mixtures. Use of surfactants differing by their alkyl chain length and number of ethylenoxide units allows us to finely control the physicochemical behavior of the alkyl ether carboxylates and therefore to adjust the properties of the resulting materials to specific needs. In detail, self-assembling of chitosan and C8EO5CH2COOH (C8E5Ac), C12EO10CH2COOH (C12E10Ac), and C12EO4.5CH2COOH (C12E4.5Ac) was investigated, as these surfactants represent limiting cases: (i) C8E5Ac is expected to have a rather high cmc, making its mixtures with chitosan relevant also when no surfactant aggregates are present in solution, (ii) C12E10Ac is expected to form spherical micelles, and, finally, (iii) C12E4.5Ac was shown to spontaneously form vesicles at the given experimental conditions,25 i.e., at 25 °C and pH = 4.0.

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Ahaw =

RT (1 + αeff ) d ln cs 1 = NA Γ NA dσ

cmc

(1)

with NA, Γ, R, T, αeff, and cs being Avogadro’s constant, surface excess, ideal gas constant, temperature, effective degree of ionization, and surfactant concentration, respectively. αeff for C18:1E9Ac, C12E10Ac, and C8E5Ac was obtained from the structure factor used in the description of the SANS data. For the remaining surfactants a value of αeff = 0.24, obtained assuming a pKa of 4.5, was used. Fluorescence spectra of solutions containing 15 nM of Nile Red were recorded on a Hitachi F4500 FL spectrometer. The excitation wavelength was 550 nm. The cmc was determined from the abrupt change of the integrated fluorescence intensity (integration was performed between 580 and 750 nm). The same results can also be obtained from the dependence of the emission peak maximum. The free standard Gibbs energy of micellization was calculated assuming the pseudophase transition model ΔGm0 = RT ln cmc

(2)

with the cmc given in mole fraction. Small Angle Neutron Scattering (SANS). SANS curves of complexes formed by chitosan and C12E10Ac were recorded on D11 at the Institut Laue-Langevin (ILL) in Grenoble, France.30 Three different configurations were used, with a wavelength of λ = 6.0 Å sample-to-detector distances (SD) of 1.2, 8, and 34 m and collimation of 4, 8, and 34 m, respectively, covering a q range of 0.02−5 nm−1, where

MATERIALS AND METHODS

Materials. Chitosan was obtained from TCI Europe. It is characterized by a viscosity-average molecular weight of 100 kDa and a degree of acetylation of 0.15 determined by 1H NMR. The polymer was purified before use. Further details can be found in ref 8. The different alkyl ether carboxylic acids are a kind gift of KAO Chemicals. Their properties are summarized in Table 1. Sample Preparation. Solutions were prepared in an acetic acid− acetate buffer at pH = 4 with a total concentration of acetate/acetic acid (Roth, 99%) of 0.2 mol kg−1. The desired pH was obtained by addition of concentrated sodium hydroxide (Fluka, puriss.) or hydrochloric acid (Merck, 37%) solutions. In this work we systematically changed the surfactant to chitosan ratio Z, defined as the ratio between the moles of deprotonable surfactants (see Mw given in Table 1) and the moles of deacetylated chitosan units (using an effective Mw of 167.3 g mol−1). Samples in the complex-rich phase were prepared by adding a concentrated surfactant solution to a chitosan solution, such that the final concentration of chitosan was 0.3 wt % and a mixing ratio just beyond the phase boundary to the unstable region. Mixtures were then

q=

4πn sin(θ /2) λ

(3)

is the magnitude of the scattering vector, n being the refractive index (1 for neutrons) and θ the scattering angle. SANS curves of complexes formed by chitosan and C8E5Ac and C12E4.5Ac were recorded on V4 at the Helmholtz-Zentrum Berlin (HZB), Berlin, Germany.31 Four configuration were used, with λ = 4.57 Å, SD of 1.33 and 6 m, and collimation of 2 and 8 m, respectively, and at SD 15.75 m collimation 16 m with λ = 7.94 and 18.1 Å. The wavelength relative fwhm was in all cases 10%. Instrumental resolution was taken into account and obtained from the beam profile and wavelength spreading. Details on data reduction 10609


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Table 2. Summary of Some Physicochemical Properties of the Surfactants Used at 25 °C and pH 4.0a short name C18:1E9Ac C18:1E5Ac C12E10Ac C12E4.5Ac C12E2.5Ac C8E5Ac

cmcST 5.5 3.7 29 15 7.7 17 000

cmcFL 4.0 0.8 23 18 7.3 18 300

cmcCiEj c

2.0 1.6d 100e 64f 33g 9900f

ΔG0m −40.0 −41.0 −35.9 −37.5 −39.2 −20.8

σ∞

αeff 0.164 0.249

0.260

32.0 30.5 33.5 28.8 27.2 33.0

Aaw h 45.0 37.0 64.4 44.4 38.7 I

ACS h 64.2

8

57.8 42.7b 52.8

ACiEj h

p

53c 38d

0.329

50.1f 35.1g

0.364 0.493 0.395

Critical micelle concentrations obtained from surface tension (cmcST), fluorescence experiments (cmcFL), and the literature values for corresponding nonionic surfactants (cmcCiEj) are given in 10−6 mol·kg−1, ΔG0m calculated from cmcST in kJ·mol−1, surface tension after the cmc σ∞ in CS mN·m−1; area per surfactant at the air−water interface Aaw h determined by surface tension (ST), at the core−shell interface Ah determined by SANS (see text for details), and area per surfactant of the corresponding non-ionic surfactant ACh iEj determined by surface tension experiments are given in c 36 d For C16E6.36 eFor Å2. The packing parameter p is dimensionless. bWeighted average of ACS h from cylindrical and ellipsoidal aggregates. For C16E9. C12E9.38 fFor C12E5.38 gfor C12E2.38 hefor C8E6.37 IData not available due to formation of microdroplets. See text for details. a

and scattering length densities and specific volumes of the different compounds can be found in the Supporting Information. SANS data were fitted in absolute units. Small Angle X-ray Scattering (SAXS). SAXS curves were recorded with a SAXSess mc2 system (Anton Paar, Austria) equipped with a CCD detector (Princeton Instruments, USA). Experiments were performed using a X-ray beam (Kα of copper, λ = 0.154 nm) collimated with a slit profile of 10 mm width resulting in a q range of 0.08−4 nm−1. Data were corrected for detector efficiency and for the smearing arising from the slit profile. Finally, data were normalized using the direct beam intensity and the scattering arising from the capillary subtracted. Light Scattering. Light scattering experiments were performed with a compact ALV/CGS-3 instrument, equipped with a He−Ne laser with a wavelength of 632.8 nm. Absolute intensities were obtained by toluene as a standard, where a Rayleigh ratio of 1.340 × 10−5 cm−1 for 25 °C and 632.8 nm was used.32 Sample transmissions were measured using a Varian Cary 50 UV−vis spectrophotometer. The apparent diffusion coefficient Dapp was obtained from the field autocorrelation function33 μ ⎞ ⎛ g(1)(q , τ ) = exp(− Dappq2τ )⎜1 + 2 τ 2⎟ ⎝ 2 ⎠

and lc/Å = 1.5 + 1.265· (nc − 1)

with nc being the number of carbons in the alkyl chain. There is good agreement between the cmc determined with the two methods, and the values are only slightly higher than those found for the corresponding nonionic surfactants36−38 due to partial deprotonation (ca. 25% at pH 4.0) of the carboxylic acids. Differently, the Aaw h are generally slightly smaller than those found for the corresponding purified nonionic surfactants, probably due to the presence of impurities, e.g., nonethoxylated fatty alcohols. C8E5Ac shows two abrupt changes in the fluorescence intensity. The solutions prepared within these two critical concentrations have a bluish or even slightly turbid appearance, and a hydrodynamic radius of ca. 200 nm is found by DLS. Therefore, we believe that in the intermediate region microdroplets of insoluble impurities are formed, which are solubilized by the surfactant once the cmc is reached. By varying the chemical composition of the surfactant, not only the cmc can be controlled but also the size and shape of the resulting micellar aggregates can too. In Figure 1 the SANS

(4)

where τ and μ2 are the delay time and the second moment around the mean, respectively. The hydrodynamic radius is then obtained by applying the well-known Stokes−Einstein equation

Rh =

kBT 6πη1Dapp

(8)

(5)

with η1 being the solvent viscosity. Experiments were performed between 23° and 130°, therefore covering a q range of 0.0054−0.024 nm−1.

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RESULTS AND DISCUSSION Pure Surfactant Solutions. Before examining in detail the structures formed by chitosan−alkyl ether carboxylates complexes, the pure surfactant solutions were characterized. The pure C18:1E2Ac solutions were not investigated in detail, as its Kraft point lies above 25 °C. The critical micelle concentrations, headgroup areas, and the packing parameter p are reported in Table 2. The packing parameter was obtained from the area per surfactant at the core−shell interface ACS h as determined by SANS and from the volume vc and length lc of the alkyl chain34,35 v p = CSc Ah ·lc (6)

Figure 1. SANS intensity as a function of magnitude q of the scattering vector for solution of different alkyl ether carboxylates and chitosan at pH 4.0. Concentrations are 0.3, 0.9, 1.1, and 2.6 wt % for Chitosan, C12E4.5Ac, C12E10Ac, and C8E5Ac, respectively. Curves are scaled for successive factors of 10. Full lines are best-fit curves calculated according to different models schematically represented in the insets; see text for more details. Chitosan and C12E10Ac solutions were recorded on D11 at ILL and C12E4.5Ac and C8E5Ac on V4 at HZB.

with vc/Å3 = 27.2 + 26.99·(nc − 1)

(7) 10610


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all cases, the same results are found: the relative solubility of the complexes increases with increasing surfactant hydrophilicity. This trend was found in several SPECs40−42 and is ascribed to the less favorable surfactant−surfactant interactions40 and to the smaller size of the micelles, i.e., to a reduced entropic force driving the association process.42,40 While this trend is found also in the examined chitosan−alkyl ether carboxylates, strong deviations are evident, which may arise from several effects: as shown before, the curvature of the surfactant aggregates strongly varies within the investigated series and it is well known that the binding of stiff polymers is favored on flat surfaces.43 This explains the relatively small solubility of C12E2.5Ac complexes (having a small headgroup and accordingly the tendency to form flat surfaces). Further differences can arise from a different degree of deprotonation of the surfactant, which also depends on the headgroup size of the surfactant. With this consideration the higher solubility of C18:1E9Ac complexes with respect to C12E10Ac can be explained. A further aspect affecting the solubility of the complexes is the structural order in the complex-rich phase, examined later in this paper. Structures Formed in Aqueous Solution. In Figure 3 the scattering patterns of chitosan complexes formed with C12E10Ac, C8E5Ac, and C12E4.5Ac are reported. It is evident that the surfactant chemical composition has a remarkably strong effect on the resulting structures. In the following the results of SANS experiments performed at pH = 4.0 and chitosan concentration of 0.3 wt % are presented, with the aim of studying in a systematic fashion the effect of the molecular surfactant structure on the chitosan/surfactant complexes. In particular, the comparison between C12E10Ac and C8E5Ac allows for investigation of the effect of the surfactant solubility, since the packing parameters of the surfactants are very close. Differently, the comparison between C12E10Ac and C12E4.5Ac highlights the effect of changing the packing parameter from ca. 0.3, typical for spheroidal micelles to beyond 0.5, typical for flat surfaces. C12E10Ac. The scattering pattern arising from chitosan− C12E10Ac complexes at pH 4 follows the same trend as observed in chitosan C18:1E9Ac mixtures.8 At low Z onedimensional ordered structures are formed, composed of N aligned micelles embedded in a cylindrical chitosan matrix44 (see schematic representation if Figure S5, Supporting Information). The large-scale behavior can be described using a fractal aggregation model. All details concerning the SANS analysis, including scattering length densities and constraints used, can be found in the Supporting Information. In the fitting procedure the number of micelles and their spacing, the fractal dimension, and cutoff length were optimized. Fit results are reported in Table S3, Supporting Information. At higher surfactant concentration, these one-dimensional suprastructures turn into a dense 3D packing of micelles, as evidenced from the correlation peak appearing at q ≈ 0.75 nm−1. Such scattering patterns can be described using a model of core−corona suprastructure with micelles densely packed within the core and a stabilizing chitosan corona (see schematic representation in Figure S6, Supporting Information). A hardsphere potential was used for describing the packing of the micelles in the core. About 104 micelle per aggregate are found, with an overall size of the core of ca. 140 nm. A schematic representation of the models used for the description of the SANS data is given in the inset of Figure 3. The analytical expressions used for data analysis, the constraints used, and the

patterns arising from different surfactant solutions at pH 4.0 are shown. C8E5Ac and C12E10Ac, having a large headgroup compared to the hydrophobic part of the molecules, form spherical micelles. Accordingly, the scattering intensity can be described with a model of core−shell spheres interacting via a hard charged spheres potential. Differently, C12E4.5Ac forms much larger aggregates, as seen by the continuous increase in scattering intensity at low q. As shown by Renoncourt et al.,25 at the current pH and concentration, C12E4.5Ac forms a mixture of large vesicles and long cylinders, presumably due to the different molecular surfactant species present. The scattering patterns were described accordingly using a model of large unilamellar bilayer vesicles and core−shell long cylinders with 60% of the surfactant forming cylindrical aggregates.39 Use of long cylinders, i.e., with the length being much longer than the radius, can be justified by the large anisotropy of the aggregates. For C12E4.5Ac and C12E10Ac the scattering arising from the free surfactant was neglected, while in the case of C8E5Ac it was described with a Gaussian chain model. Details on the SANS data analysis and the fitted parameters can be found in the Supporting Information. Best-fit and schematic representations of the structures are reported in Figure 1. The scattering pattern of chitosan at pH 4 is also included in Figure 1 and was previously described using a generalized Gaussian coil model (see Supporting Information for further details). Phase Behavior of Chitosan/Alkylethoxy Carboxylate Mixtures. The mixing ratio at which phase separation is observed, Z*, is reported in Figure 2 as a function of the

Figure 2. Critical mixing ratio Z*, i.e., Z value at which phase separation is observed, reported as a function of the cmc of the surfactant obtained at pH 4.0. Full squares are Z* values for phase separation observed immediately after mixing and full circles for phase separation observed 10 days after mixing. Full lines are only a guide for the eyes. Broken line indicates equimolarity taking into account the cmc of the free surfactant.

hydrophilicity of the surfactant, in this case described by the cmc. The reported values were recorded immediately after mixing and then again 10 days after mixing. In the Supporting Information the data are reported also as a function of the number of methylene units per ethylene oxide unit. The terms monophasic, metastable, and biphasic are used to describe the colloidal stability of the surfactant/polyelectrolyte complex. In 10611


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Figure 3. SANS intensities for mixtures of C12E10Ac, C8E5Ac, and C12E4.5Ac and chitosan at 0.3% chitosan at different Z = [−]/[+] and pH = 4. Curves are scaled by successive factors of 3. Full lines are scattering intensities calculated according to different models; see text for more details. Fitting curves for the chitosan/C12E4.5Ac data were obtained using a homogeneous vesicle model for q < 0.025 nm−1 and staked trilayered discs for q > 0.025 nm−1. C8E5Ac and C12E4.5Ac mixtures were recorded on V4 at HZB and C12E10Ac mixtures on D11 at ILL.

parameters obtained from the fitting procedure can be found in the Supporting Information. Similar structures are commonly found in block copolymer/surfactant mixtures45 and were reported also for other polysaccharide/surfactant complexes.46 An elongation of the surfactant micelles takes place upon chitosan complexation, as also observed in the case of chitosan C18:1E9Ac mixtures.8 However, due to the smaller packing parameter of C12E10Ac with respect to C18:1E9Ac the elongation is less pronounced in the former case. Interestingly, the number of micelles per cylinder (1.1−6.2) and the length of the cylinders (10−30 nm) are almost equal in mixtures of chitosan with C18:1E9Ac and C12E10Ac, despite the different alkyl chains of the surfactants, C18:1 and C12, respectively. C8E5Ac. The cmc determined for C8E5Ac is ca. 0.9 wt %, corresponding to Z ≈ 1.1. However, the presence of micellar aggregates is evident from the scattering at mid q in all spectra but for the lowest Z. We have shown before that large aggregates are found in C8E5Ac solutions below the cmc. Such large objects are also present in mixtures with chitosan before micelle formation. A similar trend as observed for C12E10Ac can be observed also for C8E5Ac, with a dense packing of micelles at high Z and formation of linearly ordered complexes at intermediate Z. Furthermore, from the scattering data at low q a clear minimum in the size of the colloidal complex at intermediate Z can be evidenced. The free surfactant concentration (fsc), not to be confused with the cmc or the critical aggregation concentration (cac), can be estimated from the scattering invariant Inv =

∫0

∞

invariant obtained from the spectra shown in Figure 3 has to be evaluated with caution, as the forward scattering intensity needed to calculate the experimental invariant via extrapolation to q = 0 using the Guinier approximationcannot be exactly determined. As the surfactant concentration increases formation of rodlike aggregates is found, as indicated by the q−1 dependence of the scattering intensity. While complexes with Z = 0.84 can be described with the model of aligned micelles, it fails when describing the scattering pattern at Z = 0.59 and a model of homogeneous cylinder has to be used instead. This is probably due to the fact that no real micellar aggregates are present, but polymer chains are bridged by hydrophobic interactions among the surfactant tails (see sketch in Figure 3). The linear ordering arises from the stiffness of chitosan and is retained also after micelle formation (0.59 < Z < 0.84). As for the case of C18:1E9Ac and C12E10Ac, after a further increase in Z the complexes collapse into a dense packing of micelles stabilized by a polymeric corona. The different structures are schematically represented in inset of Figure 3. Details on the SANS analysis and obtained parameters are reported in the Supporting Information. C12E4.5Ac. Very different scattering patterns are found in chitosan/C12E4.5Ac mixtures. All spectra show a q∼−2 power law behavior commonly found for structures with bidimensional ordering. Furthermore, for all values of Z but the smallest one (Z = 0.08) a clear correlation peak at q ≈ 0.95 nm−1 is found. Finally, a change in slope is observed at q ≈ 0.09 nm−1, indicating that the bidimensionality is lost at a scale of 2π/q ≈ 70 nm. To clarify the large-scale behavior, the q range was extended toward low q by merging the neutron scattering data with light scattering intensities. C12E4.5Ac forms, at the given experimental conditions, a mixture of vesicles and cylinders, as shown by Renoncourt et al.25 and confirmed by the SANS experiments. It is reasonable that such a mixture (cylinders and vesicles) is present also in chitosan-containing samples, with the vesicles forming ordered multilayered structures. The scattering intensity can therefore be described accordingly

n

I(q)q2dq = 2π 2

∑ i>j=1

ϕϕ (SLDi − SLDj )2 i j (9)

with n being the number of scattering domains with volume fractions ϕi. The fsc is determined within 0.1 wt % discrepancy by comparing the experimental invariant with the calculated one assuming a three-level system: (a) an anhydrous apolar phase made of aggregated surfactant alkyl chains, (b) a polar phase made of ethylene oxide units and chitosan with an hydration of 70% and 90%, respectively, and (c) the continuous phase made of the solvent plus the free surfactant chains (Figure S4, Supporting Information). It is noteworthy that the

I(q) = 1N cylPcyl(q) + 1N mlvPmlv(q) 10612

(10)


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with 1Ncyl, Pcyl(q), 1Nmlv, and Pmlv(q) being the particle number density and unnormalized scattering form factor of a long cylinder and of the multilayered vesicles, respectively. Due to the large distance between objects no structure factor was considered. Two models were used to account for the contribution of the multilayered vesicles to the scattering intensity: (i) a homogeneous vesicle model for description of the low- and mid-q data, therefore neglecting the internal structure of the vesicle wall, and (ii) large stacked trilayered disks for description of mid- and high-q data. The scattering arising from staked trilayered disks was approximated using a form factor of trilayered large disks (Ptld) and their stacking using the modified Caillé theory47,48 (SMC(q)) Pmlv(q) = Ptld(q)SMC(q) = PD′ (q)Pcs(q)SMC(q)

layers Ns. Moreover, the size of the domains is considered to be polydisperse and follow a normal distribution. Further details can be found in the Supporting Information. To limit as much as possible the number of free parameters, for the description of the scattering pattern of chitosan/ C12E4.5Ac complexes we used the values for the cylinder length L = 40 nm, radius R = 1.65 nm, and shell thickness ΔR = 1.2 nm as well as bilayer core (Tc = 1.8 nm) and shell (Tsh = 1.3 nm) thicknesses found for the pure surfactant solution. Also, the repartition of surfactant between bilayer and cylinder was kept constant and equal to 60% of the surfactant-forming cylindrical aggregates. The chitosan layer thickness (To) results from the spacing between layers and Tc and Tsh: 2To = d − Tc − 2Tsh. In order to meet the requirements for decoupling the shape and cross-section form factors a disk radius of 500 nm was used. This value, however, has no physical meaning, as the low-q part is described with a model of polydisperse homogeneous vesicles (see Supporting Information for details). Two thicknesses for the vesicle shell were used, namely, N·d and d, representing the multi- and unilamellar vesicles, respectively. The fraction of surfactant-forming unilamellar vesicles χu is obtained from SMC(q): χu = Ns/(N + Ns). Furthermore, the sizes of the multi- and unilamellar vesicle core are equal and follow a log-normal distribution (eq S47, Supporting Information). With these assumptions the fits reported in Figures 3 and S8, Supporting Information, are obtained. The parameters used are reported in Table 3. From the fit results, it can be stated that chitosan promotes formation of multiwalled vesicles, with the number of layers directly depending on the mixing ratio Z. Furthermore, the spacing between layers is independent of Z, and a total thickness of chitosan layer of less than 2 nm is found. Finally, the bending rigidity Kc can be deduced from the Caillé parameter

(11)

with P′D(q) being the shape form factor of an infinitesimally thin disk with radius RD, and Pcs(q) the cross-section form factor describing a three-layer system with thicknesses Tc, Tsh, and To. A schematic representation of the model is given in Figure 4. The Caillé theory describes the stacking of lamellae spaced by a distance d, and thermal undulations are allowed.

η1 =

πkBT 2d 2 BKc

(12)

where B is the bulk compression modulus. Assuming a constant value of B = 1014 J m−4, typically found in lipid bilayers,49 an increasing rigidity of the vesicle wall is found, ranging from 2 kBT per layer at low Z to over 10 kBT per layer at high Z. The presence of a semirigid polymer in the bilayer interstices produces an increasingly stiff vesicle wall. Similarly, membrane undulations of phospholipid vesicles were shown to be strongly reduced upon chitosan adsorption,50 which is an indication of the strong membrane−chitosan interaction. It is noteworthy that the values found at high Z are possibly underestimated, as

Figure 4. Schematic representation of the scattering model used for the description of ordered multilamellar vesicles formed in chitosan/ C12E4.5Ac complexes. Sketch is not at scale. See text for further details.

The modified Caillé structure factor is obtained considering finite sample sizes, i.e., the presence of domains characterized by a number of layers NL and the presence of uncorrelated

Table 3. Parameters Obtained from the Description of the Scattering Patterns Arising from Chitosan/C12E4.5Ac Mixtures at pH = 4.0, Chitosan Concentration of 0.3 wt %, and Increasing Surfactant Concentrationa conc.

Z

q1

NL

Ns

d

η1

Dtot

Kc/N

μ

σ

⟨Rc⟩

Rcalc h

Rexp h

0.05% 0.10% 0.15% 0.20% 0.25% 0.30%

0.08 0.17 0.25 0.33 0.42 0.51

0.946 0.932 0.951 0.946 0.950

3 4 6 6 6 7

2 3 4 3 2 1

6.3 6.30 6.34 6.40 6.43 6.45

0.080 0.040 0.035 0.030 0.025

18.9 24.8 38.0 38.4 38.6 45.2

2.1 6.5 7.6 11.0 13.4

70 65 63 55 45 45

0.65 0.65 0.65 0.60 0.50 0.50

90 80 80 70 50 50

450 430 420 280 160 160

320 310 330 260 260 260

The concentration of C12E4.5Ac is given in wt %, the peak position q1 in nm−1, the spacing d, the total thickness Dtot = Nd, and the mean vesicle size exp ⟨Rc⟩, the location parameter of the lognormal distribution μ, the calculated hydrodynamic radius Rcalc h and the experimentally determined one Rh in nm, the bending modulus per layer in kBT, the Caillé η1 parameter and the relative standard deviation σ of the log-normal distribution used are adimensional. a

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the width of the peak is limited by the instrumental resolution. With increasing surfactant concentration, i.e., increasing number of layers, a decrease in the core size is observed. However, the overall size of the vesicles Rc + N·d ≈ 100 nm remains constant. This is a rather unexpected result, as it indicates that instead of a classical layer-by-layer growth the additional layers are internalized in the vesicle. This finding can be further confirmed with dynamic light scattering experiments. The hydrodynamic radius, as obtained from polydisperse samples, can be approximated as the mean intensity weighted radius R hcalc =

∫ V 2(r )·(r + Dtot)dr ∫ V 2(r )dr

(13)

where V(r) = 4/3π((r + Dtot)3 − r3) is, in our case, the volume of a multiwalled vesicle with core radius r. The value of the hydrodynamic radius can, following eq 13, be calculated from the size distribution as obtained from the fit of the SLS-SANS data reported in Figure 3 (explicit calculation can be found in the Supporting Information). The hydrodynamic radii calculated according to eq 13 and obtained from DLS experiments are reported in Table 3. The good agreement between calculated and experimentally determined radii well support the idea of a vesicle with constant outer diameter, increasing wall thickness, and decreasing polydispersity with increasing mixing ratio Z. In particular, this demonstrates a very good agreement of the DLS and SANS/SLS data with respect to the structural picture derived. The apparent discrepancy between the size derived from DLS and static scattering can be attributed to the polydispersity of the shell thickness of the multiwalled vesicles and to the presence in solution of free chitosan chains and cylindrical aggregates. Structures Formed in the Complex-Coacervate. SANS experiments have shown an interesting structural evolution of the complexes in aqueous solutions when the surfactant concentration is increased. In all cases the shape of the surfactant aggregates is retained within the supra-aggregates. A further increase in surfactant concentration leads to phase separation of the mixtures, with formation of a liquid transparent solution and a second phase rich in polymer and surfactant (clear coacervate). The complex-rich phase can be used as a starting point for formulation of smart materials for several purposes, e.g., for responsive coatings.3,4 Accordingly, detailed knowledge of the structure-determining factors in complexes with stiff polysaccharides represents a crucial point when engineering new biofriendly materials. The microstructure of the coacervate was investigated by means of SAXS, and the scattering patterns are reported in Figure 5. They can be divided into four groups, those arising from complexes with C12E10Ac and C18:1E9Ac, with C8E5Ac, with C18:1E5Ac and C12E4.5Ac, and with C18:1E2Ac and C12E2.5Ac. C18:1E9Ac, C12E10Ac, and C8E5Ac were shown to form small globular micelles; SANS on aqueous solution of C18:1E9Ac8 and C12E10Ac complexes with chitosan show at high Z the presence of densely packed micelles in the supra-aggregate core. The dense packing of the micelle is, in these two cases, also retained in the coacervate, with the appearance of a second-order diffraction peak. Differently, the C8E5Ac micelles are not so densely packed, as indicated by the absence of a second-order peak. Furthermore, a one-dimensional suprastructure is

Figure 5. SAXS patterns of chitosan−alkyl ethylene oxide carboxylic acids found in the coacervated complexes formed at pH 4.0.

retained in the coacervate, as indicated by the q−1 dependence of the scattering intensity at low q. Soluble C12E4.5Ac complexes showed formation of ordered multilamellar vesicles. The SAXS pattern arising from the coacervate shows the presence of highly ordered lamellae, therefore retaining the layered structure of the surfactant. Retention of a flat surface is corroborated by the q−2 dependence at low q. A clear distinction between multilamellar vesicles and lamellar phase with planar membranes cannot directly be made. However, the presence of a second-order peak indicates a high number of layers involved in the structure and points toward the presence of a lamellar phase rather than a multilayered vesicle. The same applies also to C18:1E5Ac, which exhibits a similar packing parameter. Similar structures are seen in mixtures of surfactant with the tendency of forming flat surfaces and polyelectrolytes strongly differing by their persistence length, e.g., DNA and polystyrenesulfonate.51,52 Accordingly, the molecular geometry of the surfactant (as, for instance, described by the packing parameter) can be pointed out as the main driving force for formation of a lamellar solid phase. Finally, no particular long-range ordering can be seen in the coacervates formed with C18:1E2Ac and C12E2.5Ac, the most hydrophobic surfactants. In Table 4 the peak positions and the characteristic distance D* = 2π/qmax are reported for all 1 examined complexes.

■

CONCLUSIONS In this work, complexes of the biopolycation chitosan and anionic alkyl ethylene oxide carboxylates are presented. With an appropriate choice of alkyl chain length and size of the Table 4. Peak Positions and Characteristic Distances Found in Complex Coacervates of Chitosan and Different Surfactants formed at pH 4.0

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short name

−1 qmax 1 /nm

−1 qmax 2 /nm

max qmax 2 /q1

D*/nm

C18:1E9Ac C18:1E5Ac C18:1E2Ac C12E10Ac C12E4.5Ac C12E2.5Ac C8E5Ac

0.791 0.844 1.06 0.817 0.931 1.25 1.18

1.25 1.70

1.58 2.01

1.23 1.88

1.51 2.02

7.94 7.44 5.93 7.69 6.75 5.03 5.32


Langmuir

Article

Present Address

hydrophilic part, self-assembly into very different colloidal suprastructures can be controlled, ranging from small vesicles in a dense polymeric network to multilayered vesicles. We investigated in detail the structural characteristics of chitosan/alkyl ethylene oxide carboxylate mixtures as these systems seem to be promising for a variety of applications. All these investigations have been done at pH 4.0, i.e., conditions where the chitosan is almost fully charged while the surfactants are only partially charged, which means that they are a mixture of the anionic surfactant and the corresponding acid. We could show that small, globular micelles (C8E5Ac and C12E10Ac) are incorporated in the chitosan network retaining a one- or threedimensional symmetry, depending on the mixing ratio. Control over the size of the hydrophobic packets within the complexes can, for instance, be exploited for optimal solubilization of hydrophobic molecules. Furthermore, as these systems are strongly reactive upon small changes in pH,8 they be exploited for a controlled release/uptake of active molecules, such as drugs, pesticides, or pollutants. Moreover, very different types of structures are obtained from self-assembling of chitosan and large surfactant vesicles (C12E4.5Ac): multiwalled vesicles, with alternating layers of surfactant and chitosan are observed, with the number of layers being directly controlled by the mixing ratio. Accordingly, also the layer thickness and its stiffness can be accurately controlled, which makes this system highly interesting as carriers of hydrophilic molecules. Formation of multiwalled vesicles, with up to seven layers, represents a drastic improvement with respect to similar structures reported for mixtures of chitosan/ lipid vesicles.7 In these systems most of the vesicles are unilamellar, and less than 10% of the vesicles show a layer multiplicity of N > 3. Similarly, formation of multilamellar vesicles was observed in cationic dioctadecyldimethylammonium bromide vesicles and poly(acrylic acid) mixtures.53 Furthermore, from the phase diagram it is seen that the hydrophilicity of the surfactant is only partly determining the solubility of the complexes and evidenced how specific effects play also an important role. As the surfactants are very similar from a chemical perspective these effects are ascribed to the interplay between the stiffness of chitosan and the curvature of the surfactant aggregates. Finally, from characterization of the sedimented complexes we could show that the surfactant structure is retained in the solid state, allowing for preparation of materials with different properties. It is noteworthy that all employed compounds are biocompatible, affordable, and available in large quantities, hence fulfilling the important prerequisites for industrial applications, e.g., in food, agricultural, and medical industry.

■

§

European Synchrotron Radiation Facility, 6 rue Jules Horowitz, F-38043 Grenoble, Cedex 9, France.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the ILL for granted beam time, and the beam time at HZB became available due to a long-term cooperation contract between the TU Berlin and HZB. In addition, we are grateful to the TU Berlin for funding this research and Kao Chemicals for kindly providing the surfactant.

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(1) Goddard, E. D.; Ananthapadmanabhan, K. P. Interactions of surfactants with polymers and proteins; CRC Press: Boca Raton, 1993; pp 1−427. (2) Holmberg, K.; Jönsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons, Ltd: Chichester, UK, 2002; Vol. 2002; p 562. (3) Ober, C. K.; Wegner, G. Polyelectrolyte-Surfactant Complexes in the Solid State: Facile building blocks for self-organizing materials. Adv. Mater. 1997, 9, 17−31. (4) Thünemann, A. F. Polyelectrolyte-surfactant complexes (synthesis, structure and materials aspects). Prog. Polym. Sci. 2002, 27, 1473−1572. (5) Hoffmann, I.; Theile, M.; Grätz, S.; Scholz, J.; Barreleiro, P.; von Rybinski, W.; Gradzielski, M. On the influence of surfactants on the adsorption of polysaccharide-based polymers on cotton studied by means of fluorescence spectroscopy. Langmuir 2012, 28, 11400−9. (6) Pacios, I. E.; Lindman, B.; Thuresson, K. Polyelectrolytesurfactant complexes with long range order. J. Colloid Interface Sci. 2008, 319, 330−7. (7) Gerelli, Y.; Barbieri, S.; Di Bari, M. T.; Deriu, A.; Cantù, L.; Brocca, P.; Sonvico, F.; Colombo, P.; May, R.; Motta, S. Structure of self-organized multilayer nanoparticles for drug delivery. Langmuir 2008, 24, 11378−84. (8) Chiappisi, L.; Prévost, S.; Grillo, I.; Gradzielski, M. Chitosan/ Alkylethoxy Carboxylates: A Surprising Variety of Structures. Langmuir 2014, 30, 1778−1787. (9) Chiappisi, L.; Li, D.; Wagner, N. J.; Gradzielski, M. An improved method for analyzing isothermal titration calorimetry data from oppositely charged surfactant polyelectrolyte mixtures. J. Chem. Thermodyn. 2014, 68, 48−52. (10) Tolentino, A.; Alla, A.; Martínez de Ilarduya, A.; Muñoz Guerra, S. Comb-like ionic complexes of hyaluronic acid with alkyltrimethylammonium surfactants. Carbohydr. Polym. 2013, 92, 691−6. (11) Schanté, C. E.; Zuber, G.; Herlin, C.; Vandamme, T. F. Chemical modifications of hyaluronic acid for the synthesis of derivatives for a broad range of biomedical applications. Carbohydr. Polym. 2011, 85, 469−489. (12) Campo, V. L.; Kawano, D. F.; da Silva, D. B.; Carvalho, I. Carrageenans: Biological properties, chemical modifications and structural analysis − A review. Carbohydr. Polym. 2009, 77, 167−180. (13) Mourya, V.; Inamdar, N. Chitosan-modifications and applications: Opportunities galore. React. Funct. Polym. 2008, 68, 1013−1051. (14) Hoffmann, I.; Heunemann, P.; Prévost, S.; Schweins, R.; Wagner, N. J.; Gradzielski, M. Self-aggregation of mixtures of oppositely charged polyelectrolytes and surfactants studied by rheology, dynamic light scattering and small-angle neutron scattering. Langmuir 2011, 27, 4386−96. (15) Akhlaghi, S. P.; Peng, B.; Yao, Z.; Tam, K. C. Sustainable nanomaterials derived from polysaccharides and amphiphilic compounds. Soft Matter 2013, 9, 7905. (16) Chiappisi, L.; Hoffmann, I.; Gradzielski, M. Complexes of oppositely charged polyelectrolytes and surfactants - recent develop-

ASSOCIATED CONTENT

S Supporting Information *

Further experimental details, uorescence spectra and surface tension measurements, detailed description of SANS models used in this work, calculation of hydrodynamic radius, and further SANS data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 10615


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From Crab Shells to Smart Systems: ChitosanâAlkylethoxy

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