Article pubs.acs.org/Langmuir
Chitosan/Alkylethoxy Carboxylates: A Surprising Variety of Structures Leonardo Chiappisi,*,† Sylvain Prévost,†,‡ Isabelle Grillo,§ and Michael Gradzielski*,† †
Stranski Laboratorium für Physikalische Chemie und Theoretische Chemie, Institut für Chemie, Straße des 17, Juni 124, Sekr. TC7, Technische Universität Berlin, 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, we present a comprehensive structural characterization of long-term stable complexes formed by biopolycation chitosan and oppositely charged nonaoxyethylene oleylether carboxylate. These two components are attractive for many potential applications, with chitosan being a bioderived polymer and the surfactant being ecologically benign and mild. Experiments were performed at different mixing ratios Z (ratio of the nominal charges of surfactant/polyelectrolyte) and different pH values such that the degree of ionization of the surfactant is largely changed whereas that of chitosan is only slightly affected. The structural characterization was performed by combining static and dynamic light scattering (SLS and DLS) and small-angle neutron scattering (SANS) to cover a large structural range. Highly complex behavior is observed, with three generic structures formed that depend on pH and the mixing ratio, namely, (i) a micelle-decorated network at low Z and pH, (ii) rodlike complexes with the presence of aligned micelles at medium Z and pH, and (iii) compacted micellar aggregates forming a supraaggregate surrounded by a chitosan shell at high Z and pH. Accordingly, the state of aggregation in these mixtures can be tuned structurally over quite a range only by rather small changes in pH.
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INTRODUCTION In mixtures of oppositely charged surfactants and polyelectrolytes (PE), one often observes the formation of colloidal complexes.1−3 Such complexes are interesting colloidal structures because they contain hydrophobic domains stabilized by hydrophilic moieties of surfactant or residual polyelectrolyte chains, thereby constituting compartmentalized water-soluble systems. They not only are able to modify the rheology of aqueous systems substantially4 but also are interesting systems for solubilizing compounds for purposes such as drug delivery and wastewater treatment5,6 because the compartments entail locally quite different conditions of polarity. However, for many potential applications of polyelectrolyte/ surfactant complexes a high biocompatibility and the use of bioderived components is desired. This can best be achieved by employing combinations where both components are ecofriendly. Accordingly, bioderived polyelectrolytes are a natural choice for these purposes. For this reason, in the last few years intensive research has been performed on complexes formed by ionic surfactants and oppositely charged polysaccharides, such as cationically and anionically modified cellulose,4,7−10 hyaluronic acid,11,12 and chitosan.13−18 Some peculiar characteristics of these systems have been recently reviewed.19,20 Chitosan is one of the most important cationic polysaccharides. It is a partially or completely deacetylated form of chitin (Figure 1), the most abundant natural biopolymer after cellulose. In addition to its high biodegradability, availability, and low cost, another quality of chitosan is its ease of modification offered by the hydroxylic, amino, and acetylic © 2014 American Chemical Society
Figure 1. Chemical structure of chitosan.
groups.21 Unmodified chitosan is soluble in acidic aqueous solutions if sufficiently ionized, which is typically the case for pH 0.17). In contrast, Mapp w increases immediately with Z for mixtures at higher pH, and almost no difference between mixtures at pH 4.5 and 5.0 can be observed. Here it might be noted that Mw is always much higher than that of individual polymer chains, which indicates that chitosan is aggregated under all of these conditions. Similar results were obtained from the dependence of the transmission from Z, reported in Figure S5. The radius of gyration obtained using eq 2 and given in Figure 4b similarly shows two types of behavior. For complexes at pH 3.5, 3.75, and 4.0, rather constant values are observed. On the contrary, the addition of RO90 to a chitosan solution at pH 4.5 and 5.0 has a strong and similar effect on the radius of gyration, showing a well-defined minimum at Z ≈ 0.1. In general, one finds that Rg changes rather little compared to the large increase in Mapp w seen in Figure 4a, which means that the complexes become substantially more compact with increasing Z. This compaction phenomena is much more remarkable for mixtures at higher pH, as seen in Figure S6, where the ratio of 3 Mapp w and Rg (a kind of effective density) is reported as a function of Z. DLS allows us to investigate the dynamics of the systems, and important structural information can be extracted from the dependence of the mean decay time Γ̅ on q. Usually, power law behavior (i.e., Γ̅ ∝ qγ) is observed. For a diffusive mode, γ = 2 and the proportionality coefficient is the diffusion coefficient. However, different power laws are often found in polymer solutions: for example, γ = 4 for chain relaxations following the Rouse model36 and γ = 3 predicted by the Zimm model.37 The factor γ obtained for the different mixtures is reported as a function of Z for different pH values in Figure 4c. Chitosan solutions at 0.3% show a relaxation mode with γ ≈ 3, regardless of the pH (i.e., the Zimm-like behavior of a polymer in solution), as typically observed for polymers at higher concentration.38 In general, the addition of RO90 reduces the
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RESULTS AND DISCUSSION In the following section, we describe the structural characterization of mixtures of chitosan and RO90 in which the surfactant content and the pH were varied while the chitosan concentration was kept constant at 0.3%. This concentration of chitosan was chosen because we were interested in staying in the liquid state and having soluble complexes (e.g., for drug delivery). Experiments were carried out in the polyelectrolyte excess regime (i.e., Z < 1 and with an RO90 concentration well above the cmc reached at Z ≈ 5 × 10−4 with a cmc of ca. 6 μM35). A surfactant content of ca. 0.5% corresponds to a Z value of approximately 0.3. By the variation of pH one controls the degree of ionization of the surfactant and therefore the charge density of the micelles whereas the charge density of chitosan is only slightly affected within the investigated pH range, as shown in Figure S3 in the ESI. In detail, the degree of ionization of the surfactant ranges from 0.15 at pH 3.5 to 0.62 at pH 5.0, whereas that of chitosan decreases only from 0.95 to 0.85. It might be noted that the real charge density of both components in the mixtures may substantially differ from that of the pure solutions as a result of the polyelectrolyte− surfactant interactions; in particular, in this case PE and the surfactant are weak electrolytes. Several studies have shown that chitosan forms insoluble complexes when mixed with oppositely charged surfactants.15,16,18 On the contrary, the complexes formed with RO90 under the given experimental conditions are soluble over a period of at least several months. Their appearance ranges from clear to milky, depending on Z and pH (Figure 3). We 1780
dx.doi.org/10.1021/la404718e | Langmuir 2014, 30, 1778−1787
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The Rh values for mixtures at pH 4.5 and 5.0 follow a trend similar to that of the radius of gyration (Figure 4b). The very high value for the pure chitosan solution is a result of the slow network relaxation and should not be confused with a real size. For mixtures at pH 4.0, a steady decrease in Rh is found as the surfactant content is increased, which should be interpreted as the result of the stepwise disruption of the polymer network. At Z > 0.2 a diffusive mode is observed and the Rh that is found is equal to that for mixtures at higher pH and the same Z. Finally, the ratio of Rg and Rh is reported in Figure S7, and similar values are found for all mixtures, being below the value of Rg/Rh = 0.776 predicted for compact spheres. For samples showing a nondiffusive relaxation mode, this is due to the network entanglements. However, even at high pH and Z, where γ = 2, Rg/Rh is well below 0.776, indicating the formation of core−corona structures,39 with a compact core surrounded by extended PE chains. In summary, the complexation process of chitosan by RO90 as seen by light scattering can be divided into different regions: (1) below a certain Z, which depends on pH, no change is observed in Mapp w and Rg, indicating no structural rearrangement of the chitosan chains; (2) at sufficiently high surfactant content (Z), a rapid increase in Mapp w is observed, caused by an aggregation of PE chains as the consequence of the presence of bridging micelles; (3) additional micelles become incorporated into the already-existing complexes, resulting in only a small increase in molecular weight but in a compaction of the complex; and (4) at a sufficiently high Z, the network is fully disrupted and aggregates with a core−corona structure are formed, with densely packed micelles in the core. In Figure S8, values for Mapp w , Rg, γ, and Rh are reported as functions of Z normalized by the degree of ionization obtained by pH titrations. Two groups of superimposed curves are obtained, for mixtures at pH >4.0 and at pH
