Interactions between Chitosan and SDS at a Low-Charged Silica

3814

Langmuir 2008, 24, 3814-3827

Interactions between Chitosan and SDS at a Low-Charged Silica Substrate Compared to Interactions in the BulksThe Effect of Ionic Strength Maria Lundin,† Lubica Macakova,*,† Andra Dedinaite,†,‡,§ and Per Claesson†,‡ Surface Chemistry, Department of Chemistry, Royal Institute of Technology, Drottning Kristinas Va¨g 51, SE-10044 Stockholm, Sweden, Institute for Surface Chemistry, P.O. Box 5607, SE-114 86 Stockholm, Sweden, and Department of Chemistry, UniVersity of Aarhus, DK-8000 Aarhus C, Denmark ReceiVed August 28, 2007. In Final Form: October 16, 2007 The effect of ionic strength on association between the cationic polysaccharide chitosan and the anionic surfactant sodium dodecyl sulfate, SDS, has been studied in bulk solution and at the solid/liquid interface. Bulk association was probed by turbidity, electrophoretic mobility, and surface tension measurements. The critical aggregation concentration, cac, and the saturation binding of surfactants were estimated from surface tension data. The number of associated SDS molecules per chitosan segment exceeded one at both salt concentrations. As a result, a net charge reversal of the polymer-surfactant complexes was observed, between 1.0 and 1.5 mM SDS, independent of ionic strength. Phase separation occurs in the SDS concentration region where low charge density complexes form, whereas at high surfactant concentrations (up to several multiples of cmc SDS) soluble aggregates are formed. Ellipsometry and QCM-D were employed to follow adsorption of chitosan onto low-charged silica substrates, and the interactions between SDS and preadsorbed chitosan layers. A thin (0.5 nm) and rigid chitosan layer was formed when adsorbed from a 0.1 mM NaNO3 solution, whereas thicker (2 nm) chitosan layers with higher dissipation/unit mass were formed from solutions at and above 30 mM NaNO3. The fraction of solvent in the chitosan layers was high independent of the layer thickness and rigidity and ionic strength. In 30 mM NaNO3 solution, addition of SDS induced a collapse at low concentrations, while at higher SDS concentrations the viscoelastic character of the layer was recovered. Maximum adsorbed mass (chitosan + SDS) was reached at 0.8 times the cmc of SDS, after which surfactant-induced polymer desorption occurred. In 0.1 mM NaNO3, the initial collapse was negligible and further addition of surfactant lead to the formation of a nonrigid, viscoelastic polymer layer until desorption began above a surfactant concentration of 0.4 times the cmc of SDS.

Introduction Systems containing both polyelectrolytes and surfactants in aqueous solutions have attracted significant interest due to their intriguing and useful properties. Industrial applications range from personal care products and food additives to adhesion and viscosity modifiers in paints. Interactions between polyelectrolytes and oppositely charged surfactants have been extensively studied both in bulk phase and at solid-liquid and liquid-air interfaces. The topic has been reviewed in several publications.1-5 There is a growing interest toward better understanding of mixtures between surfactants and naturally occurring polymers, mainly polysaccharides, which are abundant, biocompatible, and biodegradable. Chitosan is obtained by N-deacetylation of chitin, the next most abundant natural polysaccharide after cellulose, which can be found in the shells of shrimps, crabs, and insects.6 Chitosan is a relatively stiff polyelectrolyte, which is completely soluble * Corresponding author: Tel: +46 87909911. Fax: +46-8208998. [email protected].; [email protected]. † Royal Institute of Technology. ‡ Institute for Surface Chemistry. § University of Aarhus. (1) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker, Inc.: New York, 1998; Vol. 77. (3) Nylander, T.; Samoshina, Y.; Lindman, B. AdV. Colloid Interface Sci. 2006, 123, 105. (4) Claesson, P. M.; Dedinaite, A.; Poptoshev, E. Polyelectrolyte-Surfactant Interactions at Solid-Liquid Inetrfaces Studied with Surface Force Technique. In Physical Chemistry of Polyelectrolytes; Radeva, T., Ed.; Marcel Dekker, Inc.: New York, 2001; Vol. 99. (5) Langevin, D. AdV. Colloid Interface Sci. 2001, 89, 467. (6) Shahidi, F.; Abuzaytoun, R. AdV. Food Nutr. Res. 2005, 49, 93.

under the experimental conditions used. The intrinsic pKa of chitosans amino groups is 6.0 ( 0.1 and its effective value increases with increasing degree of protonation. For chitosan with 88% degree of deacetylation, the pKa value increases from 6.0 to 6.5 upon protonation from 0 to 90%.7 Chitosan is extensively used in a range of applications such as drug formulations and antibacterial treatments8,9 and more recently in building polyelectrolyte multilayers driven by successive recharging of the surface due to the last adsorbing polyelectrolyte.10,11 Previously, we showed that chitosan adsorbed at a mica substrate is highly resistant toward desorption by sodium dodecyl sulfate, a surfactant that is commonly used in personal care products.12 In another report, we suggested the use of chitosan in protective coatings for surfaces that are often treated by anionic surfactants, where a preadsorbed layer of the glycoprotein mucin was protected against surfactant removal by a thin chitosan layer.10 In this study, several different techniques were combined to obtain a many sided view of the association between chitosan and sodium dodecyl sulfate (SDS) in bulk and at interfaces. The formation of surface-active chitosan/SDS complexes at the airliquid interface was followed by monitoring changes in surface tension upon stepwise addition of surfactants to chitosan solutions. These data were complemented by turbidity and electrophoretic (7) Rinaudo, M.; Pavlov, G.; Desbrieres, J. Polymer 1999, 40, 7029. (8) Kumar, M.; Muzzarelli, R. A. A.; Muzzarelli, C.; Sashiwa, H.; Domb, A. J. Chem. ReV. 2004, 104, 6017. (9) Janes, K. A.; Calvo, P.; Alonso, M. J. AdV. Drug DeliVery ReV. 2001, 47, 83. (10) Dedinaite, A.; Lundin, M.; Macakova, L.; Auletta, T. Langmuir 2005, 21, 9502. (11) Svensson, O.; Lindh, L.; Cardenas, M.; Arnebrant, T. J. Colloid Interface Sci. 2006, 299, 608. (12) Dedinaite, A.; Ernstsson, M. J. Phys. Chem. B 2003, 107, 8181.

10.1021/la702653m CCC: $40.75 © 2008 American Chemical Society Published on Web 03/15/2008

Interactions between Chitosan and SDS

mobility measurements, providing information on association in the bulk phase and some further insight into the behavior observed at the air/water interface. Ellipsometry and QCM-D were used in order to characterize adsorption of chitosan and SDS on solid silica substrates in terms of adsorbed amount, water content, and structure of the adsorbed layer. First, the effect of solution ionic strength on chitosan adsorption onto silica substrates was investigated. Then, the silica/chitosan/SDS interactions were elucidated for two ionic strengths. The association of SDS to chitosan in bulk solution and to preadsorbed chitosan at the silica/ liquid interface was compared in terms of critical association concentration (cac) and saturation binding of surfactants. Furthermore, the role of the substrate for the association of SDS with predsorbed chitosan was investigated by comparing association between SDS and preadsorbed chitosan at a highly charged mica substrate12 with association of SDS and preadsorbed chitosan at a silica substrate at pH 4, where silica is only weakly negatively charged. In the appendix, we present a method for determination of the adsorbed amount by a combination of ellipsometric and QCM-D data. This is a useful approach when the adsorption results in only small changes in ellipticity that do not allow a reliable determination of both thickness and refractive index of the adsorbed film. Materials and Methods Chemicals. Chitosan was purchased from Fluka (cat. No. 22741) with a molecular weight of 150 000 g/mol and a degree of deacetylation of 84.5%, containing e1% of insoluble matter. SDS, with >99% purity, was purchased from Sigma-Aldrich (cat. No. L6026). It was further purified by recrystallization from Milli-Q water, repeating the procedure three times. The purification level of SDS is crucial for the association behavior between chitosan and SDS, as shown in Appendix 3. Sodium nitrate (NaNO3), suprapur grade with >99.99% purity, and aqueous acetic acid, of pro analysis grade, were obtained from VWR and used as received. All solutions were prepared using deionized water (resistivity > 18 MΩ cm), purified with a Milli-RO 10 Plus pretreatment unit, followed by purification with a Q-PAK unit. The outgoing water was filtered through a 0.2 µm filter. The stock solution of chitosan was prepared with a concentration of 1 wt % in 1 wt % aqueous acetic acid and stirred for at least 48 h before use. The solution was diluted to 20 ppm for adsorption studies and to 200 ppm for bulk studies. All solutions were adjusted to pH 4.0 by nitric acid, HNO3. Substrates. Ellipsometry measurements were performed on thermally oxidized silicon slides covered with an approximately 30 nm silica layer (prepared by Stefan Klintstro¨m at Linko¨ping University, Sweden). QCM-D measurements were performed on AT-cut 5 MHz quartz crystals covered with approximately 50 nm of silica, purchased from Q-sense AB (Gothenburg, Sweden). Ellipsometry and QCM-D substrates were cleaned in the same way. First, they were immersed in a 2% Hellmanex-11 solution (VWR, cat. No 116900-0) for 1 h and then rinsed extensively with Milli-Q water. The surfaces were left standing in water over night. Immediately before use the surfaces were treated in an air plasma chamber for 5 min (Harrick Scientific Corp., model PDC-3XG, Ossining, NY). Quartz Crystal Microbalance-Dissipation (QCM-D). The quartz crystal microbalance used was a QCM-D from Q-sense AB (Gothenburg, Sweden) with a capacity to measure changes in frequency, ∆f, and in dissipation, ∆D, at four different frequencies simultaneously. A detailed description of the instrument has been provided by Rodahl.13 The technique is based on a circular crystal disk with metal electrodes on each side, which is made to oscillate at a resonance frequency by applying an ac voltage over the electrodes. (13) Rodahl, M.; Hook, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. ReV. Sci. Instrum. 1995, 66, 3924.

Langmuir, Vol. 24, No. 8, 2008 3815 The resonance frequency is related to the mass of the crystal, and thus any mass added to or removed from the electrode induces a frequency shift, ∆f, related to the change in sensed mass, ∆m. The Sauerbrey equation, eq 1, is valid when the adsorbed mass is small compared to the mass of the crystal and the adsorbed layer is thin, rigid, and evenly distributed over the surface.14 ∆f n

∆m ) -C

(1)

where ∆m is the sensed mass per unit area, ∆f the shift in frequency, n (n ) 1, 3, 5, 7) is the overtone number, and C is a constant that only depends on the properties of the quartz crystal, such as density and thickness. For the crystal used here, C ) 0.177 mg m-2 Hz-1. The mass calculated from the Sauerbrey equation, using the third overtone (f ∼ 15 MHz), will be referred to as the sensed mass. All overtones would give the same sensed mass if the adsorbed layer was thin and rigid. This is not necessarily the case for nonrigid layers, where viscoelastic contributions to the sensed mass become important. In a first approximation, these contributions increase with the square of the measuring frequency according to Johannsmann.15 The true sensed mass, m0, is then related to the sensed mass, m, of the adsorbed film according to eq 2

[

]

Fd24π2f2 3

m ) m0 1 + J(f)

(2)

where F is the film density, d the film thickness, f the frequency, and J is the viscoeleastic compliance, which may be frequency dependent. When the plot of m versus f2 is linear, one can consider J to be independent of frequency, and the true sensed mass, m0, is obtained by plotting the sensed masses measured with the overtones versus f2 and then interpolating it to zero frequency. We note that the true sensed mass includes the mass of the adsorbing species (the adsorbed amount) and the change in solvent mass that oscillates with the crystal due to formation of the adsorbed layer. The solvent content in the layer, ws, was determined from the difference between the true sensed QCM-D mass, m0, and the adsorbed amount determined by ellipsometry, ΓEllips, as ws )

m0 - ΓEllips m0

(3)

The sensed mass can also be affected by changes in bulk density and viscosity. In our case, with low polyelectrolyte and surfactant concentrations, these effects are small. However, various concentrations of SDS were added to a silica crystal, in the absence of preadsorbed chitosan, in order to investigate the effect of increasing the concentration of surfactant in the bulk phase. Since this surfactant does not adsorb to silica, the observed changes in frequency and dissipation are solely due to changes in bulk density and viscosity.16,17 The effective density, Feff, and effective thickness, deff, of the adsorbed chitosan layer was determined according to the procedure suggested by Ho¨o¨k et al.18 deff )

m0 ) Feff

m0 ΓEllips ΓEllips FCH + Fs 1 m0 m0

(

)

(4)

where ΓEllips is the adsorbed amount determined by ellipsometry, FCH the bulk density of chitosan (1410 kg/m3), and FS the density of the solvent (1000 kg/m3). The dissipation factor, D, is obtained from the rate of decay of the oscillation amplitude when the driving power is turned off. (14) Sauerbrey, G. Z. Phys. 1959, 155, 206. (15) Johannsmann, D.; Mathauer, K.; Wegner, G.; Knoll, W. Phys. ReV. B 1992, 46, 7808. (16) Kanazawa, K. K.; Gordon, J. G. Anal. Chem. 1985, 57, 1770. (17) Kanazawa, K. K.; Gordon, J. G. Anal. Chim. Acta 1985, 175, 99. (18) Hook, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796.

3816 Langmuir, Vol. 24, No. 8, 2008

Lundin et al.

Experimentally, the time constant, τ, for the exponential decay of the oscillating amplitude is determined and related to the dissipation as D)

1 πfτ

(5)

Generally, a flat, rigid layer has a small impact on the dissipation, while a floppy extended layer greatly increases D. The crystal is suspended in a temperature-controlled flow chamber (set to 25 ( 0.02 °C) with a sample volume of 100 µL. Measurements were initiated by establishing a baseline for frequency and dissipation in the background electrolyte. Next, a 20 ppm solution of chitosan, with the same concentration of salt as the background electrolyte, was introduced to the measurement chamber. Equilibrium was reached after 30 min of adsorption, and the polyelectrolyte present in solution was removed by flushing the flow cell with the background electrolyte solution. Next, surfactant solutions of various concentrations were introduced and interactions between the preadsorbed polyelectrolyte layer and surfactants were monitored as a function of time. Finally, the surface was rinsed with the initial electrolyte solution. Ellipsometry. The ellipsometric adsorbed amount was determined from the primary data obtained by a Multiskop instrument (Optrel GdBR, Berlin, Germany), equipped with a 532 nm laser used in null ellipsometry mode at an angle of incidence of 67°. Silica substrates were immersed in a 5 mL temperature-controlled quartz cuvette, where a magnetic stirrer mixed the solution. Chitosan layers were adsorbed from a 20 ppm solution. After adsorption equilibrium was reached (30 min), the chitosan solution was replaced by a polymerfree electrolyte solution by extensive rinsing, exchanging the cuvette volume 13.5 times. Next, SDS was added into the cuvette by stepwise addition of a solution with a concentration 10 times the cmc of SDS. The temperature was kept constant at 25 °C. The data were evaluated as follows: First, optical parameters of the substrate were characterized by measurements in two ambient media, air and aqueous electrolyte solution. The ellipsometric angles were obtained as average values measured in four zones to eliminate the effect of imperfections in the optical components. Studies of adsorption kinetics were done in one zone (zone 1) only and then corrected for imperfections through multiplication by the calculated correction factor that was obtained from the previous four-zone measurement in the electrolyte solution as the ratio between the average value and the value measured in zone 1. The procedure is described in detail by Landgren et al.19 and Tiberg et al.20 The ellipsometry technique measures changes in ellipticity of light upon reflection at an optical interface. The primary data obtained by null ellipsometry are the ellipsometric angles ψ and ∆. They are related to the complex reflectance ratio through eq 621 tan ψ ei∆ )

Rp Rs

(6)

where tan ψ corresponds to the amplitude ratio of the reflection in the direction parallel and perpendicular to the plane of incidence, and ∆ corresponds to the phase difference between them. A four-layer model (silicon, silicone dioxide, adsorbed film, ambient media) was employed in order to determine the thickness and the refractive index of the adsorbed film, assuming horizontal, optically homogeneous layers. The thickness and refractive index of the adsorbed film were varied until the complex reflectivity ratio of the model became consistent, within given errors, to that obtained from the measured values of ψ and ∆. These values were then used for calculating the adsorbed amount, ΓEllips, from the de Feijters formula22 (19) Landgren, M.; Jonsson, B. J. Phys. Chem. 1993, 97, 1656. (20) Tiberg, F.; Landgren, M. Langmuir 1993, 9, 927. (21) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarised Light, 1st ed.; North-Holland Publishing Co.: Amsterdam, 1977. (22) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759.

ΓEllips )

df(nf - na) dn/dc

(7)

where df is the average thickness of the film, nf the refractive index of the film, na the refractive index of the ambient medium, and dn/dc the refractive index increment of the adsorbent in solution. The refractive index increments for chitosan and SDS were measured with a Wyatt differential refractometer operating at a wavelength of 450 nm. The values obtained were 0.18 cm3/g for chitosan and 0.12 cm3/g for SDS (above cmc), which are in good agreement with literature values between 0.185 ( 0.003 and 0.189 ( 0.003 for chitosan with a degree of deacetylation of 88.5% and 76% respectively measured at 632.8 nm23 and the value of 0.119 measured for SDS at 632.8 nm.24 The changes in the ellipticity of light due to adsorption were in all cases very small and the described iterative procedure often resulted in unreasonably low thicknesses and high refractive indexes. Even though errors in nf and df are coupled and partly cancel each other in the calculation of the adsorbed amount,20 high deviations may have significant impact on the resulting values. Thus, in order to obtain more reliable values of the adsorbed amount, we varied only the film thickness in the model, while the refractive index was kept constant and equal to the value obtained by the effective media approximation, EMA.25 In this model, the refractive index of the layer is the effective value for the mixed layer consisting of chitosan, SDS, and solvent, and for each composition it can be calculated using the EMA; for details, see Appendix 1. For the EMA analysis, one needs to know the bulk refractive index and bulk density of chitosan and SDS. The bulk refractive index of chitosan was determined using the Optrel ellipsometer. For this purpose an approximately 100 nm thick chitosan film was spin-coated from a 10 000 ppm solution in 1% acetic acid onto an optically characterized SiO2 substrate. The film was dried in an oven at 100 °C for 2 h in order to ensure evaporation of the solvent before its refractive index was determined to be 1.54. The density of bulk chitosan was measured to be 1.41 ( 0.01 g/cm3 by mixing organic solvents with different densities and noting the liquid composition at which chitosan flakes neither sank to the bottom nor floated to the top. The bulk refractive index and density of SDS were not measured. In the calculations the refractive index of dodecane, 1.43, was used, which we suggest represents the lower estimate of the refractive index of SDS. The density of 1.1 g/cm3 was provided by the supplier. Evaluation of Ellipsometric Data for Chitosan-SDS Mixtures. The calculations are complicated by the fact that the refractive index increment used for transforming optical thickness into adsorbed amount by the de Fejters formula is different for chitosan and SDS. Thus, the effective refractive index increment is a function of the composition of the adsorbed film. Therefore, we choose to only show data calculated with a fixed refractive index increment value of 0.18, which is the value for pure chitosan. This is the maximum value of dn/dc for the mixed layer, while the minimum value can be even lower than the refractive index increment of pure SDS for certain layer structures in terms of spatial arrangement of the layer components.25 The adsorbed amount is proportional to 1/(dn/dc), and therefore, a value of 0.18 gives a lower estimate of the true adsorbed amount for mixed SDS-chitosan layers. Surface Tension Measurement. Surface tension measurements were conducted with a Sigma 70 tensiometer with a De-Nuo¨y ring,26 which reduces the errors due to analyte adsorption to the measuring device in comparison to the Wilhelmy plate method.27 A Methron Dosimat titration unit was employed in order to change the surfactant concentration in a stepwise manner. After each addition of surfactant, the solution was stirred for 60 s and then left to equilibrate for 300 s before the surface tension was measured. At each concentration, (23) Schatz, C.; Viton, C.; Delair, T.; Pichot, C.; Domard, A. Biomacromolecules 2003, 4, 641. (24) Nishikido, N.; Shinozaki, M.; Sugihara, G.; Tanaka, M.; Kaneshina, S. J. Colloid Interface Sci. 1980, 74, 474. (25) Stroud, D. Superlattices Microstruct. 1998, 23, 567. (26) Huh, C.; Mason, S. G. Colloid Polym. Sci. 1975, 253, 566. (27) Lunkenheimer, K. J. Colloid Interface Sci. 1989, 131, 580.

Interactions between Chitosan and SDS measurements were repeated until the deviation of the three last points was less than 0.1 mN/m, but not longer than 1 h. The temperature was controlled, but not adjusted, and it remained between 22 and 24 °C during experiments. The concentration of the stock SDS solution was 200 mM in 0.1 mM NaNO3 and 100 mM in 30 mM NaNO3. The concentration of chitosan was initially 200 ppm and diluted to 164 ppm upon titration. For some samples, prepared separately, the evolution of the surface tension over time was followed for up to 6 days. Turbidity. The turbidity of chitosan-SDS mixtures was measured by employing a HACH ratio turbidimeter (model 18900). Individual samples containing mixtures of chitosan and SDS were prepared for each concentration of SDS, following the procedure of adding surfactant to the polyelectrolyte solution. Samples were prepared by adding 17 mL of chitosan at a concentration of 200 ppm to each sample vial, to which a concentrated surfactant solution was added dropwise. Mixing was performed for 15 min using a shake table and the turbidity was measured 2 h, 48 h, and 6 days after mixing. All sample solutions were prepared the same way, since the mixing protocol has been shown to have a major effect on the result for mixtures of oppositely charged polyelectrolytes and surfactants that are prone to be trapped in long-lived nonequilibrium states.28,29 Electrophoretic Mobility. The electrophoretic mobility of chitosan and of a few chitosan-SDS complexes was measured by employing a Zetasizer 2000 (Malvern Instruments) equipped with a standard electrophoresis cell (ZET5104). Individual samples containing mixtures of chitosan and SDS were prepared for each concentration of SDS. Each sample was mixed for 15 min and the electrophoretic mobility was measured 2 h after mixing following exactly the same procedure as for samples prepared for turbidity measurements. The presented mobility values with error-bars are an average of 10 measurements for each sample. Prior to use the instrument was calibrated with the Malvern Zeta Potential transfer standard (code DTS-1050). All mixtures for electrophoretic mobility and turbidity measurements were prepared separately and with the same polymer and surfactant concentrations as used for surface tension measurements. Small-Angle X-ray Scattering, SAXS. The structure of 0.5 wt % chitosan solutions in the absence and presence of SDS was investigated using the modified NanoSTAR from Bruker AXS available at Aarhus University, Denmark. This instrument is a threepinhole camera with two Go¨bel mirrors to make the X-ray beam parallel and monochromatic. The detector is a two-dimensional position-sensitive gas detector (HiSTAR). The instrument and some experimental details have been described in detail by Pedersen.30 The scattered intensity is azimuthally averaged and converted to absolute scale using water as primary standard.

Results 1. Chitosan/SDS Interactions in the Bulk. Bulk association studies were performed at two electrolyte concentrations, 0.1 and 30 mM NaNO3, in which SDS in the absence of chitosan have a cmc of 8.3 and 3.3 mM, respectively. The surfactant concentration is in all cases reported as the total SDS concentration (bound and free). The electrophoretic mobility data (Figure 1) are obtained for SDS-chitosan complexes formed after addition of SDS into 200 ppm chitosan solutions. The results show that chitosan has a net positive mobility value that decreases upon addition of SDS. Charge reversal occurs between 1.0 and 1.5 mM SDS in both 0.1 and 30 mM NaNO3. Above the charge neutralization point, an increased surfactant concentration results in an increased magnitude of the negative electrophoretic mobility. (28) Naderi, A.; Claesson, P. M.; Bergstrom, M.; Dedinaite, A. Colloids Surf. A 2005, 253, 83. (29) Meszaros, R.; Thompson, L.; Bos, M.; Varga, I.; Gilanyi, T. Langmuir 2003, 19, 609. (30) Pedersen, J. S. J. Appl. Crystallogr. 2004, 37, 369.

Langmuir, Vol. 24, No. 8, 2008 3817

Figure 1. The electrophoretic mobility of chitosan and of chitosan/ SDS mixtures in 0.1 mM (open squares) and 30 mM (diamonds) NaNO3 solutions at pH 4.0 as a function of SDS concentration. The concentration of chitosan was 200 ppm. All samples were prepared separately and measured 2 h after mixing. The reported electrophoretic mobility value is an average of 10 measurements, and the error bars indicate the deviations between these measurements.

Figure 2. Surface tension and turbidity as a function of the SDS concentration in solution. The chitosan concentration was 200-163 ppm and all solutions contain 0.1 mM NaNO3 at pH 4.0. The surface tension isotherm for pure SDS (crosses), and for chitosan mixed with various concentrations of SDS (triangles) was measured with a ring. Turbidity measurements of separately prepared mixtures of chitosan and SDS measured 2 h (open squares) and 2 days (filled circles) after mixing.

Similar trends have been observed for other polyelectrolyteSDS combinations.28,31-34 2. Chitosan-SDS Complexes at the Air/Water Interface. First we note that identical surface tension values were obtained for a solution containing 200 ppm chitosan and for the chitosanfree electrolyte solution. Thus, we conclude that chitosan by itself is not surface active. The changes of surface tension upon stepwise addition of SDS in 0.1 and 30 mM NaNO3 are shown in Figures 2 and 3, respectively. At low surfactant concentrations, the surface tension reduction of the polymer-free surfactant solution is negligible. However, there is a substantial lowering of the surface tension when the (31) Fielden, M. L.; Claesson, P. M.; Schillen, K. Langmuir 1998, 14, 5366. (32) Naderi, A.; Claesson, P. M. J. Dispersion Sci. Technol. 2005, 26, 329. (33) Dedinaite, A.; Claesson, P. M. Langmuir 2000, 16, 1951. (34) Bastardo, L. A.; Meszaros, R.; Varga, I.; Gilanyi, T.; Cleasson, P. M. J. Phys. Chem. B 2005, 109, 16196.

3818 Langmuir, Vol. 24, No. 8, 2008

Figure 3. Surface tension and turbidity as a function of the SDS concentration in solution. The chitosan concentration was 200-163 ppm and all solutions contained 30 mM NaNO3 at pH 4.0. The surface tension isotherms for pure SDS (crosses), and for chitosan mixed with various amounts of SDS (black triangles) were measured with a ring. A region of unstable surface tension values for the polymer-surfactant mixture is marked with large gray triangles. Turbidity measurements of separately prepared mixtures of chitosan and SDS measured 2 h (open squares), 2 days (filled circles), and 6 days (open circles) after mixing.

nonadsorbing chitosan is present. A similar surface tension reduction at low SDS concentration (0.01 mM) in presence of chitosan has been observed previously by Babak et al.35 The synergistic effect of hydrophilic polymers on the surfactant adsorption at the air/water interface was related to adsorption in the form of polymer-surfactant complexes, which has been proven for several systems by neutron reflectivity, as recently reviewed by Penfold et al.36 The adsorption of complexes has also been observed by X-ray reflectivity37 and ellipsometry,38 where the addition of a polyelectrolyte led to thicker layers than those expected for adsorbed layers consisting only of surfactants. Clearly, association between chitosan and SDS, when the surfactant is present at low concentrations, results in the formation of highly surface-active complexes, which adsorb at the air/ water interface. 2.1. Chitosan-SDS Complexes at the Air/Water Interface in 0.1 mM NaNO3. In 0.1 mM NaNO3 (Figure 2), the surface tension decreases steeply upon SDS addition until a break point appears at approximately 0.02 mM SDS (marked with an arrow in the figure). We assign this surfactant concentration to the cac, where SDS-chitosan complexes start to associate cooperatively in the bulk solution. The surface tension decreases slower after cac as the concentration of surface active complexes does not increase to the same extent as before the cac. This part of the isotherm finishes by a sudden reduction in surface tension at an SDS concentration of 1 mM. The “jump”, which is a reproducible feature, is accompanied by a macroscopic phase separation at the air/water interface in the form of “flakes” that can be seen by the naked eye. After the jump, the surface tension value remains almost constant to about 4 mM SDS. However, one should keep in mind that the surface tension is not well defined in this region, (35) Babak, V.; Lukina, I.; Vikhoreva, G.; Desbrieres, J.; Rinaudo, M. Colloids Surf. A 1999, 147, 139. (36) Penfold, J.; Thomas, R. K.; Taylor, D. J. F. Curr. Opin. Colloid Interface Sci. 2006, 11, 337. (37) Stubenrauch, C.; Albouy, P. A.; von Klitzing, R.; Langevin, D. Langmuir 2000, 16, 3206. (38) Monteux, C.; Williams, C. E.; Meunier, J.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 57.

Lundin et al.

due to phase separation at the interface. The time dependence of the surface tension in the region between 1 and 4 mM SDS was investigated by preparing separate chitosan/SDS mixtures and recording the surface tension after different equilibration times. A small increase in surface tension, approximately 1 mN/ m, was observed when the equilibration time was increased from 5 min to 6 days (data not shown). At higher SDS concentrations, the surface tension decreases with increasing SDS concentration until a value of 39 mN/m is reached at 8.9 mM SDS and the amount of surface precipitate diminishes until it completely disappears. From this concentration the surface tension remains close to constant and equal to the value at the cmc of SDS in the absence of chitosan. Thus, this concentration is assigned to cmc in presence of chitosan. The difference between the cmc of SDS in presence and absence of chitosan corresponds to the amount of surfactant monomers associated with the polymer, estimated to be between 1.1 and 1.8 SDS molecules per segment of chitosan. The large uncertainty in this value is due to the small dip in the surface tension data (Figure 2), which complicates this estimate at low ionic strength. The observed dip was found when leaving SDS in solution for several hours under stirring and it is due to the formation of small amounts of dodecanol. When SDS solutions were prepared from the same powder and measured shortly after mixing, the dip was absent and the cmc was 8.3 mM. 2.2. Chitosan-SDS Complexes at the Air/Water Interface in 30 mM NaNO3. In 30 mM NaNO3 (Figure 3), stepwise addition of SDS to the solution results in a continuous decrease in the surface tension until a local minimum is observed at a concentration between 0.1 and 0.3 mM SDS. The cac is therefore 0.1 mM SDS according to the surface tension data. Above 0.3 mM SDS, the surface tension value increases and becomes unstable up to a concentration of about 1.4 mM SDS. In this region, which is marked with large gray triangles in Figure 3, no consistent equilibrium values were obtained, even after 1 h of measurement, and the reported values in this region have a standard deviation within 3 mN/m. At surfactant concentrations above 1.4 mM SDS, the surface tension value becomes stable again within 0.2 mN/m after 5 min. Some small precipitates were observed at the air/water interface in the SDS concentration range of 1-4 mM. The increase in surface tension is suggested to be caused by desorption of complexes from the interface as the solubility of the complexes increases. A similar increase in surface tension at intermediate surfactant concentrations was observed for mixtures of poly(dimethyldiallylammonium chloride) and SDS, and neutron reflectivity measurements confirmed that it corresponded to a decrease in the adsorbed amount of both surfactant and polyelectrolyte at the air/water interface.39 Soluble negatively charged complexes are formed at higher SDS concentrations, and SDS monomers become the dominant surface active species, which results in a surface tension reduction until a constant value is reached at approximately 4.4 mM SDS as surfactant micelles start to form in the presence of chitosan. The degree of SDS binding at the cmc was estimated to be 1.8-2.0 SDS molecules per chitosan segment from the difference between the cmc of SDS in solutions in the presence and absence of chitosan. 3. Turbidity of Chitosan-SDS Solutions. 3.1. Turbidity of Chitosan-SDS Solutions in 0.1 mM NaNO3. The turbidity of polyelectrolyte/surfactant mixtures was investigated in order to follow the aggregation and phase separation of SDS-chitosan complexes in the bulk. Results obtained in 0.1 mM NaNO3 (39) Staples, E.; Tucker, I.; Penfold, J.; Warren, N.; Thomas, R. K.; Taylor, D. J. F. Langmuir 2002, 18, 5147.

Interactions between Chitosan and SDS

solutions are shown in Figure 2. The turbidity of an initially clear chitosan solution remains low until the concentration of SDS is 0.2 mM and reaches a maximum at 0.6 mM. When the concentration of SDS is increased further, to 1.1 mM, the turbidity value decreases sharply and a precipitate appears at the bottom of the sample vial due to formation of insoluble complexes. An arrow in the figure indicates the concentration region where precipitates are formed 2 h after preparation. From 7.9 mM the samples are completely free from bottom sediment. The turbidity values are relatively stable, with minor differences between values measured 2 h and 2 days after preparation. An even longer equilibrium time, 6 days, had only little effect on the turbidity values and is therefore not included in Figure 2. By comparing turbidity and electrophoretic mobility data, it becomes clear that the decreased turbidity due to precipitation/sedimentation occurs when chitosan-SDS aggregates are close to the point of zero net charge. Furthermore, this surfactant concentration coincides with the sudden reduction in surface tension (Figure 2). Phase separation may be expected at a binding ratio close to one surfactant ion per charged polyelectrolyte segment. A 200 ppm chitosan solution corresponds to a concentration of 1.0 mM charged segments, and the fact that charge reversal occurs between 1.0 and 1.5 mM SDS indicates that, at the point of zero charge, most of the added surfactants are present in complexes. Also, the electrophoretic mobility data demonstrate that the increased turbidity values observed at high surfactant concentrations are due to formation of electrostatically stabilized dispersions of net negatively charged complexes. 3.2. Turbidity of Chitosan-SDS Solutions in 30 mM NaNO3. The turbidity of chitosan-SDS solutions in 30 mM NaNO3 solution is shown in Figure 3. The values change significantly with time, which is the major difference in comparison to the behavior in 0.1 mM NaNO3. For samples measured 2 h after preparation, the turbidity increases substantially from an SDS concentration of approximately 0.1 mM (cac in Figure 3) until a precipitate appears as flakes at the bottom of the sample vial at 0.6 mM. The concentration region where phase separation occurs in the bulk-phase 2 h after preparation is marked with an arrow below Figure 3. When leaving solutions for 2 days, a noticeable decrease in turbidity is observed for samples with SDS concentrations between 0.4 and 1.0 mM SDS, and some precipitate appears already at 0.4 mM. Clearly, sedimentation occurs despite that the complexes are net positively charged. At and above 8.0 mM SDS no precipitate is observed even after 6 days of equilibration. In the region of high SDS concentrations (above 8.0 mM), very high turbidity values are observed, which decreases with time. However, no phase separation is observed even after 6 days. Small-angle X-ray Scattering (SAXS) measurements are currently performed to identify the internal structure of the chitosan-SDS complexes, and these will be presented in a forthcoming paper. Some data obtained in 30 mM NaNO3 in the region of high turbidity are also shown in Figure 4. The relative position of the Bragg-like peaks observed in presence of SDS is consistent with a lamellar arrangement with a repeat distance of 37 Å, and the large width of the peaks indicates poor long-range order. 4. Chitosan Adsorption on Weakly Charged Silica Surfaces. 4.1. Adsorbed Amount of Chitosan. The adsorption of chitosan was carried out from a 20 ppm solution at pH 4.0 for all ionic strengths. The results are summarized in Table 1, and within experimental error the adsorbed amount of chitosan was the same from 20 and 50 ppm solutions both at 0.1 and 30 mM NaNO3 (data not shown). A chitosan concentration of 20 ppm is therefore sufficient to reach the adsorption plateau. The

Langmuir, Vol. 24, No. 8, 2008 3819

Figure 4. Small-angle X-ray scattering curves for 0.5 wt % chitosan solutions in 30 mM NaCl solutions in D2O. Results for the cases of no added surfactant (unfilled circles) and for SDS to chitosan charge ratios of 0.5 (filled circles) and 1.5 (filled squares) are shown.

ellipsometric adsorbed amount is shown as a function of background electrolyte concentration in Figure 5. When increasing the concentration of NaNO3 from 0.1 to 30 mM, the adsorbed amount of chitosan increases significantly, on average by 1.4 times, and the thickness increases from approximately 0.5 to 2 nm. At higher ionic strengths, up to 300 mM, the adsorbed amount remains the same while the thickness increases slightly. A further increase in ionic strength to 500 mM results in a small decrease in adsorbed amount and layer thickness, and no adsorption of chitosan occurs from a 1000 mM NaNO3 solution. Also included in Table 1 is the effective thickness calculated from a combination of QCM-D and ellipsometry data (eq 4), which, within experimental error, is consistent with the ellipsometer values. The results confirm the self-consistency of method for determination of the ellipsometric adsorbed amount as described in the Appendix 1. In general, the adsorbed amount of chitosan is small on the weakly negatively charged silica surface at pH 4.0, almost 10 times lower than the value of 1 mg/m2 reported on highly negatively charged mica surfaces.12 4.2. Structure of the Chitosan Layers. The results obtained with the QCM-D technique are illustrated in Figure 6 in the form of the change in dissipation per unit sensed mass (∆D/∆m ratio) and the relative solvent content in the layer as a function of the background electrolyte concentration. An increase of the ∆D/ ∆m ratio indicates formation of a less rigid layer. There is a significant difference in the structure of the chitosan layer adsorbed from a 0.1 mM NaNO3 solution compared to that from solutions of ionic strength 30 mM or higher. At 0.1 mM NaNO3 the layer is rigid with a low ∆D/∆m ratio, whereas layers adsorbed from higher ionic strength solutions are more dissipative. This is consistent with the ellipsometric results that suggest formation of a thin flat layer in 0.1 mM NaNO3 and more extended viscoelastic layers at higher ionic strengths. The chitosan layers adsorbed on silica at pH 4.0 contain a large amount of solvent. The flat layer formed at low ionic strength contains about 76% of solvent, and the extended layers contain more than 90%. The solvent content is thus 1.2 times higher for the extended layer adsorbed from 30 mM NaNO3 than for the flat layer adsorbed from 0.1 mM NaNO3. In contrast, the mean values of the layer thickness in 30 mM NaNO3 is 4-5 times thicker than the flat layer formed in 0.1 mM NaNO3. Thus, it is evident that most of the sensed solvent is not directly associated with chitosan chains, but rather have a different origin. Hence,

3820 Langmuir, Vol. 24, No. 8, 2008

Lundin et al.

Table 1. Adsorption of Chitosan on Silica at pH 4.0 in the Presence of Various Concentrations of NaNO3a NaNO3 (mM)

m0 (mg/m2)

ΓEllips (mg/m2)

water (%)

n

dEllips (Å)

deff (Å)

0.1 30 100 300 500 1000

0.39 ( 0.05 1.41 ( 0.10 2.01 ( 0.15 2.68 ( 0.37 2.45 ( 0.12 0

0.10 ( 0.02 0.14 ( 0.01 0.15 ( 0.01 0.15 ( 0.01 0.12 ( 0.01 0

76 ( 12 90 ( 2 93 ( 2 94 ( 2 95 ( 1 -

1.359 < n < 1.397 1.348 < n < 1.353 1.346 < n < 1.349 1.345 < n < 1.349 1.346 < n < 1.349 -

2