The Behavior of Chitosan in Solvents with Different Ionic Strengths

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The Behavior of Chitosan in Solvents with Different Ionic Strengths Simona Morariu,* Cristina-Eliza Brunchi, and Maria Bercea ”Petru Poni” Institute of Macromolecular Chemistry, 41-A Grigore Ghica Voda Alley, 700487 Iasi, Romania ABSTRACT: The thermodynamic behavior of chitosan in acidic aqueous solutions with ionic strengths between 0.04 × 10−2 and 24.30 × 10−2 M was investigated at 25 °C. The intrinsic viscosity and other thermodynamic parameters of chitosan with the molecular weight and the degree of acetylation of 7.14 × 105 g mol−1 and 26%, respectively, were determined and discussed by using both classical equations (Huggins and Fedors) and a new model (Wolf). At low ionic strengths of the solvent, the highest values of the intrinsic viscosity were obtained due to the expansion of the chitosan chains. Two critical concentrations, c* (which separates the dilute-semidilute regimes) and c+ (at which the dimensions of the polymer coils are considered to have shrunk to their unperturbed dimensions), were estimated, and the effect of the solvent ionic strength on their values was discussed. By increasing of the solvent ionic strength from 0.04 × 10−2 to 24.30 × 10−2 M, the persistence length decreased from 17.14 to 4.60 nm, suggesting an increase of the chitosan flexibility due to the decrease of repulsive potential between the polymer chains. The radius of gyration and the persistence length in the unperturbed state were determined as being 52.50 and 3.90 nm, respectively. The viscometric data were corroborated with those obtained by the zeta potential, conductivity, and turbidity measurements for the ionic strength of the solvent between 0.04 × 10−2 and 42.25 × 10−2 M.

1. INTRODUCTION Chitin is the second most abundant and important natural polysaccharide present in nature after cellulose and it is composed by β(1→4)D-glucosamine units with variable Nacetylation degree (DA). By de-N-acetylation of chitin, free amino groups are formed along the macromolecular chain. Generally, the polymer with DA lower than 50% is named chitosan and becomes soluble in dilute acids. At pH below 6.5 (acidic medium), the whole macromolecule becomes positively charged due to the protonation of amino groups and chitosan is converted to a weak cationic polyelectrolyte. The solubilization depends on the distribution of free amino and N-acetyl groups and increases by increasing the number of the protonated amino groups (−NH3+) on the chitosan chain. The existence of a high concentration of multivalent anions in acidic aqueous solutions of chitosan induces the physical crosslinking and the aggregation of chitosan. Also, at pH values higher than 6.5, the deprotonation of the amino groups occurs and the polymer tends to precipitate from solution. Its biological properties (biocompatibility, biodegradability, and bioadhesivity)1−7 and the ability of the positively charged backbone (controlled by pH, ionic strength, temperature, thermodynamic quality of the solvent, etc.) to interact with other macromolecules or low molecular weight compounds with negatively or neutral charged groups8−12 made efficient the application of chitosan in different domains such as cosmetics,13,14 medicine,15−17 pharmaceutical, food, industrial, and agricultural fields.18−20 Therefore, the knowledge of the structural parameters (molecular weight, degree of acetylation) as well as the physical, chemical, and biological properties is essential to choose the best chitosan sample for a particular application.21 The aim of the present study is the investigation of the ionic strength effect (I) of the solvent on the hydrodynamic properties of chitosan in solution by using different experimental techniques such as viscometry, turbidimetry, and © XXXX American Chemical Society

electrokinetic measurements. The interaction types that can occur are also discussed in accordance with the evaluated dynamic and static parameters.

2. EXPERIMENTAL SECTION 2.1. Materials. The chitosan (CS) sample was purchased from Fluka, and it was used without further purification. Acetic acid (AcOH) and sodium acetate (AcONa) with purity of 99.8% and 99%, respectively, were purchased from SigmaAldrich, Germany. 2.2. Sample Preparation. The solutions with different concentrations of chitosan were obtained under magnetic stirring for 4 h at room temperature. The acetic acid solutions with ionic strengths between 0.04 × 10−2 M (pH = 3.38) and 0.50 × 10−2 M (pH = 2.30) and the buffer solutions of acetic acid/sodium acetate with ionic strengths in the range of 4.32 × 10−2 M to 49.64 × 10−2 M (pH values from 3.74 to 4.80) were used as solvents. All solutions were prepared in bidistilled water within one day before the experimental measurements. To prevent degradation of the chitosan macromolecules, the solutions were kept in the refrigerator at temperature below 5 °C. 2.3. Measurements. Viscosity measurements were carried out in solvents with different thermodynamic qualities for chitosan, that is, aqueous solutions of acetic acid and acetic acid/sodium acetate mixture, at 25 °C (±0.01 °C), using an Ubbelohde suspended-level viscometer with 0.47 mm capillary diameter. The flow volume of the used viscometer exceeded 5 mL, making drainage errors unimportant. Flow times were obtained with an accuracy of ±0.01 s. Received: May 18, 2012 Revised: August 30, 2012 Accepted: September 6, 2012

A

dx.doi.org/10.1021/ie301309g | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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glucosamine unit (in our case 0.74), the molecular weight of the glucosamine unit (161 g mol−1) and the molecular weight of the acetylglucosamine unit (203 g mol−1), respectively. The obtained value for DP was 4152. 3.2. Effect of Solvent Ionic Strength on the Viscosity of Chitosan Solution. Figure 1 presents the variation of the

The degree of acetylation of the chitosan was established by infrared spectroscopy (IR) and nuclear magnetic resonance spectroscopy (1H NMR). The IR spectrum was recorded in the range of 4000−400 cm−1 on a Vertex 70 Bruker FT-IR spectrometer in KBr pellets. The 1H NMR spectrum was obtained by using a Bruker Avance DRX 400 spectrometer in volumetric ratio 50:50 of D2O/HCl at room temperature. To speed up the dissolution of the polymer, the sample was heated at 70 °C and the solution remained clear at room temperature during the 1H NMR analysis. The turbidity of chitosan solutions was monitored at room temperature in the wavelengths range of 400−600 nm by using a HACH 2100AN turbidimeter. The zeta potential (ζ) and the conductivity of the chitosan solutions were determined at 25 °C with a Zetasizer Nano NS (Malvern Instruments, UK). ζ was calculated from the electrophoretic mobility (μ) using the Smoluchowski relationship (ζ = ημ/ε, where η is the viscosity, ε is the dielectric constant of the medium) with the condition kDHα ≫ 1; kDH represents the Debye−Huckel parameter and α is the particle radius. The concentration of the solutions used for electrokinetic measurements was 0.4 g dL−1. pH measurements were performed at 25 °C with a Metrohm automatic instrument type Titrino 716 DMS equipped with a combined glass electrode 6.0232.100. The experimental pH values were used to estimate the concentration of ions and the ionic strength of the solvent. The ionic strength was evaluated and expressed in molar concentration, M, using the following equation: I=

1 2

∑ (ciZi 2)

Figure 1. Variation of ηsp/c as a function of polymer concentration for CS solutions in AcOH and AcOH/AcONa with different ionic strengths at 25 °C. The lines represent the extrapolation of the experimental data (obtained for the relative viscosity between 1.1 and 2) to zero polymer concentration, according to Huggins method.

reduced viscosity, ηsp/c, as a function of chitosan concentration, c, in AcOH and AcOH/AcONa aqueous solutions with different ionic strengths, at 25 °C. The [η]H values were determined in the range of relative viscosity from 1.1 to 2, where one can observe a linear decrease of the ηsp/c values with the dilution up to a certain value of concentration, depending on the ionic strength of the used solvent, from which a slightly upward curvature appears by dilution. This behavior is wellknown for polyelectrolytes in solution, a high asymptotic increase of ηsp/c occurs in the range of small polymer concentration due to a progressively enhanced of the effect of the ionizable groups along each macromolecular coils leading to the intensification of the intramolecular repulsions. In our case, the slow increase of reduced viscosity at low polymer concentrations proves the weak polyelectrolyte character of chitosan in AcOH and AcOH/AcONa aqueous solutions. The electrostatic repulsions between −NH3+ groups are stronger in the solvents with lower ionic strength and determine an expansion of chitosan chains leading to the high values of the viscosity (Table 1). The deviation from the linear dependence of the reduced viscosity with the concentration at low polymer concentration makes it difficult to determine the intrinsic viscosity by the Huggins method. For this reason, we also used other methods, such as the Fedors equation that can be applied to both uncharged and charged polymer solutions on the dilute and moderately concentrated domains:27 1 1 1 = − 1/2 c[η]F cm[η]F 2(ηr − 1) (4)

(1)

where ci is the concentration of ion i and Zi represents the charge number of ion i.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Chitosan Sample. The acetylation degree (DA) values of the chitosan sample evaluated by IR22,23 and 1H NMR24,25 methods were 26.56% and 25.80%, respectively. An average value of 26% for DA of the chitosan was taken into account. The molecular weight of chitosan was evaluated from the intrinsic viscosity, [η], determined in 0.5 M acetic acid/0.5 M sodium acetate buffer solution at 25 °C by using the Huggins equation: ηsp = [η]H + kH[η]H 2 c (2) c where ηsp/c is the reduced viscosity, kH represents the Huggins constant which offers information about the polymer interactions and the solvent quality, [η]H is the intrinsic viscosity determined by Huggins method, and c is the polymer concentration. By applying the following Mark−Houwink equation proposed by Yomota et al.:26 [η] = 0.199M v 0.59

(mL/g)

(3)

where [η]F is the intrinsic viscosity calculated by Fedors method, ηr is the relative viscosity, and cm is a maximum packing density of the polymer coils. The [η]F values for studied samples were determined from the slope of the plots (1/(2(η1/2 r −1))) versus 1/c according to eq 4 (Figure 2). The values of [η]F are in good agreement with

the viscometric molecular weight (Mv) of chitosan was established as being 7.14 × 105 g mol−1. The degree of polymerization (DP) of the chitosan sample was calculated with the relationship ((Mv)/(xM1 + (1 − x) M2)) where x, M1 and M2 are the molar fraction of the B



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Table 1. Viscometric Parameters Determined by Huggins, Fedors, and Wolf Methods for CS in Solvents with Different Ionic Strengths Huggins method (eq 2) −1

Fedors method (eq 4) −1

Wolf method (eq 5)

I × 10 (M)

[η]H (dL g )

kH

[η]F (dL g )

cm (g dL )

[η]W (dL g−1)

BW

k H + BW

0.04 0.18 0.32 0.50 4.32 8.05 16.06 24.30

55.92 37.64 30.33 25.14 9.31 11.70 9.72 7.96

0.74 0.30 0.31 0.68 0.43 0.36 0.51 0.52

58.14 37.39 30.45 25.44 9.36 11.72 9.94 8.08

0.07 0.46 0.83 0.14 0.88 1.10 0.77 0.78

57.84 37.67 30.55 25.39 9.33 11.74 9.85 8.02

−0.01 0.21 0.20 −0.07 0.12 0.17 0.08 0.05

0.73 0.51 0.51 0.61 0.55 0.53 0.59 0.57

2

−1

those evaluated by the Huggins method in the dilute regime for relative viscosity between 1.1 and 2 (Table 1).

Figure 3. Plots of ln ηr as a function of chitosan concentration for different ionic strengths of the solvents at 25 °C. The lines are calculated according to the Wolf method by means of the parameters from Table 1.

Figure 2. Variation of (1/(2(η1/2 −1))) as a function of 1/c for CS r solutions in AcOH and AcOH/AcONa with different ionic strengths at 25 °C. The lines represent the experimental data fitted with the Fedors equation.

The BW parameter, which reflects the curvature of the plot of ln ηr as a function of c, is lower than zero for ionic strengths of the solvent of 0.04 × 10−2 M and 0.50 × 10−2 M meaning that the chitosan macromolecules behave in term of hydrodynamic interactions as an uncharged polymer in a poor solvent. In acidic aqueous solution with the lowest ionic strength, the formation of the aggregates by the intermolecular interactions between chitosan chains is favored, resulting in an increase of the intrinsic viscosity. The BW parameter becomes positive for chitosan in acidic aqueous solutions with ionic strengths from about 0.05 × 10−2 M to 0.32 × 10−2 M, indicating that the interactions between the solvent molecules and polymer chains are thermodynamically favorable. The way in which BW varies with the ionic strength is opposite to that of parameter kH and the maximum values of BW correspond to the minimum values of kH (Table 1). The kH values higher than 0.5 (poor polymer−solvent interactions) suggest the formation of macromolecular aggregates as a result of the long-range interactions between the chitosan chains. The origin of these long-range interactions is not due to the extended or stretched conformation of individual chains, but rather it is likely the long-range Coulombic interaction or aggregates formation that contributes to this electrostatic viscosity behavior. The kH values around 0.3 indicate a good solvent behavior and the existence of the preponderant polymer−solvent interactions. Wolf and co-workers29,31 compared the hydrodynamic interaction parameters, kH, (from eq 2) with the BW values (from eq 5) obtained for the neutral polymers, establishing the following correlation:

Recently, a new alternative method28 was proposed for the determination of [η], from the initial slope of the dependence of ln ηr as a function of c at sufficient low shear rates and polymer solution concentrations, according to the following equation: ln ηr =

c[η]W + B W c 2[η]W [η]• 1 + B W c[η]W

(5)

where BW represents a viscometric interaction parameter and [η]• is the characteristic specific hydrodynamic volume. Although the eq 5 was initially developed for the evaluation of the intrinsic viscosity of polyelectrolytes in aqueous solutions in the presence/absence of the salts,29−31 this equation was successfully applied for polyelectrolytes in aqueous/organic solvent mixtures32,33 and for neutral (co)polymers, neutral polymer mixtures, and neutral polymers in solvents mixture.34−37 The experimental data were well fitted according to the Wolf equation (Figure 3) and the calculated values of [η]W are shown in Table 1. For all systems investigated in the present paper, [η]• can be considered zero, within the accuracy of the experimental data. By applying the three methods given by eqs 2, 4, and 5, with the limitations discussed above, the obtained intrinsic viscosity values are closed (Table 1). The Wolf method is able to describe the viscometric behavior for dilute and very dilute regimes of concentrations where the Huggins and Fedors methods cannot be applied. C


Industrial & Engineering Chemistry Research kH + B W =

1 2

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(6)

which only holds true for the range of pair interactions between the solute. The sum of the two parameters determined for all ionic strengths was around 0.5 excepting the solution corresponding to the solvent with the lowest ionic strength (Table 1). In the solvent with very low ionic strength the chitosan sample has more accentuated properties of polyelectrolyte and the relation mentioned above is not valid. Generally, the hydrodynamic interaction parameter BW reflects the curvature of the concentration dependence of ln ηr. These curves can be bent downward, corresponding to BW > 0, or upward, when BW < 0. BW is positive for almost all uncharged polymers when the relative increase in viscosity slows down as the polymer concentration rises. This behavior is typical for thermodynamically good solvents when the polymer coils shrink upon an augmentation of concentration due to the preference of contacts between solvent molecules and polymer segments.31 In the very unfavorable thermodynamic conditions, kH becomes large and BW takes the negative values. 3.3. Effect of Solvent Ionic Strength on the Chain Conformation in Perturbed and Unperturbed States. The size and the conformation of the chitosan chains in dilute solution are strongly dependent on the medium (ionic strength and pH) and the polymer structural characteristics (DA and DP). Three domains of DA were distinguished and correlated with the chitosan conformation in solution: (1) for DA < 25% chitosan exhibited a flexible conformation, (2) for 25% < DA < 50% the chitosan conformation became slightly stiffer, and (3) for DA > 50%, by increasing DP and DA, the participation of the excluded volume effect became preponderant and counterbalanced the depletion of the chains by steric effects and longdistance interactions.38 For DA < 25%, the chitosan chains are quite isolated, rather flexible and the theories related to the neutral polymers in dilute solution can be used only in the case of DP > 1300. The structural characteristics of chitosan investigated in the present work, DA = 26% and DP = 4152, allowed a discussion of the chitosan chain conformation in perturbed and unperturbed states based on the theories of the neutral polymers. In the following calculations [η]W values were considered to be more adequate because the Wolf equation is applicable over a large range of concentrations, including those for which the relative viscosity is lower than 1.2. The coil density (ρ) of the polymer chains in solution was estimated with the relationship proposed by Qian et al.:39,40 ρ=

c (1.25 + 0.5 56.4ηsp + 6.25 ) ηsp

Figure 4. Variation of ρ with the polymer concentration for CS solutions in AcOH and AcOH/AcONa with different ionic strengths at 25 °C.

[η] = [η]∞ + S ·I −1/2

(8)

The plot of [η] as a function of I−1/2 for the chitosan solutions is shown in Figure 5.

Figure 5. Dependence of intrinsic viscosity on the ionic strength. The line represents the fit of the experimental data (excepting the solutions with the solvent ionic strengths of 0.04 × 10−2 M and 4.32 × 10−2 M) according to eq 8.

Although the electrostatic contribution to the excluded volume is not completely inhibited at infinite ionic strength,42 the [η]∞ value of 6.28 dL g−1 (Figure 5) could be interpreted as the viscosity under theta conditions ([η]θ) because the electrostatic repulsions between protonated amino groups are largely suppressed at infinite ionic strength. The slope S is related to the stiffness of the molecule, B, as B=

S ([η]0.1 )1.3

(9)

where B is the stiffness parameter and [η]0.1 represents the intrinsic viscosity at I = 0.1 M. The estimated stiffness parameter was 0.063 in agreement with that reported by Rinaudo et al.43 which consider that stiffness parameter is a characteristic of the main chain independent of DA and of the solvent. In contrast, other authors44,45 found that B decreases with the increase of DA suggesting a stiffening of chitosan chains. The concentration dependence of the polymer chain dimensions in solution was determined according to the Qian and Rudin method.40 Considering the chitosan solution as a suspension of hard spheres, the radius of gyration, Rg, at different concentrations can be written as

(7)

Figure 4 shows the concentration dependence of ρ by using the experimental data from Figure 1 by means of the eq 2. The polymer coil density increases slowly by increasing the polymer concentration and the ionic strength of the medium due to the increase of the intramolecular interactions. The increase of the polymer density is more significant at I > 0.5 × 10−2 M as a result of the changes in conformation of polymer coils by adding of AcONa. The intercept of the dependence of [η] as a function of I−1/2 gives the intrinsic viscosity extrapolated to infinite ionic strength, [η]∞ according to the following relationship:41 D



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chitosan sample in the solvents with ionic strengths of 0.18 × 102 M, 0.50 × 102 M, and 8.05 × 102 M. One can observe that the polymer chain dimension decreases by increasing the polymer concentration up to a critical concentration, c+, at which the dimensions of polymer coils are considered to have shrunk to their unperturbed dimensions, Rg,c≥c+ (Table 2). Rg,c→0 is the radius of gyration when the concentration of the polymer solution approaches zero (perturbated state), and Rg,c≥c+ represents the radius of gyration in unperturbed state (theta condition) when the concentration of the polymer solution is c ≥ c+ > c*. The c+ value was determined by using a MathCad7 Professional Program from the variation of Rg as a function of c (according to eq 10) as being the concentration at which Rg begins to be constant. The Rg value for c = 0 corresponds to Rg,c→0. In Table 2 are given the chitosan chain dimensions (Rg,c→0) in solvents with different ionic strengths and the critical concentrations c* and c+ for the chitosan solutions. The critical concentration c* and c+ values and the difference between them increase with the increase of the solvent ionic strength. For the studied chitosan sample, Rg,c≥c+ was determined as being 52.50 nm. The persistence length (Lp) defined as the projection of the end-to-end distance on the direction of the first bond is equal to half of the Kuhn length, lK, for wormlike molecules. The persistence length of polyelectrolytes depends strongly on the ionic strength and pH.47,48 The lK values for the solvents with different ionic strengths were determined using a relationship proposed by Anthonsen and co-workers:45

⎛ ⎞1/3 ⎜ ⎟ 3M̅ [η] Rg = ⎜ ⎟ ⎜ 9.3 × 1024 1 + [η] − [η]θ ⎡1 − exp − c ⎤ ⎟ ⎣ ⎦ [η]θ c* ⎝ ⎠

{

( )}

(10)

where [η] and [η]θ are the intrinsic viscosities in the given solvent and under theta conditions, respectively, M represents the molecular weight, and c* is the critical concentration at which the macromolecular coils begin to overlap (the concentration which separates the dilute-semidilute regimes). The c* values (Table 2) were calculated with the following relationship:46 c* =

3ϕ′ 4πNA[η]

(11)

where ϕ′ is the Flory universal constant which is 3.1 × 1024 when [η] is expressed in mL g−1 and NA represents Avogadro’s number (6.023 × 1023). Table 2. Chain Dimensions in Perturbed State (Radii of Gyration and Persistence Lengths) and the Critical Concentrations, c* and c+, for Investigated Chitosan Sample in the Solvents with Different Ionic Strengths at 25 °C I × 102 (M)

Rg,c→0 (nm)

c* (g dL−1)

c+ (g dL−1)

Lp (nm)

0.04 0.18 0.32 0.50 4.32 8.05 16.06 24.30

110 95.4 89 83.6 59.9 64.7 61 57

0.021 0.033 0.040 0.048 0.132 0.105 0.125 0.153

0.165 0.250 0.305 0.369 0.880 0.748 0.845 0.960

17.14 12.89 11.22 9.90 5.08 5.93 5.27 4.60

lK =

6R g 2 lo DP

(12)

where lo represents the virtual bond length per monomer unit and is 0.51 nm in the case of chitosan, regardless of the molecular weight, degree of deacetylation and ionic strength,49 DP is the degree of polymerization (4152). The Lp values for the chitosan sample in solvents with different ionic strengths are shown in Table 2. The persistence length values decreased from 17.14 to 4.60 nm by increasing of the ionic strengths of the solvent from 0.04 × 10−2 M to 24.30 × 10−2 M proving the increase of the chitosan chain flexibility due to the decrease of repulsive potential between them. Although many studies were performed, the effect of the molecular weight on the chitosan chain dimensions in solution have not been completely elucidated. The values of persistence length of chitosan reported in the literature vary in the range of 4−23 nm.50 In our study, the persistence length in unperturbed state was estimated as being 3.90 nm in agreement with the values reported by Rinaudo et al.43 3.4 Effect of Solvent Ionic Strength on the Electrokinetic Parameters and Turbidimetry. Zeta potential (ζ) is a very good index of the magnitude of the electrostatic repulsive interaction between particles and it is used to predict and control dispersion stability. ζ values determined for the chitosan solutions decrease significantly from 66 to 19 mV by increasing the ionic strength up to around 5 × 10−2 M and then the decrease is slower (Figure 7). The decrease of ζ could be related with the diminution of the repulsive potential that result in the increase of flocculation or precipitation.49 Figure 8 shows the variation of conductivity of the chitosan solutions as a function of the ionic strength of the solvent. Three domains characterized by different slopes in the variation

Figure 6 shows the dependency of the radius of gyration (calculated with eq 10) on polymer concentration for the

Figure 6. Dependence of Rg on chitosan concentration for solvent ionic strengths of 0.18 × 10−2 M, 0.50 × 10−2 M, and 8.05 × 10−2 M, according to eq 10. The inset figure shows the variation of Rg at low chitosan concentrations (up to 0.25 g dL −1 ). The critical concentrations, c* and c+, are shown. E



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Figure 7. Variation of ζ for chitosan in acidic aqueous solution as a function of ionic strength values of the solvent at 25 °C. The data are fitted according to a power equation (ζ = a + bIc).

Figure 9. Variation of turbidity for chitosan in acidic aqueous solutions as a function of solvent ionic strength at 25 °C. The dotted line is calculated with an equation of the third degree.

× 10−2 M and 49.64 × 10−2 M. The values of the intrinsic viscosity were determined both by classical methods (Huggins, Fedors) and by a new equation proposed by Wolf. The Wolf method has given the intrinsic viscosity values by modeling the viscometric data in the region of very low concentrations where the classical methods cannot be applied accurately. The Huggins constant was correlated with the BW parameter from the Wolf equation, both parameters reflecting the intermolecular interactions. The intrinsic viscosity decreased and the polymer coil density increased by increasing the solvent ionic strength due to the enhancement of the polymer−polymer intermolecular interactions. The dimensions of the chitosan chains (the radius of gyration and the persistence length) in perturbed and unperturbed states were also discussed. The radius of gyration and the persistence length in the unperturbed state were determined as being 52.50 and 3.90 nm, respectively. The viscometric parameters, corroborated with zeta potential, conductivity, and turbidity measurements give information about the thermodynamic quality of the solvent.

Figure 8. Conductivity of the chitosan solutions as a function of the solvent ionic strength.

of the conductivity as a function of the solvent ionic strength were evidenced. A decrease of the slope in the dependence of conductivity on the ionic strength was observed at around 1 × 10−2 M. The presence of CH3COO− groups in the solvent with ionic strength higher than 4.32 × 10−2 M (acetic acid/ sodium acetate solution) leads to a screening effect of the repulsion interactions between positively charged −NH3+ along the chitosan chain. This induces a rod-to-coil conformation transition when the association of compact macromolecules occurs. The conductivity remains almost constant in the solvents with the ionic strength higher than 32.20 × 10−2 M. Chen et al.47 have observed at low ionic strength (1 × 10−2 M) a considerable quantity of particle structure and that the electrodynamic coupling effect predominates on the mobile behavior of the extended and congested chitosan molecule in solution. At ionic strengths higher than 20 × 10−2 M the association of chitosan macromolecules occurs and/or the quantity of the aggregates increases by increasing the solvent ionic strength.47 The existence of the three domains mentioned above was also evidenced by turbidimetry measurements (Figure 9). One can observe that the turbidity of the chitosan solutions starts to rise abruptly above 32.20 × 10−2 M due to the increase of the size and the amount of the chitosan aggregates.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +40 232 217454. Fax: +40 232 211299. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, Project No. PN-II-ID-PCE-2011-3-0199 (Contract 300/2011).

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REFERENCES

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4. CONCLUSIONS The thermodynamic properties of chitosan having the molecular weight and the degree of acetylation of 7.14 × 105 g mol−1 and 26%, respectively, were investigated at 25 °C in acidic aqueous solutions with the ionic strengths between 0.04 F



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The Behavior of Chitosan in Solvents with Different Ionic Strengths

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