Surface activity and micellization properties of chitosan-succinyl derivatives

Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 246–253

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Surface activity and micellization properties of chitosan-succinyl derivatives Chee-Chan Wang a , Li-Huei Lin a,∗ , Hsun-Tsing Lee b , Yu-Wun Ye b a b

Department of Cosmetic Science, Vanung University, 1, Van Nung Road, Chung-Li City, Taiwan, ROC Graduate Institute of Materials Science, Vanung University, 1, Van Nung Road, Chung-Li City, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 10 June 2011 Received in revised form 30 July 2011 Accepted 17 August 2011 Available online 25 August 2011 Keywords: Chitosan Surface activity Critical micelle concentration Emulsion

a b s t r a c t Surface active chitosan-succinyl derivatives were prepared by incorporating hydrophilic succinyl group and different hydrophobic alkyl chains into chitosan through covalent bonding. Chemical structures of the derivatives were characterized by FTIR spectroscopy. Surface activity and critical micelle concentration values of the chitosan-succinyl derivatives were characterized by surface tension, conductivity and fluorescence measurements. Due to the surface inactivity and insoluble in common solvent or water, the application of chitosan is still a problem. The incorporation of succinyl group and alkyl chains significantly improves the surface activity of chitosan. Moreover, increasing the hydrophobic chain length leads to a smaller particle size and a higher stability of the emulsions of chitosan-succinyl derivatives. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Chitosan, also called d-glucosamine, is a fully or partially deacetylated chitin. Chitin, a naturally occurring polysaccharide, and chitosan show many unique properties, such as biocompatibility, biodegradation capability, biological activity and low-toxicity, which have been used in recent years for the development of drug delivery systems, e.g., the preparation of microspheres, microcapsules and films for controlled release [1,2]. However, the application of neat chitosan, a linear polysaccharide, is still a problem. This is due to its water-insoluble nature. If water-soluble chitosan derivatives could be prepared in a simple way, it is expected that the biological and physiological application potentials of chitosan would be developed dramatically. In order to find more applications in pharmaceutical, medicine and food industry, many soluble chitosan derivatives were prepared [3–8]. Many hydrophobic derivatives have been prepared by attaching alkyl chains to natural polymers to meet the different needs. Magdassi et al. [9–12] linked different alkyl chains to a protein and found that the surface activity increased with the increasing chain length, as determined by surface tension, foaming and emulsifying activity measurements. Moreover, the water soluble derivatives of chitosan and chitosan-succinyl were hydrophobically modified by grafting butyl glycidol ether to produce amphiphilic chitosan derivatives [3,5]. Hydrophilic portions of the derivatives would concentrate

∗ Corresponding author. Tel.: +886 3 4515811/51736; fax: +886 3 4514814. E-mail address: [email protected] (L.-H. Lin). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.08.017

on the droplet surface to decrease the surface tension and the hydrophobic chains were aggregated to form droplets in the solutions. Although there are some papers in literature reported the study of chitosan-based surfactants, only limited papers studied their surface activity and micellization properties. In this study, surface active chitosan-succinyl surfactants were prepared by incorporating hydrophilic succinyl group and different hydrophobic alkyl chains into chitosan through covalent bonding. Surface active properties of these chitosan-succinyl surfactants were studied by means of surface tension and fluorescence spectroscopy measurements. The surface tension behavior can be explained by the free energy theory. When chitosan-succinyl surfactants dissolve in water, their molecules prefer to locate at the air/water interface in order to avoid the energetically unfavourable contact of insoluble alkyl chains and water. In other words, the free energy of an amphilic molecule located at the interface is lower than that stayed in bulk water. Adsorption of surfactants at the interface decreases the surface energy and surface tension thereby. In addition, the particle sizes and zeta potentials of these chitosansuccinyl derivatives in emulsion system were also examined.

2. Materials and methods 2.1. Materials Chitosan was obtained from ACROS USA, which has a deacetylation degree of 85% and average molecular weight of 1.5 × 105 . Reagent-grade fatty alcohols (n = 4, 6, 8, 10), succinyl anhydride, epichlorohydrin and sodium hydroxide were purchased from

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247

Step 1 O OH

HO

O

O

+

NH2

OH

O

O

O HO

n

NH

n

O

O

A

O

OH

Step 2 O

OH

Cl

+

n

n = 4,6,8,10

OH O

Cl

n

NaOH

O

+

n

NaCl

O

B

Step 3

OH O

O n

O

A +

B

HO

NH

O n

O

O

OH Fig. 1. Synthesis scheme for the chitosan-succinyl derivatives.

Hayashi Pure Chemical Co. Japan and used without further purification. The fluorescence probe, pyrene, was supplied by Fluka Chemical Co. Japan. 2.2. Synthesis of chitosan-succinyl derivative surfactants Synthesis scheme for the chitosan-succinyl derivatives is divided into three steps in Fig. 1. In step 1, chitosan (1 mol) in 2% aqueous acetic acid and succinyl anhydride (1 mol) were stirred mechanically, gradually heated to 50 ◦ C under a nitrogen atmosphere, and then maintained at that temperature for 4 h. In step 2, different chlorohydrin ethers were firstly obtained

through the reactions of epichlorohydrin with different fatty alcohols at 60 ◦ C for 3 h. Then, the resulting chlorohydrin ethers were converted to glycidyl ethers under caustic conditions. The product was precipitated with ethanol, then filtered and dried in a vacuum at 50 ◦ C. In the third step, products from the first and second steps were mixed and stirred mechanically at 110 ◦ C for 8 h. Then, ethanol was added to the reaction mixture. The insoluble impurities were removed by filtration. Subsequently, ethanol and water in the filtrate were removed with a rotary evaporator and the chitosan-succinyl derivatives were obtained. The products were further dried in a vacuum oven.

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2.3. Analysis Structures of the products were confirmed by using Fouriertransform infrared spectroscopy (FTIR). FTIR spectra were recorded in the range 4000–650 cm−1 using a Japan Spectroscopic FT/IR-3 spectrophotometer. The test compound was spread as a thin layer on a KBr tablet for examination. For each sample, 32 scans were collected with a resolution of 4 cm−1 . 2.4. Measurements Surface tensions were determined at room temperature using a Japan Kaimenkaguka CBVP-A3 surface tensiometer that was calibrated with ultra-pure water prior to use. The Pt plate was cleaned through flaming; the glassware was rinsed sequentially with tap water and ultra-pure water. The surfactant solution was freshly prepared as a stock one and then diluted to the desired concentration for each measurement. Surface tension for each concentration was measured three times; an average error of less than 0.5 dyne/cm was obtained routinely. The critical micelle concentration (cmc) and the corresponding surface tension were determined from the breakpoint of the curve with surface tension versus logarithmic concentration. Surface excess concentration of the surfactant at the air–solution interface ( ; units: mol m−2 ) was calculated by the Gibbs adsorption isotherm equation [13–16]  =−

 1   d  iRT

d ln C

where  represents the surface tension (units: mN m−1 ), R is the gas constant (8.314 J mol−1 K−1 ), T is the absolute temperature, C is the surfactant concentration, and (d/d ln C) is the slope below the cmc in the surface tension plots. The value of i represents the number of species at the interface for which the surface excess concentration changes with the surfactant concentration. The chitosan-succinyl derivatives are classified as nonionic surfactant, for mixtures of ionic and nonionic surfactants in aqueous solution in the absence of added electrolyte, so the coefficient i = 1 for the dilute solution (10−2 M or less) of chitosan-succinyl derivative surfactants [17,18]. The area occupied by a surfactant molecule at the air–solution interface, Acmc , was obtained from the saturated adsorption as follows: Acmc =

The fluorescence emission spectra of the solutions were measured by an Aminco–Bowman Series 2 photoluminescence spectrometer. The excitation wavelength was 335 nm; the emission was measured between 350 and 450 nm. Hydrophobicity was evaluated by the intensity ratio of peak 1 (I1 ) at 374 nm to peak 3 (I3 ) at 394 nm for 10−6 M pyrene in aqueous chitosan-succinyl derivative or surfactant solutions. Each pyrene solution was prepared by evaporating the solvent from 0.1 ml of 10−4 M pyrene in ethanol, adding 10 ml surfactant solution, and then sonicating it for 15 min in an ultrasonic bath. Conductivity measurements were performed using a COND 720 digital conductivity meter with a cell constant of 0.475 cm−1 . The experimental temperature was maintained at 298 K with a water bath. An exact 50 ml of distilled water was successively added into the solution for each conductivity measurement at different concentrations. The 10 wt% O/W emulsions were prepared by adding soybean oil (10 g) to the 1 wt% chitosan-succinyl derivative solutions (90 g) and then homogenizing (IKA Labortechnik Ultra-Turrax T25 homogenizer) at 11,000 rpm for 5 min [22]. The particle size distribution and volumetric average diameter of the emulsion droplets were measured by a Microtrac S3000 apparatus. A ZetaProbe (Colloidal Dynamics Co.) was used to measure the zeta potentials of the chitosan-succinyl derivative emulsions at 298 K. 3. Results and discussion 3.1. Preparation FTIR spectra of the synthesized chitosan-succinyl derivatives are shown in Fig. 2. They display bands at 3200–3550 cm−1 (O–H, stretching), 2926 cm−1 (CH2 , asymmetric), 2853 cm−1 (CH2 , symmetric), 1640–1700 cm−1 (C O, stretching). For each derivative, grafting alkyl chains to chitosan caused intensity increases of the bands at 2926 cm−1 (CH2 , asymmetric) and 2853 cm−1 (CH2 , symmetric). In addition, the chitosan-succinyl derivatives have new absorption bands at 1400–1200 and 1460 cm−1 , corresponding to the carboxyl group of the succinyl side chains. The increase of amide I peak (1643 cm−1 ) and decrease of amide II peak (1565 cm−1 ) indicated the increase of amidation by grafting succinyl chains to

1 N · cmc

Chitosan-C12

where N is Avogadro’s number and  cmc represents the surface excess concentration at the cmc. The other parameter [19,20],

in which cmc stands for the surface pressure at the cmc,  0 and  cmc stand for the respective surface tensions of pure water and surfactant solution at the cmc. The cmc indicates the maximum reduction of surface tension caused by the dissolution of surfactant molecules and is also an evaluation of the effectiveness for the surface tension lowering by a surfactant. To estimate the adsorption and aggregation properties of chitosan-succinyl derivatives in water, we used the parameter, pC20 (pC20 = − log C20 ) [21]. Here, C20 represents the surfactant concentration required to reduce the surface tension of water by 20 mN m−1 . C20 is the minimum concentration needed for a saturation of surface adsorption. The value of pC20 represents the efficiency of adsorption of surfactant molecules at the air/water interface that can be correlated with the structural factors with regard to the adsorption processes. The greater the pC20 is, the more preferred the surfactants adsorb at the air/water interface.

Chitosan-C8

Transmittance (a.u.)

cmc = 0 − cmc

Chitosan-C10

4000

Chitosan-C6

Chitosan

3000

2000

1000

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of the chitosan-succinyl derivatives.

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249

80 a

1.6

60

Fluorescence Intensity

Surface Tension (mN m-1)

70

Chitosan Chitosan-C6 Chitosan-C8

50

Chitosan-C10 Chitosan-C12

40

1.2 j

0.8

0.4

0.0 30

20

340

360

380

400

420

440

460

Emission (nm) 1E-4

1E-3

0.01

0.1

1

Fig. 4. Fluorescence spectra of Chitosan-C10 with different concentrations: (a) 0.003, (b) 0.004, (c) 0.005, (d) 0.006, (e) 0.007, (f) 0.008, (g) 0.009, (h) 0.01, (i) 0.03, and (j) 0.05 mol/dm3 .

-3

log C (mol dm ) Fig. 3. Surface tension versus concentration for the chitosan-succinyl derivatives.

chitosan [5,23]. In Fig. 2, the two peaks of the derivatives at 1107 and 1050 cm−1 were attributed to the C-CH3 stretching vibration of the 2-hydroxypropyl-3-alkoxide group. These results suggest that both the hydrophilic group (carboxyl group in succinyl chain) and hydrophobic group (alkyl chain C6–C12) were introduced to chitosan. 3.2. Surface tension Fig. 3 shows that the equilibrium surface tensions of the chitosan-succinyl surfactants are lower than that of the unmodified chitosan at all concentrations. This indicates that the chitosansuccinyl surfactants are more surface active than the unmodified chitosan. More specifically, the chitosan-succinyl surfactant with longer hydrophobic alkyl chain is more effective in reducing surface tension of the solution. This phenomenon can be easily identified after the air/water interface becomes saturated. Thus chitosan-succinyl surfactants lower down the surface tension of water as is observed in Fig. 3. But as surfactant concentration is above cmc, the lipophilic alkyl chains would move toward and locate in the interior of micelles for preventing the free energy from increasing further [24,25]. Therefore, the surface tension changes non-obvious when the surfactant concentration is well above cmc. cmc values determined from the plots of surface tension versus logrithmic concentration are listed in Table 1. The cmc values decrease in the order Chitosan-C6 > Chitosan-C8 > ChitosanC10 > Chitosan-C12. This trend is in accordance with the principle that surfactant with longer alkyl chain is more hydrophobic. Table 1 also shows that both the surface tension at cmc,  cmc , and the area occupied by a surfactant molecule at the air/water interface, Acmc , are smaller when the alkyl chain of a surfactant is longer. This indicates that there is a strong interaction between water and surfactant with long alkyl chains. Among the four surfactants in Table 1, Chitosan-C6 exhibits the

largest Acmc of 0.467 nm2 molecule−1 . Comparing this datum to 0.2 nm2 molecule−1 for the cross-sectional area of a closely packed linear aliphatic chain [15,23], it is known that the alkyl chains of Chitosan-C6 at the air/water interface are not closely packed. In addition, the surface excess concentration at cmc,  cmc , was calculated by applying the Gibbs adsorption isotherm equation given in the measurement section. Table 1 shows that the  cmc is higher when the alkyl chain of a surfactant is longer. Because of the most hydrophobic property, Chitosan-C12 with the longest alkyl chain exhibits the biggest  cmc . Values of cmc and pC20 for the chitosan-succinyl derivatives are also listed in Table 1. In principle, surface tension of a solution would decrease when surfactant molecules adsorb at the air/water interface, consequently cmc and pC20 would increase. Therefore, efficiency for interfacial adsorption of surfactants can be evaluated by cmc and pC20 . The higher the cmc and pC20 are, the more efficient the surfactant adsorption is [18,26]. Chitosan-C12 is the most efficient surfactant evaluated by cmc , pC20 and other parameters in Table 1. 3.3. Fluorescence Fluorescence spectra of Chitosan-C10 with different concentrations are shown in Fig. 4. During the measurement, pyrene was used as a probe because its fluorescence emission spectrum is very sensitive to solvent polarity, In Fig. 4, the fluorescence intensity decreases with the increase of Chitosan-C10 concentration. The decrease of fluorescence intensity is attributed to the less soluble of fluorescence probe micelles in water phase. As Chitosan-C10 concentration increases, the solution is less hydrophilic and does not favor the solubility of pyrene micelles in water phase [18,9]. Fig. 5 shows the dependence of both surface tension and fluorescence I1 /I3 ratio on the Chitosan-C10 concentration. The two curves of surface tension and fluorescence I1 /I3 ratio are quite similar. As Chitosan-C10 concentration increases, the value of I1 /I3

Table 1 Surface properties of chitosan-succinyl surfactants in aqueous solution at 27 ◦ C. Cpd.

cmc (mmol dm−3 )

 cmc (mN m−1 )

 cmc (×10−6 mol m−2 )

Acmc (nm2 molecule−1 )

cmc

pC20

Chitosan-C6 Chitosan-C8 Chitosan-C10 Chitosan-C12

8.09 7.04 6.57 5.72

37.6 37.4 34.5 32.0

3.555 3.834 4.007 4.621

0.467 0.433 0.415 0.360

35.2 35.4 38.3 40.8

1.112 1.256 1.426 1.850

250

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1.10

80 Surface Tension

1.05

I 1/I 3

Cpd.

1.00

Chitosan-C10

60 0.95 50

0.90

I1/I3

Surface Tension (mN/m)

70

Table 2 Critical micelle concentrations (CMCs) and micelle ionization degree (˛) of chitosansuccinyl derivatives in aqueous solution at 27 ◦ C.

Chitosan-C6 Chitosan-C8 Chitosan-C10 Chitosan-C12 a

0.85

40

0.80

30

0.75 20

1E-4

1E-3

0.01

0.1

1

-3

log C (mol dm ) Fig. 5. Dependence of both surface tension and fluorescence I1 /I3 ratio on the Chitosan-C10 concentration.

CMC (mmol dm−3 ) Surface tension

Conductivity

Fluorescence

˛a

8.09 7.04 6.57 5.72

10.62 9.85 9.53 9.31

9.21 8.99 8.67 8.34

0.8898 0.8902 0.9027 0.9071

From conductivity measurement.

the surfactant concentration below and above the critical micellar concentration (cmc), respectively [30,31]. Similarly, cmc values for other chitosan-succinyl derivatives were obtained and listed in Table 2. Moreover, the degree of counterion dissociation, ˛, corresponds to the average number of counterions/surfactant ion that dissociates from the micelle, can be eatimated from ratio of the respective slopes of the conductivity curve above and below cmc point. The ˛ values thus obtained for the chitosan-succinyl derivatives are also listed in Table 2. The ˛ value increases with the increase of alkyl chain length. This result is probably due to the stronger hydrophobic forces among longer alkyl side-chains. A stronger hydrophobic interaction would result in a lower charge density at the micellar surface, which inspires the dissociation of the counterions [32]. 3.5. Contact angle

Fig. 6. Conductivities of the chitosan-succinyl derivatives at various concentrations.

decreases abruptly and then gradually to 0.78. Lower I1 /I3 ratio corresponds to lower polarity of the microdomain around the pyrene molecule [27–29]. In other words, lower I1 /I3 value indicates that pyrene would prefer to escape from the micelle surface that contacts water. In this circumstance, pyrene would locate around the lipophilic alkyl chains of chitosan-succinyl derivatives. 3.4. Conductivity We used conductivity measurements to study the micellar aggregation behavior of these surface active chitosan-succinyl derivatives in aqueous solution. Fig. 6 presents the conductivities of the chitosan-succinyl derivatives at various concentrations. The conductivities increased linearly with increasing the chitosansuccinyl derivative concentrations. In addition, derivatives with longer hydrophobic chains exhibited larger conductivities; i.e., Chitosan-C12 exhibited significantly greater conductivity than other derivatives. At a high concentration of 1.0 mol dm−3 , the conductivities of the Chitosan-C12 and Chitosan-C6 solutions were 16,000 and 1200 ␮S/cm. The plots of conductivity versus concentration show a breakpoint corresponding to the micellization of amphiphilic compounds. The conductivity increases linearly with

Another surface activity of the chitosan-succinyl derivatives is its wetting ability on a solid surface. The obtained contact angles between 1 wt% aqueous chitosan-succinyl derivative solutions and a standard PMMA plate are presented in Fig. 7. It shows that pristine chitosan is a poor wetting agent as compared to the modified ones. Moreover, long alkyl side-chain improves the wetting ability of chitosan-succinyl derivatives, while Chitosan-C12 exhibited the highest wetting ability All the contact angle variations may be explained by the ability of molecules of the chitosan-succinyl derivatives in destroying the water film on the standard plate surface. The formation of water film was due to strong interaction between water molecule and plate. There are clathrate-like water structures around ether groups of the surfactants, including hydrogen bonding between water and ether oxygen as indicated in Fig. 8. The clathrate-like water or surfactant-water structures adsorbed on the standard plate surface would replace some original water film on the plate surface [33,34]. 3.6. Emulsification Volumetric average droplet diameters of chitosan-succinyl derivative emulsions with 1 wt% concentration at different stock times are presented in Fig. 9. The average droplet diameter of pristine chitosan increases significantly after a stock time of 2 h. Similarly, the average droplet diameter of Chitosan-C6 increases significantly after a stock time of 3 h. On the contrary, the average droplet diameters of other chitosan-succinyl derivative emulsions with alkyl side-chains longer than that of Chitosan-C6 increase insignificantly with the stock time. Thus Chitosan-C12, ChitosanC10 and Chitosan-C8 are more stable than Chitosan-C6 and Chitosan. Moreover, chitosan-succinyl derivative emulsion with longer alkyl side-chain has smaller average droplet diameter and is more stable until the Chitosan-C12 behaves contrarily with an increase of average droplet diameter. This droplet diameter behavior may be interpreted by the higher amount of chitosan-succinyl derivative adsorbed on oil droplets with the longer alkyl side-chain

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251

Fig. 7. Contact angles between aqueous chitosan-succinyl derivative solutions and a standard plate.

except for Chitosan-C12 with the longest alkyl side-chain. The alkyl side-chain of Chitosan-C12 is too long and hydrophobic to form more stable droplets, thus larger droplets were produced. Fig. 10 shows zeta potentials of the chitosan-succinyl derivatives at various pH values. It was found that modification changed charge number of the chitosan, the longer alkyl side-chain, the

H

H

OH

H bond

.. O ..

O

n O HO

NH

Chitosan Chitosan-C6 Chitosan-C8 Chitosan-C10 Chitosan-C12

5

Particle Size (μ m)

.. ..

O

6

4

3

2

O

1 n

O

O

OH Fig. 8. Hydrogen bonding interaction between chitosan-succinyl derivative and water.

0

1

2

3

4

5

6

Time (hours) Fig. 9. Volumetric average droplet diameters of chitosan-succinyl derivative emulsions with 1 wt% concentration at different stock times.

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References

Fig. 10. Zeta potentials of chitosan-succinyl derivative emulsions with 1 wt% concentration at different pH values.

more negative zeta potential of the chitosan-succinyl derivative. Isoelectric point (IEP) of chitosan is 5.2. The electrical charge density of pristine chitosan droplets was less negative than those of chitosan-succinyl derivative droplets. The zeta potential of pristine chitosan is more negative with higher pH value of the emulsion and was nearly invariant with pH value above 5.2. Droplet surfaces of the emulsion are presumably saturated with chitosan molecules when pH value is above 5.2. Droplet charge densities of the chitosan-succinyl derivative emulsions were negative and became more negative when pH value is above 5, suggesting the adsorption of polysaccharide derivatives on droplet surfaces [35]. For Chitosan-C12 as a typical example of chitosan-succinyl derivatives, the droplet charge density became less negative as pH value was increased from 3 to 5, but became more negative at higher pH value and remained nearly invariant at pH value above 8. The more negative zeta potential of Chitosan-C12 was due to the ionization of –COOH groups by the incoming base to become –COO-groups as pH value above 5. However, all –COOH groups in Chitosan-C12 were neutralized by the incoming base to become –COO-groups and no additional negative ions were formed as pH value above 8, so that the zeta potential remained unchanged.

4. Conclusion Water-soluble chitosan-succinyl derivatives can be hydrophobically modified by glycidyl ethers to yield amphiphilic chitosan derivatives. We investigated the adsorption and aggregation properties of the modified chitosan-succinyl surfactants. It was found that chitosan-succinyl surfactant with longer alkyl chain had lower cmc and surface tension in water, smaller area occupied by the surfactant molecule at the air/water interface, higher adsorption efficient and effectiveness of surface tension reduction. The surface activities of the modified chitosan-succinyl surfactant in aqueous solution increased with longer hydrophobic alkyl chain of the surfactants. The modified chitosan-succinyl surfactants reduced the equilibrium surface tension of water to a minimum value of 32 mN m−1 . Furthermore, we found that the surface activity of Chitosan-C12 surfactant was better than other derivative surfactants, presumably because of the stronger attractive interactions among the longer alkyl chains of C12.

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