Synthesis and emulsification properties of novel modified polyoxyethylenated stearylamine surfactants

Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 26–32

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Synthesis and emulsification properties of novel modified polyoxyethylenated stearylamine surfactants Shuenn-Kung Su a , Li-Huei Lin b,∗ , Yu-Ching Lai a a b

Department of Materials Science and Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei, Taiwan, ROC Department of Cosmetic Science, Vanung University, 1, Van Nung Road, Chung-Li City, Taiwan, ROC

h i g h l i g h t s

g r a p h i c a l

 The modified surfactants featured ester bonds reflected by signals at 1734 cm−1 in FTIR spectra.  The modified surfactant 415MA had the lowest surface concentration and surface area per molecule.  The foam height of the modified surfactants was lower than unmodified surfactants.  The modified surfactants exhibited more-negative zeta potentials than unmodified surfactants.

The modified surfactants have possessed more negative zeta potential and less isoelectric point values. The electrical charge density of modified surfactants droplets was less negative than those of unmodified surfactants droplets. We expected the surface potential to also increase upon increasing the surface concentration because an increase in surface concentration would lead to compression of the electrical double layer and a corresponding reduction in zeta potential.

a r t i c l e

a b s t r a c t

i n f o

Article history: Received 17 April 2012 Received in revised form 17 July 2012 Accepted 5 August 2012 Available online 19 August 2012 Keywords: Surfactants Polyoxyethylenated stearylamine Critical micelle concentration Fluorescence Emulsification

a b s t r a c t

We have prepared a series of novel modified polyoxyethylenated stearylamine surfactants from two polyoxyethylenated stearylamines (415ST, 430ST), maleic anhydride (MA), and sodium hydrogen sulfite. We measured the surface activities of these compounds in terms of their critical micelle concentrations (CMCs), effectiveness at minimizing surface tension ( CMC ), surface excess concentrations ( CMC ), minimum average areas per surfactant molecule (ACMC ), and standard free energies of micellization (Gm ◦ ). We studied the CMCs of the modified polyoxyethylenated stearylamine derivatives through measurements of their surface tensions, conductivities, and fluorescence (namely, the intensity ratio of the pyrene emissions at 374 and 394 nm) with respect to their concentration. In addition, we also studied the surface activity in terms of the emulsification of 10% (w/w) soybean oil in the presence of these surfactants. The smallest emulsion droplets (i.e., the most stable emulsions) were obtained using the modified surfactant 415MA. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Conventional single-chain surfactant molecules feature hydrophobic and hydrophilic parts. With increasing concentrations, they form micelles and, ultimately, lyotropic mesophases.

∗ Corresponding author. Tel.: +886 3 4515811x51736; fax: +886 3 4514814. E-mail address: [email protected] (L.-H. Lin). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.08.017

Above the critical micelle concentration (CMC), surfactants in aqueous solutions can form a variety of microstructures, including spherical, ellipsoidal, vesicular, rod-like, and thread-like forms [1,2]. The microstructures of surfactant aggregates depend on the molecular structures of the surfactants and the solution conditions (e.g., concentration, temperature) [3]. In a previous paper, we reported a novel series of ethoxylated hydroxysulfobetaines prepared from polyoxyethylenated stearylamine. We demonstrated that these surfactants had

S.-K. Su et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 26–32

low-foaming properties [4,5]. In addition, we reported that watersoluble polyesters prepared from dicarboxylic anhydride and polyethyleneglycol (PEG) exhibited excellent surface-active properties, similar to those of traditional surfactants. All of those species were nonionic surfactants containing nonionic hydrophilic ethylene oxide (EO) moieties in their structures [6–8]. It is generally accepted, however, that polyoxyethylene chain structures improve the industrial applicability of nonionic surfactants; for example, when using surfactants as dyeing auxiliaries in the textile industry, polyoxyethylene nonionic surfactants display excellent wetting, emulsifying, and dye dispersant ability and improved dyeing behavior [9–11]. In this present study, we prepared a series of novel modified polyoxyethylenated stearylamine surfactants. These surfactants feature two identical hydrophilic ethylene oxide chains and sulfonate units. Herein, we report the surface properties of these surfactants, including their surface tension, emulsification, conductivity, and fluorescence behavior.

27

Step1: O m

O

H

+

N

9

O

m+n=15,30

2

O

H

O

n

OH O

O

m

o

150 C

O O

N

9

O

O

n

OH

Step2: 2. Materials and methods OH

2.1. Materials O

O

m

Polyoxyethylenated stearylamines (415ST, 430ST), maleic anhydride (MA), sodium hydrogen sulfite, and other reagents were supplied in reagent grade by Niaon Shiyaku Industries and used without further purification. The fluorescence probe, pyrene, was supplied by Fluka Chemicals.

O O

N

9

O

+

NaHSO3

O

n

OH

2.2. Modified polyoxyethylenated stearylamine surfactants a SO 3N

The modified polyoxyethylenated stearylamine surfactants were prepared in two steps (Fig. 1). In Step 1, a polyoxyethylenated stearylamine (415ST or 430ST; 1 mol) and MA (2 mol) were mixed evenly and heated in the presence of titanium isopropyl oxide (1 g) at 150 ◦ C for 5 h. In Step 2, the mixture was cooled to 90 ◦ C and then sodium hydrogen sulfite solution (50%) was added to yield watersoluble polyesters. The raw product was dissolved in EtOH and the insoluble impurities filtered off; the solvents were evaporated from the filtrate under reduced pressure in a rotary evaporator and then the product was dried in a vacuum desiccator.

O

O

m

90oC 9

OH

O O

N O

O

n

SO N 3 a

OH

Compounds: 415MA,430MA Fig. 1. Synthesis of novel modified surfactants.

2.3. Analysis Fourier transform infrared (FTIR) spectra were recorded in the range 4000–650 cm−1 using a Japan Spectroscopic FT/IR-3 spectrophotometer. Each test compound was spread as a thin layer on a KBr tablet and then 32 scans were collected at a resolution of 4 cm−1 . The yield (%) was calculated as follows [12]: Yield(%) =

(acid values of original − acid values of end) × 100% acid values of original

The sulfonate group (SO3 Na) content was determined using the JIS K3362 method [13]. 2.4. Measurements Surface tensions were determined at room temperature using a Kaimenkaguka CBVP-A3 surface tensiometer (Japan), which 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 solution and then diluted to the desired concentration for each measurement. The surface tension was measured three times at each concentration; an average error of less than 0.5 dyne cm−1 was obtained routinely.

The CMC and the surface tension at the CMC were determined from the breakpoint of the surface tension and the logarithm of the concentration curve. The surfactant surface excess concentration at the air–solution interface ( ; units: mol m−2 ) was calculated using the Gibbs adsorption isotherm equation [14–16]:  =−

 1   d  iRT

d ln C

where  is the surface tension (mN m−1 ), R is the gas constant (8.314 J mol−1 K−1 ), T is the absolute temperature, i is the number of species at the interface whose concentration changes with the surfactant concentration (i = 1 for a gemini surfactant), C is the surfactant concentration, and (d/d lnC) is the slope below the CMC in the surface tension plots. The area occupied by the surfactant molecules at the air–water interface (ACMC ) was obtained from the saturated adsorption using the equation [10]: ACMC =

1 NCMC

where N is Avogadro’s number and  CMC is the surface excess concentration at the CMC. The value of i depends critically on the valence or stoichiometry of charge neutralization of the air–water interface (i = 1). The standard free energy of micellization per mole

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S.-K. Su et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 26–32

Table 1 The yield and sulfonate group content of modified surfactants.

Table 2 FTIR spectral data for the modified surfactants.

Compound

Yield (%)

Sulfonate group content (%)

Functional group

Vibration mode

Wave number (cm−1 )

415MA 430MA

83.8 82.4

37.0 16.2

CH2

Asymmetric stretching Symmetric stretching

2918 2864

C O C H SO3 C O C

Stretching Asymmetric stretching Stretching Stretching

1734 1460 1150–1250 1100

of monomer unit (Gm ◦ ), with reference to the standard state of the unit mole fraction for the nonionic surfactants, was calculated using the relation [8,15–18] Gm ◦ = RT ln CMC

3.2. Surface tension

The fluorescence emission spectra of the solutions were measured using 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 in terms of the intensity ratio of peak 1 (I1 ) at 374 nm and peak 3 (I3 ) at 394 nm when 10−6 M pyrene was present in the modified surfactant solutions. Each pyrene solution was prepared by evaporating the solvent from 0.1 mL of 10−4 M pyrene in EtOH, adding 10 mL of the surfactant solution, and then sonicating the mixture for 15 min in an ultrasonication bath. Foaming properties were determined using the Ross–Miles method. A modified polyoxyethylenated stearylamine surfactant solution (500 mL) was prepared and placed in a beaker. A dilute solution was dropped from a fixed height into a pool of the surfactant solution (1 g L−1 ) and then the foam height was measured. Foam production was measured in terms of the height of the foam initially produced; foam stability was measured in terms of the height after 3 min. Conductivity was measured using a COND 720 digital conductivity meter (cell constant: 0.475 cm−1 ); the experimental temperature was maintained at 298 K (water bath). To perform each series of measurements, an exact volume of distilled water (50 mL) was introduced into the bath and then the conductivity was measured. Oil-in-water (O/W) emulsions (10%, w/w) were prepared by adding soybean oil (25 g) to the novel modified surfactant solutions (225 g) and then homogenizing (IKA Labortechnik Ultra-Turrax T25 homogenizer) the mixtures (11,000 rpm, 10 min). The average diameter (by volume) and size distribution of the emulsion droplets were measured using a Microtrac S3000 apparatus. A ZetaProbe (Colloidal Dynamics) was used to measure the zeta potentials of a series of emulsions at 298 K. The surface morphologies of the emulsions were analyzed using an AETC-M100A electron microscope (AETC Toshi).

Fig. 3 reveals that the equilibrium surface tensions of the modified surfactants (415MA, 430MA) were lower than those of the unmodified surfactants (415ST, 430ST) at all concentrations, suggesting that the former were more surface-active than the latter. More specifically, the data for the modified surfactants indicated

3.1. Preparation The yield and sulfonate group content of modified surfactants are shown in Table 1. The yield of modified polyoxyethylenated stearylamine surfactants was 82–84% by the acid values measurement. The sulfonate group (SO3 Na) content decreased with the increase of the oxyethylene chain length. Fig. 2 displays FTIR spectra of the synthesized modified polyoxyethylenated stearylamine surfactants; Table 2 lists pertinent data. The typical IR spectrum of a modified polyoxyethylenated stearylamine surfactant displayed bands at 2924 (CH2 , asymmetric), 2854 (CH2 , symmetric), 1700–1750 (C O, stretching), 1460 (C H, asymmetric stretching), 1250 (C O C), and 1040–1050 ( S O) cm−1 . In addition, an absorption band appeared at 1734 cm−1 representing the ester bond in the modified polyoxyethylenated stearylamine surfactant.

Transmittance (a.u.)

430ST 415MA 430MA

4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 2. FTIR spectra of novel modified and unmodified surfactants.

65 415ST 415MA

60

430ST

Surface tension (mN/m)

3. Results and discussion

415ST

430MA

55

50

45

40

35 0.0

0.2

0.4

0.6

0.8

1.0

3

Concentration (mol/dm ) Fig. 3. Surface tensions plotted with respect to the concentrations of the modified and unmodified surfactants.

S.-K. Su et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 26–32

29

Table 3 Surface properties of modified and unmodified 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 )

Gm ◦ (kJ mol−1 )

415ST 430ST 415MA 430MA

10.23 9.883 3.140 7.169

41.43 44.19 37.32 42.26

4.58 4.62 5.76 4.93

0.363 0.360 0.288 0.337

−11.353 −11.439 −14.280 −12.234

Photophysical techniques have become powerful tools for studying micellar media and other microheterogeneous systems. The key idea in the use of luminescence probes is that certain molecules display selective affinity for a unique site of an aggregate, with the nature of the probe environment reflected in its emission properties. The main spectroscopic parameters used to characterize micellar assemblies are the excitation and emission spectral shapes, the vibrational fine structure, the quantum yields, and the degree of polarization of the emission [8,26]. These properties can be correlated to the characteristics of the microenvironments and

3.4. Foaming properties Recent advances in the design of dyeing machines, such as providing more-rapid circulation of the liquor, have resulted in foam formation commonly occurring in dye baths. This inconvenience has led to a desire for low-foaming dyeing auxiliaries.

1.20

Surface tension

60

I1/I3

1.18 1.16 1.14

55

1.12 50

1.10

I1/I3

3.3. Fluorescence properties

used to predict the existence and properties of the hydrophilic and aqueous domains. We used the emission spectrum of pyrene to evaluate the hydrophobicities of our modified polyoxyethylenated stearylamine surfactants. This approach provided a means to measure the polarity of the microenvironment within the micelles, in which the pyrene molecules were present. Our novel modified polyoxyethylenated stearylamine surfactants were modified with sodium sulfite hydrophilic groups. Fig. 4 displays the fluorescence spectra recorded to detect the emission signals of pyrene added to the solution. The emission intensities of pyrene’s first (374 nm) and third (394 nm) peaks are sensitive to its microenvironment; as a result, the ratio of the intensities of the emissions at 374 and 394 nm (i.e., I1 /I3 ) can be used to monitor the solution behavior of surfactants and polymers [27–30]. Accordingly, we evaluated the hydrophobicity of the products over a wide concentration range. The dependence of the ratio I1 /I3 on the surfactant (I) concentration was very similar to the surface tension behavior. In the region from 4 × 10−4 to 1 × 10−2 mol dm−3 , the value of I1 /I3 decreased gradually from 1.189 to 1.022 (i.e., the microenvironment experienced by the pyrene molecules became increasingly less polar). We conclude that aggregates formed in solution when the surface tension reached an equilibrium value, leading to a decrease in the value of I1 /I3 , which we could use to determine the CMC. Thus, the gradual decrease in the value of I1 /I3 indicates that the microenvironment around each pyrene molecule became increasingly more hydrophobic.

Surface Tension (mN/m)

that a hydrophilic–lipophilic balance was in operation for the amphiphilic molecules. This phenomenon was readily identified after the air–water interface had become saturated. Fig. 3 reveals that the modified polyoxyethylenated stearylamine surfactants decreased the surface tension of water. When the surfactant concentration was above the CMC, however, the lipophilic alkyl chains reoriented toward the interior of micelles to prevent any further increase in free energy [19,20]. Therefore, the changes in surface tension were non-obvious when the surfactant concentration was well above the CMC. Table 3 lists the CMCs, surface tensions at the CMCs ( CMC ), maximum surface excess concentrations ( CMC ), standard free energies (Gm ◦ ), and surface areas per molecule (ACMC ) at the air–water interface for the modified polyoxyethylenated stearylamine surfactants. The CMCs decreased in the order 415ST > 430ST > 430MA > 415MA. An increase of the oxyethylene chain length results the CMC to higher concentrations due to the increase of the surfactant solubility [21,22]. Because the surfactant possessed polyoxyethylene chain in its structure produced hydrogen bond in the solutions provided more solubility; enhancing the hydrophilicity of surfactants (by increasing their oxyethylene chain length) usually leads to a decrease in the surface activities [23]. In addition, the occupied area per molecule, which relates to the degree of packing of surfactant molecules adsorbed at the air–water interface, increased upon increasing the polyethylene oxide hydrophilic group. The area occupied by the surfactant molecule, ACMC , provides information regarding the packing of the molecules adsorbed at the air–water interface; indeed, we found that the value of ACMC decreased upon increasing the polyethylene oxide hydrophilic groups. For example, the modified surfactant (415MA) had the lowest surface concentration and the lowest surface area per molecule (0.288 nm2 molecule−1 ), presumably because these surfactant molecules packed closely at the air–water interface, due to interactions between their hydrophilic and lipophilic chains [24]. The negative values of Gm ◦ implied that the modified polyoxyethylenated stearylamine surfactants were likely to form micelles in solution and to adsorb at the air–water interface; in other words, the micellization process would be spontaneous. In addition, the values of Gm ◦ became increasingly negative upon increasing the polyethylene oxide derivatives, suggesting that a driving force for micellization or adsorption was the interactions among the hydrophilic–lipophilic chains [24,25].

1.08 45 1.06 1.04

40

1.02 35

0.000

0.002

0.004

0.006

0.008

0.010

1.00

3

Concentration (mol/dm ) Fig. 4. I1 /I3 ratios and surface tensions plotted with respect to the concentration of the modified surfactant 415MA.

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200 415ST

150

415MA

Zeta Potential (mV)

430ST 430MA

100

50

0

-50

-100 Fig. 5. Foaming properties of the modified and unmodified surfactants.

3

Fig. 5 lists the low-foaming properties of our modified surfactants. Each compound exhibited not only low foam production (measured in terms of the height of the foam initially produced) but also low-foaming stability (measured in terms of the height after 3 min). Moreover, the foam height and foam stability of the modified surfactants (415MA, 430MA) were both less than those of the unmodified surfactants (415ST, 430ST), presumably because the multiple anionic hydrophilic groups of the modified polyoxyethylenated stearylamine surfactants considerably increased the area per molecule and produced a lower cohesive force at the surface [31,32]. 3.5. Conductivity We measured conductivities to study the micellar aggregation behavior of our surface-active modified polyoxyethylenated stearylamine surfactants in aqueous solution. Fig. 6 presents the conductivities of the polyoxyethylenated stearylamine surfactant derivatives at various concentrations. The conductivity increased linearly upon increasing the concentration of each modified polyoxyethylenated stearylamine surfactant. In addition, derivatives with shorter hydrophilic chains exhibited larger conductivities; for example, the conductivity of the modified surfactant 415MA was significantly greater than those of the other products.

415ST 415MA 430ST 430MA

300

Conductivity (µs/cm)

250 200 150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

3

Concentration (mol/dm ) Fig. 6. Conductivities plotted with respect to the concentrations of the modified and unmodified surfactants.

4

5

6

7

8

9

10

pH Fig. 7. Zeta potentials of emulsions prepared at 1% (w/w) concentrations of the modified and unmodified surfactants, plotted with respect to pH.

When micelles are formed, the counterions bound to the micellar surfaces decrease the mobility of the micelles relative to the mobility in their monomeric form. Thus, some type of discontinuity should appear in a plot of conductivity versus concentration (i.e., at the concentration where the micelles begin to form) [33,34]. Using conductometry to measure the CMCs, we observed a break in the value of the conductivity that was consistent with the onset of formation of micelles. We fitted straight lines to the linear fragments above and below the break and treated the concentration at which these lines intersected, the inflection point, as the CMC [4,35]. 3.6. Emulsification We prepared 10% (w/w) O/W emulsions by adding soybean oil to solutions of the modified and unmodified surfactants and then measured their zeta potentials at various values of pH. Fig. 7 reveals that the modified surfactants possessed more-negative zeta potentials and lower isoelectric points than did their unmodified counterparts. The isoelectric point of each modified polyoxyethylenated stearylamine surfactant was 5.5. Thus, the electrical charge densities of the modified surfactant droplets were less negative than those of the unmodified surfactants. We expected the surface potential to increase upon increasing the surface concentration, because an increase in surface concentration would lead to compression of the electrical double layer and a corresponding decrease in zeta potential. Therefore, the stability of the emulsions resulted from a combination of electrical repulsion and steric stabilization [36]. Fig. 8 presents images of the emulsions prepared from the modified and unmodified surfactants at a concentration of 1% (w/w), and their average diameters. The modified surfactants displayed greater emulsifying ability in terms of their average initial droplet diameters and increase in the sharpness of their particle size distributions. In addition, the average droplet diameter of unmodified surfactants (415ST) increases significantly after a stock time of 2 h (Fig. 9). Similarly, the average droplet diameter of the unmodified surfactant 430ST decreased significantly after a stock time of 3 h. The modified surfactants 415MA and 430MA were more stable than their corresponding unmodified surfactants. Moreover, the emulsion formed by the modified surfactant 415MA having shorter hydrophilic groups (i.e., where hydrophilic–lipophilic balance played a role in the amphiphilic molecule) featured droplets of

S.-K. Su et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 26–32

smaller average diameter and was more stable. From the adsorption isotherms, it appears that the emulsions prepared using 10% (w/w) of the various modified surfactants were formed under conditions that led to full coverage of each droplet’s surface [8,37]. Microscopic images (Fig. 10) of the emulsion formed from the modified surfactant 415MA confirmed its stability and the absence of flocculation. Thus, steric stabilization was the dominant feature in the modified polyoxyethylenated stearylamine surfactant, imparting excellent stability to its O/W emulsions.

35

415ST 415MA 430ST 430MA

30

Volume (%)

25

31

20

15

4. Conclusions

10

5

0 0

1

2

3

4

5

6

Particle Size (µm) Fig. 8. Average droplet diameters (by volume) of emulsions prepared at 1% (w/w) concentrations of the modified and unmodified surfactants, plotted with respect to time.

We have prepared a series of novel modified polyoxyethylenated stearylamine surfactants through reactions involving maleic anhydride and sodium hydrogen sulfite. These novel compounds exhibit excellent surface activities, measured in terms of their surface tensions, conductivities, and foaming and emulsifying abilities. The surfactants modified with sulfonate units were more surfaceactive than their corresponding unmodified surfactants at all concentrations—presumably because fewer amphiphilic molecules reached the surface of the solution, decreasing their ability to influence the surface tension. Furthermore, the emulsification stabilities of the modified surfactants were better than those of the unmodified surfactants, presumably because of greater hydrophilic–lipophilic balance and stronger attractive interactions among the hydrophobic chains.

4500

Particle Size (nm)

References 4000

415ST 415MA 430ST 430MA

3500

3000

2500

0

1

2

3

4

Time (hr) Fig. 9. Volumetric average droplet diameters of emulsions prepared at 1% (w/w) concentrations of the modified and unmodified surfactants, plotted with respect to time.

Fig. 10. Photographic image (50×) of an emulsion of the modified surfactant 415MA.

[1] P.H. Yen, K.M. Chen, Preparation and properties of novel low-foaming dyeing auxiliaries. Part 1. Preparation and properties of ethoxylated hydroxylsulphobetaines in nylon dyeing, J. Soc. Dyers Colour. 114 (1998) 160. [2] A.S. Mohamed, M.Z. Mohamed, Preparation of novel cationic surfactants from epichlorohydrin: their surface properties and biological activities, J. Surfact. Deterg. 13 (2010) 159. [3] D.A. Jaeger, B. Li, T. Clark Jr., Cleavable double-chain surfactants with one cationic and one anionic head group that form vesicles, Langmuir 12 (1996) 4314. [4] C.C. Wang, L.H. Lin, H.T. Lee, Y.W. Ye, Surface activity and micellization properties of chitosan–succinyl derivatives, Colloids Surf. A: Physicochem. Eng. Aspects 389 (2011) 246. [5] P.H. Yen, K.M. Chen, Preparation and properties of novel low-foaming dyeing auxiliaries. Part 2. Preparation and dyeing properties of anionic derivatives of polyoxyethylenated stearylamine, J. Soc. Dyers Colour. 115 (1999) 88. [6] H.J. Liu, L.H. Lin, K.M. Chen, Synthesis and surface activity of polyethylene glycol–maleic anhydride–polydimethylsiloxane polyester surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 215 (2003) 213. [7] C.C. Lai, K.M. Chen, Preparation and surface activity of polyoxyethylene- carboxylated modified Gemini surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 320 (2008) 6. [8] L.H. Lin, Y.C. Lai, Synthesis and physicochemical properties of nonionic Gemini surfactants with a sulfonate spacer, Colloids Surf. A: Physicochem. Eng. Aspects 386 (2011) 65. [9] L.H. Lin, C.Y. Chiang, H.J. Liu, K.M. Chen, Preparation and properties of water-soluble polyester surfactants. I. Preparation and surface activity of poly(ethylene glycol)-dimethyl 5-sulfo-isophthalate sodium salt polyester surfactants, J. Appl. Polym. Sci. 86 (2002) 2727. [10] L.H. Lin, P.C. Lin, Preparation and surface activity of gelatin-derived surfactants modified with polyoxyethylene stearyl ether units, J. Appl. Polym. Sci. 121 (2011) 2993. [11] S.M. Huang, J.J. Hwang, L.H. Lin, H.J. Liu, T.C. Yeh, Interfacial properties of chitosan and nonylphenol polyoxyethylene ether, J. Appl. Polym. Sci. 116 (2010) 2227. [12] C.F.J. Kuo, L.H. Lin, M.Y. Dong, W.S. Chang, K.M. Chen, Preparation and properties of new ester-linked cleavable gemini surfactants, J. Surfact. Deterg. 14 (2010) 195. [13] F. Leicester, S.B. Hamilton, G.S. Stephen, Quantitative Chemical Analysis, Macmillan, New York, 1964, p. 510. [14] T. Yoshimura, T. Ichinokawa, M. Kaji, K. Esumi, Synthesis and surface-active properties of sulfobetaine-type zwitterionic Gemini surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 273 (2006) 208. [15] L.H. Lin, Y.S. Chou, Surface activity and emulsification properties of hydrophobically modified dextrins, Colloids Surf. A: Physicochem. Eng. Aspects 364 (2010) 55. [16] K. Nyuta, T. Yoshimura, K. Esumi, Surface tension and micellization properties of hetero gemini surfactants containing quaternary ammonium salt and sulfobetaine moiety, J. Colloid Interface Sci. 301 (2006) 267.

32

S.-K. Su et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 414 (2012) 26–32

[17] R. Zana, Critical micellization concentration of surfactants in aqueous solution and free energy of micellization, Langmuir 12 (1996) 1208. [18] V. Chauhan, S. Singh, A. Bhadani, Synthesis, characterization and surface properties of long chain ␤-hydroxy-(-alkyloxy-N-methylimidazolium surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 395 (2012) 1. [19] C. Jungnickel, J. Luczak, J. Ranke, J.F. Fernandez, A. Muller, J. Thoming, Micelle formation of imidazolium ionic liquids in aqueous solution, Colloids Surf. A: Physicochem. Eng. Aspects 316 (2008) 278. [20] L.H. Lin, C.C. Wang, K.M. Chen, P.C. Lin, Synthesis and physicochemical properties of casein-derived surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 346 (2009) 47. [21] K. Kratzat, H. Finkelmann, Influence of the molecular geometry of nonionic surfactants on surface and micellar properties in aqueous solutions, Langmuir 12 (1996) 1765. [22] I.D. Robb, P. Stevenson, Interaction between Poly (acrylic acid) and an ethoxylated nonionic surfactant, Langmuir 16 (2000) 7168. [23] F. Yang, G. Li, N. Xu, R. Liu, S.M. Zhang, Z.J. Wu, Synthesis and critical micelle concentration of a series of Gemini alkylphenol polyoxyethylene nonionic surfactants, J. Surfact. Deterg. 14 (2011) 339. [24] L. Chen, Z. Hu, H. Zhu, Z. Xue, Synthesis of cleavable aryl sulfonate anionic surfactants and a study of their surface activity, J. Surfact. Deterg. 11 (2008) 97. [25] N.A. Negm, Solubilization, surface active and thermodynamic parameters of Gemini amphiphiles bearing nonionic hydrophilic spacers, J. Surfact. Deterg. 10 (2007) 71. [26] O. Toledano, S. Magdassi, Formation of surface active gelatin by covalent attachment of hydrophobic chains, J. Colloid Interface Sci. 193 (1997) 172. [27] W. Sui, G. Song, G. Chen, G. Xu, Aggregate formation and surface activity property of an amphiphilic derivative of chitosan, Colloids Surf. A: Physicochem. Eng. Aspects 256 (2005) 29.

[28] M.A. Alcalde, A. Jover, F. Meijide, L. Galantini, N.V. Pavel, A. Antelo, J.V. Tato, Synthesis and characterization of a new gemini surfactant derived from 3␣,12␣-dihydroxy-5␤-cholan-24-amine (steroid residue) and ethylenediamintetraacetic acid (spacer), Langmuir 24 (2008) 6060. [29] T. Asakawa, T. Okada, T. Hayasaka, K. Kuwamoto, A. Ohta, S. Miyagishi, The unusual micelle micropolarity of partially fluorinated gemini surfactants sensed by pyrene fluorescence, Langmuir 22 (2006) 6053. [30] G. Bai, C. Gonc¸alves, F.M. Gamab, M. Bastos, Self-aggregation of hydrophobically modified dextrin and their interaction with surfactant, Thermochim. Acta 467 (2008) 54. [31] T. Yoshimura, K. Esumi, Synthesis and surface properties of anionic gemini surfactants with amide groups, J. Colloid Interface Sci. 276 (2004) 231. [32] C.F.J. Kuo, L.H. Lin, M.Y. Dong, W.S. Chang, K.M. Chen, Preparation and properties of new ester-linked cleavable gemini surfactants, J. Surfact. Deterg. 14 (2011) 195. [33] T. Inoue, T. Misono, S. Lee, Comment on determination of the critical micelle concentration of dodecylguanidine monoacetate (dodine), J. Colloid Interface Sci. 314 (2007) 334. [34] A. Bendjeriou, G. Derrien, P. Hartmann, C. Charnay, S. Partyka, Microcalorimetric studies of cationic gemini surfactant with a hydrophilic spacer group, Thermochim. Acta 434 (2005) 165. [35] J. Xiang, J.B. Fan, N. Chen, J. Chen, Y. Liang, Interaction of cellulase with sodium dodecyl sulfate at critical micelle concentration level, Colloids Surf. B: Biointerfaces 49 (2006) 175. [36] M.L. Jayme, D.E. Dunstan, M.L. Gee, Zeta potentials of gum arabic stabilised oil in water emulsions, Food Hydrocolloids 13 (1999) 459. [37] O. Toledano, S. Magdassi, Emulsification and foaming properties of hydrophobically modified gelatin, J. Colloid Interface Sci. 200 (1998) 235.