Cardanol was purchased from Mei Dong Bio-Logical Material Co., Ltd. in Shanghai, China. 1,3-dibromo pro- pane was obtained from Yuan Li chemical reagent company in Tianjin, China. Chlorosulfonic acid, sodium hy- droxide and methylene chloride were provided by Hua Dong chemical reagent company in Shenyang, China. Te-traethylammonium bromide was supplied by chemical engineering technology research and development center in Guangdong, China. Diethyl ether and ethylic acid was obtained from Kaitong chemical reagent factory in Tianjin, China. All solvents were analytical grade and purified before using them. And deionized water was employed for all experiments.

In order to synthesize cardanol sulfonate Gemini surfactant, several steps of chemical reaction and treatments were employed. In the first procedure, the etherification reaction was carried out to generate cardanol Gemini surfactant. And in the next procedure, sulfonation reaction is undergoing to form cardanol sulfonic acid Gemini surfactant with the products obtained from the first procedure. In the final procedure, the product generated from the last two steps, was neutralized using sodium hydroxide solution, and then filtered, distillated and obtained the purity of the final product is 82.1% by two-phase titration methods. The schematic steps for synthetic route of cardanol sulfonate Gemini surfactant are shown in the Figure 1, and all the experimental details are recorded in our previous work [7] .

The structure for designed cardanol sulfonate Gemini surfactant was confirmed by spectral data. 1 H NMR (400 MHz) spectra were recorded with solutions in CDCl₃ by Bruker DRX 400 spectrometer using tetramethylsilane as a standard. IR spectrum was obtained by pellets in KBr methods using a Bruker Vector 22 FTIR spectrometer. Surface tensions of surfactant aqueous solutions at different concentrations were measured at 25.0˚C ± 0.1˚C by a drop-volume method [8] . The surface tensions were averaged values by three times measurements. The esti- mated error of the surface tension measurements was in the 0.01 mN/m. The surface tension value was de- creased gradually with increasing the concentration of cardanol sulfonate Gemini surfactant and exhibited an in- flection point, the concentration corresponding to the inflection point is the CMC. The thermo-dynamic proper- ties of micellization in aqueous solution were studied by surface tension method.

The structure of the designed cardanol sulfonate Gemini surfactant was identified by IR spectrum and ¹H NMR spectra. The IR frequency of synthesized cardanol sulfonate Gemini surfactant are 2970 cm⁻¹, 2871 cm⁻¹, 1115 cm⁻¹, 1014 cm⁻¹, 929 cm⁻¹, 864 cm⁻¹, 723 cm⁻¹, and 662 cm⁻¹, which are the characteristic peaks of functional

groups for -CH=CH-, -S=O, -C-O-, and Ph-H. And ¹H NMR (400 MHz, CDCl₃), δ: 7.77 ~ 6.71 (d, 3H) ArH; 5.77 (m, 8H) CHCH₂; 4.02 (m, 4H) OCH₂CH₂CH₂O; 2.77 (m, 4H) CHCH₂CH; 2.50 (m, 4H) CH₂Ar; 2.04 (m, 2H) OCH₂CH₂CH₂O; 1.58 ~ 1.3 (m, 32H) CH₂; 0.90 (t, 6H) CH₃.

As is shown in Table 1, the CMC for cardanol sulfonate Gemini surfactant was 6.20 × 10⁻² mmol·L⁻¹, and γ_cmc was 36.92 mN·m⁻¹. It displays a lower surface tension as a kind of Gemini surfactants, and the critical micelle concentration is two orders of magnitudes lower than the cardanol sulfonate single chain surfactant synthesized before [7] . And the value of Γ_max of Gemini surfactant is 6.16 mol·cm⁻² and the A_min is 0.27 nm². Owing to two hydrophobic chains, it increases the distortion of the hydrophilic group. At the same time, the electrostatic inte- raction is weaken, which can restrain the hydrophilic group divorced, close and regular the arrangement of mo- lecules. High concentration of micelle leads to higher surface activity, increase the maximum surface adsorption and reduce the saturation adsorption area per surfactant molecule that compared to the conventional surfactants.

The efficiency of decreasing the surface tension can be characterized by the value of logarithm of the surfac- tant concentration pC20. The consumption of Gemini surfactant is 0.06 mmol·L⁻¹ while 4.50 mmol·L⁻¹ of mo- nomeric surfactants, which is used to reduce the surface tension of water by 20 mN·m⁻¹ (Table 1). As a result, smaller value of pC20 makes higher tendency of the surfactant to adsorb on the air-water interface, in a mean while, to form micelles easier and efficiently reduce the surface tension. It indicates that the Gemini surfactant is superior in improving the efficiency of reducing surface tension.

Base on the fundamental principle of aqueous solution thermodynamics and classical mass-action model of mi- cellization, thermodynamic parameters were calculated at 25˚C using the Equation (1)-(3):

And the Figure 2 was plotted with the natural logarithms of CMC of cardanol sulfonate Gemini surfactant shown in Table 2 and temperatures to obtain ∂lnCMC/∂T in Equation (2). Then the thermodynamic parameters (,and) can be calculated (Table 3).

^aCardanol sulfonate singlechain surfactant [7] ; ^bCardanol sulfonate Gemini surfactant.

Table 3Table 3

. Thermodynamic parameters of the micellization of designed Gemini surfactant in aqueous solution

T (K)	(kJ·mol−1)	(kJ·mol−1)	(kJ·mol−1·K−1)	(kJ·mol−1)
288.15	−32.49	7.31	0.1381	−39.79
298.15	−33.89	7.82	0.1399	−41.71
308.15	−35.33	8.36	0.1418	−43.70
318.15	−36.75	8.91	0.1435	−45.65
328.15	−38.15	9.48	0.1451	−47.61

As is shown in Table 3, all values of for micellization at 25˚C are negative. It means that the micelli- zation process of Gemini surfactants in aqueous solutions is spontaneous, and the micellization system is stable. shows that the micellization is endothermic process. is positive, which indicates that the “iceberg structure” is broken and the water molecules tend to a disordered state. With increasing of temperature, and decrease, but the value increases, which illustrate that the contribution of the entropy for changing Gibbs free energy is diminished by increasing of temperature. In another word, high temperature is the disadvantage of micellization because of the driving force of entropy is reduced.

The enthalpy-entropy compensation for the micellization process of Gemini surfactants in aqueous solutions can be illuminated by the formula below:

where T_c is the compensation temperature, it is the parameter of dehydration in the procedure of molecular ag- gregates, which represents the interaction between solute and solvent. is the value of enthalpy change, it is the parameter of aggregation for hydrophobic group of the Gemini surfactant, which reflects the interaction between the solute and solute. When is smaller, the stability of the corresponding aggregate structure is higher. When is zero, is defined as the value of entropy change. Under this situation, the micelli- zation is forced by entropy change. This result is consistent with literature studies [9] .

The relationship between the enthalpies of intention () and the entropy term () was investigated that a linear plots line was obtained with slope of unity (Figure 3). Then a linear fitting was done to calculate the compensation temperature T_c, and it was found to be 308 ± 1 K (Table 4). It illustrates that micellization of car- danol sulfonate Gemini surfactant in aqueous solution is proved to be enthalpy-entropy compensation. In a sense, compensation temperature is the key point for micellization in aqueous solution, the contributions of enthalpy or entropy are equal [10] .