Synthesis and surface activitiy of 4'-fatty amido-3'-sodium carboxylate azobenzene-4-sodium sulfonates

Zhao Yuping, Zheng Zhuoli, Tang Yu, Yang Jun, Zhang Yuanming
(Department of Chemistry, Jinan University, Guangzhou, 510632)

Supported by Natural Science Group Fund of Gonddong Province (039213)

Abstract 4'-fatty amido-3'-sodium carboxylate azobenzene-4-sodium sulfonates were synthesized by coupling, and then acylation reactions. They are proved to be surfactant by determining their surface tensions at various concentrations in aqueous solution, and investigating their adsorption isotherms. Of them the best C16 compound shows relatively good surface activity with g_cmc of 49.4mN/m at cmc of 0.024mol/L and g_min of 44.6mN/m.
Keywords azo, preparation, surface activity, adsorption isotherm

1. INTRODUCTION
Surfactant and dye are two kinds of compound applied widely. A new kind of functional compound may be expected by enduing dye with surface activity. But it is difficult to achieve this purpose owing to the complicated structure of dye, and it turns out to be a great challenging problem for the future. Few such research works could be found in the literatures ^[1].In our previous report, colored 3'-fatty amido-4'-hydroxy azobenzene-4-sulfonic acids were prepared, which showed good surface activity ^[2]. But they could only dissolve in strong alkaline solution due to the structure character with one hydroxyl and one sulfonic group. Thus their application was limited. In order to improve the water solubility of such compound, 4'-fatty amido-3'-sodium carboxylate azobenzene-4-sodium sulfonates (AFAS) were synthesized here by changing hydroxyl with carboxyl. Scheme 1 presents the synthetic route of AFAS. These compounds have also been proved to show relatively good surface activity with g_cmc of 49.4 mN/m at cmc of 0.024 mol/L and g_min of 44.6 mN/m for the best one C16 AFAS.

R = C₇H₁₅，C₉H₁₉，C₁₁H₂₃，C₁₃H₂₇，C₁₅H₃₁
Scheme 1 The synthetic route of AFAS

2. EXPERIMENTS
2.1 Reagents and instruments
All chemicals were of analytical or chemical grade, and used without further purification. Distilled water was used as solvent in determining the surface tensions of prepared compounds. IR spectra were recorded on a Germany Bruker EQUINOX 55 spectrometer (KBr). ¹H-NMR spectra were obtained on an American Varian 500WB FI-NMR spectrometer with TMS as an internal standard and deuterium substituted DMSO as solvent. Surface tension was determined by maximum bubble pressure tensiometry at room temperature.
2.2 Synthesis of 4'-amino-3'-sodium carboxylate azobenzene-4-sodium sulfonates
4.0g of anhydrous p-amino benzenesulfonic acid (0.02mol) were dissolved in 24mL solution of sodium hydroxide (w_NaOH =10%, 0.05mol). After 20mL solution of sodium nitrite (w_NaNO2=20%, 0.05mol) were added to this mixture, 34mL of sulfuric acid (w_H2SO4 =20%, 0.112mol) were gradually dropped in keeping the reacting temperature of diazotization under 5oC. Excess nitrous acid was decomposed by adding urea. After the pH of diazonium salt solution was adjusted to 8~9 by sodium carbonate, 3.2g of o-aminobenzoic acid (0.02mol) in 20mL of dilute sodium hydroxide (w_NaOH = 5%) were added slowly at 5~10oC, then the mixture was stirred for 1h by keeping the pH at 8-9. After the reaction, the solution was adjusted to pH 11, the reaction product was precipitated by adding salt, and filtered. The obtained dried khaki powder was about 6.0g. The yield was about 70.0%. The melting point of product was above 300^oC.
2.3 Synthesis of 4'-fatty amido-3'-sodium carboxylate azobenzene-4-sodium sulfonates (AFAS)
2.0g of product (0.005mol) obtained above and 0.8mL of pyridine (0.01mol) were added in distilled DMSO (20mL), then 0.0075mol of fatty acyl chloride in 10mL of ethyl acetate were dropped slowly below 10oC. The pyridine and ethyl acetate used were dried with solid potassium hydroxide and anhydrous sodium sulfate, respectively. After the addition, the reacting mixture was stirred over night at room temperature. Ethyl acetate and most of the DMSO were distilled after reaction. Claret solid was obtained after precipitated by adding acetone, filtered, and dried. Yields of C8AFAS, C10AFAS, C12AFAS, C14AFAS, and C16AFAS were 35.8%, 28.6%, 30.8%, 41.0%, and 33.0%, respectively. The melting points of the products were all above 300^oC.

3. RESULTS AND DISCUSSION
3.1 Spectral characteristics      
The data in Table 1 indicated that the AFAS compounds with different length of carbon chain displayed similar absorptive positions in IR spectra. Thus, the IR spectra could be corresponded to the structures of AFAS compounds by taking C8 AFAS as example. The absorptive band at 3430.8 cm^-1 showed the NH group. CH groups of long carbon chain were indicated at 2927.0 and 2858.0 cm^-1. Carbonyl groups of amide appeared at 1687.6 cm^-1. The vibration of the C=O bond of carboxyl group was shown at 1596.8cm^-1. The peaks at 1499.8 cm^-1 and 1460.1cm^-1 indicated C=C bond of benzene. 1403.0cm^-1 is the vibrancy band of N=N, and 1201.3cm^-1 revealed the flex vibrancy band of S=O of sulfonate group.

Table 1 IR spectrum Data of AFAS

Table 2 The ¹HNMR data of AFAS

    Table 2 showed the ¹HNMR d values of prepared compounds. The d values of CH₂ groups near carbonyl were at 2.43-2.46. The d values of next CH₂ groups were at 1.58-1.60. The other CH₂ groups located at 1.26-1.28 could not be distinguished. The CH₃ groups were indicated at 0.84-0.87. The H atoms of both phenyl groups were shown at 7.18-8.28. NH groups of amide appeared at 9.23-9.25. The results of IR and ¹HNMR spectra confirmed the prepared compounds to be the expected AFAS.
3.2 Surface activity
The surface tensions of AFAS were shown in Fig.1. The solubility of AFAS was good, thus no need of sodium hydroxide for determination of surface tension like the situation in our former work ^[2].

Fig.1 The surface tension - concentration curves

    The results in Fig.1 indicate the surface tension decreases with the increment of concentration. Obvious inflexions appear at certain concentrations for all curves, which reveal the typical characteristic feature of surfactant. From Fig.1, cmc, g_cmc and g_min have been calculated (Table 3). The cmc, g_cmc and g_min of AFAS decrease from C8 to C16. These results are consistent with the rules of homologous surfactant. The values of g_cmc and g_min indicate the ability of AFAS to decrease surface tension is not good as commercial surfactant due to their bulky hydrophilic groups, but still comparative to those results for compounds dissolved in sodium hydroxide solution ^[2].Thus, by changing hydroxyl with carboxyl, the solubility of prepared azo dyes in neutral water become better, but they can still keep relatively high surface activity, which extend their applications. By conclusion, the prepared AFAS could be thought as surfactants.

Table 3 Ability of AFAS to decrease the surface tension of solution in water

   From the structure of AFAS, they could be classed to 1~2 valent ion surfactant, so the adsorbance can be calculated by the following formula (m=3) ^[3]:
Γ=

Fig.4 presents the adsorption isotherms of AFAS. It is exhibited that the adsorbance of AFAS increases quickly in the range of low concentration, and then becomes slow and even constant, which is similar to that of typical surfactant.

Fig. 4 The adsorption isotherm of AFAS

    Saturated adsorbances (G_m) and the limiting molecular areas (A_m) of AFAS can be calculated from adsorption isotherm (Table 4). With the increment of length of alkyl chain, the saturated adsorptive concentrations of AFAS decreased gradually, limiting adsorbances to be increased, and limiting molecular areas reduced, which followed the rules of homologous surfactant. The limiting molecular areas of C8-C16 AFAS are 1.6-0.9nm², much larger than that of commercial surfactant. The possible reason is due to the large hydrophilic group of AFAS, which results in large molecular section area, thus further results in lower saturated adsorbance and surface activity.

Table 4 Saturated adsorption quantum and maximum molecular area of AFAS and LAS

REFERENCES
[1] Hori Kimihiko. JP: 0341166, 1991.
[2] Zheng Z L, Tang Y, Zhang Y M, et al. Fine Chemicals (Jingxi Huagong), 2003, 20 (10): 593-595．
[3] Prosser A J, Franses E I. Colloids and Surfaces A-Physicochemical and Engineering Aspects, 2001, 178: 1-40.

4′-脂肪酰氨基-3′-苯甲酸钠偶氮苯-4-磺酸钠盐的合成及表面性能研究
赵玉苹，郑卓丽，唐渝，杨骏，张源明
(暨南大学化学系，广州 510632)
摘要 通过先偶合和后酰化的反应合成了4′-脂肪酰氨基-3′-羧酸钠偶氮苯-4-磺酸钠，通过测定在水溶液中不同浓度下，这些化合物的表面张力和研究其吸附等温线，证明了它们是表面活性剂，其中C16的这类化合物表现出相对较好的表面活性，其cmc在0.024 mol/L时的γ_cmc为49.4 mN/m，最低表面张力γ_min为44.6 mN/m。
关键词 偶氮，合成，表面活性，吸附等温线