Synthesis of anionic carboxylate dimeric surfactants and their interactions with electrolytes

Peer review under responsibility of Taibah University.

Abstract

The present work was a systematic study of the carboxylate anionic dimerics (CAD) N-methyldodecylamine and ethylene diaminetetraacetic acid dianhydride to investigate their synthesis, characterization, surface and salt behaviour. CAD were synthesized with two solvents at various temperatures. Fourier transform infrared spectroscopy was used to identify the functional groups of CAD, and ¹H and ¹³C nuclear magnetic resonance were used to determine the type of proton and for carbon atom confirmation in the synthesized moiety, respectively. The effects of inorganic (NaCl, KCl) and organic salts (sodium salicylate and sodium benzoate) on surface activity were estimated by tensiometric methods. All the salts lowered the critical micelle concentration of the synthesized dimeric surfactant, the organic salts to a greater extent than the inorganic salts, in the sequence sodium salicylate > sodium benzoate > KCl > NaCl.

Keywords:

• Carboxylate anionic dimeric surfactant
• Surface activity
• Salt

1 Introduction

Dimeric surfactants have been the subject of much research. They usually comprise two amphiphilic moieties chemically linked to or near the head groups by a spacer [1,2]. The spacers are instrumental for the performance of dimeric surfactants, as they lower surface tension and form micelles at a very low critical micelle concentration(CMC). The surfactants also possess unusual wetting, aggregation and rheological properties [3]. Dimeric surfactants are therefore widely used in many fields, including the production of detergents and cleaning agents, pharmaceuticals, cosmetics and toiletries, gene transfection, genetics, corrosion inhibition, environmental protection and emulsion polymerization [4–7]. Carboxylate anionic dimerics (CADs) are modern members of the dimeric surfactants category. Anionic dimeric surfactants have been widely studied [8–10], but there have been few studies of anionic dimeric surfactants with carboxylate head groups [11,12].

In the present study, CADs were prepared as described previously [11], with small modifications. CADs based on ethylene diaminetetraacetic acid (EDTA) dianhydride and the secondary fatty amine N-methyldodecylamine were prepared with two solvents, methanol and ethanol, to obtain a higher yield of end-product. We also optimized the temperature of the reaction. The effects of solvent and temperature on the yield of CADs were studied at a constant molar ratio of reactants (2:1) for a constant reaction duration of 20 h. The reactants were N-methyldodecylamine and EDTA dianhydride, respectively. The surface properties of CADs in water and in the presence of inorganic and organic salts were determined. The parameters studied include CMC, surface tension (γ_cmc), efficiency in reducing surface tension (C₂₀), maximum surface excess (Γ_cmc) and the occupied area per molecule (A_cmc) at the CMC.

2 Experimental

2.1 Materials and methods

EDTA dianhydride and N-methyldodecylamine were obtained from Sigma–Aldrich. Sodium dodecylsulfate (SDS) was purchased from Merck. NaOH, NaCl, KCl, sodium salicylate (NaSal) and sodium benzoate (NaBenz) were supplied by S.D. Fine Chemicals, Mumbai, and used as received. All the solvents were of analytical grade. Deionized water was treated with KMnO₄ and redistilled. All chemicals were used without further purification.

The functional group of synthesized compounds was determined by Fourier transform infrared (FT-IR) spectroscopy (Perkin Elmer, United Kingdom) before neutralization of the compound, and the spectra confirmed formation of the amide group. The chemical structure of compounds was determined by ¹H and ¹³C nuclear magnetic resonance (NMR) (Bruker Avance-III 300 MHz).

Surface tension was measured with a du Noüytensiometer (Jencon, India) by the platinum ring detachment technique [13]. The tensiometer was calibrated against double-distilled water, and the platinum ring was completely cleaned and dried before each observation. The observations were carried out in such a way that the vertically hung ring was plunged into the solution to measure its surface tension and was then hauled out; the maximum force required to drag the ring through the interface was then expressed as the surface tension. The CMC and the surface tension at the CMC (γ_cmc) were determined by plotting the breakpoint of the surface tension against the logarithm of the concentration curve. The results were accurate within ±0.1 dyn/cm. All measurements were carried out at 25 °C.

The performance of synthesized dimerics in hard water were studied by the foaming method [14,15], in which foaming properties were evaluated by the height of foam after shaking the solution of dimeric in hard water (hardness, 160 mg/L). To assess the performance of the synthesized dimerics in hard water, the foaming power was measured for three solutions: one in 1% sodium stearate (soap) solution in hard water (A), one a 1% soap solution in double-distilled water and the third a 1% soap solution in hard water with 2.8 × 10⁻⁶ mol/L of CAD.

2.2 Synthesis and characterization of carboxylate anionic dimerics

CADs were prepared with EDTA dianhydride (10 mmol, 2.56 g) and N-methyldodecylamine (20 mmol, 3.98 g) in methanol and ethanol, with refluxing and constant stirred for 20 h at temperatures of 40 °C, 50 °C and 60 °C (Scheme 1). After the solvent had evaporated, the residue was purified with chloroform, and a white powder was obtained. Data on the synthesized compounds are reported in Table 1.

Table 1 Effect of solvent and varying temperatures on yield of synthesized dimerics (CADs).^a

Surfactants (CAD)	Temperature (°C)	Solvent	Yield (%)
ECAD1	40	EtOH	64.0
MCAD1	40	MeOH	67.6
ECAD2	50	EtOH	66.1
MCAD2	50	MeOH	80.0
ECAD3	60	EtOH	61.7
MCAD3	60	MeOH	76.6

a Reaction conditions: reactants – N-methyldodecylamine and EDTA dianhydride, molar ratio – 2:1, duration of reaction – 20 h.

Scheme 1 Synthesis pathway of CAD, 3 by N-methyldodecylamine 1 and EDTA dianhydride 2.

The FT-IR spectra of CADs prepared in methanol at 50 °C (MCAD2) (Nujol mulls, selected bands in cm⁻¹) were: 3383.62 [N–H stretching], 3176.22 [–OH stretching for the acid group], 2924.13 [–CH₂ asymmetric stretching], 2854.21 [–CH₃ symmetrical stretching], 1742.02 [C=O stretching of carboxylic acid], 1652.36 [C=O stretching of tertiary amide], 1463.54 [N–CH₃], 1377.47 [C–N stretching], 1303.46 [–CO stretching], 961.65 [–OH deformation] and 720.03 [–(CH₂)_n, skeletal]. The ¹H NMR spectra of the compounds (300 MHz, CDCl₃, δ in ppm) were: 0.889 [t, 6H, –CH₃], 1.265 [m, 36H, –(CH₂)₉], 1.485 [m, 4H –CH₂–C–N–C=O], 2.772 [m, 4H, N –CH₂–CH₂–N], 2.894 [s, 6H (C–O–N–CH₃] and 2.92–3.716 [m, 4H+4H+4H, CH₃–N–C=O, N–C–CH₂–N, N–CH₂–COO]. The ¹³C NMR spectra of the compounds (300 MHz, CDCl₃, δ in ppm) were: 14.08 [CH₃–], 22.65 [CH₃–CH₂–], 27.18 [N–CH₂–CH₂–CH₂–], 28.28 [N–CH₂–CH₂–CH₂], 29.32–29.63 [–(CH₂)₆–], 31.88 [CH₃–CH₂–CH₂–], 33.57, 34.36 [CH₃–N–CH₂], 48.19, 49.10 [N–CH₂–CH₂–N], 51.88 [CH₃–N–CH₂], 55.30, 57.41 [CH₂–CO–N+CH₂–CO–O] and 169.32 [N–C=O], 173.98, [O–C=O]. The ¹H NMR spectra of the dimeric surfactants are shown in Fig. 1.

Fig. 1 ¹H NMR spectra of anionic dimeric surfactants MCAD2.

3 Results and discussion

3.1 Effect of solvent and temperature on yield of CADs

Methanol and ethanol were used to study the effect of solvent on the yield of CADs. Synthesis was studied at a constant molar ratio of EDTA dianhydride and fatty amine of 2:1 and a constant duration of the reaction at 20 h. There action temperature was 40 °C, 50 °C or 60 °C. Quantitative observations (Table 1) indicated that methanol resulted in higher yields of CADs than ethanol.

We also studied the effect of temperature and solvent on the yield of CADs. The CADs prepared at 50 °C with methanol as the solvent (MCAD2) gave 80.0% yield. A rise in temperature from 40 °C to 50 °C increased the yield of CADs, irrespective of the solvent, whereas a further rise in temperature lowered the yield, and at 60 °C neither solvent caused an appreciable rise in yield or even a decreased yield, perhaps due to initiation of a reversible reaction. Thus, use of methanol as solvent and 50 °C as the reaction temperature are optimum for commercial production of CADs.

3.2 Surface properties

Surface tension (γ_cmc), CMC, efficiency in reducing surface tension (C₂₀), maximum surface excess (Γ_cmc), occupied area per molecule (A_cmc) and surface pressure (Π_cmc) at the CMC were estimated for MCAD2 and for conventional monomeric sodium dodecyl sulphate (SDS). The interaction of the inorganic salts NaCl and KCl and the organic salts Na Benz and NaSal on micellisation was also evaluated. The surface tension (γ) of MCAD2 and SDS in aqueous solutions depended on the concentrations of surfactants. The CMC values for both surfactants were determined from the breakpoint of the plot of surface tension against concentration. The surface tension of an aqueous solution of MCAD2 with SDS and the CMC of MCAD2 at various concentrations of inorganic and organic salts are shown in Figs. 2 and 3, respectively. Table 2 shows that the CMC of the synthesized dimeric surfactant MCAD2 was approximately 300 and 800 times less than that of the conventional monomeric surfactant SDS and of the carboxylate surfactant sodium laurate, respectively [16]. This surface-active behaviour of MCAD2 shows that it is much more effective in lowering the surface tension than monomeric analogues. The maximum surface excess or adsorption (Γ_cmc) and occupied area per molecule (A_cmc) of MCAD2 and SDS were calculated from the following equations [17]:(1) ${\Gamma }_{\text{cmc}}=\left(\frac{-1}{2.303nRT}\right){\left(\frac{d\gamma }{d\log C}\right)}_T$(1) (2) $A_{\text{cmc}}=\frac{1}{N_A{\Gamma }_{\text{cmc}}}$(2) where γ is the surface tension measured at a surfactant concentration C, T is the absolute temperature, R is the gas constant, N_A is Avogadro number and n is the number of adsorption species. For dimerics, various researchers have used 2 or 3 for n. The values of A_cmc in aqueous solution without salt (Table 2) are based upon n = 3, whereas in Table 3, n is assumed to be 1 in the presence of electrolyte [18,19].

Table 2 Micellization properties of dimeric (MCAD2) and conventional surfactants.

	MCAD2	SDS	SL^a
CMC (mmol/l)	0.025	8.1	20
γcmc (dyn/cm)	31.0	36.1	37.5
Πcmc (dyn/cm)	41.0	35.9	–
C20 (mmol/l)	0.006	2.0	–
Γcmc × 10^10 (mol/cm^2)	4.09	3.41	2.43
Acmc (nm^2 × 10^2/molecule)	40.6	48.7	69.0

a Ref. [16].

Table 3 Micellization properties of anionic dimeric surfactant (MCAD2) in different concentrations of inorganic (NaCl, KCl) and organic salts (NaBenz, NaSal).

Salt (M)	CMC (mmol/L)	γcmc (dyn/cm)	Πcmc (dyn/cm)	Γcmc × 10^10 (mol/cm^2)	Acmc (nm^2 × 10^2/molecule)
NaCl
10^−3	0.020	30.9	41.1	4.26	38.9
10^−2	0.019	30.9	41.1	4.28	38.8
5 × 10^−2	0.016	30.9	41.1	4.66	35.6
10^−1	0.014	30.8	41.2	4.69	35.4
KCl
10^−3	0.019	30.8	41.2	4.29	38.7
10^−2	0.017	30.6	41.4	4.65	35.7
5 × 10^−2	0.015	30.4	41.6	4.70	35.3
10^−1	0.013	30.4	41.6	4.78	34.7
NaBenz
10^−3	0.017	30.6	41.4	4.66	35.6
10^−2	0.016	30.5	41.5	4.68	35.4
5 × 10^−2	0.014	30.4	41.6	4.71	35.3
10^−1	0.011	30.4	41.6	5.01	33.1
NaSal
10^−3	0.014	30.3	41.8	4.74	35.0
10^−2	0.012	30.1	41.9	4.95	33.5
5 × 10^−2	0.010	30.1	41.9	5.08	32.6
10^−1	0.008	30.1	41.9	5.39	30.8

Fig. 2 Variation of the surface tension vs surfactants concentration of anionic dimeric surfactants, MCAD2 (

) and conventional monomeric surfactants, SDS (□) at 25 °C.

Fig. 3 Values of critical micelle concentration (CMC) of anionic dimeric surfactants (MCAD2) at various concentrations of inorganic and organic salts at 25 °C.

The surface pressure at the CMC (Π_cmc) were calculated for MCAD2 and SDS from the equation:(3) ${\Pi }_{\text{cmc}}={\gamma }_0-{\gamma }_{\text{cmc}}$(3) where γ₀ and γ_cmc are the surface tension of the solvent and surfactant solution at the CMC, respectively. This parameter shows maximum reduction of surface tension, and the higher the Π_cmc value, the greater the effectiveness of the surfactant.

The C₂₀ value, an alternative parameter for the surface action of any surfactant, indicates the surfactant concentration required to reduce the surface tension of the solvent by 20 dyn/cm [6,20] and was determined to measure the efficiency of MCAD2.

3.3 Performance in hard water

The synthesized dimeric surfactant MCAD2 showed good capacity in hard water (Fig. 4). As it is an EDTA-based dimeric, the sodium salt of EDTA traps the calcium ion of hard water and enhances the efficiency of these dimerics, even in hard water. The foaming method was used to evaluate the activity of MCAD2 in hard water with sodium stearate (soap). Table 4 shows that MCAD2 had a higher foam height than soap.

Table 4 Variation in foam height of dimeric surfactants (MCAD2) with sodium stearate soap.

Compound	Instantaneous foam height (mm)	Foam height after 4 min (mm)
A	47	39
B	60	55
C	85	75

Fig. 4 Foaming ability of soap in hard water (A), soap in D.D.W. (B) and soap with MCAD2 in hard water (C).

4 Conclusions

In summary, 20 h at 50 °C, with methanol as the solvent and a 2:1 molar ratio of fatty amine to EDTA dianhydride were the most favourable reaction parameters for a high yield, i.e. 80%, of prepared CADs. An earlier study reported a duration of 22 h [11], but we found that 20 h was optimal. A shorter reaction might be cost-saving for commercial production of CADs. The surfactant MCAD2 has a CMC 300 times lower than that of the conventional monomer SDS and 800 times lower than that of the carboxylate monomer sodium laurate. Both inorganic and organic salts effectively reduced the CMC of dimeric MCAD2, although organic salts decreased the CMC to a greater extent, as the benzene rings of organic salts are somewhat hydrophobic, making them more effective from the viewpoint of surface activity. This study clarified the effects of both inorganic and organic salts on the surface behaviour of carboxylate dimeric surfactants and suggests that use of a suitable organic salt could effectively reduce the CMC of surfactant formulations. With excellent surface activity, MCAD2 will have broad use in both industrial and household applications. As MCAD2 is an EDTA-based carboxylate dimeric, it can be used in detergents and may have superior performance even in hard water.

Acknowledgement

We acknowledge the Department of Science and Technology, Government of India, for providing the research grant (SR/FT/CS-043/2010) for this work.

Notes

Peer review under responsibility of Taibah University.