Synthesis and physiochemical properties of novel gemini surfactants with phenyl-1,4-bis(carbamoylmethyl) spacer

Abstract

A series of novel gemini surfactants, namely, phenyl-1,4-bis[(carbamoylmethyl) N,N-dimethylalkyl ammonium chloride)] (a, b and c), was synthesized systematically and characterized by FTIR, ¹H NMR, ¹³C NMR and MS. The surface and bulk properties were evaluated by surface tension, conductivity, viscosity, dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements. These surfactants have been found to have low surface tension (γ_CMC) values as compared to other categories of gemini cationic surfactants, and they form vesicles in solutions at low concentrations. The DLS and TEM studies showed that the aggregations of the above mentioned surfactants changed from larger globular vesicles and smaller globular micelles to network aggregates and then to globular vesicles with an increase in the surfactant concentration. It is assumed that this unusual aggregation behavior is related to the transformation of the molecular conformation of phenyl-1,4-bis(carbamoylmethyl) spacer, along with its rigidity and hydrogen-bonding capability. The thermodynamic parameters of micellization process, namely, standard Gibbs free energy (ΔG⁰_m), enthalpy (ΔH⁰_m) and entropy (ΔS⁰_m), were derived from conductivity measurements at different temperatures. The Krafft points of three gemini surfactants are very low.

---

Gemini surfactants are amphiphilic molecules containing two hydrophilic headgroups and two hydrophobic tails connected by a spacer at the two headgroups. Many studies have been carried out on gemini surfactants, focusing on their unique surface and bulk properties such as high surface activity, low critical micelle concentration (CMC), and more abundant self-assembly morphologies than the corresponding traditional single-chain surfactants.^1–4 Owing to these unique properties, gemini surfactants have been widely used as industrial detergents, in the construction of high-porosity materials, and as templates for the synthesis of nanoparticles and nanorods.^5–10 Thus, the aggregation behavior of surfactants has attracted great attention in academic research.

The chemical structures of gemini surfactants play an important role in their aggregation behavior.^11,12 In particular, the nature of the spacer group (length, flexibility, and polarity) has been shown to be of the utmost importance in determining the solution properties of aqueous gemini surfactants.^13,14 These effects lead to the synergism of two alkyl tails of a gemini molecule, change in the charge density of its headgroup, and variation of its molecular geometry due to which rich structures and morphologies of aggregates are obtained. In addition, the flexible spacer influences the above mentioned functions of the gemini mainly depending on its length.^6,15 A very long flexible spacer can bend toward the alkyl tails to meet the chemical environment around the molecule, due to which the molecule self-assembly is influenced. The effect of the short rigid spacer is almost identical to that of the flexible spacer having a similar length.¹⁶ However, the long rigid spacer yields considerably different effects compared to the flexible spacer owing to the two alkyl tails being inhibited from being too close, which leads to a column-like molecular shape and identical probability for the cis/trans configuration of the two alkyl tails around the spacer.^17,18 These factors change the aggregate morphology of the gemini surfactant with long rigid spacer, when the gemini surfactant concentration is increased or few additives are added.^19,20 The chemical modification for the spacer is also discussed, which is expected to promote molecular self-assembly, or give some new functions to the aggregates.^21–24

The most widely studied gemini surfactants are cationic alkanediyl-α,ω-bis(alkyldimethyl-ammonium) dibromides, which are referred to as C_mC_sC_m, where m and s stand for the carbon atom number in the tail alkyl chain and in the methylene spacer, respectively. Previous studies have shown that the variation of the spacer and the tail alkyl chain usually affects the aggregation behavior of C_mC_sC_m,^2,25 and C_mC_sC_m series (4 ≤ s ≤ 12, 12 ≤ m ≤ 16) tends to form higher-curvature aggregates in aqueous solutions such as spherical or elongated micelles.^26,27 Therefore, to design and synthesize a series of new gemini surfactants with phenyl-1,4-bis(carbamoylmethyl) as spacer (the phenyl possesses rigidity and N–H can easily form a hydrogen bond) tend to form larger aggregates (vesicles and network aggregations) than micelles in aqueous solutions. Their properties such as surface activity and self-aggregation behavior were investigated, which would contribute to extend their potential application such as templates for the synthesis of nanoparticles, nanorods, and mesoporous materials.

In this study, gemini surfactants phenyl-1,4-bis[(carbamoylmethyl) N,N-dimethylalkyl ammonium chloride)] (a, b and c) have been synthesized. Their surface activity, thermodynamic property and aggregation behavior were evaluated by surface tension, electrical conductivity, transmission electron microscopy (TEM) and dynamic light scattering (DLS) techniques.

Experimental

Materials and instruments

All the reagents were of analytical grade and used directly without further purification. Melting point (mp) was measured using an X-6 micro melting point apparatus, Beijing Tektronix Instrument Company. FT-IR was obtained using a Nicolet750 infrared spectrometer, American Nicolet Company; ¹H NMR and ¹³C NMR were recorded using an Avance 400 and 600 superconducting NMR instrument, Switzerland Bruker Company; DMSO and CDCl₃ were used as the solvents. Mass spectral analyses were carried out on an Agilent 7500 ESI-Ion Trap Mass spectrometer, American Agilent Company. Surface tension was tested on a K100 Tensiometer, Germany Krüss Company. Conductivity was measured on a Leici DDS-11A conductivity analyzer, Shanghai Leici Instrument Company. Krafft temperature (T_K) was determined by heating the surfactant solution until a clear solution was obtained. All solution concentrations of these surfactants were 1 wt% (i.e. well above the CMC of the investigated surfactants), evaluated using visual observation method.¹⁵ DLS measurements were performed on a Malvern Autosizer, Malvern, UK Company. TEM image was obtained with an H-7650, Hitachi Instruments Company. Relative viscosities were measured in two thermostatted Ubbelohde viscometers, Shanghai Huake equipment Company.

Synthesis of 1,4-bis(chloroethylamido)benzene

1,4-Bis(chloroethylamido)benzene was synthesized according to the literature.²⁸ 2-Chloroacetyl chloride (2 mL, 25.0 mmol) was dissolved in chloroform (15 mL), then added dropwise to a stirred solution of 1,4-phenylene diamine (1.103 g, 10.0 mmol) and pyridine (2.1 mL, 25.0 mmol) in chloroform (30 mL). A pale purple solid precipitated immediately. After stirring for 4 h under nitrogen atmosphere at room temperature, the pale purple solid was filtered, washed with saturated sodium bicarbonate solution and water, and dried, yielding 2.436 g of 1,4-bis(chloroethylamido)benzene. The synthesis of the gemini surfactants (a, b and c) is illustrated in Scheme 1.

Scheme 1 Synthetic routes of gemini surfactants (a, b and c).

Characterization of 1,4-bis(chloroethylamido)benzene

1,4-Bis(chloroethylamido)benzene. Yield: 92.98%. FTIR (KBr pellet) v (cm⁻¹): 3420 (N–H stretching, amino), 3096 (C–H stretching, benzene ring), 2952 (C–H stretching, methyl), 1668 (C=O stretching, carbonyl), 1595, 1514, 1407 (C=C stretching, benzene ring), 739 (C–Cl bending). ¹H NMR (600 MHz, DMSO), δ: 10.31 (s, 2H, NH), 7.53 (s, 4H, PhH), 4.21 (s, 4H, O=C–CH₂–).

Synthesis of gemini surfactants (a, b and c)

10.0 mmol 1,4-bis(chloroethylamido)benzene and 30 mL of N,N-dimethylformamide (DMF) were placed in a three-necked round-bottom flask, and 25.0 mmol N,N-dimethyl alkylamine (CH₃)₂NC_nH_2n+1 (n = 12, 14, and 16) was added dropwise into the flask; then, the mixture was stirred and allowed to react at 80 °C under nitrogen for 6–7 h. The resulting crude mixtures were cooled to 25 °C. DMF was removed from the crude reaction mixture under reduced pressure using a rotary flash evaporator at 80 °C. It was then allowed to cool. The solid product was purified by recrystallization from mixtures of ethyl acetate and chloroform five times to obtain each of the gemini surfactants (a, b and c) as white solids, which were then dried in a vacuum oven for 5–6 h at 40 °C. The yields were 68.82%, 71.64%, and 72.97% for a, b and c, respectively. The synthesis process is shown in Scheme 1. The structures of all the products were confirmed by characterization using FT-IR, ¹H NMR, ¹³C NMR and ESI-MS.

Characterization of gemini surfactants (a, b and c)

Phenyl-1,4-bis[(carbamoylmethyl)-N,N-dimethyldodecyl ammonium chloride)] a. White solid, mp: 204.2–205.4 °C. FTIR (KBr pellet) v (cm⁻¹): 3497 (N–H stretching, amino), 3027 (C–H stretching, benzene ring), 2920 (C–H stretching, methyl), 2851 (C–H stretching, methylene), 1680 (C=O stretching, carbonyl), 1563, 1520, 1470 (C=C stretching, benzene ring), 1418 (C–H bending, methyl), 721 (alkyl chain bending, methylene). ¹H NMR (400 MHz, CDCl₃), δ: 11.09 (s, 2H, NH), 7.51 (s, 4H, PhH), 4.87 (s, 4H, CH₂N⁺), 3.69 (br s, 4H, N⁺CH₂CH₂), 3.48 (s, 12H, N⁺CH₃), 1.80 (m, 4H, N⁺CH₂CH₂), 1.24–1.34 (m, 36H, alkyl chain), 0.87 (t, J = 6.8 Hz, 6H, CH₃). ¹³C NMR (150 MHz, CDCl₃), δ (×10⁻⁶): 161.12, 134.18, 120.58, 65.78, 63.87, 51.98, 31.90, 29.58, 29.43, 29.38, 29.32, 29.14, 26.26, 22.96, 22.68, 14.13. ESI MS (positive ions) m/z: found 615.6 for [M − 2Cl − H]⁺, 308.3 for [M − 2Cl]²⁺.

Phenyl-1,4-bis[(carbamoylmethyl)-N,N-dimethyltetradecyl ammonium chloride)] b. White solid, mp: 210.1–212.2 °C. FTIR (KBr pellet) v (cm⁻¹): 3456 (N–H stretching, amino), 3080 (C–H stretching, benzene ring), 2917 (C–H stretching, methyl), 2850 (C–H stretching, methylene), 1683 (C=O stretching, carbonyl), 1588, 1517, 1470 (C=C stretching, benzene ring), 1415 (C–H bending, methyl), 720 (alkyl chain bending, methylene). ¹H NMR (400 MHz, CDCl₃), δ: 11.11 (s, 2H, NH), 7.52 (s, 4H, PhH), 4.88 (s, 4H, CH₂N⁺), 3.69 (br s, 4H, N⁺CH₂CH₂), 3.49 (s, 12H, N⁺CH₃), 1.81 (m, 4H, N⁺CH₂CH₂), 1.21–1.35 (m, 44H, alkyl chain), 0.87 (t, J = 6.8 Hz, 6H, CH₃). ¹³C NMR (150 MHz, CDCl₃), δ (×10⁻⁶): 161.13, 134.16, 120.59, 65.80, 63.88, 51.97, 31.93, 29.67, 29.65, 29.59, 29.44, 29.39, 29.36, 29.15, 26.26, 22.96, 22.69, 14.13. ESI MS (positive ions) m/z: found 671.6 for [M − 2Cl − H]⁺, 336.3 for [M − 2Cl]²⁺.

Phenyl-1,4-bis[(carbamoylmethyl)-N,N-dimethylhexadecyl ammonium chloride)] c. White solid, mp: 204.1–205.4 °C. FTIR (KBr pellet) v (cm⁻¹): 3434 (N–H stretching, amino), 3019 (C–H stretching, benzene ring), 2955, 2921 (C–H stretching, methyl), 2851 (C–H stretching, methylene), 1682 (C=O stretching, carbonyl), 1574, 1513, 1469 (C=C stretching, benzene ring), 1404 (C–H bending, methyl), 723 (alkyl chain bending, methylene). ¹H NMR (400 MHz, CDCl₃), δ: 11.25 (s, 2H, NH), 7.55 (s, 4H, PhH), 4.89 (s, 4H, CH₂N⁺), 3.68 (t, J = 8.0 Hz, 4H, N⁺CH₂CH₂), 3.50 (s, 12H, N⁺CH₃), 1.81 (m, 4H, N⁺CH₂CH₂), 1.25–1.36 (m, 52H, alkyl chain), 0.88 (t, J = 6.8 Hz, 6H, CH₃). ¹³C NMR (150 MHz, CDCl₃), δ (×10⁻⁶): 161.12, 134.15, 120.60, 65.80, 63.89, 51.98, 31.93, 29.71, 29.69, 29.67, 29.65, 29.60, 29.45, 29.40, 29.37, 29.16, 26.27, 22.97, 22.70, 14.13. ESI MS (positive ions) m/z: found 727.7 for [M − 2Cl − H]⁺, 364.4 for [M − 2Cl]²⁺.

Krafft point

The Krafft temperature (T_K) was determined by heating the surfactant solution until a clear solution was obtained. All solution concentrations of these surfactants were 1 wt% (i.e. well above the CMC of the investigated surfactants) using visual observation method.¹⁵

Surface tension

The surface tensions of the surfactant solutions were measured by the ring method using Krüss K100 Tensiometer (Krüss, Germany). The temperature during the measurement was controlled at 25.0 ± 0.1 °C using a thermostatic bath. The tensiometer was calibrated using ultrapure water. The experiment was conducted from high concentrations to low concentrations.

Conductivity

The conductivity of surfactant solutions was measured using a DDS-11A conductivity meter (Shanghai Leici Instrument Co.) with a Shanghai Leici DJS-1 conductivity electrode. Ultrapure water was added to surfactant solutions to change the surfactant concentration. The solutions were thermostated at 25.0 ± 0.1, 35.0 ± 0.1, 45.0 ± 0.1 and 55.0 ± 0.1 °C in a thermostatic bath.

Dynamic light scattering (DLS)

DLS measurements were performed on a Malvern Autosizer (ZETASIZER Nano series Nano ZS-90, Malvern, UK) at a scattering angle of 90°. All the measurements were performed at 25.0 ± 0.1 °C.

Transmission electron microscopy (TEM)

Micrographs were obtained with an H-7650 (HITACHI Co.) transmission electron microscope at a working voltage of 100 kV. The TEM samples were prepared by the negative-staining method. Phosphotungstic acid solution (2%) was used as the staining agent. A carbon Formvar-coated copper grid (200 mesh) was placed on one drop of the sample solution for 5 min, and the excess solution was wiped with a filter paper to obtain a thin liquid film on the copper grid. Subsequently, the copper grid was placed onto one drop of phosphotungstic acid solution for 2 min. The excess liquid was wiped with a filter paper, and then the samples were dried in air.

Viscosity measurement

The relative viscosities of the surfactant solutions were measured in two Ubbelohde viscometers thermostatted at 25 °C. The viscosities of all the solutions were found to be independent of capillary diameter and thus the flow rate.

Results and discussion

Krafft point

Krafft points of three gemini surfactants (a, b and c) were measured to ensure absolute dissolution in water at the experimental temperature. The Krafft points of all the gemini surfactants (a, b and c) were determined and found to be less than 5 °C (Table 1), which are lower than that of bis-N,N,N-dodecyldimethyl-p-phenylene diammonium dichloride,²⁹ possibly resulting from the amide group being inserted into the spacer of gemini surfactants (a, b and c). Thus, the gemini surfactants have good solubility in water.

Table 1 Krafft points of three gemini surfactants (a, b and c)

Gemini surfactants	a	b	c
Krafft point (°C)	<5	<5	<5

---

Surface activity

Surface activity of gemini surfactants (a, b and c) was determined by surface tension measurements. The surface tension (γ) versus log C (i.e., C is the surfactant concentration) plots for the aqueous solutions of gemini surfactants (a, b and c) at 25 °C are shown in Fig. 1.

Fig. 1 Curves of surface tension (γ) vs. log C of gemini surfactants (a, b, and c) at 25 °C.

A plot of surface tension versus log C is often linear up to a concentration called the critical micelle concentration (CMC), at which the surfactant begins to aggregate into a micelle. From Fig. 1, it can be observed that with increasing hydrophobic chain length, the CMC value gradually decreased because of the enhanced hydrophobic interaction between the longer alkyl chains (n = 12, 14 and 16). The surface parameters, i.e., CMC, the surface tension attained at the CMC (γ_CMC), the effectiveness of surface tension reduction at CMC (π_CMC), the maximum surface excess concentration (Γ_max), the area per molecule at the interface (A_min), and the surfactant concentration required to reduce the surface tension of the solvent by 20 mN m⁻¹ (C₂₀) of three gemini surfactants are given in Table 2. The packing densities of surfactants at the air–water interface are very important to interpret the surface activities of various surfactants.¹⁴ The maximum surface excess concentration (Γ_max)³⁰ is defined as the maximum concentration of surfactant molecules at the interface of their solutions in the saturation state. It can be calculated by eqn (1).

where C is the concentration of the surfactant aqueous solution, R is gas constant (8.314 J mol⁻¹ K⁻¹), T is absolute temperature, γ denotes the surface tension, and n is the number of active species in the surfactant solution, which is equal to 3 (n = 3) in the case of gemini surfactants;²⁴ the results are listed in Table 2. It is clear from the data that the gradual increase in the hydrophobic chain length decreases the concentration of the surfactant molecules at the interface.

Table 2 Surface activity parameters of the three new gemini surfactants and corresponding single chain surfactant (a, b, and c) at 25 °C

Surfactants	CMC (mol L^−1)	γCMC (mN m^−1)	πCMC (mN m^−1)	Γmax (μmol m^−2)	Amin (nm^2)	C20 (mol L^−1)	CMC/C20	pC20
a BAC-12: dodecyl dimethyl benzyl ammonium chloride, BAC-14: tetradecyl dimethyl benzyl ammonium chloride, BAC-16: hexadecyl dimethyl benzyl ammonium chloride. The data of corresponding single chain surfactant (BAC) reported in ref. 31.
a	6.37 × 10^−4	38.97	33.03	0.93	1.78	1.43 × 10^−4	4.45	3.84
b	1.38 × 10^−4	40.70	31.30	0.82	2.03	2.94 × 10^−5	4.69	4.53
c	4.58 × 10^−5	41.31	30.69	0.65	2.56	6.46 × 10^−6	7.09	5.19
BAC-12^a	9.10 × 10^−3	38.0		1.64	1.01			2.850
BAC-14^a	1.90 × 10^−3	39.2		1.59	1.05			3.368
BAC-16^a	4.00 × 10^−4	39.6		1.46	1.14			4.134

---

The minimum area occupied per surfactant molecule (A_min) at the air–water interface is related to the maximum surface excess concentration (Γ_max), as shown in eqn (2).

where N_A is Avogadro's number.

Table 2 shows that these surfactants have a significantly lower critical micelle concentration (CMC) and a higher efficacy for reducing surface tension compared to other conventional surfactants.³¹ With increasing hydrocarbon chain length, the gemini surfactant molecules pack more loosely; thus, the γ_CMC of c compound is more than that of a or b compound.³² It is also quite clear that surface tension values of these surfactants (n = 12, 14, and 16) are lower than the corresponding gemini surfactants with large diethyl ammonium headgroups,¹⁴ which indicates that the gemini surfactants (a, b and c) have higher surface activity. The gemini surfactants (a, b and c) have two alkyl chains, dimethyl ammonium headgroups, and a spacer containing a –(C₆H₄)– and two amide groups. The γ_CMC values of Et series were slightly bigger than those of Me series; loose arrangement of molecules at the air/water interface induced by a bigger headgroup should account for this phenomenon.¹⁵ Close arrangement of gemini surfactants (a, b and c) molecules at the air/water interface, induced by a smaller headgroup and a rigid spacer, should account for the phenomenon of lower surface tension. The CMC values of the gemini surfactants (a, b, and c) are significantly lower than those of conventional surfactants. The lower CMC values of these gemini surfactants are largely attributed to the two alkyl chains and the long rigid spacer. The long rigid spacer of phenyl-1,4-bis(carbamoylmethyl) yields considerably different effects compared to the flexible spacer because two alkyl tails are inhibited from being close, which leads to a column-like molecular shape (Fig. 7a) and the identical probability for the cis/trans configuration of the two alkyl tails around the spacer (Fig. 8). The longer hydrophobic chain is more prone to bending at the air/water interface. These factors make the surfactant molecules to reach the saturated adsorption state easily at the interface. Thus, it can be seen that, for the gemini surfactants, the CMC values decrease and A_min values increase as the tail carbon number is increased from 12 to 16 (Table 2).

The gemini surfactants (a, b and c) have para-phenyl employed as the rigid spacer unit with dimethyl ammonium headgroups. Dimethyl ammonium headgroups are small. Thus, A_min values of the gemini surfactants (a, b and c) are smaller than those of cationic gemini surfactants with diethyl ammonium headgroups.¹⁴ Moreover, the longer hydrophobic chain is more prone to bending at the air/water interface, which leads to surfactant molecules occupying a larger area at the air/water interface.³³ According to Table 2 data, Γ_max decreased but A_min increased with the increase of the hydrocarbon chain length, which suggested that the gemini surfactant molecules (a, b and c) with longer hydrocarbon chains have lower packing densities at the air–water interface.

π_CMC indicates the effectiveness of surface tension reduction, and it can be used to evaluate the effectiveness of surfactant to lower the surface tension of water.³² (Here, π_CMC = γ₀ − γ_CMC, where γ₀ and γ_CMC are the surface tensions of water and the surfactant solution at the CMC, respectively.) Thus, the larger the value of π_CMC is, the higher the effectiveness of the surfactant is. pC₂₀ is obtained using eqn (3):

pC₂₀ = −log C₂₀ (3)

The values of pC₂₀ and π_CMC are listed in Table 2. The efficiency parameter indicates the adsorption behavior of surfactant molecules at the interface. The larger the value of pC₂₀, the greater is the tendency of surfactants to be adsorbed.³⁴ The variation of pC₂₀ indicated that the pC₂₀ increased with the increasing hydrocarbon chain length. However, the π_CMC decreased with the increasing hydrocarbon chain length. The results show that the gemini surfactants (a, b and c) with longer hydrocarbon chain have lesser ability to reduce surface tension. The value of CMC/C₂₀ ratio is used to determine structural factors in the adsorption and micellization process. The surfactant with a larger CMC/C₂₀ ratio has a greater tendency to adsorb at the interface than to form micelles in solution. In Table 2, it can be seen that the surfactant with the increased hydrophobic chain length was more easily adsorbed at the air/water interface than self-assembling in solution. The result is in accordance with the conclusion from the value of pC₂₀.

The chemical structures of the gemini surfactants play an important role for their surface activity. In particular, the nature of the spacer has been shown to be of the utmost importance in determining the properties of the aqueous solutions of gemini surfactants. Gemini surfactant molecules adopt different conformations, depending on the spacer (length and flexibility). Gemini surfactants (a, b and c) have unsaturated and conjugated rigid spacer. The gemini surfactants might assume a linear conformation, which would place only one of its hydrophobic tails into the air, while the other tail would be forced to dip entirely into the water (Fig. 2A). Alternatively, the horseshoe conformation would place both chains into the air (Fig. 2B), but they would be separated from one another by a distance equal to the length of the spacer. Finally, a gemini surfactant might prefer to lie flat on the water surface (Fig. 2C), an orientation that would occupy an extraordinarily large surface area per molecule. A_min values of the gemini surfactants (a, b and c) are very large (1.78–2.56 nm²), which correspond closely to 2.36 nm².³⁵ We thereby speculate that surfactant molecules are planar at the interface. The surfactant with a longer hydrocarbon chain has a lower surface concentration at the interface and a higher hydrophobic character. Therefore, hydrophobic chain length also has a great influence on the surface activity of the gemini surfactants (a, b and c). These results are in good agreement with the previously obtained surface activity parameters.

Fig. 2 Possible orientations of the gemini surfactants (a, b and c) at an air/water interface.

Thermodynamics of micellization

To obtain the micellization thermodynamics parameters of the gemini surfactants (a, b and c), the micellization behavior of the aqueous solutions of gemini surfactants (a, b and c) (25, 35, 45 and 55 °C) was investigated at different temperatures by conductivity measurement. The electrical conductivity (κ) versus concentration (C) plots for the aqueous solutions of the gemini surfactants (a, b and c) are shown in Fig. 2.

The CMC values were obtained from the intersection of two fitted lines in the conductivity region. It is observed that the CMC values of the gemini surfactants (a, b and c) increase with the increase in temperature (Fig. 3). The CMC values obtained by conductivity measurement are in good agreement with the surface tension derived data (Table 3). It is noteworthy that the CMC value of the gemini surfactants (a, b and c) obtained by this method is larger than that obtained by surface tension measurement (Table 2), which is due to the existence of non-surface-active premicellar aggregates in aqueous solutions of gemini surfactants. Similar results have been observed in another study.¹²

Fig. 3 Variation in the electrical conductivity (κ) with the concentrations (C) of gemini surfactants (a, b and c) at different temperatures (25, 35, 45 and 55 °C).

Table 3 The CMC and thermodynamic parameters of the gemini surfactants (a, b, and c) determined by the electrical conductivity method at different temperatures

Surfactants	T (K)	CMC (mmol L^−1)	β	ΔG^0m (kJ mol^−1)	ΔH^0m (kJ mol^−1)	TΔS^0m (kJ mol^−1)
a	298	1.17	0.69	−32.48	−7.38	25.11
	308	1.22	0.64	−31.37	−7.60	23.77
	318	1.35	0.63	−31.76	−8.02	23.74
	328	1.50	0.59	−31.13	−8.19	22.94
b	298	0.17	0.51	−31.67	−10.25	21.42
	308	0.18	0.48	−31.55	−10.64	20.91
	318	0.21	0.46	−31.57	−11.10	20.47
	328	0.25	0.43	−31.30	−11.53	19.77
c	298	0.10	0.50	−32.59	−11.16	21.43
	308	0.13	0.44	−31.20	−11.22	19.98
	318	0.20	0.39	−29.73	−11.36	18.37
	328	0.23	0.37	−29.67	−11.84	17.83

---

Two opposite factors operated with increasing temperature:³⁶ (1) the ordered structure of water molecules around the hydrocarbon chain may be broken, which could promote the solubilization of the surfactant monomer, and thus disfavor micellization and (2) the degree of hydrophilic hydration around the polar headgroup may decrease, thus favoring micellization. For the gemini surfactants (a, b and c), the former factor is predominant in the micellization process in the studied temperature range.

In addition, the degree of counterion dissociation (α) was calculated from the ratio of the slopes above and below CMC. The degree of counterion binding (β) could be obtained by eqn (4).³³ β values were obtained from conductivity curves and listed in Table 3.

β = 1 − α (4)

The decrease of β with the increase of temperature meant the decrease of charge density on the micelle surface, which may be caused by a reduction in aggregation number of the gemini surfactants (a, b and c) micelles.

The thermodynamic parameters for the micellization of the aqueous solutions of gemini surfactants (a, b and c) can be calculated by the phase separation model. The standard Gibbs free energy (ΔG⁰_m), enthalpy (ΔH⁰_m), and entropy (ΔS⁰_m) of micellization at different temperatures were calculated using eqn (5)–(7).

ΔG⁰_m = RT(0.5 + β)ln X_CMC (5)

where R is gas constant (8.314 J mol⁻¹ K⁻¹); T is absolute temperature; X_CMC is the CMC in molar fraction, X_CMC = CMC/55.4, where CMC is in mol L⁻¹, and 55.4 comes from 1 L of water corresponding to 55.4 mol of water at 298.15 K. Thus, when T = 308.15 K, X_CMC = CMC/55.2; when T = 318.15 K, X_CMC = CMC/55.0; and when T = 328.15 K, X_CMC = CMC/54.8. β is the degree of counterion binding to micelles. The values of dln X_CMC/dT were obtained by fitting ln X_CMC–T to a second-order polynomial.

The thermodynamic parameters, i.e., ΔG⁰_m, ΔH⁰_m and ΔS⁰_m, of the gemini surfactants (a, b and c) at different temperatures are listed in Table 3.

It can be seen that ΔG⁰_m for micellization in all the cases was negative and that the negative values decreased with the increase of temperature. The negative values of ΔG⁰_m indicated that the micellization process was a thermodynamical spontaneous process. It can also be observed that ΔH⁰_m was negative in all the cases and decreased with the increase of temperature. The negative values of ΔH⁰_m indicated that micelle formation is an exothermic process. It is quite clear from Table 3 that the values for ΔH⁰_m are smaller compared to −TΔS⁰_m, thus the micellization process is entropy driven. This is true for all the gemini surfactants investigated in this study.

Studies on aggregation behavior

To study aggregation behavior, the size distributions and morphologies of the aggregations formed for the gemini surfactants at different concentrations were investigated by DLS and TEM, respectively. The chemical structures of the gemini surfactants a and c are analogous to b. Thus, only aggregation behavior of the gemini surfactant b is shown as the example.

Fig. 4 shows the size distributions of the gemini surfactant b in solutions as observed at 5 times the CMC, 50 times the CMC and 100 times the CMC by DLS. Fig. 4 shows that the size distribution range is about from 23 nm to 400 nm at different times of the CMC.

Fig. 4 DLS measurement of the size distributions of the gemini surfactant b at different concentrations: (a) 5 times the CMC (red); (b) 50 times the CMC (black); (c) 100 times the CMC (blue); and (d) 50 times the CMC (0.1 M NaCl) (green).

Fig. 4b (black curve) shows that there is one obvious peak with an average r of about 400 nm at 50 times the CMC for the surfactant b, reflecting the occurrence of large aggregations. However, there are two peaks with an average r of about 45 nm and of about 200 nm of the surfactant b at 5 times the CMC (Fig. 4a, red curve). Usually, the relative intensity of the larger size distributions is big and the relative intensity of the smaller size distributions is small. This reflects that lots of large aggregations and a small amount of micelles coexist at 5 times the CMC. Fig. 4c (blue curve) shows that there is one peak with an average r of about 170 nm at 100 times the CMC, which is close to the big peak of Fig. 4a. Thus, large aggregations may exist at 100 times the CMC.

The abovementioned results show that the relative intensity of the large size distribution began to increase, and then decreased with the increase of the surfactant concentration (from 5 CMC to 100 CMC).

To attain a direct visualization of the abovementioned results, the morphologies of the gemini surfactant b at different concentrations were studied by TEM (Fig. 6).

It is well known that for a bilayer self-assembly, the hydrophobic chain modulates the phase behavior, whereas the headgroup determines the bilayer surface chemistry. Intermolecular H-bondings between N–H and C=O in the amide group of the spacer are responsible for the formation of spherical bilayer vesicles in dilute aqueous solutions, which has also been reported earlier.²⁴ In fact, similar H-bonding interaction has been reported for many vesicles-forming surfactants.³⁷ The spacer of surfactants (a, b and c) that contains an amide group possesses hydrogen-bonding capability. A hydrogen atom with an oxygen atom of an amide group forms an intermolecular hydrogen bonding of the surfactant molecules (Fig. 5). These H-bonding interactions near the headgroup region are able to minimize the repulsive interactions among the cationic Me₂N⁺ groups, thus ensuring higher stability to the bilayer membrane. Thus, spherical vesicles with diverse sizes were observed for the surfactant b at 5 times the CMC (Fig. 6a, the left arrow marks the small vesicle, while the right arrow marks the large vesicle), 50 times the CMC (Fig. 6c, the arrow mark), and 100 times the CMC (Fig. 6e, the arrow mark). Moreover, visibly smaller globule micelles were also present at 5 times the CMC (Fig. 6b, the arrow mark). We thereby have reached the conclusion that the vesicles and micelles coexist at low concentrations. The abovementioned results are in agreement with the DLS outcome.

Fig. 5 Schematic representation of hydrogen bonding between different surfactant molecules.

Fig. 6 TEM micrographs of (a) 5 times the CMC, bar = 200 nm; (b) 5 times the CMC, bar = 200 nm; (c) 50 times the CMC, bar = 200 nm; (d) 50 times the CMC, bar = 200 nm; (e) 100 times the CMC, bar = 200 nm; and (f) 50 times the CMC (0.1 M NaCl), bar = 200 nm.

The different spacers used will influence the molecule in a variety of ways. The distance between the charged nitrogen atoms will affect the charge density of the micelle and thus the degree of ionization. Thus, we assume that the formation of vesicles is also due to the chemical structure of the spacer of the gemini surfactant. Phenyl-1,4-bis(carbamoylmethyl) as a long rigid spacer constrains the distance between the headgroups, which inhibits the two alkyl tails from being close. Consequently, the surfactant molecules present a mainly column-like molecular configuration (Fig. 7a), which can form vesicles. In addition, a small amount of surfactant molecules also pack the hydrophobic tails tightly (Fig. 7b), which can form micelles at low concentration.¹²

Fig. 7 Probable aggregation fashions of gemini surfactants with a long rigid spacer.

The DLS and TEM studies also show that the vesicles (Fig. 6a) of various sizes and smaller micelles (Fig. 6b) transform into network aggregates (Fig. 6d) with increasing concentration of the gemini surfactant (from 5 CMC to 50 CMC). Vesicles can be cross-linked together (Fig. 6c, the arrow mark) by trans configuration (Fig. 8b) and then network aggregates could be formed at 50 times the CMC (Fig. 9). Usually, surfactants first form small aggregates at low concentrations, then the aggregates become larger with the increase of the surfactant concentration. In general, network aggregation should become larger at 100 times the CMC. However, the results obtained by this study are exactly opposite. Vesicles exist in lots of diverse sizes (Fig. 6e) rather than as larger aggregates at 100 times the CMC. This is an abnormal phenomenon. With the increase of the surfactant concentration (from 50 CMC to 100 CMC), trans configuration of alkyl tails (Fig. 8a) of surfactant molecules will transform into the cis configuration of alkyl tails (Fig. 8a). The cis configuration of alkyl tails causes the network aggregates to disappear and transform into vesicles.

Fig. 8 Cis (a) and trans (b) configurations of alkyl tails.

Fig. 9 Schematic representation of cross-linked vesicles by trans configurations of alkyl tails.

We thereby have reached the conclusion that diverse sized vesicles may arise at low concentrations as well as at high concentrations.

To further study the aggregation behavior, relative viscosity (relative to the water) of the gemini surfactant b was measured at different concentrations (Fig. 10).

Fig. 10 Relative viscosity versus concentration of aniline surfactant b at 25 °C.

Fig. 10 shows that the relative viscosity increased with rapid growth from 5 times the CMC to 50 times the CMC and with very slow growth from 50 times the CMC to 100 times the CMC. At low concentrations, the adhesion and fusion of vesicles by the trans configuration of alkyl tails led to rapid growth of viscosity. The results agree with the TEM outcome.

The effect of salt (NaCl) on network aggregates was investigated at the same time. In combination with the DLS results, it can be concluded that network aggregates were destroyed and transformed into vesicles [size distributions (about 172 nm)] and micelles [size distributions (about 23 nm) (Fig. 4d, green curve)] in the aqueous solution of these gemini surfactants with the addition of 0.1 M NaCl. The TEM and DLS results showed that the network aggregations indeed disappeared at 50 times the CMC with the addition of 0.1 M NaCl (Fig. 6f). The results show that the network aggregates have a poor salt tolerance. In addition, adding NaCl increases the concentration of counterion (Cl⁻), which inhibits the ionization of vesicles. Thus, vesicles find it difficult to form network aggregates.

Conclusions

In this study, a new series of gemini surfactants with phenyl-1,4-bis(carbamoylmethyl) has been synthesized, characterized, and investigated for their surface, thermodynamic, and thermal properties. They have lower CMC values than corresponding mono-chain surfactants, and the CMC values decreased with increasing hydrophobic chain length. The gemini surfactants with a rigid spacer and a longer hydrocarbon chain have lower packing density but higher efficiency of adsorption. They have low Krafft points, indicating that they have good solubility in water. An interesting phenomenon was revealed by the DLS and TEM studies, which reflected that the surfactant concentration, spacer and configurations strongly affect the aggregation behavior of these surfactants, i.e., diverse sized spherical vesicles and smaller globule micelles form at low concentration (5 CMC); these vesicles and micelles gradually transfer to network aggregates (50 CMC) and then to diverse sized vesicles (100 CMC) with an increase of the surfactant concentration. The salt-induced network aggregate-to-vesicles transitions were also found in these novel cationic gemini surfactants systems. It has also been observed from the thermodynamical studies that the micellization of gemini surfactants is entropy driven.

Acknowledgements

This study was supported by the Foundation of the Program for Heilongjiang Province of China (No. 12541864 & 12511591), the Science & Technology of Heilongjiang Province of China (No. B201116), the Overseas Scholars Foundation of Scientific and Technical Agency of Heilongjiang province of China (No. LC2011C17), the Overseas Scholars Foundation of Ministry of Human Resources and Social Security of the People's Republic of China (No. [2011] 508), and the National Science Foundation of China (No. 81403067).

References

1. F. M. Menger and J. S. Keiper, Angew. Chem., Int. Ed., 2000, 39, 1906.
2. R. Zana, Adv. Colloid Interface Sci., 2002, 97, 205.
3. S. S. Kim, W. Z. Zhang and T. J. Pinnavaia, Science, 1998, 282, 1302.
4. E. Alami, H. Levy and R. Zana, Langmuir, 1993, 9, 940.
5. Q. Liu, M. L. Guo, Z. Nie, J. B. Yuan, J. Tan and S. Z. Yao, Langmuir, 2008, 24, 1595.
6. M. S. Bakshi, S. Sachar, G. Kaur, P. Bhandari, G. Kaur, M. C. Biesinger, F. Possmayer and N. O. Petersen, Cryst. Growth Des., 2008, 8, 1713.
7. M. S. Bakshi, Langmuir, 2009, 25, 12697.
8. G. M. Andres, P. J. Jorge, C. A. Enrique and T. Gloria, Angew. Chem., Int. Ed., 2009, 48, 9484.
9. J. Aguado, J. M. Escola and M. C. Castro, Microporous Mesoporous Mater., 2010, 128, 48.
10. Q. R. Chen, L. Han, C. B. Gao and S. N. Che, Microporous Mesoporous Mater., 2010, 128, 203.
11. T. Lu, F. Han, G. G. Mao, G. F. Lin, J. B. Huang, X. Huang, Y. L. Wang and H. L. Fu, Langmuir, 2007, 23, 2932.
12. X. G. Liu, X. J. Xing and Z. N. Gao, Colloids Surf., A, 2014, 457, 374.
13. R. Zana, J. Colloid Interface Sci., 2002, 248, 203.
14. Q. Zhang, Z. N. Gao, F. Xu, S. X. Tai, X. G. Liu, S. B. Mo and F. Niu, Langmuir, 2012, 28, 11979.
15. T. Lu, Y. R. Lan, C. J. Liu, J. B. Huang and Y. L. Wang, J. Colloid Interface Sci., 2012, 377, 222.
16. T. Lu and J. B. Huang, Chin. Sci. Bull., 2007, 52, 2618.
17. X. Y. Wang, J. B. Wang, Y. L. Wang and H. K. Yan, Langmuir, 2004, 20, 53.
18. J. X. Zhao, Prog. Chem., 2014, 26, 1339.
19. T. Lu, F. Han, Z. C. Li and J. B. Huang, Langmuir, 2006, 22, 2045.
20. M. N. Wang, Y. X. Wang, D. F. Yu, Y. C. Han and Y. L. Wang, Colloid Polym. Sci., 2013, 291, 1613.
21. C. C. Zhou, X. H. Cheng, O. D. Zhao, S. Liu, C. J. Liu, J. D. Wang and J. B. Huang, Soft Matter, 2014, 10, 8023.
22. J. Lv, W. H. Qiao and C. Q. Xiong, Langmuir, 2014, 30, 8258.
23. S. S. Song, Q. S. Zheng, A. X. Song and J. C. Hao, Langmuir, 2012, 28, 219.
24. T. Patra, S. Ghosh and J. Dey, J. Colloid Interface Sci., 2014, 436, 138.
25. R. Zana, M. Benrraou and R. Rueff, Langmuir, 1991, 7, 1072.
26. D. Danino, Y. Talmon and R. Zana, Langmuir, 1995, 11, 1448.
27. A. B. Groswasser, R. Zana and Y. Talmon, J. Phys. Chem. B, 2000, 104, 4005.
28. H. G. Lee, J. H. Lee, S. P. Jang, H. M. Park, S. J. Kim, Y. Kim, C. Kim and R. G. Harrison, Tetrahedron, 2011, 67, 8073.
29. L. Mivehi, R. Bordes and K. Holmberg, Langmuir, 2011, 27, 7549.
30. A. Rodriguez, M. M. Graciani, M. Munoz, I. Robina and M. L. Moya, Langmuir, 2006, 22, 9519.
31. Y. Y. Zhang, X. F. Guo, L. H. Jia, L. B. Gao and P. Wu, Journal of Qiqihar University, 2014, 30, 1.
32. C. C. Ren, F. Wang, Z. Q. Zhang, H. H. Nie, N. Li and M. Cui, Colloids Surf., A, 2015, 467, 1.
33. L. M. Zhou, X. H. Jiang, Y. T. Li, Z. Chen and X. Q. Hu, Langmuir, 2007, 23, 11404.
34. L. Perez, A. Pinazo, M. J. Rosen and M. R. Infante, Langmuir, 1998, 14, 2307.
35. F. M. Menger and C. A. Littau, J. Am. Chem. Soc., 1993, 115, 10083.
36. J. Luczak, C. Jungnickel, M. Joskowska, J. Thoming and J. Hupka, J. Colloid Interface Sci., 2009, 336, 111.
37. M. Popov, C. Linder, R. J. Deckelbaum, S. Grinberg, I. H. Hansen, E. Shaubi, T. Waner and E. Heldman, J. Liposome Res., 2010, 20, 147.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13616d

---