A Comprehensive Study on the Synthesis and Micellization of Disymmetric Gemini Imidazolium Surfactants

Abstract

Two groups of disymmetric Gemini imidazolium surfactants, [C₁₄C₄C _m im]Br₂ (m = 10, 12, 14) and [C _m C₄C _n im]Br₂ (m + n = 24, m = 12, 14, 16, 18) surfactants, were synthesized and their structures were confirmed by ¹H NMR and ESI–MS spectroscopy. Their adsorption at the air/water interface, thermodynamic parameters and aggregation behavior were explored by means of surface tension, electrical conductivity and steady-state fluorescence. A series of surface activity parameters, including cmc, γ _cmc, π _cmc, pC ₂₀, cmc/C ₂₀, Γ _max and A _min, were obtained from surface tension measurements. The results revealed that the overall hydrophobic chain length (N _c) for [C₁₄C₄C _m im]Br₂ and the disymmetry (m/n) for [C _m C₄C _n im]Br₂ had a significant effect on the surface activity. The cmc values decreased with an increase of N _c or m/n. The thermodynamic parameters of micellization (ΔG ^θ_m , ΔH ^θ_m , ΔS ^θ_m ) derived from the electrical conductivity indicated that the micellization process of [C₁₄C₄C _m im]Br₂ and [C _m C₄C _n im]Br₂ was entropy-driven at different temperatures, but the contribution of ΔH ^θ_m to ΔG ^θ_m was enhanced by increasing N _c or m/n. The micropolarity and micellar aggregation number (N _agg) were estimated by steady-state fluorescence measurements. The results showed that the surfactant with higher N _c or m/n can form larger micelles, due to a tighter micellar structure.

Introduction

Gemini surfactants, “dimers” of single-tailed surfactants linked at the level of, or very close to, the head groups by a spacer group, have gained much attention in the past few decades [1–4]. Compared to the conventional single-tailed surfactants, Gemini surfactants exhibit overwhelming superiority, such as lower critical micelle concentration (cmc), higher adsorption efficiency, better wetting, foaming and lime-soap dispersing properties, and unusual rheological and aggregation properties [5–7], which makes them widely useful in skin care, petrochemistry, medicine, life science, and construction of porous materials [8–13]. Considerable effort has been exercised to design and develop a library of Gemini surfactants with diverse structure and functionality. The most investigated Gemini surfactants are the dicationic quaternary ammonium compounds with the general structure [C _m H_2m+1(CH₃)₂N(CH₂) _s  N (CH₃)₂C _n H_2n+1]Br₂, abbreviated as C _m C _s C _n Br₂ (m = n) [14, 15]. The effect of alkyl tail length and spacer group length on the surface properties and micellization process has been systematically studied. Heterocycles headgroup-based Gemini surfactants, such as pyrrolidinium, imidazolium and hexahydropyridine Gemini surfactants, have also attracted increasing interest, and different hydrophilic head groups have a significant effect on the surface adsorption and aggregation behavior of Gemini surfactants [16–21]. For example, Gemini imidazolium surfactants ([C _m C _s C _n im]Br₂, m = n), employing heterocyclic imidazole as the headgroup and methylene as the spacer group, demonstrate higher surface activity and lower cmc [22–24]. In addition, owing to the existence of imidazolium headgroup, as cationic micelle systems, they show a distinctly greater tendency to self-aggregation and thus are used as supramolecular templates in the preparation of functional materials [25]; as cationic reverse-micelle systems, they display greater solubilization ability compared with cationic Gemini ammonium surfactants [26]. They can form tighter membranes [27] and thus be applied widely in the field of biology due to the strong attraction between the imidazolium headgroup and aromatic rings through π–π interaction [28].

The disymmetric Gemini surfactants consist of two different hydrophobic chains or two different hydrophilic head groups, connected by a rigid or flexible spacer group. Owing to more controlled molecular structures, these surfactants have many peculiar properties, such as the enhanced contribution of enthalpy to the Gibbs free energy [29], higher efficiency of reducing surface tension and greater ability to the formation of micelles caused by the disymmetric Gemini surfactants with ionic–nonionic head groups [30], counterions-free systems constructed by the disymmetric Gemini surfactants containing cationic–anionic head groups [31], and various morphologies of aggregates formed by the dissymmetric Gemini surfactants making use of their different length alkyl chains [32, 33]. Currently, the synthesis and properties of the dissymmetric ammonium Gemini surfactants (C _m C _s C _n Br₂, m ≠ n) [34, 35] and the disymmetric pyrrolidinium Gemini surfactants (C _m C _s C _n PB, m ≠ n) have been researched in detail [36]. Their results indicate the disymmetric degree of the hydrophobic chains have a strong influence on the physicochemical properties of the surfactants. However, no works have focused on preparing and exploring the disymmetric Gemini imidazolium surfactants ([C _m C _s C _n im]Br₂, m ≠ n). The motivation of the present work is to further study the effect of disymmetric degree on the physicochemical properties of these surfactants and build up the structure–property relationships of them. Therefore, a series of disymmetric Gemini imidazolium surfactants with a four-methylene spacer group were synthesized and characterized by ¹H NMR and ESI–MS spectroscopy (Scheme 1). The investigated surfactants can be divided into two groups: the disymmetric [C₁₄C₄C _m im]Br₂ (m = 10, 12, 14) surfactants with 14 carbon atoms in one hydrocarbon chain, and the disymmetric [C _m C₄C _n im]Br₂ (m + n = 24, m = 12, 14, 16, 18) surfactants with the same total carbon number of 24 in the two hydrophobic chains. Their surface activity, aggregation number and thermodynamic properties of micellization were obtained by means of surface tension, steady-state fluorescence and electrical conductivity measurements. Therefore, the effect of dissymmetry of the hydrophobic chains on the surface activity and micellization process can be discussed comparatively and systematically.

Scheme 1

Synthesis route of the disymmetric Gemini imidazolium surfactants

Experimental Section

Materials

Imidazole, potassium hydroxide, dimethyl sulfoxide (DMSO), chloroform, anhydrous magnesium sulfate, ethyl acetate, acetone, hexane, methanol, and isopropanol were purchased from Chengdu Kelong Reagent. Alkyl bromide (C _n H_2n+1Br, n = 6, 8, 10, 12, 14, 16 and 18), 1, 4-dibromobutane, and pyrene (99 %) were purchased from Aladdin Reagent. Benzophenone (C.P.) was purchased from Sinopharm Reagent. Ultrapure water was utilized for preparing all the surfactant solutions.

Synthesis of N-Alkyl Imidazole

A mixture of imidazole (30 mmol, 2.04 g), potassium hydroxide (30 mmol, 1.68 g) and dimethyl sulfoxide (10 mL) was stirred for 2 h at room temperature. After that, alkyl bromide (25.0 mmol of 1-bromohexane, 1-bromooctane, 1-bromodecane, 1-bromododecane, 1-bromotetradecane, 1-bromohexadecane, or 1-bromooctadecane) was dropped in slowly and the mixture was stirred for an additional 4 h. Upon completion, water (30 mL) was added to the resulting mixture followed by extraction with chloroform (5 × 30 mL). The combined organic layer was dried over anhydrous magnesium sulfate and the filtrate was concentrated under reduced pressure. The residue was subjected to flash chromatography with ethyl acetate as eluent to give N-alkyl imidazole. The respective yields of N-hexyl imidazole, N-octyl imidazole, N-decyl imidazole, N-dodecyl imidazole, N-tetradecyl imidazole, N-hexadecyl imidazole and N-octadecyl imidazole are 84.6, 82.3, 81.2, 80.5, 80.4, 79.8 and 79.6 %. ¹H NMR (Bruker Avance III 500, 500 MHz, CDCl₃) N-hexyl imidazole: δ 7.42 (s, –NCHN–), 7.01 (s, –NCHCHN–), 6.87 (s, –NCHCHN–), 3.88 (t, –NCH ₂ –), 1.73 (m, –NCH₂ CH ₂ –), 1.25–1.27 (m, –NCH₂CH₂ (CH ₂ ) ₃ –), 0.85 (t, –CH₂ CH ₃ ). N-octyl imidazole: δ 7.44 (s, –NCHN–), 7.03 (s, –NCHCHN–), 6.88 (s, –NCHCHN–), 3.90 (t, –NCH ₂ –), 1.75 (m, –NCH₂ CH ₂ –), 1.24–1.27 (m, –NCH₂CH₂ (CH ₂ ) ₅ –), 0.85 (t, –CH₂ CH ₃ ); N-decyl imidazole: δ 7.41 (s, –NCHN–), 6.99 (s, –NCHCHN–), 6.86 (s, –NCHCHN–), 3.88 (t, –NCH ₂ –), 1.74 (m, –NCH₂ CH ₂ –), 1.21–1.25 (m, –NCH₂CH₂ (CH ₂ ) ₇ –), 0.85 (t, –CH₂ CH ₃ ); N-dodecyl imidazole: δ 7.44 (s, –NCHN–), 7.03 (s, –NCHCHN–), 6.88 (s, –NCHCHN–), 3.90 (t, –NCH ₂ –), 1.75 (m, –NCH₂ CH ₂ –), 1.23−1.27 (m, –NCH₂CH₂ (CH ₂ ) ₉ –), 0.88 (t, –CH₂ CH ₃ ); N-tetradecyl imidazole: δ 7.43 (s, –NCHN–), 7.01 (s, –NCHCHN–), 6.88 (s, –NCHCHN–), 3.89 (t, –NCH ₂ –), 1.75 (m, –NCH₂ CH ₂ –), 1.22–1.26 (m, –NCH₂CH₂ (CH ₂ ) ₁₁ –), 0.85 (t, –CH₂ CH ₃ ); N-hexadecyl imidazole: δ 7.45 (s, –NCHN–), 7.05 (s, –NCHCHN–), 6.90 (s, –NCHCHN–), 3.91 (t, –NCH ₂ –), 1.76 (m, –NCH₂ CH ₂ –), 1.25–1.32 (m, –NCH₂CH₂ (CH ₂ ) ₁₃ –), 0.87 (t, –CH₂ CH ₃ ); N-octadecyl imidazole: δ 7.44 (s, –NCHN–), 7.04 (s, –NCHCHN–), 6.89 (s, –NCHCHN–), 3.91 (t, –NCH ₂ –), 1.76 (m, –NCH₂ CH ₂ –), 1.21–1.30 (m, –NCH₂CH₂ (CH ₂ ) ₁₅ –), 0.87 (t, –CH₂ CH ₃ ).

Synthesis of 1-(4-Bromobutyl)-3-Alkylimidazolium Bromide

1,4-dibromobutane (36.0 mmol, 7.70 g) was dissolved in dry acetone (50 mL) and the obtained N-alkyl imidazole (9.0 mmol of N-hexyl imidazole, N-octyl imidazole, N-decyl imidazole, or N-dodecyl imidazole) was added gradually. The reaction mixture was refluxed at 50 °C under nitrogen atmosphere and the reaction was monitored by TLC analysis. After the reaction finished, the solvent was removed under reduced pressure, and subsequently the un-reacted 1, 4-dibromobutane was washed thoroughly with hexane. The collected viscous liquid was purified by silica column chromatography with mixtures of acetone/methanol (10:1, by vol) as eluent to afford 1-(4-bromobutyl)-3-alkylimidazolium bromide. The yields of 1-(4-bromobutyl)-3-hexylimidazolium bromide, 1-(4-bromobutyl)-3-octylimidazolium bromide, 1-(4-bromobutyl)-3-decylimidazolium bromide and 1-(4-bromobutyl)-3-dodecylimidazolium bromide are 72.5, 74.1, 76.8 and 80.3 %, respectively. ¹H NMR (500 MHz, CDCl₃) 1-(4-bromobutyl)-3-hexylimidazolium bromide: δ 10.32 (s, –NCHN–), 7.68 (s, –NCHCHN–), 7.45 (s, –NCHCHN–), 4.43 (t, –N⁺–CH ₂ –), 4.25 (t, –NCH ₂ –), 3.41 (t, –CH ₂ Br), 2.01–2.09 (m, –N⁺–CH₂ CH ₂ –), 1.79–1.94 (m, –NCH₂ (CH ₂ ) ₂ –), 1.21 (m, –N⁺–CH₂CH₂ (CH ₂ ) ₃ –), 0.79 (t, –CH₂ CH ₃ ); 1-(4-bromobutyl)-3-octylimidazolium bromide: δ 10.39 (s, –NCHN–), 7.63 (s, –NCHCHN–), 7.40 (s, –NCHCHN–), 4.47 (t, –N⁺–CH ₂ –), 4.28 (t, –NCH ₂ –), 3.45 (t, –CH ₂ Br), 2.06–2.14 (m, –N⁺–CH₂ CH ₂ –), 1.90 (m, –NCH₂ (CH ₂ ) ₂ –), 1.24 (m, –N⁺–CH₂CH₂ (CH ₂ ) ₅ ), 0.82 (t, –CH₂ CH ₃ ); 1-(4-bromobutyl)-3-decylimidazolium bromide: δ 10.31 (s, –NCHN–), 7.70 (s, –NCHCHN–), 7.46 (s, –NCHCHN–), 4.45 (t, –N⁺–CH ₂ –), 4.29 (t, –NCH ₂ –), 3.43 (t, –CH ₂ Br), 2.08 (m, –N⁺–CH₂ CH ₂ –), 1.92 (m, –NCH₂ (CH ₂ ) ₂ –), 1.26 (m, –N⁺–CH₂CH₂ (CH ₂ ) ₇ –), 0.85 (t, –CH₂ CH ₃ ); 1-(4-bromobutyl)-3-dodecylimidazolium bromide: δ 10.49 (s, –NCHN–), 7.62 (s, –NCHCHN–), 7.40 (s, –NCHCHN–), 4.48 (t, –N⁺–CH ₂ –), 4.28 (t, –NCH ₂ –), 3.45 (t, –CH ₂ Br), 2.11 (m, –N⁺–CH₂ CH ₂ –), 1.91 (m, –NCH₂ (CH ₂ ) ₂ –), 1.17–1.33 (m, –N⁺–CH₂CH₂ (CH ₂ ) ₉ –), 0.84 (t, –CH₂ CH ₃ ).

Synthesis of the Disymmetric Gemini Imidazolium Surfactants

An amount of 5.0 mmol of the obtained 1-(4-bromobutyl)-3-alkylimidazolium bromide (in which the alkyl were hexyl, octyl, decyl, or dodecyl, respectively) was dissolved in isopropanol (20 mL), followed by addition of 7.5 mmol of N-alkyl imidazole (in which the alkyl were octadecyl, hexadecyl, tetradecyl, or tetradecyl, correspondingly). The mixture was stirred at 60 °C for 48 h under nitrogen atmosphere. After removal of isopropanol, the residue was washed with ethyl acetate, purified three times by recrystallization in acetone and then dried under vacuum for 48 h, to yield the disymmetric Gemini imidazolium surfactants as white powders. The respective yields of [C₁₈C₄C₆im]Br₂, [C₁₆C₄C₈im]Br₂, [C₁₄ C₄C₁₀im]Br₂ and [C₁₄C₄C₁₂im]Br₂ are 85.5, 87.3, 88.2 and 89.0 %. ¹H NMR (500 MHz, CDCl₃) Take [C₁₄C₄C₁₀im]Br₂ as an example: δ 10.27 (s, 2H, –NCHN–), 7.96 (s, 2H, –NCHCHN–), 7.15 (s, 2H, –NCHCHN–), 4.58 (t, 4H, –N⁺–CH ₂ –), 4.24 (t, 4H, –NCH ₂ –), 2.22 (m, 4H, –N⁺–CH₂ CH ₂ –), 1.90 (m, 4H, –NCH₂ CH ₂ –), 1.19–1.37 (m, 36H, –N⁺–CH₂CH₂ CH ₂ –), 0.87 (t, 6H, –CH₂ CH ₃ ); MS (Finnigan TSQ Quantum ultra AM, ESI) [M–2Br]^2+: 264.32. [C₁₄C₄C₁₂im]Br₂: δ 10.31 (s, 2H, –NCHN–), 7.98 (s, 2H, –NCHCHN–), 7.15 (s, 2H, –NCHCHN–), 4.58 (t, 4H, –N⁺–CH ₂ –), 4.24 (t, 4H, –NCH ₂ –), 2.23 (m, 4H, –N⁺–CH₂ CH ₂ –), 1.89 (m, 4H, –NCH₂ CH ₂ –), 1.21–1.36 (m, 40H, –N⁺–CH₂CH₂ CH ₂ –), 0.87 (t, 6H, –CH₂ CH ₃ ); MS (ESI) [M-2Br]^2+: 278.30.

Synthesis of the Symmetric Gemini Imidazolium Surfactants ([C _m C₄C _m im]Br₂, m = 12, 14)

A mixture of 10 mmol of N-dodecyl imidazole or N-tetradecyl imidazole, 1,4-dibromobutane (4 mmol, 0.856 g) and isopropanol (20 mL) was stirred for 48 h at 60 °C under nitrogen atmosphere. After removal of isopropanol, the residue was washed with ethyl acetate, further purified four times by recrystallization in acetone and then dried under vacuum for 48 h. [C₁₂C₄C₁₂im]Br₂ and [C₁₄ C₄C₁₄im]Br₂ as the white powder were gained. The respective yields of [C₁₂C₄C₁₂im]Br₂ and [C₁₄ C₄C₁₄im]Br₂ are 92.4 % and 93.3 %. ¹H NMR (500 MHz, CDCl₃) [C₁₂C₄C₁₂im]Br₂: δ 10.19 (s, 2H, –NCHN–), 8.05 (s, 2H, –NCHCHN–), 7.21 (s, 2H, –NCHCHN–), 4.58 (t, 4H, –N⁺–CH ₂ –), 4.24 (t, 4H, –NCH ₂ –), 2.19 (m, 4H, –N⁺–CH₂ CH ₂ –), 1.88 (m, 4H, –NCH₂ CH ₂ –), 1.20–1.35 (m, 36H, –N⁺–CH₂CH₂(CH ₂ ) ₉ –), 0.86 (t, 6H, –CH₂ CH ₃ ); MS (ESI) [M-2Br]^2+: 264.27. [C₁₄C₄C₁₄im]Br₂ δ 10.30 (s, 2H, –NCHN–), 7.99 (s, 2H, –NCHCHN–), 7.16 (s, 2H, –NCHCHN–), 4.58 (t, 4H, –N⁺–CH ₂ –), 4.24 (t, 4H, –NCH ₂ –), 2.22 (m, 4H, –N⁺–CH₂ CH ₂ –), 1.89 (m, 4H, –NCH₂ CH ₂ –), 1.20–1.37 (m, 44H, –N⁺–CH₂CH₂(CH ₂ ) ₁₁ –), 0.87 (t, 6H, –CH₂ CH ₃ ); MS (ESI) [M-2Br]^2+: 292.32.

The Krafft temperatures of [C _m C₄C _n im]Br₂ (m + n = 24, m = 12, 14, 16, 18) were all lower than 15 °C. For [C₁₄C₄C₁₂im]Br₂ and [C₁₄C₄C₁₄im]Br₂, their solubility was not better than those of [C _m C₄C _n im]Br₂ (m + n = 24, m = 12, 14, 16, 18), but when their concentration was not higher than 2 mmol/L, they can still remain as a clear aqueous solution at 15 °C. Therefore, in our experimental range of the prepared surfactant concentration, all the experiments can be performed at or above 15 °C.

Equilibrium Surface Tension Measurements

The equilibrium surface tension was estimated by using the pendant drop method, performed on an optical contact angle measuring instrument (OCA 40; Beijing Eastern Dataphy Instruments). All measurements were taken at 25.0 ± 0.1 °C and the surfactant solutions were kept still for 30 min in order to achieve the system equilibrium.

Electrical Conductivity Measurements

The electrical conductivity of surfactant solutions at five different temperatures was obtained by a conductivity analyzer (DDS-11A; Shanghai Leici TRONY Instrument). The measurements were repeated three times for each temperature and each temperature was controlled with a precision of ±0.1 °C.

Steady-State Fluorescence Measurements

Fluorescence spectra were recorded on a FluoroMax-4 spectrofluorometer with a 1-cm² quartz cuvette. Pyrene was used as the fluorescence probe and benzophenone as the quencher. The concentration of pyrene was kept at 1.0 × 10⁻⁶ mol L⁻¹. The excitation wavelength was fixed at 335 nm and the emission spectra range was selected from 350 to 500 nm. The slit widths of excitation and emission were set at 8 and 2 nm, respectively.

Results and Discussion

Adsorption at Air/Water Interface

The adsorption at air/water interface for Gemini imidazolium surfactants, including [C₁₂C₄C₁₂im]Br₂, [C₁₄C₄C₁₀im]Br₂, [C₁₆C₄C₈im]Br₂, [C₁₈C₄C₆im]Br₂, [C₁₄C₄C₁₂im]Br₂ and [C₁₄C₄C₁₄im]Br₂, was evaluated by the equilibrium surface tension as a function of surfactant concentration (Fig. 1). The surface tension gradually decreased with the increase of surfactant concentration and then reached a plateau. The concentration of the clear breakpoint in the γ-log c curves corresponded to the critical micelle concentration (cmc), indicating the formation of micelles, and the surface tension at the cmc was defined as γ _cmc. In addition, the absence of a minimum near the breakpoint suggested a low concentraction of surface chemical impurities for the investigated Gemini surfactants [20]. In this work, the classic Gibbs adsorption theory was used to study the surface properties of the disymmetric Gemini imidazolium surfactants.

Fig. 1

Plots of surface tension vs. log c at 25 °C for the disymmetric Gemini imidazolium surfactants in aqueous solution. The error of surface tension value is ±0.2 mN/m

The maximum surface excess concentration, Γ _max, and the minimum average area occupied by a single surfactant molecule at the air/water interface, A _min, can be estimated according to the Gibbs adsorption equation [37]:

$$ \varGamma_{ \hbox{max} } = - \frac{1}{2.303nRT} \times \frac{d\gamma }{{d{ \log }c}} $$

(1)

$$ A_{ \hbox{min} } = \frac{{10^{24} }}{{N_{\text{A}} \varGamma_{ \hbox{max} } }} $$

(2)

where Γ _max is in μmol/m², R is the gas constant (8.314 J mol⁻¹ K⁻¹), T is the absolute temperature in Kelvin, γ is the surface tension in mN m⁻¹, c is the surfactant concentration and (d γ/d logc) is the slope of the linear part in the γ-log c curves where the surfactant concentration is below the cmc. In aqueous solutions of the investigated Gemini imidazolium surfactants, n is taken as 3, considering one dimeric ion and two counterions in the surfactant molecule [38]. N _A is Avogadro’s number and A _min is in nm²/molecule.

The adsorption efficiency at the air–water interface, pC ₂₀, and the surface pressure at cmc, π _cmc, are obtained from Eqs. (3, 4):

$$ {\text{p}}C_{20} = - { \log }C_{20} $$

(3)

$$ \pi_{\text{cmc}} = \gamma_{0} - \gamma_{\text{cmc}} $$

(4)

where C ₂₀ represents the surfactant concentration at which the surface tension of water is reduced by 20 mN·m⁻¹. Generally, the larger the value of pC ₂₀, the higher the adsorption efficiency of the surfactant.

γ ₀ is the surface tension of water and γ _cmc is the surface tension of surfactant solution at the cmc. The larger the value of π _cmc, the higher the effectiveness of the surfactant to lower the surface tension of water. The value of cmc/C ₂₀ is related with the structural factors in the adsorption and micellization process. The surfactant with larger cmc/C ₂₀ value a tends more to adsorb at the interface than to form micelles. Based on the γ-log c curves, the above-described physicochemical parameters for [C₁₄C₄C _m im]Br₂ and [C _m C₄C _n im]Br₂, such as cmc, γ _cmc, π _cmc, pC ₂₀, cmc/C ₂₀, Γ _max and A _min, are listed in Table 1.

Table 1 Surface activity of the disymmetric Gemini imidazolium surfactants at 25 °C

From Fig. 2a, the values of cmc for the disymmetric [C₁₄C₄C _m im]Br₂ series decreased with the increasing total carbon number in two hydrocarbon chains regardless of the degree of disymmetry, implying that the longer overall hydrophobic chain length (N _c), the higher the aggregation abilityas. The contribution of each additional methylene unit to the cmc values of [C₁₄C₄C _m im]Br₂ was higher than that of classic single-tailed surfactant homologues [C _n mim]Br [39, 40]. It can be noticed that the γ _cmc values of [C₁₄C₄C _m im]Br₂ decreased with the increasing N _c, indicating that [C₁₄C₄C₁₄im]Br₂ possessed higher surface activity, which could also be found from the changes of π _cmc and pC ₂₀ values. For the disymmetric [C₁₄C₄C _m im]Br₂ series, the changes of cmc, γ _cmc, π _cmc and pC ₂₀ values with N _c were similar to those of the disymmetric Gemini pyrrolidinium surfactants (C _m C₃C₁₄PB, m = 10, 12, 14) [36]. The cmc/C ₂₀ values increased with the increase of N _c, which suggested that, in comparison with micellization process, the adsorption at the air/water interface for [C₁₄C₄C _m im]Br₂ with longer hydrophobic chains was much easier. However, the A _min values of [C₁₄C₄C _m im]Br₂ decreased with the increasing N _c, meaning that [C₁₄C₄C₁₄im]Br₂ had a higher packing density. The hydrophobic interactions were strengthened with the increasing N _c, which could promote the surfactant molecules to pack much more densely, and consequently the A _min values will decrease. This result was different from C _m C₃C₁₄PB, and the possible reason was that the longer hydrophobic chains were more prone to bending.

Fig. 2

Effect of the overall hydrophobic chain length (a) and the disymmetry (b) on cmc

The typical effect of the disymmetry on the cmc can be found from the disymmetric [C _m C₄C _n im]Br₂ series (Fig. 2b), those surfactants with the same total carbon number of 24 in two hydrocarbon chains. Higher structural disymmetry resulted in a lower cmc by employing m/n as the degree of disymmetry and a similar conclusion was also observed in other disymmetric Gemini surfactants [29, 34, 36]. Clearly, the introduction of disymmetry to the hydrophobic chains can strengthen the aggregation behavior, and adding two methylene groups to one longer chain was more efficient in lowering the cmc than adding each of them to two chains separately. The hydrophobic interactions had two contributions to the aggregation behavior, intermolecular and intramolecular. The latter will be weaker because the spacer groups kept the two hydrophobic chains apart. For the symmetric Gemini surfactants, the hydrophobic interactions will be minimized due to the equal number of hydrophobic units for both intramolecular and intermolecular interactions. However, for the disymmetric Gemini surfactants, as m/n increased, the ratio of hydrophobic units interacting intermolecularly to those intramolecularly will increase [41]. Therefore, the overall hydrophobic interactions were gradually improved with the increasing m/n, which made a positive contribution to the formation of micelles. On the other hand, both the γ _cmc and A _min values increased with the increase of m/n, implying that the Gemini surfactants with a higher disymmetry were packed more loosely at the air/water interface. This might be attributed to the fact that the longer hydrocarbon chain for [C₁₈C₄C₆im]Br₂ with the highest disymmetry was more prone to bend at the interface, which made the surface area per molecule larger, and therefore resulted in the larger γ _cmc value. The γ _cmc and A _min values showed the opposite relation with the disymmetry in comparison with C _m C₃C _n PB (m + n = 24, m = 12, 14, 16). Furthermore, the increase in cmc/C ₂₀ indicated that the adsorption at the air/water interface was more favored over the micellization process for those surfactants with the higher disymmetry. The same conclusion could also be speculated from the variation in pC ₂₀ values.

Degree of Counterion Binding to Micelles

The degree of counterion binding to micelles (β), a fundamental feature of the ionic surfactant micelles, is certain to have an important role in the micellization process and various aggregation behaviors in aqueous solutions. An understanding of β can provide insight into the specific properties of ionic surfactants and a further investigation of new ionic Gemini surfactants. The electrical conductivity measurements are often used to determine the cmc and β values. For each of [C₁₄C₄C _m im]Br₂ or [C _m C₄C _n im]Br₂ series, Fig. 3 gives the plots of the conductivity (κ) versus surfactant concentration (c) at five different temperatures. It was evident that all plots had breakpoints, and that the concentration corresponded to the cmc. This was a result of the reduction of effective charges in the solution, due to the binding of some counterions to the micelles above the cmc. Moreover, the degree of counterion dissociation (α) can be taken as the ratio of the slopes of two fitted lines above and below the cmc. Thus, the β values could be obtained by using β = 1 − α. The cmc and β values of [C₁₄C₄C _m im]Br₂ and [C _m C₄C _n im]Br₂ series at five different temperatures are listed in Table 2. It can be seen that the cmc values for all the studied surfactants increased upon raising the temperature, and that the cmc values estimated from the electrical conductivity at 25 °C were in accordance with those obtained from the surface tension. A reasonable explanation was that the ordered water molecules around the hydrophobic chains may be broken at a higher temperature, which would be unfavorable for the formation of micelles [42]. All the values of β showed a monotonous decrease with the increasing temperature, which indicated the decrease of charge density on the micelle surface. The same trends of cmc and β values with temperature have also been observed for the disymmetric Gemini pyrrolidinium surfactants C _m C₃C₁₄PB and C _m C₃C _n PB.

Fig. 3

Plots of electrical conductivity vs. surfactant concentration at five different temperatures. The error of electrical conductivity value is ±0.2 µS/cm. [C₁₂C₄C₁₂im]Br₂ (a), [C₁₄C₄C₁₀im]Br₂ (b), [C₁₆C₄C₈im]Br₂ (c), [C₁₈C₄C₆im]Br₂ (d), [C₁₄C₄C₁₂im]Br₂ (e), [C₁₄C₄C₁₄im]Br₂ (f)

Table 2 Thermodynamic parameters of micellization for the disymmetric Gemini imidazolium surfactants at different temperatures

Thermodynamics Parameters

The thermodynamic parameters for the micellization process of [C₁₄C₄C _m im]Br₂ and [C _m C₄C _n im]Br₂ series were investigated by employing the mass action model [43]. The standard Gibbs free energy change (ΔG ^θ_m ), the standard enthalpy change (ΔH ^θ_m ), and standard entropy change (ΔS ^θ_m ) for the micellization process can be determined from Eqs. (5–7):

$$ \Delta G_{m}^{\theta } = RT\left( {1 + \beta } \right)\ln X_{cmc} $$

(5)

$$ \Delta H_{m}^{\theta } = \left[ {\frac{{\partial (\Delta G_{m}^{\theta } /T)}}{\partial (1/T)}} \right] \Delta H_{m}^{\theta } = - RT^{2} \left( {1 + \beta } \right)\frac{{d\ln \left( {X_{cmc} } \right)}}{dT} $$

(6)

$$ \Delta S_{m}^{\theta } = \frac{{\Delta H_{m}^{\theta } - \Delta G_{m}^{\theta } }}{T} $$

(7)

where R is the gas constant (8.314 J mol⁻¹ K⁻¹), T is the absolute temperature in K, β is the degree of counterion binding to micelles which was obtained previously, and X _cmc is the cmc expressed in mol fractions.

The corresponding thermodynamic parameters of micellization for all the investigated surfactants at five different temperatures are shown in Table 2. The values of ΔG ^θ_m are all negative, meaning that the aggregation for each surfactant in aqueous solution is spontaneous. Notably, both the overall hydrophobic chain length and the disymmetry degree greatly affected the thermodynamic parameters, or, in other words, the micellization process. The observed more negative ΔG ^θ_m values for [C₁₄C₄C _m im]Br₂ series with longer overall hydrophobic chains indicated a stronger tendency to micellization due to greater hydrophobic interactions. Although the changes of ΔG ^θ_m values for [C _m C₄C _n im]Br₂ series with an increase of the disymmetry were not so large, it demonstrated that the more negative ΔG ^θ_m of [C₁₈C₄C₆im]Br₂ with a higher disymmetry was more beneficial to micellization than that of [C₁₂C₄C₁₂im]Br₂. Simultaneously, the values of the standard enthalpy (ΔH ^θ_m ) for each surfactant were also negative, indicating that the formation process of micelles was exothermic, which can be attributed to possible surfactant–solvent interactions [17]. Additionally, all the ΔH ^θ_m values changed little as the temperature increased, suggesting no significant changes in the environment around the hydrophobic chains of surfactant molecule. The data from Table 2 indicated the ΔS ^θ_m values were positive in all cases and the values of TΔS ^θ_m were much larger than that of |ΔH ^θ_m |. The negative values of ΔG ^θ_m were mainly due to the larger positive ΔS ^θ_m , implying that the micellization for [C₁₄C₄C _m im]Br₂ or [C _m C₄C _n im]Br₂ series was entropy-driven. It can be concluded that the driving force of micellization process was the tendency of hydrophobic chains to transfer from the solvent to the interior of micelle. In general, there are mainly four contributions to ΔH ^θ_m for the Gemini surfactants: the van der Waals interaction between the alkyl chains, the hydrophobic interaction, the head group repulsion and the configuration of spacer chains [44]. Among these interactions, the van der Waals interaction and the hydrophobic interaction will lead to the changes in the ΔH ^θ_m values for the present investigated surfactants. As was previously discussed in our work, the hydrophobic interactions increased significantly with increasing N _c and m/n, which made the ΔH ^θ_m values more negative. Consequently, the overall hydrocarbon chain length and the degree of disymmetry had a significant effect on the contribution of ΔH ^θ_m to ΔG ^θ_m . For the [C₁₄C₄C _m im]Br₂ series, the contribution of ΔH ^θ_m to ΔG ^θ_m increased when the overall hydrocarbon chain length was increased, which was similar to those of the symmetric [C _m C₄C _m im]Br₂ and the disymmetric C _m C₃C₁₄PB. The |ΔH ^θ_m | values for [C₁₄C₄C₁₄im]Br₂ were nearly equal to the values of TΔS ^θ_m with the increase in the temperature. On the other hand, for the [C _m C₄C _n im]Br₂ series, a higher degree of disymmetry can result in a higher contribution of ΔH ^θ_m to ΔG ^θ_m , being consistent with those of the disymmetric C _m C₆C _n Br₂ [34] and the disymmetric C _m C₃C _n PB. From the data in Table 2, the changes of ΔH ^θ_m values at 25 °C were from −5.370 to −13.36 kJ/mol as the disymmetry increased. The introduction of disymmetry to the hydrophobic chain shed new light on the possibility of adjusting micellization process further.

Micropolarity and Micellar Aggregation Number

It is well known that pyrene is used as a fluorescence probe to investigate the cmc and polarity of the micellar microenvironment. Normally, the fluorescence emission spectra of pyrene have five vibration bands after the excitation at 335 nm. The ratio of fluorescence intensity (I ₁ /I ₃ ) between the first vibronic band (373 nm) and the third vibronic band (384 nm) is strongly dependent on the position of the pyrene in micelles and thus can be seen as a measure for the polarity of the micellar microenvironment [45]. Once the surfactant molecules begin to form micelles, the extreme hydrophobic pyrene molecules will transfer from the water environment into the interior hydrophobic region, which was reflected as a sharp drop in the I ₁ /I ₃ value. The concentration corresponding to the sharp drop was taken as the cmc. Figure 4 gives the relationship between the I ₁ /I ₃ ratio and the concentration of all the investigated surfactants. For the [C₁₄C₄C _m im]Br₂ or [C _m C₄C _n im]Br₂ series, the values of cmc determined from the fluorescence plots shown in Table 1 were consistent with those estimated by the methods of electrical conductivity and surface tension. The values of I ₁ /I ₃ were independent of the surfactant concentration above cmc and almost the same for the [C₁₄C₄C _m im]Br₂ or [C _m C₄C _n im]Br₂ series, suggesting that the hydrophobic chains length and the disymmetry have little effect on the micropolarity that was sensed by the pyrene. Moreover, the low I ₁ /I ₃ values indicated that the pyrene was solubilized in the palisade layer near the polar head groups [22]. On the one hand, as the N _c or m/n ratio increased, the hydrophobic interaction was gradually optimized. This was expected to result in a decrease of micropolarity, owing to the tighter packing of the surfactant alkyl chains. On the other hand, the tighter packing structure may make the pyrene slightly closer to the micellar surface and thus cause an increase of micropolarity, which compensated the first effect just discussed. As a result, the micropolarity of these investigated surfactants micelles that was sensed by the pyrene was observed to be almost the same.

Fig. 4

I ₁ /I ₃ ratio of pyrene as a function of the concentration for the disymmetric Gemini imidazolium surfactants in aqueous solution

The micellar aggregation number, N _agg, was measured by fluorescence quenching experiments [46] and calculated by Eq. (8):

$$ \ln \left( {\frac{{I_{0} }}{I}} \right) = \frac{{ N_{\text{agg}} C_{\text{Q}} }}{{C_{\text{S}} - {\text{cmc}}}} $$

(8)

where I ₀ and I are the fluorescence intensities of pyrene in the absence and presence of the quencher at a specific wavelength of 373 nm, and C _Q and C _S are the molar concentration of quencher and surfactant, respectively.

Figure 5 shows the variation of ln (I ₀/I) as a function of C _Q for [C₁₄C₄C _m im]Br₂ and [C _m C₄C _n im]Br₂ series. The concentrations were all 2.0 mmol L⁻¹ for the [C _m C₄C _n im]Br₂ series, and 1 mmol L⁻¹ for the [C₁₄C₄C _m im]Br₂ (m = 12, 14) series. Each plot was fitted into a straight line and the slope was obtained. The N _agg values of [C₁₄C₄C₁₀im]Br₂, [C₁₄C₄C₁₂im]Br₂ and [C₁₄C₄C₁₄im]Br₂ were found to be 18, 20 and 22, respectively, implying that N _agg increased with increasing N _c for the disymmetric [C₁₄C₄C _m im]Br₂ series. For the disymmetric [C _m C₄C _n im]Br₂ series, N _agg were 16, 18, 21 and 27 for m = 12, 14, 16 and 18, respectively, as listed in Table 1. Obviously, the N _agg values were dependent on m/n, increasing from 16 to 27 as m/n increased from 1 to 3. The possible explanation was that the surfactant with the higher N _c or m/n can form a more compact micellar structure, because of the enhancement in the hydrophobic interaction. Such a trend of increasing N _agg values with increasing m/n has been found in the disymmetric C _m C₆C _n Br₂. However, the opposite relationship of decreasing N _agg values with increasing N _c or m/n was observed in the disymmetric C _m C₃C₁₄PB or C _m C₃C _n PB, and this might result from the formation of premicelles. The more premicelles that could be formed, the lower the N _agg.

Fig. 5

ln (I ₀ /I) of pyrene as a function of concentration of the quencher benzophenone in the surfactant aqueous solution