Introduction

Ionic liquids (ILs) have unique and useful physico-chemical properties, e.g., negligible vapor pressure and high chemical and thermal stability; they are efficient solvents for inorganic and organic substances, and biopolymers [1, 2]. Focusing on imidazole-based ILs, the attachment of at least one long chain, e.g., C10 to C16, to the imidazolium ring results in compounds that are surface active and, hence, are termed ionic-liquid-based surfactants, ILBSs [3–25]. Owing to their structural flexibility, ILs were employed in diverse fields, including catalysis [26, 27], nanotechnology [28–30], biomedical applications [31, 32], organic synthesis [2, 26], ion-gel formation [33, 34], drug delivery, extraction, and biotechnological processes [35–40]. The aggregation behavior of ILBSs with different head groups, including amino acid cations, imidazolium, pyridinium, piperidinium, and pyrrolidinium heterocycle, was investigated [1–25, 41]. Tailor-made ILBSs were synthesized by incorporating specific functional groups in their structures, and their aggregation behaviors were compared with “conventional” surfactants [42–45].

In principle, it is possible to alter the balance between hydrophobic/hydrophilic and, where applicable, electrostatic interactions by judicious selection of the head group, the length of the hydrophobic tail, and by incorporating a functional group in the surfactant structure [43–50]. For example, Incorporation of a double bond (db) in the long alkyl chain makes the hydrophobic tail shorter, and less hydrophobic than that of the saturated counterpart. The reason is that db is shorter, and less hydrophobic than the single (C–C) bond [51, 52]. Additionally, packing of the db-carrying chains in the micellar core is hindered because of the rigidity of the unsaturated moiety [53]. Several authors have reported increase in the value of critical micelle concentration (cmc) upon introducing db, including at terminus of the hydrophobic tail, or close to the head group of ionic surfactants [54–61].

We carried out the present study as a part of our interest in assessing the effects of the presence of db on the aggregation of ILBSs; see Fig. ESM-1 (Fig. 1 of Electronic Supplementary Material). Herein, we report on the synthesis and aggregation behavior in aqueous solution at 298.15 K of the series 1-(n-alkyl)-3-vinylimidazolium bromide, CnVnImBr (n = 10, 12, 14, and 16). We studied the adsorption of these surfactants at aqueous solution/air interface and their micellization by surface tension measurements. Conductivity measurements were used to determine the values of their critical micelle concentration (cmc), degree of counter-ion binding (β), and the thermodynamic parameters of micellization (from measurements in the range 298.15 to 313.15 K). Additional information on the formed micelles, e.g., micellar aggregation number (N agg), was obtained from fluorescence quenching of micelle-solubilized pyrene. The effect of the presence of db in the head group on micellar properties was assessed by comparing the aggregation of C16VnImBr with the saturated counterpart (C16EtImBr; Et = ethyl). Where appropriate, we compare the results of the present series with those of other ILBSs, 1-(n-alkyl)-3-methylimidazolium bromides, CnMeImBr, and “conventional” cationic surfactant N-(n-alkyl)-N,N,N-trimethylammonium bromides, CnMe3ABr.

Fig. 1
figure 1

a Γ max and b A min of various surfactant series as a function of number of the number of carbon atom in the hydrophobic chain. The symbols employed are as follows: black square, CnVnImBr; red circle, CnMeImBr [8, 17]; and pink inverted triangle, CnMe3ABr [73]

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Materials and methods

Materials

1-Vinylimidazole (99 %), 1-ethylimidazole (99 %), 1-bromodecane (98 %), 1-bromododecane, (97 %), 1-bromotetradecane (98 %), and 1-bromohexadecane (97 %) were purchased from Sigma-Aldrich. Pyrene (Merck, 96 %), methyl orange (Acros, 95 %), ethyl acetate (Rankem, 99 %), and diethyl ether (Rankem, 99 %) were used as received. All aqueous solutions were prepared using deionized water.

Synthesis of ILs CnVnImBr and C16EtImBr

We synthesized the CnVnImBr surfactants as reported elsewhere [62–65] with several modifications. In brief, we slowly added (3 h), under stirring, a solution of 1-bromoalkane in ethyl acetate (23 mmol in 80 mL) to a solution of 1-vinylimidazole in the same solvent (20 mmol in 60 mL). The reaction temperature was maintained 0 °C throughout the addition. After completion, the reactant mixture was stirred for further 48 h at 50 °C. The progress of the reaction was monitored by TLC (chloroform/methanol, 4:1 by volume). After the completion of the reaction, ethyl acetate was removed and the resulting solid was washed several times with diethyl ether and then dried under reduced pressure at 50 °C until constant weight.

The synthesized ILBSs were characterized by proton nuclear magnetic resonance (1H NMR) spectroscopy (Bruker Avance-II 400 spectrometer, CDCl3), Fourier transform infrared (FTIR) spectroscopy (PerkinElmer Spectrum RX-IFTIR spectrometer; KBr pellet), and elemental analysis (Thermo Electron-Flash EA 1112- CHNS analyzer).

C10VnImBr

Light yellow liquid; yield = 82 %. 1H-NMR (δ in ppm): 0.87 (3H, t, N-(CH2)9-CH3), 1.20–1.34 (14H, m, N-CH2-CH2-(CH2)7-CH3), 1.92–1.98 (2H, m, N-CH2-CH2-(CH2)7-CH3), 4.43 (2H, t, N-CH2-CH2-(CH2)7-CH3), 5.37–5.40 (1H, dd, CH=CH2), 6.08–6.12 (1H, dd, CH=CH2), 7.49–7.55 (1H, dd, CH=CH2), 7.80 (1H, d, N-CH-CH), 8.18 (1H, d, N-CH-CH), and 10.60 (1H, s, N-CH-N). FTIR, νmax/cm−1: ν-CH2 2928, 2838, ν-CH=CH2 1650, imidazole ring 1551. Elemental analysis: Analyzed: C 57.08, H 8.55, N 8.75; calculated: C 57.14, H 8.63, N 8.88.

C12VnImBr

White solid, m.p. = 47 °C (literature m.p. = 47 °C) [66]; yield = 87 %. 1H-NMR: 0.87 (3H, t, N-(CH2)11-CH3), 1.24–1.34 (18H, m, N-CH2-CH2-(CH2)9 CH3), 1.94–1.97 (2H, m, N-CH2-CH2-(CH2)9-CH3), 4.42 (2H, t, N-CH2-CH2-(CH2)9-CH3), 5.37–5.50 (1H, dd, CH=CH2), 6.07–6.11 (1H, dd, CH=CH2), 7.49–7.55 (1H, dd, CH=CH2), 7.79 (1H, d, N-CH-CH), 8.17 (1H, d, N-CH-CH), and 10.56 (1H, s, N-CH-N). FTIR, νmax/cm−1: ν-CH2 2917, 2850, ν-CH=CH2 1650, imidazole ring 1552. Elemental analysis: Analyzed: C 59.33, H 9.03, N 8.10; calculated: C 59.47, H 9.10, N 8.16.

C14VnImBr

White solid, m.p. = 62 °C; yield: 84 %. 1H-NMR: 0.87 (3H, t, N-(CH2)13-CH3), 1.24-1.34 (22H, m, N-CH2-CH2-(CH2)11-CH3), 1.92–1.99 (2H, m, N-CH2-CH2-(CH2)11-CH3), 4.20 (2H, t, N-CH2-CH2-(CH2)11-CH3), 5.37–5.40 (1H, dd, CH=CH2), 6.05–6.10 (1H, dd, CH=CH2), 7.49–7.52 (1H, dd, CH=CH2), 7.75 (1H, d, N-CH-CH), 8.13 (1H, d, N-CH-CH), and 10.62 (1H, s, N-CH-N). FTIR, νmax/cm−1: ν-CH2 2917, 2849, ν-CH=CH2 1650, imidazole ring 1551. Elemental analysis: Analyzed: C 61.23, H 9.42, N 7.37; calculated: C 61.44, H 9.50, N 7.54.

C16VnImBr

White solid, m.p. = 69 °C; yield: 81 %. (literature mp = 69 °C [67]) 1H-NMR: 0.87 (3H, t, N-(CH2)15-CH3), 1.24–1.34 (26H, m, N-CH2-CH2-(CH2)13 CH3), 1.91–1.99 (2H, m, N-CH2-CH2-(CH2)13-CH3), 4.41 (2H, t, N-CH2-CH2-(CH2)13-CH3), 5.37–5.40 (1H, dd, CH=CH2), 6.02–6.07 (1H, dd, CH=CH2), 7.35–7.54 (1H, dd, CH=CH2), 7.68 (1H, d, N-CH-CH), 8.05 (1H, d, N-CH-CH), and 10.69 (1H, s, N-CH-N). FTIR, νmax/cm−1: ν-CH2 2917, 2849, ν-CH=CH2 1650, imidazole ring 1551. Elemental analysis: Analyzed: C 63.06, H 9.71, N 6.93; calculated: 63.14, H 9.84, N 7.01.

1-(n-Hexadecyl)-3-ethylimidazolium bromide C16EtImBr was synthesized according to the procedure reported earlier [63]. Briefly, a mixture of 1-ethylimidazole (10.4 mmol) and 1-bromohexadecane (10.4 mmol) in 120 ml of 2-propanol was maintained under reflux with constant stirring for 24 h. The solvent was removed, and the solid product was dissolved in water and extracted five times by ethyl acetate. Finally, water was evaporated under reduced pressure at 80 °C, and the product was dried in a vacuum oven for 48 h.

C16EtImBr

White solid, m.p. = 56 °C; yield: 81 %. (literature mp = 55.7 °C [63]) (white solid) 1H-NMR: 0.85 (3H, t, N-(CH2)15-CH3), 1.22–1.29 (26H, m, N-CH2-CH2-(CH2)13 CH3), 1.56 (3H, t, CH2-CH3), 1.88 (2H, m,CH3-CH2), 1.88–1.92 (2H, m, N-CH2-CH2-(CH2)13-CH3), 4.24 (2H, t, N-CH2-CH2-(CH2)13-CH3), 5.22–5.30 (1H, dd, CH=CH2), 5.98–6.02 (1H, dd, CH=CH2), 7.35–7.54 (1H, dd, CH=CH2), 7.60 (1H, d, N-CH-CH), 7.95 (1H, d, N-CH-CH), and 10.35 (1H, s, N-CH-N). FTIR, νmax/cm−1: ν-CH2 2917, 2849, ν-CH 1475, imidazole ring 1551. Elemental analysis Analyzed: C, 60.40; H, 10.60; N, 6.68. Calculated: C, 60.13; H, 10.33; N, 6.68.

Notes

Except for conductance, we carried out all measurements at 298.15 ± 0.1 K. In all cases, ILBSs were dried under reduced pressure until constant weight; deionized water was employed throughout.

Measurements

Surface tension measurements

Surface tension measurements were performed using du Noüy ring method and Krüss K9 tensiometer. The concentrations of ILBSs were increased by successive addition of their concentrated solutions. The measured surface tension values (γ ± 0.1 mN m−1) were corrected according to the procedure of Harkins and Jordan [68], built-in in the instrument software.

Conductivity measurements

Electrical conductivities (κ) were measured at four different temperatures from 298.15 to 313.15 K, in 5-K intervals, by EUTECH PC 6000 digital conductivity meter, having a sensitivity of 0.1 μS cm−1 and an accuracy of 0.5 %. The conductivity probe (EC- CONSEN 21B) has a built-in PT-100 temperature sensor; it was calibrated with aqueous KCl solutions (0.01–1.0 mol kg−1). Five measurements were made for each surfactant concentration; the uncertainty of the measurements was <0.3 %.

UV–vis measurements

We used a Varian Cary 50 spectrophotometer, equipped with a thermostated cell compartment. The absorption spectra of solutions containing MO (fixed at 20 μM) plus variable [ILBS] were registered using a 1-cm path length quartz cuvette.

Steady state fluorescence measurements

These were performed using a Jasco FP-6300 spectrofluorimeter, using a 1-cm path length quartz cuvette. Pyrene was used as the polarity probe with fixed concentration of 1 μM in all experiments to avoid perturbing the micelle. The emission spectra of pyrene were recorded in the wavelength range 350–500 nm at an excitation wavelength of 334 nm using the excitation and emission slit widths of 2.5 nm. The first (I 1) and third (I 3) vibronic peaks of pyrene appeared at 373 and 384 nm, respectively. The fluorescence intensities were corrected for the instrumental response. The value of cmc was determined from the dependence of (I 1/I 3) on log [ILBS]; see part Fig. 3b. For the aggregation number, steady state fluorescence quenching measurements were performed using pyrene as probe and cetylpyridinium chloride (CPC) as quencher, by adding a stock solution of ILBS-solubilized pyrene (2.0 × 10−6 M probe) to a stock solution of quencher in water. Prior to the measurements, the mixed solutions were stirred and equilibrated for 2 and 5 min, respectively.

Results and discussion

Notes:

  1. 1.

    The (standard) equations employed to calculate surfactant adsorption and micellization are listed in ESM.

  2. 2.

    The structural variable in the series studied is the chain length of the alkyl group (Cn). Additionally, the ILBS head group carries a db. For ease of reading, we present our data by discussing first the effect of Cn (C10–C16) on the calculated property and compare the present series, where appropriate, with other ILBSs, and conventional surfactants with the same counter-ion and Cn. In order to show the effect of the db, we compare the data for C16VnImBr with the corresponding ones for C16EtImBr. We selected this Cn because the corresponding cmc’s are low, i.e., the solutions can be considered ideal. Consequently, we can attribute any difference between the properties of C16VnImBr and C16EtImBr to the effect of the vinyl group, as compared with the ethyl moiety.

  3. 3.

    We present our data in the order of sequence of events when the ILBS is dissolved in water, i.e., adsorption at solution/air interface and then aggregation as micelles.

Adsorption at solution/air interface

Surface tension measurements provide information about surfactant adsorption at solution/air interface, including surface tension at cmc (γcmc), adsorption efficiency (pC 20), the effectiveness of surface tension reduction (πcmc), the maximum surface excess concentration (Γmax), and the minimum area occupied per ILBS molecule (A min) at solution/air interface [17, 69–72]; these data are reported in Table 1.

Table 1 Dependence of surfactant adsorption parameters at 298.15 K on Cn of ILBSs
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As shown in Table 1, the values of γcmc, a measure of surface activity, decrease as a function of increasing (Cn) the chain length of CnVnIMBr; γcmc for C16VnImBr is slightly larger than for C16EtImBr, showing that the db is less hydrophobic than the ethyl group. The values of pC 20 and πcmc of CnVnImBr increase with increasing alkyl chain length, similar to the CnMeImBr series [8, 17]. Such dependence indicates that the adsorption efficiency of ILBSs at solution/air interface increases with increasing Cn, due to an increase in the hydrophobic interactions between ILBS monomers.

The values of Γmax and A min listed in Table 1 show that the former increases and the latter decreases as a function of increasing (n), similar to CnMeImBr, and conventional cationic surfactants [8, 17, 45, 69, 73]. These results indicate the enhanced hydrophobic interactions and tight packing of the CnVnImBr monomers. In Fig. 1, we compare the results of three surfactant series, namely, CnVnImBr, CnMeImBr, and CnMe3ABr. As shown, Γmax were found to be largest and A min smallest [8, 17, 73] for CnVnImBr. This indicates that more CnVnImBr molecules adsorb at solution/air interface, which is the result of higher hydrophobic interactions in the vinyl series. On comparing the effect of unsaturation in the head group, C16EtImBr has higher Γmax than the C16VnImBr, due to larger hydrophobic character of the former surfactant.

The standard free energy of adsorption (ΔG 0ads ), which is the free energy of transfer of 1 mol of surfactant molecules from bulk solution to the surface [69], is negative. The larger values of ∣ΔG 0ads ∣ than those of ∣ΔG 0m ∣ (see Table 3) indicate more spontaneous adsorption at the solution/air interface than micellization in the bulk. The difference between ∣ΔGads0∣ of C16VnImBr and C16EtImBr can be attributed to the difference between the hydrophobic character of the vinyl and ethyl groups.

Micelle formation

Dependence of the cmc on Cn at 298.15 K

Values of cmc were determined by surface tension and conductivity measurements, and from the spectra of dissolved probes, as follows:

  • Plots of surface tension (γ) versus log [ILBS] decreased as a function of increasing the surfactant concentration until the cmc and stayed practically constant at larger log [ILBS]. Values of the cmc were taken where γ became essentially constant, as shown in of Fig. 2a.

  • Plots of solution conductivity (κ) versus log [ILBS] showed two straight lines with different slopes, intersecting at the cmc, as shown in Fig. 2b.

  • In the experiments with probes, we plotted the dependence of a spectral parameter of the probe on log [ILBS]. As shown in Fig. 3a, the dependence of the ultraviolet–visible (UV–vis) absorbance of MO on log [ILBS] is sigmoidal. We calculated the cmc from the inflection point in the derivative curve (∂(absorbance)/∂ log [ILBS]) versus log [ILBS], as shown in Fig. ESM-3. As shown in Fig. 3b, the dependence of the intensity ratio of the first and third vibronic peaks of pyrene (I 1/I 3) on log [ILBS] showed two lines, intersecting at the cmc. Table 2 shows the cmc values, calculated from the data of these independent techniques.

Fig. 2
figure 2

Representative plots of the dependence of surface tension (γ; a) and specific conductance (κ; b) on surfactant concentration in millimolars, at 298.15 K. The symbols are as follows: blue upright triangle, C14VnImBr; pink inverted triangle, C16VImBr; orange diamond, C16EtImBr; black square, C10VnImBr; and red circle, C12VnImBr

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Fig. 3
figure 3

Dependence of UV–vis absorption of solubilized MO (a) and the ratio of the first and third vibronic peaks of solubilized pyrene (I 1/I 3; b) on surfactant concentration in millimolars, at 298.15 K. The symbols are as follows: black square, C10VnImBr; red circle, C12VnImBr; blue upright triangle, C14VnImBr; pink inverted triangle, C16VnImBr; and orange diamond, C16EtImBr

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Table 2 Values of cmc determined by different experimental techniques for ILBSs, at 298.15 K
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The agreement between the results of distinct techniques is satisfactory, taking into account that these are sensitive to different aspects of the micellization process and the fact that cmc calculated from probe solubility maybe slightly different due to probe–monomer association before the cmc (vide infra the discussion on solubilization of MO). The reasons for the observed dependence of cmc on the technique employed have been discussed elsewhere [74]. For example, Mukerjee and Mysels have compiled 54 cmc’s for C16Me3ABr (measurements at 25 °C), differing, for the same technique, by 22% [75]!

Another way of assessing the effect of the vinyl moiety in the surfactant head group is Fig. 4, which depicts the cmc values of the CnVnImBr (present work), CnMeImBr [8, 17], and CnMe3ABr [69, 70, 76, 77] series. As the insert shows, the order of cmc is CnMe3ABr > CnVnImBr and, as shown above, C16EtImBr > C16VnImBr. Except for C10VnImBr, the cmc values of the CnVnImBr and CnMeImBr series are similar, i.e., the effect of the vinyl moiety on cmc is akin to introducing one carbon atom in the head group (CH3), in agreement with the less hydrophobic character of the vinyl group, relative to the ethyl group.

Fig. 4
figure 4

Dependence of the cmc values on the structure of the head group. The symbols are as follows: black square, CnVnImBr; red circle, CnMeImBr; and pink inverted triangle, CnMe3ABr

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According to the Stauff-Klevens rule [78–80], the relationship between cmc and the number of carbon atoms in the alkyl chain is given by

$$ \log cmc=A-B\kern0.5em {C}_n $$
(1)

where A and B are constants for a particular homologous series. The constant A varies with the nature and number of head groups per surfactant monomer, whereas B measures the effect of each additional methylene group on cmc. It represents the free energy of the transfer of a methylene group from bulk to micellar pseudo-phase. We applied Eq. 1 for the data obtained from conductance and obtained excellent straight lines with slopes of −0.3 ± 0.05 and correlations coefficients of ~0.999. These results are similar to those obtained for CnMeImBr and CnMe3ABr [81]. For all these series, the value of B is close to log 2, meaning that addition of a methylene group to the hydrocarbon chain decreases the cmc of by approximately a factor of 2 [82].

Dependence of cmc on the temperature: calculation of the thermodynamic parameters of micellization

From conductance data at different temperatures (298.15, 303.15, 308.15, and 313.15 K), we calculated the cmc, β, and thermodynamic parameters of micelle formation; see Table 3. For each studied ILBS, the cmc increases as a function of increasing T. The two contributing factors for such behavior are (i) decreasing hydration of the surfactant head group, favoring micellization, and (ii) breaking of the water structure around the hydrophobic part, which increases the solubilization of monomers and disfavors the micellization. In the present study, the second factor is dominant.

Table 3 Dependence of the critical micelle concentration (cmc), degree of counter-ion association (β), free energy (ΔG 0m ), enthalpy (ΔH 0m ), and entropy (ΔS 0m ) of micelle formation on the structure of the surfactants studied and the temperature (T)
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At a fixed T, the value of β increases with increasing the alkyl chain length for CnVnImBr, due to the corresponding increase in hydrophobic interactions. A similar observation was reported for the CnMeImBr and for CnMe3ABr [4, 11, 69–72, 76, 77]. For the same alkyl chain length, β follows the order CnVnImBr < CnMeImBr < CnMe3ABr [69, 70, 76, 77]. The decreasing β values with increasing T indicates that the charge density on the micellar surface is decreasing, due to increased thermal motion of the surfactant ions [36]. Recently, Kamboj et al. observed a decrease in β with increasing alkyl chain length for the morpholinium-based amide-functionalized ILBSs in aqueous media [44]. The same result was observed for conventional cationic and anionic surfactants and may be traced to increased mobility and hydration of the head-ions [44, 83–85].

The thermodynamics parameters of micellization (ΔG 0m , ΔH 0m , and (ΔS 0m ) were calculated using the pseudo phase model, and the values are listed in Table 3. The increase of ∣ΔG 0m ∣ with the chain length indicates the dominating hydrophobic interactions [35], and the order of ∣ΔG 0m ∣ is C16MeImBr ≈ C16VnImBr > C14VnImBr > C12VnImBr > C10VnImBr. Furthermore, ΔH 0m values are negative at each T (Table 3), i.e., micelle formation is an exothermic process and slightly increases with temperature, indicating that temperature has less impact on the hydrophobic part of ILBSs in aqueous solution [18]. Such phenomena occur in the system where the principle force of micellization is the London dispersion interactions [86]. The notable positive values of ΔS 0m indicate that micelle formation is entropy-driven rather than enthalpy-driven [5, 36]. After micellization, the hydrophobic part of the monomer including vinyl group interacts with each other and excludes water from the micellar core, which increases the entropy of the system [42].

From the limiting equivalent conductivity (Λ0) for CnVnImBr (Fig. ESM-2), and Λ0 for the Br− ion, 78.1 S cm2 mol−1 [85], we calculated the values of Λ0 (CnVnIm+) shown in Table 3. It decreases with increasing Cn, due to the concomitant reduction in cation mobility of ILBSs [87]. The values of Λ0 for CnVnIm+ are lower than the corresponding values for CnMeIm+ [87]. On comparing the values for Λ0 (C16VnIm+) with Λ0 (C16Eim+), it was observed that incorporating the unsaturation in the head group increases the Λ0.

Micelle aggregation numbers and properties of the interfacial region

The aggregation numbers (N agg) increase with increasing chain length from C10VnImBr to C16VnImBr; they are lower than the corresponding ones for CnMeImBr, 42, 44, 59, and 66, for Cn = 10, 12, 14, and 16, respectively [4, 8]. The same trend is observed for CnMe3ABr, 40, 55, 70, and 89, for Cn = 10, 12, 14, and 16, respectively [88]. Furthermore, N agg values are in the order C16EtImBr > C16VnImBr (Table 4). The lower N agg of CnVnImBr is attributed to the more rigid nature of the vinyl group which does not permit efficient packing in the micelle.

Table 4 Dependence of the aggregation number (N agg) and intensity ratio of the first and third vibronic peaks of pyrene (I 1/I 3) on the chain length of ILBSs, and calculated dielectric constants (ε) for the average solubilization sites of micelle-solubilized pyrene, at 298.15 K
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For pyrene solubilized in micellar CnVnImBr, with increasing the alkyl chain length from C10 to C16, I 1/I 3 values decrease from 1.28 to 1.19 (Table 4), similar to the CnMeImBr and CnMe3ABr micelles [4]. Furthermore, the order of I 1/I 3 is CnVnImBr < CnMeImBr [4], which shows that pyrene resides in less polar environment in the vinyl group containing ILBSs than in the CnMeImBr series. The I 1/I 3 value for C16VnImBr is higher than that for C16EtImBr, which confirms the less hydrophobic character of the unsaturated bearing head group.

We calculated the apparent dielectric constant (ε) of the (average) micellar solubilization site of pyrene from the following relation [89]:

$$ {l}_1/{l}_3=1.000461+0.01253\varepsilon $$
(2)

As reported in Table 4, with increasing the alkyl chain length in CnVnImBr, solubilized pyrene experiences more nonpolar environment. The unsaturation in the C16VnImBr leads to higher value for ε, as compared with that in C16EtImBr.

Solubilization of methyl orange

Analysis of the UV–vis absorbance of MO in the presence of ILBSs (both in monomer and micellar regions) gave interesting results because the dye and ILBS carry opposite charges [90–95]. In pure solvents, the sensitivity of the value λ max of MO to medium polarity is shown by the following values of λmax: 462, 442, 417, 412, and 396 nm, for water, ethylene glycol, ethanol, acetone, and heptane, respectively [94]. We used this sensitivity to assess the solubilization of MO in the surfactant solution. [90–95]. Our results are depicted in Fig. 5 and Table 5.

Fig. 5
figure 5

Dependence of UV–vis spectra and solution colors of MO on the concentration of CnVnImBr, in aqueous solution, at 298.15 K

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Table 5 Dependence of λ max of MO on [CnVnImBr] and [C16EtImBr] ILBSs in aqueous solutions at 298.15 K
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As shown by these results, λ max of the dye shows a blue shift (relative to water) as a function of increasing [C16VnImBr], most certainly because of dye–surfactant association. This anion–cation interaction decreases dye hydration, i.e., results in a decrease in the polarity of the dye solvation shell [94]. Additionally, dye–surfactant interactions result in decreasing the electrostatic repulsion between dye molecules. This possibly leads to formation of dye dimeric or trimeric species [94, 96] at concentration below the cmc, and visible change in the dye color, see Fig. 5. When more surfactant is added, MO dimers and trimers disappear [96, 97]; the dye shows red shift, and a change in absorbance (manifested by a color change).

We envisage that the MO molecule is aligned parallel with the alkyl chain of the ILBS, with its sulfonate group anchored to the cationic head group of the ILBS [91]. The decrease in λ max with increasing Cn is consistent with the more hydrophobic environment for the probe molecule as a consequence of more extensive “enclosure” by a longer alkyl tail. After the formation of the probe–ILBS complex, once the dye molecules are entrapped in the micelle, their absorbance becomes insensitive to increasing [ILBS], i.e., λ max stays constant (except for C10VnImBr). The difference between the vinyl and ethyl groups is manifested in the values of λmax, before or at the cmc.

Conclusions

Structural modification of ILBSs by incorporating unsaturation in the head group leads to an increase in cmc, relative to C16EtImBr, due to the less hydrophobic character of the vinyl group, relative to the ethyl moiety. We obtained information on the adsorption, and micellization of CnVnImBr in aqueous solutions from surface tension, conductivity, UV–vis, and fluorescence of solubilized dyes. From the results obtained, we calculated surface adsorption parameters (pC 20, π cmc, and Γ max). These are higher as compared to those for CnMeImBr, which indicates that CnVnImBr is more surface active. Thermodynamic parameters of micellization were evaluated using the temperature dependence of cmc and counter-ion binding β. The thermodynamic parameters indicated that micellization is entropy-driven. Methyl orange (MO) was used as a probe to investigate the dye aggregation behavior, and formation of MO–ILBS complexes prior to and after the cmc. Steady state fluorescence and pyrene fluorescence quenching were used to calculate the cmc and N agg of the ILBSs, respectively. Values of cmc obtained from all four techniques are in good agreement with each other. cmc’s of the studied ILBSs are lower as compared to those for CnMeImBr and CnMe3ABr. We show a graphic summary of these results in Fig. 6.

Fig. 6
figure 6

A graphic summary of our results for four surfactant series

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