Impact of organic solvents on the micellization and interfacial behavior of ionic liquid based surfactants

Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Impact of organic solvents on the micellization and interfacial behavior of ionic liquid based surfactants Sargam M. Rajput a , Utkarsh U. More a , Zuber S. Vaid a , Kamlesh D. Prajapati b , Naved I. Malek a,∗ a b

Applied Chemistry Department, S. V. National Institute of Technology, Surat, 395 007, India V.S. Patel College of Arts and Science, Bilimora, 396 321, India

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Organic solvent influences the micellization and surface activity of ionic liquid based surfactants. • Increasing organic solvent composition in water, cmc increases. • Organic solvent, disfavoring the micellization, displaces water layer at the interface. • Comparative aggregation behavior of ILBSs with conventional cationic surfactants.

a r t i c l e

i n f o

Article history: Received 26 April 2016 Received in revised form 28 July 2016 Accepted 6 August 2016 Available online 8 August 2016 Keywords: Critical micelle concentration Relative permittivity of organic solvents Ionic liquid based surfactants 1-Tetradecyl-3-methylimidazolium bromide 1-Hexadecyl-3-methylimidazolium bromide

a b s t r a c t To elucidate the role of organic solvents, in altering the micellization and interfacial behavior of Ionic Liquid based Surfactants (ILBSs), we had investigated the effect of formamide (FA), glycerol (GLY), ethylene glycol (EG) and 1-propanol (PRO) on the micellization and interfacial behavior of 1-tetradecyl-3methylimidazolium bromide (C14 mimBr) and 1-hexadecyl-3-methylimidazolium bromide (C16 mimBr) ILs, in aqueous solution by conductometric and tensiometric techniques at 298.15 K. The micellization behavior has been determined by studying the changes in critical micelle concentration (cmc), degree 0 of counter ion binding (␤), gibbs free energy of micellization (Gm ) and interfacial behavior by studying various surface parameters. Micellization behavior was mainly studied in light of altering the relative permittivity of the medium and displacement of water layer, which disfavors the micellization and as a result increases cmc. Effect of solvophobic parameter (Sp ) and packing parameter (P) on the micellization was also examined. Results have been compared with the conventional cationic surfactants with identical alkyl chain. © 2016 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author. E-mail addresses: [email protected], [email protected] (N.I. Malek). http://dx.doi.org/10.1016/j.colsurfa.2016.08.008 0927-7757/© 2016 Elsevier B.V. All rights reserved.

Ionic liquids (ILs), consisting of larger organic cations and smaller organic/inorganic anions compared to traditional salts,

S.M. Rajput et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

are of interest to the research community due to their potential green characteristics. The unique physicochemical properties of ILs make them the best alternative to the traditional volatile organic compounds (VOCs) [1–4]. The long chain imidazolium based ionic liquids possesses better surface active properties than the traditional cationic surfactants [5–7] and are termed as Ionic Liquid Based Surfactants (ILBSs). These ILBSs found applications in various fields of science, including colloidal science [8,9], catalysis [10,11], nanotechnology [12,13] and biomedicine [14,15] to name a few. Among these bunches of applications, several applications need the media with no water or having water at the minimum level [16]. Among the possible alternatives, traditional strategy to synthesize newer ILs with structural variables such as alkyl chain length variations, various cations and/or anions [17–20] does not solve the purpose. The best possible way to alter the micellization behavior of these ILBSs and make them a better choice for such applications requiring water poor media is by changing the polarity of the aqueous medium by the addition of organic solvents. The nature of surfactant-solvent interactions depends on the solvent environment and significantly influenced through changing the solvent relative permittivity. Introducing organic solvent improves the solvophobicity of the medium and makes the micellization process less favorable as compared to water. Several industrial applications use surfactants either in their aggregated form or they are subjected to the aggregation during the process. During the course of application, they come in contact with the organic solvents which have an impact on their overall aggregation behavior so on performance. Micellar liquid chromatography (MLC), dispersed or phase-separated systems, environmental pollution control, synthesis of reversible solvent-induced porous polymers, applications from battery design to reaction controlled, and extraction processes are among these applications [21,22]. MLC uses surfactants in the mobile phase (in their aggregated form). Polar and non-polar analysts are separated based on the interaction of the surfactant aggregates with the organic solvents [23]. Several conventional surfactants are been tested in MLC [24,25] but the data for the ILBSs are scared [26]. Several extraction processes, where surfactant solutions are used in the presence of organic solvents (ternary system) also find applications in recent times. The aggregated form of the surfactants reduces the efficiency of the process by partial extraction of the product. ILBSs are the best suitable alternative due to their better surface properties in such kind of applications. The knowledge of micellization and interfacial behavior of ILBSs in the presence of organic solvents could help us in designing such extraction systems [22,27]. Such systems also find their applications in high submicellar chromatography, where micellization of the surfactants occurred at high composition of the organic solvents [28]. Though several research groups had studied the influence of organic solvent on the micellization of ILBSs in aqueous medium, the data on the interfacial behavior is yet not studied systematically. Such data will provide researchers a handful of information in finding applications of ILBS. In this context, the aggregation behavior of imidazolium based ILBSs having long alkyl chain in the head group (1-dodecyl-3methylimidazolium bromide, C12 mimBr) in various water-organic solvents mixtures was reported by Wang et al. [29,30]. The study reported that with increasing the organic solvent proportion in water, there appears significant increase in cmc and decrease in size of the aggregates as well as in aggregation number (Nagg ). Recently, micellization and surface adsorption behavior of C12 mimBr was studied in aqueous solution in the presence of several organic solvents and the results have been compared with the traditional cationic surfactant with identical alkyl chain length. The results show that the solvents having larger dielectric constant act as cosolvents and the more hydrophobic solvents acts as co-surfactants [31]. To the best of our knowledge, this is the first report that com-

183

prehensively studies the effect of the organic solvent additives on IL aggregation for these two ILBSs (C14 mimBr and C16 mimBr) in aqueous solutions. Our data for C14 mimBr and C16 mimBr therefore, complete these published work on the 1-R-3-methylimidazolium bromide series [29–31], in addition to demonstrating the effects of organic solvents on (conventional) TTAB and CTAB surfactants. Han and his co-workers [32] studied aggregation behavior of ILs in organic solvents by applying several analytical tools and found that aggregation depends on the dielectric constant of the solvents. Polarized optical microscopy and small-angle X-ray scattering was used to investigate the phase behavior of ternary mixtures including IL, water and alcohols (long chain) [33]. Pino et al. [26] studied the impact of various organic solvents on the aggregation behavior of long chain aqueous ILs (1-hexadecyl-3-butylimidazolium bromide and 1,3-didodecylimidazolium bromide) solution. The aggregation behavior of 1-dodecyl-3-methylimidazolium chloride was studied in various concentrations of water-methanol mixtures and the role of methanol as co-surfactant at lower concentration and as co-solvent at higher concentration [34]. During the preparation of this manuscript, a publication appeared on the determination of cmc of reverse micelle in the anionic surfactant AOT [35] in various nonpolar hydrocarbon solvents. Day et al. studied the aggregation behavior of the anionic surfactant sodium dioctylsulphosuccinate (AOT) in the presence of water-acetonitrile mixture [36]. As per our knowledge, further literature studying the aggregation behavior of ILBSs in presence of organic solvents in aqueous medium is not available. In the present investigation, we have studied the micellization and interfacial behavior of 1-tetradecyl-3-methylimidazolium bromide (C14 mimBr) and 1-hexadecyl-3-methylimidazolium bromide (C16 mimBr) in water in the presence of different organic solvent with variable polarity (Formamide (FA), glycerol (GLY), ethylene glycol (EG) and 1-propanol (PRO)) by electrical conductivity and surface tension measurements at 298.15 K. The micellization and interfacial properties of the studied ILBSs are compared with the conventional cationic surfactants having similar carbon chain length i.e. Tetradecyltrimethylammonium bromide, (C14 TAB) and Cetyltrimethylammonium bromide, (C16 TAB). The aim of the present work is to understand the factors influencing the micellization of ILBSs in different composition of solvents. The data are helpful in various filed of applications including extraction, synthesis of various organic molecules and in various separation methods to name a few.

2. Materials and methods The cationic surfactants tetradecyltrimethyl ammonium bromide (C14 TAB) and cetyltrimethyl ammonium bromide (C16 TAB) were purchased from Aldrich LTD (≥99%) and used as received. ILBSs (C14 mimBr and C16 mimBr) were synthesized as per the procedure published elsewhere [37–40]. Briefly, N-methyl imidazole (0.10 mol) and respective alkyl bromides (0.13 mol) were taken in three necks round bottom flask and toluene as the solvent. The reaction was preceded for 24 h at 80 ◦ C by continuously observing the progress of reaction through Thin Layer Chromatography (TLC). Product was washed three times by ethyl acetate and the resulting white crystalline solids were dried in vacuum for 48 h before the preparation of the respective solution. Water content was found to be less than 100 ppm (Karl-Fischer, Metrohm, 890 Titrando). The probable impurities in the products were analyzed through 1 H NMR (Bruker 400 MHz) spectra and 99.5% purities (on mass fraction basis) were obtained. 1 H NMR data for the synthesized ILBSs are reported in ESI as Figs. S1 and S2 . Further, absence of minimum near the breakpoint in the surface tension graph confirms the absence of any surface impurity in the synthesized ILs.

184

S.M. Rajput et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

Fig. 1. Plot of specific conductance () vs concentration of [C14 mim][Br] in (a) Formamide and (b) n-Propanol solvent at various composition (䊏10 wt%, 䊉20 wt%,  30 wt% and 䊐 in water) at 298.15 K.

Fig. 2. Plots of CMC of (a) C14 TAB and (b) C14 mimBr as a function of solvent composition at 298.15 K.

The structures of the ILBSs and surfactants used in the present investigation are represented as Scheme 1 in the Electronic Supplementary information. Formamide (Merck, 99%), Glycerol (Merck, 99%), Ethylene glycol (Merck, 99%) and 1-propanol (Merck, 99%) were used as received. Double distilled deionized water of conductivity 3.1–3.4 ␮Scm−1 was used throughout the experiments.

the desired surfactant concentration. The solution was thoroughly mixed and temperature equilibrated before measuring the surface tension. The surface tension was measured with an accuracy of 0.1 mN m−1 .

2.1. Preparation of solution for measurements

3.1. Critical micelle concentration (cmc) and counter ion binding

Stock solutions of respective Cn TAB & Cn mimBr (n = 14 and 16) were prepared using double distilled deionized water. Calculated amount of aliquots were added in the predefined water-organic solvent mixture to get the final concentration of the surfactants/ILBSs in water-organic solvent mixtures.

The representative plots for the specific conductivity of the studied C14 mimBr, ILBSs in the presence of different composition of the most (FA, ␧ = 111) and the least (Pro, ␧ = 20.45) polar organic solvent used in the present investigation are drawn in Fig. 1. With increasing the FA content, conductivity increases whereas reverse trend is observed for the organic solvents having relative permittivity less than the water, i.e. PRO. This is due to the fact that with increasing relative permittivity of the solution (i.e. by FA content), cation-anion interaction in the solution decreases, which leads to the higher mobility of the monomers and hence conductivity increases. Reverse is the case for the organic solvents with lesser relative permittivity than water [41,42]. Specific conductivity of the conventional surfactants was compared with ILBSs having similar alkyl chain length (Fig. S3). With increasing the size of the cations, ionic solvation and the limiting ionic conductivity decreases, which reduces the charge transport as well as the mobility and as a result conductivity decreases in ILBSs [42]. The critical micelle concentration (cmc) values of the ILBSs (Cn mimBr, n = 14 and 16) and the conventional surfactants (Cn TAB, n = 14 and 16) in water-organic solvent mixtures with different compositions were determined from the conductivity plot. As observed in Figs. S4 (a–d), the break point in the plot, which is rou-

2.1.1. Electrical conductivity and surface tension measurements Electrical conductivities, measured through EUTECH PC 6000 with dip type conductivity probe (EC-CONSEN 21B) with cell constant of 1 cm−1 . Conductance was measured with 0.5% of accuracy and with sensitivity of 0.1 ␮Scm−1 . Experiments were performed at controlled temperature of 0.1 K using constant temperature bath. Aqueous KCl solutions of 0.01–1.0 molkg−1 concentration range were used to calibrate the probe. All the experiments were performed in triplicate and the mean values were taken for further calculation. Surface tension measurements were performed using Kruss K9 tensiometer using a platinum ring at 298.15 K ± 0.1 K. Calibration of the instrument was performed through measuring the surface tension of the double distilled deionized water (±72.5 mNm−1 at 298.15 ± 0.1 K). Aliquots from the stock solutions of surfactants/ILBSs were added to the water-organic solvent mixture to get

3. Results and discussions

S.M. Rajput et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

185

Table 1 Critical micelle concentration (cmc), counter-ion binding ␤, standard Gibbs energies of micellizationand solvophobic parameter Sp for the Cn TAB and [Cn mim][Br](n = 14 and 16) in different solvents at 298.15 K. Surfactant/ILBSs C14 TAB

System H2 O Formamide

Glycerol

EG

1-Propanol

C16 TAB

H2 O Formamide

Glycerol

EG

1-Propanol

C14 mimBr

H2 O Formamide

Glycerol

EG

1-Propanol

C16 mimBr

H2 O Formamide

Glycerol

EG

1-Propanol

Solvent (v/v) %

cmc (mmol dm−3 )

␤

G0 m (kJ mol−1 )

Sp

0 10 20 30 10 20 30 10 20 30 10 20 30

Cond. 3.66 5.55 5.89 6.75 5.50 6.39 6.73 4.66 5.71 6.39 3.97 5.03 6.77

ST 3.45 5.42 5.80 6.55 5.25 6.14 6.42 4.18 5.20 6.18 3.75 4.83 6.03

0.70 0.69 0.67 0.64 0.68 0.65 0.63 0.65 0.60 0.57 0.59 0.48 0.32

−40.40 −38.70 −37.80 −36.56 −38.61 −37.09 −36.43 −38.43 −36.41 −35.29 −37.51 −34.21 −29.45

1.000 0.945 0.889 0.832 –

0 10 20 30 10 20 30 10 20 30 10 20 30

1.00 1.55 2.33 2.98 1.37 1.94 2.17 1.14 1.26 1.96 1.11 1.21 1.41

0.92 1.51 2.29 2.90 1.06 1.13 1.80 0.98 1.06 1.13 0.90 1.01 1.11

0.44 0.42 0.40 0.36 0.34 0.31 0.23 0.29 0.28 0.22 0.25 0.22 0.18

−38.99 −36.88 −34.99 −33.21 −35.32 −34.77 −31.61 −34.55 −34.03 −30.99 −33.62 −32.39 −30.88

– 0.945 0.889 0.832 –

0 10 20 30 10 20 30 10 20 30 10 20 30

2.61 3.43 3.78 4.04 3.11 3.58 3.89 2.78 3.43 3.73 2.68 3.30 3.60

2.43 3.21 3.46 3.96 2.95 3.20 3.71 2.62 3.01 3.59 2.52 2.84 3.15

0.75 0.77 0.74 0.72 0.74 0.73 0.71 0.71 0.67 0.62 0.62 0.59 0.56

−43.22 −42.50 −41.52 −40.62 −42.22 −41.37 −40.54 −41.96 −40.13 −38.67 −39.79 −38.27 −37.25

– 0.945 0.889 0.832 –

0 10 20 30 10 20 30 10 20 30 10 20 30

0.66 0.76 0.94 1.13 0.74 0.85 1.04 0.70 0.83 1.03 0.68 0.76 0.85

0.76 0.87 0.97 1.08 0.86 0.92 1.02 0.80 0.86 0.91 0.76 0.80 0.90

0.67 0.65 0.59 0.54 0.62 0.57 0.52 0.60 0.55 0.50 0.56 0.52 0.45

−46.87 −45.76 −43.38 −41.66 −45.19 −43.24 −40.42 −44.81 −42.64 −40.18 −43.86 −42.07 −39.71

– 0.945 0.889 0.832 –

0.914 0.856 0.797 0.892 0.811 0.715

0.914 0.856 0.797 0.892 0.811 0.715

0.914 0.856 0.797 0.892 0.811 0.715

0.914 0.856 0.797 0.892 0.811 0.715

Standard uncertainties u are: u(cmc) = 0.04mmol dm−3 , u(␤) = 0.4, u(G0 m ) = 0.05 kJ mol−1 and u(T) = 0.01 K.

tinely used to determine the cmc, is not clearly defined. In this case, cmc and counter-ion binding (␤) determined by this method may lead to high degree of uncertainty. To avoid such erroneous data, we have determined the cmc and ␤ for the studied surfactants and ILBSs in different mixed solvents by the Carpena method [43]. The cmc and ␤ of Cn TAB and Cn mimBr (n = 14 and 16) in aqueous and different mixed solvents are presented in Table 1and found to be in good agreement with the literature [42,44–46]. In the present investigation, we had compared the effect of organic solvent addition on the micellization of ILBSs with traditional cationic surfactants having similar alkyl chain length and identical anion [44–48]. As reported in Table 1, cmc of the aqueous as well as the mixed solvents for ILBSs are lower than their analogous conventional surfactants, which indicate the superior

surface activity of the former [31,49]. The only structural dissimilarity is of head group, the methylimidazolium head group, which is less polar and reduces the electrostatic repulsion between the ionic head groups making the micellization process more spontaneous and resulted in lower cmc. To get further insight into the micellization behavior of ILBSs in different solvent media of varying polarity, here we have examined the effect of different organic solvent compositions at fixed temperature of 298.15 K. With increasing concentration of the organic solvents (Table 1, Fig. 2), cmc of the investigated ILBSs and cationic surfactants increases, similar to C12 TAB [48], C14 TAB [15,48–50], C16 TAB [44,50] and for various ILBSs [29–34] in the organic solvents with varying polarity [42,53–56]. The formations of aggregates in these amphiphiles are the result of the balance between the

186

Surfactant/ILBSs

System

Solvent (v/v) %

␥cmc

Пcmc (N/m)

106 max (mol/m2 )

Amin (nm2 /molecule)

G0 ads (kJ mol−1 )

P

C14 TAB

H2 O Formamide Glycerol EG 1-propanol H2 O Formamide Glycerol EG 1-propanol H2 O Formamide Glycerol EG 1-Propanol H2 O Formamide Glycerol EG 1-Propanol

0 10 20 30 10 20 30 10 20 30 10 20 30 0 10 20 30 10 20 30 10 20 30 10 20 30 0 10 20 30 10 20 30 10 20 30 10 20 30 0 10 20 30 10 20 30 10 20 30 10 20 30

40.00 41.70 41.90 42.10 41.40 41.50 41.70 40.40 40.60 40.70 31.70 31.00 28.10 40.80 42.50 42.60 42.80 41.80 41.90 42.20 41.10 41.30 41.50 32.30 31.40 28.60 39.10 37.30 37.50 37.70 36.90 37.10 37.40 36.40 36.80 36.90 33.00 29.40 26.20 38.70 36.50 36.80 37.10 36.20 36.40 36.70 35.90 36.10 36.20 29.40 28.60 25.80

32.50 18.10 18.90 21.00 22.10 19.80 18.10 28.70 27.70 24.60 10.40 8.80 7.70 31.40 17.10 17.90 20.50 21.50 19.20 18.00 28.30 26.70 24.00 10.20 8.20 7.10 33.70 22.20 23.40 25.70 26.80 24.40 23.00 33.00 31.60 28.70 9.80 9.50 9.00 34.20 22.90 23.90 26.10 27.30 25.30 23.60 33.60 31.90 29.60 12.90 10.80 9.70

2.18 1.32 1.09 0.92 1.68 1.53 1.40 1.97 1.78 1.55 1.06 0.90 0.73 2.30 1.78 1.54 1.43 2.09 1.91 1.74 2.21 2.02 1.81 1.13 0.99 0.92 1.98 1.54 1.43 1.37 1.75 1.59 1.45 1.89 1.74 1.56 0.75 0.68 0.66 2.09 1.72 1.59 1.45 1.85 1.66 1.55 1.95 1.80 1.65 0.81 0.71 0.69

0.80 1.25 1.52 1.81 0.99 1.08 1.19 0.84 0.90 1.07 1.57 1.85 2.27 0.72 0.93 1.08 1.17 0.80 0.81 0.95 0.75 0.82 0.89 1.47 1.67 1.80 0.84 1.08 1.16 1.21 0.95 1.05 1.15 0.88 0.95 1.06 2.21 2.45 2.52 0.79 0.96 1.05 1.14 0.90 1.00 1.07 0.85 0.92 1.01 2.05 2.33 2.39

−38.83 −35.67 −38.66 −43.28 −35.13 −33.74 −33.36 −36.96 −36.42 −35.78 −31.85 −29.43 −27.24 −38.55 −32.46 −32.75 −34.13 −32.72 −31.90 −30.08 −34.99 −34.79 −32.66 −30.94 −29.13 −27.59 −42.86 −39.60 −40.85 −42.34 −40.47 −40.03 −39.60 −42.53 −42.19 −41.03 −37.10 −37.00 −35.92 −46.03 −42.08 −42.48 −44.03 −43.17 −42.50 −41.28 −45.52 −44.67 −43.64 −44.98 −42.76 −39.91

0.26 0.17 0.14 0.12 0.21 0.19 0.17 0.25 0.23 0.20 0.13 0.11 0.09 0.29 0.23 0.19 0.18 0.26 0.26 0.22 0.28 0.25 0.24 0.14 0.13 0.12 0.25 0.19 0.18 0.17 0.22 0.20 0.18 0.24 0.22 0.20 0.09 0.08 0.08 0.27 0.22 0.20 0.18 0.23 0.21 0.20 0.25 0.23 0.21 0.10 0.09 0.09

C16 TAB

C14 mimBr

C16 mimBr

Standard uncertainties u are: u ( cmc ) = 0.02, u(˘ cmc ) = 0.3 mN m−1 , u(max ) = 0.05 × 10−6 mol m2 , u(Amin ) = 0.5 nm2 molecule−1 , u(G0 ads ) = 0.05 kJ mol−1 and u(T) = 0.01 K.

S.M. Rajput et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

Table 2 Various interfacial parameters (max ,Amin ,cmc ), and Gibbs Energy of Adsorption (G0 ads ) for the Cn TAB and [Cn mim][Br](n = 14 and 16) in different solvents at 298.15 K.

S.M. Rajput et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

two opposite effects including the electrostatic repulsion between the ionic head groups and/or attractive interaction between the hydrophobic chains [29,31,57,58]. Here, the elevation in the cmc with increasing the hydrophobicity of the medium (by increasing the organic solvent composition) can be explained by two factors: (i) with reducing the polarity of the medium, the repulsion between the ionic head group increases and attractive hydrophobic interaction between the alkyl chains of the surfactants increases, which eventually disfavor the micellization, and (ii) the solubility of the surfactants increases with reducing the cohesive energy density of the mixture [51]. In other words, addition of the organic solvents will diminish the hydrophobic effect by breaking the water structure and increases the cmc [59]. cmc and ␤ increases more in case of FA than in GLY, EG and PRO for the studied systems (Table 1). ␤ decreases with increasing the organic solvent composition as well as with decreasing the relative permittivity of the solvent, which indicates the less favorable aggregation process. Decrease in the ␤ values indicates the weaker binding of the anions on the aggregate surfaces. For the sake of comparison, we have computed the change in the cmc on the addition of different organic solvents at fixed concentration. For instance, for the addition of 10 v/v% of the organic solvents cmc increases 27%, 37%, 31% and 15% for FA, 23%, 27%, 19%, and 12% for GLY, 16%, 14%, 7% and 5% for EG and 8%, 11%, 3% and 3% for PRO for the C14 TAB, C16 TAB, C14 mimBr and C16 mimBr respectively. The results are not surprising as with decreasing the relative permittivity of the solution, cmc decreases in order of FA > GLY> EG> PRO. Ruiz et al. investigated similar observation for the conventional surfactant C14 TAB in different composition of the polar solvents [60]. 0 was derived for the neat Gibbs energy of micellization Gm C14 TAB and C16 TAB and found in good agreement with the literature [51,61–63]. With increasing alkyl chain length, both in 0 becomes more negative conventional surfactants and ILBSs, Gm indicating favorable aggregation in higher alkyl chain length sur0 for C TAB factants [63]. For the addition of 20 v/v% EG, the Gm 14 (−36.41 kJmol−1 ) and C16 TAB (−34.03 kJmol−1 ) are in good agree0 changes ment with previous studies [42,64]. For C16 mimBr, Gm −1 from −44.81 to −40.18 kJmol for the addition of 10 v/v% to 30 v/v% EG, which is similar to its analogous single chain cationic surfac0 for C mimBr for 10 v/v% addition tant i.e. C16 TAB (Table 1). Gm 12 of EG in aqueous solution was reported to be −36.0 kJ mol−1 [29], whereas for C14 mimBr and C16 mimBr we obtained the results to be −41.96 kJ mol−1 and −44.81 kJ mol−1 respectively, the more negative values for the higher alkyl chain length ILBSs indicate that with increasing alkyl chain length, aggregation becomes more favorable. For the addition of 10 v/v% EG, similar results for the single chain cationic surfactants, i.e. C12 TAB [67], C14 TAB [52] and C16 TAB [62] were reported. 0 with increase in the As reported in Table 1, increase in Gm organic solvent composition suggests the less favorable aggregation [29–31,34,45–48,62,63]. The reasonable argument for this is the fact that, the transfer of the alkyl chain of surfactant/ILBS, which is principally located in the bulk phase, to the micellar core and the alkyl chains in the head groups of the surfactant/ILBSs from the bulk phase into the micellar surface is less favorable [47,62–68]. This prediction has the experimental evidence by the surface tension results we obtained in the later stage of the manuscript. To understand the interaction between the alkyl chains of the amphiphiles with the solvents, it is necessary to understand the mixture hydrophobicity [57] through solvophobic parameter (Sp ) [29]. Sp values for the pure solvents are found in literature, whereas for the mixtures most recently, Wang et al. [29] have developed a correlation method for the prediction of the Sp values at any composition of the organic solvent mixtures with water. The linear relationship derived by the aforementioned authors between the

187

Sp and the volume fraction of the organic solvents added in to the water is used here (Table 1). With decreasing Sp values (Table 1), cmc increases which indicates the less favorable aggregation. With decreasing Sp values, the interaction of the ILBSs alkyl chain with organic solvents increases and increases the cmc. The knowledge of Sp values for the particular solvent composition will help us to modulate the aggregation property of the ILBSs as well as the conventional surfactants for the particular application. Sp values for the GLY are not reported due to the unavailability in the literature. In a similar fashion, the Sp values are directly proportional to the ␤ values. With increasing the solvent solvophobicity, the anions bind weakly and are responsible for the less favorable aggregation. Wang et al. [29] observed similar observation for the conventional surfactants and ILBSs respectively in various organic mixtures with varying polarity.

3.2. Adsorption at solution/air interface and surface active parameters Surface activities of surfactants and ILBSs in pure water and in water-organic solvent mixtures were evaluated by measuring surface tension at 298.15 K. In general, surfactants orient at the air-solution interface and reduces the surface tension till the concentration reaches to the maximum. The concentration after which surface tension does not decrease further is called the cmc and the corresponding surface tension is known as ␥cmc , which is the measure of the surfactant efficiency to accommodate at the airsolution interface. The less the ␥cmc , the more the surface active the amphiphile is. cmc of the surfactant and ILBSs in aqueous as well as in the mixed solvent, obtained through the  versus log [surfactant/ILBSs] plot are depicted in Table 1 and are in good agreement with the literature [64–66] and with the conductivity method used in the current investigation. The ability of the ILBSs to decrease the surface tension of water is higher than that of the corresponding conventional surfactant with the similar alkyl chain length. This fact sheds the light on the better surface activity of the latter [31]. A careful examination of Figs. S5 (a–d) reveals that as the organic solvent composition increases, surface tension decreases. The ␥cmc values for the corresponding ILBSs and conventional surfactants are listed in Table 2. Lower cmc and ␥cmc values for ILBSs than the conventional surfactants indicate the better surface activity of the former. Micelle formations in the surface-active molecules are due to the contribution of the electrostatic and hydrophobic interactions [66]. The reason for the better activity in ILBSs is relatively smaller electrostatic repulsion between the bulky head groups of the cations. The probable reasons for the reduced cmc in ILBSs are due to (i) the hydrogen bonds in the cations which are uniformly distributed and (ii) the delocalized positive charge over the imidazolium ring. These two factors reduces the electrostatic repulsion and hence the cmc [67,68]. As reported in Table 2, with increasing concentration of the organic solvents, ␥cmc increases, which indicated looser packing of the ILs monomers at the interface. This may be due to the displacement of the IL monomer from the interface [26,31,34]. Further, ␥cmc for the ILBSs at respective solvent composition are less than the conventional surfactants with the identical chain length [31]. In their comparative investigation for ILBS with common cationic surfactant, Pino et al. [26] observed two opposite trend and concluded that ␥cmc does not depend on the relative permittivity of the solvent. Various surface parameters such as, the surface pressure at the cmc or the effectiveness of surface tension reduction (cmc ), the maximum surface excess concentration ( max ) and the area occupied per molecule (Amin ) at air-water interface are calculated using the experimental surface tension data. The equations employed to

188

S.M. Rajput et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

calculate these surface parameters are listed in electronic Supplementary information with the terms involved. The values of cmc (Table 2), which depends on the hydrophobicity of the medium, decreases with increasing the organic solvent composition with lower relative permittivity than water, whereas for the FA, cmc increases with increasing its composition in water. Therefore, these ILBSs reduce the surface tension of the organic solvent less effectively with increasing the organic solvent composition in the solution [26,31]. cmc for the conventional surfactants [64] and ILBSs [66] are reported in Table 2 and are in good agreement with the literature. The efficiency of the micellization decreases in the presence of high composition of the organic solvents [67,68]. For the specific composition of the organic solvent addition, cmc for the conventional surfactants and for the ILBSs increases in order of EG > Gly > FA > Pro. The values of max and Amin for the conventional surfactants and ILBSs in aqueous as well as water-organic solvent media are reported in Table 2 and are compared with the available literature and are in good agreement [66]. The increase in the values of max with each CH2 in the alkyl chain of conventional surfactants and ILBSs indicates the higher population of the molecules at the air/water interface due to the increased hydrophobic interaction [61,67–70]. With increasing organic solvent composition and decreasing relative permittivity, max decreases and Amin shows the reverse trend than the max . The plausible reason for such behavior is the reduction of the hydrophobic interactions between the head groups of the surfactants molecules and displacement of the water layer from the air/solution interface by more hydrophobic solvent thereby inducing the weaker adsorption tendency [26,71–73]. 0 reported in Table 2 are negative, which All the values of Gads shows that adsorption takes place spontaneously. As reported 0 in Table 2, with increasing organic solvent composition, Gads becomes less negative for the solvents having relative permittivity lesser than water (i.e GLY, EG and PRO) whereas for FA, reverse 0 reflect the efficiency of the trend was observed. The values of Gads surfactant to get adsorbed at the surface, which decreases with the addition of the organic solvents [57]. The result also focuses on the stronger aggregation in case of ILBSs than the conventional surfactants and ILBSs due to the presence of hydrogen bonded network. 0 denotes the higher Further, the higher negative value for Gads efficiency of the surfactant to get adsorbed at the surface [57]. 3.3. Packing parameter (P) The structure of the surfactant self-assembly or micelles influences the performance in various applications to a larger extent. It is desirable to know the molecular structure of the surfactants which governs the application by the shape and size. In this connection, Tanford [74,75] explain various factors which influences the formation of aggregates in the solution, their shape and size and most importantly why the aggregates attain finite size and particular shape. By applying thermodynamic principals and molecular packing considerations, Israelachvili, Mitchell and Ninham [76] proposed the concept of molecular packing parameter (P) as: P=

V0 lcAmin

(6)

Where, Vo is the volume of the hydrophobic chain and can be determined by Tanford’s equation as, [74,75], Vo = (0.0274 + 0.0269 nc ), lc is the maximum effective length or called as the critical chain length and can be determined by: lc ≤ lmax ≈ (0.154 + 0.126 nc ) [67]. According to the prediction of Israelachvili et al., for P < 1/3, the structure of the micelle will be spherical, for 1/3 < P < 1/2, micelles will be nonspherical, for 1/2 < P < 1, the micellar shape will be vesicles or bilayers and for P > 1, inverted structures will be predicted.

From Table 2, it is predicted that all the conventional surfactants and ILBSs are spherical in shape [77–79]. With increasing the organic solvent content, the packing of the aggregates decreases and loosely packed structure were formed of the type spherical in nature. Sharma et al. [80] observed similar results for the addition of C14 mimBrin the conventional surfactant C14 TAB. With increasing EG and DMSO content in C12 TAB, Das et al. observed decrease in the packing of the aggregates and the shape of the aggregates were found to be spherical [57]. 4. Conclusion In the present investigation, we have examined the micellization and interfacial behavior of ionic liquid based surfactants (ILBSs), Cn mimBr (n = 14 and 16) in the presence of several organic solvents in water through conductivity and surface tension measurements at 298.15 K. cmc of the ILBSs increases by increasing the organic solvent content in aqueous solutions. Incorporating the solvents having higher relative permittivity makes the system more conductive whereas the less conductive organic solvent incorporation leads the system to be as relatively less conductive than the aqueous system with unfavorable micellization. Increasing the composition of organic solvent in the mixture weakens the binding of the anions on the aggregate surfaces and makes the micellization process less unfavorable. Similarly, the results of Gibbs energy of micellization also provide information related to the less favorable micellization behavior of the ILBSs in the presence of organic solvents. Further, the dominant interaction among the alkyl chain of the IL with the solvents of interest is indicative in the form of increased cmc with the reduced solvophobic parameters. The better surface activity of the ILBSs than the conventional cationic surfactants with same alkyl chain length is also studied in the present investigation by studying various surface parameters. Increasing the organic solvent composition in water reduces the effectiveness of the ILBSs. Higher content of the organic solvents decreases the tendency of the surfactant molecules to get adsorb on the surfaces. The results of the Gibbs energy of adsorption also shed the light of the organic solvent content in the aqueous ILBSs solution. Overall, the study denotes the role of the organic solvents on the micellization and interfacial behavior of the ILBSs in aqueous medium. The results will help the researchers to regulate the various aggregation parameters and interfacial phenomena of the ILBSs for the better tailor made properties of these new types of the surfactants which are higher surface active than the conventional cationic surfactants. The knowledge will open up the new era for the new ionic liquid–oil microemulsions systems and expand the potential applications of the ILBSs [81]. Acknowledgements N.I.M. acknowledges financial assistances through Department of Science and Technology, New Delhi (SR/FT/CS-014/2010), Institute Research Grants to the Assistant Professors by SVNIT and Council of Scientific and Industrial Research (CSIR), New Delhi (Grant No. 01 (2545)/11/EMR-II). Maulana Azad National Fellowship, (MANF-2012-13-MUS-GUJ-10818) for a research fellowship to Z. Vaid and TEQIP fellowship to U. More is kindly acknowledged here. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.08. 008.

S.M. Rajput et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 507 (2016) 182–189

References [1] P. Wasserscheid, R. Hal, A. Boesmann, Green Chem. 4 (2002) 400–404. [2] T. Welton, Chem. Rev. 99 (1999) 2071–2083. [3] Ionic Liquids in Synthesis, in: P. Wassersheid, P.T. Welton (Eds.), Wiley-VCH Weinheim, Germany, 2003. [4] J.D. Holbrey, K.R. Seddon, Clean Prod. Process. 1 (1999) 223–228. [5] J. Bowers, C.P. Butts, P.J. Martin, M.C. Vergara-Gutierrez, Langmuir 20 (2004) 2191–2198. [6] A. Modaressi, H. Sifaoui, M. Mielcarz, U. Domanska, M. Rogalski, Colloids Surf. A: Physicochem. Eng. Aspects 302 (2007) 181–185. [7] Q.Q. Baltazar, J. Chandawalla, K. Sawyer, J.L. Anderson, Colloids Surf. A: Physicochem. Eng. Aspects 302 (2007) 150–156. [8] K. Behera, N.I. Malek, S. Pandey, Chem. Phys. Chem. 10 (2009) 3204–3208. [9] S. Trivedi, N.I. Malek, K. Behera, S. Pandey, J. Phys. Chem. B 114 (2010) 8118–8125. [10] X. Wang, J. Liu, L. Yu, J. Jiao, R. Wang, L. Sun, J. Colloid Interface Sci. 391 (2013) 103–110. [11] T. Singh, K.S. Rao, A. Kumar, J. Phys. Chem. B 116 (2012) 1612–1622. [12] D. Ki-Sub Kim, H. Demberelnyamba Lee, Langmuir 20 (2004) 556–560. [13] M. Yang, P.S. Campbell, C.C. Santini, A.V. Mudring, Nanoscale 6 (2014) 3367–3375. [14] X. Yang, S. Zhang, W. Yud, Z. Liu, Lei Lei, H. NaLi, Y. Yu Zhang, Talanta 124 (2014) 1–6. [15] A.B. Khan, M. Ali, N.A. Malik, A. Ali, R. Patel, Colloids Surf. B: Bio. Interfaces 112 (2013) 460–465. [16] K. Mukherjee, D.C. Mukherjee, S.P. Moulik, J. Phys. Chem. 98 (1994) 4713–4718. [17] S.K. Mehta, K.K. Bhasin, R. Chauhan, S. Dham, Colloids Surf. A: Physicochem. Eng. Aspects 255 (2005) 153–157. [18] C.M.R. Almeida, B.F.O. Nascimento, M. Pineiro, A.J.M. Valente, Colloids Surf. A: Physicochem. Eng. Aspects 448 (2015) 279-. [19] H.U. Kim, K.H. Lim, Colloids Surf. A: Physicochem. Eng. Aspects 235 (2004) 121–128. [20] A. Rodriguez, M. Munoz, M.D.M. Graciani, M.L. Moya, J. Colloid Interface Sci. 298 (2006) 942–952. [21] A. Visser, R.P. Swaltowski, R.M. Reichert, R. Mayton, S. Sheff, A. Wierzbicki, J.H. Davis, R.D. Rogers, Environ. Sci. Technol. 36 (2002) 2523–2529. [22] J.G. Huddleston, A.E. Visser, M.W. Reichert, H.D. Willauer, G.A. Broker, R.D. Rogers, Green Chem. 3 (2001) 156–164. [23] T.M. Kalyankar, P.D. Kulkarni, S.J. Wadher, S.S. Pekamwar, J. Appl. Pharm. Sci. 4 (2014) 128–134. [24] J. Esteve-Romero, S. Carda-Broch, M. Gil-Agust, M.E. Capella-Peiro, D. Bose, Trends Anal. Chem. 24 (2005) 75–91. [25] M.J. Ruiz-angel, J.R. Torres-Lapasio, M.C. Garcia-aivarez-Coque, Anal. Chem. 80 (2008) 9705–9713. [26] V. Pino, C. Yao, J.L. Andersn, J. Colloid Interface Sci. 333 (2009) 548–556. [27] A.K. Ressmann, R. Zirbs, M. Pressler, P. Gaertner, K. Bica, Z. Naturforsch. 68 (2013) 1129–1137. [28] M.J. Ruiz-Angel, J.R. Torres-Lapasio, M.C. Garcıa-Alvarez- Coque, S. Carda-Broch, Anal. Chem. 80 (2008) 9705–9713. [29] J. Wang, L. Zhang, H. Wang, C. Wu, J. Phys. Chem. B 115 (2011) 4955–4962. [30] Q. Feng, H. Wang, S. Zhang, J. Wang, Colloids Surf. A: Physicochem. Eng. Aspects 367 (2010) 7–11. [31] T. Nazemi, R. Sadeghi, Colloids Surf. A: Physicochem. Eng. Aspects 462 (2014) 271–279. [32] W. Li, Z. Zhang, J. Zhang, B. Han, B. Wang, M. Hou, Y. Xie, Fluid Phase Equilib. 248 (2006) 211–216. [33] G. Zhang, X. Chen, Y. Zhao, Y. Xie, H. Qiu, J. Phys. Chem. B 111 (2007) 11708–11713. [34] B. Hemmateenejad, A. Safavi, S. Dorostkar, J. Mol. Liq. 160 (2011) 35–39. [35] G.N. Smith, P. Brown, C. James, S.E. Rogers, J. Eastoe, Colloids Surf. A: Physicochem.l Eng. Aspects 494 (2016) 194–200. [36] J. Dey, D. Ray, S. Kumar, N. Sultana, V.K. Aswal, J. Kohlbrecher, K. Ismail, J. Mol. Liq. 216 (2016) 450–454. [37] N.I. Malek, S.P. Ijardar, S.L. Oswal, J. Chem. Eng. Data 59 (2014) 540–553. [38] S.P. Ijardar, N.I. Malek, J. Chem. Thermodyn. 71 (2014) 236–248. [39] U. More, Z. Vaid, P. Bhamoria, A. Kumar, N.I. Malek, J. Solution Chem. 44 (2015) 850–874.

189

[40] N.I. Malek, A. Singh, R. Surati, S.P. Ijardar, J. Chem. Thermodyn. 74 (2014) 103–118. [41] Y. Marcus, The properties of solvents, in: P.G.T. Fogg (Ed.), Wiley Series in Solution Chemistry, John Wiley and Sons, Chichester, 1998, p. 94. [42] M.A. Rodriguez, M. Munoz, M.M. Graciani, M.S.F. Pachon, M.L. Moya, Colloids Surf. A: Physicochem. Eng. Aspects 298 (2007) 177–185. [43] P. Carpena, J. Aguiar, P. Bernaola-Galvan, C.C. Ruiz, Langmuir 18 (2002) 6054–6058. [44] G. D’Errico, D. Ciccarelli, O. Ortona, J. Colloid Interface Sci. 286 (2005) 747–754. [45] A. Rodrıguez, M. Mu˜noz, M.M. Graciani, M.L. Moya, J. Colloid Interface Sci. 298 (2006) 942–951. [46] C. Carnero Ruiz, J. Colloid Interface Sci. 221 (2000) 262–267. [47] M.M. Graciani, A. Rodriguez, M. Munoz, M.L. Moya, Langmuir 21 (2005) 7161–7169. [48] A. Rodrıguez, M. Munoz, M.M. Graciani, M.S. Fern, E.Z. Pachon, M.L. Moya, Langmuir 20 (2004) 9945–9952. [49] C.F. Poole, S.K. Poole, J. Chromatogr. A 1217 (2010) 2268–2286. [50] A. Rodrıguez, M.M. Graciani, M.L. Moya, Langmuir 24 (2008) 12785–12792. [51] C.C. Ruiz, L. Di´ıaz-Lo´ıpez, J. Aguiar, J. Colloid Interface Sci. 305 (2007) 293–300. [52] A. Rodriguez, M.M. Graciani, G. Fernandez, M.L. Moya, J. Colloid Interface Sci. 338 (2009) 207–215. [53] D.J. Lee, W.H. Haung, Colloid Polym. Sci. 274 (1996) 165. [54] L. Canto, M. Corti, V. Degiorgio, H. Hoffmann, W. Ulbricht, J. Colloid Interface Sci. 116 (1987) 384–389. [55] K. Aramaki, U. Olsson, Y. Yamaguchi, H. Kunieda, Langmuir 15 (1999) 6226–6232. [56] H. Wang, J. Wang, S. Zhang, X. Xuan, J. Phys. Chem. B 112 (2008) 16682–16689. [57] S. Das, S. Mondal, S. Ghosh, J. Chem. Eng. Data 58 (2013) 2586–2595. [58] S.K. Metha, K.K. Bhasin, A. Kamar, S. Dham, Colloids Surf. A: Physicochem. Eng. Aspects 278 (2006) 17–25. [59] K. Gracie, D. Turner, R. Palepu, Can. J. Chem. 74 (1996) 1616–1625. [60] F. García Sánchez, C.C. Ruiz, J. Lumin. 69 (1996) 179–186. [61] D.F. Evans, P.J. Wightman, The Colloidal Domain: Where Pysics, Chemistry and Biology Meets, VCH, New york, 1994. [62] M.L. Moya, M. Munoz, A. Rodriguez, M.M. Graciani, G. Fernandez, J. Phys. Org. Chem. 19 (2006) 676–682. [63] M.L. Moya, A. Rodriguez, M.M. Graciani, G. Fernandez, J. Colloid Interface Sci. 316 (2007) 787–795. [64] C. Das, B. Das, J. Chem. Eng. Data 54 (2009) 559–565. [65] S. Javadian, V. Ruhi, A.A. Shahir, A. Heydari, J. Akbari, Ind. Eng. Chem. Res 52 (2013) 15838–15846. [66] B. Dong, X.Y. Zhao, L.Q. Zheng, J. Zhang, N. Li, T. Inoue, Colloids Surf. A: Physicochem. Eng. Aspects 317 (2008) 666–672. [67] H.C. Zhang, K. Li, H.J. Liang, J.J. Wang, Colloids Surf.A: Physicochem. Eng. Aspects 329 (2008) 75–81. [68] M.J. Rosen, Surfactants and Interfacial Phenomena, 2nd edition, Wiely, New York, 1989. [69] B. Dong, Y. Gao, Y. Su, L. Zheng, J. Xu, T. Inoue, J. Phys. Chem. B 114 (2010) 340–348. [70] N.M. Vaghela, N.V. Sastry, V.K. Aswal, Colloid. Polym. Sci. 289 (2011) 309–322. [71] M. Anouti, J. Jones, A. Boisset, J. Jacquemin, C.C. Magaly, D. Lemordant, J. Colloid Interface Sci. 340 (2009) 104–111. [72] M.J. Jaycock, G.D. Parfitt, Chemistry of Interfaces, John Wiley and Sons, New York, 1981. [73] J.B. Huang, M. Mao, B.-Y. Zhu, Colloids Surf. A: Physicochem. Eng. Aspects 155 (1999) 339–348. [74] C. Tanford, The Hydrophobic Effect, Wiley-Inter science, New York, 1973. [75] C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes, John Wiley, New York, 1980. [76] J. Israelachvili, D.J. Mitchell, B.W. Ninham, J. Chem. Soc. Faraday Trans. 272 (1976) 1525–1568. [77] R. Nagarajan, Langmuir 18 (2002) 31–38. [78] R. Nagarajan, E. Ruckenstein, Langmuir 7 (1991) 2934–2969. [79] Y. Han, Y. Wang, Phys. Chem. Chem. Phys. 13 (2011) 1939–1956. [80] R. Sharma, S. Mahajan, R.K. Mahajan, Colloids Surf. A: Physicochem. Eng. Aspects 427 (2013) 62–75. [81] J. Eastoe, S. Gold, S.E. Rogers, A. Paul, T. Welton, R.K. Heenan, I. Grillo, J. Am. Chem. Soc. 127 (2005) 7302–7303.