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Synthesis, physicochemical characteristics and antimicrobial studies of ethyl-substituted imidazolium-based surface active ionic liquids (SAILs)
Colloid and Interface Science Communications 33 (2019) 100204
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Synthesis, physicochemical characteristics and antimicrobial studies of ethyl-substituted imidazolium-based surface active ionic liquids (SAILs)
T
Arifa Shaheena, , Rabia Arifa, Ab Waheed Mira, Sumbul Rehmanb ⁎
a b
Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India Department of Ilmul Advia, Ajmal Khan Tibbiya College, Aligarh Muslim University, Aligarh 202002, India
ARTICLE INFO
ABSTRACT
Keywords: Imidazolium Thermodynamic parameters Interfacial parameters Micellization behavior ZOI
A series containing higher members of ethyl-substituted imidazolium-based surface active ionic liquids (SAILs) [Cneim]Br, (n = 12, 14, 16) has been synthesized by a two-step reaction pathway. They were characterized by FT-IR, 1H NMR and 13C NMR and their effect on self-aggregation behavior in aqueous solution has been examined, employing conductometric and tensiometric methods besides fluorescence measurements and UVvisible spectroscopy. The conductivity data were recorded at different temperatures and thermodynamic parameters of micelle formation such as, standard Gibbs free energy ( Gmo ), standard enthalpy ( Hmo ), standard entropy ( Smo ) were evaluated. The process of micellization is entropy driven and supported by Gibbs free energy measurements. Interfacial parameters have also been evaluated by employing surface tension technique. The SAILs were also analyzed for their antimicrobial activity by taking eleven kinds of bacterial strains.
1. Introduction Ionic liquids (ILs) are used as green solvents as they give clean reactions without unwanted side products and they possess distinct place in the colloid and surface chemistry [1,2]. ILs have attractive properties as compared to conventional organic solvents including high thermal stability, large liquid limits, insufficient vapor pressure, high ionic conductivity, non-flammability and very low volatility which make them interesting and potential solvents for many other important aspects [3–5]. Therefore, they have been used as a solvent for green chemistry and find many applications in nanostructured materials, organic synthesis, catalysis and extraction processes [6,7]. The chemical structure of ILs and thus their physicochemical features can be easily modified by varying their substituent groups, cationic and counter ionic parts [8–10]. ILs having long hydrophobic chains (typically more than eight carbons) and hydrophilic head groups are generally termed as surface active ionic liquids (SAILs) which act as novel surfactants owing to their amphiphilic nature [11]. Therefore, they are assumed to exhibit the desired properties of both surfactants as well as ILs and thus can be used as drug delivery agents so as to enhance the bioavailability of amphiphilic drugs [12]. The surface active ionic liquids can also be used as good alternatives to conventional surfactants in wide applications such as solubilization [13,14], catalysis [15,16] and protein folding [17] due to their eco-friendly and amphiphilic nature. Based on various cationic head groups such as pyrrolidinium, pyridinium, ⁎
imidazolium, amino acids and piperidinium and varying length of alkyl chain, numerous SAILs have been synthesized since both these factors play an important role in aggregation behavior and the interfacial phenomenon of SAILs in aqueous solutions [18,19]. In recent years, the synthesis of imidazolium-based SAILs has attracted more attention as they have better self-aggregation property than others [19,20]. Owing to the different possible combinations of their cation-anion pairs, a wide range of SAILs with antimicrobial properties has been reported [21–23]. The pyridinium and imidazolium-based ILs exhibit antimicrobial activity like quaternary ammonium counterparts [24,25]. Determination of critical micelle concentration (CMC) is supposed to be one of the most essential criteria for comparing the effectiveness related to the required applications of SAILs in solution chemistry [26]. The hydrophobic and steric effects of polar head groups on micellization behavior of SAILs were studied by Wang et al. [27] by comparing the micellization of ([C8mim]Br), (4-m-[C8pyr]-Br) and ([C8mpyrr]Br). Similar types of studies were carried out by Kamboj's group [28] who synthesized four SAILs containing different types of cationic head groups and reported that larger head groups were more significant in enhancing surface and micellization behavior of SAILs. The aggregation behavior of 3-hexadecyl-1-methylimidazolium bromide cation, [C16mim]+with different aromatic counterions was studied by Singh and co-workers [29]. The synthesis and effect of SAILs with different aromatic counterions i.e. anisate and bromobenzoate have been done by H. Ma et al. [30].
Corresponding author. E-mail address: [email protected] (A. Shaheen).
https://doi.org/10.1016/j.colcom.2019.100204 Received 29 July 2019; Received in revised form 28 August 2019; Accepted 6 September 2019 2215-0382/ © 2019 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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through the same procedure (Scheme S1 in supplementary file). 2.3. Characterization of synthesized imidazolium-based SAILs The synthesized imidazolium-based SAILs ([C12eim]Br, [C14eim]Br and [C16eim]Br) were characterized by FT-IR, 1H NMR and 13C NMR to establish their molecular structure. IR spectra (KBr) within the range of 4000−400 cm−1 were recorded on Perkin-Elmer spectrum version 10.4.00 spectrophotometer. The 1H NMR and 13C NMR spectra were recorded at 298.15 K on a Bruker Advance II 400 NMR spectrometer in CDCl3. The FT-IR, 1H NMR and 13C NMR spectra of three synthesized SAILs are given in Supplementary file. 1-dodecyl-3-ethylimidazolium bromide [C12eim]Br: νmax/cm−1 (KBr): 3441 (O−H), 3092, 2925 (aromatic CeH stretching), 2854 (aliphatic CeH stretching), 1637 (C]C), 1506 (C]N), 1461 (CH2 bending), 1375 (CH3 bending), 1255 (NeC stretching), 1110,807, 723 e(CH2)n-, n ≥ 4. 1H NMR spectra (400 MHz, CDCl3): δ/ppm = 0.86 (3H, t, J = 7.12 Hz), 1.19–1.36 (18H, m), 1.51–1.54 (3H, t, J = 7.36 Hz), 1.69–1.86 (2H, m), 4.27–4.31 (4H, m), 6.97 (1H, s), 7.42 (1H, s), 10.58 (1H, s).13C NMR (400 MHz, CDCl3): δ/ppm = 14.06, 15.65, 22.69, 26.26, 28.79, 29.36, 29.41, 29.52, 30.32, 31.92, 33.92, 45.30, 50.15, 121.46, 121.90, 136.56. 1-tetradecyl-3-ethylimidazolium bromide [C14eim]Br: νmax/cm−1 (KBr): 3429 (OeH), 3090, 2925 (aromatic CeH stretching), 2855 (aliphatic CeH stretching), 1640 (C]C), 1565 (C]N), 1462 (CH2 bending), 1376 (CH3 bending), 1166 (NeC stretching), 1085, 727 e (CH2)n-, n ≥ 4. 1H NMR spectra (400 MHz, CDCl3): δ/ppm = 0.87 (3H, t, J = 6.08 Hz), 1.24–1.43 (22H, m), 1.58–1.60 (3H, t, J = 7.36 Hz), 1.74–1.94 (2H, m), 4.33–4.49 (4H, m), 7.58 (1H, s), 7.71 (1H, s), 10.27 (1H, s). 13C NMR (400 MHz, CDCl3): δ/ppm = 14.08, 15.67, 22.64, 26.25, 28.99, 29.31, 29.49, 29.60, 30.31, 31.87, 34.07, 45.28, 50.11, 121.95, 122.07, 136.61. 1-hexadecyl-3-ethylimidazolium bromide [C16eim]Br: νmax/cm−1 (KBr): 3422 (OeH), 3083, 2920 (aromatic CeH stretching), 2851 (aliphatic CeH stretching), 1629 (C]C), 1565 (C]N), 1466 (CH2 bending), 1378 (CH3 bending), 1165 (NeC stretching), 1092, 839, 722 –(CH2)n, n ≥ 4. 1H NMR spectra (400 MHz, CDCl3): δ/ppm = 0.87 (1H, t, J = 7.0 Hz), 1.25–1.33 (26H, m), 1.54–1.63 (3H, t, J = 7.36 Hz), 1.90–1.93 (2H, m), 4.32–4.48 (4H, m), 7.35 (1H, s), 7.47 (1H, s), 10.50 (1H, s). 13C NMR (400 MHz, CDCl3): δ/ppm = 14.09, 15.65, 22.65, 26.26, 28.85, 29.32, 29.42, 29.58, 29.66, 30.30, 31.89, 45.29, 50.13, 121.85, 121.92, 136.76.
Scheme 1. Structures of (a) [C12eim]Br, (b) [C14eim]Br and (c) [C16eim]Br.
The present work reports the synthesis of higher members of a series of ethyl group containing imidazolium-based SAILs, [Cneim]Br (n = 14, 16) for the first time. The lower 12-C member was also synthesized for comparative study with higher members of the series. The SAILs were characterized by using FT-IR, 1H NMR and 13C NMR. The synthesized SAILs were named as; 1-dodecyl-3-ethylimidazolium bromide ([C12eim]Br), 1-tetradecyl-3-ethylimidazolium bromide ([C14eim]Br), 1-hexadecyl-3-ethylimidazolium bromide ([C16eim]Br) (Scheme 1). The thermodynamic and interfacial parameters were determined using conductometric and tensiometric techniques. The aggregation behavior of the SAILs has been observed using fluorescence spectrometry, besides testing them for their antimicrobial activity. The parallel comparison of the three SAILs has been made to get an insight into the effect of their structural disparity on physicochemical properties to identify and relate the SAILs with most superior features. 2. Experimental section 2.1. Materials Imidazole (99%), cetylpyridinium chloride (98%) were purchased from Alfa Aesar, India, bromoethane (99%, CDH), 1-bromododecane (> 98%), 1-bromotetradecane (> 97%), 1-bromohexadecane (> 97%) were purchased from TCI chemical India Pvt. Ltd., acetone (99%), dichloromethane (99%), ethyl acetate (99%), KOH (pellets) (97%) were purchased from Fisher Scientific, pyrene (99%), acetonitrile (HPLC grade) (99%) were purchased from Sigma Aldrich, India. Double distilled water with specific conductivity of 1–5×10−6 S.cm−1 at 298.15 K was used to prepare all studied solutions.
2.4. Methods 2.4.1. Conductivity measurements The conductometry was accomplished on a digital conductometer (EUTECH, CON 700, Singapore) equipped with a conductivity cell devising platinized platinum electrode with cell constant of 1.01 cm−1. The electrode cell was calibrated before each experiment by using 0.1 mol Kg−1 of KCl solution. After the calibration, the CMC of synthesized SAILs were measured at three temperatures differing by 5 K. The temperature was maintained by means of a thermostat bath with a precision of ± 0.2 K. Each measurement was recorded after giving sufficient time of 2 min for stabilization of reading.
2.2. Synthesis of imidazolium-based SAILs The synthesis of all the three SAILs was done in a two step-reaction pathway. The procedure for the synthesis of [C14eim]Br has been described in detail. The first step involves the ethylation of imidazole ring (0.01 mol) in the presence of KOH (0.01 mol). The reaction was mediated in 100 ml acetonitrile at room temperature for 30 min. After that bromoethane (0.01 mol) was added dropwise with constant stirring at room temperature for 3 h. The product was filtered off from the precipitate of KBr followed by recrystallization of the final product from acetonitrile by evaporation under vacuum. Thin Layer Chromatography (TLC) was used to monitor the progress of the reaction. The second step of the reaction involves the refluxing of reaction mixture containing 1ethylimidazole (0.01 mol) and 1-bromotetradecane (0.008 mol) in dichloromethane at about 70 °C for 54 h. The reaction mixture was cooled at room temperature followed by washing about 5–7 times by means of ethyl acetate. The crude solid product was kept at room temperature for 72 h to remove the solvent residue by evaporation and get the desired product. The dodecyl and hexadecyl counterpart were synthesized
2.4.2. Surface tension measurements Tensiometric measurements of synthesized SAILs were carried out using S.D. Hardson tensiometer (Kolkata, India) at 298.15 ± 0.1 K fitted with a platinum‑iridium ring of mean circumference 6 cm. Prior to each measurement, the tensiometer was calibrated with double distilled water of surface tension equal to 72.0 mNm−1 at 298.15 K. During each measurement variable amount of the solution was added and the readings were noted by repeating the experiment three times to get reproducible values. The precision in measurement is around ± 0.1 mNm−1. 2
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2.4.3. Fluorescence measurements The fluorimetric experiments were done on Hitachi F-2700 spectrophotometer (Tokyo, Japan) at 298.15 K. During fluorescence measurements, the excitation wavelength was fixed at 337 nm and the emission spectra were recorded between 350 and 550 nm. The emission and excitation slit widths were fixed at 2.5 nm. Cetylpyridinium chloride (CPC) was taken as quencher whereas pyrene was used as fluorescence probe. A constant concentration of 1 μM pyrene solutions were prepared for the measurements.
We have chosen different temperatures (298.15–308.15 K) for studying the aggregation behavior and determining the thermodynamic parameters in aqueous solutions of SAILs (Table 1). The micellization process is influenced by two factors (1) hydrophobic-hydrophobic interactions among the tails of amphiphilic molecules (2) electrostatic interactions among the polar head groups of amphiphiles [32,33]. From the conductivity measurements, the CMC values of [C12eim]Br, [C14eim]Br and [C16eim]Br were found to be 8.08 mmol·L−1, 1.17 mmol·L−1 and 0.42 mmol·L−1 respectively at 298.15 K. However, the reported CMC values of methyl-subtituted counterparts, [C12mim] Br, [C14mim]Br and [C16mim]Br are 8.5 mmol·L−1, 2.55 mmol·L−1 and 0.61 mmol·L−1 respectively by conductivity method at the same temperature. Thus, the CMC values of ethyl-substituted SAILs are lower than those of methyl-substituted counterparts which are ascribed to the improved hydrophobicity in the former case [34,35].
2.4.4. UV- visible spectroscopy The values of CMC of synthesized SAILs were evaluated by using UV–visible spectroscopy (Perkin-Elmer, Lambda 25) in the absence of any external probe. A cuvette of 10 mm path length was used. The absorption behavior of ethyl-substituted imidazolium-based SAILs was recorded between 200 and 360 nm at 298.15 ± 0.1 K. The probe may change the structure and destabilize the micelles; hence the method adopted by using a probe is less reliable and we have not preferred it for the determination of CMC. Instead, we have determined the CMC of imidazolium-based SAILs without using a probe [31].
3.1.2. Surface tension measurements The values of CMC of these SAILs have been determined from the (γ) vs [SAILs] plots. The surface tension reduces with the added volumes of SAIL solutions and then becomes constant after certain concentration. The point of inflection in the plots of (γ) vs [SAILs] is recognized as CMC, which indicates the onset of self-aggregation of SAILs in aqueous solution and is shown in Fig. 1(b). The CMC of these SAILs obtained from tensiometry given in Table 2 show an inverse relation with hydrocarbon chain length. Furthermore, we have also evaluated interfacial parameters at 298.15 K using tensiometric data.
2.4.5. Antimicrobial studies In order to test the antimicrobial activity of synthesized SAILs, clinical isolates of both types of bacterial strains (seven gram-positive and four gram-negative), most of them being pathogenic in behavior, were selected. The bacterial strains used for the study are Staphylococcus aureus (S. aureus), Streptococcus mutans (S. mutans), Bacillus cereus (B. cereus), Streptococcus pyrogenes (S. pyrogenes), Streptococcus viridans (S. viridans), Staphylococcus epidermidis (S. epidermidis), and Corynebacterium xerosis (C. xerosis) gram-positive bacteria and Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumoniae), Proteus vulgaris (P. vulgaris) and Pseudomonas aeruginosa (P. aeruginosa) gram-negative bacterial strains. The selected bacterial strains were developed on Nutrient Agar No.2 (Himedia Labs Pvt. Ltd., Bombay, India). The antimicrobial activity of synthesized SAILs was carried out by using agar well diffusion method. After filling each Petri plate with 20 ml of agar media and swabbing the plates with bacterial colony, the wells were bored into the agar seeded plates using sterile cork borer of 6 mm diameter. Each well was filled with 100 μl solutions of the prepared compounds consisting of 1 mg SAILs in 1 ml of DMSO and left for incubation for 24 h at 37 °C. The dimethylsulphoxide (DMSO)-containing medium was used as negative control whereas Norfloxacin and Gentamicin (Himedia Labs Pvt. Ltd., Mumbai, India) were used as positive control.
3.1.3. UV–visible measurements For further confirmation of our CMC results, we have performed the quickest and simplest UV–visible spectrophotometer method. A representative absorption spectrum for [C14eim]Br is shown in Fig. 2(a). From the plots of absorbance vs [SAILs], the CMC values were obtained from the point of intersection of two straight lines as shown in Fig. 2(b) for [C14eim]Br and the observed CMC values are given in Table 2. With increasing hydrocarbon chain length, the CMC values were found to follow trend similar to the previous two techniques. The CMC values determined from above mentioned techniques were found to vary slightly showing that the CMC is a method dependent parameter [36]. 3.2. Thermodynamic properties of SAILs The conductivity data of SAILs was used to elucidate the different thermodynamic parameters in order to get a deep understanding of their micellization behavior. The degree of dissociation (α) of SAILs has been calculated from the slopes of conductivity i.e. by taking the ratio of the post-micellar (S2) to pre-micellar (S1) slopes [37]. The degree of counterion binding (β) has been evaluated by substituting the value of the degree of dissociation (α) in the following given formula and the calculated parameters are listed in Table 1. It has been found that the β values show an inverse relation with both the temperature and hydrocarbon chain length.
3. Results and discussion 3.1. Determination of self- aggregation behavior of SAILs The self-aggregation behavior of imidazolium-based SAILs in aqueous solution was determined by employing different techniques which include conductometry, tensiometry, fluorescence and UV–visible spectroscopy.
(1)
= s2 /s1
3.1.1. Conductivity measurement The plots between specific conductivity vs [Cneim]Br where n = 12, 14 and16 at different temperatures were used for determining the CMC of these SAILs and are provided in Table 1. The conductivity plots were such that two straight lines of different capacities traverse at a particular point termed as critical micelle concentration (CMC). A representative plot for [C14eim]Br is shown in Fig. 1(a). The CMC values of these three SAILs determined from conductivity plots at different temperatures increased with increasing temperature but decreased with increasing length of alkyl chain (Table 1). This behavior occurs due to the breakage of water cluster surrounding the hydrophobic chains which in turn delays the micellization process at a higher temperature.
= (1
)
(2)
The pseudo-phase model meant for 1:1 type of ionic surfactants has been used to calculate the standard Gibbs free energy of micellization ( Gmo ) [38]:
Gmo = (1 + ) RT ln XCMC
(3)
where XCMC represents the CMC in terms of mole fraction, β shows the degree of counterion binding to the micellar interface. The standard enthalpy of micellization ( Hmo ) and standard entropy of micellization ( Smo ) have been evaluated by employing the GibbsHelmholtz equation as given below: 3
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Table 1 Critical micelle concentration (CMC) determined from conductivity measurements, degree of counterion binding (β), standard Gibbs free energy of micellization ( Gmo ), standard enthalpy of micellization ( Hmo ), standard entropy of micellization (T Smo ) for SAILs at different temperatures (T/K). SAILs
[C12eim]Br [C14eim]Br [C16eim]Br
T
CMC
β
(K)
(mmol·L−1)
298.15 303.15 308.15 298.15 303.15 308.15 298.15 303.15 308.15
8.08 8.30 8.61 1.17 1.19 1.37 0.42 0.44 0.57
o Gm
0.431 0.362 0.299 0.276 0.248 0.243 0.258 0.233 0.181
(mScm-1 )
80 60
(a)
40 20
0.0
0.5
1.0
1.5
2.0
(kJmol−1)
(kJmol−1)
−31.33 −30.23 −29.18 −34.63 −33.82 −33.24 −37.99 −37.08 −34.77
−3.90 −3.95 −4.25 −5.66 −7.07 −8.25 −8.99 −9.50 −9.64
27.43 26.28 24.93 28.97 26.75 24.99 29.00 27.58 25.13
The surface parameters such as surface tension at CMC (γCMC), effectiveness of the adsorption efficiency (pC20), surface pressure at CMC (πCMC), surface excess concentration (Γmax) and minimum surface area per molecule at the interface (Amin) of three synthesized SAILs were evaluated according to the formulae described below and their values areprovided in Table 2. The adsorption efficiency (pC20) of SAILs at the interface was evaluated by employing the given Eq. (6) [39]:
2.5
C (mmol . L-1) 70 [C12 eim]Br
65
[C14 eim]Br [C16 eim]Br
60
(mNm-1 )
(kJmol−1)
3.3. Evaluation of interfacial parameters of SAILs
0
pC20 =
55 50
40 35 30 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
log [SAILs] (mmol . L ) -1
CMC
Fig. 1. (a) A representative plot of conductivity(κ) vs concentration(C) of [C14eim]Br for the determination of CMC at 308.15 K and (b) Surface tension plots against log[SAILs] for the determination of the CMC of SAILs, [Cneim]Br (n = 12,14,16) at 298.15 K.
RT 2 (1 + )d ln XCMC / dT
Smo = ( Hmo
Gmo )/T
1 log C20
(6)
where C20 is the concentration of SAILs obtained by decreasing the surface tension of pure solvent by 20 mNm−1. The pC20 values of these SAILs are given in Table 2. The increased pC20 values show that the adsorption efficiency of SAILs also increases with the increase in hydrocarbon chain length. Moreover, the surface pressure at CMC has also been evaluated by putting the value of surface tension of solvent (γ0) and those of SAILs solution at the CMC (γCMC), in Eq. (7) given below [39]:
(b)
45
Hmo =
T Smo
become more negative with increasing temperature because the structured water molecules surrounding the hydrocarbon chain of SAILs break at high temperature. From Table 1, it is clear that the large and positive entropy (T Smo ) values for all three SAILs prove that the process of micellization is entropy driven in the entire temperature range being considered. It shows that the entropy provides the force for the formation of the micelles which occurs when the hydrophobic group moves from the solvent to the micellar core and thus favors the micellization process and this process is promoted further with the increase in the hydrocarbon chain length.
cmc
100
o Hm
=
(7)
0 – CMC
The values of γCMC (Table 2) show that the SAILs become more efficient in the reduction of the surface tension with their increasing hydrophobic chain length. However, πCMC increases with the increasing hydrophobicity of SAILs which accounts for their increased efficiency with the increasing carbon chain length. The adsorption efficiency of SAILs at air-liquid interface is evaluated in terms of surface excess concentration (Γmax), through the Gibbs adsorption Eq. (8) [40]:
(4) (5)
All the thermodynamic parameters are provided in Table 1. It was observed that the values of Gmo are negative in the studied range of temperature for all the three SAILs which show the spontaneity of micellization in their aqueous solutions. The Gmo values were more negative for SAILs bearing longer hydrocarbon chain, showing that the micellization process is more spontaneous with a bigger hydrocarbon chain. The negative Hmo values exhibit that the micellization process is exothermic. Furthermore, it has also been found that the values of Hmo
max
=
1 2.303nRT
log C
T
(8)
where γ, R, T and C have their usual meanings, n stands for the number of ionic species present in the solution formed by the dissociation of SAIL molecules. The value of n is taken as 2 for SAILs ([Cneim]Br) as all the systems contain imidazole ring with one positive charge and similar counterion i.e. bromine with one negative charge on it. The values of 4
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Table 2 Critical micelle concentration (CMC), surface tension at CMC (γCMC), adsorption efficiency (pC20), surface pressure at CMC (πCMC), surface excess concentration (Γ o max), minimum surface area per molecule (Amin), standard Gibbs free energy of adsorption ( Gad ), aggregation numbers (Nagg) of SAILs at 298.15 K. SAILs
[C12eim]Br [C14eim]Br [C16eim]Br
πCMC
Γmax. 109
Amin. 105
(mNm−1)
(molm−2)
Å2
(kJmol−1)
1.71
37.83
5.62
0.295
−36.96
61
34.07
2.42
37.93
5.72
0.290
−41.26
80
31.98
2.65
40.02
5.82
0.285
−44.87
101
CMC
γCMC
(mmol·L−1)
(mNm−1)
(7.03)a (6.07)b (1.15)a (1.02)b (0.52)a (0.62 (0.62)b
34.17
pC20
o Gad
Nagg
Determined by aSurface tension, bUV‐–visible spectroscopy.
largest as compared to lower members of the series i.e. [C14eim]Br and [C12eim]Br. This indicates that the micelles of longer chain-containing [C16eim]Br SAIL are closely packed at interface due to greater van der Waals forces in the hydrophobic part in comparison to shorter chaincontaining [C14eim]Br and [C12eim]Br SAILs. Further, the Amin values (Table 2), show an opposite trend to Γmax values because the values of Amin are also influenced by the hydrophobic part of SAILs. Therefore, values of the Amin parameter are in the order; [C12eim]Br > [C14eim]Br > [C16eim]Br. This reveals that the micelles of SAILs become more compact or closely packed when we move from lower member ([C12eim]Br) to the higher member of the series ([C16eim]Br) (Scheme 2). o The standard Gibbs free energy of adsorption, Gad [42] at interface can be evaluated by using the following Eq. (10): o Gad =
Gmo
CMC max
(10)
where πCMC stands for surface pressure, a parameter which measures the effectiveness of SAILs to lower the surface tension. From Table 2 we o have observed that all the Gad values for three SAILs were negative and greater in magnitude than the Gmo values. It shows that the adsorption phenomenon is spontaneously favorable. Moreover, the higher magnitude exhibits that the adsorption process which is a surface process occurs prior to the micelle formation which is a bulk phenomenon. 3.4. Determination of aggregation numbers of SAILs Steady-state fluorescence quenching method was also used to evaluate the aggregation numbers of the three imidazolium-based SAILs in aqueous solution. The Nagg was calculated by quenching of pyrene through the CPC according to the relation given in Eq. (11) [43]:
ln
Fig. 2. (a) UV‐–visible spectra of aqueous solution of [C14eim]Br with inset image showing the magnified spectra in the wavelength range 230–360 nm and (b) Determination of CMC from the plot of Absorbance vs concentration (C) of [C14eim]Br at 298.15 K.
log C T
constant temperature. The minimum surface area of three SAILs at air-liquid interface was evaluated from the following Eq. (9) [41]:
10 20 NA max
(11)
where I0 and I show fluorescence intensities of pyrene in absence and presence of quencher respectively. [C] and [CPC] are the concentrations of the studied SAILs and quencher respectively. Fluorescence emission spectra of pyrene showing its quenching by CPC is given in Fig. 3 and the inset plot of ln(I0/I) vs [CPC] is used to determine aggregation numbers of SAILs. The plot clearly shows the linear relation for all the SAILs. The calculated values of Nagg for the three SAILs are listed in Table 2. The aggregation numbers of three SAILs increase in aqueous solution as the alkyl chain length increases within the series. The Nagg values of these SAILs were found to be higher than those reported for 1-alkyl-3-methylimidazolium chlorides [11].
( ) were obtained from the slopes of surface tension plots at a Amin =
I0 [CPC ] = Nagg I [C CMC ]
(9)
where NA represents the Avogadro's number and factor 1020 is used to convert the area from nm2 to Å2. The Γmax and Amin values are given in Table 2. The Γmax values depend upon (a) van der Waals forces between the hydrophobic parts of SAILs (b) steric factor due to the presence of bulky groups in these SAILs. The value of Γmax for [C16eim]Br is the
3.5. Antimicrobial studies The ZOI data (mm/mg) calculated by agar well diffusion method shows that all the synthesized SAILs exhibit significant activity against 5
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Scheme 2. Pictorial representation of the effect of alkyl chain length on self-aggregation behavior and micellar surface area of SAILs, [Cneim]Br (n = 12, 14, 16).
that among gram-positive strains, the SAILs, [C12eim]Br and [C14eim] Br showed effective antimicrobial activity against S. pyrogenes and C. xerosis respectively {Fig. 4(a)}. On the other hand, [C16eim]Br showed effective antimicrobial behavior against S. epidermidis and C. xerosis. Among gram-negative bacterial strains, maximum activity was shown by both [C12eim]Br and [C14eim]Br against K. pneuomoniae whereas [C16eim]Br proved to be more potent against E. coli as shown in Fig. 4(b). During the antimicrobial screening of these synthesized SAILs, we have observed that the treatments with [C12eim]Br do not let the growth of bacterial colony in most of the species; as no colonies were formed after incubation period. The positive control (Norfloxacin and Gentamicin) showed a moderate activity for all the bacterial species while DMSO, which was used as negative control did not cause any inhibition to the growth of the bacterial cultures; indicating the inert nature of DMSO at the concentration used. [C12eim]Br was more effective in controlling the bacterial growth as compared to higher chain members especially for the gram-negative strains. This may be due to most favorable hydrophilic-lipophilic balance in their structure which is a cause of their preferential adsorption at bacterial cell wall and results in their better biological activity [24]. All the SAILs were equally effective for most of the strains in controlling the bacterial growth when compared to the respective positive control (Table S1).
Fig. 3. Fluorescence emission spectra showing quenching of pyrene by CPC in aqueous solutions of SAILs with inset plots of ln I0/I vs [CPC] for the determination of aggregation numbers of SAILs [Cneim]Br (n = 12,14,16).
the studied bacterial strains except [C16eim]Br which was found to be inactive towards the gram-negative bacterium K. pneumoniae. Though the antimicrobial activity of SAILs is primarily decided by their degree of adsorption on bacterial cell wall followed by penetration inside the cell to disrupt the membrane permeability of the cell. However, the overall biological activity at a specific hydrocarbon chain length is governed by the combined effects of various physicochemical parameters like CMC, hydrophobicity, adsorption and deposition on cell wall, aqueous solubility and passage through medium. The resistance of gram-negative bacterium K. pneumoniae towards [C16eim]Br may be attributed to the restricted entry of 16-C SAILs into the bacterial cell through cell wall, which would subsequently disrupt the underlying cell membrane or peptidoglycan cell wall. Unlike the gram-negative bacteria, the cell wall of gram-positive bacteria allows the free adsorption of SAILs over its surface and finally leads them to the inner cell membrane [44]. From Table S1 (supplementary file), it is clearly observed
4. Conclusion It can be summarized that we have synthesized higher members of a series of ethyl-substituted imidazolium-based SAILs and characterized them by FT-IR, 1H NMR and 13C NMR spectroscopic techniques. The self-aggregation behavior of these synthesized SAILs was observed by different methods such as conductometric, tensiometric, fluorimetric measurements and UV–visible spectroscopy. The CMC of [C14eim]Br and[C16eim]Br show lower values as compared to [C12eim]Br, which is mainly due to the greater hydrophobicity of the former as compared to 6
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Fig. 4. Effect of different SAILs on the bacterial growth shown by Zone of inhibition (ZOI) (a) gram-positive bacterial strain (C. xerosis) and (b) gram-negative bacterial strain (E. coli) against their respective positive controls.
the latter. We have also studied the influence of temperature on CMC of SAILs by conductometric method and found that CMC is directly proportional to the temperature. The thermodynamic parameters, Gmo , Hmo and Smo , determined showed that the micellization process is thermodynamically favorable. The Γmax values for [C16eim]Br is highest and shows that the surfactant monomers are closely packed at the interface. This is also supported by its lower Amin value which also confirms the close packing of surfactant monomers of [C16eim]Br. Further, the surface and aggregation behavior of ethyl-substituted SAILs were found to be better than the methyl-substituted SAILs which is again the result of their superior hydrophobic interactions. The antimicrobial data which was determined presents significant activities of these SAILs against clinically relevant microorganisms. The antimicrobial activity of [C12eim]Br was found to be greater than the higher members of the series. The imidazolium-based SAILs possess better self-aggregation property, surface and antimicrobial activity therefore, they will provide wide applications in nanostructured materials, biocatalytic activities, pharmaceutical, agricultural, biotechnological and refinery industries.
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Declaration of Competing Interest None. Acknowledgments Authors acknowledge UGC (DRS II) and DST (FIST & PURSE), India for providing research support. Arifa Shaheen acknowledges the University Grants Commission (New Delhi, India) for the UGC-BSR Startup Grant (No.F.30-310/2016). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.colcom.2019.100204. References [1] P.S. Gehlot, H. Gupta, A. Kumar, Paramagnetic surface active ionic liquids: interaction with DNA and MRI application, Colloids Interface Sci. Commun. 26 (2018) 14–23, https://doi.org/10.1016/j.colcom.2018.07.004. [2] A. Pal, M. Saini, Effect of alkyl chain on micellization properties of dodecylbenzenesulfonate based surface active ionic liquids using conductance, surface tension and spectroscopic techniques, J. Dispers. Sci. Technol. (2019) 1–10, https://doi. org/10.1080/01932691.2019.1593859.
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Synthesis, physicochemical characteristics and antimicrobial studies of ethyl-substituted imidazolium-based surface active ionic liquids (SAILs)
T
Arifa Shaheena, , Rabia Arifa, Ab Waheed Mira, Sumbul Rehmanb ⁎
a b
Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India Department of Ilmul Advia, Ajmal Khan Tibbiya College, Aligarh Muslim University, Aligarh 202002, India
ARTICLE INFO
ABSTRACT
Keywords: Imidazolium Thermodynamic parameters Interfacial parameters Micellization behavior ZOI
A series containing higher members of ethyl-substituted imidazolium-based surface active ionic liquids (SAILs) [Cneim]Br, (n = 12, 14, 16) has been synthesized by a two-step reaction pathway. They were characterized by FT-IR, 1H NMR and 13C NMR and their effect on self-aggregation behavior in aqueous solution has been examined, employing conductometric and tensiometric methods besides fluorescence measurements and UVvisible spectroscopy. The conductivity data were recorded at different temperatures and thermodynamic parameters of micelle formation such as, standard Gibbs free energy ( Gmo ), standard enthalpy ( Hmo ), standard entropy ( Smo ) were evaluated. The process of micellization is entropy driven and supported by Gibbs free energy measurements. Interfacial parameters have also been evaluated by employing surface tension technique. The SAILs were also analyzed for their antimicrobial activity by taking eleven kinds of bacterial strains.
1. Introduction Ionic liquids (ILs) are used as green solvents as they give clean reactions without unwanted side products and they possess distinct place in the colloid and surface chemistry [1,2]. ILs have attractive properties as compared to conventional organic solvents including high thermal stability, large liquid limits, insufficient vapor pressure, high ionic conductivity, non-flammability and very low volatility which make them interesting and potential solvents for many other important aspects [3–5]. Therefore, they have been used as a solvent for green chemistry and find many applications in nanostructured materials, organic synthesis, catalysis and extraction processes [6,7]. The chemical structure of ILs and thus their physicochemical features can be easily modified by varying their substituent groups, cationic and counter ionic parts [8–10]. ILs having long hydrophobic chains (typically more than eight carbons) and hydrophilic head groups are generally termed as surface active ionic liquids (SAILs) which act as novel surfactants owing to their amphiphilic nature [11]. Therefore, they are assumed to exhibit the desired properties of both surfactants as well as ILs and thus can be used as drug delivery agents so as to enhance the bioavailability of amphiphilic drugs [12]. The surface active ionic liquids can also be used as good alternatives to conventional surfactants in wide applications such as solubilization [13,14], catalysis [15,16] and protein folding [17] due to their eco-friendly and amphiphilic nature. Based on various cationic head groups such as pyrrolidinium, pyridinium, ⁎
imidazolium, amino acids and piperidinium and varying length of alkyl chain, numerous SAILs have been synthesized since both these factors play an important role in aggregation behavior and the interfacial phenomenon of SAILs in aqueous solutions [18,19]. In recent years, the synthesis of imidazolium-based SAILs has attracted more attention as they have better self-aggregation property than others [19,20]. Owing to the different possible combinations of their cation-anion pairs, a wide range of SAILs with antimicrobial properties has been reported [21–23]. The pyridinium and imidazolium-based ILs exhibit antimicrobial activity like quaternary ammonium counterparts [24,25]. Determination of critical micelle concentration (CMC) is supposed to be one of the most essential criteria for comparing the effectiveness related to the required applications of SAILs in solution chemistry [26]. The hydrophobic and steric effects of polar head groups on micellization behavior of SAILs were studied by Wang et al. [27] by comparing the micellization of ([C8mim]Br), (4-m-[C8pyr]-Br) and ([C8mpyrr]Br). Similar types of studies were carried out by Kamboj's group [28] who synthesized four SAILs containing different types of cationic head groups and reported that larger head groups were more significant in enhancing surface and micellization behavior of SAILs. The aggregation behavior of 3-hexadecyl-1-methylimidazolium bromide cation, [C16mim]+with different aromatic counterions was studied by Singh and co-workers [29]. The synthesis and effect of SAILs with different aromatic counterions i.e. anisate and bromobenzoate have been done by H. Ma et al. [30].
Corresponding author. E-mail address: [email protected] (A. Shaheen).
https://doi.org/10.1016/j.colcom.2019.100204 Received 29 July 2019; Received in revised form 28 August 2019; Accepted 6 September 2019 2215-0382/ © 2019 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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through the same procedure (Scheme S1 in supplementary file). 2.3. Characterization of synthesized imidazolium-based SAILs The synthesized imidazolium-based SAILs ([C12eim]Br, [C14eim]Br and [C16eim]Br) were characterized by FT-IR, 1H NMR and 13C NMR to establish their molecular structure. IR spectra (KBr) within the range of 4000−400 cm−1 were recorded on Perkin-Elmer spectrum version 10.4.00 spectrophotometer. The 1H NMR and 13C NMR spectra were recorded at 298.15 K on a Bruker Advance II 400 NMR spectrometer in CDCl3. The FT-IR, 1H NMR and 13C NMR spectra of three synthesized SAILs are given in Supplementary file. 1-dodecyl-3-ethylimidazolium bromide [C12eim]Br: νmax/cm−1 (KBr): 3441 (O−H), 3092, 2925 (aromatic CeH stretching), 2854 (aliphatic CeH stretching), 1637 (C]C), 1506 (C]N), 1461 (CH2 bending), 1375 (CH3 bending), 1255 (NeC stretching), 1110,807, 723 e(CH2)n-, n ≥ 4. 1H NMR spectra (400 MHz, CDCl3): δ/ppm = 0.86 (3H, t, J = 7.12 Hz), 1.19–1.36 (18H, m), 1.51–1.54 (3H, t, J = 7.36 Hz), 1.69–1.86 (2H, m), 4.27–4.31 (4H, m), 6.97 (1H, s), 7.42 (1H, s), 10.58 (1H, s).13C NMR (400 MHz, CDCl3): δ/ppm = 14.06, 15.65, 22.69, 26.26, 28.79, 29.36, 29.41, 29.52, 30.32, 31.92, 33.92, 45.30, 50.15, 121.46, 121.90, 136.56. 1-tetradecyl-3-ethylimidazolium bromide [C14eim]Br: νmax/cm−1 (KBr): 3429 (OeH), 3090, 2925 (aromatic CeH stretching), 2855 (aliphatic CeH stretching), 1640 (C]C), 1565 (C]N), 1462 (CH2 bending), 1376 (CH3 bending), 1166 (NeC stretching), 1085, 727 e (CH2)n-, n ≥ 4. 1H NMR spectra (400 MHz, CDCl3): δ/ppm = 0.87 (3H, t, J = 6.08 Hz), 1.24–1.43 (22H, m), 1.58–1.60 (3H, t, J = 7.36 Hz), 1.74–1.94 (2H, m), 4.33–4.49 (4H, m), 7.58 (1H, s), 7.71 (1H, s), 10.27 (1H, s). 13C NMR (400 MHz, CDCl3): δ/ppm = 14.08, 15.67, 22.64, 26.25, 28.99, 29.31, 29.49, 29.60, 30.31, 31.87, 34.07, 45.28, 50.11, 121.95, 122.07, 136.61. 1-hexadecyl-3-ethylimidazolium bromide [C16eim]Br: νmax/cm−1 (KBr): 3422 (OeH), 3083, 2920 (aromatic CeH stretching), 2851 (aliphatic CeH stretching), 1629 (C]C), 1565 (C]N), 1466 (CH2 bending), 1378 (CH3 bending), 1165 (NeC stretching), 1092, 839, 722 –(CH2)n, n ≥ 4. 1H NMR spectra (400 MHz, CDCl3): δ/ppm = 0.87 (1H, t, J = 7.0 Hz), 1.25–1.33 (26H, m), 1.54–1.63 (3H, t, J = 7.36 Hz), 1.90–1.93 (2H, m), 4.32–4.48 (4H, m), 7.35 (1H, s), 7.47 (1H, s), 10.50 (1H, s). 13C NMR (400 MHz, CDCl3): δ/ppm = 14.09, 15.65, 22.65, 26.26, 28.85, 29.32, 29.42, 29.58, 29.66, 30.30, 31.89, 45.29, 50.13, 121.85, 121.92, 136.76.
Scheme 1. Structures of (a) [C12eim]Br, (b) [C14eim]Br and (c) [C16eim]Br.
The present work reports the synthesis of higher members of a series of ethyl group containing imidazolium-based SAILs, [Cneim]Br (n = 14, 16) for the first time. The lower 12-C member was also synthesized for comparative study with higher members of the series. The SAILs were characterized by using FT-IR, 1H NMR and 13C NMR. The synthesized SAILs were named as; 1-dodecyl-3-ethylimidazolium bromide ([C12eim]Br), 1-tetradecyl-3-ethylimidazolium bromide ([C14eim]Br), 1-hexadecyl-3-ethylimidazolium bromide ([C16eim]Br) (Scheme 1). The thermodynamic and interfacial parameters were determined using conductometric and tensiometric techniques. The aggregation behavior of the SAILs has been observed using fluorescence spectrometry, besides testing them for their antimicrobial activity. The parallel comparison of the three SAILs has been made to get an insight into the effect of their structural disparity on physicochemical properties to identify and relate the SAILs with most superior features. 2. Experimental section 2.1. Materials Imidazole (99%), cetylpyridinium chloride (98%) were purchased from Alfa Aesar, India, bromoethane (99%, CDH), 1-bromododecane (> 98%), 1-bromotetradecane (> 97%), 1-bromohexadecane (> 97%) were purchased from TCI chemical India Pvt. Ltd., acetone (99%), dichloromethane (99%), ethyl acetate (99%), KOH (pellets) (97%) were purchased from Fisher Scientific, pyrene (99%), acetonitrile (HPLC grade) (99%) were purchased from Sigma Aldrich, India. Double distilled water with specific conductivity of 1–5×10−6 S.cm−1 at 298.15 K was used to prepare all studied solutions.
2.4. Methods 2.4.1. Conductivity measurements The conductometry was accomplished on a digital conductometer (EUTECH, CON 700, Singapore) equipped with a conductivity cell devising platinized platinum electrode with cell constant of 1.01 cm−1. The electrode cell was calibrated before each experiment by using 0.1 mol Kg−1 of KCl solution. After the calibration, the CMC of synthesized SAILs were measured at three temperatures differing by 5 K. The temperature was maintained by means of a thermostat bath with a precision of ± 0.2 K. Each measurement was recorded after giving sufficient time of 2 min for stabilization of reading.
2.2. Synthesis of imidazolium-based SAILs The synthesis of all the three SAILs was done in a two step-reaction pathway. The procedure for the synthesis of [C14eim]Br has been described in detail. The first step involves the ethylation of imidazole ring (0.01 mol) in the presence of KOH (0.01 mol). The reaction was mediated in 100 ml acetonitrile at room temperature for 30 min. After that bromoethane (0.01 mol) was added dropwise with constant stirring at room temperature for 3 h. The product was filtered off from the precipitate of KBr followed by recrystallization of the final product from acetonitrile by evaporation under vacuum. Thin Layer Chromatography (TLC) was used to monitor the progress of the reaction. The second step of the reaction involves the refluxing of reaction mixture containing 1ethylimidazole (0.01 mol) and 1-bromotetradecane (0.008 mol) in dichloromethane at about 70 °C for 54 h. The reaction mixture was cooled at room temperature followed by washing about 5–7 times by means of ethyl acetate. The crude solid product was kept at room temperature for 72 h to remove the solvent residue by evaporation and get the desired product. The dodecyl and hexadecyl counterpart were synthesized
2.4.2. Surface tension measurements Tensiometric measurements of synthesized SAILs were carried out using S.D. Hardson tensiometer (Kolkata, India) at 298.15 ± 0.1 K fitted with a platinum‑iridium ring of mean circumference 6 cm. Prior to each measurement, the tensiometer was calibrated with double distilled water of surface tension equal to 72.0 mNm−1 at 298.15 K. During each measurement variable amount of the solution was added and the readings were noted by repeating the experiment three times to get reproducible values. The precision in measurement is around ± 0.1 mNm−1. 2
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2.4.3. Fluorescence measurements The fluorimetric experiments were done on Hitachi F-2700 spectrophotometer (Tokyo, Japan) at 298.15 K. During fluorescence measurements, the excitation wavelength was fixed at 337 nm and the emission spectra were recorded between 350 and 550 nm. The emission and excitation slit widths were fixed at 2.5 nm. Cetylpyridinium chloride (CPC) was taken as quencher whereas pyrene was used as fluorescence probe. A constant concentration of 1 μM pyrene solutions were prepared for the measurements.
We have chosen different temperatures (298.15–308.15 K) for studying the aggregation behavior and determining the thermodynamic parameters in aqueous solutions of SAILs (Table 1). The micellization process is influenced by two factors (1) hydrophobic-hydrophobic interactions among the tails of amphiphilic molecules (2) electrostatic interactions among the polar head groups of amphiphiles [32,33]. From the conductivity measurements, the CMC values of [C12eim]Br, [C14eim]Br and [C16eim]Br were found to be 8.08 mmol·L−1, 1.17 mmol·L−1 and 0.42 mmol·L−1 respectively at 298.15 K. However, the reported CMC values of methyl-subtituted counterparts, [C12mim] Br, [C14mim]Br and [C16mim]Br are 8.5 mmol·L−1, 2.55 mmol·L−1 and 0.61 mmol·L−1 respectively by conductivity method at the same temperature. Thus, the CMC values of ethyl-substituted SAILs are lower than those of methyl-substituted counterparts which are ascribed to the improved hydrophobicity in the former case [34,35].
2.4.4. UV- visible spectroscopy The values of CMC of synthesized SAILs were evaluated by using UV–visible spectroscopy (Perkin-Elmer, Lambda 25) in the absence of any external probe. A cuvette of 10 mm path length was used. The absorption behavior of ethyl-substituted imidazolium-based SAILs was recorded between 200 and 360 nm at 298.15 ± 0.1 K. The probe may change the structure and destabilize the micelles; hence the method adopted by using a probe is less reliable and we have not preferred it for the determination of CMC. Instead, we have determined the CMC of imidazolium-based SAILs without using a probe [31].
3.1.2. Surface tension measurements The values of CMC of these SAILs have been determined from the (γ) vs [SAILs] plots. The surface tension reduces with the added volumes of SAIL solutions and then becomes constant after certain concentration. The point of inflection in the plots of (γ) vs [SAILs] is recognized as CMC, which indicates the onset of self-aggregation of SAILs in aqueous solution and is shown in Fig. 1(b). The CMC of these SAILs obtained from tensiometry given in Table 2 show an inverse relation with hydrocarbon chain length. Furthermore, we have also evaluated interfacial parameters at 298.15 K using tensiometric data.
2.4.5. Antimicrobial studies In order to test the antimicrobial activity of synthesized SAILs, clinical isolates of both types of bacterial strains (seven gram-positive and four gram-negative), most of them being pathogenic in behavior, were selected. The bacterial strains used for the study are Staphylococcus aureus (S. aureus), Streptococcus mutans (S. mutans), Bacillus cereus (B. cereus), Streptococcus pyrogenes (S. pyrogenes), Streptococcus viridans (S. viridans), Staphylococcus epidermidis (S. epidermidis), and Corynebacterium xerosis (C. xerosis) gram-positive bacteria and Escherichia coli (E. coli), Klebsiella pneumoniae (K. pneumoniae), Proteus vulgaris (P. vulgaris) and Pseudomonas aeruginosa (P. aeruginosa) gram-negative bacterial strains. The selected bacterial strains were developed on Nutrient Agar No.2 (Himedia Labs Pvt. Ltd., Bombay, India). The antimicrobial activity of synthesized SAILs was carried out by using agar well diffusion method. After filling each Petri plate with 20 ml of agar media and swabbing the plates with bacterial colony, the wells were bored into the agar seeded plates using sterile cork borer of 6 mm diameter. Each well was filled with 100 μl solutions of the prepared compounds consisting of 1 mg SAILs in 1 ml of DMSO and left for incubation for 24 h at 37 °C. The dimethylsulphoxide (DMSO)-containing medium was used as negative control whereas Norfloxacin and Gentamicin (Himedia Labs Pvt. Ltd., Mumbai, India) were used as positive control.
3.1.3. UV–visible measurements For further confirmation of our CMC results, we have performed the quickest and simplest UV–visible spectrophotometer method. A representative absorption spectrum for [C14eim]Br is shown in Fig. 2(a). From the plots of absorbance vs [SAILs], the CMC values were obtained from the point of intersection of two straight lines as shown in Fig. 2(b) for [C14eim]Br and the observed CMC values are given in Table 2. With increasing hydrocarbon chain length, the CMC values were found to follow trend similar to the previous two techniques. The CMC values determined from above mentioned techniques were found to vary slightly showing that the CMC is a method dependent parameter [36]. 3.2. Thermodynamic properties of SAILs The conductivity data of SAILs was used to elucidate the different thermodynamic parameters in order to get a deep understanding of their micellization behavior. The degree of dissociation (α) of SAILs has been calculated from the slopes of conductivity i.e. by taking the ratio of the post-micellar (S2) to pre-micellar (S1) slopes [37]. The degree of counterion binding (β) has been evaluated by substituting the value of the degree of dissociation (α) in the following given formula and the calculated parameters are listed in Table 1. It has been found that the β values show an inverse relation with both the temperature and hydrocarbon chain length.
3. Results and discussion 3.1. Determination of self- aggregation behavior of SAILs The self-aggregation behavior of imidazolium-based SAILs in aqueous solution was determined by employing different techniques which include conductometry, tensiometry, fluorescence and UV–visible spectroscopy.
(1)
= s2 /s1
3.1.1. Conductivity measurement The plots between specific conductivity vs [Cneim]Br where n = 12, 14 and16 at different temperatures were used for determining the CMC of these SAILs and are provided in Table 1. The conductivity plots were such that two straight lines of different capacities traverse at a particular point termed as critical micelle concentration (CMC). A representative plot for [C14eim]Br is shown in Fig. 1(a). The CMC values of these three SAILs determined from conductivity plots at different temperatures increased with increasing temperature but decreased with increasing length of alkyl chain (Table 1). This behavior occurs due to the breakage of water cluster surrounding the hydrophobic chains which in turn delays the micellization process at a higher temperature.
= (1
)
(2)
The pseudo-phase model meant for 1:1 type of ionic surfactants has been used to calculate the standard Gibbs free energy of micellization ( Gmo ) [38]:
Gmo = (1 + ) RT ln XCMC
(3)
where XCMC represents the CMC in terms of mole fraction, β shows the degree of counterion binding to the micellar interface. The standard enthalpy of micellization ( Hmo ) and standard entropy of micellization ( Smo ) have been evaluated by employing the GibbsHelmholtz equation as given below: 3
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Table 1 Critical micelle concentration (CMC) determined from conductivity measurements, degree of counterion binding (β), standard Gibbs free energy of micellization ( Gmo ), standard enthalpy of micellization ( Hmo ), standard entropy of micellization (T Smo ) for SAILs at different temperatures (T/K). SAILs
[C12eim]Br [C14eim]Br [C16eim]Br
T
CMC
β
(K)
(mmol·L−1)
298.15 303.15 308.15 298.15 303.15 308.15 298.15 303.15 308.15
8.08 8.30 8.61 1.17 1.19 1.37 0.42 0.44 0.57
o Gm
0.431 0.362 0.299 0.276 0.248 0.243 0.258 0.233 0.181
(mScm-1 )
80 60
(a)
40 20
0.0
0.5
1.0
1.5
2.0
(kJmol−1)
(kJmol−1)
−31.33 −30.23 −29.18 −34.63 −33.82 −33.24 −37.99 −37.08 −34.77
−3.90 −3.95 −4.25 −5.66 −7.07 −8.25 −8.99 −9.50 −9.64
27.43 26.28 24.93 28.97 26.75 24.99 29.00 27.58 25.13
The surface parameters such as surface tension at CMC (γCMC), effectiveness of the adsorption efficiency (pC20), surface pressure at CMC (πCMC), surface excess concentration (Γmax) and minimum surface area per molecule at the interface (Amin) of three synthesized SAILs were evaluated according to the formulae described below and their values areprovided in Table 2. The adsorption efficiency (pC20) of SAILs at the interface was evaluated by employing the given Eq. (6) [39]:
2.5
C (mmol . L-1) 70 [C12 eim]Br
65
[C14 eim]Br [C16 eim]Br
60
(mNm-1 )
(kJmol−1)
3.3. Evaluation of interfacial parameters of SAILs
0
pC20 =
55 50
40 35 30 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
log [SAILs] (mmol . L ) -1
CMC
Fig. 1. (a) A representative plot of conductivity(κ) vs concentration(C) of [C14eim]Br for the determination of CMC at 308.15 K and (b) Surface tension plots against log[SAILs] for the determination of the CMC of SAILs, [Cneim]Br (n = 12,14,16) at 298.15 K.
RT 2 (1 + )d ln XCMC / dT
Smo = ( Hmo
Gmo )/T
1 log C20
(6)
where C20 is the concentration of SAILs obtained by decreasing the surface tension of pure solvent by 20 mNm−1. The pC20 values of these SAILs are given in Table 2. The increased pC20 values show that the adsorption efficiency of SAILs also increases with the increase in hydrocarbon chain length. Moreover, the surface pressure at CMC has also been evaluated by putting the value of surface tension of solvent (γ0) and those of SAILs solution at the CMC (γCMC), in Eq. (7) given below [39]:
(b)
45
Hmo =
T Smo
become more negative with increasing temperature because the structured water molecules surrounding the hydrocarbon chain of SAILs break at high temperature. From Table 1, it is clear that the large and positive entropy (T Smo ) values for all three SAILs prove that the process of micellization is entropy driven in the entire temperature range being considered. It shows that the entropy provides the force for the formation of the micelles which occurs when the hydrophobic group moves from the solvent to the micellar core and thus favors the micellization process and this process is promoted further with the increase in the hydrocarbon chain length.
cmc
100
o Hm
=
(7)
0 – CMC
The values of γCMC (Table 2) show that the SAILs become more efficient in the reduction of the surface tension with their increasing hydrophobic chain length. However, πCMC increases with the increasing hydrophobicity of SAILs which accounts for their increased efficiency with the increasing carbon chain length. The adsorption efficiency of SAILs at air-liquid interface is evaluated in terms of surface excess concentration (Γmax), through the Gibbs adsorption Eq. (8) [40]:
(4) (5)
All the thermodynamic parameters are provided in Table 1. It was observed that the values of Gmo are negative in the studied range of temperature for all the three SAILs which show the spontaneity of micellization in their aqueous solutions. The Gmo values were more negative for SAILs bearing longer hydrocarbon chain, showing that the micellization process is more spontaneous with a bigger hydrocarbon chain. The negative Hmo values exhibit that the micellization process is exothermic. Furthermore, it has also been found that the values of Hmo
max
=
1 2.303nRT
log C
T
(8)
where γ, R, T and C have their usual meanings, n stands for the number of ionic species present in the solution formed by the dissociation of SAIL molecules. The value of n is taken as 2 for SAILs ([Cneim]Br) as all the systems contain imidazole ring with one positive charge and similar counterion i.e. bromine with one negative charge on it. The values of 4
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Table 2 Critical micelle concentration (CMC), surface tension at CMC (γCMC), adsorption efficiency (pC20), surface pressure at CMC (πCMC), surface excess concentration (Γ o max), minimum surface area per molecule (Amin), standard Gibbs free energy of adsorption ( Gad ), aggregation numbers (Nagg) of SAILs at 298.15 K. SAILs
[C12eim]Br [C14eim]Br [C16eim]Br
πCMC
Γmax. 109
Amin. 105
(mNm−1)
(molm−2)
Å2
(kJmol−1)
1.71
37.83
5.62
0.295
−36.96
61
34.07
2.42
37.93
5.72
0.290
−41.26
80
31.98
2.65
40.02
5.82
0.285
−44.87
101
CMC
γCMC
(mmol·L−1)
(mNm−1)
(7.03)a (6.07)b (1.15)a (1.02)b (0.52)a (0.62 (0.62)b
34.17
pC20
o Gad
Nagg
Determined by aSurface tension, bUV‐–visible spectroscopy.
largest as compared to lower members of the series i.e. [C14eim]Br and [C12eim]Br. This indicates that the micelles of longer chain-containing [C16eim]Br SAIL are closely packed at interface due to greater van der Waals forces in the hydrophobic part in comparison to shorter chaincontaining [C14eim]Br and [C12eim]Br SAILs. Further, the Amin values (Table 2), show an opposite trend to Γmax values because the values of Amin are also influenced by the hydrophobic part of SAILs. Therefore, values of the Amin parameter are in the order; [C12eim]Br > [C14eim]Br > [C16eim]Br. This reveals that the micelles of SAILs become more compact or closely packed when we move from lower member ([C12eim]Br) to the higher member of the series ([C16eim]Br) (Scheme 2). o The standard Gibbs free energy of adsorption, Gad [42] at interface can be evaluated by using the following Eq. (10): o Gad =
Gmo
CMC max
(10)
where πCMC stands for surface pressure, a parameter which measures the effectiveness of SAILs to lower the surface tension. From Table 2 we o have observed that all the Gad values for three SAILs were negative and greater in magnitude than the Gmo values. It shows that the adsorption phenomenon is spontaneously favorable. Moreover, the higher magnitude exhibits that the adsorption process which is a surface process occurs prior to the micelle formation which is a bulk phenomenon. 3.4. Determination of aggregation numbers of SAILs Steady-state fluorescence quenching method was also used to evaluate the aggregation numbers of the three imidazolium-based SAILs in aqueous solution. The Nagg was calculated by quenching of pyrene through the CPC according to the relation given in Eq. (11) [43]:
ln
Fig. 2. (a) UV‐–visible spectra of aqueous solution of [C14eim]Br with inset image showing the magnified spectra in the wavelength range 230–360 nm and (b) Determination of CMC from the plot of Absorbance vs concentration (C) of [C14eim]Br at 298.15 K.
log C T
constant temperature. The minimum surface area of three SAILs at air-liquid interface was evaluated from the following Eq. (9) [41]:
10 20 NA max
(11)
where I0 and I show fluorescence intensities of pyrene in absence and presence of quencher respectively. [C] and [CPC] are the concentrations of the studied SAILs and quencher respectively. Fluorescence emission spectra of pyrene showing its quenching by CPC is given in Fig. 3 and the inset plot of ln(I0/I) vs [CPC] is used to determine aggregation numbers of SAILs. The plot clearly shows the linear relation for all the SAILs. The calculated values of Nagg for the three SAILs are listed in Table 2. The aggregation numbers of three SAILs increase in aqueous solution as the alkyl chain length increases within the series. The Nagg values of these SAILs were found to be higher than those reported for 1-alkyl-3-methylimidazolium chlorides [11].
( ) were obtained from the slopes of surface tension plots at a Amin =
I0 [CPC ] = Nagg I [C CMC ]
(9)
where NA represents the Avogadro's number and factor 1020 is used to convert the area from nm2 to Å2. The Γmax and Amin values are given in Table 2. The Γmax values depend upon (a) van der Waals forces between the hydrophobic parts of SAILs (b) steric factor due to the presence of bulky groups in these SAILs. The value of Γmax for [C16eim]Br is the
3.5. Antimicrobial studies The ZOI data (mm/mg) calculated by agar well diffusion method shows that all the synthesized SAILs exhibit significant activity against 5
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Scheme 2. Pictorial representation of the effect of alkyl chain length on self-aggregation behavior and micellar surface area of SAILs, [Cneim]Br (n = 12, 14, 16).
that among gram-positive strains, the SAILs, [C12eim]Br and [C14eim] Br showed effective antimicrobial activity against S. pyrogenes and C. xerosis respectively {Fig. 4(a)}. On the other hand, [C16eim]Br showed effective antimicrobial behavior against S. epidermidis and C. xerosis. Among gram-negative bacterial strains, maximum activity was shown by both [C12eim]Br and [C14eim]Br against K. pneuomoniae whereas [C16eim]Br proved to be more potent against E. coli as shown in Fig. 4(b). During the antimicrobial screening of these synthesized SAILs, we have observed that the treatments with [C12eim]Br do not let the growth of bacterial colony in most of the species; as no colonies were formed after incubation period. The positive control (Norfloxacin and Gentamicin) showed a moderate activity for all the bacterial species while DMSO, which was used as negative control did not cause any inhibition to the growth of the bacterial cultures; indicating the inert nature of DMSO at the concentration used. [C12eim]Br was more effective in controlling the bacterial growth as compared to higher chain members especially for the gram-negative strains. This may be due to most favorable hydrophilic-lipophilic balance in their structure which is a cause of their preferential adsorption at bacterial cell wall and results in their better biological activity [24]. All the SAILs were equally effective for most of the strains in controlling the bacterial growth when compared to the respective positive control (Table S1).
Fig. 3. Fluorescence emission spectra showing quenching of pyrene by CPC in aqueous solutions of SAILs with inset plots of ln I0/I vs [CPC] for the determination of aggregation numbers of SAILs [Cneim]Br (n = 12,14,16).
the studied bacterial strains except [C16eim]Br which was found to be inactive towards the gram-negative bacterium K. pneumoniae. Though the antimicrobial activity of SAILs is primarily decided by their degree of adsorption on bacterial cell wall followed by penetration inside the cell to disrupt the membrane permeability of the cell. However, the overall biological activity at a specific hydrocarbon chain length is governed by the combined effects of various physicochemical parameters like CMC, hydrophobicity, adsorption and deposition on cell wall, aqueous solubility and passage through medium. The resistance of gram-negative bacterium K. pneumoniae towards [C16eim]Br may be attributed to the restricted entry of 16-C SAILs into the bacterial cell through cell wall, which would subsequently disrupt the underlying cell membrane or peptidoglycan cell wall. Unlike the gram-negative bacteria, the cell wall of gram-positive bacteria allows the free adsorption of SAILs over its surface and finally leads them to the inner cell membrane [44]. From Table S1 (supplementary file), it is clearly observed
4. Conclusion It can be summarized that we have synthesized higher members of a series of ethyl-substituted imidazolium-based SAILs and characterized them by FT-IR, 1H NMR and 13C NMR spectroscopic techniques. The self-aggregation behavior of these synthesized SAILs was observed by different methods such as conductometric, tensiometric, fluorimetric measurements and UV–visible spectroscopy. The CMC of [C14eim]Br and[C16eim]Br show lower values as compared to [C12eim]Br, which is mainly due to the greater hydrophobicity of the former as compared to 6
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Fig. 4. Effect of different SAILs on the bacterial growth shown by Zone of inhibition (ZOI) (a) gram-positive bacterial strain (C. xerosis) and (b) gram-negative bacterial strain (E. coli) against their respective positive controls.
the latter. We have also studied the influence of temperature on CMC of SAILs by conductometric method and found that CMC is directly proportional to the temperature. The thermodynamic parameters, Gmo , Hmo and Smo , determined showed that the micellization process is thermodynamically favorable. The Γmax values for [C16eim]Br is highest and shows that the surfactant monomers are closely packed at the interface. This is also supported by its lower Amin value which also confirms the close packing of surfactant monomers of [C16eim]Br. Further, the surface and aggregation behavior of ethyl-substituted SAILs were found to be better than the methyl-substituted SAILs which is again the result of their superior hydrophobic interactions. The antimicrobial data which was determined presents significant activities of these SAILs against clinically relevant microorganisms. The antimicrobial activity of [C12eim]Br was found to be greater than the higher members of the series. The imidazolium-based SAILs possess better self-aggregation property, surface and antimicrobial activity therefore, they will provide wide applications in nanostructured materials, biocatalytic activities, pharmaceutical, agricultural, biotechnological and refinery industries.
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Declaration of Competing Interest None. Acknowledgments Authors acknowledge UGC (DRS II) and DST (FIST & PURSE), India for providing research support. Arifa Shaheen acknowledges the University Grants Commission (New Delhi, India) for the UGC-BSR Startup Grant (No.F.30-310/2016). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.colcom.2019.100204. References [1] P.S. Gehlot, H. Gupta, A. Kumar, Paramagnetic surface active ionic liquids: interaction with DNA and MRI application, Colloids Interface Sci. Commun. 26 (2018) 14–23, https://doi.org/10.1016/j.colcom.2018.07.004. [2] A. Pal, M. Saini, Effect of alkyl chain on micellization properties of dodecylbenzenesulfonate based surface active ionic liquids using conductance, surface tension and spectroscopic techniques, J. Dispers. Sci. Technol. (2019) 1–10, https://doi. org/10.1080/01932691.2019.1593859.
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