Micellization Behavior of Morpholinium-Based Amide-Functionalized

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Micellization Behavior of Morpholinium-Based Amide-Functionalized Ionic Liquids in Aqueous Media Raman Kamboj,† Pankaj Bharmoria,‡ Vinay Chauhan,† Sukhprit Singh,*,† Arvind Kumar,‡ Venus Singh Mithu,† and Tejwant Singh Kang*,† †

Department of Chemistry, University Grants Commission (UGC) Centre for Advanced Studies-I, Guru Nanak Dev University, Amritsar-143005, Punjab, India ‡ Academy of Scientific and Innovative Research (AcSIR) and §Salts and Marine Chemicals Division, CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific & Industrial Research (CSIR), G. B. Marg, Bhavnagar-364002, Gujarat, India S Supporting Information *

ABSTRACT: Morpholinium-based amide-functionalized ionic liquids (ILs) [CnAMorph][Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior in aqueous medium using a variety of state of the art techniques. The adsorption and micellization behavior of [CnAMorph][Br] ILs at the air−solution interface and in the bulk, respectively, has been found to be much better compared to that observed for nonfunctionalized homologous ILs and conventional cationic surfactants, as shown by the comparatively higher adsorption efficiency, lower surface tension at the critical micelle concentraiton (γcmc), and much lower critical micelle concentration (cmc) for [CnAMorph][Br] ILs. Conductivity measurements have been performed to obtain the cmc, degree of counterion binding (β), and standard free energy of micellization (ΔGm°). Isothermal titration calorimetry has provided information specifically about the thermodynamics of micellization, whereas steady-state fluorescence has been used to obtain the cmc, micropolarity of the cybotactic region, and aggregation number (Nagg) of the micelles. Both dynamic light scattering and atomic force microscopy have provided insights into the size and shape of the micelles. 2D 1H−1H nuclear Overhauser effect spectroscopy experiments have provided insights into the structure of the micelle, where [C16AMorph][Br] has shown distinct micellization behavior as compared to [C8AMorph][Br] and [C12AMorph][Br] in corroboration with observations made from other techniques.

1. INTRODUCTION Ionic liquids (ILs) have gained much attention from the scientific community around the globe in the past decade owing to their unique physicochemical properties.1,2 The possibility of fine-tuning the characteristic properties of ILs by judicious choice of their constituent ions makes them more suitable for use in diverse applications.3−11 The other attractive aspect of many of the ILs is their inherent amphiphilic nature due to the presence of a hydrophobic alkyl chain and hydrophilic headgroup, which place them in the category of surfactants. A variety of ILs have been investigated for their aggregation and self-assembly behavior in aqueous media.12−41 Maximally investigated ILs are based on the 1-alkyl-3-methylimidazolium cation ([Cnmim]+) with a variety of counterions.12−36,40 The alkyl chain length of [Cnmim]+ and the nature of the anions have been shown to affect both the micellization properties and the shape of the micelles similarly to those of conventional cationic surfactants. Recently, our research group reported [Cnmim]+-based biamphiphilic ILs, which exhibit much better micellization properties as compared to various analogous anionic surfactants.32 Some other types of ILs based on alkylpyridinium ([CnPy]+), alkylpyrrolidinium ([CnmPyrr]+), alkylpiperidinium ([Cnmpip]+), and even amino acid-based © XXXX American Chemical Society

cations have been reported, where the nature of the cationic headgroup has been found to alter the micellization behavior of ILs.20,39,42 However, no in-depth investigations have been made on the micellization behavior of morpholinium cation ([CnMorph]+)-based ILs with the exception of a single report where physicochemical interactions of cyclodextrin with morpholinium-based ILs has been described.43 Furthermore, a little effort has been made to modify the cationic headgroup of ILs, which is expected to modify the selfassembly behavior of ILs by altering the balance of hydrophobic/hydrophilic and electrostatic interactions.29,44 Another important part of any surfactant is the hydrophobic alkyl chain, which can be functionalized with a variety of functional groups, expected to alter the micellization behavior via modification of the extent of hydrophobic interactions among alkyl chains. In this regard, carboxyl-modified [Cnmim]+-based ILs have been reported which have shown enhanced surface activity over [Cnmim]+-based ILs.35 Recently, Garcia et al. reported the aggregation behavior and antimicrobial properties of esterReceived: May 17, 2014 Revised: July 24, 2014

A

dx.doi.org/10.1021/la501897e | Langmuir XXXX, XXX, XXX−XXX

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functionalized imidazolium ([CnEmim]+)-based and pyridinium ([CnEPyr]+)-based ILs.36 Ester-functionalized ILs have shown a higher adsorption efficiency at the air−solution interface while exhibiting a significantly lower critical micelle concentration (cmc) as compared to homologous ILs bearing a simple alkyl chain as the hydrophobic part. No report exists in the literature where amide-functionalized ILs have been investigated for their micellization behavior, although a variety of amide-functionalized conventional surfactants have been reported.45−47 The presence of an amide group is anticipated to affect the micellization behavior due to its ability to form a hydrogen bond. Therefore, looking at the aspects discussed above, we aimed at synthesizing and characterizing the micellization behavior of morpholinium-based amide-functionalized ILs [CnAMorph][Br] in aqueous media. The novel aspect of this work lies in the fact that amide-functionalized morpholiniumbased ILs in particular and amide-functionalized surfactant-like ILs with any other cationic headgroup have not been reported. These ILs have been found to be better surfactants as compared to homologous nonfunctionalized ILs or conventional cationic surfactants. Furthermore, a relatively hydrophobic headgroup in conjunction with a long alkyl chain appended with a Hbonding-prone amide functionality in the case of [C16AMorph][Br] has been found to form structurally different micelles as compared to other investigated counterparts. Pursuant to our continued research in this area,37,38,41 herein we report the synthesis and micellization behavior of a new series of morpholinium-based amide-functionalized ILs, [CnAMorph][Br] (4-methyl-4-(2-(octylamino)-2-oxoethyl)morpholin-4-ium bromide for n = 8, 4-methyl-4-(2-(dodecylamino)-2-oxoethyl)morpholin-4-ium bromide for n = 12, and 4methyl-4-(2-(hexadodecylamino)-2-oxoethyl)morpholin-4-ium bromide for n = 16) in aqueous medium. The role of the alkyl chain length in altering the key properties of micellization has been investigated and compared with that of homologous nonfunctionalized ILs or conventional cationic surfactants. These investigations have been carried out utilizing a set of state of the art techniques, viz., tensiometry, conductivity, isothermal titration calorimetry (ITC), steady-state fluorescence, dynamic light scattering (DLS), and atomic force microscopy (AFM) measurements. 2D 1H−1H NOESY (nuclear Overhauser effect spectroscopy) experiments have been performed to gain insight into the structure of the micelles. The observed NOESY cross-peaks in micelles of different [CnAMorph][Br] ILs supported the observations made from the other techniques very well.

Scheme 1. Molecular Structure of the Investigated ILs

under a N2 atmosphere from 30 to 400 °C with a heating rate of 10 °C min−1. Surface tension measurements were performed using a DataPhysics DCAT II automated tensiometer employing the Wilhelmy plate method within an accuracy of ±0.1 mN m−1. The specific conductance was measured using a digital conductivity meter (Systronics 308) employing a cell of unit cell constant. Measurements were performed in triplicate with an uncertainty of less than 0.5%. The isothermal titration calorimetric measurements were made using a MicroCal ITC200 microcalorimeter equipped with an instrumentcontrolled Hamilton syringe having a volume capacity of 40 μL. Steady-state fluorescence was performed using a PerkinElmer LS-55 luminescence spectrometer with pyrene as the external fluorescent probe at an excitation wavelength of 334 nm and an excitation and emission slit width of 2.5 nm. The concentration of pyrene was kept as 2.0 × 10−6 M to avoid the formation of an excimer. In a typical steadystate fluorescence titration, a stock solution of the respective IL having a concentration 10 times greater than the cmc was added to 2.5 mL of pyrene solution in a stepwise manner. Before the spectra were recorded, the sample was stirred for 2 min followed by stabilization for 5 min. A pyrene fluorescence quenching experiment was performed to obtain the aggregation number of the micelles (Nagg) using cetylpyridinium chloride as the quencher. For quenching experiments, to 2.5 mL of the stock solution of the respective IL above the cmc having 2.0 × 10−6 M pyrene in a quartz cuvette was added a stock solution of quencher in water. Prior to the measurements, the samples were stirred and equilibrated for 2 and 5 min, respectively. The concentrations of the stock solutions of the ILs used to determine Nagg are provided in a footnote of Table 2. For surface tension, conductivity, and fluorescence measurements, the temperature was controlled with a Julabo water thermostat within ±0.1 K. DLS measurements were performed using a light scattering apparatus (Zetasizer, Nano series, Nano-ZS, Malvern Instruments) equipped with a built-in temperature controller having an accuracy of ±0.1 K. The measurements were made using a quartz cuvette having a path length of 1 cm. The average of 10 measurements is reported as the experimental data. The data were analyzed using the standard algorithms and are reported with an uncertainty of less than 7%. AFM was performed using an Ntegra Aura atomic force microscope (NT-MDT, Russia) in semicontact mode using an NSG-01 silicon probe. The samples were prepared by putting a drop of the sample solution at twice the concentration of the cmc of the respective IL on a thin mica sheet followed by drying in air for 48 h prior to the measurements. 2D 1H−1H NOESY and ROESY (rotating-frame nuclear Overhauser effect spectroscopy) experiments were performed with water suppression using a standard three-pulse sequence and a mixing time of 300 ms in a D2O−H2O mixture ([D2O] = 10%, v/v). All measurements were made on a Bruker Ascend 500 spectrometer (AVANCE III HD console) using a 5 mm BBO (broad-band observe) double-channel probe equipped with z-gradients.

2. MATERIALS AND METHODS 2.1. Materials. Bromoacetyl bromide, 1-aminoooctane (>99%), 1aminododecane (>99%), and 1-aminohexadecane (>98%) were purchased from Sigma-Aldrich and used without further purification. N-Methylmorpholine was also purchased from Sigma-Aldrich (99%). Dichloromethane, acetone, and diethyl ether (AR grade) were purchased from SD Fine-Chem Ltd., Mumbai, India. Pyrene was purchased from Sigma-Aldrich and used after recrystallization from ethanol. Cetylpyridinium chloride (> 99.0%) was purchased from Sigma-Aldrich and used without further purification. The synthesis procedure of [CnAMorph][Br] ILs and characterization data are provided in Scheme S1 and Annexure S1 (Supporting Information). Scheme 1 shows the molecular structure of the investigated ILs. 2.2. Methods. Differential scanning calorimetry (DSC) was performed employing a Mettler Toledo DSC822 thermal analyzer at a scan rate of 10 °C min−1. Thermogravimetric analysis (TGA) was performed on a TGA/SDTA851 Mettler Toledo thermogravimeter

3. RESULTS AND DISCUSSIONS 3.1. Melting Point and Thermal Stability. Figure S1 (Supporting Information) shows the DSC thermograms for the investigated ILs showing melting points of 51, 57, and 58 °C for [C8AMorph][Br], [C12AMorph][Br], and [C16AMorph][Br], respectively. Here it is important to mention that although the conventional ionic surfactants are comprised of ions similar to those of ILs, they cannot be placed in the category of ILs as, in general, conventional ionic surfactants possess a relatively high melting point, usually >100 °C. The B


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alkyl chain. The decrease in cmc by a factor of 4.9 with the addition of a methyl group on going from n = 8 to n = 12 is much higher than that reported for [Cnmim]+-based ILs, where it decreases by a factor of 2.23.18 The decrease is even much higher as compared to that of conventional cationic surfactants such as alkylpyridinium bromides, alkylpyridinium chlorides, and alkyltrimethylammonium bromides, where it is around 2.20,48,49 reflecting the greater tendency of [CnAMorph][Br] ILs to form micelles with increasing alkyl chain length over nonfunctionalized ILs or conventional cationic surfactants. Furthermore, the cmc of [C16AMorph][Br] is 3.9-fold lower compared to that of its nonfunctionalized counterpart, [C16Morph][Br].43 This behavior can be assigned to the presence of the bulky morpholinium headgroup along with the presence of the amide moiety in the alkyl chain, prone to take part in H-bonding. Various thermodynamic parameters have been calculated using surface tension measurements and are provided in Table 1. As can be seen from Table 1, γ at the cmc (γcmc) for different [CnAMorph][Br] ILs follows the order [C8AMorph][Br] < [C12AMorph][Br] < [C16AMorph][Br], indicating the greater efficiency of [C8AMorph][Br] in reducing γ at the air−solution interface. For a homologous series, γcmc for [CnAMorph][Br] is comparable with that of ester-functionalized ILs ([CnEmim][Br] and [CnEPyr][Br]) and is much lower than that of nonfunctionalized [Cnmim]+- or [Cnmpip]+-based ILs.22,36,39 This indicates that the amidefunctionalized [CnAMorph][Br] ILs possess a better surface activity than other reported homologous nonfunctionalized ILs. The surface tension reduction, πcmc, and adsorption efficacy, pC20, are two additional important parameters which can be deduced from surface tension measurements as follows:50 πcmc = γ − γcmc (1)

thermal stability of the ILs under investigation in terms of temperature corresponding to the onset of weight loss (Tonset) was determined by thermogravimetric analysis. The profiles of TGA and corresponding DTG (differential thermogravimetry) profiles are provided in Figure S2 (Supporting Information). Tonset for [CnAMorph][Br] ILs is compared with that for [CnEmim][Br] and [CnEPyr][Br] as well as nonfunctionalized imidazolium ([Cnmim][Br]) and pyridinium ([CnPyr][Br]) ILs in Table S1 (Supporting Information). With the progressive elongation of the alkyl chain, Tonset increases similarly to that of nonfunctionalized ILs.1 Depending on the length of the amidefunctionalized alkyl chain, [CnAMorph][Br] ILs exhibit singlestep weight loss with the corresponding temperature of weight loss at 237 °C for [C8AMorph][Br], a two-step weight loss at 232 °C (40% mass loss) and 280 °C (52% mass loss) for [C12AMorph][Br], and a three-step weight loss at 205 °C (12% mass loss), 308 °C (62% mass loss), and 374 °C (15% mass loss) for [C16AMorph][Br]. It can be seen from Table S1 that, for a homologous series, nonfunctionalized ILs exhibit better thermal stability as compared to amide- or ester-functionalized ILs. The role of the structure of the headgroup in altering Tonset cannot be ruled out. 3.2. Tensiometry. Figure 1 shows the variation of the surface tension (γ) as a function of the concentration of

and

pC20 = −(log C20)

(2)

where γ is the surface tension of water, γcmc is the surface tension of the solution at the cmc, and C20 is the molar concentration of IL required to reduce the γ of water by 20 mN m−1. The values of pC20 and πcmc are provided in Table 1. The value of πcmc decreases progressively with an increase in the alkyl chain length, contrary to what is observed for imidazolium- or ester-functionalized imidazolium- and pyridinium-based ILs.22,36 This is in line with the variation of γcmc as a function of the alkyl chain length. It is natural to assume that, with an increase in the alkyl chain length, the conformational flexibility of [CnAMorph]+ having a bulky morpholinium headgroup in the vicinity of the amide functionality increases, thereby providing more orientational freedom to the bulky morpholinium headgroup. Therefore, such behavior can be due to a varying orientation of the bulky morpholinium headgroup

Figure 1. Variation of the surface tension, γ, in aqueous solutions of various amide-functionalized morpholinium-based ILs as a function of ln C.

[CnAMorph][Br] ILs in aqueous solutions at 298.15 K. The clear break point in the tensiometric profiles gives the cmc, the values of which are provided in Table 1. The cmc decreases with an increase in the alkyl chain length of the investigated ILs following a linear relationship between ln cmc and the number of carbon atoms in the alkyl chain, as shown in Figure S3 (Supporting Information). The cmc decreases by a factor of 4.9 on going from n = 8 to n = 12 and by a factor of 2.7 on going from n = 12 to n = 16 per unit increase in methyl groups in the

Table 1. Critical Micelle Concentration (cmc) Determined Using the Surface Tension (ST), Conductivity (Cond), Steady-State Fluorescence (Flr), and Isothermal Titration Calorimetry (ITC) and Various Parameters Obtained from ST Measurements at 298.15 K for Various ILsa cmc/mM

a

params obtained from ST measurements

IL

ST

Cond

Flr

ITC

av

γcmc

πcmc

pC20

106Γ

Amin

ΔGad°

[C8AMorph][Br] [C12AMorph][Br] [C16AMorph][Br]

58.8 3.1 0.28

78.1 5.02 0.46

63.0 3.20 0.30

69.2 2.38 0.22

67.3 3.4 0.32

25.0 29.5 39.7

47.0 42.5 32.3

3.29 3.65 4.38

1.11 1.39 1.75

149 119 95

−67.4 −68.0 −56.1

Units: γcmc and πcmc, mN m−1; 106Γ, μmol m−2; Amin, Å2; ΔGad°, kJ mol−1. C


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Figure 2. Variation of the specific conductance, κ (filled symbols), in aqueous solutions of various amide-functionalized morpholinium-based ILs as a function of the concentration: (A) [C8AMorph][Br], (B) [C12AMorph][Br], (C) [C16AMorph][Br]. Hollow symbols represent the first derivative of the specific conductance as proposed by Carpena et al.51

Figure 3. Calorimetric profiles of amide-functionalized morpholinium-based ILs in aqueous solutions at 298.15 K: (A) [C8AMorph][Br], (B) [C12AMorph][Br], (C) [C16AMorph][Br].

to that of the nonfunctionalized ILs or conventional cationic surfactants. Furthermore, the standard free energy of adsorption, ΔGads°, was calculated employing the standard methods (Table 1) and follows the order [C16AMorph][Br] < [C8AMorph][Br] ≈ [C12AMorph][Br]. For all the investigated ILs, the higher magnitude of ΔGads° compared to ΔGam° indicates that adsorption at the air−solution interface is more spontaneous than micellization in the bulk. 3.3. Conductometry. Figure 2 shows the variation of the specific conductance in aqueous solutions of the investigated ILs as a function of the concentration. A clear break point in the κ vs C profiles is indicative of the cmc. The cmc has been obtained by treating the experimental data with the method given by Carpena et al.51 The obtained cmc values are provided in Table 1 and are in line with those obtained from tensiometry. As can be seen from Figure 3A, the fitting of specific conductivity data using the method proposed by Carpena et al. shows two break points, one at a relatively lower concentration, designated as C1, and the other at a higher concentration corresponding to the cmc in the case of [C8AMorph][Br]. At C1, the formation of premicelles is considered to take place, which has been verified from the profiles of the molar conductance as a function of the square root of the concentration as shown in Figure S4 (Supporting Information). In other ILs with longer alkyl chain lengths, the absence of premicelles is determined to be due to relatively greater hydration of the cationic headgroup as indicated by steady-state fluorescence and 1H NMR spectroscopy (discussed later). The degree of counterion binding (β), which gives the

at the air−solution interface. However, pC20 increases with an increase in the alkyl chain length, similar to what is observed for nonfunctionalized morphlinium-based ILs,43 indicating an increase in the efficacy of the investigated ILs to adsorb at the air−solution interface with an increase in the alkyl chain length. The maximum surface excess concentration, Γmax, and the area occupied by a single surfactant molecule at the air− water interface, Amin, which are mainly influenced by the size of the hydrophilic group and by the length of the alkyl chain, have been estimated by employing the Gibbs adsorption isotherm to the surface tension data.50 The values of Γmax and Amin are reported in Table 1. Γmax increases and Amin decreases with an increase in the alkyl chain length of [CnAMorph][Br], similar to what is observed for nonfunctionalized ILs and conventional cationic surfactants.29,50 For the same alkyl chain length, the amide-functionalized IL [C16AMorph][Br] exhibits a relatively lower Γmax and a higher Amin as compared to its nonfunctionalized counterpart, [C16Morph][Br].43 This indicates an increased extent of packing of [CnAMorph]+ at the air− solution interface with an increase in the alkyl chain length. It seems that two balancing forces exist: one is due to the presence of the amide group in the alkyl chain, which can take part in intermolecular H-bonding, and the other is due to the presence of the electronegative oxygen atom in the headgroup carrying a lone pair of electrons, which can take part in interactional forces (electrostatic and hydrogen-bonding) with protons of other [CnAMorph]+ cations or water and repulsions with negatively charged counterions. The result of these two factors governs the surface activity in a way superior or inferior D


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Table 2. Degree of Counterion Binding (β), Change in Standard Free Energy (ΔGm°), Enthalpy (ΔHm°), and Entropy (TΔSm°) of Micellization, Polarity Indicator (I1/I3) for the Cybotactic Region of Pyrene in Micelles, and Aggregation Number (Nagg) of Micelles of Various ILs at 298.15 Ka IL

βb

ΔGm°b

ΔGm°c

ΔHm°c

TΔSm°c

I1/I3d

Naggd

[C8AMorph][Br] [C12AMorph][Br] [C16AMorph][Br]

0.54 0.62 0.30

−25.1 −37.4 −37.7

−25.5 −40.3 −40.1

−1.3 −20.2 −10.9

24.2 20.1 29.2

0.85 1.23 1.34

46 39 24

The units of ΔGm°, ΔHm°, and TΔSm° are kilojoules per mole. The concentrations of [C8AMorph][Br], [C12AMorph][Br], and [C16AMorph][Br] used to determine Nagg are 138, 13, and 2.5 mmol kg−1, respectively. bConductivity. cIsothermal titration calorimetry. dSteady-state fluorescence. a

[C16AMorph][Br], indicating that the spontaneity of micellization increases with an increase in the alkyl chain length and pointing to the dominance of hydrophobic interactions in governing the micellization process. It is observed that, on going from [C12AMorph][Br] to [C16AMorph][Br], there is not much change in ΔGm°. At this point, it is speculated that the ionic headgroup in the case of [C16AMorph][Br] coils back toward the long alkyl chain. Such a condition is assumed to reduce the electrostatic repulsion between the ionic headgroups of the investigated ILs in micelles, leading to formation of loosely packed micelles with decreased counterion binding. 3.4. Isothermal Titration Calorimetry. Parts A−C of Figures 3 and S5 (Supporting Information) show the variation of the differential enthalpy (dH) and differential power (dP) as a function of the concentration of [C 8 AMorph][Br], [C12AMorph][Br], and [C16AMorph][Br], respectively. Figure S5 shows the presence of endothermic changes well below and above the cmc for all the investigated ILs, with the exception of [C16AMorph][Br], where a small exothermic change has been observed above the cmc. After micellization, both the formation of micelles and the dilution of micelles take place side by side. This exothermic change has been assigned to the dominance of enthalpy changes occurring due to dilution of the formed micelles as compared to the enthalpy changes occurring due to micellization. Different types of enthalpograms have been classified as A, B, or C on the basis of the changes in the differential enthalpy as a function of the concentration of the surfactant at a particular temperature.52 [C12AMorph][Br] exhibits a type A enthalpogram, whereas the type of enthalpogram exhibited by [C 8 AMorph][Br] and [C16AMorph][Br] is not certain. Many of the surfactants and even nonfunctionalized ILs have been shown to exhibit a type A enthalpogram, where a constant change in dH is observed below and above the cmc with a pronounced change at the cmc.32,33,52 The enthalpogram in the case of [C8AMorph][Br] looks similar to a type C enthalpogram with the exception that here an exponential decay followed by an exponential growth in dH is observed in the very dilute concentration regime, well below the cmc. Above the cmc, dH decreases continuously and does not reach a plateau. The observed change in dH is a function of many factors such as the apparent molar enthalpy of the micelles and monomers, aggregation number (Nagg), and counterion binding, etc. Below the cmc, the observed changes in dH originate from demicellization of the concentrated stock solution during titration along with hydration of the surfactant. As the apparent molar enthalpy of micelles is more dependent on the composition of the solution as compared to that of monomers because of the higher cmc for short-chain surfactants, it is inferred that the decrease in the apparent molar enthalpy of micelles in the dilute region leads to a negative change in dP and therefore to minima at C1. At C1, the formation of premicelles seems to take place as indicated by the

fraction of counterions associated with the micelle, is obtained using the ratio of the slopes (S2/S1) of linear fragments of κ vs C profiles below (S1) and above (S2) the cmc and is provided in Table 2. The values of β for the investigated ILs follow the order [C12AMorph][Br] > [C8AMorph][Br] > [C16AMorph][Br], indicating the formation of relatively compact micelles in the case of [C12AMorph][Br] as a consequence of increased counterion binding, which shields the electrostatic interactions between the ionic headgroups effectively. The degree of counterion binding increases on going from [C8AMorph][Br] to [C12AMorph][Br] as expected; however, unexpectedly, it decreases on going from [C12AMorph][Br] to [C16AMorph][Br]. Similar observations have also been made in the case of conventional ammonium- and sulfate-based cationic and anionic surfactants, where an increase in the alkyl chain length leads to a decrease in the cmc along with a decrease in counterion binding as a consequence of an increase in the area per headgroup of the surfactant ion at the surface of the micelle.50 Furthermore, the value of β for [C16AMorph][Br] has been found to be half that observed for its nonfunctionalized counterpart, [C16Morph][Br].43 A comparatively larger decrease in the degree of counterion binding in the case of [C16AMorph][Br] is attributed to the flexibility provided by the long alkyl chain in the vicinity of the amide group. This increases the availability of the amide group to water, leading to high hydration in the headgroup region. The increased extent of hydration decreases the relative hydrophobicity of the headgroup in [C16AMorph][Br] as compared to other investigated ILs, leading to a decrease in counterion binding. This is more prone to happen in the case of ILs appended with an amide group, where the formation of intermolecular Hbonding is expected, similar to that of conventional surfactants bearing an amide group.50 For the same alkyl chain length, β follows the order [CnAMorph][Br] < [CnEmim][Br] < [CnTAB] ≈ [Cnmim][Br].29,36,50 This indicates that the decrease in the cmc of [CnAMorph][Br] ILs as compared to other nonfunctionalized ILs or conventional cationic surfactants is not due to the increased counterion binding, but is due to the relative increase in size and hydrophobicity of the morpholinium headgroup along with the possible H-bonding interactions fashioned by the presence of the amide group. Furthermore, the pseudophase model of micellization has been applied to obtain the value of the standard free energy of micellization (ΔGm°) using the following equation:50 ΔGm° = (1 + β)RT ln Xcmc

(3)

where R is the gas constant, T is the temperature (K), Xcmc is the cmc (mole fraction), and β is the degree of counterion binding. The calculated values of ΔGm° are given in Table 2. The negative free energy change, which increases in magnitude on going from [C8AMorph][Br] to [C16AMorph][Br], follows the order [C 8 AMorph][Br] < [C 12 AMorph][Br] ≈ E


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Figure 4. (A) Variation of I1/I3 of pyrene emission as a function of the concentration of amide-functionalized morpholinium-based ILs. (B) Representative pyrene emission showing quenching of emission with increasing concentration of quencher. (C) Variation of ln(I/I0) as a function of the concentration of quencher, Cq. Lines in (A) and (C) are sigmoidal and linear fits of the data, respectively, with adjusted R2 > 0.997.

is considered the cmc, whereas a double differentiation was performed to obtain the values of the cmc for [C8AMorph][Br] and [C16AMorph][Br]. The values of cmc obtained by ITC measurements are in corroboration with those obtained from surface tension and conductivity measurements within the limits of the sensitivity of the technique used. As can be seen from Table 2, ΔGm° obtained from ITC measurements is very close in magnitude to that obtained from conductivity measurements. ΔHm° is negative for all investigated ILs and follows the order [C8AMorph][Br] < [C16AMorph][Br] < [C12AMorph][Br]. The order is the same as that observed for the variation in ΔGm°. The rankings of both ΔHm° and ΔGm° presented here correspond to their absolute values. Similarly, TΔSm° follows the order [C12AMorph][Br] < [C8AMorph][Br] < [C16AMorph][Br]. Comparing the variation in ΔHm° and TΔSm° to the variation in ΔGm°, as given in Table 2, it is inferred that, although similar to the micellization of conventional surfactants and nonfunctionalized imidazolium-based ILs, the micellization of [CnAMorph][Br] ILs is an entropy-driven phenomenon at 298.15 K, but for [C12AMorph][Br], the enthalpy factor equally contributes.33 3.5. Steady-State Fluorescence. Pyrene has been used as an external fluorescent probe as the ratio (I1/I3) of the intensity of the first vibronic peak (I1) to that of the third vibronic peak (I3) of pyrene fluorescence is very sensitive to a change in the polarity of the cybotactic region and has been employed to gain information about the cmc of conventional surfactants as well as ILs.15,22,25,29,56 I1/I3 decreases on going from polar to nonpolar solvents.56 Upon formation of a micelle, the solubilization of pyrene into the micelle leads to a decrease in I1/I3 due to a change in the polarity of the cybotactic region of pyrene. Figure 4 shows various observations made from fluorescence measurements. Figure 4A shows the variation of I1/I3 as a function of the concentration in aqueous solutions of the investigated ILs. The sigmoid variation of I1/I3 is characteristic of micelle formation, where a plateau at lower concentrations of ILs followed by a sharp decrease in I1/I3 upon micellization is observed, and I1/I3 again attains a constant value at concentrations higher than the cmc. The midpoint of the transition is presented as the cmc and is provided in Table 1. The values of the cmc obtained from fluorescence measurements are in corroboration with those obtained from other techniques. I1/I3 for the different investigated ILs varies in the order [C8AMorph][Br] < [C12AMorph][Br] < [C16AMorph][Br]. The order indicates

transition in conductivity vs concentration profiles (Figure 2), fitted using the method given by Carpena et al.,51 and molar conductivity vs square root of concentration profiles (Figure S4, Supporting Information) at similar concentration. An exponential growth in dH, reaching a maximum, followed by a continuous decrease in the values of dH is very similar to that observed for conventional short-chain surfactants showing type C enthalpograms.52 Similarly, in the case of [C16AMorph][Br], a continuous exponential decay in dH is observed as a function of the concentration without showing a plateau region. The enthalpogram is similar to that observed for [C8AMorph][Br] with the exception that no minimum in dH, at very low concentrations, much below the cmc, is observed. A continuous decrease without a prominent sudden change signifies the lesser spontaneity of micellization contrary to what should be observed for ILs with long alkyl chains. This is supported by the lower β and marginally larger ΔGm° for [C16AMorph][Br] as compared to [C12AMorph][Br]. Furthermore, a lower aggregation number, Nagg (discussed later), seems to affect the nature of the enthalpogram.52 Various thermodynamic parameters such as the cmc, ΔGm°, ΔHm°, and TΔSm° have been deduced from ITC measurements and are reported in Tables 1 and 2. The value of ΔHm° in the case of [C12AMorph][Br] has been determined by a simple graphical extrapolation method as the transition in the entahlpogram upon micellization is very sharp. However, in the case of [C8AMorph][Br] and [C16AMorph], the curves do not present clear sigmoidal features and hence were analyzed by a data treatment method analogous to one suggested by different researchers.53−55 Linear fits of the data sets in the lower and upper concentration domains were performed as can be seen from Figure 3A,C. The intercepts of the two straight lines were determined, and the difference between the two intercepts yielded the enthalpy of micellization. ΔGm° along with the related entropy of micellization cannot be determined only from isothermal titration calorimetry. Therefore, it has been suggested that ΔGm° can be determined using the cmc obtained from ITC and the degree of counterion binding, β, which is equal to a parameter, 1 − p/n, used in ITC measurements,55 where p is the charge on the micelle and n is the aggregation number, obtained from other techniques. In this study, we have used β obtained from conductivity to obtain the Gibbs free energy of micellization. Using the Gibbs− Helmolthz equation, the related entropy change has been obtained. For [C12AMorph][Br], the midpoint of the transition F


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Figure 5. AFM height profile of aqueous micellar solutions of (A) [C8AMorph][Br], (B) [C12AMorph][Br], and (C) [C16AMorph][Br] and corresponding 3D AFM images of (D) [C8AMorph][Br], (E) [C12AMorph][Br], and (F) [C16AMorph][Br] at 298.15 K. The arrows in (A)−(C) indicate the presence of smaller micelles as observed from DLS measurements.

respectively. The obtained Nagg is provided in Table 2. Nagg decreases with an increase in the alkyl chain length. However, the size of the micelles increases with an increase in the alkyl chain length as revealed by DLS as discussed in the next section. This indicates the decrease in the compactness of the micelles as the alkyl chain length increases, which is also supported by increasing I1/I3 values with an increase in the alkyl chain length. Nagg for [C16AMorph][Br] has been found to be lower than that for nonfunctionalized [C16Morph][Br].43 As also discussed for conductometry (section 3.3), it is expected that coiling of the bulky morpholinium headgroup toward the longer alkyl chain occurs, especially in the case of [C16AMorph][Br], as a consequence of decreased β, which results in the formation of loose micelles. This speculation is confirmed from 2D 1H−1H NOESY experiments (discussed in section 3.7), where it is observed that the bulky headgroup associated with the long flexible alkyl chain in the case of [C16AMorph][Br] coils back toward the interior of the micelle. 3.6. DLS and AFM. DLS and AFM have provided insights into the size distribution and shape of the micelles of different [CnAMorph][Br] ILs in aqueous solutions, respectively. Parts A and B of Figure S6 (Supporting Information) show the intensity- and number-weighted size distribution profiles of micelles of the investigated ILs, respectively. The obtained Dh, both intensity percentage and number percentage, as well as polydispersity index is provided in Table S2 (Supporting Information). As can be seen from Figure S6A, two size distributions, one with Dh (at maximum scattering intensity) in the range of 2.7−23.1 nm and the other with Dh in the range of 97−146 nm, are observed. The smaller and larger Dh values are assigned to the presence of micelles and larger micellar agglomerates, respectively. As the intensity of the scattered light is directly proportional to the sixth power of the radius of a

the decreased extent of compactness of the micelles on going from [C8AMorph][Br] to [C16AMorph][Br]. I1/I3 values in the micelles indicate that the polarity of the cybotactic region of pyrene is between those of aromatic solvents and hydrocarbon solvents (I1/I3 = 0.80−1.00)56 for [C8AMorph][Br], close to that of benzyl alcohol (I1/I3 = 1.22)57 or tetrahydrofuran (I1/I3 = 1.20)56 for [C12AMorph][Br], and similar to that of dichloromethane (I1/I3 = 1.37)56 for [C16AMorph][Br]. This indicates that pyrene is solubilized in the hydrophobic core of the micelle in the case of [C8AMorph][Br], whereas its position is somewhere in the palisade layer of the micelle near the cationic headgroups in the case of [C12AMorph][Br] and [C16AMorph][Br]. Furthermore, the penetration of water into the palisade layer of the micelle, leading to comparatively higher values of I1/I3 in the case of [C12AMorph][Br] and [C16AMorph][Br], cannot be ruled out. It is reported that water can penetrate into the micelles and extend up to four carbons from the headgroup.57 The steady-state fluorescence quenching measurements using pyrene as the fluorescent probe and cetylpyridinium chloride as the quencher have been utilized to gain information about the aggregation number (Nagg) of micelles in aqueous solutions of ILs using the following equation: ln I = ln I0 − Cq /Cm = ln I0 − NaggCq /(C t − cmc)

(4)

where Cq, Cm, and Ct are the molar concentrations of the quencher and micelles and total concentration of the IL, respectively, while I and I0 are the fluorescence intensities of pyrene fluorescence in the presence and absence of quencher, respectively. Parts B and C of Figure 4 show representative plots of the change in the emission spectrum of pyrene in micellar solutions with increasing Cq and the variation of ln(I/ I0) with Cq in aqueous micellar solutions of the investigated ILs, G


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Figure 6. (A) Schematic representation of [CnAMorph]+, where n = 5, 9, and 13 for [C8AMorph][Br], [C12AMorph][Br], and [C16AMorph][Br], respectively. Selected regions of 2D 1H−1H ROESY (B) and NOESY (C, D) through-space correlation spectra of [C8AMorph][Br] (B), [C12AMorph][Br] (C), and [C16AMorph][Br] (D). (E) Overlay of the amide region of the 1D 1H spectra of [C8AMorph][Br] (black), [C12AMorph][Br] (orange), and [C16AMorph][Br] (blue). Chemically distinct protons in the 2D spectra are labeled from 1 to 9 (blue numerals) as assigned in the schematic representation shown in (A). Through-space correlations between protons at positions 1 and 4−8 in (D) are highlighted using a rectangle.

phenomenon of micellar growth is ruled out looking at the low concentration of the investigated ILs and comparatively lesser number of larger micelles. 3.7. 2D 1H−1H ROESY and NOESY. Figure 6 shows a schematic representation of the [CnAMorph][Br] molecule, the corresponding 2D 1H−1H through-space correlation spectra for [C8AMorph][Br] (ROESY) and for [C12AMorph][Br] and [C16AMorph][Br] (NOESY), and an overlay of the amide region of the 1D 1H spectra of the investigated ILs above the cmc. For [C8AMorph][Br], through-space correlation could not be obtained using NOESY, probably because of its small size, and hence, a 2D 1H−1H ROESY spectrum was recorded. As can be seen from Figure 6B,C (left panels), the amide proton (proton at position 9 in the schematic shown in Figure 6A) shows strong correlations with other protons at positions 1−3 and 4−8 in the case of [C 8 AMorph][Br] and [C12AMorph][Br]. As expected, terminal protons at position 1 do not show through-space correlations with any other proton except at position 2, suggesting the formation of a typical micellar structure with terminal protons buried in the core of the micelle. However, micelles formed by [C16AMorph][Br] were found to exhibit anomalous behavior. Here, the terminal protons at position 1 yield strong correlation with protons at positions 4−8. This indicates that the terminal and headgroup regions share close spacial proximity in [C16AMorph][Br] micelles. This unexpected observation could be atributed to the formation of a micelle where the headgroup region of the monomeric units is bent backward toward the hydrophobic alkyl chain. We suspect this bending of the headgroup occurs around the flexible amide proton region. This conclusion was made on the basis of the following observations. The amide proton in [C16AMorph][Br] micelles did not yield through-space correlations with any other proton (Figure 6C, left panel). This could be a result of conformational flexibility imparted to the amide proton due to the unexpected bending, which could cause the disappearance of through-space correlations in the NOESY spectrum. Also, the amide proton in [C16AMorph][Br] yields a sharp resonance in the 1H 1D

particle according to Rayleigh’s approximation, it is considered that larger particles scatter much more light compared to smaller ones even if the number of large particles is exceptionally less compared to the number of smaller particles.58 Therefore, for further discussion, number-weighted Dh is taken into consideration. As shown in Figure S6B, micelles grow in size with an increase in the alkyl chain length of the investigated ILs. The approximate length of the respective cation of the ILs comes out to be ∼1.4, ∼1.9, and ∼2.4 nm for [C 8 AMorph] + , [C 1 2 AMorph] + , and [C16AMorph]+. This means that the Dh of the micelle formed by [C8AMorph][Br] (Dh = 2.4 nm) and [C12AMorp][Br] (Dh = 4.6 nm) is almost double the length of [C8AMorph]+ and [C12AMorph]+, respectively, indicating the formation of tightly packed spherical micelles. However, in the case of [C16AMorph][Br], a larger Dh of 20.0 nm accounts for the formation of nonspherical large micelles as a consequence of growth of the micelles. Such larger aggregates have also been observed for a variety of ILs along with the micelles from DLS and transmission electron microscopy measurements.20 Figure 5 shows the AFM images (A−C) and corresponding 3D profiles (D−F) for [C8AMorph][Br], [C12AMorph][Br], and [C16AMorph][Br], respectively. The AFM images (Figure 5A−C) show the presence of micelles of two sizes, similar to the Dh obtained from intensity-weighted DLS measurements (Figure S6, Supporting Information). The size of the micelles increases with an increase in the alkyl chain length of [CnAMorph][Br] ILs in corroboration with the DLS measurements. A careful examination of the height profiles of the respective AFM images (Figure 5D−F) reveals that the number of smaller micelles for all the ILs under investigation is much greater compared to that of larger micellar agglomerates, in corroboration with the analysis of DLS data. For better clarity, some of the smaller micelles are marked with an arrow in Figure 5A−C. From 3D AFM profiles, it is clear that smaller micelles are spherical in shape, whereas the larger micelles are considered as nonspherical in nature. The presence of large micelles is assigned to the growth of smaller micelles. The H


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along with the relatively greater hydrophobicity and larger size of the morpholinium headgroup. The compactness of the micelles decreases with an increase in the alkyl chain length as indicated by a decreased degree of counterion binding and increased polarity of the cybotactic region experienced by pyrene in the micelle. The increase in size and decrease in Nagg with an increase in the alkyl chain length also supports this observation. For the long-chain IL [C16AMorp][Br], the bulky cationic headgroup coils back toward the interior of the micelle as revealed by 1H−1H 2D NOESY experiments. It has been found that functionalization of the alkyl chain of ILs improves their surface activity and micellization behavior; therefore, other functional groups should be appended to the alkyl chain of ILs in future investigations to achieve the best surface activity and micellization properties.

spectrum (Figure 6D, blue). The fact that we did not observe 3 N α JH H coupling in [C16AMorph][Br] micelles, which could go to zero at certain values of the backbone dihedral angle ϕ, is more strong proof that the bending occurs around the amide region only. Furthermore, a downfield shift of the amide proton in [C16AMorph][Br] in comparison with [C8AMorph][Br] and [C12AMorph][Br] suggests changes in the local structure around this region. In a nutshell, our results suggest that both [C8AMorph][Br] and [C12AMorph][Br] form typical micelles, whereas in the case of [C16AMorph][Br] the headgroup is bent backward toward the hydrophobic alkyl chain. The flexible long alkyl chain in [C16AMorph][Br] induces the conformational flexibility near the amide group region, making the amide group more available to bulk water during micellization and therefore leading to greater hydration of the cationic headgroup region as also supported by the relatively lower value of I1/I3. This results in a decreased degree of counterion binding as also discussed earlier. Upon micellization, an increase in electrostatic repulsion of the bulky morpholinium headgroup as a consequence of decreased counterion binding seems to be the driving force leading to bending of the comparatively hydrophobic headgroup toward the nonpolar microenvironment. This bending takes place around the amide group, exposing the protons at position 9 to H2O and therefore leading to the formation of loose micelles, which is also supported by the smaller β, relatively higher I1/I3 values, and large Dh. Scheme 2 depicts the possible micellar structure for the investigated ILs based on observations made from different techniques.

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ASSOCIATED CONTENT

* Supporting Information S

Synthesis and characterization of [CnAMorph][Br] ILs, DSC thermograms and TGA and DTG profiles of various [C nAMorph][Br] ILs, variation of ln cmc of various [CnAMorph][Br] ILs as a function of the alkyl chain length, variation of κ in aqueous solutions of various amidefunctionalized morpholinium-based ILs as a function of C1/2, dP profiles of amide-functionalized morpholinium-based ILs, intensity- and number-weighted size distributions in aqueous solutions of different [CnAMorph][Br] ILs obtained from DLS measurements, comparison of Tonset of various functionalized and nonfunctionalized ILs, and size from AFM measurements, Dh, polydispersity index, and Nagg of micelles of various IL surfactants. This material is available free of charge via the Internet at http://pubs.acs.org.

Scheme 2. Schematic Structures of Micelles of (A) [C8AMorph][Br], (B) [C12AMorph][Br], and (C) [C16AMorph][Br]

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +91-183-2258802, ext 3206. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to the University Grants Commission (UGC), Government of India, Wide Project Number F.20-24(12)/ 2012(BSR), and Department of Science and Technology (DST), Government of India, Wide Project Numbers SB/ FT/CS-057/2013 and SR/S1/OC-35/2010, for providing the research grants for this work. We are thankful to the UGC, India, for their UGC-CAS (Centre for Advanced Studies) program and DST, India, for the FIST (Fund for Improvement of Science & Technology Infrastructure in Universities and Higher Educational Institutions) program awarded to the Department of Chemistry, Guru Nanak Dev University, Amritsar.

Here we have observed how the functionalization of the alkyl chain of the ILs led to marked changes in their surface adsorption and micellization behavior. It is expected that this work along with the earlier reported work36 will put emphasis on the functionalization of ILs with appropriate functional groups, where a diverse and striking self-assembly behavior can be observed. Owing to their better surface activity and comparatively lower cmc, the functionalized ILs can act as a substitute for conventional ionic surfactants in various applications.

4. CONCLUSION Morpholinium-based amide-functionalized ILs [CnAMorph][Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior. The investigated ILs have shown a lower thermal decomposition temperature compared to nonfuncitonalized ILs. The investigated ILs have been found to show better surface activity compared to nonfunctionalized ILs while exhibiting a much lower cmc. This observation is assigned to the presence of intermolecular Hbonding fashioned by the presence of the amide functionality

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REFERENCES

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Micellization Behavior of Morpholinium-Based Amide-Functionalized

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