Physicochemical studies of morpholinium based ionic liquid crystals and their interaction with cyclodextrins

Fluid Phase Equilibria 361 (2014) 104–115

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Physicochemical studies of morpholinium based ionic liquid crystals and their interaction with cyclodextrins Rabia Sharma, Suruchi Mahajan, Rakesh Kumar Mahajan ∗ Department of Chemistry, UGC-Centre for Advanced Studies, Guru Nanak Dev University, Amritsar 143005, India

a r t i c l e

i n f o

Article history: Received 31 July 2013 Received in revised form 15 October 2013 Accepted 17 October 2013 Available online 29 October 2013 Keywords: Inclusion complex Aggregation number Chemical shift Packing parameter Ionic liquids

a b s t r a c t The various physicochemical properties of morpholinium based ionic liquid crystals (ILC) (N-methyl-Ntetradecylmorpholinium bromide (M14) and N-methyl-N-hexadecylmorpholinium bromide (M16)) in the absence and presence of ␤-cyclodextrin (␤-CD) and hydroxypropyl-␤-cyclodextrins (HP␤-CD) have been studied using conductance, surface tension, fluorescence and 1 H NMR measurements. The various micellar, interfacial and thermodynamic parameters such as critical micelle concentration (cmc), degree of counterion binding (g), association constants (K), surface excess concentration (max ), minimum area per molecule (Amin ), aggregation number (Nagg ), interaction parameter (ˇm ), standard Gibbs free energy o o of adsorption (Gads ) and standard Gibbs free energy of micellization (Gm ) have been evaluated for mixtures of M14/M16+ ␤-CD/HP␤-CD. Conductance study also shows that cyclodextrins (CDs) entraps in hydrophobic moieties of ILC to form inclusion complexes with stoichiometry of 1:1. The formation of inclusion complexes is confirmed by increase in cmc of synthesized ILC and changes in chemical shift of CDs and ILC protons in the presence of CDs. Forces responsible for the formation of inclusion complexes are van der Waals, hydrogen bonding, hydrophobic interactions, release of ring strain in cyclodextrin molecules and changes in solvent-surface tensions. The value of packing parameter (p > 0.5) for pure ILC which suggest a flexible bilayer arrangement for ILC. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Ionic liquid crystal (ILC) is an enthralling class of material that combines the properties of ionic liquids and liquid crystals [1]. Just as ionic liquids, the properties of ILCs can be tuned by an appropriate choice of anions and cations. These salts have been used as ordered solvents, templates for the synthesis of mesoporous and zeolitic materials, optoelectronics and dye-sensitized solar cells [2–7]. Among the known ILCs, imidazolium or pyridinium salts are the most studied compounds [8]. Only few thermotropic ILCs based on other cations have been reported [9]. Out of which morpholinium based ILCs received much attention because of their structural properties especially for the design of ILC and their mesomorphic behavior [10,11]. These are widely used for organic synthesis, heat stabilizers, and antioxidants for lubricating oils and for electrochemical purposes as corrosion inhibitors [12–14]. They have been considered in recent inocuity tests, among other commonly used ILs [15] and applied as gel polymer electrolytes [16]. Despite the argued advantage of having low vapor pressure, even the most hydrophobic ILs show some degree of solubility in water, allowing their dispersion into aquatic systems and raising

∗ Corresponding author. Fax: +91 183 225882. E-mail address: rakesh [email protected] (R.K. Mahajan). 0378-3812/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2013.10.042

concerns on its pollutant potential [17]. So, the disadvantage is that once these ILC have been used it is essential to extract these compounds and for this purpose a material having extracting property is required. Hence, the present work was aimed to resolve this scientific challenge which would provide momentous information to the environmentalists. CDs are well-known for including various guest molecules, and this unique ability makes them functional in cosmetics, pharmaceutical science, and several other fields [18,19]. CDs have homogeneous toroidal structures of different molecular sizes: most typical are cyclohexaamylose (␣-CD), cycloheptaamylose (␤-CD), and cyclooctaamylose (␥-CD). The primary hydroxyl groups are located at the wider rim and the secondary hydroxyl groups are found at the narrower rim [20,21]. The toroidal structure has a hydrophilic surface, making them water-soluble, whereas the cavity is composed of the glucoside oxygens and methylene hydrogens, giving it a hydrophobic character. As a consequence, the CDs are capable of forming inclusion complexes with compounds having a size compatible with dimensions of their cavity [22,23]. ␤-Cyclodextrin (␤-CD) is of special consideration, since the size of its cavity is suitable for many donor molecules such as the surfactants’ hydrophobic tail. Hydroxypropyl-␤-cyclodextrins (HP␤-CD) are nontoxic and biodegradable and, hence, widely usable without risk for the environment. They can be employed to remove non aqueous phase liquids from contaminated soils and

R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115

groundwater as they enhance the water solubility of nonpolar organic compounds and their biodegradation and reduce their sorption onto the soil [24,25]. Thermodynamic properties are very sensitive to the hydrophobic, hydrophilic interactions and to a lesser extent, electrostatic interactions. Therefore these properties have been revealed as suitable to study the cyclodextrins in the presence of surfactants. The complex formation of CDs and surfactants has been examined by many techniques from physicochemical point of view [26–28]. However there is no report in the literature comprises the interactions between the morpholinium ILC and CDs. As there are few reports on morpholinium salts with varying anionic species and alkyl chain length of cationic group [11,29,30]. Morpholinium ILC, N-methyl-N-tetradecylmorpholinium bromide (M14) and N-methyl-N-hexadecylmorpholinium bromide (M16) also show aggregation behavior as that of surfactants due to their amphiphilic properties. Hence, all the theories used to the study the behavior of surfactant+ CDs mixtures are applied on the ILC+ CDs systems. The objective of the present work is to study the encapsulation processes of cationic morpholinium ILC, N-methyl-N-tetradecylmorpholinium bromide (M14) and Nmethyl-N-hexadecylmorpholinium bromide (M16) into the cavity of ␤-CD and HP␤-CD and its effect on the micellization and interfacial properties of the surfactants. The various physicochemical techniques such as conductance, surface tension and fluorescence have been employed in order to predict different micellar, interfacial and thermodynamic parameters. However these techniques provide only indirect information on the molecular structure of the inclusion complexes. To understand the type of interactions and orientations of the molecules inside the cavity of CDs, 1 H NMR has been employed. The experimental results obtained through the present study might be practically useful because they will (1) assist researchers in determining whether their properties are practical for further applications, and (2) provide a useful background to investigate morpholinium ILCs with other species.

105

Table 1 Supplier and purity of chemicals used. Chemicals

Supplier

Purity (%)

Pyrene 1-Bromotetradecane 1-Bromohexadecnae 4-Methylmorpholine ␤-CYCLODEXTRINS Hydroxypropyl-␤-cyclodextrin Acetonitrile Hexadecylpyridinium chloride

Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Sigma–Aldrich Merck HIMEDIA

≥99.0 ≥99.0 ≥99.0 ≥99.0 ≥97.0 ≥98.0 ≥99.9 ≥97.0

2. Experimental 2.1. Materials Morpholinium ILC, N-methyl-N-tetradecylmorpholinium bromide (M14) and N-methyl-N-hexadecylmorpholinium bromide (M16) (Fig. 1) were synthesized and purified as described in literature [11]. The purities of the chemicals used and their suppliers are given in Table 1. All the chemicals were of analytical grade and used without further purification. Purity of the products was assessed by 1 H NMR spectroscopy and surface tension. Double distilled water having specific conductivity of range (1–2) × 10−6 S cm−1 was used for the solution preparations. The solutions were prepared by mass with an accuracy of ±0.0001 g using Sartorius analytical balance. All the measurements have been performed at 25.0 ± 0.1 ◦ C after giving overnight time for the stabilization. 2.2. Methods 2.2.1. Specific conductivity measurements The specific conductivity () measurements of morpholinium ILC (M14, M16) and their mixtures with ␤-cyclodextrin (␤-CD) and hydroxypropyl-␤-cyclodextrin (HP␤-CD) were carried out at

Fig. 1. Molecular structures and their proton group assignments of morpholinium ILC and CDs.

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2500

5000

(a) [β β − CD]/mM

2000

3000

-1

0.0 0.5 1.0 2.0 3.0

κ/ μ Scm

κ/ μ Scm

-1

4000

1000

1000

500

0

0

10

20

30

40

50

60

70

0.0 0.5 1.0 2.0 3.0

1500

2000

0

80

0

5

10

15

20

25

30

35

40

-3

-4

-3

-4

(b)

[β β − CD]/mM

C / 10 mol dm

C / 10 mol dm

T

T

Fig. 2. Plots of specific conductance () vs. total concentration (CT ) of mixtures (a) M14+ ␤-CD and (b) M16+ ␤-CD. The scales shown are for M14/M16. The values for other curves have been multiplied by a factor of 2, 4, 6 and 8 for 0.5, 1.0, 2.0 and 3.0 mM concentrations of ␤-CD, respectively, for clarity purpose.

25.0 ± 0.1 ◦ C with a digital conductivity meter (Equiptronics, Bombay, Model EQ661). A dip type conductivity cell with cell constant of 1 cm−1 was used. The specific conductivity at various concentrations of cyclodextrins was measured by successive additions of the stock solution in CDs. The reproducibility of conductivity measurements was within ±0.2%. A break in the plot of specific conductivity () vs. CT (total concentration of ILC) indicates the onset of micellization and the point of intersection referred to cmc of ILC (Figs. 2 and S1). 2.2.2. Surface tension measurements The Wilhelmy plate method was used to measure the surface tension () values of the pure amphiphile (ILC) and their mixtures with Kruss Easy Dyne tensiometer from Kruss Gmbh (Hamburg, Germany). The platinum plate used in the measurements was cleaned by washing with doubly distilled water followed by heating through alcoholic flame. The surface tension of doubly distilled water, 72.8 mN m−1 at 25.0 ± 0.1 ◦ C, was used for the calibration

purpose and also to check the cleanliness of the glassware. For cmc determination, the concentrated stock solution was added progressively to a known volume of cyclodextrin and the -values were then measured after thorough mixing and temperature equilibration. -Value showed a decrease on the addition of stock solution up to a certain concentration and then it became constant. This break point refers to cmc value, as shown in Figs. 3 and S2. It is well known that cmc derived from surface tension is sensitive to impurities. There is no evidence of minima in the region of the cmc, which indicates that the surface-active impurities are absent. The accuracy in the measurement of surface tension with the tensiometer is ±0.15 mN m−1 . 2.2.3. Fluorescence measurements Steady-state fluorescence experiments were performed with Cary Eclipse Fluorescence Spectrophotometer from Varian Ltd. using pyrene as the probe at 25.0 ± 0.1 ◦ C. Pyrene is the most effective fluorescent probe to investigate the polarity changes in the

70

70

(a)

-1

50

40

30

20 -4.5

0.0 0.5 1.0 2.0 3.0

60

0.0 0.5 1.0 2.0 3.0

γ/mNm

γ/mNm

-1

60

[β β − CD]/mM

(b)

[β β − CD]/mM

50

40

30

-4.0

-3.5

-3.0

-2.5 -3

log C / mol dm T

-2.0

20 -5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-3

log C / mol dm T

Fig. 3. Plots of surface tension () vs. logarithm of the total concentration (CT ) of interacting species at various concentration of ␤-CD: (a) M14+ ␤-CD and (b) M16+ ␤-CD.

R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115

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Table 2 Average cmc (cmcav ), degree of counterion binding (g), efficiency of adsorption (pC20 ), cmc/C20 , surface pressure at cmc (cmc ), surface excess ( max × 106 ), minimum area per molecule (Amin ) and packing parameters (p) for binary mixtures of M14/M16+ ␤-CD/HP␤-CD at various concentrations of ␤-CD and HP␤-CD. g

pC20

cmc/C20

cmc × 103 (J m−2 )

 max

3.65 3.18 3.53 3.63 4.63

0.73 0.64 0.70 0.68 0.66

3.2 3.6 3.6 3.1 2.8

4.6 7.6 9.0 2.8 2.1

41.2 39.1 45.3 40.4 36.5

2.79 2.07 2.56 4.00 5.85

0.59 0.80 0.64 0.41 0.28

0.70 0.52 0.64 1.01 1.48

M14+ HP␤-CD 0.5 1.0 2.0 3.0

3.32 4.15 4.56 4.95

0.62 0.65 0.66 0.70

3.4 3.0 2.9 2.8

5.4 2.6 2.3 2.2

31.5 35.2 39.7 36.5

1.49 2.74 5.11 5.27

1.11 0.60 0.32 0.31

0.37 0.69 1.29 1.33

M16+ ␤-CD 0.0 0.5 1.0 2.0 3.0

1.01 1.23 1.56 2.17 2.71

0.66 0.73 0.70 0.57 0.65

3.6 3.4 3.9 3.0 2.8

4.1 2.8 2.5 2.1 1.4

38.5 26.6 33.6 33.9 30.0

2.69 1.20 1.06 2.86 6.06

0.61 1.37 1.56 0.57 0.27

0.68 0.30 0.26 0.72 1.53

M16+ HP␤-CD 0.5 1.0 2.0 3.0

1.25 1.66 2.10 2.90

0.73 0.73 0.72 0.67

3.4 3.3 3.1 2.9

3.0 3.3 2.3 2.3

27.4 31.1 33.2 30.8

1.53 1.94 3.35 2.95

1.08 0.85 0.49 0.56

0.38 0.49 0.84 0.74

[CD] (mM)

cmcav

M14+ ␤-CD 0.0 0.5 1.0 2.0 3.0

a b c

a

(mM)

b

× 106 (mol m−2 )

Amin

c

(nm2 /molecule)

p

The uncertainty in these values ranges from ±0.01 to ±0.04. The uncertainty in these values ranges from ±0.02 to ±0.04. The uncertainty in these values ranges from ±0.02 to ±0.04.

systems. The concentration of pyrene proposed to use in all the measurements should be approximately equal to 10−6 mol dm−3 in order to avoid the interference of pyrene in the micelle formation process. A quartz cell having optical length of 10 mm was used for the measurements. The excitation and the emission band widths were kept at 5 nm. The excitation wavelength for the pyrene was 335 nm, while the emission spectra were recorded between 350 and 600 nm. The first (I1 ) and third (I3 ) vibronic peaks of the pyrene appeared at 373 and 384 nm, respectively. To determine cmc values, the I1 /I3 of pyrene spectrum was measured as a function of [ILC]. In all experiments the concentration of ILCs was kept 10 times above of their cmc values and was held constant to obtain the aggregation number [31]. Hexadecylpyridinium chloride (HPyCl) was chosen as a static quencher and its concentration was varied from 10 to 100 ␮M. The values of ratio of [pyrene]/[micelles] and [quencher]/[micelles] are less than 0.01 and 0.9, respectively, ensuring a Poisson distribution [31]. 2.2.4. NMR measurements 1 H NMR spectra were recorded on a JEOL-FT NMR-AL at 300 MHz. All the NMR measurements were recorded using D2 O as solvent. The NMR titration experiments were carried out by titrating one equivalent of cyclodextrins (␤-CD and HP␤-CD) with increasing equivalents of M14 and M16 solutions and using center of HDO signal (4.650 ppm) as the internal reference. ı Scale is used in order to represent the chemical shifts of various protons of cyclodextrins and morpholinium ILC. The external reference was not used to avoid possible interaction with ILC micelles. 3. Results and discussion 3.1. Micellar and interfacial properties of morpholinium ILCs with CDs The characteristic phenomena for amphiphiles in the presence of various additives are the formation of mixed micelles in the bulk solution and monolayers of mixed systems at the interface. To

analyze the mixed micellization studies various techniques such as conductivity, surface tension and fluorescence have been carried out. The values of critical micelle concentration (cmc) for M14/M16+ ␤-CD systems have been obtained from conductivity (Fig. 2) and surface tension measurements (Fig. 3). However the similar plots for the mixtures of M14/M16+ HP␤-CD are given in ESI (Figs. S1 and S2 in supplementary material). 3.1.1. Critical micelle concentration (cmc) values The cmc values for the pure morpholinium ILC (M14 and M16) have been evaluated from conductivity and surface tension techniques. Since the experimental cmc values obtained for various systems depend upon the methods, hence cmc and cmc derived parameters have been evaluated using average cmc values (cmcav ). The average cmc values for all the mixtures have been shown in Table 2. For most of the mixtures, these values are found to be higher than those of individual cmc values for morpholinium ILC which clearly indicate the delay in micellization process on the addition of ␤-CD and HP␤-CD. Also, it is clear from the Table 2 that an increase in concentration of CDs causes an increase in the value of cmc. This may be due to formation of the inclusion complex between the CD and S (it is used for any amphiphile which can show aggregation property, here for ionic liquid crystal) [25,32–34]. The process can be represented as: CD + S ↔ CDS This equilibrium gets shifted toward the complex formation on the addition of S because the association between the CD and S is more favored process than micellization. However further addition of S causes the formation of micelles when all the CD molecules present in the solution form complex with S. nS ↔ Sn The mechanism of the whole process is that initially the constant molecules of CD are present in the solution. The addition of S molecules insists the formation of CD–S complex because the tail

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3.5

group of S gets incorporated into the CD cavity easily and the association between the CD and S is stronger than the association of S molecules [32]. It leads to the decreases in free S-monomers which in turn decreases number of monomers required for the formation of micelles. However with the increase in concentration of S, the amount of free S-monomers increases and the process of micellization instigate. In this way, the value of cmc enhances for the morpholinium ILC (M14 and M16) in the presence of cyclodextrins. On comparison between ILC+ ␤-CD and ILC+ HP␤-CD systems, value of cmc for the latter mixtures are found to be more than the former one which indicate that HP␤-CD form complexes with ILC more efficiently than ␤-CD. This may be due to the presence of hydroxypropyl group which facilitates the complexation process more easily than ␤-CD and delays the formation of micelles. 3.1.2. Effect of ILC-CDs complex on the counterion binding The degree of counterion binding (g) is an important parameter, since it is the expression of how many counterions are contained in the Stern layer to counterbalance the electrostatic force that opposes the micelle formation. The value of ‘g’ of the pure morpholinium ILC and M14/M16+ CDs systems (Table 2) have been evaluated from the degree of dissociation which is further obtained from the ratio of post-to-premicellar slopes using the specific conductance () isotherms [35] (Figs. 2 and S1). Table 2 shows an increase in the ‘g’ values of morpholinium ILC (M14) with an increase in concentration of HP␤-CD molecules, indicating stabilization. This may be due to the formation of compact micelles with a high degree of counterion binding. However for other systems M14+ ␤-CD and M16+ ␤-CD/HP␤-CD the value of g shows a decrease with an increase in the concentration of cyclodextrin molecules which may be due to less compact nature of the micelle. The lower binding of counterions to micelles arises from the loose packing of the head groups and a lower surface charge density at the micelle-solution interface. In case of M16+ ␤-CD/HP␤-CD systems, this loose packing of head group is due to the more hydrophobic nature of M16 crystals. However some irregularities in g-values have also been observed in some of the systems due to the equilibrium established between inclusion complex of cyclodextrin-ILC and their individual molecules. 3.1.3. Association constants As discussed above, the cmc values of morpholinium ILC, M14 and M16 are linearly correlated with the concentration of CD which suggests that more ILC molecules form inclusion complex with CD and lead to delay in the micelles formation. From this, we can calculate the association constant (K) between S and CD in the micellar concentration by assuming a 1:1 association stoichiometry [36] as given by: CD + S ↔ CD : S K=

[CD : S] [CD][S]

or

(1) [CD : S] = K[CD][S]

(2)

[CD]t = [CD] + [CD : S]

(3)

[CD]t = [CD] + K[CD][S]

(4)

[CD]t = [CD](1 + K[S])

(5)

[S]t = [S] + [CD : S]

(6)

By substituting Eq. (2) into Eq. (6), we get [S]t = [S] + K[CD][S] K[S][CD]t 1 + K[S]

cmc/mM

M16+ BCD M16+ HCD

2.5

2.0

1.5

1.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

[CD]/mM Fig. 4. Variation of cmc (from conductivity only) for M16 in the presence of CDs.

where K, CD, S, CD:S, CDt and St are the association constant, free CD, free amphiphilic molecules, inclusion complex, total CD, and total amphiphilic concentration (ILC) at cmc, respectively. Equation (8) predicts a linear correlation between CDt and St which we also observe in Fig. 4. This leads to determination of association constants of CD:S systems and the values obtained for the mixtures of M14+ ␤-CD, M14+ HP␤-CD, M16+ ␤-CD and M16+ HP␤-CD are 0.220, 0.501, 1.896, 2.902 × 103 M−1 , respectively. The higher value obtained for the binary mixtures of M16+ CDs indicate that M16 bind more strongly with the cyclodextrins than M14. Also M16+ HCD system corresponds to the highest value of K which suggests strong interactions between the interacting species. These variations in the K value imply that the binding efficiency depends on the size of both the components as well as the chain length of the amphiphiles used. Maeso et al. [37] used X-ray crystallography for the calculation of diameter and depth of ␤-CD and HP␤-CD which were 6.5, 7.8 A˚ for both the cyclodextrins. The following equations were used by Rafati et al. [38] to calculate the length, lc , and diameter, dc , of the structural alkyl group with formulae Cn H2n+1 : ◦

lc (A) ≈ 1.5 + 1.265(nc − 1) ◦

dc (A) ≈ {[34.89 + 34.25(nc − 1)]/[1.5 + 1.265(nc − 1)]}

(9) 1/2

(10)

The length and diameter of the chain for morpholinium ILC were estimated using above equations and values obtained are 17.95, 5.17 A˚ for M14 and 19.98, 5.24 A˚ for M16, respectively. Comparison of these values with the structural characteristics of cyclodextrins reveals that there is a good match between CD molecules and ILC chain and this fitting is far better for M16 than M14. This further contributes stabilization to the M16+ CDs complexes and hence the association constants are higher for these systems. Binding of cyclodextrins with substrate suggests that van der Waals forces are important in complex formation. Other forces such as hydrogen bonding, hydrophobic interactions, release of ring strain in cyclodextrin molecules and changes in solvent-surface tensions are also responsible for the complexation process [19,20]. Hence to explore other kind of forces these systems require further investigations employing some other techniques.

(7)

By substituting Eq. (5) into Eq. (7), we get [S]t = [S] +

3.0

(8)

3.1.4. Interfacial properties at the air/water interface Figs. 3 and S2 describe the effect of various concentrations of cyclodextrins (␤-CD and HP␤-CD) on the surface tension isotherm of M14 and M16. The addition of cyclodextrins causes an increase in

R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115

surface tension for most of the systems which suggests that ILC are pulled by the hydrophobic cavity of the oligosaccharide and eliminated from the surface. Also increase in surface tension becomes more prominent as we increase the concentration of cyclodextrins, of course, due to the more exclusion of the ILC from the surface for the assembling the inclusion complex. So in the presence of cyclodextrins, the amount of ILC needed to form micelles increase. This causes delay in the process of micellization and hence the value of cmc increases (Table 2). The pC20 is efficiency of adsorption of amphiphile at the air/water interface where C20 is the concentration of an amphiphile required to reduce the surface tension by 20 mN m−1 . This can be obtained by using the equation: pC20 = − log C20

(11)

These values for pure morpholinium ILC and in the presence of cyclodextrins are listed in Table 2. It has been observed that pC20 value increases with the increase in chain length for pure ILC. This is quite expected that with the increase in hydrophobicity of the component adsorption of ILC at air/water interface increases. For M14/16+ ␤-CD/HP␤-CD systems pC20 values show an overall decrease with the increase in the concentration of cyclodextrins which is due to reduction in surface adsorption of ILC. This is because CDs used are non-surface active and do not contributing in the surface properties. cmc/C20 ratio has also been calculated which measures how far the surface tension of water can be reduced by the presence of amphiphiles. The values of cmc (the surface pressure at the cmc) were obtained by using the equation: cmc = o − cmc

(12)

where  o and  cmc are the surface tension of the solvent and mixture of ILC+ CD at cmc, respectively. The values of cmc (Table 2) decrease with increase in the chain length of ILC. As we know CDs are not surface-active so should not affect the value of surface pressure but they play a significant role in the complexation process and hence can affect the surface pressure indirectly. Because in the complex formation process between CDs and ILC, the total number of monomers of ILC available at the surface depends on the concentration of CDs. So the change in the surface pressure values is expected. For all M14/M16+ CDs systems at low concentrations of ␤-CD/HP␤-CD, cmc values increase with the increase in amount of CDs, indicating that the efficiency of systems increases (Table 2). However at higher concentration of ␤-CD/HP␤-CD, all the systems shows a reduction in cmc values that suggest decrease in their efficiency. This may be due to the reason that more amount of CD causes saturation in the solution and hence efficiency of the mixtures decreases. Also these values for M14/M16+ HP␤-CD complexes are found to be lower than M14/M16+ ␤-CD complexes which imply that the introduction of hydroxypropyl group in the system causes decrease in the efficiency of these systems. When interfacial adsorption occurs, the amphiphiles arrange themselves at the air/water interface orienting their hydrophilic head groups toward water and hydrophobic groups toward air to avail favorable thermodynamic interactions. The Gibbs surface excess concentration at the surface saturation,  max (mol m−2 ), is a useful measure of adsorption effectiveness of amphiphile at the air/water interface [39,40].  max (the maximum surface excess) values for pure morpholinium ILC (M14/M16) and their mixtures with CDs have been evaluated using Gibbs equation. max = −

1 2.303nRT



∂ ∂ log CT



(13)

where CT , n, R, and T are the total concentration of interacting species, no. of species at the air/solution interface, gas constant and temperature respectively. The slope of the tangent of ␥ versus log CT plot (Fig. 3) at the given concentration of cyclodextrin was used

109

to calculate the  max which increases with an increase in the concentration of cyclodextrins. It is clear from the Table 1 that  max values for pure M14 (2.79) is slightly larger than the M16 (2.69). A comparison of  max values of pure morpholinium ILC with conventional surfactants TTAB (2.32) and HTAB (1.12) (due to same counterion (Br- ) and chain length) has been made and is found to be higher for synthesized ILC (M14/M16) which may be due to the head group of M14 and M16 ILC [41]. In case of binary mixtures of M14/M16+ CDs, an increase in the value of surface excess has been observed with the enhancement of CDs concentration (Table 2). At low concentrations of CDs,  max values for all the systems are lower than the pure M14/M16 due to the formation of ILC+ CD complex at low amount of ILC which is not surface-active and hence does not contribute to the surface properties. However at high concentration of CD,  max values for all the systems are more than M14/M16 because once ILC+ CD complex gets formed at higher amount of ILC and the system becomes saturated, further addition of ILC increases the concentration of monomers on the air/water interface which causes an increase in the surface excess values. This is further confirmed by the values of minimum area per molecule, Amin [42,43] which are evaluated by using the following equation: 1018 (14) NA max where NA is the Avogadro’s number. These values for pure M14/M16 and their binary mixtures with CDs have been compiled in Table 2. As expected, Amin value for M16 is higher than M14 and which is due to the increase in chain length. It also suggests that the interacting components are loosely packed at the air/water interface in case of M16. As the amount of CD increases in binary mixtures of M14/M16+ CDs, the value of Amin decreases which suggests that the molecules are closely packed at the air/water interface due to the decrease in repulsive interactions between the interacting components. It is clear from Eqs. (13) and (14) that the trend obtained for surface excess and minimum area per molecule values must be opposite and the same has been observed. Amin of amphiphiles in mixtures can be used to find out the packing parameter (p) which determines the geometry of micelles and indicate the minimum sized micelles in solution. The packing parameter minimizes the Gibbs free energy of micellization and is given by Eq. (15): Amin =

p = Vo /lc Amin

(15)

where Vo is the volume of exclusion per monomer in the aggregate, given by Tanford’s formulae [44], Vo = [27.4 + 26.9(nc − 1)] 2 A˚ 3 , lc is the maximum chain length (as calculated from Eq. (9)) and nc is the number of carbon atoms in the hydrocarbon chain [45]. The value of p > 0.5 for pure morpholinium ILC, M14 and M16 which suggest a flexible bilayer arrangement for these ILC which may be due to its head group and flexible chains [46] as observed in literature also for imidazolium ILC [2]. At low concentrations of ␤-CD in M14+ ␤-CD systems the formation of vesicles has been observed [46]. The reason is that when the hydrophobic part of ILC gets imbedded in the cavity of ␤-CD it leads to the formation of complex between M14/M16+ ␤-CD in such a way that bilayer arrangement takes place. However for binary mixtures of M14+ ␤-CD/HP␤-CD at higher concentrations of CDs, value of p suggests the formation of inverted micelles (p > 1) [46]. This can be attributed to the high concentration of cyclodextrins which easily orients the systems to form reverse micelles due to increase in hydrophobic and van der Waals interactions. For M14/M16+ HP␤-CD systems at low content of CDs, value of p indicates the formation of cylindrical micelles and the structure changes to vesicles and then converts to reverse with the increase in amount of cyclodextrins. This trend has been observed for all the mixtures except for M16+ ␤-CD where at low concentration of ␤-CD, the micelles formed are spherical [46] (p < 0.33) due to the increase in Amin which may be due to increase

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Table 3 o o s ), free energy of adsorption (Gads ), surface free energy (Gmin ), aggregation number (Nagg ) and Stern–Volmer constant (Ksv ) for binary Free energy of micellization (Gm mixtures of M14/M16+ ␤-CD/HP␤-CD at various concentrations of ␤-CD and HP␤-CD. (kJ mol−1 )

(kJ mol−1 )

(kJ mol−1 )

10−3 ·Ksv (M)

[CD] (mM)

o −Gm

M14+ ␤-CD 0.0 0.5 1.0 2.0 3.0

15.5 14.1 15.0 15.1 15.8

15.4 14.0 14.9 15.1 15.7

10.5 13.1 9.7 5.9 3.9

61 66 59 48 43

1.85 1.48 1.63 1.28 1.09

M14+ HP␤-CD 0.5 1.0 2.0 3.0

14.1 15.2 16.3 16.4

14.0 15.2 16.3 16.4

17.7 9.8 4.3 4.4

47 46 45 43

1.27 1.26 1.25 1.23

M16+ ␤-CD 0.0 0.5 1.0 2.0 3.0

9.6 11.5 12.0 12.3 14.0

9.5 11.4 11.9 12.2 14.0

11.8 22.5 26.9 9.1 4.9

58 31 33 35 36

10.80 7.89 8.40 8.63 9.97

M16+ HP␤-CD 0.5 1.0 2.0 3.0

10.8 12.0 12.9 13.9

10.8 12.0 12.9 13.9

21.0 15.0 8.0 8.6

39 33 34 35

10.70 8.35 9.36 10.10

a b c d

a

o −Gads

b

s Gmin

c

Nagg

d

The uncertainty in these values ranges from ±0.01 to ±0.1. The uncertainty in these values ranges from ±0.01 to ±0.1. The uncertainty in these values ranges from ±0.1 to ±0.4. The uncertainty in these values ranges from ±0.1 to ±0.5.

in length of hydrophobic chain of M16 because of steric origin from the tails of this ILC. 3.1.5. Thermodynamic parameters To quantify the interactions between morpholinium based ILC (M14 and M16) and CDs at the air/water interface as well as in bulk various thermodynamic parameters have been calculated. Sugihara et al. [47] have proposed free energy of the given air/water s , a thermodynamic quantity for the evaluation of interface, Gmin synergism and is given by: s Gmin = Amin .cmc .NA

(16)

s Gmin

(Table 3) is regarded as the work needed to make an interface per mole of the free energy change accompanied by the transition from bulk to the surface of the solution. In other words, s , the more thermodynamically stable the lower the value of Gmin surface is formed or more surface activity is attained, same has been observed for the present mixtures which suggests enhancement in the spontaneity of process. The standard Gibbs energy of o , have been calculated using Eq. (17): micellization [48], Gm o Gm = (2 − ˛) · RT · ln CT

(17)

where ˛ is the micelle degree of dissociation and other symbols o (Table 3) are negative have their usual meaning. The values of Gm for all the mixtures which indicate that the process of micellization is spontaneous. An overall increase in the negative value of o has been noticed with the increase in concentration of CDs in Gm M14/M16+ CDs mixtures. The standard Gibbs energy of adsorption o , was calculated according to Eq. (18): [49], Gads



  cmc  o

o Gads = Gm −

max

(18)

The standard state for the adsorbed amphiphile is a hypothetical monolayer at its minimum surface area per molecule, when surface pressure is assumed to be zero. The last term (cmc /max ) in Eq.

(18) expresses the work involved in transferring the amphiphile from a monolayer at zero surface pressure to the micelle. Here for all the binary mixtures last term of Eq. (18) is very small as como which suggests that work involved in transferring pare to Gm the ILC from a monolayer at zero surface pressure to the micelle o is negligible. Gads values (Table 3) are negative and reveal that o and the process is spontaneous. On comparing the values of Gm o , very little difference has been observed for M14/M16+ ␤Gads CD mixtures. However for M14/M16+ HP␤-CD mixtures both the values are found to be same which implies that for these mixtures o . This last term in Eq. (18) does not contribute to the value of Gads may be due to the encapsulation of ILC in CDs to form inclusion complexes in such a way that no monomer is free to contribute to the surface properties.

3.2. Fluorescence measurements Fluorescence is a versatile technique for the study of static and dynamic properties of aggregated systems such as micelles. The change in the microenvironment of solution experienced by the probe (pyrene in the present study) is reflected in the relative intensity of vibronic fine structures of monomer fluorescence when pyrene get transferred from the aqueous bulk to micellar phase. Fluorescence measurements can be used to calculate the aggregation number of micelles and also in understanding the interactions between the host-guest inclusion processes.

3.2.1. Aggregation number The aggregation number (Nagg ) defines the total number of amphiphilic molecules forming pure or mixed micelles. The concentration of pyrene was kept constant in solution and the amount of quencher (HPyCl) was varied. The aggregation numbers of the pure morpholinium ILC (M14 and M16) and their mixtures with ␤-CD/HP␤-CD were determined by the static fluorescence

R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115

1.0

0.35

(a)

1

0.6

β − CD (mM) 0.0 0.5 1.0 2.0 3.0

o

0.20

o

1

0.8

0.0 0.5 1.0 2.0 3.0

0.25

ln(I /I )

(b)

β − CD (mM)

ln(I /I )

0.30

111

0.15

0.4

0.10 0.2 0.05

0.00 0.0

5.0 10

-5

0.00010

0.00015

0.00020

0.00025

0.0 0.0

-3

[Q] / mol dm

5.0 10

-5

0.00010 -3

0.00015

[Q] / mol dm

Fig. 5. Plot of ln I0 /I1 vs. [Q] at various concentrations of ␤-CD (a) M14+ ␤-CD (b) M16+ ␤-CD.

quenching method using the following equation [50]: Nagg =

ln(I0 /I1 ) · (CT − cmc) [Q ]

probe. It reflects the accessibility of the fluorophore to the quencher and can be calculated from the following relation: (19)

where I0 and I1 are the fluorescence intensities of first vibronic peak of pyrene without and with the quencher, (Q) respectively. A plot of ln (I0/ I1 ) vs. [Q] results in a slope that allows the calculation of aggregation number since the concentration of amphiphiles and cmc are known. Fig. 5(a) and (b) shows the quenching plots of binary mixtures of M14/M16+ ␤-CD for various concentrations of ␤-CD and the solid line observed here represent the best fit to Eq. (19). Similarly, linear behavior has been obtained for M14/M16+ HP␤-CD systems also (Fig. S3). Nagg values for pure morpholinium ILC and their mixtures with CDs have been reported in Table 3. It is clear that Nagg values for pure M14 and M16 are higher than their mixtures with cyclodextrins which is due to the dense packing of these amphiphiles and hence more closely packed micelle structure is formed. However in case of mixtures, close packing is restricted due to the formation of complex which reduces the availability of free ILC for the formation of micelles. A comparison between the M14+ CDs and M16+ CDs systems implies that the Nagg values for former mixtures are higher than the later one. This suggests strong interactions between M16 and CDs which leads to the formation of more stable M16+ CDs complex in comparison to the M14+ CDs complex. This fact was also supported by the values of association constants (K) for these binary mixtures. The reason behind this may be the appropriate size of M16 ionic liquid crystals which fit well into the cavity of CDs and hence generates a more stable complex. The effect of concentration of CDs has also been studied which indicates that the increasing amount of ␤-CD and HP␤-CD causes a decrease in Nagg values for M14 ILC. This can be attributed to the fact that as the concentration of CDs increase, more number of ILC tends to unites with the cavity of cyclodextrins due to stronger ILC-CD interactions than aggregation of ILC which leads to the formation of loosely packed micelles. However for M16 ionic liquid crystals, an increase in the concentration of CDs increases Nagg value to a little extent. This may be due to the fact that once a more stable complex has been formed; number of ILC available for the micelle formation enhances and tends to increase the value of Nagg . 3.2.2. Stern–Volmer constant The Stern–Volmer equation was used to get the equilibrium constant for the interaction between the quencher and the fluorescence

I0 /I1 = 1 + Ksv [Q]

(20)

where Ksv is the Stern–Volmer constant and all other symbols have their usual meanings. The values of Ksv are evaluated from the slope of Eq. (20) and listed in Table 3. The values of Ksv for pure morpholinium ILC and their mixtures with CDs are different which comprises that the hydrophobicity of micelle environment and efficiency of ILC to quench the fluorophore vary with interacting species. The quenching efficiency is related to the probability of finding both Py* and HPy+ species in a confined environment and solubilization of pyrene is restricted to the palisade layer [51]. The values of Ksv for the pure M14/M16 are found to be more than their respective mixtures which suggest that in the presence of CDs the environment becomes less suitable for an effective quenching. Further Ksv values for M14 and M14+ CDs are found to be lower than M16 and M16+ CDs, respectively, which suggest less hydrophobic environment for M14 and their mixtures. This is also supported by the values of association constants for these systems, i.e. K values are low for M14 and their respective mixtures. Even for M16 and M16+ CDs systems should have shown much higher Ksv but these values are not so high. It supports that despite the presence of strong hydrophobic environment, HPy+ ion seems unable to approach Py*. This can be attributed to the formation of complex between ILC and CDs which restricts HPy+ ions to approach Py*. Also quencher competes with ILC to form complex which limits the availability of HPy+ ions. It is also clear from Table 3 that there is an increase in the value of Ksv as the concentration of CD increases for M16+ ␤-CD/HP␤-CD binary mixtures. An enhancement in Ksv value suggests an increase in the first order quenching due to the greater probability of finding both probe and quencher in a stronger hydrophobic environment. However in case of the binary mixtures of M14+ ␤-CD/HP␤-CD, Ksv values shows an overall decrease with the increase in composition of CDs which indicate that the quencher is unable to approach Py* embedded in highly hydrophobic environment. This may be due to low stability of M14+ CDs complex which causes a reduction in hydrophobic interactions also. 3.3. NMR measurements NMR spectroscopy has been used in the analysis of structural and dynamic properties of complexes in aqueous solution. 1 H NMR

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R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115

Fig. 6.

1

H NMR titrations of ␤-CD with increasing equivalents of (a) M14 and (b) M16.

can also be used to determine the direction of penetration of guest molecules into cyclodextrin cavity [52]. Moreover, NMR on CDs chemistry is so important that no other technique give the same wealth of chemical information on the supramolecular systems. The formation of inclusion complexes with cyclodextrin is normally evidenced by changes in chemical shifts [53]. Such chemical shift changes may provide valuable insight into the molecular conformation of the inclusion complexes. The concentration of added ILC

varies from below cmc to above cmc in all the spectra of M14/M16+ CDs systems. 1 H NMR spectrum of ␤-CD, HP␤-CD and pure ILC (M14/M16) in D2 O is shown in electronic supplementary material (Figs. S4 and S5). It is clear that the hydrophobic moieties of ILC near the core of the micelles are highly shielded and therefore show resonance at lower ı values (resonances for p, q, r and s protons). However, as we move toward the head group, the presence of N and O atoms make the adjacent protons more deshielded

R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115

Fig. 7.

1

113

H NMR titrations of HP␤-CD with increasing equivalents of (a) M14 and (b) M16.

(Fig. S1) and therefore show higher ı values (resonances for t, w, u and v protons) (Fig. S5). But for ␤-CD and HP␤-CD, most of the protons appear in the higher ı values region except H7 which is in propyl-group of HP␤-CD (Fig. S4). Figs. 6 and 7 show the influence of simultaneous addition of increasing equivalents of morpholinium ILC (M14/M16 to ␤-CD in Fig. 6(a) and (b) and M14/M16 to HP␤-CD in Fig. 7(a) and (b)). It is to be mention here that in these figures we focused only on those particular regions of aliphatic sections of 1 H NMR spectra where the changes are prominent. Significant changes were observed for 1 H NMR spectra of mixtures and these upfield and downfield chemical shift values have been reported in Tables S1

(M14/M16+ ␤-CD) and S2 (M14/M16+ HP␤-CD) in supplementary material. For most of the mixtures when the concentration of ILC exceeds the value of its cmc, H3 and H5 signal disappears which indicates the formation of complex between CD and ILC (Fig. 8). This is because H3 and H5 protons are inside the cavity of CD and are intensely affected by the protons of tail of ILC (Fig. S6). The stability of the inclusion complex and the orientation of ILC can be inferred by this experiment. When ıH3 > ıH5, (here ı = ıcomplexed − ıfree ) it is considered that the inclusion of guest molecule occurs partially inside the cavity and if ıH3 < ıH5 then a total inclusion takes

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R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115

Fig. 8. Scheme showing penetration of ILC below and above cmc in CDs.

place [54]. For M14/M16+ HP␤-CD mixtures, ıH3 > ıH5 (Table S3) suggest partial inclusion of ILC in the cavity of CDs which may be due to the steric hindrance caused by the hydroxypropyl group located at the edge of the CD cavity in HP␤-CD and hence causes a restriction to the incoming guest molecule. However for mixtures of M14/M16+ ␤-CD, reverse takes place, i.e. the values of ıH3 and H5 suggest a total inclusion of the species. This may be due to the absence of bulky group on CD cavity. The change in chemical shift of H3 and H5 protons of CDs and Hp, Hq, Hr, Hs protons of ILC ensures the encapsulation of ILC in the CDs cavity. This could be due to the hydrophobic forces which play a significant role to bind the insoluble part of ILC with non-polar part of the CDs and van der Waals forces. The possible orientation of ILC is such that the hydrophobic tail of M14 and M16 gets encapsulated in the cavity of CDs and is shown in Fig. 8. As it is clear from the scheme that below cmc only complexation takes place while above cmc complexation with micellization takes place. 4. Conclusions Conductivity, surface tension, fluorescence and 1 H NMR techniques have been employed to reveal the binding behavior of cyclodextrins (␤-CD and HP␤-CD) with the synthesized morpholinium ILC (M14 and M16). The data provide much insight into the nature of these ILC as in literature this is the first report on the evaluation of various micellar, interfacial and thermodynamic parameters of morpholinium ILC. Moreover to study their interactions with CDs are important from industrial and chemical point of view also. The values of cmc for all the mixtures show an increase with the increase in concentration of CDs which suggest that the micellization process is delayed due to the formation of inclusion complexes between CDs and ILC. K values obtained for the binary mixtures of M16+ CDs is higher than M14+ CDs which indicate that M16 bind more strongly with CDs. cmc values for M14/M16+ HP␤-CD complexes are found to be lower than M14/M16+ ␤-CD complexes which imply that the introduction of hydroxypropyl group in the system causes decrease in the efficiency of these systems.  max values obtained for pure M14/M16 ILC have been compared with conventional surfactants (TTAB and HTAB) and found to be higher for M14/M16. The value of packing parameter

(p > 0.5) for these ILC suggests a bilayer arrangement for pure ILC. Nagg values for pure M14 and M16 are higher than their mixtures with CDs due to the dense packing of these ILC in the absence of CDs. Also formation of complex between CDs and ILC is the main cause for the reduction of Nagg values for mixtures. Ksv values for M14/M16+ CDs mixtures are less than the individual components due to the less hydrophobic environment experienced by the intero and Go acting components. The negative values of Gm show ads that the micelle formation and adsorption of amphiphiles at the air/water interface is energetically favorable, while a low value of s Gmin ensures the stability of mixed micelles. The formation of inclusion complexes of ILC: CD is also evidenced by changes in chemical shifts of CDs and ILC protons. Significantly, this work provides us a key step to understand the effects of CDs on the morpholinium ILC. Also the addition of CDs to various amphiphiles may solve the pharmaceutical problems such as toxicity which comes from the surface active property of amphiphiles. Moreover in the light of the above study it is demonstrated that CDs can be used for extraction purposes of venomous materials due to their complexation property. In this way, the present work would take the field forward and enlighten novel ways for researches to explore new findings. Acknowledgment Rabia Sharma thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for the award of Senior Research Fellowship. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fluid.2013.10.042. References [1] K. Binnemans, Chem. Rev. 105 (2005) 4148–4204. [2] C.K. Lee, H.W. Hsin, I.J.B. Lin, Chem. Commun. 19 (2000) 1911–1912. [3] N. Nishiyama, S. Tanaka, Y. Egashira, Y. Oku, K. Ueyama, Chem. Mater. 15 (2003) 1006–1011. [4] K.E. Strawhecker, E. Manias, Chem. Mater. 15 (2003) 844–849. [5] T. Wang, H. Kaper, M. Antonietti, B. Smarsly, Langmuir 23 (2007) 1489–1495.

R. Sharma et al. / Fluid Phase Equilibria 361 (2014) 104–115 [6] D. Haristoy, D. Tsiourvas, Chem. Mater. 15 (2003) 2079–2083. [7] N. Yamanaka, R. Kawano, W. Kubo, T. Kitamura, Y. Wada, M. Watanabe, S. Yanagida, Chem. Commun. 6 (2005) 740–742. [8] P.H.J. Kouwer, T.M. Swager, J. Am. Chem. Soc. 129 (2007) 14042–14052. [9] J. Yang, Q. Zhang, L. Zhu, S. Zhang, J. Li, X. Zhang, Y. Deng, Chem. Mater. 19 (2007) 2544–2550. [10] P. Szklarz, M. Owczarek, G. Bator, T. Lis, K. Gatner, R. Jakubas, J. Mol. Struct. 929 (2009) 48–57. [11] K. Lava, K. Binnemans, T. Cardinaels, J. Phys. Chem. B 113 (2009) 9506–9511. [12] B. Huang, Z. Li, N. Shi, X. Xu, K. Zhang, Y. Fang, Youji Huaxue 29 (2009) 770. [13] A. Choudhary, S. Agarwal, V. Sharma, Indian J. Chem. Sect. A Inorg. Bio-inorg. Phys. Theor. Anal. Chem. 48A (2009) 362–366. [14] B.-J. Gans, R. Hansel, US 2009053552 A1 20090226 (2009). [15] C. Pretti, C. Chiappe, I. Baldetti, S. Brunini, G. Monni, L. Intorre, Environ. Saf. 72 (2009) 1170–1176. [16] K.S. Kim, S.Y. Park, S. Choi, H. Lee, J. Power Sources 155 (2006) 385–390. [17] S.P.M. Ventura, A.M.M. Goncalves, T. Sintra, J.L. Pereira, F. Goncalves, J.A.P. Coutinho, Ecotoxicology 22 (2013) 1–12. [18] M.A. Hossan, K. Hamasaki, K. Takahashi, H. Mihara, A. Ueno, J. Am. Chem. Soc. 123 (2001) 7435–7436. [19] X. Du, X. Chen, W. Lu, J. Hou, J. Colloid Interface Sci. 274 (2004) 645–651. [20] R. Singh, N. Bharti, J. Madan, S.N. Hiremath, J. Pharm. Sci. Technol. 3 (2010) 171–183. [21] K.-H. Fromming, J.J. Szejtli, Cyclodextrins in Pharmacy, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994. [22] S.K. Mehta, K.K. Bhasin, S. Dham, M.L. Singla, J. Colloid Interface Sci. 321 (2008) 442–451. [23] N. Funasaki, M. Ohigashi, S. Hada, S. Neya, Langmuir 16 (2000) 383–388. [24] S. Tanada, N. Kawasaki, A.T. Hubbard (Eds.), Encyclopedia of Surface and Colloid Science, Marcel Dekker, New York, 2002. [25] R.D. Lisi, G. Lazzara, S. Milioto, N. Muratore, I.V. Terekhova, Langmuir 19 (2003) 7188–7195. [26] P. Choppinet, L. Jullien, B. Valeur, J. Chem. Soc. Perkin Trans. 2 (1999) 249–255. [27] H. Gharibi, S. Jalili, T. Rajabi, Colloids Surf. A 175 (2000) 361–369. [28] Y. Liu, Y. Liu, R. Guo, J. Solut. Chem. 40 (2011) 1140–1152. [29] M. Galinski, I. Stepniak, J. Appl. Electrochem. 39 (2009) 1949–1953.

115

[30] J.-H. Cha, K.-S. Kim, S. Choi, S.-H. Yeon, H. Lee, H.S. Kim, H. Kim, Korean J. Chem. Eng. 22 (2005) 945–948. [31] R.K. Mahajan, R. Sharma, J. Colloid Interface Sci. 363 (2011) 275–283. [32] E. Junquera, G. Tardajos, E. Aicart, Langmuir 9 (1993) 1213–1219. [33] T. Tominaga, D. Hachisu, M. Kamado, Langmuir 10 (1994) 4676–4680. [34] H.-J. Buschmann, E. Cleve, E. Schollmeyer, J. Incl. Phenom. Macrocycl. Chem. (1999) 233–241. [35] N. Jiang, P. Li, Y. Wang, H. Yan, R.K. Thomas, J. Phys. Chem. B 108 (2004) 15385–15391. [36] P. Sehgal, M. Sharma, R. Wimmer, K.L. Larsen, D.E. Otzen, Colloid Polym. Sci. 284 (2006) 916–926. [37] M.H. Maeso, S.P. Gonzalez, C.B. Diaz, E.G. Romero, Colloids Surf. A 249 (2004) 29–33. [38] A.A. Rafati, A. Bagheri, H. Houkhani, M. Zarinehzad, J. Mol. Liq. 116 (2005) 37–41. [39] M.J. Rosen, Surfactants and Interfacial Phenomenon, Wiley-Intersciences, John Wiley & Sons, New York, 2004. [40] R. Sharma, R.K. Mahajan, RSC Adv. 2 (2012) 9571–9583. [41] S.P. Moulik, Md.E. Haque, P.K. Jana, A.R. Das, J. Phys. Chem. 100 (1996) 701–708. [42] K. Anand, O.P. Yadav, P.P. Singh, Colloids Surf. A 55 (1991) 345–358. [43] S. Mahajan, R. Sharma, R.K. Mahajan, Langmuir 28 (2012) 17238–17246. [44] C. Tanford, The Hydrophobic Effect: Formation of Micelles and Biological Membranes, John Wiley & Sons, New York, 1980. [45] J.N. Israelachvili, D.J. Mitchell, B.W. Ninham, J. Chem. Soc. 72 (1976) 1525–1568. [46] D. Myers, Surfactant Science and Technology, third ed., Wiley-Intersciences, John Wiley & Sons, New Jersey, 2006. [47] J.-S. Ko, S.-W. Oh, Y.-S. Kim, N. Nakashima, S. Nagadome, G. Sugihara, J. Oleo Sci. 53 (2004) 109–126. [48] J. Aguiar, J.A. Molina-Bolivar, J.M. Peula-Garcia, C.C. Ruiz, J. Colloid Interface Sci. 255 (2002) 382–390. [49] R. Zana, Langmuir 12 (1996) 1208–1211. [50] J. Turo, M. Gratzel, A.M. Braun, Angew. Chem. Int. Ed. Engl. 19 (1980) 675–696. [51] M.S. Bakshi, N. Kaur, R.K. Mahajan, J. Photochem. Photobiol. A Chem. 183 (2006) 146–153. [52] N. Funasaki, H. Yamaguchi, S. Ishikawa, S. Neya, J. Phys. Chem. B 105 (2001) 760–765. [53] H.-J. Schneider, F. Hacket, V. Rudiger, Chem. Rev. 98 (1998) 1755–1785. [54] D. Greatbanks, R. Pickford, Magn. Reson. Chem. 25 (1987) 208–215.