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Hydrophobically Driven-Morphologically Diverse SelfAssembled Architectures of Deoxycholate and Imidazolium Based Biamphiphilic Ionic Liquids in Aqueous Medium Gurbir Singh, * Komal, Manpreet Singh, Ormanpreet Singh, and Tejwant Singh Kang J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b10161 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on December 2, 2018

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The Journal of Physical Chemistry

Hydrophobically

Driven-Morphologically

Diverse

Self-assembled

Architectures of Deoxycholate and Imidazolium Based Biamphiphilic Ionic Liquids in Aqueous Medium Gurbir Singh,a Komal,a Manpreet Singh,a Ormanpreet Singh,a Tejwant Singh Kanga,* aDepartment

of Chemistry, UGC-Centre for Advanced Studies-II, Guru Nanak Dev University, Amritsar-

143005, India

Abstract: Biamphiphilic ionic liquids (BAILs) having amphiphilic cation and as well as anion are thought to exhibit improved surface activity and colloidal stability to be utilized in different applications. For their effective use, a control over synergetic hydrophobic and electrostatic interactions between oppositely charged ions along with the possibility of tuning of hydrophobicity of the core of aggregates is required. Focusing on this, new BAILs comprising a bile salt anion, deoxycholate, [DC]-, and 1-alkyl-3-methylimidazolium cations, [Cnmim]+ (n = 2, 4, 6, 8, and 12) were synthesized and characterized for their behavior at air-solution interface as well as in bulk. The synthesized BAILs exhibit high surface activity and self-assemble in the form of different architectures ranging from nano-sheets, nano-rods and vesicles with varying hydrophobicity of the formed core of aggregates, depending on the length of alkyl chain of [Cnmim]+. Analysis of various parameters obtained from investigated techniques suggested the changing role of [Cnmim]+ from a counter-ion (n = 2 & 4) to a co-surfactant (n = 8 & 12) via a borderline case of [C6mim]+. This changeover in the nature of counter-ion controlled by hydrophobicity of alkyl chain resulted in morphological diversification in self-assembled architectures via varying set of interactions. It is believed that the present work would offer new perspectives in self-assembly phenomenon of surfactants in general and SAILs in particular to devise new strategies for inducing morphology dependent functionality in self-assembled structures of BAILs.

*To whom correspondence should be addressed: e-mail: [email protected]; [email protected] 3206

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1. Introduction In past few decades, ionic liquids (ILs), that are composed solely of ions and are liquids at temperatures at least below 100 ºC,1 have gained considerable popularity among different scientific ventures due to their remarkable physicochemical properties.2-5 This led to their utilization in various applications,6-11 and growth of IL based chemistry resulted in further categorization of ILs into different types, e.g., room-temperature ILs (RTILs),6,11 polyionic liquids (PILs),12 task-specific ILs (TSILs),13-14 energetic ionic liquids (EILs),15 and supported IL membranes (SILMs).16 Inherent amphiphilicity of various ILs, derived by their designer nature required to control their physico-chemical properties, resulted in emergence of another class of ILs called as surface active ionic liquids (SAILs)17-26 that many times exhibit better surface activity as compared to conventional ionic surfactants. A control over surface active behavior of SAILs along with other characteristic properties of self-assembly via judicious choice of cation and anion, length of alkyl chain along with a possibility of functionalization of alkyl chain further increased the interest of scientific community in SAILs. The pioneering work on self-assembly of ILs was reported by Bowers et al., wherein aggregation behavior of ILs, [C4mim][BF4], [C8mim][Cl], and [C8mim][I] was thoroughly investigated,17 which was followed by many research groups including ours. Till date, a variety of SAILs based on different ionic head groups such as imidazolium,17-26 pyridinium,27-28

pyrolidinium,28

morpholinium,29 and nicotinium30 etc., appended with different hydrocarbon chain lengths along with functionalization of alkyl chain27,30 near ionic head group, have been reported for their selfassembly behavior in aqueous medium. The effect of nature of different counter-ions on selfassembly behavior has also been reported.31 Both, the nature of cation and anion influences the aggregation behavior of SAILs, governed through mutual binding capacity, hydrophobicity and


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size of cation and anion, and studies on self-assembly behavior of imidazolium based SAILs having aromatic counter-ions has been derived.32-33 To have synergetic interactions similar to that exhibited by cat-anionic systems giving rise to better surface activity,34-36 not present in either of cationic or anionic surfactant, biamphiphilic SAILs (BAILs) comprising amphiphilic cation and anion were reported.37-39 This not only led to enhanced surface activity but also offered less complex systems than cat-anionic ones, which contains the counter-ions of respective surfactants in the colloidal system. Therefore, we conceptualize to synthesize and investigate aggregation behavior of new BAILs based on imidazolium cations, [Cnmim]+, and deoxycholate anion, [DC]-, in expectation of high surface activity and control over structure of formed aggregates via minor alterations in length of alkyl chain of [Cnmim]+. Here

in,

biamphiphilic

ionic

liquids

[BAILs],

[Cnmim][DC],

based

on

1-alkyl-3-

methylimidazolium cation, [Cnmim]+ (n = 2, 4, 6, 8 & 12) and deoxycholate anion, [DC]-, are synthesized, characterized and investigated for their self-assembly behavior in aqueous medium. It has been established that conventional surfactants with alkyl chain shorter than 8 Cs don’t selfassemble in aqueous medium where as those with 6 carbon long alkyl chain may self-assemble depending on the nature of counter-ion. 40 Therefore, it was thought that a changeover from the cations with shorter alkyl chains ([C2mim]+ and [C4mim]+) to that with relatively longer alkyl chains ([C8mim]+ and [C12mim]+) via a border line case of [C6mim]+ would alter the overall amphiphilicity of system and thus appreciable changes in characteristic properties of selfassembly were expected. The choice of bile salt as one of the component of BAILs, sodium deoxycholate (NaDC), a steroidal compound, is governed by its unusual molecular structure having hydrophobic convex and hydrophilic concave surface41,42 contrary to a polar head and non-polar aliphatic tail present in classical surfactants, that self-assemble in an unusual manner



in aqueous medium.42,43 Its biological origin and vital importance in various physiological processes that includes solubilization of water insoluble compounds,44-46 drug delivery47-48 and as model molecules used to study membrane interactions49 also instigated us to investigate BAILs having deoxycholate, [DC], as anion. Moreover, such systems would be interesting to investigate for their self-assembly behavior considering a wide dissimilarity in molecular structure of [Cnmim]+ and [DC]- (Scheme 1). It is mentioned that there are some reports wherein, micellization of NaDC in aqueous solutions of imidazolium based SAILs has been investigated,50-52 however to best of our knowledge, there exists no report on BAILs comprising [Cnmim]+ and [DC]-. The investigated system is different than reported systems in the sense that here [Cnmim]+ would act either as counter-ion or co-surfactant in the absence of other ions (Na+ and X-), which remains present in case of cat-anionic systems or investigated systems of NaDC in aqueous solution of SAILs.50-52 The presence of such ions would affect the physico-chemical properties such as degree of hydration, counter-ion binding and entropy, which plays an important role in controlling the characteristic properties of micellization.50-51 Further, it is difficult to assess the quantitative effect of such ions in these relatively complex systems. Therefore, it is expected that the present study would offer new insights into devising and controlling the aggregation behavior of BAILs to create diverse sell-assembled structures, which expectedly would find place in different applications in future. 2. Materials and methods 1-methylimidazole (≈ 99%), 1-chlorododecane (> 97%), 1-chlorooctane (≈ 99%), 1chlorohexane (≈ 99%), 1-ethyl-3-methylimidazolium chloride (≈ 98%) and sodium deoxycholate ( ≈98%) were purchased from Sigma Aldrich, and used without further purification. Butanol, methanol, acetone, ethyl acetate and diethyl ether (AR grade) were purchased from SD Fine-


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Chem Ltd., Mumbai, India. Pyrene was purchased from Sigma Aldrich and used after recrystallization from ethanol. Synthetic procedure adopted for synthesis of BAILs along with characterization data (1H NMR and HR-MS) are provided in Annexure S1 and S2 (Supporting Information, SI). The details about the methods employed for carrying out different measurements are provided in Annexure S3 (SI). Scheme 1 shows the molecular structure of different surfactant ions comprising BAILs.

Scheme 1: Molecular structure of [DC]- and [Cnmim]+ ions constituting the synthesized BAILs. Left side (light Pink in shade) of the scheme shows the hydrophobic convex and hydrophilic concave face of [DC]- with protons labeling.

3. Results and Discussion 3.1 Surface Active Nature of BAILs: Variation of log of surface tension () of aqueous solution BAILs and their precursor NaDC, as a function of concentration, is shown in Figure 1 (A-B) and Figure S1, SI respectively. Critical aggregation concentration (cac) of investigated BAILs (Table 1) is obtained as the concentration corresponding to onset of plateau region in concentration profiles of . The obtained cac follows the order: BAIL-1 > BAIL-4 > BAIL-2 ≈ BAIL-3 > NaDC > BAIL-5, which deviates from the fact that, in general, cac decreases with an increase in alkyl chain length of conventional ionic surfactants40 and SAILs.17,20,24,29,30 Even for BAILs comprising [Cnmim]+ and [C8OSO3]-, cac decreases with increase in alkyl chain length of imidazolium cation.37 The value of cac seems to be governed by [DC]- in BAIL-1 and BAIL-2



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where [Cnmim]+ acts as counter-ion and remains hydrated at the aggregate-water interface of self-assembled structures.

Figure 1 (A,B):Variation of surface tension () as a function of log of concentration of investigated BAILs at 298.15 K. Table 1: Critical aggregation concentration (cac) determined using surface tension (ST), conductivity (Cond.), steady-state fluorescence (Flr.) and isothermal titration calorimetry (ITC) measurements along with various parameters obtained from surface tension (ST) measurements for investigated BAILs and NaDC at 298.15 K. ST

Cond. C1

cac

Flr. C1

ITC cac

---

C1

5.0

Ave. cac

C1

6.0

ST cac

γcac

Пcac

pC20

106Г

Amin

4.3

26.1

45.9

3.81

2.6

69

[Na][DC]

1.91

[C2mim][DC]

3.2

2.2

5.0

1.41

5.5

1.62

4.3

1.74

4.2

41.6

30.4

3.41

0.89

184

[C4mim][DC]

2.4

1.44

5.4

1.06

5.8

2.0

5.0

1.50

4.4

42.6

29.4

3.46

0.95

174

[C6mim][DC]

2.5

1.61

6.8

0.8

6.6

1.9

6.9

1.44

5.3

40.3

31.7

3.40

1.21

137

[C8mim][DC]

2.9

0.74

3.2

1.05

2.5

1.27

2.9

1.02

2.9

36.0

36.0

4.03

0.98

170

[C12mim][DC]

0.41

0.21

0.84

0.3

0.83

0.3

0.5

0.27

0.6

33.5

38.5

4.55

1.37

120

Units for C1 and cac are mmol L-1, γcmc and Пcmc are mN.m-1, 106Г is μmol∙m-2, and Amin is Å2. Ave. represents the average value of concerned parameter obtained from different techniques.

However the increased hydrophobicity of [C4mim]+ as compared to [C2mim]+ give rise to lower cac value of BAIL-2 as compared to BAIL-1. [C6mim]+ of BAIL-3 being relatively more hydrophobic has tendency to act as a counter-ion by keeping its alkyl chain hydrophobically hydrated towards bulk water as well as a co-surfactant that could result in the formation of mixed


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aggregates with [DC]-. It is natural to believe that hydration of [C6mim]+ would opposes its insertion towards hydrophobic core of forming aggregates and delays the cac. Further a competition between hydration of cation and its hydrophobic interactions with anion become more prominent in case of BAIL-4. [C8mim]+ have enough hydrophobicity to get inserted into aggregates and could also compete with [DC]- to occupy air-water interface. This leads to delay in surface saturation and therefore, highest cac value for BAIL-4 has been observed from surface tension measurements among investigated BAILs. It is important to mention that this trend of variation in cac is not reflected from bulk behavior, where BAIL-3 exhibit highest cac (average) as obtained from different techniques. Another reason for such behavior seems to be varying nature of hydrophobic and hydrophilic surfaces of constituent ions. The hydrophilicity and hydrophobicity of [DC]- is stretched over its concave and convex surface (Scheme 1) as compared to linear chain structure of [Cnmim]+. Therefore the orientation of [DC]- at air-solution interface is expected to be completely different as compared to linear surface active anions, which in turn could affect the orientation of [Cnmim]+ cations leading to contrasting variation in cac values. With further increase in alkyl chain length of [Cnmim]+ in BAIL-5, cac decreases drastically. The obtained cac values are smaller than that observed for either of the NaDC and [C12mim][Br].24 This marks the dominance of favorable synergetic hydrophobic interactions assisted by electrostatic interactions over opposing forces of hydration and structural mismatch between [DC]- and [Cnmim]+. Similar synergistic interactions were also observed in the mixed systems of NaDC and [C12mim][Br], where around 0.5 mole fraction, the behavior of the mixed system at air-water interface is quite similar to that observed for BAIL-5.51 Surface tension at cac (cac) and effectiveness of surface tension reduction (𝛱𝑐𝑎𝑐) (Table 1) for investigated BAILs decreases and increases, respectively, following the same order as observed



Page 8 of 34

for the variation of cac. On similar lines, higher increase (6 to 10 times in magnitude) in adsorption efficiency (pC20) for BAIL-4 and BAIL-5 as compared to other BAILs is observed. This again suggests the varying role of cations from counter-ion to co-surfactant depending on the length of alkyl chain, which influences the hydrophobic interactions between [Cnmim]+ and [DC]- and hence the surface behavior. To have an understanding about the packing and orientation of BAILs at air-solution interface, saturation adsorption (Гmax) and minimum surface area per molecule (Amin) was calculated using the surface tension data via following equation:40,53 Гmax = 1023/𝑁𝐴 * 𝐴𝑚𝑖𝑛 = ―

1

𝑛𝑅𝑇 (∂𝛾/∂𝑙𝑛𝐶)𝑇

(1)

where (∂𝛾/∂𝑙𝑛𝐶) is the pre-micellar slope obtained from the γ versus natural log of concentration plot (Figure 1A-B), n is a pre-factor whose value is equal to the number of species formed in solution upon dissociation of amphiphile (n = 2) and other symbols have usual meanings. As can be seen from Table 1, the value of Amin follows the order: BAIL-1 > BAIL-2 > BAIL-4 > BAIL-3 > BAIL-5 and is indicative of enhanced packing of ions at air-solution interface with increase in alkyl chain length of [Cnmim]+. The observed trend is in line with what has been observed for variation in cac and other parameters with the exception of BAIL-4. It is natural to assume that an increase in alkyl chain length of the [Cnmim]+ cation beyond C8 favors the expulsion of hydrocarbon chain from bulk toward air-solution interface perpendicular to central axis of [DC]-. This increases the surface excess resulting in relatively tight packing of the BAIL ions. The results obtained from the surface tension measurements of the BAILs under investigation are in contrast to that reported in the literature for the similar BAILs where a linear variation in cac, pC20, Гmax and Amin has been observed with increase in the alkyl chain length of [Cnmim]+.37 A


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mismatch of hydrophobicity and hydrophilicity surfaces between cation and anion in investigated SAILs results in such variation. 3.2 Aggregation Behavior of the BAILs in Bulk: Aggregation behavior of BAILs in bulk was explored by using conductivity (Figure 2A and Figure S2A, SI) and steady-state fluorescence (Figure 2B-D) measurements. The concentrations of BAILs corresponding to a break-point in the variation of specific conductance () and midpoint of sigmoidal curve (Figure 2C) for the variation in polarity index (I1/I3) of pyrene,54 as a function of concentration of BAILs, marks the cac.

Figure 2 (A-D): Variation in (A) specific conductivity; (B) a representative pyrene fluorescence spectra; and (C) variation in the I1/I3 values as a function of concentration of different BAILs in aqueous medium at 298.15 K. (D) Logarithmic variation of (Io/I) of pyrene as a function of quencher concentration (Cq) used to obtain aggregation number of different BAILs.



The obtained values of cac are provided in Table 1 and are found to be in close agreement with those obtained from surface tension measurements, qualitatively. However, quantitatively the values obtained from γ measurements are on lower side as different techniques monitor different stages of aggregation. The variation of molar conductance (Λm) as a function of C1/2 of SAIL suggests the formation of pre-aggregates (Figure S3, SI) by BAILs whereas no pre-aggregation is observed in case of NaDC (Figure S4, SI). This suggests the role of [Cnmim]+ in the formation of pre-aggregates. The formation of pre-aggregates by investigated BAILs is supported by an initial decay in variation of I1/I3, much below cac (Figure 2C and S5, SI). The concentrations corresponding to pre-aggregate formation, marked as C1, are provided in Table 1. The tendency of BAILs to form pre-micellar aggregates seems to be decreased while moving from BAIL-1 towards BAIL-5 as indicated by decay slopes of variation in I1/I3 upon pre-aggregation (Figure S5, SI). The average values of C1 (Table 1) for different BAILs follow the same order as followed by cac. Further, the cac values obtained from the UV-Visible measurements matches well with those observed from conductivity and fluorescence measurements (Figure S6, SI). The ratio (S2/S1) of slope of observed conductivity change in post- (S2) and preaggregation (S1) region was employed to get the degree of counter-ion binding (β) (Table 2) using the relation β = 1-S2/S1.55 Thus obtained β values increases with increase in the chain length of [Cnmim]+, however the increase is not very significant while moving from BAIL-1 to BAIL-3, whereas a sharp increase is observed for BAIL-4 and BAIL-5 as compared to other BAILs. It is noteworthy that the observed small values of β in case of BAIL-1 and BAIL-2 indicate the presence of [DC]- and [Cnmim]+ cation in vicinity of each.


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Table 2. Degree of counterion binding, (β), polarity index, (I1/I3), for cybotactic region of pyrene in aggregates, standard free energy, (∆𝐺𝑜𝑎𝑔𝑔), enthalpy, (∆𝐻𝑜𝑎𝑔𝑔), and entropy, (𝑇∆𝑆𝑜𝑎𝑔𝑔)b change of aggregation at 298.15 K. IL

βa

NaDC

I1/I3b

Nagg b

0.77

8

∆𝐺𝑜𝑎𝑔𝑔a

∆𝐺𝑜𝑎𝑔𝑔c

∆𝐻𝑜𝑎𝑔𝑔c

𝑇∆𝑆𝑜𝑎𝑔𝑔c

-0.43

[C2mim][DOC]

0.07

0.80

56

-24.7

-25.2

0.41

25.6

[C4mim][DOC] kjdfkj

0.08

0.83

124

-24.8

-25.0

0.81

25.9

[C6mim][DOC]

0.2

0.86

178

-26.7

-26.7

1.31

28.0

[C8mim][DOC]

0.50

0.93

184

-36.4

-36.8

1.12

37.7

[C12mim][DOC]

0.57

1.06

289

-43.3

-45.8

-0.76

44.3

aConductivity, bSteady-state

𝑇∆𝑆𝑜𝑎𝑔𝑔

are kJ

fluorescence, cIsothermal titration calorimetry, Units of ∆𝐺𝑜𝑎𝑔𝑔, ∆𝐻𝑜𝑎𝑔𝑔 and

mol-1.

From these results, it is inferred that cac and β, necessarily could not be correlated linearly to each other as also reported in literature.56 Relatively higher hydrophobicity of [C8mim]+ and [C12mim]+ drives their hydrophobic interaction with [DC]- supported by electrostatic interactions between oppositely charged head groups. This leads to lower cac and high values of β in case of BAIL-4 and BAIL-5. Therefore, the formation of mixed aggregates, with longer alkyl chain of [C8mim]+ and [C12mim]+ inserted towards hydrophobic core of aggregates in close association with hydrophobic surface of [DC]-, is expected. On the other hand, smaller size and high hydration of [Cnmim]+ cations in case of BAIL-1 and BAIL-2 directs [Cnmim]+ cations to act as counter-ions at aggregate-water interface in contact with water and does not affect the characteristic parameters of aggregation appreciably in comparison to NaDC. The values of I1/I3 (Table 2) upon aggregation are very good indicators of polarity of the hydrophobic region of aggregates inhabited by pyrene.54 The obtained values of I1/I3 are quite low as compared to that of different reported SAILs22,24,29,30 which establishes that the core of



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aggregates formed by BAILs is more hydrophobic than ever reported. Surprisingly, I1/I3 increases with increase in alkyl chain length of [Cnmim]+ linearly, which otherwise should decrease (Figure S7, SI). This is assigned to changing role of [Cnmim]+ from counter-ion in BAIL-1, BAIL-2 to co-surfactant in case of BAIL-3, BAIL-4 and BAIL-5, which could result in reduced compactness of aggregate-water interface. This reduced compactness along with hydrophilic nature of [Cnmim]+ result in penetration of water towards palisade layer of aggregates as also reported conventional surfactants.54 Aggregation number of aggregates (Nagg) formed by the BAILs were determined using steadystate fluorescence quenching experiment using pyrene as fluorescent probe and cetylpyridinium chloride as quencher using the following relation:57 ln 𝐼 = ln 𝐼𝑜 ― 𝐶𝑞/𝐶𝑚 = 𝑙𝑛𝐼𝑜 ― 𝑁𝑎𝑔𝑔𝐶𝑞/ (𝐶𝑡 ―𝑐𝑎𝑐)

(2)

where I0 and I are the fluorescence intensities of pyrene fluorescence in the absence and the presence of quencher. Cq, Cm, and Ct are the molar concentrations of quencher, aggregates and total concentration of BAIL, respectively. The observed results are shown in Figure 2D and thus obtained Nagg are provided in Table 2. Nagg is found to increase with lengthening of alkyl chain of [Cnmim]+ cation, which along with increasing I1/I3 suggest a decrease in compactness of aggregates while going from BAIL-1 to BAIL-5. 3.3. Size and shape of the aggregates: Turbidity measurements have been performed to have an idea about the size of the aggregates and the variation in the turbidity as a function of concentration of investigated BAILs is provided in the Figure S8, SI. BAIL-1, BAIL-2 and BAIL-3 seems to self-assemble in the form of relatively smaller aggregates as indicated by negligible change in transparency of solutions and the small change in the turbidity profile. On the other hand, increased turbidity and translucent nature of solutions in case of BAIL-4 and


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BAIL-5 suggested the presence of vesicles (Figure S8, SI). Dynamic light scattering (DLS) and transmission electron microscopic (TEM) measurements have been carried out to have precise information about the size and shape of formed aggregates (Figure 3).

Figure 3 (A-F): (A) Intensity-weighed size distribution (at twice of cac); (B-F) TEM images of the aggregates formed by the BAILs as (B) BAIL-1; (C) BAIL-2; (D) BAIL-3; (E) BAIL-4; and (F) BAIL-5. Inset of B shows the selected area electron diffraction pattern of the thin nano sheets formed by BAIL-1 and inset of C and D shows the enlarged images of spindles and vesicles.

BAIL-1 forms aggregates with hydrodynamic diameter (Dh) ≈ 40-50 nm (Figure 3A) and are confirmed to be very thin nano-sheets (NSs) with an average size ≈ 30-40 nm from TEM measurements (Figure 3B). The formed NSs are found to be highly crystalline in nature as observed from high resolution TEM (Figure S9, SI) and is confirmed from selected area electron diffraction (SAED) (Figure 3B). Bright diffraction points in form of rings observed in the SAED pattern indicates about the crystalline nature of sheets (Inset Figure 3B) where d values 4.45, 2.59, 1.83, and 0.89 Å has been calculated from these diffraction spots. The obtained d values are



very close to the reported d values for the ordered bilayers formed by the mixture cholesterol and lecithin.58 Considering the structural similarity between [DC]- and cholesterol, such ordered arrangement of the BAIL-1 ions in 2D linear fashion could have resulted in the formation of sheets by layer by layer arrangement of the linear 2D segment, on one another. A bimodal distribution in the intensity weighed Dh was observed for the BAIL-2 and BAIL-3 (Figure 3A), and is in line with formation of elongated/rod shaped aggregates in the solution.33 The rolling pin like aggregates in case of BAIL-2 with a length ≈ 60-80 nm and breadth ≈ 5 nm (Figure 3C), and perfect rod-like aggregates with a length ≈ 80-200 nm and breadth ≈ 10 nm in case of BAIL-3 (Figure 3 D) are observed. The width of nano-aggregates about ≈ 5 and 10 nm, which is relatively more as compared to extended molecular length of ions, further supports that the selfassembled structures could be hierarchal self-assembled bilayer structures. The formation of vesicles is observed in case of BAIL-4 and BAIL-5 (Figure 3E and F) with a size ≈130-170 nm and ≈350-500 nm, respectively, at concentrations much lower than that reported for formation of vesicles in mixed system of NaDC and [C12mim][Br].50 The comparable size of [C8mim]+ and [C12mim]+ with that of [DC]- anion could have facilitated their packing in the bilayers as well as helped each other in attaining the required curvature for the formation of vesicles. The large size of the vesicles in case of SAIL-5 could be due to the large size and increased hydrophobicity of the [C12mim]+ that favors the sustainability of large curvature. The Atomic force microscopy (AFM) additionally confirms the formation of vesicles (Figure S10(A-F), SI). Further stability of the formed aggregates has been investigated using zeta potential (ζ) measurements and ζ values for BAIL-1 ≈ -37.2, BAIL-2 ≈ -36.4, BAIL-3 ≈ -35.8, BAIL-4 ≈ -37.7, and BAIL-5 ≈ -20.9 mV have been observed. The large negative ζ values indicate good colloidal stability of formed


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aggregates. In this way, the role of alkyl chain length of [Cnmim]+ cation in deciding the shape and size of the aggregates formed by the BAILs under investigation, is established. 3.4. Molecular packing and internal structure of aggregates: In past, nuclear magnetic resonance (NMR) spectroscopy has proved very helpful in predicting the position and orientation of surfactant ion59, SAIL ions20,29,30 as well as relative arrangement and orientation of the cation and anion in self-assembled form of BAILs.37 Following this, 1H and 1H-1H 2D NMR spectroscopy measurements were employed to elucidate relative position, orientation of alkyl chain of [Cnmim]+ and [DC]- and to gain further other insights about the packing of ions in the aggregates. 3.4.1. 1H NMR of different protons of [DC]-: 1H NMR spectra of NaDC and all the BAILs under investigation at a concentration below and above the cac are given in Figure 4 and Figure S11 (SI).

Figure 4: 1H NMR spectra of BAIL-3, BAIL-4 and BAIL-5 under investigation at the concentration below and above cac at 298.15 K.



The assignment of resonance peaks to different protons of [DC]- has been done as per literature reports.60 Magnitude of the change in chemical shift values (Figure 5) of protons in 1H NMR spectra indicates about the change in electronic environment of protons as a consequence of aggregation. A downfield shift upon aggregation of NaDC has been observed for the protons of three methyl groups (H21, H19 and H18) present at the convex hydrophobic face of the [DC](Figure S11,SI) following the order: H21 > H19 > H18. The position of the methyl groups toward the hydrophilic environment in the aggregates seems to drive such change in chemical shift.61

Figure 5 (A-B): Magnitude of change in chemical shift (Δδppm= δaggregate- δmonomer) for different protons of (A) [DC]- and (B) [Cnmim]+ of BAILs obtained from 1H NMR measurements at 298.15 K.

One can’t expect very large values of change in chemical shift (Δδppm= δaggregate- δmonomer) for the methyl group protons present on the hydrophobic face of [DC]- due to its structural specificity, where hydrophilicity in the molecule is not concentrated on one point but runs along the large face of the molecule. The hydrophilic face moves parallel to the hydrophobic face in the molecule and both the faces are not very far from each other. The protons at H18, which undergo a downfield shifted in case of the NaDC, experiences up-field shifted upon aggregation in case of BAIL-1. It points toward the presence of H18 in hydrophobic environment of the formed


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aggregates. However, the protons at H19 does not show any change in chemical shift values. Other protons at H3 and H12, which experienced an up-field in NaDC, experience a similar upfiled shift with relatively large magnitude in case of BAIL-1. These changes suggest that the arrangement, position and orientation of the [DC]- in the aggregates formed by BAIL-1 are different than those formed by the NaDC owing to difference in size and hydrophilicity of respective counter-ions. The probable arrangement of ions and the formation of thin nano-sheets of BAIL-1 is described in scheme 2.

B

A

C

Scheme 2 (A-C): Schematic showing the formation of (A) thin nano sheets in case of BAIL-1; (B) compact vesicles for BAIL-4 where red arrows shows the close packing of the [DC]- along back to back and head to tail arrangement in bilayer and yellow circles shows the interaction of alkyl chain of [C8mim]+ with the hydrophobic face of the [DC]- and (C) large vesicles in case of BAIL-5 where red arrows depict the large separation between the [DC]- and similarly yellow circles shows the interaction of alkyl chain of [C12mim]+ with the hydrophobic face of the [DC]-. Red arrows in C and D mark the distance between hydrophobic components of respective BAILs.



δppm values for different protons of [DC]- in BAIL-2 follow similar trend as observed for BAIL1, however a slight increase in the magnitude of Δδppm values (Figure 5A) is observed. This could be due to similarity in position and arrangement of ions resulting in closely related structure of aggregates among these two BAILs as observed from TEM measurements (Figure 3). Based on this, the pointed edged near hollow rod like structures observed in case of BAIL-2 (Figure 3C) are thought to be formed via wrapping of thin nano-sheets which are structurally similar to that formed by BAIL-1 (Figure 3B). A sharp increase is observed in the magnitude of Δδppm values for the protons at H3, H12, H18, and H19 as we move from BAIL-2 to BAIL-4, however the order of up-field shift for different set of protons remains the same i.e. H3 > H12 > H18 > H19. The penetration of long alkyl chain of [Cnmim]+ (n = 6, 8 and 12) towards hydrophobic region of aggregates while keeping cationic head group in contact with water near hydrophilic surface of [DC]- in self-assembled structures increases the hydrophobicity around convex face of [DC]- and results in relatively large up-field shift for the protons at H3, H12, H18 and H19. These protons in case of BAIL-4 shows maximum up field shift, which could be due to enhanced hydrophobic interactions between alkyl chain of [C8mim]+ with hydrophobic face of [DC]- (Scheme 2B, yellow circles) and relatively tight packing of the [DC]- along its hydrophobic face ( Scheme 2B, red arrows) in the formed vesicles. Further the peak broadening calculated as difference in peak width at half height (v1/2, in ppm) of a proton signal in monomer and aggregated form (Δv1/2 = v1/2(aggregate) - v1/2(monomer)) is observed to be quite significant for the protons H18 (0.0035), H19 (0.0038), H21(0.0045), H3(0.0052) and H12 (0.0019) of [DC]─ as well for Hc (0.0124), Hd (0.0001), He (0.0065) and Hf (0.0068) protons of [C8mim+] in case of BAIL-4. Similar large values for peak broadening has been observed for BAIL-5 protons, H18 (0.0051), H19 (0.0053), H21(0.0036), H3(0.001) and H12 (0.0044) of [DC]─ as well for Hc (0.001), Hd (0.003), He


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(0.0104) and Hf (0.0104) protons of [C812mim+]. Hence this significant peak broadening (Figure 4) in case of BAIL-4 and BAIL-5 supports the formation of vesicles62 as inferred from DLS and TEM measurements (Figure 3E). However in case of BAIL-5, protons at H3, H12, H18, H19 experiences relatively less up-field shift as compared to that observed in case of BAIL-4 (Figure 5A), which suggests the loose packing of BAIL ions in vesicles. Longer alkyl chains of [C12mim]+ cations pushes [DC]- anions apart from each other in formed bilayers (Scheme 2C, red arrows) giving rise to relatively loose packing of ions resulting in formation of large vesicles as compared to that formed by BAIL-4. Further, the proximity of protons at H21 to the carboxylate group of [DC]-, which upon aggregation constraints this methyl group in the hydrophilic environment near imidazolium head groups of cation, results in a down field shift for protons at H21 for all investigated BAILs. The loose packing of ions in the large vesicles of the BAIL-5 is further confirmed by the 1H-1H 2D NOESY measurements (discussed later). 3.4.2. 1H NMR of different protons of [Cnmim]+: Protons of imidazolium head group of [Cnmim]+ i.e. Hd, He, Hf, Hg undergoes downfield shift following the order: Hg > He > Hf ≈ Hd ≈ He in case of all the investigated BAILs. The magnitude of downfield shift increases with increase in the chain length of [Cnmim]+ cation (Figure 5B). It is observed that the magnitude of downfield shift for different protons is very less while moving from BAIL-1 to BAIL-3 and indicates negligible change in chemical environment of the head group protons upon aggregation. This again corroborates that short chain imidazolium cations acts as counter-ions and remains in contact with water at aggregate-water interface. A large downfield shift for protons at Hg and He in case of BAIL-4 and BAIL-5 could be due to formation of H-bond between these protons and carboxylate groups of [DC]-. This in turn helps both the ions to achieve ordered packing required to form bilayer vesicles.



3.4.3. 1H-1H 2D NOESY measurements: To get concrete evidence about molecular arrangement of ions in aggregates formed by different BAILs, 2D 1H-1H NOESY measurements were performed. 1H-1H 2D NOESY spectrum of BAIL-1 at concentration twice of its cac (Figure 6A,B and Figure S12, SI) shows strong correlation peaks for the methyl group protons H18, H19 and H21 present at hydrophobic face of [DC]-, which confirms their close proximity in the aggregated structure. Similar correlation has also been observed in case of NaDC (Figure S13AB, SI) as the proximity of hydrophobic groups is highly probable in rod shaped aggregates formed from the combination of helically arranged [DC]-.61

. Figure 6 (A-D): Zoomed out portion of the 1H-1H 2D NOESY spectra of BAIL-1 showing the cross peaks for (A) H18, H19 and H21 protons; and (B) between H3 and H18, H19, H21 protons of [DC]-. Zoomed portion of the spectra of BAIL-4 showing the cross peaks for the interaction of (C) C8 alkyl chain protons of [C8mim]+ with H18, H19 and H21 protons of [DC]- and (D) between H3 and H18, H19, H21 protons of [DC]-.


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Correlation peaks between H3 and H21 methyl group protons of [DC]- are also observed in case of BAIL-1, which are not present in case of NaDC. These protons are on opposite ends of [DC]and their through space correlation could only be possible by head to tail arrangement or parallel stacking of the [DC]- with each other in the form of thin nano-sheets (Figure 3A), while exposing their hydrophilic faces to water as shown in Scheme 2A. Hydrogen bonding similar to the one observed in the helical rod like structures of NaDC61 between the carboxylate groups and the hydroxyl groups on the hydrophilic face of [DC]- might have resulted in the formation relatively thick nano-sheets via stacking of the nano sheets upon one another (Scheme 2A). Further in case of BAIL-2 (Figure S14A-B, SI), similar cross peaks but relatively weaker in magnitude has been observed for the all the protons of [DC]- as observed for BAIL-1 and NaDC.On the other hand, no cross peaks were observed for the alkyl chain protons of the [C4mim]+ and different protons of [DC]-. This shows that aggregates formed by BAIL-1 and BAIL-2 have similar internal structure however adopts a different morphology owing to difference in hydrophobicity of alkyl chain of cation. This supports the formation nano spindles in BAIL-2 via coiling of the nano-sheets similar in structure to that observed in case of BAIL-1. The correlation between H18, H19 and H21 protons of [DC]- indicates the relative compact arrangement of [DC]- in the aggregates of BAIL-3 (Figure S15A-B, SI), which is supported by relatively large upfield shift for these protons upon aggregation (Figure 5A). The alkyl chain protons of [C6mim]+ interact with the methyl group protons H18, H19 and H21 present on the hydrophobic face of [DC]- suggesting the involvement of the [C6mim]+ as a co-surfactant with [DC]-. In case of BAIL-4, besides strong correlation peaks between H18, H19 and H21, cross peaks for the alkyl chain protons Hb of [C8mim]+ cation with H18, H19 and H21 of the DC anion (Figure 6 C-D and Figure S16, SI) are observed. This strongly supports the notion observed from 1H NMR measurements that the alkyl



chain of [C8mim]+ is in close proximity to the hydrophobic face of the [DC]- (Scheme 2B). Further a strong correlation between the protons at H19 and H21 along with convincing cross peaks among H3 and H21 protons indicates a compact back to back and head to tail arrangement of [DC]- in the vesicles formed by BAIL-4 (Scheme 2B, red arrows). Similar to that observed in case of BAIL-4, correlation peaks for the alkyl chain protons Hb of [C12mim]+ with methyl protons at H18, H19 and H21 of [DC]- are observed (Figure S17A-B, SI) in case of BAIL-5. However no correlation peak has been observed for H21 methyl group protons with H3 and H19 methyl group protons present at the backbone of the [DC].- That points toward the loose packing of [DC]- in the large vesicles formed by the BAIL-5 and supports the observations made from 1H NMR measurements. 3.5 Thermodynamics of Aggregation: The variation in the differential enthalpy (dH) and differential power (dP) as the function of concentration of BAILs obtained from ITC measurements is shown in Figure 7(A-F) and Figure S18A-E (SI).

Figure 7 (A-E): (A-E) shows enthalpy changes in aqueous solutions of various BAILs from BAIL-1 to BAIL-5 respectively and (F) variation of various thermodynamic parameters as function of length of alkyl chain of [Cnmim]+ among investigated BAILs at 298.15 K. with quencher concentration (Cq).


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A hump (Figure 7) in low concentration region, which is in line with concentration (C1) corresponding to formation of pre-aggregates as obtained from conductivity and fluorescence measurements, is observed similar to that observed for some SAILs reported earlier.63 Except BAIL-1 and BAIL-5, thus obtained enthalpograms are nearly sigmoidal in shape, and resembles with Type A enthalpograms.64 The observed deviation from Type A is due the special structural features of the [DC]- as suggested by Type C64 enthalpogram observed in case of NaDC (Figure S19, SI), which is quite similar to that observed in case of BAIL-1. The observed variation between two is assigned to varying nature of Na+ and [C2mim]+ in case of NaDC and [C2mim][Dc], respectively. The addition of BAILs to water was accompanied by endothermic change in case of BAIL-1 and BAIL-5 (Figure S18, SI). Further, the magnitude of corresponding enthalpy changes decreases after aggregation. It is established that many factors such as partial molar enthalpies of the aggregate and the monomer, β, aggregation number (Nagg), the dilution and dissociation of the aggregates on addition of stock solution of the BAILs 64-68 govern overall enthalpy change. In case of BAIL-1, which exhibit stronger electrostatic interactions (endothermic) between constituent ions, as observed from conductivity measurements, dominates the enthalpy change. On the other hand, greater extent of hydrophobic hydration of long alkyl chain of [C12mim]+ in BAIL-5 below cac along with electrostatic interactions seems to govern the enthalpy changes. After cac, although the enthalpy change remain endothermic but it decreases in magnitude (Figure S16A and E, SI). This is assigned to electrostatic repulsions between the formed aggregates and dilution of aggregates (exothermic) with increase in concentration. On the other hand, BAIL-2, BAIL-3 and BAIL-4 exhibit almost identical enthalpograms (Figure 7 B-D and Figure S18, SI), where the extent of exothermic change decrease with increase in concentration



of respective BAILs and becomes almost constant after cac. Below cac, similar to that reported in literature,66 disaggregation of aggregates (exothermic) seems to dictate the enthalpy changes, whereas stronger electrostatic interactions between [DC]- and [Cnmim]+ dominates the enthalpy over other endothermic factors, after cac. Using the standard procedures,64,65,67 the values of cac (Table 1) and ΔHoagg (Table 2) are obtained from enthalpograms. All the investigated BAILs except BAIL-5 exhibit endothermic enthalpy of aggregation (ΔHoagg), whose magnitude increases while going from BAIL-1 to BAIL-3 following the order: BAIL-1 < BAIL-2 < BAIL-4 < BAIL-3. The increased hydrophobic interactions (exothermic) between [DC]- anions in NaDC and similar interactions between relatively hydrophobic [C12mim]+ and [DC]- in BAIL-5 along with the release of solvating water molecules (exothermic) upon aggregation turns overall enthalpy of these two system to be negative. With increase in alkyl chain length of [Cnmim]+, the extent of dehydration (endothermic) and enhanced electrostatic (endothermic) forces upon aggregation results in large endothermic changes. ΔHoagg observed for BAIL-1 and BAIL-2 is almost equal to that observed for NaDC (Table 2), which could be due to a similar pathway of aggregation among these amphiphiles. The observed smaller endothermic change in case of BAIL-4 supports the presence of stronger inter-ionic and intra-ionic hydrophobic interactions, which leads to formation of vesicles. The hydrophobic interactions in case of BAIL-5 seem to be greater than that observed in case of BAIL-4 owing to longer alkyl chain length of cation as suggested by relative large exothermic change in enthalpy. Further, increased mismatch in size compatibility between cation and anion along with enhanced hydration of loosely formed vesicles by BAIL-5 adds to exothermic enthalpy change.


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Goagg and TSoagg have been calculated employing standard procedure66 utilizing  and cac values from conductivity and ITC measurements, respectively (Table 2). Thus obtained values of TSoagg are almost same for BAIL-1 and BAIL-2 and a marginal increase is observed while moving from BAIL-2 to BAIL-3. This again supports the results obtained from NMR measurements that [Cnmim]+ cation of BAIL-1 and BAIL-2 remains hydrated, whereas BAIL-3 aggregates are relatively less hydrated. Abrupt increase in TSoagg values while moving from BAIL-3 to BAIL-4 is observed, where relatively less increase in TSoagg values is observed while moving from BAIL-4 to BAIL-5. This is indicative of role of dehydration of alkyl chain upon formation of aggregates and supports the observation made form NMR spectroscopy that BAIL4 forms much compact vesicles as compared to loose and hydrated vesicles formed by BAIL-5. The observed negative values of Goagg (Table 2, Figure 7F) suggest the spontaneity of aggregation, which is maximum in case of BAIL-4 and BAIL-5. It is observed that the entropy factor is major contributor to overall values of Goagg as the values of ΔHoagg are quite small and the variation of Goagg follows same trend as followed by TSoagg. In this way, the varying role of cation from counter-ion to a co-surfactant depending on the length of alkyl chain in BAILs having structurally unique [DC]-, which governs the diversification among shape and size of selfassembled structures is established. Conclusion: Different BAILs based on bile salt surfactant anion, deoxycholate anion, [DC]-, and 1-alkyl-3mehtylimidazolium cation, [Cnmim]+ (n = 2, 4, 6, 8 and 12), are found to self-assemble in aqueous medium. The role of [Cnmim]+ as counter-ion (n = 2 and 4), as a co-surfactant (n = 6) along with enhanced hydrophobic interactions synergistically supported by electrostatic interactions between constituent ion governs the cac. The nature of cation accompanied by



structural

mismatch

between

[DC]-

and

[Cnmim]+

Page 26 of 34

cation

effects

the

hydrophobicity/hydrophilicity balance and results in formation of diverse morphological aggregates by investigated BAILs. The aggregates formed by [C2mim][DC], [C4mim][DC], and [C6mim][DC] are although inherently similar in molecular arrangement, however differs in hierarchical self-assembly in the form of nano-spindles and nano-rods in case of [C4mim][DC], and [C6mim][DC], respectively, as compared to nano-sheets in case of [C2mim][DC]. On the other hand, BAILs having cation with longer alkyl chain i.e. [C8mim][DC] and [C12mim][DC] forms vesicles. Formation of such diverse morphological architectures by changing alkyl chain length in quite unique and the observed structures are very different to that observed in case of NaDC.61 It is expected that the present work along with previous reports on BAILs37-39 would provide a new dimension to conceptualize and synthesize new BAILs to achieve higher-ordered self-assembled structures similar to those obtained via ionic self assembly69-70 that can be used for different applications. Such type of systems would be highly useful for solubilization of hydrophobic compounds/drugs owing to greater hydrophobicity of core of aggregated structures provided the BAILs comprises of non-toxic bio-based components. Supporting Information: Annexure S1-S3: Synthesis and Characterization of BAIL, Methods employed; Figure S1-S19: Surface tension measurement of NaDC in aqueous solution, specific conductance of BAILs and NaDC, Variation of molar conductance of BAILs and NaDC as a function of square root of concentration, Variation in the I1/I3 and absorbance values as function of concentration of different BAILs, Plot of I1/I3 values at twice the cac values of BAILs as function of chain length imidazolium cation of the BAILs, Turbidity profiles as the function of concentration of the BAILs, HR TEM of BAIL-1and AFM of BAIL-4 and BAIL-5, 1H NMR spectra of NaDC,


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BAIL-1 and BAIL-2 at the concentration below and above cac, 2D 1H-1H NOESY spectra of the BAIL-1, BAIL-2, BAIL-3, BAIL-4, BAIL-5 and NaDC, Differential power profiles of different BAILs, Variation of enthalpy changes in aqueous solution of NaDC as a function of concentration at 298.15 K. Acknowledgement: This work was supported by the DST, Govt. of India wide project number EMR/2017/002656. Gurbir Singh (09/254(0278)/2018-EMR-I) and Komal are thankful to CSIR and UGC, respectively, Govt. of India, for providing fellowship.

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