Thermodynamic and computational study of isomerism effect at micellization of imidazolium based surface-active ionic liquids: Counterion structure

Journal Pre-proof Thermodynamic and computational study of isomerism effect at micellization of imidazolium based surface-active ionic liquids: Counterion structure

Isidora Čobanov, Bojan Šarac, Žiga Medoš, Aleksandar Tot, Milan Vraneš, Slobodan Gadžurić, Marija Bešter-Rogač PII:

S0167-7322(19)34239-4

DOI:

https://doi.org/10.1016/j.molliq.2019.112419

Reference:

MOLLIQ 112419

To appear in:

Journal of Molecular Liquids

Received date:

28 July 2019

Revised date:

7 December 2019

Accepted date:

27 December 2019

Please cite this article as: I. Čobanov, B. Šarac, Ž. Medoš, et al., Thermodynamic and computational study of isomerism effect at micellization of imidazolium based surfaceactive ionic liquids: Counterion structure, Journal of Molecular Liquids(2020), https://doi.org/10.1016/j.molliq.2019.112419

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© 2020 Published by Elsevier.

Journal Pre-proof

Thermodynamic and computational study of isomerism effect at micellization of imidazolium based surface-active ionic liquids: Counterion structure Isidora Čobanova, Bojan Šaraca, Žiga Medoša, Aleksandar Totb, Milan Vranešb, Slobodan Gadžurićb, Marija Bešter-Rogača* a

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, 1000

of

Ljubljana, Slovenia b

ro

Faculty of Science, Department of Chemistry, Biochemistry and Environmental Protection,

re

-p

University of Novi Sad, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia

*

lP

To whom the correspondence should be addressed:

na

E-mail: [email protected] (M. B.-R.)

Jo

ur

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Slovenia

Abstract

1

Journal Pre-proof In this work the influence of benzoate, ortho-, meta- and para-hydroxybenzoate anions, as counterions, on micellization process of 1-dodecyl-3-methyl-imidazolium cation in water is investigated by isothermal titration calorimetry (ITC) at temperatures between 278.15 and 318.15 K. ITC experimental data were analyzed by the help of an improved mass-action model, yielding the values of critical micelle concentration, cmc, the degree of counterion binding, , aggregation number, n, standard heat capacity, ΔMcp, enthalpy, ∆MHθ, entropy, ∆MSθ, and Gibbs free energy, ∆MGθ of micellization. It was found that the investigated systems behave mainly like common ionic surfactants and already investigated SAILs: the micellization process of

of

investigated systems is entropically driven at low temperatures, whereas at high temperatures the

ro

enthalpic contribution becomes equally important. The last is especially important in the case of ortho-hydroxybenzoate, which incorporates into micellar structure affecting also

the entropy-

re

influence considerably the micellization process.

-p

enthalpy compensation. But evidently, the presence and position of –OH group in the counterion

lP

ΔMcp values were further discussed in the light of the removal of water molecules from contact with nonpolar surface area upon micelle formation. All the values are negative, the most in the

na

case of benzoate anion, which could be ascribed to the absence of –OH group. To refine the thermodynamic parameters obtained from ITC, the molecular simulations were

ur

performed. First, it is shown that the binding energies between anion and cations increase in the order from benzoate to para-, meta- and ortho-hydroxybenzoate, which coincide with ∆MHθ

Jo

values. Second, it is demonstrated that water around counterions is strongly perturbed leading to differences in ΔMcp.

Keywords: surface-active ionic liquids, micellization, thermodynamics, isothermal titration calorimetry, hydroxybenzoate

2

Journal Pre-proof 1. Introduction Recently, it was demonstrated on the dodecyltrimethylammonium chloride (DTAC) in the 0.01 M solutions of sodium orto-, meta- and para-hydroxybenzoates, that even small changes in the structure of cations of added salt, such as the position of the substituent on an aromatic ring, considerably influence the micellization process [1]. However, it should be kept in mind, that the micellization of DTAC in that case was studied in the presence of NaCl, which also considerably affects the self-aggregation process [2]. Thus, the main aim of the present work is to study the influence of isomerism of counterions on micellization of surfactant in water without any added

of

compound. For this purpose, surface-active ionic liquids (SAILs) turned out as a very suitable

ro

system.

Namely, due to the possible variations in the structure, SAILs were proved as excellent model

-p

systems for investigation of micellization process in the last years. Many studies have been

re

devoted to the influence of alkyl chain length, different charged head groups and functional groups on micellization properties of SAILs in aqueous solutions [3, 4, 5, 6, 7, 8, 9, 10, 11, 12,

lP

13].

It was reported, that the micellization behaviour of SAILs in water is in many ways similar to

na

conventional surfactants [10]: the critical micelle concentration, cmc, is decreasing with the length of hydrophobic chain and expresses a minimum in the temperature dependence and the

ur

micellization process is endothermic at low temperatures and exothermic at high temperatures.

Jo

Nevertheless, the extremely important influence of counterions on the micellization process of imidazolium-based SAILs was demonstrated clearly [10, 14, 15, 16] It was confirmed, that the hydrophobicity of counterions evidently contributes to the heat capacity change and the water accessible surface area removal upon burial of non-polar group from the contact with water at micellization process [10]. Even more, it was shown that counterions with hydrophobic character are partially incorporated in the micelles [17]. The attention of many researchers has drawn also the diverse applications of SAILs: from their use as modulators of corrosion inhibition [18], in micellar catalysis [19], to the potential use as drug delivery agents [20]. Furthermore, the pharmaceutical potential of SAILs has been explored lately, since some studies have shown that they act against bacteria and fungi [21, 22]. In 3

Journal Pre-proof addition, specific ILs with biologically active ions were synthesized and shown to keep the biological activity of both the original cation and anion, therefore acting as active pharmaceutical ingredients [23,24]. For instance, salicylate (ortho-hydroxybenzoate) SAILs have been studied for antimicrobial and biocide applications and showed that they can act as antimicrobials [25]. To prove the influence of isomerism of counterions at micellization 1-dodecyl-3methylimidazolium ([C12mim]) benzoate ([Bz]), ortho- ([o-HBz]), meta- ([m-HBz]), and para([p-HBz]) hydroxybenzoate (Scheme 1) were synthesized. The micellization was studied by isothermal titration calorimetry (ITC) as one of the most valuable techniques for thermodynamic

of

analysis. The experiments were performed within the temperature range of 278.15 to 318.15 K in

ro

step of 10 K. The corresponding standard thermodynamic parameters of micellization (enthalpy, ΔMH; Gibbs free energy, ΔMG; entropy, ΔMS; heat capacity change, ΔMcp) were estimated by

-p

fitting of the one-step (one equilibrium) model equation based on the mass-action model to the

re

experimental data [11]. In addition, ΔMcp was further correlated to the changes in solventaccessible surface upon micelle formation. To elucidate more about the interactions between the

Jo

ur

na

lP

cation and anions (counterions) in the systems computational simulations were also performed.

Scheme 1. Structures of investigated systems: a) [C12mim]+; b) [Bz]– c) [o-HBz]–; d) [m-HBz]– e) [pHBz]–

4

Journal Pre-proof 2. Experimental section 2.1. Materials All chemicals for preparation of ionic liquids were used as received without further purification. Millipore ultrapure water for preparation of solution is applied. The summary of the provenance and purity of the chemicals is given in Table S1 in Supplementary data. 2.2. Synthetic procedures 1-dodecyl-3-methylimidazolium, [C12mim][Bz], was synthesized by mixing the equimolar

of

amounts of 1-dodecyl-3-methylimidazolium chloride and sodium benzoate. Both components were dissolved in acetone and the mixture was stirred in a round-bottom flask for 5 hours. The

ro

resulting white precipitate (sodium chloride) was removed by filtering under vacuum and the

-p

clear, pale yellow liquid was obtained. In order to remove the acetone, the IL was heated for 30 min at 343.15 K under vacuum. After achieving a constant mass, the obtained product was

re

additionally dried under the vacuum. A similar procedure was used for all the other ILs synthesized in this work. Apart from the starting compound, sodium benzoate, different starting

lP

compounds were used to obtain different ILs. Therefore, instead of sodium benzoate, sodium 2hydroxybenzoate, sodium 3-hydroxybenzoate and sodium 4-hydroxybenzoate were used to

na

obtain [C12mim][o-HBz], [C12mim][m-HBz] and [C12mim][p-HBz], respectively. The residual

ur

chloride in the samples was tested by AgNO3. 2.3. Isothermal titration calorimetry (ITC)

Jo

The heat changes associated with (de)micellization were measured using a VP-ITC microcalorimeter (MicroCal Inc., Malvern, UK). 1.00 g of distilled and degassed water was titrated with the surfactant solution added by a 300 μL syringe in temperature intervals between 278.15 and 318.15 K. Successive aliquots of 3 μL of surfactant solution were injected at 10-15 min intervals by a motor-driven syringe into the calorimeter cell under constant stirring at 300 rpm. The concentrations of surfactant in the stock solutions were 10-15 times higher than their cmc (Table S2). The area under the peak following each injection of the surfactant solution is proportional to the heat effect expressed per mole of added surfactant, obtained by integration of the raw signal (example in Figure S1a) and b) of the Supplementary data). The concentration after each addition was calculated in a mole of surfactant per kg of solution (applied in the thermodynamic analysis) from the known starting mass of the water in the cell, the concentration 5

Journal Pre-proof of the stock solution in the syringe. We assumed the density of the solution was equal to the density of pure water (applied on graphs). 2.4. Computational simulations The theoretical investigation of ionic liquids was performed using Schrödinger Suite package. To perform Density Functional Theory (DFT) calculations Jaguar software was applied. The B3LYP exchange-correlation functional with empirical correction for dispersion (B3LYP-D3) has been used together with 6-31+G(d,p) basis set [26,27]. The geometry optimizations were followed by

of

harmonic frequency analysis to ensure that the obtained structures were true minima due to the

investigated using the method of Johnson et al [28].

ro

absence of imaginary wavenumbers. Intermolecular non-covalent interactions (NCI) have been

-p

Further, molecular dynamics (MD) simulations were performed using Desmond program. The OPLS force field was used [29]. The simulations were performed in NPT canonical ensemble.

re

The temperature was set to 308.15 K while pressure was 1·105 Pa, controlled by the Nose-

lP

Hoover barometer. The periodic cubic boundary conditions were employed. The Ewald method was used for computing long-range electrostatic interactions with no truncation in investigated systems. The overall time of simulations was 30 ns. The equilibrium was obtained after 10 ns,

na

which was not considered in further analysis. The trajectories are integrated via the Verlet

Jo

and further analyzed.

ur

leapfrog algorithm combined with the quaternion method for rotations with a time step of 2 fs

3. Thermodynamics of Micellization Micellization is a well-known process where monomers form spherical structures due to the polar duality of molecules or ions which form monomers. The experimental ITC data in this work were fitted with the one-step model [11]. In fact, micellization should be regarded as a multistep process or stepwise association with a series of equilibria with aggregation numbers starting from 2 to infinity and each with its own thermodynamic parameter. However, first, most of the formulations are unfavourable and therefore irrelevant for thermodynamic studies, and second, defining thermodynamic parameters for every aggregation number is highly impractical due to the requirement of too many parameters at fitting the experimental data. In our recent work we introduced the two-step model [30], which turned out as more appropriate for 6

Journal Pre-proof description of micellization process of surfactants with shorter alkyl chains (with 10 or less C atoms). However, the one-step model, used also in our previous work on SAILs [11] and at sodium dodecanoate [30], describes successfully the micellization of systems with longer alkyl chains (with 12 or more C group) and slightly higher aggregation numbers. Therefore, also in present study one-step model was applied. The simplest micellization model of a cationic surfactant can be represented with an equilibrium KM   n A-  nC+  Cn A  n (1- )n

of

(1)

where C+ represents the monomeric state of surfactant, A- the corresponding counterions, β is the

ro

degree of counterion binding, n aggregation number and CnAβn(1-β)n+ the micellar aggregate (M)

-p

with an effective charge (1-)n+. The equilibrium between species can be expressed by the

a a a

M n n C A



x x x

M n n C A

K

(2)

lP

KM =

re

apparent equilibrium constant, KM,

where the activities of micelles, aM, free monomers, aC, and free counterions, aA, are

na

approximated by the molar fractions of micelles, xM, free monomers, xC, and free counterions, xA, assuming the activity coefficients forming the constant Kγ = 1. The value for KM is obtained

RT ln KM n

(3)

Jo

Δ MGθ = -

ur

from the standard Gibbs free energy of micellization, ΔMG,

and determines the concentration of each species at a given total concentration of surfactant, c. The amounts of the free counterions, nA, free monomers, nC and micelles, nM can be calculated having the information on concentration of the solution (mole of surfactant per kg of solution), total mass and molar masses of solvent and surfactant (see Supplementary data, p. S9). The enthalpy of solution can be written as ' ' H = nsol H sol + nS H CA + nM nΔ M H θ  2 RT 2 BCA bC nC  RT 2 1+ (1-  )n  BMA bM nM θ

(4)

where ΔMH is the enthalpy of micellization. Equation 4 is applicable to any calorimetric experimental method. The second part of the equation introduces a non-ideal contribution, 7

Journal Pre-proof typical for solutions of ions (coefficients BCA', BMA') [31]. Detailed derivation of the equation 4 with a corresponding explanation for all of the symbols may be seen in the Supplementary data, p. S10. In ITC experiments, the measured heat changes

ΔH =

q nS,stock

=

H - H 0 - Hstock nS,stock

(5)

are the result of three contributions, namely the enthalpy of the stock solution, Hstock, as well as

of

the enthalpies of the solution in the cell before, H0, and after addition, H, divided by the total

ro

amount of surfactant added, nS,stock.

-p

By combining Eqs. (4) and (5), the final form of the model equation is given as

   

re

2 2  nC2 nC,0 nC,stock ΔnM RT 2 θ ' ΔH = ΔM H  2 BCA  m Δn nS,stock  sol msol,0 msol,stock

  

(6)

lP

2 2  nM2 nM,0 nM,stock RT 2 ' BMA 1+ (1-  )n   m nS,stock  sol msol,0 msol,stock

na

where ΔnM/Δn is the change in the amount of micelles upon addition. A detailed derivation with an explanation of the symbols and indexes is given in the Supplementary data, p. S11. The

ur

enthalpy change in Eq. (6) depends upon the amounts of micelles and free surfactant before and

Jo

after the addition of the stock solution, their amounts in the stock solution itself and the mass of solvent in all three solutions. The value of the cmc for each of the investigated systems at a given temperature was estimated numerically from the inflection point of the line representing the dependence of ΔnM/Δn on concentration. In this work, a global fitting of the model equation to the corresponding experimental curves at all examined temperatures (see Supplementary data, p. S12) reduces their correlations significantly. However, some parameters (n, β, ∆MGθ) tend to have a greater error due to the fact that they are determined by the shape of the enthalpograms and not by absolute values of ∆H.

8

Journal Pre-proof 4. Results and Discussion The dependence of experimental heat of dilution, ΔH, on surfactant concentration (enthalpogram) for titration of [C12mim][p-HBz] in water in the investigated temperature range is shown in Fig. 1 a). Similar enthalpy patterns were also obtained for other investigated systems (Fig. S2 in the Supplementary data). In Fig. 1 b) the comparison of enthalpograms for all investigated systems at 298.15 K is presented.

20

b)

20

of

a)

[C12mim][p-HBz] 278.15 K 288.15 K 298.15 K 308.15 K 318.15 K

ro

[C12mim][Bz] [C12mim][o-HBz] [C12mim][m-HBz] [C12mim][p-HBz]

10

-p

10

15

H/kJ mol-1

H/kJ mol-1

15

5

re

5

0

0

0

5

10

15

c/ mmol dm-3

20

lP

-5

0

5

10

15

20

c/ mmol dm-3

na

Fig. 1: a) Temperature-dependent enthalpograms for [C12mim][p-HBz]; b) enthalpograms for

ur

[C12mim][X] at 298.15 K in water. Solid lines represent the fits according to the model function (Eq. (6)).

Jo

As can be seen from Figs. 1 and S2 in Supplementary data, the experiment can be described as follows: the surfactant in solution initially undergoes full demicellization, followed by monomer and counterion dilution and their mutual interactions. All effects are accompanied by nearly constant heat uptake or release (depending on temperature), until the cmc of surfactant is reached resulting in inflection point. Afterwards, solution undergoes mainly micellar dilution (and for the studied systems the micellization contribution is negligible) therefore the change in enthalpy remains nearly constant with increasing surfactant concentration. Evidently, investigated systems, in general, obey the same pattern as it has been observed for classical surfactants [30,32,33,34] and already studied SAILs [10,11]. The values of ΔMH were simultaneously determined by the fitting procedure together with the degree of the counterion 9

Journal Pre-proof binding, , aggregation number, n, standard heat capacity, ΔMcp and Gibbs free energy, ∆MGθ, of micellization. In Table 1 the values for all studied systems at 298.15 K are listed together with the entropy contributions, whereas in Table S3 in Supplementary data cmc and all thermodynamic functions are presented at all investigated temperatures together with coefficients required to account for ionic interactions for all systems at all investigated temperatures. In additon, n as well as ΔMcp of investigated systems are presented in Figure S3 also. For comparison, available literature data for [C12mim][o-HBz] are added from different experimental

Jo

ur

na

lP

re

-p

ro

of

setup and model applied [10].

10

Journal Pre-proof Table 1. Aggregation numbers for micelles, n, degree of counterion binding, , critical micelle concentration, cmc, Gibbs free energy, ∆ MGθ, standard enthalpy ∆MHθ, entropy contributions, T∆MSθ, standard heat capacity change upon micelle formation, ΔMcp, for the investigated systems at the temperature of 298.15 K; the temperature T*(cmc), at the minimum critical micelle concentration, cmc* and T0 at ∆MHθ = 0. At [C12mim][oHBz] the comparison with the literature data is added.a

[C12mim][Bz]

-682  20

298  2

4.3  0.1

296.6  0.3

32  1 32.6  0.1b

-551  20 -526  2b

279  20 275.5b

24  1 28  2

-486  10 -530  10

279  12 289  7

∆MHθ

T∆MSθ

18  2

0.6  0.2

4.35  0.10

-34  4

-1.2  0.1

33  4

-46  1 -45.7  0.1b

-14.0  0.6 -13.15 0.05b

-34  1 -34  2

-9.1  0.1 -6.4  0.2

0.90  0.05 0.92  0.01c

[C12mim][m-HBz] [C12mim][p-HBz]

13  1 10  1

0.5  0.1 0.6  0.2

4.12  0.10 5.36  0.20

r P

l a

ref [10] from ITC, the values for ∆MGθ and T∆MSθ were recalculated to rational scale

n r u

ref [10] from conductivity ref [35] from surface tension measurements

Jo

11

f o

o r p

e

Units: ∆MGθ, ∆MHθ, T∆MSθ, kJ mol-1; ΔMcp, J K-1 mol-1; T*, T0, K; cmc, mmol dm-3

d

T0

∆MGθ

19  2 9.3  0.8b

c

cmc*

cmc

[C12mim][o-HBz]

b

T*(cmc)

β

2.01  0.10 1.83  0.09b 1.89c 1.59d

a

ΔMcp

n

1.8  0.2 1.66 0.09b

3.8  0.1 5.2  0.2

273  2 273.2b 279.5  0.3 286.6  0.4

Journal Pre-proof The temperature dependence of cmc shows a typical U-shaped form (Fig. 2 a)) reaching the minimum value cmc* at the temperature T*. Values of T* were obtained from the corresponding derivatives of the polynomials cmc = A+BT+CT2, whereas cmc* were calculated from the polynomial equations, setting T = T*. All the values are listed in Table 1, whereas the coefficients of the polynomial are given in Table S4 in Supplementary data. As it is evident from Table 1, T* is close to temperature T0, where ∆MHθ = 0 for all investigated systems, but not for [C12mim][o-HBz], despite the fact, that values of β, cmc, cmc*, T* and T0 for [C12mim][o-HBz] are in reasonable agreement with our previous results [3]. The micellization process for [C12mim][o-HBz] is namely exothermic in the whole investigated

of

temperature range, but with linear extrapolation we can estimate T0 = 2732 K (where the experiment is not possible) easily. On the other hand, the minimum in the cmc = f(T) is less

ro

expressed in the case of [C12mim][o-HBz] – we can see almost the plateau below 290 K and

-p

thus the estimation of T* from the first derivative is less reliable.

re

The aggregation numbers are in the case of [m-HBz] and [p-HBz] lower comparing to other

lP

two ILs probably due to larger steric hindrances of solvated –OH group during micellization

a) 7

Jo ur

cmc/ mmol dm-3

6 5 4 3 2 270

280

290

300

310

320

T/ K

b) 20

y = Intercept + B1*x^1 + B2*x^2

Equation

No Weighting

Weight Residual Sum of Squares

0.00197

Adj. R-Square

0.99627 Value

Standard Error

Residual Sum of Squares Adj. R-Square

7.43732

-0.85437

0.04997

B2

0.00148

8.37879E-5

y = Intercept + B1*x^1 + B2*x^2

Equation Weight

128.62407

B1

No Weighting

Equation Weight

[C12mim][p-HBz]

Equation

y = a + b*x

Weight

No Weighting

Value

Adj. R-Square

0.00808

0.00144

1.35526E-5

p

y = Intercept + B1*x^1 + B2*x^2 No Weighting

-20

Intercept

71.12132

B1

-0.48247

0.08383

B2

8.64286E-4

1.40553E-4

y = Intercept + B1*x^1 + B2*x^2

Weight

No Weighting

-30 270

0.98524

Value

41.89402

280

Standard Error

9.38702

B1

-0.28707

0.06307

B2

5.14286E-4

1.05753E-4

Intercept

290

300

T/ K

310

320

0.45594

-0.531

0.00153

0.003 -0.99999 0.99998 Intercept

0.29849

-0.487

1E-3

y = a + b*x

Weight

No Weighting

Pearson's r Adj. R-Square

0.003 -0.99999 0.99999 Value

o

corresponding polynomial fits, cmc=A+BT+CT2. Coefficients are given in Table S4 in Supplementary data. b) ∆MHθ. Solid lines represent linear fit.

As it is evident from Figure 2 b) and Table S3 in Supplementary data, the micellization process of most SAILs is endothermic at low temperatures and becomes exothermic at high temperatures, phenomenon usual for ionic surfactants. This behaviour could be ascribed to

Standard Error

136.13905

Slope

Equation Residual Sum of Squares

Standard Error

151.93765

Slope

Value

Fig. 2. Temperature dependence of a) cmc for investigated SAILs in water. Solid lines represent the

12

0.99997

Weight

m

0.00313

Intercept

0.007 -0.99999

y = a + b*x

Adj. R-Square

12.47599

m

0.00361

No Weighting

Pearson's r

Standard Error

Equation

1.0762

-0.689

Equation Residual Sum of Squares

Value

Standard Error

204.32535

Slope

Value

1.20298

-0.85462

B2

0.99072

Intercept

Standard Error

131.52319

B1

0.00553

o

Bz

Pearson's r

-10

0.99989

[C12mim][m-HBz]

Residual Sum of Squares

Value

0.039 -0.99996

Adj. R-Square

0.99965

Adj. R-Square

Adj. R-Square

No Weighting

[C12mim][o-HBz]

0

Residual Sum of Squares

Residual Sum of Squares

y = a + b*x

Weight

Pearson's r

5.14286E-5

Intercept

Bz

Equation Residual Sum of Squares

[C12mim][Bz]

10

Intercept p

H/kJ mol-1

na

(more details will be given in continuation).

Intercept Slope

Standard Error

146.53025

0.29849

-0.535

1E-3

Journal Pre-proof the delicate balance between hydrophobic (de)hydration of nonpolar parts (endothermic) and counterion binding (exothermic) upon micellization, where former effect diminishes with temperature [36]. Such balance is also reflected in the above-mentioned U-shape dependence where its minimum (and ∆MHθ = 0) moves to lower temperatures according to the increasing prevalence of electrostatic contribution due to a counterion binding (). The last is highest in the case of [C12mim][o-HBz] (Table 1) which consequently leads to exothermic enthalpies in the whole investigated temperature range. It is also well-known that [o-HBz] provokes the formation of more compact elongated micelles by stronger interaction of [o-HBz] with surfactant molecule (which is evident also from Figure 4) and its subsequent incorporation

of

resulting in the highest exothermicity of micellization process in the case of [o-HBz]

ro

[37,38,39].

The main driving force for the formation of micelles is the apparent disaffinity of water and

-p

the nonpolar (interacting) surfaces known as hydrophobic effect. This effect is reflected in the

re

value of ΔMcp, which for micellization is always highly negative (Table 1) and can be ascribed to the removal of water molecules from the contact with nonpolar surface area upon

lP

micelle formation [40]. In our recent work we showed, that values of ΔMcp are dependent on the counterions, especially if they can incorporate into the micelles [10].

na

By modelling the micellization processes as a transfer of surfactant molecules into the micellar phase, the heat capacity can be expressed in terms of the change of water accessible

Jo ur

surface areas, derived by Spolar et al [41] from protein folding  M cθp (J K 1mol1 )  1.34 (0.33) Anp

(7



where Anp (in Å2) stands for the change of water accessible nonpolar surface area (ASA) upon protein folding (we presume that only nonpolar tail of cation is dehydrated). According to Richards [42, 43, 44], water accessible surface area of a methylene group is 30 Å2 and 88 Å2 for a methyl group, giving the value for hydrophobic C12-alkyl chain 418 Å2. We can presume that upon micellization of [C12mim][Bz] also the nonpolar part of the counterion is dehydrated, having ASA around 114 Å2 [45]. Since only 60 % of the counterions are incorporated into the micellar structure (Table 1), we obtain, by applying Eq. (7), ΔMcp = – 650 (± 160) J K-1 mol-1. This value is in good agreement with our experimental value. As shown in Table 1, the values of cp fall (in absolute sense) in order [Bz]>[o-HBz]>[p13

Journal Pre-proof HBz]>[m-HBz], which was already observed for alkylpiridinium surfactants [46]. Most interestingly, it is a relatively high value for [Bz] compared to [o-HBz]. It was already shown that all the counterions are partly incorporated into the micellar structure [1,47,48], where – COO– and –OH groups are located near the micellar surface. The difference in cp can be partly ascribed to the polar nature of –OH group. Since it's strongly hydrated, it presents a steric hindrance upon micellization and some water still maintains in the micelles, which lowers the value of Anp and consequently cp. Besides that, the overall non-polar character of all [HBz] is lower than in the case of [Bz], meaning that the number of watermediated hydrophobic interactions (hydrophobic hydration) is also lower (more details in

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continuation).

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The comparison of thermodynamic functions at 298.15 K is given in Figure 3 where the interplay between enthalpy and entropy resulting in the negative values of Gibbs free energy,

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characteristic of spontaneous processes at constant p and T, is evident. The reported errors in

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Table S3 in Supplementary data correspond to the precision of the experimental data and the

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fitting procedure assuming the proposed model is correct.

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[C12mim][p-HBz]

TMS

[C12mim][m-HBz]

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MH G

[C12mim][o-HBz]

[C12mim][Bz] -50

-40

-30

-20

-10

0

kJmol-1

10

20

30

40

Fig. 3. Thermodynamic parameters of micellization for investigated systems in water at 298.15 K: standard enthalpy, ∆MHθ, Gibbs free energy, ∆MGθ, and entropy contributions, T∆MSθ, for micellization at investigated systems in water as obtained by the fitting procedure.

14

Journal Pre-proof Micellization of SAILs is entropically driven process at low temperatures, whereas at high temperatures the entropy and enthalpy contribute almost equally to the Gibbs free energy (Table S3). The main contribution to the entropy change comes from desolvation of the cations and anions upon micellization, which is comparable among the investigated systems, leading to small differences in T∆MSθ (Fig. 3). The difference is more noticeable in the case of enthalpies of micellization (∆MHθ) where the highest enthalpy change is found for [oHBz]. This is connected to the interactions between the cations and anions upon micellization. From enthalpy-entropy compensation (EEC) plot (figure S4 in SI) the linear relationship

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between the changes in these thermodynamic functions is evident, what was found as typical

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for micellization process and represented usually by the relationship

(8)

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Δ M H 0  Δ M H c  Tc Δ M S 0

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where Tc is called the compensation temperature and can be obtained from the slope of the

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straight line with MHc as the intercept [49, 50]. Tc often was interpreted as a characteristic of solute-solvent interactions, i.e., proposed as a measure of the “desolvation” part of the process of micellization and correlates to thus to the dehydration of the nonpolar parts of the

na

self-assembled molecules [50]. MHc should characterize the solute-solute interactions and is

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considered as an index of the “chemical” part of the process of micellization, connected to the formation of micelle of monomers [50]. Here, the values of Tc = 2927, 2927, 2896, 3007 K and MHc = -33.70.8, -45.7 0.8, -33.20.5, -34.60.6 kJmol-1 were obtained for [C12mim][Bz], [C12mim][o-HBz], [C12mim][m-HBz] and [C12mim][p-HBz], respectively Recently, it has been demonstrated [51] that Tc should be close to the central value of the temperature of investigated temperature interval, what also here is the case. But it is also stated that Tc does not provide any additional information about the systems [51], thus it will be not discussed further. It was assumed [50], that MHc represents the strength of the solute-solute interactions at surfactant aggregation when the entropic contribution to micellization is zero i.e., Δ M S o  0 . The increase in MHc (less negative values) should correspond to a decrease in the stability

15

Journal Pre-proof of the structure of the micelles. For surfactants possessing C12-alkyl chain, value of MHc = ~ -35 kJ mol-1 is reported [50]. MHc values obtained here are close to it, except for [C12mim][o-HBz], where the value is lower (more negative). Thus, it can be assumed that the self-assembled structures of [C12mim][o-HBz] are more densely packed than at other studied counterions, what is in agreement with the assumption of the insertion of [o-HBz]- in the micelle. In order to better explain differences in the values of ∆MHθ and ΔMcpθfor investigated ionic liquids, the computational simulations were performed. As the first step, DFT calculations

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were performed for ionic liquid ion pair, to understand basic interactions between cation and

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various anions. In Figure 4 the optimized structures of each ionic liquids, along with noncovalent interaction (NCI) and molecular electrostatic potential (MEP) surfaces are

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presented. As can be seen, the ionic liquids [C12mim][Bz] and [C12mim][p-HBz] form 5 NCI, mainly between imidazolium ring and –COO– group of corresponding anion. On the other

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hand, for [C12mim][o-HBz] and [C12mim][m-HBz] beside imidazolium ring, the dodecyl

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alkyl chain also contribute to interactions between cation and anion, resulting in 8 and 7 NCI, respectively. The –OH group on ortho and meta position cause different orientation of alkyl side chain than in case of para and non-substituted analogues. As can be seen from Figure 4,

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dodecyl chain is mainly positioned only around –COO– groups of [Bz] and [p-HB] anions. In contrast, in ionic liquids [C12mim][o-HBz] and [C12mim][m-HBz], side chain of cation is

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oriented in front of whole anion ring, resulting in additional stabilization through van der Waals interactions.

16

Journal Pre-proof Ebin = –63.21 kJ mol–1

a)

Ebin = –129.46 kJ mol–1

b) Ebin = –74.97 kJ mol–1

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Ebin = –106.31 kJ mol–1

d)

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c)

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Fig. 4. Visualization of non-covalent interactions between cation and anion along with MEP surfaces: a) [C12mim][Bz] b) [C12mim][o-HBz]; c) [C12mim][m-HBz]; d) [C12mim][p-HBz]. Values of binding energy (Ebin) between cation and anion of investigated ionic liquids are listed in white squares.

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To prove the stated observations, the molecular dynamics simulations of each pure ionic liquid were performed. From obtained results, radial distributive functions (RDFs) between center of the mass of cation ring and center of the mass of adequate anion are calculated and presented in Fig. 5. As can be seen, the [o-HBz] anion is closest and with highest probability to be found around imidazolium ring, due to ortho position of substituted hydroxyl group. Further, the [m-HBz] anion is shifted to longer distance, but with significantly higher g(r) values compared to other two anions. The RDFs for [C12mim][Bz] and [C12mim][p-HBz] are similar, indicating that introduction of –OH group on para position does not influence noteworthy to potential interactions between cation and anion. Moreover, binding energy between cation and anions are calculated and presented in Fig. 4. As can be seen, the strength of interactions (absolute value of Ebin) between cation and anion follow the trend: [o-HBz]> [m-HBz]> [p-HBz]> [Bz]. All obtained results indicate that the introduction of –OH group on ortho and meta position promotes cation-anion interactions and stability of ionic liquids. Moreover, the hydroxyl group on para position has insignificant effect on cation-anion 17

Journal Pre-proof interactions comparing to non-substituted anion. Interestingly, the trend observed for Ebin is the same as for ∆MHθ from which we can conclude that interactions between cations and anions play a crucial role during micellization of these systems.

0.7 0.6

g (r)

0.5 0.4

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0.3

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0.2 0.1

0

2

4

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0.0

6

8

10

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r/Å

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Fig. 5. RDFs of cation ring center of mass interaction with anion's center of mass: black ([Bz]–), red ([o-HBz]–), blue ([m-HBz]–), green ([p-HBz]–)

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Figure 6 presents the spatial distribution functions (SDF) of water molecules around anions before cmc obtained by the MD simulations. It is clearly evident that water molecules around

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–COO– and –OH groups of counterions are strongly perturbed and are oriented differently than in the bulk. It is well-known that non-polar and polar surfaces induce the reordering of water molecules but in a different manner. According to Sharp and Madan [52], water around non-polar solutes is less perturbed than water around polar solutes in terms of hydrogen bonding of surrounding water. Such perturbation of hydrogen bonding leads to greater energy microstates of water around both types of solutes, but since the perturbation is weak in the case of non-polar solutes, the water can easily fluctuate between these microstates. This also means that it has higher heat capacity than bulk water, since it has higher reachable energy states. On the contrary, polar solutes strongly perturb the water; water molecules are strongly associated with the polar solute and therefore cannot fluctuate between these states, meaning that such water has lower heat capacity than bulk water. In the case of [HBz], the presence of –OH group, therefore, increases the polar character of free ions (and decreases overall nonpolar character) and consequently decreases the heat capacity of surrounding water. This can 18

Journal Pre-proof be further confirmed by comparing partial molar heat capacities, c p , of the counterions. These are all positive, which is distinctive of hydrophobic hydration, where the values fall in order [Bz]>[o-HBz]>[p-HBz]>[m-HBz] [53]. A similar observation was made using dielectric relaxation spectroscopy (DRS) when one of the methyl groups on the headgroup of decyltrimethylammonium chloride (DeTAC) was replaced with a short alkylene linker terminated by an –OH group. The amount of water slowed down by the free cation, a feature typical for hydrophobic hydration, decreased from 28 to 7 [34]. Similar decrease, although less pronounced, was also observed for systems with ether or ester terminal functional group. During micellization, the water around cations and counterions is partly removed into the

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bulk, as the latter are also incorporated into the micellar structure. Therefore, it is expected

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that heat capacity change upon micellization, cp, is most negative in the case of [Bz] since the free ion has the largest hydrophobic hydration. We can conclude that hydrophobic

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hydration plays the most important role during the micellization of the studied systems and is the main driving force (reflected in the large entropic part). The enthalpic part in these

re

systems is quite large compared to the surfactants with inorganic counterions and can be

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ascribed to the direct ion-ion interactions which also take an important part in the process of

a)

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na

micellization.

b)

c)

d)

Fig. 6. SDFs of water molecules around ionic liquids ions: a) [HB]–, b)[o-HB]–; c) [m-HB]–; d) [pHB]–

19

Journal Pre-proof 5. Conclusions In present work, the effect of isomerism of counterions on micellization process of 1dodecyl-3-methyl-imidazolium cation in water is investigated by isothermal titration calorimetry (ITC) in temperature range from 278.15 K to 318.15 K. Counterion structure was varied from benzoate anion to ortho-, meta- and para-hydroxybenzoate anions. ITC experimental data were analyzed by the help of an improved mass-action model, in order to obtain values of critical micelle concentration, cmc, the degree of counterion binding, , aggregation number, n, standard heat capacity, ΔMcp, enthalpy, ∆MHθ, entropy, ∆MSθ, and

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Gibbs free energy, ∆MGθ of micellization.

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The micellization process of 1-dodecyl-3-methyl-imidazolium cation in water was detected as endothermic at low temperatures and exothermic at high temperatures which could be

-p

referred to the delicate balance between hydrophobic hydration of nonpolar parts (endothermic) and counterion binding (exothermic) during the micellization process.

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Enthalpies of micellization were found to fall in order [Bz]>[p-HBz]>[m-HBz]>[o-HBz]. It is

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in accordance with a fact that [o-HBz] provokes the formation of more compact micelles by stronger interaction (and subsequent incorporation) of [o-HBz] with surfactant molecule,

shown by MD simulations).

na

resulting in the highest exothermicity of micellization process in the case of [o-HBz] (also

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In order to better understand the interactions between cation and different anions, computational simulations were performed. It can be seen that [C12mim][Bz] and [C12mim][p-HBz] form non-covalent interactions, mainly between imidazolium ring and – COO– group of corresponding anion. On the other hand, the –OH group on ortho and meta position cause different orientation of alkyl side chain, which additionally contributes to interactions between cation and anion. Also, binding energy between cation and each anion was calculated. It can be seen, that the strength of interactions between cation and anions follow the trend: [o-HBz]> [m-HBz]> [p-HBz]> [Bz]. All obtained results indicate that the introduction of –OH group on ortho and meta position promotes cation-anion interactions and stability of ionic liquids. Moreover, the hydroxyl group on para position has insignificant effect on cation-anion interactions comparing to non-substituted anion. Furthermore, it could be noted that the values of cp fall (in absolute sense) in order [Bz]>[o-HBz]>[p-HBz]>[m-HBz], where relatively high value for [Bz] compared to [o-HBz]

20

Journal Pre-proof is noticeable. The difference in cp can be partly ascribed to the steric hindrance of the polar and strongly hydrated –OH group, leaving part of the micellar structure still hydrated. On the other hand, the presence of –OH group also decreases the overall non-polar surface of the counterions, and, consequently hydrophobic hydration, so it is expected that ∆Mcp is most negative in the case of [Bz] as counterion.

6. Acknowledgements The work was supported by the Ministry of Education, Science and Technological

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Development of the Republic of Serbia under contract number ON172012. I.Č. is grateful to

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Public Scholarship, Development, Disability and Maintenance Fund of the Republic of Slovenia for the grant enabling her research work and doctoral studies at the University of

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Ljubljana. The support of the Slovenian Research Agency through Grant No. P1-0201 is

re

acknowledged also.

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7. Supplementary data

NMR spectra, thermodynamics of micellization, additional figures and data tables.

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Supplementary data associated with this article can be found in the online version.

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Journal Pre-proof Author Statement Isidora Čobanov: Experiment, Data gathering and analysis, Writing - original draft, Writing - review & editing Bojan Šarac: Experiment, Data gathering and analysis, Writing - original draft, Writing review & editing Žiga Medoš: Data analysis, Software, Writing - original draft, Writing - review & editing

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Aleksandar Tot: Synthesis and analysis of compounds, DFT&MD simulations, Writing -

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original draft

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Milan Vraneš; Synthesis and analysis of compounds, DFT&MD simulations, Writing -

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original draft

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Slobodan Gadžurić: Synthesis and analysis of compounds, DFT&MD simulations, Writing original draft

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Writing - review & editing.

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Marija Bešter-Rogač: Conceptualization, Funding acquisition, Methodology, Validation,

22

Journal Pre-proof Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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ro

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Journal Pre-proof

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Graphical abstract

24

Journal Pre-proof Highlights

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  

Micellization of 1-dodecyl-3-methyl-imidazolium cation in water was investigated As counterions, benzoate, ortho-, meta- and para-hydroxybenzoate anions, were taken The standard thermodynamic parameters of micellization process are derived To refine the thermodynamic parameters, the molecular simulations were performed Calculated binding energies at anion and cations coincide with found energetics

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 

25

Journal Pre-proof References [1] B. Šarac, G. Meriguet, B. Ancian and M. Bešter-Rogač, Salicylate Isomer-Specific Effect on the Micellization of Dodecyltrimethylammonium Chloride: Large Effects from Small Changes, Langmuir, 29 (2013) 4460-4469. [2] A. Kroflič, B. Šarac, M. Bešter-Rogač, Influence of the alkyl chain length, temperature, and added salt on thethermodynamics of micellization: Alkyltrimethylammonium chlorides in NaCl aqueous solutions, J. Chem. Thermodynamics, 43 (2011) 1557-1563.

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