Crystallization and Glass-Forming Ability of Ionic Liquids: Novel

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On the Crystallization and Glass-Forming Ability of Ionic Liquids: Novel Insights into their Thermal Behavior Ana I. M. C. Lobo Ferreira, Ana Rodrigues, Miguel Villas, Emilia Tojo, Luis Paulo N. Rebelo, and Luis M.N.B.F Santos ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04343 • Publication Date (Web): 06 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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ACS Sustainable Chemistry & Engineering

On the Crystallization and Glass-Forming Ability of Ionic Liquids: Novel Insights into their Thermal Behavior Ana I.M.C. Lobo Ferreira,*,†,‡,§ Ana S.M.C. Rodrigues,† Miguel Villas,  Emília Tojo,  Luís Paulo N. Rebelo,§ Luís M.N.B.F. Santos*,†

† CIQUP,

Departamento de Química e Bioquímica, Faculdade de Ciências da

Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal.

‡

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.

§

LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal.



Department of Organic Chemistry, Universidade de Vigo Marcosende, As Lagoas, Vigo, 36210, Spain

ACS Paragon Plus Environment

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Corresponding author: [email protected]; [email protected]


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ABSTRACT

This work presents an extended study of the thermal behavior of the alkyl and dialkylpyridinium derivatives of the bis(trifluoromethylsulfonyl)imide ionic liquids series, using a high resolution power compensation differential scanning calorimetry (DSC) equipment. Temperatures, enthalpies, entropies, and the heat capacity change associated with the glass transition, as well as cold crystallization, solid-solid transitions, and melting, are used to evaluate their ability to form a glass and crystallize. The effects of the cation isomerization and the alkyl chain length increase were used to investigate the nature of the irregular thermal behavior of ionic liquids in general, and to establish the link between their thermal properties and nanostructuration. The observed V-shape profile of the melting temperatures versus the alkyl side-chain length is interpreted as a consequence of the balance between the initial decrease of the magnitude of the electrostatic interaction and the regular increase of the dispersive van der Waals forces. The observed differentiation between the thermal behavior below and above the critical alkyl size, CAS, is analyzed and compared with the regular behavior of both the n-alkanes


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and n-alkanols. Above the CAS, the entropy and enthalpy profiles present trends similar to those observed in the alkane and alcohol series, contrasting with those observed in the molten salts-like region. These results and evidences are a strong support to the interpretation of the effect of the ILs nanostructuration in the physical-chemical properties. We found a great similarity between the thermal behavior of the imidazolium and pyridinium cation core, highlighting the strong ability of ILs to form a nanostructured network with distinguishable polar and nonpolar domains in the liquid phase.

Keywords: Ionic Liquids; Phase Transitions; Glass; Enthalpy; Nano structuration; Melting; CAS; Entropy; Solid; Crystallization; Heat Capacity


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INTRODUCTION

Due to the tuning capability of the ionic liquids (ILs) physicochemical properties they have been investigated in numerous research fields.[1-3] ILs exhibit low melting temperatures and in some cases only glass formation is detected, which is driven by the decrease of the electrostatic interactions density, and disruption of the packing efficiency due to the increased charge delocalization and cation/anion asymmetry. The increased irregular shape and size of the substituents in the ions contributes to the rise of rotational degrees of freedom, leading to the appearance of polymorphism. The thermal behaviour of ILs has been a subject of research since decades. However, most of the work reported until now is mainly related to imidazolium- or pyridinium-based ILs combined with [Cl]-, [BF4]-, [PF6]- or [NTf2]-.[4-14] Despite of the intense research, many data found in the literature are not of the highest quality due to the use of different experimental methodologies (e.g., scanning rates, annealing periods, sample size, thermal history and sample purity).[12-15] It is well accepted that ILs are structured fluids due to the complexity of their organization and diversity of the nature of their molecular interactions, showing high- and low-charge density regions.[16-19] The understanding of the nanostructuration of ILs and its intrinsic relation to most of their physicochemical properties are fundamental to achieve the possibility of tuning an ionic liquid to a specific functionality or application without endless screening/(trial and error). The effects of nanostructuration have been detected in several physicochemical properties, such as heat capacities,[12,20-23] densities, surface tensions

[24,25]

and viscosities,[21,26-32] thermal

behavior,[12,13,33-36] vapor pressures,[37-40] among others.[41] For all these properties a trend shift was observed at the same critical value for the number of carbons of the alkyl chain N = 6, named as the Critical Alkyl Size (CAS).[12,13,27,37,42,43] Pyridinium based ILs have a high potential of applicability[44,45] due to their high thermal stability and the aromatic character of the cation,


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exhibiting physical-chemical properties very similar to those of the imidazolium cation analogues.[46-49] Nevertheless, the replacement of the imidazolium by the pyridinium cation modifies the structural organization of the liquid phase, affecting the physico-chemical properties. The larger polar cation size of the latter increases the distance between the ions resulting in a decrease in the electrostatic interaction energy.[50,51] However, pyridinium-based ILs have found to be associated with a higher cohesion energy than that of imidazolium counterparts, which is linked to the greater cation-cation dispersive interactions and a distinct charge distribution of the cation.[31,42,52,53] For the 1-ethyl-2-alkylpyridinium series [1C22CnPy][NTf2] (n=2-10) the heat capacities, at T = 298.15 K, densities, volatilities viscosities and their dependence on temperature have been previously studied,[31,42] Brennecke et al.[54] have studied the thermal behaviour, heat capacities and

viscosities

for

some

series

of

pyridinium-based

ionic

liquids

including

the

1-hexylpyridinium bistriflimide. Oliveira et al.[27] investigated the structural and positional isomerism influence on the physical properties: density and viscosities of six pyridinium NTf2based ionic liquids. A comprehensive study of the liquid-liquid equilibrium between water and isomeric pyridinium-based ionic liquids has been performed by Freire et al.,[55] in order to understanding the isomeric effects on the pyridinium-based cation and the impact of the alkyl side chain length. The volatilities of the 1-alkylpyridinium-based ILs family have been published by Rocha et al.[56] The effect of the alkyl chain increase, that of the ortho- and para- positions, and the cation’s symmetry on the thermal behaviour has been rationalized by means of a comparative analysis with the imidazolium bistriflimide ILs.[14]


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This article reports the synthesis of new pyridinium bistriflimide based ILs and a comprehensive study on their thermal behavior exploring the effect of the alkyl chain increase, and ortho- or paraposition substitutions in the cation (illustrated in Figure 1).

[1CnPy]+

[1C22CnPy]+

[1C24CnPy]+

R N+

[1Cn4CnPy]+

[NTf2]-

R N+

R

N+

N+

O S F3C O

R

N

O S O CF3

R

Figure 1. Schematic representation of the studied alkylpyridinium derivatives. The [NTf2]- Ionic liquids series. R ranging from C2 to C9.

EXPERIMENTAL SECTION

Synthesis and Characterization of the Ionic liquid samples The alkylpyridinium bis(trifluoromethylsulfonyl)imide, [1CnPy][NTf2], ionic liquids were acquired from IOLITEC with stated purity above 99%. The synthesis of the 1-ethyl-2alkylpyridinium bis(trifluoromethylsulfonyl)imide, [1C22CnPy][NTf2], ionic liquids series has already been reported elsewhere.[42] The new 1-ethyl-4-alkylpyridinium, [1C24CnPy][NTf2], and 1,4-dialkylpyridinium, [1Cn4CnPy][NTf2], bis(trifluoromethylsulfonyl)imide ionic liquids were prepared in a three-step process following the general route shown in Figure 2. Different alkyl chains were introduced in position 4 of the pyridine ring by metalation of commercial 4methylpyridine (1), using lithium diisopropylamide as a base, and subsequent alkylation with the


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appropriate alkyl halide. 4-alkylpyridines “2-7” were then reacted with bromoethane, to give the corresponding 1-ethyl-4-alkylpyridinium bromides “8-13”, while 4-alkylpyridines “3-7” were reacted with the appropriate 1-bromoalkanes to give the corresponding 1,4-dialkylpyridinium bromides “14-18”. Subsequent metathesis with the bis(trifluoromethane)sulfonimide lithium salt afforded the desired 1-ethyl-4-alkylpyridinium bis(trifluoromethane)sulfonimides “19-24” and 1,4-dialkylpyridinium bis(trifluoromethane)sulfonimides “25-29” with high purity (99%) and an overall yield around 82%. Their structures were confirmed by 1H-NMR, 13C-NMR and ESI-HRMS. General procedures and spectral data are described in more detail in the supporting information.

Figure 2. Synthetic pathway for the synthesis of [1C24CnPy][NTf2] and [1Cn4CnPy][NTf2]

The ILs samples were dried under reduced pressure (0.1 Pa) for a minimum of 48 h at moderate temperature (303 K for crystalline samples; 323 K and constant stirring for liquid samples) to reduce the presence of water or other volatile components. The water mass fraction content was determined in a 151 Metrohm 831 Karl Fischer coulometer, using a Hydranal-152, Coulomat AG


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from Riedel-de Haën. This process was performed systematically, before any experimental measurement. The purity of all the ionic liquid samples were assigned above 99 % (in mass fraction) based on a set of the following different analytical characterization criterions: 1H-NMR & 13C-NMR spectra in full agreement with the chemical formula and absence of any other detectable unexpected peak; no detection of bromide anion (based on the AgNO3 test); mass spectra analysis (ESI-HRMS) in full agreement with the molecular formula ESI-HRMS with absence of any trace of other compounds; water content below 100 ppm.

Thermal behavior and phase transition study The thermal behavior study was done in a Perkin Elmer power compensation differential scanning calorimeter DSC (Pyris Diamond DSC), additional details in SI. Briefly, it consists in the sample heating up to 330 K, followed by a quenching step (cooling at 50 K∙min-1) to 173 K. Afterwards, the ILs samples were heated (5 K∙min-1) to promote the glass transition followed by (if the case) a cold crystallization. An isothermal step was performed after the IL crystallization in order to guarantee the full crystallization of the IL (the procedure was repeated and the sample annealed until no sign detection of crystallization or phase reorder). This was followed by cooling (50 K∙min-1) and heating (5 K∙min-1) cycles in the crystallization region, exceeding the glass transition and below the temperature of melting to assure complete crystallization before the melting. A final scan at 5 K∙min-1 was performed to determine the temperatures and enthalpies of the solid-solid and the isotropization of the phase transitions. All runs were performed in the same DSC instrument, using identical experimental conditions. Additional details about the methodology and thermal behavior of each sample are described and presented as supporting


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information. The values of the relative atomic masses of the elements were used according to the recommendations made by the 2013 IUPAC Commission.[57]

RESULTS AND DISCUSSION Temperature and heat capacity changes at the glass transition, crystallization behaviour (solidsolid and fusion transitions) temperatures and enthalpies and entropies of melting of the studied ILs were measured in a power compensation differential scanning calorimeter, DSC (additional details in the Experimental Session). In this study we adopt the same experimental methodology as described elsewhere.[14] The melting temperatures (Tm) and the solid-solid transition temperatures (Tss) were taken as the onset temperature of the endothermic peak on heating. Cold crystallization temperature (Tcc) was determined as the onset temperature of an exothermic peak on heating from a subcooled liquid to a crystalline solid, and the glass transition temperature (Tg) was obtained considering the midpoint of the heat capacity change on heating from a glass to a liquid. Experimental results concerning the thermal behaviour and the thermograms of the studied ILs series are presented in detail as SI. o

The values of glass transition temperatures, Tg, melting temperatures, Tm, enthalpies, ∆lsHp,m (sum o

of all enthalpic contributions until isotropization), and entropies, ∆lsSp,m (sum of all entropic contributions until isotropization) of melting corrected to the reference temperature of 298.15 K are summarized in tables 1 to 4, for the [1CnPy][NTf2], [1C22CnPy][NTf2], [1Cn4CnPy][NTf2] and [1C24CnPy][NTf2], respectively. The heat capacity corrections of the enthalpies and entropies of melting to the reference temperature of 298.15 K, for each series of ILs, were calculated using the equations (1) and (2), respectively:


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∆lsHop,m(298.15) = ∆lsHop,m(𝑇m) + ∆lsCop,m(Tm) ⋅ (298.15 - Tm)

(1)

(2)

∆lsSop,m(298.15) = ∆lsHop,m(𝑇m) / 𝑇m + ∆lsCop,m(Tm) ⋅ ln(298.15 / 𝑇m)

where, ∆lsCop,m(Tm) is the molar heat capacity change between the solid and the liquid, (at the fusion temperature). It was estimated based on the experimental value of the molar heat capacity o

change at the glass transition ∆lglCp,m(Tg) as indicated in equations (3) and (4) ∆lsCop,m(𝑇m) = Cop,m(l, 𝑇m) ― Cop,m(s, 𝑇m) o

o

(3)

o

l ∆glCp,m(𝑇𝑔 ) = Cp,m(l, 𝑇𝑔 ) ― Cp,m(gl, 𝑇𝑔)

(4)

taking into consideration the typical relation between heat capacities changes expressed in equation (5), ∆lsCop,m(𝑇m)≅1.25 ∙ ∆lglCop,m(𝑇g)

(5)

The relation expressed by equation (5) has been found both in our experimental results as well as, in the experimental data reported by Paulechka et al.[12-14, 34-36] and it applies for ionic liquids with a low-T melting point. The assigned uncertainty of (20%) of ∆lsCop,m(𝑇m) estimated from equation (5) has a negligible effect in the differential analysis of the ILs thermal behavior due to the small difference between the melting temperatures and the reference temperature of 298.15 K.


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Table 1. Experimental glass and melting temperatures, Tg and Tm, respectively, glass transition l o heat capacity change,  glCp,m , enthalpies,  H , and entropies of melting,  S , corrected at 298.15 K for the [1CnPy][NTf2] ILs series.[a]

T/K

Ionic Liquid

[1C2Py][NTf2] 302.5 ± 0.5 (Tm) [1C3Py][NTf2] 317.3 ± 0.5 (Tm) [1C4Py][NTf2] 190.1 ± 0.5 (Tg) 295.3 ± 0.5 (Tm)

l

o

l

o

s

m

s

m

l

Tg/Tm

0.64

[1C5Py][NTf2] 192.5 ± 0.5 (Tg) 269.5 ± 0.5 (Tm)

0.71

[1C6Py][NTf2] 193.6 ± 0.5 (Tg) 268.8 ± 0.5 (Tm)

0.72

[1C7Py][NTf2] 193.8 ± 0.5 (Tg) 265.4 ± 0.5 (Tm)

0.73

[1C8Py][NTf2] 194.1 ± 0.5 (Tg) 262.8 ± 0.5 (Tm)

0.74

[1C9Py][NTf2] 194.8 ± 0.5 (Tg) 264.8 ± 0.5 (Tm)

0.74

[a]

o

ΔglCp,m ―1 ―1 JK ∙ mol

l

l

o

kJmol

―1

o

Δ𝑠Sm

Δ𝑠Hm

JK

―1

mol

21.6 ± 0.7 22.2 ± 0.7

72 ± 3 70 ± 3

27.3 ± 0.7

93 ± 3

26.8 ± 0.7

99 ± 3

20.6 ± 0.7

76 ± 3

20.4 ± 0.7

76 ± 3

26.8 ± 0.7

101 ± 3

31.8 ± 0.7

122 ± 3

―1

91

115

115

128

116

146

∆sHom and ∆ Som corresponds to the sum of enthalpies and entropies changes contributions from the most l

l s

l

stable crystal to the isotropic liquid, respectively, and corrected to T = 298.15 K by using the estimated ∆

gl

o Cp,m


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Table 2. Experimental glass temperature, Tg, and glass-liquid transition heat capacity change,  C , for the [1C22CnPy][NTf2] series.[a] l

o

gl

p,m

Ionic Liquid

Tg / K

ΔlglCop,m / JK ―1 ∙ mol

[1C22C2Py][NTf2] [1C22C3Py][NTf2]

191.3 ± 0.5 196.2 ± 0.5

119 124

[1C22C4Py][NTf2]

197.5 ± 0.5

119

[1C22C5Py][NTf2]

197.1 ± 0.5

128

[1C22C6Py][NTf2]

198.2 ± 0.5

155

[1C22C7Py][NTf2]

198.9 ± 0.5

153

[1C22C8Py][NTf2]

200.1 ± 0.5

178

[1C22C9Py][NTf2]

200.1 ± 0.5

190

[1C22C10Py][NTf2]

201.3 ± 0.5

184

[a]

―1


l s

l

stable crystal to the isotropic liquid, respectively, and corrected to T = 298.15 K by using the estimated ∆ o [b] Cp,m Ratio of Tg/Tm.

gl

Table 3. Experimental glass and melting temperatures, Tg and Tm, respectively, glass-liquid l o transition heat capacity change,  C , enthalpies,  sHm , and entropies of melting,  S , corrected to 298.15 K for the [1Cn4CnPy][NTf2] ILs series.[a] Ionic Liquid

l

o

l

o

gl

p,m

s

m

T/K

Tg/Tm

[1C44C4Py][NTf2] 192.1 ± 0.5 (Tg) 282.4 ± 0.5 (Tm)

0.68

[1C54C5Py][NTf2] 192.9 ± 0.5 (Tg)

0.66

ΔlglCop,m JK ―1 ∙ mol

ΔlsHom ―1

kJmol

―1

Δl𝑠Som JK ―1mol

110 24.0 ± 0.7

85 ± 3

23.3 ± 0.7

80 ± 3

291.1 ± 0.5 (Tm)

27.4 ± 0.7

94 ± 3

[1C74C7Py][NTf2] 304.9 ± 0.5 (Tm)

35.4 ± 0.7

116 ± 3

[1C84C8Py][NTf2] 312.6 ± 0.5 (Tm)

43.7 ± 0.7

140 ± 3

113

291.2 ± 0.5 (Tm) [1C64C6Py][NTf2] 193.5 ± 0.5 (Tg)

[a]

0.66

―1

116


l s

l


gl

o Cp,m


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Table 4. Experimental glass and melting temperatures, Tg and Tm, respectively, glass-liquid transition heat capacity change,  C , enthalpies,  H , and entropies of melting,  S , corrected to 298.15 K for the [1C24CnPy][NTf2] ILs series.[a] Ionic Liquid

l

o

l

o

l

o

gl

p,m

s

m

s

m

T/K

Tg/Tm

[1C24C3Py][NTf2] 189.4 ± 0.5 (Tg) 277.8 ± 0.5 (Tm)

0.68

[1C24C4Py][NTf2] 188.9 ± 0.5 (Tg)

0.75

ΔlglCop,m JK ―1 ∙ mol

0.70

[1C24C6Py][NTf2] 189.3 ± 0.5 (Tg)

0.70

0.68

[a]

0.69

―1

JK ―1 ⋅ mol

―1

24.5 ± 0.7

88 ± 3

23.0 ± 0.7

90 ± 3

23.0 ± 0.7

85 ± 3

23.9 ± 0.7

91 ± 3

28.3 ± 0.7

107 ± 3

33.9 ± 0.7

125 ± 3

104

118

118

277.7 ± 0.5 (Tm) [1C24C8Py][NTf2] 189.9 ± 0.5 (Tg) 273.9 ± 0.5 (Tm)

kJmol

132

268.2 ± 0.5 (Tm) [1C24C7Py][NTf2] 189.9 ± 0.5 (Tg)

―1

Δl𝑠Som

107

252.9 ± 0.5 (Tm) [1C24C5Py][NTf2] 188.9 ± 0.5 (Tg) 270.3 ± 0.5 (Tm)

Δl𝑠Hom

135


l s

l


gl

o Cp,m

The uncertainties of the experimental results were assigned on the basis on the extended standard deviation of the experimental and the calibration results, and are reported as twice the standard deviation of the mean. It is interesting to observed that the ratio Tg/Tm (from 0.64 to 0.75) is in relatively good agreement with the Beaman-Kauzmann rule (Tg/Tm  2/3)[58-60] for the typical onecomponent glass-forming liquid.


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Whether some liquids (or homologous series within a family of compounds), including polymers, molecular liquids, inorganic salts, protic and aprotic ILs might, instead, tend to present a Tg/Tm  3/4 rule is under deep debate.[61-64] Figure 3 depicts the glass temperature, Tg, as a function of the total number of carbon atoms in the alkyl chains, N, of the pyridinium-based ILs: [1CnPy][NTf2], [1C22CnPy][NTf2], [1C24CnPy][NTf2] and [1Cn4CnPy][NTf2] as well as for the imidazolium series, [C1Cnim][NTf2],[14] and [1Cn3Cnim][NTf2].[14] In the glass, packing requirements are virtually not existent, so the relative stability of the glass will be driven by the magnitude of the bulk interactions and the inability to generate both internal rotational degrees of freedom and external rotations and translations. The observed glass temperature of the pyridinium series is significantly higher than the analogous imidazolium series. For the pyridinium series glass temperature follows the order: [1C24CnPy][NTf2] < [1Cn4CnPy][NTf2] < [1CnPy][NTf2] < [1C22CnPy][NTf2] which reflects the isomerization effect. It was found that the more hindered cation series [1C22CnPy][NTf2], presents the higher glass stability, in line with the lower ability to have internal rotational of the alkyl groups. The glass Tg dependence on the chain length was found to be regular (small and positive) for N > 6 for all the alkylpyridinium series.


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Figure 3. Glass transition temperature, Tg, as a function of N (total number of carbons atoms) of the series: [1CnPy][NTf2] (), (N=n); [1C24CnPy][NTf2] (), (N=n+2); [1Cn4CnPy][NTf2] (), (N=1n+4n). Literature data for the imidazolium series, [C1Cnim][NTf2] (), (N=n+1) and [1Cn3Cnim][NTf2] (), (N=1n+3n).[14] The border grey-white background indicates the region of trend change. Dashed lines are merely guides to the eye with no physical meaning.

o

The molar heat capacities change at the glass-liquid transition temperature, ∆lglCp,m, are presented in figure 4 for the [1CnPy][NTf2], [1C22CnPy][NTf2], [1C24CnPy][NTf2] and [1Cn4CnPy][NTf2] IL series, as a function of the total number of carbon atoms, N, together with data of the imidazoliumbased ILs.[14] Similar to what has previously been observed for the imidazolium ILs series, the contribution of the methylene group, –CH2–, was found to be in the order of 10 JK-1mol-1 in agreement with the expected heat capacity increment from glass/solid to liquid phase of the methylene –CH2– group contribution. Figure 5 depicts the experimental melting temperatures of [1CnPy][NTf2], [1C24CnPy][NTf2], and [1Cn4CnPy][NTf2] series as a function of the total number of carbon atoms in the alkyl side chains (N), together with the literature values for [C1Cnim][NTf2],[14]


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[1Cn3Cnim][NTf2][14] n-alkanes[65] and n-alkanols[65] series, 2  N  20. We could not find any evidence of crystallization in the [1C22CnPy][NTf2] series.

Figure 4. Heat capacity change at the glass transition temperatures,  C as a function of N (total l

o

gl

p,m

number of carbon atoms) of the series: [1CnPy][NTf2] (), (N=n); [1C22CnPy][NTf2] (), (N=n+2); [1C24CnPy][NTf2] (), (N=n+2); [1Cn4CnPy][NTf2] (), (N=1n+4n). Literature data for the imidazolium series, [C1Cnim][NTf2] (), (N=n+1) and [1Cn3Cnim][NTf2] (), (N=1n+3n).[14] Straight and dotted lines are guides to the eye.


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Figure 5. Melting transition temperatures, Tm, as a function of N (total number of carbons atoms) of the series: [1CnPy][NTf2] (), (N=n); [1C24CnPy][NTf2] (), (N=n+2); [1Cn4CnPy][NTf2] (), (N=1n+4n). Literature data for [C1Cnim][NTf2] (), (N=n+1), and [1Cn3Cnim][NTf2] (), (N=1n+3n).[14] n-Alkanes: , 2 N 20,[65] and n-alkanols: x, 2 N 20.[65] The border grey-white background indicates the region of trend change. Dashed lines are merely guides to the eye with no physical meaning.

In this series only the glass transition was detected. The inability of the [1C22CnPy][NTf2] series to crystalize is pretty amazing and it seems to be a consequence of the weakening of the electrostatic interactions, the hindrance stress between the alkyl tails in the neighboring position and the decrease of the cation’s symmetry of this series when compared with those of the [1CnPy][NTf2], [1C24CnPy][NTf2] or [1Cn4CnPy][NTf2]. Overall, the initial decrease in the melting temperatures (short alkyl chains) is explained by the decrease in the solid stability compared with the liquid phase, due to the decrease of the electrostatic interactions between the ion pairs arising from the increase in the alkyl chain length, with a correspondent increase in the conformational


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entropy. Above the N = 6, the effect of adding a methylene –CH2– group in the melting temperature becomes significantly more regular and identical to that observed in the n-alkanes or n-alkanols.[65] In a previous work[66] some of us suggested for the first time that comparisons between the behavior of ionic liquids and that of n-alkanols were worth to be checked out. All the isomeric series are converging to an identical trend, which gradually approaches the trend/behavior of the n-alkane and n-alkanols series. The observed V-shaped profile for the melting temperatures is also quite similar to that observed in others series of ILs,[12-14] highlighting the trend shift, so-called critical alkyl size (CAS),[14,27,37,38,42,43] due to the intensification of the nanostructuration in the liquid phase with the formation of polar domains (mainly dominated by the cation-anion continuous network interactions) and non-polar segregation region (alkane-like domain, with predominant van der Waals interactions). The observed cool crystallization temperature, Tcc (details in Supporting Information Table SI.1-3) is found to be in the range of 200 to 260 K which are typically the temperatures associated with the thermal motion needed to the alkyl group rotation.[67] o

o

The graphical representations of the ∆lsHp,m and ∆lsSp,m, at T = 298.15 K, of the ILs series as a function of the number of carbon atoms in the alkyl side chains of the cation, N, are presented in Figure 6. It is observed an initial decrease in the enthalpies of melting for the ILs with the shorter alkyl chain length due to an increase of the hindering effect of adding a –CH2– group that leads to a decrease of the localized electrostatic interaction. Above the critical alkyl size, CAS, a regular increase in the enthalpy and entropy profiles is observed which arise from the formation of alkyl nonpolar domains very similar to those observed in the alkane and alcohols series. For the shorter alkyl chain length the entropy contribution per methylene group, –CH2–, is smaller when compared to that observed for the ILs with long alkyl chains, indicating a very small perturbation in the solid-


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liquid entropy change with the increase of the alkyl chain length. The profile of the melting temperatures, ought to be analyzed based on the balance between the enthalpic and entropic contribution as expressed in equation (3):

l

Tm =

∆sH(T) l

∆sS(T)

(3)

As expected, in the short alkyl chain region, the melting temperatures of the ionic liquids are significantly higher than in the alkanes and alcohols series, which is ruled by their higher enthalpy of melting. In the long alkyl chain length region, the enthalpies and entropies of melting are identical but slightly lower than those observed in the n-alkanes and n-alkanols series. This is an indication that the melting of the ionic liquids is essentially related to the increase of mobility of the nonpolar domains, in which, the cohesive interaction associated with the polar network is essentially preserved in the liquid phase. In this region, the slightly higher temperatures of melting observed in the ionic liquids are driven by their lower entropy of melting. A differentiation between the asymmetric [1CnPy][NTf2] and [1C24CnPy][NTf2], and the more symmetric [1Cn4CnPy][NTf2] IL series is observed for both the enthalpy and the entropy of melting. This differentiation is similar to that observed previously for the asymmetric [CnC1im][NTf2] versus the symmetric [1Cn3Cnim][NTf2] series.[14]


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Figure 6. Enthalpies ∆sHom (I) and entropies ∆sSom (II) of melting transition corrected for T = 298.15 K of the [1CnPy][NTf2] (), (N=n), [1C24CnPy][NTf2] (), (N=n+2) and [1Cn4CnPy][NTf2] (), (N=1n+4n), series as a function of the N (total number of carbons atoms). Literature values for [C1Cnim][NTf2] (), (N=n+1), and [1Cn3Cnim][NTf2] (), (N=1n+3n).[14] n-Alkanes: , 2 N 20,[65] and n-alkanols:[65] x, 2 N 20. The border grey-white background indicates the region of trend change. Dashed lines are merely guides to the eye with no physical meaning l

l


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The many aspects of the thermal behaviour along the [1CnPy][NTf2], [1C22CnPy][NTf2], [1C24CnPy][NTf2] and [1Cn4CnPy][NTf2] ionic liquids series was used to explore the effect of the alkyl chain length and that of isomerization in the nanostructuration of these salts. The glass transition, Tg, melting Tm, and cold crystallization, Tcc, temperatures, the enthalpy, entropy and heat capacity change involved in these transitions were determined with high reproducibility using a high-resolution power-compensation DSC apparatus and a well-defined methodology. The melting temperatures, Tm, along the alkyl series present a V-shape profile that is explained by the balance between the initial strong decrease of the electrostatic interaction potential and the regular increase of the van der Waals interactions with the increasing size of the alkyl side chain of the cation. A significant differentiation of the thermal behaviour below and above the critical alkyl size, CAS, was observed. Above the critical alkyl size, a regular increase in the entropy and enthalpy profiles presents a trend similar to that observed in both the n-alkane and n-alkanols series. The observed results constitute a strong support to the molecular interpretation of the nature and origin of the ILs nanostructuration. However, there are issues still to definitely be bared: (i) the apparent complete absence of melting/crystallization in ILs such as [1C22CnPy][NTf2], and (ii) the “forbidden land” for crystallization around the CAS for other typical ILs – the crystallization gap.


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Supporting Information Experimental description, ionic liquids synthesis, 1H and 13C NMR spectra, DSC experimental results, DSC thermograms.

Author Information Corresponding Author *E-mail: [email protected]; [email protected]

Author Contributions All authors contributed equally.

Acknowledgements We wish to thank Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and the European Social Fund (ESF) for their financial support CIQUP, University of Porto (Projects Pest-C/QUI/UI0081/2013 and NORTE-01-0145-FEDER-000028, SAM Sustainable Advanced Materials). A.I.M.C.L.F. and A.S.M.C.R. thank FCT for the PostDoc: SFRH/BPD/84891/2012 and for the PhD Research Grant: SFRH/BD/81261/2011, respectively.


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[55] Freire, M. G.; Neves, C. M. S. S.; Shimizu, K.; Bernardes, C. E. S.; Marrucho, I. M.; Coutinho, J. A. P. ; Canongia Lopes, J. N.; Rebelo, L. P. N. Mutual solubility of water and structural/positional isomers of n-alkylpyridinium-based ionic liquids. J. Phys. Chem. B 2010, 114 (6), 15925-15934, DOI 10.1016/j.jct.2005.03.013. [56] Rocha, M. A. A.; Santos, L. M. N. B. F. First volatility study of the 1-alkylpyridinium based ionic liquids by knudsen effusion. Chem. Phys. Lett. 2013, 585, 59-62, DOI 10.1016/j.cplett.2013.08.095. [57] Wieser, M. E.; Holden, N.; Coplen, T. B.; Böhlke, J. K.; Berglund, M.; Brand, W. A.; DeBiev̀re, P.; Gröning, M.; Loss, R. D.; Meija, J. et al. Atomic Weights of the Elements 2011 (IUPAC Technical Report). Pure Appl. Chem. 2013, 85 (5), 1047-1078, DOI 10.1351/PAC-REP-13-03-02. [58] Kauzmann, W. The nature of the glassy state and the behavior of liquids at low temperatures. Chem. Rev. 1948, 43 (2), 219-256, DOI 10.1021/cr60135a002. [59] Gutzow, I.; Dobreva, A. Structure, thermodynamic properties and cooling rate of glasses. J. Non-Cryst. Solids 1991, 129 (1-3), 266-275, DOI 10.1016/0022-3093(91)90103-D. [60] Beaman, R. G. Relation between (apparent) second- order transition temperature and melting point. J. Polym. Sci. 1952, 9 (5), 470-472, DOI 10.1002/pol.1952.120090510. [61] Angell, C. A. On the importance of the metastable liquid state and glass transition phenomenon to transport and structure studies in ionic liquids. I. Transport properties. J. Phys. Chem. 1966, 70 (9), 2793-2803, DOI 10.1021/j100881a014. [62] Angell, C. A. Thermodynamic aspects of the glass transition in liquids and plastic crystals. Pure & Appl. Chem. 1991, 63 (10), 1387-1392, DOI 10.1351/pac199163101387. [63] Ping, W.; Paraska, D.; Baker, R.; Harrowell, P.; Angell, C. A. Molecular engineering of the glass transition: glass-forming ability across a homologous series of cyclic stilbenes. J. Phys. Chem. B 2011, 115 (16), 4696-4702, DOI 10.1021/jp110975y. [64] Belieres, J. P.; Angell, C. A. Protic ionic liquids: preparation, characterization, and proton free energy level representation. J. Phys. Chem. B 2007, 111 (18), 4926-4937, DOI 10.1021/jp067589u. [65] Costa, J. C. S.; Mendes, A.; Santos, L. M. N. B. F. Chain length dependence of the thermodynamic properties of n-alkanes and their monosubstituted derivatives. J. Chem. Eng. Data 2018, 63 (1), 1-20, DOI 10.1021/acs.jced.7b00837. [66] Canongia Lopes, J. N.; Cordeiro, T. C.; Esperanca, J. M. S. S.; Guedes, H. J. R.; Huq, S.; Rebelo, L. P. N.; Seddon, K. R. Deviations from ideality in mixtures of two ionic liquids containing a common ion. J. Phys. Chem. B 2005, 109 (8), 3519-3525, DOI 10.1021/jp0458699.


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For Table of Contents Use Only.

N_ Alkanes

Insights into their Thermal Behavior. V-shape profile of the melting temperatures vs the alkyl side-chain. Temperatures, entropies and enthalpies of melting converges to n-alkanes derivatives.


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Crystallization and Glass-Forming Ability of Ionic Liquids: Novel

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