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Critical Role of the Spacer Length of Gemini Surfactants on the Formation of Ionic Liquid Crystals and Thermotropic Behavior Ricardo M.F. Fernandes, Yujie Wang, Pedro B. Tavares, Sandra C.C. Nunes, Alberto A. C. C. Pais, and Eduardo F. Marques J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08618 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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The Journal of Physical Chemistry

Critical Role of the Spacer Length of Gemini Surfactants on the Formation of Ionic Liquid Crystals and Thermotropic Behavior

Ricardo M.F. Fernandes1, Yujie Wang1,2, Pedro B. Tavares3, Sandra C.C. Nunes4, Alberto A.C.C. Pais4, Eduardo F. Marques1*

1

CIQUP, Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, Rua do Campo Alegre, s/n, 4169-007 Porto, Portugal. 2

Henan Institute of Science & Technology, School of Chemistry & Chemical Engineering, Xinxiang 453003, Henan, People’s Republic of China.

3

CQVR - Centro de Química de Vila Real, Departamento de Química, Universidade de Trás-os-Montes e Alto Douro, 5000-801 Vila Real, Portugal.

4

CQC - Centro de Química de Coimbra, Department of Chemistry, University of Coimbra, Rua Larga 3004-535, Coimbra, Portugal.

*email: [email protected]

1

ACS Paragon Plus Environment


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Abstract Numerous reports have shown that the self-assembling properties of 12-s-12 bis(quaternary ammonium) gemini surfactants in aqueous solution are significantly influenced by s, the number of methylene groups in the covalent spacer. However, the role played by s on the phase behavior of the single compounds has not been investigated in a similarly systematic way. Here, we report on the thermotropic phase behavior of the anhydrous compounds with s = 2-6, 8, 10 and 12, resorting to differential scanning calorimetry (DSC), polarized light microscopy (PLM) and X-ray diffraction (XRD). All of the compounds show a stepwise melting behavior, decomposing at 200 ºC. As the spacer length increases non-monotonic trends are observed for the thermodynamic parameters of the thermotropic phase transitions, mesophase formation and solid-state d00l spacings. In particular, the number and type of mesophases (ordered smectic phases and/or fluid smectic liquid crystals) depend critically on s. Further, upon heating molecules with s < 8 decompose before the liquid phase, while those with long spacers, s = 812, reach the isotropization (clearing) temperature hence forming both ionic liquid crystals and ionic liquid phases. We demonstrate that the melting behavior and type of ionic mesophases formed by gemini molecules can be usefully manipulated by a simple structural parameter like the length of the covalent linker.

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1. Introduction Thermotropic liquid crystals (TLCs), and thermotropic mesophases in general, are intermediate phases that form between the solid and the isotropic liquid phase of a material (a single component or a mixture of components) upon heating, at constant pressure.1-4 TLCs can often respond to external stimuli such as mechanical stress or electrical fields, thereby changing their optical properties (e.g. birefringence and color).5,6 These organized structures have had a great impact in technology, being pivotal in the development of displays, sensors and new optoelectronic devices.1,3,7 TLCs have also drawn interest in other materials applications, e.g. as components in functional textile fibers8 and functional nanocomposites with carbon nanomaterials.9-11 Fundamental studies have focused on the development and characterization of novel mesogenic molecules (and their mixtures) and structure-function relationships, which no rarely foster the expansion of applications.3,4 In the last two decades, ionic liquid crystals which are liquid crystalline phases containing cations and anions (like the gemini mesogens investigated in this work) have also been subject to intense research and growing interest.12,13 These materials combine the properties of conventional uncharged TLCs, such as optical anisotropy, and some of the properties of ionic liquids such as ionic conductivity. The

gemini

surfactants

known

as

alkanediyl-α,ω-bis(alkyldimethylammonium

bromides), commonly designated by n-s-n (where n and s are the number of carbon atoms of the surfactant alkyl tail and alkyl spacer, respectively) are an important class of surfactants due to their aggregation properties and many applications in colloid science and technology.14,15 Gemini surfactants have been extensively investigated regarding their interfacial properties, thermodynamics of micellization and self-assembled structures in solution.14-21 In contrast, the thermal phase behavior of the anhydrous compounds has been much less studied, with only scarce reports available.22-25 One could anticipate that the mesogenic properties of these

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compounds will be highly dependent on the structural degree of freedom provided by the spacer length and flexibility. For amphiphilic mesogens containing alkyl chains, any mesophase always contains some degree of structural order of headgroups and structural disorder of the chains (positional, orientational, or conformational).1,26,27 The type of chain disorder depends inter alia on chain length, headgroup composition, and original crystalline lattice structure. Different models for thermally-induced chain disordering have been proposed on the basis of experimental data,27-32 computer simulations33-36 and thermodynamic considerations.33,37-42 For most surfactants, where chain-chain interactions are governed by weak dispersion forces, the first thermotropic mesophase is usually associated with chain melting or partial chain melting.27,30 However, the formation of TLCs depends also on headgroup interactions and positional rearrangements, which in turn depend on factors such as polarity, charge density43 and hydrogen bonding.30,40 The influence of headgroup interactions is demonstrated by the fact that in binary surfactant mixtures, the number and thermal range of mesophases is expanded compared to the individual compounds.21,43-45 Regarding n-s-n bis-quat gemini surfactants, early work showed that these ionic compounds could withstand long heating periods below 200 ºC with no detectable decomposition but in the 120-180 ºC range the formation of TLC was absent.23 Later, Fuller et al observed for 15-s-15 (s = 1, 2, 3, and 6) and θ > 100 ºC, a so-called viscous neat mesophase and specifically for 15-3-15 and 15-6-15 also smectic A mesophases before liquid formation.24 Asymmetric 12-s-14 gemini with s = 2, 6, and 10 were shown to possess complex solid polymorphism and rich mesomorphism from crystal to smectic TLC phases, and it was implied that the latter was a consequence of tail length asymmetry.25 However, smectic TLCs were found for symmetric n-2-n gemini with n = 12, 14, 16, and 18, on the basis of DSC and PLM.21 A subsequent study for 12-2-12 reported the phase sequence soft crystal→SmC→SmA→liquid 4


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phase/decomposition.46 The gemini 14-2-14 with different counterions was also reported to form TLCs.47 Mesomorphism has been further described for 12-6-12 and 14-6-14 imidazoliumbased gemini (SmA phases)48 and n-3-n type (with n = 10, 12, 14, 16) with a hydroxyl group in the spacer (SmB and SmC phases).49 Despite all the work cited above, a comprehensive and coherent investigation of the role of spacer length on the thermotropic mesopmorphism of gemini surfactants involving a large number of molecules is still lacking. In this work, the main goal was to carry out a systematic investigation on the effect of spacer length variation of 12-s-12 surfactants (Figure 1) upon the thermal phase behavior and formation of ionic liquid-crystalline phases. The thermodynamic parameters for the phase transitions were determined by DSC, while polarizing light microscopy allowed the detection of mesophase textures and respective phase assignment. XRD was used to gain insight to the solid-state structure and further unravel the differences in molecular packing between the compounds. Further, we assess the relation between spacer length, molecular packing and thermodynamic phases, based on our data and the available literature.

s

Figure 1. Molecular structure of 12-s-12 gemini surfactants, where s in the number of methylene groups in the spacer (s = 2, 3, 4, 5, 6, 8, 10 and 12 are herein investigated). 5



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2. Experimental Section 2.1 Materials. The 12-s-12 gemini surfactants were synthesized according to the method originally described by Menger et al.50 For 12-2-12, N,N,N´,N´-tetramethylethylenediamine (99 %, Aldrich) and 1-bromododecane (97 %, Aldrich) were used as received. For the remaining compounds, 1-dimethylaminododecane (97 %, Fluka) and the appropriate dibromoalkanes were used, namely 1,3-dibromopropane (99 %, Aldrich), 1,4-dibromobutane (99 %, Aldrich), 1,5-dibromopentane (97 %, Aldrich), 1,6-dibromohexane (96 %, Aldrich), 1,8dibromooctane (98 %, Aldrich), 1,10-dibromodecane (97 %, Aldrich), 1,12-dibromododecane (98 %, Aldrich). All the surfactants were twice recrystallized with an acetone-methanol mixture. The high purity of the compounds was ascertained by NMR and mass spectrometry; further, conductivity measurements showed that the critical micelle concentration of these compounds (see Supporting Information, Figure S1 and Table S1) are in good agreement with reference values from the literature.51,52 2.2 Differential Scanning Calorimetry (DSC). DSC scans were performed using a Setaram DSC141 differential calorimeter, previously calibrated for temperature and energy.53 A mass of 5-10 mg of solid compound was weighed to aluminum pans and an empty pan was used as reference. Heating-cooling cycles were performed at a scanning rate of 3 °C.min-1 in a temperature range of 20-250 °C, with nitrogen (p = 0.3 bar) as sweeping gas. Five independent essays were run for each compound and average values for the thermodynamic parameters of the phase transitions were determined (Table 1); the uncertainties for the phase transition temperature values are within ± 0.5 ºC, and for the transition enthalpy and entropy values within ± 5 %. 2.3 Polarizing Light Microscopy (PLM). Characterization of thermal behavior, with assignment of different birefringent textures, was carried out by inspecting glass-cover slip 6


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preparations of each gemini compound on the polarized light microscope. An Olympus BX51 microscope, equipped with a Linkam THMS-600 heating stage with a temperature control of about ±0.1 ºC, was used. The micrographs were obtained with an Olympus C-5060 Wide-Zoom digital camera. Typically, 3-5 independent preparations were analyzed for each compound with heating-cooling cycles, at different rates (0.1 – 10 ºC.min-1). 2.4 X-ray Diffraction (XRD). The x-ray powder diffraction spectra of the solid state gemini compounds were recorded at room temperature with a PANalytical X’Pert MPD diffractometer using the λ = 0.154 nm Kα line of a Cu anode (Bragg–Brentano geometry) equipped with a X’Celerator detector. The spectra were obtained from 10 to 95° (2θ), using a step of 0.017° and 100 s/step.

3. Results and Discussion 3.1. Thermal behavior and trends in thermodynamic parameters. To obtain a detailed characterization of the thermal behavior of the 12-s-12 gemini compounds, an extensive DSC study was carried out. Representative thermograms are shown in Figure 2 and the respective thermodynamic parameters for the phase transitions are presented in Table 1. The peaks are numbered according to increasing temperature for easier identification, and in the case of resolved peaks, an apparent enthalpy for the global transition is presented and no transition entropy is given. As can be seen, all compounds show several phase transitions denoting a gradual and complex melting process. Further, it is obvious that the incremental length of the spacer has a marked effect on the thermal behavior. Noteworthy, all the compounds undergo chemical degradation for θ ≥ 200 ºC (see Supporting Information, Figure S2) and hence the DSC heating traces are shown only up to 200 ºC. We could therefore confirm previous work that reported degradation for some of these 7



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compounds.21,23-25,44 Another conspicuous feature common to all compounds is the presence of one main endothermic peak. Inspection of the full thermograms and the enthalpies involved allows inferring that this peak corresponds to partial melting of the surfactant alkyl chains.27,54 The phase formed immediately after this peak is here designated by M1 (mesophase 1) and is an ordered smectic phase (either a soft crystal or a hexatic phase)2, having long range orientional order presumably with the headgroups positions fixed by the strong ionic interactions and alkyl chains partially molten. Another important observation is that the only compounds that undergo a phase transition to an isotropic liquid phase (denoted as Iso, in Table 1), are the longest spacer ones, namely 12-8-12, 12-10-12 an 12-12-12, whereas all the remaining compounds decompose before they melt to the liquid. For s = 2, 3, 4 and 12, one or more peaks appear at lower temperature than the main (strongest) peak. The 12-4-12 case is particularly significant since a complex structure with three coalesced peaks is observed, implying successive phase transitions in the crystalline region within a narrow temperature range (θ = 47-67 ºC), i.e. complex solid polymorphism. Compounds 12-3-12 and 12-6-12 also show resolved peaks. The enthalpy of the Cr → M1 transition (including all solid-solid transitions), which dominates over other enthalpy values, is plotted as function of s in Figure 3 for comparisons. There is first a decreasing trend in enthalpy (and apparently linear) as s increases from 2 to 8, followed by increasing enthalpy from s = 8 to 12. This trend shift can be understood considering that from s = 2 to 8 there is a decrease in the ionic crystal character of the compound, which is offsetting the increasingly stronger dispersion forces. That the shorter spacer compounds can attain high temperatures without melting to the liquid phase suggests this ionic character, favored by the closer proximity of the charged groups. For the longer spacers, the trend could be explained by increasing dispersion forces related to both higher

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number of CH2 groups and a more efficient chain packing resulting from the increasing spacer flexibility.

Figure 2. DSC thermograms for the 12-s-12 gemini surfactants.

!"# In Figure 3, the overall solid-to-liquid transition enthalpy, ∆!"# !" 𝐻! , and entropy, ∆!" 𝑆! , is

plotted as function of s for the gemini with spacer 8-12, the only compounds that melt to the liquid phase before decomposing, yielding an apparent linear trend. This linearity points to a constant contribution to melting per CH2 in the spacer. The slopes of the linear fits give a

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Table 1. Thermodynamic data* for phases transition for 12-s-12 gemini surfactants (phase notation: Cr, crystalline phases; M1-M3, ordered smectic mesophases; SmC and SmA, smectic C and C liquid crystalline phases; I, isotropic liquid phase) Compound

peak no.

Phase transition

θ / ºC

∆trsHm/

∆trsSm /

kJ·mol-1

J·K1·mol-1

1

Cr-Cr(1)

58.2

4.2

12.7

2

Cr-M1

97.3

43.3

116.9

3

M1-M2

105.2

12.0

31.7

4

M2-SmC; SmC-SmA

165.5;166-167

23.8

54.3

12-3-12

1-2

Cr-Cr(1); Cr-M1

75.0; 77.6

43.71

-

12-4-12

1-3

Cr-Cr(1)

48.8; 54.3; 56.7

18.9(2)

-

4

Cr-Cr

(1)

73.3

0.3

0.8

5

Cr-M1

83.9

15.7

44.0

6

M1-M2

116.6

0.2

0.5

12-5-12

1

Cr-M1

99.9

58.34

156.4

12-6-12

1

Cr-M1

91.1

31.9

87.6

12-2-12

12-8-12

12-10-12 12-12-12

(1)

(2)

(2)

2-3

M1-M1’, M1’-M2

108.8; 113.0

17.5

-

4

M2-M3

163.1

9.4

21.5

1

Cr-M1

75.1

26.5

76.1

2

M1-M2

146.5

3.7

8.8

3

M2-SmA; SmA-Iso

189-190

0.1(2)

ca. 0.2

1

Cr-M1

87.7

57.4

157.2

2

M1-SmA; SmA-Iso

144-145

15.6(2)

ca. 37

1

Cr-Cr(1)

40.7

10.6

33.8

2

Cr-M2

83.0

108.1

303.5

3

SmC-SmA

114.4

7.6

19.6

4

SmA-Iso

134.3

16.9

41.4

Transition between different crystalline phases. Total value for the complex peak.

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∆trsHm / kJ.mol-1

150

M1

∆crHm ∆Hmain

400

I

∆Htotal ∆crHm

100

∆Smain ∆crSm

300 200

50

100 0

0

2

4

6

8

10

12

∆trsSm / J.K-1.mol-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14

spacer s

Figure 3. Molar enthalpy for Cr→M1 phase transition (¡ ), molar enthalpy (●) and molar entropy (s ) for Cr→I phase transition as function of s, number of CH2 groups in the covalent spacer of 12-s-12 gemini.

melting enthalpy and entropy per CH2 in the spacer of ca. 26 kJ.mol-1 and 70 J.K-1.mol-1, respectively. These values are several times higher than the ∆H ≈ 3 kJ.mol-1 and ∆S ≈ 10 J.K1.

mol-1 per CH2 reported for melting of aliphatic chains55-57, suggesting that the gemini

I spacer.plays a critical role in the cohesive forces of the lattice. Plotting ∆ cr H m vs. the total

number of CH2 units (i.e., spacer plus the two long long chains) yields also a roughly linear variation with 28 kJ.mol-1 and 78 J.K-1.mol-1 per CH2 in line with the values above. Taken together, these results suggest that for the long spacers (s ≥ 8) the effect of stronger dispersion forces associated with an increasing number of CH2 groups is also accompanied by a more efficient packing of the chains (tails and spacer). 3.2. Phase Behavior and Formation of Ionic Liquid Mesophases. The characterization of the thermal behavior was further complemented with microscopy observations of the textures under plane-polarized light, for the assignment of the different phases.2 Figure 4 summarizes the results obtained and should be followed alongside the description of the birefringent textures (Figures 5 and 6) and associated phases. M1, M2 and M3 indicate ordered smectic mesophases (soft crystals or hexatic phases)1,2; SmC and SmA 11



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indicate the fluid smectic liquid-crystalline phases smectic C and smectic A, respectively; Cr denotes solid crystalline phases and Iso the isotropic liquid phase. Starting at 20 ºC and up to the transition temperature to phase M1, all the 12-s-12 compounds exhibit crystalline solid phases (region Cr) and in some cases (12-2-12, 12-3-12 and 12-4-12) solid-solid transitions. All these phases appear as weakly birefringent crystallites under the microscope and no discernible changes are seen when solid-solid transitions are crossed.

Figure 4. Thermal phase behavior of 12-s-12 gemini compounds. Notations are: Cr, crystalline solid phase region; M1, mesophase 1; M2, mesophase 2; M3, mesophase 3; SmA, smectic A liquid crystalline phase; SmC, smectic C liquid crystalline phase; Iso, isotropic liquid phase.

When the temperature overcomes the main peak into M1, partial loss of birefringence occurs in general and this is particularly evident for the compounds with s = 2 - 6. The M1 phase can be characterized as a solid-like phase that upon applied shear on the slide-cover slip 12


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preparation develops incipient, low-birefringence textures, with either marble-like (Figure 5A) or granite-like (Figure 5B) appearance. When obtained under cooling from isotropic melt for 12-10-12, this phase appears with a more finely defined texture, in the form of big striated feather-like (Figure 5C) or striated fan-like (Figure 6F) domains. For s = 3 and 5, this phase has a wide thermal stability range and in the case of 12-5-12 is in fact the only mesophase forming before decomposition.

A

12-3-12, 100 ºC

B B"

12-8-12, 115 ºC

C C"

12-10-12, 130 ºC

D D"

12-12-12, 90 ºC

E

12-2-12, 160 ºC

F

12-12-12, 100 ºC

G

12-2-12, 175 ºC

H

12-4-12, 170 ºC

I

12-8-12, 185 ºC

Figure 5. Characteristic polarized light microscopic textures found for the gemini and their respective phase assignments: A) marble-like, M1 (12-3-12, 150 ºC, on heating); B) granitelike, M1 (12-8-12, 115 ºC, on heating); C) big striated feather-like domains, M1 (12-10-12, 130 ºC, on cooling); D), tile-like, M2 (12-12-12, 90 ºC, on cooling); E) Schlieren texture, SmC (160 ºC, 12-2-12 ºC, cooling); F) fine flakes, SmC (12-12-12, 100 ºC); G) focal conics or

13



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Maltese crosses, SmA (12-2-12, heating, 175 ºC); H) oily streaks, SmA (12-4-12, 170 ºC, on heating); H) lancets, SmA (12-8-12, 185 ºC, on cooling). Magnification: 400 x.

The next mesophase, M2, which occurs for s = 2, 4, 6, 8 and 12, is more birefringent and slightly less “hard” than the previous one. For s = 8 and s = 12 it appears as an incipient patchy pattern (Figure 5D) or a clearer cotton-like patchy texture (Figure 6C) on cooling from the isotropic liquid and fluid smectic phases. For 12-12-12, an important difference to other

12-8-12 on cooling A 180 ºC B

150 ºC

C

140 ºC

12-10-12 on cooling 145 ºC E D

135 ºC

F

120 ºC

Figure 6. Illustrative polarized light microscopic textures obtained upon slowly cooling from the isotropic melt (liquid phase) for 12-8-12 (A-C) and 12-10-12 (D-E): A), fine lancets of SmA phase at 180 ºC; B) coexistence of SmA lancets (left of dashed line) and M2 patterned “cotton-like” domains (right) at 150 ºC; C) fully developed M2 “cotton-like” patchy texture at 140 ºC; D) SmA thin flakes inside liquid droplets, at 145 ºC; E) coexistence of lancets and M1 fan-like domains at 135 ºC; F) fully developed M1 fan texture at 120 ºC that becomes striated upon further cooling to room temperature.

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compounds is that M2 is the first mesophase to form above the Cr, and no M1 is present; this assignment was made on the basis of the strong DSC peak (≈ 300 kJ.mol-1) and the stripped cotton-like texture presented. Above M2, the type of mesophases that is produced depends on the spacer length. For the shortest and longest spacers, 12-2-12 and 12-12-12, there is a similar pattern: a SmC liquid crystal first form seemingly with Schlieren texture (Figure 6E),2,45 followed by a SmA liquid crystal appearing either as oily streaks or fan-shaped domains with focal conics; 12-12-12 then melts to the Iso phase. For 12-4-12, SmC is absent and there is a direct M2 → SmA transition close to 200 ºC (no DSC peak was detected, though). For 12-6-12, there is a M2 → M3 transition, where the latter shows as an incipient ill-defined texture more elastic than viscous in character, suggesting that this phase is still fairly ordered. For 12-8-12, there is an M2 → SmA → Iso sequence, while 12-10-12 shows an M1 → SmA → Iso sequence. The textures observed for the SmA phase vary depending on the spacer: for spacers 2, 3 and 4, focal conics (Figure 5G) and oily streaks (Figure 5H) are typically observed 12-3-12; for spacers 8, 10 and 12 it is more common to observe lancets on cooling (Figure 5E) or fine flakes right before isotropization to the liquid (Figure 6D). It is worthwhile to mention that the most discernible smectic textures were observed for s = 8, 10, and 12 developing on cooling from the liquid phase after several heating-cooling cycles. Annealing could also eventually lead to the development of the textures assigned to the ordered mesophases (M1 and M2). This is illustrated in Figure 6 with some specific runs. On cooling slowly (0.1 ºC/min) from the melt, 12-8-12 formed SmA lancets down to 180 ºC (Figure 6A), followed by a transient coexistence of lancets (to the left) and an incipient cottonlike patchy texture attributed to M1 (to the right); at 140 ºC, the fully developed M1 texture was obtained, remaining undercooled until room temperature. For 12-10-12, on cooling, SmA thin flakes inside pseudoisotropic droplets at 145 ºC (Figure 6D) evolved slowly to a transient 15



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mixture M1 big incipient fan-like domains and remaining lancets at 135 ºC (Figure 6E), and eventually to a more well-defined texture of fan or “palm-leaf” domains attributed to the M2 phase at 120 ºC (Figure 6F). Small visual changes marked, sometimes, a phase transition to the solid phase region such as the appearance of striated domains or cracks. 3.3 X-ray Diffraction Study of Solid State. To gain further insight on the differences in behavior between these compounds, structural information for the solid state at room temperature of all the gemini surfactants was obtained from powder X-ray patterns. Figure 7 shows illustrative patterns for all compounds except 123-12 and 12-5-12 (for concision). The patterns are rather complex, but Bragg reflection peaks were observed corresponding to the (00l) reflections, and hence to a smectic layering of the molecules as indicated in Figure 7. From these Bragg peaks, we could determine the interplanar distances, d00l and plotted them vs. the spacer length as shown in Figure 8. We note here that the 12-2-12 pattern is similar to that previous published by Berthier et al.58 Clearly, there is a non-monotonic and quite remarkable variation of d-spacing with the spacer length, which is qualitatively in line with our observations so far. The d-spacing displays a sharp increase going from s = 2 to s = 3 and then it increases only marginally in the region s =3-6. Subsequently, it drops for s = 8 and 12, while s = 10 unexpectedly remains in line with the 3-6 trend. If we take into consideration (i) the length of a 12-carbon alkyl chain, 1.67 nm, (ii) the ionic radii of a bromide ion, 0.11 nm, and (iii) an arrangement of bilayers (tail to tail positioning) with tails perpendicular to the headgroup plane and without interdigitation, then a d-spacing of ca. 3.5 nm ought to be obtained.46 This is a much higher value than any of those shown in Figure 8 and hence such a structure is ruled out. In fact, several authors have proposed, on the basis of XRD data, that n-2-n bis-quat gemini in the crystalline lattice have their alkyl tails in a trans conformation with respect to the spacer plane.46,58,59 Jurasin et al. have also proposed that the tails interdigitate and pack with a tilt angle with respect to the 16


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Figure 7. Illustrative X-ray diffraction patterns of gemini surfactants, 12-2-12, 12-6-12 and 1212-12, showing some of the (00l) peaks from which interplanar distances were calculated

Figure 8. Smectic (interlayer) repeat distance, d00l, as a function of the number of carbons, s, in the spacer alkyl chain.

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bromide ion planes.46 Figure 8 implies that there are significant differences in packing according to spacer, because no regularity in the smectic periodicities is observed. This point will be further addressed in the following section. 3.4 Molecular Structure and Molecular Packing. We have so far shown that 12-s-12 molecules exhibit smectic liquid crystalline structures. These structures are among the most frequently found TLCs for amphiphiles.2,27,60 Figure 9 depicts in a schematic way quasi-2D smectic arrangements that we posit as possible for the gemini molecules (for simplicity the bromide counterions were omitted). In general, smectic LC structures consist of bilayers either parallel, like those in A1, or tilted with an angle (α) with respect to layer normal (n), like in A2, depending on the possible mismatches between the cross section areas of the headgroups and the alkyl tails. In the case of highly interdigitated tail arrangements, A3 and A4, the layers are best described as monolayers. Naturally, intermediate arrangements between A1-A2 and between A3-A4 may exist. Regarding fluid smectic phases, the tails may also be parallel (SmA) or tilted (SmC) with respect to the normal. If the headgroups arrange into some positional ordering within the bilayer plane, in ordered smectic phases (hexatic phases), the structures are referred to SmB if the chains are perpendicular to the headgroup plane, SmI if the chains are tilted to the apex of the in-plane hexagon, and SmF if the chains have a tilt to their side.2 For the gemini amphiphiles, as already pointed out, the results for the d-spacings of (Figure 8) indicate that none of the smectic arrangements from s = 2 to s = 12 consists of bilayers of cis conformers of the type in A1, which would require d001 ~ 3.5 nm. For spacer 2, we found d001 = 1.54 nm, in agreement with that reported by Jurasin et al.46 On the basis of XRD data and geometric considerations, these authors proposed a B4 arrangement for 12-2-12, consisting of tilted and interdigitated chains in trans conformation; this seems plausible and would require α ∼ 19º. Analyzing now spacers 3-6 and spacer 10, they have similar d values in 18


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the range 2.4 - 2.8 nm and d increases slowly with increasing s; this suggests a common type of packing. Arrangements A3-A4 and B1 are ruled out because the respective d value is not compatible, while A2 implies d values independent of s. We therefore propose that arrangement B2 is the most likely, because it is both consistent with the d values and the respective dependence on s. This arrangement implies, however, tilt angles of about 60º. Large values of tilt angles (> 50º) in smectic LC layers, though not very common, have been reported before.61-65 Concerning spacers 8 and 12, and taking into account their similar d values and similar thermal behavior to 12-2-12, an arrangement of trans tails with interdigitation as in type B4 is plausible. Because d remains essentially constant, there should be a decreasing tilt angle α with increasing s (from 20º for s = 8 to 13º for s = 12). The fact that 12-10-12 is out of line with the packing trends of 12-8-12 and 12-12-12 stands out, but with the available data we cannot easily interpret it.

Figure 9. Possible packing configurations for the smectic layers in the 12-s-12 gemini, based on a cis (A) or trans (B) tail conformations. For A arrangements: A1, upright tails with no interdigitation (tail-to-tail); A2; tilted tails with no interdigitation; A3, upright tails with full 19



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interdigitation (tail-to-head); A4, tilted tails with full interdigitation. For B arrangements: B1, upright tails with no interdigitation (tail-to-tail); B2, tilted tails with no interdigitation; B3 upright tails with interdigitation (tail to head); B4, tilted tails with interdigitation (tail-to-head). For simplicity, bromide counterions are omitted.

For s = 8, 10 and 12, it is interesting to note that the isotropization temperature, corresponding to the SmA → Iso transition (Table 1), decrease with increasing spacer length, in the sequence 190, 145 and 134 ºC. This means that the liquid phase is thermodynamically stable at gradually lower temperature as s increases. Yet, the overall Cr → Iso enthalpy and entropy increase sharply with s as patent in Figure 3. This apparent paradox may be explained in thermodynamic and molecular packing grounds. If the Cr → Iso process were a single step (i.e. a direct solid-liquid transition), then the melting temperature Tf defined as !"# 𝑇! = ∆!"# !" 𝐻 ∆!" 𝑆 would yield values of 83, 103 and 86 ºC for s = 8, 10 and 12, respectively.

This sequence is in qualitative agreement with the transition temperatures for the strongest enthalpy peak for these compounds — 75, 88 and 83 ºC. These observations suggest that (i) the Cr → M1 (Cr → M2 for 12-12-12) transition is associated with chain melting; (ii) this transition is the main structural disorder event in the full solid-liquid transition (iii) more flexible spacers tends to promote tighter molecular packing in the solid state and hence higher chain melting temperature; (iv) the remaining loss of positional order associated with liquid formation requires comparatively lower energy. Further, the maximum in temperature for 1210-12 seems to be qualitatively in line with the outlying d-spacing and solid-state packing for this compound. If one takes the ratio ∆H/∆S for the full M1/Iso process (M2/Iso for 12-12-12) the sequence 149, 145 and 129 ºC is obtained, in qualitative agreement with the decrease in isotropization temperature. This suggests that after the alkyl chains become fluid, the transition to the liquid phase is more dominated by entropic than enthalpic effects. In fact, longer and 20


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more flexible alkyl spacers possess higher conformational entropy and as a consequence the liquid phase of the compounds is thermodynamically stabilized at lower temperature. Finally, we note that for 12-10-12 and especially for 12-12-12, the formation of fluid smectic liquid crystals at relatively low temperatures (considering that the compounds are paraffinic salts) is particularly interesting.7,13 These phases possess the common features of nonionic liquid crystals (e.g. structural and optical anisotropy) but contain mobile anions and cations like traditional ionic liquids, and could for instance be explored for the development of TLC-based sensors.7,13 One of the significant aspects of our study is that the simple control of the covalent spacer length in gemini mesogens seems to provide a way of tailoring thermotropic ionic liquid crystals. Investigations of gemini compounds with even longer spacers beyond 12 methylene units and the further unveiling of molecular packing and mesogenic effects (and possibly new shifts in trends) would be relevant and interesting.

4. Conclusions All of the 12-s-12 gemini compounds herein investigated show a complex melting process forming a number of thermotropic mesophases and undergoing decomposition at ca. 200 ºC. Significantly, the incremental increase of the number of methylene units in the spacer of causes marked non-monotonic trends on the thermal phase behavior, mesophase formation and solidstate packing. The molecules can be broadly subdivided in three groups according to the thermotropic behavior displayed: (i) molecules with short spacer, s = 2-4, form fluid smectic phases at high temperature (> 160 ºC) but decompose before isotropization; (ii) molecules with intermediate spacers, s = 5-6, form only ordered smectic phases and also decompose before the liquid phase; (iii) in contrast, molecules with long spacer ones with s = 8-12 form fluid smectic phases and fully melt to the isotropic liquid phase. For the latter, the isotropization temperature decreases as the spacer length increases, implying that the liquid phase is stabilized. XRD data 21



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for the solid phases indicate arrangements of smectic layers for all of the compounds but also reveal important differences in packing. The Bragg peaks for s = 2 yielding d00l ≈ 1.5 nm and for the long spacers 8 and 12 yielding d00l ≈ 2.0 nm are consistent with trans conformers packing in tilted monolayers with deep interdigitation. In contrast, the peaks for spacers 3-6 giving d00l ~2.4-2.6 nm and for spacer 10 point to trans conformers packed in highly tilted bilayers with no interdigitation. We have thus that the melting process of gemini compounds can be controlled through the number of methylene units, with longer spacers (s ≥ 8) yielding fluid smectic liquid crystals and liquid phases well below decomposition temperature.

Acknowledgements. CIQUP acknowledges financial support from FEDER/COMPETE and FCT through grants UID/QUI/00081/2013, POCI-01-0145-FEDER-006980 and NORTE-010145-FEDER-000028. CQC and FCT are also acknowledged for financial support through the Post-Doctoral grant SFRH/BPD/71683/2010, Project PEst-OE/QUI/UI0313/2014 and POCI01-0145-FEDER-007630. CQVR acknowledges financial support from FEDER/COMPETE and FCT through project UID/QUI/00616/2013. Supporting Information: Critical micelle concentration data of the gemini surfactants as obtained from conductimetry (Figure S1, Table S1); thermogravimetric analysis data (Figure S2).

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448x251mm (72 x 72 DPI)


Critical Role of the Spacer Length of Gemini ... - ACS Publications

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