Article pubs.acs.org/Langmuir
Structural Transitions of CTAB Micelles in a Protic Ionic Liquid Carlos R. López-Barrón† and Norman J. Wagner*,‡ †
ExxonMobil Chemical Company, Baytown, Texas, 77520, United States Center for Neutron Science, Chemical Engineering, University of Delaware, Newark, Delaware 19716, United States
‡
S Supporting Information *
ABSTRACT: Micellar solutions of hexadecyltrimethylammonium bromide (CTAB) in a protic ionic liquid, ethylammonium nitrate (EAN), are studied by shear rheology, polarizing optical microscopy (POM), conductivity measurements, and small angle neutron scattering (SANS). Three concentration regimes are examined: A dilute regime (with concentrations [CTAB] < 5 wt %) consisting of noninteracting spherical micelles, a semidilute regime (5 wt % ≤ [CTAB] ≤ 45 wt %) where micelles interact via electrostatic repulsions, and a concentrated regime (45 wt % < [CTAB] ≤ 62 wt %) where a reversible, temperature-dependent isotropic (L1) to hexatic (Hex) phase transition is observed. The L1− Hex transition, which has been predicted but not previously observed, is characterized by (1) a sharp increase in the shear viscosity, (2) the formation of focal conical birefringence textures (observed by POM), and (3) enhancement of the crystalline order, evidenced by the appearance of Bragg reflections in the SANS profiles. Ionic conductivity is not sensitive to the L1−Hex transition, which corroborates the absence of topological transitions.
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by Greaves and Drummond.11 Of particular interest here is the self-assembly of hexadecyltrimethylammonium bromide (CTAB) in EAN. From surface tension measurements, Evans and co-workers12 determined the critical micelle concentration (CMC) of CTAB in EAN at 50 °C (CMCCTAB/EAN = 1.8 × 10−2 mol/L) and the Krafft point (TK = 48 °C). The CMC is 1 order of magnitude larger than that observed in water (CMCCTAB/H2O = 9.64 × 10−4 mol/L22). From the change of CMC with (alkyltrimethylammonium bromide) surfactant chain length, they calculated the free energy of transfer of a methylene group from EAN to the micelle interior (−370 cal/ mol), which is about half the free energy for a similar transfer from water to the micelle (−680 cal/mol).12 This indicates that the solvophobic effect is weaker in EAN than in water, and therefore, self-assembly and phase transitions are expected to occur at higher surfactant concentrations in EAN than in water. These studies have recently been extended to ionic surfactants at the interface of ionic liquids.23 The formation of different lyotropic liquid crystals (LLC) for both ionic and nonionic surfactants in several protic ionic liquids has been reported in the literature.13,15,17,18,24−28 Evans and co-workers reported formation of a lamellar phase by lipids in EAN.24 Araos and Warr studied the micelle and LLC formation of nonionic polyoxoethylene alkyl ether surfactants in EAN.25,26 Greaves and co-workers reported the formation of
INTRODUCTION Ionic liquids (IL) are organic salts with low melting point (mp < 100 °C) composed entirely of poorly coordinated ions. Therefore, they have the potential to be highly polar yet noncoordinating solvents.1 ILs with mp below room temperature are referred as room temperature ionic liquids (RTIL). The remarkable set of physical properties of ILs (e.g., negligible vapor pressure, high chemical and thermal stability, good solvation properties, high ionic conductivity with wide electrochemical windows2−4) makes these compounds very attractive for a number of applications.5−9 Generally, ILs can be classified as protic ionic liquids (PILs) and aprotic ionic liquids (AILs).4 PILs are formed by proton tranfer of a Brønsted acid to a Lewis base. In this work, we use the most well-known PIL, namely, ethylammonium nitrate (EAN, mp =14 °C), whose finding dates back to 1914.10 The three protons on the cation and the three oxygens on the anion provide the basis for an extensive hydrogen-bonded network in EAN.11 Consequently, many properties associated to the hydrogen bonding in water are observed in EAN as well (e.g., micelle and liquid crystals formation,11−18 negative enthalpies and entropies of gas dissolution, and formation of a three-dimensional hydrogen bonded network12,19). Similar to aqueous solutions, the self-assembly of amphiphilic molecules in EAN is driven by the solvophobic effect, that is, the (entropically favorable) tendency of solvophobic molecules to form aggregates of like molecules, due to the release of solvent structured around the solvophobic tails of the surfactant.11,20,21 A thorough review of the self-assembly of several amphiphiles in both protic and aprotic ILs was recently given © 2012 American Chemical Society
Received: June 1, 2012 Revised: August 6, 2012 Published: August 8, 2012 12722
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phase sequence: Isotropic (I) → SmA → SmB → crystal E. Other authors have predicted the sequence: I → nematic (N) → SmB via Monte Carlo or molecular dynamics simulations.56,57,61 Using a Landau (mean field) theory analysis, Das and Mukherjee predicted the direct I → SmB transition when the system is close to a I-N-SmB or I-SmA-SmB triple point, i.e., when temperatures of the I ↔ N(SmA) and N(SmA) and N(SmA) ↔ SmB phase transitions coincide.58 Near the triple point, the N and SmA phase regions shrink and finally disappear, leading to the I → SmB transition. Using Monte Carlos simulations, Matinez-Haya and Cuetos61 studied the istropic to crystal-like hexatic phase (I ↔ Hex) transition near the I-N-Hex and I-SmA-Hex triple points. The suppression of the N or SmA phases has also been observed by molecular dynamics simulation56 and Monte Carlo simulations57 at temperatures below the triple point or on compression. To our knowledge, no experimental evidence of the direct transitions I → SmB or I → Hex has been reported to date. In this paper we present POM, SANS and rheological measurements that support the first evidence of a direct isotropic to hexatic transition in a surfactant/ionic liquid system.
hexagonal, cubic, and lamellar lyotropic liquid crystalline phases of CTAB solutions in a number of protic ionic liquids.13 Formation of cubic, hexagonal, lamellar, and reverse bicontinuous cubic phase in mixtures of the amphiphilic block copolymer EO20PO70EO20 (P123) and EAN was reported by Zhang and co-workers.27 Zhao et al.15 mapped a series of LLC phases formed in 1-hexadecyl-3-methylimidazolium chloride (C16mimCl)/EAN solutions. Ma et al.28 observed an hexagonal phase in a oleyl polyoxyethylene (10) ether (Brij 97) in EAN. Recently, Wang et al. observed the formation of a reverse hexagonal phase of three gemini surfactants in EAN.18 We have also investigated the phase behavior of the double tail surfactant, didodecyldimethylammonium bromide (DDAB), in EAN.17 A sponge (L3) phase was observed in a broad window in the DDAB/EAN phase diagram, followed by a transition to a lamellar phase at higher concentrations. Using cross-polarized optical microscopy and penetration scans, Greaves and co-workers qualitatively identified hexagonal, cubic, and lamellar LLC phases in CTAB/EAN solutions at temperatures above 58 °C.13 However, the compositions at which phase transitions occur and structural details of the micellar aggregates cannot be determined by penetration scans and has not been studied to date. In this paper, we report a small angle neutron scattering (SANS) study of CTAB/EAN solutions in the concentration range: 1 wt % ≤ [CTAB] ≤ 62 wt %. Aggregation number, micelle size and shape, and a transition to interacting micelles are characterized by the analysis of the SANS measurements of dilute and semidilute solutions. At higher concentrations, a reversible, temperaturedependent isotropic to hexatic phase transition (Figure 1) is studied by SANS, shear rheology, dc-conductivity, and polarizing optical microscopy (POM).
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EXPERIMENTAL SECTION
Materials. Hexadecyltrimethylammonium bromide, CTAB, was purchased from Sigma-Aldrich (99%) and used as received. Ethylammonium nitrate (EAN), supplied by Iolitec, was dried by heating to 100 °C under nitrogen atmosphere for at least 24 h. The water content, after drying, was 0.13 wt % (measured by Karl Fischer titration). Deuterated EAN (dEAN) was prepared as previously reported (by three times mixing with equimolar quantities of D2O and redrying17). The neutron scattering length density of dEAN (ρdEAN = 3.09 × 10−6 Å−2 at 55° and 3.06−6 Å−2 at 80°) is calculated from the mass density (1.237 g/cm3 at 55 °C and 1.203 g/cm3 at 80 °C), measured with a density meter (DMA 5000M, Anton Paar), and considering that only 86% of the amino-hydrogens are substituted by deuterium.29 CTAB/EAN solutions with compositions ranging from 2 to 62 wt % were prepared by weighing the designed amounts of CTAB and EAN in stoppered glass vials, followed by many cycles of vortex mixing/centrifugation until clear homogeneous solutions were obtained. Solutions prepared with dEAN were used for SANS measurements. Determination of Phase Transitions. CTAB/EAN solutions were put in a thermostatted oil bath at 150 °C. The bath was cooled down to 35 °C, with steps of 5 °C, and equilibrated for 12 h at each temperature. Formation of LLC phases and solid crystals was determined by visual inspection through crossed polarizers. Accurate determination of the type of LLC phase and phase boundaries was carried out by means of polarizing optical microscopy (POM), small angle neutron scattering (SANS), oscillatory shear rheology, and conductivity measurements. POM was carried out using a polarizing inverted microscope (IX2 Olympus) with a homemade aluminum plate whose temperature is controlled by circulating hot water (from a circulating bath) through inner channels. Small Angle Neutron Scattering. SANS experiments were carried out on the NG-7 30 m beamline at the NIST Center for Neutron Science (Gaithersburg, MD). Three different instrument configurations (wavelength λ = 6 Å, wavelength spread Δλ/λ = 0.11, sample to detector distances of 1, 4, and 15.3 m with lenses) to cover a q-range of 1 × 10−3 to 0.5 Å−1. Standard 1 mm thick titanium cells with quartz windows were used to contain the samples. Azimuthally averaged raw scattering data were corrected for background radiation, sample transmission, sample thickness, and detector sensitivity using IGOR macros available from NIST30 to obtain one-dimensional plots of absolute intensity, I, versus scattering wave vector, q. The total background was calculated from the slope of the plot Iq4 versus q4,31,32 and subtracted from all SANS spectra.
Figure 1. Schematic representation of the hexatic (Hex) ↔ isotropic (L1) transition observed in CTAB/EAN solution.
The hexatic (hex) mesophase, first described by Halperin and Nelson,53 has hexagonal symmetry in its crystalline ground state. This phase is characterized by short-range positional order and quasi-long-range bond-orientational-order (BOO). A three-dimensional (3D) liquid crystal (LC) phase consisting of interacting two-dimensional stacked hexatic layers was proposed by Birgeneau and Litster.72 This phase, called hexatic B (HexB) or hexatic smectic B (SmB), has long-range 3D BOO, and similar to the Smectic A (SmA) mesophase, has a layered structure and molecular orientation perpendicular to the smectic plane. Pindak et al.50 reported the first experimental evidence of a SmB phase on n-hexyl-4′-n-pentyloxybiphenyl-4carboxylate (65OBC). They observed the thermodynamic 12723
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SANS Data Analysis. SANS spectra of micellar solutions are described by the function33 I(q) = nV 2Δρ2 P(q) S(q)
polydisperse distribution of spheres with uniform SLD (ρCTAB = −2.95 × 10−7 Å−2) with no micelle−micelle interactions (eq 2) are shown as solid lines in Figure SI1 (in the Supporting Information). The distribution of radii is a Schulz distribution.39 The model fits well the SANS data for compositions below 5 wt %. At [CTAB] = 5 wt %, the first sign of intermicellar correlation peak is observed and the model for noninteracting spheres can no longer fit the data. Hence, a transition from noninteracting to interacting micelles is identified at ∼5 wt %. Structural parameters of CTAB micelles (listed in Table 1) are obtained by fitting the micelle volume fraction, ϕ, the mean
(1)
where n is the micellar number density, V is the volume of the micelle, and Δρ is the difference in scattering length density (SLD) between scatterers (micelles) and the solvent. P(q) is the form factor, which describes the scattering from individual micelles, and S(q) is the structure factor, which contains information of intermicellar interactions. Two form factors are considered, namely, spherical and ellipsoid. The form factor for a monodisperse spherical particle of radius R, with uniform SLD is34 2 ⎡⎛ 3 ⎞⎛ sin(qR ) cos(qR ) ⎞⎤ ⎟⎥ P(q) = ⎢⎜ ⎟⎜ − ⎢⎣⎝ qR ⎠⎝ (qR )2 qR ⎠⎥⎦
Table 1. Structural Parameters of Spherical CTAB/dEAN Micelles in the Dilute Regime
(2)
and for an ellipsoid of revolution with semiaxes R, R, and ζR, with uniform SLD is35
P(q) =
∫0
π /2
F(q , r(R , ζ , α))2 sin α dα
1 1.5 2 3 4 5
(3)
where
r(R , ζ , α) = R(sin 2 α + ζ 2 cos2 α)1/2
ϕ
composition, CTAB wt %
(4)
1.91 3.92 8.40 16.7 21.4 29.2
× × × × × ×
−3
10 10−3 10−3 10−3 10−3 10−3
R, Å
PDI
Nagg
19.2 19.4 19.1 18.7 18.6 18.5
0.094 0.093 0.094 0.095 0.095 0.095
64 66 63 60 59 58
and F(q , R ) =
radius, R, and polydispersity, PDI. The aggregation number (also shown in Table 1) can be computed as
3[sin(qR ) − qR cos(qR )] 3
(qR )
(5)
Nagg = (4/3)πR3/v
To include interparticle interference effects due to screened Coulombic repulsions between charged micelles, the Hayter and Penfold screened Coulomb structure factor, SSC(q), is used.36 Algorithms to compute SSC(q) using the dielectric constant of the solvent (ε = 26.2 for EAN37) and the salt concentration are implemented in the NIST SANS analysis package and described in Hammouda’s SANS toolbox38 and in the original publication.36 Rheology. Rheological characterization was carried out using a strain-controlled rheometer (ARES-G2, TA Instruments) with cone and plate geometry. To ensure homogeneity, the samples were loaded at 120 °C and then quenched to the testing temperature. Steady shear measurements both ascending from 1 to 500 s−1 and descending from 500 to 1 s−1 were performed at temperatures ranging from 130 to 40 °C. Dynamic frequency sweeps both forward (from 1 to 300 rad/s) and backward (from 300 to 1 rad/s) were carried out at temperatures below and above the liquid−gel transition. The results shown in this paper correspond only to the forward tests, since negligible hysteresis was observed in both the oscillatory and the steady flow measurements. Ionic Conductivity Measurements. The conductivity of EAN and three CTAB/EAN solutions (with compositions: 50, 51, and 52.5 wt %) was measured using a conductivity meter (CM, Oakton CON11) at temperatures ranging from 95 to 40 °C. The CM was put into glass vials filled with the CTAB/EAN solutions and immersed in a thermostatted oil bath. The samples were equilibrated for 2 h at the each temperature before reading the conductivity values. Prior to the measurements, the CM was calibrated using KCl conductivity standards (Aldrich).
(6)
where v is the volume of a simple hydrocarbon tail, which for C16 is 460 Å3.40 The aggregation number is comparable to that reported for aqueous CTAB micelles (59−9041−44). From the difference between the total surfactant volume fraction and the micelle volume fraction (obtained by SANS) for the 1 wt % solution, we estimate that EAN contains 0.84 wt % of dissolved CTAB, which is in agreement with the CMC reported previously (2.23 × 10−3 mole fraction or ∼0.75 wt %12). Semidilute Regime. SANS spectra of representative CTAB/dEAN solutions with compositions ranges 17 wt % ≤ [CTAB] ≤ 45 wt % are shown in Figure 2. A well-defined peak is observed in all the spectra which is characteristic of repulsive interactions. Solid lines in Figure 2a are fits to the spherical form factor (eq 2) with a screened Coulomb structure factor.36 For compositions above 28 wt %, the spherical form factor does not fit the SANS data, due to an extra shoulder that appears at q > 0.2 Å (see Figure 2b). For these data, we attempted to use cylindrical and ellipsoid form factors for the fitting. The latter (eqs 3−5) resulted in better fittings; hence, it was used here and the results are presented as solid lines in Figure 2b. For this calculation, it was assumed that the minor axis of the ellipsoid was equal to the radius of the spherical micelle at 28 wt % (17.6 Å), leaving the major axis of the ellipsoid as an adjustable parameter. These fits also include the screened Coulomb structure factor.36 Table 2 shows the structural parameters obtained from the fittings in the semidilute composition range. These results indicate a shape transition from spherical to ellipsoidal micelles at [CTAB] ∼ 29 wt % ± 1 wt %. Notice that the radius ratio (ζ = major axis length/minor axis length) in this range is ≲2.2, before transitioning to an hexatic phase (see next section). This is in contrast with the behavior of CTAB in water, where the transition is to very long cylindrical micelles, with contour length of 223 nm.45 This difference is also evident in the rheological behavior and in visual observations under cross-
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RESULTS AND DISCUSSION Three CTAB concentration regimes were identified and characterized on the basis of SANS analysis, POM textures, rheology, and conductivity measurements. These regimes are dilute (spherical, noninteracting micelles), semidilute (spherical and ellipsoidal interacting micelles), and concentrated (highly interacting normal micellar phase and a hexatic phase). Dilute Regime. Figure SI1 (in the Supporting Information) shows SANS spectra of CTAB/dEAN solutions at 55 °C, with compositions ranging from 1 to 5 wt %. Best fits to a model for 12724
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Figure 2. SANS spectra for CTAB/EAN solutions at concentrations indicated, measured at 55 °C. Lines in (a) give best fits to the sphere form factor eq 2 and in (b) to the ellipsoid form factor (eqs 3−5), both with screened Coulomb interactions.
Table 2. Structural Parameters of CTAB/dEAN Micelles in the Semidilute Regime ϕ
composition,CTAB wt % 17 20 22 24 26 28 30 40 45
8.02 9.08 9.90 11.1 11.9 13.3 16.3 21.8 23.0
× × × × × × × × ×
10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2
R, Å
ζ
shape
Nagg
d, Å
18.3 17.6 17.9 17.9 17.3 17.6 17.6 17.6 17.6
1 1 1 1 1 1 1.63 2.19 2.09
sphere sphere sphere sphere sphere sphere ellipsoid ellipsoid ellipsoid
57 50 52 52 47 50 79 106 102
60.6 55.2 55.5 54.1 50.3 50.0 46.2 42.4 41.2
polarized light. The aqueous CTAB solutions show flowbirefringence and highly viscoelastic properties near the isotropic−nematic transition,45,46 whereas CTAB/EAN solutions are optically isotropic with Newtonian fluid behavior in the vicinity of the isotropic−hexatic transition. The aggregation number for the ellipsoidal micelles is calculated with eq 6 and considering the equivalent spherical radius, Rsph = (ζR)1/3, where R is minor axis length of the ellipsoid of revolution. The intermicellar distance, d (calculated from d = 2π/q*, where q* is the peak position), decreases at higher concentrations (see inset in Figure SI2 in the Supporting Information), which strengthens the intermicelle repulsive interactions. At [CTAB] = 28 wt %, the micelles are very close to each other (d ∼ 2.8R) and strongly interacting. These interactions can be reduced by growing the micellar size along one axis, resulting in a sphere to ellipse, or rodlike, transition. The shape transition goes hand in hand with an increase in the aggregation number (Table 2). Increasing the concentration further does not result in further elongation (as is the case of aqueous CTAB micelles47−49); rather, a direct transition to a hexatic phase occurs at [CTAB] ≥ 50%, as described below. Concentrated Regime. At concentrations above 49 wt % (at 55 °C), a second peak at the position √3q* (where q* is the position of the major peak) starts to appear in the scattering profile (see arrows in Figure SI2 in the Supporting Information), which suggests an enhancement in the microstructural order. A weak third peak at √4q* is observed in the 52 wt % solution, which is not very well-defined due to the incoherent scattering from the solvent. The position of the peaks, relative to q*, indicates that the ellipsoidal micelles form an hexagonally ordered mesostructure, which, together with the POM results, leads us to conclude that this is a hexatic (Hex) phase, as explained below. The transition from isotropic (normal micellar) phase, L1 to Hex phase occurs at higher compositions when the temperature is increased. As shown in
the right-hand side of Figure SI2 (in the Supporting Information), at 80 °C, the second peak is first observed in the 52 wt % solution. The intensity upturn with slope of −4 at low q-values, observed at [CTAB] ≥ 49 wt %, for T = 55 °C, and [CTAB] ≥ 55 wt %, for T = 80 °C, may be due to grain boundaries between ordered regions. The inset in Figure SI2 (in the Supporting Information) shows the intermicellar distance as a function of volume fraction, ϕ. This plot includes all the samples studied in the semidilute and concentrated regimes. As expected for the disordered (liquid) state, the scaling relation d ∝ ϕ−1/3 is obeyed. A different swelling law is observed in the region of the hexatic phase, namely, d ∝ ϕ−2/3, which indicates that swelling occurs in two dimensions, perpendicular to the ellipsoid major axis. Optical Changes during the L1 → Hex Transition. The isotropic to hexatic transition is also marked by changes in the optical, viscoelastic, and conductivity properties of the solutions. Measurements of these properties were used to identify the transition line in the phase diagram shown in Figure 3, as described below. Figure 4 shows POM micrographs and photographs of the solutions (as seen through crosspolarized backlight) containing a 52 wt % CTAB/EAN solution at different temperatures. At temperatures above the L1 → Hex transition, the samples are optically isotropic (Figure 4a) and they display high fluidity. At temperatures in the Hex region, the solution displays intense birefringence and the fluidity drops dramatically; that is, the system becomes a self-sustaining gel (Figure 4c, right). Figure 4b shows a snapshot of the first formation of the birefringence textures during cooling from the L1 region toward the Hex region in the phase diagram. The conical fanlike birefringence (Figure 4c, left) and the hexagonal order deduced from SANS indicate that the ellipsoidal micelles arrange to form a hexatic phase.50,51 12725
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Figure 3. Phase behavior of CTAB in EAN. Square symbols with error bars indicate the temperature ranges where two phases are observed under cross-polarized light and POM. Circles indicate the transition temperature measured with dynamic rheology (arrows in Figure SI3 in the Supporting Information). Diamonds indicate the temperatures where the Krafft transition is observed. The inset shows the transition temperature as a function of CTAB mole fraction for CTAB/EAN and CTAB/dEAN solutions, as determined by rheology. The concentration scales in wt % for both systems are included to facilitate comparison with other plots in the paper.
Note that, due to the formation of very long flexible rods (i.e., wormlike micelles, WLM), in analogous aqueous salt-free CTAB solutions,45,46 an intermediate nematic (N) phase is observed in CTAB/water solutions, followed by transiton to a normal hexagonal (H1) liquid crystalline phase, consisting of very long rods or threads of CTAB packed in a hexagonal array,48,52 at higher concentrations. The hexagonal lattice in H1 gives this phase long-range orientational order. We rule out the possibility of the H1 phase in the concentrated CTAB/EAN solutions due to the following reasons: (1) CTAB/EAN solutions form ellipsoids, rather than long rods, before the phase transition, (2) the N phase is not observed as a intermediate phase, and (3) no long-range BOO was observed in the CTAB/EAN liquid crystal phase (evidenced by the weak reflection in the SANS patterns), which is consistent with the fact that the hexatic phase has quasi-long BOO.53 On the other hand, the SmB phase has also hexagonal order; however, an additional diffraction peak, corresponding to the intersmectic layer distance, is absent in the SANS profiles (Figure SI2). Therefore, the SmB phase is ruled out as well, and the hexatic phase is conclusively identified as the liquid crystal mesophase. By means of temperature ramps, the L1 → Hex transition was characterized as the temperature where the birefringence texture first appears (in a cooling ramp, see Figure 4). These values are indicated as empty squares in Figure 3. The L1 → Hex transition is schematically depicted in Figure 1. At temperatures below 50 °C, the fanlike birefringence texture disappears, and is replaced by needlelike crystals, as observed in the POM micrographs (Figure 4d, left). This transition, marked with empty diamonds in Figure 3, is the Krafft temperature, TK, below which solid surfactant crystals are formed and phase
Figure 4. Change in birefringence during phase transitions for the 52 wt % CTAB/EAN solution observed by POM (left) and in vials through cross-polarized light (right). (a) At 80 °C, optically isotropic samples are observed; (b) at 70 °C, birefringence textures start to appear; (c) at 55 °C, well-defined conical fan-like birefringence textures are observed; and (d) at 40 °C, birefringence textures disappear and are replaced by needlelike solid crystals. 12726
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Figure 5. Rheological signals of the L1 → Hex transition in a 52 wt % CTAB/EAN solution. (a) Dynamic moduli as a function of frequency measured at 55 and 80 °C with a strain amplitude of 10%. (b) Steady state shear viscosity as a function of shear rate at temperatures spanning the L1 → Hex transition.
L1−N−SmB triple point.56 The high concentrations at which the phase transition occurs in our system ([CTAB] > 50 wt %, see Figure 3) could lead to a high osmotic pressure in the EAN phase, which could be the cause of the suppression of a intermediate (N or SmA) phase. Here we report three experimental observations that support the direct L1 → Hex transition. First, the POM micrographs show a direct formation of fanlike birefringence texture with focal conical defects including out-of-phase correlations (highcontrast bright-dark faces in Figure 4c). The typical textures of the N phase (i.e., marbled textures or oily streaks) and/or the SmA phase (i.e., focal conical textures with smooth parabolic defects)51 are not observed as intermediate states. Second, neither N nor SmA have hexagonal order in any plane. Therefore, the fact that the second peak at √3q* appears right below the transition temperature (see Figure SI2 in the Supporting Information), suggests direct formation of the hexatic phase. Third, both N and SmA phases behave as viscoelastic fluids,62 whereas the hexatic phase shows an elastic response, which is due to the internal friction of the long-range 3D BOO.59,60 Cagnon and Durand, for instance, measured the stress response to dynamic shear parallel to the layers in the SmA and SmB phases of butyloxybenzylidene-octylamine.59 They found that the SmA phase behaves as a viscous fluid with high friction between the layers, but with finite relaxation time, whereas the rheological behavior of the SmB phase is that of a 3D crystal, with plastic deformation at low frequencies (i.e., the material has a yield stress). This is consistent with the sharp increase in viscosity (see Figure SI3 in the Supporting Information) and the sudden transition to a self-sustaining gel (Figure 4, right) observed in the CTAB/EAN solutions. Ionic Conductivity of Concentrated CTAB/EAN Solutions. Ion conductivity, σ, is sensitive to topological changes in ionic liquid crystals.63−65 Moreover, conductivity measurement is a common method to determine Krafft temperature of solution of ionic surfactants.66 We measured the conductivity of the pure EAN and three CTAB/EAN solutions with concentrations and temperatures covering the three microstructural regimes shown in the phase diagram (i.e., L1, Hex, and solid crystals). The results are given in Figure 6a. The conductivity of pure EAN is 1 order of magnitude larger than that of the solutions in the L1 and Hex regimes, which indicates that micelles have an electrical insulation effect. Below the Krafft temperature, σ shows a sharp decrease due to phase
separation occurs. The Krafft temperature reported here is in agreement with previous reports at lower compositions.12 Rheological Signals of the L1 → Hex Transition. The linear viscoelasticity of the solutions was characterized by means of dynamic frequency sweeps. Figure 5a shows the dynamic moduli as a function of frequency for the 52 wt % CTAB/EAN solution in the L1 (80 °C) and Hex (55 °C) regions of the phase diagram. In the isotropic region, the moduli show the typical frequency dependence of a viscoelastic liquid in the terminal regime, namely, G′ ∝ ω2 and G″ ∝ ω, that is, the system behaves as a Newtonian liquid.54 In the Hex region, the frequency dependence corresponds to that of a physical (soft) gel, namely, G′ ∼ G″ ∝ ωb (with b = 0.65 for this sample).55 The L1 → Hex transition is also observed in steady flow measurements. Figure 5b shows that the system behaves as a Newtonian liquid (i.e., the viscosity is shear rateindependent) at T ≤ 70 °C and as a shear thinning fluid at T ≥ 70 °C. In order to accurately measure the L1 → Hex transition, steady flow measurements at temperatures ranging from 40 to 120 °C were performed using a shear rate of 1 s−1. The viscosity as a function of temperature for pure EAN and solutions with different concentrations are shown in Figure SI3 (in the Supporting Information). At concentrations below 50 wt %, the viscosity increases monotonically upon cooling, whereas at [CTAB] > 50 wt % a sharp viscosity increase is observed at temperatures marked by the arrows. These temperatures are indicated as empty circles in the phase diagram (Figure 3) and are in good agreement with the L1 → Hex transition observed via POM (empty squares in Figure 3). The inset in Figure 3 shows the L1 → Hex transition temperatures, as measured by rheology, for CTAB solution both in EAN and dEAN. A weak isotope effect is observed in the L1 → Hex transition by substituting EAN for dEAN. Discussion of the L1 → Hex Transition. The L1 → Hex transition is not a common thermodynamic phase sequence in liquid crystals. Either the N phase, the SmA phase, or a sequence of both phases is normally observed upon cooling from the isotropic state and before reaching the Hex or SmB phases.53,56−61 However, as discussed above, both the L1 → Hex and L1 → SmB transitions have been theoretically predicted to occur near L1−N (or SmA)−Hex (or SmB) triple points. Miguel et al. showed that the N phase is not stable on compression, in which case the SmB is directly formed near the 12727
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Bη and Bσ are related to the entropic barrier of molecular motion68 and ionic conduction,69 respectively. Hence, Bη and Bσ can be used to obtain pseudoactivation energies for viscosity, E aσ = −RB σ, and for ionic conductivity, E aη = RB η , respectively.17 Solid lines in Figure SI3 (in the Supporting Information) and Figure 6a are fits to eq 8 for pure EAN and CTAB/EAN solutions. Table 3 shows the VTF parameters obtained from Table 3. VTF Parameters of Viscosity and Conductivity for EAN and CTAB/EAN Solutions sample EAN 15 wt % CTAB 40 wt % CTAB 50 wt % CTAB 51 wt % CTAB 51.5 wt % CTAB 52 wt % CTAB 53 wt % CTAB 54 wt % CTAB 55 wt % CTAB
−4
9.8 4.3 2.4 2.4 2.2
× × × × ×
10 10−4 10−4 10−4 10−4
1.4 1.4 1.1 1.2
× × × ×
10−4 10−4 10−4 10−4
Eaη, kJ/mol 3.34 5.42 8.82 9.71 9.86
σ0, μS/cm 1.4 × 10
−5
5.6 × 10−4 5.2 × 10−4 4.8 × 10−4
Eaσ, kJ/mol 1.54
3.00 2.81 2.60
10.7 10.7 11.3 11.2
the fittings along with the calculated activation energies. The fitted Vogel temperature for EAN is very close to its glass transition temperature (Tg,EAN = −95 °C70). Since there is no available information of the effect of CTAB on the glass transition of EAN, we set T0 = T0,EAN for all the fittings. Eaη is found to be at least two times larger than Eaσ. This behavior is due to the high ionicity of EAN, and has been observed in other molten salts.37,71 As expected, the activation energy for viscosity increases with the surfactant concentration, due to the restriction to flow imposed by interacting micelles. Interestingly, two regimes of linear growth are observed (see Figure 7): At concentrations
Figure 6. (a) Conductivity and (b) obstruction factor as a function of temperature and concentration for pure EAN and CTAB/EAN solutions (symbols correspond to the same compositions in both plots). Solid lines are fits to the VTF model (eq 8).
separation.66 The low conductivity of the solutions, compared to pure EAN, is due to two main effects, namely, the trivial decrease in ion concentration and the tortuosity effect, which is due to the presence of nonconductive micellar aggregates. The latter effect can be quantified via the obstruction factor, which is defined as63 σ ϑ≡ (1 − ϕ)σEAN (7) where the term (1 − ϕ) accounts for the fact that the effective volume for ion transport is the solvent volume. Figure 6b shows that ϑ increases monotonically with temperature at T > TK. Interestingly, the change in ϑ with temperature at the L1 → Hex transition is gradual, rather than abrupt (as is the case for the elastic modulus and the birefringence). This could indicate that, although the nanostructural order is enhanced upon cooling, the topology remains unchanged. That is to say, no aggregation or breakage of micelles occurs upon the phase transition. In this scenario, the conductivity decrease, with temperature, could be simply due to a decrease in ion mobility and/or to solvation of the micelle surface,67 that is, binding of EAN ions to the palisade layer. Viscosity and Conductivity Activation Energies. It has been shown before that the temperature dependence of the viscosity and conductivity of EAN do not follow a typical Arrhenius behavior.17 Rather, these dependences can be described by the Vogel−Fulcher−Tammann (VTF) equation68
Figure 7. Activation energy of viscosity for CTAB/EAN solutions as a function of concentration. Solid lines are linear fits with the indicated slopes.
close to the L1 → Hex transition ([CTAB] > 51 wt %), the concentration dependence of Eaη is stronger than in the dilute and semidilute regimes, as quantified by the slopes indicated in Figure 7. This can be rationalized by the enhanced packing of ellipsoidal micelles. Counterintuitively, the activation energy for ionic conductivity, along with the values of σ and ϑ (see Figure 6), decreases with CTAB concentration near the L1 → Hex transition. This implies that the formation of the hexatic phase goes hand in hand with a decrease in tortuosity, thereupon ion transport is eased.
σ = σ0 exp[Bσ /(T − T0)] η = η∞ exp[Bη /R(T − T0)]
η∞, Pa·s
(8)
where T0 is the Vogel temperature at which the viscosity becomes infinite and the conductivity drops to zero. The prefactors η∞ and σ0 are fitting parameters, and the constants 12728
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(7) Han, X.; Armstrong, D. W. Ionic Liquids in Separations. Acc. Chem. Res. 2007, 40, 1079−1086. (8) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Ionic Liquid (Molten Salt) Phase Organometallic Catalysis. Chem. Rev. 2002, 102, 3667− 3692. (9) Kunz, W.; Zemb, T.; Harrar, A. Using Ionic Liquids to Formulate Microemulsions: Current State of Affairs. Curr. Opin. Colloid Interface Sci. 2012, 17, 205−211. (10) Walden, P. Ueber die Molekulargrosse und Elektrische LeitfAhigkeit Einiger Geschmolzener Salze (Molecularweights and electrical conductivity of several fused salts). Bull. Acad. Imp. Sci. St.Petersbourg 1914, 8, 405−422. (11) Greaves, T. L.; Drummond, C. J. Ionic Liquids as Amphiphile Self-Assembly Media. Chem. Soc. Rev. 2008, 37, 1709−1726. (12) Evans, D. F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. Micelle Formation in Ethylammonium Nitrate, a Low-Melting Fused Salt. J. Colloid Interface Sci. 1982, 88, 89−96. (13) Greaves, T. L.; Weerawardena, A.; Fong, C.; Drummond, C. J. Many Protic Ionic Liquids Mediate Hydrocarbon-Solvent Interactions and Promote Amphiphile Self-Assembly. Langmuir 2007, 23, 402− 404. (14) Atkin, R.; Fina, L.-M. D.; Kiederling, U.; Warr, G. G. Structure and Self Assembly of Pluronic Amphiphiles in Ethylammonium Nitrate and at the Silica Surface. J. Phys. Chem. B 2009, 113, 12201− 12213. (15) Zhao, Y.; Chen, X.; Wang, X. Liquid Crystalline Phases SelfOrganized from a Surfactant like Ionic Liquid C16mimCl in Ethylammonium Nitrate. J. Phys. Chem. B 2009, 113, 2024−2030. (16) Fernández-Castro, B.; Méndez-Morales, T.; Carrete, J.; Fazer, E.; Cabeza, O.; Rodríguez, J. R.; Turmine, M.; Varela, L. M. Surfactant Self-Assembly Nanostructures in Protic Ionic Liquids. J. Phys. Chem. B 2011, 115, 8145−8154. (17) López-Barrón, C. R.; Basavaraj, M. G.; DeRita, L.; Wagner, N. J. Sponge-to-Lamellar Transition in a Double-Tail Cationic Surfactant/ Protic Ionic Liquid System: Structural and Rheological Analysis. J. Phys. Chem. B 2012, 116, 813−822. (18) Wang, X.; Chen, X.; Zhao, Y.; Yue, X.; Li, Q.; Li, Z. Nonaqueous Lyotropic Liquid-Crystalline Phases Formed by Gemini Surfactants in a Protic Ionic Liquid. Langmuir 2012, 28, 2476−2484. (19) Allen, M.; Evans, D. F.; Lumry, R. Thermodynamic Properties of the EthylammoniumNitrate + Water System: Partial Molar Volumes, Heat Capacities, and Expansivities. J. Sol. Chem. 1985, 14, 549−560. (20) Chandler, D. Interfaces and the Driving Force of Hydrophobic Assembly. Nature 2005, 437, 640−647. (21) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed; Academic Press: Burlington, MA, 2011. (22) Ribeiro, A. C. F.; Lobo, V. M. M.; Valente, A. J. M.; Azevedo, E. F. G.; da G Miguel, M.; Burrows, H. D. Transport Properties of Alkyltrimethylammonium Bromide Surfactants in Aqueous Solutions. Colloid Polym. Sci. 2004, 283, 277−283. (23) Chen, L. G.; Bermudez, H. Solubility and Aggregation of Charged Surfactants in Ionic Liquids. Langmuir 2012, 28, 1157−1162. (24) Evans, D. F.; Kaler, E. W.; Benton, W. J. Liquid Crystals in a Fused Salt: β,γ -Distearoylphosphatidylcholine in N-Ethylammonium Nitrate. J. Phys. Chem. 1983, 87, 533−535. (25) Araos, M. U.; Warr, G. G. Self-Assembly of Nonionic Surfactants into Lyotropic Liquid Crystals in Ethylammonium Nitrate, a Room-Temperature Ionic Liquid. J. Phys. Chem. B 2005, 109, 14275−14277. (26) Araos, M. U.; Warr, G. G. Structure of Nonionic Surfactant Micelles in the Ionic Liquid Ethylammonium Nitrate. Langmuir 2008, 24, 9354−9360. (27) Zhang, G.; Chen, X.; Zhao, Y.; Ma, F.; Jing, B.; Qiu, H. Lyotropic Liquid-Crystalline Phases Formed by Pluronic P123 in Ethylammonium Nitrate. J. Phys. Chem. B 2008, 112, 6578−6584. (28) Ma, F.; Chen, X.; Zhao, Y.; Wang, X.; Li, Q.; Lv, C.; Yue, X. Nonaqueous Lyotropic Liquid Crystal Fabricated by a Polyoxy-
CONCLUSIONS Structural transitions in CTAB/EAN solutions were characterized by means of various experimental techniques. These transitions include noninteracting to interacting micelles, sphere-to-ellipsoidal shape transition, and normal micellar (L1) phase to hexatic phase. The L1 to Hex transition has been predicted, but not observed previously in any surfactant solution. This transition is enabled in the ionic liquid by the high surfactant concentration at the phase boundary. Due to the weaker solvophobic effect in EAN as compared to water, the surface curvature of the micelles in the ionic liquid does not decrease as much as in water. Hence, micelle elongation is less dramatic, resulting ellipsoidal micelles, instead of wormlike micelles. Moreover, the stronger charge screening in EAN (compare to salt-free water) allows the micelles to approach closer to each other before transiting to a liquid crystalline state. These two effects, combined, explain the phase behavior of CTAB/EAN solutions described here, as well as the formation of a stable sponge (L3) phase and the transition to a lamellar phase at high concentrations in the DDAB/EAN system.17 The fact that the obstruction factor does not show a sharp increase with temperature at the L1 → Hex phase transition indicates that the topology of the system does not change; that is, the micelles are simply rearranged into a more ordered state without forming aggregates or surface defects.
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ASSOCIATED CONTENT
S Supporting Information *
SANS spectra for CTAB/EAN solutions in the dilute and concentrated regimes (Figures SI1 and SI2, respectively) as well as a plot of viscosity as a function of temperature for pure EAN and CTAB/EAN solutions (Figure SI3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This manuscript was prepared under cooperative agreement 70NANB7H6178 from the National Institute of Standards and Technology (NIST), U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors and do not necessarily reflect the views of NIST or the U.S. Department of Commerce.
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