Chloroform Reverse Micelles at Above- and

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Nature of CTAB/Water/Chloroform Reverse Micelles at Above- and Subzero Temperatures Studied by NMR and Molecular Dynamics Simulations Lubica Klíčová,† Eva Muchová,‡ Peter Šebej,† Petr Slavíček,‡ and Petr Klán*,† †

Department of Chemistry and RECETOX, Masaryk University, Kamenice 5, 625 00 Brno, Czech Republic Department of Physical Chemistry, University of Chemistry and Technology, Technická 5, 16628 Prague 6, Czech Republic

‡

S Supporting Information *

ABSTRACT: The nature and stability of cetyltrimethylammonium bromide (CTAB) reverse micelles in chloroform formed above the critical micellar concentration at above- and subzero temperatures were examined by NMR and molecular dynamics simulations. The experiments showed that the supercooled micellar water pool becomes unstable upon cooling to relatively high temperatures (253 K), and smaller micelles are formed. Upon freezing at lower temperatures (233 K), micelles become completely frozen and remain intact in the solution. With an average hydrodynamic radius of approximately 1.3 nm, we estimate that the water pool contains approximately 50 water molecules, which is well below the onset of ice crystal formation. To support the experimental results, molecular dynamics simulations were used to model the structure of CTAB/water/chloroform reverse micelles of different sizes. The MD simulations show that the reverse micelles contain a water pool with bromide anions residing on its surface and their shape is nonspherical, especially in the case of larger water pools. Upon fast freezing, the mobility of the water molecules is suppressed, and the pool becomes more spherical.

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INTRODUCTION Water in all forms is one of the most abundant molecules on Earth and its role is widely appreciated, yet there are still surprising gaps in our knowledge on its behavior. One of the questions attracting continuously wide interest is the nature of liquid water below the normal melting point. Bulk water can remain in the liquid state below its melting point, but it cannot be supercooled below the homogeneous nucleation temperature (at around TH = 235 K).1 However, the liquid state can be maintained even below this temperature in finite-size water particles.2 This nanoconfined, deeply supercooled water exhibits surprising properties. For example, it has been suggested that deeply supercooled liquid water has a density minimum.3 Confined water can be found in protein pockets or in synthetic nanopores. It has been hypothesized that water compartmentalization represents a mechanism for the cryogenic protection of organisms.4 Finite-size water particles, such as nanometer-sized water aerosols, were shown to be critical for the nucleation processes in the atmosphere.5 Reverse micelles, self-organized assemblies of amphiphilic molecules in nonpolar solvents, serve as a useful model for confined water in the condensed phase.6 The polar heads of amphiphiles are oriented toward the water cores and their hydrophobic chains form the outer shells. The nature and behavior of reverse micelles at ambient temperatures have been a subject of many investigations. Recently, we have validated © XXXX American Chemical Society

Eicke’s association model for CTAB/water/chloroform reverse micelles, according to which micelles are formed by a structural reorganization of linear associates within the apparent critical micelle concentration (cmc).7 Various factors can affect the lowest temperature to which water can be cooled before freezing to ice. The volume and spatial confinement8 as well as the presence of charged interfaces 9 are among the most important parameters determining the supercooling/crystallization process.10 Only a few studies have been performed to study AOT reverse micelles or proteins at subzero temperatures. Flynn and co-workers have reported that, at relatively low subzero temperatures, a supercooled water core of anionic AOT reverse micelles stays liquid and micelles become unstable,11 which is manifested by water shedding, that is the loss of water from the core until equilibrium is established.11,12 It was hypothesized that the entropically favorable encapsulation does not always contribute enough to the Gibbs energy to keep reverse micelles intact.11 These authors have also demonstrated that the water loading (w; the ratio of the molar concentrations cwater(core) and camphiphile(micelle)) of reverse micelles is influenced by the ionic strength of the aqueous phase. Kevan and co-workers have reported that water shedding can be prevented by shock Received: May 14, 2015 Revised: June 29, 2015

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DOI: 10.1021/acs.langmuir.5b01776 Langmuir XXXX, XXX, XXX−XXX

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Langmuir cooling of a micellar system to 77 K.13 Nucci and Vanderkooi have shown that water cores of a reverse AOT micellar system in n-pentane can freeze at 238 K independently on the waterto-surfactant molar ratio (x).14 Bright and co-workers have studied AOT reverse micelles in n-heptane using fluorescence probes at subzero temperatures.12 Their steady-state fluorescence emission and fluorescence anisotropy studies indicated that freezing occurs within the water pool at temperatures between 263 and 213 K and is affected by x. It has also been reported by Dokter and co-workers that fast freezing to 180 K does not affect the structure of reverse AOT micelles for water loadings smaller than 3.5.15 The structure of the cores of these small micelles, containing approximately 150 water molecules, resembled an amorphous form of ice. Recently, Suzuki and Yui demonstrated that loss of confined water in AOT reverse micelles upon freezing can be prevented by a combination of rapid cooling and a small sample cell size that allows for the crystallization of water pools with larger radii (over 2.1 nm).16 Attenuated total reflection infrared spectroscopy (ATR-IR) measurements revealed that the frozen pool exhibits features similar to the spectrum of metastable cubic ice (Ic), and also that an ice−water coexistence phase is formed during melting.17 The interactions of confined micellar water with the ionic amphiphile headgroups and the presence of counterions affect the structure of the micellar cores and their dynamics.18 For example, interfacial water molecules in AOT reverse micelles form rigid hydrogen bonds with the anionic sulfonate headgroups,19 whereas hydrogen bonding of interfacial water in cetyltrimethylammonium bromide (CTAB) reverse micelles is less directional due to the large polarizable cloud of the bromide ion.20 The ammonium headgroup has a negligible primary hydration capacity, as observed in didodecyldimethylammonium bromide-water systems where no “interfacial” water was detected.21 The behavior of cationic reverse micelles at subzero temperatures has hardly been studied yet. Only one work reports that CTAB/n-hexanol/alkane reverse micelles are stable above 263 K.22 This work is a follow-up of our previous study,7 in which we determined the boundary conditions of the stability of CTAB reverse micelles in chloroform. Here, we investigate the nature, size and dynamics of the water pool of such micelles in the temperature range of 303−233 K using 1H NMR spectroscopy. The NMR experiments are supported by molecular dynamics (MD) simulations, which already proved to be useful for investigating the structure of reverse micelle at ambient conditions;23−26 yet only little work has been done for reverse micelles at subzero temperatures. Here, we use MD simulations to reveal how the structural and dynamical properties of the CTAB/water/chloroform micelles vary with temperature.

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whole time of equilibration). For experiments lasting over 1 day, the NMR tubes were sealed. 1 H NMR spectra were obtained on 300 and 500 MHz spectrometers at different temperatures. The initial and final spectra of an individual sample were always acquired at 30 °C. The temperature of a sample was equilibrated for at least 3 min after the given temperature in the NMR probe was set. 1 H NMR was also used to evaluate the diffusion coefficients of reverse micelles in chloroform. The 2D DOSY NMR experiments were carried out on a 500 MHz spectrometer at 303 K, employing a PABBO probe-head equipped with the z-gradients. The experiments consisted of several measurements of a pulsed gradient stimulated echo (PGSTE) sequence with a longitudinal eddy current delay, two bipolar gradient pulse pairs of smoothed-square shape and two additional spoiling gradients, placed in the delay, and the Z-period of the diffusion time in the pulse sequence. The resulting data were obtained as a set of the 1D measurements differing in the gradient strength value, which were linearly modified from 2 to 95% of the maximum gradient strength (a gradient calibration constant) in 32 gradient steps, causing a gradual attenuation of the signal. The longitudinal relaxation time was 18.8 s for water in the case of gradient calibration experiments and to 3 s for CTAB in the case of diffusion coefficient measurements. The gradient system was calibrated by measuring the diffusion coefficient of H2O in D2O at 298 K using a previously reported value of 1.872 × 10−9 m2 s−1.27 The corrected gradient calibration constant was determined before each measurement and was found to be in the range of 5.01 to 5.43 G cm−1. The intensity and/or peak area decay curves for CTAB protons with chemical shifts of 3.49, 1.74, 1.25, or 0.88 ppm were fitted independently. The diffusion coefficient was calculated using the equation: S(G)/S(0) = exp[−γ2δ2G2D(Δ − δ/3 − τ/2)],28 where S(G) is the signal at a gradient amplitude G, S(0) is the signal at zero gradient, γ is the gyromagnetic ratio of a proton, δ is a duration of the magnetic field gradient pulse, Δ is the diffusion time, and τ is the time between the two gradient pulses in the bipolar gradient pair. The diffusion delay Δ was set to 50 ms. The gradient pulse length of δ/2 was set in the range of 1.6−2 ms, whereas the spoil gradient pulse length was set in the range of 0.6−1 ms. The mean self-diffusion coefficient of the reverse micelles was determined as an average of 19 values of the diffusion coefficients measured for 5 independent samples. The Size of Reverse Micelles. In order to obtain qualitative information about the size of reverse micelles, the Stokes−Einstein equation was used, and the hydrodynamic radius RH was calculated according to RH = kBT/6πηD, where η is the solvent viscosity. A rather low volume fraction of the dispersed phase Φ (0.0425) was obtained by the equation Φ = (Vwater+ nCTABvCTAB)/Vtotal, where Vwater is the volume of water, nCTAB is the number of moles of CTAB, and vCTAB is the molar volume of CTAB (363 mL mol−1 at room temperature29). It was not necessary to make a correction for collisions.30 The value of RH is an upper limit because the obstruction effect has not been taken into account; it is very small (within the measurement error of the diffusion coefficient) for the volume fraction used. For the present purpose, water assemblies in the micellar core were assumed to be spherical, monodispersed, and separated from the homogeneous organic solvent by a monolayer of surfactant molecules, and all water in the sample was uniformly distributed inside the reverse NMR NMR micelles (for wobs water, core = cwater, core/cCTAB = x = 3.4). These assumptions allowed us to calculate the aggregation number Ns (the number of molecules present in a micelle once the cmc is reached) of the prepared reverse micelles together with the amount of water molecules Nw inside their water pools. The hydrodynamic radius RH of a reverse micelle (the volume of one CTAB molecule is Vs= 0.469 nm3,31 and the volume of one water molecule is Vw = 0.03 nm332) were calculated from the following system of two linear equations and two variables, Ns and Nw: VRM = Vs + Vw = NsVs + NwVw, where VRM = (4/3)πRH3 is the volume of one reverse micelle, and Ns = Nw/w. The amount of water molecules Nw was then expressed as Nw = (4/3)πRH3w/(Vs + Vww).

MATERIALS AND METHODS

Materials. Cetyltrimethylammonium bromide (CTAB; >99.0%) and chloroform-d (99.8%) were used as purchased. Chloroform-d was stored in amber bottles over flame-dried molecular sieves (3 Å) with a piece of silver foil as a stabilizer. 1 H NMR and Diffusion Coefficient Measurements. The chloroform-d solutions were prepared by direct weighing of CTAB (cCTAB = 0.1 mol dm−3) into NMR tubes. Water was subsequently added to this solution to adjust the water-to-surfactant molar ratio, x = analytical to 3.4.7 The mixture was then vigorously agitated. A canalytical water /cCTAB sealed capillary tube containing a chloroform-d solution of tetramethylsilane or dichloromethane was added as an internal standard. All samples were equilibrated for 14 h prior to the measurements (the NMR signal of water remained unchanged during B


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intramicellar exchange is faster50 than the time resolution of the NMR spectroscopy. For x > 4, the phase separation is observed during the sample preparation before equilibrium is reached, which is manifested by the presence of a separated aqueous layer and the signal of bulklike water at δwater,bulk ∼ 4.7 ppm.51 In order to evaluate time needed for equilibration in an unperturbed mixture, a sample (cCTAB = 0.1 mol L−1 in dry chloroform-d) with 4 μL of water initially added to the surface of a CTAB solution was prepared without shaking. Subsequently, the complete incorporation of water into the reverse CTAB micelles was accomplished in approximately 200 h. The incorporation rate constants were calculated from the water loading wobs water,core values obtained from the relative water and CTAB concenNMR NMR 7 obs trations, wobs water,core = cwater,core/cCTAB, when wwater,core increased from 0 to the maximum value of 3.1. Biexponential fitting of the data then provided two rate constants of k = 1.0 × 10−4 and 8.1 × 10−6 s−1 (Figure S2). In the next step, the temperature of an equilibrated sample was decreased from 303 to 253 K in 10 min and was kept there for 2 h. The initial NMR signal of core water at δwater,core ∼ 3.7 ppm at 303 K was slightly shifted upon cooling to 253 K to ∼4 ppm (Figure 1, black empty circles). The observed water

Using simple geometric considerations, the radius of the water pool Rw could be expressed as Rw = [3Ns(wVw + VBr−)/4π]1/3,32 where Rw is given in nm, and VBr− represents the partial molecular volume of the surfactant counterion Br− (VBr− = 0.0493 nm3 at 25 °C33). The spherical surface area σa, occupied by each cationic surfactant ion at the water pool surface, was then obtained according to σa/nm2 = 4πRw2/Ns. Molecular Dynamics Simulations. Molecular dynamics simulations were performed with the GROMACS 4.5.3 code.34 A standard nonpolarizable force field, comprising point charges and Lennard− Jones potentials, was used to account for intermolecular interactions. The rigid SPC/E model for water35 was applied. Chloroform was modeled using the OPLS nonbonded parameters36 (σ = 0.3800 nm, ε = 0.3269 kJ mol−1, and charge 0.420 e for the CH group; σ = 0.3470 nm, ε = 1.2560 kJ mol−1, and charge −0.140 e for the Cl group). The force field of the CTAB molecule was constructed with the GROMOS-87 with corrections to nonbonded parameters for atoms,37 with the geometries and charges taken from the DFT/ BLYP/6-31g* calculation using a CHelpG population analysis (see the Supporting Information, Table S1). Nonbonding parameters for the bromide ion were taken from the literature.38 The simulations box was prepared as follows. The bromide ions and CTAB cationic heads were randomly distributed on the surface of ice particles. The cluster was optimized in the gas phase and subsequently immersed in a box containing 512 chloroform molecules. The clusters were then equilibrated for 20 ns. The productions runs were 20 ns long, using a time-step of 1.5 fs. Most of the simulations were performed at 300 K; however, the behavior of the reverse micelles was also studied at 273 and 200 K. Using the previously described force field, chloroform remains liquid at all studied temperatures. Periodic boundary conditions were applied with the forces being cutoff at 19 Å. The simulation was run in the NpT ensemble. The Berendsen thermostat with the time constant of 0.5 ps and the Berendsen barostat with the time constant of 1 ps were used. It is well-known that the lack of polarization can seriously affect the structure of dissolved electrolytes, particularly those with a low charge density.39 A simple remedy for this problem was recently introduced within the concept of electronic continuum correction (ECC).40 To account for the screening effect of the electronic polarization, the charges should be scaled by a factor of 1/√εel, where εel is the optical part of the dielectric constant. The value of this parameter is almost equal for most solvents, for example, 1.78 for water (at 298 K) and 2.09 for chloroform.41 In our simulations, the value of 2.09 for chloroform as a solvent was used. Note that the water charges were not scaled as their charges are fitted to the experiment and have to be considered as effective charges. Simulations with scaled atomic charges were frequently used in different fields, such as the simulations of ionic liquids,42 electrolyte solutions,43 or interfaces.44,45 The ECC framework brings a justification for this scaling.46 The MSMS (Maximal Speed Molecular Surface) code was used to calculate the surface and volume of the water pool in the reverse micelle.47

Figure 1. Dependence of wobs water,core (black full circles; right ordinate), wobs water,bulk (red full triangles; right ordinate), δwater,core (black empty circles; left ordinate), and δwater,bulk (red empty triangles; left ordinate) (bottom graph) on temperature (blue solid line; upper graph) and time (abscissa) for the CTAB/water/chloroform-d solution (canalytical CTAB = 100 mmol dm−3) with the initial x = 3.4. The dashed lines are shown to guide the eye.

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loading wobs water,core, calculated from the relative water and CTAB NMR NMR 7 concentrations, wobs water,core = cwater,core/cCTAB, dropped concomitantly from 3.0 to 0.5 (Figure 1, black full circles). Simultaneously, the NMR signal of bulklike water,52 δwater,bulk = 5.2−6.0 ppm, appeared (Figure 1, red empty triangles). The corresponding bulklike water concentration, wwater,bulk = NMR cNMR water,bulk/cCTAB = 0.26, gradually (in ∼50 min) disappeared (Figures S3 and S4), whereas several minor water signals appeared (Figures S5 and S6). After warming of the sample to 303 K, the chemical shift δwater,core decreased, and the water loading wobs water,core increased to 1.0, which was lower by a factor of ∼3 compared to the initial value determined before the cooling cycle started. When the sample temperature was kept at 303 K without agitation for ∼50 h, the water loading slowly rose to wobs water,core = 2.8 biexponentially with rate constants of the same magnitude as those measured in the model experiment

RESULTS AND DISCUSSION Temperature Range: 303−253 K. Chloroform-d solutions of CTAB reverse micelles with a concentration of 100 mmol dm−3, which is well above the critical micellar concentration of ∼40 mM,7 and the water-to-surfactant molar ratio of x = 3.4, were prepared in NMR tubes, and the solutions were left to equilibrate at 303 K for 14 h. The corresponding 1 H NMR spectrum contained a single averaged water signal at δwater,core ∼ 3.7 ppm of the micellar core7 (Figure S1) which is consistent with NMR observations that water encapsulated in the reverse micelles exists as a single pseudophase48 below the phase separation limit of the water-to-surfactant molar ratio of x ∼ 4 at 303 K.7 Such an averaging of the water signal may be caused by the confinement of core water molecules49 or their C


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Langmuir described above (4.9 × 10−4 s−1 and 2.0 × 10−6 s−1; Figure S2). These values differ by factors of ∼4 (the faster component is faster, the slower component is slower) compared to those of a model experiment (see above), but they are still in the same order of magnitude. We hypothesize that the formation of initial linear associates7 corresponds to the fast process observed, whereas the subsequent formation of micelles is characterized by the slow component. It is obvious that such rate constants should depend on the experimental conditions, such as the size of phase-separated water droplets, the CTAB concentration, and temperature. It is thus probable that water expelled from micelles floats in the form of very small droplets throughout the sample volume and is incorporated in the micelles faster than phase-separated water. It is to be noted that the micelles precipitated upon cooling to 253 K in some experiments, which was evidenced by disappearance of the CTAB NMR signal. At temperatures below 273 K, a supercooled state can be formed in small volumes of encapsulated water, characterized by a hydrogen bonding rearrangement and manifested, for example, by an increase in the δwater,core values.53 In addition, due to decreased mobility of water molecules in the supercooled state, several water populations inside the reverse micelles may be observable because their lifetime exceeds the time window of an NMR measurement11 (a specific ice−water coexistence has also been observed during melting of frozen AOT micelles by IR spectroscopy17). A small shift of δwater,core from 3.7 to ∼4.1 ppm (Figure 1) can be related in part to these phenomena as this value dropped again upon warming the sample back to 303 K. A significant decrease of the observed water loading wobs water,core upon cooling to 253 K by a factor of 6 must be attributed to water shedding, the loss of water content from a reverse micelle under perturbing conditions,11,12 which could eventually be circumvented at larger water pool radii.17 This water loss was observed already at x = 3.4 which is the value at which AOT reverse micelles are usually stable while their core stays liquid (above 218−238 K).12,14,15 The stability of cationic CTAB reverse micelles when the water pool is not frozen must thus be lower compared to that of the analogous AOT micelles. The lack of strong headgroup hydration21 can be related to a lower enthalpic stabilization of the CTAB reverse micelles at subzero temperatures and the specific electronic9 and steric properties of the CTAB headgroups and the bromide counterion.20 It is thus possible that the amphiphile heads serve as a nucleating agent. Expelled water gradually segregated at 253 K, which was observed as a bulklike water signal by NMR (Figure 1, red empty triangles), and changed to free ice crystals floating in the sample upon freezing, being apparent to the bare eye but invisible by NMR (loss of the signal). This caused an irreversible change of the water loading noticeable after the temperature quickly increased above 273 K. Only a small part of the encapsulated, probably supercooled water, remained visible by NMR at 253 K (wobs water,core = 0.5). Freezing of the remaining encapsulated water inside the reverse micelle is only one possibility of the NMR signal loss (water shedding or precipitation of micelles could be other reasons). If micelles kept the initial water loading upon cooling to 253 K, a water pool signal should be observed immediately after the temperature was increased over 0 °C. As only ∼30% of the initial core water was recovered upon heating, micelles had to shed water to become smaller and more stable at the given conditions. Indeed, the subsequent equilibration required tens

of hours at 303 K, and its kinetics (Figure S2) corresponded to that of a model equilibration experiment for x = 3.4 (Figure S2). The experimental temperature of 253 K is still higher than that of homogeneous nucleation (225−232 K) found in the case AOT reverse micelles possessing a water pool size equal to 1.2−3.4 nm,3 and comparable to the heterogeneous nucleation in the presence of a nucleating agent (241−266 K) when the water pool size is 1.2−2 nm.3 In the next step, we thus investigated the dynamics of CTAB micelles upon rapid freezing at lower temperatures. Temperature Range: 303−233 K. The solutions of reverse CTAB micelles in chloroform-d (canalytical = 100 mmol CTAB dm−3, x = 3.4) in NMR tubes equilibrated for 14 h were exposed to a temperature cycle, in which the temperature oscillated between 303 and 233 K (Figure 2). The δwater,core

Figure 2. Dependence of wobs water,core (black full circles; right ordinate), the CTAB amount (aCTAB, red full squares; normalized; right ordinate) and δwater,core (black empty circles; left ordinate) (bottom graph) as a function of temperature (blue solid line, upper graph) and time = 100 (abscissa) for a CTAB/water/chloroform-d solution (canalytical CTAB mmol dm−3) with the initial x = 3.4. The dashed lines are shown to guide the eye.

signal at 3.4 ppm (w ∼ 3.4; Figure 2, black empty circle) almost disappeared (w → 0) upon relatively fast (3 min) initial cooling to 233 K. Several very weak signals (w ≤ 0.04) appeared, but the signal of CTAB almost disappeared (Figure S7). Water loading and the observed chemical shift reverted back to nearly the initial values of wobs water,core ∼ 3.3 and δwater,core = 3.2 ppm upon the first warming step to 303 K. Subsequently, slightly reduced values of wobs water,core∼ 3.0 and δwater,core = 3.0 ppm (∼90% recovery) were observed in the end of the second cooling/ warming cycle at 303 K; the signal of CTAB exhibited the same temperature change as that observed during the first cycle. Disappearance of the NMR signals of both water and CTAB molecules inside reverse micelles at 233 K, nearly a full and repetitious recovery of the water loading, as well as all other important NMR parameters determined upon warming to 303 K and essentially undetected water shedding imply that, in contrast to cooling at 253 K, micelles were fully frozen at 233 K and retained their initial size. A lower hydration of the ammonium headgroup21 and a higher orientational mobility of counterion-bound water in CTAB reverse micelles20 are probably the major factors that influence the rigidity of CTAB reverse micelles at low temperatures. In addition, a higher viscosity of chloroform at 233 K might also contribute to their stabilization. The presence of multiple populations of water of negligible concentrations that coexisted at 233 K, D


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Figure 3. Typical snapshots for a micelle containing (a) 50 water molecules and (b) 216 water molecules. Atoms of water, bromide and CTAB are represented as spheres whereas chloroform covalent bonds are shown as lines. Nitrogen atoms of CTAB are depicted in blue, carbon atoms in cyan, and bromide ions in pink. Radial distribution functions for water and the bromide ion with respect to the center of mass of the water pool are demonstrated for micelles containing either (c) 50 or (d) 216 water molecules at 300 K.

characterized by δ = 1.85 (in the range typical for dissolved water in chloroform7) and δ = 7.8 and 7.9 ppm (possibly a supercooled aqueous phase), appeared and subsequently disappeared upon warming. An analogous stabilizing effect at lower temperatures, enhanced by an increased ionic strength of the water pool, has been observed in the case of AOT reverse micelles by Flynn and co-workers,11 or when the water pool radius of AOT micelles was larger than 1 nm to allow ice formation.16 Therefore, the size of CTAB micelles was evaluated for comparative purposes. Size of Micelles and Properties of the Water Core. The diffusion coefficients of reverse-micellar aggregates in equili= 100 brated CTAB/water/chloroform-d solutions (canalytical CTAB mmol dm−3, x = 3.4) obtained using the 2D DOSY NMR experiments were found to be (3.58 ± 0.80) × 10−10 m2 s−1 as an average over the measurements of five independent samples at 303 K. The corresponding calculated hydrodynamic radius, RH, was then (1.26 ± 0.28) nm.54 When the sample underwent two cooling/warming cycles (233/263 K), RH dropped to (1.06 ± 0.28) nm that is attributed to partial water shedding. Using geometrical considerations, the initial reverse micelles contained (50 ± 30) water molecules in the water pool with an aggregation number of (15 ± 8). The water pool radius is then Rw = (0.82 ± 0.20) nm and the spherical surface area σa = (0.55 ± 0.15) nm2. For example, the magnitude of RH, determined by SAXS, SANS, and DLS, for typical AOT reverse micelles with w ∼ 5 in i-octane or n-heptane is approximately 2.5 nm;55 RH = 0.9 nm was estimated in the case of CTAB/n-hexanol/water microemulsion with a volume fraction ϕ = 0.4 and w = 7.2 using the pulsed-gradient stimulated-echo NMR experiments.30 Thus, RH for our CTAB/water/chloroform system is of the same magnitude. The higher values of RH reported by us

previously7 were not observed under the experimental conditions in this work. Since the mass fraction of CTAB was equal to 2.4 wt %, a mean Ns value of 15 was calculated, and for the RH deviations, Ns values of 7 and 27 were found. Similar Ns values in the range of 5−20 have been determined for 0−15 wt % of CTAB in CTAB/n-hexanol/water reverse micelles.56 The calculated CTAB headgroup area of 0.55 ± 0.15 nm2 and Rw = 0.82 nm are fairly comparable to the parameters obtained for other systems investigated. For example, the parameters Ns = 65, Rw = 1.46 nm, and σa = 0.41 nm2 were calculated for CTAB reverse micelles in chloroform/i-octane (2:1, v/v) in the case of 0.75 mol dm−3 CTAB (w = 5) in the presence of a fluorescence probe at 298 K,32 whereas the headgroup area of CTAB in normal (oil-in-water) micelles was determined to be 0.64 nm2.57 In our system, the values of Rw = 0.82 nm and RH = 1.26 nm correspond to a surfactant shell of a thickness of 0.44 nm, which is considerably lower than the length of a fully extended hydrocarbon chain of ∼2.2 nm.57 It suggests that the surfactant chains could be twisted toward the surface of the water pool; however, the MD simulations do not support this interpretation. If CTAB reverse micelles with a hydrodynamic radius of 1.26 nm contain only ∼50 water molecules, this number is insufficient to form ice upon freezing;8 instead, water could be vitrified as the viscosity increases.58 It is thus possible that amorphous frozen water is formed inside the CTAB reverse micelles upon cooling to 233 K, although the Rw value of 0.82 nm is only slightly lower than that limiting the water pool crystallization (Rw > 1 nm) to form cubic ice Ic in AOT micelles.16 We therefore decided to perform molecular E


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Langmuir dynamics simulations to better understand the structural parameters and dynamics of CTAB micelles in chloroform. Molecular Dynamics Simulations. We considered micelles with two sizes of the water pool containing either 50 or 216 water molecules. The smaller system corresponds to the size determined by our experiment; the larger one was chosen to investigate how the properties of the system evolve with the micelle size. The same fraction x = 3.4 as that in the experiments was kept constant during all the simulations (specifically, the smaller system contained 50 water molecules, 15 bromide ions and 15 CTAB molecules; the larger system contained 216 water molecules, 63 bromide ions and 63 CTAB molecules). Typical snapshots for the smallest and largest micelle are shown in Figure 3a and b. Visual inspection of these structures demonstrates that some hydrocarbon chains point toward the bulk solution whereas some chains are not fully extended and approach the water pool. On average, the alkyl chains are however rather straight (see also the Supporting Information, Figure S8). We also observed that particularly the system with a smaller water pool is not fully encapsulated by the surfactant. The sparse coverage of the water pool by CTAB can explain the observed smaller effective length of the micelles. A critical step in the simulations is a proper choice of the force field. This can be exemplified on distributions of the bromide ions in the water pool. Figure 3c and d show the radial distribution functions of water and the bromide ions with respect to the center of mass of the water pool. We immediately observed a dramatic difference between the nonpolarizable force field and the ECC based calculations. The radial distribution functions for water as a solvent and the bromide ion almost coincided for both investigated sizes. This indicates that the bromide ions are fully dissolved in the water pool. On the other hand, we clearly observed a surface excess of the bromide ions for the ECC calculations. In fact, the bromide ions were fully expelled from the water pool and the observed nonzero intensity at r distances close to zero resulted from a nonspherical shape of the water pool. The strong surface preference for heavier halide solvation upon adding the polarization effects was previously observed for finite size clusters and aerosols59 as well as the air−water interfaces.60 More relevant in our context is the increase of an interfacial halide anion concentration observed at the interface between the aqueous phase and hydrophobic surfaces.44 In this case, the effect calculated within the ECC model agreed with explicitly polarizable calculations; the authors also showed that the ECC calculations agree with experimental data. The ECC model thus seems to be suitable for our micellar systems. Based on the ECC simulations, we conclude that the water pool is formed by a compact hydrogen-bonded network of the water molecules. The bromide ions are placed on the surface of the cluster, surrounded by the cetyltrimethylammonium heads and chloroform. The radial distribution functions were also consistent with the experimentally determined radius of the micelles (Rw = (0.82 ± 0.20) nm that corresponds to the reduced water density for the smaller cluster). In the subsequent step, we asked what the structure of the reverse micelles is in particular, what the shape of the water pool is. The sphericity of the water pool is visualized here by displaying semiaxes a, b, and c that are related to the principal moments of inertia: I1 =

I2 =

1 M (a 2 + c 2 ) 5

I3 =

1 M (b 2 + c 2 ) 5

The three semiaxes would be identical for a spherical object. Another quantitative criterion is based on calculating the solvent-excluded surface (SES) and the corresponding volume. We can define a ratio s as

s=

SSES Ssphere

where SSES is the SES and Ssphere is the surface calculated from the SES-related volume considering a spherical shape of the object. This ratio decreases for a more spherical object, with a lower surface-to-volume ratio. Figure 4 displays both of these

Figure 4. (a) Time evolution of the semiaxes (a, b, c) along the trajectories for micelles containing 50 (brown, dark green, green) or 216 water molecules (red, orange, yellow) at 300 K. (b) Time evolution of s for reverse micelles containing 50 (red) or 216 (black) water molecules at 300 K and for pure water clusters containing 50 (green) or 216 (brown) water molecules at 300 K. Semiaxes a,b,c are shown in nm units.

criteria for the two sizes of the water pools investigated here. Inspection of the semiaxes (Figure 4a) shows that the water pool is rather nonspherical, with a roughly prolate shape. A similar picture is obtained from the s parameter. Let us first consider pure water clusters in chloroform. Since water is only poorly soluble in chloroform, we can expect that pure water clusters immersed in chloroform tend to form spherical particles. Figure 4b demonstrates a relatively small value of the s parameter irrespective of the cluster size, indicating a spherical shape of the particle. Once a micelle is formed, it loses the spherical shape. This effect is even more pronounced for larger micelles as can be seen in Figure 4: the s parameter is

1 M (a 2 + b 2 ) 5 F


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function of time. Note that for the condensed phase, the slope of the MSD should be proportional to the diffusion coefficient. Figure 6 shows the MSD of different molecular units of the micelle as a function of time for micelles containing 50 water molecules at different temperatures.

significantly higher for a larger micelle (black curve in Figure 4b), and the semiaxes do not coincide as in the smaller micelles. Clearly, an onset of the solubilization of the water cluster is observed here. It should be mentioned that nonspherical shapes of larger micelles are found also for other reverse micelle systems.25 Interestingly, a highly nonspherical shape of water particles is observed also in the gas phase water particles formed in a supersonic expansion.61 As the micelles formed in this study are rather small, we made the approximation that their shape is spherical while processing the experimental data. Let us now explore how the structure of the micelle changes with decreasing temperature. In the simulation, cooling will always be ultrafast and we can thus observe only a nonequilibrium situation. We compared the properties of water/CTAB/chloroform micelles at the temperatures of 300 and 200 K. The latter temperature is slightly below the melting temperature of chloroform; however, the system stayed liquid with the force field used in this study. Figure 5 shows the

Figure 5. (a) Semiaxes (a, b, c) for micelles containing 50 waters at 300 K (red, orange, yellow) and 200 K (blue, violet, maroon). (b) Time evolution of s for micelles containing 50 water molecules at 300 K (black) and 200 K (green). Semiaxes a,b,c are shown in nm units.

Figure 6. MSD for three different temperatures: 200 K (blue), 273 K (green), and 300 K (red) as a function of time for micelles containing 50 water molecules for (a) water, (b) chloroform, and (c) terminal CH3 groups. Data for water and terminal CH3 were obtained from a progressive fit, for chloroform from an unfitted trajectory.

structural parameters characterizing the shape of the water pool, the semiaxes a, b, c and the ratio s. From both parameters, we observe (i) a slower dynamics of the structural fluctuations at lower temperatures and (ii) more spherical shapes of the water pools at the lower temperature. The latter finding suggests a tendency for a phase separation of water under these conditions. The central question is what is the state of the water pool at a lower temperature? The chloroform stays liquid; yet, it is unclear whether the small water particles immersed in chloroform freeze or the thermal agitation of the surrounding chloroform keeps it in a liquid state. Water clusters might form a cubic ice16 even though it is uncertain whether such small particles can form true ice structures.8 Nevertheless, the water pool can still form an amorphous solid phase. The character of the cluster can be revealed by looking at the mean square displacement (MSD) of the water molecules as a

Note that, for water molecules, the MSD grows linearly only for a short time before MSD turns over and saturates to give a finite value. This reflects a finite size of the water particles immersed in chloroform. The water molecule can diffuse freely only within the water pool, the maximum displacement is thus restricted by the size of the water cluster. For a liquid, the value at which MSD saturates can be considered as the measure of the effective size of the water pool. We observed that (⟨R2⟩)1/2 is approximately 1 nm for a micelle with 50 water molecules. At 300 K, the water pool is liquid and the diffusion of water molecules is possible. The initial slope is related to the diffusion coefficient of water in the water pool. We found no qualitative difference upon decreasing the temperature from 300 to 273 K. However, it should be mentioned that the ice described with G


Langmuir the SPC/E water model melts well below this temperature. Freezing the cluster to 200 K brings a qualitative change. The observed dynamics is much slower which is particularly apparent in the beginning. On the other hand, the mobility of the terminal CH3 group decreases much less with the decreasing temperature, whereas chloroform still stays liquid even at such low temperatures. The molecular dynamics simulations have thus demonstrated that upon fast freezing, the mobility of the water molecule, and to a lesser extent also of the CTAB, is much suppressed. This may have caused the disappearance of the NMR signal in our experiments with rapid cooling. The water pool simultaneously becomes more spherical which suggests a tendency for a phase separation. However, a phase separation and possible water shedding are well beyond the time scale of our MD simulations.

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSION In this work, we investigated the behavior of CTAB/water/ chloroform reverse micelles (w ∼ 3.4) at subzero temperatures using NMR. The experiments provided evidence that supercooled water inside the micellar water pool became unstable at 253 K, shed water, and new smaller micelles (w ∼ 1) were formed. A part of the encapsulated water was frozen at 253 K, which is well above the temperature of homogeneous nucleation, indicating that the positively charged surface might induce a heterogeneous nucleation of the water pool. The stability of cationic CTAB reverse micelles is thus lower than the stability of anionic AOT reverse micelles, probably due to a lower enthalpic stabilization caused by a lower surface hydration of the CTAB ammonium headgroups. By cooling to 233 K, the reverse micelles became completely frozen and stayed intact in the solution. Such reverse micelles with an average hydrodynamic radius of 1.26 nm at 30 °C contain ∼50 water molecules in the water core, which is well below the onset of an ice crystal formation, thus the frozen water should be in an amorphous form. The structure of the CTAB/water/chloroform reverse micelles was further studied by molecular dynamics simulations. It turned out that the micelles are formed by an aqueous pool with bromide anions residing on its surface. Such micelles tend to form nonspherical structures, especially for larger water pools. Upon fast freezing, the mobility of the water molecules is suppressed. This is true also for the terminal CH3 group of the CTAB molecule, however, with a slower decrease of the diffusion coefficient with decreasing temperature. Larger cooled CTAB/water/chloroform micelles tend to become more spherical. ASSOCIATED CONTENT

S Supporting Information *

NMR experiments; parameters of the force field. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01776.

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Support for this work was provided by the Czech Science Foundation (15-12386S). The RECETOX research infrastructure was supported by the projects of the Czech Ministry of Education (LO1214) and (LM2011028). P.Š. was supported by the project “Employment of Best Young Scientists for International Cooperation Empowerment” (CZ.1.07/2.3.00/ 30.0037) cofinanced from European Social Fund and the state budget of the Czech Republic. The authors express their thanks to Lukás ̌ Maier and Václav Havel for their help with the NMR. Peter Štacko, Zdeněk Moravec, and Jakob Wirz are acknowledged for fruitful discussions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +420-54949-4856. Fax: +420-54949-2443. Notes

The authors declare no competing financial interest. H


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Chloroform Reverse Micelles at Above- and

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