DOI: 10.1021/cg101061p
On the Use of Ionic Liquids To Tune Crystallization
2011, Vol. 11 684–691
Magdalena Kowacz,* Patrick Groves, Jose M. S. S. Esperanc-a, and Luı´ s Paulo N. Rebelo* Instituto de Tecnologia Quı´mica e Biol ogica, Universidade Nova de Lisboa, Av. da Rep ublica, 2780-157 Oeiras, Portugal Received August 13, 2010; Revised Manuscript Received December 10, 2010
ABSTRACT: The precipitation of barium sulfate (barite) in aqueous solutions of ionic liquids (ILs) has been used as a model system to derive fundamental relationships between crystal nucleation phenomena and some properties of those liquid salts. A systematic dependence between the size of the precipitating crystalline particles and the limiting molar conductivity of ILs has been found. The unraveled correlation has been interpreted in the light of the effect of ILs on water structure dynamics in hydration shells of barite building units and in the bulk solution and respective consequences for crystal nucleation. The response of the crystal formation process to the presence of specific ILs has been used to extract information about some particular properties of ILs (ion association and hydration) that modify the crystal hydration environment.
Introduction Application of additives to affect crystal growth and nucleation is a common approach in crystallization strategies. Nevertheless, the interpretation of specific effects of a given additive on crystal behavior is often a challenging task, especially if the former is a complex organic molecule. The most common explanation is based on the assumption that there exist some specific interactions (e.g., steric fit) between the additive and particular sites at the crystal surface.1,2 Because of such a specificity, the implementation of experimental results can remain restricted to the particular system under study. However, it has been shown that the effect of additives (organic molecules and simple ionic salts) on crystal nucleation and growth can often arise from their influence on the water structure dynamics.3-7 This idea has been known in protein chemistry for a long time and is expressed in the so-called Hofmeister series, whose origin at the molecular level is nonetheless still matter of scientific debate.8,9 To this end, the study of aqueous solutions containing ionic liquids and inorganic salts has proven to reveal unexpected features.10 In general, one should expect that precipitation phenomena and liquidliquid splitting where, for instance, inorganic salts and ionic liquids interact with each other11-15 should share at least a few common features. Therefore, it has been realized that similar approaches can be used to understand the systematic effect of inorganic salts and the influence of organic species on the behavior of inorganic ionic crystals.3-6,16 The current findings suggest that it is possible to selectively affect the kinetics of nucleation and growth of those crystals, change their morphology or tune impurity incorporation by the use of ionic species to modify hydration environment.3,4,6 If similar relationships apply to the solution-solid phase transition of common smallmolecule ionic substances and of organic macromolecules,17 then inorganic crystals can add to our understanding of crystallization of more complex organic systems. Recently, there has been an increasing amount of research concerning the application of ionic liquids (organic salts that are liquid at room temperature) as additives or new reaction *To whom correspondence should be addressed. E-mail: (M.K.)
[email protected]; (L.P.N.R.)
[email protected]. pubs.acs.org/crystal
Published on Web 01/19/2011
media in crystallization processes.18 They have been shown to affect polymorphism,19 template nanoparticles formation,20-23 advance protein crystallization,24-27or improve the quality and purity of molecular crystals.28 Ionic liquids (ILs) have gained special attention from the scientific community, due to the fact that they are considered a perspective “green” alternative to substitute traditional organic solvents,29 and offer a unique capability to fine-tune their physical and chemical properties by appropriate selection of the cation and anion.30 On the one hand, the huge variety of possible combinations of cation and anion composing ILs constitutes an advantage, but on the other hand, it might pose difficulties whenever the resulting properties are not fully known. The aim of this work is to test the hypothesis relating the effect of additives on crystal behavior with water structure dynamics to obtain some insight into the mechanism underlying the influence of ILs on crystal formation in aqueous media. The precipitation of an inorganic ionic crystal (barium sulfate) in IL solutions serves as a model system to derive some fundamental relationships between specific properties of ILs and their effect on crystallization phenomena. Furthermore, the response of the crystal behavior to the presence of a particular IL is used to extract some information about the specific properties of the latter that affect the crystal hydration environment. Experimental Section Ionic Liquids. In this work, we have employed the following ionic liquids: 1-ethyl-3-methylimidazolium chloride, [EMIM]Cl (io-li-tec, > 98%), bromide, [EMIM]Br (Fluka, g 97%), thiocyanate, [EMIM][SCN] (io-li-tec, > 98%), acetate, [EMIM][Ac] (io-li-tec, > 98%), tetracyanoborate, [EMIM][(CN)4B] (Merck, g 99%), ethanesulfonate, [EMIM][C2SO3] (synthesized in our lab), propylsulfonate, [EMIM][C3SO3] (synthesized in our lab), butylsulfonate, [EMIM][C4SO3] (synthesized in our lab), 2-hydroxyethyl-trimethylammonium chloride, [Ch]Cl (Sigma, g 98%), acetate, [Ch][Ac] (Solchemar, 95%), and methanesulfonate, [Ch][C1SO3] (synthesized in our lab). The structures of the cations and anions are presented in Figure 1. The ionic liquids were chosen based both on their water solubility and distinct hydration characteristics of the anions. Therefore, anions of different hydration properties were selected (positively hydrated [Ac]- and negatively hydrated Cl-, Br- and [SCN]-).31 The meaning of these hydration characteristics is further r 2011 American Chemical Society
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Figure 1. Structures of the cations and anions composing the ionic liquids used in this study and corresponding abbreviations. provided in the text. For some anions, their a priori assumed hydration characteristics could merely be inferred from their structures; their actual behavior was later confirmed by the experiments. Prior to their use, all ionic liquids were purified by vacuum evaporation, typically during several hours at 1 Pa and 330 K. Stock aqueous solutions of 0.1 M concentration (0.05 M in the case of [EMIM][(CN)4B] due to its low solubility) were prepared in bidistilled water and further used to make working solutions of desired concentration for both precipitation experiments and conductivity measurements. Conductivity of ILs. Electrical conductivities of aqueous solutions of ILs were measured using a CDM210 conductivity meter from MeterLab. The electrode was calibrated against solutions of KCl. Before measurements, solutions of the studied ILs, in the concentration range between 0.001 and 0.1 M (average concentration increment of 0.002 and 0.01 M for IL concentrations up to and above 0.01 M, respectively), were placed in a thermostatted water bath at 22 °C and left to thermally equilibrate for about 1 h. The conductivity values determined within the specified concentration range were used to calculate limiting (infinite dilution) molar conductivities of ILs by fitting the data to the empirical equation:32,33 Λ ¼ Λ0 - Ac1=2 þ Bc - Cc3=2 þ :::
ð1Þ
where Λ and Λ0 are the molar and limiting molar conductivity, respectively, and c is the electrolyte concentration. Heat Capacity of Ionic Liquid Solutions. Measurements of the heat capacities of ionic liquid aqueous solutions have been performed using a differential scanning calorimeter from TA Instruments (model DSC
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Q200) and using a modulated-temperature scanning differential calorimetry method (MDSC). The method and theory of MDSC have already been presented by numerous publications (ref 34 and references therein). The MDSC experiments have been performed in the 283.15-333.15 K temperature range at a constant heating rate of 2 K 3 min-1, temperature amplitude of (0.5 K, and modulation period of 100 s. Diffusion Coefficient of Water: An NMR Study. Samples of ionic liquids of 0.5 mL in the 0.005-2 M range were prepared in 1:9 H2O/ D2O (99.9% D2O, Cortec), placed in 5 mm NMR tubes (New Era NE-HL5 tubes, Cortec), and analyzed by 1D 1H NMR and 2D DOSY spectroscopy on a 500 MHz NMR spectrometer (Bruker) with the manufacturer’s standard software (Topspin 2.1). A standard 1H pulse sequence (Bruker program: zg) was used to obtain reference 1H NMR spectra of the IL solutions. A 2D DOSY ste-type DOSY experiment with bipolar gradients (standard Bruker program: stebpgp1s) was used with the DOSY parameters Δ = 200 ms and δ = 2 ms. A 5-95% gradient range of the z-gradient amplifier, delivering a maximum 53.5 G/cm power, was defined in the file ‘difflist’. Thirty-two diffusion experiments were obtained with a quadratic spacing of gradient power, that is, the majority of data was collected with low gradient powers during which the H2O signal decays with several higher powered points that define the ILs with slower diffusion coefficients and slower signal decay. The parameters were optimized to simultaneously monitor the diffusion coefficient of the H2O signal and the nonexchangeable protons present in the majority of cations and anions of the ILs in their monomeric states. The data were processed using the standard Bruker DOSY processing software and the diffusion data presented in a 1024 data point window, over a -8.5 to -10.0 log scale. The individual, calculated diffusion coefficients in each column were summed (weight averaged) by the command f1sum over the complete chemical shift range and the average diffusion coefficients of H2O and the IL components were taken from the peak positions in the obtained 1D diffusion trace. Precipitation of Barium Sulfate. The experiments consisted of precipitating barium sulfate from aqueous solutions of ionic liquids (ILs). The precipitation of barium sulfate was induced by mixing two working solutions containing Ba2þ and SO42- ions, respectively. Five milliliters of an aqueous solution containing 0.04 M of Na2SO4 and 0.05 M of ionic liquid was placed in a reaction vessel and subsequently 100 μL of the 0.1 M aqueous solution of BaCl2 was injected into the vessel. Immediately after the nucleation process had been induced by mixing of the working solutions, an aliquot of 1 mL of the mixture was retrieved and diluted with 4 mL of Milli-Q water in order to harvest further particle growth and prevent aggregation. Each experiment was repeated at least in triplicate. Program PHREEQC35 was used to calculate the solution supersaturation index SI = log(IAP/Ksp) (IAP, ion activity product; Ksp, solubility product) with respect to barium sulfate in an electrolyte solution of 0.05 M concentration. To perform the calculation, NaCl was used as a model background electrolyte (as there is no possibility to introduce ILs into the simulation). The predicted supersaturation describes the state of the system under assumption of complete mixing of working solutions. The actual initial supersaturation degree is expected to be higher as a result of nonimmediate mixing of solutions. Particle Size by Dynamic Light Scattering. The size distribution of the precipitated particles in quenched solutions (mixtures diluted with water in a 1:4 proportion) was immediately monitored by dynamic lights scattering (DLS) with the use of Zetasizer Nano device from Malvern Instruments. Herein, the reported size of the barite crystals which were grown in the respective IL solutions is described by the mean diameter of the particles as measured with DLS and is given as an average of the mean diameters from experiments performed at particular working conditions. Precipitate Analysis with Two-Dimensional X-ray Diffractometer. The nanometer-sized precipitated particles have been analyzed using a single crystal diffractometer equipped with CCD. The acquired data have been processed with XRD2DScan, software specially designed for displaying and analyzing two-dimensional diffraction patterns of polycrystalline samples collected using area detectors.36 Crystal Morphology by Scanning Electron Microscopy (SEM). The morphology of the precipitated particles has been examined by
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SEM. The precipitate has been washed abundantly with Milli-Q water, air-dried, and gold-coated prior to SEM analysis.
Results The estimated degree of supersaturation of our experimental solutions with regard to barium sulfate, SI = 4.12, should ensure precipitation by a homogeneous nucleation mechanism.37 The growth of the particles was quenched immediately after the nucleation event had been induced under high supersaturation conditions. Therefore, the resulting size of the precipitated particles is expected to be determined by the characteristics of the nucleation stage: a large number of small nuclei or a low density of larger nuclei generated in the system, corresponding to a high or low nucleation rate, respectively. The precipitate has been confirmed to be crystalline barium sulfate by X-ray diffractometry. SEM analysis of the crystals have shown that there were no morphological differences between the precipitated barite particles at different experimental conditions (different ILs); otherwise, the results of the DLS measurements could have been affected. Representative SEM images are presented in Figure 2. The nucleation characteristics of barite (as designated by crystal size) have been correlated with two properties of IL solutions: conductivity and water diffusivity. The relationship between the mean size of the barite particles nucleated in IL solutions and the limiting molar conductivity (Λ0) of respective ILs in presented in Figure 3. For a given group of ILs (distinguished by characteristics of the anions), the crystal size decreases (nucleation rate increases) with decreasing Λ0. Different trends of this dependence are defined by anions of specific hydration characteristics (the exact meaning of these hydration properties is explained in the Discussion). Figure 4 shows that there is a linear correlation between the size of the barite crystals and the effect of respective ILs on the diffusion of water (expressed by the ratio of water diffusion in IL-free system (D0) to its diffusion in IL solution (D)). The more hampered the water mobility in the system, the smaller the size of the precipitating barite particles, an indication of higher nucleation rates. Figure 4 clearly shows that the [EMIM][SCN] solution is an outlier. Here, the particle size is markedly lower than (and supposedly the nucleation rate is greater than) that expected from the general behavior. The effect of the ILs on water diffusion (D0/D) also expresses their influence on solvent viscosity as resulting from the Stokes-Einstein equation: D ¼ kB T=6πηr
ð2Þ
where D is the diffusion coefficient of a spherical particle (we consider diffusion of water molecules), kB is the Boltzmann’s constant, T is the absolute temperature; r is the radius of the sphere (here an assumption is made that the hydrodynamic radius of water does not depend on the solution’s composition), and η is the viscosity of the solution (dependent on the ionic liquid present in solution). Consequently D0/D ≈ η/η0, where the index “0” designates the IL-free system. Therefore, the observed dependence of the barite nucleation characteristics on water dynamics in IL-containing solutions indicates that the nucleation rate of barite increases (particles size diminishes) with increasing viscosity of the solution. This is also in agreement with the fact that the specific conductivity of different electrolyte solutions at a given concentration is inversely proportional to their viscosity39 and in our experiments, for each particular group of ionic liquids, the nucleation
Figure 2. SEM images presenting the morphology of the precipitated barite crystals in (a) [Ch][Ac] and (b) [EMIM][Ac] solutions. Crystals which were precipitated in the presence of all the other ILs exhibited virtually the same morphological features.
rate of barium sulfate is augmented as conductivity decreases (Figure 3). Discussion To understand the observed correlation between the barite crystal nucleation and the particular characteristics of IL solutions;electrical conductivity and water diffusivity (and solution viscosity);a molecular approach is needed with respect to both, the nucleation event and the phenomena that are expressed in measurable properties of aqueous solutions of ILs. It is worth noting that our data, indicating an increase in the ionic crystal nucleation rate with increasing solvent viscosity, set a counterintuitive relationship because nucleation kinetics is proportional to the diffusion of the building units40 and, according to the Stokes law, the more viscous the solvent the more retarded the mobility (μ) of charged particles:41 μ ¼ ze=πrη
ð3Þ
where ze is the charge of the particle, r is the hydrodynamic radius of the particle, and η is the viscosity of the solvent. The relationship observed in this work is the opposite of that reported for colloidal system crystallization, where nucleation kinetics is inversely proportional to the viscosity due
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Figure 3. Size of the barite crystals precipitated in aqueous solutions of ILs as a function of the limiting molar conductivity (Λ0) of the ILs. The dashed line indicates the size of the particles precipitated in an IL-free system.
Figure 4. Dependence of the size of barite crystals precipitated in IL solutions on the effect of respective ILs on the diffusion of water (D is represented by the diffusion coefficient of water in 1 M solutions of ionic liquids. For [EMIM][C2SO3] and [EMIM][C3SO3], the values of D at 1 M are extrapolated from measurements at lower concentrations). The values of D correspond to 1 M ionic liquid solutions although the precipitation experiments were performed at 0.05 M. This discrepancy results from the fact that, in order to observe representative differences in the effect of ILs on the dynamics of water, NMR experiments have to be performed at sufficiently high solute concentration38 (values of D at the several concentrations which were investigated are provided as Supporting Information).
to the retarded particle motion in viscous solution.42 Thus, the explanation for our results cannot be found within the framework of continuum theories. It needs due consideration of the relation between the diffusion of charged particles and other molecular level phenomena which are responsible for the observed viscosity of a multicomponent aqueous systems. Nucleation in Aqueous Solution: Frequency of Water Exchange. For nucleation to occur, the building units of the crystal (here Ba2þ and SO42- ions) need to diffuse to meet each other in solution, and at least partially dehydrate. At the molecular level, diffusion of ions in aqueous solution is determined by the frequency of water exchange around them (frequency of dehydration).43-47 The water exchange depends on the competition between the tendency of the ion to align water dipoles in its electric field and the tendency of the aqueous solvent to preserve its structure.48 Background ionic species present in solution can affect both ion-solvent and solvent-solvent interactions.49
In an electrolyte solution, the potential energy of a water molecule aligned in an ion electric field (here we consider Ba2þ and SO42-) is lowered by the attractive interaction between the partial charge of the water dipole and the unlike electric field of background ions50,51 (here components of ILs). As a result, the position of a water of hydration of an ion immersed in electrolyte solution is stabilized and the residence time of this water increases in electrolyte solution in comparison to that of pure water.52 The extent of this stabilization depends on the distribution of background ions in solution. If they are close to each other, their electric fields are screened and exert less effect on the water dipoles oriented in hydration shells of the other ions. The other factor that determines dehydration rates of ions, namely, the affinity of water to other solvent molecules, is also modified by background ionic species present in solution. The ions of high charge density (high charge to radius ratio) that bind water strongly in their electric fields (hydrophilic ions) or
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large, weakly charged ions that reinforce water structure due to hydrophobic hydration (cage formation) retard water mobility in their vicinity with respect to water mobility within the bulk hydrogen-bonded network of other waters. Such retardation of water mobility is called “positive hydration”. In contrast, relatively small ions of low charge density break H-bonds of water in their vicinity and increase water mobility with respect to its mobility in the bulk. Such a phenomenon is called “negative hydration”.53-56 The higher the affinity of water to the background ions (the more positive their hydrophilic hydration) or to the other water molecules in solution (e.g., as a result of structure reinforcement by hydrophobic hydration), the higher the competition for water of hydration between bulk solvent (here IL solution) and other ions immersed in solution (crystal building units). This mechanism facilitates the water exchange around the respective ions. For barium sulfate, it has been shown that the kinetics of homogeneous nucleation as well as the two-dimensional nucleation at the crystal surface are limited by the dehydration rates of the crystal building units.3,4,6 On the basis of the measurements of the enthalpy of solution of BaCl2 in different chloride salts, it has been concluded that the frequency of water exchange around Ba2þ can be modified by background electrolytes depending on the distribution of background ions in solution and their hydration characteristics6 (for details, see Supporting Information). Molecular simulations have also shown that the surface nucleation of barium sulfate is limited by the energy barrier of the desolvation of the barium cation.57 This barrier is determined by the energy of striping water molecules from the ion hydration shell and this process is facilitated by the tendency of water to preserve its hydrogen bonding. Piana and collaborators57 have found that the presence of sulfate anions helps the barium cation desolvation. This can be explained by the strong hydration of the sulfate anion and its competition for solvent molecules with Ba2þ. Conductivity and Water Structure Dynamics. Having in mind that the dehydration rate of the crystal building units is the rate-limiting factor for barite nucleation, we will now consider the nature of the conductivity phenomenon at a molecular level in order to understand the observed correlation between barite nucleation rate and Λ0 of ILs. Conductivity of an electrolyte at infinite dilution depends on diffusion of the ions, as expressed by the Nernst-Einstein equation for a symmetric electrolyte, z2 F 2 0 ð4Þ ½D þ þ D0- Λ0 ¼ RT where z is the charge of the ion, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and D0 is the diffusion coefficient of the cation and anion at infinite dilution. Since the diffusion of ions in aqueous solution is determined by water structure dynamics around them,34-38 the Λ0 expresses the effect of those ions on water mobility. The more retarded the water mobility in the vicinity of the ions (a situation which one can find either when there is higher affinity of the ions to the water molecules or when molecules’ hydrophobicity increases), the slower their diffusion. Apart from the intrinsic characteristics of the ions (such as charge density), the diffusion of the ions is affected (retarded) by the presence of counterions that stabilize orientation of water in ion hydration shells.43-45 The extent of this effect depends on the distribution of ions in a particular electrolyte solution. At higher concentration, the tendency of ions to associate results in the formation of
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neutral ion pairs, a phenomenon that obviously reduces the amount of charged species, and therefore also reduces the electrical conductivity. It is interesting to realize that even at close to infinite dilution (where ion pairing is not expected) the conductivity tracks information about the tendency of ions to stay either apart or in a closer vicinity of each other. This is due to the mutual effect of ions on their hydration and thus diffusion in aqueous solution. Conductivity as a Probe of the Effect of ILs on Barite Nucleation. If we now recall that the frequency of water exchange around barite building units (which is the rate-limiting factor for nucleation) is affected by both the affinity of water to other solvent molecules (background ions or other waters) and by the distribution of background ions in solution, the properties that are explicitly expressed by conductivity, then the observed correlation between the rate of barite nucleation and Λ0 becomes understandable. For a given family of ILs, the size of barite particles (diminishing with increasing nucleation rate) becomes smaller with decreasing conductivity, therefore with decreasing water mobility and/or increasing ion association in solution (Figure 2). Retardation of water in the vicinity of other solvent molecules (water or ionic liquid ions) results in a higher competition for water of hydration between the barite building units and the bulk of the solvent (IL solution), thus, helping the dehydration of crystal building units and the nucleation process. The association of ionic liquid ions in solution screens their charges and reduces the electrostatic stabilization of water in hydration shells of ions building the crystal (here Ba2þ and SO42-), therefore aiding dehydration and nucleation events. It has been shown that at low ionic strength (up to about 0.01 M) the effect of background electrolyte on dehydration rates of other ions immersed in an electrolyte solution is determined mainly by electrostatics (ion distribution), while at higher ionic strength (as applied in this work) it is controlled by the hydration properties of the background ions (water mobility).4,6 As a result, in our experiments, we can find a linear correlation between the nucleation rates of barite (as judged by the particle size) and the water structure dynamics in respective solutions of ILs (Figure 4). Nevertheless, there is an exception: the nucleation rate of barite in [EMIM][SCN] aqueous solution is higher than that predicted by the effect of the IL on water diffusion. Our findings suggest that this fact is a consequence of a significant association of [EMIM][SCN] with the formation of a neutral species, a phenomenon that is not reflected in the effect of the IL on water dynamics, but is captured by conductivity. The tendency of [EMIM][SCN] to associate in aqueous solution is confirmed by the fact that [EMIM][SCN] and water mixtures show positive excess molar volumes over the whole concentration range and positive excess molar enthalpies of mixing58 and our data on heat capacity of ILs (see Supporting Information). It appears that the correlation between the kinetics of barite nucleation and the bulk water structure dynamics can be broken by specific interactions between the ions composing the ionic liquid. Yet, the electrical conductivity remains a sensitive probe of the effect of ILs on crystal nucleation rate. Hydration and Association of ILs. The effect of ILs on barite nucleation (denoted by conductivity vs particle size) follows different trends, each of them linked to the hydration characteristics of the IL anions (Figure 3). Our findings suggest that this is due to the distinct tendency of the ions of particular hydration to associate, which results in the nonlinear dependence of Λ0 on water structure dynamics
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around ions. The [SCN]- and halide anions (Cl-, Br-) are negatively hydrated species31 and define one of the trends. Another trend is delineated by the acetate and [(CN)4B]anions. [Ac]- is known to be a positively hydrated anion31 (due to its relatively high charge density). To the best of our knowledge, the hydration characteristics of the tetracyanoborate anion have not yet been established. However, our results suggest that the water dynamics in the vicinity of this ion can be retarded possibly due to its relatively large size and low charge, which can induce a hydrophobic hydration effect.55,59 This conclusion is further supported by data on the heat capacity of [EMIM][(CN)4B] solutions (see Supporting Information).60 The intermediate trend in the effect of ILs on the barite nucleation is defined by the sulfonate anions that can be identified as species of mixed hydration properties. They are composed of hydrophobic aliphatic chain that retard water mobility as a result of cage formation and the -SO3 group that breaks H-bonds of water in its vicinity and increases water mobility.61,62 Similarly hydrated species tend to associate in solution.62-64 Both [EMIM]þ and [Ch]þ have a hydrophobic ethyl (short) chain and a hydrophilic part: the imidazole ring and the hydroxyl group, respectively. Both cations are then expected to retard water mobility in their vicinity and associate preferentially with anions that exert a similar effect on water structure dynamics. This is reflected in the fact that, apart from the relatively small difference in water dynamics, there is a significant drop in conductivity on going from ILs containing negatively hydrated anions that are not supposed to associate with positively hydrated cations to ILs bearing positively hydrated anions that are expected to be more associated with [EMIM]þ and [Ch]þ in solution (Figure 5). Such a conclusion is supported by the results of other studies that show that the association of 1-alkyl-3-methylimidazolium cations with anions decreases with increasing negative hydration of the anion.65,66 The intermediate region of conductivity versus water dynamics (thus, an expected intermediate mutual affinity of the ions) is represented by ILs with anions of mixed hydration characteristics. In Table 1, cations and anions of ionic liquids are classified according to their hydration properties as inferred from our experiments. Hydration Properties of ILs and Crystal Nucleation. In solutions of ILs bearing anions of mixed hydration characteristics, two distinct regimes can be distinguished with respect to the effect of IL on barite nucleation. For the ILs with anions of chain length up to two carbons (C1-C2), the size of the crystal nuclei is larger, and for those of chain length of three and four carbons (C3-C4) smaller, than the size of nuclei formed in ILs bearing positively hydrated anions (Figures 3 and 4). This suggests that in the C1-C2 region, negative hydration of the -SO3 part of the anion prevails, while in the C3-C4 region, the net hydration of the anion is dominated by hydrophobicity of the carbon chain. Such a conclusion is consistent with the fact that water mobility in the vicinity of differently hydrated species decreases in the order: negatively hydrated > hydrophilic > hydrophobic species67,68 (NMR data in this work); therefore, the latter are expected to be the most effective in assisting the nuclei formation from ions affected by their hydration. It is interesting to note that only in ILs containing anions with dominating hydrophobic hydration the size of the precipitating barite particles is smaller than in a pure water system. This suggests that these ILs in fact induce nucleation. Therefore, it can be concluded that the effect of hydrophobic
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Figure 5. Dependence of the limiting molar conductivity (Λ0) on the relative water diffusivity (D0/D) in solutions of ILs of different hydration characteristics (D is represented by the diffusion coefficient of water in 1 M solutions of ILs). Note that the conductivity of [EMIM][SCN] is lower than expected due to its specific association (H-bonding58). Table 1. Hydration Characteristics of ILs Cations and Anions positive hydration [(CN)4B][Ac][EMIM]þ [Ch]þ
mixed hydration negativea [C1SO3][C2SO3]positivea [C3SO3][C4SO3]-
negative hydration [SCN]BrCl-
a For anions of mixed hydration, either negative or positive hydration may prevail (see Hydration Properties of ILs and Crystal Nucleation).
ions on water structure dynamics prevails over their electrostatic influence on crystal building units (increased hydration and charge screening) making the nucleation events more frequent in comparison to IL-free solutions. Conclusions Our model system;precipitation of barium sulfate in ILcontaining solutions;provides further insights into the fundamentals of the nucleation phenomena in aqueous ionic solutions on one hand, and serves as a relatively simple probe of hydration and association characteristics of ionic liquids on the other. Some peculiarities of the behavior of ILs, extracted from our crystallization experiments, were further confirmed by the NMR study and heat capacity measurements as well as by the relationship between the diffusivity of water and the conductivity of the IL solutions. To the best of our knowledge, this is the first study showing a systematic dependence of the size of precipitating crystalline particles on a given measurable property (conductivity) of IL solutions and providing a theoretical explanation for the observed correlation. The revealed relationship defines the effect of specific properties of ILs (hydration and association) on the hydration characteristics of other ionic species present in solution (herein, barite building units). The established correlation should be treated as a model that bears qualitative information and it is important to realize that the response of the system to changes in hydration induced by ILs depends on the properties of the particular system. For instance, it has been shown that for the case of barium sulfate the increased hydration of the ions building the crystal stabilizes them in solution and assists the dissolution process,4,5 while for calcium carbonate, the opposite has been found.69 The reasoning lies on the fact that the more ordered hydration shells of calcite building units induce
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adverse entropy penalties which dominate over the enthalpic contribution. This knowledge can have potential implications, for instance, in the full understanding of liquid-solid phase transitions of macromolecules, which are known to be often entropy driven processes. We have observed that the trend established in this work, relating the hydration and association of ILs, on the one hand, with crystal nucleation, on the other, applies also to the crystallization of a protein (work in progress; manuscript in preparation). The systematic dependence of the crystallization characteristics on particular properties of ILs can be helpful in selecting/designing ionic liquids with a predictable effect on crystal nucleation to be applied in crystallization strategies. On the other hand, response of the precipitation reaction to the particular IL can provide some information about specific properties of IL that can be of interest for other applications. Acknowledgment. The authors thank FCT for grant SFRH/BPD/63554/2009 and the Research Executive Agency for Marie Curie Reintegration grant PERG05-GA-2009249182 and acknowledge the assistance of the Lisbon Small Molecule X-ray Data Collection Service and, in particular, Dr. Isabel Bento. The authors are especially grateful to Dr. Alejandro R. Navarro for providing the software for XRD data analysis and to Dr. Ana B. Pereiro for her assistance in parts of the experimental work. The National NMR Network (REDE/1517/RMN/2005) was supported by POCI 2010 and Fundac-~ ao para a Ci^encia e a Tecnologia. Supporting Information Available: Data on the diffusivity of ILs cations and anions together with the corresponding diffusion coefficient of water obtained by NMR and heat capacity measurements. This material is available free of charge via the Internet at http://pubs.acs.org.
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