Article pubs.acs.org/crystal
Molecular Paradigm Dependent Nucleation in Urea Aqueous Solution Xiaoyan Chen,†,‡ Congting Sun,‡ Sixin Wu,*,† Yingning Yu,‡ and Dongfeng Xue*,‡ †
The Key Laboratory for Special Functional Materials of MOE, Henan University, Kaifeng, Henan 475004, China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
‡
S Supporting Information *
ABSTRACT: During the nucleation process in urea aqueous solution, rigid urea molecules are linked by hydrogen bonding without any changes in molecular symmetry, which makes it difficult to track the nucleation by identifying the symmetric variation of urea in a solution system. In this work, combining in situ Raman and infrared spectroscopy was found to be powerful to observe the fine variations of different groups such as CO, CN, NH2, and OH in urea molecules at the nucleation stage. The dehydration of the hydrated urea and the aggregation between urea molecules were experimentally confirmed in the nucleation. According to the evolution of vibration bands of these typical groups, three states can be clearly distinguished, i.e., hydrated monomers, prenucleation clusters, and crystalline nuclei. In the initial period of nucleation, hydrated urea monomers and clusters coexist in the solution. With an increasing the size of urea aggregations, prenucleation clusters are formed, which then transform into crystalline nuclei within a very short time. During this period, a rapid structural adjustment occurs, which can be identified by the dramatic variations of both wavenumbers and vibration intensity of constituent groups in urea molecules. Finally, crystalline urea nuclei are formed and grow, during which the recorded Raman and attenuated total reflection-infrared spectral signals remain unchanged. Our present work will deepen the understanding of the nucleation of rigid molecules nearly without symmetric variations in a solution system.
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INTRODUCTION Nucleation serves as a critical stage in materials formation, which directly determines their crystallographic structures and attracts much attention in the community.1,2 At the nucleation stage, the system has to overcome a free energy barrier in order for a first-order phase transition.3 In the liquid-to-solid transition, the process occurs between phases with different symmetries, and it is thus inherently a multidimensional process, in which the symmetric broken is involved. This allows researchers to track the nucleation process by identifying the symmetric evolution of constituent groups. In the homogeneous nucleation process, materials components transform from disordered to short-range ordered and final long-range ordered arrangements in different size regimes. These shortrange ordered intermediates as precursors often influence the afterward crystalline nuclei.4 Clarifying the short-range ordered intermediates will facilitate the exploration of nucleation; however, it is still lacking the effective experimental tools in resolving the local structure of the amorphous phase. Urea is ubiquitous in nature and has been widely used as reagents, catalysts, nonlinear optical materials, self-assembly architectures in supramolecular structures, carriers for controlled-release, anion recognition, amination substrate, and structural modification in organic chemistry.5,6 Moreover, urea is also biologically important as an end product of human and animal metabolism of the circulatory system; therefore, it maintains the balance of osmotic pressure in an organism.7 Uncovering the formation mechanism of crystalline urea will © XXXX American Chemical Society
promote the application of urea in materials design and the clarification of its biological role in metabolism. The structural details of urea in both aqueous and crystalline states are initially needed. It is well-known that the urea molecule possesses a planar structure with C2v symmetry and has three coordination sites under an extensive hydrogen bond network in the crystalline phase.8 Aqueous urea solutions exhibit some special features; for example, they have prevented micelle formation, enhanced the solubility of hydrocarbons, and denatured proteins.9,10 Extensively studies on the urea aqueous solution have been carried out on the basis of theoretical simulations and experimental observations, such as molecular dynamics,11,12 neutron scattering, X-ray, NMR spectroscopy, and molecular vibration spectroscopy.13−16 Previous studies indicate that urea molecules alter the water structure by disrupting the hydrogen bond network of water;17,18 however, another view shows no or only a negligible effect of urea on the hydrogen bond of water molecules, and the generation of dimers or higher aggregations between urea molecules by intermolecular hydrogen bonding.19,20 A recent IR study on the urea−water solutions system confirms that urea has no influence on the structure of water, but strong coupling exists in urea−water solutions. 21 Furthermore, Bakker et al. have proven that urea does not change the strength of hydrogen−bond interactions between Received: January 25, 2017 Revised: March 13, 2017 Published: March 31, 2017 A
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were recorded with the solution of 1 cm−1 in the frequency range 200−4000 cm−1. The data were recorded using the NGSLabSpec software. Raman spectra were collected during the experiments to identify the chemical composition and group symmetry in the liquid droplet. In situ ATR-IR spectral experiments were carried out by using an ATR cell with diamond wafer in Nicolet 6700 FT−IR spectrometer at 20 °C. The absorption spectral range is 4000−525 cm−1. A new ATRIR spectrum of urea aqueous solution was collected per 40 s, and successive IR spectra can be obtained by increasing the measurement time until the urea aqueous solution transforms into a crystalline solid completely (Figures S4 and S5). Four typical statuses involve in the measurement process, i.e., aqueous, prenucleation, nucleation, and crystalline states. IR spectroscopy can provide the hydrogen bonding characteristics in the formation of a crystallographic structure framework.
water molecules, but a small fraction of water molecules can be strongly immobilized by urea through intermolecular hydrogen bonds.22 Moreover, urea can substitute for water in the hydrogen-bonded network without breaking the tetrahedral, hydrogen-bonded structure of water.23 The hydrogen bonds between water molecules are even proven to be stronger than urea−water hydrogen bonds, which will enhance the selfaggregation of urea.24 The interaction between urea and water molecules can be used to judge the aggregation of urea in nucleation. Before the formation of crystalline urea in aqueous solution, a change of symmetry is involved for urea aggregations. Molecular vibration spectroscopy serves as an effective approach to identify the molecular symmetry since group theory has been applied to the spectroscopy.25,26 The structure of urea in different states as well as vibrational assignments has been clarified by combining both IR and Raman spectra and the Hartree−Fock method.27,28 It is generally accepted that the vibration strength of water is fairly strong, and it is difficult to distinguish the shape and frequencies of vibration bands of H2O molecules in urea solution, owing to the overlapping region of IR bands of urea and water in IR spectra. Fortunately, Raman spectroscopy has the advantage of weak Raman scattering in water.29 Generally, the symmetry of constituent groups will be varied accompanied by the nucleation, and tracking the symmetric evolution of a constituent group acts as an effective approach to identify the phase transition of clusters or prenucleation clusters.30−32 However, rigid urea molecules are linked by hydrogen bonding without any changes in molecular symmetry in nucleation. This creates difficulty in tracking the nucleation by identifying the symmetric variation of urea in a solution system. In this work, in situ Raman and attenuated total reflection-infrared (ATR-IR) spectroscopy studies of urea aqueous solution were carried out to clarify the nucleation process from a microscopic viewpoint. On the basis of time-dependent molecular vibration spectra, we illustrate the fine variations of CO, CN, NH2, and OH in urea molecules during the nucleation process. Combined with the electronegativity scale and the fine variations of vibration bands for urea molecules, the nucleation approach and the mesoscale structural details in the phase transition have been identified.
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RESULTS AND DISCUSSION In an aqueous solution system, urea (CO(NH2)2) molecules will transform to crystalline nucleus with the P−421m space group once the solution reaches its supersaturated state. It is notable that the urea molecule is rigid, which remains C2v symmetry in the nucleation process. On the basis of the group theory, no obvious exchanges would be shown in molecular vibration spectra. However, variations exist in the Raman vibration bands of urea at the nucleation stage (Figures 1 and
EXPERIMENTAL SECTION
The concentration of saturated urea aqueous solution is about 9.90 mol/L at 20 °C. Urea aqueous solution with a concentration of 6.67 mol/L was prepared at the room temperature of 20 °C. The structural evolution time during urea nucleation is longer for such a solution than that of the solutions with other concentrations. The longer structural evolution time can help us to capture more structural information on the mesoscale structures during nucleation. On the other hand, the temperature should be fixed, since the supersaturation of urea aqueous solution increases with increasing the temperature. That is, the nucleation time of urea is dependent on the temperature, which will influence the number of time-dependent molecular vibration spectra and the structural information deduced form these spectra. In situ Raman spectra of urea solution were recorded using a Jobin-Yvon Horiba T64000 Raman triple grating spectrometer (Horiba Ltd., France) with green line of Ar+ laser (514.5 nm radiation). Before Raman measurements, the frequency was calibrated by the Raman band of silicon at 520 cm−1. A 5 μL urea solution was added on the glass substrate, and the diameter of urea droplets is about 3.5 mm. The laser was then focused in the center of the aqueous droplet, and successive Raman measurements were carried out with the mapping mode per 1 min (Figures S1−S3). The Raman spectra
Figure 1. Raman spectra evolution of NCN and NCO bending vibrations under different hydrogen bonding in the nucleation of urea solution. (A) Time-dependent Raman spectra of NCN and NCO bending vibrations in the nucleation of urea solution. (B) H−O···H− N−C substituted by CO···H−N−C hydrogen bonding during urea nucleation from aqueous solution.
2). As shown in Figure 1, the Raman vibration band at ∼526 cm−1 can be assigned to NCN bending vibration, and the vibration band at ∼590 cm−1 can be assigned to NCO bending vibration.33,34 In the aqueous state, free urea molecule is surrounded by water molecules, and the electrostatic interaction between H2O and urea molecules leads to a more isotropic hydrogen bonding environment. In such a case, both NCN and NCO Raman bands of urea molecules with C2v symmetry can be clearly distinguished in the spectrum. On the basis of Pauling’s electronegativity (EN) scale,35 the EN of O is larger than N; therefore, NCO vibration energy is stronger than B
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weight of CO···H−N−C becomes larger in hydrogen bonding. The Raman band vs(CN) may become more sharp, which can be attributed to the enhanced symmetry of urea in the period of urea nucleation (Figure 2A). Once urea nuclei is formed, a three-dimensional CO···H−N−C hydrogen bonding net will be generated (as schemed by Figure 2E). In this status, CO···H−N−C accounts for the largest proportion in hydrogen bonding (after 10 min), which promotes the shift of vs(CN) toward a higher wavenumber at ∼1008 cm−1 (Figure 2A). Later, with the crystalline urea nuclei grows, its specific surface area gradually decreases, leading to the decreased percentage of H−O···H−N−C, and Ramanactive vs(CN) band further shifts to 1009 cm−1. In order to further support the results deduced from in situ Raman spectroscopy, we also carried out in situ ATR-IR experiments. As shown in Figure 3, IR absorption bands at
Figure 2. Raman spectra evolution of NCN stretching vibration with aggregation between urea molecules during nucleation. (A) Timedependent Raman spectra of NCN stretching vibration in the nucleation of urea solution. (B) Hydrated urea monomer. (C) Hydrated urea dimer. (D) Prenucleation clusters formed by the aggregation of urea molecules. (E) Crystalline nucleus.
that of NCN; i.e., the wavenumber of δ(NCO) is higher than that of δ(NCN). With the volatilization of H2O from the solution, urea molecules begin to aggregate, leading to the breaking of some CO···H−O hydrogen bonding between urea and water, and the formation of CO···H−N−C between urea molecules. Because EN(O) is larger than EN(N), CO··· H−N−C hydrogen bonding is stronger than CO···H−O. Consequently, δ(NCO) bands will shift toward a lower wavenumber. On the other hand, some H−O···H−N−C will be substituted by CO···H−N−C hydrogen bonding. EN(C) is larger than EN(H), and CO···H−N−C hydrogen bonding is weaker than H−O···H−N−C. This will result in the shift of δ(NCN) toward a higher wavenumber. With the proceeding of nucleation, only one Raman vibration band appears at ∼549 cm−1. Such a change in the Raman spectra implies the variation of the chemical environment of the urea molecule. Two Raman modes are observed for the urea molecule with a nonplanar geometry in solution, whereas only one Raman mode indicates the crystalline urea molecule with a strictly planar geometry in the solid state. Figure 1 shows the aggregation between urea molecules by dehydration. In the Raman spectrum of crystalline urea, another dramatic vibration band is vs(CN) (Figure S1).34 We therefore track the variation of the vs(CN) band during the nucleation process of urea in an aqueous solution system. As shown in Figure 2, the vs(CN) Raman band appears at ∼1001 cm−1 in solution status. In the initial period of nucleation (at the time before 2 min), hydrated urea molecules exist in dilute solution, as illustrated by Figure 2B. With increasing the solution concentration, (CO(NH2)2)n clusters are formed by hydrogen bonding between urea molecules (Figure 2C). In such a case, a few H−O···H−N−C hydrogen bonds are substituted by CO··· H−N−C. Weaker CO···H−N−C hydrogen bonding helps to increase the vibration energy of CN group in the urea molecule. When the urea solution is saturated and even supersaturated with prolonging the measurement time to 8 min, the hydrated cluster becomes larger (Figure 2D). The
Figure 3. Time-dependent ATR-IR absorption spectra of urea solution in the wavenumber region of 1300−1800 cm−1. The blue dot lines are drawn to guide the assignment of different IR absorption bands in urea.
∼1460 cm−1 can be assigned to asymmetric stretching vibration of the CN group (vas(CN)).36 With nucleation proceeding in urea aqueous solution, it can be found that the position of vas(CN) shifts toward lower wavenumbers (Figure 4). This is induced by the formation of CO···H−N−C hydrogen bonding between urea molecules in the nucleation stage. Moreover, the ratio of the relative absorption intensity between vas(CN) and δas(NH2) increases from 0.27 to 0.82 from hydrated molecules to crystalline urea (Figure 5A). IR absorption bands at ∼1660 cm−1 can be assigned to stretching vibration of the CO group (v(CO)).37 Once the nucleation clusters are formed, v(CO) becomes more dramatic and shifts from 1659 to 1672 cm−1. This can be attributed to the decreased vibration energy of the CN group (wavenumber decreases from 1461 to 1456 cm−1). The vibration characteristics of both CO and CN groups in urea molecules are consistent with the results obtained from Raman spectra. In the time-dependent Raman spectra, nearly no stretching vibration information on NH2 group in urea aqueous solution was obtained; in situ ATR-IR spectroscopy was therefore used to observe the evolution of stretching vibration bands of NH2. C
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Figure 4. Position of IR absorption bands of CO, NH2, and CN groups in urea molecules at the nucleation stage. According to the evolution CO, NH2, and CN vibration bands, three states can be clearly distinguished, and the black dashed lines are drawn to guide the partition of hydrated monomers, prenucleation clusters, and crystalline nuclei.
Figure 6. ATR-IR spectra evolution of the NH2 group in urea under the breakage of H−O···H−N−C and the formation of CO···H−N− C hydrogen bonding. (A) Time-dependent ATR-IR absorption spectra of NH2 group during urea nucleation. The blue dot lines are drawn to guide the assignment of different IR absorption bands in urea. (B) Transformation from H−O···H−N−C to CO···H−N−C hydrogen bonding during the nucleation process in urea aqueous solution. (C) Transformation from CO···H−O to CO···H−N−C hydrogen bonding during the nucleation process in urea aqueous solution.
vas(NH2) and vs(NH2) absorption bands shift toward lower wavenumbers. As shown in Figure 7, there are two variation
Figure 7. Time-dependent position of IR absorption bands of CO, NH2, and CN groups in the urea molecule at the nucleation stage. The black dashed lines are drawn to guide the partition of hydrated monomers, prenucleation clusters, and crystalline nuclei. Figure 5. Ratio of IR absorption intensity between NH2 and CN groups in urea molecules at the nucleation stage. (A) Relative intensity ratio between vas(CN) and δas(NH2). (B) Relative intensity ratio between δas(NH2) and δs(NH2). The black dashed lines are drawn to guide the partition of hydrated monomers, prenucleation clusters, and crystalline nuclei.
trends for stretching vibrations of the NH2 group. This can be used to point whether urea prenucleus are formed. In aqueous status, NH2 groups are bonded with H2O molecules to form hydrated urea. With the aggregation between urea molecules, H−O···H−N−C hydrogen bonding is broken, while CO··· H−N−C hydrogen bonding is formed, resulting the increased vibration energy of NH2 group. Accompanied by this process, both vas(NH2) and vs(NH2) bands shift toward lower wavenumbers. Moreover, the decrease degree of vas(NH2) is larger than that of vs(NH2). Once urea solution enters in prenucleation stage, the variation trend is different from that in
As shown in Figure 6, the time-dependent ATR-IR spectra of urea solution can provide obvious structural information about the NH2 group. In the aqueous state, IR bands at ∼3486 and 3352 cm−1 can be assigned to asymmetric and symmetric stretching vibration bands of NH2 groups in urea, i.e., vas(NH2) and vs(NH2).37 With the proceeding of nucleation, both D
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The ATR-IR absorption band at ∼3226 cm−1 can be assigned to the vibration of OH groups, i.e., v(OH) (Figure 6). In the dilute solution, v(OH) mainly originates from H2O, hydrogen bonding between H2O molecules, and hydrogen bonding between urea and H2O. With the solution concentrating, the amount of H2O and hydrogen bonding between H2O molecule decreases, while the amount of H−O···H−N−C and CO··· H−N−C hydrogen bonding increases. Since EN(O) is larger than EN(N), H−O bonding is stronger than H−N bonding, and H−O···H−O is weaker than H−O···H−N. Under the greater interaction of H−O···H−N, OH vibration energy in H2O becomes weaker, and v(OH) will shift toward a lower wavenumber. Figure 7 shows the shift of the position of v(OH) from 3226 to 3220 cm−1 before constructing the prenucleation clusters. Once prenucleation clusters are formed, the amount of CO···H−N−C hydrogen bonding increases, whereas the amount of other hydrogen bonding decreases. In the transformation from the prenucleation cluster to the crystalline nucleus, hydrogen bonding guides urea crystallized in a tetragonal system with space group of P−421m. This requires two types of CO···H−N−C hydrogen bonding between urea molecules: one links urea molecules along coaxial direction, and the other one links urea molecules in the same crystallographic a−b plane (Figure S6). These two types of hydrogen bonding guide the arrangement of urea molecules. Consequently, two v(OH) bands appear in the IR spectra after the formation of urea prenucleation clusters. Moreover, Figure 8A shows that the IR absorption intensities of v(OH) and vas(NH2) decrease from 1.28 to 0.68 during the whole nucleation process. This may be attributed to the continuously decreased H2O amount in the urea solution system.
solution. This can be attributed to the rapid structural adjustment. In this stage, it can be found that the vas(NH2) shift toward lower wavenumbers more dramatically, while the position of vs(NH2) remains nearly unchanged. During the nucleation process, the relative absorption intensity between vas(NH2) and vs(NH2) increases from 0.70 to 1.03, i.e., increases 0.33 (Figure 8B). Moreover, the IR absorption
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CONCLUSION
In this work, in situ Raman and ATR-IR spectroscopy were carried out to clarify the nucleation process of urea aqueous solution from the microscopic viewpoint. On the basis of timedependent molecular vibration spectra, we illustrate the fine variations of CO, CN, NH2, and OH in urea molecules during the nucleation process. Combined with the electronegativity scale and the fine variations of vibration bands for urea molecules, it can be deduced that the broken of H−O··· H−N−C hydrogen bonding and the formation of CO···H− N−C hydrogen bonding promote the generation of crystalline urea nuclei. The whole process can be divided into three stages. The first stage is the transformation from hydrated urea molecules to hydrated urea aggregations; the vibration bands of CN, NH2, and OH groups shift toward lower wavenumbers. In the second stage, prenucleation clusters are formed, and then they transform into crystalline nuclei in a short time. During this period, rapid structural adjustment occurs, which can be identified by the dramatic variations of both wavenumbers and vibration intensity of CO, CN, NH2, and OH groups in urea molecules. In the last stage, the crystalline urea nuclei are formed and grow, and there are nearly no changes in molecular vibration spectra. Our present work will deepen the understanding of the nucleation of rigid molecules without symmetric variation in a solution system and promote the application of urea in materials design as well as the clarification of its biological role in metabolism.
Figure 8. Ratio of IR absorption intensity between NH2 and OH groups in the urea molecule at the nucleation stage. (A) Relative intensity ratio between v(OH) and vas(NH2) in the urea molecule. (B) Relative intensity ratio between vas(NH2) and vs(NH2) in the urea molecule. The black dashed lines are drawn to guide the partition of hydrated monomers, prenucleation clusters, and crystalline nuclei.
bands at ∼1624 and ∼1600 cm−1 respectively belong to the asymmetric and symmetric deformation vibration modes of −NH2 in urea molecule, i.e., δas(NH2) and δs(NH2), bending δ(NH2) (Figure 3). Similarly, Figure 4 shows that both δas(NH2) and δs(NH2) bands shift toward lower wavenumbers with the proceeding of nucleation in urea solution. This can be attributed to the formation of CO···H−N−C hydrogen bonding, which is induced by the aggregation of urea molecules. Moreover, the ratio between the IR absorption intensities of δas(NH2) and δs(NH2) decreases from 1.18 to 0.82 from hydrated molecules to crystalline urea (Figure 5B), that is, decreasing 0.36, which is in balance with that of NH2 stretching vibrations. It can also be divided into three stages, solution state, prenucleation state, and crystalline state. For the urea solution with the concentration of 6.66 M, before 800 s, urea molecules exist in the solution in the form of hydrated molecule and aggregation states. In the time from 800 to 920 s, prenucleation clusters transform into crystalline nucleus in the urea solution system. After 920 s, urea nuclei grow and become pure crystalline urea without H2O at 1120 s. E
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00128. Additional Figures S1−S6 depicting experimental results (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(D.X.) E-mail:
[email protected]. *(S.W.) E-mail:
[email protected]. ORCID
Congting Sun: 0000-0002-6949-6417 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant Nos. 51125009, 91434118, and 21401185), National Natural Science Foundation for Creative Research Group (Grant No. 21521092), Hundred Talents Program of Chinese Academy of Sciences, and Jilin Province Science and Technology Development Project (Grant Nos. 20170101092JC and 20160520006JH) is acknowledged.
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