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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Molecular Simulation Studies on the Growth Process and Properties of Ammonium Dinitramide Crystal Xinjian Chen, Lichao He, Xiangrong Li, Zhiyong Zhou, and Zhongqi Ren J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00120 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry

Molecular Simulation Studies on the Growth Process and Properties of Ammonium Dinitramide Crystal Xinjian Chen, Lichao He, Xiangrong Li, Zhiyong Zhou*, and Zhongqi Ren* College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

ABSTRACT ： Ammonium dinitramide (ADN) is an energetic material and an oxidizer. It is a relatively new environmentally friendly oxidizer component of propellant without halogens and carbon elements. However, ADN has high hygroscopicity property when exposed to high humidity air, restricting its military applications. In this study, the important crystal faces and crystal habit of ADN in vacuum were predicted. According to the prediction results of ADN crystal habit, the hygroscopicities and sensitivities of ADN crystal faces were investigated and compared to determine total ADN crystal properties. Using the grand canonical Monte Carlo method, the adsorption capacities of water molecules on ADN crystal surfaces were predicted to determine which surface of ADN crystal attracts moisture in air. Solubility parameter of ADN crystal was calculated, and interaction type was investigated to explain ADN dissolution characteristic. Furthermore, solubility parameter of 1,3,5,7Tetranitro-1,3,5,7-tetraazacyclooctane (HMX) was also calculated and compared with experimental values to verify accuracy of calculation method. Before the calculations of crystal properties and crystal growth were performed, density, crystal parameters and distribution of bond lengths were calculated and analyzed to obtained precise calculation parameters. In the study of ADN crystal growth process, calculation models

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were constructed to investigate the impurity effect, temperature effect, and solvent effect on crystal growth, and molecular dynamics simulations were performed. Based on Burton-Cabrera-Frank spiral growth theory, the adsorption energies of ammonium dinitramide on flat and stepped surfaces were calculated to investigate crystal growth from the viewpoint of thermodynamics. INTRODUCTION Ammonium dinitramide (ADN, NH4NO2NNO2) is a recently discovered energetic compound. ADN is an ionic compound containing an ammonium cation (NH4+) and a dinitramide anion [(NO2NNO2)−]. Its crystal structure was determined by Gilardi et al. and can be found in the Cambridge Crystallographic Database (CCDC 854801).1 ADN has high performance characteristics and good thermal stability compared with other oxidizer components of propellant, such as ammonium perchlorate. As an energetic material with high oxygen balance, ADN is being increasingly used as oxidizer component of propellant, such as solid rocket and military propellants, and it has attracted much attention in recent years. For energetic materials, in addition to requiring high performance and low sensitivity, researchers need to take the hygroscopicity into consideration when evaluating the explosive performance. Hygroscopicity is the ability of a solid to adsorb water from air in the external environment. Controlling the environmental humidity is an important issue in energetic material formulation and manufacturing owing to the possibility of deliquescence under high humidity conditions. Severe hygroscopicity performance leads to the need for strict storage conditions under low humidity, which


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limits the range of applications. For example, because of its severe hygroscopicity performance, ADN deliquesces and does not work when exposed to high humidity air (relative humidity 50%) for 10 h. The hygroscopicity of ADN limits its applications as an oxidizer component of propellant.2–5 Therefore, it is necessary to investigate why ADN has severe hygroscopicity and which face(s) of the ADN crystal possess strong hygroscopicity properties. In 2013, Wang et al.6 investigated the structure and hygroscopicity of ADN by quantum calculations and molecular simulations. They found that the interaction between ADN and water molecules is dominated by electrostatic and orbital interactions, and among the (100), (010), and (001) surface, the (001) surface of ADN interacts the most with H2O. This is important because knowing the hygroscopicity resistant surfaces means that the proportions of exposed hygroscopicity resistant surfaces can be increased by controlling ADN crystal growth to improve ADN hygroscopicity performance. Therefore, Prilling and recrystallization in different solvents as common methods can reduce the adsorption capacity of water in the air. In 2015, Lan et al.7 use a combined experimental and simulation approach to investigate the crystal morphology of ADN. They found that ADN crystals are mainly composed of the (010), (011), (110), (100), (121), and (031) crystal faces and solvent molecules play an important role in determining the crystal morphology in the process of crystallization. Furthermore, they explained crystal morphology of ADN and calculated interaction between solvents and ADN molecules using Dreiding force field. In recent years, many crystal morphologies have been predicted by the Bravais–Friedel–



Donnay–Harker (BFDH) model and crystal growth method.8–12 Investigation of ADN crystal growth is challenging and novel, because differing from most energetic materials, ADN is an ionic energetic compound containing two ions, and a large number of papers have reported a series of molecules as growth unit.13– 17

The anion and cation in ADN crystal determine the crystal properties and crystal

growth. The experimental phenomena were predicted, and some experimental results verify the simulation results. This study is to determine ADN crystal properties, including impact sensitivity and hygroscopicity. Dissolution characteristics of ADN is explained by computational solubility parameters. ADN crystal habit were investigated, and the important crystal surfaces were determined. To perform better experiments in the future, ADN crystal growth process and suitable ADN growth conditions were investigated by simulations. This work will provide substantial insight into ADN hygroscopicity and crystal growth process prior to experimental work. All of the calculations were performed with Morphology, Sorption, Dmol3, Forcite, Amorphous cell modules in Materials Studio 8.0 (Accelrys Inc., San Diego, USA). COMPUTATIONAL METHODS AND DETAILS Validation of calculation parameters. Since density is an indicator of atomsatoms interactions between the same or different molecules, the comparison of calculated and experimental values of density is an effective way to obtain a suitable force field and charge calculation method.18 Dreiding, Universal, COMPASS, Cvff and pcff force fields were the objects of our selection. Comparison of experimental density


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of ADN crystal and calculated densities of 50 ADN molecule clusters by various force fields is listed in Table 1. It can be seen that the computational results obtained by Dreiding force field are very close to experimental density with −0.28% relative error, indicating that Dreiding force field is suitable for simulation of ADN. Dreiding force field is a generic force field and is useful for predicting dynamics of organic, biological, and main group inorganic molecules.19 In order to verify the accuracy of calculation parameters, Dreiding force field and Gasteiger charge calculation method were used to optimize experimental crystal parameters of ADN. Table 2 shows the comparison of experimental and calculated values of ADN crystal parameters and bond lengths. Because of structure symmetry, ADN bonds can be classified as three types, such as NN bond, N-O bond and N-H bond. It can be seen that the calculated values of crystal parameters and bond lengths are close to experimental values in Dreiding force field. The distribution analyses of calculated bond lengths of ADN are shown in Figures 1 and 2. The model which contains many ADN molecules (Figure 3) was used to analyze the bond length. The special force field was used to optimize the crystal structure for calculating crystal parameters of ADN. In conclusion, computational densities, bond lengths and crystal parameters of ADN were compared with experimental values, proving that the computational parameters were available in this work. ADN Crystal Morphology Prediction in Vacuum. The Morphology module in Materials Studio was used to investigate ADN crystal morphology in vacuum. In the crystal morphology prediction, the BFDH growth morphology method was used to predict ADN crystal morphology.20–22 The BFDH method assumes that the dhkl spacing



is inversely proportional to the perpendicular distance from the center of the crystal to the corresponding face, which can be considered to be a measure of the relative growth rate for simulation of the crystal morphology.23 In the BFDH morphology prediction, the minimum dhkl was set to 1.30 Å and the maximum h/k/l values were set to 3/3/3. The maximum number was set to 200. Another crystal growth morphology prediction method is the attachment energy (AE) method.24–26 This model assumes that the growth rate of a crystal surface R is proportional to Eatt. The growth morphology was calculated by the Morphology module using Dreiding force field and current charges at fine quality because of unstable surface with large charges in the calculation. Water Sorption Capacities of ADN Crystal Surfaces. Ito27 simulated adsorption of water molecules to crystal facets and found that water is more capable of adsorbing to sodium valproate than L-arginine valproate by comparing the adsorption energy per site. Xiong et al.28 investigated the methane adsorption behavior in slit-like chlorite nanopores using the GCMC method, and they discussed the influences of the pore size, temperature, water, and composition on methane adsorption to chlorite. Chen et al.29 synthesized a –SO3–H-modified mesoporous silica adsorbent with water sorption capacity and fast desorption kinetics and investigated the adsorbent by a combined experimental and numerical approach. Using the insight from these studies, we first calculated the water-saturated adsorption capacities of the important ADN surfaces. Adsorption of water to ADN is a physical surface adsorption process. Surface adsorption process models were constructed to investigate water adsorption. From the viewpoint of thermodynamics, an equilibrium state will exist where the material and


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adsorbed water reach a saturation state. In 1961, Gibbs30 defined the “adsorbed phase” as the actual amount of gas minus the amount of gas which would be presented in the same space at the prevailing bulk density of the gas to avoid the question: how close to the surface must a molecule be in order to be classified as adsorbed. In 2001, Talu and Myers31 studied molecular simulation of adsorption by Gibbs dividing surface. The calculation method is as follows:

nex  nab -gVa

(1)

where quantity 𝑛ex is the saturated adsorption capacity, quantity 𝑛ab is called as the absolute adsorption capacity, 𝜌g is the density of the equilibrium gas phase determined by independent measurements of its equation of state, and 𝑉a is the adsorbed phase volume. The experimental water adsorption rate of the sample at a specific temperature, humidity and moisture adsorption time is given by Eq (2). W

m1  m 0 100% m

(2)

where m1 is the mass of the sample and sample bottle after constant temperature *adsorption, m0 is the mass of the sample and sample bottle before constant temperature adsorption, and m is the mass of the sample. First, a 2 × 2 × 2 supercell of ADN crystal was constructed. The supercell was cleaved with 1.0 factional thickness to create the required important surfaces. To avoid the additional free boundary effect, a 10 Å vacuum slab and repetitive unit were added. After testing many force fields and charge calculation methods, the Dreiding force field and Gasteiger charge calculation method were the most suitable for the adsorbed



water calculations at the specified temperature and relative humidity. The amount of adsorbed H2O was obtained when the potential energy in the simulation was stable. According to Eqs. (1) and (2), the calculated saturated adsorption amount was converted to the mass of each substance, and the mass of adsorbed water is defined as the saturated water adsorption rate. The (100), (010), and (001) faces are common surfaces in all crystal structures. The important faces of ADN crystal are (100), (020), (011), (110), and (11−1). The Sorption module in Materials Studio software was used to calculate the different crystal surface sorption capacities of ADN at 308.15K, 0.1 MPa, and 50% relative humidity. The temperature was set to 308.15 K, the vapor fugacity was set to 2.813 kPa, and the saturated vapor pressure of water was set to 5.626 kPa. Because of the low pressure system, the fugacity approximates the pressure in air. The ensemble was μVT, where the chemical potential μ, volume V, and temperature T are constant in the simulation process. Since the final state of water molecules adsorption should be a thermodynamic gas-solid equilibrium state, the chemical potential μ should be a constant in equilibrium state. Furthermore, the ensemble of μVT was used to calculate how many water molecules were adsorbed on the surface of ADN crystal. Relationship between Surface and Impact Sensitivity of ADN. The anisotropic impact sensitivities of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) for the different surfaces were calculated to explore the relationship between surface and impact sensitivity.32 Different surfaces have important influences on impact sensitivity. The band gap is the difference between the lowest point of the conduction band and the


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highest point of the valence band. A larger band gap leads to a higher impact sensitivity and a higher impact sensitivity implies a smaller h50. Based on density functional theory, band gaps of ADN anisotropic surfaces were studied by constructing surface model to determine which crystal surface of ADN is with the lowest impact sensitivity. The five important faces of the ADN crystal structure were cleaved, and a vacuum slab of 15 Å was added. The calculations were performed with the DMol3 module in Materials Studio with the task set to energy, the quality set to medium, the core treatment set to all electrons, the DND 3.5 basis set, and the GGA- Beeke, Lee-YangParr (BLYP) functional set. After the performances of some functions were checked, the BLYP hybrid function is available to calculate electronic properties.33-35 The calculation task was performed with the band structure. The arrangements of ADN molecules are different for the different surfaces (Figure 4). ADN Solubility Parameter and Dissolution Characteristics. The solubility parameter is a measure of the interaction between molecules. The solubility characteristics of substances in different solvents can be predicted according to the solubility parameters. In this work, Solubility parameters of ADN and HMX were calculated. The solubility parameter 𝛿 is defined as E   =  cohesive   V 

0.5

(3)

where V is the molar volume and Ecohesive is the cohesive energy. The cohesive energy consists of two parts: electrostatic interaction energy and van der Waals interaction energy. The cohesive energy is equal to the electrostatic interaction energy plus the van der Waals interaction energy:



Ecohesive =Eelectrostatic  Evdw

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(4)

where 𝐸electrostatic and 𝐸vdw are the electrostatic energy and van der Waals energy, respectively. The potential, electrostatic, van der Waals, and hydrogen bonding energy components were obtained from the simulation results file. Using the Amorphous Cell tool, 50 ADN molecules were added to the amorphous cell. After geometry optimization, a molecular dynamics (MD) simulation was performed using the NPT ensemble (constant number of particles, pressure, and temperature). The task was set to the Forcite module cohesive energy density task. MD simulations with the NPT and NVT (constant number of particles, volume, and temperature) ensembles were used for the density and solubility parameter estimation at 298 K and 0.1 MPa. The total simulation time was 500 ps, and the final 100 ps was analyzed. After validation of calculation parameters, Dreiding force field and Gasteiger charge calculation method were selected. The cutoff distance, spline width, and buffer width of the atom-based method were set to 18.5, 1.0, and 0.5 Å, respectively. The final 1000 structures were selected to calculate the solubility parameter. ADN Crystal Growth Process. There are many factors that affect the crystal growth process, such as the temperature, impurity effect, and solvent effect. MD simulations were performed to investigate the factors that affect crystal growth. Taking ADN (100) surface as an example, a model was constructed. The process for constructing the model is shown in Figure 5. The structure of the growth face was constructed by cleaving ADN crystal along the indices and the normal to the growth face was aligned along the z direction. After ADN structure optimization, MD


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simulations was performed. To investigate the effect of temperature on crystal growth, isopropanol was selected as the solvent and the temperature was set to 288, 298, 308, 318, 328, and 338 K. To investigate the impurity effect, methanol was used as the solvent and ammonium nitrate, which is a common impurity in ADN preparation process, was added to investigate whether it affects crystal growth. Many solvents were selected to investigate the solvent effect on ADN crystal growth. The diffusion coefficient D is a measurement of crystal growth rate: 2 1 d n D  lim  ri  t   ri  0  t  6 dt i 1

(5)

where ri(t) is the position vector of the ith particle and the angular brackets denote an ensemble average. From the viewpoint of thermodynamics, the adsorption energy of the dinitramide anion determines the rate of crystal growth. For a single molecule, the interaction energy is the adsorption energy:

Eint =Etotal -Esurface -Emolecule

(6)

where Eint is the interaction energy between the molecule and the surface and Etotal, Esurface, and Emolecule are the total potential energy of the simulation system, surface potential energy of ADN crystal surface, and the potential energy of the dinitramide anion, respectively. RESULTS AND DISCUSSION ADN Crystal Habit Prediction in Vacuum. Experimentally prepared ADN crystals usually have needle-like or lamellate or polyhedral structures,7 which is determined by the added conditions of solvents or additives, which is called solvent



effect. In our work, to determine ADN crystal important surface, the BFDH and growth morphology methods were used to investigate ADN morphology in vacuum. According to the calculated results (Tables 3 and 4), ADN crystal has a polyhedral shape (Figure 6). There are five important crystal faces of ADN crystal: (100), (020), (110), (011), and (11−1). According to the crystal habit prediction in vacuum, the crystal structure was cleaved to investigate the crystal surface properties, such as the hygroscopicity and impact sensitivity. Adsorption Capacity of water molecule on ADN Crystal Surface. Based on the prediction results, the important ADN planes are (100), (020), (110), (011), and (11−1). The hygroscopicity values of these crystal faces were calculated. Eight models were constructed to investigate the relationship between the crystal face and the hygroscopicity (Figure 7). The adsorption capacities of water molecules on crystal surface are given in Table 5. The density of air was estimated to be 1.13 × 10−3 g/cm3 according to an empirical equation. Using Eqs. (1) and (2), from the number of adsorbed water molecules, mass of ADN molecule, and space volume, the moisture adsorption rates were calculated to analyze the surface hygroscopicity performance. From the results, the (100) and (020) surfaces with saturated moisture adsorption rates of 14.56% and 11.82% are hygroscopicity resistant surfaces. The crystal properties always are the sum of crystal surface properties. Therefore, according to our simulation results, increasing the facet areas of (100) and (020) will reduce the adsorption capacity of water in the air. In addition to the important ADN crystal faces, the low-index crystal faces, such


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as (100), (010), and (001), always occupy a large amount of total crystal surface. Therefore, the adsorption capacities of water molecules on (100), (010), and (001) surfaces were calculated to approximately predict the crystal hygroscopicity. From the results listed in Table 5, the (010) crystal face has a small number of adsorbed water molecules, with 31 water molecules adsorbed on the crystal surface. Therefore, the (010) crystal face is a hygroscopicity resistant face compared with the (100) and (001) faces. At 308.15 K and 50% relative humidity, ADN saturated water adsorption rate should be in the range 6.82–22.30%. Three factors affect the hygroscopicity results. First factor is the calculation parameters. Hygroscopicity results vary widely when hygroscopicity is calculated in different force fields. Suitable force field can be a good description of ADN system. The second factor is model. The depth of ADN crystal surface, quantity of ADN molecules and vacuum slab are uncertain at the beginning of calculation. It is found that different depths of ADN crystal surface and vacuum slabs will lead to different hygroscopicity results owing to different size scales. Therefore, constructing an appropriate model is very important for studying hygroscopicity. Thus, after experimental hygroscopicity data were obtained, appropriate model was constructed to investigate which face was with high or low hygroscopic properties. Third factor is moisture adsorption mechanism. The hygroscopic mechanism of ADN is from surface adsorption into liquid membrane diffusion under high humidity and temperature conditions. The clusters of water molecules on the surface of ADN can form a water membrane. Thus, high temperature increases ADN moisture transmission rate and



hygroscopicity of ADN, revealing high sensitivity of ADN on temperature and humidity. In the GCMC simulations, water molecules prefer to be close to the ammonium cations of ADN molecules. This means that the ammonium cation is a hygroscopic group and can easily adsorb H2O molecules. According to the simulation results, ADN hygroscopicity mechanism involves water molecules attaching to the ammonium cations on the surface of ADN. Relationship Between Surface and Impact Sensitivity of ADN. The band gap structures of the different surfaces of the crystal are given in Table 6. Enlarging the band gap will increase the impact sensitivity of the crystal face. The differences in the band gaps of the important ADN surfaces are small and the differences in the sensitivities are also small. The order of the sensitivities of the important surfaces is (020) ≈ (110) > (11−1) > (100) > (011). For decreasing the sensitivity of ADN or pursuing a high h50 as a starting point, the proportions of the (011), (100), and (11−1) crystal faces should be increased, and the proportions of (020), (110) should be decreased. ADN Solubility Parameter and Dissolution Characteristics. The above results show that Dreiding force field with the Gasteiger charge calculation method can well describe ADN system. HMX has been used as the subject in many MD simulations.9 Thus COMPASS Force field is available for HMX system. According to the suitable calculation scheme, the solubility parameters of ADN and HMX were calculated (Table 7). The calculated solubility parameter of HMX is very close to experimental value


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with −0.74% relative error. For ADN system, the experimental solubility parameter has not been reported, and the solubility parameter was calculated for the first time. The electrostatic solubility is a large component of the solubility parameter, which means that very large electrostatic interactions occur in ADN system. The non-bonding interaction energy is −19316.24 kcal/mol, the electrostatic interaction energy is -18912.1kcal/mol, and hydrogen bonding interaction energy is −836.86 kcal/mol, and the van der Waals energy is 404.14 kcal/mol (Figure 8). The electrostatic interaction energy includes the hydrogen bond energy. The order of nonbonding interactions is electrostatic interaction energy > van der Waals interaction energy. ADN is an ionic compound and contains ionic bonds. Hydrogen bonding interactions involve electrostatic interaction of X–H to strongly polar atoms, such as oxygen and sulfur, so the electrostatic energy is the main part of the non-bonding energy. According to the principles of similarity and compatibility, ADN crystal has good solubility in solvents where hydrogen bonds form, such as water and methanol. In contrast, ADN crystal has poor solubility in solvents without oxygen and sulfur elements, such as benzene and carbon tetrachloride. According to the results, ADN hygroscopicity is mainly for the electrostatic force between the ammonium ions and oxygen atoms of water in air. The ammonium ion has a strong hydrogen bonding interaction with the dinitramide ion (Figure 3). The blue lines show the hydrogen bonding interactions. The hydrogen atoms of the ammonium ion interact with the oxygen atoms of water to form hydrogen bonds. Simulation of ADN Crystal Growth Process. Crystal growth is a complicated



process and it is influenced by many factors. The ammonium cation is the growth site in the crystal growth process simulation, as shown in Figure 9. The dinitramide anion near the solute molecules gradually moves close to the ammonium cation in the simulation and adsorbs on the crystal face. According to the diffusion coefficient of dinitramide anion on the crystal face, the growth rate of ADN crystal was predicted from the viewpoint of kinetics. Based on Burton-Cabrera-Frank spiral growth theory,37crystal growth occurs by formation of a small monolayer-thick island with molecules occupying adjacent lattice sites that is of sufficient size for it to be energetically favorable to add new molecules to the edge of the island and thus grow to a complete new layer on the crystal. Two models of the perfect face and the face with a step were constructed to investigate the crystal growth process. The dinitramide anion adsorption energy was calculated for the two models to investigate the rate of crystal growth from the viewpoint of thermodynamics and the results are listed in Table 8. According to thermodynamic analysis of the crystal interface, the higher the adsorption energy of the crystal growth unit, the faster the crystal growth rate. The adsorption energies of the smooth sites on the five crystal planes are higher than those of the kink sites, which indicates that the growth rate of the crystal is controlled by the kink position. For the flat crystal surfaces, the crystal growth rates are in the order (100) > (110) > (020) ≈ (11−1) > (011). For the stepped crystal surfaces, the crystal growth rates are in the order (100) > (110) ≈ (020) > (11−1) > (011). By comparing the growth rates of the flat and stepped crystal surfaces, the relationships between the growth rate of the crystal plane at the kink site and the growth rate at the flat site are essentially the


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same. Ammonium nitrate is one of common impurities in the preparation of ADN. To investigate the effect of impurities on ADN crystal growth, a small amount of ammonium nitrate was added to the crystal growth model. The mean square displacement (MSD) of the dinitramide ion was investigated in methanol solvent (Figure 10). As everyone known, the quantity of ammonium nitrate increases in the preparation, which is harmful to total performance. However, ammonium nitrate increases the diffusion coefficient of ADN crystal surface, accelerating ADN crystal growth. ADN crystal growth process was investigated by MD simulation. In ADN crystal, the ammonium cation is a growth site and an adsorption site. The solvent and dinitramide anion tend to attach to the ammonium cation, and the ammonium cation breaks away from the solvent on the surface of ADN crystal. The relationship between temperature and MSD is shown in Figure 11. The diffusion coefficients of the dinitramide anion on the crystal surfaces were calculated. With increasing temperature, the ratio between the MSD and time increases, indicating that the diffusion coefficient increases. Generally, since the number of crystal nucleus formed in high temperature will be larger than that formed in low temperature during crystal growth process, the obtained crystal particle size will be small when crystal growth is processed in high temperature. Therefore, a suitable high temperature is beneficial for ADN crystal growth. Solvent effect is also an important factor on crystal growth. At 298 K temperature,



different solvents were added to the simulation systems to investigate the solvent effect on crystal growth from the viewpoint of dynamics (Figure 12). The MSD comparison order is as follows: acetone > ethanol > DMSO ≈ 1-butanol ≈ acetonitrile > butyrolactone > ethyl acetate. From the simulations, for the same degree of supersaturation, ADN crystal grows faster in acetone than in the other solvents. CONCLUSIONS Crystal growth and the properties of ADN have been investigated by molecular simulations. The ADN crystal habit was predicted by the BFDH and growth morphology method. ADN has five important crystal faces: (100), (020), (011), (110), and (11−1). The five important crystal surfaces were investigated in terms of their impact sensitivity and hygroscopicity by constructing surface models. The adsorption capacities of water molecules on ADN crystal surfaces were calculated for the first time. Furthermore, from the simulation results, ADN has two important hygroscopicity resistant faces: (100) and (020). As for impact sensitivity of ADN faces, the (020) and (110) crystal faces possess high impact sensitivity. ADN system was simulated to investigate the non-bonding interactions and ADN dissolution characteristics. The electrostatic interaction energy is the main part of the non-bonding interaction energy. According to the principles of similarity and compatibility, ADN has good solubility in solvents where hydrogen bonds form and ADN has poor solubility in solvents without oxygen and sulfur elements. Moreover, water adsorbs to ADN in air because of electrostatic interactions between ADN and water. The ADN crystal growth process was then simulated under different conditions,


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such as with additives, at different temperatures, and in different solvents. From the simulation results, it is concluded that a small amount of ammonium nitrate accelerates ADN crystal growth. The temperature effects on the growth rate and the size of crystal were investigated, and a suitable high temperature is beneficial to ADN crystal growth. As for solvent effect, ADN crystal grows fast in acetone than in other solvents. ADN crystal growth is controlled by the kink position on the crystal surface. These results will lead to a deeper understanding of ADN crystal hygroscopicity and crystal growth process. AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

ORCID Zhongqi Ren: 0000-0002-2571-5702 Zhiyong Zhou: 0000-0001-6436-1399 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We thank Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript. We thank the supports by Xi’an modern chemistry research institute and High performance computing platform of Beijing University of Chemical Technology. This work was supported by the National Natural



Science Foundation of China (21576010, 21606009, U1607107 and U1862113) and Beijing Natural Science Foundation (2172043). The authors gratefully acknowledge these grants. REFERENCES (1) Gilardi, R.; Filppen-Anderson, J.; George, C.;.Butcher, R. J. A new class of flexible energetic salts: the crystal structures of the ammonium, lithium, potassium, and cesium salts of dinitramide. J. Am. Chem. Soc. 1997, 119, 9411-9416. (2) Cui, J. H.; Han, J. Y.; Wang, J. G.; Huang, R. Study on the crystal structure and hygroscopicity of ammonium dinitramide. J. Chem. Eng. Data 2010, 55, 3229-3234. (3) Yedukondalu, N.; Ghule, V. D.; Vaitheeswaran, G. High pressure structural, elastic and vibrational properties of green energetic oxidizer ammonium dinitramide. J. Chem. Phys. 2016, 145, 1-28. (4) Bunte, G.; Ncumann, H.; Antes, J.; Krause, H. H. Analysis of ADN, its precursor and possible by-products using ion chromatography. Propell. Explos. Pyrot. 2002, 27, 119-124. (5) Venkatachalam, S.; Santhosh, G.; Ninan, K. N. An overview on the synthetic routes and properties of ammonium dinitramide (ADN) and other dinitramide Salts. Propell. Explos. Pyrot. 2004, 29, 178-187. (6) Wang, F.; Liu, H.; Gong, X. D. A theoretical study on the structure and hygroscopicity of ammonium dinitramide. Struct. Chem. 2013, 24, 1537-1543. (7) Lan, Y. H.; Zhai, J. X.; Li, D. H.; Yang, R. J.. The influence of solution chemistry on the morphology of ammonium dinitramide crystals. J. Mater. Sci. 2015, 50, 4933-


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Table Captions Table 1. Comparison of experimental and calculated densities of ADN Table 2. Comparison of experimental and calculated values of ADN crystal parameters and bond lengths Table 3. Results of crystal morphology calculated by BFDH method Table 4. Results of crystal morphology calculated by growth morphology method Table 5. Saturated moisture adsorption on ADN crystal faces Table 6. Calculated band gap on ADN surface Table 7. Comparison of experimental and calculated solubility parameters Table 8. Adsorption energy of dinitramide anion on the crystal surface



Page 26 of 47

Table 1. Comparison of experimental and calculated densities of ADN Calculated density (g/cm3)

Experimental density (g/cm3) ADN

1.812

Dreiding 1.807(0.28%)

COMPASS 1.98(9.27%)

Universal

pcff

cvff

1.51(-

1.71(-

1.59(-

16.45%)

5.35)

12.25)

Note: the parenthesis in Table 1 means relative error.


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Table 2. Comparison of experimental and calculated values of ADN crystal parameters and bond lengths d(N-N) (Å) d(N-O) (Å) d(N-H) (Å)

a(Å)

b(Å)

c(Å)

Experimental

1.370

1.232

0.867

6.914

11.787

5.614

Calculated

1.285

1.202

1.012

6.937

11.826

5.633

RE(%)

-6.20

-2.44

16.72

0.33

0.33

0.34



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Table 3. Results of crystal morphology calculated by BFDH method Indices of crystal face

Multiplicity dhkl (Å) Distance (Å) Total facet area (%)

(1 0 0)

2

6.80

14.70

21.53

(0 2 0)

2

5.89

16.97

20.10

(1 1 0)

4

5.89

16.98

20.61

(0 1 1)

4

5.00

20.00

29.50

(1 1 -1)

4

4.39

22.80

8.26


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Table 4. Results of crystal morphology calculated by growth morphology method Indices of crystal face

dhkl (Å)

Attach energy

Total facet area (%)

(kcal/mol) (1 0 0)

6.80

-18.18

33.86

(0 2 0)

5.89

-21.12

26.47

(1 1 0)

5.89

-24.57

6.24

(0 1 1)

5.00

-28.12

27.72

(1 1 -1)

4.39

-32.72

5.71



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Table 5. Saturated moisture adsorption on important ADN crystal faces Number of

Saturated

Average

adsorbed

moisture

adsorption

water

adsorption rate

heat

molecules

(%)

(kcal/mol)

(1 0 0)

66

14.56

21.85

3.41-35.71

(0 2 0)

27

11.82

17.57

1.71-23.71

(1 1 0)

97

21.26

23.11

1.71-38.91

(0 1 1)

98

21.46

20.75

2.61-36.21

(1 1 -1)

116

25.33

18.98

1.71-31.21

(0 1 0)

31

6.82

19.32

1.01-25.81

(0 0 1)

102

22.30

14.25

1.91-36.91

Indices of Importance crystal face


Adsorption heat (kcal/mol)

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Table 6. Calculated band gap on ADN surface

Band gap (ev)

(1 0 0)

(0 2 0)

(1 1 0)

(0 1 1)

(1 1 -1)

2.74

2.01

2.01

2.95

2.73



Page 32 of 47

Table 7. Comparison of experimental and calculated solubility parameters ADN

HMX

Experimental δ (Mpa0.5)36

—

26.800

Calculated δ (Mpa0.5)

88.832

26.602

RE(%)

—

-0.74

Electrostatic solubility (Mpa0.5)

86.193

20.192


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Table 8. Adsorption energy of dinitramide anion on the crystal surface Indices of

Adsorption energy

Adsorption

Relative

Relative

crystal

in smooth site

energy in kink

growth rate in

growth rate

face

(kcal/mol)

site (kcal/mol)

smooth site

in kink site

(1 0 0)

-212.56

-129.90

1.00

1.00

(0 2 0)

-130.47

-86.32

0.61

0.66

(0 1 1)

-124.52

-65.53

0.59

0.50

(1 1 0)

-172.53

-86.32

0.81

0.66

(1 1 -1)

-130.13

-71.93

0.61

0.55



Figure Captions Figure 1. Distribution analysis of N-H bond length in ADN model. Figure 2. Distribution analyses of N-O and N-N bond lengths in ADN model. Figure 3. Hydrogen bond interaction in ADN crystal. Figure 4. Models of ADN surface for calculating impact sensitivity. Figure 5. Construction process of crystal growth model. Figure 6. Prediction of ADN crystal habit in vaccum. Figure 7. Snapshots of ADN crystal face low energy in moisture adsorption. Figure 8. Non-bond interaction in ADN crystal. Figure 9. Face growth process of ADN crystal. Figure 10. Effect of impurity on crystal growth. Figure 11. Effect of temperature on crystal growth process. Figure 12. Effect of solvent on ADN crystal growth.


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Page 35 of 47

15.0

12.5

P (1/Angstrom)

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


10.0

7.5

5.0

2.5

0.0 0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

Bond length (Angstrom)

Figure 1. Distribution analysis of N-H bond length in ADN model.



15.0

N-O 12.5

P (1/Angstrom)

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

Page 36 of 47

10.0

7.5

N-N 5.0

2.5

0.0 1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

1.45

Bond length (Angstrom)

Figure 2. Distribution analyses of N-O and N-N bond lengths in ADN model.


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Figure 3. Hydrogen bond interaction in ADN crystal.



Figure 4. Models of ADN surface for calculating impact sensitivity.


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Figure 5. Construction process of crystal growth model.



Figure 6. Prediction of ADN crystal habit in vaccum.


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Figure 7. Snapshots of ADN crystal face low energy in moisture adsorption.



Page 42 of 47

Van der waals energy H-bond interaction energy Electrostatic interaction energy Non-bond energy

-20000

-15000

-10000

-5000

Energy

Figure 8. Non-bond interaction in ADN crystal.


0

5000

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Figure 9. Face growth process of ADN crystal.



80

5NH4NO3/20ADN

20ADN

60

MSD (Å2)

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

Page 44 of 47

40

20

0 300

350

400

450

Time (ps)

Figure 10. Effect of impurity on crystal growth.


500

Page 45 of 47

80 70 60

MSD (Å2)

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


50

288K 298K 308K 318K 328K 338K

40 30 20 10 0 300

350

400

450

t (ps)

Figure 11. Effect of temperature on crystal growth process.


500


50

40

MSD (Å2)

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

Page 46 of 47

isopropanol ethanol acetone acetonitrile 1-butanol DMSO butyrrolactone ethyl acetate

30

20

10

0 300

350

400

450

Time (ps)

Figure 12. Effect of solvent on ADN crystal growth.


500

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