The Study of Co-Crystallization of Trinitrophenol (TNP) and Ammonium Nitrate (AN) As a Potential Method for Enhanced Stability

The Study of Co-Crystallization of Trinitrophenol (TNP) and Ammonium Nitrate (AN) As a Potential Method for Enhanced Stability

¹Aliyu, AO., ¹Nwaedozie, J. M., ¹Awe, F. E., *²Mohammed, Y. M

¹Department of chemistry, Nigeria Defence academy post graduate school Kaduna state, Nigeria

²Department of Armament Engineering, Airforce Institute of technology Kaduna, Nigeria

*Corresponding Author

DOI: https://doi.org/10.51584/IJRIAS.2025.100500035

Received: 16 April 2025; Accepted: 28 April 2025; Published: 05 June 2025

ABSTRACT

This study explores the co-crystallization of trinitrophenol (TNP) and ammonium nitrate (AN) using a slow solvent evaporation method at a 1:1 molar ratio. The co-crystal formation and its potential for improved explosive stability were investigated. Characterization techniques including Fourier-Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), and Differential Scanning Calorimetry (DSC) were employed. FTIR analysis revealed new peaks corresponding to functional group interactions between TNP and AN, indicating co-crystal formation. XRD confirmed the presence of new diffraction peaks not observed in the individual components, further supporting co-crystallization. DSC analysis demonstrated a significant shift in thermal behaviour compared to TNP and AN. The co-crystal exhibited a glass transition, melting point, and decomposition temperature distinct from the single components. These observations suggest interactions between TNP’s -OH groups and Ammonium NH₄⁺ ions, leading to the formation of a more stable ammonium picrate co-crystal. The altered thermal profile indicates a potential improvement in the co-crystal’s stability compared to the individual explosives.

Key words: co-crystal. Energetic material, trinitro phenol, ammonium nitrate, thermal profile.

INTRODUCTION

The quest to understand and control the behavior of energetic materials, which is the power houses behind explosives, propellants, and oxidizers, has been a continuous struggle for national defense sectors worldwide. Traditionally, advancements have relied on diverse synthesis techniques, leading to the discovery of iconic explosives like black powder, TNT, and RDX. However, this approach often comes at the cost of sacrificing stability or safety (e.g., nitroglycerine) (Ahmed et al.,2018).

One such example is trinitrophenol, also known as picric acid. While boasting explosive power comparable to TNT, its sensitivity to shock, heat, and friction in its dry state makes it a precarious proposition. Paradoxically, when wet, it becomes remarkably safe to handle. This inherent instability, coupled with its reactivity towards metals (forming salts), hinders its practical use. This research delves into the potential of subjecting trinitrophenol to co-crystallization with ammonium nitrate, aiming to create a more stable and explosively superior material with altered thermal properties (Zongwei et al., 2016).

The realm of energetic materials typically relies on established methods to enhance power and stability. These often involve modifying the chemical structure of existing compounds, searching for denser polymorphs (crystal structures), or formulating composite materials. However, co-crystallization offers a novel and potentially more systematic approach. This technique strategically combines existing molecules into a single crystal structure. Excitingly, the co-crystal can offer new and desirable properties based on the easily measurable solubility data of its individual components ( Han et al.,2017).

This research explores the potential of co-crystallization as a path towards a new generation of energetic materials that not only pack a punch but also prioritize safety and stability. By harnessing the combined knowledge of existing energetic materials and the innovative power of co-crystallization, we can embark on a new chapter in the ongoing quest to unlock the secrets of these remarkable materials (Yadav et al., 2015).

CHEMICALS AND METHOD

Chemicals

Ammonium nitrate, Trinitro phenol, acetone, methanol, ethanol, di-chloromethane all AR grade was obtained from CONSAT chemical shop Kaduna and Lagos, Nigeria.

Co-crystalization

Ammonium Nitrate and Picric Acid NH₄NO₃/C₆H₃N₃O₇ at a molar ratio of 1:1 (balanced stoichiometric equation) was prepared using slow evaporation method. A 0.259g of NH₄NO₃ and 0.74 of C₆H₃N₃O₇ was weighed and added into a 100ml beaker. 65ml of acetone was measured and also added into the 100 ml beaker containing the mixture of NH₄NO₃ and C₆H₃N₃O₇. The choice of acetone as a solvent is because it can dissolve both samples without using other solvent. Stirring commences using the stirrer and after 15 minutes during the process 15ml of distilled water was added to the mixture and the stirring continued till it dissolved completely. The beaker was removed from the water bath and covered with a filter paper which was kept in an oven for a week during which crystals was formed and dried at room temperature.

FTIR Analyses

The Cocrystal, ammonium nitrate and tri -nitro phenol were characterized by Agilent technology Cary 630 FTIR. The scanning range of the IR spectra was 525-4000 cm⁻¹. Experiments were performed at room temperature.

XRD analysis

The Co-crystal, potassium chlorate and Ammonium nitrate samples were characterized by X-ray diffraction (XRD) using a Empeyrean Pan-Analytical Advance XRD. Each sample was scanned over the 2θ angular range 5°-80° with a sampling interval of 0.2°. The XRD reflections were obtained at room temperature (25 °C).

DSC analysis Differential Scanning Calorimetry

(DSC) analyses of the dried Co-crystal samples, ammonium nitrate and potassium chlorate were carried out using a METLER star (SW 13.00)  and Chip DSC-10 Linseisinstrument at a heating rate of 10 K/min. The samples were heated from 30 °C to 150 °C. The mass of the samples used was 1.000 mg. The system was in a nitrogen atmosphere.

RESULT AND DISCUSSION

FTIR Analysis

The assignments for the major characteristic bands are listed in Table 1.0. The results indicated that several characteristic absorption peaks are highly sensitive to the structure changes of the co-crystal in the IR spectra. NH₄NO₃ has bands at 3231.6 cm-¹, 3045.2 cm-¹, 827.5 cm-¹, and 1289.7 cm-¹ O-H stretching , C=C stretching , C=H bending and C=C bending and . However, these bands shifted to 3239.1 cm-¹  and 3101.1 cm-¹, 1867.4 cm-¹  and 827.5 cm-¹, 782.7 and 700.7, 916.9 cm-1, respectively in the co-crystal. Similarly, some characteristic absorption peaks of C₆H₃N₃O₇ 3104.9, 916.9, 831.2 and 700.7, 823.7, 1364.2 and 1763.0 cm-¹ also shift after crystallization and also the co-crystal exhibited some bands of N-H stretching 3425.4 cm-¹, CC stretching 2113.4 cm-¹, C=C stretching 1628.8 cm-¹, O=H bending 1423.8 and 1312.0 cm-¹, N-H bending 1524.5 cm-¹, C-O stretching 1259.8 cm-¹, C-N stretching 1334.4 cm-¹ which did not appear in the band of NH₄NO₃ and C₆H₃N₃O₇. Those shifts in phenomena may be caused by the hydrogen bond interactions involved in co-crystal formation which changes the symmetry characteristic which is similar when compared with the work Jin-ting  et al, 2015 carried out on cocrystallization of  3- nitro- 1,2,4- triazol-5 –one (NTO) and 5,6,7,8-tetrahydroterazolo(1,5-b(1,2,4)-triazine(TZTN) at a molar ratio of 1:1.

Table 1 shows the FTIR bands, Assignments of NH₄NO₃, C₆H₃N₃O₇ and NH₄NO₃/C₆H₃N₃O₇

Xrd Analysis

The XRD diffractogram of NH₄NO₃, C₆H₃N₃O₇ and co-crystal (NH₄NO₃/ C₆H₃N₃O₇ 1:1 SLOW) are presented in Table 2 . The diffractogram of NH₄NO₃ exhibited characteristic peak of 2Ѳ with some selected values at 18.0881, 24.5084, 26.3488, 28.2676 29.2147 and 31.2757 which are not present in the co-crystal pattern while C₆H₃N₃O₇ exhibited characteristics peak of 2Ѳ with some selected values at 9.3970, 14.0291, 16.2646, 18.6813, 19.0188, 19.2708 which are also not present in the co-crystal pattern. The co-crystal exhibited new peak of 2Ѳ values at 13.9970 and 17.8743 13.9970 17.8743, 19.4140, 23.7984, 23.9260, 32.9365, 37.9099, 41.2689, 46.9912, 48.5101, 48.9905, 57.2375, 59.7379 , 65.7211 which are not in the pattern of the co-formers, that indicates a new peak have been formed after crystallization. The work shows some similarities when compared with the work of Chongwei et al, 2017 worked on Nano-CL-20/HMX Cocrystal Explosive for Significantly Reduced Mechanical Sensitivity. The XRD shows all diffraction peaks of raw HMX appeared in CL-20/HMX cocrystals pattern. However, diffraction peaks of raw CL-20 did not appear in pattern of CL-20/HMX cocrystal explosive at 12.5° and 13.8°. Compared with patterns of raw CL-20 and raw HMX, at 11.4° and 13.2°, respectively, new diffraction peaks appeared on CL-20/HMX cocrystals pattern. Changes showed that prepared samples did not simply mix with CL-20 and HMX, but they interacted to form new cocrystals.

Table 2 shows the XRD 2Ѳ peaks values of NH₄NO_3, C₆H₃N₃O₇ and NH₄NO₃/C₆H₃N₃O₇1:1

DSC Analysis

Fig 1.a:  DSC spectra of trinitro Phenol (C₆H₃N₃O₇)Fig .

1.b:  DSC spectra of ammonium nitrate

Fig 1.c: DSC spectra of C₆H₃N₃O₇/NH₄NO₃cocrystal  1:1 slow

Table 3. endothermic, exothermic peak values and melting point for C₆H₃N₃O_7, NH₄NO_3, 1:1 cocrystal slow

The DSC curve as shown in Fig. 1a-c and thermal values in table 3, displayed a curve of single component of C₆H₃N₃O₇, NH₄NO₃ and co-crystal of C₆H₃N₃O₇/NH₄NO₃ 1:1 which are different from each other.  The C₆H₃N₃O₇/NH₄NO₃ co-crystal presents a unique thermal property by exhibiting a shift of the base line indicating  a glass point peak at 130.49 ⁰C,  an endothermic peak value of 215.7 ⁰C was observed which signify the melting point of the crystal before decompose and also an exothermic peak values of 301.45 ⁰C was displayed indicating the temperature at which it decompose which is different from the value of single component C₆H₃N₃O₇ and NH₄NO₃ , where C₆H₃N₃O₇ displayed an endothermic peak of 129.7 ⁰C  indicating a melting  stage of the crystal while  an exothermic peak of 291.50 ⁰C was displayed which indicate the decompose temperature without undergoing co-crystallizaation, while NH₄NO₃  displayed an endothermic peak of 54.7 ⁰C as a phase change and an endothermic peak of 127.9 and 170.5 ⁰C, indicating the melting of the crystal leading to an  endothermic peak value of 237.5 ⁰C which decompose without heat. The endothermic peak value displayed by the single component indicate to be lower  for C₆H₃N₃O₇ compared to the co-crystal value while NH₄NO₃  gave a value higher than the co-crystal and also the co-crystal displayed an exothermic peak value that did not appear  on the single component and indicated to be higher than the single components this behavior observed could be as a result of the interaction between NH₄ClO₄ and NH₄NO₃  which altered the phase change resulting to co-crystallization thereby altering the thermal property resulting to the values obtained. This behaviuor was observed to be in line with the work of Dong et al 2015 were they cocrystalize picric acid and trinitrotoluene and the work of Zhi et al.,2016.

Molecular Modeling of Picric acid and Ammonium nitrate

Fig. 2 and 3 show 2d, and 3d view of Hydrogen bond interaction of the picric acid (TNP) – C₆H₃N₃O₇ trinitrophenol and NH₄NO₃ ammonium nitrate using discovery studio virtualizer.

The stability and characteristics of the co-crystallized compounds are greatly influenced by the hydrogen bond interactions between C₆H₃N₃O₇ (trinitrophenol) and NH₄NO₃ (ammonium nitrate).  Discovery Studio Virtualizer was used in this work to display the hydrogen bond interactions, which shed light on the molecular connections between these oxidizer and explosive compounds. The complex nature of molecular interactions in these systems is highlighted by the hydrophobic pi-alkyl interactions and hydrogen bonding between TNP C₆H₃N₃O₇ and AN NH₄NO₃ with particular amino acid residues, including SER110, LIG1, and VAL109.  The observed hydrogen bond distances, which range from 1.76269 Å to 5.28963 Å, indicate different interaction strengths; stronger bonds are generally indicated by shorter distances.

At a distance of 1.94802 Å, as seen in Fig 2 and 3 the hydrogen bond interactions between C₆H₃N₃O₇ and NH₄NO₃ with SER110 is very significant because it represents a stable connection that adds to the overall stability of the co-crystallized compounds.  The structural integrity of the co-crystallized system is further improved by the hydrophobic pi-alkyl contact with LIG1 and the carbon-hydrogen bond interaction with VAL109. The significance of hydrogen bonding interactions in stabilizing energetic materials is shown by the production of ammonium picrate as a result of these interactions between C₆H₃N₃O₇ and NH₄NO₃.  The hydrogen bond interactions found in this study provide insight into the underlying molecular mechanisms controlling the creation and stability of ammonium picrate, which is well-known for its explosive and stable characteristics.

Furthermore, knowing the precise chemical interactions that exist between C₆H₃N₃O₇ and NH₄NO₃ not only helps to explain how they co-crystallize, but it also provides important information for the creation of new energy materials with specific characteristics.

CONCLUSION

The investigation into co-crystallization as a method to modify the properties of trinitrophenol (TNP) yielded promising results. Utilizing slow solvent evaporation, a co-crystal with ammonium nitrate was successfully formed.  FTIR analysis confirmed interactions between the hydroxyl groups of TNP and the ammonium ion (NH₄⁺), suggesting hydrogen bond formation. Differential Scanning Calorimetry (DSC) revealed a significant alteration in thermal behavior.

The co-crystal exhibited a glass transition temperature of 130.49 °C, a melting point of 215.7 °C, and a decomposition temperature of 301.45 °C. These values were all notably higher compared to those observed for the individual components, indicating enhanced stability of the co-crystal. X-ray Diffraction (XRD) further corroborated the formation of a new crystalline phase, potentially an ammonium picrate co-crystal, resulting from the interaction between TNP’s hydroxyl groups and NH4⁺. Also the Discovery Studio Virtualize corroborated the formation of hydrogen bonding leading to the formation of a new crystalline phase by displaying a molecular model of trinitrophenol TNP and ammonium nitrate. The model shows angle formation showing the hydrogen bonding through cocrystallization. These findings highlight the potential of co-crystallization as a technique to not only modify TNP’s thermal properties but also potentially reduce its corrosive nature, paving the way for its safer future applications.

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