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. 2024 Apr 15;9(17):18893–18900. doi: 10.1021/acsomega.3c08695

ReaxFF-Based Molecular Dynamics Study of the Mechanism of the Reaction of N2O4 with H2O

Yi Guo †, Gan Tian †, Xinlong Chang †,*, Zhanmei Tang ‡, Zhiyong Huang †, Dejun Liu †, Xinzhi Yang †
PMCID: PMC11064440  PMID: 38708236

Abstract

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During long-term storage of the liquid propellant N2O4, it absorbs H2O to form the N2O4(H2O)n system, and this in turn generates HNO3, HNO2, and other substances in the storage tank because of corrosion, which seriously affects the performance of weaponry. In this work, we carried out computational simulations of N2O4 with different masses of water based on ReaxFF, analyzed the reaction intermediates and products, and investigated the mechanism of the reaction of N2O4 with H2O and of N2O4(H2O)n. The results show that the reaction product ω(HNO3+HNO2) undergoes a rapid growth in the early stage of the reaction and then tends toward dynamic equilibrium; the potential energy of the system decreases with the increase of ω(H2O), the reaction rate increases, and the rate of decomposition of HNO2 to form HNO3 increases. When ω(H2O) is 0.2 or 1.0%, the intermediate products are N2O4H2O or N2O4(H2O)2, respectively, and the reaction proceeds along two paths; when ω(H2O) ≥ 2.0%, N2O4(H2O)3 appears as the intermediate product, HNO3 and HNO2 are directly produced in one step, and a stable current loop can be formed within the whole system.

1. Introduction

N2O4 is often used as an oxidizer in two-component liquid propulsion systems in the first and second stages of launch vehicles, owing to its high density, high specific impulse, and strong oxidizing properties.1 In the case of long-term storage, N2O4 absorbs H2O from the surrounding environment to form the N2O4(H2O)n system, which in turn generates HNO3, HNO2, and other substances, accelerating the corrosion of the storage tank and causing structural damages and propellant leakage, which are hazardous and threaten the safety of the equipment.2 Therefore, the study of the reaction mechanism of N2O4 and H2O is of great significance for the structural design of weapons and equipment as well as for long-term storage safety.

At present, the test and analysis methods for N2O4 are mainly based on the Chinese military standard GJB1673–93; further, owing to the limitation of the instruments, the accuracy of test results is poor, and only parameters such as the equivalent water content of N2O4 with a water content of ≤0.4% can be determined, whereas determination of the content of HNO3, HNO2, and other substances is impossible. In addition, N2O4 is a volatile, reddish-brown transparent liquid at room temperature with strong oxidizing and toxic properties.3 Therefore, the design and performance of the experiments are challenging.

In principle, quantum chemistry (QC) is applicable to all chemical systems but not to large systems owing to the high computational effort.4 Although conventional force fields such as MM3,5,6 DREIDING,7 and EFF8 can be applied in the case of large systems, they can only describe the interactions between molecules and those within condensed phase systems. ReaxFF is a bond order-based reaction force field, originally proposed by van Duin in 2001, that has no energy or force discontinuities, even during a reaction.9 Thus, ReaxFF can describe the formation and dissociation of chemical bonds. The classical treatment of reactive chemistry made available by the ReaxFF method has enabled numerous studies of phenomena occurring on scales that were previously inaccessible via computational methods. In particular, ReaxFF is capable of modeling reaction events involving reactions at the interface between the solid, liquid, and gas phases, which is possible because the ReaxFF description of each element can be transferred across phases. For example, the same system of mathematical forms is used for an oxygen atom, whether that oxygen is in the gas phase as O2, in the liquid phase within an H2O molecule, or bound in a solid oxide. This transferability coupled with lower computational costs that allow for longer simulation time scales enables ReaxFF to consider phenomena that depend not only on the reactivity of the species involved but also on dynamic factors such as diffusivity and solubility that affect how species migrate through the system. This allows ReaxFF to model complex processes involving multiple stages in contact with each other.10 Zhao11 and Liu12 et al. simulated the combustion and explosion of N2O4 and combustant based on the ReaxFF molecular dynamics simulation method, but the system remains in a state of rapid warming and long-term maintenance of the high temperatures. At present, few studies have focused on the reaction mechanism of N2O4 with H2O, and these were mainly based on the density functional theory (DFT) to explore the isomerization and reaction process.13−24 Only a limited number of tests have been performed by AFRPL,25 which is not enough to perfect the reaction path.

In this study, ReaxFF molecular dynamics simulation was used to simulate the intermediates and products of the reaction of N2O4 with different mass fractions of H2O, and combined with the results of the previous work of the group, the reaction course of the N2O4(H2O)n system was investigated based on DFT, to analyze the reaction mechanism between N2O4 and H2O at room temperature.

2. Computational Method

Molecular dynamics studies of N2O4 and H2O (ω = 0.2–8%) were based on the results of Brown C.T.25 and the Chinese military standard GJB1673–93. The mixing ratios of N2O4 and H2O are shown in Table 1. The initial configurations of N2O4 and H2O were optimized using Materials Studio software.26 The initial reactants were randomly inserted into the simulation box, and then, the periodic system was constructed using the amorphous cell module. A conjugate gradient method was adopted for the initial system to perform energy minimization calculations, which facilitate the subsequent relaxation operation. According to the conclusions of Miller,14 N2O4 and H2O were subjected to 90 ps isothermal isotropic (NVT) kinetic equilibrium with a step size of 0.3 fs at a temperature of 293 K. Temperature control was achieved using the Berendsen method with a temperature damping coefficient of 0.1 ps. Atomic and molecular species and dynamic trajectories were determined every 100 steps. All MD simulations were performed in the Lammps package27 using the ReaxFF method.28 In order to verify the reaction products, the polarization curves for a model Al alloy in N2O4 with different ω(H2O) values were tested. Further, the reaction process of N2O4 with different ω(H2O) was explored in conjunction with the previous work of the group.29

Table 1. Mixing Ratio of N2O4 to H2O.

water content/wt % 0.2 0.6 1 2 4 6 8
N2O4/n 4900 1600 950 480 235 155 112
H2O/n 50 50 50 50 50 50 50
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3. Results and Discussion

3.1. Reaction Products HNO3 and HNO2

According to the conclusions of Liu,1,2 the main corrosive intermediates generated by the reaction of N2O4 with H2O are HNO3 and HNO2, and their mass fractions depending on the H2O amount reacting with N2O4 are shown in Figure 1. In the initial stage of the reaction, ω(HNO3+HNO2) rapidly increases, and then, its mass fraction is gradually stabilized, reaching dynamic equilibrium, with a further increase in the amount of formed products in the system. In order to verify the corrosion intensity of H2O-containing N2O4, a certain type of aluminum alloy was selected to carry out the polarization curve test, the results of which are shown in Figure 2. The electrochemical corrosion current densities obtained from the fitting are listed in Table 2. When ω(H2O) < 1.0%, the measured polarization curves were extremely unstable, and no obvious equilibrium electrode potential appeared. As shown in Figure 1, the least amount of HNO3 + HNO2 was generated. Since N2O4 is not conductive, this also led to an uneven distribution of the generated ions in the solution, which were in a free state and could not form a stable current loop. With the increase in ω(H2O), the number of ions generated within the N2O4(H2O)n system gradually increased, and as shown in Figure 2, ω(H2O) = 1.0% is the detection limit using the polarization curve. When ω(H2O) > 1.0%, the polarization curve tends to be regular, indicating that a sufficient number of ions exist within the solution to form a stable current loop. Further, when ω(H2O) > 2.0%, as shown in Table 2, the corrosion current density changes abruptly, and its corresponding polarization curve is compared to ω(H2O) = 2.0%. Further, a significant passivation area and pitting potential Eb of about 0.3 V, combined with the results in Figure 1, indicate that passivation occurs on the surface of the specimen when ω(H2O) > 2.0%, while the dissolution film formation equilibrium reaction occurs at a higher corrosion rate.

Figure 1.

Figure 1

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ω(HNO3+HNO2) during the reaction of N2O4 with different ω(H2O).

Figure 2.

Figure 2

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Polarization curves of the type XX aluminum alloy for the reaction of different ω(H2O) ions with N2O4.

Table 2. Corrosion Current Density of the Type XX Aluminum Alloy for the Reaction of Different ω(H2O) with N2O4.

water content/wt % 0.6 1 2 4 6 8
Icorr (A/cm2)   1.54 × 10–8 3.50 × 10–8 4.97 × 10–6 7.30 × 10–6 7.06 × 10–5
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Based on the results in Figures 1 and 2, the changes in the potential energy (ΔEp) during the reaction of N2O4 with ω(H2O) = 0.2, 1.0, 2.0, and 6.0% were analyzed as shown in Figure 3, with the unit au/atom meaning the total potential energy of the system divided by the total number of atoms. When N2O4 comes into contact with H2O, the potential energy of the whole system decreases dramatically, and an exothermic chemical reaction occurs. Subsequently, the generated HNO2 absorbs heat and undergoes decomposition that leads to a rise in the potential energy of the system; eventually, when the reaction equilibrium is reached in the system, its potential energy also reaches a stable state. Moreover, with the increase in ω(H2O), the potential energy of the system decreases, the rate of decrease is the highest in the initial stage, and the final equilibrium state is also lower, which indicates that the reaction rate is faster, and the energy released is also larger.

Figure 3.

Figure 3

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Potential energy changes for the reaction of different amounts of H2O with N2O4.

Figure 4 shows the variation in the amount of HNO3 and HNO2 during the course of the reaction between N2O4 and different ω(H2O). As shown in Figure 4, the production of HNO2 can be roughly divided into three phases: a period of rapid increase in the amount of HNO2 produced, a period of slow decrease in the amount of HNO2 produced, and a stabilization period; on the other hand, the production of HNO3 has only a period of gradual increase in the amount of HNO3 produced and a stabilization period. In the early stage of the reaction, the production rate and quantity of HNO2 are higher than that of HNO3, but as the reaction proceeds, HNO2 is further decomposed into HNO3 and NO: 3HNO2 → HNO3 + 2NO + H2O.

Figure 4.

Figure 4

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Variation in amount of HNO3 and HNO2 during the course of the reaction of N2O4 with different ω(H2O).

Consistent with the conclusions of AFRPL,25 when ω(H2O) = 0.2%, the amount of HNO3 and HNO2 is almost the same after the reaction is stabilized. When ω(H2O) is increased, the yield of HNO2 always remains low because the decomposition of HNO2 produces H2O that continues to react with N2O4: N2O4 + H2O → HNO3 + HNO2.

Therefore, HNO2 does not decompose completely, and the whole system remains in a dynamic equilibrium. However, the reaction time required for the amount of HNO3 to exceed that of HNO2 continuously reduces. As shown in Figure 3d, the amount of HNO3 generated exceeds that of HNO2 at T = 1200 fs, indicating that the reaction rate of the system increases with ω(H2O), which is also verified by the results of Multer.14

3.2. Intermediate N2O4(H2O)n

The reaction between N2O4 and H2O first produces the N2O4(H2O)n complex due to hydrogen bonding,14 which further affects the course of the reaction depending on the value of n.16 Therefore, the intermediates formed with ω(H2O) = 0.2, 1.0, 2.0, and 6.0% were analyzed. Figure 5 shows the mass fraction of the intermediate product N2O4(H2O)n for different ω(H2O). As shown in Figure 5a, at ω(H2O) = 0.2%, the intermediate product is only N2O4(H2O), the yield of which increases rapidly and remains stable for a long period of time with the mass fraction maintained at about 0.12%. As ω(H2O) continues to increase, as shown in Figure 5b, a small amount of N2O4(H2O)2 appears, but the main intermediate product is still N2O4(H2O). Then, as shown in Figure 5c, the yield of N2O4(H2O) further increases at ω(H2O) = 2.0% and the yield of N2O4(H2O)2 tends to be stabilized; N2O4 combines with H2O in the local region of the system and produces N2O4(H2O)3, but the yield of N2O4(H2O)3 is unstable and low. When ω(H2O) = 6.0%, the mass percentage of its intermediate product N2O4(H2O)2 increases dramatically, and although its yield exceeds that of N2O4(H2O), the N2O4(H2O) content does not change substantially with respect to that corresponding to ω(H2O) = 2.0%; this indicates that when ω(H2O) = 6.0%, the N2O4(H2O) content in the system is saturated and that some N2O4 would bind more H2O. Therefore, the yield of N2O4(H2O)2 increases dramatically and stabilizes; at the same time, after the system reaches equilibrium, the yield of N2O4(H2O)3 also tends to stabilize to 1.0%.

Figure 5.

Figure 5

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Mass fraction of the reaction intermediate N2O4(H2O)n for the reaction of N2O4 with different ω(H2O).

3.3. Reaction Mechanism

The results of the previous study of this group show that the reaction of one N2O4 molecule with different amounts of H2O result in complexes with different structures,29 as shown in Figure 6. One N2O4 molecule combines with one H2O molecule to form N2O4(H2O), which has only one stable structure, namely, IM1, as shown in Figure 6a. However, when one N2O4 molecule combines with two H2O molecules, two structures are formed: two H2O molecules located on the same side of N2O4, as shown in Figure 6b, and two H2O molecules located on both sides of N2O4, as shown in Figure 6c. Similarly, the combination of one N2O4 molecule with three H2O molecules produces N2O4(H2O)3 with two structures, i.e., three H2O molecules located on the same side of N2O4, as shown in Figure 6d, and two H2O molecules on one side of N2O4 and one H2O molecule on the other side, as shown in Figure 6e. When H2O is located on the same side of N2O4, the number of interconnected chemical bonds is fewer and the bond length is shorter, so the reaction potential energy of IM2 and IM4 is lower and the molecular structure is more stable than those of IM3 and IM5. Thus, in summary, N2O4 reacts with H2O and its intermediate products are IM1, IM2, and IM4.

Figure 6.

Figure 6

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Structure of N2O4(H2O)n (n = 1–3).

After the formation of the intermediate product N2O4(H2O)n, there are usually two alternatives for the next reaction: the direct decomposition of N2O4(H2O)n to form HNO3 + HNO2 and the final formation of HNO3 + HNO2 from N2O4(H2O)n after t-ONONO2-(H2O)n. From Sections 3.1 and 3.2, the reaction mechanisms of N2O4 with ω(H2O) = 0.2, 1.0, 2.0, and 6.0% are analyzed.

3.3. 1

As can be seen from Figure 5, at ω(H2O) = 0.2%, the only intermediate product is IM1. The reaction can proceed along both paths simultaneously because the difference between the potential energy surfaces for the two reaction paths of IM1 is small,29 as shown in Figure 7a: (I) One –NO2 in IM1 rotates along the N–N bond, which causes the shift of H2O and the formation of t-ONONO2–H2O. Subsequently, the N1–O4 bond breaks, and – NO3 and −NO combine with ionized H and −OH in H2O to form HNO3 and HNO2, respectively. (II) IM1 generates HNO3 + HNO2 in one direct step, and the N–N bond in N2O4 breaks directly to form two −NO2, and one combines with the detached H of H2O to form HNO2, while the other −NO2 combines with HNO2 remaining in H2O to form HNO2. NO2 combines with the remaining −OH in H2O to form HNO3.

3.3. 2

Figure 7.

Figure 7

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Reaction mechanism of N2O4 with H2O.

When ω(H2O) = 1.0%, the main intermediate product of the reaction between N2O4 and H2O is still IM1, and its main reaction mechanism is the same as that in Figure 7a. However, a small amount of IM2 appears, and the difference between the potential energy surfaces for the two reaction paths is small. The main reaction paths are shown in Figure 7b: (I) Both of the H2O molecules in IM2 are involved in the reaction course, and t-ONONO2-(H2O)2 is in the octameric ring structure. With the further reaction of t-ONONO2-(H2O)2, proton transfer occurs in the two H2O molecules during the process; O7 and H10 of one H2O (O7) combine with NO to form HNO2, and H12 in the other H2O (O10) combines with NO3 to form HNO3. The remaining H8 and O10 of the two H2O molecules recombine with H11 to form a new H2O molecule. (II) Similar to pathway 2 of IM1 in Figure 7a, only one H2O molecule is involved in the reaction, which ultimately produces the product directly. Further, the molecule O10 in the other H2O interacts with H8 and O7 to form the O–H–O hydrogen bonds. According to Miller et al.,14t-ONONO2-(H2O)n exhibits equilibrium geometry when n ≤ 2, and the bonding in this molecule is partly ionic but mainly covalent. Therefore, in Figure 2, the polarization curves are not regular for ω(H2O) ≤ 1.0%, and the solution system cannot form a stable current loop.

3.3. 3

When ω(H2O) = 2.0%, the main intermediate product is still IM1, but small amounts of IM2 and IM4 are present. The reaction mechanism of IM1 and IM2 is presented in the previous section. According to Miller and previous work,14,29 in N2O4(H2O)n, the two-path reaction potential of IM4 reaches 28.4 KJ·mol–1 at n = 3, and the reaction rate constant is higher by 2 orders of magnitude compared to that at n < 3. Further, the formation of t-ONONO2-(H2O)2 is accompanied by proton transfer as the reaction proceeds. Luo et al.24 confirmed that hydrogen bonding and a polar environment are prerequisites for intermolecular proton transfer. Moreover, the proton transfer occurring in t-ONONO2 dominates the production of – OH, again supported by experimental results,30 leading to an increasingly shorter duration of existence of t-ONONO2 as an increasing amount of H2O is added. All charge transfer leaps lead to a partial reverse transfer of charge from NO3– to NO+, which causes difficulty in breaking the ON–ONO2 bond. Therefore, with ionic bonding being predominant in the N2O4(H2O)3 system, IM4 can be determined to preferentially cause the direct generation of HNO3 and HNO2 in one step, as shown in Figure 7c. Therefore, within the solution system, the number of ions reaches a stable value as per the polarization curve.

3.3. 4

When ω(H2O) = 6.0%, its intermediate product species are the same as those when ω(H2O) = 2.0%, but the dominant ones are IM1 and IM2; further, the number of IM4 is not negligible and is qualitatively higher than that at ω(H2O) = 2.0%. The dramatic increase in the number of IM4 leads to an enhancement in the corrosion current density by about 2 orders of magnitude and the appearance of passivation regions; these results again validate the experimental results in Section 3.1. Therefore, the reaction mechanism at ω(H2O) = 6.0% corresponds to that in Figure 7a–c.

4. Conclusions

  • (1)

    At the beginning of the reaction, the reaction product ω(HNO3 + HNO2) rapidly increases and then approaches dynamic equilibrium; further, ω(H2O) increases, the potential energy of the system decreases, the rate of the reaction increases, and the rate of decomposition of HNO2 to form HNO3 increases.

  • (2)

    With ω(H2O) = 0.2%, one N2O4 molecule combines with only one H2O molecule to produce N2O4·H2O; as ω(H2O) increases, small amounts of N2O4(H2O)2 and N2O4(H2O)3 appear as intermediates. At ω(H2O) = 6.0%, the yield of the intermediate product N2O4(H2O)2 begins to exceed that of N2O4H2O, and the N2O4(H2O)3 content reached 1.0%.

  • (3)

    With ω(H2O) = 0.2 and 1.0%, the intermediate product N2O4(H2O)n in the system, i.e., N2O4(H2O)n, reacts along two different paths when n = 1 or 2. When ω(H2O) ≥ 2.0%, the appearance of N2O4(H2O)n in the system tends to produce the final product in one direct step.

Acknowledgments

The authors are very grateful to Associate Researcher Guo Wei and engineers Peng Zheng and Jiang Manzi of the Beijing Institute of Aerospace Testing and Technology, China, for their assistance in this research.

Author Contributions

Y.G.: conceptualization, investigation, methodology, and writing—original draft. G.T.: project administration and validation. X.C.: funding acquisition and writing—review and editing. Z.T.: electrochemical test. Z.H.: data curation and formal analysis. D.L.: resources and supervision. X.Y.: software and validation.

This research was funded by the National Natural Science Foundation of China (NO.52272446) and the Natural Science Foundation of Shaanxi Province (NO.2021JM-250).

The authors declare no competing financial interest.

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