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. 2025 Nov 14;10(46):55234–55241. doi: 10.1021/acsomega.4c09652

Effect of Substituent Groups on the Antioxidant Performance of Diphenylamine Derivatives and Structural Optimization on the Basis of Theoretical Calculations

Changqing Miao †, Aimin Chen †, Wenbo Li †, Mengfan Zhang †, Yingying Deng †, Guole Bai †, Zhihua Ma †, Zhiying Ma †,‡,*, Tan Dang †,*
PMCID: PMC12658683  PMID: 41322559

Abstract

Lubricating oils that reduces friction and wear are widely used in mechanical engineering but generally suffer oxidation and degradation. In this study, a series of diphenylamine antioxidants with different substituted alkyl groups were synthesized via the Ullmann method. The antioxidative performance and lubricating properties in both nonpolar and polar-based oils were investigated, and their enhancement mechanism was discussed. The results showed that in polar base oils, the oxidation induction time (OIT) was prolonged from 40 min in PAO and 48 min in DIOS to 3235 and 3198 min, respectively, depending on the alkyl groups. As the alkyl chain increases, the polarity of diphenylamines gradually decreases, thus enhancing the compatibility and the ability to trap generated free radicals. Similar enhancement was also achieved in nonpolar oils, showing the maximum OIT of up to 3235 and 1524 min in PAO and liquid paraffin, respectively, but independent of the length of the substituted alkyl chains since the stability of the nitrogen-containing radicals was barely influenced. Besides improved antioxidative performance, a ∼50% reduction in wear loss was determined after the addition of as-prepared antioxidants, which prevented the degradation of lubricating oils and the formation of sludge and carbon deposition. This work provides a theoretical basis for the structure design of antioxidants with high performance and offers experimental guidance for their applications in lubricating oils with different polarities.

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1. Introduction

The friction between sliding components consumes energy, generates friction heat, and finally leads to mechanical wear, which is accelerated by the environmental temperature. , Therefore, reducing friction is imperative to prolong the service life of mechanical equipment and save energy. The design and application of lubricating materials is an effective way to reduce friction and wear, and many types of lubricating materials, such as solid, liquid, and gaseous lubricating materials, have been developed. , Among them, lubricating oil is widely used in many fields for lubrication, cooling, cleaning, sealing and buffering functions.

However, lubricating oil is affected by environmental temperature, metal, water and oxygen, which results in rapid oxidation and deterioration, thus resulting in the loss of lubricating function and the production of a large amount of sludge. The addition of antioxidants to lubricating oil is an important solution for preventing severe oxidation. , Since they were first proposed in the early 1800s, organic sulfides with antioxidant capacity have been prepared by heating untreated mineral oil with sulfur and adding it to mineral lubricating oils; however, their use has been limited by severe corrosion induced by contact with copper. After more than 200 years of development, different types of antioxidants based on hindered phenols, amines, sulfides, sulfur nitrides, and organophosphates have been discovered. ,

As early discovered antioxidants, amines exhibit excellent antioxidant capacity, especially at high temperatures. Many amine antioxidants, such as diphenylamine N-phenyl-α,β-naphthylamine, substituted N,N’-diphenyl-p-phenylenediamine and their derivatives, have been reported. , The synergistic effect of amine antioxidants with other antioxidants or metal deactivators has been reported. Among the known amine antioxidants, diphenylamine is the most important for its antioxidant properties at high temperatures, albeit with the production of excessive sludge. Recently, alkyl diphenylamine with better sludge control ability was developed and widely used in engine oils and industrial lubricants. Jin et al. synthesized a polyphenolic antioxidant and reported that the product had excellent compatibility with dinonyldiphenylamine and ZDDP in ester oil. The relationship between concentration and antioxidant performance was further revealed. At present, the alkylation of diphenylamine is achieved mainly through the Fu alkylation reaction. Typically, a mixture containing products is obtained and highly relies on experience. The key factors in the synthesis of alkyl diphenylamine have not been well revealed, limiting the production and application of alkyl diphenylamine with high oxidative performance and a high cost ratio.

In this study, a series of monosubstituted diphenylamines with uniform structures were synthesized via the Ullmann method. The chemical structure of the fabricated antioxidants was determined. The antioxidative performance of both nonpolar and polar oils was investigated together with their lubricating properties. Moreover, the effects of polarity, the dissociation energy of nitrogen–hydrogen bonds and the stability of nitrogen-containing free radicals on the antioxidative capacity of substituted diphenylamines were explored via Gaussian calculations in depth.

2. Experimental Section

2.1. Materials

The commercial antioxidants diphenylamine and 4-methyldiphenylamine, denoted as 1a and 1b, respectively, were commercially provided by Aladdin Reagent Company (Shanghai, China) and used without further purification. The synthetic ester base oil di-iso-octyl sebacate (DIOS) provided by the Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences (Lanzhou, China) is chemically pure. The epoxidized soybean oil was commercially supplied by Ji’nanYongchen Chemical Company Limited. Polyalphaolefin (PAO) and liquid paraffin (LP) were obtained from Sigma-Aldrich (China).

2.2. Preparation

Briefly, in an anhydrous and anaerobic environment, bromobenzene, p-alkyl aniline-substituted aniline, 1,4-dioxaneandthe catalyst were stirred at 110 °C in a Hiddink reaction bottle for 6–24 h. The resulting mixtures were cooled to room temperature and collected via column chromatography method in detail.

Diphenylamines with different alkyl substituents, ranging from methyl (−CH3) to octyl (−C8H17) groups, were prepared via the route schematically shown in Figure . The obtained products were denoted as 1c, 1d, 1e, 1f, 1g, 1h and 1i.

1.

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Synthesis routine and chemical structure of diphenylamine (1a), 4-methyldiphenylamine (1b) and synthesized diphenylamine derivative antioxidants, denoted as 1c–1i.

2.3. Characterization

The diphenylamine derivatives were characterized by nuclear magnetic resonance spectroscopy (NMR) with an AVANCE 400 instrument (Bruker, Germany) and high-resolution mass spectrometry (HRMS) with an AB SCIEX 4000 QTRAPMALDI-TOF/TOF-MS instrument (Bruker, Germany). The conditions of the NMR and HRMS characterization are detailed in the Supporting Information.

2.4. Tribological and Antioxidationtests

The tribological properties of 1a, 1b, 1c, 1d, 1e, 1f, 1g, 1h and 1i in several base oils at a constant concentration of 0.5% were evaluated by a UMT-5 tester (Bruker, America). Five milliliters of mixture oil was added between the ball-on-disc sliding pairs comprising a GCr15 steel ball and an LF5 alumina alloy disc. The reciprocating sliding test was conducted under a load of 5 N with a stroke of 5 mm and a frequency of 10 Hz at room temperature for 30 min.

The antioxidation behavior was evaluated via a rotating bomb oxidation test (RBOT) at 150 °C. The test methods are detailed elsewhere. The average value obtained from three or more tests was reported.

2.5. Theoretical Calculation

By using Gaussian software, the structures of several alkylated diphenylamines were optimized via the exchange correlation method B3LYP, basis set def2-TZVPD, and the dispersion correction method. − Then, Multiwfn software was used to obtain the molecular polarity index (MPI). , The SMD solvent model was used to simulate the ethyl acetate environment, and the structures of alkylated diphenylamine (named R-NR’-H) and its free radicals (R-NR’ and H) were optimized at the D3-B3LYP/def2-TZVPD level of theory, thus obtaining the thermodynamic parameters and spin population of unpaired electrons.

3. Results and Discussion

3.1. Chemical Structure

The synthesis process and designed chemical structure are shown in Figure . The NMR and HRMS spectra of the obtained antioxidants, denoted as 1a–1i, are shown in the Supporting Information.

3.2. Antioxidation Behavior

Rotary oxygen bomb tests simulating the working conditions of lubricating oil were conducted. − The antioxidation performance of the nonpolar oils of LP and PAO is shown in Figure .

2.

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Oxidation induction time of the prepared antioxidants in the nonpolar oils of (a) PAO and (b) LP at a concentration of 0.013 mol/L.

Typically, the weight concentration of antioxidants in lubricating oils ranges from 0.5 wt % to 2.0 wt %. In this study, the lower limiting value of 0.5 wt % (equivalent to 0.013 mol/L) was selected and remained the same for all the tested oils. Pure PAO with low polarity has a short oxidation induction time (OIT) of ∼40 min at 150 °C. After the addition of the 1a–1i antioxidants, the OIT increased to 336, 868, 1650, 2022, 2480, 2684, 2754, 2863, and 3235 min, respectively, which is consistent with the increase in the number of alkyl chains on the structure of diphenylamine. A similar increasing trend was also found for LP without any polar functional groups. As shown in Figure b, the OIT of pure LP at 150 °C in the rotary oxygen bomb test was ∼18 min. After the addition of 1a–1i antioxidant, the OIT gradually increased, reaching a maximum value of 1524 min, which was strongly dependent on the alkyl length in substituted diphenylamine.

To evaluate the antioxidative performance of polar lubricants, DIOS and soybean oil were also selected as the base oils. The results are shown in Figure . As a polar lubricating oil containing multiple ester bonds, pure DIOS has an OIT of ∼48 min, which is prolonged to 3098 min with the addition of diphenylamines, indicating that diphenylamine effectively improved the antioxidation ability of DIOS. However, the OIT is close for antioxidants denoted as 1b–1i, being in the narrow range from 2940 to 3198 min. The antioxidant capacity in DIOS is essentially the same; that is, increasing the length of the substituted carbon chain does not improve their antioxidant performance in DIOS, differing from the results obtained in nonpolar LP and PAO6.

3.

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Oxidation induction time (OIT) of the prepared antioxidants in the polar oils of (a) DIOS and (b) soybean oil at a concentration of 0.013 mol/L.

For soybean oil with higher polarity, the OIT was determined to be 21 min without any antioxidants. After the addition of 1a, the OIT was extended to 203 min, which was 10-fold greater than that of pristine soybean oil. After the addition of 1b-1i at the same concentration, the OIT was essentially the same at ∼200 min. This result indicated that antioxidants from 1a-1i prolonged the oxidation induction period of soybean oil by approximately 10-fold independent of the length of the alkyl chains on diphenylamine, which is similar to the DIOS results.

The prolonged OIT suggests enhanced antioxidant performance in both polar and nonpolar oils, whereas the effects of alkyl groups differ. In nonpolar oils such as LP and PAO, the OIT increases with longer alkyl chains. The incorporation of synthesized diphenylamine derivatives in polar oils also leads to increased OIT, but the value is independent of the alkyl chain.

3.3. Tribological Performance

The friction coefficient and wear rate of metallic couples lubricated with LP and DIOS oils containing a constant weight concentration of 0.5% antioxidants are shown in Figure . In LP, the synthesized antioxidants exhibit a slight friction-reducing effect, decreasing the friction coefficient from 0.135 for the base oil to 0.113–0.130, which is dependent on the antioxidants. A decrease in the wear rate ranging from 9 to 49% was also detected, indicating that the obtained antioxidants also exhibit lubricating performance. A similar enhancement in polar DIOS oil was also found but seems independent of the exact type of antioxidant.

4.

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Friction coefficient and wear rate of friction pairs lubricated by (a) LP and (b) DIOS containing 0.5% antioxidants.

The worn surface morphologies of the LF5 alumina alloy, which is easily oxidized during the wear process, were observed, and typical optical images are shown in Figure . For friction couples lubricated by LP, parallel plowing alongside the sliding direction was observed, as were dark and wide strips assigned to the oxidation products. When antioxidants with longer alkyl chains were added, both abrasion wear and oxidation on the alumina surface were inhibited, generating a smoother worn surface with only a few dark dots. The synthesized antioxidants not only increase the resistance to friction and wear but also protect the friction surface from severe oxidation of metallic friction pairs.

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Worn morphologies of LF5 alumina alloy lubricated by DIOS base oil and 0.5% antioxidants of (b) 1a, (c) 1b, (d) 1c, (e) 1d, (f) 1e, (g) 1f, (h) 1g and (i) 1h.

3.4. Antioxidant Mechanism and Structure Optimization

Generally, the performance of antioxidants is affected by many factors, such as their solubility and environmental conditions. , It can be concluded from Figures – that the OIT of LP and polyalphaolefin gradually increased from 1a to 1i with longer alkyl substituted groups, which has a limited influence on the polar oils of diisooctyl sebacate and primary epoxide soybean. The molecular polarity indices (MPIs) of compounds 1a–1i possibly determine the OIT and were evaluated via theoretical calculations via Multiwfn software , on the basis of the optimized structures at the D3-B3LYP/def2-TZVPD level of theory. The number of carbon atoms is shown in Figure .

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Number of carbon atoms in the optimized molecular structure.

The MPI is defined as eq :

MPI=(1/A)∬S|V(r)|dS 1

where A is the molecular surface area, V is the electrostatic potential of the molecule, and the MPI is obtained by integrating the molecular surface S.

The larger the MPI is, the greater the overall polarity of the molecule. The calculated MPIs are listed in Table . With increasing alkyl chain length, MPI decreased from 7.72 to 6.13 kcal/mol, indicating that the polarity of the molecule decreased with increasing alkyl chain length. According to the similarity compatibility principle, with increasing number of alkyl chains, the compatibility of diphenylamine derivatives in nonpolar lubricating oil significantly increases, which is consistent with the experimental results of OIT.

1. Molecular Polarity Index (MPI), Enthalpy of Formation (Δ f H 298.15 K) for the Neutral Molecules and Free Radicals, and Dissociation Energy (ΔH 298.15 K).

  MPI △ f H 298.15k(R -NR’- H) △ f H 298.15k(R-NR’) △ f H 298.15k(H) △ f H 298.15k
1a 8.31 –518.7039 –518.0712 –0.4995 83.59
1b 7.72 –558.0113 –557.3805 –0.4995 82.43
1c 7.43 –597.3154 –596.6845 –0.4995 82.52
1d 7.15 –636.6200 –635.9891 –0.4995 82.49
1e 6.92 –675.9242 –675.2933 –0.4995 82.46
1f 6.69 –715.2284 –714.5975 –0.4995 82.47
1g 6.49 –754.5325 –753.9017 –0.4995 82.45
1h 6.30 –793.8367 –793.2058 –0.4995 82.44
1i 6.13 –833.1408 –832.5099 –0.4995 82.44
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However, the increase in the length of the alkyl chains is not prominent in the polar lubricants, which is related to the working mechanism of diphenylamine. Diphenylamine is generally considered to prevent oil oxidation by eliminating generated free radicals cyclically. In principle, diphenylamine antioxidants react with free radicals in lubricating oils, resulting in the loss of hydrogen atoms to form nitrogen-containing free radicals, which terminate the free radical chain reaction of the lubricant, preventing further oxidation. , The formation of stable intermediates against free radicals is the key factor determining antioxidative performance.

To evaluate the stability of the diphenylamine radicals, bond dissociation energies of the N–H bonds in compounds 1a–1i were investigated, which were denoted as R-NR’-H, where R stands for alkyl benzene and R’ for phenyl. ASMD solvent model was used to simulate the ethyl acetate environment and optimize the structures of R-NR’-H, R-NR’ free radicals, and H free radicals at the theoretical level of D3-B3LYP/def2-TZVPD. The obtained thermodynamic parameters and spin population of unpaired electrons are listed in Table . The N–H bond dissociation energies from 1a to 1i are 83.59, 82.43, 82.52, 82.49, 82.46, 82.47, 82.45, 82.44, and 82.44 kcal/mol, respectively, decreasing slightly with increasing number of alkyl chains. This finding indicates that the energy absorbed by the breakage of nitrogen and hydrogen bonds on diphenylamine structures with nonpolar alkyl chains is roughly the same in a polar environment.

2. Spin Population on the Atoms in 1a’–1i’.

  1a’ 1b’ 1c’ 1d’ 1e’ 1f’ 1g’ 1h’ 1i’
C1 0.0046 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045
C2 0.1166 0.1007 0.1013 0.1007 0.1003 0.1003 0.1000 0.0999 0.0999
C3 0.0046 0.0064 0.0065 0.0066 0.0067 0.0067 0.0067 0.0068 0.0068
C4 0.1171 0.1274 0.1251 0.1254 0.1258 0.1256 0.1260 0.1260 0.1260
C5 0.1023 0.0910 0.0914 0.0910 0.0907 0.0907 0.0905 0.0904 0.0904
C6 –0.0193 –0.0172 –0.0173 –0.0172 –0.0172 –0.0172 –0.0172 –0.0171 –0.0171
C7 –0.0191 –0.0168 –0.0169 –0.0168 –0.0167 –0.0167 –0.0167 –0.0166 –0.0166
C8 0.1022 0.0905 0.0911 0.0906 0.0903 0.0904 0.0901 0.0900 0.0900
C9 0.1025 0.1048 0.1091 0.1094 0.1095 0.1095 0.1097 0.1097 0.1097
C10 –0.0191 –0.0216 –0.0197 –0.0197 –0.0196 –0.0204 –0.0196 –0.0196 –0.0196
C11 –0.0193 –0.0190 –0.0206 –0.0205 –0.0205 –0.0197 –0.0205 –0.0205 –0.0205
C12 0.1026 0.1140 0.1088 0.1090 0.1091 0.1090 0.1092 0.1093 0.1093
N13 0.3935 0.3851 0.3859 0.3855 0.3854 0.3854 0.3854 0.3854 0.3854
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The isosurfaces of the spin density are depicted in Figure , and the obtained spin population values of different carbon atoms are listed in Table , which further reveals the spin distribution of the nitrogen-containing free radicals. The unpaired electrons in the 1a’–1i’ series of free radicals are mainly concentrated on nitrogen atoms, with specific values of 0.3935, 0.3851, 0.3859, 0.3855, 0.3854, 0.3854, 0.3854, 0.3854, and 0.3854, respectively. In addition, there is also a small alpha single electron distribution on sp2 hybrid carbon atoms, which indicates that unpaired electrons can exist stably once they are formed. The sp3 C in the alkyl chain merely participates in molecular conjugation and disperses the electron density on the nitrogen radical, leading to poor conjugation ability and weak polarity. The electron cloud density on the nitrogen atom in the 1a’–1i’ radical is quite close, indicating similar stability and antioxidant capacity in polar lubricants.

7.

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Isosurfaces of the spin density (isovalue = 0.01), green and blue correspond to the parts where the spin density is positive (alpha electron density) and negative (beta electron density), respectively.

The results of theoretical calculation are consistent with the experimental results and provide an in-depth insight into the antioxidant mechanism. It is illustrated that the antioxidative performance is determined by both the N–H bond dissociation energies and influenced by the polarity matching between antioxidants and base oils. In polar lubricating oils such as DIOS and soybean oil, the generally prolonged OIT after the addition of diphenylamines is attributed to the similar dissociation energy, which is barely influenced by the substituted alkyl groups. However, in nonpolar oils, the influence of polarity overwhelms and results in gradually increased OIT with the increasing alkyl chain length since the compatibility is simultaneously enhanced. The structure design and lubricating oil polarity is two key factors to optimizing the performance of diphenylamine antioxidants. ,

4. Conclusions

In this study, a series of diphenylamine-based antioxidants with different substituted alkyl groups ranging from methyl to octyl were synthesized via the Ullmann method. The antioxidative performance together with the tribological properties of typical polar and nonpolar oils was investigated. Special focus was given to the relationship between substitute groups of diphenylamine and working performance from a microscopic perspective on the basis of Gauss calculations. The developed anoxidants show potential applications in environment friendly oils such as electrolyte, hydraulic fluids, printing ink and metal working fluid while the long-term aging performance and comparison with commercial antioxidant additives need to be further revealed. The main conclusions can be drawn as follows.

  • 1

    In nonpolar base oils of PAO and LP, the oxidation-induced time (OIT) was prolonged from 40 and 18 min after the addition of diphenylamine-based antioxidants, reaching maximum values of 3235 and 3198 min for octyl-substituted diphenylamine, respectively.

  • 2

    In polar oils, the OIT generally increased from 48 and 21 min to ∼3000 min and ∼200 min in DIOS and soybean oils, respectively, but was independent of the alkyl groups in the structure of diphenylamine.

  • 3

    The increase in nonpolar oils was attributed to the greater compatibility induced by longer alkyl groups, which was characterized by a higher molecular polarity index. However, the length of the alkyl group had a limited effect on the stability of nitrogen-containing radicals, leading to a similar increase in OIT in polar oils.

  • 4

    The tribological performance was slightly enhanced by the addition of synthesized oxidants, characterized by a reduction of ∼50% in wear rate owing to preventing the degradation of lubricating oils, the formation of sludge and carbon deposition and severe oxidation of metallic friction pairs.

Supplementary Material

ao4c09652_si_001.pdf (569.5KB, pdf)

Acknowledgments

The authors acknowledge the support provided by the Natural Science Foundation of Henan (242300420047), Key Research Programs of Higher Education Institutions in Henan Province (22A480007), Key Science & Technology Specific Projects of Xinxiang (ZD2020008), and Key Research Projects of Higher Education Institutions in Henan Province (22B480004).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c09652.

  • 1H NMR, 13C NMR, and HRMS spectra for synthesized antioxidants denoted as 1c–1i (PDF)

The authors declare no competing financial interest.

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