Low-Temperature Dielectric Spectroscopy Characterization of the

Jan 24, 2017 - In this study, the two-channel and differential dielectric spectroscopy (TD-DES) technique has been applied to study different degrees ...
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Low-Temperature Dielectric Spectroscopy Characterization of the Oxidative Degradation of Lubricating Oil Yingzhong Gong,† Liang Guan,*,† Xinlu Feng,† Jian Zhou,† Xian Xu,‡ and Liguang Wang† †

Department of Oil Application and Management, Logistical Engineering University, Shapingba District, Chongqing 401311, China Oil Technical Supervision Office, Logistics Support Department, Beijing 100036, China

‡

ABSTRACT: In this study, the two-channel and differential dielectric spectroscopy (TD-DES) technique has been applied to study different degrees of oxidative degradation for two series of simulated oxidized lubricating oils at low temperatures ranging from 20 °C to −55 °C. The deep oxidation, general oxidation, nitration, and sulfation products of the degraded lubricating oil were precisely identified by the Fourier transform infrared spectroscopy (FT-IR) and increased with the level of oxidation, which agreed well with the TD-DES data at room temperature (20 °C). For the severely degraded lubricating oil, the TD-DES real and imaginary data and relaxation characteristic changes from 20 °C to −55 °C were dramatically reduced; the interlacing characteristics of Cole−Cole plots at low temperatures could be ascribed to the formation of high-molecular-weight products during the oxidative degradation process. It was found that the two-dimensional (2D) synchronous and asynchronous dielectric spectroscopy were able to qualitatively describe the degrees of simulated oxidative degradation and the formation of highly oxidized products, as well as to explain the polarization relaxation mechanism of degraded lubricating oil. The partial least-squares (PLS) and multilinear-PLS (N-PLS) regression results indicated that 2D synchronous and asynchronous dielectric spectroscopy could better predict the FT-IR deep oxidation, general oxidation, nitration, and sulfation peak areas than TD-DES real data at 20 °C with regard to lower root-mean-square error of cross-validation (RMSECV), better correlation coefficients (R), and smaller predicted errors. ters.7,22−25 Of all these methods mentioned above, FT-IR and dielectric (impedance) techniques were the most widely used technologies in oxidative degradation analysis.26 As a rapid, automated, and high-throughput quantitative analysis tool, FT-IR presents characteristic infrared absorption peaks associated with the oxidation, nitration, and sulfation products during the oxidative degradation of lubricating oil,16,27 which is consistent with the issued standard test methods as ASTM D7414, D7624, and D7415, respectively. These characteristic spectra peaks were often used to monitor the trends of oxidative degradation, including the determination of TAN and TBN in oil.28−32 A quantitative model was developed by Dong et al.32 between the characteristic absorption of carboxylic acid and TAN. Three mathematical models were established by Adams et al.29 to qualitatively and quantitatively predict TAN and antioxidant concentration, viz, principal component analysis (PCA), partial least-squares (PLS), and two-dimensional (2D) FT-IR correlation spectroscopy. An insite investigation by attenuated total reflectance was carried out by Purushothaman et al.28 into the depletion of detergents and antiwear agents during oxidative degradation. According to Guan et al.,9,33 the appearance of characteristic 1774 cm−1 carboxyl absorption for seriously degraded oil was most likely due to the formation and accumulation of highly oxidized polar products. Dielectric spectroscopy (DES) (also called electrical impedance spectroscopy, EIS) is an important technique to

1. INTRODUCTION It is well-known that the oxidative degradation is one of the most important quality characteristics involved in the drain intervals of lubricating oil. This process is usually accompanied by the depletion of additives and degradation of base oils,1−3 during which undesirable polar oxidation products are formed, such as aldehydes, ketones, carboxylic acids, and polymers (ester, resin, and asphaltene). 4,5 These newly formed degradation products could lead to a gradual change of the physicochemical properties of the lubricating oil, viz., the decrease of total base number (TBN) could be observed in the initial stage of oxidative degradation, followed by the increase of total acid number (TAN) when additives were depleted and insufficient to neutralize the acidic products, and, lastly, the highly oxidized polymers and sludges could lead to a dramatically increase of kinematic viscosity (KV) and insoluble contents (IC).6−9 These properties are often assessed according to standard test methods such as ASTM D974 (for TAN), D445 (for KV), and D893 (for IC). In addition to these standard procedures, voltammetry10 and Fourier transform infrared spectroscopy (FT-IR) have been used to measure the additive depletion; the induced currents,11 iridium oxide chronopotentiometry sensor (iridium oxide sensors),12 dielectric constant,13 ion-selective thick-film ruthenium oxide pH electrode,14 and FT-IR15,16 used to monitor the changes in TAN and TBN; the surface acoustic wave,17 masssensitive quartz crystal microbalances,18 and vibrating piezoelectric cantilever beam19 used to investigate the KV change; the flow injection analysis with visible spectrometry20,21 used to detect the IC trend; and various electrical chemical sensors used to correlate with the TAN, TBN, and KV parame© 2017 American Chemical Society

Received: October 25, 2016 Revised: December 3, 2016 Published: January 24, 2017 2501

DOI: 10.1021/acs.energyfuels.6b02795 Energy Fuels 2017, 31, 2501−2512

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Energy & Fuels Table 1. Oxidation Duration and FT-IR Peak Areas for Oxidative Degradation of Lubricating Oil Samples Series 1

Series 2

FT-IR Peak Area (abs/0.1 mm)

FT-IR Peak Area (abs/0.1 mm)

No.

OD (min)

A1

A2

A3

A4

No.

OD (min)

A1

A2

A3

A4

Ox1_0 Ox1_1 Ox1_2 Ox1_3 Ox1_4 Ox1_5 Ox1_6 Ox1_7 Ox1_8 Ox1_9 Ox1_10 Ox1_11 Ox1_12

0 1230 2550 4280 6140 8090 9720 11 570 13 310 15 050 17 090 18 480 20 180

0.63 0.66 1.05 1.61 2.23 3.40 4.37 5.78 7.18 8.68 10.13 11.46 12.80

6.38 7.71 9.80 12.18 15.13 18.77 21.59 25.11 28.66 31.73 35.51 38.25 41.71

2.76 3.38 3.65 3.87 4.56 4.93 5.39 5.86 7.13 7.46 8.62 9.34 10.32

10.90 14.16 16.81 18.98 22.63 25.06 27.21 29.57 37.87 36.69 39.78 41.15 43.23

Ox2_0 Ox2_1 Ox2_2 Ox2_3 Ox2_4 Ox2_5 Ox2_6 Ox2_7 Ox2_8 Ox2_9 Ox2_10 Ox2_11 Ox2_12

0 1860 3540 5400 7140 9000 10 800 12 600 15 600 18 030 20 910 24 030 26 430

0.43 0.52 0.67 0.83 1.10 1.40 1.53 1.99 3.55 5.47 8.36 11.45 13.77

3.49 4.78 5.53 6.32 6.89 7.69 8.30 9.41 15.24 21.12 29.29 37.14 42.97

1.69 2.19 2.28 2.55 2.44 2.60 2.75 2.79 3.57 4.15 5.09 6.24 7.21

8.57 11.48 12.42 13.38 13.83 14.51 15.02 15.57 18.44 20.72 24.35 27.65 30.15

air flow continuously bubbled into the oils could effectively stir and saturate these samples to different levels of degradation. Approximately 150−180 mL of oil samples were taken for analysis at oxidation duration intervals, which were named Series 1 (Shell Rimula R3 Turbo CH-4 15W-40 series) and Series 2 (Shell Helix HX2 SG/CD 15W-40 series), respectively. The oxidation degradation (OD) for these samples were recorded in minutes, as shown in Table 1. 2.2. FT-IR Spectra Acquisition. A PerkinElmer Model 400 FT-IR spectrometer with 0.1 mm spacer well-polished potassium bromide (KBr) measuring cell was used to obtain the FT-IR spectra of the degraded lubricating oil samples. All spectral data were acquired in the wavenumber range of 4000− 400 cm−1 with 2 cm−1 resolution and six scans at room temperature. 2.3. DES Apparatus and Methods. The two-channel and differential dielectric spectroscopy (TD-DES) analyzer, made by Logistical Engineering University, was employed to collect the TD-DES raw real and imaginary data at the frequency ranging from 1 to 100 kHz with precision of 0.5 Hz, and the excitation voltages ranging from 0.1 to 23 Vpp with precision of 0.001 Vpp.33 The TD-DES device unified both the two-channel and differential measurement methods and the efficient impedance/dielectric measurement function of AD5933 impedance converter chip. The excitation voltages were applied simultaneously to the sample and reference sensors, followed by the differential operation, which could significantly improve the sensitivity of dielectric measurement. The TD-DES raw real and imaginary data were the real and imaginary part of response signal (the dielectric response of sample under excitation voltages after differential treatment), respectively, after discrete Fourier transform without calibration.33 The samples were detected by a pair of planar interdigital capacitor sensors (sample sensor and reference sensor) with gold-plated copper electrode, which had a round shape with an outer diameter of 33 mm and comblike grid with finger width and spacing of 450 μm.33 All the TD-DES data were acquired at temperatures from 20 °C to −55 °C at the interval of 15 °C, with the sine wave excitation voltage of 18 Vpp at the frequency ranging from 1 kHz to 95.5 kHz with an interval of 0.5 kHz. The reference samples for Series 1 and 2 were the corresponding fresh oils Ox1_0 and Ox2_0, respectively. The temperature was controlled with a precision of ±0.1 °C using

explore the polarization mechanism and relaxation behavior of dielectric materials, which could monitor the oxidative degradation process of lubricating oil arising from the polarity increase of oxidized products.7−9 Lvovich et al. studied the influence of electrode electrochemical cells on the high-resistive lubricating oil,34,35 applied the DES technique to monitor the oxidative degradation and condition of lubricating oil,36−38 and designed and fabricated electrical chemical sensors to detect the TAN based on the their EIS characteristics.7,12,14 Perez et al. designed and applied cylindrical capacitors to monitor the condition of the lubricating oil.39 Guan et al. noted that DES could quantitatively and accurately determine the TAN and IC during simulated oxidative degradation.9 However, most research to date has been limited to studying the DES characteristics of lubricating oil at fixed temperature and focusing on establishing the DES relationship with routine physical and chemical properties. Although the overall oxidation degradation levels could be estimated according to the dielectric increase (decrease) and relaxation changes, the internal structure and distribution of oxidation products from degraded lubricating oil could not be reflected. Note that DES itself has great potential in composition, structure, and phase transition analysis, according to the relaxation characteristics;40 rich information could be obtained with regard to the phase transition temperature and structure characteristic of crystal,41 and the interaction of heterogeneous system42 under various temperatures. Based on this background, the two-channel and differential dielectric spectroscopy (TD-DES) was applied to study the relaxation characteristics of degraded lubricating oil under various temperatures. The 2D correlation analysis of temperature-dependence TD-DES data aimed to further understand the change of oil compositions and the formation of oxidation products during the oxidative degradation process.

2. MATERIAL AND METHODS 2.1. Samples. The lubricating oil samples (2.6 L) were subjected to different degrees of oxidation, using an oxidative degradation device that we designed ourselves. This device includes a heating control module, an air pump, a flowmeter, and a sample holder (a 3 L three-necked distillation flask was equipped with an air flow tube, a temperature sensor, and a condenser), which serves to oxidize the oil samples at specified temperature (160 ± 2 °C) and air flow (∼500 mL/min). The 2502

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Figure 1. FT-IR spectra of samples: (a) Series 1 and (b) Series 2.

Figure 2. FT-IR peak areas trends with oxidation duration of samples: (a) Series 1 and (b) Series 2.

and predicted FT-IR peak areas were calculated to evaluate the prediction effect of the predictor vectors on response variables.

the Jiminuo JMBD8013 system (made by Suzhou Jiminuo Instrument Co., Ltd., China). 2.4. Data Processing Methods. Two-dimensional (2D) dielectric correlation spectroscopy, including the 2D synchronous and 2D asynchronous spectra, was adopted for qualitative data analysis of TD-DES data obtained at different temperatures. This method, originated from nuclear magnetic resonance (NMR) spectroscopy, was widely used in infrared (IR), Raman, and near-infrared (NIR) spectroscopy,43 which could reveal correlations between spectral changes and deconvolve overlapping peaks. In this paper, the 2D synchronous and asynchronous dielectric spectra were calculated by TD-DES real data within the temperature range from 20 °C to −55 °C to qualitatively describe the degree of the oxidative degradation and the distribution of formed oxidation products. Partial least-squares (PLS)44 and multilinear partial leastsquares (N-PLS)45 regression models were adopted to quantitatively analyze the effects of TD-DES on predicting the degrees of oxidative degradation. The response variables in PLS and N-PLS were FT-IR deep oxidation, general oxidation, nitration, and sulfation peak areas. The predictor vectors were TD-DES real data for PLS, and 2D synchronous (asynchronous) dielectric data for N-PLS, respectively. The leave-one-out cross-validation (LOOCV)44 was applied to verify the prediction accuracy of PLS and N-PLS models due to the limitation of samples (each series has only 13 samples). The root-mean-square error of cross-validation (RMSECV), correlation coefficient (R), and predicted errors between measured

3. RESULTS AND DISCUSSION 3.1. FT-IR Analysis. The FT-IR spectra of oxidation Series 1 and Series 2 are shown in Figures 1a and 1b, respectively. As shown in Figure 1, four distinct peak features could be observed in the vibration bands: the υ(CO) stretching vibrations centered at 1774 and 1708 cm−1, the υ(NOx) stretching vibration centered at 1604 cm−1, and the υ(SOx) stretching vibration centered at 1158 cm−1, which were called deep oxidation, general oxidation, nitration, and sulfation, respectively. As a characteristic peak of oxidation, the strong υ (CO stretching vibration) centered at 1708 cm−1 was accompanied by another peak centered at 1774 cm−1, which emerged obviously and increased rapidly after sample Ox1_3 and Ox2_7 in Series 1 and Series 2, respectively. The characteristic absorptions in the carbonyl region of 1708 cm−1 and carboxyl region of 1774 cm−1 were usually analyzed together as oxidation. The appearance of 1708 cm−1 absorption was likely due to the formation and accumulation of aldehydes and ketones, while the 1774 cm−1 peak was ascribed, to a large extent, to the further oxidized carboxylic acids, esters, lactones, etc.46,47 The induction and vibration coupling effect of electronwithdrawing groups directly connected to the CO of the further oxidized products, such as −OH, OR′, OC−O, etc., may shift the carbonyl (CO) characteristic absorption peaks (1708 cm−1) toward high frequencies (1774 cm−1). 2503

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Figure 3. TD-DES spectra of samples at 20 °C: real plots ((a) Series 1 and (b) Series 2), imaginary plots ((c) Series 1 and (d) Series 2), and Cole− Cole plots ((e) Series 1 and (f) Series 2).

range of 1760−1670 cm−1, nitration (A3) in the range of 1640−1600 cm−1, and sulfation (A4) in the range of 1180− 1120 cm−1, were calculated by Matlab R2013b programming, according to eq 1, and the baseline points m and n were determined by minima FT-IR spectra absorbance at 2200 to 1900 and 650 to 550, respectively, according to ASTM E2412.27 The four characteristic FT-IR peak areas for the oxidative degradation of lubricating oil samples are given in Table 1 and the trends with oxidation duration were shown in Figure 2. According to the increasing rate of the peak areas of deep oxidation, general oxidation, nitration, and sulfation shown in Table 1 and Figure 2, the FT-IR peak areas of the series 1 samples increased uniformly (no inflection point was observed) while those of the Series 2 samples exhibited two stages of oxidative degradation (with an induction period of ∼12 600

The corrected FT-IR absorbance peak area was fundamental to the direct trend analysis of oxidative degradation of lubricating oil. For a characteristic FT-IR peak in the range from a to b, the corresponding baseline points were m and n (n < min (a and b), m > max (a and b)), the corrected absorbance peak area A was calculated by using eq 1: b

A=

b

∑ Absi − ∑ Babsi i=a

i=a

⎛ ⎞ Absm − Absn ⎜Babsi = Absn + (i − n)⎟ ⎝ ⎠ m−n

(1)

where i is the wavenumber; Absi and Babsi are the FT-IR absorbance of spectra and baseline at the ith wavenumber, respectively. The corrected absorbance areas for deep oxidation (A1) in the range of 1820−1760 cm−1, general oxidation (A2) in the 2504

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Figure 4. (a, c, e) TD-DES real spectra and (b, d, f) TD-DES imaginary spectra of Series 1 samples at temperatures of 20 °C to −55 °C: Ox1_0 (panels (a) and (b)); Ox1_3 (panels (c) and (d)); and Ox1_12 (panels (e) and (f)).

component breakdown, or the combustion byproducts of gasoline and diesel fuel for the lubricating oils used.27 As can be seen from Figure 2 and Table 1, the characteristic peaks of deep and general oxidation presented the most sensitive responses to different degradation levels for both series of oil samples. As depicted by Figure 2, the deep oxidation peak areas (blue line) increased more rapidly and faster than general oxidation (black line) after sample Ox1_4 of series 1 and sample Ox2_7 of series 2. The possible reason could be that the deep oxidation products, especially the polar compounds such as gum and asphaltenes, were gradually formed and accumulated after the inflection points. The composition changes of the lubricant species could be further distinguished with the subsequent dielectric spectroscopy technique. 3.2. TD-DES Analysis. 3.2.1. TD-DES Characteristics of the Oxidative Degradation of Lubricating Oil at Room

min). The increasing trend of the FT-IR peak areas at the early oxidation stage of Series 2 samples (Ox2_0 to Ox2_7) was much slower than that at the later stage (Ox2_8 to Ox2_12). The possible reason could be that the fresh oil of the Series 2 (from Shell Helix HX2 SG/CD 15W-40) samples had better antioxidation performance and, therefore, longer induction period value. Although the four characteristic peak areas presented consistent features of increasing trend with the increase of degradation degree, the absorption peaks due to the CO stretching vibration could better clarify the changes associated with the process of oxidative degradation. The absorption peaks centered at 1708 and 1774 cm−1 were involved with series of complex reactions, including auto-oxidation, condensation, and polymerization,48 while the absorption peaks of nitration and sulfation degradation products may come from the lubrication 2505

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Figure 5. Cole−Cole plots of Series 1 samples ((a) Ox1_0, (c) Ox1_2, (e) Ox1_3, (g) Ox1_7, and (i) Ox1_12) and Series 2 samples ((b) Ox2_0, (d) Ox2_7, (f) Ox2_8, (h) Ox2_10, and (j) Ox2_12) at temperatures of 20 °C to −55 °C. 2506

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Energy & Fuels Temperature. The TD-DES raw real and imaginary data and complex plane dielectric diagrams (TD-DES real on the x-axis and imaginary on the y-axis, also called Cole−Cole plots) of degraded oil samples from Series 1 and Series 2 which, measured at room temperature (20 °C), are shown in Figure 3. As shown in Figure 3, with the increasing degree of oxidation, the TD-DES real data (Figures 3a and 3b) increased in the low-frequency ranges of 10−60 kHz, and decreased in the high-frequency ranges of 80−95 kHz; meanwhile, the peaks at ∼10 kHz increased in intensity and shifted to low frequencies. The TD-DES imaginary data (Figures 3c and 3d) remained almost unchanged at 1−35 kHz but increased at 40−95 kHz as the oil samples degraded further. The Cole−Cole plots of Series 1 (Figure 3e) and Series 2 (Figure 3f) samples with elliptical shapes were deviated from the typical empirical equations such as Cole−Cole, Cole− Davidson, and Havriliak−Negami, according to the semicircular arc rules. The plot was obtained based on the two-channel and differential measurement method and the uncalibrated TDDES real and imaginary data. TD-DES relaxation characteristic differences can be observed in the low-frequency ranges from the Cole−Cole plots, which were gradually expanding outward at lower frequencies as the oxidation duration increased. Lubricating oil usually contained large numbers of nonpolar base oils and small amounts of functional polar additives. During the degradation process of simulated oxidation, a small fraction of light components could be evaporated out of the oil system, and high-molecular-weight compounds with large viscosity value could be generated when the additives were completely depleted. The positive response of the relaxation characteristics to the composition changes in Figure 3 did prove that the TD-DES technique can be a useful tool for monitoring the degradation process of lubricating oil. It can be clearly seen from Figure 3 that the TD-DES real and imaginary data, and relaxation characteristics, changed faster for Series 1 (samples Ox1_0 to Ox1_7) at the early stage of oxidation, and the same held true for Series 2 (samples Ox2_8 to Ox2_12) at the fast stage of oxidation, which showed good agreement between the TD-DES data and the FT-IR results mentioned above for Series 2 samples. Moreover, the shift to lower relaxation frequencies with the increase of oxidation level could be due to the longer relaxation times needed to reach a new equilibrium of polarization under sinusoidal excitation, when various polar compounds with high molecular weights and large viscosity values were formed. 3.2.2. Low-Temperature TD-DES Characteristics of Oxidative Degradation of Lubricating Oil. To get more structure and composition information about the degraded oil samples, the TD-DES technique was applied to different temperatures, ranging from 20 °C to −55 °C to analyze the degradation characteristics. The temperature and frequency dependence of the TD-DES real and imaginary data for Series 1 samples with different degrees of oxidation are shown in Figure 4. As shown in Figure 4, when the temperature decreased from 20 °C to −55 °C, the TD-DES real and imaginary data showed obvious changes in low frequencies (1−30 kHz) while remaining almost unchanged in high frequencies. The TDDES real data (Figures 4a, 4c, and 4e) decreased in the lowfrequency ranges of 1−10 kHz, and increased in the frequency ranges of 15−50 kHz with the decrease of temperature, and meanwhile, the peaks at ∼10 kHz increased in intensity and shifted to high frequencies. The TD-DES imaginary data

(Figures 4b, 4d, and 4f) decreased with decreasing temperature at 1−30 kHz. As the degree of oxidation was increased, the temperature dependence of the TD-DES data was significantly reduced, especially for real data in frequency ranges of 10−60 kHz and the peaks at ∼10 kHz. The temperature dependences of Ox1_0, Ox1_3, and Ox1_12 were shifted to narrower frequencies as the degree of oxidation increased, viz, 10−60, 10−50, and 10−40 kHz, respectively. As the molecular mobility of the oil system was significantly reduced at low temperatures, the molecular motion could not keep pace with the change of sinusoidal at high frequencies. Therefore, the low-temperature effects on TD-DES real and imaginary data were focused mainly in low frequencies; inversely, the oxidative degradation effects were imparted to the whole frequencies (Figures 3a and 3b). The TD-DES real and imaginary data changes with decreasing temperatures were similar for Series 1 and Series 2 oil samples, which could be clearly reflected in the relaxation characteristics (Cole−Cole plots) derived from the TD-DES real and imaginary data. Typical temperature dependence of relaxation characteristics for both series are shown in Figure 5. As can be seen from Figure 5, the temperature-dependent relaxation characteristics of Series 1 and 2 samples were dramatically changed in the low frequencies. Temperature showed pronounced effects on the relaxation characteristics of the fresh and slightly oxidized oil samples of both series (Ox1_0, Ox1_2; Ox2_0, Ox2_7). For the fresh oil samples (Ox1_0, Ox2_0; Figures 5a and 5b), the Cole−Cole plots expanded outward with the decreasing temperature, and similar trend could be observed for oil samples that were slightly oxidized (Ox1_2, Ox2_7; Figures 5c and 5d). For the fresh and slightly oxidized oil samples (Ox1_0, Ox1_2; Ox2_0, Ox2_7), few polar compounds were formed in the oil system, according to the FT-IR results. The outward expansion trends of the Cole−Cole plots with the decreasing temperature could be ascribed to the significant decrease of the molecular mobility of the oil system by the low temperature ramped down, and a longer relaxation time may be needed to reach a re-equilibrium of polarization under sinusoidal; therefore, the corresponding relaxations were in low frequencies and changed significantly. However, the temperature dependence of the relaxation characteristics was greatly weakened when the oil system was significantly degraded. The Cole−Cole plot of Ox1_3 (Figure 5e) and Ox2_8 (Figure 5f) were interlaced to each other at −40 °C and −55 °C, and these interlacing characteristics were shifted to lower frequencies and higher temperatures for those seriously degraded samples (Figures 5g−j, corresponding to samples Ox1_7, Ox1_12; Ox2_10, Ox2_12), distinctly different from the relaxation characteristics of the fresh and slightly oxidized oil samples. According to the FT-IR results, the highmolecular-weight oxidation products such as gum and asphaltenes could be formed and aggregated in the base oil when the oil samples were severely degraded. These highmolecular-weight polar compounds could decrease the intermolecular thermal motion and weaken the temperature dependence of the relaxation characteristics. In conclusion, the low-frequency changes of TD-DES real and imaginary data and relaxation characteristics for the Series 1 and 2 samples were dependent on both the external temperature exerted and the formation of the oxidized polar compounds. For the slightly oxidized oil samples, the low 2507

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Figure 6. Two-dimensional (2D) synchronous and asynchronous correlation dielectric spectra of samples: (a, c) Series 1 and (b, d) Series 2.

temperature hindered the molecular motion of oil samples, which was responsible for the changes of TD-DES real and imaginary data in low frequencies and the relaxation characteristics. For the seriously degraded oil samples, the increased viscosity and polarity by the highly oxidized polar compounds hindered the molecular mobility to a greater extent, which could weaken the dependence of temperature and decrease the change in relaxation characteristics. The TD-DES at varied low temperatures did provide more information about the degrees of oxidative degradation and the composition characteristics of oil samples associated with the degradation process. 3.2.3. 2D Dielectric Correlation Spectroscopy Characteristics of Oxidative Degradation of Lubricating Oil. To better understand the degree of oxidation and composition characteristics of oil samples under low temperatures, the 2D synchronous and asynchronous dielectric spectra calculated by TD-DES real data at temperatures from 20 °C to −55 °C for both series are presented in Figure 6. According to the synchronous 2D spectra shown in Figures 6a and 6b, there were two characteristic auto peaks (5−15 kHz, 5−15 kHz) and (65−80 kHz, 65−80 kHz) and a pair of cross peaks at (8−12 kHz, 55−90 kHz) and (55−90 kHz, 8−12 kHz) for both series of oil samples.

A wider frequency range of synchronous auto peaks reflected a higher degree of oxidative degradation. The shape and frequency ranges of samples Ox1_0 to Ox1_2 and Ox2_0 to Ox2_7 were significantly different from that of samples Ox1_3 to Ox1_12 and Ox2_8 to Ox2_12. This finding agreed well with the above-mentioned FT-IR results that deep oxidation products increased more rapidly after Ox1_4 and Ox2_7 for Series 1 and Series 2, respectively, and Series 2 entered into advanced stages of oxidative degradation after Ox2_7. The slightly oxidized oil samples could be easily distinguished from highly degraded ones. The synchronous cross peaks were associated with the relaxation mechanism of lubricating oils and in similar response to the degradation degrees of oil samples as the synchronous auto peaks. The gradual and consistent changes of synchronous cross peaks indicated that the relaxation mechanism of slightly oxidized and seriously degraded oil samples were similar. The polar oxidation products from the highly oxidized oil samples did not change the relaxation mechanism, which differed from the admixture of water into lubricating oil that altered the polarization mechanism of the oil system.33 The synchronous 2D spectra indicated that both auto and cross peaks correlated 2508

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Figure 7. TD-DES spectra prediction of FT-IR peak areas: (a, b) deep oxidation; (c, d) general oxidation; (e, f) nitration; and (g, h) sulfation. (Data for Series 1 samples are shown in panels (a), (c), (e), and (g); data for Series 2 samples are shown in panels (b), (d), (f), and (h).) 2509

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In contrast to the TD-DES real data obtained at room temperature (20 °C), the 2D dielectric correlation spectroscopy obtained at different temperatures (from 20 °C to −55 °C) showed better prediction effects and could qualitatively and quantitatively analyze the degradation degrees of oil samples and obtain the composition information on the degraded products.

well with the oxidation degradation degrees described by FT-IR peak areas and TD-DES raw data in 20 °C. Figures 6c and 6d showed the asynchronous 2D spectra characteristics of the oxidative degradation of lubricating oil, with a pair of cross peaks for Series 1 (10−40 kHz, 35−95 kHz) and Series 2 (35−95 kHz, 10−40 kHz), respectively. Unlike the synchronous 2D spectra shown in Figures 6a and 6b, a higher degree of oxidative degradation presented a narrower frequency range of the asynchronous cross peaks, which eventually disappeared in sample Ox1_3 and Ox2_8 for Series 1 and Series 2, respectively. For the fresh and slightly oxidized samples (Ox1_0 to Ox1_2; Ox2_0 to Ox2_7), the cross peaks in asynchronous 2D spectra were due to the fact that the TD-DES response to different components in oil system was not at the same speed at different temperatures. The newly formed lower contents of polar components did not prevent the relative motion of the components, whereas the low temperature hindered the molecular mobility of the oil system, leading to much slower motion speed from that at high temperature. The larger the mobility difference, the wider the cross peaks. In the seriously oxidized samples (Ox1_3 to Ox1_12; Ox2_8 to Ox2_12), however, the formation of highly degraded polar compounds prevented the relative molecule motion of the oil system. The motion speed uniformity at different temperatures could explain the narrowing and disappearance of cross peaks in the asynchronous 2D spectra of the highly degraded oil samples. 3.3. Quantitative Analysis of Oxidative Degradation. For quantitative analysis of the oxidative degradation of lubricating oil, TD-DES real data obtained at 20 °C and 2D synchronous (asynchronous) dielectric data obtained at temperatures from 20 °C to −55 °C were employed to establish a correlation with FT-IR peak areas (including deep oxidation, general oxidation, nitration, and sulfation) and make a prediction by PLS (for TD-DES real data obtained at 20 °C) and N-PLS (for 2D synchronous (asynchronous) dielectric data) models; the data process methods were mentioned in section 2.4, and the results (including RMSECV, R, predicted error bars, and the fitted line between predicted and measured peak areas) are shown in Figure 7. We can see from Figure 7 that both the TD-DES real data and 2D synchronous (asynchronous) dielectric data could well predict the FT-IR deep oxidation (Figures 7a and 7b), general oxidation (Figures 7c and 7d), nitration (Figures 7e and 7f), and sulfation (Figures 7g and 7h) peak areas: a good linear relationship could be observed between predicted and measured (calculated) FT-IR oxidation peak areas with a correlation coefficient of R > 0.9100. The 2D synchronous data (denoted by inverted blue triangles) and asynchronous (denoted by green diamonds) have lower RMSECV, better R, and smaller predicted errors (error bars), compared with TDDES real data (represented by black triangles), to predict the FT-IR deep oxidation, general oxidation, nitration, and sulfation peak areas for both series of oil samples. The results of TD-DES real data from Series 1 samples to predict the FT-IR deep oxidation, general oxidation, nitration, and sulfation peak areas were weaker than that from Series 2 samples. The TD-DES data trends of Series 2 samples at 20 °C showed good agreement with the FT-IR results mentioned in section 3.2.1. The predicted errors of TD-DES real data were relatively larger in the case of Ox1_4 and Ox1_5 for prediction of deep oxidation, general oxidation, nitration, and sulfation, and, in the case of Ox2_12, for prediction of deep oxidation.

4. CONCLUSIONS The degradation processes of two series of lubricating oil samples were investigated by means of the two-channel and differential dielectric spectroscopy (TD-DES) technique at temperatures ranging from 20 °C to −55 °C. The characteristic Fourier transform infrared (FT-IR) peak areas of deep oxidation, general oxidation, nitration, and sulfation increased as the degree of degradation of the oil samples increased. An advanced stage of oxidation could be observed after sample Ox2_7 in Series 2, which led to the rapid formation and accumulation of polar oxidation products. The TD-DES real and imaginary data and relaxation characteristics at 20 °C showed good agreement with the FT-IR oxidation peak areas. The TD-DES spectra at 20 °C to −55 °C indicated that both series of oil samples showed temperature-dependent relaxation characteristics. The TD-DES real and imaginary data and relaxation characteristic changes in low frequencies were dependent on both the exerted temperature and the increased viscosity and polarity by the highly oxidized polar compounds, which could lead to a long relaxation time to reach the new equilibrium under sinusoidal excitation. The interlacing behavior of the Cole−Cole plots and the shift to lower frequencies at low temperatures provided strong evidence for the TD-DES relaxation characteristics of seriously degraded oxidation products. According to the two-dimensional (2D) synchronous dielectric spectra, a higher degree of oxidative degradation presented a wider frequency range of synchronous auto peaks and a narrower frequency range of asynchronous cross peaks. 2D asynchronous dielectric spectra cross peaks were present in narrower frequency ranges and finally disappeared when highly degraded polar compounds were formed. Although both results of TD-DES real data obtained at 20 °C and 2D synchronous (asynchronous) dielectric data obtained at 20 °C to −55 °C showed good consistency, the 2D synchronous (asynchronous) could better predict the FT-IR peak areas (deep oxidation, general oxidation, nitration, and sulfation) than TD-DES real data, with regard to lower root-mean-square error of crossvalidation (RMSECV), better correlation coefficients (R), and smaller predicted errors.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 023 86731450. E-mail: [email protected]. ORCID

Yingzhong Gong: 0000-0003-0228-3852 Liang Guan: 0000-0002-6922-1691 Jian Zhou: 0000-0001-9357-2929 Notes

The authors declare no competing financial interest. 2510

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ACKNOWLEDGMENTS This work was funded by National Natural Science Foundation of China (No. 21205136), Chongqing Science & Technology Commission Frontier and Applied Basic Research Project (general) of China (No. cstc2014jcyjA50007) and Chongqing Graduate Research & Innovation Project of China (No. CYB14101).

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