Abstract:
Alkylthioperoxydiphosphates having an alkyl group between about 4 and 18 carbons, a decomposition temperature between about 150 and 300° C. and a wear volume between about 0.0025 and 0.0015 mm 3  in HFRB testing, and methods of making same. Lubricant additives that includes the alkylthioperoxydiphosphates.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional application Ser. No. 61/642,921 filed on May 4, 2012. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Zinc dialkyldithiophosphates (ZDDPs) have been the primary anti-wear and antioxidant additives in engine oil and other industrial lubricant formulations for more than 60 years. Over the past half century, considerable research efforts have been focused on the characterization of ZDDP tribofilm to elucidate its chemical composition and molecular structure, and the mechanism of tribofilm formation. Although the exact pathways leading to the formation of anti-wear film on metal surfaces are not well understood due to the complicated characteristics of reactions in lubricant formulations, it is generally accepted that the thermal, hydrolytic, and oxidative decomposition of ZDDP is involved. In addition it is understood that the degraded intermediates of ZDDP are actively involved with the formation of protective tribofilms on metal surfaces, which prevents the direct contact of metal surfaces in boundary condition, and therefore reduces wear. 
         [0003]    During the investigation of the lubrication mechanism of ZDDP, many attempts to simulate combustion engine conditions and the identification of decomposed intermediates have been reported. It is generally accepted now that the rapid decomposition of ZDDP generates both soluble and insoluble organic and inorganic phosphorus intermediate compounds. These soluble products identified by  31 P-NMR may include (RO) 2 P(═S)SSP(═S)(OR) 2 , (RO) 2 P(═S)SR, (RO) 2 P(═S)SH, (RO) 3 P═S, (RS) 3 P═O, and (RO) 3 P═O. The insoluble residue may consist of zinc sulfate, polyphosphate, and/or pyrophosphate compounds. 
         [0004]    Over the past two decades, environmental concerns have been the major driving force for replacing ZDDP with zinc-free, lower phosphorus, and lower sulfur anti-wear additives, at least partially, especially in the automobile industry. Ashless lubricant additives are a potential alternative to ZDDP, and provide necessary protection against wear. Earliest research work and application of ashless lubricant additive goes back to the 1940s. In general, based on the elements contained in their chemical structures, ashless lubricant additives can be classified as phosphorus additives, e.g. phosphate esters, phosphites, sulfur additives, e.g. sulfurized olefins, and additives with multiple elements, e.g. alkyldithiophosphates, dithiocarbamates, and amine phosphates. More recently, some novel ashless anti-wear and/or extreme-pressure additives containing other elements, i.e. boron and fluorine, have been reported. 
         [0005]    The lubrication mechanism of ashless additives is more complicated than that of ZDDP due to the variety of chemical structures. It is believed that ashless additives function by either promoting physical absorption of additives to metal surfaces, or accelerating chemical reactions with iron to form protective tribofilms. Investigation of chemical composition of tribofilms via modern surface techniques, such as X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge structure (XANES), X-ray photoemission electron microscopy (XPEEM), and Fourier transform infra-red (FT-IR) spectroscopy, shows iron polyphosphates and sulfates are generated by tributyl thiophosphate. Iron sulfide together with iron polyphosphates and sulfates were found in the tribofilm formed by alkyldithiophosphates. 
         [0006]    Alkylthioperoxydiphosphates have been identified as the principal decomposed product of ZDDP (Willermet, P. A. and Kandah, S. K. Lubricant degradation and wear v. reaction products of Zinc dialkyldithiophosphate and peroxy radicals. ASLE Trans, 27, 67-72 (1984)). An earlier study reported the synthesis of alkylthioperoxydiphosphates with short alkyl chains (≦C 10 ) by I 2  oxidation, and subsequently evaluated their anti-wear properties on four-ball machine (Sarin, R. et al. Synthesis and performance evaluation of O,O-dialkylphosphorodithioic disulphides as potential antiwear, extreme-pressure and antioxidant additives. Tribol Int, 26, 389-94 (1993)). 
         [0007]    It would be useful to have better anti-wear lubricant additives, particularly which are zinc-free and contain less phosphorus and sulfur. 
         [0008]    It would be useful to have a synthetic method for making alkylthioperoxydiphosphates having various alkyl chain lengths through H 2 O 2  oxidation reactions, rather than using I 2  oxidation. 
         [0009]    It would be useful to have alkylthioperoxydiphosphates with alkyl chain lengths greater than ten carbons, and a method of making same. 
       SUMMARY OF THE INVENTION 
       [0010]    The invention is directed to alkylthioperoxydiphosphates having an alkyl group between about 4 and 18 carbons, or more preferably between about 13 and 18 carbons. The alkylthioperoxydiphosphates have a decomposition temperature between about 150 and 300° C. and a wear volume between about 0.0025 and 0.0015 mm 3  in HFRB testing. 
         [0011]    The invention further is directed to methods of making alkylthioperoxydiphosphates having an alkyl group between about 4 and 18 carbons by a two step process involving oxidizing an O,O-dialkyl dithiophosphoric acid with H 2 O 2 . 
         [0012]    In another embodiment, the invention is a lubricant additive that includes an alkylthioperoxydiphosphate. Desirably the alkylthioperoxydiphosphate alkyl groups have between about 4 and 18 carbons, and more preferably about 13 to 18 carbons. The alkylthioperoxydiphosphates have a decomposition temperature between about 150 and 300° C. and a wear volume between about 0.0025 and 0.0015 mm 3  in HFRB testing. 
         [0013]    The alkylthioperoxydiphosphate is desirably tridecylthioperoxydiphosphate, 1-methyl dodecylthioperoxydiphosphate, tetradecylthioperoxydiphosphate, or octadecylthioperoxydiphosphate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a  1 H NMR spectrum of tridecylthioperoxydiphosphate. 
           [0015]      FIG. 2  is a  31 P NMR spectrum of tridecylthioperoxydiphosphate 
           [0016]      FIG. 3  is a  1 H NMR spectrum of 1-methyldodecylthioperoxydiphosphate 
           [0017]      FIG. 4  is a  31 P NMR spectrum of 1-methyldodecylthioperoxydiphosphate. 
           [0018]      FIG. 5  illustrates FT-IR spectra of tridecylthioperoxydiphosphate and 1-methyldodecylthioperoxydiphosphate. 
           [0019]      FIG. 6  is TGA plots illustrating the thermal stability of four alkylthioperoxydiphosphates in a nitrogen environment. 
           [0020]      FIG. 7  is a bar graph showing calculated wear volumes for various alkylthioperoxydiphosphates. 
           [0021]      FIG. 8  illustrates the P L-edge FLY spectra of pure ZDDP and octadecylthioperoxydiphosphate powder along with model compounds FePO 4 , Zn 3 (PO 4 ) 2  and Fe 4 (P 2 O 7 ) 3 . 
           [0022]      FIG. 9  illustrates the S L-edge FLY spectra of pure ZDDP and octadecylthioperoxydiphosphate together with model compounds Fe 2 (SO 4 ) 3 , FeSO 4 , ZnSO 4 , FeS 2 , FeS and ZnS. 
           [0023]      FIG. 10  shows the P L-edge FLY spectra of thermal films formed at 30 minutes of thermal exposure. 
           [0024]      FIG. 11  illustrates the S L-edge FLY spectra of thermal films formed at 30 minutes of thermal exposure. 
           [0025]      FIG. 12  shows the P K-edge TEY XANES spectra of the model compounds Zn 3 (PO 4 ) 2  and FePO 4  along with thermal films formed from ZDDP, tridecylthioperoxydiphosphate, and 1-methyldodecylthioperoxydiphosphate at 30 minutes of thermal exposure. 
           [0026]      FIG. 13  illustrates the S K-edge TEY XANES spectra of model compounds ZnSO 4 , ZnS, FeS 2 , FeS, FeSO 4  and Fe 2 (SO 4 ) 3  along with thermal films formed from ZDDP, tridecylthioperoxydiphosphate, and 1-methyldodecylthioperoxydiphosphate at 30 minutes of thermal exposure. 
           [0027]      FIG. 14  illustrates the P L-edge TEY spectrum of tribofilms from ZDDP, octadecylthioperoxydiphosphate, 1-methyldocecylthioperoxydiphosphate, and tridecylthioperoxydiphosphate. 
           [0028]      FIG. 15  illustrates the P L-edge FLY spectrum of tribofilms from ZDDP, octadecylthioperoxydiphosphate, 1-methyldocecylthioperoxydiphosphate, and tridecylthioperoxydiphosphate. 
           [0029]      FIG. 16  illustrates the S L-edge TEY spectrum from ZDDP and octadecylthioperoxydiphosphate, 1-methyldocecylthioperoxydiphosphate, and tridecylthioperoxydiphosphate. 
           [0030]      FIG. 17  illustrates the S L-edge FLY spectrum from ZDDP and octadecylthioperoxydiphosphate, 1-methyldocecylthioperoxydiphosphate, and tridecylthioperoxydiphosphate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    The present disclosure may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The present disclosure is illustrated in the accompanying drawings, throughout which like reference numerals may indicate corresponding or similar parts in the various figures. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the disclosure. Given the following enabling description, the novel aspects of the present disclosure should become evident to a person of ordinary skill in the art. 
         [0032]    The general structure of alkylthioperoxydiphosphates is illustrated below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0033]    Alkylthioperoxydiphosphates having an alkyl chain length between 4 and 18 carbons were synthesized via a two-step reaction, O,O-dialkyl dithiophosphoric acids were obtained by mixing phosphorus pentasulfide with appropriate alcohols at an elevated temperature in toluene; this facile reaction usually gives a quantitative yield. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0034]    The oxidation of O,O-dialkyl dithiophosphoric acids was carried out by adding H 2 O 2  solution (37%) into a vigorously stirred O,O-dialkyl dithiophosphoric acid/ice mixture held in an ice bath. The pathway of the reaction is outlined below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0035]    Alkylthioperoxydiphosphates having various alkyl constituents from 4 to 18 carbons were synthesized and characterized. The compounds and their general characteristics are shown in Table 1. All reactions gave fairly good yields. 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Yield 
                 Physical property 
               
               
                 Alkylthioperoxydiphosphate 
                 R (# of carbons) 
                 (%) 
                 (Melting point ° C.) 
               
               
                   
               
             
             
               
                 Butylthioperoxydiphosphate 
                 —CH 2 CH 2 CH 2 CH 3  (4) 
                 90 
                 Colorless liquid 
               
               
                 1-Methyl 
                 —CH(CH 3 )CH 2 CH 3  (4) 
                 92 
                 Colorless liquid 
               
               
                 propylthioperoxydiphosphate 
               
               
                 1,3-Dimethyl 
                 —CH(CH 3 )CH 2 CH(CH 3 )CH 3  (6) 
                 91 
                 Colorless liquid 
               
               
                 butylthioperoxydiphosphate 
               
               
                 2-Ethyl 
                 —CH 2 CH(CH 2 CH 3 )CH 2 CH 2 CH 2 CH 3  (8) 
                 93 
                 Colorless liquid 
               
               
                 hexylthioperoxydiphosphate 
               
               
                 Octylthioperoxydiphosphate 
                 —CH 2 (CH2) 6 CH 3  (8) 
                 93 
                 Colorless liquid 
               
               
                 1-Methyl 
                 —CH(CH 3 )CH 2 (CH2) 4 CH 3  (8) 
                 92 
                 Colorless liquid 
               
               
                 heptylthioperoxydiphosphate 
               
               
                 Tridecylthioperoxydiphosphate 
                 —CH 2 (CH 2 ) 11 CH 3  (13) 
                 86 
                 Colorless liquid 
               
               
                 1-Methyl 
                 —CH(CH 3 )CH 2 (CH 2 ) 9 CH 3  (13) 
                 89 
                 Colorless liquid 
               
               
                 dodecylthioperoxydiphosphate 
               
               
                 Tetradecylthioperoxydiphosphate 
                 —CH 2 (CH 2 ) 12 CH 3  (14) 
                 90 
                 Colorless liquid 
               
               
                 Octadecylthioperoxydiphosphate 
                 —CH 2 (CH 2 ) 16 CH 3  (18) 
                 91 
                 White solid 
               
               
                   
                   
                   
                 (37-38° C.) 
               
               
                   
               
             
          
         
       
     
         [0036]      1 H NMR,  31 P NMR, Fourier transform infra-red (FT-IR), Electrospray Ionization Time of Flight Analysis (ESI-TOF), and Matrix Assisted Laser Desorption Ionization Mass Spectroscopy (MALDI-MS) were used to confirm the structures of the synthesized compounds. The results are shown in the experimental section. 
         [0037]    Volatility and thermal degradation play a critical role in the anti-wear performance of lubricant additives. Accordingly, thermal stability of the synthesized compounds was measured. Alkylthioperoxydiphosphates prepared from primary alcohols demonstrated slightly better thermal stability than those from secondary alcohols, and compounds with long chain length (&gt;13 carbon atoms) are thermally stable under nitrogen beyond 200° C. In a study of thermal decomposition of ZDDP using DSC it was shown that in a flowing nitrogen environment ZDDP decomposes between 170-200° C. depending on the heating rate, with a higher heating rate (5° C./minute) yielding a decomposition endotherm at 200° C. and a slower heating rate (0.1° C./minute) yielding a decomposition endotherm centered around 170° C. The alkylthioperoxydiphosphates can be synthesized with tunable decomposition temperatures, including within the range exhibited by ZDDP, determined by the alkyl chain lengths and whether the alkyl chain has a primary or secondary structure. 
         [0038]    High-frequency reciprocating ball (HFRB) tests were carried out to evaluate anti-wear performance of alkylthioperoxydiphosphates, and test conditions are listed in Table 2. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Parameter 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 Test duration 
                 60 min 
               
               
                   
                 Load 
                 1 Kg (9.8 N) 
               
               
                   
                 Stroke Length 
                 2.5 mm 
               
               
                   
                 Stroke Frequency 
                 50 Hz 
               
               
                   
                 Temperature 
                 100° C. 
               
               
                   
                 Ball 
                 52100 Steel (6.25 mm diameter) 
               
               
                   
                 Disk 
                 52100 Steel 
               
               
                   
                   
               
             
          
         
       
     
         [0039]    Topography of the wear scars was measured using the Veeco Wyko NT9100 Optical Profiler. The results indicate that formulations with alkylthioperoxydiphosphates of long alkyl chain length produce significantly lower wear volume in comparison to ZDDP at the same phosphorus level. The topographical image derived from optical profilometry for ZDDP in base oil shown a wear scar with a maximum depth of 10 μm which remains more or less uniformly deep along the length of the wear scar. However the wear scar from tridecylthioperoxydiphosphate has a maximum depth of 3.5 μm with many regions much shallower than that. 1-methyl dodecylthioperoxydiphosphate exhibits the best wear performance with a maximum depth of only 2 μm for similar test conditions. The differentiation in extent of wear is also reflected by the amount of debris build up on the sides of the wear track with ZDDP exhibiting the largest build up and the 1-methyl dodecylthioperoxydiphosphate exhibiting the smallest build up. Calculated wear volume for all the formulations and ZDDP is shown in  FIG. 7  and Table 3. Note that the maximum depth numbers are different from maximum height of the profile R t  in the table. Due to the edges build up caused by debris, maximum height of the profile R t  is calculated from maximum peak height and maximum valley depth and the value is higher than the estimated maximum depth which derived from the original surface to the deepest valley. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
                 Wear Volume 
                 Maximum Height of the 
               
               
                 Formulation 
                 (mm 3 ) 
                 Profile R t  (μm) 
               
             
          
           
               
                 Name 
                 Measurement 
                 Average 
                 Measurement 
                 Average 
               
               
                   
               
             
          
           
               
                 ZDDP 
                 0.0075 
                 0.0045 
                 28.69 
                 23.42 
               
               
                   
                 0.0021 
                   
                 20.32 
               
               
                   
                 0.0039 
                   
                 21.26 
               
               
                 Octylthioperoxy- 
                 0.0006 
                 0.0003 
                 3.61 
                 4.70 
               
               
                 diphosphate 
                 0.0001 
                   
                 4.09 
               
               
                   
                 0.0001 
                   
                 6.41 
               
               
                 1-Methyl 
                 0.0002 
                 0.0005 
                 11.35 
                 10.49 
               
               
                 heptylthioperoxy- 
                 0.0007 
                   
                 8.17 
               
               
                 diphosphate 
                 0.0006 
                   
                 11.96 
               
               
                 Tridecylthioperoxy- 
                 0.0009 
                 0.0013 
                 7.27 
                 6.67 
               
               
                 diphosphate 
                 0.0014 
                   
                 8.09 
               
               
                   
                 0.0017 
                   
                 4.65 
               
               
                 1-Methyl 
                 0.0007 
                 0.0013 
                 4.00 
                 6.96 
               
               
                 dodecylthioperoxy- 
                 0.0020 
                   
                 4.28 
               
               
                 diphosphate 
                 0.0012 
                   
                 12.61 
               
               
                 Octadecyl- 
                 0.0009 
                 0.0011 
                 9.13 
                 7.50 
               
               
                 thioperoxy- 
                 0.0008 
                   
                 7.16 
               
               
                 diphosphate 
                 0.0016 
                   
                 6.20 
               
               
                   
               
             
          
         
       
     
         [0040]    Scanning electron microscopy using secondary electrons was used to examine local morphology of the wear scar in all compositions. The morphology and characteristics of the wear track provides insight to explain the wear behavior of the different chemistries. Under the test conditions used, it is evident that the ZDDP exhibits a higher level of abrasive wear and tribofilm pullout that is likely responsible for its relatively poor performance in comparison to the different alkylthioperoxydiphosphates. All of the alkylthioperoxydiphosphates exhibit a small pad like morphology, which is discontinuous on the wear surface. In some cases such as octylthioperoxydiphosphate and 1-methyl dodecylthioperoxydiphosphate the pads grow to become more continuous across the wear surface. The wear test with octylthioperoxydiphosphate had the best wear performance, but cannot be directly correlated to the tribo pad size as the 1-methyl dodecylthioperoxydiphosphate had larger pad sizes but a slightly higher wear volume. However, none of the alkylthioperoxydiphosphates exhibited the kind of abrasive wear seen in ZDDP resulting in significantly improved wear performance. 
         [0041]    XANES has been used extensively in the past to examine the chemical structure of tribofilms formed when ZDDP and ashless thiophosphates are used as an anti-wear agent. Accordingly, XANES was used to examine the tribofilms of the present compounds as well as thermal films. Both P and S L-edge and P and S K-edge XANES were examined for thermal films. The P K-edge provides insight into the formation of phosphates while the S K-edge provides information on the sulfates and sulfides that form. The TEY at the P and S K-edge typically provides information from the top 50-100 nm of the thermal films which is deeper than what is acquired from the L-edge. The K-edge has been used extensively in the past to study tribofilms and is well suited to study thermal films as well. Results of XANES are thoroughly discussed in the experimental section. 
         [0042]    In brief, XANES analysis of thermal films suggests the formation of phosphates of Zn when ZDDP is used and of Fe when alkylthioperoxydiphosphates are used. However, the severe oxidative conditions of the test result in sulfur being oxidized into sulfates in the thermal films. The K-edge spectra indicate small presence of sulfides in the thermal films as well. 
         [0043]    XANES analysis of tribofilms suggests the formation of polyphosphates of iron near the surface of tribofilms from alkylthioperoxydiphosphates while Zn 3 (PO 4 ) 2  is present when ZDDP is used. In addition in the near surface region both ZDDP and alkylthioperoxydiphosphates have a mixture of sulfides with ZDDP having ZnS and alkylthioperoxydiphosphates having FeS. 
       Experimental 
       [0044]    The examples serve to further illustrate the invention, to provide those of ordinary skill in the art with a complete disclosure and description of how the compounds, compositions, articles, devices, and/or methods claimed herein are made and evaluated, and are not intended to limit the scope of the invention. In the examples, unless expressly stated otherwise, amounts and percentages are by weight, temperature is in degrees Celsius or is at ambient temperature, and pressure is at or near atmospheric. 
         [0045]    All reagents were purchased from commercial suppliers and were used without purification unless otherwise specified. Reactions involving air- or water-sensitive compounds were conducted in oven-dried (overnight) glassware under an atmosphere of dry argon. Neutral-100 base oil and zinc dialkyldithiophosphates (ZDDP) were obtained from Chevron Oronite (Richmond, Calif., USA). 
         [0046]    Nuclear Magnetic Resonance (NMR) spectra ( 1 H,  13 C, and  31 P) were recorded on JEOL eclipse instrument at 500, 125, and 202 MHz respectively using CDCl 3  as solvent and TMS as reference. 
         [0047]    Melting points were obtained in capillary tubes on a Mel-Temp II apparatus, and the thermometer was uncorrected. 
         [0048]    Fourier transform infrared (FT-IR) spectra were obtained on a Bruker Vector 22 FT-IR spectrometer, using KBr pressed pellets for solids or neat films between KBr plates for liquids and oils, and are reported in cm −1  with a resolution of 4 cm −1 . 
         [0049]    High-resolution electrospray ionization time-of-flight (ESI-TOF) experiments were performed on an Agilent ESI-TOF mass spectrometer at Scripps Center for Mass Spectrometry (La Jolla, Calif. 92037). Sample was electrosprayed into the TOF reflectron analyzer at an ESI voltage of 4000V and a flow rate of 200 micro liters/minutes. 
         [0050]    MALDI-TOF spectra were obtained on Applied Biosystems Voyager-STR Mass spectrometer. 
         [0051]    The thermo gravimetric analysis (TGA) measurements were performed using a TA 2050 thermo gravimetric analyzer (TA Instruments) at a heating rate of 5° C./min under nitrogen. 
         [0052]    All column chromatography separations were performed on Sorbent Technologies silica gel (standard grade, 60A, 32-63 μm). 
         [0053]    HFRB (High Frequency Reciprocating Ball) tests were performed on a home-built HFRB machine with test condition: 1.0 Kg Load, 100° C., 50 Hz, 1 hour and travel distance of 5 mm and a ball diameter of 6.25 mm. Both the moving ball and stationary disc were made of 52100 steel with the ball having a hardness of 55 HRc and the stationary disc having a hardness of 40 HRc. 
         [0054]    Wear volume was measured by Veeco Wyko NT9100 Optical Profiler System equipped with Vision® software. 
         [0055]    Scanning electron microscopy (SEM) images were obtained with Hitachi S-3000N Variable Pressure SEM. 
         [0056]    X-ray absorption near-edge structure (XANES) spectra were obtained at VLS-PGM beamline at the Canadian Light Source (Saskatoon, SK, Canada). A 100 μm slit size was used with a 0.1 eV step size, the energy sweep for the absorption spectra was between 160-190 eV for sulfur L-edge and 130-155 eV for phosphorous L-edge. The PGM entry slit size was 200 μm by 200 μm. 
         [0057]    The P and S K-edge spectra were acquired at the double crystal monochromator (DCM) beamline at Synchrotron Radiation Center, Madison Wis. The P K-edge spectra were acquired between 2110-2200 eV with an energy increment of 0.15 eV and the S K-edge spectra was acquired between 2460-2510 eV with an energy increment of 0.15 eV. All measurements were normalized to the incident flux I 0  and the background subtracted. 
       Synthesis of Compounds 
       [0058]    Preparation of alkylthioperoxydiphosphates was achieved via a two-step reaction. O,O-dialkyl dithiophosphoric acid was obtained by mixing phosphorus pentasulfide with alcohol at an elevated temperature in toluene. The oxidation of O,O-dialkyl dithiophosphoric acid was carried out by adding H 2 O 2  solution (37%) into a vigorously stirred O,O-dialkyl dithiophosphoric acid/ice mixture held in an ice bath. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0059]    The synthesis of octylthioperoxydiphosphate was performed as follows. 1.75 mL hydrogen peroxide solution (37% in H 2 O) was added slowly into a vigorously stirred 3.6 g (7.3 mmol) O,O-dioctyldithiophosphoric acid/50 g ice mixture in an ice bath. After addition of H 2 O 2 , the mixture was kept in an ice bath for another 4 hours, then allowed to naturally warm up to room temperature overnight. The reaction mixture was extracted with chloroform (50 ml×3), and combined organic layers were washed with water and brine, then dried over magnesium sulfate. Removing solvent under vacuum affords crude product as a pale yellow oil. The crude product was further purified by column chromatography (silica gel, hexane) to yield 3.5 g octylthioperoxydiphosphate as colorless oil. 
       NMR of Alkylthioperoxydiphosphates 
       [0060]      1 H and  31 P NMR spectra were obtained on a JEOL eclipse +  NMR spectrometer at operating frequency of 500 and 202 MHz, respectively. The sweep width of  1 H was 7508 Hz (−2.5-12.5 ppm), and chemical shifts were calibrated by tetramethylsilane (TMS). The sweep width of  31 P was 50760 Hz (−75-175 ppm), and chemical shifts were externally calibrated by phosphoric acid. All NMR experiments were performed at room temperature of 21° C. 
         [0061]      1 H and  31 P NMR spectra for tridecylthioperoxydiphosphate and 1-methyldodecylthioperoxydiphosphate are shown in  FIGS. 1-4 .  1 H NMR spectrum of tridecylthioperoxydiphosphate ( FIG. 1 ) exhibits two sets of multiplets with chemical shifts of 4.18-4.24 and 4.06-4.13 (ppm), respectively, corresponding to the four sets of CH 2  (methylene group) protons next to oxygen atoms in these four tridecyoxyl chains. Neighboring oxygen atoms result in a less shielded proton, and multiplicity is due to spin coupling between —O—CH 2 — and its neighboring —CH 2 — with different chemical shifts of 1.71-1.76 (ppm). The methylene protons of the other ten —CH 2 — groups of the tridecyoxyl chain give a broadened multiplet with chemical shifts ranging from 1.23-1.39 (ppm), and the end methyl protons show a triplet (0.88 ppm) with a H—H coupling constant of 6.87 Hz. Integration of proton peaks gives a ratio that perfectly matches that of tridecyoxyl chains. 
         [0062]    The  31 P NMR spectrum of tridecylthioperoxydiphosphate ( FIG. 2 ) shows only one P resonance peak due to tridecylthioperoxydiphosphate present in solution is in rapid dynamic equilibrium on the NMR time-scale yielding a time-averaged spectrum. 
         [0063]      1 H NMR spectrum of 1-methyldodecylthioperoxydiphosphate ( FIG. 3 ) exhibits one set of multiplet with chemical shifts of 4.69-4.72 (ppm) corresponding to the four-methine protons in these four methyldodecyoxyl chains. The neighboring four methylene (—CH 2 —) protons give two sets of multiplets with chemical shifts of 1.70-1.76, and 1.53-1.61 (ppm). The methyl protons next to methine groups overlap with other methylene protons to afford a broadened multiplet of 1.23-1.40 (ppm), and the four end methyl groups yield a triplet of 0.88 (ppm) with a spin coupling constant of 6.87 (Hz), which resulted from H—H spin coupling with neighboring methylene protons. 
         [0064]      31 P NMR spectrum of 1-methyldodecylthioperoxydiphosphate ( FIG. 4 ) exhibits a more complicated pattern than that of tridecylthioperoxydiphosphate due to the introduction of four chiral 1-methyldodecyoxyl chains and the pseudo asymmetry of phosphorus atoms which results in 1-methyldodecylthioperoxydiphosphate existing in several diastereomeric forms. 
       FT-IR of Alkylthioperoxydiphosphates 
       [0065]    FT-IR has been used extensively to examine the oxidative stability and structure of lubricant additives. FT-IR spectra of tridecylthioperoxydiphosphate and 1-methyldodeeylthioperoxydiphosphate are illustrated in  FIG. 5 . Strong absorption arising from alkyl (aliphatic) C—H stretching vibrations occurs in the 2925, 2855 cm −1  region, and C—H bending vibrations are seen at 1463, 1379 cm −1 . Absorption at 989 cm −1  corresponds to P—O—C stretching vibrations. A strong absorption from P═S stretching occurs around 723 cm −1 . These stretching and bending vibrations are present in all of the synthesized compounds. 
       Electrospray Ionization Time of Flight Analysis (ESI-TOF) And Matrix Assisted Laser Desorption Ionization Mass Spectroscopy (MALDI-MS) 
       [0066]    Time-of-flight (TOF) mass analysis coupled with two “soft” ionization techniques, electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) have been widely used in mass spectrometry owing to their excellent sensitivity and broad mass range. Compared with other traditional ionization methods, ESI and MALDI generate few fragments, which is ideal for molecular peaks detection in mass spectrometry. In ESI, an analyte solution is forced through a capillary surrounded by nebulizing gas in the presence of an electric field. As the solution exits the capillary, an aerosol of charged droplets forms. Droplets in the aerosol shrink as the solvent evaporates, and undergo “Columbic explosion” to generate analyte ions into vapor phase at various charge states. The MALDI method employs a chromophoric matrix in which the analyte is dissolved. The analyte-matrix mixtures are subject to pulsed laser irradiation, and energy absorbed by matrix causes the analyte molecules to be ionized and ejected from the matrix into the mass spectrometer. 
       Spectral Data 
       [0067]    Listed below is the spectral data from NMR and FT-IR for all the compounds synthesized as well as ESI-TOF and MALDSI-MS spectral data for select compounds. 
       Butylthioperoxydiphosphate 
       [0068]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.20-4.26 (m, 4H), 4.08-4.15 (m, 4H), 1.70-1.76 (m, 8H), 1.39-1.47 (m, 8H), 0.96 (t, J=6.87 Hz, 12H).  13 C-NMR (125 MHz, CDCl 3 ): 68.7, 68.6, 32.02, 31.96, 18.8, 13.7.  31 P-NMR (202 MHz, CDCl3): 86.1. FT-IR (KBr): 2961, 2934, 2874, 1464, 1382, 1148, 1015, 979, 902, 855, 803, 737, 646, 520 cm −1 . 
       1-Methylpropylthioperoxydiphosphate 
       [0069]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.64-4.73 (m, 4H), 1.71-1.81 (m, 4H), 1.61-1.69 (m, 4H), 1.394 (d, J=6.0 Hz, 3H), 1.390 (d, J=6.0 Hz, 3H), 1.37 (d, J=6.0 Hz, 6H), 0.97 (t, J=7.3 Hz, 6H), 0.96 (t, J=7.3 Hz, 6H).  13 C-NMR (125 MHz, CDCl 3 ): 79.42, 79.37, 79.31, 79.27, 30.40, 30.36, 30.10-30.18 (m), 20.9, 20.8, 9.6, 9.54, 9.51.  31 P-NMR (202 MHz, CDCl 3 ): 83.9, 83.8, 83.7, 83.3, 83.1, 83.1, 82.6, 82.5, 82.4. FT-IR (KBr): 2974, 2936, 2880, 1462, 1381, 1173, 1013, 975, 856, 814, 761, 646, 522 cm −1 . 
       1,3-Dimethylbutylthioperoxydiphosphate 
       [0070]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.76-4.84 (m, 4H), 1.69-1.80 (m, 8H), 1.41 (d, J=6.4 Hz, 6H), 1.36-1.39 (m, 6H), 1.29-1.35 (m, 4H), 0.91-0.97 (m, 24H).  13 C-NMR (125 MHz, CDCl 3 ): 76.7-76.9 (m), 46.9, 46.8, 46.63, 46.59, 24.6, 24.47, 24.45, 22.90, 22.85, 22.80, 22.77, 22.55, 22.14, 21.84.  31 P-NMR (202 MHz, CDCl 3 ): 84.1, 84.0, 83.8, 83.2, 83.1, 83.0, 82.9, 82.3, 82.1, 82.0. FT-IR (KBr): 2960, 2932, 2871, 1467, 1382, 1121, 976, 790, 640, 541 cm −1 . 
       2-Ethylhexylthioperoxydiphosphate 
       [0071]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.10-4.16 (m, 4H), 3.98-4.04 (m, 4H), 1.64-1.69 (m, 4H), 1.27-1.47 (m, 32H), 0.89-0.93 (m, 24H).  13 C-NMR (125 MHz, CDCl 3 ): 71.2, 40.0, 39.9, 39.8, 30.1, 30.0, 29.0, 28.9, 23.44, 23.40, 23.0, 14.1, 11.0, 10.9.  31 P-NMR (202 MHz, CDCl 3 ): 86.7-86.9 (m). FT-IR (KBr): 2960, 2930, 2862, 1462, 1380, 1003, 870, 661, 512 cm −1 . 
       Octylthioperoxydiphosphate 
       [0072]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.18-4.25 (m, 4H), 4.07-4.13 (m, 4H), 1.71-1.77 (m, 8H), 1.28-1.40 (m, 40H), 0.89 (t, J=6.4 Hz, 12H).  13 C-NMR (125 MHz, CDCl 3 ): 69.0, 68.9, 31.9, 30.1, 30.0, 29.3, 29.2, 25.6, 22.7, 14.2.  31 P-NMR (202 MHz, CDCl 3 ): 86.0. FT-IR (KBr): 2955, 2926, 2856, 1464, 1380, 989, 856, 647, 510, 499 cm −1 . 
       1-Methylheptylthioperoxydiphosphate 
       [0073]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.69-4.77 (m, 4H), 1.71-1.77 (m, 4H), 1.53-1.61 (m, 4H), 1.29-1.40 (m, 44H), 0.87-0.90 (m, 12H).  13 C-NMR (125 MHz, CDCl 3 ): 78.4, 78.3, 78.2, 37.60, 37.55, 37.50, 37.29, 31.8, 29.23, 29.16, 25.19, 25.16, 22.7, 21.6, 21.4, 14.2.  31 P-NMR (202 MHz, CDCl 3 ): 84.2, 84.0, 83.9, 83.4, 83.3, 83.2, 83.1, 82.8, 82.5, 82.4. FT-IR (KBr): 2930, 2858, 1462, 1380, 1121, 973, 803, 643, 519 cm −1 . 
       Tridecylthioperoxydiphosphate 
       [0074]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.18-4.24 (m, 4H), 4.06-4.13 (m, 4H), 1.71-1.76 (m, 8H), 1.23-1.39 (m, 80H), 0.88 (t, J=6.87 Hz, 12H).  13 C-NMR (125 MHz, CDCl 3 ): 68.91, 68.87,32.0, 30.0, 29.9, 29.73, 29.70, 29.64, 29.57, 29.4, 29.2, 25.6, 22.7, 14.1.  31 P-NMR (202 MHz, CDCl 3 ): 86.0. FT-IR (KBr): 2954, 2925, 2855, 1463, 1378, 989, 858, 723, 648, 514 cm −1 . ESI-TOF (High accuracy) calculated for C 52 H 108 O 4 P 2 S 4  as 987.6678 (MH + ), found 987.6632. 
       1-Methyldodecylthioperoxydiphosphate 
       [0075]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.69-4.72 (m, 4H), 1.70-1.76 (m, 4H), 1.53-1.61 (m, 4H), 1.23-1.40 (m, 84H), 0.88 (t, J=6.87 Hz, 12H).  13 C-NMR (125 MHz, CDCl 3 ): 78.3, 78.2, 37.60, 37.56, 37.5, 37.3, 32.0, 29.4-29.8 (m), 25.3, 25.2, 22.8, 21.6, 21.4, 14.2.  31 P-NMR (202 MHz, CDCl 3 ): 84.1, 83.9, 83.8, 83.4, 83.3, 83.2, 83.1, 82.8, 82.6, 82.4. FT-IR (KBr): 2925, 2854, 1465, 1380, 972, 800, 644, 524 cm −1 . ESI-TOF (High accuracy) calculated for C 52 H 108 O 4 P 2 S 4  as 987.6678 (MH + ), found 987.6663. 
       Tetradecylthioperoxydiphosphate 
       [0076]    Colorless oil.  1 H-NMR (500 MHz, CDCl 3 ): 4.18-4.24 (m, 4H), 4.06-4.13 (m, 4H), 1.70-1.76 (m, 8H), 1.26-1.40 (m, 92H), 0.88 (t, J=6.9Hz, 12H).  13 C-NMR (125 MHz, CDCl 3 ): 68.9, 68.8, 32.0, 30.1, 30.0, 29.81, 29.80, 29.78, 29.7, 29.6, 29.5, 29.3, 25.6, 22.9, 14.2.  31 P-NMR (202 MHz, CDCl 3 ): 86.0. FT-IR (KBr): 2955, 2922, 2854, 1463, 1380, 989, 852, 722, 648, 512 cm −1 . 
       Octadecylthioperoxydiphosphate 
       [0077]    White solid. Melting point: 37-38° C.  1 H-NMR (500 MHz, CDCl 3 ): 4.18-4.25 (m, 4H), 4.06-4.13 (m, 4H), 1.71-1.76 (m, 8H), 1.22-1.40 (m, 124H), 0.88 (t, J=6.9 Hz, 12H).  13 C-NMR (125 MHz, CDCl 3 ): 69.0, 68.9, 32.0, 30.05, 29.99, 29.7-29.8 (m), 29.7, 29.6, 29.4, 29.3, 25.6, 22.8, 142  31 P-NMR (202 MHz, CDCl 3 ): 86.0. FT-IR (KBr): 2919, 2850, 1466, 1381, 989, 852, 722, 647, 537, 508 cm −1 . MS (MALDI-TOF): m/z 1267.84 (MH + ). 
       Thermal Stability of Alkylthioperoxydiphosphate 
       [0078]    Volatility and thermal degradation play a critical role in the anti-wear performance of lubricant additives. Thermal stability of alkylthioperoxydiphosphates was studied by thermo gravimetric analysis (TGA) under nitrogen atmosphere at a heating rate of 5° C./minute. The results are shown in  FIG. 6 . 
       High-Frequency Reciprocating Ball (HFRB) Test 
       [0079]    High-frequency reciprocating ball (HFRB) tests were carried out to evaluate anti-wear performance of alkylthioperoxydiphosphates, and test conditions are listed in Table 2 above. All formulations were tested at 0.1 wt % P level in base oil. 52100 steel disks were ground with 100, 240, 400 and 600 grit sandpapers, followed by polishing with 25, 5, and 0.5 μm Al 2 O 3  aqueous solutions to achieve reproducible surface condition. Each lubricant formulation was consecutively tested three times on HFRB to obtain wear results. 
         [0080]    Topography of the wear scars were measured using the Veeco Wyko NT9100 Optical Profiler. Calculated wear volume for all the formulations is shown in  FIG. 7 . 
       XANES Analysis of Thermal Films 
     P And S L-Edge XANES of Thermal Films 
       [0081]    Thermal films were deposited at a temperature of 160° C. with air being bubbled into the oil mixture to ensure the harsh oxidizing environments seen in combustion engines. Thermal films were grown for periods of 30 minutes under these conditions. In order to ensure that the observed XANES spectra were indeed from films formed on the surface, reference spectra of the pure compounds used in this study were acquired at the P and S L-edge.  FIG. 8  shows the P L-edge FLY spectra of pure ZDDP and octadecylthioperoxydiphosphate powder along with model compounds FePO 4 , Zn 3 (PO 4 ) 2  and Fe 4 (P 2 O 7 ) 3 . The absorption edges for the pure antiwear additives are well to the left of the model compounds and it is also evident that the edge for the Zn is at lower energy in comparison to the two iron phosphates. This is because in the model compounds ZDDP and ashless thiophosphate, P is coordinated with two oxygen and two sulfur atoms, whereas in the case of FePO 4  and Zn 3 (PO 4 ) 2  the P is coordinated with 4 oxygen atoms. The difference in peak positions between the ZDDP and octadecylthioperoxydiphosphate and the FePO 4  and Zn 3 (PO 4 ) 2  arises from this difference in electronegativity between O and S. 
         [0082]      FIG. 9  is the S L-edge FLY spectra of pure ZDDP and octadecylthioperoxydiphosphate together with model compounds Fe 2 (SO 4 ) 3 , FeSO 4 , ZnSO 4 , FeS 2 , FeS and ZnS. It is easy to distinguish between the sulfates and the sulfides due to the distinguishing peaks at lower energy in the sulfides that are absent in the case of the sulfates. The large difference in energies associated with the sulfides and sulfates can be attributed to the big difference in the oxidation state of S in the sulfide to the sulfate, e.g. −2 in FeS to +6 in FeSO 4 . In addition the sulfide antiwear additives have their primary edge at lower energies and the post edge structure is very diffuse. 
         [0083]      FIG. 10  shows the P L-edge TEY spectra of thermal films formed at 30 minutes of thermal exposure. It is clearly evident that the primary absorption edge present at 139 eV corresponds to Zn 3 (PO 4 ) 2  (see  FIG. 8 ). In an earlier study it was shown that the presence of pre-edge peaks prior to the white line for the P L-edge are characteristic of the longer chain Zn-polyphosphates with the relative peak heights representative of the polyphosphate chain length. In an earlier study of thermal films formed from ZDDP at 150° C. it was shown that it took over 4 hours to develop longer chain polyphosphates of Zn. In the current case, even though temperatures of 160° C. were employed in the thermal film study the absence of any pre-edge structure in the ZDDP thermal film indicates that the phosphates in these thermal films are short chain and have not cross-linked or chain extended. The P L-edge peaks for the three ashless additives, tridecylthioperoxydiphosphate, 1-methyldodecylthioperoxydiphosphate and octadecylthioperoxydiphosphate have their edges at the same location as the thermal films of ZDDP. The Fe 4 (P 2 O 7 ) 3  model compound shown in  FIG. 8  has a binding energy of 139.3 eV which is right at the location of the absorption edge of the thermal films. In addition, the presence of a faint pre-edge structure in the thermal film from 1-methyldodecylthioperoxydiphosphate suggests the possible formation of longer chain polyphosphates of iron. The temperature used in this study of 160° C. helps in the cross linking of the phosphate films of the shorter alkyl chain length antiwear chemistries. The thermal stability of tridecylthioperoxydiphosphate and octadecylthioperoxydiphosphate in a nitrogen environment are very similar as illustrated in the TGA plot in  FIG. 6  however in an oxidizing environment it is believed that the longer alkyl chain length in the latter molecule may result in larger stability and incomplete decomposition as evidenced by the noisy spectra from the thermal film for the octadecylthioperoxydiphosphate compound. 
         [0084]      FIG. 11  is the S L-edge TEY spectra of the thermal films formed on the surface for the same conditions described for the P L-edge spectra. The position of the peaks clearly indicates that the S is present in the form of sulfates and not sulfides in all three cases. The positions of the main absorption edges for ZnSO 4  and FeSO 4  are identical and it is not possible to distinguish between the two, however, in the case of ZDDP there is a small amount of sulfide that is not present in the case of the alkylthioperoxydiphosphates. The minimal amount of sulfides in the films suggests that the active oxidative conditions preclude the reduction of the sulfates to sulfides at the surface. Even in this case it is evident from the weak signal and larger noise to signal ratio for the octadecylthioperoxydiphosphate thermal film that the higher stability of this compound in an oxidizing environment results in incomplete decomposition and thinner thermal films. 
       P And S K-Edge XANES of Thermal Films 
       [0085]    The P K-edge provides insight into the formation of phosphates while the S K-edge provides information on the sulfates and sulfides that form. The TEY at the P and S K-edge typically provides information from the top 50-100 nm of the thermal film which is deeper than what is acquired from the L-edge. The K-edge has been used extensively in the past to study tribofilms and is well suited to study thermal films as well.  FIG. 12  shows the P K-edge TEY XANES spectra of the model compounds Zn 3 (PO 4 ) 2  and FePO 4  along with thermal films formed from ZDDP, tridecylthioperoxydiphosphate, and 1-methyldodecylthioperoxydiphosphate. The difference in white line energy between Zn 3 (PO 4 ) 2  and FePO 4  is about 1.5 eV with Zn 3 (PO 4 ) 2  located at 2151.5 eV and FePO 4  located at 2153 eV. Another distinguishing feature between Zn 3 (PO 4 ) 2  and FePO 4  is the presence of a small pre-edge at 2149 eV in the case of FePO 4 . The ZDDP thermal film has a white line which aligns perfectly with Zn 3 (PO 4 ) 2  which indicates that the substrate does not play a significant role in the formation of the thermal film and the film is largely composed of decomposition products of ZDDP. On the other hand the thermal film from the tridecylthioperoxydiphosphate and 1-methyldodecylthioperoxydiphosphate appear to have white lines that match the peaks from FePO 4 . The absence of the weak pre-edge before the primary peak does not preclude the presence of FePO 4 . As detailed in the L-edge spectra it is highly unlikely that the peaks present are from un-decomposed compounds because their white lines lie at lower energy. 
         [0086]      FIG. 13  illustrates the S K-edge TEY XANES spectra of model compounds ZnSO 4 , ZnS, FeS 2 , FeS, FeSO 4  and Fe 2 (SO 4 ) 3 . In addition, this figure has the spectra for the thermal films from ZDDP, tridecylthioperoxydiphosphate, and 1-methyldodecylthioperoxydiphosphate. The model compounds clearly illustrate the difference in the spectra of the sulfides and sulfates. The white lines for the sulfates lie around 2482 eV with the energy for ZnSO 4  slightly lower than the Fe-sulfates. On the other hand, the sulfides of both Zn and Fe have very sharp characteristic features at lower energies between 2470-2475 eV. All three thermal films indicate that the dominant peak is associated with the sulfates in a fashion similar to what is seen in the L-edge spectra indicating the oxidizing environment results in oxidation of the sulfur species in the thermal films. The ZDDP thermal film shows a small pre-edge at 2473 eV which arises from a contribution from ZnS while the 1-methyldodecylthioperoxydiphosphate thermal film shows a small bump at 2472 eV that arises from FeS 2 /FeS. This indicates that most of the thermal film is made up of sulfates deeper in the thermal films and some sulfides are present as well. 
       XANES Analysis of Tribofilms 
       [0087]    Subsequent to the HFRB test the tribological specimens were preserved with a layer of base oil (sulfur free) on the surface until they were examined at the Canadian Light Source. Total electron yield (TEY) and fluorescent yield (FLY) spectra were acquired. The L-edge spectra are much more sensitive to the surface in comparison to the K-edge spectra and TEY spectra for the L-edge of P and S provide information from the top about 5 nm of the tribofilm while FLY spectra provides information from the top about 50 nm of the tribofilm. As tribofilms are generally less than 100 nm in thickness, these two edges provide useful information on the distribution of P and S in the tribofilm. 
         [0088]      FIGS. 14 and 15  are the P L-edge TEY and FLY spectra of tribofilms from ZDDP, octadecylthioperoxydiphosphate, 1-methyldocecylthioperoxydiphosphate, and tridecylthioperoxydiphosphate. It is evident from the TEY spectra that near the surface there is formation of phosphate based tribofilm with the ZDDP film composed of a short chain zinc phosphate. In an earlier study in a pin on disc configuration which results in lower Hertzian contact loads it was shown that when ZDDP was used at higher temperatures and longer rubbing time it was possible to form longer chain Zn polyphosphates. On the other hand the tribofilms for the three alkylthioperoxydiphosphates in this study have edges at energies lower than their thermal film counterparts and match the edge corresponding to Fe 4 P 2 O 7 —a medium chain polyphosphate. However, the broad nature of the peaks suggests that it is made up of a mixture of polyphosphates as opposed to a single chemistry. The FLY spectra of the tribofilms indicate that in the case of ZDDP the tribofilm is still comprised largely of short chain Zn phosphates. On the other hand the very noisy and low intensity of the spectra for the three alkylthioperoxydiphosphates suggests that deeper in the film the contribution from the polyphosphate film is less significant. 
         [0089]      FIGS. 16 and 17  are the S L-edge TEY and FLY spectra from ZDDP and the three alkylthioperoxydiphosphates detailed in the earlier section. The TEY spectra indicate that in the case of ZDDP and in the three alkylthioperoxydiphosphates there is a strong contribution from the formation of the sulfides, associated with the peak at 162 eV. In addition, there is evidence for the presence of sulfates as well in the tribofilm. The smaller presence of sulfates in the ZDDP tribofihn may be attributed to the fact that ZDDP also serves as an antioxidant in addition to its performance as an antiwear agent. The FLY spectra indicates that deeper into the tribofilm in both the case of ZDDP and ashless additives sulfur is preferentially present in the form of sulfates. 
         [0090]    Modifications and variations of the present invention will be apparent to those skilled in the art from the forgoing detailed description. All modifications and variations are intended to be encompassed by the following claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety.