Introduction: One of the important properties of lubricating base oils is their oxidation stability. It is well established that the oxidation stability of base oils is highly dependent on their composition, particularly the sulfur and aromatic hydrocarbon content. To achieve maximum oxidation stability of lubricating oils, the concept of optimum aromaticity was introduced by Van Fuchs and Dimond in 1942 (1), and this concept has been recently confirmed even for oils containing antioxidants (2, 3).

Antioxidants are generally considered as free radical inhibitors or peroxide decomposers and can vary widely in their chemical structures. Three types of additives have been shown to be effective in controlling the degradation of lubricating oils: radical scavengers, hydroperoxide decomposers, and synergistic mixtures of both. Polyfunctionalized phenols linked to heterocyclic derivatives have also been evaluated for their antioxidant properties (4).

Organic sulfur compounds are commonly incorporated into lubricating formulations for their extreme pressure and anti-wear properties, as well as for enhancing oxidation stability (5, 6).

The aim of this study is to investigate the effect of phase transfer catalysts on the yield of dibenzyl-S-phenyl thio glycol nitrile and its derivatives when used as antioxidants. Additionally, a comparison between the synthesized compounds and a commercially available antioxidant (IRCANOX L 135–CIBA) was conducted.

Experimental

The paraffinic gas oil has physic chemical properties were listed in Table 1

Table 1- characteristics of the gas oil

Property	Test method	Gas oil
Specific gravity  60/60F	IP160/60F	0.88
Pour point C	IP 15/67 (86)	17
Cloud point  C	IP 219182	26
Viscoosity at 40 C cST	ASTM D 445	28.7
Total sulphur wt%	IP 266/87	0.39
n-paraffin wt%	ASTM 3238/85	65.5
Naphthenes wt%	ASTM 3238/85	23.5
Aromatic wt%	ASTM 3238/85	10.8
Carbon residue wt%	ASTM D 524	1.6
Sulphur  content wt%	ASTM D 4294	1.5
Ash content wt%	ASTM D 482	0.03

Preparation antioxidants dibenzyl-S-phenyl thio glyconitrile  ( A):

C₆H₅ SCH₂ CN  + 2 C₆H₅CH₂Cl+Na OH      C₆H₅ S C(CH₂ C₆H₅) (CH₂ C₆H₅) CN  

S-Phenyl thio glyconitrile  (7.5 gm 0.05 mol), benzyl chloride 14.4 g 0.011 mol, 50 % aqueous NaOH  20 mol, phase transfer catalyst ( benzyl triethanol ammonium chloride  prepared by Omar et al (6), 0.25 gm were stirred vigorously under nitrogen , the mixture was stirred for three hours. Then diluted with water, prtoduct dibenzyl -S -phenyl thio glyconitrile, filtererd and cristalized from methanol, mp. 152 C , with yield  85 % and purity 98%  (7).

Preparation antioxidants dibenzyl phenyl glyconitrile  with yield 90% and purity 97% ( B):

C₆H₅ CH₂ CN  + 2 C₆H₅CH₂Cl+Na OH     C₆H₅ C(CH₂ C₆H₅)₂ CN  

Oxidation test was carried out at 120 C according to ASTM D 943 standard method. The dose of antioxidants were added in different concentrations ranging from 10^-5  to 0.1mole/ L . The oil samples after 12,24,48 and 72 hours of oxidation time were analyzed for total acid number.

Surface and interfacial tension measurement

Surface tension of different concentration for 10^-5  to 0.1mole/ L of the synthesized antioxidants was measured by using a tensiometer Kruss model 8451 in petroleum ether at 30 nC according to Omar et al  (6).

Results and discussions

The yields of alkylation product in the presence benzyl triethyl ammonium chloride as the catalyst are listed in the following table:

Additive	Percentage of yield %
A	82
B	87

The yield was found to be much higher using benzyl chloride in alkylation and faster. These results suggest that the product distribution is thermodynamically with highly reactive benzyl chloride but kinetically controlled in reaction with less reactive compound. It found that, the yield with benzyl chloride increased with increasing amounts of NaOH up to 2 moles per mole of phenyl acetonitrile, beyond which no further increase was observed. Also increasing the concentration of the catalyst benzyl triethyl ammonium chloride lead to increase the yield of product until the critical concentration 0.025 mole per mole of phenyl acetonitrile as shown in the following table (2 )

Table 2 Effect of concentration of phase transfer catalyst on the yield of product at concentration 2 mole / mol/ reactant:

Concentration of PHC Mole/mole of reactant	Yield %
PHC	A	B
0.015	72	80
0.02	82	91
0.025	81	87

To suggest the mechanism of additive as antioxidant and improving the oxidation stability of oil sample, it must be studied the physical properties of oil and additives in oil phase.  Values of the surface tensions at 25 C obtained for various concentrations of additives A and B in petroleum ether are shown graphically in figure 1.

From the intersection point in the vs log c curves, the critical micelle concentration (CMC) was determined. Surface properties were calculated according to Omar (2001) (6). The results indicate that the CMC decreases and depends on the molecular weight of the additive, suggesting that the structure of the alkyl group plays a key role in the micellization process (Figure 1). The effectiveness (π_cmc), which corresponds to the maximum surface excess and minimum surface area for the additives under study, was calculated following Omar et al. (6).

Table 3  Surface and thermodynamic  of synthesized additives.

Additive	CMc	πcmc (mN/m2)	πmax  × 10-4 mol/ml	Amin	∆Gmic KJ/mol	∆Gads KJ/mol	πmax  × 10-4 mol/ml
A	5×10-3	12	1.21	0.18	-13.2	-19.2	1.21
B	1×10-3	17	1.5	0.25	-18.11	-23.6	1.5

Analysis of the data in Table 3 shows that the synthesized Additive B exhibits the highest effectiveness and the smallest surface area. This indicates that Additive B is the most efficient, favoring adsorption during micellization and producing a greater reduction in surface tension. The results also reveal that changes in the hydrophobic moieties of the additives influence their micellization. Furthermore, the standard free energies of micellization (∆G_mic) and adsorption (∆G_ads) demonstrate that Additive B micellizes faster than Additive A and forms more stable micelles. In other words, Additive B preferentially undergoes micellization rather than adsorption at the interface. The minimum area per molecule also increases with the hydrophobic portion of the molecule, reflecting the transfer of methylene groups from the oil phase to the micelle interior. Introduction of additional methylene groups enhances micellization and micelle stability.

Table 4  Viscosity and total acid numbers of oil after oxidations tests.

Test	30h	50h	70h
Viscosity (cSt@ 100 0C)	30	32.8	34
Total acid number, mg, KOH/g	2.5	3.8	5.2

The physicochemical properties of the oil are presented in Table 1. Aromatic hydrocarbons constitute approximately 10% of the oil, while polar hetero compounds are negligible, and the carbon residue content is low. Oxidation tests were performed by heating the oil at 120 °C and evaluating oxidation stability through viscosity measurements at 100 °C and total acid number (TAN) over time. The results in Table 4 show that both viscosity and TAN increase with oxidation time, likely due to free radical formation and the generation of sludge and acidic compounds.

Table 5 Effect of different concentrations of additives on viscosity.

Compound	Viscosity
	10-4 mol/L			mol/L			5×10-3 mol/L
	30h	50h	70h	30h	50h	70h	30h	50h	70h
B	28	30	31	29.2	3`1.5	33.5	29.5	31.5	33.5
A	27	28	30	28	29	31.5	29	31	33

The synthesized Additive B was compared with Additive A as antioxidants. Kinematic viscosity at 100 °C and TAN were used as complementary evaluation parameters. The amount of oxidation products formed was determined by fractionation of the oil before and after oxidation. Changes in viscosity after oxidation are shown in Table 5. Significant differences were observed between Additives A and B. Oxidation slightly increased viscosity due to the formation of carboxylic acids; however, both additives effectively slowed the increase in viscosity over time.

The oxidation inhibition efficiency of the additives increased with concentration and reached an optimum at the critical micelle concentration (CMC) for each additive. Beyond the CMC, the oxidation stability remained unchanged. A strong correlation exists between the oxidation stability of the oil and the CMC of each additive. These findings are supported by the TAN measurements in Table 6, which show that the additives inhibit the increase of acid number during oxidation. Additive B demonstrated the best performance at 10⁻³ mol/L relative to its CMC.

Table 6 Effect of different concentrations of additives on total acid number.

Compound	Total acid number
	30h	40h	50h	60h	70h
A	1.5	2	2.8	3	3.2
B	0.6	1.5	1.4	1.3	1.2

The results suggest that the antioxidant mechanism of both additives involves micelle formation in the oil phase. These micelles interact with polar oxygen-containing products formed during oxidation, sequestering them in their core and thereby inhibiting the propagation step of the reaction. This process reduces the concentration of free radical intermediates. Additionally, Additive B forms micelles at lower concentrations and with greater stability than Additive A, as confirmed by surface and thermodynamic properties (Table 3). This points to a novel antioxidant mechanism based on the surface properties of the additives.

Further studies are needed to investigate the synergistic effects between the two additives and the structure of the micelles.

Conclusions
Oil-soluble antioxidants prepared via phase transfer catalysts represent a novel type of inhibition: micelle-based inhibition. Free radical species are sequestered within the micelle core. Surface and thermodynamic parameters effectively predict and control oil oxidation stability and are dependent on the CMC of each additive. Future work will explore the synergistic effects between additives and their micelle structures.

References:

[1] Von Fuchs, G.H., Diamonid, H., Ind. Eng Chem., 34, 1942, 927-937.

[2] Igarashi, J, Yoshida, T., Watanbe, H. Am. Chem. Scoc., Div. Pet. Chem. 42, 1997, 211-217.

[3] Maleville, X., Faure, D., Legros, A.;Hipeaux, J.C. Lubr. Sci., 9 (1996),1, 3-60.

[4] Elsayed, H., El Ashry, Mohamed E. El-rafi., Mohamed H. Elnagdi. Handy H. Abou—Elnag., Wedad M. A. Abdelazim, Yasser M., Boghdad. Jordon Journal of chemistry. 4, 2009, 223-231.

[5] Born, M., Parc, G., Paque, D. J. chimie Physique. 84, 2, 1987.

[6] Omar A.M.A. J. of adsorption science & techenology. 19, 2001, 91. [7] M. Makoza, E. Bialecka., M. ludwickow. Tetrahedron Lett. 1972, 2391