Study on some azo liquid crystals as antioxidants for local base oil

Article abstract of DOI:10.1080/15421406.2018.1560917

Four new azo compounds namely, 2-(sec-butyl)-4-((4-methoxyphenyl)diazenyl) phenyl-4-(hexyloxy)benzoate (I6_a), 2-(sec-butyl)-4-(p-tolyldiazenyl)phenyl-4-(hexyloxy) benzoate (I6_b), 4-((4-bromophenyl)diazenyl)-2-(sec-butyl)phenyl 4-(hexyloxy)benzoate (I6_c) and 2-(sec-butyl)-4-((4-nitrophenyl)diazenyl)phenyl 4-(hexyloxy)benzoate (I6_d) were synthesized. Their structure elucidated via Elemental Analysis, FT-IR, ¹H-NMR and Mass Spectroscopy. Their mesophase behavior was investigated by Differential Scanning Calorimetry (DSC). They were tested as antioxidants for local base oil. The efficiency of them was investigated via monitoring the oxidation reaction through the change in total acid number (TAN) and viscosity. The obtained results showed that, the efficiency of these compounds was ranked as follows I6_d > I6_c > I6_b > I6_a.

Full text of DOI:10.1080/15421406.2018.1560917

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Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Study on some azo liquid crystals as antioxidants
for local base oil
Ashraf M. Ashmawy, Sayed K. Attia, Maher I. Nessim, El sayed M. Elnaggar &
Ali A. El-Bassoussi
To cite this article: Ashraf M. Ashmawy, Sayed K. Attia, Maher I. Nessim, El sayed M. Elnaggar
& Ali A. El-Bassoussi (2018) Study on some azo liquid crystals as antioxidants for local base oil,
Molecular Crystals and Liquid Crystals, 668:1, 78-90, DOI: 10.1080/15421406.2018.1560917
To link to this article: https://doi.org/10.1080/15421406.2018.1560917
Published online: 03 Apr 2019.
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2018, VOL. 668, NO. 1, 78–90
https://doi.org/10.1080/15421406.2018.1560917
Study on some azo liquid crystals as antioxidants for local
base oil
Ashraf M. Ashmawya, Sayed K. Attiab, Maher I. Nessimb, El sayed M. Elnaggara, and
Ali A. El-Bassoussib
b
aChemistry Department, Faculty of Science (boys), AL-Azhar University, 11884, (EGYPT); Evaluation and
Analysis Department, Egyptian Petroleum Research Institute (EGYPT)
KEYWORDS
ABSTRACT
Liquid crystals; Lubricating
oil; TAN; viscosity;
antioxidants and DSC
Four new azo compounds namely, 2-(sec-butyl)-4-((4-methoxyphe-
nyl)diazenyl) phenyl-4-(hexyloxy)benzoate (I6a), 2-(sec-butyl)-4-(p-tol-
yldiazenyl)phenyl-4-(hexyloxy) benzoate (I6b), 4-((4-bromophenyl)
diazenyl)-2-(sec-butyl)phenyl 4-(hexyloxy)benzoate (I6c) and 2-(sec-
butyl)-4-((4-nitrophenyl)diazenyl)phenyl 4-(hexyloxy)benzoate (I6d)
were synthesized. Their structure elucidated via Elemental Analysis,
FT-IR, 1H-NMR and Mass Spectroscopy. Their mesophase behavior
was investigated by Differential Scanning Calorimetry (DSC). They
were tested as antioxidants for local base oil. The efficiency of them
was investigated via monitoring the oxidation reaction through the
change in total acid number (TAN) and viscosity. The obtained
results showed that, the efficiency of these compounds was ranked
as follows I6d > I6c > I6b > I6a.
1. Introduction
Lubricating oil plays a key role in internal-combustion engines. It consists of complex
mixtures of hydrocarbons. Engine lubricants are used to reduce the frictions of the
mobile components and to keep the different elements clean. The engine lubricating oil
ageing process is a very complex process during which degradation of the base oil and
depletion of its additives take place simultaneously1,2
.
Degradation of engine oils may affect engine properties, for example through forma-
3–9
tion of combustion chamber deposits (CCDs)
and particulate emissions10. Engine oil
is degraded due to oxidation and heat 11,12. The oxidation of engine oil can be studied
using several methods 13,14. Oxidation is one of the most important reactions causing
degradation of engine oils during service. Oil oxidation leads to formation of acidic
products, insoluble materials, and sludge, and increase in viscosity All of these undesir-
able changes are affected by other concurrent processes occurring in an operating
engine such as thermal degradation, mechanochemical reactions, and metal catalysis.
Therefore addition of antioxidants is the suitable way to protect the lubricant from oxi-
dative degradation during industrial application15
Some of chemical species may act as antioxidants or may be converted to antioxi-
dants during use. Both hydro peroxide-decompose and radical-trapping antioxidant
CONTACT Ashraf M. Ashmawy
ß 2019 Taylor & Francis Group, LLC
ashraf_ashmawy2002@azhar.edu.eg, ashraf_ashmawy2002@yahoo.com

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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
79
Scheme 1. Preparation of liquid crystal.
activity may be provided by these components. The nature and amount of these antioxi-
dants depend upon the crude oil source and the method of refining 15. There are large
groups of liquid crystals (LCs) can undergo crosslinking in the main surface and in the
peripheries and possess the properties of liquids, owing to their chemical structure at
certain thermodynamic regimes. Depending on their nature, the structural features of
LCs and the nature of the solids, the surface treatment molecules of the LCs may have
different orientations with respect to the main surfaces.
16
Previous research has shown that small additions of LCs to lubricating oils lowered
considerably the friction of solids. These features along with the ability of certain sub-
stances to dissolve in mineral or synthetic oils attract researchers to investigate their
lubricating mechanisms 17. Liquid crystal compounds used in the literature 4,4’-disubsti-
tuted azo benzenes as precursors of new organic compounds, and we have recently opti-
mized their synthesis and purification by using supercritical fluid extraction. So these
groups of liquid crystalline substances are readily available at a relatively low cost.
Cyanoaryls particularly 4-pentyl, 4-cyanobiphenyl and 4-octyl, 4-cyanobiphenyl have
been previously studied for their tribological properties 18. In our work we synthesized
four chiral azo liquid crystals and studied them as antioxidants for Egyptian base
oil sample.
2. Experimental
2.1. Raw material
Sample of Hydro finished base oil (HBS) was delivered from Co-Operative Petroleum
Company, Cairo, Egypt. Chemicals, (4-aniline derivatives, 2-Secbutyl phenol, ethyl-4-
hydroxybenzoate, Dicyclohexyl carbodiimide (DCC), Dimethylamino pyridine (DMAP)
were purchased from Merck, and Fluka chemical companies. Solvents purchased from
sigma-Aldrich chemical company. Chemicals and solvents were used without further
purification.
2.2. Preparation of azo phenol derivatives
Synthesis of the four azo compounds took place through three steps according to
scheme 1:

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80 A. M. ASHMAWY ET AL.
Scheme 2. Structure of azo ester compounds.
Step one: Preparation of 4-(hexyloxy) benzoic acid:
4-(hexyloxy) benzoic acid was prepared by refluxing ethyl 4-hydroxybenzoate and the
appropriate 1-bromhexane (1:1) in presence of alcoholic potassium hydroxide for three
hours. The ester was then saponified to the corresponding acids using alcoholic potas-
sium hydroxide followed by dilute hydrochloric acid as the method described previ-
19
ously
.
Step two: Preparation of 4-(4-Substituted phenylazo) phenol:
20
Four azo phenols were prepared as reported previously
Step three: Preparation of I6a-d
.
.
One molar equivalent of both the 4-(4-substituted phenyl azo) phenol and 4-(hexy-
loxy) benzoic acid were dissolved in methylene dichloride. To the mixed solution,
(DCC), (1.1 mole.) and (DMAP), (as catalyst) were added. The solution was left over-
night with stirring at room temperature. The solution was filtered off and the solute
21
was distilled off and recrystallized from acetic acid
.
Where X ¼ CH3O-, 2-(sec-butyl)-4-((4-methoxyphenyl)diazenyl)phenyl-4-(hexyloxy)
benzoate (I6a).
CH3-, 2-(sec-butyl)-4-(p-tolyldiazenyl)phenyl-4-(hexyloxy) benzoate (I6b).
Br-, 4-((4-bromophenyl)diazenyl)-2-(sec-butyl)phenyl 4-(hexyloxy)benzoate.
NO2-, 2-(sec-butyl)-4-((4-nitrophenyl)diazenyl)phenyl 4-(hexyloxy)benzoate.
2.3. Characterization of the prepared compounds
The chemical structure of the prepared compounds were well established by Elemental
analysis;(CHNS-932(LECO)Vario Elemental Analyzer), IR; Perkin-Elmer FT-IR type
1
1650 spectro-photometer " Model "Vector 22", H-NMR; (type Varian 300 MHz with
the TMS as internal standard zero compound and Mass spectroscopy; using direct inlet
unit (D1-50) of SHIMADZU GC/MS-QP5050A. Calorimetric investigation was made
using a polymer laboratories differential scanning Calorimeter, PL-DSC, England, with
pure grade nitrogen with heating rate 20 ꢀC/min.
2.4. Evaluation of the prepared compounds as antioxidants
Evaluation of the prepared compounds as antioxidants was carried out according to
ASTM D-943 method. The cell contained 200 ml base oil in the static mode, and copper
and iron wires were used as catalysts. The base oil samples were subjected to oxidation
at 120 ꢀC with a pure oxygen with flow rate (0.1 liter/hour) up to 96 hours. The