Preparation and evaluation of some novel liquid crystals as antioxidants

Article abstract of DOI:10.1080/15421406.2016.1264240

Four liquid crystal compounds of the form 2-sec-butyl-4-[(4-X-phenyl) diazenyl) phenyl-4-(octadecyloxy] benzoate symbolized as I18_a, I18_b, I18_c, and I18_d were prepared in which the substituent (X) was taken CH

Full text of DOI:10.1080/15421406.2016.1264240

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Preparation and evaluation of some novel liquid
crystals as antioxidants
A. M. Ashmawy, M. I. Nessim, E. M. Elnaggar & D. I. Osman
To cite this article: A. M. Ashmawy, M. I. Nessim, E. M. Elnaggar & D. I. Osman (2017)
Preparation and evaluation of some novel liquid crystals as antioxidants, Molecular Crystals and
Liquid Crystals, 643:1, 188-198, DOI: 10.1080/15421406.2016.1264240
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1264240
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Date: 24 February 2017, At: 09:27

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Preparation and evaluation of some novel liquid crystals as
antioxidants
A. M. Ashmawya, M. I. Nessimb, E. M. Elnaggara, and D. I. Osmanb
aChemistry Department, Faculty of Science (boys), AL-Azhar University, Cairo, Egypt; bAnalysis & Evaluation
Department, Egyptian Petroleum Research Institute, Cairo, Egypt
ABSTRACT
KEYWORDS
Antioxidants and DSC; liquid
crystals; lubricating oil; TAN
Four liquid crystal compounds of the form 2-sec-butyl-4-[(4-X-phenyl)
diazenyl) phenyl-4-(octadecyloxy] benzoate symbolized as I18a, I18b,
I18c, and I18d were prepared in which the substituent (X) was taken
CH3O-, CH3-, Br-, and -NO2, respectively. Characterization of prepared
1
compounds is done using FT-IR, H-NMR, mass spectroscopy, and ele-
mental analysis. Their mesophase was investigated by differential scan-
ning calorimetry. Their antioxidant efficiency for Egyptian lubricating
base oil was tested via monitoring the oxidation reaction through the
change in total acid number. The obtained results showed that the effi-
ciency of these compounds was ranked as follows I18d > I18c > I18b >
I18a.
Introduction
The most important parameter that affect the lube oil degradation is its oxidation stability
[1]. Oxidation products are the primary cause of metal corrosion, viscosity increasing, and
sludge formation in lubrication systems [2]. An investigation of the chemical nature of these
products was undertaken to provide a better understanding of primary oxidation and the
subsequent behavior of these primary oxidation products [1, 2].
Oil deterioration results in a loss of lubrication, with the signals showed by the appearance
of sludge. Efforts aimed to reduce sludge formation may extend the life span of the lubricant
and prevent failure in service. The latter, however, cannot be achieved without the under-
standing of the processes of sludge formation and the nature of the oxidation products [3].
Although qualitative information is available, which reports the presence of acids, aldehydes,
ketones, esters, and lactones in the oxidation of used crankcase lube oils, information on the
extent of oxidation is scant [4–7].
Today, many efforts have been made papers, Conference recommendations, or books and
inventions in an attempt to use monomeric liquid crystals (LCs) whether as lubricants or as
additives for lubricating oils. The improvement of the efficiency of lubricants is of important
technological and economic relation as it is estimated that 50% of the energy consumption is
wasted as friction [8, 9].
CONTACT Ashraf M. Ashmawy
AL-Azhar University, , Egypt.
ashraf_ashmawy@yahoo.com
Chemistry Department, Faculty of Science (boys),
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
189
LCs are soft materials that spontaneously form several types of ordered structures and
thermally change their physicochemical properties [10]. Since the time of their discovery,
LCs have been very extensively investigated. The exploration of LCs included molecules with
different shapes (rod-like, disc-like). Also there are different possibilities of substitution and
combination of linking units in designing of new molecular structures [11].
The ordered LCs have received much attention, as potential lubricants, due to the long-
range orientation of their molecules [9, 12–17]. Most LCs studied until present are ther-
motropic phases constituted by polar neutral organic molecules [18]. The goal of our present
work is to prepare new monomeric (LCs), azo phenol derivatives, and applying them as good
antioxidants.
Experimental
Raw material
Samples of hydro-finished base oil were delivered from Co-operative Petroleum Company,
Cairo, Egypt. All other reagents were purchased from Merck, Aldrich, and Fluka chemical
companies.
Preparation of azo phenol derivatives
The preparation of 2-sec-butyl-4-[(4-X-phenyl) diazenyl) phenyl-4-(octadecyloxy] benzoate
took place through three steps according to Scheme 1:
Scheme . Preparation of additives (Ia–Id).

190
A. M. ASHMAWY ET AL.
Step one: Preparation of 4-(octadecyloxy) benzoic acid:
4-(octadecyloxy) benzoic acid was prepared from ethyl 4-hydroxybenzoate and the appro-
priate 1-bromooctadecane; the ester was then saponified to the corresponding acids
using alcoholic potassium hydroxide, by the method described previously [19].
Step two: Preparation of 4-(4-substituted phenyl azo) phenol:
Two azo phenols were prepared as reported previously [20].
Step three: 4-(octadecyloxy) benzoic acid with 4-(4-substituted phenyl azo) phenol:
One molar equivalent of both the 4-(4-substituted phenyl azo) phenol and 4-
(octadecyloxy) benzoic acid were dissolved in methylene dichloride. To the resulted
solution, dicyclohexylcarbodiimide and 4-(dimethylamino) pyridine were added as a
catalyst, and the solution was left to stand overnight with stirring at room temperature.
The solution was filtered off and the solute was distilled off and the residue recrystallized
by acetic acid [21].
Characterization of the prepared compounds
The chemical structure of the prepared compounds was well established by elemental anal-
yses using CHNS-932 (LECO)Vario Elemental Analyzers, IR; using A Perkin-Elmer FT-IR
type 1650 spectro-photometer; “Model Vector 22,” Nuclear Magnetic Resonances (NMR),
1
using H-NMR (type Varian 300 MHz) with the TMS as internal standard zero com-
pound, mass spectroscopy was one using direct inlet unit (D1-50) of SHIMADZU GC/MS-
QP5050A,and calorimetric investigation was made using a polymer laboratories differential
scanning calorimeter, PL-DSC, England, with nitrogen as a pure gas. Typical heating and cool-
ing rates were 20 K/min, and sample masses were 1–2 mg.
Evaluation of the prepared compound as antioxidant
Evaluation of the prepared compounds as antioxidants was carried out according to ASTM D-
943 method. Where the cell contained 200 mL base stock 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 L/hr) up to 96 hr. The prepared compounds were added
with different percents. The change in total acid number (TAN) of oil samples were examined
(after 24, 48, 72, and 96 hr) according to ASTM test methods D-664.
Results and discussion
The physicochemical properties of local base stock used in this study represented in Table 1.
Characterization of compounds I18a–d
1
The prepared compounds were characterized by elemental analysis, FT-IR, H-NMR, mass
spectroscopy. Their mesophase behavior was investigated by differential scanning calorimetry
(DSC). The chemical structure of compounds I18a–d was elucidated using the following.
Elemental analysis
Elemental analysis was performed for the synthesized azophenols I18a–d, and the obtained
results are shown in Table 2. The data obtained from Table 2 show that the calculated values
of the elements were in good compatibility with that measured.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
191
Table . The physicochemical properties of the base stock.
Test
Result
Test method
ASTM D-
Density @ .°C, g/L
Pour point, °C
Viscosity at –°C
. 
Zero
.
.
ASTM D-
ASTM D-
ASTM D-
ASTM D-
ASTM D-
ASTM D-
ASTM D-
ASTM D-
ASTM D-
ASTM D-
ASTM D-
—
Viscosity index (VI)
Total acid number (TAN)
Sulfur content, wt%
Color
Ash content, wt%
Copper corrosion
Flash point, °C

.
.
.
.
Ia

.
.
Aniline point, °C
Molecular weight
Table . Elemental analysis of newly prepared compounds, Ia–d
.
Analysis calculation (found)
H% Br%
C%
N%
Cpd.
Calc.
Obs.
Calc.
Obs.
Calc.
Obs.
Calc.
Obs.
Ia
Ib
Ic
Id
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
—
—
.
—
—
.
.
.
.
.
.
.
.
.
.
—
—
Infrared spectra of compounds Ia–d
Infrared absorption bands for compounds I18a–d are given in Table 3. From Table 3, small
shifts were observed in the carbonyl absorption bands with the effect of the alkoxy-chain
length (O-C18). Moreover, it can be noted that the ester C=O absorption bands are not greatly
affected by the nature of the substituents (MeO-, Me-, Br-, or NO2-). This can be attributed
to their weak effect on the polarization of the ester C=O group through the phenylazo group.
Nonetheless, the ester oxygen absorption bands are affected by the electronic nature of MeO-,
Me-, Br-, or NO2-. Thus, in the electron withdrawing (NO2-) substituted derivative I18d, con-
jugative interaction takes place between NO2- and the lone pair of the ester oxygen, via the
phenylazo moiety (Fig. 6).
H-NMR spectroscopy
Chemical shift of the prepared compounds are given in Tables 4–7.
Table . Infrared spectra of Ia–d
.
Stretching of vibrations (v) in cm−
Compound
CH aromatic
CH aliphatic
N=N
C=O
C-O-C
Br
NO
Ia
Ib
Ic
Id




















—
—

—
—
—
—


192
A. M. ASHMAWY ET AL.
Table . Chemical shifts of compound Ia.
H+ types
a (d)
b (d)
c (d)
d (s)
e (d)
f (d)
g (d)
h )t)
Chemical shift
H
Chemical shift
.
i (m)
.
.
j (m)
.
.
k (d)
.
.
k− (t)
.
.
l (t)
.
.
m (m)
.
.
n (m)
.
.
o (t)
.
+ types
Table . Chemical shifts of compound Ib.
H+ types
a (d)
b (d)
c (d)
d (s)
e (d)
f (d)
g (d)
h (t)
Chemical shift
H
Chemical shift
.
i (m)
.
.
j (m)
.
.
k (d)
.
.
k− (t)
.
.
l (t)
.
.
m (m)
.
.
n (m)
.
.
o (t)
.
+ types
Mass spectroscopy
Molecular ion peak of the prepared compounds are given in Table 10 and Figures 1–4. From
the previous table and figures we noted that the molecular ion peaks were in accordance with
the calculated molecular weight for all the prepared compounds.
Differential scanning calorimetry
In order to investigate the effect of terminal substituents (X) on the mesophase behavior
of compounds I18a–d the number of carbon atoms in the alkoxy substituent kept constant
(C18H37O-). The substituent X varied between CH3O-, CH3-, Br-, and NO2-. Table 8 com-
pares the transition temperatures of the four compounds (I18a–d).
From tables we observed proton (a) for I8a-I18d possess high chemical shift because of its
attachment to the carbonyl ether group (withdrawing group). The effect of the azo group on
the protons (b) gave a high chemical shift. The presence of the bromide and nitro group in
compounds I18a-I18d gave very high chemical shift (8.463ppm and 8.383ppm) respectively

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
193
Table . Chemical shifts of compound Ic.
H+ types
a (d)
b (d)
c (d)
d (s)
e (d)
f (d)
g (d)
Chemical shift
H
Chemical shift
.
i (m)
.
.
j (m)
.
.
.
l (t)
.
.
m (m)
.
.
n (m)
.
.
o (t)
.
+ types
k (d)
.
k− (t)
.
Table . Chemical shifts of compound Id.
H+ types
a (d)
b (d)
c (d)
d (s)
e (d)
f (d)
g (d)
Chemical shift
H
Chemical shift
.
i (m)
.
.
j (m)
.
.
.
l (t)
.
.
m (m)
.
.
n (m)
.
.
o (t)
.
+ types
k (d)
.
k− (t)
.
Where (s) singlet, (d) doublet, (t) triplet, (m) multiplet and σ chemical shift.
Figure . Mass spectra of the prepared compound (Ia).

194
A. M. ASHMAWY ET AL.
Figure . Mass spectra of the prepared compound (Ib).
Figure . Mass spectra of the prepared compound (Ic).
because of the high attractive power. Furthermore, the ester group in compounds I18c-I18d
effect the value of protons (b) [8.129 and 8.129] respectively. In case of all four compounds
I18a-I18d the aromatic protons (g) have the lowest (δ) values due to the presence of the ether
oxygen. Despite, the aliphatic protons (i, j, k, k-, l, m, n and o) have different (δ) values accord-
ing to their position to the ether bond. The j protons Ina have higher (δ) value 3.305 ppm, than
I18a (δ) 3.209ppm, due to ether oxygen in the methoxy group.
Figure . Mass spectra of the prepared compound (Id).
Table . Comparison of the transition temperatures of the compounds (Ia–d).
Compound
Ts-s
Tm
TA-N
Tc
Ia
Ib
Ic
Id
.
.
.
—
.
.
.
.
—
—
.
—
.
.
.
.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
195
Figure . Dependence of Tc on the polarizability anisotropy ࢞α× of the Car-x bonds for series Ia–d
.
Figure . Conjugative interaction within the nitro-substituted homologue Id.
Figure . Conjugative interaction within the methoxy substituted homologue Ia.
It is well recognized that the stability of the mesophase would be augmented by an increase
in the polarity of the mesogenic part of the molecule (Fig. 5). Thus in the nitro analogue, the
polarity of the mesogenic portion as a whole, Fig. 6, should be increased, which stabilized
the mesophase. This heuristics is in accordance with the observed results, Table 8, that the
nitro analogue has the highest clearing (Tc) temperature. On the contrary, the molecules of
the methoxy and methyl analogues (Fig. 7) are expected to possess lower dipolar character
that would lead to derivatives with lower Tc values.
Evaluation of the synthesized compounds as antioxidants using change total acid
number
The TAN defined the amount of potassium hydroxide in milligrams that is needed to neutral-
ize the acids in 1 g of oil. It is important to valuate the amount of additive reduction, acidic
contamination, and oxidation of lubricant degradation. It is an important quality measure-
ment of crude oil and used as a guide in the quality control of lubricating oil formulations. It
is also sometimes used as a measure of lubricant degradation in service. The TAN value indi-
cates to the crude oil refinery the potential of corrosion problems. Testing for TAN is essential
to maintain and protect equipment, preventing damage in advance. TAN testing is a measure
of both the weak organic acids and strong inorganic acids present within oil. The TAN is an

196
A. M. ASHMAWY ET AL.
analytical test to determine the deterioration of lubricants. The more acidic a lubricant is, the
more degradation occurs. As a fluid degrades, the levels of corrosive acids increase, along with
the danger of component failure.
The TAN value itself cannot be used to predict the corrosive nature of an oil, as the test
only measures the amount of acid in a sample, not the specific quantities of different acidic
compounds in the sample. Two samples might have the same TAN value, but one has high
levels of corrosive acids while the other much lower levels of the same corrosive acids. An
increase in viscosity and the formation of gums and resins are two other negative effects which
can be attributed to an increased TAN value. A rise in TAN is indicative of oil oxidation due to
time and/or operating temperature. Trend as well as absolute values should be used to monitor
TAN levels.
The synthesized antioxidants were added to the base stock. The blends obtained were sub-
jected to severe oxidation conditions at 120oC. Samples were taken at specific time intervals
(24, 48, 72, and 96 hr) of oxidation. Usually the TAN of the oil increases by increasing the oxi-
dation time. The increment of TAN values is due to the oxidation processes which produce
peroxides. These peroxides undergo further reaction to form a variety of oxygenated com-
pounds such as alcohols, aldehydes, and ketones. The TAN is affected by the formation of
carboxylic acids after prolonged oxidation and increases with increasing carbonyl formation
which deteriorates the lubrication ability of the oil [22, 23].
1. Initiation:
RH+I·→ R·+ IH
2. Propagation:
R·+O2 → ROO
ROO·+ RH → ROOH + R
3. Termination:
R·+ R·→ R-R
ROO·+ ROO → Stable products
From Table 9, Figs. 8–10, we can see that generally the TAN of the oxidized base stock
increases by increasing the duration time. First of all, after 24 hr, the TAN decreases by
Table . Variation of total acid number, TAN, with and without additives.
Base Stock + Additive × 
Base oil + a
Base oil + b
Base oil + c
Base oil + d
Time
(hours)
Basea
stock




































































a
The TAN of the base stock at room temperature is . mg KOH/mg sample.
Table . Mass spectroscopy for Ia-d
Compound
Molecular formula
m/z obs.
m/z cal.
Ia
Ib
Ic
Id
CHNO
C NO
CHBrNO
.
.
.
.
.
.

H
CHNO
.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
197
Figure . Variation of total acid number (TAN) of base oil without and with  ppm Ia–d additives.
Figure . Variation of total acid number (TAN) of base oil without and with  ppm Ia–d additives.
Figure . Variation of total acid number (TAN) of base oil without and with  ppm Ia–d additives.
increasing the concentration of the additive added from each of the four compounds used.
The base oil + I18d showed the best results.
Increasing the oxidation period to 48, 72, and 96 hr, the TAN also decreases by increasing
the concentration dose of any additive used. Again the base oil + I18d gave the best results.
Looking in Table 9, we can see that the base oil +I18a shows TAN at 100 ppm after 24 hr,
base oil + I18b shows the same at 500 ppm after 48 hr while the base oil + I18d shows the same
after 96 hr. Using 500 ppm from 18a after 24 hr, 100 ppm from 18c after 48 hr or 500 ppm after
72 hr and 500 ppm from 18d after 96 hr, one can notice that TAN of 0.032 mg KOH/g sample
were obtained from each. Although the I18c gave the best condition, TAN of 0.20 mg KOH/g

198
A. M. ASHMAWY ET AL.
sample, by using 500 ppm after 24 hr, but this compound showed less efficiency compared
with I18d at 48, 72, and 96 hr.
The results obtained after 24 hr from 18a using 100 ppm, after 48 hr from 18b using
500 ppm and from 18d after 96 hr using 250 ppm are the same (TAN 0.35 mg KOH/g
sample). But, although it is more economical to use low concentration from the additive
added, it is more efficient to use high concentration to run more operating time. So the 18d
is more efficient because it gave the same results after long operating period.
Conclusions
The results obtained in this work indicate the following:
1. Four prepared compounds had properties of LCs.
2. In our study, LCs give more efficiency as antioxidant.
3. The prepared antioxidants proved to be successful in enhancing the oxidation stability
of the base stock.
4. The above (total acid number) data reveal that the most effective concentration is
500 ppm.
5. From the previous data we noted that the inhibition efficiency of the two prepared
antioxidants is ranked as follows: I18d > I18c> I18b > I18a.
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