الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The present work is aimed to prepare different cationic surfactants based
on natural source. One of the most widespread compounds in the plant
kingdom is Cinnamaldehyde, which is present in the bark of several species
of Cinnamomum and is modified to produce three cationic cinnmaldehyde
derived Schiff bases surfactants. The second category of the prepared
cationic surfactants is based on Cinnamic acid which is present in coffee
beans, tea, mate, cocoa, apples and pears, berries, citrus, grape, brassicas
vegetables, spinach, beetroot, artichoke, potato, tomato, celery, faba beans,
and cereals. Caffeic acid, which is a naturally phenolic compound and a
member of flavonoids presents in coffee, olive oil, white wine, cabbage etc,
is modified to produce the third category of cationic surfactants.
Then application of the prepared compounds as corrosion inhibitors for
carbon steel in 1 M HCl. Finally, correlating the experimental data to the
theoretical calculations of quantum chemical parameters using Density
Functional Theory (DFT).
The synthesis of the first category is carried out by two steps, The first is
the preparation of Schiff base through the condensation reaction of
cinnmaldehyde with N,N-Dimethylethylenediamine in ethanol for six hours,
then quaternization of the prepared Schiff base with ( decyl, dodecyl and
hexadecyl) bromide for 48 hours in ethanol to give N,N-dimethyl-N-(2-((3-
phenylallylidene)amino)ethyl)decan-1-aminiumbromide (Ia), N,Ndimethyl-
N-(2-((3-phenylallylidene)amino)ethyl)dodecan-1-aminiumbromide (Ib), N,N-dimethyl-N-(2-((3-
phenylallylidene)amino)ethyl)hexadecan-1-aminiumbromide (Ic).
The synthesis of the two categories from cinnamic and caffeic acid
is carried out by esterification of these two acids with N, N-Dimethyl
ethanolamine in xylene. The prepared esters 2-(dimethylamino)ethyl
cinnmate/caffeate were quaternized with each of ( decyl, dodecyl and
hexadecyl) bromide for 48 hours in ethanol to give N-(2-
(cinnamoyloxy)ethyl)-N,N-dimethyldecan-1-aminium bromide (IIa), N-(2-
(cinnamoyloxy)ethyl)-N,N-dimethyldodecan-1-aminium bromide (IIb), N-
(2-(cinnamoyloxy)ethyl)-N,N-dimethylhexadecan-1-aminium bromide (IIc)
and (E)-N-(2-((3-(3,4-dihydroxyphenyl)acryloyl)oxy)ethyl)-N,Ndimethyldecan-
1-aminium bromide (IIIa), (E)-N-(2-((3-(3,4-
dihydroxyphenyl)acryloyl)oxy)ethyl)-N,N-dimethyldodecan-1-aminium
bromide (IIIb) and (E)-N-(2-((3-(3,4-dihydroxyphenyl)acryloyl)oxy)ethyl)-
N,N-dimethylhexadecan-1-aminium bromide (IIIc), respectively.
The chemical structures of the prepared compounds were confirmed
by FTIR and 1H NMR.
The surface activity of the prepared cationic surfactants was
evaluated using surface tension technique. The surface parameters
including surface tension, efficiency, maximum surface excess, critical
micelle concentration, effectiveness, and minimum surface area were
determined at room temperature. The length of the hydrophobic chain has
an effect on their surface activity as the surface tension decreases
considerably by increasing their hydrophobic chain length. By focusing on
the head group and by fixing the alkyl chain length, CMC values for the
prepared imine cationic surfactants I (a-c) showed the largest reduction in
CMC compared to II (a-c) and III (a-c) with the same length of carbon chain. caffeic acid -derived surfactants III (a-c) showed a higher CMC
values than cinnamic acid-derived surfactants II (a-c) at any length of the
hydrocarbon chain, Due to the presence of a hydroxyl group, such slight
increase in CMC values is related to the hydrophilicity of the polar head.
max increased gradually by increasing the number of methylene group from
10 to 16, max Ic > Ib > Ia; IIc > IIb > IIa; IIIc > IIIb > IIIa; That
indicates the high accumulation of the compounds (Ic, IIc and IIIc) with
the longest alkyl chain length so, the formed adsorbed monolayer is
expected to be highly dense. Depending on the polar head, max I (a-c) > III
(a-c) > II (a-c) with the same alkyl chain length. III (a-c) with two
hydroxyl groups have higher values than II (a-c). This is due to the effect of
the hydroxyl groups which increase the possibility of forming hydrogen
bonds between the polar heads of surfactant molecules at the interface,
giving higher max values.
The prepared cationic surfactants were evaluated as corrosion
inhibitors using different techniques:
i. Weight loss measurements
The data revealed that, the inhibition efficiency of the prepared cationic
surfactants I (a-c), II (a-c) and III (a-c) increased with increasing the
concentration and the hydrophobic chain length.
Increasing the temperature increases the inhibition efficiency of the
prepared cationic surfactants I (a-c), II (a-c) and III (a-c) indicating that
these inhibitors are adsorbed on the steel surface by chemical adsorption.
The adsorption of the studied inhibitors on the steel surface in 1 M HCl
solution obeys the Langmuir adsorption isotherm. The values of ΔGo
ads are around - 40 kJ mol-1 revealing that the inhibitor
molecules are adsorbed onto the metal surface by chemical adsorption.
Increasing negativity of ΔGo
ads values by increasing the alkyl chain length
of the different inhibitors is assigned to the role of these chains in the
adsorption of the inhibitor molecules on the metal–solution interface.
The positive values of ΔHo
ads illustrates that, the adsorption of the
inhibitors is an endothermic process pointing to increasing the inhibition
efficiency with increasing the temperature. Also, ΔS°ads values have
positive sign in the presence of the inhibitors referring to increasing
disorder.
The apparent activation energy, Ea values in presence of the prepared
inhibitors is lower than the blank indicating chemical adsorption
mechanism.
ii. Potentiodynamic polarization measurements
Both the cathodic and the anodic reactions were suppressed with the
addition of the prepared inhibitors, indicating that these compounds reduce
effectively the anodic dissolution and also retard the hydrogen evolution
reaction.
The presence of the inhibitors leads to a slight shift of corrosion potentials
(Ecorr) towards the noble direction compared to that of the blank solution,
Ecorr shifts were less than 85 mV suggesting that the prepared inhibitors I
(a-c), II (a-c) and III (a-c) are mixed type inhibitors.
The corrosion current density (Icorr) decreases with increasing the
concentration of the inhibitors, while the inhibition efficiency increases
with increasing the concentration indicating that the inhibitor molecules were adsorbed on the metal surface, giving wider surface coverage so, these
compounds were acting as adsorption inhibitors.
iii. Electrochemical impedance spectroscopy (EIS)
The shape of Nyquist plots in the inhibited and uninhibited solutions are
the same indicating unchanged mechanism of corrosion.
The Nyquist plots contain one capacitive loop and the diameters of the
capacitive loops increase by increasing the inhibitors concentration in the
medium, This can be attributed to increasing the values of the charge
transfer resistances (Rct) for the formed protected layers on the carbon steel.
The bode plots for the prepared inhibitors are characterized by two time
constant.
Inhibition mechanism
The first step in the mechanism of action of surfactant molecules as
corrosion inhibitors is the adsorption onto the metal surface. The adsorption
process is influenced by the nature and the surface charge of the metal, the
chemical structure of the surfactant, the type of surfactant and the kind of
the corrosive medium. The adsorption of organic molecules on solid
surfaces cannot be considered only a purely physical or a purely chemical
adsorption phenomenon.
The adsorption of the prepared cationic surfactants on the carbon steel
surface in 1.0 M HCl solution takes place on the metal surface by the
electrostatic interaction between the charged surfactant molecules and the
charged steel surface through the counter Br- ion and the quaternary
nitrogen atom (N+). Br- ion adsorbed on the anodic sites to minimize the
anodic dissolution while N+ adsorbed on the cathodic sites to decrease the
hydrogen evolution. Also, all the prepared cationic surfactants contain a ordination bonds between these heteroatoms and metal atoms), which
covers the entire surface quickly and blocks the access to the active site of
corrosion on the surface. Besides, intermolecular H-bonds are formed via
hydroxyl groups, which increase the stability of the protective layer.
The inhibition efficiency for all the prepared inhibitors increases with
increasing the hydrophobic chain length so, the prepared inhibitors with 16
carbon atoms Ic, IIc and IIIc are the highest in inhibition efficiency. The
inhibition efficiency for the prepared inhibitors Ic, IIc and IIIc at room
temperature is 94.34, 92.74 and 94.12 %, respectively. It is clear that the
inhibition efficiency for the inhibitors Ic is higher than IIc due to the
presence of the azomethine group (–CH=N–) as the donatin of nitrogen is
more than that of oxygen while the efficiency of Ic is close to IIIc due to
the presence of oxygen and two (OH) which increase the donation. We can
say in general that the inhibition efficiency of Ic and IIIc is very close and
higher than IIc.
Quantum chemical technique used to relate the inhibition efficiency
to the molecular structure of the prepared inhibitors. The theoretical results
show that the behavior of energy gap and adsorption energy confirmed the
sequence of the percentage inhibition efficiency obtained by chemical and
electrochemical measurements.