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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. |