untitled ISSN 2 Preparat experim quantum Mushtaq Je Education Direct * Corresponding Tel.: +964.40.770 ARTICLE INF DOI: 10.5155/eu Received: 23 Ma Received in revis Accepted: 26 Jun Published online Printed: 30 Sept KEYWORDS Mild steel Q235 Thermodynamic Quantum chemis Corrosion inhibi Adsorption isoth Heterocyclic com 1. Introducti Corrosion through the environment. explain the quantum chem using organi inhibitor mol a protective corrosive ion inhibitors ha attempts hav with propert geometrical d atoms, metal structure [3‐ and oxygen metals and structure of can be obtain chemistry suc gap (ΔE = ELU 2153‐2249 (Prin tion of no mentally an m chemist erri Meften torate of Basrah, M author at: Educati 05639149. Fax: +96 FORMATION urjchem.8.3.229-2 ay 2017 sed form: 26 June ne 2017 e: 30 September 20 tember 2017   c stry itors herm mpounds ion n is the degr reaction chem . Theoretical ch mechanism o mical calculatio c inhibitors m ecules on the m film on met ns to it. Gene ve been based ve been made ties of organic dimension, π‐b ionization pote 5]. Heterocycli atoms are goo alloys in var compounds an ned through th ch as HOMO en UMO‐EHOMO) have Eu t) / ISSN 2153‐2 ht Europ ovel comp nd theore try Ministry of Educatio ion Directorate of B 64.40.7705639149. 239.1589 2017 017 radation of m mical or electr hemistry has be of corrosion i ons [1,2]. Inhibi mostly depends metal surface, th tal surface m erally, selection d on an empiric to correlate i c molecules (i bonding, electr ential, steric ef ic compounds od corrosion i rious aggressiv nd the electron heoretical calcu ergy, LUMO en e been found t uropean Journal Europe 2257 (Online)  2 ttp://dx.doi.org/10 pean Jo Journal web pounds, ch etically as on, Basrah, 61001, I Basrah, Ministry of E‐mail address: m ABSTRACT To inhibit corr compounds w ((4‐nitropheny (A1), and (5‐(4 were experime 1×10‐1 M to 1×1 activation ener adsorption. On such as quantu Lowest unoccu potential, ΔEB ionization pote effect on the co for studied inh methods of co prepared throu results indicate steel corrosion inhibitors conc Cite this: Eur. J materials prope rochemical wit een used recent inhibition, suc ition of corrosio s on adsorptio hus the formati inimizes acces n of these org cal approach, m inhibition effic nhibitors) suc ron pairs in h ffects and mole containing nitr nhibitors for m ve media [6]. nic parameters ulations of quan ergy and the en to be quite usef of Chemistry 8 ( ean Journal of Ch 2017 Atlanta Pub 0.5155/eurjchem ournal bpage: www. haracteriz s inhibitor Iraq f Education, Basrah, ushtak9791@yaho rosion of the mil were used, nam yl)diazennyl)phe 4‐(1,3,5‐dithiazi entally evaluate 10‐5 M at 5 hour rgy, standard fre n the other hand um parameters i upied molecular ack‐donation, globa ential, electro ne orrosion rate ha hibitors on mil ompounds prep ugh several step e that the studied n in cooling wat centration and de J. Chem. 2017, 8( erties th its tly to h as on by on of ion of ss of ganic many iency h as, etero ecular rogen many The that ntum nergy ful to inve and have meta effec temp effec diffu usef inhib tion mea < 0 chem inter stud com inhib inve 2. Ex 2.1. (3) (2017) 229‐2 hemistry lishing House LLC m.8.3.229-239.15 of Che eurjchem.com zation an rs through , 61001, Iraq. oo.ca (M.J. Meften). ld steel Q235 ty mely (3‐(2‐hydro enyl)dihydro‐2H nan‐5‐yl)phenyl ed by weight lo s, and theoretica ee energy of adso d, they were theo including Highe orbital (LUMO) al hardness, glo gativity and num as been studied a d steel surface aration A1 and ps, and A2 thro d inhibitors exhi er systems, and ecreased with te (3), 229‐239 estigate the rea its electronic s e been calculate al surface and i ct on the rate perature on ne ct on the pot usion [12]. Kin ful tools for c bitor on the m process [13,14 ans chemisorpti (exothermic p misorptions or rest for using dy has been mpounds and t bitors through estigated throug xperimental Materials 239 C ‐ All rights rese 589 emistry m d studyin h thermo ype in cooling w oxy‐3‐methoxyp H‐pyrrolo[3,4‐d]i l)‐5‐pentyl‐1,3,5 ss method at d ally through the orption, enthalp oretically studie st occupied mol energy, energy obal softness, mber of transferr at 25, 35, 45, 55 obeyed Langm d A2 are differe ough the domin ibit good perform d inhibition effic emperature rise. action mechan states [7‐10]. T ed to explain th its discussion [1 electrochemica eutral corrosion tential of oxy netic and ther larifying the a metal surface, th 4], if the ΔHads > ions, if the nega process) may in mixture of bot inhibitors to r aimed to p their experime weight loss m gh thermodyna rved ‐ Printed in y ng dynamic water systems, tw phenyl)‐5‐(4‐nitr isoxazole‐4,6(5H 5‐dithiazinan‐5‐i deference conce rmodynamic fun py of adsorption ed through quan lecular orbital (H y gap, dipole mo global electrop red electrons. Th 5 and 65 °C, and muir adsorption ent from each o reaction (by mance as an inh iency increasing . nisms of inhibi he thermodyna he adsorption p 11]. Temperatu al corrosion. Th n solutions has gen depolariza modynamic pa adsorption beh hus they explai 0 (endothermi ative heat of ad nvolve either p th [15]. Due to reduce metals c prepared new entally investi method and also mic and quantu the USA and wo heterocyclic rophenyl)‐2‐(4‐ H,6aH)‐dione) ium (A2). They entrations from nctions, such as and entropy of ntum chemistry, HOMO) energy, ment, chemical philicity index, he temperature the adsorption isotherm. The other, A1 was two step). The hibitors for mild g with increase tor molecules amic functions process on the ure has a great he increase in s a favourable ation and its arameters are haviour of an in the adsorp‐ c process) this sorption ΔHads physisorption, the continued corrosion this heterocyclic igation as an o theoretically um chemistry. 230 Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 Corrosion tests were performed on a freshly specimens of mild steel Q235 type and equipped from company Hebei Jimeng Yongxing Flange Pipe Fittings Co., Ltd, China. Its chemical composition is 0.16 %C, 0.53 %Mn, 0.30 %Si, 0.025 %P, 0.015% S and 98.97 %Fe [16]. Used specimens in weight loss experiments have been cut mechanically to 6.0×4.0×0.5 cm for length, width and thickness respectively, so its area is 58 cm2. Cooling water systems have been equipped from the petrochemical plant, and all chemical materials were procured from Sigma‐Aldrich, Merck and BDH companies. 2.2. Corrosion measurements Mild steel specimens have been polished by using a series of silicon carbide papers SiC (120, 320, 800 grit size) then rinsed in distilled water, acetone and dried in a desiccators to be used in weight loss experiments. After weighing the specimens accurately in digital balance they have been immersed in cooling water as a corrosive medium for 5 hours in absence and presence of inhibitors A1 and A2 at different concentrations and temperatures. Then, the specimens were taken out of a corrosive medium and washed by distilled water in order to remove products of corrosion. After that they were dried in a desiccator and weighed again. The difference in weight before and after immersion represents weight loss 2.3. Synthesis and characterization of 3‐(2‐hydroxy‐3‐ methoxyphenyl)‐5‐(4‐nitrophenyl)‐2‐(4‐((4‐nitrophenyl) diazennyl)phenyl)dihydro‐2H‐pyrrolo[3,4‐d] isoxazole‐ 4,6(5H,6aH)‐dione (A1) Step 1: Synthesis of phenyl hydroxyl amine: Nitro benzene (6.6 mL) has been added to 4 g of ammonium chloride and 128 mL water, the mixture was stirred in beaker at 50‐60 °C. Then, 9.4 g of zinc powder was added to the mixture for 15 min then left for 30 min. After that it has been filtered and saturated by using sodium chloride. It was extracted by using chloroform and the organic layer precipitated by using a hexane and kept it in a dark cool place (109 g/mol). Step 2: Synthesis of N‐(2‐hydroxy‐3‐methoxybenzylidene) aniline oxide: 2‐Hydroxy‐3‐methoxybenzeldehyde (0.76 g; 0.005 mol) and phenyl hydroxyl amine (0.545 g; 0.005 mol) prepared in Step 1 have been dissolved in 100 mL of absolute ethanol for 2 hours within refluxing. The precipitate has been formed and cooled, then it was filtered to produce N‐(2‐ hydroxy‐3‐methoxybenzylidene)aniline oxide. Step 3: Synthesis of 1‐(4‐nitrophenyl)‐1H‐pyrrole‐2,5‐ dione: A mixture consists of p‐nitro aniline (1.38 g; 0.01 mol) and of maleic anhydride (0.98 g; 0.01 mol) have been stirred for about 2 hours in 50 mL ethanol, then it was filtered to produce precipitate (precipitate of intermediate state). Acetic anhydride (10.5 mL) and sodium acetate (1.09 g) have been added to 4.94 g of formed precipitate (precipitate of intermediate state) and was stirred for about 1.5 hours in water bath at 90 °C, then it was cooled for room temperature to produce 1‐(4‐nitrophenyl)‐1H‐pyrrole‐2,5‐dione. Step 4: Synthesis of heterocyclic compound: N‐(2‐hydroxy‐ 3‐methoxybenzylidene)aniline oxide (1.215 g; 0.005 mol) prepared in Step 2 and 1‐(4‐nitrophenyl)‐1H‐pyrrole‐2,5‐ dione (0.935 g; 0.005 mol) prepared in Step 3 have been blending at a refluxing in 150 mL chloroform for 5 hours, then this mixture was cooled and precipitated by using hexane, and finally it was filtered to produce 3‐(2‐hydroxy‐3‐methoxy phenyl)‐5‐(4‐nitrophenyl)‐2‐phenyldihydro‐2H‐pyrrolo[3,4‐ d]isoxazole‐4,6(5H,6aH)‐ dione. Step 5: Synthesis of diazonium salt: p‐Nitro aniline (0.138 g; 0.001 mol) in 0.5 mL HCl with 20 mL water at 3 °C has been dissolved (I). Sodium nitrite (0.08 g) in 10 mL water at 3 °C has been dissolved (II). The solutions I and II were blending at 3 °C to produce diazonium salt. Step 6: Synthesis of final compound (Isoxazolidine): Finally, 0.461 g of heterocyclic compound prepared in Step 4 has been blending with diazonium salt prepared in Step 5 in stirred at 3 °C, and neutralized by using hydrochloric acid, then left for 24 hours at 3 °C, after that the mixture was filtered to produce 3‐(2‐hydroxy‐3‐methoxyphenyl)‐5‐(4‐ nitrophenyl)‐2‐(4‐((4‐nitrophenyl)diazennyl)phenyl)dihydro‐ 2H‐pyrrolo[3,4‐d]isoxazole‐4,6(5H,6aH)‐dione (A1). Scheme 1 illustrates synthesis mechanism of compound A1. Color: Brown. Yield: 45%. M.p.: 211‐213 °C. FT‐IR (KBr, , cm‐1): 3321 (br, OH), 3070 (CH‐aromatic), 2982 (CH‐aliphatic), 1703 (C=O), 1600 (C=C), 1188 (C‐O), 1452 (N=N), 1396, 1429 (CH‐ bending), 1097 (C‐N). 1H NMR (500 MHz, CDCl3, δ, ppm): 1.691 (s, 3H, CH3), 2.390‐2.488 (d, 1H, N‐CH), 3.331‐3.450 (d, 1H, O‐ CH), 4.291‐4.700 (m, 1H, CH‐CH‐C=O), 6.11 (br, s, 1H, OH), 7.491‐8.089 (m, 15H, Ar‐H), 7.12 (s, CDCl3) (Figures 1 and 2) [17‐19]. 2.4. Synthesis and characterization of 5‐(4‐(1,3,5‐dithiazin an‐5‐yl)phenyl)‐5‐pentyl‐1,3,5‐dithiazinan‐5‐ium (A2) This compound was prepared in one pot through domino reaction. One pot contains mixture (4.86 mL, 60 mmol) of aqueous solution 37% formaldehyde, 100 mL methanol and (1.08 g; 10 mmol) of 1,4‐diamino benzene, which stirred at room temperature for 2 hours, then 40 mmol H2S gas was added to the mixture for 1 hour and filtered to produce white precipitate. After that 10 mmol of 1‐chloropentane and 100 mL of absolute ethanol have been added to the whole mixture at a refluxing for 5 hours to produce 5‐(4‐(1,3,5‐dithiazinan‐5‐ yl)phenyl)‐5‐pentyl‐1,3,5‐dithiazinan‐5‐ium (A2). Scheme 2 illustrates synthesis mechanism of compound A2. Color: White. Yield: 63%. M.p.: 165‐167 °C. FT‐IR (KBr, , cm‐1): 3041 (CH‐aromatic), 2960 (CH‐aliphatic), 1109 (C‐N), 1597 (C=C), 680 (br, C‐S), 1407 (CH‐bending ). 1H NMR (500 MHz, CDCl3, δ, ppm): 0.921‐1.031 (t, 3H, CH3), 1.211‐1.410 (m, 2H, CH3‐CH2), 2.144‐2.291 (t, 4H, CH3‐CH2‐CH2‐CH2), 2.821‐2.911 (t, 2H, N+‐ CH2‐CH2), 3.521 (s, 4H, N+‐CH2‐S), 4.375 (s, 4H, N‐CH2‐S), 5.481 (br, s, 4H, S‐CH2‐S), 7.200 (s, CDCl3), 7.600‐7.811 (m, 4H, Ar‐H) (Figures 3 and 4) [17‐19]. 3. Results and discussion 3.1. Effect of inhibitors concentration In experiments of weight loss, mild steel coupons have been completely immersed in 200 mL of cooling water in open beaker, size 250 mL, for 5 hours in absence and presence compounds at different concentrations. The beaker has been inserted into a water bath for keeping the temperature at 298 K. From the weight loss results obtained, inhibition efficiency % IE, corrosion rate CR and degree of surface coverage θ have been calculated by using Equations 1, 2 and 3, respectively [20]. IE% 100 (1) CR (2) Surface conerage (3) where Wblank and Winhibitor are the weight loss of mild steel in absence and presence of inhibitors respectively, and θ is the degree of metal surface coverage by inhibitors. From the results shown in Table 1, the decrease of both weight loss and corrosion rate with concentration increase can be observed. Figures 5 and 6 have been shown decrease of corrosion rate and increase efficiency of inhibition with increasing concentration of compounds A1 and A2. Meften / E Fi F F F European Journa igure 1. 1H NMR s Figure 2. FT‐IR sp Figure 3. 1H NMR s Figure 4. FT‐IR sp l of Chemistry 8 pectrum of hetero pectrum of heteroc spectrum of hetero pectrum of heteroc (3) (2017) 229‐2 cyclic compound A yclic compound A1 ocyclic compound A yclic compound A2 239 A1. 1. A2 2. 231 232 Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 Scheme 1 The reason is due to the increased amount of the used inhibitor, thereby increasing the metal surface area which covered by molecules of inhibitor; therefore, the adsorption process increases and prevents the arrival of corrosive material to surface, and finally increases the efficiency of inhibition. Also, perhaps the reason is due to the numerous presence from action centers in the structures of compound A1 and A2 such as double bonds, π‐bonds and electron pairs of atoms N, S and O, therefore are likely to π‐electrons and non‐bonding electrons in hetero atoms can provide suitable places for direct absorption on metal surface which based on of donor acceptor interactions between π‐electrons with non‐ bonding electrons and vacant d‐orbitals of mild steel atoms, hence adsorption process facilitated and inhibition efficiency increased [21‐24]. The values of inhibition efficiency and corrosion rate obtained at different concentrations indicated to decrease clearly in the corrosion processes of mild steel in presence of inhibitors, thus the inhibition efficiency increases to reach the higher value 94.98% of compound A1 and 91.64% of compound A2 at 1×10‐1 M. 3.2. Thermodynamic functions and temperature effect Study temperature effect on the metals corrosion is very complex because many of changes occur on the metal surface such as, rapid etching, desorption of inhibitor and the inhibitor itself may undergo decomposition. Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 233 Table 1. Corrosion parameters of mild steel in cooling water at deferent concentrations of compounds A1 and A2 at 298 K for 5 hours. Inhibitors Concentration (M) Weight (g) Corrosion rate (CR) (mg/cm2 h) Inhibition efficiency (%) θ Blank 0.00 0.1954 6.7×10‐4 ‐ ‐ A1 1×10‐5 0.0834 2.8×10‐4 57.31 0.5731 1×10‐4 0.0618 2.1×10‐4 68.65 0.6865 1×10‐3 0.0421 1.4×10‐4 79.10 0.7910 1×10‐2 0.0221 7.6×10‐5 88.65 0.8865 1×10‐1 0.0098 3.3×10‐5 94.98 0.9498 A2 1×10‐5 0.0804 2.7×10‐4 59.70 0.5970 1×10‐4 0.0628 2.1×10‐4 68.65 0.6865 1×10‐3 0.0441 1.5×10‐4 77.61 0.7761 1×10‐2 0.0242 8.3×10‐5 87.61 0.8761 1×10‐1 0.0165 5.6 x10‐5 91.64 0.9164 H2N NH2 + (37%) CH3OH 2 h O N O O O N S N S S S N 1-Chloropentane C2H5OH 5 h S N S S S N CH3 O H H H2S 1 h Scheme 2 Figure 5. Variation of corrosion rate and inhibition efficiency against the different concentrations of compound A1 at 298 K. Temperature is an important kinetic factor and it influences the corrosion rate of metals and reduces inhibitors adsorption on surface of metals. In order to study the effect of temperature and elucidate that on the corrosion rate and efficiency of inhibition, the experiments have been conducted in 298, 308, 318, 328 and 338 K with and without inhibitors for 5 hours. Results in Tables 2 and 3, and Figure 7 have been shown that the temperature has a great effect on corrosion rate that increases with temperature rises with and without inhibitors. Besides, the raise of temperature leads to a decrease of inhibition efficiency and the reason is probably increasing the kinetic energy of inhibitor molecules and move away from the metal surface, and finally this leads to an increase in rate of corrosion and decrease of efficiency. Also, decrease of inhibition efficiency perhaps back to weakening of physical adsorption which occurs on the metal surface, or may be due to increased rate of desorption inhibitor from the mild steel surface gradually at high temperatures. All results have been shown decreases of inhibition efficiency with tempe‐ rature raise [25‐30]. The corrosion inhibition of mild steel by using prepared compounds could be well explained through thermodynamic functions, the enthalpy of adsorption ΔH°, standard free energy of adsorption ΔG° and entropy of adsorption ΔS°. These functions have been calculated to elucidate the inhibitors action in inhibition process and that is through transition state Equation 4, also activation energy Ea of the corrosion process of mild steel has been calculated by using Equation 5 [31]. exp ∆ exp ∆ (4) Ln (5) 0.00067 0.00028 0.00021 0.00014 0.000076 0.000033 0 57.31 68.65 79.1 88.65 94.98 0 10 20 30 40 50 60 70 80 90 100 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 -0.020 0.000 0.020 0.040 0.060 0.080 0.100 0.120 In h ib it io n e ff ic ie n cy % C o rr o si o n r at e (m g /c m 2 . h ) Concentration (M) Corrosion rate Inhibitive efficiency 234 Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 Table 2. Corrosion rate of mild steel in cooling water with presence of inhibitor A1. Corrosion rate (mg/cm2 h) of compound A1 at different temperatures Concentration (M) 298 K 308 K 318 K 328 K 338 K 0.00 6.7×10‐4 7.9×10‐4 9.9×10‐4 1.9×10‐3 3.3×10‐3 1×10‐5 2.8×10‐4 4.3×10‐4 6.4×10‐4 8.4×10‐4 9.9×10‐4 1×10‐4 2.1×10‐4 3.1×10‐4 4.2×10‐4 6.2×10‐4 8.1×10‐4 1×10‐3 1.4×10‐4 2.6×10‐4 3.2×10‐4 5.4×10‐4 6.8×10‐4 1×10‐2 7.6×10‐5 9.1×10‐5 9.8×10‐5 4.2×10‐4 5.5×10‐4 1×10‐1 3.3×10‐5 5.2×10‐5 7.1×10‐5 2.1×10‐4 3.2×10‐4 Table 3. Corrosion rate of mild steel in cooling water with presence of inhibitor A2. Corrosion rate (mg/cm2 h) of compound A2 at different temperatures Concentration (M) 298 K 308 K 318 K 328 K 338 K 0.00 6.7×10‐4 7.9×10‐4 9.9×10‐4 1.9×10‐3 3.3×10‐3 1×10‐5 2.7×10‐4 5.1×10‐4 6.3×10‐4 7.9×10‐4 9.2×10‐4 1×10‐4 2.1×10‐4 4.8×10‐4 5.7×10‐4 6.6×10‐4 7.9×10‐4 1×10‐3 1.5×10‐4 3.7×10‐4 4.5×10‐4 5.8×10‐4 6.2×10‐4 1×10‐2 8.3×10‐5 9.7×10‐5 2.9×10‐4 3.8×10‐4 4.4×10‐4 1×10‐1 5.7×10‐5 7.7×10‐5 2.1×10‐4 3.0×10‐4 3.9×10‐4 where CR is the corrosion rate, R is the gases constant 8.314 J/mol.K, T1 and T2 are the absolute temperatures, h is the Plank's constant 6.626176×10‐34 J.s and N is Avogadro's number 6.02252×1023 1/mol. Ea is the activation energy for corrosion process, ΔH° is the enthalpy of adsorption, ΔS° is the entropy of adsorption. r1 and r2 are corrosion rate at deferent temperatures [24,32,33]. The Δ ° has been calculated by using Equation of Gibbs‐Helmholtz (6) depending on ΔH° and ΔS° functions [34‐36]. ∆ ∆ T∆ (6) Figure 6. Variation of corrosion rate and inhibition efficiency against the different concentrations of compound A2 at 298 K. Figure 7. Variation of corrosion rate of mild steel in cooling water at optimum concentration 1×10‐1 M of compounds A1 and A2 at different temperatures. On the light of plots of log (CR/T) versus 1/T, the straight lines have been obtained as shown in Figures 8 and 9. From slope and intercept the ΔH° and ΔS° have been calculated as shown in Equations 7 and 8, respectively [25,37,38]. Slope ∆ . (7) Intercept log ∆ . (8) It is clear from results shown in the Table 4 and Figures 8 and 9, the entropy in the inhibitor presence A1 at concent‐ rations 1×10‐1 M and 1×10‐2 M has been increased in compa‐ rison with the blank sample that indicates the increased disorderliness in going from reactant to compounds activation, whereas in other concentrations the decrease of entropy (large negative values) have been noted, this means that in the rate determining step represented that there is an association rather than dissociation. In some concentrations the values of entropy remains almost constant (slight change) in compa‐ rison with blank. The values of ΔS° are negative for both the inhibited and uninhibited systems, this might interpret inhibitors adsorption on mild steel surface through quasi‐ substitution process between the heterocyclic compounds in the aqueous phase [Org.(in aqueous)] and water molecules on mild steel surface [H2O(ads)] as shown Equation 9 [25,35,39‐ 41]. Org.molecules H O . → Org. molecules . H O (9) The positive values of enthalpy ΔH° means the dissolution process of mild steel which is endothermic suggesting that the dissolution of mild steel in presence of inhibitors are slow, also it means that adsorption which occurred on the mild steel surface is chemical adsorption. The ΔH° values in inhibitors presence are lower mostly than that in its absence, this due to the decrease of the energy barrier of corrosion reaction occurring on the mild steel surface [42‐44]. The activation energy values Ea in presence of inhibitors are greater than its absence, this indicates that the corrosion reaction of mild steel has been inhibited by using compounds A1 and A2. The increase of activation energies values signify that the adsorption which has been occurred is physical adsorption. The change in values of Ea at inhibitors presence is not at the same pace due to the modification of mechanism of corrosion process and its decrease [38,45,46]. The ΔG° values listed in Table 4 are positive and increase with increasing both of concentration and temperature. This indicates that the compounds A1 and A2 are stable inhibitors on the surface of mild steel [34,38]. 0.00067 0.00027 0.00021 0.00015 0.000083 0.000056 0 59.7 68.65 77.61 87.61 91.64 0 10 20 30 40 50 60 70 80 90 100 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 0.0008 -0.020 0.000 0.020 0.040 0.060 0.080 0.100 0.120 In h ib it io n e ff ic ie n cy % C o rr o si o n r at e (m g /c m 2 . h ) Concentration (M) Corrosion rate Inhibitive efficiency 0.0000011 0.0005011 0.0010011 0.0015011 0.0020011 0.0025011 0.0030011 0.0035011 295 300 305 310 315 320 325 330 335 340 C o rr o si o n r at e (m g /c m 2 h ) Temperature (K) A1 A2 Blank Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 235 Table 4. Thermodynamic functions of mild steel dissolution in cooling water in absence and presence of compounds A1 and A2 at deferent concentrations and temperatures. Inhibitor concentration (M) Temperature (K) Compound A1 Compound A2 Ea (kJ/mol) ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/mol·K) Ea (kJ/mol) ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/mol.K) 0.00 (Blank) 12.51 91.19 31.30 ‐201.56 12.51 91.19 31.30 ‐201.56 1×10‐5 298 32.66 92.91 24.08 ‐231.26 48.45 92.52 21.30 ‐239.82 308 95.22 94.91 318 97.53 97.30 328 99.84 99.69 338 102.15 102.08 1×10‐4 298 29.68 93.67 25.73 ‐228.64 63.03 93.07 22.75 ‐236.72 308 95.95 95.43 318 98.23 97.79 328 100.51 100.15 338 102.79 102.51 1×10‐3 298 47.23 94.49 30.13 ‐216.74 68.67 94.04 25.50 ‐230.04 308 96.65 96.34 318 98.81 98.64 328 100.97 100.94 338 103.13 103.24 1×10‐2 298 13.73 96.50 41.08 ‐186.17 11.82 95.64 31.57 ‐215.08 308 98.36 97.79 318 100.22 99.94 328 102.08 102.09 338 103.94 104.24 1×10‐1 298 34.64 98.51 46.96 ‐173.72 22.89 96.63 37.03 ‐200.96 308 100.24 98.63 318 101.97 100.63 328 103.70 102.63 338 105.43 104.63 Table 5. Shows the adsorption parameters of inhibitors A1 and A2 in cooling water at 298 K. Inhibitors Adsorption isotherm r2 Slope Intercept Kads ΔG° (kJ/mol) A1 Langmuir 0.999990 1.04848 0.000185 5405.40 ‐31.245 A2 Langmuir 0.999991 1.08855 0.000179 5586.59 ‐31.327 Figure 8. Transition state plots for corrosion rates of mild steel in cooling water with presence of compound A1 at different concentrations. Figure 9. Transition state plots for corrosion rates of mild steel in cooling water with presence of compound A2 at different concentrations. 3.3. Behaviour of adsorption isotherm It is essential to know the mode of adsorption inhibitors because the adsorption isotherms provide valuable basic information on the interaction between the inhibitors and metal surface, and the adsorption mechanism depends on the electronic characteristics of the inhibitor, the nature of metal surface and temperature. The experimental data has been applied on various adsorption isotherms and the Langmuir adsorption isotherm has been found to be the best description of inhibitors studied through the relationship between concentration versus (concentration/θ) which has been calculated by using Equation 10 [32,45,47]. (10) where Cinh is the inhibitor concentration, θ is the fraction of surface covered, ΔG°ads is the standard free energy of adsorption, R is the gases constant 8.314 J/mol.K, T is the temperature in Kelvin and Kads is equilibrium constant of adsorption reaction and from intercept line on (concentration/θ) axis has been calculated as shown in Equation 11. Besides, standard free energy of adsorption has been calculated by the following Equation 12 [34,47,48]. Intercept (11) ∆ R T ln 55.5 (12) Results of adsorption behavior have been listed in Table 5 and Figure 10. Infer that the plots of Cinh/θ versus Cinh produce the straight lines. Thus the average correlation coefficient 0.999990, 0.999991 and slopes 1.04848, 1.08855 have been calculated for both compounds A1 and A2, respectively, indicating that the adsorption of compounds 3‐(2‐hydroxy‐3‐ -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 2.9 3.0 3.1 3.2 3.3 3.4 L o g (C R /T ) 1000/T ( K-1) Blank 1x10-1 1x10-2 1x10-3 1x10-4 1x10-5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 2.9 3.0 3.1 3.2 3.3 3.4 L o g (C R /T ) 1000/T ( K-1) Blank 1x10-1 1x10-2 1x10-3 1x10-4 1x10-5 236 Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 methoxyphenyl)‐5‐(4‐nitrophenyl)‐2‐(4‐((4‐nitrophenyl)diaz ennyl)phenyl)dihydro‐2H‐pyrrolo[3, 4‐d]isoxazole‐4, 6 (5H, 6aH)‐dione and 5‐(4‐(1,3,5‐dithiazinan‐5‐yl)phenyl)‐5‐pentyl‐ 1,3,5‐dithiazinan‐5‐ium have been subject Langmuir adsorp‐ tion. The high values of the equilibrium constant of adsorption (Kads) of compounds A1 and A2 are 5405.40 and 5586.59, respectively, reflecting the high capacity for adsorption these compounds on mild steel surface in cooling water. Thus the values of ΔG°ads are negative ‐31.245 and ‐31.327 kJ/mol this suggests that a mixed type of adsorption has been occurred on surface which involves both physiosorption and chemisorp‐ tion [38,47,49,50]. The negative values of ΔG°ads, which obtained, indicate to spontaneous adsorption of the inhibitors on the mild steel surface, and a low negativity refers to electrostatic interactions between inhibitor and the charged metal surface [51]. 3.4. Quantum chemistry methodology Recently, the density functional theory (DFT) has been used to analyze the properties of inhibitor and describe inhibition of metal surfaces; it is one of theoretical models used in explaining the chemistry and physics of solids. Furthermore, DFT is considered a very useful technique to analyse the experimental data, thus in the present investigation, the quantum chemical has been calculated by using density functional theory for explaining the experi‐ mental results obtained and to further give insight into the inhibitors action of compounds A1 and A2 on the mild steel surface [52,53]. On the other hand, the quantum chemical results show ability of molecular structure for donation and back donation between the molecule and the metal surface [3]. Quantum parameters of heterocyclic compounds included EHOMO, ELUMO, S, ΔEgap, ΔN, ω, χinh, ƞinh, μ and ΔEBack‐donation. The number of transferred electrons (ΔN), denote the absolute electro negativity of inhibitor molecule (χinh) and the global hardness of the inhibitor molecule (ƞinh) have been calculated by using Equations 13, 14 and 15, respectively [54‐56]. ΔN (13) χinh (14) ηinh (15) According to Koopman’s theorem, I is ionization potential which represents the (‐EHOMO), and A is electron affinity which represents the (‐ELUMO). The theoretical values used of χFe and ƞFe are 7.0 and 0 eV/mol, respectively, according to Pearson’s scale. The energy gap (ΔEgap) has been calculated by using Equation 16, the inverse of the global hardness (ƞ) designated as the global softness (S) as shown in Equation 17, the global electrophilicity index (ω) has been calculated by using Equation 18 [50,54‐57]. Δ (16) S ƞ (17) ω ƞ (18) The HOMO is the orbital that could act as an electrons donor since it is the outermost (highest energy), in other words it is orbital which contains electrons. The LUMO is the orbital that could act as the electron acceptor since it is the innermost (lowest energy), in other words it is orbital that has room to accept electrons. ΔEBack‐donation is directly proportional to the hardness (ƞ) of the molecule, and calculated as indicated in Equation 19 [3,54]. ∆ = ƞ (19) The relation between the Fukui function f (r) and the local softness S(r) is given in Equation 20 [9,54]. (20) From this relation it can be concluded that local softness S(r) is directly related with Fukui function f (r) closely, and it plays an important role in the field of chemical reactivity. The chemical reactivity of different sites of molecules is evaluated through Fukui indices. It is defined by for nucleophilic attack [9,54]. (21) For electrophilic attack: q q (22) where qN, qN−1 and qN+1 are the electronic population of the atom K in neutral, cationic and anionic systems respectively. Ionization potential (I) has been calculated as indicated in Equation 23 [53,57]. I (23) The relationship between the EHOMO and the inhibition efficiency can be represented by Equation 24 [6]. 6.64 10 ln eff% 142.29 (24) Figures 11 and 12 show that the relationship between EHOMO values and inhibition efficiency is proportional relationship, the other words, increase of inhibition efficiency causes increase of EHOMO values due to the increasing activation energy in presence of inhibitors as shown in Table 4. Also the positive values of EHOMO indicate to the ability of inhibitors to adsorb on the mild steel, and the type adsorption is chemisorptions [1,3,6]. Figure 10. Langmuir isotherm adsorption of heterocyclic compounds A1 and A2 on the surface of mild steel in cooling water at 298 K. Frontier molecular orbital theory is useful and important in predicting adsorption centers of the inhibitor molecules responsible for the interaction with metal surface atoms. According to the molecular orbital theory of chemical reactivity, transitions of electron is due to interaction between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for reactants [54,55]. y = 1.048x + 0.000 R² = 0.999991 y = 1.088x + 0.000 R² = 0.999990 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 C o n ce n tr at io n ( 1 /ɵ ) Concentration (M) A1 A2 Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 237 Table 6. Quantum chemical parameters of studied compounds, calculated using B3LYP/6‐31G (d,p). Quantum parameters Compound A1 Compound A2 EHOMO (eV) ‐9.698 ‐5.0458 ELUMO (eV) ‐1.726 ‐1.6890 ΔEGap (eV) 7.972 3.3568 ΔEBack‐donation (eV) ‐0.9965 ‐0.4196 Dipole moment μ (Debye) ‐5.712 ‐3.3674 Transferred electrons ΔN 0.1615 1.0821 Electro negativity χ (eV/mol) 5.712 3.3674 Global hardness ƞ (eV/mol) 3.986 1.6784 Global softness S (eV/mol) 0.2508 0.5958 Global electrophilicity ω 4.0926 3.3780 Ionization potential I (eV) 9.698 5.0458 Electron affinity A (eV) 1.726 1.6890 Chemical potential б (eV) ‐5.712 ‐3.3674 Figure 11. Relationship between EHOMO and inhibition efficiency of compound A1 at 298 K. Figure 12. Relationship between EHOMO and inhibition efficiency of compound A2 at 298 K. Heterocyclic compounds which have been studied differ in atoms number of carbon, nitrogen, oxygen and sulfur; also they differ in the number of aromatic rings. Figures 13‐15 the optimized molecular structure and electronic distribution of HOMO and LUMO were showed, thus it was observed that these distributions were different, indicating the inhibition efficiency would be sensitive for it. For the compound A1, the HOMO density have been mainly concentrated on the nitrogen atoms, oxygen atom and benzene rings as shown in Figure 14, while LUMO it was distributed on the nitrogen atoms and oxygen atoms as well as on one benzene ring, thus unoccupied 3d‐orbitals of Fe atoms can accept electrons from inhibitor molecule mainly using the nitrogen atoms, oxygen atom and benzene rings to form a coordinate bond, also the molecule of inhibitor can accept electrons from anti‐bonding orbitals of Fe atoms to form back‐donation bond. As for the compound A2, the HOMO density have been mainly concentrated on the benzene ring, while LUMO it was distributed on the nitrogen atom and sulfur atoms, thus unoccupied 3d‐orbitals of Fe atoms can accept electrons from inhibitor molecule using the electrons of benzene ring to form a coordinate bond, also the molecules of inhibitor can accept electrons from anti‐bonding orbitals of Fe atoms to form back‐donation bond. The high values of EHOMO indicate a tendency of molecule to donate electrons to appropriate acceptor molecules with low energy. Similarly, the value of ELUMO reflects the ability of the molecule to accept electrons. The low values of ΔEgap imply that the good inhibition efficiency and they imply that the excitation energy to remove electron from the last orbital will be low. This indicates that the molecules of inhibitors A1 and A2 are more polarized with low in kinetic stability and will be termed as soft molecules or soft‐soft interactions. Based on the results shown in Table 6, it was observed that the values of ELUMO are low which indicates that the ability of inhibitor molecules on that accept electrons. Through looking at the distribution of LUMO in Figure 15 it was noted that molecule of A1 inhibitor can easily accept electrons from the occupied 4s‐orbital of Fe atoms by the benzene ring, oxygen atoms and nitrogen atoms to form binding forces between the inhibitor molecule and Fe atoms on the mild steel surface, while molecule of A2 inhibitor can easily accept electrons of occupied 4s‐orbital of Fe atoms by nitrogen atom and two sulfur atoms. Consequently this electronic acceptance could help to formation more stable bonds between inhibitor molecules and surface of mild steel [3,25,35,58‐62]. According to the Lukovits’s, if the value of the transferred electrons (ΔN) is less than 3.6, the inhibition efficiency increases through the increase of electron‐donating ability of these inhibitors to the metal surface [54]. In this study it was observed that the values of ΔN are less than 3.6. They are 0.1615 and 1.0821 for compounds A1 and A2, respectively, as shown in Table 6. This explains that inhibition efficiency has been increased with the increase of electron‐donating ability. In another words, the ΔN values correlates strongly with experimental inhibition efficiency, hence the highest fraction of electrons transferred is associated with inhibitor which has the least efficiency (A2), while the least fraction of electrons transferred is associated with the inhibitor which has the best efficiency (A1) [3,54]. If the value of ΔEBack‐donation < 0 and value of global hardness η > 0, this implies that charge transfer to a molecule followed by a back‐donation from the molecule, hence it is possible to compare the stabilization among inhibiting molecules, the interaction will be occurred with same metal, then global hardness decreases. At the meanwhile the results of this study were identical for it as shown in Table 6, if they were observed that the values of global hardness (η) are 3.986 and 1.6784 (η > 0) while the values of ΔEback‐donation are ‐0.9965 and ‐0.4196 (ΔE < 0) for compounds A1 and A2, respectively. Fukui indices were used for the local reactivity description thus they can be only used for comparing atoms centers within the same molecule, also widely used to describe the selective site for soft‐soft interaction, as well as, they were used for predicting the preferred site of electrophilic attack, whereas global softness allow the comparing between similar 57.31 68.65 79.1 88.65 94.98 40 50 60 70 80 90 100 0.09700 0.09710 0.09720 0.09730 0.09740 0.09750 0.09760 In h ib it io n e ff ic ie n cy (% ) EHOMO (eV) 59.7 68.65 77.61 87.61 91.64 40 50 60 70 80 90 100 0.09715 0.09720 0.09725 0.09730 0.09735 0.09740 0.09745 0.09750 In h ib it io n e ff ic ie n cy (% ) EHOMO (eV) 238 F Fig atoms for diff measures the an additional have been tra it will be mor low, and conv high, in this 4.0926 for co Dipole mome (Dipole mom moment (μ) i of a corrosio polarity of c inhibitor pola The high va adsorption pr surface [1,3,5 compounds p surface shoul literature, it will lead to t metallic surfa was noted th Figu Figure 14. The high gure 15. The lowe ferent molecule e stabilization o l number from ansferred from re reactive and versely will be study it was o ompound A1 an ent was calcula ment = ‐ Elec s an index used on inhibition p covalent bond arity related in alue of dipole rocesses among 55], it is genera possessing hig ld lead to better was noted tha the accumulati ace(adsorption hat dipole mom A1 ure 13. Optimized A1 hest occupied mole A1 est unoccupied mo es. Global electr f energy when transferred el corrosion envi a good nucleop a good electrop bserved that th nd 3.3780 for co ated according ctro negativity) d for the predic process, also it d, and further distribution of moment incre g molecules of ally agreed that gh dipole mom r inhibition effic t low value of ion of inhibito occurs) [57], h ment values for Meften / Europe d molecular structu ecular orbital (HOM lecular orbital (LU rophilicity inde the system acq ectrons (ΔN) w ironment, there phile when its v phile when its v he value of (ω ompound A2 [9 to the relation ) [63]. The d ction of the dire t is the measu rmore, it desc f molecule elect eases probably inhibitor and m adsorption of p ments on the m ciency [55]. In o dipole momen r molecules on hence in this stu compounds A1 ean Journal of Ch ures of compounds MO) density of com UMO) density of com x (ω) quires which efore, value value ω) are 9,54]. nship dipole ection re of cribes trons. y the metal polar metal other nt (μ) n the udy it 1 and A2 a com stee inhib A2 a repo 4. Co T com phen pyrr thiaz sess exce syste incre raise mole posi occu occu emistry 8 (3) (20 s A1 and A2 by usi mpounds A1 and A mpounds A1 and A are low as show mpound A1 has l surface comp bition efficiency and this under orted. onclusions The main con mpounds 3‐(2 nyl)‐2‐(4‐((4‐ni rolo[3,4‐d]isoxa zinan‐5‐yl)phen high inhibitio ellent inhibitors ems. Inhibitio easing of conce e for both c ecules subject tive values o urred, and the urred, therefor 017) 229‐239 A2 ng B3LYP/6‐31G ( A2 A2 using DFT at the A2 A2 using DFT at th wn in Table 6, t more inclinatio pared with com y of compound rlines experime clusions draw ‐hydroxy‐3‐m itrophenyl)diaz azole‐4,6(5H,6a nyl)‐5‐pentyl‐1 on characteristi s for mild steel on efficiency entration, and ompounds. Th to Langmuir of ΔH° mean Ea values me re they infer (d,p). e B3LYP/6‐31G(d,p e B3LYP/6‐31G (d therefore it can on to the adsor mpound A2, a A1 is higher th ental results th n from this s ethoxy phen zennyl)phenyl) aH)‐dione and 1,3,5‐dithiazinan ics, indicating l Q235 type in has been in decreased with he adsorption adsorption i chemical ad an physical ad that the m p). d,p). conclude that rb on the mild nd finally the han compound hat the earlier tudy are two yl)‐5‐(4‐nitro dihydro‐2H‐ 5‐(4‐(1,3,5‐di n‐5‐ium, pos‐ that they are cooling water creased with h temperature of inhibitor sotherm. The sorption was dsorption was ixed type of Meften / European Journal of Chemistry 8 (3) (2017) 229‐239 239 adsorption has been occurred on the mild steel surface. Also, the negative values of ΔG°Ads refer that the mixed type of adsorption was occurred. The positive values of enthalpy suggested that dissolution of mild steel in presence of inhibitors is slow. 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