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Effectiveness of O-bridged Cationic Gemini Surfactants as Corrosion Inhibitors for Stainless Steel in 3M HCL

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Effectiveness of O-bridged cationic gemini surfactants as corrosion inhibitors for stainless steel in 3M HCl: experimental and theoretical studies
Four novel cationic gemini surfactants with a spacer functionalised by an oxygen atom were synthesised, characterised using FTIR and 1H NMR spectroscopy and tested as corrosion inhibitors for stainless steel in 3M HCl solution. The corrosion inhibition efficiency was determined using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS). The polarisation measurements showed that inhibitors act as mixed inhibitors. The results indicated that the highest inhibition efficiency was reached around the values of the critical micellar concentration (CMC) for individual inhibitors.The compound with twelve carbon atoms in the alkyl chains and hydroxyethyl groups linked to the positively charged nitrogen atoms appeared to be the most effective inhibitors showing 98% at concentration
3 mM. The efficiencies obtained from the EIS measurements were in good agreement with those obtained from the polarisation techniques. Following corrosion testing  the surface of the stainless steel samples were studied by confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). The Density Functional Theory (DFT) was applied for theoretical studies of the inhibitors. Correlation between experimental and theoretical results was carried out and based upon Density Functional Theory (DFT) analysis.
Gemini surfactants, Corrosion inhibitors, Stainless steel

  1. Introduction

Stainless steel (SS) is widely used in different areas of industry: chemical, food [1,2], pharmaceutical, petrochemical [3], pulp and paper for piping systems, heat exchangers, tanks, structural material [4] and process vessels [5,6]. Stainless steel is a popular choice for the applications due to its excellent mechanical properties [5] and good corrosion resistance [7], these latter being associated with formation of passive layer on its surface [8]. The corrosion resistance of SS is limited by its susceptibility to passive film breakdown in aggressive environments, especially in acid mediums. In this case, breakdown can lead to pitting corrosion which is one of the most dangerous forms of localized corrosion [8].
Acid solutions are generally used in industry for cleaning, pickling, descaling, oil well acidification and petrochemical processes [9–11]. Hydrochloric acid is one of the most widely used acid solutions and its concentration is usually between 5 and 15% [12].
There are several strategies employed to prevent corrosion, where one of the most efficient approaches is to use corrosion inhibitors (CIs) [8,13]. Corrosion inhibitors are chemical compounds which, when added in small amounts can reduce or inhibit metal dissolution [14–16]. Surfactants are a large group of highly efficient and commonly used CIs which change the electrochemical behaviour of the metal due to the interactions between the surfactant molecules and the metal surface [16] leading to a reduction of the corrosion properties of the metal [17]. It has been estimated that the US demand for surfactants as CIs will rise 4,1% per year to USD$ 2.5billion in 2017. In 2012, the market demand of surfactants as inhibitors for petrochemical industry was 26,6% [6].
Among the many surfactants available, quaternary ammonium salts (QAS) have aroused great interest because of their inhibition efficiency at lower concentration [18], easy production and low price [17]. They consist of a positively charged nitrogen atom (hydrophilic part) connected with four substituents, where at least one of these is a long alkyl chain (hydrophobic part) [6]. The mechanism of corrosion inhibition is based on the adsorption of inhibitor molecules onto the metal surfaces, replacing water molecules and forming a film which protects the metal against deterioration [6,10]. This is a physicochemical process. Physical adsorption is connected with electrostatic interaction between molecules and chemisorption with donor-acceptor interactions [15,19]. Adsorption is influenced by different factors such as: kind of metal and the surface charge of it [20], the chemical structure of the organic inhibitor (electron density, molar mass, hydrophobicity, chain length, type of counterion and steric effect) [21], type of aggressive media (pH and/or the electrode potential of the metal) [15,22,23]. Compounds with multiple bonds, aromatic rings and functional groups containing heteroatoms [24] are found to be more effective as metal CIs [20]. The electron-rich structures can share free electrons with the free d-orbital of metals [19].  Inhibitor efficacy also depends on the character of hydrophilic and hydrophobic groups. The degree of protection against corrosion increases, i.e., inhibition efficacy, with the number of carbon atoms in the alkyl chain [25].
Recently, greater attention has been focused on novel type of surfactants notably: dimeric quaternary ammonium salts [26]. These compounds were identified by Bunton and named gemini surfactants in 1991 by Menger and Littau [27]. Gemini surfactants are made up of two classical surfactants (two hydrophilic and two hydrophobic parts) bonded together by a spacer [28], which can be rigid or flexible [29]. As a result of their structure gemini surfactants exhibit superior properties compared to conventional types such as lower critical micelle concentration, increased ability to lowering surface tension and higher biological activity [26,30]. As a result, they are characterised by higher inhibition efficiency at a lower concentration compared to their monomeric analogues [20]. Gemini surfactants are also less toxic than their monomeric analogues which is important from the environmental point of view [31] and strictly corresponds to Green Chemistry philosophy.
Critical micelle concentration (CMC) is a key parameter when considering the use of surfactants for corrosion inhibition, as it is related to the efficiency of CIs [18]. CMC is the critical concentration when surfactant molecules begin forming micelles. Around this concentration surfactants tend to adsorb onto the exposed surface and form a protecting monolayer [30,32]. The CMC values decrease with increasing the number of carbon atoms in the alkyl chain and the spacer and with increasing number of positively charged nitrogen atoms [33]. The CMC for gemini surfactants may be up to hundreds of times lower than those of monomeric quaternary ammonium salts [34], hence they have the potential to operate as efficient CIs.
Gemini surfactants have been studied as CIs for mild and carbon steel, especially in acid solutions [35–41] however there is a lack of information on the performance for inhibiting corrosion of stainless steel, hence the motivation for the current study. Here we investigated, the efficiency of four novel gemini cationic surfactants as CIs of stainless steel in 3M HCl by potentiodynamic polarisation and electrochemical impedance spectroscopy. Quantum chemical calculations were used to investigate the theoretical ability of the compounds to inhibit the corrosion process.

  1. Experimental methods and materials
    1. Materials

The working electrode was made of stainless steel (AISI 304). The chemical composition of the working electrode (weight %): Cr 17-20, Mn < 2, Ni 8-11, C < 0.08, Fe balance.

  1. Inhibitors

Synthesis of 3-oxa-1,5-pentamethylene-bis(N-alkyl-N,N-dimethylammonium chloride)
(12-O-12, R=C12H25; 18-O-18, R=C18H37)
Gemini surfactants were prepared from bis(2-chloroethyl)ether (4.7 mmol) and corresponding amine (N,N-dimethyl-N-dodecylamine for 12-O-12 and N,N-dimethyl-N-octadecylamine for 18-O-18; 28.2 mmol) in n-propanol under reflux for 30 h. After reflux, 40 mL of acetone was added and a white, solid product was obtained. The raw product was washed with diethyl ether.
Synthesis of 3-oxa-1,5-pentamethylene-bis(N-alkyl-N-hydroxyethyl-N-methylammonium chloride) (12-MOH-O-MOH-12, R=C12H25; 18-MOH-O-MOH-18, R=C18H37)
Gemini surfactants were prepared from bis(2-chloroethyl)ether (7 mmol) and corresponding amine (N-hydroxyethyl-N-dodecyl-N-methylamine for 12-MOH-O-MOH-12 and
N-hydroxyethyl-N-octadecyl-N-methylamine for 18-MOH-O-MOH-18; 23 mmol) respectively in n-propanol under reflux for 30 h. After reflux, 60 mL of acetone was added and a white, solid product was obtained. The raw product was washed with diethyl ether.
The chemical structure of the synthesised novel cationic gemini surfactants, which was characterised by FTIR and 1H NMR is given in Figure 1.

  1. Electrolytes

The test electrolyte, 3M HCl was prepared by dilution of analytical grade HCl (32%) with distilled water. Solutions with different concentrations of inhibitors were prepared from the solution of 3M HCl. The range of concentrations for compound with twelve carbon atoms in the alkyl chain was: 1-5 mM and for those with eighteen carbon atoms was 0.005-0.1 mM. Note: all experiments were carried out at room temperature in a naturally aerated test solution.

  1. FTIR spectroscopy

The FTIR (KBr) spectra of the synthesised inhibitors were performed using a BrukerTM model FT-IR IFS 66/s.

  1. 1H-NMR spectroscopy

The 1H NMR spectra of the synthesised gemini surfactants were performed using Varian model VNMR-S 400 MHz with TMS (tetramethylsilane) as an internal standard.

  1. Polarisation measurements

The polarisation measurements were carried out using a conventional three-electrode cell with a platinum counter electrode and a saturated calomel electrode (SCE) as a reference electrode and a working electrode of stainless steel AISI 304 plate (2.5×1 cm). All the polarisation curves were recorded using a potentiostat (VersastatTM 4.0). Individual anodic and cathodic potentiodynamic polarisation measurements were obtained by changing the electrode potential automatically from the OCP to -300 mV vs the OCP and from the OCP to +300 mV vs OCP, respectively, with a scan rate 0.5 mV s-1. Linear polarisation resistance measurements (LPR) were obtained by changing the potential from -20 to 20 mV versus the measured OCP with a scan rate of 10 mV min-1.

  1. Electrochemical impedance spectroscopy (EIS)

EIS measurements were carried out using a potentiostat (VersastatTM 4.0) and controlled with corrosion analysis software (VersaStudio). Impedance spectra were obtained over a frequency range of 100 kHz to 10 mHz with a 10 mV sine wave as the excitation signal at open circuit potential. A SCE electrode was used as reference and a platinum electrode as the counter electrode.

  1. Confocal laser scanning microscopy (CLSM)

After corrosion testing, the surface of the 304 stainless steel samples after the test was observed using a Keyence VK-X200K 3D Laser Scanning Microscope in order to identify areas that would be selected for further examination using the SEM.

  1. Scanning electron microscopy (SEM)

The morphology of the 304 SS surface after the corrosion test was also studied by scanning electron microscopy (SEM) using a FEI Quanta 200 operating at 20 kV and different magnification.
2.8. Computational approaches
Quantum chemical calculations were carried out using a GAUSSIAN 09’ program [42]. PM5 (Parameterized model number 5) semiempirical calculations were performed using the WinMopac 2003 program [43]. In all cases full geometry optimization of O-bridged cationic gemini surfactants was carried out without any symmetry constraints [44,45]. The geometries obtained were further optimized and calculated with DFT at B3LYP/6-31 G(d,p) level of the theory. Electronic parameters of the investigated gemini surfactants were calculated, notably: energy of the molecule (E), energy of the highest occupied molecular orbital (EHOMO), energy of the lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE), chemical hardness (η), softness (σ), ionization potential (IP), electron affinity (EA), electronegativity (χ), the fraction of electron transferred (ΔN) and dipole moment (μ).

  1. Results and discussion
    1. Characterisation of the synthesised inhibitors
      1.    FTIR spectroscopy

FTIR spectra of the gemini surfactants exhibited the bands at 3467-3417 cm­-1 ν (H2O), 3310-3288 cm-1 ν (O-H involved in hydrogen bonds), 2960-2845 cm-1 ν (C-H), 1474-1378 cm-1 δ (-CH2– and -CH3 groups), 1272-1250 cm-1 ν (C-N) and  1143-1127 cm-1 ν (C-O).
The FTIR spectra of 12-O-12 is presented in Figure 2.

  1.    1H-NMR spectroscopy

The 1H-NMR of 12-O-12 (CDCl3, TMS, ppm): 0.88 (t, 6H, -CH2CH3), 1.25 (m, 36H,
-CH2(CH2)9CH3), 1.72 (m, 4H, -NCH2CH2-), 3.39 (s, 12H, -N(CH3)2), 3.58 (t, 4H,
-NCH2CH2-), 3.97 (t, 4H, -NCH2CH2O-), 4.26 (t, 4H, -NCH2CH2O-).
The 1H-NMR of 18-O-18 (CDCl3, TMS, ppm): 0.88 (t, 6H, -CH2CH3), 1.26 (m, 60H,
-CH2(CH2)15CH3), 1.74 (m, 4H, -NCH2CH2-), 3.47 (s, 12H, -N(CH3)2), 3.68 (t, 4H,
-NCH2CH2-), 4.04 (t, 4H, -NCH2CH2O-), 4.30 (t, 4H, -NCH2CH2O-).
The 1H-NMR of 12-MOH-O-MOH-12 (CDCl3, TMS, ppm): 0.88 (t, 6H, -CH2CH3), 1.26
(m, 36H, -CH2(CH2)9CH3), 1.72 (m, 4H, -NCH2CH2-), 3.38 (s, 6H, -N(CH3)2), 3.59 (t, 4H,
-NCH2CH2-), 3.78 (t, 4H, -NCH2CH2O-), 3.96 (t, 4H, -NCH2CH2OH), 4.07 (t, 4H,
-NCH2CH2O-), 4.23 (t, 4H, -NCH2CH2OH), 5.61 (t, 2H, -CH2OH).
The 1H-NMR of 18-MOH-O-MOH-18 (CDCl3, TMS, ppm): 0.88 (t, 6H, -CH2CH3), 1.26
(m, 60H, -CH2(CH2)15CH3), 1.72 (m, 4H, -NCH2CH2-), 3.38 (s, 6H, -N(CH3)2), 3.61 (t, 4H,
-NCH2CH2-), 3.78 (t, 4H, -NCH2CH2O-), 3.97 (t, 4H, -NCH2CH2OH), 4.07 (t, 4H,
-NCH2CH2O-), 4.23 (t, 4H, -NCH2CH2OH), 5.59 (t, 2H, -CH2OH).

  1. Corrosion behaviour
    1.    Potentiodynamic polarisation measurements

Figures 3-6 show Tafel plots for the stainless steel in 3M HCl at different concentrations of prepared cationic gemini surfactants. The corrosion parameters: corrosion current density (icorr, μA cm-2), corrosion potential (Ecorr, mVSCE), cathodic Tafel slope (bc, mV), anodic Tafel slope (ba, mV) are presented in Table 1. It was observed that both the cathodic and anodic curves showed lower current density in the presence of the inhibitors. For the uninhibited solution corrosion current density was 2550 μA cm-2  whereas for solutions with inhibitors: icorr=111.70 μA cm-2 for 12-O-12 (2mM), 96.74 μA cm-2 for 12-MOH-O-MOH-12 (3 mM), 122.13 μA cm-2 for 18-O-18 (0.01mM) and 1037.20 μA cm-2 for 18-MOH-O-MOH-18 (0.01 mM).  Adding inhibitors shifted the anodic and cathodic curves toward more positive potentials and made the metal more resistance to corrosion. This may be due to adsorption of the inhibitor on the metal surface and means that the inhibitors act as mixed inhibitors i.e. inhibitors reduce the anodic dissolution of stainless steel and retard the cathodic hydrogen evolution reaction. Moreover, after adding inhibitor, the branches have similar shape to those one without inhibitor. It indicates that the mechanism of the corrosion reaction is not changed [46].   For 18-O-18 (0.01 mM) the biggest change in anodic and cathodic slopes is observed, which is in a good agreement with the results of inhibition efficiency (95%). For 12-O-12 and 12-MOH-O-MOH-12, a change of the cathodic slope is greater than that observed for the anodic slope. For 18-MOH-O-MOH-18 only a slight shift of both slopes is observed.
Inhibition efficiency (IE%) was obtained from the following equation (1) [9]:
IE% = [1-(i/i0)]x100    (1)
Where i0 and i are the corrosion current densities in the absence and presence of the inhibitor.
The values of corrosion current density (icorr) were calculated from the transformation of the well known Stern-Geary equation  (2) [9]:
icorr = (babc/2.303(ba+bc))/Rp  (2)
Data in Table 1 shows that the inhibition efficiency increased with increasing concentration of inhibitor, reaching the maximum around a value of critical micelar concentration (CMC) and staying constant, which means that a stable adsorbed layer of inhibitor molecules is formed. Interfacial aggregation reduces surface tension and it is related to corrosion inhibition [47]. Above CMC, adding additional molecules leads to formation of micelles and it does not alter the surface tension and corrosion current density [47]. The values of CMC for tested gemini surfactants are presented in Table 2 [28].
The CMC values decreases with the increase in the length of the hydrophobic group, by more than 20-times.
12-O-12 reached 96% of inhibition efficiency when its concentration was 2 mM, which was higher compared to 12-MOH-O-MOH-12 at the same concentration (IE%=83%). The latter reached the best efficiency result (98%) at 3 mM. In the case of 12-O-12 elongating the alkyl chain from 12 carbon atoms to 18 (18-O-18) led to a high inhibition efficiency at 200-times lower concentration (0.01 mM). This phenomenon was not observed for the other pair of inhibitors. Elongating alkyl chains from 12 carbon atoms (12-MOH-O-MOH-12) to 18 (18-MOH-O-MOH-18) decreased the inhibition efficiency from 98% (3 mM) to 59% (0.01 mM).

  1.    Open circuit potential

Open circuit potential measurements were carried out for a stainless steel in 3M HCl in presence and absence of the tested gemini surfactants at different concentrations at room temperature. Figure 7 presents the relation between OCP and time. At the beginning the blank potential value was -401 mV vs SCE and after 24h reached -355 mV vs SCE. Shifting the potential towards more noble direction is associated with corrosion product forming on the metal surface partially protecting the surface from corrosion [48]. Adding inhibitors shifted the potential of the corrosion process towards more positive values than the potential registered in uninhibited system. The lowest value of OCP (after 24 hours) was reached for 12-MOH-O-MOH-12 (3 mM), which is in good agreement with polarisation measurements. This surfactant is characterised by presenting the highest inhibition efficiency.

  1.    Electrochemical impedance spectroscopy

Figure 8 shows the impedance diagrams (Nyquist plots) for the stainless steel in 3M HCl solution without and with the synthesised inhibitors after 24 hours of immersion.
The charge transfer resistance values (Rct) were calculated from fitting the experimental data to the equivalent circuit model which is shown in Figure 9 [49]. The equivalent circuit obtained consists of a solution resistance (Rs) and a constant phase element (CPE) Q in parallel with a resistor (Rct).
The CPE is used instead of a pure capacitor for fitting purposes and takes into account the non ideal capacitive behaviour of the system [46]. The CPE of each semi-circle was converted into a pure double layer capacitance (Cdl) using the following equation (3) [50]:
Cdl = ((Q*Rct)1/n)/Rct   (3)
Where n is the phase shift, represents about the degree of non-ideality in capacitive behaviour [46].
The inhibition efficiency (IE%) was calculated using charge transfer resistance according to the following equation (4) [46]:
IE% = ((Rct-Rct0)/Rct0)x100 (4)
Where Rct and Rct0 are the charge transfer resistance values with and without inhibitors for stainless steel in 3M HCl, respectively.
Corrosion rate was calculated using following equation (5) [51]:
Corrosion rate = icorr*Aw*10*3.15*107/nFd  (5)
Where icorr is the current density [A/cm2], Aw – 56 g/mol1 of iron, F- Faraday’s constant (96500 C/mol1), n=2, d – density of stainless steel (7.93g/cm3), 3.15*107– one year in seconds and 10- is to obtain results in mm [51]. The current density was calculated from equation (2).
The results of the EIS are presented in Table 3. The values of charge transfer resistance Rct in the presence of an inhibitor are greater than for the uninhibited solution, which indicates a reduction in the stainless steel corrosion rate. The double layer capacitance, Cdl, values decrease in the presence of inhibitors, reaching the lowest value around the CMC values. For 12-O-12 the double layer capacitance decreased from 4890 to 796 μF/cm2. This decrease is considered to relate to the replacement of the water molecules at the electrode surface by the inhibitor molecules, decreasing the extent of metal dissolution and reducing the interfacial surface [9]. The double layer between metal surface and the solution is considered as an electrical capacitor [48] and the capacitance reduces due to decreasing in local dielectric constant and/or increase in the thickness of the electrical double layer according to the Helmholtz model [52] (6):
Cdl=ɛɛ0A/d      (6)
Where ɛ – the dielectric constant of the medium, ɛ– vacuum permittivity. A – the electrode surface area and d – thickness of the protective layer [52].
The decrease of the double layer capacitance suggests the inhibitors act by adsorption at the metal/solution interface [53].
The inhibition efficiency of the synthesised gemini surfactants obtained from the electrochemical impedance measurements are in good agreement with the values obtained from the polarisation measurements.

  1. Confocal laser scanning microscopy

After the 24 hours test with the AISI 304 immersed in the solutions of 3M HCl without inhibitors and with the most efficient concentration of inhibitors the surface of the samples was analysed using confocal laser scanning microscopy. The roughness (Ra) of the stainless steel surfaces was determined (Table 4) using the VK Analyzer. The inhibited steel surface is smoother than the uninhibited surface indicating that the presence of a protective layer of adsorbed molecules prevented corrosion by the acid, inferring that the gemini surfactants tested hinder the dissolution of iron, and thereby, reduces the rate of corrosion of stainless steel in 3M HCl.

  1. Scanning electron microscopy

In order to characterise the effect of the synthesised gemini surfactants on the corrosion resistance of the AISI 304 stainless steel SEM micrographs of the sample surfaces in the absence and presence of the most efficient concentration of the surfactants after immersion of 24 hours in 3 M HCl (Figure 10) were obtained. Figure 10: (a) represents the bare steel surface (before immersion); (b) shows the steel surface after immersion in 3M HCl solution without surfactants; and (c), (d), (e) and (f) show the steel surface immersed in 3M HCl solution with 12-MOH-O-MOH-12 (3 mM), 18-O-18 (0.01 mM), 12-O-12 (2 mM) and 18-MOH-O-MOH-18 (0.01 mM) respectively. The morphology of the steel surface shown in Figure 10 (b) indicate a heavily corroded surface. These observations indicate that the steel surface was damaged by the acid in the absence of surfactants. On the other hand, Figure 10 (c), (d) and (e) appears to be less corroded in the presence of surfactants compared with that of the surface immersed in acid medium alone, which is attributed to the formation of a protective layer by the gemini surfactants. This indicates that tested gemini surfactants reduce the corrosion rate of stainless steel in 3M HCl solution. However, the image in Figure 10 (f) shows the poorest performance where the surface exhibits greater corrosion compared to surfaces exposed to other inhibitor system. This characterisation, which is independent in nature from electrochemical tests is in agreement with the electrochemical testing further confirming the performance of the inhibitors tested.

  1. Adsorption isotherm

In order to understand the mode of adsorption of the corrosion inhibitors onto the SS surface, the data obtained from EIS measurements have been fitted to different theoretical adsorption isotherms, namely, Langmuir and Freundluich. Analysis of the results suggest they are best fitted by the Langmuir adsorption isotherm equation(7) [47]:
C/θ = (1/Kads) + C  (7)
Where C is the inhibitor concentration [mol/dm3], Kads is the adsorptive equilibrium constant and θ is the surface coverage calculated from equation(8) [54]:
θ = IE%/100   (8)
Where IE% is inhibition efficiency [%].
The plots of C/θ versus C for two inhibitors are presented in Figure 11. The linear regression coefficient is almost equal to 1 which indicates that CIs formed a single molecule adsorbed layer on the SS surface and there was no interaction between adsorbed molecules [55].
The values of Kads were calculated from the reciprocal of the intercept [46]. The adsorptive equilibrium constant values are presented in Table 5. The large values indicate a strong adsorption of the tested inhibitors on the surface of stainless steel in 3M HCl.
Gibbs free energy (ΔGads) can be calculated with the following equation(9) [55]:
ΔGads = -RT ln(55.5Kads)  (9)
Where R is the gas constant (8.314 J mol/K), T is temperature (K) and the value 55.5 is the molar concentration of water. It was observed that ΔGads values for tested gemini surfactants are negative (Table 5). This indicates that the adsorption process is spontaneous and the adsorbed layer is stable [46,47]. Generally, ΔGads values up to -20kJ mol-1 are consistent with the electrostatic interaction between inhibitor molecules and the charged metal, physisorption. Those more negative than -40 kJ/mol involve sharing or transfer of electrons, chemisorption [46,47,55]. The values of ΔGads for 12-O-12, 12-MOH-O-MOH-12 and 18-MOH-O-MOH-18 ranged between -31.05 and -38.47 kJ/mol which suggests the adsorption process involves two type of interaction, chemisorption and physisorption. Whereas for 18-O-18 the value is -46.45 kJ/mol, indicating that it is typical of chemisorption [55].
3.6. Semiempirical calculations
Quantum chemical calculations can provide information about  the electronic structure and reactivity of the molecules. Calculated parameters can be  connected with the ability to prevent the corrosion process [56].
PM5 semiempirical calculations were performed using the WinMopac 2003 program [43]. Representative compounds of 12-O-12, 18-O-18, 12-MOH-O-MOH-12 along with 18-MOH-O-MOH-18 are shown in Figure 12. The final heat of formation (HOF) for all compounds is presented in Table 6. In O-bridged cationic gemini surfactants the lowest HOF value is observed for derivatives with a hydroxyl group, i.e., 12-MOH-O-MOH-12 and 18-MOH-O-MOH-18. The substitution of hydrogen atoms hydroxyl groups in methyl group on average lowers HOF by about 90 kcal/mol. In turn, the highest HOF is observed for derivatives without additional polar groups, such as 12-O-12 and 18-O-18. On the other hand, the elongating the alkyl chain affects the reduction of the HOF value. The distances between the quaternary nitrogen atom and the chloride anion are 3.70–4.66 Å; additionally the distances between the quaternary nitrogen atoms and the oxygen atoms are 2.89–3.35 Å.
The spatial arrangement and interaction of compound 12-MOH-O-MOH-12 are shown in Figure 13. The final heat of formation is –2206.3370kcal/mol, and the distances between the individual molecules are 19.1 and 8.9 Å, respectively. In contrast, the heat of formation of an anti supramolecular structure of 12-MOH-O-MOH-12 is –2094.2154kcal/mol, which is less profitable from the energetic point of view.
Based on the PM5 calculations, it is observed that the conformations off the molecules are controlled by the electrostatic interaction between the positively charged nitrogen atom and the chloride anions. This is a very good confirmation of the conclusion that interactions reduce HOF. Moreover, the observations are important when the corrosion protection is considered as the electrostatic interactions are responsible for the adsorption process.
Relating the HOF values with corrosion inhibition efficiency it is observed that 12-O-12 has the highest heat of formation value (-164.6531 kcal/mol) indicating the compound is the most reactive. This is in an agreement with electrochemical results where 12-O-12 achieved 95% at 2 mM. 18-MOH-O-MOH-18 has the lowest HOF value (–328.3942 kcal/mol) suggesting the compound is the most stable as monomer. This observation is also in a good agreement with experimental data as 18-MOH-O-MOH-18 turned out the least efficient corrosion inhibitor, IE=65% at 0.05 mM.
3.7. DFT calculations
Recently density functional theory (DFT) have been applied for predicting a theoretical ability to inhibit corrosion process according to some quantum chemical parameters [57].
Some calculated theoretical parameters are presented in Table 7.
The values of the energy of molecules are negative indicating that surfactants are thermodynamically stable. However, the values for C12 salts are more negative suggesting they are more reactive than their C18 analogues indicating better inhibition ability.
The energy of the highest occupied molecular orbital (HOMO) is related to the ability of a molecule to donate electrons to the free d orbital of a metal. Compounds with higher EHOMO are more capable of donating electrons [12]. The energy of the lowest unoccupied molecular orbital (LUMO) is connected with the ability to accept electrons from the metal. Lower values indicate higher tendency of accepting electrons. For all synthesised the values of EHOMO are negative which indicates the adsorption process is rather physical than chemical. That observation is in a good agreement with the observation made from the isotherm fitting which indicates rather physical than chemical adsorption.
The energy gap (ΔE=ELUMO– EHOMO) is another important parameter [12]. The lower ΔE, the more reactive molecule is. From the Table 7 it is observed that 12-O-12 should be the most efficient inhibitor and the worse inhibitor should be 18-MOH-O-MOH-18 due to the highest value of the energy gap.
The dipole moment (μ) is another important parameter which gives information about polarity in a bond [12]. The higher value of μ, the stronger dipole-dipole interactions with the metal surface resulting in stronger adsorption. From the tested surfactants 18-MOH-O-MOH-18 has the highest dipole moment.
Chemical hardness (η) and softness (σ) provide information about the resistance of a molecule to charge transfer and about the capacity of a molecule to receive electrons. They are calculated from the following equations [12]:
η = -1/2 (EHOMO – ELUMO)  (10)
σ = 1/η  (11)
The higher σ value suggests softer nature of the molecule and greater tendency to donate electrons to the metal [12]. The softness of the tested gemini surfactants is similar.
HOMO and LUMO energies can be used to calculate ionization potential (IP) and electron affinity (EA)  by the following equations:
IP = -EHOMO  (12)
EA = -ELUMO  (13)
The calculated values are used to estimate the electronegativity (χ) [58]:
χ = (IP + EA)/2  (14)
High value of χ suggest strong ability to attract electrons from the metal which leads to greater interactions and higher corrosion protection. Electronegativity values of 12-MOH-O-MOH-12 and 18-MOH-O-MOH-18 are a little bit higher than for 12-O-12 and 18-O-18.
The last parameter which can be calculated is the fraction of electron transferred (ΔN):
ΔN = χFe – χinh / 2(ηFe – ηinh)  (15)
where χFe equals 7eV and ηFe = 0 [12]. If ΔN > 0 the electron are transferred from the molecule to the metal and if ΔN<0 from the metal to the molecule. For all tested gemini surfactants the values of the fraction of electron transferred are negative indicating transferring from the metal to the molecules.
When comparing the experimental results to theoretical calculations, good correlation can be found. The electrons parameters for C18 compounds indicate higher ability to prevent corrosion and it is observed, they prevent corrosion at lower concentrations. However, the values of energy of the molecules are more negative indicating they are less reactive than C12 salts and this may explain why they are less effective than compounds with twelve carbon atoms in the hydrophobic chains. Moreover, it is important to remember that calculations are made for a single molecule and do not include interactions between them as well as any steric effects which have a significant impact on corrosion inhibition, especially for 18-MOH-O-MOH-18. This compound does not achieve corrosion inhibition efficiency as high as 18-O-18, as a result of the steric effects and the interaction between hydrophobic tails, as discussed above.

  1. Conclusions
  • The chemical structure of novel synthesised surfactants was confirmed by FTIR and 1H NMR.
  • High inhibition efficiencies are observed around the critical micellar concentration values – the inhibition efficiency increases with the increase of concentration and reaches the maximum value near the CMC.
  • In the presence of the inhibitors, the open circuit potential of the AISI 304 stainless steel is shifted towards more positive values when compare to that of the potential of the blank sample (no inhibitor).
  • The addition of inhibitors to the HCl shifts both the cathodic and anodic curves towards lower current densities, which suggest that the inhibitors act as mixed type inhibitors; reducing the anodic dissolution of stainless steel and retarding the cathodic hydrogen evolution reaction.
  • EIS measurement results indicate that the resistance of the stainless steel electrode reaches the highest values around the surfactant’s CMC values. The decrease of the electrochemical double layer capacitance of the tested surfactants may be attributed to the replacement of the water molecules at the electrode surface by the inhibitor molecules of lower dielectric constant through an adsorption process.
  • The adsorption of cationic gemini surfactants on the SS surface from hydrochloric acid solutions obeys the Langmuir adsorption isotherm model. For three of them (12-O-12, 12-MOH-O-MOH-12 and 18-MOH-O-MOH-18) the adsorption process involves chemisorption and physisorption and for 18-O-18 involves typical chemisorption.
  • Experimental results appear to be in good agreement with the theoretical calculations.
  1. Acknowledgement

This work has been supported by the National Centre for Research and Development (Poland; TANGO1/266340/NCBR/2015).
O. K. thanks to the University of Manchester for the provision of laboratory facilities.

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List of Figures
Figure 1. Structures of synthesised inhibitors.
Figure 2. FTIR spectra of 3-oxa-1,5-pentamethylene-bis(N,N-dimethyl-N-dodecylammonium chloride).
Figure 3. Tafel plots for 12-O-12 with different concentrations.
Figure 4. Tafel plots for 18-O-18 with different concentration.
Figure 5. Tafel plots for 12-MOH-O-MOH-12 with different concentrations.
Figure 6. Tafel plots for 18-MOH-O-MOH-18 with different concentrations.
Figure 7. Open circuit potential-time curves for stainless steel (AISI 304) electrode immersed in 3M HCl in the absence and presence of gemini surfactants.
Figure 8. Nyquist plots for stainless steel in 3M HCl in the absence and presence of synthesised inhibitors; HCl 3M; 12-O-12 2mM; 18-O-18 0.01mM; 12-MOH-O-MOH-12 3 mM; 18-MOH-O-MOH-18 0.01 mM.
Figure 9. The equivalent circuit for the studied system (* RP= Rct + Rs).
Figure 10. SEM images of stainless steel surfaces: (a) before immersion; (b) after immersion in 3M HCl; (c) after immersion in 3M HCl with 12-MOH-O-MOH-12; (d) after immersion in 3M HCl with 18-O-18, (e) after immersion in 3M HCl with 12-O-12, (f) after immersion in 3M HCl with 18-MOH-O-MOH-18.
Figure 11. Langmuir isotherm adsorption model of 12-O-12 and 12-MOH-O-MOH-12 on the stainless steel surface in 3M HCl.
Figure 12. Molecular models of 12-O-12, 18-O-18, 12-MOH-O-MOH-12 and18-MOH-O-MOH-18 calculated by the PM5 method.
Figure 13. Supramolecular models of 12-MOH-O-MOH-12 calculated by PM5 method for eight molecules.
List of Tables
Table 1. Electrochemical polarisation parameters and the inhibition efficiencies for AISI 304 in 3M HCl in the absence and presence of different concentrations of synthesised inhibitors at room temperature. Data obtained after 24 hrs immersion.
Table 2. The values of critical micellar concentration.
Table 3. EIS parameters for corrosion of stainless steel in 3M HCl in absence and presence of different concentrations of the synthesised gemini surfactants after 24 hours of immersion.
Table 4. The roughness of the stainless steel samples before and after (24 hrs) immersion in 3M HCl without and with tested corrosion inhibitors (the most efficient concentrations).
Table 5. Thermodynamic parameters of adsorption on stainless steel surface in 3M HCl containing different concentrations of the corrosion inhibitors.
Table 6. Heat of formation (HOF) [kcal/mol] of 12-O-12, 18-O-18, 12-MOH-O-MOH-12 and 18-MOH-O-MOH-18.
Table 7. Quantum chemical parameters calculated using the Gaussian 09 program package with B3LYP/6-31 G (d, p) basis for the tested gemini surfactants.

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