The adsorption on copper of 2-Mercaptobenzothiazole (2-MBT), a heterocyclic compound member of the tiazole family, has been investigated at different concentrations (1×10−1 to 1×10−6 M) in water, employing the Electrochemical Quartz Crystal Microbalance (EQCM). The frequency response over time was obtained for each concentration, showing a defined exponential behavior at higher concentrations (1×10−1, 1×10−2 and 1×10−3 M), which was fitted to the Langmuir adsorption isotherm with a good correlation coefficients (
Corrosion is a serious problem in many fields of industry, destroying the metal components in structures, electronic devices, vehicles, etc., as a consequence has a great impact on the economy of any country. Thus, the protection of metals and alloys is of paramount importance for large industrial conglomerates and the search for effective means to prevent corrosion is a very active area.
Copper and its alloys are extensively used in industrial applications, ranging from construction to electronics, this metal is present in almost every aspect of modern life (Davis,
Organic inhibitors are compounds that, applied in very small concentrations, are capable of effectively reduce the corrosion rate. Many of them are heterocyclic compounds consisting of a π-system and/or containing O, N, or S heteroatoms (Schmitt,
However, the kinetics of the formation process of this protective film remains unresolved. There are several techniques that can be used to study the adsorption process of organic molecules on metal substrates.
The most notable techniques are: measurement of mass change, using the electrochemical quartz crystal micro-balance (EQCM), measurement of contact angle, ellipsometry, reflectance absorption infrared spectroscopy (RA-IRS), surface plasmon resonance (SPR), electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM)(Subramanian and Lakshminarayanan,
The EQCM is a valuable tool that enables the acquisition of relevant data about the adsorption of chemical species (molecules or ions) on metallic substrates in the microscale and characterization of their adsorption kinetics (Eickes
The aim of this work is to enlarge the information about the adsorption of 2-Mercaptobenzothiazole (2-MBT, C7H5-NS2) see
Schematic molecular structure of 2-MBT.
Commercially available reactive grade (96%) Aldrich 2-Mercaptobenzothiazole, was used without further purification to prepare solutions with distilled water. The concentration range of 2-MBT was 1×10−6 to 1×10−1 M (pH = 6.5).
The EQCM working electrodes are manufactured by Gamry Instruments and they consist of AT-cut quartz crystals coated with Cu, having an active area of 0.205 cm2 and a sensitivity factor
During 2-MBT adsorption process, conducted at 22 °C, EQCM 10M Gamry frequency monitor was used to record the frequency response during 3600 seconds, inside a Gamry Vista Shield Faraday Cage.
Frequency variations (
where
As can be seen the frequency shift (
where
being
Surface coverage (
where
where
The frequency response for the adsorption of 2-MBT on copper is shown in
Frequency response over time of 1×10−1 M 2-MBT on copper.
Frequency response over time of 1×10−2 M 2-MBT on copper.
Frequency response over time of 1×10−3 M 2-MBT on copper.
At 1×10−2 to 1×10−6 M 2-MBT concentrations (
Desorption region increases as the concentration of 2-MBT decreases. It is clearly visible that a three step process takes place. In the first step 2-MBT molecules reach the metallic surface and force water molecules to desorb (initial desorption).
The second step is the adsorption of 2-MBT molecules at the active copper sites that are now vacant, due to the water desorption. It is considered that initially 2-MBT molecule is adsorbed in the metal surface through the exo-sulfur atom (Oshawa and Süetaka,
The final step of the adsorption is the rearranging of 2-MBT molecules on the metal surface, as proposed in (Kokalj
Due to this fact, the new active sites are rapidly occupied by other inhibitor molecules available in the solution.
However, when 2-MBT concentration is too low (i.e., 1×10−4 to 1×10−6 M), there are not enough molecules to adsorb and compensate the mass loss, due to water desorption and thus, the resulting frequency response corresponds only to a desorption process (
Frequency response over time of 1×10−4 M 2-MBT on copper.
Frequency response over time of 1×10−5 M 2-MBT on copper.
Frequency response over time of 1×10−6 M 2-MBT on copper.
Thiol-Thione tautomeric equilibrium (Oshawa and Süetaka,
At concentration higher than 1×10−4 M 2-MBT (
The frequency response of the three higher concentrations (1×10−1 up to 1×10−3 M 2-MBT) was fitted to the Langmuir kinetic model (
The fitted curves are presented in
Fit of frequency response data to Langmuir adsorption isotherm (
Fit of frequency response data to Langmuir adsorption isotherm (
Fit of frequency response data to Langmuir adsorption isotherm (
Langmuir fit parameters as a function of 2-MBT concentration [M]
2-MBT (M) | R2 | K’ | km (s−1) | t0 (s) |
---|---|---|---|---|
1×10−1 | 0.91 | 1689.40 | 1.60×10−3 | −575.94 |
1×10−2 | 0.97 | 1090.64 | 1.58×10−3 | −186.56 |
1×10−3 | 0.98 | 399.53 | 2.48×10−4 | 120.48 |
The correlation coefficients show a very good agreement between the experimental data and the Langmuir isotherm.
Linear fit for the concentration dependence of
Previously reported investigation (Karpovich and Blanchard,
According to
Fraction of surface coverage of copper surface
2-MBT (M) |
|
---|---|
1×10−1 | 0.50 |
1×10−2 | 0.09 |
1×10−3 | 0.01 |
The free energy of adsorption was found to be −5.59 kJ mol−1. It is considered that a chemisorbed molecule must have a free energy of −40 kJ mol−1 (Atkins,
2-MBT adsorbs on copper following the Langmuir adsorption isotherm. The initial desorption process observed in the concentration range 1×10−3 to 1×10−6 M is probably due to water molecules being substituted by 2-MBT molecules at the metal surface. The kinetic parameters of 2-MBT adsorption process showed a very low rate in the range of 2.48×10−4 to 1.60×10−3 s−1 and low surface coverage of 0.01 to 0.50. These facts are probably due to the lack of aggressive ions, which can deteriorate the copper patina, forming copper cations and exposing a pure metal surface to 2-MBT. Copper cations make possible the interaction with the anionic form of 2-MBT, leading to strong interactions, obtaining as a consequence higher adsorption rate and better surface coverage. Our research results showed that the calculated free energy of adsorption (−5.59 kJ mol−1) corresponds to a physisorption process, probably of electrostatic nature for the interaction between 2-MBT and copper surface in aqueous solution.
The authors would like to express their gratitude to the Mexican Council of Science and Technology (CONACYT), grant 179110, for the financial support of this study.