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Synthesis, characterization and anticorrosion potentials of chitosan-g-PEG assembled on silver nanoparticles
Accepted Manuscript Title: Synthesis, characterization and anticorrosion potentials of chitosan-g-PEG assembled on silver nanoparticles Author: Hassan H.H. Hefni Eid M. Azzam Emad A. Badr M. Hussein Salah M. Tawfik PII: DOI: Reference:
S0141-8130(15)30167-7 http://dx.doi.org/doi:10.1016/j.ijbiomac.2015.11.073 BIOMAC 5578
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
23-10-2015 18-11-2015 25-11-2015
Please cite this article as: H.H.H. Hefni, E.M. Azzam, E.A. Badr, M. Hussein, S.M. Tawfik, Synthesis, characterization and anticorrosion potentials of chitosan-g-PEG assembled on silver nanoparticles, International Journal of Biological Macromolecules (2015), http://dx.doi.org/10.1016/j.ijbiomac.2015.11.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, characterization and anticorrosion potentials of chitosang-PEG assembled on silver nanoparticles Hassan H.H. Hefni, Eid M. Azzam, Emad A. Badr, M. Hussein , Salah M. Tawfik*
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Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt. * Corresponding author: [email protected], Tel. 002-01273615278.
Abstract
Chitosan (Ch) grafted with poly(ethylene glycol) (Ch-g-mPEG) were synthesized
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using mPEG with molecular weights 2000 g/mol. The synthesized Ch-g-mPEG was characterized using gel permeation chromatography (GPC), fourier transform infrared
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spectroscopy (FTIR), proton nuclear magnetic resonance (1HNMR), and X-ray diffraction (XRD) techniques. Ch-g-mPEG silver nanoparticles has been synthesized
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and characterized by high-resolution transmission electron microscopy (HRTEM) and energy dispersive analysis of X-rays (EDAX). The synthesized Ch-g-mPEG and its nanostructure were examined as corrosion inhibitors for carbon steel in 1 M HCl
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solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results revealed that the inhibition efficiency obtained by Ch-g-mPEG self-assembled on silver nanoparticles is greater than that
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obtained by Ch-g-mPEG only. Potentiodynamic polarization results reveal that the
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synthesized compound could be classified as mixed-type corrosion inhibitors with predominant control of the cathodic reaction. The results of EIS indicate that the both
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charge transfer resistance and inhibition efficiency tend to increase by increasing the inhibitor concentration. Keywords:
Chitosan; Poly(ethylene glycol); silver nanoparticles; carbon steel; corrosion inhibition; electrochemical impedance spectroscopy 1. Introduction Steel is widely used as part of our life in different applications in automotive, household appliances, machinery and heavy construction such as the marine, petroleum and chemical industries which make it very important to research corrosion and protection of iron and its alloys. The most issue of iron is how to improve its resistivity against corrosion phenomena, which is one of the major reasons for industrial accidents and consumption of material resources [1,2]. There are different types of corrosive media such as salts, acids, humidity and microorganisms [3]. It is well established that corrosion problems may be caused by microorganisms, 1 Page 1 of 29
acidification, formation of salt water and corrosive gases in numerous systems within the petroleum industry [3–5]. Hydrochloric acid solutions are widely used among these various acids for stimulating carbonate-based reservoirs like lime stone and dolomite [5]. Moreover, bacteria called sulfate-reducing bacteria (SRB) are always
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the responsible of these problems [6]. Several studies have examined the relationship between the structure of the inhibitor molecule and its efficiency [5-9] but much less attention has been paid to the dependence of the protection efficiency on the size of the
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inhibitor molecule and the electronic distribution in the inhibitor molecule. With the rapid advancement of nanotechnology, thin films of thickness in the micron and nanometeric
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scales are increasing their popularity in scientific and technological applications [10].Nanoparticles have high tendency to interact with each other to form agglomeration [11]. Their unique properties are mainly due higher surface area of the nanosized particles
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in compare to microsized caused by their large surface area to volume ratio [12]. There are various reports concerning improving corrosion resistance using nanoparticles such
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as; TiO2 [13], Cu2O [14], ZnO [15], ZrO2 nanoparticles [16], Fe3O4 [17], SiO2 [18] and organoclay nanoparticles [19]. Migahed et al. studied electrochemical behaviour of carbon steel in acid chloride solution in the presence of dodecyl cysteine hydrochloride
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self-assembled on gold nanoparticles. The relatively high value of (Kads) in case of
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dodecyl cysteine hydrochloride self-assembled on gold nanoparticles reveals a strong interaction between the inhibitor molecules and the metal surface [20]. Azzam and Abd
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elaal studied corrosion inhibition of the self-assembled Ag nanoparticles films on the surface of carbon steel [21]. Atta et al. studied the synthesis and application of hybrid polymer composites based on silver nanoparticles as corrosion protection for line pipe steel. Results revealed that polarization measurements indicate that the AgNPs and
hybrid polymer acts as a mixed type-inhibitor and the inhibition efficiency increases with inhibitor concentration. Generally, various stabilizers (surfactants, gemini surfactants, polymers, triblock polymers, proteins and carbohydrates) have been used in the synthesis and characterization of different shaped and sized of advanced silver nanomaterials [23, 24].
A number of naturally occurring polymers have been investigated and have been reported to show promising results as metal corrosion inhibitors in different corrosive environments. For instance the corrosion inhibiting effect of chitosan has been reported for mild steel and copper in acid medium [25-28]. Glucose, gellan gum, and hydroxylpropyl cellulose have been assessed as green inhibitors for cast iron in acidic
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environment by means of chemical and electrochemical techniques [29]. To our knowledge, no other research has been published until now referring to the use of chitosan grafted copolymer assembled on silver nanoparticles as corrosion inhibitor for carbon carbon steel in 1 M HCl solution. Therefore, the idea of the present work is
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to use silver modified nanoparticles with chitosan grafted copolymer as corrosion inhibitors in acidic media. The nanoparticles were characterized by TEM and EDX techniques.
Moreover,
polarization
curve
and
electrochemical
impedance
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spectroscopy techniques were employed to investigate the corrosion inhibition
efficiencies of the synthesized Ch-g-PEG and its nanostructures for carbon steel in 1
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M HCl solution.
2. Experimental method
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2.1. Materials
Chitosan with molecular weight (MW: 109.050 kDa, and deacetylation degree >80%) was prepared in our laboratory [25] and Monomethylated PEG (mPEG) with
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molecular weights 2000 g/mol (mPEG2000) were supplied by Aldrich chemicals. All other materials and reagents used in this study were of analytical grade.
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The corrosion measurements were performed on carbon steel samples with the following composition (wt%): 11% C, 0.45% Mn, 0.04% P, 0.05% S, 0.25% Si, and
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the reminder is Fe. For the electrochemical studies, the specimens were covered with an epoxy resin leaving an exposed surface area of 1 cm2 in aqueous solution. The
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specimens used for the analysis were polished with a series of emery paper (320– 1200), washed with double-distilled water, degreased with acetone and dried at room temperature.
2.2. Solutions
The corrosive solution was a 1.0 M hydrochloric acid solution diluted from concentrated acid (37%, Merck) with double-distilled water. This solution was used as blank. Concentration range of used synthesized inhibitor varied from 5 x 10-5 to 1 x 10-3 M for corrosion measurements. All tests were performed in non-deareated solutions under unstirred conditions at 25 ◦C.
2.3. Synthesis of mPEG-aldehyde (mPEG-CHO) PEG-aldehyde was synthesized via oxidation of PEG with DMSO/acetic anhydrate according references [30,31]. 5 ml of acetic anhydrate was added under N2 atmosphere to 32 ml anhydrous DMSO containing 10 g mPEG (Mw =2000 g/mol)
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and 6% CHCl3 and the mixture was stirred for 9 h at room temperature (20 ◦C). The reaction mixture was then poured into 400 ml diethyl ether. The precipitate was filtered, dissolved in chloroform and recrystallization twice by the use of diethyl ether to give PEG-aldehyde with molecular weight of 2000 g/mol.
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2.4. Synthesis of Ch-g-PEG compound Ch-g-PEG compound was synthesized according the method of Harris et al. [30].
Chitosan (0.5 g) was dissolved in an aqueous solution of 2 % V/V acetic acid by
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vigorously stirring to obtain a solution with a concentration of 2 %, filtered through
polyester cloth to remove residues of insoluble particles and an aqueous solution of
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PEG aldehyde (mPEG-CHO) with MW = 2000 g/mol (2.9 g) was added dropwise during stirring for 30 min at room temperature [31, 32]. Then the pH of chitosan/PEG
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aldehyde solution was increased by gradually adding Na2CO3 until pH = 6. After 1 h NaBH4 (0.183 g) was added and the mixture was stirred for 5 h at 55 ◦C. The precipitate was obtained by pouring the reaction mixture into a saturated ammonium
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sulfate solution [33]. The material from the inner solution was freeze-dried and washed with ethanol and acetone in order to remove the remaining mPEG that did not reacted. After drying in vacuum the obtained white powder was Ch-g-PEG.
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2.5. Synthesis of the Silver Nanoparticles (AgNPs)
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Silver nanoparticles colloidal solution was synthesized via chemical reduction method. In a typical experiment 100 ml of AgNO3 solution (1x10-3 M) was heated to
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boiling. To this solution 10 ml of 1 % tri-sodium citrate was added drop by drop. The solution was mixed vigorously and heated until the yellow colour obtained. Then it was removed from the heating element and stirred until cooled to room temperature [34].
2.6. Synthesis of the Ch-g-PEG nanostructure (Ch-g-PEG/AgNPs) The silver nanoparticles solution 20 ml (1x10-3 M) was mixed with 5 ml solution of
the synthesized Ch-g-PEG (1.5%) in distilled water. The mixture was stirred continuously for 24 h till the colour change. The resulting solution was used for TEM image and energy dispersive X-ray [35]. 2.7. Characterization of Chitosan-g-PEG 2.7.1. Gel permeation chromatography (GPC) The molecular weight determinations were carried out by gel permeation chromatography (GPC) using a Supremamax 3000 column (Polymer Standard Service, Mainz, Germany) with 2 % CH3COOH/0.2 M buffer (CH3COONa) as an 4 Page 4 of 29
eluent (1 ml/min). The standard pullulans (Mw of 11,800, 47,300, 112,000, and 780,000) were used for calibration. 2.7.2. Fourier transform-infrared spectroscopy (FT-IR) Fourier transform infrared (FTIR) spectra was recorded for the synthesized Ch-g-PEG
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and compared with chitosan on an ATI Mattson Infinity Series TM, Bench top 961 controlled by win first TM V2.01 software (Egyptian Petroleum Research Institute ‘‘EPRI’’) at 25 °C. About 2 mg of sample with 100 mg of KBr was fully grinded and
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‘mixed. The mixed samples were pressed into pills with a compressor and prepared
pellets were used for studies. All spectra were scanned against a blank KBr pellet
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back-ground in the range of 4000–400 cm−1with resolution of 4.0 cm−1. 2.7.3. Nuclear magnetic resonance (NMR)
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The 1H-NMR was measured in DMSO-d6 by Spect Varian, GEMINI 200 (1H 200 MHz) (Micro-analytical Center, Cairo University). The synthesized inhibitors were dissolved in DMSO-d6 solvent.
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2.7.4. X_ray diffraction
The XRD analysis was carried out on an X_ray diffract meter (D/Max2500VB2+/Pc, X_Ray.
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Rigaku Company, Tokyo, Japan) with a Cu detector using 1.54 Å wavelength of the
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2.8. Characterization of Chitosan-g-PEG assembled on silver nanoparticles 2.8.1. Transmission electron microscope (TEM)
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A convenient way to produce good TEM samples is to use copper grids. A copper grid pre-covered with a very thin amorphous carbon film. To investigate the prepared AgNPs using TEM, small droplets of the liquid were placed on the carbon-coated grid. A photographic plate of the transmission electron microscopy employed on the present work to investigate the microstructure of the prepared samples. Nanoparticle size was determined by using TEM model ‘‘Jeol JeM – 2100 (Japan)’’ (Egyptian Petroleum Research Institute ‘‘EPRI’’) 2.8.2. Energy dispersive X-ray (EDX) spectroscopy The energy-dispersive X-ray (EDX) spectroscopy was recorded with an EDX detector (Oxford LINKISIS 300) equipped on a transmission electron microscope (TEM, Hitachi S-520) operated at 10 kV accelerating voltage. 2.9. Corrosion studies Electrochemical measurements were carried out using a Voltalab 40 Potentiostat PGZ 301 and a personal computer was used with Voltamaster 4 software at 25 oC and 5 Page 5 of 29
frequency response analyzer in a three-electrode arrangement. Carbon steel was taken as the working electrode, platinum as the counter electrode (The main function of the counter electrode is to provide the location of the second electron transfer reaction. Important parameter of the counter electrode is the surface area. It is required (area)
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large enough to support the current generated for the working electrode the surface area of the used platinum electrode is 5 cm) and saturated calomel electrode as reference electrode. The working electrode was held in the electrolyte solution for 60
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min before each measurement for attaining a steady-state potential. For polarization studies, the potential was swept from cathodic direction to anodic direction with
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potential (−1000 mV to +100 mV) at a scan rate of 1 mV/s. The point of intersection of extrapolated tafel lines gives the value of corrosion current density Icorr and
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corrosion potential Ecorr. The inhibition efficiency is calculated using the corrosion current densities values in the absence and presence of various concentrations of the inhibitor. The electrochemical impedance spectroscopy (EIS) measurements were
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performed in the frequency range 100 kHz to 30 mHz, using AC signals of amplitude of 5 mV peak to peak. All the impedance values were measured at open circuit potential. Nyquist plot represents the results from EIS measurements. The charge
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Nyquist plot [36-40].
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transfer resistance (Rct) and capacitance of double layer (Cdl) were calculated from the
3. Results and Discussion
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3.1. Synthesis and characterization of Ch-g-PEG Chitosan is a linear polysaccharide composed of randomly distributed -(1–4)-linked
d-glucosamine and N-acetyl-d-glucosamine. Chitosan contains mainly hydroxyl and amino repeating in its. The hydroxyl end groups of PEG are difficult to react with chitosan at normal condition to prepare the grafted compound (Ch-g-PEG). Furthermore, in this case since PEG is a bifunctional reagent crosslinked macromolecules could be prepared. In order to avoid this mono methyl ether of PEG was used, which contains only one hydroxyl group in the macromolecular chain. For a successful grafting reaction of PEG into chitosan, PEG monomethyl ether was oxidized in a first stage to PEG-aldehyde with acetic anhydride and DMSO according to the method described by Sugimoto [30,31]. The room temperature was preferred for the oxidation reaction since there might be a chance of excessive reaction [32]. After the successful synthesis of mPEG-CHO with molecular weights 2000 g/mol,
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PEG can be reacted with chitosan in order to prepare the grafted compound. The synthetic method used for the N-PEGylation of chitosan by the use of reductive amination is demonstrated in Fig. 1. One step-reductive amination, also known as Borch reduction, is the reaction between an amino group of a primary or secondary
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amines and aldehyde group in the presence of a reducing agent [33]. During the second step of the procedure, imine compound is created and reduction reaction follows, leading to the Ch-g-mPEG graft copolymers. In both cases of the produced
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copolymers the reducing agent is added to the reaction mixture drop wise in a time period of 1 h in order to avoid precipitation of chitosan due to high alkalinity of
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NaBH4. Grafting of chitosan-g- PEG was confirmed as the following:. 3.1.1. GPC Data
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From the GPC data, we found that the molecular weights of chitosan and Ch-g- PEG compounds are 109.050, and 356.808 kDa respectively. The increase in molecular weight of Ch-g-PEG over that of chitosan indicated the formation of new products.
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3.1.2. FT-IR Spectroscopy
Comparative IR spectra of chitosan and the synthesized Ch-g-PEG are shown in Fig. 2. To the spectra of grafted chitosan the characteristic peaks of chitosan and mPEG
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can be identified. The characteristic peaks of PEG at 1110 cm−1 (C-O stretch) and
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2900 cm−1 (C-H stretch) appear more intense than those of chitosan at the grafted compound. Chitosan characteristic bands appear at 3380 cm−1 (O-H stretch
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overlapped with N-H stretch) and 2860 cm−1 (C-H stretch) [41]. On the contrary other
two characteristic peaks of chitosan at 1607 cm−1 and 1400 cm−1, that correspond to Amide I and Amide II, respectively, show a quite different behavior. The absorbance of Amide II, at 1400 cm−1, seems to remain unaffected, a fact that seems to be
reasonable as this peak is attributed to the stretching vibration of the C-O group of the acetylated amino groups of chitosan, which is not affected during the modification procedure of chitosan. The absorbance peak of bending primary amine of chitosan at 1590 cm−1, is almost disappeared and shifted to 1535 cm−1 due to formation of secondary amine of Ch-g-PEG. This characteristic groups are attributed to the bending vibration of N-H group of chitosan (NH2), and this constitutes strong evidence of the extend degree of reaction between the free NH2 groups of chitosan and the aldehyde group of mPEGCHO [42]. 3.1.3. 1HNMR Spectroscopy
7 Page 7 of 29
The characterization of the grafted compound is completed through the 1HNMR spectra. The characteristic proton signals of Ch-g-PEG appeared in the range of 3.5– 4.0 ppm (Fig. 3). The peak at around 3.36 ppm is attributed to the proton signal of methoxyl of CS-g-PEG. Peaks of the glucosamine residues at 4.99 ppm for the
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anomeric proton on C-1, and at 2.50 ppm for the proton on C-2. The peak for the methyl protons of the N-acetylglucosamine residues appears at 1.63 ppm. The Ch-gPEG showed new peak at 1.23 ppm (Fig. 3) assigned to the grafted of chitosan with
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mPEG2000 [43] 3.1.4. XRD analysis
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Typical XRD patterns of chitosan and Ch-g-PEG2000 are shown in Fig. 4. XRD pattern of neat Chitosan (Ch) showed that is in an amorphous to partially crystalline state. This observation is in accordance with Nunthanid [44] who reported a peak at
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approximately 12◦ (2θ) corresponding to hydrated crystals and one at 18◦ (2θ) corresponding to anhydrous crystals. On the other hand the XRD pattern of Ch-g-
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PEG has three strong characteristic crystalline peaks at 2θ deg 10.3◦, 19.9◦ and 29.4◦ that seem to be the characteristic peaks of mPEG but broadened due to the disrupt of
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PEG crystalline structure from amorphous chitosan [45] .The characteristic peaks of Ch-g-PEG are recorded, which however in all samples are broadened, in comparison
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with the patterns of the neat materials. Furthermore, these peaks have lower intensity, a fact that justifies that the crystal structure of grafted compound is disrupted probably
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due to distribution intermolecular hydrogen bond of chitosan units by PEG as previously reported [46].
3.2. Characterization of Ch-g-PEG nanoparticles loaded with AgNPs 3.2.1. Transmission electron microscope (TEM) The size and morphology of the synthesized silver nanoparticles and the Ch-g-PEG assembled on silver nanoparticles were investigated by TEM as shown in Fig.5. It was seen from TEM images that the silver nanoparticles were predominantly spherical in shape and polydispersed. The size of pure AgNPs is 7-15 nm according to the TEM image, and that was consistent with the published data for the synthesis of silver nanoparticles by the chemical reduction method [47]. Fig.5. shows the spherical silver nanoparticles with a corona of the Ch-g-PEG ligands, which related to the selfassembling of the Ch-g-PEG materials on the silver nanoparticles. The TEM images showed good stabilization of silver nanoparticles due to interaction with the Ch-g8 Page 8 of 29
PEG materials. The effect of hydrophilic PEG2000 chain length of the Ch-g-PEG on the stabilization of AgNPs was observed where the surface aggregates capped on silver surface should provide more effective stabilization with presence the PEG chain .
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3.2.2. Energy dispersive X-ray (EDX) spectroscopy EDX spectroscopy results confirmed the significant presence of pure silver with no other contaminants. The optical absorption peak at 3 keV, is typical for the absorption
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of metallic silver nanoparticles due to surface plasmon resonance [48]. Fig.6. show
the EDX analysis of the AgNPs stabilized by the synthesized Ch-g-PEG. The EDX
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spectrum showed a strong and typical optical absorption peak at approximately 3 keV, which was attributed to the SPR of the metallic Ag nanocrystals [48]. This result
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indicated that AgNPs were formed in the reaction medium. Beside of Ag there are others bands for other elements peaks which appeared due to the scattering caused by the compounds that are bound to the surface of silver which indicating that the used
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Ch-g-PEG act as stabilized agents for silver nanoparticles. 3.3. Corrosion studies
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3.3.1. Potentiodynamic polarization measurements
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Nanometer-size noble metals such as Ag with large specific surface areas exhibit notable properties which are different from their corresponding bulk materials. Silver (Ag) is a
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noble metal with an inert chemical reactivity in its bulk form and is listed below hydrogen in the activity series of metals. It is well known that bulk Ag cannot react with hydrochloric acid (HCl). If the reaction between Ag nanoparticles and HCl can occur, the product will be silver chloride (AgCl), which is an insoluble precipitate and can be easily collected for characterization. It was previously reported that, high chemical reactivity of Ag nanoparticles was observed in the reaction with hydrochloric acid. The
potentiodynamic polarization curves for carbon steel in 1.0 M HCl without and with different concentrations of Ch-g-PEG and its nanostructure with silver nanoparticles (AgNPs) are shown in Figs. (7,8). As could be seen in the figure, both the cathodic and anodic (more pronounced on the cathodic) current densities decrease considerably on the introduction of grafted chitosan derivatives into the aggressive medium (1.0 M HCl). Also the corrosion potential (Ecorr) in the presence of Ch-g-PEG and Ch-gPEG/AgNPs derivatives is slightly displaced towards the negative direction compared to the blank solution. The cathodic polarization curves is also observed to give rise to 9 Page 9 of 29
parallel Tafel lines indicating that there is no modification of the hydrogen evolution reaction process on the introduction of Ch-g-PEG derivatives into the corrosive medium. This also suggests that Ch-g-PEG and Ch-g-PEG/AgNPs inhibits carbon steel corrosion by simply blocking the reaction sites without affecting the actual
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reaction mechanism [49]. Values of electrochemical parameters derived from the polarization measurements namely corrosion current density (Icorr), corrosion potential (Ecorr), anodic (βa) and cathodic (βc) Tafel slopes, surface coverage and inhibition
the
following
equation
[50]:
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from
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efficiency (η %) are listed in Table 1. Inhibition efficiency, (ηp, %), was obtained
where icorr and i0corr are uninhibited and inhibited corrosion current densities,
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respectively, determined by extrapolation of Tafel lines to the corrosion potential. Data in the Table 1 show that Icorr decreased in the presence of Ch-g-PEG and Ch-g-
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PEG/AgNPs compared to the blank solution and further decreases as the concentration of Ch-g-PEG and Ch-g-PEG/AgNPs increased. This is an indication that Ch-g-PEG and Ch-g-PEG/AgNPs inhibited the acid-induced corrosion of carbon
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steel. Inspection of the Table 1 also reveals noticeable changes in both the anodic and
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cathodic Tafel slopes. It is clear that the presence of the inhibitors causes a markedly decrease in the corrosion rate. The increase in inhibition efficiency is associated with
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a shift of both cathodic and anodic branches of the polarization curves towards lower current densities, together with a negative shift in Ecorr, suggest that the Ch-g-PEG and
Ch-g-PEG/AgNPs inhibitors act as mixed type inhibitors with predominantly cathodic; i.e. meaning inhibitors reduce the anodic dissolution of mild steel and retards the cathodic hydrogen evolution reaction, but the effect on the cathodic hydrogen evolution reactions surface is more than the anodic dissolution reactions. Also the corrosion rate is observed to decrease in the presence of Ch-g-PEG and Chg-PEG/AgNPs in comparison to its absence indicating decreased metal dissolution and further decreases with increasing concentration. As also shown in Table 1, inhibition efficiency increased with increase in concentration of Ch-g-PEG and Ch-gPEG/AgNPs and at the same inhibitor concentration the inhibition efficiency of Ch-gPEG assembled on silver nanoparticles greater than that of Ch-g-PEG. 3.3.2. Electrochemical impedance spectroscopy (EIS)
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The impedance responses of low carbon steel in 1 M HCl in the absence and presence of Ch-g-PEG and its nanostructure with AgNPs is depicted in Figs. (9,10). It could be observed from the Nyquist plots that the impedance responses of carbon steel in the acid medium changed on addition of the inhibitor. The Nyquist plot is characterized
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by one semicircle capacitive loop corresponding to one time constant in the Bode plot suggesting that the corrosion of carbon steel is controlled by a charge transfer process.
The diameter of the semicircles in the Nyquist plot and the magnitude of Bode
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modulus are observed to increase with increasing concentration of Ch-g-PEG and Chg-PEG/AgNPs indicating the formation of an adsorption film on the steel surface. In
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all cases, the Nyquist plot in Figs. (9,10) are not perfect semicircles but depressed with center under the real axis. These kinds of deviations are often referred to as the
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frequency dispersion of interfacial impedance [51]. The anomaly is usually attributed to the inhomogeneity of the electrode surface arising from surface roughness or interfacial phenomena [51]. The equivalent circuit (EC) model shown in Fig. 11 was
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used to model the physical processes taking place at the steel /solution interface. The EC consists of solution resistance (Rs), charge transfer resistance (Rct) and constant phase element (CPE). The CPE is substituted for the capacitive element to give a
ZCPE = Q-1(jω )-n
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equation:
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more accurate fit as specified in the CPE
impedance shown in the following
(2)
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where Q and n stand for the CPE constant and exponent respectively, j = (–1) is an imaginary number and ω is the sine wave modulation angular frequency in rad s–1
(ω= 2πf; where f is the frequency in Hz). The corresponding impedance parameters
obtained are listed in Table 2. Data in the table show that Ch-g-PEG and Ch-gPEG/AgNPs compounds caused an increase in the Rct value and a corresponding decrease in Cdl. Such an increase in the Rct value, synonymous with an increase in the
diameter of the semicircle in the Nyquist plot as well as the increase in the magnitude of the absolute impedance in the Bode plot point towards improved corrosion resistance due to the corrosion inhibiting action of Ch-g-PEG and Ch-g-PEG/AgNPs derivatives. The Cdl was computed using the expression [52]:
where fmax the frequency at which the imagiary component of the impedance is a maximum. 11 Page 11 of 29
In case of the electrochemical impedance spectroscopy, the inhibition efficiency was calculated using charge transfer resistance according to the following equation [52]:
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where Rct and R°ct are the charge transfer resistance values without and with inhibitor for carbon steel in 1 M HCl, respectively.
The observed decrease in the values of Cdl, which normally arises from a decrease in
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the dielectric constant and/or an increase in the double-layer thickness, is as a result of adsorption of alginate derivatives onto the metal/electrolyte interface [53].
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For more complicated system, Bode plots and phase angle can give more information. The Bode plots of the synthesized inhibitor Ch-g-PEG and Ch-g-
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PEG/AgNPs are presented in Figs. (12,13). The low frequency impedance modulus Zmod is one of the parameters which can be easily used to compare corrosion resistance of different samples. A larger Zmod demonstrates a better protection
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performance [54].
In Figs. (12, 13), it is shown that Zmod increases as a function of the concentration of the synthesized inhibitor Ch-g-PEG and Ch-g-PEG/AgNPs. These phenomena can be
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explained as follows, the high frequency phase angle range (105–104 Hz) of the
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impedance spectra corresponds to the properties of an outer layer, the middle frequency range (104–102 Hz) reflects the properties of an inner barrier layer, while
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the low frequency range (less than 102 Hz) corresponds to the properties of the
double-electrical layer information. Therefore, the high frequency phenomenon may due to the thickness increase of the outer porous layer, and the middle frequency phenomenon can be attributed to the penetration of active chloride ions and water through the defect of the prepared inhibitors inner barrier layer, though the whole effect induced the increase of Zmod. As seen from Figs. (12, 13), Bode plots refer to the existence of an equivalent
circuit that contains a single constant phase element in the metal/solution interface. The increase of absolute impedance at low frequencies in Bode plot confirmed the higher protection with increasing the concentration of the prepared inhibitors, which is related to adsorption of the inhibitors on the carbon steel surface [55]. The phase angle plots for the carbon steel in the presence and absence of synthesized inhibitors Ch-g-PEG and Ch-g-PEG/AgNPs concentrations in 1.0 M HCl 12 Page 12 of 29
solution are given in Figs. (12, 13). As seen the increasing of the inhibitors concentration in the test solution indicated superior inhibitive behavior due to adsorption of the metal surface of more prepared inhibitors molecules at higher concentrations. Furthermore, the depression of phase angle at relaxation frequency
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occurs with decreasing of inhibitors concentration which indicated the decrease of capacitive response with the decrease of inhibitors concentration. Such a phenomenon could be attributed to higher corrosion activity at low concentrations of inhibitors.
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3.4. Mechanism of inhibition
Results obtained from all the experimental techniques reveal that addition of Ch-g-
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PEG and Ch-g-PEG/AgNPs derivatives to the acid corrosive medium retarded the corrosion of carbon steel. The corrosion inhibition process no doubt occurs by virtue
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of adsorption mechanism where the oxygen and nitrogen heteroatoms present in Chg-PEG and Ch-g-PEG/AgNPs structure serves as adsorption centers to contain excessive negative charges. In acidic medium, the anodic dissolution of iron is 2+
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accompanied by the cathodic hydrogen evolution reaction as follows: ---
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Fe • Fe2+ + 2e--(5) + --2H + 2e • H2• (6 ( 6) Potentiodynamic polarization results indicate that Ch-g-PEG and Ch-g-PEG/AgNPs follow mixed inhibition mechanism. The chemical structure of Ch-g-PEG derivatives
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repeat unit (Fig.1) reveals the presence of ammonium–NH, –HO(CH2CH2O)mCH3 and –OH functional groups. It could be assumed that the cationic form of Ch-g-PEG and
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Ch-g-PEG/AgNPs may be adsorb on the cathodic sites of carbon steel and inhibits the cathodic hydrogen evolution reaction. On the other hand, –OH groups have lone pairs of electrons and can be adsorbed on the anodic sites of the metal surface via interaction with the vacant d-orbital of iron and inhibits the anodic metal dissolution reaction. These processes effectively help in isolating the metal surface from the aggressive ions present in the acid medium and thereby inhibiting corrosion. It is also known that in the acid corrosive medium, the inhibitor (Ch-g-PEG and Ch-gPEG/AgNPs) may be protonated and exists as polycation. However, the specific adsorption of chlorides ions (Cl–) and replacing hydroxyl groups on the metal surface would render the metal surface negatively charged [56-59]. Hence Ch-g-PEG and Chg-PEG/AgNPs in the form of polycation will be adsorbed on negatively charged metal surface and inhibit corrosion.
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When iron is immersed in the Ch-g-PEG and Ch-g-PEG/AgNPs solution, the protective film is formed through the hydroxyl group of the chitosan adsorbed on the iron surface. But the film is not compact because there are some defects in it. When iron specimen is immersed in the silver nanoparticles solution, the adsorption maybe
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happened with two patterns. One is made by monomethylated PEG, the other is that silver nanoparticles maybe adsorbed on the iron surface directly, and fill up some defects made by monomethylated PEG molecules. Interactions may happen between
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the silver nanoparticles and the iron atoms because of the active properties of the nanoparticles [60, 61].
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4. Conclusions
PEG grafted into chitosan backbone was easefully prepared using Borch reduction
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process, as was characterized by GPC, FTIR, 1HNMR and XRD spectroscopy. These materials are able to prepare nanoparticles via self-assembled on silver nanoparticles using chemical reduction method. The nanostructures of chitosan grafted PEG with
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AgNPs were confirmed using TEM and EDX techniques. TEM image shown that the size of the synthesized nanoparticles is 7-14 nm. Electrochemical techniques of
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monitoring corrosion approach were employed to evaluate the potential of Ch-g-PEG and Ch-g-PEG/AgNPs derivatives as an inhibitor for carbon steel corrosion in acidic
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medium. Results obtained show that Ch-g-PEG and Ch-g-PEG/AgNPs act as good inhibitor for acid-induced corrosion of carbon steel. Corrosion inhibition effect was
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found to be inhibitor concentration and structure; inhibition efficiency increased with increasing
Ch-g-PEG
and
Ch-g-PEG/AgNPs
concentration.
Potentiodynamic
polarization studies reveal that Ch-g-PEG and Ch-g-PEG/AgNPs functions as a mixed-type inhibitor but under cathodic control. The inhibition efficiency of Ch-gPEG as corrosion inhibitor enhanced via assembled of Ch-g-PEG on silver nanoparticles to form Ch-g-PEG/AgNPs nanostructure materials.
5. References
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[38] N.A.Negm, F.M.Ghuiba, S. M. Tawfik, Corrosion Science 53 (2011) 3566–3575. [39] I.Aiad, S. M.Tawfik, S. M.Shaban, A.Abd-Elaal, M.El-Shafie, Journal of Surfactants and Detergents17 (2014)391-401. [40] S. M. Shaban, A.Sayed, S. M.Tawfik, A.Abd-Elaal, I. Aiad, Journal of Industrial and Engineering Chemistry 19 (2013) 2004. [41] M. H. M. Hussein, M. F. El Hady, W. M. Sayed, and H. Hefni, Polymer science Ser. A, 54 (2012)113–124. [42] N.Bhattarai, H.R.Ramay, J.Gunn, F.A.Matsen, M.Zhang, J.Control. Release 103(2005) 609–624. 16 Page 16 of 29
[43] G. Rocasalbas, S. Tourino, J. Torresb , T. Tzanov, J. Mater. Chem. B, 1(2013) 1241 [44] J.Nunthanid, S.Puttipipatkhachorn, K.Yamamoto, G.E.Peck, Drug. Dev. Ind. Pharm. 27(2001)143–157.
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[54] S. A.Umoren, M. M.Solomon, U. M.Eduok, I. B.Obot, A. U. Israel, Journal of
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Environmental Chemical Engineering, 2(2), (2014)1048-1060. [55] M.A. Hegazy, Ali M. Hasan, M.M. Emara, Mostafa F. Bakr, Ahmed H. Youssef, Corros. Sci. 65 (2012) 67–76.
[56] S. M. Tawfik, Journal of Molecular Liquids 207(2015) 185-194. [57] E. A. Badr, Journal of Industrial and Engineering Chemistry 20 (2014) 3361– 3366.
[58] S. Ghareba, S. Omanovic, Corros. Sci. 52 (2010) 2104. [59] S.M. Tawfik, N. A. Negm, Res Chem Intermed (2015): DOI 10.1007/s11164015-2233-9. [60] H. Ma, S. Chen, G. Liu, J. Xu, Min Zhou, Applied Surface Science 252 (2006) 4327–4334. [61] A. A. Abd-Elaal, S. M. Tawfik, S. M. Shaban, Applied Surface Science, 342( 2015) 144–153.
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mPEG
Ac2O
O=CH-CH2-CH2-O-(CH2CH2O)m-CH3
DMSO, CHCl3
mPEGCH=O
mPEGCH=O
HO O 1
n
H
NHR2
H
H
O H n
mPEG-g-Chitosan
R2= H, COCH3, -CH2-CH2-O-(CH2CH2O)m-CH3
Ac ce p
te
d
Chitosan R1= H or COCH3
H
an
NHR
H
HO
NaBH3CN
H
M
H
HO
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H O
H O HO
cr
H OH
H OH
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HO-CH2-CH2-O-(CH2CH2O)m-CH3
Fig. 1. Synthetic route via the two-step reaction for the synthesis Ch-g-PEG.
18 Page 18 of 29
ip t cr us
Ac ce p
te
d
M
an
1535
Fig. 2. FT-IR spectra of chitosan and Ch-g-PEG compounds
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Ac ce p
Fig. 3. 1H NMR spectra of Ch-g-PEG compounds
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Ac ce p
te
d
M
Fig. 4. X-ray powder diffraction patterns of chitosan and Ch-g-PEG.
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ip t cr us an M d te Ac ce p
Figure 5. TEM image of the synthesized silver nanoparticle assembled on Ch-gPEG compound
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Figure 6. EDX of silver nanoparticles assembled on Ch-g-PEG compound
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Fig. 7. Polarization curves for carbon steel in 1 M HCl in the absence and
Ac ce p
te
d
M
presence of different concentrations of Ch-g-PEG at 25 oC.
24 Page 24 of 29
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Ac ce p
te
d
M
Fig. 8. Polarization curves for carbon steel in 1 M HCl in the absence and presence of different concentrations of Ch-g-PEG/ AgNPs at 25 oC.
25 Page 25 of 29
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Fig. 9. Nyquist plots for CS in 1 M HCl in absence and presence of different
Ac ce p
te
d
M
concentrations of Ch-g-PEG compound at 25 OC
26 Page 26 of 29
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Fig. 10. Nyquist plots for CS in 1 M HCl in absence and presence of different
Ac ce p
te
d
concentrations of Ch-g-PEG/AgNPs at 25 OC
Fig. 11. Suggested equivalent circuit model for the studied systems.
27 Page 27 of 29
ip t cr us an M Ac ce p
te
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Fig. 12.Bode and Phase angle plots for carbon steel in 1 M HCl in absence and presence of different concentrations of the synthesized Ch-g-PEG compound
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Fig. 13.Bode and Phase angle plots for carbon steel in 1 M HCl in absence and presence of different concentrations of the synthesized Ch-g-PEG/AgNPs
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S0141-8130(15)30167-7 http://dx.doi.org/doi:10.1016/j.ijbiomac.2015.11.073 BIOMAC 5578
To appear in:
International Journal of Biological Macromolecules
Received date: Revised date: Accepted date:
23-10-2015 18-11-2015 25-11-2015
Please cite this article as: H.H.H. Hefni, E.M. Azzam, E.A. Badr, M. Hussein, S.M. Tawfik, Synthesis, characterization and anticorrosion potentials of chitosan-g-PEG assembled on silver nanoparticles, International Journal of Biological Macromolecules (2015), http://dx.doi.org/10.1016/j.ijbiomac.2015.11.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, characterization and anticorrosion potentials of chitosang-PEG assembled on silver nanoparticles Hassan H.H. Hefni, Eid M. Azzam, Emad A. Badr, M. Hussein , Salah M. Tawfik*
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Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt. * Corresponding author: [email protected], Tel. 002-01273615278.
Abstract
Chitosan (Ch) grafted with poly(ethylene glycol) (Ch-g-mPEG) were synthesized
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using mPEG with molecular weights 2000 g/mol. The synthesized Ch-g-mPEG was characterized using gel permeation chromatography (GPC), fourier transform infrared
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spectroscopy (FTIR), proton nuclear magnetic resonance (1HNMR), and X-ray diffraction (XRD) techniques. Ch-g-mPEG silver nanoparticles has been synthesized
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and characterized by high-resolution transmission electron microscopy (HRTEM) and energy dispersive analysis of X-rays (EDAX). The synthesized Ch-g-mPEG and its nanostructure were examined as corrosion inhibitors for carbon steel in 1 M HCl
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solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results revealed that the inhibition efficiency obtained by Ch-g-mPEG self-assembled on silver nanoparticles is greater than that
d
obtained by Ch-g-mPEG only. Potentiodynamic polarization results reveal that the
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synthesized compound could be classified as mixed-type corrosion inhibitors with predominant control of the cathodic reaction. The results of EIS indicate that the both
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charge transfer resistance and inhibition efficiency tend to increase by increasing the inhibitor concentration. Keywords:
Chitosan; Poly(ethylene glycol); silver nanoparticles; carbon steel; corrosion inhibition; electrochemical impedance spectroscopy 1. Introduction Steel is widely used as part of our life in different applications in automotive, household appliances, machinery and heavy construction such as the marine, petroleum and chemical industries which make it very important to research corrosion and protection of iron and its alloys. The most issue of iron is how to improve its resistivity against corrosion phenomena, which is one of the major reasons for industrial accidents and consumption of material resources [1,2]. There are different types of corrosive media such as salts, acids, humidity and microorganisms [3]. It is well established that corrosion problems may be caused by microorganisms, 1 Page 1 of 29
acidification, formation of salt water and corrosive gases in numerous systems within the petroleum industry [3–5]. Hydrochloric acid solutions are widely used among these various acids for stimulating carbonate-based reservoirs like lime stone and dolomite [5]. Moreover, bacteria called sulfate-reducing bacteria (SRB) are always
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the responsible of these problems [6]. Several studies have examined the relationship between the structure of the inhibitor molecule and its efficiency [5-9] but much less attention has been paid to the dependence of the protection efficiency on the size of the
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inhibitor molecule and the electronic distribution in the inhibitor molecule. With the rapid advancement of nanotechnology, thin films of thickness in the micron and nanometeric
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scales are increasing their popularity in scientific and technological applications [10].Nanoparticles have high tendency to interact with each other to form agglomeration [11]. Their unique properties are mainly due higher surface area of the nanosized particles
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in compare to microsized caused by their large surface area to volume ratio [12]. There are various reports concerning improving corrosion resistance using nanoparticles such
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as; TiO2 [13], Cu2O [14], ZnO [15], ZrO2 nanoparticles [16], Fe3O4 [17], SiO2 [18] and organoclay nanoparticles [19]. Migahed et al. studied electrochemical behaviour of carbon steel in acid chloride solution in the presence of dodecyl cysteine hydrochloride
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self-assembled on gold nanoparticles. The relatively high value of (Kads) in case of
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dodecyl cysteine hydrochloride self-assembled on gold nanoparticles reveals a strong interaction between the inhibitor molecules and the metal surface [20]. Azzam and Abd
Ac ce p
elaal studied corrosion inhibition of the self-assembled Ag nanoparticles films on the surface of carbon steel [21]. Atta et al. studied the synthesis and application of hybrid polymer composites based on silver nanoparticles as corrosion protection for line pipe steel. Results revealed that polarization measurements indicate that the AgNPs and
hybrid polymer acts as a mixed type-inhibitor and the inhibition efficiency increases with inhibitor concentration. Generally, various stabilizers (surfactants, gemini surfactants, polymers, triblock polymers, proteins and carbohydrates) have been used in the synthesis and characterization of different shaped and sized of advanced silver nanomaterials [23, 24].
A number of naturally occurring polymers have been investigated and have been reported to show promising results as metal corrosion inhibitors in different corrosive environments. For instance the corrosion inhibiting effect of chitosan has been reported for mild steel and copper in acid medium [25-28]. Glucose, gellan gum, and hydroxylpropyl cellulose have been assessed as green inhibitors for cast iron in acidic
2 Page 2 of 29
environment by means of chemical and electrochemical techniques [29]. To our knowledge, no other research has been published until now referring to the use of chitosan grafted copolymer assembled on silver nanoparticles as corrosion inhibitor for carbon carbon steel in 1 M HCl solution. Therefore, the idea of the present work is
ip t
to use silver modified nanoparticles with chitosan grafted copolymer as corrosion inhibitors in acidic media. The nanoparticles were characterized by TEM and EDX techniques.
Moreover,
polarization
curve
and
electrochemical
impedance
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spectroscopy techniques were employed to investigate the corrosion inhibition
efficiencies of the synthesized Ch-g-PEG and its nanostructures for carbon steel in 1
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M HCl solution.
2. Experimental method
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2.1. Materials
Chitosan with molecular weight (MW: 109.050 kDa, and deacetylation degree >80%) was prepared in our laboratory [25] and Monomethylated PEG (mPEG) with
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molecular weights 2000 g/mol (mPEG2000) were supplied by Aldrich chemicals. All other materials and reagents used in this study were of analytical grade.
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The corrosion measurements were performed on carbon steel samples with the following composition (wt%): 11% C, 0.45% Mn, 0.04% P, 0.05% S, 0.25% Si, and
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the reminder is Fe. For the electrochemical studies, the specimens were covered with an epoxy resin leaving an exposed surface area of 1 cm2 in aqueous solution. The
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specimens used for the analysis were polished with a series of emery paper (320– 1200), washed with double-distilled water, degreased with acetone and dried at room temperature.
2.2. Solutions
The corrosive solution was a 1.0 M hydrochloric acid solution diluted from concentrated acid (37%, Merck) with double-distilled water. This solution was used as blank. Concentration range of used synthesized inhibitor varied from 5 x 10-5 to 1 x 10-3 M for corrosion measurements. All tests were performed in non-deareated solutions under unstirred conditions at 25 ◦C.
2.3. Synthesis of mPEG-aldehyde (mPEG-CHO) PEG-aldehyde was synthesized via oxidation of PEG with DMSO/acetic anhydrate according references [30,31]. 5 ml of acetic anhydrate was added under N2 atmosphere to 32 ml anhydrous DMSO containing 10 g mPEG (Mw =2000 g/mol)
3 Page 3 of 29
and 6% CHCl3 and the mixture was stirred for 9 h at room temperature (20 ◦C). The reaction mixture was then poured into 400 ml diethyl ether. The precipitate was filtered, dissolved in chloroform and recrystallization twice by the use of diethyl ether to give PEG-aldehyde with molecular weight of 2000 g/mol.
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2.4. Synthesis of Ch-g-PEG compound Ch-g-PEG compound was synthesized according the method of Harris et al. [30].
Chitosan (0.5 g) was dissolved in an aqueous solution of 2 % V/V acetic acid by
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vigorously stirring to obtain a solution with a concentration of 2 %, filtered through
polyester cloth to remove residues of insoluble particles and an aqueous solution of
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PEG aldehyde (mPEG-CHO) with MW = 2000 g/mol (2.9 g) was added dropwise during stirring for 30 min at room temperature [31, 32]. Then the pH of chitosan/PEG
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aldehyde solution was increased by gradually adding Na2CO3 until pH = 6. After 1 h NaBH4 (0.183 g) was added and the mixture was stirred for 5 h at 55 ◦C. The precipitate was obtained by pouring the reaction mixture into a saturated ammonium
M
sulfate solution [33]. The material from the inner solution was freeze-dried and washed with ethanol and acetone in order to remove the remaining mPEG that did not reacted. After drying in vacuum the obtained white powder was Ch-g-PEG.
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2.5. Synthesis of the Silver Nanoparticles (AgNPs)
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Silver nanoparticles colloidal solution was synthesized via chemical reduction method. In a typical experiment 100 ml of AgNO3 solution (1x10-3 M) was heated to
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boiling. To this solution 10 ml of 1 % tri-sodium citrate was added drop by drop. The solution was mixed vigorously and heated until the yellow colour obtained. Then it was removed from the heating element and stirred until cooled to room temperature [34].
2.6. Synthesis of the Ch-g-PEG nanostructure (Ch-g-PEG/AgNPs) The silver nanoparticles solution 20 ml (1x10-3 M) was mixed with 5 ml solution of
the synthesized Ch-g-PEG (1.5%) in distilled water. The mixture was stirred continuously for 24 h till the colour change. The resulting solution was used for TEM image and energy dispersive X-ray [35]. 2.7. Characterization of Chitosan-g-PEG 2.7.1. Gel permeation chromatography (GPC) The molecular weight determinations were carried out by gel permeation chromatography (GPC) using a Supremamax 3000 column (Polymer Standard Service, Mainz, Germany) with 2 % CH3COOH/0.2 M buffer (CH3COONa) as an 4 Page 4 of 29
eluent (1 ml/min). The standard pullulans (Mw of 11,800, 47,300, 112,000, and 780,000) were used for calibration. 2.7.2. Fourier transform-infrared spectroscopy (FT-IR) Fourier transform infrared (FTIR) spectra was recorded for the synthesized Ch-g-PEG
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and compared with chitosan on an ATI Mattson Infinity Series TM, Bench top 961 controlled by win first TM V2.01 software (Egyptian Petroleum Research Institute ‘‘EPRI’’) at 25 °C. About 2 mg of sample with 100 mg of KBr was fully grinded and
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‘mixed. The mixed samples were pressed into pills with a compressor and prepared
pellets were used for studies. All spectra were scanned against a blank KBr pellet
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back-ground in the range of 4000–400 cm−1with resolution of 4.0 cm−1. 2.7.3. Nuclear magnetic resonance (NMR)
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The 1H-NMR was measured in DMSO-d6 by Spect Varian, GEMINI 200 (1H 200 MHz) (Micro-analytical Center, Cairo University). The synthesized inhibitors were dissolved in DMSO-d6 solvent.
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2.7.4. X_ray diffraction
The XRD analysis was carried out on an X_ray diffract meter (D/Max2500VB2+/Pc, X_Ray.
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Rigaku Company, Tokyo, Japan) with a Cu detector using 1.54 Å wavelength of the
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2.8. Characterization of Chitosan-g-PEG assembled on silver nanoparticles 2.8.1. Transmission electron microscope (TEM)
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A convenient way to produce good TEM samples is to use copper grids. A copper grid pre-covered with a very thin amorphous carbon film. To investigate the prepared AgNPs using TEM, small droplets of the liquid were placed on the carbon-coated grid. A photographic plate of the transmission electron microscopy employed on the present work to investigate the microstructure of the prepared samples. Nanoparticle size was determined by using TEM model ‘‘Jeol JeM – 2100 (Japan)’’ (Egyptian Petroleum Research Institute ‘‘EPRI’’) 2.8.2. Energy dispersive X-ray (EDX) spectroscopy The energy-dispersive X-ray (EDX) spectroscopy was recorded with an EDX detector (Oxford LINKISIS 300) equipped on a transmission electron microscope (TEM, Hitachi S-520) operated at 10 kV accelerating voltage. 2.9. Corrosion studies Electrochemical measurements were carried out using a Voltalab 40 Potentiostat PGZ 301 and a personal computer was used with Voltamaster 4 software at 25 oC and 5 Page 5 of 29
frequency response analyzer in a three-electrode arrangement. Carbon steel was taken as the working electrode, platinum as the counter electrode (The main function of the counter electrode is to provide the location of the second electron transfer reaction. Important parameter of the counter electrode is the surface area. It is required (area)
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large enough to support the current generated for the working electrode the surface area of the used platinum electrode is 5 cm) and saturated calomel electrode as reference electrode. The working electrode was held in the electrolyte solution for 60
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min before each measurement for attaining a steady-state potential. For polarization studies, the potential was swept from cathodic direction to anodic direction with
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potential (−1000 mV to +100 mV) at a scan rate of 1 mV/s. The point of intersection of extrapolated tafel lines gives the value of corrosion current density Icorr and
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corrosion potential Ecorr. The inhibition efficiency is calculated using the corrosion current densities values in the absence and presence of various concentrations of the inhibitor. The electrochemical impedance spectroscopy (EIS) measurements were
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performed in the frequency range 100 kHz to 30 mHz, using AC signals of amplitude of 5 mV peak to peak. All the impedance values were measured at open circuit potential. Nyquist plot represents the results from EIS measurements. The charge
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Nyquist plot [36-40].
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transfer resistance (Rct) and capacitance of double layer (Cdl) were calculated from the
3. Results and Discussion
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3.1. Synthesis and characterization of Ch-g-PEG Chitosan is a linear polysaccharide composed of randomly distributed -(1–4)-linked
d-glucosamine and N-acetyl-d-glucosamine. Chitosan contains mainly hydroxyl and amino repeating in its. The hydroxyl end groups of PEG are difficult to react with chitosan at normal condition to prepare the grafted compound (Ch-g-PEG). Furthermore, in this case since PEG is a bifunctional reagent crosslinked macromolecules could be prepared. In order to avoid this mono methyl ether of PEG was used, which contains only one hydroxyl group in the macromolecular chain. For a successful grafting reaction of PEG into chitosan, PEG monomethyl ether was oxidized in a first stage to PEG-aldehyde with acetic anhydride and DMSO according to the method described by Sugimoto [30,31]. The room temperature was preferred for the oxidation reaction since there might be a chance of excessive reaction [32]. After the successful synthesis of mPEG-CHO with molecular weights 2000 g/mol,
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PEG can be reacted with chitosan in order to prepare the grafted compound. The synthetic method used for the N-PEGylation of chitosan by the use of reductive amination is demonstrated in Fig. 1. One step-reductive amination, also known as Borch reduction, is the reaction between an amino group of a primary or secondary
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amines and aldehyde group in the presence of a reducing agent [33]. During the second step of the procedure, imine compound is created and reduction reaction follows, leading to the Ch-g-mPEG graft copolymers. In both cases of the produced
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copolymers the reducing agent is added to the reaction mixture drop wise in a time period of 1 h in order to avoid precipitation of chitosan due to high alkalinity of
us
NaBH4. Grafting of chitosan-g- PEG was confirmed as the following:. 3.1.1. GPC Data
an
From the GPC data, we found that the molecular weights of chitosan and Ch-g- PEG compounds are 109.050, and 356.808 kDa respectively. The increase in molecular weight of Ch-g-PEG over that of chitosan indicated the formation of new products.
M
3.1.2. FT-IR Spectroscopy
Comparative IR spectra of chitosan and the synthesized Ch-g-PEG are shown in Fig. 2. To the spectra of grafted chitosan the characteristic peaks of chitosan and mPEG
d
can be identified. The characteristic peaks of PEG at 1110 cm−1 (C-O stretch) and
te
2900 cm−1 (C-H stretch) appear more intense than those of chitosan at the grafted compound. Chitosan characteristic bands appear at 3380 cm−1 (O-H stretch
Ac ce p
overlapped with N-H stretch) and 2860 cm−1 (C-H stretch) [41]. On the contrary other
two characteristic peaks of chitosan at 1607 cm−1 and 1400 cm−1, that correspond to Amide I and Amide II, respectively, show a quite different behavior. The absorbance of Amide II, at 1400 cm−1, seems to remain unaffected, a fact that seems to be
reasonable as this peak is attributed to the stretching vibration of the C-O group of the acetylated amino groups of chitosan, which is not affected during the modification procedure of chitosan. The absorbance peak of bending primary amine of chitosan at 1590 cm−1, is almost disappeared and shifted to 1535 cm−1 due to formation of secondary amine of Ch-g-PEG. This characteristic groups are attributed to the bending vibration of N-H group of chitosan (NH2), and this constitutes strong evidence of the extend degree of reaction between the free NH2 groups of chitosan and the aldehyde group of mPEGCHO [42]. 3.1.3. 1HNMR Spectroscopy
7 Page 7 of 29
The characterization of the grafted compound is completed through the 1HNMR spectra. The characteristic proton signals of Ch-g-PEG appeared in the range of 3.5– 4.0 ppm (Fig. 3). The peak at around 3.36 ppm is attributed to the proton signal of methoxyl of CS-g-PEG. Peaks of the glucosamine residues at 4.99 ppm for the
ip t
anomeric proton on C-1, and at 2.50 ppm for the proton on C-2. The peak for the methyl protons of the N-acetylglucosamine residues appears at 1.63 ppm. The Ch-gPEG showed new peak at 1.23 ppm (Fig. 3) assigned to the grafted of chitosan with
cr
mPEG2000 [43] 3.1.4. XRD analysis
us
Typical XRD patterns of chitosan and Ch-g-PEG2000 are shown in Fig. 4. XRD pattern of neat Chitosan (Ch) showed that is in an amorphous to partially crystalline state. This observation is in accordance with Nunthanid [44] who reported a peak at
an
approximately 12◦ (2θ) corresponding to hydrated crystals and one at 18◦ (2θ) corresponding to anhydrous crystals. On the other hand the XRD pattern of Ch-g-
M
PEG has three strong characteristic crystalline peaks at 2θ deg 10.3◦, 19.9◦ and 29.4◦ that seem to be the characteristic peaks of mPEG but broadened due to the disrupt of
d
PEG crystalline structure from amorphous chitosan [45] .The characteristic peaks of Ch-g-PEG are recorded, which however in all samples are broadened, in comparison
te
with the patterns of the neat materials. Furthermore, these peaks have lower intensity, a fact that justifies that the crystal structure of grafted compound is disrupted probably
Ac ce p
due to distribution intermolecular hydrogen bond of chitosan units by PEG as previously reported [46].
3.2. Characterization of Ch-g-PEG nanoparticles loaded with AgNPs 3.2.1. Transmission electron microscope (TEM) The size and morphology of the synthesized silver nanoparticles and the Ch-g-PEG assembled on silver nanoparticles were investigated by TEM as shown in Fig.5. It was seen from TEM images that the silver nanoparticles were predominantly spherical in shape and polydispersed. The size of pure AgNPs is 7-15 nm according to the TEM image, and that was consistent with the published data for the synthesis of silver nanoparticles by the chemical reduction method [47]. Fig.5. shows the spherical silver nanoparticles with a corona of the Ch-g-PEG ligands, which related to the selfassembling of the Ch-g-PEG materials on the silver nanoparticles. The TEM images showed good stabilization of silver nanoparticles due to interaction with the Ch-g8 Page 8 of 29
PEG materials. The effect of hydrophilic PEG2000 chain length of the Ch-g-PEG on the stabilization of AgNPs was observed where the surface aggregates capped on silver surface should provide more effective stabilization with presence the PEG chain .
ip t
3.2.2. Energy dispersive X-ray (EDX) spectroscopy EDX spectroscopy results confirmed the significant presence of pure silver with no other contaminants. The optical absorption peak at 3 keV, is typical for the absorption
cr
of metallic silver nanoparticles due to surface plasmon resonance [48]. Fig.6. show
the EDX analysis of the AgNPs stabilized by the synthesized Ch-g-PEG. The EDX
us
spectrum showed a strong and typical optical absorption peak at approximately 3 keV, which was attributed to the SPR of the metallic Ag nanocrystals [48]. This result
an
indicated that AgNPs were formed in the reaction medium. Beside of Ag there are others bands for other elements peaks which appeared due to the scattering caused by the compounds that are bound to the surface of silver which indicating that the used
M
Ch-g-PEG act as stabilized agents for silver nanoparticles. 3.3. Corrosion studies
d
3.3.1. Potentiodynamic polarization measurements
te
Nanometer-size noble metals such as Ag with large specific surface areas exhibit notable properties which are different from their corresponding bulk materials. Silver (Ag) is a
Ac ce p
noble metal with an inert chemical reactivity in its bulk form and is listed below hydrogen in the activity series of metals. It is well known that bulk Ag cannot react with hydrochloric acid (HCl). If the reaction between Ag nanoparticles and HCl can occur, the product will be silver chloride (AgCl), which is an insoluble precipitate and can be easily collected for characterization. It was previously reported that, high chemical reactivity of Ag nanoparticles was observed in the reaction with hydrochloric acid. The
potentiodynamic polarization curves for carbon steel in 1.0 M HCl without and with different concentrations of Ch-g-PEG and its nanostructure with silver nanoparticles (AgNPs) are shown in Figs. (7,8). As could be seen in the figure, both the cathodic and anodic (more pronounced on the cathodic) current densities decrease considerably on the introduction of grafted chitosan derivatives into the aggressive medium (1.0 M HCl). Also the corrosion potential (Ecorr) in the presence of Ch-g-PEG and Ch-gPEG/AgNPs derivatives is slightly displaced towards the negative direction compared to the blank solution. The cathodic polarization curves is also observed to give rise to 9 Page 9 of 29
parallel Tafel lines indicating that there is no modification of the hydrogen evolution reaction process on the introduction of Ch-g-PEG derivatives into the corrosive medium. This also suggests that Ch-g-PEG and Ch-g-PEG/AgNPs inhibits carbon steel corrosion by simply blocking the reaction sites without affecting the actual
ip t
reaction mechanism [49]. Values of electrochemical parameters derived from the polarization measurements namely corrosion current density (Icorr), corrosion potential (Ecorr), anodic (βa) and cathodic (βc) Tafel slopes, surface coverage and inhibition
the
following
equation
[50]:
us
from
cr
efficiency (η %) are listed in Table 1. Inhibition efficiency, (ηp, %), was obtained
where icorr and i0corr are uninhibited and inhibited corrosion current densities,
an
respectively, determined by extrapolation of Tafel lines to the corrosion potential. Data in the Table 1 show that Icorr decreased in the presence of Ch-g-PEG and Ch-g-
M
PEG/AgNPs compared to the blank solution and further decreases as the concentration of Ch-g-PEG and Ch-g-PEG/AgNPs increased. This is an indication that Ch-g-PEG and Ch-g-PEG/AgNPs inhibited the acid-induced corrosion of carbon
d
steel. Inspection of the Table 1 also reveals noticeable changes in both the anodic and
te
cathodic Tafel slopes. It is clear that the presence of the inhibitors causes a markedly decrease in the corrosion rate. The increase in inhibition efficiency is associated with
Ac ce p
a shift of both cathodic and anodic branches of the polarization curves towards lower current densities, together with a negative shift in Ecorr, suggest that the Ch-g-PEG and
Ch-g-PEG/AgNPs inhibitors act as mixed type inhibitors with predominantly cathodic; i.e. meaning inhibitors reduce the anodic dissolution of mild steel and retards the cathodic hydrogen evolution reaction, but the effect on the cathodic hydrogen evolution reactions surface is more than the anodic dissolution reactions. Also the corrosion rate is observed to decrease in the presence of Ch-g-PEG and Chg-PEG/AgNPs in comparison to its absence indicating decreased metal dissolution and further decreases with increasing concentration. As also shown in Table 1, inhibition efficiency increased with increase in concentration of Ch-g-PEG and Ch-gPEG/AgNPs and at the same inhibitor concentration the inhibition efficiency of Ch-gPEG assembled on silver nanoparticles greater than that of Ch-g-PEG. 3.3.2. Electrochemical impedance spectroscopy (EIS)
10 Page 10 of 29
The impedance responses of low carbon steel in 1 M HCl in the absence and presence of Ch-g-PEG and its nanostructure with AgNPs is depicted in Figs. (9,10). It could be observed from the Nyquist plots that the impedance responses of carbon steel in the acid medium changed on addition of the inhibitor. The Nyquist plot is characterized
ip t
by one semicircle capacitive loop corresponding to one time constant in the Bode plot suggesting that the corrosion of carbon steel is controlled by a charge transfer process.
The diameter of the semicircles in the Nyquist plot and the magnitude of Bode
cr
modulus are observed to increase with increasing concentration of Ch-g-PEG and Chg-PEG/AgNPs indicating the formation of an adsorption film on the steel surface. In
us
all cases, the Nyquist plot in Figs. (9,10) are not perfect semicircles but depressed with center under the real axis. These kinds of deviations are often referred to as the
an
frequency dispersion of interfacial impedance [51]. The anomaly is usually attributed to the inhomogeneity of the electrode surface arising from surface roughness or interfacial phenomena [51]. The equivalent circuit (EC) model shown in Fig. 11 was
M
used to model the physical processes taking place at the steel /solution interface. The EC consists of solution resistance (Rs), charge transfer resistance (Rct) and constant phase element (CPE). The CPE is substituted for the capacitive element to give a
ZCPE = Q-1(jω )-n
te
equation:
d
more accurate fit as specified in the CPE
impedance shown in the following
(2)
Ac ce p
where Q and n stand for the CPE constant and exponent respectively, j = (–1) is an imaginary number and ω is the sine wave modulation angular frequency in rad s–1
(ω= 2πf; where f is the frequency in Hz). The corresponding impedance parameters
obtained are listed in Table 2. Data in the table show that Ch-g-PEG and Ch-gPEG/AgNPs compounds caused an increase in the Rct value and a corresponding decrease in Cdl. Such an increase in the Rct value, synonymous with an increase in the
diameter of the semicircle in the Nyquist plot as well as the increase in the magnitude of the absolute impedance in the Bode plot point towards improved corrosion resistance due to the corrosion inhibiting action of Ch-g-PEG and Ch-g-PEG/AgNPs derivatives. The Cdl was computed using the expression [52]:
where fmax the frequency at which the imagiary component of the impedance is a maximum. 11 Page 11 of 29
In case of the electrochemical impedance spectroscopy, the inhibition efficiency was calculated using charge transfer resistance according to the following equation [52]:
ip t
where Rct and R°ct are the charge transfer resistance values without and with inhibitor for carbon steel in 1 M HCl, respectively.
The observed decrease in the values of Cdl, which normally arises from a decrease in
cr
the dielectric constant and/or an increase in the double-layer thickness, is as a result of adsorption of alginate derivatives onto the metal/electrolyte interface [53].
us
For more complicated system, Bode plots and phase angle can give more information. The Bode plots of the synthesized inhibitor Ch-g-PEG and Ch-g-
an
PEG/AgNPs are presented in Figs. (12,13). The low frequency impedance modulus Zmod is one of the parameters which can be easily used to compare corrosion resistance of different samples. A larger Zmod demonstrates a better protection
M
performance [54].
In Figs. (12, 13), it is shown that Zmod increases as a function of the concentration of the synthesized inhibitor Ch-g-PEG and Ch-g-PEG/AgNPs. These phenomena can be
d
explained as follows, the high frequency phase angle range (105–104 Hz) of the
te
impedance spectra corresponds to the properties of an outer layer, the middle frequency range (104–102 Hz) reflects the properties of an inner barrier layer, while
Ac ce p
the low frequency range (less than 102 Hz) corresponds to the properties of the
double-electrical layer information. Therefore, the high frequency phenomenon may due to the thickness increase of the outer porous layer, and the middle frequency phenomenon can be attributed to the penetration of active chloride ions and water through the defect of the prepared inhibitors inner barrier layer, though the whole effect induced the increase of Zmod. As seen from Figs. (12, 13), Bode plots refer to the existence of an equivalent
circuit that contains a single constant phase element in the metal/solution interface. The increase of absolute impedance at low frequencies in Bode plot confirmed the higher protection with increasing the concentration of the prepared inhibitors, which is related to adsorption of the inhibitors on the carbon steel surface [55]. The phase angle plots for the carbon steel in the presence and absence of synthesized inhibitors Ch-g-PEG and Ch-g-PEG/AgNPs concentrations in 1.0 M HCl 12 Page 12 of 29
solution are given in Figs. (12, 13). As seen the increasing of the inhibitors concentration in the test solution indicated superior inhibitive behavior due to adsorption of the metal surface of more prepared inhibitors molecules at higher concentrations. Furthermore, the depression of phase angle at relaxation frequency
ip t
occurs with decreasing of inhibitors concentration which indicated the decrease of capacitive response with the decrease of inhibitors concentration. Such a phenomenon could be attributed to higher corrosion activity at low concentrations of inhibitors.
cr
3.4. Mechanism of inhibition
Results obtained from all the experimental techniques reveal that addition of Ch-g-
us
PEG and Ch-g-PEG/AgNPs derivatives to the acid corrosive medium retarded the corrosion of carbon steel. The corrosion inhibition process no doubt occurs by virtue
an
of adsorption mechanism where the oxygen and nitrogen heteroatoms present in Chg-PEG and Ch-g-PEG/AgNPs structure serves as adsorption centers to contain excessive negative charges. In acidic medium, the anodic dissolution of iron is 2+
M
accompanied by the cathodic hydrogen evolution reaction as follows: ---
d
Fe • Fe2+ + 2e--(5) + --2H + 2e • H2• (6 ( 6) Potentiodynamic polarization results indicate that Ch-g-PEG and Ch-g-PEG/AgNPs follow mixed inhibition mechanism. The chemical structure of Ch-g-PEG derivatives
te
repeat unit (Fig.1) reveals the presence of ammonium–NH, –HO(CH2CH2O)mCH3 and –OH functional groups. It could be assumed that the cationic form of Ch-g-PEG and
Ac ce p
Ch-g-PEG/AgNPs may be adsorb on the cathodic sites of carbon steel and inhibits the cathodic hydrogen evolution reaction. On the other hand, –OH groups have lone pairs of electrons and can be adsorbed on the anodic sites of the metal surface via interaction with the vacant d-orbital of iron and inhibits the anodic metal dissolution reaction. These processes effectively help in isolating the metal surface from the aggressive ions present in the acid medium and thereby inhibiting corrosion. It is also known that in the acid corrosive medium, the inhibitor (Ch-g-PEG and Ch-gPEG/AgNPs) may be protonated and exists as polycation. However, the specific adsorption of chlorides ions (Cl–) and replacing hydroxyl groups on the metal surface would render the metal surface negatively charged [56-59]. Hence Ch-g-PEG and Chg-PEG/AgNPs in the form of polycation will be adsorbed on negatively charged metal surface and inhibit corrosion.
13 Page 13 of 29
When iron is immersed in the Ch-g-PEG and Ch-g-PEG/AgNPs solution, the protective film is formed through the hydroxyl group of the chitosan adsorbed on the iron surface. But the film is not compact because there are some defects in it. When iron specimen is immersed in the silver nanoparticles solution, the adsorption maybe
ip t
happened with two patterns. One is made by monomethylated PEG, the other is that silver nanoparticles maybe adsorbed on the iron surface directly, and fill up some defects made by monomethylated PEG molecules. Interactions may happen between
cr
the silver nanoparticles and the iron atoms because of the active properties of the nanoparticles [60, 61].
us
4. Conclusions
PEG grafted into chitosan backbone was easefully prepared using Borch reduction
an
process, as was characterized by GPC, FTIR, 1HNMR and XRD spectroscopy. These materials are able to prepare nanoparticles via self-assembled on silver nanoparticles using chemical reduction method. The nanostructures of chitosan grafted PEG with
M
AgNPs were confirmed using TEM and EDX techniques. TEM image shown that the size of the synthesized nanoparticles is 7-14 nm. Electrochemical techniques of
d
monitoring corrosion approach were employed to evaluate the potential of Ch-g-PEG and Ch-g-PEG/AgNPs derivatives as an inhibitor for carbon steel corrosion in acidic
te
medium. Results obtained show that Ch-g-PEG and Ch-g-PEG/AgNPs act as good inhibitor for acid-induced corrosion of carbon steel. Corrosion inhibition effect was
Ac ce p
found to be inhibitor concentration and structure; inhibition efficiency increased with increasing
Ch-g-PEG
and
Ch-g-PEG/AgNPs
concentration.
Potentiodynamic
polarization studies reveal that Ch-g-PEG and Ch-g-PEG/AgNPs functions as a mixed-type inhibitor but under cathodic control. The inhibition efficiency of Ch-gPEG as corrosion inhibitor enhanced via assembled of Ch-g-PEG on silver nanoparticles to form Ch-g-PEG/AgNPs nanostructure materials.
5. References
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ip t
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ip t
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cr
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ip t
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te
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17 Page 17 of 29
mPEG
Ac2O
O=CH-CH2-CH2-O-(CH2CH2O)m-CH3
DMSO, CHCl3
mPEGCH=O
mPEGCH=O
HO O 1
n
H
NHR2
H
H
O H n
mPEG-g-Chitosan
R2= H, COCH3, -CH2-CH2-O-(CH2CH2O)m-CH3
Ac ce p
te
d
Chitosan R1= H or COCH3
H
an
NHR
H
HO
NaBH3CN
H
M
H
HO
us
H O
H O HO
cr
H OH
H OH
ip t
HO-CH2-CH2-O-(CH2CH2O)m-CH3
Fig. 1. Synthetic route via the two-step reaction for the synthesis Ch-g-PEG.
18 Page 18 of 29
ip t cr us
Ac ce p
te
d
M
an
1535
Fig. 2. FT-IR spectra of chitosan and Ch-g-PEG compounds
19 Page 19 of 29
ip t cr us an M d te
Ac ce p
Fig. 3. 1H NMR spectra of Ch-g-PEG compounds
20 Page 20 of 29
ip t cr us an
Ac ce p
te
d
M
Fig. 4. X-ray powder diffraction patterns of chitosan and Ch-g-PEG.
21 Page 21 of 29
ip t cr us an M d te Ac ce p
Figure 5. TEM image of the synthesized silver nanoparticle assembled on Ch-gPEG compound
22 Page 22 of 29
ip t cr us an M d
Ac ce p
te
Figure 6. EDX of silver nanoparticles assembled on Ch-g-PEG compound
23 Page 23 of 29
ip t cr us an
Fig. 7. Polarization curves for carbon steel in 1 M HCl in the absence and
Ac ce p
te
d
M
presence of different concentrations of Ch-g-PEG at 25 oC.
24 Page 24 of 29
ip t cr us an
Ac ce p
te
d
M
Fig. 8. Polarization curves for carbon steel in 1 M HCl in the absence and presence of different concentrations of Ch-g-PEG/ AgNPs at 25 oC.
25 Page 25 of 29
ip t cr us an
Fig. 9. Nyquist plots for CS in 1 M HCl in absence and presence of different
Ac ce p
te
d
M
concentrations of Ch-g-PEG compound at 25 OC
26 Page 26 of 29
ip t cr us an M
Fig. 10. Nyquist plots for CS in 1 M HCl in absence and presence of different
Ac ce p
te
d
concentrations of Ch-g-PEG/AgNPs at 25 OC
Fig. 11. Suggested equivalent circuit model for the studied systems.
27 Page 27 of 29
ip t cr us an M Ac ce p
te
d
Fig. 12.Bode and Phase angle plots for carbon steel in 1 M HCl in absence and presence of different concentrations of the synthesized Ch-g-PEG compound
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ip t cr us an M Ac ce p
te
d
Fig. 13.Bode and Phase angle plots for carbon steel in 1 M HCl in absence and presence of different concentrations of the synthesized Ch-g-PEG/AgNPs
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