Synthesis of nonionic amphiphilic chitosan nanoparticles for active corrosion protection of steel

Journal of Molecular Liquids 211 (2015) 315–323

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Synthesis of nonionic amphiphilic chitosan nanoparticles for active corrosion protection of steel Ayman M. Atta a,b,⁎, Gamal A. El-Mahdy a,c, Hamad A. Al-Lohedan a, Abdel-Rahman O. Ezzat a a b c

Surfactants research chair, Chemistry Department, College of Science, P.O. Box-2455, King Saud University, Riyadh 11451, Saudi Arabia Petroleum Application Department, Egyptian Petroleum Research Institute, Cairo 11727, Egypt Chemistry Department, Helwan University, Helwan, Cairo 11795, Egypt

a r t i c l e

i n f o

Article history: Received 2 May 2015 Received in revised form 12 July 2015 Accepted 13 July 2015 Available online xxxx Keywords: Chitosan Amphiphilic nanoparticles Acid corrosion Polarization Steel EIS Inhibition

a b s t r a c t Here we report new successful work for synthesis of amphiphilic chitosan (CS) nanogels in water using surfactant free method. In this respect, CS was amidated with unsaturated fatty acids such as oleic and linolenic acids to prepare hydrophobic CS. The CS fatty amides were grafted with polyoxyethylene aldehyde monomethyl ether prepared via Schiff base condensation to prepare CSLA-MPEG and CSOA-MPEG amphiphilic CS surfactants. The produced surfactants were converted to nanoparticles using methylene chloride as emulsifying solvent followed by crosslinking with sodium tripolyphosphate. The chemical structure of the prepared CS amphiphilic surfactants were characterized by FTIR and 1HNMR analyses. The particle size distribution of the prepared nanoparticles was characterized by DLS measurements. The surface activities and contact angle measurements of the amphiphilic CS nanoparticles solutions were measured using drop shape analyzer (DSA). The corrosion inhibition measurements of CSLA-MPEG and CSOA-MPEG nanoparticles on steel in 1 M hydrochloric acid solution were investigated using electrochemical polarization and impedance spectroscopy (EIS) methods. © 2015 Published by Elsevier B.V.

1. Introduction Chitosan ( CS) is one of the most promising biodegradable polysaccharide that produced from deacetylated product of chitin that found naturally in the shells of shrimp and crab, cell walls of fungi and cuticles of insects [1]. CS and its chemically modified derivatives have wide advanced applications in the field of medicine, cosmetics, and food and textile industries [2–4]. CS has different functional groups such as the hydroxyl, amide and amine groups which can be modified through grafting or crosslinking techniques [5]. There are a few reports for converting of CS to amphiphilic surfactants [6–8]. CS is an attractive material to apply as green corrosion inhibitor because of its properties such as wound healing, immunological activity, low toxicity, biocompatibility, and biodegradability [7–9]. CS-based coatings were suggested as “green” alternatives for active corrosion protection of metals [10]. It is expected that the CS amphiphilic surfactants can be assembled and good adhered as thin films to reduce water and aggressive species permeability towards metal substrates to increase the coating stability as well. The scientists have devoted themselves in searching new modification methods and new applications of CS as such as attractive applied ⁎ Corresponding author at: Surfactants research chair, Chemistry Department, College of Science, P.O. Box-2455, King Saud University, Riyadh 11451, Saudi Arabia. E-mail address: [email protected] (A.M. Atta).

http://dx.doi.org/10.1016/j.molliq.2015.07.035 0167-7322/© 2015 Published by Elsevier B.V.

materials. The main goal of modifications is to alter the functional groups of CS to match specific applications [11]. The modification techniques to amplify applications of CS were often based on modification of the two hydroxyl groups at 6-site and amino group at 2-site [12]. There are few researches reported to synthesize cationic and ionic water soluble CS derivatives [13,14]. The literature survey indicated that there is no any nonionic amphiphilic CS surfactants were prepared. Based on the analysis above, we attempted to find a simple and convenient approach to synthesize new amphiphilic CS derivatives containing hydrophilic and hydrophobic moieties to cater for specific applications such as anticorrosion for steel in aqueous acidic medium. In our previous works [15–18], it was found that the nanomaterials showed good performances as corrosion inhibitors for steel in aggressive corrosive medium due to their ability to form thin film coats and their superior surface activities. Moreover, it was found that the formation of crosslinked nanogels produced uniform chemical resistance thin films to cover metal surfaces without defects [15–17]. CS nanomaterials as green biomaterials attracted great attentions in the environmental applications [19,20]. The CS nanomaterials have been used as protective materials for metal substrates [21–23]. In this work, we chose fatty acids such as oleic and linoleic acids as hydrophobic moieties to graft CS amino group and polyoxyethylene terminated with aldehyde group as hydrophilic moiety to get active corrosion inhibitors. The remained amine groups are chemically crosslinked with sodium tripolyphosphate to produce amphiphilic nanogels to apply as

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corrosive protective nanomaterials. The electrochemical anticorrosion behavior of the prepared derivatives for steel in 1 M HCl was investigated through different electrochemical techniques.

2. Experimental 2.1. Materials Chitosan (CS) of 100 mesh, degree of deacetylation 90% and molecular weight 28 kDa, was produced from crab shell, and 1-ethyl-3-(3dimethylaminopropyl) carbodiimide (EDC), oleic acid (OA) and linoleic acid (LA) were obtained from Aldrich Chemicals Co. All solvents were analytical grade and purchased from Aldrich Chemicals Co. and used without further purification. Polyethylene glycol monomethylether (MPEG 550, molecular weight 550 obtained from Aldrich Chemical Co.) was oxidized to produce MPEG-aldehyde according to the previous work [24]. Test specimens used in this work were cut from steel rode with the following composition (0.14% C, 0.57% Mn, 0.21% P, 0.15% S, 0.37% Si, 0.06% V, 0.03% Ni, 0.03% Cr and Fe balance).

2.2. Preparation of amphiphilic CS surfactant Amphiphilic nonionic CS surfactant was prepared by amidation of NH2 group of CS with unsaturated fatty acids followed by Schiff base condensation with MPEG-aldehyde. In this respect OA or LA was amidated to CS using EDC as a catalyst. CS (1 g) was dissolved in 1% aqueous acetic acid solution (100 mL). Methanol (85 mL) was added dropwise to the reaction with stirring. The OA or LA dissolved in methanol was added to the CS solution at 0.34 mol/mol glucosamine residue. EDC (15 mL) dissolved in methanol (0.07 g/L) was added dropwise to the reaction mixture with stirring at room temperature. The mole ratio of LA or OA to EDC was 1. The reaction mixture was stirred for 24 h at room temperature and poured into 200 mL of methanol/ammonia solution (70/30, v/v) with stirring. The precipitate was filtered, washed with distilled methanol, ether, and dried under vacuum for at 20 °C. The amidated CS with OA and LA were designated as COA and CLA, respectively. COA or CLA was condensed with MPEG-aldehyde to produce CS Schiff base surfactant. In this respect, COA or CLA was dissolved in 100 mL water, MPEG550-CHO aldehyde was added to the solution, and the solution was then adjusted to pH 7 with saturated sodium carbonate. The reaction mixture was stirred at about 5 °C for 24 h. The solution containing CSLA-MPEG and CSOA-MPEG was precipitated with child acetone and dried in vacuum oven at 25 °C for 24 h. The condensed CSOA and CSLA derivatives with MPEG550CHO aldehyde were designated as CSOA-MPEG and CSLA-MPEG, respectively.

2.3. Preparation of as CSOA-MPEG and CSLA-MPEG nanoparticles CSLA-MPEG or CSOA-MPEG (0.05 g) was dissolved in 10 mL of 0.1 M acetic acid solution. Methylene chloride (served as an oil phase with concentration range of 5%, v/v) was added to the CSLA-MPEG or CSOA-MPEG solution using a DI-25 basic Yellow-line, IKA (Germany) homogenizer (rotor–stator) with an 18 mm head operating at 13,000 rpm for five minutes. The methylene chloride was separated at 20 °C using rotary evaporator under vacuum for 30 min. Sodium tripolyphosphate 2 mL (8% w/v solution) was added under constant stirring at ambient temperature. The mixture was allowed to mix for further 30 min. Thereafter, the mixture was centrifuged for 15 min at 18 °C at a speed of 14,000 rpm. The supernatant was discarded and the microparticles washed severally with water, are dried in a vacuum oven at 20 °C for 24 h.

2.4. Characterization 1 HNMR (400 MHz Bruker Avance DRX-400 spectrometer; Bruker Analytische Messtechnik, Karlsruhe, Germany) was used to investigate the chemical structures of the prepared CS derivatives using CDCl3 as solvent and tetramethylsilane (TMS) as an internal reference. FTIR spectra were analyzed with a Nicolet FTIR spectrophotometer using KBr in a wavenumber range of 4000–500 cm−1 with a resolution accuracy of 4 cm−1. All samples were ground and mixed with KBr and then pressed to form pellets. The surface tension measurements were measured at 25 °C by means of the pendent drop technique using drop shape analyzer model DSA-100 (Kruss, Germany). Particle size distribution of nanoparticle suspensions were measured at 25 °C using (Particle Sizing Systems PSS.NICOMP 380 ZLS, USA). Particle suspensions were diluted with deionized water or 0.05 M NaCl prior to measurement.

2.5. Electrochemical studies Electrochemical tests were performed in a corrosion cell with a three-electrode system using Solartron 1470E (multichannel system) as electrochemical interface and the Solartron 1455A as frequency response analyzer. The reference electrode was a saturated calomel electrode (SCE). A platinum electrode was used as auxiliary electrode. Steel specimen was used as a working electrode. Potentiodynamic polarization curves were obtained at a scan rate of 1 mV/s. Polarization curves were collected and analyzed using CorrView, CorrWare software. Electrochemical impedance spectroscopy (EIS) tests were conducted with an ac amplitude of 10 mV in the frequency range from 10 kHz to 10 mHz. The Nyquist representations of the impedance data were analyzed with Z-View software and fitted to an appropriate equivalent circuit model. 3. Results and discussion 3.1. Synthesis and characterization of amphiphilic CS nanoparticles Amphiphilic surfactants based on CS as natural product are the main target of the present work due to their high surface activity, high antimicrobial activity and non-toxicity that were not reported elsewhere. It is expected that the incorporation of hydrophobic moieties of long chain fatty acids such as oleic and linoleic acids substituents and hydrophilic moieties based on polyoxyethylene will enhance the surface activity of CS [12,25,26]. Furthermore, the presence of a considerable percentage of free hydroxyl and amine groups on this natural polymer endows its anticorrosion properties [26]. The presence of both amine and hydroxyl groups attracted great attention in searching new modification methods and new applications. The modification techniques for CS were often handled at the two active sites: amino group at 2-site and hydroxyl group at 6-site. In this respect, EDC can be used to form CS amide using OA and LA as illustrated in Scheme 1. It was previously reported that EDC as catalyst can be used to form amide groups between CS and organic acids without forming spacer groups [27]. The yields of CSOA and CSLA were 78 and 89% (w/w), respectively. The high yield of CSLA can be attributed to high reactivity and compatibility between LA and CS which increased by increment the unsaturation degree [28]. 1 HNMR analysis can be used here to determine the degree of CS amidation with OA and LA. In this respect, the 1 HNMR spectra of CS, CSOA and CSLA were represented in Fig. 1a–c. The amidation content of the CS was determined using 1H NMR to calculate the integration ratio of CH_CH groups in the oleyl substituent (δ5.3, 2H of OA and 4H of LA) to the CS methylene group (CH2–OH, δ 3.2 ppm, 4H). The amidation percentages of CS with OA and LA were determined as 45.3 and 55.6%. This data indicated that the increment degree of unsaturation of LA increases the compatibility between CS and LA

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Scheme 1. Synthesis of CSLA-MPEG and COLA-MPEG surfactants.

due to increment of acid solubility in the reaction medium acidic water/isopropanol solvent. The amidation of LA and OA with CS amine group was confirmed by appearance of new peaks at 5.5 and 1.5 and 0.98 ppm which attributed to CH_CH, (CH2)n and CH3, respectively. More details 1H NMR of CS and hydrophobically modified CS and carboxymethyl CS with acid chloride was previously reported [29–31]. The second step to prepare amphiphilic CS after modification with LA or OA is grafting of CSOA or CSLA with MPEG550-CHO. The reaction was completed through Schiff base condensation of CS remained amine with aldehyde group of MPEG550-CHO as illustrated in Scheme 1. The molar ratio of MPEG550-CHO/amino group content was 2:1. The chemical structures and the grafting degrees of substitution of CSOA-MPEG or CSLA-MPEG can be determined from 1HNMR analysis. The 1HNMR spectrum of CSOA-MPEG was selected and represented in Fig. 2. The grafting percentages can be determined by comparing the integration intensity of CH_N (1H) of MPEG-550 and CH_CH group of OA and LA that appeared at 5.8 and 4.5 ppm, respectively. The grafting degree of CSOA-MPEG and CSLA-MPEG were 20.8 and 25.4%, respectively. This data indicated that approximately 25 and 10% of amine groups of CSOA and CSLA were not amidated or grafted with OA or MPEG550-CHO. This behavior was referred to CSOA and CSLA as macromolecules cannot easily diffuse in the water solution. Accordingly, Schiff base reaction was not fully completed. It was found that increasing the MPEG550-CHO mole content more than CSOA and CSLA increases the reaction probability to condense aldehyde with the remained CSOA and CSLA amine groups. The 1H NMR spectrum of grafted polymer CSOA-MPEG, Fig. 1d, can be used to confirm the formation of Schiff base between CS NH 2 and CHO of MPEG550-CHO. Typical peaks appeared in Fig. 1 of CSOA and CSLA of the ring methane and methylene protons of CS saccharide units and methylene groups of MPEG are same to that represented in the previous work [32,33]. The appearance of peaks at 2.9 ppm (2H) attributed to C–NH2 indicated that there are some amine groups which were not amidated or grafted with OA, LA or MPEG550-CHO. Moreover, the linkage between CSLA and CSOA and

MPEG-CHO was confirmed by the appearance of a peak of CH2 connected to azomethine \\N_CCH2O\\ at 2.6 ppm. The grafting of MPEG550-CHO with CSLA is confirmed by using The FT-IR spectroscopy as represented in Fig. 3. The band at 3433 cm−1 is due to the OH stretching vibration. The pronounced bands at 3100 cm−1, 2931 cm−1 are referred to many CH olefin and CH aliphatic stretching vibration of OA, MPEG and CS. The characteristic absorption bands at 2750 cm−1 (represented CH aldehyde) of MPEGCHO were disappeared. The appearance of new band at 1638 cm− 1 (C_N stretching vibration) indicated that aldehyde has been successfully involved in the reaction. The increase of the amide II band at 1680 cm−1 in the IR spectra of the CSLA-MPEG and CSOA-MPEG grafts confirms the formation of an amide linkage between amino groups of CS and carboxyl groups of LA and OA. The third step to prepare amphiphilic CSLA-MPEG and CSOA-MPEG nanogels is based on coiling of CSLA-MPEG and CSOA-MPEG followed by crosslinking of the remained amine groups using solvent evaporation method without surfactants. In this respect, methylene chloride was selected as a solvent due to its high rapid rate ability to diffuse into the aqueous phase as non-solvent facilitating particle formation upon evaporation [34]. In this respect, the aqueous solutions of (1% CH3COOH) CSLA-MPEG and CSOA-MPEG grafts were held under vacuum for 30 min at 20 °C to remove methylene chloride followed by addition of 1 mL of 0.25% sodium tripolyphosphate (STPP) solution as a crosslinking reagent [35]. The volume percentage of methylene chloride is the controlling factor to achieve uniform CS nanogels because it acts as non-solvent for CSLA-MPEG and CSOA-MPEG. The volume percentage of methylene chloride to CSOA-MPEG and CSLA-MPEG was changed from 4 to 8% to form slightly turbid solution. The high volume percentages of methylene chloride for CSLA-MPEG can be attributed to its high solubility in aqueous solution than CSOA-MPEG due to high content of MPEG-550 and degree of unsaturation of LA. The methylene chloride as solvent accelerates the micelle formation of CSLA-MPEG and CSOAMPEG in which the linoleyl and oleyl groups directed to the interior due to hydrophobic interaction between these groups while the MPEG

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Fig. 1. 1HNMR spectra of a) CS, b) CSOA and c) CSLA.

Fig. 2. 1HNMR spectrum of CSLA-MPEG.

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Fig. 3. FTIR spectra of a) CSOA-MPEG surfactant and b) CSOA-MPEG nanoparticles.

directed to exterior of micelles due to hydrogen bond interaction with water. After the removal of the methylene chloride by vaporization under vacuum, the ordered micelles were formed with the linoleyl and oleyl groups. It is expected that the remained amine of CSLAMPEG and CSOA-MPEG micelle will quaternize with CH3COOH to form acid salt which crosslinked by forming a complex with STPP. The remained amine groups 25 and 10% of amine groups of CSOA-MPEG and CSLA-MPEG, were not amidated or grafted with MPEG550-CHO, are used to form crosslinked CSOA-MPEG and CSLA-MPEG nanogels, respectively. Accordingly, the degree of crosslinking for CSOA-MPEG and CSLA-MPEG nanogels is 25 and 10%, respectively. This fact is proved from the chemical structure of the crosslinked CSLA-MPEG and CSOAMPEG as confirmed by FTIR analysis. In this respect, spectrum of CSLAMPEG nanoparticles was selected as representative sample and illustrated in Fig. 3b. It was observed that the characteristic spectrum of the crosslinked polymer is almost the same as the grafted polymer. The slightly shift of absorption bands to lower wave-numbers indicated the higher interactions between functional groups which referred to complete crosslinking of the CSLA-MPEG and CSOA-MPEG remained amine groups. The size and particle size distribution of (1 g/L of 0.1 M acetic acid solution) CSOA-MPEG and CSLA-MPEG nanoparticle micelles were determined by DLS measurements as represented in Fig. 4a, and b. The data indicated that CSLA-MPEG (Fig. 4a) has two peaks in the histogram, and the majority number of the particles was around 125 nm (94%) and small numbers of large particles 385 nm (6%) were formed due to aggregation between the small particles. Fig. 4b shows different size and particle size distribution for CSOA-MPEG crosslinked particles. It confirmed that the small particles with size 133 nm have the percentage of 36% and large particles aggregated with sizes of 425 nm have 64% content. This behavior indicated that CSOA-MPEG nanoparticles can form aggregates more than CSLA-MPEG. This can be referred to steric hindrance repulsion force between LA due to high double bond contents more than OA which inhibited the formation of aggregates between CSLA-MPEG nanoparticles. Moreover, the presence of double bond enhances stabilization of nanoparticles by preventing the aggregation process among the hydrocarbon chains as a result of repulsive forces between π electrons of double bonds. The CSOA-MPEG nanoparticles suffer from some aggregations in aqueous media because it is difficult to make complete dispersion after complete precipitation. Also, all nanoparticles have wide size distribution. 3.2. Surface properties of amphiphilic CS nanoparticles The chemical structure of CSOA-MPEG and CSLA-MPEG nanoparticle determined by 1HNMR and FTIR indicated the presence of both hydrophobic and hydrophilic moieties which alter their solubility and dispersibility in aqueous medium. In this respect, the surface activities of the CSOA-MPEG and CSLA-MPEG surfactants and crosslinked

Fig. 4. DLS data of a) CSLA-MPEG and b) CSOA-MPEG nanoparticles at 25 °C.

nanoparticles can be determined by measuring their surface tensions in aqueous medium at different concentrations (c; mol/L). The relations between surface tension (γ, mN/m) and ln (c; mol/L) for CSOA-MPEG and CSLA-MPEG at 25 °C were not presented here for brevity. The data indicated that the microgels reached the equilibrium after different time intervals when the concentration was lowered than (0.01 wt.%). This means that the adsorption of nanoparticles at liquid/air interfaces occurred when the surface concentration is sufficiently high due to an evolving contact network. The critical micelle concentrations (cmc, mol/L) or the critical aggregation concentrations of CSOA-MPEG and CSLA-MPEG surfactants and crosslinked CSOA-MPEG and CSLA-MPEG nanoparticles were determined at concentration which the γ of surfactants started to increase after stability at high concentration. The surface tension at cmc designated as γcmc was used to determine the effectiveness of the surface tension reduction (πcmc) which determined from relation; πcmc = γo − γcmc , where γo is the surface tension of water without surfactants (72.1 mN/m at 25 °C). The data of cmc, γcmc and πcmc were calculated and listed in Table 1. Careful inspection of data indicated that CSLA-MPEG nanoparticles have high affinity to reduce the surface tension of water than CSLA-MPEG surfactants and CSOA-MPEG due to high surface activity. These results demonstrate that the presence of high percentages of hydrophobic substituent made the resulting derivatives become amphiphilic polymers which can assemble on the surface with their hydrophobic chains pointing to the air and the hydrophilic backbone on the surface, leading to reduction of the surface tension as illustrated in Scheme 2. The chemical structure of CSLA-MPEG and CSOA-MPEG contain different types of hydrophobic and hydrophilic moieties that are responsible for their aggregation in aqueous solution or adsorption at interfaces (Scheme 2). It can proposed that the hydrophobic moieties (oleyl and linoleyl groups) can be hydrophobically interacted and oriented to interior in the bulk to aggregates or oriented outside the aqueous solution when adsorbed at interface (Scheme 2b). In contrast, the hydrophilic moieties of CSLA-MPEG and CSOA-MPEG, hydroxyl, oxyethylene, and amide groups, can form hydrogen bonds with water when diffused or suspended in water. A possible explanation seems reasonable to propose that surface tension data involves two processes the adsorption of nanogels at the interface and the unfolding of surface tails (hydrophobe) and loops to cover the entire interface. The

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Table 1 Surface activities of CSLA-MPEG and CSOA-MPEG at 25 °C. Designation

cmc mol/L × 104

γcmc mN/m

Δγ mN/m

(−∂γ/∂lnc)

Γmax ×1010 mol/cm2

Amin nm2/molecule

−ΔGagg kJ mol−1

−ΔGads kJ mol−1

CSLA-MPEG CSLA-MPEG nanoparticles CSOA-MPEG CSOA-MPEG nanoparticles

4.4 2.1 1.5 0.75

46.2 38.5 48.2 42.1

25.9 33.6 23.9 30.0

7.5 14.1 6.5 9.1

3.13 5.89 2.72 3.8

0.53 0.28 0.61 0.44

19.14 20.82 21.54 23.84

27.41 26.49 30.32 31.79

data of surface active parameters, Table 1, confirm that increasing the degree of unsaturation in alkyl group of fatty acid of the hydrophobic part enhances generally the surface activity of these nanogels. This can be referred to the repulsion between double bonds increased with increasing the degree of saturation which increased the interaction of hydrophilic groups of CSLA-MPEG with water. The ability of CSOA-MPEG and CSLA-MPEG nanogels to adsorb at water/air interface can be estimated from surface tension measurements. The adsorption parameters such as the concentration of surfactants at the water-air interface Гmax. (mol/cm2) and the minimum area per molecule Amin. (nm2/mol) at the aqueous–air interface can be used to investigate the ability of the prepared compounds to adsorb at interfaces. Гmax. was calculated from equation: Гmax. = −[(δγ / δlnc ) / RT], where (δγ / δlnc) is the slope of the plot of c versus ln c at constant temperature (T) and R is the gas constant (in J mol−1 K−1). The area per molecule at the interface provides information on the degree of packing and the orientation of the adsorbed surfactant molecules, when compared with the dimensions of the molecule as obtained from models. From the Гmax, the area per molecule Amin. at the interface is calculated using the equation: Amin. = 1016 / ГmaxN; where N is the Avogadro's number. The values of (δγ / δlnc), Гmax and Amin were calculated for CSOAMPEG and CSLA-MPEG surfactants and crosslinked nanoparticles and listed in Table 1. As shown in Table 1, high value of Гmax means that more particles adsorbed on the surface of the solution, which also means a lower surface tension. The lowest value of Amin (0.28 nm2/ molecule) suggests higher adsorption of CSLA-MPEG nanoparticles that oriented away from the liquid in a more tilted position (Scheme 2). However, a low Amin data suggest complete surface coverage with the formation of flexible CSLA-MPEG chains at interface. Moreover, it can be expected that there are some interactions between nanoparticles due to the rough surface of the particles as well as attractions between the hydrophilic arms may be responsible for the higher ability to reduce the surface tension. The relation between the surface tension and aging times indicated that the surface tension reached equilibrium and its value was not increased again with time which indicated on the formation of flexible film of microgel at interfaces without thinning or the rupture of the film [24].

The free energy of micellization (ΔGmic) or aggregation (ΔGagg) CSOA-MPEG and CSLA-MPEG surfactants and crosslinked nanoparticles was calculated from equation: ΔG mic = − RT ln cmc. The free energy of adsorption, ΔG ads , was calculated as: ΔG ads = ΔGmic − 0.6023Aminπcmc . The values of ΔGads and ΔGmic were calculated and listed in Table 1. The values of ΔGads and ΔGmic are close to each other with slight more negativity of ΔGads indicating that the adsorption of the prepared surfactants at interfaces is slightly preferable. However, the adsorption and micellization can be associated at the same time. All ΔGads. values are more negative than ΔGmic. indicating that the adsorption of microgels at the air/water interface is associated with a decrease in the free energy of the system. This may be attributed to the effect of steric factor on inhibition of aggregation more than its effect on adsorption process. 3.3. Polarization studies Potentiodynamic polarization curves for steel in 1 M HCl solution without and with different concentration of CSLA-MPEG and CSOAMPEG nanoparticles are shown in Fig. 5 a and b, respectively. Nanomaterials attracted great attention as self-healing materials for steel [36,37]. These materials possess superior characteristics due to their high efficacies to inhibit both cathodic and anodic corrosion. In the present work, it is evident that both the anodic and cathodic reactions are inhibited. If the change in Ecorr value was more than 85 mV, the inhibitor can be recognized as an anodic or cathodic type inhibitor [38]. The values of related electrochemical parameters, i.e., corrosion current density (icorr), Ecorr (vs. SCE), cathodic Tafel slope (bc) and anodic Tafel slope (ba) deduced from the polarization curves for CSLA-MPEG and CSOA-MPEG nanoparticles are summarized in Tables 2 and 3, respectively. From Tables 2 and 3 it was found that the Ecorr value changes with a maximum shift in Ecorr less than 85 mV indicating mixed mode of corrosion [39] Therefore, CSLA-MPEG and CSOA-MPEG nanoparticles behave as mixed-type inhibitors, which preferentially restrained the anodic process of corrosion. It is clear from Tables 2 and 3 that the value of corrosion current density (icorr) noticeably decreases in the presence of inhibitors. The results may be attributed to the formation of a protective film on steel surface, which in turn created a barrier film between steel

Scheme 2. Micellization and adsorption of CSLA-MPEG or CSLA-MPEG at interface.

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Fig. 5. Polarization curves for concentrations of a) CSLA-MPEG, b) CSOA-MPEG nanoparticles; Nyquist plot for steel in 1 M HCl solution containing different concentrations of c) CSLA-MPEG and d) CSOA-MPEG nanoparticles.

and corrosive medium. The inhibition efficiency IE% was calculated from polarization measurements according to the following equation [40,41]:

nanoparticles effectively reduce the corrosion of steel in 1 M HCl solution even used in small concentration.


 IE% ¼ 1– icorrðinhÞ =icorrðuninhÞ  100

3.4. Electrochemical impedance spectroscopy measurements

ð1Þ

where icorr(uninh) and icor(inh) are the corrosion current density values in the absence and presence of inhibitor, respectively. The values of IE% were calculated at different inhibitor concentrations and were presented in Table 2 and 3 for CSLA-MPEG and CSOA-MPEG nanoparticles, respectively . According to data of Table 2 and 3, the corrosion current density decreases substantially leading to high inhibition efficiency of CSLA-MPEG and CSOA-MPEG nanoparticles with the increasing of inhibitor concentration. These results are indicative of the adsorption of CSLA-MPEG and CSOA-MPEG nanoparticles on the steel surface. The results of potentiodynamic reveal that CSLA-MPEG and CSOA-MPEG

Nyquist plots of steel in 1 M HCl solution without and with different concentrations of CSLA-MPEG and CSOA-MPEG nanoparticles are shown in Fig. 5 c and d, respectively. It is seen that the Nyquist diagram shows a capacitive loop, which suggest that the corrosion of steel in test solution is mainly controlled by charge transfer process [42,43]. The diameter of the capacitive loop in the presence of inhibitors is bigger than that in the uninhibited solution and increases with the inhibitors concentrations. The equivalent circuit model employed to fit the obtained impedance data consists of solution resistance (Rs), the charge transfer resistance (Rct) and double layer capacitance (Cdl). The impedance parameter such as charge transfer resistance (Rct) and double layer

Table 2 Inhibition efficiency values for steel in 1 M HCl with different concentrations of CSOAMPEG nanoparticles calculated by polarization and EIS methods.

Table 3 Inhibition efficiency values for steel in 1 M HCl with different concentrations of CSLAMPEG nanoparticles calculated by polarization and EIS methods.

Polarization method

Blank 50 ppm 150 250

EIS method

Polarization method

Ba (mV)

Bc (mV)

Ecorr (V)

icorr μA/cm2

IE%

Rct Ohm

Cdl (μF/cm2)

IE%

69 59 60 47

120 176 125 132

−0.3955 −0.3425 −0.3578 −0.3333

839 68 38 23

– 91.8 95.4 97.2

1.80 23 53.2 67.8

334 108 104 101

– 92.1 96.6 97.3

Blank 50 ppm 150 250

EIS method

Ba (mV)

Bc (mV)

Ecorr (V)

icorr μA/cm2

IE%

Rct Ohm

Cdl (μF/cm2)

IE%

69 56 74 81

120 113 85 80

−0.3955 −0.4094 −0.4427 −0.4478

839 82 62 58

– 90.2 92.6 93.0

1.80 19.9 25.2 27.8

334 116 113 110

– 90.9 92.8 93.5

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Scheme 3. Interactions between CSLA-MPEG or CSLA-MPEG and steel surface.

Fig. 6. Contact angle measurements of CS amphiphiles at the steel surfaces.

3.5. Contact angle and SEM data

capacitance (Cdl) are listed in Tables 2 and 3 for CSLA-MPEG and CSOAMPEG nanoparticles, respectively. It is seen that the Rct value increases and the Cdl value decreases by the addition of inhibitors. A large charge transfer resistance (Rct) is associated with a slower corroding system. An increase in charge transfer resistance values could be ascribed to the adsorption of inhibitor at steel–acid interface, which effectively blocked the active sites on steel surface and hence enhances the corrosion resistance of steel in acidic medium. The decrease in Cdl values with an increase in the inhibitor concentration, suggesting that either thickness of protective layer increased, or local dielectric constant of film decreased, or both occurred simultaneously [44]. The values of percentage inhibition efficiency (IE%) were calculated from the values of Rct according to the following equation: IE% ¼ ½1−ðR ct =Rct Þ  100

ð2Þ

where Rct and R°ct are the charge-transfer resistance with and without inhibitor, respectively. The inhibition efficiency increases with increasing inhibitor concentration due to more and more coverage of steel surface with the inhibitor concentrations. The inhibition efficiencies calculated from EIS showed the same trend as those obtained from potentiodynamic polarization (Tables 2 and 3).

The interaction between the steel surface and dispersed CSLA-MPEG and CSOA-MPEG nanoparticles can be examined by contact angle measurements and scan electron microscope micrographs. The contact angles data between steel and different concentrations of CSLA-MPEG and CSOA-MPEG nanoparticles in 1 M HCl were measured and represented in Fig. 6. The SEM micrographs of CSLA-MPEG nanoparticles (100 ppm) on the steel surface before and after immersion in 1 M HCl were selected and represented in Fig. 7 a and b. It is well known that, the contact angle (θ) formed between corrosive media and steel represented the interaction between the steel and corrosive environments [45]. Careful inspection of contact angle data (Fig. 6) indicate that the contact angle data increased with increment of CSLA-MPEG and CSOA-MPEG nanoparticle concentrations. This was referred to the adsorption of the CSLA-MPEG and CSOA-MPEG nanoparticle and the formation of hydrophobic film at the steel surface [44]. Moreover, the contact angles increased when CSOA-MPEG used at high concentration but it was highly reduced at lower concentrations. This can be referred to the formation of more hydrophobic film when CSOA-MPEG was used instead of CSLA-MPEG which agrees with the data reported on the surface activity. The formation of well adsorbed film of CSLAMPEG was indicated from increment of contact angle data at lower concentration of CSLA-MPEG than CSOA-MPEG. These data agree in harmony with the data reported before for increasing of contact angles of metal surface with increment corrosion inhibitor concentrations [46].

Fig. 7. SEM micrographs of steel a) immersed in 100 ppm of CSLA-MPEG nanoparticles and b) after immersion in 1 M HCl of 100 ppm of CSLA-MPEG for 24 h.

A.M. Atta et al. / Journal of Molecular Liquids 211 (2015) 315–323

SEM micrographs of CSLA-MPEG nanoparticles (Fig. 7) confirm the formation of the films on the steel surface. Inspection of Fig. 7a showed smooth and cleaner surface and produced film at the surface of steel which inhibits the corrosion of steel. Moreover, minor pits are observed on the metal surface after immersion in 1 M HCl. This behavior suggested that CSLA-MPEG nanoparticles do mitigate steel corrosion rate with complete inhibition all over the steel surface. It was previously reported that, the CS polymer backbone can be hydrolyzed in different acid conditions [47,48]. The present work aims to form crosslinked CSLA-MPEG and CSOA-MPEG nanogels to prevent the hydrolysis of amide, azomethine and CS polymer backbone. It is necessary to determine the resistivity of CSLA-MPEG and CSOAMPEG to hydrolysis in 1 M HCl and to determine the interactions between steel surfaces and CSLA-MPEG and CSOA-MPEG nanogels, In this respect, FTIR spectra of CSLA-MPEG and CSOA-MPEG were recorded after removing the layer on the steel after 72 h immersion in 1 M HCl. The FTIR data were not changed and all bands appeared in Fig. 3 appeared without changes. The bands at 3433, 3100, 1680 and 1638 cm−1 (attributed to OH CH olefin, CONH and CH_N stretching vibration, respectively) are shifted to 3417, 3050, 1650 and 1580 cm−1, respectively. The slight shift of these bands indicating the involvement of these groups in the adsorption of CSLA-MPEG and CSOA-MPEG. The appearance of these bands confirms the stability of CSLA-MPEG and CSOA-MPEG towards HCl hydrolysis. Accordingly, the interactions of CSLA-MPEG and CSOA-MPEG with steel surface can be illustrated in the Scheme 3. It can be confirmed that the interaction of the lone pair of electrons on azomethine, ethoxy, amine salts, amide and hydroxyl groups with iron on mild steel surface are responsible for their corrosion protection efficiencies. These interactions via crosslinking decrease the diffusion of HCl in the networks and steel surfaces to prevent both hydrolysis of CS backbone grafts and corrosion of steel [9]. 4. Conclusions 1. CS amphiphiles were prepared by introducing unsaturated fatty acids (OA and LA) as hydrophobic chains and polyethylene glycol as hydrophilic chains. The amidation percentages of CS with OA and LA were determined as 45.3 and 55.6%. The high yield of CSLA can be attributed to high reactivity and compatibility between LA and CS which increased by increment the unsaturation degree that increased with incorporation of LA instead OA. 2. The grafting degree of CSOA-MPEG and CSLA-MPEG was 20.8 and 25.4%, respectively. This data indicated that approximately 25 and 10% of amine groups of CSOA and CSLA were not amidated or grafted with OA or MPEG550-CHO. 3. CSLA-MPEG showed high surface activity more than CSOA-MPEG. 4. The electrochemical results reveal that CSLA-MPEG and CSOA-MPEG nanoparticles are effective corrosion inhibitor and reduces the corrosion of steel in 1 M HCl solution even used in small concentration. 5. CSLA-MPEG and CSOA-MPEG showed good resistance to acid hydrolysis due to crosslinking and interactions with the steel surfaces. 6. The formation of a surface film containing the inhibitor decreases the active surface area on the steel susceptible to acid corrosion. Acknowledgment The project was financially supported by King Saud University, Vice Deanship of Research Chairs.

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