Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx

G ModelJIEC 3968 No. of Pages 8

Electrochemical characterization and thermodynamic tendencyof β-Lactoglobulin adsorption on 3D printed stainless steel

M.J.K Lodhia, K.M. Deenc,d, Zia Ur Rahmana,b, Ameeq Farooqd, Waseem Haidera,b,*a School of Engineering and Technology, Central Michigan University, Mt. Pleasant, MI, USAb Science of Advanced Materials, Central Michigan University, Mt. Pleasant, MI, USAcDepartment of Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, CanadadCorrosion Control Research Cell, Department of Metallurgy and Materials Engineering, University of the Punjab, Lahore 54590, Pakistan

A R T I C L E I N F O

Article history:Received 23 October 2017Received in revised form 3 April 2018Accepted 20 April 2018Available online xxx

Keywords:3D printed 316L stainless steelβ-LactoglobulinElectrochemicalPolarization resistanceThermodynamic parameters

A B S T R A C T

The electrochemical behavior of 3D printed 316L stainless steel (3DPSS) was investigated in phosphatebuffer saline (PBS), containing β-Lactoglobulin (BLG). The charge transfer resistance (Rct) was decreasedfrom 629 to 40 k V cm2with an increase in temperature from 293 to 323 K in BLG free electrolyte. At eachtemperature, the exponential decrease in the polarization resistance (Rp) of 3DPSS was a function of BLGconcentration. The significant decrease in activation energy ‘Ea’ (�15 kJ mol�1) upon adding 0.06 g/L BLGattributed to the charge transfer process involved in the adsorption process. The results were further usedto quantify the thermodynamic parameters.© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

journal homepa ge: www.elsev ier .com/locate / j ie c

Introduction

Austenitic stainless steel is a workhorse material in variousindustries, including but not limited to biomedical, food anddairy [1–4]. This material has diverse range of applications, dueto its fair corrosion resistance, good mechanical strength andexcellent biocompatibility [5–7]. In order to maximize theindustrial applications of stainless steel, improved corrosionresistance of this material has always been a focus of theresearch community. To enhance the corrosion resistance ofstainless steel, surface modification, application of coatings anduse of different inhibitors have been widely discussed in theliterature [8–11]. Recently, by tailoring the microstructure ofstainless steel, the improvement in the corrosion resistance hasalso been reported [12,13]. However, all these processes requireadditional tooling or mechanical treatments, which may not beapplicable to many components involving complex geometries.Additive manufacturing or 3D-printing is an emerging metal-lurgical process which has established a drastic impact on theexisting technology due to the development of the mostintricate parts with great time efficiency [14,15]. It is a layer-by-layer manufacturing process, involving a localized melting

* Corresponding author at: School of Engineering and Technology, CentralMichigan University, Mt. Pleasant, MI, USA.

E-mail address: haide1w@cmich.edu (W. Haider).

https://doi.org/10.1016/j.jiec.2018.04.0271226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Publis

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemicaladsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), http

and extremely high solidification rate that can result into verynon-conventional microstructure compared to the wroughtcounterpart [16,17]. Wang et al. [18] reported that 316L stainlesssteel parts with more strength and appreciable ductility can bemanufactured via 3D printing approach, attributing to thesignificant modification of microstructure. Sander et al. [19]mentioned that the pitting resistance of 3D printed 316Lstainless steel was significantly higher compared to theconventional 316L stainless steel. In essence, the fundamentalunderstanding of the material behavior is dependent on itsmicrostructural features which are also influenced by themanufacturing and processing methods. Furthermore, materialsbehavior is also strongly dependent on its working environment.

In the food, biomedical and pharmaceutical industries, theadsorption of proteins on the surface of metallic components is animportant issue [20–24]. Proteins adsorption can subsequentlyalter the corrosion behavior of the metallic components due to theformation of biofilm on the surface [25]. The adsorption of proteinon the surface is an instantaneous, heterogeneous and a complexphenomenon which may involve electrostatic, hydrophobic and/orhydrogen bonding interactions [26]. The quantity and nature of theprotein adsorption is dependent on the surface properties of thematerial i.e. wettability, polar interaction and surface topography[27,28]. Proteins may tend to denature or nature on the surfacewhich may catalyze, moderate or mediate the chemical, electro-chemical and/or physical reactions [29–31]. Temperature, pH, ionicstrength of the solution and structural stability of the protein are

hed by Elsevier B.V. All rights reserved.

characterization and thermodynamic tendency of β-Lactoglobulins://doi.org/10.1016/j.jiec.2018.04.027

2 M.J.K. Lodhi et al. / Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx

G ModelJIEC 3968 No. of Pages 8

some other key factors which also play an important role on thestability of the surface and protein structure. The adsorptionbehavior of the proteins on metallic surface is considered achemical interaction, in which the building blocks of amino acids(proteins) interact with hydroxylated sites on the metal surface[32–34]. There are some reports available in the literatureaddressing the interaction of various proteins with stainless steeland its implications [25,35,36]. Thermal instability of the proteinscould also lead to the biofouling of the equipment, particularlyused in the food industries [37,38]. The decrease in corrosionresistance of the metallic parts was also found to be related withthe physisorption and irreversible chemisorption of the proteins[33,37]. Adsorption of protein through chemisorption processdecreases the activation energy which may result due to thedecrease in the polarization resistance (accelerated dissolution ofthe metals). The change in corrosion behavior due to proteinadsorption could further affect the surface affinity, structuralrearrangement, thermodynamic and kinetic properties of thematerial [39–41].

The protein, β-Lactoglobulin (BLG) is one of the most prevalentingredients in the mammalian milk and has been implicated in thefouling of the metallic parts in the dairy industry. With a molarmass of 18,328 Da and isoelectric point of 5.13, it is found naturallyas dimer of two monomeric subunits, non-covalently bonded inthe bovine milk [42]. In the pH range of 6.0–7.4, the BLG transformsfrom native to reversible denatured state known as the “TanfordTransition” [43]. This research aims to elucidate the electrochemi-cal response of 3D printed 316L stainless steel (3DPSS) inphosphate buffer saline containing varying concentration of BLGprotein. The main objective of this research work is to highlight theinfluence of BLG protein on the corrosion behavior of (3DPSS). Theelectrochemical impedance spectroscopy and potentiodynamicpolarization tests were performed to observe the effect oftemperature on the adsorption characteristics of BLG protein.The electrochemical stability and thermodynamic parameters forthe 3D printed 316L stainless steel are also evaluated for the veryfirst time.

Experimental

The 3D printed 316L stainless steel (3DPSS) samples used in thisstudy were fabricated using Renishaw AM250 unit equipped withytterbium continuous laser beam (wavelength 1060 nm) devel-oped at 200 W. All the samples were built under same operatingconditions with a hatch distance of 100 mm and layer thickness of30 mm. The manufactured samples were circular disks having1.58 cm diameter and 0.5 cm thickness. In order to ensure thechemical composition of the build samples, the energy dispersiveX-ray spectroscopy analysis was carried out and the results arepresented in Table 1. Before performing all the tests, the surface ofeach sample was polished using SiC papers (180–1200 grit size)under running water. To obtain mirror like surface, the sampleswere then polished on the canvas by using 3 mm fine diamondsuspension. Furthermore, the samples were degreased via ultra-sonication in ethanol for 15 min and rinsed in deionized waterbefore drying in N2 gas stream. The samples were stored in adesiccator for further use.

The phosphate buffer saline (PBS) solution was prepared bydissolving PBS tablets (Sigma Aldrich) in deionized water, which

Table 1Chemical composition of 3D printed 316L stainless steel used in this study.

Elements Cr Ni Mn C Mo S P Fe

Wt.% 18.1 11.2 1.8 0.03 2.5 0.03 0.05 Balance

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemicaladsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), http

was used as electrolyte. The chemical composition (in g/L) of PBSwas 0.8 NaCl, 0.35 NaHCO3, 0.2 MgSiO4, 0.14 CaCl2, 0.06 KH2PO4

and 0.06 Na2HPO4. Afterwards, the known concentration of 0.02,0.04 and 0.06 g/L β-Lactoglobulin (Sigma Aldrich) was added intothe PBS solutions for electrochemical testing.

Electrochemical tests were performed in three electrode cellsystem connected with the Gamry interface 1000E potentiostat. Inthis cell, polished 3DPSS samples (exposed surface area = 1.27 cm2)were used as working electrodes. The saturated calomel electrode(Eo = +240 mV vs SHE) and a high purity (99.9%) platinum coil wereused as reference and counter electrodes, respectively. All the testswere conducted in the naturally aerated electrolytes. The experi-ments were performed at different temperatures (293, 303, 313and 323 K). The open circuit potential (OCP) of the electrode wasmeasured initially for 4 h to achieve the 0.01 mV/min potentialstability. The impedance spectra were recorded in the frequencyrange of 100 kHz–1 mHz by applying 5 mV r.m.s amplitudesinusoidal AC signal at OCP (0 V DC potential). The potentiody-namic polarization scans were obtained within �0.5 V to +1.0 V (vs.OCP) potential range at 0.5 mV/s scan rate.

Results and discussion

Electrochemical impedance spectroscopy (EIS)

In order to analyze the electrochemical processes occurring atthe electrode/electrolyte interface, electrochemical impedancespectroscopy (EIS) tests were conducted. The samples wereimmersed in the electrolyte for 4 h prior to EIS scans to achievequasi-steady state conditions at the electrode/electrolyte interface.The Bode plots are shown in Fig. 1 presenting the experimentalresults for 3DPSS in PBS electrolyte containing varying amount ofBLG at different temperatures. The measured impedance spectra,as a function of frequency are related with the current responseassociated with the reversible charge transfer and adsorptionprocesses occurring at the electrode surface, when a smallsinusoidal AC potential is applied.

Different model circuits are presented in the literature formechanistic interpretation of impedance data [44–47]. To simulatethe adsorption behavior of protein at the metal surface, theequivalent electrical circuit (EEC) model includes two timeconstants as proposed by Omanovic and Roscoe [47]. This EECmodel incorporates two hierarchical parallel constant phaseelements in series with the solution resistance (Rs) as shown inFig. 2. This can be used to predict the interfacial adsorption/desorption behavior of BLG and its interaction with 3DPSS surface.The high frequency impedance was attributed to the charging/discharging of double layer (Ydl) and charge transfer resistance(Rct). The low frequency impedance was represented by ‘Yf’ and ‘Rf’indicating the capacitive and resistive behavior related to passivefilm, respectively. The constant phase elements were incorporatedbecause of the charge distribution within the double layer (due tospecifically adsorbed species) and on the passive film. Thedeviation of impedance slope from �1 and the phase angle from�90� in the Bode plots suggest the annexation of constant phaseelements in the EEC. In this case, these effects can be related withthe surface characteristics i.e. double layer formation andspecifically adsorbed species in association with the uniformityof passive film [47].

The second time constant corresponded to the dielectriccharacteristics of the passive film and the associated processesinvolved with the transport of electroactive species to/from theinherent oxide film attributing to its dissolution tendency. Table 2provides the quantitative information of the impedance behaviormeasured from the fitting of simulated EEC model over theexperimental spectra.

characterization and thermodynamic tendency of β-Lactoglobulins://doi.org/10.1016/j.jiec.2018.04.027

Fig. 1. Bode plots of 3DPSS samples in PBS solution containing varying concentrations of BLG at different temperatures (a) 293 K (b) 303 K (c) 313 K (d) 323 K.

Fig. 2. Equivalent electrical circuit (EEC) model used to simulate the impedancespectra.

M.J.K. Lodhi et al. / Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx 3

G ModelJIEC 3968 No. of Pages 8

The impedance plots clearly depict the presence of threedifferent frequency dependent regions as shown in Fig. 1. In highfrequency regime, the phase angle is almost zero and thecorresponding current response is in phase with the applied ACpotential perturbation. This region is depicting the solutionresistance between the reference and working electrode. A linearrelationship between an impedance and frequency is observed inthe medium frequency range with a slope (n1) of 0.91 and phaseangle of approximately �82�, depicting the capacitive response ofelectrode surface within this frequency range. In lower frequencyregime, the decrease in the slope of the impedance curve and anincrease in the phase angle are indicating the faradaic activity bythe ionic species over the surface passive film. In EEC model,faradaic activity is represented as capacitive and resistive responseof the passive film. Therefore, the magnitude of real impedancevalue at the lower frequency is designated as ‘Rf’ [48].

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemicaladsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), http

The variation in impedance spectra of 3DPSS is directly relatedwith the interfacial faradaic reactions, particularly associated withthe adsorption/desorption characteristics of BLG and its effect onthe barrier characteristics of the passive film. The interaction ofBLG with the 3DPSS surface, as a function of its concentration wasestimated from ‘Rct’ and ‘Rf’ as given in Table 2. The relatively largepolarization resistance (Rp) of 3DPSS in BLG-free solution predictsthe negative impact of protein on its electrochemical response. Inother words, the adsorption/desorption of BLG involved the chargetransfer phenomenon that adversely affected the barrier character-istics of the passive film. This involve the release of metal ions fromthe surface into the electrolyte and that may be deleterious interms of product quality and pose serious threat to the equipmentin the food processing industry. Moreover, with increase intemperature (from 293 K to 323 K) and concentration of BLG(from 0 g/L to 0.06 g/L), the ‘Rp’ was decreased, which representsincreased adsorption tendency of the BLG molecules throughcharge transfer process (chemisorption) over the 3DPSS surface.The ‘Ydl’ and ‘Yf’ indicates the amount of charge relaxed within thedouble layer and the interaction of specifically adsorbed specieswith the surface passive film, respectively. The significant variationin the ‘Ydl’ and ‘Yf’ as a function of BLG concentration andtemperature was observed, which validates the influence ofprotein adsorption on the surface. At 293 K, the larger value of‘Ydl’ and ‘Yf’ in the BLG containing electrolytes is affiliated with thestrong interaction of BLG with the active sites available on thesurface. However, the interaction of BLG with electrode surface andthe behavior of localized defects in the passive film are difficult tobe differentiated from these results. With increase in temperature

characterization and thermodynamic tendency of β-Lactoglobulins://doi.org/10.1016/j.jiec.2018.04.027

Table 2The parameters obtained from the fitting of experiment impedance spectra with the EEC model.

Electrolyte (g/L) Rs (V cm2) Ydl (mSsn/cm2) Rct (kV cm2) Yf (mSsn/cm2) Rf (kV cm2) Rp (V cm2) Goodness of fit

293 K0.00 18.35 35 629 15 692 1.32 � 106 5.14 �10�4

0.02 14.70 45 812 45 572 1.38 � 106 2.57 � 10�4

0.04 18.70 47 72 30 627 699 � 103 3.67 � 10�4

0.06 10.40 55 9 63 622 631 �103 6.41 �10�4

303 K0.00 5.47 48 359 6.6 578 937 � 103 0.82 � 10�4

0.02 10.24 55 428 14 288 716 � 103 3.20 � 10�4

0.04 6.61 28 34 25 589 623 �103 0.20 � 10�4

0.06 3.25 44 115 16 568 683 � 103 2.75 �10�4

313 K0.00 6.069 39 168 52 305 473 � 103 5.14 �10�4

0.02 10.75 48 243 50 159 402 � 103 6.80 � 10�4

0.04 2.91 49 4.61 12 409 414 �103 1.83 � 10�4

0.06 2.32 46 347 205 13 360 � 103 3.8 � 10�3

323 K0.00 2.92 48 40 4.9 239 279 � 103 0.38 � 10�4

0.02 4.19 53 435 805 20 455 �103 0.61 �10�4

0.04 2.55 65 267 95 62 329 � 103 0.90 � 10�4

0.06 5.17 49 3.68 3.78 284 288 � 103 0.99 � 10�4

4 M.J.K. Lodhi et al. / Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx

G ModelJIEC 3968 No. of Pages 8

and BLG concentration, the decrease in the ‘Yf’ is in synergism withdecrease in ‘Rct’ and could also be associated with the degradationof BLG molecules and their relatively high desorption tendency.The interaction between BLG and 3DPSS surface leads to theformation of soluble metal complexes. This process initiates thedeterioration of the passive film, which could be another reason forlow ‘Rct’. Similarly, with increase in temperature and BLGconcentration, variation in the ‘Rp’ is also observed. Thesevariations could be a result of competitive adsorption of BLG asa function of its concentration or due to a possible change in itsmolecular structure at different temperatures.

Potentiodynamic polarization

As indicated by the EIS results, the addition of BLG in anelectrolyte significantly influenced the electrochemical responseof 3DPSS. In order to further validate this behavior, potentiody-namic polarization scans (PPS) were also obtained. The PPS of3DPSS are shown in Fig. 3. Similar cathodic polarization curvesshowed that increase in BLG concentration and temperature didnot affect the reduction reactions (reduction of dissolved oxygenand water) at the surface. For instance, the addition of BLG does notshow any change in the cathodic interaction between electrolyteand electrode under applied conditions. However, the addition ofBLG significantly influenced the anodic behavior of 3DPSS asdepicted in Fig. 3. At low anodic potentials (<0.3 V vs. SCE), thepolarization trends could be associated either with the uniformdissolution of the electrode surface or to the oxidative adsorptionof BLG at the electrode surface. Qualitatively, this can also bepredicted from the anodic polarization curves that addition of BLGin the PBS significantly influenced the surface characteristics of3DPSS. The increase in couple current (ic) and decrease in couplepotential (Ec) below �0.3 V (vs. SCE) also indicated its favorableinteraction with the surface. The anodic behavior below 0.3 V (vs.SCE) could also be associated with the oxidation of BLG and/or itsdissociation into small fragments at the surface of 3DPSS. Underthese conditions, the product species may not be stable and coulddissolve in the electrolyte by forming metal complexes. This mayalso cause localized dissolution of the surface from the active siteswithin the defective passive film. The dissolution of 3DPSS cannotbe neglected in this potential region but almost similar anodictrends beyond this potential (>0.3 V vs. SCE) indicated the

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemicaladsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), http

uniformity in the passive film independent to the BLG concentra-tion and temperature.

The couple current density and couple potential were measuredfrom the intersection of interpolated linear cathodic and anodicpolarization curves. It has been suggested from the potentiody-namic curves that addition of BLG in the electrolyte significantlyinfluenced the surface characteristics of 3DPSS and may initiatelocalized reactions at the surface [49]. The polarization resistance(Rp) has been calculated from the ‘ic’ values using the Stern–GearyEq. (1) [50].

ic ¼ babc2:303 ba þ bcð Þ

1Rp

ð1Þ

Here ‘ba’ and ‘bc’ are the anodic and cathodic Tafel slopes,respectively. The value of these slopes and ‘ic’ were used tocalculate ‘Rp’.

To further elaborate the effect of BLG on the electrochemicalbehavior of 3DPSS as a function of temperature, Fig. 4 is presented.The decrease in ‘Rp’ with increase in temperature and BLGconcentration indicated the high charge transfer rate associatedwith the adsorption/desorption of BLG. This resulted in theenhanced dissolution tendency of the surface [50]. Similar findingsabout the adsorption–desorption (via oxidative dissociation) ofBLG and other proteins on the wrought stainless steel has also beenreported in the literature [42,51].

By using the data presented in Fig. 4, the activation energy (Ea)was calculated to estimate the adsorption behavior of BLG on3DPSS. The dependence of dissolution rate on the temperature wasdetermined by using the Arrhenius equation (Eq. (2)).

ic ¼ Ae�EaRT ð2Þ

where ‘Ea’ (J mol�1) is the activation energy for surface dissolution,‘T’ (K) is the absolute temperature, ‘R’ (8.314 J mol�1 K�1) is theuniversal gas constant, ‘A’ is the pre-exponential factor and ‘ic’ (A/cm2) is the couple current density.

Arrhenius plots for 3DPSS immersed in BLG-free and BLGcontaining electrolytes are shown in Fig. 5a.

The values of the ‘Ea’ were calculated from the slopes ofArrhenius curves (Fig. 5a). It has been evaluated that the activationenergy decreased (Fig. 5b) at high concentration of BLG, indicatingthat the dissolution tendency of the surface was enhanced throughoxidative dissociation of protein molecules. A decrease of

characterization and thermodynamic tendency of β-Lactoglobulins://doi.org/10.1016/j.jiec.2018.04.027

Fig. 4. The effect of BLG concentration in the PBS solution and temperature on thepolarization resistance (Rp) of the 3DPSS.

Fig. 3. Potentiodynamic polarization scans of 3DPSS at different temperatures (a) 293 K (b) 303 K (c) 313 K (d) 323 K.

M.J.K. Lodhi et al. / Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx 5

G ModelJIEC 3968 No. of Pages 8

approximately 15 kJ mol�1 in the ‘Ea’ was calculated with theaddition of 0.06 g/L BLG in the PBS. A notable decrease in the ‘Ea’with the addition of BLG also suggests that the adsorption/desorption of BLG on the 3DPSS is an electrochemical process [49].This finding further supported EIS simulation data as explainedabove. Earlier literature also reported that the adsorption ofprotein on platinum, gold, titanium and stainless steel duringanodic polarization results from the interaction of carboxylatefunctional groups of amino acids on the active sites at metallicsubstrates [52,53]. The possible reaction defining the adsorption ofBLG on the 3DPSS is shown in Eq. (3) [47].

B þ nS ! BðSÞn;ads þ ne ð3Þ

where ‘B’ is the adsorbed protein (BLG) molecule, ‘S’ is the surfaceof 3DPSS, ‘n’ is the number of active sites on the electrode surfaceavailable for carboxylate interaction (or one could say ‘n’ is thenumber of transferred electrons).

The affinity of the protein towards the metallic surface ispossible due to electrostatic–hydrophobic interaction and/orhydrogen bonding. Protein molecule is composed of polar andnon-polar groups [54–56]. The hydrophobic interaction occursbetween non-polar groups [57–61]. When proteins are in wet

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemical characterization and thermodynamic tendency of β-Lactoglobulinadsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2018.04.027

Fig. 5. (a) Arrhenius plot of 3DPSS in PBS solution containing various concentrations of BLG, (b) activation energy of the 3DPSS calculated from Arrhenius plots and by usingEq. (2).

6 M.J.K. Lodhi et al. / Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx

G ModelJIEC 3968 No. of Pages 8

condition, there is a tendency for an electrostatic interactionbetween the polar groups and water molecules, while repellingnon-polar groups away [62]. Two different viewpoints have beenpresented in the literature about the interaction of protein towardsthe metal surface. In the first view, it is believed that theelectrostatic interaction has a major effect over the hydrophobicinteraction [63]. In the second view, the hydrophobic interactionhas a dominant effect on the protein adsorption [64]. Theelectrostatic interaction is related to the charge on both, protein(at isoelectric point) and adsorbent surface, which is dependent onthe pH value of an electrolyte. An isoelectric point for BLG andstainless steel is 5.13 and 8.5, respectively [42,65]. Therefore, BLGcarries a net negative charge in PBS (at pH = 7.4). Hence, it can beassumed that an electrostatic interaction is caused between thenegatively charged carboxylate groups of the protein and thepositively charged active sites on 3DPSS surface. Similar con-clusions about protein interaction on the metallic surfaces isreported in literature [52,65–67]. Furthermore, literature alsosuggests the use of Langmuir isotherm calculations (as given inEq. (4)) to predict the adsorption characteristics of the protein onthe metallic surfaces [42,47,65,68].

G ¼ BadsGmaxc

1 þ Badscð4Þ

where, ‘G’ (mol cm�2) represent the amount of adsorbed proteinmolecules (related to surface coverage), ‘Bads’ (cm3mol�1)

Fig. 6. (a) Langmuir isotherms obtained at different temperatures (b). Gibbs fr

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemicaladsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), http

represents the affinity of the adsorbate molecules, ‘Gmax’ (molcm�2) represents the full surface coverage by the adsorbedmolecules, and ‘c’ (mol cm�3) corresponds to the concentrationof adsorbate molecules in the electrolyte. Rearranging theLangmuir isotherm relation would give Eq. (5):

cG

¼ 1BadsGmax

þ cGmax

ð5Þ

For a Langmuir isotherm to be valid for this system the plot ofequilibrium concentration ‘c’ vs. ‘c/G’ should yield a straight line,from which ‘Gmax’ and ‘Bads’ values can be derived using the slopeand intercept of the line, respectively. Fig. 4 shows the dependenceof ‘Rp’ on the equilibrium concentration of BLG. The magnitude ofthe ‘Rp’ after being corrected from the BLG free solution (Rp,0) foreach electrolyte with (Rp,a) obtained after BLG addition and can beexpressed as (G = 1/Rp,c= 1/Rp,a� 1/Rp,0). This corrected polariza-tion resistance can be correlated to the amount of protein adsorbed(1/Rp.c / G) [51].

Fig. 6aindicates the linear dependence of c*Rp,c G versusequilibrium concentration c with the correlation coefficient valueof 0.9971. The value Gmax was calculated using the slope of the lineand the affinity of the adsorbent molecules (Bads) from theintercept. The Bads at constant temperature is defined as:

Bads ¼1

cSolvente

�DGadsRT

� �ð6Þ

ee energy (DG) calculated at each temperature from experimental results.

characterization and thermodynamic tendency of β-Lactoglobulins://doi.org/10.1016/j.jiec.2018.04.027

M.J.K. Lodhi et al. / Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx 7

G ModelJIEC 3968 No. of Pages 8

where ‘Csolvent’ represents the molar concentration of solvent, thevalue for molar concentration of water is (55.5 mol/L), ‘DGads’

(J mol�1) is the Gibbs free energy of adsorption, ‘R’ (J mol�1 K�1)represents the gas constant and ‘T’ (K) is the temperature on theabsolute scale.

By using Eq. (6), the Gibbs free energy (DGads) was calculated atdifferent temperatures. The value of ‘DGads’ in the absence of BLGwas determined to be �24.54 kJ/mol at 293 K. An increasing trendin the ‘DGads’ (more negative) with the concentration of BLG in PBSalso validated its preferential adsorption at the surface of 3DPSS athigh temperature as shown in Fig. 6b. The obtained value for‘DGads’ also confirmed that the adsorption reaction is a chemi-sorption process involving charge transfer between BLG and themetal surface. The increasing trend in the free energy with theaddition of BLG in electrolyte indicates its spontaneous adsorptionover the surface of 3DPSS, which is in confirmation with theconclusions drawn from EIS and PPS results.

It can be observed that ‘DGads’ increased to more negativevalues when the temperature was increased from 293 K to 313 Kbut the slight decrease in ‘DGads’ at 323 K suggests the variation inthe molecular structure of the BLG [42,69,70].

From Eq. (7), the values of entropy (DS) and enthalpy (DHads) ofBLG adsorption reaction on 3DPSS was calculated by plotting DGads

versus temperature (293 K–313 K). From the slope and intercept ofthe curve, the value of ‘DS’ and ‘DHads’ was calculated to be�165 J mol�1 K�1 and 23.7 kJ mol�1, respectively.

DGads = DHads� TDS (7)

The ‘DHads’ is the combination of various enthalpies attributingto the structural rearrangement in the protein molecule ‘DHstr.pr’,ions incorporation in the adsorbed layer ‘DHion’, overlap of electricfield ‘DHel’, dissociation/association of proton on the proteinsurface with charged groups ‘DHH

+’, and to the changes associatedwith the hydration state of metal surface ‘DHhyd’ [71], asrepresented in Eq. (8).

DHads ¼ DHstr:pr þ DHion þ DHel þ DHHþ þ DHhyd ð8ÞAs the value of ‘DHads’ is relatively very small so it is not possible

to calculate the individual values of the enthalpies on the right-hand side of the Eq. (8). The changes in the enthalpy valuesassociated to all these processes are very important in determiningthe sign and the magnitude of change in adsorption enthalpy(DHads). However, the large value obtained for the change inentropy suggests the adsorption behavior of BLG on 3DPSS surfaceis driven by the significant change in the entropy for the adsorptionreaction under applied experimental conditions.

Conclusions

The electrochemical response of 3DPSS in PBS solution,containing varying concentrations (0, 0.02, 0.04 and 0.06 g/L) ofBLG at different temperatures (293, 303, 313 and 323 K) is reported.The decrease in charge transfer resistance with an increase intemperature and BLG concentration was observed from impedancespectroscopy results. The potentiodynamic polarization curvessuggested the variation in the kinetic behavior of the anodicpolarization as a function of both temperature and BLG concen-tration. Furthermore, the decrease in activation energy from22.44 kJ mol�1 at 0 g/L BLG to 7.23 kJ mol�1 at 0.06 g/L BLGconfirmed the favorable tendency of protein adsorption on the3DPSS surface. In addition, the obtained results were describedusing Langmuir isotherm and ‘DGads’ was calculated. The change inGibbs free energy from �24.54 J mol�1 at 293 K to �27.18 J mol�1 at323 K suggested the preferential adsorption of BLG on 3DPSSsurface at higher temperature. Thus, it can be concluded that the

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemicaladsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), http

adsorption of BLG is facilitated at higher temperature and resultedin the increased corrosion tendency of the 3DPSS surface.Moreover, the change in entropy (�165 J mol�1 K�1) and enthalpyof adsorption (23.7 kJ mol�1) on 3DPSS was also calculated.

References

[1] V. Hynninen, et al., Sci. Rep. 6 (July (1)) (2016) 29324.[2] Z. Ur Rahman, K.M. Deen, L. Cano, W. Haider, Appl. Surf. Sci. 410 (July) (2017)

432.[3] M. Navarro, A. Michiardi, O. Castan, J.A. Planell, J. R. Soc. Interface 5 (2008) 1137.[4] M. Talha, C.K. Behera, O.P. Sinha, Mater. Sci. Eng. C 33 (7) (2013) 3563.[5] Y.Y. Chen, Y.M. Liou, H.C. Shih, Mater. Sci. Eng. A 407 (1–2) (2005) 114.[6] C.C. Mohan, et al., Nanoscale 7 (2) (2015) 832.[7] M. Koyama, et al., Science (80-.) 355 (6329) (2017) 1055.[8] S. Hejazi, et al., J. Ind. Eng. Chem. 25 (May) (2015) 112.[9] K.-K. Chew, S.H.S. Zein, A.L. Ahmad, D.S. McPhail, M.F. Abdullah, J. Ind. Eng.

Chem. 19 (4) (2013) 1123.[10] M. Shabani-Nooshabadi, M.S. Ghandchi, J. Ind. Eng. Chem. 31 (November)

(2015) 231.[11] N. Soltani, N. Tavakkoli, M. Khayat Kashani, A. Mosavizadeh, E.E. Oguzie, M.R.

Jalali, J. Ind. Eng. Chem. 20 (September (5)) (2014) 3217.[12] S.V. Muley, A.N. Vidvans, G.P. Chaudhari, S. Udainiya, Acta Biomater. 30 (2016)

408.[13] X.Y. Wang, D.Y. Li, Electrochim. Acta 47 (24) (2002) 3939.[14] A. Vyatskikh, S. Delalande, A. Kudo, X. Zhang, C.M. Portela, J.R. Greer, Nat.

Commun. 9 (1) (2018) 593.[15] D. Herzog, V. Seyda, E. Wycisk, C. Emmelmann, Acta Mater. 117 (2016) 371.[16] S.A. Khairallah, A.T. Anderson, A. Rubenchik, W.E. King, Acta Mater. 108 (2016)

36.[17] L.E. Rännar, A. Koptyug, J. Olsén, K. Saeidi, Z. Shen, Addit. Manuf. 17 (October)

(2017) 106.[18] Y.M. Wang, et al., Nat. Mater. 17 (2017) 63–71.[19] G. Sander, et al., J. Electrochem. Soc. 164 (March (6)) (2017) C250.[20] H. Noh, S.T. Yohe, E.A. Vogler, Biomaterials 29 (May (13)) (2008) 2033.[21] M. Beuf, et al., Heat Exch. Fouling Clean. Fundam. Appl. RP1 (2003) 99.[22] R. Rosmaninho, et al., J. Food Eng. 80 (June (4)) (2007) 1176.[23] O. Santos, T. Nylander, M. Paulsson, C. Trägårdh, J. Food Eng. 74 (4) (2006) 468.[24] P. Roach, D. Farrar, C.C. Perry, J. Am. Chem. Soc. 127 (22) (2005) 8168.[25] Y. Hedberg, X. Wang, J. Hedberg, M. Lundin, E. Blomberg, I. Odnevall Wallinder,

J. Mater. Sci. Mater. Med. 24 (4) (2013) 1015.[26] N. Barnthip, P. Parhi, A. Golas, E.A. Vogler, Biomaterials 30 (33) (2009) 6495.[27] M. Alkan, Ö. Demirbaş, M. Do�gan, O. Arslan, Microporous Mesoporous Mater.

96 (November (1–3)) (2006) 331.[28] M. Liu, Y. Zhang, M. Wang, C. Deng, Q. Xie, S. Yao, Polymer (Guildf) 47 (May

(10)) (2006) 3372.[29] S. Karimi, T. Nickchi, A.M. Alfantazi, Appl. Surf. Sci. 258 (June (16)) (2012) 6087.[30] F. Zhang, J. Pan, P.M. Claesson, Electrochim. Acta 56 (January (3)) (2011) 1636.[31] C.K. Kang, Y.S. Lee, J. Mater. Sci. Mater. Med. 18 (7) (2007) 1389.[32] Y.S. Hedberg, M.S. Killian, E. Blomberg, S. Virtanen, P. Schmuki, I. Odnevall

Wallinder, Langmuir 28 (November (47)) (2012) 16306.[33] D. Costa, P.-A. Garrain, B. Diawara, P. Marcus, Langmuir 27 (March (6)) (2011)

2747.[34] C. Rubio, D. Costa, M. Bellon-Fontaine, P. Relkin, C. Pradier, P. Marcus, Colloids

Surf. B Biointerfaces 24 (April (3–4)) (2002) 193.[35] C.-K. Kang, Y.-S. Lee, J. Mater. Sci. Mater. Med. 18 (June (7)) (2007) 1389.[36] T. Sakiyama, J. Tomura, K. Imamura, K. Nakanishi, Colloids Surf. B Biointerfaces

33 (January (2)) (2004) 77.[37] I.B. Beech, J.A. Sunner, K. Hiraoka, Int. Microbiol. 8 (September (3)) (2005) 157.[38] P. de Jong, Trends Food Sci. Technol. 8 (December (12)) (1997) 401.[39] M. Rabe, D. Verdes, S. Seeger, Adv. Colloid Interface Sci. 162 (February (1–2))

(2011) 87.[40] R.A. Hartvig, M. Van De Weert, J. Stergaard, L. Jorgensen, H. Jensen, Langmuir

27 (6) (2011) 2634.[41] C. Kotsmar, et al., Adv. Colloid Interface Sci. 150 (1) (2009) 41.[42] S. Omanovic, S.G. Roscoe, J. Colloid Interface Sci. 227 (July (2)) (2000) 452.[43] J.A. Thomson, B.A. Shirley, G.R. Grimsley, C.N. Pace, J. Biol. Chem. 264 (20)

(1989) 11614.[44] P. Electrochemistry, C. Elements, C. Equivalent, C. Models, Appl. Note AC 286

(1) (2010) R491.[45] B.-Y. Chang, S.-M. Park, Annu. Rev. Anal. Chem. (Palo Alto Calif.) 3 (2010) 207.[46] M. Ates, Prog. Org. Coat. 71 (1) (2011) 1.[47] S. Omanovic, S.G. Roscoe, Langmuir 15 (23) (1999) 8315.[48] E. Barsoukov, J.R. Macdonald, Impedance Spectroscopy: Theory, Experiment

and Applications, (2005) .[49] M.A.A. Rahim, H.B. Hassan, M.W. Khalil, Materwiss. Werksttech. 28 (April (4))

(1997) 198.[50] ASTM G102 - 89(2015)e1 Standard Practice for Calculation of Corrosion Rates

and Related Information from Electrochemical Measurements, (Online).Available: https://www.astm.org/Standards/G102.htm. (Accessed: 17 August2017).

[51] N.P. Cosman, K. Fatih, S.G. Roscoe, J. Electroanal. Chem. 574 (January (2)) (2005)261.

[52] S.M. MacDonald, S.G. Roscoe, Electrochim. Acta 42 (January (8)) (1997) 1189.

characterization and thermodynamic tendency of β-Lactoglobulins://doi.org/10.1016/j.jiec.2018.04.027

8 M.J.K. Lodhi et al. / Journal of Industrial and Engineering Chemistry xxx (2018) xxx–xxx

G ModelJIEC 3968 No. of Pages 8

[53] A. Horbett Thomas, Biomater.: Interfacial Phenom. Appl. 199 (199) (1982) 233.[54] V.V. Loladze, D.N. Ermolenko, G.I. Makhatadze, J. Mol. Biol. 320 (2) (2002) 343.[55] P.L. Privalov, G.I. Makhatadze, J. Mol. Biol. 232 (2) (1993) 660.[56] K. Illergård, A. Kauko, A. Elofsson, Proteins Struct. Funct. Bioinform. 79 (1)

(2011) 79.[57] J.A. Queiroz, C.T. Tomaz, J.M.S. Cabral, J. Biotechnol. 87 (2) (2001) 143.[58] J.A. Queiroz, C.T. Tomaz, J.M. Cabral, J. Biotechnol. 87 (2) (2001) 143.[59] J.L. Fausnaugh, L.A. Kennedy, F.E. Regnier, J. Chromatogr. A 317 (C) (1984) 141.[60] A. Jungbauer, C. Machold, R. Hahn, J. Chromatogr. A 1079 (1–2) (2005) 221

Special issue.[61] J.A. Queiroz, C.T. Tomaz, J.M.S. Cabral, J. Biotechnol. 87 (2) (2001) 143.[62] Z. Wang, Y. Yan, L. Qiao, Colloids Surf. B Biointerfaces 156 (August) (2017) 62.

Please cite this article in press as: M.J.K. Lodhi, et al., Electrochemicaladsorption on 3D printed stainless steel, J. Ind. Eng. Chem. (2018), http

[63] I. Lee, J.W. Lee, A. Stubna, E. Greenbaum, J. Phys. Chem. B 104 (11) (2000) 2439.[64] R.D. Tilton, C.R. Robertson, A.P. Gast, Langmuir 7 (November (11)) (1991) 2710.[65] D.C. Hansen, G.W. Luther, J.H. Waite, J. Colloid Interface Sci.168 (November (1))

(1994) 206.[66] Ian G.N. Wylie, Sharon G. Roscoe, J. Bioelectrochem. Bioenergetics 28 (October

(3)) (1992) 367.[67] N. Chandrasekaran, et al., Chem. Eng. Res. Des. 91 (September (9)) (2013) 1674.[68] D.R. Jackson, S. Omanovic, S.G. Roscoe, Langmuir 16 (12) (2000) 5449.[69] J. Visser, T.J.M. Jeurnink, Exp. Therm. Fluid Sci. 14 (4) (1997) 407.[70] S.G. Roscoe, K.L. Fuller, G. Robitaille, J. Colloid Interface Sci. 160 (October (1))

(1993) 243.[71] C.A. Haynes, W. Norde, Colloids Surf. B Biointerfaces 2 (July (6)) (1994) 517.

characterization and thermodynamic tendency of β-Lactoglobulins://doi.org/10.1016/j.jiec.2018.04.027