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Adsorption-based Synthesis of Magnetically Responsive and Interfacially-Active Composite Nano Particles for Dewatering of Water-in-Diluted Bitumen Emulsions Chen Liang, Xiao He, Qingxia Liu, and Zhenghe Xu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01187 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on July 9, 2018
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Adsorption-based Synthesis of Magnetically Responsive and Interfacially-Active Composite Nano Particles for Dewatering of Water-in-Diluted Bitumen Emulsions
Chen Liang†, ǂ, Xiao He†, ǂ, Qingxia Liu† and Zhenghe Xu†, §
†
Department of Chemical and Materials Engineering and Department of Mechanical Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada
§
Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China
KEYWORDS Carboxymethyl cellulose, ethylcellulose, magnetic separation, oil-water separation, green chemistry ABSTRACT
Magnetically-responsive and interfacially-active composite nano particles comprising an inner magnetic core and an outer layer of interfacially-active material were prepared by first priming iron oxide (Fe3O4) nano particles with sodium carboxymethyl cellulose (CMC), followed by the adsorption of ethyl cellulose (EC) coating on the CMC-primed nano particles. In contrast to the previous preparation methods, synthesizing magnetically-responsive and interfacially-active composite nano particles by sequential adsorption of contrasting cellulosic materials without unnecessary derivatization reactions is much simpler and more energy efficient while generating less waste. The resulting composite nano particles are interfacially active and thus can be effectively partitioned at the oil-water interface. Once the nano particles attached to the interface, 1 ACS Paragon Plus Environment
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multiphase materials will be magnetically-tagged and effectively manipulated or isolated by an applied magnetic field. The sequential adsorption of CMC and EC on iron oxide surface was investigated using quartz crystal microbalance with dissipation monitoring (QCM-D). The adsorption of CMC on bare iron oxide surface can not only enhance the subsequent adsorption of EC on CMC-primed iron oxide surface, but also drastically improved colloidal stability of iron oxide dispersions. Quick phase separation of emulsions, oily rag layers and sludge was achieved by applying the magnetically-responsive and interfacially-active composite nano particles.
GRAPHIC ABSTRACT:
SYNPOSIS: Simple synthesis method for magnetic and interfacially-active composite particles based on cellulose without generating unnecessary chemical waste 2 ACS Paragon Plus Environment
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INTRODUCTION Formation of emulsion droplets, rag layers and sludge is often inevitable during various stages of materials processing for desired properties and petroleum production due to the immiscibility of co-produced water with oil. And the salt contained in dispersed aqueous phases of emulsions can corrode industrial equipment and poison catalyst during oil refinery process. So, strategies for breaking emulsions are required as most industrial processes inevitably create such stable emulsions when multiple phases are present. Traditional methods such as heating and addition of chemicals are not energy saving and environmentally friendly [1]. Therefore, economical and effective methods for phase separation in oilsands industry are necessary
[2]
. Magnetic
separation/breaking of biphasic material is possible using magnetically-responsive and interfacially-active particles that readily attach to the interface of two immiscible liquids [3-6]. The magnetic particles partitioned at the interface impart magnetic susceptibility to the tagged droplets and/or particles as such that the emulsified droplets, rag layer particles and sludge are subsequently concentrated or broken under an applied magnetic field. Compared with the traditional demulsification methods, interfacially-active nano particles grafted with nontoxic CMC and EC are environmentally friendly and can demulsify emulsions more effectively [4,5], with the feature of recycling to reduce the consumption of once-through chemicals. Superparamagnetic nano particles are prepared by co-precipitation, thermal decomposition, hydrothermal synthesis or from microemulsions [7,8,12]. Nano particles are typically more reactive than their bulk counterparts due to high specific surface area, and therefore require either protection or stabilization to prevent undesirable adsorption and reactions. Common materials used to modify nano particles for this purpose include surfactants, polymers, noble metals, silica and carbon materials
[7,8]
. The type of materials coated on the surface decides the variety of 3 ACS Paragon Plus Environment
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materials that can be used for the subsequent modification of the surface, as the materials to be coated should have the ability to bind or graft onto the materials that are already coated on the surface. Cellulose is a natural and degradable polymer consisting of repeating anhydroglucose units, which is generally used as a structural material. The abundance of intra- and inter-molecular hydrogen bonds is believed to be the main cause of limited solubility of hydrophilic cellulose in water, although the hydrophobic interaction may also be a significant contributor [9]. To produce materials with unique properties and functionalities, cellulose containing three hydroxyl groups in a unit is often chemically modified to produce derivatives such as cellulose ether and/or cellulose ester which forms one of the basis for the current approach of preparing magnetically responsive and interfacially active composite nano particles. The surface of iron oxide, more specifically magnetite (Fe3O4) particles, acquires surface charges in an aqueous environment. Functional groups known to bind chemically to the surface of magnetite particles include phosphates, sulphates and carboxylates
[7]
. Carbohydrates such as
starch and dextran on the other hand are used as dispersants of hematite in reverse flotation of silica using cationic collectors
[10]
. Carboxymethyl cellulose (CMC) is therefore anticipated to
adsorb on iron oxide nano particles due to its cellulosic structure and the presence of multiple anionic carboxylic acid functional groups, which is investigated by QCM-D studies [11]. In a previous study, ethylcellulose (EC) was found to be an effective demulsifier for breaking water-in-diluted bitumen emulsions
[11,13-16]
. EC-grafted magnetic nano particles prepared by
chemical functionalization showed an enhanced removal of emulsified water droplets from oil by promoting droplet coalescence under external magnetic field. Furthermore, the EC-grafted magnetic nano particles could be readily separated from the multiphase mixture using a magnet 4 ACS Paragon Plus Environment
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and subsequently recycled after cleaning. However, these particles were prepared using complex synthesis procedures, requiring several different chemical reactions in different solvents, as shown in Figure 1A. The synthesis of the interfacially-active magnetic particles requires comparably complex chemical derivation and surface modification. Specifically, the iron oxide nano particles are first coated with a thin layer of silica, followed by chemical reactions to graft interfacially-active substances such as polymeric demulsifiers [17]. In this paper, we demonstrate a simple and environmentally friendly method of producing magnetically-responsive and interfacially-active particles by sequential adsorption of contrasting cellulosic materials of carboxymethyl cellulose (CMC) and ethylcellulose (EC), as shown in Figure 1B. Using these two different cellulosic materials with opposite hydrophobicity to make iron oxide nano particles interfacially active does not require chemical derivatization and generates little waste [18]. This kind of nano particles could be used in petroleum industry for enhancing oil-water phase separation, removing rag layer and petroleum sludge from W/O emulsions.
EXPERIMENTAL SECTION Materials. Sodium carboxymethyl cellulose (Sigma-Aldrich; molecular weight: 250 000 g/mol; degree substitution: 0.7), ethylcellulose (Sigma-Aldrich; ethoxy content: 42%), iron oxide nano particles (MAG or Fe3O4; Sigma-Aldrich; < 50 nm), toluene (Fischer Scientific; ACS grade), acetone (Fischer Scientific; ACS grade), methanol (Fischer Scientific; ACS grade), ethanol (Commercial Alcohols; 99%), 2-propanol (Fischer Scientific; ACS grade) were used as received without further purification. Both vacuum distillation tower feed bitumen and plant recycle process water were provided by Syncrude Canada Ltd. and used to prepare process water-in5 ACS Paragon Plus Environment
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diluted bitumen emulsions. For better comparison, the pH of the process water was adjusted before emulsification to 8.9 which is the same as the pH of the process water used in the previous studies [4,13]. Deionized water (>18.0 MW/cm) used in the other types of experiments was prepared using Thermo Fischer Barnstead Nanopure ultrapure water purification system.
Figure 1 – Schematics of a magnetically-responsive and interfacially-active composite nano particle prepared from iron oxide (Fe3O4) using two different approaches for separation of biphasic materials: A) chemical functionalization of amine-grafted and silica-coated magnetic nano particles using chemically-activated ethylcellulose (EC);1 or B) sequential adsorption of carboxymethyl cellulose (CMC) and ethylcellulose (EC) onto magnetic nano particles proposed in this study. 6 ACS Paragon Plus Environment
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Preparation of Magnetically Responsive and Interfacially-Active Composite Nano Particles. Magnetic iron oxide (Fe3O4) nano particles were first washed with acetone and dried under nitrogen flow. Washed particles were dispersed in the aqueous solution using an ultrasonic dismembrator (Fisher Scientific Model 500). A dilute (1.0 wt%) CMC aqueous solution was prepared and added to the prepared magnetic nano particle dispersions. The mixture was placed in an ultrasonic bath for 30 min to prime magnetic nano particles with CMC. The CMC-primed magnetic nano particles were washed three times with deionized water and subsequently three times with ethanol. After each washing, the particles were collected using a strong permanent magnet. Magnetic nano particles primed with CMC were dispersed in a 1.0 wt% EC in toluene solution and then placed in the ultrasonic bath for another 30 min. EC-coated magnetic particles were washed three times with toluene and three times with ethanol. After each washing, particles were collected using the permanent magnet. Recovered particles were dried in a vacuum oven at 105°C. To prepare suspensions of the synthesized nano particles for characterization or use in separation of biphasic systems, the particles were first wetted by a small amount of methanol and subsequently diluted with deionized water. The stable dispersion was achieved first using an ultrasonic dismembrator with amplification of 35% for 15 min and subsequently an ultrasonic bath for 20 min. A small drop of the dispersion was transferred to a 1 mM KCl aqueous solution for zeta-potential measurement at ambient temperature. Characterization. Zeta-potential of solid particles was measured using the Malvern Nano Zetasizer. The infrared spectra of celluloses and synthesized particles were measured using FTIR spectrometer (Cary 670 – Agilent technologies) with an attenuated total reflection (ATR) accessory. The spectra over a wavenumber range from 4000 to 550 cm-1 were collected at a 7 ACS Paragon Plus Environment
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spectral resolution of 4 cm-1. FE-SEM was operated at 5 kV with a working distance of 6 mm to determine the size and morphology of nano particles. A thermo-gravimetric analyzer (Q200, TA Instruments) was used to determine the amount of cellulose materials coated on the solid particles by increasing the temperature from room temperature to 800 ºC at a heating rate of 20 ºC/min in a N2 atmosphere. Water content of the treated emulsions was determined by KarlFischer titration using an automatic coulometric titrator (G.R. Scientific Cou-Lo 2000). Critical Surface Tension. To confirm the biwettable nature of the synthesized nano particles, the critical surface tension of the particle was determined by film flotation experiments using methanol in water solutions [19], with the surface tension of the solution ranging from 22.5 mN/m for pure methanol to 72.8 mN/m for pure water. Solutions containing different proportions of methanol and water were prepared to adjust the surface tension of solutions. A known quantity of solid particles was placed on the top of the solutions containing different proportions of methanol and water. After 30 s, any particles remaining on the surface of the solution were physically separated and dried overnight in a vacuum oven at 105°C under reduced pressure. The critical surface tension of synthesized particles was determined when half of the particles remained on the surface of the solution. Quartz Crystal Microbalance with Dissipation Monitoring. Iron oxide (Fe3O4)-coated sensors were purchased from Q-Sense, whose composition was determined using X-ray fluorescence spectroscopy (XRF). A 1.0 wt% CMC aqueous solution and a 1.0 wt% EC in toluene solution were prepared separately. The pH of the aqueous CMC solution was adjusted to 5.5, at which carboxylic acid is deprotonated (ionized) while iron oxide surface is positively charged. The prepared solutions flowed sequentially over the sensor surface at 0.150 ml/s flow rate, while the
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resonance frequency and dissipation were monitored to determine the adsorption of CMC and EC on Fe3O4 surfaces. Separation of Solids from Oil Sand Ore. As an example of applying the synthesized interfacially active and magnetically responsive nano particles to separation of water from waterin-oil emulsions containing fine solids as encountered in crude oil production, fine solids from the heaviest crude oil formations (oil sands) were used as an extreme case of separation difficulties. In this case, 100 g of oil sands sample was defrosted, manually broken up and washed with a mixture of toluene/2-propanol. Mineral particles of sizes greater than approximately 5 mm were removed using a sieve. The remaining suspension of under size particles was placed in an ultrasonic bath for 30 min. Solids were further separated based on their sizes using standard mesh sieves and additional 2-propanol. Particles obtained were washed with toluene, separated using a centrifuge and dried in an oven at 105°C under reduced pressure. Contact Angle Measurement. The static contact angle of water droplets was measured using Krüss DSA instrument. Small water droplets (3 – 4 mm in diameter) held on the tip of a micro syringe needle were brought in contact with iron oxide sensor surfaces. Upon the contact with the sensor surface, the water droplet detached from the needle and spread on the sensor surface. The contact angle of the water droplet on the sensor surface was measured through the water phase immediately after the three phase contact line stopped moving and monitored for 100 s. The contact angle measured as such represents the advancing equilibrium contact angle or static advancing contact angle. Interfacial Tension Measurement. The interfacial tension of water/toluene interfaces in the presence of different nano particles (MAG, MAG-CMC and MAG-CMC-EC) was measured by pendant drop method using a Theta Optical Tensiometer (T200 Biolin Scitific). Magnetic nano 9 ACS Paragon Plus Environment
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particles were first dispersed in toluene with a concentration of (0.5 mg/ml) using an ultrasonic bath for 15 min. Then, an oil droplet containing dispersed particles with volume around 25 µL was generated and suspended on the tip of the micro syringe needle in water phase. The toluenewater interfacial tension was measured at room temperature once oil droplets contacted with water phase. The measurement lasted 300 s to determine dynamic toluene-water interfacial tension. Characterization of Particle Magnetization. The magnetization of the bare and synthesized iron oxide particles was determined using a Quantum Design PPMS magnetometer. The measurement was conducted at room temperature with a magnetization field strength cycling from 0 Tesla to 4 Tesla, and then decreasing to - 4 Tesla and increasing back to 0 Tesla. Preparation of Biphasic Material. To test versatility of our synthesized MAG-CMC-EC nano particles for breaking water-in-oil petroleum emulsions, three different types of emulsions were prepared as follows. The water content in the prepared emulsions stabilized by fine solids from the oil sands ore was accurately determined by Coulometric Karl-Fisher titration. 1. The first type of water-in-mineral oil emulsion containing 1 wt% fine solids recovered from the oil sand ore with particle sizes less than 44 µm, 5 wt% process water and 94 wt% mineral oil was prepared using a high-speed homogenizer operated at 10,000 rpm for 30 s. The solids were first dispersed in mineral oil, assisted by ultra-sonication in an ultrasonic bath for 5 min prior to emulsion preparation. The emulsions were prepared for investigating demulsification of MAGCMC-EC nano particles in treating W/O Pickering emulsions stabilized by fine solids. 2. The second emulsion sample was prepared using the same procedures as described above, but contained 5 wt% solids of sizes between 44 – 90 µm, 85 wt% mineral oil and 10 wt% process 10 ACS Paragon Plus Environment
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water. The emulsions prepared as such contained densely packed sludge (rag layer) that allowed us to test the ability of MAG-CMC-EC on treating oil sludge. 3. The third type of emulsions was prepared at an oil to water mass ratio of 1. Two concentrations of solids were used in preparation of emulsions: 0.5 wt% fine solids of sizes less than 44 µm and 5.0 wt% coarse solids of sizes between 44 – 90 µm. The solids isolated from the oil sands ore were first dispersed in naphtha-diluted bitumen at the naphtha to bitumen mass ratio of 0.65, assisted by ultra-sonication in an ultrasonic bath for 5 min. An equal mass of water in reference to the oil was added to the prepared solid-in oil suspension and the biphasic mixture was mechanically agitated for 30 s to form emulsions for dewatering tests by MAG-CMC-EC nano particles. This set of emulsions were much closer to the real water-in-diluted bitumen emulsions encountered in oil sands industry and represent the worst scenario of petroleum emulsions. Preparation and Demulsification of Process Water-in-diluted Bitumen Emulsions. The process water-in-diluted bitumen emulsions were prepared with process water and naphthadiluted bitumen. For naphtha-diluted bitumen, the mass ratio of naphtha/bitumen was 0.65. The naphtha-diluted bitumen was first shaken in a mechanical shaker for 4 h at 200 cycles/min. Using a homogenizer (PowerGen homogenizer, 125 W), 2.2 g process water was then well mixed with 42 g diluted bitumen at 30,000 rpm for 3 min. The process water-in-diluted bitumen emulsions had a water content of 5 wt% which was measured using an automatic coulometric titrator (G.R. Scientific Cou-Lo 2000). For demulsification, 150 mg MAG-CMC-EC nano particles were first dispersed in proper amount of naphtha by ultrasonic bath for 15 min and then mixed using Vortex Mixer (Thermo Fisher Scientific, MAXI MIX Plus) with 10 g process water-in-diluted
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bitumen emulsions for 3 min. A hand magnet was then placed on the bottom of the vial for 10 min to remove the magnetic nanoparticle-tagged water droplets from the emulsions. Separation of W/O emulsions and petroleum sludge by MAG-CMC-EC nano particles. In contrast to composite absorbent particles made from CMC and EC as reported previously [17], water-absorption by MAG-CMC-EC nano particles was limited to less than 1 wt% of its dry mass, as designed due to the low CMC content of coated particles and the shield of coated EC layer. Our objective is to laden magnetically responsive and interfacially active nano particles at oil-water interface due to particles’ interfacial activity and hence to achieve effective removal of water droplets by magnetic separation. Separation of multiphasic material using magneticallyresponsive and interfacially-active MAG-CMC-EC nano particles was demonstrated using different oil-water systems, including a solids-stabilized water-in-mineral oil emulsion and a high-solid content mineral oil sludge. In this case, a water-in-mineral oil emulsion (25 ml total volume of 5 wt.% water) stabilized by 1 wt% fine solids recovered from an oil sand ore was prepared.
RESULT AND DISCUSSION Field Emission Scanning Electron Microscopy (FE-SEM) Imaging. FE-SEM micrographs of bare iron oxide particles, MAG-CMC and MAG-CMC-EC nano particles are shown in Figure 2. As shown in Figure 2 A, the size of bare iron oxide particles is less than 100 nm and severe particle aggregation is observed. Compared with bare iron oxide, aggregation of surfacemodified MAG-CMC particles shown in Figure 2 B is reduced significantly, indicating a successful surface modification of original iron oxide nano particles by CMC adsorption. MAGCMC-EC aggregates shown in Figure 2 C are much smaller than MAG-CMC aggregates shown 12 ACS Paragon Plus Environment
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in Figure 2C, demonstrating the successful coating of EC on MAG-CMC nano particles. MAGCMC-EC particles are more difficult to aggregate because highly soluble EC in naphtha on the particle surfaces induces steric repulsion and hence prevents these particles from aggregation in organic solvent.
Figure 2. Field-emission scanning electron microscopy micrographs of A) bare iron oxide particles, B) MAG-CMC particles and C) MAG-CMC-EC particles at 20,000x amplification, showing much less aggregation of MAG-CMC-EC particles than MAG-CMC particles and bare iron oxide particles. The scale bar is 200 nm. Fourier Transform Infrared (FTIR) Spectroscopy. As shown in Figure 3 A, the spectrum of iron oxide nano particles (MAG) has distinct bands at 3424 cm-1, 632 cm-1 and 602 cm-1. The band at 3424 cm-1 is attributed to the stretching vibrations of -OH bonds on iron oxide surfaces and bands at 632 cm-1 and 602 cm-1 are resulted from the vibration of Fe-O bonds which were also observed in the spectrum of MAG-CMC and MAG-CMC-EC, as shown in Figure 3B. The band at 1636 cm-1 indicates the presence of H2O in the sample. For the spectrum of CMC, the broad band around 3361 cm-1 represents the stretching vibration of hydrogen-bonded –OH groups, while the characteristic absorption band at 2882 cm-1 is related to stretching of C-H bonds in CH2 groups. The strong peak at 1591 cm-1 is attributed to stretching of carboxyl groups. The bands at 1415 cm-1 and 1323 cm-1 are assigned to in-plane stretching vibration of –OH 13 ACS Paragon Plus Environment
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bonds and stretching vibration of C-H bonds of CMC, respectively. The strong band at 1022 cm-1 stands for the C-O stretching vibration of polysaccharide skeleton [20].
Figure 3. (A) FTIR spectra of MAG-CMC-EC, MAG-CMC, MAG, EC and CMC. (B)A narrow scan about the FTIR spectra of MAG-CMC-EC, MAG-CMC and MAG confirming the Fe-O bands and (C) A narrow scan about the FTIR of MAG-CMC-EC, MAG-CMC and MAG confirming the coating of EC and CMC. After coating of CMC onto iron oxide surface (MAG-CMC), the presence of weak bands at 1415 cm-1 and 1323 cm-1 suggests the successful coating of CMC, as shown in Figure 3C. It is interesting to note that weak bands at 1415 cm-1 and 1323 cm-1 are still present while strong bands at 1591 cm-1 disappeared in the spectrum of MAG-CMC, accompanied by the appearance of a band at 1630 cm-1. Such spectral change is attributed to the binding of carboxyl groups in CMC with iron oxide surfaces. Compared with the original bands of CMC, the corresponding 14 ACS Paragon Plus Environment
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bands on MAG-CMC are weaker due to the adsorption of only a small amount of CMC on iron oxide, which is shown later in the QCM-D and TGA measurements. For EC, the strong peaks at 2974 cm-1 and 2872 cm-1 represent stretching vibrations of C-H bonds and peak at 1375 cm-1 is attributed to bending vibration of C-H bonds. After coating of EC onto MAG-CMC nano particles, the weak bands of 2974 cm-1, 2872 cm-1 and 1375 cm-1 were observed, suggesting the successful coating of EC on MAG-CMC. Compared with MAG-CMC, stronger peaks between 1109 cm-1 and 1070 cm-1 induced by ring stretching indicate more ring-structured polymers on iron oxide particles, which is further investigated and confirmed in the QCM-D and TGA experiments. On the other hand, the shift of band at 1631 cm-1 from –COO- on MAG-CMC to 1605 cm-1 on MAG-CMC-EC indicates the binding of EC with CMC. CMC/EC adsorption studied by QCM-D. As the basis of our novel synthesis, the sequential adsorption of CMC and EC was first investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). A change in the resonance frequency of the QCM-D sensor occurs when adsorption or desorption of a material onto or from the sensor surface occurs. The dissipation factor of the QCM-D sensor provides the information on viscoelastic properties of the adsorbed film. QCM-D is therefore a powerful tool to provide in situ and real time information on CMC/EC adsorption on desired sensor surfaces from solutions of suitable physicochemical properties. In this study, sensors coated with a thin layer of iron oxide films were sequentially exposed to CMC and/or EC solutions. The QCM-D results of CMC adsorption by flowing a 1.0 wt% aqueous CMC solution are shown in Figure 4. Upon switching to 1.0 wt% CMC aqueous solution at point A after establishing a stable baseline by flowing pure DI water as background solution, a significant decrease in frequency by 20 Hz 15 ACS Paragon Plus Environment
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was observed, accompanied by an increase in dissipation by 24 x10-6. After reaching the steady state of CMC adsorption, washing of the adsorbed CMC by deionized water as indicated by point B led to a significant increase in frequency by 17 Hz, accompanied by a significant decrease in dissipation to 0.35x10-6, leading to an overall decrease in frequency by only 3 Hz. The results suggest a limited irreversible adsorption of CMC on the Fe3O4-coated QCM-D sensor surface, most likely as a result of water being a good solvent of CMC that led to very thin layer of chemisorbed carbolic groups on active sites of iron oxides. The thickness and mass surface coverage of the layer on iron oxide surface shown in Table 1 indicate that there is just a single chemisorbed layer of carboxylic layer, which will be confirmed later on.
Figure 4. Change in resonance frequency and dissipation of QCM-D sensor as CMC adsorbs onto an iron oxide-coated sensor surface by: (A) flowing a 1.0 wt% CMC in water solution and (B) washed with deionized water.
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In contrast, direct adsorption of EC in toluene on Fe3O4-coated sensor surface resulted in a decrease in frequency by 139 Hz and an increase in dissipation by 32x10-6, as shown in Figure 5. The results indicate much more EC adsorbed from toluene solutions on to iron oxide surfaces. Interestingly, washing with solvent led to a slight further decrease in frequency by 9 Hz, accompanied by a slight increase in dissipation to 32x10-6. Such slight decrease in frequency could be attributed to the trap of more toluene in adsorbed EC layer and adsorbed EC layer in pure toluene will be more swollen, which would lead to an increase in dissipation as observed. This contrast result clearly shows stronger interactions of iron oxide surfaces with EC in toluene than with CMC in water. The amount of cellulosic materials adsorbed on iron oxide-coated QCM-D sensor surfaces (i.e., EC in toluene and CMC in water) is calculated by applying the viscoelastic (Voigt) model to the frequency change of third overtones and the results are summarized in Table 1[21-23].
Figure 5. Change in resonance frequency and dissipation factor of QCM-D sensor as EC adsorbs directly on iron oxide-coated sensor surface by flowing a 1.0 wt% EC in toluene solution (A) and washed with toluene (B).
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The results in Table 1 show only a small amount (56±9 ng/cm2) of CMC adsorbed on iron oxide surfaces from its aqueous solutions, which is attributed to strong hydration of CMC in water and hence weak intermolecular forces among CMC molecules in aqueous solutions to prevent multilayer adsorption. Such CMC adsorption made the iron oxide sensor surface more hydrophilic as anticipated, decreasing the contact angle from original 5 ± 1° to complete wetting (less than 1°) after adsorption of 56±9 ng/cm2 CMC. Nevertheless, the strong attachment of CMC once adsorbed, although in small amount, is confirmed by remaining CMC attached to iron oxide surfaces due to chemical binding of carboxylic acid with iron, which was reported by Pensini et al.
[27]
A much stronger adsorption (2065±233 ng/cm2) of EC on iron oxide surface
from its toluene solution was observed. Hydroxyl groups on EC are known to adsorb on iron oxide surface through multiple hydrogen bonds between EC and iron oxide surfaces. More importantly, intermolecular hydrogen bonds among hydroxyl groups in organic solvent could lead to multilayer adsorption. Such adsorption of EC on the iron oxide sensor surface increased the contact angle from original 5.0 ± 1.0° to 60 ± 3.0° as anticipated for coatings of water insoluble EC on a hydrophilic iron oxide surface. Despite the fact that only a small amount of CMC adsorbed on iron oxide surfaces (A to C in Figure 6), the adsorption of EC increased from 2100±230 ng/cm2 on bare iron oxide sensor surfaces (A to B in Figure 5) to 2800±240 ng/cm2 on CMC-primed iron oxide sensor surface (C to D in Figure 6), representing a 33% increase in EC adsorption. More interestingly, the layer thickness of EC remained the same for both cases as shown in Table 1, suggesting a more collapsed configuration of EC on CMC primed iron oxide surfaces than on bare iron oxide surfaces. This finding indicates a stronger binding of EC with CMC than with bare iron oxide surfaces. It is interesting to note a corresponding increase in contact angle from a complete 18 ACS Paragon Plus Environment
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Energy & Fuels
wetting (less than 1.0º) to a contact angle of 59 ± 2.0°, upon adsorption of 2800±240 ng/cm2 EC on CMC primed iron oxide surfaces. Table 1. Amount of cellulosic material adsorbed on an iron oxide coated QCM-D sensor surface, determined using quartz crystal microbalance with dissipation monitoring (QCM-D). Sensor surface Adsorption sequence
Surface coverage** Adsorbed layer Contact Angle*** (ng/cm2) thickness** (nm) (degree)
EC in toluene
2100 ± 230
23 ± 2.0
60 ± 3.0
CMC in DI water
44 ± 14
< 1.0
