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Research Papers

# Methods for Separation of Copper Oxide Nanoparticles From Colloidal Suspension in DodecaneOPEN ACCESS

[+] Author and Article Information
Mohammed H. Sheikh

Department of Aerospace
Engineering and Mechanics,
The University of Alabama,
Tuscaloosa, AL 35487-0280
e-mail: mdharoon85@gmail.com

Mem. ASME
Department of Aerospace
Engineering and Mechanics,
The University of Alabama,
Tuscaloosa, AL 35487-0280
e-mail: msharif@eng.ua.edu

1Corresponding author.

Manuscript received December 22, 2013; final manuscript received March 12, 2014; published online April 4, 2014. Assoc. Editor: Jung-Chih Chiao.

J. Nanotechnol. Eng. Med 5(1), 011001 (Apr 04, 2014) (9 pages) Paper No: NANO-13-1088; doi: 10.1115/1.4027219 History: Received December 22, 2013; Revised March 12, 2014

## Abstract

Phase change materials (PCM) are used in many energy storage applications. Energy is stored (latent heat of fusion) by melting the PCM and is released during resolidification. Dispersing highly conductive nanoparticles into the PCM enhances the effective thermal conductivity of the PCM, which in turn significantly improves the energy storage capability of the PCM. The resulting colloidal mixture with the nanoparticles in suspension is referred to as nanostructure enhanced phase change materials (NEPCM). A commonly used PCM for energy storage application is the family of paraffin (CnH2n+2). Mixing copper oxide (CuO) nanoparticles in the paraffin produces an effective and highly efficient NEPCM for energy storage. However, after long term application cycles, the efficiency of the NEPCM may deteriorate and it may need replacement with fresh supply. Disposal of the used NEPCM containing the nanoparticles is a matter of concern. Used NEPCM containing nanoparticles cannot be discarded directly into the environment because of various short term health hazards for humans and all living beings and unidentified long term environmental and health hazards due to nanoparticles. This problem will be considerable when widespread use of NEPCM will be practiced. It is thus important to develop technologies to separate the nanoparticles before the disposal of the NEPCM. The primary objective of this research work is to develop methods for the separation and reclamation of the nanoparticles from the NEPCM before its disposal. The goal is to find, design, test, and evaluate separation methods which are simple, safe, and economical. The specific NEPCM considered in this study is a colloidal mixture of dodecane (C12H26) and CuO nanoparticles (1–5% mass fraction and 5–15 nm size distribution). The nanoparticles are coated with a surfactant or stabilizing ligands for suspension stability in the mixture for a long period of time. Various methods for separating the nanoparticles from the NEPCM are explored. The identified methods include: (i) distillation under atmospheric and reduced pressure, (ii) mixing with alcohol mixture solvent, and (iii) high speed centrifugation. These different nanoparticle separation methods have been pursued and tested, and the results are analyzed and presented in this article.

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## Introduction

Phase change materials (PCM) are used in thermal energy storage applications where energy is stored (as latent heat of fusion) by melting the PCM and is released during solidification [1-3]. Dispersing highly conductive nanoparticles into the PCM enhances the effective thermal conductivity of the PCM, which in turn significantly improves the energy storage capability [4-6]. The resulting colloidal mixture with the nanoparticles in suspension is referred to as nanostructure enhanced phase change materials (NEPCM). A commonly used PCM for energy storage application is the family of paraffin (CnH2n+2). Mixing copper oxide (CuO) nanoparticles (treated with surfactants for stability) in paraffin produces a stable and highly conductive NEPCM for energy storage. However, after long term application cycles, the functionality of the NEPCM deteriorates and it is required to replace it with fresh supply. Disposal of the used NEPCM containing the nanoparticles is a matter of great concern as it cannot be discarded directly into the environment because of the short and long term environmental and health hazards [7-10]. Due to the widespread application potential of NEPCM, it is very important to develop proper technology to separate the nanoparticles before the disposal of the NEPCM. This is the motivation behind the present study.

Separation of particles/impurities from a solid–liquid mixture has been an age old problem. Various separation methods have been developed for various types of applications over the years. The separation method for a specific class of solid–liquid mixture depends on various factors such as the size of the particles, type/class of the liquid, mass/volume fraction of the particles in the mixture, and physical properties of the base liquid and the particles. Filtration, distillation, centrifugation, electrophoresis, magnetic separation, chromatography, and chemical methods are among the most widely used methods to separate particulate matter from liquid–solid mixture. Every separation method has various operating requirements and the most critical ones being physical, chemical, and electrical properties of entities to be separated and the mixture to be processed [11-16]. By studying and analyzing such properties of NEPCM, various methods have been attempted. In this work, the following methods: distillation, centrifugation, and use of alcohol mixture solvents, which have produced successful results, have been reported along with brief description of the methods followed by analysis of the results and conclusion.

## NEPCM Configuration

In this research, dodecane (C12H26) was used as the PCM base fluid and CuO nanoparticles were employed to enhance thermal conductivity of dodecane. Ninety-nine percent pure technical grade dodecane was used to obtain the NEPCM. Dodecane is a colorless liquid with a slight gasolinelike odor, has density of 0.753 g/cm3, and boiling point of 216 °C [17]. The copper oxide nanoparticles are of approximately spherical shape whose size varies from 5 to 15 nm with majority of them being about 9 nm. The density of CuO is 6.31 g/cm3 and they appear black in color. The nanoparticles used in this research was manufactured and provided by the Chemistry Department of the Auburn University [18].

Mixing the bare nanoparticles in the base fluid does not create a stable colloidal suspension. The particles coagulate together and quickly precipitate under the action of gravity because of the much higher density of the nanoparticles compared to that of the base fluid. In order to disperse the nanoparticles evenly in the base fluid and to achieve a stable nano-colloid, it is required to prevent agglomeration of particles using suitable treatment of the particles. As the CuO particles do not carry any charge leading to electrically and magnetically neutral buoyant behavior, a viable method is to restrict the agglomeration by placing a barrier between two approaching particles. This is achieved by coating the nanoparticle surface with stabilizing surfactant also known as ligands which act like a cushion among the particles. Sodium oleate (C18H33NaO2) is used as the stabilizer [18]. This forms numerous ligands on the particle surface which has a thread/string like structure, schematically shown in Fig. 1. The resulting coated nanoparticles have average mass composition of 69 ± 1.4% copper oxide and 31 ± 1.4% sodium oleate. The polar head of these ligands associates with particle surface, whereas the nonpolar tail interacts with the base fluid. The ligands on particle surface provide a physical barrier (cushion) which prevents particle to particle contact and subsequent agglomeration. Furthermore, the stringlike ligands exert additional opposing viscous drag on the particles against gravitational precipitation. Thus, a highly stable colloidal mixture of the nanoparticles and base fluid is obtained where the particles remain in the desirable homogeneous suspension for a long time with no or negligible precipitation [18]. This form of stabilization is known as steric stabilization.

## Experimental Methods and Setup

###### Preparation of the NEPCM.

Nearly homogeneous NEPCM samples were prepared by dispersing desired amount of CuO nanoparticles into the base PCM, i.e., dodecane. The samples were prepared by following a mass fraction approach. In this approach, density of the nanoparticles and dodecane was used to find a required nanoparticle concentration (by mass) for a particular volume of NEPCM. The volume fraction, ϕvol, of CuO nanoparticles required for the preparation of NEPCM is calculated using law of mixtures given byDisplay Formula

(1)$φvol=wpρp/(wpρp+wbfρbf)$

where w and ρ are the weight and density and the subscripts p and bf are referred to nanoparticles and base fluid, respectively. In case the corresponding weight or mass fraction, ϕwt, are of interest, the following relation is used for conversion for a two component system:Display Formula

(2)$φwt=φvolρp/[φvolρp+(1-φvol)ρbf]$

The total mass of the particles, mp, required to be mixed in a certain volume of the base fluid, Vbf, to get the mass fraction, ϕwt, is calculated asDisplay Formula

(3)$mp=Vbfρbfφwt/(1-φwt)$

After determining the mass of the nanoparticles required, next step followed is dispersing this nanoparticle mass in required volume of dodecane, then heating and stirring vigorously at 60 °C on a hot-plate magnetic stirrer (SP131325Q, Thermo Fisher, Dubuque, IA), shown in Fig. 2, for 30 min to attain a stable colloidal suspension. This mixing in combination with heating produced a stabilized NEPCM, which appeared like a black ink and was used for further experiments. Using the oleate stabilized CuO nanoparticles, long term stability of colloidal suspensions in various hydrocarbons (hexane, octane, dodecane, and eicosane) was observed both qualitatively and quantitatively for mass fraction up to 20% corresponding to volume fraction of about 3% [18].

In this work, only dodecane was considered as the base fluid to prepare the NEPCM and to conduct different trials. Table 1 presents mass of nanoparticles required to mix in 100 ml of the base fluid for different mass concentrations.

###### Atmospheric Pressure Distillation.

Generally, distillation is used to separate one component liquid from a liquid mixture containing components of different volatilities or boiling points. In the simplest form of distillation, the liquid mixture is heated in a container until the liquid component with lower boiling point is vaporized and then subsequently condensed to yield the distillate [19-22]. The distillation process can also be used to separate suspended solid particles from a colloidal mixture. In the present context, the separation of the nanoparticles from the NEPCM was accomplished by evaporating the dodecane (base fluid) at 218 °C using a distillation process. In this study, a standard laboratory distillation unit was used to carry out the distillation process at atmospheric pressure. An energy meter measured the amount of energy consumed during each experiment. Figure 3(a) depicts the 100 ml NEPCM with a 1% by mass concentration of the CuO nanoparticles. The black color of the colloid is due to the presence of black CuO nanoparticles. Figure 3(b) shows the nanoparticle residue left in heating flask after completion of distillation. Nanoparticles were deposited in the form of a thin layer at the bottom of the flask.

###### Vacuum Distillation.

The boiling point of a liquid varies depending upon the surrounding environmental pressure. A liquid at high pressure has a higher boiling point than when the liquid is at atmospheric pressure. Conversely, at lower pressure or under vacuum, boiling point of liquid is lower than that at atmospheric pressure conditions. Distillation has long been criticized as slow and highly energy intensive process. Hence, in order to make it more energy efficient, distillations were performed at reduced pressure. To perform distillation under reduced pressure, a complete new setup was required, as the distillation unit for this purpose has to be airtight and of sufficient strength to withstand the applied vacuum. Hence, a new vacuum distillation unit with vacuum pump of 1/6 HP rating, 2.4 CFM free air displacement, and 10 Pa ultimate vacuum was used. All the standard procedures and safety measures were followed while conducting the trials. Figure 4 shows the vacuum distillation setup used to conduct trials.

###### Chemical Treatment of the NEPCM.

It is hypothesized that, if the NEPCM is mixed with another heavier liquid in which the NEPCM base fluid is soluble, then a portion of the base fluid will be dissolved into the heavier liquid and settle down at the bottom, leaving a more concentrated NEPCM layer on top. The concentrated top layer can then be separated out and further processed. Based on literature survey and consultation with the experts in chemistry, different heavier solvents were selected to verify the hypothesis. Initial attempts were made using methanol, ethanol, and mixture of methanol and propanol as solvents. Equal volume of NEPCM and solvent were added in a test tube and were shaken for 30 min at 450 rpm and 34 °C using a Controlled Environment Incubator Shaker unit, available at the University of Alabama Biology Department. Three samples of alcohols containing pure methanol, pure ethanol, and a 50–50% mixture (by volume) of propanol and methanol were tested. Out of three samples tested, the 50–50% mixture of propanol and methanol showed satisfactory results.

###### Centrifugation of the NEPCM.

Due to the density gradient between CuO nanoparticles (6.31 g/cm3) and base fluid dodecane (0.753 g/cm3), it is anticipated that centrifugation at high speeds would precipitate the nanoparticles. Initial trials were performed on a micro-centrifuge at speed up to 10,000 rpm, and no precipitation was observed for centrifugation duration of 30 min. Hence, in order to precipitate the nanoparticles out of the suspension, it was deemed necessary to apply larger centrifugal forces to overcome the stabilization effect. This force was applied using higher centrifugation speeds on a large size centrifugation machine. A SORVALL RC6 super-speed centrifuge floor model with maximum speed rating of 21,000 rpm which is equivalent to over 50,000 × gravity or relative centrifugal force (RCF) was used to conduct high speed trials in conjunction with SM-24 rotor which is fixed angle rotor. Initial trials were done for up to 6 h duration at 18,000 rpm which showed partial precipitation of nanoparticles indicating the RCF for this duration is not enough to overcome stabilization completely. Further trials were done using 0.5%, 1%, 2%, and 5% concentration (by mass) of the NEPCM for the same operating conditions of 18,000 rpm for 19.5 h, 24 h, and 48 h.

## Results and Discussion

###### Distillation.

Figure 5 shows distillate collected after atmospheric pressure and vacuum pressure distillation, respectively. Distillate collected resembles to pure colorless dodecane used to prepare the nano-colloid. As NEPCM exhibits black color due to presence of CuO nanoparticles, the colorless distillate indicates the absence of any nanoparticles.

Table 2 summarizes the data collected using the atmospheric pressure as well as vacuum distillation processes. A total of six trials were performed for different combinations of the NEPCM volume and nanoparticles concentration for atmospheric pressure distillation. The mass concentration of nanoparticles chosen was 0%, 1%, and 3%. A trend was observed for the time required and energy consumed in relation to the amount of NEPCM used. Also it was observed that for the same volume, the NEPCM with higher concentration of nanoparticles distilled faster than the NEPCM with lower nanoparticle concentration. This is mainly due to the fact that, higher the concentration of the CuO nanoparticles in the NEPCM, greater is the thermal conductivity of NEPCM and faster is the distillation. The vacuum distillation trials were conducted at 33.6 kPa vacuum. For a 100 ml volume of NEPCM, it was found that, as the amount of nanoparticles increased, the time required for vacuum distillation decreased which resulted in less power consumption for both conditions. This is consistent with the fact that high heat conducting nanoparticles present in NEPCM increases the rate of distillation. The final volume of distillate measured was found to be lower than the initial volume before distillation due to the removal of nanoparticles and loss of dodecane vapors.

Figure 6 presents the distillation data for atmospheric pressure and vacuum distillation processes on total energy and total time required basis. For vacuum distillation, the energy meter was connected to the distillation heater and the vacuum pump in series, so the energy meter readings were the total energy for heater and pump both. The total energy data obtained from the energy meter readings, however, include the heat losses from the distillation heater and operational losses in the vacuum pump. It is evident that vacuum distillation is much more efficient process to carry out distillation of NEPCM, as it consumes lower energy and takes lesser time than distillation at atmospheric pressure. It is found that for the same volume and nanoparticle concentration of NEPCM distilled, vacuum distillation consumes about 60% less energy as well as time. Though initial equipment cost for vacuum distillation setup is higher than that for atmospheric distillation, the operating cost for vacuum distillation is much lower. Thus, it can be concluded that for long term applications and higher volumes of the NEPCM to be processed, vacuum distillation is a better alternative.

To verify the nanoparticle removal efficiency of the distillation processes, Scanning Electron Microscope (SEM) was utilized. All samples were prepared on a standard nickel grid having carbon coating and were dried for 48 h before imaging. Figure 7 shows comparison of SEM images taken for the base fluid (pure dodecane), NEPCM before distillation, distillate at atmospheric pressure, and distillate collected by vacuum distillation. Figure 7(a) shows the SEM image of the 99% pure dodecane, which has been used as a base fluid for the NEPCM preparation. The structure of carbon coating on the grid is clearly discernible from this image which shows no signs of any impurities present. Figure 7(b) shows the structure of the NEPCM containing 0.5% (by mass) concentration of the CuO nanoparticles. The lump of nanoparticles sticking to the carbon web structure is clearly visible in the image. As 0.5% by mass is a higher concentration especially after preparing SEM sample where it is dried which makes nanoparticles to agglomerate. These two images in Figs. 7(a) and 7(b) were compared with the SEM images of the distillate samples. Figure 7(c) shows the image of the distillate collected from the atmospheric pressure distillation, while Fig. 7(d) shows the image of the distillate collected from the vacuum distillation. No trace of the nanoparticle is detected in the distillate in these images asserting that the distillation results in complete separation of nanoparticles from the NEPCM.

###### Chemical Treatment of the NEPCM.

Figure 8 shows the results obtained using the alcohol mixture. During sample preparation and before shaking, it was found that due to high density, the colorless alcohol mixture sits at the bottom of the test tube occupying half of the filled tube length while the top half is filled by the black colored NEPCM. After shaking, it was observed that the volume of the black colored liquid part shrank while the volume of the colorless liquid part increased substantially. This is because a portion of the colorless dodecane (base fluid) in the NEPCM got dissolved into the colorless alcohol mixture and settled down at the bottom. Thus, the process leads to a concentrated NEPCM layer on top.

To obtain a quantitative data about the amount of dodecane dissolved using the procedure described above, 5 ml of NEPCM (containing 0.5% mass fraction of nanoparticles) was added to 5 ml of alcohol mixture and experiment was repeated. Figure 9 (top row) shows the results of first trial after shaking at 450 rpm for 30 min. The concentrated NEPCM on top after shaking was pipetted out and was found to be 3.75 ml in volume. Using this concentrated volume, the experiment was repeated again using another 5 ml of alcohol mixture and it was found that the NEPCM got concentrated to 2.5 ml as shown also in Fig. 9 (bottom row).

Though visible observations indicate some amount of dodecane has been mixed with alcohol and clear layer shown in Fig. 9 does not contain any nanoparticles, experimental evidences are required to support this claim. Hence, in order to confirm complete removal of nanoparticles after processing with alcohol mixture, pipetted out sample of the clear alcohol and dodecane solution from the bottom was analyzed using UV-visible spectrophotometer. UV-visible spectrometry was selected to perform the analysis over other methods like electron microscopy due to its simplicity, versatility, speed, accuracy, and cost-effectiveness. A Varian Cary 100 UV-Vis spectrophotometer was employed to determine absorption wavelengths. It is a double beam, recording spectrophotometer controlled by a computer operating under Windows 2000 and WinUV software. It hosts tungsten halogen as a visible light source and deuterium arc for ultra-violet light and has wavelength range from 190 to 900 nm. A 1 cm cell was used to feed the sample in the sample holder. A wavelength range of 300 nm to 800 nm was used to capture the data. UV-Vis scan rate was set to 600 nm/min and data interval was 1 nm.

Figure 10 shows UV-Vis spectrum of the collected sample, which is compared with baseline (dodecane) spectrum and a 0.01 wt. % NEPCM spectrum. Due to the presence of the nanoparticles, the NEPCM spectrum shows increasing absorbance values as the wavelength is varied from 800 nm to 300 nm, indicating the presence of light absorbing nanoparticles. For the sample spectrum (Run1), no variation of the absorbance with respect to the wavelength is observed. Also the sample absorbance spectrum resembles closely with pure dodecane spectrum which provides concrete evidence that sample is free from any nanoparticles after treatment with the alcohol mixture.

It can be concluded from these trials that every mixing and shaking step yields about 25% more concentrated nanofluid. The trial was done with a mixture of alcohol containing 50–50% of methanol and propanol. The concentrated NEPCM collected after a few cycles of mixing and shaking can be further processed by other separation methods such as distillation for complete separation of the nanoparticles with significant reduction in energy and time. The total energy consumed to run the shaker unit for each 30 min time period of shaking was estimated to be 0.6 kW h.

###### Centrifugation.

Figure 11 shows the results for two centrifugation trials, showing partial precipitation of the nanoparticles. It is observed that centrifugation could not remove all of the nanoparticles from the given sample volume, as all the centrifuged samples have reddish color indicating some of the nanoparticles are still in suspension. As the pure dodecane is colorless, a colored centrifuged sample indicates presence of nanoparticles. Since the centrifugal force is proportional to the particle mass, the larger particles are precipitated and the smaller ones remained trapped in suspension. The nanoparticles still in suspension after the centrifugation giving the reddish color to the centrifuged samples are of lower size in the nanoparticle size range of about 5–7 nm. Being light in weight, the exerted centrifugal force on them was lesser and they could not be precipitated. Figure 12 shows SEM image of un-precipitated particles after trial 1, which indicates these are particles of size less than 10 nm.

Centrifugation method is effective when the centrifugal force overcomes the steric stabilizing effect on the particles. The centrifugal force depends on the centrifuge speed and the mass of the particle, which, in turn depends on the density and size of the particles. Thus, for particles with given density at a given rotational speed, there is a minimum threshold value of the particle size below which the centrifugal method is not effective for particle precipitation. Based on the experiments conducted and data analyzed, it is found that, under the specified conditions, the applied centrifugal force was capable of separating nanoparticles of size larger than 10 nm. This is indicated in Fig. 12, which shows that particles of size of 5–9 nm still remain in suspension after the centrifugation. Ultra-high speed centrifuges capable of applying greater centrifugal force may bring higher yield by separating smaller size (<10 nm) particles as well. The wt. % of the nanoparticles is not a significant parameter for centrifugation and nanoparticle size alone governs the centrifugation effectiveness. This is also evident from particle removal efficiency values mentioned in Table 3 which shows for the same centrifugation duration, different wt. % yields almost same respective particle removal efficiency.

Similar trials were conducted for 0.5%, 1%, and 2 wt. % concentrations of 12 ml of the NEPCM for 24 h and 48 h at 18,000 rpm in order to investigate the effect of the duration of centrifugation. Figure 13(a) shows three samples (1, 2, and 3) of concentration 0.5%, 1%, and 2%, respectively, before centrifugation. After 24 h of centrifugation at 18,000 rpm, most of the nanoparticles were precipitated as shown in Fig. 13(b). At this point, 1 ml of centrifuged NEPCM sample was collected from each of the three samples for analysis and comparison purposes. Samples in Fig. 13(b) were centrifuged again for additional 24 h and after end of 48 h sample conditions are presented in Fig. 13(c). General visual observation of Fig.13(c) indicates that longer centrifugation duration of additional 24 h at 18,000 rpm results in no further precipitation of the nanoparticles because samples in Figs. 13(b) and 13(c) bear similar color intensity.

In order to quantify the particle removal efficiency of the centrifugation trials and to record the difference in concentration after 24 and 48 h of centrifugation, a concentration based analysis was done where the concentrations of the NEPCM before and after centrifugation were used to calculate efficiency of particle removal by centrifugation. The concentration of a particular NEPCM sample was known before centrifugation. To determine concentration of NEPCM after centrifugation, “Calibration Curve” or “Internal Standard” approach using UV-Vis photo-spectroscopy was employed. A calibration curve is a graph of concentration versus absorptivity for the NEPCM solution. To obtain a calibration curve, several samples of known (dilute) concentrations were prepared. The known concentration range was selected such that the unknown concentration of the NEPCM sample after centrifugation falls in the middle of this range. The color of centrifuged NEPCM of unknown concentration was used as reference to prepare the NEPCM samples of the known concentration bracket. The absorbance spectra were recorded using the UV-Vis spectrometer for each sample with known concentration as well as for the corresponding centrifuged sample of unknown concentration. The value of the maximum absorbance (Amax) of each of the spectrum curve and the corresponding wavelength (λmax) was determined from the record. The calibration curves for the maximum absorbance versus the known sample concentration are then plotted. Linear curve fits through the data points provided the calibration equation for the unknown concentration as a function of the known maximum absorbance. This was done to determine the post-centrifugation concentration of the 1.0 wt. % (sample 2 in Fig. 13) and 2.0 wt. % (sample 3 in Fig. 13) NEPCM after 24 h and 48 h of centrifugation. Thus, a total of 4 cases were analyzed to determine the concentration of the NEPCM after centrifugation. The absorbance spectra for two representative cases are shown in Figs. 14(a) and 14(b). Figure 14(a) shows the absorbance spectra curves to determine the concentration of 2 wt. % NEPCM subjected to 24 h of centrifugation at 18,000 rpm while Fig. 14(b) shows the absorbance spectra to determine the concentration of 1 wt. % NEPCM subjected to 48 h of centrifugation at 18,000 rpm. The corresponding linear fitted calibration curves for these cases are shown in Fig. 15.

Using the calibration curve, concentration (wt. %) of the NEPCM after centrifugation were determined and compared with initial concentration before centrifugation. Table 3 presents summary of data collected for two centrifugation samples. For the same centrifugation speed, in this case, 18,000 rpm equivalent to 40,173× gravity (which is the maximum attainable speed with the available equipment), 24 h period resulted in about 95% efficiency which remained almost same without any significant further improvement for additional 24 h centrifugation period. This is evident from the overall efficiency values presented in Table 3. Referring to Figs. 13(b) and 13(c), remaining nanoparticles are of lower size range (diameter < 10 nm), hence they need stronger centrifugation force to bring them out of suspension to make them precipitate and achieve separation. The primary hypothesis, applying same centrifugation force for longer duration should result in improved centrifugation efficiency, proved to be wrong. It may also be mentioned that the dilute NEPCM after centrifugation can be further processed by other methods for complete separation of the nanoparticles.

###### Comparison of the Nanoparticle Separation Methods.

Based on the experiments conducted and the analysis performed and presented here, separation methods have been compared on the basis of economical and functional parameters. Table 4 presents the comparison summary. The comparison has been done strictly referring to the equipment used and practices followed to conduct trials.

## Conclusions

Referring to the results presented here and the comparison of different methods presented in Table 4, the following conclusions can be drawn:

1. (1)As evident from the results presented, distillation at atmospheric pressure is one of highly efficient process to separate nanoparticles from NEPCM. For the setup used to conduct trials, the equipment cost was low and it did not require any material during operation other than the sample. Also it was able to handle moderate volumes of NEPCM to process, which resulted in high overall efficiency. For larger NEPCM volumes to process, distillation units on industrial scale are available and have been used successfully for other different applications. Hence, distillation at atmospheric pressure qualifies as one of the best method to separate nanoparticles from NEPCM. However, it has one inherent limitation of slow speed and comparatively high energy consumption.
2. (2)Vacuum distillation demands negative pressure setup which adds to the equipment cost. However, while conducting experiments, it was observed that operating costs in terms of energy consumed and time taken to process a given NEPCM sample was much lower (60% less) compared to distillation at atmospheric pressure. This mode of distillation also does not mandate any additional material requirements. Moderate volumes (about 1000 ml) of NEPCM can be easily processed using the lab scale unit utilized in this work. For higher NEPCM volume, industrial scale units can be utilized. Similar to distillation at atmospheric pressure, vacuum distillation also yields 100% efficiency.
3. (3)Chemical (alcohol) treatment was proved to be another efficient process to separate nanoparticles from NEPCM. This method does not demand any special setup as only test-tubes and beakers were used to conduct experiments. However, it requires a shaking unit capable of producing required momentum to attain desired result, which adds to the equipment cost. As only mixture of inexpensive alcohols is required, its material requirement cost is low. Presented trials were conducted to prove the functionality of the approach and record the data to provide grounds for further research. Hence, low volumes of NEPCM were processed. However, using appropriate equipment, this approach can be manipulated to process higher NEPCM volumes. Further processing, such as distillation, of the concentrated NEPCM collected after a few cycles of mixing and shaking, for complete removal of the nanoparticles, will bring substantial savings in cost (time and energy).
4. (4)Centrifugation is another potential method to achieve nanoparticles separation from NEPCM. Referring to the data presented in Table 4, it can be concluded that for the maximum relative centrifugation force used (40,173 × gravity), this method yields about 95% separation efficiency. This also clarifies that the NEPCM separation efficiency of centrifugation is a factor of centrifugal force (which depends on the particle density and size, and rotational speed) and not of duration of centrifugation or wt. % of the nanoparticles in the colloid. This method bears high equipment cost and does not demand any additional materials for NEPCM processing. Even though the trials were conducted using low volumes of NEPCM, advanced centrifuges offer high volume processing capabilities. One of the advantages of centrifugation over other methods is the preservation of the stabilizing ligands on the precipitated nanoparticles, which may be redispersed back, and hence centrifugation facilitates nanoparticle reclamation. Furthermore, the dilute NEPCM after centrifugation can be further processed by other methods for complete separation of the nanoparticles.

Further testing of several other nanoparticle separation methods, such as nanoparticle surfactant destabilization using chemicals, silica column chromatography, adsorbance of nanoparticles on silica particle surfaces, and filtration, are continuing and the results will be presented in forthcoming papers.

## Acknowledgements

Dr. German Mills, Chemistry Department, Auburn University, Dr. Jay Khodadadi Mechanical Engineering Department, Auburn University, and Dr. Hank Heath, Biology Department, University of Alabama have been consulted on many occasions during the progress of this study.

This material is based upon the work supported by the US Department of Energy DOE) under Award no. DE-SC0002470 (http://www.eng.auburn.edu/nepcm). This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise do not necessarily constitute or imply its endorsement, recommendation, or favouring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Lam, K. F., Sorensen, E., and Gavriilidis, A., 2011, “Towards an Understanding of the Effects of Operating Conditions on Separation by Microfluidic Distillation,” Chem. Eng. Sci., 66(10), pp. 2098–2106.
Xiong, B., Cheng, J., Qiao, Y., Zhou, R., He, Y., and Yeung, E. S., 2011, “Separation of Nanorods by Density Gradient Centrifugation,” J. Chromatogr., A, 1218(25), pp. 3823–3829.
Chen, H., Kaminski, M. D., Ebner, A. D., Ritter, J. A., and Rosengart, A. J., 2007, “Theoretical Analysis of a Simple Yet Efficient Portable Magnetic Separator Design for Separation of Magnetic Nano/Micro-Carriers From Human Blood Flow,” J. Magn. Magn. Mater., 313(1), pp. 127–134.
Liu, F., 2009, “Using Micellar Electrokinetic Chromatography for the Highly Efficient Preconcentration and Separation of Gold Nanoparticles,” J. Chromatogr., A, 1216(12), pp. 2554–2559.
Van der Bruggen, B., Mänttäri, M., and Nyström, M., 2008, “Drawbacks of Applying Nanofiltration and How to Avoid Them: A Review,” Sep. Purif. Technol., 63(2), pp. 251–263.
Vertellus, 2005, “Material Safety Data,”
Clary, D. R., and Mills, G., 2011, “Preparation and Thermal Properties of CuO Particles,” J. Phys. Chem., C, 115(5), pp. 1767–1775.
Coker, A. K., 2010, “Distillation: Part 1: Distillation Process Performance,” Ludwig's Applied Process Design for Chemical and Petrochemical Plants, 4th ed., Vol. 2, Gulf Professional Publishing, Boston, MA, pp. 1–268.
Yang, D., Martinez, R., Fayyaz-Najafi, B., and Wright, R., 2010, “Light Hydrocarbon Distillation Using Hollow Fibers as Structured Packings,” J. Membr. Sci., 362(1–2), pp. 86–96.
Sánchez, L. M. G., Meindersma, G. W., and Haan, A. B., 2009, “Potential of Silver-Based Room-Temperature Ionic Liquids for Ethylene/Ethane Separation,” Ind. Eng. Chem. Res., 48(23), pp. 10650–10656.
Shi-Chang, W., 1987, “Ten Years' Development on Distillation in China,” Desalination, 64, pp. 211–215.
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## References

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Liu, M., Saman, W., and BrunoF., 2012, “Review on Storage Materials and Thermal Performance Enhancement Techniques for High Temperature Phase Change Thermal Storage Systems,” Renewable Sustainable Energy Rev., 16(4), pp. 2118–2132.
Khodadadi, J. M., Fan, L., and Babaei, H., 2013, “Thermal Conductivity Enhancement of Nanostructure-Based Colloidal Suspensions Utilized as Phase Change Materials for Thermal Energy Storage: A Review,” Renewable Sustainable Energy Rev., 24, pp. 418–444.
Fan, L., and Khodadadi, J. M., 2011, “Thermal Conductivity Enhancement of Phase Change Materials for Thermal Energy Storage: A Review,” Renewable Sustainable Energy Rev., 15(1), pp. 24–46.
GunawanC., Teoh, W. Y., Marquis, C. P., and Amal, R., 2011, “Cytotoxic Origin of Copper(II) Oxide Nanoparticles: Comparative Studies With Micron-Sized Particles, Leachate, and Metal Salts,” ACS Nano, 5(9), pp. 7214–7225. [PubMed]
Heinlaan, M., Kahru, A., Kasemets, K., Arbeille, B., Prensier, G., and Dubourguier, H., 2011, “Changes in the Daphnia Magna Midgut Upon Ingestion of Copper Oxide Nanoparticles: A Transmission Electron Microscopy Study,” Water Res., 45(1), pp. 179–190. [PubMed]
Manusadžianas, L., Caillet, C., Fachetti, L., Gylytė, B., Grigutytė, R., Jurkonienė, S., Karitonas, R., Sadauskas, K., ThomasF., Vitkus, R., and Férard, J. F., 2012, “Toxicity of Copper Oxide Nanoparticle Suspensions to Aquatic Biota,” Environ. Toxicol. Chem., 31(1), pp. 108–114. [PubMed]
Wang, Z., Li, N., Zhao, J., White, J. C., Qu, P., and B.Xing, 2012, “CuO Nanoparticle Interaction With Human Epithelial Cells: Cellular Uptake, Location, Export, and Genotoxicity,” Chem. Res. Toxicol., 25(7), pp. 1512–1521. [PubMed]
Liu, F. K., Ko, F. H., Huang, P. W., Wu, C. H., and Chu, T. C., 2005, “Studying the Size/Shape Separation and Optical Properties of Silver Nanoparticles by Capillary Electrophoresis,” J. Chromatogr., A, 1062(1), pp. 139–145.
Lam, K. F., Sorensen, E., and Gavriilidis, A., 2011, “Towards an Understanding of the Effects of Operating Conditions on Separation by Microfluidic Distillation,” Chem. Eng. Sci., 66(10), pp. 2098–2106.
Xiong, B., Cheng, J., Qiao, Y., Zhou, R., He, Y., and Yeung, E. S., 2011, “Separation of Nanorods by Density Gradient Centrifugation,” J. Chromatogr., A, 1218(25), pp. 3823–3829.
Chen, H., Kaminski, M. D., Ebner, A. D., Ritter, J. A., and Rosengart, A. J., 2007, “Theoretical Analysis of a Simple Yet Efficient Portable Magnetic Separator Design for Separation of Magnetic Nano/Micro-Carriers From Human Blood Flow,” J. Magn. Magn. Mater., 313(1), pp. 127–134.
Liu, F., 2009, “Using Micellar Electrokinetic Chromatography for the Highly Efficient Preconcentration and Separation of Gold Nanoparticles,” J. Chromatogr., A, 1216(12), pp. 2554–2559.
Van der Bruggen, B., Mänttäri, M., and Nyström, M., 2008, “Drawbacks of Applying Nanofiltration and How to Avoid Them: A Review,” Sep. Purif. Technol., 63(2), pp. 251–263.
Vertellus, 2005, “Material Safety Data,”
Clary, D. R., and Mills, G., 2011, “Preparation and Thermal Properties of CuO Particles,” J. Phys. Chem., C, 115(5), pp. 1767–1775.
Coker, A. K., 2010, “Distillation: Part 1: Distillation Process Performance,” Ludwig's Applied Process Design for Chemical and Petrochemical Plants, 4th ed., Vol. 2, Gulf Professional Publishing, Boston, MA, pp. 1–268.
Yang, D., Martinez, R., Fayyaz-Najafi, B., and Wright, R., 2010, “Light Hydrocarbon Distillation Using Hollow Fibers as Structured Packings,” J. Membr. Sci., 362(1–2), pp. 86–96.
Sánchez, L. M. G., Meindersma, G. W., and Haan, A. B., 2009, “Potential of Silver-Based Room-Temperature Ionic Liquids for Ethylene/Ethane Separation,” Ind. Eng. Chem. Res., 48(23), pp. 10650–10656.
Shi-Chang, W., 1987, “Ten Years' Development on Distillation in China,” Desalination, 64, pp. 211–215.

## Figures

Fig. 1

(a) Schematic diagram of a nanoparticle with long ligands coated on its surface serving as the stabilizing cushion layer and (b) Ligands on particle surface to provide a physical barrier (cushion) which prevents particle contact and subsequent agglomeration

Fig. 2

NEPCM preparation

Fig. 3

NEPCM before and after distillation

Fig. 4

Laboratory vacuum distillation unit

Fig. 5

Collected distillate; (a) atmospheric pressure distillation and (b) vacuum distillation

Fig. 6

Distillation data comparison on total energy and total time basis

Fig. 7

SEM images of: (a) dodecane, (b) NEPCM, (c) distillate after atmospheric pressure distillation, and (d) distillate after vacuum distillation; the 1 μm scale is shown at the lower right corner of (b)

Fig. 8

NEPCM and alcohol mixture

Fig. 9

Successive reduction of NEPCM volume by mixing and shaking in alcohol mixture

Fig. 10

UV-Vis spectrum of samples. Baseline is for pure dodecane and Run1 is for the clear liquid after first mixing as shown in Fig. 9.

Fig. 11

Centrifugation: Nanoparticle precipitation. Sample: volume: 13 ml, speed: 18,000 rpm, duration of centrifugation: 19.5 h. (a) 0.5% conc. (by mass) repeated once, (b) 2% conc. (by mass), and (c) 5% conc. (by mass).

Fig. 12

SEM image of the centrifuged sample (a) in Fig. 11; the 10 nm scale is on the lower right corner

Fig. 13

(a) Before centrifugation, (b) after 24 h of centrifugation, and (c) after 48 h of centrifugation

Fig. 14

(a) UV-Vis spectra of 2 wt. % NEPCM and sample 3 (after 24 h) and (b) UV-Vis spectra of 1 wt. % NEPCM and sample 5 (after 48 h)

Fig. 15

Calibration curve for centrifugation; (a) sample 2 and (b) sample 3

## Tables

Table 1 Mass of CuO nanoparticles required
Table 2 Distillation data summary
aReading from the energy meter. For vacuum distillation, the energy meter was connected in series with both the distillation heater and the vacuum pump.
Table 3 Centrifugation (18,000 rpm) efficiency results
Table 4 Separation methods comparison

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