Preparation and stability of silver/kerosene nanofluids
© Li and Fang; licensee Springer. 2012
Received: 22 March 2012
Accepted: 2 July 2012
Published: 2 July 2012
A series of silver nanoparticles surface-coated with di-n-dodecyldithiophosphate, di-n-cetyldithiophosphate, or di-n-octadecyldithiophosphate have been prepared and have good dispersity in alkanes or kerosene. Stable silver nanofluids can be formed in alkanes or kerosene with the surface-coated silver nanoparticles. Thermal stability of the silver nanofluids has been measured at different temperatures. The effects of the silver nanoparticles on the thermal oxidation of kerosene have been investigated at different temperatures. The coatings can be released from the surface of the silver nanoparticles above 150°C, giving oxygen access to the silver core and inhibiting the kerosene oxidized by oxygen.
Nanofluids, nanometal, or metal oxide particles suspended in traditional fluids (water, ethylene glycol, and engine oils) are receiving much more attention in recent years because of the advantages of their heat transfer capacities and broad application prospects [1–5]. Metal nanoparticles with low dispersity in paraffin oils or nonpolar organic solvents show their limited applications. Nanofluid production faces major challenges, such as poor dispersity of metal nanoparticles, agglomeration of particles in base fluids, and the rapid settling of particles in fluids .
Silver nanoparticles have been applied in many science and technology areas such as antimicrobial materials, photonics, surface-enhanced Raman scattering, electronics, and catalysis [6–11]. Sometimes, silver nanoparticles with low dispersity in oils or nonpolar organic solvents show their limited applications. Surfactants and extractants such as oleic acid [7, 8], alkylthiol [9, 10], alkylamine , and di(2-ethylhexyl) phosphoric acid  have been employed to coat or modify the surfaces of the metal nanoparticles to improve the dispersity in paraffin or oils. Dialkyl dithiophosphates (DDP, (RO)2PS2−), which are composed of nonpolar alkyl hydrocarbon tails and polar head groups, are appropriate for coordinating with the metal nanoparticle surfaces. Dialkyl dithiophosphates and their derivatives are versatile ligands. Metal complexes with DDP have been widely used in application as antioxidants, lubricant additives, and bioactive agents [13–15].
This work describes the preparation of a series of surface-coated silver nanoparticles using dialkyl dithiophosphates with different hydrocarbon chain lengths (C12, C16, C18) as efficient bonding ligands. Differences of the nanoparticle size, size distribution, and hydrophobicity have been investigated. The thermal stability of silver-kerosene nanofluids is studied. The effects of the silver nanoparticles on the thermal oxidation stability of kerosene are discussed by monitoring the hydroperoxide formation during the thermal oxidation. The results may provide important information on the study and application of additives of oil and fuel.
Silver nitrate, ascorbic acid, cyclohexylamine, and ethanol which are analytically pure reagents were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The coating ligands, pyridinium di-n-dodecyldithiophosphate (DDP12), pyridinium di-n-cetyldithiophosphate (DDP16), and pyridinium di-n-octadecyldithiophosphate (DDP18) were synthesized in our laboratory according to the literature procedures .
Preparation of nanoparticles
The following typical preparation process is described. A mixture is formed with 0.5 mmol dialkyl dithiophosphate dissolved in a mixed solvent of deionized water, ethanol, and cyclohexylamine. The silver nitrate solution (2 mmol, 20 mL) was added into the above mixture, which was heated up to 60°C under stirring. Excess ascorbic acid (0.05 g/mL, 20 mL) was then added, and the reaction was allowed to last for 3 h under vigorous stirring. After that, the mixture was moved to be cooled and kept at the ambient temperature. The powders were separated by centrifugation and washed with ethanol and water several times. Finally, the surface-coated nanoparticles were dried in a vacuum oven at 40°C. A series of nanoparticles were synthesized by similar preparation methods with different ligands and reaction temperatures. The nanoparticles are dispersed in nonpolar solvents such as chloroform, dichloromethane, or kerosene to form different nanofluids or colloids.
The X-ray diffraction (XRD) patterns of silver nanoparticles were recorded by employing a Bruker D8 Advance X-ray diffractometer (Bruker Optik GmbH, Ettlingen, Germany) with monochromatized Cu Kα radiation (λ = 1.5405 Å). A transmission electron microscope (TEM; CM-200, FEI-Philips, Hillsboro, OR, USA) was used to determine the size and morphology of the silver nanoparticles. Samples were dispersed in dichloromethane and prepared by placing a drop on a carbon-coated standard copper grid. The particle size distribution was analyzed by measuring over 200 particles from the TEM micrographs and using a digital micrograph software. The silver substrates were prepared by depositing several drops of silver nanoparticle suspension over glass slides and waiting for solvent evaporation. The metal content of the silver nanoparticles was determined by thermogravimetry (TG) analysis. Measurements of the particle absorption band were performed using a UV-1770 spectrophotometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan) equipped with 1.0-cm quartz cells at 25°C.
The silver-kerosene nanofluids (0.1 mass% silver nanoparticles) were thermally oxidized in an isothermal apparatus . Each test tube containing a 100-mL sample of silver nanofluids was placed in the heated test well. The flow meter was used to regulate the oxygen flow with the rate of 30 mL/min into each sample through a gas dispersion tube. The investigated samples were subjected to thermal oxidation at 120°C, 140°C, and 150°C. A small aliquot (<0.5 mL) of the samples was removed from the test tubes at fixed time intervals for the hydroperoxide measurements. The hydroperoxides formed in the samples during the thermal oxidization process were determined by measuring the absorption spectra of the iodine-starch solutions [16, 17] using ultraviolet–visible (UV–vis) spectrometry.
Results and discussion
XRD, TEM, TG, and UV–vis analyses of silver nanoparticles
The silver nanoparticles, AgDDP16 and AgDDP18, were synthesized by a similar procedure for AgDDP12. The growth of the metallic silver core was suppressed by the addition of dithiophosphates. In the preparation process, dithiophosphates coated quickly on the surface of the silver core, and the silver nanoparticles with coating layer have different solubilities in the reaction system, which is a mixture of water, ethanol, and cyclohexylamine. The solubility of the three dialkyl dithiophosphates of various chain lengths (C12, C16, C18) in the reaction system was different at 60°C. The dithiophosphate ligands with longer chain length (DDP16 and DDP18) are not completely soluble in the mixed solution of cyclohexylamine/water at 60°C. It had difficulty in capping the surface of the newly generated particles, which may make the silver nanoparticles to be partly coated, and the particle size of AgDDP16 and AgDDP18 was inhomogenous. So, the size of the silver particles was not controlled well by DDP16 and DDP18. At 60°C, AgDDP18 separated more quickly than AgDDP16 and AgDDP12. Therefore, the size distributions of the silver nanoparticles coated with longer chain ligands were wider than those coated with shorter chain ligands at 60°C.
Metal contents of the nanoparticles prepared at different temperatures
Prepared temperature (°C)
Mental content (%)
Thermal stability of the silver nanofluids
Effects of silver nanoparticles on the thermal oxidation stability of kerosene
For the reduction synthesis of silver nanoparticles, we use silver nitrate, ascorbic acid, cyclohexylamine, and dialkyl dithiophosphates of various chain lengths (DDP12, DDP16, DDP18), respectively, to serve as precursor, reducing agent, additive, and surface coating reagent. The effects of the chain length of dialkyl dithiophosphate on size and dispersibility of the obtained silver nanoparticles are different. The silver nanoparticles coated with DDP show typical hydrophobicity. DDP12 is better than DDP16 and DDP18 for coating and forming small and uniform silver nanoparticles. The silver nanofluids are stable in a certain time limit at high temperatures because of the surface coating of the ligands. The silver nanoparticles surface-coated by dithiophosphates with good oil dispersity are thus suitable for preparing oil-based nanofluids, which is in favor of the thermal oxidation stability of oil at higher temperatures.
The authors are grateful for the financial supports from the National Natural Science Foundation of China (No. 21103129 & 20973154).
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