The Extension of Colloid Chemistry from Aqueous to Non-Aqueous Media with Application to Nanofluid Research

Abstract

Nanofluids are colloids whose design is specific to the study and application of heat transfer systems. Metals typically have thermal conductivities which are several times higher than that of traditional cooling liquids such as water or ethylene glycol. The suspension of metal particles in a cooling fluid is expected to yield a nanofluid with enhanced thermal transport properties relative to the base solvent. During the course of the current study, several colloidal suspensions were prepared and the thermal properties of the resulting suspensions were evaluated when appropriate. Due to the surplus of literature available which describes aqueous colloids, the current work will diverge and place the bulk of its emphasis on suspensions in nonpolar solvents. Cyclodextrins are cyclic molecules composed of glucose units. The inner cavity of cyclodextrins is noted for its ability to form stable inclusion complexes with a wide variety of guests. A cyclodextrin-glucose host-guest complex was prepared and utilized as both a salt reductant and a particle stabilizer in the generation of aqueous metal colloids including Ag, Au, Pd, and Pt. The resulting colloids demonstrated remarkable stability—3 years and running, in some cases—and have been evaluated for thermal conductivity. Evaluation of the reaction products when the complex is used to reduce Pd2+ demonstrated a unique comproportionation reaction in which the guest undergoes a two electron oxidation to produce a Pd atom. The resulting atom reduces a neighboring Pd2+ ion to yield two Pd+ ions. The monovalent species, in contrast to Pd2+, can then oxidize the host to form atoms which rapidly aggregate to yield particles. Highly stable, crystalline copper(II) oxide particles were prepared which can be isolated as a powder and redispursed in low dielectric media such as hydrocarbons or chloroform. Mass concentrations of up to 20% (1.65 M) were achieved in octane, dodecane, and eicosane and remained stable for at least ten days at room temperature as observed by visible spectroscopy. Quasi-spherical particle shape was observed with the largest fraction possessing a diameter of 9 nm and 90% of the population existing within the range of 5 to 15 nm. The colloidal systems were characterized using FAA, XRD, TEM, UV-Vis, DSC, and a simple device inspired by Newton’s Law of cooling which was employed to measure cooling/heating rates. Thermodynamic measurements of sodium oleate-stabilized CuO particles suspended in dodecane and eicosane reveal a decrease in Cp, , and cooling/heating rates of the resulting colloid with large increases in particle mass concentration. Irradiation with 350 nm photons of anhydrous, air-free octane or toluene solutions of copper(II) oleate containing benzophenone as a photosensitizer and oleoylsarcosine as a stabilizer resulted in metallic Cu particles with nanometer dimensions. Evidence is presented that implicates the hydrocarbon as the predominant H-atom donor in the generation of reductive benzophenone ketyl radicals and a kinetic model is constructed to rationalize the rate dependencies with respect to the Cu2+/Cu+ step. Rates of both Cu2+ consumption and Cu formation vary linearly with light intensity and exhibit a first-order dependence on benzophenone concentration but the latter step shows little dependence otherwise. The initial rate of reactant consumption decreases with increasing concentration of cupric ions or sarcosine. Quenching of the excited state of benzophenone by the stabilizer occurs with a rate constant of k4 = 1.6 × 105 M-1 s-1 and is explained by the formation of a contact ion pair between the reduced chromophore and oxidized sarcosine which ultimately decays by back electron transfer. The rate decrease induced by cupric ions results from the quenching of the excited state of benzophenone by the copper(II) complex with a rate constant of k5 = 6.1 × 105 M-1 s-1. The resulting colloids proved to be stable in an anaerobic environment for at least a month and require more than 12 hours to oxidize upon exposure to air. Upon removal of the octane solvent, the particles can be redispersed in a variety of low dielectric media such as chloroform, carbon tetrachloride, hexane, or toluene. UV irradiation of octane solutions containing Ag neodecanoate, Pd(acac)2, or Pt(acac)2 in the presence of benzophenone and oleoyl sarcosine resulted in crystalline metal particles. Rates of metal formation in the absence of BP for Pd(acac)2 and Pt(acac)2 were ri = 3.4×10-8 M/s and ri = 4.7×10-8 M/s, respectively, which are 2-4 times slower than the analgous reactions conducted in the presence of the chromophore. The direct irradiation of Ag(OOR), on the other hand, resulted in no reaction. In the presence of BP, silver atoms were formed with a rate constant of 4.2×10-7 M/s. The resulting octane colloids were evaluated for enhancements in thermal conductivity (TC) using the Thermal HotDisk method. Increases in krel of up to 10% were observed for the Ag and Pt systems at [M] = 5 mM which are far larger than what Maxwell's theory predicts for a colloid of such low volume fraction (~5×10-5 vol%).