Phase Transfer Properties of Silver Nanoparticles
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Silver nanoparticles are synthesized by reduction of silver nitrate in a toluene solution of tetra-n-octyl ammonium bromide (TAB) and dodecanethiol (DDT). The nanoparticles consist of a silver core with DDT chemically bound at the particle surface. TAB remains associated with the particles and provides them extra stability. These passivated silver nanoparticles are dissolved in dry chloroform and various phase transfer phenomena across the liquid-liquid interface are investigated. When a mixture of the polar solvent mixture ethanol/water is added to the chloroform solution containing Ag nanoparticles, two liquid layers form, with the denser and relatively apolar chloroform at the bottom. When the top aqueous layer (EtOH/H2O) contains 11-mercaptoundecanoic acid (MUA), exchange of DDT with MUA occurs, leaving negatively charged ligand chains. However, with TAB present, the nanoparticles are not extracted to the aqueous layer. When the nanoparticles are washed with ethanol, the concentration of TAB can be reduced. When these washed particles are dissolved in chloroform and added to the aqueous solution of MUA, extraction occurs. When fully extracted, HCl can be added to neutralize the MUA passivation layer of the nanoparticles, forcing them back into the apolar chloroform layer again. To alter the surface chemistry of the passivation layer, Ag nanoparticles were refluxed in excess DDT to increase the number of thiol ligands bound to the silver particle. These particles are not extracted by MUA. Quantitative measurements and interpretations of these effects are presented.